BINDING OF HARD AND SOFT METAL-IONS TO RHIZOPUS-ARRHIZUS BIOMASS

The greater the covalent index value of a metal ion, the greater its p otential to form covalent bonds with biological ligands, In this study , freeze-dried Rhizopus arrhizus biomass was tested for its potential to adsorb the hard metal ion Sr2+ and the borderline metal ions Mn2+, Zn2+, Cd2+, Cu2+, and Pb2+ from aqueous solutions. Equilibrium metal u ptake values increased in the order: Sr2+ < Mn2+ < Zn2+ < Cd2+ < Cu2< Pb2+, and were positively correlated with the covalent index of the metal ions. Equilibrium was rapid and 95% complete within 5 min of con tacting the metal ions with the biomass. The potential of the test ion s to displace preloaded borderline metals ions from the biomass increa sed with increasing covalent index. In the case where the hard metal i on Sr2+ was preloaded, the displacement potential of the borderline te st ions decreased with increasing covalent index, except for Pb2+, whi ch totally displaced the preloaded Sr2+. The potential of a preloaded test ion to inhibit the adsorption of another test ion was also invest igated and similar trends to the displacement studies were observed. A s a consequence of test ion adsorption, Ca2+ and Mg2+ displacement fro m the biomass ligands was observed for each test ion and H+ displaceme nt was observed for the borderline test ions only. Overall, the hard m etal Sr2+ was found to exhibit ionic binding only, whereas the borderl ine test ions exhibited a significant degree of covalent binding.