KINETICS OF SELENATE AND SELENITE ADSORPTION DESORPTION AT THE GOETHITE WATER INTERFACE

Kinetics and mechanisms of selenate and selenite adsorption/desorption at the goethite/water interface were studied by using pressure-jump (p-jump) relaxation with conductivity detection at 298.15 K. A single relaxation was observed for seienate SeO42− adsorption. This relaxation was ascribed to SeO42− on a surface site through electrostatic attraction accompanied simultaneously by a protonation process. The intrinsic rate constant for adsorption (log [formula omitted] = 8.55) was much larger than that for desorption (log [formula omitted] = 0.52). The intrinsic equilibrium constant obtained from the kinetic study (log [formula omitted] = 8.02) was of the same order of magnitude as that obtained from the equilibrium study (log [formula omitted] = 8.65). Unlike SeO42−, selenite adsorption on goethite produced two types of complexes, XHSeO30 and XSeO3−, via a ligand-exchange mechanism. Double relaxations were attributed to two reaction steps. The first step was the formation of an outer-sphere surface complex through electrostatic attraction. In the second step, the adsorbed selenite ion replaced a H2O from the protonated surface hydroxyl group and formed an inner-sphere surface complex. A modified triple layer model (TLM) was employed to describe the adsorption phenomena. The intrinsic equilibrium constants obtained from the equilibrium modeling (log Kint = 20.42 for XHSeO30 and 15.48 for XSeO3−) and from the kinetic studies (log Kint = 19.99 for XHSeO30 and 16.24 for XSeO3−) were similar, which further verified the hypothesized reaction mechanism. © 1990, American Chemical Society. All rights reserved.