Construction of a stable and superhydrophilic PHMG/TA/Fe(III) composite coating-modified membrane for for oil–water separation

Polyvinylidene fluoride (PVDF)/polyhexamethylene guanidine (PHMG)/tannic acid (TA) coatings have some degree of hydrophilicity but lack stability. To improve the hydrophilicity and stability of antimicrobial PVDF/PHMG/TA membranes, Fe(III) is introduced into the composite system. As a weakly acidic hydrophilic nanoion, Fe(III) can enhance the hydrophilicity and resistance to contamination of the membranes. Furthermore, the stability can be enhanced because a three-dimensional cross-linked coating structure is formed by the complexation of Fe(III), TA and PHMG. This work investigates the effects of different tannic acid concentrations and pH conditions on the hydrophilicity, anti-pollution and stability of the modified coatings. The surface morphology of the membranes was analyzed by field-emission scanning electron microscopy. Additionally, the hydrophilicity and the antipollution performance were evaluated based on the water contact angle, submerged oil contact angle, emulsion flux and flux recovery rate. The stability was evaluated by the changes in flux recovery rate and water contact angle before and after emulsion scouring and ultrasonic treatment. The results show that the modified membranes have favorable hydrophilicity and stability after the addition of Fe(III). The water contact angle can be reduced to 19°, the pure water flux can reach 13,850 L·m−2·h−1, and the emulsion flux can reach 2650 L·m−2·h−1. Moreover, 99.68% flux recovery was achieved after 7 days of deionized water flushing, and the water contact angle remained almost unchanged.