Zinc Adsorption and Diffusion in Goethite Pastes

Summary Zinc diffusion in goethite (α‐FeOOH) was examined theoretically and experimentally. Concentration profiles of zinc diffusion after various periods of time were determined in physically and chemically defined oxide pastes using a tracer technique. Apparent diffusion coefficients were calculated as a function of diffusible zinc concentration from these results and compared with values obtained indirectly from slopes of adsorption isotherms and experimentally determined impedance factors of the conducting media. To deal with a wide range of experimental conditions, a model for calculating zinc adsorption was applied and evaluated. Changes in pH across the diffusion profile due to zinc transport were postulated to account for some of the results. Where these effects were minimal, the agreement between the diffusion coefficients obtained by the direct and indirect methods was satisfactory. The contribution of surface diffusion to the total transport of zinc in the goethite paste was estimated to be very small.