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In this study, the mesoporous material NCNT was prepared by treating carbon nanotubes (CNT) with hydrazine and subsequently loaded with Cu-Fe layered double hydroxide (CuFeLDH) to create a multiphase catalyst (CuFeLDH-NCNT). Its application as a multiphase catalyst was investigated in an ultrasound-assisted Fenton process for ciprofloxacin (CIP) degradation in aqueous solution. In addition, the impacts of catalyst dosage, ultrasonic power, H2O2 dosage, and beginning pH on CIP removal efficiency were carefully evaluated to maximize the removal efficiency of CIP. The findings indicated that the elimination rate of the initial CIP concentration of 20 mg/L surpassed 94.66% after a mere 100 min, while the TOC degradation rate was 70.4%. The high removal rate was due to the synergistic action between the nanoparticles, H2O2, and ultrasonography. The degradation intermediates of CIP were examined, and putative degradation pathways and mechanisms were postulated.

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  • Preparation of CuFeLDH–NCNT material as an efficient catalyst for ultrasound-assisted Fenton-like reaction.

  • Enhanced catalytic performance of H2O2 by the synergistic effect of CuFeLDH–NCNT.

  • Effective degradation of CIP by CuFeLDH–NCNT/US/H2O2.

  • Good stability and reusability of CuFeLDH–NCNT, with a low metal leaching rate.

ciprofloxacin, CuFeLDH, Fenton-like, ultrasonic
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The excessive use of antibiotics in personal care products (PPCPs), animal disease prevention, aquaculture, and animal husbandry can dramatically impact the ecological environment and human health even at minimum doses (Wang et al. 2017; Zhang et al. 2021; Vasseghian et al. 2023). Ciprofloxacin (CIP) is one of the most common antibiotics. The chief sources of CIP contamination include domestic sewage, hospital wastewater, and wastewater from the pharmaceutical industry (Liu et al. 2022a). In pharmaceutical wastewater, its concentration can reach 6 mg/L–31 mg/L (Dang et al. 2023), their production and usage are both at the forefront of the world, which would cause substantial harm to the immune systems of both ecosystems and consumers. Due to CIP's high ecotoxicity and structural complexity, typical physical, chemical, and biological treatment procedures make removing it from aquatic habitats challenging (Shah et al. 2019; Shah et al. 2020).

To successfully reduce antibiotic pollution in water resources (Alonso et al. 2018), the Fenton reaction with H2O2 oxidation is regarded as a feasible and valuable technique for degrading harmful and refractory contaminants (Shah et al. 2018), and complex organic pollutants can be converted into substances of lesser harm and even into inorganic materials such as CO2 and H2O. However, classic homogeneous Fenton technology is constrained by characteristics such as the massive amount of Fe2+ supplied, the rigorous pH value (around 2.8), and the difficulty of recycling the catalyst. These factors severely limit their application in crucial parts. Multiphase Fenton catalysts have been created to mitigate these harmful effects, including metal oxides (Chen et al. 2017) and metal sulfides (Liu et al. 2018).

Layered double hydroxides (LDHs) are widely employed because of their high stability, non-toxicity, good exchange performance for anions, low cost, and simple production process (Salem et al. 2023). CuFeLDH is the catalyst of choice in this investigation for the target pollutants' degradation. Moreover, the transition metal copper speeds up the conversion of Fe(III) to Fe(II), and its distinct lamellar structure increases the contact area with the target contaminants. However, due to the interlayer electrostatic forces of LDH, it is prone to stacking or aggregation, which usually results in the covering of catalytic active sites and a decrease in specific surface area, lowering its catalytic activity (Chen et al. 2019; Yin et al. 2021; Boumeriame et al. 2022). Therefore, numerous ways have been devised to modify LDHs, principally by doping them with different semiconductors and immobilizing them on the needed substrates to optimize their nanocatalyst activity (Ni et al. 2018). At the same time, new nanocarbon materials have introduced more alternatives for multifunctional catalyst carriers, including graphene, carbon nanotubes (CNTs), and biochar (Gautam et al. 2022). Among them, CNTs possess distinct advantages such as a substantial specific surface area, excellent thermal stability, high conductivity, and a multitude of surface functional groups, making them well-suited for the development of LDH. Not only can they reduce the accumulation of LDH, but they can also promote electron transport during the reaction process and expedite the reaction rate (Gautam et al. 2022; Liu et al. 2022b). In addition, CNTs can potentially increase their dispersibility in water by doping nitrogen (Hu et al. 2023).

Some researchers have revealed that ultrasound (US) has a tremendous penetrating ability in liquid media (Khatri et al. 2016), and its cavitation effect has been widely exploited in organic wastewater treatment. The cavitation bubbles formed by ultrasonic irradiation experience three stages: nucleation, expansion, and implosion. In the implosion stage, the bubbles burst violently, generating high temperatures and pressure in the liquid. The result of this process is the production of local hotspots with high pressure and temperatures (Tyagi et al. 2014; Wang et al. 2020). These hotspots can accelerate the breakdown rate of H2O2 and water molecules (Gholami et al. 2020). Hence, they enhance the production of reactive radicals •OH and foster the advancement of the Fenton-like reaction (Equations (1) and (2)).
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(2)

This work produced carbon nanotube-loaded LDH as a Fenton-like catalyst utilizing hydrothermal synthesis and selected ultrasonic irradiation as the enhancing method for the Fenton-like reaction, using CIP as the target pollutant. The •OH formed in the Fenton-like response can selectively oxidize and degrade CIP and its intermediates. By characterizing CuFeLDH–NCNT, comparing different oxidation processes, and analyzing the impacts of various operational parameters like catalyst concentrate, ultrasonic power, H2O2 concentration, and initial pH on CIP degradation, we aim to unravel the function of free radicals and evaluate the metal leaching and stability of CuFeLDH–NCNT. The overall performance of CuFeLDH–NCNT is assessed. Finally, the various processes and paths for CIP oxidation and degradation are addressed.

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Materials

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Ciprofloxacin (C17H18FN2O3, purity: 98%) was obtained from Maclean Biochemical Technology Co., Ltd (located in Shanghai, China), hexafluoride nitrate copper (Cu(NO3)2·6H2O, purity: 99%), nonahydrate nitrate iron (Fe(NO3)3·9H2O, purity: 98.5%), urea (CH4N2O, purity: 99%), ammonium fluoride (NH4F, purity: 96.5%), sulfuric acid (H2SO4, purity:98%), sodium hydroxide (NaOH, purity: 98.5%), anhydrous ethanol (EtOH, purity: 99.7%), and tert-butyl alcohol (TBA, purity: 95%) were all obtained from Guang Fu Fine Chemical Research Institute (located in Tianjin, China), raw multi-wall carbon nanotubes (MWCNTs, purity: 95%) were obtained from Maclean Biochemical Technology Co., Ltd (located in Shanghai, China).

Modification of CNT

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Initially, a particular amount of raw CNTs was combined with concentrated nitric acid and mixed in an ultrasonic cleaner for 1 h. Subsequently, the combination was rinsed with distilled water until it reached a neutral pH level. The modified CNTs were placed in a vacuum dryer and dried for 12 h at 60 °C. Ultimately, the pulverized powder was positioned in a muffle furnace and calcined for 3 h at 400 °C. This procedure tries to eliminate contaminants from the CNTs and graft oxygen groups onto the CNTs.

Additionally, 300 mg of the treated CNTs were added to a mixed solution of deionized water and hydrazine in a 1:1 ratio and heated under reflux for 6 h at 60 °C. After being washed neutral, it was transferred to a vacuum drier for drying, and the sample was labeled as NCNT.

Preparation of CuFeLDH–NCNT

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CuFeLDH–NCNT composites were prepared using the hydrothermal method in a 100-mL autoclave lined with PTFE. A specific amount of Cu (NO3)2·6H2O, Fe (NO3)3·9H2O, CH4N2O, NH4F, and 0.5 g of NCNT were mixed in deionized water (70 ml), with a Cu/Fe molar ratio of 2:1, and the mixed solution was magnetically stirred for 30 min. Then, the combined solution was moved to the autoclave and maintained for 24 h at 90 °C. Finally, the nanocomposite was washed clean and transferred to vacuum drying for drying, and the final nanocomposite was CuFeLDH–NCNT (Equations (3)–(5)). The FeLDH and CuFeLDH were synthesized using the same method as the control experiments.
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Characterization of materials

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This experiment characterized and analyzed the surface morphology of NCNT, CuFeLDH, and CuFeLDH–NCNT using a scanning electron microscope (SEM, Zeiss Sigma 300, Germany). X-ray powder diffraction (XRD, Bruker D8 Advance, Germany) determined the crystallinity and purity of materials. The main functional groups of the samples were analyzed using a Fourier transform infrared spectrometer (FT-IR et al. iS20, USA). The main elements of CuFeLDH–NCNT nanomaterials, such as iron, copper, and carbon, were confirmed using X-ray photoelectron spectroscopy (XPS, PHI5000 Versaprobe III, Japan). Different materials' electrochemical impedance spectroscopy (EIS) was analyzed using an electrochemical workstation (CHI660, China). The total organic carbon (TOC) was analyzed using a TOC analyzer (Multi C/N 3100, Germany). The primary radicals responsible for the ultrasonic-Fenton-like system were inferred from the radical scavenging experiments as •OH. CIP's intermediate products and possible degradation pathways were inferred using a liquid chromatography-mass spectrometry (LC-MS, Ultimate 3000 UHPLC-Q, US) instrument.

CIP degradation experiment

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The catalytic activity of synthetic samples in the CIP ultrasonic-Fenton-like system was investigated. Typically, under room temperature conditions, the required CuFeLDH–NCNT, H2O2, and 250 mL of 20 mg/L CIP solution are added separately into 250 mL conical bottles, and the pH is adjusted using 0.1 mol/L H2SO4 and NaOH solutions. Then, the key was irradiated with an ultrasonic bath (YM-060S, China), and under the action of ultrasonic-Fenton-like, the CIP solution began to degrade. Before the reaction, the material is pre-treated in a constant-temperature water bath shaker at 160 rpm for 20 min to achieve adsorption equilibrium and then placed in the ultrasonic bath for reaction. During the measurement, 5 mL of the reaction solution was extracted at 0, 20, 30, 40, 50, 60, 80, and 100 min, and a specific volume of NaOH solution was incorporated to bring the reaction to an end. Subsequently, the CIP concentration was determined employing a UV-visible spectrophotometer (UV-2800A, China) at a wavelength of 277 nm. The degradation efficiency and reaction rate constant were calculated according to Equations (6) and (7):
(6)
(7)
and represent the initial and CIP concentrations measured at different times (t), respectively; all experiments were repeated, and the error bars represent the standard errors of the repeated experiments.
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Material characterization

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SEM images

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Figure 1 exhibits the SEM images of NCNT, CuFeLDH, and CuFeLDH–NCNT nanocomposite. The images displayed in Figure 1(a) and 1(d) indicated that NCNT exhibited a distinctive tubular structure, indicating that the pretreatment did not alter the overall design of the original CNT, hence rendering it an excellent carrier for loading CuFeLDH. Figure 1(b) and 1(e) depicts the morphology of CuFeLDH, which was synthesized from layered nanoplates into microscale spheres. The SEM images of CuFeLDH–NCNT nanocomposite (Figure 1(c) and 1(f)) substantiated the successful integration of CuFeLDH-modified microscale spheres and NCNTs.
Figure 1
SEM images of NCNT (a, d), CuFeLDH (b, e), and CuFeLDH–NCNT (c, f).
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SEM images of NCNT (a, d), CuFeLDH (b, e), and CuFeLDH–NCNT (c, f).

Figure 1
SEM images of NCNT (a, d), CuFeLDH (b, e), and CuFeLDH–NCNT (c, f).
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SEM images of NCNT (a, d), CuFeLDH (b, e), and CuFeLDH–NCNT (c, f).

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XRD patterns

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Figure 2 describes the XRD patterns of NCNT, CuFeLDH, and CuFeLDH–NCNT samples (a)–(c). Distinct characteristic peaks were observed at 2θ = 26.03° and 43.35°, typical characteristic diffraction peaks of NCNT. The characteristic peaks at 17.41°, 24.23°, 31.64°, and 35.52° were characteristic diffraction peaks of Cu (OH)2 and Fe (OH)3. In the CuFeLDH–NCNT sample, the characteristic diffraction peak of NCNT was significantly weakened, indicating that LDH nanoparticles completely cover NCNT. It may be attributed to the fact that the surface of NCNT was doped with many oxygen-containing groups, which carried negative charges, while the LDH metal ions carried positive charges. These charges interacted electrostatically, forming a stable nanocomposite material. In addition, the LDH in the nanocomposite maintained its characteristic diffraction peaks, indicating that the CuFeLDH–NCNT nanocomposite also retained the structure of CuFeLDH.
Figure 2
XRD patterns of (a) NCNT, (b) CuFeLDH, and (c) CuFeLDH–NCNT.
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XRD patterns of (a) NCNT, (b) CuFeLDH, and (c) CuFeLDH–NCNT.

Figure 2
XRD patterns of (a) NCNT, (b) CuFeLDH, and (c) CuFeLDH–NCNT.
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XRD patterns of (a) NCNT, (b) CuFeLDH, and (c) CuFeLDH–NCNT.

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FT-IR spectra

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The FT-IR spectra of CNT, NCNT, CuFeLDH, and CuFeLDH–NCNT samples are shown in Figure 3(a)–3(d). In NCNT, CuFeLDH, and CuFeLDH–NCNT samples, a distinct O–H stretching peak could be observed (Sharifi-Bonab et al. 2020). Moreover, there was only a weak C = O peak (1,449 cm−1) in the CNT sample; the spectrum of the NCNT sample exhibited C = N stretching peaks at 1,550 cm−1 and C = O stretching peaks at 1,654 cm−1 (Mondal et al. 2015; Li et al. 2022), indicating the successful modification of CNT. However, no obvious C = N stretching peak was observed in the CuFeLDH–NCNT sample, possibly due to its low nitrogen content. The FT-IR spectrum of CuFeLDH–NCNT was very similar to that of the CuFeLDH precursor. Therefore, the 518 and 697 cm−1 peaks in the low-frequency region represent the Cu–O and Fe–O stretching peaks (Bloch et al. 2011; Chen et al. 2011), respectively. Additionally, the peaks at 1,617, 1,384, and 817 cm−1 may be caused by the stretching vibrations of C = O, C–N, and C–O (Fang et al. 2019), respectively.
Figure 3
FT-IR spectra of (a) CNT, (b) NCNT, (c) CuFeLDH, and (d) CuFeLDH–NCNT.
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FT-IR spectra of (a) CNT, (b) NCNT, (c) CuFeLDH, and (d) CuFeLDH–NCNT.

Figure 3
FT-IR spectra of (a) CNT, (b) NCNT, (c) CuFeLDH, and (d) CuFeLDH–NCNT.
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FT-IR spectra of (a) CNT, (b) NCNT, (c) CuFeLDH, and (d) CuFeLDH–NCNT.

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Analysis of surface chemical composition

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The Fe 2p XPS spectrum is depicted in Figure 4(a), revealing energy peaks at 711.3 and 723.9 eV that correspond to the two spin orbitals of Fe 2p. The peaks at 710.1 and 723.4 eV demonstrated the presence of Fe2+ in the composite material. Likewise, the peaks at 712.6 and 725.8 eV implied the existence of Fe3+ (Bagus et al. 2021). As illustrated in the figure, the results indicated that Fe primarily exists as FeO, Fe2O3, and Fe (OH)3 forms on the surface of CuFeLDH–NCNT.
Figure 4
(a) Fe 2p, (b) Cu 2p, (c) C 1s, and (d) O 1s XPS spectra of the CuFeLDH–NCNT.
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(a) Fe 2p, (b) Cu 2p, (c) C 1s, and (d) O 1s XPS spectra of the CuFeLDH–NCNT.

Figure 4
(a) Fe 2p, (b) Cu 2p, (c) C 1s, and (d) O 1s XPS spectra of the CuFeLDH–NCNT.
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(a) Fe 2p, (b) Cu 2p, (c) C 1s, and (d) O 1s XPS spectra of the CuFeLDH–NCNT.

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The Cu 2p spectrum was presented in Figure 4(b), where the energy peaks at 935.1 and 961.9 eV correspond to the binding energies of the Cu2p1/2 and Cu2p3/2 spin orbitals, respectively. Additionally, the peaks at 923.9 and 952.5 eV demonstrated the presence of Cu+ in the composite material (Xu et al. 2016), and the peaks at 934.5 and 954.3 eV indicated the presence of Cu2+ (Liu et al. 2021). The results revealed that Cu primarily existed in the forms of Cu2O, Cu (OH)2, and CuO on the surface of CuFeLDH–NCNT.

A characteristic peak in the C1s spectrum (Figure 4(c)) appeared at 284.8, representing graphitic sp2-carbon. The 286.1 and 290.1 eV peaks represented C–O and O–C = O (Singh et al. 2014; Liu et al. 2017), respectively. Carbon-based materials helped disperse and stabilize CuFeLDH and enhanced the electron transfer between the material surface and pollutant molecules. In the O1s XPS spectrum (Figure 4(d)), the same types of C–O bonds were observed as in the C1s XPS spectrum, including peaks at 531.9 and 533.6 eV, respectively, corresponding to C–O and O–C = O bonds (Luo et al. 2020). The peak at 530.3 eV corresponded to the lattice oxygen bound to metal cations, confirming the presence of oxides.

EIS spectra

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As demonstrated in Figure 5, the EIS Nyquist plots of the electrodes reveal a direct proportional relationship between the diameter of the semicircle and the charge transfer resistance (Yu et al. 2019). Moreover, compared to NCNT and CuFeLDH, CuFeLDH–NCNT exhibits the lowest RCT, suggesting that incorporating NCNT can enhance electron transfer at the CuFeLDH interface. This improvement in electron transfer kinetics contributes to an accelerated degradation reaction.
Figure 5
EIS spectra of NCNT, CuFeLDH, and CuFeLDH–NCNT.
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EIS spectra of NCNT, CuFeLDH, and CuFeLDH–NCNT.

Figure 5
EIS spectra of NCNT, CuFeLDH, and CuFeLDH–NCNT.
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EIS spectra of NCNT, CuFeLDH, and CuFeLDH–NCNT.

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The effects of the operational parameters

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Removal of CIP in various systems

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The ultrasonic-assisted Fenton-like behavior of different systems was evaluated by CIP degradation. As shown in Figure 6(a), the adsorption of CuFeLDH–NCNT onto CIP reached 7.34% within 100 min, while the removal rate of CIP in the CuFeLDH–NCNT/US system was 16.58% within 100 min, indicating a relatively weak oxidation ability of ultrasound. After combining with H2O2 to form a Fenton-like reaction, the removal rate of CIP reached 71.59% within 100 min, suggesting that CuFeLDH–NCNT shows strong catalytic performance toward H2O2. In the CuFeLDH/US/H2O2 system, the removal rate of CIP was 80.49%, which might be related to the strong metal aggregation and lack of catalytic active sites. As could be seen from the figure, NCNT had a strong adsorption capacity and a removal effectiveness of 77.51%. The removal rate of CIP in the FeLDH-NCNT/US/H2O2 system reached 73.97%, indicating that FeLDH-NCNT positively affected the Fenton-like system. In the CuFeLDH–NCNT/US/H2O2 system, the removal rate of CIP was the greatest, reaching 94.66%. This could be attributed to the combined effect of ultrasound and the Fenton-like reaction.
Figure 6
(a) Performance of removing CIP by various reaction systems and (b) CIP removal rate and TOC removal rate of the CuFeLDH–NCNT, NCNT, and CuFeLDH.
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(a) Performance of removing CIP by various reaction systems and (b) CIP removal rate and TOC removal rate of the CuFeLDH–NCNT, NCNT, and CuFeLDH.

Figure 6
(a) Performance of removing CIP by various reaction systems and (b) CIP removal rate and TOC removal rate of the CuFeLDH–NCNT, NCNT, and CuFeLDH.
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(a) Performance of removing CIP by various reaction systems and (b) CIP removal rate and TOC removal rate of the CuFeLDH–NCNT, NCNT, and CuFeLDH.

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In addition, during the process of oxidative removal of pollutants, the mineralization degree was one of the indicators for evaluating pollutant degradation performance, and TOC values could reflect the mineralization degree of this indicator. As indicated in Figure 6(b), the TOC degradation rates of NCNT and CuFeLDH were 22.7 and 42.3%, respectively. The TOC degradation rate of CuFeLDH–NCNT was 70.4%, indicating a high mineralization rate, which could effectively degrade the large molecular organic compounds of CIP into a certain amount of small molecular organic compounds.

The effect of catalyst dosage

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Figure 7(a) illustrates the removal efficiency of CIP with different dosages of CuFeLDH–NCNT at a pH of 3 and an H2O2 concentration of 5 mM. The adsorption experiment in the first 20 min showed minimal effectiveness, indicating that adsorption cannot effectively remove CIP. As the dosage of the catalyst increased from 0.04 to 0.2 g/L, the removal efficiency of CIP demonstrated an upward trend, and the highest removal efficiency of CIP is 91.96% at a dosage of 0.2 g/L. The increase in catalyst dosage led to a greater number of catalytic active sites, which contributed to the observed effect and could accelerate the generation of •OH. In addition, the increase in catalyst dosage could considerably increase the adsorption of CIP. Nonetheless, as the catalyst dosage escalated to 0.4 g/L, the CIP removal efficiency diminished, leading to a reduction in the removal rate to 85.22%, respectively. The findings suggested that an excessive amount of catalyst was not beneficial for generating additional •OH radicals through the decomposition of H2O2, and that an excessive catalyst could, in fact, increase the transfer resistance (Wang et al. 2020), as well as the excess Fe2+ and Fe3+ on the catalyst surface, which could remove H2O2 and •OH (Equations (8) and (9)) (Acisli et al. 2017). Therefore, the optimal catalyst dosage for the experiment was selected as 0.2 g/L.
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Figure 7
The influences of (a) catalyst amount, (b) ultrasonic power, (c) H2O2 concentration, and (d) initial concentration on the effect of CuFeLDH–NCNT/US/H2O2. Experimental conditions: [CIP] = 20 mg/L, initial pH = 3, T = 25 °C, [catalyst] = 0.2 g/L, [H2O2] = 7.5 mM, and ultrasonic power = 300 W (except for the parameters illustrated in figures).
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The influences of (a) catalyst amount, (b) ultrasonic power, (c) H2O2 concentration, and (d) initial concentration on the effect of CuFeLDH–NCNT/US/H2O2. Experimental conditions: [CIP] = 20 mg/L, initial pH = 3, T = 25 °C, [catalyst] = 0.2 g/L, [H2O2] = 7.5 mM, and ultrasonic power = 300 W (except for the parameters illustrated in figures).

Figure 7
The influences of (a) catalyst amount, (b) ultrasonic power, (c) H2O2 concentration, and (d) initial concentration on the effect of CuFeLDH–NCNT/US/H2O2. Experimental conditions: [CIP] = 20 mg/L, initial pH = 3, T = 25 °C, [catalyst] = 0.2 g/L, [H2O2] = 7.5 mM, and ultrasonic power = 300 W (except for the parameters illustrated in figures).
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The influences of (a) catalyst amount, (b) ultrasonic power, (c) H2O2 concentration, and (d) initial concentration on the effect of CuFeLDH–NCNT/US/H2O2. Experimental conditions: [CIP] = 20 mg/L, initial pH = 3, T = 25 °C, [catalyst] = 0.2 g/L, [H2O2] = 7.5 mM, and ultrasonic power = 300 W (except for the parameters illustrated in figures).

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Therefore, when optimizing the catalyst dosage, it was necessary to consider both the CIP removal rate and the occurrence of side reactions, avoiding excessive consumption of H2O2 and reducing the generation of •OH.

The effect of ultrasonic power

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Figure 7(b) investigates the impact of ultrasonic power on the efficiency of CIP removal by CuFeLDH–NCNT/H2O2/US under an H2O2 dosage of 5 mM. The first 20 min saw an adsorption effectiveness of less than 10%. Following the H2O2 addition, under different ultrasonic powers, the removal rates of CIP were 73.21, 91.96, and 84.89%, respectively. As the ultrasonic power surged from 150 to 300 W, the rate of CIP removal experienced a significant acceleration. It may be due to the increasing cavitation intensity with the higher power, leading to more •OH radicals' decomposition. However, more extensive ultrasonic powers also adversely affected the reaction, causing decreased bubble size and cavitation intensity, resulting in fewer •OH radicals generated and reduced interaction between the target pollutants and •OH radicals (Petrier & Francony 1997). Therefore, this study chose 300 W as the optimal ultrasonic power in subsequent experiments.

The effect of H2O2 concentration

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Figure 7(c) illustrates the influence of H2O2 concentration on the degradation of CIP. The adsorption contributed little to the removal of CIP within the first 20 min, but the removal efficiency significantly increased after the addition of H2O2. As the H2O2 concentration escalated from 5 to 7.5 mM, both the removal of CIP, reaching 94.66%, respectively. It was because the increased concentration of H2O2 could accelerate the generation rate and reaction rate of •OH radicals, thereby effectively degrading CIP. The removal efficiency slightly decreased when the H2O2 concentration increased between10 and 20 mM, implying that only a portion of the H2O2 could react with the active sites of the catalyst to form •OH radicals when the catalyst dosage remained constant. In addition, when the H2O2 concentration was excessive, not only would it undergo self-decomposition, Nonetheless, it would concurrently interact with •OH radicals to create radicals possessing inferior oxidation and less reactivity (Equations (10)–(13)) (Dükkanci 2018; Mao et al. 2019). Therefore, 7.5 mM H2O2 was the optimal concentration for subsequent experiments.
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The effect of initial concentration of CIP

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Figure 7(d) indicates that the increase in CIP initial concentration caused a decrease in CIP removal, but the adsorption effect was not significantly affected. When the initial concentrations of CIP were 10 and 20 mg/L, the former had a relatively fast reaction rate within the first 50 min, but the final removal was similar for both concentrations. When the initial concentration of CIP exceeded 20 mg/L, the degradation impact of CuFeLDH–NCNT decreased with the increase in pollutant concentration. Moreover, the final degradation rate was over 85%, demonstrating a reasonable CIP removal rate. The degradation process of CIP was primarily carried out by heterogeneous catalysis, with the quantity of catalytic active sites participating in the Fenton-like reaction on the finite catalyst surface also being finite. As the CIP molecules populated the catalyst's active centers, an excess of CIP molecules engulfed the catalytic active sites, inhibiting the formation of •OH radicals and amplifying the reaction resistance, thereby affecting its degradation efficiency. Moreover, as the initial concentration of CIP increased, more intermediate products were formed in the reaction system, resulting in an excessive consumption of •OH radicals and accordingly diminishing the removal efficiency of CIP.

Effect of pH

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As depicted in Figure 8(a), the initial pH had a minor effect on the adsorption efficiency. The highest removal efficiency of CIP was achieved at pH = 3 in the CuFeLDH–NCNT system. It was more advantageous for the synthesis of •OH in acidic circumstances, which aided in the CIP's breakdown. Competitive side reactions were more likely to happen as the pH increased (Yang et al. 2021). Furthermore, the lower oxidation potential of •OH in alkaline circumstances hindered the CIP degradation process (Zhang et al. 2022). Overall, the CuFeLDH–NCNT system exhibited an acceptable CIP removal rate of over 65% in a pH range of 3–7. Besides, the pH-pzc of CuFeLDH–NCNT was 6.8 (Figure 8(b)). In addition, the amounts of Fe and Cu present in the reaction solution under different pH conditions were measured within 100 min in this study. Figure 8(c) demonstrated that the total Fe and Cu concentrations in the leaching solution at 100 min positively correlated with the initial pH value. At a pH of 3, the leaching of Fe and Cu in the reaction solution after the Fenton-like reaction was 0.149 and 0.207 mg/L, respectively. Under a pH = 5 condition, the leaching of Fe and Cu was 0.035 and 0.108 mg/L, respectively. At pH = 7 and 9, the stability of CuFeLDH–NCNT was relatively high, and the ion leaching was almost negligible. These values comply with the Chinese National Standard (GB3838-2002) and do not cause secondary pollution.
Figure 8
(a) The initial pH of the solution; (b) pH-pzc; and (c) the total concentration of leached Fe and Cu ions at different initial pH.
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(a) The initial pH of the solution; (b) pH-pzc; and (c) the total concentration of leached Fe and Cu ions at different initial pH.

Figure 8
(a) The initial pH of the solution; (b) pH-pzc; and (c) the total concentration of leached Fe and Cu ions at different initial pH.
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(a) The initial pH of the solution; (b) pH-pzc; and (c) the total concentration of leached Fe and Cu ions at different initial pH.

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Effect of inorganic anions and stability of CuFeLDH–NCNT

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Numerous inorganic anions can concurrently exist in wastewater and potentially impact the Fenton catalytic action. The influence of three coexisting anions (Cl−, ⁠, and ⁠) on the catalytic oxidation of CIP by CuFeLDH–NCNT/US/H2O2 at a concentration of 10 mM was investigated. As illustrated in Figure 9(a), the efficacy of CIP removal under the influence of Cl−, ⁠, and was 83.33, 70.61, and 43.94%, respectively. It may be because these anions have a specific quenching effect on the •OH radicals and can also generate fewer reactive radicals (Equations (13)–(16)) (Zheng et al. 2019).
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Figure 9
(a) Coexisting anions on the CIP degradation and (b) periodic experiment of CuFeLDH–NCNT; XPS spectra of (c) Cu 2p and (d) Fe 2p of CuFeLDH–NCNT before and after the treatment.
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(a) Coexisting anions on the CIP degradation and (b) periodic experiment of CuFeLDH–NCNT; XPS spectra of (c) Cu 2p and (d) Fe 2p of CuFeLDH–NCNT before and after the treatment.

Figure 9
(a) Coexisting anions on the CIP degradation and (b) periodic experiment of CuFeLDH–NCNT; XPS spectra of (c) Cu 2p and (d) Fe 2p of CuFeLDH–NCNT before and after the treatment.
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(a) Coexisting anions on the CIP degradation and (b) periodic experiment of CuFeLDH–NCNT; XPS spectra of (c) Cu 2p and (d) Fe 2p of CuFeLDH–NCNT before and after the treatment.

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The practical application of nanocomposites depended on their stability; thus, the reusability and stability of nanocomposites play a crucial role in practical applications. Continuous degradation experiments of CIP using CuFeLDH–NCNT under the same conditions. Each experiment lasted for 100 min. Following each experimental run, the composite material was removed, cleansed, centrifuged, and dried prior to initiating the subsequent cycle of the experiment. As demonstrated in Figure 9(b), after five cycles, the removal rate of CIP still reached 85.7% using CuFeLDH–NCNT, indicating that the material had good stability. Furthermore, the utilization of H2O2 was evaluated by calculating the ratio between the initial H2O2 dosage and the actual H2O2 intake (ΔH2O2). Table 1 indicates that as the number of cycles increased, H2O2 utilization likewise declined, which could be related to a decrease in active sites on the composite material's surface. Nevertheless, after five cycles, the utilization could still reach 71.07%, demonstrating CuFeLDH–NCNT 's strong H2O2 catalytic activity and capacity to efficiently remove CIP.

Table 1

The rates of H2O2 utilization in the cycle experiment

Cycle indexTime (min)CIP removal (%)ΔH2O2 (mM)ηH2O2 (%)
First 100 94.66 6.73 89.73 
Second 100 91.96 6.22 82.93 
Third 100 89.89 5.98 79.73 
Fourth 100 87.25 5.65 75.33 
Fifth 100 85.71 5.33 71.07 
Cycle indexTime (min)CIP removal (%)ΔH2O2 (mM)ηH2O2 (%)
First 100 94.66 6.73 89.73 
Second 100 91.96 6.22 82.93 
Third 100 89.89 5.98 79.73 
Fourth 100 87.25 5.65 75.33 
Fifth 100 85.71 5.33 71.07 
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After the reaction, the relative ratio of Fe3+ and Cu+ species decreased by 10.28 and 18.36%, respectively (Figure 9(c) and 9(d)). The rapid transformation between Fe2+ to Fe3+ and Cu+ to Cu2+ is the primary step for activating H2O2. The study further validated the synergistic interaction between Fe2+/Fe3+ and Cu+/Cu2+, which contributed to enhancing the catalytic potency and employment of H2O2, selectively transforming H2O2 into •OH radicals.

The TOC removal rate of the CuFeLDH–NCNT/US/H2O2 system compared with other systems

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The system was compared with other heterogeneous ultrasound-assisted Fenton processes in terms of catalyst type, target pollutant concentration, H2O2 concentration, and TOC degradation rate. As shown in Table 2, the ultrasound-assisted Fenton process is also applicable to other types of pollutants. Compared with the reported catalyst materials, the experiment used a lower concentration of H2O2, showing an excellent TOC removal rate.

Table 2

Comparison of TOC removal rates in various ultrasound-assisted Fenton process

CatalystConcentrationH2O2 (mM)Removal efficiency of TOC (%)Reference
Mn3O4-TA@FeNiB CIP (5 mg/L) 10 16.12% (60 min) Dang et al. (2023)  
Activated carbon/magnetite 4-chloropheno (250 mg/L) 25.55 79% (78 min) Haghmohammadi et al. (2023)  
Fe2 + Organophosphorus (50 mg/L) 150 29.9% (60 min) Wang & Shih (2015)  
FeII-MIL-88B/GO/P25 TC (10 mg/L) 20 52.2% (60 min) Geng et al. (2022)  
Copper film BPA (0.1 mM) 100 39.54% (180 min) Chu et al. (2021)  
SmFeO3/Ti3C2Tx TC (10 mg/L) 2 46% (90 min) Mahmoudi et al. (2022)  
CuFeLDH–NCNT CIP (20 mg/L) 7.5 70.4% (100 min) This work 
CatalystConcentrationH2O2 (mM)Removal efficiency of TOC (%)Reference
Mn3O4-TA@FeNiB CIP (5 mg/L) 10 16.12% (60 min) Dang et al. (2023)  
Activated carbon/magnetite 4-chloropheno (250 mg/L) 25.55 79% (78 min) Haghmohammadi et al. (2023)  
Fe2 + Organophosphorus (50 mg/L) 150 29.9% (60 min) Wang & Shih (2015)  
FeII-MIL-88B/GO/P25 TC (10 mg/L) 20 52.2% (60 min) Geng et al. (2022)  
Copper film BPA (0.1 mM) 100 39.54% (180 min) Chu et al. (2021)  
SmFeO3/Ti3C2Tx TC (10 mg/L) 2 46% (90 min) Mahmoudi et al. (2022)  
CuFeLDH–NCNT CIP (20 mg/L) 7.5 70.4% (100 min) This work 
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Practical application and catalytic mechanisms

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We evaluated the practical application of the CuFeLDH–NCNT/US/H2O2 system by collecting water samples from the Yellow River (Lanzhou, China), lake water (Lanzhou University of Technology, Lanzhou, China), and purified water. After letting the water samples stand still and filtering them, we conducted experiments. As shown in Figure 10(a), the removal rate of CIP in other water types is lower than that in deionized water. The lowest efficiency was observed in Yellow River water, which may be due to the complexity of its components, which contained many inorganic anions that inhibited the reaction (as shown in Figure 9(a)). The finding suggested that the CuFeLDH–NCNT/US/H2O2 system exhibits remarkable practicality.
Figure 10
Effects of different water types (a) and different scavengers (b) on CIP degradation in the CuFeLDH–NCNT/US/H2O2 system.
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Effects of different water types (a) and different scavengers (b) on CIP degradation in the CuFeLDH–NCNT/US/H2O2 system.

Figure 10
Effects of different water types (a) and different scavengers (b) on CIP degradation in the CuFeLDH–NCNT/US/H2O2 system.
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Effects of different water types (a) and different scavengers (b) on CIP degradation in the CuFeLDH–NCNT/US/H2O2 system.

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The primary reactive oxygen species (ROSs) were identified through radical scavenging experiments in an ultrasonic-Fenton-like system for the degradation of CIP. In this investigation, TBA was used as a clearing agent for •OH, and benzoquinone (BQ) was used as a clearing agent for the superoxide radical (⁠⁠). The addition of TBA and BQ was 15 mM. As shown in Figure 10, TBA significantly reduced the degradation rate of CIP, with a degradation rate of 29.54%, while the removal effect of BQ on CIP was insignificant. It indicated that the •OH radical played the most crucial role in the degradation process of CIP.

Possible degradation pathways

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As illustrated in Figure 11, the intermediate products of CIP degradation were examined utilizing LC-MS, and the possible degradation pathways of CIP were presented. According to the intermediate products obtained, CIP's degradation pathway could be divided into four routes. In Pathway I, CIP was hydroxylated to form P1 (m/z = 348), and then P1 lost a hydroxyl group and water to transform into P2 (m/z = 288) (Chen et al. 2018). Subsequently, P2 closed its alkyl group and underwent amination to generate P3 (m/z = 245). Then, the pyrimidine ring of the CIP molecule was broken to form P4 (m/z = 362), which lost groups such as ‘CH2CH2NH2’ to generate a smaller molecular weight, P5 (m/z = 291). In Pathway II, P6 (m/z = 225) was obtained from P5 by losing formaldehyde, followed by the oxidation of P6 to P7 (m/z = 199). P7 was further converted into P8 (m/z = 111) by losing amino and carboxyl groups. In Pathway III, P5 lost a carboxyl group and was transformed into P9 (m/z = 219) by formaldehyde, and in Pathway Ⅳ, P5 also generated P10 (m/z = 245) through decarboxylation and defluorination reactions. Finally, these intermediates were oxidized into CO2 and H2O by ROSs such as •OH radicals.
Figure 11
Proposed pathways of CIP degradation in the CuFeLDH–NCNT/H2O2/US system.
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Proposed pathways of CIP degradation in the CuFeLDH–NCNT/H2O2/US system.

Figure 11
Proposed pathways of CIP degradation in the CuFeLDH–NCNT/H2O2/US system.
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Proposed pathways of CIP degradation in the CuFeLDH–NCNT/H2O2/US system.

Close modal
Figure 12(a) shows that ln(C/C0) has a linear relationship with reaction time t, indicating that the degradation of CIP over time followed a pseudo-first-order reaction kinetics model. A potential mechanism for the removal of CIP using the CuFeLDH–NCNT/H2O2/US system was suggested (Figure 12(b)). In the initial stage, within the first 20 min of adsorption, CIP molecules were efficiently adsorbed on CuFeLDH–NCNT, leading to electron transfer between them and resulting in partial self-degradation of CIP (Liang et al. 2021). Upon the addition of H2O2, the surface of CuFeLDH–NCNT and a small quantity of leached copper and iron ions could catalyze intra-molecular electron transfer to generate •OH. Both copper and iron exhibited similar reactions, activating the rate constant (K = 1.0 × 104 m−1 s−1) of H2O2 to produce •OH which was higher than iron (K = 63–76 m−1 s−1) (Pham et al. 2013; Bokare & Choi 2014). Simultaneously, electron transfer took place from Cu+ and Fe3+, and the regeneration of Cu2+ and Fe2+ could be accomplished via redox reactions (Equations (8) and (16)–(19)). It facilitated the Fe3+/Fe2+ and Cu2+/Cu+ redox cycles, allowing for the continual synthesis of •OH radicals. These reactions significantly enhanced the utilization of H2O2, and by consistently extracting electrons from the metal center of CuFeLDH–NCNT, H2O2 was selectively reduced to •OH radicals. Furthermore, adding US could boost the mass transfer efficiency and nanoparticle agglomeration at the solid-liquid interface, which could significantly increase the Fenton-like process's catalytic capacity. The •OH radicals generated by the reaction attacked CIP and intermediate products, ultimately transforming them into CO2 and H2O (Equation (20)).
(16)
(17)
(18)
(19)
(20)
Figure 12
(a) Degradation kinetic curves and (b) schematic diagrams of the proposed mechanism involved in CIP degradation by CuFeLDH–NCNT/US/H2O2.
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(a) Degradation kinetic curves and (b) schematic diagrams of the proposed mechanism involved in CIP degradation by CuFeLDH–NCNT/US/H2O2.

Figure 12
(a) Degradation kinetic curves and (b) schematic diagrams of the proposed mechanism involved in CIP degradation by CuFeLDH–NCNT/US/H2O2.
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(a) Degradation kinetic curves and (b) schematic diagrams of the proposed mechanism involved in CIP degradation by CuFeLDH–NCNT/US/H2O2.

Close modal
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In this paper, a novel and efficient CuFeLDH–NCNT nanocomposite was successfully synthesized using the hydrothermal method. The catalytic activity of the CuFeLDH–NCNT nanocomposite for CIP degradation was investigated. The nanocomposite was characterized using SEM, XRD, FI-IR, XPS, and EIS. The results revealed that the removal rate was highest when the dosage of CuFeLDH–NCNT catalyst was 0.2 g/L, ultrasonic power was 300W, H2O2 dosage was 7.5 mM, and pH = 3, reaching up to 94.66% and TOC degradation rate of 70.4%. Compared to other systems, the CuFeLDH–NCNT/US/H2O2 system exhibits the most significant removal effect for CIP and demonstrates excellent synergistic effects among the three components. Radical scavenging and inorganic ion experiments indicated that •OH radicals played a dominant role in the oxidative degradation of CIP. In addition, electron transfer between metal ions and redox cycling also promoted the degradation of CIP. However, some metal ions still had a leaching phenomenon, which may be the reason for the slight decrease in the stability of the catalyst. Subsequent studies will continue to investigate the causes of metal leaching to improve the strength of motivation. These findings provide new research possibilities for generating innovative Fenton-like catalysts and provide selectivity for removing cyclopropyl quinoline antibiotics from water, thereby achieving efficient environmental remediation methods.

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This work was supported by the National Natural Science Foundation of China (Grant No. 22166023), the Industrial Support Plan Project of Colleges and Universities in Gansu Province (Grant No. 2023CYZC-30), and the Talents Innovation and Entrepreneurship Project of Lanzhou (Grant No. 2021-RC-20).

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All relevant data are included in the paper or its Supplementary Information.

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The authors declare there is no conflict.

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