Anion Adsorption by Soils and Soil Materials

Abstract

This chapter reviews the work done with soils and with hydrous oxides of iron and aluminum, with particular reference to mechanisms and structural aspects of anion adsorption. Anion adsorption on clay minerals and calcium carbonate is also discussed. The determination of adsorption isotherms is one of the most useful experimental procedures in the study of the interaction of anions with hydrous oxides or soils. Anion adsorption involves an electrostatic interaction and some chemical interaction between the surface and the ion. The Langmuir model is incomplete in describing this adsorption because it takes no account of charge. The adsorption of anions is dependent on pH, with maximum adsorption usually occurring for fully dissociated ions at low pH where the surface becomes positively charged, owing to protonation of surface M·OH groups. Maximum adsorption of anions of incompletely dissociated acids occurs at a pH value close to the pK of the acid. Anion adsorption reactions in soil are complicated by competition for adsorption sites from anions, including carboxylates, and also by the presence of cations, such as calcium and aluminum.