Processes occurring at mineral-water interfaces play critical roles for regulating the composition of surface and groundwater, for soil development, and for the availability of plant nutrients. Sulfate adsorption at three pH levels was conducted on y-AI203 and kaolinite. The adsorption isotherms were described well by the simple Langmuir, two-site Langmuir, Freundlich, and Temkin equations. The capacity of SO42-adsorption for y-AI203 was five times greater than for kaolinite, indicating the difference in reactive site density between y-Ab03 and kaolinite. Mathematical analyses for the adsorption isothenns demonstrated that S042- may not be adsorbed on the d-plane, i.e., in the diffuse layer, whereas both outer- and inner-sphere complexation mechanisms predicted S042- adsorption equally well. / Ph. D.