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

Abstract

Environmental context. The use of groundwater with elevated concentrations of arsenic for drinking, cooking or irrigation has resulted in the worst mass poisoning in human history. This study shows that organic compounds that can be found in arsenic rich subsurface sediments may be used by indigenous microorganisms, contributing to the release of arsenic from the sediments into the groundwater. This study increases our understanding of the range of organic substrates (and their sources) that can potentially stimulate arsenic mobilisation into groundwaters.Abstract. Microbial activity is generally accepted to play a critical role, with the aid of suitable organic carbon substrates, in the mobilisation of arsenic from sediments into shallow reducing groundwaters. The nature of the organic matter in natural aquifers driving the reduction of As V to As III is of particular importance but is poorly understood. In this study, sediments from an arsenic rich aquifer in Cambodia were amended with two 13 C-labelled organic substrates. 13C-hexadecane was used as a model for potentially bioavailable long chain n-alkanes and a 13 C-kerogen analogue as a proxy for non-extractable organic matter. During anaerobic incubation for 8 weeks, significant Fe III reduction and As III mobilisation were observed in the biotic microcosms only, suggesting that these processes were microbially driven. Microcosms amended with 13 C-hexadecane exhibited a similar extent of Fe III reduction to the non-amended microcosms, but marginally higher As III release. Moreover, gas chromatography-mass spectrometry analysis showed that 65 % of the added 13 C-hexadecane was degraded during the 8-week incubation. The degradation of 13 C-hexadecane was microbially driven, as confirmed by DNA stable isotope probing (DNA-SIP). Amendment with 13 C-kerogen did not enhance Fe III reduction or As III mobilisation, and microbial degradation of kerogen could not be confirmed conclusively by DNA-SIP fractionation or 13 C incorporation in the phospholipid fatty acids. These data are, therefore, consistent with the utilisation of long chain n-alkanes (but not kerogen) as electron donors for anaerobic processes, potentially including Fe III and As V reduction in the subsurface.

Biomarker-indicated extent of oxidation of plant-derived organic carbon (OC) in relation to geomorphology in an arsenic contaminated Holocene aquifer, Cambodia

“…The correlation between grainsize and CPI shows that sandy sediments primarily contain thermally mature OC which previous studies have shown to be bioavailable but at relatively low concentrations 12–14,16,17,50 . The results shown in Fig.…”

“…Previous studies have already shown that thermally mature (petroleum-derived) OC exists in the sands and is bioavailable, yet the concentrations are very low 11–14,16,17,50 . These results show that a much greater reservoir of plant derived OC exists in the clays both from the early Holocene (>7,000 years BP) and more recent Holocene (<2,000 years BP).…”

Microbial Community Structure and Arsenic Biogeochemistry in Two Arsenic-Impacted Aquifers in Bangladesh

“…Thus, there is a consensus that arsenic is naturally released from sediments to water under microbially induced reducing conditions within the aquifer sediments ( 10 – 12 ). Furthermore, previous studies suggest that the biogeochemical cycling of iron and arsenic, specifically the reduction of mineral assemblages containing Fe(III) and sorbed arsenic(V), plays a critical role in the mobilization of arsenic in aquifers ( 13 – 18 ). Although most of these studies have been based on “lab microcosm” incubations, the precise mechanisms of arsenic release in situ remain poorly constrained.…”

“…Although a wide range of organisms carry the arsenic resistance operon, including many that are not implicated in arsenic mobilization, a narrow distribution of organisms can respire As(V) ( 27 – 29 ). Laboratory incubations (or “microcosms”) using sediments supplied with the addition of 13 C-labeled carbon sources have also suggested that the expression of arrA could be an important factor in controlling the high concentrations of arsenic in aquifers ( 14 , 30 – 33 ). However, only a few studies have examined arsenic reduction by microorganisms in the aquifers with geogenic arsenic via the direct analysis of the field samples, which lack exogenous carbon sources and exhibit lower rates of metabolism ( 34 , 35 ).…”

Foreword to the Research Front on ‘Arsenic Biogeochemistry and Health'

“…[4] Wovkulich et al investigated how the injection of oxalic acid can enhance the remediation of arsenic at an arsenic-contaminated field site [5] whereas Ruzoulis et al used an approach employing 13 C-labelled organic compounds to evaluate the reduction of Fe III and As V in arsenic contaminated sediments. [6] The second part of this Research Front deals with studies on arsenic removal and the prediction of arsenic contamination in the environment. Corsini et al studied the effectiveness of different sorbents and biological oxidation on the removal of arsenic from groundwater, [7] whereas Voegelin et al [8] and Wenk et al [9] show results for arsenic removal from water with two different kinds of household filters.…”