Hydrogel of the Supramolecular Complex of Graphene Oxide and Sulfonatocalix[4]arene as Reusable Material for the Degradation of Organic Dyes: Demonstration of Adsorption and Degradation by Spectroscopy and Microscopy.

Industrial modernization causes severe contamination of water resources due to which the presence of organic dyes poses a great threat to human life. To address this, we have synthesized a hydrogel GSCg using graphene oxide (GO), sulphonatocalix[4]arene (SC4a), and l-Cys by heating at 90 °C for 30 min and characterized by analytic, spectroscopy, and microscopy techniques. The GSCg possessing porous structure and adsorbs all three types of dyes, viz., eosin yellow (anionic), neutral red, and methylene blue (cationic), as shown by scanning electron microscopy, and the adsorption kinetics are addressed. The dye adsorbed by the gel (dye@GSCg) has been degraded by the treatment of Cu2+/N2H4, which regenerates the gel. The regenerated gel has been demonstrated for further cycles of adsorption followed by degradation. Alternatively, the degradation of the organic dyes was also demonstrated by an in situ approach by taking GO, SC4a, l-Cys, and the organic dye together and subjecting the mixture to hydrothermal conditions and the process leaves out free gel (GSCg d). This was proven to be true in the case of each of the 12 dyes studied individually and also for their mixture, supporting that this methodology can be employed for large scale purification of contaminated water with high efficiency. GSCg d was repeatedly used for the adsorption and degradation (with the use of Cu2+/N2H4) cycles wherein the gel does not lose its adsorption capability even after several cycles. Therefore, {GO···SC4a} hybrid is a smart, sustainable, and reusable material suitable for the purification of water contaminated with industrial organic dye effluents.

Introduction

Industries based on textile, paper, plastic, leather, and cosmetics continuously discharge several contaminants into the environment, in which the major proportion is organic dyes. This causes severe pollution and thereby poses a great threat to the life of plants, animals, and human.[1,2] These contaminants are carcinogenic and mutagenic and cause respiratory diseases, skin dermatitis, and eye irritation.[3] The sustainable means of purification of such water resources from the contamination of organic dyes is in high demand. In the literature, various photocatalysts, reagents like H2O2 and NaBH4 are used for the degradation of organic dyes.[4−9] For adsorption of dyes, the adsorbents, such as nanomaterials based on proteins and organic molecules, hydrogels, zeolites, carbon nanotubes, activated carbon, functionalized graphene, and chitosan, are reported in the literature.[10−20] Among these, graphene/graphene oxides (GOs) are widely utilized materials because of their large surface area, high mechanical strength, and thermal stability and hence are used in designing different materials suitable for application in optoelectronics, biosensors, catalysis, and electrochemistry.[21−26]

On the other hand, the hydrogels are generally formed by cross-linking of the corresponding monomer unit and these are used in drug storage and delivery, sensing, and water purification.[27−31] The hydrogels give an advantage with regard to water purification as the material can be easily separated out from the medium after the treatment. For additional advantages, such as high surface area and easy separation, the graphene-oxide-based hydrogel would be of immense importance and such a material has been synthesized for the adsorption of organic dyes.[30,31] Therefore, in this paper, we report a porous hydrogel (GSCg) synthesized from the functionalization of graphene oxide (GO) with sulphonatocalix[4]arene (SC4a) at a relatively lower gelator concentration. The details of the adsorption and degradation of organic dyes have been demonstrated by analytic, spectroscopy, and microscopy techniques, and our methodology has been applied successfully to purify bulk water containing 12 dyes together.

Results and Discussion

Formation of the Gel, GSCg

The functionalization of GO and/or rGO renders it to interact with organic molecules through noncovalent interactions.[32−34] Therefore, GO is expected to form a supramolecular assembly with SC4a to give {GO···SC4a} via π···π interactions extended between the π segment of SC4a and the highly conjugated π system of GO. All of this is expected to simplify the requirements for the gelation. Thus, both GO and SC4a were mixed together in aqueous form, and the gelation was initiated by the addition of l-Cys followed by heating. The use of l-Cys in the gelation stems from a literature report where the use of a mixture of GO, l-Cys, and ammonia resulted in the formation of the gel in 3 h when heated at 90 °C.[33] After going through the corresponding experimental iterations given in Figure S01, the optimal parameters for the GSCg formation are determined and these are (i) a 1:3 w/w ratio of GO vs SC4a, (ii) 2 mg/mL gelator concentration, (iii) 30 min of reaction time, and (iv) 3 mg of l-Cys as binder. The well-formed gel is separated out of the aqueous medium.

The modification of GO by SC4a decreases the gelation time drastically from 3 h that is reported in the literature to 30 min in the present case. No ammonia was used, and only a low concentration of l-Cys was used in our case. The decrease in the gelation time is primarily attributed to the enhanced interactions between GO and SC4a. It is known in the literature that the heating of l-Cys releases H2S,[35,36] which acts as a reducing agent and will covalently cross-link the {GO···SC4a} hybrid via the sulfide (C–S–C) linkage, as depicted in Figure a, and the gel turns less hydrophilic and hence separates out of the medium. The control experiment was carried out at room temperature with only the GO but without SC4a or l-Cys did not yield any gel. The gel was confirmed by inverting the glass slide and sustains a weight ∼2500 times its own weight with a ∼40% compression in the height, as can be noticed from Figure .

Figure 1

(a) Schematic representation for the formation of the gel GSCg. Photographs of the gel GSCg on the glass slide (b) before and (c) after inversion. Photographs showing the stress sustainability of GSCg: (d) before (normal state), (e) during, and (f) after the stress is removed. Stress of 50 g of weight is applied.

Characterization of the Gel, GSCg

The GSCg gel was characterized by analytic, spectroscopy, microscopy, and diffraction techniques. The thermogravimetric analysis (TGA) reveals that almost 90% of the total weight of the gel is water (Figure a). Further, the gel was freeze-dried to remove the water and was analyzed. Both SC4a and GO exhibited ∼20 and ∼15% weight losses, respectively, up to 100 °C, which correspond to the loss of water. An additional 30% weight loss was observed in both the GO (up to 330 °C) and SC4a (up to 270 °C) due to pyrolysis of the functional groups. However, in the case of the gel GSCg, the weight loss is ∼75% up to 300 °C. The functionalization of GO with SC4a is supported by comparing the IR spectra of GO and GSCg (Figure b). The peak corresponding to the C=C stretching frequency was shifted from 1620 to 1626 cm–1 on going from GO to the gel GSCg. The presence of νS–O vibration of the sulfonato group at 1040 cm–1 in the gel indicates the attachment of SC4a to graphene oxide. As given in Figure c, the dispersion of GO shows two absorption bands, one at 230 nm and the other at 300 nm, whereas SC4a shows single absorption band at 277 nm. In the case of GSCg, only one absorption band was observed at 282 nm, supporting the presence of SC4a. The diminishing of the absorption band observed at 300 nm supports that the GO is successfully modified to its reduced state. The X-ray diffractograms clearly support that the SC4a is highly crystalline and GO is partially crystalline as compared with the gel (Figure d). The X-ray photoelectron spectroscopy (XPS) spectrum of S 2p of {GO···SC4a} hybrid exhibits two bands, one at 166.4 (2p3/2) and the other at 167.7 eV (2p1/2), and these peaks correspond to the ‘S’ from sulfonato groups. In addition, the gel shows a new band at 163.4 eV (Figure e), which supports the presence of the sulfide linkage (C–S–C) that results in the formation of a graphene oxide-based three-dimensional (3-D) architecture. Whereas the G and D Raman bands of graphene oxide are observed at 1590 and 1350 cm–1, respectively, in GO, these were shifted to 1575 and 1355 cm–1 in the GSCg gel (Figure f). The 15 cm–1 blue shift observed in the G band is attributed to the thermal reduction of GO during the gelation. Even the integrated intensity ratio (ID/IG) increases from 1.05 to 1.23 upon gelation, which is attributed to an increase in the number of sp3 defects.[37]

Figure 2

(a) TGA thermogram GO (red line), SC4a (blue line), hydro-GSCg (cyan line), and dried GSCg (black line). (b) Fourier transform infrared (FTIR) spectra, (c) absorption spectra, and (d) X-ray diffraction (XRD) pattern of GO, SC4a, and GSCg. (e) XPS spectra in the sulfur region for {GO···SC4a} (upper panel) and GSCg (lower panel). (f) Raman spectra of GO (lower panel) and GSCg (upper panel). Deconvoluted peaks are seen. The color code is the same for (a)–(d).

Scanning electron microscopy (SEM) images of GSCg were recorded in the cryo mode to obtain its surface morphology, and the pore structure of GSCg is clearly seen in the images given in Figure a. The size of the pores varies from 0.3 to 1.6 μm with a weighted average of 0.84 μm (Figure b). To support that the 3-D pore network is formed through the C–S–C link, elemental distribution, including of sulfur, was mapped by ESEM and the micrographs clearly show the distribution of all constituting elements, viz., C, O, and N, including S, all over the surface (Figure c–h).

Figure 3

Microscopy data for the gel GSCg: (a) SEM micrograph, (b) histogram of pore size distribution, and (c) ESEM micrograph. Elemental mapping: (d) oxygen, (e) carbon, (f) sulfur, (g) nitrogen, and (h) merged.

Adsorption Properties of Organic Dyes by GSCg

Owing to the porous nature of the gel, GSCg has been studied for its ability to adsorb organic dyes. Since GSCg exhibited adsorption of organic dyes irrespective of their charge, two cationic (MB, CV), two anionic (MO, EY), and one neutral dye (NR) were chosen for the study. Photographs of the vials given in Figure a reveal that all five dyes studied are decolorized after the adsorption by GSCg. The dye-adsorbed gel is referred as dye@GSCg, where the dye used is MB, CV, NR, MO, or EY. The decrease in the absorbance of the supernatant solution with time shows that the dye is adsorbed by the gel (Figures b,c and S02). According to the absorption spectral data obtained, it is understood that almost 80–90% of all dyes are removed from solutions within 10 h of incubation (Figure d). The time taken for 50% removal (T50, min) of the dye follows a trend, viz., EY (220) > MO (169) ∼ MB (168) > NR (90) > CV (25), suggesting that the process is not dependent on the charge of the dye and there is a 9-fold decrease in the T50 on going from EY to CV (Figure e). At saturation (i.e., 3× highest T50), the percent of dye removal is marginally higher in the case of cationic and neutral dyes as compared with the anionic ones.

Figure 4

(a) Photographs of the vials before (upper panel) and after (lower panel) the adsorption of the dyes over a period of 10 h by the gel GSCg. Time-dependent (0–600 min) adsorption spectra of the supernatant solutions upon incubation with GSCg (b) for MB and (c) for EY. (d) Percentage removal of dyes from the supernatant upon treatment with GSCg. (e) Histogram of T50 (time taken for 50% adsorption) vs the studied organic dyes. (f) Absorption spectra of dispersed gels of dye@GSCg. Color code: EY (black), MO (red), NR (lime green), CV (blue), and MB (sky blue). SEM micrographs: (g) only GSCg, (h) NR@GSCg, (i) MB@GSCg, (j) MO@GSCg, (k) EY@GSCg, and (l) CV@GSCg.

The adsorption isotherm data for the EY@GSCg, NR@GSCg, and MB@GSCg fits to both the Langmuir as well as Frendluich isotherm models (Figure S03), whereas a better fit was observed in the case of Langmuir model (Table S1). The adsorption kinetic data for all of these three dyes follow a pseudo second order adsorption rate (Figure S04 and Table S1). qmax values obtained from the Langmuir model are 230, 526, and 426 mg/g for MB, NR, and EY, respectively. These are in good agreement with the experimentally obtained values of 225, 491, and 405 mg/g, respectively. The maximum adsorption capacity for GSCg was compared to that of already known graphene/ graphene oxide-based hydro-/aerogels in the literature, as given in Table . Whereas some of the literature is concerned with the adsorption of the cationic dyes,[38−41] a few papers report the adsorption of cationic as well as anionic dyes.[42−44] The GSCg reported in this paper shows the adsorption of all three charge types of dyes and exhibits a higher qmax as compared with these reports in the literature.

Table 1

Comparison of the Maximum Adsorption Capacity (qmax) of the Gel GSCg to That of Other Graphene-Based Hydro/Aerogels Reported in the Literature

adsorbent	organic dye	adsorption capacity (mg/g)	references
rGO/AG hydrogel	cationic (MG)	242	(38)
GO/AG hydrogel	cationic (MG)	186
GO/PEI hydrogel	cationic (MB)	∼325a	(39)
	cationic (RhB)	∼125a
reduced graphene oxide hydrogel	cationic (MB)	7.85	(40)
	cationic (RhB)	29.44
mGO	cationic (MB)	188.32	(41)
mGO/PVA-CG	cationic (MB)	270.94
graphene/Ag3PO4	cationic (MB)	84	(42)
	anionic (MO)	40
graphene oxide—chitosan hydrogel	cationic (MB)	390	(43)
	anionic (EY)	326
ascorbic acid reduced graphene hydrogel	cationic (MB)	∼170a	(44)
	anionic (MO)	∼150a
graphene oxide—SC4a hybrid hydrogel (GSCg)	cationic (MB)	225	this work
	anionic (EY)	405
	neutral (NR)	526
GSCgd	cationic (MB)	305	this work
	anionic (EY)	515
	neutral (NR)	610

The maximum adsorption capacity value was not given in the main text of the corresponding paper. Therefore, the same has been extrapolated from Figures S1 and 7, respectively, in the case of refs (39) and (44).

Characterization of Dye@GSCg

The absorption spectra of dye@GSCg showed the bands corresponding to the dye (400–660 nm) as well as to SC4a (280 nm) (Figure f). The GSCg exhibit well defined the 3-D pore network on the basis of SEM micrographs. The dye-adsorbed gel exhibits fibril-like structures along the 3-D pore network of GSCg, as shown in the case of GSCg, MB@GSCg, MO@GSCg, NR@GSCg, EY@GSCg, and CV@GSCg (Figure g–l) by SEM.

To support that the dye was captured by the gel, i.e., dye@GSCg, FTIR, XRD, and Raman spectra were recorded and compared to those of the controls. In the fingerprint region in FTIR spectra, some additional peaks (Figure a,d) were observed upon the adsorption of EY and NR. The powder XRD for EY@GSCg supports the presence of both components when compared with the XRD pattern of EY and GSCg (Figure b), and similar features were observed even for NR@GSCg (Figure e). In Raman spectra, two distinct bands at 642 and 714 cm–1 were observed in the case of EY and the same were also present in EY@GSCg. Along with this, some additional peaks were present in the characteristic D and G bands of EY@GSCg and NR@GSCg as compared with only GSCg (Figure c,f).

Figure 5

Spectroscopy studies for the adsorption of EY by the gel GSCg: (a) FTIR, (b) XRD pattern, and (c) Raman spectra. (d)–(f) are the same as (a)–(c), respectively, where the dye is NR instead of EY. Color code: GSCg (black), dye (blue), and dye@GSCg (red).

Recyclability of Adsorption vs Degradation of Organic Dyes by GSCg

The recyclability has been demonstrated in the case of the MB dye. To reuse the gel GSCg for the adsorption of another cycle of organic dyes, MB@GSCg was dipped in a solution of CuSO4·5H2O (15 mM, 1.8 mL) to which 200 μL of hydrazine was added and rotated on a tube rotator for 5 h. The dye adsorbed by the gel is degraded at this stage, as can be noticed from the SEM micrographs given in Figure d upon comparison with Figure c. At this stage, the gel was washed with water and reused for the adsorption of MB again. The whole protocol was repeated five times, and no change in the adsorption capacity of GSCg was observed, as understood from Figure a,b.

Figure 6

Recyclability: (a) UV–vis absorption spectra of supernatant solution in the case of MB upon adsorption by the gel GSCg over a period of 10 h. Color code: initial (black), cycle_1 (red), cycle_2 (blue), cycle_3 (dark cyan), cycle_4 (magenta), and cycle_5 (dark yellow). (b) Histogram of the percentage of dye adsorbed vs number of adsorption–degradation cycles for MB by the gel GSCg. SEM micrographs for the MB@GSCg (c) before and (d) after the degradation of the dye carried out in presence of Cu2+/N2H4.

In Situ Degradation of Organic Dyes

To degrade the organic dyes in situ, the gelation protocol given in the Experimental Section was followed with the addition of dye solution. Thus, the ingredients, viz., {GO···SC4a} hybrid and l-Cys in presence of the dye, when heated at 90 °C result in the gelation while degrading the dye simultaneously. From the photographs of the vials given in Figure , the color of the respective dye is clearly visible before the gelation, whereas the supernatant solution is completely decolorized upon gelation. On the basis of extensive studies carried out, it is understood that the dye is in fact degraded during this process. The gel formed during the in situ degradation of the dyes is referred as GSCgd, where d stands for in situ degradation. The degradation of the dyes has been proven by the following measurements.

Figure 7

Photographs of the vials containing [{GO···SC4a} + l-Cys + dye] before (upper panel) and after degradation of dyes along with formation of GSCgd (lower panel).

Based on Supernatant Solution

The time-dependent degradation of EY has been monitored by a decrease in the absorbance with time, whereas no change was noticed in the case of the control (Figures a,b and S05). The histogram clearly shows that more than 70% of the dye is degraded within 2 min and further it degrades by >95% within 4 min (Figure c). The control experiment that is carried out without the use of SC4a but only using {GO + l-Cys} leads to only 70% of degradation within 4 min. All this supports that the {GO···SC4a} hybrid is certainly better and acts faster in degradation as compared with GO alone. Effectively, the dye degradation is increased to about 150% on going from only GO to the hybrid {GO···SC4a} whereas l-Cys is used in both cases.

Figure 8

Time-dependent absorption spectra of the supernatant solution in the case of EY upon treatment at 90 °C: (a) for [{GO···SC4a} + l-Cys] and (b) for [SC4a + l-Cys]. (c) Histogram of time-dependent (0–4 min) percentage degradation for EY upon treatment with [l-Cys] (black bar), [l-Cys + SC4a] (red bar), [l-Cys + GO] (blue bar), and [l-Cys + {GO···SC4a}] (olive bar).

Based on GSCgd Formed

In all 12 dyes studied, no band corresponding to the dye was observed in the supernatant after the degradation (Figures a,b and S06) and the Figure c provides a histogram of all results together. The absorption spectral measurements of dispersed gel showed only one band at 282 nm that corresponds to the SC4a and no band corresponding to the dye was observed (Figure d), supporting that the dye has been degraded whereas SC4a is intact.

Figure 9

Absorption spectra of the supernatant solution of dye upon treatment with {GO···SC4a} with l-Cys at 90 °C (a) for MB and (b) for NR. (c) Histogram of percentage degradation of dyes upon formation of GSCgd. (d) Absorption spectra for the dispersion of GSCgd gel formed upon in situ degradation: BB (black line), CR (red line), EY (blue line), MB (dark cyan line), MO (magenta line), NR (dark yellow line), R6G (navy line), RB (wine line), AO (pink line), bromophenol blue (BPB; olive line), CV (royal line), and RhB (orange line). SEM micrographs of GSCgd gel formed upon in situ degradation for (e) CV, (f) MB, (g) NR, (h) MO, and (i) for EY.

Based on the Morphology of GSCgd by SEM

The SEM micrograph showed porous network of the GSCg filled with fibrillar structures when adsorbed by the dyes, viz., MB, CV, NR, MO, and EY, whereas upon degradation, the gels showed only a porous structure that is not filled with any fibrils. All of this supports that the dyes are degraded during the gelation shown in Figure e–i.

Based on Mass Spectral Studies

As the dye is degraded during the formation of the gel, the supernatant solution was checked to identify the degraded products by mass spectrometry. In the case of RhB, a peak corresponding to the intact dye was observed at m/z = 443.23 (Figure a) before the degradation whereas peaks were exhibited at m/z = 123.04, 138.99, 149.02, 179.08, and 241.02 after the degradation and these peaks correspond to the oxidatively degraded products given in Figure b. Similar mass spectral studies were carried out in the case of four other dyes, viz., BB, CR, MO, and MB, and the peaks corresponding to the degraded products and their structures are given in Figure c,d. After the degradation, no peak corresponding to the intact dye was observed.

Figure 10

Electrospray ionization mass spectrometry (ESI-MS) spectra: (a) for RhB and (b) for the degraded products of RhB. (c) Table for the mass spectral data obtained for the degradation in the case of the five dyes. (d) Chemical structures of the degraded products of the dyes as given in (c). Raman spectra of the dye (blue), dye@GSCg (red), and dye-degraded gel GSCgd (black) in the case of (e) EY and (f) CV.

Based on Raman Spectroscopy

The in situ degradation of dyes has been supported by Raman spectroscopy, and this is clearer upon comparing the spectra of simple dyes to those of the gel after adsorption (dye@GSCg) followed by degradation (GSCgd). It can be seen from Figure e,f that the peaks corresponding to EY and CV are present in the gel after adsorption whereas no such peak is observed in the case of the GSCgd.

Degradation of the Mixture of Dyes

Since several dyes are always present together in contaminated water, degradation experiments were carried out for different combinations of organic dyes, viz., A + A, A + C, A + N, N + N, N + C, C + C, and A + N + C, where A, N, and C stand for anionic, neutral, and cationic dyes, respectively. It can be seen from Figures a,b and S07 that even the mixture of 12 dyes taken in a total volume of 500 mL degrades immaterial of the charge nature of the dye taken in the mixture for the study. This is prepared by taking 20 mL of 500 mg/L solution of each dye and diluting the mixture to give a final volume of 500 mL. To this mixture, the {GO···SC4a} hybrid and l-Cys were added and instantaneously heated at 90 °C to avoid any precipitation. After heating, the reaction mixture was filtered from the dispersed gel material and the filtrate was colorless (Figure c), which supports that this material can be used for a large scale water treatment to remove organic dye contaminants.

Figure 11

UV–vis spectra of the supernatant solution for the mixture of dyes: (a) A + C and (b) N + C. Here, A, C, and N refer to anionic (EY), cationic (MB), and neutral dye (NR). (c) Photograph of decolorization of the mixture of 12 dyes (EY, MO, CR, BPB, RB, MB, R6G, CV, RhB, AO, BB, and NR) upon treatment with [{GO···SC4a} + l-Cys] at 90 °C.

Readsorption of Organic Dyes by GSCgd

The gel GSCgd has been formed when the in situ dye degradation experiment was carried out, and this is true in the case of all 12 organic dyes studied. The GSCgd formed during the degradation of EY was further checked for the adsorption of organic dyes. Just like the gel GSCg, the gel GSCgd was able to remove all three types of dyes, viz., cationic, anionic, and neutral, from their aqueous solutions to an extent of >90% (Figures a–c and S08). The SEM micrographs of dye@GSCgd show a fibril structure along with porous network of the gel (Figure d–f).

Figure 12

Time-dependent absorption spectra of the supernatant solution for the dyes upon incubation with the gel GSCgd over a period of 10 h: (a) EY, (b) NR, and (c) MB. SEM micrographs for the dye@GSCgd with the dyes: (d) EY, (e) NR, and (f) MB.

Recyclability of Adsorption vs Degradation of Organic Dyes by GSCgd

To reuse the gel MB@GSCgd, the dye should be degraded and the same is achieved by the treatment of {CuSO4 + N2H4}. The SEM micrographs given in Figure c,d clearly support the dye degradation. This gel GSCgd was further used for five adsorption–degradation cycles and it was found that its adsorption capacity does not alter significantly even after performing all of these cycles (Figure a,b).

Figure 13

Recyclability: (a) UV–vis absorption spectra of the supernatant solution for MB upon adsorption over a period of 10 h by GSCgd. Color code: initial (black), cycle_1 (red), cycle_2 (blue), cycle_3 (dark cyan), cycle_4 (magenta), and cycle_5 (dark yellow). (b) Histogram of the percentage removal of dye vs number of adsorption–degradation cycles in the case of MB by GSCgd. SEM micrographs for the MB@GSCgd (c) before and (d) after the degradation of MB carried out in the presence of Cu2+/N2H4.

Comparison of qmax of the Gel GSCg with GSCgd

Materials for 12 different adsorbents have been generated by carrying out in situ dye-degradation that we reported in this paper, wherein all three types of dyes have been used. To compare the adsorption efficiency among these adsorbents, nine different qmax values were derived by using three different gels. Thus, the gels formed during the degradation of EY (anionic), NR (neutral), and MB (cationic) were demonstrated separately to obtain qmax and the corresponding data are given in Table (Figure S09). This data resulted in almost 15–30% of increase in qmax values as compared to those of the gel GSCg. All of this has been compared with the data reported in earlier papers in the literature, as given in Table . Thus, almost 100 mg/g of additional dye has been removed by using GSCgd. This is much greater than that using GSCg.

Conclusions and Comparisons

The supramolecular assembly of sulphonatocalix[4]arene-coated graphene oxide {GO···SC4a} hybrid has been successfully employed to make the hydrogel (GSCg) when reacted with l-Cys at 90 °C. The {GO···SC4a} hybrid forms a good dispersion in water, and l-Cys would easily link these hybrids using their −COOH and −NH2 groups. At high temperature, l-Cys releases H2S that acts as a reducing agent and leads to the formation of 3-D network of {GO···SC4a} hybrid via C–S–C linkage resulting in the formation of the gel. During the gelation, some of the oxygen functional groups are reduced, resulting in a decrease in the hydrophillicity of the gel and hence it separates out from water. The hydrogel formed has been fully characterized using TGA, XRD, XPS, and Raman spectroscopy, and the porous structure of the gel has been well confirmed by SEM. The GSCg adsorbs all five dyes irrespective of the nature of their charge of whether these are anionic, cationic, or neutral. The maximum adsorption capacities (qmax) were 225, 526, and 405 mg/g for MB, NR, and EY, respectively. The qmax observed by GSCg is comparable or better when compared to that of the graphene/graphene oxide-based gels reported in the literature, as given in Table . Comparison of this data reveals that only our gel GSCg is able to remove all three types of dyes, viz., anionic, cationic, and neutral, with rather high qmax values whereas no other reported graphene hydro-/aero-gel has all of these qualities together.

As per the detailed studies carried out in the case of the dyes, viz., MB, NR, and EY, the corresponding data follow Langmuir adsorption isotherm and pseudo second-order kinetics. In the SEM micrographs, the dye-adsorbed gel (dye@GSCg) exhibited fibril structures in association with its porous network as a characteristic signature of the adsorption of the dye. The absorption spectra of dye@GSCg gel showed bands corresponding to both SC4a and the dye, supporting the presence of the dye in the gel. Since the gelation took place with partial reduction of graphene oxide, organic dyes are oxidatively degraded during the gelation (GSCgd). This kind of degradation is green since no harmful radiations and/or hazardous oxidizing/reducing agents were used in the process. The degradation of the organic dyes has been proven by different spectroscopy and microscopy techniques. To scale up the in situ dye degradation process, 500 mL of contaminated water possessing 12 different organic dyes has been purified and showed the applicability of the {GO···SC4a} hybrid for the purification of bulk contaminated water. The gel formed during the in situ degradation of EY was further used for the adsorption of organic dyes and it was found that this gel GSCgd retains all its dye adsorption characteristics similar to the other gel GSCg. As given in Scheme , both GSCg and GSCgd are equally efficient in the degradation of dyes and in both cases, the gels are recyclable.

Scheme 1

Flowchart Showing that the Functional Behavior is Similar among GSCg and GSCgd Gels in Their Action in Degrading Organic Dyes

In summary, the {GO···SC4a} hybrid and the gels GSCg and GSCgd reported in this paper are ideal materials for the removal and degradation of dyes for new generation water purification systems.

Experimental Section

Synthesis and Characterization of p-Sulfonatocalx[4]arene (SC4a)

para-Sulfonatocalix[4]arene (SC4a) was synthesized according to the reported procedure,[45] and the product was characterized by ESI-MS, 1H, and 13C NMR (Figure S10). ESI-MS: HRMS [M + H]+ (m/z) for C28H24O16S4 = 745.002 (calcd), 745.004 (obtained). 1H NMR (400 MHz, D2O): δ = 7.51 (s, 8H, ArH), 3.92 (s, 8H, ArCHAr). 13C NMR (100 MHz, D2O): δ = 156, 133, 131, 126, 32.

Synthesis of 3-D Graphene Hydrogel (GSCg) Based on {GO···SC4a} Hybrid

GO was synthesized by the chemical modification of graphite according to the modified Hummer’s method[46] and was characterized by absorption, XRD, and IR spectroscopy and the morphology, by SEM and TEM (Figure S11). GO (50 mg) was dispersed in 5 mL of milli Q water using bath sonication. To this dispersion, 150 mg of SC4a was added, followed by sonication for 30 min and then stirring for 24 h to result in the formation of {GO···SC4a} hybrid. Three mg of l-Cys and 400 μL of Milli Q water were added to 100 μL of {GO···SC4a} hybrid to get a final concentration of 2 mg/mL. The mixture was sonicated for 2 min, followed by heating at 90 °C for 30 min without stirring. All of this leads to the formation of GSCg hydrogel, and the gel was washed thoroughly with milli Q water. To optimize the parameters affecting the gelation, different experiments were carried out and the optimum conditions were worked out. The formation of the gel was identified by visual observation as well as by physical tests.

Characterization of the Gel (GSCg)

Photographs were taken during the formation of the gel GSCg by a digital camera and were processed using ImageJ software. XPS was carried out on AXIS Supra instrument of Kratos Analytical with an analysis chamber pressure of <2.0 × 10–9 Torr and take-off angle of 90°. The X-ray diffraction measurements were carried out using an X’Pert Pro instrument by PANalytical with the X-ray generator functioning at 40 KV and 30 mA of current. To find the features of D and G bands, Raman spectra were measured. The surface morphology of the gel was studied by a JSM-7600F field emission gun scanning electron microscope in cryo mode. Before recording the images, the sample was frozen in liq. N2 and was fractured and sputtered for 30 s.

Dye Adsorption by GSCg

Two milliliters of 100 mg/L aqueous solution of an organic dye was added to the GSCg, and 100 μL was taken out after a fixed interval of time for absorption measurements. The dyes studied were methyl orange (MO, anionic), eosin yellow (EY, anionic), neutral red (NR, neutral), methylene blue (MB, cationic), and crystal violet (CV, cationic). The amount of dye being adsorbed was derived from the absorbance data collected at their respective λmax values. The adsorption capacity qe, amount of dye adsorbed in milligrams per unit mass of the adsorbent in grams, has been calculated as qe = (co – ce) × V/m, where co and ce are the initial and equilibrium concentrations (mg/L) of the dye, V is the volume in liters, and “m” is the mass of the dried adsorbent in grams. The adsorption data for the dye has been fitted to both the Langmuir and Freundlich adsorption isotherm models.

In Situ Dye Degradation

Five hundred milligrams per liter of different dyes, EY, MO, congo red (CR, anionic), bromophenol blue (BPB, anionic), rose bengal (RB, anionic), MB, rhodamine6G (R6G, cationic), CV, rhodamine b base (RhB, cationic), acridine orange (AO, neutral), bismarck brown (BB, neutral), and NR, were used (Scheme S01) as the contaminants present in water for the formation of the gel while carrying out in situ degradation of the dyes. In the degradation experiment, a protocol similar to that used for making the gel was adopted but the water is replaced by the dye solution. Both formation of the gel and decolorization of dyes due to the degradation were followed by recording their photographs with a digital camera. The degradation was monitored by absorption spectroscopy, and the degraded products were identified by ESI-MS.