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Calcium and Sulfate Retention by Two Oxisols of the Brazilian Cerrado

Dep. of Agronomy, Bradfield Hall, Cornell Univ., Ithaca, NY

ABSTRACT

The ability of two representative oxisols from the Cerrado region of Brazil to adsorb Ca²⁺ and SO^2-₄, when in their natural acidic state, was determined by using dissolved CaCl₂, K₂SO₄ and CaSO₄ salts as adsorbates. Calcium adsorption from CaCl₂ and CaSO₄ was nearly linearly dependent on the equilibrium Ca²⁺ concentration, and was most successfully fitted to a Freundlich equation. Conversely, sulfate adsorption from K₂SO₄ and CaSO₄ was curvilinearly dependent on equilibrium SO^2-₄ concentration, conforming more closely to Langmuir adsorption behavior. Calcium adsorption was explained by nonspecific electrostatic attraction to soil clays, and did not appear to be adsorbed to any significant degree by the native organic matter. Sulfate adsorption was attributed at least in part to a ligand exchange mechanism, resulting in a measured increase in the negative charge of the soil surfaces. Simultaneous adsorption of Ca²⁺ and SO^2-₄ from CaSO₄ solutions caused an enhancement in retention of both ions at higher concentrations, despite undersaturation with respect to gypsum precipitation. A mechanism of CaSO⁰₄ ion pair adsorption on mineral surfaces is proposed in which the presence of one ion facilitates adsorption of the other.

Contribution from the Dep. of Agronomy, Cornell Univ. Research supported in part by the TropSoil project.

Received for publication June 15, 1988.

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