RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The adsorption of cadmium onto goethite in the presence of citric acid was measured as a function of pH and cadmium concentration at 25 °C. Potentiometric titrations were also performed on the system. Cadmium adsorption onto goethite was enhanced above pH 4 in the presence of 50 μM, 100 μM and 1 mM citric acid. While there was little difference between the enhancements caused by 50 and 100 μM citric acid below pH 6, above pH 6 further enhancement is seen in the presence of 100 μM 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⁺ ↔ [(SOH)(SOH₂⁺)--LCd⁻]⁰,
SOH + Cd²⁺ + L³⁻ + 2 H⁺ ↔ [SLH⁻--Cd²⁺]⁺ + H₂O,

and

SOH + Cd²⁺ + L³⁻ + 2OH³⁻ ↔ SOH--Cd²⁺ ↔ [SOCd⁺--LCdOH²⁻]⁻

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.