Phosphate Adsorption by Soils: II. Reactions in Tropical Acid Soils

Abstract

Abstract Phosphate adsorption by several Hawaiian and Indian soils, and the relation of phosphate adsorption to hydroxyl, sulfate, and silicate ions released in two Hawaiian soils were studied. Adsorption isotherms of some of the soils show an abrupt increase in phosphate adsorption at high concentrations. The isotherms were analyzed applying a binary Langmuir equation (assuming two types of sites). The curve predicted by the Langmuir parameters describes the portion of the isotherm where adsorption seems to be mainly by surface ligand exchange (at low levels of phosphate adsorbed). Where additional reactions occur the experimental points deviate from the predicted curve. Phosphate adsorption is associated with increased pH and sulfate release at low levels of phosphate adsorbed and increased silicate release throughout. The observations suggest that at low concentrations phosphate exchanges with (i) adsorbed sulfate and adsorbed silicate, and (ii) with water and hydroxyls of metal hydrous oxides and edge aluminum of clays. At high concentrations additional phosphate is adsorbed by displacing the structural silicate of clays. The increase in phosphate adsorption by structural silicate release, over that of surface exchange reactions, is about 50 and 25% in two soils containing kaolinite and allophane clays respectively.