The greater the covalent index value of a metal ion, the greater its potential 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 uptake 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 contacting the metal ions with the biomass. The potential of the test ions to displace preloaded borderline metals ions from the biomass increased with increasing covalent index. In the case where the hard metal ion Sr2+ was preloaded, the displacement potential of the borderline test ions decreased with increasing covalent index, except for Pb2+, which totally displaced the preloaded Sr2+. The potential of a preloaded test ion to inhibit the adsorption of another test ion was also investigated and similar trends to the displacement studies were observed. As a consequence of test ion adsorption, Ca2+ and Mg2+ displacement from the biomass ligands was observed for each test ion and H+ displacement was observed for the borderline test ions only. Overall, the hard metal Sr2+ was found to exhibit ionic binding only, whereas the borderline test ions exhibited a significant degree of covalent binding.