Synergistic degradation of levofloxacin (LEV) by Cu²⁺-activated peroxymonosulfate (PMS) under hydrodynamic cavitation (HC): Efficiency and mechanistic insights

Graphical abstract

Abstract

To effectively eliminate excess antibiotics from aqueous environments and to mitigate the dissemination of antibiotic resistance genes (ARGs), this study proposes a novel degradation system that activates peroxymonosulfate (PMS) through a synergistic combination of hydrodynamic cavitation (HC) and divalent copper ions (Cu²⁺). Levofloxacin (LEV) is employed as the representative target contaminant to evaluate the system’s performance. HC has emerged as a promising technique for pollutant removal. In this study, the localized high-temperature and high-pressure conditions generated by HC not only partially activated PMS but also facilitated its interaction with Cu²⁺ ions, leading to a pronounced synergistic enhancement in sulfate radical (SO₄^•−) generation and efficient pollutant degradation. Under optimized HC/Cu²⁺/PMS conditions (Cu²⁺ = 5 mM, PMS = 2.5 mM, inlet pressure = 0.15 MPa, pH = 10), complete removal of LEV (30 mg/L) was achieved within 50 min. This study elucidates the degradation mechanisms and pathways of LEV within the coupled HC/Cu²⁺/PMS system and evaluates the ecological safety of its degradation intermediates using the U.S. EPA’s T.E.S.T. (Toxicity Estimation Software Tool). Furthermore, the system’s applicability was validated through degradation experiments involving a range of representative pollutants, demonstrating its broad-spectrum effectiveness. Crucially, the HC/Cu²⁺/PMS system demonstrated a superior cavitation yield (2.78 × 10⁻⁵ mg/J) and a low electrical energy per order (EE/O) of 229.48 kWh/m³, highlighting its high energy efficiency and practical potential for sustainable wastewater treatment. The experimental results emphasize the system’s strong potential for the effective removal of organic pollutants from water, offering a novel and sustainable approach for advanced water treatment.

1. Introduction

Since their discovery in the 1920 s, antibiotics have undergone extensive development and application across various sectors [1]. However, with the increasing use of antibiotics, their environmental impact has become increasingly evident [2]. According to statistics, by 2020, antibiotic consumption in China reached approximately 8990 billion defined daily doses [3]. Excessive antibiotics and their metabolic intermediates are increasingly discharged into lakes, reservoirs, and even groundwater, and their chemically stable structures hinder natural degradation processes [4]. As a result, these compounds persist and accumulate in aquatic environments, posing significant threats to ecosystems and facilitating the emergence and spread of antibiotic-resistance genes (ARGs) [5], which are considered a potential hazard in water bodies.

Levofloxacin (LEV) is a widely used third-generation fluoroquinolone with broad-spectrum antimicrobial activity [6]. However, due to the extensive use and incomplete metabolic elimination of LEV, substantial residues have been detected in various aquatic environments and treated wastewater effluents [7]. Various approaches, such as physical adsorption [8], bioremediation [9], photocatalysis [10], and electrochemical catalysis [11], have been explored for the removal of LEV. However, these methods often face limitations, including incomplete degradation and prolonged treatment times. Therefore, in this study, LEV is selected as the model pollutant to develop a more efficient and rapid strategy for antibiotic removal.

Advanced oxidation processes (AOPs) are widely regarded as promising technologies for wastewater treatment [12,13]. While hydroxyl radical (HO^•)-based AOPs (HR-AOPs) have demonstrated high efficacy [14,15], HO^• can undergo side reactions with various substances, forming H₂O₂ or HO₂·, thereby reducing reaction stability and efficiency [16]. In contrast, sulfate radical (SO₄^•−) based AOPs (SR-AOPs) have recently gained attention for their ability to degrade pollutants selectively [17,18], owing to the higher redox potential of SO₄^•− (E₀ = 2.5–3.1 V) and their longer half-life compared to HO^• [19], alongside their effectiveness across a wider pH range [20,21]. SO₄^•− are typically generated through the activation of persulfate (PS), a compound that remains relatively stable at room temperature and requires external energy input for activation. Common activation techniques include plasma, thermal, and ultraviolet (UV) methods [[22], [23], [24], [25]]; however, these approaches often demand substantial energy, limiting their practical and economic feasibility.

Hydrodynamic cavitation (HC) is an emerging green process intensification (PI) technology characterized by the rapid collapse of microbubbles upon reaching their maximum size [26]. This implosion releases intense localized energy, generating transient high-temperature, high-pressure zones, commonly referred to as hot spots [27]. It has been demonstrated that the extreme conditions generated by HC, localized high temperatures and pressures, can effectively cleave chemical bonds in PS, thereby enhancing its activation and facilitating the generation of a variety of reactive radicals. Specifically, Anvar et al. [28] investigated the synergistic use of HC and a cobalt tungstate (CoWO₄) photocatalyst to activate peroxymonosulfate (PMS) for the removal of cloxacillin (CLX), achieving 95.68 % degradation within 60 min under optimised conditions. The same research group further combined HC with CoFeO photocatalysts under UVC irradiation to activate peracetic acid (PAA) for bisphenol A (BPA) degradation, attaining 97.29 % removal within 60 min [29]. Similarly, Elham et al. [30] developed an integrated HC/PMS/UVC system to effectively degrade carbamazepine (CBZ), achieving 73 % degradation within 90 min. The activation of PS is facilitated by the extreme conditions generated through HC, including localized high temperatures and pressures. Simultaneously, HC significantly enhances the mass transfer of reactive species within the liquid phase, thereby improving the overall efficiency of pollutant degradation in the system.

Meanwhile, transition metals, such as iron (Fe), copper (Cu), and cobalt (Co), have been widely employed for PS activation due to their high catalytic efficiency and low material cost [31]. Among them, Cu²⁺ has attracted attention due to its excellent pH adaptability and lower toxicity. [32]. It plays a key role in PS activation by generating potent oxidants and reactive intermediates [33]. Cu activates PS via redox reactions; Cu²⁺ readily forms an inactive species in the reaction and precipitates, leading to deactivation. This reduces the effective availability of Cu and limits its overall utilization efficiency. It has been well established that external energy (such as ultrasound) can address this issue by promoting the valence cycle of copper ions, thereby enhancing its catalytic efficiency [34,35]. In this context, HC may serve as an effective method of PS activation, operating through a mechanism similar to ultrasound but offering greater scalability and energy efficiency. However, to the best of our knowledge, systematic studies on this specific application remain lacking. Despite the individual promise of HC and Cu²⁺ in PS activation, their synergistic combination, particularly in relation to the underlying mechanism, degradation pathways, and energy efficiency for fluoroquinolone antibiotics, remains largely unexplored. While HC has been coupled with other oxidants and catalysts, a systematic investigation into the HC/Cu²⁺/PS ternary system is lacking. It is hypothesized that HC can not only directly activate PS but also overcome Cu²⁺ deactivation by promoting its redox cycle via cavitation-induced energy, thereby establishing a sustained, highly efficient catalytic system.

In this study, we integrated HC with copper ions (Cu²⁺) to activate PMS for the degradation of LEV. We investigated the synergistic effects of the HC/Cu²⁺/PMS system on pollutant removal efficiency. Initially, the degradation efficiencies of various systems (HC, Cu²⁺, HC/PMS, Cu²⁺/PMS, and the combined HC/Cu²⁺/PMS) were compared. Subsequently, the performance of the HC/Cu²⁺/PMS system was systematically evaluated under different operational parameters, including the Cu²⁺ to PMS molar ratio, PMS dosage, inlet pressure, solution pH, and initial LEV concentration. Finally, the potential degradation mechanism, practical applicability, and economic feasibility of the system were analyzed. The findings of this study aim to provide a novel and efficient approach to removing organic pollutants from aqueous environments.

2. Materials and methods

2.1. Chemicals

Levofloxacin (LEV, C₁₈H₂₀FN₃O₄), sodium peroxymonosulfate (KHSO₅, PMS), copper sulfate pentahydrate (CuSO₄·5H₂O), acetonitrile (C₂H₃N), formic acid (CH₂O₂), methanol (MeOH, CH₄O), tert-Butanol (TBA, C₄H₁₀O), sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), malachite green (MG, C₂₃H₂₅ClN₂), rhodamine B (RhB, C₂₈H₃₁ClN₂O₃), and doxycycline hydrochloride (DH, C₂₂H₂₄N₂O₈·HCl) were all obtained from Shandong Chengchuan Life Science and Technology Co. All chemicals were of analytical reagent (AR) grade and used without further purification. Ultrapure water was utilized in all experimental procedures.

2.2. Apparatus setup

The schematic diagram of the setup is shown in Fig. 1. The experimental system comprises a customised 25-litre inlet tank with a cooling water reservoir, a 3-kW centrifugal pump (CMI20-30 T, China Linxiao Pumps Co.), and a Venturi tube, all interconnected with 304-grade stainless steel piping. The Venturi device was fabricated from polymethyl methacrylate (PMMA) and featured an inlet diameter of 25 mm, a throat diameter of 4 mm, and a throat length of 2 mm. The flow area at the throat was approximately 12.57 mm², with a contraction angle of 45°, an expansion angle of 12°, and corresponding contraction and expansion lengths of 25.35 mm and 99.90 mm, respectively. The flow rate was monitored using a flowmeter (model LW-DN15, Pufit Instrument Co.) installed upstream of the Venturi. Flow regulation was achieved using ball valves, while two pressure gauges (model PCM-300, Xuansheng Instrumentation Co.) were installed upstream and downstream of the Venturi to monitor pressure. Transient cavitation phenomena within the Venturi were captured using a high-speed camera (VEO 1310, Phantom High Speed Co.). Based on the measured pressure and flow data, the cavitation number (σ) was calculated using Eq. (1) under various operating conditions to evaluate the cavitation intensity and regime.

$$\sigma =\frac{p_{\text{out}}-p_{\text{v}}}{\frac{1}{2}\rho {\text{v}}_{\text{th}}^2}$$ (1)

where σ is the cavitation number, p_out is the downstream pressure of the liquid (Pa), p_v is the saturated vapor pressure of the liquid (Pa), ρ is the density of the liquid (kg/m³), and v_th is the fluid velocity at the Venturi throat (m/s). In this study, cavitation numbers ranged from 0.41 to 0.84 across different operating conditions.

Fig. 1.

Schematic diagram of the HC system and Venturi structure. F: flowmeter; P₁, P₂: pressure gauges; V₁, V₂, V₃, and V₄: ball valves; C: cooling water tank.

2.3. Experimental procedure

For each experimental run, 10 L of LEV solution with a defined initial concentration (10, 20, 30, and 40 mg/L) was prepared. These concentrations were selected to simulate high pollutant loads typically observed in antibiotic-rich wastewaters (e.g., from hospitals or pharmaceutical industries) and to evaluate the degradation kinetics and treatment capacity of the HC/Cu²⁺/PMS system under challenging conditions [36,37]. Subsequently, predetermined amounts of PMS and CuSO₄·5H₂O were added, and the cavitation system was activated to initiate continuous circulation and solution degradation. At predetermined time intervals (0, 5, 10, 20, 30, 40, 50, and 60 min), 5 mL aliquots of the reaction mixture were withdrawn, and 200 μL of methanol was immediately added to each sample to quench residual radicals and terminate the reaction. Upon completion of the experiment, the residual LEV concentration was quantified using an HPLC (model LC-2050C, Shimadzu). The analysis was performed with a mobile phase consisting of 0.1 % formic acid and acetonitrile (80:20, v/v) at a flow rate of 0.8 mL/min. The injection volume was 10 μL, the column temperature was maintained at 30 °C, and detection was performed at 293 nm. To elucidate the LEV degradation pathway, degradation intermediates were identified using a quadrupole time-of-flight liquid chromatography-mass spectrometry (LC-MS) instrument (6200 series TOF/6500 series Q-TOF). To investigate the LEV degradation pathway, mass spectrometry was performed in positive electrospray ionisation (ESI) mode over an m/z range of 20–500. The concentrations of MG, RhB, and DH were monitored at their respective maximum absorbance wavelengths of 265, 554, and 275 nm, respectively, using a UV–Vis spectrophotometer (DR6000, Hash). All experiments were conducted in triplicate to ensure reproducibility. Throughout the experimental runs, tap water was continuously supplied to the cooling water tank to maintain the system temperature at 25 ± 5 °C.

2.4. Data analysis

To quantitatively assess the degradation efficiency, the degradation rate (η) was calculated using the following Eq. (2):

$$\eta =\left(\frac{C_0-C_{\text{t}}}{C_0}\right)\times 100\%$$ (2)

where C₀ and C_t represent the concentrations of LEV at the initial time and at time t (mg/L), respectively, the degradation kinetics of LEV were investigated by fitting experimental data to zero-order, first-order, and second-order kinetic models, as described by Eqs. (3), (4), and (5), respectively [38]:

$$K_{0}t=C_{\text{0}}\text{-}{C}_t$$ (3)

$${\text{K}}_1\text{t}=\text{ln}\frac{{\text{C}}_0}{{\text{C}}_t}$$ (4)

$$K_{2}t\text{= (1/}{C}_t-\text{1/}{C}_{\text{0}})\times {\text{10}}^{-\text{3}}$$ (5)

where K₀, K_1, and K₂ are the apparent kinetic rate constants for the zero-order, first-order, and second-order kinetic models, respectively, and t denotes the reaction time in minutes.

As illustrated in Fig. 2(a), the degradation of LEV follows first-order kinetics, with a coefficient of determination (R²) of 0.976, indicating a good fit. In first-order reactions, the time required for a given proportional decrease in solute concentration is determined exclusively by the rate constant [39]. This model is particularly useful for comparing apparent degradation rates under different operating conditions. Given that radical-induced reactions primarily occur at the gas–liquid interface of cavitation bubbles, the process may also be interpreted through the lens of a surface-mediated Langmuir-Hinshelwood (L-H) model (Eq. (6)), as observed in analogous cavitation-based systems (Fig. 2(b)) [27]. Although the excellent first-order fit suggests that adsorption is not the rate-limiting step, the L-H model provides valuable mechanistic insight into interfacial processes, while the first-order model remains a practical and robust empirical tool for efficiency quantification.

$$\frac{\text{1}}{K_{\text{obs}}}=\frac{\text{1}}{k_c{K}_{\mathit{LH}}}+\frac{\text{1}}{k_c}{\text{[LEV]}}_{\text{0}}$$ (6)

where [LEV]₀ is the initial LEV concentration, K_obs is the first-order rate constant, K_LH is the L-H adsorption equilibrium constant, and k_c is the surface reaction rate constant.

Fig. 2.

(a) Linear fitting results based on different kinetic models. (b) Langmuir-Hinshelwood (L-H) kinetic plot of LEV degradation. Reaction conditions: [LEV]₀ = 10 mg/L, [Cu²⁺]₀ = 5 mM, [PMS]₀ = 2.5 mM, P_in = 0.15 MPa, pH = 10, T = 25 °C.

The synergistic effect of the combined system was further assessed using the synergy index (SI), calculated as follows (Eq. (7)):

$$\text{SI}=\frac{{\text{K}}_{HC\text{/}AOP\text{s}}}{{\text{K}}_{\mathit{HC}}+{\text{K}}_{AOP\text{s}}}$$ (7)

where K_HC/AOPs, K_HC, and K_AOPs represent the first-order constants (min⁻¹) for the coupled HC and advanced oxidation process (HC/AOPs), the standalone HC system, and the individual advanced oxidation process (AOP), respectively. An SI value less than 1 indicates antagonism between the combined processes, suggesting mutual inhibition. Conversely, an SI value greater than 1 signifies a synergistic interaction. The magnitude of the SI quantifies the extent of inhibition or enhancement among the integrated treatment methods [40].

In HC systems, the number of liquid passes through the cavitation zone is a key performance parameter, as it directly quantifies cumulative cavitation exposure [X, Y]. The number of passes (N) was calculated using the following Eq. (8) [40]:

$$N=\frac{Q}{V}t$$ (8)

where Q is the volumetric flow rate (m³/h), and V is the total solution volume (m³).

To assess the economic efficiency of the degradation process, the cavitation yield (CY) and electrical energy per order (EE/O) were calculated using Eqs. (9), (10), respectively [41,42]:

$$\text{CY (mg/J) =}\frac{\text{Amount of pollutant degraded}}{\mathit{Pt}}$$ (9)

$$\text{EE/O}=\frac{\mathit{Pt}·1000}{V·60\times \log \left(\frac{C_0}{C_t}\right)}$$ (10)

where P is the input power (kW) of the system (i.e., the rated power of the pump), and V is the solution volume (L).

The biological toxicity of LEV and its degradation products was evaluated using T.E.S.T. (Version 5.1.2) software. Quantitative structure–activity relationship (QSAR) methods were applied to identify potential toxic components within the target compounds.

3. Results and discussion

3.1. Cavitation characteristics of the Venturi reactor

In HC applications, selecting an appropriate reactor configuration is critical for achieving effective treatment. Commonly used HC reactors include orifice plates, Venturi tubes, vortex diodes, and rotational devices [43]. Among these, the Venturi reactor has garnered considerable attention owing to its operational simplicity, mechanical robustness, and consistent performance [44]. In a Venturi reactor, as the fluid passes through the converging section, its velocity increases, resulting in a corresponding drop in pressure. When the local pressure falls below the liquid’s saturated vapor pressure, cavitation nuclei begin to grow and form vapor cavities. As the fluid enters the diverging section of the Venturi, the pressure recovers, leading to the collapse of vapor cavities, which generate intense shear forces and induce sonochemical effects. Venturi reactors offer distinct advantages in cavitation generation as an optimized throat geometry promotes maximal cavity formation. At the same time, the gradual pressure recovery downstream enables full development of the cavities prior to collapse [44,45]. Table 1 presents the flow characteristics of the Venturi under five operating conditions, corresponding to cavitation numbers (σ) ranging from σ = 0.41 to 0.84 and inlet pressures P_in of 0.04, 0.08, 0.12, 0.16, and 0.20 MPa. The results reveal a clear positive correlation between increasing inlet pressure and both fluid velocity and flow rate. A decrease in the cavitation number or an increase in inlet pressure results in higher fluid velocity and flow rate, thereby enhancing cavitation intensity [46]. Furthermore, cavitation intensity can be effectively tuned by modifying geometric parameters such as the throat diameter-to-length ratio, cross-sectional profile, and surface characteristics of the Venturi [47].

Table 1.

Flow characteristics of the Venturi. P_in: inlet pressure; Q: flow rate; v_in: upstream velocity; and l_c,max: maximum cavity length.

σ	Pin (MPa)	Q (m3/h)	vin (m/s)	vt (m/s)	lc,max (mm)
0.84	0.04	0.68	0.39	15.10	7.45
0.67	0.08	0.77	0.43	16.91	14.75
0.55	0.12	0.84	0.48	18.59	19.07
0.46	0.16	0.92	0.52	20.29	23.86
0.41	0.20	0.98	0.55	21.57	28.57

Cavitation visualization experiments were performed at 40,000 f.p.s to observe cavity formation and development under five operating conditions, as shown in Fig. 3. At lower inlet pressures (Fig. 3(a)–(c)), the cavitation zones appear confined to a localized region, and the duration of cavity development is relatively short. The analysis focuses on the primary flow regime, where minor cavitation shedding is observed at lower inlet pressures (Fig. 3(a)), primarily driven by the reduced upstream pressure. Under these conditions, a shorter separation bubble forms than at higher inlet pressures. Increasing the inlet pressure markedly enhances cavitation generation, as indicated by the enlarged cavitation zones and higher vapor fraction observed in (Fig. 3(d)–(e)) [48]. Under these conditions, the attached cavity progressively expands, occupying a greater portion of the near-wall region. Moreover, the higher flow rate resulting from increased inlet pressure leads to a greater number of fluids passing through the Venturi per unit time. Consequently, the degradation rate increases with the inlet pressure.

Fig. 3.

Cavitation development in the Venturi under different inlet pressures: (a) 0.04 MPa (σ = 0.41), (b) 0.08 MPa (σ = 0.46), (c) 0.12 MPa (σ = 0.55), (d) 0.16 MPa (σ = 0.67), and (e) 0.20 MPa (σ = 0.84).

3.2. Synergistic effect of HC, Cu²⁺, and PMS

To evaluate the synergistic effects of HC, Cu²⁺, and PMS on LEV degradation, a series of comparative experiments was conducted under a consistent sub-optimal yet representative operating condition. The systems tested included individual HC and PMS treatments, as well as combined configurations: HC/PMS, Cu²⁺/PMS, and HC/Cu²⁺/PMS. As shown in Fig. 4, the degradation of LEV in the first four systems progressed relatively slowly over 60 min, with degradation rates remaining below 40 %, as summarized in Table 2. Compared with the minimal degradation achieved by the sole PMS system (2.02 %), the HC/PMS system demonstrated significantly enhanced performance, with a degradation rate of 37.80 % and a synergy index (SI) of 1.85. This result suggests that HC facilitates PMS activation and enhances the interaction between LEV and reactive species through sonochemical effects [49]. Furthermore, coupling HC with the Cu²⁺/PMS system resulted in a substantial enhancement in degradation efficiency, achieving a LEV removal rate of 97.74 % within 60 min and an SI of 5.23.

Fig. 4.

Degradation of LEV under different treatment systems. Reaction conditions: [LEV]₀ = 30 mg/L, [Cu²⁺]₀ = 3 mM, [PMS]₀ = 1.5 mM, P_in = 0.1 MPa, pH = unadjusted, T = 25 °C, and V = 10 L. (Error bars represent standard deviations from triplicate measurements (n = 3)).

Table 2.

Degradation effectiveness of different treatment systems. K_obs: apparent first-order rate constant.

System	η (%)	Kobs (×10−3 min−1)	SI
HC	18.77	4.9	/
PMS	2.02	0.32	/
HC/PMS	37.80	9.64	1.85
Cu2+/PMS	12.20	2.75	/
HC/Cu2+/PMS	97.94	40.04	5.23

The experimental results indicate that Cu²⁺ activate PMS via redox-based electron transfer reactions (Eq.(11)) [50,51]. Simultaneously, HC not only directly initiates PMS activation and generates SO₄^•− [52], but also creates transient high-temperature, high-pressure microenvironments that serve as an external energy source to accelerate Cu²⁺-mediated reactions [39]. It is noteworthy that the initial reaction kinetics exhibit a distinct biphasic behaviour. During the first 20 min, the HC/PMS system shows a significantly higher degradation rate, whereas the HC/Cu²⁺/PMS system demonstrates a temporary inhibition. This initial inhibition is attributed to the rapid consumption of PMS by Cu²⁺, leading to its decomposition into less reactive species such as SO₄²⁻ via non-radical pathways (Eq. (12)) [51]. As a result, the effective utilisation of PMS is transiently reduced. As the reaction progresses, the intensified effects of HC, including the generation of extreme local temperatures and pressures, enhanced interfacial mass transfer through microjets, and the cumulative energy of repeated bubble collapses, begin to dominate the system. These effects promote (i) the redox cycling of copper ions and (ii) the synergistic activation of the remaining PMS. Consequently, the generation rate of reactive radicals increases significantly in the later stages, resulting in a reversal of the initial kinetic inhibition. This dynamic evolution aligns well with the competitive-synergistic kinetic mechanisms reported for transition-metal-activated PMS systems [52,53].

$${\text{Cu}}^{\text{2 +}}{\text{+ HSO}}_{\text{5}}^{\text{-}}\stackrel{\text{HC}}{\to }{\text{Cu}}^{\text{3 +}}{\text{+ SO}}_4^{·-}{\text{+ OH}}^{-}$$ (11)

$${\text{Cu}}^{\text{2 +}}{\text{+ HSO}}_{\text{5}}^{-}\stackrel{\text{HC}}{\to }{\text{Cu}}^{\text{3 +}}{\text{+ SO}}_{\text{4}}^{2-}{\text{+ HO}}^{·}$$ (12)

The HC/Cu²⁺/PMS system exhibited the highest observed degradation rate constant for LEV (K_obs = 0.04004 min⁻¹) among all tested systems. The corresponding SI of 5.23 confirms a strong positive synergistic interaction (SI > 1) between cavitation and chemical activation pathways. These results clearly demonstrate that the HC/Cu²⁺/PMS system significantly enhances PMS activation, achieving outstanding synergistic performance.

3.3. Effect of operating parameters on LEV degradation

3.3.1. Inlet pressure

The inlet pressure of the Venturi plays a key role in regulating cavitation intensity, thereby influencing the efficiency of LEV degradation. To investigate this effect, experiments were performed at four distinct inlet pressures of 0.10 (σ = 0.59), 0.15 (σ = 0.48), 0.20 (σ = 0.39), and 0.30 MPa (σ = 0.30) under controlled conditions: initial LEV concentration ([LEV]₀) of 30 mg/L, unadjusted pH, and a constant temperature of 25 °C. The degradation of LEV over reaction time is shown in Fig. 5. The results indicate that increasing the inlet pressure from 0.10 to 0.15 MPa enhances LEV degradation efficiency, with the removal rate rising from 18.77 % to 21.78 %. However, further increases in pressure to 0.20 and 0.30 MPa result in a decline in degradation performance, indicating that excessively high inlet pressures may suppress effective cavitation activity. To account for the varying flow rates, the degradation data were replotted as a function of the number of passes (Fig. S1(a)). This representation confirms that overall degradation efficiency scales with cumulative cavitation exposure. Notably, the system operated at 0.15 MPa consistently achieved the highest degradation per pass, followed by 0.10, 0.20, and 0.30 MPa, demonstrating that 0.15 MPa provides optimal cavitation intensity. The corresponding single-pass performance shown in Fig. S1(b) further supports this conclusion [54].

Fig. 5.

Effect of inlet pressure on LEV degradation. Reaction conditions: [LEV]₀ = 30 mg/L, pH = unadjusted, T = 25 °C, V = 10 L. (Error bars represent standard deviation from triplicate measurements (n = 3)).

Within a defined operational window, increasing the inlet pressure intensifies cavitation by amplifying the pressure differential and turbulence within the Venturi flow field [55]. This promotes the nucleation of cavitation bubbles, which occur when the local static pressure drops below the liquid’s saturated vapor pressure. When the inlet pressure exceeds the optimal threshold, cavitation bubbles tend to coalesce, reducing the intensity of their collapse and, consequently, suppressing the generation of reactive radicals [56]. Notably, the optimal inlet pressure is not universal and varies considerably depending on the reactor configuration and the physicochemical characteristics of the target pollutants. For example, Pooja et al. [57] reported improved benzene degradation in an HC/air system as the inlet pressure increased from 1.8 to 2.4 bar; however, further increases beyond 2.4 bar led to a decline in performance, highlighting an optimal pressure range for effective cavitation. In the case of p-nitrophenol degradation using HC, Mauro et al. [58] observed a declining trend in CY at inlet pressures exceeding 0.45 MPa. Similarly, Jongbok et al. [59] reported inhibitory effects in an HC/PS system for bisphenol A degradation when the inlet pressures surpassed 0.5 MPa. Based on these observations and the current experimental findings, an optimal inlet pressure of 0.15 MPa was selected for subsequent experiments to ensure effective cavitation performance.

$${\text{H}}_2\text{O}\stackrel{\text{HC}}{\to }{\text{HO}}^{·}+{\text{H}}^{·}$$ (13)

3.3.2. Concentration ratio of Cu²⁺ to PMS

In the HC/Cu²⁺/PMS system, the concentration of Cu²⁺, which directly reacts with PMS, plays a critical role in modulating both PMS activation and LEV degradation. To evaluate this effect, the influence of Cu²⁺ on PMS concentration ratio was investigated under controlled conditions: initial LEV concentration ([LEV]₀) of 30 mg/L, PMS concentration ([PMS]₀) of 1.5 mM, P_in of 0.15 MPa, unadjusted pH, and a T of 25 °C. As illustrated in Fig. 6(a), Cu²⁺:PMS molar ratios of 1:1, 1.5:1, 2.0:1, and 2.5:1 yielded LEV degradation efficiencies of 79.38 %, 82.87 %, 97.94 %, and 93.26 %, respectively. A similar trend was observed in the corresponding pseudo-first-order degradation rate constants (Fig. 6(b), indicating that an optimal Cu²⁺:PMS ratio of 2.0:1 maximises system performance. The experimental results confirm the crucial catalytic role of Cu²⁺ in PMS activation. As a transition metal ion, Cu²⁺ promotes redox reactions by facilitating electron transfer processes, thereby generating SO₄^•−, as illustrated in Eq. (11). At a Cu²⁺: PMS molar ratio of 1:1, the system exhibited a relatively low LEV degradation rate, indicating inadequate activation of PMS and limited generation of reactive radicals, likely due to the insufficient concentration of Cu²⁺ required to drive effective redox processes.

Fig. 6.

(a) Effect of the concentration ratio of Cu²⁺ to PMS on LEV degradation. (b) Corresponding degradation rate constants for the reaction are shown in (a). Reaction conditions: [LEV]₀ = 30 mg/L, [PMS]₀ = 1.5 mM, P_in = 0.15 MPa, pH = unadjusted, T = 25 °C, V = 10 L. (Error bars represent standard deviation from triplicate measurements (n = 3)).

As the Cu²⁺:PMS molar ratio increases, the number of active sites available for PMS-Cu²⁺ interactions also rises, thereby enhancing PMS activation and improving LEV degradation efficiency, which reaches its peak at a ratio of 2:1 [60,61]. However, further increases beyond this optimal ratio result in a decline in performance, likely due to excessive Cu²⁺ leading to scavenging of reactive radicals or formation of less reactive species. This effect arises because an excessive concentration of Cu²⁺ leads to the overproduction of reactive radical species (Eqs. (14), (15)) [62], thereby increasing the likelihood of radical–radical recombination, promoting self-quenching, and ultimately reducing the effective concentration of oxidants available for LEV degradation [32]. Based on this observation, a Cu²⁺: PMS molar ratio of 2: 1 was employed in the subsequent experiments.

$${\text{Cu}}^{\text{2 +}}{\text{+ HSO}}_{\text{5}}^{\text{-}}\stackrel{\text{HC}}{\to }{\text{Cu}}^{+}{\text{+ SO}}_{\text{5}}^{·-}{\text{+ H}}^{+}$$ (14)

$${\text{SO}}_{\text{5}}^{·-}{\text{+ H}}_2\text{O}\to {\text{HSO}}_{\text{4}}^{\text{-}}{+}^{\text{1}}{\text{O}}_2$$ (15)

The influence of the Cu²⁺ to PMS concentration ratio was further investigated to optimize catalytic efficiency. However, beyond performance, the environmental fate of Cu²⁺ must be critically evaluated for practical application. By adjusting the pH of the solution, a significant portion of Cu²⁺ undergoes precipitation in alkaline conditions as Cu(OH)₂ or CuO, substantially reducing its bioavailability and acute toxicity [32]. This intrinsic precipitation serves as a primary, cost-effective mitigation step. For the remaining soluble fraction, tertiary treatment options such as adsorption on biochar or metal–organic framework (MOF) materials and ion-exchange processes offer reliable, scalable removal [8,34]. Thus, the potential environmental impact of residual Cu²⁺ is not prohibitive but manageable. Future scale-up efforts will include copper speciation analysis and integration of a polishing step to ensure environmental compliance.

3.3.3. Concentration of PMS

In the HC/Cu²⁺/PMS system, PMS serves as the primary source of reactive radicals. To evaluate the influence of PMS concentration on LEV degradation, experiments were conducted using various PMS concentrations (0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mM) under controlled conditions: initial LEV concentration ([LEV]₀) of 30 mg/L, Cu²⁺: PMS molar ratio of 2: 1, P_in of 0.15 MPa, unadjusted pH, and a constant temperature of 25 °C. As shown in Fig. 7(a), the LEV degradation rate initially increased and then declined with rising PMS concentration. Within the tested range, the enhancement in degradation can be attributed to the increased activation of PMS, leading to the generation of SO₄^•− and, to a lesser extent, HO^• (as described by Eqs. (11), (12)). Notably, complete LEV degradation (100 %) was achieved at the PMS concentrations of 2.0, 2.5, and 3.0 mM. The corresponding pseudo-first-order degradation rate constants were 0.0287, 0.0502, and 0.0370 min⁻¹, respectively (Fig. 7(b)). The highest rate constant observed at 2.5 mM indicates the optimal PMS concentration for maximum reaction kinetics, while the slight decline at 3.0 mM suggests possible radical-scavenging or self-quenching effects that limit further enhancement in degradation efficiency. This phenomenon can be attributed to the excessive generation of radicals at high PMS concentrations, which promotes self-scavenging reactions among the radicals themselves (Eqs. (16), (17), and (18)). Furthermore, H₂O₂ formed as a by-product can react with HO^• (Eqs. (19), (20)), thereby consuming reactive species and contributing to the observed decline in degradation efficiency [32,63]. Additionally, excessive PMS may directly react with reactive radicals (Eqs. (21), (22)) [32] or interact with solution constituents to form inhibitory by-products (Eqs. (23), (24)) [62], both of which can adversely impact the degradation rate. This inhibitory behavior of surplus PMS has been documented in previous studies. For example, Carmen et al. [64] investigated PS activation using thermal, alkaline, and iron salt methods and reported that the optimal degradation rate of p-nitrophenol was achieved at a PS concentration of 6.4 g/L. However, further increases in PS concentration led to a decline in performance due to radical scavenging effects. Similarly, Luo et al. [65] employed a Co-based MOF as a catalyst to activate PMS for the degradation of ciprofloxacin, demonstrating that excessive oxidant dosing can limit treatment efficiency. They observed that the maximum degradation efficiency was achieved at a PMS concentration of 1 mM, with higher concentrations leading to a decline in performance. Collectively, these studies demonstrate that increasing PMS concentration can effectively enhance pollutant degradation within an optimal range; however, excessive PMS levels may lead to inhibitory effects, ultimately diminishing overall degradation efficiency. Therefore, in practical applications, the PMS concentration must be carefully optimized based on the specific characteristics of the target pollutant and treatment conditions to achieve maximum degradation efficiency while minimizing unwanted side reactions or reagent waste.

$${\text{SO}}_{\text{4}}^{·-}+{\text{SO}}_{\text{4}}^{·-}\to {\text{S}}_2{\text{O}}_{\text{8}}^{\text{2 -}}$$ (16)

$${\text{HO}}^{·}{\text{+ HO}}^{·}\to {\text{H}}_2{\text{O}}_2$$ (17)

$${\text{SO}}_{\text{4}}^{·-}{\text{+ HO}}^{·}\to {\text{HSO}}_{\text{5}}^{-}$$ (18)

$${\text{H}}_2{\text{O}}_2{\text{+ HO}}^{·}\to ·{\text{HO}}_2{\text{+ H}}_2\text{O}$$ (19)

$${\text{HO}}^{·}+·{\text{HO}}_2\to {\text{H}}_2{\text{O + O}}_2$$ (20)

$${\text{HSO}}_{\text{5}}^{\text{-}}{\text{+ HO}}^{·}\to {\text{SO}}_{\text{5}}^{·-}{\text{+ H}}_2\text{O}$$ (21)

$${\text{HSO}}_{\text{5}}^{\text{-}}{\text{+ SO}}_{\text{4}}^{·-}\to {\text{HSO}}_{\text{4}}^{\text{-}}{\text{+ SO}}_5^{·-}$$ (22)

$${\text{HSO}}_{\text{5}}^{-}\to {\text{SO}}_{\text{5}}^{2-}{\text{+ H}}^{+}$$ (23)

$${\text{HSO}}_{\text{5}}^{\text{-}}{\text{+ SO}}_{\text{5}}^{\text{2 -}}\to {\text{HSO}}_{\text{4}}^{\text{-}}{\text{+ SO}}_{\text{4}}^{\text{2 -}}{+}^{\text{1}}{\text{O}}_2$$ (24)

Fig. 7.

(a) Effect of PMS concentration on LEV degradation. (b) Corresponding degradation rate constants for the reactions are shown in (a). Reaction conditions: [LEV]₀ = 30 mg/L, [Cu²⁺]₀:[PMS]₀ = 2:1, P_in = 0.15 MPa, pH = unadjusted, T = 25 °C, V = 10 L. (Error bars represent standard deviation from triplicate measurements (n = 3)).

3.3.4. Initial solution pH

Solution pH plays a crucial role in governing both the chemical speciation of reactants and the stability of reactive radicals in AOPs [66]. To assess its effect on LEV degradation, experiments were conducted at initial pH values of 4, 6, 8, and 10 under controlled conditions: initial LEV concentration ([LEV]₀) of 30 mg/L, [Cu²⁺]₀ of 5 mM, [PMS]₀ of 2.5 mM, P_in of 0.15 MPa, and T of 25 °C. As shown in Fig. 8(a), complete LEV degradation was achieved across all tested pH conditions; however, the initial pH significantly influenced the degradation rate. As illustrated in Fig. 8(b), the pseudo-first-order degradation rate constants increased from 0.0159 to 0.0393 min⁻¹ with increasing initial pH, indicating enhanced reaction efficiency at higher pH. These results suggest that under acidic conditions, elevated concentrations of H⁺ ions can react with reactive radicals such as (SO₄^•− and HO^•) (Eqs. (25), (26)) [67], thereby reducing the availability of oxidants for LEV degradation. In contrast, under alkaline conditions, PMS predominantly exists in its monoanionic (HSO₅⁻) and dianionic (SO₅²⁻) forms, which are less stable and more prone to decomposition, facilitating the generation of reactive species [68].

$${\text{SO}}_{\text{4}}^{·-}{\text{+ H}}^{+}{\text{+ e}}^{\text{-}}\to {\text{HSO}}_{\text{4}}^{·-}$$ (25)

$${\text{HO}}^{·}{\text{+ H}}^{+}{\text{+ e}}^{\text{-}}\to {\text{H}}_2\text{O}$$ (26)

Fig. 8.

(a) Effect of initial pH on LEV degradation. (b) Corresponding degradation rate constants of the reactions in (a). Reaction conditions: [LEV]₀ = 30 mg/L, [Cu²⁺]₀ = 5 mM, [PMS]₀ = 2.5 mM, P_in = 0.15 MPa, T = 25 °C, V = 10 L. (Error bars represent standard deviation from triplicate measurements (n = 3)).

In addition, variations in pH influence the speciation of LEV [69], as illustrated in Fig. 9. At pH values below its first pKa (pKa₁), LEV predominantly exists in the protonated form (H₂L⁺), where the piperazinyl group is fully protonated, resulting in a cationic species. Between pk_a1 and pk_a2, LEV primarily exists as a zwitterion (HL^±) or in its neutral form (HL⁰), depending on the degree of ionization of its functional groups. When the pH exceeds pk_a2, the carboxylic acid group of LEV deprotonates, forming an anionic species [70]. These pH-dependent changes in functional group ionization influence both the structural stability and the chemical reactivity of LEV. At lower pH values, LEV carries a stronger positive charge due to protonation, leading to electrostatic repulsion with the positively charged Cu²⁺ and reducing the effective collision frequency with reactive radical species, thereby diminishing degradation efficiency. When the pH is between LEV’s pk_a1 and pk_a2 values or exceeds neutral pH (pH > 7), most of the carboxyl groups become deprotonated, leading to a reduction in the molecule’s net positive charge and a gradual shift toward a negatively charged state [71]. The increased negative charge enhances electrostatic attraction to positively charged species, such as Cu²⁺ and radical intermediates, thereby facilitating stronger interactions and promoting more efficient degradation.

Fig. 9.

Molecular structure of LEV at different pH values, illustrating its protonation and deprotonation states.

3.3.5. Initial concentration of LEV

Under the optimized conditions ([Cu²⁺]₀ = 5 mM, [PMS]₀ = 2.5 mM, p = 0.15 MPa, and pH = 10), the influence of initial LEV concentration on degradation performance was assessed using concentrations above typical environmental levels, specifically, 10, 20, 30, and 40 mg/L [7,72]. Fig. 10 presents the degradation profiles and corresponding rate constants, revealing a decreasing trend in degradation efficiency with increasing initial LEV concentration. Complete degradation of LEV at an initial concentration of 10 mg/L was achieved within approximately 40 min, whereas 60 min were required to achieve the same outcome at a higher initial concentration of 40 mg/L. A similar trend has been reported in previous studies [73,74], indicating a negative correlation between the initial pollutant concentration and the degradation rate. This reduction in efficiency is primarily due to the competitive consumption of reactive radicals by both LEV and its intermediate degradation products, thereby limiting the availability of oxidants for complete mineralisation. Under identical operating conditions, the total amount of reactive radicals generated in the HC/Cu²⁺/PMS system remains constant. While a higher initial LEV concentration may improve the overall radical utilisation, the ratio of available radical reaction sites to LEV molecules decreases [75], thereby reducing degradation efficiency due to insufficient oxidant availability per pollutant molecule.

Fig. 10.

(a) Effect of initial LEV concentration on its degradation efficiency. (b) Corresponding degradation rate constants of the reactions are shown in (a). Reaction conditions: [Cu²⁺]₀ = 5 mM, [PMS]₀ = 2.5 mM, P_in = 0.15 MPa, pH = 10, T = 25 °C, V = 10 L. (Error bars represent standard deviation from triplicate measurements (n = 3)).

3.4. Identification of the active substances

The primary radical species involved in the activation of PMS are SO₄^•− and HO^•, along with other reactive oxygen species such as singlet oxygen (¹O₂) and superoxide anion (O₂^−•). These reactive species contribute collectively to the degradation of LEV in the HC/Cu²⁺/PMS system. To determine the individual contributions of SO₄^•− and HO^•, radical quenching experiments were conducted using TBA and MeOH as scavengers, respectively. TBA exhibits high reactivity toward HO^• (K_{HO•, TBA} = (3.8–7.6) × 10⁸ M⁻¹s⁻¹) but low reactivity toward SO₄^•− (K_{SO4•−, TBA} = (4–9.1) × 10⁵ M⁻¹s⁻¹), whereas MeOH quenches both radicals, with rate constants of K_{SO4•−, MeOH} = 3.2 × 10⁶ M⁻¹s⁻¹ and K_{HO•, MeOH} = 9.7 × 10⁶ M⁻¹s⁻¹ [76]. LEV degradation experiments were performed under standard conditions in the presence of 100 mM of each scavenger. As shown in Fig. 11(a), the addition of either TBA or MeOH resulted in a notable decrease in LEV degradation after 60 min of reaction compared to the control without scavengers. MeOH exhibited greater inhibition than TBA, suggesting that SO₄^•− plays a dominant role in the degradation process. The degradation rate constants under different quenching conditions, as presented in Fig. 11(b), were used to estimate the contributions of SO₄^•− and HO^• to LEV degradation quantitatively using the following Eqs. (27), (28), respectively [77]. The results indicate that SO₄^•− contributed 50.38 % while HO^• contributed 26.72 % to the total degradation. Although a minor fraction of LEV may be degraded by other ROS such as ¹O₂ or O₂^−•, SO₄^•− remains the most significant reactive species in the system.

$${\text{HO}}^{·}contribution\text{(\%)}=\frac{{\text{K}}_{\mathit{obs}}-{\text{K}}_{\mathit{obs}+TBA}}{{\text{K}}_{\mathit{obs}}}\times 100\text{\%}$$ (27)

$${\text{SO}}_{\text{4}}^{·-}\text{contribution(\%)}=\frac{{\text{K}}_{\mathit{obs}+TBA}-{\text{K}}_{\mathit{obs}+MeOH}}{{\text{K}}_{\mathit{obs}}}\times 100\text{\%}$$ (28)

Fig. 11.

(a) Effect of radical scavengers (TBA and MeOH) on LEV degradation. (b) Corresponding degradation rate constants. Reaction conditions: [LEV]₀ = 30 mg/L, [Cu²⁺]₀ = 5 mM, [PMS]₀ = 2.5 mM, P_in = 0.15 MPa, pH = 10, T = 25 °C, V = 10 L. (Error bars represent standard deviation from triplicate measurements (n = 3)).

3.5. Mechanistic insights into LEV degradation under PMS activation

In the HC/Cu²⁺/PMS system, the collapse of cavitation bubbles leads to an intense energy release, which can initiate a cascade of complex chemical reactions. Among these, radical-mediated pathways are of particular interest to this study due to their critical role in pollutant degradation. Extensive research on HC has led to a more comprehensive understanding of the underlying reactions. During the rapid formation and collapse of cavitation bubbles, localized zones of extremely high temperatures and pressures are created. These extreme conditions induce the pyrolysis of water molecules, generating highly reactive H^• and HO^•. A portion of the generated radicals recombines within the cavitation bubbles, while others diffuse into the surrounding liquid phase [29]. At the gas–liquid interface, a high concentration of diffused radicals accumulates, where they may undergo self-reactions or interact with other dissolved chemical species in the bulk solution [78]. In contrast to the extreme conditions inside the bubbles, the surrounding liquid region remains at ambient temperature. Here, solutes that have not penetrated the bubble core can still react with various radical species, contributing to the overall chemical transformation.

Based on the mechanistic insights discussed above, a plausible pathway illustrating the interactions among HC, Cu²⁺ ions, PMS activation, and the subsequent degradation of LEV is proposed in Fig. 12. The energy released during the collapse of cavitation bubbles generates extreme local conditions (temperatures > 5000 K and pressures > 1000 atm), which drive the homolytic cleavage of water molecules (Eq. (13)), forming reactive radicals. These radicals subsequently undergo recombination reactions, leading to the in situ formation of H₂O₂ (Eq. (17)) [27]. The introduced Cu²⁺ ions initiate a catalytic redox cycle. They can either participate in a pseudo-Fenton reaction with the generated H₂O₂ (Eq. (29)) or react directly with PMS (Eq. (11)), producing SO₄^•−, HO^•, and Cu³⁺ [79]. This process involves electron transfer from Cu²⁺, promoting its oxidation (${\text{Cu}}^{\text{2 +}}\to {\text{Cu}}^{\text{3 +}}+{\text{e}}^{\text{-}}$). Simultaneously, under the influence of cavitation-induced extreme conditions, PMS is activated via electron acceptance (Eq. (30)), further accelerating SO₄^•− generation [80]. The Cu³⁺ species formed are highly oxidising and thermodynamically unstable. They can be readily reduced back to Cu²⁺ by reacting with PMS (Eq. (31)) [81], thus preventing catalyst deactivation. Additionally, Cu²⁺ may also directly activate PMS under the influence of cavitation, further enhancing SO₄^•− generation (Eqs. (14), (32)) [31]. This dynamic redox cycling among Cu⁺/Cu²⁺/Cu³⁺ species establishes a highly efficient catalytic loop, maximising radical generation and promoting sustained catalytic activity of Cu²⁺. It is important to acknowledge that the proposed involvement of Cu³⁺, while consistent with the enhanced degradation performance and supported by previous studies on copper-activated PMS systems, is inferred indirectly in this work. Therefore, its specific participation in the HC-based process, although highly plausible within the established catalytic cycle, is partly speculative and warrants further investigation using advanced in situ spectroscopic techniques. Ultimately, LEV undergoes oxidative degradation via successive reactions with reactive radicals, forming low-molecular-weight organic intermediates, which are subsequently mineralised to CO₂ and H₂O.

$${\text{Cu}}^{\text{2 +}}{\text{+ H}}_2{\text{O}}_2\to {\text{Cu}}^{\text{3 +}}{\text{+ HO}}^{·}{\text{+ OH}}^{\text{-}}$$ (29)

$${\text{HSO}}_{\text{5}}^{\text{-}}{\text{+ e}}^{\text{-}}\to {\text{SO}}_{\text{4}}^{·-}{\text{+ OH}}^{\text{-}}$$ (30)

$${\text{Cu}}^{\text{3 +}}{\text{+ HSO}}_{\text{5}}^{\text{-}}\to {\text{Cu}}^{\text{2 +}}{\text{+ SO}}_{\text{5}}^{·-}{\text{+ H}}^{+}$$ (31)

$${\text{Cu}}^{+}{\text{+ HSO}}_{\text{5}}^{\text{-}}\stackrel{\text{HC}}{\to }{\text{Cu}}^{\text{2 +}}{\text{+ SO}}_{\text{4}}^{·-}{\text{+ OH}}^{\text{-}}$$ (32)

Fig. 12.

Proposed reaction mechanism for LEV degradation in the HC/Cu²⁺/PMS system.

3.6. Proposed degradation pathway of LEV

Due to the highly reactive and non-selective nature of oxidative radical species, AOPs often generate a wide range of intermediate compounds during the degradation of pollutants [82]. Therefore, LC-MS analysis was conducted to identify the molecular structures of these intermediates and to elucidate the potential degradation pathway of LEV. Table 3 summarises the molecular structures of the 10 intermediates characterised. Fig. S2 presents the LC-MS spectrum of the solution after 50  min of LEV degradation.

Table 3.

Degradation by-products in the HC/Cu²⁺/PMS system.

Compound	m/z	Molecular formula	Molecular structure
LEV	362	C18H20FN3O4
L1	318	C17H20FN3O2
L2	274	C15H17N3O2
L3	250	C13H17N3O2
L4	103	C4H9NO2
L5	341	C18H19N3O4
L6	212	C11H9NO2
L7	150	C9H10O2
L8	367	C16H21N3O7
L9	308	C15H21N3O4
L10	72	C3H8N2

Based on the identified intermediates, the proposed degradation pathway of LEV is illustrated in Fig. 13, outlining the sequential transformation steps inferred from the LC-MS analysis. The degradation of LEV involved multiple transformation reactions, including decarboxylation, demethylation, defluorination, de-piperazinylation, and hydroxylation/dehydrogenation. Pathway Ⅰ begins with a decarboxylation step, leading to the formation of intermediate L1 ([M + H]⁺ m/z 318) [83]. Subsequently, intermediates L2-L4 (([M + H]⁺ m/z 274), ([M + H]⁺ m/z 250), ([M + H]⁺ m/z 103)) [[84], [85], [86], [87]] were formed through a series of radical-induced reactions, including demethylation, defluorination, piperazine ring cleavage, and hydroxylation. In parallel, defluorination and dehydrogenation of the LEV core structure led to the formation of intermediate L5 ([M + H]⁺ m/z 341) [85]. Intermediate L6 ([M + H]⁺ m/z 212) [88] was generated through a combination of decarboxylation, cleavage of the piperazine ring, and demethylation of the morpholine moiety. L7 ([M + H]⁺ m/z 150) resulted from the rupture of the morpholine ring. Intermediates L8 ([M + H]⁺ m/z 367) and L9 ([M + H]⁺ m/z 308) were derived from the parent LEV molecule via hydroxylation and decarboxylation reactions. L10 ([M + H]⁺ m/z 72) was formed through piperazine ring cleavage, followed by methylation and dehydrogenation. Ultimately, all intermediate compounds were further degraded into smaller molecules, including CO₂, H₂O, and various inorganic ions [86].

Fig. 13.

Proposed degradation pathway of LEV in the HC/Cu²⁺/PMS system.

3.7. Applicability of the HC/Cu²⁺/PMS system

Given the wide diversity of pollutants present in natural ecosystems, it is essential to assess the applicability of the HC/Cu²⁺/PMS system across a range of model contaminants to evaluate its broader usability and effectiveness. Therefore, the system's degradation performance was evaluated under optimal operating conditions (10 L capacity) using three representative pollutants, MG, RhB, and DH, at their respective initial concentrations. As shown in Fig. 14, the degradation rates of MG, RhB, and DH reached 94.22 %, 100 %, and 99.11 %, respectively, within 60 min. These results demonstrate the high efficiency and scalability of the HC/Cu²⁺/PMS system. In comparison, Hu et al. [89] reported a 92.7 % degradation of MG in a 250 mL solution using an EDTA-Fe(III)-based Fenton process over 90 min, highlighting the superior performance of the current system in both reaction rate and treatment volume. Gu et al. [90] reported nearly 90 % degradation of RhB in a 6 mL solution using laser-induced cavitation after 60 min. Similarly, Sun et al. [91] achieved 92.18 % degradation of doxycycline hydrochloride in a 50 mL solution over 120 min using photocatalysts based on TiO₂-anchored iron tailings, with approximately 80 % removal observed at 60 min. The promising results from these lab-scale (10 L) applicability tests demonstrate the broad potential of the HC/Cu²⁺/PMS system. The core components of this technology, including the centrifugal pump and Venturi reactor, are inherently modular and scalable to larger treatment volumes. Future efforts will address key engineering challenges in scale-up, including optimising reactor geometry for intensified cavitation, improving energy utilisation through process integration, and evaluating system performance in real wastewater environments, where matrix effects and radical scavenging by background constituents may influence degradation efficiency.

Fig. 14.

Degradation effectiveness of different pollutants using the HC/Cu²⁺/PMS system. Reaction conditions: [MG]₀ = [RhB]₀ = [DH]₀ = 30 mg/L, [Cu²⁺]₀ = 5 mM, [PMS]₀ = 2.5 mM, P_in = 0.15 MPa, pH = 10, T = 25 °C, V = 10 L. (Error bars represent standard deviation from triplicate measurements (n = 3)).

3.8. Economic feasibility of the HC/Cu²⁺/PMS system

HC demonstrates substantial potential in wastewater treatment. For its successful practical implementation, it is crucial to conduct a comprehensive assessment of both treatment efficacy and operational costs associated with HC-based AOPs. In this study, HC-based treatment systems were comparatively evaluated using CY (Eq. (9)) and EE/O (Eq. (10)) as key performance indicators. As summarized in Table 4, under operating conditions optimized for LEV degradation, the HC/Cu²⁺/PMS system demonstrated superior energy efficiency, achieving the lowest EE/O value of 229.48 kWh/m³ and the highest CY of 2.78 × 10⁻⁵ mg/J. This work represents the first report of the successful application of an HC-based process for the degradation of LEV, a significant advancement given the antibiotic's environmental persistence. As shown in Table 5, the coupled system employed in this study exhibits excellent performance compared to other HC-based water treatment approaches. Notably, in contrast to LEV degradation systems reported in previous studies, the present system achieved complete LEV degradation within 60 min, representing a highly efficient and desirable treatment outcome [92,93]. A comparison of degradation data for other pollutants studied in this work supports these findings. These performance metrics confirm the system’s dual advantage of high degradation efficiency and cost-effectiveness, positioning it as a promising and practical solution for large-scale wastewater remediation.

Table 4.

Performance metrics (η, CY, and EE/O) for different HC-based systems used in this study.

Systems	Pollutant	η (%)	CY (mg/J)	EE/O (kWh/m3)
HC	LEV	18.77	5.21 × 10−6	2363.27
HC/PMS	LEV	37.80	1.05 × 10−5	1201.24
HC/Cu2+/PMS	LEV	100	2.78 × 10−5	229.48
HC/Cu2+/PMS	MG	94.22	2.62 × 10−5	220.27
HC/Cu2+/PMS	RhB	100	2.78 × 10−5	180.85
HC/Cu2+/PMS	DH	99.11	2.75 × 10−5	105.11

Table 5.

Comparison of pollutant degradation performance in different HC-based systems reported in the literature.

Method	Pollutant	V (L)	P (kW)	t (min)	η (%)	CY (mg/J)	EE/O (kWh/m3)	Ref.
Orifice plate (HC/α-Fe2O3/PS)	Sulfadiazine (SDZ)	5	1.1	90	81.24	1.68 × 10−5	203.57	[55]
Venturi (HC/PC)	Tetracycline (TC)	4	7.5	90	78.20	2.32 × 10−6	4311.37	[94]
Orifice (HC/Fe0/PS)	Tetracycline (TC)	1	3	30	97.80	7.24 × 10−6	250.43	[56]
Orifice (HC/PS)	Bisphenol A (BPA)	10	1.1	120	81.28	1.03 × 10−5	343.41	[59]
Venturi (HC/PDS)	Atrazine (ATZ)	3	0.25	60	95.00	1.33 × 10−6	213.33	[26]
Venturi (HC)	Rhodamine B (RhB)	4	3	120	38.7	7.17 × 10−7	6620.69	[95]
Venturi (HC/PC)	Doxycycline hydrochloride (DH)	5	1.5	60	97.6	2.71 × 10−5	185.54	[96]

3.9. Toxicity assessment

To validate the experimental feasibility of the HC/Cu²⁺/PMS system, toxicity analysis was conducted for LEV and its major degradation intermediates [97]. The predicted endpoints included developmental toxicity, mutagenicity, the 96 h LC₅₀ for Fathead minnow, and the 48 h LC₅₀ for Daphnia magna. As shown in Fig. 15(a), the majority of intermediates exhibited lower developmental toxicity and comparable or reduced mutagenicity relative to LEV. Additionally, most intermediates showed higher LC₅₀ values for Fathead minnow (Fig. 15(c)), indicating reduced acute toxicity. Only a few compounds (L1, L3, L7) displayed slightly higher toxicity (Fig. 15(d)) toward Daphnia magna [98], while others (L8 and L9) could not be reliably predicted due to high water solubility [99]. Overall, these results indicate that the degradation pathway effectively reduces the ecological toxicity of LEV, supporting the environmental safety of the process [100].

Fig. 15.

Toxicity of LEV and its degradation intermediates. (a) Developmental toxicity. (b) Mutagenicity. (c) LC50 for Fathead Minnow (96 h). (d) LC50 for Daphnia magna (48 h).

4. Conclusion

This study systematically explored the synergistic effects, degradation kinetics, and underlying reaction mechanisms of a novel HC/Cu²⁺/PMS system for the degradation of LEV. The results demonstrate that integrating HC significantly enhances LEV degradation efficiency compared to the standalone Cu²⁺/PMS system. The optimal operating conditions were identified as an inlet pressure of 0.15 MPa, a Cu²⁺ to PMS molar ratio of 2: 1, a PMS concentration of 2.5 mM, and an initial solution pH of 10. The combination of HC and Cu²⁺ effectively enhances the interaction between Cu²⁺ and PMS, facilitating PMS activation through the localized high temperatures and pressures generated by cavitation bubble collapse, as well as the improved mass transfer induced by HC. This synergistic effect leads to a substantial increase in LEV's degradation efficiency. Degradation experiments were conducted to assess the system’s applicability using MG, RhB, and DH as representative pollutants. The results confirmed that the system exhibits excellent degradation performance across a broad spectrum of contaminants. Economic analysis suggests that integrating multi-system synergistic components significantly enhances cost-effectiveness compared to single-system approaches. Despite the high degradation efficiency, the environmental implications of residual Cu²⁺ and degradation intermediates must be considered. In the HC/Cu²⁺/PMS system, copper serves as an active catalyst, and under alkaline conditions (pH = 10), a considerable fraction precipitates as Cu(OH)₂ or CuO. To ensure a benign effluent, cost-effective post-treatment methods such as biochar adsorption or ion exchange are recommended. Future studies will focus on quantifying copper speciation, verifying toxicity trends of intermediates, and conducting pilot-scale trials to assess system scalability, energy demand, and operational feasibility. Overall, the HC/Cu²⁺/PMS system offers a promising and energy-efficient approach for antibiotic degradation, provided that these limitations are systematically addressed in subsequent research.

CRediT authorship contribution statement

Zheng Li: Writing – original draft, Investigation, Formal analysis, Data curation. Weibin You: Methodology. Sivakumar Manickam: Writing – review & editing. Haiyan Bie: Writing – review & editing. Wenlong Wang: Writing – review & editing. Xun Sun: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: One of the authors is an Executive Editor of this journal, Sivakumar Manickam. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article: