RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Kinetics and mechanisms of selenate and selenite adsorption/desorptionat 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 selenate SeO₄²⁻ adsorption. This relaxation was ascribed to SeO₄²⁻ on a surfacesite through electrostatic attraction accompanied simultaneously by a protonation process. The intrinsic rate constant for adsorption (log k₁^int = 8.55) was much larger than that for desorption (log k₋₁^int = 0.52). The intrinsic equilibrium constant obtained from the kinetic study (log k_kinetic^int = 8.02) was of the same order of magnitude as that obtained from the equilibrium study (log k_model^int = 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 k^int = 20.42 for XHSeO₃⁰ and 15.48 for XSeO₃⁻) and from the kinetic studies (log k^int = 19.99 for XHSeO₃⁰ and 16.24 for XSeO₃⁻) were similar, which further verified the hypothesized reaction mechanism.