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 goethit e (alpha-FeOOH) were investigated using a pressure-jump (p-jump) relax ation technique, information provided by this technique was used to el ucidate the fate of arsenate and chromate in natural environments. Che mical relaxations resulting from rapidly induced pressure changes were monitored via conductivity detection. The adsorption/desorption of th ese oxyanions on goethite involved a double relaxation event. The prop osed mechanism far the adsorption of arsenate and chromate on goethite is a two-step process resulting in the formation of an inner-sphere b identate surface complex. The first step, associated with the fast tau values, involved an initial ligand exchange reaction of aqueous oxyan ion species H2AsO4- or HCrO4- with OH ligands at the goethite surface forming an inner-sphere monodentate surface complex. The subsequent st ep, associated with the slow tau values, involved a second ligand exch ange reaction, resulting in the formation of an inner-sphere bidentate surface complex. Overall, the results suggest that chromate may be th e more mobile of the two oxyanions in soil systems.