CALCIUM-INDUCED SULFATE ADSORPTION BY SOILS

Gypsum (CaSO4.2H2O) is used in agriculture both as a source of calcium (Ca2+) and sulfate (SO42-) and as an amendment to improve soil structure. We examined the effect of Ca2+ on the adsorption of SO42- in variable-charge soils. Sulfate adsorption measurements from batch and column experiments showed that SO42- adsorption increased with increasing adsorption of Ca2+. The increase in SO42- adsorption per unit increase in Ca2+ adsorption was 12 times more in soils containing Fe and Al hydrous oxides as the major variable-charge component than in soils dominated by organic matter. In soils containing Fe and Al hydrous oxides, specific adsorption of Ca2+ increased the positive charge and thereby induced further adsorption of SO42-. At low levels of solution Ca2+ (< 0.003 mol L-1), most of the increase in SO42- adsorption (85-98%) due to Ca2+ adsorption could be attributed to the increase in positive charge. At higher Call concentration (0.00-30.015 mol L-1), the increase in positive charge accounted for up to only 75% of the increase in SO42- adsorption. The remaining increase in SO42- adsorption is attributed to the coadsorption of Ca2+ and SO42- as a CaSO40 ion pair. In soils with organic matter as the major variable-charge component, Ca2+ is complexed by organic ligands. Calcium complex formation through electrostatic attraction does not create positive sites and this may be the reason for the absence of Ca2+-induced SO42- adsorption in these soils.