The phenomenon of selective retention of cations from aqueous solutions by microbial biomass has been termed biosorption. Samples of waste microbial biomass, originating from industrial fermentations and biological waste water treatment plants, have been tested for their uranium and thorium biosorption potential. Optimum biosorption conditions have been identified. Rhizopus arrhizus was identified as the biomass presenting the highest U or Th uptake capacity, in excess of 170 mg/g. The effect of solution co-ions (namely Fe('+2), Zn('+2)) on the equilibrium biosorptive uptake capacity of Rhizopus arrhizus has been examined. The study of the rapid kinetics of U and Th biosorption has been initiated. Accumulated experimental and theoretical information led to the formulation of a biosorption mechanism hypothesis for the systems U or Th - Rhizopus arrhizus. Biosorption of both U and Th by Rhizopus arrhizus occurs mainly in the cell wall of the mycelium. Complexation by the cell wall chitin, adsorption, and hydrolysis of the complex are the processes participating in the proposed mechanism hypotheses.

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