Characterisation of dissolved thioarsenite complexes and precipitated arsenic species in a low-pH sulphate-reducing bioreactor

Some mining effluents and mine drainage waters are acidic and highly polluted with arsenic. A bio-process performing the precipitation of arsenic sulphide would be very attractive, because the final solid waste would contain up to 60% arsenic in weight. In the framework of Biomine European Integrated project (contract NMP2-CT-2005-500329), a bacterial population able to reduce sulphate at pH 4.5 was selected and used to inoculate column bioreactors that were continuously fed with arsenic in acidic conditions. Arsenic precipitates with sulphide in acidic and low-sulphide solutions. With excess sulphide, soluble thioarsenite species are expected to form.The present work showed that sulphate-reducing bacteria can mediate the precipitation of arsenic sulphide in acidic conditions. The main part of the precipitate is composed of amorphous orpiment, however crisrtallised phases could be detected. The new method developed here to analyse of thioarsenite species in solution will facilitate the monitoring of anaerobic bioreactors treating arseniccontaining effluents and will allow the biogeochemical modelling of such systems.