Pergamon PH: S0043--1354(97)00273-X War. Res. Vol. 32, No. 2, pp. 400-406, 1998 © 1998Elsevier ScienceLtd. All rights reserved Printed in Great Britain 0043-1354/98$19.00 + 0.00 THE REMOVAL AND RECOVERY OF CADMIUM FROM DILUTE AQUEOUS SOLUTIONS BY BIOSORPTION AND ELECTROLYSIS AT LABORATORY SCALE T. J. BUTTER I, L. M. EVISON K, I. C. HANCOCK 2, F. S. HOLLAND 3, K. A. MATIS ~, A. PHILIPSON 2, A. I. SHEIKH l and A. I. ZOUBOULIS' ~Department of Civil Engineering, University of Newcastle, Newcastle upon Tyne NEI 7RU, U.K., 2Department of Microbiology, The Medical School, University of Newcastle, Newcastle upon Tyne, NE2 4HH, U.K., 3Electrochemical Techniques Limited, 5 Castleton Road, Hazel Grove, Stockport SK7 6LB, U.K. and 'Department of Chemistry, Aristotle University, GR-54006 Thessaloniki, Greece (First received September 1996; accepted in revisedform August 1997) Abstraet--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 cat- ions 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 con- tains only 10/~g 1-1 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 ehited, 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 duate with respect to the original wastewater. Finally, the cadmium is recovered from the eluate by electrolysis using a Rotating Cathode Cell, resulting in cad- mium powder and cadmium-depleted electrolyte which is recycled as the duant. This process is there- fore capable of achieving very effective cadmium removal and produces only dean water and solid metal, both commercially useful products. © 1998 Elsevier Science Ltd. All rights reserved Key words---cadmium recovery, biosorption, Streptomyces, flotation, sedimentation, metal desorption, electrolysis, Rotating Cathode Cell, clean technology, water re-use INTRODUCTION The contamination of water by toxic heavy metals is a world-wide environmental problem. For this reason, the discharge of industrial wastewaters con- taining heavy metals to waterways or sewerage sys- tems is stringently regulated to reduce environmental impacts. Discharges containing cad- mium, in particular, are strictly controlled due to the highly toxic nature of this element and its ten- dency to accumulate in the tissues of living organ- isms (Friberg et aL, 1992). Because of the importance of cadmium as a pollutant, recognised in Europe by its inclusion on the EU List I (Council of the European Communities, 1976), this metal was chosen to test the efficacy of a new treat- ment process for industrial wastewater and other contaminated waters. Chemical precipitation is the most commonly used method for the removal of dissolved metals from wastewaters. Alternative processes include reverse osmosis, electrodialysis, ion exchange, and passive treatment using engineered wetlands (Noyes, 1991; Brierley et aL, 1989). These methods primarily result in the transformation of the dis- solved metals into a more concentrated and man- ageable form prior to final disposal, usually to landfill. However, the metals can continue to rep- resent a further long term environmental hazard. As a result, the disposal of such wastes is becoming increasingly expensive as costly environmental pro- tection measures are imposed. A more logical approach to the treatment of metal-contaminated waters is to combine metal removal for clean up purposes with metal recovery, the metal then being available for reuse in industry. By adopting such a strategy the wastewater is con- verted into two valuable end products, the clean water and the metal, whilst environmental liabilities are greatly reduced. Expenditure on waste disposal is therefore also minimised. The new process initially utilises biosorption onto a free cell suspension of dead bacterial biomass for cadmium removal. The biomass is then separated from the metal-depleted aqueous phase and the bound metal is concentrated through an elution, or 400