Calcium‐Induced Sulfate Adsorption by Soils

<jats:title>Abstract</jats:title> <jats:p> Gypsum (CaSO <jats:sub>4</jats:sub> ·2H <jats:sub>2</jats:sub> O) is used in agriculture both as a source of calcium (Ca <jats:sup>2+</jats:sup> ) and sulfate (SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> ) and as an amendment to improve soil structure. We examined the effect of Ca <jats:sup>2+</jats:sup> on the adsorption of SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> in variable‐charge soils. Sulfate adsorption measurements from batch and column experiments showed that SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> adsorption increased with increasing adsorption of Ca <jats:sup>2+</jats:sup> . The increase in SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> adsorption per unit increase in Ca <jats:sup>2+</jats:sup> 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 Ca <jats:sup>2+</jats:sup> increased the positive charge and thereby induced further adsorption of SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> . At low levels of solution Ca <jats:sup>2+</jats:sup> (<0.003 mol L <jats:sup>−1</jats:sup> ), most of the increase in SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> adsorption (85–98%) due to Ca <jats:sup>2+</jats:sup> adsorption could be attributed to the increase in positive charge. At higher Ca <jats:sup>2+</jats:sup> concentration (0.003–0.015 mol L <jats:sup>−1</jats:sup> ), the increase in positive charge accounted for up to only 75% of the increase in SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> adsorption. The remaining increase in SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> adsorption is attributed to the coadsorption of Ca <jats:sup>2+</jats:sup> and SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> as a CaSO <jats:sup>0</jats:sup> <jats:sub>4</jats:sub> ion pair. In soils with organic matter as the major variable‐charge component, Ca <jats:sup>2+</jats:sup> 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 Ca <jats:sup>2+</jats:sup> ‐induced SO <jats:sup>2−</jats:sup> <jats:sub>4</jats:sub> adsorption in these soils. </jats:p>