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Aqueous Boron Removal by Using Electrospun Polyvinyl Alcohol (PVA) Mats; A Combined Study of IR/ Raman Spectroscopy and Computational Chemistry Kwan Sik Lee, Ki Heon Eom, Jun-Heok Lim, Hyunwook Ryu, Suhan Kim, Dong-Kyu Lee, and Yong Sun Won J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12578 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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The Journal of Physical Chemistry

Aqueous Boron Removal by Using Electrospun Polyvinyl Alcohol (PVA) Mats; A Combined Study of IR/Raman Spectroscopy and Computational Chemistry

Kwan Sik Lee,†,* Ki Heon Eom,‡,* Jun-Heok Lim,‡ Hyunwook Ryu,§ Suhan Kim,§ Dong-Kyu Lee,† and Yong Sun Won†,**

†

Department of Engineering Chemistry, College of Engineering, Chungbuk National University, Cheongju, Chungbuk 28644, Republic of Korea

‡

Department of Chemical Engineering, Pukyong National University, Busan 48547, Republic of Korea §

Department of Civil Engineering, Pukyong National University, Busan 48513, Republic of Korea

ABSTRACT. In this study, we report the use of a novel and efficient method to remove aqueous boron by using electrospun, water-resistant polyvinyl alcohol (PVA) mats stabilized in methanol. The removal of the primary aqueous boron species as (B(OH)3), was accomplished by chemical adsorption in reactions with –OH (hydroxyl) groups on the PVA mat surface. The chemical adsorption of B(OH)3 was qualitatively confirmed by the analysis of IR and Raman spectra. The bands, corresponding to the molecular vibration modes of chemically bonded boron in PVA, were identified by using the frequency calculation from the computational chemistry for the first time. The adsorption capacities of PVA mats for aqueous boron were then quantitated at a low boron concentration (range: 0.0010~0.0025 g of aqueous boron per g of PVA mats) by the Carmine method. The PVA mats were prepared by a well-established electrospinning technique,

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which make these substrates promising potential candidates for use as boron-selective sorbent media in applications such as reverse osmosis desalination processes.

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INTRODUCTION Aqueous boron removal is still an issue, even in well-established seawater reverse osmosis (SWRO) processes for desalination, because of the size of the primary aqueous boron species, A major issue with single-stage RO desalination technology is that boron contaminants exceed WHO recommendations for drinking water. This happens because the boron rejection feature of the membrane is low, thus enabling boric acid (B(OH)3) to seep through the pores in a non-ionic way.1,2 A two-pass RO process is thus generally used where a part of the stream is bypassed to increase the rejection of aqueous boron by converting B(OH)3 into borate ion (B(OH)4-) under high pH conditions.3 B(OH)4- is far more selectively rejected by commercial negatively-charged RO membrane surfaces, but the process is complicated by accompanying alkalinization and neutralization.4 Therefore, commercial SWRO processes could be further improved by the development of any highly efficient boron-selective membrane/material used at neutral pH. Many efforts have been made to improve the aqueous boron removal process, including the development of effective sorbent media for B(OH)3.5 Most of the sorbents use the functionalization of the resin surfaces with hydroxyl (-OH) groups; as an example, commercially available boron- selective resins have been developed based on a macroporous polystyrene (PS) matrix with multiple surface –OH groups generated after chloromethylation and amination with N-methyl glucamine.6 The mechanism of B(OH)3 adsorption is nothing but the chemical reaction between B(OH)3 and surface –OH groups as shown in Reaction (1).7 Depending on the pH, the reaction can proceed further to generate borate species.

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This mechanism, in fact, is exactly the same for the complexation of polyols and B(OH)3. We recently confirmed that the simple addition of polyols (xylitol and mannitol) possessing multiple –OH groups generates larger complexes easily rejected by SWRO membranes.8 And it was also suggested that the forward reaction is thermodynamically favorable based on the energetics calculated by computational chemistry.9 Also, Raman spectroscopy indirectly confirmed the preference of B(OH)3 complex formation with polyols over its conversion into B(OH)4-.10 Thus, we have used the same mechanism to focus on a novel boron-selective sorbent media which are simply prepared and easily incorporated into commercial RO processes. Polyvinyl alcohol (PVA) is known to have multiple –OH groups and Yao et al. proposed the preparation of PVA mats by electrospinning.11 They also solved the problem of dissolution of asprepared PVA mats in water by post-treatment with methanol to stabilize the mats and prevent disintegration upon contact with water. Electrospinning is a well-established, easy-to-control technique for producing nano- or microfibers. Effective complexing of B(OH)3 to electrospun PVA mats via -OH functionalities may make these substrates good candidates as boron-selective sorbent media in RO membrane systems because of their enhanced mechanical characteristics. In this study, we have prepared water-resistant PVA mats by electrospinning and stabilization in methanol. Then, we have performed qualitative and quantitative evaluations of the adsorption capacity of a PVA mat for aqueous boron; Infrared (IR) and Raman spectra were used to qualitatively confirm the formation of B-O bonds as a result of the reaction of B(OH)3 and –OH groups of PVA. Data from a vibrational frequency algorithm generated by computation chemistry confirmed the interpretations of the IR and Raman spectra. These results demonstrate to the best of our knowledge the first effective removal of aqueous boron by stabilized PVA mats. [G G


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Furthermore, the enhanced boron-adsorption capacity of stabilized PVA mats was quantitatively shown by a corresponding drop in aqueous boron in solution.

EXPERIMENTAL Electrospinning of PVA mats. We have followed the procedure reported by Yao et al..11 Aqueous PVA solutions (PVA powder/water = 1/9 by weight) were prepared by dissolving PVA powder (Sigma-Aldrich, number average MW 130,000 g/mol) in deionized water at 80 °C with constant stirring for 24 h. After the solution had cooled to room temperature, Triton X-100 (Aldrich) was added, and the mixture was stirred for 15 min. For PVA electrospinning, we have used a commercial setup (ESP-100D, NanoNC, Korea) with a 25-kV positive voltage at a constant feed rate of 2.0 mL/h for 10 mL of aqueous PVA solution (see Figure 1S in supporting information). The configuration of the typical electrospinning setup is also available elsewhere.11-14 The electrospun PVA mats were stabilized against disintegration in water by a post-treatment in methanol for 24 h. Then, the methanol-treated PVA mats were dried in a ventilated hood at room temperature for 24 h. The morphologies of as-prepared and methanoltreated PVA mats were measured by a scanning electron microscope (HITACHI, S-2700, Japan). IR/Raman spectroscopies. To confirm the reaction of B(OH)3 and –OH groups of PVA, two complementary spectroscopies were employed, IR (Perkin Elmer, Spectrum-X, USA) and Raman (Lambda Solutions, Inc., Dimension-P1, USA). The methanol-treated PVA mats were cut by roughly 10 cm × 10 cm and weighed. After dipping in 0.1 M aqueous solution of B(OH)3 for 30 secs, the mats were dried in a ventilated hood at room temperature for 24 h before IR and Raman measurements. The aqueous solution of B(OH)3 with a high concentration (0.1 M) was used to enhance the IR and Raman spectra by securing sufficient reaction degree. The pH of the \G G



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0.1 M B(OH)3 solution was maintained at 7.0 by using 5 wt.% NaOH aqueous solution to emulate the pH condition of seawater. The adsorption capacity of PVA mats. To evaluate the adsorption capacity of PVA mats for aqueous boron, we used 10 ppm aqueous solution of B(OH)3 (Sigma-Aldrich, Co., LLC, USA). Three PVA mat samples with different masses (0.04, 0.08, and 0.12 g) were prepared and dipped into 100 ml of the aqueous solution of B(OH)3 for 10 min. The following relation gives the adsorption capacity.

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where, W, Cinitial, Cfinal, MPVA, and Msolution are the adsorption capacity of PVA mats for aqueous boron (g/g), the initial boron concentration (ppm), the final boron concentration (ppm), the weight of the PVA mats, and the solution volume (100 ml), respectively. The initial boron concentration was 10 ppm as mentioned above and the final boron concentration means the concentration of the B(OH)3 solution left after PVA mat samples were dipped for 10 min. The pH of the solution was ~7.0, not affected because of the very low concentration of B(OH)3. The boron concentrations before and after dipping were measured by Carmine method using a spectrophotometer (Hach, DR3900, Germany) according to Standard Methods for the Examination of Water and Wastewater (4500-B).15 The boron concentration in aqueous solution was measured by the reaction of aqueous boron with carminic acid in the presence of sulfuric acid to change the reddish color to bluish one by forming a complex compound (see Figure 2S in supporting information).16 The boron reagents for this measurement were prepared by dissolving powder pillows (BoroVer®3 Boron Reagent, Hach Co., Ltd, USA) and sulfuric acid with ACS ]G G


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grade (Hach Co., Ltd, USA). The intensity of the color is directly proportional to the aqueous boron concentration. Average value of multiple measurements of the intensity of color was used for the analysis. Computational chemistry. As mentioned in the Introduction, the PVA mats are used in aqueous boron removal for the first time in our best knowledge. No previous reports are thus available so that we had to introduce a more fundamental approach to the interpretation of measured IR and Raman spectra. The frequency calculation from the computational chemistry is known to produce theoretical IR and Raman spectra, and corresponding molecular vibration modes. In other words, molecular vibration modes relevant to the B-O bonds as a result of the reaction of B(OH)3 and –OH groups of PVA have to be identified to confirm the B(OH)3 adsorption on the surface of PVA mats qualitatively. Technically, we emulated PVA polymer, an infinite chain, with an oligomer chain possessing 10 repeating units (see Figure 3Sa in supporting information). Then, assuming that the reaction between two neighboring –OH groups of PVA oligomer and B(OH)3 releases two water molecules as products,9 the final geometry of the complex with B(OH)3 chemically adsorbed (see Figure 3Sb in supporting information) was proposed. The details for the reactions are available in our previous report.9 Full geometry optimization was performed with the Gaussian 09W software (Gaussian Inc., USA), with B3LYP density functional theory (DFT) model chemistry and 6-31G(d) basis set.17 Harmonic frequency calculation was then followed to locate all molecular vibration modes of the initial PVA oligomer (see Figure 3Sa) and the complex of PVA and B(OH)3 (see Figure 3Sb). The GaussView software (Gaussian Inc., USA) was used for the visualization of the results.

RESULTS AND DISCUSSION ^G G



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Electrospun PVA mats. The electrospun PVA mats look like fabrics by visual appearance, but micro-fibers are interwoven inside as shown in Figure 1. The morphological characteristics are not changed a lot, regardless of the methanol stabilization. Although the mechanism of the methanol stabilization has not yet been completely clarified, the as-prepared PVA mats were quickly dissolved in water as expected but the methanol-stabilized PVA mats retained their fibrous characteristics in water. Moreover, we found that when the as-prepared PVA mats were immersed in 0.1 M aqueous solution of B(OH)3, the mats were not only dissolved but the color of the mats was also sharply enhanced. It seemed that some reactions occurred to protect the PVA mats. One proposition is that B(OH)3 reacts quickly with –OH groups on the surface of PVA mats to inhibit further disintegration reactions between –OH groups and water. Meanwhile, Yao et al. speculated that methanol treatment increased the degree of crystallinity and hence the number of physical cross-links in the electrospun PVA fibers.11 In other words, the removal of residual water within the fibers by methanol replaces PVA-water hydrogen bonding by intermolecular polymer hydrogen bonding resulting in additional crystallization.11 These two speculations would secure the stability of electrospun PVA mats in practical water-based applications. The specific surface area of the electrospun PVA mats was not sufficiently high enough to be detected by N2 adsorption/desorption analysis due to the large porosity as shown in Figure 1. However, the density of PVA mats is controllable, if necessary, by adjusting the operating parameters of electrospinning process.

Figure 1.

Detection of B(OH)3 chemical adsorption on PVA mats. To identify the B-O bonds as a _G G


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result of B(OH)3 and –OH groups of PVA, IR and Raman spectroscopies were employed. Figure 2 shows the IR spectra for methanol-stabilized PVA mats before and after dipping in 0.1 M aqueous solution of B(OH)3. The comparison indicates the only major difference at ~665 cm-1. In fact, the IR spectra for as-prepared PVA mats before and after dipping showed almost the same trend (see Figure 4S in supporting information).

Figure 2.

The harmonic frequency calculation from the computational chemistry was then employed to identify the band at ~665 cm-1, which appears only after dipping in an aqueous solution of B(OH)3. A molecular vibration mode probably corresponding to the band was located and visualized in Figure 3. Its frequency was calculated as 665.69 cm-1. As expected, there exists a molecular vibration-relevant to B-O bonds as a result of B(OH)3 and –OH groups of PVA. The displacement vectors show in- and out-of-plane bending vibration to the plane formed by boron at the center and three neighboring oxygens. It is consistent with the fact that IR signal interacts with dipole moment change. In the other hand, Raman signal interacting with polarization change would correspond to the stretching vibration of three B-O bonds.

Figure 3.

For comparison, the Raman spectra are given in Figure 4 for methanol-treated PVA mats before and after dipping in 0.1 M aqueous solution of boric acid. Because Raman signal is very weak, the Raman spectra are not as sharp as IR spectra. Nonetheless, two major differences were `G G



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found at ~776 and ~1258 cm-1. As expected, the harmonic frequency calculation confirmed that two bands are all related to the stretching vibration of B-O bonds as shown in Figure 5. Their calculated frequencies were 770.35 and 1216.76 cm-1, a little deviated from experimental values. It is because we have only considered a limited PVA oligomer chain for frequency calculation instead of the infinite PVA chain (see Figure 3S). In summary, IR and Raman measurements combined with the computational chemistry confirmed the adsorption capacity of the electrospun PVA mats for B(OH)3 qualitatively.

Figure 4. and Figure 5.

The adsorption capacity of electrospun PVA mats for aqueous boron. As shown in Eq. (2), the adsorption capacity of the PVA mats for aqueous boron were measured. Three different weights (0.04, 0.08, and 0.12 g) of the PVA mats were used, and three samples were measured for each weight after dipping. Figure 6 shows how much aqueous boron is adsorbed per 1g of the PVA mats. As expected, the (Cinitial – Cfinal) value was increased with the weight of the PVA mats, and a linear relationship was found (see Figure 6a). The slope was 22.5 ppm per g of PVA mats, corresponding to a W value of 0.00225 g of aqueous boron per g of PVA mats. However, considering the sensitivity in the measurement of low boron concentrations, it is safe to say that the adsorption capacity ranges in 0.0010~0.0025 g of aqueous boron per g of PVA mats, as shown in Figure 6b.

Figure 6.

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CONCLUSIONS Our study represents the first successful removal of aqueous boron using electrospun PVA mats. To secure the stability of the PVA mats in water (not to be dissolved in water), a posttreatment with methanol was applied. The adsorption capacity of the PVA mats for aqueous boron was quantitatively evaluated by the measurements of the boron concentrations in aqueous solution of boric acid before and after dipping the PVA mats. It ranges in 0.0010~0.0025 g of aqueous boron per g of PVA mats. Then, IR and Raman measurements were supported to clarify whether aqueous boron species are chemically adsorbed on the PVA mats. The bands corresponding to the vibration modes relevant to B-O bonds as a result of B(OH)3 and –OH groups of PVA were clearly identified by the aid of the computational chemistry. The proven adsorption capacity of the PVA mats, as well as the facility of the preparation of the mats by a commercially available electrospinning technique, pose a great potential of their application in reverse osmosis (RO) processes for desalination.

ASSOCIATED CONTENT Supporting Information Experimental setup of electrospinning, formation of boron-containing complex with Carmine indicator, oligomer PVA chain possessing 10 repeating units and the complex generated by the reaction of B(OH)3 and two neighboring –OH groups (for harmonic frequency calculations), IR spectra of as-prepared and methanol-stabilized PVA mats before and after dipping in 0.1 M aqueous solution of B(OH)3 (Figures 1S-4S)

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AUTHOR INFORMATION Corresponding author **

Yong Sun Won, E-mail: [email protected]. Tel.: +82 51 629 6431.

Notes *

Kwan Sik Lee and Ki Heon Eom have contributed equally.

ACKNOWLEDGEMENTS This research was supported by a Research Grant of Pukyong National University (CD-20160620).

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REFERENCES [1] Redondo, J.; Busch, M.; De Witte, J.P. Boron removal from seawater using FILMTECTM high rejection SWRO membranes. Desalination 2003, 156, 229-238. [2] Sagiv, A.; Semiat, R. Analysis of parameters affecting boron permeation through reverse osmosis membranes. J. Membr. Sci. 2004, 243, 79-87. [3] Glueckstern, P.; Priel, M. Optimization of boron removal in old and new SWRO systems. Desalination 2003, 156, 219-228. [4] Greenlee, L.F.; Lawler, D.F.; Freeman, B.D.; Marrot, B.; Moulin, P. Reverse osmosis desalination: Water sources, technology, and today’s challenges. Water Res. 2009, 43, 23172348. [5] Melnyk, L.; Goncharuk, V.; Butnyk, I.; Tsapiuk, E. Boron removal from natural and waste waters using combined sorption/membrane process. Desalination 2005, 185, 147-157. [6] Kabay, N.; Sarp, S.; Yuksel, M.; Kitis, M.; Koseoglu, H.; Arar, O.; Bryjak, M.; Semiat, R. Removal of boron from SWRO permeate by boron selective ion exchange resins containing N-methyl glucamine groups. Desalination 2008, 223, 49-56. [7] Darwish, N.B.; Kochkodan, V.; Hilal, N. Boron removal from water with fractionized Amberlite IRA743 resin. Desalination 2015, 370, 1-6. [8] Park, B.S.; Lee, J.S.; Kim, M.S.; Won, Y.S.; Lim J.-H.; Kim, S.H. Enhanced boron removal using polyol compounds in seawater reverse osmosis processes. Desal. Water Treat. 2016, 57, 7910-7917. [9] Kim, M.-K.; Eom, K.H.; Lim, J.-H.; Lee, J.-K.; Lee, J. D.; Won, Y.S. Simple boron removal from seawater by using polyols as complexing agents: A computational mechanistic study. Korean J. Chem. Eng. 2015, 32, 2330-2334.

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[10] Eom, K.H.; Jeong, H.C.; An, H.Y.; Lim, J.-H.; Lee, J.-K.; Won, Y.S. Removal of aqueous boron by using complexation of boric acid with polyols: A Raman spectroscopic study. Korean Chem. Eng. Res. 2015, 53, 808-813. [11] Yao, L.; Haas, T. W.; Guiseppi-Elie, A.; Bowlin, G.L.; Simpson D.G.; Wnek, G.E. Electrospinning and stabilization of fully hydrolyzed poly(vinyl alcohol) fibers. Chem. Mater. 2003, 15, 1860-1864. [12] Wu, L.; Yuan, X.; Sheng, J. Immobilization of cellulose in nanofibrous PVA membranes by electrospinning. J. Membr. Sci. 2005, 250, 167-173. [13] Yang, E.; Qin, X.; Wang, S. Electrospun crosslinked polyvinyl alcohol membrane. Mater. Lett. 2008, 62, 3555-3557. [14] Son, W.K.; Youk, J.H.; Lee, T.S.; Park, W.H. Effect of pH on electrospinning of poly(vinyl alcohol). Mater. Lett. 2005, 59, 1571-1575. [15] Clesceri, L.S.; Greenberg, A.E.; Eaton, A.D. Standard method for the examination of water and wastewater, 22th ed., Water Environment Federation, Washington, DC, 2012. [16] Nies´cior-Browin´ ska, P.; Zakrzewska-Koátuniewicz, G.; Chajduk, E. The recovery of boric acid from PWR reactor cooling water and wastewater by using micellar-enhanced ultrafiltration. Membr. Membr. Processes Environ. Prot. 2014, 119, 17–26. [17] Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A. et al. Gaussian 09, Revision C.01, Gaussian, Inc.: Wallingford CT, 2009. [18] Kabay, N.; Bryjak, M.; Hilal, N. Boron separation processes; Elsevier, Amsterdam, 2015; pp. 58.

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LIST OF FIGURES

Figure 1. SEM micrographs of as-prepared PVA mat (a) and methanol-stabilized PVA mat (b). Figure 2. IR spectra of methanol-stabilized PVA mats before and after dipping in 0.1 M aqueous solution of boric acid. Figure 3. A molecular vibration mode relevant to the frequency of ~665 cm-1 (displacement vectors included). Red, white, gray, and incarnadine balls indicate oxygen, hydrogen, carbon, and boron, respectively. Figure 4. Raman spectra of methanol-stabilized PVA mats before (left) and after (right) dipping in 0.1 M boric acid aqueous solution. Figure 5. Molecular vibration modes relevant to the frequencies of ~776 cm-1 (left) and 1253 cm-1 (right) (displacement vectors included). Red, white, gray, and incarnadine balls indicate oxygen, hydrogen, carbon, and boron, respectively. Figure 6. Adsorption capacity of the PVA mats for aqueous boron; (a) (Cinitial – Cfinal) vs. MPVA and (b) W vs. MPVA

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Figure 1. (a)

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Figure 2. beforedipping

afterdipping

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Figure 4.

Figure 5.

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