ARS | Publication request: ARSENATE AND CHROMATE RETENTION MECHANISMS ON GOETHITE. II. KINETIC EVALUATION USING A PRESSURE-JUMP RELAXATION TECHNIQUE

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 1996
Publication Date: N/A

Interpretive Summary: Arsenate and chromate are specifically adsorbing anions that are toxic to animals at elevated concentrations. Toxic concentrations can occur in agricultural soils and irrigation waters. A better understanding of the adsorption behavior of these ions is necessary. Adsorption of arsenate and chromate by iron oxide was investigated under changing conditions of solution pH and ionic strength. Our results show that chromate is more tightly held than arsenate. Our results will benefit scientists who are developing models of arsenate and chromate movement in arid zone soils. The results can be used to improve predictions of arsenate and chromate behavior in soils and thus aid action and regulatory agencies in the management of soils and waters which contain elevated concentrations of arsenate and chromate.

Technical Abstract: The rapid kinetics of arsenate and chromate adsorption/desorption on goethite (gamma-FeOOH) were investigated using the pressure-jump (p-jump) relaxation technique. The adsorption/desorption of these oxayanions 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 tau values, involves an initial ligand exchange reaction of aqueous oxyanion species H2AsO4- or HCrO4-with goethite, forming an inner-sphere monodentate surface complex. The subsequent step, associated with the slow tau values, involves a second ligand exchange reaction, resulting in the formation of an inner-sphere bidentate surface complex. For both oxyanions, the rate constants for desorption were slower than the rate constants for adsorption and therefore the desorption process was the rate limiting step. In addition, results suggest that chromate may be the more mobile of the two oxyanions in soil systems.