Cadmium, zinc, copper, silver and chromium(III) removal from wastewaters by Sphaerotilus natans

Living cells of Sphaerotilus natans are used for heavy metal's (Cd, Zn, Cu, Ag, and Cr) removal from aqueous solutions simulating the polluting power of acid industrial wastewaters. At low metal concentrations (<25 mg/l) this microorganism is able to remove within 8-15 days Cd, Zn, Cu, and Ag with excellent yields (from 81 to 99%) often increasing with starting metal concentration. The yield observed for Cr(III) removal, never exceeding 60%, is not appreciably influenced by the starting biomass level; in addition, the time necessary to reach the equilibrium concentration is always remarkably longer (>30 days) than for the other metals. At much higher concentrations, the removal of all the metals is strongly affected in terms of both yield reduction and increase in the time necessary to reach the equilibrium concentrations. Under the hypothesis of mass transfer limitation, the kinetic study of batch runs suggests that metal diffusion from the bulk to the surface of S. natans clumps could be responsible not only for the simple biosorption of the tested metallic micronutrients or abiotic metals, but even for the cell penetration by ions of biological significance, like Mg2+ and Fe3+.