Biodegradable and biocompatible temperature sensitive triblock copolymer hydrogels as draw agents for forward osmosis

Accepted Manuscript Biodegradable and Biocompatible Temperature sensitive Triblock Copolymer Hydrogels as Draw Agents for Forward Osmosis Ramesh Nakka, Alka A. Mungray PII: DOI: Reference:

S1383-5866(16)30496-8 http://dx.doi.org/10.1016/j.seppur.2016.05.021 SEPPUR 13016

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

9 December 2015 29 April 2016 25 May 2016

Please cite this article as: R. Nakka, A.A. Mungray, Biodegradable and Biocompatible Temperature sensitive Triblock Copolymer Hydrogels as Draw Agents for Forward Osmosis, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur.2016.05.021

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Biodegradable and Biocompatible Temperature sensitive Triblock Copolymer Hydrogels as Draw Agents for Forward Osmosis Ramesh Nakka, Alka A. Mungray* *Department of Chemical Engineering, Sardar Vallabhbhai National Institute of Technology (SVNIT), Surat-395007, Gujarat, India.

Abstract Recently, polymer hydrogels have been studying as a draw agent in forward osmosis (FO) due to their low toxicity, reusability and dewatering abilities. In this research work, we studied five triblock copolymer hydrogels as draw agents in FO process. Hydrogels were synthesized by ring opening polymerization of monomers D,L-lactide, Glycolide, methyl ether poly ethylene and stannous octane as catalyst. Prepared hydrogel named as “poly(ethylene glycol-[DL-lactic acid-co-glycolic acid]-b-ethylene glycol) (PEG-PLGA-PEG).” Different weight percentages i.e., 0.09 and 0.18 wt% of Graphene oxide (GO) and Graphene (Gr) were incorporated in hydrogels and named as “PEG-PLGA-PEG/ GO-0.09wt%, PEG-PLGAPEG/GO-0.18wt%, PEG-PLGA-PEG/G -0.09wt% and PEG-PLGA-PEG/G-0.18wt%. Different types of characterization technique were used to characterize the prepared hydrogels i.e., HNMR spectrophotometers, Fourier transform infrared spectrometer (FTIR), Scanning electron microscopy (SEM), Molecular weights (MWs) measured Gel Permeation Chromatography (GPC), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). All the five prepared hydrogels were tested as draw agents in FO process to check the effect of GO and G on swelling properties, water fluxes, and water recovery. After FO experiments results advocate, GO incorporation in polymer hydrogels increase the swelling ratios and thus yields higher water fluxes among all hydrogels. By using PEG-PLGA-PEG/GO-0.09wt% and PEG-PLGA-PEG/GO0.18wt% hydrogels as draw agents, deionized (DI) water and 2000 ppm NaCl as feed solutions, the average water fluxes obtained are 0.68 LMH, 0.57 LMH and 0.55 LMH, 0.48 LMH, respectively. All prepared hydrogels remained active up to two times regeneration and these hydrogels were shown negligible reverse solute diffusive flux through the FO membrane. Key words: Triblock copolymers; Forward Osmosis; Desalination; Biodegradable; Draw agent *Corresponding author. Tel.: 91 261 2201716, 91 261 2201901; fax: +91 261 2201605. E-mail addresses- [email protected] (Alka A. Mungray); [email protected] 1

1. Introduction One of the most significant challenges of this century is fulfilling the increasing water demand for drinking water supplies, food production and other industrial needs to support the enormous population growth [1,2]. To meet fresh water demand there is a need to find out the best process with energy efficient that should give water with drinking standards. Lots of research is going on membrane based separation processes. For this purpose reverse osmosis (RO) is currently the most commonly used desalination technologies because of its many merits over other conventional thermal desalination technologies [3]. RO requires large amounts of electricity and extremely high hydraulic pressures to force water across the membrane while rejecting salt and other contaminants. Drinking water production and industrial necessaries water from waste or contaminated water is a major challenge because it will helpful to reduce volume of waste water. Forward osmosis (FO) can do better than RO in terms of energy consumption and fouling [4]. Forward osmosis (FO) being a membrane based separation process; it deploys osmotic pressure difference to be the driving force for water permeation. In which semipermeable membrane acts as a separation medium [5]. A typical FO separation involves the feed solution, i.e. saline water, passing through a semipermeable membrane in another section containing draw agent (having high osmotic pressure in comparison with saline water/feed water) which flows on the other side of the membrane. Water gets transported from the feed side to the draw agent side through the membrane because of the osmotic pressure difference [5, 6]. However, the separation of water from draw agent to retrieve pure water and regenerate draw agent is also important in FO. Hence there is a niche of developing a good drawing agent [4]. One of the key challenges in FO process is the limited choice of efficient and economic draw agent. While selecting the draw agent, it is desired to meet basic criteria i.e., high water permeation rate, it should generate high osmotic pressure, easy water recovery, reusable, non toxic, compatible with membrane surface, and cost effective [6,7]. Development of a novel draw agent has attracted attention of science community so far with incorporation of inorganic and organic salts [7-12]. Ammonia-carbon dioxide among them appears to be promising [13,14]. Few studies have been conducted with novice compounds such as hexavalent phosphazene salts [15], strong polyelectrolytes [16] and 2-methylimidazole-based compounds [17] to counteract 2

with the problems such as low regeneration energy efficiency and counter diffusion. Inspite of elevated osmotic pressure, their separation/regeneration/recycle, viscosity, or dissociation etc. are the pitfalls [15-17]. Cross-linked by polymer chains with three-dimensional network structures is the characteristic of polymer hydrogels. Presence of a large number of hydrophilic groups is the reason of why polymer hydrogels absorb a lot of water [5]. The presence and dissociation of ionic species within the polymer hydrogel are responsible to swell and develop a higher internal osmotic pressure. Water absorbing properties of the networks results from the presence of functional groups such as Carboxylic (-COOH), Hydroxylic (-OH), Amidic (-CONH) and Sulphonic (-SO3) [2]. Recently, researchers are focusing on polymer hydrogels as a draw agent in FO process. Smart polymer hydrogels exhibit sensitivity towards various parameters like temperature, pH, magnetic field, electric field, ionic strength, light, etc. [18-22]. A novel class of draw media, thermosensitive polymer hydrogels is sensitive to heat [23]. Polymer hydrogels have been claimed to provide a sufficient driving force to draw water from high salinity seawater across membrane by virtue of high osmotic pressure (2.7 MPa at 27°C). Swollen hydrogel was dewatered at 50°C and no by-product was produced, making it a great advantage during regeneration of drinking water. Polymer hydrogel series hence gained a stature of effective draw agent ever since in FO processes [5,23-27]. Synthetic polymer hydrogels are having following disadvantages like non-biodegradability, high energy consumption for water recovery [28], toxicity and non-biocompatibility. Hence, bio polymer based hydrogel will be highly required to overcome the above disadvantages. Temperature sensitive triblock copolymer hydrogels are good in biodegradability and temperature responsive properties. A new species called “triblock copolymers”, consisting of A block covalently bonded (by ester link) with B-Block and vice versa form an ABA and BAB type hydrogel. Copolymer exhibits variable viscosity, it flows freely at low temperature and may form a gel at 35-400C. Poly([DL-lactic acid-co-glycolic acid]-b-ethylene glycol- [DL-lactic acidco-glycolic acid]) PLGA-PEG-PLGA) or poly(ethylene glycol-[DL-lactic acid-co-glycolic acid]b-ethylene glycol) (PEG-PLGA-PEG), are a kind of block copolymers composed of hydrophobic poly(DL-lactic acid-co-glycolic acid) (PLGA) segments and hydrophilic poly(ethylene glycol) 3

(PEG) segments. Copolymer molecules remain in solution because of hydrophilic PEG which is associated with hydrophobic PLGA segments by crosslinks. PEG and water molecules form hydrogen bonds association at lower temperate and hence favor aqueous solution. At higher temperature, hydrophobic forces of PLGA dominate the hydrogen bonds cause solution-gel transition. Unlike to the homopolymers of lactic acid (polylactide) and glycolic acid (polyglycolide), which show poor solubility. Thus, this makes it a suitable candidate to be used in the synthesis of various thermo responsive hydrogels [29, 30]. The high molecular weight along with high PLGA contents makes the block polymer insoluble in water, although they tend to swell in aqueous media. Block copolymers with hydrophilic and hydrophobic blocks are capable of forming physical crosslink in water by means of hydrophobic interaction, crystalline micro domains or chain entanglement [31]. Hydrophobic domains, because of physical interaction, retain micro domains swollen and together maintaining the integrity of polymer network in aqueous media. The elastic and viscoelastic properties make these copolymers soft for easy casting and thermally curable in spite of weak and reversible physical associations. Graphene oxide is a two-dimensional carbon material with a large number of hydrophilic oxygenated functional groups i.e, carboxyl groups, epoxy, hydroxyl and carbonyl etc. These groups enhance miscibility with polymer matrix and make more hydrophilic composites. Moreover, the thermal stability and mechanical strength of the hydrogels may be improved due to the outstanding thermal resistance and mechanical properties of graphene [32]. In literature, many papers are available on “PEG-PLGA-PEG hydrogel. But all are used only in the pharmaceutical field as drug targeting systems. This combination is never used in forward osmosis (FO) as a draw agent. Therefore, an attempt was made to evaluate the performance of these (biodegradable and biocompatible) graphene and graphene oxide incorporated PEG-PLGA-PEG hydrogels in FO instead of synthetic polymer based hydrogel. We prepared five different hydrogels with incorporation 0.09 and 0.18 wt% of Graphene (G) and Graphene oxide (GO) i.e., (PEG-PLGA-PEG), PEG-PLGA-PEG/GO-0.09wt%, PEG-PLGAPEG/GO-0.18wt%,

PEG-PLGA-PEG/G-0.09wt%,

PEG-PLGA-PEG/G-0.18wt%.

These

hydrogels were characterized by different techniques like, H-NMR, FTIR, SEM, GPC, XRD, swelling ratios, water fluxes, water recovery and regeneration. 4

2. Materials and Methods 2.1 Materials Monomer D,L-lactide (3,6-Dimethyl-1,4-dioxane-2,5-dione) was purchased from SigmaAldrich, USA. Glycolide (≥99%) (1,4-Dioxane-2,5-dione) was purchased from Sigma (Life science), Netherlands. Methylether polyethylene glycol (mPEG, Mn 550g/mol) and Dibutyltin Diacetate were purchased from Aldrich, USA. Polymerization catalyst used as stannous 2-ethylhexanoate (92.5-100%) was purchased from Sigma (Life science), Japan. Coupling agent as hexamethylene diisocyannate (HMDI) (≥99%) was purchased from Sigma (Life science), Germany.

Solvents used in this study; diethyl ether, hexane, and anhydrous toluene were

purchased from RANKEM, India. Graphene Oxide (GO) and Graphene were purchased from Adnano Technologies, India. Size of GO was between 0.8-1.6 nm and surface area was 400 m2/g. Graphene size was between 5-10 nm and surface area was 310 m2/g. For FO tests, sodium chloride (NaCl) purchased from FINAR chemicals Ltd, India. Commercial FO membrane purchased by Hydration Technologies Innovations (HTI) (Albany, OR) was used to test the performance of prepared hydrogel in forward osmosis process. This FO membrane was made of cellulose triacetate with an embedded polyester screen mesh. Deionized (DI) water produced by milliQ, Millipore-India was used for the experiments.

2.2 Synthesis of Triblock Copolymers The synthesis of pure PEG-PLGA-PEG triblock copolymers was performed through a ring-opening copolymerization. A dried three-neck round-bottom flask was used, which equipped with a condenser. Poly(ethylene glycol)(2g, M w 550 g/mole), D,L-lactide(5g), and glycolide (1.66g) were polymerized and catalyzed by 0.03g of stannous 2-ethyl hexanoate, followed by elevation reaction temperature at 150°C up to 8 hr. Diethyl ether was used to wash (thrice) PEG-PLGA products followed by vacuum drying at room temperature. PEG-PLGA copolymer triblock synthesized by dissolving PEG-PLGA copolymer having terminal hydroxyl groups in toluene (20 mL), followed by introduction of HMDI (170 µL) and dibutyltin diacetate (0.7 µL) as a coupling solution. Reaction of hydroxyl groups on PLGA end were and HMDI at 60°C for 8 h produces triblock copolymer PEG-PLGA-HMDI-PLGA-PEG. This triblock 5

copolymer termed henceforth as “PEG-PLGA-PEG”. For preparation, it was washed using diethyl ether/n-hexane (v/v 1/1) followed by vacuum drying for 48 h to result a final product [33,34]. The Graphene and GO incorporated polymer hydrogels were prepared as similar procedures described above. 0.09 and 0.18 wt% of both of GO and Graphene were added to monomers respectively. After the addition of monomers, catalyst, and GO/Graphene particles was vigorously stirred. The reaction temperature was elevated and kept at 150°C for 8 h. Similar procedure was followed as like pure PEG-PLGA-PEG triblock copolymer. Photographs of PEGPLGA-PEG, PEG-PLGA-PEG/GO-0.18wt%, and PEG-PLGA-PEG/G-0.18wt% are shown in Fig.1.

2.3 Characterization of hydrogels H-NMR spectrophotometer is useful to determine the chemical structure and composition of triblock copolymers. Spectra were recorded at ambient temperature with a CDCl3 as the solvent, and chemical shifts (δ) were given in ppm using tetra methyl silane as an internal standard. Fourier transform infrared spectrometer (FTIR, Perkin Elmer, USA) was used for the structural confirmation of prepared pure polymer and composite triblock copolymer hydrogels. The spectrum range was 4000-400 cm-1. Scanning electron microscopy (Hitachi series-3400 N (SEM, EDAX XL-30)) was used to examine the morphology of normal hydrogel (before swelling) characterized by scanning electron microscopy. Triblock copolymers molecular weights (MWs) were measured by Gel Permeation Chromatography (GPC) (GPC, Perkin Elmer, turbo matrix 400 model), equipped with a refractive index detector. Tetrahydrofuran was used as the mobile phase at a flow rate of 1.0 mL min−1 at 35 °C. A 1.0% (w/v) polymer solution (20 μL) was injected for each measurement. MWs were calibrated by monodisperse polystyrene standards.

6

To evaluate the dispersion of the GO and G in hydrogels, X-ray diffraction (XRD) was performed using XRD instrument (Rigaku Model, Miniflex serial No. BD111915, Japan). The diffraction patterns of a scan range between 10° to 80° were recorded with a scanning rate of 3° min−1 at room temperature. Thermal analysis was employed to estimate the thermal stability of the prepared hydrogels with thermogravimetric analysis (TGA) (Perkin Elmer pyris1 TGA) at a heating rate of 10 °C/min under nitrogen atmosphere in temperature ranging from 45 to 700 °C. All the specimens were first dried in vacuum to remove moisture before the TGA characterization. 2.3.1 Equilibrium swelling ratio measurements Equilibrium swelling ratios (ESR) was used to determine by using weighed method for the prepared hydrogels. Pre-weighed dry hydrogel pieces were kept in a specialized mesh bag made of nylon, followed by immersion in DI water (at 20°C) for 72 h till no increment in swelling was observed in hydrogels [28]. The following equation (1) was used to calculate the ESR; ESR =

……………………………………………….. (1)

Where Wd (g) = weight of the initial dry hydrogel; and Ws (g) = weight of the hydrogel after immersion. 2.3.2 Swelling ratio measurement

To study the swelling ratio, 1 g of the prepared semi solid hydrogel were placed into identical nylon mesh bags immersed into 100 ml of DI water at 20°C [35]. The alteration in weight hydrogels was recorded at discrete times. The following equation (2) was used to calculate the swelling ratio (Q) of hydrogels at a certain time [5]. Q

……………………………………….………… (2)

7

Where Wd (g) = weight of the initial dry hydrogel and Wt (g) = weight of the of hydrogel at time t (h). 2.3.3 Deswelling and reswelling measurements In the dewatering process, all prepared hydrogels (1g) were allowed to swell in 100 ml distilled water and left to achieve equilibrium swelling. These swollen hydrogels were then dried in natural solar radiation and also in convection oven at 60°C for 2 min and their weight was measured. The water recovery rate (WR) was calculated by below equation;

WR (t) % =

……………………..…………. (3)

Where Wt (g) = weight of the hydrogel at given instant; Wd (g) = initial weight of the dry hydrogel; We (g) = weight of swollen hydrogel equilibrium (at 20 °C). Swollen hydrogel water was recovered by centrifugation with 13,000 rpm for 10 min. For reswelling test, all the dried hydrogel samples (1 g) obtained in dewatering test were thoroughly dried in oven till constant weight was obtained. Then these were again soaked in 100 mL of distilled water to achieve the swelling equilibrium and swelling ratios were measured using equations 1 and 2 [ 3 6 ] . Similar procedure was repeated for three times to evaluate the reswelling capabilities of hydrogels.

2.4 FO process A bench scale FO system was used to evaluate the performances of prepared hydrogels (Fig. 2). The FO membrane soaked in DI water for 60 min was installed in the permeation cell with pressure retarded osmosis (PRO) or forward osmosis (FO) mode to initiate the test. The feed solution was placed on the active layer side of the membrane in PRO mode (the active layer). The feed solution face against the active layer of the membrane was in FO mode [28]. DI water and 2000 ppm saline solutions were the feed solution (FS). The weighed prepared 8

hydrogels were used as the draw agent. The feed solution tank was mounted on a scale, the water flux, JV (LMH) is nothing but change in weight of tank (ΔW, g) as determined by equation (4) [36]; Jv

………………..…………………………. (4)

Where ΔW = feed solution decreasing water weight because of water permeating through the FO membrane over a predetermined interval Δt (h) during the FO process; A (m2) = effective area of membrane used in the FO permeate cell, and ρ (g/L) = feed solution density and usually assumed as 1000 g/L, the density of water.

3. Results and Discussion 3.1 Characterization of Hydrogels 3.1.1 H-NMR The H-NMR spectrum of a PEG-PLGA-PEG, PEG-PLGA-PEG/G-0.18wt% and PEGPLGA-PEG/GO-0.18% triblock copolymers were shown in Figs. 3(a) to 3(c). The peaks were obtained at 5.20 ppm (CH of DLLA), 4.80 ppm (CH2 of GA), 3.65 ppm (CH2 of ethylene glycol), 3.38 ppm (CH3 of PEG end group) and 1.55 ppm (CH3 of DLLA) [34,37.38]. Proton signals of the methyl group of PLGA and the methylene group of PEG were observed as sharp peaks in CDCl3 [37].

3.1.2 FT-IR The infrared spectra of PLGA-PEG-PLGA, PEG-PLGA-PEG/G-0.18wt% and PEGPLGA-PEG/GO-0.18wt% were presented in Fig.4. 1455 and 1384 cm-1 peaks, represents the D,L-LA methyl group, were assigned to the characteristic of polymer’s methyl group. Peaks at 1096 and 2881 cm-1 were attributed for C–O–C group and polymer’s methylene group near to oxygen. Hence, it was confirmed that PEG ether linkage exists in the polymer. The peak at 1758 cm-1 was attributed to carbonyl group in polymer. The peak at 3432 cm-1 was attributed to terminal hydroxyl group of LA or GA structure piece in the polymer and the peak at 1189 cm -1 9

was attributed for C–O linkage of ester in polymer. So, it could be proven that in polymer product existed ester groups which were formed by inducting LA and GA into PEG. These results confirmed that PEG reacted with LA and GA, as a result, PEG-PLGA-PEG was produced [39, 40]. For Graphene/GO composite hydrogels C=O group observed at 1725 cm-1 and ends at 1635 cm-1 [41].

3.1.3 SEM Water absorbency of hydrogels and its retention rate depend on the porosity and layered surface of the hydrogels. Hence, hydrogel microstructure morphologies are one of the important properties [42]. Surface morphology of PEG-PLGA-PEG, PEG-PLGA-PEG/GO-018% and PEG-PLGA-PEG/G-0.18% hydrogels are shown in Fig.5 (a) to (c). All three hydrogels are shown porous structure in SEM figure. PEG-PLGA-PEG/GO-0.18wt% and PEG-PLGA-PEG/G0.18wt% incorporated hydrogels are showing the even distribution of GO and Graphene particles. Therefore, the SEM images are blacker and porous than the pure hydrogel. In Fig 5 (b) and (c), pores are available as dark black dots but those are not clearly visible. This may be due to the incorporation of G and GO in hydrogel, which filled the pores and reduced the size of pores.

3.1.4 GPC Gel permeation chromatography was used to obtain the molecular weight and molecular weight distribution of triblock copolymers. The molecular weight can control by changing monomer to initiator (PEG) ratio in the ring opening polymerization step [34]. Coupling of diblock copolymers to make triblock copolymers doubled the molecular weight as confirmed by GPC chromatogram and weight distributions shown in Fig. 6. From the figure, it can be easily seen that narrow peak shift from PEG-PLGA-PEG, PEG-PLGA-PEG/G and PEG-PLGAPEG/GO. This may be due the addition of G and GO in PEG-PLGA-PEG hydrogel. Summary of molecular weight average (Mw), molecular number average (Mn), and polydispersity index (PDI) of the triblock copolymers is given in Table 1. The polydispersity index (PDI) is the ration of weight-average MW over number-average Mn (Mw/Mn), which was around 1.3 for PEG10

PLGA-PEG/GO. This type of PDI value indicated that purity was sufficiently high to investigate its physical–chemical properties [37].”

3.1.5 XRD The crystalline or amorphous behavior of three triblock copolymers was analyzed by XRD measurements (Fig.7). The XRD patterns of PEG-PLGA-PEG, PEG-PLGA-PEG/G0.18wt% and PEG-PLGA-PEG/GO-0.18% were performed to confirm pure polymer, Graphene and GO in the hydrogel structure. The peaks of triblock copolymer are shown in Fig.7. The XRD spectra measured in a range of 0 to 80°, with a scanning step of 3° min-1 at room temperature. The XRD pattern of triblock copolymers just exhibited a broad amorphous peak [37]. PEG– PLGA-PEG presented a sticky paste in the bulk state and has shown broad region from broad region from 10° to 25º [43]. 2θ value for GO and Graphene incorporated hydrogels is 23º. These results are in accordance with the previously reported studies [18, 44]. These results are showing that Graphene and GO are properly distributed in hydrogel.

3.1.6 TGA Thermo-gravimetric analysis (TGA) was used to investigate the thermal decomposition behavior of the prepared hydrogels. Samples were heated under a nitrogen atmosphere from room temperature to 700 °C [26]. It can be seen in Fig. 8, that the thermal decomposition of PEG-PLGA-PEG/G-0.18wt% composite triblock copolymer passes through three stages upon heating from 45 °C to 700 °C. At the initial stage, up to 100 °C, only 1.8% wt loss was observed in the hydrogel showing removal of surface water molecules that are held in the material. The significant weight loss (53%) was observed from 200 to 250 °C, which was due to the degradation of poly(D,L-lactide-co-glycolide) (PLGA) [40]. The weight loss about 30% was observed from 250 °C to 500 °C, due to the degradation of PEG in triblock copolymers. Graphene decomposition was also observed at around 300 °C [45]. By the addition of graphene, onset degradation temperature increased and thus the thermal stability of the hydrogel also increase. PLGA-PEG/G-0.18wt% shows good/higher thermal stability in the temperature range of 400–700 °C due to the formation of more interfacial interactions between PEG-PLGA-PEG and graphene through hydrogen bonding. 11

3.2 Swelling properties of hydrogels 3.2.1 Swelling ratios The degree of swelling a hydrogel is a function of its structural (cross-linking density, polymer network thickness and relaxation rate) and chemical (degree of solubility and ionic strength) characteristics [27]. Swelling properties are important for hydrogels to get absorb more water from feed solution and this will lead higher water fluxes in FO. Due to the porous construction of hydrogels, they swell in the presence of fluids and absorb large quantities of water, thereby making the system comprehensively biocompatible [46]. Swelling ratios of PEGPLGA-PEG,

PEG-PLGA-PEG/GO-0.09wt%,

PEG-PLGA-PEG/Gr-0.09wt%,

PEG-PLGA-

PEG/GO-0.18wt% and PEG-PLGA-PEG/Gr-0.18wt% hydrogels are studied and shown in Fig.9. It can be seen that the swelling rate of the hydrogels is increases with time up to eight hours, then after hydrogels become saturated. Initially, hydrogels are taking time to swell and then started swelling faster up to eight hours of experiment. Also, the GO homogeneously dispersed in the matrix, which might influenced the microstructure of the polymer networks because of GO exceptional nanostructure. This could also influence the swelling capacities. Besides, there might be some synergetic intermolecular interactions between polymer networks and GO for holding water or other aqueous fluids, leading to the increment in the swelling ratios [32]. GO incorporated hydrogels shown higher swelling ratios compared to pure polymer and Graphene incorporated hydrogels. Greater swelling ratios can be achieved by the increment in loading of carbon fillers/GO. GO incorporated in polymer matrix, hydrophilic surface properties of carbon fillers is the key reason for improvement of swelling ratios. The incorporation of inorganic carbon fillers with polar surfaces is also responsible for increasing the accessibility of the charged groups within the hydrogel, and therefore the osmotic pressure [5, 26, 36, 47]. Swollen hydrogels photographs are shown in below Fig.10. The equilibrium swelling ratios of PEG-PLGA-PEG/GO-0.09wt% and PEG-PLGA-PEG polymer hydrogels are 140 and 120 respectively, shown in Fig.11. PEG-PLGA-PEG/G-0.09wt% and PEG-PLGA-PEG/G-0.18wt% hydrogels are shown lower ESR compared to pure polymer and GO incorporated hydrogels.

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3.2.2 Deswelling Hydrogels with good dewatering properties is most important in forward osmosis. Here, natural solar irradiation, conventional heating and centrifugation dewatering methods studied and shown in Fig. 12. Among these three methods, centrifugation has given maximum water recovery i.e. 98%. Centrifugation operated at 13000 rpm for 10 min. For conventional dewatering of hydrogels, heated at 60 °C for 2 min and water was recovered in the form of 90% liquid and remaining in the form of vapor phase. Water released from hydrogels was due to the thermosensitive property. If we increase temperature from 30°C to 60 °C (lower critical solution temperature (LCST) to upper critical solution temperature (UCST)) copolymer transfers from solution to gel. Solar dewatering shown good dewatering rates due to the addition of carbon particles or GO/Graphene to the hydrogels increase the solar absorption and thus increase the temperature of the composite hydrogels [2, 26]. In the solar dewatering tests, 2 g of swollen hydrogel was placed in natural solar irradiation with 984 W/m2 intensity, average relative humidity = 71% average wind velocity =18 km/hr. In solar dewatering, GO effectively absorbs the sunlight and then converts it to heat to increase the temperature of the hydrogels, thus accelerating the water release process in GO based hydrogel [36]. In other methods of dewatering also GO based hydrogel may contribute to the greater water recovery rate. This may be due to the fast water transport through the twodimensional interspaces between the GO sheets and polymer network [36]

3.3 Water flux evaluation Because of swelling pressure of polymer hydrogels, water can naturally be driven through semipermeable membranes and hydrated salt ions are rejected. Recovery of calculated amount of water is possible by application of external stimuli, making polymer hydrogels a potential for reuse in FO process. PEG-PLGA-PEG, PEG-PLGA-PEG/GO-0.09wt%, PEGPLGA-PEG/GO-0.18wt%, PEG-PLGA-PEG/G-0.09wt% and PEG-PLGA-PEG/G-0.18wt% hydrogels were tested as draw agents in FO process with DI water and 2000 ppm NaCl as feed solutions. Studied 24 h FO process to obtain water fluxes by using 1g of each draw agent and shown in Fig. 13 and 14. Results shows that for both DI water and 2000 ppm NaCl feed solutions, PEG-PLGA-PEG/GO-0.09wt% draw agent showing good water fluxes. After addition 13

of hydrophilic carbon microparticles, enhancement in swelling (osmotic) pressures resulted in higher water fluxes in the FO process by using composite GO and carbon fillers. GO contains plenty of hydrophilic groups, which dramatically increase the density of the hydrophilic groups of the polymer networks. As a result, the osmotic pressure of the nanocomposites increased during the swelling process and respectively water flux enhanced [32]. The incorporation of hydrophilic carbon particles in polymer hydrogels inducing a higher driving force in the FO process [26] resulted in higher swelling ratio (pressures) of the resulting polymer composite hydrogels. Addition of Graphene to the polymer hydrogels shows the lower water fluxes and swelling ratios in FO process. This may be because of incorporation of Graphene lead to increase the hydrophobicity of polymer hydrogel. If we increase the percentage incorporation of GO and Graphene in to the triblock copolymer hydrogels beyond this range (0.09-0.18) wt%, lead to lowering the water fluxes and swelling ratios in the FO process. After four hr, hydrogels started rising water fluxes and then decreased. As the process proceeds the osmotic pressure across the membrane goes on decreasing, thereby decreasing driving force and water flux. The average fluxes obtained for both DI water and 2000 ppm NaCl as feed solutions by using PEG-PLGA-PEG, PEG-PLGA-PEG/GO-0.09wt%, PEG-PLGA- PEG/GO0.18wt%, PEG-PLGA-PEG/G-0.09wt% and PEG-PLGA-PEG/G-0.18wt% hydrogels are shown in Table 2. Among all these hydrogels, GO incorporated and pure hydrogels are showing good water fluxes compared to the Graphene incorporated hydrogels. For both feed solutions water fluxes of hydrogels that are decreasing in the order of PEG-PLGA-PEG/GO-0.09wt% >PEGPLGA-PEG/GO-0.18wt% > PEG-PLGA-PEG > PEG-PLGA-PEG/G-0.09wt% > PEG-PLGAPEG/G-0.18wt%. In case of desalination increasing the feed solution led to decrease the water fluxes of hydrogels. In FO test there was no initial water flux up to four hours of starting experiment, after four hours of experiment feed solution started coming to draw agent. This may be because osmotic pressure depends on the dissociation number of solute and not on their chemical identity; it is a colligative property of [48]. Therefore, for a given mass of solute, larger dissociation molecule/ion size means lower resultant osmotic pressure. Dry hydrogel particles do not add up any osmotic pressure since they do not dissolve in solution. However, dry hydrogel particles with lapse of time to become wet and swell significantly on their contact with sufficient 14

amount of moisture or water [49].

By considering above swelling ratios and water fluxes,

PEG-PLGA-PEG, PEG-PLGA-PEG/GO-0.09wt% and PEG-PLGA-PEG/Gr-0.09wt% hydrogels are considered for recycle in FO process. From this recycle experiments, we observed that increasing number of cycles decreasing the water fluxes and swelling ratio. The average water fluxes for PEG-PLGA-PEG, PEG-PLGA-PEG/GO-0.09wt% and PEG-PLGA-PEG/G-0.09wt% hydrogels are 0.43 LMH, 0.55 LMH and 0.33 LMH respectively (Fig.15).

4. Conclusion The present study gives the synthesis of biodegradable and biocompatible hydrogels as draw agent in forward osmosis. We prepared different types of biodegradable hydrogels, but mainly we focused on PEG-PLGA-PEG, PEG-PLGA-PEG/GO-0.18wt% and PEG-PLGA-PEG/G-0.18wt%. For these hydrogels structural conformation was obtained from H-NMR, FT-IR, GPC, XRD, TGA and SEM. We have investigated the influence of feed solution, feed concentration, and graphene oxide (GO)/graphene (G) concentration in forward osmosis desalination by utilizing these hydrogels. The results revealed that considering PEGPLGA-PEG/GO-0.09wt% polymer hydrogels as draw agent was a desirable option for FO processes. Compared with pure polymer hydrogels, GO based hydrogel has higher the swelling ratio, water flux and water recovery. The incorporation of Graphene into the polymer hydrogel decreases the swelling ratios and water fluxes of FO process. Pure polymer hydrogels also have well in hydrophilic nature and this will lead to give good water fluxes, swelling ratios and water recovery. For PEG-PLGA-PEG/GO-0.09wt% we got average 24 h water fluxes with DI water and 2000 ppm NaCl as feed solutions, 0.67 LMH and 0.55 LMH. These FO water fluxes are among highest reported. For recovery of water from hydrogels we studied natural solar irradiation, centrifugation and conventional heating methods. Among these, conventional heating and centrifugation methods are good in water recovery. Centrifugation gave 98% water recovery for all types of hydrogel. Thus, this work report novel biodegradable and biocompatible, graphene and graphene oxide based hydrogel in FO process.

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Acknowledgment Second author acknowledge the financial support from the Institute (SVNIT, Surat) Research Grant {Dean (R&C) /1503/2013-14} from the project entitled “A revolutionary approach for regeneration of graphene based membrane and draw agent in UASB-EFO (Up flow anaerobic sludge blanket-Electric field attached, Forward Osmosis) for the production of pure water from municipal wastewater” for carrying out research work.

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List of Figures and Tables: Fig.1: Photographs of PEG-PLGA-PEG, PEG-PLGA-PEG/G and PEG-PLGA-PEG/GO hydrogel

.

Fig. 2: Laboratory scale forward osmosis batch setup. Fig. 3: proton-NMR spectra of (a) PEG-PLGA-PEG (b) PEG-PLGA-PEG/G-0.18wt% (c) PEGPLGA-PEG/GO-0.18wt% triblock copolymers in CDCl3. Fig. 4: FT-IR spectra of (a) PEG-PLGA-PEG/GO-0.18wt%, (b) PEG-PLGA-PEG/G-0.18wt%, (c) PEG-PLGA-PEG. Fig. 5: SEM images of (a) PEG-PLGA-PEG, (b) PEG-PLGA-PEG/GO-0.18wt%, (c) PEGPLGA-PEG/G-0.18wt%. Fig. 6: GPC traces of the triblock copolymers (a) PEG-PLGA-PEG, (b) PEG-PLGA-PEG/G, (c) PEG-PLGA-PEG/GO. Fig. 7: XRD patterns of (a) PEG-PLGA-PEG, (b) PEG-PLGA-PEG/GO-0.18wt%, (c) PEGPLGA-PEG/G-0.18wt%. Fig. 8: TGA of PEG-PLGA-PEG/G-0.18wt% hydrogel. Fig. 9: Swelling ratios of PEG-PLGA-PEG hydrogels. Fig. 10: PEG-PLGA-PEG and PEG-PLGA-PEG/GO-0.09wt% hydrogel after swelling. Fig. 11: Equilibrium swelling ratios of hydrogels. Fig. 12: Dewatering of hydrogels (a) PEG-PLGA-PEG, (b) PEG-PLGA-PEG/GO-0.18wt%, (C) PEG-PLGA-PEG/G-0.18wt% Fig. 13: The water fluxes in a 24 hr FO process by using PEG-PLGA-PEG polymer hydrogels as draw agent and DI water as feed solution. Fig. 14: The water fluxes in a 24 hr FO process by using PEG-PLGA-PEG polymer hydrogels as draw agent and 2000 ppm NaCl as feed solution. Fig. 15: Water fluxes for 2000 ppm NaCl feed solution by using recycled hydrogels as a draw agent.

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List of Tables Table 1: Summary of molecular weight average (Mw), molecular number average (Mn), and polydispersity index (PDI) of the triblock copolymers. Table 2: Average 24 hr water fluxes of triblock copolymer hydrogels as draw agent, DI water and 2000 ppm NaCl as feed solutions in FO process.

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

Table 1. Summary of molecular weight average (Mw), molecular number average (Mn), and polydispersity index (PDI) of the triblock copolymers. Triblock Block copolymers

MW

Mn

PDI (Mw/Mn)

PEG-PLGA-PEG

3403

951

3.580

PEG-PLGA-PEG/G-0.18wt%

2934

121

24.225

PEG-PLGA-PEG/GO-0.18wt%

1156

887

1.303

Table 2. Average 24 hr water fluxes of triblock copolymer hydrogels as draw agent, DI water and 2000 ppm NaCl as feed solutions in FO process. DI water (LMH)

2000 ppm NaCl (LMH)

PEG-PLGA-PEG

0.50

0.43

PEG-PLGA-PEG/GO-0.09 wt%

0.68

0.55

PEG-PLGA-PEG/GO-0.18 wt%

0.57

0.48

PEG-PLGA-PEG/G-0.09 wt%

0.38

0.34

PEG-PLGA-PEG/G-0.18 wt%

0.36

0.28

39

Highlights: 

Biodegradable and biocompatible temperature sensitive hydrogels were prepared.



Prepared hydrogels were used as draw agents for FO desalination process.



Prepared hydrogels are temperature responsive and good in dewatering properties.



Well in hydrophilic behavior, this can lead higher water fluxes.

40