Advanced oxidation processes (AOPs) have been universally recognized strategy for the treatment of organic contaminants [1]. The traditional Fenton can non-selectively remove the organics by hydroxyl radical (HO^•), which is generated from the ferrous ion (Fe²⁺) activated hydrogen peroxide (H₂O₂) process at acidic conditions (Eqs. 1–3) [2]. Nevertheless, there are a few inherent limitations for the Fenton reaction, including the narrow application range of pH [3,4], the slow ferric ion (Fe³⁺)/Fe²⁺ circulation rate [5,6], and the instability of H₂O₂ [7]. As a solid oxidant, calcium peroxide (CaO₂) is easier to store and transport, which can slowly release H₂O₂ (Eq. 4) [8]. The decomposition rate of CaO₂ could be controlled by adjusting pH, which could avoid the disproportionation of H₂O₂ [9]. Therefore, CaO₂ is considered as an effective substitute for H₂O₂ [10].

On the other side, as another common transition metal, cupric ion (Cu²⁺) is ubiquitous in multiple waterbodies [11,12]. Cu²⁺ has a larger solubility (K_{sp Cu(OH)2} = 4.8 × 10⁻²⁰) than that of Fe³⁺ (K_{sp Fe(OH)} = 2.8 × 10⁻³⁹) [13], and the circulation of Cu²⁺/cuprous ion (Cu⁺) is easier than that of Fe³⁺/Fe²⁺ due to the lower standard potential (E⁰ (Fe³⁺/Fe²⁺) = 0.77 V vs. SHE, E⁰ (Cu²⁺/Cu⁺) = 0.17 V vs. SHE) [14], which means the Cu-based Fenton process is superior with a greater pH tolerance and better catalytic activity (Eqs. 5 and 6) than the Fe-based Fenton method [11]. Therefore, if the Cu²⁺ existed in water matrix can be in-situ used to activate Fenton/Fenton-like reactions, which could put forward to the practicality of the homogeneous Cu-based Fenton-like system. However, similar to the conventional Fenton reaction, the slow reduction of Cu²⁺ to Cu⁺ (k = 1 L mol⁻¹ s⁻¹) remains the rate-limiting step [15,16]. Thus, exploring a skill to accelerate the Cu²⁺/Cu⁺ circulation is imperative for promoting this Fenton-like reaction.

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The complexation of Cu²⁺ with organic ligands could alter the activity of Cu²⁺/Cu⁺ couple, and stabilize Cu ions in terms of electronic property and steric effect [12]. Moreover, some Cu²⁺ complexes could efficiently trigger H₂O₂ or persulfates to produce radicals, for degrading diverse organic pollutants [17]. However, some organic ligands, such as ethylenediaminetetraacetic acid [18,19], pyridine [17], and ethylenediamine [20], they are themselves contaminants, which not only increase the amount of organic matter and bring secondary pollution, but also quench free radicals by their self-degradation effect [21]. Hence, searching an environmentally friendly organic ligand is the key for improving homogeneous Cu-based Fenton-like system.

Plant-derived polyphenols belong to natural organic matter (NOM) and also low-molecular-weight organic acids, which are commonly found in the natural environments, including water [11,22]. The hydroxyl groups of polyphenols have the property of strong electron donor [23], which could mediate the Cu²⁺/Cu⁺ cycle and then reduce Cu²⁺ to Cu⁺ [11,24], triggering a series of chain reactions (Eqs. 5-11) to generate HO^•, superoxide (O₂^•−), and singlet oxygen (¹O₂) [25,26]. Tartaric acid (TA) is a typical polyphenol, comes from grape and tamarind [10,27], which could be strongly complexed with Fe ions and regulate their redox cycling [28,29]. Nevertheless, there has been an obvious knowledge gap in the function of TA complexed with Cu ions for Fenton/Fenton-like processes.

Based on above, the in-situ simultaneous utilizations of Cu²⁺ and NOM in water might realize the most efficient implementation for Fenton/Fenton-like reactions [30]. Thus, in this study, the effectiveness of Cu²⁺-TA complex for catalyzing CaO₂-based Fenton-like system was verified for metronidazole (MTZ) elimination. The systematical study centered around the followings: (1) Determine the influences of key factors (the dosages of Cu²⁺, TA, and CaO₂, and pH) of the ternary processes; (2) investigate the catalytic function of Cu²⁺-TA complex; (3) explore the evolution and contribution of reactive oxygen species (ROS) during this Cu-based Fenton-like reaction.

The specific chemicals of the work were recorded in Text S1 (Supporting information). The batch experiment was conducted in a 150 mL glass vessel under electromagnetic stirring at room-temperature. The desired concentration of MTZ was configured with phosphate buffer, which fixed the initial pH of the reaction solution. TA and cupric chloride (CuCl₂) were successively added into 100 mL of MTZ solution, the experiment was triggered once added the desired concentration of CaO₂. All the tests were conducted at least in twice. MTZ concentration was detected through UV spectroscopy at 320 nm [13,31]. Tert-butyl alcohol (TBA), l-Histidine (l-His), and p-benzoquinone (p-BQ) were selected as the ROS scavenger, and the generation of ROS was detected by electron spin resonance technique (ESR). The concentration of H₂O₂ was determined by potassium titanium oxalate colorimetry [13]. The content of Cu ions was measured by neocuproine spectrophotometry [32]. Electro-chemical workstation was applied to conduct the cyclic voltammetry measurement [13]. MTZ decomposed intermediates were determined by high performance liquid chromatography-mass spectrometry (HPLC-MS). The detailed analysis and operation were listed in Text S2 (Supporting information).

The enhancement effect of TA on Cu²⁺/CaO₂ was evaluated by comparing the degradation efficiencies of MTZ in diverse processes. Fig. 1a shows the removal of MTZ was negligible in Cu²⁺/CaO₂ after 60 min. Nevertheless, the elimination of MTZ augmented to 78.7% in the Cu²⁺/TA/CaO₂ system. The apparent rate constant (k_obs) of MTZ degradation in Cu²⁺/TA/CaO₂ process was 225 times higher than that in Cu²⁺/CaO₂ in the first 10 min, and 25 times higher in the following 50 min reaction (Fig. 1b). The decrease of k_obs during the following 50 min might be ascribed to the quickly falling H₂O₂. Furthermore, the insignificant degradation of MTZ was observed in TA/CaO₂ and Cu²⁺/TA systems, which indicated that TA could not activate CaO₂ directly, and CaO₂ was the motivity of MTZ removal [33]. UV–vis spectrum was employed to verify the formation of Cu²⁺-TA complex. As depicted in Fig. 1c, compared with the spectra of Cu²⁺ and sole TA, an emerging absorption peak was appeared around 213 nm in the mixed sample of Cu²⁺ and TA, which testified the production of Cu²⁺-TA metal organic ligand. TA might serve as the chelating agent to mediate the redox potential of Cu ions and promote the generation of ROS [18], therefore enhancing the performance of the Cu²⁺/CaO₂ system.

Besides, Fig. 1d shows the Cu²⁺/TA/CaO₂ process also had the remarkable capability for other contaminants removal, including rhodamine b (RhB), sulfamethoxazole (SMX), methyl orange (MO), tetracycline (TC), and methylene blue (MB). Furthermore, Fig. S1 (Supporting information) shows that 34.6% of chemical oxygen demand (COD) removal ratio was reached for domestic sewage by the synergistic process after 180 min reaction. Besides, the total organic carbon (TOC) removal achieved 20.2% after 180 min (Fig. S2 in Supporting information). Thus, the ternary system possessed an extensive applicability and actual potential for removing organic pollutants in wastewater.

The influences of specific reaction parameters on the elimination of MTZ in the Cu²⁺/TA/CaO₂ system were systematically investigated, including Cu²⁺ dosage (Fig. 2a), TA dosage (Fig. 2b), and CaO₂ concentration (Fig. 2c). The results manifested that Cu²⁺, TA, and CaO₂ were all indispensable factors, the absence of any one of them would seriously affect the degradation of MTZ. Furthermore, the higher dosages of Cu²⁺ and CaO₂ would accelerate the generation of HO^• to trigger the self-quenching reaction (Eq. 9) to restrain the elimination of MTZ. TA as an organic compound would compete HO^• with MTZ, hence the excess TA also hindered the removal of MTZ. Therefore, after the comprehensive investigation, the optimal experimental dosages were as follows: 0.1 mmol/L Cu²⁺, 2 mmol/L TA, and 2 mmol/L CaO₂. Moreover, the ternary process indicated a wide pH tolerance, as described in Fig. 2d, MTZ could be eliminated efficiently as the pH values increased from 3 to 8. Above results proved the benign catalytic ability of Cu²⁺-TA complex.

Moreover, the resistant ability for common anions (chloridion (Cl⁻), sulfate ion (SO₄²⁻), nitrate ion (NO₃⁻), and bicarbonate ion (HCO₃⁻)) on the Cu²⁺/TA/CaO₂ system was investigated (Fig. S3 in Supporting information). The complexing system represented a great resistant ability for different anions expect HCO₃⁻. Carbonate radical anion (CO₃^•−) with lower redox potential (1.59 V) would be generated by HCO₃⁻ quenching HO^• (Eqs. 12–14), which would inhibit the degradation of MTZ [34,35].

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To investigate the role of TA in the enhanced Cu²⁺/CaO₂ process, the change of Cu²⁺/Cu⁺ cycle and H₂O₂ concentration during the reaction were compared in Cu²⁺/CaO₂ and Cu²⁺/TA/CaO₂. As displayed in Fig. 3a. 0.19 mg/L of Cu⁺ was produced in Cu²⁺/CaO₂ system, the weak reduction of Cu²⁺ to Cu⁺ might be the reason for hindering MTZ degradation. However, the Cu²⁺ was almost completely converted into Cu⁺ in the present of TA (Fig. 3b), which testified that the Cu²⁺-TA complex could effectually accelerate the reduction of Cu²⁺. In addition, the consumption of H₂O₂ in different systems was also investigated (Fig. 3c). Firstly, 5.88 mmol/L H₂O₂ could be released by CaO₂ alone, and only 0.2 mmol/L of H₂O₂ was decomposed in Cu²⁺/CaO₂. In Cu²⁺/TA/CaO₂ system, H₂O₂ could be consumed rapidly, and the concentration of H₂O₂ was declined from 5.82 mmol/L to 0.05 mmol/L after 60 min. These results proved that the Cu²⁺-TA complex could availably promote the disintegration of H₂O₂.

To demonstrate the redox ability of Cu²⁺-TA complex, the cyclic voltammetry (CV) was conducted in the Cu²⁺ system with or without TA. As shown in Fig. 3d, the peaks of A₁ and A₂ were owed to the oxidation of Cu⁺ to Cu²⁺, while C₁ and C₂ were the reducing peaks of Cu²⁺ to Cu⁺ [13]. The higher peak potential in the mixed Cu²⁺ and TA solution (A₂, C₂) than that in the without TA (A₁, C₁) indicated that the Cu²⁺-TA complex had the greater redox capacity. Hence, TA chelated with Cu²⁺ in the Cu²⁺/TA/CaO₂ could reduce the Cu²⁺/Cu⁺ redox potential to improve the Cu²⁺/Cu⁺ cycle, enhancing the H₂O₂ disintegration and the ROS production [13].

The primary ROS in the Cu²⁺/TA/CaO₂ were clarified firstly by quenching experiment. TBA was applied to quench HO^• (k_HO = 6 × 10⁸ L mol⁻¹ s⁻¹) [36,37]. As described in Fig. 4a, MTZ removal efficiency significantly declined to 24.2% under 2 mmol/L TBA, indicating that HO^• played the dominant role for MTZ elimination. Moreover, p-BQ and l-His were selected to suppress O₂^•− and ¹O₂ (k_O, _p-BQ = (0.9–1) × 10⁹ L mol⁻¹ s⁻¹, k_¹, _l-His = 3.2 × 10⁷ L mol⁻¹ s⁻¹), separately [38–40]. As shown in Figs. 4b and c, the slight quenching effect of p-BQ and l-His illustrated that O₂^•− and ¹O₂ both played the minor role in this system. Ulteriorly, ESR was employed to confirm the existence of ROS. 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) was used to trap HO^• and O₂^•−, 2,2,6,6-tetramethylpiperidine (TEMP) was employed to capture ¹O₂ [38,41]. As presented in Figs. 4d– f, the typical signals of DMPO-HO^•, DMPO-O₂^•−, and TEMP-¹O₂ adducts were detected in both processes with and without TA. Of note, the signal intensity of Cu²⁺/TA/CaO₂ was obviously stronger than that of Cu²⁺/CaO₂, which affirmed the role of TA on the enhancement of ROS generation in this Cu-based Fenton-like reaction.

The generated HO^• exhibited triple roles in this system: (1) contributing to the degradation of MTZ; (2) reacting with the excess H₂O₂ to generate the secondary O₂^•− and ¹O₂ (Eqs. 9–11); (3) attacking the electrophilic TA into the small molecule. Furthermore, Cu⁺-TA also reacted with O₂ to promote the production of O₂^•− (Eq. (15)), and the generated Cu²⁺-TA reacted with the excess H₂O₂ to sustain the Cu²⁺/Cu⁺ cycle (Eq. 16) [42]. However, O₂^•− and ¹O₂ could not effectively remove MTZ due to the low oxidation capacity (k_{O2^•−, MTZ} = 0 L mol⁻¹ s⁻¹, k_{¹O2, MTZ} = 1.5 × 10⁸ L mol⁻¹ s⁻¹), so HO^• was the dominant ROS for MTZ elimination (k_{HO^•, MTZ} = 6 × 10⁹ L mol⁻¹ s⁻¹) in the ternary system, followed by ¹O₂ and O₂^•− [43,44].

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According to the result of HPLC-MS, the possible degradation pathways of MTZ during Cu²⁺/TA/CaO₂ were proposed. As revealed in Fig. 5, firstly, (2-methyl-5-nitroimidazol-1-yl) acetic acid (P1, m/z = 185) might be produced by the N-ethanol group oxidation of MTZ [45,46]. Next, HO^• attacked the N-acetic acid group of P1 and generated 2-methyl-5-nitro-1H-imidazole (P3, m/z = 127) [45,47], or P1 underwent hydroxylation and ring opening reaction to form 3-carbonyl propylene glycol 2-oleic acid (P4, m/z = 118) [45,48]. In another pathway, 3-(2-hydroxy-ethyl)-2-methyl-3H-imidazole-4-ol (P2, m/z = 141) was formed by the direct hydroxylation of nitro group [45,49]. The crack of N-acetic acid group and nitration reaction were occurred on P2 to form byproduct P5 (m/z = 113) [50]. In addition, the biotoxicity of MTZ and its elimination byproducts was evaluated by toxicity assessment software, the results demonstrated that the oral rat LD50 (Fig. S4a in Supporting information) and the developmental toxicity (Fig. S4c in Supporting information) of part intermediates were higher than that of MTZ. However, the bioaccumulation factor (Fig. S4b in Supporting information) and the mutagenicity (Fig. S4d in Supporting information) of the intermediates were all decreased after the treatment of the synergic process. Therefore, the Cu²⁺/TA/CaO₂ process could decrease the biotoxicity of MTZ to some extent, and appropriate prolongation of reaction might be required to minimize toxicity in practical treatment.

To sum up, the conceivable mechanism of TA enhanced Cu²⁺/CaO₂ system can be summarized as follows. In the Cu²⁺/CaO₂ system, the sluggish Cu²⁺/Cu⁺ cycle hindered the activation of H₂O₂ (Eqs. 5 and 6), then hampered the elimination of MTZ (Fig. 1a). Upon the TA addition, Cu²⁺ was rapidly complexed with the two hydroxyl groups of TA to form a soluble Cu²⁺-TA complex (Eq. 17, Fig. 1c), which could decrease the redox potential of Cu²⁺/Cu⁺ (Fig. 3d). The Cu²⁺-TA could be reduced by H₂O₂ to form the reductive Cu⁺-TA, which could improve the reduction of Cu²⁺ (Eq. 16, Fig. 3b) and the activation of H₂O₂ (Eq. 18, Fig. 3c) [42]. Next, the production of various ROS was enhanced by the complexation, such as HO^•, O₂^•−, and ¹O₂ (Figs. 4d–f), and the removal of MTZ was also promoted. Overall, Cu⁺-TA was the critical species to manipulate this ternary system, and TA severed as the electron donor to promote the electron transfer in metal-ligand and enhance the generation of ROS.

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In summary, the complexation of Cu²⁺ and TA could efficaciously catalyze CaO₂ oxidation of MTZ in the pH extent of 3–8. The addition of TA could accelerate the regeneration of Cu⁺, which was driven from the formed ligand of Cu²⁺ and TA. Furthermore, the Cu²⁺-TA complex could motivate the H₂O₂ disintegration and the ROS production. HO^• was proved to be the main ROS for MTZ degradation, followed by ¹O₂ and O₂^•−. MTZ decomposition pathways mainly included hydroxylation, ring opening, attack on specific group, and nitration reactions. This work proved that TA could be a potential chelating agent to promote Fenton-like reaction for degrading different organic contaminants, which provided a referable method for improving the performance of Cu-based Fenton-like system.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank the supports from the National Natural Science Foundation of China (No. 51908485), the Central Guidance on Local Science and Technology Development Fund of Hebei Province (No. 226Z3603G), Hebei Province Foundation for Returnees (No. C20210502).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109185.