Polyethersulfone Nanofiltration Membrane Incorporated With Silicon Dioxide Prepared by Phase Inversion Method for Xylitol Purification

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1 Polyethersulfone Nanofiltration Membrane Incorporated With Silicon Dioxide Prepared by Phase Inversion Method for Xylitol Purification Polyethersulfone Nanofiltration Membrane Incorporated With Silicon Dioxide Prepared by Phase Inversion Method for Xylitol Purification Khalefa A. Faneer, Rosiah Rohani*, and A. Wahab Mohammad Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, UKM Bangi, Selangor, Malaysia Summary Xylitol purity is essential for a high value product obtained from biomass fermentation. Common biomass fermentation from sugar cane bagasse, corncobs or rice husk source in the presence of yeast produces xylitol mixture (containing xylose, arabinose), thus, various methods have been used for the purification such as crystallization and adsorption. However, membrane technique is of interest due to the operational simplicity and flexibility, relatively high selectivity and permeability for the transport of specific components. Nanofiltration (NF) membrane is targeted for the membrane application based on the ranges of molecular weight of the mixture components that fall in NF (between 200 to 2000 g/mol). In this paper, a new and efficient NF membrane used for purifying xylitol was synthesized from polyethersulfone (PES) and PES incorporated with silicon dioxide ( ) (NP) of 5 wt.% via phase inversion technique. These membranes have been characterized for their chemical, physical and morphological properties and their performances have been evaluated in the dead end filtration to obtain the pure water flux and filtration performance using xylitol mixture. EDX showed the presence of NP on the membrane for PES/ membrane but none in PES membrane. The membrane permeation properties improved also when has been incorporated to the PES membrane. The hydrophilicity of the PES/ membrane measured by contact angle improved from 79.7±0.65 o to 59.1±0.15 o for PES/ and PES membranes, respectively. The water flux has enhanced for PES/ membrane from to L/m 2 h measured at 4 bar. Therefore, in terms of flux and contact angle, the synthesized membrane of PES/ was found to be more effective compared to pure PES membrane. Keywords: PES; ; Xylitol; Nanofiltration; Membrane 1. Introduction Concentrated xylitol is important for its value in the pharmaceutical, medicine and food industries due to its low calories and ideal sweetener for diabetics since its metabolism does not require insulin 1. Xylitol can be produced in two ways using xylose: (1) via biotechnological process or (2) chemical hydrogenation 2. xylitol yield is in the range of 65-85% when produced via biotechnological process, and the yield is lower at 50 60% when it is produced chemically 3. Xylitol is either crystallized (>75%) or adsorbed (>65%) for purifying the component for a higher value *Corresponding author: rosiah@ukm.edu.my Smithers Information Ltd., 2016 product 4,5. Moreover, due to the fermentation broth complexity, the purity grade of xylitol obtained by direct crystallization is not acceptable (around 75%) 6. For these reasons, purification of the fermented broth prior to crystallization is required to produce high purity xylitol 4. Hence, for this current study, membrane separation technology is not proposed as an alternative technique, but as a unit in the system to raise up the xylitol purity. In terms of xylitol selling price, NF membrane technology is considered economically effective method compared to other techniques i.e. evaporation, the international price of raw xylose processing might bring down the price to four or five times when NF was used 7. Recently, membrane technology plays an important role in purification and separation technologies for various applications 8,9. This is because the technology is simple and has a flexible operation and it is also compatible with various applications 8. It has high selectivity and absorptivity for specific components to transported, requires low energy, stable under a spectrum of operating conditions and compatible with the environment. The interest in the research has recently developed a new and better asymmetric NF membrane that characteristics performed and improved better. This membrane has also improved in its thermal, radiation and resistant to the environment and stable 10. Purification of xylitol using membrane is based on its molecular weight cut off Polymers & Polymer Composites, Vol. 24, No. 9,

2 Khalefa A. Faneer, Rosiah Rohani, and A. Wahab Mohammad (MWCO), NF membrane has MWCO between g/mol 7, allowing the retention of soluble compounds in xylitol mixture with a molecular weight (MW) up to g/mol. Therefore, NF is suitable since the main compounds in the fermentation broth include monosaccharides, aliphatic acids, furan derivatives and phenolic compounds. The MW of inhibitors is between 40 and 120 g/mol, while that of monosaccharides is more than 150 g/mol and xylitol is g/mol in particular 7. The utilization of polymer based membrane for purifying xylitol is repeatedly reported 6,7. One of the promising polymer used is polyethersulfone (PES), which has exceptional thermal and mechanical stability in addition to good chemical resistance 11. Besides, PES is able to give an advanced degree of chain rigidity because of its regular and polar backbone 12 thus making membrane performance improved. The task to improve PES hydrophilicity has involved great effort such as mixing with hydrophilic polymer polymerized surface graft, coating and adding inorganic fillers 11,13. The hydrophobic membrane can be changed using including, TiO 2, Al 2 O 3 etc. to form hydrophilic membrane 14. Jiang et al. 15 reported that hydrophilic have improved the hydrophilicity of the membrane due to the recovery of higher water flux. The altered ultrafiltration PES membrane was found to be appropriate as pretreatment process during surface water NF 15. On response surface plots, the diverse percentages of the of silica affected the membrane s penetrability and salt rejection significantly. Silica NP was the main factor (i.e. doped solution composition) affecting the permeability of PSF membrane when up to 12 wt.% of NP were added to the membrane 16. Thus, this work aimed to synthesize and characterize phase inversion PES NF membrane for xylitol purification application, which has been incorporated with to enhance the membrane permeation properties as well as the membrane morphology. 2. Materials and Method 2.1 Materials Polyethersulfone (PES) granule, (Goodfellow) as the membrane based polymer. Silicon dioxide as (99.5%, 20 nm) (Nanoamor). The solvent used is N-methyl-2-pyrrolidone (NMP) with analytical purity 99.7% (Fluka, Germany) and distilled water was used as non-solvent. Xylitol, xylose and arabinose powder 99% (Acros Organic) have been used as received. 2.2 Membrane Preparation The phase inversion method was used to alter these two membranes as presented in Table 1. Initially, for the PES/ membrane synthesis, stirring of with 10% of NMP was carried out for 4 h and followed by sonication for 1 h in order to overcome aggregation and intensify its dispersal in the doped solution. Each polymer doped solution then stirred for 4 h to get a homogenous solution. Next, the doped solution was left overnight for degassing purposes. Elcometer 4340 Automatic Film Applicator with speed at 76 mm/s used to cast the membrane. A suitable amount of suspension cast using a casting knife set at 200 µm onto a glass plate at ambient atmosphere (27 o C) has been carried out. The membranes have been submerged in distilled water for 2 h before they were immersed in distilled water for solvent exchange. Table 1. Polymer doped solution composition and parameters PES (wt.%) NMP (wt.%) (wt.% of PES) The synthesized membranes were then stored in distilled water at room temperature prior to use. 2.3 Membrane Characterization Contact Angle The hydrophilicity of the membranes (with and without ) was measured by using contact angle. The water droplet and the membrane surface s contact angle have been measured after the membranes were dried for 48 h. Rame-Hart model 200 standard contact angle goniometer has been used with DROPimage Standard Software with an accuracy of o. The medium used to measure the contact angle was deionized water and air at ambient temperature (25 28 o C) Energy-Dispersive X-ray Spectroscopy The line scan of the spectrum of energy dispersion of X-ray (EDX) has been used to investigate the presence on the surface of the synthesized membrane. The equipment is Oxford Instruments, Model: x-max, UK and the scan rate was at 10 KV Membrane Surface Morphology Surface morphology of original and modified membranes was investigated by atomic force microscopy (AFM). Digital instrument (Scanning Probe Microscope, NTEGRA Prima, NT- MDT, and Russia) was used to obtain the membranes surface roughness by surface image analysis. The measurement details were semi-contact mode and NSG01 (golden silicon) probe. 2.4 Membrane Filtration Study Permeation of pure water flux and xylitol solution through the PES and Temperature ( o C) Stirring speed (rpm) Polymers & Polymer Composites, Vol. 24, No. 9, 2016

3 Polyethersulfone Nanofiltration Membrane Incorporated With Silicon Dioxide Prepared by Phase Inversion Method for Xylitol Purification PES/ membranes were carried out in a dead end filtration (Sterlitech HP4750, Sterlitech Corporation, USA). The acquired membrane sheets were cut into circle with the area of 14.6 cm 2. Primarily, all membranes were compacted at 12 bar until steady water flux achieved (approx. 30 min) using deionized water prior to filtration. Three different pressures (4, 6 and 8 bar) applied to calculate pure water flux (PWF) from Equation (1): (1) where J w is the water flux (L m -2 h -1 ), V is the permeate volume (L), A is the membrane area (m 2 ), t is the time (h). 2 g/l of xylitol mixture was used at 4 bar to measure filtration properties of both membranes. In order to minimize experimental errors, 3 to 5 samples have been collected and the average flux was reported. Table 2. The contact angle of PES and PES/ membranes Membrane type Additive (wt.%) Contact angle (theta) PES ± 0.65 o PES 59.1 ± 0.15 o was added to increase polyamide/ membrane hydrophilicity 18. From this result, the contact angle of PES and PES/ membranes was found less than 90 o, therefore, both membranes are classified as hydrophilic membranes; the PES membrane is categorized as hydrophilic while PES- membrane as semi hydrophilic EDX Analysis To prove the presence of, energydispersive X-ray spectroscopy (EDX) has been used to confirm the existence of. Figure 1(i) and (ii) showed the EDX spectra of PES/ and PES membranes, respectively. Obviously, the PES/ membrane has a new peak (Si) located at 1.7 kev while Figure 1. EDX analysis of the membranes synthesized from (i) PES/ and (ii) PES 3. Results and Discussion 3.1 Membrane Characterization Contact Angle Hydrophilicity of the material reveals the contact angle value of the membrane, whereas the hydrophobicity increases, the contact angle of the droplets with the surface increases. Surfaces with contact angles greater than 90 are labeled as hydrophobic 17. The arranged PES contact angles of the membrane and PES- membrane have been measured and the analysis results are showed in Table 2. It is evident that when was added to PES membrane, the contact angle improved from 79.7±0.65 o to 59.1±0.15 o. This is because after was added into the polymer-doped solution, nano- particles will spread among PES chain segments. The chain segment cannot spread during membrane formation, and consequently, pore size diminished. The same phenomena were reported where the nano- Polymers & Polymer Composites, Vol. 24, No. 9,

4 Khalefa A. Faneer, Rosiah Rohani, and A. Wahab Mohammad none in pure PES membrane. Initially 5% of was incorporated to the membrane, this high amount may cause aggregation, and NPs may not distributed uniformly at high load. Li et al. 19 reported the use of 3% NPs in the phase inversion membrane making and found that this amount has led to heterogeneous distribution of NPs and at the same time NPs become agglomerated, which also showed that the percentage of has significantly influenced the agglomeration phenomena. Meanwhile the atomic percentage of Si in the PES/ membrane was found to be 1.25%. This low percentage observed because of leaching out of the membrane matrix during fabrication i.e. in wet phase inversion process. Additionally, the lower percentage of may affect the flux and membrane morphology Filtration and Separation Experiments Pure Water Flux Figure 2 illustrated the pure water flux through the PES and PES/ membranes at different pressures (4, 6 and 8 bar). The addition of 5 wt.% of NPs to the PES significantly improved the water flux. For instance, at 4 bar the pure water flux of the PES/ membrane doubled from to L/m 2 h. With the increase in pressure from 4 to 6 to 8 bar, the flux also increased. Addition or increasing of NP amount will increase water flux 14. Ng et al. 16 also stated that the increment of silica NP led to an increase in the membrane permeability. Moreover, the higher weight percentage of the silica NP may have increased the membrane permeability 20. In this work, the filtration study of xylitol mixture was carried out at 4 bar for 1 h to evaluate the membrane separation. To reach that, a model solution of xylitol, xylose and arabinose has been used at 2 g/l concentration. The xylitol mixture flux of pure PES membrane was obtained at 3.10 L/ m 2.h, while for PES/ membrane, the value had increased to 4.26 L/m 2 h. The xylitol mixture fluxes are much lower than the pure water flux due to the presence of sugars in the solution compared to the sugar-free (pure) water that hinder the water to easily permeate through the membranes, as expected. Ananth et al. 21 reported that when the amount of NPs with additives Tetraethyl orthosilicate (TEOS) were increased from 7.5 to 10 wt.%, the solution separation performance have increased as well. The modification of membrane structure by adding NP leads to enhance the membrane flux and hydrophilicity. When the amount of Graphene Oxide (GO) in the membrane is 0.3 wt.%, the water flux reaches a maximum that is nearly twice of that of polysulfone membrane 22. Among various parameters, pore size, surface charge and degree of hydrophilicity are the most effective parameters that will affect the rejection percentage 23. Accordingly, the NP size plays an important role to raise up or drop down the flux or rejection, the bigger size of pores leads to higher rejection Membrane Surface Analysis Membrane surface analysis was carried out via testing the membrane morphological properties using AFM (method refer to Section 2.3.3). Figure 3 (i) to (iv) showed the comparison of the 2-D and 3-D AFM images of PES and PES/ membranes. It is perceivable from the images that the incorporation of has reduced the surface roughness. The membrane roughness can be obtained from the root mean square roughness (RMS). The membrane roughness is proportional to membrane pore size; changes in roughness can be due to the changes in pore size 24,25. RMS results showed the decreasing in membrane surface roughness from nm to nm after the NPs incorporation. Khulbe et al. 26 have obtained similar results where their membrane s roughness decreased from 6.4 to 2.3 nm. This decrease in roughness is a proof of decreasing in membrane pore size 26. Based on the AFM results, they seemed to support the filtration study where surface roughness decreased from to nm after adding which imply an improvement in membrane morphology, In addition, water flux has an indirect relationship with NP size while by using a small size of, the flux will be higher Conclusions In conclusion, PES membranes incorporated with/without of NPs have successfully been fabricated via phase inversion technique. The characterizations of the membranes with/without of were investigated. The key conclusions are listed below: Figure 2. Water flux relationship with pressure between PES/PES+ membranes 806 Polymers & Polymer Composites, Vol. 24, No. 9, 2016

5 Polyethersulfone Nanofiltration Membrane Incorporated With Silicon Dioxide Prepared by Phase Inversion Method for Xylitol Purification Figure 3. AFM images of (i) PES membrane in 2-D (ii) PES membrane in 3-D (iii) PES/ membrane in 2-D and (iv) PES/ membrane in 3-D have a notable effect on PES membrane properties where the contact angles increase forward to be hydrophilic. The additions of to PES membrane significantly influence the flux positively. The flux doubled when 5 wt.% of added. NP size has clear impact on the flux and rejection. The bigger size will increase the rejection. Acknowledgments The Authors are thankful to the LRGS for their financial support (LRGS/2013/ UKM-UKM/PT/03), GGM/074/2013 from UKM and to technical staff of the Department of Chemical and Process Engineering, UKM. References 1 Tochampa, W., Optimal fed-batch control of xylitol production. Kasetsart University Rafiqul, I. and A. Sakinah, International Food Research Journal. 19, (2012): Nigam, P. and D. Singh, Process Biochemistry. 30, (1995): Mussatto, S.I., et al., Journal of Chemical Technology and Biotechnology. 81, (2006): Misra, S., et al., Separation and Purification Technology. 78, (2011): Affleck, R.P., Recovery of xylitol from fermentation of model hemicellulose hydrolysates using membrane technology, 2000, Virginia Polytechnic Institute and State University. 7. Murthy, G., et al., Separation and purification technology. 44, (2005): da Silva, S.S. and A.K. Chandel, D-Xylitol. Springer SHIPENG, S., Fabrication of Nanofiltration Hollow Fiber Membranes for Sustainable Pharmaceutical Manufacture, Peinador Dávila, R.I., Zhao, C., et al., Progress in Materials Science. 58, (2013): Jalil, S., A. Ismail, and S. Hashim, Vatanpour, V., et al., Journal of Membrane Science. 375, (2011): Shen, J.-n., et al., Chemical Engineering Journal. 168, (2011): Polymers & Polymer Composites, Vol. 24, No. 9,

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