EFFECT OF PERCHLORATE, NITRATE, CHLORIDE AND PH ON SULFATE ADSORPTIONBY VARIABLE-CHARGE SOILS

The adsorption of sulfate at different pH in the presence of different concentrations of sodium perchlorate, sodium nitrate or sodium chlori de by two variable charge soils with contrasting surface charge proper ties was studied. The adsorption decreased with the increase in pH. Fo r the Ferralsol carrying net positive surface charge at natural pH inc reasing the concentration of NaClO4 or NaNO3 resulted in a decrease in sulfate adsorption at low pH and an increase in sulfate adsorption at high pH. There was an intermediate pH al which the salt concentration had no effect on adsorption. This point of zero salt effect (PZSE) on adsorption for NaClO4 and NaNO3 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 wi th a net negative surface charge at natural pH a PZSE on adsorption oc curred at pH 5.7 only in the presence of NaClO4. The adsorption-pH cur ves converged at pH 6.4 in the presence of NaNO3 and were nearly paral lel to one another in the presence of NaCl. These results were explain ed by the combined effect of competitive adsorption and the change in electrical potential at the adsorption plane, It was suggested that th e more abundant the positive surface charge relative to negative surfa ce 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 io ns were more effective in inducing the increases in negative surface c harge and surface potential of soils than nitrate and perchlorate ions .