Hybrid PK-P/Fe₃O₄ Catalyst Derived from Pumpkin Peel (Bio-Waste) for Synozol Red KHL Dye Oxidation Under Photo-Fenton Reaction

Abstract

This study introduces a novel photocatalyst derived from pumpkin peel bio-waste, calcined at 200 °C and incorporated with magnetite nanoparticles to form a hybrid PK-P/Fe₃O₄ catalyst. The material was characterized using X-ray diffraction (XRD), diffuse reflectance spectra (DRS), and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) mapping to confirm its structure and elemental distribution. The catalyst was applied for the photo-Fenton degradation of Synozol Red KHL dye under natural solution conditions (pH 5.7). Optimal parameters were achieved with a 20 mg/L catalyst and 200 mg/L H₂O₂, resulting in complete dye removal within 25 min of irradiation. The PK-P/Fe₃O₄ catalyst exhibited excellent reusability, retaining 72% removal efficiency after 10 successive cycles. Kinetic analysis confirmed a first-order model, while thermodynamic evaluation revealed a non-spontaneous, endothermic process with a low activation energy barrier, indicating energy-efficient dye degradation. These findings highlight the potential of bio-waste-derived PK-P/Fe₃O₄ as a sustainable, low-cost, and highly effective catalyst for treating dye-polluted wastewater under photo-Fenton conditions.

1. Introduction

The release of industrial wastewater discharge into natural ecosystems represents a major source of pollution and poses serious ecological and aesthetic threats to both human beings and aquatic environments [1,2]. Such discharges also create risks of bioaccumulation [3], whereby contaminants enter the food chain and ultimately affect human health. Certain pollutants are associated with severe outcomes, including allergies and cancer [4,5]. Therefore, wastewater must be treated before discharge to safeguard both the environment and public health.

One of the industrial sectors producing massive amounts of contaminated aqueous effluent is the textile industry. The discharge of textile dyes into the environment poses serious ecological and health challenges due to their complex aromatic molecular structures, high stability, and resistance to biodegradation [6]. Dye-containing effluents are often highly colored, reducing light penetration in water bodies and thereby disrupting photosynthetic activity in aquatic plants and algae [7]. Many dyes and their degradation by-products are toxic, mutagenic, or even carcinogenic, posing risks to aquatic organisms and, through bioaccumulation, to humans as well [8]. In addition, untreated textile wastewater typically contains high levels of chemical oxygen demand (COD), total dissolved solids (TDS), and heavy metals, which further deteriorate water quality and harm aquatic ecosystems [9]. Persistent contamination can destabilize food chains, reduce biodiversity, and impair ecosystem balance, while also limiting the usability of water resources for drinking, irrigation, and industrial purposes. These impacts highlight the urgent need for sustainable and effective wastewater treatment strategies to mitigate the environmental burden caused by textile dye effluents.

Wastewater treatment typically relies on conventional methods such as physical, chemical, and biological processes, including adsorption on activated carbon, membrane filtration, coagulation–flocculation, and biological degradation [10,11]. While these techniques are effective in removing suspended solids, turbidity, and biodegradable organics, they often face significant drawbacks when applied to recalcitrant pollutants such as synthetic dyes, pharmaceuticals, and aromatic compounds. For example, adsorption only transfers contaminants to another medium without degradation, biological treatments are ineffective against non-biodegradable compounds, and chemical coagulation generates large amounts of sludge that pose secondary disposal issues [12,13]. To overcome these limitations, advanced oxidation processes (AOPs) have been developed, which rely on the generation of highly reactive hydroxyl radicals (•OH) capable of non-selectively oxidizing and mineralizing a wide range of persistent organic pollutants [14,15]. Among AOPs, the Fenton reaction, which is based on the catalytic decomposition of hydrogen peroxide (H₂O₂) in the presence of ferrous ions (Fe²⁺), has received particular attention due to its simplicity, high efficiency, and ability to operate under mild conditions [16]. The Fenton process not only achieves rapid decolorization and degradation of complex dye molecules but also reduces toxicity by converting pollutants into less harmful intermediates, thereby offering a promising and sustainable alternative to conventional treatment methods [17].

Despite its proven efficiency, the conventional Fenton process suffers from several limitations that restrict its large-scale applications. One major drawback is its requirement for an acidic medium (optimal pH is always in the acidic range 2.5–3.0), which necessitates continuous pH adjustment and increases operational costs [18]. In addition, the process generates large amounts of iron-rich sludge due to the reaction between Fe²⁺ and Fe³⁺, creating secondary disposal and environmental challenges [19]. Furthermore, the narrow working pH range, difficulty in catalyst recovery, and relatively high operating costs hinder its industrial-scale adoption. Moreover, the requirement for fresh chemicals, particularly ferrous salts and hydrogen peroxide, adds to the overall cost of treatment, making the process less economically viable for long-term or large-volume wastewater treatment [20]. These limitations have driven interest in modified systems such as photo-Fenton, electro-Fenton, and heterogeneous Fenton, which aim to reduce chemical use, reduce sludge generation, widen the applicable pH range, and improve overall process sustainability [21].

On the other hand, porous carbon-based substances have gained significant attention due to their high surface area, shortened diffusion pathways, and reduced diffusion limitations [22]. Recently, the fabrication of porous carbon materials from diverse biomass precursors—including cotton pulp, lignin, silk, fruit waste, and peanut shells—has become a prominent research direction in the wastewater treatment field [23,24]. These biomass-derived carbons generally exhibit high efficiency in removing contaminants from wastewater. When subjected to pyrolysis at elevated temperatures (700–900 °C), such materials develop hydroxyl and carboxyl functional groups on their surfaces [25,26,27,28,29]. These groups enhance adsorption capacity and may promote selective pollutant uptake [26,30,31,32]. Therefore, porous carbon electrodes derived from biomass not only deliver improved treatment performance but also offer the advantages of low cost and reduced environmental toxicity.

Efficient biowaste management plays a crucial role in the industrial sector, as its utilization can deliver environmental and economic merits while enhancing the overall value of production [27,33]. By-products from fruit and vegetable processing, as well as from juice and jam manufacturing, are especially noteworthy since they serve as valuable sources of compounds. Non-edible portions of fruits, such as peels and seeds, are often rich in phenolic and bioactive compounds [34,35,36,37,38,39]. Pumpkin peel contains high levels of these molecules and, in addition to its well-recognized antioxidant activity, has been identified as a promising natural preservative [30,40,41,42,43]. Recent investigations have also shown that waste materials can be modified into functional adsorbents and catalysts for wastewater treatment [31,44,45,46,47,48]. Bio-char is highly porous, enriched with hydroxyl and carboxyl functional groups [49], and well-suited for applications in adsorption [50], catalyst support, and wastewater treatment. This highlights their potential use in the wastewater management industry as natural alternatives to synthetic substances, contributing to safer and more sustainable strategies.

Biomass materials investigated so far face major limitations for large-scale industrial application due to the scarcity and high cost of raw materials, uneven geographical distribution, and challenges related to cultivation, transportation, and storage. But pumpkins waste peels stand out as a more suitable precursor due to their availability worldwide. They are abundant in sugars and starches and offer favorable properties for global production as porous carbon-based substances with outstanding properties characteristics [51,52,53,54].

Herein, the current investigation deals with the converting biomass waste pumpkin peels (PK-P) into a valuable substance with specific characteristics. The material is valorized through simple techniques. Next, magnetite is prepared as an environmentally benign material and added to PK-P as a composite photocatalyst. Further, the composite characterization is also assessed using XRD and SEM morphology. Synozol Red KHL is oxidized, and the removal efficiency related to the operating variables is investigated. Hence, the suggested system is introduced as a modified Fenton system to overcome the demerits of conventional Fenton oxidation to reach a win-win ecological system.

2. Results and Discussion

2.1. Catalyst Characterization

Crystalline structure:

The X-ray diffraction (XRD) spectra of PK-P-bio-char/magnetite and pristine magnetite nanoparticles were obtained using an X-ray diffractometer (XRD, D8-Find, Bruker, Madison, WI, USA) with CuKα radiation (1.5418 Å), operating at a current of 40 mA and voltage of 40 kV, and a step filter of 0.01° to analyze the crystalline nature. The data is presented in Figure 1. As shown in Figure 1a, the diffraction peaks correspond well to the magnetite crystal structure (JCPDS No. 89-4319) [46]. Also, Figure 1b represents magnetite augmented with PK-P-bio-char/magnetite.

In both graphs (a) and (b) in Figure 1, the patterns of the same spinel phase appear, which signifies the presence of magnetite nanoparticles. The characteristic reflections appear at the 2 theta values of 30.2°, 35.5°, 43.2°, 57.0°, and 62.6° 2θ, which index to the corresponding planes of (220), (311), (400), (511), and (440) of the cubic spinel lattice. The magnetite XRD pattern (Figure 2a) reflects a sharp intense peak on low background evidence, which reflects a good crystallinity as well as a relatively large crystallite size. Figure 2b, which represents the XRD graph of the PK-P-bio-char/magnetite pattern, exhibits the same peak positions as exhibited for magnetite (Figure 1a), but a more pronounced broadening, lower intensity, and a higher diffuse background [47]. Furthermore, a notable weak hump near the 2 thetas of 20–30° might indicate the presence of an amorphous component, increased microstrain, or poorly crystalline structures. Such amorphous components might be referred to as the carbon matrix or poorly ordered phase. However, it is notable that there are no additional peaks attributable to impurity iron oxides. Thus, the results verify phase purity [48]. It is worth noting that the crystallinity is retained and decreases in the composite PK-P-bio-char/magnetite material.

Moreover, the XRD pattern of the PK-P-bio-char/magnetite material after dye loading and cyclic use is displayed in Figure 1c. It is clear from the graph that the magnetite peaks still appear at the same 2θ positions, but their heights decrease. When the PK-P-bio-char/magnetite was loaded with Synozol Red, the XRD pattern retained the characteristic reflections of magnetite at 2θ ≈ 30.1°, 35.5°, 43.1°, 53.5°, 57.1°, and 62.7°, confirming the presence of the crystalline phase of Fe₃O₄, which remained intact and integrated with the PK-P-bio-char in the material. However, a noticeable decrease in the relative peak heights was observed, accompanied by a slight broadening of the diffraction peaks. This attenuation in intensity can be attributed to the coverage of magnetite crystallites by the amorphous dye layer, which absorbs and scatters incident X-rays, thereby lowering the diffracted signal. In addition, the baseline of the diffractogram was slightly elevated with the appearance of a broad hump around 22–26°, corresponding to the amorphous contribution from the bio-char matrix and the adsorbed organic dye molecules [48,49]. These changes indicate that dye loading does not alter the crystalline structure of magnetite but introduces additional amorphous content and surface disorder, consistent with the successful adsorption of Synozol Red onto the composite surface.

Morphological characterization:

The SEM micrograph in Figure 2 displays the morphological characteristics of the bio-char-derived material incorporated with magnetite nanoparticles. The morphology was imaged by field emission scanning electron microscopy (FE-SEM) using Quanta FEG 250 (FEI Company, Hillsboro, OR, USA). Also, dispersive X-ray spectroscopy (EDX) for elemental analysis was performed using the same instrument (Quanta FEG 250).

The SEM micrograph of magnetite nanoparticles (Figure 2a) exhibits a dense, cauliflower-like carpet of near-spherical nanoparticles. Also, it is notable that they are agglomerated into larger clusters. The particles possess a rough surface with a non-faceted texture and non-uniform ordering. The pronounced aggregation is associated with the magnetically driven properties. During material drying, a porous hierarchical texture forms, providing a high external surface area that is beneficial for wastewater adsorption or catalytic oxidation.

Furthermore, the average particle size was measured to be approximately 45 nm, which is signifies a narrow size distribution, reinforcing their classification as nanocomposites (<100 nm) (Figure 2b).

The composite image exhibits a heterogeneous and irregular surface with a wrinkled texture, and visible layered and fragmented sheet-like structures (Figure 3). Such morphology indicates the occurrence of a highly porous material, which is a characteristic feature of bio-char-derived materials. The sheet-like and wrinkled texture supports a large specific surface area and, thereby, abundant active sites. Hence, the material could be categorized as favorable for adsorption and catalytic applications.

Also, dark regions appear in the micrograph, signifying the presence of microporous and mesoporous cavities, whereas the brighter areas correspond to denser carbon-rich or possibly iron-rich spheres. Furthermore, it is notable that the rough and fractured appearance improves the potential of the material to act as an efficient catalyst support substance, because it facilitates the uniform dispersion and incorporation of magnetite nanoparticles. Notably, visible structural defects, cracks, and pores signify that the pyrolysis and composite preparation processes are successful in developing a porous carbonaceous matrix. This porous nature not only endorses better mass transfer and dye contaminants diffusion in polluted aqueous effluents but also enhances the accessibility of reactive sites. Therefore, such reactive sites are highly beneficial for photo-Fenton catalytic reactions, and the performance of the modified Fenton reaction is maximized.

Overall, the SEM image verifies that the prepared PK-P-bio-char-incorporated magnetite nanocomposite retains the desirable surface roughness and porosity essential for high-performance wastewater treatment applications.

Elemental Composition:

An energy-dispersive X-ray spectroscopy (EDS) spectrum that illustrates the elemental composition of the PK-P-bio-char/magnetite nanocomposite was also obtained to investigate the material’s composition. The data displayed in Figure 3a indicate the intensity of the detected X-rays from the material. Notably, sharp peaks are observed, corresponding to specific elements, verifying the presence of specific material derived from the pumpkin peel treatment and pyrolysis. The spectrum reveals prominent peaks for carbon (C), nitrogen (N), oxygen (O), aluminum (Al), sulfur (S), phosphorus (P), and iron (Fe). It is noteworthy that the most intense peak corresponds to carbon, indicating that carbon is the dominant element in the PK-P-bio-char/magnetite nanocomposite. In contrast, nitrogen, oxygen, and other detected elements were present in lower concentrations. However, it is essential to mention that the notable presence of aluminum, sulfur, phosphorus, and iron suggests the possibility of the existence of possible inorganic compounds or complex substances in the material. Overall, the EDS analysis confirms the multi-elemental nature of the prepared sample, with a high carbon content, validating the successful incorporation of bio-char in the composite material.

EDX elemental mapping was also obtained, and the data is displayed in Figure 3b. Different colors in the displayed image correspond to the spatial distribution of specific elements in the PK-P-bio-char/magnetite nanocomposite sample. The legend at the bottom of Figure 3b represents the occurrence of identified elements: phosphorus (P), sulfur (S), potassium (K), aluminum (Al), iron (Fe), nitrogen (N), oxygen (O), and carbon (C). Notably, a red signal, representing carbon, dominates the image, suggesting that the sample is primarily a carbon-rich material; this observation is consistent with the carbonaceous or possibly organic-based nature of the substance. Other elements also appear, including oxygen (blue) and nitrogen (orange), which are distributed in patches. Such distribution indicates surface oxidation or the incorporation of heteroatoms. Notably, iron (pink) appears more sparsely but in localized regions, which could correspond to embedded nanoparticles or possible mineral inclusions. Nevertheless, the overall mapping exhibits a heterogeneous distribution. It is worth mentioning that the carbon matrix serves as the main structural component, whereas other elements are dispersed or clustered in specific regions, reflecting the complex multiphase composition.

Diffuse reflectance spectra, DRS:

The absorption spectrum of the PK-P-bio-char/magnetite composite is presented in Figure 4. The DRS profile of the PK-P-bio-char/magnetite composite shows broad absorption in the visible region (approximately 400–800 nm), which is a characteristic feature of magnetite nanoparticles. The initial absorption plateau around 350–400 nm can be attributed to charge transfer transitions in the Fe–O bonds. A slight incline near 500 nm indicates scattering effects or overlapping transitions, while the strong increase in absorption above 600 nm reflects the narrowing of the optical band gap induced by the incorporation of magnetite into the pumpkin peel-derived composite. Additionally, the optical absorption spectra were analyzed using a plot of $\left(F\right(R)hv{)}^{{\displaystyle \frac{1}{2}}}$ as a function of the energy of a photon $\left(hv\right)$, where $F\left(R\right)$ follows the Kubelka–Munk relation [54]. This relation is expressed as $F\left(R\right)=(1-{R)}^2/2R$, where R is the diffuse reflectance. The band gap energy (E_g) was estimated by extrapolating the absorption edge to the photon energy $\left(hv\right)$ axis through a linear fit. The optical band gap was determined to be approximately 2.49 eV.

2.2. Dye-Containing Wastewater Oxidation

2.2.1. Effects of Different Treatment Systems

To evaluate the performance of the proposed novel system, various Synozol Red KHL treatment systems were compared simultaneously to investigate their oxidation performance as well as the time of reaction. Such a comparison verifies the efficiency of the proposed system with the conventional process, and the data exhibited in Figure 5. PK-P-bio-char/magnetite, as a novel catalyst, was subjected to degradation experiments under the initiation of the ultraviolet source. The data exhibited in the graph reveal that the H₂O₂/UV, PK-P-bio-char/magnetite/UV, and photo-Fenton PK-P-bio-char/magnetite/H₂O₂/UV were compared, and the experimental results showed that the highest efficiency was achieved by the combined photo-Fenton system PK-P-bio-char/magnetite/H₂O₂/UV. The results clearly demonstrate that the dual treatment achieved superior removal efficiency, reaching 95% under the experimental conditions (pH 5.7, catalyst 10 mg/L, and hydrogen peroxide 100 mg/L).

The plot shows that oxidation time is approximately 30 min under UV irradiance for all the applied systems: dark Fenton, H₂O₂/UV, and the PK-P-bio-char/magnetite/UV photo-Fenton catalyst. It is noteworthy that the hybrid PK-P-bio-char/magnetite/UV curve drops steeply within the initial 5 min of irradiance, followed by a gradual drop in oxidation efficiency. Also, the pronounced early drop and slower oxidation for the PK-P-bio-char/magnetite/UV system are associated with the strong synergistic effect between UV irradiation and the magnetite photo-Fenton reaction supported on bio-char derived from PK-P waste. Furthermore, the system is superior compared to other oxidation systems and might be linked to the higher OH radical yield associated with the Fenton reaction.

The system was compared to hydrogen peroxide and the solo magnetite system. It is noteworthy that the combined system benefits from rapid adsorption/pre-concentration, efficient •OH generation, and sustained redox cycling. However, the other routes are limited by slower radical generation. This data is in accordance with the previous data in the literature on treating various contaminants in wastewater [14,33].

2.2.2. Effect of Photo-Fenton Operating Parameters

Effect of Hydrogen Peroxide:

Hydrogen peroxide may also decompose prematurely under non-optimal conditions, reducing efficiency. In addition, the process sometimes achieves incomplete mineralization, producing toxic intermediates that require further treatment. Hydrogen peroxide concentration was varied from 50 to 300 mg/L to investigate its effect on the oxidation and removal of Synozol Red KHL dye, whereas all other parameters—including 50 ppm dye concentration, 20 mg/L of the catalyst, and the original pH of 5.7 of the dye solution—were kept constant. The results are displayed in Figure 6. All experiments displayed a superior oxidation tendency during the initial period of irradiation, with a fast drop within the first 5–10 min. Subsequently, a slower plateau approach was commonly observed, consistent with Fenton/photo-Fenton oxidation for Synozol Red KHL dye removal. It is noteworthy that adsorption and oxidation reactions occur simultaneously due to the presence of the bio-char/magnetite catalyst. Notably, as the reaction proceeds, hydrogen peroxide is consumed, and thereby the rate of oxidation decreases.

PK-P-Bio-Char/Fe₃O₄ catalyst:

The impact of the PK-P-bio-char/Fe₃O₄ catalyst on the photo-Fenton reaction was assessed by varying its concentration in the oxidation system reaction. Figure 7 illustrates the effect of the catalyst in Fenton’s oxidation. The catalyst concentration ranged from 5 to 30 mg/L. The data show a drop in pollutant concentration during the initial oxidation period, followed by a slower decline. This is linked to the increase in the catalyst concentration, which results in superior oxidation efficiency, which increased from 40 to 100% removal with the catalyst increasing from 5 to 20 mg/L. This could be attributed to the active sites of the substance being filled, with intermediates accumulating in the reaction medium rather than oxidizing the pollutants. When •OH radicals are generated at an optimal value, it leads to higher removal efficiency. However, a further increase in the catalyst concentration to 30 mg/L results in a decline in the oxidation efficiency. This is attributed to the radicals/active sites becoming limiting and light screening or mass-transfer effects appearing. The available data aligns with the expectation that a high concentration of hydrogen peroxide will produce a significant amount of •OH radicals [44], thereby enhancing the dye removal rate up to a point, beyond which hydrogen peroxide contributes minimally. Thus, this investigation confirms that H₂O₂ must be optimized for best performance.

pH-value effect:

pH is signified as a worthy adjustable variable in the photo-Fenton reaction, since the pH in the reaction is considered to influence the H₂O₂ reagent decomposition and the hydrolytic speciation of metal ions. In this regard, the pH value in the system is varied to evaluate the best operating performance, whereas the hydrogen peroxide value (200 mg/L) and PK-P-bio-char/Fe₃O₄ catalyst (20 mg/L) are kept constant in all pH conditions of the systems. The data displayed in Figure 8 exhibited a strong pH dependence of Synozol Red KHL dye oxidation and thereby removal in the photo-Fenton system. The process is superior and reaches complete dye removal (100%) when the pH is the natural pH of the dye (5.7). Also, the acidic pH offers superior efficiency that also achieves almost complete removal of the dye (99%). This might be attributed to the Fenton chemistry where Fe²⁺/Fe³⁺ cycling and •OH production is maximized, and iron remains soluble under the given pH value. At pH 5.7, the curve drops very quickly in the first 5–10 min. Similarly, the adsorption could take place onto the bio-char support, demonstrating that near-neutral operation is feasible. However, at neutral pH 7.0, Synozol Red KHL dye oxidation markedly declines and becomes slower due to the Fe³⁺ hydroxy species precipitating. Thereby, the result is a reduction in the photo-Fenton activity and limited surface redox cycling. Also, when the pH is raised to the alkaline range 8.0, the reaction is strongly inhibited due to iron precipitation. Also, new intermediates appeared in the reaction medium that could be scavenging the •OH radicals such as OH⁻ species [12,33]. Furthermore, H₂O₂ could be decomposed into the less reactive HO₂⁻/O₂•⁻ pathway that leads to the formation of the hydroperoxide (•HO₂) inactive radical [22]. The hydroperoxide radical and the superoxide radical anion are signified as the strongest reaction inhibitors that neutralize the oxidizing forms of metal cations in aquatic solution. Overall, acidic conditions yield the highest radical flux and the fastest mineralization. Also, near-neutral pH benefits from adsorption plus heterogeneous photo-Fenton. But the alkaline pH conditions suppress the process. The data exhibited for the experiment runs verifies that even though natural dye pH or acidic pH is favorable, it is noteworthy that the oxidation could be efficient at varied pH ranges. This investigation is in accordance with the previous work [17,20] that confirmed the pH dependence of Fenton’s reaction and other work [22] that reported that carbon-based treatment is also significant with respect to the pH value.

Temperature effect

To achieve effective treatment of textile effluents and ensure real-life applicability, it is crucial to evaluate the impact of temperature to understand its effectiveness in the oxidation system. Since discharged effluents may vary in temperature, the temperature influence is investigated by varying the temperature from room temperature (32 °C) to 40, 50, and 60 °C, whereas the hydrogen peroxide value (200 mg/L) and PK-P- bio-char/Fe₃O₄ catalyst (20 mg/L) are kept constant at pH 5.7. The data displayed in Figure 9 revealed that at all temperatures, the dye concentration drops sharply within the initial 5 min of oxidation time. This fast drop in the preliminary irradiance time suggests a high availability of active sites and/or OH hydroxyl radical species in the initial step of the oxidation process. Also, the oxidation tendency increases as the temperature increases. Complete dye removal is achieved (100%) with oxidation of Synozol Red KHL dye, and when the temperatures 60 °C, the oxidation is completed within only 20 min compared to 25 min at room temperature, with notable residual concentration even at 15 min. Notably, all temperature systems reached full degradation by 25–30 min, but higher temperatures significantly accelerated the oxidation rate. This could be associated with the enhanced molecular collisions as well as the increased oxidative •OH radical yield, which increases at high temperature. Thus, the data revealed that operating at elevated temperatures can markedly improve treatment efficiency, though energy considerations should be addressed for large-scale applications. These data are in accordance with previous work [44] using dual adsorption and Fenton reaction with activated carbon and Fe³⁺ composite, which addressed the influence of temperature on such treatment.

Dye loading:

Synozol Red K-HL dye oxidation is influenced by the initial dye concentration, and it is essential to study this effect since, in the real world, industrial effluents might be discharged at varied loads. In this context, the dye concentration ranging from 20 to 200 mg/L is investigated, whereas all other operational parameters are kept constant (hydrogen peroxide value at 200 mg/L, PK-P-bio-char/Fe₃O₄ catalyst at 20 mg/L, and pH 5.7). The photo-Fenton-based PK-P-bio-char/Fe₃O₄ oxidation process is evaluated at different dye concentrations. The experimental data displayed in Figure 10 reveals that at low dye concentrations of 20 ppm and 40 ppm, the oxidation rate is significantly high and proceeds at a rapid rate, achieving complete removal within 25 and about 40 min, respectively. However, at the high dye load of 80 ppm, the removal is significantly slower compared to low loads, with only partial oxidation of 25% after 60 min of irradiance time. Nevertheless, at the highest concentration of 200 ppm dye load, the process revealed the lowest efficiency, with over 50% of the dye persisting even after 60 min.

This above-mentioned observation indicates that higher dye concentrations hinder oxidation efficiency. Such a trend might be due to the insufficient radical availability relative to dye molecules at higher concentrations since the same amounts of Fenton reagent are added even though the dye load is low or high. Thus, the same quantity of •OH radicals is produced, and the dye molecules increase [24]. Moreover, radical scavenging effects could occur, whereas extra dye molecules compete with intermediates, thereby decreasing the effective oxidation [22,44]. Furthermore, several researchers [13,25,44] have mentioned that the possibility of ultraviolet light penetration into the reaction medium is reduced due to the shadowing effect of the dye in the solution at high loads. Also, mass transfer resistance persists at high concentrations, limiting the oxidation reaction from occurring efficiently. These findings highlight that optimal performance is achieved at moderate dye concentrations, while sufficient •OH radical species are available in the reaction medium, ensuring a rapid and complete oxidation rate. Thus, this investigation emphasizes the importance of dose optimization in practical wastewater treatment applications.

2.3. Comparative Investigation with Cited Literature

Synozol Red K-HL dye oxidation was investigated in the current work and compared with the available literature. According to the data exhibited in

Table 1. Comparison of various types of wastewater loaded with Synozol Red KHL dye treatment-based catalysis reactions.

Type of Process/ Catalyst	Removal Efficiency	Operating Conditions	Oxidation Time	Reference
Heterogeneous photo-Fenton/Fe3O4 nanoparticles	80%	pH 3–4, UV irradiation, H2O2 100 mg/L, catalyst 0.5 g/L	60–90 min	[35]
Photocatalysis/Fe-doped TiO2	86%	pH 5.0–7.0, UV light, catalyst 0.5 g/L	120 min	[42]
Photocatalysis/ZnO nanoparticles	83%	pH 6.0, solar/UV light, catalyst 1.0 g/L	90–120 min	[43]
Adsorption + Fenton/activated carbon/Fe3+ composite	81–94%	pH 3.0, H2O2 50–80 mg/L, catalyst 1.0 g/L	40–70 min	[44]
Biomass-derived photo-Fenton/pumpkin peel–Magnetite (PK-P/Fe3O4)	100%	pH 5.7, H2O2 100 mg/L, UV irradiation, catalyst 10 mg/L	25 min	Current work

, although the homogeneous Fenton (Fe²⁺/H₂O₂) remains one of the most superior processes, achieving a high rate of dye oxidation within minimal time (>90%)—with reaction times ranging from 30 to 60 min—it possesses some limitations. Its drawbacks include the requirement for a strictly acidic pH medium and by-product sludge formation. But heterogeneous catalysts, including Fe₃O₄ nanoparticles and Fe-doped TiO₂, afford comparable removal efficiencies (80–90%), with the advantage of easier recovery and reusability opportunities, although at the expense of longer reaction times. ZnO nanoparticles exhibit similar performance under solar/UV irradiation, suggesting a sustainable alternative; nevertheless, stability and photo-corrosion remain challenges.

Activated carbon/Fe³⁺ composites provide a dual treatment of adsorption and oxidation reactions. Hence, the removal yield is high and could reach a removal efficiency of more than 85–95% due to the synergistic effect. However, it is worth mentioning that regeneration of the adsorbent might be costly. Notably, the biomass-derived PK-P/Fe₃O₄ material demonstrates efficient removal, achieving complete dye removal comparable to previous conventional systems. Moreover, the current system offers the advantages of low cost, sustainability, and valorization of agricultural waste. Thus, such a study highlights the potential of integrating bio-waste-derived catalysts with photo-Fenton processes as an environmentally friendly alternative to traditional techniques of wastewater treatment.

2.4. Oxidation Mechanism

Synozol Red KHL dye oxidation by applying the modified photo-Fenton system using pumpkin peel–magnetite (PK-P/Fe₃O₄) is investigated through the reaction mechanism (as displayed in Figure 11). In such a reaction, the photocatalytic oxidation and degradation pathway of the dye (Synozol Red KHL)-contaminated wastewater is proposed by the use of pumpkin peel–magnetite (PK-P/Fe₃O₄) as a heterogeneous catalyst under ultraviolet assisted photo-Fenton environments. Primarily, Synozol Red KHL dye molecules are adsorbed onto the porous prepared bio-char-based bio-waste material incorporated with magnetite nanoparticles (PK-P-bio-char/Fe₃O₄) through electrostatic interactions and hydrogen bonding, which improves the Synozol Red KHL contaminant loading near catalytic sites. Under ultraviolet illumination, the exposed H₂O₂ peroxide might decompose, either directly or via catalytic Fe²⁺/Fe³⁺ redox cycling on the PK-P/Fe₃O₄ surface, thereby producing highly reactive oxidative radical species. These radicals include hydroxyl radicals (•OH) and hydroperoxyl radicals (•HO₂). These radicals attack the Synozol Red KHL molecules, cleaving azo bonds and aromatic rings; the result is fast decolorization due to the dye molecules’ reduction. Subsequent oxidation converts intermediate compounds into low molecular weight organic acids and eventually into CO₂, H₂O, and inorganic ions species such as SO₄²⁻ and NO₃⁻. The porous bio-char matrix of PK-P-bio-char not only improves the adsorption and electron transfer, but also helps produce Fe²⁺ from Fe³⁺, thus sustaining the catalytic cycle. Finally, after the oxidation treatment reaction, the magnetite-based composite can be easily separated magnetically for reuse. Thus, this catalyst demonstrates an efficient, eco-friendly solution that aligns with the 3 R criteria of “Recover, Recycle and Reuse” as a sustainable option for wastewater treatment.

2.5. Oxidation Kinetics

Generally, real-life industrial applications require process design aspects for reactor design and system control, which can be evaluated through the study of system kinetic modeling. Hecne, in this contextt, the oxidation of Synozol Red KHL dye-containing wastewater is considered a function of time under various temperatures. Three general kinetic models—0th-, 1st-, and 2nd-order [51,52]—are applied by transforming their equations into linearized forms, as displayed in

Table 2. Kinetic models and their parameter values for the PK-P-bio-char/Fe₃O₄-based photo-Fenton oxidation kinetics.

Model and Equation *	Model Parameters	Values
		T, °C
		32 °C	40 °C	50 °C	60 °C
0th-order model $C_t=C_0-k_{0}t$	k0 (min−1)	0.68	0.65	0.67	0.66
	t1/2 (min)	14.70	15.38	14.92	15.15
	R2	0.85	0.78	0.58	0.73
1st-order model $C_t=C_0-e^{k_{1}t}$	k1 (min−1)	0.064	0.0211	0.015	0.0103
	t1/2 (min)	67.28	46.2	32.84	10.82
	R2	0.93	0.96	0.95	0.87
2nd-order model $\left({\displaystyle \frac{1}{C_t}}\right)=\left({\displaystyle \frac{1}{C_0}}\right)-k_{2}t$	k2 (L·mg−1·min−1)	0.0123	0.0382	0.0185	0.0177
	t1/2 (min)	4.065	1.31	2.70	2.82
	R2	0.88	0.86	0.51	0.79

* C₀ and C_t are the concentration at time 0 and time t; k₀, k_1, and k₂ are the kinetic constants of 0th, 1st and 2nd orders, respectively.

.

R² values (correlation coefficients) were applied to assess the fit of the compared models of 0th-, 1st-, and 2nd-order kinetics. The best-fitting model is determined by the highest correlation coefficient value between the empirical and theoretical data. Initially, the relationship between various operating temperatures and time for 0th-, 1st-, and 2nd-order reaction rates was plotted, and the data is displayed in Figure 12a, b, and c, respectively. Furthermore, the relationship between various operating temperatures and time for 0th-, 1st-, and 2nd-order oxidation rates were investigated (Table 2).

According to the data tabulated in Table 2 and the graphs exhibited in Figure 12, to identify the best-fit kinetic model, the correlation coefficients were compared. The compared models of 0th-, 1st-, and 2nd-order kinetics models were evaluated, and the 0th- and 2nd-order models were excluded due to their relatively low correlation coefficient values (0.58–0.85 and 0.51–0.88 for 0th and 2nd order, respectively). But the values of 0.87–0.96, which are comparatively higher, correspond to the 1st-order model.

As presented in Table 2, k₁ decreased to 0 as the temperature increased from 32 °C to 60 °C, and the half-lives of the reaction time also declined with the rise in temperature. These findings suggest that the oxidation process is strongly influenced by temperature. This indicates that the reaction rate accelerates as the temperature rises, consistent with the temperature relationship. Similar findings have been reported by different researchers, who stated that 1st-order kinetics in Fenton oxidation has been observed for dyes such as Reactive Red SBE [51], highlighting that the reaction order may vary depending on the dye structure and system conditions, while other studies have reported that the follows 2nd-order kinetic models [52,53].

2.6. Thermodynamic Investigation

To evaluate the feasibility of implementing the photo-Fenton oxidation reaction for system design, a thermodynamic investigation was conducted. The key thermodynamic parameters were predicated to investigate the nature of the reaction. The Arrhenius equation, based on the 1st-order rate constant $k_1=A{e}^{{\displaystyle \frac{{-E}_a}{RT}}}$ was linearized by taking its natural logarithm and then plotted, as displayed in Figure 12, where A is the pre-exponential factor, E_a 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). The linear plot (Figure 13) estimates the activation energy, which was determined to be 43.22 kJ·mol⁻¹. This relatively low E_a confirms that the oxidation of the dye via the photo-Fenton process proceeds with a minimal energy barrier.

To gain deeper insight into the temperature effect, other thermodynamic parameters, including the Gibbs free energy of activation (ΔG′), activation enthalpy (ΔH′), and activation entropy (ΔS′), were also investigated, with the results summarized in

Table 3. Thermodynamic parameters for Synozol Red KHL dye oxidation for the PK-P-bio-char/Fe₃O₄-based photo-Fenton.

Temperature, °C	Ln k2	Ea, kJ mol−1	ΔG′, kJ mol−1	ΔH′, kJ mol−1	ΔS′, J mol−1
32	−2.74	43.22	80.06	40.74	−131.51
40	−3.85		86.82	40.63	−147.5
50	−4.199		90.59	40.54	−154.95
60	−4.57		94.52	40.46	−162.35

. The variation of these parameters with temperature highlighted the dependence of the system-based photo-Fenton oxidation on the 1st-order kinetic constants. The Gibbs free energy of activation (ΔG′) was calculated using Eyring’s equation, $k_1={\displaystyle \frac{k_{B}T}{h}}{e}^{(-{\displaystyle \frac{\Delta G^{\prime }}{RT}})}$, where k_B and h are Boltzmann and Planck’s constants, respectively. Additionally, the activation enthalpy (ΔH′) and entropy (ΔS′) were derived from the relations $\Delta H^{\prime }=E_a-RT$ and $\Delta S^{\prime }=(\Delta H^{\prime }-\Delta G^{\prime })/T$. These findings provide a comprehensive understanding of the thermodynamic behavior of the system, offering valuable insights into the design and optimization of the photo-Fenton oxidation process.

As summarized in Table 3, the positive Gibbs free energy values indicate that the PK-P-bio-char/Fe₃O₄-based photo-Fenton oxidation process is a non-spontaneous type of oxidation, with the degree of non-spontaneity increasing at higher temperatures. However, the lowest Gibbs free energy values were observed at ambient temperature, suggesting greater molecular freedom of the dye and more effective interaction with hydroxyl radicals (•OH). These findings confirm that increasing the temperature up to 60 °C improves the degradation efficiency of Synozol Red KHL dye. Additionally, the positive activation enthalpy values reveal that the oxidation reaction is endothermic in nature, while the negative activation entropy values further support the non-spontaneous characteristics of the PK-P-bio-char/Fe₃O₄-based photo-Fenton system.

2.7. Reusability and Sustainability

To verify the catalyst sustainability, PK-P-bio-char/magnetite as a based photo-Fenton system is evaluated by catalyst recovery through filtration, thoroughly washing it with distilled water to remove residual impurities, and then oven-drying at 150 °C for one hour. The catalyst is thereafter used for successive cycles of oxidation. The data are exhibited in Figure 14. The results demonstrated that the catalyst maintained effective performance over multiple uses, sustaining high activity for up to 10 consecutive cycles. This data verifies the catalyst stability and potential for repeated applications. The fresh catalyst achieved 100% removal efficiency, which slightly decreased to 98% in the first cycle and 93% in the second cycle, indicating minimal loss of activity during early reuse. However, a gradual decline in performance was observed with successive cycles, dropping to 82% by the fifth cycle and 70% by the tenth cycle. However, it is essential to mention that, although the oxidation rate remains high even with cyclic use, there is a reasonable reduction in the activity of PK-P-bio-char/Fe₃O₄ material due to partial deactivation or loss of active sites, since the surface is filled with the dye molecules and minor leaching of active components occurs during repetitive catalyst use. Despite this reduction, maintaining 70% efficiency after 10 cycles highlights the catalyst’s superior stability and recyclability, supporting it as a highly promising candidate for cost-efficient and sustainable wastewater treatment applications.

Furthermore, the kinetic investigation across cycles (fresh, 1st to 10th cycle) is conducted. The concentration profiles for representative cycles fit well to the first-order model according to the correlation coefficient values, and the data is tabulated in

Table 4. First-order kinetic model and its parameter values for the cyclic PK-P-bio-char/Fe₃O₄-based photo-Fenton oxidation use.

Cycle	Removal (%)	k1 (min−1)	R2
Fresh use	100	0.064	0.93
1st cycle	98	0.056	0.94
2nd cycle	93	0.053	0.91
3rd cycle	91	0.046	0.92
4th cycle	87	0.040	0.90
5th cycle	82	0.034	0.87
6th cycle	80	0.030	0.86
7th cycle	77	0.027	0.91
8th cycle	75	0.026	0.93
9th cycle	72	0.022	0.88
10th cycle	70	0.018	0.96

. The first-order kinetic rate constant decreased with the reuse cycle. This behavior is attributed to progressive surface coverage by reaction intermediates and minor loss of accessible iron sites, which collectively slow •OH generation and substrate access. This data reflects a gradual deactivation of the catalyst surface during successive runs. Also, the observed decline in k₁ values, as displayed in Table 4, is primarily attributed to the progressive surface coverage by reaction intermediates, which limits the availability of active sites and the minor loss of accessible Fe sites [54]. Thus, overall, the oxidation reaction has reduced efficiency in hydroxyl radical (•OH) generation. Nevertheless, the catalyst retained appreciable activity even after 10 cycles, confirming its potential for repeated use despite reduced efficiency.

2.8. Radical Scavenging Test

Overall, hydroxyl (•OH) and hydroperoxyl (HO₂•) radicals are the most quantitatively significant reactive species in the catalytic oxidation system. In particular, the hydroxyl radical (•OH) is known to be a highly effective oxidant for a wide range of pollutant molecules. Thus, in order to verify the significant effects and the role of these radicals and other charge carriers in the oxidation process, radical scavenging experiments were performed using specific quenching agents: isopropyl alcohol (IPA) as an •OH scavenger, ammonium oxalate (AO) as a hole (h⁺) scavenger, and silver nitrate (AgNO₃) as an electron (e⁻) scavenger for Synozol Red KHL dye [2].

Figure 15 presents the results of the scavenger experiments performed under the optimal operating conditions of the PK-P/Fe₃O₄ based photo-Fenton system. When isopropyl alcohol (IPA) was introduced as an •OH scavenger, the degradation efficiency of Synozol Red KHL dye dropped drastically from nearly 100% to 40%. This sharp decline highlights the dominant role of hydroxyl radicals (•OH) in the oxidation of Synozol Red KHL dye over PK-P/Fe₃O₄. In comparison, the addition of electron (AgNO₃) and hole (AO) scavengers produced less pronounced effects, lowering the efficiency to 56% and 59%, respectively. These results confirm that although multiple reactive species are involved, •OH radicals are the primary contributors in Synozol Red KHL dye degradation within our system. This could be attributed to the fact that the PK-P/Fe₃O₄ composite generates multiple reactive species during the oxidation process, with •OH radicals dominating the oxidation of Synozol Red KHL. However, the photogenerated holes (h⁺) exert a minimal influence on dye oxidation. The scavenger experiments clearly demonstrate that hydroxyl radicals (•OH) are the predominant reactive species responsible for the degradation of Synozol Red KHL dye in the PK-P/Fe₃O₄-based photo-Fenton system. Although electrons (e⁻) and photogenerated holes (h⁺) also participate in the process, their contributions are comparatively minor. These findings confirm that the superior oxidation performance of PK-P/Fe₃O₄ is mainly attributed to its ability to effectively generate •OH radicals, which drive the rapid breakdown of the dye molecules.

3. Materials and Methodology Investigation

3.1. Materials

Wastewater

A synthetic and commercial type of textile dye is introduced in the current work to simulate industrial discharges. An aqueous dye solution is initially prepared though 1000 ppm of stock solution from such dye by dissolving a certain amount in distilled water. Next, a subsequent dilution is carried out to investigate the effects of different concentrations of the dye on the oxidation efficiency in the range of 100–500 ppm. The pH of the wastewater is signified as the natural pH of the effluent and recorded at 5.7. However, to adjust the pH (using a digital pH meter, ADWA, Szeged, Hungary) of such wastewater into different values, successive diluted concentrations of H₂SO₄ and NaOH are used to adjust the solution according to the studied values. All the chemicals used were of analytical grade and applied as purchased without further purification.

PK-P-Bio-Char/Fe₃O₄

Pumpkin-derived bio-char is prepared by simple treatments of pumpkin peels (PK-P). The treatment is designed to convert bio-waste into a porous, carbon-rich substance. Primarily, pumpkin PK-P peel residue peels were collected from a local shop. Subsequently, they were initially washed thoroughly with distilled water to remove any dirt or impurities on its surface. Thereafter, the material is was air-dried for 7 days. Then, the material obtained was rinsed with distilled water again to remove any dust prior to being subjected to oven drying at 80 °C until a constant weight was achieved.

The dried material was ground using a pestle and mortar to attain small particles and then sieved to achieve a uniform size, which enhances the efficiency of subsequent carbonization. Next, the powdered biomass was subjected to calcination. To illustrate, the dried powder was gradually calcined to the desired temperature of 400 °C at a heating rate of 10 °C/min in a furnace. The pyrolysis was continued for 2 h to ensure complete carbonization. The resultant material is referred to as pumpkin peel bio-char, PK-P-bio-char.

The co-precipitation method is applied as a simple, cost-effective, and high yield process. In the procedure, the molar stoichiometric ratio of 1:2 from the sources of Fe₂(SO₄)₃ and FeSO₄ are added and employed by dissolving in distilled water. The pH is elevated to 12.0 by the gradual addition of sodium hydroxide solution under continuous stirring at 80 °C. The result is a precipitate of magnetite nanoparticles. Next, the attained substance of precipitate is exposed to successive washing with distilled water until it reaches the neutral water pH of 7.0. The collected nanoparticles are then dried overnight in an electric oven at 60 °C. Then, the resultant magnetite is mixed with the prepared pumpkin peel bio-char. 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 energy-dispersive X-ray spectroscopy (EDX) analysis, and their corresponding mapping is conducted to verify its structural and chemical properties.

3.2. Experimental Methodology

A certain amount (100 mL) of the model synthetic dye solution is poured into the jar test. When the pH adjustment to a certain level is required in such a case, it is adjusted using acid/base prior to being poured into the experiment. Then, a certain amount of catalyst reagent PK-P-bio-char/Fe₃O₄ is added to the reaction and subjected to ultrasonic dispersion before the hydrogen peroxide reagent is added to initiate the reaction. Afterwards, the mixture is exposed to ultraviolet UV radiation emitting 254 nm illumination while stirring is continued. The graphical representation of the experimental set-up is displayed in Figure 16.

Through the experiments, the samples are taken periodically for the object of analysis. The samples initially are filtered through a micro-filter and then exposed for spectrophotometric analysis using a Unico UV-2100 spectrophotometer (UNICO, Dayton, NJ, USA) model, with some modification.

For the kinetic experiments, a series of batch experiments were conducted in a thermostatically controlled reactor at different temperatures (32, 40, 50, and 60 °C) to examine the influence of thermal variation on the reaction rate. The initial concentration of the Synozol Red KHL solution was fixed at 20 ppm, and the catalyst dosage and oxidant concentration were maintained constant throughout the tests to ensure comparability. During each run, aliquots of the reaction mixture were withdrawn at regular time intervals (5 min) for subsequent analysis using a UV–vis spectrophotometer at the maximum absorbance wavelength of the dye. The concentration at each time point was calculated using a pre-established calibration curve. To ensure reproducibility, all experiments were carried out in triplicate, and the reported values represent the average of three independent trials with standard deviations. Kinetic data were then evaluated according to different models.

4. Conclusions

A novel PK-P/Fe₃O₄ hybrid photocatalyst derived from pumpkin peel bio-waste and magnetite nanoparticles was successfully applied for the photo-Fenton oxidation of Synozol Red KHL dye. The catalyst demonstrated rapid and complete dye removal, excellent stability, and significant reusability, confirming its promise for repeated applications. Kinetic and thermodynamic evaluations further supported its efficiency, showing first-order behavior and an energy-efficient endothermic process. These findings establish bio-waste-derived PK-P/Fe₃O₄ as a sustainable, low-cost, and highly effective material for wastewater treatment. Thus, this work demonstrates that pumpkin peel-derived PK-P/Fe₃O₄ is a sustainable, low-cost, and energy-efficient photocatalyst with strong potential for scalable solar-driven wastewater treatment. Hence, future studies should focus on pilot-scale validation and the utilization of renewable solar irradiation to enhance the practicality and environmental benefits of this approach.