Enhanced Diclofenac Photomineralization under Solar Light Using Ce_1−xZn_xO_2−x Solid Solution Catalysts: Synergistic Effect of Photoexcited Electrons and Oxygen Vacancies

Abstract

The present work describes the synthesis, characterization, and photomineralization activity of synthesized Ce_1−xZn_xO_2−x solid solution catalysts allowing the degradation of diclofenac as a model of anti-inflammatory medicines in water. The oxygen-deficient photocatalyst Ce_1−xZn_xO_2−x (CeZnx), produced by mixing ZnO and CeO₂, is characterized for its crystallographic parameters, specific surface area, and morphology. Photomineralization activity determination using TOC analysis shows efficient diclofenac photo-oxidation under sunlight. Moreover, the results indicate that the coexistence of Zn²⁺ and Ce⁴⁺ and the oxygen vacancies rate in CeZnx solid solution are key factors for strong drug mineralization. Ultimately, CeZn0.1, which is one of the photocatalysts synthesized in the present work, represents a cheap and efficient reagent for organic matter photomineralization in wastewater.

1. Introduction

Detection of pharmaceutical residues in aquatic environments is a major concern because of their potential impacts on ecosystems and human health [1]. Pharmaceuticals end up in wastewater in several ways: fecal excretion of medicines and their metabolites by patients; care facilities’ untreated effluent released into rivers and carried over to STEPs; and industrial effluent from pharmaceutical production sites. Although in developed countries such effluence undergoes on-site wastewater treatment before being released into rivers, in emerging countries, which produce numerous generics, no strict regulations currently exist for the removal of pharmaceuticals before their environmental release. Hence, wastewater contains various medicines, mostly antibiotics, anti-inflammatories, and analgesics. Moreover, several drug residues are undegraded in the final STEPs effluent and are released into the surface water. Sodium diclofenac is a major pharmaceutical detected in many wastewater treatment plant effluents [2]. This medicine is a nonsteroidal anti-inflammatory drug commonly used for rheumatoid arthritis and osteoarthritis [3]. It is considered a persistent toxic substance causing adverse effects on terrestrial and aquatic flora [4]. As a result, efficient and sustainable technologies such as coagulation, flocculation, electrochemical treatment, filtration, and adsorption are urgently needed to oxidize this pollutant in wastewater. However, many of these processes have high treatment costs and are inefficient for mineralizing and completely degrading the molecule.

Many advanced oxidation processes (AOPs) have been used to remove diclofenac from wastewater [3,4,5]. Among them, heterogeneous photocatalysis is efficient for diclofenac mineralization [5,6]. Moreover, it is a sustainable, effective, and nonselective water purification and disinfection method, allowing the mineralization of most organic compounds [7]. The process is based on light absorption by semiconductors such as TiO₂, ZnO, Fe₂O₃, and CdS [7,8,9,10,11]. TiO₂ is the main semiconductor used in heterogeneous photocatalysis because of its high activity, stability, and chemical inactivity. ZnO is an alternative to TiO₂ as it has similar properties, but it displays reduced production costs, higher electronic conductivity resulting in faster charge transfer of photogenerated species to the surface, and lower recombination rates than TiO₂ [12,13]. However, the photocatalytic properties of ZnO depend on particle size, morphology [14,15], and photocorrosion sensitivity, which limit its activity towards recalcitrant pollutants [16]. ZnO stability and reactivity can be improved by creating heterojunctions with other semiconductors [17,18,19,20]. Cerium oxide (CeO₂) is a good electron acceptor and excellent oxygen storage medium [21], showing a similar energy level (E_g) to ZnO [22]. In addition, this mineral is characterized by high thermal stability, abundance, nontoxicity, and low cost. It has been widely applied in water–gas conversion reactions, three-way automotive catalysts, fuel cells, and oxygen sensors [23]. Recent studies have reported potential photocatalytic activity since CeO₂ shows a UV-vis response thanks to abundant oxygen vacancies and high redox capacity, allowing the efficient oxidation of wastewater organic matter and hydrogen production using water [23].

Interfacial CeO₂-ZnO heterojunctions should be possible, due to their appropriate band structures allowing high photocatalytic efficiency with high redox capacity and efficient photoinduced charge carrier separation (Figure 1). Although some ZnO/CeO₂ composites have been developed for photocatalytic applications [24,25], identification of the photocatalytic mechanism for pharmaceutical removal in water by mineralization remains a major challenge. Intermediate product formation after photocatalysis, metal ions release, and weak bonds between the two oxides in composites appear to be limiting factors for photocatalytic applications.

The UV photocatalytic activity of CeO₂/ZnO composites has shown an efficiency of 67.4% for methylene blue degradation [26]. In addition, significant photocatalytic activity increase has been demonstrated by intraband exciton transfer in the removal of phenol and its derivatives [27]. Reports on antibiotic photomineralization using CeO₂/ZnO composites are rare and limited to the degradation of carbamazepine [28], nizatidine [29], levofloxacin [27], acetaminophen [2], and ciprofloxacin. None of these studies has specified antibiotic mineralization mechanisms on CeO₂/ZnO heterostructures, and reactions have generally been limited by toxic by-products. In most studies of heterogeneous catalysis, no measurements have confirmed the complete degradation of organic pollutants. The photodegradation of some toxic chemicals has been achieved under UV irradiation, but the few studies performed under visible light have shown poor photocatalytic efficiency. The limited mineralization activity of CeO₂/ZnO heterostructures has been linked to the nature of the heterojunctions between the two oxides and the number of radicals produced under UV light. Various techniques have been used for synthesizing ZnO/CeO₂ composites, such as hydrothermal/solvothermal and microwave synthesis, electrodeposition, precipitation, physical vapor deposition, chemical vapor deposition, micellar method, and template-assisted synthesis, etc. To stabilize Zn in CeO_2, and limit its release, a Ce_1−xZn_xO_2−x solid solution was prepared via a soft solution chemical route in the presence of citric acid as complexing agent for metal cations. This solid solution allows the construction of heterojunctions in the Ce_1−xZn_xO_2−x oxygen vacancies to enhance O₂ chemisorption and Ce⁴⁺↔Ce³⁺ electron conduction, further leading to easy photoexcited electron transfer [23]. Furthermore, abundant oxygen gaps can reduce electron-hole recombinations and enhance the photocatalytic mineralization of organic species. Moreover, photocatalytic reactions supported on ZnO, CeO₂, and their composite materials have only been carried out under ultraviolet light although there is the possibility of studying photo-excitation under visible light [23,24,25,26,27]. While most catalytic efficiencies obtained under UV are said to be good, the process is expensive, whereas the photoexcitation effect of visible/solar light is highly beneficial and can lead to significant degradation rates.

In the present work, several substitution rates of Ce⁴⁺ by Zn²⁺ in Ce_1−xZn_xO_2−x are calculated. The relationship between the photoexcitation mechanism, pollutant mineralization capacity, and photocatalyst microstructure are discussed. To the best of our knowledge, the photocatalytic behavior of either CeO₂/ZnO composites or a Ce_1−xZn_xO_2−x solid solution towards nonsteroidal anti-inflammatory drugs in water, especially sodium diclofenac, has never been described. Therefore, the present work is a pioneer study for diclofenac photomineralization by Ce_1−xZn_xO_2−x solid solution catalysts under visible light.

2. Results and Discussion

2.1. Structural Analysis

X-ray diffraction (XRD) analysis of the materials ZnO, CeO₂, and the solid solution Ce_1−xZn_xO_2−x was performed to identify the corresponding crystal structures (Figure 2). The diffraction pattern of ZnO calcined at 700 °C shows a zincite structure, while CeO₂ has a fluorine structure without any secondary phase in both cases. Combining the two oxides ZnO and CeO₂ produces the solid solution material Ce_1−xZn_xO_2−x (x = 0.1, 0.2, 0.3, 0.4) (Figure 2), with a crystalline structure typical of CeO₂. It should be noted that when the calcination temperature increases, crystallinity improves with a slight shift of the XRD peaks towards the large angles while preserving the fluorine structure. The quantity of CeO₂ being preponderant in the materials prepared, they retain the physical properties of cerium oxide. Indeed, the melting temperature of CeO₂ (2400 °C) is higher than that of ZnO (1975 °C), which suggests good thermal resistance for CeO₂ due to its higher crystallinity than ZnO. These explanations may favor the unique appearance of CeO₂ peaks on the diffractogram of Ce_1−xZnxO_2−x composite materials [23]. The variation in intensity may be due to the differences in diffusion coefficient between the two cations Ce⁴⁺ and Zn²⁺ (D_Ce > D_Zn) in the final product structure.

In order to further determine the structural characteristics, the microstrain (δ) and dislocation density (ε) were calculated using Equations (1) and (2) as follows:

$$\epsilon =\frac{\beta }{4tan\theta }$$

(1)

$$\delta =\frac{1}{D^2}$$

(2)

The average crystallite size (D) was obtained by the Debye–Scherrer Equation (3):

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

(3)

where λ is the X-ray wavelength, θ is a specific angle, β is the width at half maximum (FWHM) for the anatase peak (111), and k is the constant depending on the shape of the crystallites (k is 0.9 when the particles are spherical).

According to the results in Figure 3, the average crystallite size slightly increases with calcination temperature and the inserted Zn content. This promotes the electronic and optical properties of the Ce_1−xZn_xO_2−x solid solution, giving a periodic arrangement of Ce and Zn atoms in the fluorine crystal not found in CeO₂-ZnO composites reported elsewhere [27,30,31]. This arrangement of atoms affects the microstrain (δ) and dislocation density (ε) (

Table 1. Crystallographic parameters of synthesized materials.

		2θ (Degree)	Microstrain (ε)	Dislocation Density (δ)
CeO2		28.16	0.617	1.820
	0.1	28.58	0.589	1.505
Ce1−xZnxO2−x	0.2	27.98	0.544	1.439
	0.3	28.10	0.518	1.301
	0.4	28.61	0.438	0.783
ZnO		36.17	0.236	0.441

). Thus, a periodic heterojunction in the Ce_1−xZn_xO_2−x crystal lattice can be established.

Surface area and pore size are among the key indicators of good photocatalytic efficiency of materials. Thus, texture parameters of the different photocatalysts calcined at 700 °C were determined using nitrogen (N₂) adsorption–desorption data at 77 k (Figure 4). The mesoporosity of the surface of the studied materials was confirmed by the presence of hysteresis. As shown in

Table 2. Specific surface area (S_BET), pore volume (V_p), and average pore diameter (D_p) of the oxides ZnO, CeO₂, and CeZn0.1.

	ZnO	CeO2	CeZn0.1
SBET (m2·g−1)	0.65	6.44	8.05
Vp (cm3·g−1)	0.006	0.039	0.069
Dp (nm)	70.25	29.62	40.89

, the BET surface area of cerium oxide is 6.44 m²·g⁻¹, while the surface area of pure zinc oxide is 0.65 m²·g⁻¹. Interestingly, the surface area is higher for CeZn0.1 (S_BET = 8.05 m²·g⁻¹) than for ZnO or CeO₂. This increase could be due to better dispersion and a decrease in the ZnO size inside the CeO₂ matrix, allowing the extension of the network through hybrid bonds such as Zn-O-Ce. Pore size distribution shows average pore diameters of 30, 70, and 41 nm for CeO₂, ZnO, and CeZn0.1, respectively (Table 2).

The surface morphologies of the ZnO, CeO₂, and CeZnx oxides calcined at 700 °C were studied by SEM (Figure 5). ZnO particles are in the form of irregularly shaped elongated hexagons and quasispherical aggregated nanoparticles, while CeO₂ nanoparticles appear as amorphous agglomerates. However, the addition of ZnO to CeO₂ seems to affect the morphology and size of the resulting nanocomposites. Figure 5c,d clearly shows a decrease in the size of the elongated CeZn0.1 hexagons with a slight increase in porosity.

XPS analysis was performed to determine the surface composition as well as the oxidation state of each material. This analysis shows the presence of O-Ce (Figure 6D) and O-Zn bonds in the CeO₂ modified by ZnO. Figure 6A below shows high-resolution XPS spectra of C 1s, Zn 2p, O 1s, and Ce 3d, providing the elemental valence state and surface composition of the ZnO-CeO₂ heterojunctions. Figure 6B shows two main peaks on the high-resolution spectrum of C 1s, observed at 284.4 and 288.7 eV, which can be attributed to the binding energies of the adventitious carbon (C-C) and the C=O bond, respectively. According to Figure 6C showing the high-resolution Zn 2p spectrum, the Zn 2p core level of hexagonal ZnO exhibits two fitting peaks located at around 1044 and 1021 eV, attributed to the Zn 2p_1/2 and Zn 2p_3/2 orbitals, respectively. This result indicates that the chemical valence of Zn at the surface of the ZnO-CeO₂ material is 2 (+2 oxidation state). The difference in binding energy between the Zn 2p_1/2 and Zn 2p_3/2 orbitals is 23 eV. This confirms the presence of Zn²⁺ ions. Figure 6D shows that the deconvolution of the O 1s peak demonstrates a binding energy band at 529.93 eV, which can be attributed to O²⁻ ions in the oxygen-associated cubic structure of the lattice, while the band located at 532.14 eV can be associated with the O²⁻ ions found in oxygen-deficient regions with unusually weak coordination [31,32]. The peak at ∼528.93 eV corresponds to the binding energy of the C=O double bond. Figure 6E shows the deconvolution of the Ce3d region, with the six typical peaks characteristic of Ce3d. The peaks designated V (884.01 eV), V′ (889.3 eV), V″ (899.9 eV), U (902.66 eV), U′ (908.60 eV), and U″ (918.20 eV) have been assigned to the Ce⁴⁺ state. Here, V and U correspond to the spins of the 3d_5/2 and 3d_3/2 orbitals, respectively.

2.2. Optical Properties

The optical properties of the prepared materials were determined by UV-vis diffuse reflectance spectroscopy (DRS) (Figure 7). Depending on the Zn content in the CeZnx solid solution, a distinct red shift of the UV-vis spectra was observed. The band gap energy (E_g) of the synthesized photocatalysts was calculated using the Tauc method, based on the assumption that the energy dependence of the absorption coefficient (α) can be expressed by the following equation [31]:

$$\alpha =\frac{{A(h\nu -Eg)}^{\gamma }}{h\nu }$$

where α is the optical absorption coefficient, (hν) is calculated using the wavelength (hν = 1236/wavelength), the exponent can take the values 1/2, 2, 3/2, and 3 for directly allowed, indirectly allowed, directly forbidden, and indirectly forbidden transitions, respectively. The Tauc plot of (αhν)² as a function of hν allows the nature of the optical absorption transition to be determined.

Results indicate that the gap energies of ZnO and CeO₂ are 3.22 eV and 3.18 eV, respectively, while the band gap of CeZnx materials is approximately 2.8 eV, except for CeZn40 which has a bandgap of 2.68 eV. The narrowing of the band gap is closely related to the concentration of oxygen vacancies when Zn is inserted into the Ce_1−xZn_xO_2−x solid solution [23]. These results also indicate that CeZnx catalysts can extend the absorption slope to larger wavelengths. Consequently, these materials can be photoactivated under sunlight, which is very beneficial for improving photocatalytic mineralization properties.

2.3. Photocatalytic Mineralization of Diclofenac

The photocatalytic efficiency of Ce_1−xZn_xO_2−x solid solution materials (termed CeZnx) compared to CeO₂ and ZnO oxides was assessed for the degradation and mineralization of diclofenac in aqueous solution under UV-visible irradiation (λ_max = 300–800 nm). In the absence of the catalyst, diclofenac was stable during sunlight irradiation, and its photolysis was insignificant. It should be noted that photocatalysis is coupled to pollutant adsorption at the catalyst surface after diffuse migration close to this surface, and degradation rate is proportional to the recovery rate of active sites on the catalyst surface. Tests performed in the dark showed no significant reduction in the initial diclofenac concentration. The decrease of diclofenac concentration observed under sunlight showed the photocatalytic efficiency of the various produced photocatalysts. Lower photocatalytic activity was observed with CeZnx, solid solution materials than with pure oxides (ZnO and CeO₂) (Figure 8a). In contrast, variations in total organic carbon showed higher mineralization activity with CeZnx solid solution catalysts than with ZnO (no mineralization) and CeO₂ (less than 20% mineralization after 2 h irradiation) (Figure 8b). Optimal mineralization was observed with the Ce_0.9Zn_0.1O_1.9 catalyst, which showed about 65.50% diclofenac mineralization and photodegradation after only 2 h irradiation.

This result strongly suggests that the small fraction of Zn in Ce_0.9Zn_0.1O_1.9 is offset by the presence of vacancies acting as determining factors in the essential step of drug mineralization. Indeed, while the insertion of Zn in the CeO₂ structure leads to structural defects, mineralization increases when the amount of inserted Zn decreases and is optimal with the CeZn0.1 catalyst.

Heat treatment of Ce_0.9Zn_0.1O_0.9 has a significant effect on its photocatalytic activity (Figure 9). Catalysts calcined at 500 or 700 °C show the highest photocatalytic activity (Figure 9a) and drug mineralization (Figure 9b). The improved photocatalytic performance could be due to the heterojunction between the Ce-O and Zn-O bonds and the presence of oxygen vacancies that narrow the band gap of the CeZnx solid solution materials.

Due to the complexity of the photodegradation process, the mechanisms of action, as well as the relative roles of the different reactive species, are not yet sufficiently elucidated. In the mechanism of photocatalysis, the oxidation of organic compounds can be carried out either by oxidizing radicals (^•OH) produced by photoinduced positive charge carriers (h⁺) or directly by the latter.

It is well known that -OH radicals are not only formed via holes in the valence band (VB) but also via electrons in the conduction band (CB). When oxygen O₂ is available and adsorbed on the surface of the catalyst, it can scavenge electrons from the VB to form superoxide radicals O₂^•−. To clarify whether the degradation mechanism of diclofenac involves ^•OH or ^•O₂⁻ radicals, or holes, we performed scavenging experiments using specific inhibitors for each of these active species. In this study, benzoquinone (BQ), ammonium oxalate (OA), and isopropanol (IPA) were added to the reaction solutions as scavengers for O₂^•− radicals, h⁺ holes, and ^•OH radicals, respectively, at a concentration of 5 mmol/L before the catalyst addition. We used the same degradation process detailed earlier. After 2 h irradiation, the concentration of diclofenac decreased by 67% without scavengers and by 54, 48, and 37% with inhibition of ^•OH radicals, h⁺ holes, and O₂^•− radicals, respectively (Figure 10). These results show that the relevance of active species in diclofenac photocatalysis is as follows: O₂^•− radicals > h⁺ holes > ^•OH radicals.

In photocatalysis, the superoxide anion radical is formed by electrons and dissolved oxygen at the catalyst conduction band, while the hydroxyl radical is generated from reactions between adsorbed water (or hydroxide ions) and holes at the valence band. Holes generated by the absorption of light at an appropriate wavelength can react directly with adsorbed diclofenac but also with water, leading to competition between these two reactions. Our results show that the indirect reduction of diclofenac via superoxide radical by electrons is the main photocatalytic mechanism. These results are in agreement with Liu et al. [33,34,35] who also concluded that O₂^•− was the main reactive species for the degradation of diclofenac.

3. Materials and Methods

3.1. Materials

All reagents were of analytical grade and used without further purification. Zinc acetate dihydrate (≥99.0%), cerium (III) chloride heptahydrate (99.9%), citric acid (99%), sodium diclofenac (Aldrich 98%), and ammonium hydroxide were supplied by Sigma Aldrich (Taufkirchen, Germany).

3.2. Synthesis of ZnO, CeO₂ and Ce_1−xZn_xO_2−x Solid Solutions

Synthesis of ZnO nanoparticles was carried out by coprecipitation. First, 3 g of zinc acetate was dissolved in 20 mL of distilled water under magnetic stirring for 30 min. Then, 3.2 g of sodium carbonate was dissolved in 100 mL of water and added dropwise to the zinc acetate solution. The resulting mixture was stirred for 3 h with pH control at pH 7. The resulting white precipitate was filtered and dried in an oven at 105 °C. The obtained powder was then calcined at 700 °C in a muffle oven for 2 h. It was ground with an agate mortar and stored in an airtight container at room temperature for further analysis.

Preparation of CeO₂ was carried out by dissolving 1.5 g of cerium chloride (CeCl₃) and 0.7698 g of citric acid in 20 mL of distilled water under magnetic stirring for 30 min. Then, ammonium hydroxide (NH₄OH) was added dropwise to the mixture at room temperature. The produced precipitate was left to ripen for 3 h at room temperature and subsequently dried at 105 °C in an oven for 24 h without solid–liquid separation to preserve the metal ion content introduced into the final product. Finally, the collected powder was ground and calcined for 2 h, at temperatures between 500 and 800 °C.

The solid solution Ce_1−xZn_xO_2−x (x = 0, 0.1, 0.2, 0.3, and 0.4) materials were obtained by a similar procedure to that of pure CeO₂, except variable amounts of preformed ZnO powder were added to the solution containing Ce³⁺ ions. Finally, the samples were calcined at 500, 60, 700, and 800 °C.

3.3. Methods of Characterization

X-ray diffraction patterns were recorded using a Ragaku Miniflex I diffractometer (Rigaku Corporation, Tokyo, Japan) (CuKα cathode, λ = 0.154056 nm). Crystallite size was estimated by applying the Debye–Scherrer equation. SEM analyses were performed using an ion beam scanning electron microscope with EDX (JEOL 6700F, JEOL Ltd., Tokyo, Japan) equipped with a field emission gun with an extraction potential of 2.5 kV. The specific surface area (S_BET) was calculated using the Brunauer–Emmett–Teller (BET) equation from the physisorption of N₂ at 77 K. X-ray photoelectronic spectroscopy (XPS) measurements were performed using an ultra-high vacuum (UHV) spectrometer equipped with a VSW-class WA hemispheric electron analyzer. The X-ray source was a double Al Kᾳ (1486.6 eV) aluminum anode as incident radiation. The general high-resolution spectra were recorded in constant energy mode (100 and 20 eV, respectively). In order to correct for shifts in binding energy due to electrostatic charge, the C1s peak at 284.9 eV was used as the internal reference, characteristic of sp²-hybridized C. The background was subtracted according to the Shirley method.

3.4. Evaluation of Photocatalytic Performance

First, 100 mg of the photocatalyst (0.5 g/L) was added to sodium diclofenac solution (200 mL, 10 mg/L) in a 400 mL Pyrex beaker. The suspension was stirred in the dark for 30 min to reach the adsorption–desorption equilibrium. Then, the solution was irradiated using a SUNTEST CPS+ solar simulator with air-cooled 1500 W xenon lamp (765 W/m², λ_max = 300–800 nm). The distance between the light source and the suspension was 20 cm. At regular irradiation intervals, samples were taken with a syringe followed by filtration with 0.22 μm millipore filters and then analyzed with a UV-vis spectrophotometer (ZUZI Spectrophotometer 4211/50, Auxilab, Navarra, Spain) at 276 nm. The total organic carbon (TOC) concentration was assessed using a Shimadzu TOC-L (Shimadzu Corporation, Kyoto, Japan). To demonstrate the efficiency of the catalyst, direct photolysis of sodium diclofenac in water was performed by solar irradiation. This was carried out before any photocatalytic experiments to evaluate its contribution to the degradation of the chosen drug under the same operating conditions recommended for photocatalysis.

4. Conclusions

In the present study, ZnO, CeO_2, and solid solution Ce_1−xZn_xO_2−x photocatalysts were synthesized and characterized. X-ray diffraction showed that CeO₂ and CeZnx (x = 0.1, 0.2, 0.3, and 0.4) display a fluorine structure, while ZnO has a zincite structure without any secondary phase. The fluorine structure of CeZn0.1 remained stable after calcination between 500 and 800 °C, except for a slight shift of the lines towards large angles. In addition, XPS analysis showed several peaks attributable to the divalent character of Zn²⁺ and mainly to the quadrivalent character of Ce⁴⁺, highlighting the Zn-O and Ce-O bonds. The lower gap energies of CeZnx solids, as compared to ZnO and CeO₂, are related to the presence of vacancies and the electron transfer between the cerium atoms. A synergy between the photodegradation and mineralization processes was carried out on diclofenac under sunlight.