Distinct Effect of Conjugation Units in Covalent Organic Frameworks on Iodine and Gold Adsorption

The capture of radioactive iodine and recovery of precious metal gold have become hot research topics in recent years. Covalent organic frameworks (COFs) linked by imine bonds are ideal adsorbents for iodine and gold due to their abundance of N sites. The N site as an electron-deficient group has been well-discussed; therefore, in the present work, we explored the effect of its conjugation degree on gold and iodine adsorption by modulating the electron-rich unit. As the degree of donor conjugation increases (COF-Be, COF-Na, and COF-An), the adsorption of gold rises (492.0, 711.3, and 1080.7 mg/g), which is the opposite for iodine (3.8, 2.9, and 2.02 g/g). By characterization and theoretical calculations, the increase in the conjugation degree enhances the reduction of gold by COFs. In contrast, the decrease in the density of N sites and the decrease in the pore volume led to a decrease in the adsorption capacity for iodine. The work clarifies the differences in the adsorption processes of gold and iodine by these three COFs and provides a new idea to optimize the adsorption capacity of gold by adjusting the degree of donor conjugation of COFs.