Influence of colloids and colloids’ coagulation on selenite sorption

Abstract Colloids play a crucial role in the sorption and migration of contaminants in the environment，while up to today, limited information exists about the influence of bentonite colloids on the sorption of selenite due to the weak sorption capacity of bentonite colloids to anions. Since bentonite will be used as backfilling materials in granite hosted repositories, and colloids would be generated when bentonite contacts with groundwater, the investigation of the sorption of selenite onto bentonite colloids and the effects of colloids coagulation on the sorption of selenite onto the colloids are badly needed. In this work, the distribution of selenite in the bentonite/deionized (DI) water system and the influence of Beishan (potential granite-hosted geologic repository of China) groundwater on the distribution are investigated. The results show that in the bentonite/DI water system, about 40% of the selenite is adsorbed onto the colloids at pH~5. Selenite sorption onto colloids reaches maximum at pH~5 as a result of the synergy effects of charge of the edge sites and size of the colloids. However, high concentration of colloids cannot be generated or stable in Beishan groundwater. When the bentonite colloids coagulate, the sorption sites of the colloids for selenite decrease, which leads to selenite sorption decreasing. When the colloids bearing selenite coagulate, most of the selenite adsorbed onto the colloids would be released into the solution, and the sorption of selenite onto the colloids at pH~5 decreases from 70% to 10−20%. Selenite sorption onto coagulated colloids is mainly controlled by the surface complex of selenite and divalent cations in the system, which is different from that onto bentonite colloids (controlled by inner-complexation).