Study On The Sorption Of U(Ⅵ), Th(Ⅳ) And Eu(Ⅲ) Onto Phosphate

Insoluble phosphates are components of soil and rocks with good sorption capacity and expected to play an important role in the deep geological disposal of high level nuclear waste. In this dissertation, three insoluble phosphate, zirconium diphosphate (ZrP2O7), zirconium oxophosphate (Zr2O(PO4)2) and thorium phosphate diphosphate (Th4(PO4)4P2O7), were chosen as sorbent to study the sorption of U(VI), Th(IV) and Eu(III).The phosphates were characterized by using SEM, XRD and FT-IR in detail and the point of zero charges were estimated by mass titration. The sorption of U(VI), Th(IV) and Eu(III), from aqueous solution on the phosphates was investigated by using a batch technique as a function of contact time, pH, solid content, ionic strength and foreign ions.The sorption behavior of UO22+ on the two kinds of zirconium phosphate, 2 and ZrP2O7, and the effects of simple organic compounds were studied in the fourth chapter. The results indicated that the sorption of uranyl ions on the phosphates was strongly dependent on pH and weakly dependent on ionic strength at moderate concentration (10-1～10-3 mol/L). There were different sorption edges of the two phosphates and the sorption edge of ZrP2O7 was at lower pH than that of Zr2O(PO4)2, which was determined possibly by the mole ratio(r= P/Zr). However there were no significant influences on the sorption of UO22+ on ZrP2O7 and Zr2O(PO4)2 in the presence of fulvic acid and the simple organic compounds.The sorption processes of U(VI) onto ZrP2O7 and Th(IV) onto Zr2O(PO4)2 were investigated in the fifth and sixth chapter, respectively. The results showed that sorption processes were influenced by time and can be described accurately by the pseudo-second order rate model. The sorption processes were strongly affected by pH, solid-to-liquid ratio (m/V), the species of electrolyte in solution and fulvic acid (FA) but was insensitive to ionic strength. The sorption amounts increased as the temperature of the system increased and sorption processes were spontaneous and endothermic. The presence of phosphate or sulfate caused opposite effects on the sorption processes. The sorption amounts decreased significantly in the presence of citrate while enhanced in the presence of FA.The sorption behaviors of U(VI) and Th(IV) on Th4(PO4)4P2O7 were studied in the seventh chapter,. The results indicated that the sorption processes were strongly affected by pH and sorption edges of U(VI) and Th(IV) on Th4(PO4)4P2O7 were different. The sorption edge of Th(IV) on Th4(PO4)4P2O7 was at lower pH than that of U(VI) and the sorption amount of Th(IV) on Th4(PO4)4P2O7 was great. The sorption amounts of U(VI) and Th(IV) on Th4(PO4)4P2O7 were enhanced in the presence of FA. There were opposite effects of U(VI) and Th(IV) on Th4(PO4)4P2O7 in the same electrolyte solution and can be thought as different characteristic of U(VI) and Th(IV) in solution.The sorption of Eu(III) on Th4(PO4)4P2O7 was investigated in the eighth chapter. The sorption of Eu(III) on the phosphates was strongly dependent on pH and there was great effect in the presence of citrate. The sorption amounts of Eu(III) on Th4(PO4)4P2O7 can be enhanced in the presence of sulfate and phosphate and the sorption of sulfate and phosphate on Th4(PO4)4P2O7 can be found. There were opposite effects of the Eu(III) desorption from Th4(PO4)4P2O7 in the presence of sulfate and phosphate when the citrate was added into the solution system. The sorption of Eu(III) on Th4(PO4)4P2O7 can be enhanced in the presence of FA and the sorption of FA on Th4(PO4)4P2O7 also increased with the concentration of FA in the solution increasing.