Microwave-Assisted Rapid Synthesis of Reduced Graphene Oxide-Based Gum Tragacanth Hydrogel Nanocomposite for Heavy Metal Ions Adsorption

Reduced graphene oxide (RGO) was synthesized in this research via Tour’s method for the use of filler in the hydrogel matrix. The copolymerization of N,N-dimethylacrylamide (DMA) onto the gum tragacanth (GT) was carried out to develop gum tragacanth-cl-N,N-dimethylacrylamide (GT-cl-poly(DMA)) hydrogel using N,N’-methylenebisacrylamide (NMBA) and potassium persulfate (KPS) as cross-linker and initiator correspondingly. The various GT-cl-poly(DMA) hydrogel synthesis parameters were optimized to achieve maximum swelling of GT-cl-poly(DMA) hydrogel. The optimized GT-cl-poly(DMA) hydrogel was then filled with RGO to form reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide (GT-cl-poly(DMA)/RGO) hydrogel composite. The synthesized samples were used for competent adsorption of Hg2+ and Cr6+ ions. Fourier transform infrared, X-ray powder diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy were used to characterize the gum tragacanth-cl-N,N-dimethylacrylamide hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite. The experiments of adsorption-desorption cycles for Hg2+ and Cr6+ ions were carried out to perform the reusability of gum tragacanth-cl-N,N-dimethylacrylamide hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite. From these two samples, reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide exhibited high adsorption ability. The Hg2+ and Cr6+ ions adsorption by gum tragacanth-cl-N,N-dimethylacrylamide and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide were best suited for pseudo-second-order kinetics and Langmuir isotherm. The reported maximum Hg2+ and Cr6+ ions adsorption capacities were 666.6 mg g-1 and 473.9 mg g-1 respectively.


Introduction
Continuous industrialization leads to the excessive release of toxic pollutants into various sources of water. The heavy metal poisoning of water has now become a pandemic concern due to its dangerous impacts to human health because these pollutants are non-degradable, poisonous, cancer-causing agent and are hard to separate from water [1]. For metal uptake from water, several approaches have been established, among them, treatment via adsorption is the most appealing one. Adsorption technology is widely used for removing pollutants due to its simple operation, cost and easy implementation [2]. Specifically, hydrogels based on biopolymer have now become very useful in adsorptive wastewater treatment [3]. Gum tragacanth based hydrogel is highly adsorptive because of the presence of hydroxyl (-OH) and carboxyl (-COOH) groups [4,5]. It is a renewable, cost-effective and environmentally friendly polysaccharide that can be easily polymerized to form cross-linked structures [6][7][8].
Gum tragacanth is commonly found in the sap of different legumes in the Middle East. The biological source of gum tragacanth is a plant named Astragalus gummifer. It is a complex mixture of polysaccharides including bassorin and tragacanthin units. When mixed with water, gum tragacanth produces a colloidal hydrosol. The bassorin unit can (composed of 60-70% of the compound) swells to form a gel [9]. Mallakpour et al. reported the glutaraldehyde cross-linked gum tragacanth/CaCO 3 hydrogel composite as an adsorbent for the abstraction of Pb 2+ ion [10]. Moghaddam et al. synthesized methoxyl gum tragacanth-glutamic acid/polyacrylamide hydrogel via electron beam radiations as an adsorbent for trapping uranium ions from toxic uranium solution [11].
The adsorption and stability of hydrogel can be improved by using reduced graphene oxide as filler in the hydrogel matrix. Reduced graphene oxide (RGO) can result in high C/O with better mechanical strength [12]. The reduced graphene oxide is partially decorated with an oxygen-rich functional group that acts as active sites for interaction. The high RGO surface, large porosity and defect sites are the features that help pollutants adsorption [13]. Sahraei et al. reported adsorption of Cr 6+ metal using chitosan/reduced-graphene oxide/montmorillonite composite hydrogel. The composite hydrogel showed maximum Cr 6+ absorption of 87.03 mg g −1 [14]. Zhuang et al. synthesized molybdenum disulfide/RGO hydrogel as an adsorbent for mercury ions removal [15].
The previously reported works (Table 1) have not comprehensively considered the factors responsible for the high adsorption capability of the adsorbent. In this work, we achieved a better adsorption capacity of 666.6 mg g −1 and 473.9 mg g −1 for mercury and chromium ions within less time using a low adsorbent dose. Specifically, prepared reduced graphene oxide incorporated gum tragacanth cross-linked poly N,N-dimethylacrylamide hydrogel composite shows a very high adsorption percentage of 99% for mercury metal ion under optimal conditions (adsorbent dose = 0.035 g and time = 270 min, T = 25 • C, the concentration of mercury solution = 20 ppm) which means it is highly efficient for mercury adsorption. Also, compared to recently reported studies, we are able to synthesize our adsorbents in very short period (90 s) with high swelling percentage ( Table 2) using microwave radiations. This is one of the key points where our synthesis part shows novelty. Hence, we developed the simple and fast synthetic route for the preparation of efficient, sustainable and eco-friendly graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel with high adsorption rate for heavy metal ions.  In this work, we developed first-time gum tragacanth-cl-N,N-dimethylacrylamide hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite for adsorption of Hg 2+ and Cr 6+ . The RGO was synthesized from graphite and incorporated in GT-cl-poly(DMA) hydrogel matrix to increase the adsorption efficiency. The sorption study was explained by kinetic and isotherm models. The effect of pH, adsorbent dose and RGO loading on adsorption were performed. The gum tragacanth-cl-N,N-dimethylacrylamide hydrogel was systematically designed based on swelling. The adsorbed samples were desorbed successfully by using 0.1 M HNO 3 and used further for adsorption experiments.

Synthesis of Reduced Graphene Oxide (RGO)
For the synthesis, a mixture of 50 mL concentrated H 2 SO 4 and 5.5 mL of H 3 PO 4 was taken in a beaker. Then, 1.0 g of graphite powder and KMnO 4 (4.0 g) were added to the mixture after maintaining 10 • C under magnetic stirring in ice bath. The mixture was heated to 35 • C governed by a water bath. After 2 h of stirring, the reaction mixture was sonicated 10 times with the help of ultrasonicator. To stop the reaction, deionized water (200 mL) and 1 M NaOH solution were added dropwise to maintain the pH = 6 of solution mixture. The reaction process was then followed with the addition of H 2 O 2 (15 mL) which led to a change in suspension color to yellow. The mixture was kept overnight. Thereafter, the solution of ascorbic acid (0.227 mol L −1 ) was added dropwise under magnetic stirring at 95 • C. In this step, the color of the solution changes slowly from greenish-yellow to black. Finally, the mixture was allowed to settle down for 1 h. Black precipitates were formed which were then centrifuged and washed several times using ethanol.

Synthesis of Gum Tragacanth-cl-N,N-dimethylacrylamide (GT-cl-poly(DMA)) Hydrogel
Microwave-assisted copolymerization method was used in the synthesis of gum tragacanth-cl-N,N-dimethylacrylamide hydrogel [5]. In a typical reaction, 0.5 g of gum tragacanth (GT) (in 11 mL of deionized water) was taken in 50 mL beaker, stirred until GT was uniformly mixed with distilled water. After this, KPS (10 × 10 −1 mol L −1 ) and NMBA (5.8 × 10 −1 mol L −1 ) were added into the GT solution. Magnetic stirring was continued to get a homogenous mixture and then 4.4 × 10 −1 mol L −1 of DMA was added in this mixture. The solution mixture was placed under microwave radiations for 90 s to generate active radicals needed for the initiation of the polymerization reaction. The prepared gel was washed using acetone and dried inside the preheated (50 • C) hot air oven for 24 h. For the preparation of reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite, 0.5 g of GT was added in a solution of RGO (0.005-0.025 g in 11 mL of deionized water). Physical agitation was applied until the mixture becomes homogenously uniform. Thereafter, the mixture was treated similarly following the procedure in Section 2.3. The incorporation of RGO was confirmed physically by monitoring the color change from light orange to black, as presented in Scheme 1. The optimized quantities that are used in the preparation of hydrogels are given in Table 3.

Swelling Study
Various parameters such as initiator concentration (KPS) solvent volume, time, cross-linker concentration (NMBA), microwave power, monomer concentration (DMA) and amount of RGO were optimized to obtain the maximum swelling percentage. The swelling percentages of gum tragacanth-cl-N,N-dimethylacrylamide and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogels in deionized water were examined for a fixed period of 16 h. The pre-weighed dry piece of the sample was added in 50 mL of distilled water for 16 h. Then, swelled hydrogel was weighed. The swelling percentage was calculated using Equation (1) [28]: where, W s = weight of swelled gum tragacanth-cl-N,N-dimethylacrylamide hydrogel, W d = weight of dry gum tragacanth-cl-N,N-dimethylacrylamide hydrogel.

Adsorption of Hg 2+ and Cr 6+
The batch adsorption analyses were performed in 150 mL beaker using adsorption shaker (200 rpm) at pH of 5.5 and 3.5 for Hg 2+ and Cr 6+ removal respectively. For more illustration, 0.010-0.070 g of adsorbents were used in Hg 2+ and Cr 6+ ions solution (50 mL, 20 ppm) at 25 • C for fixed period. After adsorption, the mixture was filtered to determine the concentration of Hg 2+ and Cr 6+ ions using 1,3-diphenylcarbazide method [14]. The concentration of adsorbed Hg 2+ and Cr 6+ was calculated by evaluating the absorbance of heavy metal ion solution using UV-Vis spectrophotometer at 532 nm and 370 nm respectively. The amount of adsorbed Hg 2+ and Cr 6+ was calculated by Equation (2) [28]: where q e = equilibrium gum tragacanth-cl-N,N-dimethylacrylamide and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide adsorption capacity, C o = initial Hg 2+ and Cr 6+ concentration (mg L −1 ), C e = equilibrium Hg 2+  The maximum adsorption-desorption rate was found at optimized 0.1 M HNO 3 . The Hg 2+ and Cr 6+ loaded GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite were desorbed by using 100 mL of 0.1 M HNO 3 followed by neutralization with 0.1 M NaOH. Finally, the desorbed adsorbent was washed by distilled water and dried at room temperature for further adsorption of Hg 2+ and Cr 6+ .

Mechanism for Synthesis of Gum Tragacanth-cl-N,N-dimethylacrylamide Hydrogel
In this work, gum tragacanth-cl-N,N-dimethylacrylamide hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite were prepared by radical copolymerization of DMA and GT in the presence radical initiator (KPS) and the cross-linking action of NMBA. Under microwave radiations, KPS was decomposed and radical ions were generated [25]. These primary free radicals led to the generation of DMA monomer radical (through addition reaction with KPS) and GT alkoxy radical (through abstraction of hydrogen by KPS). The grafting of DMA radical and GT alkoxy radical was carried through radical copolymerization reaction (Scheme 1). The crosslinker NMBA led to the cross-linkages between different chains to facilitate the construction of three-dimensional gum tragacanth-cl-N,N-dimethylacrylamide hydrogel polymeric network. Finally, the dispersion of RGO led to the generation of reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite.

Optimization of Swelling for Reduced Graphene Oxide Incorporated
The swelling percentage of GT-cl-poly(DMA) hydrogel was affected by the concentration of KPS and results are shown in Figure 1a. The maximum swelling of 565.5% was observed at 10 × 10 −1 mol L −1 of KPS. Below this concentration (<10 × 10 −1 mol L −1 ), the swelling percentage was lower due to inadequate initiator, which was unable to produce appropriate active sites on GT-cl-poly(DMA).

Reaction Time
The GT-cl-poly(DMA) hydrogel showed the highest swelling percentage (657.8%) at 90 s ( Figure 1b). The swelling percentage was decreased from 90 s to 130 s, this might be due to the formation of excess branched chains that could inhibit the expansion of the polymer.

Solvent
The solvent volume was varied from 5 mL to 13 mL in the formation of GT-cl-poly(DMA) hydrogel ( Figure 1c). The maximum swelling (784.4%) was obtained at solvent volume of 11 mL. At higher solvent volume beyond 11 mL, the swelling percentage of GT-cl-poly(DMA) hydrogel was decreased, the excess solvent volume lowered the concentration of KPS, DMA and NMBA resulted in poor degree of polymerization.

Microwave Power
The swelling percentage was maximum at 20% of microwave power for GT-cl-poly(DMA) hydrogel ( Figure 1d). The swelling percentage was lower at microwave power above 20%. This was due to the formation of excess radical led to increase the homo-polymerization rate reaction. Hence, the microwave power was kept at 20% for further synthesis reaction of GT-cl-poly(DMA) hydrogel.

Monomer (DMA) Concentration
The  Figure 1g shows the influence of RGO loading on the swelling of GT-cl-poly(DMA) hydrogel. The rise in the amount of RGO from 0.005 g to 0.020 g was attributed to an increase in swelling percentage. This was due to an increase in hydrophilic group and surface area of GT-cl-poly(DMA) on the incorporation of RGO. Any further increase in RGO loading (> 0.020 g) was found to decrease the swelling percentage of GT-cl-poly(DMA) hydrogel. This might be attributed to the increased cross-linking density of composite hydrogel networks and the aggregations of excessive RGO in the hydrogel matrix.

FTIR
The FTIR graphs of samples are presented in Figure 2. In spectrum of RGO (Figure 2a), broadband of nearly 3358 cm −1 can be attributed to -OH stretching mode. The peak at 1421 cm −1 corresponds to the carboxylic acid and peak at 1625 cm −1 belongs to the -C=C group in the aromatic rings. The peak at 1128 cm −1 is due to -C-O stretching in the C-OH functional groups of RGO [29]. The shifting of -C-O stretching from 1128 cm −1 to 1125 cm −1 in GT-cl-poly(DMA)/RGO hydrogel composite is related to the successful incorporation of RGO in GT-cl-poly(DMA) hydrogel. The bands at 1638 cm −1 and 1748 cm −1 correspond to asymmetric stretching of the carboxylate group and asymmetric stretching of C=O in galacturonic acid respectively [30], peak at 1142 cm −1 ascribed to antisymmetric vibrations of C-O-C linkage in glycosidic groups [31]. The asymmetric stretching of C=O shows shifting of peaks from 1748 cm −1 to 1750 cm −1 after the crosslinking of poly(DMA). The GT-cl-poly(DMA)/RGO hydrogel composite (Figure 2a) shows shift related to asymmetric stretching of C=O from 1750 cm −1 to 1758 cm −1 suggesting interaction between RGO and GT-cl-poly(DMA) hydrogel. The peaks at 1608 cm −1 and 1410 cm −1 in GT-cl-poly(DMA) hydrogel ascribed to stretching vibrations of poly(DMA) amide group [32]. These stretching vibrations of poly(DMA) show peak shifting from 1608 cm −1 to 1612 cm −1 and from 1410 cm −1 to 1403 cm −1 in GT-cl-poly(DMA)/RGO hydrogel composite which confirms the changes in the structure of poly(DMA) after RGO incorporation. In GT-cl-poly(DMA)/RGO, the broadband of O-H stretching vibration shifted from 3403 cm −1 to 3382 cm −1 which may be attributed to the RGO interaction with GT-cl-poly(DMA) through intermolecular hydrogen bonds. The peaks intensity of GT-cl-poly(DMA)/RGO hydrogel composite is slightly lower than the GT-cl-poly(DMA) hydrogel, which also confirms the RGO dispersion in GT-cl-poly(DMA)/RGO hydrogel composite. The absorption band at 2910 cm −1 was attributed to stretching vibrations of aliphatic C-H [10]. Also, peaks at 1048 cm −1 and 1052 cm −1 in the spectra of hydrogels correspond to the -C-O bending. After the adsorption of Hg 2+ and Cr 6+ on GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite, peak due to carboxylate groups was shifted from 1612 cm −1 to 1621 cm −1 and the intensity of the peaks decreases (Figure 2b). The peaks at 1410 cm −1 and 1048 cm −1 were shifted to 1403 cm −1 and 1061 cm −1 respectively, which was probably due to the interactions of metal ions to the active site of adsorbent. The peaks intensity of Hg 2+ loaded GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite was lower than the Cr 6+ loaded GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite, which supports higher Hg 2+ adsorption than Cr 6+ adsorption.

XRD
The XRD pattern of GT, gum tragacanth-cl-N,N-dimethylacrylamide hydrogel (GT-cl-poly(DMA)), RGO and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite (GT-cl-poly(DMA)/RGO) is shown in Figure 3. The RGO formation was confirmed by the characteristic peak at 2θ = 24.3 • [33]. On applying Bragg's law, the calculated interlayer spacing of RGO was 0.367 nm. Another peak of RGO at 2θ = 43.6 • corresponded to the fingermark of graphite indicating the regeneration of graphitic onto RGO [34]. According to Scherrer's formula the calculated particle size of RGO at 2θ = 24.3 • was 0.894 nm. In the case of GT, the diffraction peak occurred at 2θ = 22.9 • and 26.2 • , exhibited semi-crystalline in nature [35]. The slight shifting of the peak at 2θ = 26.6 • in XRD pattern of GT-cl-poly(DMA) hydrogel confirms the crosslinking of DMA with polysaccharide by destroying semi-crystalline structure into the amorphous structure. The broad peak in GT-cl-poly(DMA)/RGO hydrogel composite indicates the poor ordered arrangement of RGO in GT-cl-poly(DMA) hydrogel supported by SEM morphology.

SEM
Microscopic images of RGO, gum tragacanth-cl-N,N-dimethylacrylamide hydrogel and reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite are shown in Figure 4. In RGO, Figure 4a shows the aggregated wrinkled structure, which means particles were closely associated. The RGO morphology showed the formation of agglomerated RGO with estimated average grain size of 20-25 nm. Figure 4b shows the distribution of certain bulges on a quite smooth, porous and compact surface of gum tragacanth-cl-N,N-dimethylacrylamide hydrogel.
After the incorporation of RGO, reduced graphene oxide incorporated gum tragacanth-cl-N,N-dimethylacrylamide hydrogel composite showed ( Figure 4c) the rough and irregular surface with reduced size which was beneficial in fast adsorption of Hg 2+ and Cr 6+ .

Influence of GT-cl-poly(DMA) Hydrogel and GT-cl-poly(DMA)/RGO Hydrogel Composite Dose for Removal of Hg 2+ and Cr 6+
The effect of adsorbents dosages (0.015 -0.065 g) on the removal of metal ions are represented in Figure 6e,f. The adsorption percentage was increased by increasing the adsorbent dosage. This was due to the existence of more adsorption sites with enhanced dose of adsorbent. The removal efficiencies of Hg 2+ were found to be 86.4% and 98.4% by GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite correspondingly at dose of 0.035 g. The reported Cr 6+ ion removal percentages were 77.2% and 82.3% by using GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite respectively at optimized dose of 0.045 g. Thus, 0.035 g (for Hg 2+ ) and 0.045 g (for Cr 6+ ) were the ideal doses used for experiments.

Adsorption Kinetics
The pseudo first-order rate equation is given as: where, q e and q t are the adsorption capacity at equilibrium (mg g −1 ) and time t respectively and K 1 is the pseudo first order kinetics rate constant. The pseudo second-order rate equation is given as: where K 2 is the pseudo second-order kinetics rate constant. The removal mechanism for Hg 2+ and Cr 6+ by GT-cl-poly(DMA) and GT-cl-poly(DMA)/RGO were solved by different kinetic models as given in Equations (3) and (4). The parameters (pseudo-first-order: R 2 , K 1 , q e ) were calculated from Figure 7a,b ( Table 4). The parameters (pseudo-second-order: K 2 , q e ) and correlation coefficient (R 2 ) were calculated from Figure 7c,d ( Table 4). The higher R 2 values for the pseudo-second-order kinetic model supports Hg 2+ and Cr 6+ ions adsorption onto GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite through the pseudo-second-order kinetic model.

Adsorption Isotherms
The Langmuir model is expressed according to Equation (5) as: where C e is the equilibrium concentration of metal ions solution, q e is the amount of equilibrium adsorbed metal ions, q m is maximum adsorption capacity and b is the Langmuir isotherm constant. The separation factor R L of Langmuir isotherm was examined by using Equation (6) as: where C o represent the initial concentration of metal ions. The R L values show the nature of adsorption i.e. irreversible (R L = 0), linear (R L = 1), unfavorable (R L > 1) and favorable (0 < R L < 1). The Freundlich isotherm model is given by Equation (7) as: where K F and n are Freundlich constants and indicate the adsorption capacity and adsorption intensity of adsorbent respectively. The interaction between adsorbent (GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite) and adsorbate (Hg 2+ and Cr 6+ ) was explained through isotherms model Equations (5) and (7). The Langmuir parameters were calculated from the graph between C e /q e and C e (Figure 8a-d) and presented in Table 5. The Freundlich parameters were determined from the graph of lnq e vs lnC e (Figure 9a-d) and depicted in Table 5. For the Langmuir isotherm, the higher R 2 suggests that the Langmuir isotherm was best suited for the removal of Hg 2+ and Cr 6+ ions on GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite. For Hg 2+ , GT-cl-poly(DMA) and GT-cl-poly(DMA)/RGO showed higher removal capacity of 625 mg g −1 and 666.6 mg g −1 respectively. Similarly, for Cr 6+ , the maximum reported removal capacities were 401.6 mg g −1 and 473.9 mg g −1 by GT-cl-poly(DMA) hydrogel and GT-cl-poly(DMA)/RGO hydrogel composite respectively.

Relationship between the Adsorption and Swelling
For the investigation of the correlation between the swelling of GT-cl-poly(DMA)/RGO hydrogel composite and adsorption of the Hg 2+ and Cr 6+ onto GT-cl-poly(DMA)/RGO hydrogel composite, the adsorption percentage was determined using aqueous metal solution (20 mg L −1 ) and the swelling experiments were performed in distilled water. The relationship between the adsorption values versus swelling values is presented in Figure 10. It is clear from Figure 10 that the adsorption percentage is directly proportional to the swelling percentage of the adsorbent. The adsorption percentages for Hg 2+ and Cr 6+ were increased from 78.9% to 90.7% and 29.8% to 38.4% respectively when the swelling percentage of GT-cl-poly(DMA)/RGO rise from 834.6% to 971.9%.

Conflicts of Interest:
The authors declare no conflict of interest.