A sustainable approach for the adsorption of methylene blue from an aqueous background: an adsorbent based on DES/CGS modified GO@ZrO₂

Abstract

Gemini surfactants (GSs) and deep eutectic solvents (DESs) belong to two important classes of industrially important materials which can be used to modify the performances of other entities where they are used for functionalization. A graphene oxide-zirconium oxide (GO@ZrO₂) nanocomposite has been synthesised and modified by using a cationic gemini surfactant (CGS, butanediyl-1,4, bis(N,N-hexadecyl ammonium) dibromide (16-4-16)) or by using a well-known DES (reline, choline chloride : urea, molar ratio 1 : 2). The adsorbent materials were characterized by various physicochemical techniques (FTIR, XRD, TEM, SEM-EDX, and TGA). Methylene blue (MB), a well-known industrially important colouring material, has been used as a model adsorbate to investigate its adsorption/removal from aqueous solution by using the above-modified nanocomposites (NCs, CGS-GO@ZrO₂ and DES-GO@ZrO₂). The adsorption process follows the Langmuir model (R² ≈ 0.995) together with pseudo-second order rate kinetics. Adsorption variables were optimised in the light of [NC], [MB], pH, and contact time. DES-GO@ZrO₂ has been found to be a better candidate for the fast removal of MB (∼100% at 20 mg L⁻¹, 5 m with 2 mg ml⁻¹ DES-GO@ZrO₂) when compared with other similarly modified materials. To economize the method, desorption of adsorbed MB (performed by using ethanol) is necessary. It has been found that the DES-GO@ZrO₂ performs efficiently even after 5 adsorption–desorption series. The findings of the present study can have potential applications in developing an economic strategy for the purification of industrial dye effluents with a concomitant redressal of aquatic pollution.

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Sustainability spotlight

Industrial effluents containing colour and colouring materials pose a significant threat to the aquatic ecosystem, with dyes from textile industries being a major contributor. To save running water sources from contamination, it is desirable to search for effective strategies for removing dyes from industrial wastewater. In response to this challenge, a sustainable and economically viable answer emerges in the form of modified graphene-based composites. This innovative idea not only offers valuable insights but also presents a practical solution for mitigating the harmful effects of dye effluents. By adopting the methodology presented here, one can actively conserve the already present limited potable water resources on mother earth.

1 Introduction

The last 4–5 decades witnessed a sharp increase in the fundamental progress and prospects of various kinds of materials such as nanomaterials, electronic materials, solvent materials, associated materials, membrane materials, and porous materials among others.^1–11 Among these materials, carbon allotropes attract special attention due to their novel properties and potential application in various fields of life.^12–16 Graphene is the most sought-after carbon allotrope, both in its pure form and when incorporated into composite materials or utilized in its derived forms, in various areas of scientific and engineering research.^4,17–20 Graphene is a uni-layer bi-dimensional surface of carbon atoms chemically bonded in the sp² configuration with a hexagonal pattern (benzene ring).^17,18 However, graphene oxide (GO) is preferred over graphene due to the presence of functional groups, though graphene has exceptional mechanical, electrical, and thermal properties.²¹

GO spontaneously distributes in an aqueous medium facilitating polluted water treatment.^22,23 Furthermore, GO shows high electronic mobility imparted from oxygenated moieties at the basal plane and edges.²⁴ However, high surface energy results in agglomeration and lower dispersibility as well as applicability.²⁵ Such inefficiencies of GO can be resolved by its fusion with some metallic oxides by anchoring a nanocomposite (NC) into its skeleton which may address and modify the existing problem and properties, respectively.^26–30 Nanocomposites are known as multicomponent materials with different phase nano-domains.³¹ Compared with GO, the above nanocomposites show specific architectural morphology and photochemical characteristics and exhibit good performance towards water treatment technologies.³² Among various metal oxides used for composite formation, zirconia (ZrO₂) has attracted researchers due to its passive nature and lower reductive potential together with lower processing cost.³³ It has been reported that GO-based metal composites are preferably used for the removal of cationic pollutants while very rarely used for the anionic moieties.³⁴ Currently, NCs are modified with a variety of compounds/mixtures (synthetic and natural polymers and surfactants) owing to their field of application.^34–41

In the above context, surfactants are preferred as modification agents due to their simple synthesis process, improved crystallinity/thermal stability, prevention of agglomeration, and the increased surface area and controlled porosity of the resulting modified material.^42–47 Cationic surfactants have found use as preferred modifiers in comparison to other charge type ones.^45,48–50 Recently, a cationic gemini surfactant (CGS) was reported as a modification material.^51–56 CGSs are known for showing improved structural and surface/solution properties over conventional cationic surfactants.^57–65 These research studies show that CGSs can be used to modify metal oxide or GO to facilitate the adsorption/degradation phenomenon. The utilization of CGS modified nanocomposites, based on metal oxide and GO, has been reported only a few times.^40,66

A new solvent material denoted as deep eutectic solvent (DES), has been introduced as a substitute for ionic liquids with added superior properties.^67–70 DES is now gaining momentum as a potential modifier in nanomaterial synthesis, processing, and functionalization of NCs.^71–78 These materials are safe, accessible, green, and environmentally friendly, and show increased affinity towards the materials of interest.^76,79 On the basis of the above survey, it seems of genuine interest to compare the adsorption efficiencies of CGS and DES modified NCs.

Saving pure water bodies, for the next generation, on the earth, is the biggest challenge to the research community.^80–84 World bank data show that textile waste (e.g., colour or colour-causing material) contributes ∼20% of the total industrial water pollution.⁸⁵ Thus, the colour/colouring compound eradication from the effluent is the first and foremost challenge faced by industries worldwide. This boosts the search for an optimised treatment method in order to diminish colouring pollutants from the effluent, before mixing in the regular potable water streams. NCs have been shown to be used as a potential material to solve the above mentioned problem using various modifications.^26,86–90 Owing to the economic and synthetic characteristics of ZrO₂ and DES, respectively, both are chosen for NC synthesis with GO and for further functionalization (DES-GO@ZrO₂). Moreover, another similar composite has been synthesised and modified with CGS using a method reported elsewhere.⁴⁰ Methylene blue (MB) is used as a model dye for the adsorption/kinetic study. Butanediyl-1,4, bis(N,N-hexadecyl ammonium) dibromide (16-4-16) and reline are used as the CGS and DES, respectively, for the current study. Reline is a well-known DES that can be obtained by mixing (1 : 2, molar ratio) choline chloride (ChCl), as a hydrogen bond acceptor (HBA), and urea, as a hydrogen bond donor (HBD).⁹¹ Modified NCs were characterized by different methods such as Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and thermogravimetric analysis (TGA). Various experimental conditions are optimised (NC doses, contact time, pH, initial adsorbate content, etc.) in order to realize the maximum removal of dyes from the aqueous medium. Probably, such type of comparative work on CGS/DES modified NCs has been performed for the first time. The strategies reported here may find application in reducing water pollution and cleaning the environment in general.

2 Materials and methods

2.1 Chemicals

GO, CGS, and DES (reline) are synthesised and characterized using the same procedures as reported previously.^40,58,92 Zirconium acetate [Zr(CH₃COO)₂] (99%) has been purchased from Loba Chemie, India, and used as received. ChCl (99%) has been obtained from TCI (India) and was used after vacuum drying (40 ± 0.5 °C). Urea has been obtained (99%) from Sigma Aldrich and used with no further purification. Methylene blue (MB), of microscopic grade (98%), was obtained from Loba Chemie, India. Distilled water of specific conductance ∼1 μS cm⁻¹ was used throughout. All the other reagents and chemicals used are of AR grade. The chemical structures of 16-4-16 (CGS), reline (DES), and MB are given in Scheme 1.

Scheme 1 Chemical structures of various materials used in the study.

2.2 Preparation of GO@ZrO₂ NC

GO was synthesised by using a well-established modified Hummers method.^93,94 Synthesis of GO@ZrO₂ involves the following steps: GO has been dispersed in an aqueous solution (1 g in 100 ml) through sonication for 30 m. The resulting dispersion was mixed with aqueous zirconium acetate (1 g in 100 ml) and stirring was performed for 30 m. Subsequently, the mixture was again sonicated for 1 h and 20 ml 1 M NaOH was added gradually. The resulting mixture was heated to 100 ± 0.1 °C and stirred for another 2 h. A colour change from greenish yellow to black indicates the formation of the dispersed aqueous GO@ZrO₂ NC. The latter was filtered and the solid mass was washed thrice with distilled water and then ethanol (50 ml each). The material, GO@ZrO₂ NC, was dried in a vacuum oven (100 ± 0.5 °C) for 12 h which resulted in a black shiny powder.

2.3 Modification of GO@ZrO₂ using CGS

The above synthesised GO@ZrO₂ was modified with CGS using the following procedure. GO@ZrO₂ (3 g/100 ml) was stirred in aqueous CGS solution (0.1 g/100 ml). The resulting mixture was sonicated for 5 m and then gently stirred for another 2 h at room temperature. The resulting dispersion was subsequently centrifuged (3000 rpm for 10 m) and washed thrice with distilled water (50 ml each time). This gives a CGS-modified NC (CGS-GO@ZrO₂) after drying in a vacuum oven at 65 ± 0.5 °C for 12 h.

2.4 Modification of GO@ZrO₂ using reline as the DES

1 g of GO@ZrO₂ was sonicated with 10 ml of DES (reline) for 30 m to get a homogenous dispersion. This dispersion has been transferred into a round bottom flask and then stirred at 80 ± 0.1 °C for 2 h. The mixture was filtered and washed thrice with distilled water followed by ethanol (50 ml of each). This DES-modified NC (DES-GO@ZrO₂) was dried in a vacuum oven for 24 h at 80 ± 0.5 °C.

2.5 Instrumentation for characterisation of the synthesised/modified NCs

X-ray diffraction (XRD) spectra of GO@ZrO₂, CGS-GO@ZrO₂, and DES-GO@ZrO₂ powder NCs have been recorded on an X-ray diffractometer (SmartLab, Rigaku Corporation, Japan, operating at 40 kV) equipped with a Cu laser source at a scanning rate of 2° m⁻¹ within a range of 5° to 80°. FT-IR spectra of various NCs were recorded in the range of 400–4000 cm⁻¹ on an FTIR-8400S, Shimadzu, Japan. TEM has been used to obtain morphological information by using a TEM-1400 PLUS, Jeol India Pvt. Ltd., India. For the TEM grid preparation, 1 mg of each NC sample has been sonicated in 20 ml of pure ethanol for 30 m using a tip sonicator, followed by placing two drops of the resulting suspension on a carbon-coated copper grid (200 mesh). The prepared TEM grid was air-dried for 30 m at room temperature before the TEM imaging experiment. The surface morphologies and elemental composition of the NC samples were studied using SEM (Hitachi SU70 SEM, Japan) at 15 kV coupled with EDX (EDX-8100, Shimadzu, Japan). Thermal stabilities of the synthesised/modified NCs were estimated using the TGA technique (TGA 50, Shimadzu, Japan) by collecting thermograms in the temperature range between 25 and 700 °C (heating rate of 10 °C m⁻¹) under a nitrogen atmosphere.

2.6 Dye adsorption study

Adsorption data are acquired by adopting a batch experiment approach (in triplicate). The concentration of MB has been quantified using an external calibration method (λ_max = 663 nm). A good correlation line (not shown) has been obtained (R² ∼ 0.999) and is thus used for quantifying MB adsorption. The solution of dye (20 ml) of an appropriate concentration (10–1000 mg L⁻¹) was mixed with different composite dosages (1–15 mg ml⁻¹) in quick-fit glass bottles. The mixture was kept under ambient conditions for different periods of time depending upon the nature of the study (adsorption or kinetics). The equilibrated (after the stipulated interval) mixture was centrifuged before UV-vis investigation (UV-1800, Shimadzu, Japan). The composite adsorption capacity (Q) has been computed using the following expression,where C_i and C_t are the initial and at time t concentrations of MB, respectively, V is the volume of MB solution and w is the weight of the modified composite (adsorbent). Furthermore, the percentage of dye removal (P) has been estimated by using the following mathematical expression,

2.7 Determination of pH at the point of zero charge (pH_pzc)

The salt addition method⁹⁵ was used to determine the pH_pzc of DES-GO@ZrO₂ and CGS-GO@ZrO₂ using 0.1 M KNO₃ solution at 30 ± 0.1 °C. The total volume of the solution was adjusted exactly to 20 ml in a conical flask by adding 0.1 M KNO₃. The initial pH (pH_initial) of the solutions was adjusted between 2.0 and 12.0 by adding 0.1 M HCl and 0.1 M NaOH solutions, respectively. The solutions of different pH_initial values were then mixed with 50 mg NC, and the suspension was allowed to stand for 24 h. The final pH (pH_final) of the supernatant liquid was noted for each solution. The difference between the pH_initial and pH_final (ΔpH = pH_initial − pH_final) was then plotted against pH_initial. The point of intersection of the resulting curve with the abscissa, at which ΔpH = 0, gives the pH_pzc.

2.8 Dye re-adsorption study

The adsorbed MB has been separated from the adsorbent by washing (3 times) with 50 ml ethanol each time. With each washing, the adsorbent was filtered. The washed adsorbent was then dried in a vacuum oven at 80 °C for 12 h. The so-obtained recharged DES-GO@ZrO₂ has been used (up to the 5th cycle) for the re-adsorption of MB. R_e-adsorption efficiency (R_e) has been computed by using the below-mentioned expression,where Q_r and Q_f are adsorption capacities with recharged and fresh DES-GO@ZrO₂.

3 Results and discussion

3.1 Characterisation of the synthesised/modified NCs

XRD spectra of all the NCs are shown in Fig. 1. The broadness of NC spectra indicates poor crystallinity (amorphousness) of all NCs. When XRD data were compared with those of pure GO and pure ZrO₂ (Fig. S1, ESI†), the composite material has been found to be more amorphous than the individual components. Similar types of signals were observed for the controlled deposition of ZrO₂ on graphene nanosheets in earlier studies.^96,97Fig. 1 also shows that amorphousness more or less remained similar even after modification with CGS or DES. In an earlier study, it has been shown that a surfactant modified NC-material shows more amorphousness than pure GO and corroborates the present data.⁹⁸

Fig. 1 XRD spectra of the modified nanocomposite(s) and its precursor.

FT-IR spectra of GO@ZrO₂, CGS-GO@ZrO₂, and DES-GO@ZrO₂ are compiled in Fig. 2. In the case of CGS-GO@ZrO₂, the appearance of peaks at around 2923 and 2847 cm⁻¹ indicates the existence of C–H stretching vibration bands and confirms the presence of the CGS alkyl group and hence the modification of GO@ZrO₂ by CGS. Similarly, in the case of DES-GO@ZrO₂, peaks appear at 3019 and 3261 cm⁻¹ which are expected to be from the vibrations of H-bonds in O–H and/or N–H, as reported earlier.⁹⁹ Furthermore, Zr–O bands appeared at 635 cm⁻¹ and 762 cm⁻¹. The absorption band which appeared at 2900 cm⁻¹ indicates a small alkyl chain of the choline part of reline present at the surface of DES-GO@ZrO₂.

Fig. 2 FT-IR spectra of GO@ZrO₂ and its modified forms with CGS and DES nanocomposites.

TEM images of pure GO@ZrO₂ and modified NCs are shown in Fig. 3. Modified composite images (Fig. 3b and c) show wrinkles and folding on the outer part of the original composite. The sheets are comparatively well separated for DES-GO@ZrO₂ than for CGS-GO@ZrO₂ (for the same magnification).

Fig. 3 TEM images of (a) GO@ZrO₂, (b) CGS-GO@ZrO₂, and (c) DES-GO@ZrO₂.

SEM-EDX data are given in Fig. 4 to further ensure the modification of GO@ZrO₂. The elemental analysis data are summarized in the ESI (Table S1†). The presence of nitrogen and chlorine atoms in the modified composites shows the functionalization of GO@ZrO₂ with CGS or DES (as the case may be). The data are indicative of the successful functionalization of the GO@ZrO₂ NC.

Fig. 4 SEM-EDX images of (a) GO@ZrO₂, (b) DES-GO@ZrO₂ and (c) CGS-GO@ZrO₂.

TGA thermograms and derivative thermograms for pure and functionalized nanocomposites are provided in Fig. 5a and b, respectively. Thermal stability of GO@ZrO₂ increases with both CGS/DES modified NCs. This additional thermal stability in the presence of both DES and CGS indicates that the decomposition of labile surface oxygens is restricted. This surface stability can be further utilized for higher temperature adsorption if needed.

Fig. 5 (a) Thermograms and (b) derivative thermograms of GO@ZrO₂ and CGS/DES modified nanocomposites.

3.2 Influence of [MB]

Removal of MB (by an adsorption process) from aqueous dye solution (without agitation) was studied in the presence of a fixed dose of the composite (GO@ZrO₂ (2 mg ml⁻¹), CGS-GO@ZrO₂ (10 mg ml⁻¹) or DES-GO@ZrO₂ (2 mg ml⁻¹)). The percentage of MB removal (P) in all the cases (after 60 m, followed by centrifugation) has been plotted against different fixed [MB] (20–100 mg L⁻¹), and data are depicted in Fig. 6. The perusal of data (Fig. 6) suggests that modification improves the P value compared to that observed with GO@ZrO₂. It may be mentioned that DES-GO@ZrO₂, even having 5 times lower quantity, competes well for the initial lower [MB] (∼40 mg L⁻¹). However, at further higher [MB], the P values show distinct differences for the above two modified NCs. Preliminary adsorption data show the competitiveness of DES modified NC with the CGS one. In this experiment, the best performances of MB adsorption were found at 20 mg L⁻¹ and 50 mg L⁻¹ for DES-GO@ZrO₂ and CGS-GO@ZrO₂, respectively. Therefore, these concentrations were taken to optimise the composite dosages.

Fig. 6 Variation of % MB removal from background solution (20 ml, after 60 m) with different initial concentrations of MB (20–100 mg L⁻¹) at 30 ± 0.1 °C: GO@ZrO₂ (2 mg ml⁻¹); DES-GO@ZrO₂ (2 mg ml⁻¹) and CGS-GO@ZrO₂ (10 mg ml⁻¹).

3.3 Influence of the composite load

The above fixed MB concentrations were used to study the influence of the composite load on the absorbability of MB from an aqueous solution. The NC load has been varied to determine the appropriate content of the adsorbent in the removal of MB (after 60 m) together with Q. Data for DES-GO@ZrO₂ and CGS-GO@ZrO₂ are shown in Fig. 7. A perusal of the data indicates that a near complete MB adsorption has been observed at/above 2 mg ml⁻¹ with the former NC while similar MB adsorption was observed at/above 10 mg ml⁻¹ for the latter NC (CGS-GO@ZrO₂). Therefore, 2 mg ml⁻¹ composite load and 20 mg L⁻¹ MB concentration have been fixed for DES-GO@ZrO₂ while 10 mg ml⁻¹ composite load and 50 mg L⁻¹ MB concentration were fixed for CGS-GO@ZrO₂ to study the influence of pH.

Fig. 7 Optimisation of composite dosages for MB adsorption (20 ml solution, after 60 m): (a) DES-GO@ZrO₂ (with 20 mg L⁻¹ MB) and (b) CGS-GO@ZrO₂ (with 50 mg L⁻¹ MB) at 30 ± 0.1 °C.

3.4 Influence of pH

pH variation has been accomplished by varying the concentration of NaOH/HCl depending upon the pH range (basic or acidic). With an increase in pH, MB molecules will exist in both charged and uncharged forms. The variation of P vs. pH for the above-mentioned systems (Section 3.2, after 60 m) is shown in Fig. 8. P has been found to be lower in a higher acidic range and increases with an increase in pH and shows distinctly higher MB adsorption (or P) at pH 10 for DES-GO@ZrO₂. This may be interpreted in terms of the conversion of the cationic MB form into the neutral MB form. The latter form may be driven towards the NC surface due to the hydrophobic attraction of the dye towards modified NC surfaces. Fig. 9 shows the variation of ΔpH vs. pH_initial and resulted in pH_pzc zero between pH 7 to 8. Beyond this pH range the NC surface will be negatively charged and start attracting the cationic form of the dye and be responsible for a sudden increase in the P value from pH 8–10. This could also be understood in light of the fact that the pK_a value of MB is 3.8.¹⁰⁰ The increase in the P value with pH finds support in an earlier study in which MB adsorption has been conducted on a GO-based composite.²⁶ However, the amount of MB adsorbed from pH 4–8 for CGS-GO@ZrO₂ is nearly constant. The former form will interact electrostatically with the CGS-GO@ZrO₂ surface while the latter one will interact hydrophobically. The overall adsorption might be the result of the above two interactions and works oppositely for CGS-GO@ZrO₂ and is responsible for a level-off in the P value. This is indeed observed in Fig. 8.

Fig. 8 (a) Influence of pH (2–10) on MB adsorption (20 ml solution, after 60 m) by DES-GO@ZrO₂ (2 mg ml⁻¹ dosage, with 20 mg L⁻¹ MB) and (b) CGS-GO@ZrO₂ (10 mg ml⁻¹ dosage, with 50 mg L⁻¹ MB) at 30 ± 0.1 °C.

Fig. 9 Variation of ΔpH vs. pH_intial for DES-GO@ZrO₂ (50 mg/20 ml KNO₃ solution) and CGS-GO@ZrO₂ (50 mg/20 ml KNO₃ solution) at 30 ± 0.1 °C.

3.5 Influence of contact time: adsorption kinetics

In order to understand the kinetics of the adsorption process, influences of contact time and [MB] have been seen at a fixed concentration of composite (2 or 10 mg ml⁻¹) and pH (=10). Q values at different time intervals (Q_t) are plotted against time (t) and such profiles are shown in Fig. 10a & b. The data have been used to obtain Q_e (amount of MB adsorbed per gram of the composite after attaining equilibrium). Various kinetic models (pseudo-first order, pseudo-second order, and intraparticle diffusion model) have been applied to reveal adsorption kinetics. The well-known Lagergren equation is widely used for liquid and solid systems to deal with pseudo-first order kinetics.¹⁰¹

Fig. 10 Variation of adsorption capacity (Q_t) with contact time for various starting MB concentrations (10–1000 mg L⁻¹): (a) DES-GO@ZrO₂ (2 mg ml⁻¹ dosage, 20 ml solution of MB) and (b) CGS-GO@ZrO₂ (10 mg ml⁻¹ dosage, 20 ml solution of MB) at 30 ± 0.1 °C.

3.5.1 Pseudo-first order kinetic model.

ln(Q_e − Q_t) = ln Q_e − k₁t (4)

where k₁ (min⁻¹) represents the pseudo-first order rate constant for the MB adsorption. Q_e and k₁ can be obtained from the slope and intercept of log(Q_e − Q_t) vs. t plots (Fig. S2a & b†). The computed data related to this model are compiled in Table 1. A perusal of Fig. 10 clearly depicts that kinetic data cannot be fitted in the present model (with both the NCs, Fig. 10a & b, Table 1) and hence other models are also checked. The square of the correlation coefficient (R²) values, for different initial MB concentrations, are much lower than one and hence denies the appropriateness of the pseudo-first order kinetic model. This is also confirmed due to distinct variations in the value of experimentally obtained Q_e and theoretically obtained Q_e (Table 1).

Table 1 Fitted kinetic data in different models for DES-GO@ZrO₂ and CGS-GO@ZrO₂

Adsorbent (mg L^−1)	Pseudo-first order				Pseudo-second order			Intra-particle diffusion
	Q e (exp) (mg g^−1)	Q e (cal) (mg g^−1)	k 1 (min^−1)	R ^2	Q e (cal) (mg g^−1)	k 2 (g mg^−1 min^−1)	R ^2	k i	C i	R ^2
DES-GO@ZrO 2
20	9.88	9.92	0.087	0.885	9.94	1.065	0.999	2.412	2.69	0.620
40	19.93	18.76	0.035	0.239	20.99	0.019	0.993	2.272	5.88	0.765
60	29.84	22.55	0.596	0.113	31.66	0.033	0.967	2.197	5.99	0.921
80	39.89	23.56	0.113	0.387	41.52	0.004	0.979	2.687	11.21	0.857
100	47.85	25.36	0.067	0.209	54.43	0.001	0.896	3.584	3.02	0.968
CGS-GO@ZrO 2
10	0.86	0.745	0.011	0.909	0.98	0.668	0.988	0.233	0.10	0.603
50	3.28	3.184	0.063	0.864	33.70	0.001	0.913	0.897	0.47	0.529
100	9.33	10.14	0.011	0.655	10.81	0.154	0.999	1.395	3.55	0.861
1000	72.13	58.35	0.034	0.716	72.83	0.002	0.998	3.663	28.71	0.825

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3.5.2 Pseudo-second order kinetic model. In the process of finding the correctness of the kinetic model, the data of MB adsorption, with both NCs, were fitted to the pseudo-second order kinetic equation,¹⁰²where k₂ (g mg⁻¹ min⁻¹), the second order rate constant, and Q_e have been computed from a straight-line plot (Fig. S3a & b)† of t/Q_tvs. t (using the slope and intercept). When t → 0, the adsorption rate becomes the initial adsorption speed. The data computed for this model are compiled in Table 1. Better R² values (∼1) indicate the suitability of the pseudo-second order kinetic model to understand the adsorption kinetics of MB on both modified NCs (CGS-GO@ZrO₂ and DES-GO@ZrO₂).

3.5.3 Intraparticle diffusion kinetic model. Another kinetic model has also been used to interpret adsorption behaviour. This model also considers the diffusion process of the adsorbate in addition to concentration variation. The well-known expression¹⁰³ of the present model is as follows,

Q_t = k_it^1/2 + C_i (6)

k_i is the intraparticle velocity constant (mg g⁻¹ min^1/2) and C_i is a constant that is related to boundary layer thickness/diffusion. Q_tvs. t_½ provides a linear plot (Fig. S4a & b)† whose slope and intercept can be used to obtain k_i and C_i. The computed values of k_i, C_i, and R² are summarized in Table 1. Looking at the fitted data and R² values (far less than 1), one can be observe that pore/surface diffusion is not the only factor but other routes such as film diffusion may also be involved.

From a perusal of all the fitted data in different kinetic models with both the NCs (Table 1), it can be seen that better fitting (Fig. S3†) and acceptable R² values are produced with the pseudo-second order kinetic model, which hints that this model is probably followed in the present adsorption of MB on both the modified NC surfaces.

3.6 Adsorption isotherm

The applicability of different adsorption isotherm models (at 30 ± 0.1 °C) has been checked by considering Freundlich, Langmuir, and Temkin models.

3.6.1 Freundlich isotherm. According to this adsorption model, the surface of the composite is heterogeneous and assists in multilayer adsorption. Mathematically, the Freundlich isotherm model can be expressed¹⁰⁴ as under,where K_FI (mg g⁻¹) and n are the Freundlich adsorption constant and adsorption potency, respectively. n decides the spontaneity and reversibility of the process (0 < 1/n < 1, spontaneous; 1/n > 1, non-spontaneous; 1/n = 1, non-reversible). K_FI and n can be computed from the intercept and slope of the plots of log Q_evs. log C_e (Fig. S5†) where C_e is the equilibrium [MB]. The related data are summarized in Table 2. The poor R² values clearly indicate the non-applicability of the Freundlich adsorption model and deny the multilayer adsorption.

Table 2 Fitted adsorption data of MB on DES-GO@ZrO₂ and CGS-GO@ZrO₂ using various models

Isotherms	Parameters	DES-GO@ZrO2	CGS-GO@ZrO2
Freundlich	K FI (mg g^−1)	14.24	1.51
	n	0.15	0.73
	R ^2	0.936	0.885
Langmuir	Q m (mg g^−1)	22.94	15.91
	b (L mg^−1)	4.731	0.127
	R ^2	0.971	0.995
	R EP	0.0104–0.0021	0.4405–0.0078
Temkin	K T (L g^−1)	359.50	3.50
	2.303 RT/b	2.595	2.251
	R ^2	0.979	0.816

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3.6.2 Langmuir isotherm. Since the Freundlich adsorption isotherm model fails to fit the present adsorption data of MB, therefore, other models were tested to know the exact mechanism of MB adsorption. Another well-known adsorption model namely, the Langmuir isotherm,¹⁰⁵ has been considered to fit. The Langmuir isotherm is represented by,where Q_m is the content of MB to form a uni-layer (mg g⁻¹) and b (L mg⁻¹) is a constant which represents adsorption energy. A plot of 1/Q_evs. 1/C_e results in a straight line with a slope (gives 1/b Q_m) and intercept (1/Q_m) (Fig. S6†). The values of R² were found to be reasonably good and are compiled with other adsorption constants of this model in Table 2. These data indicate the suitability of the Langmuir model for MB adsorption on modified NCs. This was further confirmed by the determination of the equilibrium adsorption parameter (REP) which is related to b and C_i by using the following mathematical expression,

R _EP gives an idea about the favourability of adsorption equilibrium. The ranges of R_EP for different initial [MB] are also tabulated in Table 2. R_EP varies between 0.01 and 0.0021 (for DES-GO@ZrO₂) and 0.4405–0.0078 (for CGS-GO@ZrO₂) which proves that monolayer adsorption is taking place spontaneously. Furthermore, NCs provide nearly homogeneous surfaces.

3.6.3 Temkin isotherm. To ensure the applicability of the Langmuir model, the adsorption data were also fitted into another well-known model, known as the Temkin isotherm.¹⁰⁶ This model asserts that the heat of adsorption lowers linearly during the transfer of the adsorbate toward the adsorbent surface, and the model can be expressed mathematically as,

The term K_T (L g⁻¹) is the Temkin constant related to the energy involved in the binding of MB to modified NC. The value of RT/b gives insight into the nature of the adsorption process. These values were obtained from the slope and intercept of the plot of Q_evs. log C_e (Fig. S7†). If RT/b > 0, the process can be said to be exothermic and vice versa, while when RT/b < 0, means that heat is absorbed during adsorption, i.e., an endothermic process. The values of RT/b, KT, and R² are also included in Table 2.

A perusal of isotherm parameters (Table 2) of different adsorption models (mentioned above) indicates that both Langmuir (monolayer adsorption) and Temkin (exothermic) models are equally followed by the MB adsorption data obtained with CGS-GO@ZrO₂ and DES-GO@ZrO₂ as adsorbents. This indicates that monolayer adsorption of MB is taking place together with heat release. Therefore, the binding strength of MB to DES-GO@ZrO₂ seems higher than to CGS-GO@ZrO₂.

3.7 Comparison of MB adsorption with similar composite materials

Table 3 shows the compilation of the adsorption/removal data of MB with similar composites.^107–110 Among two modified composites (for faster removal), DES-GO@ZrO₂ has been found to be superior to CGS-GO@ZrO₂. A perusal of comparative data shows that DES-GO@ZrO₂ shows good adsorption efficiency (close to 100%) within a short time (5 m) though the DES-GO@ZrO₂ dose was slightly towards the higher side. However, the adsorption time column in Table 3 clearly shows that the DES-GO@ZrO₂ can be a preferred candidate for the faster removal of MB or similar materials (yet to be checked). Based on this observation, DES-GO@ZrO₂ has been chosen for the recyclability study.

Table 3 Comparison of removal efficiency of DES-GO@ZrO₂ and CGS-GO@ZrO₂ with similar materials reported in the literature

Nanocomposite names	Adsorbent dosage (mg mL^−1)	Initial MB concentration (ppm)	Adsorption time (min)	Removal efficiency (%)	Ref.
DES-GO@ZrO2	2	20	5	∼100%	This work
CGS-GO@ZrO2	10	10	5	82%
Fe3O4−xGO	0.28	150	180	97.5%	71
CS/Fe3O4/GO	0.4	5	1200	∼25%	109
MGC/GO	1	30	840	96.7%	108
GO/Fe3O4/SiO2	0.25	3.2	10	∼65%	110

---

3.8 Recyclability/reusability study

For an adsorbent, the potential is not only determined by its adsorption/removal efficiency but also by reusability (to control the economy of the process) one after another cycle.^111–113 Here, MB adsorbed DES-GO@ZrO₂ has been recycled by washing it with various solvents (water, 0.1 M HCl, 0.1 M NaOH, methanol, and ethanol). The washing ability of adsorbed MB is shown in Fig. S8.† Ethanol has been found to be a better solvent to desorb MB from the said composite (R_e ∼ 86%). Fig. 11 shows the adsorption–desorption efficiency of the MB adsorbed DES-GO@ZrO₂. It can be noticed that the composite can be efficiently recycled/reused for at least 5 successive cycles with ethanol as the recharging solvent. The efficiency of the process corroborates the physical interaction involved in the adsorption process of MB. Better adsorption capabilities even after 5 cycles suggest that DES-GO@ZrO₂ can perform as a potential adsorbent for MB or MB derivatives.¹¹⁴

Fig. 11 Recyclability study of DES-GO@ZrO₂ using 50 ml of ethanol for each cycle (up to 5 cycles).

3.9 Mechanism of MB adsorption

Preliminary TEM data (Fig. 3) show that ZrO₂ sits on the surface of GO sheets with a homogenous distribution. CGS and DES contain certain groups which can interact with the GO@ZrO₂ composite via electrostatic and hydrophobic interactions. MB will be adsorbed on the modified surface of the composite via various interactions represented in Scheme 2. Various active groups are available on the modified NCs which perform an important role in the MB adsorption process. The nature of the NCs is shifted towards hydrophobic (due to hydrocarbon chains available on CGS or DES) which consequently contributes to attracting MB molecules towards the adsorbent surface.

Scheme 2 Depiction of MB adsorption on modified NCs via different modes of interactions.

4 Conclusion

DES-GO@ZrO₂ has been found to be distinctly better than CGS-GO@ZrO₂ for adsorption/removal from an aqueous solution of MB. The adsorption process has been found to be driven by electrostatic/hydrophobic interactions of MB with NC surfaces. The results of the adsorption process follow the Langmuir adsorption isotherm model indicating the homogeneity of the NC surfaces. DES-GO@ZrO₂ shows complete adsorption (within 5 m) for MB when compared with other similar adsorbents (Table 3). Kinetic study showed that adsorption-time data follow pseudo-second order kinetics. The recyclability experiment showed that DES-GO@ZrO₂ performs well even after the 5th cycle. Ethanol has been found to be a good recharging solvent for the said composite. Furthermore, the information can be used to produce potential commercial filters involving DES-GO@ZrO₂ for the faster/effective removal of coloured dyes or colouring material before discarding into running streams (control of water pollution to make it potable for humans, aquatic habitats together with irrigation). The spectrum of application can be enlarged by performing similar studies with other industrial effluents, e.g., paint, textile, leather, or paper.

Abbreviations

DES	Deep eutectic solvent
GO	Graphene oxide
MB	Methylene blue
CGS	Cationic gemini surfactant
NC	Nanocomposite

Conflicts of interest

There is no conflict of interest.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors are thankful to the Heads, Applied Chemistry Department and Chemical Engineering Department, Faculty of Tech. & Engg. (FTE), The Maharaja Sayajirao University of Baroda, Vadodara, India, for the research facilities. Vishwajit Chavda is thankful to the SHODH programme (Ref. no. 202001720059) of the Government of Gujarat for providing financial support. Special thanks are due to the Head, Department of Applied Physics, Faculty of Tech. & Engg., Maharaja Sayajirao University of Baroda, Vadodara, India, for acquiring XRD data (Model: Rigaku SmartLab SE) on the XRD instrument purchased under the DST-FIST program (No. SR/FST/PS-II/2017/20).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3su00236e

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