Effect of perchlorate, nitrate, chloride and pH on sulfate adsorption by variable-charge soils

Abstract

The adsorption of sulfate at different pH in the presence of different concentrations of sodium perchlorate, sodium nitrate or sodium chloride by two variable charge soils with contrasting surface charge properties was studied. The adsorption decreased with the increase in pH. For the Ferralsol carrying net positive surface charge at natural pH increasing the concentration of NaClO 4 or NaNO 3 resulted in a decrease in sulfate adsorption at low pH and an increase in sulfate adsorption at high pH. There was an intermediate pH at which the salt concentration had no effect on adsorption. This point of zero salt effect (PZSE) on adsorption for NaClO 4 and NaNO 3 was 5.5 and 5.8, respectively. In the presence of NaCl the adsorption-pH curves converged at pH 7 at which the soil practically did not adsorb sulfate. For the Ferric luvisol with a net negative surface charge at natural pH a PZSE on adsorption occurred at pH 5.7 only in the presence of NaClO 4. The adsorption-pH curves converged at pH 6.4 in the presence of NaNO 3 and were nearly parallel to one another in the presence of NaCl. These results were explained by the combined effect of competitive adsorption and the change in electrical potential at the adsorption plane. It was suggested that the more abundant the positive surface charge relative to negative surface charge of a soil at natural pH, the more likely the occurrence of a point of zero salt effect on sulfate adsorption, and that chloride ions were more effective in inducing the increases in negative surface charge and surface potential of soils than nitrate and perchlorate ions.