Kinetics of Selenate and Selenite Adsorption/Desorption at the Goethite/Water Interface

w Kinetics and mechanisms of selenate and selenite adsorption/desorption a t the goethitelwater interface were studied by using pressure-jump (p-jump) relaxation with conductivity detection at 298.15 K. A single relaxation was observed for selenate Se0:adsorption. This relaxation was ascribed to Se0,2on a surface site through electrostatic attraction accompanied simultaneously by a protonation process. The intrinsic rate constant for adsorption (log k';"t = 8.55) was much larger than that for desorption (log l ~ $ ~ = 0.52). The intrinsic equilibrium constant obtained from the kinetic study (log Piketic = 8.02) was of the same order of magnitude as that obtained from the equilibrium study (log F:tdel = 8.65). Unlike Se0,2-, selenite adsorption on goethite produced two types of complexes, XHSe030 and XSeOC, 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 H 2 0 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 P" = 20.42 for XHSeO,O,and 15.48 for XSeOC) and from the kinetic studies (log Kmt = 19.99 for XHSe0,O and 16.24 for XSe03-) were similar, which further verified the hypothesized reaction mechanism.