A novel sulfidogenic process via sulfur reduction to remove arsenate in acid mine drainage: Insights into the performance and microbial mechanisms

Biological sulfidogenic processes based on sulfate-reducing bacteria (SRB) are not suitable for arsenic (As)-containing acid mine drainage (AMD) treatment because of the formation of the mobile thioarsenite during sulfate reduction. In contrast, biological sulfidogenic processes based on sulfur-reducing bacteria (S0RB) produce sulfide without pH increase, which could achieve more effective As removal than the SRB-based process. However, the reduction ability and toxicity tolerance of S0RB to As remains mysterious, which may substantially affect the practical applicability of this process when treating arsenate (As(V))-containing AMD. Thus, this study aims to develop a biological sulfur reduction process driven by S0RB, and explore its long-term performance on As(V) removal and microbial community evolution. Operating under moderately acidic conditions (pH=4.0), the presence of 10 mg/L As(V) significantly suppressed the activity of S0RB, leading to the failure of As(V) removal. Surprisingly, a drop in pH to 3.0 enhanced the tolerance of S0RB to As toxicity, allowing for efficient sulfide production (396±102 mg S/L) through sulfur reduction. Consequently, effective and stable removal of As(V) (99.9 %) was achieved, even though the sulfidogenic bacteria were exposed to high levels of As(V) (42 mg/L) in long-term trials. Spectral and spectroscopic analysis showed that As-bearing sulfide minerals were present in the bioreactor. Remarkably, the presence of As(V) induced notable changes in the microbial community composition, with Desulfurella and Clostridium identified as predominate sulfur reducers. The qPCR result further revealed an increase in the concentration of functional genes related to As transport (asrA and arsB) in the bioreactor sludge as the pH decreased from 4.0 to 3.0. This suggests the involvement of microorganisms carrying asrA and arsB in an As transport process. Furthermore, metagenomic binning demonstrated that Desulfurella contained essential genes associated with sulfur reduction and As transportation, indicating its genetic potential for sulfide production and As tolerance. In summary, this study underscores the effectiveness of the biological sulfur reduction process driven by S0RB in treating As(V)-contaminated AMD. It offers insights into the role of S0RB in remediating As contamination and provides valuable knowledge for practical applications.

Fundamentals and application in phytoremediation of an efficient arsenate reducing bacterium Pseudomonas putida ARS1

Arsenic is a ubiquitous environmental pollutant. Microbe-mediated arsenic bio-transformations significantly influence arsenic mobility and toxicity. Arsenic transformations by soil and aquatic organisms have been well documented, while little is known regarding effects due to endophytic bacteria. An endophyte Pseudomonas putida ARS1 was isolated from rice grown in arsenic contaminated soil. P. putida ARS1 shows high tolerance to arsenite (As(III)) and arsenate (As(V)), and exhibits efficient As(V) reduction and As(III) efflux activities. When exposed to 0.6 mg/L As(V), As(V) in the medium was completely converted to As(III) by P. putida ARS1 within 4 hr. Genome sequencing showed that P. putida ARS1 has two chromosomal arsenic resistance gene clusters (arsRCBH) that contribute to efficient As(V) reduction and As(III) efflux, and result in high resistance to arsenicals. Wolffia globosa is a strong arsenic accumulator with high potential for arsenic phytoremediation, which takes up As(III) more efficiently than As(V). Co-culture of P. putida ARS1 and W. globosa enhanced arsenic accumulation in W. globosa by 69%, and resulted in 91% removal of arsenic (at initial concentration of 0.6 mg/L As(V)) from water within 3 days. This study provides a promising strategy for in situ arsenic phytoremediation through the cooperation of plant and endophytic bacterium.

Enterobacter sp. E1 increased arsenic uptake in Pteris vittata by promoting plant growth and dissolving Fe-bound arsenic

Microbes affect arsenic accumulation in the arsenic-hyperaccumulator Pteris vittata, but the associated molecular mechanism remains uncertain. Here, we investigated the effect of Enterobacter sp. E1 on arsenic accumulation by P. vittata. Strain E1 presented capacities of arsenate [As(V)] and Fe(III) reduction during cultivation. In the pot experiment with P. vittata, the biomass, arsenic content, and chlorophyll content of P. vittata significantly increased by 30.03%, 74.9%, and 112.1%, respectively. Strikingly, the water-soluble plus exchangeable arsenic (WE-As) significantly increased by 52.05%, while Fe-bound arsenic (Fe-As) decreased by 29.64% in the potted soil treated with strain E1. The possible role of activation of arsenic by strain E1 was subsequently investigated by exposing As(V)-absorbed ferrihydrite to the bacterial culture. Speciation analyses of As showed that strain E1 significantly increased soluble levels of As and Fe and that more As(V) was reduced to arsenite. Additionally, increased microbial diversity and soil enzymatic activities in soils indicated that strain E1 posed few ecological risks. These results indicate that strain E1 effectively increased As accumulation in P. vittata mainly by promoting plant growth and dissolving soil arsenic. Our findings suggest that As(V) and Fe(III)-reducer E1 could be used to enhance the phytoremediation of P. vittata in arsenic-contaminated soils.

Environmental Mn(II) enhances the activity of dissimilatory arsenate-respiring prokaryotes from arsenic-contaminated soils

Many investigations suggest that dissimilatory arsenate-respiring prokaryotes (DARPs) play a key role in stimulating reductive mobilization of As from solid phase into groundwater, but it is not clear how environmental Mn(II) affects the DARPs-mediated reductive mobilization of arsenic. To resolve this issue, we collected soil samples from a realgar tailings-affected area. We found that there were diverse arsenate-respiratory reductase (arr) genes in the soils. The microbial communities had high arsenate-respiring activity, and were able to efficiently stimulate the reductive mobilization of As. Compared to the microcosms without Mn(II), addition of 10 mmol/L Mn(II) to the microcosms led to 23.99%-251.79% increases in the microbial mobilization of As, and led to 133.3%-239.2% increases in the abundances of arr genes. We further isolated a new cultivable DARP, Bacillus sp. F11, from the arsenic-contaminated soils. It completely reduced 1 mmol/L As(V) in 5 days under the optimal reaction conditions. We further found that it was able to efficiently catalyze the reductive mobilization and release of As from the solid phase; the addition of 2 mmol/L Mn(II) led to 98.49%-248.78% increases in the F11 cells-mediated reductive mobilization of As, and 70.6%-104.4% increases in the arr gene abundances. These data suggest that environmental Mn(II) markedly increased the DARPs-mediated reductive mobilization of As in arsenic-contaminated soils. This work provided a new insight into the close association between the biogeochemical cycles of arsenic and manganese.

Anaerobic methane oxidation coupled to arsenate reduction in paddy soils: Insights from laboratory and field studies

Anaerobic methane oxidation (AOM) coupled to nitrate, sulfate and iron has been most extensively studied. Recently, AOM coupled with arsenate reduction (AOM-AsR) was demonstrated in laboratory microcosm incubation, however whether AOM-AsR is active in the field conditions remains elusive. Here, we used 13C-labeled methane (13CH4) to investigate the AOM-AsR process in both anaerobic microcosms and field conditions with identical soils. Our results revealed the occurrence of AOM-AsR in the field, but AOM-AsR in the field was not as active as that which occurred in the laboratory (AOM-AsR contributed approximately 33.87% and 80.76% of total As release in the field and laboratory studies, respectively). This occurred because the laboratory setting provided a more suitable condition for the AOM-AsR process. Moreover, the results suggested that the relative abundance of mcrA from the ANME-2d was the most abundant. Our results clearly demonstrate that the AOM-AsR is active in both the laboratory and field conditions. Moreover, the results highlight the potential risk the AOM-AsR for pose for As contamination in rice paddies.

An Insight into Efflux-Mediated Arsenic Resistance and Biotransformation Potential of Enterobacter Cloacae RSC3 from Arsenic Polluted Area

Indiscriminate discharge of heavy metals/metalloids from different sources into the sustainable agro-ecosystem is a major global concern for food security and human health. Arsenic (As), categorized as group one human carcinogen is a quintessential toxic metalloid that alters the microbial compositions and functions, induce physiological and metabolic changes in plants and contaminate surface/ground water. The management of arsenic toxicity, therefore, becomes imminent. Acknowledging the arsenic threat, the study was aimed at identifying arsenic resistant bacteria and evaluating its arsenic removal/detoxification potential. Of the total 118 bacterial isolates recovered from arsenic rich environment, the bacterial strain RSC3 demonstrating highest As tolerance was identified as Enterobacter cloacae by 16S rRNA gene sequence analysis. Enterobacter cloacae tolerated high concentration (6000 ppm) of As and exhibited 0.55 h-1 of specific growth rate as calculated from growth kinetics data. Strain RSC3 also displayed varying level of resistance to other heavy metals and many antibacterial drugs in plate bioassay. The bacterial strain RSC3 possessed gene (arsC) which causes transformation of arsenate to arsenite. The arsenate uptake and efflux of the bacterial cells was revealed by high throughput techniques such as AAS, SEM/TEM and EDX. The simultaneous As reducing ability, and multi metal/multi-antibiotics resistance potentials of E. cloacae provides a promising option in the microbes based remediation of As contaminated environments.

Effect of speciation and composition on the kinetics and precipitation of arsenic sulfide from industrial metallurgical wastewater

Precipitation of arsenic as As₂S₃ produces little waste sludge, has the potential for low chemical consumption and for selective metal(loid) removal. In this study, arsenic removal from acidic (pH 2), metallurgical wastewater was tested in industrially relevant conditions. Sulfides added at a S:As molar ratio of 2.5 and 5 resulted in removal of 99% and 84% of As(III) and As(V). Precipitation of As₂S₃ from the As(III) and industrial wastewater containing 17% As(V) was nearly instantaneous. For the synthetic As(V) solution, reduction to As(III) was the rate limiting step. At a S:As ratio of 20 and an observed removal rate (k₂ = 4.8 (mol L^-1) h^-1), two hours were required to remove of 93% of arsenic from a 1 g As L^-1 solution. In the case of As(V) in industrial samples this time lag was not observed, showing that components in the industrial wastewater affected the removal and reduction of arsenate. Speciation also affected flocculation and coagulation characteristics of As₂S₃ particles: As(V) reduction resulted in poor coagulation and flocculation. Selective precipitation of arsenic was possible, but depended on speciation, S:As ratio and other metals present.

The biotransformation of arsenic by spent mushroom compost - An effective bioremediation agent

Spent mushroom compost (SMC) is a lignocellulose-rich waste material commonly used in the passive treatment of heavy metal-contaminated environments. In this study, we investigated the bioremediation potential of SMC against an inorganic form of arsenic, examining the individual abiotic and biotic transformations carried out by SMC. We demonstrated, that key SMC physiological groups of bacteria (denitrifying, cellulolytic, sulfate-reducing, and heterotrophic) are resistant to arsenites and arsenates, while the microbial community in SMC is also able to oxidize As(III) and reduce As(V) in respiratory metabolisms, although the SMC did not contain any As. We showed, that cooperation between arsenate and sulfate-reducing bacteria led to the precipitation of As_xS_y. We also found evidence of the significant role organic acids may play in arsenic complexation, and we demonstrated the occurrence of As-binding proteins in the SMC. Furthermore, we confirmed, that biofilm produced by the microbial community in SMC was able to trap As(V) ions. We postulated, that the above-mentioned transformations are responsible for the sorption efficiency of As(V) (up to 25%) and As(III) (up to 16%), as well as the excellent buffering properties of SMC observed in the sorption experiments.

strain PVR-YHB1-1: Characterization and genome analysis

Arsenate (AsV) reduction in bacteria is essential to alleviate their arsenic (As) toxicity. We isolated a Bacillus strain PVR-YHB1-1 from the roots of As-hyperaccumulator Pteris vittata. The strain was efficient in reducing AsV to arsenite (AsIII), but the associated mechanisms were unclear. Here, we investigated its As resistance and reduction behaviors and associated genes at genome level. Results showed that the strain tolerated up to 20 mM AsV. When grown in 1 mM AsV, 96% AsV was reduced to AsIII in 48 h, with its AsV reduction ability being positively correlated to bacterial biomass. Two ars operons arsRacr3arsCDA and arsRKacr3arsC for As metabolisms were identified based on draft genome sequencing and gene annotations. Our data suggested that both operons might have attributed to efficient As resistance and AsV reduction in PVR-YHB1-1, providing clues to better understand As transformation in bacteria and their roles in As transformation in the environment.

The role of arsenic resistant Bacillus aryabhattai MCC3374 in promotion of rice seedlings growth and alleviation of arsenic phytotoxicity

The biological agents have been utilized as an affordable alternative to conventional costly metal remediation technologies for last few years. The present investigation introduces arsenic (As) resistant plant growth promoting rhizobacteria (PGPR) isolated from the As-contaminated agricultural field of West Bengal, India that alleviates arsenic-induced toxicity and exhibited many plant growth promoting traits (PGP). The isolated strain designated as AS6 has identified as Bacillus aryabhattai based on phenotypic characteristics, physio-biochemical tests, MALDI-TOFMS bio-typing, FAME analysis and 16S rDNA sequence homology. The strain found to exhibit five times more resistance to arsenate than arsenite with minimum inhibitory concentrations (MIC) being 100 mM and 20 mM respectively. The result showed that accumulation of As was evidenced by SEM- EDAX, TEM-EDAX studies. The intracellular accumulation of arsenic was also confirmed as in bacterial biomass by AAS, FTIR, XRD and XRF analyses. The increased rate of As (V) reduction by this strain found to be exploited for the remediation of arsenic in the contaminated agricultural field. The strain also found to exhibit important PGP traits viz., ACC deaminase activity (2022 nmol α-ketobutyrate/mg protein/h), IAA production (166 μg/ml), N₂ fixation (0.32 μgN fixed/h/mg proteins) and siderophore production (72%) etc. Positive influenced of AS6 strain on rice seedlings growth promotion under As stress was observed considering the several morphological, biochemical parameters including antioxidants activities as compared with an uninoculated set. Thus this strain might be exploited for stress amelioration and plant growth enhancement of rice cultivar under arsenic spiked agricultural soil.

Characterising microbial reduction of arsenate sorbed to ferrihydrite and its concurrence with iron reduction

A series of model anoxic incubations were performed to understand the concurrence between arsenate and ferrihydrite reduction by Shewanella putrefaciens strain CN-32 at different concentrations of arsenate, ferrihydrite and lactate, and with given ΔG_rxn for arsenate and ferrihydrite reduction in non-growth conditions. The reduction kinetics of arsenate sorbed to ferrihydrite is predominately controlled by the availability of dissolved arsenate, which is measured by the integral of dissolved arsenate concentrations against incubation time and shown to correlate with the first order rate constants. High lactate concentrations slightly slowed down the rate of arsenate reduction due to the competition with arsenate for microbial contact. Under all experimental conditions, simultaneous arsenate and ferrihydrite reduction occurred following addition of S. putrefaciens inoculums and suggested no apparent competition between these two enzymatic reductions. Ferrous ions released from iron reduction might retard microbial arsenate reduction at high arsenate and ferrihydrite concentrations due to formation of ferrous arsenate. At high arsenate to ferrihydrite ratios, reductive dissolution of ferrihydrite shifted arsenate from sorption to dissolution and hence accelerated arsenate reduction. The interaction between microbial arsenate and ferrihydrite reduction did not correlate with ΔG_rxn, but instead was governed by other factors such as geochemical and microbial parameters.

Effect of extracellular electron shuttles on arsenic-mobilizing activities in soil microbial communities

Microbially mediated arsenate (As(V)) and Fe(III) reduction play important roles in arsenic (As) cycling in nature. Extracellular electron shuttles can impact microbial Fe(III) reduction, yet little is known about their effects on microbial As mobilization in soils. In this study, microcosm experiments consisting of an As-contaminated soil and microbial communities obtained from several pristine soils were conducted, and the effects of electron shuttles on As mobilization were determined. Anthraquinone-2,6-disulfonate (AQDS) and riboflavin (RF) were chosen as common exogenous and biogenic electron shuttles, respectively, and both compounds significantly enhanced reductive dissolution of As and Fe. Accumulation of Fe(II)-bearing minerals was also observed, which may lead to re-immobilization of As after prolonged incubation. Interestingly, Firmicutes-related bacteria became predominant in all microcosms, but their compositions at the lower taxonomic level were different in each microcosm. Putative respiratory As(V) reductase gene (arrA) analysis revealed that bacteria closely related to a Clostridia group, especially those including the genera Desulfitobacterium and Desulfosporosinus, might play significant roles in As mobilization. These results indicate that the natural soil microbial community can use electron shuttles for enhanced mobilization of As; the use of this type of system is potentially advantageous for bioremediation of As-contaminated soils.

Phytochelatins play key roles for the difference in root arsenic accumulation of different Triticum aestivum cultivars in comparison with arsenate uptake kinetics and reduction

In the previous studies, we have found that arsenic (As) accumulation in roots of bread wheat (Triticum aestivum L.) seedlings were significantly different among different wheat cultivars, and As(V) tolerant wheat cultivars have much higher capacities of root As accumulation. However, the reason for the difference remains unclear. Four wheat cultivars with high (MM45 and FM8) or low (QF1 and HM29) levels of arsenic (As) accumulation were selected to investigate the relationship between root As(V) uptake kinetics and root As accumulation. MM45 and HM29 were also used to examine As(V) reduction ability and non-protein thiol (cysteine [Cys], glutathione [GSH], and phytochelatins [PCs]) concentrations in wheat seedlings. MM45 had the lowest Michaelis-Menten constant (K_m) and maximum influx rate (V_max). No difference in the K_m values was found among the three other cultivars. No difference in As(V) reduction capacity was observed between MM45 and HM29. GSH and PC₂ were significantly induced by 10 μM As(V) in roots of wheat seedlings, particularly in MM45. Synthesis of GSH and PCs was completely suppressed in the presence of l-buthionine sulfoximine (BSO), a specific inhibitor of γ-glutamylcysteine synthetase. BSO markedly decreased the As tolerance of wheat seedlings and decreased the accumulation of As in roots, but increased As accumulation in shoots. No significant difference in As concentrations was found between MM45 and HM29 under the BSO treatment. GSH and PCs are the reason why As accumulation and As(V) tolerance differ in roots of different wheat cultivars.

Draft genome sequence of Arthrobacter sp. strain B6 isolated from the high-arsenic sediments in Datong Basin, China

Arthrobacter sp. B6 is a Gram-positive, non-motile, facultative aerobic bacterium, isolated from the arsenic-contaminated aquifer sediment in the Datong basin, China. This strain displays high resistance to arsenic, and can dynamically transform arsenic under aerobic condition. Here, we described the high quality draft genome sequence, annotations and the features of Arthrobacter sp. B6. The G + C content of the genome is 64.67%. This strain has a genome size of 4,663,437 bp; the genome is arranged in 8 scaffolds that contain 25 contigs. From the sequences, 3956 protein-coding genes, 264 pseudo genes and 89 tRNA/rRNA-encoding genes were identified. The genome analysis of this strain helps to better understand the mechanism by which the microbe efficiently tolerates arsenic in the arsenic-contaminated environment.

Characterization of Roseomonas and Nocardioides spp. for arsenic transformation

The metalloid arsenic predominantly exists in the arsenite [As(III)] and arsenate [As(V)]. These two forms are respectively oxidized and reduced by microbial redox processes. This study was designed to bioprospect arsenic tolerating bacteria from Lonar lake and to characterize their arsenic redoxing ability. Screening of sixty-nine bacterial species isolated from Lonar lake led to identification of three arsenic-oxidizing and seven arsenic-reducing species. Arsenite oxidizing isolate Roseomonas sp. L-159a being closely related to Roseomonas cervicalis ATCC 49957 oxidized 2mM As(III) in 60h. Gene expression of large and small subunits of arsenite oxidase respectively showed 15- and 17-fold higher expression. Another isolate Nocardioides sp. L-37a formed a clade with Nocardioides ghangwensis JC2055, exhibited normal growth with different carbon sources and pH ranges. It reduced 2mM As(V) in 36h and showed constitutive expression of arsenate reductase which increased over 4-fold upon As(V) exposure. Genetic markers related to arsenic transformation were identified and characterized from the two isolates. Moderate resistance against the arsenicals was exhibited by the two isolates in the range of 1-5mM for As(III) and 1-200mM for As(V). Altogether we provide multiple evidences to indicate that Roseomonas sp. and Nocardioides sp. exhibited arsenic transformation ability.

Comparison of arsenate reduction and release by three As(V)-reducing bacteria isolated from arsenic-contaminated soil of Inner Mongolia, China

Arsenic (As) contamination has become a worldwide environmental problem: arsenite (As(Ⅲ)) especially has posed a major threat to human health. Here, we report the first three isolates of anaerobic As(Ⅴ)-reducing bacterial strains (strains JQ, DJ-3 and DJ-4) from a soil sample containing 48.7% of total As in the form of As(III) collected in Chifeng, Inner Mongolia, China. Strains JQ, DJ-3 and DJ-4 were phylogenetically closely related to Bacillus, Desulfitobacterium and Exiguobacterium, respectively. Among these strains, JQ and DJ-3 have the arsC gene, DJ-4 possesses the arrA gene. The three strains could all resist and reduce high concentrations of As(Ⅴ) under anoxic conditions. The order of resistance to As(Ⅴ) was DJ-3 > JQ > DJ-4. Strain DJ-3 not only possesses the strongest resistance to As(Ⅴ) but could also reduce 53% of the As(Ⅴ) to As(III) in the treatment of 60 mM As(Ⅴ) in 5 d. All three strains could release As from goethite; strain DJ-4 has the highest ability to promote the release of As (90.5%) from goethite. These results suggested that strains JQ, DJ-3 and DJ-4 may play an important role in the mobilization and transformation of As in soil.

An arsenate-reducing and alkane-metabolizing novel bacterium, Rhizobium arsenicireducens sp. nov., isolated from arsenic-rich groundwater

A novel arsenic (As)-resistant, arsenate-respiring, alkane-metabolizing bacterium KAs 5-22^T, isolated from As-rich groundwater of West Bengal was characterized by physiological and genomic properties. Cells of strain KAs 5-22^T were Gram-stain-negative, rod-shaped, motile, and facultative anaerobic. Growth occurred at optimum of pH 6.0-7.0, temperature 30 °C. 16S rRNA gene affiliated the strain KAs 5-22^T to the genus Rhizobium showing maximum similarity (98.4 %) with the type strain of Rhizobium naphthalenivorans TSY03b^T followed by (98.0 % similarity) Rhizobium selenitireducens B1^T. The genomic G + C content was 59.4 mol%, and DNA-DNA relatedness with its closest phylogenetic neighbors was 50.2 %. Chemotaxonomy indicated UQ-10 as the major quinone; phosphatidylethanolamine, phosphatidylglycerol, and diphosphatidylglycerol as major polar lipids; C_16:0, C_17:0, 2-OH C_10:0, 3-OH C_16:0, and unresolved C_18:1 ɷ7C/ɷ9C as predominant fatty acids. The cells were found to reduce O₂, As⁵⁺, NO₃^-, SO₄^2- and Fe³⁺ as alternate electron acceptors. The strain's ability to metabolize dodecane or other alkanes as sole carbon source using As⁵⁺ as terminal electron acceptor was supported by the presence of genes encoding benzyl succinate synthase (bssA like) and molybdopterin-binding site (mopB) of As⁵⁺ respiratory reductase (arrA). Differential phenotypic, chemotaxonomic, genotypic as well as physiological properties revealed that the strain KAs 5-22^T is separated from its nearest recognized Rhizobium species. On the basis of the data presented, strain KAs 5-22^T is considered to represent a novel species of the genus Rhizobium, for which the name Rhizobium arsenicireducens sp. nov. is proposed as type strain (=LMG 28795^T=MTCC 12115^T).

Arsenic redox transformation by Pseudomonas sp. HN-2 isolated from arsenic-contaminated soil in Hunan, China

A mesophilic, Gram-negative, arsenite[As(III)]-oxidizing and arsenate[As(V)]-reducing bacterial strain, Pseudomonas sp. HN-2, was isolated from an As-contaminated soil. Phylogenetic analysis based on 16S rRNA gene sequencing indicated that the strain was closely related to Pseudomonas stutzeri. Under aerobic conditions, this strain oxidized 92.0% (61.4μmol/L) of arsenite to arsenate within 3hr of incubation. Reduction of As(V) to As(III) occurred in anoxic conditions. Pseudomonas sp. HN-2 is among the first soil bacteria shown to be capable of both aerobic As(III) oxidation and anoxic As(V) reduction. The strain, as an efficient As(III) oxidizer and As(V) reducer in Pseudomonas, has the potential to impact arsenic mobility in both anoxic and aerobic environments, and has potential application in As remediation processes.

Biomineralization of arsenate to arsenic sulfides is greatly enhanced at mildly acidic conditions

Arsenic (As) is an important water contaminant due to its high toxicity and widespread occurrence. Arsenic-sulfide minerals (ASM) are formed during microbial reduction of arsenate (As(V)) and sulfate (SO4(2-)). The objective of this research is to study the effect of the pH on the removal of As due to the formation of ASM in an iron-poor system. A series of batch experiments was used to study the reduction of SO4(2-) and As(V) by an anaerobic biofilm mixed culture in a range of pH conditions (6.1-7.2), using ethanol as the electron donor. Total soluble concentrations and speciation of S and As were monitored. Solid phase speciation of arsenic was characterized by x-ray adsorption spectroscopy (XAS). A marked decrease of the total aqueous concentrations of As and S was observed in the inoculated treatments amended with ethanol, but not in the non-inoculated controls, indicating that the As-removal was biologically mediated. The pH dramatically affected the extent and rate of As removal, as well as the stoichiometric composition of the precipitate. The amount of As removed was 2-fold higher and the rate of the As removal was up to 17-fold greater at pH 6.1 than at pH 7.2. Stoichiometric analysis and XAS results confirmed the precipitate was composed of a mixture of orpiment and realgar, and the proportion of orpiment in the sample increased with increasing pH. The results taken as a whole suggest that ASM formation is greatly enhanced at mildly acidic pH conditions.

Microbiology of inorganic arsenic: From metabolism to bioremediation

Arsenic (As) contamination of drinking water and soils poses a threat to a large number of people worldwide, especially in Southeast Asia. The predominant forms of As in soils and aquifers are inorganic arsenate [As(V)] and arsenite [As(III)], with the latter being more mobile and toxic. Thus, redox transformations of As are of great importance to predict its fate in the environment, as well as to achieve remediation of As-contaminated water and soils. Although As has been recognized as a toxic element, a wide variety of microorganisms, mainly bacteria, can use it as an electron donor for autotrophic growth or as an electron acceptor for anaerobic respiration. In addition, As detoxification systems in which As is oxidized to the less toxic form or reduced for subsequent excretion are distributed widely in microorganisms. This review describes current development of physiology, biochemistry, and genomics of arsenic-transforming bacteria. Potential application of such bacteria to removal of As from soils and water is also highlighted.