Pergamon 0045-6535(95)00168-9 Chemosphere, Vol. 31, No. 4, pp. 3093-3104, 1995 Elsevier Science Ltd Printed in Great Britain 0045-6535/95 $9.50+0.00 PHOTO-OXIDATIVE DISSOLUTION OF CdS(s): THE EFFECT OF COMPLEXING AGENTS Allen P. Davis Environmental Engineering Program Department of Civil Engineering University of Maryland College Park, MD 20742. Y. H. Hsieh t and C.P. Huang2 Environmental Engineering Program University of Delaware Newark, DE 19716 (Received in Germany 15 May 1995; accepted 7 June 1995) ABSTRACT The photo-oxidative dissolution of cadmium sulfide has been investigated in the presence of several complexing organic and inorganic compounds under oxygen-saturated conditions. At pH 4 and 7, aromatic organic ligands enhance the cadmium dissolution rate by a factor of 5-6 over that without the organic present; EDTA, NTA, and citric and oxalic acids have little effect. Sulfite and hydrogen sulfide/bisulfide slightly enhance the dissolution. Conversely, at pH 10, EDTA and NTA produce a significant increase in dissolved cadmium (factor of approximately 18) while the other compounds exert little effect. These phenomena are explained by ligand surface complexation, and both surface and homogeneous ligand-enhanced dissolution. INTRODUCTION Organic substances modify the transport of heavy metals in aquatic systems by forming both soluble and interfacial complexes. Correspondingly, the presence of organic acids in metal sulfide dominant systems (i.e., anaerobic sediments and sludges) may have profound effects on metal reaction equilibria and kinetics. Aqueous r Present address: Department of Environmental Engineering, National Chung Hsing University, Republic of China. 2 To whom correspondence should be addressed. 3093