EFFECT OF INORGANIC AND ORGANIC LIGANDS ON THE MECHANISM OF CADMIUM SORPTION TO GOETHITE - ROLE OF COMPLEXATION AND ADSORPTION

Abstract

Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic data for Cd2+ sorbed on goethite in the presence of phosphate, sulphate and humate indicate that cadmium is surrounded by a first shell of ~6 +/- 1 O atoms at 2.3 +/- 0.1 #9. A second shell of approximately 2 Fe atoms at 3.8 +/- 0.1 #9 is also observed. These interatomic distances suggest that Cd2+ is inner-spherically bound by bidentate double-corner-sharing adsorption on the (110) surface of goethite. In contrast, Cd2+ appears to form a precipitate in the presence of citrate and oxalate. The enhancement of Cd2+ adsorption in the presence of sulphate and phosphate is solely by electrostatic interaction, implying that the ligands sorb to sites other than those occupied by Cd2+. No ternary complexes are observed in the presence of these 2 inorganic ligands.