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 removal and recovery of cadmium from dilute aqueous solutions. Metal removal is achieved by biosorption of the metal cations onto a free cell suspension of dead Streptomyces biomass in a stirred tank reactor. The solids are then separated from the aqueous phase by flotation or sedimentation. The resulting water, which contains only 10 mu g l(-1) residual cadmium, can be safely discharged. The solids are diverted to a filtration unit where they become immobilised as a filter cake. The biomass 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 in intimate contact between the eluant and the solids. The elution step regenerates the biomass for subsequent biosorption steps and also greatly concentrates the cadmium in the eluate with respect to the original wastewater. Finally, the cadmium is recovered from the eluate by electrolysis 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 removal and produces only clean water and solid metal, both commercially useful products. (C) 1998 Elsevier Science Ltd. All rights reserved.