Organophilic bentonites as adsorbents for radionuclides I. Adsorption of ionic fission products

Abstract The retardation of radionuclides by engineered clay barriers is primarily controlled by the sorption potential of the mineral constituents. Adsorption and desorption experiments were performed with I−, TcO4−, Cs+ and Sr2+ on MX-80 Wyoming-bentonite treated with hexadecylpyridinium (HDPy+) in amounts equivalent to 20%–400% of the cation exchange capacity (CEC). Bidistilled water, synthetic ground water (SGW), and sea water with half of the ionic strength (Sea/2) were used as equilibrium solutions, and 125 I , 95 m Tc , 134 Cs and 85 Sr were employed as tracers. In HDPy-bentonite, I− and TcO4− exhibited increasing adsorption (characterized by the distribution ratio, Rd), while Cs+ and Sr2+ ions showed decreasing adsorption with increasing organophilicity. The extent of the adsorption as well as the reversibility of the binding processes was influenced by the chemical composition of the equilibrium solutions. These effects were more pronounced in the case of the cationic fission products compared to the anions investigated. Generally, sorption and desorption were linear over a wide concentration range of the carrier ions investigated, indicating that adsorption was independent from the sorbate concentrations (up to ∼10−1 mmol g−1 organo-bentonite). This suggests ion exchange as the principal sorption mechanism. Adsorption capacities for the anions investigated were estimated to be ∼0.5 molc kg−1, for the cations to be ∼0.1 molc kg−1 depending on the HDPy+ loading of the samples. In case of cationic radionuclides, Cs+ was preferentially adsorbed in competition with the bivalent Sr2+ ions in both the untreated and modified samples. The results of this study may be applicable to predict the long-term behaviour of fission products in waste disposal sites equipped with clay barriers.