The need for bioaugmentation after thermal treatment of a TCE-contaminated aquifer: Laboratory experiments

A microcosm study was conducted to evaluate the need for bioaugmentation after a thermal treatment to anaerobically dechlorinate trichloroethene (TCE) to ethene. The microcosms were either: heated to 100 degrees C and slowly cooled to simulate thermal remediation while bioaugmenting when the declining temperature reached 10 degrees C; or kept at ambient groundwater temperatures (10 degrees C) and bioaugmented for comparison. Aquifer samples from three sediment locations within a TCE-polluted source zone were investigated in duplicate microcosms. In biostimulated (5 mM lactate) and heated microcosms, no conversion of TCE was observed in 4 out of 6 microcosms, and in the remaining microcosms the dechlorination of TCE was incomplete to cDCE (cis-dichloroethene). By comparison, complete TCE dechlorination to ethene was observed in 4 out of 6 heated microcosms that were bioaugmented with a highly enriched dechlorinating mixed culture, KB-1, but no electron donor, and also in 4 of 6 microcosms that were augmented with KB-1 and an electron donor (5 mM lactate). These data suggest that electron donor released during heating, was capable of promoting complete dechlorination coincident with bioaugmentation. Heated microcosms demonstrated less methanogenesis than unheated microcosms, even with elevated H2 concentrations and addition of KB-1, which contains methanogens. This suggests that the heating process suppressed the native microbial community, which can decrease competition with the bioaugmented culture and increase the effectiveness of dechlorination following a thermal treatment. Specifically, cDCE removal rates were four to six times higher in heated than unheated bioaugmented microcosms. This study confirms the need for bioaugmentation following a laboratory thermal treatment to obtain complete dechlorination of TCE.     

Use of principal component analysis to profile temporal and spatial variations of chlorinated solvent concentration in groundwater

This work aims at evaluating spatial distribution patterns of concentration variations for chlorinated solvents in groundwater, based on principal component analysis and geographic information system (GIS) tools. The study investigates long-time series of chlorinated solvent concentrations in groundwater measured for 18 contaminated industrial sites. The characterization of contaminant plumes and delineation of pollutant sources are essential for choosing appropriate monitoring and remediation strategies, as contaminated groundwaters are characterized by complex patterns of spatial and temporal concentration variability, with wide unpredictable fluctuations over time. The present work describes the results of a new exploratory statistical method called the Variability Index Method (VIM) applied to environmental data to assess the performance of using concentration variations as molecular tracers to reveal aquifer dynamics, industrial impacts, and point sources for contamination plumes. The application of this method provides a useful assessment of controls over contaminant concentration variations as well as support for remediation techniques.     

Integration of traditional and innovative characterization techniques for flux-based assessment of Dense Non-aqueous Phase Liquid (DNAPL) sites

Key attributes of the source zone and the expanding dissolved plume at a trichloroethene (TCE) site in Australia were evaluated using trends in groundwater monitoring data along with data from on-line volatile organic compound (VOC) samplers and passive flux meters (PFMs) deployed in selected wells. These data indicate that: (1) residual TCE source mass in the saturated zone, estimated using two innovative techniques, is small ( 10 kg), which is also reflected in small source mass discharge ( 3 g/day); (2) the plume is disconnecting, based on TCE concentration contours and TCE fluxes in wells along a longitudinal transect; (3) there is minimal biodegradation, based on TCE mass discharge of  6 g/day at a plume control plane  175 m from source, which is also consistent with aerobic geochemical conditions observed in the plume; and (4) residual TCE in the vadose zone provides episodic inputs of TCE mass to the plume during infiltration/recharge events. TCE flux data also suggest that the small residual TCE source mass is present in the low-permeability zones, thus making source treatment difficult. Our analysis, based on a synthesis of the archived data and new data, suggests that source treatment is unwarranted, and that containment of the large TCE plume ( 1.2 km long,  0.3 km wide; 17 m deep;  2000–2500 kg TCE mass) or institutional controls, along with a long-term flux monitoring program, might be necessary. The flux-based site management approach outlined in this paper provides a novel way of looking beyond the complexities of groundwater contamination in heterogeneous domains, to make intelligent and informed site decisions based on strategic measurement of the appropriate metrics.     

Model-based evaluation of controlled-release systems in the remediation of dissolved plumes in groundwater

Controlled-release, semi-passive reactive barrier systems have been recently developed as a long-term treatment option for controlling the spread of contaminant plumes in groundwater. This paper describes a new computer code, and applies it to study coupled processes of solute release, reaction, and mass transport in an in situ remediation scheme using the controlled release of potassium permanganate. Confidence with the modeling approach was developed by model verifications and simulating results of a pilot-scale test-cell experiment. Sensitivity analyses indicated the possibilities of treatment inefficiencies due to inability of transverse dispersion to mix the permanganate (MnO(4)(-)) within the zone of reaction, fluctuations in source strength due to variations in flow velocity, and the small length of treatment zone due to strong soil utilization of MnO(4)(-). Although problems associated with the fluctuating source strength and strong soil utilization can be addressed by optimizing the release rate, the inefficiency of transverse dispersion to create mixing could pose a serious limitation. Through a series of model simulations, a system of injection/withdrawal wells in a doublet arrangement was developed to facilitate lateral spreading and mixing of MnO(4)(-). A well-mixed, stable MnO(4)(-) zone with predetermined size (DxL=8m x 2m) and concentration ranges (1.5-20 mg l(-1)) was created by four 1-day injection/withdrawal pumping periods over 24 d. This type of mixing zone may persist for many years with periodic well mixing and replacements of exhausted controlled-release forms. Coupled use of the generalized code with field hydrologic data will help to optimize the design and operation of controlled-release systems in practice.     

Derivation of aquatic predicted no-effect concentration (PNEC) for 2,4-dichlorophenol: comparing native species data with non-native species data

2,4-Dichlorophenol (2,4-DCP) is known as an important chemical intermediate and an environmental endocrine disruptor. There is no paper dealing with the predicted no-effect concentration (PNEC) of 2,4-DCP, mainly due to shortage of chronic and site-specific toxicity data. In the present study, toxicity data was obtained from the tests using six Chinese native aquatic species. The HC₅ (hazardous concentration for 5% of species) was derived based on the constructed species sensitivity distribution (SSD), which was compared with that derived from literature toxicity data of non-native species. For invertebrates, the survival no-observed effect concentrations (NOECs) were 0.05 and 1.00 mg L⁻¹ for Macrobrachium superbum and Corbicula fluminea, respectively. NOECs based on fishes’ growth were 0.10, 0.20 and 0.40 mg L⁻¹ for Mylopharyngodon piceus, Plagiognathops microlepis and Erythroculter ilishaeformis, respectively. For aquatic plant Soirodela polyrhiza, NOEC based on concentration of chlorophyll was 1.00 mg L⁻¹. A final PNEC calculated using the SSD approach with a 50% certainty based on different taxa ranged between 0.008 and 0.045 mg L⁻¹. There is no significant difference between HC₅ derived from native and that from non-native taxa.     

High temperature dependence of 2,4-dichlorophenoxyacetic acid degradation by Fe3+/H(2)O(2) system

This study demonstrates the importance of reaction temperature on the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D). In addition, we provide a mechanistic explanation for the temperature dependence of 2,4-D degradation. Thermal enhancement of 2,4-D degradation and H(2)O(2) decomposition was measured in the absence and in the presence of the z.rad;OH scavenger (t-butanol). The half-life for 2,4-D degradation was reduced by more than 70-fold in the absence of t-butanol, and by more than 700-fold, in the presence of t-butanol, when the reaction temperature was increased from 10 to 50 degrees C. In addition, similar temperature relationships were found for H(2)O(2) decomposition. The major reason for the high temperature dependence of the Fe(3+)/H(2)O(2) system in the case of 2,4-D degradation is due to the dependence of the initiation reaction of the Fe(3+)/H(2)O(2) system (i.e., Fe(3+)+H(2)O(2)-->Fe(2+)+HO(2)(z.rad;)+H(+) upon temperature), which is entirely consistent with the kinetics of the activation energy. In the presence of a z.rad;OH scavenger, the initiation reaction of the Fe(3+)/H(2)O(2) system became a determining factor of this temperature dependence, whereas in the absence of z.rad;OH scavenger, several other radical reactions played a role and this result in an apparent decrease in the activation energy for 2,4-D degradation. Moreover, the enhanced 2,4-D removal at higher temperatures did not alter H(2)O(2) utilization. The practical implications of the thermal enhancement of the Fe(3+)/H(2)O(2) system are discussed.     

Influence of various reaction parameters on 2,4-D removal in photo/ferrioxalate/H(2)O(2) process

The influence of various reaction parameters on herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) removal were examined in the photo/ferrioxalate/H(2)O(2) system, with regard to: (1) sulfate, phosphate, and z.rad;OH scavenger, as solution constituent; and (2) light intensity, ferrioxalate, H(2)O(2), and oxalate concentration, as operating parameter. In terms of 2,4-D removal, the photo/ferrioxalate/H(2)O(2) system has always been superior to the photo/Ferric ion/H(2)O(2) system, despite the high presence of anions (sulfate 100 mM, phosphate 1 mM) or z.rad;OH scavenger. Not only the rate of 2,4-D removal, but also the decomposition rate of H(2)O(2) and oxalate proportionally increase with light intensity. The ferrioxalate concentration determines the light absorption fraction, and thus, controls the rates of 2,4-D removal, and the decomposition of H(2)O(2) and oxalate, are predicted from kinetic formulations. The optimal concentration of H(2)O(2) and oxalate, according to the extent of the z.rad;OH scavenging reaction with these reagents, has been demonstrated for 2,4-D removal. It was found that an increasing oxalate concentration, which bears the burden of increased dissolved organic carbon (DOC), does not occur. This is because its decomposition, as a result of the photochemical reduction of the ferric oxalate complex, results in a decrease of the equivalent DOC. The importance of the key reaction factors to be considered, when applying this system to real wastewater treatment, is also discussed.     

Hydrogeochemical comparison and effects of overlapping redox zones on groundwater arsenic near the Western (Bhagirathi sub-basin, India) and Eastern (Meghna sub-basin, Bangladesh) margins of the Bengal Basin

Although arsenic (As) contamination of groundwater in the Bengal Basin has received wide attention over the past decade, comparative studies of hydrogeochemistry in geologically different sub-basins within the basin have been lacking. Groundwater samples were collected from sub-basins in the western margin (River Bhagirathi sub-basin, Nadia, India; 90 samples) and eastern margin (River Meghna sub-basin; Brahmanbaria, Bangladesh; 35 samples) of the Bengal Basin. Groundwater in the western site (Nadia) has mostly Ca-HCO(3) water while that in the eastern site (Brahmanbaria) is much more variable consisting of at least six different facies. The two sites show differences in major and minor solute trends indicating varying pathways of hydrogeochemical evolution However, both sites have similar reducing, postoxic environments (p(e): +5 to -2) with high concentrations of dissolved organic carbon, indicating dominantly metal-reducing processes and similarity in As mobilization mechanism. The trends of various redox-sensitive solutes (e.g. As, CH(4), Fe, Mn, NO(3)(-), NH(4)(+), SO(4)(2-)) indicate overlapping redox zones, leading to partial redox equilibrium conditions where As, once liberated from source minerals, would tend to remain in solution because of the complex interplay among the electron acceptors.     

Fine route for an efficient removal of 2,4-dichlorophenoxyacetic acid (2,4-D) by zeolite-supported TiO2

Zeolites HY, Hbeta and HZSM-5 with different physico-chemical properties were chosen as support for TiO2 to illustrate their adsorption, dispersion and electronic structure in photocatalysis. The extent of TiO2 loading was monitored by XRD and BET surface area measurements. The adsorption capacity of HY zeolite was found to be high and hence chosen for further modification to continue the investigation. Photodegradation kinetics were carried out with 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution. The extent of 2,4-D degradation on TiO2/HY loading revealed the importance of adsorption in photocatalysis. Mineralisation studies on all three zeolites with 1 wt.% TiO2 loading demonstrated the good dispersion properties of TiO2/HY. Its photocatalytic activity was found to be excellent with formulated 2,4-D. Comparison of relative photonic efficiencies demonstrated that supported photocatalysts exhibited higher activity than some of the commercial photocatalysts. The high activity of supported TiO2 is due to synergistic effects of improved adsorption of 2,4-D and efficient delocalisation of photogenerated electrons by zeolite support.     

Impacts of COD and DCP loading rates on biological treatment of 2,4-dichlorophenol (DCP) containing wastewater in a perforated tubes biofilm reactor

Biofilm processes offer considerable advantages for biological treatment of chlorophenol containing wastewaters since such industrial effluents are difficult to treat by conventional activated sludge processes. A rotating perforated tubes biofilm reactor (RTBR) was developed and used for treatment of 2,4-dichlorophenol (DCP) containing synthetic wastewater. Effects of COD and DCP loading rates on COD, DCP and toxicity removals were investigated. Percent COD removal decreased and effluent COD increased with increasing COD and DCP loading rates due to toxic effects of high DCP content in the feed. DCP and toxicity removals showed similar trends. As the DCP loading rate increased the effluent DCP content increased yielding high toxicity levels in the effluent. COD and DCP loading rates should be below 90gCODm(-2)d(-1) and 2.8gDCPm(-2)d(-1) in order to obtain more than 90% DCP and toxicity removals. However, DCP loading rates lower than 1gDCPm(-2)d(-1) are required to obtain more than 90% COD removal. Empirical equations were developed to estimate percent COD, DCP and toxicity removals as functions of COD and DCP loading rates. The coefficients of the empirical equations were determined by using the experimental data. Empirical model predictions for percent COD, DCP and toxicity removals were in good agreement with the experimental data.     

Validation of regression models for nitrate concentrations in the upper groundwater in sandy soils

For Dutch sandy regions, linear regression models have been developed that predict nitrate concentrations in the upper groundwater on the basis of residual nitrate contents in the soil in autumn. The objective of our study was to validate these regression models for one particular sandy region dominated by dairy farming. No data from this area were used for calibrating the regression models. The model was validated by additional probability sampling. This sample was used to estimate errors in 1) the predicted areal fractions where the EU standard of 50 mg l⁻¹ is exceeded for farms with low N surpluses (ALT) and farms with higher N surpluses (REF); 2) predicted cumulative frequency distributions of nitrate concentration for both groups of farms.Both the errors in the predicted areal fractions as well as the errors in the predicted cumulative frequency distributions indicate that the regression models are invalid for the sandy soils of this study area.     

Improving the knowledge of pesticide and nitrate transfer processes using age-dating tools (CFC, SF6, H) in a volcanic island (Martinique, French West Indies)

Numerous successful examples of CFC and SF₆ groundwater dating applications were recently published. However the proposed CFC/SF₆ method needs various hydrodynamic parameters that are not always available. In order to predict groundwater-quality trends in areas where the hydrogeological context is poorly known, a dating method using tritium, CFC and SF₆ was successfully implemented in Martinique. Hydrogeological understanding is limited in this volcanic island where groundwater contamination by pesticides and nitrate has been recently proven in various areas. A negative correlation was observed between nitrate concentrations and groundwater ages while pesticide contamination showed a more complex schema. Consequently the presence of old groundwater clearly explained the absence or low pesticide and nitrate concentrations in some areas. However a possible degradation of the water quality is to be feared in the future. In view of the relatively long transfer times and the complexity of the remobilization processes of solutes, the expected effects of any modifications in the use of fertilizers, or of changes in pesticide-use legislation, would take a long time to become apparent.     

Reductive dechlorination of chlorinated methanes in cement slurries containing Fe(II)

Degradative solidification/stabilization (DS/S) is a novel remediation technology that combines chemical degradation with conventional solidification/stabilization. The applicability of the Fe(II)-based DS/S to treating chlorinated alkanes was tested by characterizing degradation reactions of carbon tetrachloride (CT) and its daughter products in cement slurries containing Fe(II). Degradation kinetics of CT and chloroform (CF) were generally very rapid with reaction rates comparable to rates that can be obtained with zero-valent iron. Dechlorination reactions of CT proceeded primarily via a hydrogenolysis pathway, which yielded CF and methylene chloride (MC) as major products and chloromethane and methane as minor products. However, reaction pathways other than hydrogenolysis also appeared to be important at very high pH conditions. MC apparently was resistant to dechlorination reactions over a period of about two months. Kinetics of CT and CF transformation were strongly dependent on pH with an optimal value around 13, which was higher than found previously for PCE. When the initial CF concentration varied between 0.01 and 1 mM, and the Fe(II) dose was 104 mM, pseudo-first-order kinetics generally described the degradation reactions of CF. However, there was also some indication of substrate saturation kinetics in these experiments. This suggests that a saturation model would better describe the kinetics in systems with higher concentration of substrates or lower concentration of the reactive surfaces.     

可渗透反应墙技术的水质净化特性

通过中试研究考察了可渗透反应墙(PRB)技术的动态水质净化特性。结果表明:(1)在动态条件下,PRB中试系统中COD、NH4+-N、NO3--N、TN、TP去除率随水力停留时间(HRT)的延长而增加。当HRT为7.00h时,COD、NH4+-N、TN、TP去除率分别为59.4%、26.9%、76.6%、62.2%、82.0%。HRT的延长使难生物降解的有机物也得到部分降解,PRB中试系统中铁屑及新生态的[H]、铁离子的氧化还原作用可以大幅度提高水的可生化性。(2)PRB技术对COD、NH4+-N、TN、TP的去除主要发生在反应区前半段,而且PRB中试系统中COD、NH4+-N、TN、TP沿程的变化情况可用指数方程来描述,动态模型预测曲线拟合较好。同时,对于实际PRB技术工程,PRB技术选择适当的反应介质及介质配比是关键环节。     

pH dependence of persulfate activation by EDTA/Fe(III) for degradation of trichloroethylene

The ability of free ferrous ion activated persulfate (S₂O₈²⁻) to generate sulfate radicals (SO₄⁻) for the oxidation of trichloroethylene (TCE) is limited by the scavenging of SO₄⁻ with excess Fe²⁺ and a quick conversion of Fe²⁺ to Fe³⁺. This study investigated the applicability of ethylene-diamine-tetra-acetic acid (EDTA) chelated Fe³⁺ in activating persulfate for the destruction of TCE in aqueous phase under pH 3, 7 and 10. Fe³⁺ and EDTA alone did not appreciably degrade persulfate. The presence of TCE in the EDTA/Fe³⁺ activated persulfate system can induce faster persulfate and EDTA degradation due to iron recycling to activate persulfate under a higher pH condition. Increasing the pH leads to increases in pseudo-first-order-rate constants for TCE, S₂O₈²⁻ and EDTA degradations, and Cl generation. Accordingly, the experiments at pH 10 with different EDTA/Fe³⁺ molar ratios indicated that a 1/1 ratio resulted in a remarkably higher degradation rate at the early stage of reaction as compared to results by other ratios. Higher persulfate dosage under the EDTA/Fe³⁺ molar ratio of 1/1 resulted in greater TCE degradation rates. However, increases in persulfate concentration may also lead to an increase in the rate of persulfate consumption.     

Application of nanoscale zero valent iron (NZVI) for groundwater remediation in Europe

Purpose  

Nanoscale zero valent iron (NZVI) is emerging as a new option for the treatment of contaminated soil and groundwater targeting mainly chlorinated organic contaminants (e.g., solvents, pesticides) and inorganic anions or metals. The purpose of this article is to give a short overview of the practical experience with NZVI applications in Europe and to present a comparison to the situation in the USA. Furthermore, the reasons for the difference in technology use are discussed.     

Organochlorine contaminants in coastal marine ecosystems of southern Alaska: inferences from spatial patterns in blue mussels (Mytilus trossulus)

We measured the concentrations and chemical structures of persistent organochlorines (OCs) in blue mussels (Mytilus trossulus) from 44 sites across southwest and southeast Alaska in an effort to determine both the sources of these compounds and the extent to which this region might be contaminated. High PCB concentrations were detected at Amchitka, Adak, and Unalaska Islands (83, 430, and 2800 μg kg⁻¹ dry weight, respectively) in the Aleutians with relatively low concentrations elsewhere (7.1-51 μg kg⁻¹ dry weight). Heavy PCB congener profiles (indicative of localized point sources) characterized the high concentration sites whereas distinctly lighter congener profiles (indicative of atmospheric transport) characterized the lower concentration sites. Elevated PCB concentrations at Adak were restricted to a small area along the island’s eastern shore, suggesting either limited dispersion or rapid dilution of these compounds. More uniform chlorinated pesticide concentrations among the collection sites suggests that these compounds are entering the Aleutian ecosystem from distant sources. Pesticide concentrations correlated significantly with seabird density across the islands we sampled, thus identifying biological transport as a delivery mechanism of these compounds to the Aleutian archipelago. Our findings do not implicate persistent organochlorines as a significant factor in the recent pinniped and sea otter population declines across southwest Alaska.     

Removal of dissolved organic matter in municipal effluent with ozonation, slow sand filtration and nanofiltration as high quality pre-treatment option for artificial groundwater recharge

In the paper the combination process of ozonation, slow sand filtration (SSF) and nanofiltration (NF) was investigated with respect to dissolved organic matter (DOM) removal as high quality pre-treatment option for artificial groundwater recharge. With the help of ozonation leading to breakdown of the large organic molecules, SSF preferentially removes soluble microbial by-product-like substances and DOM with molecular weight (MW) less than 1.0 kDa. NF, however, removes aromatic, humic acid-like and fulvic acid-like substances efficiently and specially removes DOM with MW above 1.0 kDa. The residual DOM of the membrane permeate is dominated by small organics with MW 500 Da, which can be further reduced by the aquifer treatment, despite of the very low concentration. Consequently, the O₃/SSF/NF system offers a complementary process in DOM removal. Dissolved organic carbon (DOC) and trihalomethane formation potential (THMFP) can be reduced from 6.5 ± 1.1 to 0.7 ± 0.3 mg L⁻¹ and from 267 ± 24 to 52 ± 6 μg L⁻¹, respectively. The very low DOC concentration of 0.6 ± 0.2 mg L⁻¹ and THMFP of 44 ± 4 μg L⁻¹ can be reached after the aquifer treatment.     

Assessing the transformation of chlorinated ethenes in aquifers with limited potential for natural attenuation: added values of compound-specific carbon isotope analysis and groundwater dating

The evaluation of biotransformation of chlorinated ethenes (CEs) in contaminated aquifers is challenging when variable redox conditions and groundwater flow regime are limiting factors. By using compound-specific stable carbon isotope analysis (C-CSIA) and ³H-³He based groundwater dating, we assessed three CE-contaminated field sites that differed in groundwater flow velocities, redox conditions, and level of contamination. CE isotopic signatures and carbon isotopic mass balances were applied to quantify CE transformation, whereas groundwater dating allowed determining degradation timescales and assessing hydrodynamic regimes. The combination of these techniques enabled at all field sites to indicate zones within the aquifers where CE dechlorination preferably occurred, sometimes even to metabolites of no toxic concern. However, the natural transformation processes were insufficient to mitigate the entire CE contamination at the studied sites. Such situations of limited transformation are worldwide far more common than sites where optimal natural (mainly redox) conditions are enabling complete CEs degradation. Despite such constraints for natural transformation, this study showed that even under non-favorable biogeochemical CEs degradation, the combination of CSIA and groundwater dating provide valuable information to the understanding of the fate of the CEs, thus, being an important contribution in the definition of efficient remediation measures at any given biogeochemical conditions.     

A novel application of H2O2-Fe(II) process for arsenate removal from synthetic acid mine drainage (AMD) water

Batch experiments were carried out to investigate the influences of H₂O₂/Fe(II) molar ratio, pH, sequence of pH adjustment, initial As(V) concentration, and interfering ions on As(V) removal in H₂O₂-Fe(II) process from synthetic acid mine drainage (AMD). The optimum H₂O₂/Fe(II) molar ratio was one for arsenate removal over the pH range of 4-7. Arsenate removal at pH 3 was poor even at high Fe(II) dosage due to the high solubility of Fe(III) formed in situ. With the increase of Fe(II) dosage, arsenate removal increased progressively before a plateau was reached at pH 5 as arsenate concentration varied from 0.05 to 2.0 mg L⁻¹. However, arsenate removal was negligible at Fe/As molar ratio <3 and then experienced a striking increase before a plateau was reached at pH 7 and arsenate concentration ≥1.0 mg L⁻¹. The co-occurring ions exerted no significant effect on arsenate removal at pH 5. The experimental results with synthetic AMD revealed that this method is highly selective for arsenate removal and the co-occurring ions either improved arsenate removal or slightly depressed arsenate removal at pH 5-7. The extended X-ray absorption fine structure (EXAFS) derived As-Fe length, 3.27-3.30 Å, indicated that arsenate was removed by forming bidentate-binuclear complexes with FeO(OH) octahydra. The economic analysis revealed that the cost of the H₂O₂-Fe(II) process was only 17-32% of that of conventional Fe(III) coagulation process to achieve arsenate concentration below 10 μg L⁻¹ in treated solution. The results suggested that the H₂O₂-Fe(II) process is an efficient, economical, selective and practical method for arsenate removal from AMD.