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Article

Ultrasonic-Cavitation-Enhanced Biodegradation of Ciprofloxacin: Mechanisms and Efficiency

by
Qianheng Wen
1,
Qiwei Peng
2,
ThuThi Pham
2 and
Xiwei He
2,*
1
School of Geography and Environmental Science, University of Southampton, Southampton SO17 1BJ, UK
2
School of Environment, Jiangsu Engineering Lab of Water and Soil Eco-Remediation, Nanjing Normal University, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(16), 2495; https://doi.org/10.3390/w17162495
Submission received: 11 July 2025 / Revised: 13 August 2025 / Accepted: 19 August 2025 / Published: 21 August 2025
(This article belongs to the Special Issue Application of Microbial Technology in Wastewater Treatment)
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Abstract

Ciprofloxacin (CIP), a persistent fluoroquinolone antibiotic, poses serious environmental concerns due to its low biodegradability and widespread presence in aquatic ecosystems. This study investigates the synergistic application of low-frequency ultrasonic cavitation and biological treatment to enhance CIP removal efficiency. Experiments have shown that under the optimal biological treatment conditions (6 g/L sludge concentration, pH 8), single biological treatment for 48 h can only remove 41.9% CIP and 24.9% total organic carbon (TOC). Ultrasonic pretreatment was conducted under varying frequencies and pH conditions to determine optimal cavitation parameters, while biodegradation performance was evaluated at different sludge concentrations and pH levels. Results indicated that in 10 mg/L CIP wastewater under alkaline conditions (pH 9.0), CIP and TOC removal efficiencies reached 58.9% and 35.2%, respectively, within 30 min using 15 kHz ultrasound irradiation. When ultrasonic pretreatment was followed by biological treatment, overall removal rates increased to 96.3% for CIP and 90.4% for TOC, significantly outperforming either method alone. LC-MS analysis identified several degradation intermediates during ultrasonic pretreatment, revealing key transformation pathways such as piperazine ring cleavage, hydroxylation, and defluorination. Furthermore, toxicity evaluation using the T.E.S.T. model confirmed a substantial reduction in ecological risk after ultrasonic treatment. Overall, the combined ultrasonic–biological process offers a cost-effective and environmentally sustainable strategy for the efficient removal of fluoroquinolone antibiotics from wastewater.

1. Introduction

Ciprofloxacin (CIP), a broad-spectrum and highly effective fluoroquinolone antibiotic, is widely used in clinical medicine, animal husbandry, and aquaculture [1,2]. However, the presence of CIP in water environments poses serious environmental risks [3,4]. Its stable molecular structure resists natural degradation, leading to long-term accumulation [5]. These residues can disrupt microbial communities, promote antibiotic resistance, and bioaccumulate through the food chain, ultimately threatening human health [6,7]. Studies have shown that CIP exhibits significant developmental toxicity to aquatic organisms [8]. Moreover, the public health crisis caused by antibiotic resistance is projected to result in over 10 million deaths globally each year by 2050 [9,10]. Therefore, developing efficient technologies to remove residual CIP from water is critically important for controlling antibiotic resistance, maintaining ecosystem stability, and ensuring drinking water safety.
The traditional biodegradation method, as a key technology for treating antibiotic wastewater, has been widely used due to its low cost and environmental friendliness [11,12]. However, CIP’s highly stable quinolone ring and strong antibacterial properties significantly inhibit microbial metabolism [13,14]. As a result, conventional biological treatments exhibit low degradation efficiency for CIP [15,16]. Research indicates that CIP removal rates in typical activated sludge systems generally remain below 40%, with relatively long degradation times, which limits practical applications [17,18]. Therefore, single biological treatments struggle to effectively remove CIP, necessitating enhanced pretreatment or combined technologies to improve overall degradation performance.
Ultrasonic cavitation technology leverages the synergistic effects of mechanical forces and cavitation to efficiently disrupt the stable functional groups in CIP molecules [19,20]. The mechanical effect induces intense vibration of water molecules, accelerating molecular displacement and collisions that physically damage CIP structures [21,22]. Meanwhile, cavitation generates localized high-temperature and high-pressure microenvironments during bubble collapse. This, combined with strong oxidative free radicals such as hydroxyl radicals (·OH), leads to chemical bond cleavage in CIP [23,24,25,26]. Under optimized conditions, CIP degradation rates can exceed 85% within 60 min [27,28]. However, individual ultrasonic degradation may produce various organic intermediates, which, if discharged untreated, pose potential secondary pollution risks [29,30]. To address this, a combined ultrasonic–biological treatment process has been proposed. This strategy first employs ultrasonic cavitation to efficiently break down CIP molecules and eliminate their antibacterial properties, followed by biological treatment to further degrade and mineralize residual intermediates [28]. This combined approach significantly improves overall removal efficiency while reducing system energy consumption. It offers a cost-effective and environmentally friendly solution for managing antibiotic pollution in water bodies.
Currently, research on combined ultrasonic cavitation and biological degradation of CIP remains limited, particularly regarding process parameter optimization and elucidation of synergistic degradation mechanisms. Therefore, this study aims to explore the degradation performance of ultrasound–biological synergistic treatment of CIP under different ultrasound frequencies and pH conditions. It also further analyzes the degradation intermediates and potential ecological risks, with the goal of providing theoretical support and practical references for the engineering application of this process.

2. Materials and Methods

2.1. Chemicals

Ciprofloxacin (analytical grade, ≥98%), sodium hydroxide (analytical grade, ≥96%), and hydrochloric acid (analytical grade, 36.0–38.0%) were purchased from China National Pharmaceutical Group Corp. (Beijing, China). Chromatographic-grade formic acid and acetonitrile were obtained from Agilent Technologies (Santa Clara, CA, USA). All reagents were used as received without further purification. Solutions were prepared with deionized water.

2.2. Experimental Procedure

2.2.1. Biodegradation

The activated sludge used in the experiments was collected from the aerobic tank of a municipal wastewater treatment plant and pretreated through sedimentation and short-term acclimation to preserve its biological activity. All experiments were carried out in 250 mL Erlenmeyer flasks, each containing 100 mL of synthetic wastewater spiked with CIP at an initial concentration of 10 mg/L. To simulate conventional biological treatment conditions, the flasks were placed on a thermostatic shaker and incubated at room temperature (approximately 25 ± 1 °C) with continuous shaking at 150 rpm. In biological treatment processes, sludge concentration and pH are critical factors influencing the degradation efficiency of organic compounds. These parameters affect overall treatment performance by modulating microbial abundance and metabolic activity. To systematically assess their impact, this study conducted a series of comparative experiments with sludge concentrations ranging from 1.5 to 6.0 g/L and pH values from 5 to 9, based on typical operational conditions in municipal wastewater treatment plants. Water samples were collected at designated time intervals to measure CIP concentrations and total organic carbon (TOC) levels. All degradation experiments were conducted through three independent parallel trials. The data are presented as mean values, with error bars representing the standard deviation to indicate the degree of data dispersion.

2.2.2. Ultrasonic Degradation

To systematically investigate the effects of ultrasonic conditions on the degradation behavior of CIP, this study designed ultrasonic degradation experiments varying ultrasonic frequency (15–30 kHz) and initial pH (4–10), aiming to evaluate CIP degradation efficiency and organic mineralization capacity under different reaction conditions. The ultrasonic system comprises an ultrasonic generator (Jinan Precision Ultrasound Equipment Co., Ltd., Jinan, Shandong, China), a transducer (Hangzhou Success Ultrasound Technology Co., Ltd., Hangzhou, Zhejiang, China), and a titanium alloy probe (Xi’an Sonacell Ultrasonics Co., Ltd., Xi’an, Shaanxi, China), with a rated output power of 600 W and a frequency adjustable within the low-frequency range to meet the requirements for strong cavitation effects. The duty cycle was set at 66.7% to maintain a dynamic balance between cavitation bubble formation and collapse, thereby enhancing ultrasonic energy utilization efficiency [31]. During the experiments, the ultrasonic probe was vertically fixed at the center of a 5 L quartz reactor. To control the system temperature, a condenser device was externally connected to the transducer to stabilize the temperature throughout the operation and prevent cavitation efficiency fluctuations caused by temperature increases.
In the reaction system, the initial CIP concentration was set at 10 mg/L, with an effective reaction volume of 3 L. The actual ultrasonic intensity within the system was determined to be 300 W/cm2 by calorimetric measurement, with calorimetric efficiency variations not exceeding ±2% across different temperatures [32,33]. Samples were collected every 10 min during the reaction to measure CIP concentration and TOC levels, thereby assessing both degradation and mineralization. To exclude potential errors caused by volatile losses during ultrasonic treatment, a portable volatile organic compound (VOC) detector (PGM-7300, RAE Systems, San Jose, CA, USA) was used for periodic monitoring of the gas phase above the reactor. Results indicated VOC concentration fluctuations within ±5%, suggesting negligible volatilization of organics during the experiments. To ensure the reliability and reproducibility of the results, all experiments were conducted in triplicate, followed by statistical analysis.

2.2.3. Ultrasound-Assisted Biological Degradation

Although both biological treatment and ultrasonic cavitation can be independently applied for pollutant removal, they often face limitations in practical applications, such as low degradation efficiency, extended reaction times, and incomplete mineralization. To address these challenges, this study developed a combined process integrating ultrasonic cavitation pretreatment with biological degradation, aiming to improve the overall removal efficiency of CIP. CIP-containing wastewater was first subjected to ultrasonic pretreatment under optimized conditions (treatment duration: 30 min; frequency: 15 kHz; pH: 9). The pretreated solution was subsequently transferred to a biological treatment stage (operational parameters: 24 h treatment duration, 6 g/L sludge concentration, pH 8) utilizing activated sludge for the further removal of residual contaminants. This combined process was designed to systematically evaluate the feasibility and effectiveness of the ultrasound–biological degradation pathway in enhancing CIP removal performance.

2.3. CIP Concentration and TOC Analysis

The concentration of CIP was determined by measuring absorbance at its characteristic wavelength of 277 nm using a UV-9000 ultraviolet spectrophotometer. Concentrations were calculated based on a standard calibration curve. TOC content was measured with a TOC analyzer (Multi N/C 3100, Analytik Jena, Jena, Germany).

2.4. Degradation Product Analysis

Mass spectral information of CIP degradation products was obtained using a high-performance liquid chromatography–mass spectrometry system (ZQ2000, Waters, Milford, MA, USA) equipped with an electrospray ionization (ESI) source [34,35]. Samples were separated on a Waters Sunfire C18 column (4.6 × 100 mm, 3.5 μm particle size) maintained at 30 °C, with an injection volume of 10 μL. The mobile phase consisted of 1.0% formic acid in water (A) and 1.0% formic acid in acetonitrile (B), applied using a gradient elution at a flow rate of 0.8 mL/min. The gradient program was as follows: 0–7 min, 95% A/5% B; 7–12 min, 5% A/95% B; 12–12.2 min, 5% A/95% B; 12.2–15 min, 95% A/5% B. Mass spectrometry analysis was performed in positive ion mode, scanning over an m/z range of 50 to 1500. The operating parameters were set as follows: ion source temperature at 140 °C, desolvation temperature at 350 °C, capillary voltage at 3000 V, cone voltage at 10 V, extractor voltage at 5 V, RF lens voltage at 0.3 V, desolvation gas flow rate at 800 L/h, and cone gas flow rate at 50 L/h.

2.5. Toxicity Assessment

The Toxicity Estimation Software Tool (T.E.S.T., version 5.1.2) utilizes big data processing and statistical analysis to identify potential toxic groups within target compounds [36]. Users can input compound information by entering the CAS number or manually drawing the chemical structure. Subsequently, they select the prediction endpoints and methods and specify the output path. Upon completion of the prediction, the software automatically generates a detailed results report.

3. Results and Discussion

3.1. Effects of Different Factors on the Biodegradation Efficiency of CIP

3.1.1. Effect of Sludge Concentration

Figure 1a presents the time-dependent degradation profiles of CIP under different sludge concentrations. A certain degree of CIP removal observed in the inactivated sludge group during the first 4 h was likely due to physical adsorption. In contrast, the activated sludge groups showed significantly higher degradation efficiencies, which increased with sludge concentration. After 48 h, CIP removal rates reached 22.8%, 31.6%, and 36.9% for the low, medium, and high concentration groups, respectively. Previous studies have reported that elevated sludge loads can raise microbial density and enzyme activity, thereby accelerating the degradation of antibiotic contaminants [5]. Moreover, higher sludge concentrations may provide more extracellular polymeric substances (EPSs), which improve CIP adsorption and capture and promote its transport into microbial metabolic pathways [37]. Therefore, it can be concluded that biodegradation was the dominant mechanism of CIP removal in this system. Sludge concentration played a crucial regulatory role by affecting microbial population, metabolic activity, and biofilm structure, thereby significantly influencing the kinetics of the degradation process.
Figure 1b shows that the overall trend of TOC removal generally aligns with the degradation trend of CIP. This indicates that while sludge removes CIP, it also helps purify its transformation intermediates and other organic components in the system. After 48 h, the TOC removal rates were 14.2%, 19.6%, and 23.0%, respectively, significantly lower than the corresponding CIP degradation rates. This suggests that although CIP is biodegradable, its degradation may produce intermediate metabolites that are not readily or fully mineralized, causing a delayed TOC reduction [38].
Previous studies have reported that CIP degradation can generate complex compounds such as nitrogen-containing heterocyclic intermediates and carboxylated derivatives. These compounds strongly resist biodegradation, limiting further TOC removal [39]. Additionally, microbial degradation of antibiotics often involves initial bond cleavage and partial oxidation, while complete mineralization to CO2 and H2O typically requires longer reaction times or assistance from other treatments, such as advanced oxidation processes [40].

3.1.2. Effect of pH

At a fixed sludge concentration of 6 g/L, a series of experiments was conducted by adjusting the initial pH of the reaction system to 5, 6, 7, 8, and 9. The degradation efficiency of CIP and the removal of TOC were evaluated under each condition. As shown in Figure 2, pH exhibited a significant effect on the biodegradation behavior of CIP. Under acidic conditions (pH 5 and 6), the degradation rate of CIP was considerably lower. This decrease may be attributed to the inhibitory effect of low pH on microbial activity within the sludge. Previous studies have shown that acidic environments can disrupt normal microbial metabolism and lead to the denaturation of extracellular enzymes, thereby limiting the decomposition of complex organic compounds [41]. Furthermore, in acidic media, CIP is more likely to become protonated, which increases its permeability through microbial membranes and its toxicity, thus further reducing its biodegradability [42].
In contrast, alkaline conditions enhanced the degradation of CIP. At pH 8, the 48 h degradation rate reached 41.9%, which was higher than that observed at pH 7. This improvement may be due to the increased negative charge of functional groups in EPSs under alkaline conditions, which enhances the binding between CIP and EPSs. This promotes CIP adsorption and accumulation within the sludge, facilitating its biodegradation [43]. In addition, higher pH levels may stimulate the activity of specific metabolic enzymes, such as monooxygenases and dioxygenases, which are responsible for cleaving and degrading CIP molecular structures [44]. However, further increasing the pH to 9 resulted in a slight decline in degradation efficiency, with a 48 h removal rate of 40.6%. This reduction may be due to physiological stress imposed on key microbial populations, suppressing their growth and metabolic activity. While elevated pH can accelerate certain chemical reactions, overly alkaline environments may disrupt membrane potential and interfere with intracellular enzyme systems, thereby counteracting the benefits of high pH [45]. Despite the slight decrease at pH 9, the overall removal efficiency remained higher than under neutral or acidic conditions, indicating that the stimulatory effect of alkalinity was still predominant.
The TOC removal trend closely mirrored that of CIP degradation. Under pH 8 conditions, the TOC removal rate reached 24.9%, which was higher than that under neutral (23.0%) and acidic conditions (20.3% at pH 5). This suggests that a moderately alkaline environment not only facilitates the initial breakdown of CIP but may also promote the further decomposition and mineralization of its metabolic intermediates. In summary, pH exerted a notable influence on both CIP degradation and TOC removal, with pH 8 identified as the optimal condition. This improvement likely results from the combined effects of enhanced microbial activity and multiple degradation pathways. A moderately alkaline environment not only improves the adsorption and transformation of the parent compound but also creates favorable conditions for the mineralization of intermediate products. These findings support the feasibility of enhancing antibiotic removal efficiency by regulating pH levels.

3.2. Effect of Different Factors on Ultrasonic Cavitation Degradation of CIP

3.2.1. Effect of Frequency

As shown in Figure 3a, the degradation rate increased linearly during the first 30 min at all tested frequencies. This trend suggested that the initial degradation was primarily driven by cavitation-induced free radical attacks and mechanical shear forces [46]. As the reaction progressed, the degradation rate slowed, likely due to reduced cavitation intensity and a decrease in substrate concentration [47]. Lower frequencies (such as 15 kHz and 20 kHz) resulted in higher degradation efficiencies. After 60 min of treatment, the degradation rates at 15 kHz and 20 kHz reached 63.8% and 62.8%, respectively—both significantly higher than the 55.9% observed at 30 kHz. This enhancement can be attributed to the stronger cavitation effects generated at lower frequencies, which produce localized high-energy zones and increase the yield of hydroxyl radicals (·OH), thereby accelerating the degradation process [48]. Previous studies have also confirmed that low-frequency, high-intensity ultrasound promotes the generation of reactive species and facilitates the activation of degradation pathways [49].
The TOC removal results, as shown in Figure 3b, provide additional insight into the extent of mineralization. In general, TOC removal rates were substantially lower than CIP degradation rates. This indicates that a significant portion of intermediate products accumulated during the reaction and required further transformation to achieve complete mineralization. At 15 kHz, TOC removal reached 31.5% after 30 min and 48.4% after 60 min, which were markedly higher than those observed at higher frequencies. These results further support the effectiveness of low-frequency ultrasound in promoting the deep oxidation of organic pollutants. Previous studies suggest that ultrasonic degradation often leads to the formation of stable compound intermediates more resistant to further oxidation, which explains the observed delay in TOC removal compared to CIP degradation [50,51,52]. Taken together, these findings demonstrate that low-frequency, high-intensity ultrasound not only enhances the initial breakdown of CIP molecules but also facilitates the progressive conversion and eventual elimination of intermediate degradation products.

3.2.2. Effect of pH

To further investigate the influence of environmental pH on the ultrasonic degradation of CIP, a series of experiments was conducted under the optimal ultrasonic frequency of 15 kHz by adjusting the initial pH values from 4 to 10. As shown in Figure 4a, CIP degradation was most efficient under alkaline conditions (pH 8–10), with the highest removal rate of 73.4% observed at pH 9 after 60 min. Alkaline conditions enhance free radical generation and activity, optimize ciprofloxacin ionization, and strengthen cavitation effects, thereby improving its ultrasonic degradation efficiency [53]. However, a slight decrease in degradation efficiency was noted at pH 10, suggesting that excessive alkalinity may exert an inhibitory effect on the reaction process. Excessively alkaline conditions may inhibit ultrasonic degradation by promoting free radical scavenging, altering pollutant ionization and cavitation behavior, and causing intermediate accumulation [54]. In contrast, degradation efficiency drops significantly under acidic conditions, reaching only 57.3% removal at pH 4. This trend can be attributed to the impact of pH on cavitation intensity and the generation of reactive species. Previous studies have indicated that alkaline conditions promote the formation and stabilization of hydroxyl radicals (·OH), thereby enhancing oxidative capacity. In contrast, under strongly acidic conditions, excess protons (H+) may react non-selectively with hydroxyl radicals (·OH), decreasing the effective radical concentration and suppressing the degradation process [53,54].
As shown in Figure 4b, the trend of TOC removal generally mirrored that of CIP degradation. Under pH 9, TOC removal reached 55.6% at 60 min, the highest among all tested conditions. Additionally, the slightly reduced TOC removal at pH 10 compared to pH 9 may be attributed to alterations in the charge state of reactants or the promotion of side reactions under highly alkaline conditions [55]. In summary, maintaining a moderately alkaline environment (pH 8–10) is favorable for improving both the degradation and mineralization efficiency of pollutants in ultrasonic cavitation systems. These findings offer valuable theoretical support for the further optimization of operating conditions in advanced oxidation processes.

3.3. Ultrasonic Pretreatment Enhances CIP Biodegradation Efficiency

Figure 5a presents the biodegradation curves of CIP solution after 30 min of ultrasonic pretreatment under optimal conditions (ultrasound frequency of 15 kHz, pH 9). The results indicate that the removal rates of CIP and TOC continuously increased throughout the reaction, reaching 96.3% and 90.4%, respectively, after 24 h. This demonstrates that ultrasonic pretreatment significantly enhanced the biodegradability of CIP, accelerating both its degradation rate and organic carbon mineralization [56]. The mechanism likely involves the high temperature and pressure generated by cavitation bubble collapse, which break aromatic structures, promote chain and ring cleavage, and produce smaller intermediates more readily utilized by microorganisms [56,57,58].
Figure 5b further compares the degradation performances of different treatment methods. Compared to biological treatment alone (B) or ultrasonic treatment alone (U), the combined process (U+B) showed significant improvements in both CIP and TOC removal rates. This suggests that ultrasonic pretreatment not only overcomes the slow degradation and low efficiency of biological methods but also facilitates further mineralization of recalcitrant intermediates produced during ultrasonic degradation [59]. A cost comparison with the conventional Fenton process (Table 1) demonstrates that the combined process not only sustains high removal efficiency but also offers significant economic advantages and cost-effectiveness. By integrating physicochemical pretreatment with microbial metabolism, this synergistic approach achieves more efficient and comprehensive pollutant removal. It presents a practical and viable technical solution for the advanced treatment of highly toxic and refractory organic contaminants, with promising potential for widespread application.
To further evaluate the anti-interference level of the process, we supplemented degradation experiments with influent from wastewater treatment plants as the background; its water quality information is summarized in Table S1. As shown in Figure S1, under identical operational conditions, the CIP removal efficiency was 67.2%, which was notably lower than the 96.3% observed in synthetic wastewater. This reduction primarily stems from matrix complexity: competing organics (COD = 254.2 mg L−1) scavenged oxidative radicals during ultrasonication, while variable nutrients/inhibitors in actual wastewater suppressed microbial activity. Although these results validate the technical feasibility of our approach in real matrices, we emphasize that comprehensive optimization (e.g., extended ultrasound duration or biomass acclimation) remains essential for diverse wastewater types.
Moreover, the applicability of the proposed technology was also explored by treating other quinolone antibiotics, such as ofloxacin (OFX) and norfloxacin (NFX). The results demonstrated that the combined ultrasonic and biological treatment approach exhibited excellent degradation performance for these organic pollutants. As indicated in Figure S2, under the same operating conditions, the degradation efficiencies of OFX and NFX reached 98.9% and 94.2%, respectively. This indicated that our proposed methodology possesses broader applicability.

3.4. Molecular Mechanisms by Which Ultrasonic Pretreatment Enhances CIP Biodegradation Efficiency

3.4.1. Transformation Pathways of Ultrasonic Degradation Products

In this study, to elucidate the molecular mechanisms through which ultrasonic pretreatment enhances the biodegradation efficiency of CIP, CIP’s degradation products generated in the ultrasonic cavitation system were identified and analyzed based on LC-MS mass spectra (Figures S3–S14 and Table 2). Based on the obtained results, the possible transformation pathways of CIP were subsequently proposed (Figure 6). The results indicate that under ultrasonic cavitation, CIP transforms into diverse intermediates via multiple pathways, highlighting its complex structural breakdown and reconstruction during the sonochemical process [60].
One major degradation pathway initiates with the ring-opening reaction of the piperazine ring. Under attack by highly oxidative species such as hydroxyl radicals (·OH), the C–N bonds in the CIP molecule cleave, forming an open-ring intermediate P1 (C13H17FN2O, m/z = 237) [61]. This intermediate further undergoes defluorination and decarboxylation via oxidation and dehydrogenation, sequentially producing P2 (C10H14N2, m/z = 163) and P3 (C10H14N2O2, m/z = 195) [62]. This series of transformations reflects the progressive simplification of the CIP structure, reducing molecular stability and thus enhancing its biodegradability by microorganisms.
Another degradation pathway primarily involves molecular rearrangement and hydroxylation modifications [24]. In this pathway, CIP undergoes side-chain reconstruction with the introduction of hydroxyl groups, producing products P4 (C17H19FN2O3, m/z = 319) and P5 (C16H19FN2O, m/z = 275). The aromatic ring core structure remains intact, with modifications occurring only at the substituent positions. Product P4 is further converted into a deeply oxidized product P6 (C15H15FN2O4, m/z = 307) through additional oxygenation reactions [63]. Simultaneously, the aromatic ring core may also undergo ring-opening or cleavage reactions, yielding degraded products with disrupted structures such as P7 (C14H14N2O2, m/z = 243), P8 (C13H11NO3, m/z = 230), and P9 (C10H6FNO3, m/z = 208), indicating significant damage to CIP’s core structure under ultrasonic treatment [64].
As the reaction progresses, some intermediates continue to undergo backbone cleavage and oxidation, eventually forming low-molecular-weight mineralization products such as P10 (C5H7NO2, m/z = 114), P11 (C4H7NO2, m/z = 102), and P12 (C3H4O2, m/z = 73). The final products are mainly small organic acids and nitrogen-containing compounds, indicating that ultrasonic cavitation can effectively convert CIP from a large organic pollutant into smaller, more bioavailable molecules for microorganisms. This transformation significantly improves the overall efficiency of subsequent biodegradation [65].

3.4.2. Toxicity Reduction of Ultrasonic Degradation Products

To further clarify the molecular mechanisms behind ultrasonic pretreatment’s enhancement of CIP biodegradation efficiency, this study systematically evaluated the toxicity characteristics of CIP and its ultrasonic degradation products. Toxicity predictions for CIP and its major ultrasonic degradation intermediates were conducted using the T.E.S.T. model. The ecological parameters evaluated included the 96 h median lethal concentration (LC50) for Fathead Minnow, the 48 h LC50 for Daphnia magna, bioaccumulation factors, and developmental toxicity.
The results indicated that the parent CIP compound exhibited strong acute toxicity, with a Fathead Minnow 96 h LC50 of 0.19 mg/L, classifying it as highly toxic (Figure 7a) [66]. After ultrasonic degradation, most intermediate products showed significantly increased LC50 values, indicating a marked reduction in toxicity. Small molecule products such as P10, P11, and P12 exhibited toxicity levels within the low-risk range. Similarly, the 48 h LC50 results for Daphnia magna (Figure 7b) demonstrated that the majority of intermediates were less toxic than the parent CIP compound, further confirming the detoxification efficacy of the ultrasonic cavitation process [67,68].
Moreover, CIP itself showed high potential for bioaccumulation and developmental toxicity risks (Figure 7c,d). However, the main intermediates generated during ultrasonic treatment generally exhibited lower bioaccumulation potential, and their predicted developmental toxicity values also decreased substantially. Notably, terminal products such as P10 and P11 posed minimal ecological risk, indicating improved environmental compatibility. These findings suggest that ultrasonic cavitation not only accelerates the structural breakdown of CIP but also significantly reduces its ecotoxicity during degradation, thereby providing a safer and more controllable foundation for subsequent biological treatment processes [69].

4. Conclusions

Ultrasonic pretreatment significantly enhanced the removal of ciprofloxacin (CIP) by increasing its biodegradability and reducing its toxicity. The combined ultrasonic and biological treatment demonstrated markedly better performance than either method alone.
  • Optimal ultrasonic conditions were identified as 15 kHz frequency and pH 9, under which 58.9% of CIP degradation and 35.2% of total organic carbon (TOC) removal were achieved within 30 min.
  • The combined treatment process achieved removal rates of 96.3% for CIP and 90.4% for TOC after 24 h.
  • LC-MS analysis revealed that ultrasonic cavitation induced ring cleavage, hydroxylation, defluorination, and the formation of low-molecular-weight organic acids in CIP.
  • Toxicity assessments indicated significant reductions in acute toxicity, bioaccumulation potential, and developmental toxicity for CIP and its degradation products.
Future research should prioritize scaling the process to continuous-flow and pilot-scale systems, evaluating the long-term impacts of degradation intermediates on activated sludge performance, and validating environmental safety through comprehensive ecotoxicological testing. These efforts will be essential to advance the practical application of this technology.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w17162495/s1: Figure S1: Effect of influent from wastewater treatment plants as background solution on removal rate of CIP; Figure S2: Degradation of OFX and NFX using the combined ultrasonic and biological treatment approach; Figure S3: Mass spectrum of the intermediate product “P1” generated during ultrasonic cavitation degradation; Figure S4: Mass spectrum of the intermediate product “P2” generated during ultrasonic cavitation degradation; Figure S5: Mass spectrum of the intermediate product “P3” generated during ultrasonic cavitation degradation; Figure S6: Mass spectrum of the intermediate product “P4” generated during ultrasonic cavitation degradation; Figure S7: Mass spectrum of the intermediate product “P5” generated during ultrasonic cavitation degradation; Figure S8: Mass spectrum of the intermediate product “P6” generated during ultrasonic cavitation degradation; Figure S9: Mass spectrum of the intermediate product “P7” generated during ultrasonic cavitation degradation; Figure S10: Mass spectrum of the intermediate product “P8” generated during ultrasonic cavitation degradation; Figure S11: Mass spectrum of the intermediate product “P9” generated during ultrasonic cavitation degradation; Figure S12: Mass spectrum of the intermediate product “P10” generated during ultrasonic cavitation degradation; Figure S13: Mass spectrum of the intermediate product “P11” generated during ultrasonic cavitation degradation; Figure S14: Mass spectrum of the intermediate product “P12” generated during ultrasonic cavitation degradation; Table S1: Conventional water quality parameters for influent from wastewater treatment plants; Table S2: Kinetic models of CIP biodegradation under different sludge concentrations; Table S3: Kinetic models of CIP biodegradation at different pH levels; Table S4: Kinetic models of CIP sonodegradation at different ultrasonic frequencies; Table S5: Kinetic models of CIP sonodegradation at different pH levels.

Author Contributions

Conceptualization, Q.W.; methodology, Q.W. and Q.P.; validation, Q.P. and T.P.; formal analysis, Q.W. and T.P.; investigation, Q.W. and Q.P.; data curation, Q.W. and Q.P.; writing—original draft preparation, Q.W.; writing-review and editing, Q.W. and X.H.; visualization, Q.P. and T.P.; supervision, X.H.; project administration, X.H.; funding acquisition, X.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (52470220).

Data Availability Statement

The original contributions presented in this study are included in the article and supplementary materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 02495 g001
Figure 1. Effect of sludge concentration on the biodegradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
Figure 1. Effect of sludge concentration on the biodegradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
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Figure 2. Effect of pH on the biodegradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
Figure 2. Effect of pH on the biodegradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
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Figure 3. Effect of frequency on high-intensity ultrasonic cavitation degradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
Figure 3. Effect of frequency on high-intensity ultrasonic cavitation degradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
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Water 17 02495 g004
Figure 4. Effect of pH on high-intensity ultrasonic cavitation degradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
Figure 4. Effect of pH on high-intensity ultrasonic cavitation degradation of CIP. (a) CIP removal rate; (b) TOC removal rate.
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Figure 5. Effects of ultrasonic pretreatment on CIP biodegradation performance. (a) Biodegradation curves of CIP and TOC; (b) removal rates of CIP and TOC under optimal conditions of different processes. Note: B, U, and U+B represent (i) biological degradation for 24 h—sludge concentration 6 g/L, pH 8; (ii) ultrasonic degradation for 60 min—frequency 15 kHz, pH 9; (iii) ultrasonic pretreatment for 30 min + biological degradation for 24 h—frequency 15 kHz, pH 9 + sludge concentration 6 g/L, pH 8.
Figure 5. Effects of ultrasonic pretreatment on CIP biodegradation performance. (a) Biodegradation curves of CIP and TOC; (b) removal rates of CIP and TOC under optimal conditions of different processes. Note: B, U, and U+B represent (i) biological degradation for 24 h—sludge concentration 6 g/L, pH 8; (ii) ultrasonic degradation for 60 min—frequency 15 kHz, pH 9; (iii) ultrasonic pretreatment for 30 min + biological degradation for 24 h—frequency 15 kHz, pH 9 + sludge concentration 6 g/L, pH 8.
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Figure 6. Transformation pathways of ultrasonic degradation products of CIP.
Figure 6. Transformation pathways of ultrasonic degradation products of CIP.
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Figure 7. Toxicity assessment of CIP and its ultrasonic degradation products. (a) LC50 for Fathead Minnow (96 h); (b) LC50 for Daphnia magna (48 h); (c) bioaccumulation factors; (d) developmental toxicity.
Figure 7. Toxicity assessment of CIP and its ultrasonic degradation products. (a) LC50 for Fathead Minnow (96 h); (b) LC50 for Daphnia magna (48 h); (c) bioaccumulation factors; (d) developmental toxicity.
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Table 1. Cost calculation of various treatment processes.
Table 1. Cost calculation of various treatment processes.
MethodMass Removed (g/m3)Cost (in USD) per g of CIP Removal
Chemical CostEnergy CostTotal Cost
U+B9.630.2080.0110.219
Fenton9.000.3100.0130.324
Note: Unit cost of power = 0.06 USD/kWh; power for pilot plant = 3.5 kW; unit cost of FeSO4 = 1 USD/kg; unit cost of H2O2 = 2 USD/kg.
Table 2. Identification of intermediate products from ciprofloxacin degradation by ultrasonic cavitation.
Table 2. Identification of intermediate products from ciprofloxacin degradation by ultrasonic cavitation.
CompoundRetention (min)Experimental Mass (m/z)Chemical
Formula		Molecular Structure
P11.046237C13H17FN2OWater 17 02495 i001
P24.934163C10H14N2Water 17 02495 i002
P34.527195C10H14N2O2Water 17 02495 i003
P46.767319C17H19FN2O3Water 17 02495 i004
P56.508275C16H19FN2OWater 17 02495 i005
P63.823307C15H15FN2O4Water 17 02495 i006
P76.471243C14H14N2O2Water 17 02495 i007
P89.415230C13H11NO3Water 17 02495 i008
P98.212208C10H6FNO3Water 17 02495 i009
P105.101114C5H7NO2Water 17 02495 i010
P119.711102C4H7NO2Water 17 02495 i011
P121.41673C3H4O2Water 17 02495 i012
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Wen, Q.; Peng, Q.; Pham, T.; He, X. Ultrasonic-Cavitation-Enhanced Biodegradation of Ciprofloxacin: Mechanisms and Efficiency. Water 2025, 17, 2495. https://doi.org/10.3390/w17162495

AMA Style

Wen Q, Peng Q, Pham T, He X. Ultrasonic-Cavitation-Enhanced Biodegradation of Ciprofloxacin: Mechanisms and Efficiency. Water. 2025; 17(16):2495. https://doi.org/10.3390/w17162495

Chicago/Turabian Style

Wen, Qianheng, Qiwei Peng, ThuThi Pham, and Xiwei He. 2025. "Ultrasonic-Cavitation-Enhanced Biodegradation of Ciprofloxacin: Mechanisms and Efficiency" Water 17, no. 16: 2495. https://doi.org/10.3390/w17162495

APA Style

Wen, Q., Peng, Q., Pham, T., & He, X. (2025). Ultrasonic-Cavitation-Enhanced Biodegradation of Ciprofloxacin: Mechanisms and Efficiency. Water, 17(16), 2495. https://doi.org/10.3390/w17162495

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