Surface Complexation and Its Impact on Geochemical Kinetics*

The weathering of rocks, the formation of soils, the alteration and dissolution of sediments are a consequence of surface reac-. tions. Furthermore, many redox processes such as the oxidation of V02+, Mn2+ and Fe2+, the ncn-biotic degradation of organic substances and photosensitized processes are catalyzed by surfaces. The electric double layer theory, despite its efficiency in quantifying certain phenomena of colloid stability, has limitations because it neglects chemical speciation at the surface and does not provide information on the chemical structure of the interfacial region. The surfaces of naturally occurring solids are characterized by functional groups, e. g., OHgroups on the surface of hydrous oxides ar on organic surfaces. Specific adsorption of or interaction with H+, OH-, metal ion s and ligands occurs through coordination at the surface; inner-sphere surface complexes can be formed. The form of occurrence of the individual compounds (speciation) needs to be known in order to understand their reactivity; especially the geometry of the coordination shell of surface sites or of reactants at surfaces is a prerequisite for interpreting reaction rates occuring at the particle-water interface. Some case studies on the oxidation of Mn2+ and V02+ and on the dissolution of hydrous oxides and silicates are presented. In each case, the kinetics of the processes and how it is affected by solution variables such as H+ and ligands (such as oxalate and other diar hydroxy-carboxylates) are explained by simple mechanistic models that involve the coordination at the mineral-solution interface. Simple rate laws are derived illustrating the rates' dependence on the concentration (activity) of surface species.