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Abstract Membrane-based filtration is a promising technique to treat the enormous amounts of oily wastewater, specifically those with micron-sized oil droplets. However, the understanding on the effect of the surfactants, that are inevitably present to stabilize the oil emulsion, on the filtration performance remains poor. This study aimed to investigate the effect of surfactant type (namely, non-ionic Tween 20, positively charged CTAB, and negatively charged SDS) on filtration flux and membrane fouling during the microfiltration of the surfactant-stabilized oil emulsion with mean droplet sizes of approximately 20 μm. The Optical Coherence Tomography (OCT) was employed to quantify the evolution of fouling, and both the DLVO and XDLVO models were used to quantify the oil droplet-membrane and oil droplet-deposited layer interaction energies. Two key understanding on the correlation between the DLVO and XDLVO predictions with flux and fouling were obtained. Firstly, the steep flux enhancement vis-à-vis a DI water feed by the feed containing CTAB-stabilized oil emulsion was tied to the attractive interaction of the surfactant with the membrane, as evident from the DLVO model. This attraction was not related to the extent of membrane fouling, which was relatively lesser for the CTAB-stabilized oil emulsion. Secondly, the extent of membrane fouling was tied to the repulsive energy magnitudes rather than attractive ones, specifically in that the least repulsive energy values of the Tween 20 – stabilized oil emulsion was linked to the most extensive fouling. Highlights • Differences in flux, fouling and interaction energy due to surfactant type. • Surfactants adsorbed onto membrane to give the same charge sign as the oil emulsion. • Correlate DLVO and XDLVO profiles with flux and fouling trends. • Attractive surfactant-membrane energy linked to flux enhancement. • Greater repulsive interaction energy tied to lesser extent of fouling. [ABSTRACT FROM AUTHOR]

Membrane-based filtration is an emerging technique for processing oily wastewater. Improving the efficiency of oil-water separation via membranes entails novel techniques for studying the complex interactions between the oil droplets and membrane underlying fouling. This paper presents the first study that applies optical coherence tomography (OCT) to the characterization of membrane fouling by oil emulsions. A series of dead-end filtration experiments was performed to characterize the rejection of oil droplets (~ 10 µm; hexadecane) by the membrane structure (0.45 µm PVDF) via three-dimensional (3D) OCT scanning in real time. The experimental results were compared with the control experiments with ~ 10 µm glass beads to identify the optical artifacts. Both the external and internal fouling by the oil droplets were successfully revealed by analyzing the variation in OCT intensity at various layers that were mathematically defined in terms of the coordinate surfaces parallel to, and above and below, the feed-membrane interface. The evolution of membrane fouling was quantified by evaluating the fraction of fouling voxels as a function of time at varied depths. This study demonstrates that the OCT-based characterization has the potential to shed light on the complex interactions occurring in oil-water separations via membrane filtration, particularly by providing real-time non-invasive monitoring of internal fouling, the understanding of which is valuable for both fundamental research and practical applications. [ABSTRACT FROM AUTHOR]

Some inconsistencies were found in the literature on the effect of surfactant type on membrane fouling by surfactant-stabilized oil emulsions. For instance, a recent study showed that the fouling trends revealed by optical coherence tomography (OCT) were incongruent with the flux decline trends. This study was targeted at providing more mechanistic insights on the evolution of the distinct membrane layers (namely, diffusion polarization, membrane skin and membrane microporous substrate layers) via electrical impedance spectroscopy (EIS). The three surfactants investigated were cationic cetyltrimethylammonium bromide (CTAB), anionic sodium dodecyl sulphate (SDS) and non-ionic Tween 20, the oil was hexadecane, and the ultrafiltration membrane was polyethersulfone (PES). The Tween 20-stabilized oil emulsions exhibited a greater flux decline than that of SDS, which agreed well with the relative shifts of the Nyquist plots and increase in conductance with time. Interestingly, the CTAB-stabilized oil emulsions exhibited a unique flux decline trend, in which the flux decline was the least in the initial 0.5 h then the worst beyond 0.75 h. Despite the negligible flux decline initially, EIS results indicated significant shifts of the Nyquist plots and increase of the conductance with time, which was tied to the extensive adsorption of CTAB. As filtration progressed beyond 0.5 h, the conductance and capacitance of the membrane skin layer in the case of CTAB-stabilized oil emulsions became respectively higher and lower than that in the case of CTAB alone, signifying that the deposition of oil emulsions had become more dominant than that of surfactants. Therefore, this study provided clear evidence that the surfactant oppositely charged to the membrane (i.e., CTAB) adsorbed extensively initially to give negligible flux decline, which was not the case for the surfactants that were similarly negatively charged like the membrane. This only lasted for 0.5 h beyond which fouling became pronounced per dictated by DLVO interaction energy. Image 1 • Understanding of fouling behaviors of oil emulsions stabilized by three surfactants. • CTAB-stabilized oil emulsions gave distinctly different flux decline trends. • EIS revealed extensive CTAB adsorption initially caused negligible flux decline. • Only CTAB caused capacitance to increase, tied to polarity change of the membrane. • Surfactants oppositely charged to the membrane deterred fouling only initially. [ABSTRACT FROM AUTHOR]

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. • Novel grafting of zwitterionic SBMA on hydrophobic PVDF membrane for MD applications. • Modified membranes consistently out-performed pristine ones for five foulants. • Reduction of hydrophobic-hydrophobic affinity underlie superior performance. • Electrostatic interactions deficient to explain fouling/wetting trends among foulants. • Greater ratios of zeta potentials of foulant to membrane gave better performance. [ABSTRACT FROM AUTHOR]