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 liberat ion of phosphate by immobilized cells of a Citrobacter; insoluble meta l phosphate is retained at the cell surface. Lanthanum uptake was abol ished at pH 4, but the activity of the mediating phosphatase was reduc ed by only 50% at this pH. At pH 5 phosphate release was largely unaff ected 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 phos phatase activity gave only a two-fold increase in metal removal al pH 5 by immobilized cells. The precipitated species was identified as LaP O4; the poor removal seen at low pH was attributed to inefficient deso lubilization of lanthanum phosphate. In addition, some contributory bi ochemical factors were identified. The apparent K-m (K-m app) of the w hole-cell enzyme for p-nitrophenyl phosphate was little-affected by th e 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-activit y relationship, in accordance with a Michaelis-Menten description of t he bioreactor. It is concluded that although reduced metal uptake at l ow pH is mainly attributable to chemical effects, these can be exacerb ated according to the choice of substrate supporting the desolubilizat ion reaction: p-nitrophenyl phosphate is the better substrate for effi cient metal desolubilization at high how rates at low pH values.