Calcium and sulfate retention by two Oxisols of the Brazilian Cerrado
Marcano-Martinez, E. | McBride, M.B.
The ability of two representative oxisols from the Cerrado region of Brazil to adsorb Ca2+ and SO4(2-), 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 SO4(2-) 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 SO4(2-) from CaSO4 solutions caused an enhancement in retention of both ions at higher concentrations, despite undersaturation with respect to gypsum precipitation. A mechanism of CaSO4(0-) ion pair adsorption on mineral surfaces is proposed in which the presence of one ion facilitates adsorption of the other.
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