Oxygen as an exchangeable ligand in soil

Authors

• Erkki Aura Department of Agricultural Chemistry, University of Helsinki 00710 Helsinki 71

DOI: https://doi.org/10.23986/afsci.72028

Abstract

An attempt was made to clarify the adsorption of oxygen ligands onto the Al- and Fe-oxides in the soil. In the ligand exchange the central ions Al3+ and Fe3+ of the oxides are Lewis acids or electron acceptors and the ligands are Lewis bases or electron donors. The binding of ligand oxygen by the central ion is primarily dependent on the nucleophilic strength of the ligand oxygen. The proton is a hard Lewis acid as are Al3+ and Fe3+. The pKa-value of the acid corresponding to the anion shows the ability of ligand oxygen to bind a proton and also the ability to bind Al3+ and Fe3+. The greater the nucleophilic strength of the ligand, the more covalent the nature of the bond between ligand oxygen and the central ion. However, the covalent character of this bond does not make the adsorption of a ligand by oxides an exothermic reaction. The entropy changes in adsorption determine the exchange equilibrium of the anions. The nucleophilic strength of ligand oxygen also determines the rate of exchange. If an anion has a high nucleophilic strength, it is rapidly adsorbed by oxides but the desorption is slow. The theory explains the relationship between the adsorption of anions to oxides and the pH-value in the equilibrum solution. The theory also illuminates the factors, which determine the leaching of an anion from the soil and the uptake rate of an anion from the soil by plants.