Independent Acidic pH Reactivity of Non-Iron-Fenton Reaction Catalyzed by Copper-Based Nanoparticles for Fluorescent Dye Oxidation

Abstract

The process of hydrogen peroxide decomposition, facilitated by copper oxide nanoparticles, produces reactive oxidants that possess the ability to oxidize multiple pollutants. CuO/Cu₂O hybrid nanoparticles were successfully synthesized through a thermal decomposition route and applied as a heterogeneous catalytic oxidant for a fluorescent dye, namely Basic Violet 10 (BV10) dye. The microstructure and morphology of the prepared catalyst were evaluated via X-ray diffraction (XRD) and a field-emission scanning electron microscope (FE-SEM), respectively. The produced nanoparticles (NPs) were induced through ultraviolet light as a green photodecomposition technology. The system parameters were investigated, and the optimal initial NP concentration, H₂O₂ concentration, and pH were assessed. The highest removal rate corresponding to 82% was achieved when 40 and 400 mg/L of NPs and H₂O₂ were introduced, respectively. The system could operate at various pH values, and the alkaline pH (8.0) was efficient in proceeding with the oxidation system that overcomes the limitation of the homogeneous acidic Fenton catalyst. The introduced catalyst demonstrated consistent sustainability, achieving a notable removal rate of 68% even after six consecutive cycles of use. This innovative technique’s accomplishment examines the feasibility of utilizing copper as a replacement for iron in the Fenton reaction, demonstrating efficacy over an extended pH range. Finally, the temperature effectiveness of the reaction showed that the reaction is exothermic in nature, working at a low energy barrier (20.4 kJ/mol) and following the pseudo-second-order kinetic model.

1. Introduction

Inevitable water pollution due to industrial activities is significant in modern societies worldwide [1]. Commonly, the unprecedented upsurge in ecosystem pollution is a threat not only to human health but also to the environment and aquatic life [2,3]. According to industrial discharge records, dyes rank among the most significant chemicals contributing to water pollution. Following the processing of textiles, a considerable amount of wastewater is generated. Such wastewater is distinguished by high levels of dissolved and suspended content [4,5,6]. Dyes constitute one of the major groups of organic complexes which instigate contaminated wastewater discharge that causes massive environmental damage [7,8]. The textile industry is one of the most polluting industries in dye processing in terms of the volume of effluent produced as well as its characteristics due to the resulting high organic matter content in it [9,10,11]. In the textile manufacturing process, dyeing is a vital operation, yet it frequently produces effluent that is considered unacceptable. Industrial discharges, when introduced into the environment, are classified as restricted toxic materials and represent a serious hazard to wildlife [12]. Thus, treating such effluents is a must. However, the safe treatment and nontoxic disposal of such hazardous industrial wastewater is an unresolved challenge [13,14,15,16].

Numerous conventional techniques have been applied for treating aqueous industrial discharge, especially dye-contaminated water. Physical [14], chemical [15,16,17], and biological [18] treatments are suitable candidates. For instance, adsorption, filtration, oxidation, and sonication were introduced [19,20,21]. However, seeking an environmentally eco-friendly, efficient system is still gaining scientists’ interest.

In the present scientific society, the elaboration of environmentally benign catalytic materials is of significant magnitude. The most valuable features of such green substances for industrial applications are their potential to work under mild conditions and their sustainable use [22,23,24,25]. Among the numerous conventional candidates, the chemical oxidation systems that utilize highly oxidizing (^•OH) radicals are attaining considerable attention [26,27]. Advanced oxidation processes (AOPs), which encompass various chemical oxidation techniques, have proven to be effective in degrading toxic pollutants into benign end products such as carbon dioxide and water, utilizing an efficient and economically viable method [3,28]. Among the various AOP systems, the Fenton reaction is a considerable candidate to substitute the non-green systems due to its quick and efficient treatments [29,30,31,32]. However, it is noteworthy to mention that the conventional homogenous Fenton reaction that is based on the catalytic decomposition of H₂O₂ by Fe^2+/3+ to produce the active species “^•OH radicals” still possesses some limitations [33,34,35]. Accordingly, since the system might work with a limited pH (3.0) value, the system after the reaction requires extra treatment prior to the final effluent discharge [36]. Also, dissolved iron species in large quantities must also be purified [37]. Thus, the overall Fenton process is costly, which affects the practical applications in real life [38,39,40,41]. Hence, the synthesis and exploration of suitable materials to substitute the conventional iron catalyst as a Fenton source is essential; this is gaining both the academic and industrial sectors’ interest.

Introducing nanoparticles (NPs) as an alternative heterogeneous catalyst source to prompt the catalytic activity of chemical reactions is a viable option [5,42,43,44]. There have been several recent studies focusing on metal-based nanocatalysts and atomic catalysts in the context of dye treatment [45,46,47]. Among numerous heterogeneous catalysts, copper-based substances attained substantial interest since their outstanding catalytic characteristics and the system could lead to oxidation at a near-neutral pH value [48]. However, the introduction of nanoparticles still has some limitations due to their costly development and synthesis [49]. Thus, the synthesis of such nanoparticles using a facile and rapid methodology is an inexpensive method essential for mass-scale production. In this regard, to overcome such challenges, introducing a thermal decomposition technique might overcome such limitations [12,50]. Thus, an alternative heterogeneous copper oxide-based Fenton system is a suitable candidate to overcome the system limitations. As previously reported [24], the thermal decomposition of copper (II) acetate monohydrate results in CuO/Cu₂O hybrid porous nanoparticles. The porous nature of these CuO/Cu₂O nanoparticles allows for an effective interaction between the photocatalysts and the organic dye.

In this regard, researchers are focused on substituting homogenous Fenton systems with heterogeneous ones to overcome the systems’ drawbacks. This alternative nano-copper-based heterogeneous Fenton system has gained substantial importance due to its selective reaction and its ability to work in a wide pH range. Furthermore, Cu-based nanoparticles are both cost-effective and widely accessible, given their natural abundance and availability. The present research introduces copper-based nanoparticles (NPs) that are stimulated by ultraviolet radiation to increase the generation rate of ^•OH radicals. This enhancement allows for the selective and swift oxidation of Basic Violet 10 (BV10), a fluorescent dye found in wastewater. The influence of operating parameters on the oxidation system was explored, and the optimal operating conditions have been highlighted.

2. Results and Discussion

2.1. Characterization of Microstructure and Morphology of Hybrid CuO/Cu₂O Nanoparticles

FE-SEM was used to examine the microstructure and morphology of the CuO/Cu₂O hybrid nanostructure powder. Figure 1 presents the FE-SEM micrographs of the analyzed powder, demonstrating the aggregation of porous particles. The produced CuO/Cu₂O nanoparticles manifested in the shape of a rosette-like gypsum rose. An identical morphology was achieved through the thermal decomposition of CuAc₂·H₂O at 400 °C and 500 °C, as documented in our prior publication [24].

Figure 2 shows the X-ray diffraction pattern of the CuO/Cu₂O nanostructure. The XRD patterns of the CuO/Cu₂O nanostructure are in a 2θ range from 25° to 70°. As shown in Figure 2, the detected peaks of the diffraction pattern are similar to those obtained from the standard data that include peaks of the CuO (Ref code #00-001-1117) and Cu₂O (Ref code #01-071-4310) phases. The planes (110), (-111)/(002), (111)/(200), (-112), (-202), (020), (202), (-113), (002)/(-311), and (220)/(113) of the CuO phase are at 32.8, 35.9, 39.0, 46.6, 49.2, 53.8, 58.6, 61.8, 66.5, 68.3, 73.7, and 75.4° planes, respectively [51]. Such outcomes point to the development of CuO in its monoclinic structure crystallites. The peaks at 2θ values of 29.7, 36.8, and 42.6° correspond to (110), (111), and (200) planes of the cubic structure of Cu₂O. This indicates that the synthesis of the CuO/Cu₂O nanostructure nanoparticles has been successfully achieved.

The analysis of the obtained X-ray diffraction pattern has revealed the formation of crystalline phases with CuO as the tenorite phase and Cu₂O as the cuprite phase. The phase fractions were 64.2% and 35.8% for the CuO and Cu₂O phases, respectively. The average crystallite size was calculated by the Scherrer equation for both phases. The average crystallite size values calculated via the Scherrer equation showed a decline, a phenomenon that is associated with an increase in dislocation, lattice strain, and the FWHM [52,53]. The average crystallite size was about 24.2 nm and 10.2 nm, and the average values of lattice microstrains of 0.0024 and −0.003 were obtained for the CuO and Cu₂O phases, respectively.

The evaluation of X-ray peak intensities and numbers indicated that the most prevalent phase is CuO, succeeded by the Cu₂O phase. The identification of these two phases was anticipated due to the oxidation of copper on the surface, which is in agreement with previously reported research findings [24,54,55].

To validate the findings obtained from the XRD results, FTIR spectroscopy of CuO/Cu₂O hybrid nanoparticles was conducted. The FTIR spectrum of the CuO/Cu₂O nanoparticles is depicted in Figure 3. Figure 3 reveals the formation of a broad band around 3441 cm⁻¹ that corresponds to O–H stretching vibrations. Additionally, the peak at 1630 cm⁻¹ indicates the presence of water molecules.

Furthermore, another broad band observed at 1048 cm⁻¹ is associated with C–C stretching vibrations. The nanoparticles display four prominent peaks at 797, 613, 530, and 436 cm⁻¹ which are characteristic of Cu–O vibrations and are consistent with values reported in the literature [56,57]. In the current investigation, the Cu-O peaks observed show a downshift relative to those previously reported in the literature, which is explained by the influence of nanoscale effects [57,58]. The identification of specific Cu-O peaks in the FTIR spectrum serves as evidence of the successful synthesis of CuO/Cu₂O nanoparticles.

The valence states and chemical composition of the produced sample’s surface were investigated by XPS measurements. The existence of CuO is demonstrated by the two peaks observed at approximately 936.1 and 956 eV, which are attributed to the Cu²⁺ 2p_3/2 and Cu²⁺ 2p_1/2 peaks of Cu²⁺, respectively, as illustrated in Figure 4a. Nevertheless, the presence of Cu₂O in the final product is indicated by the peaks at around 934.3 and 954.3 eV, which match the Cu⁺ 2p_3/2 and Cu⁺ 2p1/2 characteristic peaks of Cu⁺, respectively. The increased binding energies of the satellite peaks to approximately 942.4, 944.9, and 962.9 eV, as compared to the main peaks, are likely due to the empty Cu₃d₉ shell of Cu⁺. It has been verified that the synthetic sample does, in fact, contain Cu₂O [59]. Figure 4b demonstrates that the O 1s core-level spectrum is broad and has two peaks. The signal at 530.7 eV is due to O²⁻ in CuO. The other peak at the greatest binding energy, 532.5 eV, is attributed to O¹⁻ in Cu₂O [60]. The XPS results show that the sample is constituted of CuO and Cu₂O, with atomic percentages of 56.8% and 43.2%, respectively. This result refers to the CuO composition created with a larger ratio than the Cu₂O composition, which confirms the conclusion drawn from the XRD results.

2.2. Different Oxidation Processes and Reaction Times

In order to simultaneously evaluate the comparison between different oxidation processes for BV10 elimination and degradation as well as the operating time assessment, various operational systems were employed. These included the usage of solo Cu-based nanoparticles, H₂O₂, Cu-based nanoparticles/H₂O₂, and Cu-based nanoparticles/H₂O₂/UV, all tested in a natural solution with a dye pH (6.1). According to the preliminary study, the optimal dose of hydrogen peroxide oxidant was 400 mg/L, and CuO/Cu₂O hybrid nanoparticles were added at 40 mg/L. The data displayed in Figure 5 show that BV10 dye removal is hardly seen with the solo system of the Cu-based nanoparticles, which identified ultraviolet illumination as very weak regarding breaking the dye bonds. Nevertheless, a reversible effect is detected in the Cu-based nanoparticles/H₂O₂ and Cu-based nanoparticles/H₂O₂/UV processes. Furthermore, Cu-based nanoparticles/H₂O₂/UV showed a superior oxidation capability that exceeded 35%. The modified Fenton system that employs CuO/Cu₂O hybrid nanoparticles shows its potential as an alternative approach. However, the effectiveness of dye removal in this CuO/Cu₂O hybrid nanoparticle treatment system is considerably reduced due to the nonexistence of the oxidant hydrogen peroxide. This could indicate that hydrogen peroxide induces the CuO/Cu₂O nanoparticles to generate ^•OH radicals [12,61]. However, it is essential to point out that remarkably high oxidation efficiency is reached in the early stages, which is subsequently followed by a decrease in efficiency as time progresses.

Noticeably, as the reaction time advances, a plateau is observed, signifying that the CuO/Cu₂O nanoparticles have become saturated with the fluorescent BV10 dye molecules, thereby inhibiting the formation of any further radicals in the reaction medium. An investigation of lessening efficiency with time has also been previously reported when oil-polluted water was subjected to treatment with Fenton’s reagent [30].

2.3. Effect of BV10 Dye Concentration

To further reach practical and large-scale application, it is crucial to study the initial pollutant load, which will inform the system alignment with real discharge conditions. In this concept, the effect of the initial BV10 dye concentration on the Cu-based nanoparticles/H₂O₂/UV Fenton system technique was examined, and the results are displayed in Figure 6. It is evident from the data shown that elevating the initial concentration of BV10 dye from 5 mg/L to 40 mg/L while keeping the solution pH at its natural level of 6.1 leads to a considerable reduction in the dye removal efficiency, decreasing from 64% to only 8%.

Such a trend might be attributed to the relationship between the amount of hydroxyl radicals, which signifies the horsepower in the oxidation reaction [62], and the concentration of BV10 dye present in the aqueous solution. Thereby, the ^•OH that is mainly responsible for oxidation in the system is formed in a certain amount in the reaction media. Hence, at a constant ^•OH concentration, the increase in the BV10 dye concentration decreases the relative radical concentration; thereby, the overall efficiency declines. At elevated initial concentrations of fluorescent dye, ranging from 20 mg/L to 40 mg/L, the radicals may not effectively oxidize the dye. This inefficiency arises from the availability of additional dye molecules in the reaction medium that can compete for oxidation [63]. This is consistent with prior literature findings [29], which also discuss the treatment of effluent containing the dye Synozol Red KHL through a heterogeneous Fenton reaction. Moreover, it is apparent that the condensed dye solution could potentially hinder the ultraviolet illumination in the reaction media [64].

2.4. Effect of Amount of H₂O₂

The presence of hydrogen peroxide in the Fenton reaction is an essential parameter. Since such a reagent induces the oxidation catalyst, it affects the amount of generated ^•OH radicals [24]. The data displayed in Figure 7 indicate that increasing the H₂O₂ dosage in the Fenton-based Cu system affects the rate of BV10 reduction and thereby mineralization. Also, a significant improvement in the oxidation system is observed regarding BV10 dye removal with a H₂O₂ dose of 400 mg/L. This phenomenon could be attributed to the fact that ^•OH radicals, which are chiefly responsible for dye oxidation, are generated through H₂O₂ decomposition under UV illumination, as shown in Equation (1) [24]. Furthermore, the reaction between Cu-based nanoparticles and H₂O₂ results in the generation of extra ^•OH radicals that attack the chemical bonds in dye molecules (Equations (2) and (3)). Subsequently, this leads to the generation of reaction intermediates that are eradicated by further extra hydroxyl radical ^•OH species. Subsequently, the end products materialize as water and carbon dioxide [24,29]_.

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{UV} \mathrm{induction}\to 2^{·}\mathrm{OH}$$

(1)

$${\mathrm{Cu}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Cu}}^{+}+{\mathrm{HO}}_2^{•}+{\mathrm{H}}^{+}$$

(2)

$${\mathrm{Cu}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Cu}}^{2+}+{}^{•}\mathrm{O}\mathrm{H}+{\mathrm{OH}}^{-}$$

(3)

In the meantime, an additional increase in the hydrogen peroxide reagent dose does not reflect the further conversion of BV10 dye yield. This might be attributed to the high hydrogen peroxide oxidant dose; the peroxide reagent acts as an ^•OH radical scavenger rather than a generator, as exhibited in Equations (4) and (5) [16,27]. Furthermore, recombination reactions of ^•OH hydroxyl radicals yielding H₂O₂ arise, contributing to the promotion of more ^•OH radical scavenging competence (Equation (6)) [26].

$${\mathrm{H}}_2{\mathrm{O}}_2+{}^{•}\mathrm{O}\mathrm{H}\to {\mathrm{HO}}_2^{•}+{\mathrm{H}}_2\mathrm{O}$$

(4)

$${\mathrm{HO}}_2^{•}+{}^{•}\mathrm{O}\mathrm{H}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2$$

(5)

$${}^{•}\mathrm{O}\mathrm{H}+{}^{•}\mathrm{O}\mathrm{H}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(6)

But a similar tendency in the cumulative reaction rate with the collective hydrogen peroxide reagent is achieved for the Cu-based nanoparticle catalytic system until a certain limit, which is followed by a decrease in the oxidation rate. The Cu-based nanoparticles augment hydrogen peroxide under UV illumination, leading to a 25% increase when the peroxide reagent is increased to 800 mg/L.

As displayed in Equations (2) and (3), Cu⁺ triggers H₂O₂ that is not responsible for the direct development of ^•OH radicals but nonetheless is involved the production of an excess oxidation state of copper, supposedly Cu²⁺. These exist at a lower reaction rate compared to the strong oxidizing species’ ^•OH radicals, which are not selective. Accordingly, such radicals’ species challenge ^•OH radicals in BV10 dye oxidation. The result suggests that H₂O₂ should be added at an optimal concentration to enhance the dye oxidation efficiency. In this regard, 400 mg/L was selected to be the optimal dose to be added in the current investigation.

2.5. Cu-Based Nanoparticles’ Concentration

Figure 8 exhibits the effect of various Cu-based nanoparticle catalyst concentrations on the effectiveness of BV10 dye removal. It can be detected from the data that for the CuO/Cu₂O hybrid nanoparticle system, the BV10 dye removal is significantly increased by the escalation of the CuO/Cu₂O hybrid nanoparticles’ load from 10 to 40 g/L. Nevertheless, beyond such concentration, the BV10 dye oxidation rate drops with the increase in the concentration of Cu-based nanoparticle catalyst, which is attained as the saturation trend.

Commonly, at higher Cu-based nanoparticle catalyst doses, extra active sites are produced that might accelerate the creation of hydroxyl ^•OH radicals. Accordingly, this result promotes inclusive BV10 dye removal efficacy [25]. But after a certain limit, with the auxiliary upsurge in the CuO/Cu₂O hybrid nanoparticle catalyst concentration, the oxidation system is retarded. Such a rise in the oxidation rate stems from the fact that, at a constant concentration of H₂O₂, the amount of H₂O₂ peroxide reagent is not sufficient to be decayed by CuO/Cu₂O hybrid nanoparticles. Hence, excessive copper ions might act as ^•OH scavengers rather than as generators, as displayed in Equations (7) and (8). Consequently, the qualified ^•OH radicals’ dose deteriorates, resulting in a reduction in overall BV10 dye removal and oxidation tendency. Consequently, it might be concluded that the optimum Cu-based nanoparticle catalyst concentration under UV illumination is 40 mg/L, and the removal efficacy reached 35%. Different authors treating various types of wastewater previously reported this phenomenon [12,24,25,26,27].

$${\mathrm{Cu}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{O}}_2^{-}+2{\mathrm{H}}^{+}+{\mathrm{Cu}}^{+}$$

(7)

$${\mathrm{Cu}}^{2+}{+}^{·}\mathrm{OH}\to {\mathrm{Cu}}^{2+}+{\mathrm{OH}}^{-}$$

(8)

2.6. Effect of Initial pH Value

Commonly and previously reported in the literature [50], the oxidation reaction via the classical Fenton system is notably affected by the pH of the reaction media. The system is greatly induced at the optimum pH value, and the ideal oxidation is attained when the aqueous pH of the solution is about 3.0. In this regard, the effect of the pH on the introduced modified Fenton system is assessed.

To test the effect of pH of the CuO/Cu₂O hybrid nanoparticle system-based Fenton reaction, the wastewater pH was adjusted before conducting the UV illumination, and it was adjusted to the desired values according to the catalytic reaction. As presented in Figure 9, the aqueous solution pH value has been assessed in an investigation that extended from the acidic to the alkaline range. A slight increase in the BV10 removal rate was observed with the pH increase, and the maximum removal was attained at the alkaline pH 8.0 value. However, it is essential to mention that the BV 10 fluorescent dye could be oxidized under various values of pH but with varied removal efficiency. This observed phenomenon can be attributed to the pH of the aqueous solution, which simulates the catalytic breakdown of H₂O₂ and the hydrolytic speciation of CuO/Cu₂O hybrid nanoparticle ions. Therefore, extra ^•OH radicals exist at higher pH, and the oxidation reaction reached 82%. Such observation of the working pH of a Fenton Cu-based nanoparticle system is not limited to pH 3.0 and is in accordance with previous work reported using phenolic compound oxidation by a Cu²⁺/H₂O₂ system [24]. Such investigation enlarges the real application of the Fenton reaction since it could be independent of acidic solution pH (about 3.0).

This phenomenon could be associated with extra species which might be formed in the reaction medium such as metal–oxo or metal–hydroxo compounds that are interconvertible species and are related to fast hydration and dehydration equilibrium reactions. Such oxo species (Cu=O⁺), as exhibited in Equation (9), could be converted to hydroxyl complexes, Cu(OH)₂⁺. It is possible that the hydroxo complexes are the primary forms of high-valent metal species generated via the Cu-based Fenton-like system, which are pH-dependent species. Generally, the oxidation power of metal–hydroxo complexes declines with the increase in hydroxo ligand concentrations since it serves as an electron donor [10,65].

$${\mathrm{Cu}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{Cu}{\left(\mathrm{OH}\right)}_2^{+}$$

(9)

2.7. CuO/Cu₂O Hybrid Nanoparticles’ Sustainability

It is an essential feature to evaluate the catalyst’s sustainability through checks on its reusability to assure its catalytic affinity and reactivity after its use for pollutant oxidation. Initially, CuO/Cu₂O hybrid nanoparticle catalytic regeneration involves the collection of these nanoparticles at the conclusion of each wastewater treatment cycle. Following collection, the nanoparticles are filtered and subjected to three consecutive washings with distilled water. Subsequently, they are gathered and dried in an electric oven at a temperature of 105 °C for a duration of one hour. Afterwards, the regenerated catalyst is subjected again to treatment, and the successive cyclic results are displayed in Figure 10.

As shown in Figure 10, upon multiple successive uses, the catalytic reactivity of the Cu nanoparticles does not show a distorted efficiency. A slight decrease in the performance activity and dye removal rate is attained. For fresh catalyst use, the BV10 removal efficiency reached 82%, and upon successive catalyst recycles, it declined to only 64% after the sixth cycle. Therefore, this might be attributed to the CuO/Cu₂O hybrid nanoparticles’ active sites, which are occupied by some organic intermediates. Such coverage prevents these active centers from generating ^•OH, thereby hindering their ability to interact with the dye molecules for mineralization [24]. Hence, the overall reaction efficiency has declined, but it is noteworthy to mention that even though the removal rate has declined, the pollutants are still oxidized at a high removal rate. Such results confirm the sustainability of this material as an oxidative catalyst.

2.8. Temperature Effects on Kinetics and Thermodynamics

Aqueous solution temperature is considered an essential feature since it might affect the oxidation reaction rates. To explore such an influence of temperature on the reaction kinetics, BV 10 dye oxidation experiments over a temperature range from 32 °C to 60 °C were undertaken. The data of such experiments are displayed in Figure 11. There was a noticeable decline in the effectiveness of BV 10 oxidation, and the removal efficacy decreased from 82% to 69% over the temperature range investigated. Such data could be attributed to the hydrogen peroxide decomposing into O₂ and H₂O, thereby deteriorating the reaction oxidation affinity [44].

It is noteworthy to mention that, according to the above-mentioned results, although the reaction rates are normally more efficient at lower temperatures, the removal did not decline much when the temperature was elevated. Such data extend the affinity of the CuO/Cu₂O hybrid nanoparticle-based Fenton catalyst since it might work in a wide reaction range when verified and facilitate its real applications.

Previous studies have shown that temperature might have a minor effect on Fenton systems when compared to other reaction variables. Another author reported that the optimal temperature for Fenton systems could range from 17 °C to 38 °C [40].

The reaction kinetics and thermodynamics of BV 10 fluorescent dye oxidation through the modified Cu-based Fenton system are highlighted for practical application suggestions. In the current work, the photo-oxidation kinetics of the present catalyst were assessed for various temperatures. Then, zero-, pseudo-first-, and second-order reaction kinetics were examined in relation to the removal of BV 10 dye using the modified Fenton oxidation system by applying the models of zero-order [26], first-order [10,24], and pseudo-second-order kinetic models in their linear form [24].

By plotting equations of the three models, the kinetic parameters were investigated, and the data are tabulated in

Table 1. Parameters of first- and pseudo-second kinetic models for BV10 dye removal through Cu-based Fenton system.

T,	Zero-Order Reaction Kinetics			Pseudo-First-Order Reaction Kinetics $({\mathit{C}}_{\mathit{t}}={\mathit{C}}_{\mathit{o}}-{\mathit{e}}^{{\mathit{k}}_1\mathit{t}})$			Pseudo-Second-Order Reaction Kinetics $(\left(\frac{1}{{\mathit{C}}_{\mathit{t}}}\right)=\left(\frac{1}{{\mathit{C}}_0}\right)-{\mathit{k}}_2\mathit{t})$
K	k0,	R2	t1/2, min	k1,	R2	t1/2, min	k2,	R2	t1/2, min
	min−1			L·mg−1min−1			L·mg−1min−1
305	0.0023	0.89	48.47	0.032	0.94	21.65	0.399	0.96	11.23
313	0.0031	0.89	35.96	0.0286	0.93	24.23	0.311	0.97	14.41
323	0.0028	0.90	39.82	0.0249	0.93	27.83	0.238	0.97	18.84
333	0.0031	0.85	37.16	0.0226	0.88	30.66	0.204	0.95	21.98

. The evaluation of kinetic parameters and the regression coefficients (R²) showed that the most appropriate kinetic model that modeled the BV 10 oxidation through the Cu-based modified system corresponds to a pseudo-second-order model. Also, the pseudo-second-order reaction constant, k₂, is significantly affected by the reaction temperature, decreasing with increasing temperature, from 0.399 to 0.204 L mg⁻¹ min⁻¹ over the investigated temperature range. Such results are linked to the fact that at lower temperatures, the ^•OH radicals in the reaction medium are produced via the catalytic induction of Cu nanoparticles with H₂O₂. Such radicals are elevated at low temperatures, and this means that the reaction rate is elevated [28]. Another essential kinetic variable that is also investigated is the half-life of a reaction, t_1/2, which is described as the time essential for the reactant concentration to reduce to half of its initial value, C_o [30].

Table 1 exhibits the calculated t_1/2 as a function of the reaction temperature; the t_1/2 upsurges with rising temperature. Briefly, it has been stated that BV 10 dye oxidation by CuO/Cu₂O hybrid nanoparticles in a modified Fenton’s reagent follows a pseudo-second-order reaction model for the temperature range investigated. A pseudo-second-order kinetic model was previously reported by other authors for the treatment of pesticide-containing wastewater by the Fenton system [12].

To further understand the oxidation system for real applications, thermodynamic variables were also quantified for the modified Cu-based Fenton system. Using the Arrhenius formula for the kinetic rate constant, $k_2=A{e}^{{\displaystyle \frac{-E_a}{RT}}}$, the activation energy (E_a) of the BV 10 oxidation can be computed [30], where R is the gas constant (8.314 J mol⁻¹ K⁻¹); T is the temperature in Kelvin; and A is the pre-exponential factor that is constant regarding temperature. Taking the natural log of the Arrhenius formula yields a straight line for the plot of $\ln k_2$ versus 1/T, whose slope is $-E_a/R$, which can be used to calculate $E_a$. The activation energy of the process showed that it could be conducted at a low energy barrier, recorded as 20.44 K J mol⁻¹.

Furthermore, other thermodynamic factors, including the enthalpy of activation (∆H’), the entropy of activation (∆S’), and the free energy of activation (∆G’), were also estimated through the application of Eyring’s equation $(k_2={\displaystyle \frac{k_{B}T}{h}}{e}^{(-{\displaystyle \frac{\Delta G^{\prime }}{RT}})})$ using the $E_a$ and $k_2$ values [24]. Then, the activation enthalpy was estimated from the relation ($\Delta H^{\prime }=E_a-RT$), and further, the entropy of activation, $\Delta S^{\prime }=\left(\Delta H^{\prime }-\Delta G^{\prime }\right)/T$, was calculated, where k_B is the Boltzmann constant and h is Planck’s constant.

Thereby, the thermodynamic parameters for BV 10 dye oxidation have been projected appropriately and are tabulated in

Table 2. Thermodynamic parameters for BV10 dye oxidation by Cu-based Fenton system.

Temperature, K	Ln k2	Ea,	∆G’,	∆H’,	∆S’,
		kJmol−1	kJmol−1	kJmol−1	Jmol−1
305	−0.92	20.44	77.08	17.90	−194.04
313	−1.17		79.82	17.83	−198.04
323	−1.44		83.17	17.75	−202.55
333	−1.59		86.26	17.67	−205.99

. Investigating the results in Table 2 exhibited that the positive values of $\Delta H^{\prime }$ across the investigated temperature range indicate that the reaction is exothermic. Moreover, $\Delta G^{\prime }$ exhibited positive values, which means that the system is of a non-spontaneous nature. This finding could be explained by the development of a well-solvated configuration between the BV 10 dye molecules and hydroxyl radicals, which is additionally reinforced by a negative entropy of activation [44]. Also, the results verify the minimal energy barrier required to proceed with the reaction (20.44 kJ/mol).

2.9. Comparative Investigation

A comparison of the oxidation efficiency using the modified Fenton system (based on a hybrid copper system) with other various treatment technologies is displayed in

Table 3. Summary of comparison of the current study with the relevant literature regarding treating various effluents through Fenton-based systems.

Wastewater	Catalyst	Oxidation Time	Operating Conditions	Treatment Efficiency	Ref.
BV 10 dye wastewater	CuO/Cu2O hybrid nanoparticles	60 min	pH 8.0, catalyst 40 mg/L, H2O2 400 mg/L	82%	Current work
Organics in wastewater	FeSO4·7H2O	280 min	pH 3.0, catalyst 20 mg/L, H2O2 50 mg/L	80%	[66]
Oily wastewater	FeSO4·7H2O	-	catalyst 0.08 g/L, H2O2 1 g/L	87%	[67]
Dye stuff wastewater	FeSO4·7H2O	-	pH 2.8, Na2SO4 60 mg/L, H2O2 68.4 mM	86%	[68]
Organics in wastewater	Natural iron	360 min	pH 5.0, catalyst 25 mg/L	50%	[69]
Surfactant wastewater	FeCl3	2 min	pH 3.0	89%	[69]
Dye stuff wastewater	α-Fe2O3	60 min	pH 3.0, catalyst 40 mg/L, H2O2 400 mg/L	60%	[24]
Pesticide wastewater	n-CuO	60 min	Catalyst 75 mg/L, H2O2 395 mg/L, pH 6.5	85%	[70]
Pesticide wastewater	Cu/Cu2O/CuO	-	H2O2 5000 mg/L, catalyst 3.0 g/L, pH 6.5, microwave power 400 W	91%	[24]
Cephalexin wastewater	Na2SO4	15 min	Catalyst 0.05 M, pH 3.0, electric current 125 mA	100%	[71]
Amoxicillin wastewater	Nano-Fe2O3	60 min	Catalyst 0.01, pH 3, electric current 300 mA	98.2%	[72]
Tetracycline wastewater	Pyrite	20 min	Catalyst 0.05 M, pH 3, 300 mA	100%	[73]

. It can be concluded that hybrid copper-based Fenton oxidation exhibited an efficient treatment, reaching 83% of dye removal. Some other treatments showed complete pollutant removal. It is noteworthy to mention that other treatments may use extra activation for the system such as electro-Fenton treatments or may use an acidic pH which requires post-treatment that makes the process costly. Also, some treatments require a longer time for treatment. These drawbacks do not apply to the current oxidation technology recommended for dye removal. Additionally, the recommended heterogeneous process is environmentally friendly compared to the other listed techniques in Table 3 since it is a recyclable and sustainable treatment according to the above-mentioned data.

3. Experimental Section

3.1. Materials

3.1.1. Wastewater

Basic Violet 10, C₂₈H₃₁CIN₂O₃, dye was applied as a model pollutant and used as received without any further purification or treatment. A stock solution of 1000 ppm was prepared, and further dilution was conducted when required to attain the desired concentrations. A volume of 100 mL of aqueous Basic Violet 10 dye solution was used in all the treatment experiments, and the system was subjected to treatment under ultraviolet illumination.

3.1.2. Catalyst and Reagents

CuO/Cu₂O hybrid nanostructure powder was synthesized by the thermal decomposition of copper (II) acetate monohydrate, CuAc₂·H₂O. This conventional preparation method has been described elsewhere [17,24]. The typical route involves introducing 3 g of CuAc₂·H₂O into a covered 50 mL alumina crucible. After that, we placed the crucible into an oven at 410 °C for 3 h. After slowly cooling in air overnight, we collected the reaction product. Next, we introduced it as the catalyst for the Fenton reaction.

Hydrogen peroxide (30% w/v) was applied to the aqueous system to initiate the Fenton reaction as a dual oxidation system. Diluted sodium hydroxide (NaOH) and sulfuric acid (H₂SO₄), both of analytical grade supplied by Sigma-Aldrich, were used for wastewater pH adjustment to the required values.

3.2. Characterization

The phase structure of the produced copper oxide-based nanoparticles was examined using an X-ray diffractometer XRPhillips X’pert, MPD 3040, PANalytical X’Pert PRO MRD, (PANalytical Inc., Westborough, MA, USA) with a monochromatic CuKa source (λ = 0.15406 nm). For the analysis of X-ray diffraction (XRD), diffracted intensities were recorded from 10 to 80 degrees using a step-scan mode with 0.02-degree increments. The FTIR transmittance spectra of the materials were acquired using a JASCO (FT/IR-4100) spectrometer, (Jasco Inc., Easton, MD, USA) across the wavenumber range of 400 to 4000 cm⁻¹. The morphology of the produced Cu oxide nanoparticles was examined using a Quanta FEJ20 field-emission scanning electron microscope (FE-SEM, Thermo Fisher Scientific Inc., Hillsboro, OR, USA). An X-ray photoelectron spectroscopy (XPS) analysis was performed utilizing a Thermo Scientific™ K-Alpha™ microfocused monochromator XPS spectrometer (Waltham, MA, USA), functioning at a maximum energy of 4 keV.

3.3. Procedures and Analyses

Initially, a 100 mL volume of BV10 wastewater samples was placed in a container. Subsequently, the amounts of the prepared nano-copper particles as the source of the Fenton reaction that is induced with the addition of H₂O₂ were checked at certain values. Ultraviolet illumination was applied to enhance the Fenton reaction, attained through a UV lamp emitting a 253.7 nm wavelength (15 W, 230 V/50 Hz). When it was necessary to adjust the pH, the desired pH values of the solution were determined using diluted H₂SO₄ or NaOH solutions before incorporating the designated amounts of Fenton’s reagent. Following this, the aqueous effluent undergoing treatment was periodically analyzed. Initially, the samples were filtered by a microfilter to remove the Cu-NPs. Then, the remaining dye residue was measured by recording its absorbance at 554 nm using a UV–visible spectrophotometer (Unico UV-2100 spectrophotometer, United Products & Instruments Inc., Dayton, NJ, USA, with modification). The pH of the aqueous dye solution was continuously monitored and adjusted as necessary using a digital pH meter (AD1030, Adwa instrument, Szeged, Hungary), with the appropriate quantities of H₂SO₄ or NaOH solutions being added. A graphical illustration of the treatment steps is shown in Figure 12.

4. Conclusions

The application of the Fenton reaction induced by an ultraviolet illumination technique in the treatment of BV 10 dye wastewater as a catalytic oxidation system is assessed. CuO/Cu₂O hybrid nanoparticles were synthesized using a thermal decomposition technique. XPS findings revealed that the sample consists of 56.8% CuO and 43.2% Cu₂O, suggesting CuO predominates, aligning with XRD analysis. CuO/Cu₂O NPs were applied for BV 10 dye oxidation as an oxidation system. The experimental data expose that the excess presence of Cu-based nanoparticles is preferred; these were oxidized and increased the ^•OH radicals’ production. Consequently, the maximal dye oxidation is reached at 82% when the optimal dose of 400 mg/L of hydrogen peroxide and 40 mg/L of CuO/Cu₂O hybrid nanoparticles is subjected to the reaction medium. Also, the system could work at various pH ranges, and an initial pH of 8.0 increases the removal rate in comparison to an acidic medium. The sustainable cyclic use showed that the catalyst is still reactive even after the sixth cyclic use, which showed that the heterogeneous system is preferred since it widens the system’s application. Extra study is needed to grasp practical-scale application, lessening the feasible cost using solar energy as the basis of the ultraviolet light to initiate the system with minimal cost, a shortened treatment period, and environmental friendliness.