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 substance is a microbial exudate that has been shown to enhance the solubility of gibbsite and goethite, form stable aqueous complexes with Al, Fe, and Ca metal species, and to compete with PO₄, AsO₄, and SO₄ ligands for adsorption sites on gibbsite and kaolinite. This present study examined the adsorption of kG by goethite, as influence 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 ligands decreased kG adsorption throughout the pH 4 to 10 range. However, SO₄ only decreased kG retention in the pH < 6 range. The 1-pK basic Stern surface complexation model, coupled with a chemical model that considered the monodentate ≡FeOkG^0.5–(s) and the bidentate ≡Fe₂O₂H_–1kG^–(s) surface species was used to successfully describe the experimental adsorption data. In general, the kG adsorption isotherms were H-type, and they were well-described by the Langmuir and the Dubinin-Radushkevich (D-R) isotherm models. The adsorption of kG increased with increasing temperature and increasing ionic strength. Adsorption was endothermic and entropically-driven, and the computed ΔH_ad values were similar to those reported for other specifically-adsorbed ligands, ranging from 22.32 to 53.7 kJ mol^–1.  The 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.