Ultrafast Dye Removal Using Ionic LiquidâGraphene Oxide Sponge

Research Article pubs.acs.org/journal/ascecg

Ultrafast Dye Removal Using Ionic Liquid−Graphene Oxide Sponge Rahul Zambare,†,‡ Xiaoxiao Song,‡ S. Bhuvana,‡ James Selvaraj Antony Prince,*,‡ and Parag Nemade*,†,§ †

Department of Chemical Engineering, Institute of Chemical Technology (ICT), Nathalal Parekh Marg, Matunga, Mumbai 400019, India ‡ Environmental and Water Technology Centre of Innovation (EWTCOI), Ngee Ann Polytechnic, Singapore 599489, Singapore § Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology (ICT), Nathalal Parekh Marg, Matunga, Mumbai 400019, India S Supporting Information *

ABSTRACT: A methylimidazolium ionic-liquid-functionalized graphene oxide (mimGO) sponge was prepared by facile amidation reaction followed by freezedrying. Covalent functionalization of graphene oxide (GO) was confirmed by SEM, XRD, XPS, FT-IR, Raman and TGA. The mimGO sponge was employed to remove direct red 80 (DR80) dye from aqueous solutions. Results showed exceptional affinity for adsorption of azo dye direct red 80 (DR80) due to the charge-induced adsorption aided by protonated amine and cationic methylimidazolium ionic liquid pendant groups. Detailed adsorption characteristics of mimGO sponge including operational parameters, adsorption kinetics and adsorption isotherms were investigated. Adsorption on the mimGO sponge followed pseudo-second-order kinetic model, and the equilibrium capacity was described by the Langmuir adsorption model. An ultrahigh adsorption rate of 588.2 mg/(g·min) and equilibrium adsorption capacity of 501.3 mg/g for DR80 were observed for the mimGO sponge, which is much higher than that of ethylenediamine functionalized GO, unmodified GO and commercial activated carbon. Further, desorption of 99.4% of DR80 was achieved at even higher rate in aqueous solution of pH 12. The dye removal efficiency of the mimGO sponge remains at 99.2% after four adsorption−desorption cycles. Combining the ultrafast adsorption kinetics with high removal capacity and good recyclability, the mimGO sponge has great potential for effluent treatment applications. KEYWORDS: Graphene oxide−ionic-liquid-based adsorbent, Direct red 80 dye, Ultrafast adsorption, Recyclability

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INTRODUCTION Graphene-based carbon materials are regarded as highly efficient adsorbents for dyes and heavy metal ions.1−8 A variety of pollutants can be adsorbed to the surface due to interaction with the functionalities endowed on graphene nanosheets. Thanks to the high density of oxygen-containing functional groups (hydroxyl, epoxy and carboxylic), abundant sites are available for adsorption of dyes9,10 and heavy metal ions.11 Likewise, amine groups exhibit electrostatic interactions with negatively charged groups on the dye molecules.10,12−15 Typically, the interactions between dye molecules and graphene nanosheets are (1) electrostatic interactions; (2) hydrogen bonding; (3) π−π electron stacking; (4) physical adsorption.6,7 Despite these beneficial factors the interactions are relatively weak and a long equilibrium period is needed before satisfactory adsorption can be achieved.9,10,16 Moreover, the graphene-based materials tend to aggregate in the aqueous solution, which reduce accessible active sites for adsorption, lowering capacity during reuse.3,6,7,17,18 On the application end, dye wastewater has characteristics such as large volume, low-to-medium dye concentration (20− 100 ppm) and complexity of components.19,20 Dye molecules, © 2017 American Chemical Society

with small Stokes radius, pose difficulty for membrane separation.21,22 Additionally, the tendency of dyes to foul polymeric membranes make membrane separation a costly technology. Alternatively, high selectivity adsorptive materials are preferred for their cost-efficiency characteristics.23 The high selectivity materials enable valuable dyes to be preferably adsorbed and hence recovered from the wastewater. An ideal dye-adsorptive material should have following desired properties: (1) fast adsorption rate; (2) high adsorption capacity; (3) high selectivity for dye molecules; and (4) ease of regeneration and reuse.22,24 Ionic liquids (ILs) are considered as highly efficient environmental-friendly solvents for liquid−liquid extraction25,26 and adsorption (supported ionic liquids)16,27−31 of organic dyes and heavy metals due to the presence of strong electrostatic attractions from the polar functionalities, ease of operation, tunable and high solubility, etc.32,33 However, one of the biggest challenges facing the use of IL-based agents is Received: March 21, 2017 Revised: May 15, 2017 Published: May 19, 2017 6026

DOI: 10.1021/acssuschemeng.7b00867 ACS Sustainable Chem. Eng. 2017, 5, 6026−6035

Research Article

ACS Sustainable Chemistry & Engineering

immersed into a liquid nitrogen bath for freeze-drying process. A solidified mimGO/water biphasic system was transferred into a freezedryer (Scanvac Coolsafe, Denmark) under vacuum (less than 0.4 mbar and freeze-dried for 3 days to obtain a hierarchical mimGO sponge. Characterization. The structure of mimGO was determined using Fourier transform infrared spectrophotometry (FT-IR) (IR Prestige21 Shimadzu). X-ray diffraction (XRD) (D8 ADVANCE, Bruker) with monochromatized Cu Kα radiation (λ = 1.5406 Å) was used to identify the stacking in adsorbents. Raman spectra were obtained using a Raman spectrophotometer having a laser excitation wavelength of 532 nm (Senterra, Bruker) to verify any defects and structural changes upon functionalization of GO. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Escalab 250Xi equipped with a monochromatic Al Kα X-ray source. Binding energies were referenced to the adventitious hydrocarbon C 1s line at 284.6 eV and curve fitting of XPS spectra was done using CasaXPS software. Thermal stability of adsorbent was measured using thermogravimetric analyzer (TGA/DSC 1, Star system, Mettler-Toledo) by applying the heating rate of 10 °C/min under nitrogen atmosphere with a flow rate of 50 mL/min. Physical morphology of adsorbent was characterized by field emission scanning electron microscopy (FE-SEM, JEOL, JMS6400, Japan). Elemental distribution mapping of the adsorbent was carried out using energy dispersive X-ray (EDX) analysis. Nitrogen adsorption−desorption isotherms were obtained at 77 K after simple N2 gas flushing of the sample without any vacuum at 80 °C for 12 h using surface area and porosimetry system (ASAP 2020, Micromeritics). The Brunauer−Emmett−Teller (BET) model was applied to adsorption isotherm branch in the relative pressure range (P/P0) below 0.25 to calculate surface area. The concentration of dye solutions before and after adsorption was determined using doublebeam UV−vis spectrometer (UV-1800, Shimadzu). Surface charge of the adsorbent in water dispersion (0.1 mg/mL) at different pH values was estimated using ζ-potential measurements (Nano ZS90, Malvern Instruments). Dye Adsorption Experiments. A stock solution of Direct Red 80 (DR80) containing 1000 ppm of DR80 was prepared by dissolving DR80 in DI water. The stock solution was diluted to the required concentration for adsorption experiments. 1 M HCl and 1 M NaOH were used for pH adjustment using a pH meter (Accumet XL600 Fisher Scientific). All adsorption experiments were carried out at ambient temperature (23 °C). To study the effect of the operational parameters on dye removal, various experiments were performed as follows; adsorbent loading (0.1−1 g/L), solution pH (2−10), dye concentration (25−100 ppm). For adsorption kinetics studies, various adsorbent amounts (0.1−1 g/L) were added to different 250 mL conical flasks containing 100 mL of DR80 aqueous solution (50 ppm, pH 2). The flasks was kept on an orbital shaker (NB-101S, N-BIOTEK) rotated at 150 rpm to ensure complete mixing. Samples were collected at predetermined time intervals (2.5 to 90 min) and filtered through 0.2 μm polypropylene membrane syringe filter. The ability of polypropylene membrane to adsorb DR80 is ignored because their adsorption capacity was measured to be negligible. UV−vis absorption spectra were recorded at 540 nm and compared with the established calibration curve to obtain the concentration of dye in particular solution. All adsorption experiments were repeated two times, and the average value of measurement is reported. % dye removal and dye adsorption capacity were calculated using mass balance equations:

difficulty in the recovery of the solvent or the loss of supported IL during operation.26,34 We have designed a highly efficient adsorbent that addresses the issue of loss of IL by covalently linking IL to graphene oxide. This novel material maximizes the benefit by combining the high surface area of GO with abundant IL functionalities. Moreover, ILs significantly increase the stability of GO nanosheets, a critical parameter for long-term reuse.35 Cationic imidazole ring with alkyl chain on GO offers π−π and hydrogen bond interactions and increase electrostatic intersheet repulsion, which improve the adsorption capacity.36 Lastly, the hierarchical structured GO to be separated due to its lateral dimensions with ease using microfiltration membranes. We synthesized ionic-liquid-functionalized graphene oxide (mimGO) by amidation. The mimGO sponge was subsequently prepared by freeze-drying. The mimGO sponge showed the ultrafast removal of azo dye, direct red 80 (DR80) from aqueous solution. The adsorbent can be separated easily and regenerated for adsorption. Kinetic and equilibrium studies on the adsorption of DR80 on mimGO sponge were carried out to obtain insights about the adsorption mechanism. Electrostatic, hydrogen bond and π−π interactions between the mimGO sponge and DR80 due to the presence of protonated amines on mimGO are responsible for the exceptional adsorption capacity of anionic DR80 molecules.

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EXPERIMENTAL SECTION

Materials. Graphite powder (98%) and ethanol (reagent grade) purchased from Sigma-Aldrich were used for the synthesis of the ionic liquid. 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride (EDCI) purchased from Tee Hai Chem Pte Ltd. Singapore. N-Hydroxysuccinimide (NHS) and dimethylformamide (DMF) were purchased from Sigma-Aldrich were used for functionalization of graphene oxide. Commercial activated carbon (AC, ColorSorb 620) was used for dye adsorption comparison studies. Dye (Direct Red 80) was obtained from Sigma-Aldrich. All chemicals were used without further purification. Adsorbent Synthesis. Synthesis of Methylimidazolium Ionic Liquid (mimNH2). The mimNH2 was synthesized according to the literature.35 1-Methylimidazole (80 mmol, 6.57 g) and 3-bromopropylamine hydrobromide (80 mmol, 17.51 g) were added to 200 mL of ethanol under refluxing conditions in nitrogen atmosphere for 24 h, and the product was purified by recrystallization. Synthesis of mimNH2 Functionalized Graphene Oxide (mimGO). Graphene oxide was prepared using modified Hummer’s method from graphite powder as the starting material.37,38 A detailed synthesis procedure is provided in the Supporting Information. The mimGO was synthesized by an amidation reaction between carboxylic acid functionalized GO and mimNH2.39 30 mg of GO was dispersed in DMF using ultrasonication for 2 h, GO suspension was then cooled to 0 °C, and 3 mmol each of EDCI and NHS was added and the mixture followed by stirring for 2 h at 0 °C. The mixture was then allowed to reach room temperature. 3 mmol of mimNH2 was added to the reaction mixture under constant stirring, and stirring was continued for another 22 h at room temperature. The mimGO was separated from the reaction mixture by centrifugation, washed with DI water and acetone and was subsequently dried in a freeze-dryer. Preparation of mimGO Sponge. The mimGO sponge was prepared by freeze-drying technique.40 The mimGO was dispersed in DI water (1 mg/mL) by sonication for 3 h. A polypropylene tube, containing a dispersed solution of mimGO in water, was vertically

⎛ C − Ct ⎞ %dye removed = ⎜ 0 ⎟ × 100 ⎝ C0 ⎠

qe =

(C0 − Ce)V m

(1)

(2)

Where C0 and Ce (ppm) are initial and equilibrium concentration of the dye, respectively. Ct (ppm) is the dye concentration in aqueous solution at time t (min). qe (mg/g) is the amount of dye adsorbed at 6027


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ACS Sustainable Chemistry & Engineering equilibrium per unit mass of the adsorbent, m is the mass of adsorbent (g) used and V is the volume of solution (L). Minimum time required to reach equilibrium and optimum adsorbent amount were obtained from adsorption kinetics curves. The adsorption studies in the control experiments, effect of initial solution pH and feed concentration on dye removal capacity were carried out using a similar procedure using 0.4 g/L optimum adsorbent loading. Adsorption behavior and maximum adsorption capacity of mimGO was obtained from adsorption equilibrium experiments using series of different concentration of dye aqueous solution ranging from 25 to 400 ppm. Ethylenediamine-functionalized graphene oxide (EDA-GO), unmodified GO sponge and commercial activated carbon (AC) were used as adsorbents for DR80 removal for comparison of adsorption capacity. EDA-GO was synthesized according to reported literature.39 The EDA-GO sponge and GO sponge were prepared using the same procedure as that for the mimGO sponge and used for adsorption batches. Desorption Experiments. For desorption studies, 0.4 g/L of adsorbent was added to a conical flask containing 100 mL of DR80 aqueous solution (25 ppm, pH 2). After a predetermined equilibrium time, dye laden adsorbent was separated from the aqueous solution. Aqueous solution of pH 12 was used as eluent to regenerate the mimGO sponge. Dye loaded adsorbent was washed with 10 mL of aqueous solution of pH 12 (10% volume of total adsorption batch) to desorb the dye and separated from the solution. The concentration of dye released was determined as mentioned earlier. The percentage dye desorption was calculated using the mass balance equation:

total concentration after elution desorption (%) = × 100 total conentration before elution

Figure 1. (a) SEM image of GO sponge. (b) SEM image of mimGO sponge. The insets display the actual photographs of GO sponge and mimGO sponge, respectively. (c) XRD spectra of graphite, GO sponge and mimGO sponge. (d) SEM-EDX nitrogen mapping of mimGO sponge.

Scheme 1 and its molecular structure was confirmed using 1H NMR analysis [1H NMR (200 MHz, DMSO): δ 8.12 (s, 1H), 6.71−6.63 (d, 2H), 2.73 (s, 3H)]. These results show the successful synthesis of mimNH2. GO was prepared using a modified Hummer’s method. The mimNH2 was grafted on GO surface through the coupling of amine groups of mimNH2 and carboxylic acid groups of GO as shown in Scheme 2, introducing cationic imidazole ring and amine groups onto the GO, which was expected to improve dye adsorption efficiency. The mimGO sponge was freeze-dried in liquid N2, resulting in a porous spongy structure (surface area = 31.71 m2/g, pore volume = 0.22 cm3/g and pore diameter = 46.5 nm) that could be dispersed easily in aqueous solutions. Detailed N2 adsorption−desorption isotherm curves are shown in Figure S1. The SEM image of GO (Figure 1a) shows relatively smooth and compact structure due to the interaction of various oxygencontaining functional groups, whereas mimGO sponge in Figure 1b exhibits hierarchical spongy structure, preventing agglomeration of GO sheets, due to porous structure. Variation in stacking characteristics of GO on ionic liquid functionaliza-

(3)

Subsequently, the adsorbent was washed with DI water until supernatant became neutral. After washing, the regenerated adsorbent was dried for 12 h in a freeze-dryer and subjected to the further adsorption−desorption cycles.

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RESULTS AND DISCUSSION Synthesis and Characterization of Adsorbents. Imidazolium ionic liquid (mimNH2) was synthesized as illustrated in Scheme 1. Synthesis of mimNH2

Scheme 2. Synthesis of mimGO

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Figure 5. (a) ζ-Potential of mimGO sponge at different pH values. (b) Effect of dye solution pH on dye removal efficiency. (mimGO sponge dosage = 0.4 g/L, 50 ppm solution, T = 23 °C).

Figure 2. (a) Survey XPS spectra of GO sponge and mimGO sponge. (b) C 1s XPS spectra of GO sponge (c) C 1s XPS spectra of mimGO sponge. (d) N 1s XPS spectra of mimGO sponge. Figure 6. (a) Effect of mimGO sponge dosage on the removal of DR80. (50 ppm solution, pH 2, T = 23 °C). (b) Pseudo-second-order kinetic plot of DR80 adsorption at varying mimGO sponge loading.

tion was characterized using XRD as shown in Figure 1c. A sharp peak at 2θ = 10.1°, d-spacing of 0.85 nm, without a graphitic peak at 2θ = 26.7° confirmed the successful synthesis of GO. Functionalization of GO with mimNH2 increased the dspacing to 0.92 nm, indicating insertion of ionic liquid moieties in the interlayer space between GO sheets. Increased d-spacing and presence of ionic liquid sites on mimGO sponge was expected to enhance its dispersion in the aqueous dye solution and thus improving adsorption efficiency. The distribution of nitrogen in mimGO sponge was characterized by EDX and is shown in Figure 1d. Bright spots in the image show uniform dispersion of nitrogen functionalities on the mimGO sponge. This uniform grafting of imidazolium ionic liquid on lateral dimension of GO sheets is expected to enhance its dispersion. Survey XPS spectra of mimGO sponge (Figure 2a) shows a nitrogen peak at ∼401.6 eV binding energy (3.44 at. %), confirming covalent functionalization of GO with mimNH2. In Figure 2b, high-resolution C 1s XPS spectra of GO sponge show three major components. The peak at ∼284.6 eV is due to the CC bonds (sp2 hybridized graphene carbon atoms); the peak at ∼286.9 eV is due to the CO bonds and the peak at ∼288.2 eV is assigned to CO bonds. Whereas C 1s XPS spectra of mimGO sponge as shown in Figure 2c displayed a little reduction of GO. The C/O ratio was determined from the ratio of fractional areas of C 1s and O 1s peaks and used to

Figure 3. (a) FT-IR spectra of GO sponge, mimGO sponge, DR80 and DR80 laden mimGO. (b) Raman spectra’s of GO sponge and mimGO sponge.

Figure 4. Adsorption rate of DR80 onto different adsorbents (adsorbent dosage = 0.4 g/L, 50 ppm solution, pH 2, T = 23 °C). The inset photograph shows decrease in DR80 concentration using mimGO sponge adsorbent.

Table 1. Initial Rate of DR80 Adsorption Based on Pseudo-Second-Order Kinetic Parameters onto Different Adsorbent at 0.4 g/L Loading Adsorbent

qe (exp.) (mg/g)

k2 (g/(mg·min))

qe (cal.) (mg/g)

R2

V0 (mg/(g·min))

mimGO sponge EDA-GO sponge GO sponge AC

127.3 79.6 20.1 20.8

0.036 0.011 0.104 0.050

128.2 80.0 19.8 21.1

0.9996 0.9877 0.9944 0.9922

588.2 73.0 40.5 22.2

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ACS Sustainable Chemistry & Engineering Table 2. Kinetic Parameters for Adsorption of DR80 onto mimGO Sponge at Different Adsorbent Loading pseudo-first order

pseudo-second order

adsorbent loading (g/L)

qe (exp.) (mg/g)

qe (cal.) (mg/g)

k1

R2

qe (cal.) (mg/g)

k2

R2

0.1 0.25 0.4 0.5 1

211.3 168.5 127.3 103.0 47.4

201.8 135.6 34.3 6.1 3.6

0.229 0.221 0.294 0.163 0.139

0.9079 0.9408 0.6498 0.2574 0.301

217.4 172.4 128.2 103.1 47.4

0.004 0.006 0.036 0.235 0.557

0.9885 0.9935 0.9996 1 1

Figure 7. (a) Effect of initial DR80 dye concentration on dye removal efficiency (mimGO sponge dosage 0.4 g/L, pH 2 and T = 23 °C). (b) Equilibrium adsorption capacity of mimGO sponge plotted against initial DR80 concentration. (c) Langmuir adsorption isotherm for adsorption of DR80 onto mimGO sponge.

Table 3. Isotherm Parameters for Adsorption of DR80 onto mimGO Sponge at Equilibrium Condition Langmuir model

Freundlich model

Tempkin model

D-R model

dye

qmax

KL

R2

1/n

KF

R2

B

KT

R2

qs

E

R2

DR80

500.0

3.3

0.9999

0.26

8.5

0.6785

71.1

10.8

0.8611

236.1

100.0

0.1061

(CS stretching) and a peak at 1109 cm−1 (asymmetric sulfonate stretching), indicating an interaction between DR80 and mimGO sponge.43 Moreover, the intensity of band due to asymmetric sulfonate stretching (in DR80 spectra) and CN stretching (in mimGO sponge spectra) was reduced, which proved the interactions between adsorbed dye anions (−SO3−) and ionic liquid cations (N+) and protonated amines (H+) on mimGO sponge carried out through the sulfonate groups.43 Further, Raman spectra was used to observe the effect of ionic liquid functionalization on GO (Figure 3b). GO sponge displayed an eminent broad G band at 1584.5 cm −1 corresponding to E2g mode for in-plane vibrations of sp2hybridized carbon atoms. A prominent D band at 1338 cm−1 corresponding to A1g symmetry mode associated with structural defects and amorphous carbon was also observed. Slight broadening of the D band at 1341 cm−1 and G band at 1574.5 cm−1 was observed postfunctionalization. The G band position of mimGO sponge was red-shifted down (10 cm−1) with almost the same intensity ratio (ratio = ID/IG, 0.98 to 1.00) as observed for GO sponge, indicating a little reduction in material. Grafting of amine moieties on GO induces the positive inductive effect of amine group attached to the GO surface, which acts as an electron donor leading to a downshift in the G band position.44,45 The successful functionalization of GO with mimNH2 was also confirmed by thermogravimetric analysis (TGA) as shown in Figure S2. GO was thermally unstable and lost mass upon heating even below 100 °C due to evaporation of surface moisture and water content. Two significant drops in mass around 200 and 300 °C occurred. The former was due to the

define the reduction of GO. An increase in C/O ratio of mimGO sponge (3.20) compared to that of GO sponge (1.98) due to the elimination of few oxygen functional groups. In C 1s XPS spectra of mimGO sponge, a substantial decrease in the amount of C in carboxylic acid groups was observed. Two carbon−nitrogen peaks, CN (∼285.4 eV) and NCO (∼288.7 eV) appeared after functionalization. CN and N CO binding energies are due to the formation of amide bonds between mimNH2 and the carboxylic acid groups present on the surface of GO. These C bound nitrogen components were also confirmed by the high-resolution N 1s XPS spectra of mimGO sponge as shown in Figure 2d, which contain two major peaks: one at ∼401.6 eV corresponding to imidazole ring nitrogen, and another shoulder at a lower binding energy of ∼399.6 eV corresponding to amide nitrogen. Changes in the characteristic absorption of the functional groups were characterized by FT-IR spectroscopy as shown in Figure 3a. A prominent band at 1728 cm−1 (CO stretching), 1628 cm−1 (aromatic CC stretching) and a peak at 3416 cm−1 (OH stretching vibration) confirmed the successful synthesis of GO. The FT-IR spectra of mimGO sponge showed the amide-carboxyl stretching at 1640 cm−1; peaks at 1550 and 1090 cm−1 were attributed to amide NH bending and CN stretching, respectively, confirming the formation of the amide linkage between GO and mimNH2, in agreement with the XPS results. Additionally, a new peak at 1167 cm−1 was observed due to the in-plane asymmetric stretching arising from imidazolium ring,41,42 as an evidence of attachment of mimNH2 to GO surface. DR80-laden mimGO sponge showed new peaks at 1473 cm−1 (aromatic ring vibrations), 614 cm−1 6030


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ACS Sustainable Chemistry & Engineering Table 4. DR80 Dye Maximum Adsorption Capacities of State-of-the-Art Adsorbents adsorbent

adsorption capacity (mg/g)

adsorbent loading (g/L)

mimGO sponge

501.3a

0.4

2

23

97.9a

0.4

2

23

17.4b 120.5b

0.3 0.4

7.5 7.5

25 25

GO sponge CNT surfactantfunctionalized CNT surfactantfunctionalized AC amino-silica NPs sodium alginatetitania NPs soy meal hull canola hull mixture almond shells orange peel egg shell membrane polyurethane foam Mentha pulegium cross-linked chitosan beads TEPA modified peanut shell TETA modified peanut shell DETA modified peanut shell

pH at qmax

temp. (°C)

reference present study present study 50 50

b

0.2

6.5

20

43

41.2b 163.9b

0.4 0.5

2 2

25 25

51 52

178.6b 8.7b 22.4b

0.3 2.0 3.2

2 2.5 6

20 25 20

53 54 55

21.0b 161.3b

4.0 0.8

2 2

25 20

56 57

4.5b

5.3

2

25

58

b

52.4 610.5b

2.0 60.0

2.5 5.5

25 37

59 60

b

0.2

2

45

61

500.0b

0.2

2

45

61

b

0.2

2

45

61

274.7

561.8

502.5

Figure 9. (a) Photo of original DR80 aqueous solution (25 ppm, pH 2). (b) Supernatant of the dye aqueous solution after adsorption by mimGO sponge. (c) Supernatant of the dye aqueous solution of pH 12 after desorption.

Figure 10. Removal efficiency of DR80 at 25 ppm initial concentration on mimGO sponge in different cycles.

decomposition of the labile oxygen-containing functional groups, yielding CO, CO2 and steam, whereas the latter was ascribed to pyrolysis of GO’s carbon skeleton.46 mimGO, having fewer thermally labile oxygen functional groups, showed a gradual mass loss in the range of 150−300 °C that was

a Experimentally obtained maximum adsorption capacity. bMaximum adsorption capacity obtained from Langmuir adsorption isotherm.

Figure 8. Adsorption mechanism of DR80 on the mimGO sponge. 6031


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mimGO can be used in the harsh environments due to its high acid tolerance. Optimization of adsorbent dosage is crucial to determine the minimum amount required for reaching the maximum adsorption. DR80 adsorption kinetics on varying mimGO sponge dosage was investigated. Figure 6a shows that the dye removal efficiency of DR80 increased, as the mimGO sponge dosage increased. At a mimGO sponge loading of 0.4 g/L and above, about 99.9% DR80 was removed from 50 ppm solution in 20 min of contact time. Considering the dye adsorption behavior and removal efficiency, the optimum adsorbent loading was maintained at 0.4 g/L for further adsorption experiments. To interpret dynamic adsorption characteristics of DR80 on the mimGO sponge, two kinetic models, pseudo-first order and pseudo-second order, were used to investigate adsorption mechanism and rate controlling step. Detailed analysis of kinetic equations are given in the Supporting Information. Figure 6b shows a linear plot for adsorption of DR80 on the mimGO sponge for pseudo-second-order kinetic model, which gave good agreement between (R2 = 0.9996) the experimental and calculated adsorption capacity, qe, dye adsorbed at equilibrium (Table 2). Therefore, surface adsorption is the rate-limiting step in the DR80 removal process. Further, Figure 7a reveals that as the initial dye solution concentration increased from 25 to 100 ppm, DR80 removal efficiency decreased from 99.9% to 78.0% (after 20 min of contact time). Because the occupation of vacant surface sites on the adsorbent in initial stage is very fast but, as time progresses, adsorption becomes more difficult due to repulsive forces between adsorbed dye molecules and bulk phase dye molecules.9 Conversely, the higher initial concentration of dye solution (up to 400 ppm) increased the dye adsorption capacity per unit mass of adsorbent (Figure 7b), which is further explored in equilibrium adsorption study for fitting different adsorption isotherms. Adsorption isotherm models explain how dye molecules and adsorbent interacts with each other. To provide insights into adsorption behavior, four isotherm models, Langmuir, Freundlich, Tempkin and Dubinin−Radushkevich (D-R), were applied to equilibrium adsorption experimental data. Details of the isotherm models equation are given in the Supporting Information. The adsorption isotherm parameters estimated from isotherm models are listed in Table 3. Among four isotherm models studied, the Langmuir isotherm model showed a best fit (R2 = 0.9999) for adsorption of DR80 onto the mimGO sponge. Therefore, monolayer adsorption of the DR80 molecule has occurred on structurally and energetically homogeneous active sites present on the outer surface of the mimGO sponge. As illustrated in Figure 7c, the maximal DR80 adsorption capacity of the mimGO sponge using the Langmuir isotherm model has been calculated to be 500.0 mg/g, which is in good agreement with experimentally observed adsorption capacity (501.3 mg/g) (Table 4). Moreover, the mimGO sponge showed higher adsorption capacity for azo dyes compared to other hierarchical carbon−nitrogen-based hybrid materials.49 This high adsorption capacity can be attributed to abundant cationic functional sites present on mimGO sponge. Adsorption Mechanism. The charge-induced adsorption mechanism of DR80 on the mimGO sponge is illustrated in Figure 8. The chemical structure of DR80 dye contains multiple azo-linkages having rich aromatic rings and anionic sulfonic acid ions, whereas the hierarchical mimGO sponge has cationic

attributed to the decomposition of surface-attached mimNH2. The detailed mass loss percentage at different temperature interval was calculated, and the results are shown in Table S1. Removal of Dye by Adsorbent. The mimGO sponge having cationic imidazole ring and presence of amine functionalities can be adopted as a promising adsorbent for anionic dye removal. The efficiency of mimGO sponge for removal of direct red 80 (DR80, chemical information and calibration curve are given in Table S2 and Figure S3, respectively) from aqueous solutions was studied. Once the mimGO sponge was dropped in the DR80/aqueous solution, it immediately absorbed the dye. This ultrafast adsorption of DR80 on the mimGO sponge achieved equilibrium in just 10 min, which was much faster than those of previously reported ionic-liquid-functionalized carbon-based adsorbents.16,36,47 Figure 4 shows that about 99.9% of DR80 was removed from aqueous solution within 20 min at ambient temperature. After separation of mimGO sponge, the color of DR80/aqueous phase turned colorless (inset of Figure 4). Moreover, EDA-GO sponge, GO sponge and commercially available activated carbon (AC), used for comparison, showed a much lower DR80 removal (68.1% for EDA-GO sponge, 15.3% for GO sponge and 16.0% for AC in 20 min of contact time), due to lack of cationic ionic liquid functionalities and aggregation of the material in solution. The data was fitted to a pseudosecond-order kinetic model, and the parameters are presented in Table 1. Initial adsorption rate of DR80 on mimGO sponge, EDA-GO sponge, GO sponge and AC was 588.2, 73.0, 40.5 and 22.2 mg/(g·min), respectively. These results demonstrate the effect of imidazolium ionic liquid for DR80 adsorption. The ultrafast adsorption phenomenon can be explained by stronger driving forces between anionic sulfonic acid on DR80 molecules and cationic ionic liquid functional sites (protonated amine groups) of the mimGO surface under acidic environment. Charge-induced adsorption driving forces between adsorbent and adsorbate are influenced by the surrounding environment.10,48 Therefore, the effect of the initial solution pH on the removal of DR80 was further investigated and discussed based on point of zero charge (pHzpc) of the mimGO sponge. pHzpc is the pH at which the adsorbent surface charge becomes zero. It was determined by measuring the ζ-potential of the mimGO sponge at different pH values, shown in Figure 5a. The pHZPC of the mimGO sponge was 7.7, indicating that positively charge groups are predominant on the surface of the adsorbents, and below pH = 7.7, the adsorbent is most suitable for the adsorption of an anionic dye. Further, Figure 5b shows that the adsorption of DR80 on mimGO was remarkably pH dependent. The decrease in solution pH below the pHzpc value drastically increased the DR80 removal efficiency of the mimGO sponge. After 20 min of contact time, the DR80 dye removal percentage at pH 2 is 99.9%, whereas at pH 4 it reached up to 44.3%. Further, dye removal efficiency of the mimGO sponge at pH 5.9 (without pH adjustment of DR80 aqueous solution) reached to 31.2%, and at pH 7 it was about 24.8%, whereas almost negligible adsorption was observed at, above pHzpc value, pH 12. At a solution pH lower than pHzpc of the mimGO sponge, the synergetic effect on DR80 dye removal occurred due to cationic charge on both imidazole rings and protonated amine groups. In acidic conditions, sulfonic groups on DR80 are mainly in the form of SO3−, and cationic imidazolium rings and protonated amine groups on mimGO facilitate the strong electrostatic interactions. It is expected that 6032



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imidazolium ionic liquid, amide linkages and hydroxyl functionalities. The mimGO sponge acquires positive charges at neutral-low pH (below pHPZC) due to the protonation of ionic liquid moieties, amide linkages as well as unreacted carboxylic groups. Because of strong electrostatic interactions between sulfonic acid groups from DR80 and mimGO sponge, the rate of adsorption is enhanced. Hydrogen bonds between hydroxyl and nitrogen containing groups in both DR80 and mimGO sponge exist. Further, π−π interactions between the phenyl rings of DR80 molecules and mimGO surface also contribute toward adsorption. Therefore, dominant electrostatic interactions, enhanced hydrogen-bonding and π−π interactions contribute to ultrafast adsorption and high adsorption capacity for DR80 on the mimGO sponge. Although π−π interactions and hydrogen bonds exist in unmodified GO, the electrostatic interactions between sulfonic acid groups from DR80 and nitrogen groups in the mimGO sponge are absent, contributing to a lower rate of adsorption and lower adsorption capacity, as was observed. Desorption and Reuse of Adsorbent. The larger lateral dimension of mimGO enables the hierarchical structure to be separated and easily regenerated. Performance evaluation and regeneration ability of the adsorbent was carried out by performing DR80 adsorption−desorption cycles at fixed adsorbent dose of 0.4 g/L, initial dye concentration 25 ppm at pH 2. Initially, adsorption of DR80 onto the mimGO sponge at predetermined equilibrium time was carried out and the adsorbent was separated. Desorption of 99.4% of the loaded dye was achieved in aqueous solution of pH 12. The corresponding photograph of dye solutions in adsorption− desorption process is shown in Figure 9. The elution of the DR80 from the mimGO sponge can be explained on the basis of deprotonation of the imidazolium cations and amine groups under alkaline conditions. The number of negatively charged sites were increased and electrostatic repulsion exists between the negatively charged surface of the adsorbent and anionic dye. Further, Figure 10 shows the removal percentage of DR80 at the first adsorption−desorption cycle is 99.9%, and after the fourth cycle is 99.2%. These adsorption−desorption results demonstrate that the mimGO sponge remained effective even after multiple reuses.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00867. Synthesis of graphene oxide, detail of adsorption kinetic models and adsorption isotherm models, TGA analysis result of GO sponge and mimGO sponge, chemical information on DR80 dye, N2 adsorption−desorption isotherms of mimGO sponge, calibration curve of DR80 dye aqueous solution (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*J. Antony Prince. E-mail: [email protected]; Tel: +65 64607569. *P. R. Nemade. Email: [email protected]; Tel: +91 (22) 3361 2027. ORCID

Rahul Zambare: 0000-0003-1024-3252 James Selvaraj Antony Prince: 0000-0001-7213-8226 Parag Nemade: 0000-0003-4680-1084 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a grant from the Singapore National Research Foundation under the Incentive for Research and Innovation Scheme administered by PUB (1501-IRIS-05). R.S.Z. and P.R.N. thank the University Grants Commission of India for support under UGC-Special Assistant Program and UGC-Faculty Research Promotion Scheme.

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REFERENCES

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CONCLUSIONS We synthesized a hierarchical graphene-based adsorbent (mimGO sponge) via a facile amidation reaction between imidazolium-based ionic liquid and carboxylic acid groups on GO surface. Compared with other conventional adsorbents, the mimGO sponge displayed an ultrafast dye adsorption rate of 588.2 mg/(g·min), adsorption capacity of 501.3 mg/g and facile regeneration. Anionic DR80 dye can be effectively removed (99.9%) at low pH, which follows the charge-induced adsorption mechanism with the aid of protonated amine groups and cationic imidazolium ionic liquid on the mimGO sponge. The mimGO was easily regenerated under the alkaline conditions and dye removal efficiency was 99.2% after the fourth adsorption cycle. In view of the simple preparation method of the adsorbent, its reusability and its ultrafast adsorption rate, it is expected that a versatile mimGO sponge can be applied for wastewater remediation through different techniques. 6033


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Ultrafast Dye Removal Using Ionic LiquidâGraphene Oxide Sponge

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