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Effects of the presence of a composted biosolid on the metal immobilizing action of an urban soil
Abstract
The influence of a composted biosolid from urban wastewater treatment on the retention and solubility of Cu, Pb or Zn added to a soil was studied by batch adsorption/desorption experiments, equilibrating both materials and their mixtures with solutions containing various metal concentrations. The composted biosolid adsorbed less Cu or Pb and slightly more Zn than the soil, and thus caused a noticeable decrease in the retention of Cu or Pb and an increase in Zn adsorption by soil-biosolid mixtures, but these effects in the mixtures were not additive for any metal. The pH effects were studied by means of (log metal concentration)/pH diagrams. It was shown that Cu behaviour was different from that of the other metals: the relation between pH and Cu concentrations suggested similar solubilities in the presence of the biosolid and the mixtures, whereas the biosolid-free soil gave data located on a region of the diagram corresponding to slightly lower solubility. In the case of Pb or Zn, the data for the biosolid were located in a region of the diagram corresponding to clearly lower solubilities than those for the biosolid/soil mixtures. It was concluded that the biosolid has little effect on metal solubility when it is mixed with the soil in the proportions used here.
... The retention of heavy metals, such as Zn, may be due to stabilization in the media through repeated wetting and drying cycles (Han et al., 2001) and the formation of chelates with organic materials (Kadlec and Knight, 1996). Additionally, in one study, composted biosolids were found to lower the solubility of Zn and increase Zn retention (Madrid and Florido, 2010). Overall, the green roof retained over 65% of the Zn input from precipitation (Table 5). ...
... The excavation in the surrounding areas may be responsible for the re-mobilization of the Pb (Turer et al., 2001) from the soil to the atmosphere and subsequent scavenging and deposition in precipitation . Similar to Zn, the composted biosolids may contribute to the retention of Pb in the green roof (Madrid and Florido, 2010). No significant differences in total Hg concentrations were found between precipitation, green roof, and control roof runoff (Table 4). ...
a b s t r a c t Runoff quantity and quality from a 248 m 2 extensive green roof and a control were compared in Connecti-cut using a paired watershed study. Weekly and individual rain storm samples of runoff and precipitation were analyzed for TKN, NO 3 + NO 2 –N, NH 3 –N, TP, PO 4 –P, and total and dissolved Cu, Pb, Zn, Cd, Cr, and Hg. The green roof watershed retained 51.4% of precipitation during the study period based on area extrap-olation. Overall, the green roof retained 34% more precipitation than predicted by the paired watershed calibration equation. TP and PO 4 –P mean concentrations in green roof runoff were higher than in precip-itation but lower than in runoff from the control. The green roof was a sink for NH 3 –N, Zn, and Pb, but not for TP, PO 4 –P, and total Cu. It also reduced the mass export of TN, TKN, NO 3 + NO 2 –N, Hg, and dissolved Cu primarily through a reduction in stormwater runoff. Greater than 90% of the total Cu, Hg, and Zn concen-trations in the green roof runoff were in the dissolved form. The growing media and slow release fertilizer were probable sources of P and Cu in green roof runoff. Overall, the green roof was effective in reducing stormwater runoff and overall pollutant loading for most water quality contaminants.
... Higher value of Pb and Cd could be due to tendency of Pb to strongly adhere to particles [42], Pb and Cd are associative [43]. Cd, Zn and Pb which were not detected in TR runoff sample may be due to the tendency of this roof type to retain heavy metals, owing to stabilization of the media through repeated wetting, drying cycles and formation of chelates with organic materials [37,44], since composited biosolids have been found to lower the solubility of Zn and increases its retention [45]. ...
Samples collected from different runoffs; AR, GMR, TR, ASR, and DR for aluminum roof, galvanized metal roof, thatch roof, asbestos roof and ambient rainfall respectively were analyzed for pH, conductivity, turbidity, TDS, TSS, NO 3 ⁻ , PO 4 ³⁻ , Cd, Cu, Fe, Zn and Pb. The result indicates that the mean concentration of the parameters analyzed ranged from 5.8± 0.39 –7.10±0.70, 22.25±11.70-79.99± 3.40μScm ⁻¹ , 1.47±0.43 - 46.53±1.60mg/l, 11.90±0.93 - 59.83±1.62NTU,15.53±0.70 - 204.53±5.08mg/l, 0.93±0.06 - 2.55±0.13 mg/l, 1.33±0.22 - 7.30±0.57mg/l respectively for pH, conductivity, TDS, turbidity, TSS, PO 4 ³⁻ and NO 3 ⁻ , and the levels of the heavy metals (in mg/l); Cd, Cu, Fe, Zn and Pb ranged from 0.0023±0.001– 0.0521±0.004, 0.052±0.01–0.2483±0.02, 0.0348±0.01–1.1120±0.07, 0.0161±0.01-0.8093±0.02 and 0.0106±0.01-0.0499±0.002 respectively. Ranking of the heavy metal in roof runoff is in the order; Fe>Zn>Cu>Cd>Pb. The result compared with WHO standard showed elevated level of the parameters analyzed with Cd and Pb exceeding the limit. Though the result of this study showed some variability which is an indication of the type of roofing material; air quality of the environment and industrial activity going on in the area. It could be deduced from the result that roof runoff may be a non point source of environmental pollution owning to the release of heavy metals and other pollutants into the environment, and increased concentration of some of the pollutants as reported by this study suggests that roof runoff water could impact negatively to the environment and if consumed without being treated may be injurious to human health.
... However, 2% bone char significantly reduced the content of acid extractable Pb in soil, with little influence on Zn [14]. At present, the main modifiers are calcium-rich substances [15], acid-base regulators [16], chelating agents, organic acids, and surfactants [17], sludge [18], and peat soil [19]. The mushroom residue exhibited many of the characteristics typical of other organic waste byproducts, such as high porosity and organic matter content [20]. ...
At present, the application of phytoremediation technology in the ecological remediation of heavy metal tailings is receiving more and more attention. In this study, the physiological and biochemical response and tolerance mechanism of woody plant Nerium indicum to Pb and Zn under different proportions of inorganic modifier calcium carbonate (C1: 5%, C2: 10%, C3: 15%) and organic modifier mushroom residue (M1: 10%, M2: 20%, M3: 30%) was compared. The results showed that the pH value has a trend of C group > M group > CK group and organic matter has a trend of M group > CK group > C group. Phosphatase activity and catalase activity has a trend of M group > C group > CK group, but catalase was more vulnerable to the calcium carbonate concentration. Both modifiers can promote the transformation of Pb, Zn, Cu, and Cd in tailings to more stable organic bound and residual states. However, the stabilization effect of mushroom residue is better, and its stability is Pb, Zn > Cd, Cu. Both modifiers can increase the biomass of Nerium indicum and the modification effect of mushroom residue is better than calcium carbonate. Pb/Zn content and accumulation in Nerium indicum organs showed root > stem > leaf in all groups. Compared with the CK group, the enrichment coefficient of Pb/Zn in C1 and M1 groups decreased, while the translo-cation factor of Pb/Zn in C1 and M1 groups increased. With the increase in modifier concentration, the enrichment coefficient increases about 1.75~52.94%, but the translocation factor decreases rapidly (20.01~64.46%). Clearly, both the calcium carbonate and mushroom residue amendment could promote the growth ability of Nerium indicum in lead-zinc tailings and strengthen the phytoreme-diation potential.
... Among these techniques, chemical immobilization by means of soil amendments has been investigated as a technique for a wide range of contaminated sites (Scanferla et al, 2009). A number of natural or synthetic materials, such as phosphate rocks (Mignardi, Corami and Ferrini, 2012), zeolites (Kosobucki, Kruk and Buszewski, 2008;Shi et al., 2009), municipal biosolids (Madrid and Florido, 2010), red mud (Garau et al., 2007;Liu, Naidu and Ming, 2011) or carbonates (Pérez-Sirvent et al, 2011) have been tested to evaluate their ability to immobilize pollutants. ...
Mitigation of environmental issues and decontamination of land by using a variety of industrial minerals mainly immobilizes contaminants in solution, whether in polluted water or contaminated soils and sediments. That is because a variety of minerals, in particular carbonates, clays and zeolites, have a significant capacity to ad/absorb harmful ions and to react to form precipitates. Phosphates, iron hydroxides, some silica minerals and lime-derived products can also be used to solve environmental issues. This paper focuses on the amendment of soils and sediments affected by the impact of mining or by waste landfills, with some reference to the use of minerals in cultural heritage conservation. An example of actions leading to the amelioration of contaminated land is provided after describing some current pilot experiences on highly polluted wastes derived from mining in SE Spain.
... Higher value of Pb and Cd could be due to tendency of Pb to strongly adhere to particles [42], Pb and Cd are associative [43]. Cd, Zn and Pb which were not detected in TR runoff sample may be due to the tendency of this roof type to retain heavy metals, owing to stabilization of the media through repeated wetting, drying cycles and formation of chelates with organic materials [37,44], since composited biosolids have been found to lower the solubility of Zn and increases its retention [45]. ...
Samples collected from different runoffs; AR, GMR, TR, ASR, and DR for the aluminum roof, galvanized metal roof, thatch roof, asbestos roof, and ambient rainfall respectively were analyzed for pH, conductivity, turbidity, TDS, TSS, NO3-, PO43-, Cd, Cu, Fe, Zn and Pb. The result indicates that the mean concentration of the parameters analyzed ranged from 5.8± 0.39 –7.10±0.70, 22.25±11.70 -79.99± 3.40μScm-1, 1.47±0.43 - 46.53±1.60mg/l, 11.90±0.93 -59.83±1.62NTU,15.53±0.70 - 204.53±5.08mg/l, 0.93±0.06 - 2.55±0.13 mg/l, 1.33±0.22 -7.30±0.57mg/l respectively for pH, conductivity, TDS, turbidity, TSS, PO43-- and NO3-, and the levels of the heavy metals (in mg/l); Cd, Cu, Fe, Zn and Pb ranged from 0.0023±0.001–0.0521±0.004, 0.052±0.01–0.2483±0.02, 0.0348±0.01–1.1120±0.07, 0.0161±0.01- 0.8093±0.02 and 0.0106±0.01- 0.0499±0.002 respectively. Ranking of the heavy metal in roof runoff is in the order; Fe>Zn>Cu>Cd>Pb. The result compared with WHO standard showed elevated level of the parameters analyzed with Cd and Pb exceeding the limit. Though the result of this study showed some variability which is an indication of the type of roofing material; air quality of the environment and industrial activity going on in the area. It could be deduced from the result that roof runoff may be a non-point source of environmental pollution owing to the release of heavy metals and other pollutants into the environment, and increased concentration of some of the pollutants as reported by this study suggests that roof runoff water could impact negatively to the environment and if consumed without being treated may be injurious to human health.
... In recent decades, many remediation technologies have been applied to deal with contaminated soils and waters. Among the techniques proposed, chemical immobilisation using soil amendments has been investigated for a wide range of contaminated sites [3] and several materials, such as phosphate rocks [4], zeolites [5,6], municipal biosolids [7], red mud [8,9] and carbonates have been tested. ...
The leachability and ecotoxicity of potentially toxic elements (PTE) eluting from technosols formed of soils affected by mining activities and limestone filler were evaluated. A total of four contaminated soils affected by opencast mining were selected and mixed with limestone filler at three percentages: 10, 20 and 30 %, providing 12 stabilised samples. Total and soluble PTE content (As, Cd, Cu, Fe, Pb and Zn) was determined in all the samples and the Microtox® bioassay was applied to determine the ecotoxicological effect. The stabilised material had a neutral pH and low soluble PTE concentration. Moreover, the ecotoxicological assay indicated the presence of low toxicity levels in the stabilised samples. The applied bioassay can be considered a good tool for the screening of PTE contamination in areas affected by mining activities, while providing information about possible attenuation processes.
... Among such remediation techniques, chemical immobilisation by means of soil amendments has been investigated as an alternative technique for a wide range of contaminated sites (Scanferla et al. 2009). A number of natural or synthetic materials, such as phosphate rocks (Mignardi et al. 2012), zeolites (Kosobucki et al. 2008;Shi et al. 2009), municipal biosolids (Madrid and Florido 2010), red mud (Garau et al. 2007;Liu et al. 2011) and carbonates have been tested in order to evaluate their ability to immobilise toxic metal(loid)s. Limestone filler is a good choice for remediation processes because of its low permeability and low solubility, it has a high degree of physicalchemical stability, it is non-toxic and it has a finely divided calcium carbonate content (Pérez-Sirvent et al. 2007, 2011Martínez-Sánchez et al. 2008). ...
Purpose
The objective of this work was to evaluate the effectiveness of a plant bioassay (Phytotoxkit®) for screening ecotoxicological risks in sediments affected by mining activities.
Materials and methods
A total of 42 sediment samples affected by mining activities were studied, including 39 sediment samples from the Sierra Minera, Spain, an area affected by old extraction procedures, and three sediments from an area affected by opencast mining. These three samples were then mixed with limestone filler at 10, 20 and 30 %, providing nine stabilised samples. The total and soluble metal(loid) content (As, Cd, Cu, Fe, Pb and Zn) was determined in all samples, and the Phytotoxkit® bioassay was applied to determine the ecotoxicological effect of this procedure.
Results and discussion
The stabilised material had a neutral pH and low soluble metal(loid) concentration, similar to that of samples in which a natural attenuation process had taken place because of mixing with surrounding carbonate-rich materials. An ecotoxicological survey identified the low toxicity levels of the stabilised samples.
Conclusions
The applied bioassay is a good tool for screening metal(loid) contamination in areas affected by mining activities, since it provides information on both natural and simulated attenuation processes. The mixing of sediments with limestone filler could be applied to the remediation of zones affected by mining activities, because the toxicological effect on the tested organisms in the stabilised sediments was reduced significantly and the metal(loid) content was diminished.
... As a working definition for EPA to use in risk assessment and risk management decision-making, bioavailability of metals is the extent to which bioaccessible metals adsorb onto or absorb into and across biological membranes of organisms [11]. Bioavailability can be minimized through chemical and biological immobilization of metal(loid)s using a range of inor-ganic compounds, such as lime and phosphate (P) compounds (e.g., apatite rocks), and organic compounds, such as 'exceptional quality' biosolid which meets the highest level of quality in three categories: (1) reduction of pathogens; (2) reduction of vector attraction (odor); and (3) low concentrations of specific metals [12][13][14][15]. The more localized metal contamination found in urban environments (e.g., Cr contamination in timber treatment plants) is remediated by metal mobilization processes that include phytoremediation (including phytovolatilization) and chemical washing [16][17][18]. ...
As land application becomes one of the important waste utilization and disposal practices, soil is increasingly being seen as a major source of metal(loid)s reaching food chain, mainly through plant uptake and animal transfer. With greater public awareness of the implications of contaminated soils on human and animal health there has been increasing interest in developing technologies to remediate contaminated sites. Bioremediation is a natural process which relies on soil microorganisms and higher plants to alter metal(loid) bioavailability and can be enhanced by addition of organic amendments to soils. Large quantities of organic amendments, such as manure compost, biosolid and municipal solid wastes are used as a source of nutrients and also as a conditioner to improve the physical properties and fertility of soils. These organic amendments that are low in metal(loid)s can be used as a sink for reducing the bioavailability of metal(loid)s in contaminated soils and sediments through their effect on the adsorption, complexation, reduction and volatilization of metal(loid)s. This review examines the mechanisms for the enhanced bioremediation of metal(loid)s by organic amendments and discusses the practical implications in relation to sequestration and bioavailability of metal(loid)s in soils.
Ni, Cr, Pb, Cd, Cu and Zn Geo-accumulation indexes revealed that industrial activities and used TWW contributed to soil pollution, and water irrigation decreased always this contamination. After long-term use of different water types, high perturbation of metal redistribution have taken. Ground water use changed all metal redistributions in irrigated soil among various soils-solid and soil-solution fractions, compared with non-irrigated soil. Slightly acid water use transferred some metals from different solid phase components into soluble and exchangeable fractions. However, TWW use transformed some Ni, Cr, Cd, Cu and Zn quantities from soluble and exchangeable fractions to the less labile fractions, particularly into organic matter fraction. Reuse of conventional water within the same soil decreased the whole soil redistribution index values indicating tendency to return to the pattern of distribution of groundwater irrigated soil.
Diuron desorption and mineralisation were studied on an amended and artificially contaminated soil. The amendments used comprised two different composted organic residues i.e., sewage sludge (SS) mixed with pruning wastes, and urban solid residues (USR), and two different solutions (with inorganic salts as the micronutrients and hydroxypropyl-β-cyclodextrin (HPBCD)). After applying micronutrients to activate the soil flora, 15.5% mineralisation could be reached after 150days, indicating that the soil has a potential capacity to mineralise the herbicide through biostimulation-assisted attenuation. Diuron mineralisation was also improved when HPBCD solutions were applied. Indeed, the extent of herbicide mineralisation reached 29.7% with this application. Moreover, both the lag phase and the half-life time (DT50) were reduced to 33 and 1778days, respectively, relative to the application of just micronutrients (i.e., 39 and 6297days, respectively). Organic amendments were also applied (i.e., USR and SS) on the contaminated soil: it was found that the diuron mineralisation rate was improved as the amendment concentration increased. The joint application of all treatments investigated at the best conditions tested was conducted to obtain the best diuron mineralisation results. The micronutrient amendment plus 4% USR or SS amendment plus HPBCD solution (10-fold diuron initially spiked) caused an extent of diuron mineralisation 33.2 or 46.5%, respectively.
The term “trace elements” generally includes elements (both metals and metalloids) that occur in natural and perturbed environments in small amounts and that, when present in sufficient bioavailable concentrations, are toxic to living organisms (Adriano 2001). This group includes both biologically essential [e.g., cobalt (Co), copper (Cu), chromium (Cr), manganese (Mn), and zinc (Zn)] and nonessential [e.g., cadmium (Cd), lead (Pb), and mercury (Hg)] elements. The essential elements (for plant, animal, or human nutrition) are required in low concentrations and hence are known as “micro nutrients.” The nonessential elements are phytotoxic and/or zootoxic and are widely known as “toxic elements” (Adriano 2001). Both groups are toxic to plants, animals, and/or humans at exorbitant concentrations (Alloway 1990; Adriano 2001). Heavy metal(loid)s, which include elements with an atomic density greater than 6 g cm-3 [with the exception of arsenic (As), boron (B), and selenium (Se)] are also considered to be trace elements.
Soil chemists have long-recognized that knowledge of the elemental composition of soils is generally of little use in assessing the availability of these elements to plants. An obvious illustration of this principle is the common occurrence of Fe and Mn deficiency in plants despite the relatively high levels of Fe and Mn in many soils. For this reason, chemical soil tests have relied on measurement of extractable or “labile” fractions of elements. Such tests are empirical and provide little basis to relate metal extractability to the chemical forms of the metal in the soil. As soils are increasingly used in our society for purposes other than agriculture, the frequency and extent of soil contamination by toxic metals will increase. Empirical relationships may have to be replaced by a more fundamental understanding of the soil processes controlling metal solubility to prevent practices that could have deleterious effects on soil productivity and environmental quality.
Concerns exist over the long-term availability of trace metals in biosolids-amended soils. The objective of this study was to quantify extractable forms of trace metals in biosolids-amended, continuously cultivated soils after cessation of sewage sludge applications and to determine their bioavailability to romaine lettuce (Lactuca sativa L.). Trace metals in surface soils from two biosolids-amended sites were speciated into increasingly stable chemical fractions using a series of sequential extractions that were operationally defined as exchangeable (Exch), specifically adsorbed (SA), Fe-Mn oxide and acid replaceable (Ox/AR), residual organic (R-Org), and residual inorganic (R-In). Romaine lettuce was grown on the soils to determine heavy metal bioavailability. In control and biosolids-amended soils, 60 to 75% of Cd was found in the more easily extracted Exch and SA forms, but the percentage was significantly greater in the biosolids-amended soils. Biosolids applications also increased the percentages of Ni and Zn in the Exch and SA fractions. Biosolids applications had little effect on the Exch and SA fractions of Cr and Pb. Greater than 75% of Cr, Cu, Ni, Pb, and Zn were found in the relatively stable Ox/AR, R-Org, and R-In fractions of control and biosolids-amended soils. Concentrations of Cd, Ni, and Zn in aboveground lettuce tissue, and to a lesser extent, Cu and Cr, were significantly increased by biosolids applications, but Pb uptake was not affected. Cadmium, Cr, and Cu concentrations in lettuce were highly correlated (P < 0.001) to metal concentrations in one or more of the Exch, SA, and Ox/AR soil chemical fractions. Results of this study show that 15 yr after biosolids applications, the relative bioavailability of biosolids- applied heavy metals was Cd >> Zn ≤ Ni > Cu >> Cr > Pb.
The use of three inorganic materials as potential immobilizers of metals in soils has been studied by monitoring metal availability
by EDTA extraction, the Simple Bioaccessibility Extraction Test (SBET) and extraction with a mixture of organic acids (OA).
The SBET test was the most suitable for risk assessment in soils of recreational areas. The materials were a 4A-type zeolite,
tri-calcium phosphate and ‘slovakite’, a synthetic sorbent developed for remediation of metal-polluted soils. Adsorption/desorption
experiments of metals by the isolated materials showed that all materials caused a strong retention of metals from solutions,
with negligible release by dilution. When added to soils of three parks, zeolite and, to a much lesser extent, slovakite caused
some increase in soil pH. Despite this increase of pH, zeolite is often the least effective amendment for decreasing metal
availability estimated by any method, and even sometimes seems to cause some increase, as well as an increase of soil electrical
conductivity. In contrast, slovakite causes a decrease of available metals as estimated by EDTA and SBET, but by SBET the
effect seems to be steadily reduced after the first samplings, so that after 300days the metals extracted by this method
are very similar to the data for the blanks. Despite the differences in pseudo-total metal contents, few differences are noticeable among parks. In general, these amendments are scarcely efficient in
the case of neutral urban soils like those studied here. Other techniques are needed for controlling and, eventually, decreasing
metal pollution hazard in soils of recreational areas.
This study was performed on 21 soils with the aim of establishing whether Pb and Ni adsorption/desorption parameters could
be considered as good indicators of the risk of groundwater pollution. Results showed that high pH values in soil caused a
totally irreversible Pb adsorption, thus excluding any risk of Pb groundwater pollution. Sorption/desorption studies, quantified
by the desorption index (DI), showed that Ni retention was only partly affected by the basic pH values but it was also due
to the electrostatic attraction processes occurring on soil surfaces, as demonstrated by the partial reversibility of the
Ni sorbed. This justifies possible risks of Ni groundwater pollution. The results of a monitoring research confirmed these
findings. Results suggested that the adsorption/desorption parameters, namely DI, are promising indicators to predict the
risk of groundwater pollution from metals in calcareous soils.
The potential use of processed solid residue of olive mill products (SROOMP) to treat drinking water containing several heavy metals in trace concentrations, namely Cr(III), Ni(II), Pb(II), Cd(II), and Zn(II), was explored. Different experimental approaches including equilibrium batch mode experiments, scanning electron microscopy (SEM), and X-ray fluorescence (XRF) were used to explore the feasibility of this material as an adsorbent for the removal of these heavy metals from aqueous solutions. Results indicate that SROOMP can be used to remove Pb(II) and Zn(II) from aqueous solutions by adsorption (data best fit Fruenlich isotherm) but does not remove the other heavy metals investigated in this study.
The desorption characteristics of lead in two variable charge soils (one developed from Arenaceous rock (RAR) and the other derived from Quaternary red earths (REQ)) were studied, and the effects of pH value, organic acid, and competitive ions were examined. Desorption of Pb(2+) decreased from nearly 100.0 to 20.0% within pH 1.0-4.0 in both soils, and then the decrease diminished at pH > 4.0. Organic ligands at relatively low concentrations (< or =10(-3) mol L(-1)) slightly inhibited Pb(2+) desorption, but enhanced Pb(2+) desorption at higher concentrations. In this study, citric acid or acetic acid at higher concentrations (>10(-3) mol L(-1)) had the greatest improvement of Pb(2+) desorption, followed by malic acid; and the smallest was oxalic acid. Desorption of the adsorbed Pb(2+) increased greatly with increasing concentrations of added Cu(2+) or Zn(2+). Applied Cu(2+) increased Pb(2+) desorption more than Zn(2+) at the same loading.
We evaluated the effects of different amendments and/or a plant cover on reclamation of a trace element contaminated soil. Seven treatments were established: four organic (leonardite (LEO), litter (LIT), municipal waste compost (MWC), biosolid compost (BC)), one inorganic (sugar beet lime (SL)) and two controls (control without amendment but with Agrostis (CTRP) and control without amendment and without Agrostis (CTR)). Results showed that total organic C was significantly higher in organic treatments in all samplings. Water-soluble C was lower in CTR compared to other treatments, but no significant differences were observed between organic treatments and SL and CTR. SL, BC and MWC treatments increased soil pH and reduced 0.01 M CaCl2-extractable Cd, Cu and Zn concentrations more efficiently, especially in the first 2 years. At the end of the experiment 0.01 M CaCl2-extractable trace element concentrations were similar in all treatments. 0.01 M CaCl2-extractable As and Pb were below the detection limit. Addition of amendments showed no clear reduction in 0.05 M EDTA-extractable trace element concentrations and some amendments even increased 0.05 M EDTA-extractable As and Cu with time. Pseudo-total trace element concentrations were higher for As in controls. On the other hand, mean values of Cu and Zn were higher in MWC treatment. BC and SL treatments also showed higher Zn mean concentration than controls. No amendment effect was observed for Cd and Pb.
Use of a harmonised sampling regime has allowed comparison of concentrations of copper, chromium, nickel, lead and zinc in six urban parks located in different European cities differing markedly in their climate and industrial history. Wide concentrations ranges were found for copper, lead and zinc at most sites, but for chromium and nickel a wide range was only seen in the Italian park, where levels were also considerably greater than in other soils. As might be expected, the soils from older cities with a legacy of heavy manufacturing industry (Glasgow, Torino) were richest in potentially toxic elements (PTEs); soils from Ljubljana, Sevilla and Uppsala had intermediate metal contents, and soils from the most recently established park, in the least industrialised city (Aveiro), displayed lowest concentrations. When principal component analysis was applied to the data, associations were revealed between pH and organic carbon content; and between all five PTEs. When pH and organic carbon content were excluded from the PCA, a distinction became clear between copper, lead and zinc (the "urban" metals) on the one hand, and chromium and nickel on the other. Similar results were obtained for the surface (0-10 cm depth) and sub-surface (10-20 cm depth) samples. Comparisons with target or limit concentrations were limited by the existence of different legislation in different countries and the fact that few guidelines deal specifically with public-access urban soils intended for recreational use.
Metals in urban soils might be transferred to humans via ingestion, dermal contact, or breathing, especially to children due to the “hand to mouth” activity during outdoor activities in playground and recreational areas. This involuntary soil ingestion depends on soil adherence to skin; it is known that the adhesion process tends to exclude particles greater than 50 μm, so the fraction below this diameter would be the most dangerous for health. The aim of this work was to study the “availability”, estimated by the EDTA extraction, and “oral bioaccessibility”, estimated by the Simple Bioaccessibility Extraction Test (SBET), of several metals in urban soils of two European cities (Sevilla and Torino), as related to the soil particle size distribution. Torino and Sevilla showed different levels of metal contents, availability, and bioaccessibility. In Torino, the finer particles showed metal enrichment of Cu, Zn, and, to a lesser extent, Pb, whereas in Sevilla, all of the studied metals showed this enrichment compared to the whole soils. The whole soil cannot be used as a good general indicator of the bioaccessibility of metals in the finest fractions of the soil. Metal availability was higher in the clay fraction (EDTA. A different origin is attributed to bioaccessible metal forms. Anthropic influence seems more important in determining metal availability and bioaccessibility in urban soils of both cities than the different geological or industrial characteristics.
The ability of Neem oil cake (NOC), a bio-waste material to remove Cu(II) and Cd(II) ions from aqueous solution was investigated. The influence of pH, sorbent dose, concentration, temperature, and contact time on the sorption of Cu(II) and Cd(II) on NOC has been studied by batch process. The sorption was found to be maximum in acidic medium and increases with increase in initial concentration. Percent deviation (P) and regression coefficient (r2) for Langmuir and Freundlich sorption isotherms shows better fitting of the models. Thermodynamics parameters indicate that the process is endothermic and spontaneous. Activation energies for Cu(II) and Cd(II) were calculated from Dubinin–Redushkevich (D–R) isotherms are high 8.3–8.7 and 7.17–11.4 kJ mol−1 showing that chemisorption is involved. The sorption kinetics was found to follow pseudo-second-order model.
Summary When limed farmland is converted to forestry cadmium (Cd), copper (Cu) and other heavy metals can become mobile because of acidification and increased concentration of dissolved humic substances. The influence of pH and dissolved organic C on amounts and rates of Cd and Cu release was investigated in a cultivated soil by extraction with ∼ 1 m m hydrochloric acid at pH 3 with and without dissolved organic C in the batch mode with weekly replacement of the extraction solution. After 88 weeks, 35–50% of aqua regia-extractable Cd was extracted; addition of 10 m m dissolved organic C had no effect on the amount dissolved, but it increased the initial rate of release because the organic matter buffered the suspension at a lower pH. The solubility and release rate of Cd decreased as the geochemically active fraction was depleted. This suggests that Cd occurs in the soil in a continuum of binding strengths ranging from readily available to strongly bound forms. The repeated extractions resulted in distribution coefficients (Kd dis) that have log-linear relationships with pH. This allows prediction of Cd solubility during acidification of soil. Dissolved organic C enhanced the release of Cu from less than 8% (without) to more than 20% (with) of aqua regia-extractable Cu. Total contents of Cd and Cu cannot be used as measures of the metals' availability during acidification of former limed farmland. Predictions of availability should be based on the solubility as a function of pH and the degree of Cd or Cu depletion from the geochemically active fraction in soil.
This chapter focuses on the nature of interaction among trace metals in soil solution, dissolved organics in soil solution, and solid surfaces. The interaction between metal cations and dissolved polyfunctional organic compounds of low molecular weight is important because of its role in mineral-weathering and soil-forming processes and its potential role in heavy metal contamination of soil and groundwater. The chapter presents the organics and metals in the soil solution. Dissolved organics that interact with soil constituents and trace metal ions are of two major kinds: a range of low-molecular-weight organic acids—including polyphenols, simple aliphatic acids, amino acids, sugar acids, and hydroxamate siderophores; and a series of soluble humic/fulvic acids. Numerous environmental issues arise in relation to the interaction of metal ions with soluble organics. Some of these include the phytoavailability of metals, plant nutrient availability, toxicological effects of coordinated metal ions on aquatic and marine organisms, and transport of contaminants, particularly in relation to implications for surface and groundwater quality and soil genesis. All of these issues are highly dependent on the nature and concentration of the contaminant in the soil solution phase. Extant research indicates that low-molecular-weight ligands in soil solution may either enhance or retard reactions with solid surfaces—depending on the functional groups on the organic molecule, soil surface properties, and soil solution conditions. It is imperative that increased research efforts be devoted to evaluating the effects of these organics on metal reactions in the soil.
【Summary】 Humic substances (HSs) are by far the most abundant of the organic components of nature, and are present in all soils and natural waters that contain organic matter. The more widely accepted values for organic carbon (OC) in soil organic matter (SOM) are in the range of (14~15)×10 17
Regression is frequently abused in soil research. Its proper use is for statistical prediction. It may also be used to calculate equations for calibration A regression equation may be used to express a functional relation between two soil variables that are thought to be dated by some simple mathematical law but only where one of the variables is known exactly. In most other circumstances regression is inappropriate. Where departures from a functional relation are a result of errors of measurement and sampling fluctuation it should be replaced by a structural analysis to find the best equation. Where the underlying relation is truly bivariate it should be described as such.
La regression est fréquemment utilisée de manière abusive dans les travaux de recherche en science du sol, car souvent fondée sur l'emploi d'un modèle de regression à effets fixes. Ce dernier est approprié pour des problèmes d'estimation statistique. Il peut aussi être employé pour déterminer des équations d'étalonnage. Le modèle de regression à effets fixes sert également à exprimer une relation fonctionnelle entre deux variables supposées liées par une loi mathématique simple, mais seulement si l'une des variables est connue sans erreur. Dans la plupart des autres cas, ce modèle de régression est inadéquat. Quand les écarts entre les observations et une relation fonctionnelle proviennent d'erreurs de mesure ou de fluctuations dues à l'échantillonnage, il faut le remplacer par un modèle fonctionnel pour déterminer l'équation la meilleure. Quand la relation fonctionnelle sous-jacente relie réellement deux variables aléatoires, il faut la décrire en conséquence.
Soils of the urban area of Sevilla have been scarcely studied, especially concerning their concentrations of potentially toxic metals. A previous paper has shown that moderate pollution exists in soils from some public green areas of the city, and a common pattern was found in the distribution of the concentrations of some particular metals. The present paper is aimed at (i) determining possible seasonal changes in the measured concentrations; (ii) checking possible effects of the observed pollution on plant metal contents, and (iii) finding possible relations between metals by statistical techniques.
Disposal of sewage sludge creates the potential for heavy metal accumulation in the environment. This study assessed nine soils currently used as Dedicated Land Disposal units (DLDs) for treatment and disposal of municipal sewage sludge in the vicinity of Sacramento, California. Adsorption characteristics of these soils for Cd, Cu, Ni, Zn, Pb, and Cr were studied by simultaneously mixing these elements in the range of 0–50μmol L−1 with sludge supernatant and reacting with the soil using a soil:supernatant ratio of 1:30, pH=4.5 or 6.5, and constant ionic strength (0.01M Na-acetate). The concentration of metals in the supernatant was determined after a 24 hr equilibration period. Adsorption isotherms showed that metal sorption was linearly related to its concentration in the supernatant solution. The distribution coefficient Kd
(Kd
= concentration on solid phase/concentration in solution phase) was computed as the slope of the sorption isotherm. The distribution coefficients were significantly correlated to soil organic matter content for Ni, Cu, Cd, and Pb at pH 4.5 and for Ni, Cu, Zn, and Cd at pH 6.5. There was also a correlation between Kd
and soil specific surface area but no relationship to other soil properties such as CEC, clay content, and noncrystalline Fe and Al materials. Therefore, soil organic carbon and surface area appear to be the most important soil properties influencing metal adsorption through formation of organo-metal complexes. The Kd
values for all elements were higher at pH 6.5 than at 4.5. Selectivity between metals resulted in the following metal affinities based on their Kd
values: Pb>Cu>Zn>Ni>Cd≈Cr at pH 4.5 and Pb>Cu≈Zn>Cd>Ni>Cr at pH 6.5.
Sorption of heavy metals to organic matter and mineral soil constituents can hardly be separated experimentally. Here we studied
the retention capacity of organic matter and minerals from soils in a long-term field experiment in which the organic carbon
content had been altered, but the mineral phase had remained constant over time. The sorption of Cu, Cd and Zn showed a non-additive
contribution of soil organic matter and minerals to the sorption capacity of soil. Sorption on organic matter exceeded mineral
sorption from 6 to 13 times. This is the first time that sorption to soil organic matter is quantified in bulk soils.
Dissolved organic matter (DOM) is one of the important factors affecting metal mobility and phytotoxicity in the soils receiving sewage sludge. The aim of this study was to investigate the effects of DOM from anaerobically digested dewatered sludge on Cd and Zn sorption by three different soil types (calcareous clay loam, calcareous sandy loam and acidic sandy loam) of different physico-chemical properties through batch studies. The addition of DOM significantly reduced the Cd and Zn sorption capacity by a factor of 2.1–5.7 for Cd and 2.3–13.7 for Zn for these three soils as seen by their K values in the Freundlich equation compared to the control receiving no DOM, suggesting that DOM had a stronger inhibitory effect on Zn sorption than that of Cd. The reduction in metal sorption caused by DOM was very apparent in the pH range of 5 to 8, with a maximum inhibition on metal sorption occurring at pH 7–7.5 especially for Zn but the effect was minimal at lower pH. At a DOM concentration of < 200 mg C l− 1, Cd and Zn sorption by all the three soils decreased with an increase in DOM concentration. At each given DOM concentration, the inhibition of metal sorption of the different soil types increased in the following order: acidic sandy loam < calcareous sandy loam < calcareous clay loam. DOM derived from sludge would significantly reduce metal sorption and increase its mobility through the formation of soluble DOM–metal complexes and poses risk of metal leaching and phytotoxicty in near-neutral and alkaline soils.
Heavy metals which accumulate in soils may be harmful to soil and its boundary ecosystems. In this paper a unified risk assessment and risk management concept is proposed followed by a discussion on its practical implementation. To assess and manage risk, the application of a three-level evaluation system is presented, incorporating the degree of metal contamination. Levels for guide values, trigger values and clean up values are used. In order to assess exposure to heavy metals, three metal fractions like mobile, mobilisable and pseudo total metal fractions are introduced. Exceeding trigger and guide values at a site may be harmful to the risk receptors. Adequate site-specific mild remediation measures aim to diminish or eliminate risk without adversely affecting basic functions of soil. The importance of mobile and mobilisable metal concentrations is discussed in relation to the development of ecological and economical sound remediation techniques.
A 30-month experiment was carried out in field containers to evaluate the potential of various amendments and/or a plant cover (Agrostis stolonifera L.) for reclamation of a soil contaminated with As, Cd, Cu, Pb and Zn. Five amended treatments with plant were established: leonardite (LEO), litter (LIT), municipal waste compost (MWC), biosolid compost (BC) and sugar beet lime (SL). Two controls were also set up: control without amendment but with plant (CTRP) and control without amendment and without plant (CTR). A widely accepted sequential extraction procedure was used to assess changes in trace element pools in soil following remediation. Results showed that trace element distribution in soil was highly influenced by the origin of the contamination (mine sulphides): trace elements were mainly associated with the residual fraction (in form of sulphides) and to a lesser extent with the reducible fraction, where Fe oxides and oxyhydroxides are expected, as a consequence of sulphide oxidation. The addition of amendments and/or establishment of a plant cover showed no clear effect on Cd and Pb distribution. The influence on As fractionation was also small and restricted to the most labile fraction, where higher concentrations of As were found in SL, MWC and BC. In contrast, Cu concentrations in the most labile fraction decreased in MWC, BC, LEO and LIT, while Cu recovery within the organic matter-associated fraction increased in these treatments. Zinc concentration in the most labile fraction was significantly lower in CTRP than in CTR, whereas Zn recovery in this fraction increased in MWC due to Zn input. In some cases redistribution of amendment-derived trace elements from slow-releasing fractions to the residual pool seemed to occur. Since the increase in As and Zn only accounted for a small proportion of the total pool and soil quality improved in treated soils, the use of soil amendments and/or revegetation might be a reliable option for long-term reclamation of soils moderately contaminated with trace elements. Nonetheless, high quality by-products should be employed to minimize trace element input to soil.
The elemental composition, patterns of distribution and possible sources of street dust are not common to all urban environments, but vary according to the peculiarities of each city. The common features and dissimilarities in the origin and nature of street dust were investigated through a series of studies in two widely different cities, Madrid (Spain) and Oslo (Norway), between 1990 and 1994. The most comprehensive sampling campaign was carried out in the Norwegian capital during the summer of 1994. An area of 14 km2, covering most of downtown Oslo and some residential districts to the north of the city, was divided into 1 km2 mapping units, and 16 sampling increments of approximately 150 g were collected from streets and roads in each of them. The fraction below 100 μm was acid-digested and analysed by ICP-MS. Statistical analyses of the results suggest that chemical elements in street dust can be classified into three groups: “urban” elements (Ba, Cd, Co, Cu, Mg, Pb, Sb, Ti, Zn), “natural” elements (Al, Ga, La, Mn, Na, Sr, Th, Y) and elements of a mixed origin or which have undergone geochemical changes from their original sources (Ca, Cs, Fe, Mo, Ni, Rb, Sr, U). Soil resuspension and/or mobilisation appears to be the most important source of “natural” elements, while “urban” elements originate primarily from traffic and from the weathering and corrosion of building materials. The data for Pb seem to prove that the gradual shift from leaded to unleaded petrol as fuel for automobiles has resulted in an almost proportional reduction in the concentration of Pb in dust particles under 100 μm. This fact and the spatial distribution of Pb in the city strongly suggest that lead sources other than traffic (i.e. lead accumulated in urban soil over the years) may contribute as much lead, if not more, to urban street dust.
This paper describes changes in retention of Cu and Zn in laboratory experiments by a sandy soil that had been amended in the field with different composted wastes. The amounts of the metals retained increased as a result of the amendments, especially after two years. Desorption of the sorbed metals was always negligible, regardless of the treatment. The proportion of Cu retained was considerably higher than that of Zn, suggesting a higher affinity of the soil for the former. The greater sorption in the amended soils indicates a build-up of fresh sites for metal retention. The use of 'log(activity) vs. pH' plots showed that the nature of the surfaces retaining metals on the untreated and amended soils is different. At comparable pH values, the amended soils gave higher solution metal concentrations. Some of the possible environmental consequences of the use of these amendments for remediation purposes are discussed.
A study on olive mill residues (OMR) as copper adsorbing material is reported in this work. A rough characterization of this waste material has been performed, by microanalysis and SEM pictures. Sorption tests with suspended OMR evidenced copper removal from solution, of about 60% in the investigated experimental conditions. The COD release in solution was also monitored during biosorption. Considering that it was significant, OMR washings with water were performed before biosorption. In this case the COD release in solution was reduced to less than 600 mg/L after two washings, while the OMR metal sorption properties did not change. Regenerated residues by acid solutions gave a copper removal of about 40%, in the same experimental conditions of the first adsorption test: regeneration with EDTA at different concentrations suggested that it presents a damage of adsorption active sites. On the other hand, the use of HCl and CaCl(2) led to completely regenerate the biosorbent material. Tests were also performed with a column filled with 80 g of OMR and the breakpoint was demonstrated to take place after that about 1L solution was treated in the investigated experimental conditions. Regeneration tests permitted to demonstrate that a concentration factor of about 2 can be obtained in no-optimized conditions, highlighting the possibility of using OMR for the treatment of metal bearing effluents. The main advantage of the process would be the "low cost" biosorbing material, considering that it represents a waste in the olive oil production.
Organic acids are commonly produced and exuded by plant roots and soil microorganisms. Some of these organic compounds are effective chelating agents and have the potential to enhance metal mobility. The effect of citrate and salicylate on the leaching of lead in soil was investigated in a laboratory experiment. In short-term batch experiments, adsorption of lead to soil was slightly enhanced with increasing salicylate concentration (500-5000 microM) but decreased significantly in the presence of citrate. These observations suggested that citrate may enhance Pb leaching, but this was not observed in the column study. Soluble Pb in the presence and absence citrate or salicylate (up to 5000 microM) was added to soil columns at a moderate flow rate, but no Pb was observed to emerge from the soil in any of the soil columns. Rapid biodegradation of citrate in soil eliminated potential complexing ability. Breakthrough of Pb from soil was noted only when using small columns at high flow rates (>20 pore volumes per day). Under these conditions of physical and chemical non-equilibrium, citrate was not degraded and significantly enhanced Pb mobility. As in the batch adsorption experiments, the presence of salicylate reduced Pb leaching. Considering the extreme conditions required to induce Pb leaching, it is likely that Pb will remain relatively immobile in soil even in the presence of a strong complexing agent such as citrate.
Fifty-two samples of surface soils were taken in the urban area of Seville, to assess the possible influence of different land uses on their metal contents and their relationship with several soil properties. The samples corresponded to five categories or land uses: agricultural, parks, ornamental gardens, riverbanks, and roadsides. Sequential extraction of metal according to the procedure proposed by the former Community Bureau of Reference (BCR) was carried out, and pseudo-total (aqua regia soluble) metal contents were determined. Lower organic C, total N and available P and K contents were found in riverbank samples, probably due to the lack of manuring of those sites, left in a natural status. In contrast, significantly higher electrical conductivity was found in those sites, due to the tidal influence of the nearby Atlantic Ocean. Other land uses did not show significant differences in the general properties. Concentrations of Cu, Pb and Zn, both aqua-regia soluble and sequentially extracted, were clearly higher in soils from ornamental gardens, whereas the concentrations in the riverbank samples were slightly lower than the other categories. In contrast, other metals (Cd, Cr, Fe, Mn, Ni) were uniformly distributed throughout all land uses. A strong statistical association is found among the concentrations of Cu, Pb, Zn and organic C, suggesting that the larger contents of these metals in ornamental gardens are partly due to organic amendments added to those sites more frequently than to other kinds of sites. Considering the conclusions of previous studies, heavy traffic can also contribute to those ;urban' metals in urban soils. Periodic monitoring of the concentrations of urban metals in busy city centres and of the quality of amendments added to soils of recreational areas are recommended.
The efficiency of parthenium weed as an adsorbent for removing Cd(II) from water has been studied. Parthenium is found to exhibit substantial adsorption capacity over a wide range of initial Cd(II) ions concentration. Effect of time, temperature, pH and concentration on the adsorption of Cd(II) was investigated by batch process. Pseudo-first-order and Pseudo-second-order models were evaluated. The kinetics data for the adsorption process obeyed second-order rate equation. The equilibrium data could be described well by the Langmuir and Freundlich isotherms. Thermodynamic parameters such as DeltaH degrees , DeltaS degrees and DeltaG degrees were calculated. The adsorption process was found to be endothermic and spontaneous. The maximum adsorption of Cd(II) ions (99.7%) in the pH range 3-4 indicated that material could be effectively utilized for the removal of Cd(II) ions from wastewater. The desorption studies showed 82% recovery of Cd(II) when 0.1 M HCl solution was used as effluent.
Phytoremediation of metal-polluted soils can be promoted by the proper use of soil amendments and agricultural practices. A 4-year phytoremediation programme was applied to a site affected by the toxic spill of pyrite residue at Aznalcóllar (Spain) in 1998, contaminated with heavy metals (Zn, Cu, Pb, Cd) and arsenic. This consisted of active phytoremediation, using organic amendments (cow manure and compost) and lime and growing two successive crops of Brassica juncea (L.) Czern., followed by natural attenuation without further intervention. Changes in soil pH, extractable metal and As concentrations, organic carbon content and microbial biomass was evaluated. The initial oxidation of metal sulphides from pyrite residues released soluble metals and reduced soil pH to extremely acidic values (mean 4.1, range 2.0-7.0). The addition of lime (up to 64 t ha(-1)) increased soil pH to adequate values for plant growth, resulting in a significant decrease in DTPA-extractable metal concentrations in all plots. The natural attenuation phase showed also a decrease in extractable metals. Organic treatments increased the soil total organic carbon, which led to higher values of microbial biomass (11.6, 15.2 and 14.9 g kg(-1) TOC and 123, 170 and 275 microg g(-1) biomass-C in control, compost and manure plots, respectively). Active phytoremediation followed by natural attenuation, was effective for remediation of this pyrite-polluted soil.
The presence of heavy metals in the sludges produced in wastewater treatment plants restricts their use for agricultural purposes. This study looks at different types of sludge (aerobic, anaerobic, unstabilised, sludge from a waste stabilisation pond, sludge from an extended aeration plant and heat treated sludge) and compares the distribution of heavy metals with the treatment that they have undergone. In addition, the total quantity of metals (Cd, Cr, Cu, Ca, K, Fe, Mg, Ni, Na, Pb and Zn) and some agronomic parameters necessary for characterising a sludge as suitable for use as amendment were determined. The BCR method for heavy metal speciation was followed. Principal component analysis (PCA) was applied in order to obtain more information about metal speciation in the sewage sludges. It was confirmed that the concentration of heavy metals did not exceed the limits set out by European legislation and that the stabilisation treatment undergone by the sludges strongly influenced the heavy metal distribution and the phases to which they were associated. The waste stabilisation pond sludge, which has undergone a higher degree of mineralisation than the others, shows a lower metal bioavailability index since practically all the heavy metals in it are associated to the oxidisable and residual fraction. On the other hand the unstabilised sludge, which, along with that exposed to extended aeration, contains the highest accumulations of heavy metals in the most easily assimilable fractions.
Biosolid application to soil may be a supply of nutrients and micronutrients but it may also accumulate toxic compounds which would be absorbed by crops and through them be incorporated to the trophic chain. The present study deals with the effect of biosolid application on Cr, Cu, Pb, Ni, and Zn in agricultural soils. The procedure used is sequential extraction so that the availability of those metals may be estimated and related to their bioavailability as determined through two indicator plants grown in greenhouse: ryegrass (Lolium perenne L.) and red clover (Trifolium pratense). Results showed that biosolid application to soil increased total Cu and Zn content. Sequential extraction showed that the more labile Zn fractions increased after biosolid application to soil. This was confirmed when assessing the total content of this metal in shoot and root of the plants under study, since a higher content was found in plant tissues, while no significant differences were found for Cu, Cr, Ni, and Pb.
Microwave-enhanced Chemistry
- H M Kingston
- S J Haswell
H.M. Kingston, S.J. Haswell, Microwave-enhanced Chemistry, American Chemical Society, Washington, DC, 1997.
Metal retention and mobility as influenced by some organic residues added to soils: a case study
- L Madrid
L. Madrid, Metal retention and mobility as influenced by some organic residues
added to soils: a case study, in: H.M. Selim, I.K. Iskandar (Eds.), Fate and Transport of Heavy Metals in the Vadose Zone, CRC Press, Boca Raton, FL, 1999, pp.
201-223.
- W L Lindsay
W.L. Lindsay, Chemical Equilibria in Soils, John Wiley & Sons, New York, 1979.