Synthesis and characterization of rGO/ZrO₂ nanocomposite for enhanced removal of fluoride from water: kinetics, isotherm, and thermodynamic modeling and its adsorption mechanism

Abstract

A nanocomposite of rGO/ZrO₂ prepared by a simple hydrothermal method using GO and ZrOCl₂·8H₂O has been successfully utilized for the removal of fluoride from aqueous solutions by adsorption. The synthesized nanocomposite was characterized by various techniques, such as FT-IR, XRD, SEM, EDX, TGA, XPS, Raman spectroscopy and BET surface area measurement. Various process parameters viz. rGO/ZrO₂ dose, initial fluoride concentration, temperature and pH, which influence the removal of fluoride, were studied and it was found that the maximum uptake capacity of the nanocomposite was 46 mg g⁻¹ at 30 °C, pH 7, rGO/ZrO₂ dose of 0.5 g L⁻¹ and initial fluoride concentration of 25 mg L⁻¹. The rGO/ZrO₂ possesses a high surface area (632 cm² g⁻¹) and maximum adsorption occurs at neutral pH and ambient temperature. Therefore, rGO/ZrO₂ can be used for the adsorption of fluoride without much alteration in the quality of drinking water. The experimental data was applied to various kinetics, isotherm and thermodynamic studies. The monolayer adsorption of fluoride followed a pseudo-second order kinetic model, which was found to be spontaneous and endothermic in nature. The results obtained from the current adsorption system might be helpful for designing a continuous column system for the treatment of fluoride contaminated water.

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1. Introduction

Fluoride is necessary for bone formation and prevents tooth decay when its concentration is present within the permissible limits (WHO recommends 1.5 mg L⁻¹). An excess amount of fluoride causes detrimental effects on human health leading to dental and skeletal fluorosis, brittle bones, osteoporosis, and arthritis.¹ In addition to these deleterious effects, it can also induce cancer, immunological problems and birth defects.^2,3 Water gets polluted with fluoride by weathering of fluoride-containing minerals, such as fluorite, apatite, rock phosphate and topaz,⁴ and discharges from industries as large amounts of fluorochemicals are being used in various industries, such as semiconductor manufacturing, electroplating, coal, ceramic production and aluminium smelting.⁵ An excess concentration of fluoride in drinking water is a serious problem in many parts of the world.⁶ Various technologies have been reported previously and are available for abatement of fluoride from water, such as coagulation and precipitation^7–9 membrane processes,^10,11 adsorption, electrochemical treatments^12,13 and ion-exchange.¹⁴ Among them, the adsorption process is generally accepted as the cheapest and most efficient method for the removal of fluoride from water.¹⁵

Zirconium oxide is an important transition metal oxide with high chemical inertness, non-toxic nature and insoluble in water, which makes it more environmentally friendly.¹⁶ Besides all these properties, it is highly biocompatible, cheap to produce, thermally stable and electrochemically active.¹⁷ Similar to alumina,^18,19 zirconium(IV) also exhibits specific binding affinity toward fluoride through Lewis acid–base (i.e., metal–ligand) interactions²⁰ and its insolubility under a wide pH range indicates its great applicability under different solution chemistries. Various carbonaceous materials, such as carbon nanotubes and activated carbon, are quite effective for facile defluoridation of water owing to their high specific surface area, surface functional groups and requisite pore size distribution. Recently, graphene oxide (GO) has emerged as an attractive member of the carbon family because of its high surface area and the presence of various functional groups (hydroxyl, epoxy groups and carboxylic).^21,22 Graphene has also been reported to be an effective adsorbent for the adsorption of fluoride with an uptake capacity of 17.65 mg g⁻¹.²³ A novel metalloporphyrin-grafted graphene oxide has been utilized as a sensor for fluoride in aqueous solution.²⁴ Hence, it is possible that a suitable modification of graphene oxide could also assist the adsorption of fluoride with good adsorption capacity. It is well documented that graphene oxide acts as an excellent host for nanoparticles and in turn the nanoparticles provide structural rigidity by inhibiting the different layers of graphene from restacking. There are many composites comprising graphene oxide and metal oxide, viz. manganese oxide-coated graphene oxide²⁵ (adsorption capacity of 11.93 mg g⁻¹) and basic aluminium sulfate@graphene hydrogel²⁶ (adsorption capacity of 33.4 mg g⁻¹) have been reported, which showed good adsorption capacity towards fluoride. Therefore, the incorporation of ZrO₂ into graphene oxide may show a significantly high fluoride adsorption capacity.

In the present investigation, a nanocomposite of reduced graphene oxide and ZrO₂ nanoparticles (rGO/ZrO₂) has been prepared by the hydrothermal route for potential application towards fluoride removal. Since Zr⁴⁺ is a hard acid and F⁻ ion is a hard base, it is anticipated that there may be strong interactions between these two oppositely charged ionic species. Moreover, the graphene oxide, which gets reduced during the reaction, would also contribute in fluoride adsorption as its surface become less negative, which facilitates the interaction of fluoride with the ZrO₂ nanoparticles on the rGO surface. Furthermore, the effects of various process parameters (pH, adsorbent dose, fluoride concentration, temperature and contact time) on the adsorption were investigated. To explore the adsorption mechanism, detailed thermodynamics, isotherm and kinetic studies were also conducted.

2. Materials and methods

2.1. Materials

All the chemicals involved in this study were of analytical grade. Graphite powder (mesh size 150 μm), zirconyl chloride octahydrate (ZrOCl₂·8H₂O), phosphoric acid (H₃PO₄), sulfuric acid (H₂SO₄) and potassium permanganate (KMnO₄) were purchased from Sigma-Aldrich. An aqueous stock solution containing 1000 mg L⁻¹ of fluoride was prepared by dissolving the desired amount of sodium fluoride (NaF) into deionized double distilled water (DW). All the other solutions were prepared in DW water throughout the experiments. The working solutions for the adsorption experiments were prepared from the stock solution (NaF) as per the requirements. During the adsorption experiments, 0.1 N NaOH and 0.1 N HCl were used for the adjustment of pH.

2.2. Preparation rGO/ZrO₂ nanocomposite

GO was synthesized by modified Hummers' method by oxidizing the graphite powder.²⁷ In the typical process of GO synthesis, graphite powder (3 g) and KMnO₄ (15 g) were mixed in a solution containing concentrated H₂SO₄/H₃PO₄ (360 : 40 mL) in a 9 : 1 ratio. The resulting mixture was stirred at 50 °C for 12 h then cooled to room temperature. The cooled reaction mixture was then decanted into a beaker containing ice. Further, 30% H₂O₂ (3 mL) was added to it and subjected to stirring for another 2 h. The solid black compound thus obtained was washed with HCl (5%) 3–4 times as well as with double distilled water repeatedly till the pH become neutral. For each wash, the material was suspended through ultrasonication and recollected through centrifugation at 10 000 rpm for 15 min. Then, the obtained product was vacuum dried for 24 h. The rGO/ZrO₂ nanocomposite was prepared by a simple one-step hydrothermal method in which ZrOCl₂·8H₂O acted as the ZrO₂ precursor and GO as the supporting material. In this procedure, 0.07 g of GO was dispersed uniformly into 50 mL of DW by ultrasonication for 2 h. In another beaker, 30 mL of an aqueous solution of ZrOCl₂·8H₂O (0.03 M) was taken and stirred vigorously for 30 min. After that, the GO suspension was added to the solution of ZrOCl₂·8H₂O with stirring. The final mixture was ultrasonicated for 1 h followed by addition of 1 mL of hydrazine hydrate to it, and the mixture was placed in a stainless steel Teflon-lined autoclave at 180 °C for 18 h. The black colored composite thus obtained was separated by centrifugation and then repeatedly washed with distilled water and subsequently by ethanol. In the last step, the collected rGO/ZrO₂ nanocomposite was dried overnight at 120 °C. The rGO and ZrO₂ nanoparticles were also prepared by the same method but without the addition of ZrOCl₂·8H₂O and GO.

2.3. Characterization of rGO/ZrO₂

The Fourier Transform-InfraRed (FT-IR) spectra of GO and rGO/ZrO₂ were recorded in the range of 4000–400 cm⁻¹ using a Spectrum 100, PerkinElmer spectrometer. The type of functional groups and their oxidation states were determined by X-ray Photoelectron Spectroscopy (XPS) analysis using Mg Kα (1253.6 eV) radiation as the X-ray source with an AMICUS, Kratos Analytical. A Shimadzu Quanta 200 F Scanning Electron Microscopy (SEM) equipped with Energy-Dispersive X-ray Spectroscopy (EDS) was used to observe the morphology and composition of the GO and rGO/ZrO₂. The structural characteristic of the prepared GO and rGO/ZrO₂ were studied by X-ray diffraction (XRD) pattern recorded in the 2θ range of 5–60° using a Rigaku Miniflex II X-ray diffractometer with Cu Kα (λ = 1.5406 Å) radiation and an Ni filter. The X-ray scan rate was 6° min⁻¹ with a step size of 0.02°. The specific surface area (SSA) of rGO/ZrO₂ was measured by Brunauer–Emmett–Teller (BET) method through nitrogen adsorption–desorption isotherm conducted using Micrometrics, USA, FAP 2020 Model. Thermogravimetric Analysis/Differential Scanning Calorimetry (TGA) analysis was carried out with an STA 6000 PerkinElmer. The concentration of fluoride in the solution was measured using a Thermo Orion expandable ion analyzer EA 940 connected with a fluoride-selective electrode. During the measurement of the fluoride concentration, the buffer TISAB III was used to maintain optimum pH.

2.4. Batch adsorption studies

A series of adsorption experiments were performed in batch mode with 25 mL of a solution of desired fluoride concentration in 100 mL Erlenmeyer flasks. To each Erlenmeyer flask, the calculated amount of rGO/ZrO₂ was added and sealed with a lid. Further, flasks were shaken in a thermostatic orbital shaker (at 100 rpm) for a specified time. Afterward, rGO/ZrO₂ was separated by centrifugation at 5000 rpm for 10 min and the supernatant liquid was collected for analyzing the residual concentration of fluoride. The uptake capacity of rGO/ZrO₂ was calculated using the following equation.²⁸where C_i is the initial concentration and C_t (mg L⁻¹) represents the fluoride concentration after time t (min). q stands for the uptake capacity (mg g⁻¹). V (L) is the volume of the fluoride solution and W (g) stands for the amount of nanocomposite added in the adsorption experiments.

The effect of process parameters on adsorption was investigated by varying pH from 2 to 10, contact time from 0 to 90 min, rGO/ZrO₂ dose from 0.1 to 1 g L⁻¹, initial fluoride concentration from 5 to 50 mg L⁻¹ and temperature from 10 to 50 °C. All the experiments were performed at least thrice and the average values were used for the calculation. For further analysis, only significant values and range of parameters were discussed in this study.

2.5. Kinetic studies

The adsorption process is time-dependent and is affected by the chemical and physical properties of the adsorbent. Therefore, kinetic studies were conducted to explore the mechanism of adsorption and to investigate the rate determining step of fluoride adsorption onto the rGO/ZrO₂. In this study, various kinetic models, such as pseudo-first order, pseudo-second order, mass transfer, intraparticle diffusion and Richenberg, were used to study the different kinetic parameters.

2.5.1. Pseudo-first order kinetic model. The equation for the pseudo-first-order kinetic model can be expressed as follows:²⁹where q_e and q_t are the amounts of fluoride adsorbed per unit weight of adsorbent (mg g⁻¹), i.e. uptake capacity at equilibrium and at time t, respectively. k_s stands for equilibrium rate constant, which is calculated from the slope of the graph plotted between log(q_e − q_t) vs. t (min).

2.5.2. Pseudo-second order kinetic model. The linear expression for pseudo-second-order kinetic model can be given as follows:in which k′₂ stands for the equilibrium rate constant for the pseudo second order kinetic model, which can be calculated from the graph of t/q_t vs. t (min).

2.5.3. Mass transfer study. The adsorption process involves the transfer of pollutant species from solution phase to the adsorbent surface. There are three steps involved in the transfer of adsorbate from the liquid phase to a solid adsorbent surface.³⁰

1. Transportation of adsorbate species from solution to boundary film.

2. External diffusion: adsorption of pollutant species on the adsorbent surface by diffusion of adsorbate from boundary film to the adsorbent surface.

3. Intraparticle diffusion: transfer of adsorbate to the internal pore and spaces present in between the active sites of the adsorbent.

4. Adsorption and desorption of the adsorbate at the active site of the adsorbent surface.

From the above four steps, an inference can be drawn that step 1 cannot be considered as a rate limiting step because the process of adsorption takes place in hydro-dynamic conditions, which inhibits the establishment of the concentration gradient in between the solution phase and the boundary film. Further, step 4 is deliberated as rapid enough so it cannot take part in rate determination. Therefore, one of the steps, either external diffusion or intraparticle diffusion, will be the slowest step, which may act as the rate controlling step of this adsorption process.³⁰ So the contribution of both these steps is studied by the linear form of the Mckay et al. model given as follows:

where C_i and C_t represent the initial concentration of fluoride (mg L⁻¹) and concentration after time t (min), respectively. The m denotes adsorbent mass per unit volume (g L⁻¹), k stands for the Langmuir constant and S_s is the specific surface area of adsorbent per unit volume of the reaction mixture (cm⁻¹). β_t stands for the coefficient of external mass transfer (cm² s⁻¹).

2.5.4. Intraparticle diffusion. In the process of adsorption through batch mode, there is a possibility of diffusion of the adsorbent into the intraparticle spaces or pores of the solid adsorbent. This process is often considered as the rate determining step of adsorption. The intraparticle diffusion rate constant (k_id) at different temperatures can be determined by the Weber–Morris model, which can be expressed as follows:³¹

q_t = k_idt^0.5 + C (v)

where k_id (mg g⁻¹ h^−0.5) is determined by the slope of the respective plot of q_t vs. t^0.5.

2.5.5. Richenberg model. This expression is applied to check whether the current adsorption process involves external diffusion or intraparticle diffusion, which can be given by the following equation:where G = q_e/q_t and B_t is the mathematical function of G, which can be evaluated from respective value of G by the following equation:

B_t = −0.4977 ln(1 − G) (vii)

The slope of the plot of B_t vs. t gives an idea about the true rate controlling step of adsorption. If the plot does not pass through the origin, then both external diffusion and intraparticle diffusion jointly determine the rate of adsorption.³²

2.6. Isotherm studies

Different isotherm models were used to investigate the distribution of adsorbate between the liquid and the solid phase at equilibrium. Therefore, for scheming of an efficient adsorption system, the correct adsorption isotherm should be applied. For the current adsorption system, three isotherm models were applied, viz. Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherms, to evaluate the equilibrium data of the adsorption of fluoride.

2.6.1. Langmuir isotherm. This isotherm model describes the process of uptake of adsorbate on a homogenous surface in a monolayer fashion where there are no interactions between two adsorbate molecules.³³ Langmuir isotherms may be expressed in linear form and written as follows:where q⁰ and b are termed as isotherm constants and can be calculated by the slope and intercept of the plot between C_e/q_e and C_e, respectively.

2.6.2. Freundlich isotherm. The Freundlich isotherm model assumes multilayer adsorption that occurs on the heterogeneous surface of the adsorbent. It can be represented in linear form as follows:³⁴

Isothermic constants k_f and n were calculated by the intercept and slope of the plot of log C_e vs. log q_e.

2.6.3. Dubinin–Radushkevich sorption. In order to investigate the essence of adsorption, the Dubinin–Radushkevich (D–R) isotherm was applied to the adsorption equilibrium data. This isotherm is applied to study the adsorption free energy. The linear equation associated with this isotherm can be written as:

ln q_e = ln X_m − βF² (x)

where F stands for Polanyi potential. X_m and β were obtained from the intercept and slope of the plot of ln q_e vs. F². β (isotherm constant) is related to mean free energy (E) of adsorption according to the following equation:where the value of E can be defined as free energy associated with the transfer of 1 mol of adsorbate to the adsorbent surface from infinity. Depending upon the value of E, the type of process involve in adsorption, either physisorption or chemisorption, can be determined.³⁵

2.7. Thermodynamics studies

The thermodynamic study was used to examine the thermodynamic feasibility of the process and to provide an idea about the Gibbs free energy change associated with the adsorption process. Various thermodynamic parameters (ΔG⁰, ΔS⁰, and ΔH⁰) were used to describe whether the process is endothermic or exothermic and feasible or not. These thermodynamic parameters can be calculated from the following equations:³⁶

ΔG = −RT ln k_c (xiv)

where C_ae (mg L⁻¹) denotes the equilibrium adsorbate concentration, C_e is the concentration of adsorbate in bulk at equilibrium (mg L⁻¹), T (in Kelvin) stands for the absolute temperature and R is the universal gas constant (8.314 J mol⁻¹).

3. Results and discussion

3.1. Characterization of rGO/ZrO₂

3.1.1. FTIR study. The FTIR analysis provided information on the different types of functional groups present on the adsorbent. The FTIR analysis of ZrO₂, GO and rGO/ZrO₂ are shown in Fig. 1. In the spectra, a broad and strong absorption band at approximately 3266 cm⁻¹ attributed to the O–H stretching vibration of water molecules present on the surface of nanocomposite was observed. The spectrum of ZrO₂ presents a peak for Zr–O stretching vibration at 619 cm⁻¹.³⁷ The FTIR spectrum of GO represents the characteristics peaks at 1724 cm⁻¹ (C=O stretching vibration of –COOH), 1618 cm⁻¹ (O–H bending vibration of water molecules and C–C skeleton vibration of GO), 1396 cm⁻¹ (O–H bending vibration of C–OH and –COOH), 1245 cm⁻¹ (C–O stretching vibration of ethers and epoxy) and 1062 cm⁻¹ (C–O–C stretching vibration). In the spectrum of the rGO/ZrO₂ nanocomposite, the typical stretching vibration peak of C=O shifted from 1724 to 1741 cm⁻¹ and its intensity also decreased appreciably, which was due to the interaction of the C=O group and Zr.³⁸ The peaks below 700 cm⁻¹ belong to the characteristic peaks of ZrO₂.³⁷ So, it can be concluded that GO is successfully decorated by ZrO₂ nanoparticles.

Fig. 1 FT-IR spectra of GO, rGO/ZrO₂ and ZrO₂.

3.1.2. XRD studies. To study the crystalline structure of GO, rGO, ZrO₂ and rGO/ZrO₂ were characterized by XRD analysis, as presented in Fig. 2. In the XRD pattern of GO, the most evident diffraction peak at 2θ was found at 10.8° for the 002 reflection plane, which corresponds to d-spacing of 0.85 nm.³⁰ This d-spacing is significantly expanded as compared to that of 0.34 nm for pristine graphite, which revealed that the natural graphite is successfully oxidized into GO.³⁹ The pattern of rGO showed a characteristic broad peak at 24.2°. The crystal structure of the ZrO₂ nanoparticles showed characteristic peaks corresponding to the tetragonal structure (JCPDS card, file no. 89-7710) at 2θ = 29.9°, 34.7°, 43.02°, 52.2°, whereas the XRD pattern of the rGO/ZrO₂ nanocomposite did not show any characteristic peaks at the low angle, suggesting that GO was well reduced during the process.⁴⁰ The XRD pattern of rGO/ZrO₂ also showed the characteristic peaks corresponding to the tetragonal structure of ZrO₂ as well as for the rGO. The above data showed all the featured peaks corresponding to rGO and ZrO₂, which suggested that ZrO₂ was successfully grafted on the rGO.

Fig. 2 XRD pattern of GO rGO, ZrO₂ and rGO/ZrO₂.

3.1.3. XPS studies. The XPS spectra of rGO/ZrO₂ provide information about the elements present in the nanocomposite. Fig. 3a represents the wide scan XPS spectrum of rGO/ZrO₂ exhibiting the characteristic peaks of C1s, O1s and Zr3d, which show the presence of carbon, oxygen and zirconium elements in the nanocomposite. The core levels C1s XPS spectrum of GO (Fig. 3c) exhibited four type of functionalized carbon atoms viz. C–C/C=C (284.7 eV), C–OH (286.7 eV), C=O (288.1 eV) and C=O–OH (289.1 eV). Fig. 3d presents the C1s spectrum of rGO/ZrO₂ having the same functionalized carbon analogous to the GO nanosheets but their intensities were weaker those that of GO, which indicates that GO was successfully reduced. An additional peak at 285.6 eV corresponding to the C of the C–N bond of hydrazine appeared in Fig. 3d, which suggested that the GO was deoxygenated by the hydrothermal process.³⁸ Fig. 3e presents the XPS spectrum of Zr, which shows peaks at 183.5 and 185.7 eV corresponding to the binding energies of Zr3d_5/2 and Zr3d_3/2, respectively.

Fig. 3 XPS spectra: (a) wide scan spectra of rGO/ZrO₂, (b) wide scan spectra of rGO/ZrO₂ after fluoride adsorption, (c) core level C1s spectrum of GO, (d) core level C1s spectrum of rGO/ZrO₂ and (e) core level Zr3d spectrum of rGO/ZrO₂.

In order to study the mechanism of fluoride removal and interaction between fluoride and adsorbent, XPS analysis was conducted before and after the adsorption. A wide scan XPS spectrum of rGO/ZrO₂ nanocomposite after fluoride adsorption is shown in Fig. 3b. On comparing Fig. 3a with 3b, it was noticed that a new peak appeared in Fig. 3b at the binding energy of 684.75 eV, which is attributed to F1s. Moreover, in the spectrum of Zr (Fig. 3e), shifting in the peak of Zr3d from 184.4 eV to 184.8 eV was also observed, which indicated the interaction between fluoride and Zr.⁴¹ The peaks corresponding to the Zr3d_3/2 and Zr3d_5/2 broadened and shifted toward higher binding energy owing to the interaction with fluoride, which signified the formation of new zirconium complexes with fluoride such as ZrF₄ or Zr-oxyfluorides. In this process, zirconium bonded to a more electronegative element, which caused the greater electron withdrawal from zirconium. Therefore, the reduction in electron density on zirconium taken place, leading to shifting of the peaks.⁴²

3.1.4. SEM and EDX studies. The SEM image of the GO (Fig. 4a) showed the characteristic crumpled, wrinkled and sheet-like arrangement while the SEM image of rGO/ZrO₂ (Fig. 4b) shows a new morphology with a uniform distribution of numerous ZrO₂ nanoparticles on the surface of the rGO sheets. The high-magnification SEM image of the rGO/ZrO₂ nanocomposite is shown in Fig. 4c, which clearly indicates that a large quantity of ZrO₂ nanoparticles of average size 35 nm are homogeneously anchored onto the surface of rGO. Fig. 5 presents the elemental mapping analysis of rGO/ZrO₂ for C, Zr, and O, which also revealed that the ZrO₂ nanoparticles were evenly distributed on the surface of rGO. Furthermore, Fig. 6a and b show the EDX spectra of GO and rGO/ZrO₂, respectively. The spectra of GO exhibited signals for C and O while the signals for C, O and Zr in the rGO/ZrO₂ spectrum suggested the incorporation of ZrO₂ on the GO surface. The EDX spectrum of rGO/ZrO₂ after fluoride adsorption (Fig. 6c) shows the signal for fluoride, which advocated the adsorption of fluoride on the surface of rGO/ZrO₂.

Fig. 4 (a) SEM image of GO, (b) SEM image of rGO/ZrO₂ at low magnification and (c) SEM image of rGO/ZrO₂ at high magnification.

Fig. 5 Elemental mapping images of (a) C, (b) O and (c) Zr element.

Fig. 6 EDS spectra of (a) GO, (b) rGO/ZrO₂ and (c) rGO/ZrO₂ after fluoride removal.

3.1.5. Raman analysis. Raman analysis is a very important means of characterization of carbon-based materials, particularly the sp³ and sp² hybridized carbon atoms that are present in the graphene oxide. The synthesized rGO/ZrO₂ and GO were studied by Raman analysis and the results are presented in Fig. 7. The spectra displayed the typical pattern of GO with the characteristic D and G bands at 1362 and 1599 cm⁻¹, respectively.⁴³ After rGO/ZrO₂ formation, the G band shifted to the lower frequency region, i.e. blue shift, whereas the D band was unchanged as in GO, which confirmed the composite formation as well as reduction of GO.⁴⁴

Fig. 7 Raman spectra of GO and rGO/ZrO₂.

3.1.6. TGA analysis. The thermal stability of rGO/ZrO₂ was investigated through TGA analysis. Fig. 8 displays the TGA curve of the rGO/ZrO₂. In the first stage, the nanocomposite showed 7% weight loss in the temperature range of 110–220 °C, which was due to the removal of the physically adsorbed water molecules. However, in the second step, 8% weight loss was observed between temperature ranges of 220–600 °C, which was attributed to the removal of crystallization water. Therefore, the prepared adsorbent, i.e. rGO/ZrO₂, is found to be stable up to 600 °C. As a result, rGO/ZrO₂ can be effectively used for fluoride removal as most of the adsorption experiments were performed at temperatures below 50 °C.

Fig. 8 TGA curve of rGO/ZrO₂.

3.1.7. Determination of pHzpc. The pHzpc is the pH at which the surface of the adsorbent behaves as electrically neutral. For the current study, we used NaCl (0.01 N) solution and the pH was adjusted from 2 to 12 using NaOH and HCl.⁴⁵ For this purpose, 50 mL of NaCl solution and 0.01 g of rGO/ZrO₂ were added into each flask. The flasks were left for 24 h and the final pH was measured. Then the graph between pH initial and pH final was plotted (see Fig. 9a). The pH at which the two curves intersects is considered as pHzpc.

Fig. 9 (a) pHzpc of rGO/ZrO₂ and (b) N₂ adsorption desorption isotherm of rGO/ZrO₂.

3.1.8. Surface area analysis. The porous characteristic of rGO/ZrO₂ was measured by nitrogen adsorption–desorption at 77 K, which is shown in Fig. 9b. The nitrogen adsorption–desorption isotherms were utilized in the Brunauer–Emmett–Teller (BET) model and the SSA was calculated to be 632.67 m² g⁻¹.

3.1.9. Mechanism of rGO/ZrO₂ nanocomposite formation. All the characterizations discussed above evidently confirmed that the ZrO₂ nanoparticles had been successfully grafted onto the rGO sheets. The synthesis process of the rGO/ZrO₂ nanocomposite is illustrated in Scheme 1. When the ZrOCl₂ solution was mixed with an aqueous solution of GO, the tetramer complex [Zr₄(OH)₈(H₂O)₁₆]⁸⁺ formed as a major species.⁴⁶ The tetramer has 8 hydroxo bridges and 16 coordinated water molecules, which bind on the surface of GO owing to the abundant negative charges and the plentiful oxygen-containing functional groups on the surface and edges of the GO sheets. When this solution was mixed into a strong basic solution, the tetramer complex released certain amounts of H⁺ ions from the coordinated water in terms of the following equation.

[Zr(OH)₂·4(H₂O)]₄⁸⁺ → [Zr(OH)_2+x·(4 − x)(H₂O)]₄^(8−4x)+ + 4xH⁺. (xvi)

Scheme 1

When the aqueous solution of ZrOCl₂·8H₂O is heated, there is a shift in the equilibrium towards the right side and the [Zr(OH)_2+x·(4 − x)H₂O]₄^(8−4x)+ concentration tends to increase. It is well known that both these complexes viz. [Zr(OH)₂·4H₂O]₄⁸⁺ and [Zr(OH)_2+x·(4 − x)H₂O]₄^(8−4x)+ are covered by plenty of OH groups, which can react with the carboxylic groups present on the surface of the GO forming C–O–Zr by esterification.³⁷ Later on, as the reaction proceeds the Zr⁴⁺ complex ions change into stable ZrO₂ nanoparticles by nucleation and further growth. Simultaneously, in the presence of hydrazine hydrate, GO is reduced to rGO. Ultimately, the ZrO₂ nanoparticles were formed and uniformly attached to the rGO nanosheets to form the nanocomposite.

3.2. Effect of pH on uptake capacity of rGO/ZrO₂ and mechanism of fluoride adsorption

The effect of pH on fluoride adsorption was studied at five different pH levels between 2 to 10 by keeping other parameters like dose (0.5 g L⁻¹), temperature (30 °C) and initial fluoride concentration (25 mg L⁻¹) constant. Fig. 10a explains the effect of pH on the uptake capacity of rGO/ZrO₂ and the maximum adsorption capacity was achieved at neutral pH, i.e. 7. Further, the pHzpc was found to be 7.3, which revealed that at pH > pHzpc the surface of the adsorbent acquired a negative charge whereas at pH < pHzpc the adsorbent surface became positively charged. Thus, the rGO/ZrO₂ surface was favourable for fluoride adsorption at pH below 7.3. The reduced uptake capacity observed at lower pH was due to the protonation of fluoride and the formation of hydrofluoric acid.²⁵ Moreover, the adsorption capacity of rGO/ZrO₂ falls considerably at pH > 7. The reason for the decline in uptake capacity can be accounted for by the fact that at higher pH the adsorbent surface retains a negative charge (pH > pHzpc), which tend to repel the negatively charged fluoride electrostatically. In addition, the concentration of OH⁻ also increased, which competed with the fluoride for the active binding sites of the adsorbent.

Fig. 10 Effect of parameters on the uptake capacity: (a) pH, (b) contact time at different rGO/ZrO₂ doses, and (c) comparison of uptake capacities of GO, rGO and rGO/ZrO₂.

The mechanism by which fluoride tends to adsorb by rGO/ZrO₂ can be explained mainly by the HSAB principle. According to this principle, Zr⁴⁺ is a hard acid and it has a high affinity for fluoride, which is a hard base.⁴⁷ In the aqueous phase, ZrO₂ exists in hydroxide form, which interacts with fluoride and exchanges the hydroxide ion with it.

3.2.1. Mechanism of adsorption⁴⁸. 1. By electrostatic interaction.

At low pH region protonation of the surface occurs.

≡Zr–OH + H⁺ → ≡Zr–OH₂⁺ (pH < pHzpc) (xvii)

At high pH region deprotonation of the surface occurs.

≡Zr–OH + OH⁻ → ≡Zr–O⁻ + H₂O (pH > pHzpc) (xviii)

2. By ligand exchange or surface complexation.

≡Zr–OH + F⁻ → ≡Zr–F + OH⁻ (xix)

≡Zr–OH₂⁺ + F⁻ → ≡Zr–OH₂⁺F⁻ (xx)

3. By hydrogen bonding.

O–H + F⁻ → O–H⋯F⁻ (xxi)

The mechanism of adsorption is illustrated in Scheme 2. Overall, it can be concluded that ligand exchange (complex formation), hydrogen bonding and electrostatic interaction were the factors responsible for the adsorption process.

Scheme 2

3.3. Effect of contact time and rGO/ZrO₂ dose on the uptake capacity

The effect of contact time on the uptake capacity of rGO/ZrO₂ was investigated at doses of 0.4, 0.5 and 0.6 g L⁻¹. The experiments were carried out at an initial fluoride concentration of 25 mg L⁻¹, temperature of 30 °C and pH 7. Fig. 10b represents the effects of contact time at rGO/ZrO₂ doses of 0.4, 0.5 and 0.6 g L⁻¹, which indicated that initially the rate of adsorption was very rapid because the active binding sites of rGO/ZrO₂ were free and the concentration gradient of fluoride was also high. However, as the time advanced, the rate of adsorption decreased owing to the reduction in the number of free binding sites and the concentration gradient of fluoride. It was also observed that the equilibrium time did not depend on the rGO/ZrO₂ dose. Therefore, the equilibrium time was 50 min at all rGO/ZrO₂ doses. The uptake capacity increased slightly on increasing the dose from 0.4 g L⁻¹ to 0.5 g L⁻¹, whereas on increasing the adsorbent dose from 0.5 g L⁻¹ to 0.6 g L⁻¹ the uptake capacity decreased remarkably (Fig. 10b). The reduction in uptake capacity occurs at higher dose since the uptake capacity depends on the adsorbate-to-binding sites ratio, which decreased on increasing the rGO/ZrO₂ dose from 0.5 to 0.6 g L⁻¹. That's why at higher rGO/ZrO₂ doses the fluoride became insufficient to cover all the binding sites of the adsorbent. Furthermore, control experiments were also carried out to compare the uptake capacity of GO and rGO (reduced graphene oxide) with the prepared rGO/ZrO₂ nanocomposite (Fig. 10c). It was found that rGO showed a lower uptake capacity (13.8 mg g⁻¹) than rGO/ZrO₂ but more than that of GO (6.4 mg g⁻¹). GO contains plenty of oxygen functional groups, which make it negatively charged and in turn repel the negatively charge fluoride ions. Whereas rGO showed an improved uptake capacity (13.8 mg g⁻¹) because most of its oxygen functional groups get reduced, resulting in a higher uptake capacity than that of GO. Furthermore, on comparing rGO with the rGO/ZrO₂ nanocomposite, it was observed that uptake capacity of the nanocomposite was dramatically increased, which supports the usefulness and effectiveness of the as-synthesized composite.

3.4. Effect of initial concentration and temperature on the uptake capacity

In order to study the effect of initial concentration of fluoride on the uptake capacity of rGO/ZrO₂, experiments were carried out by varying the initial concentration from 5 to 50 mg L⁻¹. The effect of concentration was measured at three different temperatures, 20, 30 and 40 °C, while keeping the rGO/ZrO₂ dose (0.5 g L⁻¹) and pH (7) constant. The results presented in the Fig. 10c clearly indicate that the uptake capacity increased with the increase in initial concentration up to 25 mg L⁻¹. After that, no significant increase in uptake capacity was observed with any further increase in the initial concentration. Therefore, the 25 mg L⁻¹ concentration of fluoride was sufficient to cover the binding sites present on the 0.5 g L⁻¹ rGO/ZrO₂ dose. It was also observed from Fig. 8c that the uptake capacity increased from 38 to 46 mg g⁻¹ on increasing the temperature from 20 to 30 °C, whereas further increasing the temperature up to 40 °C resulted in a decrease in the uptake capacity from 46 to 41.5 mg g⁻¹. Therefore, temperature above 30 °C did not support the adsorption significantly owing to the increased randomness of ions in solution phase, which became a limiting factor and neutralized the favorable effect of temperature.³⁰

A comparison of the rGO/ZrO₂ nanocomposite with formerly reported adsorbents for the removal of fluoride is presented in Table 1.^{23,25,26,49–54}

Table 1 Comparison of rGO/ZrO₂ nanocomposite with previously reported adsorbents for the removal of fluoride in terms of uptake capacity

S. no.	Adsorbent	Uptake capacity	pH	Reference
1	Graphene	17.65	7	23
2	MOGO	11.93	5.5	25
3	BAS@GHG	33.4	7.2	26
4	Nano alumina	14	6.15	45
5	ZrWP	2	3	46
6	Zr-CCB	4.8	7	47
7	Zirconium oxide	19	4.75	48
8	Fe–Zr hybrid oxide	7.5	6.8	49
9	Zr(IV) impregnated into collagen fiber	41.4	5.5	50
10	rGO/ZrO2	46	7	Present work

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3.5. Kinetic studies

In this investigation, various kinetic models were employed to determine the rate controlling step and the mechanism of the adsorption process at different adsorbent doses. The kinetic experiments were conducted at three variable doses of 0.4, 0.5 and 0.6 g L⁻¹.

3.5.1. Pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order kinetic model was examined by plotting log(q_e − q_t) vs. t (min). The plots were approximately linear at all doses. The kinetic model parameters and respective correlation coefficients (R²) are listed in Table 1. For the pseudo-second-order model, parameters and R² were calculated by the linear plot of t/q_t vs. t (min) (Fig. 11a). Results are summarized in Table 2. From the study, it was observed that pseudo-second-order corresponded to an appreciably linear plot with a high value of R² in comparison to that of the pseudo-first-order kinetic model, which revealed that the kinetics of the current adsorption system can be explained in a better way with a pseudo-second-order kinetic model. Thus, it can be concluded that the rate of the adsorption is proportional to the square of free binding sites (q_e − q_t)² on the adsorbent. Further, there is an increase in the rate constant of the pseudo-second-order model with increase in adsorbent dose, which provides evidence of the applicability of the pseudo-second-order kinetic model (Table 2).

Fig. 11 (a) Fluoride ion concentrations at different temperatures. Kinetic models plotted at different rGO/ZrO₂ doses: (b) pseudo-second-order, (c) mass transfer plot at different rGO/ZrO₂ doses, (d) intraparticle diffusion plot at different rGO/ZrO₂ doses and (e) Richenberg model plot at different rGO/ZrO₂ doses.

Table 2 Kinetic parameters for the adsorption of fluoride at different rGO/ZrO₂ doses

Model parameters	Fluoride concentration
	0.4 g L^−1	0.5 g L^−1	0.6 g L^−1
Pseudo-first-order
K1 (min^−1)	0.0741	0.074	0.069
qe (exp.) (mg g^−1)	45	46	40
qe (cal.) (mg g^−1)	48.8	50.03	41.98
R^2	0.981	0.985	0.972
Pseudo-second-order
k′2	0.0024	0.0024	0.0034
qe (mg g^−1)	48.5	49.75	43
R^2	0.990	0.992	0.997
Mass transfer
βt × 10^−3 (cm^2 s^−1)	2.5	4.6	3.6
R^2	0.972	0.984	0.980
Intraparticle diffusion
kid (mg g^−1 min^−0.5)	6.42	6.64	5.68
C	2.03	1.73	1.65
R^2D	0.986	0.984	0.989

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3.5.2. Mass transfer studies. The mass transfer coefficient β_t (3.7 × 10⁻³, 6.7 × 10⁻³, 5.2 × 10⁻³) of the Mckay et al. model was determined by the slopes of the plots of ln((C_t/C_i) − 1/(1 + mK)) vs. t (min) at different doses (0.4, 0.5, 0.6 g L⁻¹), which are shown in Fig. 11b. The high value of mass transfer coefficient suggested a rapid mass transfer from the boundary film to the adsorbent surface. Therefore, rGO/ZrO₂ can be used for remediation of fluoride from water. Moreover, owing to the high value of β_t, it cannot be considered as a rate controlling step.

3.5.3. Intra particles diffusion model. The plot of q_t vs. t^0.5 (min) shown in Fig. 11c represents the corresponding Weber–Morris model to examine the probability of intraparticle diffusion for the adsorption process of fluoride onto the rGO/ZrO₂. The plots were found to be significantly linear, and k_id at different doses was determined by their respective slopes. If the plot of q_t vs. t^0.5 is a straight line and passes through the origin, then the intraparticle diffusion is considered as the rate controlling step.⁵¹ In this adsorption system, although these plots are linear they did not pass through origin, which indicates that the adsorption of fluoride occurred via a complex mechanism and both external mass transfer and intraparticle diffusion play a significant role in the rate determining step. This observation was further proved by the value of the intraparticle diffusion coefficient (D), which can be calculated by the following equation⁵⁵where r (cm) stands for the average radius of the adsorbent particle and t^0.5 (min) is the time required for half of the adsorption. It was reported earlier that if the D value is in the order of 10⁻¹¹ cm² s⁻¹ then intraparticle diffusion is the rate determining step.⁵⁶ In the present adsorption system, the value of D (1.47 × 10⁻⁹ cm² s⁻¹) obtained was in the order of 10⁻⁹, which is more than 10⁻¹¹ cm² s⁻¹, indicating that intraparticle diffusion was not the only rate controlling step.

3.5.4. Richenberg model. Fig. 11d presents the plot of the Richenberg model [B_t vs. t (min)] at various adsorbent doses, which showed considerable linearity with the R² values of 0.9781, 0.9842 and 0.9905 at corresponding doses of 0.4, 0.5 and 0.6 g L⁻¹, respectively. The plots are linear but did not pass through origin, which is indicative of the fact that intraparticle diffusion is not the only rate controlling step and this is in agreement with the results obtained with the Weber–Morris model.⁵⁷

3.6. Isotherm studies

Different isotherm models were applied to find a suitable isotherm that can better explain the adsorption process. The experimental data were subjected to Freundlich, Langmuir and D–R isotherm models. The experiments were carried out at three different temperatures, 20, 30 and 40 °C, by varying the initial fluoride concentration from 5 to 50 mg L⁻¹ and keeping the adsorbent dose (0.5 g L⁻¹) and pH (7) constant.

3.6.1. Freundlich isotherm. The Freundlich constant (n and k_f) and R² were obtained from the graph of log C_e vs. log q_e at different temperatures and are summarized in Table 3. It was found that values of n ranged from 1 to 10, which is indicative of favorable adsorption of the fluoride on the rGO/ZrO₂.⁵⁸ Moreover, the value of k_f (sorption capacity) was found to increase on increasing the temperature from 20 to 30 °C, which suggested the endothermic nature of adsorption. However, k_f did not increase on further increasing the temperature from 30 to 40 °C. Thus, 30 °C is considered as the optimum temperature for the adsorption of fluoride in this adsorption system.

Table 3 Parameters of Langmuir, Freundlich and D–R isotherms for the adsorption of fluoride at different temperatures

Temperature	Freundlich parameters			Langmuir parameters			D–R parameters
	kf (mg g^−1)	n	R^2	q^0 (mg g^−1)	b (L mg^−1)	R^2	Xm (mmol g^−1)	E (kJ mol^−1)	R^2
20 °C	16.59	3.33	0.60	40.49	0.0007	0.99	0.995	14.43	0.99
30 °C	28.18	5.12	0.65	47	0.0001	0.99	0.995	9.12	0.99
40 °C	20.89	3.80	0.893	43.3	0.0003	0.99	0.995	10	0.99

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3.6.2. Langmuir isotherm. The Langmuir isotherm constants q⁰ and b were calculated from the slopes and intercepts, respectively, from the plot of C_e/q_e vs. C_e (Fig. 12a) and the values are presented in Table 3. This isotherm showed a good fit to the experimental data with high R² values, which showed that the adsorption process was monolayered and takes place at homogenously energized active bonding sites (Table 3). The uptake capacity of rGO/ZrO₂ was found to be 46.9 mg g⁻¹ at 30 °C and pH 7.0. The values of q⁰ increased with the increase in temperature, which suggested the endothermic nature of the adsorption process. Additionally, the dimensionless separation factor (R_L) was also calculated, which gives an idea about the reaction and whether the adsorption process was favorable or not.⁵⁹ It was calculated by the following expression:
where b and C₀ are the Langmuir constant and initial concentration of fluoride, respectively. If the value of R_L is in the range 0 < R_L < 1 it indicates favorable adsorption. The value of R_L for the rGO/ZrO₂ calculated was 0.0399, indicating that fluoride adsorption by the rGO/ZrO₂ was favorable.

Fig. 12 Equilibrium isotherm studies at temperatures of 20, 30 and 40 °C. (a) Freundlich isotherm plot and (b) D–R isotherm plot.

3.6.3. D–R isotherm. To evaluate the D–R isotherm parameters, graphs were plotted between ln q_e vs. F² (Fig. 12b) and were found to be significantly linear at all the temperatures. The parameters of this isotherm, such as energy of adsorption E (kJ mol⁻¹), β (mol² kJ⁻²) and R², are listed in Table 3. The significant high value of R² observed at all the mentioned temperatures suggested the applicability of the D–R isotherm. The calculated value of E (kJ mol⁻¹) was found to be in the range of chemisorption. Therefore, it is concluded that adsorption of fluoride on the rGO/ZrO₂ was chemical in nature.

3.7. Thermodynamics studies

The thermodynamic parameters (ΔG⁰, ΔH⁰, ΔS⁰) calculated from the slopes and intercepts of the Van't Hoff plot (Fig. 13) are summarized in Table 4. The ΔG⁰ for the adsorption of fluoride on the rGO/ZrO₂ was calculated at temperatures of 293, 298 and 303 K, which are listed in Table 4. The negative value of ΔG⁰ and the positive value of ΔH⁰ (kJ mol⁻¹) suggested that the adsorption of fluoride was a spontaneous process and endothermic in nature. It was also found that the negative value of ΔG⁰ increased with the increase in temperature, which indicated that spontaneity of the reaction increased with the increase in temperature, i.e. a high temperature was favorable for the adsorption. The ΔS⁰ was also found to be positive for this adsorption system, which suggested that randomness was increased during the adsorption process. This was due to the fact that the desorption process also takes place during adsorption, which increased the entropy (ΔS⁰) at the solid–liquid interface.^60,61

Fig. 13 Van't Hoff plot for the adsorption of fluoride on rGO/ZrO₂.

Table 4 Thermodynamic parameters for the adsorption of fluoride on rGO/ZrO₂

Temperature	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (kJ mol^−1 K^−1)
20 °C	−2.80464	0.07698	0.272008
25 °C	−3.99468
30 °C	−5.52854

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3.8. Desorption and re-adsorption capacity

To make a cost effective, efficient and user-friendly adsorbent, the regeneration of the adsorbent is necessary for its further reuse for adsorption. Desorption studies were carried out by using the fluoride-adsorbed rGO/ZrO₂ adsorbent. First, the fluoride-adsorbed rGO/ZrO₂ is generated by adsorbing 25 mg L⁻¹ fluoride solution on 0.5 g L⁻¹ rGO/ZrO₂ at pH 7. After the equilibration, the residue was filtered and the filtrate was measured for fluoride content. Then this fluoride-adsorbed rGO/ZrO₂ was subjected for desorption studies by maintaining the pH values (8–13) by addition of 0.1 M NaOH solution.

Fig. 14a shows that up to pH 8, no fluoride comes into the solution. However, the desorption efficiency increases to 97.3% of fluoride as the pH increases to pH 12. The rGO/ZrO₂ adsorbent performed well after reuse (Fig. 14b) with a slight decrease in its efficiency. The percentage of adsorption of fluoride was found to be 92.0%, 88.0%, 79.0%, 68.0%, 59.0%, respectively, for the 1st, 2nd, 3rd, 4th and 5th cycles of fluoride operation.

Fig. 14 (a) Desorption of fluoride from loaded rGO/ZrO₂ at various pH values. (b) Regeneration results for rGO/ZrO₂ over five cycles.

4. Conclusions

The rGO/ZrO₂ nanocomposite was prepared by a simple hydrothermal method using GO and ZrOCl₂·8H₂O, and the nanocomposite showed high surface area and remarkable adsorption capacity for the removal of fluoride from water. The maximum uptake capacity of rGO/ZrO₂ was found to be 46 mg g⁻¹ at an initial fluoride concentration of 25 mg L⁻¹, rGO/ZrO₂ dose of 0.5 g L⁻¹, temperature of 30 °C and pH 7. The establishment of equilibrium of fluoride removal within 50 min suggested that the current adsorption system was fast. The kinetics of the adsorption process were also investigated and it was found that the adsorption process followed a pseudo-second-order kinetic model, which is in accordance with the above fact. The external mass transfer studies revealed that the removal of fluoride would have occurred by the adsorption process. The application of the intraparticle diffusion model and the Richenberg model indicated that the adsorption process was governed by intraparticle diffusion, which was not the sole rate controlling step. The equilibrium data fit well with Langmuir isotherm, which confirmed that the adsorption of fluoride on the energetically heterogeneous active sites of the rGO/ZrO₂ was monolayer. The negative values of ΔG⁰ recommended the spontaneity of the process whereas the positive value of ΔH⁰ suggested an endothermic process for the adsorption of fluoride. These batch mode results concluded that the rGO/ZrO₂ is a potential adsorbent for fluoride removal from water at ambient conditions, i.e. temperature 30 °C and neutral pH. These obtained results can be helpful for designing continuous column studies for the treatment of fluoride contaminated water using rGO/ZrO₂ as the adsorbent.

Acknowledgements

Authors SM and VK would like to acknowledge the MHRD, New Delhi, India for providing financial assistance. The authors are also thankful to the Director, Indian Institute of Technology (BHU), Varanasi, India for providing infrastructure and the central instrumentation facilities centre (CIFC).

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