The mechanism of cadmium removal from aqueous solution by nonmetabolizing free and immobilized live biomass of Rhizopus oligosporus

A preliminary study on the removal of cadmium by nonmetabolizing live bioma ss of Rhizopus oligosporus from aqueous solution is presented. The equilibr ium of the process was in all cases well described by the Langmuir sorption isotherm, suggesting that the process was a chemical, equilibrated and sat urable mechanism which reflected the predominantly site-specific mechanism on the cell surface. A curve of Scatchard transformation plots reflected th e covalent nature of Cd2+ adsorption by the cells. The maximum cadmium upta ke capacities were 34.25 mg/g for immobilized cells and 17.09 mg/g for free cells. Some factorial experiments in shake flasks were performed in order to investigate the effect of different initial cadmium concentrations and b iomass concentrations on the equilibrium. Experimental results showed a rev erse trend of the influence of the immobilized and free biomass concentrati on on the cadmium specific uptake capacity. The immobilized cells had a hig her specific cadmium uptake capacity with increasing biomass concentrations compared to free cells. In a bioreactor, the cadmium uptake capacity of im mobilized cells (q(max) = 30.1-37.5 mg/g) was similar to that observed in s hake flask experiments (q(max) = 34.25 mg/g) whereas with free cells the bi oreactor q(max) of 4.8-13.0 mg/g; was much lower than in shake flasks (q(ma x) = 17.09 mg/g), suggesting that cadmium biosorption by immobilized cells of R. oligosporus might be further improved in bigger reactors. EDAX and tr ansmission electron microscopic experiments on the fungal biomass indicated that the presence of Cd2+ sequestrated to the cell wall was due to bioadso rption.