The effect of zinc on the biological reduction of hematite

by Stone, James J.

The impact of zinc on the reductive dissolution of hematite (?-Fe2O3) by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens strain CN32 was studied. Experiments were conducted with a suspension of hematite (2.0 g L -1) in 10 mM PIPES (pH 6.8) and H2 as an electron donor under non-growth conditions (108 cell mL -1). Experiments were also conducted with ferric citrate (2 mM) and nitrate (20 mg L -1 NO3-N) to evaluate the effect of zinc with soluble electron acceptors. The net effect of zinc was measured based upon the change of rate or extent electron acceptor consumption. To further our understanding, additional hematite bioreduction experiments were performed using anthraquinone-2,6-disulfonate (AQDS), a soluble electron shuttling agent, ferrozine, a strong Fe(II) complexant, and natural organic material (NOM). All amendments were found to increase zinc inhibition compared to noamendment biotic controls. Ferrozine and AQDS addition decreased Fe(II) sorption and increased zinc sorption. Increased zinc inhibition with ferrozine was a result of complexation of surface bound Fe(II) that subsequently allowed additional zinc sorption to cell and hematite surfaces. AQDS addition also increased surface sorbed zinc. Increased zinc inhibition during NOM addition was attributed both the Me(II) complexation capacity and the surface sorption affinity of the NOM. Taken together, the results show that surface sorbed zinc was a more potent inhibitor of hematite bioreduction than free zinc, especially in the presence of the amendments studied. iv