Studies On U（Ⅵ）, Th（Ⅳ） And Eu（Ⅲ）Adsorption On SiO₂, TiO₂and Na-Bentonite In Presence Of Fulvic Acid

The safety disposal of high-level radioactive waste (HLW) is a key restriction on sustainable development of nuclear fission energy and deep geological disposal is one of the most promising strategies for HLW. Humic substances (i.e., fulvic acid, humic acid and humines) exist widely in the environment of planning repositories for HLW and have significant influences on the adsorption of radionuclides on solid media in both near-field and far-field of repositories. Although chemical models are bases of performance assessment of a geological repository for HLW, there are still significant difficulties to quantitatively describe the adsorption of radionuclides in the presence of humic substances.In the present dissertation, the adsorption of U(VI), Th(IV) and Eu(III) on three adsorbants (i.e., SiO2, anatase (TiO2) and Na-bentonite (Na-BNT)), which are relevant to the environment of a geological repository, in the presence of fulvic acid (FA) were studied. Based on macroscopic data of adsorption, surface complexation models (SCMs) were constructed to describe quantitatively the experimental data and to support safety assessment of geological repository.Ⅰ. Study on Eu(Ⅲ) adsorption on SiO2in the presence of phosphate and FA.(ⅰ) It was found that all binary adsorption systems can be described by double-layer models (DLMs). For Eu(Ⅲ) adsorption, quantitative description was made by a reaction between Eu3+and surface hydroxyl ((?)SOH) to form an inner-sphere surface complex,(?)SOEu2+. For phosphate adsorption, two reactions for surface complexes,(?) SOH2P O42-and (?)SONaHPO42-, gave reasonable interpretation of the experimental data.(ⅱ) The adsorption of FA was interpreted by Stockholm Humic Model (SHM) based on general parameters for reactions between FA and H+. The description of FA adsorption at0.1M ionic strength was realized by a ligand exchange reaction between FA and=SOH.(ⅲ) Base on proposed DLMs for Eu(Ⅲ) and phosphate, the adsorption in ternary system, Eu(Ⅲ)/P(V)/SiO2, was interpreted by the formation of ternary surface complex,(?)SOEuHPO4. According to DLM for Eu(Ⅲ) and SHM for H+and Eu3+, the adsorption at0.1M ionic strength in ternary system Eu(Ⅲ)/FA/SiO2was interpreted by considering two the formation of ternary surface complexes,≡SOEuHFA2+and≡SOEuFA+.(ⅳ) In the presence of both FA and phosphate, it was found that complexation between FA and Eu(Ⅲ) was more stronger than that between phosphate and Eu(Ⅲ). It was not necessary to consider new surface reaction to interpret the experimental data.1. U(Ⅵ) adsorption on TiO2in the presence of FA.The adsorption of U(Ⅵ) on TiO2in the presence of FA was based on two sub-models. One was NIC A-Donnan model for interaction between FA and UO22+in which generic parameters were used and the other was Constant Capacitance Model (CCM) for acid-base reactions on TiO2. The adsorption of FA was interpreted by three surface reactions for (≡SOH)FAH,(≡SOH)FA-and ((?) SO)FA2-, whereas for U(Ⅵ) adsorption in binary system,(≡SO)2UO2,(≡SO)2(UO2)2(OH)2and≡SO (U02)3(OH)5were used to describe the experimental data. The adsorption in ternary system was described quantitatively by two additional reactions to form ternary surface complexes,≡SOUO2FAH and≡SO(UO2)2(OH)2FA. The former was responsible for U(Ⅵ) adsorption in low pH range, whereas the latter for that in high pH range.Ⅲ. The adsorption of U(Ⅵ) and Th(Ⅳ) on Na-BNT in the presence of FA.(ⅰ) U(Ⅵ) adsorption on Na-BNT in the absence FA was quantitatively interpreted by a cation exchange reaction ((≡X)2UO2) and two surface complexation reactions (≡)AlOUO2+and≡AlO (U02)3(OH)5) based on a DLM for acid-base reactions on Na-BNT. With a similar method, the adsorption of Th(Ⅳ) on Na-BNT was described by two reactions (=XTh3+and (≡AlO)2Th2+).(ⅱ) The adsorption of FA on Na-BNT was described by two surface reactions (=AlOHFA and≡AlOFA-) base on SHM.(ⅲ) The adsorption of U(Ⅵ) in the presence of FA was described by considering ternary surface complex≡AIOUO2HFA2+based on the models for binary systems.In brief, both the experimental data and the chemical models in this dissertation have great significance for safety assessment of deep geological repository in China.