Material balance associated with phosphate sorption by amorphous clays, silica-alumina gel and synthetic goethite

The material balance of all the chemical species associated with the phosphate sorption reaction by two amorphous clays, silica-alumina gel and synthetic goethite was investigated adjusting the initial pH to 4.0, 5.0, 6.0, and 7.0. Phosphate sorption was found to decrease with increasing initial pH. During phosphate sorption, the equilibrium pH rose, adsorption of Cl⁻ decreased to a state of negative adsorption, adsorption of Na⁺ increased markedly, silicate was released, and Al in the solution was precipitated. With increasing initial pH, the amount of apparent released OH⁻ increased, the amount of Na⁺ adsorption increased, and the amount of released silicate decreased. The overall relationship of the material balance is explained in terms of the charge balance associated with phosphate sorption.