Assessment of fouling in native and surface-modified water purification membranes
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Fouling is a major obstacle to the implementation of membranes in water purification applications. Hydrophilization of the membrane surface tends to mitigate fouling because hydrophobic interactions between foulants and the membrane are reduced. Polydopamine was deposited onto membranes to render their surfaces hydrophilic. The chemical structure of polydopamine, which was previously ambiguous, was investigated by many spectroscopic techniques. While previously thought to consist of covalently-linked monomers, polydopamine was found to be an aggregate of partly-oxidized dopamine units linked by strong, non-covalent secondary interactions. Polydopamine was also used as a platform for the molecular conjugation of other anti-fouling materials, such as poly(ethylene glycol), to the membrane surface. Membrane fouling was assessed by constant permeate flux crossflow filtration with an oil/water emulsion feed. The threshold flux--the flux at which the rate of fouling significantly increases--was determined by a well-established flux stepping technique. Membrane resistance evolution during fouling was compared for constant flux and constant transmembrane pressure operation using unmodified membranes. Below the threshold flux (slow fouling), good agreement in resistance evolution was found between the two operational modes; above the threshold flux, significant deviation was observed. The effect of polydopamine and polydopamine-g-poly(ethylene glycol) surface modifications was studied under constant flux crossflow fouling conditions. The surface modifications were found to increase the membrane resistance, resulting in higher transmembrane pressures in the modified membranes than in the unmodified membranes at fluxes below the threshold flux. Modified membranes were also compared to unmodified membranes with the same pure water permeance (same initial resistance). In this case, the modified membranes had lower transmembrane pressures during fouling than the unmodified membranes, suggesting that a preferred method of membrane surface modification is to begin with a membrane of higher permeance than required, and then surface-modify it to achieve the desired permeance. The efficacy of polydopamine and polydopamine-g-poly(ethylene glycol) surface modifications in reducing biofouling was also evaluated. Modified membranes showed reduced protein and bacterial adhesion in short-term tests, which are commonly used to assess biofouling propensity. However, long-term operation under hydrodynamic conditions mimicking those of an industrial module showed no benefit of the hydrophilic coatings in limiting biofouling.