新型石墨烯及其复合物对铀等放射性元素的分离与富集

Extraction of uranium and other radionuclides from waste water plays an important role in recyling limited uranium resources and protecting environment from radioactive pollution. This project aims at the development of novel graphene-based material for application in this field. Graphene is a newly-discovered carbon nanomaterial, possessing many unique physicochemical properties, such as single-atom thickness, two-dimentional structure, large specific surface area and high electronic conductivity. Some metal and metal oxide nanoparticles (e.g. Fe, TiO2, Fe2O3, and MnO2), effective to sorb radioactive nuclides, could be assembled evenly onto graphene nanosheets, thus forming high-quality inorganic nanocomposites; Highly efficient organic ligands of uranium and other radionuclides (e.g. TBP, CMPO and R-BTP) can also be tightly attatched to graphene nanosheets in a manner of either covalent modification, or π-π stack and hydrophobic interactions and such inorganic-organic hybrids could be excellent candidates of solid-phase-extractants for spent fuel reprocessing under strongly acidic conditions. Additionally, graphene electrodes will be prepared to study its performance to remove uranium from waste water by an electrosorptive technique, which is becoming popular recently due to its energy-saving and clean uniqueness. Then, the sorption behavior of uranium and other radionuclides on the above mentioned graphene-based material will be investigated systematically, including the influences of solution pH, ionic strength, interfering ions and so on. The sorption mechanism between metal ions and graphene sorbents will be analyzed as well with powerful spectrometries, such as EXAFS and XPS. On the basis of this study, we can be clear of the application potential of various graphene nanocomposites in nuclear waste management and environmental remediation.

从水体中分离回收铀等放射性核素既可充分利用有限铀资源，又可解决放射性核素环境污染问题，意义重大。本项目拟合成新型石墨烯材料并探讨其在该领域的应用。利用石墨烯独特的二维平面结构、超常的比表面积和电子传输性能以及多种可供选择的功能化途径，将活性金属或氧化物纳米颗粒（如Fe、TiO2、Fe2O3和MnO2等）分散到石墨烯单原子片层制备大容量、高选择性的无机吸附剂；将铀等放射性元素高效有机配体（萃取剂，如TBP、CMPO和R-BTP等）共价接枝或以非共价键作用（π-π堆积、疏水作用等）牢固负载于石墨烯片层制备性能优异的固相萃淋剂；开发高品质石墨烯电极并研究其在电吸附处理放射性废液中的应用。然后，系统研究石墨烯材料对铀等核素离子吸附属性及机理，包括溶液酸度、离子强度和干扰离子等对吸附的影响。在此基础，提出基于石墨烯复合物和石墨烯电极的核素提取与分离的概念流程，作为乏燃料后处理与环境污染治理的参考。

随着我国核能快速发展，产生了大量放射性废水，同时核能也面临着铀资源短缺问题，因此自废水或污染环境中去除及富集铀等核素对于环境保护及核能可持续发展至关重要。本项目开发了新型石墨烯材料在该领域的应用，主要做了三方面的研究：i）氧化石墨烯（GO）吸附铀和钍。研究发现GO上丰富含氧官能团对铀和钍有很高亲和性，在pH 4.0条件下GO对铀的最大吸附容量高达299 mg/g，pH 2.6时对钍的最大吸附容量为214.6 mg/g，与其他纳米材料相比优势明显。GO对两种核素的吸附都具有pH依赖-离子强度不依赖特征，表明金属离子在GO表面形成了内配位络合物，红外及同步辐射X射线吸收精细结构谱（EXAFS）表征也证明了这一结论。GO经脱附处理对Th的再吸附可达到原来的90%，说明GO循环利用性能优异；ii）纳米铁颗粒及其石墨烯复合物去除厌氧水体中的铀。研究表明纳米铁可作为污染地下水修复的可渗透反应墙技术活性介质，在pH 5-9范围内高效去除水体（纯水、1.0 M NaHCO3、模拟地下水或10 ppm腐殖酸）中铀。纯水pH 5.0条件下，纳米铁对铀的最大吸附容量高达8.2 g/g。对吸附后材料进行SEM、XRD、X射线近边吸收谱（XANES）表征及(NH4)2CO3浸提实验发现反应机理为：当铁充足时，U(VI)被部分还原为U3O7沉积在铁颗粒表面，随着反应进行，U(IV)及铁腐蚀产物不断累积，铁表面钝化，U(VI)水解沉淀析出，并诱导铁核溶解，直到铁核耗尽水解终止，达到最大吸附。与石墨烯复合后，纳米铁颗粒更分散、活性更高，去除铀的动力学更快，固定的铀中U(IV)比例也更高，有利于铀在地质环境长期固定；iii）磁性TiO2/Fe3O4及TiO2/石墨烯/Fe3O4光催化还原固定铀。发现光照下TiO2/Fe3O4去除铀的动力学较纯TiO2大幅提高，并且可在pH 3.3-9.3范围内清除铀，虽然pH>5.3时，黑暗预平衡阶段已有近80%铀吸附；离子强度对铀的去除影响较小；体系含有机物可实现两种污染物的协同去除；多种镧系及过渡金属离子共存时，仅Cr(VI)显著抑制铀的去除。TiO2/石墨烯/Fe3O4复合物中，Fe3O4的光溶解受到抑制，有利于材料的循环利用。综上所述，这些石墨烯材料在放射性废水处理及环境污染修复领域具有很好效果及应用前景。