A COMPARATIVE-STUDY OF CADMIUM UPTAKE BY FREE AND IMMOBILIZED CELLS FROM ACTIVATED-SLUDGE

Cadmium biosorption by free and immobilized cells from activated sludge was studied in batch experiments to determine the effects of immobilization on the biosorption properties of the microorganisms. The method of immobilization consisted of the entrapment of the microorganisms in calcium alginate beads. In a first step, the time course of cadmium uptake and the effects of pH and temperature on biosorption were studied in each case. Then the biosorption pattern and the effect of biosorbent concentration were investigated. It was found that the magnitudes of the effects of pH and temperature on biosorption were reduced by the entrapment of the cells, but the major effects of the entrapment consisted of a very high reduction of the biosorption rate and a modification of the biosorption pattern. On the one hand, cadmium biosorption by free cells followed the pattern of the Freundlich isotherm over wide ranges of cell and cadmium concentrations in solution. On the other hand, biosorption by the beads could be described by the Freundlich isotherm only when bead concentration was kept constant because the parameters K and n varied with bead concentration. Finally, it was found that, as expected, the entrapment of the cells significantly reduced their intrinsic biosorption capacity due to a loss of available binding sites. For cases where biosorption is the primary objective, free cell systems should therefore be preferred to entrapped cell processes to remove cadmium from aqueous effluents by using activated sludge. Biosorption isotherms show that cadmium can be biosorbed up to more than 20,000 times above water concentrations utilizing free cells at 30-degrees-C and pH 6.6. However, the numerous advantages of immobilized cells in reactor operation should also be considered to select the appropriate technology.