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Interactions Between Organic Compounds, Minerals, and Ions in Volcanic-ash-derived Soils: II. Effects of Organic Compounds on the Adsorption of Phosphate¹

ABSTRACT

The effects of organic compounds on phosphate adsorption by volcanic-ash-derived soils were studied using benzoate, p-OH benzoate, salicylate, and phthalate, and by humic and fulvic acids extracted from a surface soil sample of a Typic Dystrandept. The adsorption of simple organic anions that are specifically adsorbed block to some extent the adsorption sites for nonspecifically adsorbed anions such as chloride and/or nitrate. But, organic anions did not compete for or block adsorption sites for phosphate anions, because of the much higher affinity of phosphates for the adsorption sites in these volcanic-ash-derived soils.

Because of the characteristics of humic acid and noncrystalline alumino-silicates ("allophane") colloidal particles, humic acid-allophane interactions are probably mainly between external surfaces because of steric hindrance. Hence, if there are blocking effects of humic acid on phosphate adsorption, they would affect only part () of the reactive surfaces of the "allophane." The complexes formed between humic acid and soluble Al ions extracted from "allophane" are active in phosphate adsorption, and this reactivity depends on the OH/Al ratio of the complexed Al_x(OH)_y^(3x-y) species.

¹ Contribution from the Department of Inorganic and Analytical Chemistry, Facultad de Ciencias Quimicas, Universidad de Chile; and Dept. of Soil Science and Agricultural Engineering, Univ. of California, Riverside 92502. Part of a dissertation submitted by the senior author in partial satisfaction of the requirements for a Ph.D. degree in Soil Science. Support of the Convenio Universidad de Chile-University of California Program is gratefully acknowledged.

² Former Graduate Student, Professor of Soil Science (deceased), and Professor of Soil Science, respectively.

Received for publication May 13, 1974. Accepted for publication February 19, 1975.