ELECTROLYTE CONCENTRATION AND COMPOSITION EFFECTS ON SULFATE SORPTION BY 2 SPODOSOLS

Few reports exist on the influence of electrolyte concentration and composition on SO4 retention, although sorption experiments have been conducted with a variety of supporting electrolytes. Sulfate sorption by the B horizon of two forest Spodosols from Quebec was measured as a function of supporting electrolyte concentration and cationic composition, solution pH, and reaction time. The solutions had an ionic strength (I) of = 0 (deionized H2O), 0.001, 0.01, 0.1, and 1.0 M NaCl, 0.01 M KCl, and 0.01 M CaCl2. Increasing the ionic strength of the electrolyte (NaCl) always decreased SO4 sorption at pH 4.25. It was proposed that increased Cl ion competition for sorption sites and the formation of NaSO4- aqueous complexes (2.8, 14, and 55% of total SO4 at 0.01, 0.1, and 1.0 M NaCl) contributed to the observed trend. Increasing electrolyte concentration also lowered SO4 sorption by both horizons in the range of pH 3.0 to 5.2. Moreover, the influence of pH on sorption was more pronounced at low electrolyte concentration. At a constant I value, the cationic composition of the electrolyte also had an impact on SO4 sorption. The presence of Ca2+ and K+ favored sorption, compared with Na+. A mechanism involving the formation of a surface complex between Ca2+ or K+ and SO42- is proposed to explain the experimental data. The results of this study support the hypothesis that the composition and concentration of the electrolyte influence SO4 sorption by Spodosols and that both should be considered when comparing the SO4-sorption capacity of soil materials from distinct experiments or when projecting laboratory results to field conditions.