Coating zwitterionic amino acid l -DOPA to increase fouling resistance of forward osmosis membrane

Desalination 312 (2013) 82–87

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Coating zwitterionic amino acid L-DOPA to increase fouling resistance of forward osmosis membrane Anh Nguyen, Sara Azari, Linda Zou ⁎ SA Water Centre for Water Management and Reuse, University of South Australia, Adelaide SA 5095, Australia

H I G H L I G H T S ► ► ► ►

The The The The

forward osmosis membrane surface was coated zwitterionic L-DOPA. hydrophilicity and charge of the membrane porous layer have been changed. modified membranes in PRO mode showed less organic fouling in FO experiments. direct and easy coating of L-DOPA has the potential for industrial scale up.

a r t i c l e

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Article history: Received 26 May 2012 Received in revised form 20 November 2012 Accepted 30 November 2012 Available online 22 December 2012 Keywords: Forward osmosis Membrane fouling resistance L-DOPA Hydrophilicity Surface modification

a b s t r a c t In this study, the HTI Forward Osmosis (FO) membrane surfaces were modified by the deposition of poly amino acid 3-(3,4-Dihydroxyphenyl)-L-alanine (L-DOPA), a zwitterionic polymer on the membrane surface in order to enhance the fouling resistance of the FO membranes. The modification took place on the porous layer/side of the membranes. The modified membrane surfaces became more hydrophilic with a reduced initial water contact angle. Accelerated fouling experiments were conducted in pressure retarded osmosis (PRO) mode, in which the porous layer of membrane faced the feed solution. The feed solution comprised alginic acid sodium salt (AAS) and calcium ions (CaCl2). The filtration results proved an effective improvement of fouling resistance of the L-DOPA coated membranes. The membrane samples with a 12-hour coating achieved 30% less fouling compared to the uncoated sample. The L-DOPA coating method is simple and direct; it has the potential for scaled up industrial applications. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Forward osmosis (FO) is a membrane process that uses natural osmotic pressure of a concentrated draw solution to drive water across a semi-permeable membrane from the feed solution [1,2]. As an emerging technology, FO brings many advantages over pressure-driven processes like reverse osmosis (RO), microfiltration (MF) and nanofiltration (NF) [3], include: less energy input [4], lower fouling tendency and/or easier fouling removal [5,6], and higher water recovery [7]. In the last decade, FO technology has been studied in the application of wastewater treatment [6,8], seawater desalination [9,10], power generation [11–13] and liquid food processing [14,15]. The current FO process uses an asymmetric/composite membrane consisting of a dense active layer and a porous support layer [16]. The performance of FO membranes is affected by which of these layers is facing the feed solution. When the feed is facing the active layer, the membrane orientation is referred as FO mode; when the feed is facing the support layer, the membrane orientation is referred as pressure retarded osmosis (PRO).

⁎ Corresponding author. Tel.: +61 8 8302 5489; fax: +61 8 8302 3386. E-mail address: [email protected] (L. Zou). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.11.038

Although FO is considered as a promising technology, it still faces some problems that challenge the application of FO in real-world water purification [5]. These challenges include, internal concentration polarisation (ICP), membrane fouling, reverse solute diffuse as well as the need for the development of both better FO membranes and draw solutes. In general, fouling is a major and unavoidable problem that limits performance of all membrane processes including FO. Membrane fouling in FO was observed in various experiments. Organic compounds, such as proteins, polysaccharides, humic acids and extracellular polymeric substances, tend to adsorb on the membrane surfaces to form a gel layer or biofilm, which not only significantly promotes the bacteria growth but also causes severe reduction in water flux. Due to fouling, membrane-based processes require higher energy inputs, frequent chemical cleaning and premature membrane replacement, which lead to higher operational costs. As no hydraulic pressure is applied in the osmotic-driven membrane processes, membrane fouling in the FO process is different from that in pressure-driven membrane processes. Previous research has reported that the fouling in the FO process is lower than others, but it still can severely limit FO membrane performance [2,17]. Several anti-fouling surface modification methods have been developed in recent years, but most of surface modification studies focus on pressure-driven membranes (e.g. UF, NF and RO), and none of the

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surface modification studies have been reported for FO membranes. Current fouling resistant coating materials for pressure driven membranes can be classified into two categories: (i) anionic coating, such as poly(acrylic acid) coated on the surfaces of membranes, providing strong electrostatic repulsion as well as strong steric repulsion that prevents the adsorption of negatively charged organic matter [18]; and (ii) hydrophilic polymer coating, such as polyethylene glycol (PEG) which prevents the fouling by hydrating the surface via a hydrogen bond network [3,19,20]. The zwitterionic coating has been introduced as new and promising candidates for surface modification. Zwitterionic coating can bind water molecules even stronger than hydrophilic materials via electrostatic interactions [21–23]. Recent studies have developed innovative techniques to synthesize zwitterionic polymers on the surface. However, most of them involve a multistep process which may be harsh to the delicate structure of the membranes. L-DOPA is a zwitterionic (redox functional amino acid) inspired by the adhesive proteins found in marine mussels [24]. The L-DOPA is able to self-polymerise in aqueous solutions and form a strong attachment to a wide range of substrates [23]. Our previous work demonstrated that L-DOPA coating on the RO membranes significantly enhanced membrane hydrophilicity and membrane fouling resistance which motivated us to investigate L-DOPA coating on the FO membranes. In this study, the commercial FO membrane surface was modified by L-DOPA coating in order to reduce the membrane fouling. Fig. 1 is a schematic diagram of the structure of zwitterionic poly L-DOPA attached on the FO membrane via in-situ polymerisation. It is hypothesized that the coated membrane becomes strongly hydrated and the hydrated layer prevents the accumulation and adsorption of solutes on the membrane surface. The PRO mode was used for this accelerated fouling study. PRO mode can provide higher flux however; it suffers from a higher fouling rate than the FO mode which is due to the lack of hydrodynamic shear force in the membrane porous layer [5]. The enhancement in the membranes' fouling resistance was investigated by observing the filtration flux. Alginic acid sodium salt (AAS) was used as the model organic foulant. AAS has been widely used in membrane fouling studies. In addition, Mi et al. [2] showed that alginate and the calcium chloride solution caused a higher rate of fouling on the FO membranes than the bovine serum albumin (BSA) or Aldrich humic acid (AHA). Their results also showed that the fouling with alginate is not dependent on hydrodynamic shear force and makes

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it a suitable choice for studying the membrane fouling. Similar to alginate fouling in RO membrane systems, the presence of calcium ions enhances the alginate fouling for FO membrane by increasing of intermolecular adhesion between alginate molecules and resulting in a formation of gel network on membrane surfaces [2]. 2. Materials 2.1. FO membranes Flat sheet FO membranes were provided by Hydration Technologies Innovation (USA). The commercially available FO membranes were made of cellulose triacetate (CTA) and supported by embedded polyester mesh. These membranes are comparable to other semi-permeable membranes used in the pressure-driven processes, and have been considered the best available membranes for current FO applications [16]. 2.2. Foulant solution AAS extracted from brown algae was purchased from Sigma-Aldrich (Australia and New Zealand). A solution of 1 g/L AAS was prepared by dissolving the chemical in deionised water for over 16 h, to ensure that it was completely dissolved, and storing it in the fridge at 4 °C to prevent bacteria growth. It is known that the presence of calcium ions resulted in more severe flux decline during alginate fouling due to the formation of a cross-linked alginate gel layer on the membrane surface [2]. Therefore the stock solution was then added by 200 mg/L CaCl2 to promote synergistic fouling effects in FO process. 2.3. Coating materials 3-(3,4-Dihydroxyphenyl)-L-alanine (L-DOPA) (formula weight (FW) 197 g) and Tris (hydroxymethyl) aminomethane (Tris) buffer were obtained from Sigma-Aldrich (Australia and New Zealand). In oxidative environment the catechol part of the L-DOPA can be oxidized to form a Dopa quinone. The primary roles of adhesive bonding can be assigned to the reverse dismutation reaction between catechol and o-quinone form of DOPA. Dopa quinone can further react to form poly L-DOPA

Fig. 1. Schematic of surface adsorption resistance for organic matter imparted by the zwitterionic poly(L-DOPA) coating.

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oligomers which firmly attach to the surface. The method of surface coating will be described further in the following section. 2.4. Feed and draw solution 2 M sodium chloride (NaCl) solution was used as the draw solution for all FO experiments. It was prepared from analytical reagent grade NaCl (Ajax Finechem Pty. Ltd., Australia). The concentration of the draw solution was maintained as constant by using a dosing pump that was activated by an on-line conductivity sensor. The feed solution for FO fouling experiments contained 1 g/L AAS, a model foulant, and 200 mg/L CaCl2 which enhances the intermolecular adhesion between alginate molecules that result in increased alginate fouling.

in similarly range used in other studies [25,26]. The draw concentration was maintained constantly at 2 M NaCl by a conductivity sensor and a dosing pump, which automatically pumps the NaCl solution from a more concentrated reservoir to the draw solution reservoir once the conductivity of the draw solution falls below the required value. This setup helped to keep the osmotic pressure constant to avoid the reduction of water flux due to the dilution of draw solution. During the experiments, weights of permeate were recorded every minute using a digital balance and a computer. The water flux (Jw) was calculated by measuring the weight change of the permeate during each experimental time period as follows in equation [7]: Jw ¼ ΔWeight=ðWaterdensity  Effectivearea  ΔTimeÞ:

3.3. Organic fouling and cleaning experiments

3. Methods 3.1. Surface coating A 10 mM Tris–HCl buffer solution was prepared in advance to be used as a solvent for L-DOPA. The concentration of L-DOPA solution was 2 g/L. The pH of the buffer solution was also adjusted at 8.0. Only the porous side of FO membranes was coated by using a flat sheet membrane cell that had symmetric channels. A peristaltic pump allowed the solution to flow and circulate over one side of the membrane in the cell. The L-DOPA solution was circulated in the cell for a pre-designed coating time. The coating time varied from 1 h to 12 h. When coated, the membranes were removed from the cell, carefully washed by deionised water at least three times and kept in the fridge at 4 °C.

The FO membranes were firstly stabilised for at least 30 min, with deionised water as the feed and 2 M NaCl as the draw solution, until achieving the stable water flux. Then, the pure water flux was recorded as the average value of 30 min. The next stage was the fouling filtration, which used a mixture of AAS and CaCl2 solution as the feed and 2 M NaCl as the draw solution, which lasted for three hours. The water flushing method was applied for the cleaning process which was also conducted in our previous studies [7]. In the cleaning process, both the feed and draw solutions were changed into deionised water to rinse the membrane at a cross-flow rate of 30.0 cm/s. After one hour rinsing, the pure water flux of the membrane was measured again with deionised water as the feed solution and 2 M NaCl as the draw solution. The pure water flux was calculated again to examine the removability of the fouled FO membranes.

3.2. Experimental setup of accelerated fouling on FO membranes 3.4. FO membrane characterisation The schematic diagram of the FO experimental system is illustrated in Fig. 2. This setup is similar to that used in our previous study [7,16,23]. It includes a membrane holding cartridge with dimensions of 2, 80 and 60 mm for height, length and width, respectively; a digital balance; two peristaltic pumps; a feed tank and a draw tank; and an in-stream conductivity sensor (Fig. 2). To reduce external concentration polarisation on both sides of the membrane, plastic mesh spacers were employed. All the experiments were conducted in PRO mode, in which the active dense layer of the FO membrane faces the draw solution. The cross-flow rates of the draw and the feed solution were at 25.0 cm/s which were optimised according to our previous studies [7,16] and also

3.4.1. Surface hydrophilicity Water contact angle analysis was used to determine the hydrophilicity of membrane surface. It was measured by the captive bubble method using a Data-Physics OCA15 Contact Angle Analyser (Data-Physics Instruments GmbH, Filderstadt, Germany). The technique was similar to what explained by Grundkeet al. [27] that included three steps. First of all, the membranes were fixed horizontally on a substrate, and then carefully immersed in a chamber with water. After that, a microsyringe was lowered into the water where a drop was formed on the syringe tip, snapped from the tip and allowed to rise to the membrane–water interface. The contact

Fig. 2. Schematic diagram of the laboratory-scale FO system [7].

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3.4.3. Zeta potential measurements An Anton Paar SurPASS Electrokinetic Analyser (Anton Paar Austria) was employed to determine the Zeta potential values of uncoated and coated FO membranes. The adjustable gap cell apparatus was used, the streaming current was measured and the Helmholtz–Smoluchowski approximation was applied to evaluate the data. All experiments were performed from high to low pH, which ranged from 8.5 to 3 using 10 mM KCl solution as the background solution. The initial pH was adjusted around 8.5 using 0.1 M NaOH. After that, the tests were carried out by automatically dosing the solution with 0.1 M HCl until the pH was reduced to 3. The instrument was set to measure the data at 0.5 pH intervals. Four measurements were recorded and the average values with error bars in each pH value were reported. 4. Results and discussion 4.1. Surface characterization 4.1.1. Surface hydrophilicity It is well known that membrane surface hydrophilicity significantly affects the membrane fouling resistance property [23,28]. Thus, contact angle analysis was performed to examine the hydrophilicity of membrane surfaces. The studied surface has a porous structure which is rough and heterogeneous in the dry stage. According to Grundke et al. [27], in this case, conventional contact angle measurements do not provide reproducible contact angle values. Therefore, the more suitable captive bubble method was carried out for measuring more accurate contact angle data. Fig. 3 shows the contact angle results of uncoated and coated membranes, it was evident that the membrane wettability increased after the coating. As can be seen from this figure, the contact angle was decreased as the coating time was increased, from initial 48° for untreated sample down to 44° for 4 h and to 38° for 12 h coated membranes. This can be explained by the existence of both positively and negatively charged moieties of L-DOPA, which strongly interact with water via an ionic–dipole interaction. These two full charges within the L-DOPA molecule greatly increased its hydrophilicity. As the coating time was increased, the more interactions occurred between L-DOPA moieties with membrane surfaces, and more improvement of membrane surface hydrophilicity was achieved. 4.1.2. Surface functional groups The ATR-FTIR spectra of the unmodified and modified L-DOPA coating membranes are shown in Fig. 4. The functional groups of FO membrane surfaces and L-DOPA are similar, such as \C_O, \O\H or \C\O\C\. The characteristic broad band for the O\H group appears at approximately 3500 cm−1. The band appearing at approximately 1700 cm−1 was identified as corresponding to the C_O group while the peak at around 1200 cm−1 was assigned to the \C\O\C bond, and the band appearing at approximately 1000 cm−1was assigned to the \C\O\C bond in the pyranose ring [29]. It is noted that there was a decrease in

Contact angle (degree)

40 30

48

42

38

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4h coating

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20 10 0

Fig. 3. Contact angles of untreated, 4 h and 12 h L-DOPA coated membranes.

the intensities of the band of the O\H group of modified membranes, the longer the coating time, the more the decrease of the intensities, particularly the 12 h L-DOPA coated membrane showed the most decrease. This suggested that there were chemical interactions between \O\H-group on FO CTA membrane surfaces and catechol group of L-DOPA, which has a very active property. 4.1.3. Surface colour changes Fig. 5A shows the results of UV–Vis absorbance of the untreated membrane and L-DOPA coated membranes. As can be observed in the figure, the original FO membranes have almost zero absorption in the visible range, due to its white and smooth surface. After coating with L-DOPA, all the samples showed increased absorbance in the visible range. The longer the coating time was, the more absorbance increased, where the 12 h coating sample has higher adsorption than 4 h and 1 h coating membranes. It also can visually observe that the coated membranes become dark in colour (Fig. 5B). 4.1.4. Surface charge Zeta potential values of uncoated and coated FO membranes are illustrated in Fig. 6. The Zeta potential of uncoated FO membrane was slightly positive at a pH of three but more negative at a pH of seven. Meanwhile, the L-DOPA coated membranes were negatively charged. As can be observed from Fig. 6, the L-DOPA coated membranes showed higher negative Zeta potential than that of uncoated membranes. This is due to the fact that, although L-DOPA has the positively charged NH3+ groups and the negatively charged carboxylic acid (COO−) groups, the latter contributed more to the overall charges of the coated membranes surfaces. This

d c b Transmitter

3.4.2. UV–vis absorption and FTIR spectra Since the coated L-DOPA turned the membrane surface into a dark brown colour, UV–Vis adsorption analyses were carried out by using Varian Cary 100 UV–Vis spectrophotometer, equipped with diffuse reflectance accessories to examine the adsorption of L-DOPA on modified membranes. An ultraviolet to visible range of 200–800 nm was used as the scanning wavelength for the measurements. ATR-FTIR spectroscopy with a Spectrum100 spectrometer (Perkin Elmer) was also employed to characterise the membrane surface. The spectra were collected over the range of 4000–400 cm−1. The measurement was conducted five times for each sample to ensure the consistent result.

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a -OH 12h coating

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-C=O –C-O-C

4000 3885 3770 3655 3540 3425 3310 3195 3080 2965 2850 2735 2620 2505 2390 2275 2160 2045 1930 1815 1700 1585 1470 1355 1240 1125 1010 895

angles of pendant drop were finally calculated by OCA software. Each measurement was conducted at least five times and the average values were calculated.

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Fig. 4. ATR-FTIR spectra of (a) original membrane, (b) membrane with 1 h DOPA coating, (c) membrane with 4 h DOPA coating and (d) membrane with 12 h DOPA coating.

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A

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Fig. 7. Normalised flux of the original membrane and L-DOPA coated membranes.

Fig. 5. (A) UV–Vis absorbance of untreated membrane and L-DOPA coated membranes and (B) visible surface colour change of virgin and coated membranes.

result also confirmed the successful L-DOPA coating on membrane surfaces. 4.2. Effect of L-DOPA coating on the fouling behaviour Filtration experiments were conducted to investigate the fouling behaviour changes of coated FO membranes. The results are shown in Fig. 7. The draw solution concentration was kept constant at 2 M NaCl, while the feed is a mixture of AAS and CaCl2 that was reported to be able to accelerate the onset of fouling on the membrane. The normalised fluxes (J/Jo) were plotted to demonstrate the membrane fouling changes and flux recovery ratios. As can be observed from

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Fig. 7, after 180 min of FO filtration, the uncoated sample showed a 70% flux reduction. Whereas, L-DOPA coated samples experienced less flux reduction caused by fouling than the untreated samples, which only showed a 40% reduction for the 12-hour coating, a 50% reduction for the four-hour coating and about 60% for the one hour of coating. The sample coated with L-DOPA for 12 h achieved 30% less fouling compared to the uncoated sample. It indicates that with L-DOPA coating, higher fouling resistant membranes are achieved. This can be explained by two possible mechanisms; (1) reducing the adhesion force between membrane material and the foulants; (2) reducing the internal concentration polarisation. Since ICP cause the accumulation of solutes in the boundary layer adjacent to the membrane surface, it can promote their attachment to the membrane surface. Therefore, reduction in ICP for L-DOPA coated membranes can improve the fouling resistance of the membranes. An improvement of the flux recovery of the modified membranes was achieved. Fig. 8 shows that after 1 h hydraulic cleaning of membranes with deionised water, the unmodified membranes regained only 85% of the initial flux, while a slightly higher recovery of 90% was achieved for the modified membranes. This also indicated that the fouling layer on modified membranes was more loosely attached than that of the unmodified one. It should be noted that in this study fouling experiments were conducted with the porous layer of the FO membranes facing the feed (PRO mode) and it is known that the membrane recovery in the PRO mode is generally lower than the FO mode (with the supporting layer facing the feed).

-10

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pH Fig. 6. Zeta potential of uncoated and L-DOPA coated FO membranes.

Initial flux

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Water flux Jo (L/m2h)

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Recovery flux

19 18 17 16 15 Untreated

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Fig. 8. Initial and recovered flux cleaned by water of virgin and L-DOPA coated membranes.

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5. Conclusions The commercial CTA FO membranes were modified by direct coating of zwitterionic L-DOPA on the porous side of the membrane. The modified membrane surfaces became more hydrophilic with reduced water contact angle. During the accelerated fouling experiments, the L-DOPA coated samples experienced less flux reduction caused by on-set of fouling than the untreated membranes. For example, the sample with a 12-hour coating of L-DOPA achieved 30% less fouling than untreated membrane samples. The coated membranes achieved 90% recovery by hydraulic cleaning with water. In summary, L-DOPA modification showed effective improvement of fouling resistance of FO membranes. Since the direct coating of L-DOPA is simple and easy to operate without the requirement of high-energy input, it has the potential to be scaled up for industrial in situ application. Acknowledgements The authors appreciate the University of South Australia for providing research funding for this project. References [1] W.C.L. Lay, T.H. Chong, C.Y. Tang, A.G. Fane, J. Zhang, Y. Liu, Fouling propensity of forward osmosis: investigation of the slower flux decline phenomenon, Water Sci. Technol. 61 (2010). [2] B. Mi, M. Elimelech, Chemical and physical aspects of organic fouling of forward osmosis membranes, J. Membr. Sci. 320 (2008) 292–302. [3] S. Chen, L. Li, C. Zhao, J. Zheng, Surface hydration: Principles and applications toward low-fouling/nonfouling biomaterials, Polymer 51 (2010) 5283–5293. [4] R.L. McGinnis, M. Elimelech, Energy requirements of ammonia–carbon dioxide forward osmosis desalination, Desalination 207 (2007) 370–382. [5] B. Mi, M. Elimelech, Organic fouling of forward osmosis membranes: fouling reversibility and cleaning without chemical reagents, J. Membr. Sci. 348 (2010) 337–345. [6] A. Achilli, T.Y. Cath, E.A. Marchand, A.E. Childress, The forward osmosis membrane bioreactor: a low fouling alternative to MBR processes, Desalination 239 (2009) 10–21. [7] S. Zhao, L. Zou, Effects of working temperature on separation performance, membrane scaling and cleaning in forward osmosis desalination, Desalination 278 (2011) 157–164. [8] R.W. Holloway, A.E. Childress, K.E. Dennett, T.Y. Cath, Forward osmosis for concentration of anaerobic digester centrate, Water Res. 41 (2007) 4005–4014. [9] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717.

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