Non-porous covalent organic polymers enable ultrafast removal of cationic dyes via carbonyl/hydroxyl-synergetic electrostatic adsorption

Abstract

Rapid and highly efficient removal of organic dyes from polluted water is of great significance for environmental protection and water resources management. Herein, an anionic covalent organic polymer (COP), denominated as COPOH+CO, with the fastest cationic dye adsorption kinetics so far was prepared and packed into solid phase extraction cartridges for rapid and automatic water treatment. As-prepared COPOH+CO contains abundant hydroxyl and carbonyl groups. Compared to COPOH and COPCO, other two COPs containing only hydroxyl and carbonyl, respectively, COPOH+CO gives a maximum adsorption capacity of 813 mg·g−1 towards cationic dye methylene blue (MB), which is about 4.6 times the sum of those of COPOH (42.4 mg·g−1) and COPCO (135 mg·g−1), suggesting the existence of 1 + 1 > 2 synergy between carbonyl and hydroxyl groups. Such a synergy is well demonstrated by theoretical calculation, which shows that the mutual penetration distance of electrostatic potentials in MB/COPOH+CO complex (4.722 Å) is much larger than those in MB/COPOH (0.443 Å) and MB/COPCO (1.602 Å). Although the rapid synthesis endows COPOH+CO with very poor porosity, the resulted surface adsorption avoids the long-term mass transfer, thus giving an ultra-fast adsorption kinetics with a so far fastest adsorption rate constant of 0.17 g·mg−1·s−1. This work demonstrates that by using the synergetic effect of surface functional groups, rapidly synthesized non-porous COPs may work as promising adsorbents for fast adsorption of targets.