Adsorption behavior of Se(IV) in calcium-based (natural) bentonite and sodium-based (engineered modified) bentonite

Abstract Bentonite plays a significant role as a buffer backfill material in geological disposal due to its unique properties that are crucial for the safe containment of high-level radioactive waste. In this work, the impact of various factors on the adsorption of selenite (Se(IV)) by both calcium-based and sodium-based bentonite was examined using batch experiments. As a result, both types of bentonite achieve adsorption equilibrium with Se(IV) within approximately 120 h. The adsorption capacity decreases with increasing initial concentration of Se(IV), while it is positively influenced by longer contact times and higher temperatures. The partition coefficient ( K d ) exhibits a complex relationship with pH, initially decreasing, then increasing, and finally decreasing again. Additionally, ionic strength significantly affects the adsorption process. These observations suggest that Se(IV) adsorption on bentonite is pH-dependent and involves a non-spontaneous heat-absorbing process, likely a chemical reaction dominated by the ion-exchange form of chemisorption in a monomolecular layer. This process aligns well with the isothermal adsorption model and the simulated secondary kinetics of the Langmuir model. Furthermore, the adsorption of Se(IV) is notably hindered by the increased electrostatic repulsion between the negatively charged bentonite surface and the negatively charged species HSeO 3 2− and SeO 3 2− . The Fourier transform infrared spectra showed that the positions, intensities and shapes of the spectral characteristic peaks of the functional groups before and after adsorption did not change significantly.