Arsenate and Chromate Retention Mechanisms on Goethite. 2. Kinetic Evaluation Using a Pressure-Jump Relaxation Technique

The kinetics of arsenate and chromate adsorption/desorption on goethite (α-FeOOH) were investigated using a pressure-jump (p-jump) relaxation technique. Information provided by this technique was used to elucidate the fate of arsenate and chromate in natural environments. Chemical relaxations resulting from rapidly induced pressure changes were monitored via conductivity detection. The adsorption/desorption of these oxyanions on goethite involved a double relaxation event. The proposed mechanism for the adsorption of arsenate and chromate on goethite is a two-step process resulting in the formation of an inner-sphere bidentate surface complex. The first step, associated with the fast τ values, involved an initial ligand exchange reaction of aqueous oxyanion species H₂AsO₄^- or HCrO₄^- with OH ligands at the goethite surface forming an inner-sphere monodentate surface complex. The subsequent step, associated with the slow τ values, involved a second ligand exchange reaction, resulting in the formation of an inner-sphere bidentate surface complex. Overall, the results suggest that chromate may be the more mobile of the two oxyanions in soil systems.