Sorption behavior and mechanism of U(VI) on Tamusu clay in the presence of U(VI)-CO3 complexes

The sorption behavior of U(VI) on Tamusu clay sampled from a pre-selected high-level radioactive waste (HLW) disposal site in Inner Mongolia (China) was studied systematically in the U(VI)–CO3 solution at pH 7.8 by batch experiments. The results demonstrated that the distribution coefficients (Kd) decreased with the increasing values of pHinitial, [U(VI)]initial, and ionic strength, but increased with the extended time and the rising temperature. The sorption was a pH-dependent, heterogeneous, spontaneous, and endothermic chemical process, which could be better described by Freundlich isothermal model and pseudo-second-order kinetic model. The presence of humic acid (HA) or fulvic acid (FA) significantly inhibited the U(VI) sorption, due to the enhanced electrostatic repulsion between the negatively charged HA/FA adsorbed on the clay surface and the negative U(VI) species, as well as the well dispersed HA/FA aggregates in solution wrapping the U(VI) species. The FTIR and XPS spectra indicated that the HCO3− groups on the surface of Tamusu clay after hydroxylation and the ‒OH groups in HA/FA were involved in the U(VI) sorption. The results reported here provide valuable insights into the further understanding of U(VI) migration in geological media.