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 pollut ing power of acid industrial wastewaters. At low metal concentrations (<25 mg/l) this microorganism is able to remove within 8-15 days Cd, Z n, Cu, and Ag with excellent yields (from 81 to 99%) often increasing with starting metal concentration. The yield observed for Cr(III) remo val, never exceeding 60%, is not appreciably influenced by the startin g biomass level; in addition, the time necessary to reach the equilibr ium concentration is always remarkably longer (>30 days) than for the other metals. At much higher concentrations, the removal of all the me tals is strongly affected in terms of both yield reduction and increas e in the time necessary to reach the equilibrium concentrations. Under the hypothesis of mass transfer limitation, the kinetic study of batc h runs suggests that metal diffusion from the bulk to the surface of S . natans clumps could be responsible not only for the simple biosorpti on of the tested metallic micronutrients or abiotic metals, but even f or the cell penetration by ions of biological significance, like Mg2and Fe3+.