Zwitterionic grafting of sulfobetaine methacrylate (SBMA) on hydrophobic PVDF membranes for enhanced anti-fouling and anti-wetting in the membrane distillation of oil emulsions

Membrane distillation (MD) is promising for treating the vast amounts of oily wastewater, but unfortunately membrane fouling and wetting remain major impediments. This study aimed to firstly assess a novel means of modifying membranes to mitigate fouling/wetting by grafting the hydrophobic PVDF membrane with zwitterionic sulfobetaine methacrylate (SBMA) polymers, which have been shown to offer significant fouling resistance in ultrafiltration, but their efficacy has not been verified in MD. Characterization studies indicate that, relative to the pristine membranes, the modified membranes had enhanced hydrophilicity/oleophobicity, elements corresponding to SBMA according to the XPS spectra, a denser layer at the surface corresponding to the SBMA layer per the SEM images, nearly constant porosity, less negative zeta potential values and slightly higher fluxes for 5 g/L NaCl feeds. These characteristics enabled the modified membranes to give lesser flux declines and conductivity increases than the pristine membranes for the range of foulants investigated. The second aim of the study was to further understand the mechanisms underlying fouling and wetting. One suite of experiments involved high concentrations (2000 ppm) of two surfactant-free oil emulsions (namely, hexadecane and soy bean oil), while another suite involved hexadecane emulsions stabilized by three surfactants of different charges (namely, SDS, CTAB and Tween 20). Results indicate that the relative extents of fouling/wetting among the different foulants cannot be explained by electrostatic interactions or surface tension of the feed (tied to liquid entry pressure) or reduction of hydrophobic affinity alone, but the enhanced hydrophilicity and greater absolute ratios of the zeta potential of the oil emulsion to the membrane conferred by the zwitterionic SBMA layer were keys to mitigating fouling and wetting.