THE REMOVAL AND RECOVERY OF CADMIUM FROM DILUTE AQUEOUS-SOLUTIONS BY BIOSORPTION AND ELECTROLYSIS AT LABORATORY-SCALE

A multi-stage process has been developed at laboratory scale for the r emoval and recovery of cadmium from dilute aqueous solutions. Metal re moval is achieved by biosorption of the metal cations onto a free cell suspension of dead Streptomyces biomass in a stirred tank reactor. Th e solids are then separated from the aqueous phase by flotation or sed imentation. The resulting water, which contains only 10 mu g l(-1) res idual cadmium, can be safely discharged. The solids are diverted to a filtration unit where they become immobilised as a filter cake. The bi omass is then eluted, using an electrolyte solution as the eluant, in order to desorb the bound cadmium ions from the biomass. The eluant is drawn through the biomass filter cake under a soft vacuum resulting i n intimate contact between the eluant and the solids. The elution step regenerates the biomass for subsequent biosorption steps and also gre atly concentrates the cadmium in the eluate with respect to the origin al wastewater. Finally, the cadmium is recovered from the eluate by el ectrolysis using a Rotating Cathode Cell, resulting in cadmium powder and cadmium-depleted electrolyte which is recycled as the eluant. This process is therefore capable of achieving very effective cadmium remo val and produces only clean water and solid metal, both commercially u seful products. (C) 1998 Elsevier Science Ltd. All rights reserved.