The Adsorption of 2-Ketogluconate by Goethite

Low-molecular-mass organic acids play an important role in many soil processes, including mineral dissolution and nutrient mobilization and uptake. One such organic acid is 2-ketogluconate (kG). This study examined the adsorption of kG by goethite, as influenced by temperature, pH, and ionic environment. Adsorption edge studies showed that kG adsorption was a function of pH but independent of ionic strength. Ketogluconate adsorption also resulted in a negative shift in the surface charge characteristics of goethite. The inclusion of PO4 and AsO4 decreased kG adsorption throughout the pH 4 to 10 range; however, SO4 only decreased kG retention in the pH <6 range. The basic Stern surface complexation model that considered the monodentate FeOkG(0.5)-(s) and the bidentate Fe(2)O(2)H(-1)kG(-)(s) surface species was used to successfully describe the experimental adsorption data. In general, the kG adsorption isotherms were well described by the Langmuir and Dubinin-Radushkevich (D-R) isotherm models. The adsorption of kG increased with increasing temperature and generally with increasing ionic strength at low values of surface coverage (the adsorption maxima were not affected by ionic strength). Adsorption was endothermic and entropically driven, and the computed enthalpy values were similar to those reported for other specifically adsorbed ligands, ranging from 22.32 to 53.70 kJ mol(-1). The characteristic energy of adsorption computed from the D-R isotherm model ranged from 12.84 to 18.33 kJ mol(-1), indicating chemical adsorption. Results from this study are consistent with those of previous research, indicating that kG is retained by the goethite surface via inner sphere mechanisms and that kG may play an important role in soil chemical processes.