Microbially mediated reduction of Fe-III and As-V in Cambodian sediments amended with C-13-labelled hexadecane and kerogen

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 C-13-labelled organic substrates. C-13-hexadecane was used as a model for potentially bioavailable long chain n-alkanes and a C-13-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 C-13-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 C-13-hexadecane was degraded during the 8-week incubation. The degradation of C-13-hexadecane was microbially driven, as confirmed by DNA stable isotope probing (DNA-SIP). Amendment with C-13-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 C-13 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.