Sorption Of U(Ⅵ), Th(Ⅳ) And Eu(Ⅲ) On Mineralogical Compoenents Of Granite And Bentonite

The safe disposal for high level radioactive waste (HLRW) is one of the most significant issues that should be taken into consideration for nuclear energy development. The backfill materials (bentontite) and host rocks (the most potential choice in China is granite) play critical roles in radionuclides retardation. Thus the migration of radionuclides in bentonite and granite media is an ongoing research subject in site selection as well as safety evaluation of geological repository. However, the detailed interaction between radionuclides and granite was still veiled due to the mineralogical complexity of granite, therefore, the understanding of sorption behavior and mechanism on individual mineralogical components are potential and necessary to predict the effectiveness of granite-based geological repositories.In this dissertation, the sorption of U(VI), Th(IV) and Eu(III) on mineralogical components of granite (mica, feldspar and quartz) and bentonite were investigated by batch sorption experiments and spectroscopic technologies. The effects of pH, ionic strength, temperature, humic substances were studied in detail. The sorption structures and mechanisms were discussed by identifying surface species with the aid of XPS and TRLFS. The primary aim for this work is to further understand the physico-chemical behavior of U(VI), Th(IV) and Eu(III) in real granite media, to provide basic reference not only for reliable predictive migration model, but also for safety assessment of HLRW geological repository.1) The sorption and surface complexation of uranium(VI) on phlogopite were studied by batch sorption and cryogenic time-resolved laser fluorescence spectroscopy. Batch sorption experiment showed that the sorption of U(VI) on phlogopite was strongly dependent on pH while minimally affected by ionic strength. Fluorescence spectral data indicated the formation of multiple inner-sphere surface species, including=SOUO2+,=SO(UO2)2(OH)2CO3-and=SOUO2(CO3)x1-2x, with their abundances varying as a function of pH. A portion of U(VI) precipitated as uranyl oxyhydroxides at high pH (9-11). HA made little difference at low pH while suppressed the U(VI) sorption on phlogopite from pH4to12. Fluorescence spectral results indicated the formation of multiple surface and aqueous uranium(VI)-humate species with abundances varying as a function of pH, the HA coordinated U(VI) preferred directly binding on surface site rather than via HA. The sorption isotherms showed that the sorption of U(VI) on phlogopite increased as temperature raising, suggesting that U(VI) sorption on phlogopite was endothermic. Spectral signatures suggested that the primary U(VI) surface complexes remained the same at all temperatures examined. The presence of HA increased U(VI) desorption without changing surface species before and after desorption.2) The sorption of Eu(III) on muscovite was investigated by batch sorption experiments and room temperature time-resolved laser induced fluorescence spectroscopy. The effects of pH, ionic strength, the presence of PO43-, SO42-and humic acid (HA) were investigated in detail. The results showed that Eu(III) sorption on muscovite was strongly dependent on pH and ionic strength, the ion exchange and outer-sphere complexation controlled sorption at lower pH while inner-sphere complexation predominated at higher pH range. The presence of PO43-enhanced Eu(III) sorption by forming=SO-PO43--Eu ternary surface complexes, while SO42-reduced Eu(III) sorption by forming soluble complexes. The fluorescence spectral data showed that the number of hydration water in first coordination sphere of Eu(III) decreased with increasing pH, suggesting that the outer-sphere complexes dominated at lower pH, and the inner-sphere complexes prevailed with pH increasing. The complex mode evolved from monodentate toward multidentate as pH increase.3) The sorption of U(VI) onto muscovite was studied by batch technique. The effects of contact time, pH, background electrolyte, ionic strength, temperature and the presence of humic acid (HA) were investigated in detail. The results demonstrated that the sorption tended to equilibrium after about12hours, and sorption data could be fitted well by psedo-second-order kinetic reaction. The sorption of U(VI) onto muscovite strongly depended on pH, while slightly relied on ionic strength and background electrolyte types, suggesting that the inner-sphere complexation was the predominant mechanism in the sorption process. High temperature was favorable for U(VI) sorption, and the isotherms could be described better by Langmuir model than Freundlich and D-R model. Humic acid had positive impact on sorption under acid ambient condition.4) The immobilization of Eu(III) on potassium feldspar (K-feldspar) was studied under various pH values, temperatures, counter ions, and organic ligand. The results showed that sorption of Eu(III) on K-feldspar was significantly increased with an increase of pH, and high ion strength reduced Eu(lII) sorption to some extent, suggesting that a relative complicated sorption mechanism referring in ion exchange, outer-and inner-sphere complexes, and precipitation to control Eu(III) sorption behaviors at various conditions. Humic acid (HA) increased the sorption of Eu(III) on K-feldspar at low pH range while inhibited Eu(III) sorption to a large extent at alkaline condition. In the presence of HA, high ionic strength could promote Eu(III) immobilization on K-feldspar, whilst the sorption was restricted obviously by NaCl in the binary system. The sorption was deeply dependent on foreign anions, F-and PO43-dramatically enhanced Eu(III) sorption on K-feldspar while SO42-and CO32-had negative effects on Eu(III) sorption. The notable influence of temperature suggested that higher temperature was favorable for Eu(III) sorption. Eu(III) sorption isotherms on K-feldspar could be described better by Freundlich model. The XPS analysis indicated that Eu(III) tended to form hydrates at high concentration, and the high temperature was also helpful for Eu(IH) hydrolysis.5) The influence of background components on sorption of Th(IV) to Gaomiaozi-bentonite (GMZ-bentonite) was investigated using batch sorption experiments. The results showed that K+, Mg2+and Ca2+obviously restrained Th(IV) sorption in various degrees. Anions (such as NO3-, ClO4-and Br-) made unobvious difference, while F-and PO43-dramatically enhanced Th(IV) sorption. SO42-had a negative effect on Th(Ⅳ) sorption. The positive effects of humic acid (HA) and fulvic acid (FA) at low pH range were mainly attributed to the strong complex ability of humic substances, while Th(IV) sorption decreased quickly from-90%to-40%in the higher pH range. The notable influence of temperature suggested that high temperature was favorable for Th(IV) sorption to GMZ-bentonite. Isotherms of Th(IV) sorption to GMZ-bentonite could be described by Langmuir model better than Freundlich model. The thermodynamic results indicated that the sorption of Th(IV) on bentonite was a spontaneous process.