Asymmetric reverse osmosis sulfonated poly(arylene ether sulfone) copolymer membranes

Author's Accepted Manuscript

Asymmetric reverse osmosis sulfonated poly(arylene ether sulfone) copolymer mem-branes

Derek M. Stevens, Bill Mickols, Caleb Funk

PII: S0376-7388(13)00846-6DOI: http://dx.doi.org/10.1016/j.memsci.2013.10.042Reference: MEMSCI12485

To appear in: Journal of Membrane Science

Received date: 14 August 2013Revised date: 30 September 2013Accepted date: 16 October 2013

Cite this article as: Derek M. Stevens, Bill Mickols, Caleb Funk, Asymmetricreverse osmosis sulfonated poly(arylene ether sulfone) copolymer mem-branes, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2013.10.042

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http://dx.doi.org/10.1016/j.memsci.2013.10.042






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Asymmetric reverse osmosis sulfonated poly(arylene ether sulfone) copolymer membranes Derek M. Stevensa,*, Bill Mickols1, Caleb Funka aDow Water and Process Solutions, Edina, MN 55439 1Present address, Phillips 66, Bartlesville, OK

*Corresponding author: Tel.: +1 952 897 4262; Fax: +1 952 914 1086; E-mail address: [email protected] Abstract Reverse osmosis membranes based on partially disulfonated copolymers of biphenol and aryl sulfone (BPS) have been studied in an asymmetric architecture. In these films, BPS represents both the porous support and solute rejection layer of the membrane. A procedure was developed to fabricate these asymmetric films through phase inversion of BPS solutions. Recipe optimization and the inclusion of several post-treatment steps to densify the rejection layer and heal defects gave the largest improvements in salt passage and are described in detail. A strong dependence of the final salt passage on ionic strength and temperature of the annealing solution is demonstrated. Under 2000 ppm NaCl and 15.5 bar test conditions, membranes were developed with salt passage from 0.34–1.81% across a flux range of 2.5–20 lmh (1.5–12 gfd). This performance represents a step change in the capability of BPS membranes for reverse osmosis. To achieve competitiveness with commercial polyamide membranes, several challenges required for continued improvements and commercialization are discussed. Keywords

� Reverse osmosis � Asymmetric membranes � Sulfonated poly(arylene ether sulfone) � Monovalent and divalent rejection

1. INTRODUCTION

Over the last several decades, polyamides have emerged as the dominant material for reverse osmosis (RO) membranes, owing to unrivaled flux and rejection performance.[1, 2] Despite their impressive credentials, these materials exhibit several shortcomings, particularly in their thermal stability and chemical susceptibility to chlorine. Because chlorine is intentionally added as a biocide to many water treatment streams, improved chlorine tolerance has long been an area of focused research interest in the reverse osmosis industry.[3] Nitrogen atoms are the most chlorine sensitive sites in polyamides.[4] While chemical modification to typical polyamide structures has been explored to mitigate this susceptibility, another route is the development of membranes from different polymer families. Polysulfone materials represent one such promising group. These amorphous materials have high toughness and thermal stability, are soluble in

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common organic solvents, and exhibit excellent chlorine tolerance.[5] Unfortunately, polysulfones are also hydrophobic, translating to low water fluxes in reverse osmosis membranes. Accordingly, polysulfones in the water purification industry are most frequently used as ultrafiltration membranes, or alternatively as ultrafiltration support scaffolds for the interfacial polymerization of polyamides. Sulfonation is a common method used to improve water permeability of aromatic polymers. Successfully demonstrated in polysulfone over 30 years ago, this process is generally accomplished by treating the material, either in bulk or membrane form, with sulfuric acid or sulfur trioxide complexes.[6-9] Sulfonation occurs “at random”, taking place on the least stable, most reactive sites of the polymer chain. As such, the stability and reproducibility of this traditional route is questionable,[9] and several examples of systematic ionic clustering have been observed.[10, 11] Today, sulfonated polyethersulfone membranes with high salt passage are commercially available for nanofiltration applications,[12] but comparable sulfonated products meeting the salt rejection benchmarks for reverse osmosis established by commercial aromatic polyamides have yet to be developed. A unique class of sulfonated polysulfones, random copolymers of sulfonated poly(arylene ether sulfone), have garnered considerable interest since their first introduction by McGrath and coworkers over 10 years ago.[13] BPS, the polymer studied in this report, is a classic example of such materials. As shown in Figure 1, BPS is derived from biphenol, and comprises a random mixture of disulfonated and non-sulfonated monomers. The molar fraction of disulfonated monomers can be easily controlled to produce random copolymers which are hydrophilic and highly water permeable (x�1) or hydrophobic and highly water impermeable (x�0). In RO membranes, higher degrees of disulfonation improve permeability and water swelling at the expense of higher salt passage.[14] Thus, the optimum balance of permeability and rejection occurs at intermediate degrees of sulfonation.

Figure 1. Chemical structure of BPS. “x” represents the fraction of repeat units in the polymer chain which contain disulfonated monomer.

Previous work in both the external literature[14-16] and our laboratory has shown BPS-20 (where x = 20% disulfonation) to be a promising candidate for achieving membranes with reasonable rejection. In these early studies, BPS RO membranes were fabricated and tested in either dense film or thin film composite (TFC) configurations. Dense films are a good method to study inherent performance in the absence of defects, but their thick cross-sections (typically 10 �m or more) make them too resistive to achieve realistic water flux. Thin film composites improve the flux by placing a thin (ideally 200 nm or less) discriminating layer on top of a thicker (40–100 �m) yet highly permeable porous support. Microscopic defects in the rejecting layer can become an issue with this strategy, especially given the laboratory coating techniques typically employed (e.g., brush and dip coating). Another disadvantage of the TFC approach is that the solvent used to coat BPS must wet but not dissolve the ultrafiltration support. The default choice for support is polysulfone, and few orthogonal solvents exist between BPS and polysulfone, especially at high molecular weights.[17]

Asymmetric membranes are a third coating option for BPS. While structurally similar to thin film composites, the entire film in an asymmetric membrane consists of a single polymer material. These films have an open, porous back side which transitions with decreasing pore size to the top surface. For utility

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as ideal RO membranes, the top surface should terminate as a continuous, pore-free, solute rejecting layer, known as a “skin.” The interior bulk should contain interconnected pores with high permeability, low resistance, and sufficient structural stability. Macrovoids may be present, provided they do not penetrate to the front surface or otherwise disrupt the stability of the film. The back surface should also be open and porous, allowing permeate to easily exit the membrane. Asymmetric membranes are formed through the well-known phase inversion process, also referred to as coagulation, diffusion induced phase separation, or simply phase separation.[18-20] In this process, the polymer is cast onto a surface as a homogeneous solution in good solvent and subsequently immersed in a bath of nonsolvent for coagulation. After immersion, diffusion of solvent out of the film and nonsolvent into the film takes place, and the composition within the film begins to vary in both time and distance from the membrane-nonsolvent interface. Even if phase separation occurs readily, ideal asymmetric structures are by no means guaranteed. Conditions must be engineered and optimized for each system to obtain the desired film morphology. In addition to the binodal and gelation lines commonly shown in ternary polymer/solvent/nonsolvent phase diagrams, a variety of additional kinetic and thermodynamic properties can dictate the morphology of asymmetric structures. Some examples are: solution viscosity, diffusion coefficients between solvent and nonsolvent, temperature, initial composition, additives, etc. Asymmetric membranes have been widely studied since they were first demonstrated by Loeb and Sourirajan in 1963 with cellulose acetate films.[21-23] After more than 40 years of research, an abundance of literature is available to help aid in the design of asymmetric formation processes for new materials, like BPS. Phase inversion of both polysulfone and, to a lesser extent, sulfonated polysulfone polymers has also been examined. Key learnings from these works for BPS will be detailed throughout this report, which describes the development of BPS-20 in an asymmetric membrane geometry suitable for reverse osmosis. Here, we have found an optimized casting dope coagulated in an isopropanol quench bath followed by several post-treatment steps produces near ideal asymmetric BPS-20 films.

2. EXPERIMENTAL

2.1 Materials

Several batches of BPS-20 copolymer with similar molecular weight and reported intrinsic viscosities of ~0.70–0.75 were purchased from Akron Polymer Systems. As purchased, the sulfonate counterion of the polymer is primarily potassium. Most membranes were made from polymer with molecular weight distribution: Mn = 17.9 kg/mol, Mw = 54.6 kg/mol, PDI = 3.05. The distributions were measured with size exclusion chromatography equipped with a refractive index detector against poly(ethylene oxide) standards. Dimethylformamide containing 4 g/liter LiNO3 was used as eluent. The poly(vinyl alcohol) used in coating post-treatments was purchased from Sekisui Specialty Chemicals with a typical molecular weight of approximately 125 kg/mol. 2.2 Asymmetric Membrane Fabrication

A thorough examination of casting techniques and conditions was explored to find ideal protocols for asymmetric membrane fabrication. The important aspects of this work are described throughout this report. Because asymmetric processing consists of several consecutive steps, the experiments performed in this study have been inherently iterative. The range of protocols described in this section can be viewed as a baseline for understanding the experiments in the rest of this report. Ammonium nitrate (NH4NO3) was dissolved in dimethylacetamide (DMAc) to a concentration ranging from 0 to less than 10%. In some cases, aniline was also included as an additive up to 0.5%. BPS-20 was

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added to this solution and left to slowly mix until completely dissolved, producing a final casting solution containing typically 24.5% polymer. Unless otherwise noted, all solution concentrations expressed are in units of wt/vol%, with volume representing the added amount of DMAc only. This solution was then cast by hand at ambient temperature onto a cleaned glass plate with a 10 mil doctor blade. Before casting, the glass plate was cleaned thoroughly with soap and water, acetone, and isopropanol and subsequently blown clear of particles and other contaminants with a nozzle of pressurized air. The cast film and plate were then immediately exposed to two IR heat lamps for a specified period of time to induce partial evaporation of solvent and densification of the film-air interface. A similar evaporation step could be accomplished by placing the cast film inside a convection oven warmed to 40–50 °C. Ambient humidity of the casting environment is an important consideration for the evaporation period. While water does not function ideally as a coagulation medium, it is still a nonsolvent, capable of altering phase separation in BPS-20 solutions. High humidity levels in the air can result in enough vapor absorption to induce some phase separation during the evaporation period. Most experiments in this report were performed during the winter and early spring months, when the water content in the air is low. Films cast during the summer months with high humidity partially phase separated during the evaporation step, and resulted in poor membrane salt passage. To get around this issue, membranes can successfully be fabricated by casting and evaporating in a glove box pumped with a constant flow rate of dry air. The dry box may have a lower convective air flow across the membrane surface, and evaporation conditions must be optimized to account for any change in the evaporation rate of solvent. After the evaporation period, the castings were quenched into a bath of pure isopropanol at room temperature and left to phase separate. The phase separation time was kept constant at 30 min for the sake of consistency, but visual phase separation was typically complete after as little as 30 sec. During coagulation, the polymer generally lifts off the glass plate. The opaque, phase separated films were then removed from the bath and solvent exchanged with pure water for 30 min. After this step, samples were either further modified by additional post-treatments, or subjected directly to testing and characterization. Several post-treatments are described in this report. In the most common treatment, membranes were soaked face down in a concentrated, 33 wt% aqueous solution of sodium nitrate. This treatment was performed for 15 min near the boiling point of the electrolyte solution, typically ranging from 105–120 °C. 2.3 Reverse Osmosis Testing

Films were cut into individual rectangular samples with a pair of scissors, with each casting yielding as many as 5 to 6 individual samples. Reverse osmosis performance was measured in a cross-flow filtration system with an active area of approximately 2x10-3 m2 per sample and typical feed velocity of approximately 1.5 m/sec. The free surface of all cast films was oriented toward the feed. The backside of each casting sample was placed against a sheet of a commercial nonwoven PET paper to provide structural support to the film during pressurization. After loading, the permeate was generally allowed to equilibrate for 1 hr or more, depending on the test pressure and flux of the individual samples. Unless otherwise noted, standard tests were performed using a feed of 2000 ppm NaCl at 25 °C, pH 8, and 15.5 bar (225 psi). All results were normalized to the conditions stated, to account for deviations in temperature, feed concentration, and pressure drop down the cross-flow apparatus. In standard tests using NaCl only, a conductivity meter was used to analyze feed and permeate NaCl concentrations. For tests containing both Na+ and Ca2+, the feed and permeate cation concentrations were determined using inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) measurements. Flux measurements are reported primarily in units of liters per square meter of membrane per hour (lmh), but in certain cases have also been expressed in gallons per square foot of membrane per day (gfd) to provide additional clarity. The flux and salt passage data

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presented here have not been adjusted to compensate for any concentration polarization present during the cross-flow test. 3. RESULTS AND DISCUSSION

We begin with the optimization of asymmetric formation and post-treatment conditions, organized in sequence according to fabrication step. The end of this section summarizes the overall reverse osmosis performance and prospects for further improvement in flux. 3.1 Phase Inversion Conditions

3.1.1 Selection of solvent, nonsolvent, and polymer concentration

All of the castings described in this report are made in solutions of DMAc. This solvent yielded the best membrane performance under the range of conditions examined, but similar film structures could be obtained with other polar aprotic solvents, such as dimethylformamide and N-methylpyrrolidinone. Because membrane microsctructure relies on both equilibrium phase behavior of the ternary system and liquid diffusion rates, it comes as no surprise that solvent choice can so drastically affect membrane performance. Additional range finding may uncover sets of conditions where one of these alternative solvents rivals or perhaps even outperforms DMAc. Water is an excellent nonsolvent for solutions of traditional polysulfone. The phase behavior of such systems can change drastically, however, when the polymer is sulfonated. With increasing degree of disfulfonation, the water uptake of BPS copolymers increases,[13, 14] and water becomes less effective as a nonsolvent. BPS-100, in fact, is a water soluble polymer.[13] For BPS-20, coagulation in RO water always leads to unstable structures that appear more gelled than phase separated. Dried films are transparent when coagulated in water, indicating no fine pore structure, but opaque when coagulated in lower MW alcohols such as isopropanol, indicating a stable phase separated structure. This behavior is in contrast to reports of asymmetric films from sulfonated polysulfone, where coagulations in both water and solutions of water-solvent-electrolyte have been reported.[24-26] Isopropanol has proven the most effective nonsolvent for BPS-20 in this work, producing the best reverse osmosis performance of the systems we have examined, including water, acetone, and butanol. Figure 2 shows the effect of polymer concentration on asymmetric membrane flux and salt passage. The membranes were cast from a solution containing 3.9% ammonium nitrate in DMAc and coagulated immediately in isopropanol without an evaporation period. Polymer concentrations beyond 33% were not realistically obtainable, due to the high viscosity and length of time required for dissolution. As expected, membrane flux at 15.5 bar drops steadily with polymer concentration: 70–75 lmh at 18.5% down to less than 5 lmh at 32.7%. Salt passages for all membranes are quite poor, but drop from 50–60% at 18.5% polymer to ~30% at 24.5% polymer. The flux loss at the highest polymer concentrations is sharp, but is not reflected by any improvement in membrane salt passage. Consequently, 24.5% was identified as the approximate optimum in polymer concentration.

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Figure 2. Flux and salt passage of asymmetric BPS-20 films as a function of polymer concentration. Membranes were cast with ~4% ammonium nitrate in the polymer solution and were coagulated in isopropanol with no evaporation period. Tests were performed at 15.5 bar with no post-treatments.

3.1.2 Electrolyte in the casting solution

The amide solvents are noted for their ability to dissolve high concentrations of salts in anhydrous solvents. Because of this, additives in amide solvents can include simple salts such as lithium chloride, lithium nitrate, sodium nitrate and ammonium nitrate, which are not soluble in simple hydrocarbons or halogenated hydrocarbons. The literature on membrane formation using phase inversion is rife with examples of additives used in casting solutions to enhance membrane performance.[24, 25, 27-34] One particularly relevant U.S. patent, issued in 1974 to Rhone-Poulenc S.A., describes the use of amine or quaternary ammonium salts in the casting solution of sulfonated polysulfone asymmetric membranes coagulated in water.[24] Motivated by this work, BPS-20 asymmetric membranes coagulated in isopropanol were formed with ammonium nitrate (NH4NO3) in the casting solution. SEM cross-sections of these films are shown in Figure 3. All castings were made with a 30 min evaporation period under a pair of IR heat lamps (as described later). In the absence of NH4NO3, phase separation in the IPA baths sets in relatively slowly, over the course of about 60 sec, and asymmetric membranes with skins several microns thick are formed. This structure (Figure 3a) is a classic prototype of an asymmetric film, but such a thick skin is impractical for reverse osmosis. As the amount of NH4NO3 in the casting solution increases, phase separation sets in 30 sec or less, and a thinner skin is formed. At 7.8% NH4NO3, however, the membrane structure is destroyed, evidenced by an SEM cross-section (Figure 3d) full of large pores, several microns in size, from top to bottom. The initial casting solution is less transparent, and some phase separation can even be visually observed during evaporation before the isopropanol quench. Naively, a shorter phase separation time due to the addition of ammonium nitrate could signify a reduced distance between the initial casting composition and the binodal curve. However, limited solubility information collected to date indicates that moderate amounts of NH4NO3 (2–4%) actually increases the size of the one phase region of the phase diagram (i.e., the system becomes more tolerant to IPA rather than less tolerant). If that is the case, kinetics may be responsible for the differences seen in Figure 3. With no electrolyte, the coagulation time is longest and the skin is thickest, perhaps indicative of slow nonsolvent influx into the membrane relative to solvent outflux. If solvent leaves faster than nonsolvent enters, the film densifies rather than phase separating, and the result is a thick skin and slow coagulation

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time. In the presence of electrolyte, if the nonsolvent influx increases, the skin will have less time to form and phase separation will occur more quickly. Finally, increased ambient humidity uptake may explain the observation of some visible phase separation during the evaporation step at the highest levels of electrolyte.

Figure 3. SEM cross-sections of BPS films made with increasing concentrations of ammonium nitrate in the casting dope. All films were cast from a solution containing 24.5% BPS-20 in DMAc, evaporated for 30 min under a pair of IR heat lamps, coagulated in IPA, and post-treated by boiling in concentrated sodium nitrate. The bottom of each image represents the separating layer, or polymer-nonsolvent interface during the quench.

Moderate compositions of NH4NO3 are well suited for reverse osmosis membranes. Cross-sections of these films generally have larger pores at the polymer-glass interface and in the bulk which transition to a somewhat finer structure topped by a skin at the polymer-IPA interface (Figures 3 and 4). As seen in Table 1, an optimum in salt passage for the films in Figure 3 occurs near 3.9% NH4NO3. Owing to the decrease in skin thickness, membrane flux goes up from 0.24 lmh with no additive to 19.2 lmh at 3.9% NH4NO3. At 7.8%, the large pore structure increases salt passage by over a factor of 50, while flux also

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suffers. A casting solution containing 3.9% NH4NO3 was selected as the optimum value for further studies.

Table 1. Membrane performance for BPS films made with varying concentrations of ammonium nitrate in the casting dope.

NH4NO3Conc.

(wt/vol%)aFlux (lmh)b Salt Passageb Coagulation

time Skin Morphology

0.0% 0.24 ± 0.00 n/a ~ 60 sec thick (~microns) 2.0% 4.0 ± 0.78 0.95 ± 0.17% ~ 30 sec thin (~nanometers) 3.9% 19.2 ± 4.8 0.89 ± 0.22% � 30 sec thin (~nanometers) 7.8% 17.0 ± 8.0 49.5 ± 13.9% < 30 sec not distinguishable

aAll samples were post-treated in NaNO3:water (1:2) at 115 °C for 15 min. bTest conditions: 41.4 bar (600 psi), 2000 ppm NaCl. Additional SEM images of films with 3.9% NH4NO3 are shown in Figure 4. Note that these membranes were fabricated under near ideal casting and post-treatment conditions and, as such, represent the structure of some of the highest rejection asymmetric membranes in this study. Details of the casting conditions and post-treatment steps are described in the remainder of this report. These films are typically around 80 microns thick. Macrovoids are often observed in cross-sections but are not always present (contrast Figures 3c and 4a). An exhaustive study of macrovoid formation has not been performed for this system. However, the amount of solvent evaporation taking place at the casting surface may be a controlling factor in the presence of these voids. Top and cross-sectional views of the polymer-IPA interface show a sharp transition from fine pore structure to a nearly defect free skin. The skin is too thin to precisely measure, but Figures 4c and 4d suggest a thickness of less than 50 nm. While the polymer-IPA interface is largely defect free, circular holes hundreds of nanometers in size sparsely populate this surface. From these images, it is difficult to ascertain whether the holes terminate or penetrate fully into the asymmetric pore structure. The back side of the film, Figure 4b, is surprisingly lacking in porosity. While the dominant source of resistance is presumed to come from the surface skin, the potential importance of non-ideal bulk porosity, bulk thickness, and back-side resistance cannot be ignored. These aspects have not been thoroughly explored and remain open routes to improve the asymmetric membrane permeability.

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Figure 4. SEM microgaphs of optimized BPS-20 films made with 3.9% NH4NO3 in the casting solution. Films were cast from a DMAc solution containing 24.5% polymer, evaporated in air for 30 minutes under a set of IR heat lamps, coagulated in IPA, and post-treated by boiling in concentrated sodium nitrate.

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3.1.2 Additional additives

A limited sampling of other additives has been examined as part of the casting solution, with statistically inconclusive results. For example, addition of 0.5% aniline to the casting solution appeared to improve flux, albeit along the existing trade-off curve shown later in Figure 8. This effect was not always reproducible. The remaining experiments discussed in this report were made from batches either with or without 0.5% aniline, again without a conclusive difference. Variability in the drying conditions discussed in the next section are believed to be the primary reason for inconsistencies between measurements.

3.1.3 Evaporation Period

The partial evaporation of solvent between casting and coagulation is of paramount importance in membrane performance. Figure 5 shows membranes with no post-treatment cast in a fume hood and dried under a pair of IR heating lamps for 0, 30 and 60 min. Membrane flux drops consistently with evaporation time as solvent evaporates from the film and the surface layers become enriched in polymer. Salt passage shows an optimum with dry time at around 30 min. Based on readings from a thermocouple mounted to a glass plate, we estimate the temperature of the film when exposed to the IR lamps to be about 31–36 °C. For a standard casting solution containing 24.5% BPS-20, the evaporation rate of solvent during this evaporation step is linear, and amounts to approximately 0.07 grams/(m2·sec). Several castings were alternatively dried in a convection oven at 40–50 °C. These films required shorter evaporation time periods, and often produced membranes with lower flux and lower salt passage. The exact conditions of the evaporation step are believed to significantly impact membrane performance. A trade-off between flux and salt passage is commonly observed, as illustrated later in Figure 8.

Figure 5. Flux and salt passage for asymmetric BPS-20 films as a function of evaporation time before coagulation. Tests performed at 15.5 bar and 2000 ppm NaCl with no post-treatments.

3.2 Post-Treatment

3.2.1 Annealing

Thermal annealing is a common post-treatment used to enhance the performance of RO membranes. Previous work in the patent literature describes significant improvement in asymmetric sulfonated polysulfone membranes through heat treatment in concentrated sodium nitrate.[24, 25] Such a treatment works exceptionally well for asymmetric BPS-20 membranes.

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A select summary of the different post-treatment steps employed for these membranes is shown in Figure 6, for membranes cast from 24.5% polymer solutions with 3.9% NH4NO3 additive and tested at 15.5 bar. Membranes cast with no post-treatment generally give fluxes in the range of 15–40 lmh, with salt passages ranging from 4–13%. Boiling in concentrated sodium nitrate reduces the flux to a typical range of 3.5–17 lmh while salt passage also falls to around 2% or less. All of these membranes were coagulated in isopropanol, but were subsequently solvent exchanged with water for 30 min before starting the post-treatment. It is believed the annealing promotes osmotic dewatering by dehydrating small pores and regions of water swollen polymer left after coagulation and solvent exchange. The dehydration occurs due to the strong osmotic pressure difference between water in the polymer pores and water in the post-treatment solution (33 wt% NaNO3). A 15 min post-treatment was applied for the sake of consistency, but a structural collapse and “shrinking” of the film can be visually observed almost immediately after immersing the coupon in the electrolyte solution. The result is a tighter membrane containing less water and probably less free volume. Such free volume effects can be quantified spectroscopically and have been shown to correlate well with water and salt permeability.[35]

Figure 6. Change in membrane performance with different post-treatment conditions. All films were cast from 24.5% BPS-20 with 3.9% added ammonium nitrate. Reverse osmosis tests were performed at 15.5 bar, 2000 ppm NaCl.

The improvement is identical if films are post-treated in hot solutions of other electrolytes. Figure 6 shows a broad range of nitrates: NaNO3, NH4NO3, LiNO3, Ca(NO3)2. Thus, the diameter or the valency

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(mono vs divalent) of the counter-ion of the polymer sulfonation does not greatly control the flux or salt passage of the resultant membrane. Though not shown here, we also see the similar improvement for films post-treated in concentrated chlorides and iodides. The key factor for this process to work is solubility in water. The chosen electrolyte must be soluble enough to dissolve concentrations high enough to achieve sufficient osmotic pressure. To further substantiate this claim, we see that films simply boiled in RO water show no improvement in performance. Films boiled in ethylene glycol, on the other hand, actually swell the membrane, giving drastic increases in flux and salt passage. Finally, the dehydration process of simply allowing the membranes to air dry overnight produces improvements similar (albeit never as effective in salt passage) to those membranes boiled in concentrated electrolyte. In addition to the concentrated electrolyte, thermal energy is required to drive the dehydration and structural reorganization of the film. Figure 7 shows the effect of solution temperature on membranes soaked in 1:2 NaNO3 solution. Salt passage performance steadily improves from soaking at ambient temperature (no improvement) to near boiling, which typically corresponds to a solution temperature of 105–120 °C at this concentration of electrolyte.

Figure 7. Effect of post-treatment annealing temperature in NaNO3:water (1:2) solution. Tested at 15.5 bar, 2000 ppm NaCl.

3.2.2 Coating

In addition to heat treatments, membrane performance can be improved substantially by intentionally fouling the surface with a coating solution. For the membranes in this report, we expect sealing of surface defects and pinholes (see Figure 4) to be the main reason for improvement. The asymmetric membranes were coated after NaNO3 heating post-treatment with poly(vinyl alcohol) (PVA). The PVA was applied by recirculating a 250 ppm solution through a cross-flow test bench at 15.5 bar for 1 hour. The bench was then flushed clean with RO water before running a standard NaCl test. Figure 8 illustrates the stark improvement in performance. On average, membrane flux drops by only about 4%, while simultaneously improving salt passage by 34%. Figure 8 plots a series of the best performing membranes in this study with and without PVA coating. The PVA essentially shifts the operating line (i.e., trade-off curve between flux and rejection) down, allowing an improved range of capability. Several days of flushing and testing show little to no change in performance, indicating the coating solution to be stable over the time frames examined in this report.

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Figure 8. Flux and salt passage at 15.5 bar for BPS-20 membranes: a) before and after PVA coating, b) average of optimized recipe after PVA coating, along with the best combinations of flux and rejection observed under those conditions.

3.3 RO Performance

3.3.1 Flux and NaCl Rejection

For the optimum recipe established in this work, membranes were quenched in pure isopropanol from DMAc solutions containing 24.5% BPS-20 and 3.9% NH4NO3, with or without 0.5% aniline. The films were dried under heat lamps or a convection oven for 10–30 min before thermal annealing in NaNO3 solution. The post-treatment coating was applied in situ by recirculating 250 ppm PVA in the testing bench feed water for approximately 1 hour. As mentioned previously, the membranes examined in this work exhibit an inherent trade-off between flux and rejection, evidenced by the “operating line” in Figure 8. Movement up and down this line can be seen even in repeated samples taken from the same optimized procedure. Much of the variability is believed to come from the manual nature of the hand-casting method employed, as well as local differences in casting thickness and drying rate along a single cast film. Fluctuations from day to day in

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ambient humidity, temperature, heat lamp and convection oven settings will widen the range of performance even further. Nevertheless, expressing the RO capability of the optimized recipe with a single set of average metrics yields a flux of 9.9 ± 4.1 lmh and salt passage of 1.12 ± 0.40%, as tested under feed conditions of 15.5 bar and 2000 ppm NaCl. The asymmetric fabrication method developed in this report represents a clear performance improvement, in both flux and rejection, over previous reports of BPS membranes.[5, 14-17, 24, 25, 29] These earlier studies have focused on either dense film or thin film composite (TFC) BPS membranes. TFC architectures can reduce the resistive layer and increase flux. However, the laboratory casting techniques employed to date have been unsuccessful in preventing significant defects in such thin barrier coatings, and the salt passages suffer heavily. The asymmetric route clearly appears to combine the best of both worlds: a thick film resilient to defects joined with a thin upper skin capable of higher flux. Formation of the skin during coagulation in solution makes the top surface inherently more robust to many of the common defects which plague coated TFCs, such as external contamination, breaks in meniscus, or ribbing and cracking during drying. TFCs formed through interfacial polymerization, such as polyamide RO membranes, share many of these same advantages. 3.3.2 Mixed Feeds

Increased monovalent salt passage in the presence of mixed valency feeds is a well known weakness of negatively charged membranes. In these materials rejection is strongly driven by ionic repulsion between negative charge on the polymer and anions in the feed (e.g., Cl-). In such situations, the divalent cation binds strongly to the negative charge on the polymer, screening the repulsive benefit which would otherwise take place between the negative charge on the polymer and the anions in the feed. The reduction in ionic repulsion causes anion passage to increase. Cations must also pass to maintain charge neutrality, and monovalent cations like Na+ pass preferentially over divalent cations. The end consequence for reverse osmosis is that monovalent salt passage, such as Na+, will increase when divalents, such as Ca2+, are present in the feed. This phenomenon, previously identified and well documented for sulfonated polysulfone membranes,[5] limits RO viability, as the feed streams in most applications have at least some level of hardness. As seen in Figure 9, the asymmetric BPS-20 membrane demonstrates a substantial rise in Na+ passage with increasing amounts of Ca2+ in the feed, compared to the relatively flat response seen by a standard DOW FILMTECTM BW30 membrane. The inset to Figure 9 also shows NaCl passage for pure NaCl feeds of 850 and 2000 ppm. The asymmetric BPS film is more sensitive to feed ionic strength than BW30, but the rise in observed Na+ passage (~0.34 %SP/1000 ppm NaCl) is substantially less than is realized by holding the feed NaCl constant and adding CaCl2 (~4.1 %SP/1000 ppm CaCl2). This limitation of BPS may be of paramount importance, given that many of the applications in which a chlorine-resistant membrane would be valuable contain significant quantities of divalent cations.

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Figure 9. Sodium ion rejection for feeds containing mixed cationic valency. NaCl and CaCl2 were used at a feed pressure of 27.6 bar (400 psi). The feed concentration of Na+ was kept constant at ~390 ppm for all measurements used in the graph. Ionic strength dependency alone can be seen in the inset table, which compares salt passage for feeds of 850 and 2000 ppm NaCl in the absence of CaCl2.

While the asymmetric BPS film is more sensitive to ionic strength and hardness than BW30, both the starting salt passage and magnitude of these changes are less than has previously been reported for sulfonated polysulfone.[5] In that work, NaCl passage in the absence of any divalents was 3% or more, with passage increasing by around 4% SP/1000 ppm NaCl, approximately 10 times higher than the asymmetric BPS films examined here. Those sulfonated polysulfones also showed a degradation in Na+ passage beyond 10% with sufficient amounts of Ca++ added to the feed. A possible explanation for these differences is that sulfonation on BPS polymers occur in pairs but is statistically random, whereas direct sulfonation of polysulfone is not randomized and can result in localized regions of interconnected negative charge aggregates.[10, 11] Alternatively, or in addition to the polymer structural differences, the low starting salt passage obtained through this asymmetric process may play a key role. Efficient packing of the polymer and plugging of defects, resulting in low starting salt passage, could lead to better performance in this type of charged polymers and could explain the improved ionic strength and hardness performance. The BPS asymmetric films operate with NaCl rejection on par to standard reverse osmosis membranes. These rejections have been achieved only after careful optimization of the membrane structure (casting conditions, osmotic tightening, defect sealing). Compared to films with different structure and perhaps higher salt passage, the relative importance of the membrane charge on salt rejection may differ. Indeed, some evidence in the literature suggests the response of BPS membranes to mixed feeds can vary widely with film structure.[36] More work is clearly needed in these areas, but such results hold promise that undesirable responses to mixed feeds in charged membranes such as BPS could perhaps be controlled by properly engineering the structure or chemistry of the film. Perhaps additional refinement in casting conditions to further tighten the skin rejecting layer could be pursued to reduce the dependence of salt rejection on membrane charge. Alternatively, improvements may also be possible by increasing the separation distance between the two sulfonation sites in each repeat unit of the polymer, to guarantee greater distance between all sulfonation sites.

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3.4 Prospects for Higher Flux

Despite the improvements realized through asymmetric films in this study, BPS membranes still lag far behind the performance of commercial brackish water polyamide RO membranes, which operate in the neighborhood of at least 42 lmh (25 gfd) and 1% salt passage at 15.5 bar (225 psi). Several strategies exist for increasing the flux of these membranes to a competitive level while also maintaining high rejection. Several aspects of the asymmetric casting have not been investigated thoroughly and merit further study. Excess resistance in the film can be minimized by reducing the bulk film thickness, opening up the pores of the back surface, or even perhaps switching the casting substrate from uniform glass to a structured web. Improvements in the casting and/or coagulation solutions can also be made, perhaps by including additives or solvent/nonsolvent mixtures to enhance porosity. As stated previously, the permeability of BPS and similar materials increases with the amount of sulfonation on the polymer chain.[8, 14] The tradeoff of this benefit, of course, is higher salt passage. Only membranes made from BPS-20 were used in this study, and it is worthwhile to investigate the flux-rejection tradeoff that can be realized by fabricating asymmetric membranes with higher percent disulfonation, or perhaps other materials with higher inherent permeability. 4. CONCLUSIONS

Fabrication routes for asymmetric membranes using 20% disulfonated BPS copolymers have been explored. For BPS-20, the key steps in phase inversion (solution composition, casting, drying, coagulation, and post-treatment) have all been studied to produce recipes which yield high rejection and flux. In the casting, a solution in DMAc of 24.5% BPS-20 with added ammonium nitrate is used, where the ammonium nitrate serves to minimize the skin thickness of the final asymmetric membrane. While the casting and drying steps show room for improvement, it is clear that an evaporation step to densify the top surface is crucial in forming effective selective layers. After coagulation in a bath of isopropanol, several post-treatment steps are employed to further improve membrane performance. Boiling in concentrated electrolytes, such as sodium nitrate, collapses pores and produces near defect free skins by dehydrating the membrane. In-situ coating with poly(vinyl alcohol) further serves to plug up any remaining holes or other defects, resulting in a highly rejecting surface. Using these procedures, the BPS-20 asymmetric membranes described here exhibit improved flux and rejection performance over other reported casting methods and sulfonated materials. Two key hurdles remain for this technology. Despite the improvements, asymmetric BPS-20 membranes remain at a significant flux disadvantage relative to benchmark commercial polyamides. Further improvement in casting conditions and/or polymer chemistry are required to reduce this gap. Additionally, the negative charge of BPS causes increasing passage Na+ ions when divalent cations are present in the feed. The asymmetric BPS-20 films appear more tolerant to both ionic strength mixed feeds than previous work on sulfonated polysulfone, but the issue remains critical, as it limits the rejection potential of these membranes in mixed feed waters. Further improvements in the membrane structure may be a route to correct this important failing.

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5. ACKNOWLEDGEMENTS

This work was supported by Dow Water and Process Solutions. The authors would like to acknowledge the assistance of Joe Kiefer for size exclusion chromatography measurements and Jim Thorpe for ICP analysis.

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Highlights� Asymmetric reverse osmosis membranes were with a disulfonated copolymer of biphenol and

aryl sulfone � Casting conditions were tuned to provide the optimum balance between water permeability and

salt rejection � Membranes were post-treated by annealing and coating to further improve rejection � Salt rejection in feeds contained mixed valency was examined

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Asymmetric reverse osmosis sulfonated poly(arylene ether sulfone) copolymer membranes

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