Effective removal of Cr(VI) from aqueous solutions by covalent organic framework modified with the hyper-crosslinked aniline polymer: Adsorption performance and mechanism

Due to their carcinogenicity and persistence, water pollution of heavy metals, particularly chromium (VI), presents a key environmental problem. The attractive properties of a covalent organic framework make it a promising adsorbent for removing pollutants from water. Nevertheless, the lack of diversity and number of functional groups and the insufficient stability of the covalent organic skeleton limit its potential for adsorptive removal of specific metal ions, such as hexavalent chromium Cr(VI). A series of COFs@HPAN composed of protonated hypercrosslinking aniline polymer (HPAN) and COFs were synthesized via Friedel-Crafts reaction of dimethoxymethane with aniline in the presence of COFs. The modification with COFs not only provides HPAN with a higher specific surface area and improves its dispersibility, but it can also result in COFs@HPAN surface rich in functional groups. The structure of COFs@HPAN is verified by different characterization methods, such as FTIR, XRD, SEM, BET, and so on. 1610 cm -1 (-NH 2 ), 1657 cm -1 (C=N), 3388 cm -1 (N-H), and 2904 cm -1 (-CH 2 ) confirms the successful introduction of HPAN at COFs@HPAN. The XRD pattern of COFs@HPAN shows excellent crystal structure with distinct characteristic peaks at 21.1o and 26.8 o . The surface area, pore size, and pore volume of COFs@HPAN are 9.48 m 2 /g, 12.5 nm, and 0.04 cm 3 /g, respectively. SEM image shows COFs@HPAN contains numerous small pores, along with a high surface area and porosity, making it highly effective for Cr(VI) adsorption. The impact of time, initial pH, temperature, initial concentration, and ionic strength on Cr(VI) adsorption capacity was explored through batch tests. The adsorption is spontaneous and endothermic and follows Freundlich's isothermal adsorption and quasi-second-order model, suggesting chemisorption. At 318 K, the highest adsorption capacity of Cr(VI) was 412.26 mg/g. Additionally, COFs@HPAN has good anti-interference and reusability. Except for CO 3 2- , SO 4 2- , and PO 4 3- , the presence of coexisting anions did not significantly affect Cr(VI) adsorption, and the adsorption capacity remained well above 166 mg/g after 10 cycles. Electrostatic adsorption, chelation, and reduction are the predominant processes regulating Cr(VI) removal. XPS analysis demonstrated a nearly 77 % reduction of Cr(VI) to Cr(III) through electron transfer from the electron-donating functional groups such as -NH 2 . The results show that COFs@HPAN have good Cr(VI) adsorption capacity and can be used for the treatment of Cr(VI) in acidic wastewater.