15 th International Biohydrometallurgy Symposium (IBS 2003) September 14-19, Athens, Hellas "Biohydrometallurgy: a sustainable technology in evolution" 711 The role of metal-organic complexes in the treatment of chromium containing effluents in biological reactors E. Remoudaki, A. Hatzikioseyian, F. Kaltsa and M. Tsezos School of Mining and Metallurgical Engineering, Heroon Polytechniou 9, 157 80 Zografou, Athens, Greece Tel: +30 210 772 2271 / + 30 210 772 2172 Fax: +30 210 772 2173 e-mail: remound@metal.ntua.gr, artin@metal.ntua.gr, tsezos@metal.ntua.gr Abstract Industrial effluents containing Cr(VI) introduce significant toxicity in the environment. Treatment purposes are (i) the reduction of toxic chromium (VI) to the less toxic Chromium(III) and (ii) the precipitation of trivalent chromium. Metabolically mediated reduction of hexavalent chromium by microbial biomass is now well documented and, if applied successfully in an effluent treatment scheme, offers an efficient and low cost alternative avoiding the consumption of chemicals and energy. Positive results of complete biological reduction of hexavalent chromium are reported in the literature from the successful operation of pilot scale biological reactors. Organic compounds are often simultaneously present with chromium, in the solution under treatment, having three possible origins: (i) organic compounds co-existing with chromium in the effluent (e.g. leather tanning), (ii) organic compounds metabolically produced by the microbial biomass used for chromium (VI) reduction (EPS and other organic molecules), (iii) excess of nutrients added in the system to support microbial growth during treatment. The formation of complexes between these organic compounds and trivalent chromium alters the solubility behavior of Cr(III) inhibiting the precipitation of chromium. In this paper, the results of a systematic experimental study of the solubility of trivalent chromium in the presence of selected organic compounds, representative of the above three possible origins (organic acids, amino-acids, proteins and biomass growth nutrients) are presented. The organic compound concentration threshold, above which the solubility behavior of trivalent chromium becomes different from that reported from simple chromium (III) aqueous solutions, was experimentally determined for each one of the compounds tested. Keywords: metal-organic complexes, chromium, solubility curves 1. INTRODUCTION Treatment purposes of industrial effluents containing Cr(VI) are (i) the reduction of toxic chromium (VI) to the less toxic Chromium(III) and (ii) the precipitation of trivalent chromium. Biological reduction of Cr(VI) is an attractive alternative as it can be efficient and low cost avoiding the consumption of chemicals and energy.