Uranium competitive uptake experiments by Rhizopus arrhizus were carried out at three different solution pH levels and in the presence of different concentrations of competing aluminum ions in order to examine the competing ion effect. The coion effect became more pronounced as the coion concentration in solution and pH level increased. A preliminary examination of the effect of aluminum on the rate of uranium uptake was also completed. Results showed that the presence of aluminum does not interfere with the kinetics of uranium uptake by R. arrhizus. Electron microscopic and energy dispersive X‐ray analyses were also performed on samples of the biomass. The combination of spectral data and the information from the equilibrium studies and the kinetic studies suggested that aluminum interferes with the uranium biosorptive uptake capacity of R. arrhizus by the precipitation of a metastable amorphous hydroxy polymeric precipitate through a mechanism we refer to as steric competition. © 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 16–27, 1997.

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: Immobilized Rhizopus arrhizus biomass was studied in a continuous sorption and desorption mode in order to identify factors that a †ect the long term uranium biosorptive uptake capacity performance of the immobilized biomass. Laboratory-scale continuous operation pilot plant experiments were performed using synthetic uranyl nitrate and industrial uranium mine leachate solutions. Analysis of the liquid solutions indicated that the immobilized Rhizopus arrhizus biomass successfully recovered all of the uranium from the dilute (less than 500 mg U dm~3) solutions. All uranium can subsequently be eluted, yielding highly concentrated uranium eluates. The immobilized Rhizopus arrhizus biomass maintained its uranium biosorptive uptake capacity over 12 successive sorptionÈelution cycles when synthetic uranyl nitrate solutions were used. However, when used with mine leachate solutions, an 18% reduction in the uranium biosorptive uptake capacity occurred within the Ðrst four adsorptionÈ elution cycles. Spectral analysis indicated that, during continuous use and reuse, the immobilized biomass retained its structural integrity. EDAX, scanning and transmission electron microscopic techniques employed on the microbial biomass suggested that the presence of aluminium interferes with the uranium biosorption process. Spectral analysis also indicated that the presence of silicon enhances the negative e †ect of the presence of aluminium on the uranium biosorptive uptake capacity of the immobilized Rhizopus arrhizus biomass particles.