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The formation and activity of an As(III)-oxidising biofilm in a bioreactor, using pozzolana as bacterial growth support, was studied for the purpose of optimising fixed-bed bioreactors for bioremediation. After 60 days of continuous functioning with an As(III)-contaminated effluent, the active biofilm was found to be located mainly near the inflow rather than homogeneously distributed. Biofilm development by the CAsO1 bacterial consortium and by Thiomonas arsenivorans was then studied both on polystyrene microplates and on pozzolana. Extra-cellular polymeric substances (EPS) and yeast extract were found to enhance bacteria attachment, and yeast extract also appears to increase the kinetics of biofilm formation. Analysis of proteins, sugars, lipids and uronic acids indicate that sugars were the main EPS components. The specific As(III)-oxidase activity of T. arsenivorans was higher (by ninefold) for planktonic cells than for sessile ones and was induced by As(III). All the results suggest that the biofilm structure is a physical barrier decreasing As(III) access to sessile cells and thus to As(III)-oxidase activity induction. The efficiency of fixed-bed reactors for the bioremediation of arsenic-contaminated waters can be thus optimised by controlling different factors such as temperature and EPS addition and/or synthesis to increase biofilm density and activity.

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A process for the precipitation of trivalent arsenic sulphide in sulphate-reducing condition at low pH would be very attractive due to the high arsenic content (60%) in the final precipitate. A bacterial consortium able to reduce sulphate at pH 4.5 served to inoculate a column bioreactor that was continuously fed with As(V) or As(III), glycerol as energy source, at pH values between 2 and 5. The best efficiency, in terms of residual As concentration in the outlet, was obtained with the lowest feed pH, i.e. pH 2. However, in these conditions, the bacterial activity inside the bioreactor was isolated in higher pH zones. The diversity, functionality and evolution of the consortium colonizing the bioreactor were characterized by means of biomolecular tools, in relation with operating parameters (pH, As, sulphide). Fermentative and sulphate-reducing bacteria were present both in the liquid phase and onto the filling material. All sulphate-reducers that were detected belonged to the Desulfosporosinus genus. Spectroscopic analyses of the solid phases that precipitated into the bioreactor revealed the presence of amorphous precursors of orpiment of nano-size.