Release of arsenite, arsenate and methyl-arsenic species from streambed sediment affected by acid mine drainage: a microcosm study

The (bio-)geochemical processes driving As mobilisation from streambed sediments affected by acid mine drainage (AMD) were investigated, and the structure of the bacterial community associated with the sediments was characterised. Microcosm experiments were set up to determine the effect of oxygen, temperature (4 and 20 degrees C) and microbial activity on As mobilisation from contrasting sediments collected during high- (November 2011) and low- (March 2012) flow conditions in the Amous River, that received AMD. Distinct bacterial communities thrived in the two sediments, dominated by Rhodobacter spp., Polaromonas spp. and Sphingomonads. These communities included only few bacteria known for their capacity to interact directly with As, whereas biogeochemical processes appeared to control As cycling. Major As mobilisation occurred in the As-III form at 20 degrees C in anoxic conditions, from both November and March sediments, as the result of successive biotic reductive dissolution of Mn- and Fe-oxyhydroxides. The later process may be driven by Mn- and Fe-reducing bacteria such as Geobacter spp. and possibly occurred in combination with microbially mediated As-V reduction. The involvement of other bacteria in these redox processes is not excluded. Biomethylation occurred only with the sediments collected at low-flow during oxic and anoxic conditions, although no bacteria characterised so far for its ability to methylate As was identified. Finally, sorption equilibrium of As-V onto the sediment appeared to be the main process controlling As-V concentration in oxic conditions. Comparison with field data shows that the later process, besides biomethylation, may be of relevance to the As fate in AMD-affected streams.