Experimental and theoretical diffusivities of Cd and Sr in hydrous ferric oxide

Oxides of manganese, aluminum, and especially iron are important sorbents for inorganic contaminants. The sorption process can be characterized by two steps. The first step is a rapid, reversible reaction between the bulk aqueous phase and external surfaces. The slow, second step is the rate limiting mechanism wherein the contaminant diffuses through small pores along surface sites. Isotherm and constant boundary condition studies were conducted to evaluate the sorption process. Best fit experimental surface diffusivities ranged from 10^-14 to 6 x 10^-13 cm²/s. Using site activation theory and assuming a sinusoidal potential field along the pore surface, theoretical surface diffusion coefficients were estimated from the adsorption enthalpy.