Sulfate retention by kaolinitic alfisols from Nigerian savanna

The low amount of available SO4 in savanna Alfisols necessitates investigation of mechanism of SO4 retention and availability in these soils. The study examines the mechanism and SO4 retention capacity of kaolinitic Alfisols from Nigerian savanna. Soil samples from 0- to 20-cm depth from three cultivated plots and a native site were equilibrated with varying amounts of SO4 (0-0.003 M) in three supporting electrolytes, 0.01 M CaCl2, KCl, and NaCl, and three ionic strengths, 0.0 1, 0.1, and 0.2 M KCl at pH 5.6+/-0.2, above the point of zero salt effect (PZSE) and point of zero net charge (PZNC) of the soils determined by potentiometric titration and ion adsorption method, respectively. The results of the study indicated that the soils had high SO4 retention capacity ranging from 15 to 17 mmol kg(-1) soil, and the sorption isotherms were described by a modified Freundlich equation. Sulfate sorption was, however, not significantly affected by ionic composition and strength. If the forces of SO4 adsorption were simply electrostatic, increasing ionic strength would have significantly decreased SO4 adsorption above the PZSE or PZNC of the soils. The study suggested that SO4 retention by kaolinitic Alfisols might involve inner-sphere complexation despite the difficulty of making mechanistic, molecular level interpretation from gross solution data of whole soils.