Adsorption of Pb(II) on covalent organic frameworks: Overlooked sorption sites formed in the post-synthetic modification process

The post-synthesis strategy is frequently employed to create functional covalent organic frameworks (COFs) for the removal of heavy metals. Different post-synthesis methods can yield structurally distinct side chains. However, the potential adsorption role of these side chains is often overlooked, even though they can provide efficient adsorption sites that significantly enhance the adsorption performance of functional COFs for targeted pollutants. In this study, two post-modification reactions were conducted to introduce functional groups into the original COFs, namely acetylene COFs and ethylene COFs (referred to as COFV1 and COFV2). This resulted in the synthesis of modified MCOFV1 and MCOFV2 with emerging triazole and thioether groups, respectively. Subsequently, these modified COFs were characterized and employed to investigate the influence of the newly formed structures on the adsorption of Pb(II). Density functional theory (DFT) calculations were carried out to explore the adsorption mechanism and the binding energies between COFs and Pb(II). The results indicate that after modification, the specific surface area of COFV1 and COFV2 decreased by 8.08 and 2.05 times, reaching 204.9 m(2) g(-1) for MCOFV1 and 587.5 m(2) g(-1) for MCOFV2. However, their Pb(II) equilibrium adsorption capacity significantly increased, with MCOFV1 achieving 52.1 mg/g (11.5 times higher than COFV1) and MCOFV2 achieving 31.1 mg/g (1.6 times higher than COFV2) based on the pseudo-first-order model. This indicates that the triazole group, which is produced in the post-modification reaction, demonstrates greater efficacy in the removal of Pb(II) as compared to the thioether group. This study reveals that efficient bifunctional groups can be formed through suitable post-modification methods, enhancing the adsorption performance of COFs in environmental remediation.