Synergistic Effects of UV Radiation and H₂O₂ on Chloramphenicol Degradation by REE-Based Catalysts

Abstract

The persistent occurrence of antibiotics like chloramphenicol (CAP) in aquatic systems poses serious environmental and public health risks. This study investigates the photocatalytic degradation of CAP using cerium oxide (CeO₂), lanthanum oxide (La₂O₃), and lanthanum-doped cerium oxide (Ce_xLa_yO_2−δ), synthesized via co-precipitation. The catalysts were tested under a solar simulator, UV-A, and UV-C radiation, both with and without hydrogen peroxide (H₂O₂). Structural characterization confirmed successful synthesis of nanometric catalysts, with La doping causing lattice expansion in CeO₂ and a reduction in crystallite size (from 27 nm in CeO₂ to ~20 nm in doped samples). Photolysis alone achieved limited CAP removal (~34–35%), while photocatalysis with La₂O₃ under UV-A and UV-C improved removal up to 58% and 55%, respectively. Complete degradation was obtained with La₂O₃ under UV-C in the presence of H₂O₂ within 15 min. Pareto analysis highlighted the dominant effect of the interaction between radiation and H₂O₂ (43%), while the catalyst type contributed minimally (0.23%). These findings confirm the potential of REE oxides, especially La₂O₃, in advanced oxidation processes and underscore the importance of light source and radical generation over catalyst selection alone.

1. Introduction

Over the last few decades, the contamination of aquatic ecosystems has emerged as a critical global concern. One of the most pressing issues in this context is the presence of contaminants of emerging concern (CECs) in water bodies, derived from domestic and industrial activities. CECs are defined as “naturally occurring, manufactured or anthropogenic chemicals or materials that have recently been detected or are suspected to be present in various environmental compartments”, and “are believed to pose potential risks to human health or the environment but are not yet regulated by established standards or guidelines” [1]. This broad classification includes pharmaceuticals and personal care products, endocrine-disrupting compounds, flame retardants, and pesticides. Among pharmaceuticals, antibiotics are the most frequently detected compounds [2]. Global antibiotic consumption increased by 65% between 2000 and 2015, and it is predicted to rise to 200% by 2030 relative to 2015 levels [3]. Antibiotic consumption is directly linked to the emergence of antibiotic resistance bacteria, which poses a serious and persistent threat to public health, with significant associated morbidity and mortality [4].

Conventional wastewater treatment plants, based on activated sludge processes, are generally unable to completely remove CECs, resulting in their continuous release into the aquatic environment [5]. In this context, advanced oxidation processes (AOPs) have gained attention as promising alternatives for the degradation of these persistent pollutants, allowing non-selective oxidation of organic contaminants in wastewater. Among AOPs, photocatalysis, based on the use of semiconductors under light irradiation, has proven to be an effective method for removing a wide range of CECs from aqueous matrices due to the generation of hydroxyl radicals, which are powerful oxidants able to degrade a wide range of organic contaminants [6].

In recent years, oxides of rare earth elements (REEs) have attracted significant attention as promising materials for enhancing photocatalytic efficiency. Due to their unique physicochemical characteristics, such as high surface area, reactivity, and redox potential, REEs are suitable for catalyzing the formation of reactive oxygen species (ROS) under UV irradiation [7]. Among REEs, cerium (Ce) has been the most extensively studied, followed by lanthanum (La). Cerium oxide (CeO₂) and lanthanum oxide (La₂O₃) are novel catalytic materials that exhibit remarkable photocatalytic performance due to their unique electronic and optical properties, particularly related to their 4f electronic transitions [8,9]. The reported band gaps of CeO₂ and La₂O₃ are 3.2 eV and 5.2 eV, respectively [10,11]. Their high photocatalytic activity is primarily attributed to the presence of oxygen vacancies on the surface and the efficient redox cycling between Ce⁴⁺/Ce³⁺ and La³⁺/La²⁺, which facilitates the formation and elimination of oxygen defects [12,13]. The general photocatalytic degradation mechanism of pollutants by REE oxides can be described by the following reactions [14]:

$$REEoxides+hv\to REEoxides(e_{\left(CB\right)}^{-}+h_{\left(VB\right)}^{+})$$

$${HO}^{-}+h_{\left(VB\right)}^{+}\to {HO}^{•}$$

$$O_2+e_{\left(CB\right)}^{-}\to O_2^{•-}$$

$$O_2^{•-}+H^{+}\to {HO}_2^{•}$$

$${HO}_2^{•}+{HO}_2^{•}\to O_2+H_2{O}_2$$

$$H_2{O}_2+O_2^{•-}\to {HO}^{•-}+{HO}^{•}+O_2$$

$${HO}^{•}+Organicpollutants\to Intermediates\to {CO}_2+H_{2}O+Products$$

Additionally, both Ce and La oxides can act as photo Fenton-like catalysts in the presence of hydrogen peroxide, promoting H₂O₂ decomposition and the generation of radicals that further enhance pollutant degradation [15,16].

Doping CeO₂ with La has been reported to increase photocatalytic activity. The incorporation of La³⁺ ions into the CeO₂ lattice (replacing Ce⁴⁺ ions) generates additional oxygen vacancies to maintain charge balance, thereby enhancing catalytic performance [17].

In this study, CeO₂, La₂O₃, and lanthanum-doped cerium oxide (Ce_xLa_yO_2−δ) were synthesized via a simple precipitation method and evaluated for the photocatalytic degradation of chloramphenicol (CAP), a broad-spectrum antibiotic commonly used to treat various bacterial infections ranging from conjunctivitis to meningitis. CAP has been detected in municipal sewage, wastewater treatment plant effluents, river water, and river sediment worldwide [18]. It is known for its high toxicity, with well-documented adverse effects on aquatic organisms and algae [19]. Several studies dealing with the photocatalytic degradation of CAP have been performed in the last few decades, as shown in Table S1. To the best of our knowledge, only one previous study investigated CAP removal using CeO₂ [20], and this work represents the first investigation of CAP photocatalytic degradation using La₂O₃ and Ce_xLa_yO_2−δ.

2. Results and Discussion

2.1. Characterization of REE Oxides

The SEM analysis of the CeO₂ sample revealed a compact material with a regular morphology characterized by flat regions (Figure 1A,D). The La₂O₃ samples exhibited a homogenous morphology (Figure 1B), showing the typical powder-like appearance (Figure 1E) reported in a previous study [21]. The morphology of Ce_xLa_yO_2−δ (Figure 1C) was very similar to that observed in CeO₂; however, some porous regions, indicative of particle aggregation, were also observed.

The results of the EDX analyses are reported in

Table 1. Elemental composition from EDX analysis: The results are expressed as atomic percentage (At%); standard deviations are reported for each value.

	Ce	O	La	C	Ag
CeO2	39 ± 11	53 ± 11	---	6 ± 2	2.3 ± 0.5
La2O3	---	57 ± 4	28 ± 6	14 ± 6	1.6 ± 0.3
CexLayO2−δ	37 ± 7	53 ± 8	1.4 ± 0.2	7 ± 2	2.1 ± 0.9

. In the CeO₂ sample, cerium and oxygen were the most abundant elements, with a Ce/O atomic ratio of 0.8 ± 0.4, which is higher than the expected stoichiometric value of 0.5. For the La₂O₃ samples, lanthanum and oxygen were the predominant elements, although the measured La/O ratio (0.5 ± 0.1) was lower than the expected stoichiometric value of 0.67. EDX analysis of the Ce_xLa_yO_2−δ sample revealed high concentrations of cerium and oxygen, along with a small amount of lanthanum. The stoichiometric coefficients were calculated using the elemental composition (as atomic percentage—At%) measured from the EDX analysis. More in detail, starting from the formula Ce_xLa_yO_2−δ, (i.e., the general formula of the lanthanum-doped CeO₂), the following equation was applied: y = (La At%)/(Ce At% + La At%) and x = 1 − y. The coefficient for oxygen was calculated as (2 − δ) = (100 − (Ce At% + La At%))/(O At%). The calculation was repeated for each of the spot EDX analyses performed on the sample, the results were averaged, and the standard deviations were determined. The stoichiometry coefficients resulted in x = 0.96 ± 0.01, y = 0.04 ± 0.01, and δ = 0.82 ± 0.04.

EDX analyses were performed on distinct morphological features of Ce_xLa_yO_2−δ, such as smooth areas and particulate (Figure 2). The composition of the flat regions was consistent with the average composition of the entire sample, whereas the particle aggregates displayed higher oxygen content and slightly lower cerium content. In all samples, the detected silver and carbon were attributed to the conductive silver coating and to the carbon tape used for SEM preparation, respectively.

XRD patterns of the synthesized REE oxides powders are shown in Figure 3A. The CeO₂ sample exhibited characteristic diffraction peaks at 28.51°, 33.07°, 47.43°, 56.33°, 59.06°, 69.38°, 76.66°, 79.09°, and 88.70° [22]. The XRD pattern of La₂O₃ displayed peaks corresponding to the presence of La(OH)₃ [23] (Figure 3B). The Ce_xLa_yO_2−δ sample exhibited a diffraction pattern similar to that of CeO₂; however, a slight shift in the peaks towards lower angles was observed (Figure 3C), attributed to the substitution of Ce⁴⁺ ions (ionic radius: 87 pm) by larger La³⁺ ions (ionic radius: 103 pm), resulting in lattice expansion [24].

The average crystallite size of CeO₂ was estimated to be 27 nm, while that of La₂O₃ and Ce_xLa_yO_2−δ waws approximately 20 nm. These values are consistent with previous studies employing co-precipitation method for the synthesis of cerium and lanthanum oxides [17,25,26].

2.2. Photolysis and Photocatalytic Degradation of CAP

The results of CAP photolysis under solar, UV-A, and UV-C radiation are reported in Figure 4. After 120 min, all photolysis experiments resulted in similar CAP removal efficiencies, with overall removal of 34–35%, regardless of the radiation source.

Degradation of CAP by direct photolysis is known to be challenging due to its low quantum yield (approximately 3%, independent of wavelength) [27,28]. For example, da Rocha et al. [29] reported only ~5% and ~25% removal of CAP after 2 h under solar and UV-C irradiation, respectively, using a 20 mg/L solution. Higher removal efficiencies (up to 21.8% with solar and 83.3% with UV-C) were achieved only after extended photolysis duration (up to 12 h).

Table 2. CAP removal by photolysis reported in the literature.

Initial CAP Concentration (mg/L)	Radiation	Time (min)	Removal	Reference
20	Solar	720	21.8%	[29]
	UV-C		83.3%	[29]
200	Solar	60	34%	[30]
25	UV-C	120	40%	[31]
9.69	UV-C	120	20.3%	[32]
16.16	Solar	100	21.9%	[33]
0.969	UV-A	30	20%	[34]
5	UV	60	40%	[35]
1	Solar simulator	120	34%	This study
	UV-A		34%
	UV-C		35%

summarizes relevant literature data on CAP photolysis. The removal observed in this study, using UV-C, is comparable with those observed in studies [30,31,32], where UV-C lamps were also used and the same treatment time was established. It is worth noting that the initial concentration was relatively higher (25 and 9.69 mg/L, respectively).

Figure 4 shows the CAP degradation results using REE oxides under various radiation sources. No significant CAP adsorption on the catalysts was detected during the initial 60 min dark phase.

Under solar irradiation, CAP removal reached approximately 31–33% after 120 min, suggesting that photolysis was the dominant process (Figure 4A). This observation aligns with prior reports stating that CeO₂ and La₂O₃ mainly absorb UV light [26].

Under UV-A irradiation, photocatalytic treatment led to CAP removals of 43% with CeO₂, 58% with La₂O₃, and 34% with lanthanum-doped cerium oxide after 120 min (Figure 4B). Photocatalysis under UV-C resulted in CAP removals of 49%, 55%, and 47% with CeO₂, La₂O₃, and Ce_xLa_yO_2−δ, respectively (Figure 4C).

Figure 5 presents the results obtained by combining REE oxides with H₂O₂.

In dark conditions, minimal CAP degradation was observed after 120 min, with removal rates ranging from 2% (H₂O₂/CeO₂) to 12% (H₂O₂/La₂O₃) (Figure 5A), consistent with previous findings [30,36].

Under UV irradiation, H₂O₂ photolysis generates hydroxyl radicals (HO^•), enhancing CAP degradation:

$$H_2{O}_2+hv\to 2{HO}^{•}$$

In UV-A experiments, CAP removals reached 79% with H₂O₂ alone, and 74%, 87%, and 76% with H₂O₂ combined with CeO₂, La₂O₃, and lanthanum-doped cerium oxide, respectively (Figure 5B). The presence of H₂O₂ significantly enhanced CAP removal, improving photocatalytic efficiency by 29–42%.

Under UV-C irradiation, rapid CAP degradation occurred within 30 min in the presence of H₂O₂ (Figure 5C). The higher efficiency under UV-C is due to the stronger absorption of H₂O₂ at shorter wavelengths (200–350 nm), promoting rapid photolysis [37]. Notably, complete CAP degradation was achieved within 15 min using the H₂O₂/UV-C/ La₂O₃ system.

A previous study [17] showed that a synergistic effect was observed for La-doped cerium oxide in terms of removal of methylene blue when the La percentage was 10% (removal of 99.99% after 180 min). In our case, the doping level of 10% was not achieved, suggesting that further optimization of the doping level may be needed to enhance photocatalytic efficiency.

The removal results for all treatments after 15 min are summarized in the heatmap shown in Figure 6.

In most cases, CAP degradation followed pseudo-first-order kinetics, described by:

$$ln\left({\displaystyle \frac{C_t}{C_0}}\right)=-k_{obs}t$$

(1)

where $C_t$ and $C_0$ are the CAP concentration at time $t$ and 0, respectively, and $k_{obs}$ is the observed rate constant for the photodecomposition reaction. The $k_{obs}$ values, obtained from the slope of the linear plots, are reported in

Table 3. Pseudo-first order kinetics constant for the performed experiments.

Process	kobs (min−1)	R2
Solar	0.0032	0.98
UV-A	0.0032	0.98
UV-C	0.0030	0.89
Solar/CeO2	0.0029	0.99
Solar/La2O3	0.0030	0.96
Solar/CexLayO2−δ	0.0030	0.97
UV-A/CeO2	0.0044	0.99
UV-A/La2O3	0.0067	0.98
UV-A/CexLayO2−δ	0.0034	0.98
UV-C/CeO2	0.0057	0.99
UV-CLa2O3	0.0067	1.00
UV-C/CexLayO2−δ	0.0052	0.99
UV-A/H2O2	0.1280	1.00
UV-A/H2O2/CeO2	0.1090	0.99
UV-A/H2O2/La2O3	0.0174	1.00
UV-A/H2O2/CexLayO2−δ	0.0117	1.00
UV-C/H2O2	0.2669	1.00
UV-C/H2O2/CeO2	0.2683	1.00
UV-C/H2O2/La2O3	0.4188	0.99
UV-C/H2O2/CexLayO2−δ	0.2700	1.00

.

The relative contribution of each factor and their interactions on CAP degradation was evaluated from through Pareto chart (Figure 7). The most influential factor was the interaction between the radiation source (A) and the H₂O₂ addition (C), accounting for 43% of the observed variance. This reflects the high effectiveness of UV-C combined with H₂O₂, whereas limited degradation occurred under dark conditions. The catalyst type exhibited the least significant effect (0.23%), confirming similar performances among the tested oxides.

3. Materials and Methods

3.1. Synthesis of RRE Oxides

The REE oxides were synthetized following a co-precipitation method, as described by Ref. [38]. Cerium(III) nitrate hexahydrate (Ce(NO₃)₃ · 6H₂O, CAS number: 10294-41-4, purchased by Merck) and lanthanum(III) nitrate hexahydrate (La(NO₃)₃ · 6H₂O, CAS number: 10277-43-7, purchased by Merck) were individually dissolved in 50 mL of deionized water (ρ > 5 MΩ cm, obtained by Milli-Q Elix^®, Millipore, Merck, Milan, Italy) to achieve a final concentration of 0.2 mol/L. To prepare the lanthanum-doped CeO₂ (Ce_xLa_yO_2−δ), both precursors were mixed, with Ce(NO₃)₃ accounting for 90% and La(NO₃)₃ for 10% of the total molar concentration. A doping percentage of 10% was previously reported as optimal for enhancing photocatalytic activity in methylene blue degradation [17]. The solutions were stirred with a magnetic stirrer for 30 min at room temperature. Aqueous ammonia was then added dropwise until the pH reached between 8 and 9, initiating the precipitation of the metal oxides. Stirring continued for an additional 15 min after the precipitation began. The resulting precipitate was collected in centrifuge tubes and centrifuged at 3000 rpm for 10 min, followed by three washing cycles with deionized water. The washed samples were then dried in an oven at 110 °C for 8 h and subsequently ground into fine powders. Finally, the powders were calcined at 850 °C for 2 h in air.

The surface morphology of the synthesized oxides was analyzed using a scanning electron microscope (SEM) (LEO Evo 50, Carl Zeiss AG, Oberkochen, Germany). The elemental composition of the samples was determined using an energy-dispersive X-ray spectroscopy (EDX) system (7650 INCA x-sight, Oxford Instruments, High Wycombe, UK), coupled with the SEM. Before the SEM-EDX analysis, the samples were mounted on aluminum stubs using a double-stick carbon tape and sputter-coated with silver to enhance surface conductivity and minimize charging effects. All SEM analyses were performed at an acceleration voltage of 20 kV, with a working distance of 8.5 mm and a probe current of 348 pA. SEM images were acquired using secondary electron detection. EDX data acquisition and processing were conducted using Oxford Inca software (v. 4.14). Elemental quantification was performed via spot analysis at seven different locations on each sample, and the results were averaged. The results of the EDX analyses were also used to calculate the stoichiometry coefficients of the investigated Ce_xLa_yO_2−δ compound.

X-ray diffraction (XRD) analysis was conducted to characterize the crystalline structure of the REE oxides using a D2 Phaser diffractometer (Bruker, Milan, Italy), with CuKα radiation (λ = 1.5418 Å), 2θ range of 20–90°, voltage of 30 kV, and current of 10 mA. The average crystallite size was calculated using the Scherrer equation (Equation (2)):

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

(2)

where $D$ is the average crystallite size (nm), $K$ is the particle shape factor (taken as 0.89), λ is the X-ray wavelength corresponding to the Cu-Kα irradiation, β is the full width at half maximum (FWHM) of the diffraction peaks (in radians), and θ is the Bragg diffraction angle [25,26].

3.2. Degradation Experiments

For each test, 100 mL of a 1 mg/L CAP solution were placed under stirring in a double walled beaker, with a continuous flow of water to maintain a constant temperature and prevent evaporation. Three different light sources were employed to provide irradiation: a solar simulator (I = 250 W/m², λ = 300–800 nm), a UV-A lamp (I = 28 W/m², λ = 370 nm), and a UV-C lamp (I = 5 W/m², λ = 253.7 nm). A schematic representation of the photocatalytic reactor setup is presented in Figure S1. In photolysis experiments, the solution was irradiated for 120 min without any catalysts. For photocatalytic experiments, 0.5 g/L of either CeO₂, La₂O₃, or Ce_xLa_yO_2−δ was added to the solution. Initially, the suspension was kept in the dark for 60 min to establish adsorption–desorption equilibrium, after which the selected light source was switched on. For experiments involving hydrogen peroxide (H₂O₂ 30%, CAS number: 7722-84-1, purchased by Merck), H₂O₂ was added to achieve a final concentration of 340 mg/L.

Tests were carried out using H₂O₂ alone, or in combination with 0.5 g/L of either CeO₂, La₂O₃, or Ce_xLa_yO_2−δ, under both dark and irradiated (UV-A and UV-C) conditions. Each experiment was conducted for 120 min. At predetermined time intervals (5, 10, 15, 30, 60, and 120 min), 1.5 mL aliquots were withdrawn and immediately filtered through a syringe filter with a pore size of 0.45 μm (Millipore, Merck, Milan, Italy). Sodium thiosulfate pentahydrate (Na₂S₂O₃· 5H₂O, CAS number: 10102-17-7, Sigma Aldrich, Milan, Italy) was used to quench residual H₂O₂ in the samples.

CAP concentration was analyzed using High Performance Liquid Chromatography (HPLC) (UltiMate™ 3000 Basic Automated System, Thermo Scientific, Monza, Italy), equipped with a reversed-phase C18 analytical column (Luna^®, 5 μm, 150 × 4 mm, Phenomenex, Castel Maggiore, Bologna, Italy) and a spectrophotometer as a detector. The chromatographic conditions were as follows: mobile phase consisting of 40% of an A solution (H₂O + H₃PO₄ 0.2%) and 60% of a B solution (CH₃OH), flow rate of 1.5 mL/min, column temperature of 25 °C, detection wavelength at 275 nm, injection volume of 50 µL, and a total run time of 5 min.

The effects of each factor (i.e., radiation source, catalyst, and presence of H₂O₂), as well as their interactions, were evaluated through Pareto analysis, which determined the relative importance of each factor in the removal of the target compound. The percentage contribution of each factor was calculated according to Equation (3).

$$P_i={\displaystyle \frac{b_i^2}{\sum b_i^2}}\times 100(\mathrm{if}i\ne 0)$$

(3)

where P_i is the percentage contribution of factor i, and b_i represents the estimated effect of each factor i [39].

4. Conclusions

This work demonstrated the potential of rare earth element (REE) oxides for the photocatalytic degradation of chloramphenicol (CAP), particularly under UV-assisted advanced oxidation conditions. The synthesized materials—CeO₂, La₂O₃, and Ce_xLa_yO_2–δ—were characterized by SEM-EDX and XRD. Doping CeO₂ with La resulted in smaller crystallite size and lattice expansion, confirming structural modification of the host lattice.

Photocatalytic tests revealed that photolysis alone was insufficient for CAP degradation (<35%), while UV-A and UV-C irradiation in the presence of REEs enhanced removal efficiency. Among the tested REEs, La₂O₃ under UV-C in the presence of H₂O₂ was the most effective, achieving complete CAP degradation within 15 min. However, La doping did not lead to a synergistic effect under the tested conditions, suggesting that further optimization of the La/Ce ratio is necessary.

Kinetic analysis confirmed pseudo-first-order degradation in most treatments, and Pareto analysis clearly indicated that the combination of UV radiation and H₂O₂ addition was the primary driver of CAP degradation, while the catalyst type had only a marginal effect. These findings suggest that operational parameters such as light wavelength and oxidant presence may outweigh the influence of catalyst composition—though the optimization of nanoengineered materials can play a significant role in fine-tuning the performance of AOPs.