Constant flux crossflow filtration evaluation of surface-modified fouling-resistant membranes
Daniel J. Miller, Sirirat Kasemset, Lu Wang, Donald R. Paul, Benny D. Freeman n
Department of Chemical Engineering, Center for Energy and Environmental Resources, and Texas Materials Institute, The University of Texas at Austin, 200 E. Dean Keeton Street, Stop C0400, Austin, TX 78712, USA a r t i c l e i n f o
Received 23 June 2013
Received in revised form 2 October 2013
Accepted 5 October 2013
Available online 18 October 2013
Threshold flux a b s t r a c t
The surfaces of polysulfone ultrafiltration membranes were modified with polydopamine and polydopamine-g-poly(ethylene glycol) hydrophilic coatings. Unmodified and modified membranes were challenged with a soybean oil emulsion feed at six different permeate fluxes in constant flux crossflow filtration fouling studies. The threshold flux was determined for each membrane. Above the threshold flux, modified membranes generally exhibited lower transmembrane pressures than unmodified membranes. However, below the threshold flux, modified membranes had higher transmembrane pressures than unmodified membranes, likely due to a decrease in permeance resulting from the surface modification. To account for this difference in permeance, polydopamine-g-poly(ethylene glycol) modified membranes were compared to membranes with a thicker polydopamine coating and to an unmodified membrane with a smaller pore size than that used for the surface modified membranes; in this way, all three membranes had similar pure water permeances. In this case, the unmodified membrane exhibited a much higher transmembrane pressure during fouling than the modified membranes, so when membranes of the same permeance are compared, the surface modifications improved fouling resistance. Therefore, a potential strategy to achieve fouling resistance in a membrane of a desired flux and rejection is to modify the surface of a more permeable (and perhaps lower rejection) membrane, thereby making the resulting modified membrane fouling-resistant but leaving it with the desired flux and rejection characteristics. & 2013 Elsevier B.V. All rights reserved. 1. Introduction
Fouling is a significant challenge when using polymer membranes in water purification applications [1–3]. Many porous membranes are made of hydrophobic polymers by a phase inversion process , so hydrophobic solutes in feed water, such as emulsified oils, readily foul such membranes via strong hydrophobic interactions . Such fouling typically decreasesmembrane productivity, requiring increased energy expenditure or cleaning frequency . Surface modification is a common approach to control membrane fouling [6–8]. Because many polymeric membranes are hydrophobic, such surface modifications are frequently directed towards making the membrane surface more hydrophilic . Hydrophilic surfaces are hypothesized to attract a strongly-bound layer of water molecules, whichmay act a buffer to the adhesion of hydrophobic foulants . Historically, hydrophilic surface modifications may take the form of thin, highly-permeable dense films, such as crosslinked poly(ethylene glycol) (PEG)-based coatings [1,2,5,10], or grafted molecules, such as PEG brushes [11–13] or zwitterionic polymers [14,15]. Typically, such modifications reduce the membrane permeance, since the coating or grafted material contributes to the overall mass transfer resistance of the membrane [5,10,13,16–18].
Polydopamine (PDA) deposition has recently been used to hydrophilize a variety of materials , including many types of polymeric membranes [20–23]. PDA may be deposited under mild conditions from buffered, aqueous dopamine solution and forms a robust, thin layer on nearly any surface in contact with the solution [19,24]. The effects of polydopamine on the hydrophilicity, charge, and surface roughness of polymer films and polymeric membranes have been reported previously [19–21,23,25–32]. For example, we have previously reported a substantial increase in hydrophilicity when PDA films were deposited on polysulfone UF membranes . PDA can also act as a surface primer, facilitating the grafting of other molecules to a PDA-modified surface [19,33]. Poly(ethylene glycol) (PEG) is of particular interest due to its widespread use as a foulingresistant surface modification agent [11,34]; poly(ethylene glycol) monoamine (PEG-NH2) may be easily grafted to PDA-modified surfaces . Many types of membranes, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), have shown improved resistance to fouling by emulsified oil
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Journal of Membrane Science 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.037 n Corresponding author. Tel.: þ1 512 232 2803; fax: þ1 512 232 2807.
E-mail address: firstname.lastname@example.org (B.D. Freeman).
Journal of Membrane Science 452 (2014) 171–183 after modification with PDA or PDA-g-PEG [21,29]. Furthermore, the non-specificity of PDA deposition allows for modification of entire membrane modules post-fabrication, as has been recently demonstrated in a report describing purification of flowback water from hydraulic fracturing . In that study, commercial polyacrylonitrile
UF modules and composite polyamide RO modules were modified with PDA in situ by filling the feed side of the modules with buffered dopamine solution, permitting hydrophilization of the membrane surfaces, feed spacers, and other wetted parts. UF modules showed improved fouling resistance and enhanced recovery after typical chemical cleaning procedures .
Like most literature studies reporting the fouling performance of surface-modified membranes, studies on PDA and PDA-g-PEGmodified membranes have focused on constant transmembrane pressure experiments, where the transmembrane pressure (TMP) is fixed, and fouling is evaluated by monitoring permeate flux decline over time [21,29]. This technique is widely used in laboratory fouling studies [10,16,21]. However, many industrial membrane-based water purification systems operate at constant permeate flux to provide a steady rate of product water production with a given membrane area [36,37]. In this operational mode, the permeate flux is fixed, and the TMP increases over time, as the membrane fouls, to maintain the desired permeate flux . Aside from the similarity to industrial practice, constant flux operation has several other benefits. As permeate flux declines in a constant