Kinetic analysis of the bacterial reduction of goethite

The kinetics of dissimilatory reduction of goethite (alpha -FeOOH) was stud ied in batch cultures of a groundwater bacterium, Shewanella putrefaciens, strain CN32 in pH 7 bicarbonate buffer. The rate and extent of goethite red uction were measured as a function of electron acceptor (goethite) and dono r (lactate) concentrations. Increasing goethite concentrations increased bo th the rate and extent of Fe(lll) reduction when cell and lactate concentra tions were held constant. However, constant initial reduction rates were ob served after normalization to the Fe(ll) sorption capacity of FeOOH, sugges ting that the bacterial reduction rate was first order with respect to surf ace site concentration. Increasing the lactate concentration also increased the rate and extent of FeOOH reduction. Monod-type kinetic behavior was ob served with respect to lactate concentration. Fe(ll) sorption on FeOOH was well-described by the Langmuir sorption isotherm. However, the Fe(ll) sorpt ion capacities hyperbolically decreased with increasing FeOOH concentration (10-100 mM) implying aggregation, while the affinity constant between Fe(l l)and goethite was constant (log K approximate to 3). Evaluation of the end states of the variable FeOOH and lactate experiments when iron reduction c eased indicated a consistent excess in reaction free energy of -22.7 kJ/mol . This value was remarkably close to the minimum value reported for bacteri a to mediate a given reaction (-20 kJ/mol). X-ray diffraction (XRD) and sca nning electron microscopy (SEM) indicated that siderite (FeCO3) was the onl y biogenic Fe(ll) solid formed upon Fe00H bioreduction. A kinetic biogeoche mical model that incorporated Monod kinetics with respect to lactate concen tration, first-order kinetics with respect to goethite surface concentratio n, 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 wi th variable FeOOH and lactate concentrations.