Highly efficient and selective removal of Cr(VI) by covalent organic frameworks: Structure, performance and mechanism

In consideration of increasingly serious environmental issues, and concern for the environmental impact by heavy metals, materials have been developing to act as adsorbents with high capacity and selectivity for metal ions in the environment. Covalent organic frameworks (COFs) are potential materials for addressing these environmental challenges as their molecular skeletons can be aimed at particular application by predesigned affine functional groups. Here, two COFs namely COF1 and COF2 with different hydroxyl group distributions were successfully synthesized from 1,3,5-triaminobenzene (TAB) and 2,3-dihydroxyterephthalaldehyde (2,3-Dha) or 2,5-dihydroxyterephthalaldehyde (2,5-Dha), respectively. COF2 exhibited a high adsorption capacity of 649.35 mg g−1 for Cr(VI) in water, performing better than COF1 and most of previously reported adsorbents. COF1, however, performed better than COF2 at a low Cr(VI) concentration (<20 mg L−1), while COF2 was more efficient at higher concentration due to the more available and homogenic sites provided by the para –OH functional group. Both COFs showed a good performance in removing Cr(VI) within a wide solution pH, and exhibited high selectivity for Cr(VI) in mixed ion solutions. According to FTIR and XPS spectra analysis, the synergism from electrostatic interaction and intramolecular hydrogen bond mainly devoted to the capture of Cr(VI) in acidic or slightly basic solution. Based on the aforementioned research, designing COFs with high selectivity and adsorption capacity for the targeted containments would be a tendency in the further application of adsorption technology.