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The precipitation of sulfate as a result of acid rain appears to decrease the mechanical stability of clay soils. The sulfate anions are said to have a dispersive effect on clays through edge adsorption analogous to the action of phosphate. A comparative study was therefore made of the interaction of chloride, sulfate, and phosphate with montmorillonite and kaolinite. In addition, some measurements were performed with gibbsite, the surface of which may be considered to be a model for clay mineral edge surfaces, where Al-OH groups are exposed. The adsorption of chloride and sulfate was measured by the liquid scintillation technique, using the radiotracers 36C1 and 35S.

Measurements of critical coagulation concentrations, Bingham yield stresses, and relative sediment volumes show that the dispersive power of sulfate is intermediate between that of chloride and phosphate. Adsorption data show that little sulfate is adsorbed compared with phosphate. Therefore, the dispersive power of sulfate, especially at concentrations >0.1 N may be partly due to a secondary effect, i.e., divalent anions reduce the cation activity and thereby the flocculating power of cations more than do monovalent anions. The effect of H+ that accompanies sulfate in acid rains has not yet been investigated; however, the present study suggests that if chloride in clays is exchanged by equivalent amounts of sulfate, the clay will become more dispersed.

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