Removal of Tetracycline via Ultraviolet-Activated Peroxyacetic Acid: Performance and Mechanism

Abstract

To address the worsening environmental pollution caused by the large-scale release of tetracycline (TC) into the environment, this study developed an advanced oxidation system utilizing ultraviolet (UV)-activated peroxyacetic acid (PAA) for the removal of TC. The results showed that the UV/PAA system exhibited markedly enhanced performance compared to individual treatments. Under identical conditions (1.0 mM PAA, 400 W UV irradiation), the TC removal rates by PAA alone and UV irradiation alone were 25.80% and 55.05%, respectively. In contrast, the combined UV/PAA system achieved a significantly higher degradation efficiency of 79.77%, which was 3.09 times and 1.45 times higher than that of PAA and UV processes alone. This superior performance is attributed to the generation of highly reactive species within the system. The degradation process followed pseudo-first-order kinetics. An increase in TC concentration led to a decrease in degradation efficiency, whereas elevating the PAA dosage or light intensity increased the concentration of radicals in the system, thereby enhancing removal performance. Overall degradation efficiency was slightly higher under alkaline conditions compared to acidic conditions, while neutral conditions resulted in slower degradation rates. Among coexisting anions, HCO₃⁻ and H₂PO₄⁻ inhibited TC degradation, SO₄²⁻ and Cl⁻ exhibited negligible effects, and NO₃⁻ promoted the degradation of TC. Radical quenching experiments confirmed that hydroxyl radicals (·OH) were the dominant reactive species, working together with superoxide anion radicals (O₂·⁻) and singlet oxygen (¹O₂) to drive TC degradation in the UV-activated PAA system. Experiments conducted in real water matrices demonstrated that the system could effectively degrade TC in ultrapure water, tap water, and campus lake water, highlighting its strong environmental adaptability. These findings provide both technical support and a theoretical foundation for the treatment of antibiotic pollutants.

1. Introduction

With the increasing annual consumption of pharmaceuticals, antibiotics and their metabolites are entering aquatic environments through pathways such as domestic sewage and medical wastewater [1,2,3]. Among these, tetracycline antibiotics—including tetracycline (TC), chlortetracycline (CTC), oxytetracycline (OTC), and others—are widely used in medical, aquaculture, livestock, and related industries [4]. Currently, tetracycline antibiotics have been detected in water bodies worldwide [5]. The continuous accumulation of antibiotic residues in water and soil can trigger the spread of antibiotic resistance genes (ARGs) and cause ecotoxicological effects, posing serious threats to environmental safety and human health [6,7]. For instance, tetracycline antibiotics can impair the membrane permeability of freshwater green algae, weakening the uptake and transport of cellular materials and ultimately inhibiting algal growth [8]. Exposure of fish embryos to low concentrations of tetracycline has been shown to significantly reduce hatching rates and increase the incidence of developmental deformities in larvae [9]. The persistent release and accumulation of tetracycline antibiotics in the environment also promote the proliferation of antibiotic-resistant bacteria (ARB) and ARGs [10,11], which may diminish the therapeutic efficacy of drugs against pathogens and pose risks to aquatic ecosystems and human health [12,13]. Therefore, developing environmentally friendly and cost-effective technologies for tetracycline removal is of great significance for safeguarding water environment security and public health.

Peracetic acid (PAA) is an organic peroxyacid with strong disinfection capabilities and a relatively high redox potential (1.06–1.96 V), enabling it to directly oxidize and degrade certain organic pollutants in water with notable selectivity [14,15]. To further enhance its pollutant removal efficiency, researchers have explored various activation methods involving external energy input or catalysts [16,17]. Energy-based activation primarily includes thermal and radiation (e.g., ultraviolet, UV) approaches, while catalytic activation mainly involves metal-based and carbon-based catalysts [18,19,20]. Daswat and Mukhopadhyay demonstrated the use of UV-activated PAA for chlorophenol removal, finding that parameters such as UV intensity and PAA concentration influenced degradation efficiency in industrial chlorophenol-containing wastewater [21]. Rizzo et al. reported that PAA could be applied in tertiary municipal wastewater treatment, with UV/PAA exhibiting higher efficiency than solar/PAA in reducing the release of emerging contaminants and antibiotic-resistant bacteria into ecosystems [22]. Yao et al. investigated the degradation of tetracycline hydrochloride via ultrasound-activated PAA, showing that ultrasonic irradiation at 1625 W/L effectively activated PAA, achieving 99.4% degradation within 30 min, where CH₃COO· and ¹O₂ were identified as the primary reactive species [23]. Among these methods, UV activation is widely adopted due to its operational simplicity, absence of additional chemical activators—which avoids secondary pollution risks—and its low-carbon, environmentally friendly operation [24]. However, there is still a lack of comprehensive and in-depth understanding of how complex water matrix components (such as specific anions and natural organic matter) interfere with the UV/PAA system [25,26].

This study focuses on tetracycline (TC) as the target contaminant and aims to systematically evaluate the degradation efficiency of TC in a UV/PAA system. The influences of key operational parameters—including TC concentration, PAA dosage, UV intensity, solution pH, common anions, and real water matrices—on TC removal performance are investigated. In addition, the quenching agent method, which is easy to operate and cost-effective, was selected to elucidate the degradation mechanism of TC in the UV/PAA system.

2. Materials and Methods

2.1. Chemicals

Tetracycline hydrochloride (C₂₂H₂₄N₂O₈·HCl, TC, 99%) was obtained from Merck. Sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O), sodium sulfate (Na₂SO₄), sodium bicarbonate (NaHCO₃), sodium chloride (NaCl), sodium dihydrogen phosphate dihydrate (NaH₂PO₄·2H₂O), sodium nitrate (NaNO₃), peracetic acid solution (C₂H₄O₃), manganese(II) sulfate monohydrate (MnSO₄·H₂O), sulfuric acid (H₂SO₄), ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O), potassium iodide (KI), potassium permanganate (KMnO₄), methanol (CH₄O), ascorbic acid (C₆H₈O₆), and histidine (C₆H₉N₃O₂) were all of analytical grade and supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water used in the experiments was produced by a Molresearc 1010A water purification system (Shanghai Mole Scientific Instrument Co., Ltd., Shanghai, China). All other chemicals employed were of guaranteed reagent (GR) grade or higher.

2.2. Apparatus

An ultraviolet–visible spectrophotometer (752 N, Shanghai Yidian Analytical Instrument Co., Ltd., Shanghai, China), electronic balance (DHG-9023A, Shanghai Precision Laboratory Equipment Co., Ltd., Shanghai, China), photochemical reaction instrument (HF-GHX-VI, Shanghai Hefan Instrument Co., Ltd., Shanghai, China), digital pH meter (PHS-3E, Shanghai Yidian Scientific Instrument Co., Ltd., Shanghai, China), digital magnetic stirrer (ZGCJ-3A, Shanghai Zigui Instrument Co., Ltd., Shanghai, China), ultrapure water system (UPT-11-40, Chengdu UP Instrument Co., Ltd., Chengdu, China), micropipettes (100–1000 μL, Thermo Scientific, Waltham, MA, USA), and electric thermostatic drying oven (Model 202, Beijing Yongguangming Medical Instrument Co., Ltd., Beijing, China) were used.

2.3. Experimental Procedure

In each experiment, 100 mL of tetracycline solution (20 mg L⁻¹) was placed in a 100 mL quartz test tube. After switching on the mercury lamp (the main emission wavelength is 365 nm) and adjusting the power to 400 W, 1 mM peracetic acid (PAA) solution was added. The distance between the light source and the reaction solution is 100 mm. The working voltage is 110 ± 10 V, and a current controller controls the power. The working current varies with the change in power in the experimental conditions. The main emission wavelength of the UV lamp is concentrated at 365 nm. The emission spectrum of the light was shown in Figure S1. Samples were withdrawn at reaction times of 0, 15, 30, 45, and 60 min and immediately quenched by adding 0.2 mL of sodium thiosulfate solution (0.1 mol L⁻¹). The potential environmental impact or toxicity of tetracycline solution after oxidative degradation process was evaluated by luminescent bacterial toxicity assay (Vibrio fischeri).

2.4. Analytical Methods

The concentration of peracetic acid (PAA) was determined by titration according to the Chinese National Standard method for peroxyacetic acid solutions (GB/T 19104-2021 [27]). The concentration of tetracycline (TC) was measured using a UV-Vis spectrophotometer by recording the absorbance at 355 nm. The absorbance values of TC before and after the reaction were obtained at this wavelength and then converted into concentration values using the linear regression equation derived from the TC calibration curve. The removal efficiency (%) of TC by PAA was calculated based on the change in TC concentration before and after degradation:

$$\mathrm{R}={\displaystyle \frac{C_0-C_t}{C_0}}\times 100\%$$

(1)

where R represents the removal efficiency of TC, C₀ is the initial TC concentration, and C_t is the TC concentration at reaction time t.

3. Results and Discussion

3.1. Removal Efficiency of TC in Different Systems

The degradation performance of TC under individual UV irradiation, individual PAA treatment, and the combined UV/PAA system was evaluated at an initial TC concentration of 20 mg L⁻¹, a PAA dosage of 1 mM, and a light intensity of 400 W (As shown in Figure 1). The results indicate that the degradation efficiency of TC in the PAA-only system was 25.80%. Photolysis of TC occurred under UV irradiation alone, leading to a degradation efficiency of 55.05% in the UV-only system. When PAA was introduced into the UV system, the degradation efficiency of TC increased to 79.77%. The results indicate that UV/PAA has a good synergistic effect. In the UV/PAA system, reactive radicals such as ·OH are generated, which enhance the oxidative capacity and consequently improve the degradation of TC [26,28]. The initial pH of the TC solution was approximately 5.0. After the addition of PAA, the pH dropped to ~3.5 due to the acidity of the PAA solution. The pH value remained stable at 3.5 during the reaction process and did not fluctuate significantly over time. The mineralization rate of the reaction solution is approximately 30.4% after 60 min. The results of UV-Vis spectroscopic (Figure S2) analysis show that the characteristic absorption peaks of tetracycline (357 nm and 275 nm) attenuated rapidly and even disappeared in the initial stage of the reaction (within 15 min), which confirms the destruction of its conjugated chromophore structure [4]. Meanwhile, the persistent absorbance signal in the low-wavelength region indicates that transformation by-products with different structures were generated during the degradation process, rather than direct mineralization [5]. The results of the toxicity test (Vibrio fischeri) showed that inhibition rates of tetracycline solution after oxidative degradation at 0 min, 15 min, 30 min, 45 min, and 60 min are 43%, 49%, 47%, 45%, and 35%, respectively. There is a trend of toxicity increasing first and then decreasing. Meanwhile, the TOC analysis results showed that TC was not completely mineralized during the oxidation process (about 30%) (Figure S4). This toxic change may be caused by intermediate products in the reaction.

3.2. Effect of PAA Dosage on TC Removal

The influence of PAA dosage on TC removal was investigated under fixed conditions: TC concentration of 20 mg L⁻¹ and a light intensity of 400 W. As shown in the corresponding Figure 2, the degradation efficiency of TC increased steadily with increasing PAA dosage. At PAA dosages of 0.5, 1.0, 1.5, and 2.0 mM, the TC degradation efficiencies reached 70.65%, 79.71%, 85.40%, and 90.26%, respectively. The degradation kinetics followed pseudo-first-order behavior, and the apparent rate constant (k_obS) increased from 0.0201 min⁻¹ to 0.0375 min⁻¹ as the PAA dosage rose. The higher PAA dosage contributed to greater generation of reactive species, thereby enhancing both the degradation rate and overall removal efficiency of TC [29].

3.3. Effect of TC Concentration on Removal Performance

Experiments were conducted with a fixed PAA dosage of 1 mM and a light intensity of 400 W, while the TC concentration was varied at 10, 20, 30, and 40 mg L⁻¹. The results (Figure 3) indicated that higher TC concentrations significantly constrained the degradation capacity of the system. The corresponding degradation efficiencies were 87.91%, 79.85%, 66.37%, and 57.11%, respectively. The apparent rate constant k_obS decreased from 0.0345 min⁻¹ at 10 mg L⁻¹ TC to 0.0138 min⁻¹ at 40 mg L⁻¹ TC. At elevated TC concentrations, the limited PAA availability hindered the generation of sufficient radicals for complete degradation, and the radical formation rate could not meet the demand for full TC degradation, ultimately leading to reduced degradation efficiency and rate. These findings highlight the need to adjust the PAA dosage according to the actual pollutant concentration in water to ensure that radical production matches the contaminant load, thereby achieving efficient degradation [30].

3.4. Effect of Light Intensity on TC Removal

When the TC concentration was 20 mg L⁻¹ and the PAA dosage was 1 mM, light intensities of 200 W, 300 W, 400 W, and 500 W were applied (Figure 4). The corresponding TC degradation efficiencies were 43.23%, 55.81%, 79.50%, and 89.82%, respectively. Under high light conditions of 500 W, when the reaction time is extended to 120 min, the removal rate can be increased to 98.85% (Figure S3). As the light intensity increased from 200 W to 500 W, the apparent rate constant (k_obS) rose from 0.0088 min⁻¹ to 0.0377 min⁻¹. Thus, higher light intensity resulted in more effective TC removal and faster reaction rates in the UV/PAA system. This enhancement is attributed to the greater photon energy available at higher light intensities, which increases the number of PAA molecules activated per unit time, accelerates PAA photolysis, and promotes the generation of highly oxidative radicals such as ·OH, thereby strengthening the oxidative capacity and advancing the degradation process [31]. In order to balance the degradation performance with the economic feasibility of practical operation, this study ultimately determined 400 W as the optimal condition.

3.5. Effect of Anions on TC Removal

The influence of various anions on TC degradation in the UV/PAA system was evaluated under the following conditions: TC concentration of 20 mg L⁻¹, PAA dosage of 1 mM, and light intensity of 400 W (Figure 5). After adding 1 mM of SO₄²⁻, HCO₃⁻, Cl⁻, H₂PO₄⁻, or NO₃⁻, the TC degradation efficiencies were 76.56%, 76.86%, 75.91%, 70.52%, and 85.11%, respectively. The corresponding k_obS values were 0.0235 min⁻¹, 0.0206 min⁻¹, 0.0237 min⁻¹, 0.0201 min⁻¹, and 0.0302 min⁻¹. NO₃⁻ clearly promoted TC degradation, while the other anions exhibited inhibitory effects, with the inhibition strength following the order: H₂PO₄⁻ > Cl⁻ > HCO₃⁻ > SO₄²⁻. As a photosensitive compound, NO₃⁻ can be activated under UV irradiation to generate ·OH, which works synergistically with other reactive species to enhance TC degradation. SO₄²⁻ showed a minor impact, likely because it does not readily react with ·OH. The decrease in degradation efficiency upon adding Cl⁻ is due to its reaction with ·OH to form ClOH·⁻, which reduces the available ·OH concentration in the UV/PAA system [32]. The addition of HCO₃⁻ lowered both degradation efficiency and reaction rate, probably because HCO₃⁻ acts as a ·OH scavenger, diminishing the system’s oxidative capacity [33,34]. Among the anions tested, H₂PO₄⁻ resulted in the slowest degradation rate because ·OH reacts with H₂PO₄⁻ to produce HPO₄·⁻, a less reactive radical that consumes ·OH without contributing significantly to degradation.

3.6. Effect of Solution pH on TC Removal

The influence of solution pH on TC degradation in the UV/PAA system was examined under the following conditions: TC concentration of 20 mg L⁻¹, PAA dosage of 1 mM, and light intensity of 400 W (Figure 6). At pH 3.0, 5.0, 7.0, and 9.0, the TC degradation efficiencies were 72.82%, 72.35%, 69.21%, and 75.61%, respectively, with corresponding apparent rate constants (k_obS) of 0.0213 min⁻¹, 0.0210 min⁻¹, 0.0190 min⁻¹, and 0.0234 min⁻¹. Removal efficiency was slightly higher at pH 9.0 than at pH 3.0 or 5.0, while the neutral condition (pH 7.0) showed the lowest performance. At pH > 8.2, PAA undergoes simultaneous decomposition and hydrolysis, yielding acetic acid and H₂O₂. The UV/H₂O₂ system also generates ·OH and other reactive radicals, further contributing to TC removal. Additionally, TC can undergo epimerization and degradation under alkaline conditions (pH 9.0), accelerating its dissociation. Under acidic conditions, PAA hydrolysis is limited, resulting in only slight inhibition of TC degradation. The slower degradation observed at neutral pH is attributed to faster hydrolysis of PAA, which reduces the generation of reactive radicals.

3.7. Effect of Real Water Matrices on TC Removal

Differences in water composition may affect the degradation efficiency of TC. Experiments were performed under the same operational conditions (20 mg L⁻¹ TC, 1 mM PAA, 400 W light intensity) using ultrapure water, campus lake water (Central South University of Forestry and Technology, Changsha, China), and tap water (Changsha, China). The k_obS values were 0.0255 min⁻¹, 0.0234 min⁻¹, and 0.0214 min⁻¹, with corresponding degradation efficiencies of 79.21%, 75.86%, and 73.40%, respectively (Figure 7). In all real water matrices, TC removal exceeded 70%, indicating that the system maintains robust performance under varying environmental conditions and possesses a degree of resistance to matrix interference. Compared to ultrapure water, the degradation efficiency was slightly reduced in tap water and campus lake water, likely due to the complex composition of natural waters—such as natural organic matter, inorganic ions, and suspended solids—which can scavenge reactive radicals formed in the UV/PAA system. Common inorganic anions (e.g., HCO₃⁻, Cl⁻) present in natural water bodies may inhibit the activity of radicals and affect the stability of the reaction.

3.8. Investigation of UV/PAA Reaction Mechanism

To elucidate the reaction mechanism, 5 mM L-histidine (L-His), 5 mM ascorbic acid (Asa), and 1 M methanol (MeOH) were separately added to the system as scavengers for specific reactive species (Figure 8). Methanol effectively quenches ·OH and can be used to identify its presence, although its addition may accelerate PAA decomposition to some extent. Ascorbic acid scavenges superoxide radical (O₂·⁻), while L-histidine quenches singlet oxygen (¹O₂). In the control experiment (without scavenger), the degradation efficiency was 79.20% with a rate constant of 0.0251 min⁻¹. Upon addition of methanol, L-histidine, and ascorbic acid, the degradation efficiencies decreased to 45.64%, 54.85%, and 49.15%, with rate constants of 0.0102 min⁻¹, 0.0128 min⁻¹, and 0.0110 min⁻¹, respectively. These results indicate that ·OH serves as the primary reactive species responsible for TC degradation in the UV/PAA system, with contributions from ¹O₂ and O₂·⁻ collectively driving the degradation of TC under UV-activated PAA conditions. Under acidic conditions in this study, PAA exists predominantly in its molecular form, and ·OH is mainly derived from its direct photolysis. The generated ·OH can further induce the formation of O₂·⁻ and ¹O₂ through chain reactions. Under neutral/alkaline conditions, the PAA anion (CH₃C(O)OO⁻) is dominant, and its activation pathway results in a distinct distribution of primary free radicals and subsequent ROS generation. Unlike most studies conducted under neutral/alkaline conditions [6], where the generation of reactive oxygen species (ROS) relies primarily on the activation of ionic PAA⁻, interactions with transition metals, or reactions with background matrices, this work uncovers a distinctly different dominant pathway under acidic conditions: ROS are mainly derived from the direct photolysis of molecular PAA, and are subsequently proliferated and transformed through ·OH-induced chain reactions and free radical disproportionation processes.

$${\mathrm{CH}}_3\mathrm{C}\left(\mathrm{O}\right)\mathrm{OOH}\stackrel{\mathrm{h}\mathsf{\nu }}{\to }{\mathrm{CH}}_3\mathrm{C}\left(\mathrm{O}\right)·+·\mathrm{OH}$$

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

$${\mathrm{HO}}_2^{·}\rightleftharpoons {\mathrm{O}}_2^{·-}+{\mathrm{H}}^{+}$$

$$2{\mathrm{O}}_2^{·-}+2{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2{+}^1{\mathrm{O}}_2$$

$$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\left(\mathrm{O}\right)\mathrm{OOH}+\mathrm{O}{\mathrm{H}}^{-}\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{C}\left(\mathrm{O}\right)\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}$$

$${\mathrm{CH}}_3\mathrm{C}\left(\mathrm{O}\right){\mathrm{OO}}^{-}\stackrel{\mathrm{hv}/·\mathrm{OH}}{\to }{\mathrm{CH}}_3\mathrm{C}\left(\mathrm{O}\right)·+{\mathrm{O}}_2^{·-}/·\mathrm{OH}$$

(2)

4. Conclusions

(1)

The UV/PAA system demonstrated significantly enhanced degradation efficiency for TC compared to individual UV or PAA treatment, achieving a removal rate of up to 79%. This confirms that the synergistic interaction between UV and PAA enhances oxidative capacity and enables efficient TC decomposition.

(2)

As the TC concentration increases, degradation becomes more challenging. Higher PAA dosage and greater light intensity promote the generation of reactive radicals (e.g., ·OH), thereby accelerating the degradation rate and improving removal efficiency.

(3)

The solution pH influences the stability of PAA and consequently affects degradation performance. Under neutral pH conditions, PAA decomposition leads to relatively lower radical generation efficiency, resulting in slower degradation rates.

(4)

NO₃⁻ promotes TC degradation in the system, as it can be photoactivated under UV irradiation to produce reactive radicals such as ·OH, thereby enhancing degradation efficiency. Other anions inhibit the reaction system, with the order of inhibitory effect being: H₂PO₄⁻ > Cl⁻ > HCO₃⁻ > SO₄²⁻.

(5)

Radical quenching experiments indicate that ·OH serves as the primary reactive species responsible for TC degradation in the UV/PAA system, working together with ¹O₂ and O₂·⁻ to drive the degradation of TC under UV-activated peracetic acid conditions.

In addition, identification of reaction intermediates and in-depth ecological toxicity testing are necessary in subsequent work. This study lays a scientific foundation for the application of advanced oxidation technology based on PAA in water pollution control.