Arsenic release and attenuation in low organic carbon aquifer sediments from West Bengal

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.

Microbially mediated reduction of FeIII and AsV in Cambodian sediments amended with 13C-labelled hexadecane and kerogen

“…2), confirming that these processes are biologically driven, as shown previously. [19,20,22] The 13 C-hexadecane-amended microcosms showed similar levels of Fe III reduction and a noticeable increase in As III release into the porewaters, in comparison to the 'sediment only' incubations (Fig. 2).…”

“…C 17 , C 18 , C 19 and C 20 ) with concentrations between 1.3 and 66 mg g À1 of sediment. The background sedimentary content of n-alkanes was 3.7 mg g À1 of sediment (sum of C 20 to C 34 ; data not shown).…”

“…[20,24] As V reduction was shown repeatedly in a series of microcosm studies set up with sediments from arsenic affected aquifers, where simple organic proxies such as acetate or lactate were used to stimulate microbial mobilisation of arsenic. [15,19,20,22,25,26] Molecular studies in some of these systems revealed the presence of known Fe III and As V respiring bacteria, as well as arsenate respiratory reductase gene sequences. [19,[25][26][27] Nevertheless, the identification of the active members of the microbial communities in these complex sedimentary environments is challenging, especially when investigating specific microbe-substrate interactions.…”

“…[15,19,20,22,25,26] Molecular studies in some of these systems revealed the presence of known Fe III and As V respiring bacteria, as well as arsenate respiratory reductase gene sequences. [19,[25][26][27] Nevertheless, the identification of the active members of the microbial communities in these complex sedimentary environments is challenging, especially when investigating specific microbe-substrate interactions. Thus, in some studies, the use of 13 C-labelled organic carbon substrates (mainly acetate) in combination with molecular microbiology techniques (DNA stable isotope probing (DNA-SIP), followed by DNA fractionation and 16S rRNA gene cloning and sequencing) have allowed the identification of the active members of the microbial community during As III mobilisation.…”

“…[11][12][13][14][15][16][17][18] One of these processes, microbial reduction of As V , is widely accepted as being an important mechanism of As III release in shallow reducing aquifers. [19][20][21][22][23] Bioavailable organic carbon serves both as a carbon source and electron donor for the microbial respiratory reduction of sedimentary As V , which serves as an electron acceptor, resulting in the release of As III , which is more mobile, to the aqueous phase. [20,24] As V reduction was shown repeatedly in a series of microcosm studies set up with sediments from arsenic affected aquifers, where simple organic proxies such as acetate or lactate were used to stimulate microbial mobilisation of arsenic.…”