Functional Ionic Liquid-Capped Graphene Quantum Dots for

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Functional Ionic Liquid-Capped Graphene Quantum Dots for Chromium Removal from Chromium Contaminated Water Ammavasi Nagaraj,† Murugan A. Munusamy,‡ Abdullah A. Al-Arfaj,‡ and Mariappan Rajan*,† †

Biomaterials in Medicinal Chemistry Laboratory, Department of Natural Products Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai, 625021, India ‡ Department of Botany and Microbiology, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

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S Supporting Information *

ABSTRACT: New functional ionic liquid (IL) capped graphene quantum dot (GQD) was synthesized as an adsorbent for the removal of toxic heavy metal chromium(VI) ion (Cr6+). The physicochemical properties of the adsorbent (IL-GQD) were investigated by Fourier transform infrared, Xray diffraction, atomic force microscopy, Raman, scanning electron microscopy, energy dispersive X-rays, and transmission electron microscopy analyses. The adsorption parameters, namely equilibrium time, solution pH, competing co-ions, dosage, and initial chromium concentration, were optimized for maximum Cr6+ adsorption. The maximum adsorption capacity reached 934.62 mg/g at 40 min in neutral pH; this is much better than most of the other adsorbents reported earlier. In addition, the effect of pH in solution was investigated in the range of 3.0−12.0. The result showed that the lower pH value was found to favor the adsorption. The adsorption kinetics and isotherms fitted well with Langmuir isotherm model and pseudo-second-order kinetic model. The thermodynamic studies indicated that the chromium adsorption process followed a spontaneous and endothermic model. This new functionalization of ionic liquid moieties into graphene quantum dot provides excellent results for the removal of toxic Cr6+. This can be utilized for field applications to reduce the chromium concentration to below the tolerance limit (>0.05 mg L−1). a result of its productivity and ease.9 Carbon-based graphene materials are viewed as exceptionally productive adsorbents for heavy metals.10−12 An assortment of contaminations can be adsorbed to the surface because of cooperation with the functionalities of graphene nanosheets. On account of the high thickness of oxygen-containing functional groups (hydroxyl, epoxy, and carboxylic), abundant sites are available for adsorption of heavy metals.13−17 Ordinarily, the associations between heavy metals and graphene nanosheets are brought about by electrostatic attraction,18,19 membrane filtration,20 ion exchange,21,22 and adsorption.23 In spite of these advantageous factors, the interaction is moderately weak and a long equilibrium period is required before satisfactory adsorption can be achieved.24 In addition, the graphene-based materials tend to aggregate in the aqueous solution which decreases the active sites for adsorption, lowering the reusable capacity. On account of the high removal capacity of carbon-based materials and aggregating nature of graphene-based materials in aqueous solution,carbon dot have attracted attention recently for the removal of heavy metals. In recent advances of graphene

1. INTRODUCTION Water bodies are polluted by different pollutants such as organic materials, inorganic compounds, dye molecules, heavy metals, and oil, etc.1The toxins present in the water bodies seriously affect human life and aquatic environmental system. Among these toxic pollutants, heavy metals such as mercury, cadmium, arsenic, lead and chromium, etc. are exceptionally poisonous and nonbiodegradable. Chromium(VI) (Cr6+) is a standout among the most widely recognized heavy metals utilized in different enterprises such as leather industries, metal cleaning, electroplating, mining etc. Chromium causes cancer by acting as a mutagenic agent on the living system when it exceeds the permissible limit of >0.05 mg L−1. So it is essential to remove chromium from contaminated water.2 Prevention and treatment of Cr6+ contaminated water bodies are necessary for environmental protection.3 Many technologies have been developed for the removal of toxic Cr6+ ions such as ion exchange,4 adsorption,5 catalytic reduction,6 electrochemical precipitation,7 and membrane filtration.8 These strategies, aside from adsorption, are nontemperate with inconveniences; for example, fragmented metal expulsion, high reagent cost, vitality necessities and age of lethal ooze or other waste items that require advanced transfer or treatment. The adsorption strategy remains the most favored technique as © XXXX American Chemical Society

Received: October 3, 2018 Accepted: December 11, 2018

A

DOI: 10.1021/acs.jced.8b00887 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Schematic representation of adsorbent IL-GO synthesis.

adsorption of Cr6+ on IL-GQD were carried out to obtain insights into the adsorption mechanism. Electrostatic, ion exchange and hydrogen bond between the IL-functionalized GQD and Cr6+ occurred due to the presence of protonated amines on IL-GQD which were responsible for the exceptional adsorption capacity of Cr6+ in drinking water.

materials such graphene quantum dot (GQD) was used in environmental applications, because GQD particularly shows unique electrical and optical properties because of the quantum confinement effect. GQD is low cost compared to other carbon-based nanomaterials, chemically inert, and has low toxicity in nature. Owing to the quantum confinement impact, GQD shows a size-dependent band gap. Such a band gap makes the adsorption of toxic ions or molecules easier.25 The surface of the GQD is given a greater number of oxygen containing functional groups such as carboxyl and hydroxyl, and accordingly the GQD can be stably dispersed into water under neutral conditions.26 Carboxyl and the hydroxyl groups on the surface of the GQD and most heavy metal ions can be realized; agglomeration can be caused, and the metals can be precipitated with GQD. In this way, the GQD can be utilized for purification of industrial wastewater containing heavy metals.27 Ionic liquids (ILs) are considered highly effective ecofriendly solvents for liquid−liquid extraction and adsorption28,29 of heavy metals because of the strong electrostatic attractions from the polar functionalities, tunability and high dissolvability, ease of operation, and so on.30 In any case, one of the greatest difficulties confronting the utilization of ILbased solvents is the trouble in the recovery of the solvent or the loss of sustained IL during operation. We have designed a highly efficient adsorbent that addresses the issue of loss of IL by covalently linking IL to GQD. This material maximizes the benefit by combining the high surface area of GQD with abundant IL functionalities. Moreover, ILs significantly increase the stability of GQD, a critical parameter for longterm reuse.31 A cationic imidazole ring with alkyl chain on GQD offers π−π and hydrogen bond interactions and increases electrostatic interlayer repulsion, which improves the adsorption capacity. Lastly, the hierarchically structured GQD can be separated with ease due to its lateral dimensions using microfiltration membranes. We synthesized ionic-liquid-capped graphene quantum dot (IL-GQD) by amidation followed by freeze-drying; it is the easiest method to condense the carboxylic group with amine.32 The IL-GQD showed ultrafast removal of chromium (6+) from aqueous solution. Kinetic and equilibrium studies on the

2. MATERIALS AND METHODS Graphite powder from Sigma-Aldrich was used as a starting material for synthesis of graphene oxide. Potassium dichromate (K2Cr2O7), hydrogen peroxide (H2O2) (30%), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrochloric acid (HCl) (37%), and sulfuric acid (H2SO4) were purchased from Sisco Research Laboratory (SRL), Bangalore. 1-Methylimidazole, 3-bromopropylamine hydrochloride (>98%) and ethanol (reagent grade) purchased from SigmaAldrich were used for the synthesis of the ionic liquid. 1-(3(Dimethylamino) propyl)-3-ethyl carbodiimide hydrochloride (EDC·HCl) was purchased from Central Drug House, New Delhi. N-Hydroxysuccinimide (NHS) and dimethylformamide (DMF) were purchased from Alfa Aesar, Bangalore; these were used for functionalization of graphene quantum dot. All chemicals were used without further purification. The Cr6+ stock solution was prepared using potassium dichromate with double deionized (DDI) water. 2.1. Synthesis of Ionic Liquid. The ionic liquid was synthesized according to the previous report by Zhou et al., (2016).33 Methylimidazole (80 mmol) and 3-bromopropylamine hydrochloride (60 mmol,) were added to 100 mL of ethanol under refluxing conditions at 70 °C in nitrogen atmosphere for 36 h, and the product was purified by recrystallization. 2.2. Synthesis of Graphene Oxide. The graphene oxide (GO) was prepared by the well-known earlier reported procedure.34 About 6 g of graphite powder was treated with a mixture of concentrated H2SO4/HNO3 (3:1) for 24 h at ambient environment (27 °C). The material was washed with Milli Q water, dried, and the exfoliation was performed at 800 °C. One gram of graphite was sonicated with acetone for 1 h. The powdered graphite was dried and further oxidized to GO on the basis of the Hummers method.35 The method involves B



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Figure 2. Schematic representation of adsorbent IL-GQD synthesis.

constant stirring, and stirring was continued for 24 h at room temperature (27°C). The IL-GO and IL-GQD were separated from the reaction mixture by centrifugation and then washed with DDI water and acetone. The product was subsequently dried in a freeze-dryer. The schematic synthetic route of the ionic liquid solvent and adsorbents is given in Figures 1 and 2. 2.5. Chromium Adsorption Experiment. The synthesized GO, IL-GO, GQD, and IL-GQD adsorbents were used for the removal of Cr6+ through the batch adsorption mode. About 0.1 g of the synthesized adsorbents was added to 100 mL of a 100 mg L−1 initial chromium solution in an iodine flask. After that, the mixture was shaken at a speed of 200 rpm using a thermostat shaker at room temperature (27 °C). All Cr6+ adsorption studies except the pH experiment were carried out at neutral solution pH. Then, the adsorbent was filtered by Whatman 42 filter paper, and the amount of Cr 6+ concentration in the aqueous solution was measured by using a UV−visible spectrophotometer (Shimadzu, UV-1600) at λmax value of 540 nm using a 1,5-diphenylcarbazide reagent. The adsorption capacity of the adsorbents was tested with various adsorption parameters such as adsorbent dosage, different initial solution pH, equilibrium time, initial chromium concentration, different temperature, and influence of other ions. The temperature influenced adsorption studies were conducted for the synthesized adsorbents at different temperature conditions such as 303, 313, 323, and 333 K with various initial chromium concentrations such as 80, 100, 120, and 140 mg L−1. The Cr6+ adsorption capacity was calculated using the following equation:

the treatment of graphite with approximately 50 mL of concentrated H2SO4 and 6 g of KMnO4 as the oxidizing agent maintaining a temperature between 0−5 °C. The temperature of the mixture was raised to 95 °C followed by treatment with 25 mL of 30% H2O2. The resulting solid was filtered and washed with 5% HCl, and the filtrate was checked for the presence of sulfate. The GO was further washed thoroughly with Milli Q water and dried in a hot air oven at 60 °C for 24 h. 2.3. Synthesis of Graphene Quantum Dot. The graphene quantum dot was synthesized by chemical ablation (acidic oxidation).36 Graphene sheets (GSs) were obtained by thermal deoxidization of GO sheets in a furnace at 200−300 °C for 2 h with a heating rate of 5 °C min−1. In a nitrogen atmosphere, 0.10 g of GSs was oxidized by 10 mL of concentrated H2SO4 and 30 mL of HNO3 for 2 h under ultrasonication (20 kHz, 130W). The mixture was then diluted with 250 mL of deionized water and filtered. A 0.5 g sample of oxidized GSs was redispersed in 40 mL of deionized water, and the pH was adjusted to 8.0 with diluted NaOH. The suspension was transferred into a Teflon autoclave and heated at 250 °C for 12 h. After being cooled to room temperature, the resulting black suspension of GQD was filtered by Whatman 42 filter paper and dried in a hot air oven at 80 °C. 2.4. Synthesis of Ionic Liquid-Capped Graphene Quantum Dot. The IL-GO and IL-GQD were synthesized by an amidation reaction between carboxylic acid functionalized GO and GQD with ionic liquid.37 A 30 mg sample of GO or GQD was dispersed separately in DMF using probe ultrasonication (Vibra Cell Sonics), for 2 h by a 5 min on−30 s off method with 20 kHz, 130 W. The GO or GQD suspension was then cooled to 0 °C, and 3 mmol each of EDC and NHS was added, and the mixture was stirred for 3 h at 0 °C. The mixture was then allowed to reach room temperature (27 °C). Ionic liquid (3 mmol) (methyl imidazole and 3-bromopropylamine hydrochloride) was added to the GO or GQD under

Cr(VI) adsorption capacity =

Co − Ce V (mg/g) W

(1)

where Co and Ce are the initial and equilibrium Cr 6+ concentrations (mg L−1), V is the volume of the Cr6+ solution taken (L) and W is the mass of the synthesized adsorbent (g). C



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Figure 3. 1H NMR spectrum of synthesized ionic liquid.

Figure 4. 13C NMR spectrum of synthesized ionic liquid.

All the adsorption experiments were carried out in duplicate, and the average values were reported. The aqueous solution pH was adjusted using an Orion Bench top multipara meter kit (VERSA STAR 92 model) with a pH electrode. The significant adsorption isotherm data of Cr6+ adsorption onto the synthesized IL-GQD adsorbent were obtained using Freundlich, Langmuir, and D−R isotherms. Furthermore, the standard mathematical tools such as standard deviation (sd), chi-square analysis (χ2) and regression correlation coefficient (r) were calculated in order to check the most suitable isotherm model for chromium adsorption. 2.6. Desorption Studies. For desorption studies, 0.1 g/L of adsorbent was added to a conical flask containing 100 mL of chromium aqueous solution. After a predetermined equilibrium time, chromium adsorbent was separated from the aqueous solution. Aqueous solution of pH 12.0 was used as eluent to regenerate the IL-GQD. Chromium loaded adsorbent was washed with 10.0 mL of aqueous solution of pH 12.0 (10% volume of total adsorption batch) to desorb chromium and separated from the solution. The concentration of chromium released was determined as mentioned earlier. The percentage of chromium desorption was calculated using the following equation:

desorption efficiency (%) concentration after elution 100 = concentration before elution

(2)

Subsequently, the adsorbent was washed with DDI water until the supernatant became neutral. After being washed, the regenerated adsorbent was dried for 12 h in a freeze-dryer and subjected to the further adsorption−desorption cycles. 2.7. Characterization Studies. The 1H and 13C NMR of the synthesized ionic liquid were recorded in DMSO solvent by (Bruker, 400 MHz) NMR spectrometer. Fourier-transform infrared (FT-IR) spectroscopy was used to characterize all the synthesized products. For all spectra, the % transmittance was plotted as a function of the wavenumber (cm−1). FT-IR spectra of the adsorbent samples were recorded on a Brukertensor 27 spectrometer operated in the attenuated total reflectance mode in the range of scanning wave numbers 4000−400 cm−1 with 32 scans per sample cycle at a resolution of 4 cm−1. The morphology studies of the adsorbent were done by transmission electron microscopy (TEM) images which were obtained with a high-resolution transmission electron microscope (model Tecnai Philips F30, FEI Co., Hillsboro), and atomic force microscope (AFM, model BT 02218, D



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Figure 5. FTIR spectrum of (A) (a) graphite, (b) GO, (c) IL-GO, (d) Cr6+ adsorbed IL-GO; and (B) (a) GQD, (b) IL-GQD, (c) Cr6+ adsorbed IL-GQD.

Figure 6. (A) XRD spectrum of (a) graphite, (b) GO, (c) and IL-GO adsorbent; (B) XRD spectrum of (a) GQD (b) IL-GQD and (c) Cr6+ adsorbed IL-GQD.

given in Figure 5A,B. No important characteristic peaks were seen; a broad peak at 1300−1200 cm−1 was due to the C−O stretching vibration which confirmed the graphite sheet formation (Figure 5A-a). As seen in Figure 5A-b the peaks appeared at 1628 cm−1(−CC−), 1728 cm−1(CO), and 3472 cm−1(−OH) which indicated the confirmation of successful synthesis of GO.38 The FT-IR spectra of IL-GO are shown in Figure 5A-c; the amide-carboxyl stretching was seen at 1650 cm−1; peaks at 1557 and 1082 cm−1 were attributed to amide N−H bending and C−N stretching, respectively, confirming the formation of the amide linkage between GO and the ionic liquid. Additionally, a new peak at 1967 cm−1 was observed due to the in-plane asymmetric stretching arising from the imidazolium ring,39 an evidence of attachment of ionic liquid on the GO surface. Figure 3A-d shows the spectrum of the Cr6+ adsorbed IL-GO; in this spectrum all the characteristic peaks of IL-GO were present (1094 cm−1, 1580 cm−1, 1659 cm−1, 1961 cm−1) but a new peak appeared at 903 cm−1 which was responsible for Cr6+; the hydroxyl group intensities of IL-GO (3493 cm−1) decreased into 3368 cm−1 in Cr6+ adsorbed IL-GO spectrum which also confirmed the Cr6+ adsorption of IL-GO adsorbent.40 Moreover after the adsorption of Cr6+ ion by IL-GQD adsorbent, the intensity of the C−N stretching band was reduced, which proved that the interactions between adsorbed

Switzerland). The scanning electron microscopy (SEM) micrograph, EDAX, and elemental mapping studies were done using a VEGA 3 TESCON model. The powder X-ray diffraction (XRD) studies were performed using the dried adsorbent samples with an X-ray diffractometer (XPERT-PRO with 2 microangle) equipped with a conventional Cu Kα X-ray radiation (40 kV, 30 mA) source and a Bragg diffraction setup. The Raman spectrum of the adsorbent was studied by Raman spectrometer (RAM HR 800) with the excitation source of He−Ne laser having the power of 17 MW.

3. RESULTS AND DISCUSSION 3.1. 1H and13C NMR Spectra of Synthesized Ionic Liquid. The synthesized ionic liquid was confirmed by 1H and 13 C NMR. Figure 3 shows the 1H NMR spectrum of synthesized ionic liquid; the chemical shift values of 1H NMR (400 MHz, DMSO), δ(ppm) were 7.82(d, 1H), 7.37(s, 1H), 7.82(d, 1H), 3.8(t, 2H), 3.35(s, 1H), 2.5−2.0(m, 6H). The 13C NMR spectrum of synthesized ionic liquid is given in Figure 4; the chemical shift values of 13C NMR (400 MHz, DMSO), δ (ppm) were 14.6. 16.6, 21.3, 38.3, 123.6, 127.8, 147.6. 3.2. FTIR Analysis of the Synthesized Adsorbent. The FTIR spectra of pristine graphite, GO, IL-GO, GQD, IL-GQD, and after Cr6+ adsorbed IL-GO & IL-GQD adsorbents are E



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Figure 7. Raman spectrum of (a) GO, (b) IL-GO, (c) GQD, and (d) IL-GQD adsorbents.

Figure 8. AFM images of (A) GO; (B) IL-GO adsorbents.

was confirmed by the appearance of new peak at 907 cm−1 which was due to the complex formation of Cr6+ with IL-GQD. 3.3. XRD Analysis. The synthesized adsorbents (pristine graphite, GO, IL-GO, GQD, IL-GQD, and after Cr6+ adsorbed IL-GQD) crystalline properties were characterized by powder X-ray diffraction analysis. A sharp peak at 2θ = 25.7° with 001 plane (JCPDS card no: 75-1621) indicated the graphite peak (Figure 6A-a).37 A sharp peak at 2θ = 10.4° with 002 plane (JCPDS card no: 75-2078) indicated the confirmation of successfully formed GO (Figure 6A-b).33 The peaks appearing at 2θ = 10.3° (001), 23.7°, 34.2°, 39.3°, 50.6°, 71.2° indicated the corresponding peaks of IL-functionalized GO (Figure 2Ac). The GO plane at 2θ = 10.3° (001) was not disturbed by the interaction of IL which indicated that the GO properties were retained and the crystalline nature was increased by the

chromium anion and ionic liquid cations were carried out through the complex formations.29 Figure 5B shows the FTIR spectrum of (a) GQD, (b) IL-GQD and Cr6+ adsorbed ILGQD. Figure 5B-a shows the responsible peaks of GQD such as 1631 cm−1 for CC, 1725 cm−1 for CO, and 3478 cm−1 for the −OH group present in the GQD. Figure 5B-b shows the corresponding peaks of GQD with new peaks of 1557 cm−1 for the N−H group, and 1074 cm−1 for the C−N group present in the ionic liquid. The chromium adsorbed IL-GQD spectrum is given in Figure 5B-c; this spectrum contained the basic peaks of 3453 for −OH (intensity of the hydroxyl group decreased due to chromium adsorption), and 1649 cm−1 for carboxyl stretching, 1553 cm−1 for N−H group; a peak appeared at 1956 cm−1 due to the in plane asymmetric stretching arising from ionic liquid. The chromium adsorption F



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GO showed the crumbled sheet-like morphology; this was due to the ionic liquids placed in the interlayer gaps of graphene oxide; so the cleared sheet-like morphology of GO was changed into compressed-sheet like morphology (Figure 9C). The morphology of the Cr6+ adsorbed IL-GO is given in Figure 9D, the Cr6+ ions were placed in the IL-GO sheets; the Cr6+ ions are denoted by green color circles (Figure 9E). 3.7. TEM Analysis. The morphological analyses of the assynthesized GQDs and those of the products were carried out using TEM analysis. Figure 10 shows the TEM, SAED patterns, and size distribution histogram images of assynthesized GQDs, GQDs along with the IL-GQD, and Cr6+ adsorbed IL-GQD. Figure 10A shows the clear dot-like morphology for the nanoparticles of GQD. Figure 8D shows the ionic liquids capped on the surface of the GQD; it shows the black color clusters surrounded on the GQD, and the ionic liquids are indicated by yellow color arrows. Figure 10G shows the tangent dark particles; this confirmed that the Cr6+ was adsorbed by the adsorbent IL-GQD. The particle size distribution obtained from TEM analysis are shown in Figure 10C,F,I) for GQD, IL-GQD, and Cr6+ adsorbed IL-GQD, respectively. The average particle size and standard deviation were calculated directly from the individual sizes, which also were in good agreement with the results. Also, the size distributions for all the samples were narrow. The sizes of the synthesized GQD, IL-GQD, and Cr6+ adsorbed IL-GQD were calculated to be 5.6 ± 0.9, 5.8 ± 0.7, and 6.2 ± 0.3 nm, respectively. The HRTEM images of the samples and the SAED pattern showed clear lattice fringes and semicrystallinity nature of adsorbent (Figure 10 (B,E,H for GQD, IL-GQD, and Cr6+ adsorbed IL-GQD, respectively). The results confirmed the formation of GQD and IL-GQD, and Cr6+ adsorption by IL-GQD was confirmed. the semicrystalline nature of the adsorbents supported more interaction with the adsorbent compared to the crystalline adsorbents. Figure 11 shows the EDAX spectra of (a) GO, (b) IL-GO, (c) Cr6+ adsorbed ILGO, (d) GQD, (e) IL-GQD, (f) Cr6+ adsorbed IL-GQD. Figure 11 (c and f) show the characteristic peak of Cr6+ ion. This confirmed that the IL-GQD adsorbed the Cr6+ ions from aqueous solution. Figure 12 shows the elemental mapping image of the Cr6+ adsorbed IL-GQD adsorbent. 3.8. Effect of Contact Time. The effect of Cr6+ adsorption was carried out on the adsorbents of GO, IL-GO, GQD, and IL-GQD at different contact times in the range from 10−100 min using 0.1 g of the adsorbent with 100 mL of 100 mg L−1 initial chromium concentration at a solution pH of 7.0, 300 K. Figure 13 shows the Cr6+ adsorption results of GO, IL-GO, GQD, and IL-GQD adsorbent with reference to contact times based on the absorbance of UV visible spectrum for concentration change versus the contact times. The results clearly showed that all the synthesized adsorbents’ adsorption capacity rapidly increased with an increase in contact time. The maximum Cr Cr6+ adsorption capacity of adsorbents such as GO, IL-GO, GQD, and IL-GQD were 75%, 79%, 84%, and 93%, respectively. On increasing the contact time, IL-GQD attained saturation at 40 min, while the GQD, IL-GO, and GO attained saturation at 60 min. However, all these adsorbents showed significant adsorption capacity over a contact time of 100 min; the IL-GQD attained the equilibrium at 40 min, which might be due to the active sites of IL-GQD being completely occupied in a short time. Hence, the optimum contact time for the adsorption process of IL-GQD was fixed

interaction of IL on the GO surface The amorphous nature of synthesized GQD was further confirmed by the SAED pattern (Figure 6B-a); the crystallinity nature was increased by the addition of ionic liquid as in IL-functionalized GO (Figure 6Bb) and the characteristic peaks of ionic liquid appeared at 2θ = 24.3°, 33.4°, 42.6°, 51.1°, 55.6°. A new peak appeared at 2θ = 46.2° with the characteristic peaks of IL-GQD (Figure 6B-c) which confirmed that the Cr6+ was adsorbed by the IL-GQD by weak electrostatic forces of attraction. The modification of GQD with ionic liquid increased the d spacing to 0.87 nm, indicating insertion of ionic liquid moieties in the interlayer space between GQD layers.37 3.4. Raman Analysis. The significant structural changes that occurred during the chemical processing from pristine graphite to GQD, and then GQD to IL-GQD, are reflected in their Raman spectra analyses (Figure 7). In the Raman spectrum of GQD, the G band was broadened and shifted to 1544 cm−1.41 In addition, the D band at 1237 cm−1 became prominent, indicating the reduction in size of the in-plane sp2 domains (ID/IG ratio is 0.80), possibly due to the extensive oxidation from graphite. The Raman spectrum of the reduced IL-GQD also contained both D and G bands (at 1262 and 1567 cm−1, respectively); however, with an increased D/G intensity ratio compared to that in GQD, it was merely equal (ID/IG = 0.80).37 This result showed that the characteristics of GQD remained also in IL-GQD. 3.5. AFM Analysis of Synthesized Adsorbent. AFM images of GO and IL-GO exfoliated by the ultrasonic treatment at concentrations of 1 mg/mL in water always revealed the presence of dot with uniform thickness. While a pristine graphene sheet is atomically flat with a well-known van der Waals thickness of 0.34 nm, GO is expected to be “thicker” due to the presence of covalently bound oxygen and the displacement of the sp3-hybridized carbon atoms slightly above and below the original graphene plane. The 2 and 3dimensional images of GO and IL-GO are shown in Figures 8 panels A and B, respectively. 3.6. SEM Analysis. The morphological changes were studied by SEM micrographs; the images are shown in Figure 9. The pristine graphite (Figure 9A) showed plate-like morphology, whereas the converted graphene oxide showed clear sheet-like morphology (Figure 9B). The synthesized IL-

Figure 9. SEM images of (A) graphite, (B) GO sheet, (C) GO, (D) IL-GO, and (D and E) Cr6+ adsorbed IL-GO. G



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Figure 10. (A,D,G) TEM images of GQD, IL-GQD, and Cr6+ adsorbed IL-GQD, respectively; (B,E,H) SAED pattern of GQD, IL-GQD, and Cr6+ adsorbed IL-GQD, respectively; and (C,F,I) size distribution histogram of GQD, IL-GQD, and Cr6+ adsorbed IL-GQD, respectively.

Figure 11. EDAX images for (a)GO, (b)IL-GO, (c) Cr6+ adsorbed IL-GO, (d) GQD, (e) IL-GQD, and (f) Cr6+ adsorbed IL-GQD.

dosage required for Cr6+ removal of the prepared adsorbents such as GO, IL-GO, GQD, and IL-GQD. The different dosage (5−30 mg) levels of the synthesized adsorbents were added into 100 mL of the initial chromium solution and their

as 40 min and for other adsorbents such as GQD, IL-GO, and GO it was fixed as 60 min.42 3.9. Influence of Adsorbent Dosage. The dosage experiment was used to conclude the optimum amount of H



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Figure 12. Elemental mapping images for Cr6+ adsorbed IL-GQD.

Figure 13. (a,b) UV−visible spectrum of Cr6+ adsorption concentration of adsorbents IL-GO and IL-GQD with reference to contact time; (c) cumulative Cr6+ adsorption with reference to contact time images of the adsorbents GO, IL-GO, GQD, and IL-GQD at 300 K.

the dosage exceeded 0.1 g. Hence, for further Cr6+ adsorption studies, 0.1 g of dosage was fixed for all the synthesized adsorbents.43 3.10. Effect of Initial Concentration. We examined the effect of varying the initial concentration of chromium ions in Cr6+ contaminated water on the adsorption efficiency, using values from 2.0 mg·L−1 to 10 mg·L−1 (Figure 15). At an initial

adsorption capacity was assessed (Figure 14). Chromium adsorption capacity was increased with an increase in the adsorbent dosage of GO, IL-GO, GQD, and IL-GQD. This is because, when the adsorbent dosage increases, the number of active sites increase, which leads to high chromium adsorption capacity. Furthermore, all the synthesized adsorbents possessed no significant change in their adsorption capacity when I



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Figure 14. (a,b) UV−visible spectrum of Cr6+ adsorption concentration of adsorbents IL-GO and IL-GQD with reference to adsorbent dosage; (c) cumulative Cr6+ adsorption with reference to adsorbent dosage images of the adsorbents GO, IL-GO, GQD, and IL-GQD at 300 K.

concentration of chromium ions of 2.0 mg·L−1 the adsorption efficiencies were 51.7% for GO, 54.2% for IL-GO, 61.6% for GQD, and 66.2% for IL-GQD, respectively. When the concentration of chromium ions was raised to 4.0 mg·L−1, the efficiency improved to 55.6% (GO), 58.5 (IL-GO), 66.1% (GQD), and 74.3% (IL-GQD). When the initial concentration was 10 mg·L−1, the chromium removal efficiency was 73.2% for GO, 78% for IL-GO, 84.9% for GQD, and 92% for IL-GQD. In this study, we noted that the adsorption efficiency of the adsorbents such as GO, IL-GO, GQD, and IL-GQD increased with increasing concentrations of chromium ions. The synthesized adsorbents were able to adsorb above 10 mg·L−1 chromium contaminated water. 3.11. Effect of pH. The surface Coulombic force of the adsorbent is extremely pH dependent; so its impact on the adsorption process was investigated in the solution in the range of pH 3.0 to 11.0. During the investigation, the pH of the solution was adjusted by 0.1 M NaOH/HCl; the adsorbent dosage was fixed as 15 mg, and all other parameters were kept constant. The effect of pH on the chromium adsorption results of the adsorbents are presented in Figure 14. Figure 14a,b shows the UV visible spectrum of chromium concentration in solution with various pH solutions of IL-GO and IL-GQD adsorbent. Figure 16c shows the cumulative chromium concentration of the adsorbents GO, IL-GO, GQD, and ILGQD with various pH solutions. By comparing the acidic and basic pH ranges, the adsorption capacity was found to be high at the acidic rather than in the basic. This may be due to the presence of positively charged ammonium ions present on the surface of the synthesized adsorbent at acidic pH.44 Since the surface of the adsorbent becomes positively charged, it attracts

the negatively charged chromate ions toward itself electrostatically. At pH 3, high chromium adsorption capacities of 74.2%, 77.4%, 87.1%, and 92.6% were observed for GO, ILGO, GQD, and IL-GQD, respectively. In general, the surface of the adsorbent is positively charged due to the lower pH zero point charge (pHzpc) than neutral pH of the adsorbent (pH > pHzpc), and negatively charged means that the pHzpc is greater than neutral pH (pH < pHzpc). The pHzpc value of IL-GQD adsorbent was found to be 6.7 (Figure 16d). The observed pHzpc value of the IL-GQD adsorbent was less positively charged due to the adsorbent having more electropositive magnesium ions. The positive charge on the surface easily attracts the chromium ions from chromium contaminated water.45 On comparison of the adsorbents, GO, IL-GO, GQD, and IL-GQD, GQD had less adsorption capacity than IL-GQD due to the additional functional groups and surface area. The functional group becomes highly protonated in acidic pH, which can attract the chromate ions through the electrostatic attraction. Moreover, GO, IL-GO, and GQD showed very low Cr6+ adsorption capacity compared to IL-GQD adsorbent.46−50 This may be due to the presence of free carboxylic groups, which can react with the anion, and the complete dissociation of the functional groups may take place at around pH 4.5. This experiment demonstrates that the synthesized adsorbents can remove Cr6+, not only from acidic medium, but also from neutral and basic medium. 3.12. Effect of Foreign Ions. The ground potable water possesses chromium ions with background ions such as sulfate, nitrate, magnesium, calcium, sodium, bicarbonate, and chloride. These background ions may hamper the adsorption J



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Figure 15. (a,b) UV−visible spectrum of Cr6+ adsorption concentration of adsorbents IL-GO and IL-GQD with reference to initial concentration; (c) cumulative Cr6+ adsorption with reference to initial concentration images of the adsorbents GO, IL-GO, GQD, and IL-GQD at 300 K.

3.13. Selectivity of the Best Adsorbent. The Cr6+ removal capacity of the synthesized four adsorbents such as GO, IL-GO, GQD, and IL-GQD was examined by batch adsorption studies. On the basis of the batch adsorption studies, the adsorbent IL-GQD showed marvelous Cr6+ removal capacity than other adsorbents such as GO, IL-GO, GQD. The selection of best adsorbent for Cr6+ adsorption was optimized for Cr6+ removal from water, and IL-GQD adsorption efficacy was compared with GO, GO-IL, and GQD adsorbent. Therefore, the other studies such as isotherm model, kinetic analysis, thermodynamic parameter determination, and field studies were carried out only for IL-GQD adsorbent. 3.14. Adsorption Isotherm. To quantify the adsorption capacity of IL-GQD adsorbent for the removal of chromium, the three most commonly used isotherms, namely, Freundlich51 Langmuir52 and Redlich−Peterson53 were adopted. The graphical plots of the adsorption isotherm models are shown in Figure 18: (a) Freundlich isotherm model; (b) Langmuir isotherm model; (c) Redlich−Peterson isotherm model, respectively. 3.14.1. Freundlich Isotherm. The linear form of the Freundlich isotherm model can be represented by

capacity of the synthesized adsorbents. Therefore, chromium adsorption capacity in the presence of foreign ions was studied on the synthesized GO, IL-GO, GQD, and IL-GQD adsorbents at neutral solution pH, with the initial Cr6+ concentration as 100 mg L−1, with 200 mg L−1 of other coion concentration at equilibrium contact time; the other parameters were kept constant. Figure 17 shows the results of the competing ions on the adsorption capacity of the synthesized adsorbents. The competing cations such as Ca2+, Mg2+, and Na+ ions did not significantly affect chromium adsorption on the synthesized adsorbents. This may be due to the adsorption process being carried out at neutral solution pH, such that at acidic pH conditions the adsorbent surface becomes protonated, which results in repulsion between the positive surface and competing cations. The anions, such as Cl−, SO42− and NO3−, slightly affect the chromium adsorption capacity on the synthesized adsorbents, but the bicarbonate ion imposes high competition with chromate during adsorption. This is because the HCO3− ion would release the hydroxyl group during the hydrolysis of NaHCO3, thus the solution pH increases, which in turn decreases the chromium adsorption capacity, and also due to similar size of HCO3− with that of HCrO4−. The IL-GQD adsorbent possesses higher adsorption capacity than GQD, IL-GO, and GO. The IL-GQD possessed good adsorption capacity than other adsorbents such as GO, IL-GO, and GQD.

log qe = log kF + K

1 log Ce n

(3) DOI: 10.1021/acs.jced.8b00887 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Figure 16. (a,b) UV−visible spectrum of Cr6+ adsorption concentration of adsorbents IL-GO and IL-GQD with reference to various pH; (c) cumulative Cr6+ adsorption with reference to various pH images of the adsorbents GO, IL-GO, GQD, and IL-GQD at 300 K; (d) pH-zero point charge (pHzpc) images of IL-GQD adsorbent.

Ce C 1 = + e qe Q 0b Q0

(4)

where Q0 is the amount of adsorbate at monolayer coverage (mg·g−1), which gives the maximum adsorption capacity of the adsorbent, and b (L·mg−1) is the Langmuir isotherm constant that relates to the energy of adsorption. A linear plot is obtained for IL-GQD adsorbent when Ce/qe is plotted against Ce which gives Q0 and b values from the slope and intercept, respectively, and the calculated results are listed in Table S1. The increase in values of Q0 with the rise in temperature indicates that the uptake amount of chromium increased with the rise in temperature. The value of b increases with the increase in temperature. It also confirms the endothermic nature of the chromium adsorption reaction. The RL values lying between 0 and 1 indicate favorable adsorption for all the temperatures studied. 3.14.3. Redlich−Peterson Isotherm. The linear form of the Redlich−Peterson isotherm model can be represented by

Figure 17. Effect of competing ions on Cr6+ adsorption with adsorbents GQD and IL-GQD.

ij K C yz logjjjj R e − 1zzzz = β log Ce + log αR j q z k e {

where qe is the amount of chromium ions adsorbed per unit weight of the adsorbent (mg·g−1), Ce is the equilibrium concentration of chromium ions in solution (mg·L−1), kF is a measure of adsorption capacity, and 1/n is the adsorption intensity. The linear plot of log qe vs log Ce indicates the applicability of Freundlich isotherm. The values of 1/n and kF of IL-GQD adsorbent are listed in Table S1. The values of 1/n are lying between 0 and 1 and the n value lying in the range of 1−10 confirms the favorable conditions for adsorption. With the rise in temperature, the kF values increase, which indicates that the chromium uptake by IL-GQD adsorbent is an endothermic process. 3.14.2. Langmuir Isotherm. The linear form of the Langmuir isotherm model can be represented by the equation,

(5)

where KR = Qb (L/g) and Q and b are the Langmuir adsorption capacity (monolayer) and adsorption energy, respectively. The αR is the exponent of the Redlich−Peterson isotherm. The values of the Redlich−Peterson isotherm constants, αR and β, were calculated from the slope and intercept of the plot between log (KRCe/qe − 1) vs log Ce. The higher r values of isotherms (Langmuir, Freundlich, and Redlich−Peterson) show that the well fit of the experimental values obtained with the Freundlich isotherm was more L



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Figure 18. Graphical plots of (a) Freundlich isotherm model; (b) Langmuir isotherm model; (c) Redlich−Peterson isotherm model.

either by particle diffusion56 or intraparticle diffusion57 control. Both particle and intraparticle diffusion models were used to describe the chromium removal by IL-GQD adsorbent. The respective straight line plots of ln(1 − Ct/Ce) vs t and qt vs t0.5 indicate the applicability of both particle and intraparticle diffusion models. The kp, ki, and r values at three different temperatures, namely, 303, 313, and 323 K for both particle and intraparticle diffusion models are illustrated in Table S2. The r values obtained for both the particle and intraparticle diffusion models are almost comparable and suggest that the chromium diffusion on IL-GQD adsorbent follows both models. 3.16. Thermodynamic Studies. The temperature effect of chromium adsorption IL-GQD adsorbent was carried out at three different temperatures, 303, 313, and 323 K. The important thermodynamic parameters such as Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were determined (Table S3). The values of ΔG° were negative for the synthesized adsorbents implying that the adsorption process was spontaneous. The values of enthalpy change (ΔH°) and entropy change (ΔS°) were obtained by the slope and intercept of the plot between ln K and 1/T.58 The values of ΔH° were positive, which highlighted that the chromium adsorption was endothermic in nature. Furthermore, the positive values of ΔS° indicated the good binding affinity of the synthesized adsorbents with chromium ions and increased randomness at the adsorbent−adsorbate interface during the adsorption process.59

suitable than those of the Langmuir and Redlich−Peterson isotherms. 3.15. Kinetic Models. The two main types of adsorption kinetic models namely reaction-based and diffusion-based models were adopted to fit the experimental data. The graphical plots of kinetic models are shown in Figure 19: (a) pseudo-first-order kinetic model; (b) pseudo-second-order kinetic model; and (c) intraparticle diffusion model. 3.15.1. Reaction-Based Models. The most commonly used pseudo-first-order54 and pseudo-second-order55 models were employed to explain the solid/liquid adsorption. The linear plots of log (qe− qt) against t gives a straight line indicating the applicability of the pseudo-first-order model. The slope of the straight line plot of log (qe − qt) against t adsorption at different temperatures, namely, 303, 313, and 323 K gave the value of the pseudo-first-order rate constant (kad) (Table S2). In addition, the fitness of the data and the values of qe, k, h, and r of the pseudo-second-order model were obtained from the plots of t/qt vs t for chromium adsorption at different temperatures, namely, 303, 313, and 323 K of IL-GQD adsorbent (Table S2). The values of qe increased with the increase in temperature indicating that the chromium adsorption increased with the rise in temperature. The higher r values obtained for pseudo-second-order model than for the pseudo-first-order model indicated the applicability of the pseudo-second-order model. 3.15.2. Diffusion Based Model. For a solid−liquid adsorption process, the solute transfer is usually characterized M



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Figure 19. Graphical plots of (a) pseudo-first-order kinetic model; (b) pseudo-second-order kinetic model; and (c) intraparticle diffusion model.

Figure 20. Plausible Cr6+ adsorption mechanism of IL-GQD adsorbent.

3.17. Chromium Removal Mechanism. The chargeinduced adsorption mechanism of chromium on the IL-GQD adsorbent is illustrated in Figure 20. The IL induced the GQD of imidazolium ionic liquid, amide linkages, and hydroxyl functionalities. The long chain protonated amine arranged itself with the alkyl groups pointing away, in this way encouraging the head gathering (NH+) to interact electrostatically with hexavalent chromium oxyanion. The IL-GQD adsorbent acquired positive charges at neutral-low pH

(below pHzpc) due to the protonation of ionic liquid moieties, amide linkages as well as unreacted carboxylic groups. Because of strong electrostatic interactions between the oxonium group from chromate and IL-GQD, the rate of adsorption was enhanced. Hydrogen bonds between hydroxyl and nitrogen containing groups in IL-GQD adsorbent existed. The Cr6+ efficiency of the adsorbent IL-GQD was compared with already reported adsorbents and it was given in Table 1. N



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Table 1. Comparison of Cr6+ Removal Capacity of IL-GQD Adsorbent with Other Adsorbents no.

sdsorbent name

1

expoliated graphene oxide ionic liquid (Aliquate 336) composite60 magentic chitosan/graphene oxide-ionic liquid composite61 ionic liquid modified reduced graphene oxide62 graphene oxide and magnesium oxide nanohybrid composite63 reduced graphene oxide nickel oxide nanocomposite64 reduced graphene oxide/polypyrrole and Fe3O4 magnetic composite65 magnetic β-cyclodextrin graphene oxide nanocomposite66 ionic liquid modified metal organic framework67 quaternary ammonium and quaternary phosphonium ionic liquid functionalized silica material68 functional ionic liquid capped graphene quantum dots

2 3 4 5 6 7 8 9 10

Cr6+ removal capacity (mg·g−1) 285.71 145.35 232.55 190 198 293.3

Figure 22. Desorption studies of the adsorbents GO, IL-GO, GQD, and IL-GQD.

120.0 285.71 15.29

concentration was measured using a UV−visible spectrophotometer. The results showed that GO, IL-GO, GQD, and ILGQD could be used for up to five cycles. The regeneration performances of the adsorbents were greater than 80% after five cycles. Hence, the prepared adsorbents could be utilized for up to five cycles without any loss in their removal efficiency in order to reduce the cost of the adsorbents. 3.20. Field Trial of the Synthesized Adsorbent. The evaluation of Cr6+ uptake performance in existing Cr6+ contaminated water was carried out using GQD and ILGQD adsorbents. About 0.1 g of the synthesized adsorbent was added to 100 mL of a chromium contaminated water sample (collected from Madura Coats Private Limited, Madurai, Tamil Nadu, India) and shaken until equilibrium time at room temperature. The final chromium concentration was measured; the results are shown in Table S4. The results showed the complete removal of Cr6+ from the field water sample by the synthesized adsorbents. In addition, a significant amount of other water quality parameters were also reduced by the synthesized adsorbents. The results indicated the suitability of GQD and IL-GQD for Cr6+ removal in field conditions.

934.61 (present study)

3.18. Selectivity of Cr6+ over Cr3+ by IL-GO and ILGQD Adsorbent. The selectivity of Cr6+ over Cr3+ by the adsorbent is most important in field water trial since the real sample contains both Cr6+ and Cr3+ ions. The Cr3+ ion is not hazardous but Cr6+ is highly hazardous; so the adsorbent selectively adsorbing the Cr6+ ion is an important factor. In a real sample the Cr3+ ion exists in cationic form, but the Cr6+ ion exists as anionic form (HCrO42−). The synthesized IL-GO and IL-GQD have basically positively charged ionic liquid cation (NH4+); this positive group will make the surface of the adsorbent positive; its pH zero point charge (pHzpc = 6.7) value also indicates the surface of the IL-GO and IL-GQD adsorbent to be positive. So the positively charged IL-GQD adsorbent will selectively adsorb the anionic Cr6+ ion and not the cationic Cr3+ ion. This was confirmed by the UV visible spectroscopy (Figure 21). Cr6+ showed the peaks at λmax value of 540 nm but Cr3+ did not show absorbance peak (Figure 21). 3.19. Desorption Studies of the Synthesized Adsorbent. The regeneration study was carried out for the exhausted GO, IL-GO, GQD, and IL-GQD adsorbent using NaOH as an eluting solution (Figure 22). About 0.1 g of Cr6+ adsorbed synthesized adsorbent was added to 100 mL of 0.5 mol NaOH solution and stirred well for 60 min. Then, the Cr6+

4. CONCLUSION We synthesized carbon-based adsorbents such as GO, GO-IL, GQD, and IL-GQD via a facile and cost-effective method. IL functionalization was successfully carried out by amidation reaction between imidazolium-based ionic liquid and carboxylic acid groups on the GQD surface. Compared with other conventional adsorbents, the IL-GQD showed ultrafast Cr6+ adsorption capacity of 934.62 mg/g and facile regeneration. The equilibrium data of the IL-GQD adsorbent were best fitted with Langmuir isotherm. Thermodynamic parameters suggested the spontaneous and endothermic nature of Cr6+ adsorption. The kinetic studies showed that pseudo-secondorder model fitted well with experimental data. The IL-GQD adsorbent removed chromium by electrostatic attraction, complexation, ion-exchange, and hydrogen bonding mechanism. The reusability and regeneration studies of the IL-GQD adsorbent indicated that they can be reused up to five cycles. The field trial results indicated that the IL-GQD adsorbent could be effectively applied for chromium removal. The ILGQD adsorbent is a potential adsorbent for the removal of Cr6+ from the chromium contaminated water.

Figure 21. Selectivity of Cr6+ over Cr3+ by adsorbent GQD and ILGQD. O



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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00887. Procedure for the determination of pHzpc of the ILGQD adsorbent; tables of isotherm studies, kinetic studies, thermodynamic studies, and field studies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 9488014084. ORCID

Mariappan Rajan: 0000-0001-9569-8744 Funding

M. Rajan is grateful to the University Grants Commission (UGC), Government of India, for providing financial support under the schemes of “UGC-MRP Grants” MRP (ref: F. No. 43-187/2014 (SR)) and Department of Science and Technology, Science and Engineering Research Board (ref: YSS/2015/001532; New Delhi, India) and also acknowledges the DST-PURSE program for the purchase of FT-IR, SEM. The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through Research Group Project No. RG-1435-057. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to extend their sincere appreciation to RSSU at King Saud University for their technical support. REFERENCES

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