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Biosorption, the process in which live or dead microbes are used for removal of metal from their dilute solution, has emerged as a viable alternative to conventional scavenging techniques. Biosorption process have been employed in pilot plant scale studies for metal r e m~v a l .~ However, very little is known about the mechanism of metal binding by such microbes. Such an understanding is of scientific and engineering interest.The interaction between proteins and metals being well known and well e~tablished,~ many workers in the field of biosorption have attributed the metal binding capacity of microbes to the proteins associated with them.3,6 However, little experimental evidence is available to support this. Esser and Brunnert' isolated a cadmium binding protein from the fruiting bodies of the mushroom Agaricus bisorpus, but the contribution of the protein to total metal uptake was only 20%, indicating the involvement of other group(s) as major contributor(s) to metal accumulation. Sulfhydral groups were shown to be involved in metal binding by an algal cell Chlorella vulgaris. Experiments conducted, though indicating involvement of sulfhydral groups, did not rule out the possibility of involvement of other groups.Tsezos and Volesky* have reported results of a systematic study carried out to delineate the mechanism of biosorption of uranium by Rhizopus arrhizus. Based on electron microscopy, X-ray energy dispersion analysis, electron paramagnetic resonance and infrared spectrascopic studies, they proposed three processes responsible for metal scavenging:1. The first process is a complex formation between dissolved uranium ionic species and chitin present in the cell wall of R. arrhizus. Uranium coordinates to the amino nitrogen of the chitin crystallates and is retained within the cell wall of the mycelium. 2. The second process is adsorption of additional uranium by chitin network close to that complexed by chitin nitrogen.

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