Efficient adsorption of malachite green dye using novel reduced graphene oxide/β-cyclodextrin epichlorohydrin composite: batch and fixed-bed studies

ABSTRACT A novel ternary composite (RGO-βCD-ECH) consisting of epichlorohydrin (ECH) cross-linked beta-cyclodextrin polymer (βCD-ECH) with reduced graphene oxide (RGO) as a carrier was synthesised and used for the removal of malachite green (MG) dye from water. The adsorption performance was systematically investigated in batch mode, and the optimised parameters were as follows: initial MG dye concentration 100 mg/L, solution pH 8.0, contact time 90 min, adsorbent dosage 0.4 g/L, and temperature 30°C. The adsorption data follow the Langmuir model (R2 = 0.998) and the maximum sorption capacity (qm) was 902.125 mg/g. The kinetic data of adsorption follows the pseudo-second-order kinetic model (R2 = 0.999). The positive value of ΔHo = 102.047 kJ/mol indicates the endothermic nature of the adsorption process. The effects of bed height (2–4 cm) and MG dye flow rate (5–15 mL/min) were investigated in the fixed-bed column study. The exhaustion time (te), breakthrough time (tb), and bed capacity (qbed) of the column increases with bed height increases, and the value decreases with flow rate increases. The most important feature of this adsorbent is it could efficiently and easily regenerate from the MG dye loaded composite with a negligible effect on percentage removal. These characteristics of these novel composites could exhibit a bright future in the application of MG dye removal from wastewater.


Introduction
Nowadays, wastewater treatment is an important and large area of research worldwide because of the sudden reduction in freshwater reservoirs.Water shortage is a rising global problem that is not limited to the developing world.The continuous discharge of pollutants from the aqueous environments leads to a serious deterioration in the quantity and quality of usable water resources [1].Several industrial discharges including food producers, electroplating factories, textile companies, plants, dye factory released wastewater that pollutes the water resources [2].The effluents coming from the textile dye industry contain various complex mixtures such as organic compounds, salts, acids, dye bases, auxiliary chemicals, and occasionally heavy metals [3].These harmful dye pollutants simplicity of obtainment, and porous structure, βCD becomes an appropriate adsorbent for the treatment of wastewater.However, the βCD molecule is water-soluble, which limits its application as a material for water treatment.Therefore, the polymerisation of βCD was done to form a water-insoluble polymer by a cross-linking reagent.Among the cross-linkers, epichlorohydrin is chosen as a cross-linker for synthesising the waterinsoluble polymer [24].
A lot of current research emphasises the use of graphene and βCD-based composites as an effective adsorbent for treating MG dye from aqueous solution.Ghahramani et al. have synthesised PANI-coated RGO as an adsorbent having a maximum capacity of 666.7 mg/g at pH 7, and it fits Langmuir isotherm [25].Wang et al. have synthesised magnetic-βCD-GO for the treatment of MG dye from wastewater.The adsorption process fits the Langmuir model with 990.1 mg/g capacity at pH 7.0 [26].Crini et al. synthesised a cyclodextrin-based material for MG dye removal with a maximum capacity of 91.9 mg/g [27].Kinani et al. have synthesised GO-tannic acid composite, having 283.4 mg/g adsorption capacity at pH 8.0, time 2 hr, and Temperature 298 K [28].Zhang et al. have synthesised GO in cellulose bead for the remediation of MG dye with maximum capacity 30.091 mg/g at pH 7, 1 hr time, and Temperature 298 K [29].
These results motivate the feasibility of RGO-βCD-ECH composite as an excellent and low-cost adsorbent for MG dye removal from an aqueous solution in batch and fixed-bed column study.The adsorbent material was successfully synthesised and characterised using FESEM, EDAX, FTIR, XRD, RAMAN, BET surface area analyser, and Zeta potential.The batch adsorption experiments were explored to know the experimental conditions on adsorption efficiency.The effect of adsorbent bed height and the adsorbate flow rate was evaluated in a continuous fixed-bed column study to provide the basic scale-up data.An attempt has been made to fit the data obtained from continuous column study with the different mathematical models published in the literature.Additionally, the adsorptiondesorption study was also explored to prove the long-term use of RGO-βCD-ECH composite as an adsorbent material in the adsorption system.

Synthesis of graphene oxide (GO) and reduced graphene oxide (RGO)
For the synthesis of GO, the Modified Hummer's method was used.The synthesis procedure follows both exfoliation and oxidation of graphite powder [30].The detailed procedure followed in a 1 L beaker, H 3 PO 4 (40 mL) and H 2 SO 4 (360 mL) were mixed in the ratio of 1:9 and kept for 30 minutes of stirring then graphite powder (5 g) was added and stirred for 45 minutes [18].After that, very slowly added KMnO 4 (18 g) to the above mixture as the above reaction is exothermic, to maintain the temperature below 10°C an ice bath was used.After 6hrs of vigorous stirring, a dark green solution appears.Add 9 mL of H 2 O 2 to stop the reaction, and another 15-minute stirrer was done to mix the H 2 O 2 properly and exfoliate the graphene sheet, then 400 mL of DIW was added into the above solution.The temperature rose above 100°C after the addition of DIW, and yellowish-brown colour solution appears after 24 hr of cooling.To remove the unreacted residuals, the solution was centrifuged at 5000 rpm for 10 minutes and repeatedly rinsed with DIW until the pH became neutral.Then the solution was put in an oven at 60°C for 12 hr for drying to produce GO powder.
The procedure for synthesis of RGO followed 5 g GO, and 500 mL DIW was added in a beaker and stirred for 30 minutes to produce GO suspension.The pH of the suspension increased to 10 by adding either 0.1 M NaOH or 0.1 N HCl solution.Hydrazine monohydrate (5 mL) as a reducing agent was added at 65°C and stirred for another 4 hr to reduce GO to RGO.The product was centrifuged to eliminate the DIW from the solution.The black suspension was then put an oven for drying at 70°C to obtain RGO powder.

Synthesis of beta-cyclodextrin epichlorohydrin (βCD-ECH) polymer
Synthesis of the βCD-ECH polymer was done in a cross-linking reaction using ECH as a cross-linker [31].In brief, the procedure follows βCD (10 g) comprehensively mixed with 20 mL NaOH solution (50% w/w) in a 250 mL beaker and continuously stirred for 12 hrs at 25°C.Then 6 mL ECH was added to the above mixture and stirred for 40 min at 400 rpm.To stop the reaction, 15 mL acetone was added, and the resulting residue was put in an oven at 50°C overnight.To neutralise the solution, 19.9 mL of 6 N HCL was added to the solution.For moisture evaporation, the obtained solution was put in a tray dryer at 60°C.In the above-obtained residue, 44 ml of ethanol/methanol was added.The white precipitate separated from the above mixture by decantation and put in an oven at 60°C for 24 h.Finally, the synthesised βCD-ECH polymer size was reduced and stored in a desiccator.

Synthesis of RGO-βCD-ECH composite
A wet chemical procedure was used for the preparation of the RGO-βCD-ECH composite, and in our previous work, the detailed procedure was discussed [32].Briefly, RGO (125 mg) was completely mixed in 100 mL DIW by sonication.Add βCD-ECH polymer (2 g) to the above solution and stir for 30 min, after that, a homogeneous solution is produced.After that, add 5 mL of NH 4 OH into the above homogeneous solution and sonicate for 90 min.After sonication, a black suspension is produced, constantly washing with ethanol and DIW to remove the unreacted components.The synthesised residue was put in an oven around 50°C for drying and then stored for further use.The schematic presentation for the RGO-βCD-ECH composite synthesis is shown in Fig. S1.

Characterisation
Scanning electron microscopy (SEM) was used to analyse the surface and morphological characterisation of RGO, GO, RGO-βCD-ECH composite, and βCD-ECH polymer by JSM-5900 LV, Japan, under an operating voltage of 20 kV.X-ray diffraction (XRD) was measured using a D/MAX Model D-5000 Diffractometer between 2θ of 2-60° obtained at a scan rate of 5°/min on the diffractometer with Co Kα radiation and wavelength of 0.174 nm at room temperature.The Raman spectra were obtained using the Raman Micro spectrometer (Renishaw).Fourier-transform infrared spectroscopy (FTIR) data were obtained by using KBr pellets in a Perkin-Elmer spectrum in the 4000-400 cm −1 frequency range.The pore information and surface area of the RGO-βCD-ECH composite was determined by the Brunauer-Emmett-Teller (BET) method at 77 K using an ASAP 2000 surface area analyser.The Zeta potential of the composite was measured in the pH ranges from 1 to 10 using the Malvern zeta metre, and it is used to measure the point of neutral charge on the surface of the composite.Zeropoint charge (ZPC) was measured by plotting the pH of the solution with the Zeta potential.

Batch adsorption experiments
The RGO-βCD-ECH composite was used as an adsorbent for the batch adsorption experiments with 50 mL of MG dye solution in 250 mL conical flasks in a shaker with 150-RPM speed.Standard solutions having concentrations (100-500 mg/L) were prepared by diluting the stock solution.The adsorption parameters such as pH of the solution, temperature, adsorption time, and adsorbent dosage were varied from 5 to 10, 20°C to 40°C, 0 to150 min, and 10-35 mg, respectively.Standard acid of 0.1 M HCl or base of 0.1 M NaOH was used to adjust solution pH.A centrifuge was used for the separation of adsorbent from the solution after adsorption.A UV-VIS spectroscopy (Shimazu UV-3150 spectroscopy) was used to determine the remaining concentration of MG dye at a wavelength of 616 nm.The percentage removal (%R), uptake at equilibrium (q e ), and at time t (q t ) were calculated using the formula presented in Supplementary Material.

Fixed-bed column studies
The fixed-bed adsorption column (FBAC) studies were implemented using a lab-scale Perspex column with an internal dia. of 1.5 cm and a height of 20 cm.Glass wool was placed on the top and bottom of the bed to ensure packing and to prevent the loss of the bed material.The column was packed with the RGO-βCD-ECH composite to obtain the desired bed height (equiv.to 2, 3, and 4 cm) at 303 K, keeping influent flow rate and MG dye concentration constant at 5 mL/min and 100 mg/L.The effect of adsorbate flow rate was measured from 5 to 15 mL/min, keeping bed height constant at 4 cm.The adsorption experiments were performed by pumping the solution in an upward flow model using a peristaltic pump at optimum pH 8.0 and temperature 303 K.At 60 min time interval, the effluent samples were collected and measured by UV/Vis spectrophotometer to know the remaining dye concentration.The mathematical relation for the calculation of Fixed bed column parameters are present in supplementary materials

Characterisation of RGO-βCD-ECH composite
Figure 1 shows the SEM image of βCD-ECH, RGO, GO, and RGO-βCD-ECH to find out the dispersion of RGO in the βCD-ECH surface and the resulting polymer surface.In GO (Figure 1a), the oxygen-containing functional groups made the crumbled sheets stuck together.The GO sheets are randomly aggregated, the surface is very rough, there is heterogeneous surface structure, wrinkles are present, and the sheets are thin.In the SEM image of RGO (Figure 1b), the topographic characteristics evolved to a certain degree to form a well-developed mesopore.Also, it is observed that the surface of RGO is corrugated, highly wrinkled, irregular structure, and the sheets are less aggregate [33].It is seen that the cross-linked βCD-ECH are irregular solid particles, and the surface is porous, which confirms that in the composite, the main adsorption mechanism cannot be a nanopore structure [34].The βCD-ECH polymer surface (Figure 1c) is porous, regular shaped, and smooth because of cross-linked networks.However, the shape of βCD is a parallelogram, and the particles are three-dimensional, suggesting that the polymer is successfully synthesised.After introducing RGO sheets onto the polymer agglomeration surface starts, the synthesised RGO-βCD-ECH composite (Figure 1d) looks different from the pure βCD-ECH.
To determine the crystal structure and the interlayer spacing (d-spacing) of the synthesised material, XRD analysis is used.0.334 nm at a 2θ value of 26.62° [35].After introducing oxygen functionalities, in GO, the graphitic peak shifts to 2θ = 12.12° having d-spacing 0.83 nm confirms the successful exfoliation of graphite powder to synthesise GO.For RGO, this peak is obtained at a 2θ = 29.68°and d spacing of 0.35 nm, confirming GO reduction to RGO.Also, for both RGO and GO, one small peak could be seen at 42.05° and 41.7°, which occurred due to the turbostratic disordered nature of carbonaceous materials [36].Two comprehensive peaks at 36.72° and 53.4° are observed in the polymer and composite XRD spectra because of the amorphous nature of βCD-ECH.FTIR spectra are used to obtain information about the interactions between βCD-ECH and RGO nanosheets.Figure 2(b) shows the FTIR spectra of βCD-ECH, RGO, RGO-BCD-ECH, and GO.In the FTIR spectra of GO, various oxygen-containing functional groups are present: -OH stretching vibration at 3429 cm −1 , C-O vibration at 1051 cm −1 , C = O in carboxylic groups at 1738 cm −1 , C-O-C vibration at 1178 cm −1 , C-OH vibration at 1404 cm −1 , confirms synthesis of GO from graphite powder [37].For RGO peaks are observed at 1620, 1192, and 2361 cm −1 which belongs to C = C, C-O epoxide group vibration, and C ≡ C alkyne stretching indicates very few oxygen-containing groups are available on the RGO surface.Lower absorption peaks for βCD-ECH are presented at 943, 857, 756, and 707 cm −1 confirms the presence of ring vibration, and the most significant peaks are seen at 3418, 1713, 1368, and 1084 cm −1 because of -OH group vibration, C = O carbonyl stretching, C-OH hydroxyl vibration, and C-O epoxide group vibration.All the βCD-ECH polymer absorption peaks have been present in the composite spectra.The -OH hydroxyl stretching in the composite is shifted to a lower wavenumber value because of the hydrogen bond formation between RGO and βCD -ECH polymer.The above results suggest that the βCD-ECH polymers are formed hydrogen bonding with the surface of RGO to form the composite.The similar peak-shaped vibration of βCD-ECH and RGO-βCD-ECH implies that the composites keep the original characteristic structure.
The Raman spectra of βCD-ECH, RGO, RGO-βCD-ECH, and GO are shown in Figure 2(c).It is observed that after the modification of RGO, some structural changes is observed on the surface of the RGO-βCD-ECH composite.The spectra of RGO-βCD-ECH, GO, and RGO show two strong peaks observed at 1590 and 1343 cm −1 can be allotted to G and D bands, respectively.The D band occurs due to defective amorphous graphitic structures, whereas the G band is because of the E 2g phonon of sp 2 atoms of C = C bonds.In the spectra of βCD-ECH polymer, no G and D band peaks are observed.The other peaks in the βCD-ECH polymer are glucose ring vibration at 947 cm −1 , single bonds C-C at 1130 cm −1 , C-O at 1080 cm −1 , C-H deformation (1330-1400 cm −1 ).However, the peak at 2913 cm −1 is due to asymmetric and symmetric C-H stretching in composites, confirming the successful composite synthesis.The intensity ratio (I D /I G ) value indicates the sp 2 /sp 3 carbon ratio, for GO, this value is 1.08, and for RGO, it is relatively higher 1.33 because of the increase in defect, removal of functional groups, restoration of carbon network, greater discontinuity in the graphene layers [38].The value is 1.05 for the RGO-βCD-ECH composite, and this reduction is due to the addition of functional groups, cavity formation, and decrease in defects.
The N 2 desorption/adsorption isotherm and Barrett-Joyner-Halendra (BJH) pore size distribution of RGO-βCD-ECH composite are shown in Figure 2(d,e).The size distribution plot shows that between 3.08 and 23.88 nm, sharp peaks are formed, which indicates that the synthesised RGO-βCD-ECH composite is mesoporous.The surface area, pore volume and mean pore diameter values are 35.22 m 2 /g, 0.085 cm 3 /g, and 2.987 nm, respectively, which is less than the values of RGO.This decrease of value confirms that the βCD-ECH polymer is successfully assembled on the RGO surface.From Figure 2(e), it is observed that the peaks are formed for composite between 2.86 and 18.65 nm, indicating mesopore on the surface.
Figure 2(f) shows the Zeta potential of the RGO-βCD-ECH composite, which is used to evaluate the surface charge and electrical potential.It is detected that when the value of solution pH, increases zeta potential value decreases, suggesting that the protonation degree of the present in composite reduces and the surface becomes positive.The pH at which the zeta potential value is zero is called the point of zero charges (PZC), and for the RGO-βCD-ECH composite, it is 6.55.This value suggests that for pH value less than 6.55, the composite surface is positive, and for pH greater than 6.55, the surface is negatively charged, which attracts more and more MG dye on the surface of the composite through the electrostatic attraction force.

Effect of solution pH
In the adsorptive remediation of dye molecules from wastewater, the pH of the solution plays an important role in keeping the overall alkalinity or acidity of the solution.Fig. S2(a) shows the effect of solution pH on percentage removal (%) of MG dye using RGO-βCD-ECH.The pH of the solution varies from 5.0 to 10.0 by using either NaOH (0.1 M) or HCl (1 N) solution.The batch experiments are carried out at 30°C temperature, time (90 min), 100 mg/L initial MG dye concentration, and 20 mg adsorbent dosage.The results show that as the solution's pH increased up to 8.0, the removal percentage (%) increased, and the maximum value is achieved at pH 8.0.The phenomena are explained by electrostatic action and repulsion between the dimethylamine group of MG dye and the protonated adsorption sites of composites.At low pH, the repulsion is higher, which affects the process of adsorption.In addition, it is difficult for the positively charged dye molecules to form a host-guest inclusion with βCD cavity at a low pH value.However, with the increased pH value, the positively charged MG dye gradually becomes neutral, and the hydroxyl, carboxyl, and epoxy groups of RGO-βCD-ECH composite interact with MG dye molecules through electrostatic force, Van der Waals forces, and hydrogen bonding, which enhances the adsorption capacity.However, after the pH value exceeds 8.0, the values of removal percentage and adsorption capacity gradually decrease as some of the functional groups become protonated, which increases the repellent force.Based on the above results, pH 8.0 was chosen as the optimum for our experiment.

Effect of adsorbent dosage
Fig. S2(b) shows the influence of RGO-βCD-ECH composite dosage on MG dye removal (%R) and adsorption capacity (q t ) ranging from 10 to 50 mg, keeping other adsorption parameters constant.As observed from Fig. S2(b), the %R of MG dye increases from 85.321 to 98.845% when the adsorbent dosage increases from 10 mg to 20 mg, and after that, it reaches a plateau, and then no such increase occurs.It is also observed that the uptake capacity of MG dye decreases as the amount of composite dosage increases.Because initially, more vacant activation sites are available on the adsorbent surface, so more dye molecules are adsorbed.More adsorbent sites remain unsaturated with an increase in dosage, decreasing the q t value.The maximum MG dye removal is achieved at 20 mg of composite; therefore, 20 mg is chosen as the optimum adsorbent dosage for the next adsorption experiments.

Effect of MG dye concentration and contact time
The effect of initial MG dye concentration and adsorption time is investigated using RGO-βCD-ECH composite, keeping other parameters constant such as adsorption temperature (30°C), adsorbent dosage (20 mg), and solution pH (8.0) and the results are shown in Fig. S2(c).It is seen that the removal rate is more at the initial stage of the adsorption, and then the rate gradually decreases.Because initially, more active vacant adsorption sites are present for the MG dye adsorption and with an increase in time, the active sites are progressively filled with MG dye molecules.From Fig. S2(c), the removal efficiency is observed at a low initial concentration, which gradually decreases with an increase in concentration.As for low initial concentration, the ratio of the accessible active sites of adsorbent to the initial number of ions is high; therefore, the removal efficiency is higher.Whereas for higher concentration, quick saturation occurs on limited active adsorbent sites, reducing the removal efficiency [39].

Effect of solution temperature
The temperature effect on adsorption of MG dye on RGO-βCD-ECH is investigated from 30°C to 40°C, keeping other adsorption parameters constant.It is observed that (Fig. S(2d)) as the temperature increased the %R also increased, which confirms that the adsorption process is endothermic in nature.As with an increase in temperature, the kinetic energy of the MG dye molecule increases, which enhances the mobility of the dye molecules, so the %R increases.The MG dye removal is increased at a higher rate up to 30°C, but after that, the removal is not so high, as a result, the optimal value is 30°C (303 K) for this adsorption process.

Isotherm study
The isotherm study in the batch sorption process is used to calculate the maximum monolayer sorption capacity.It also describes how the adsorbent interacts with pollutants.Also, it is used to study the circulation of ions between the solid and liquid phases at a constant temperature and equilibrium [40].Here, we have explored the batch adsorption process with three typical isotherm models: Temkin, Freundlich, and Langmuir isotherm model to fit the experimental data.
The assumption for Langmuir isotherm is that the adsorption occurs on a homogeneous surface having a single layer of adsorption (monolayer), there is no diffusion of molecules, and the area of adsorption is uniform.The interaction between the adsorbate molecules is absent, and only one molecule can accommodate each adsorption site [41].Freundlich model is described to be an empirical equation applicable for multilayer adsorption.The assumption for this isotherm is that the interaction between adsorbent surface and adsorbed molecules occurs on a heterogeneous surface, and the energy is distributed non-uniformly.In Freundlich isotherm, the adsorption sites are occupied first, having stronger binding energy, and towards the completion of the adsorption process, the energy decreases exponentially [42].The Temkin isotherm ignores the extremely low and high concentrations pollutants and is appropriate for an intermediate concentration range.This model assumes that for tall the molecules present in the layer, the heat of the adsorption process decreases linearly rather than logarithmically with coverage [43].The linear equations of the above-described three models (Langmuir, Freundlich, and Temkin) are given by in Eqn. 1, 2, 3.
where K L (L/mg) is the Langmuir isotherm constant related to adsorption enthalpy, q m is the maximum monolayer adsorption capacity (mg/g), k F ((mg/g)(mg.L −1 ) -1/nF ) is the Freundlich constant, 1/n F is the heterogeneity factor, b T (J/mol) represents the Temkin constant related to the heat of adsorption, T is the absolute temperature (K), R is the universal gas constant (8.314J/mol.K), and A T (L/mg) is the binding energy.The non-linear plots of the above-described isotherm models is shown in Figure 3(a).If 1/n F is nearly equal to zero, then heterogeneous occurs in the adsorption process, and for 0 < 1/n F <1, the adsorption process becomes favourable.For 1/n F is close to unity, chemisorption occurs on the adsorption process, whereas for 1/n F >1, cooperative adsorption occurs.In this adsorption process, the value of 1/n F is 0.291, which confirms favourable process.If b T <40 J/mol, then Physiosorption occurs in adsorption, whereas for 40< b T <80, both Physiosorption and Chemisorption occur [44].Here in this adsorption process, b T is 22.751 J/mol which confirms Physiosorption is the dominant mechanism.
The parameters of the above-described models are presented in Table 1, it is evident that the regression coefficient (R 2 ) is maximum for the Langmuir model (0.998), which indicates a good linear fit, whereas for Temkin, and Freundlich are 0.991, and 0.892, respectively.Thus the models are in the order of Freundlich < Temkin < Langmuir.The evaluated maximum monolayer capacity (q m ) for MG dye remediation is about 902.125 mg/g at 303 K.The adsorption capacity of some reported graphene-based adsorbents is summarised in Table S2.

Adsorption kinetic study
Adsorption kinetic can help to know the dynamics and mechanism of the sorption process.The kinetic study of MG dye is also essential for designing and modelling the adsorption process by selecting optimal parameter conditions.Well-established kinetic models, Intraparticle diffusion (IPD), pseudo-first-order (PFO), and pseudo-second-order (PSO) models are used to analyse data obtained from the batch experimental study to determine the parameters.The linear form of PFO and PSO can be expressed as where q e is the uptake amount of MG dye onto RGO-βCD-ECH composite at equilibrium (mg/g), k 1 is the pseudo-first-order rate constant (min −1 ), and k 2 is the pseudo-secondorder rate constant (g.mg −1 .min−1 ).The parameters mentioned above are evaluated from the linearised plot (Figure 3(b,c) and are presented in Table 2.It is confirmed that the R 2 value is high for all the concentrations, and the evaluated MG dye uptake (q cal ) is very close to the experimental value (q exp ), which suggests the applicability of this model for this adsorption process.
Since none of the above kinetic models can identify the particular diffusion mechanism, Weber's Intra-particle diffusion (IPD) model is studied to find out the controlling ratelimiting steps.It is established that the uptake value at time t, varied proportionally with t 1/2 , and can be written as [41]: where C i (mg/g) is the sorption capacity related to the boundary layer thickness for stage i, and k id (mg.g −1 .min−1 ) is the IPD model rate constant.The IPD plot for all the concentrations (100 to 500 mg/L) is shown in Figure 3(d).The constants C i and k id are evaluated from the IPD plot and presented in Table S3.In the IPD model, if the plot passes through the origin and linear, IPD is considered as the only rate-determining step in adsorption.It is seen that none of the diffusion plots is linear, and it did not pass the origin (C i = 0), which confirms more than one rate-limiting steps are available rather than a single rate-limiting step.It is observed that three linear sections are available in all the concentrations.The first section is for film diffusion, where MG dye molecules start diffusing from the aqueous solution to the adsorption sites dispersed on the composite surface.In the next linear section, the MG dye molecules start diffusion into the composites micropore, mesopore, and macropore.In the final linear section, adsorption equilibrium starts, and Intraparticle diffusion decreases [45].All the parameters of the IPD are given in Table S3, and it is observed that the rate is in the order of k 1d > k 2d > k 3d , which confirmed that in the above adsorption process, both film diffusion and IPD occur.

Thermodynamics study
Thermodynamics analysis of the adsorption process is done to evaluate the parameters such as entropy change (∆S o ), enthalpy change (∆H o ), and Gibbs free energy (∆G o ) by using the following equations [46]: where, K d is the adsorption coefficient, ρ is the solution density (g/L), and R is the universal gas constant (8.314J/mol.K).Fig. S3(a) gives the plot of ln K d vs. 1/T, and the parameters like ∆S o and ∆H o are determined from the linearisation of the plot.Despite the ∆S o and ∆H o depending on temperature, the value did not change over a narrow temperature range from (293-313 K) considered as temperature-independent constants.The positive value of ΔH o (102.047kJ/mol) calculated from the Van't Hoff plot confirms the endothermic nature of the adsorption.The ΔG o value decrease from −17.79 to −25.97 kJ/mol for a range of 293-313 K, these -ve values indicates that at higher temperatures, the MG dye adsorption is spontaneous and more favourable [47].The +ve value of ΔS o (409.306J/ mol.K) listed in Table S4 indicates that randomness increased during adsorption at the solid/solution interface and the affinity of RGO-βCD-ECH composite towards the MG dye decreases.

Regeneration of MG dye loaded adsorbents
From an economical and practical application point of view desorption and regeneration are the most important factors for advanced adsorbents.These two factors of the adsorbent could reduce the cost of the material significantly.For this adsorption desorption-adsorption process, the experiments are performed at pH 8.0, temperature 303 K, concentration 100 mg/L, time 90 min, and adsorbent dosage 20 mg.The regeneration study is carried out using a mixture of acetic acid with low boiling points polar solvent such as methanol or ethanol having volume ratio (V:V = 1:9) as desorption solvent.The removal percentage is calculated in each cycle and the results (Fig. S(3b)) showed that with increasing cycle the performance decreases.Because after adsorption of MG dye, a large number of chemical reactions occurred on the surface of RGO-βCD-ECH composite, and due to this, the pore structure of composite collapsed which reduces the adsorption capacity moderately after regeneration.After five cycles, the MG dye removal remains high at 90.3%.These regeneration results indicate that the RGO-βCD-ECH composite could be cost-effective, efficient, and potential adsorbent for dye removal.

Effect of bed height
The breakthrough (BT) performance of MG dye at three different bed heights (Z) using novel RGO-βCD-ECH composite as bed material is given in Figure 4(a).The fixed-bed parameters are kept constant such as solution pH (8.0), MG dye concentration (100 mg/L), an initial flow rate of MG dye (5 mL/min), and temperature (303 K).The results exhibited that with an increase in Z value from 2 to 4 cm, the exhaustion time (t e ) and breakthrough time (t b ) got extended from 117 to 380 min, and 890-1620 min (Table 3), respectively.Furthermore, the volumetric flow rate of MG dye at a higher bed height is higher than the lower bed height.Because a mass transfer zone (MTZ) occurs during the adsorption process in the column, bed, so the RGO-βCD-ECH composite spends more time in contact with the solution and travels a greater distance from the inlet to the bed exit.The higher bed height slowly saturates, while the smaller bed height saturates faster.As a result, the BT curves shifted away from the origin with an increase in Z value.The q bed and %R values also increased from 320.63 to 358.91 mg/g, and 48.19 to 58.11%, respectively, for bed height 2-4 cm.The empty bed contact time (EBCT) increased from 0.706 to 1.413 min, with the Z value increasing from 2 to 4 cm.These results indicate that maximum adsorption occurs at a higher Z value, so the subsequent experiments using Z = 4 cm are carried out.

Effect of MG dye flow rate
The adsorption of MG dye in an FBAC is studied at three different flow rates: 5, 10, and 15 mL/ min by using composite as a bed material to know the effect of flow rate on the adsorption performance.The other column parameters such as MG dye concentration (100 mg/L), bed height (4 cm), pH of the solution (8.0), and temperature (30°C) are kept constant.The BT curve of all the three flow rates is shown in Figure 4(b).It is observed that the t b of the FBAC is 109, 251, and 380 min corresponding to the flow rates of 15, 10, and 5 mL/min, respectively, and the saturation times are 630, 990, and 1620 min, respectively.At a low flow rate, the t e of the FBAC is maximum, which suggests that the pollutant MG dye has enough time to enter into the composites internal pores, which enhances the column's adsorption performance [32].From Table 3, it is witnessed that the increased MG dye flow rate reduces the %R and q bed from 58.11 to 41.36% and 358.91 to 298.13 mg/g, respectively.This decrease in %R and q bed is due to the less contact time between the adsorbent and MG dye.The EBCT of the column decreased from 1.413 to 0.471 min, with flow rate increased from 5 to 15 mL/min.Table 3

Decolourization of real wastewater in a fixed-bed column
The nature and properties of a laboratory-made single component aqueous solution are entirely different from real industrial wastewater as it contains various other types of ions and complex components.The adsorption of real textile effluent will give a clear viewpoint of applying the RGO-βCD-ECH composite in a fixed-bed column study.The real wastewater for this experiment is collected from the local textile industry, and the physicochemical properties are presented in Table S5.The experimental operating conditions are temperature (30°C), influent flow rate (5 mL/min), column bed height (4 cm), and solution pH (8.0).The treatment procedure for this multi-component real wastewater is similar to a single component aqueous solution, and the result is shown in Figure 4(c).The performance of the column has resulted in the decolourisation of 60.25% from real wastewater.The above result is lower than the single-component aqueous solution due to suspended solids, organic compounds, and coexisting ions in wastewater that interacts with MG dye for adsorption.The successful treatment of real wastewater using RGO-βCD-ECH composite suggests the practical application of this composite as an adsorbent for industrial purposes.

Conclusions
A simple wet chemical strategy successfully synthesises the RGO-βCD-ECH composite material.Batch adsorption studies confirmed that adsorption parameters play an essential role in the process of adsorption, such as solution pH, concentration of MG dye, amount of adsorbent dosage, temperature, and adsorption time.The equilibrium data followed the Langmuir model and the maximum uptake is about 902.125 mg/g.The rate of adsorption followed the Pseudo-second-order model with little deviation of the calculated values from the experimental values.From the intra-particle diffusion analysis, it is confirmed that both intra-particle and film diffusion are controlled the sorption mechanism.The positive value of ΔS o suggests the randomness at the solution/solid interface, while the positive value of ΔH o suggests endothermic process occurs.The negative values of ΔG o confirmed that the adsorption process is spontaneous in nature.The synthesised composite effectively regenerated from the MG dyeloaded adsorbent using a solution of ethanol or methanol with acetic acid to remove 90.3% after five adsorption-desorption cycles.The column study confirmed that the parameters like q bed , t e , t b , and %R decrease as the flow rate decreases, whereas the value increases with the height of the bed increases, the value increases.The FBAC performs best at higher bed heights (4 cm) and lower flow rates (5 mL/min) such as q bed = 361.02mg/g, t e = 1620 min, t b = 360 min, and %R = 65.24.Both Thomas and Y-N models are better fitted with the experimental data obtained from the lab-scale model.For real wastewater, the composite can adsorb up to 60.25% and these results exhibit a bright future of novel RGO-βCD-ECH composite for the application of wastewater treatment.

Figure 2 (Figure 1 .
Figure1shows the SEM image of βCD-ECH, RGO, GO, and RGO-βCD-ECH to find out the dispersion of RGO in the βCD-ECH surface and the resulting polymer surface.In GO (Figure1a), the oxygen-containing functional groups made the crumbled sheets stuck together.The GO sheets are randomly aggregated, the surface is very rough, there is heterogeneous surface structure, wrinkles are present, and the sheets are thin.In the SEM image of RGO (Figure1b), the topographic characteristics evolved to a certain degree to form a well-developed mesopore.Also, it is observed that the surface of RGO is corrugated, highly wrinkled, irregular structure, and the sheets are less aggregate[33].It is seen that the cross-linked βCD-ECH are irregular solid particles, and the surface is porous, which confirms that in the composite, the main adsorption mechanism cannot be a nanopore structure[34].The βCD-ECH polymer surface (Figure1c) is porous, regular shaped, and smooth because of cross-linked networks.However, the shape of βCD is a parallelogram, and the particles are three-dimensional, suggesting that the polymer is successfully synthesised.After introducing RGO sheets onto the polymer agglomeration surface starts, the synthesised RGO-βCD-ECH composite (Figure1d) looks different from the pure βCD-ECH.To determine the crystal structure and the interlayer spacing (d-spacing) of the synthesised material, XRD analysis is used.Figure2(a) shows the XRD pattern of RGO-βCD-ECH, βCD-ECH, RGO, and GO.A sharp peak is obtained for graphite powder with d-spacing

Table 2 .
Pseudo-first order and pseudo-second-order kinetic model parameters for MG dye adsorption onto RGO-βCD-ECH composite.

Table 3 .
Process parameters of fixed-bed column for MG dye adsorption onto RGO at various operating conditions.