Fabrication of anti-fouling and self-cleaning PHI modified PVDF membranes for high-flux dye removal

The large pore size of ultrafiltration membranes presents a challenge in rejecting small molecules and the accumulated contaminants on the membrane surface severely restrict the treatment efficiency and shorten the lifespan of separation membranes. Herein, poly(heptazine imide) (PHI) is utilized as a modifier to fabricate anti-fouling and self-cleaning polyvinylidene fluoride (PVDF) membranes for high-flux dye removal. The introduction of PHI does not affect the rejection performance of bovine serum albumin (>97%), whilst improving the water permeability and mechanical strength of membranes. The anti-fouling ability is also significantly enhanced with a flux recovery ratio of 91.61%. In addition, the rejection performance of PHI modified PVDF (PHI/PVDF) membranes for anionic dyes, especially for those of low molecular weight, is obviously improved. The rejection ratios of Congo red (CR) and orange G are 99.8% and 87.4%, respectively, and rejection performance for methyl orange is increased from 22.0% (pure PVDF membrane) to 62.5% (M3 membrane with 3 g PHI added). Furthermore, in the presence of anionic dye (such as CR), 99.3% of methylene blue, 91.8% of malachite green and 95.9% of basic red 46 can be rejected and approximately 613 L m^-2 of 50 mg per L CR solution can be processed after 2 h of operation, with the rejection ratio consistently above 98%. The accumulated CR dyes on PHI/PVDF membranes can be easily self-cleaned within 60 min by the H₂O₂-assisted photocatalytic reaction, effectively solving the problem of membrane fouling.