Using polyelectrolyte coatings to improve fouling resistance of a positively charged nanofiltration membrane

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Abstract

In this study, a coating technique was applied to prepare an antifouling nanofiltration membrane. We first designed a positively charged nanofiltration (NF) membrane by chemical modification of P84 copolyimide using branched polyethylenimine (PEI). Then, a layer of water-soluble polymers was adsorbed onto the membrane surface in a dynamic manner. With such coatings, membrane surface properties such as hydrophilicity, roughness and charge were modified to give improved resistance to fouling. Depending on the coating materials, the coating layer may be erasable or inerasable. For example, the neutral polymer polyvinyl alcohol (PVA) may be adsorbed onto the membrane surface by hydrogen bonding. Such interaction becomes weakened during acid cleaning so that the PVA layer can be detached. Thus, if membrane fouling occurs, the PVA layer and attached foulants can be removed by acid cleaning to refresh the membrane. Negatively charged polymers such as polyacrylic acid (PAA) and polyvinyl sulfate (PVS) can be adsorbed onto the membrane surface by electrostatic force. Such strong interactions made the coating layers stable during acid cleaning. However, these coating layers permit removal of the foulants by a simple treatment with acid.

Introduction

Membrane processes are often hindered by fouling due to a buildup of the material being rejected [1]. As fouling progresses, membrane productivity declines; higher pressures and thus more energy must be expended to achieve the desired throughput. Cleaning strategies must be implemented to remove foulant material and restore productivity. In many cases, however, the fouling is irreversible and the membrane elements must be replaced [2]. Organic impurities in the water such as proteins, humic substances, and polysaccharides have been implicated as strong, irreversible foulants [3]. Such foulants can adsorb to the membrane surface due to hydrophobic interactions, hydrogen bonding, van der Waals attractions, and electrostatic interactions [4]. Therefore, an effective method to reduce fouling is to modify the membrane surface to minimize these adsorptive interactions between foulant and membrane [3], [4], [5].

Several surface characteristics of membranes are known to be strongly related to fouling such as hydrophilicity, roughness and charge [6], [7]. It has been generally acknowledged that membranes with hydrophilic surfaces are less susceptible to fouling [4], [8], [9]. Membranes with rougher surfaces are observed to be more favorable for foulant attachment resulting in more extensive fouling and faster fouling rates [7], [10], [11], [12], [13]. For charged organic compounds, electrostatic attraction or repulsion forces between the component and the membrane, which depend on pH, influence the degree of fouling; when the surface and foulant have similar charge, foulant adsorption is reduced [14], [15]. Therefore, development of membranes with a smooth hydrophilic surface that has a surface charge similar to the foulant seems to be desirable for antifouling purposes [16].

Over the last few years researchers have devised various strategies to modify membrane surfaces, which can be generally divided into two categories: grafting and coating [4]. In grafting, hydrophilic species such as polyethylene glycol and polyacrylic acid can be covalently bonded to the membrane surface by chemical, low-temperature plasma, or photochemical techniques [17], [18], [19], [20], [21]. Although surface grafting has shown high fouling resistance, the technique leads to a permanent change of membrane chemistry and properties. For example, membrane permeability may be reduced because the grafting layer adds an extra hydraulic resistance [6], [17]. Grafting may also increase manufacturing costs due to process complications, time consumption, and extensive use of organic solvents and monomers [4]. Compared to grafting, coating a thin layer of water-soluble polymers or surfactants from solution by physical adsorption provides certain distinct advantages for surface modification [22], [23], [24], [25]. Adsorbed coatings are relatively simple to apply, the process can be performed in existing membrane installations, and the structure of the membrane is not likely to be affected by the adsorbed molecules. In addition, the type of coating can be tailored to the specific application of interest [25]. Until now, most studies of adsorbed coatings have focused on ultrafiltration membranes. Preparation of antifouling nanofiltration membranes using the coating technique has not been reported.

Depending on adsorption affinity with the membrane surface, the adsorbed coating layer can be stable or erasable. Thin films formed through layer-by-layer (LbL) deposition of positively and negatively charged polyelectrolytes show good stability due to electrostatic attraction among the membrane surface and the deposited layers [26]. These multilayer polyelectrolyte films have previously been deposited on ultrafiltration substrate membranes to prepare composite membranes for nanofiltration [26], [27]. Although antifouling properties of the LbL membranes are still rarely reported [28], this type of membrane may potentially have high fouling resistance due to the hydrophilicity of the applied polyelectrolytes and controllable surface charge [29]. On the other hand, hydrogen-bonded layer-by-layer films have attracted considerable attention in recent years because the hydrogen bonding can be altered by changes in solution pH and thus the films can be erased and replaced [30], [31], [32], [33], [34]. When applying this erasable film to the membrane surface, any foulant material that deposits on top of the film could be effectively removed by detaching the film from the membrane surface, which leaves behind a clean membrane. It may be much easier and more cost-effective to remove and replace the film instead of replacing the membrane. To the authors’ knowledge, such an application has not been previously proposed.

In a previous study, positively charged nanofiltration membranes were developed by chemical modification of P84 copolyimide membranes using branched polyethylenimine (PEI) [35]. The PEI-modified polyimide membranes (P84-PEI membranes) have a highly cross-linked structure which makes these membranes stable in various operating environments including high temperature (100 °C), organic solvents, and mild acid and base (2  pH  10). Meanwhile, due to the Donnan repulsion effect, these membranes showed efficient removal of multivalent heavy metal ions (>95%) and may potentially be used for treatment of industrial wastewater.

In the present study, the P84-PEI membranes were further modified by adsorption of a layer of polymers from a dilute solution in a dynamic manner [36]. Polyvinyl alcohol (PVA), polyacrylic acid (PAA) and polyvinyl sulfate-potassium salt (PVS), were tested as modifying agents. The effect of these coatings on membrane surface roughness and hydrophilicity was characterized by atomic force microscopy (AFM) and contact angle measurements. The charge and pore size of the unmodified and modified membranes were qualitatively analyzed using ions and uncharged sugars as probes. Stability of the coating layers during acid or base cleaning was studied by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Fouling experiments were carried out in a cross-flow setup using three model foulants, bovine serum albumin (BSA), sodium alginate (SA) and humic acid sodium salt (HA) representative of proteins, polysaccharides and natural organic matter (NOM). The effect of the polymer coatings for fouling prevention was tested with respect to both the reduction of flux decline and the cleanability by acid after fouling.

Section snippets

Chemicals

P84 powder was purchased from HP Polymer Inc. and dried in vacuum at 160 °C for 12 h before use. PEI (MW: 25,000), PVA (MW: 89,000–98,000), PAA (MW: 250,000) and PVS (MW: 170,000) were purchased from Aldrich. Three dilute coating solutions with concentrations of 50 mg/L were prepared by dissolving PVA, PAA or PVS in deionized (DI) water at 80 °C overnight. The PVS solution was filtered before use to remove the insoluble impurities. The pH of the PVA, PAA and PVS solutions were 5.7, 4.7, and 5.8,

Surface chemistry of the P84-PEI NF membrane

Fig. 1 shows the chemical reaction between P84 copolyimide and PEI. It can be seen that the resultant P84-PEI NF membrane contains carbonyl, amide, amine, ammonium and phenyl groups on the surface. Among them, the amine and ammonium groups provide positive charge on the membrane surface [35]. Therefore, this membrane could adsorb negatively charged foulants via electrostatic attraction. Common foulants like BSA, HA and SA contain carboxylic groups and thus are negatively charged at neutral pH

Conclusions

In this study we used three water-soluble polymers PVA, PVS and PAA to form a protective coating layer on the surface of a positively charged NF membrane to improve membrane fouling resistance. After applying these coatings, pore size was reduced, permeation flux decreased, and rejection to uncharged sugars and charged salts increased. Hydrophilicity and smoothness were improved by these coatings, which was favorable for fouling resistance. Surface charge varied with different coating

Acknowledgments

This work was supported by The WaterCAMPWS, a Science and Technology Center of Advanced Materials for the Purification of Water with Systems under the National Science Foundation agreement number CTS-0120978. SEM analysis was carried out in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439.

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