Toward Improved Boron Removal in RO by Membrane Modification

ARTICLE pubs.acs.org/est

Toward Improved Boron Removal in RO by Membrane Modification: Feasibility and Challenges Roy Bernstein,†,‡ Sofia Belfer,† and Viatcheslav Freger*,†,‡,§ †

Zuckerberg Institute for Water Research, Ben-Gurion University of the Negev, P.O. Box 635, Sde-Boqer 84990, Israel Unit of Environmental Engineering and §Department of Biotechnology, Ben-Gurion University of the Negev, P.O. Box 635, Beer-Sheva 84105, Israel.

‡

bS Supporting Information ABSTRACT: Membrane modification by concentration polarization (CP)-enhanced radical graft polymerization using a dilute aqueous solution of appropriate monomer was examined as a method for increasing rejection of boric acid by reverse osmosis (RO) membranes. On the basis of suggested physicochemical rationales a number of monomers were examined in order to determine those with the lowest affinity toward boric acid as compared to water. The improvement in the modified membrane performance was mainly attributed to sealing less selective areas (“defects”) inherently present in the original low pressure RO (LPRO) membranes. However, the effect clearly differed for different monomers. Among the examined monomers glycidyl methacrylate (GMA) exhibited the lowest affinity and the largest improvement in removal of boric acid along with a moderate loss of permeability and slightly improved NaCl rejection. Modification of LPRO membrane thus resulted in a membrane with a permeability in the brackish water RO (BWRO) range but with removal of boric acid and salt superior to those reported for most commercial BWRO membranes.

’ INTRODUCTION Today RO/NF desalination usually employs polyamide membranes that are cost-effective and combine a high NaCl rejection and good permeability.1 However, one major drawback of RO membranes is poor removal of small uncharged molecules such as boric acid at pH below its pKa 2,3 or small organics.4,5 Sea water contains boron (B) as boric acid at concentrations of 5
7 ppm. Although its toxicity to humans is uncertain, the WHO recommended guideline for maximum B concentration in potable water is 0.5 ppm. Boric acid is an essential micronutrient for plants. However, it becomes toxic above 0.5
1 ppm depending on their boron tolerance 6,7 and therefore, the recommended B concentration in countries, where the desalinated water may be used for irrigation, is 0.3
0.5 ppm in the permeate line.8 Sea water (SWRO) and brackish water (BWRO) polyamide membranes have B rejection of 80
93% and 30
80%, respectively, consequently, a single-pass RO process is usually unable to remove boron down to the standard. Boron removal can be improved by various pre- and post-treatment techniques 9 and by double-pass RO filtration. In the latter, the pH of the permeate from the first pass, after removal of all hardness, is raised to 10, resulting in boric acid dissociation and its rejection rises to 95
98%. Since the salinity of permeate is already low, more permeable BWRO membranes are usually used in the second pass.9
11 Removal of B using a second pass or alternative method increases water cost by approximately 10
20%,12 providing a strong incentive to improve B rejection of RO membranes up to a level where the second pass would be unnecessary. Boric acid is a weak acid with pKa1 ≈ 8.6 in seawater.13 Since most water used in desalination has pH 7.5
8,2 boric acid in r 2011 American Chemical Society

seawater is mainly undissociated and its passage through RO membranes is affected mostly by size exclusion and hindrance (molecular friction) which are not strong because the B(OH)3 molecule is small; its Stokes radius is only about 0.155 nm, as calculated from the diffusion coefficient 1.41  10
9 m2/sec in water at pH 7.14 Hyung and Kim noted that boric acid has a much lower rejection than monovalent ions with comparable Stokes radii.15 This emphasizes the fact that electrostatic exclusion mechanisms (Donnan and dielectric) responsible for the high salt rejection become inactive for undissociated acids. Sagiv and Semiat 12 suggested that another factor for the high passage of boric acid through RO membranes could be hydrogen bonding between its three hydroxyl groups and the bound water in the membrane (hydrogen bridges), which enhances association and drag of B(OH)3 by water. Surface modification was demonstrated to be a useful approach to improve filtration selectivity of membranes and reducing their propensity to fouling and biofouling.16,17 Recently, we have introduced a facile modification technique
concentration polarization (CP)-enhanced radical graft polymerization
for modification of dense membranes.18 In this method, the undesired effect of CP becomes beneficial and the reaction is selectively enhanced at the membrane surface by filtering a monomer solution through the membrane. As a result, the required concentration of reagents and undesired homopolymerization in the bulk solution are largely reduced. The method is Received: November 29, 2010 Accepted: March 3, 2011 Revised: March 2, 2011 Published: March 21, 2011 3613

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Table 1. Monomers and Solvents Used in the Solubility Tests and Their Characteristics

a

See ref 21. b See ref 22. c See ref 23.

particularly advantageous for monomers slowly reacting or sparingly soluble in aqueous solutions thus expanding the range of usable monomers. Ben David et al. demonstrated that the filtration selectivity of NF membranes toward a number of endocrine-disrupting compounds could be significantly increased using this technique.19 The present study analyzed the feasibility and challenges of reducing passage of boric acid through RO membranes by means of modification using CP-enhanced graft-polymerization. First, some general guiding thermodynamic and molecular principles for selecting most beneficial monomers were formulated. Based on these principles, several candidate monomers were chosen, analyzed and used for modifying a commercial low pressure RO (LPRO) membrane, allowing comparison between the expected and actual effect of modification on membrane performance. Ultimately, the best-performing modified membranes were compared to their commercial counterparts to evaluate the potential and limitations of the proposed approach.

’ EXPERIMENTAL SECTION Materials. All chemicals were purchased from Aldrich and used without purification. Double-distilled deionized water (DDW) was used in all experiments. The fully aromatic polyamide (PA) membranes LE and SW30 (Dow-Filmtec) and ESPA1 (Hydranautics) were kindly supplied by the manufacturers and stored as described in.18 Before modification all

membranes were first wetted with 50% ethanol and then with water to ensure complete pore filling. It was verified that such wetting did not affect the membrane performance. The membranes were then tested at pressure 20 bar for water permeability using DDW and for rejection of boric acid and NaCl using a solution of 1.5 g/L NaCl and 5 ppm B at pH 7
7.3. Membranes that showed NaCl rejection below 95% were discarded. When necessary, pH was adjusted to 7 using a NaOH solution. Modification Procedure. Table 1 lists the monomers used for modifications. The CP-enhanced surface grafting of membranes was carried out as described elsewhere.18 Briefly, an RO membrane (LE, Dow-Filmtec) was mounted in a dead end cell (see below) and tested for performance. Thereafter, the cell was filled with a solution of a monomer (see Table 1) and initiators and modification was carried out at pressure 20 bar for 30 min. At the end of the modification, without disassembling the cell, the membrane was washed several times with DDW, cleaned by shaking for 24 h in 50% (v/v) ethanol to remove adsorbed homopolymer, washed again with water and examined for B and NaCl rejection and permeability. For the conversion of grafted glycidyl methacrylate (using 2.3 mM GMA for grafting) to diol (GM) or sulfonate (GMS) the membrane was unmounted and the conversion was carried out using epoxy-ring-opening reactions of hydrolysis or sulfonation, respectively (see the Supporting Information). Membrane Testing. The filtration tests were performed in a 150 mL nitrogen-pressurized dead-end stirred cell having a 3614

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membrane area of 12.4 cm2. Water permeability (Lp) was calculated by collecting and weighing the permeate. NaCl concentration in the feed and the permeate was determined from electric conductance of the solutions. B concentration was measured using 2 methods: inductively coupled plasma (ICP280S, Varian) and the azomethine-H spectrophotometric method.20 The difference between the results did not exceed 5%. The passages of salt and boric acid were calculated using the relation P ¼ Cp =Cf

ð1Þ

where Cp and Cf are the permeate and feed concentrations, respectively. Surface Characterization of Membranes. Attenuated total reflection (ATR) FTIR spectra (average of 40 scans at 4 cm
1 resolution) were recorded on a Vertex 70 FTIR spectrometer (Bruker) using a Miracle ATR attachment with a one-reflection diamond-coated KRS-5 element (Pike). Degree of grafting (DG) was defined as:18 DG ¼

Imon Imem

ð2Þ

where Imon is the intensity of the 1724
1730 cm
1 band assigned to carbonyl group and characteristic of acrylic monomers and polymers, and Imem is the intensity of the 1586 cm
1 bands of polysulfone (part of the original membrane). The reported DGs are the average of at least 5 spots on each sample for at least 5 different samples. Contact angles of water were measured using the sessile drop method using an OCA-20 contact angle analyzer (DataPhysics) equipped with a video camera, image grabber and data analysis software. Every measurement was repeated at least 3 times and averaged for at least 7 drops (0.3 μL) on each membrane sample. The standard deviation was typically less than 15% of the measured value. Partitioning of Boric Acid between Water and Organic Liquids. Partitioning of boric acid between water and organic liquids (solvents and monomers, Table 1) was estimated by mixing 1 g of boric acid, 2.5 mL of water, 10 mL of organic solvent, and shaking the mixture for 24 h at room temperature. Because the tests were only meant to provide an indication of the trend in partitioning, the high proportion of boric acid was taken to allow facile determination of the boric acid concentration in the organic phase. The boric acid concentration in the water phase was approximately its solubility limit (40 g/L). The boric acid concentration in the organic phase was measured by taking a small sample from the organics phase, diluting it 100 to 1000 times with DDW and measuring B concentration as described above. The sorption selectivity factor S was estimated as follows S¼

KB Kw

ð3Þ

where KB is the distribution coefficient of boric acid between the organic and water phases (calculated assuming 40 g/L boric in water phase) and Kw is the water fraction in the organic phase assumed to represent the water distribution coefficient (i.e., assuming that the water phase is pure water). The values of Kw for solvents were taken from the literature,22,23 while for the monomers they were measured using the Karl Fischer titration.

’ RESULTS AND DISCUSSION Rationales for Selection of Monomers for Graft-Polymerization. Search for the most selective monomer for modification

must account for the fact that boric acid is only twice the size of water in terms of Stokes radius and has 3 hydroxyl groups which can form up to 6 hydrogen bonds with water, resulting in strong association.24 It is well-known that membrane filtration selectivity, i.e., solute passage relative to water passage, is determined by frictional (hindrance) as well as thermodynamic factors.25 Hindrance of the solute by the membrane matrix, including both diffusion and convection (i.e., drag by water), is larger for solutes than for small water molecules. It steeply rises when the ratio between the solute size and pore size of the membrane approaches 1.26,27 In this respect a tighter grafted layer is expected to be more beneficial. The CP-enhanced modification technique applied in this work seems to be well suited for this purpose, since it maximizes the monomer concentration and tightens the graftpolymer during graft-polymerization, particularly over “defect” areas of the original active layer.18 Cross-linking used in some cases to stabilize the grafted layer and ensure its covalent bonding to the membrane should also help tighten its structure. These effects of hindrance in the grafted layer may then contribute to filtration selectivity. However, alone they are unlikely to improve on the original RO membranes, especially, in the case of SWRO, in which the dense and rigid polyamide layer is relatively thick and contains very few “defects”.1 Nevertheless, a certain potential for improving boron rejection derives from the fact that the polyamide may not be optimal from the thermodynamic viewpoint. Rationales may then be proposed for choosing an optimal monomer based on available molecular interactions or exclusion mechanisms. Although the strongest electrostatic interactions become inactive in the case of undissociated boric acid, among the remaining types of interactions the following could be considered for uncharged solutes: (a) Steric (size) exclusion, which is usually considered jointly with hindrance, since they both follow a qualitatively similar trend. The above discussion of hindrance then fully applies to steric exclusion as well and implies that this effect alone would be insufficient. (b) Salting out that occurs within polymers containing charged groups. For example, grafting negatively charged sulfopropyl methacrylate on a NF membrane was shown to be effective in excluding organics.19 Unfortunately, a large content of charged groups may result in excessive swelling, weakening hindrance and steric exclusion. More importantly, salts such as sulfate, known to cause a strong salting out of organics from water, do not salt out boric acid; in fact, boric acid in water is salted in.28,29 Indeed, it is shown below that addition of charged groups to the grafted layer reduces removal of boric acid. This suggests that the strong association between water and boric acid (see below) and relatively high polarity of the latter inactivate this mechanism as well. (c) van der Waals, i.e., dispersive (based on molecular polarity/polarizability), hydrogen bonding, and hydrophobic interactions.30 It may be suggested that disruption of association between boric acid and water, possibly combined with strong size exclusion, could be the key to improving filtration selectivity. For instance, it is known that proximity of a hydrophobic surface affects hydrogen bonds in water. A classic example is the 3615

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Figure 1. (a) Boric acid partitioning versus water partitioning for tested liquid monomers (empty symbols) and solvents (solid symbols) presented in Table 1. The solid line is a linear fit. (b) Sorption selectivity S vs versus water partitioning for the same liquids, and (c) S vs the polarity parameter π* from Table 1 for solvents. Solid and open symbols show solvents and monomers, respectively.

structure-breaking effect of large hydrophobic ions in water.31 This implies that hydrophobicity of the grafted layer might be beneficial, provided it is not excessive and preserves a reasonable combination of water flux and salt removal.32 Another potential way to suppress association of water and boric acid, yet keep water permeability high, is to search for appropriate types of hydrogen-bonding sites in the polymer. Boric acid has lower proton content, hence, it could have a lower ability than water to donate hydrogen bonds, despite the similar to water overall hydrogen-bonding capacity, donating and accepting. As a result, dominance of hydrogen-bond-accepting sites in the membrane might be beneficial for B removal. Similar argument was used in connection with protein adsorption on surfaces.33 Obviously, boric acid is much smaller and more abundant in hydrogen bonding sites than proteins, yet this principle could be another useful rationale and in combination with a moderate hydrophobicity and size exclusion could provide another incremental improvement. To test these hypotheses several moderately hydrophobic organic solvents and liquid acrylic monomers, listed in Table 1, were examined for water and boric acid partitioning between the organic and aqueous phases. Solvents are used here as indicators because their characteristics are well-known. All liquids have limited solubility in water and contain hydrogen-bond-accepting groups (ether, ester, epoxy and keto), except for butanol which contains a hydrogen-bond-donating OH group. For solvents, the hydrogen bonding character could also be quantified by the parameters R and β (R = 0 and β = 0 indicating an inability to accept and donate H-bonds, respectively) introduced by Kamlet et al. 34 and compiled for many solvents by Marcus,21 see Table 1. The partitioning results are summarized in Figure 1. Figure 1a demonstrates that partitioning coefficient of boric acid (KB)

Figure 2. Membrane permeability versus boric acid passage following modification with different monomers. The different points for GMA, EMA, and METMAC are measured at different modifications. Testing conditions [B] = 5 ppm, pH 7
7.3, pressure 20 bar. Each symbol represents at least 5 results, the standard deviation of the B passage and permeability is within 20% of the shown value. Arrows indicate the shift of performance after conversion of GMA graft-polymer to GMS and GM.

shows a strong linear correlation with partitioning (solubility) of water (Kw). This correlation could indicate association of water and boric acid within organic medium. On the other hand, there are noticeable deviations from linear relation, clearly seen in Figure 1b, displaying the sorption selectivity S = KB/Kw. Surprisingly, S does not show correlation to the chemical structure, hydrogen bonding character (R and β) or solubility 3616

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Figure 3. Membrane permeability and boric acid passage following modification with GMA. Modificaion conditions: pressure 20 bar, reaction time 30 min. Filtration conditions: [B] = 5 ppm, pH 7.

of water in the organic phase. However, Figure 1c shows that S of solvents has a clear correlation with the parameter π*, which was defined by Kamlet et al. 34 in order to represent the polarizability/ polarity, i.e., dispersive interactions in the solvent. It may then be hypothesized that the nonsteric KB reflects a superposition of two factors: Kw, which is related to overall water uptake by the polymer (resulting from all polymer
water interactions), and S, which seems to be positively correlated mainly with polarizability and generally should be lower for hydrophobic polymers. This suggests that low polarity/polarizability, i.e., hydrophobicity of the monomer used for modification could be beneficial in reducing boric acid passage. Note that passage of boric acid within a polymer should be proportional to either KB or S, depending on whether the mechanism is diffusion or convection, respectively.25 Because it is not clear which contribution is dominant in the boric acid transport, it is uncertain which indicator, KB or S, is relevant. Nevertheless, it is evident from the results in Figure 1 that both KB and S indicate that GMA has a low affinity to boric acid and is a promising monomer for modification. Membrane Modification Using Different Monomers. As shown in the next section, some loss of water permeability is inevitable when modification is used for changing filtration selectivity. Therefore, a highly permeable LPRO membrane, LE, was chosen under a premise that it would yield membranes with permeabilities typical of BWRO or lower; thus, their performance may be compared to that of commercial BWRO and SWRO membranes. In addition to the three monomers tested for boric acid partitioning (see above), two neutral hydrophilic monomers HEMA and PEGMA and a positively charged METMAC were also used. Although EMA, GMA and EEMA are sparingly soluble in water, the CP-enhanced technique allowed their modification in aqueous solution at very low monomer concentrations 0.7
10 mM, in some cases with addition of a small proportion (0.5
1%) of a cross-linker, as described here and in ref 18. Figure 2 presents pristine and modified LE membranes on a plot of boric acid passage versus permeability. Modified membranes of several DG are shown for GMA, EMA, and METMAC monomers and demonstrate that the performance is affected by DG as well as by the type of monomer. The shift in performance after converting a GMA-modified membrane to diol (GM) and sulfonate (GMS) is also indicated. Although the DG in Figure 2

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varied for different monomers, the general trend is well observed. Modification with hydrophilic and charged monomers HEMA, PEGMA, and MATMEC mainly resulted in lower permeability due to the extra resistance of the modification layer. However, it did not improve and sometimes even increased the B passage, thus, the overall membrane performance became poorer. The hydrophobic monomer EEMA had almost no effect on passage of boric acid, whereas EMA reduced it from an average of 42 to 30
35%, but only at a high DG and with a 30
50% loss of permeability. This change would still not be beneficial because the permeability was affected more than the boric acid passage. Clearly, the largest improvement was obtained after modification with GMA, which is analyzed in more detail in the next section. Note that the trend of the B rejection for EMA, EEMA and GMA modifications correlates well with the sorption selectivity S (Figure 1b) rather than with Kw (Figure 1b). This may suggest that transport of boric acid is dominated by convection, i.e., drag by water. The inferior performance of membranes modified with hydrophilic monomers HEMA and PEGMA supports the premise that excessive hydrophilicity/polarity is detrimental for boron removal. This conclusion as well as the negative effect of negatively charged groups (cf. salting-in of boric acid by sulfate salts in water28,29) was also directly demonstrated by postconverting the epoxy groups of the grafted GMA units to a highly hydrophilic hydrogen bonddonating diol (GM) and negatively charged sulfonate (GMS). The high hydrophilicity of these derivatives is confirmed by measurements of contact angle that drops from 40 and 42° for the original and GMA-modified LE membrane respectively, to 20° for diol and 10
15° for sulfonate. Figure 2 shows that the GMA-modified membrane performs better than its diol and sulfonate derivatives obtained by postconversion of GMA. Furthermore, it may be noted that both GMA and its two derivatives GM and GMS seem to perform somewhat better than other monomers with close characteristics or structure (e.g., cf. GM and HEMA in Figure 2). It implies that there might be an additional effect specific for GMA. For instance, although ATRFTIR spectra presented in the Supporting Information show intact epoxy groups after GMA grafting, GMA could have enhanced adhesion to polyamide through reaction of some epoxy groups with amine groups within polyamide. Nevertheless, in the absence of direct evidence, one may reasonably explain the observed trends within the present thermodynamic arguments. Effect of Degree of Grafting: The Case of GMA. Figure 3 shows the relation between the passage of boric acid (PB) and Lp and DG of GMA, which was varied by changing the GMA concentration during grafting (see the Supporting Information). As DG increases, both PB and Lp first drop precipitously and then the decrease slows down and moderate above DG about 1. This can be explained by assuming that the effect is primarily achieved through the so-called “caulking” mechanism proposed previously.19 The model put forward in ref 19 describes the change of the resistance of the modified membrane (R) to pressure-driven flow or solute diffusion relative to the initial resistance (R0). It assumes that the fast effect is first produced through sealing less selective areas (“defects”). As the modification progresses, an extra layer is grafted onto the regular dense polyamide causing the slower change. The initial sealing is described by the following approximate linearized expression: R 1 þ a0 X  1 þ a0 Xð1
bÞ, ða0 X , 1Þ  R0 1 þ ba0 X 3617

ð4Þ

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Figure 4. Boric aicd passage and membrane premeability of commercial BWRO (gray squares) and SWRO (black triangles) elements as reported by the manufacturers, Empty triangles show results for commercial membranes tested in dead-end cells and empty circles show the dead-end cell results for LE membranes modified with GMA to a different degree (DG = 0.67 and 1.1).

whereas for the subsequent slow change it becomes R 1  ð1 þ a00 XÞ, ða0 X . 1Þ R0 b

ð5Þ

Here X is a parameter that expresses the progress of modification (e.g., DG or monomer concentration), a0 and a00 reflect the intrinsic resistance of the graft polymer (to water flow or solute diffusion) relative to the underlying polymer, “defect” or regular, respectively, and b < 1 is the fraction of the total flow or solute diffusion flux in the original membrane passing through the regular (nondefect) areas. The definition of “defects” implies a0 . a00 . Because the Lp is inversely proportional to resistance and the PB is, approximately, the ratio of the B permeability (reciprocal resistance) and water flux,19 it approximately holds that Lp ðmodifiedÞ 1 PB ðmodifiedÞ 1 þ aw X ¼ and ¼ ð6Þ 1 þ aw X PB ðoriginalÞ 1 þ aB X Lp ðoriginalÞ where a is either (1
b)a0 or a00 , depending on the regime (eq 4 or 5), and indices “w” and “B” designate water and boric acid. Equation 6 explicitly shows that altering the performance will always require a finite X. The modification will be beneficial, if PB drops more than Lp, in which case eq 6 yields the condition 1 þ aB X > ð1 þ aw XÞ2

ð7aÞ

Equations 6 and 7a assume a diffusive transport of solute, known to be dominant for salts.25,35 However, the correlation of PB with S rather than with Kw (see above) does not rule out that the B transport is dominated by convection. In such a case, as easily shown, PB(modified)µ(1 þ aBX)
1 and eq 7a will be replaced with eq 7b 1 þ aB X > 1 þ aw X

ð7bÞ

In either case eqs 7 explicitly state that in order to improve filtration selectivity by modification, aB needs to be significantly larger than aw, i.e., the selectivity of the graft-polymer must exceed that of the underlying polymer. Apparently, this condition is easily met in the defect-sealing regime (eq 4), since the

“defects” are less selective than regular polyamide. Once most “defects” have been sealed, aB decreases and the effect of further modification on B removal becomes smaller and even negative. Indeed, when DG exceeded about 2 (beyond the range of Figure 3), PB started to increase. Modification was also accompanied by a small improvement in NaCl rejection, from 96.5 ( 1% to 97.5 ( 1%. However, the improvement was already achieved at the lowest GMA concentration (0.75 mM) and further modification had no effect. This result also agrees with the caulking mechanism. It must be stressed that structural differences between the regular and “defect” areas might be fairly minor. Yet, there may be significant differences in selectivity, due to the high sensitivity of steric exclusion and hindrance to the ratio of the solute and pore radii when it approaches 1.26,36 Although polyamide is nonporous, the use of an effective pore radius and hindered transport theory (HTT) 26,36 is fairly common and adequate,27 especially, when partitioning is properly addressed.37,38 Based on the measured B passage and water flux and realistic values for relevant membrane characteristics, the pore radius of LE membrane was estimated to be about 0.175 nm (see the Supporting Information). Given the radius of boric acid 0.155 nm, estimates using HTT presented in the Supporting Information show that the observed change in B passage may be explained by a small reduction of the effective radius of modified polyamide to just 0.165 nm. Such a change could be obtained by sealing of the areas with larger effective pore radii in the active layer.

’ COMPARISON OF MODIFIED AND COMMERCIAL MEMBRANES Because the B passage of the modified membrane is correlated with water permeability and the same is true for most RO membranes, a plot similar to Figure 2 is a consistent way to compare the performance of modified and commercial membranes tested at pressures 10
16 bar. Figure 4 makes such a comparison between different commercial SWRO and BWRO membranes by Dow-Filmtec, Toray, and Hydranautics and GMA-modified LE membranes. The B passage of BWRO membranes (as commercial spiral-wound elements), which is usually not cited by manufacturers, was taken from the literature.39
42 Compared to the commercial BWRO membranes, the GMAmodified membranes show a lower B passage in the BW range, i.e., for permeabilities Lp from about 2 to 4 L m
2 h
1 bar
1. This demonstrates that modification is beneficial and membranes modified with GMA have a more favorable combination of performance characteristics than commercial membranes in this range. An exception is BW30HR-440i, the only commercial membrane for which the B passage is cited by the manufacturer. Data obtained for some commercial membranes (SW30, LE and ESPA1) using dead-end cells and testing conditions utilized in this work show somewhat higher Lp than for elements but a similar B passage. Note also that reported results for elements showed that B passage is fairly insensitive to pressure43 within the pressure range involved here (10 to 20 bar). This eliminates the possibility that the higher B passage of commercial BW membranes could be due to different testing conditions.44 At lower permeabilities, i.e., in the SWRO range (Lp < 2 L m
2 h
1 bar
1), the B passage of the modified membrane was not found better than that of the commercial SWRO ones. This result can be once more rationalized by assuming that modification reliably 3618

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Environmental Science & Technology seals “defects” inherently present in more open BW and LPRO membranes; however, the dense and thick SWRO membranes have inherently less defects thereby the benefits of modification are largely lost. Clearly, much optimization and thorough search for more selective monomers are still needed to achieve a major improvement in B rejection. Nevertheless, the present results demonstrate that tailoring performance using surface modification could be feasible. This may be assisted by using rationales and quantitative relations presented in this report. The CP-enhanced graft-polymerization offers a particularly facile route, well-suited for this purpose, including in situ modification of whole commercial elements, which is currently underway and will be reported separately.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details on grafting of GMA layer and its conversion to GM and GMS and their characterization by ATR-FTIR (part S1), variation of DG with GMA concentration (S2), and pore radius estimates using HTT (S3) (PDF).This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ972-8-6563523. Fax: þ972-8-6563503. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank A. Mamoutov for her assistance with analytical measurements. Dow-Filmtec and Hydranautics are gratefully acknowledged for supplying membrane samples. This work was supported by a grant from the Water Authority of Israel. ’ REFERENCES (1) Freger, V. Swelling and morphology of the skin layer of polyamide composite membranes: an atomic force microscopy study. Environ. Sci. Technol. 2004, 38, 3168–3175. (2) Redondo, J.; Busch, M.; De Witte, J. P. Boron removal from seawater using FILMTECTM high rejection SWRO membranes. Desalination 2003, 156, 229–238. (3) Kabay, N.; G€uler, E.; Bryjak, M. Boron in seawater and methods for its separation-A review. Desalination 2010, 261, 212–217. (4) Kimura, K.; Amy, G.; Drewes, J. E.; Heberer, T.; Kim, T. U.; Watanabe, Y. Rejection of organic micropollutants (disinfection byproducts, endocrine disrupting compounds, and pharmaceutically active compounds) by NF/RO membranes. J. Membr. Sci. 2003, 227, 113–121. (5) Steinle-Darling, E.; Zedda, M.; Plumlee, M. H.; Ridgway, H. F.; Reinhard, M. Evaluating the impacts of membrane type, coating, fouling, chemical properties and water chemistry on reverse osmosis rejection of seven nitrosoalklyamines, including NDMA. Water Res. 2007, 41, 3959–3967. (6) Nable, R. O.; Ba~nuelos, G. S.; Paull, J. G. Boron toxicity. Plant Soil 1997, 193, 181–198. (7) Hilal, N.; Kim, G.; Somerfield, C. Boron removal from saline water: A comprehensive review. Desalination, 2010; DOI:10.1016/j. desal.2010.05.012. (8) Glueckstern, P.; Priel, M. Optimization of boron removal in old and new SWRO systems. Desalination 2003, 156, 219–228.

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(9) Xu, Y.; Jiang, J. Q. Technologies for boron removal. Ind. Eng. Chem. Res. 2008, 47, 16–24. (10) Glueckstern, P.; Priel, M. Optimization of boron removal in old and new SWRO systems. Desalination 2003, 156, 219–228. (11) Magara, Y.; Tabata, A.; Kohki, M.; Kawasaki, M.; Hirose, M. Development of boron reduction system for sea water desalination. Desalination 1998, 118, 25–33. (12) Sagiv, A.; Semiat, R. Analysis of parameters affecting boron permeation through reverse osmosis membranes. J. Membr. Sci. 2004, 243, 79–87. (13) Hansson, I. A new set of acidity constants for carbonic acid and boric acid in sea water. Deep-Sea Res. 1973, 20, 461–478. (14) Park, J. K.; Lee, K. J. Diffusion Coefficients for Aqueous Boric Acid. J. Chem. Eng. Data 1994, 39, 891–894. (15) Hyung, H.; Kim, J. H. A mechanistic study on boron rejection by sea water reverse osmosis membranes. J. Membr. Sci. 2006, 286, 269–278. (16) Belfer, S.; Fainshtain, R.; Purinson, Y.; Gilron, J.; Nystr€om, M.; M€antt€ari, M. Modification of NF membrane properties by in situ redox initiated graft polymerization with hydrophilic monomers. J. Membr. Sci. 2004, 239, 55–64. (17) Rana, D.; Matsuura, T. Surface Modifications for Antifouling Membranes. Chem. Rev. 2010, 110, 2448–2471. (18) Bernstein, R.; Belfer, S.; Freger, V. Surface Modification of Dense Membranes Using Radical Graft Polymerization Enhanced by Monomer Filtration. Langmuir 2010, 26, 12358–12365. (19) Ben-David, A.; Bernstein, R.; Oren, Y.; Belfer, S.; Dosorez, C.; Freger, V. Facile surface modification of nanofiltration membranes to target the removal of endocrine-disrupting compounds. J. Membr. Sci. 2010, 357, 152–159. (20) Gupta, S.; Stewart, J. The extraction and determination of plant-available boron in soils. Schweiz. Landwirtsch. Forsch. 1975, 14, 153–159. (21) Marcus, Y. The properties of organic liquids that are relevant to their use as solvating solvents. Chem. Soc. Rev. 1993, 22, 409–416. (22) Flick, E. W. Industrial Solvents Handbook., 5th ed.; William Andrew Publishing /Noyes Data Corporation:Westwood, NJ, 1998. (23) Arce, A.; Blanco, A.; Souza, P.; Vidal, I. Liquid-liquid equilibria of the ternary mixtures waterþ propanoic acidþmethyl ethyl ketone and waterþpropanoicþ acid methyl propyl ketone. J. Chem. Eng. Data 1995, 40, 225–229. (24) Dordas, C.; Brown, P. Permeability of boric acid across lipid bilayers and factors affecting it. J. Membr. Biol. 2000, 175, 95–105. (25) Kedem, O.; Freger, V. Determination of concentration-dependent transport coefficients in nanofiltration: Defining an optimal set of coefficients. J. Membr. Sci. 2008, 310, 586–593. (26) Deen, W. Hindered transport of large molecules in liquid-filled pores. AIChE J. 1987, 33, 1409–1425. (27) Nghiem, L. D.; Sch€afer, A. I.; Elimelech, M. Removal of natural hormones by nanofiltration membranes: measurement, modeling, and mechanisms. Environ. Sci. Technol. 2004, 38, 1888–1896. (28) Linke, W. F. Seidell, A. Solubilities: Inorganic and Metal-Organic Compounds: A Compilation of Solubility Data from the Periodical Literature; American Chemical Society: Washington, D.C., 1958. (29) Chanson, M.; Millero, F. J. The solubility of boric acid in electrolyte solutions. J. Solution Chem. 2006, 35, 689–703. (30) Israelachvili, J. N. Intermolecular and Surface forces, 2nd ed.; Academic Press: London, 1992. (31) Robinson, R. A. Stokes, R. H. Electrolyte Solutions, 2nd ed.; Dover Publications: Mineola, NY, 2002. (32) Strathmann, H.; Michaels, A. Polymer
water interaction and its relation to reverse osmosis desalination efficiency. Desalination 1977, 21, 195–202. (33) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. A Survey of Structure- Property Relationships of Surfaces that Resist the Adsorption of Protein. Langmuir 2001, 17, 5605–5620. 3619

dx.doi.org/10.1021/es103991u |Environ. Sci. Technol. 2011, 45, 3613–3620

Environmental Science & Technology

ARTICLE

(34) Kamlet, M. J.; Abboud, J. L.; Taft, R. The solvatochromic comparison method. 6. The. pi.* scale of solvent polarities. J. Am. Chem. Soc. 1977, 99, 6027–6038. (35) Bason, S.; Oren, Y.; Freger, V. Ion transport in the polyamide layer of RO membranes: composite membranes and free-standing films. J. Membr. Sci. 2010, 367, 119–126. (36) Bungay, P. M.; Brenner, H. The motion of a closely-fitting sphere in a fluid-filled tube. Int. J. Multiphase Flow 1973, 1, 25–56. (37) Verliefde, A. R. D.; Cornelissen, E. R.; Heijman, S. G. J.; Hoek, E. M. V.; Amy, G. L.; Bruggen, B. V.; van Dijk, J. C. Influence of solutemembrane affinity on rejection of uncharged organic solutes by nanofiltration membranes. Environ. Sci. Technol. 2009, 43, 2400–2406. (38) Bason, S.; Kaufman, Y.; Freger, V. Analysis of Ion Transport in Nanofiltration Using Phenomenological Coefficients and Structural Characteristics. J. Phys. Chem. B 2010, 114, 3510–3517. (39) Prats, D.; Chillon-Arias, M.; Rodriguez-Pastor, M. Analysis of the influence of pH and pressure on the elimination of boron in reverse osmosis. Desalination 2000, 128, 269–273. (40) Xu, P.; Drewes, J. E.; Heil, D. Beneficial use of co-produced water through membrane treatment: technical-economic assessment. Desalination 2008, 225, 139–155. (41) Redondo, J.; Busch, M.; De Witte, J. P. Boron removal from seawater using FILMTECTM high rejection SWRO membranes. Desalination 2003, 156, 229–238. (42) Murray-Gulde, C.; Heatley, J. E.; Karanfil, T.; Rodgers, J. H. Performance of a hybrid reverse osmosis-constructed wetland treatment system for brackish oil field produced water. Water Res. 2003, 37, 705–713. (43) Arias, M. F. C.; iBru, L. V.; Rico, D. P.; Galvan, P. V. Reduccion de boro en aguas procedentes de la desalacion. Ph.D. Dissertation, University of Alicante, Alicante, Spain, 2010. (44) Van Wagner, E. M.; Sagle, A. C.; Sharma, M. M.; Freeman, B. D. Effect of crossflow testing conditions, including feed pH and continuous feed filtration, on commercial reverse osmosis membrane performance. J. Membr. Sci. 2009, 345, 97–109.

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dx.doi.org/10.1021/es103991u |Environ. Sci. Technol. 2011, 45, 3613–3620