Charge Relationships of Sulfate Sorption by Soils
1987
Marsh, K. B. | Tillman, R. W. | Syers, J. K.
An investigation into the effect of pH on surface charge and the sorption of sulfate has highlighted a direct and very close relationship between sulfate sorbed by soils and surface positive charge. The addition of lime to contrasting soils decreased phosphate and sulfate sorption, decreased surface positive charge and increased surface negative charge in all cases. Liming had a proportionally much larger effect on sulfate sorption than on phosphate sorption. In both 0.1 M NaCl and 0.025 M CaCl₂ the amounts of sulfate sorbed by soils were highly correlated (r = 0.98***) with the measured amounts of surface positive charge. When surface positive charge was low, little or no sulfate was sorbed by soils from either medium. These findings point to a predominantly electrostatic link between sulfate and the surface, with the changes in sulfate sorbed being dependent on changes in surface positive charge. However, sorption was not entirely nonspecific because there was a strong selectivity for sulfate over chloride and sulfate was sorbed from 0.025 M CaCl₂ in excess of the amounts of surface positive charge initially present. This “superequivalent” sorption did not occur to any great extent when the surface carried no charge. Results suggest that the sorption of sulfate by soils can largely be explained by electrostatic considerations, with sorption occurring in a plane distinct from the surface, but closer than the plane of sorption of nonspecifically sorbed ions, such as chloride.
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