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Anion Adsorption by Allophanic Tropical Soils: II. Sulfate Adsorption
Author(s) -
Gebhardt H.,
Coleman N. T.
Publication year - 1974
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1974.03615995003800020017x
Subject(s) - adsorption , sulfate , chemistry , soil water , inorganic chemistry , leaching (pedology) , environmental chemistry , geology , organic chemistry , soil science
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 H 2 SO 4 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 1 N KNO 3 . It was completely displaced by 1 N NH 4 OAc, 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 NO 3 .