PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance

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Abstract

This study focuses on the use of surface-coated reverse osmosis (RO) membranes to reduce membrane fouling in produced water purification. A series of crosslinked PEG-based hydrogels were synthesized using poly(ethylene glycol) diacrylate as the crosslinker and poly(ethylene glycol) acrylate, 2-hydroxyethyl acrylate, or acrylic acid as comonomers. The hydrogels were highly water permeable, with water permeabilities ranging from 10.0 to 17.8 (L μm)/(m2 h bar). The hydrogels were applied to a commercial RO membrane (AG brackish water RO membrane from GE Water and Process Technologies). The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 μm thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater, and coating the membranes appeared to increase salt rejection, in contrast to predictions from the series-resistance model. Zeta potential measurements showed a small reduction in the negative charge of coated membranes relative to uncoated RO membranes. Model oil/water emulsions were used to probe membrane fouling. Emulsions were prepared with either a cationic or an anionic surfactant. Surfactant charge played a significant role in membrane fouling even in the absence of oil. A cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), caused a strong decline in water flux while an anionic surfactant, sodium dodecyl sulfate (SDS), resulted in little or no flux decline. In the presence of DTAB, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, its water flux gradually decreased to 74% of its initial value after 24 h. DTAB-fouled membranes had lower salt rejection than membranes not exposed to DTAB. In contrast, SDS-fouled membranes had higher salt rejection than membranes not exposed to SDS, with rejection values increasing, in some cases, from 99.0 to 99.8% or higher. In both surfactant tests, coated membranes exhibited less flux decline than uncoated AG RO membranes. Additionally, coated membranes experienced little fouling in the presence of an oil/water emulsion prepared from DTAB and n-decane. For example, after 24 h the water flux of the AG RO membrane fell to 26% of its initial value, while the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value.

Introduction

Over 41% of the world population currently lives in water-stressed regions, and with the global population projected to increase by 80 million people per year, this percentage is expected to rise [1]. Only 2.5% of the world's water is available as fresh water, and most of this water is trapped in glaciers or resides far below the earth's surface [2]. Therefore, to meet the growing demand for water, new resources are of significant interest.

One potential water source is purified produced water. Produced water is a waste by-product generated during oil and gas production [3]. Over 15 billion barrels of produced water are generated in the United States annually [3]. Produced water is a complex mixture that can contain emulsified oil, water, soluble organics, particulate matter, and high levels of salt [3]. Produced water composition varies over the lifetime of the well and depends on geographical location [3]. Cost-effective treatment of produced water could generate a new water resource for beneficial uses such as agricultural irrigation, power generation, and even human consumption [3].

Produced water treatment using membrane-based methods has been considered [4], [5], [6]. Typically, membranes are used in multi-step processes, with reverse osmosis (RO) membranes providing one of the final purification steps after extensive pre-treatment by microfiltration, ultrafiltration, and/or nanofiltration membranes [4]. These attempts have shown some promise, but are limited by the need to carefully control pre-treatment conditions to prevent catastrophic fouling of downstream RO membranes [4], [5].

A fouling-resistant RO membrane could increase the efficiency of produced water treatment. Fouling occurs when constituents, such as emulsified oil droplets, deposit on or inside the membrane, markedly reducing permeate flow and decreasing membrane lifetime. Several variables are often cited as major contributors to membrane fouling: surface hydrophilicity, surface charge, and surface roughness [6], [7], [8]. Surface hydrophilicity is related to the affinity of a surface for water over oil [9]. Surface charge typically arises from ionization of chemical groups on the membrane surface and may promote membrane fouling if there are contaminants in the feed solution of opposite charge to that of the membrane [10]. Membrane surface roughness may be influenced by the manufacturing process [8]. Generally, a smoother surface is expected to experience less fouling, presumably because foulant particles are more likely to be entrained by rougher, peak-and-valley topologies than by smoother membrane surfaces [8].

One proposed method for creating a fouling-resistant RO membrane is to apply a smooth coating of a hydrophilic, neutrally charged material [6], [11], [12]. Previous attempts at preparing such membrane coatings used polyether–polyamide commercial block copolymers to coat UF and RO membranes [6], [12]. These coatings were effective in reducing membrane fouling by oil/water emulsions, but the coating materials were only soluble in organic solvents, making it necessary to use membrane substrates that were not chemically attacked by the coating solution during the coating process. Additionally, the water permeability of the coating materials was limited by the essentially impermeable polyamide block in the copolymers. Previous work from this laboratory used crosslinked poly(ethylene oxide) as fouling-resistant coatings for UF membranes [11]. However, the effect of the coating on salt rejection was not addressed, and fouling in the presence of charged surfactants was not explored.

In this study, poly(ethylene glycol) (PEG)-based materials were considered as potential RO membrane coatings for use in applications such as produced water filtration. The water and NaCl transport properties of free-standing films of these PEG-based materials have already been reported [13]. Here, the effect of coating chemistry on water flux and salt rejection of coated RO membranes is presented. Also, the effect of membrane coating chemistry on fouling resistance to charged surfactants and charged surfactant emulsions is explored.

Section snippets

Solution-diffusion model

Transport through reverse osmosis membranes is typically described using a solution-diffusion model [14], [15]. The governing equation describing water flux is [14], [15]:Jw=Lp(ΔpΔπ)where Jw is volumetric water flux (L/(m2 h)), Lp is membrane permeance (L/(m2 h bar)), Δp is the applied transmembrane pressure difference (bar), and Δπ is the osmotic pressure difference between the feed and the permeate solutions (bar).

For a nonporous, defect-free RO membrane, Lp depends on the physical

Materials

Commercial thin film composite polyamide RO membranes were kindly supplied by GE Water and Process Technologies (Minnetonka, MN). The AG RO membrane is a brackish water membrane typically used in spiral-wound modules. According to the product specifications for AG RO spiral-wound modules, the membrane permeance ranges between 2.6 and 3.9 L/(m2 h bar), with an average of 3.1 L/(m2 h bar) [18]. At an applied transmembrane pressure difference of 15.5 bar (225 psig), these permeance values correspond to

Hydrogel characterization

Table 1 summarizes the water and salt transport properties of free-standing films of the hydrogels used in this study. The copolymer densities reported in Table 1 decrease with increasing chain length, consistent with shorter pendant chains promoting more efficient packing and greater hydrogen bonding, as suggested previously in the literature [30], [31]. The densities of PEGDA and 50PEGA hydrogels are approximately equal, suggesting that the chains in the 50PEGA copolymer are long enough to

Conclusions

PEG-based hydrogels were synthesized and applied as coatings to commercial AG RO membranes. The hydrogels have high water uptake and water permeability, making them interesting candidates for fouling-resistant coating materials. Water flux of coated membranes is less than that of uncoated membranes. However, in contrast to expectations from a series-resistance model, which is based upon a coating of a thin layer of poorly salt-rejecting hydrogel on top of a nonporous, high rejection

Acknowledgements

The authors gratefully acknowledge partial support of this work by the U.S. Department of Energy (DOE) under DE-FC26-04NT15547, by the Office of Naval Research (ONR) under 140510771 and 140510158, and by the National Science Foundation (NSF) under CBET-0553957. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE, ONR or NSF. Also, the authors would like to recognize Dr. Hugo Celio of the

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