Reductive dissolution of biogenic manganese oxides in the presence of a hydrated biofilm

The chemical reactivity of the biogenic Mn oxide produced by Pseudomonas putida strain MnB1 was assessed in the presence of bacterial cells and biofilm through reductive dissolution experiments at pH 4.4, 5.2, and 6.0. The reactivity of the biogenic Mn oxide was compared to that of a synthetic (chemically produced) birnessite at pH 6.0. In addition, the effect of bacterial cells and biofilm on the reactivity of the synthetic birnessite was examined at pH 6.0. One-to-one (1:1) mole per mole reactions between total ascorbic acid species, [asc]_TOT, and Mn(IV), and 1:2 mole per mole reactions between [asc]_TOT and Mn(IV) were proposed to explain the observed deviation from the oxidation-reduction reaction stoichiometry, Mn(IV)O_2(s) + [asc]_TOT ⇒ Mn(II)_(aq) + products. The observed stoichiometry of the oxidation-reduction reactions was consistent with the known average oxidation number of Mn in the synthetic birnessite studied. The average oxidation number of Mn in the biogenic Mn oxide, calculated from the molar ratio of [asc]_TOT oxidized to Mn dissolved, was in agreement with previous measurements of Mn average oxidation number (3.91-4.00). The rate coefficient for reductive dissolution of synthetic birnessite was equal to that of the biogenic Mn oxide at pH 6.0. The rate coefficient for reductive dissolution of the biogenic Mn oxide was found to decrease with solution pH. The bacterial cells and biofilm interfered with the net reductive dissolution of synthetic birnessite, perhaps by intercepting the Mn_(aq)⁺² reaction product.