Volatile radioactive elements, primarily 129I and 131I, in the form of molecular iodine (I2) or organic iodides (mainly CH3I), are harmful and must be removed before discharging the nuclear off-gas. Despite the efforts devoted to developing novel adsorbents for iodine capture, certain critical issues have been overlooked. From the perspective of system evaluation, most of the adsorbents were evaluated using a static system, where the testing conditions are irrelevant to practical applications and cannot be regulated flexibly, resulting in the lack of structure-function relationship under near-realistic conditions. From the perspective of material design, an ideal iodine adsorbent should meet the following requirements: (i) chemically and thermally stable; (ii) have a remarkable adsorption capacity for low-concentration iodine; (iii) exhibit easy recyclability to decrease the operating cost. However, previous studies did not consider these important factors when examining whether the adsorbent has practical value.
In this dissertation, we developed a series of efficient iodine adsorbents, including physical adsorbent (all-silica zeolite EMM-17) and chemical adsorbents (MOF MFU-4l and its analogues), and evaluated their performance under different testing conditions using a dynamic adsorption system to gain a deeper insight into the structure-function relationship under near-realistic conditions. As a result, all-silica EMM-17 zeolite outperforms previously reported zeolites in terms of gravimetric and volumetric adsorption capacity, due to the combination of optimal pore size, high pore volume, strong hydrophobicity, and suitable particle morphology. MFU-Zn-SCN exhibits an ultrahigh CH3I breakthrough uptake at 150 ℃, 0.01 bar (0.41 g g-1) due to the strong nucleophilicity of SCN-. Moreover, due to the reversibility of the coordination bond, it can be recycled at least three times without obvious performance deterioration via simple ion exchange after CH3I adsorption. For Cu(I)-MFU-4l, the strong non-dissociative chemisorption of open Cu(I) sites towards CH3I enables its excellent CH3I uptake at 150 ℃, moreover, open Cu(I) sites can effectively interact with I2 through a redox reaction, converting the original N3-Cu(I) to N3Cu(II)-I. The high tunability of its secondary building units also allows the I2 loaded Cu(I)-MFU-4l to be easily recycled by ion exchange, maintaining the uptake efficiency at least 60 % over three cycles.
Chem Ph.D Candidate supervised by Professor Yu Han