Redox-controlled sorption behavior of Re(VII) on bulk solid and colloidal bentonite

This study investigates the sorption behavior of Re(VII) onto bulk solid and colloidal phases of Bentonil-WRK bentonite under various reducing conditions. Reducing systems were established with Na2S2O4, disodium anthraquinone-2,6-disulfonate, and DB-3 groundwater from the KAERI Underground Research Tunnel (KURT). Batch sorption experiments were conducted for 2500 h with an initial Re(VII) concentration of 10−6.5 M and a solid-to-liquid ratio of 0.5 g/L. Measured pH and Eh values indicated the reduction of Re(VII) to Re(IV), consistent with thermodynamic predictions and confirmed by X-ray photoelectron spectroscopy. Sorption efficiency increased markedly after 500 h, highlighting the governing role of Re redox chemistry in such conditions. Faster sorption occurred in the DB-3 system, where Fe(II) ions and sulfur-bearing colloids enhanced Re retention. Sorption kinetic analysis indicated a redox-controlled sorption behavior of Re(VII) onto bulk solid and colloidal bentonite under the investigated reducing conditions. The results are expected to support more reliable predictions of the migration and retardation of redox-sensitive anionic radionuclides (e.g., Tc, for which Re serves as a chemical surrogate) in the reducing subsurface environment.