3.1 Characterization of the nZVC
FESEM and XRD were used to characterize the morphological properties and phase structure of nZVC powders. As shown in Fig. 1(a-c), FESEM images revealed that nZVC powders consist of many irregular clumps, which are composed of spherical particles of different sizes. As seen in Fig. 1e, nZVC particles have a diameter of approximately 30 nm. Furthermore, as shown in Fig. 2a, three diffraction peaks at 43.40°, 50.54°, and 74.22° can be visible in the XRD pattern of nZVC, which correspond to several major Cu0 diffraction peaks (JCPDS-PDF#04-0836), indicating that nZVC powders are made chiefly of Cu0(Zhou et al. 2018a). Also, minor peaks at 36.48°, 42.40°, and 61.42°, as well as those at 37.50°, 46.00°, and 48.48° corresponded well to the standard pattern of Cu2O (JCPDS-PDF#05-0667) and Cu2S ((JCPDS-PDF#33–0490), respectively. Furthermore, EDS analysis shows that the nZVC contains a small amount of O, and S elements(Fig. 1d), which may be some Cu2O coating attached to the surface of nZVC and a few Cu2S impurities in nZVC powders(Sousa et al. 2019). However, the XRD pattern of nZVC did not display the expected CuO diffraction peaks. Thus, Cu0 made up the bulk of nZVC powders (Zhang et al. 2021b).
3.2. Performance and mechanism of nZVC/O2 system
The performances of different systems were compared for ENR degradation, as shown in Fig. 2b. The ENR degradation was found to be less than 10% within 60 minutes in the absence of nZVC, indicating that dissolved O2 alone could not effectively degrade ENR. With nZVC addition, the elimination efficiencies of ENR significantly increased to 86.42% within 60 min. Meanwhile, a contrast experiment of nZVC under anaerobic conditions was carried out, 20 minutes of nitrogen aeration were performed prior to the addition of nZVC to remove dissolved oxygen from the solution, and the reactor was sealed during the process. The change curve of the anaerobic reaction system indicated that the nZVC had negligible adsorption capacity to ENR. Therefore, the degradation of ENR in nZVC/O2 system might be due to the reaction between nZVC and O2 to produce a series of ROS, and the catalyst was an essential factor for efficient degradation.
In order to further explore the ROS that plays a leading role in the degradation process, the concentrations of Cu+ and H2O2 produced at different reaction times in nZVC/Air system were determined by spectrophotometry. As shown in Fig. 2c, H2O2 is indeed produced in the reaction system, and its concentration varies between 0.05-0.65mM. The contents of Cu+ and H2O2 increased rapidly within 0–10 minutes and reached the maximum value about 10 minutes later, and then gradually decreased.
In general, hydrogen peroxide (H2O2), hydroxyl (HO•), and superoxide (\({\text{O}}_{\text{2}}^{{\bullet }\text{-}}\)) are the possible ROS in zero-valent metal/O2 system (Liu et al. 2021b; Li et al. 2021), and their generation route has been reported as follows: to begin, nZVC leaching in acidic conditions generates Cu+ and Cu2+ as Eqs. (2)-(3); simultaneously, O2 is spontaneously reduced to H2O2 (Eq. (4)); then, some Fenton-like reactions occurred, such as the interaction between Cu+ and H2O2 (Eq. (5)) that resulted in the production of HO•, or the reaction between Cu2+ and H2O2 (Eq. (6)), which gave \({\text{O}}_{\text{2}}^{{\bullet }\text{-}}\). Meanwhile, the latter would be further transformed into the hydroperoxyl radical (\({\text{HO}}_{\text{2}}^{{\bullet }}\)) in acidic media (Eq. (7)).
$${\text{Cu}}^{\text{0}}\rightleftarrows {\text{Cu}}^{\text{+}}\text{ + }{\text{e}}^{\text{-}}{\text{E}}^{\text{0}}\text{=-0.528 V}$$
2
$${\text{Cu}}^{\text{0}}\rightleftarrows {\text{Cu}}^{\text{2+}}\text{ + 2}{\text{e}}^{\text{-}}{\text{E}}^{\text{0}}\text{=-0.334 V}$$
3
$${\text{O}}_{\text{2}}\text{+2}{\text{H}}^{\text{+}}\text{+2}{\text{e}}^{\text{-}}\rightleftarrows {\text{H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{E}}^{\text{0}}\text{=+0.695 V}$$
4
$${\text{Cu}}^{\text{+}}\text{+}{\text{H}}_{\text{2}}{\text{O}}_{\text{2}}\rightleftarrows {\text{Cu}}^{\text{2+}}\text{ +HO}\text{•}\text{+ }{\text{HO}}^{\text{-}}$$
5
$${\text{Cu}}^{\text{2+}}\text{+}{\text{H}}_{\text{2}}{\text{O}}_{\text{2}}\rightleftarrows {\text{Cu}}^{\text{+}}\text{ +}{\text{O}}_{\text{2}}^{{\bullet }\text{-}}\text{+ }{\text{H}}^{\text{+}}$$
6
\({\text{O}}_{\text{2}}^{{\bullet }\text{-}}\text{+}{\text{H}}^{\text{+}}\rightleftarrows {\text{HO}}_{\text{2}}^{{\bullet }}\) (pKa = 4.8) (7)
Although all of these radical species can then take part in ENR destruction, HO• radicals had the largest redox potential and were the most reactive to organic molecules (Zhang et al. 2021a), therefore possibly played a more dominant role in ENR degradation. To validate this hypothesis, quenching experiments were done with two well-known scavengers present, TBA for HO• and p-BQ for \({\text{O}}_{\text{2}}^{{\bullet }\text{-}}\). As can be seen in Fig. 2e, when TBA was present, the efficiency with which ENR was degraded dropped to approximately 15%, whereas the inhibitory impact of p-BQ was diminished, and the rate at which ENR was removed was around 58%. Subsequently, EPR spectra with DMPO spin trapping adducts was adopted to confirm the existence of these ROS intuitively. A four-line spectrum with relative intensities of 1:2:2:1 was found from nZVC aqueous dispersions, indicating the production of HO• (Fig. 2f). Besides, six characteristic peaks of DMPO-\({\text{O}}_{\text{2}}^{{\bullet }\text{-}}\) adducts were also identified (Fig. 2g).
Additionally, under aerobic air circumstances, the oxidation capacity of the nZVC acidic system was significantly impacted by the presence of Cu+ and Cu2+, as shown by Eqs. (2)-(7). Binding energies of the copper after using (Fig. 2d) at roughly 932.2 eV and 935.3 eV were assigned to the Cu 2p3/2 orbital in Cu2O and CuO, respectively, revealing the co-existence of Cu+ and Cu2+ in the system.
3.3. Effect of different conditions on ENR degradation
3.3.1. Effect of initial pH
As known, pH is a crucial aspect concerning the elimination performance of the oxidation unit. Herein, the impact of initial pH on ENR degradation was examined by introducing varied quantities of HCl or NaOH to the reaction system at 15°C. As shown in Fig. 3a, the nZVC/O2 degradation system was active between the pH ranges of 3.08 and 8.96, and lower pH values aided the degrading process, while almost no degradation was performed at a pH of 10.98. Besides, Cu concentration in the solution after the reaction was measured using ICP-OES, and the results are presented in Fig. 3b. With the increasing pH value, Cu concentration significantly decreased, and trace Cu was detected under alkaline conditions. Moreover, it was discovered that the final pH was higher following the reaction, regardless of the starting pH, which was attributed to the consumption of H+ ions.
Since H+ could dissolve the oxide layer on the surface of the zero-valent metal (Deng et al. 2019; Sun et al. 2021), it could be envisaged that hydronium ion dissolved the oxide layer on the surface of Cu0 and promoted degradation of ENR. Furthermore, high concentrations of H+ may enhance the formation of H2O2, which in turn promotes the degradation of organic matter (Zhou et al. 2016; Guo et al. 2021). Under strong alkaline conditions, OH- would react with Cu+ to generate cuprous hydroxide (CuOH), which could spontaneously decompose into Cu2O and H2O, significantly inhibiting the ENR degradation (Zhang et al. 2021a, b). In summary, acidic condition facilitated the ENR removal. Wen et al. (Wen et al. 2014) observed comparable results when utilizing ZVC to degrade diethyl phthalate, while Sousa et al. (Sousa et al. 2019) discovered a drop in the proportion of ciprofloxacin degradation when applying nZVC. Additionally, the change in pH of the solution was observed, and an increase was detected following the reaction. This suggests that H+ was used in the nZVC/O2 system, as the rise in pH indicates. Although the dissolved Cu concentrations in acidic solutions were higher than those allowed in the Chinese discharge standard for wastewater (2.0 mg·L-1), which may raise questions about the nZVC application Cu+/Cu2+ might be readily eliminated by precipitation techniques, such as combining with a solution containing sulfide or phosphorus and adjusting the pH value (Hollanda et al. 2019). The previous methodology was used for this investigation, and over 99.98% of Cu in the solution was precipitated. Subsequent experiments were conducted with initial pH of 3. Over 99.98% of the Cu in the solution was precipitated using the former method used in the present investigation. In subsequent trials, an initial pH of 3 was used.
3.3.2. Effect of nZVC dosage
The impact of nZVC dose (0.10–0.90 g·L− 1) on ENR degradation at an initial pH of 3 is shown in Fig. 3c. Within 60 minutes, the elimination efficiency of ENR improved from 57.23–99.70% as the dosage of nZVC went from 0.10 g·L− 1 to 0.90 g·L− 1. Furthermore, the pseudo-first-order kinetic was employed to calculate the ENR degradation rate constant, k. As shown in Fig. 3d, the k value was 0.01, 0.03, 0.06, 0.15, and 0.22 min− 1with nZVC dosage of 0.10, 0.30, 0.50, 0.70, and 0.90 g·L− 1, respectively. According to Yang et al. (Yang et al. 2022b) and Zhang et al. (Zhang et al. 2021b), the increasing degradation with ZVC concentration was likely due to the enhanced production of Cu+/Cu2+, which could promote the reaction of molecular oxygen with water to form ROS.
3.3.3. Effect of temperature
The catalysis of molecular oxygen and the generation of ROS are processes that are greatly influenced by temperature. The influence of temperature on ENR degradation in the range of 15–45°C was investigated, and Fig. 4a-b reveals that the temperature of the reaction played a significant role in ENR degradation. At higher reaction temperatures, higher degradation efficiency occurred. The ENR degradation at 15, 25, 35, and 45℃ within 30 min was approximately 83.24%, 88.33%, 99.51%, and 99.85%, respectively. Moreover, with the increasing reaction temperature, the degradation of ENR significantly accelerated. ENR degradation in each temperature was accompanied by pseudo-first-order kinetic, and the kobs increased from 0.06 to 0.14 min− 1 with enhancing reaction temperature. These results may be explained by the higher dissolution of Cu and generation of ROS at higher reaction temperatures (Deng et al. 2019), which could further promote the degradation of ENR. Satisfactory ENR degradation could be received at 35℃.
3.3.4. Effect of co-exiting anions and NOM
Commonly, certain anions and NOM coexisted in natural water, which may affect the system's oxidation reaction performance. In order to validate the influence of typical anions (Cl−, NO3−, SO42−) and NOM (humic acid) on ENR elimination by the nZVC/O2 system, tests were conducted. According to the results presented in Fig. 4c, the presence of Cl− had a positive impact on the ENR degradation, but NO3−and SO42− exhibited little influence, implying that they do not interfere in the system. These findings align with the phenomena described by Wen et al. (Wen et al. 2014) and Sousa et al. (Sousa et al. 2019). The Cl− accelerated the degradation reaction, as it could stabilize Cu+ and prevent Cu2+ oxidation (Zhang et al. 2020c). Cu+ would react with H2O2 and O2 already there to make HO• and \({\text{O}}_{\text{2}}^{{\bullet }\text{-}}\). Significantly detrimental effects were detected for humic acid, mainly owing to rivalry between humic acid and ENR for HO• radicals(Zhuan and Wang 2020).
3.4. Identified of ENR degradation products
The complexity of partially oxidized manufacturing products during ENR oxidation was essential to considering the reaction mechanism. The use of HPLC-Q-TOF-MS is possible to track these plausible oxidation products. The reactor did not find ENR after 60 minutes. The potential metabolites of ENR were screened and analyzed with secondary mass spectrometry based on variations in sample primary mass spectra. The probable molecular structure of the substance was estimated following the structure of the parent compound. While, several oxidation transformation products were identified with the help of Analyst TF 1.7.1 software in the nZVC/O2 system, as shown by peak deconvolution and alignment of the chromatograms obtained from the treated and control samples (Table 1 and Table S1). Fig. S1 describes the hypothesized structures for the fragment ions of the protonated ENR, and Fig. S2 presents a summary of the MS/MS spectra of the primary ENR dissociation products as determined by HPLC-Q-TOF-MS.
The structure of ENR is based on cyclopropyl at the N1 position, fluorine substituent at C6, N-ethyl piperazine at C7, and 4-oxo-quinoline-3-carboxylic acid (Fig. S3). According to previous literature (Sturini et al. 2010; Morales-Gutiérrez et al. 2014; Yang et al. 2016), the quinolone ring and the piperazine ring of the molecule are both potential sites for ENR degradation. The accurate mass data analysis of the MS/MS spectrum of ENR can prove that the carboxyl substituent of ENR at C3 is easily lost, and the loss of CO2 and H2O results in fragmented molecules of m/z 316 and m/z 342 (Fig. S2). Additionally, the further loss may also include the cleavage of the piperazinyl moiety, and the loss of the piperazine ring to different degrees forms fragment molecules of m/z 245 and m/z 286 substituted by N-vinyl and N-amino vinyl at C7, respectively. It is possible to classify this as secondary fragmentation. At the very end, the cyclopropyl group is lost, splitting from m/z 245 to m/z 203. (Fig. S1)
The identification of ENR's degradation products is given a lot of attention. Studies on ENR have indeed been reported in the presence of biochar (Xiao et al. 2020), graphene-WO3 nanocomposites (Guo et al. 2019), UV (Guo et al. 2017), lead dioxide anode (Wang et al. 2017), ferrate(VI) (Yang et al. 2016), and peroxymonosulfate (Zhou et al. 2018b). The degradation products of ENR were analyzed based on their accurate-mass data of the MS/MS spectra as well as ENR, and all results summarized in Fig. S2.
3.5. Proposed reaction pathway for ENR degradation in nZVC/O2 system
The major ENR degradation pathway may be mainly piperazine side-chain oxidation (Junza et al. 2016; Zhang et al. 2021a). Therefore, the generated transformation products would be ascribed to the cleavage of piperazine rings. In fact, the sequence of 2H loss as a transition state of the following degradation product is noteworthy to notice; accurate-mass measurements support this pattern. (P6 in Fig. S1 and Fig. S2). P6 ([M + H]+ with m/z 358) exhibits subsequent addition of two OH leading to fragment ions with m/z 392 at the presence of nZVC. Immediately afterward, the cleavage of the piperazine ring, two N-C = O species (P5, m/z = 390) would be formed, but two times CO may lose the piperazine ring and generate an amino group (P1, m/z = 334). After that, the loss of CH = N-C2H5N and 2H, and the addition of O would be formed and generate an HN-C = O species group (P8, m/z = 291). In the end, the loss of CO would be formed and generate an amine species group (P7, m/z = 263). It should be mentioned that m/z = 263 was the most generally reported intermediate product in ENR oxidative degradation such as electrochemical degradation (Wang et al. 2017), peroxymonosulfate (Zhou et al. 2018b; Nihemaiti et al. 2020), and ferrate (Yang et al. 2016, 2022a). A possible reaction mechanism for ENR degradation has been suggested, based on the chromatogram and mass spectrum obtained in conjunction with the current literature. In Fig. 5, the proposed manufacturing goods were named "P" (where P means proposed). The conversion diagram of several main dissociation products of ENR by HPLC-Q-TOF-MS is described in Fig. S1, and their MS/MS spectra of primary dissociation products of ENR Fig. S2.
Utilizing HPLC-Q-TOF-MS, several transformation products were also detected during ENR degradation. The piperazine ring and carboxylic group of ENR were the main sites in the degradation process. ENR could be mineralized to utilize CO2, H2O, and NO3− through oxidation of the piperazine ring or hydroxylation(Wang et al. 2017). Notably, removing of -COOH on the quinolone ring or the substitution of HO• was also one of the processes of ENR degradation. Generally, the piperazine ring could be hydroxylated to break the N-C bond (Pathway Ⅰ). This degradation pathway involves the loss of the piperazinyl group. The secondary amine N and tertiary amine N at the piperazine ring were easily attacked by •OH, which may form two hydroxyls product (P3 m/z = 392). This intermediate formed the P3 by further decarboxylation reaction. Subsequently, the CH2−NH-C2H5 at C7 was cleaved and further oxidized with the loss of H to generate P8 (m/z = 291). P7 (m/z = 263) had the same product, which may prove that the complete loss of the piperazine ring was caused by the loss of the CO fragment in P7 (m/z = 263). Similarly, cleavage of the piperazine ring has been reported by various authors (Jiang et al. 2016; Fang et al. 2021; Shu et al. 2021). Pathway II may be hydrolyzed and form P4 (m/z = 376). In pathway II, a piperazine ring was attacked by HO• and produced the hydroxylated intermediate.