Ion exchange/complexation of the uranyl ion by Rhizopus biosorbent

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Abstract
Nonliving biomass of nine Rhizopus species effectively sequestered the uranyl ion from solution, taking up 150–250 mg U/g dry cells at 300 ppm U equilibrium concentration in solution, and 100–160 mg U/g dry cells with 100 ppm U in solution. The affinity of this biosorbent for the uranyl ion was found to be affected by timing of harvesting and medium composition. Uptake of the uranyl ion by nonliving biomass of Rhizopus oligosporus was due to ion exchange or complexation, since binding was reversed by the addition of complexing ligands or the reduction of pH to a value less than 2. Uptake isotherms were interpreted in terms of a model of multiple equilibria. At pH ≤ 2, or in the presence of NO, Cl−, SO, or EDTA (ethylenediamine–tetra-acetate), the quantity of UO that was bound was a constant fraction of that bound at pH 4 in the absence of ligands. This action indicated simple competition for uptake sites between H3O+ and UO for uptake sites, or for UO between the biomass and ligands in solution. If oxalate or thiocyanate was present, however, the complexed species were sequestered by the biomass. Biomass of Rhizopus arrhizus, which grew as pellets, was subsequently used in a packed sorption column where it exchanged hydrogen ions for uranyl ions (2 H+: 1 UO). Concentrated uranyl solutions were eluted in sulfuric or nitric acids, and the biomass was reused eight times with no apparent deterioration of the biosorbent.