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The current study aimed to understand both external and internal membrane fouling by three proteins with different net charges, namely, negatively charged pepsin and bovine serum albumin (BSA), as well as positively charged lysozyme. Polycarbonate track-etched (PCTE) membranes were used. Per electrostatic attraction, the flux decline was the worst for lysozyme, which is attributed by the fouling model to the greatest pore blockage (α) and pore constriction (β), and by field-emission scanning electron microscope (FESEM) and optical coherence tomography (OCT) to the most extensive external fouling. Between pepsin and BSA, BSA gave worse flux decline despite its more negative net charge. The fouling model indicates that BSA gave greater pore blockage (α) and denser internal cake (R c / R m), while the quartz crystal microbalance with dissipation (QCM-D) indicates a rigid cake structure. Notably, despite monotonic flux decline with filtration, the OCT fouling voxel trends show significant fluctuations, which has not been reported before and thus signify the unique behavior of protein foulants in straight-through pores. Specifically, the trends below and above the −4.5 μm layer (i.e., 4.5 μm below the feed-membrane interface) are perfectly opposite, indicating the non-uniform protein deposits slipping downwards in the membrane pores as filtration progressed. The dynamic movements of the protein cakes unveiled here warrant more understanding in future studies. • Surprising fluctuations of OCT fouled voxels despite monotonically decreasing flux. • Dynamic downward motion of fouled protein deposit in the straight-through pores. • Delayed fluctuations by BSA tied to densest and most rigid cake. • Worst flux decline by lysozyme due to high pore blockage and pore constriction. [ABSTRACT FROM AUTHOR]

Membrane fouling is inevitable for liquid membrane-filtration processes. While the relationship between surface free energy and membrane fouling is now well-established for aqueous feeds, such a link remained missing for the emerging organic solvent applications. In this study, three organic solvents (namely, methanol, ethyl acetate and acetonitrile) and three colloidal foulants (namely, silicon dioxide (SiO 2), titanium dioxide (TiO 2) and aluminum (Al)) were investigated using the same polyacrylonitrile (PAN) ultrafiltration membrane at five transmembrane pressures (100–500 kPa). The zeta potentials were measured, and the Gibbs free energy of interfacial interactions for both foulant – membrane and foulant – foulant were quantified via the DLVO and XDLVO models to correlate with the fouling trends. Surprisingly, although the XDLVO-based interaction energy is generally acknowledged to correlate well with membrane fouling trends for aqueous feeds, the results here indicate that qualitative agreement with flux decline, cake resistance and cake compressibility was poor. Zeta potentials also exhibited poor predictive capability. Only the DLVO-based interaction energy values agreed with the flux decline trends, but nonetheless failed for cake compressibility trends. This study underscore the different fouling behaviors between aqueous versus organic solvent environments, particularly in the limited capacity of the XDLVO model and zeta potential values to provide insights on the fouling mechanisms. • Flux decline trends agreed with DLVO-based foulant-membrane interaction energy. • Some cake resistance agreed with DLVO-based foulant-foulant interaction energy. • Cake compressibility disagreed with foulant-foulant DLVO, XDLVO and zeta potentials. • XDLVO known to predict fouling well for aqueous, but did poorly for organic solvent. • DLVO provided better membrane fouling predictability than XDLVO or zeta potential. [ABSTRACT FROM AUTHOR]

In this study, we investigated and compared the performance (permeate flux, solute rejection and membrane fouling) of an ultrafiltration membrane and a nanofiltration membrane with water and formamide as solvent. The dextran solutions with water as solvent had lower electrical conductivity and usually greater surface charge compared to that in formamide, which facilitated the electrostatic interactions taken place. As a result, faster flux decline and greater fouling resistance were usually observed in water-based solutions in both UF and NF with the same dextran used as solute. In UF, the solute rejection usually increased as the solvent switched from formamide to water, since stronger electrostatic interactions tended to impede the transport of solute through membrane. Opposite trend was found with NF, that formamide usually led to higher solute rejection compared to water as solvent. Despite the differences of surface charge altered the membrane performance prominently, no quantitative correlation was found between permeate flux and zeta potential values. The analysis using a modified Hermia's pore blocking model indicates that the fouling mechanisms could be altered as the change of solute to membrane pore size ratio, surface charge of membranes and solutes, as well as the solvent properties. [ABSTRACT FROM AUTHOR]