Biogenic Synthesis Based on Cuprous Oxide Nanoparticles Using Eucalyptus globulus Extracts and Its Effectiveness for Removal of Recalcitrant Compounds

Abstract

Recalcitrant compounds resulting from anthropogenic activity are a significant environmental challenge, necessitating the development of advanced oxidation processes (AOPs) for effective remediation. This study explores the synthesis of cuprous oxide nanoparticles on cellulose-based paper (Cu₂O@CBP) using Eucalyptus globulus leaf extracts, leveraging green synthesis techniques. The scanning electron microscopy (SEM) analysis found the average particle size 64.90 ± 16.76 nm, X-ray diffraction (XRD) and Raman spectroscopy confirm the Cu₂O structure in nanoparticles; Fourier-transform infrared spectroscopy (FTIR) suggests the reducing role of phenolic compounds; and ultraviolet–visible diffuse reflectance spectroscopy (UV-Vis DRS) allowed us to determine the band gap (2.73 eV), the energies of the valence band (2.19 eV), and the conduction band (−0.54 eV) of Cu₂O@CBP. The synthesized Cu₂O catalysts demonstrated efficient degradation of methylene blue (MB) used as a model as recalcitrant compounds under LED-driven visible light photocatalysis and heterogeneous Fenton-like reactions with hydrogen peroxide (H₂O₂) using the degradation percentage and the first-order apparent degradation rate constant (k_app). The degradation efficiency of MB was pH-dependent, with neutral pH favoring photocatalysis (k_app = 0.00718 min⁻¹) due to enhanced hydroxyl (·OH) and superoxide radical (O₂·⁻) production, while acidic pH conditions improved Fenton-like reaction efficiency (k_app = 0.00812 min⁻¹) via ·OH. The reusability of the photocatalysts was also evaluated, showing a decline in performance for Fenton-like reactions at acidic pH about 22.76% after five cycles, while for photocatalysis at neutral pH decline about 11.44% after five cycles. This research provides valuable insights into the catalytic mechanisms and supports the potential of eco-friendly Cu₂O nanoparticles for sustainable wastewater treatment applications.

1. Introduction

In modern industry, rapid industrialization has led to a concerning proliferation of recalcitrant contaminants. These contaminants include a diverse mixture of chemicals such as pesticides, dyes, antibiotics, and various organic compounds. This rapid increment of pollutants poses substantial risks not only to human health but also to the delicate balance of aquatic ecosystems [1]. Among these pollutants, dyes stand out due to their presence across a spectrum of industries including textiles, food, plastics, printing, leather, cosmetics, and pharmaceuticals. The extensive utilization of dyes inevitably translates into the generation of voluminous wastewater streams contaminated with these colorants, presenting a formidable challenge due to their inherent toxicity and mutagenicity [2]. Given the threat posed by water contamination, there exists an urgency for the development and deployment of efficacious and environmentally sustainable technologies for treating water contaminated with dyes and other pollutants. Traditional methodologies for wastewater treatment typically require a multifaceted approach, incorporating mechanical, biological, physical, and chemical processes adapted to the unique composition of the wastewater under treatment. These processes may include filtration, flocculation, sterilization, or chemical oxidation of organic contaminants, all aimed at achieving the desired treatment outcomes while adhering to stringent environmental standards [3]. Recent scientific attempts have increasingly turned their focus towards advanced oxidation processes (AOPs) as a promising route for the treatment of contaminated water [4]. AOPs, characterized by the in situ generation of highly reactive oxygen species (ROS), such as ·OH and O₂·⁻, offer a pathway towards significantly enhancing degradation efficiency. Moreover, these processes hold the prospect of effecting complete conversion of targeted pollutants into benign byproducts, including carbon dioxide (CO₂), water (H₂O), and mineral acids [4].

The homogeneous Fenton reaction, which involves the reaction of Fe²⁺ and H₂O₂ to produce ·OH, is a well-studied AOP. However, it has some drawbacks, such as its reliance on acidic pH for optimal effectiveness [5,6]. To address this issue, researchers have explored the use of peroxymonosulfate instead of H₂O₂ [7] or molecules from the catechol family to enhance ·OH production and generate other reactive species at a more neutral pH [8]. Despite these efforts, the problem has not been fully resolved. Within the realm of AOPs, considerable attention has been directed towards the development of heterogeneous visible-light photocatalysis and specific Fenton processes that demonstrate promise under mild operating conditions [2,9]. The search for effective catalysts is pivotal for the practical application and scalability of these processes. Notably, copper has emerged as a particularly promising candidate for Fenton-like reactions, owing to its versatile valences and favorable redox characteristics [2]. In contrast to conventional iron-based Fenton processes, copper catalysis offers the distinct advantage of reduced metal solid waste generation, thereby aligning with the principles of sustainable chemistry [10,11]. Semiconductor photocatalysis has garnered significant attention owing to its potential to catalyze the decomposition of organic pollutants into innocuous compounds through the generation of ROS [12]. This process involves the illumination of semiconductor particles in water, leading to the creation of electron-hole pairs (e⁻/h⁺). While metal oxide semiconductors such as titanium dioxide (TiO₂) and zinc oxide (ZnO) have historically dominated the field [13], its conventional activation has been by ultraviolet (UV) lamps for photocatalytic degradation. However, the use of UV lamps poses inherent challenges such as high energy consumption and the presence of hazardous mercury content [14]. LEDs have been developed as a cost-effective and eco-friendly light source. LEDs offer flexibility, eliminating the shape constraints of UV lamps and can emit light in visible or near-ultraviolet wavelengths for reactor design [3]. Cuprous oxide (Cu₂O) emerges as a candidate for visible-light-driven photocatalysis due to its direct band gap falling within the visible light spectrum, low toxicity, adjustable band gap, and effective molecular oxygen adsorption, which render it well-suited for applications in visible-light-driven processes [13,15]. Plant extracts have shown great potential as a sustainable method for synthesizing nanoparticles [16]. The abundance of phytochemicals, such as sugars, phenolic compounds, and proteins, in these extracts serves as a natural reducing agent for metal ions [16]. This makes it a more environmentally friendly alternative to traditional physical and chemical synthesis methods. Among the various plant extracts, Eucalyptus stands out as a particularly attractive candidate due to its widespread availability and high phenolic content [16,17]. However, there is a gap in knowledge regarding the use of Eucalyptus globulus extracts specifically for Cu₂O nanoparticles. Even more, while there is a significant amount of research on the use of plant extracts for Cu₂O synthesis [18,19,20], the specific role of phytochemicals in the formation and stabilization of Cu₂O remains unclear. Designing reactors for AOPs, particularly those integrating nanoparticles, necessitates meticulous consideration of numerous factors. Two primary approaches, namely suspended particle reactors and fixed-catalyst reactors, offer distinct advantages and challenges. While suspended particle reactors excel in contaminant-to-surface mass transfer, concerns regarding nanoparticle loss and separation complexities persist. Immobilizing photocatalysts on substrates as cellulose supports promise for easing recovery and enhancing reusability [21,22]. However, this approach introduces challenges associated with mass transfer limitations and geometric complexities for light exposition, necessitating continued exploration and innovation in reactor design. There is substantial evidence that Cu₂O nanoparticles are effective in removal dye pollutants via photocatalysis [13,15,18,20,22,23,24] and heterogeneous Fenton-like reactions [2,23,25] and that immobilized nanomaterials offer advantages in these processes [2,13,21,22,23,26]. There have been studies that have utilized Cu₂O nanoparticles in suspension, synthesized with ascorbic acid [27,28]. These studies have shown varying levels of MB removal, ranging from 10% to 83%, when using Cu₂O concentrations of 400 and 600 mg/L, respectively. Similar evidence for varying efficiencies can be found when using Cu₂O in heterogeneous Fenton-like systems using high concentrations of H₂O₂, high concentrations of Cu₂O, and low concentrations of MB [10,29]. However, Cu₂O nanoparticles synthesized from E. globulus extracts and immobilized on cellulose-based paper have not been explored. This gap leaves uncertainties regarding pH impact, LED-visible radiation efficiency in photocatalysis, and potential reactor designs. Thus, immobilizing Cu₂O on cellulose supports, with a lower concentration of Cu₂O than other suspension systems studied, could enhance both photocatalytic and Fenton-like reactions improving the degradation of organic pollutants.

In this study, we successfully synthesized Cu₂O nanoparticles on cellulose-based paper supports using E. globulus extracts. The Cu₂O nanoparticles showed photocatalytic activity under LED-visible light and catalytic activity in Fenton-like reactions at acidic and neutral pH levels. These properties were tested in a custom reactor for methylene blue removal, revealing significant performance in acidic and neutral environments. The processes were effective over five cycles, highlighting the potential of these sustainable materials for treating dye-contaminated wastewater. This research offers a sustainable approach for developing novel materials for wastewater treatment.

2. Results and Discussion

2.1. Characterization of Nanoparticles

2.1.1. Synthesis of Nanoparticles Supported

Figure 1a shows photographic images of pristine cellulose paper as support and Figure 1b shows the cellulose-based paper with CuNPs. The color change in the cellulose-based paper, typical color of Cu₂O [30,31], suggests an effective formation of Cu₂O nanoparticles.

2.1.2. SEM Analysis

The SEM image of cellulose-based paper is shown in Figure 2a. Figure 2b shows synthesized nanoparticles with a uniform distribution on the paper support. Figure 2c shows that size of Cu₂O@CBP varied from 32.3 to 142 nm, with a mean diameter of 64.90 ± 16.76 nm, indicating the formation of primarily nanoparticles.

2.1.3. Raman Analysis

Figure 3a shows the Raman spectra of Cu₂O@CBP, ranging from 50 to 1160 cm⁻¹. Signals at 90, 107, 127, 158, 228, 311, 419, 450, 495, 528, 548, 612, 634, 658, 790, 827, and 1114 cm⁻¹ are exhibited for Cu₂O@CBP, confirming the synthesis of Cu₂O nanoparticles [32,33,34,35,36,37,38] according to the results obtained from the XRD analyzes. Additional peaks at 179, 198, 332, 336, 385, 458, 571, 589, 687, 697, 721, 753, 774, 807, 842, 875, 938, 961, 970, 992, 1020, 1042, 1135, and 1150 cm⁻¹ are related to phenolic compounds [39,40]. Peaks at 252, 349, 916, 1075, and 1095 cm⁻¹ are related to cellulose in paper [41,42]. TGA analysis shows a mass loss stage around 340 °C in Cu₂O@CBP samples corresponding to the phase transition from Cu₂O to CuO [43], which was corroborated by Balık et al. [38] using XRD, FTIR, and Raman spectroscopy (Figure S1).

2.1.4. XRD Analysis

Figure 3b shows the XRD analysis for cellulose-based paper and Cu₂O@CBP, which reveals diffraction peaks at 2θ values of 29.53°, 36.37°, 42.32°, and 61.39°. These peaks correspond to the lattice planes (110), (111), (200), and (220), respectively, and are consistent with the monoclinic Cu₂O phase (JCPDS N° 05-0667) [44]. It is worth noting that no other signals related to other copper-derived nanoparticle structures were observed despite the fact that this is a green synthesis method. According to the Debye–Scherrer equation, the average crystalline size of Cu₂O@CBP is estimated to be 78.74 nm, which closely matches the particle size obtained from SEM analysis. A diffraction peak close to 34° is typical of cellulose [45].

2.1.5. FTIR Analysis

Figure 4 shows the FTIR spectrum for pristine paper and Cu₂O@CBP.

Table 1. FTIR bands in pristine paper and Cu₂O@CBP.

Wavenumber (cm−1)		Vibration	Reference
Pristine Paper	Cu2O@CBP
3339	-	Intra-molecular hydrogen bonding C(3)OH⋯O(5) or C(6)O⋯(O)H	[46]
3298	3310	Inter-molecular hydrogen bonding C(3)OH⋯C(6)O	[46]
2904	2927	Asymmetric stretching vibration of –CH2	[47]
2853	2853	Symmetric stretching vibration of –CH2	[47]
-	1740	C=O stretching	[48]
1642	-	O–H bending of adsorbed water	[46]
-	1626	C=C of aromatic ring	[49]
1454	1456	O–H in-plane deformation	[46]
1432	1430	CH2 scissoring	[50]
1371	1368	C–OH bending	[48]
1340	-	C–O stretching	[48]
1317	-	C–N stretch of aromatic amines	[51]
1205	-	C–O stretching	[46]
1162	1162	Anti-symmetrical bridge C–O–C stretching	[50]
1106	1102	C–OH bending	[49]
1056	1057	Stretching vibration of C–O–C in the pyranose skeletal ring	[18]
1032	1030	C–OH groups of cellulose	[46]
1003	-	C–O–H stretching vibration	[52]
988	988	C–O and ring stretching modes	[46]
902	899	Glycosidic deformation –C1–O–C4 characteristic of the β-glycosidic bond of cellulose	[50]
814	−	Plane bending of =C–H	[53]

summarizes the main signals of the pristine paper and Cu₂O@CBP by FTIR.

The vibrational bands at 3339, 3298, 2904, 2853, 1454, 1371, 1340, 1317, 1205, 1162, 1056, 1003, 988, 902, and 814 cm⁻¹ in pristine paper are attributed to cellulose and show a decreased intensity in Cu₂O@CBP. This reduction is due to the interaction of cellulose with nanoparticles [54] and the interaction of cellulose chemical groups with Cu₂O nanoparticles [45]. Additionally, the decrease in signals may be attributed to the interaction of phytochemicals in E. globulus extract with cellulose functional groups [55].

FTIR analysis of Cu₂O@CBP reveals additional bands that are not present in pristine paper samples. For example, the 1626 cm⁻¹ band, which is characteristic of phenolic compounds, is likely due to their interaction with Cu₂O nanoparticles [56]. Another band appears near 1740 cm⁻¹, which is attributed to C=O bonds. One possible explanation for this phenomenon is the oxidation of -OH groups in catechol-type phenolic compounds during the reduction of metallic ions [57], such as Cu²⁺ to Cu⁺, in the synthesis of Cu₂O [58]. Additionally, there are slight shifts in the cellulose bands at 1106 and 1032 cm⁻¹, indicating possible interactions between cellulose functional groups and Cu₂O nanoparticles or phytochemicals from the E. globulus extract [45,59].

2.1.6. Optical Analysis

Figure 5a shows the reflectance diffuse and Tauc plot used to determine the optical band-gap values (2.73 eV) of Cu₂O@CBP [60].

To determine the conduction and valence band positions of Cu₂O on Cu₂O@CBP, the following empirical relations can be used (Equations (1) and (2)) [61]:

$$E_{CB}=\mathsf{\chi }-E^{\mathrm{e}}-0.5E_g$$

(1)

$$E_{VB}=E_{CB}+E_g$$

(2)

where χ is the absolute electronegativity of semiconductor which defined as the arithmetic mean of the atomic electron affinity and the first ionization energy (5.32 eV for Cu₂O), E_g is the bandgap of the semiconductor, and E^e is the energy of free electrons on the hydrogen scale (~4.5 eV), E_CB is the conduction band potential, and E_VB is the valence band potential [62]. For Cu₂O@CBP, the calculated values are E_CB = −0.54 eV and E_VB = 2.19 eV (Figure 5b). To generate reactive oxygen species (ROS) via redox reactions, the conduction band of Cu₂O must be above the reduction potential of O₂/O₂·⁻ and H₂O₂/O₂, and the valence band must be below the oxidation potential of OH⁻/·OH [63]. Figure 5b shows that electrons in the conduction band of the synthesized Cu₂O nanoparticles can be captured by O₂ to produce O₂·⁻ and H₂O₂, while holes in the valence band can react with OH⁻ to produce ·OH. The band gap energy of Cu₂O is typically around 2.0–2.2 eV [22,26,64]. However, Cu₂O samples synthesized on paper exhibit an unusual increase in band gap energy. This shift is primarily due to changes in particle size [64,65,66], exposed crystalline facets [65,67,68], ligand interactions [69,70], and material defects [71,72,73]. Evidence shows the effect of Cu₂O nanoparticle sizes, with band gap energy impacting optical properties [65]. The authors have reported that through their analysis of different sizes and structures of Cu₂O nanoparticles, they have found that a decrease in nanoparticle size leads to an increase in the band gap for each exposed facet of the crystalline structure. Specifically, they have observed that Cu₂O nanoparticles with the (111) facet and diameters of 85 and 117 nm have band gaps of 2.70 and 2.61 eV, respectively, which align with the results of their study. These findings indicate that both particle size and exposed facets have a significant impact on the band gap of the synthesized Cu₂O nanoparticles.

2.2. Identification of Biomolecules Involved in the Synthesis of Cu₂O Nanoparticles

2.2.1. Spectrophotometric Analysis of Extracts before and after Formation Cu₂O Nanoparticles

Changes in the phenolic compounds, reducing sugars, proteins, FRAP, and CUPRAC of E. globulus extract before and after the synthesis of Cu₂O on paper are shown in Figure 6. No significant changes in the concentrations of reducing sugars or proteins were observed between the initial and post-synthesis stages of Cu₂O nanoparticle production. Studies indicate that these proteins do not actively participate in the reduction of Cu²⁺ to Cu₂O or other nanoparticles [74,75,76]. While some authors attribute a reducing function to sugars in leaf extracts during Cu₂O synthesis [19,20], others suggest sugars primarily serve a stabilizing role [74,77]. In contrast, significant changes were observed in the concentrations of phenolic compounds, which play crucial roles as reducers and stabilizers in Cu₂O nanoparticle synthesis [49,74,77,78,79]. This is supported by FRAP and CUPRAC analyses, highlighting the antioxidant capacity of plant extracts and the presence of phenolic compounds [80] and underscoring their crucial role in the formation of Cu₂O nanoparticles.

2.2.2. FTIR Analysis of Extracts before and after Formation of Cu₂O Nanoparticles

A comparative experiment was conducted to examine the formation of Cu₂O nanoparticles on paper substrates using E. globulus extract. The aim of the experiment was to understand the role of plant extract components in the synthesis process. FTIR measurements were used to identify the primary functional groups involved in Cu₂O formation. Figure 7 shows the FTIR spectra of E. globulus extract before and after Cu₂O synthesis on paper. Significant decreases were observed in signals around 3375 cm⁻¹ (alcoholic and phenolic -OH groups) [81], 2936 cm⁻¹ (C–H asymmetric stretching) [81], 1700 cm⁻¹ (C=O stretching), 1610 cm⁻¹ (C=C stretching) [81], 1350 cm⁻¹ (C–H bending) [58], 1450 cm⁻¹ (C=C aromatic ring stretching) [49], 1400 cm⁻¹ (C-OH stretching) [49], 1250 cm⁻¹ (C–O stretching in aromatic esters) [81], and 1060 cm⁻¹ (C–O–C asymmetric stretching) [77]. These signals, attributed to various phenolic compounds in E. globulus extracts, decrease after nanoparticle synthesis, indicating their involvement in the process.

2.3. Photocatalytic and Fenton-Like Degradation of MB

The degradation efficiency of MB by Cu₂O@CBP was evaluated under photocatalysis and heterogeneous Fenton-like reaction conditions at pH 3.0 and 7.0 to compare their performance. Figure 8a illustrates the photocatalytic degradation kinetics of MB catalyzed by Cu₂O@CBP at pH 3.0 and 7.0. Notably, Cu₂O@CBP at pH 7.0 demonstrates the highest photocatalytic efficiency, with an apparent rate constant (k_app) of 0.00718 min⁻¹, compared to 0.00194 min⁻¹ at pH 3.0 (Figure 8c). This highlights the significant impact of pH on the photocatalytic process. The enhanced degradation of MB at basic pH is attributed to stronger electrostatic interactions between the negatively charged Cu₂O and the cationic MB. Similar findings suggest that these interactions facilitate greater MB adsorption on the Cu₂O surface, thereby increasing the degradation rate [82]. Furthermore, dark adsorption experiments confirm that MB adsorption is more substantial at pH 7.0 than at pH 3.0, supporting the proposed mechanism. This pH-dependent behavior is consistent with observations in systems using other adsorbents for MB removal [83,84] as well as Cu₂O nanoparticles for the removal of various contaminants [85].

Figure 8b illustrates the degradation of MB through a heterogeneous Fenton-like process using Cu₂O@CBP at pH 3.0 and 7.0, highlighting the significant impact of pH on the reaction. The highest apparent rate constant (k_app) is observed for Cu₂O@CBP at pH 3.0 (0.00812 min⁻¹), followed by Cu₂O@CBP at pH 7.0 (0.00273 min⁻¹) (Figure 8d). This trend is consistent with other studies that have utilized Cu₂O for Fenton-like reactions [2,23,86,87,88,89]. The enhanced degradation at acidic pH can be attributed to several factors: (i) acidic conditions promote the dissolution of Cu₂O, resulting in the formation of Cu(I) ions that readily react with H₂O₂ to produce ·OH radicals [23,86,87,88], (ii) at higher pH levels, H₂O₂ decomposes, reducing its effectiveness in generating ·OH radicals [23,89], and (iii) ·OH radicals may react with OH⁻ ions at high pH, decreasing the availability of ·OH for MB degradation [90].

In this study, it is important to note that the Cu₂O@CBP systems do not achieve the same efficiency in dye removal as other similar studies [91]. Several studies have utilized Cu₂O or Cu₂O-based nanoparticles for photocatalytic removal of MB under visible radiation.

Table 2. Comparison of photocatalysis under visible radiation for methylene blue degradation using Cu₂O-based nanoparticles.

Catalyst	Nanoparticles Synthesis	Time (min)	MB (mg/L)	Removal (%)	Ref.
Cu2O@CBP (~50 mg/L of Cu2O)	Green synthesis	120	50	57.71	This study
Cu2O (5 mg/L)	NaBH4 reduction	120	1000	70	[82]
Cu2O (3150 mg/L)	Green synthesis	80	5	60	[92]
Cu/Cu2O (250 mg/L)	Green synthesis	150	10	<10	[29]
Cu2O (400 mg/L)	Ascorbic acid	120	10	~10	[27]
Cu2O on film	Electrodeposition	150	6.25	<10	[93]
Cu/Cu2O/CuO (1000 mg/L)	Chemical synthesis	120	10	46	[94]
Cu2O thin films	Magnetron sputtering	105	50	69.6	[95]
Cu2O thin film	Electrodeposition	120	5	62	[96]
Cu2O (600 mg/L)	Ascorbic acid	105	10	83	[28]
Cu2O	Green synthesis	150	10	78.9	[97]
Cu2O-octaedral (60 mg/L)	Membrane-assisted precipitation	120	50	68	[10]
Cu2O-spherical (60 mg/L)	Membrane-assisted precipitation	120	50	49	[10]
Cu2O-micro-octahedrons (60 mg/L)	Membrane-assisted precipitation	120	50	33	[10]
Cu2O-microspheres (60 mg/L)	Membrane-assisted precipitation	120	50	6.5	[10]
Cu2O (50 mg/L)	Green synthesis	180	1000	~60	[98]

provides a summary of these investigations, including the type and concentration of the catalyst (if reported), synthesis method, duration of MB removal, and percentage of MB removal achieved. When comparing the efficiencies and experimental conditions of MB removal achieved in this work with those reported in other studies using Cu₂O-based nanomaterials, notable differences emerge. Specifically, the photocatalytic processes for MB removal (Table 2) demonstrate comparable or superior efficiencies in the system investigated here, especially considering the low mass of Cu₂O used and the high concentration of MB treated. Additionally, a similar compilation to the previous one was conducted using research on MB removal through heterogeneous Fenton-like systems catalyzed by Cu₂O or Cu₂O-based nanoparticles.

Table 3. Comparison of Fenton-like heterogeneous reaction for methylene blue degradation using Cu₂O-based nanoparticles.

Catalyst	Nanoparticles Synthesis	Time (min)	MB (mg/L)	H2O2 Concentration	Removal (%)	Ref.
Cu2O@CBP (~50 mg/L of Cu2O)	Green synthesis	120	50	0.05 M	67.10	This study
Cu/Cu2O (250 mg/L)	Green synthesis	150	10	0.55 M	~70	[29]
Cu2O (60 mg/L)	Membrane- assisted precipitation	120	50	0.1 M	74	[10]
Cu2O (200 mg/L)	Sol-gel	130	10	12.63%	~75	[99]
Cu2O/Cu (300 mg/L)	D-glucose	60	10	9.8 × 10−3 M	28	[100]

presents the type of nanoparticle, the synthesis method, the duration of MB removal, the initial MB concentration, the initial H₂O₂ concentration, and the percentage of MB removed. When comparing similar systems for MB removal via heterogeneous Fenton-like reactions, the efficiencies observed in this study are also comparable or superior. This is particularly significant given the lower concentration of H₂O₂ used in this research compared to other studies (Table 3), highlighting the effectiveness of our approach despite lower Cu₂O concentrations.

2.4. Reusability of Photocatalytic and Fenton-Like Degradation of MB

Reusability studies were conducted to assess the reusability of Cu₂O@CBP for MB dye removal through photocatalysis at pH 7.0 and Fenton-like reactions at pH 3.0 (Figure 9). The results showed a decrease in photocatalytic degradation efficiency from 57.71% to 46.27% after five cycles (Figure 9a), while Fenton-like degradation efficiency dropped from 67.10% to 44.34% (Figure 9b). It was observed that the photocatalytic system maintained a more sustained efficiency, with a less significant decline in MB degradation compared to the Fenton-like system. The performance of Cu₂O@CBP in Fenton-like systems was lower, which could be attributed to either greater Cu₂O dissolution at acidic pH [25], or the reaction of Cu⁺ on the Cu₂O surface with H₂O₂, resulting in reduced Cu⁺ availability, as it converts to Cu²⁺ [101]. A possible explanation for the decline in performance of both processes could be attributed to the inadequate washing of Cu₂O@CBP between cycles. This may result in dye particles remaining attached to the surface of the nanoparticles, affecting their effectiveness in the subsequent cycle.

2.5. Reactive Species in Photocatalytic and Fenton-Like Degradation of MB

To clarify the mechanism behind the photocatalytic and Fenton-like degradation of MB, we investigated the reactive species involved (Figure 10). Instead of measuring the degradation efficiency in a solution containing both MB and Cu₂O@CBP, we used scavengers to detect the initial steps of photocatalysis, such as the generation of electrons (e⁻) and holes (h⁺), and to identify radicals such as ·OH and O₂·⁻. Specifically, we used CrO₃ as an e⁻ scavenger, Na₂C₂O₄ as an h⁺ scavenger, p-benzoquinone (BQ) as an O₂·⁻ scavenger, and isopropanol (IP) as an ·OH scavenger.

In pH 7.0 photocatalytic systems, the addition of Na₂C₂O₄ had a minimal effect on the degradation efficiency of MB, reducing it by only 13.43% for Cu₂O@CBP compared to systems without scavengers (Figure 10a). However, the use of CrO₃ significantly reduced the degradation efficiency by 39.27%, indicating effective migration of photoexcited electrons to the Cu₂O surface. This suggests that reduction reactions are more critical than oxidation reactions, as the minimal impact of Na₂C₂O₄ suggests low availability of positive holes (h⁺) on the Cu₂O surface. Additionally, the presence of BQ resulted in a 29.83% decrease in MB degradation efficiency, while IP caused a substantial reduction of 52.45%. This highlights the primary role of ·OH in MB degradation compared to O₂·⁻. These findings are consistent with research that observed that ·OH and electrons are the main species responsible for MB degradation using a CuO/Cu₂O heterostructure under visible light [102].

The low presence of h⁺ on the Cu₂O surface suggests that ·OH, the key species in MB degradation, are produced through parallel reactions (Equations (3)–(9)) rather than the direct oxidation of hydroxide ions (OH⁻) by h⁺ [24,82,102].

$${\mathrm{Cu}}_2\mathrm{O}+\mathrm{h}\mathsf{\nu }\to {\mathrm{Cu}}_2\mathrm{O} ({\mathrm{h}}^{+}+{\mathrm{e}}^{-})$$

(3)

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{O}}_2{·}^{-}$$

(4)

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

(5)

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

(6)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-}\to ·\mathrm{OH}+{\mathrm{OH}}^{-}$$

(7)

$${\mathrm{H}}_2{\mathrm{O}}_2\to 2·\mathrm{OH}$$

(8)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2{·}^{-}\to ·\mathrm{OH}+{\mathrm{OH}}^{-}+{ \mathrm{O}}_2$$

(9)

A study was conducted by Chu and Huang [103] on the facet-dependent photocatalytic properties of Cu₂O using scavengers. Their results and trends were similar to those found in the present study for Cu₂O nanoparticles with a predominantly octahedral structure. Their findings suggest that the primary source of photocatalytic activity in octahedral Cu₂O is the migration of electrons (e⁻) to the catalyst surface, which facilitates the production of O₂·⁻ and ·OH radicals. This is in contrast with the weak migration of holes (h⁺) to the surface.

In the Fenton-like system at pH 3.0, the addition of BQ, an O₂·⁻ scavenger, resulted in a 17.71% decrease in MB degradation efficiency for Cu₂O@CBP compared to systems without scavengers (Figure 10b). Conversely, the addition of IP, an ·OH scavenger, resulted in a more significant reduction of 50.45%. This suggests that ·OH plays a more crucial role in MB degradation than O₂·⁻, which is consistent with previous findings in Cu₂O/H₂O₂ systems at acidic pH [23,100]. The reasons for this behavior include: (i) ·OH has a higher redox potential (1.5–2.8 V) compared to O₂·⁻ (O₂·⁻/H₂O₂ = +0.94 V, O₂/O₂·⁻ = −0.35 V) [104,105], (ii) The reaction between Cu⁺ and H₂O₂ to produce ·OH and Cu²⁺ (Equation (10)) occurs at a faster rate (k = 4 × 10⁵ M⁻¹ s⁻¹ at pH 6–8) compared to the reaction between Cu²⁺ and H₂O₂ (Equation (11)) to produce O₂·⁻ and Cu⁺ (k < 1 M⁻¹ s⁻¹ at pH 7) [106], and (iii) H₂O₂ interaction on the Cu₂O surface leads to ·OH generation through O-O bond cleavage [23]. These reactions could involve both surface-bound copper ions (=Cu⁺ and =Cu²⁺) and dissolved ions.

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

(10)

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

(11)

Overall, these insights into the roles of reactive species and the effect of pH provide a comprehensive understanding of the mechanisms driving the photocatalytic and Fenton-like degradation of MB in the presence of Cu₂O@CBP (Figure 11).

3. Materials and Methods

3.1. Preparation of E. Globulus Extracts

To prepare the E. globulus extracts, fresh leaves were collected from Concepcion in southern Chile, thoroughly washed with deionized water to remove any surface impurities, and then air-dried at room temperature. The dried leaves were ground into a fine powder using a mechanical grinder. An aqueous extract was prepared by boiling the leaf powder (60 g/L) in deionized water at 60 °C for 20 min. The mixture was allowed to cool to room temperature and then filtered through a 0.45 µm membrane filter to remove any solid residues. The resulting filtrate was stored at 2 °C for subsequent use in the synthesis of Cu₂O nanoparticles [107].

3.2. Synthesis of Cu₂O Nanoparticles on Cellulose Paper

The synthesis of Cu₂O nanoparticles on cellulose paper was achieved through a multi-step process. First, 4 × 4 cm pieces of cellulose paper were immersed in 50 mL of a 0.5 M CuCl₂ solution and stirred (200 rpm) at 60 °C for 30 min. The paper was then removed and the CuCl₂ solution was mixed with 10 mL of a 1 g/50 mL NaOH solution to precipitate Cu(OH)₂. The paper pieces were re-immersed in this Cu(OH)₂ suspension and stirred (200 rpm) for an additional 30 min at 60 °C. After removing the paper, it was placed in 25 mL of the prepared E. globulus extract and stirred (200 rpm) at 60 °C for 30 min to facilitate the reduction of Cu(OH)₂ to Cu₂O nanoparticles (approximate pH of 8.5). The paper was then washed thoroughly with deionized water and dried at 60 °C. Cellulose-based paper with Cu₂O is named as Cu₂O@CBP.

3.3. Characterization Techniques

To investigate the morphology and elemental composition of the synthesized Cu₂O nanoparticles, scanning electron microscopy (SEM) was employed. The SEM images were captured using a Hitachi SU3500 SEM (Bruker, Germany).

X-ray diffraction (XRD) was utilized to determine the crystalline structure of the Cu₂O nanoparticles. XRD patterns were obtained using a Bruker D4 ENDEAVOR X-ray diffractometer with Cu Kα radiation. The operating voltage and current were set at 40 kV and 20 mA, respectively. Diffraction peak intensities were recorded within a 10–80° (2θ) range, with increments of 0.02° and a count duration of 0.3 s per increment. The average crystallite size was calculated using the Debye–Scherrer equation (Equation (12)).

$$\mathrm{D}={\displaystyle \frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}}$$

(12)

Raman spectroscopy was performed using a LabRam HR Evolution Horiba Jobin Yvon spectrometer equipped with a 633 nm laser, focusing the laser spot with an Olympus 10× VIS optical lens and a NUV camera (B/S UV 50/50 + Lens F125 D25).

Fourier-transform infrared spectroscopy (FTIR) was conducted to identify functional groups before and after the synthesis, and the interactions between the Eucalyptus extract and the Cu₂O nanoparticles. FTIR spectra were obtained using a Perkin Elmer Spectrum Frontier/Spotlight 400 Microscopy System with a linear array of mercury–cadmium–tellurium (MCT) detectors.

The optical properties of the nanoparticles were evaluated using UV-Vis diffuse reflectance spectroscopy (DRS). DRS spectra were recorded with a Jasco V-750 UV–Visible spectrophotometer equipped with a Jasco ISV-922 integrating sphere. The bandgap energy of the Cu₂O nanoparticles was determined from Tauc plots derived from the DRS data.

Thermogravimetric analysis (TGA) was used to assess the thermal stability of the Cu₂O nanoparticles on cellulose paper. TGA measurements were performed using a Perkin-Elmer STA 6000 instrument, heating the samples at a rate of 15 °C/min under a nitrogen flow of 40 mL/min from 25 °C to 600 °C.

Phenolic compounds, reducing sugars, and proteins were analyzed in the E. globulus extracts before and after synthesis of Cu₂O nanoparticles [107]. Phenolic compounds were measured by Folin–Ciocalteu method and expressed as gallic acid equivalents. The reducing sugars were analyzed using the DNS method and calculated as glucose equivalents. Protein in extracts was measured using the Bradford method quantified using bovine albumin serum equivalents. Phytochemicals in the E. globulus extract with reducing capacity in the formation of Cu₂O nanoparticles were measured by the ferric-reducing antioxidant power method (FRAP) and cupric reducing antioxidant capacity (CUPRAC) [80].

3.4. Photocatalytic and Fenton-Like Degradation of MB

The photocatalytic and Fenton-like activities of the Cu₂O nanoparticles were evaluated using MB as a model pollutant. For photocatalytic degradation, 20 mL of a 50 mg/L MB solution in a glass reactor containing a 4 × 4 cm piece of the Cu₂O supported on cellulose paper was placed in 3D printed supports (Figure 12). The reactor was stirred for 120 min in the dark to achieve equilibrium. Subsequently, the reactor was irradiated with LED light (wavelength range: 400–700 nm) for 120 min while being continuously stirred. The effect of pH on photocatalytic activity was studied by adjusting the solution to pH 3.0 and 7.0 using HCl and NaOH. Samples were taken at regular intervals, and the concentration of MB was measured using a UV–Vis spectrophotometer (Jasco, V-750, Tokyo, Japan) at 664 nm.

For Fenton-like degradation, 20 mL of a 50 mg/L MB solution was added to a glass reactor containing the Cu₂O supported on cellulose paper placed in printed supports (Figure 12). The reactor was stirred for 120 min in the dark to achieve equilibrium. The reaction was started by adding 50 mM of H₂O₂. The pH of the solution was adjusted to 3.0 and 7.0 to study its effect on the degradation process. Samples were taken at regular intervals, and the concentration of MB was measured using a UV–Vis spectrophotometer (Jasco, V-750) at 664 nm.

The degradation percentage of MB and the first-order apparent degradation rate constant (k_app) were calculated using Equations (13) and (14) respectively:

$$\mathrm{MB} \mathrm{degradation} \left(\%\right)=\left({\displaystyle \frac{C_0-C_t}{C_0}}\right)\times 100$$

(13)

$$\ln \left({\displaystyle \frac{C_t}{C_0}}\right)=K_{app}t$$

(14)

where C₀ is the initial MB concentration, C_t is the remaining MB concentration after “t” min, and k_app (min⁻¹) is the first-order apparent rate constant for MB degradation by photocatalysis and Fenton-like reaction.

3.5. Reusability Studies

To assess the reusability of the Cu₂O nanoparticles, the photocatalytic and Fenton-like MB degradation experiments were repeated for five cycles. After each cycle, the Cu₂O supported on cellulose paper was washed with deionized water and dried at 50 °C before being reused in the next experiment. The degradation efficiency of MB was calculated for each cycle to evaluate the stability and reusability of the catalysts.

3.6. Reactive-Species-Trapping Experiments

Reactive-species-trapping experiments were conducted to identify the primary reactive species involved in the degradation processes. Specific scavengers were used: isopropanol (IP) for ·OH, p-benzoquinone (BQ) for O₂·⁻, Na₂C₂O₄ for holes (h⁺) and CrO₃ for electrons (e⁻). The experiments were performed under the same conditions as the photocatalytic and Fenton-like degradation tests, with the addition of appropriate scavengers [103].

4. Conclusions

This research demonstrates the potential of immobilizing Cu₂O nanoparticles on cost-effective supports such as paper cellulose-based materials through an eco-friendly synthesis method. The synthesis of Cu₂O@CBP using E. globulus extracts resulting in nanoparticles with a mean size of 64.9 nm. The degradation of MB was more effective using Cu₂O@CBP in both photocatalysis (57.71% at pH 7.0, k_app = 0.00718 min⁻¹) under visible light and Fenton-like reactions (67.1% at pH 3.0, k_app = 0.00812 min⁻¹). Reusability studies have shown that Cu₂O@CBP can be recycled multiple times with high efficiency. However, in Fenton-like systems, the efficiency declines at a faster rate (22.76% after 5 cycles) compared to photocatalysis (11.44% after five cycles). Although other systems utilizing Cu₂O-based nanoparticles have demonstrated successful removal of MB through photocatalysis and Fenton-like reactions, this study highlights the effectiveness of Cu₂O@CBP. Therefore, for the removal of similar recalcitrant compounds in acidic contaminated water, the use of Cu₂O@CBP in Fenton-like systems would be more advantageous, while for near-neutral-pH contaminated water, the utilization of Cu₂O@CBP in photocatalysis is recommended. These findings enhance the future applicability of these technologies, providing a sustainable and versatile approach to water purification and environmental remediation. In the future, we plan to investigate the impact of additional variables on the removal of contaminants. We will also examine the effectiveness of Cu₂O@CBP in photocatalysis and heterogeneous Fenton-like reactions for other dyes or contaminants. Additionally, we will explore the role of copper ion leaching, particularly in heterogeneous Fenton-like processes. Furthermore, we aim to identify the specific reasons for the decrease in removal effectiveness after multiple uses.