Sorption Of U(Ⅵ)on Mineralogical Components Of Granite

The safety and reasonable treatment of high level radioactive waste (HLRW) is a key factor for the sustainable development of Chinese nuclear power program in future. Beishan (Gansu Provience, China) has been selected to be a primary site for the geological reporistory of HLRW. Granite is the nartual barrier for Beishan area, therefore the studying on the sorption and desorption of U(Ⅵ) on the quartz, K-feldspar and Na-feldspar which are the main components of the Beishan granite becomes very important to the performance and safety assessment of deologocal repository. In this work, the sorption and desorption of U(Ⅵ) was investigated under various physiochemical conditions (such as pH, ion strengthen, humic substances, temperature etc.) to discuss the sorption mechanism and speciation of U(Ⅵ).1) The sorption of U(Ⅵ) on quartz was strongly denpent on pH, increase quickly with the increase in pH at acidic condition; however the sorption of U(Ⅵ) was decreased with the pH increase above pH 7.0. No ion strength dependence was observed in our experiment condition, which indicated that the surface complexation might be the main sorption mechanism for U(Ⅵ) sorption on quartz. The sorption isotherm of U(Ⅵ) can be simulated very well using Lanmuir model, suggesting the mono-layer sorption for U(Ⅵ) on quartz. U(Ⅵ) sorption on quartz is favor to higher temperature and is belonging to the spontaneous and endothermic processs.2) In comparisonwith quartz, the equilibtium time for U(Ⅵ) sorption on feldspar is longer, about 20 hours, and following the pseudo-second-order rate equation. As the solid-to-liquid ratio increased, the sorption U(Ⅵ) can be increased a lot because more sorption sites were available at large solid-to-liquid ratio. The sorption of U(Ⅵ) on Na-feldspar is increasing with pH increase from 0% to 90%, however U(Ⅵ) sorption was declining at high pH, which can be attributed to the soluble U(Ⅵ) species in aqueous such as carbonates andhydrolysis. For K-feldspar, no declining trend was found in the whole pH range, which is quitedifferent from the cases of Na-felspar and quartz. It is possibly due to the differences in the chragre dendsity and structure of sorbents. Our sorption/desorptionexperiments showed that U(Ⅵ) sorption on feldspar is irreversible process, which is benefit to the hysteresis of U(Ⅵ) in the granite. High temperature can enhanceU(Ⅵ) sorption on feldspar, and thermodynamic paramenters showed U(Ⅵ) sorption is sapantaneous and endothermic process. Our results clearly suggest that the inner-sphere surface complexes are the main sorption species of U(Ⅵ) on feldspar, whereas the contribution of outer-sphere surface complexes and ion exchange could be negligible.3) Compared to quartz, the sorptionability and capacity of feldspar to U(Ⅵ) are much larger, which is main contributor to thehysteresis of U(Ⅵ) in granite.