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 = k(2)(0)[equivalent to Fecys(-)] + k(2)(1)[equivalent to Fecys(0)], where k(2)(0) and k(2)(1) are the corresponding rate constants for the cysteine surface species equivalent to Fecys(-) and equivalent to Fecys(0), respectively. The pi-I-dependent dissolution behaviour of HFO suggested that k(2)(0)[equivalent to Fecys(-)] much greater than k(2)(1) [equivalent to Fecys(0)]. A value of 6.83 x 10(-2) s(-1) as the lower limit for k(2)(0) was obtained. These two surface species were related by the following proton complexation equilibrium expression: equivalent to Fecys(-) + H+ (k-1s) reversible arrow (k1s) equivalent to Fecys(0). A log K-int(s) 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 equivalent to Fecys(-) species was at maximum. Experiments in the presence of phosphate indicated that at equilibrium concentrations as low as 50 mu M, 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. (C) 1997 Academic Press.