Triazole and methylthio modified covalent organic frameworks for enhancing Hg(II) adsorption from water

The presence of mercury (Hg(II)), a highly toxic pollutant in the aquatic environment, motivates researchers to use an effective method to remove it from water. Herein, we synthesized COF-TAZ and COF-TOE via postmodification with 3-mercapto-1,2,4-triazole and 4-(methylsulfanyl)thiophenol to form triazole group and methylthio group, respectively, and followed by characterization with SEM, FTIR, and XRD, etc. The adsorption behaviors of Hg(II) on two modified COFs were evaluated by adsorption kinetic models using the pseudo-first order, pseudo-second order and intra-particle diffusion models, as well as isotherm models, i.e., Langmuir and Freundlich models. COF-TAZ and COF-TOE manifested maximum experimental uptake capacity of 108.09 mg g-1 and 104.85 mg g-1, respectively. Notably, COF-TAZ exhibited commendable binding selectivity towards Hg (II) and displayed a good recycling ability, with a removal percentage still above 90% after five cycles. Especially, the adsorption mechanisms of COF-TAZ and COF-TOE were clarified by FTIR, XPS and DFT calculations, suggesting that electrostatic interaction and chelation were involved in the adsorption process. This study shows that functionalized COFs have good application prospects in effective removal of heavy metals in water.