Calcium-Induced Sulfate Adsorption by Soils

Abstract

Gypsum (CaSO4·2H2O) is used in agriculture both as a source of calcium (Ca2+) and sulfate (SO2−4) and as an amendment to improve soil structure. We examined the effect of Ca2+ on the adsorption of SO2−4 in variable-charge soils. Sulfate adsorption measurements from batch and column experiments showed that SO2−4 adsorption increased with increasing adsorption of Ca2+. The increase in SO2−4 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 SO2−4. At low levels of solution Ca2+ (<0.003 mol L−1), most of the increase in SO2−4 adsorption (85–98%) due to Ca2+ adsorption could be attributed to the increase in positive charge. At higher Ca2+ concentration (0.003–0.015 mol L−1), the increase in positive charge accounted for up to only 75% of the increase in SO2−4 adsorption. The remaining increase in SO2−4 adsorption is attributed to the coadsorption of Ca2+ and SO2−4 as a CaSO04 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 SO2−4 adsorption in these soils.