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Bivalent Ion Adsorption by an Oxisol

Inst. for Inorganic, Analytical, and Physical Chemistry, Univ. of Bern, CH-3000 Bern 9, Switzerland

Garrison Sposito^*

Dep. of Plant and Soil Biology, Univ. of California, Berkeley, CA 94720

* Corresponding author.

ABSTRACT

Surface excesses of the bivalent ions Ca²⁺, Mg²⁺, and SO^2-₄ were measured on a Brazilian Oxisol suspended in Ca(ClO₄)₂), Mg(ClO4)₂, or Li₂SO₄ solution. The surface excess of Ca²⁺ increased proportionally with pH in the range 2.5 to 6.0 and was independent of ionic strength in the range 26 to 67 mol m^–3. The surface excess of Mg²⁺ as a function of pH, however, showed a significant dependence on ionic strength in the range from 34 to 176 mol m^–3. The surface excess of SO^2-₄ showed a dependence on ionic strength in the range 12 to 37 mol m^–3 if pH was less than the point of zero net charge (PZNC = 4.2), but showed no dependence on ionic strength if pH was above the PZNC. All three bivalent ions showed significant adsorption above the PZNC of the soil. These trends were interpreted conceptually to suggest that, although the three bivalent ions may form both outer-sphere and inner-sphere surface complexes, the latter are more important for Ca²⁺ than for Mg²⁺ or SO^2-₄.

Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California at Riverside.

Received for publication October 19, 1987.

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