新型二维/三维石墨烯基宏观体对放射性元素分离和富集的研究

Effective treatments of radioactive wastewater promote sustainable development of nuclear energy. This project aims at the development of novel graphene-based macrostructures (GMs) for application into this field. It’s known that graphene nanosheets can self-assemble into a three-dimensional (3D) network, characterized by large surface area and macropores, or into an orderly layered structure, forming a thin film. Such assembly is believed to facilitate practical application of graphene material. In this project, functionalized graphene-based macroscopic adsorbents will be prepared by using one-step self-assembly or post-decoration method, in which, highly efficient organic extractants (e.g. TBP, CMPO, and R-BTP) for radioactive nuclides, active polymers (e.g. chitosan, polyethylenimine (PEI), and phenolic resin), or inorganic adsorbents (e.g. zeolite, MnO2, zero-valent iron, and TiO2) are decorated on the surfaces of network’s graphene nanosheets. Adsorption capacities and kinetics of these composite materials for nuclides (e.g. UO22+, Th4+, Cs+, Sr2+, Co2+) are expected to be enhanced greatly, and after treatments, they are easily separated from water phase. Graphene-based macroscopic filtration membranes, containing different components (Ca2+, small organic molecule, carbon nanotube (CNT), TiO2, PEI, or zeolite nanoparticles) will be prepared by using vacuum filtration or layer by layer deposition technique. The stability of membrane structure is expected to be very good, and the spacing between graphene layers and functionalities of the composite membranes could be fine tuned. High water flux and excellent rejection of nuclides or organic substrate are also expected. Porous carbon with hierarchical pores (micropore-mesopore-macropore) can be directly prepared by introducing carbon precursors such as phenolic resin and CNTs into the 3D graphene network, followed by pyrolysis at high temperature. These composite materials process large specific surface area and excellent conductivity. They can be used as electrode materials in an electrosorption system directly without any other additives, and they are expected to show improved performances in the removal of radioactive nuclides, including higher adsorption capacities and faster kinetics. Finally, the removal behavior of radioactive nuclides by the above mentioned GMs will be investigated systematically, including the influence of solution pH, ionic strength, interfering ions and so on. On the basis of this research, we can be clear of the application potential of various graphene macroforms in nuclear waste management and environmental remediation.

放射性废水有效处理关系到核能可持续发展。本项目拟开发新型石墨烯宏观体（大孔网络及有序层状结构）在该领域的应用。i)利用一步自组装或分步法，将放射性核素高效有机配体、活性高分子或无机吸附剂负载于网络结构石墨烯片层表面，得到性能优异的宏观吸附剂，吸附后便于回收； ii）采用抽滤或层层组装技术，制备系列石墨烯基过滤膜（含不同插层组分）。膜结构稳定、具不同膜内通道尺寸及一定抗污性。水通量大、对核素离子或有机物截留率高；iii）石墨烯大孔结构上，引入高分子碳前驱体或碳纳米管，经高温热解，直接得到多级孔碳（微孔-介孔-大孔）。比表面积高、传质快、导电性好，电吸附中直接作为阴极，显著提高核素离子吸附容量及动力学。系统研究这些宏观体对核素的分离、富集行为，包括核素离子浓度、溶液pH、离子强度、干扰离子等影响。在实验结果基础上，提出基于石墨烯基宏观体核素分离、提取流程，作为放射性废水处理、环境污染修复参考。

利用壳聚糖（CS）或聚乙烯亚胺（PEI）诱导氧化石墨烯（GO）自组装，制备了两种GO复合气凝胶（GO/CS和GO/PEI），活性位点丰富、传质迅速，在实际使用中易于操作及回收。GO/CS能在很宽pH范围内富集铀，如pH3.5、pH5.0和pH8.3时，对铀的最大吸附容量分别为200、319.9和384.6mg/g。IR、XPS及EXAFS表征表明GO/CS羧基、羟基和胺基与铀形成了内配位表面络合物，而且随pH升高，配位基团由羧基向胺基转换。GO/CS对模拟海水（~pH8.2）中3.52、14.39及35.57µg/L铀提取率≥97.1%，在已知材料中极具竞争力，GO的复合抑制了CS晶化，使得CS-NH2及其C3位-OH充分暴露应是主要原因。另外，GO/CS在pH6.0对铕最大吸附容量为150mg/g，在pH3.0对钍最大吸附容量为220mg/g，此时含氧功能基团应发挥着更大作用。GO/PEI含有丰富的胺基，自由胺基能配位UO22+，质子化胺基可以离子交换机理吸附ReO4-/TcO4-，从而实现阴阳离子共吸附。GO/PEI在pH5.0时对铀的最大吸附容量高达629.5mg/g，在pH8.3和C0(U)≥100mg/L时，铀酰形成水解沉淀，归因于PEI的弱碱性。pH3.5时，对铼的最大吸附容量为262.6mg/g，有一定选择性。GO/PEI还能在含有Sr2+、过渡金属及镧系离子的模拟核工业废水中高选择性富集铀，突显含氮配体在选择性分离方面的优势，在铀污染的饮用水处理及海水提铀中同样表现优异。TiO2/Fe3O4/石墨烯复合物，石墨烯有效分流TiO2光生电子，将Fe3O4光溶解降低76%，极大改善光催化剂循环利用稳定性及磁分离性能。电吸附方面筛选了能在该领域应用的集流体材料，并采用电沉积法在碳纤维上原位组装还原氧化石墨烯（RGO）水凝胶，不需冻干，可直接作为阴极使用，水通道得到很好地保留。该电极能在-0.4V（vs. Ag/AgCl（satd. KCl））电位，电吸附2.5h清除体系~60%的铀，对应571mg/g的“吸附容量”，且复用性能稳定。模拟地下水及放射性废水处理中，通过调节电位也能实现铀的选择性富集。进一步复合ZIF-8和ZIF-67基多孔碳构筑多级孔结构，电极比表面积及电吸附除铀效率显著提高，在污染环境修复及放射性废水处理中具有广阔应用前景。