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Distribution of arsenic (III), arsenic (V) and total inorganic arsenic in porewaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada
Abstract
Inorganic arsenic species were measured in the porewaters collected from eighteen piezometers installed between 3 and 91.4 m below ground in a thick till and clay-rich aquitard sequence located in southern Saskatchewan to investigate the distribution of, and controls on arsenic speciation in the sequence. Aqueous concentrations of As(V), As(III) and total As are in the range of 0.31-97, 0.71-21 and 3.2-98 ppb, respectively. Profiles of As(III) and As(V) concentration distribution with depth broadly track that of total As: erratic increases to 15.2 m, then more uniform concentrations to 88 m. Aqueous arsenic is accumulated at the upper redox transition zone (6–14 m). The alkaline porewater at 91.4 m contains the highest concentrations of As(V) and total As, which might result from the facilitated desorption of arsenate from the host solid due to decrease of positive surface charge of the oxides in alkaline solution. The ratio of As(V)/As(III) is greater than unity in the uppermost oxidized porewater (3 m), less than unity from 4.6 to 71.6 m, and greater than unity in the lowest four porewaters (76.2 to 91.4 m). In the 3 m porewater low As(III) but high As(V)/As(III) is due to the oxidized nature of the near surface weathered till. The high As(V)/As(III) in the deepest porewater at 91.4 m likely results from the enhanced and heterogeneous oxidation of As(III) to As(V) on clay mineral surfaces in the alkaline solution. Total As and arsenic speciation may not be controlled by As, Fe or Mn concentrations in the host till or clay. Dissolved As(V) and total As positively covary with aqueous chloride, whereas dissolved As(III) is independent of aqueous chloride. Aqueous As(III), and to a less extent As(V) and total As are positively correlated with dissolved Mn in the till. In the clay, aqueous As(V) and total As show strong negative covariation with Mn. However, aqueous As(III), As(V) and total As exhibit almost no correlation with total dissolved Fe in the till. The As(V)/As(III) ratio has strong negative correlation with dissolved Mn, but positive covaration with dissolved chloride. Generally good agreement between the redox potentials (Eh) calculated from aqueous As(V) and As(III) concentrations and those measured by a Pt electrode throughout most of the unoxidized till suggests the suitability of using As(V)-As(III) redox couple as a redox indicator for the studied aquitard system. However, large negative bias of the calculated Eh from the measured Eh in the oxidized till/upper unoxidized till and the clay is attributed to errors associated with the field measurements of Eh.
... As (V) is primarily present as H 2 AsO 4 − , whereas the HAsO 4 2− form dominates at higher pH ( Fig. 12.3). On the other hand for As (III), at pH < 9.2, the uncharged arsenite species (H 3 AsO 3 ) dominate in the reducing conditions (Brookin 1988;Yan et al. 2000;Smedley and Kinniburgh 2002). The distribution of As species as a function of pH redox conditions (Eh) is summarized in Fig. 12.2 (Brookin 1988;Yan et al. 2000). ...
... On the other hand for As (III), at pH < 9.2, the uncharged arsenite species (H 3 AsO 3 ) dominate in the reducing conditions (Brookin 1988;Yan et al. 2000;Smedley and Kinniburgh 2002). The distribution of As species as a function of pH redox conditions (Eh) is summarized in Fig. 12.2 (Brookin 1988;Yan et al. 2000). In addition, the presence of differential species of As (III) and As (V) concerning changes in pH is depicted in Fig. 12.3. ...
... Further, HAsS 2 can also be formed in the presence of sulfur at lower pH conditions (Scow et al. 1981). It has been reported that anionic As species in water acts as ligands as they can form bonds with organic sulfur, carbon, or nitrogen (Brookin 1988;Yan et al. 2000;Smedley and Kinniburgh 2002). As (V) is reported to react with reduced nitrogen groups, while As (III) binds with sulfhydryl groups such as organic di thiols, cysteine, protein, and enzyme (Kumaresan and Riyazuddin 2001). ...
Arsenic (As) pollution in aquatic environment has become one of the most severe ecological problems affecting the provision of clean drinking water in many countries. To combat this, numerous physicochemical methods have been developed including adsorption, ion exchange, biosorption, solar stills, etc. However, the barrier to the successful deployment of these methods lies in the differential removal and disposal efficiency of As species/wastes generated during the treatment. Plants and algae are currently considered as efficient biotechnological tools for safe As remediation from contaminated soil and water sources. In the current chapter, we will focus on algal (micro and macro)-based As bioremediation mechanism and the influence of environmental factors on its uptake from contaminated aquatic systems. Utilization of algae for As bioremediation has an edge over other conventional technologies as it can efficiently accumulate and metabolize all the As species with adequate efficiency, along with generation of biomass that can be used as biofertilizers and biofuels. Recent studies have shown that algal strains can grow in 500–2000 mg per liter of As waters and can remediate a substantial quantity by rewiring their cellular physiology. In a nutshell, the chapter provides a detailed mechanistic overview of algal-based eco-friendly As mitigation processes for generating sustainable environmental solutions.
... For example, at lower pH, As (V) is likely to be present in the form of H 2 AsO 4 − which is expected to be adsorbed better than HAsO 4 2− that are likely to be present at higher pH (i.e., pH 8). (Smedley and Kinniburgh, 2002;Yan et al., 2000;Dixit and Hering et al. 2003). ...
... Both redox potential and pH influence the As speciations. In this study, redox-sensitive elements are Fe, sulfur (S), and As. the pH range of 6-8 (<pH 9), the As (III) is likely to be present in the form of uncharged arsenite (H 3 AsO 3 ) under reducing conditions and negatively charge arsenate (H 2 AsO 4 − and HAsO 4 2− ) under oxidizing condition (Yan et al., 2000;Smedley and Kinniburgh, 2002;Raychoudhury et al., 2015). Generally, groundwater is present in the suboxic condition (E H ranging between − 200 mV and 200 mV and pH in the range of 6-8, Fig. 5c). ...
The objectives of the study are (i) to assess arsenic immobilization efficiency by in-situ synthesized FeS within the porous media under varying solution chemistry and (ii) the performance evaluation of FeS-sand matrix in removing arsenic under natural groundwater conditions. To achieve the objectives, column experiments were performed where FeS was synthesized within the porous media in-situ. Then the As (III) or As (V) solution was injected within the column, and the effluents were collected to measure arsenic (As) and iron (Fe) concentrations. The groundwater samples were collected from seventeen locations in West Bengal, India, and were characterized. The As removal efficiency by FeS-sand matrix from the selected groundwater samples was studied in the batch and column system. The result showed that a significant amount of arsenite [As (III), 71.7–74.9%] was immobilized in the FeS synthesized porous media. The solution pH and the concentration of As (III) have an insignificant effect on immobilizing As (III) in the porous media system in the pH range of 6–8. Arsenate [As (V)] immobilization was not very promising under the reducing condition by in-situ synthesized FeS in the porous media. The As (V) removal was slightly better at pH 6 (26.9%) compared to pH 8 (11.7%). The occurrence of As in the groundwater (ranging from 0.78 μg/L to 376 μg/L) of the region was found to be associated with aluminum (Al) and Fe. The As removal by FeS-sand matrix showed better performance at pH 6 (40.7 ± 8.6%) compared to that of pH 8 (29.2 ± 15.6%) in the batch system. The immobilization of As increased significantly (83%) when the As-contaminated groundwater (376 μg/L) was injected into the porous media containing in-situ synthesized FeS. The concentration of Fe also reduced significantly (<160 μg/L) at the effluent after 2 PVs, indicating a limited risk of release of Fe. Overall, it could be stated that in-situ synthesized FeS in the porous media has the potential to immobilize As under a reducing environment within a range of pH under the groundwater conditions.
... Arsenic (As) pollution has become a global environmental concern and long-term intake of high-arsenic groundwater is threatening human health seriously [1,2]. Commonly, in an oxidizing environment, arsenate of H 2 AsO 4 − is the dominant arsenic species under pH of lower than 6.9 [3], but in a reducing environment, the dominant arsenic species become arsenite of H 3 AsO 3 0 at a pH of lower than 9.2, and arsenite can transform into AsO 4 3− at pH of higher than 9.2 [3,4]. However, when sulfur or insoluble sulfide coexists with arsenite or arsenate, oxygen-bonded arsenic will be substituted by sulfur to form As-SH and/or As=S substructures, which is named as thioarsenic, commonly including thioarsenite (TAs III ) and thioarsenate (TAs V ) [5][6][7]. ...
... Arsenic (As) pollution has become a global environmental concern and long-term intake of high-arsenic groundwater is threatening human health seriously [1,2]. Commonly, in an oxidizing environment, arsenate of H 2 AsO 4 − is the dominant arsenic species under pH of lower than 6.9 [3], but in a reducing environment, the dominant arsenic species become arsenite of H 3 AsO 3 0 at a pH of lower than 9.2, and arsenite can transform into AsO 4 3− at pH of higher than 9.2 [3,4]. However, when sulfur or insoluble sulfide coexists with arsenite or arsenate, oxygen-bonded arsenic will be substituted by sulfur to form As-SH and/or As=S substructures, which is named as thioarsenic, commonly including thioarsenite (TAs III ) and thioarsenate (TAs V ) [5][6][7]. ...
Monothioarsenate (MTAsV) is one of the major arsenic species in sulfur- or iron-rich groundwater, and the sediment adsorption of MTAsV plays an important role in arsenic cycling in the subsurface environment. In this study, batch experiments and characterization are conducted to investigate the sorption characteristic and mechanism of MTAsV on natural sediments and the influences of arsenite and arsenate. Results show that MTAsV adsorption on natural sediments is similar to arsenate and arsenite, manifested by a rapid early increasing stage, a slowly increasing stage at an intermediate time until 8 h, before finally approaching an asymptote. The sediment sorption for MTAsV mainly occurs on localized sites with high contents of Fe and Al, where MTAsV forms a monolayer on the surface of natural sediments via a chemisorption mechanism and meanwhile the adsorbed MTAsV mainly transforms into other As species, such as AlAs, Al-As-O, and Fe-As-O compounds. At low concentration, MTAsV sorption isotherm by natural sediments becomes the Freundlich isotherm model, while at high concentration of MTAsV, its sorption isotherm becomes the Langmuir isotherm model. The best-fitted maximum adsorption capacity for MTAsV adsorption is about 362.22 μg/g. Furthermore, there is a competitive effect between MTAsV and arsenate adsorption, and MTAsV and arsenite adsorption on natural sediments. More specifically, the presence of arsenite greatly decreases MTAsV sorption, while the presence of MTAsV causes a certain degree of reduction of arsenite adsorption on the sediments before 4 h, and this effect becomes weaker when approaching the equilibrium state. The presence of arsenate greatly decreases MTAsV sorption and the presence of MTAsV also greatly decreases arsenate sorption. These competitive effects may greatly affect MTAsV transport in groundwater systems and need more attention in the future.
... Inorganic arsenic compounds, which are anions, are more toxic than organic compounds, and trivalent species (As III) are more toxic than pentavalent species (As V) [19]. As III act as a cross-linking agent by binding up to three monothiol molecules, such as the antioxidant GSH (glutathione), and this arsenic-protein binding often triggers cellular responses such as oxidative stress [44][45][46] . ...
... The 48 h EC50 indicated that for this type of test C. vulgaris was more sensitive to As (36, 22 µg L −1 for both speciation models), followed by Zn (437.6 µg L −1 based on the chemical speciation estimated with Eh-pH diagram and 2.984 µg L −1 based on chemical speciation estimated with Visual MINTEQ). Yan [44] and Guo et al. [45] hypothesized that HAsO 4 2− , which is a molecular analogue of phosphate (HPO 4 2− ), can compete for phosphate anion transporters (transporter proteins). Once in the cell, As (V) can be readily converted to As (III), the more toxic of the two forms. ...
Microalgae growth inhibition assays are candidates for referent ecotoxicology as a fundamental part of the strategy to reduce the use of fish and other animal models in aquatic toxicology. In the present work, the performance of Chlorella vulgaris exposed to heavy metals following standardized growth and photosynthesis inhibition assays was assessed in two different scenarios: (1) dilutions of single heavy metals and (2) an artificial mixture of heavy metals at similar levels as those found in natural rivers. Chemical speciation of heavy metals was estimated with Visual MINTEQ software; free heavy metal ion concentrations were used as input data, together with microalgae growth and photosynthesis inhibition, to compare different effects and explain possible toxicity mechanisms. The final goal was to assess the suitability of the ecotoxicological test based on the growth and photosynthesis inhibition of microalgae cultures, supported by mathematic models for regulatory and decision-making purposes. The C. vulgaris algae growth inhibition test was more sensitive for As, Zn, and Pb exposure whereas the photosynthesis inhibition test was more sensitive for Cu and Ni exposure. The effects on growth and photosynthesis were not related. C. vulgaris evidenced the formation of mucilaginous aggregations at lower copper concentrations. We found that the toxicity of a given heavy metal is not only determined by its chemical speciation; other chemical compounds (as nutrient loads) and biological interactions play an important role in the final toxicity. Predictive mixture effect models tend to overestimate the effects of metal mixtures in C. vulgaris for both growth and photosynthesis inhibition tests. Growth and photosynthesis inhibition tests give complementary information, and both are a fast, cheap, and sensitive alternative to animal testing. More research is needed to solve the challenge of complex pollutant mixtures as they are present in natural environments, where microalgae-based assays can be suitable monitoring tools for pollution management and regulatory purposes.
... Sediment pore waters Some high concentrations of As have been found in pore waters extracted from unconsolidated sediments and often form sharp contrasts to the concentrations observed in overlying surface waters [48]. Yan et al. [49] found As concentrations in the range 3.2-99 μg L À1 in pore waters from clay sediments in Saskatchewan, Canada. Even higher concentrations can be found in pore waters from sediments affected by mining contamination (tailings, mineral-rich deposits). ...
... There is much evidence for cycling of As between shallow sediment pore waters and overlying surface waters in response to temporal variations in redox conditions [49,53]. ...
Geogenic arsenic (As) contamination is an emerging issue worldwide as > 200 million people are at the stake of As toxicity. Arsenic exists in organic and inorganic forms while inorganic forms are more toxic and dominated in nature. Biogeochemical cycling of As in the aquatic environment depends on pH, redox potential, microbes and in particular algae, as well as its interaction with different elements (e.g. iron and sulphur). Dissolved organic carbon, oxides of iron and manganese and sulphur species play a significant role in As release and speciation in water systems. This chapter gives a brief overview of source and distribution of As, describes role of As species including the thiolated As forms in aqueous systems. We also address the key functions of microorganisms with a particular emphasis on algae and its effects on As speciation, as algae plays a vital role in the biogeochemical cycling of As in aquatic environments, albeit these aspects are partially studied and explored.
... The aqueous speciation of As (i.e., its partitioning across the various dissolved forms in which it can exist) plays a pivotal role in the biogeochemical cycling and behaviour of As in the environment [13,5,14]. The most widely recognized inorganic As species include arsenate (As[V]) and arsenite (As[III]) [2]. ...
Thiolated arsenic (As) compounds have been identified in various natural and engineered environments worldwide and are important for the biogeochemical cycling of As, yet quantitative data regarding their stability and transformation rates remains scarce. This study investigates the oxidation kinetics of mono-, di-, and tri-thioarsenate at varying pH, Fe, and (thio-)As concentrations in the aqueous phase. Experiments conducted over four weeks revealed that all thioarsenates were oxidized faster at lower pH, with rates of up to several μmoles/L/d at a pH of 3. Trithioarsenate demonstrated approximately two orders-of-magnitude faster oxidation rates than di-and monothioarsenate and these rates exhibited a higher sensitivity to pH and dissolved As and Fe concentrations. The presence of Fe enhanced the oxidation rates of trithioarsenate but had less impact on di-and monothioarsenate. Kinetic data were subsequently used to parameterize oxidation rate equations and determine reaction orders, and to calibrate a kinetic model that was leveraged to determine rate constants. The fundamental insights and kinetic parameters derived for thio-As oxidation in this study are important for predicting the mobility of thio-As compounds and for assessing the potential environmental impacts of As across ambient aquatic systems.
... As (III) is the dominant species in anaerobic environments such as groundwater. Redox potentials and pH of the environment are the most important parameters for the control of As species [16]. In order to accurately predict the movements of As in the ground, it is necessary to know that the water chemistry of As to determine its transitions between liquid and solid phases. ...
Arsenic (As) contamination in water sources is a significant global issue due to its detrimental health effects. Various As removal technologies have been developed to mitigate this problem and ensure safe drinking water for affected populations. This critical review provides an in-depth analysis of As removal technologies, evaluating their effectiveness, limitations, and potential environmental impacts. The review focuses on commonly used methods such as coagulation-filtration, oxidation, lime softening, and adsorption, ion exchange, and membrane processes, while also exploring emerging technologies. The study critically examines the advantages and disadvantages of each technology, considering factors such as efficiency, cost, operational complexity, waste generation, and system scalability. Furthermore, the environmental implications associated with treatment residuals and energy consumption are discussed. The review highlights the importance of selecting appropriate As removal technologies based on site-specific conditions and the need for long-term sustainability. It also emphasizes the need for ongoing research and development to improve existing technologies and explore innovative approaches to As removal. Overall, this critical review provides valuable insights into As removal technologies, facilitating informed decision-making for effective mitigation of As contamination in water sources.
... Dissolved As in groundwater systems has been reported from different parts of Canada, and these were mainly focused on localized high As sites (Boyle et al., 1998;Grantham and Jones, 1977;McGuigan et al., 2010;Meranger et al., 1984;Wang and Mulligan, 2006;Yan et al., 2000). Previous studies reported the natural occurrences of As in groundwater of Canada but these were generally within the safe drinking limit (10 μg/L), except for very limited areas in British Columbia (Rivera, 2014). ...
The geological setting of an area plays a critical role in the transfer and ultimate distribution of hydrochemical constituents present in groundwater. In southern Ontario, Canada, the present physiography was significantly influenced by glacial processes during the Quaternary period. The heterogeneous nature and complex pattern of shallow subsurface glacial overburden sediments, likely affect the fate of different groundwater constituents. In this study, arsenic (As) and fluoride (F−) concentrations from 515 water wells, that are constructed within overburden sediment, were analyzed with the physiographic map of southern Ontario along with other related variables. Geospatial mapping and several spatial statistical analyses were performed to examine the possible geological influence on As and F− distribution and water-well susceptibility at a regional scale. Key findings suggest four physiographic settings were significant variables influencing the distribution of As and F− in differently constructed bored/dug and drilled wells. Bored/dug wells in Bevelled Till Plains and bored/dug wells in Undrumlinized and Bevelled Till Plains were found to be relatively susceptible to As and F− contamination respectively. In contrast, bored/dug and drilled wells in Drumlinized Till Plains and Drumlins and drilled wells in Sand Plains seemed to be relatively safe from F− and As respectively. The statistical regression analyses suggested that other variables, such as the application of phosphate fertilizer and the textures of till, influenced the spatial distribution of As and F− as well as which types of wells (bored/dug or drilled) were impacted. The geospatial mapping and statistical cluster analysis indicated that the possible sources of elevated As and F− in drilled wells are the clasts of underlying bedrock. The relationship between physiographic settings and impacted overburden wells in southern Ontario provides planners with an approach to water-well susceptibility assessments at the regional scale, which in turn can guide further local analysis for water resource management.
... Under oxidizing conditions, H 2 AsO 4 1is dominant at low pH (less than approximately pH 6.9), whereas at higher pH, HAsO 4 2becomes dominant. Under reducing conditions at pH less than approximately pH 9.2, the uncharged arsenite species H 3 AsO 4 0 will predominate (Yan et al. 2000). In the present study, HAsO 4 2is the dominant As species in the groundwater samples. ...
Forty-six soil and groundwater samples were collected from the agricultural farms of the Gulf of Aqaba coast. Additionally, 24 granitic and marine sedimentary rock samples were collected from the study area. The collected samples were analyzed for As, Al, Au, B, Ba, Be, Fe, Sb, Se, Sn, Ti, and V using inductively coupled plasma mass spectrometry. Levels of the studied metals in the groundwater samples lie within the acceptable limits of the World Health Organization (WHO). The rock samples exhibit a significant variation in mean metal content from one rock type to another. Concentrations of As and B in the soil samples were determined to be higher than those of Canadian Soil Quality Guidelines (CSQG) and were primarily due to agricultural and seawater inputs. Chemical weathering of various rock units also plays a significant role. The calculations of geoaccumulation index are found to be more reliable than of those of enrichment factor for Arsenic contamination levels assessment. The study area is not significantly affected by As contamination. The correlation coefficient analysis results for the soil and groundwater data reveal a variable degree of correlations between As and other metals in the study area.
... In extremely acidic and basic conditions, H3AsO4 and AsO4 -3 forms are predominant, respectively. Therefore, no-load arsenite species (H3AsO3) are dominant under reducing conditions that are less than approximately pH 9.2 (Brookins, 1988;Yan et al., 2000;Smedley ve Kinniburgh, 2002; Figure 2 and 3). Figure 2. Redox potential of As species in water systems (at 25 °C and 1 bar pressure) and the diagram of change between As (Eh) and pH (Brookins, 1988). ...
... This means that the surface of the MGOCS is positively charged (caused by protonation) when the pH value is below 9.46 and is negatively charged when the pH value is higher than 9.46. Furthermore, As (III) mainly exists as the neutral form (H 3 AsO 3 ) in the aqueous phase below pH 9.2 and the anionic (H 2 AsO 3 − ) form in the pH range of 9.2-12.1 [53,54]. Therefore, MGOCS can adsorb a great number of As(III) from an aqueous solution through surface complexation and electrostatic attraction when the pH value is below 10. ...
A magnetite graphene oxide chitosan (MGOCS) composite microsphere was specifically prepared to efficiently adsorb As(III) from aqueous solutions. The characterization analysis of BET, XRD, VSM, TG, FTIR, XPS, and SEM-EDS was used to identify the characteristics and adsorption mechanism. Batch experiments were carried out to determine the effects of the operational parameters and to evaluate the adsorption kinetic and equilibrium isotherm. The results show that the MGOCS composite microsphere with a particle size of about 1.5 mm can be prepared by a straightforward method of dropping FeCl2, graphene oxide (GO), and chitosan (CS) mixtures into NaOH solutions and then drying the mixed solutions at 45 °C. The produced MGOCS had a strong thermal stability with a mass loss of <30% below 620 °C. The specific surface area and saturation magnetization of the produced MGOCS was 66.85 m²/g and 24.35 emu/g, respectively. The As(III) adsorption capacity (Qe) and removal efficiency (Re) was only 0.25 mg/g and 5.81% for GOCS, respectively. After 0.08 mol of Fe3O4 modification, more than 53% of As(III) was efficiently removed by the formed MGOCS from aqueous solutions over a wide pH range of 5–10, and this was almost unaffected by temperature. The coexisting ion of PO4³⁻ decreased Qe from 3.81 mg/g to 1.32 mg/g, but Mn²⁺ increased Qe from 3.50 mg/g to 4.19 mg/g. The As(III) adsorption fitted the best to the pseudo-second-order kinetic model, and the maximum Qe was 20.72 mg/g as fitted by the Sips model. After four times regeneration, the Re value of As(III) slightly decreased from 76.2% to 73.8%, and no secondary pollution of Fe happened. Chemisorption is the major mechanism for As(III) adsorption, and As(III) was adsorbed on the surface and interior of the MGOCS, while the adsorbed As(III) was partially oxidized to As(V) accompanied by the reduction of Fe(III) to Fe(II). The produced As(V) was further adsorbed through ligand exchange (by forming Fe–O–As complexes) and electrostatic attraction, enhancing the As(III) removal. As an easily prepared and environmental-friendly composite, MGOCS not only greatly adsorbs As(III) but also effectively removes Cr(VI) and As(V) (Re > 60%) and other metals, showing a great advantage in the treatment of heavy metal-contaminated water.
... The mobility of arsenic forms in water depends heavily on pH, Eh conditions and the existence of different chemical types [26,27]. The speciation of arsenic is sensitive to the pH conditions, oxidizing and reducing conditions [28]. However, under oxidizing conditions, H 2 AsO 4 − is the dominant species at pH values lower than 6.9, while at higher pH conditions, HAsO 4 2becomes dominant. ...
Contamination of arsenic (As) in water, especially groundwater, has been recognized as a major problem across the world. The presence of arsenic in groundwater has become a global problem in the past decades. Health risks have also been reported for many years. Different areas of the world are affected by arsenic contamination of groundwater, the largest population at risk in Bangladesh, followed by West Bengal in India. Arsenic concentrations in drinking water cause severe health effects on human, more than 150 million people worldwide. The current drinking water standard regulation has become strict and requires a reduction in arsenic content. Therefore, the treatment of arsenic contaminants can be the only effective option to reduce health risks. This review paper briefly describes arsenic sources, arsenic chemistry, arsenic contamination in groundwater, its impact on human health and many conventional as well as advanced techniques that are used to remove arsenic from water.
... The negative effect of a pH higher than 7 on the increase of available As has already been reported [35,36]. In turn, another key factor in the mobilization and immobilization of As is the phosphorus (P) concentration [37]. We found positive correlations between the concentrations of As and P (0.79, p < 0.01). ...
... The upper shallow and main aquifers [20] can yield large quantities of water. However, it is not entirely suitable for development because of quality issues, especially where arsenic contamination in shallow groundwater and saline water intrusion makes the water unusable for human consumption [21][22][23][24][25][26] in the coastal belt. The current coastal groundwater situation is strongly impacted by historical sea-level rise (SLR) and proximity to the sea, particularly in the low-elevation central part of the delta [27,28]. ...
Salinity causes a hostile environmental impact throughout the year in the coastal region of Bangladesh, and its severity increases day by day. Because of upstream freshwater flow reduction and massive groundwater extraction, salinity has increased substantially over the last three decades. Moreover, arsenic contamination in shallow groundwater makes the groundwater unsuitable for potable use. Consequently, the coastal area suffers from acute storage of safe water supply. Salinity also negatively impacts human activities, livelihood, agricultural production, and the aquatic ecosystem. Though the shallow aquifer contains high salinity and a small amount of Arsenic (As), the very shallow aquifer (within 3m to 8m) contains fresh water in many areas in the rainy season due to the direct recharge of rainwater. However, rainfall recharge varies significantly depending on the geological and hydrogeological settings. Specifically, up to 50% of annual rainfall is stored in shallow aquifers of Quaternary sands through direct infiltration. The research’s principal objective is to identify the safe and sustainable drinking water source in the arsenic and saline-prone coastal region. Groundwater samples were collected from the different locations of the study area during both dry and wet seasons and examined seasonal variations in groundwater table and salinity levels. The chemical analyses and Physico-chemical parameters indicate that the groundwater samples are suitable for drinking. Except for some groundwater samples from the wet season, the salinity of all samples was under the allowable limit for Bangladesh (<2000 µS/cm), and the targeted aquifer was almost arsenic (50 µg/l) free. Therefore, a comprehensive analysis has been made to accomplish the study goals. Particularly, the groundwater’s electrical conductivity (EC) values of most samples were measured within the limit of fresh or brackish water (<2000 μS/cm). Overall, the results indicate the prospect of a very shallow aquifer as a source of freshwater for drinking purposes throughout the year, considering both arsenic and salinity, which effectively solve the freshwater shortage, especially in the saline-arsenic prone area.
... Bacteria can also reduce arsenate to arsenite in anoxic environment [6]. Redox potential and pH are driving forces in controlling As speciation [47]. When oxidizing conditions occur, H2AsO4is predominant at pH lower than 7. HAsO4 2becomes predominant at higher pH. ...
Arsenic contamination in drinking water poses worldwide threat to public health and requires emergency actions in some parts of the world. Several technologies have been used to overcome arsenic contamination issues and to meet the arsenic concentration limitations for public health. In this study, research tendencies on arsenic removal technologies were evaluated. A total of 4083 publications, published between 1970 and 2019, on arsenic removal from drinking water, groundwater and wastewater were retrieved from Web of Science (WoS) database. A bibliometric analysis was carried out and word frequency along with visualization map analysis were used to provide a quantitative analysis, and an overview on the current research trends and research prospects. The results showed that annual output of the “arsenic removal” subject increased significantly after the year 2000. “Article” was the most preferable publication type, and “Journal of Hazardous Materials” had the highest publication number. The most productive country in terms of number of total articles on arsenic removal was China. Also, the South-East Asian countries highly contributed to the literature. “Adsorption” was found to be the most frequently researched arsenic removal technology and nanotechnology plays a significant role in the adsorption development.
... In creeks and rivers, arsenic(V) is generally the predominant species (Pettinea et al., 1992) over arsenic (III). The concentrations and relative proportions of arsenic(V) and arsenic(III) vary according to sediment characteristics, redox conditions, the hydrological regime and biological activity (Alcaine et al., 2020;Smedley et al., 2002;Tufano et al., 2008;Yan et al., 2000;Yu et al., 2018). Generally, arsenic in creeks is from the weathering of pyrite-related sediment oxidization (Seidel et al., 2007), which results in the acidification of the creek water. ...
The release of arsenic and related species from mining activities has been investigated widely at both seasonal and diel scales, contributing to the understanding of arsenic cycles, its ultimate fate, and enabling accurate estimates of arsenic flux in specific areas. To enrich the research in this area, a case study was undertaken in Huangshui Creek, Hunan province, China. Here, arsenic is present in the sediment at the Creek entrance to a reservoir and in the widely developed alkali realgar(α-As4S4)–calcite(CaCO3)–dolomite[CaMg(CO3)2] strata (pH 7–11). Water from different levels in the Huangshui Creek, the Creek/reservoir entrance, and the downstream reservoir together with corresponding sediments were collected and analyzed. The local algae were separated and cultured. A diel variation of arsenic (688 ug/L in AM 3:50–1152 ug/L in PM 19:50) was observed in the Creek. The largest difference in arsenic concentration between the upper and lower water body was at the mixed creek/reservoir site (364 ug/L). Laboratory experiments showed that arsenic release from Creek sediment and pristine realgar was 1.3–2.7 times and 2.0–2.3 times at 25 and 37 °C, respectively, than low-temperature samples (8 °C) over 24 h. However, temperature variation is not the only factor controlling arsenic release from Huangshui Creek. Batch experiments show that both sediment and pristine realgar can release arsenic(III). In addition, the presence of bicarbonate promotes arsenic(V) release by 15.2–24.3 times for the sediment and by 1.7–3.4 times for pristine realgar compared to the control, though it restrains arsenic(III) release. High levels of algae have a complex effect on arsenic release; it increases arsenic(V) release by accelerating dissolution of realgar but decreases arsenic(III) release through adsorption. The field observations—variation of bicarbonate (67 mg/L in day and 201 mg/L in night) and chlorophyll-a (0.06–0.87)—support that both dissolved bicarbonate and algae affect arsenic concentration. These factors establish a circadian rhythm in the Creek, which coupled with arsenic release, ultimately affect the fate of arsenic.
... As reported by Smedley and Kinniburgh (2002), environmental conditions such as Eh and pH are the major controls on the speciation of As. H 2 AsO 4− constitutes the major species at pH < 6.9, while at increasing pH (pH > 6.9), HASO 4 2− is the dominant species (Yan et al., 2000). Under the present groundwater pH condition of the study area, H 2 AsO 4 2− , was identified as the dominant species accounting for about 73% of the total As. ...
The indiscriminate discharge of industrial effluents from beverage industries into nearby streams and lands within the Udi area of Enugu State has resulted in a decline in the drinking water quality. In this study, geochemical, indexical, statistical and spatiotemporal models were generated to assess the drinking water quality of the area. Seventeen borehole samples were randomly collected across the area and examined for 18 chemical parameters using standard methods. Results revealed that the groundwater is slightly acidic. Hydrochemical modeling identified Ca2+–Mg2+–Cl−–SO42− as the major hydrogeochemical facies and Ca2+–Mg2+ and Cl−–SO42− as the dominant water type, implying that the water is permanently hard and unfit for laundry. The hydrochemical model, PHREEQC, was used in trace element species assessment. Results from the model revealed that trace elements were immobile under the prevailing pH conditions owing to the presence of limiting mineral phases (e.g. sulphates and carbonates). The pollution index of groundwater revealed that 29.5% of the water samples recorded high pollution, hence are unsuitable for drinking, while 17% recorded insignificant pollution and were adjudged fit for drinking. Similarly, water quality index and GIS-based spatiotemporal analysis revealed that 64.7% of boreholes around the northeastern, west-central and southern parts of the area are unsuitable for drinking, however, boreholes within the central parts are drinkable. The groundwater flow map showed that groundwater flow in the area is predominantly from the northeastern to the southwestern direction. Hence, untreated wastewater and industrial effluents disposal sites should be restricted to the southern parts of the study area.
... In addition, H 2 AsO 4 À is the predominant at pH < 6.9 in the oxidizing conditions, whilst arsenite exists as HAsO4 2À at pH > 6.9 with the possibility of the presence of both H 3 AsO 4 and AsO 4 3À at extremely acidic and alkaline medium, respectively. However, arsenite species are major forms under reducing conditions (Yan et al., 2000). ...
With globally increased human population and industrialization, the natural sources of water are reduced and then contaminated. Therefore, development of advanced technologies for the efficient water treatment is becoming of the scope of each of the nation. One of the cost-effective and well-known technologies for wastewater treatment is adsorption of contaminants by natural biopolymer like chitosan (CS) due to its unique features such as availability, biodegradability, biocompatibility, eco-friendly and low-cost production. However, Cs suffers considerable limitations such as low adsorption capacity, low surface area and limited reusability. Thence, this review intended to provide an overview for recent advances of chitosan-based adsorbents that established better adsorption activities towards various hazard heavy metals, including: As(III), As(V), Cu(II), Cr(VI), Pb(II) and Cd(II) ions. In addition, the capabilities of chitosan-based adsorbents for the adsorptive removal of anions including phosphates and nitrates were discussed. Besides, the suggested adsorption mechanisms of these contaminants onto chitosan-based adsorbents and the research conclusions for the optimum conditions of the adsorption processes were explained in light of the currently reported studies. Furthermore, to emphasize the foremost research gaps and future potential trends that could inspire further researchers to find out the best solutions for water treatment problems.
... A bioreactor of fixed-bed with up-flow was combined with oxidation of arsenite, and then, the removal of arsenic using physiochemical zerovalent iron was developed by . Arsenite oxidation was done by using already extracted oxidative bacteria which were restrained Inonotus hispidus (macrofungus) The biosorption capacity was found to be 51.5 and 59.6 mg g − 1 for As(III) and As(V), respectively at Optimum conditions of pH 6 for As(III) and pH 2 for As(V), contact time of 30 min and temperature of 20 °C Seawater As(III) and As(V), 10 mg L −1 (Sarı and Tuzen 2009) A. niger (coated with iron oxide) Maximum removal (approximately 95% As(V) and 75% As(III)) obtained at pH 6 being incubated on a platform shaker (175 rpm) for 12 h Laboratory culture As(III) and As(V), 100 μg L −1 (Dobrevski et al. 1986;Ghosh and Yuan 1987;Singh et al. 1988;Yadava et al. 1988;Fox 1989;Clifford 1990;Maeda et al. 1992;Zouboulis et al. 1993;Peräniemi et al. 1994;Yan et al. 2000;Ramaswami, Tawachsupa et al. 2001;Zhang et al. 2003;Kim and Benjamin 2004;Baciocchi et al. 2005;Jegadeesan et al. 2005;Amin et al. 2006) Ion exchange resin Well-defined medium and capacity; pH independent; exclusive ion specific resin to remove arsenic High-cost medium; high-tech operation and maintenance; regeneration creates a sludge disposal problem; As(III) is difficult to remove; life of resins Iron coated sand Cheap; no regeneration is required; remove both As(III) and As(V), Relatively well known and commercially available Not standardized; produces toxic solid waste Needs replacement after four to five regeneration Activated alumina NA produces toxic solid waste Needs replacement after four to five regeneration (Amirtharajah and O'melia 1990;Benefield and Morgan 1990;Cheng et al. 1994;Edwards 1994;Hering et al. 1996;Hering et al. 1997;McNeill and Edwards 1997;Pande et al. 1997;Jiang and Graham 1998;Balasubramanian and Madhavan 2001;Johnston et al. 2001;Wang and Tang 2001;Han et al. 2002;Khan et al. 2002;Meng et al. 2002;Katsoyiannis 2002, Ghosh et al. 2003;Yuan et al. 2003;Kumar et al. 2004;Wickramasinghe et al. 2004;Parga et al. 2005;Hansen et (Kang et al. 2000;Han et al. 2002;Ning 2002;Ballinas et al. 2004;Velizarov et al. 2004;Wickramasinghe et al. 2004;Weng et al. 2005;Chung et al. 2006;Kim et al. 2006;,یمالغ یراتخم et al. 2006;Iqbal et al. 2007) Electrodialysis ...
Arsenic-contaminated water is a major concern in many areas worldwide, causing several diseases such as cancer. There is therefore a need for advanced methods to clean waters because conventional methods have drawbacks such as generation of hazardous sludge, heavy operation and high costs. Here we review arsenic sources, chemistry, toxicity and remediation methods. We discuss also sociological aspects of arsenic prevention. Sources include surface water, groundwater and seawater. Methods include bioremediation, phyto-remediation and biofilters. Sociological aspects are public awareness, sharing information on arsenic-free water sources, removing As at the household level, building a community plant and training facilitators.
... As(III) exists as H 3 AsO 3, but it can also be found as H 2 AsO 3 − , HAsO 3 2− , and AsO 3 3− ions; As(V) exists as H 3 AsO 4 , but it can also be found as H 2 AsO 4 − , HAsO 4 2− , and AsO 4 3− anions [11,12]. Both arsenic types can coexist as vertically distributed in groundwater [8,13], but the As(III) type is usually the prevailing one [14]. Hence, groundwater treatments must be capable of removing both arsenic species. ...
Arsenic is an inorganic pollutant that, depending on oxidation–reduction and pH level conditions, may be found in natural waters in two variants: As(III) and As(V). Any treatment to effectively remove arsenic from water will be conditioned by the presence of one or both variants. In this context, this study assesses using electrochemically produced Fe(VI) with Fe(III) to remove As(III), As(V), and their combinations from the Synthetic Bangladesh Groundwater (SBGW) containing anions that interfere with iron-based arsenic removal processes. The combined use of Fe(VI) and Fe(III) allowed us to remove the total arsenic below the 10 mg L⁻¹ threshold established by the World Health Organization and Peruvian regulations for drinking water. An optimum combination of 1 mg L⁻¹ of Fe(VI) and 30 mg L⁻¹ of Fe(III) was identified and tested on the removal of four different proportions of As(III):As(V) for two total concentrations: 500 and 250 mg L⁻¹. There were no significant differences in the final removal values under the different proportions of As(III):As(V) for each total concentration, with a final removal average of 99.0% and 96.9% for the 500 and 250 µg L⁻¹ concentrations, respectively.
... Wells with lateral positions within the Valley Trough or wells with deeper screens are more likely to encounter clay lenses, which could explain the larger proportion of wells with increasing arsenic concentrations in this area. If the clay lenses encountered by these wells have high arsenic pore water, then that pore water may be drawn out during pumping (Yan et al., 2000;Stopelli et al., 2020;Mozumder et al., 2020;Mihajlov et al., 2020) or expelled from compaction (Erban et al., 2013;Smith et al., 2018). ...
In the San Joaquin Valley (SJV), California, about 10% of drinking water wells since 2010 had arsenic concentrations above the US maximum contaminant level of 10 μg/L. High concentrations of arsenic are often associated with high pH (greater than 7.8) or reduced geochemical conditions. Although most wells have low arsenic (<3 μg/L) and do not have changing arsenic concentrations, this study found that most wells with concentrations above 10 μg/L had arsenic trends. Overall, about 24% of wells had time-series trends since 2010 and 59% had paired-sample trends since 2000. Most wells had decreasing arsenic trends, even in wells with higher arsenic concentrations. These wells often had co-detections of increasing nitrate and sulfate trends that reflect oxic groundwater likely derived from agricultural recharge. Wells with increasing arsenic trends were deeper or located in the valley trough where aquifer materials are more fine-grained and where reducing conditions favor arsenic mobility. Wells with arsenic trends also tend to be clustered near areas of higher well density. Groundwater pumping in these areas has likely increased the contribution of younger, more oxic groundwater in wells with declining arsenic or, less frequently, increased the contribution of higher pH or reduced groundwater in wells with rising arsenic. Projections of arsenic trends indicate that 37 wells with high arsenic presently will be below 10 μg/L in ten years. Unfortunately, these improvements will be largely offset by 31 wells that are expected to increase above 10 μg/L in addition to expected rises in nitrate in wells where arsenic decreased. This study shows how human-altered flow systems can impact the natural geochemical character of water in both beneficial and deleterious ways.
... The pore waters from the unconsolidated sediments have been reported to have elevated As levels and it can increase and may be higher compared to the overlying surface waters (Rahman et al., 2014). Yan et al. (2000) found that concentration pore water As from clay deposits in Saskatchewan, Canada varied between 3.2 and 99 μg L −1 . Relatively higher As concentration was observed in pore water of sediments contaminated by anthropogenic activities (e.g., tailings, mineral-rich deposits). ...
The contamination of aquatic systems with arsenic (As) is considered to be an internationally-important health
and environmental issue worldwide, affecting over 115 countries globally. Arsenic contamination of aquatic
ecosystems is a global threat as it can enter the food chain from As-rich water and cause harmful impacts on the
humans and other living organisms. Although different factors (e.g., pH, redox potential, iron/manganese oxides,
and microbes) control As biogeochemical cycling and speciation in water systems, the significance of algal species
in biotransformation of As is poorly understood. The overarching attribute of this review is to briefly elaborate
various As sources and its distribution in water bodies and factors affecting As biogeochemical behavior in aqueous
ecosystems. This review elucidates the intriguing role of algae in biotransformation/volatilization of As in
water bodies under environmentally-relevant conditions. Also, we critically delineate As sorption, uptake, oxidation
and reduction pathways of As by algae and their possible role in bioremediation of As-contaminated water
(e.g., drinking water, wastewater). The current review provides the updated and useful framework for government
and water treatment agencies to implement algae in As remediation programs globally.
... Moreover, Fe concentrations do not correlate well with As throughout the area. Notably poor correlations between Fe and As may reflect a re-oxidation of Fe (II) due to subsurface distribution of redox potential, which mainly depends upon local lithological and biogeochemical conditions and distribution of redox zones (Yan et al., 2000). ...
Alluvial aquifers are the main source of groundwater worldwide. In Hyderabad area of Sindh province, aquifers are naturally polluted by arsenic (As) like other alluvial aquifers of the world. Present study was carried out to decipher the mobilization mechanism of arsenic in Holocene aquifers of Indus river basin, where a large population is at the risk of arsenic ingested diseases. Fifty groundwater samples were collected from Hyderabad and its surrounding areas to examine their physical, chemical and microbiological characteristics. In 80% of the groundwater samples, TDS is above the WHO limit. Dominant (40%) hydrofacies in groundwater of study area is NaCl, which shows water-rock interaction and cation exchange mechanism. In order to investigate the source of arsenic, eleven near-surface soil samples were also collected and analyzed for elemental and mineral composition using XRD and AES techniques. Correlation Coefficient, Principal Component Analysis (PCA) and multivariate statistical analyses were used to interpret the data. Arsenic ranges between 10-150 µg/L in groundwater, while in soil samples it ranges from 77 and 137µg/kg. Findings showed that arsenic is mobilized in the alluvial aquifers of Indus river through dissolution/ leaching of iron oxyhydroxides under anoxic conditions. Arsenic is mainly leached from mica and phlogopite under reducing conditions. Meandering of Indus river through different historical time periods is an important factor for the distribution of redox zones created by mirco-biodegradation of organic matter rich with clayey sediments. Irrigation return flow, infiltration of sewerage in groundwater and unlined sanitation are also important anthropogenic factors for creating local anoxic conditions to mobilize arsenic in groundwater.
... Commonly, the major species of inorganic arsenic in groundwater are arsenite and arsenateJekel and Amy, 2006 When pH value is lower than 6.9 in an oxidizin environment, the dominant arsenic is arsenate of H 2 AsO 4 − , which can transform into HAsO 4 2with the increase of pH value (Cullen and Reimer, 1989). In a reducing environment, when pH value is lower than 9.2, the dominant arsenic is arsenite of H 3 AsO 3 0 and when pH value is larger than 9, the dominant species changes to arsenate of AsO 4 3- (Yan et al., 2000). Recently, many studies show that arsenic and sulfur usually co-exist in groundwater environment, and oxygen-bonded arsenic will be substituted by sulfur, thereby forming As-SH and/or As=S substructures, which are known as thioarsenic compounds of thioarsenite and thioarsenate (Burton et al., 2013;Hartig and Planer-Friedrich, 2012;Stucker et al., 2014). ...
Thioarsenic is one of the major arsenic species recently detected in high-arsenic groundwater, and precise detection of thioarsenic is very important for understanding arsenic transport in the environment. However, the existing methods usually involve complicated operation procedures at a high cost. In this work, a cost-effective new method was developed for direct measurement of monothioarsenate (MTA) using Liquid Chromatography Hydride-Generation Atomic Fluorescence (LC-HGAFS), and it was applied to study MTA adsorption on the sand, soil, and goethite. The standard MTA sample was prepared using As2O3, NaOH, and sulfur, and its purity was determined at different reaction temperatures to identify the thermal effect. Results showed that the major component of the prepared MTA sample was Na3AsSO3•7H2O and its purity was higher than 98% with only one impurity of arsenite. With the decrease of reaction temperature, the purity of the MTA sample decreased and both arsenite and arsenate impurities were present in samples. When MTA concentrations were at the range of 18 to 360 μg/L, it had an excellent linear relationship with the peak area with a coefficient determination (R²) greater than 0.9998 and an analytical detection limit of 33 μg/L. For three water samples containing different concentrations of arsenite, arsenate, and MTA, the relative errors between the detected and actual values were ranged from -0.11% to 8.21%. The adsorption of MTA on the sand, soil, and goethite reached equilibrium at 4 hr, 8 hr, and 8 hr, respectively, and their maximum adsorption capacities were 122.47, 226.73 and 1979.75 μg/g, respectively. MTA adsorptions on sand and soil fitted well with Langmuir as well as Freundlich models, with R² values greater than 0.98 and 0.96, respectively. MTA adsorption on goethite fitted better into the Freundlich model with an R² value of 0.9863. Besides, the increasing pH values can suppress MTA adsorption on these three media because iron oxide is the major factor controlling the MTA adsorption on them.
... One of the factors is the speed of establishing equilibria, and this question is debatable for various forms of arsenic. Some anomalies of arsenic behavior are explained by kinetics of oxidation-reduction reactions (Cherry et al. 1979;Yan et al. 2000). ...
... The negative effect of a pH higher than 7 on the increase of available As has already been reported [35,36]. In turn, another key factor in the mobilization and immobilization of As is the phosphorus (P) concentration [37]. We found positive correlations between the concentrations of As and P (0.79, p < 0.01). ...
The circular economy seeks to minimize the use of raw materials and waste generation. In this context, here we addressed the use of dunite mining tailings and subproducts to stabilize metal(oid)s in polluted soils. We first characterized the dunite mining tailings and subproducts, and a paradigmatic polluted soil in depth to determine their chemical and mineralogical properties. Experimental trials using Brassica juncea L. were performed to evaluate the impact of the two materials on vegetation growth, edaphic properties and pollutant stabilization yields. To this end, the plants were grown over 75 days in 1 kg pots containing the polluted soil amended with the dunite materials. Notably, both amendments caused a dramatic decrease in the available Zn and a moderate reduction in available Cu, Cd and Pb. In contrast, the concentration of available As was not modified. The cation exchange capacity (CEC) was improved by treatment with the amendments, allowing an increase in the biomass harvested. The immobilization mechanism achieved was probably due to an increase in pH and CEC. In conclusion, the dunite tailings and subproducts could be effective amendments for stabilizing polluted soil. This work paves the way for additional studies with distinct types of soils and conditions.
... Moreover, Fe concentrations do not correlate well with As throughout the area. Notably poor correlations between Fe and As may reflect a re-oxidation of Fe (II) due to subsurface distribution of redox potential, which mainly depends upon local lithological and biogeochemical conditions and distribution of redox zones (Yan et al., 2000). ...
Alluvial aquifers are the main source of groundwater worldwide. In Hyderabad area of Sindh province, aquifers are naturally polluted by arsenic (As) like other alluvial aquifers of the world. Present study was carried out to decipher the mobilization mechanism of arsenic in Holocene aquifers of Indus river basin, where a large population is at the risk of arsenic ingested diseases. Fifty groundwater samples were collected from Hyderabad and its surrounding areas to examine their physical, chemical and microbiological characteristics. In 80% of the groundwater samples, TDS is above the WHO limit. Dominant (40%) hydrofacies in groundwater of study area is NaCl, which shows water-rock interaction and cation exchange mechanism. In order to investigate the source of arsenic, eleven near-surface soil samples were also collected and analyzed for elemental and mineral composition using XRD and AES techniques. Correlation Coefficient, Principal Component Analysis (PCA) and multivariate statistical analyses were used to interpret the data. Arsenic ranges between 10-150 µg/L in groundwater, while in soil samples it ranges from 77 and 137µg/kg. Findings showed that arsenic is mobilized in the alluvial aquifers of Indus river through dissolution/ leaching of iron oxyhydroxides under anoxic conditions. Arsenic is mainly leached from mica and phlogopite under reducing conditions. Meandering of Indus river through different historical time periods is an important factor for the distribution of redox zones created by mirco-biodegradation of organic matter rich with clayey sediments. Irrigation return flow, infiltration of sewerage in groundwater and unlined sanitation are also important anthropogenic factors for creating local anoxic conditions to mobilize arsenic in groundwater.
... The presence and mobility of As and V in groundwater are regulated mainly by the pH and Eh conditions (Appelo and Postma, 2005;Lee et al., 2008). Under conditions of positive Eh and pH higher than 6.5, these elements are found as oxyanions (Rango et al., 2013;Wehrli and Stumm, 1989;Wright and Belitz, 2010;Yan et al., 2000). ...
Contamination of groundwater in different parts of the world is a result of natural and/or anthropogenic sources, leading to adverse effects on human health and the ecosystem. In Península Valdés, where groundwater is the only source of supply, high concentrations of As and F-were registered. Since it is a region without industrial activity, an analysis of possible natural sources of contamination is necessary. The aim of this study is to analyse the hydrological processes that determines the presence and mobilization of those elements through the analysis of the mineralogy of the aquifer sediments and the ionic water relationships. The productive aquifer, dominated by psamites, coquinas and siltstone is located between 29 and 42 m below ground surface. The hydrochemistry studied from 105 sampling points, shows that groundwater is dominated by Na-Cl ions and, in the fresh water sectors, the ionic type is Na-HCO3 to Na-Cl. In 17 of these samples, Zn, Cr, Mn, As, V, Sr, Fe, F ions were measured and As and F contents above the potability limit were recorded. These contents vary between 0.01 and 0.40 mg/L in As and between 0.31 and 4 in F-which are both associated with elevated V values. The optical petrographic microscope observations and the X-ray diffraction measurements show that the sediments are dominated by volcanic lithic fragments, volcanic glass shards and quartz, plagioclase, pyroxenes and magnetite clasts. The scanning electron microscopy, combined with the energy dispersive X-ray analysis, shows that the highest concentrations of As are associated with volcanic shards and iron oxides. The combined analysis of all these elements leads to conclude that the processes which explain the presence of those ions are a result of the interaction of groundwater with the components of the aquifer sediments. At alkaline pH, the high solubility of the amorphous silica of vitreous shards allows the release of As, V and F-ions towards the solution. Thus, adsorption-desorption processes can also control the presence of these ions in groundwater. Both As and V (in solution in the form of oxyanions) can be adsorbed by iron oxides, while F-anions have more affinity to be adsorbed by the carbonate facies, some of them re-precipitated as a result of the increase in pH. The identified hydrogeological processes provide information for the planning of water purification measures that tend to improve the water resources management in a large arid region of Patagonia.
Graphene oxide chitosan composite (GOCS) is recognized as an environmentally friendly composite adsorbent because of its stability and abundant functional groups to adsorb heavy metals, and Fe-Mn binary oxides (FMBO) have attracted increasing interest due to their high removal capacity of As(III). However, GOCS is often inefficient for heavy metal adsorption and FMBO suffers poor regeneration for As(III) removal. In this study, we have proposed a method of dopping FMBO into GOCS to obtain a recyclable granular adsorbent (Fe/MnGOCS) for achieving As(III) removal from aqueous solutions. Characterization of BET, SEM-EDS, XRD, FTIR, and XPS are carried out to confirm the formation of Fe/MnGOCS and As(III) removal mechanism. Batch experiments are conducted to investigate the effects of operational factors (pH, dosage, coexisting ions, etc.), as well as kinetic, isothermal, and thermodynamic processes. Results show that the removal efficiency (Re) of As(III) by Fe/MnGOCS is about 96 %, which is much higher than those of FeGOCS (66 %), MnGOCS (42 %), and GOCS (8 %), and it increases slightly with the increasing molar ratio of Mn and Fe. This is because amorphous Fe (hydro)oxides (mainly in the form of ferrihydrite) complexation with As(III) is the major mechanism to remove As(III) from aqueous solutions, and it is accompanied by As(III) oxidation mediated by Mn oxides and the complexation of As(III) with oxygen-containing functional groups of GOCS. Charge interaction plays a weaker role in As(III) adsorption, therefore Re is persistently high over a wide range of pH values of 3-10. But the coexisting PO43- can greatly decrease Re by 24.11 %. As(III) adsorption on Fe/MnGOCS is endothermic and its kinetic process is controlled by pseudo-second-order with a determination coefficient of 0.95. Fitted by the Langmuir isotherm, the maximum adsorption capacity is 108.89 mg/g at 25 °C. After four times regeneration, there is only a slight decrease of <10 % for the Re value. Column adsorption experiments show that Fe/MnGOCS can effectively reduce As(III) concentration from 10 mg/L to <10 μg/L. This study provides new insights into binary polymer composite modified by binary metal oxides to efficiently remove heavy metals from aquatic environments.
The rapid growth of industrialization and urbanization has increased the risk of water pollution worldwide. In the current scenario the water pollution due to the heavy metals reached to the alarming stage. The non‐biodegradable nature of heavy metals and their easy bioaccumulation on the body of the living organism causes chronic health issues in human beings as well as animals. Arsenic (As) and chromium (Cr) are the most toxic heavy metal ions, which are readily present in the aquatic system. The toxicity of the arsenic and chromium depends upon the speciation of these metal ions in the aquatic system. Chromium mainly exists in aqueous system as Cr(III) and Cr(VI), among which Cr(VI) is in highly noxious oxidation state and Cr(III) is required in trace amount for the biological function. Arsenic exists in four oxidation states, i.e. −3, 0, 3, and 5, while As(III) and As(V) are in the common oxidation state in aqueous system. As(III) is in a highly dangerous oxidation state of arsenic than As(V). In this chapter discussion has been carried out on the speciation of chromium and arsenic in aqueous system. In addition to this, the overview of the biogeochemical cycle of these metals ions is also provided.
Release of contaminants from aquifers at the coastal area is of increasing concern, but remains unclear due to the complex groundwater dynamics and hydrochemistry. Specifically, frequently occurring seawater intrusion and the subsequent engineering measures of managed aquifer recharge (MAR) could alter the groundwater regime, which might affect the fate and behaviors of contaminants. In this work, we investigated the transport and transformation of arsenic (As) in the coastal aquifer at the scenario of seawater intrusion followed by the injection-based MAR process. Results showed that seawater intrusion induced 10.3% more release of aqueous As in aquifers, which was attributed to the competitive desorption as a result of elevated anion concentration and pH, and the reduction of As(V) to As(III) due to the reduced redox potential and enriched As-reducing bacteria. Furthermore, seawater intrusion inhibited the recrystallization of iron (hydr)oxides and instead facilitated its conversion to iron sulfide with lower affinity to As. The subsequent MAR introduced oxygenated recharge water into aquifers and increased the redox potential, leading to the dissolution of iron sulfide followed by formation of amorphous iron (hydr)oxides. However, the competitive desorption of As with rich HCO3⁻ under increased pH dominated continuous increase in the aquifer aqueous As during MAR process. A constructed numerical model for describing As transport based on the experimental data showed that As transported along the interface between seawater and freshwater, and MAR enhanced the release of As and expanded the spread range of As. Our findings reveal that both seawater intrusion and subsequent MAR could cause the release, transport, and transformation of As, which provides new insight on the understanding of geochemical process of As in coastal aquifers.
The chemistry and occurrence of arsenic (As) worldwide are described in this chapter, mainly in waters. Most environmental As contamination is natural, but an important amount originates from anthropogenic activities. This issue leads millions of human beings to health complications by ingesting poisoned water and meals, coming from soils contaminated with As or irrigated with water polluted with As. This leads to millions of people's health complications by consumption of poisoned water and food produced using water contaminated with As. The worldwide occurrence of As is also described. At present, over 220 million people coming from more than 105 countries are exposed to As pollution. The most important affected countries are Argentina, Bangladesh, Cambodia, Canada, Chile, China, Germany, Hungary, India, Japan, Laos, Mexico, Nepal, Nicaragua, Pakistan, Poland, Romania, Taiwan, Thailand, United Kingdom, United States, and Vietnam, i.e. countries that present the highest concentrations. Arsenic occurs naturally in several chemical forms of different toxicity; the inorganic and predominant oxidation states are the trivalent (As(III)) and the pentavalent (As(V)) forms, which are also the most toxic, especially As(III). Arsenic is widely distributed on the earth. The occurrence and mobilization of As in natural media and their biological impact on organisms are also important issues. The element is part of more than 200 minerals; the natural As mostly comes from hydrothermal mineral deposits, volcanic rocks, marine sedimentary rocks, ash, geothermal waters, and fossil fuels (e.g. coal and oil). Anthropogenic activities (agriculture, mining, glass and ceramics industry, feed additives of poultry and swine, electronics, pharmaceuticals, coal combustion, etc.) also contribute importantly. The toxicity of As coming from different chemical forms is also discussed together with its implications on human health.
The Gazlıgöl geothermal field located in 20 km northern of Afyonkarahisar in Western Anatolia region of Turkey is one of the most important geothermal areas in the Afyon-Akşehir Graben on the basis of potential. The present study has been performed to investigate the hydrogeochemical and isotopic characteristics and evaluate the origin, reservoir temperature and processes controlling the chemical composition of thermal waters of the Gazlıgöl geothermal field. Discharge temperature, electrical conductivity and pH of the thermal waters vary from 58.5 to 74 °C, 3910 µS/cm to 4050 µS/cm and, 7.05 to 7.54, respectively. The Gazlıgöl thermal waters are chemically of Na-HCO3 type, with high salinity, while the cold waters have mainly Ca-Mg-HCO3, Ca-Na-HCO3 and Ca-Mg-HCO3-SO4 types, with low salinity. Geochemical processes controlling chemical composition of the thermal waters are mainly water-rock interactions including dissolution and/or weathering of mostly silicates and partially carbonates and ion-exchange reactions. Besides, the bacterial sulfate reduction is another main process leading to the depletion of SO4 in the thermal water. Higher contents of some minor elements in the thermal waters, such as F, B, Li, As and Sr, are probably derived from enhanced water-rock interaction, and the minor elements can be regarded as indicator of residence times and flow paths.
Geothermometer applications including quartz geothermometers and SiO2-K²/Mg, mineral saturation state and silica enthalpy mixing models provide the most reliable estimations of reservoir temperature in the varying of 100-150 °C for the Gazlıgöl thermal waters. Accordingly, the Gazlıgöl geothermal system can be classified as a low-moderate geothermal resource. The isotope data (δ¹⁸O, δ²H and ³H) of the thermal waters point to their deep circulating meteoric origin, and let estimating of recharge elevation varying from 1300 to 1400 m.a.s.l. These elevations suggest the mountainous region in the north-northeastern of the study area as the recharge area of the geothermal system. Long-term circulation of meteoric waters within the aquifer is confirmed by low tritium ratios (< 1 TU) of the thermal waters although the fluids do not achieve thermodynamic equilibrium. Based on the all gathering data, a conceptual model of the functioning of geothermal system has been constructed. Accordingly, geothermal fluid is heating at deep zones due to the high geothermal gradient caused by active fault systems and heated waters rise toward the shallow levels through faults and fractures. During moving to the surface, they undergo conductive cooling by mixing with the cold groundwater at shallow depths and during contacting with the colder rocks.
Microbial-mediated redox reactions typically control the solute mobilization in groundwater systems, in which the clay aquitard is usually a triggering factor of the release of the dissolved organic carbon, ammonium, and iron into the aquifer during pore water discharge and clay compaction. However, solute mobilization inside the aquitard during clay compaction is generally mistaken for a similar water-rock interaction process in aquifers controlled by microbial-mediated redox reactions. Through a simulation experiment on argillaceous sediment compaction, we tested the currently accepted solute mobilization mechanism and determined that the variation in the mineral structure dominates it. The variation in the mineral structure occurs in low symmetry minerals, such as clay minerals, and is controlled by the sediment moisture content, the liquid-plastic limit, and the effective stress. When the sediment moisture content decreases to below the plastic limit through pore water discharge and compaction, sudden changes in the mineral structure occur, releasing iron and capturing ammonium through variations in the relative position of the Si-O tetrahedrons and isomorphism of similar atoms without the participation of microbial-mediated redox reactions. These results suggest that the biochemical reactivities of organic carbon, iron, and ammonium are sometimes overestimated, i.e., when the role of the physical processes is ignored, in solute mobilization during clay compaction, which warrants more attention and investigation.
The Microbacterium foliorum species is known for its ability to produce siderophores which, in synergy with the plant root, can effectively affect the rates of arsenate-to-arsenite mobilization, arsenic uptake, and translocation. This study investigates M. foliorum, a plant growth-promoting bacterium (PGPB), in the absorption of arsenic (As) by Melastoma malabathricum plant from contaminated soil. The current study demonstrates M. foliorum phytoremediation’s efficacy in terms of As tolerance, removal, and toxicity in the M. malabathricum plant. For an observation period of 90 days, growing plants were treated with M. foliorum in arsenic-contaminated soil. The morphological trait, pH of soil, and potential advantage induced by M. foliorum on M. malabathricum were evaluated. Plants did not display any noticeable signs of toxicity, however, the root and stem length were significantly increased in the presence of M. foliorum. The Bioconcentration Factor (BCF) was increased in plants inoculated with M. foliorum by 0.3 times, the Transfer Factor (TF) of As in the M. malabathricum plants decreased, whether in the presence or absence of PGPB. The As phytoremediation treatment with M. foliorum also enhanced uptake of As in root (by 26%) and shoot (by 22%) than in the other two (A2 and control) phytoremediation treatments. This shows that M. foliorum inoculation reduced As toxicity through substantial reduction of the adverse stress effects, increased stem and root fresh and dry weight.
Acid mine drainage (AMD) of the abandoned coal mines of the Kizelovsky coal basin (the Urals, Russia) is one of the worst natural disasters in the region. Acidic sulphate waters with a high content of metals freely flow into the surface water bodies. Arsenic, found in elevated concentrations in AMD, is an element of concern due to its potential toxicity to humans and animals. The aim of this work is determination of chemical speciation of inorganic arsenic in AMD as well as the surface water and groundwater affected by mine drainage, and assessment the natural removal of As from mine drainage due to adsorption on precipitated hydrous ferric oxide (HFO). Geochemical speciation (PHREEQC) revealed that arsenic occurs in all water samples as As(V). Surface complexation model shows that, HFO induced by the natural attenuation process may remove 46–85% of total arsenic in AMD and only 28% in polluted groundwater (on average).
Arsenic is a toxic carcinogen mostly found in subsurface environments. It is released into groundwater mostly via natural geological and hydrological processes. Consumption of such contaminated water leads to serious health crises for mankind. Developing countries in South Asia, particularly the rural regions, have been severely affected by this environmental phenomenon. In India, government authorities have implemented several remedial measures to provide safe drinking water to the rural habitations, ranging from utilization of surface water bodies (e.g., rivers, ponds, etc.) for piped water supply schemes and establishment of groundwater treatment units. This article attempts to review the scientific literature describing the critical situation in India's fourth most populous state of West Bengal and the engineering advances made in the mitigation. The issue of safe disposal or stabilization of arsenic wastewaters generated from the treatment units is often not emphasized in policy discussions. In a concluding note, this article proposes innovations necessary for achieving effective arsenic mitigation in the region that involve both groundwater remediation and waste management in tandem for sustainability.
This study investigated the arsenide removal by using mesoporous CoFe2O4/graphene oxide nanocomposites based on batch experiments optimized by artificial intelligence tools. These nanocomposites were prepared by immobilizing cobalt ferrite on graphene oxide and then characterized using various techniques, including small angle X-ray diffraction, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. Artificial intelligence tools associated with response surface methodology were employed to optimize the conditions of the arsenide removal process. The results showed that back propagation neural network combined with genetic algorithm was suitable for the arsenide removal from aqueous solutions by the nanocomposites based on the minimum average values of absolute errors and the value of R2. The optimal values of the four variables (operating temperature, initial pH, initial arsenide concentration, and contact time) were found to be 25.66 °C, 7.58, 10.78 mg/L and 46.41 min, and the predicted arsenide removal percentage was 84.78%. The verification experiment showed that the arsenide removal percentage was 86.62%, which was close to the predicted value. Three evaluation methods (gradient boosted regression trees, Garson method and analysis of variance) all demonstrated that the temperature was the most important explanatory variable for the arsenide removal. In addition, the arsenide removal process can be depicted with pseudo-second-order kinetics model and Langmuir isotherm, respectively. The thermodynamics investigation disclosed that the adsorption process was of a spontaneously endothermic nature. In summary, this study showed that ANN-GA was an efficient and feasible method in determining the optimum conditions for arsenic removal by CoFe2O4/graphene oxide nanocomposites.
Arsenic (As) is an element naturally present in some rock formations, in soils and water.
Innumerable studies demonstrate that As is ultra toxic: a dose of 125 milligrams can kill a human adult.
Consumed in minimal portions throughout time, as occurred to the Bengalese, is also lethal. The poison accumulates in the organism and the symptoms of intoxication take up to two decades, on average, to manifest their presence. The main objective of this study was: to investigate and prove natural contamination by As in the soils and groundwaters in part of the urban area of the city of Ouro Preto (MG). The presence and evolution of As in the groundwaters of four water collection areas used by the population of the city of
Ouro Preto is a natural, temporal and localized contamination peculiar to the area studied. As is a toxic element and extremely aggressive to human health, and the As content in the waters used by part of the population of the city of Ouro Preto is greater than the values established by the state and federal environmental control and sanitation agencies. In the collection areas studied, the waters analyzed presented marked variations in As concentrations. In dry periods with reduction in the water level in the aquifers, there
is reduction in As concentration. However, in the rainy season, rainfall mobilizes and leaches As to the environment, increasing its concentrations in groundwaters. The species of As present are controlled by the Eh – pH conditions. The As5+ species of the oxyanions (H2AsO4 - and HAsO4
2- ) are predominant in the Eh and pH conditions of the groundwaters analyzed. The waters analyzed are from oxygen-poor environments, nevertheless from aquifers near to contact with the atmosphere, which aids in understanding of the existing
hydrogeological model. Thus, it is ascertained that the waters have their origin in rainwater, with circulation
nearer to the topographical surface. Less time of water remaining in the aquifers due to topographical and hydrogeological conditions, and a greater rate of renewal of groundwaters result in a greater quantity of As to be carried by the surface, subsurface and groundwater flows. In the city of Ouro Preto, the soils of the neighborhoods Alto da Cruz, Padre Faria, Piedade, and Antônio Dias, present elevated total As contents that
greatly surpass the maximum admissible value for any type of use. In general, the As contents varied from 6 to 925 mg.kg-1. Dust from contaminated soils may have toxic effects if inhaled or ingested by human beings, particularly children, who are more susceptible to this type of contamination due to their habits.
Contamination of waters and soils by As is derived from mineral paragenesis of some types of rocks, hydrological system, geochemical environment, and hydrogeological and topographical control. For the scenario studied, the assessment of risk to human health arising from exposure to As, and the values estimated for receptors (carcinogenic and non-carcinogenic effects) were considered unacceptable, with the greatest levels of risk being for children exposed through ingestion and dermal contact with soil and groundwater.
Arsenic (As) bioremediation has been an economical and sustainable approach, being practiced widely under several As-contaminated environments. Bioremediation of As involves the use of bacteria, fungi, yeast, plants, and genetically modified organisms for detoxification/removal of As from the contaminated site. The understanding of multi-factorial biological components involved in these approaches is complex and more and more efforts are on their way to make As bioremediation economical and efficient. In this regard, we systematically reviewed the recent literature (n=200) from the last two decades regarding As bioremediation potential of conventional and recent technologies including genetically modified plants for phytoremediation and integrated approaches. Also, the responsible mechanisms behind different approaches have been identified. From the literature, it was found that As bioremediation through biosorption, bioaccumulation, phytoextraction, and volatilization involving As-resistant microbes has proved a very successful technology. However, there are various pathways of As tolerance of which the mechanisms have not been fully understood. Recently, phytosuction separation technology has been introduced and needs further exploration. Also, integrated approaches like phytobial, constructed wetlands using As-resistant bacteria with plant growth-promoting activities have not been extensively studied. It is speculated that the integrated bioremediation approaches with practical applicability and reliability would prove most promising for As remediation. Further technological advancements would help explore the identified research gaps in different approaches and lead us toward sustainability and perfection in As bioremediation.
Groundwater with low levels of arsenic (As) in deep aquifers has been overexploited for decades in many regions (such as South Asian and American countries), resulting in the compaction of clayey aquitards and the release of pore water with As into deeper aquifers. However, the release mechanism of arsenic during clayey aquitards compaction is poorly understood. An indoor compaction experiment using muddy sediments was conducted to identify the As-releasing mechanisms during clayey aquitards compaction. The chemical characteristics and As species in pore water and sediment samples collected at different stages during the compaction experiment were analyzed. Initially, the reductive dissolution of iron oxides was a key process controlling As release during compaction. With increased pressure, As desorption from Fe (hydr)oxides became more important than the reductive dissolution as the main driver for As release. At the end of the compaction, the release of As was weak and the dominant process was desorption of As from clay or carbonate minerals. Our estimate result in the Jianghan Plain suggested that As concentration release from aquitard compaction by overexploited into groundwater would be 9.33–118.09 μg/L.
Threats due to insufficient, inadequate and costlier methods of treating contaminants such as arsenic have emphasized the significance of optimizing and managing the processes adopted. This study was aimed at the complete elimination of arsenic from an aqueous medium with minimum energy consumption using the electrocoagulation process. Arsenic removal around 95% was rapidly attained for optimized conditions having a pH of 7, 0.46 A current intensity, 10 mg/L initial concentration and only 2 min of applied time duration using the energy of 3.1 watt-hour per gram of arsenic removed. Low values of applied current for longer durations resulted in the complete removal of arsenic with low energy consumption. Various hydroxide complexes including ferrous hydroxide and ferric hydroxide assisted in the removal of arsenic by adsorption along with co-precipitation. Surface models obtained were checked and found with a reasonably good fit having high values of coefficient of determination of 0.933 and 0.980 for removal efficiency and energy consumption, respectively. Adsorption was found to follow pseudo-first-order kinetics. Multivariate optimization proved it as a low-cost effective technology having an operational cost of 0.0974 Indian rupees (equivalent to USD 0.0013) per gram removal of arsenic. Overall, the process was well optimized using CCD based on response surface methodology.
Arsenic (As) is a pollutant of major concern worldwide, posing as a threat to both human health and the environment. Phytoremediation has been proposed as a viable mechanism to remediate As-contaminated soil environments. Pot experiments were performed to evaluate the phytoextraction efficiency of As by Pteris vittata, a known As hyperaccumulating fern, from soil amended with different concentrations of arsenate [As(V)] and arsenite [As(III)], the more common, inorganic As forms in soil. The greatest accumulation of As (13.3 ± 0.36 g/kg Dwt) was found in fronds of plants grown in soil spiked with 1.0 g As(V)/kg. The maximum As-bioaccumulation factor (27.3 ± 1.9) was achieved by plants grown in soil amended with 0.05 g As(V)/kg. A total of 864 bacterial cultures were isolated and examined for their ability to enhance phytoremediation of As-contaminated soils. Traits examined included tolerance to As (III and V), production of siderophores, and/or ability to solubilize calcium phosphate and indole acetic acid (IAA) production. A culture-based survey shows greater numbers of viable and As-resistant bacteria were found in the rhizosphere of As-grown plants compared to bulk and unplanted soils. The percentage of bacteria resistant to As(V) was greater (P < 0.0001) than those resistant to As(III) in culture medium containing 0.5, 1, 1.5, and 2 g As/L. Higher (P < 0.0001) percentages of siderophore producing (77%) and phosphate solubilizing (61%) bacteria were observed among cultures isolated from unplanted soil. About 5% (44 of 864) of the isolates were highly resistant to both As (III) and As (V) (2 g/L), and were examined for their As-transformation ability and IAA production. A great proportion of the isolates produced IAA (82%) and promoted As (V)-reduction (95%) or As(III)-oxidation (73%), and 71% exhibited dual capacity for both As(V) reduction and As(III) oxidation. Phylogenetic analysis indicated that 67, 23, and 10% of these isolates belonged to Proteobacteria, Actinobacteria, and Firmicutes, respectively. Analysis of the 16S rRNA gene sequences confirmed that these isolates were closely related to 12 genera and 25 species of bacteria and were dominated by members of the genus Pseudomonas (39%). These results show that these isolates could potentially be developed as inocula for enhancing plant uptake during large scale phytoremediation of As-impacted soils.
Sulfuric acid plant wastewater is characterized by low pH and high concentrations of sulfate and metals, including arsenic, which is a toxic element that requires specific treatment. This study aimed to investigate the sulfuric acid plant wastewater treatment using NF by evaluating the main NF operational conditions (feed pH, applied pressure and permeate recovery rate) and the preliminary capital and operational cost. The results showed the NF potential to treat the wastewater. The retention of higher valence counterions to balance the retained co-ion loads was the most important mechanism observed. By combining the best operational conditions (feed pH = 2, applied pressure = 10 bar and permeate recovery rate = 45%), retention of 94% of sulfate, 98% of calcium, 85% of As III and 75% of As V was achieved and permeate could be reused in industry. The NF system had its CapEx estimated at US 99,000 year ⁻¹ could be achieved with the industrial reuse of the permeate, which could reduce the water consumption by approximately 551,880 m ³ year ⁻¹ .
Reactive oxygen species (ROS) such as the free radicals (e.g. hydroxyl, nitric acid, superoxide) cause damage to lipids, proteins and DNA. Increased production of ROS occurs from pollution. Process of removal or neutralization of ROS is achieved through antioxidants enzyme defense systems and provide homeostasis within biological systems. Aerobic organisms have complex antioxidant systems using enzymatic and non-enzymatic antioxidants to prevent overproduction of ROS. This study examined the toxic effects of arsenic and zinc on Eastern oysters, their interaction and resulting enzymatic responses. Cellular damage as indicated with lipid peroxidation and antioxidant defensive enzyme activities (superoxide dismutase, SOD; glutathione peroxidase, GPX and catalase, CAT) were measured in the hepatopancreas of Eastern oysters exposed to single and combined treatments of arsenic and zinc for 30 days. The results showed either arsenic or zinc exposure significantly increased the lipid peroxidation and triggered antioxidant defenses. Activities of antioxidant enzymes (SOD, GPX and CAT) were markedly elevated upon expose of As or Zn. However, at the presence of Zn, As toxicity expressed as lipid oxidation significantly decreased as well as accordingly decreased activities of antioxidant enzymes. This revealed that the presence of Zn showed a significantly antagonistic effect on arsenic toxicity in Eastern oysters from Northern Gulf of Mexico.
Oxidation-reduction (redox) potentials calculated from arsenic speciation showed fair correlation with measured Pt-electrode potentials (Eh) in shallow groundwaters in east-central Illinois. The observed bias in calculated potentials relative to measured Eh values could not be explained by analytical imprecision. Redox potentials calculated from Fe concentrations showed better correlation with measured potentials than the As potentials. Arsenic speciation may be a useful supplement to Eh measurements and concentrations of other solutes, but is probably not a good indicator of redox conditions when used alone.
Arsenic species, iron, and manganese distributions were studied in the oxidizing and reducing waters of Lake Pavin, a small and well-stratified crater lake situated in the Massif Central range (France). Arsenate and arsenite concentrations versus depth do not reflect the expected thermodynamic equilibria, indicating a slow and incomplete response to the redox conditions. The occurrence of arsenic in the anoxic zone results from transport on a particulate phase, due to adsorption onto iron and manganese oxides and probably to incorporation in phytoplanktonic organisms.
A detailed hydrogeochemical investigation of 14 km of stream waters contaminated by acid mine drainage in Shasta County, California provides insight into the equilibrium transformations of iron during the oxidative weathering of pyritic ore. Over 60 water analyses covering a range of pH, redox state, total iron concentration, temperature and ionic strength were processed with the computerized chemical model WATEQ2 so that activities of free ions and complexes and saturation indices could be determined. The results demonstrate (a) measured Eh values agree quite well with Eh values calculated from the ferrous/ferric activity ratio, (b) the dominant complexes in acid mine waters are FeSO//4 degree , FeSO//4** plus , Fe (SO//4)//2** minus , Fe (OH)**2** plus , Fe//2 (OH)//2**4** plus and Fe (OH)//2** plus and (c) acid mine waters low in pH (1 to 2) and high in reduced iron approach saturation with respect to melanterite whereas progressive downstream oxidation and dilution (pH equals 2 to 3. 5) of these waters forces these waters to become saturated with respect to amorphous iron hydroxide.
Sorption of momomethyl arsonic acid (MMAA), dimethyl arsinic acid (DMAA), and arsenate on anaerobic bottom sediments from the Menominee River, Wisconsin are described by Langmuir isotherms. These results were incorporated into a kinetic model of arsenic species transformation which takes sorption into account. Model predictions were found to be sensitive to the sediment water content and GAMMA infinity , the adsorptive capacity of the sediment. Demethylation of MMAA and DMAA was observed in sediment incubation experiments. The predictions of the sorption/kinetic model were in good agreement with the results of the incubation experiments.
Abstract The influence of redox environments and/or pH on the mobility of arsenic through sand columns was studied by using waters of different redox characteristics for elution and by comparing the elution profiles of As(V) to that of As(III). The mobility of arsenic was affected by each of the above parameters, by the amount of arsenic loads onto the columns and the nature of the column materials. Solution studies have shown that both As(III) and As(V) from complexes with Fe(III); the solubility of the complexes being dependent upon the oxidation state of the arsenic and the solution pH.
Detailed vertical profiles of dissolved Cl- and its isotopes
(36Cl and δ37Cl) provided new information on
the origin and systematics of this conservative tracer in pore waters of
a thick aquitard system. The aquitard system consists of surficial
Quaternary clay-rich till (80 m thick) deposited 30-20 kyr B.P.,
overlying Cretaceous marine clay (76 m thick) deposited ˜71 Ma.
The distribution of Cl-, δ37Cl, and
Br- showed the presence of five distinct end-members for
Cl-: the top of the unoxidized till, a regional aquifer
underlying the Cretaceous clay, two localized geological heterogeneities
(sand streaks) in the till, and glacial meltwater emplaced with the till
and still present at depths of between 36 and 60 m. Numerical
simulations of the transport of Cl- from the sand streaks
indicated that this geochemical profile has been developing throughout
most of the Holocene. The 36Cl measurements showed that the
age of the dissolved Cl- in the upper Cretaceous clay is
likely between 0.75 and 1.9 Myr. The 36Cl measurements
further suggested that the dissolved Cl- in the till was not
directly derived from the underlying Cretaceous clay. Finally, it was
not possible to quantify the effects of isotopic fractionation of
37Cl relative to 35Cl because of diffusion in this
aquitard system.
The Late Cretaceous Fox Hills Formation and the basal portion of the
overlying Hell Creek Formation constitute an important aquifer in the
Fort Union coal region. Throughout most of southwestern North Dakota and
northwestern South Dakota the aquifer is at depths ranging from 1000 to
2000 ft, except for exposures along the Cedar Creek anticline. Water
flows in the aquifer from southwest to northeast, with flow rates of a
few feet per year. The recharge and discharge areas of the aquifer are
separated by a north-south trending transition zone in which significant
changes in water chemistry occur. Dissolved constituents in the recharge
area (the western part of the study area) are Na+ = 18
mmol/l, Cl- = 0.7 mmol/1, SO42- = 2.7
mmol/1, and HCO3- = 13 mmol/l
(δ13C = -12‰) with pH = 8.5. Ca2+,
Mg2+, and K+ are each less than 0.1 mmol/l,
dissolved O2 = 0, and traces of H2S and
CH4 are present. Computer modeling and carbon isotope data
suggest the following reactions in the recharge area. CO2
derived from lignitic carbon reacts to dissolve carbonate minerals, with
cations then being exchanged for Na+ on clay minerals. The
high pH in the aquifer is the result of buffering by carbonate-ion
exchange equilibria. In the discharge area, pH values have declined to
8.3, Cl- has increased from 0.7 to 5.5 mmol/l, with a
parallel increase in Na+ SO42- has
essentially disappeared, HCO3- has increased from
13 to 21 mmol/l (δ13C = -9‰), CH4 has
attained concentrations greater than 0.5 mmol/l, and small amounts of He
are present. Traces of H2S are present, and Ca2+,
Mg2+, and K+ concentrations remain low throughout
the aquifer: These changes can be accounted for by reactions in the
aquifer: (1) sulfate reduction to pyrite with lignitic material as the
carbon source and (2) continuous buffering of pH by the carbonate-ion
exchange equilibria. Chemical and hydrologic data suggest that the
increase in NaCl results from upward movement of small volumes of water
into the Fox Hills aquifer from the transition zone eastward. Redox
reactions in the aquifer are closely analogous to those observed in pore
waters of reducing marine sediments. Reactions approach but do not
achieve true thermodynamic equilibrium. Measurements of redox potential
suggest a downgradient decrease in redox potential. The measurements are
not amenable to quantitative interpretation.
The Milk River artesian aquifer underlies 15 000 km2 of southern Alberta, Canada. It consists of thin (30-75 m thick) sandstone and is confined above by the Pakowki shale (typically 120 m thick) and below by the Colorado shale. The aquifer subcrops in southern Alberta and northern Montana. Groundwater movement is to the north, west and east from the outcrop area (dominant recharge area). Cl- and I- concentrations increase in the direction of flow from less than 0.05 and 0.001 mmol/L, respectively, near the recharge area to more than 140 and 0.15 mmol/L at the northern edge of the aquifer. Similarly, waters become more enriched in oxygen 18 and deuterium from less than -21.0 and -167‰ near the recharge area to values approaching -8.0 and -70.0‰ in the north. Isotope values in the recharge area plot on the global meteoric water line indicate that the the recharge waters are isotopically unaltered meteoric waters. Downgradient from the recharge area the data deviate from the meteoric water line (slope of 6.3 instead of 8.0). Three mechanisms have been advanced to explain the origin of the chemical and isotopic patterns: the introduction of connate formation water through the Colorado shale and subsequent mixing with infiltrating meteoric water; a finite source of meteoric recharge mixing with more saline water in the aquifer; and chemical and isotopic enrichment due to ion filtration. -from Authors
Diagenetic Fe and As samples have been obtained from 12 lakes on the Canadian Shield by deposition on Teflon collectors inserted vertically in the surficial sediments. Dissolution of the deposits showed that only arsenate was present, i.e. arsenite concentrations were undetectable. Laboratory batch experiments indicated that As(III) is oxidized rapidly by Fe oxyhydroxides. Apparent equilibrium constants for the absorption of As have been calculated from the Fe and As concentrations dissolved from the deposit of diagenetic material and As concentration in the water overlying the lake sediments. These field‐derived equilibrium constants agree well with laboratory‐derived equilibrium constants for the absorption of arsenate on amorphous Fe oxyhydroxides; the results taken together suggest that the latter laboratory constants can be used for modeling adsorption of inorganic As on Fe oxyhydroxides in natural waters.
The fate of As in soils is regulated mostly by its participation in sorption reactions and redox transformations. Few studies have examined the rate of arsenite and arsenate reduction or the extent to which these redox transformations may be affected by sorption reactions. The objective of this study was to examine changes in solution concentrations of H3AsO30 and H2AsO4- in two soils subjected to prolonged flooding. The soils, which differed in H3AsO30 and H2AsO4- sorption capacities, were flooded by suspending 1 g of soil in 25 mL of a solution containing 0.01 M CaCl2 and 1 g D-glucose kg-1. The suspensions were amended with NaAsO2 or Na2HAsO4 · 7H2O and were incubated for 0.5 h to 20 d. Changes in solution chemistry (electrode potential [Eb], pH, and dissolved Fe, Mn, H3AsO30, and H2AsO4-) were observed with time. Sorption processes controlled the dissolved concentrations of H3AsO30 and H2AsO4MIN during initial stages of flooding. When anaerobic conditions were achieved, dissolution of Fe and Mn oxyhydroxides occurred, causing desorption of H3AsO30 and H2AsO4-. In NaAsO2-amended suspensions, desorbed H3AsO30 disappeared from solution within 10 d. In Na3HaSO4--amended suspensions, desorbed H2AsO4- also disappeared within 10 d. Concurrent with the disappearance of H2AsO4- was the appearance of H3AsO30, indicating that H2AsO4- was rapidly reduced to H3AsO30. First-order plots of H3AsO30 and H2AsO4- disappearance had a linear relationship. Rates of desorption and disappearance of H3AsO30 and H2AsO4- were slower in the soil with higher adsorption capacity, suggesting that sorption processes may influence redox transformations of As oxyanions.
Transport and geochemical processes controlling the chemical and isotopic composition of pore waters in the upper 45 m of a thick, surficial, clay-rich till (Battleford Formation) were studied. The upper 3 m of the till is oxidized and fractured. The remaining 77 m of the till (the aquitard) is nonfractured and unoxidized. Concentrations of total dissolved solids, SO42-, Na+, Mg2+, and K+ decrease with depth through the unoxidized zone to a depth of about 15 m below ground, below which the concentrations remain constant. A similar trend was observed for alkalinity; however, the decrease in concentration occurs over 20 m. In contrast to these ions, Ca2+ concentrations increase with depth through the upper 20-m unoxidized zone, below which the concentrations remain relatively constant. The distribution of dissolved ions shows the presence of two end-members: elevated solute concentrations in the oxidized zone attributed to geochemical weathering that occurred since the start of the Holocene and connate Pleistocene-age water at depths between 20 and 45 m. Abiotic and biotic geochemical reaction rates in the aquitard were shown to be extremely slow or not occurring.
The sorption of arsenic (as arsenate), phosphate, DSMA (disodium methanearsonate) and the sodium salt of cacodylic acid (hydroxydimethylarsine oxide) by 16 Mississippi River alluvial flood plain soils was measured in a slurry experiment in which initial pH and solute molar solution concentrations for all four solutes were identical. Sorption of the two organoarsenical herbicides was strongly correlated with arsenate and phosphate sorption, and sorption of all four species was strongly correlated with clay and iron oxide contents of the soils. In 10-3 to 10-4 M solution, the arsenicals are more strongly sorbed than phosphate with sorption increasing in the order P < cacodylate < arsenate methylarsonate.
The μg/l to mg/l range of arsenic As3+ and As5+ (III) and (V) is found in most natural waters and wastewaters. Experimental results show that arsenic removal can be best accomplished in the As5+ state. As5+ is effectively adsorbed between pH 4 and 7 by activated alumina and bauxite, and between pH 3 and 5 by activated carbon. H2AsO4- is found to be the major species removed by slightly positive or neutral charge surfaces. For AS3+ adsorption, neutral H3AsO3 is found to be removed by the relatively neutral charge surfaces. The adsorption of As4+ and As5+ follows the Langmiur isotherm. The rates of adsorption for both As3+ and As5+ decrease with increasing ionic strength.
Recent improvements in sample collection and analytical techniques have suggested that As(III) is more prevalent in groundwater than previously believed. Indeed, reducing conditions in alluvial aquifers supplying single families may result in significant exposures to naturally occurring As(III). These results are noteworthy because As(III) is both more toxic and more mobile in the environment than As(V). The literature contains contradictory information concerning the appropriate preservation and analytical techniques for determining As(III). It appears that several previously reported occurrences of As(V) may have been predominantly As(III), but the samples were either not preserved or analyzed properly. For example, separation of arsenic species by ion exchange is apparently necessary to obtain reliable analytical results for certain environmental samples. The problems encountered with investigating As(III) in the environment are due to the complex series of geochemical reactions undergone by arsenic. The complexity of these reactions and the variable experimental conditions used by different investigators have resulted in widely different conclusions concerning both the nature of arsenic adsorption reactions and reaction kinetics. Moreover, it appears that biological reactions may play a role in certain ecosystems. In general, the mechanism promoting the mobility of As(III) in groundwater is the onset of reducing conditions in alluvium in which iron oxides have sorbed arsenic. Such conditions may result in concentrations of arsenic in groundwater as high as several hundred micrograms per liter.
Porewater REE concentrations vary by an order of magnitude over 45 vertical m in a thick clay-rich till aquitard in southern Saskatchewan. To address controls on aqueous REE, the till was disaggregated into seven size fractions (> 850, 425-850, 295-425, 180-295, 150-180, 75-150, and < 75 μm). Each fraction was sequentially leached with water (L1); 1 M NaOAc (pH 5) (L2); 0.25 M NH 2OH · HCl (L3); 1 M NH 2OH · HCl (L4); 12 M HCl + KClO 3 + 4 M HNO 3 (L5); and the residue digested in Na 2O 2 (L6). Aqueous leachate of the < 75 μm size fraction has a near flat REE pattern at ~0.1 PAAS and carries 99% of REE, relative to the total from all L1 leached size fractions. Aqueous leachates of the coarser size fractions have flat patterns 1 to 2 orders of magnitude lower. Leachate L2 extracts metals held electrostatically on inorganic or organic material: the REE also plot at ~0.1 PAAS with a convex up pattern and a maximum at Gd. Leachate L3 of amorphous Fe- and Mn-oxyhydroxides carries more REE than L1, L2 or L4, and like L2, has a mildly convex up pattern. There are lower REE abundances associated with crystalline Fe- and Mn-oxides (L4) than with amorphous counterparts, and HREE are fractionated and depleted. Leaching of organic matter (L5) produces REE patterns comparable to those of L4. The residue (L6) has convex down REE patterns at 0.1 to 0.4 PAAS with prominent positive Eu anomalies from plagioclase feldspar, excepting the < 75 μm size fraction. Natural porewaters have mildly fractionated patterns with LREE and MREE depletion versus convex up patterns for leachates (L1, L2 and L3). LREE and MREE may readily sorb onto clays, or clay coatings, in the till, and be desorbed during leaching.
Samples collected from a 170 m borehole penetrating till, bentonitic clay, and sand at the Birsay site, Saskatchewan, Canada, has been studied as a first step for understanding the controls on dissolution and transport of trace elements, including REEs, in porewaters of a clay-rich aquitard. The REE budgets of the till, clay, and sand are dominated by apatite and monazite, with sporadic zircon and Y-phosphate. Apatites contain 0.3 to 1.7 wt.% REE, and monazites 12 to 55 wt.%. All three units are characterized by unusual bulk sample REE patterns, with maxima at Eu–Gd, convex down patterns from La–Sm with minima at Ce, and variably fractionated HREE. Apatites feature maxima at Eu–Gd. Consequently the complex patterns arise from variable modal proportions of apatite, monazite, and plagioclase feldspar. Relative to Post-Archean Average Shale (PAAS), the till is SiO2 and CaO rich with about half the contents of Al2O3, TiO2, and Fe2O3. These compositional relationship stem from relatively high proportions of quartz and carbonate, but low modal abundances of ferromagnesian and Ti-oxide minerals. Similarly, the till has Cs, Co, Ni, Sc, V, Cu, Pb and Zn contents at about half PAAS. The bentonite clay has slightly higher SiO2 and CaO contents than PAAS, with lower abundances of most other trace elements, including REE which plot at 0.7 to 0.9 PAAS. The major and trace element composition is similar to continental arc andesites, and the clay is interpreted to have a mixed provenance of arc andesite and eroded underlying sands. The composition of the till appears to reflect mixed provenance: erosion of the underlying Cretaceous clay and sand, Paleozoic carbonate rocks, and the Precambrian Shield. The silicious Ardkenneth sand has lower concentrations of REE and most trace elements compared to the clay, but the most fractionated REE patterns. The clay and till have inherited more subdued REE patterns in part from the underlying sand unit. Compositional variations between the units are marked by an upturn of Nb, Th, U and REE concentrations at the till–clay interface (80.2 m), and decrease of these elements at the clay–sand boundary (156.2 m). Ce and Eu anomalies vary erratically with depth: the anomalies appear to reflect the degree of sedimentary sorting, which in turn controls the modal abundances of detrital monazite, apatite, zircon, and plagioclase feldspar. Zr/Zr*, Hf/Hf* and Zr/Hf ratios are higher in the till than the clay and sand, indicating an additional Zr (Hf)-bearing phase with a distinctively high Zr/Hf ratio. The observed small compositional variations in the till and clay are not reflected in variations of geotechnical properties. Given that the REE budget of the till and clay are dominated by highly insoluble trace minerals, it is unlikely that the REE patterns of porewaters will be controlled by bulk sample REE concentrations. Weathering of the upper 3 m of the till has oxidized Fe2+-bearing silicates, oxides, and sulphides, and dissolved carbonates relative to the underlying unoxidized till, weathering has not disturbed major, REE or other trace element compositions.
The mobility of arsenic in the sediment of Lake Ohakuri, a hydroelectric lake on the Waikato River in New Zealand, was studied during the stratification period in 1985/1986. During stratification arsenic was released from the solid phase in the sediments to the interstitial water whence it migrated upwards and accumulated in the upper layers of the sediment. In all sediment cores examined in this study over 90% of arsenic in interstitial waters was present as arsenic(III), an indication that reduction from arsenic(V), the predominant form adsorbed from the lakewater, was virtually quantitative. When conditions at the sediment-water interface became anoxic arsenic(III) diffused across the interface into the hypolimnion where it was stable at least until turnover. Since arsenic(III) is reputedly the most toxic form of arsenic the almost quantitative conversion of arsenic(V) to arsenic(III) in the sediments appears to be of primary importance in the environmental cycle of arsenic in lakes and freshwaters.
Trace-element analysis, correlation statistics, depositional environments, syngenetic, diagenetic, and epigenetic minor element
enrichment, samples mainly from United States
Findings on methylation of tin, mercury, and arsenic in the environment are reviewed. The development of separation and detection systems in trace analysis has made it possible to determine the concentration of these methylated forms in the environment. Abiological methylation mechanisms are similar for these three metals, and methylcobalamin and methyl iodide are thought to be the major methyl donors in the environment. Photochemical reaction and transalkylation produce methylmetals. Humic and fulvic acids are the factors affecting methylation. The research on biological methylation started from incubation with polluted water and sediments, and some exceptional reports cast doubt as to whether it was really biomethylation. Some organisms that can form methyl metals from their metal forms have been separated, and the mechanism has been investigated using the pure culture.Methylation of tin and mercury increases the toxicity of their original metal forms, while methylation of arsenic lowers its toxicity. Methyl metals also have an important role in the geochemical cycle with its higher volatility or lipophilicities. The distribution and ecotoxicities of these methylated compounds in the environment are also discussed.
A detailed vertical profile of the stable isotope deltaD in pore water was measured through a thick aquitard system consisting of surficial Quaternary clay-rich till (80 m thick) and an underlying Cretaceous marine clay (76 m thick). Numerical modeling was used to simulate one-dimensional (vertical) groundwater flow and transport of deltaD. Best fit simulations to the data provided an independent estimate of long-term groundwater velocity through the aquitard and estimates of the timing of late Pleistocene and Holocene events. Best fit simulations to the measured isotope profile across the till-clay interface yielded a groundwater velocity of 0.75-1.0 m per 10 ka for a transport time of between 20 ka and 30 ka. The estimate of velocity agreed well with that calculated from hydraulic data and suggested that hydraulic conductivities of these aquitards are independent of volume tested. The 20-30 ka time frame required for the deltaD profile to develop across the till-clay interface reflects the timing of till deposition and shows that the till is the Battleford Formation, a younger till than previously believed. Numerical transport modeling of deltaD in the upper 30 m of the profile yielded a nonunique fit. Assuming a similar groundwater velocity to that determined across the till-clay interface, a best fit was obtained for a transport times of 7.5-10 ka. This range compared favorably with that reported for the start of the Holocene (about 10 ka B.P.). This study shows that the application of deltaD, and by analogy delta18O, to the study of thick aquitard systems not only can provide independent, long-term estimates of very low groundwater velocities but can also provide insight into the timing of major geologic events such as glaciations.
Although the thermodynamically based concept of oxidation-reduction potential has for many decades been an accepted tool for interpretation of the chemistry of hydrochemical systems, attempts at measurement of actual redox levels in natural waters have been fraught with difficulty. Existing methods of measurement involve use of potential-sensing inert metal electrodes or analytical determination of redox-indicator species such as dissolved O2 or Fe2+ or redox couples such as SO2−4-HS− and HCO−3-CH4. As a result of recent advances in analytical methods, it is now possible to determine the concentrations of both As(III) and As(V) at sufficiently low levels so that the apparent redox condition, as pE or Eh, can be computed from measured concentrations of As(III) and As(V) species. The arsenic pE or Eh domain obtained using published thermodynamic data for As species and the assumption of redox equilibrium, provides a basis for obtaining an indication of redox levels within the central portion of the redox field for natural waters. The redox domain for the As couple is largest at high total dissolved As concentrations, but even at concentrations as low as 1–10 μg/l the domain has significant extent.Oxidation and reduction of As(III) and As(V) in laboratory trials with redox agents common to natural waters, such as O2, H2S and Fe, suggests that oxidation or reduction of As species in natural waters occurs at rates sufficiently slow to enable water samples to be collected, transported and analysed before excessive change in species distribution takes place, but rapid enough for As species to adjust to the dominant redox condition of the water if periods of years or longer are available for equilibration. Because of the long equilibration time and the position of the pE-pH domain for the As couple, groundwater is best suited for use of As as a redox indicator.
Estudia las formas en que los metales se incorporan al suelo, llegandose a establecer que puede ser por adsorcion o incorporacion de los iones al suelo y por retencion. La incorporacion de los diferentes iones al suelo no es arbitraria y depende de la constante de formacion del complejo metal-superficie y del pH en el sistema. Asi, por ejemplo, las tierras alcalinas como Na+, Ca2+ y Mg2+ se adicionan al suelo por medio de procesos de intercambio ionico; los cationes monovalentes de peso molecular mas grande como K+, Rb+ y Cs+ pueden adsorberse directamente a las arcillas minerales y los elementos hidrolizables forman oxidos o hidroxidos
Clay-rich glacial till and Cretaceous clay are common throughout the Interior Plains of North America. Quantifying groundwater flow through these aquitards has implications for solute transport in aquitards and protection of underlying aquifers. Groundwater flow through a two-tiered aquitard system was investigated using laboratory and field methods at a test site in Saskatchewan, Canada. The aquitard system consists of 80 m of uniform, plastic clay-rich Battleford till (deposited 12-18 ka BP) disconformably overlying 77 m of late Cretaceous plastic marine clay (Snakebite Member, deposited 70-72 Ma BP). The upper 3-4 m of till is oxidized and fractured whereas the remainder is unoxidized. For the scales investigated, results suggested that hydraulic conductivity, K, is independent of scale for relatively thick till and clay bedrock deposits. Analysis of slug tests in the unoxidized till and laboratory tests on cores of unoxidized till yielded geometric mean K values of 5.4 × 10-11 and 2.7 × 10-11 m/s, respectively. Laboratory K tests of clay samples yielded a geometric mean K of 4.3 × 10-12 m/s. Bulk K of the clay was estimated to be 2.3 × 10-12 m/s assuming steady-state flow through the till and clay. The present-day groundwater velocity through the aquitard system was estimated to be between 0.5 and 0.8 m/10 ka downward based on the measured K values, measured hydraulic gradients, and measured porosities. Results suggested that pore water in much of the till was introduced during or shortly after glaciation.Key words: hydrogeology, aquitards, Cretaceous clay, Battleford till, hydrogeologic properties, geotechnical properties.
The arsenic cycle was investigated over 1.5 yr in a eutrophic Swiss lake with a seasonally anoxic hypolimnion. Arsenate, As(V), was predominant throughout the entire water column when the lake was well‐mixed in winter. During summer stratification, arsenate disappeared in the epilimnion and was replaced by arsenite, As(III), but in the anoxic hypolimnion As(V) remained the dominant species. Reduction of As(V) in the hypolimnion occurred only late in the stagnation phase, in the presence of sulfide. The dominant redox reactions of inorganic As in the epilimnion were biologically mediated reduction by phytoplankton in summer and oxidation of As(III) by Mn oxides in fall. Approximately 50% of total dissolved As in the epilimnion and hypolimnion was refractory to the direct hydride generation method. The composition of particles, collected regularly in sediment traps, indicated that Fe oxides were the main scavengers for inorganic As. Nevertheless sedimentation was only a minor sink of inorganic As.
Contaminated sediments from the Milltown Reservoir in western Montana release arsenic and various heavy metals (e.g., Cu, Cd, Pb, Zn, Mn) into an underlying alluvial aquifer as redox conditions in the sediments change with seasonally fluctuating water levels. Porewater analyses indicate that sulfate is depleted with depth. In this study, the feasibility of inducing As(III) precipitation through bacterial reduction of sulfate was evaluated in laboratory microcosms established under strictly anaerobic conditions. As(lII), Fe(II), and sulfate concentrations were routinely monitored in the aqueous phase as sulfate was reduced to sulfide. Both As(III) and Fe(II) concentrations in the sediment microcosms decreased as sulfide was made available. Energy‐dispersive x‐ray (EDS) analysis indicated that some of the arsenic was precipitated as an iron‐arsenic‐sulfide solid phase. The precipitation of arsenic observed in this laboratory study suggests that bacterial sulfate reduction may be a process by which heavy metals are immobilized in sediments; however, even though the Milltown sediments contained sulfate‐reducing bacteria, their activity appears to be both sulfate and carbon limited.
Adsorption isotherms in solutions with ionic strengths of 0.01 at 25° C were measured over the arsenite concentration range 10-7-10-5 M and the pH range 4-10. These isotherms obeyed equations of the Langmuir type. Curves of arsenite removed by iron hydroxide from a constant volume of solution, as a function of pH, go through a maximum at approximately pH 7. The pH of the zero point of charge of the suspension was measured as a function of the amount of adsorption of arsenite and was found to decrease as more arsenite adsorbed.
The influence of redox potential and pH on arsenic speciation and solubility was studied in a contaminated soil. Alterations in the oxidation state of arsenic, and influenced by redox potential and pH, greatly affected its solubility in soil. At higher soil redox levels (500-200 mV), arsenic solubility was low and the major part (65-98%) of the arsenic in solution was present as As(V). An alkaline pH, or the reduction of As(V) to As(III), released substantial proportions of arsenic into solution. Under moderately reduced soil conditions (0-100 mV), arsenic solubility was controlled by the dissolution of iron oxyhydroxides. Arsenic was coprecipitated (as As(V)) with iron oxyhydroxides and released upon their solubilization. Upon reduction to -200 mV, the soluble arsenic content increased 13-fold as compared to 500 mV. The observed slow kinetics of the As(V)-As(III) transformation and the high concentrations of Mn present indicate that, under reduced soil conditions, arsenic solubility could be controlled by a Mnâ(AsOâ)â phase.
Due to extremely slow water recovery rates in aquitards and high contamination potential from sealing materials, installation of piezometers in aquitards for geochemical studies requires specialized construction and careful sampling techniques. Few methods have been demonstrated for obtaining representative ground water samples for geochemical parameters from piezometers in aquitards. Here we implement and evaluate an aquitard piezometer installation and ground water chemistry sampling strategy and show that the use of an inert gas pocket in piezometer construction can be used to delay seal contamination for at least three years and avoid oxygenation and disturbance of downhole redox conditions. Major ion analyses did not change appreciably through the standing water columns in the aquitard piezometers over time; however, reliable measurements of typically unstable geochemical parameters (dissolved oxygen, pH, and oxidation-reduction potential) were best obtained using downhole, in situ instrumentation, provided there was at least 2 to 10 m of standing water in the piezometer.
Natural occurrences of ground water with moderate (10 to 50 micrograms per liter) to high (greater than 50 micrograms per liter) concentrations of arsenic are common throughout much of the Western United States. High concentrations of arsenic are generally associated with one of four geochemical environments: (1) basin-fill deposits of alluvial-lacustrine origin, particularly in semiarid areas, (2) volcanic deposits, (3) geothermal systems, and (4) uranium and gold-mining areas. These findings are based on an extensive literature review, compilation of unpublished reports and data, and the review of data bases containing more than 7,000 analyses of ground-water samples for arsenic. In the first two environments, arsenic appears to be associated with sediments derived, in part, from volcanic rocks of intermediate to acidic composition. Dissolved arsenic concentrations in water from volcanic aquifers in the same regions, however, may be low (less than 10 micrograms per liter). Solid phases (minerals, amorphous solids, and sedimentary organic matter) that supply the dissolved arsenic have not been identified in most areas. Alluvial and lacustrine sedimentary deposits appear to be an important source of arsenic in volcanic areas (such as Lane County, Oregon) and in areas underlain by basin-fill deposits (such as Carson Desert in Nevada and the Tulare Lake basin in California). Mobilization of arsenic in sedimentary aquifers may be, in part, a result of changes in the geochemical environment due to agricultural irrigation. In the deeper subsurface, elevated arsenic concentrations are associated with compaction caused by groundwater withdrawals.
A primary source of dissolved inorganic carbon (DIC) in the Black Creek aquifer of South Carolina is carbon dioxide produced by microbially mediated oxidation of sedimentary organic matter. Groundwater chemistry data indicate, however, that the available mass of inorganic electron acceptors (oxygen, Fe(III), and sulfate) and observed methane production is inadequate to account for observed CO2 production. Although sulfate concentrations are low (approximately 0.05–0.10 mM) in aquifer water throughout the flow system, sulfate concentrations are greater in confining-bed pore water (0.4–20 mM). The distribution of culturable sulfate-reducing bacteria in these sediments suggests that this concentration gradient is maintained by greater sulfate-reducing activity in sands than in clays. Calculations based on Fick's Law indicate that possible rates of sulfate diffusion to aquifer sediments are sufficient to explain observed rates of CO2 production (about 10−5 mmoll−1 year−1), thus eliminating the apparent electron-acceptor deficit. Furthermore, concentrations of dissolved hydrogen in aquifer water are in the range characteristic of sulfate reduction (2–6 nM), which provides independent evidence that sulfate reduction is the predominant terminal electron-accepting process in this system. The observed accumulation of pyrite- and calcite-cemented sandstones at sand-clay interfaces is direct physical evidence that these processes have been continuing over the history of these sediments.
Arsenic has accumulated in the different compartments of Moira Lake for the last 160 years, since mining and mineral processing began in the area. The annual total fluvial input of arsenic (As) to the lake is approximately 3.5 tonnes. The dissolved As concentrations in the water show seasonal differences, with an average concentration of 62 μg/L during the summer, and 22 μg/L in winter. The As bound to particles represent approximately 8% of the total As burden in the water column. Arsenic bound to the organic humic, fulvic and lipid fractions represents approximately 1% of the total As in Moira Lake waters. The dissolved As profile of interstitial waters are characterized by subsurface maxima with concentrations four to six times greater than the lake waters. Inorganic As makes up the majority of the As in all the porewater profiles examined. The upward diffusion of As-enriched interstitial waters can explain the distribution of As(III) and As(V) in the overlying lake water. The diffusion upward through the hypolimnion of the arsenite originated in the interstitial water explain the thermodynamic disequilibrium in the distribution of As species in the surface waters of Moira Lake.
The equilibrium activities of 39 dissolved species in the system C-N-S-H2O are computed as a function of pe− and pH at 25°C. For pe-pH conditions encountered in natural waters the predominant dissolved species are CH4, H2CO3, HCO3−, CO32−, NH4OH, NH4+, N2, NO3−, H2S, HS− and SO42−. Organic compounds of higher molecular weight, dissolved or particulate, are thermodynamically unstable in most natural environments.The distribution of dissolved species for the decomposition reactions of carbohydrate (CH2O) and alanine (C3H7O2N) in sea water are computed, assuming equilibrium among the decomposition products. The decomposition, in a closed system, of 0.1 gram-atom of organic carbon per liter of solution produces: a. (CH2O)—mCH4 ~ 10−2, mHCO3− ~ 10−1, mHS− ~ 10−2.5, mNH4+ ~ 10−3, pH ~ 6.5; b. (C3H7O2N)—mCH4 ~ 10−2, mHCO3− ~ 10−1, mHS− ~ 10−1.5, mHN4+ ~ 10−1.5, pH ~ 7.3. In the course of these reactions, the oxidation potential changes by only 50–60 mV after consumption of the initial oxygen. A mathematical expression for the homogeneous pe buffer capacity, βE, is derived. Data from five reducing marine environments show that the predominant dissolved species tend to approach equilibrium, with the exception that the concentration of dissolved methane in certain environments is considerably higher than the predicted equilibrium concentration. Values of βE range from 10−3 to 1.2 for the environments studied.
A 91.4-m thick porewater profile of the rare earth elements (REEs) is presented from a thick clay-rich Pleistocene till (0–80 m) and Cretaceous clay (80–156 m) aquitard sequence, Saskatchewan, Canada. The upper 3–4 m of till is weathered. Absolute aqueous concentrations of the REEs vary by more than one order of magnitude. Aqueous REE concentrations decrease with depth through the weathered till to a depth of about 15 m below ground, below which, the concentrations in the unweathered till remain relatively consistent. The only porewater sample from the clay (91.4 m) has the highest total REE concentration. Elevated REE concentrations in the oxidized zone attributed to geochemical weathering. There is no significant seasonal variation in aqueous REE concentrations within the water columns in the piezometers. Redox conditions and solution complexation reactions exert predominant controls on the aqueous REE concentrations. The majority of REE patterns are coherent, characterized by overall negatively fractionated REE, with convex-down profiles from La to Pr or Nd, and convex-up from Nd to Tb. In the porewater sample from the clay, the LREEs are an exception, with a flat normalized profile. HREE/LREE fractionations in the porewaters are depth-dependent and are mainly controlled by solution complexation with dissolved sulfate, phosphate, free carbonate ions and DOC. Porewaters in the unweathered till (5–76.2 m) are characterized by negative Ce anomalies, which may reflect in part, negative Ce anomalies in the host till geochemistry. The negative Ce anomalies are magnified relative to the host tills during water–rock interaction and REE transportation. Porewaters from the weathered till exhibit strong positive Ce anomalies relative to PAAS, the host rocks and their leachates. The Ce anomalies are predominantly controlled by the redox conditions and solution complexation. The large positive Ce anomalies in these porewaters are probably caused by the oxidation of Ce(III) to Ce(IV) that was stabilized in solution through complexation with high contents of inorganic and organic ligands. Positive Eu anomalies relative to PAAS, the host rocks and leachates of the host rocks, result from hydrolysis of plagioclase feldspar. The limited depth profile for total dissolved REEs across the weathered–unweathered till boundary is retarded with respect to that previously established for Na+, K+, Mg2+, SO42−, and TDS, suggesting that chemical reactions control the migration of dissolved REEs.
Adsorption isotherms in solutions with ionic strengths of 0.01 at 25°C were measured over the arsenite and arsenate concentration range 10−7−10−3 M and the pH range 4–10. At low concentrations, these isotherms obeyed equations of the Langmuir type. At higher concentrations the adsorption isotherms were linear, indicating the existence of more than one type of surface site on the amorphous iron hydroxide adsorbent. Removal of arsenite and arsenate by amorphous iron hydroxide throughout the concentration range were determined as a function of pH. By careful selection of the relative concentration of arsenic and amorphous iron hydroxide and pH, removals on the order of 92% can be achieved.
Aquitards exert significant influence on the hydrogeochemistry of aquifer systems. This influence is manifested somewhat differently depending on the relative position of aquitards within a system. In the deeper regimes, they are influential in the origin and distribution of brines and the development of geopressured zones. In intermediate regimes, they form multi-layered aquifer systems and provide a source of reactive minerals and exchangeable ions. In shallow regimes, aquitards can influence the topography and drainage patterns; this influences the relationship between the water table and the potentiometric surface of confined aquifers, controls the rates of infiltration and discharge, and controls whether the geochemical system is open or closed to exchange of carbon dioxide gas. In coastal areas, aquitards can determine the depth of the saltwater-freshwater interface, its distance from the shoreline, and the position of the mixing zone that causes geochemical alteration of minerals and development of porosity.
Fixation, speciation, and mobilization of sediment arsenic (As) during sediment-water interactions were studied. Emphasis was placed on transformation and fixation of As(V) in anaerobic sediment, long-term (6 months) release of naturally occurring and added As, and sediment properties affecting the mobilization of As(V), As(III), and organic As. Arsenic added to sediment became associated with relatively immobile iron and aluminum compounds. Addition of As(V) to sediments prior to anaerobic incubation also resulted in accumulation of As(III) and organic As in the interstitial water and exchangeable phases of anaerobic sediments. Arsenic was mobilized from sediment over both the short and long term. Short-tern releases were related to As concentrations in the interstitial water and exchangeable phases. Long-term net mass releases were related to sediment total iron, extractable iron, or CaCOâ equivalent concentration. 52 references, 3 figures, 11 tables.
The speciation of arsenic in the environment is controlled by reduction, methylation, and oxidation processes and is therefore influenced by redox conditions. However, in the lakes studied, speciation was found to be far from thermodynamic equilibrium. A Superfund site is a major source of As to the watershed, with the Aberjona River being the main conduit for As found in the Mystic Lakes. Total As concentrations in the water column decrease downstream, from >100 nM in the Hall's Brook Storage Area (HBSA), where As(III) enters via reducing groundwater, to <20 nM in the Lower Mystic Lake (LML); the sediments of all the lakes are sinks for As. Biologically mediated reduction, at rates of 0.2-0.5% of the total As day-1, and methylation, at rates of 0.4-0.6% of the total As day-1, occur in the mixed layers of these lakes. However, these processes are slow or absent in the hypolimnion, allowing As(V) to accumulate in seasonally anoxic hypolimnetic waters. High micromolar concentrations of As, predominantly As(III), persist in the saline, sulfidic monimolimnion of the LML. The production of As(III) and methylated As species in the water column of these lakes inhibits the removal of As to the sediments, thereby increasing the mobility of As in this watershed.
Intravascular stents have been developed to address acute arterial closure and restenosis, the major limitations of percutaneous transluminal coronary angioplasty (PTCA). Metallic stents in human clinical trials have shown efficacy in treating acute closure and, in selected patients, lowering the restenosis rate. This review delineates the characteristics of the ideal stent and examines ongoing clinical trials that are evaluating various stent prototypes. Developmental stents that use radiopaque and bioabsorbable materials are presented. The potential role for intravascular stents as vehicles for localized drug delivery and gene therapy is discussed.
Adsorption and oxidation reactions of arsenite (As(III)) at the mineral-water interface are two important factors affecting the fate and transport of arsenic in the environ ment. Numerous studies have concluded that As(III) is more soluble and mobile than arsenate (As(V)) in soils, though very little experimental work has demonstrated the differences in reactivity and stability of As(III) and As(V) at the mineral-water interface. In this investigation, As(III) adsorption on kaolinite, illite, montmorillonite, and amorphous aluminum hydroxide (am-Al(OH)3) was studied as a function of pH and ionic strength and was compared with As(V) adsorption. High-performance liquid chromatog raphy-hydride generation atomic absorption spectrophotometry (HPLC-HGAAS) was employed for direct determination of As(III) and As(V). In addition, surface complexation modeling was used to describe As(III) and As(V) adsorption on the four minerals. It was revealed that alkaline solutions (pH > 9) without mineral solids caused homogeneous oxidation of As(III) to As(V). In addition, recovery of adsorbed As from As(III)-treated clay mineral solids showed that oxidation of As(III) to As(V) was enhanced by heterogeneous oxidation on kaolinite and illite surfaces.
Arsenic has both metallic and nonmetallic properties and is a member of the nitrogen family. It occurs as a free element and combined form being widely distributed in sulfide ores. The poisonous character of arsenic allowed for its use as a herbicide, cattle and sheep dips, and insecticides. The ubiquity of arsenic in the environment, its biological toxicity, and its redistribution are factors evoking public concern. This review will cover the chemistry of arsenic and methods curently used to speciate the prevalent chemical forms in various environments. The major focus of this review is on the biological transformations of arsenic in both the terrestrial and aquatic systems.
Inorganic arsenic (In-As) is known to be a human carcinogen, causing lung cancer by inhalation and skin cancer by ingestion. Ecologic studies in Taiwan have found a dose-response relation between ingestion of In-As from drinking water and bladder cancer, but questions have been raised concerning the validity and generalizability of the findings. Several areas of Argentina have had high exposures to arsenic from naturally contaminated drinking water, particularly the eastern region of the province of Córdoba. In this study, we investigated bladder cancer mortality for the years 1986-1991 in Córdoba's 26 counties, using rates for all of Argentina as the standard for comparison. Bladder cancer standardized mortality ratios (SMRs) were consistently higher in counties with documented arsenic exposure. We grouped counties into low-, medium-, and high-exposure categories; the corresponding SMRs [with 95% confidence intervals (CI)] were 0.80 (95% CI = 0.66-0.96), 1.42 (95% CI = 1.14-1.74), and 2.14 (95% CI = 1.78-2.53) for men, and 1.21 (95% CI = 0.85-1.64), 1.58 (95% CI = 1.01-2.35), and 1.82 (95% CI = 1.19-2.64) for women. The clear trends found in a population with different genetic composition and a high-protein diet support the findings in Taiwan.
A method has been developed for determination of (ultra)trace amounts of As(III) and As(V) in water by flow injection on-line sorption preconcentration and separation coupled with inductively coupled plasma mass spectrometry (ICPMS) using a knotted reactor (KR). The determination of As(III) was achieved by selective formation of the As(III)-pyrrolidine dithiocarbamate complex over a sample acidity range of 0.01-0.7 mol L-1 HNO3, its adsorption onto the inner walls of the KR made from 150-cm-long, 0.5-mm-i.d. PTFE tubing, elution with 1 mol L-1 HNO3, and detection by ICPMS. Total inorganic arsenic was determined after prereduction of As(V) to As(III) in a 1% (m/v) L-cysteine-0.03 mol L-1 HNO3 media. The concentration of As(V) was calculated by difference (the total inorganic arsenic and As(III)). Owing to the group-specific character of the chelating agent, and the use of an efficient rinsing step before elution, the interferences encountered in conventional ICPMS from common major matrix, alkali and alkaline earth metals, and chlorides were eliminated. The presence of organoarsenic species such as monomethylarsonate and dimethylarsinate in water samples had no effect on the results of As(III) and As(V). Thus, the method can be applied to the speciation analysis of inorganic arsenic at submicrogram per liter levels in aqueous solutions with high total content of dissolved solid and/or high content of chlorides. Using a preconcentration time of 60 s and a sample flow rate of 5 mL min-1, an enhancement factor of 22 was achieved in comparison with conventional ICPMS. The time required for a single determination was 200 s. The detection limits (3s) was evaluated to be 0.021 microgram L-1 for As(III) and 0.029 microgram L-1 for total inorganic arsenic. The precision for 14 replicate determinations of 1 microgram L-1 As(III) was 2.8% (RSD) with drift correction and 3.9% (RSD) without drift correction. The concentrations of As(III) and As(V) in synthetic mixtures obtained by the present method were in good agreement with expected values. Results obtained by the proposed method for total arsenic in a river water reference material agreed well with certified and recently reevaluated values. The method was also applied to the speciation analysis of inorganic arsenic in porewaters.
To investigate the distribution of microbial biomass, populations and activities within a clay-rich, low hydraulic conductivity (10-11 to 10-12 m s-1) aquitard complex, cores were aseptically obtained from a series of overlying clayey deposits; a fractured till, unfractured till (20-30 ka BP), a disturbed interfacial zone (20-30 ka BP), and a Cretaceous clay aquitard (71-72 Ma BP). The results of confocal microscopy studies, culture methods, molecular approaches, and extractive fatty acid analyses all indicated low bacterial numbers that were non-homogeneously distributed within the sediments. Various primers for catabolic genes were used to amplify extracted DNA. Results indicated the presence of eubacterial 23S rDNA, and the narH gene for nitrate reductase and ribulose-1,5-biphosphate carboxylase (RuBP carboxylase). Although there was no evidence of limitation by electron acceptors or donors, sulfate-reducing bacteria were not detected below the fractured till zone, using PCR, enrichment, or culture techniques. Denaturing gradient gel electrophoresis (DGGE) analyses indicated differences in community composition and abundance between the various geologic units. Results of FAME analyses of sediments yielded detectable extractable fatty acids throughout the aquitard complex. Bacterial activities were demonstrated by measuring mineralization of (14C) glucose. Porewater chemistry and stable isotope data were in keeping with an environment in which extremely slow growing, low populations of bacteria exert little impact. The observations also support the contention that in low permeability sediments bacteria may survive for geologic time periods.
Computer modeling of 611 high-quality analyses of normal ground waters from diverse geographic areas reveals that aqueous
oxidation-reduction reactions are generally not at equilibrium. Multiple redox couples present in individual samples yield
computed Nernstian Eh (redox potential) values spanning as much as 1000 millivolts. The computed Eh values do not agree with
each other, nor do they agree with the single "master" value measured in the field with a platinum electrode. Because of internal
disequilibrium, the use of any measured Eh value as input to equilibrium hydrogeochemical computer models will generally yield
misleading results for normal ground waters.