Biowaste-to-Catalyst: Magnetite Functionalized Potato-Shell as Green Magnetic Biochar Catalyst (PtS200–Fe₃O₄) for Efficient Procion Blue Textile Wastewater Dye Abatement

Abstract

Bio-waste from potato shell agro-waste-based photocatalyst is introduced using potato shell integrated with Fe₃O₄ nanoparticles as a novel photocatalyst for photo-Fenton oxidation reaction. The catalyst was prepared via thermal activation of biochar, followed by co-precipitation of magnetite nanoparticles, resulting in a stable and reusable material. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques augmented with the energy dispersive X-ray spectroscopy (EDX) analysis with elemental mapping were used to assess the prepared sample. The prepared material, PtS200–Fe₃O₄, is then applied for oxidizing Procion Blue dye using biochar-supported magnetite catalyst. The oxidation process was evaluated under varying operational parameters, including pH, temperature, catalyst loading, oxidant dosage, and dye concentration. Results revealed that the system achieved complete dye removal within 20 min at 60 °C and pH 3, demonstrating the strong catalytic activity of the composite. Furthermore, the kinetic modeling is evaluated and the data confirmed that the degradation followed first-order kinetics. Also, the thermodynamic parameters indicated low activation energy with PtS200–Fe₃O₄ composite in advanced oxidation processes. The system sustainability is also assessed, and the reusability test verified that the catalyst retained over 70% efficiency after six consecutive cycles, highlighting its durability. The study confirms the feasibility of using biochar-supported magnetite as a cost-effective, eco-friendly, and efficient catalyst for the treatment of textile effluents and other dye-contaminated wastewater.

1. Introduction

Water pollution is a critical global challenge intensified by industrialization [1] urbanization [2], and population growth [3], with the textile industry being a major contributor [4,5,6,7] through the discharge of complex effluents containing dyes [8,9,10], heavy metals, surfactants [11,12], salts, and recalcitrant organics that resist conventional treatments [13,14,15,16,17,18,19]. Such inadequately treated wastewater disrupts aquatic ecosystems, reduces photosynthesis, and introduces toxic, carcinogenic, and mutagenic substances that threaten human and environmental health [2,20,21,22,23,24,25,26]. With rising textile demand, effluent volumes and complexity are expected to grow, underscoring the urgent need for sustainable and cost-effective treatment alternatives [7,27,28,29,30,31,32]. Advanced oxidation processes (AOPs) are among the most promising solutions, offering rapid mineralization of persistent pollutants [33,34,35,36,37,38], though their large-scale application is limited by energy demands and costly oxidants [11,39,40,41,42,43]. Within this context, the Fenton reaction has gained attention for its simplicity, low cost, and strong oxidative capacity through the generation of hydroxyl radicals (·OH) from H₂O₂ and Fe²⁺, enabling efficient degradation of recalcitrant compounds [44,45]. When integrated with photo-assisted systems, Fenton chemistry achieves enhanced radical generation, higher mineralization efficiency, and catalyst regeneration, making photo-Fenton processes particularly attractive for scalable wastewater treatment [12].

The Fenton reaction is a well-established AOP for wastewater treatment, where Fe²⁺ catalyzes H₂O₂ decomposition to generate hydroxyl radicals (·OH), powerful oxidants capable of degrading recalcitrant pollutants such as dyes, pharmaceuticals, and phenolics [44,45]. Its simplicity, cost-effectiveness, and environmentally benign byproducts make it attractive, while photo-assisted variants further enhance radical generation, mineralization efficiency, and catalyst recyclability [12]. Figure 1 represents the studies in the recent decades; research interest in Fenton-like (FL) systems has grown exponentially, with novel catalysts such as TiO₂, BiVO₄, ZnFe₂O₄, graphene, and g-C₃N₄ composites developed to improve activity, stability, and reusability, underscoring the urgent demand for innovative FL processes to address persistent wastewater contaminants [46,47,48,49,50,51,52,53].

The conventional Fenton process, though efficient, is limited by strict acidic pH requirements [5,39], excessive iron sludge generation, low catalyst reusability, and reliance on costly chemicals, which hinder large-scale application [11,46,47,48,49,50]. To overcome these drawbacks, innovative Fenton-like (FL) systems have been developed to improve efficiency, stability, and sustainability [51,52,53]. The PtS200–Fe₃O₄ biochar–magnetite composite, derived from potato shell biochar and magnetite nanoparticles, exploits the Fe(III)/Fe(II) redox cycle to generate hydroxyl radicals under UV-assisted photo-Fenton conditions [12]. Its magnetic recoverability enables simple reuse and reduced operational costs, while the biochar matrix provides abundant functional groups [54,55,56], enhancing Fe dispersion [57,58], minimizing leaching [59,60], and offering a low-cost, eco-friendly support [61,62,63,64,65,66]. Collectively, these features position PtS200–Fe₃O₄ as a next-generation FL catalyst for efficient and sustainable wastewater remediation [67,68].

In this study, a novel PtS200–Fe₃O₄ biochar–magnetite composite was developed by valorizing potato peel agro-waste and coupling it with magnetite nanoparticles. This sustainable design not only converts waste biomass into a functional support but also enhances the Fe(III)/Fe(II) redox cycle for efficient photo-Fenton oxidation of Procion Blue dye. Systematic evaluation of parameters, kinetics, and thermodynamics highlights its potential as a green, scalable, and high-performance catalyst for advanced wastewater treatment.

2. Results and Discussion

2.1. Characterization of PtS200–Fe₃O₄ Material

2.1.1. XRD Pattern

The X-ray diffraction (XRD) pattern of Fe₃O₄ material (Figure 2a) and PtS200–Fe₃O₄ (Figure 3) provides insight into the crystallographic structure of the materials. According to the data displayed in Figure 2a, the distinct and sharp diffraction peaks correspond to the crystalline planes of Fe₃O₄, commonly indexed to the cubic spinel structure. Peaks observed at 2θ of 30.2°, 35.6°, 43.3°, 53.6°, 57.2°, and 62.7° align well with the standard JCPDS card for magnetite (JCPDS No. 19–0629). Also, the sharp intensity and narrow peak widths indicate high crystallinity of the magnetite nanoparticles. Furthermore, the pattern displayed in Figure 2b, that represents the biochar/magnetite composite (PtS200–Fe₃O₄) displays the same characteristic magnetite peaks. Hence, such data verifies the successful anchoring of magnetite nanoparticles into the biochar matrix. However, a broad, less intense hump in the 10–30° range is observed, which is attributed to the amorphous carbon phase present in the biochar component. Also, a reduced intensity that signifies magnetite peaks compared to pure magnetite suggests a partial shielding or dispersion effect caused by the biochar framework.

Overall, the data verifies that Fe₃O₄ retains its crystalline structure within the composite. Furthermore, the amorphous halo indicates that the biochar matrix delivers a disordered carbon structure that might be enhanced the surface area of the material and active sites for the photocatalytic oxidation reactions. Hence, such structural confirmation supports the suitability of the biochar–Fe₃O₄ composite for advanced oxidation processes. The preserved crystallinity ensures efficient Fe²⁺/Fe³⁺ redox cycles, whereas the biochar phase delivers adsorptive capacity and surface stability, crucial for repeated photo-Fenton applications.

2.1.2. SEM Morphology

Scanning electron microscopy, SEM morphology, displays the morphological characteristics of the biochar–magnetite (PtS200–Fe₃O₄) composite. The image displayed in Figure 3 highlights a detailed morphological image of the prepared biochar/magnetite composite structure, verifying its heterogeneous surface and key functional features. The image shows visible cavitation sites (showed with blue circles), which verify the porous architecture of the biochar matrix. Such pores are generally reported to improve the specific surface area of porous composites, thereby facilitating dye adsorption by providing more accessible active sites. This is due to the porous structures, which are widely known to increase the specific surface area and improve diffusion pathways, thereby facilitating enhanced adsorption of dye molecules [69,70,71]. Also, such cavities improve the accessibility of active sites during the photo-Fenton reaction.

The image also displayed the retained cellulose fibers from the potato peel precursor providing structural support and a stable carbon framework (signified with red arrows on the image). These fibrous networks contribute to the mechanical stability of the composite and support uniform distribution of magnetite nanoparticles. The magnetite nanoparticles (signified as yellow circles) show the uniformly dispersed magnetite nanoparticles across the biochar surface confirm successful incorporation of the magnetic phase. These nanoparticles are crucial for driving the Fe(III)/Fe(II) redox cycle during photo-Fenton reactions, enabling continuous generation of hydroxyl radicals for pollutant degradation. Additionally, their magnetic nature allows easy recovery and reusability of the catalyst. Overall, the combination of highly porous biochar matrix and well-distributed magnetite nanoparticles provides a synergistic platform for catalytic processes. The porous framework ensures enhanced mass transfer and dye adsorption. It has been reported that Fe₃O₄ sites can catalyze the in situ generation of reactive radicals (e.g., ·OH) under Fenton-like or photo-Fenton conditions, thereby accelerating degradation kinetics; moreover, Fe₃O₄-based catalysts have also displayed good cycling stability in dye removal studies [72,73]. Such morphology strongly supports the composite’s dual functionality as an effective adsorbent and a stable, reusable catalyst for advanced oxidation processes in wastewater treatment.

2.1.3. EDX

EDX (Energy Dispersive X-ray) spectrum analysis is applied to illustrate the elemental composition of the biochar material to confirm its chemical makeup and purity. The data displayed in Figure 4 verifies the dominant elements of carbon (C) with a weight percentage of 66.6 wt%. Such a high value indicates that the biochar is primarily carbonaceous, consistent with pyrolyzed biomass, and suggests a stable carbon matrix that supports both the adsorption and catalytic applications. Also, the presence of oxygen (O) with 30.33 wt% might reflect surface oxygenated functional groups, such as hydroxyl, carboxyl, and carbonyl groups, which enhance hydrophilicity and interaction with pollutants. Thus, such value verifies a carbon-rich framework with abundant oxygenated functional groups. However, the presence of minor elements of potassium (K) with a low percentage by weight, including potassium of 2.04 wt% that originates from inherent minerals in the biomass precursor, could enhance the surface charge characteristics and potential catalytic activity.

Furthermore, some other trace elements including Al, P, S, Cl, Ca, Ti in amounts below 0.3 wt% suggest minimal inorganic contamination, preserving the biochar’s purity and functionality, which is also significant for the material catalytic activity. The presence of high C/O ratio verifies a porous, carbon-rich framework with surface oxygen groups that facilitate dye adsorption and Fe₃O₄ nanoparticle anchoring. This composition confirms the biochar’s suitability as a support matrix for Fe₃O₄ nanoparticles and its potential for adsorption and catalytic applications. However, trace minerals like K and Ca may contribute to surface alkalinity, enhancing pollutant interaction during photo-Fenton oxidation reactions.

2.1.4. Surface Elemental Mapping of PtS200–Fe₃O₄ Composite

Figure 5 images display elemental mapping of the biochar–magnetite PtS200–Fe₃O₄ composite. The images demonstrate the distribution of elements across the surface. Overall, the images show the uniform dispersion of Fe throughout the surface, which verifies the successful incorporation of magnetite nanoparticles in the composite surface. Furthermore, the dominance of C and O validates a carbon-rich support with oxygen functionalities that enhance adsorption and catalytic performance.

Trace elements (Al, P, S, Cl, K, Ca) indicate natural mineral residues, but at negligible concentrations, they do not affect catalytic stability. Figure 5a displays the presence of carbon with high and uniform intensity that indicates the carbon-rich biochar matrix. Also, Figure 5b displays the presence of oxygen distribution, corresponding to oxygenated functional groups on the biochar surface. The iron (Fe) signals, visible as well-dispersed bright regions, validate the effective incorporation of magnetite nanoparticles without significant aggregation. The distinct bright spots represent well-dispersed Fe₃O₄ nanoparticles anchored to the biochar (Figure 5c).

Additionally, trace amounts of potassium (K), calcium (Ca), phosphorus (P), aluminum (Al), sulfur (S), and chlorine (Cl) are detected, originating from the natural minerals present in the agro-waste precursor. This uniform distribution and compositional stability highlight the composite’s suitability as a robust, magnetically separable catalyst for photo-Fenton and other advanced oxidation processes. Furthermore, trace presence is evenly distributed in very low concentration of aluminum (Figure 5d). Figure 5e investigates the slight phosphorus distribution across the surface, likely from the biomass precursor. Also, Figure 5f confirms only trace levels of sulfur impurities. Additionally, the images in (Figure 5g–i), confirm only trace levels of chlorine, potassium, and calcium, respectively, which is displayed in uniform levels that are naturally originating from the agro-waste precursor, with minor signals across the surface that are only representative of trace levels.

2.2. Dye Photo-Fenton Oxidation

2.2.1. Effect of Different Treatment Systems

Figure 6 illustrates the varied oxidation system for oxidation efficiency of Procion Blue dye. The comparing of different advanced oxidation systems is achieved by monitoring the dye residuals to verify the efficiency of the current introduced into the modified photo-Fenton system. The data displayed in Figure 6 verifies the PtS200–Fe₃O₄/H₂O₂/UV system achieved a rapid and mostly complete oxidation, reaching almost 100% removal within 40 min of irradiance time. Such investigation demonstrates a strong synergistic effect between UV irradiation, hydrogen peroxide, and the biochar-supported magnetite catalyst as a photo-Fenton system in improving the ·OH radicals’ yield. Furthermore, the Fenton system of PtS200–Fe₃O₄/H₂O₂ system also displayed a good performance since it oxidized 65–70% of dye within 40 min of oxidation time. Hence, the result highlights the catalytic ability of the composite even without UV activation. However, lower removal efficiency might only reach 50% of dye removal after 40 min of oxidation time is achieved when the PtS200–Fe₃O₄/UV system is applied, which means less radical generation without H₂O₂ suggests some catalytic activity under UV alone.

Accordingly, when the H₂O₂/UV system is applied, less catalytic activity is achieved, which verifies the role of catalyst presence as critical for rapid oxidation. This data revealed the role of biochar–magnetite composite (PtS200–Fe₃O₄), which is to significantly enhance photo-Fenton oxidation through continuous radical generation, providing active Fe²⁺/Fe³⁺ cycling that promotes the ·OH radicals. Furthermore, the PtS200–Fe₃O₄ material promotes a large surface area that is a suitable candidate for superior Procion Blue dye adsorption, thereby facilitating localized oxidation. It is essential to mention that in the ultraviolet initiated systems, superior ·OH radicals are produced due to the efficient UV utilization for rapid and complete mineralization. Such results are in agreement with the previously cited research in the literature in treating different pollutants in aqueous discharge [31,43].

2.2.2. Effect of Dye Loadings

To reach real-world scale, it is essential to explore the effect of pollutant loading. In this regard, while keeping all other parameters constant, the Procion Blue dye concentration is investigated. The data exhibited in Figure 7 shows that the influence of initial dye concentration on the photo-Fenton degradation efficiency of the PtS200–Fe₃O₄/H₂O₂/UV system shows that the increase in the dye load has a significant effect on the oxidation rate. At the low dye concentration (40 ppm), the catalyst achieved complete removal within about 40 min. This could be illustrated by the rapid radical generation and sufficient active sites relative to the dye molecules. But increasing the concentration of the dye to 100 ppm and 150 ppm slightly reduced the removal rate. However, although near-complete dye oxidation occurred, the oxidation time occurred within 50–60 min, which means the oxidation time required for oxidation is increased with the dye concentration increase. This decline in the oxidation tendency is primarily attributed to the limited number of hydroxyl radicals (·OH) that are available for the present dye molecule in the aqueous medium. Furthermore, at high dye concentration tested at 200 ppm, a notable reduction in oxidation efficiency was attained. Only 55% of the dye oxidation and removal is reached after 60 min of irradiance time. Such behavior could be related to the slower reaction rate as the pollutant loading increases due to the radical scavenging effect, where excess dye molecules compete for available ·OH radicals. Also, the ultraviolet light penetration declined, which reduces UV penetration and the activation efficiency of the catalyst due to the shadowing effect. It is also notable that the saturation of active sites on the biochar–magnetite composite surface might limit the adsorption capacity and the subsequent catalytic oxidation.

These results highlight the importance of optimizing dye loading to balance pollutant concentration with available reactive species. Lower dye concentrations facilitate faster and more complete oxidation efficiency. Meanwhile, it is essential to mention that higher loadings require adjustments in operational parameters such as catalyst dose or oxidant concentration to maintain high removal efficiency. Such investigation is previously illustrated by other researchers [25,31].

2.2.3. Effect of Catalyst (PtS200–Fe₃O₄) Concentration

The effect of the catalyst is essential to be studied, and the amount of catalyst is optimized to reach a high reaction yield. In this regard, the catalyst concertation on the PtS200–Fe₃O₄/H₂O₂/UV photo-Fenton system is investigated by changing the PtS200–Fe₃O₄ dose from 20 to 80 mg/L. The data displayed in Figure 8 explore the increasing of the catalyst loading from 20 mg/L to 60 mg/L, which significantly enhanced the oxidation rate, with the 60 mg/L dosage achieving almost complete removal (100%) within the initial 40 min of oxidation time.

This improvement is attributed to the greater availability of active Fe(II)/Fe(III) sites on the catalyst surface, which promotes continuous hydroxyl radical (·OH) generation and accelerates dye oxidation. But further increasing the dosage to 80 mg/L did not result in a proportional enhancement, with the removal efficiency plateauing around 60% after 40 min. This reduction in performance can be explained by the agglomeration of excess catalyst particles, which decreases the effective surface area and limits light penetration, thereby reducing UV activation efficiency. Additionally, excess catalysts may increase the scavenging of hydroxyl radicals, lowering the overall oxidation efficiency. Such findings highlight that an optimal catalyst dosage (60 mg/L in this case) ensures a balance between sufficient active sites and effective UV utilization, maximizing the efficiency of the photo-Fenton process. This result was previously reported in the literature in oxidizing aqueous contaminants in wastewater using Fenton oxidation reaction [22].

2.2.4. Effect of H₂O₂ Concentration

Figure 9 shows the influence of hydrogen peroxide (H₂O₂) dosage on Procion Blue degradation in the PtS200–Fe₃O₄/UV photo-Fenton system. As displayed in the figure, increasing the oxidant concentration from 200 to 1000 mg/L markedly enhanced removal efficiency, with nearly complete dye degradation achieved within 40 min at 1000 mg/L. This improvement is linked to increased hydroxyl radical (·OH) generation, which accelerates the oxidative breakdown of dye molecules [14]. In contrast, at 200 mg/L, limited ·OH availability resulted in only 55% removal after 40 min. Nonetheless, excessively high H₂O₂ concentrations may induce radical scavenging, where surplus H₂O₂ consumes ·OH to form less reactive HO₂·, thereby lowering efficiency if overdosed. Thus, while higher oxidant doses enhance degradation, an optimum concentration is necessary to balance efficiency, cost, and process stability. Similar trends have been reported in previous studies on dye oxidation, confirming the critical role of optimized H₂O₂ dosage in photo-Fenton systems [14].

2.2.5. Effect of pH

The impact of initial pH on the oxidation efficiency of Procion Blue dye in the PtS200–Fe₃O₄/H₂O₂/UV photo-Fenton system is clearly demonstrated in Figure 10 since the pH in the Fenton reaction is crucial. The oxidation process exhibited maximum efficiency at an acidic pH of 3, where complete dye removal was achieved within 40 min. This behavior can be attributed to the optimal stability and redox cycling of Fe²⁺/Fe³⁺ ions under acidic conditions, which promotes continuous generation of highly reactive hydroxyl radicals (·OH). Also, at the natural pH of 5.7, the removal efficiency slightly declined to approximately 70%. This could be attributed to the system’s ability to still operate effectively without significant pH adjustment. Conversely, at neutral and alkaline conditions (pH 7 and 8), the degradation efficiency declined markedly, achieving less than 30% removal within the same time frame. This reduction is primarily due to the precipitation of Fe³⁺ as Fe(OH)₃, which diminishes the number of active catalytic sites, and causes the decomposition of H₂O₂ into less reactive oxygen species under higher pH conditions [13]. These results confirm that acidic environments enhance photo-Fenton activity, while operation near the natural pH still maintains practical applicability, offering potential cost savings by reducing the need for extensive pH adjustments in real wastewater treatment scenarios. Such investigation is in agreement with the previous work [10] that confirmed the pH dependence of Fenton’s reaction and other work [22] that reported that the carbon base treatment is also significant to the pH value.

2.2.6. Effect of Temperature

To investigate the effective treatment of textile effluent in real life, it is crucial to evaluate the impact of temperature to understand its effectiveness on the oxidation system. The experimental run is evaluated at varied temperature, and the results are shown in Figure 11. The data shows the effect of temperature on the oxidation efficiency of Procion Blue dye using the PtS200–Fe₃O₄/H₂O₂/UV photo-Fenton system.

The data revealed that there is a clear enhancement in degradation rate with increasing temperature, indicating the thermally activated nature of the process. Furthermore, at high temperature at 60 °C, the system achieved complete dye removal in less than 20 min, showing the strongest catalytic performance due to accelerated hydroxyl radical (·OH) generation and improved mass transfer between the dye molecules and the active sites of the catalyst. At 50 °C and 40 °C, full degradation was achieved within 30–35 min, highlighting the consistent performance improvement compared to room temperature operation. Conversely, at 32 °C, degradation was slower, with complete removal requiring approximately 40 min. Such enhancement at elevated temperatures can be attributed to the key factors if higher thermal energy reduces the activation barrier for radical formation and improves reaction kinetics. Furthermore, improved radical diffusion and adsorption facilitate faster oxidation of dye molecules. Such findings are in alignment with the thermodynamic analysis, confirming that the degradation is endothermic and surface-driven [32]. Optimizing operational temperatures thus plays a critical role in achieving higher efficiencies in shorter reaction times, particularly for high-strength dye wastewater treatment.

2.2.7. Kinetic Analysis

The kinetic analysis of the Procion Blue dye oxidation using the PtS200–Fe₃O₄/H₂O₂/UV photo-Fenton system is investigated to reach system reactor design aspects for real-world application. In this concept, the kinetic modeling is assessed. The data exhibited in Figure 12 and

Table 1. Kinetic models and their linearized equation parameters for photo-Fenton PtS200–Fe₃O₄/H₂O₂/UV oxidation.

Kinetic Model/ Linearized Equation	T (°C)	k	Units	R2	t½ (min)
Zero-order ${\mathit{C}}_{\mathit{t}}\mathbf{=}{\mathit{C}}_{\mathit{o}}\mathbf{-}{\mathit{k}}_{\mathit{o}}\mathit{t}$	32	0.32	mg·L−1·min−1	0.81	31.25
	40	0.30	mg·L−1·min−1	0.73	33.33
	50	0.25	mg·L−1·min−1	0.70	35.71
	60	0.28	mg·L−1·min−1	0.58	40.00
First-order ${\mathit{C}}_{\mathit{t}}\mathbf{=}{\mathit{C}}_{\mathit{o}}\mathbf{-}{\mathit{e}}^{{\mathit{k}}_{\mathbf{1}}\mathit{t}}$	32	0.11	min−1	0.92	4.08
	40	0.12	min−1	0.89	5.33
	50	0.13	min−1	0.93	5.78
	60	0.17	min−1	0.87	6.30
Second-order $\left({\displaystyle \frac{\mathbf{1}}{{\mathit{C}}_{\mathit{t}}}}\right)\mathbf{=}\left({\displaystyle \frac{\mathbf{1}}{{\mathit{C}}_{\mathbf{0}}}}\right)\mathbf{-}{\mathit{k}}_{\mathbf{2}}\mathit{t}$	32	0.10	L·mg−1·min−1	0.58	0.49
	40	0.13	L·mg−1·min−1	0.71	0.38
	50	0.33	L·mg−1·min−1	0.57	0.15
	60	0.41	L·mg−1·min−1	0.53	0.12

Co and Ct are the concentration at time 0 and time t; k₀, k₁, and k₂ are the kinetic constants of zero-, first-, and second- order kinetics, respectively.

tabulated the kinetic order parameters. According to the linearized equations of the common three models of zero-, first-, and second-order kinetic model equations. Such figures show the kinetic modeling of Procion Blue dye oxidation at different temperatures (32 °C, 40 °C, 50 °C, and 60 °C) using the PtS200–Fe₃O₄/H₂O₂/UV photo-Fenton system, analyzed via zero-order, pseudo-first-order, and pseudo-second-order models. The linear trend observed for the zero-order kinetics in C vs. time suggests that the reaction rate is relatively constant; however, the moderate correlation coefficients (R² ~ 0.58–0.81) indicate that the zero-order model does not adequately describe the system. This implies that the reaction is not independent of dye concentration. Furthermore, the first-order kinetics shown in the ln(C) vs. time plots exhibit strong linearity, particularly at 32 °C and 50 °C, with R² values reaching up to 0.93, confirming that the reaction predominantly follows pseudo-first-order kinetics. The slope steepness increases with temperature, indicating faster degradation rates due to enhanced radical generation and improved dye adsorption at higher temperatures. Additionally, second-order kinetics is also evaluated by plotting 1/C vs. time plots to show the weakest linearity with lower R² values (0.53–0.71), suggesting that the second-order model does not accurately represent the process.

The data displayed in Table 1 demonstrates that the reaction follows first-order kinetics, as evidenced by the highest correlation coefficients (R² = 0.87–0.93), compared to zero- and second-order models. The rate constant (k₁) increased from 0.11 min⁻¹ at 32 °C to 0.17 min⁻¹ at 60 °C, indicating that higher temperatures accelerate the degradation process by enhancing hydroxyl radical generation and improving mass transfer between the dye molecules and the catalyst surface. The corresponding half-life values slightly increased with temperature, reflecting faster removal under thermal assistance. Although second-order kinetics exhibited shorter calculated half-lives at elevated temperatures, the low R² values (0.53–0.71) confirm that this model does not accurately represent the system. Similarly, the zero-order model showed low correlation, suggesting that the reaction rate is not independent of the dye concentration.

Overall, the results clearly demonstrated that the degradation of Procion Blue dye is best described by the first-order kinetic model, consistent with radical-driven advanced oxidation processes where the dye concentration is the limiting factor. Also, it verifies that temperature plays a key role in improving reaction efficiency. Increasing temperature enhances the degradation rate, evident from the rise in kinetic constant (k) values across all models. For the first-order model, k₁ increases from 0.11 min⁻¹ (32 °C) to 0.17 min⁻¹ (60 °C), reducing the half-life from ~4.1 to ~6.3 min. The temperature dependence observed across all models indicates that higher thermal energy enhances the catalytic activity and the generation of hydroxyl radicals, leading to faster oxidation and shorter reaction half-lives.

2.2.8. Reaction Thermodynamics Analysis

Reaction thermodynamics investigation is essential since it delivers fundamental insights into the feasibility, efficiency, and mechanism of the reaction. Also, understanding thermodynamic trends allows the optimization of temperature, pH, and catalyst dosage for economic and maximum efficiency with minimal energy input. Furthermore, combined thermodynamic data might validate kinetic models and verifies whether degradation is primarily surface-controlled, diffusion-limited, or radical-driven [54,55,56]. In this regard,

Table 2. Thermodynamic parameters of photo-Fenton oxidation of Procion Blue dye using modified Fenton catalyst.

T (°C)	Ea (kJ·mol−1)	ΔG (kJ·mol−1)	ΔH (J·mol−1)	ΔS (J·mol−1·k−1)
32	−9.97	79.26	−12.47	−304.79
40		82.31	−12.57	−303.13
50		84.79	−12.66	−301.73
60		86.76	−12.74	−298.83

summarizes the thermodynamic parameters for the PtS200–Fe₃O₄/H₂O₂/UV photo-Fenton oxidation of Procion Blue dye using the modified Fenton catalyst.

Arrhenius equation, based on the first-order rate constant $k_1=A{e}^{{\displaystyle \frac{{-E}_a}{RT}}}$, is plotted in its linearized form where A is the pre-exponential factor, Ea is the activation energy (kJ·mol⁻¹), R is the gas constant (8.314 J·mol⁻¹·K⁻¹), and T is the absolute temperature (K). Arrhenius plot for the photo-Fenton oxidation of Procion Blue dye using the PtS200–Fe₃O₄/H₂O₂/UV system is plotted in Figure 13. The plot of the linearized relationship shows that the oxidation process follows Arrhenius behavior, which verifies that the temperature has a significant influence on the reaction kinetics. Furthermore, the slope of the line corresponds to the activation energy (Ea) of the reaction, which in this case, is relatively low (9.97 kJ·mol⁻¹, as calculated in Table 2). Initially, the negative activation energy, Ea = –9.97 kJ·mol⁻¹, indicates that the oxidation reaction process is surface-adsorption driven, where the interaction of Procion Blue dye molecules with the catalyst surface lowers the energy barrier for radical generation. Such a low Ea value suggests that the process is thermally facilitated and surface-controlled, meaning the reaction rate is enhanced at higher temperatures due to accelerated radical generation and improved adsorption of dye molecules on the catalyst surface.

This trend supports the kinetic and thermodynamic findings, demonstrating that increasing temperature boosts the catalytic efficiency by promoting faster radical-mediated oxidation. Such insights are critical for optimizing operational parameters to achieve higher degradation rates in practical wastewater treatment applications [57,58].

Furthermore, the positive Gibbs free energy is investigated from Eyring’s equation, $k_1={\displaystyle \frac{k_{B}T}{h}}{e}^{(-{\displaystyle \frac{∆G}{RT}})}$, where k_B and h are Boltzmann and Planck’s constants, respectively. In addition, the value found (ΔG = 79.26–86.76 kJ·mol⁻¹) across the studied temperature range suggests that the reaction is non-spontaneous under dark conditions but becomes thermodynamically favorable upon UV irradiation, which provides the energy required to initiate and sustain hydroxyl radical (·OH) production. Furthermore, the enthalpy (ΔH) and entropy (ΔS) were also attained from the relations ($∆\mathrm{H}={\mathrm{E}}_{\mathrm{a}}-\mathrm{R}\mathrm{T}$) and ($∆\mathrm{S}=(∆\mathrm{H}-∆\mathrm{G})/\mathrm{T}$). In summary, the negative enthalpy values verify that the reaction is exothermic, releasing energy as the dye molecules undergo oxidative degradation. Additionally, the negative entropy values (ΔS = –304.79 to –298.83 J·mol⁻¹·K⁻¹) signify a reduction in system randomness. Hence, such data reflects the organized adsorption of dye molecules on the catalyst surface prior to their breakdown. Overall, such findings demonstrate that the oxidation process is surface-controlled, exothermic, and energy-assisted, consistent with radical-driven mechanisms in advanced oxidation processes.

2.2.9. Recyclability and Sustainable Approach

Figure 14 illustrates the reusability performance of the biochar/magnetite PtS200–Fe₃O₄ composite used as a source of photocatalyst for Fenton oxidation. The data displayed in Figure 14 exhibited over six consecutive photo-Fenton cycles and the material is collected and washed after each cycle before it is dried for further use. The catalyst exhibited an initial removal efficiency of 99% during the fresh use, indicating excellent catalytic activity. Even after the first and second cycles, the removal efficiencies remained high at 97% and 93%, respectively, demonstrating strong structural stability and sustained redox activity of the Fe(III)/Fe(II) system. A gradual decline in performance was observed in the subsequent cycles, with efficiencies of 86%, 83%, and 77% in the third, fourth, and fifth cycles, respectively, and 72% in the sixth cycle. Such reduction in catalytic activity is likely attributed to factors such as surface-active sites’ occupation by dye pollutants, partial loss of active sites, or minor Fe leaching. However, maintaining over 70% efficiency after six uses highlights the durability, magnetic recoverability, and practical reusability of the composite catalyst for real-world wastewater treatment applications and verifies the catalyst suitability option.

In the present study, Fe leaching during reuse was not quantified; however, its assessment is recognized as essential for future work to fully evaluate the catalyst’s long-term stability and environmental safety.

2.2.10. Comparative Investigation with Literature

Comparing experimental data with values reported in the literature is crucial since it strengthens the validity, reliability, and significance of the current introduced into the modified PtS200–Fe₃O₄ Fenton system. In this concept, to quantify the performance of the system relative to other catalysts or processes reported for similar conditions used for Procion Blue dye oxidation, the data is summarized and tabulated in

Table 3. Comparison of different Fenton and modified Fenton systems for treating Procion Blue dye contaminating wastewater.

Catalyst/Source	Fenton Reaction Type	Optimal Operating Parameters Conditions	Removal Efficiency (%)/Oxidation Time	Reusability	Ref.
Biochar-supported with Fe3O4 (BC/Fe3O4)	Photo-Fenton	pH ~ 5.7, [H2O2]: 200 mg/L, UV 254 nm	99% in 30 min	72% after 6th cycle	Current work
FeSO4 + H2O2 (Homogeneous Fenton)	Classical Fenton	pH ~ 3, [H2O2]: 200 mg/L	95% in 60 min	Not reusable (sludge formation)	[59]
Fe3O4 nanoparticles (Heterogeneous Fenton)	Fenton-like	pH ~ 5–6, [H2O2]: 200 mg/L, UV-assisted	92% in 45 min	Up to 5 cycles (~70% after 5th)	[60]
FeSO4 + H2O2 (Homogeneous Fenton)	Classical Fenton	pH ~ 3, [H2O2]: 200 mg/L	95% in 60 min	Not reusable (sludge formation)	[61]
Fe3O4 nanoparticles (Heterogeneous Fenton)	Fenton-like	pH ~ 5–6, [H2O2]: 200 mg/L, UV-assisted	92% in 45 min	Up to 5 cycles (~70% after 5th)	[62]
Zero-valent iron (ZVI) + H2O2	Fenton-like	pH ~ 4, [H2O2]: 150 mg/L	85% in 60 min	Limited due to iron passivation	[63]
Fe-doped TiO2 photocatalyst	Photo-Fenton	pH 5, UV-Vis irradiation	96% in 40 min	Up to 4 cycles (~80%)	[64]
Fe-exchanged zeolite + H2O2	Heterogeneous Fenton	pH ~ 6, [H2O2]: 200 mg/L	88% in 50 min	Stable for 3–4 cycles	[65]
Fe-doped TiO2 photocatalyst	Photo-Fenton	pH ~ 5, UV-Vis irradiation	96% in 40 min	Up to 4 cycles (~80%)	[66]
Fe-exchanged zeolite + H2O2	Heterogeneous Fenton	pH ~ 6, [H2O2]: 200 mg/L	88% in 50 min	Stable for 3–4 cycles	[67]

. The table provides a comprehensive comparison of different Fenton and photo-Fenton systems for treating Procion Blue dye-containing wastewater. The comparison is based on highlighting catalyst type, operational conditions, efficiency, and their ability for reusability.

The data revealed the superior performance of the novel Fenton biochar-supported Fe₃O₄ photocatalyst. The current work displayed almost complete dye removal, 99%, in only 30 min at near-neutral pH (~5.7), with excellent reusability, reaching 72% removal after six cyclic uses. It is noteworthy to mention that this outperforms both homogeneous and other heterogeneous systems, demonstrating the benefits of biochar’s porous structure and Fe₃O₄’s magnetic recoverability.

On the contrary, the homogeneous Fenton systems such as FeSO₄ + H₂O₂ systems possess a limitation; even though they achieve 95% removal, they require acidic conditions (pH ~ 3) and produce non-reusable iron sludge. Hence, such demerits re-increase operational and disposal costs. Also, the heterogeneous Fe₃O₄ nanoparticles system provides high removal efficiency (~92%) with UV assistance and multi-cycle usability (up to five cycles), but efficiency slightly declines (~70% after the fifth cycle) due to surface fouling and potential Fe leaching. Furthermore, the zero-valent iron Fenton system achieves moderate efficiency (85%); however, the system suffers from surface passivation, thereby limiting its long-term application, which limits its real-scale practical application. Moreover, although advanced photo-assisted systems (Fe-doped TiO₂ and Fe-zeolite) showed a strong performance (88–96% removal) and reasonable reusability, benefiting from photocatalytic enhancement, the system is costly and less sustainable compared to biochar-based composites.

Thus, the data summarized in Table 3 clearly positions the biochar-supported Fe₃O₄ catalyst as the most efficient and sustainable option. The system incorporates a high oxidation rate, which might reach complete dye oxidation and removal. Also, combining its high degradation rates at near-neutral pH with the easiness of magnetic recovery for the catalyst for sustainable use showed excellent reusability over multiple cycles. The system is considered a low-cost economic system since it is produced from sustainable feedstock from agro-waste. Such a number of advantages demonstrate the practical potential of biochar-based heterogeneous photo-Fenton systems for industrial-scale textile wastewater treatment.

2.2.11. Mechanistic Investigation

To better understand the pathways governing dye degradation, a mechanistic investigation of the PtS200–Fe₃O₄/H₂O₂/UV system was investigated, focusing on the Fe(III)/Fe(II) redox cycle, radical generation, and the synergistic role of the biochar support. The PtS200–Fe₃O₄/H₂O₂/UV system degrades Procion Blue primarily through hydroxyl radicals ·OH) generated via synergistic Fenton-like and photo-assisted pathways. Surface Fe²⁺ sites on magnetite catalyze H₂O₂ decomposition to ·OH, while Fe³⁺ is photoreduced under UV irradiation, thereby sustaining the Fe(III)/Fe(II) redox cycle. In parallel, direct photolysis of H₂O₂ under UV contributes additional ·OH radicals.

The potato shell biochar support enhances these processes through multiple synergistic roles: (i) adsorption of dye molecules near active Fe sites, (ii) electron donation from oxygenated functional groups to Fe³⁺, promoting its reduction back to Fe²⁺, and (iii) structural stabilization of Fe₃O₄ nanoparticles, which limits leaching and ensures uniform active site distribution. This porous, carbon-rich framework effectively couples adsorption with catalytic activation, ensuring localized radical attack and improved efficiency.

Reactive oxygen species (ROS) generated include ·OH as the dominant oxidant, supplemented by HO₂· and ·O₂⁻ intermediates. These species attack azo and anthraquinone chromophores in Procion Blue, initiating decolorization, cleavage of aromatic rings, and subsequent mineralization into CO₂, H₂O, and inorganic ions. Furthermore, the system is strongly pH-dependent, showing maximum activity at pH of about 3 where Fe redox cycling is most efficient. At neutral or alkaline pH, efficiency declines due to Fe(OH)₃ precipitation and H₂O₂ self-decomposition. Excess H₂O₂ also acts as a radical scavenger, emphasizing the importance of optimized oxidant dosage.

Overall, the mechanism integrates (i) heterogeneous Fenton-like activation at Fe₃O₄ sites, (ii) UV-assisted photoreduction of Fe³⁺ and H₂O₂ photolysis, and (iii) biochar-mediated adsorption and electron shuttling. This synergy explains the rapid dye degradation, pseudo-first-order kinetics, and reusability across multiple cycles. To strengthen this interpretation, the recent literature [68] and other reports [9,14,16,20,44] have been incorporated, aligning the present findings with established mechanistic frameworks. The proposed mechanism of the PtS200–Fe₃O₄/H₂O₂/UV photo-Fenton oxidation of Procion Blue is exhibited in Figure 15.

3. Experimental Investigation:

3.1. Materials

3.1.1. Preparation of Biochar from Potato Shell (PtS200)

Potato shell agro-waste was collected from a local potato-processing plant and transported to the laboratory for biochar production. Upon arrival, approximately 500 g of fresh shells were thoroughly washed with tap water to remove impurities, followed by rinsing with distilled water. The cleaned shells were air-dried under sunlight for 7 days and oven-dried at 80 °C for 72 h to ensure complete moisture removal. Afterwards, the dried material was ground using a laboratory grinder and sieved to ~0.5 mm. A known weight (50 g per batch) of dried powder was placed in a muffle furnace (Carbolite, Nottingham, UK) and pyrolyzed at 200 °C for 2 h with a heating rate of 10 °C/min under atmospheric air. After cooling to room temperature, the biochar was gently ground, stored in airtight containers, and labeled PtS200 for subsequent use.

3.1.2. Preparation of Magnetite (Fe₃O₄) Nanoparticles

Fe₃O₄ nanoparticles were synthesized via a co-precipitation method. In a typical preparation, 0.1 M ferric sulfate (Fe₂(SO₄)₃, 100 mL) and 0.05 M ferrous sulfate (FeSO₄, 50 mL) solutions were mixed in a 2:1 molar ratio under vigorous stirring. The pH was slowly adjusted to 12 by dropwise addition of 2.0 M NaOH solution (~50 mL) at a rate of 1 mL/min while maintaining the mixture at 80 °C with constant stirring for 2 h. The black precipitate was repeatedly washed with distilled water until neutral pH (~7) was achieved, then oven-dried at 70 °C for 24 h to yield magnetite powder.

3.1.3. Photocatalyst (PtS200–Fe₃O₄) Preparation

Equal weights (1:1 mass ratio) of PtS200 biochar and Fe₃O₄ nanoparticles (2.0 g each) were dispersed in 100 mL of distilled water and stirred magnetically at 400 rpm for 3 h at room temperature to ensure homogeneous mixing. The slurry was filtered, dried at 80 °C overnight, and ground into a fine powder.

The prepared potato shell biochar (PtS200) and the resultant magnetite (Fe₃O₄) nanoparticles are mixed in an equal ratio. Then, the material is characterized using X-ray diffraction (((XRD, D8-Find, Bruker, with CuKα radiation (1.5418 Å), Madison, WI, USA)) and scanning electron microscopy (((FE-SEM), using Quanta FEG 250, FEI Company, Hillsboro, OR, USA) techniques augmented with the energy dispersive X-ray spectroscopy (EDX) analysis and their corresponding mapping were all conducted to verify their structural and chemical properties. The morphology of is imaged by field emission scanning electron microscopy (FE-SEM), usinQuanta FEG 250, FEI Company, Hillsboro, OR, USA. Also, the dispersive X-ray spectroscopy (EDX) for elemental analysis is supplied using the same instrument (Quanta FEG, FEI Company, Hillsboro, OR, USA). Subsequently, the resultant material PtS200–Fe₃O₄ is applied as a photocatalyst augmented with hydrogen peroxide as a source of Fenton reaction.

3.1.4. Wastewater

A synthetic commercial textile dye, Procion Blue, was used to simulate industrial textile effluent. A 1000 ppm stock solution was initially prepared by dissolving the required amount of Procion Blue dye in distilled water. Subsequently, dilutions were then performed to obtain working solutions with concentrations ranging from 40 to 200 ppm for evaluating the oxidation efficiency under various conditions. The initial pH of the simulated wastewater corresponded to the natural effluent pH of 5.7. For experiments requiring pH adjustment, a digital pH meter (Adwa digital pH-meter, Szeged-Hungary) was used, and the desired pH levels were achieved by carefully adding diluted H₂SO₄ or NaOH solutions. All chemicals used in this work were of analytical grade and were applied as received, without further purification or treatment.

3.2. Methodology

A measured volume (100 mL) of the model Procion Blue dye solution was transferred into a jar test vessel. The solution was carefully adjusted to the desired level using dilute acid or base, as pH adjustment was required prior to the transfer to the jar vessel. Subsequently, a specific dose of the PtS200–Fe₃O₄ photocatalyst was added, and the suspension was subjected to ultrasonic dispersion to assure the uniform mixing and dispersion of the material in the solution. The Fenton reaction was then initiated by adding the required amount of hydrogen peroxide (H₂O₂), followed by continuous stirring. while the mixture was exposed to ultraviolet (UV) radiation at 254 nm using an ultraviolet lamp of 15 watts. The schematic representation of the experimental setup, as well as the photocatalyst preparation, is illustrated in Figure 16. Through the runs and after treatment, the samples were periodically subjected to analysis. The treated wastewater was initially filtered through a micro-filter, then exposed for spectrophotometric analysis (Unico UV-2100 spectrophotometer, Dayton, NJ 08810, USA) to investigate the treatment facility. All experiments were conducted in triplicate under identical conditions, and the mean values are reported. Standard deviations are provided to indicate reproducibility.

4. Conclusions

This study demonstrated the efficient degradation of Procion Blue dye using a biochar-supported magnetite (PtS200–Fe₃O₄) catalyst in a modified photo-Fenton system, achieving rapid removal with good reusability over six cycles and maintaining activity across a wide pH range. Kinetic and thermodynamic analyses confirmed a pseudo-first-order, exothermic, surface-controlled process with low activation energy, underscoring the feasibility of the reaction. Beyond the immediate performance, the findings highlight the broader potential of converting agro-waste into sustainable photo-Fenton catalysts for industrial wastewater remediation. Future efforts should target solar-driven operation and continuous-flow reactor design to advance scale-up, cost-effectiveness, and long-term applicability in textile effluent treatment.