Solid-solid reaction synthesis of covalent organic framework as a stable and highly active photo-catalyst for degradation of sulfathiazole in industrial wastewater

Large-scale preparation of stable and efficient covalent organic frameworks (COFs) for photo-catalysts is challenging. In this work, an environmentally-friendly ball milling method was applied to synthesize COFs (TpMA) by use of appropriate symmetry combinations (C3 + C3) of building blocks (i.e., melamine (MA) and 1,3,5-Triformylphloroglucinol (Tp)). Characterization confirmed that TpMA were successfully synthesized and exhibit thin ribbon-like structure with wrinkles and rough edges. A concentration of 120 mg/L sulfathiazole, could be completely degraded in the presence of 30 mg TpMA under visible light (λ > 420 nm). The rate of photo-degradation was 0.1498 min−1, which was more than three times than that of TpMA prepared by solvothermal method. Compared with commercial g-C3N4 and self-prepared g-C3N4, TpMA with “g-C3N4 active center-photo-eletron shift platform-electron withdrawing unit” triadic structure shows greater photo-catalytic activity, which exhibited rapid charge separation and electron transport capabilities, and low recombination rates of electron-hole pairs. In addition, TpMA retained 88.5% photocatalytic activity after four cycles under the same experiment conditions, which indicates TpMA had outstanding stability and recyclability. Superoxide radical (·O2−) was determined to play a dominant role in photo-degradation of sulfathiazole through radical scavenging experiments. TpMA was used to treat two industrial wastewaters including cellulose-containing wastewater and polyvinyl alcohol-containing wastewater to determine the practicality of TpMA. The results showed that TpMA possessed favorable anti-interference ability and high photocatalytic activity, which indicates TpMA has practical application potential. The method of ball milling has broad application prospects in the fields of large-scale preparation of COFs, which is conducive to bottom-up preparation of COFs with excellent degradation capacity of organic pollutants.