Probing the mechanisms of Fe(III)/As(V) reduction and As mobilisation using mineral-coated sands; impact of electron donor treatments.

The release of geogenic arsenic into groundwater, driven by reductive dissolution of Fe(III)/As(V) oxide phases, poses a severe health risk to millions in South and Southeast Asia. However, the microbes and electron donors responsible for the reductive dissolution remain unclear, due to complex a(biotic) interactions in sediments (traditionally used in microcosm incubation studies). In this study, indigenous microbial communities were sampled from arsenic-prone aquifers in Kandal Province, Cambodia, by filtering groundwater through sands coated with Fe(III)/As(V) minerals. This provided a streamlined inocula to study fundamental Fe(III)/As(V) reduction processes in controlled laboratory experiments. Anoxic incubations with contrasting electron donors suggested that biolabile organics are the main drivers of Fe(III) and As(V) reduction in the sampled aquifers, but methane can also contribute to Fe(III) reduction (at a slower rate) in the absence of labile organics. Known Fe(III)-reducing bacteria (e.g. Geobacter and Geothrix) were implicated in Fe(III)/As(V) reduction. Methane-driven Fe(III) reduction appeared to be mediated by proteobacterial methanotrophs (e.g., Methylomonas and Methylosinus), either directly or via symbiotic interactions with Geobacter through labile organic intermediates (suggested by acetate generation) highlighting the flexibility of proteobacterial methanotrophs under anoxic conditions. No methane-driven As(V) reduction was implicated in this study, while nominal As(V) reduction driven by aquatic organics (sorbed from the groundwater during filtration) was evident in control incubations suggesting some decoupling between Fe(III) and As(V) reduction. Furthermore, the sand filtration approach offers a promising method for producing simplified inocula for further studies of microbe-organic-mineral interactions in arsenic-prone aquifers and other complex biogeochemical systems.