Studying the Retention of Multivalent Pollutants in Bentonite

Radionuclides, even in low concentration, form a potential threat to the environment and, therefore, to humans due to their strong radiation and long half-life times. Water sources are among the targets of contamination by radionuclides. However, for protection of the environment, the removal and recovery of such pollutants represent an important issue of special interest. Adsorption of pollutants on a solid matrix is an effective method used for heavy metal removal from aqueous solutions. In many countries such as Sweden, Canada, France, Spain, Japan etc., the design of radioactive repositories in clay sediments is based on the combination of natural and man-made barriers to gain long-term isolation of nuclear waste. One strong alternative is to put the nuclear waste into copper containers embedded in bentonite and are placed underground. In this work, the focus is on studying the retention of multivalent pollutants in bentonite clay. The sorption of trivalent and tetravalent radioactive pollutants from sea water on bentonite has been studied using Grand canonical Monto Carlo simulation. Primitive model has been adopted where the water is included by its dielectric constant. Series of simulations have been performed with systematic variations of different parameters to investigate the sorption behavior of the system; these parameters including concentration of multivalent ion in the bulk solution, surface charge density of the plates, valency of radioactive ions, temperature of the system and ionic strength. As a result of this study, the average concentration of multivalent ions in electrical double layer and the retention coefficient are found to be strongly affected by aforementioned parameters. Specifically, the average concentration of multivalent ions in the electrical double layer was found to be increased by increasing the surface charge density, the bulk concentration of radioactive ions and the valency. However, this average concentration was found to be decreased by increasing temperature and ionic strength. On the other hand, the retention coefficient was found to be decreased by increasing the bulk concentration of multivalent species, ionic strength and temperature, although it shows incremental behavior by increasing surface charge density and valency. The predictions of this study about the sorption behavior of multivalent ions well agrees with several experimental results.