Probing the adsorption behavior and mechanism of NO2 and NH2 functionalized covalent organic frameworks (COFs) for removal of bisphenol A

In order to efficiently remove bisphenol A (BPA) known as a typical endocrine disrupting chemical (EDC) from the water environment, the adsorption capacities of two different groups of covalent organic frameworks (COFs) materials for BPA were investigated. COF(NO2) was directly synthesized by solvothermal method, and COF(NH2) was obtained by reducing COF(NO2) with tin chloride. The prepared COFs were characterized by scanning electron microscopy (SEM), Brumaire-Emmett-Teller theory (BET), X-ray powder diffraction (XRD), and Fourier transform-infrared (FTIR), and X-ray photoelectron spectroscopies (XPS). The effects of initial pH value and ion concentration of the solution were analyzed through batch adsorption experiments. The density functional theory (DFT) calculation of adsorption energy and XPS analysis before and after adsorption were performed to reveal the adsorption mechanisms of BPA by two COFs. The COFs materials achieved the best adsorption performance for BPA in the weak acid of aqueous solution. Although the specific surface area of COF(NH2) was lower, the adsorption capacity of COF(NH2) (190.22 mg/g) was higher than that of COF(NO2) (155.14 mg/g) at 298 K. In terms of mechanism, COF(NH2) favored BPA removal from aqueous solution was mainly due to the formation of hydrogen bonds between amino groups and phenolic hydroxyl groups and the synergistic effect of Lewis acid-base action between amino groups and BPA. This study offers insight into future appropriate selection of COFs functional groups with removal of bisphenol A from wastewater.