Sorption Of Eu(Ⅲ),Th(Ⅳ),U(Ⅵ)on Iron Oxides And Np(Ⅴ)on Na-Bentonite

The safe and proper treatment/disposal of high level radioactive waste (HLW) is one of the most important factors that should be taken into consideration during the development of nuclear energy. The study of radionuclides sorption on different media of multiple-barrier in the HLW disposal repository is of great importance for the performance and safety assessment of the HLW disposal repository. For recent years, the main concern was radionuclides sorption and diffusion in the buffer/backfilled materials as well as the surrounding rock. However, the study of sorption behaviors of radionuclides on the waste container and the corrosion products is limited. In this study, the sorption behaviors of radionuclides were studied in detail under various physicochemical conditions (pH, ionic strength, solid dose, adsorbate concentration, humic acid, temperature and so on). Models were set to discuss the sorption mechanism and the adsorbed species of radionuclides. In addition, the buffer/backfilled materials were studied for the sorption of radionuclide with low affinity for many adsorbents. The obtained data are important for understanding the behaviors of radionuclides in natural environment and retardation of radionuclides by the media in HLW disposal repository.(1) Iron oxides were characterized with SEM、XRD and BET in detail. The acid-Base surface chemistry of the iron oxides was determined by potentiometric titration, the site concentration and the intrinsic acidity constants of the protonation/deprotonation process were calculated. Both ≡FesOH and ≡FewoH are the main sorption sites for iron oxides. The concentrations of ≡FewoH2+, ≡FesOH and≡FesOH2+ decrease with an increase in pH, and the species≡FewOH-and=FesOh-gradually prevail.(2) Effects of pH, Eu(III) concentration, ionic strength, temperature and humic acid (HA) on Eu(III) sorption to iron oxides were investigated in detail. The sorption of Eu(Ⅲ) to iron oxides was significantly dependent on pH and weakly dependent on ionic strength, and higher temperature was gainful to Eu(III) sorption. In the presence of HA, Eu(III) sorption was enhanced significantly at tow pH; whilst obvious negative effect was observed IN higher pH range. Below 12 mg/L HA, HA could obviously enhance Eu(III) sorption to iron oxides, nevertheless Eu(Ⅲ) sorption decreased steeply with increasing HA while HA exceeded 12 mg/L. The results were helpful for understanding radionuclides behaviors in natural environment.(3) Batch technique was applied to investigate the sorption of Th(Ⅳ) on iron oxides. Influence of equilibrium time, pH, ionic strength, solid content, HA, foreign ions, and temperature were studied in detail. It was found that Th(Ⅳ) sorption on iron oxides was significantly dependent on pH but weakly on ionic strength in the whole observed pH range. Kinetics estimation showed that the sorption of Th(Ⅳ) is chemical sorption and that intra-particle diffusion is the only rate limitation step. The influence of foreign monovalent cations on Th(IV) sorption followed the sequence of K+> Na+> Li+, and Ca2+, Mg2+, Cu2+ were all found to restrict Th(Ⅳ) sorption significantly. The influence of Cl- and CLO4- on Th(Ⅳ) sorption were similar, while NO3-and SO42- restrained the sorption obviously. In the presence of PO43-, Th(Ⅳ) sorption on iron oxides was enhanced to a great extent. In acidic conditions, HA promoted the sorption of Th(Ⅳ) on iron oxides, whilst obvious negative effect was observed in higher pH range. Thermodynamic estimation showed that the sorption Th(Ⅳ) is an endothermic process and is in favor of high temperature. Isotherms of Th(Ⅳ) sorption on iron oxides could be described by Langmuir model well, indicating that the sorption of Th(Ⅳ) on iron oxides was a spontaneous process.(4) The immobilization of U(Ⅵ) at solid/water interface is an important process related to its transportation and migration in environment, which is dominantly controlled by the sorption behavior of U(Ⅵ). In this study, the sorption of U(Ⅵ) on Fe(Ⅱ) and Fe(Ⅲ) oxy-hydroxides prepared using co-precipitation method was studied at various physicochemical conditions such as pH, ionic strength, humic acid (HA), and temperature. The results showed that the sorption of U(Ⅵ) is chemical sorption and that intra-particle diffusion is the rate limitation. U(Ⅵ) sorption on the iron oxy-hydroxides was significantly dependent on pH but weakly on ionic strength in the whole observed pH range. At acidic condition, the presence of HA can enhance U(Ⅵ) sorption to a large extent, a clear inhibition trend, however, can be observed under alkaline condition with respect to the soluble U(Ⅵ)-HA complexes. Thermodynamic estimation showed that the sorption of U(Ⅵ) is an endothermic process and is in favor of high temperature. Surface complexation model suggested three dominant mono-dentate inner-sphere complexes of≡ SSOUO2+ (logK= 1.65),≡SwOUO2OH0 (logK=-8.00) and≡ SWOUO2(CO3)23’(logK= 17.50) to contribute to the U(VI) sorption on the iron oxy-hydroxides over the observed pH range.(5) A method was developed for the determination of 237 Np with liquid scintillation counting (LSC) in the batch experiment of neptunium sorption onto bentonite. Before we use the pulse shape analysis (PSA) technique to discriminate α/β emission, a new approach was developed to set the optimum PSA by measuring a mixed α/β emitters sample and a background sample. The mathematic treatment of neptunium spectrum indicated that at the selected PSA-level of 38 approximately 86% of the total emission is detected, which is suitable for the determination of samples in the batch sorption experiments. Moreover, we confirmed that, at mass to volume radio 0-10 g/L the bentonite suspension in samples has a little impact on LSC determination. Thus, sorption percentage can be easily obtained by counting equivalent amount of suspension and supernatant from the same sample. Therefore, 237Np samples of batch sorption experiments could be detected with LSC quickly and effectively.(6) Effects of ionic strength, pH, temperature, humic acid (HA) and adsorbent content on Np(Ⅴ) sorption to Na-bentonite were investigated in detail. The sorption of Np(Ⅴ) to Na-bentonite was significantly dependent on pH and independent of temperature. In the presence of HA, Np(Ⅴ) sorption was enhanced significantly at low pH; whilst obvious negative effect was observed in higher pH range. Np(Ⅴ) sorption followed the Freundlich isotherm indicating a heterogeneous sorption of Np(Ⅴ) on the surface of Na-bentonite. The surface complexation model (SCM) suggests sorption of Np(Ⅴ) on Na-bentonite was mainly dominated by ion exchange at low pH values, and two dominant monodentate inner-sphere complexes of= SiONpO20 (logK=-4.55) and ≡AIo(NpO2OH)-(logK=-13.80) contributed to Np(Ⅴ) sorption on Na-bentonite over high pH range.