Enhanced adsorption selectivity of bisphenol analogues by tuning the functional groups of covalent organic frameworks (COFs)

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Bisphenol analogues (BPs) are widely used as plasticizers and can be released during the aging and degradation of microplastics. Their persistence in water can cause serious harm to the ecosystem and human health. To improve the capture ability of COFs toward these estrogen-like toxins from water, amino group-functionalized COFs (COFs-TpBD(NH2)2) were constructed from nitro group COFs (COFs-TpBD(NO2)2) via the reduction of nitro to amino groups, and the adsorption behaviors for the five BPs (BPA, BPF, BPC, BPS and 4-CP) were compared. The focus was laid on the role of functional-group-tuning in the changes of adsorption capacity, selectivity and mechanisms of the COFs absorbents. The results showed that TpBD(NH2)2 has a higher adsorption capacity and better adsorption selectivity for most BPs than TpBD(NO2)2. COFs with nitro and amino groups show the best adsorption selectivity for BPC (KF = 6.71 min−1) and BPF (KF = 9.49 min−1), respectively. Chemisorption dominates the adsorption of the two COFs, and internal particle diffusion is the rate-determining step. The adsorption behavior difference between the two COFs was ascribed to the successful conversion of functional groups of the COFs from nitro to amino groups, which was proven by FT-IR, SEM, PXRD, and BET characterization results. Adsorption of BPA, BPC and BPF by TpBD(NO2)2 is positively related to hydrophobic interactions (represented by log Kow), but the adsorption mechanism of TpBD(NH2)2 was mainly attributed to the electrostatic interaction, as evidenced by the zeta potential and pKa. Hydrogen bonds were proven to be a critical factor that affects the adsorption of BPS and 4-CP by COFs. This study on the appropriate selection of COFs functional groups can provide insight into the future design of adsorbents and the prevention of BPs pollution release from microplastics.