Omniphobic membrane via bioinspired silicification for the treatment of RO concentrate by membrane distillation

Membrane distillation (MD) is promising for the reclamation of reverse osmosis (RO) concentrate ascribed to the advantages of moderate membrane fouling, space-saving and theoretical 100% rejection to non-volatile substances. However, wetting issues remain great challenges for the hydrophobic porous membrane in the treatment of industrial wastewater by MD. In this study, a novel omniphobic membrane is developed via bioinspired silicification and fluorosilanization. The effects of polydopamine/polyethylenimine (PDA/PEI) co-deposition and silicification time on the morphology, pore structure, wettability and separation performance of the membrane are systematically investigated. The results show that the membrane surface is evenly covered by a rough silica layer after bioinspired silicification. The omniphobic membrane exhibits excellent anti-wetting properties with high liquid entry pressure (326.4 kPa) and contact angles to liquids with various surface tensions such as water (162.50°), diiodomethane (149.20°), mineral oil (135.97°), etc. The omniphobic modification has little effect on the pore structure but increases the mechanical strength of membranes significantly. The MD operation results reveal that the virgin membrane is rapidly wetted by a feed solution of 0.1 mM sodium dodecyl sulfate (SDS) in 35 g/L NaCl, while the omniphobic membranes maintain stable MD performance over a 420 min operation against the feed with a SDS concentration of up to 0.3 mM. The omniphobic membrane shows a superior separation performance in the treatment of RO concentrates from dyeing and printing industry without wetting and fouling observed. Meanwhile, high rejections to salts, SDS, metals, water quality indexes (COD, TOC, TN, NH3–N) and organic matters are achieved, demonstrating a high efficiency of the omniphobic membrane for the reclamation of RO concentrates.