The Adsorption of 2-Ketogluconate by Goethite

The low-molecular-mass-organic acid, 2-ketogluconate (kG), has been found to exist in significant concentrations in the rhizosphere of agricultural soils. Adsorption of kG by constant potential mineral surfaces may influence soil chemical processes. This study examines influence of temperature, pH, and ionic environment on the adsorption of kG by goethite. When kG was present on the surface of goethite, the pH_pzc decreased by approximately 0.4 pH units, from pH 7.6 to pH 7.2, suggesting that kG is chemisorbed to the goethite surface via ligand exchange mechanisms. The adsorption of kG by goethite is a function of pH and ionic strength. In pH 5 systems, the adsorption of kG increased with decreasing ionic strength at 25°C and 45°C, while at 7°C adsorption was noted to decrease slightly with decreasing ionic strength. The shape of the kG adsorption isotherms indicates high-intensity retention, suggesting inner-sphere surface complexation. In pH 9 systems, the adsorption of kG increased with increasing ionic strength at all temperatures, also suggesting inner-sphere complexation. The adsorption of kG was also a function of temperature. Adsorption increased with increasing temperature, indicating that the adsorption process was endothermic. Thermodynamic evaluations also showed that the heat of adsorption was consistent with a chemisorption process and that kG adsorption was entropically driven, which is characteristic of ligand exchange processes. Results from this study are consistent with those of previous research and suggest that kG is retained by the goethite surface via ligand exchange mechanisms.