Performance enhancement of the membrane distillation process via membrane surface modification for surfactant-containing feed water

Abstract

With its many advantages, direct-contact membrane distillation (DCMD) appears to hold potential for the recovery of high quality water from different industrial effluents. Porous hydrophobic polyvinylidene fluoride (PVDF) membranes have been extensively used in DCMD operations. 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. The mechanisms involved are not fully understood due to a lack of study on the interaction between different low surface tension contaminants and the membrane surface in the DCMD domain. To address these challenges, this work aims at developing a fundamental understanding of the relationship between surfactant-stabilized oil-in-water (O/W) emulsions and a PVDF membrane surface in DCMD operations. The results reveal that surfactant concentration and hydrophobicity had an influence on membrane fouling and wetting behaviors of these emulsions. Notably, surfactants with a lower hydrophilic-lipophilic balance value could make the PVDF membrane surface less hydrophobic and cause less severe fouling by restraining the adsorption of oil droplets on the membrane surface. These findings suggest that membrane surface modification is required to achieve fouling- and wetting-resistant properties for robust long-term applications.

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. In this work, two composite hollow fiber membranes were fabricated, one by single-step co-deposition of polydopamine (PDA)/polyethylenimine and the other via accelerated oxidant-induced PDA deposition onto the surface of a commercial hydrophobic PVDF substrate. The successful deposition was verified using different characterization techniques. These composite membranes exhibited Janus wettability with its modified surface being 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 both modified membranes were investigated via bench-scale DCMD experiments by feeding a series of low surface tension solutions. In comparison to the pristine PVDF membrane, the modified membranes demonstrated excellent fouling- and wetting-resistant properties in different surfactant solutions as well as O/W emulsions. This was ascribed to the formation of an interfacial hydration layer and deposition of functional groups within the membranes’ rough hierarchical structures. The PDA-decorated membrane was also used for seawater desalination, during which it maintained a stable flux and high salt rejection rate. Furthermore, the PDA-decorated membrane presented 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 both modified membranes for extended DCMD applications such as water recovery from industrial wastewater containing low surface tension substances.