Structure-activity relationship of covalent organic frameworks (COFs) for gaseous iodine adsorption

Radioactive iodine (such as 129 I and 131 I) is one of the major gaseous contaminants resulting from the utilization of nuclear energy and/or nuclear accidents.During the initial dissolution process in the spent fuel reprocessing, for example, most of the iodine is released into the off-gas system in the form of highly volatile I 2 and small amounts of organic iodine compounds (such as methyl iodide, ethyl iodide, butyl iodide).The remaining iodine in the dissolution solution exists in the forms of I 2 , IO 3 -, I -, IO -, and iodine colloids (AgI, PdI 2 ).When the iodine in the dissolution solution enters the subsequent solvent extraction process, it can lead to solvent degradation, organic iodine formation, and reduction in the quality of uranium and plutonium products.Therefore, during the dissolution stage of spent fuel elements, it is generally preferred to convert as much iodine in the dissolution solution as possible into I 2 and drive it into the off-gas.Overall, in the current nuclear fuel cycle system, radioactive iodine produced by nuclear fission mainly exists in the gaseous form.In the event of an accident, this radioactive iodine can easily leak, which presents a potential threat to the environment and public health due to its long half-life (e.g., 129 I has a half-life of 1.5710 7 years), high mobility, ease of dispersion through air and water, and tendency to accumulate in living organisms [1].Consequently, the development of efficient and recyclable adsorbent materials for the capture and fixation of radioactive iodine has become a research priority in the fields of nuclear environmental remediation, nuclear emergency response, and nuclear fuel cycle management.Covalent organic frameworks (COFs), as a novel class of crystalline porous materials, have demonstrated promising potential for the capture of gaseous radioactive iodine due to their distinctive physicochemical properties [2,3].Specifically, COFs are composed of organic monomers connected by covalent bonds, providing high chemical and thermal stability.And the highly conjugated structure of COFs can significantly improve their resistance to irradiation [4].These enable them to maintain structural integrity and functionality under various extreme environments, making them suitable for handling complex radioactive contamination scenarios.This enables them to maintain structural integrity and functionality under various extreme environments, making them suitable for handling complex radioactive contamination scenarios.Furthermore, the designability of COF skeletons allows for the