LANTHANUM ACCUMULATION FROM ACIDIC SOLUTIONS USING A CITROBACTER SP, IMMOBILIZED IN A FLOW-THROUGH BIOREACTOR

A biotechnological process for the uptake of metals from solution was evaluated at low pH. Metal uptake is mediated by the enzymatic liberation of phosphate by immobilized cells of a Citrobacter; insoluble metal phosphate is retained at the cell surface. Lanthanum uptake was abolished at pH 4, but the activity of the mediating phosphatase was reduced by only 50% at this pH. At pH 5 phosphate release was largely unaffected as compared to that at pH 7, but La removal was reduced. Growth of the strain under conditions which gave a four-fold increase in phosphatase activity gave only a two-fold increase in metal removal al pH 5 by immobilized cells. The precipitated species was identified as LaPO4; the poor removal seen at low pH was attributed to inefficient desolubilization of lanthanum phosphate. In addition, some contributory biochemical factors were identified. The apparent K-m (K-m app) of the whole-cell enzyme for p-nitrophenyl phosphate was little-affected by the pH, but the K-m app for glycerol 2-phosphate was increased at pH 5. This influences the activity of the bioreactor at high flow rates and is reflected in the gradient seen on plots of the In flow rate-activity relationship, in accordance with a Michaelis-Menten description of the bioreactor. It is concluded that although reduced metal uptake at low pH is mainly attributable to chemical effects, these can be exacerbated according to the choice of substrate supporting the desolubilization reaction: p-nitrophenyl phosphate is the better substrate for efficient metal desolubilization at high how rates at low pH values.