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

For many plant nutrients and pollutants, the reaction with soil is bet ween ions and the surfaces of variable-charge components of soil. Beca use the reaction involves ions, it changes both the charge on the surf aces and the PH. These changes have effects analogous to the lateral i nteractions which may occur for reactions between uncharged molecules and surfaces. There may also be true heterogeneity. For short-term rea ction, iron oxide surfaces appear to be heterogeneous with respect to reaction with cations but not anions. This may be caused by Fe vacanci es that are incompletely balanced by protons leading to sites of more- negative charge. Long-term reaction also shows evidence of heterogenei ty in that both cations and anions may continue to react with iron oxi des for a long period. For cations, this may be by diffusion via vacan cies; for anions, it may be by diffusion into imperfections between do mains. For soils, models which include heterogeneity plus slow diffusi on into the particles can explain most of the characteristics of react ion.