EFFECTS OF SURFACE HETEROGENEITY ON ION ADSORPTION BY METAL-OXIDES AND BY SOILS

For many plant nutrients and pollutants, the reaction with soil is between ions and the surfaces of variable-charge components of soil. Because the reaction involves ions, it changes both the charge on the surfaces and the PH. These changes have effects analogous to the lateral interactions which may occur for reactions between uncharged molecules and surfaces. There may also be true heterogeneity. For short-term reaction, iron oxide surfaces appear to be heterogeneous with respect to reaction with cations but not anions. This may be caused by Fe vacancies that are incompletely balanced by protons leading to sites of more-negative charge. Long-term reaction also shows evidence of heterogeneity in that both cations and anions may continue to react with iron oxides for a long period. For cations, this may be by diffusion via vacancies; for anions, it may be by diffusion into imperfections between domains. For soils, models which include heterogeneity plus slow diffusion into the particles can explain most of the characteristics of reaction.