Study of Surfactant Modified Bentonite (SMB) to Enhance Adsorption Capacity for Radioactive Iodine Species

Abstract

Bentonite is widely-used as a qualified engineering barrier in radioactive waste repositories because it has low permeability and high adsorption capacity for cationic radionuclides. However, bentonite is not an effective sorbent for anionic radionuclides because of the permanent negative charge on the bentonite surface. Therefore, the main goal of this study was to increase the adsorption capacity of bentonite for iodide as a representative of anionic radionuclides. Bentonite was modified using a cationic surfactant of hexadecyltrimethylammonium bromide (HDTMA-br) to change the surface charge of bentonite to positive. Based on the cationic exchange capacity (CEC) result, SMB was modified with 50% CEC-SMB, 100% CEC-SMB, and 200% CEC-SMB.Batch adsorption results of iodide in solution (pH=8 and ionic strength=NaNO3 0.001M) indicated that SMB improved iodide adsorption distribution (Kd) from 0 L kg-1 to 3186 L kg-1, compared to bentonite without modification. The iodide sorption capacity of SMB increased as surfactant amount was added. The effects of temperature and irradiation conditions representative of a high-level radioactive waste repository were also tested for surfactant-modified bentonite (SMB) upon iodide removal. A higher temperature condition of 90 °C decreased iodide uptake of SMB by about 4 times than that at a temperature of 25 °C. Irradiation of 60-Co decreased iodide uptake of SMB by approximately 1~2 times independent of iodide concentration compared with an unirradiated sample at the same condition. Analysis of 27Al-NMR and 29Si-NMR analyses demonstrated no structural change for bentonite and SMB before and after irradiation

whereas, the FTIR result demonstrated a bentonite chemical-bonding change after 106Gy irradiation. The oxidation state of iodide adsorbed on SMB did not change based on XANES analysis. In summary, we observed significantly improved iodide adsorption capacity of SMB, and suggested that SMB has the potential to be used as an engineering barrier material in radioactive waste repositories. Among SMB, 100% SMB is suitable from an economic perspective, considering adsorbed surfactant amount and anionic adsorption capacity.