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Electrolyte Concentration and Composition Effects on Sulfate Sorption by Two Spodosols

Départment de Géographie, Université de Montréal, C.P. 6128, Succ. A, Montréal, Quebec, H3C 3J7, Canada

*Corresponding author.

ABSTRACT

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 NaSO^-₄ 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 SO^2-₄ 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.

Received for publication October 15, 1990.

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