*Article* **Photo-Fenton Catalyzed by Cu2O/Al2O3: Bisphenol (BPA) Mineralization Driven by UV and Visible Light**

**Oscar Olea-Mejia 1,\* , Sharon Brewer <sup>2</sup> , Kingsley Donkor <sup>2</sup> , Deysi Amado-Piña <sup>1</sup> and Reyna Natividad 1,\***


**\*** Correspondence: oleaoscar@yahoo.com.mx (O.O.-M.); rnatividadr@uaemex.mx (R.N.)

**Abstract:** This work aimed to demonstrate Cu2O/Al2O<sup>3</sup> as a catalyst of the photo-Fenton process in the UV and visible spectra. Cu2O nanoparticles were synthesized by laser ablation in liquid and supported on Al2O<sup>3</sup> . The catalytic activity of the resulting solid was assessed in the mineralization of bisphenol A (BPA). The studied variables were type of Al2O<sup>3</sup> (*α* and *γ*), Cu content (0.5 and 1%), and H2O<sup>2</sup> concentration (1, 5, and 10 times the stoichiometric amount). The response variables were BPA concentration and total organic carbon (TOC) removal percentage. The presence of Cu2O nanoparticles (11 nm) with an irregular sphere-like shape was confirmed by transmission electron microscopy (TEM) and their dispersion over the catalytic surface was verified by energy-dispersed spectroscopy (EDS). These particles improve ·OH radical production, and thus a 100% removal of BPA is achieved along with ca. 91% mineralization in 60 min. The BPA oxidation rate is increased one order of magnitude compared to photolysis and doubles that for H2O<sup>2</sup> + UV. An increase of 40% in the initial oxidation rate of BPA was observed when switching from α-Al2O<sup>3</sup> to γ-Al2O<sup>3</sup> . 4-hydroxybenzaldehyde, 4-hydroxybenzoic acid, acetaldehyde, and acetic acid are the BPA oxidation by-products identified using LC/MS and based on this a reaction pathway was proposed. Finally, it was also concluded that the synthesized catalyst exhibits catalytic activity not only in the UV spectrum but also in the visible one under circumneutral pH. Therefore, Cu2O/Al2O<sup>3</sup> can be recommended to conduct a solar photo-Fenton reaction that can degrade other types of molecules.

**Keywords:** BPA degradation; photo-Fenton; nanocomposite catalysts; bisphenol A mineralization; solar photo-Fenton

### **1. Introduction**

Bisphenol A (BPA) is ubiquitous in the environment. Potential sources of human exposure to bisphenol A are air, water, soil, sediments, indoor dust, and human tissues. Bisphenol A is an emerging environmental contaminant. The continuous release of bisphenol A into the environment results in the continuous exposure of both plant and animal life to it. Furthermore, BPA has been determined to be an endocrine disruptor chemical, potentially associated with numerous diseases such as breast cancer, infertility, cognitive dysfunction, diabetes, cardiovascular diseases, and obesity. As a response to these concerns, several countries have banned the use of BPA in various consumer products. The abovementioned effects of BPA on human health and the environment prompted us to choose it as the research object in the present work and develop the method presented here, which can be used to remove or minimize its presence [1,2].

There are various techniques to remove pollutants from contaminated streams; these include adsorption [3,4], biodegradation, and thermal destruction, among others. However, these methods also have important disadvantages, such as only transferring the pollutant from one material to another, high energy requirements, and slow process. In this context, advanced oxidation processes (AOPs) are known to be highly efficient in removing

**Citation:** Olea-Mejia, O.; Brewer, S.; Donkor, K.; Amado-Piña, D.; Natividad, R. Photo-Fenton Catalyzed by Cu2O/Al2O3: Bisphenol (BPA) Mineralization Driven by UV and Visible Light. *Water* **2022**, *14*, 3626. https:// doi.org/10.3390/w14223626

Academic Editors: Huijiao Wang, Dionysios (Dion) Demetriou Dionysiou and Yujue Wang

Received: 30 September 2022 Accepted: 7 November 2022 Published: 10 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

organic contaminants from water, the Fenton process being one that has been extensively studied [5]. However, the Fenton reaction takes place in highly acidic conditions, and iron salts are dissolved to produce the desired ·OH radicals. Evidently, the disadvantages of this technique are the loss of the iron catalyst (homogeneous catalysis) as well as the need to reverse the pH to an acceptable level after the reaction. This is usually conducted by the addition of HCl, which indicates a new problem—the increase of purification cost.

Therefore, different AOPs have been developed to overcome these disadvantages, including ozonation [6–8], heterogeneous catalysis/photocatalysis [9,10], bioprocesses [11–13], and Fenton-like reactions [14–16]. Among these methods, the heterogeneous photo-Fentonlike is a promising one. It has the advantages of re-using catalysts and can be conducted at circumneutral pH. Here, the generation of the ·OH radicals is due to the presence of H2O2, UV light, and the metal ion that is capable of facilitating the radical production.

The degradation of bisphenols using Fenton-like processes has been studied extensively in recent years, and several methods have been proposed, including sono-photo-Fenton [17] and photo-Fenton using UV [18–24] and visible radiation [10,25,26]. Furthermore, the use of metallic nanoparticles in Fenton-like reactions has also been studied recently, with iron being the most-used metal [6,8,15,27–33]. Copper has also been used but to a much lesser extent [34,35], and its use in the degradation of bisphenols has been barely assessed [18,36]. Therefore, copper was elected in this work because of its potential as a Fenton-like reactant and due to the very little information regarding its use in the degradation of bisphenols.

Metallic and Cu oxide nanoparticles can be produced by several methods. In this work, Cu2O nanoparticles were produced via laser ablation in liquid (LAL). LAL is a physical method that can produce nanoparticles from a pure metal target immersed in a liquid medium. In short, a high-energy pulsed laser is focused on the surface of the target, the material is abruptly heated, and plasma is formed. Subsequently, the plasma condenses in the form of nanoparticles inside the liquid [37]. The main advantage when using LAL is that the obtained nanoparticles are free from any chemical by-products [38,39]. This is relevant to catalytic applications because the surface contamination-free nanoparticles are readily available to interact with the chemical species in the reaction of interest.

This work aimed to investigate the effect of the amount of H2O<sup>2</sup> and the amount of Cu incorporated in the catalysts in the degradation of BPA via a photo-Fenton-like reaction using two types of commercially available Al2O3. This is a well-known catalytic support that is relatively low-cost, with no environmental issues. From an engineering point of view, the support of nanoparticles is desirable to ease the catalyst separation. Other Cu composites have been used successfully to this purpose [18].

### **2. Materials and Methods**

### *2.1. Cu Nanoparticle Preparation and Characterization*

A Continuum Surelite II laser system was used at a wavelength of 1064 nm, pulse duration of 6 ns, and repetition rate of 10 Hz. The metal used was a 2.5 cm diameter Cu target (99.99% purity, J.K. Lesker Co., Jefferson Hills, PA, USA). The liquid medium was deionized water. The full procedure has already been published in previous works. In short, 5 mL of water was added to a Pyrex glass containing the Cu target and irradiated for the time necessary to produce the required amount of Cu nanoparticles (about 20 min). The energy output from the laser system was 90 mJ/pulse. The amount of nanoparticles was calculated from the weight difference of the target before and after the laser ablation process. A JEOL JEM-2100 transmission electron microscope (TEM) was used to study the size and shape of the nanoparticles. The same equipment was used to perform high-resolution TEM (HRTEM) to elucidate the crystal structure and metallic state of the nanoparticles.

### *2.2. Catalyst Preparation and Characterization*

Two types of alumina (Al2O3) were used in this work to study the possible effects of the crystalline phase and the size and shape of the alumina powder on BPA degradation efficiency. Powders of α-Al2O<sup>3</sup> and γ-Al2O<sup>3</sup> (Sigma Aldrich, Burlington, MA, USA, 99% purity, 1–10 µm particle size) were used. Both types of alumina were treated prior to the addition of the nanoparticles with a solution of 1M NaOH (Sigma-Aldrich) at 60 ◦C for 1 h to improve the interaction with the Cu nanoparticles. After that, the alumina was dried and added to the Cu nanoparticle colloid and stirred for 1 h. Finally, the water was evaporated at 80 ◦C to obtain the dried Cu/Al2O3. The Cu concentrations used in this study were 0, 0.5, 1, and 2% weight percentages. The catalysts were studied using scanning electron microscopy (SEM), and a JEOL JSM-6510LV with an energy-dispersive X-ray spectroscopy (EDS) probe attached to it to confirm the presence of the Cu nanoparticles on the surface of the catalyst.

### *2.3. BPA Degradation Procedure*

The BPA degradation was studied using a photo-Fenton-like reaction in an annular batch reactor with 50 mL of reaction volume. The UV irradiation was performed with a monochromatic lamp with an emission of 254 nm that was placed at the center of the reactor. All the reactions were carried out at a constant temperature of 26 ◦C, controlled with a water bath. The pH was not adjusted at any time and the initial pH was 6.7. The amount of catalyst was 10 mg for all experiments, and only the Cu nanoparticle concentration in the catalyst was varied, as mentioned above. The catalyst was added to the BPA solution (initial BPA concentration = 50 mg/L) and dispersed throughout the reaction via magnetic stirring. Finally, hydrogen peroxide was added to the reaction system right before the UV lamp was turned on. The amount of peroxide was varied by 1, 5, and 10 times the stoichiometric amount according to the total BPA mineralization reaction.

To establish the activity of the catalyst under visible light, the same aforementioned procedure was followed, but instead of using a 254 nm wavelength lamp, the reactor was surrounded by 4 lamps emitting light with a wavelength λ > 400 nm; 3 were placed at the perimeter of the reactor while a fourth one was placed on top of it. The lamps were TL5 Essential 14W/840, Philips Lighting.

### *2.4. Analytical Methods*

Aliquots were taken from the reactor at regular times and centrifuged in capillary electrophoresis (CE) vials to gather the amount of catalyst in the bottom of the vial. It is important to mention that a test reaction with no UV light (only hydrogen peroxide and catalyst) was performed and no BPA degradation was registered over 1 h of reaction; therefore, it is safe to assume that even though there might be a very small amount of catalyst in the extracted aliquots there was no reaction taking place during the analytical procedures.

Capillary electrophoresis (CE) was used to study the degradation of BPA. A Beckman P/ACE System MDQ capillary electrophoresis unit was used for all CE analysis. A polyimide-coated silica capillary (50 mm long, 50 µm inner diameter) was used inside a cartridge configured for UV detection. BPA was detected via direct absorbance at 214 nm, 25 kV normal polarity, and a constant temperature of 25 ◦C. Before each sample injection, the capillary was rinsed with a 0.1 M NaOH solution for 5 min, with deionized water for 5 min, and with the run buffer for 8 min. The run buffer was a sodium dodecyl sulfate—tetrahydrate sodium borate (20 mM–60 mM, respectively) aqueous solution with a pH of 9.5.

Total organic carbon (TOC) measurements were performed after 20, 40, and 60 min of reaction in all cases to determine the mineralization percentage reached in each case. A TOC-VCSH analyzer (Mendel Scientific, Houston, TX, USA) was used for this purpose. The samples were sparged for 2 min with dried air to remove inorganic CO<sup>2</sup> from the ambient prior to the catalytic oxidation at 680 ◦C.

Liquid chromatography coupled with mass spectroscopy (LC/MS) was used to determine the possible oxidation intermediates at the end of the BPA degradation reactions. For this purpose, an HPLC system (Agilent Series 1200) was used, coupled with an LC/MS Q-TOF mass spectrometer (Agilent 6530) equipped with an electrospray ionization (ESI) source (gas temperature, 350 ◦C; drying gas, 8 L/min; nebulizer, 10 psig; sheath gas temperature, 350 ◦C; sheath gas flow, 8 L/min; Vcap, 3500 V). The analyses were carried out in the positive ion mode and the mass spectra were collected between 40 and 250 m/z. A sample volume of 10 µL was injected into the liquid chromatograph with a flow rate of 0.5 mL/min. Separation was achieved in a Zorbax Extend-C18 column (Agilent, Mississauga, ON, Canada) at a constant temperature of 25 ◦C. The mobile phase had two components: Component A was pure LC/MC-grade water with a pH of 2 adjusted with phosphoric acid, and Component B was pure methanol. A gradient elution referred to Component B (%) was used: 0% for 15 min, 60% for 1 min, 40% for 5 min, and finally 0% for 9 min.

To determine the concentration of H2O<sup>2</sup> over time, a colorimetric method was applied [40]. More details about this technique can also be found in [41]. A spectrophotometer was used to determine the absorbance of samples at 408 nm, where maximum absorbance was observed.

### **3. Results**

### *3.1. Cu Nanoparticles and Catalyst Characterization*

Figure 1a shows the TEM image of a typical sample of Cu2O nanoparticles produced by LAL. The particles have an average size of 11 nm and an irregular sphere-like shape. To determine the chemical state and crystalline phase of the nanoparticles, high-resolution TEM (HRTEM), as seen in Figure 1b, and electron diffraction (Figure 1c) were performed. The interplanar distance from the HRTEM image is 0.24 nm, which can be ascribed to the 111 plane in the crystalline phase of Cu2O according to the JPCDS 01-075-1531 card. To corroborate this, Figure 1c shows the diffraction rings that correspond to the 110, 111, 200, and 211 planes of Cu2O. It is important to note that even though the target was pure Cu, Cu2O nanoparticles were obtained, which means that the particles oxidize when in contact with water or air after being produced by laser ablation. *Water* **2022**, *14*, x FOR PEER REVIEW 5 of 15

**Figure 1.** TEM image (**a**), HRTEM (**b**), and electron diffraction pattern (**c**) of the Cu2O nanoparticles obtained via laser ablation in liquid. **Figure 1.** TEM image (**a**), HRTEM (**b**), and electron diffraction pattern (**c**) of the Cu2O nanoparticles obtained via laser ablation in liquid.

> EDS was conducted to determine the presence of Cu2O nanoparticles on the surface of the alumina particles. Figure 2a,b show the SEM and Cu mapping for the 1% Cu/α-Al2O<sup>3</sup> catalyst. As can be seen, Cu is uniformly present throughout the sample. The same observations are true for the 1% Cu/γ-Al2O<sup>3</sup> catalyst (Figure 2c,d) as well as for the rest of the prepared catalyst (not shown). Having the Cu2O nanoparticles all over the catalyst surface is very important because they can achieve their function more efficiently during the BPA degradation process, which will be further discussed in a later section.

**Figure 2.** SEM images and Cu mapping of 1%Cu2O/α-Al2O3 catalyst (**a**,**b**) and 1%Cu2O/γ-Al2O3 cat-

The degradation of BPA using UV light only, UV/H2O2, and UV/H2O2/catalyst was tested to assess the possible contribution of each component in the reaction. It can be observed in Figure 3 that all the experiments led to BPA degradation, with differences in the rate and level of degradation achieved. With direct photolysis it was possible to degrade BPA to a concentration of 22.5 ppm. When H2O2 was added to the reaction (10 times the stoichiometric amount of the complete mineralization reaction), the BPA concentration was as low as ~1 ppm for the same reaction time (15 min), which means that almost complete degradation was reached. This considerable effect was achieved because it is known that UV light is able to form ·OH radicals from H2O2 (reaction 1 in Scheme 1). However, when the catalysts were added to the reaction, the complete BPA degradation occurred at shorter times. By comparing the initial BPA reaction rates (−rBPA,o) reported in Table 1, it can be concluded that the two types of Cu2O/Al2O3 catalyst show a similar behavior, γ-Al2O3 being better than α-Al2O3 by about 40%. This initial reaction rate corresponds to the first minute of reaction, where the hydroxyl radicals are being mainly consumed by BPA; after this reaction time, it can be observed in Figure 3 that there was a change in the slope of the BPA concentration profiles of the heterogeneous processes. This change can be ascribed to the competition of the oxidation by-products with the BPA for hydroxyl radicals. It can also be observed in Table 1 that the initial BPA oxidation rate aided by the catalyst

alyst (**c**,**d**).

*3.2. BPA Degradation Results*

**Figure 1.** TEM image (**a**), HRTEM (**b**), and electron diffraction pattern (**c**) of the Cu2O nanoparticles

**Figure 2.** SEM images and Cu mapping of 1%Cu2O/α-Al2O3 catalyst (**a**,**b**) and 1%Cu2O/γ-Al2O3 catalyst (**c**,**d**). **Figure 2.** SEM images and Cu mapping of 1%Cu2O/α-Al2O<sup>3</sup> catalyst (**a**,**b**) and 1%Cu2O/γ-Al2O<sup>3</sup> catalyst (**c**,**d**).

### *3.2. BPA Degradation Results*

obtained via laser ablation in liquid.

*3.2. BPA Degradation Results* The degradation of BPA using UV light only, UV/H2O2, and UV/H2O2/catalyst was tested to assess the possible contribution of each component in the reaction. It can be observed in Figure 3 that all the experiments led to BPA degradation, with differences in the rate and level of degradation achieved. With direct photolysis it was possible to degrade BPA to a concentration of 22.5 ppm. When H2O2 was added to the reaction (10 times the stoichiometric amount of the complete mineralization reaction), the BPA concentration was as low as ~1 ppm for the same reaction time (15 min), which means that almost complete degradation was reached. This considerable effect was achieved because it is known that UV light is able to form ·OH radicals from H2O2 (reaction 1 in Scheme 1). However, when the catalysts were added to the reaction, the complete BPA degradation occurred at shorter times. By comparing the initial BPA reaction rates (−rBPA,o) reported in Table 1, it can be concluded that the two types of Cu2O/Al2O3 catalyst show a similar behavior, γ-Al2O3 being better than α-Al2O3 by about 40%. This initial reaction rate corresponds to the first minute of reaction, where the hydroxyl radicals are being mainly consumed by BPA; after this reaction time, it can be observed in Figure 3 that there was a change in the slope of the BPA concentration profiles of the heterogeneous processes. This change can be ascribed to the competition of the oxidation by-products with the BPA for hydroxyl radicals. It can also be observed in Table 1 that the initial BPA oxidation rate aided by the catalyst The degradation of BPA using UV light only, UV/H2O2, and UV/H2O2/catalyst was tested to assess the possible contribution of each component in the reaction. It can be observed in Figure 3 that all the experiments led to BPA degradation, with differences in the rate and level of degradation achieved. With direct photolysis it was possible to degrade BPA to a concentration of 22.5 ppm. When H2O<sup>2</sup> was added to the reaction (10 times the stoichiometric amount of the complete mineralization reaction), the BPA concentration was as low as ~1 ppm for the same reaction time (15 min), which means that almost complete degradation was reached. This considerable effect was achieved because it is known that UV light is able to form ·OH radicals from H2O<sup>2</sup> (reaction 1 in Scheme 1). However, when the catalysts were added to the reaction, the complete BPA degradation occurred at shorter times. By comparing the initial BPA reaction rates (−rBPA,o) reported in Table 1, it can be concluded that the two types of Cu2O/Al2O<sup>3</sup> catalyst show a similar behavior, γ-Al2O<sup>3</sup> being better than α-Al2O<sup>3</sup> by about 40%. This initial reaction rate corresponds to the first minute of reaction, where the hydroxyl radicals are being mainly consumed by BPA; after this reaction time, it can be observed in Figure 3 that there was a change in the slope of the BPA concentration profiles of the heterogeneous processes. This change can be ascribed to the competition of the oxidation by-products with the BPA for hydroxyl radicals. It can also be observed in Table 1 that the initial BPA oxidation rate aided by the catalyst was one order of magnitude higher than that with UV and hydrogen peroxide and with UV only. Table 1 also presents the pseudo-first order kinetic constants for each process; it can be observed that the specific rate constant for α-Al2O<sup>3</sup> was about 70% higher than that obtained with UV/H2O<sup>2</sup> and was more than twice that when the support was γ-Al2O3. Although the determination coefficient, R<sup>2</sup> , is still reasonably good for the heterogeneous process, it can be observed that is not as good as for the homogeneous case. This could be ascribed to not incorporating in an explicit way the competition of the organic molecule with the catalyst for the photons in the applied kinetic power law model [42].

degradation of intermediates until complete mineralization.

**Time (min) Photolysis**

*Water* **2022**, *14*, x FOR PEER REVIEW 6 of 15

**Figure 3.** BPA degradation under different experimental reaction conditions. **Figure 3.** BPA degradation under different experimental reaction conditions. responsible for complete mineralization of BPA through reaction 3 in Scheme 1 [46–48].

after 1 h of reaction was for the experiment when only UV light was used. This is in agreement with other reports where the activation of the BPA molecule by UV light has been suggested [48]. On the other hand, the highest mineralization was reached when the catalyst with Cu2O nanoparticles was used. Therefore, the synthesized catalyst enhances reaction 2 (Scheme 1) in a way that not only hastens BPA degradation, but it also aids the

constant (k), and determination coefficient (R2) under different experimental conditions.

with the catalyst for the photons in the applied kinetic power law model [42].

was one order of magnitude higher than that with UV and hydrogen peroxide and with UV only. Table 1 also presents the pseudo-first order kinetic constants for each process; it can be observed that the specific rate constant for α-Al2O3 was about 70% higher than that obtained with UV/H2O2 and was more than twice that when the support was γ-Al2O3. Although the determination coefficient, R2, is still reasonably good for the heterogeneous process, it can be observed that is not as good as for the homogeneous case. This could be ascribed to not incorporating in an explicit way the competition of the organic molecule

**Table 1.** Percent of BPA mineralization, initial BPA oxidation rate (−rBPA,o), pseudo-first order kinetic

**BPA Mineralization Percentage (%)**

**UV + H2O2 +**

**UV + H2O2 + 1% Cu2O/γ-Al2O3**

**Scheme 1.** Reactions occurring at the catalyst surface and in solution in the studied photo-Fenton process catalyzed by Cu2O/Al2O3. **Scheme 1.** Reactions occurring at the catalyst surface and in solution in the studied photo-Fenton process catalyzed by Cu2O/Al2O<sup>3</sup> .

Figure 3 shows BPA degradation based solely on the BPA peak intensity measured in the CE electropherograms; however, it is not enough to determine the extent of com-**Table 1.** Percent of BPA mineralization, initial BPA oxidation rate (−rBPA,o), pseudo-first order kinetic constant (k), and determination coefficient (R<sup>2</sup> ) under different experimental conditions.


In these cases, the Cu<sup>+</sup> ion plays a crucial role in the reaction, because the OH radicals can also be formed from the Cu ions in a similar way that Fe ions can form these radicals in a conventional Fenton reaction. Cu<sup>+</sup> can react with H2O<sup>2</sup> to form OH and <sup>−</sup>OH radicals (reaction 2 in Scheme 1 [43–45]), and it is well-known that the OH radicals are the main responsible for complete mineralization of BPA through reaction 3 in Scheme 1 [46–48].

Figure 3 shows BPA degradation based solely on the BPA peak intensity measured in the CE electropherograms; however, it is not enough to determine the extent of complete mineralization of the molecule. For this purpose, Table 1 shows the TOC results for the same set of experiments. As expected, all the experiments show different mineralization values, following the tendency from Figure 3. The lowest mineralization achieved after 1 h of reaction was for the experiment when only UV light was used. This is in agreement with other reports where the activation of the BPA molecule by UV light has been suggested [48]. On the other hand, the highest mineralization was reached when the catalyst with Cu2O nanoparticles was used. Therefore, the synthesized catalyst enhances reaction 2 (Scheme 1) in a way that not only hastens BPA degradation, but it also aids the degradation of intermediates until complete mineralization.

Figure 4 shows the consumption of hydrogen peroxide during three processes: when only adding BPA (BPA + H2O2); when adding BPA, UV light, and H2O2; and when the catalyst was added to this system. In the first case, there was no consumption of H2O2, and this in agreement with the BPA concentration profiles under these conditions. When the BPA + H2O<sup>2</sup> was irradiated with UV light, an important consumption of H2O<sup>2</sup> was observed (red dots); this was from its decomposition, as shown in Scheme 1. However, when the catalyst was added, 1500 mg/L of H2O<sup>2</sup> were consumed within the system, which was significantly higher than the H2O<sup>2</sup> consumed when the catalyst was absent. This is evidence of BPA oxidation occurring via the hydroxyl radicals produced in reactions 1 and 2 (see Scheme 1) as reported by [48]. Nevertheless, because of the narrow Cu2O bandgap, a photocatalytic effect of the cupreous oxide cannot be ruled out [18]. It is worth noticing that the radicals produced are not only consumed in the degradation of BPA but also in the oxidation of the by-products. *Water* **2022**, *14*, x FOR PEER REVIEW 8 of 15

**Figure 4.** Hydrogen peroxide consumption profiles of BPA alone (BPA + H2O2), BPA and UV light (BPA + UV + H2O2), and by BPA, UV light, and Cu/Al2O3 (BPA + UV + Cat + H2O2). **Figure 4.** Hydrogen peroxide consumption profiles of BPA alone (BPA + H2O<sup>2</sup> ), BPA and UV light (BPA + UV + H2O<sup>2</sup> ), and by BPA, UV light, and Cu/Al2O<sup>3</sup> (BPA + UV + Cat + H2O<sup>2</sup> ).

Scheme 2 depicts a plausible BPA degradation pathway via hydroxyl radicals. The identification of acetic acid, acetaldehyde, 4-hydroxybenzaldehyde, and 4-hydroxybenzoic acid was possible by means of LC/MS. Scheme 2 depicts a plausible BPA degradation pathway via hydroxyl radicals. The identification of acetic acid, acetaldehyde, 4-hydroxybenzaldehyde, and 4-hydroxybenzoic acid was possible by means of LC/MS.

**Scheme 2.** Degradation of bisphenol A by a photo-Fenton reaction catalyzed by Cu2O/Al2O3: pro-

OH OH

**Bisphenol A**

**HO.**

<sup>+</sup> H3C OH

O

O

**4-Hydroxybenzoic acid**

OH

**Acetic acid**

posed reaction pathway*.* 

O

OH

+

OH

**HO.**

H

**Acetaldehyde**

H

O

H

**4-Hydroxybenzaldehyde**

H

**CO2** + **H2O**

**HO.**

zoic acid was possible by means of LC/MS.

O2

O2

0

 BPA+H2 O2 BPA+UV+ H2

BPA+ UV+Cat+ H2

500

1000

H2O2 (mg/L)

1500

2000

**Scheme 2.** Degradation of bisphenol A by a photo-Fenton reaction catalyzed by Cu2O/Al2O3: proposed reaction pathway*.*  **Scheme 2.** Degradation of bisphenol A by a photo-Fenton reaction catalyzed by Cu2O/Al2O<sup>3</sup> : proposed reaction pathway.

**Figure 4.** Hydrogen peroxide consumption profiles of BPA alone (BPA + H2O2), BPA and UV light

Scheme 2 depicts a plausible BPA degradation pathway via hydroxyl radicals. The identification of acetic acid, acetaldehyde, 4-hydroxybenzaldehyde, and 4-hydroxyben-

(BPA + UV + H2O2), and by BPA, UV light, and Cu/Al2O3 (BPA + UV + Cat + H2O2).

0 10 20 30 40 50 60

Time (min)

Because the proposed degradation pathway is via hydroxyl radicals, it is expected to observe some similarities with other reported proposals. The first step expected is a hydroxyl radical attack of the electron-rich carbon positions. According to calculations made by [46], such positions correspond to C3 and C8. The calculated frontier density for these positions was 0.206 and 0.207, respectively. The hydroxyl radical attack of C3 may actually lead to 4-hydroxybenzaldehyde, which was identified in this work by LC/MS. It is worth pointing out that, in the context of BPA oxidation by hydroxyl radicals, this intermediate has only been identified in the photocatalytic process conducted by [46]. The hydroxyl radical attack of the electron-rich positions also leads to the appearance of the phenyl group [47] that undergoes hydroxylation to form hydroquinone. According to the literature, benzoquinone is then produced via hydroquinone dehydrogenation. Then the ring opens from further attack by hydroxyl radicals [41,47], and acetaldehyde and acetic acid appear. It is worth noting that, as in other reports, other intermediates could be present at the end of the reaction; however, they were not detected, most likely because the concentration was lower than the detection limit.

Scheme 3 depicts the three plausible structures of BPA depending on pH. This directly impacts the observed intermediates and the efficiency of the process. In this work, the initial pH was 6.7 (BPA is a weak organic acid) and it only went down to around 6.3 at the end of the experiments. Therefore, structure (A) in Scheme 3 is the one reacting in the studied system.

the studied system.

tration was lower than the detection limit.

**Scheme 3.** BPA structures and pKa: (**A**) Neutral, (**B**) pKa,1 = 9.59, (**C**) pKa,2 = 10.2 [48]*.* **Scheme 3.** BPA structures and pKa: (**A**) Neutral, (**B**) pKa,1 = 9.59, (**C**) pKa,2 = 10.2 [48].

#### *3.3. Effect of H2O2 Concentration 3.3. Effect of H2O<sup>2</sup> Concentration*

From the previous results it is clear that H2O2 is important for the degradation of BPA, and three different concentrations of this component were tested with only UV light and no catalyst. The assessed concentrations were 1 time (1X), 5 times (5X), and 10 times (10X) the amount of H2O2 theoretically needed according to the stoichiometry of the complete mineralization reaction of BPA. It can be seen in Figure 5 that the lowest degradation occurred when using 1X H2O2. On the other hand, there seems not to be a considerable difference between the 5X and 10X cases, and it could be safely assumed that it is practically the same to use 5X or 10X of H2O2. However, the TOC results in Table 2 indicate that the mineralization achieved with 10X of H2O2 was considerably higher. This means that the rate of BPA degradation was very similar, but in the case of 10X the extra H2O2 was From the previous results it is clear that H2O<sup>2</sup> is important for the degradation of BPA, and three different concentrations of this component were tested with only UV light and no catalyst. The assessed concentrations were 1 time (1X), 5 times (5X), and 10 times (10X) the amount of H2O<sup>2</sup> theoretically needed according to the stoichiometry of the complete mineralization reaction of BPA. It can be seen in Figure 5 that the lowest degradation occurred when using 1X H2O2. On the other hand, there seems not to be a considerable difference between the 5X and 10X cases, and it could be safely assumed that it is practically the same to use 5X or 10X of H2O2. However, the TOC results in Table 2 indicate that the mineralization achieved with 10X of H2O<sup>2</sup> was considerably higher. This means that the rate of BPA degradation was very similar, but in the case of 10X the extra H2O<sup>2</sup> was reacting to degrade the intermediates faster. *Water* **2022**, *14*, x FOR PEER REVIEW 10 of 15

> It is worth pointing out that H2O2 was also being generated within the system by means of reactions 4, 5, and 6 [43,49], depicted in Scheme 1. This is plausible because of the highly energetic lamp that was being used. Figure 6 shows the H2O2 generation under

> with the lamp after 60 min was about 1.75 mg/L. It is worth clarifying that this was only part of the generated amount since there was a fraction being dissociated by UV light.

> **Table 2.** Percentage of BPA mineralization under different H2O2 concentrations and UV light. CBPA,o

**Time (min) 1X H2O2 5X H2O2 10X H2O2** 2.3 16.8 17.5 15.5 25.1 50.5 20.6 50.0 80.9

**Figure 6.** Production of hydrogen peroxide under different reaction conditions.

0 10 20 30 40 50 60

Time (min)

*3.4. Effect of Cu2O Concentration on the Catalyst*

0.0

0.5

1.0

H2O2 (mg/L)

1.5

2.0

Because the proposed degradation pathway is via hydroxyl radicals, it is expected to observe some similarities with other reported proposals. The first step expected is a hydroxyl radical attack of the electron-rich carbon positions. According to calculations made by [46], such positions correspond to C3 and C8. The calculated frontier density for these positions was 0.206 and 0.207, respectively. The hydroxyl radical attack of C3 may actually lead to 4-hydroxybenzaldehyde, which was identified in this work by LC/MS. It is worth pointing out that, in the context of BPA oxidation by hydroxyl radicals, this intermediate has only been identified in the photocatalytic process conducted by [46]. The hydroxyl radical attack of the electron-rich positions also leads to the appearance of the phenyl group [47] that undergoes hydroxylation to form hydroquinone. According to the literature, benzoquinone is then produced via hydroquinone dehydrogenation. Then the ring opens from further attack by hydroxyl radicals [41,47], and acetaldehyde and acetic acid appear. It is worth noting that, as in other reports, other intermediates could be present at the end of the reaction; however, they were not detected, most likely because the concen-

Scheme 3 depicts the three plausible structures of BPA depending on pH. This directly impacts the observed intermediates and the efficiency of the process. In this work, the initial pH was 6.7 (BPA is a weak organic acid) and it only went down to around 6.3 at the end of the experiments. Therefore, structure (A) in Scheme 3 is the one reacting in

**Figure 5.** BPA degradation with different concentrations of H2O2. **Figure 5.** BPA degradation with different concentrations of H2O<sup>2</sup> .

= 50 mg/L, pHo = 6.7, Wcat = 0.0 g/L.

 blank (water) water+Cat water+ UV water+UV+Cat


**Table 2.** Percentage of BPA mineralization under different H2O<sup>2</sup> concentrations and UV light. CBPA,o = 50 mg/L, pHo = 6.7, Wcat = 0.0 g/L. the highly energetic lamp that was being used. Figure 6 shows the H2O2 generation under different operating conditions. It can be observed that the accumulated amount of H2O2

It is worth pointing out that H2O2 was also being generated within the system by means of reactions 4, 5, and 6 [43,49], depicted in Scheme 1. This is plausible because of

**Figure 5.** BPA degradation with different concentrations of H2O2.

*Water* **2022**, *14*, x FOR PEER REVIEW 10 of 15

It is worth pointing out that H2O<sup>2</sup> was also being generated within the system by means of reactions 4, 5, and 6 [43,49], depicted in Scheme 1. This is plausible because of the highly energetic lamp that was being used. Figure 6 shows the H2O<sup>2</sup> generation under different operating conditions. It can be observed that the accumulated amount of H2O<sup>2</sup> with the lamp after 60 min was about 1.75 mg/L. It is worth clarifying that this was only part of the generated amount since there was a fraction being dissociated by UV light. **Time (min) 1X H2O2 5X H2O2 10X H2O2** 20 2.3 16.8 17.5 40 15.5 25.1 50.5 60 20.6 50.0 80.9

**Figure 6.** Production of hydrogen peroxide under different reaction conditions. **Figure 6.** Production of hydrogen peroxide under different reaction conditions.

### *3.4. Effect of Cu2O Concentration on the Catalyst*

*3.4. Effect of Cu2O Concentration on the Catalyst* Based on the results from the previous section, γ-Al2O<sup>3</sup> was elected as the support material to study the effect of Cu2O concentration on the degradation of BPA. Figure 7 shows the BPA degradation for γ-Al2O<sup>3</sup> catalysts with different quantities of Cu2O added. As can be seen, when pure γ-Al2O<sup>3</sup> was used without Cu2O nanoparticles, the degradation curve was very similar to the case of H2O<sup>2</sup> + UV light (Figure 5), which means that the γ-Al2O<sup>3</sup> acted only as a support for Cu2O nanoparticles and did not contribute to any catalytic function, nor hinder the reaction. On the other hand, when Cu2O nanoparticles were added to the surface of the catalyst, a faster degradation of BPA was observed. This reinforces the hypothesis that the Cu2O nanoparticles help degrade the BPA by creating ·OH groups via mechanisms such as the one proposed in reaction 2 in Scheme 1. Furthermore, there was no notable difference when 0.5% or 1.0% of Cu2O was added to the catalyst, since the degradation profiles in both cases are very similar. In addition, the TOC results for these two cases show that the final mineralization after 1 h of reaction was similar (Table 3). This could be explained in terms of nanoparticle agglomeration on the catalyst surface: when loading a material with nanometric nanoparticles, there is a certain concentration where the particles start to agglomerate instead of being dispersed evenly on the surface of the

material. If this is true, then the total surface area available on the nanoparticles does not grow proportionally with the number of nanoparticles loaded to the material. ticles does not grow proportionally with the number of nanoparticles loaded to the material.

Based on the results from the previous section, γ-Al2O3 was elected as the support material to study the effect of Cu2O concentration on the degradation of BPA. Figure 7 shows the BPA degradation for γ-Al2O3 catalysts with different quantities of Cu2O added. As can be seen, when pure γ-Al2O3 was used without Cu2O nanoparticles, the degradation curve was very similar to the case of H2O2 + UV light (Figure 5), which means that the γ-Al2O3 acted only as a support for Cu2O nanoparticles and did not contribute to any catalytic function, nor hinder the reaction. On the other hand, when Cu2O nanoparticles were added to the surface of the catalyst, a faster degradation of BPA was observed. This reinforces the hypothesis that the Cu2O nanoparticles help degrade the BPA by creating ·OH groups via mechanisms such as the one proposed in reaction 2 in Scheme 1. Furthermore, there was no notable difference when 0.5% or 1.0% of Cu2O was added to the catalyst, since the degradation profiles in both cases are very similar. In addition, the TOC results for these two cases show that the final mineralization after 1 h of reaction was similar (Table 3). This could be explained in terms of nanoparticle agglomeration on the catalyst surface: when loading a material with nanometric nanoparticles, there is a certain concentration where the particles start to agglomerate instead of being dispersed evenly on the surface of the material. If this is true, then the total surface area available on the nanopar-

*Water* **2022**, *14*, x FOR PEER REVIEW 11 of 15

**Figure 7.** BPA degradation with catalysts of different Cu2O nanoparticle concentrations. **Figure 7.** BPA degradation with catalysts of different Cu2O nanoparticle concentrations.


**Table 3.** Percent of mineralization (TOC) of BPA by γ-Al2O<sup>3</sup> with different Cu2O content.

### 60 79.6 88.3 90.7 *3.5. Effect of Radiation Source Wavelength*

*3.5. Effect of Radiation Source Wavelength* This variable was studied by using lamps emitting light in the visible spectrum (Vis light). Figure 8 shows the effect of different reaction variables on the TOC removal percentage after 60 min of reaction time. According to the results depicted in Figure 8, the removal of BPA by adsorption was only 5% and the removal by interaction with hydrogen peroxide was only 2.5% (gray bar). However, the TOC removal increased when Vis light was added to the system, which enhanced the TOC removal up to 18%. By comparing the results with Vis and the ones with Vis + H2O2, it can be concluded that dissociation of H2O2 This variable was studied by using lamps emitting light in the visible spectrum (Vis light). Figure 8 shows the effect of different reaction variables on the TOC removal percentage after 60 min of reaction time. According to the results depicted in Figure 8, the removal of BPA by adsorption was only 5% and the removal by interaction with hydrogen peroxide was only 2.5% (gray bar). However, the TOC removal increased when Vis light was added to the system, which enhanced the TOC removal up to 18%. By comparing the results with Vis and the ones with Vis + H2O2, it can be concluded that dissociation of H2O<sup>2</sup> does not occur readily or does not proceed at all under visible light, and therefore the addition of hydrogen peroxide alone under visible light does not improve TOC removal. On the other hand, when the catalyst and hydrogen peroxide were added to the system, the TOC removal significantly increased up to 40 and 45% for the 1% and 2 w% Cu catalyst, respectively. This is evidence of the activity of the catalyst under visible light, which is a desirable characteristic in a catalyst for the sake of sustainability. The activity of Cu2O under visible light was expected due to its narrow bandgap (2.2 eV) [18].

under visible light was expected due to its narrow bandgap (2.2 eV) [18].

does not occur readily or does not proceed at all under visible light, and therefore the addition of hydrogen peroxide alone under visible light does not improve TOC removal. On the other hand, when the catalyst and hydrogen peroxide were added to the system, the TOC removal significantly increased up to 40 and 45% for the 1% and 2 w% Cu catalyst, respectively. This is evidence of the activity of the catalyst under visible light, which is a desirable characteristic in a catalyst for the sake of sustainability. The activity of Cu2O

**Figure 8.** Effect of visible light on TOC removal percentage. **Figure 8.** Effect of visible light on TOC removal percentage.

### **4. Conclusions**

**4. Conclusions** Cu2O nanoparticles were obtained via laser ablation in liquid (LAL) and supported on Al2O3. This material catalyzes the photo-Fenton process to remove bisphenol A (BPA) under UV light as well as under visible light at circumneutral pH, and the need for acidification is eliminated. The BPA oxidation rate is one order of magnitude higher when using the catalyst than when applying only UV light and is more than twice that achieved when H2O2 is only dissociated by light. The BPA oxidation rate is affected by the crystalline structure of the catalyst support, Al2O3, and the mineralization extent is directly related to the initial H2O2 concentration. The highest mineralization percentage (ca. 91%) was found using 1%Cu2O/γ-Al2O3, 10 times the stoichiometric amount of H2O2 at an initial Cu2O nanoparticles were obtained via laser ablation in liquid (LAL) and supported on Al2O3. This material catalyzes the photo-Fenton process to remove bisphenol A (BPA) under UV light as well as under visible light at circumneutral pH, and the need for acidification is eliminated. The BPA oxidation rate is one order of magnitude higher when using the catalyst than when applying only UV light and is more than twice that achieved when H2O<sup>2</sup> is only dissociated by light. The BPA oxidation rate is affected by the crystalline structure of the catalyst support, Al2O3, and the mineralization extent is directly related to the initial H2O<sup>2</sup> concentration. The highest mineralization percentage (ca. 91%) was found using 1%Cu2O/γ-Al2O3, 10 times the stoichiometric amount of H2O<sup>2</sup> at an initial pH of 6.7, and catalyst loading of 0.2 g/L.

pH of 6.7, and catalyst loading of 0.2 g/L. After 1 h of treatment, the remaining compounds in solution were acetic acid, acetal-After 1 h of treatment, the remaining compounds in solution were acetic acid, acetaldehyde, and in smaller quantities 4-hydroxybenzaldehyde and 4-hydroxybenzoic acid.

dehyde, and in smaller quantities 4-hydroxybenzaldehyde and 4-hydroxybenzoic acid.

**Author Contributions:** Conceptualization, O.O.-M., R.N. and K.D.; methodology, O.O.-M., R.N., K.D., S.B. and D.A.-P.; validation, O.O.-M., R.N., K.D. and S.B.; investigation, O.O.-M., R.N., K.D., S.B. and D.A.-P.; writing—original draft preparation, O.O.-M. and R.N.; writing—review and editing, O.O.-M., R.N., K.D. and S.B.; supervision, O.O.-M. and R.N.; project administration, O.O.-M. and R.N.; funding acquisition, O.O.-M. and R.N. All authors have read and agreed to the published **Author Contributions:** Conceptualization, O.O.-M., R.N. and K.D.; methodology, O.O.-M., R.N., K.D., S.B. and D.A.-P.; validation, O.O.-M., R.N., K.D. and S.B.; investigation, O.O.-M., R.N., K.D., S.B. and D.A.-P.; writing—original draft preparation, O.O.-M. and R.N.; writing—review and editing, O.O.-M., R.N., K.D. and S.B.; supervision, O.O.-M. and R.N.; project administration, O.O.-M. and R.N.; funding acquisition, O.O.-M. and R.N. All authors have read and agreed to the published version of the manuscript.

version of the manuscript. **Funding:** This research received no external funding.

**Funding:** This research received no external funding. **Data Availability Statement:** Not applicable.

**Data Availability Statement:** Not applicable. **Conflicts of Interest:** The authors declare no conflict of interest.

### **Abbreviations List and Nomenclature**


### **References**

