The role of sulfate as a competitive inhibitor of enzymatically-mediated heavy metal uptake by Citrobacter sp: implications in the bioremediation of acid mine drainage water using biogenic phosphate precipitant

Heavy metals can be removed from solution via biocrystallization with enzym atically liberated inorganic phosphate, according to Michaelis-Menten kinet ics, in free whole cells and cells immobilized within polyacrylamide gel in a flow-through reactor. Sulfate is a competitive inhibitor of phosphate re lease and a predictive model was developed and shown to describe the effect of sulfate on the efficiency of phosphate release by flow-through columns. The inhibitory effect was substantially less than anticipated in the case of metal removal by the columns. In the case of lanthanum removal metal rem oval efficiency was restored by increasing the substrate concentration in a ccordance with model predictions. In the case of uranyl ion its removal wit h an equivalent substrate supplement increased the activity by 20% over the initial value at a limiting flow rate. Since the initial loss in activity in the presence of 40 mmol dm(-3) SO42- (approximately twice the K-i value) was only approximately 20% with both metals this was considered to be a mi nor problem for bioprocess application. In confirmation, calculations made from a published 'case history' of application of the system to the bioreme diation of acid mine drainage water (AMD) containing 0.22 mmol dm(-3) of ur anyl ion and 35mmoldm-3 of SO42- showed that the benchscale model is a good representation of performance under actual load conditions. (C) 1999 Socie ty of Chemical Industry.