Bifunctional hydrogel coatings for water purification membranes: Improved fouling resistance and antimicrobial activity
Research highlights
► Bifunctional PEGDA-RNH3Cl hydrogels exhibited extremely high water uptake and permeability. ► The hydrogel coated UF membranes showed outstanding protein- and oil-fouling resistance. ► The ammonium moiety in the hydrogel coatings strongly restricts cell-initiated biofilm growth.
Introduction
Membrane treatment processes, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), have been widely used to purify water as a result of their simple operation and competitive energy efficiency [1]. In particular, low pressure UF membranes, with physical pores in the size range of 0.001–0.1 μm, have become more attractive for various industrial applications such as oil/water emulsion separation [2], [3] beverage production [4], dairy/food processing [5], and membrane bioreactor (MBRs) waste water treatment [6], [7] UF is also considered an effective pretreatment solution for RO-desalination processes to remove turbidity and microbiological contaminants [8].
In general, hydrophobic polymers, such as polysulfones (PSF), polyethersulfones (PES), polyvinylidenefluoride (PVDF) and polyacrylonitrile (PAN), are often used as separation layers of commercial UF membranes due to their strong chemical and mechanical stability. However, one of the major issues with using hydrophobic separation layers for membranes is the strong interaction with hydrophobic chemicals and biological contaminants, including natural organic matters (NOM), emulsified oils, suspended solids and microorganisms [9], [10], [11], [12]. This causes significant internal (pore clogging) and external (surface deposition) membrane fouling, resulting in increased operational costs arising from higher transmembrane pressure requirements, and more frequent membrane cleaning and membrane replacement [11], [13], [14].
Therefore, recent research has focused on surface hydrophilization of UF membranes to improve inherent membrane anti-fouling properties. Surface-initiated grafting of hydrophilic functional monomers has been reported as one way to modify hydrophobic UF membranes. For surface grafting, UF membranes have to be pretreated with UV, plasma, or chemical agents to create reactive sites on the membrane surface, and in most cases, grafting occurs only on the top-surface of membranes so that internal pores are still susceptible to fouling [15], [16], [17]. Self-segregation of hydrophilic moieties has been demonstrated as an effective way for membrane surface and internal pore modification. In this method, an amphiphilic copolymer composed of hydrophilic and hydrophobic moieties is blended with a membrane casting solution containing a hydrophobic base material. During the phase inversion process, the amphiphilic copolymer typically segregates to the polymer–water interfaces to afford a hydrophilic surface [18], [19]. Alternatively, UF membranes can be coated with a highly hydrophilic dense layer to form a composite membrane [20], [21], [22]. These materials act as hydrogels due to their ability to absorb large amounts of water and they provide increased fouling resistance without significant reduction of the water flux relative to the original UF membranes. The dense hydrophilic coating layer acts as a barrier to chemical and biofoulants and thus has the potential to eliminate internal pore clogging as well as to reduce external cake-formation.
As an exemplary anti-fouling material, polyethylene glycol (PEG) has been widely used to modify hydrophobic UF membranes because it has no charge, is highly hydrophilic in nature, and uniquely able to coordinate to surrounding water molecules through hydrogen bonding. The hydrated PEG typically has large excluded volume to repel organic or bio-foulants [21], [23], [24], [25]. However, although PEG effectively mitigates the deposition of organic and bio-molecules, it slowly loses its resistance to cell attachment when exposed to microbial cells over long periods of time [25] which, in turn, promotes the growth of biofilms on the membrane surface. To prevent biofilm growth by microbial cells, cationic ammonium salts have been immobilized on membrane surfaces due to their antimicrobial activity [26]. In this case, the positive charge on ammonium units plays an important role to inhibit cell growth, but charged materials are often fouled by other contaminants with opposite charge. Therefore, dilution of positive charges by hybridizing the ammonium compounds with other hydrophilic neutral materials may help to minimize the deposition of organic- and bio-contaminates (including charged species) while retaining antimicrobial activity.
Here we have developed UV-cured, bifunctional hydrogel coatings comprised of both PEG and a charged compound containing ammonium moiety. The PEG matrix affords a fouling resistant-, inert-surface to prevent the deposition of organic and biocontaminants, and the ammonium salts provide the antimicrobial activity to effectively inhibit cell-initiated biofilm growth. In order to find the optimum hydrogel coating composition for efficient antifouling/antimicrobial activities, free-standing hydrogel films were prepared by varying the weight ratio between the PEG-containing monomer and an ammonium compound, as well as by controlling the amount of water added to the prepolymerization mixture. The intrinsic properties of the bifunctional hydrogel films, such as water uptake, water permeability, and molecular weight cutoff (MWCO), were thoroughly characterized, and the antifouling efficiency of the hydrogel coated UF membranes were evaluated by cross-flow filtration of a synthetic oil–water emulsion and bovine serum albumin (BSA) solution. The antimicrobial activity of the bifunctional hydrogel materials were also studied using Escherichia coli K12 (E. coli K12) that is known to be abundant in municipal wastewater.
Section snippets
Materials
Poly(ethylene glycol) diacrylate (PEGDA, Mn = 700 g/mol), poly(ethylene oxide) (Mn = 1,000,000 g/mol), 2-aminoethyl methacrylate hydrochloride (99%), 1-hydroxycyclohexyl phenyl ketone (99%), bovine serum albumin (BSA), and phosphate buffered saline (pH 7.4) were purchased from Sigma–Aldrich and used as received. Polysulfone (PSF) ultrafiltration (UF) membranes were purchased from Sepro Membranes, Inc. Vegetable oil (Wesson) was purchased from a local grocery store. DC193 surfactant was obtained from
Intrinsic properties of free-standing hydrogel films
Free-standing hydrogel films (ca. 200 μm thick) were prepared from hydrophilic polyethylene glycol diacrylate (PEGDA) and the cationic functional monomer, 2-aminoethyl methacrylate hydrochloride (RNH3Cl) via UV curing in the presence of a photoinitiator. Fig. 1 shows the chemical structures of both monomers. The composition ratio of PEGDA and RNH3Cl were varied from 1:1 (w:w) to 3:1 (w:w), and water content in the prepolymerization mixture was also varied from 60 wt% to 80 wt% to control the
Conclusions
We describe, herein, the formation of UV-cured bifunctional hydrogel coatings comprising hydrophilic polyethylene glycol diacrylate (PEGDA) and a cationic functional monomer with an ammonium salt (RNH3Cl). The hydrophilic nature and large exclusion volume of the crosslinked PEG matrix contributes in repelling organic and/or biofoulants, and the side chains comprising ammonium moieties effectively inhibit bio-film growth.
The water uptake and permeability of the UV-cured bifunctional hydrogel
Acknowledgements
This work was performed under a joint development agreement between IBM Research and the King Abdulaziz City for Science & Technology (KACST). The authors thank Linda Sundberg (IBM Almaden Research Center) for contact angle analysis.
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Current address: IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, United States.