金属有机框架复合材料对放射性碘的吸附机制研究进展

Research progress on adsorption mechanism of radioactive iodine by metal-organic framework composites

【目的】以金属有机框架(metal-organic framework,MOF)复合材料作为吸附剂高效吸附核电站运行、核燃料后处理、核医学过程以及核事故等所泄漏的放射性碘,研究MOF复合材料对放射性碘的吸附机制,消除环境中的放射性碘污染。【研究现状】综述了近年来MOF复合材料吸附放射性碘的基于化学反应、强化MOF空间结构和电子转移的吸附机制。MOF中掺杂银、铜和铋金属或金属氧化物生成的MOF复合材料具有较大的接触面积和较多的活性位点,铋掺杂MOF复合材料对气态I_(2)的吸附性能最优。在MOF复合材料中掺杂多孔材料能增大孔隙体积;高温热解MOF复合材料使得活性位点均匀分布并提高利用率;在MOF复合材料中嵌合纳米复合膜或离子液体能够与吸附物充分接触,回收性和重复利用性好;改变MOF的金属节点或共轭同类MOF衍生出的MOF复合材料,可增强对放射性碘的物理吸附性能。MOF复合材料可以经过碳化作用增强与放射性碘的电荷转移作用,I_(2)能以络合物的形式吸附在MOF复合材料的活性位点上。【结论与展望】虽然对MOF复合材料对放射性碘的吸附机制的研究取得重要进展,但仍然面临着吸附容量小、吸附速率慢、回收利用性差及活性位点利用率低等问题。提出应进一步研究铋掺杂MOF复合材料,探究在高温条件下对放射性碘的吸附机制,采取适当措施减少金属掺杂MOF复合材料对环境的二次污染。

Significance Radioactive iodine,generated from nuclear power plant operations,nuclear fuel reprocessing,nuclear medicine processes,and nuclear accidents,mainly exists in the form of gaseous iodine(I_(2)).Gaseous I_(2) may form iodomethane(CH_(3)I)with other volatile organic compounds and hydrocarbons in the gas stream.Once inhaled,the gaseous I_(2) and CH_(3)I convert into forms such as 131I and 129I in the human body,leading to consequences such as metabolic imbalance,thyroid cancer,and leukemia after continuous accumulation.A small portion of radioactive iodine exists in the form of solid I_(2) and I^(-).Metal-organic framework(MOF)composites are often used to adsorb various forms of radioactive iodine because of their good modifiability,large adjustable pore size,and high thermal stability.Therefore,it is of great significance to thoroughly investigate the mechanisms of effective adsorption of radioactive iodine by MOF composites.Progress The study reviews three types of adsorption mechanisms of radioactive iodine by MOF composites:chemical reaction,carrier spatial structure,and electron transfer.MOF composites doped with metals or metal oxides such as silver,copper,and bismuth have larger contact areas and more active sites.During the adsorption of radioactive iodine,these composites form compounds such as BiI3,BiOI,AgI,and CuI through chemical reactions with radioactive iodine,facilitating subsequent radionuclide solidification and other treatments.Bismuth-doped MOF composites are cost effective,low in toxicity and have the best adsorption performance for gaseous I_(2).Strengthening the spatial structure of MOF can improve the adsorption capacity of MOF composites for radioactive iodine.Effective methods include:doping porous materials into the MOF composites to increase the pore volume;conducting high-temperature pyrolysis to stabilize the structure,prevent collapse,and improve the uniform distribution and utilization of active sites,thus enhancing the reusability,acid resistance,and selective adsorption;integrating nanocomposite membranes or ionic liquids(ILs)with MOF composites to ensure that the adsorbent is in full contact with radioactive iodine,which is able to facilitate the recovery and reuse of adsorbents and can yield the highest I_(3)^(-)adsorption;changing the metal nodes of the MOF or conjugating similar MOF to derive new MOF composites,enhancing the physical adsorption performance of radioactive iodine.In electron transfer-based adsorption mechanisms,MOF composites can be carbonized to enhance the charge transfer interactions with radioactive iodine,enabling I_(2) to be adsorbed in the form of charge transfer complexes at the active sites of MOF composites.Conclusions and Prospects Although important progress has been made in the adsorption mechanisms of radioactive iodine by MOF composites,challenges such as low adsorption capacity,slow adsorption rate,poor recyclability,and low utilization of active sites still exist.To improve the performance of MOF composites for radioiodine adsorption,further studies are needed on bismuth-doped MOF composites to achieve higher adsorption capacity for gaseous I_(2) through a combination of chemical adsorption and physical adsorption.Given that the water vapor temperature in nuclear fuel reprocessing can reach up to 150℃,it is essential to investigate the adsorption mechanisms of the MOF composites under high-temperature conditions.Since the study of non-radioactive 127I cannot reflect the radiation resistance of MOF composites,the adsorption performance of MOF composites under actual irradiation should be explored.In addition,while enhancing the stability and adsorption performance of metaldoped MOF composites,it is of great importance to minimize the secondary pollution caused by them.