Novel Hybrid rGO-BC@ZrO₂ Composite: A Material for Methylene Blue Adsorption

Abstract

This study reports the preparation of a novel hybrid composite and its application in adsorption. For this composite preparation, zirconia (ZrO₂) was precipitated onto an integrated framework of reduced graphene oxide (rGO) and black cumin (BC) seeds. Characterization using Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, and transmission electron microscopy confirmed the successful incorporation of ZrO₂ nanoparticles (5–20 nm) into the integrated carbon framework of rGO and seed powder. The microscopic analysis further revealed that the ZrO₂ NPs were dispersed throughout the integrated rGO-BC framework. Using the rGO-BC@ZrO₂ composite, methylene blue dye was decontaminated from water through a batch adsorption process. The rGO-BC@ZrO₂ composite achieved 96% MB adsorption at an adsorbent dose of 2.0 g/L, and nearly 100% when the adsorbent concentration was 3.0 g/L. Modeling of the experimental adsorption values was also established to verify the adsorption viability and mechanism. Thermodynamic modeling confirmed the feasibility and spontaneity of the present batch adsorption process. Isotherm modeling, which showed its compatibility with the Freundlich isotherm, suggested multilayer adsorption. rGO-BC@ZrO₂ demonstrated good persistence and reusability for methylene blue for up to five consecutive adsorption cycles. Thus, this study presents optimistic results regarding water purification.

1. Introduction

Residential and industrial activity worldwide generates large volumes of wastewater containing pollutants such as dyes, heavy metals, explosives, biotic contaminants, chlorinated and fluorinated hydrocarbons, radioactive substances, and toxic anions [1]. Dyes are complex organic molecules that are a particular concern because they are widely employed in the paper, textile, plastics, cosmetics, and food industries [2,3], with approximately 7.0 × 10⁵ tons manufactured annually [3]. The textile industry uses more than approximately 10,000 tons of dyes [4], with 10–15% of the total volume released into wastewater [5]. A significant proportion of these dyes eventually enters natural water bodies, with potentially toxic effects for humans, aquatic animals, and plants [6,7,8].

Methylene blue (MB; C₁₆H₁₈N₃SCl; molecular weight = 319.65 g/mol) is a common dye first synthesized by Heinrich Caro in 1876 that produces an intense blue color at very low concentrations in water [9]. It is highly stable, has an aromatic heterocyclic structure, and does not easily degrade. When MB enters a body of water, it can reduce the aesthetics and block the penetration of sunlight, thus disrupting the aquatic ecosystem [9]. MB dye may also have toxic and carcinogenic effects on humans and aquatic organisms [10]. Therefore, there is a strong need for water treatment strategies that can remove industrially derived water contaminants such as dyes [11].

Various treatment technologies have been developed for this purpose, including adsorption [12,13], membrane separation [14], photo-degradation [15], and advance oxidation processes [16]. Most water treatment methods attempt to eliminate hazardous contaminants, especially for water that is intended for drinking. For MB removal, materials such as biomass [17], activated carbon [18], and metal oxide nanoparticles [15] have been developed and used to remove MB dye from wastewater.

Adsorption is a particularly effective approach in this context because it is capable of removing hazardous pollutants [10,11,12]. It is also an inexpensive and easy-to-operate water treatment method [10,11,12]. Metal oxide nanoparticles (MONPs) with various functional groups have been developed for use as adsorbents for the removal of pollutants from water [19,20]. In this respect, zirconium oxide (ZrO₂) is one of the most desirable adsorptive materials because it has high biocompatibility and chemical stability [21,22]. When an organic framework such as graphene oxide (GO) was combined with ZrO₂ [23], the resulting composite showed improved adsorption capacity for removing toxic pollutants from water.

Notably, graphene is a well-known carbon allotrope that has been employed in water purification systems due to its high mechanical stability, hydrophobicity, large surface area, and high elastic modulus [24]. Rich oxygen-containing functional groups have been combined with graphene to produce GO and reduced GO (rGO) [25,26]. GO has a honeycomb-like sheet structure surrounded by numerous functional groups (including hydroxyl, carboxylic, and epoxide groups), which makes it highly hydrophilic. The hydrophilic nature of GO can be altered through functionalization, leading to the formation of hydrophobic rGO [24,25,26].

rGO has also received significant attention for use in water treatment due to its hydrophobic structure, superior physiochemical properties, and enhanced surface functionalities [25]. The integration of metal oxides with rGO leads to the formation of hybrid inorganic–organic materials with a higher surface area, a highly porous structure, and abundant oxygen-containing functional groups [25,26,27]. These properties enhance the effectiveness of hybrid materials in environmental applications, including dye removal [25]. Attempts were also made to combine a ZrO₂-based trimetallic oxide with rGO (Ag₂O-Al₂O₃-ZrO₂/rGO) for removing pollutants [27], which showed better adsorption results compared to unmodified rGO. Thus, ZrO₂ or ZrO₂-based MONPs can be ideal candidates for integration with GO or rGO for wastewater treatment; however, the cost and biocompatibility of such materials are still challenging concerns.

In view of the demand for inexpensive and biocompatible adsorbents, GO or rGO has also been integrated with several other inexpensive organic frameworks for water treatment, including cellulose [28], chitosan [29], and other organic materials [30]. These integrated materials have shown better adsorption results than virgin particles [24,26,27] and have also reduced costs by increasing the adsorbent yield. Thus, such integrated materials have opened a new path in water treatment technology.

Since cellulosic plant materials such as leaves and seeds are natural, non-toxic, cheap, and easily available organic materials, these materials are currently considered good options to be used as adsorbents by integrating them with rGO [30,31]. Considering these advancements, in the present study, an integrated framework of rGO and Nigella sativa (black cumin, BC) seeds was developed, and ZrO₂ NPs were incorporated into this framework. BC seed powder, which is cultivated worldwide, in combination with MONPs has already demonstrated significant potential for the removal of various charged dye molecules from water [32].

In previous studies, manganese oxide (MnO₂) [30] and manganese ferrite (MnFe₂O₄) [31] have been incorporated into an integrated framework of rGO-BC seeds for water purification. The current study represents a new effort based on previous studies [30,31]. No previous study has investigated the combination of rGO, BC seed powder, and ZrO₂ NPs to create a nanohybrid composite (rGO-BC@ZrO₂ NHC) for water decontamination.

Hence, the objective of this study was to develop a novel rGO-BC@ZrO₂ composite through simple chemical precipitation, in which ZrO₂ was precipitated on the integrated surface of rGO and BC seed powder. After structural and morphological confirmation of the composites using various spectroscopic and microscopic techniques, the adsorption activity of rGO-BC@ZrO₂ was investigated under various conditions for methylene blue dye through a batch adsorption process. The experimental data obtained for MB adsorption were also validated through various isothermal and kinetic models.

2. Materials and Methods

2.1. Chemicals

The Chemicals Utilized in This Study Were as Follows

Precursor salt for the preparation of composites: Graphite Fine Powder (C; Mol. Wt. = 12.01; purity = 99.5%; source = Central Drug House Pvt. Ltd., Delhi, India) and zirconium oxy chloride dihydrate (ZrOCl₂. 2H₂O; Mol. Wt. = 322.25; purity = 99%; source = Merck, Mumbai, Maharashtra, India). Acid and base to maintain the reaction and/or solution pHs: Hydrochloric acid (HCl; Mol. Wt. = 36.46; purity = 37%; source = Sigma Aldrich, Mumbai, Maharashtra, India) and sodium hydroxide (NaOH; Mol. Wt. = 40.00; purity = 97%; source = Sigma Aldrich, Mumbai, Maharashtra, India). Adsorbate: Methylene blue (C₁₆H₁₈ClN₃S; Mol. Wt. = 319.85; purity = 82%; source = Sigma Aldrich, Mumbai, Maharashtra, India). Solvent/washing and/or dissolving: Double-distilled water (H₂O; Mol. Wt. = 18.00; purity = 100%; source = laboratory-made). Black cumin seeds: Seeds of black cumin (BC) bought from a nearby market around Jamia Millia Islamia campus, New Delhi.

2.2. Preparation of rGO

This study is a continuation of previous studies in which we prepared rGO-MnO₂-BC [30] and rGO-MnFe₂O₄-BC [31]. Therefore, a stock of washed BC seeds and rGO was prepared as in those studies.

The preparation of rGO from graphite powder was performed using Hummers’ process as reported in our previous study [30,31] and herein, shown by the flow chart presented below (Scheme 1).

2.3. Preparation of rGO-BC@ZrO₂

Briefly, a suspension of BC seed powder and rGO was produced by sonicating washed BC seeds (0.5 g) and rGO (0.5 g) in 300 mL of de-ionized water for 15 min. Following this, a 0.1 M ZrOCl₂ (100 mL) solution was added gradually to the BC-rGO suspension for 30 min under continuous stirring at 60 °C. Subsequently, an 8 M NaOH solution was gradually added to the suspension to adjust the pH to approximately 10.5–11.0. The reaction was allowed to continue for an additional 15 min, leading to the formation of a blackish precipitate of rGO-BC@ZrO₂. The precipitate was then allowed to cool to room temperature. It was subsequently filtered, thoroughly cleaned with demineralized water several times, and then dried at room temperature for 48 h. The dried rGO-BC@ZrO₂ NHC was then characterized and utilized in water cleaning.

2.4. Characterization

All of the characterization methods employed to characterize the prepared samples are illustrated in

Table 1. Instrumentations used in the characterization of rGO-BC@ZrO₂.

Analysis	Characterization Techniques	Instrumentations (Model)	City/Country
Functional group analysis	Fourier-transform infrared (FT-IR) spectroscopy	Vertex 70V, Bruker Optik, FTIR Spectrometer	Ettlingen, Germany
Crystal phase and size analysis	Powder X-ray diffraction analysis	Rigaku Smart Lab Guidance, Rigaku, X-ray Diffractometer	Tokyo, Japan
Surface morphology analysis	Scanning electron microscopy (SEM)	SIGMA VP, Zeiss, SEM Microscope	Oberkochen, Germany
	Transmission electron microscopy (TEM)	F30 S-Twin, Tecnai, FEI, TEM Microscope	Hillsboro, OR, USA
Elemental composition	Energy-dispersive X-ray spectrometer (EDAX)	SIGMA VP, Zeiss	Oberkochen, Germany

.

2.5. Zero-Point Charge Determination

To understand the nature of the functional groups of an adsorbent, it is very important to find its zero-point charge (pH-zpc). The mechanism of adsorption of a pollutant on the surface of an adsorbent in different pH environments can be explained through the pH-zpc. The pH-zpc is the point where the positive and negative charges present on the surface of an adsorbent are equal, i.e., the surface has a neutral charge. The charge on the surface of the adsorbent can be controlled by changing the pH (above or below the pH-zpc) of the solution mixture containing the adsorbent. The pH-zpc was calculated in the present study. The well-known salt addition method was used to determine the pH-zpc [31]. Using this method, 20 mL of 0.2 mol of KNO₃ solution was taken in a series of 100 mL Ermalnayer flasks, and then the pH of these solutions was adjusted from 2 to 10 (initial pH). After adjusting the pH, 20.0 mg rGO-BC@ZrO₂ was added to each Ermalnayer flask containing 20 mL of 0.2 mol of KNO₃ solution. The mixtures were then shaken on a water bath shaker for 6 h at 200 rpm and 27 °C. After shaking, the final pH of all the solutions was noted. Then, the pH-zpc was calculated by plotting the graph of the pH change and final pHs. According to a previous study [31], the pH where the lines intersect was considered the pH-zpc.

2.6. Preparation of Stock Solution of MB Dye

To investigate the adsorption activity of the prepared composite, MB dye was removed from its aqueous solution. The stock aqueous solution of MB was prepared in the laboratory by dissolving 1.0 g of MB dye powder in 1.0 L of double-distilled water. The prepared stock aqueous solution was then diluted to the required concentration using the dilution law.

2.7. Batch Adsorption Experiments

A total of 10 mL of MB dye solution with an initial concentration of 10 ppm prepared from a stock solution was used as the solute. A fixed amount of rGO-BC@ZrO₂ (ranging from 0.5 to 3.0 g/L) was added to the solution, which was placed in a reciprocating shaker for 90 min at room temperature at a neutral pH. The concentration of un-adsorbed MB was quantified spectrophotometrically using a UV–Vis spectrophotometer (T80-UV/VIS, PG instruments Ltd., Leicestershire, England) (at λ max = 665 nm) by correlating the absorbance with the adsorbate concentration via a calibration curve (using the Beer–Lambert law).

Equations (1) and (2) were used to investigate the adsorption % (removal efficiency) and equilibrium adsorption capacity (Q_e, mg/g), respectively [11,12].

$$\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\left(\%\right)=\left({\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{{\mathrm{C}}_0}}\right)\times 100$$

(1)

$$\mathrm{A}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}\left({\mathrm{Q}}_{\mathrm{e}};{\displaystyle \frac{\mathrm{m}\mathrm{g}}{\mathrm{g}}}\right)={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\times \mathrm{V}}{\mathrm{m}}}$$

(2)

Here, C_o (mg/L) = concentration before adsorption, C_e (mg/L) = concentration after adsorption, V (L) = volume of the MB solution, Q_e (mg/g) = equilibrium adsorption capacity, and m = rGO-BC@ZrO₂ mass.

The obtained MB adsorption data were analyzed using thermodynamic, isotherm, and kinetic models, with the equations for these models provided in the below sections.

2.8. Thermodynamics of MB Adsorption on rGO-BC@ZrO₂

For thermodynamic and isotherm analysis, the effect of the temperature on the percentage removal of MB was investigated at 27, 35, and 45 °C with a 10 mg/L MB solution and 2.0 g/L rGO-BC@ZrO₂.

The thermodynamic parameters, namely ΔG° (kJ/mol) = Gibbs free energy, ΔH° (kJ/mol) = enthalpy change, and ΔS° (kJ/mol/K) = entropy change, reflect the spontaneity, heat change during the adsorption process, and disorder/randomness, respectively. These parameters were calculated by using the Van ’t Hoff equation (Equation (3)) [33]:

$$∆\mathrm{G}=-\mathrm{R}\mathrm{T} \mathrm{l}\mathrm{n}\mathrm{K}\mathrm{e}°$$

(3)

From the third law of thermodynamics,

$$∆\mathrm{G}=∆\mathrm{H}-\mathrm{T}∆\mathrm{S}$$

(4)

The combination of Equations (3) and (4) leads to Equation (5), as follows:

$$\mathrm{l}\mathrm{n}\mathrm{K}\mathrm{e}°=-{\displaystyle \frac{∆\mathrm{H}°}{\mathrm{R}}}.{\displaystyle \frac{1}{\mathrm{T}}}+{\displaystyle \frac{∆\mathrm{S}°}{\mathrm{R}}}$$

(5)

The plot of ln(Ke°) versus 1/T gives a value of ΔS° and ΔH° through the intercept and slope, respectively.

Here, T is temperature (in Kelvin), R is a universal gas constant (8.314 J/K/mol), and Ke° is a thermodynamic equilibrium constant, which can be calculated as follows (Equation (6)) [33]:

$$\mathrm{K}\mathrm{e}°={\displaystyle \frac{\left[1000\times \mathrm{K}\mathrm{g}\times \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{r} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}\right]\left[\mathrm{A}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}\right]°}{\mathsf{\gamma }}}$$

(6)

where γ is the coefficient of activity (dimensionless), [Adsorbate]° is the standard concentration of the adsorbate, and Kg is the equilibrium constant of the best-fitted model (for this study, we used the Sips constant, Ks in L/mol).

2.9. Isotherms of MB Adsorption on rGO-BC@ZrO₂

Three major isotherms, Langmuir (Equation (7)), Freundlich (Equation (8)), and Sips (Equation (9)), were used for MB adsorption (

Table 2. Results of isothermal analysis for MB adsorption on rGO-BC@ZrO₂.

S. No.	Isotherm Model	Parameters	Temperature (K)
			300	308	318
1.	Langmuir isotherm Qe = (Qmax.KLCe)/(1 + KLCe) (Equation (7a))	Qmax (mg/g)	23.391	24.622	25.435
2.		KL (L/mg)	0.358	0.248	0.185
3.		RL (dimensionless) (RL = 1/(1 + KLCo) (Equation (7b))	0.217	0.287	0.350
4.		χ2	0.068	0.050	0.032
5.		SSR	5.613	4.083	2.528
6.	Freundlich isotherm Qe = KFCe1/n (Equation (8))	KF ((mg/g)(L/mg)n)	7.482	6.602	5.603
7.		n (dimensionless)	2.698	2.481	2.243
8.		χ2	0.011	0.018	0.015
9.		SSR	0.921	1.459	1.240
10.	Sips isotherm Qe = (QsKsCens)/(1 + KsCens) (Equation (9))	Qs (mg/g)	38.831	42.810	39.944
11.		Ks (L/mg)	0.227	0.166	0.141
12.		ns (dimensionless)	0.575	0.599	0.675
13.		χ2	0.001	0.005	0.003
14.		SSR	0.145	0.436	0.276

) [34,35]. The Langmuir and Freundlich models represent the adsorption of pollutants on homogeneous and heterogeneous surfaces, respectively. Through the Langmuir isotherm, the maximum monolayer adsorption capacity (Q_max, (in mg/g)), adsorption strength (Langmuir constant, K_L (in L/mg)), and favorability of adsorption (separation factor, R_L (dimensionless) (if R_L = 0, irreversible adsorption process; if 0 < RL < 1, favorable adsorption process; and if RL > 1, unfavorable process)) can also be determined. Meanwhile, the Freundlich isotherm plays an important role in determining the favorability of dye adsorption on heterogeneous surfaces (Freundlich constant, n (dimensionless) (if n = 1–10, it reflects the favorability of the adsorption process on the heterogeneous surface of the adsorbent)) and the adsorption capacity on heterogeneous surfaces (Freundlich constant, K_F (in (mg/g) (L/mg)ⁿ)). The Sips isotherm model was also used for this analysis. The Sips model represents a combination of the Langmuir and Freundlich isotherm models. Through the exponential constant (ns, (dimensionless)) of the Sips model, the fitting of the Langmuir (monolayer adsorption) and/or Freundlich (multilayer adsorption) isotherms can be confirmed. If the value of ns is less than 1 (range 0–1), it refers to multilayer adsorption on a heterogeneous surface, while if ns = 1 or greater than 1, it efficiently confirms monolayer adsorption on a homogeneous surface [35]. The maximum adsorption capacity (Q_s, (in mg/g)) and adsorption intensity (Sips constant, Ks (in L/mg)) can also be obtained from the Sips model. It has been shown in many previous studies that a non-linear plot gives better and more meaningful results than a linear isotherm [34,35]; therefore, in this study, a non-linear plot was used for the isotherm. The non-linear equations of the Langmuir, Freundlich, and Sips isotherms are represented through equation numbers 5–7 given in Table 2.

2.10. Kinetics of MB Adsorption on rGO-BC@ZrO₂

The kinetics underlying the sorption of MB ions were investigated using various kinetic models (Table 2) [34,35]. The non-linear pseudo-first-order (PFO) kinetic model (Equation (10)) and pseudo-second-order (PSO) kinetic model (Equation (11)) were employed to investigate the kinetics of the present MB sorption. To explain the favorable bonding between the MB ions and rGO-BC@ZrO₂, the sorption data were further investigated using the non-linear Elovich kinetic model (Equation (12)).

PFO indicates that adsorption depends only on the surface of the adsorbent and reflects physical adsorption, while PSO assumes that adsorption depends on the specificity of the pollutants along with the adsorbent surface. The PSO kinetic model reflects chemical adsorption. The Elovich model is used to confirm the chemical nature of adsorption. In this, alpha (α) and beta (β) are meaningful units which indicate the absorption rate and desorption rate, respectively.

To understand the adsorption mechanism of the present system, the Weber–Morris (WM) model, also known as the interparticle diffusion (IPD) model, was applied. IPD can be represented by mathematical question 13. This relationship gives a non-linear plot. It is a very important model for finding the adsorption rate because the steps of adsorption take place through mass transfer, film diffusion, and interparticle diffusion. To find out which step is the slow step and governs the reaction, the IPD model is applied.

If the intercept, i.e., the C value, is positive, it means that, along with interparticle diffusion, film diffusion also occurred, whereas if the value of C is zero, then it can be stated that the existing interparticle diffusion was responsible for the adsorption of MB.

3. Results and Discussion

3.1. Characterization of rGO-BC@ZrO₂

3.1.1. pH-zpc Result

For rGO-BC@ZrO₂, the pH-zpc was determined to be ~6.2 (Figure 1). This means that above this pH, the rGO-BC@ZrO₂ surface will be negative (-COO⁻, -OH⁻) due to deprotonation, whereas at a pH below 6.2, the adsorbent surface will be positive (-COOH₂⁺, -OH₂⁺) due to protonation. This analysis helps in understanding the adsorption process in real water.

3.1.2. Fourier-Transform Infrared (FTIR) Analysis

The integrated rGO-BC framework was used as an organic template for the formation and dispersion of ZrO₂ NPs. The resulting rGO-BC@ZrO₂ composite was highly stable, with each component enhancing its overall functionality. The presence of several functional groups in the prepared composite, attributed to the organic content of rGO and BC, was confirmed based on the observed FTIR spectrum. For this FTIR analysis, ~10 mg of rGO-BC@ZrO₂ was grounded with KBR and pressed to make pallets. The FTIR spectrum for rGO-BC@ZrO₂ was analyzed from 4000 to 400 cm⁻¹ using an FTIR spectrometer (Vertex 70V, Bruker Optik, Germany) (Figure 2).

These peaks were in good agreement with our previously published studies [30,31]. In the FTIR spectrum for rGO-BC@ZrO₂, the broad peak around ~3290 cm⁻¹ was attributed to hydroxyl (O–H) stretching [26,36]. Additionally, the peaks at ~2920 and ~2850 cm⁻¹ revealed –C–H stretching [36]. These peaks affirmed the organic content of the integrated rGO-BC containing O–H functional groups on the surface of the NHC [26,32,36]. The peak observed at around ~1552 cm⁻¹ indicated N–H stretching vibrations, likely due to the presence of amide groups in the BC seed powder [36,37]. Symmetrical –C–O stretching associated with amide II functional groups or ethereal components was indicated by the peak at ~1377 cm⁻¹ [32,36]. The peaks appearing at ~1321 cm⁻¹ and ~1016 cm⁻¹ corresponded to the deformation of –OH groups in C–OH and–C–O stretching, respectively [32,36]. The zirconia (Zr–O) bond was confirmed by the peak assigned at ~418 cm⁻¹ [38]. FTIR analysis showed several functional groups. These functional groups mainly consisted of hydroxyl, carbonyl, and amine groups. These groups were basically due to the protein structure and cellulosic surface of the BC seeds. In addition to this, rGO was also responsible for the hydroxyl and carbonyl groups. Therefore, these massive functional groups obtained in the present adsorbent are proved to be very useful for the adsorption application of the prepared composite.

3.1.3. X-Ray Diffraction (XRD) Analysis

The XRD pattern for rGO-BC@ZrO₂ was investigated to gain insight into its structural characteristics. Peaks were observed at 2θ angles of 10–80° (Figure 3). The XRD pattern for rGO (Figure S1; blue line) displayed a peak at 25.15° (2θ) corresponding to the (002) plane [30,39], while the peak in Figure S1 (red line) was attributed to the cellulosic surface of the BC seed powder [30,31,32]. The presence of ZrO₂ NPs in the crystal structure was confirmed in both the tetragonal and monoclinic phases [39,40]. The XRD pattern of rGO-BC@ZrO₂ contained distinct diffraction peaks at around 28, 32, 37, 50, 56, and 60° (2θ) corresponding to the (111), (101), (200), (220), (221), and (311) planes, respectively. The diffraction planes confirmed the presence of ZrO₂ NPs in the NHC, aligning closely with standard reference data (JCPDS card No. 38–1842). Additionally, the rGO-BC@ZrO₂ composite exhibited specific peaks corresponding to the integrated rGO-BC (002) and ZrO₂, clearly indicating the integration of ZrO₂ NPs within the integrated rGO-BC framework [30,39]. These observations were in agreement with the XRD spectra for rGO and BC seed powder [30,39]. However, in the rGO-BC@ZrO₂ composite, the intensity of this peak was reduced, which was likely due to the interaction between BC and rGO supporting the formation of ZrO₂ within the integrated rGO-BC framework [30,31,39]. It can be observed from the present XRD analysis that the present adsorbent was not very crystalline but amorphous in nature. As has been shown in many previous studies [41], adsorbents with low crystallinity may prove to be good for adsorption application, while on increasing the crystallinity, the adsorption efficiency may decrease. Hence, it can be stated that the present adsorbent may prove to be better for adsorption application.

3.1.4. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray (EDX) Analysis

SEM was employed to capture detailed images of the surface morphology of the rGO-BC@ZrO₂ composite (Figure 4a,b). These images revealed NHC particles with an irregular surface and slight agglomeration, while ZrO₂ NPs were observed within the integrated rGO-BC carbon framework. In the composite, rGO-BC enhanced the dispersion of NPs. The composite particles exhibited slight agglomeration due to electrostatic or non-electrostatic bonding interactions. The black rGO-BC@ ZrO₂ exhibited a rough structure, which is likely to contribute to enhanced adsorption performance [42]. The elemental composition of the prepared NHC was determined using EDX analysis (Figure 5), revealing the presence of C (13.25 wt.%), N (1.23 wt.%), O (77.12 wt.%), and Zr (8.39 wt.%). The C, N, and O were attributed to the rGO-BC carbon framework, while the presence of Zr was due to the ZrO₂ NPs in the composite.

3.1.5. Transmission Electron Microscopy (TEM) Analysis

The microstructure of the synthesized NHC was examined using TEM analysis (Figure 6a,b). The TEM images revealed that ZrO₂ NPs were irregularly deposited within the integrated rGO-BC carbon framework (Figure 6b). This distribution was attributed to the heterogeneous and irregular surface of the NHC. As shown in Figure 6a, the wrinkled sheet-like structure of rGO (green arrows) could be clearly observed on the composite surface. The BC surface can be seen through the yellow arrows. The average ZrO₂ particle size (red arrows) in the NHC was 5–20 nm.

3.2. Adsorption Analysis

3.2.1. Results of Batch Adsorption Experiments

The results of the batch adsorption experiments for various variables are shown in Figure 7a–d. The effect of the adsorbent dosage on the removal efficiency (Equations (S1) and (S2)) is presented Figure 7a, showing that the lowest amount of adsorbent tested (0.5 g/L) removed ~25% of the MB from the solution, rising to ~96% with an increase in the dose of rGO-BC@ZrO₂ to 2.0 g/L. This rise may have been due to an increase in the number of adsorption sites with higher adsorbent dosages. Increasing the adsorbent dosage further increased the removal efficiency to ~100%. This high percentage removal of MB (~96%) with relatively low adsorbent dosages (2.0 g/L) suggests that rGO-BC@ZrO₂ is a highly efficient adsorbent for MB. The incorporation of ZrO₂ NPs, which have a large surface area [43], into the integrated rGO-BC carbon framework may have improved the surface properties of the NHC. Based on these results, rGO-BC@ZrO₂ was used at a dosage of 2.0 g/L for further testing [30].

The pH of a solution affects the degree of surface charge ionization, the speciation of the adsorbate molecules, and the pollutant removal efficiency of the adsorbent [32,44]. The pH range of industrial effluents contaminated with dye typically varies between 4 and 12 [45,46]. This range depends on the specific types of dye and chemicals used in the industrial process. Therefore, in the present study, MB adsorption onto rGO-BC@ZrO₂ was assessed for a pH range of 2 to 10 (Figure 7b). A decrease in MB adsorption was observed in an acidic pH range of 2–6, with higher adsorption observed for the pH range of 8–10. pH 10 was found to be most suitable for maximum adsorption of MB.

This adsorption trend can be explained as follows: MB is a cationic dye with a pKa value of ~3.8 [47]. Therefore, MB retains a predominant positive charge in solution. At an acidic pH (or a pH lower than the pH-zpc) where the chances of protonation of the functional groups on the adsorbent surface are very high, the surface becomes positively charged (as -COOH₂⁺ and -OH₂⁺) [30,32]. In contrast, the deprotonation process occurs primarily at the surface of the adsorbent at alkaline pH levels (or a pH higher than the pH-zpc). This leads to the generation of negative charges on the functional groups present on the adsorbent surface. In addition to this, the concentration of negative hydroxyl ions (-OH⁻) also increases on the adsorbent surface. Therefore, based on this, it can be demonstrated that at an acidic pH, the cationic MB⁺ shows repulsive activity with the protonated (positively charged) rGO-BC@ZrO₂ surface, which ultimately decreases the sorption of MB on the protonated rGO-BC@ZrO₂ surface (Figure 7b) [30,31]. Meanwhile, the deprotonation of the functional groups of rGO-BC@ZrO₂ facilitates an increase in MB adsorption via the electrostatic interaction of the cationic MB⁺ with deprotonated (negatively charged) rGO-BC@ZrO₂.

3.2.2. Temperature Effect and Thermodynamics

MB sorption was found to be the highest at 27 °C (Figure 7d), possibly due to the balance between kinetic energy and specific interactions [48]. It can also be seen in Figure 7d that as the temperature increases, the removal efficiency decreases, confirming the physisorption of MB. The main reason for the decrease in removal efficiency may be the shrinkage and alteration of active sites [49], as well as the weakening of physical bonds (especially non-electrostatic) at higher temperatures [50,51]. Additionally, upon an increase in the kinetic energy at higher temperature, a specific interaction does not take place efficiently [48].

Thermodynamic calculations produced a ΔG° of −27.919, −27.868, and −28.343 kJ/mol at 27, 35, and 45 °C, respectively (Figure 7e). These negative values for ΔG° confirmed the thermodynamic feasibility and spontaneity of MB adsorption at these temperatures. The exothermic nature of the adsorption process was further confirmed by the negative value for ΔH° (−20.619 kJ/mol). Here, the value of ΔH° was less than −84 kJ/mol, confirming that the present MB adsorption was controlled by physisorption [51]. The positive value for ΔS (+24.053 kJ/mol/K) confirmed the lowering in the disorder at the solid–liquid interface [51,52].

3.2.3. Isotherm Analysis

The results obtained through the three isotherms (Langmuir, Freundlich, and Sips) are shown in Table 2. The values of the error functions (SSR and χ²) were found to be lower for the calculated adsorption capacities using the Freundlich model compared to the Langmuir model (Figure 8). This confirms that in the present study, the calculated adsorption capacities using the Freundlich isotherm plot show more agreement with the experimental adsorption capacity values than the Langmuir plot. Hence, it can be stated that the present adsorption majorly followed the Freundlich isotherm. Therefore, the adsorption was multilayered.

From the Langmuir isotherm, the Q_max values were found to be in the range of ~23.0–25.0 mg/g over a range of temperatures. The values of R_L measured through the Langmuir equation were in the range of 0–1, confirming the favorability of the present adsorption process over a range of temperatures. The value of b decreased with increasing temperature, which means that the adsorption intensity decreased with temperature [53]. This might be due to the weakening of non-electrostatic bonding (particularly H bonding) at higher temperatures [50,51].

The value of K_F obtained from the Freundlich isotherm was further found to be decreased with increased temperatures (Figure 8; Table 2). The values of n were measured in the range of 1 to 10, which shows the favorability of MB adsorption on a heterogeneous surface, and this was in accordance with the SEM images (Figure 4a,b) of rGO-BC@ZrO₂. However, the values of 1/n here were lower than the value that is affirming for chemisorption [34].

It is evident through the FTIR analysis of rGO-BC@ZrO₂ that the present adsorbent, which is made of a natural biomaterial (BC seeds), has several functional groups. These functional groups are dominantly responsible for the heterogeneous surface of any adsorbent, i.e., such adsorbents predominantly show a multilayer adsorption pattern. Therefore, there is no doubt that the best isotherm in the present study is the Freundlich isotherm. However, to further confirm the fitting of the Freundlich isotherm in the present MB adsorption study, the Sips model was also used. The Sips model fits better than the Langmuir and Freundlich models to the experimental data (Figure 8). It can be seen from the results of the Sips isotherm model (Table 2) that the values of ns were much lower than 1, which confirms the deviation from the Langmuir isotherm and that the current MB adsorption was governed through multilayer adsorption on the heterogenous surface of rGO-BC@ZrO₂. The maximum adsorption capacity Q_s through the Sips model was found to be ~39.0 mg/g at 27 °C. The value of K_s decreased with increasing temperature, suggesting a decrease in the adsorption intensity with temperature.

3.2.4. Optimization of Contact Time and Adsorption Kinetics and Mechanism

The experimental results indicated that ~99% of MB was eliminated within 75 min before reaching equilibrium (Figure 7c). It can be seen from Figure 7c that ~60% of the MB dye was removed in the first 15 min, but thereafter the adsorption rate slowed down significantly as the contact time passed. The availability of numerous unoccupied sites on the surface of rGO-BC@ZrO₂ facilitated the rapid mass transfer of MB for the first 15 min, after which the availability of active sites on the surface decreased and adsorption slowed down. With this result, it can be concluded that the current MB adsorption process is completed in many steps. Therefore, kinetic models were also used to understand this MB adsorption process.

The results of the non-linear kinetic analysis for MB adsorption on the rGO-BC@ZrO₂ surface are shown in

Table 3. Results of kinetic analysis for MB adsorption on rGO-BC@ZrO₂.

S. No.	Kinetic Model (Experimental Qe = 4.951 mg/g)	Parameter	MB
1.	PFO (Qt = Qe (1 − e−K1t)) (Equation (10))	K1 (/min)	0.057
2.		Qe (cal) (mg/g)	4.950
3.		SSR	0.033
4.		χ2	0.0009
5	PSO Qt = (k2Qe2t)/(1 + K2Qet) (Equation (11))	K2 (g/mg/min)	0.014
6.		Qe (cal) (mg/g)	5.641
7.		SSR	0.070
8.		χ2	0.002
9.	Elovich Qt = 1/β ln(αβt + 1) (Equation (12))	α (mg/g/min)	1.686
10.		β (g/mg)	1.021
11.		SSR	0.291
12.		χ2	0.008
13.	IPD Qt = Kipdt0. 5+ C (Equation (13))	Kipd (mg/g/min0.5)	0.440
14.		C (mg/g)	0.913
15.		SSR	2.829
16.		χ2	0.078

. The large difference between the experimental Q_e value and calculated Q_e value (extracted from the non-linear PSO kinetic model; Table 3) and the higher error functions for the PSO non-linear plot (Figure 9) suggested that the existing MB sorption experimental data did not satisfy the PSO model. Meanwhile, the PFO plot (Figure 9) returned the lowest error functions and gave a calculated Q_e close to the experimental Q_e (Table 3), which showed that the PFO non-linear kinetic model was more suitable for describing MB adsorption on rGO-BC@ZrO₂. The fitting of the PFO model suggested that the MB adsorption was controlled by physisorption. The present kinetic results showed good agreement with previously reported kinetic results [54].

From the Elovich non-linear plot (Figure 9), the value of α for the adsorption of MB ions was found to be higher than β, indicating that the rate of MB adsorption was higher than the rate of desorption from rGO-BC@ZrO₂ [30,31], which confirms the feasibility of the present MB adsorption process (Table 3). The Elovich model also showed significant fitting to the MB experimental adsorption data, confirming that chemisorption was also significantly responsible for MB adsorption.

Additionally, it can be seen here that the graph of IPD gives a positive value of C, which confirms that in the present MB adsorption study, film diffusion also governed the MB adsorption along with IPD [34]. This result confirms that in the present MB adsorption study, along with diffusion, non-electrostatic (physical bonding) and electrostatic (chemical bonding) interactions were also responsible for MB adsorption on the rGO-BC@ZrO₂ surface [34].

Overall, from these thermodynamic, isotherm, and kinetic results, it can be concluded that physiochemical sorption was responsible for MB adsorption on the rGO-BC@ZrO₂ surface, in which physical adsorption predominated.

3.3. Regeneration and Cyclic Use of rGO-BC@ZrO₂

The regeneration of any adsorbent is very important to make it sustainable and affordable. A regenerated absorbent can be reused for multiple cycles, which reduces its overall cost to a great extent. Therefore, in this study, the regeneration and reusability of rGO-BC@ZrO₂ were also examined for several cycles under optimal conditions.

For this study, firstly, the exhausted adsorbent (MB-loaded rGO-BC@ZrO₂) was regenerated. To regenerate the exhausted adsorbent, MB-loaded rGO-BC@ZrO₂ was re-collected after each experiment, and it was ensured that the amount of dye-loaded rGO-BC@ZrO₂ was approximately 20 mg. The re-collected dye-loaded rGO-BC@ZrO₂ (20 mg) was added to 0.2 M HCl solution and magnetically shaken in a water bath for 90 min at room temperature. The composite was separated from the dye solution and then washed with distilled water to eliminate any unconsumed HCl, dried, and reused for the second adsorption cycle under the optimal conditions. This process was repeated for five adsorption–desorption cycles. The results of the regeneration and reuse test analysis are shown in Figure 10, which shows that the removal efficiency of rGO-BC@ZrO₂ for MB dye was reduced only by ~40%, even after the fifth consecutive cycle. These results suggest the stability of rGO-BC@ZrO₂ over five consecutive adsorption cycles in removing MB dye from water.

3.4. Performance Evaluation of rGO-BC@ZrO₂ for MB Adsorption and Comparative Study

For any adsorbent, comparing it with a reported adsorbent is not an easy task because the adsorption capacity of any adsorbent depends on different variables such as the content of dyes, dyes’ specification, type of water, pH of the water, adsorbent dosage, contact time, and adsorption temperature. Therefore, comparing one adsorbent with another adsorbent only through the adsorption capacity is not meaningful as mentioned in a previous study [31]. Therefore, researchers measure the partition coefficient (PC) with various factors other than the adsorption capacity and compare it with the given value, which is a less biased comparison. Equations (14a) and (14b) for calculating the partition coefficient are given here [31]:

$$PC={\displaystyle \frac{Q_{\mathrm{e}}}{C_{\mathrm{e}}}}$$

(14a)

$$PC={\displaystyle \frac{Q_{\mathrm{e}}}{C_{\mathrm{o}}{Q}_{\mathrm{t}}}}$$

(14b)

where Q_t and Q_e are the adsorption capacities at the given time and equilibrium, and C_o and C_e are the initial and final concentrations of MB in the solution.

The PC values obtained under different conditions for our previous adsorbents and current adsorbent are shown in

Table 4. Performance analysis of rGO-BC@ZrO₂ for MB adsorption.

S. No.	Adsorbent	Initial Concentration (mg/L)	Adsorption Dose (g/L)	Equilibrium Adsorption Capacity (mg/g)	Maximum Adsorption (Langmuir) Capacity (mg/g)	PC (L/g)	Reference
1.	Fe2O3-SnO2/BC	10	2	4.90	58.82	23.89	[55]
2.	MnO2/BC	10	1	9.83	185.185	57.48	[56]
3.	MnFe2O4/BC	10	3	3.31	10.07	52.57	[57]
4.	Acid washed BC	10	1	9.81	73.53	51.63	[58]
5.	Ag-Ag2O/ZrO2/GL	10	2	4.95	43.95	50.62	[59]
7.	BC-GO@Fe3O4	10	1	9.90	87.71	77.65	[60]
8.	MnFe2O4/rGO-BC	10	1	9.86	74.62	72.03	[30]
9.	rGO-BC/MnO2	10	1	9.68	232.5	30.46	[31]
10.	rGO-BC@ZrO2	10	2	4.95	23.39	51.40	This study

.

It can be seen from Table 4 that the PC of the present adsorbent is comparatively better than that of some previous adsorbents. In addition to this, the equilibrium adsorption capacity is also seen to be better for the present adsorbent than some previous adsorbents (Table 4). It can be stated that the present adsorbent showed better removal efficiency for MB dye.

4. Conclusions

rGO-BC@ZrO₂ was synthesized using a simple co-precipitation method and characterized using FTIR, XRD, SEM, EDX, and TEM, with the results confirming that a highly functionalized rGO-BC@ZrO₂ NHC was obtained. Reduced graphene oxide and seed powder were used to incorporate ZrO₂ NPs, and the resulting rGO-BC@ZrO₂ was investigated for the adsorptive removal of methylene blue dye in batch experiments. Key operating parameters were optimized to achieve higher methylene blue adsorption. Isotherm analysis showed the best fitting with the Sips model. In the present study, methylene blue adsorption showed multilayer adsorption on the heterogeneous surface of rGO-BC@ZrO₂. The process was exothermic and spontaneous. The present adsorption process was physico-chemical, with physical adsorption being the predominant factor. The fitting of the pseudo-first-order kinetic model confirmed the physical adsorption. The comparative study proved the present adsorbent to be a better adsorbent than previous adsorbents. In future, the adsorption capacity and competitive study of the present adsorbent will be explored in natural water. Along with this, a study will also be conducted to improve its stability. Collectively, the data obtained from the experiments established that the use of the rGO-BC@ZrO₂ NHC is a beneficial strategy for the removal of water pollutants.