Modeling the adsorption of Cd(II) onto goethite in the presence of citric acid

The adsorption of cadmium onto goethite in the presence of citric acid was measured as a function of pH and cadmium concentration at 25 degreesC. Potentiometric titrations were also performed on the system. Cadmium adsorption onto goethite was enhanced above pH 4 in the presence of 50 muM, 100 muM and 1 mM citric acid. While there was little difference between the enhancements caused by 50 and 100 muM citric acid below pH 6, above pH 6 further enhancement is seen in the presence of 100 muM citric acid. When 1 mM citric acid was present, the enhancement of cadmium adsorption was greater below pH 6, with increased Cd(II) adsorption down to pH 3.5. However, above pH 6, 1 mM of citric acid caused slightly less enhancement than the lower citric acid concentrations. ATR-FTIR spectra of soluble and adsorbed citrate-cadmium species were measured as a function of pH. At pH 4.6 there was very little difference between the ternary Cd(II)-citric acid-goethite spectrum and the binary citric acid-goethite spectrum. However, spectra of the ternary system at pH 7.0 and 8.7 indicated the presence of additional surface species. Further analysis of the spectra suggested that these were metal-ligand outer sphere complexes. Data from the adsorption experiments and potentiometric titrations of the ternary Cd(II)-citric acid-goethite system were fitted by an extended constant-capacitance surface complexation model. The spectroscopic data were used to inform the choice of surface species. Three reactions in addition to those for the binary Cd(II)-goethite and citric acid-goethite systems were required to describe all of the data. They were 2SOH + Cd2+ + L3- + H+ reversible arrow [(SOH)(SOH2+)--LCd-](0), SOH + 2Cd(2+) + L3- + 2H(+) reversible arrow [SLH- --Cd2+](+) + H2O, and SOH + 2Cd(2+) + L3- + 2OH(-) reversible arrow [SOCd+ --LCdOH2-](-) + H2O. Neither the spectroscopy nor the modeling suggested the formation of a ternary inner-sphere complex or a surface precipitate under the conditions used in this study. (C) 2003 Elsevier Inc. All rights reserved.