No abstract

Microbes may play a key role in the mobilization of arsenic present in elevated concentrations within the aquifers extensively exploited for irrigation and drinking water in West Bengal, Bangladesh, and in other regions of South‐East Asia. Microcosm experiments using Cambodian sediments (which are also representative of other similar reducing aquifers containing arsenic‐rich waters) show that arsenic release and iron reduction are microbially mediated and demonstrate that the type of organic matter present, not necessarily the total abundance of organic matter, is important in controlling the rate and magnitude of microbially mediated arsenic release from these aquifer sediments. The possible role of naturally occurring petroleum in stimulating this process is also demonstrated. In addition to acting as an electron donor, certain types of organic matter may accelerate arsenic release by acting as an electron shuttle, indicating a dual role for organic matter in the process. The results also suggest that the fine‐grained sediment regions of these aquifers are particularly vulnerable to accelerated arsenic release following the introduction of labile organic carbon.

Poorly crystalline Fe(III) oxyhydroxides, ubiquitously distributed as mineral coatings and discrete particles in aquifer sediments, are well-known hosts of sedimentary As. Microbial reduction of these phases is widely thought to be responsible for the genesis of As-rich reducing groundwaters found in many parts of the world, most notably in Bangladesh and West Bengal, India. As such, it is important to understand the behavior of As associated with ferric oxyhydroxides during the early stages of Fe(lll) reduction. We have used X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) to elucidate the changes in the bonding mechanism of As(III) and As(V) as their host Fe(III) oxyhydroxide undergoes bacterially induced reductive transformation to magnetite. Two-line ferrihydrite, with adsorbed As(III) or As(V), was incubated under anaerobic conditions in the presence of acetate as an electron donor, and Geobacter sulfurreducens, a subsurface bacterium capable of respiring on Fe(lll), but not As(V). In both experiments, no increase in dissolved As was observed during reduction to magnetite (complete upon 5 days incubation), consistent with our earlier observation of As sequestration by the formation of biogenic Fe(III)-bearing minerals. XAS data suggested that the As bonding environment of the As(III)-magnetite product is indistinguishable from that obtained from simple adsorption of As(lll) on the surface of biogenic magnetite. In contrast, reduction of As(V)-sorbed ferrihydrite to magnetite caused incorporation of As5+ within the magnetite structure. XMCD analysis provided further evidence of structural partitioning of As5+ as the small size of the As5+ cation caused a distortion of the spinel structure compared to standard biogenic magnetite. These results may have implications regarding the species-dependent mobility of As undergoing anoxic biogeochemical transformations, e.g., during early sedimentary diagenesis.

Previous work has shown that microbial communities in As-mobilizing sediments from West Bengal were dominated by Geobacter species. Thus, the potential of Geobacter sulfurreducens to mobilize arsenic via direct enzymatic reduction and indirect mechanisms linked to Fe(III) reduction was analyzed. G. sulfurreducens was unable to conserve energy for growth via the dissimilatory reduction of As(V), although it was able to grow in medium containing fumarate as the terminal electron acceptor in the presence of 500 M As(V). There was also no evidence of As(III) in culture supernatants, suggesting that resistance to 500 M As(V) was not mediated by a classical arsenic resistance operon, which would rely on the intracellular reduction of As(V) and the efflux of As(III). When the cells were grown using soluble Fe(III) as an electron acceptor in the presence of As(V), the Fe(II)-bearing mineral vivianite was formed. This was accompanied by the removal of As, predominantly as As(V), from solution. Biogenic siderite (ferrous carbonate) was also able to remove As from solution. When the organism was grown using insoluble ferrihydrite as an electron acceptor, Fe(III) reduction resulted in the formation of magnetite, again accompanied by the nearly quantitative sorption of As(V). These results demonstrate that G. sulfurreducens, a model Fe(III)-reducing bacterium, did not reduce As(V) enzymatically, despite the apparent genetic potential to mediate this transformation. However, the reduction of Fe(III) led to the formation of Fe(II)-bearing phases that are able to capture arsenic species and could act as sinks for arsenic in sediments.The mobilization of arsenic from sediments to drinking water constitutes a major toxic hazard to millions in Bangladesh and West Bengal. A number of mechanisms have been proposed for the release of arsenic into the groundwater in Bengal shallow alluvial sedimentary aquifers (1,3,8,11,12,18,21,(33)(34)(35)39), including the oxidation of arsenic-rich pyrite in aquifer sediments, driven by lowering of the water level by abstraction, and then penetration of the aquifer by oxygen (8,11,12), or the reductive dissolution of arsenic-rich iron-oxyhydroxides, driven by the microbial consumption of sedimentary organic matter in anoxic groundwater (33,34,39). The latter mechanism has received recent support as the dominant mechanism for groundwater arsenic contamination (3,20,21,39).In a recent microcosm-based study (21), we provided the first direct evidence of the role of indigenous metal-reducing bacteria in the formation of toxic, mobile As(III) in sediment from the Ganges Delta. The study showed that addition of acetate to anaerobic sediments, as a proxy for organic matter and a potential electron donor for metal reduction, resulted in stimulation of the microbial reduction of Fe(III), followed by As(V) reduction and release of As(III). Culture-dependent techniques confirmed a role for Fe(III)-reducing bacteria in As release, while PCR studies showed that the microbial communities in these sediments were ...

The health of millions is threatened by the use of groundwater contaminated with sediment-derived arsenic for drinking water and irrigation purposes in Southeast Asia. The microbial reduction of sorbed As(V) to the potentially more mobile As(III) has been implicated in release of arsenic into groundwater, but to date there have been few studies of the microorganisms that can mediate this transformation in aquifers. With the use of stable isotope probing of nucleic acids, we present evidence that the introduction of a proxy for organic matter ( 13 C-labeled acetate) stimulated As(V) reduction in sediments collected from a Cambodian aquifer that hosts arsenic-rich groundwater. This was accompanied by an increase in the proportion of prokaryotes closely related to the dissimilatory As(V)-reducing bacteria Sulfurospirillum strain NP-4 and Desulfotomaculum auripigmentum. As(V) respiratory reductase genes (arrA) closely associated with those found in Sulfurospirillum barnesii and Geobacter uraniumreducens were also detected in active bacterial communities utilizing 13 C-labeled acetate in microcosms. This study suggests a direct link between inputs of organic matter and the increased prevalence and activity of organisms which transform As(V) to the potentially more mobile and thus hazardous As(III) via dissimilatory As(V) reduction.Arsenic poisoning of groundwater used for drinking and irrigation is a global issue, with the risk of harmful human exposure occurring at numerous locations across the Americas, Asia (most notably West Bengal and Bangladesh [7,33,34]), and also central Europe (16). Many recent studies have reported arsenic-enriched groundwater within the Ganges-Brahmaputra-Meghna Delta (e.g., see references 3 and 36), with more than 35 million people at risk of arsenic poisoning in Bangladesh alone (34). The weathering of arsenic-rich minerals prevalent in the Himalayas and their gradual transport and deposition in the alluvial deltas below, followed by microbially mediated arsenic solubilization, are thought to be major mechanisms of arsenic mobilization into aquifers within the region. Conditions similarly conducive to the development of arsenic-enriched groundwater are thought to be present within the Red River (4) and Mekong River (28) deltas of Southeast Asia, where elevated concentrations of arsenic have also recently been reported. The present study focuses on the potential causes of changes in arsenic mobility within subsurface sediments taken from the Mekong River Basin, Cambodia, where many tens of thousands of inhabitants could be at risk of exposure to hazardous levels of arsenic (28).The mechanism of arsenic release from aquifer sediments has been a topic of intense academic debate (2,22,26,33,43). However, a consensus is developing around the concept of microbially mediated release of arsenic from sediment-bound hydrated ferric oxides as the dominant mechanism of mobilization into groundwater systems of the Ganges Delta (2, 12, 13). These microbial processes may be sustained by predominantly s...

No abstract

High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal-reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters.

International audienceArsenic concentrations in shallow, reducing groundwaters in Bengal, Southeast Asia, and elsewhere constitute a major hazard to the health of people using these waters for drinking, cooking, or irrigation. A comparison of occurrences in the Ganges-Brahmaputra, Mekong, and Red River basins shows that common geological characteristics include (1) river drainage from the rapidly weathering Himalayas, (2) rapidly buried organic-bearing and relatively young (ca. Holocene) sediments, and (3) very low, basin-wide hydraulic gradients. Anaerobic microbial respiration, utilizing either sedimentary or surface-derived organic carbon, is one important process contributing to the mobilization of arsenic from host minerals, notably hydrous iron oxides. In spite of the paucity of data from before the extensive development of groundwater pumping in these areas, there is sufficient evidence to make a prima facie case that human activity might exacerbate arsenic release into these groundwaters. The difficulties in implementing comprehensive groundwater remediation suggest serious attention should be given to developing treatment technologies for alternative surface-water supplies