Research progress on adsorption mechanism of radioactive iodine by metal-organic framework composites
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(I2).Gaseous I2 may form iodomethane(CH3I)with other volatile organic compounds and hydrocarbons in the gas stream.Once inhaled,the gaseous I2 and CH3I convert into forms such as 131I and 129I in the human body,leading to consequences such as metabolic imbalance,thyroid cancer,and leuke-mia after continuous accumulation.A small portion of radioactive iodine exists in the form of solid I2 and I-.Metal-organic frame-work(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 com-pounds such as BiI3,BiOI,AgI,and CuI through chemical reactions with radioactive iodine,facilitating subsequent radionu-clide solidification and other treatments.Bismuth-doped MOF composites are cost effective,low in toxicity and have the best adsorption performance for gaseous I2.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 distri-bution and utilization of active sites,thus enhancing the reusability,acid resistance,and selective adsorption;integrating nano-composite 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 I3-adsorption;changing the metal nodes of the MOF or conjugating similar MOF to derive new MOF composites,enhancing the physical adsorption perfor-mance of radioactive iodine.In electron transfer-based adsorption mechanisms,MOF composites can be carbonized to enhance the charge transfer interactions with radioactive iodine,enabling I2 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 I2 through a combination of chemical adsorp-tion 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 metal-doped MOF composites,it is of great importance to minimize the secondary pollution caused by them.
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