Preparation of magnetic covalent triazine frameworks by ball milling for efficient removal of PFOS and PFOA substitutes from water

Abstract

Difficult separation of covalent organic frameworks (COFs) greatly limits their application in wastewater treatment. For the first time, the ball-milling method was used to prepare recyclable magnetic covalent triazine frameworks (CTF/Fe₃O₄), and little influence on the structure and adsorption performance of CTFs after ball-milling was observed. We synthesized crystalline CTF1 and amorphous CTF2 materials and then investigated the effect of CTF/Fe₃O₄ mass ratios on the removal of two typical PFOS and PFOA substitutes including sodium p-perfluorous nonenoxybenzene sulfonate (OBS) and hexafluoropropylene oxide trimer acid (HFPO-TA). CTF2/Fe₃O₄ exhibited higher adsorption performance than CTF1/Fe₃O₄ due to the additional quaternary amine groups as highly efficient adsorption sites, achieving adsorption capacities of 1.18 mmol g⁻¹ for OBS and 1.02 mmol g⁻¹ for HFPO-TA, with their adsorption equilibrium within 3 h. The adsorption mechanisms were investigated through Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), and anionic exchange, electrostatic attraction and accumulation on the air bubble surfaces on the adsorbent were proposed. Additionally, the spent CTF2/Fe₃O₄ was well regenerated by the mixture of NaCl and methanol solution and showed relatively stable adsorption for OBS and HFPO-TA within 5 cycles. This study provides a feasible method to make recyclable COF powders for efficient removal of pollutants from wastewater.