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Inspired by mussels’ byssus with remarkable adhesive power that is neither degraded nor deformed in the marine environment, dopamine coating has emerged as an option for membrane surface modification. With its many advantages, direct-contact membrane distillation (DCMD) appears to hold potential for the recovery of high quality water from produced water. However, membrane fouling and pore wetting are challenging issues of long-term DCMD operations due to the presence of oils and surfactants in these waters. Hence, it is paramount to develop robust membranes with anti-fouling and anti-wetting properties for effective produced water treatment. In this study, we fabricated a composite hollow fiber membrane by single-step co-deposition of polydopamine (PDA)/polyethylenimine (PEI) onto the outer surface of a commercial hydrophobic polyvinylidene fluoride (PVDF) substrate. The successful co-deposition was verified using different characterization techniques. The composite membrane exhibited Janus wettability (Janus membranes have opposing properties at an interface) with its modified outer surface being in-air hydrophilic/underwater superoleophobic for preventing organics adhesion while insuring that unmodified pores beneath the surface remained hydrophobic for vapor transport. The anti-fouling and anti-wetting properties of the modified membrane were investigated via DCMD experiments by feeding a series of low surface tension solutions. In comparison to the pristine PVDF membrane, the modified membrane exhibited promising wetting resistant property against different surfactant types and excellent fouling resistant property against nonionic and cationic surfactants. Moreover, the modified membrane presented a promising long-term performance when feeding a cationic surfactant-stabilized petroleum-in-water emulsion mimicking produced water, during which a stable flux and excellent permeate quality were maintained throughout 7 days of operation. The efficacious combined effects of a hydration layer and protonated amine-functional groups on the PDA/PEI grafted layer could prevent membrane fouling and pore wetting. The results suggest that the mussel-inspired composite PVDF membrane could potentially be used for long-term water recovery from produced water via DCMD. [ABSTRACT FROM AUTHOR]

Porous hydrophobic polyvinylidene fluoride (PVDF) membranes have been extensively used in direct-contact membrane distillation (DCMD) processes. However, these PVDF membranes are vulnerable to membrane fouling and pore wetting in low surface tension feeds, restricting its application for water recovery from challenging industrial wastewaters. Therefore, it is of paramount importance to engineer fouling- and wetting-resistant MD membranes for robust long-term applications. In this study, a superoleophobic composite hollow fiber membrane with sandwich structure has been developed via accelerated oxidant-induced polydopamine (PDA) deposition on both the outer and inner surfaces of a commercial hydrophobic PVDF membrane under slightly acidic conditions (pH = 5). The modified surface prevents organics adhesion ascribing to its underwater superoleophobicity while the unmodified pores remain hydrophobic for vapor transport. The long-term robustness of the PDA-decorated membrane in highly saline feeds containing low surface tension contaminants has been evaluated via bench-scale DCMD experiments. In contrast to the pristine PVDF membrane, the PDA-decorated membrane exhibits excellent fouling- and wetting-resistant properties in different surfactant solutions as well as oil-in-water emulsion. The PDA-decorated membrane has also been used for seawater desalination, during which it maintains a stable flux and high salt rejection rate. Furthermore, the PDA-decorated membrane presents a flux enhancement of up to 70% over the pristine PVDF membrane in 3.5 wt% NaCl solution at 333 K. This study demonstrates the potential of the PDA-decorated membrane for extended DCMD applications such as water recovery from industrial wastewater containing low surface tension substances. [ABSTRACT FROM AUTHOR]