Decreased leachability of arsenic linked to biological oxidation of As(III) in solid wastes from bioleaching liquors

Bioleaching of gold-bearing sulfidic ore produces liquors containing several grams per litre of arsenic, with a significant proportion of As(III). The stability of As in the solids wastes resulting from lime treatment of bioleaching effluents increases with the As(V)/As(III) ratio. Biological oxidation of As(III) can be applied during and/or after lime treatment. This biological oxidation step decreases significantly the leachability of the final solid wastes. When the As(III) bio-oxidation was performed during lime treatment, the total As released after one month of leaching was decreased by 99%, between pH 3.6 and pH 5.2, and by 90% between pH 5.2 and pH 6.2. When As(III)-oxidising bacteria were added after lime treatment, the decrease of As leachability linked to bacterial activity was rapid and durable at pH 4. At pH 7, the behaviour of inoculated and not inoculated systems was similar after 11 days of contact. Our results suggest that bacterial activity has been maintained for a long time after inoculation, during the course of the leaching experiments, and that this persistence of As(III)-oxidising process favoured a long-term stability of wastes.

Research Highlights
► Inoculated bacteria oxidise AsIII during lime treatment of bioleaching liquor.
► Inoculation during lime treatment decreases leachability of As from solid wastes.
► Inoculation after lime treatment decreases leachability of As from solid wastes.
► Bacterial AsIII-oxidising activity is maintained during leaching experiments.