| 铀与环境典型矿物的吸附/共沉淀研究 | |
| Alternative Title | Adsorption/Coprecipitation of Uranium on Typical Environmental Minerals |
牛智伟 | |
| Subtype | 博士 |
| Thesis Advisor | 吴王锁 |
| 2020-06-06 | |
| Degree Grantor | 兰州大学 |
| Place of Conferral | 兰州 |
| Degree Name | 理学博士 |
| Degree Discipline | 放射化学 |
| Keyword | 铀 典型矿物 环境 吸附 共沉淀 |
| Abstract | 放射性核素在环境中的分布、迁移和转化直接影响其环境风险和生态毒性。在地质环境中，放射性核素可发生吸附-解吸、沉淀-溶解、氧化-还原等复杂的物理、化学以及生化反应，从而使放射性核素的地球化学行为变得复杂，为准确描述和预测放射性核素的环境行为带来较大挑战。地质介质组成复杂，其组分对放射性核素的吸附和阻滞能力具有显著差异。对放射性核素有较强阻滞能力的典型介质，如富钛相、方解石、云母矿物等，能够对放射性核素富集、迁移、转化、归趋等环境行为起到至关重要的主控作用。然而，这些典型介质与放射性核素的相互作用机制仍不清晰，一方面，放射性核素除吸附作用之外，仍可通过其他方式与介质发生作用另一方面，放射性核素与环境介质作用，会对环境介质的结构和性质产生影响。因此，深入研究环境典型介质与放射性核素的相互作用和微观机制，可为构建可准确描述和预测放射性核素环境行为的模型提供最直接的理论支撑，对于深入讨论放射性核素的地球化学行为和环境风险具有重要的指导意义，而且对于放射性废物地质处置安全评价具有现实意义。 本论文主要研究典型放射性元素铀在典型环境介质（TiO2、方解石和黑云母）上的吸附行为和微观机制，探讨广泛环境条件下（酸度、盐度、温度、共存离子和共存配体等）U(VI)的吸附规律，借助光谱技术从分子尺度上探讨U(VI)在典型环境介质表面的吸附/共沉淀行为和微观机制，获得铀的吸附规律、赋存状态、微观结构、以及环境介质本身的结构变化信息，以期更加全面深入地认识铀的环境行为和主控因素，为铀的地球化学模型的构建提供依据，为放射性污染治理、安全处置和安全评价提供解决策略。 环境介质中的富钛相对放射性核素的吸附具有突出的富集作用，第二章采用批式法和光谱技术对U(VI)在锐钛矿（TiO2）上的吸附行为进行详细研究，考察了平衡时间、pH、固液比、共存离子、胡敏酸（HA）及温度对U(VI)在TiO2上吸附行为的影响。实验结果表明，U(VI)在TiO2上的吸附符合准二级动力学，且受离子强度和pH值影响显著。在低pH值时，HA促进U(VI)在TiO2上的吸附在高pH下，HA对U(VI)的吸附有明显的抑制作用，且HA浓度越大这种促进和抑制作用越明显。吸附体系温度升高有利于U(VI)在TiO2上吸附的进行，且热力学拟合结果表明，U(VI)在TiO2上的吸附是自发吸热的过程。值得注意的是，高背景电解质浓度对U(VI)在TiO2上的吸附具有一定的促进作用，这是由于在高离子强度下，TiO2易通过(001)面发生聚集，使(101)面暴露，暴露的(101)面对U(IV)具有一定的还原作用。XPS光谱证实了U(VI)在高离子强度下可以被还原为U(IV)，因此在高离子强度下，部分U(VI)在(101)面上被还原，从而促进了U(VI)在TiO2表面的吸附和还原。 方解石具有较强的热力学稳定性，在近地表环境中大量存在，第三章结合批式实验和先进的光谱学方法，从分子水平上研究了方解石对U(VI)的吸附行为以及CaCO3形成过程中U(VI)的掺入对球霰石和方解石形成的调控机制，评估了U(VI)浓度和陈化时间对CaCO3从球霰石相到方解石相重结晶过程的影响。吸附等温线证实，当pH从6.5增加到10.0时，U(VI)在方解石上的吸附量增加，在高pH条件下形成三碳酸铀酰。同时，随着U(VI)吸附量的增加，水相中Ca2+的浓度也呈线性增加，表明不同pH时U(VI)的吸附机理发生了显著变化，可能涉及离子交换和表面络合作用，U(VI)-碳酸盐物种可能进入方解石晶格代替了Ca原子的位置。U(VI)-CaCO3共沉淀研究发现，在共沉淀过程中U(VI)的去除速率在最开始的200 h内随着陈化时间的增长而逐渐下降，陈化时间超过200 h时则显著增大，这与共沉淀过程中Ca(II)浓度的变化趋势一致。并且通过扫描电镜（SEM），X射线衍射（XRD），X射线吸收近边缘结构（XANES）和傅里叶变换衰减全反射红外光谱（ATR-FTIR）分析证实，U(VI)掺入CaCO3的晶格至少经历了三个阶段：球霰石、球霰石/方解石过渡、以及方解石阶段。此外，随着U(VI)浓度的增加，从球霰石到方解石的转变需要更长的时间，且过渡态存在的时间也越长，这表明U(VI)的存在可以稳定球霰石相，增长其稳定存在的时间。延伸X射线吸收精细结构（EXAFS）则证实，在U(VI)掺入CaCO3的过程中，铀酰的局部结构发生了变化，U(VI)的种态与碳酸铀酰类似，但与方解石中高pH范围内U(VI)的主要水相物种三碳酸铀酰不同。 花岗岩体系中黑云母对放射性核素的富集和阻滞具有突出的作用，第四章研究了黑云母对U(VI)的吸附作用，探讨了酸度、离子强度、温度、有机质等环境因素对U(VI)吸附的影响，借助X射线光电子能谱（XPS）、红外光谱（FT-IR）和延伸X射线吸收精细结构（EXAFS）深入讨论了U(VI)在黑云母上吸附的形态和机制。结果表明，U(VI)在黑云母表面的吸附受pH影响显著，离子强度对吸附无明显影响，黑云母边位点与U(VI)形成内层配合物，层间位点对U(VI)的吸附几乎无贡献。磷酸根和胡敏酸（HA）的存在会显著促进U(VI)在黑云母表面的吸附，U(VI)与磷酸根形成表面三元配合物或表面难溶物而显著促进U(VI)的吸附，HA由于其易吸附于黑云母表面，吸附态HA为U(VI)的进一步吸附提供大量的官能团和活性位点，在表面形成三元配合物而促进吸附。高温条件有利于U(VI)在黑云母上吸附的进行。光谱结果显示，由于黑云母含铁量较高，部分U(VI)可被黑云母结构中的Fe(II)还原，在黑云母表面形成U(V)/U(IV)矿化产物或纳米团簇物。 |
| Other Abstract | The distribution, migration and transformation of radionuclides in the environment directly affect their environmental risks and ecotoxicity. In geological environment, the adsorption-desorption, precipitation-dissolution, oxidation-reduction and other reactions of radionucliedes will occur, which make their geochemical behavior more complex, and bring great challenges to accurately describe and predict the environmental behavior of radionuclides. However, the compositions of geological medium are complex, and the ability to absorb and block radionuclides is significantly different for each component. Typical media, such as titanium rich phase, calcite and mica minerals, which have strong retardation ability to radionuclides, play an important role in controlling the environmental behaviors such as radionuclide retardation, migration and transformation. However, the interaction mechanism between these typical media and radionuclides is still unclear. On the one hand, radionuclides can interact with the media in other ways besides adsorptionon the other hand, the interaction between radionuclides and environmental media will affect the structure and properties of environmental media. Therefore, in-depth study on the interaction and micro-mechanism between typical environmental media and radionuclides can provide the most direct theoretical support for the construction of a model that can accurately describe and predict the environmental behavior of radionuclides, which is of great guiding significance for in-depth discussion of the geochemical behavior and environmental risk of radionuclides, as well as for the safety assessment of geological disposal of radioactive wastes. This thesis mainly focused the adsorption behavior and micro-mechanism of radioactive uranium on typical environmental media (TiO2, calcite, biotite), discussed the adsorption law of U(VI) under a wide range of environmental conditions (acidity, salinity, temperature, coexisting ions and coexisting ligands, etc.), and investigated the adsorption/coprecipitation behavior and micro mechanism of U(VI) on the surface at a molecular scale by means of spectral technology. By observing the mechanism, the adsorption law, occurrence state, microstructure of uranium, and the structural change of medium itself was revealed. The corresponding results provided comprehensive and in-depth understanding on the environmental behavior, provided a basis for the construction of uranium geochemical model, and provided solutions for radioactive pollution control, safety disposal as well as safety evaluation. In chapter 2, the adsorption behavior of U(VI) on TiO2 was studied in detail by batch method and spectroscopic technique. The effects of equilibrium time, pH, solid-liquid ratio, coexisting ions, humic acid (HA) and temperature on the adsorption behavior of U(VI) on TiO2 were investigated. The experimental results showed that the adsorption of U(VI) on TiO2 conformed to the quasi second-order kinetics, and was significantly affected by the ionic strength and pH value. HA promoted the adsorption of U(VI) on TiO2 at low pH value, and showed a significant inhibition on the adsorption of U(VI) at high pH value. With the greater HA concentration, more obvious promotion and inhibition was observed. High temperature was more favorable for the adsorption of U(VI) on TiO2, and the thermodynamic fitting results showed that the adsorption of U(VI) on TiO2 was a spontaneous endothermic process. High background electrolyte concentration promoted the adsorption of U(VI) on TiO2, which was due to the aggregation of TiO2 through (001) surface under high ionic strength. More (101) surface was exposed, and the exposed (101) surface showed a certain reduction effect on uranium immoblization. The XPS spectrum analysis confirmed that U(VI) was reduced to U(IV) at high ionic strength, so at high ionic strength, part of U(VI) was reduced on (101) surface, thus promoting the removal of U(VI). Calcite has a stronger thermodynamic stability, which exists widely in the near surface environments. In chapter 3, the adsorption and incorporation behavior of U(VI) on calcite was concerned, the effect of U(VI) incorporation on the formation of CaCO3 were studied by batch experiments and advanced spectroscopy. The effects of U(VI) concentration and aging time on the recrystallization of CaCO3 from aragonite to calcite were evaluated. The adsorption isotherms showed that when pH increased from 6.5 to 10.0, the adsorption capacity of U(VI) on calcite increased, the uranyl carbonate species was formed at high pH. At the same time, with the increase of adsorption capacity of U(VI), the concentration of Ca2+ in the aqueous phase also increased linearly, which indicated that the adsorption mechanism of U(VI) changed significantly at different pH, which involved ion exchange and surface complexation. The U(VI)-carbonate species may enter calcite lattice to replace Ca atom. The coprecipitation of U(VI)-CaCO3 showed that the removal rate of U(VI) decreased with the increase of aging time in the first 200 hours, and increased significantly when aging time was more than 200 hours, which was consistent with the change trend of Ca(II) concentration in coprecipitation process. The results of scanning electron microscope (SEM), X-rays diffraction (XRD), X-ray absorption near-edge structures (XANES) and fourier transform attenuated total reflection infrared spectrum (ATR-FTIR) analysis showed that three stages was passed after adding U(VI) into CaCO3: vaterite, vaterite/calcite and calcite. In addition, with the increase of U(VI) concentration, the transition from aragonite to calcite took longer time, which indicated that the existence of U(VI) could stabilize the aragonite phase and increased the stable existence time. extended X-ray absorption fine structure (EXAFS) confirmed that in the process of U(VI) incorporation into CaCO3, the local structure of uranyl changed. The species of U(VI) was similar to uranyl carbonate, but different from the main aqueous species of U(VI) in the high pH range of calcite. Biotite plays an important role in the enrichment and retardation of radionuclides in granite system. In chapter 4, the adsorption of U(VI) on biotite was studied. The influence of environmental factors such as acidity, ionic strength, temperature and organic matter on the adsorption of U(VI) was discussed. Sorption mechanism was discussed in depth with the aid of X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FT-IR) and extended X-ray absorption fine structure (EXAFS). The results showed that the adsorption of U(VI) on the biotite surface was significantly affected by pH, the ionic strength showed little effect on the adsorption, the inner sphere complex formed on the edge sites of biotite, the interlayer site showed little contribution to the adsorption of U(VI). The presence of phosphate and humic acid (HA) significantly promoted the adsorption of U(VI) by the formation of ternary surface complex on the surface. HA was easy to be adsorbed on the surfacee, and the adsorbed HA provided a large number of functional groups and active sites for the further adsorption of U(VI) by forming ternary complex. High temperature was favorable for the adsorption of U(VI) on biotite. The spectral results showed that part of U(VI) was reduced by Fe(II) in the structure of biotite, forming U(V)/U(IV) mineralized products or nano clusters on the surface of biotite. |
| Pages | 120 |
| URL | 查看原文 |
| Language | 英语 |
| Document Type | 学位论文 |
| Identifier | https://ir.lzu.edu.cn/handle/262010/464153 |
| Collection | 核科学与技术学院 |
| Affiliation | 核科学与技术学院 |
| First Author Affilication | School of Nuclear Science and Technology |
| Recommended Citation GB/T 7714 | 牛智伟. 铀与环境典型矿物的吸附/共沉淀研究[D]. 兰州. 兰州大学,2020. |
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