Synthesis and Characterization of Eco-Friendly Fe₃O₄/g-C₃N₄/rGO Nanocomposites for Photodegradation of Methylene Blue Dye

Sanju Mahich^1,2, Kundan Singh Shekhawat³, Shubham Gupta¹, Anuj Kumar⁴, Sanjay Kumar Swami⁵, Jaya Mathur³, Vijay Devra², Amanpal Singh¹

¹Department of Physics, University of Rajasthan, Jaipur, Rajasthan, India.
²Janki Devi Bajaj Government Girls College, Kota, Rajasthan, India.
³Department of Chemistry, University of Rajasthan, Jaipur, Rajasthan, India.
⁴Department of Physics, J.C. Bose University of Science and Technology, YMCA, Faridabad, Haryana, India.
⁵School of Advanced Materials, Green Energy and Sensor Systems, Indian Institute of Engineering Science and Technology, Shibpur, Botanic Garden, Howrah, West Bengal, India.

DOI: 10.4236/anp.2025.144008                

Abstract

Fe₃O₄/g-C₃N₄/rGO nanocomposites were synthesized as visible light-powered, eco-friendly photocatalysts for treating dye-polluted wastewater. Methylene blue (MB) served as the model contaminant. The composite aimed to improve charge separation and light harvesting by integrating g-C₃N₄ with conductive rGO and catalytically active Fe₃O₄. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDX) were employed to analyze the microstructure, morphology, and elemental composition, as well as to assess compositional uniformity. Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analysis confirmed the mesoporous nature of the Fe₃O₄/g-C₃N₄/rGO composite, with a specific surface area of 25.55 m²/g, a pore volume of 0.0236 cc/g, and an average pore diameter of 3.37 nm. UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) revealed that the nanocomposites possess enhanced absorption in the visible-light region, characterized by a distinct absorption edge around 475 nm. Photocatalytic tests under visible light irradiation demonstrated a remarkable degradation efficiency of 99.53% for MB dye at pH 11, significantly outperforming the individual components. Liquid Chromatography-Mass Spectrometry (LC-MS) confirmed the presence of intermediate products, supporting a stepwise degradation mechanism of MB through demethylation and oxidative reactions.

Keywords

Environmental Remediation, Methylene Blue, Mesoporous, Fe₃O₄/g-C₃N₄/rGO Nanocomposite

1. Introduction

Clean water is essential for all living organisms, yet its quality is increasingly affected by pollution from industrialization and rising living standards. Toxic synthetic pollutants from industrial and domestic wastewater significantly disrupt ecosystems and pose serious risks to human health, making pollution control a critical global challenge. Each day, approximately 2 million tons of waste are discharged into water systems, with 17% - 20% of industrial wastewater attributed to synthetic dyes, according to World health organization statistics [1] [2]. Industries such as textiles, printing, and tanneries are major sources of dye wastewater pollution. Synthetic dyes, including MB, are particularly hazardous due to their toxicity and potential carcinogenicity, highlighting the urgent need for their removal from industrial effluents [3]. The development and implementation of effective wastewater treatment technologies are therefore essential to eliminate these persistent, non-biodegradable dyes and enable safe reuse of treated water.

Various chemical, physical, and biological treatment methods have been developed for pollutant removal. However, they are often limited in effectiveness, primarily transferring contaminants between phases and incurring higher costs. In contrast, advanced oxidation processes (AOPs) have gained attention for the treatment of organic pollutants with low biodegradability. AOPs generate highly reactive oxygen species (ROS) in aqueous solutions, enabling the complete mineralization of pollutants and providing an efficient approach for their removal [4].

Traditional AOPs are often limited by the high cost of catalysts such as H₂O₂ and O₃, as well as the need for complex equipment, leading to elevated operational costs [5]. To address these challenges, photo-induced catalytic degradation has attracted significant attention. Photocatalytic dye degradation provides an efficient, energy-saving, and cost-effective method for breaking down dyes and organic contaminants into less harmful products [6]. Solar photocatalysis harnesses sunlight to activate photocatalysts, offering an environmentally sustainable approach to wastewater treatment. Upon light irradiation, photocatalysts generate electron-hole pairs that drive chemical reactions, ultimately leading to pollutant mineralization [7]. Consequently, the development of high-performance, efficient, and environmentally benign photocatalytic materials has become a central focus of research in this field. The reported photocatalytic efficiencies of different catalysts for MB degradation are summarized in Table 1.

Table 1. Overview of reported photocatalysts and their efficiencies toward MB degradation.

S. No.	Photocatalyst	Synthesis Method	Light source	Time (min.)	Degradation (%)	References
1.	TE-g-C3N4	Thermal exfoliation of bulk g-C3N4	UV	60	92	[8]
2.	Fe3O4/hTiO2/g-C3N4	Reflux-precipitation and Sol-gel method	Xe lamp. 500 W	120	95	[9]
3.	1 C−1Fe3O4/g-C3N4	Wet impregnation method.	Visible	180	92	[10]
4.	ZnWO4/g-C3N4	Hydrothermal approach	Visible	120	92.9	[11]
5.	Zeolite-supported g-C3N4/ZnO/CeO2	Solid-state method	visible	180	95.89	[12]
6.	Ag-Ag2SeO3/Ppy	Precipitation	Visible	25	90.51	[13]
7.	BiVO4	Co-precipitation	sun light	80	86	[14]
8.	La0.75Ca0.25MnO3	Wet chemical method	Visible	100	68.52	[15]
9.	CeO2-NPs/GO/PAM	Polymerization	UV-A	90	90	[16]
10.	Fe/g-C3N4/rGO	Green Synthesis	Visible (100 W)	30	97.65	This work

Graphitic carbon nitride (g-C₃N₄) is a graphene-like 2D polymer composed of carbon and nitrogen atoms. It has attracted considerable attention as a visible-light-responsive photocatalyst due to its large surface area, high porosity, thermal and chemical stability, non-toxicity, low cost, and facile synthesis from abundant resources [1]. It is a metal-free, n-type semiconductor possessing a band gap around 2.97 eV [17]. However, g-C₃N₄ suffers from rapid electron-hole recombination, low selectivity, poor solar light absorption and weak redox capability, which limit its photocatalytic efficiency. To overcome these limitations, researchers have combined g-C₃N₄ with other functional materials, such as carbon-based materials, metal or metal oxide NPs, and co-catalysts, or employed doping and functionalization strategies [18].

Among carbon-based materials, graphene or reduced graphene oxide (rGO) has been widely used as a support to improve the photocatalytic activity of semiconductors. rGO possesses a unique two-dimensional honeycomb structure with outstanding electrical, optical, mechanical, and thermal properties. Incorporation of photoactive nanomaterials onto rGO sheets efficiently suppresses electron-hole recombination, allowing photogenerated electrons to participate in oxidative reactions and enhancing overall photocatalytic performance. Therefore, rGO serves as an excellent platform for supporting g-C₃N₄, facilitating charge transport, and improving photocatalytic efficiency [19].

Fe₃O₄ NPs are commonly employed in environmental remediation due to their high adsorption capacity, photocatalytic potential, abundance, low cost, low toxicity, notable catalytic activity and facile synthesis. When combined with g-C₃N₄ and rGO, Fe₃O₄ forms heterojunctions that enhance spatial charge separation, further reducing electron-hole recombination [20].

The integration of g-C₃N₄, rGO, and Fe₃O₄ into a single nanocomposite exploits the synergistic effects of each component: g-C₃N₄ serves as the primary photocatalyst generating electron-hole pairs under visible light, rGO enhances electron transport and suppresses charge recombination, and Fe₃O₄ promotes efficient charge separation at the heterojunction interface and provide more active sites. This interfacial synergy improves the photocatalytic efficiency while preserving the chemical stability and environmental compatibility of the g-C₃N₄ framework, resulting in a sustainable and high-performance photocatalyst system [21].

This study reports the synthesis and characterization of eco-friendly Fe₃O₄/g-C₃N₄/rGO nanocomposites as visible-light-responsive photocatalysts for the degradation of MB dye. The photocatalytic activity was evaluated under visible light by varying key parameters, including initial dye concentration, catalyst dosage and solution pH. The results demonstrate the effectiveness of Fe₃O₄/g-C₃N₄/rGO composites in enhancing visible-light-driven photocatalysis, offering a promising approach for mitigating dye pollution in aquatic environments.

2. Materials and Methods

2.1. Synthesis of Fe₃O₄ NPs through Green Tea Leaves Extract

Green tea extract was prepared by heating 4 g of tea leaves in 100 mL deionized water at 60˚C for 15 min, followed by filtration. For Fe₃O₄ synthesis, 0.811 g FeCl₃ was dissolved in 50 mL of the extract, sonicated for 1 h, and the resulting precipitate was filtered, washed with ethanol and deionized water, oven-dried at 70˚C, and ground into fine powder.

2.2. Synthesis of g-C₃N₄

The g-C₃N₄ was synthesized by heating 10 g of melamine in a covered crucible at 650˚C for 4 h, with a heating rate of 5˚C/min in air, followed by natural cooling, grinding, and storage [22].

2.3. Synthesis of GO

GO was synthesized using an improved Hummers’ method. Briefly, 1 g graphite was mixed with H₂SO₄/H₃PO₄ (120:15 mL) under ice bath conditions (<5˚C), followed by slow addition of 6 g KMnO₄ and stirring for 30 min. The mixture was then heated to 35˚C and stirred for 12 h. After cooling, DI water was added while maintaining < 60˚C, and oxidation was terminated by adding H₂O₂. The product was filtered, washed with HCl (1:10) and DI water until neutral pH, air-dried for 5 - 6 days and ground into GO powder [23].

2.4. Synthesis of Fe₃O₄/g-C₃N₄/rGO Nanocomposites

The Fe₃O₄/g-C₃N₄/rGO nanocomposites were synthesized using green tea extracted Fe₃O₄ NPs, along with g-C₃N₄ and GO. To prepare the composite material, 0.2 g of GO was dispersed in 50 mL of extract and ultrasonicated for 15 min. Subsequently, 0.2 g of g-C₃N₄ was added to the solution, and the ultrasonication process was repeated for an additional 15 min. Following this, 0.8 g of FeCl₃ was introduced into the mixture, which was stirred at room temperature for 1 h. The resulting mixture was then washed thoroughly with deionized (DI) water and ethanol before being dried in an oven at 80˚C for 4 h.

2.5. Characterization

The crystalline phases of synthesized NPs were examined using Rigaku make automated multipurpose X-ray diffractometer (model: SMARTLAB) in the 2θ range of 10˚ - 80˚ Surface functional groups of the synthesized NPs were analyzed using a FTIR spectrum 2 (PerkinElmer) with measurements recorded in the range 4000 - 400 cm⁻¹. UV-Vis diffusion reflectance spectra were acquired using Shimadzu UV-2600 to evaluate optical absorption properties. Surface morphology and elemental composition were characterized using filed emission scanning electron microscope (JEOL JSM-7610 F PLUS). The BET analysis was conducted using a NovaTouch LX2 gas sorption instrument (Quantachrome Instrument) to evaluate the specific surface area and pore size distribution. LC-MS analysis was conducted using a Waters Micromass Q-Tof Micro to investigate dye degradation. UV-Vis absorption spectra were acquired using an Agilent Carry 5000 spectrophotometer.

2.6. Photocatalytic Degradation of MB Dye

MB was employed as a model dye pollutant to assess the visible-light-driven photocatalytic activity of Fe₃O₄ and Fe₃O₄/g-C₃N₄/rGO. In a typical experiment, 10 mg of catalyst was dispersed in 50 mL of a 20 ppm MB solution contained in a 250 mL beaker. Experimental conditions, including solution pH, dye concentration, and catalyst dosage, were systematically optimized to achieve maximum degradation efficiency. Prior to irradiation with a 100 W visible light source, the suspension was stirred in the dark to establish adsorption-desorption equilibrium. At specific time intervals, 5 mL aliquots were withdrawn and centrifuged at 6000 rpm for 1 min to separate the photocatalyst. The concentration of MB was then monitored by recording UV-Vis absorption spectra at 617 nm. The photocatalytic degradation efficiency was calculated using the following expression [24].

$C_{deg}=\frac{C_0-C_t}{C_0}\times 100\%=\frac{A_0-A_t}{A_0}\times 100\%$ (1)

here $C_0$ , $C_t$ and $A_0$ , $A_t$ represent the concentrations (mg/L) and absorbance of MB dye at the initial time and at the time “t’” respectively.

3. Results and Discussion

The XRD spectra of the Fe₃O₄/g-C₃N₄/rGO composite, as shown in Figure 1, exhibits distinct reflections that confirm the successful integration of the three components. A weak peak at 11.12˚ is indexed to the (001) plane of GO, indicating the presence of partially oxidized graphitic domains that were not completely reduced during synthesis. The broad feature around 22˚ corresponds to the (002) plane of reduced graphene oxide (rGO), typically associated with disordered graphitic layers and amorphous carbon phases [25] [26]. The diffraction peak at 12.88˚ is due to g-C₃N₄ (100), while the strong peak at 28.0˚ corresponds to the (002) plane of g-C₃N₄ [27]. Additional reflections at 31.73˚, 42.70˚, and 57.72˚ are corresponding to the (220), (400), and (511) planes of Fe₃O₄ NPs, although the most intense Fe₃O₄ (311) reflection at 35.4˚ is suppressed, likely due to low oxide loading, nanoscale broadening, or overlap with the carbonaceous background. The Fe₃O₄ peaks also appear diminished, indicating poor crystallinity and fine dispersion within the composite [28]. These observations collectively validate the coexistence of g-C₃N₄, rGO, Fe₃O₄ and highlight strong structural interactions among them, confirming the successful synthesis of the ternary composite. The crystallite size of the NPs was calculated using the Debye-Scherrer equation.

$D=\frac{0.9\lambda }{\beta \text{cos}\theta }$ (2)

where D is the crystallite size, λ is the X-ray wavelength (1.54060 Å), β is the full width at half maximum (FWHM), and θ is the Bragg angle of the diffraction peak. Based on this analysis, the average crystallite size of the Fe₃O₄/g-C₃N₄/rGO nanocomposites was estimated to be 58.5 nm from the (002) reflection, as summarized in Table 2.

Figure 1. XRD spectra of Fe₃O₄/g-C₃N₄/rGO nanocomposites.

Table 2. Experimental XRD data of Fe₃O₄/g-C₃N₄/rGO nanocomposites.

Sample	Peak	Position (˚2θ)	FWHM (˚2θ)	d-spacing (A˚)	Crystallite size (nm)
rGO	(001)	11.12	0.81	7.94	58.5
	(002)	21.80	0.0010	4.07
g-C3N4	(100)	12.8806	0.62	6.87
	(002)	28.0000	0.14	3.18
Fe3O4	(220)	31.7362	0.14	2.81
	(400)	42.7090	0.15	2.12
	(511)	57.7200	0.1	1.60

Figure 2. FTIR analysis of Fe₃O₄/g-C₃N₄/rGO nanocomposites.

The FTIR spectrum of the Fe₃O₄/g-C₃N₄/rGO composites (Figure 2) displays characteristic vibrational features corresponding to all three components. A broad absorption band in the 3000 - 3300 cm⁻¹ region is attributed to O-H and N-H stretching, associated with surface hydroxyl and amine groups [29]. Peaks at 1631, 1564, 1456, 1400, and 1315 cm⁻¹ correspond to C=N and C-N stretching, as well as aromatic skeletal vibrations from the g-C₃N₄ framework and residual functional groups on rGO [30]. In the 1200 - 1000 cm⁻¹ region, bands at 1204, 1132, and 1084 cm⁻¹ are assigned to C-N and C-O stretching modes. A sharp peak at 807 cm⁻¹ confirms the presence of triazine ring bending vibrations. The peak at 718 cm⁻¹ is likely due to N-H wagging or ring deformation. Notably, multiple bands between 500 and 400 cm⁻¹ are attributed to Fe-O stretching vibrations, indicating the incorporation of Fe₃O₄ [30]. The spectrum confirms the successful formation of the composite, with all components interacting through non-covalent interactions without structural degradation [29].

Figure 3. (a - b) FESEM micrographs of the Fe₃O₄/g-C₃N₄/rGO nanocomposites (c - f) elemental distribution maps for carbon (C), nitrogen (N), oxygen (O), and iron (Fe), respectively (g) EDS spectrum of the Fe₃O₄/g-C₃N₄/rGO nanocomposites.

FESEM images of the Fe₃O₄/g-C₃N₄/rGO composite (Figure 3(a), Figure 3(b)) display a wrinkled, sheet-like layered morphology characteristic of g-C₃N₄ and rGO. The introduction of rGO increases surface roughness and sheet separation, providing favorable sites for the uniform dispersion of Fe₃O₄ nanoparticles (NPs). The fine granular structures observed on the surface correspond to Fe₃O₄ NPs anchored within the matrix. EDS as shown in Figure 3(g) confirmed the presence of Fe (2.2 wt%), while elemental mapping (Figures 3(c)-(f)) revealed a homogeneous distribution of Fe, C, N and O throughout the composite. These findings confirm the successful incorporation of Fe₃O₄ NPs without noticeable aggregation, ensuring uniform integration of all components while maintaining the layered structure.

Figure 4. (a) N₂ adsorption-desorption isotherm (b) BET analysis for surface area estimation and (c) BJH pore size distribution curve of Fe₃O₄/g-C₃N₄/rGO nanocomposites.

Nitrogen adsorption-desorption analysis was performed to study the pore characteristics and surface area of the Fe₃O₄/g-C₃N₄/rGO ternary nanocomposite. The isotherm (Figure 4(a)) exhibited a type IV curve with an H3 hysteresis loop, as classified by IUPAC, which is typical of mesoporous materials with slit-like pores arising from layered structures such as rGO and g-C₃N₄. The BET plot (1/[W(P₀/P) − 1] vs. P/P₀) in the relative pressure range of 0.05 - 0.3 showed excellent linearity (R² = 0.98399), confirming the accuracy of the surface area estimation (Figure 4(b)).

The calculated BET surface area of the nanocomposite was 25.546 m²/g, indicating the availability of sufficient active surface sites. The average pore volume and diameter, obtained from BJH analysis, were 0.0236102 cc/g and 3.37 nm, respectively, which further confirm its mesoporous nature.

The BJH pore size distribution curve (Figure 4(c)) revealed a broad range of pores with higher intensity in the lower pore diameter region, signifying abundant narrow mesopores and interparticle voids. These mesoporous features, coupled with the moderate surface area, are beneficial for photocatalysis as they enhance dye diffusion and adsorption at reactive sites [31].

Figure 5. UV-DRS spectra of synthesized Fe₃O₄/g-C₃N₄/rGO nanocomposites.

The optical absorption properties of the Fe₃O₄/g-C₃N₄/rGO nanocomposites were investigated using UV-DRS (Figure 5). The spectrum exhibits a shoulder peak at 274 nm, attributed to π→π* transitions of the aromatic C=C bonds in the rGO sheets [32]. The absorption peak at ~350 nm corresponds to the π→π* transitions in g-C₃N₄ [33]. Moreover, adjacent absorption at 415 - 430 nm is observed, which are associated with Fe₃O₄ nanoparticles. The broad absorption can be ascribed to the synergistic effect of Fe₃O₄ NPs and rGO sheets interacting with the g-C₃N₄ matrix, thereby improving the light-harvesting potential of the composite. This enhancement promotes the generation of more photoinduced charge carriers under visible irradiation, which contributes to its high photocatalytic performance [34].

3.1. Photocatalytic Degradation Analysis

Control studies confirmed that MB remained stable in both light and dark conditions in the absence of a catalyst, showing negligible self-degradation. Figure 6(a), Figure 6(b) display the UV-Vis absorption spectra of MB recorded at different irradiation times in the presence of Fe₃O₄/g-C₃N₄/rGO, where a gradual decrease in the absorption peak intensity is observed with increasing exposure duration. The corresponding degradation efficiencies are summarized in Table 3 and plotted in Figure 6(c), further demonstrating the superior activity of the ternary composite. The visible fading of MB during the degradation process is shown in Figure 6(d).

Figure 6. Time dependent UV-vis. absorption spectra of MB dye with (a) Fe₃O₄ (b) Fe₃O₄/g-C₃N₄/rGO nanocomposites and (c) calculated photocatalytic degradation efficiency of MB dye under visible light (d)visual demonstration of MB dye decolorization over time using Fe₃O₄/g-C₃N₄/rGO nanocomposites.

Fe₃O₄ NPs achieved a degradation efficiency of 47.86% within 30 min, whereas the Fe₃O₄/g-C₃N₄/rGO nanocomposites showed a much higher efficiency of 97.65% under the same conditions. The relatively low activity of Fe₃O₄ can be explained by its small surface area, low electrical conductivity, short hole diffusion length, and rapid electron-hole recombination, all of which limit the formation of ROS such as hydroxyl radicals (OH•) and superoxide anions (${\text{O}}_2^{-}$ ) that are required for dye degradation [35]. In contrast, the ternary Fe₃O₄/g-C₃N₄/rGO composite exhibits markedly enhanced photocatalytic activity due to the synergistic interactions among its constituents, which increases surface area and promotes efficient charge separation, thereby enhancing electron-hole pair generation and transfer [36]. It is well established that pristine g-C₃N₄ alone suffers from fast charge carrier recombination and limited visible-light utilization, which restrict its photocatalytic performance [1]. Incorporation of rGO provides an efficient electron-accepting and transporting pathway, thereby suppressing recombination and accelerating electron transfer for redox reactions [37]. Simultaneously, Fe₃O₄ nanoparticles act as electron mediators, promoting charge separation and extending light absorption [20]. Furthermore, the formation of heterojunction interfaces facilitates directional charge migration and sustains continuous ROS production, ultimately resulting in efficient methylene blue degradation [38] [39].

Table 3. MB dye degradation efficiency (%) using Fe₃O₄ and Fe₃O₄/g-C₃N₄/rGO nanocomposites.

% Photodegradation of MB dye
Time (min)	Fe3O4	Fe3O4/g-C3N4/rGO
5	16.49	53.92
10	27.48	74.54
15	36.44	85.92
20	46.94	91.83
25	47.51	97.13
30	47.86	97.65

3.2. Photocatalytic Degradation Pathway

Figure 7. LC-MS spectra of MB dye during photocatalytic degradation using Fe₃O₄/g-C₃N₄/rGO nanocomposites.

The photocatalytic degradation of MB using Fe₃O₄/g-C₃N₄/rGO nanocomposites under visible light was investigated through LC-MS analysis. The spectra (Figure 7) revealed distinct m/z peaks at 318, 303, 257, 243, 229, 200, 159, 124, and 93, along with the disappearance of the parent MB m/z peak at 284. This observation confirms that MB molecules undergo progressive decomposition into smaller intermediates. The detected fragments indicate that the degradation pathway involves successive demethylation steps, disruption of the aromatic structure, and further oxidation. The proposed mechanism (Figure 8) illustrates the gradual transformation of MB into low-molecular-weight intermediates, ultimately leading to complete mineralization into CO₂ and H₂O [40] [41]. These findings confirm that ROS predominantly govern the photocatalytic degradation mechanism.

Figure 8. Proposed photocatalytic degradation pathway of MB with Fe₃O₄/g-C₃N₄/rGO nanocomposites.

3.3. Factors Affecting the Photocatalytic Degradation of MB Dye

3.3.1. Effect of Catalyst Loading

To determine the optimum catalyst dosage for MB dye degradation, experiments were performed by varying the amount of Fe₃O₄/g-C₃N₄/rGO nanocomposite. At an initial dye concentration of 20 ppm and optimized pH under visible light, catalyst dosages of 5, 10, and 15 mg were evaluated, achieving degradation efficiencies of 80.26%, 97.65%, and 97.87% within 30 min, respectively (Figure 9(a)). The results show that increasing the catalyst loading enhances degradation up to a certain level, owing to the greater availability of active sites that accelerate dye adsorption and photodegradation. However, beyond the optimal dosage of 10 mg, only marginal improvement was observed. This slight decline in efficiency can be attributed to particle agglomeration at higher concentrations, which decreases the effective surface area, restricts access to active sites, and increases light scattering, thereby limiting photon penetration and suppressing the generation of ROS [42], [43]. Thus, optimizing catalyst dosage is essential to balance active site availability and light utilization, ensuring maximum photocatalytic efficiency while avoiding the drawbacks of excessive loading.

Figure 9. Variation in MB dye degradation efficiency over time using Fe₃O₄/g-C₃N₄/rGO nanocomposites under different conditions: (a) catalyst dosage, (b) initial dye concentration, and (c) pH values.

3.3.2. Effect of Dye Concentration

The initial concentration of the dye is a key factor influencing photocatalytic performance. To assess this parameter, MB solutions with concentrations of 10, 20, and 30 ppm were treated using 10 mg of Fe₃O₄/g-C₃N₄/rGO catalyst in 50 mL solution under visible light at the optimized pH. After 30 min of irradiation, the degradation efficiencies were 95.89%, 97.65%, and 84.17%, respectively, with 20 ppm showing the highest efficiency and therefore selected as the optimum concentration for subsequent studies (Figure 9(b)). At lower concentrations, the limited availability of dye molecules results in insufficient adsorption, leaving many active sites unoccupied. At moderate concentrations, a balance between dye availability and catalyst active sites enhances adsorption and degradation efficiency [44]. However, at higher concentrations, excessive dye molecules saturate the catalyst surface, while the increased optical density restricts photon penetration, thereby suppressing the generation of ROS and reducing overall photocatalytic activity [45]. These findings underline the necessity of optimizing the initial dye concentration to achieve maximum degradation efficiency.

3.3.3. Effect of pH

The pH of the dye solution is a crucial parameter governing photocatalytic degradation, as it influences both the surface charge of the photocatalyst and the ionization state of dye molecules. To examine this effect, the Fe₃O₄/g-C₃N₄/rGO nanocomposite was tested for the pH-dependent degradation of cationic MB dye (20 ppm, 10 mg catalyst). The degradation efficiency increased progressively with pH, achieving 67.40%, 88.68%, 97.65%, 98.00%, and 99.53% at pH 3, 5, 7, 9, and 11, respectively, after 30 min of visible-light irradiation (Figure 9(c)). The enhanced activity in alkaline media can be ascribed to two main factors: 1) strong electrostatic attraction between positively charged MB molecules and the negatively charged catalyst surface (due to rGO and g-C₃N₄), and 2) favorable generation of ROS such as OH• and ${\text{O}}_2^{-}$ , which accelerate oxidative degradation reactions [46]-[48]. These findings emphasize the significance of pH optimization according to the ionic characteristics of the target pollutant to achieve maximum photocatalytic performance of Fe₃O₄/g-C₃N₄/rGO nanocomposites.

3.4. Reusability

Figure 10. MB dye degradation efficiency (%) over successive photocatalytic runs using Fe₃O₄/g-C₃N₄/rGO nanocomposites.

The long-term reusability of a photocatalyst is an essential parameter for its practical deployment in environmental remediation. To evaluate this, the Fe₃O₄/g-C₃N₄/rGO nanocomposite was tested under optimized conditions (20 ppm dye, 10 mg catalyst, pH 7) for five consecutive cycles. After each run, the photocatalyst was collected by centrifugation, thoroughly washed with deionized water and ethanol, dried, and reused under identical conditions. Following 30 min of visible-light irradiation, the degradation efficiencies were recorded as 97.65%, 95.80%, 93.93%, 92.22%, and 90.72% for the 1^st to 5^th cycle, respectively (Figure 10), indicating only a modest decline of ~7%. The slight reduction in activity can be attributed to the deposition of intermediate by-products on the catalyst surface, which may partially block active sites, as well as minor material loss during recovery and washing steps [49] [50]. Overall, the nanocomposite exhibited excellent durability and recyclability, confirming its strong potential for sustained and efficient dye degradation under visible light.

4. Conclusion

This work reports the successful fabrication and application of Fe₃O₄/g-C₃N₄/rGO ternary nanocomposites as efficient visible-light-driven photocatalysts for the degradation of MB dye in aqueous solution. The nanocomposites were synthesized via a simple route using melamine-derived g-C₃N₄, rGO nanosheets and Fe₃O₄ NPs. In this hybrid system, g-C₃N₄ served as the primary photocatalytic matrix, rGO functioned as a conductive network enabling fast electron transport, while Fe₃O₄ NPs provided additional active sites and enhanced interfacial charge transfer. Structural and morphological characterizations confirmed the successful integration of all components, uniform distribution, and a mesoporous framework with a surface area of 25.55 m²/g and an average pore diameter of 3.37 nm. Photocatalytic studies demonstrated that the ternary nanocomposite displayed significantly higher degradation activity than its individual constituents, owing to the synergistic interactions facilitating effective charge separation and migration. Under optimized conditions, the material achieved 99.53% MB degradation within 30 min at pH 11, while maintaining high efficiency (97.65%) even under neutral pH, highlighting its practical potential. In addition, the photocatalyst retained 90.72% efficiency after five consecutive runs, indicating excellent reusability and stability. Overall, Fe₃O₄/g-C₃N₄/rGO emerges as a durable, sustainable, and cost-effective photocatalyst for visible-light-assisted wastewater remediation. Moreover, future research may focus on optimizing large-scale synthesis and evaluating performance in complex real wastewater systems to assess its practical feasibility for industrial applications.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Authors’ Contributions

Sanju Mahich: Conceptualization, Methodology, Writing-original draft. Kundan Singh Shekhawat: Investigation, Methodology. Shubham Gupta: Data analysis. Anuj Kumar: Validation, Visualization. Sanjay Kumar Swami: Writing - review & editing, Validation. Jaya Mathur: Methodology, Supervision, Investigation. Vijay Devra: Supervision. Amanpal Singh: Conceptualization, Writing - review & editing, Supervision.

Ethical Approval

Authors have read all the ethics carefully to publish and agree with the terms and conditions of the journal.

Consent to Participate

Not applicable

Consent to Publish

The authors give their consent to publish the manuscript in this journal after acceptance.

Data Availability Statement

All data supporting the findings of this study are included within the article.

Acknowledgements

The authors express their sincere gratitude to Manipal University Jaipur and Malaviya National Institute of Technology (MNIT), Jaipur, Rajasthan, India, for providing access to characterization facilities.

Conflicts of Interest

The authors have no relevant financial or non-financial interests to disclose.

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