Kinetic analysis of the bacterial reduction of goethite

The kinetics of dissimilatory reduction of goethite (alpha -FeOOH) was studied in batch cultures of a groundwater bacterium, Shewanella putrefaciens, strain CN32 in pH 7 bicarbonate buffer. The rate and extent of goethite reduction were measured as a function of electron acceptor (goethite) and donor (lactate) concentrations. Increasing goethite concentrations increased both the rate and extent of Fe(lll) reduction when cell and lactate concentrations were held constant. However, constant initial reduction rates were observed after normalization to the Fe(ll) sorption capacity of FeOOH, suggesting that the bacterial reduction rate was first order with respect to surface site concentration. Increasing the lactate concentration also increased the rate and extent of FeOOH reduction. Monod-type kinetic behavior was observed with respect to lactate concentration. Fe(ll) sorption on FeOOH was well-described by the Langmuir sorption isotherm. However, the Fe(ll) sorption capacities hyperbolically decreased with increasing FeOOH concentration (10-100 mM) implying aggregation, while the affinity constant between Fe(ll)and goethite was constant (log K approximate to 3). Evaluation of the end states of the variable FeOOH and lactate experiments when iron reduction ceased indicated a consistent excess in reaction free energy of -22.7 kJ/mol. This value was remarkably close to the minimum value reported for bacteria to mediate a given reaction (-20 kJ/mol). X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicated that siderite (FeCO3) was the only biogenic Fe(ll) solid formed upon Fe00H bioreduction. A kinetic biogeochemical model that incorporated Monod kinetics with respect to lactate concentration, first-order kinetics with respect to goethite surface concentration, a Gibbs free energy availability factor, the rates of Fe(ll) sorption on goethite and siderite precipitation, and aqueous speciation reactions was applied to the experimental data. Using independently estimated parameters, the developed model successfully described bacterial goethite reduction with variable FeOOH and lactate concentrations.