Modelling U(VI) reduction by Pseudomonas stutzeri

Abstract

Pseudomonas stutzeri, a facultative anaerobe sourced from soil in a uranium mine in Limpopo, South Africa, reduced uranium-6 (U(VI)) to uranium-4 (U(IV)) in batches under a relatively high initial U(VI) concentration ranging from 30 to 400 mg/L (pH 5 to 6). U(VI) reduction was rapid during the first 4 to 6 h of incubation followed by slower reduction rates at incubation times longer than 6 hours. Equilibrium conditions were only obtained after incubation for 24 h. The reduced U(VI) was recovered in solution as a hydroxide which was determined to consist of U(IV). A kinetic model based on enzyme kinetics produced the best fit of the optimised model to experimental data of U(VI) versus time in batch cultures of Pseudomonas stutzeri. The parameters: maximum specific uranium-6 reduction rate coefficient (ku), half velocity concentration (Ku), and uranium-6 reduction capacity of cells (Tc) were estimated using the data from the 200 mg/L batch. The parameters obtained in the 200 mg/L batch were then used to simulate the concentration in the other batches at lower and higher initial U(VI) concentrations. The reduction capacity Tc remained stable but a loss of accuracy was observed with increasing initial U(VI) concentration. Numerous batch experiments were conducted to establish kinetic parameters that will be used later for scale-up purposes. The knowledge gained from such processes will be of practical value in predicting effluent response to diverse loading conditions.