Extraction of heavy metals from soils using biodegradable chelating agents

Metal pollution of soils is widespread across the globe, and the clean up of these soils is a difficulttask. One possible remediation technique is ex-situ soil washing using chelating agents. Ethylenediaminetetraacetic acid (EDTA) is a very effective chelating agent for this purpose but has the disadvantage that it is quite persistent in the environment due to its low biodegradability. The aim of our work was to investigate the biodegradable chelating agents [S,S]-ethylenediaminedisuccinic acid (EDDS), iminodisuccinic acid (IDSA), methylglycine diacetic acid (MGDA), and nitrilotriacetic acid (NTA) as potential alternatives and compare them with EDTA for effectiveness. Kinetic experiments showed for all metals and soils that 24 h was the optimum extraction time. Longer times only gave minor additional benefits for heavy metal extraction but an unwanted increase in iron mobilization. For Cu at pH 7, the order of the extraction efficiency for equimolar ratios of chelating agent to metal was EDDS > NTA> IDSA > MGDA > EDTA and for Zn it was NTA > EDDS > EDTA >MGDA > IDSA. The comparatively low efficiency of EDTA resulted from competition between the heavy metals and co-extracted Ca. For Pb the order of extraction was EDTA > NTA >EDDS due to the much stronger complexation of Pb by EDTA compared to EDDS. At higher concentration of complexing agent, less difference between the agents was found and less pH dependence. There was an increase in heavy metal extraction with decreasing pH, but this was offset by an increase in Ca and Fe extraction. In sequential extractions EDDS extracted metals almost exclusively from the exchangeable, mobile, and Mn-oxide fractions. We conclude that the extraction with EDDS at pH 7 showed the best compromise between extraction efficiency for Cu, Zn, and Pb and loss of Ca and Fe from the soil.     

Uptake of metals during chelant-assisted phytoextraction with EDDS related to the solubilized metal concentration

The use of chelants to enhance phytoextraction is one method being tested to make phytoextraction efficient enough to be used as a remediation technique for heavy metal pollution in the field. We performed pot experiments with sunflowers in order to investigate the use of the biodegradable chelating agent SS-EDDS for this purpose. We used singly and combined contaminated soils (Cu, Zn) and multimetal contaminated field soils (Cu, Zn, Cd, Pb). EDDS (10 mmol kg(-10 soil) increased soil solution metals greatly for Cu (factor 840-4260) and Pb (factor 100-315), and to a lesser extent for Zn (factor 23-50). It was found that Zn (when present as the sole metal), Cu, and Pb uptake by sunflowers was increased by EDDS, butin multimetal contaminated soil Zn and Cd were not. EDDS was observed in the sunflower roots and shoots at concentrations equal to metal uptake. The different metal uptake in the various soils can be related to a linear relationship between Cu and Zn in soil solution in the presence of EDDS and plant uptake, indicating the great importance of measuring and reporting soil solution metal concentrations in phytoextraction studies.     

Influence of EDDS on metal speciation in soil extracts: measurement and mechanistic multicomponent modeling

The use of the [S,S]-isomer of EDDS to enhance phytoextraction has been proposed for the remediation of heavy metal contaminated soils. Speciation of metals in soil solution in the presence of EDDS and dissolved organic matter (DOM) received, however, almost no attention, whereas metal speciation plays an important role in relation to uptake of metals by plants. We investigated the influence of EDDS on speciation of dissolved metals in batch extraction experiments using fourfield-contaminated soils with pH varying between 4.7 and 7.2. Free metal concentrations were determined with the Donnan membrane technique, and compared with results obtained with the chemical speciation program ECOSAT and the NICA-Donnan model using a multicomponent approach. Addition of EDDS increased total metal concentrations in our soil extracts by a factor between 1.1 and 32 (Al), 2.1-48 (Cu), 1.1-109 (Fe), 1.1-5.5 (Ni), and 1.3-17 (Zn). In general, Al, Cu, Fe, and Zn had the largest total concentrations in the EDDS-treated extracts, but the contribution of these metals to the sum of total metal concentrations varied significantly between our soils. Free metal concentrations varied between 7.0 and 8.9 (pCd2+), 3.9-9.9 (pCu2+), 6.3-10.2 (pNi2+), and 5.2-7.0 (pZn2+). Addition of EDDS decreased free metal concentrations by a factor between 1.4 and 1.9 (Cd), 3.4-216 (Cu), 1.3-186 (Ni), and 1.3-3.3 (Zn). Model predictions of free metal concentrations were very good, and predicted values were mostly within 1 order of magnitude difference from the measured concentrations. A multicomponent approach had to be used in our model calculations, because competition between Fe and other metals for binding with EDDS was important. This was done by including the solubility of metal oxides in our model calculations. Multicomponent models can be used in chelant-assisted phytoextraction experiments to predict the speciation of dissolved metals and to increase the understanding of metal uptake by plants.     

Induced phytoextraction/soil washing of lead using biodegradable chelate and permeable barriers

Chelate-induced remediation has been proposed as an effective tool for the extraction of lead (Pb) from contaminated soils by plants. However, side-effects, mainly mobilization and leaching of Pb, raise environmental concerns. Biodegradable, synthetic organic chelate ethylenediaminedisuccinic acid (EDDS), and commonly used ethylenedimanetetraacetic acid (EDTA) were used for induced phytoextraction with a test plant Brassica rapa and in situ washing of soil contaminated with 1350 mg/kg of Pb. Horizontal permeable barriers were placed 20 cm deep in soil columns and tested for their ability to prevent leaching of Pb. The reactive materials in the barriers were nutrient enriched vermiculite, peat or agricultural hydrogel, and apatite. EDTA and EDDS addition increased Pb concentrations in the test plant by 158 and 89 times compared to the control, to 817 and 464 mg/kg, respectively. In EDTA treatments, approximately 25% or more of total initial soil Pb was leached in single cycle of chelate addition. In EDDS treatments, 20% of the initial Pb was leached from columns with no barrier, while barriers with vermiculite or hydrogel and apatite decreased leaching by more than 60 times, to 0.35%. 11.6% of total initial Pb was washed from the soil above the barrier with vermiculite and apatite, where almost all leached Pb was accumulated. Results indicate that use of biodegradable chelate EDDS and permeable barriers may lead to environmentally safe induced Pb phytoextraction and in situ washing of Pb.     

Estimate of heavy metal contamination in soils after a mining accident using reflectance spectroscopy

The possibility to adapt chemometrics approaches for the quantitative estimation of heavy metals in soils polluted by a mining accident was explored. In April 1998, the dam of a mine tailings pond in Aznalcóllar (Spain) collapsed and flooded an area of more than 4000 ha with pyritic sludge contaminated with high concentrations of heavy metals. Six months after the end of the first remediation campaign, soil samples were collected for chemical analysis and measurement of visible to near-infrared reflectance (0.35-2.4 microm). Concentrations for As, Cd, Cu, Fe, Hg, Pb, S, Sb, and Zn were well above background values. Prediction of heavy metals was achieved by stepwise multiple linear regression analysis (MLR) and an artificial neural network (ANN) approach. It was possible to predict six out of nine elements with high accuracy. Best R2 between predicted and chemically analyzed concentrations were As, 0.84; Fe, 0.72; Hg, 0.96; Pb, 0.95; S, 0.87; and Sb, 0.93. Results for Cd (0.51), Cu (0.43), and Zn (0.24) were not significant. MLR and ANN both achieved similar results. Correlation analysis revealed that most wavelengths important for prediction could be attributed to absorptions features of iron and iron oxides. These results indicate that it is feasible to predict heavy metals in soils contaminated by mining residuals using the rapid and cost-effective reflectance spectroscopy.     

Modeling the transport of metals with rate-limited EDTA-promoted extraction and dissolution during EDTA-flushing of copper-contaminated soils

EDTA-flushing of artificially contaminated soils enhanced Cu extraction but also induced mineral dissolution simultaneously. The mobilization and transport of these metal-EDTA complexes was investigated with column experiments. A quantitative transport model was proposed for simulating the experimental breakthrough curves of Cu, Fe, Al, and Ca. The rate-limited EDTA-promoted extraction and dissolution could be described by respective second-order kinetic terms, which were necessary for explaining the time-dependent depletion of extractable metals (sorbed and indigenous) in soils with continuous EDTA-flushing. Simultaneous simulation of extraction of sorbed Cu and dissolution of soil Fe, Al, and Ca is more conceptually accurate than individual modeling of each metal because the latter approach tends to overestimate the concentration of free EDTA during transport and thus underestimate the rate coefficients of EDTA-promoted dissolution. The fitted rate coefficients of Cu were about an order of magnitude larger than those of Fe and Al; these values probably reflect Cu extraction from weakly sorbed fractions and Fe and Al dissolution from amorphous oxides. The apparent retardation of Fe, Al, and Ca transport had to be taken into account by empirical determination, which was attributed to the metal lability in soils and thermodynamics of surface complexation.     

Formation and dissolution of single and mixed Zn and Ni precipitates in soil: evidence from column experiments and extended X-ray absorption fine structure spectroscopy

The stability and the formation and dissolution kinetics of mixed trace metal precipitates in soils are currently unknown. The objective of this study was to investigate slow sorption and release processes of Zn and Ni in a loamy soil using a combination of soil column experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. To investigate slow sorption processes, the soil material was packed into columns and leached with 5400 pore volumes of 10(-2) M CaCl2 solutions containing either ZnCl2 (5.2 x 10(-5) M) or NiCl2 (5.2 x 10(-5) M) or both ZnCl2 and NiCl2 (5.2 x 10(-5) M each). The Zn and Ni concentrations in the column effluents were monitored. The metal breakthrough curves showed that slow sorption processes lead to metal retention, whereby Zn was more strongly retained than Ni. In the experiment with both Zn and Ni present, amounts of Zn and Ni similar to those in the experiments with either Zn or Ni alone were retained. Analysis of soil samples by EXAFS spectroscopy showed that layered double hydroxide (LDH)-type precipitates had formed in all columns and that a mixed ZnNi-LDH had formed in the presence of both Zn and Ni. The dissolution of those precipitates under acidic conditions was assessed by subsequent leaching of the columns with a 10(-2) M CaCl2 solution at pH 3.0 (approximately 3000 pore volumes). When only Zn was present, 95% of the retained Zn was leached at pH 3. In contrast, only 23% of the retained Ni was leached in experiments with Ni alone. When Zn and Ni were present, 90% of the retained Zn and 87% of the retained Ni were released upon acidification. EXAFS analysis revealed that the LDH phases in the Zn experiment and the Zn-Ni experiment had been completely dissolved, while the LDH phase formed in the Ni experiment was still present. The higher resistance of Ni-LDH against dissolution at low pH could also be shown in dissolution studies with synthetic Zn-LDH, Ni-LDH, and ZnNi-LDH. Our results suggest that the individual rates at which Zn and Ni cations enter into the LDH structure determine the composition of the mixed ZnNi-LDH precipitate, and that the LDH composition determines the rate at which the LDH phase dissolves under acidic conditions.     

Effect of accelerated aging of MSWI bottom ash on the leaching mechanisms of copper and molybdenum

The effect of accelerated aging of Municipal Solid Waste Incinerator (MSWI) bottom ash on the leaching of Cu and Mo was studied using a "multisurface" modeling approach, based on surface complexation to iron/aluminum (hydr)oxides, mineral dissolution/precipitation, and metal complexation by humic substances. A novel experimental method allowed us to identify that the solid/liquid partitioning of fulvic acids (FA) quantitatively explains the observed beneficial effect of accelerated aging on the leaching of Cu. Our results suggestthat iron/aluminum (hydr)oxides are the major reactive surfaces that retain fulvic acid in the bottom ash matrix, of which the aluminum (hydr)oxides were found to increase after aging. A new modeling approach, based on the surface complexation of FA on iron/aluminum (hydr)oxides is developed to describe the pH-dependent leaching of FA from MSWI bottom ash. Accelerated aging results in enhanced adsorption of FA to (neoformed) iron/aluminum (hydr)oxides, leading to a significant decrease in the leaching of FA and associated Cu. Accelerated aging was also found to reduce the leaching of Mo, which is also attributed to enhanced adsorption to (neoformed) iron/aluminum (hydr)oxides. These findings provide important new insights that may help to improve accelerated aging technology.     

Leaching of heavy metals from contaminated soils: an experimental and modeling study

In this paper, we characterize the leaching of heavy metals (Ni, Cu, Zn, Cd, and Pb) from eight contaminated soils over a wide range of pH (pH 0.4-12) using an original approach based on batch pH-static leaching experiments in combination with selective chemical extractions and geochemical modeling. The leached concentrations of the heavy metals are generally much lower than the total concentrations and show a strong pH dependency, resulting in "V-shaped" leaching curves with orders of magnitude changes in solution concentrations. The "multisurface" model used incorporates adsorption to dissolved and solid organic matter (NICA-Donnan), iron/aluminum (hydr)oxide (generalized two-layer model) and clay (Donnan model). These models were applied without modifications, and only the standard set of binding constants and parameters was used (i.e., without any fitting). The model predictions of heavy metal leaching are generally adequate and sometimes excellent. Results from speciation calculations are consistent with the well-recognized importance of organic matter as the dominant reactive solid phase in soils. The observed differences between soils with respect to element speciation in the solid phase correspond to the relative amounts of the reactive surfaces present in the soils. In the solution phase, complexes with dissolved organic matter (DOM) are predominant over most of the pH range. Free metal ions (Me2+) are generally the dominant species below pH 4. The combination of the experimental and modeling approach as used in this study is shown to be promising because it leads to a more fundamental understanding of the pH-dependent leaching processes in soils. The "multisurface" modeling approach, with the selected sorption models, is shown to be able to adequately predict the leaching of heavy metals from contaminated soils over a wide range of conditions, without any fitting of parameters.     

Assessing the origin and fate of Cr, Ni, Cu, Zn, Pb, and V in industrial polluted soil by combined microspectroscopic techniques and bulk extraction methods

The major geochemical forms of Cr, Ni, Cu, Zn, Pb, and V in a soil from an industrial polluted site in the south of Italy were determined by means of synchrotron X-ray microanalytical techniques such as coupled micro-X-ray fluorescence/micro-X-ray diffraction and micro-X-ray absorption near edge structure spectroscopy in combination with bulk extraction methods (sequential extraction procedures, EDTA extractions, and toxicity leaching characteristic procedure tests). Cr, Ni, Zn, and Cu were found in spinel-type geochemical forms (chromite, trevorite, franklinite, zincochromite, and cuprospinel) and often in association with magnetite and hematite. Vwas mainly present as V(V) associated with iron-oxides or in the form of volborthite [Cu3(OH)2V2O7.2H2O]. Pb was speciated as minium (Pb3O4), lanarkite [Pb2O(SO4)], and, in association with Cr(VI), as crocoite (PbCrO4). In general, despite a high total concentration, metals appear to be speciated for the most part as rather insoluble geochemical forms. However, particular attention should be paid to Zn, Cu, V, and Pb that show non-negligible mobilizable fractions. On the basis of the geochemical forms identified, among others, two major former industrial activities were tentatively ascribed as being responsible for the observed major pollution: polyvinyl chloride and cement-asbestos productions.     

Dispersion of heavy metals (metalloids) in soils from 800-year-old pollution (Mont-Lozère, France)

Numerous palaeo metallurgical sites (n = 70) characterized by slag presenting a homogeneous typology have been reported on the Mont-Lozère Massif (Southern France). These activities took place in the medieval period. The silicated slag matrix comprises mainly Pb (25%), Sb (0.4%), and several thousand parts per million of As, Cu, and Zn. Soil samples were collected in and around two sites, to understand the dispersion mechanism affecting the slag tailings through use of metal concentrations and lead isotopic compositions. The majority of polluted soil samples show high enrichment factors (EF) for Pb and Sb, slightly lower EFs for Cu, and much lower EFs for As and Zn. We show that this "old" metal pollution was physically dispersed, through erosion of workshop soils and slag tailings, in a restricted area (ca. 200 m down slope form the site). There is no evidence for massive leaching of slag metals by soil waters, except for Zn. Thus, the pollution is mainly due to the metal-making process, i.e., smoke-fallout, pieces of ore, the crackling of smelting ore outside the oven during reduction, and charcoal, etc. The lead isotopic compositions of the soils define a binary mixing trend between local granite or background soil and slag (which represent the workshop soil). Simple mass balance equations using either Pb isotopes or Pb concentrations suggest that between 40 and 100% of the total Pb in soils comes from the Medieval workshop pollution, leaving any later pollution negligible. The large number of sites on the Mont-Lozère means this medieval pollution is significant and poses a real environmental risk.     

Thermally induced changes in metal solubility of contaminated soils is linked to mineral recrystallization and organic matter transformations

Soils are biogeochemical systems under continual modification by biological and chemical processes. Trace element solid-solution partitioning is thus influenced by long-term changes to these solid phases. We study Pb, Cd, Zn, and Cu solution speciation and solid-phase dynamics in two soils of volcanic origin (Te Akatea and Egmont, high in noncrystalline aluminosilicates), an oxisol from Brazil (Oxisol, high in oxides of Al and Fe), and several sludge-treated soils (labeled NYS soils, high in organic materials). Total soluble (by ICP) and labile (by ASV) concentrations of Pb, Cd, Zn, and Cu were determined after incubation of the soils for about 1.5 yr at room (23 degrees C) and elevated (70 degrees C) temperatures. Changes occurring to the solid phases were monitored by FTIR and extraction with oxalate and pyrophosphate. It is shown that induced hydrolysis or decomposition of organic materials in soils results in increases in both labile and total soluble concentrations of Pb, Cd, Cu, and Zn in solution. Labile and total soluble concentrations of Cu and Zn increase concomitantly with dissolved organic carbon (DOC); the nonlabile soluble fraction also increases with increasing DOC. Similarly, the concentration of Cd and Pb in solution increases with increasing DOC; however, most soluble Cd and Pb is asv-labile. Only in the Egmont soil (mineralogy dominated by proto-imogolite allophane) was reduced Pb solubility observed after prolonged equilibration and heating. Lead solubility increased after partial crystallization of amorphous minerals in the Te Akatea and the Oxisol. Thus, for most of the metal-soil systems studied, prolonged thermal treatment at 70 degrees C increased total soluble and asv-labile metals, suggesting that aging effects on metals in contaminated soils could release metals to labile forms in some cases.     

Leaching of arsenic from granular ferric hydroxide residuals under mature landfill conditions

Most arsenic bearing solid residuals (ABSR) from water treatment will be disposed in nonhazardous landfills. The lack of an appropriate leaching test to predict arsenic mobilization from ABSR creates a need to evaluate the magnitude and mechanisms of arsenic release under landfill conditions. This work studies the leaching of arsenic and iron from a common ABSR, granular ferric hydroxide, in a laboratory-scale column that simulates the biological and physicochemical conditions of a mature, mixed solid waste landfill. The column operated for approximately 900 days and the mode of transport as well as chemical speciation of iron and arsenic changed with column age. Both iron and arsenic were readily mobilized under the anaerobic, reducing conditions. During the early stages of operation, most arsenic and iron leaching (80% and 65%, respectively) was associated with suspended particulate matter, and iron was lost proportionately faster than arsenic. In later stages, while the rate of iron leaching declined, the arsenic leaching rate increased greater than 7-fold. The final phase was characterized by dissolved species leaching. Future work on the development of standard batch leaching tests should take into account the dominant mobilization mechanisms identified in this work: solid associated transport, reductive sorbent dissolution, and microbially mediated arsenic reduction.     

Multielementary (Cd, Cu, Pb, Zn, Ni) Stable Isotopic Exchange Kinetic (SIEK) method to characterize polymetallic contaminations

A new method is proposed to precisely and simultaneously quantify the exchangeable pool of metals in soils and to describe its reactivity at short- and long-term. It is based on multielementary Stable Isotopic Exchange Kinetics (multi-SIEK), first validated by a comparison between two monoelementary radioactive ((109)Cd*, (65)Zn*) IEK experiments, a mono- ((106)Cd) and multi- ((62)Ni, (65)Cu, (67)Zn, (106)Cd, (204)Pb) SIEK. These experiments were performed on a polluted soil located near the Zn smelter plant of Viviez (Lot watershed, France). The IEK results obtained for Cd and Zn were consistent across the experiments. (109)Cd*, (65)Zn* IEK, and multi-SIEK were then applied on 3 non- and moderate impacted soils that also provided consistent results for Cd and Zn. Within these experimental conditions, it can be concluded that no competition occurs between Cd, Zn, and the other metals during SIEK. Multi-SIEK results indicate that the isotopically exchangeable pool of Ni, Zn, and Cu are small (E(Ni), E(Zn), and E(Cu) values up to 17%) whatever the pollution degree of the soils considered in this study and whatever the duration of the interaction. On the contrary, Cd displays the highest E values (from 35% to 61% after 1 week), and E(Pb) displays a maximum value of 26% after 1 week. The multi-SIEK provides useful information on metal sources and reactivity relationship. Ni would be located in stable pedogenic phases according to its very low enrichment factor. The low E(Zn) and E(Cu) are consistent with location of Zn and Cu in stable phases coming from tailings erosion. Though Pb enrichments in soils may also be attributed to tailings particles, its larger exchangeable pool suggests that the Pb-bearing phases are more labile than those containing Zn and Cu. The high mobility of Cd in upstream soils indicates that it has been mostly emitted as reactive atmospheric particles during high temperature ore-treatment.     

Assessment of heavy metals remobilization by fractionation: comparison of leaching tests applied to roadside sediments

The pollution emitted by traffic activities and road maintenance is an area of great interest as contaminants can be transported to roadside sediments and pose a risk to environmental and human health. In the presentwork, deposited pollution in roadside sediments has been assessed by sampling along a highly traveled highway in Barcelona and the surrounding area. The available amounts of the heavy metals was determined by applying different leaching tests and calculating the concentration enrichment ratio (CER) and the environmental concentration guideline values (ECG). To gain information on the heavy metals (HMs) fractionation, the sequential extraction scheme (SES), established by the Standard Measurement and Testing (SM&T), was implemented, and the results were compared with those obtained by single leaching tests. An anthropogenic enhancement of certain metals was observed after considering both the CER and ECG values. However, if only ECG values were considered, an overestimation of the anthropogenically enhanced pollutants was obtained due to disregarding geochemical and particle size variability. CER values provide a more realistic assessment by determining different levels of anthropogenic impact. Thus, CER values suggest a minimum anthropogenic apportion for metals such as Cd, Cr, and Ni, whereas different situations from significant to extreme anthropogenic contribution were observed for Zn, Pb, and Cu. These results have been complemented by other leaching tests that minimize the time-consuming environmental evaluation. In this study, HCI extraction produces suitable results for a quick screening since they correlate well with the corresponding SES: Cu(r2 = 0.798), Pb(r2 = 0.958) and Zn(r2 = 0.901). Mild extractants have been observed to be limited to highly polluted samples due to their low leaching power. The information obtained following this procedure helps to identify hazardous areas that need a remedial strategy.     

European atmospheric pollution imported by cooler air masses to the Eastern Mediterranean during the summer

In this study we combine Pb isotopes, Pb, Ni, Cu, Zn, and several major metal concentrations, identification of the aerosol particles, and synoptic and back trajectory analyses to obtain direct evidence for the extent and nature of mixing between Middle Eastern and European sources emissions of metals and aerosols. During the summer months aerosols collected in Israel are highly polluted by metals (EF(Ni) = 120, EF(Cu) = 320, EF(Zn) = 30, EF(Pb) = 540; average values). The fraction of European Pb of mostly industrial sources is 61 +/- 21% in Jerusalem, and the fraction of European Cu, Zn, Ni, and aerosols should be on the same order. Whenever a steep pressure gradient is built between the barometric trough originating from the Persian Gulf and the subtropical ridge along the African coast, stronger westerly winds and cooler temperatures (deep Persian Trough) prevail overthe Middle East, and higher amounts of European pollution are observed in the atmosphere (74 +/- 13%). On the other hand, when the Persian Trough is in its shallow mode, the proportion of European pollution is lower (45 +/- 18%, based on Pb isotopes). This study demonstrates that atmospheric pollution over the East Mediterranean region during the summer is influenced not only by local atmospheric dispersion conditions but also by the ability of the atmosphere to inherit a significant proportion of pollutants from European sources.     

Bioremediation process for sediments contaminated by heavy metals: feasibility study on a pilot scale

The core stages of a sediment remediation process--the conditioning of dredged sludge by plants and the solid-bed leaching of heavy metals using microbially produced sulfuric acid--were tested on a pilot scale using a highly polluted river sediment. Conditioning was performed in 50 m3 basins at sludge depths of 1.8 m. During one vegetation period the anoxic sludge turned into a soil-like oxic material and became very permeable to water. Reed canary grass (Phalaris arundinacea) was found to be best suited for conditioning. Bioleaching was carried out in an aerated solid-bed reactor of 2000 L working volume using oxic soil-like sediment supplemented with 2% sulfur. When applying conditioned sediment, the oxidation of easily degradable organic matter by heterotrophic microbes increased the temperature up to 50 degrees C in the early leaching phase, which in turn temporarily inhibited the sulfur-oxidizing bacteria. Nevertheless, most of the metal contaminants were leached within 21 days. Zn, Cd, Mn, Co, and Ni were removed by 61-81%, Cu was reduced by 21%, while Cr and Pb were nearly immobile. A cost-effectiveness assessment of the remediation process indicates it to be a suitable treatment for restoring polluted sediments for beneficial use.     

XAS evidence of As(V) association with iron oxyhydroxides in a contaminated soil at a former arsenical pesticide processing plant

The molecular-level speciation of arsenic has been determined in a soil profile in the Massif Central near Auzon, France that was impacted by As-based pesticides by combining conventional techniques (XRD, selective chemical extractions) with X-ray absorption spectroscopy (XAS). The arsenic concentration is very high at the top (>7000 mg kg(-1)) and decreases rapidly downward to a few hundreds of milligrams per kilogram. A thin layer of schultenite (PbHAsO4), a lead arsenate commonly used as an insecticide until the middle of the 20th century, was found at 10 cm depth. Despite the occurrence of this As-bearing mineral, oxalate extraction indicated that more than 65% of the arsenic was released upon dissolution of amorphous iron oxides, suggesting a major association of arsenic with these phases within the soil profile. Since oxalate extraction cannot unambiguously distinguish among the various chemical forms of arsenic, these results were confirmed by a direct in situ determination of arsenic speciation using XAS analysis. XANES data indicate that arsenic occurs mainly as As(V) along the soil profile except for the topsoil sample where a minor amount (7%) of As(III) was detected. EXAFS spectra of soil samples were fit by linear combinations of model compounds spectra and by a shell-by-shell method. These procedures clearly confirmed that As(V) is mainly (at least 80 wt %) associated with amorphous Fe(III) oxides as coprecipitates within the soil profile. If any, the proportion of schultenite, which was evidenced by XRD in a separate thin white layer, does not account for more than 10 wt % of arsenic in soil samples. This study emphasizes the importance of iron oxides in restricting arsenic dispersal within soils following dissolution of primary As-bearing solids manufactured for use as pesticides and released into the soils.     

江西抚州烟区土壤及烟叶重金属污染状况评价

为了弄清江西抚州烟区土壤和烟叶中As、Cd、Cr、Cu、Ni、Pb、Zn和Hg等重金属含量和污染状况,运用内梅罗指数和相关分析法,对这些重金属元素在土壤和烟叶中的含量进行了测定。结果表明,该烟区土壤中重金属含量总体上低于我国土壤二级标准（GB15618-1995）,但其污染程度已处于警戒线水平,Cd和Hg为烟区土壤主要风险因子。土壤和烟叶中重金属含量顺序分别为Zn> Pb> Cr> Cu> Ni> As> Cd> Hg和Zn> Cu> Pb> Cd> Cr> Ni> As> Hg,土壤中Hg、Cd、As和烟叶中Ni、Cr变异系数均较大。烟叶中重金属富集系数顺序为Cd> Zn> Cu> Hg> Ni> Cr> Pb> As,烟叶Cd富集系数高达11.67,表明烟草属于Cd强烈富集作物。