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Lysimeter trials to assess the impact of different flood-dry-cycles on the dynamics of pore water concentrations of As, Cr, Mo and V in a contaminated floodplain soil
Abstract
We hypothesize that the dynamics of water soluble arsenic (As), chromium (Cr), molybdenum (Mo), and vanadium (V) in soils might be controlled by the period of flooding due to changes of redox potential (EH), pH, and carriers of metals such as dissolved organic carbon (DOC), iron (Fe), manganese (Mn) and sulfate (SO42 −). Therefore, we aimed to assess the impact of different flood–dry-cycles on the temporal dynamics of pore water concentrations of As, Cr, Mo and V as affected by changes of soil EH/pH and dynamics of DOC, Fe, Mn and SO42 − in a contaminated floodplain soil collected at the Elbe River (Germany). For this purpose a specific groundwater lysimeter technique with two separate lysimeters which served as replicates was used. The groundwater level inside the lysimeters was controlled to simulate different flood–dry-cycles sequentially as follows: the long term (LT) includes 94 days of flooding followed by similar drying term. The short term (ST) comprises 21 days flooding followed by a similar drying term and was repeated six times. The entire experimental period (LT_ST) was about 450 days. The presented data are mean values of both lysimeters.
Supplementary resource (1)
Data
February 2014
... The changes in soil pH and Eh will alter the solubility and availability of heavy metal(loid)s through a series of complex chemical reactions. Under flooded soil, the elevated sorption on amorphous (hydr)oxides of Fe/Mn and/or OM, as well as precipitation with sulfide, might have contributed to a decline in bioavailability of a majority of metals (Cd, Cu, Ni, Pb, and Zn) (Svetlana et al., 2017;Izquierdo et al., 2017;Yunyu et al., 2015), whilst the reductive dissolution of Fe-Mn-(hydr) oxides and OM decomposition also releases concomitant metal(loid)s into solution (Sheng-Huie et al., 2015;Yoshio et al., 2004;Shaheen et al., 2014b). Additionally, under low Eh or alkaline pH conditions, a significant increase of arsenic solubility in soil was observed due to both the reduction of As (V) to As (III) with the latter being largely more toxic and soluble than the former, and the solubilization of ferric arsenate (Yoshio et al., 2004;Shaheen et al., 2014b;Anamika et al., 2017). ...
... Under flooded soil, the elevated sorption on amorphous (hydr)oxides of Fe/Mn and/or OM, as well as precipitation with sulfide, might have contributed to a decline in bioavailability of a majority of metals (Cd, Cu, Ni, Pb, and Zn) (Svetlana et al., 2017;Izquierdo et al., 2017;Yunyu et al., 2015), whilst the reductive dissolution of Fe-Mn-(hydr) oxides and OM decomposition also releases concomitant metal(loid)s into solution (Sheng-Huie et al., 2015;Yoshio et al., 2004;Shaheen et al., 2014b). Additionally, under low Eh or alkaline pH conditions, a significant increase of arsenic solubility in soil was observed due to both the reduction of As (V) to As (III) with the latter being largely more toxic and soluble than the former, and the solubilization of ferric arsenate (Yoshio et al., 2004;Shaheen et al., 2014b;Anamika et al., 2017). In contrast, redox process takes places reversibly under oxidizing conditions (non-flooded soil). ...
... The increase in As concentration with flooding duration was mainly attributed to the reducing of arsenate to arsenite under flooded conditions. However, under an alkaline soil condition in the present study, it is expected that As would be released substantially from soil components, particularly in flooded soil, consequently increasing available As (Shaheen et al., 2014b;Nanthi et al., 2014). But this case did not occur, possibly because As was co-precipitated with the iron oxyhydroxides under moderately reduced conditions (Nanthi et al., 2014). ...
Continuous flooding has been widely used in paddy field to decrease the accumulation of heavy metal(loid)s by rice due to their decreased solubility and bioavailability of most heavy metal(loid)s under water flooding condition. A field experiment with six drainage treatments during grain-filling stage was performed to investigate the influence of different water flooding conditions on availability of cadmium (Cd), lead (Pb), chromium (Cr), arsenic (As), and mercury (Hg) and their accumulation by rice in alkaline paddy soil. The results showed at the end of experiment, the availability of Cd and Pb in soil with continuous flooding for above 25 days after full heading significantly decreased compared with draining immediately after full heading of rice. But some increase of the As availability in soil with different water regimes was observed at 15 days after beginning of experiment where the As availability was below its detection limit. Meanwhile, the concentrations of Cd and Cr both in rice grains and straws were decreased evidently with prolonged flooding at rice filling stage. The concentrations of Pb and Hg in grain were all below the detection limit, but increased in straw with draining 15 days later compared with draining immediately after full heading of rice. However, water regimes during grain-filling stage had little effect on As uptake by rice in alkaline paddy soil. Moreover, this study also discovered that organic matter may played a critical role in controlling availability of heavy metal(loid)s in alkaline soil. This work demonstrated that at alkaline paddy soil, maintaining water flooding until 5 days before harvest in rice grain-filling stage was an effective method to improve rice safety without decreasing yields in paddy field polluted with Cd, Cr, and As, but careful consideration is required for Pb and Hg.
... For the investigation of element and ion concentration changes in pore water, lysimeter studies can be used, which allows element monitoring in soil columns of intact soil structure, as well as a close monitoring of various related parameters such as the Eh (Meissner, Rupp, & Haselow, 2020;Rupp, Meissner, & Leinweber, 2018;Rupp, Rinklebe, Bolze & Meissner, 2010;Shaheen, Rinklebe, Rupp, & Meissner, 2014). Usually, leachate of whole soil columns has been investigated (Andersson, Bergström, Ulén, Djodijic, & Kirchmann, 2015;Siddique, Robinson, & Alloway, 2000), and studies analyzing pore water from different soil depths have most often been done with suction cups or suction plates (Koch, Kahle, & Lennartz, 2019;Siemens & Kaupenjohann, 2002). ...
... In fact, it leveled out with time, meaning that Eh changes became smaller with each reoccurring high-water event. This phenomenon was also observed for Depth 2 and 3 of the upper and mid-slope profiles, as well as for Depth 2 of the toe slope profile, and was noted before in a lysimeter study conducted by Shaheen et al. (2014) investigating heavy metals. In the present study, as well as in the study by Shaheen et al. (2014), it may be explained by a general decrease in dissolved C concentration over time, which could be due to an exhaustion of the labile soil organic matter pool, as well as a decrease or change in microbial biomass due to reoccurring rewetting events (Chen, Lai, Zhao, Li, & Lin, 2016;Van Gestel, Merckx, & Vlassak, 1993). ...
... This phenomenon was also observed for Depth 2 and 3 of the upper and mid-slope profiles, as well as for Depth 2 of the toe slope profile, and was noted before in a lysimeter study conducted by Shaheen et al. (2014) investigating heavy metals. In the present study, as well as in the study by Shaheen et al. (2014), it may be explained by a general decrease in dissolved C concentration over time, which could be due to an exhaustion of the labile soil organic matter pool, as well as a decrease or change in microbial biomass due to reoccurring rewetting events (Chen, Lai, Zhao, Li, & Lin, 2016;Van Gestel, Merckx, & Vlassak, 1993). This explanation is well in line with declining CO 2 respiration flushes frequently observed during soil DRW cycles in laboratory studies (Chow, Tanji, Gao, & Dahlgren, 2006;Fierer & Schimel, 2002;Mikha, Rice, & Milliken, 2005) and suggests a relationship between CO 2 flush and Eh intensity. ...
For arable soils, it is not clear whether closing a controlled drainage system leads to P mobilization due to water table rise and associated changes in redox‐induced biogeochemical processes. Therefore, we investigated P mobilization at different redox conditions using three spring barley (Hordeum vulgare L.) cropped lysimeters filled with monoliths of arable northeastern German soil profiles. Pore water samples were collected weekly from three different depths, and dissolved (<0.45 μm) element concentrations of total C, P, Al, Fe, Mn, Ca, Mg, and K, as well as inorganic and organic C (DIC and DOC) and P (Pi and Po), SO4²⁻–S, and NO2⁻–N and NO3⁻–N were determined. The total P concentration in pore water collected from a given lysimeter at a given time was 1.8 mg P L⁻1 maximum. Organic P concentrations in subsoil solutions were positively correlated with Fe concentrations. Grain yield of spring barley ranged between 5.6 and 6.5 Mg ha⁻1, and total biomass P uptake was negatively correlated with the stable P stocks of the soil profiles. Results suggest that reductive conditions in subsoils led to dissolution of pedogenic Fe‐(oxy)hydroxides and release of Po compounds, the latter of which were more important for biomass P uptake than P released from stable P compounds. Overall, closing the drainage at the field site could represent a moderate P mobilization risk, which would probably be lower compared with a P mobilization risk caused by a heavy rainfall event.
... Remobilisation of PTEs from sediments into the overlying water column during a flooding event depends on the flood regime; the frequency of these intense floods which flush or remobilise contaminated material as well as the duration or alternation of the flood with dry spells (Arnell et al., 2015;Foulds et al., 2014;González-Alcaraz and van Gestel, 2015). While research has suggested that the longer the flood duration, the greater the metal mobility (Shaheen et al., 2014a(Shaheen et al., , 2014b, Stafford et al. (2018) suggest that even short periods of soil saturation can have an influence the solubility of PTEs. ...
... There are important redox sensitive PTEs for which the oxidation state has a large influence on solubility and mobility. For example, Cr(VI) is more mobile than Cr(III), but As(V) is less mobile than As(III) (Frohne et al., 2015;Rinklebe et al., 2016;Schulz-Zunkel et al., 2015;Shaheen et al., 2014b;Yang et al., 2015). Speciation of PTEs within the environment has a distinct influence upon their behaviour; specifically, reactivity, toxicity, mobility and bioavailability within the floodplain (Du Laing et al., 2009;Gambrell, 1994;Hooda, 2010;Rodgers et al., 2015). ...
... These processes include; sorption, desorption, dissolution and precipitation (Puchalski, 2003;Wijngaard et al., 2017). Subsequently, PTEs are redistributed into different geochemical fractions, associated with other soluble species, released from the soil matrix into the soil solution or porewater, and transferred through the ecosystem and food web to other terrestrial or riparian areas downstream from the floodplain; thus potentially becoming a risk to human and environmental health (Adamo et al., 2014;Adewuyi and Osobamiro, 2016;Baran and Tarnawski, 2015;Dang et al., 2002;Du Laing et al., 2009;Rinklebe et al., 2016;Schulz-Zunkel et al., 2015;Shaheen et al., 2014aShaheen et al., , 2014bSizmur et al., 2011). Sorption processes that control PTEs mobility and bioavailability in soil are affected by the soil pH, redox and their interactions with other ions and substances present in soil solution (Antoniadis et al., 2018;Frohne et al., 2011;Ostergren et al., 2000;Violante, 2013). ...
The frequency and duration of flooding events are increasing due to land-use changes increasing run-off of precipitation, and climate change causing more intense rainfall events. Floodplain soils situated downstream of urban or industrial catchments, which were traditionally considered a sink of potentially toxic elements (PTEs) arriving from the river reach, may now become a source of legacy pollution to the surrounding environment if PTEs are mobilised by unprecedented flooding events.
When a soil floods, the mobility of PTEs can increase or decrease due to the net effect of five key processes; (i) the soil redox potential decreases which can directly alter the speciation, and hence mobility, of redox sensitive PTEs (e.g. Cr, As), (ii) pH increases which usually decreases the mobility of metal cations (e.g. Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, Zn²⁺), (iii) dissolved organic matter (DOM) increases, which chelates and mobilises PTEs, (iv) Fe and Mn hydroxides undergo reductive dissolution, releasing adsorbed and co-precipitated PTEs, and (v) sulphate is reduced and PTEs are immobilised due to precipitation of metal sulphides. These factors may be independent mechanisms, but they interact with one another to affect the mobility of PTEs, meaning the effect of flooding on PTE mobility is not easy to predict. Many of the processes involved in mobilising PTEs are microbially mediated, temperature dependent and the kinetics are poorly understood.
Soil mineralogy and texture are properties that change spatially and will affect how the mobility of PTEs in a specific soil may be impacted by flooding. As a result, knowledge based on one river catchment may not be particularly useful for predicting the impacts of flooding at another site. This review provides a critical discussion of the mechanisms controlling the mobility of PTEs in floodplain soils. It summarises current understanding, identifies limitations to existing knowledge, and highlights requirements for further research.
... Several authors have undertaken experiments ex-situ that simulate flooding conditions and observed that the inundation of floodplain soils re-mobilises TEs, such as Ni, Cr, Cu, Pb and Zn in the soil (Tack 2006;Du Laing 2007;Shaheen 2014bShaheen , 2014cSchulz-Zunkel 2015;Izquierdo 2017). Therefore, the effect that future climate and land-use change will have on flooding and hence TE mobilisation may turn floodplain soils into intermittent sources of pollution to rivers (Coulthard 2003;Dennis 2003). ...
... Previous studies have involved ex-situ laboratory studies of disturbed soils which have been dried and homogenized (Du Laing 2007;Schulz-Zunkel 2015;Shaheen 2017a), albeit with some limited exceptions (Shaheen 2014b(Shaheen , 2014c. Homogenisation removes important elements of soil architecture such as macropores, plants roots, and stones which are likely to have a particularly significant effect on the rhizosphere and the microbiology of the soil. ...
... A further limitation of many studies is that changes in mobility are often attributed to a change in the speciation of a redox sensitive element (e.g. As or Cr), but this is mostly based on inference alone rather than actual measurements of speciation (Shaheen 2014b;Rennert 2017). As well as being crucial to the understanding of TE mobility, different species often have different levels of toxicity. ...
Floodplains downstream of urban catchments are sinks for potentially toxic Trace Elements (TEs). An intensification of the hydrological cycle and changing land use will result in floodplains becoming inundated for longer durations in the future. We collected intact soil cores from a floodplain meadow downstream of an urban catchment and subjected them to an inundation/drainage cycle in the laboratory to investigate the effect of flood duration on TE concentrations in the soil porewater. The porewater concentrations of Ni, Cr and Zn increased, while Cu and Pb decreased, with flood duration. All the Cr present in porewaters was identified as Cr(III). Cu concentrations increased after drainage, but Pb mobility remained suppressed. Both pH and dissolved organic carbon (DOC) increased with flood duration but were lower in treatments that were drained for the longest (which were also the treatments flooded for the shortest duration). The porewater concentrations of Cr and Ni decreased after drainage to levels below that observed prior to inundation, mirroring the DOC concentrations. We conclude that the duration of floodplain inundation does have an influence on the environmental fate of TEs but that flooding does not influence all TEs in the same way. The implications of an intensification of the hydrological cycle over the coming decades are that floodplains may become a source of some TEs to aquatic and terrestrial ecosystems.
... Arsenic is generally known to show higher concentrations in aqueous solutions under low E h than high E h conditions. Shaheen et al. [4] proposed three possible mechanisms to explain the behavior of the higher As concentration under reducing conditions in their flood-dry-cycle lysimeter experiments: 1) direct reduction of As(V) to more soluble As(III) in the presence of higher DOC, 2) release of As due to the dissolution of adsorbing phases such as Feand Mn-oxides and oxyhyroxides, organic matter, and sulfate, and 3) increase of pH under high Eh. ...
... For example, geochemical conditions play an important role in determining As sequestration via the formation of insoluble arsenic sulfide, or as release due to the formation of aqueous thioarsenates under sulfate- reducing conditions [6,24]. The importance of redox conditions for the mobility of As has been investigated in many previous studies that have proved the interrelationship between As and other redox sensitive environmental factors using laboratory experiments subjected to redox cycles [3,4,12,[25][26][27]. Couture et al. [12] investigated the influence of successive redox oscillations on the fate of aqueous contaminants (e.g., Cr, As, Se, Sb, and U) in natural soil systems. ...
... Their studies revealed that such redox oscillations lead to an on-off switch mobility behavior among the major and trace elements. Based on the laboratory experiments by LeMonte et al. [25] simulating sea water inundation, high SO 4 2− concentrations affect the decrease of soluble As by suppressing dissimilatory reduction of As(V) and Fe(III), and causing the precipitation of Fe-S, As-S, or Fe-As-S as a result of preferential or concurrent SO 4 2− reduction. Although many studies have been conducted on Fe-S-As interrelationships under changing redox cycles, the fate and speciation of As and Fe in solid phases has not yet been intensively investigated. ...
Changes in the saturation degree of aquifers control the geochemical reactions of redox-sensitive elements such as iron (Fe), sulfur (S), and arsenic (As). In this study, the effects of redox conditions and the presence of Fe and S on the behavior of As in a soil environment were investigated by observation in a batch experimental system. Arsenic was stable on Fe(III) solid surface in an oxidizing environment but was easily released into the aqueous phase following the reductive dissolution of Fe during an anoxic period. The alternating redox cycles led to a change in the concentrations of Fe, S, and As in both the aqueous and solid phases. The composition of Fe minerals changed to a less crystalline phase while that of solid phase As changed to a more reduced phase in both the As-contaminated natural soil and FeS-amended soil batch systems. This tendency was more prominent in the batch containing higher amounts of total Fe and S. These results show that a redox cycle can increase the possibility of As contamination of groundwater during dissolution and reprecipitation of Fe minerals and simultaneous microbial reduction of S and/or As species.
... Previous works that studied As mobilisation and its speciation mainly were in temperate countries and ecosystems such as flood plains (Frohne et al., 2011;Shaheen et al., 2014aShaheen et al., , 2016, agricultural soils (Beiyuan et al., 2017), and coastal sediments (Lemonte et al., 2017), with limited focus on tropical rainforest environments and As-contaminated abandoned mine spoils. Such detailed investigation focusing on the biogeochemistry and potential governing factors controlling the availability of As under varying soil redox conditions in Ghana will aid to effectively plan mitigation measures against As release into watercourses in gold mining areas. ...
... Gu et al., 2019). Rinklebe et al. (2016) and Shaheen et al. (2014a) have also reported that reductive dissolution of Fe(hydro) oxides under low E H led to the release of associated As into the soil solution. Positive correlation between As and Fe, as well as influence of Fe on As solubility under redox conditions has been reported by other authors (e.g. ...
Arsenic (As) redox-induced mobilisation and speciation in polluted gold mine sites in tropical climates largely remains unknown. Here, we investigated the impact of changes in soil redox potential (EH) (-54 mV to +429 mV) on mobilisation of As and its dominant species in an abandoned spoil (total As = 4,283 mg/kg) using an automated biogeochemical microcosm set-up. Arsenic mobilisation increased (85-137 mg/L) at moderately reducing conditions (-54 mV to + 200 mV)), while its reduced (6-35 mg/L) under oxic conditions (+200 to +400 mV). This indicates the high risk of As potential loss under reducing conditions. The mobilisation of As was governed by the redox chemistry of Fe. XANES analysis showed that sorbed-As(V)-goethite, sorbed-As(III)-ferrihydrite, scorodite and arsenopyrite were the predominant As species in the mine spoil. As(V) dominated at oxic conditions and As(III) predominated at moderately reducing conditions, which may be attributed to either inability of arsenate bacteria to reduce As or incomplete reduction. Lower Fe/As molar ratios during moderately reducing conditions show that the mine spoil may migrate As to watercourses during flooding, which may increase the hazardous effects of this toxic element. Therefore, encouraging aerobic conditions may mitigate As release and potential loss from the mine field.
... Previous works that studied As mobilisation and its speciation mainly were in temperate countries and ecosystems such as flood plains (Frohne et al., 2011;Shaheen et al., 2014aShaheen et al., , 2016, agricultural soils (Beiyuan et al., 2017), and coastal sediments (Lemonte et al., 2017), with limited focus on tropical rainforest environments and As-contaminated abandoned mine spoils. Such detailed investigation focusing on the biogeochemistry and potential governing factors controlling the availability of As under varying soil redox conditions in Ghana will aid to effectively plan mitigation measures against As release into watercourses in gold mining areas. ...
... Gu et al., 2019). Rinklebe et al. (2016) and Shaheen et al. (2014a) have also reported that reductive dissolution of Fe(hydro) oxides under low E H led to the release of associated As into the soil solution. Positive correlation between As and Fe, as well as influence of Fe on As solubility under redox conditions has been reported by other authors (e.g. ...
Redox-induced release dynamics of arsenic (As) in an abandoned geogenic arsenic-contaminated gold mine spoil in Ghana has never been studied. Therefore, our aim is to investigate the effects of varied soil redox conditions on the mobilisation of As in an abandoned highly contaminated gold mine spoil (with 4,283 mg As/kg soil) using an automated biogeochemical microcosm set-up. We also studied the impact of redox potential (EH)-dependent changes of pH, Fe, Mn, Al, S, Cl-, SO42-, DOC, DIC, DC, DN and SUVA on the release dynamics of As. The XANES spectroscopy analysis results indicated that Scorodite (FeAsSO4) and arsenopyrite (FeAsS) are the two major As-containing minerals in the studied mine spoil. Geochemical fractionation using sequential extraction procedure indicates greater proportions of the extracted As in the amorphous iron oxide fraction III (1390.13 mg kg-1, 32.5% of the total As) and residual fraction V (2591.67 mg kg-1, 60.5% of the total As). Concentrations of dissolved Fe and SUVA were higher during reducing conditions and decreased under oxidising conditions and both showed negative significant relationships with EH (EH and SUVA: r = -0.76, P < 0.01; EH and Fe: r = -0.75; P < 0.01). Mobilisation of As was greater under reducing conditions (dissolved As = 136.68 mg/L) than in oxidising environments (dissolved As = 8.06 mg/L). The release of As under low EH can be explained by the associated reductive dissolution of Fe oxides, as demonstrated by the high positive significant relationship between Fe and As (r = +0.97, P < 0.01). Dissolved As release dynamics can also be linked to desorption of aromatic carbon compounds on the surfaces of dissolved organic carbon, as demonstrated by the high positive significant correlation between SUVA and As (r = +0.573, P < 0.01). Further, the release dynamics of dissolved As was also affected by changes in pH (r = -0.4, P < 0.05), but were not affected by redox-induced dynamics of Mn, Al, S, Cl-, SO42-, DOC, DIC, DC, DN-. We conclude that conditions such as flooding and high rainfall in this contaminated mine spoil could create reducing conditions, leading to reductive dissolution of the arsenopyrite As-bearing primary mineral and may lead to higher As release into the groundwater, translocation into the food chain with potential impacts on human health.
... DOM interacts with As directly through various mechanisms and indirectly via interactions with mineral oxides. For example, in waterlogged systems, DOM can affect As concentrations and the associated dynamics by triggering redox reactions and reductive dissolution of As from As-bearing mineral oxide phases (Shaheen et al., 2014(Shaheen et al., , 2016Rinklebe et al., 2016a;Stern et al., 2018;Verbeeck et al., 2020). Moreover, DOM influences As transport by forming binary and ternary colloids/complexes with As in solution or by competing with As for sorption sites on mineral surfaces (Sharma et al., 2010). ...
... In addition, the reduced soil Eh induces corresponding As redox transitions (Rinklebe et al., 2016a;El-Naggar et al., 2019). Consequently, As concentrations are higher under long-term reducing conditions due to the outlined reduction of potential As-retaining phases; as the reduction of Fe, DOM, SO 4 − 2 , and Mn may be kinetically constrained, for example, due to a redox buffer effect or due to surface limitations, a time lag occurs (Shaheen et al., 2014). ...
High concentrations of arsenic (As) in groundwater threaten the environment and public health. Geogenically, groundwater As contamination predominantly occurs via its mobilization from underground As-rich sediments. In an aquatic ecosystem, As is typically driven by several underlying processes, such as redox transitions, microbially driven reduction of iron (Fe) oxide minerals, and release of associated As. Notably, dissolved As mobilized from soils and sediments exhibits high affinity for dissolved organic matter (DOM). Thus, high DOM concentrations can increase As mobility. Therefore, it is crucial to understand the complex interactions and biogeochemical cycling of As, DOM, and Fe oxides. This review collates knowledge regarding the fate of As in multicomponent As–DOM–Fe systems, including ternary complexes involving both Fe and DOM. Additionally, the release mechanisms of As from sediments into groundwater in the presence of both Fe and DOM have been discussed. The mechanisms of As mobilization/sorption at the solid–water interface can be affected by negatively charged DOM competing for sorption sites with As on Fe (oxy)(hydr)oxides and may be further modified by other anionic ubiquitous species such as phosphate, silicic acid, or sulfur. This review emphasizes the need for a comprehensive understanding of the impact of DOM, Fe oxides, and related biogeochemical processes on As mobilization to aquifers. The review identifies important knowledge gaps that may aid in developing applicable practices for preventing the spread of As contamination in aquatic resources and traditional soil management practices.
... 29 It has been reported that the solubility of V increases under a high soil pH, such as those in neutral or alkaline soils, while it usually decreases at a lower soil pH. 12,30,31 The higher pH of the soils treated with HWB, compared to other treatments, could lead to the formation of more mobile forms of V and the predominance of oxoanions, such as vanadate (e.g., H 2 VO 4 − ), 11 consequently increasing the V solubility of the soil. ...
... Fe (hydro)oxides play an important role in bounding/occluding V; therefore, the chemistry of Fe may affect the release of V into the soil solution. 9,31,32 Impact of Biochar Application Dose on V Mobilization. Both application doses (2.5 and 5%) of RHB induced higher (P < 0.05) water-extractable V than that for control (SS), with no significant differences between both doses. ...
... Forms soluble HgS 2 2− anion in strongly alkaline soil. (Alloway, 2013;Beckers and Rinklebe, 2017;Tchounwou et al., 2012) Ni -Oxidation states: Can occur in several oxidation states: +1 to +4, but only Ni (+1, +2, and +3) are stable over a wide range of pH and redox conditions found in soil environments; -Sorption: High affinity for soil OM. (Alloway, 2013;Shaheen et al., 2014c;Tchounwou et al., 2012) Pb -Sorption: Strongly bound to humic matter (organic-rich soil) and to Fe oxides (mineral soil). ...
... Table 1 summarizes chemical behaviors of key PTEs under various environmental conditions. Redox conditions strongly affect the geochemical behavior of PTEs that have multiple oxidation states such as As, Cr, Mn, Mo, Se, U, and V (Alloway, 2013;Middelburg et al., 1988;Rinklebe et al., 2016a;Rinklebe et al., 2016b;Rinklebe et al., 2016d;Shaheen et al., 2014b;Shaheen et al., 2014c). PTE mobility and speciation indicates ecosystem toxicity. ...
Soil contamination by potentially toxic elements (PTEs) has led to adverse environmental impacts. In this review, we discussed remediation of PTEs contaminated soils through immobilization techniques using different soil amendments with respect to type of element, soil, and amendment, immobilization efficiency, underlying mechanisms, and field applicability. Soil amendments such as manure, compost, biochar, clay minerals, phosphate compounds, coal fly ash, and liming materials are widely used as immobilizing agents for PTEs. Among these soil amendments, biochar has attracted increased interest over the past few years because of its promising surface properties. Integrated application of appropriate amendments is also recommended to maximize their use efficiency. These amendments can reduce PTE bioavailability in soils through diverse mechanisms such as precipitation, complexation, redox reactions, ion exchange, and electrostatic interaction. However, soil properties such as soil pH, and clay, sesquioxides and organic matter content, and processes, such as sorption/desorption and redox processes, are the key factors governing the amendments' efficacy for PTEs immobilization in soils. Selecting proper immobilizing agents can yield cost-effective remediation techniques and fulfill green and sustainable remediation principles. Furthermore, long-term stability of immobilized PTE compounds and the environmental impacts and cost effectiveness of the amendments should be considered before application.
... Redox potential may alter the surroundings of biochar particles because of cations and anions, organic compound-rich solutions, and macro-and mesopore diffusion of the biochar. Biochar has a high adsorption capacity, a large surface area, and contains many volatile and aromatic components as well as inorganic compounds (El-Naggar et al., 2018;Shaheen et al., 2014). The redox activity of biochar depends on its surface functionality when treated to ecosystems in various redox conditions (Awad et al., 2018). ...
... Nowadays, many techniques are available for vanadium removal, including adsorption [21,22], chemical reduction and precipitation [17,23], ion exchange [24,25], microbial reduction [26][27][28], biological remediation [14,[29][30][31][32][33][34][35] and electrochemical reduction [36,37]. Some of these will produce vanadium-containing solid waste, from which utilization as a source of vanadium is not environmentally friendly and unfeasible. ...
In this paper, histidine was applied to treat vanadium-containing wastewater. Several independent experimental parameters, including H2SO4 concentration, dosage of histidine, reaction time and reaction temperature, were investigated and optimized through response surface methodology. The influence on the reduction process decreased in the following order: dosage of histidine > reaction temperature > reaction time. The reduction efficiency could be achieved at 95.77% under the following reaction conditions: H2SO4 concentration of 0.2 mol/L, reaction temperature of 90 °C, dosage of histidine at n(His)/n(V) = 3.6, reaction time of 60 min and stirring rate at 500 rpm. The reduction kinetics was followed successfully with the pseudo-first-order kinetics model and the Ea for reduction of vanadium was calculated to be 25.31 kJ/mol. The reduction kinetics was affected by these factors and the kinetics model could be described by an Equation. This paper provides a versatile strategy for treatment of wastewater containing V(V) and shows a bright tomorrow for wastewater treatment.
... Iron minerals passivate As through surface charge adsorption, hydrogen bonding with oxygen-containing functional groups and internal surface chelation with surface hydroxyl groups [31]. When Fe(III) is reduced to Fe(II), Fe -As precipitates dissolve and As ions are released owing to the loss of solid particle binding, which decreases the ability of iron minerals to fix As [45]. In this study, MPs inhibited the reduction of goethite and the desorption of As, thereby enhancing the adsorption and immobilization performance of goethite. ...
... At the same time, riparian soils are potential secondary sources of heavy metals when physicochemical conditions change (Xie et al., 2014;Chen et al., 2016). Water level, associated with the intensity and duration of flooding events, could significantly affect the transformation, migration and bioavailability of metals (Pavlović et al., 2016), being redox potential and pH the variables that mainly regulate metal partitioning between sediments or soil particles and the water column (Schulz-Zunkel and Krueger, 2009;Rinklebe et al., 2011;Shaheen et al., 2014). Additionally, mobilization of sediment-associated heavy metals from floodplain soils to the river water is also controlled by erosion during flooding episodes or runoff water during the rainy season (Tang et al., 2014). ...
Sesbania virgata is a pioneer shrub from the Fabaceae family, native to riparian environments in northeast of Argentina, southern of Brazil and Uruguay. In peri-urban riparian soils, metal contamination is a frequent problem, being its bioavailability partly determined by the stabilization time and frequency of contamination events. The effect of time elapsed between chromium (Cr) soil enrichment and plant seeding and Cr doses on S. virgata tolerance and metal absorption were evaluated. Treatments were developed by adding Cr (80–400
ppm) to the soil and allowing two days or fifteen months to elapse before sowing, and a control treatment without Cr addition. After 150 days from seeding, bioaccumulation and translocation factors, growth parameters (dry biomass and its aerial/radical allocation pattern, stem length and its elongation rate), morphological parameters (root volume and leaf area), and physiological parameters (chlorophyll content) of the specimens were determined. The emergence of S. virgata was inhibited since 150 ppm when Cr was added to the soil two days before seeding, with Cr accumulation in roots starting at 80 ppm (17.4 ± 2.5 mg kg 1). Under 15 months of metal stabilization, S. virgata plants survived across the entire range of Cr doses tested, with accumulation in roots since 100 ppm (35.5 ± 0.2 mg kg 1) and metal translocation to aerial tissues only under 400 ppm. The results obtained showed that S. virgata did not have high BCF and TF values, suggesting that it cannot be classified as bioaccumulator of Cr under the tested conditions. However, its presence in environments contaminated with Cr can be beneficial, as it helps to stabilize the metal in the soil.
... The reductive dissolution of metals (Fe/ Mn) causes solubilization of previously co-sorbed/precipitated trace metals with Fe and Mn oxides. These results are in good agreement with Shaheen et al. (2014aShaheen et al. ( , 2014b, Frohne et al. (2014) and Rinklebe et al. (2016) who have also reported mobilization of trace metals in floodplain and paddy soils because of reductive dissolution of metal oxides. ...
Flooding is known to solubilize soil nutrients, particularly those associated with redox‐sensitive metals (Fe/Mn). Both soil flooding and drying are becoming more common due to climate change, but it is not clear how soil drying prior to flooding influences nutrient solubilization in soils, compared with flooding of already moist soils. This study was designed to examine how soil drying followed by extended flooding might influence solubilization of micronutrient metals (Fe, Mn, Cu, Co, Zn and Ni). A series of laboratory mesocosm experiments was carried out by flooding samples of two contrasting grassland soils, which had each been either dried (40°C for 10 days) or kept at field moisture conditions (25%, w/w). The flooding of dried soils generally resulted in higher concentrations of the micronutrients (Mn, Co, Ni and Cu) in the water columns relative to their moist‐flooded counterparts. The results demonstrate that the flooding‐induced variations in pH and redox potential influence solubilization of micronutrients in the soils. The mobilization of Co and Ni appeared to be controlled by redox‐driven reductive dissolution of Fe/Mn minerals. This was supported by significant ( p < .001) negative correlations between redox and metals: Co ( r = −.712), Ni ( r = −.784) and the positive correlations between Fe and metals: Co ( r = .763) and Ni ( r = .714) and between Mn and other metals: Co ( r = .909) and Ni ( r = .811). However, there were no significant correlations of Zn and Cu with Fe and Mn. The results suggest that soil drying followed by flooding has the potential to promote greater solubilization of soil micronutrients compared with flooding of moist soils, with potential implications for soil fertility and catchment water quality under future changes in weather patterns driven by climate change.
... The reduction in chlorophyll content was as reported by some other researches [35] observed that Cd is more toxic to reduce the chlorophyll content compared to Pb. The addition of EDTA also caused a significant reduction due to more uptake of Cd and Pb which ultimately reduce the chlorophyll content which is as in the agreement with the findings of [36]. A decrease of water content in plants under the influence of HM was observed by many researchers [37]. ...
Heavy metals are contaminants of much environmental apprehension, as they are hazardous to human being and other biota. The chelates react with the heavy metals and free that metal from the cation exchange sites and resulting the metal chelated species and move readily into the soil. For this purpose, a hydroponic stud was conducted in wire house of Saline Agricultural Research Center to monitor the potential of Maize (Zea mays L.) cultivars against cadmium (Cd) and lead (Pb) toxicity. A chelated source Ethylene Diamine Tetra Acetic acid (EDTA) was used to evaluate the uptake of Cd and Pb. The experiment was comprised of nine treatments. As T1= Control, T2= Cd 25 ppm, T3= Pb 50 ppm, T4= Cd 50 ppm, T5= Pb 100 ppm, T6= Cd 25 ppm+EDTA, T7= Cd 50 ppm+EDTA, T8= Pb 50 ppm+EDTA and T9= Pb 100 ppm+EDTA. Two maize cultivars Syngenta-8711 and 33H-25 were used. It was concluded that the parameters like shoot fresh and dry weight, chlorophyll content (SPAD value) and relative water content decreased under toxic level of Cadmium (Cd) and Lead (Pb). The addition of EDTA increased the uptake of Cd and Pb which ultimately reduced the overall the plant growth. Among the maize hybrids, maximum Cd concentration was observed in Syngenta-33-H-25 while minimum in Syngenta-8711 at all cadmium stress levels. At all the lead stress levels the maximum Pb concentration was observed in Syngenta-33-H-25. The minimum Pb concentration was recorded in Syngenta-8711 at all lead stress levels.
... Paddy soils undergo an anoxic-oxic cycle, which can modify the physical, chemical, and biological properties of soil and the occurrence of metal (hydr) oxide organic matter, sulfate, and N species (Khaokaew et al., 2011;Pan et al., 2014;Shaheen et al., 2014). In this study, a complex interplay between redox conditions and Fe/C/N/S led to diverse reactions that determined the fate of Cr in basalt-derived paddy soils. ...
Although basalt-derived paddy soils contain high Cr levels and pose serious environmental risks, the mechanisms controlling Cr transformation during anoxic–oxic alternation remain unclear. In this study, the major geochemical processes controlling the release, geochemical speciation, and Cr redistribution during anoxic–oxic alternation were investigated in three basalt-derived paddy soils containing 114, 268, and 429 mg/kg of Cr. The exchangeable and carbonate-bound Cr (available Cr fractions) increased under anoxic phase but decreased under oxic phase, indicating the reversibility of Cr availability. The relationships between the dynamic changes in Fe/C/N/S and Cr transformations were also revealed. Fe–Mn oxide-bound Cr remained relatively stable during anoxic–oxic alternation, indicating that Cr may be released through reductive dissolution of Fe(III) (hydr)oxides while it was incorporated into the structure of Fe(III) (hydr)oxides through Fe(II)-catalyzed recrystallization under anoxic phase. The organic matter and sulfide-bound Cr increased under anoxic phase but decreased under oxic phase. During the anoxic period, organic matter can form complexes with Cr, and sulfate reduction may induce the formation of Cr sulfide. Based on elementary reactions, kinetic models were established to quantitatively describe Cr transformation during anoxic–oxic alternation in basalt-derived paddy soils. This study revealed the vital factors controlling Cr transformation and redistribution, which are useful for Cr risk assessment of basalt-derived paddy soils.
... Lysimeters are used to collect soil solutions for analysis and can offer a path for investigating soil solution properties in their natural state. The study of soil solution hydrochemistry through lysimeters helps in understanding nutrient mobility and dynamics (e.g., Carey 2003;Johnson et al. 2018;Petrash et al. 2019;Makowski et al. 2020;McDowell and Potter 2022), and can be used for evaluating the prevalence of toxicants in soils (e.g., Shaheen et al. 2014;Worrall et al. 1999). Many previous studies used lysimeters for developing the understanding on pollution processes, composition, and fluxes of vadose waters (Goss et al. 2010). ...
Soil solution chemistry depends largely on mineralogy and organic matter properties of soil horizons with which they interact. Differing lithologies within a given catchment area can influence variability in soil cation exchange capacities and affect solute transport. Zero-tension and tension lysimeters were used to evaluate the fast transport of solutes in the topsoil vs. slow diffusional matrix flow at the subsoil of three contrasting lithology catchments in a mid-elevation mountain forest. Our aim was to test the feasibility of lysimeters’ hydrochemical data as a gauge for legacy subsoil pollution. Due to contrasting lithologies, atmospheric legacy pollution prevailing at the soil-regolith interface is differently yet consistently reflected by beryllium, lead, and chromium soil solution concentrations of the three catchments. Geochemical (dis)equilibrium between the soil and soil matrix water governed the hydrochemistry of the soil solutions at the time of collection, potentially contributing to decreased dissolved concentrations with increased depths at sites with higher soil pH. A complementary isotopic δ¹⁸O runoff generation model constrained potential seasonal responses and pointed to sufficiently long water-regolith interactions as to permit important seasonal contributions of groundwater enriched in chemical species to the topsoil levels. Our study also reflects subsoil equilibration with atmospheric solutes deposited at the topsoil and thus provides guidance for evaluating legacy pollution in soil profiles derived from contrasting lithology.
... Moreover, Eh has shown a negative correlation with the concentrations of dissolved vanadium in multiple sites across the world . Last, the possibility of a more soluble vanadium species (vanadium (+4)) dominating the media could also contribute to explaining the seasonal changes in the study areas (Frohne et al., 2015;Shaheen et al., 2014b). ...
Vanadium is a component of different natural and industrial products and a widely used metal, which, nonetheless, has only garnered attention in recent years owing to its potential risks. Six sampling trips were conducted over different seasons and years, collecting 108 samples from rivers and 232 from the bays and analyzed using high-precision inductively coupled plasma mass spectrometry. This study investigated the sources, spatiotemporal characteristics, and risks of vanadium in the aquatic ecosystems of two typical bays of the Northwest Pacific that have strong links with vanadium-related industries. Likewise, the health and ecological risks were assessed using probabilistic and deterministic approaches. Overall, vanadium concentrations were higher in Jiaozhou Bay (JZB: 0.41-52.7 μg L-1) than in Laizhou Bay (LZB: 0.39-17.27 μg L-1), with concentrations higher than the majority of the worldwide studies. Vanadium-realted industries significantly impacted (p < 0.05) the metal concentrations in the rivers with 54.22% (40.73-150%) and 54.45% (27.66%-68.87%) greater concentrations in JZB and LZB rivers. In addition, vanadium exhibited significant seasonal variation, and higher values were quantified during the monsoon period at LZB owing to the greater catchment area. Impacted by smaller freshwater inputs, the post-monsoon period had substantial impacts on JZB, and vanadium in the rivers and bays was significantly higher during the winter. Despite some concentrations being higher than that indicated in the drinking water guidelines established by China, vanadium presents low to null risks to the population as per both approaches. Last, species with limited resilience are likely to face medium to high risks, with an incidence of 65-93% using the probabilistic method and 52-97% using the deterministic assessment.
... Floodwater V concentrations in Lakeland and Osborne soils were <30 and <50 μg L −1 , respectively, whereas respective pore water concentrations were 12-fold and 5-fold higher than the corresponding floodwater concentrations. Previous studies with microcosm experiments also reported higher pore water V concentrations with short-term flooding (Shaheen, Rinklebe, Rupp, et al., 2014). High aqueous V concentrations in unamended and zeolite-amended soils could be a concern due to toxic impacts on sensitive aquatic species as the thresholds for aquatic species range from 1.2 to 80 μg L −1 (Abernathy et al., 2021). ...
Addition of manganese(IV) oxides (MnO2) and zeolite can affect the mobility of As and V in soils due to geochemical changes that have not been studied well in calcareous, flooded soils. This study evaluated the mobility of As and V in flooded soils surface‐amended with MnO2 or zeolite. A simulated summer flooding study was conducted for 8 weeks using intact soil columns from four calcareous soils. Redox potential was measured in soils, whereas pH, major cations, and As and V concentrations were measured biweekly in pore water and floodwater. Aqueous As and V species were modeled at 0, 4, and 8 weeks after flooding (WAF) using Visual MINTEQ modeling software with input parameters of redox potential, temperature, pH, total alkalinity, and concentrations of major cations and anions. Aqueous As concentrations were below the critical thresholds (<100 μg L⁻¹), whereas aqueous V concentrations exceeded the threshold for sensitive aquatic species (2–80 μg L⁻¹). MnO2‐amended soils were reduced to sub‐oxic levels, whereas zeolite‐amended and unamended soils were reduced to anoxic levels by 8 WAF. MnO2 decreased As and V mobilities, whereas zeolite had no effect on As but increased V mobility, compared to unamended soils. Arsenic mobility increased under anoxic conditions, and V mobility increased under oxic and alkaline pH conditions. Conversion of As(V) to As(III) and V(V) to V(IV) was regulated by MnO2 in flooded soils. MnO2 can be used as an amendment in immobilizing As and V, whereas the use of zeolite in flooded calcareous soils should be done cautiously.
... Moreover, together with the increasing oxidation state, the highest toxicity was observed. As stated by Haluschak et al. (1998), reducing conditions results in the immobilization of V. On the other hand, analysis of Eh impact on V dynamics in different floodplain soils from the USA (Shaheen et al. 2015) and Germany Shaheen et al. 2014) showed the increased concentration of dissolved V under reducing conditions and decreased under high Eh. Such effect was explained as the oxidation of more soluble V (+ IV) to less soluble V (+ V), together with increasing Eh. ...
Metal pollution of the environment remains a very important topic for scientific discussion. Vanadium (V) is one of the toxic elements with the most extensive distribution in nature. Despite the vast use of V in heavy industries, its presence in the environment can be harmful to living organisms. Soil can be polluted by V released from both natural and anthropogenic sources. The high mobility of V from soil to plants directly affects humans. The current review provides an overview of the impact of certain soil biological-chemical properties on the bioavailability, mobility, and toxicity of V. Although some aspects are well documented, such as pH, Eh, or SOM, there are points that need to be analyzed and described in greater detail. An important aspect that requires further investigation is the effect of vanadium on microorganisms and, more precisely, on the soil processes they carry out. It can be assumed that, analogically to other heavy metals (e.g., Pb, Zn, Ni, Cd), it can impair certain reactions (methanotrophy, methanogenesis), which can have a negative impact on the environment. So far, there are no studies referring to this subject in the soil environment.
... The lower Cr concentrations (39.0 μg L −1 ) at high E H (+240 mV) in the end of the incubation time than the concentrations (623.0 μg L −1 ) at high E H (+200 mV) in the beginning of incubation may indicate that the potential mobile of Cr was released under high E H in the first ten days; then, Cr 6+ might be reduced to Cr 3+ at low E H in the middle incubation period (10-20 days); thereafter, Cr 3+ was slowly oxidized to Cr 6+ under high E H again in the last period (20-30 days). The lower mobilization of Cr in the second oxic cycle than the first one can be explained by the slow kinetics of redox reaction of Cr in the soil environment, as also assumed by Rinklebe and Du Laing (2011) and Shaheen et al. (2014c). ...
The release dynamics and mobilization of geogenic Ni, Co, and Cr in serpentine paddy soils under fluctuating redox conditions have not yet been well studied. Here we investigated the release dynamics of Cr, Co, and Ni and controlling factors (e.g., Fe, Mn, Mg, Cl⁻, PO4³⁻, SO4²⁻, and dissolved organic carbon (DOC)) in a geogenic-contaminated serpentine soil under wide range of redox potential (EH) changes. The effects of re-oxidation process have been also investigated. The soil was incubated for 28 days and EH was controlled from oxidation (+200 mV) to reduction (−200 mV) and re-oxidation (+240 mV) using a microcosm setup in duplicates. The slurry pH increased, along with decreasing EH. The average concentration of dissolved Co (17.1–23.6 μg L⁻¹) decreased under low EH/high pH and vice versa. The average concentration of dissolved Cr decreased sharply from 624 μg L⁻¹ to 54.4 μg L⁻¹ with decreasing EH from +200 mV to 0 mV and the associated increase of pH from 7.8 to 8.5; then, it was constant around 24.5 μg L⁻¹. Concentration of dissolved Ni was lower (73.5–84.6 μg L⁻¹) under high EH at the first week of incubation; then, increased to 108.5 μg L⁻¹ under low EH (−200 mV); thereafter, increased more at the end up to 124.5 μg L⁻¹ at high EH (+240 mV), because of the pH decrease. A factor analysis identified that Cr and Co formed one group with Mn and Mg, while Ni was clustered together with Cl⁻, DOC, and SO4²⁻. This indicates that the redox-induced release dynamic of Cr and Co was mainly governed by MnMg compounds, while the release of Ni was mainly affected by the aliphatic compounds of DOC and the redox chemistry of chlorides and sulfur in this soil. The re-oxidation increased the mobilization of Ni and Co and did not affect the release of Cr. These findings suggest that the redox-induced mobilization of geogenic Co, Ni, and Cr from soil to water in serpentine rice soils should be considered due to the high solubility and thus the associated bioavailability and potential environmental and human health risks, when such metal-enriched soils will be used for agricultural flood-dry cycle systems.
... These results indicated that the environmental toxicity of Cr was restricted by soil water content (Cheng et al., 2014). Shaheen et al. (2014) concluded that the dynamic migration of Cr was related to the duration of soil exposure to floods, because drivers of Cr element mobility required a certain amount of time to provoke reactions under shifting conditions. Soil Cr pollution in the experiment was caused by the leaching of Cr slag. ...
Soil contamination with potentially toxic element such as chromium (Cr) poses a threat to the environment and human health. The environmental toxicity of Cr is related not only to the total Cr content but also to the distribution of Cr fractions. In this study, laboratory simulation experiments were conducted to explore the characteristics of Cr fractions and responses of the functional microbial community during dynamic leaching and static drying processes. The results showed that acid-soluble Cr and reducible Cr transformed into other relatively stable fractions under dry conditions, and ammonium nitrogen promoted the transformation. Nitrate-nitrogen was significantly positively correlated with Cr fractions in the wet stage ( p < 0.05), while ammonium nitrogen showed the same relation in the dry process. Analysis of the microbial community showed that the bacterial and fungal genera Flavihumibacter, Altererythrobacter, Methylobacillus, Flavisolibacter, Lysobacter, and Cladosporium were related to the Cr fractions (acid-soluble Cr, reducible Cr, and oxidizable Cr) under wet conditions, while the microbial genera Ellin6067, MND1, and Ramlibacter were related to Cr fractions under dry conditions. Moreover, the proliferation of the functional microbial genera Methylobacillus, Ellin6067, and MND1 related to Cr fractions in the wet–dry conversion process alleviated the environmental toxicity of Cr. These findings provide useful information for the remediation of Cr-contaminated soils by monitoring the distribution fractions of Cr and the functional microbial community under wet–dry conditions.
... Floodplain soils are semi-terrestrial comprised of periodic deposition of suspended river sediments during flood events (Rinklebe et al. 2007;Luster et al. 2014) and with fluctuating soil moisture conditions depending on water table (Gutknecht et al. 2006;Shaheen et al. 2014). Floodplain hydrology is therefore a major factor impacting the soil microbial community (Bossio and Scow 1995;Rinklebe and Langer 2010). ...
Soil physico-chemical characteristics of floodplains, particularly hydrology, influence microbiological activity. As such, each river floodplain system has a unique physico-chemical dynamic that in turn supports the microbial community. The Mapire River floodplain is a complex system in which seasonal flood pulses cause changes in the soil physico-chemical variables. We examined how these temporal and spatial differences are associated with the microbiological activity along a seasonally flooded gradient at the mouth of the Mapire River (Lower Orinoco, Venezuela). Soil samples were collected during three different seasons by a systematic sampling at 4 points of the gradient, defined by the intensity of flooding. The physico-chemical parameters of the soil were determined and related by the density and physiological profile of the microbial community through multivariate permutation analysis and gradient analysis. The results indicate that there is a spatial gradient determined by soil clay content and a temporal gradient influenced by moisture and total organic carbon. Significant differences were found among soil zones and seasons, with the interaction of both factors also significant. It was observed that microbial activity is decisive in phosphorus dynamics, even during flooding. It is concluded that amid the complex interactions between biotic and abiotic factors, microbial communities are able to respond to changes in the physico-chemical soil environment and maintain their activity throughout the hydroperiod.
... Many methods have been developed for vanadium removal. Biological remediation came into view due to its low cost and potential applications for in situ remediation [13,[29][30][31][32][33][34][35], but knowledge was limited on their interaction during the process, as well as their biogeochemical cycling in groundwater. Another low-cost and easy-operation technology is adsorption, which had been widely applied [36][37][38][39][40]. Additionally, many materials were evaluated, such as zeolite, chitosan, biochar, and orange peel [41][42][43][44][45][46]. ...
Water pollution deteriorates ecosystems and is a great threat to the environment. The environmental benefits of wastewater treatment are extremely important to minimize pollutants. Here, the oxalic acid used as reductant was used to treat the wastewater which contained high concentration of vanadium (V). Nearly 100% of vanadium was efficiently reduced at selected reaction conditions. The optimization results simulated by response surface methodology (RSM) analysis indicated the parameters all had significant effects on the reduction process, and followed the order: dosage of oxalic acid > reaction temperature > reaction time > initial pH of vanadium-containing wastewater. The reduction behavior analysis indicated that the pseudo first-order kinetics model could describe well the reduction process with Ea = 42.14 kJ/mol, and was described by the equation as followed: −LnC=K0·[pH]0.1016·[n(O)/n(V)]2.4569·[T]2.2588·exp(−42.14/T)·t.
... In various arable soils (e.g., in Northern Germany), high precipitation plus restricted drainage may cause an increase in the groundwater table (Zimmer et al., 2016), which can lead to a low soil redox potential (E H ) and vice versa. This reversible process may affect the water regime, and thus affect the mobilization of nutrients and pollutants due to redox changes (from low E H (high water table) to high E H (low water table and vice versa) (Shaheen et al., 2014a;Baumann et al., 2021a). Consequently, mobilization of nutrients in arable soils with redoximorphic characters is of increasing interest from agro-environmental point of view. ...
Sustainable engineering and management of hydromorphic arable soils need deep knowledge about the redox-mediated interactions between nutrients and soil colloids. Consequently, we examined the redox-mediated interactions of P with metal oxides and organic carbon (OC) in toe-, mid-, and upper-slope arable soils under dynamic redox changes using geochemical (biogeochemical microcosm), spectroscopic (XANES), and molecular (quantum chemical calculations (QCC)) approaches. We controlled the redox potential (EH) in two directions i.e., 1) slowly oxidizing direction (SOD; EH increased from −286 to +564 mV); and 2) slowly reducing direction (SRD; EH decreased from +564 to −148 mV). In the SOD of all soils, P, Fe²⁺ and OC mobilized at EH ≤ 200 mV, due to the pH decrease from 7.2 to 4.1 and dissolution of Fe-oxyhydroxides/carbonates, as indicated by the decrease of Fe–P and Ca–P determined by P–K-edge-XANES. At EH > 200 mV, P immobilized due to the strong P binding with Fe³⁺ as suggested by QCC. In the SRD of mid-slope-soil, P immobilized with decreasing EH, due to pH increase and P retention by aromatic carbon and/or precipitation by carbonates, as supported by increase of organic-P and Ca–P. These findings help for management of P in arable soils.
... Floodplain soils are semi-terrestrial comprised of periodic deposition of suspended river sediments during ood events (Rinklebe et al. 2007;Luster et al. 2014) and with uctuating soil moisture conditions depending on water table (Gutknecht et al., 2006;Shaheen et al. 2014). Floodplain hydrology is therefore a major factor impacting the soil microbial community (Bossio and Scow 1995;Rinklebe and Langer 2010). ...
Soil physico-chemical characteristics of floodplains, particularly hydrology, influence microbiological activity. As such, each river floodplain system has a unique physico-chemical dynamic that in turn supports the microbial community. The Mapire River floodplain is a complex system in which seasonal flood pulses cause changes in the soil physico-chemical variables. We examined how these temporal and spatial differences are associated with the microbiological activity along a seasonally flooded gradient at the mouth of the Mapire River (Lower Orinoco, Venezuela). Soil samples were collected during three different seasons by a systematic sampling at 4 points of the gradient, defined by the intensity of flooding. The physico-chemical parameters of the soil were determined and related by the density and physiological profile of the microbial community through multivariate permutation analysis and gradient analysis. The results indicate that there is a spatial gradient determined by soil clay content and a temporal gradient influenced by moisture and total organic carbon. Significant differences were found among soil zones and seasons, with the interaction of both factors also significant. It was observed that microbial activity is decisive in phosphorus dynamics, even during flooding. It is concluded that amid the complex interactions between biotic and abiotic factors, microbial communities are able to respond to changes in the physico-chemical soil environment and maintain their activity throughout the hydroperiod.
... But it should be noted that these findings are valid under the stable soil conditions of Fluvisol during the controlled pot experiment, and the field conditions of Fluvisols may significantly vary during the episodic changing of flooding-drying cycles with serious impacts on soil conditions affecting the mobility of trace elements [35,48]. The experimental settings rather simulated the field conditions during an equilibration period of a dry cycle. ...
The paired Fluvisol and cereal samples in both the field screening and controlled experiments are reported to elucidate the soil–crop relationship for As, Cd, and Pb in relation to changing contamination levels. Significant varietal differences in plant uptake were observed for crop type (barley, triticale) and the harvested part of the crop (oat shoots and grain). When parametrizing the stepwise regression models, the inclusion of soil properties often improved the performance of soil–crop models but diverse critical soil parameters were retained in the model for individual metal(loid)s. The pH value was often a statistically significant variable for Cd uptake. For As and Pb, the more successful model fit was achieved using the indicators of quantity or quality of soil organic matter, but always with lower inherent model reliability compared to Cd. Further, a single correlation analysis was used to investigate the relationship between extractable metal concentrations in soil solution and their crop accumulation. For Cd, there were strong intercorrelations among single extractions, the NH4NO3 extraction stood out with perfect correlation with plant uptake in both experiments. For As and Pb, the CaCl2 and Na2EDTA solutions outperformed other single extractions and were the better choice for the assessment of depositional fluvial substrates.
... When paddy soil is under reduced conditions as a result of continuous flooding, more Cr(VI) is reduced with a decrease in soil Eh, whereas under IF conditions, as a result of alternate flooding and drainage practices, much less Cr(VI) is reduced to Cr(III), which probably resulted from the alternating soil redox status between reducing and oxic conditions. Previous studies have demonstrated that soil Eh values that are affected by water management have major impacts on Cr(VI) reduction in paddy soils (Shaheen et al., 2014). Further, research has found that soil pH could affect Cr(VI) reduction because the reduction of soluble Cr(VI) to soluble Cr(III) is a pH-dependent proton consumption reaction (Honma et al., 2016;Li et al., 2019). ...
Chromium (Cr) contamination in rice poses a serious threat to human health. Therefore, we conducted pot experiments to investigate the influence of water management regimes on the formation of iron plaque on rice roots, and its effect on the accumulation and translocation of Cr in rice grown on contaminated soil. The results showed that water management regimes, including continuous and intermittent flooding, exerted notable effects on soil solution concentrations of Cr(VI) and Cr(III) through changes in redox potential, pH, and dissolved Fe(II) concentrations. In particular, 69.2% – 71.8% of Cr(VI) was reduced to Cr(III) under continuous flooding, whereas only 33.3% – 38.6% was reduced under intermittent flooding conditions. Additionally, continuous flooding created a rhizosphere environment favorable to the formation of iron plaque. The amount of iron plaque formed increased by 28.2%–47.2% under continuous flooding conditions as compared with that under intermittent flooding conditions. Moreover, compared with intermittent flooding, under continuous flooding, more Cr (18.0%–23.9%) was adsorbed in the iron plaque, thereby sequestering Cr and reducing its mobility. The Cr concentrations in rice root, straw, husk, and grain under continuous flooding conditions were, respectively, 32.0% – 36.5%, 32.7% – 36.3%, 34.2% – 46.9%, and 25.4% – 37.7% lower than those under intermittent flooding conditions. Therefore, continuous flooding caused a substantial decrease in the Cr concentrations in rice tissues, as well as an increased distribution of Cr in the iron plaque that acted as a barrier to reduce Cr transfer to the rice roots. These results indicate that continuous flooding irrigation was effective in minimizing the accumulation of Cr in rice plants, as it not only enhanced Cr(VI) reduction in the soil but also improved the blocking capacity of the iron plaque.
... The values become almost constant after the second week of reduction; the average is 391 ± 39 mg l −1 in LF soil and 561 ± 45 mgl −1 in DT soil. This behaviour was already observed in similar situations and is probably due to both the dissolution of mobile hydrophilic DOC compounds during the first days and to the release of the organic matter (OM) bound to the oxyhydroxides surfaces (Shaheen et al. 2014;Pan et al. 2014). Iron and Mn minerals strongly adsorb OM, but once the soil is water-saturated, the increase in pH and the anoxia diminish the positive surface charge of the oxyhydroxides and force the microorganisms to switch and use Mn and Fe as electron acceptors, causing the release of the associated OM. ...
Metal-contaminated mining soils pose serious environmental and health risks if not properly managed, especially in mountainous areas, which are more susceptible to perturbation. Currently, climate change is leading to more frequent and intense rain events, which cause flooding episodes, thereby altering soil redox equilibria and contaminants stability. We evaluated the potential release of Zn and Cd (two of the most common inorganic contaminants) and the factors regulating their solubility and speciation in two heavily contaminated soils representative of a Zn-mining area. The soils were flooded under aerobic (for 24 h) and anaerobic (for 62 days) conditions using mesocosm experiments, sequential extractions, and geochemical modelling. Leaching trials under aerobic conditions showed a high release of Zn and Cd (10 times the legislative limits), with metals possibly migrating via water infiltration or runoff. Under anaerobic conditions Zn and Cd were initially released. Then, solution concentrations decreased gradually (Zn) or sharply (Cd) until the end of the experiment. Sequential extractions and multisurface modelling indicated that both metals precipitated mainly as carbonates. This was confirmed by a geochemical multisurface modelling, which also predicted the formation of sulphides after 60 days in one soil. The model calculated metals to be preferentially complexed by organic matter and well predicted the observed soil solution concentrations. The results showed that during flooding episodes contaminants could be promptly transferred to other environmental compartments. The use of multisurface modelling coupled with laboratory experiments provided useful indications on the potential release and speciation in case of anoxic conditions.
... Redox potential has a significant effect on controlling the biogeochemical behaviour of redox-sensitive metal such as V, Cr, and Fe in soil (Borch et al., 2010;Rinklebe et al., 2017). Many studies have shown the impact of E h on the redox behaviour of V in different floodplain soils collected from Elbe, Wupper, and Mississippi river sediments (Shaheen et al., 2014a;Frohne et al., 2015;Shaheen et al., 2016). Vanadium(V) is easily reduced to V(IV) in the presence of electron donors such as organic compounds (Gustafsson, 2019). ...
Vanadium(V) is an important component of industrial activities, while it may pose toxic hazards to plants, animals, and humans at high levels. Owing to its various uses in numerous industrial processes, high amount of V is released into the soil environment. Previous literature has focused on the biogeochemistry and ecotoxicity of V in soil-plant system. Consequently, this overview presents its source, fate, phyto-uptake, phyto-toxicity, detoxification, and bioremediation based on available data, especially published from 2015 to 2020. Vanadium occurs as various chemical forms (primarily as V(V) and V(IV)) in the soil environment, and its biogeochemical behaviour is easily influenced by soil conditions including redox potential, soil pH, organic matter, and microorganisms. Vanadium mainly accumulates in plant roots with very limited translocation to shoots. However, plants such as dog’s tail grass and green bean are reported to accumulate high levels of V in aboveground tissues. An insight into the processes and mechanisms that allow plants to absorb and translocate V in soil-plant system is also stressed in this overview. In plants, low levels of V have beneficial effects on plant growth and development. Nevertheless, excessive V provokes numerous deleterious effects including reducing seed germination, inhibiting root and shoot growth, depressing photosynthesis, interfering with nutrients uptake, inducing overgeneration of ROS, and leading to lipid peroxidation. Mechanisms related to detoxification strategies like sequestration in root system, compartmentation in vacuoles and cell wall, and antioxidant defence systems to endure V-induced toxicity in plants are discussed as well. The detailed knowledge of bioremediation involved in the cleanup of V-contaminated soils would immensely help understand and improve the remediation process. Furthermore, this overview outlines several research gaps requiring further investigation in order to advance our understanding of the biogeochemical roles of V in soil-plant systems.
... The reduction in chlorophyll content was as reported by some other researches [35] observed that Cd is more toxic to reduce the chlorophyll content compared to Pb. The addition of EDTA also caused a significant reduction due to more uptake of Cd and Pb which ultimately reduce the chlorophyll content which is as in the agreement with the findings of [36]. A decrease of water content in plants under the influence of HM was observed by many researchers [37]. ...
... Soil vanadium pollution occurs in main manufacturing locations in the region , while soil near the Panzhihua vanadium smelter presents the highest environmental risk with high level of vanadium determined in both topsoil and subsoil (Lu et al., 2020;Teng et al., 2011a;Zhang et al., 2019b). Spatial vanadium distribution in response to soil properties are also reported in other representative regions, such as Germany, Egypt and Greece (Shaheen et al., 2014b;Shaheen and Rinklebe, 2017;Tsadilas and Shaheen, 2010). Vanadium content variation is also studied under controlled condition in an automated biogeochemical microcosm apparatus (Shaheen et al., 2014a). ...
Whereas the adverse effects of vanadium released from smelting activities on soil microbial ecology have been widely recognized, little is known about spatiotemporal vanadium distribution and microbial community dynamics in typical contaminated sites. This study describes vanadium contents associated with health risk and microbial responses in both topsoil and subsoil during four consecutive seasons around an ongoing-production smelter in Panzhihua, China. Higher levels of vanadium concentration exceeding soil background value in China (82 mg/kg) were found close to the smelter. Vanadium concentrations decreased generally with the increase in distance to the smelter and depth below surface, as soil vanadium pollution is induced mainly by atmospheric deposition of vanadium bearing dust during smelting. Residual fraction was the predominated vanadium form in soils, with pronounced increase in bioavailable vanadium during rainfall period due to frequent drought-rewetting process. Topsoil close to the smelter exhibited significant contamination, inducing high probability of adverse health effects. Spatiotemporal vanadium distribution creates filtering effects on soil microorganisms, promoting metal tolerant genera in topsoil (e.g. Microvirga) and subsoil (e.g. Bacillus, Geobacter), which is the key in maintaining the community structure by promoting cooperative relation with other taxa. Our results reveal spatiotemporal vanadium distribution in soils at site scale with potential health risk and microbial responses, which is helpful in identifying severe contamination and implementing bioremediation.
... For example, when oxygen depletes in a soil system, microorganisms use electrons from Mn(III, IV)-or Fe(III)-oxides that play essential roles in sequestering toxic elements like Sb (Mitsunobu et al., 2006;Leuz et al., 2006;Hockmann et al., 2014Hockmann et al., , 2015. The release of Sb to aqueous phase is then observed under metal oxide-reducing conditions (Leuz et al., 2006;Han et al., 2019a;Shaheen et al., 2014). ...
This study examined the geochemical behavior of antimony (Sb) in a vegetated contaminated soil column consisting of unsaturated rhizosphere and a waterlogging layer. The results showed a reducing condition (Oxidation-Reduction Potential (ORP) of -171 mV) was formed in about 5 days in the waterlogging zone. The amount of Sb released was higher under the oxidizing unsaturated-rhizosphere compared to that in the waterlogging zone possibly because of the weaker affinity of Sb(V) to Mn- and/or Fe-oxides in soil. The fraction of Sb(III) in the dissolved total Sb increased with time when soil redox states were subjected to a further reduction. Solid phase Sb K-edge X-ray absorption spectroscopy (XAS) of soils showed that Sb(III) fraction of the deeper layer soil increased while the unsaturated upper soil solely composed Sb(V). In this study, 250 mg/kg of Sb pollution did not significantly affect plant growth and no significant transport of Sb occurred from the soil to plant. However, changes in redox conditions within the soil column induced a shift in soil microbial communities. Consequently, the importance of redox states of soil on geochemical behavior of Sb and the effects of soil flooding or waterlogging deserve attention in the management of Sb-contaminated soil.
... Application of biochar to Cr-contaminated soils and sediments increase the content of soil organic matter (SOM) and thus can promote adsorption of Cr(VI) (Antoniadis et al. 2017(Antoniadis et al. , 2018. The involved mechanisms are as follows: On the one hand, SOM promotes microbe growth, thereby motivating biotic reduction of Cr(VI) (Palansooriya et al. 2019); on the other hand, it creates a reduced condition and alters redox potential through proliferation of the microbes (Bolan et al. 2003b), facilitating the reduction of Cr(VI) to Cr(III) (Banks et al. 2006;Shaheen et al. 2014;Rinklebe et al. 2016); lastly, Dong et al. (2014) found that dissolved organic matter (DOM) derived from biochar can act as both electron acceptor and donor, and it was a better reductant than oxidant because it has more positive effect on Cr(VI) reduction (Kunhikrishnan et al. 2017). ...
Chromium (Cr) is a common environmental contaminant due to industrial processes and anthropogenic activities such as mining of chrome ore, electroplating, timber treatment, leather tanning, fertilizer and pesticide, etc. Cr exists mainly in both hexavalent [Cr(VI)] and trivalent [Cr(III)] form, being Cr(VI) with non-degradability and potential to be hidden, thereby affecting surrounding environment and being toxic to human health. Therefore, researches on remediation of Cr pollution in the environment have received much attention. Biochar is a low-cost adsorbent, which has been identified as a suitable material for Cr(VI) immobilization and removal from soil and wastewater. This review incorporates existing literature to provide a detailed examination into the (1) Cr chemistry, the source and current status of Cr pollution, and Cr toxicity and health; (2) feedstock and characterization of biochar; (3) processes and mechanisms of immobilization and removal of Cr by biochar, including oxidation–reduction, electrostatic interactions, complexation, ion exchange, and precipitation; (4) applications of biochar for Cr(VI) remediation and the modification of biochar to improve its performance; (5) factors affecting removal efficiency of Cr(VI) with respect to its physico-chemical conditions, including pH, temperature, initial concentration, reaction time, biochar characteristics, and coexisting contaminants. Finally, we identify current issues, challenges, and put forward recommendations as well as proposed directions for future research. This review provides a thorough understanding of using biochar as an emerging biomaterial adsorbent in Cr(VI)-contaminated soils and wastewater.
Emerging contaminants (ECs) pose a growing threat to the agricultural ecosystems and human health. Biochar (BC)
may be applied for the remediation of ECs in soils and water. There are some research papers that have been pub
lished about the potentiality of BC for the remediation of ECs in soils and water; however, there have been no critical
and comprehensive review articles published on this topic up to now. Therefore, this review explores the application
of pristine and modified BC for the remediation of various emerging inorganic contaminants (EICs), including vana
dium (V), antimony (Sb), thallium (Tl), mercury (Hg), fluoride (F−), and rare earth elements (REEs) in soils and water.
The review explores the specific mechanisms by which BC removes these EICs from water and soil. The roles of ion
exchange, complexation, electrostatic interactions, and precipitation in the removal of these EICs from water by pris
tine and functionalized BC have been reviewed and discussed. Particular attention is also paid to the interaction
and potential immobilization of those EICs in soils with pristine and functionalized BC, highlighting some applicable
strategies for treating EIC-contaminated soils, particularly paddy soils, aiming to mitigate the associated ecological
and human health risks. Finally, the potential environmental implications and further research on the applications
of pristine and functionalized BC for remediation of EICs in water and soils have been summarized. This article pro
vides a comprehensive overview on the potential applications of different pristine and engineered BCs for the sustain
able remediation of EICs contaminated soils and water
Rice cultivation under flooded conditions usually leads to a high accumulation of arsenic (As) in grains. Sulphur and iron played vital roles in affecting the bioavailability of As in the soil-rice system. Herein, using pot experiments, we investigated the effects of persulphate (PS) and ferrous (Fe2+) on the transfer and accumulation of As in the soil-rice system under flooded conditions. The concentration of As and Fe in soil porewater declined with continuous flooding. Persulphate/ferrous addition significantly inhibited the formation of iron plaque and the transfer of As to the aboveground tissues of rice. The total As, dimethylarsinicacid (DMA), As (III), and As (V) in grains significantly decreased by 49∼75%, 60∼89%, 20∼24%, and 35∼36%, respectively, by persulphate/ferrous application. Furthermore, a decrease of As in husk, leaf, and, stem was also found in persulphate and ferrous treatment. To some degree, the Fe2+ can facilitate the decreased efficiency of As accumulation and translocation in rice tissue. The present study's results demonstrated that applying persulphate/Fe2+ could effectively alleviate the excessive accumulation of As in rice grains in the soil-rice system under flooding conditions.
While Arsenate [As (V)] is the predominant species under aerobic conditions (Xu et al., Environ Sci Technol 42:5574–5579, 2008; Li et al., Environ Sci Technol 43:3778–3783, 2009b); in soil solutions, it may be as high as 5–20% (Khan et al., Environ Sci Technol 44:8515–8521, 2010), under typical flood conditions. Arsenic contamination of groundwater is a geogenic process. Oxidation of arsenopyrites or reduction of ferric oxyhydroxide or both forms an important pathway for As release in the groundwater. Iron and arsenic-bearing minerals formed in situ or brought along by rivers, combined with sulphur and form arsenopyrite (FeAsS).
Immobilization of vanadium (V) in soils is one option to prevent groundwater contamination and plant uptake. Phytoremediation, microbial remediation, and chemical stabilization using soil amendments are among the leading environmentally friendly and economically feasible techniques in V remediation. Soil amendments were used to reduce V mobility by immobilizing it in the soil matrix through chemical stabilization, while bioremediation methods such as phytoremediation and microbial remediation were used to remove V from contaminated soils. Vanadium exists in several species and among them V⁵⁺ species are the most prevalent, toxic, and soluble form and present as a negatively charged ion (H2VO4⁻ and HVO4²⁻) in oxic soils above pH 4. Amendments used for chemical stabilization can change the physicochemical properties enhancing immobility of V in soil. The pH of the soil environment, point of zero charge of the colloid surface, and redox conditions are some of the most important factors that determine the efficiency of the amendment. Commonly used amendments for chemical stabilization include biochar, zeolites, organic acids, various clay minerals and oxides of elements such as iron, titanium, manganese, and aluminum. For bioremediation, chelating agents and microbial communities are used to mobilize V to enhance phyto-or microbial-extraction procedures. The objectives of this review were to discuss remediation methods of V while considering V speciation and toxicity in soil, and soil amendment application for V removal from soil. The information compiled in this review can guide further research on soil amendments for optimal V remediation in largely contaminated industrial sites.
Soil solution chemistry depends largely on mineralogy and organic matter properties of soil horizons with which they interact. Differing lithologies within a given catchment area can influence variability in soil cation exchange capacities and affect solute transport. Zero-tension and tension lysimeters were used to evaluate fast transport of solutes in the topsoil vs. slow diffusional matrix flow at the subsoil of three contrasting lithology catchments in a mid-elevation mountain forest. Our aim was to test the feasibility of lysimeters hydrochemical data as a gauge for legacy subsoil pollution. Due to contrasting lithologies, atmospheric legacy pollution prevailing at the soil-regolith interface is differently yet consistently reflected by beryllium, lead, and chromium soil solution concentrations of the three catchments. Geochemical (dis)equilibrium between the soil and soil matrix water governed the hydrochemistry of the soil solutions at the time of collection, potentially contributing to decreased dissolved concentrations with increased depths at sites with higher soil pH. A complementary isotopic data constrained potential seasonal responses and pointed to sufficiently long water-regolith interactions as to permit important seasonal contributions of groundwater enriched in chemical species to the topsoil levels. Our study also reflects subsoil equilibration with atmospheric solutes deposited at the topsoil, and thus provides guidance for evaluating legacy pollution in soil profiles derived from contrasting lithology.
We provide an overview of arsenic (As) from gold mining spoils, tailings disposal sites, and mining degraded soils and propose sustainable soil remediation options to mitigate mobilization and human health impacts. In situating the As problem in a broader science, concepts related to As chemistry, As pollution, As mobilization, and As toxicity are discussed. Relying on empirical data from mine sites and nearby communities in southwestern Ghana and crucial scholarship and scientific literature, we report high concentration of As in six media comprising soil (tailings, farms, and mining sites), water (surface and groundwater), water sediments (rivers and streams), food (meat and fish), plants (vegetation and food crops/fruits), and human (urine and blood samples). Soil, water, and urine are the top three media that report the highest and most siginificant concentration of As with levels exceeding established recommended threshold limits. Additionally, we identify and discuss the gaps in As research in Ghana and provide recommendations on sustainable strategies for cleaning contaminated sites.
In Ghana, gold mining exists in two major scales- large and small. Despite its economic benefits, the sector presents huge environmental costs. This study examined possible pollution with potentially toxic elements in gold mine sites, dominant species, potential mobility, soil and human health impacts, and possibilities for remediation. The results showed that the gold mining sites are heavily-contaminated with As (max. 8.4 g/kg). Arsenic contamination in the sites and surrounding were related to their Fe oxide and sulphide contents. The As contents exhibited high potential mobility. The soil-human health hazard assessment demonstrated that the sites posed extreme health threats to the residents in the mining region. Therefore, soil remediation measures need to be taken to safeguard public health of the residents. Native plants (e.g., Chromolaena odorata) that grow in these polluted sites are capable of cleaning As. Also, Fe-rich amendments can be used to decrease As-associated risks.
In this era, climate change is considered one of the greatest sustainability challenges and environmental threats facing our global society. Atmospheric warming from anthropogenic greenhouse gases (GHGs) will persist many centuries and continue to change in the global climate system. Evidently, both frequency and intensity of extreme climate events and natural disasters have been observed at the regional, continental, and global scales over comparable time periods, especially the additional warming of 1.5–2.0 °C. In terms of terrestrial biological systems, all microbial mechanisms have caused several changes in the global climate system (i.e. soil carbon and nitrogen cycling, terrestrial biogenic fluxes of GHGs). Although much attention has been paid to the linkage between microbial population and soil GHG fluxes, the significance of microorganism responses to climate-related environmental stress has remained neglected. The overall aim of this chapter is, therefore, to highlight the consequences of global climate change on the performance of bioremediation treatment processes. All potential effects of climatic parameters, such as increased atmospheric temperature and elevated CO2 levels, extreme precipitations, soil moisture, soil warming, water stress (drought) on the microbiological mechanisms (i.e. microbial diversity, structure, physiological change, etc.), fate and behavior of contaminants, the ability of plant to uptake the toxic contaminants the environment, and also the efficiency of bioremediation were critically addressed. Some bioremediation techniques (i.e. phytoremediation) were also emphasized and considered for the impacts of combined climatic stress on soil microbe–plant interactions (i.e. bioavailability and potential mobility of contaminants, etc.). Some sustainable bioremediation options in the climate change era and issues for future research were further discussed.KeywordsBioremediationContaminantsGlobal climate changeImpactsPlantsMicroorganismsSoil
Soil degradation is an exceedance of the capacity and resiliency of soil for providing functions and ecosystem services. It is a complex ongoing phenomenon threatening humans’ livelihoods and our future on earth. Knowledge gain can help to find solutions for monitoring, preventing and combating soil degradation. In this chapter we address the essence, causes, extent, features and implications of various types of chemical and biological soil degradation. The aspects of chemical degradation, such as pollution, acidification, salinization, nutrient depletion and eutrophication are characterized shortly; for biological degradation, harm to soil microbiota and biodiversity, and soil organic matter depletion are considered. Progress in monitoring and modelling or forecasting these types of degradation is also shown. Soils of drylands, the Arctic and all man-made soils are hotspots of chemical and biological degradation. As chemical and biological degradation processes in the microscale are lingering and interacting, they need better awareness and monitoring approaches. Highly developed laboratory methods of soil chemical and biological analyses are existing, but screening methods that work under field conditions are comparatively rare. Biological soil degradation needs further evidence-based research and high-precision data for understanding and combating processes. Crucial questions such as calculation of carbon sequestration potential of agricultural soils and assessment of desertification processes should be better explored to bridge science-policy gaps.
Cement-based solidification/stabilization (S/S) technology is often used to remediate chromium (Cr) contaminated soils. The valence state and mobility of Cr in soils are closely related with redox potential (EH). However, Cr mobilization from the solidified soils influenced by EH has received little attention. In this study, semi-dynamic leaching tests and the toxicity characteristic leaching procedure (TCLP) were performed on a S/S treated Cr contaminated soil under various EH conditions. The effective diffusion coefficient and leachability index were obtained from the leaching data to investigate the leaching behavior of Cr from the S/S treated soil. Speciation of Cr remained in the sample after the leaching process was obtained through the sequential extraction procedures. The results show that an increase in EH increases the effective diffusion coefficient of Cr and, therefore, the amount of Cr leached. This result is attributed to immobile Cr(III) being oxidized to highly mobile Cr(VI). The leachability index results indicate that the cement solidification of Cr contaminated soil may not be appropriate under oxidizing conditions. For the TCLP and sequential extraction procedures, the leached amount of Cr exhibits a strong dependence on EH. As EH increases, the content of Cr remaining in the soil in unstable phases reduced, and more Cr was released to leachant.
The influence of sediment redox potential and pH on As and Se speciation and solubility weas studied. Hyco Reservoir (North Carolina) sediments were equilibrated under controlled redox (500, 200, 0, and {minus}200 mV) and pH (5, natural, and 7.5) conditions. Redox potential and pH affected both speciation and solubility of As and Se. Under oxidized conditions As solubility was low and 87% of the As in solution was present as As(V). Upon reduction, As(III) became the major As species in solution, and As solubility increased. Total As in solution increased approximately 25 times upon reduction to {minus}200 mV. No organic arsenicals were detected. In contrast to As, Se solubility reached a maximum under highly oxidized (500 mV) conditions and decreased significantly upon reduction. Selenium (VI) was the predominant dissolved Se species present at 500 mV. At 200 and 0 mV, Se(IV) became the most stable oxidation state of Se. Under strongly reduced conditions ({minus}200 mV) oxidized Se species were no longer detectable and Se solubility was controlled by the formation of elemental Se and/or metal selenides. Biomethylation of Se was important under oxidized and moderately reduced conditions (500, 200, and 0 mV). More alkaline conditions (pH 7.5) resulted in both greater As and Se concentrations in solution. Dissolved As and Se increased up to 10 and 6 times, respectively, as compared to the more acidic equilibrations.
Due to their high degree of reactivity, Mn oxides in soil systems may exert a greater influence on trace metal chemistry than that suggested by their relatively low abundance. In particular, Mn is the only known oxidizer of trivalent Cr in soils. We investigated soil properties that influence the Cr oxidizing capacity of Mn oxides in eight well-aerated high Mn soils. Total and easily reducible Mn abundance were quantified by extraction with 1.5 M NH 2OH·HCl and 0.02 M hydroquinone. Relative average Mn oxidation state in soil samples was determined by x-ray absorption near edge structure spectroscopy (Mn-XANES) main edge energy position. Soils ranged in percentage of NH2OH·HCl-extractable Mn between 0.14 and 1.27, pH between 4.4 and 7.2, and percentage of C between 9.0 and 27.2. Manganese-XANES spectra showed that most of the study soils had a high Mn(IV)/Mn(III) ratio with edge energy position intermediate to that of a synthetic birnessite and a synthetic pyrolusite. In these high Mn soils, Mn-XANES edge energy was positively correlated with soil pH, suggesting a linear increase, over the normal range of soil pH, in the Mn(IV)/Mn(III) ratio of soil oxides. Soils with more total reducible Mn generally demonstrated greater net Cr(VI) production, but this pattern was moderated by soil pH and relative Mn oxidation state. High Mn soils with low pH and Mn oxidation state were weaker Cr oxidizers than their Mn abundance would suggest. Our data provide evidence that greater Mn abundance and greater Mn(IV)/Mn(III) ratio in soil Mn oxides enhances Cr oxidation.
Purpose
Iron’s fluctuation between the II (ferrous) and III (ferric) oxidation states has been coined as the “FeIII–FeII redox wheel.” Numerous studies have coupled the “iron redox wheel” with the biogeochemical cycle of carbon (C), nitrogen (N), sulfur (S), or phosphorus (P) individually in soils or sediments, but evidence suggests that the FeIII–FeII redox wheel drives the biogeochemical cycles interactively in a fluctuating redox microenvironment. The interactions of the FeIII–FeII redox wheel with the biogeochemical cycles of C, N, S, and P in the fluctuating redox environments were reviewed in this paper.
Discussion
In this review, we discuss the importance of iron with regard to each of the biogeochemical cycles individually as well as interactively. The importance of crystalline and non-crystalline FeIII (hydr)oxides is highlighted as they serve as terminal electron acceptors for organic matter mineralization and N and S transformation and also act as sorbents for dissolved P compounds. Mechanically, electron transfer from organic matter to FeIII (hydr)oxides via organic matter oxidation, oxidation of NH
4+ to NO
2−, formation and oxidation of Fe sulfide minerals in the S cycle, and P transformation were discussed to couple with the FeIII–FeII redox wheel.
Conclusions
The knowledge gaps are identified at the end of the review. The natural environmental relationships still require further studies that link the iron redox wheel as a driver of the biogeochemical cycles of C, N, S, and P. Anthropogenically altered environments (nutrient and metal elevation, global warming, and acidification) require intensive studies to allow for improved integrated modeling of global C, N, S, and P biogeochemical cycles driven by the FeIII–FeII redox wheel.
In forest ecosystems, organic solutes play prominent roles in pollutant and nutrient transport. This study, conducted in subalpine forested Humaquepts in Switzerland, investigated the influence of redox conditions and flow processes on dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). In the mineral soil, concentrations of DOC were higher under reduced than under oxidized conditions. They averaged 1.2 mmol DOC L-1 in the reduced mineral soil at a 100-cm depth. A close correlation between DOC and dissolved Fe concentrations (r2 = 0.83; p < 0.001) suggests that reductive dissolution of Fe-oxides was the major reason for the low retention. However, during aerobic sampling of soil solution from the reduced mineral soil, DOC coprecipitated with Fe. This suggests that the DOC input from reduced mineral soils to aerobic stream water is lower than expected from the high DOC concentrations in the soil. Organic N was the major form of total dissolved N at all soil depths. In the reduced subsoil, dissolved N was completely organically bound. This was probably due to both an immobilization of inorganic N and a low retention of DON. During storms, DOC concentrations increased rapidly with increasing discharge in the subsurface flow. At peak flow, DOC concentrations were doubled, compared with base flow. The molar UV absorptivity of DOC in the subsurface flow corresponded closely to that of the topsoil. These findings suggest that DOC concentrations and properties are sensitive to flow velocities, and that DOC is preferentially transported to the subsoil at high discharge.
Chromium, in the trivalent form (Cr(III)), is an important component of a balanced human and animal diet and its deficiency causes disturbance to the glucose and lipids metabolism in humans and animals. In contrast, hexavalent Cr (Cr(VI)) is highly toxic carcinogen and may cause death to animals and humans if ingested in large doses. Recently, concern about Cr as an environmental pollutant has been escalating due to its build up to toxic levels in the environment as a result of various industrial and agricultural activities. In this review, we present the state of knowledge about chromium mobility and distribution in the environment and the physiological responses of plants to Cr with the desire to understand how these processes influence our ability to use low cost, environmentally friendly biological remediation technologies to clean up Cr-contaminated soils, sediments, and waters. The use of biological remediation technologies such as bioremediation and phytoremediation for the cleanup of Cr-contaminated areas has received increasing interest from researchers worldwide. Several methods have been suggested and experimentally tested with varying degrees of success.
Bacterial reduction of Fe- and Mn-oxides was studied in a surfacehorizon of a New-Caledonian Ferralsol in batch experiments. Twotreatments were imposed containing different sources of organicmatter (soil organic matter with or without glucose addition) tolink organic matter biodegradation with reduction process. Theconcomitant solubilization of Ni and Co was also studied. Resultsshowed that anaerobic Fe- and Mn-reducing bacterial activity wasresponsible for Fe- and Mn-oxide solubilization by anaerobicrespiration or fermentation. When C was more available, oxidereduction was enhanced. Mn-oxide appeared as the major reduciblephase and metal source rather than goethite. Co and Ni weresolubilized with Fe and Mn but their amounts in solutiondecreased at the end of experiment. The bioavailability of heavymetals in this soil was increased by biological reduction but waslimited by adsorption or precipitation phenomena.
The potential for using concentrations of dissolved H-2 to determine the distribution of redox processes in anoxic groundwaters was evaluated. In pristine aquifers in which standard geochemical measurements indicated that Fe(III) reduction, sulfate reduction, or methanogenesis was the terminal electron accepting process (TEAP), the H-2 concentrations were similar to the H-2 concentrations that have previously been reported for aquatic sediments with the same TEAPs. In two aquifers contaminated with petroleum products, it was impossible with standard geochemical analyses to determine which TEAPs predominated in specific locations. However, the TEAPs predicted from measurements of dissolved H-2 were the same as those determined directly through measurements of microbial processes in incubated aquifer material. These results suggest that H-2 concentrations may be a useful tool for analyzing the redox chemistry of nonequilibrium groundwaters.
Wetland ecosystems maintain a fragile balance of soil, water, plant, and atmospheric components in order to regulate water flow, flooding, and water quality. Marginally covered in traditional texts on biogeochemistry or on wetland soils, Biogeochemistry of Wetlands is the first to focus entirely on the biological, geological, physical, and chemical processes that affect these critical habitats. This book offers an in-depth look at the chemical and biological cycling of nutrients, trace elements, and toxic organic compounds in wetland soil and water column as related to water quality, carbon sequestration, and greenhouse gases. It details the electrochemistry, biochemical processes, and transformation mechanisms for the elemental cycling of carbon, oxygen, nitrogen, phosphorus, and sulfur. Additional chapters examine the fate and chemistry of heavy metals and toxic organic compounds in wetland environments. The authors emphasize the role of redox-pH conditions, organic matter, microbial-mediated processes that drive transformation in wetlands, plant responses and adaptation to wetland soil conditions. They also analyze how excess water, sediment water, and atmospheric change relate to elemental biogeochemical cycling. Delivering an in-depth scientific examination of the natural processes that occur in wetland ecosystems, Biogeochemistry of Wetlands comprises a key perspective on the environmental impact of pollutants and the role freshwater and coastal wetlands play in global climate change.
Trace elements occur naturally in soils and some are essential nutrients for plant growth as well as human and animal health. However, at elevated levels, all trace elements become potentially toxic. Anthropogenic input of trace elements into the natural environment therefore poses a range of ecological and health problems.
As a result of their persistence and potential toxicity, trace elements continue to receive widespread scientific and legislative attention. Trace Elements in Soils reviews the latest research in the field, providing a comprehensive overview of the chemistry, analysis, fate and regulation of trace elements in soils, as well as remediation strategies for contaminated soil
The book is divided into four sections:
• Basic principles, processes, sampling and analytical aspects: presents an overview including general soil chemistry, soil sampling, analysis, fractionation and speciation.
• Long-term issues, impacts and predictive modelling: reviews major sources of metal inputs, the impact on soil ecology, trace element deficient soils and chemical speciation modelling.
• Bioavailability, risk assessment and remediation: discusses bioavailability, regulatory limits and cleanup technology for contaminated soils including phytoremediation and trace element immobilization.
• Characteristics and behaviour of individual elements
Written as an authoritative guide for scientists working in soil science, geochemistry, environmental science and analytical chemistry, the book is also a valuable resource for professionals involved in land management, environmental planning, protection and regulation.
Reviews
"This book is very suitable for soil scientists involved in soil contamination, but also for research chemists, geochemists, agronomists, environmental scientists, ecotoxicologists, and professionals who deal with contaminated soils." (Anal Bioanal Chem, February 2011)
“This is a substantial textbook, divided into four carefully balanced sections, with contributions from 39 international authors. With 38 other authors to organise, the Editor deserves only congratulations. The book is certainly fit for purpose, the production quality is excellent, and I shall have no problem recommending it to our students of Soil Chemistry and Environmental Science.” (European J. Soil Sci., 2010 doi: 10.1111/j.1365-2389.2010.01302.x)
Biogeochemical Factors Governing Co, Ni, Se, and V Dynamics in Periodically Flooded Egyptian North Nile Delta Rice Soils
Abstract
The mobility of water soluble cobalt (Co), nickel (Ni), selenium (Se), and vanadium (V) was determined in fluvial and lacustrine soils used for rice (Oryza sativa L.) production in the northern portion of the Egyptian Nile Delta. The impact of redox potential (EH), soil pH, dissolved organic carbon (DOC), dissolved aromatic carbon compounds (DAC), iron (Fe), manganese (Mn), and sulfate (SO42-) on the dynamics of the studied metals was quantified in soil suspensions using an automated biogeochemical microcosm apparatus. The experiment was conducted stepwise from reducing (-307 mV) to oxidizing (+564 mV) soil conditions. We found a significantly positive correlation between soil EH and pH in both soils. Concentrations of Co, Ni, Se, DOC, Fe, and Mn were higher under reducing conditions than under oxidizing conditions. This suggests that the changes of EH/pH, Fe, Mn, and DOC might be linked to the dynamics of Co, Ni, and Se in both soils. The specific UV absorbance (SUVA) and concentrations of V were higher under oxidizing conditions than under reducing conditions especially in the fluvial soil. This result implies that release of aromatic carbon compounds might be related to release of V in this soil. Our findings suggest that a release of Co, Ni, Se, and V in temporally flooded rice soils should be considered due to increased mobility and the potential environmental risks including food security in using metal-enriched soils for flooded agricultural systems.
Key Words: Redox potential (EH); dissolved organic carbon (DOC); Specific UV absorbance (SUVA), Trace elements; Wetland soils
The mobility of water soluble cobalt (Co), nickel (Ni), selenium (Se), and vanadium (V) was determined in fluvial and lacustrine soils used for rice (Oryza sativa L.) production in the northern portion of the Egyptian Nile Delta. The impact of redox potential (EH), soil pH, dissolved organic carbon (DOC), dissolved aromatic carbon compounds (DAC), iron (Fe), manganese (Mn), and sulfate (SO42-) on the dynamics of the studied metals was quantified in soil suspensions using an automated biogeochemical microcosm apparatus. The experiment was conducted stepwise from reducing (-307 mV) to oxidizing (+564 mV) soil conditions. We found a significantly positive correlation between soil EH and pH in both soils. Concentrations of Co, Ni, Se, DOC, Fe, and Mn were higher under reducing conditions than under oxidizing conditions. This suggests that the changes of EH/pH, Fe, Mn, and DOC might be linked to the dynamics of Co, Ni, and Se in both soils. The specific UV absorbance (SUVA) and concentrations of V were higher under oxidizing conditions than under reducing conditions especially in the fluvial soil. This result implies that release of aromatic carbon compounds might be related to release of V in this soil. Our findings suggest that a release of Co, Ni, Se, and V in temporally flooded rice soils should be considered due to increased mobility and the potential environmental risks including food security in using metal-enriched soils for flooded agricultural systems.
Quantification of soil water flow is a prerequisite to accurate prediction of solute transfer within the unsaturated zone. The monitoring of these fluxes is challenging because the results are required to answer both scientific and practical questions regarding protection of groundwater, sustainable management of agricultural, forestry, mining or set-aside industrial areas, reducing leachate loss from landfills or explaining the fate of environmentally harmful substances. Both indirect and direct methods exist for estimating water-flux rates and have been used with varying success. In Europe, the use of direct lysimetry methods for measuring water and solute fluxes in soils has increased in recent years. This technique ensures reliable drainage data, but requires relatively large investment and maintenance expenses. Other research groups, especially in the USA, have developed alternative techniques. In this paper we compare the functioning of a passive-wick sampler, especially the deep-drainage meter type (DDM), with two different types of drainage lysimeters (weighing and non-weighing) under field conditions in Germany for the measurement period from May 2004 until April 2009. The study showed that under sandy soil conditions no significant differences occurred between the measurements from DDM and both drainage lysimeter types. Only in periods with increased precipitation was there a tendency of drainage over-estimation by the DDM in comparison with the lysimeters tested. For longer periods, no significant differences in the amount of drainage or the pattern of drainage formation were found between weighing and non-weighing gravitation lysimeters. The practical use of DDMs is restricted because the groundwater level must be >2 m from the soil surface. Suggestions are made for the technical improvement of the DDM as well as the testing of the device with more cohesive soils.
Soils and irrigation drainwaters from the west side of the San Joaquin Valley, California have elevated levels of Mo, U, B, V, As, and Se. Much of the drainwater is disposed of in evaporation ponds that may be periodically dried, resulting in cyclic changes in the redox status of the pond sediments. A laboratory incubation study was conducted to evaluate the effects of redox status on the solubility of trace elements in saline sediments and soils from the San Joaquin valley. The elements U and Mo were mobilized under oxidizing conditions, while Fe, Mn, Ni, V, and As were more soluble under reducing conditions. Reduction and precipitation of Mo, apparently as MoS[sub 2], was observed from 1 to 10 days after flooding, but when reaerated the Mo resolubilized in less than 1 day. The reductive dissolution of Fe- and Mn-oxyhydroxides released adsorbed B, which resulted in an increased solution concentration of B. Low redox conditions in drainwater ponds may reduce the solution concentrations of U, Mo, and Se, thereby reducing their chances of entering the food chain through algae uptake. However, low redox conditions favor the solubilization of As and V, which may pose an alternative threat to wildlife visiting the ponds. 16 refs., 7 figs., 1 tab.
Potential mechanisms for the lack of Fe(II) accumulation in Mn(IV)‐con‐taining anaerobic sediments were investigated. The addition of Mn(IV) to sediments in which Fe(III) reduction was the terminal electron‐accepting process removed all the pore‐water Fe(II), completely inhibited net Fe(III) reduction, and stimulated Mn(IV) reduction. In a solution buffered at pH 7, Mn(IV) oxidized Fe(II) to amorphic Fe(III) oxide. Mn(IV) naturally present in oxic freshwater sediments also rapidly oxidized Fe(II). A pure culture of a dissimilatory FE(III)‐ and Mn(FV)‐reducing organism isolated from the sediments reduced Fe(III) to Fe(II) in the presence of Mn(IV) when ferrozine was present to trap Fe(II) before Mn(IV) oxidized it. Depth profiles of dissolved iron and manganese reported in previous studies suggest that Fe(II) diffusing up from the zone of Fe(III) reduction is consumed within the Mn(IV)‐reducing zone. These results demonstrate that preferential reduction of Mn(IV) by Fe(III)‐reducing bacteria cannot completely explain the lack of Fe(II) accumulation in anaerobic, Mn(IV)‐containing sedments, and indicate that Mn(IV) oxidation of Fe(II) is the mechanism that ultimately prevents Fe(II) accumulation.
Two long-term submerged Eutric Gleysols (GLe) and two short-time flooded Eutric Fluvisols (FLe) at the Elbe River (Germany) with high organic carbon contents (Corg between 4.9 and 11.6%) were selected to evaluate dehydrogenase activity (DHA), soil microbial carbon (Cmic), Cmic/Corg ratio, and phospholipid fatty acids (PLFA) to characterize and discriminate these soils with microbial parameters. DHA, Cmic and PLFA-biomass as well as Cmic/Corg ratios are considerable lower in GLe's than in FLe's what seem to be an unspecific response of aerobic soil microorganisms on the long flooding period and the resulting short time for development after last flooding. Cmic/Corg ratios are low in comparison to terrestrial soils. PLFA profiles were dominated by saturated fatty acids (FA). Principal component analyses (PCA) with the identified FAs revealed a clear discrimination among the four floodplain soils. In GLe's, all groups of PLFA, inclusive monounsaturated FA, are lowest and in FLe's highest. Polyunsaturated FA fungi biomarker (18:2ω6,9c) was very low in GLe1 and could not be detected in GLe2. The environmental conditions which microorganisms are exposed seem to be disadvantageous for fungi in these long submerged soils.
Abbreviations: DOC, dissolved organic carbon; EH, redox potential; GC, gas chromatograph; OM, organic matter. Six soils used for rice (Oryza sativa L.) production were incubated using an automatic micro- cosm system. Production of trace gases (CO2, CH4, and N2O) and transformation of N, S, and metals (Fe and Mn) were studied in soil suspensions incubated from reducing to oxidiz- ing conditions. Results show that soil pH variation was inversely correlated to soil redox potential (EH) change (P < 0.01). Soil CO2 production exponentially increased with soil EH increase. In contrast, soil CH4 production and DOC showed an exponential decrease with soil EH increase. Without the presence of soil oxidants, methanogenesis occurred across the entire EH range, with probable H2-supported methanogenesis at higher soil EH conditions constituting up to 20% of total CH4 production. The CH4 compensation point, where CH4 concentration became constant due to equilibrium between CH4 production and con- sumption, exponentially decreased with soil EH increase. At pH 7, the critical EH above which soils consumed atmospheric CH4 varied among the soils, but was generally >400 mV. Signifi cant N2O production was observed between 200 and 500 mV. Nitrifi cation could also contribute to N2O production when EH is >500 mV, a possible critical EH for the initiation of nitrifi cation. The critical EH for substantial immobilization of Fe and Mn was estimated to be around 50 and 250 mV, respectively. The intermediate EH range (approximately −150 to 180 mV) provided optimum conditions for minimizing cumulative global warming poten- tial resulting from CO2, CH4, and N2O production in soils. Our results have implications in interpreting the overall benefi ts of soil C sequestration efforts.
【Summary】 Humic substances (HSs) are by far the most abundant of the organic components of nature, and are present in all soils and natural waters that contain organic matter. The more widely accepted values for organic carbon (OC) in soil organic matter (SOM) are in the range of (14~15)×10 17
Geochemical assessment has become a cost-effective and highly accurate tool for estimating metal contamination, especially in those cases where the level of contamination is not considered severe. Difficulties frequently arise in attempting to discriminate between pristine metal concentrations and low-level environmental impacts. As all example, vanadium contamination is frequently associated with coal and petroleum by-products; however air and water contamination pathways are also possible. The purpose of this investigation was to characterize the V and Fe concentration relationship among a wide variety of soil types and to formulate an estimate of the pristine V concentrations in these soils. If a linear relationship may be established between Fe and V then geochemical analysis of impacted soils may discriminate between V as a natural background component and anthropogenic V Forty-six moderately well-drained to well-drained soil profiles having cambic, argillic or calcic soil horizons were characterized,for Fe and V using an aqua-regia digestion to determine if these soils exhibited a one-to-one correspondence between V and Fe. Such a correspondence was authenticated for the majority of these soils and may be used to discriminate between natural and anthropogenic V The presence of argillic, calcic or fragipan horizons did not reduce the one-to-one correspondence between V and Fe, suggesting that these soil processes did not selectively partition either V or Fe. The method needs to be further evaluated for soils having anoxic soil conditions, lithologic discontinuities and other specific pedogenic processes.
The dissolved state of chromium in seawater has long been studied and in particular which species, trivalent or hexavalent, is predominant. Research results are not, however, consistent, although the method used has usually been the same—using ferric hydroxide as coprecipitation carrier (Table 11–7). We have studied the coprecipitation behaviour of chromium with ferric hydroxide and other metal hydroxides in the presence of naturally existing inorganic and organic ions to attain an accurate analytical method for determining chromium content, as well as to estimate its behaviour in natural waters8,9. We report here that neither Cr(VI) nor organic species coprecipitate with ferric hydroxide in seawater; Cr(VI) can be quantitatively captured by the coprecipitation with bismuth hydroxide without any pretreatment (such as reduction); and the role of manganese oxide should be considered in addition to the amount of dissolved oxygen to understand the redox system of chromium in natural waters. The inconsistency of the past research may therefore be partly because the presence of organic species was not taken into account and was analysed as either Cr(VI) or Cr(III) or overlooked entirely.
Dissolved organic matter (DOM) in soils plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize the recent literature about controls on DOM concentrations and fluxes in soils. We focus on comparing results between laboratory and field investigations and on the differences between the dynamics of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP).
Both laboratory and field studies show that litter and humus are the most important DOM sources in soils. However, it is impossible to quantify the individual contributions of each of these sources to DOM release. In addition, it is not clear how changes in the pool sizes of litter or humus may affect DOM release. High microbial activity, high fungal abundance, and any conditions that enhance mineralization all promote high DOM concentrations. However, under field conditions, hydrologic variability in soil horizons with high carbon contents may be more important than biotic controls. In subsoil horizons with low carbon contents, DOM may be adsorbed strongly to mineral surfaces, resulting in low DOM concentrations in the soil solution. There are strong indications that microbial degradation of DOM also controls the fate of DOM in the soil.
Laboratory experiments on controls of DOM dynamics have often contradicted field observations, primarily because hydrology has not been taken into account. For example, laboratory findings on the effects of plant species (conifer vs. deciduous) on DOM release from forest floors and on the effects of substrate quality (e.g.: C/N ratio) or pH on DOC concentrations were often not confirmed in field studies. The high adsorption capacity of soil clay minerals and oxides for DOM shown in laboratory studies may not control the transport of DOM in soils in the field if macropore fluxes dominate under field conditions. Laboratory findings about the biodegradability of DOM also await verification under field conditions.
Studies that include DON and DOP dynamics in addition to DOC are few. The rate of release and the fate of DOC, DON, and DOP in soils may differ to a far greater extent than previously assumed. Controls established for DOC might thus be not valid for DON and DOP.
Despite intensive research in the last decade, our knowledge of the formation and fate of DOM in soils and its response to changing environmental conditions is still fragmented and often inconsistent. Predictions at the field scale are still very uncertain, and most of the information available today is the result of studies on temperate soils and forest ecosystems. Thus, future research on controls of DOM dynamics should be extended to soils under different land uses and in other climate zones. Emphasis should also be given to: (i) the effects of soil organic matter properties on the release of DOM (ii) environmental factors controlling DOM quantity and quality (iii) the assessment of biological versus physico-chemical controls on the release and retention of DOM in soils, and (iv) the differences between DOC, DON, and DOP. Finally, if our goal is to predict DOM concentrations and fluxes in soils, future research on the controls of DOM dynamics should have a strong focus on field studies.
Vanadium (V) is a trace element involved in soil pollution, originating from either soil parent material or anthropogenic sources. The aim of this study was to investigate the distribution of total and ammonium bicarbonate-DTPA-extractable (AB-DTPA) V in soil profiles of representative Greek and Egyptian soils and their relationships to soil properties. Twenty-one soil profiles from Egypt and Greece (representing the main soil orders, that is, Entisols (developed on fluvial, lacustrine, and marine deposits) and Aridisols for Egyptian soils and to the soil orders Entisols, Alfisols, Inceptisols, Vertisols, Mollisols, and Histosols for Greek soils) were sampled and analyzed for total and AB-DTPA-extractable V, and the relationship of V levels to soil properties were examined.
Total V concentrations ranged from 23 mg kg−1 in the marine deposits to 179 mg kg−1 in the lacustrine deposits. Total V levels significantly positively correlated to clay and silt content, cation exchange capacity, and free iron and manganese oxides and were negatively correlated to sand, organic matter, and calcium carbonate content. The AB-DTPA-extractable V varied from 0.55 mg kg−1 in the Greek Entisol to 4.4 mg kg-1 in the Egyptian lacustrine deposits and were significantly positively correlated with total V concentration, soil pH, clay and silt content, and cation exchange capacity (positively) and negatively correlated with sand content. Distribution of total and AB-DTPA-extractable V related mainly to particle size distribution, sesquioxides content, and soil pH. These results suggest that V could be a concern for many of the soils studied because in a large number of samples, V concentration values exceeded the international regulatory standards for remediation.
Changes in concentrations and properties of dissolved organic matter (DOM) caused by oxygen deficiency are poorly understood. We estimated the influence of redox conditions on DOM dynamics in the field, sampling soil solutions at different depths of three soils (Humic and Histic Gleysol, Chromic Cambisol) along a soil catena in the cool-humid Black Forest (Germany) over a period of 2 years. We measured dissolved organic carbon (DOC) and determined the specific absorbance at 280 nm and two humification indices derived from fluorescence spectra to describe the aromaticity and complexity of DOM. Redox potential (Eh) was monitored continuously in situ. In the forest floor, DOC concentrations ranged independent of soil organic matter content and redox regime between 40 and 60 mg C L-1. DOC concentrations in all soils decreased with depth, accompanied by a decrease in DOM aromaticity and complexity. In the mineral subsoil, DOC concentrations, aromaticity, and DOM complexity were smallest in the aerobic soil (Chromic Cambisol; Eli > 500 mV) and largest in the most anaerobic soil (Histic Gleysol; Eli < 100 mV). Large DOM retention in the aerobic soil could be related to high contents of Fe oxides, highlighting their importance for DOM adsorption. Despite significantly reduced DOM retention under anaerobic conditions, it remains relatively large because the main DOM adsorbents changed from Fe oxides under oxic conditions to clay minerals, which were about 100 times more abundant under anaerobic conditions than Fe oxides. We found indications that biodegradation of DOM contributes more to DOM retention under anaerobic conditions, and we conclude that large DOM fluxes from anaerobic forest soils are the result of limited DOM adsorption in the subsoil rather than large DOM release from the topsoil.
Inorganic arsenite (As(III)) and arsenate (As(V)) are toxic pollutants that are transported in surface and ground waters and can adsorb on soil and sediment mineral surfaces. Because of the importance of adsorption reactions in determining the overall mobility of arsenic, we investigated the adsorption of As(III) and As(V) on three soils at varying As concentration, pH, and ionic strength. The objectives were to characterize and compare As(III) and As(V) adsorption on three arid-zone soils from California(Wasco, Fallbrook, and Wyo) and to determine the relationship between soil properties and adsorption. Chromatographic speciation of As(III)/(V) revealed that the three soils contained low levels of background As(V). Oxidation of added As(III) to As(V) was not detected below pH 8 in soil suspensions during 16-h adsorption experiments; however, As(III) oxidation was detected at higher adjusted pH. The soil with the highest citrate-dithionite extractable Fe and %clay (Wyo) had the highest affinity for As(III) and As(V) and displayed adsorption behavior similar to pure ferric oxide. Adsorption isotherms indicated that As(V) species adsorbed more strongly than As(III) under most conditions. However, a pH-dependent reversal in the relative affinity of As(III) and As(V) for the soils was observed at low As surface coverage.
Experiments were conducted to identify and quantify biogeochemical processes controlling chromium redox chemistry in a seasonally flooded Lower Mississippi Valley forested wetland. Chromium speciation, transformations, and solubility were studied in the overlying floodwater column and in the wetland soil. In the floodwater column, chromium chemistry was regulated by Cr(VI) diffusion into the soil, Cr(III) precipitation, complexation by organic ligands, sorption on suspended solids, and Cr(III) to Cr(VI) oxidation. Wetland soil redox level determined the rate and capacity of the soil to assimilate and retain chromium. Under oxidized and moderately reduced (+500 to +100 mV) soil conditions, chromium behavior was dominated by Cr(VI) sorption and reduction of Cr(VI) to Cr(III). Under more reduced soil redox (<+100 mV) levels, chromium chemistry and solubility was controlled by the chemical reduction of Cr(VI) by soluble ferrous iron. Results obtained suggest that the studied bottomland hardwood wetland or floodplain will serve as a sink for chromium entering the wetland, thereby reducing the contamination of downstream ecosystems.
IntroductionGeogenic OccurrenceSources of Soil ContaminationChemical Behavior in SoilsRisks from Arsenic and Antimony in SoilsConclusions and Future Research NeedsReferences
The dependency of the retention of dissolved organic carbon (DOC) on mineral phase properties in soils remains uncertain especially at neutral pH. To specifically elucidate the role of mineral surfaces and pedogenic oxides for DOC retention at pH 7, we sorbed DOC to bulk soil (illitic surface soils of a toposequence) and corresponding clay fraction (< 2 mum) samples after the removal of organic matter and after removal of organic matter and pedogenic oxides. The DOC retention was related to the content of dithionite-extractable iron, specific surface area (SSA, BET-N-2 method) and cation exchange capacity (pH 7). The reversibility of DOC sorption was determined by a desorption experiment. All samples sorbed 20-40% of the DOC added. The DOC sorption of the clay fractions explained the total sorption of the bulk soils. None of the mineral phase properties investigated was able to solely explain the DOC retention. A sorption of 9 to 24 mug DOC m(-2) indicated that DOC interacted only with a fraction of the mineral surface, since loadings above 500 mug m(-2) would be expected for a carbon monolayer. Under the experimental conditions used, the surface of the silicate clay minerals seemed to be more important for the DOC sorption than the surface of the iron oxides. The desorption experiment removed 11 to 31 % of the DOC sorbed. Most of the DOC was strongly sorbed.
Different methods exist for measuring soil water and solute fluxes in and below the root zone and have been critically reviewed.
Besides indirect methods (e.g. water balance, tensiometer, time domain reflectometry – TDR, frequency domain reflectometry
– FDR, environmental tracer) direct methods (e.g. drainage-type lysimeter, water fluxmeter) have a long tradition and have
been successfully used in seepage research. A large weighable out door lysimeter is the best method for obtaining reliable
data about seepage water quantity and quality, but it involves significant investment and additional expenses for maintenance.
To tackle this problem new methods for the vertical collection of large volume soil monoliths (up to 6m3) as well as for the horizontal collection (up to 6m3) have been developed. For the placement of the lysimeter a container lysimeter unit was constructed, which is cheaper than
a conventional steel or concrete cellar. Furthermore, the technical design of the newly developed lysimeter types as a weighable
gravitation lysimeter, a weighable groundwater lysimeter and a lateral flow lysimeter are presented.
Two long-term submerged Eutric Gleysols (GLe) and two short-term flooded Eutric Fluvisols (FLe) with high organic carbon contents
(Corg between 5.1 and 12.9%) were selected to characterize soil microbial communities at the Elbe River (Germany). Measurements
included dehydrogenase activity (DHA), soil microbial carbon (Cmic), soil basal respiration (BR), metabolic quotient (qCO2), Cmic/Corg ratio, and phospholipid fatty acids (PLFA). PLFA biomass, DHA, and Cmic/Corg ratios were considerable lower in GLe’s than in FLe’s. Whereas the BR as well as qCO2 were higher in GLe’s what seems to be an unspecific response of aerobic soil microorganisms to the long flooding period and
the resulting short time for development following flooding. Cmic/Corg ratios were low in comparison to terrestrial soils. PLFA profiles were dominated by saturated fatty acids (FA). Principal
component analyses (PCA) of FAs revealed clear differences among the four floodplain soils. In GLe’s all fractions of PLFAs
were lower than in FLe’s. Polyunsaturated FA biomarkers (18:2ω6,9c) were 10 times lower in GLe’s. Our results indicate that
the environmental conditions in which microorganisms are exposed (i.e., long term soil inundation and anoxia) seem to be disadvantageous
for fungi.
The Elbe River, Germany, has received heavy metals and arsenic from the discharge of urban industrial, and agricultural effluent. During periods of inundation, these contaminants were transported with water into floodplain ecosystems, where they settled and accumulated predominantly in depressions and low-lying terraces. Markedly elevated arsenic concentration in soil solution during floods exceeded the inspection value of 10 μg L−1 of the German soil protection ordinance. Highly variable hydrological conditions in floodplains can affect the dynamics of pollutants. The study of processes controlling the dynamics of pollutants is challenging because the results are required to answer both scientific and practical questions regarding protection of groundwater and plants, sustainable management of floodplains or explain the fate of environmentally harmful substances.Our experiments in small groundwater lysimeter and biogeochemical microcosms tended to yield similar results regarding the functional relationships among the investigated site parameters. But the results of the field experiments, carried out at a floodplain site of the middle course of the Elbe River, Germany, are often characterized by complex and varying factors. Whereas arsenic tended to be mobilized during flooding due to decreasing redox potential (EH), chromium showed the opposite trend, with peak concentrations at the highest EH values. Our approach at three different spatiotemporally scale levels, ranging from 23 days (microcosms) to two-and-a-half years (field soil hydrological facility) allows us to overcome process interferences observed in field studies.
Although direct microbial reduction of Cr(VI) and U(VI) is known, few studies have examined the kinetics and the underlying mechanisms of the reduction of these contaminants by different natural organic matter (NOM) fractions in the presence or absence of microorganisms. In this study, NOM was found to chemically reduce Cr(VI) at pH 3, but the reduction rates were negligible at pH ∼7. The abiotic reduction of U(VI) by NOM was not observed, possibly because of the presence of small amounts of nitrate in the reactant solution. However, all NOM fractions, particularly the soil humic acid (HA), enhanced the bioreduction of Cr(VI) or U(VI) in the presence of Shewanella putrefaciens CN32. The reduction rates varied greatly among NOM fractions with different chemical and structural properties: the polyphenolic-rich NOM-PP fraction appeared to be the most reactive in abiotically reducing Cr(VI) at a low pH, but soil HA was more effective in mediating the microbial reduction of Cr(VI) and U(VI) under anaerobic, circumneutral pH conditions. These observations are attributed to an increased solubility and conformational changes of the soil HA with pH and, more importantly, its relatively high contents of polycondensed and conjugated aromatic organic moieties. An important implication of this study is that, depending on chemical and structural properties, different NOM components may play different roles in enhancing the bioreduction of Cr(VI) and U(VI) by microorganisms. Polycondensed aromatic humic materials may be particularly useful in mediating the bioreduction and rapid immobilization of these contaminant metals in soil.
Aggregating floodplain soil- and substrate-types allows a simple and practicable transfer of their properties, with little loss of information. Spatial distribution of floodplain soils along the Central Elbe River with increased concentrations of nutrients and heavy metals tend to be determinable and definable. Soils of the low-lying terraces (“Tschernitzen” consisting of floodplain silt and loam) and soils in depressions (“Gleye” consisting of floodplain clay and silt) have the largest concentrations of nutrients and pollutants because they reveal high humus content and fine mineral particles as a result of sedimentation through frequent or extended flooding periods with low flow-rates. Proper knowledge of the distribution of floodplain soil- and substrate-types allows predicting spatial distributions of concentrations of nutrients and pollutants, although high variability within soil types and transitions between them remains a challenge to be solved.
Trace metals associate with Fe(III) oxides as adsorbed or coprecipitated species, and consequently, the biogeochemical cycles of iron and the trace metals are closely linked. This communication investigated the solubilization of coprecipitated Co(III) and Ni(II) from goethite (α-FeOOH) during dissimilatory bacterial iron reduction to provide insights on biogeochemical factors controlling trace-element fluxes in anoxic environments. Suspensions of homogeneously substituted Co-FeOOH (50 mmol/L as Co0.01Fe0.99OOH; 57Co-labeled) in eight different buffer/media solutions were inoculated with a facultative, metal-reducing bacteria isolated from groundwater (Shewanella putrefacians CN32), and incubated under strictly anaerobic conditions for periods up to 32 days. Lactate (30 mmol/L) was provided as an electron donor. Growth and non-growth promoting conditions were established by adding or withholding PO4 and/or trace metals (60Co-labeled) from the incubation media. Anthraquinone disulfonate (AQDS; 100 μmol/L) was added to most suspensions as an electron shuttle to enhance bacterial reduction. Solutions were buffered at circumneutral pH with either PIPES or bicarbonate buffers. Solid and liquid samples were analyzed at intermediate and final time points for aqueous and sorbed/precipitated (by HCl extraction) Fe(II) and Co(II). The bioreduced solids were analyzed by X-ray diffraction and field-emission electron microscopy at experiment termination. Ni-FeOOH (Ni0.01Fe0.99OOH) was used for comparison in select experiments. Up to 45% of the metal containing FeOOH was bioreduced; growth-supporting conditions did not enhance reduction. The biogenic Fe(II) strongly associated with the residual Fe(III) oxide as an undefined sorbed phase at low fractional reduction in PIPES buffer, and as siderite (FeCO3) in bicarbonate buffer or as vivianite [Fe3(PO4)2 · 8H2O] when P was present. Cobalt(III) was reduced to Co(II) in proportion to its mole ratio in the solid. The release of bioreduced Co(II) to the aqueous phase showed complex dependency on the media and buffer composition and the fractional reduction of the Co-FeOOH. In most cases Co(II) was solubilized in preference to Fe(II), but in select cases it was not. These differences were rationalized in terms of competitive adsorption reactions on the residual Fe(III) oxide surface and coprecipitation in biogenic Fe(II) solids. The bioreduced Co-FeOOH surface showed unexpectedly high sorption selectivity for the biomobilized Co(II). The bioreductive solubilization of Ni(II) from Ni-FeOOH was comparable to Co-FeOOH. Our results indicate that Fe(III)-oxide-entrained trace metals can be mobilized during bacterial iron reduction leading to a net increase, in most cases, in aqueous metal concentrations. The enhancement in trace-metal aqueous concentration, e.g., in groundwater, may proportionally exceed that of Fe(II).
Experiments were conducted to compare the affinity and reactivity of three different minerals for natural organic matter (NOM) in forest floor leachate (FFL) from hardwood and pine forests. The FFLs were acidic (pH 4) with ionic strengths of 1.4 mM (hardwood) and 1.1 mM (pine), and they contained larger organic molecules (weight average molecular weights [Mw] = 5–6 kDa) than has been reported recently for surface waters using similar methods. A synthetic diluent solution was prepared to match the inorganic chemistry of the FFL and to provide a range of initial dissolved organic carbon (DOC) concentrations (0–140 g C m−3) for reaction with goethite (α-FeOOH), birnessite (δ-MnO2) and smectite (montmorillonite, SWy-2) in suspension, and in corresponding blanks.A variety of macroscopic and spectroscopic methods were employed to show that reaction with the three minerals resulted in distinctly different NOM adsorption, fractionation and transformation patterns. Goethite exhibited a steep initial slope in the adsorption isotherm and a maximum retention of 10.5 g C kg−1. The isotherm for montmorillonite was more linear, but equal amounts of C were adsorbed to goethite and montmorillonite (per unit sorbent mass) at maximum DOC. Whereas preferential uptake of high Mw, aromatic constituents via ligand exchange was observed for goethite, compounds of lower than average Mw were retained on montmorillonite and no preference for aromatic moieties was observed. Birnessite, which has an isoelectric point of pH < 2, retained low amounts of organic C (<2 g C kg−1) but exhibited the highest propensity for oxidative transformation of the NOM. The data indicate that fractionation behavior of NOM is dependent on mineral surface chemistry in addition to sorbent affinity for organic C. This work also emphasizes the fact that abiotic transformation reactions must be considered in studies of NOM interaction with Fe(III) and Mn(IV) containing solid phases.
Microbial communities in floodplain soils are exposed to periodical flooding. A long-term submerged Eutric Gleysol (GLe), an intermediate flooded Eutric Fluvisol (FLe), and a short-time flooded Mollic Fluvisol (FLm) at the Elbe River (Germany) with similar organic carbon contents (Corg) between 8.1% and 8.9% were selected to test the quality of phospholipid fatty acids (PLFA), soil microbial carbon (Cmic), basal respiration (BR), metabolic quotient (qCO2), and Cmic/Corg ratio to characterize and discriminate these soils with microbial parameters.
Chromium is a redox active metal that persists as either Cr(III) or Cr(VI) in the environment. These two oxidation states have opposing toxicities and mobilities: Cr(III) is rather benign and immobile in soils while Cr(VI) is toxic and readily transported. Reactions influencing Cr chemistry in soils and waters must be known in order to predict and understand the fate of this potentially hazardous element. Reactions at the solid-water interface have important consequences on the bioavailability (sorption reactions) and hazard (redox reactions) of Cr. Accordingly, this paper describes surface reactions that influence Cr chemistry in soils. Specifically, retention reactions of Cr(III) and Cr(VI) are described, e.g., adsorption of Cr(III) and Cr(VI) on goethite, along with interfacial redox reactions, e.g., Cr(III) oxidation by manganese oxides and Cr(VI) reduction by Fe(ll). The influences of organic chelates on these reactions are also detailed. Direct evidence on the specific reactions of Cr at the solid-water interface are provided; techniques used in this paper to detail the reactions of Cr include X-ray absorption fine structure spectroscopy, scanning probing microscopies, and high-resolution transmission electron microscopy.
In wetlands, large quantities of dissolved organic matter (DOM) are solubilized under reducing conditions. Controlled incubations of a wetland soil were performed under oxic and anoxic conditions to investigate the extent to which the following processes account for this phenomenon: i) production of organic metabolites by microbes during soil reduction; ii) release of organic matter (OM) from Mn- and Fe-oxyhydroxides that undergo reductive dissolution; and iii) desorption of OM from soil minerals due to pH changes. Anaerobic incubation releases 2.5% of the total soil organic carbon (OC) as dissolved organic carbon (DOC), and is accompanied by a pH rise from 5.5 to 7.4 and by the soil Mn- and Fe-reduction. The three processes above all take place. However, anaerobic incubation at a constant pH of 5.5 (preventing OM desorption) releases only 0.5% of the total soil OC, while aerobic incubation at pH 7.4 (preventing Mn- and Fe-reduction) releases 1.7% of the total soil OC. By contrast, aerobic incubation at pH 5.5 (preventing both Mn- and Fe-reduction and pH rise) does not solubilize any DOC. The DOC released is markedly aromatic, indicating little contribution from microbial metabolites, but, rather, the presence of microbes leading to OM mineralization. The pH rise is the key factor controlling OM solubilization under reducing conditions. This rise of pH accounts for > 60% of the total released DOC, which is not due to reductive dissolution as such.
The various pathways for the oxygenation of ferrous iron and for the dissolution of Fe(III) (hydr) oxides, especially by reducing ligands with oxygen donor atoms in thermal and photochemical processes, are assessed on the basis of laboratory experiments for application to natural systems. The typically large specific surface area of Fe-bearing solids in natural systems and the ability of these surfaces to interact chemically (surface complexation, ligand exchange) with reductants and oxidants facilitates electron transfer as well as dissolution and precipitation.Adsorption of Fe(II) to particle surfaces (complexation with surface hydroxyl groups) enhances the oxygenation rate of Fe(II) in a similar way as hydrolysis in solution (complexation with OH− ions). Surface processes (and not transport processes) control the dissolution kinetics. The rate of dissolution is proportional to the surface complexes formed on the surface of Fe(III) (hydr)oxides. Thus, a reductant, such as ascorbate, exchanges electrons with a surface Fe(III) ion subsequent to its inner-sphere coordination to the oxide surface. The Fe(II) thereby formed becomes more easily detached from the surface.Complex formation reactions of Fe(III) and Fe(II) with organic and inorganic ligands to form solute and solid complexes makes it possible that electron cycling of Fe(III)-Fe(II) transformations can occur over the entire EH range within the stability of water (EH from −0.5 V to +1.1 V). Solid and solute Fe(II) complexes with silicates, with hydrous oxides (e.g., Fe3O4), and with sulfides are very efficient reductants from a thermodynamic as well as from a kinetic point of view.Photosynthetic processes occurring on some inorganic Fe-bearing surfaces (semiconductors) and with iron species may be looked at as “primitive” alternatives or precursors to biological photosynthesis. In light-induced reductive dissolution of Fe(III) (hydr)oxides, dissolved Fe(II) (example: reduction of solid Fe(III) phases with an organic ligand such as oxalate) is formed. In the heterogeneous photoredox reaction, the inner-sphere surface coordination of the electron donor to the oxide surface is essential for the efficiency of the electron transfer.
Leaching of dissolved organic carbon (DOC) from the forest floor is an important C flux and influences other biogeochemical fluxes in forests. To determine what controls the quantity and quality of the DOC produced, we examined the effects of microclimate on DOC production in Oa-horizon material from a red spruce forest. Samples incubated under different temperature and moisture conditions were leached with a mechanical-vacuum extractor every 7 days for 8 to 10 weeks, or once after 1 to 14 days. The concentration and, in some cases, the composition of the DOC in the extracts were measured. Production of DOC in dry samples (moisture content from 0.5 to 1.7 g g−1) was approximately 1.2 mg g−1 in the first week, but declined by 77% over 8 weeks. In sieved samples, production declined to 40% of initial values, whereas production in unsieved, moist samples declined by less than 30%. In wetter samples (moisture content from 1.8 to 5 mg g−1) DOC production increased by approximately 0.1 mg g−1 week−1 for every g g−1 increase in moisture content. The production of DOC increased exponentially with temperature, with Q10 values of 1.7 for soil with a moisture content of 2.5 g g−1, and 2 for wetter material. The composition of the DOC extracted from the driest samples suggested disruption of microbial biomass. Wetter incubation conditions increased the proportion of hydrophobic acids, whereas warmer incubation conditions increased the proportion of hydrophilic acids. The production of DOC was relatively fast in the first 2 days of incubation, and then slowed to approximately 90 μg g−1 week−1. Production rates in the first 2 days of incubation were higher under warmer conditions. Replicated experiments were useful in constructing precise curves relating the response of DOC production and composition to temperature and moisture.