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Anion Adsorption by Allophanic Tropical Soils: II. Sulfate Adsorption¹

ABSTRACT

Volcanic ash-derived soils (Andepts) from Mexico, Colombia, and Hawaii have large capacities for sulfate adsorption: 10–20 meq/100 g for surface soils and 15–60 meq/100 g for subsoils. The sulfate adsorption capacity is pH-dependent. For a typical case, B-horizon material from San Gregorio, Michoacan, Mexico, capacity in meq/100 g was 13 at pH 6.3, 22 at pH 5.1, 38 at pH 4.4, and 48 at pH 4.1. Sulfate adsorption was accompanied by and dependent upon the simultaneous adsorption, or consumption, of protons. Increasing solution concentration of sulfate beyond 5–10 meq/liter at a given pH resulted in relatively minor additional uptake by the soil. At a given pH, sulfate adsorption capacities, expressed as mmole/100 g soil, nearly equaled Cl adsorption maxima. This, together with the consumption of 1 meq H for each mmole of HCl or H₂SO₄ adsorbed, suggests that the two anions are adsorbed on the same sites and that site protonation is a prerequisite for adsorption.

Sulfate adsorbed by the San Gregorio soil at pH 4 was strongly bound against hydrolysis (removal by water leaching) and was only partly displaced by 1N KNO₃. It was completely displaced by 1N NH₄OAc, pH 7. Sulfate may be adsorbed by Andepts, specifically, by ligand exchange. Its affinity for soil is at least 10 times that of nonspecifically adsorbed anions such as Cl and NO₃.

¹ Contribution from the Dep. of Soil Science and Agr. Eng., Univ. of California, Riverside 92502. Support from the National Science Foundation, under Research Grant NSF-GB-11711-(Pratt), is gratefully acknowledged.

² Postgraduate Research Soil Scientist and Professor, respectively, Dep. of Soil Science and Agr. Eng., Univ. of California, Riverside.

Received for publication September 10, 1973. Accepted for publication September 17, 1973.