Reductive Dissolution of Fe(III) (Hydr)oxides by Cysteine: Kinetics and Mechanism

Cysteine was used as a model reductant to gain further insight into the kinetics of bacterially mediated iron reduction. Our experimental data and modeling results indicated that the reductive dissolution of hydrous ferric oxide (HFO) takes place via surface complex formation of cysteine and corresponds to the rate law d[Fe(II)]/dt = k20[ identical withFecys-] + k21[ identical withFecys0], where k20 and k21 are the corresponding rate constants for the cysteine surface species identical withFecys- and identical withFecys0, respectively. The pH-dependent dissolution behavior of HFO suggested that k20[ identical withFecys-] >> k21[ identical withFecys0]. A value of 6.83 x 10(-2) s-1 as the lower limit for k20 was obtained. These two surface species were related by the following proton complexation equilibrium expression: identical withFecys- + H+ right arrow over left arrow ks-1ks1 identical withFecys0. A log Kints value of 7.5 was estimated for this equilibrium relationship, indicating a reduction of 2.8 pH units in the acidity constant of cysteine's amino group, following adsorption onto HFO. The reductive dissolution rate of HFO exhibited a maximum of 3.3 x 10(-8) mol s-1 m-2 at pH 8.3, corresponding to the pH value where the concentration of identical withFecys- species was at maximum. Experiments in the presence of phosphate indicated that at equilibrium concentrations as low as 50 μM, this ligand brings about more than a sixfold reduction in the rate of dissolution of HFO by cysteine. Dissolution experiments with other iron oxide phases showed the following order for the reductive dissolution rates: HFO > lepidocrocite > goethite. Copyright 1997 Academic Press. Copyright 1997Academic Press