MURDOCH RESEARCH REPOSITORY

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1 MURDOCH RESEARCH REPOSITORY Yan, Y., Li, D., Simon, G. and Wang, H. (2012) Investigation on swelling and solar dewatering of composite polymer hydrogels as draw agents. In: Proceedings of CHEMECA 2012, September, Wellington, New Zealand. It is posted here for your personal use. No further distribution is permitted.

2 INVESTIGATION ON SWELLING AND SOLAR DEWATERING OF COMPOSITE POLYMER HYDROGELS AS DRAW AGENTS Yajing Yan, 1 Dan Li, 2,3* George P. Simon, 1 Huanting Wang, 2* 1 Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia 2 Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia; Huanting.Wang@monash.edu 3 Environmental Engineering, School of Environmental Science, Murdoch University, Murdoch, WA 6150, Australia; L.Li@murdoch.edu.au ABSTRACT We reported our investigation on the swelling and solar dewatering behaviours of carbonincorporated composite polymer hydrogels as draw agents in forward osmosis (FO) desalination. Particle sizes and loadings of carbon fillers inside composite polymer hydrogels affected their swelling ratios and dewatering rates. By incorporating smaller sizes and higher loadings of carbon fillers inside polymer matrix, the swelling ratios and dewatering rates of resulting composite hydrogels were enhanced. The swelling ratio of 5PNIPAM-2CS composite polymer hydrogel with 2 µm carbon fillers and a monomer/filler mass ratio of 5 : 2 was 21, which was 163% greater than that of plain PNIPAM polymer hydrogel. After exposing swollen 5PNIPAM-2CS composite hydrogel to 1 kw/m 2 simulated solar irradiation, the dewatering rate was 21% higher as compared with that of pure PNIPAM. INTRODUCTION Forward osmosis (FO) is the membrane separation process in which the osmotic pressure difference serves as the driving force and a semipermeable membrane acts as a separation medium. (Li et al., 2011b) In a typical FO separation, feed solution, i.e. saline water, passes across one side of a semipermeable membrane, and a draw agent of high osmotic pressure passes across the other side of the membrane. Due to the naturally driven osmotic flow, water permeates through the membrane from the feed solution to the draw agent. (Cath et al., 2006, Li et al., 2011a, Li et al., 2011b) After FO process, it is necessary to separate the draw agent from water product. So far, FO separation has shown a number of advantages. Especially, as compared with reverse osmosis membrane process, FO separation requires much lower or no pressure, which shows great potential to reduce energy in desalination. (Cath et al., 2006) In FO separation, the draw agent must exhibit a high osmotic pressure and the capacity to be effectively separated from the water product after FO process. The use of composite polymer hydrogels by adding carbon fillers was reported as draw agents for the first time in (Li et al., 2011a, Li et al., 2011b) The black carbon particles, which are fabricated from the lowtemperature hydrothermal carbonization reaction, exhibit good hydrophilicity and polar surfaces. (Li et al., 2011b, Sevilla and Fuertes, 2009, Sun and Li, 2004, Li et al., 2011c, Titirici et al., 2008) The incorporation of those carbon particles into polymer hydrogels has been proven to enhance the swelling ratios (pressures) of resulting composite hydrogels, thus higher fluxes in FO process. Furthermore, when the simulated solar irradiation is used as a stimulus, (Li et al., 2011b) the black carbon fillers dispersing inside the composite hydrogels can effectively absorb solar energy and heat up the polymeric matrix, leading to the separation of hydrogel draw agent and water product. (Zeng et al., 2011, Li et al., 2011b) 1

3 However, there is still a need to further improve swelling ratios (pressures) of polymer hydrogels and their dewatering rates. In this paper, we investigated the effect of sizes and loadings of carbon fillers on the swelling ratios and dewatering rates of carbon-incorporated composite polymer hydrogels. Our study showed that the swelling ratios and water dewatering rates of resulting composite hydrogels could be improved by incorporating smaller sizes and greater loadings of carbon particles into polymer matrix. EXPERIMENTAL Hydrophilic carbon fillers were synthesized by low-temperature hydrothermal carbonization reaction. Typically, 12 g of sucrose was completely dissolved in 30 g of deionized (DI) water, which was sealed into a Teflon stainless autoclave and hydrothermally reacted in a pre-heated oven at 180 ⁰C for 18 h. After the reaction, the autoclave was cooled down to room temperature. The fabricated black powders were then separated by centrifugation, repeatedly washed by DI water and dried in an oven at 80 ⁰C. The obtained carbon powders were detonated as C-L. Similarly, carbon particles, which were denoted as C-S, were prepared by heating a solution, in which 3 g of sucrose was dissolved in 30 g of DI water, at 180 ⁰C for 18 h. Polymer hydrogels were fabricated by free-radical polymerization. Monomer N- isopropylacrylamide (NIPAM, Sigma-Aldrich), crosslinker N,N -methelenebisacrylamide (MBA, Sigma-Aldrich) and initiator ammonium persulfate (APS, Sigma-Aldrich) were completely dissolved in water in a PP bottle. The mass ratio of NIPAM, MBA and APS is 36.7 : 1.0 : 1.5. The enclosed PP bottle was heated at 70 ⁰C for polymerization. The obtained polymer hydrogels were washed by water and dried in an oven, which were then fractioned into powders. The resulting polymer powders were denoted as PNIPAM. The carbonincorporated composite polymer hydrogels were prepared using the similar procedures as described above. The monomer, crosslinker and initiator were dissolved in water firstly, in which the dried carbon particles, C-L or C-S, were dispersed under ultrasonication. The resulting solution with carbon particles was heated at 70 ⁰C for polymerization. The fabricated carbon-polymer composite hydrogels with 20 : 1 and 5 : 2 mass ratio of NIPAM and C-L carbon fillers were denoted as 20PNIPAM-CL and 5PNIPAM-2CL, respectively. Similarly, the produced carbon-polymer composite hydrogels with 20 : 1 and 5 : 2 mass ratio of NIPAM and C-S carbon fillers were denoted as 20PNIPAM-CS and 5PNIPAM-2CS, respectively. After sputter-coating the samples with gold, scanning electron microscope (SEM) images at magnifications ranging from 3,000 to 13,000 were taken by desktop scanning electron microscope (Phenom) and JSM-7001F microscope (JEOL) at 5 kv. A swelling test was conducted by placing 0.5 g of pure or composite PNIPAM polymer hydrogel powders into a dialysis bag which was then immersed into DI water for 3 days. The swelling ratio (Q) of polymer hydrogel was expressed as, where M s is the mass of swollen hydrogel (g) and M d is the mass of dried hydrogel (g). 2

4 After the swelling test, the swollen hydrogel was taken out and placed under a solar simulator (CHF-XM500, Beijing Trusttech Technology) at a light intensity of 1 kw/m 2 for 1 hour. The temperature of polymer hydrogel was measured by using a thermocouple (38XR-A, AMPROBE) every 10 mins. Dewatering rate (D) was calculated by where W l is the weight of water loss during the solar dewatering test (g), which is the difference of the weight of swollen hydrogel before and after the exposure to the sunlight over a given period of time (e.g. 10 min, 20 min, 30 min, 40 min, 50 min and 60 min); W o is the weight of the water contained in the swollen gel before the dewatering test (g). RESULTS AND DISCUSSION Fig. 1 shows the SEM images of carbon fillers C-S (a) and C-L (b) prepared by lowtemperature hydrothermal carbonization reaction. Both of the carbon particles have smooth surfaces. C-S carbon particles exhibit average size 2 µm; and the average particle size of C-L is around 11 µm. Fig. 1: SEM images of carbon fillers C-S (a) and C-L (b) (scale bar = 10 µm). Fig. 2: Optical microscope images of PNIPAM (a), 20PNIPAM-CL (b), and 5PNIPAM-2CL (c) (scale bar = 50 µm); the insets are digital photos of PNIPAM, 20PNIPAM-CL, and 5PNIPAM-2CL, respectively. 3

5 Fig. 2 shows the digital photos and optical microscope images of plain polymer hydrogel powders (PNIPAM) and composite polymer hydrogel powders with the incorporation of carbon fillers C-L (20PNIPAM-CL and 5PNIPAM-2CL). In the digital photos, plain PNIPAM polymer hydrogel powders show white polymer colour. With the increasing loadings of carbon fillers inside the polymeric matrix, the colour of resulting composite polymer hydrogels changes from brown (20PNIPAM-CL) to black (5PNIPAM-2CL). In the optical microscope images, PNIPAM (Fig. 2a) exhibits clear and transparent morphology. Black carbon fillers are observed dispersing throughout the clear polymer matrix in the optical microscope images of composite polymer hydrogels, 20PNIPAM-CL (Fig. 2b) and 5PNIPAM-2CL (Fig. 2c). Fig. 3: SEM images of PNIPAM (a), 20PNIPAM-CS (b), 5PNIPAM-2CS (c), 20PNIPAM- CL (d) and 5PNIPAM-2CL (e) (scale bar = 1 µm). Fig. 3 exhibits the SEM images of PNIPAM (a), 20PNIPAM-CS (b), 5PNIPAM-2CS (c), 20PNIPAM-CL (d) and 5PNIPAM-2CL (e) at high resolution. Plain PNIPAM particles (Fig. 3a) show rough surfaces. Macroporous structures are observed in the composite polymer hydrogels, e.g. 20PNIPAM-CS (Fig. 3b) and 5PNIPAM-CS (Fig. 3c), 20PNIPAM-CL (Fig. 3d) and 5PNIPAM-CL (Fig. 3e). These findings are consistent with the results in our previous studies. (Li et al., 2011a, Li et al., 2011b) The presence of macrospores may be related to the properties of carbon fillers and polymers. It has been proven that the carbon particles synthesized by low-temperature hydrothermal carbonization of sucrose exhibit very polar surface structures with carbonyl and hydroxyl functions, aliphatic double bonds. (Li et al., 2011b, Sevilla and Fuertes, 2009, Sun and Li, 2004, Li et al., 2011c, Titirici et al., 2008) The polymerization and drying temperatures in the fabrication of carbon-incorporated composite polymer hydrogels are above the lower critical solution temperature (LCST) of PNIPAM, which is around 32 ⁰C. The presence of hydrophobic isopropyl groups in the backbones of PNIPAM may make the polymer hydrogels difficult in retaining a good contact 4

6 with the carbon fillers with hydrophilic surfaces. This results in the formation of pores around the carbon fillers after polymerization. Tab. 1: Swelling ratios of pure and composite polymer hydrogels, PNIPAM, 20PNIPAM-CL, 5PNIPAM-2CL, 20PNIPAM-CS and 5PNIPAM-2CS. Polymer PNIPAM 20PNIPAM- 5PNIPAM- 20PNIPAM- 5PNIPAMhydrogel CL 2CL CS 2CS Swelling ratio Tab. 1 shows the swelling ratios of pure and composite polymer hydrogels, PNIPAM, 20PNIPAM-CL, 5PNIPAM-2CL, 20PNIPAM-CS and 5PNIPAM-2CS, respectively. The swelling ratios of 20PNIPAM-CL and 5PNIPAM-2CL are 9 and 18, as compared with 8 for the swelling ratio of pure PNIPAM. Therefore, the incorporation of carbon fillers in polymer matrix leads to the increase of swelling ratios of resulting hydrogels. Furthermore, with a greater amount of carbon fillers incorporated in polymer matrix, the resulting composite hydrogels show enhanced swelling ratios. Particle sizes are another parameter to affect the swelling ratios of composite hydrogels, especially for those with the high loading of carbon fillers, e.g. 5PNIPAM-2CS and 5PNIPAM-2CL with a monomer/filler mass ratio of 5 : 2. The swelling ratio of 5PNIPAM-2CS by incorporating 2-µm C-S carbon fillers is 21, which is 17% higher than that of 5PNIPAM-2CL with the addition of 11-µm C-L carbon fillers. However, when the mass ratio of monomer and carbon is 20 : 1, there is almost no change of swelling ratios observed between 20PNIPAM-CL and 20PNIPAM-CS. These enhancements may be attributed to the addition of carbon fillers with polar functional groups and the porosity of hydrogel structures. (Li et al., 2011b) The high polar hydrophilic surfaces arising from carbon fillers enhance the water uptake and swelling behaviour of hydrogels. (Gu and Ye, 2009) Furthermore, the creation of macropores in PNIPAM composite hydrogels increases the swelling rates. (Kabiri and Zohuriaan-Mehr, 2004) Fig. 4 and 5 show the changes of temperatures inside the plain and composite polymer hydrogels after the exposure to 1 kw/m 2 simulated solar irradiation at different periods; and their corresponding dewatering rates. With the increasing exposure of swollen pure or composite polymer hydrogels, which are collected after the swelling test, the temperatures of resulting hydrogels increase, resulting in greater dewatering rates. For instance, the initial temperature of PNIPAM polymer hydrogels is 17 ⁰C before the dewatering test. After the exposure to solar irradiation for 10 min, 30 min, and 60 min, the temperature of PNIPAM changes to 24 ⁰C, 29 ⁰C, and 34 ⁰C, respectively. Not surprisingly, the corresponding dewatering rates of swollen PNIPAM are enhanced, which are 15%, 28% and 43%, respectively. With the incorporation of carbon fillers, the temperatures of composite polymer hydrogels are higher than those inside the plain PNIPAM hydrogel (Fig. 4), due to the good light absorbing capacity of black carbon. (Zeng et al., 2011) It results in the increase of dewatering rates of composite polymer hydrogels, when compared with those of swollen pure PNIPAM (Fig. 5). Furthermore, the composite polymer hydrogels with greater loadings of carbon particles show higher temperatures during their exposure to solar irradiation. As a result, the dewatering rate of 5PNIPAM-2CL is 32% after 30-min exposure to 1 kw/m 2 solar irradiation, which is 3% and 2% higher than that of PNIPAM and 20PNIPAM-CL. Interestingly, after exposing the composite hydrogels to the solar irradiation (1 kw/m 2 ) for 30 min, the temperature inside 5PNIPAM-2CS by incorporating 2-µm C-S carbon fillers (40 ⁰C) is slightly greater than that of 5PNIPAM-2CL with the addition of 11-µm C-L carbon fillers 5

7 (38 ⁰C). This finding suggests that the incorporation of smaller carbon particles may enhance the light absorbing capacity of resulting composite hydrogels, thus dewatering rates. Fig. 4: Temperatures of pure and composite polymer hydrogels (PNIPAM, 20PNIPAM-CL, 5PNIPAM-2CL, 20PNIPAM-CS and 5PNIPAM-2CS) after the exposure to 1 kw/m 2 simulated solar irradiation. Fig. 5: Dewatering rates of pure and composite polymer hydrogels (PNIPAM, 20PNIPAM- CL, 5PNIPAM-2CL, 20PNIPAM-CS and 5PNIPAM-2CS) after the exposure to 1 kw/m 2 simulated solar irradiation. Herein, we have proven the feasibility by varying particle sizes and loadings of carbon fillers to produce the composite polymer hydrogels with different swelling ratios and dewatering rates. Based on our previous studies, (Li et al., 2011a, Li et al., 2011b) the use of polymer hydrogels with high swelling ratios and dewatering rates as draw agents are expected to generate high fluxes and water recovery rates in FO desalination. Therefore, the current subject of our ongoing study is to further optimize the structures and properties of carbonincorporated composite hydrogels and fabricate highly efficient draw agents in FO desalination. CONCLUSIONS In conclusion, we have demonstrated that particle sizes and loadings of carbon fillers in the composite polymer hydrogels affected the swelling ratios of resulting composite hydrogels and their dewatering rates. By incorporating smaller sizes and greater loadings of carbon fillers inside polymer matrix, the swelling ratios and dewatering rates of composite hydrogels were enhanced due to the polar surfaces and good light absorbing property arising from the 6

8 black carbon fillers which were fabricated from low-temperature hydrothermal carbonization. In particular, 5PNIPAM-2CS composite polymer hydrogel exhibited swelling ratio 21 and dewatering rate 52% after 1-h exposure to 1 kw/m 2 simulated solar irradiation, which were 163% and 21% greater than the swelling ratio and dewatering rate of plain PNIPAM. Therefore, in relative to plain polymer hydrogel, the composite polymer hydrogels with the incorporation of carbon fillers are preferred by considering both the FO and dewatering processes in FO desalination. Our study suggests that the carbon-incorporated composite polymer hydrogels show great potential as draw agents in an efficient FO desalination application. ACKNOWLEDGEMENTS This work was supported by the Australian Research Council (DP ). REFERENCES CATH, T. Y., CHILDRESS, A. E. & ELIMELECH, M Forward osmosis: Principles, applications, and recent developments. J. Membr. Sci., 281, GU, Y. & YE, L Study on the polyvinylalcohol/montmorillonite composite hydrogel. Polym.-Plast. Technol., 48, KABIRI, K. & ZOHURIAAN-MEHR, M. J Porous superabsorbent hydrogel composites: Synthesis, morphology and swelling rate. Macromol. Mater. Eng., 289, LI, D., ZHANG, X., YAO, J., SIMON, G. P. & WANG, H. 2011a. Stimuli-responsive polymer hydrogels as a new class of draw agent for forward osmosis desalination. Chem. Commun., 47, LI, D., ZHANG, X., YAO, J., ZENG, Y., SIMON, G. P. & WANG, H. 2011b. Composite polymer hydrogels as draw agents in forward osmosis and solar dewatering. Soft Matter, 7, LI, M., LI, W. & LIU, S. 2011c. Hydrothermal synthesis, characterization, and KOH activation of carbon spheres from glucose. Carbohydr. Res., 346, SEVILLA, M. & FUERTES, A. B Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides. Chem. Eur. J., 15, SUN, X. & LI, Y Colloidal carbon spheres and their core/shell structures with noblemetal nanoparticles. Angew. Chem., 116, TITIRICI, M.-M., ANTONIETTI, M. & BACCILE, N Hydrothermal carbon from biomass: a comparison of the local structure from poly- to monosaccharides and pentoses/hexoses. Green Chem., 10, ZENG, Y., YAO, J., HORRI, B. A., WANG, K., WU, Y., LI, D. & WANG, H Solar evaporation enhancement using floating light-absorbing magnetic particles. Energy Environ. Sci., 4, BIOGRAPHY OF THE PRESENTER Dr Dan Li is a lecturer in Environmental Engineering at Murdoch University. Her current research interests include the development of advanced materials and membranes for gas separation, wastewater treatment, seawater desalination, and other applications. 7