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20 May 2026

Fe-Cu Co-Doping Enhanced Peroxymonosulfate Activation for the Degradation of Dimethyl Carbonate in Lithium-Ion Battery Recycling Wastewater

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1
School of Environment Science and Spatial Informatics, China University of Mining and Technology, Xuzhou 221008, China
2
State Key Laboratory of Coking Coal Resources Green Exploitation, China University of Mining and Technology, Xuzhou 221116, China
*
Author to whom correspondence should be addressed.
Catalysts2026, 16(5), 479;https://doi.org/10.3390/catal16050479
  • Editorial Note: Due to an editorial processing error, this article was incorrectly included within the Special Issue Catalytic Materials for Hazardous Wastewater Treatment upon publication. This article was removed from this Special Issue’s webpage on 21 May 2026 but remains within the regular issue in which it was originally published. The editorial office confirms that this article adhered to MDPI's standard editorial process (https://www.mdpi.com/editorial_process).
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Abstract

The lithium battery recycling industry is developing rapidly, and the rapid oxidation and degradation of dimethyl carbonate (DMC) in the wastewater generated by this industry is of crucial importance. In this study, Fe and Cu dopants were controlled and the C-SiO2 framework with porous structures was constructed to synthesize FeCuC-SiO2 and C-SiO2 catalysts. The former could achieve 91.65% of DMC degradation within 60 min through peroxymonosulfate (PMS) activation, and the degradation rate was increased to 4.44 times compared to C-SiO2 without Fe and Cu doping. And under optimized conditions, a DMC degradation rate of 90.57% can be achieved within 10 min by FeCuC-SiO2. The catalyst has good stability and the catalytic activity can be maintained during reuse process for five times with over 70% of DMC degradation rate, 58.9% of mineralization rate, and a relatively low amount of metal leaching. Moreover, the degradation rate can still remain above 70% with the existence of impurity anions, demonstrating a strong salt resistance. Hydroxyl radicals (OH•), sulfate radicals (SO4•−), and 1O2 were found to dominant the reaction in the FeCuC-SiO2-PMS system, which were involved in both free radical and non-free radical pathways and led to excellent catalytic oxidation performance and environmental adaptability. In general, a novel design for a Fenton-like catalyst was presented, providing a theoretical basis for the improvement of oxidation efficiency and the regulation of reaction pathways in Fenton-like reactions.

1. Introduction

The lithium-ion battery (LIB) industry has experienced rapid growth in recent years. In 2024, the total LIB production of China reached 1170 GWh, and the total number of new energy vehicles in the province exceeded 2.69 million only in Jiangsu Province. The extraction and recovery of valuable metals, such as lithium, nickel, cobalt, and manganese, which from the cathode materials of spent LIBs, represent one of the most effective strategies for mitigating resource scarcity and alleviating associated environmental pollution. Currently, the majority of LIB recycling enterprises employ hydrometallurgical leaching processes for lithium extraction. The wastewater generated from these processes is classified as a typical refractory industrial effluent, characterized by high salinity, high chemical oxygen demand (COD), and inherently poor biodegradability. Carbonate esters present in the electrolyte, including ethylene carbonate, propylene carbonate, and dimethyl carbonate (DMC), constitute a primary source of COD and pose significant threats to ecological systems. Consequently, the degradation of carbonate ester pollutants in lithium extraction wastewater has emerged as an urgent environmental remediation priority.
Current treatment methodologies for lithium extraction wastewater encompass coagulation–precipitation, adsorption, advanced oxidation processes (AOPs), and microbial degradation. Among these, AOPs constitute a key technology for organic pollutant abatement, primarily operating via the generation of oxidizing reactive species such as OH• and SO4•− to achieve efficient COD reduction. AOPs offer distinct advantages, including rapid degradation kinetics, high mineralization efficiency, broad pollutant compatibility, and operational simplicity. In particular, heterogeneous Fenton-like catalytic systems based on transition metal-catalyzed, atomically doped catalysts for the activation of oxidants have become a focal point of research. These systems are distinguished by their fast degradation rates, wide operational pH value, robust oxidative capacity and stability, absence of iron sludge formation, suitability for in situ chemical oxidation, and enhanced safety during transport and storage [1,2,3]. The primary reactive species generated within the catalytic system are OH•, SO4•−, and 1O2, all of which possess strong oxidizing capabilities. Reactions mediated by OH• and SO4•− proceed via radical-based pathways, whereas those involving 1O2 and direct electron transfer operate through non-radical mechanisms. Notably, non-radical pathways exhibit superior adaptability, enhanced selectivity, and higher oxidant utilization efficiency compared to their radical counterparts, and thus have garnered substantial research interest in the field of advanced oxidation, but it is difficult to be accurately controlled [4,5,6,7].
Sarath et al. [8] employed an Fe2+-activated persulfate (PS) system for the removal of magenta dye from aqueous solutions, achieving a maximum decolorization efficiency of 96.33% and a total organic carbon (TOC) removal efficiency of 55.23% within 30 min under optimal conditions. Song et al. utilized nanoscale zero-valent iron (nZVI) to activate PS for the in situ remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil, attaining an 82.21% removal efficiency for PAHs [9]. Wang et al. [10] investigated trace Cu-activated PMS for the degradation of organic contaminants, including benzotriazole, 2,4-dichlorophenol, 1,4-dioxane, and toluene, under conditions simulating natural water bodies or wastewater. Their findings indicated that the Cu(I)/Cu(II)/Cu(III) redox cycle functions as the dominant electron shuttle, with the system exhibiting enhanced performance in neutral to alkaline environments [11]. Furthermore, the activation of PMS by MnO2 for bisphenol A degradation has been reported to proceed not via radical or 1O2 pathways, but rather through direct oxidation by a surface complex formed between PMS and MnO2 [12]. Ren et al. [13] synthesized a Cu-containing metal–organic framework (MOF) material featuring ordered nanopores and a narrow pore size distribution, subsequently loaded with nano-Fe2O3, for the degradation of bisphenol A.
The utilization of a single metal species in isolation is often associated with limitations such as low active site utilization efficiency and suboptimal electron mediation capacity. To address these constraints, Zhao et al. [1] synthesized Fe-Co-C/N materials and identified multiple active site configurations, including Fe-N3, Co-N3 coordination moieties, and Fe-Co alloy phases. The encapsulation of these sites within a protective carbonaceous network conferred strong resistance to metal leaching. Concurrently, the underlying alloy phases induced functionalization of the carbon overlayer, thereby facilitating electron transfer for PMS activation. Density functional theory (DFT) calculations further revealed that the catalyst surface promotes elongation of the O-O bond in PMS, thereby lowering the energy barrier for bond cleavage. Zhong et al. synthesized FeSe-C materials and enhanced their catalytic activity through the deliberate introduction of Se vacancies. The incorporation of Se vacancies was found to modulate the surface electronic structure and local coordination environment, which not only promoted efficient charge transfer but also reduced both the adsorption energy and the activation reaction barrier, thereby significantly enhancing PMS activation [14]. Consequently, multi-metal doping has emerged as a favored strategy among researchers, as it enhances the electron-mediating capability of catalysts through synergistic and size effects [15,16,17,18,19,20,21].
In the present study, C and SiO2 were employed as catalyst supports, and the catalytic performance was optimized through the controlled doping of Fe and Cu. Advanced instrumental characterization techniques were utilized to elucidate the surface morphology, crystalline structure of active sites, elemental chemical states, pore architecture, and specific surface area of the prepared catalysts. The catalytic performance and the influence of operational parameters were systematically evaluated through DMC degradation experiments. Catalyst stability and operational safety were assessed via cyclic reuse tests, coexisting anion interference studies, metal leaching measurements, and mineralization efficiency determinations. Quenching experiments and electron spin resonance (ESR) spectroscopy were conducted to identify the dominant reactive species, revealing a hybrid radical–non-radical oxidation mechanism governed by OH•, SO4•−, and 1O2. This work provides a novel catalyst design paradigm for the rapid degradation of DMC and offers new mechanistic insights into the construction of synergistic dual-pathway reaction systems for advanced wastewater treatment.

2. Results and Discussion

2.1. Characterization

As can be seen from the FSEM images (Figure 1a), abundant nano-scale pore structures with good connectivity are observed on the surface and interior of the FeCuC-SiO2 catalyst, which is conducive to the mass transfer process. The framework structure is uniform and stable without any debris. From the EDS results of FeCuC-SiO2, it is obviously that Fe and Cu are uniformly doped on the surface of the catalyst, with contents of 0.72% and 0.38%, respectively. Additionally, the content of C on the surface is 50.26%, while the contents of Si and O are 9.69% and 38.95%, respectively, it can be inferred that the main component of the framework is C-SiO2. From Figure 1b, it can be observed that the pore structure of C-SiO2 with poor connectivity has significantly decreased, but cracks are generated. Moreover, about 51.38% of C is detected by EDS analysis, which is 5.3 times that of FeCuC-SiO2 catalyst. And the content of Si is 14.76%, indicating that the surface of the SiO2 framework is covered by a large amount of C compounds, resulting in the low exposure degree of the SiO2 framework.
Figure 1. FSEM images and EDS mapping of (a) FeCuC-SiO2 and (b) C-SiO2.
XRD is used to characterize the crystal structure generated on the surface of the catalysts and the results are shown in Figure 2a. For FeCuC-SiO2 and C-SiO2 catalysts, nine peaks at 21.98°, 28.44°, 31.46°, 36.08°, 42.66°, 44.84°, 47.06°, 48.61°, and 57.08° are assigned to the (1 0 1), (1 1 1), (1 0 2), (2 0 0), (2 1 1), (2 0 2), (1 1 3), (2 1 2), and (3 0 1) crystal planes of SiO2 with the space group of P4-12-12 (PDF#39-1425). This confirms that the main component of catalyst skeleton is SiO2, which is consistent with the results of FSEM-EDS. Otherwise, the peaks centered at 31.77°, 36.25°, 47.54°, 56.60°, and 62.86° belong to FeCuC-SiO2 are corresponding to the (0 0 6), (0 1 2), (1 0 4), (0 1 5), and (0 1 8) planes of CuFeO2 crystals with the space group of R3m (PDF#85-0605) [22,23,24]. This indicates that Fe and Cu are uniformly doped on the surface of the SiO2 framework of FeCuC-SiO2 in the form of CuFeO2 crystal.
Figure 2. (a) XRD spectra, (b) FTIR spectra, (c) N2 adsorption–desorption isotherms, and (d) pore size distribution curves of FeCuC-SiO2 and C-SiO2.
FTIR spectra is shown in Figure 2b. It can be seen that the functional groups on the surfaces of the two catalysts are relatively simple. The peaks at about 470, 800 cm−1 corresponds to the Si-O bending vibration and Si-O-Si symmetric stretching vibration; the peaks at 1080 and 1620 cm−1 are attributed to Si-O-Si stretching vibrations and OH bending vibrations, respectively [25,26,27,28]. The peak at 3440 cm−1 corresponds to the stretching vibration of O-H bond [29,30], which is attributed to the surface moisture. It indicates that there are no impurity functional groups on the catalyst surface.
The specific surface area and pore structure of the catalyst are important to the catalytic reaction, which can be characterized by N2 adsorption–desorption isotherms and pore distribution curves. From Figure 2c, the presence of mesopores in FeCuC-SiO2 and C-SiO2 catalysts is confirmed by the hysteresis loops of Type IV, which is benefited for the adsorption and mass transfer [31,32]. Moreover, the specific surface area of FeCuC-SiO2, which were calculated by the BET method, was 129.99 m2/g, which is 72.4% higher than that of C-SiO2 (75.39 m2/g), implying a stronger surface adsorption ability and more reactive sites for reaction. The pore size distribution was shown in Figure 2d; the average pore diameter (7.75 and 6.83 nm, BJH method) and the micropore size (1.46 and 1.13 nm, HK method) of FeCuC-SiO2 and C-SiO2 catalysts are relatively similar.
The XPS results shows that the C 1s peaks (Figure 3a) of FeCuC-SiO2 and C-SiO2 catalysts at 284.8 eV, 286.5 eV, and 288.5 eV correspond to C-C, C-O, and C=O, respectively, with the similar proportions [29]. It indicates that the doping of Fe and Cu has a relatively minor effect on the chemical state of C. Additionally, the O1s peaks (Figure 3b) of FeCuC-SiO2 at 531.3 eV and 533.1 eV correspond to O-Fe/Cu (25.40%) and O-C (74.60%), respectively [22], and only the peak correspond to O-C is found in C-SiO2. Moreover, Fe 2p peaks are shown in Figure 3c and the binding energy of the main peaks at 711.6/724.7 eV and 714.5/727.6 eV correspond to Fe2+ 2p3/2/Fe2+ 2p1/2 and Fe3+ 2p3/2/Fe3+ 2p1/2 with the proportions of Fe2+ (68.12%) and Fe3+ (31.88%) in FeCuC-SiO2, respectively [33,34]. Cu 2p peaks are shown in Figure 3d and the absence of peak at 708 eV indicates that no Fe0 is generated in FeCuC-SiO2 catalyst. In addition, the peaks with the binding energies of 932.3/952.0 eV and 934.2/954.0 eV correspond to Cu+ 2p3/2/Cu+ 2p1/2 and Cu2+ 2p3/2/Cu2+ 2p1/2 with the content of 78.47% and 21.53% in FeCuC-SiO2 catalyst [29,30,35]. It indicates that Fe and Cu have been successfully doped, and different valence oxide species with active properties have been formed. The results are consistent with the results of FTIR, FSEM-EDS, and XRD.
Figure 3. XPS spectra of (a) C 1s and (b) O 1s for and C-SiO2, (c) Fe 2p, and (d) Cu 2p of FeCuC-SiO2.

2.2. Catalytic Oxidition and Stability Evaluation

In this study, DMC was employed as the targeted contaminant to evaluate the PMS activation by FeCuC-SiO2 and C-SiO2 catalysts. In order to estimate the influence of the Fe and Cu doping, different catalysts were used and the results are shown in Figure 4a. The DMC removal efficiency in 60 min of FeCuC-SiO2-PMS is 91.65%, respectively, which is 4.44 times that of C-SiO2-PMS systems (20.66%), indicating the better catalytic ability of FeCuC-SiO2. It is due to that Fe and Cu are transition metals, which can act as active sites to participate in electron mediation, thereby enabling the activation of PMS and the degradation of DMC [36]. To test the effect of the catalyst dosage, PMS concentration, temperature, and the initial pH of the solution on the catalytic activity of FeCuC-SiO2, batches of experiments were conducted. As the dosage of catalyst increased from 0.05 to 0.1, 0.2, and 0.3 g/L, the degradation ratio increased from 65.14% to 91.65%, 90.96%, and 76.54% (Figure 4b), demonstrating that the appropriate increase in dosage can effectively enhance the catalytic reaction, but too much catalyst will actually inhibit the reaction. This can be attributed to the positive correlation between catalyst dosage and the number of active sites: a greater number of active sites generates more reactive species, thereby enhancing the degradation of DMC. However, when the catalyst dosage is excessively high, the overproduced reactive species undergo self-quenching, which leads to suppression of the degradation [37].
Figure 4. (a) DMC degradation in FeCuC-SiO2 and C-SiO2 systems ([DMC] = 30 mg/L, [Catalyst] = 0.2 g/L, [PMS] = 2 mM, T = 35 °C, initial pH = 3.0). Effect of (b) catalyst dosage ([PMS] = 2 mM, T = 35 °C, initial pH = 3.0), (c) PMS concentration ([Catalyst] = 0.2 g/L, T = 35 °C, initial pH = 3.0), (d) initial pH ([Catalyst] = 0.2 g/L, [PMS] = 2 mM, T = 35 °C), and (e) temperature ([Catalyst] = 0.2 g/L, [PMS] = 2 mM, initial pH = 3.0).
As shown in Figure 4c, with the concentration of PMS increases from 0.5 to 1, 1.5, and 2 mM, the degradation rates of DMC in FeCuC-SiO2-PMS system after 60 min raises from 68.41% to 69.56%, 81.36%, and 91.65%, respectively, indicating that the concentration of PMS has a positive correlation with the generation of reactive species, which dominate the DMC degradation [38]. Additionally, the degradation rates at initial pH values of 3, 5, 7, and 9 after 60 min were 91.65%, 70.66%, 66.54%, and 79.66% (Figure 4d), respectively, revealing that the FeCuC-SiO2 catalyst exhibits better activity under acidic conditions. This is attributed to the enhancement of metal ions leaching with the facilitating of acidic environment and thereby increasing the reaction rate [39]. Additionally, As the temperature raised from 15 to 25, 35, and 45 °C, the degradation rates of DMC after 60 min are 49.36%, 91.65%, 93.65%, and 95.10%, respectively (Figure 4e). It can be seen that, when the reaction temperature is higher than 25 °C, the degradation rate does not show a significant increase with the rise in temperature; it is speculated that, when the temperature exceeds 25 °C, although the generation of free radicals increases substantially, the self-quenching reactions are also dramatically intensified, resulting in a lower oxidant utilization efficiency and consequently no significant enhancement of the degradation reaction [40,41]. Importantly, 90.57% of DMC was estimated in 10 min in FeCuC-SiO2 system, which is the best result in this study, demonstrating an excellent catalytic performance. Performances of different catalysts based on Fe-Cu doping in previous studies are shown in Table 1.
Table 1. Performance of different catalysts based on Fe-Cu doping in previous studies.
This study verified the changes in the reaction activity of the FeCuC-SiO2 catalyst for degrading DMC in a PMS environment through five cycles of experiments. In Figure 5a, the DMC degradation rates for the five-cycle experiments were 94.22%, 89.47%, 80.94%, 74.62%, and 71.46% respectively. This indicates that the catalyst activity significantly decreased with the increase in the number of uses, and the rate of decline slowed down after the fourth use. Meanwhile, through the analysis of the dissolution of Fe and Cu during the cycle process (Figure 5b), the dissolution amounts of Fe were determined to be 283.6, 127.1, 98.9, 113.4, and 65.8 ug/L respectively, and the dissolution amounts of Cu were 373.5, 314.8, 230.0, 172.2, and 219.1 ug/L, respectively. It was found that the dissolution amount decreased rapidly at first and then leveled off, which was in agreement with the results of the aforementioned cyclic experiments. The main reason for the attenuation of its activity is that the dissolution of unstable metal ions on the catalyst surface leads to a decrease in the number of active sites. The later stabilization is due to the relatively stable remaining metal active sites. Furthermore, the mineralization rate of pollutants can indicate the extent of the reaction in the catalytic system (Figure 5c). The mineralization rates of DMC in the five cycles of experiments were 55.4%, 47.8%, 57.1%, 34.7%, and 58.9%, respectively, indicating that the mineralization rate was relatively stable during the cyclic experiments and the oxidation degree of the reaction system changed little. The influence of co-existing anions on the reaction system was studied by adding different anions (Figure 5d). With the high concentration (200 mM) of Cl−, CO32−, HPO42−, and NO3−, the degradation rates of DMC were 87.33%, 70.96%, 75.46%, and 80.22%, respectively. Compared with the 91.65% in the blank, they decreased by 4.32%, 20.69%, 16.19%, and 11.43%, respectively, indicating that the FeCuC-SiO2 catalyst has a strong resistance to Cl− and a relatively weak resistance to CO32− and HPO42−.
Figure 5. (a) Cycling experiment, (b) concentration of leaching Fe and Cu, (c) mineralization rate of DMC during the cycling experiment of FeCuC-SiO2 system, and (d) co-existing anions ([Catalyst] = 0.2 g/L, [PMS] = 2 mM, T = 35 °C, initial pH = 3.0, [anions] = 200 mM).

2.3. Reactive Oxidation Species Identification

In order to distinguish the reactive species generated during the activation of PMS by the two catalysts and their reaction proportions in the oxidation process, quenching experiments using different types of quenching agents were arranged. Briefly, MeOH can serve as a quencher for OH• and SO4•−, while tert-butyl alcohol (TBA) is less effective at quenching SO4•− compared to MeOH, but has a higher quenching reaction rate for OH• (k = 3.8 × 109 M−1 s−1–7.6 × 109 M−1 s−1), making it suitable as a quencher for OH•. FFA is an effective scavenger of singlet oxygen (k = 1.2 × 108 M−1 s−1). p-BQ can act as a quencher for O2•− (k = 9.7 × 108 M−1 s−1) [49,50,51,52]. The experimental results of the FeCuC-SiO2-PMS and C-SiO2-PMS systems are presented in Figure 6a,b. The four quenching agents have significant inhibitory effects on the reaction. Under the conditions of MeOH, TBA, FFA, and p-BQ, the degradation rate of DMC in the FeCuC-SiO2-PMS system decreased from 91.65% in the control experiment to 49.76%, 54.13%, 56.51%, and 58.81%, respectively. For the C-SiO2-PMS system, the degradation rate of DMC decreased from 20.65% (blank) to 11.63%, 15.46%, 13.52%, and 13.84%. The obvious inhibitory indicates that OH•, SO4•−, O2•−, and 1O2 are all generated and participate in the degradation of DMC in both catalyst systems. For the FeCuC-SiO2-PMS system with higher reactivity, different concentrations of quenching agents were used to further verify the contribution of active species. As shown in Figure 6c, with the concentration of MeOH increasing from 100 to 200, 500, and 1000 mM, the degradation rate of DMC decreased from 91.65% in the blank group to 61.51%, 60.21%, 49.76%, and 33.54%, showing an increasing trend of inhibition and indicating that more OH• and SO4•− are quenched with the increase in MeOH concentration. The quenching results of TBA are shown in Figure 6d. With the concentration of TBA increasing from 100 to 200, 500, and 1000 mM, the degradation rate of DMC decreased from 91.65% (blank group) to 72.96%, 71.65%, 60.86%, and 54.13%. It is clarified that the dominant radical is OH•, with a higher generation amount than SO4•−. The quenching experimental results of different concentrations of FFA are established in Figure 6e. With the concentration of FFA increasing from 100 mM to 200, 500, and 1000 mM, the degradation rate of DMC decreased from 91.65% in the blank group to 69.52%, 67.21%, 56.51%, and 49.63%, indicating that 1O2 also plays a crucial role in the reaction. Figure 6f shows the quenching results of p-BQ with different concentrations. With the concentration of p-BQ increasing from 1 mM to 2, 5, and 10 mM, the degradation rate of DMC decreased from 91.65% in the blank group to 77.14%, 71.65%, 57.24%, and 58.81%; the quenching effect increased with the concentration, indicating the generation of O2•−. Overall, the dominant reactive species generated by the FeCuC-SiO2-PMS system are OH•, 1O2, and O2•− and there is a relatively small amount of SO4•− involved.
Figure 6. DMC degradation in (a) FeCuC-SiO2 and (b) C-SiO2 systems using different quenching agents ([DMC] = 30 mg/L, [catalyst] = 0.2 g/L, [PMS] = 2 mM, [MeOH] = [FFA] = 0.5 M, [TBA] = 1 M, [p-BQ] = 10 mM. T = 35 °C, initial pH = 3) and different concentrations of (c) MeOH, (d) TBA, (e) FFA, and (f) p-BQ.
In order to make an accurate determination of the generation of reactive species in the reaction systems, ESR was employed using DMPO and TEMP as the scavengers for OH•, SO4•−, and 1O2 analysis. From the ESR results (Figure 7a,b), the existence of DMPO-OH• and DMPO-SO4•− signals in the FeCuC-SiO2/PMS system demonstrates the generation of OH• and SO4•− [50,53]. Furthermore, as time passes, the generation of SO4•− decreases, while the content of OH• increases and becomes the dominating factor. However, there are no DMPO-OH• and DMPO-SO4•−signals in the C-SiO2/PMS system, revealing the non-radical reaction. From Figure 7c,d, the existence of characteristic peaks of TEMP-1O2 demonstrates the dominating role of 1O2 pathway [49,53,54,55]; in FeCuC-SiO2/PMS system and as time passes, the signal becomes stronger and the yielded quantity increases. On the contrary, no TEMP-1O2 was generated within the C-SiO2/PMS system, indicating that no 1O2 was involved in the reaction. In general, OH• and 1O2 pathways dominated the catalytic oxidation reaction.
Figure 7. ESR spectra of DMPO-OH• and DMPO-SO4•− with DMPO as trapping agent in (a) FeCuC-SiO2/PMS and (b) C-SiO2/PMS systems, and TEMP-1O2 with TEMP as trapping agent in (c) FeCuC-SiO2/PMS and (d) C-SiO2/PMS systems.

2.4. Catalytic Mechanism

Fe and Cu atoms are the main active sites of FeCuC-SiO2 catalyst, which exist in the form of CuFeO2 crystals. Based on the experimental and instrumental analysis results, the DMC degradation mechanism in the FeCuC-SiO2/PMS system can be inferred and is shown in Figure 8. Two oxidation reaction pathways exist in this system. Firstly, PMS was adsorbed on the surface of CuFeO2 crystal. Through the electron-mediated process of Fe-O-Cu and the electron-donating effect of Fe and Cu atoms, electrons were transferred to PMS, leading to the break of O-O bond and the generation of OH• and SO4•−. The free radicals have strong oxidizing capacity and the DMC molecules were attacked and the electrons were seized, causing the DMC to decompose, and further mineralization occurred with the generation of CO2 and H2O. This process belongs to the free radical pathway. Secondly, after PMS was adsorbed, due to the size effect and electron effect on the catalyst surface, PMS was decomposed and 1O2 was generated as a strong oxidant, which has a longer lifetime and environmental adaptability compared with OH• and SO4•−. The DMC molecule can be oxidized and then mineralized with the bond breaks causing by 1O2. In conclusion, the FeCuC-SiO2 catalyst achieves an efficient degradation process of DMC through the synergistic pathway with 1O2 and free radicals. Compared to the single free radical pathway of the traditional Fenton reaction, it has better environmental adaptability.
Figure 8. Mechanism of FeCuC-SiO2/PMS/DMC system.

3. Materials and Methods

3.1. Chemicals

All chemical reagents employed in the experiments were of analytical grade and used as received without further purification. 2,6-Diaminopyridine, colloidal silica, sodium hydroxide (NaOH), ammonium persulfate ((NH4)2S2O8), methanol (MeOH), ethanol (EtOH), ferric nitrate nonahydrate (Fe(NO3)3·9H2O), cupric nitrate trihydrate (Cu(NO3)2·3H2O), hydrazine hydrate, hydrofluoric acid (HF), sodium thiosulfate (Na2S2O3), tert-butyl alcohol (TBA), furfuryl alcohol (FFA), p-benzoquinone (p-BQ), sodium chloride (NaCl), sodium nitrate (NaNO3), sodium carbonate (Na2CO3), and disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O) were procured from commercial sources. The suppliers included Nanjing Chemical Reagent Co., Ltd. (Nanjing, China), Xilong Scientific Co., Ltd. (Shantou, China), Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China), and Aladdin Industrial Corporation (Shanghai, China).

3.2. Catalyst Preparation

2,6-Diaminopyridine (10.9 g), ammonium persulfate (5.7 g), and NaOH (4 g) were dissolved in 200 mL of deionized water under stirring in a water bath maintained at 15 °C. The mixture was subsequently transferred, together with the magnetic stirrer, to a refrigerator set at 2 °C and allowed to react under continuous stirring for a period of 12 h. The resulting suspension was subjected to centrifugation, and the recovered solid was dried under vacuum at 105 °C to afford a brown polymeric product. This polymer was then redissolved in 300 mL of methanol (MeOH), to which 50 g of colloidal silica, 0.8 g of Fe(NO3)3·9H2O, 0.48 g of Cu(NO3)2·3H2O, and 10 mL of hydrazine hydrate were added. The resulting mixture was stirred gently at 60 °C until complete evaporation of the solvent yielded a dry residue. The solid residue was scraped from the vessel, ground into a fine powder, and subjected to calcination in a tubular furnace under a nitrogen atmosphere. The thermal treatment protocol was as follows: heating from ambient temperature to 300 °C at a ramp rate of 5 °C·min−1, followed by a 2 h isothermal hold; subsequent heating to 900 °C at the identical ramp rate, with a further 2 h dwell. Upon natural cooling of the furnace to room temperature, the resulting black particulate material was collected. To partially remove the silica framework and thereby enhance porosity, the solid was immersed in 100 mL of 10% hydrofluoric acid (HF) under ultrasonication for 10 min. The material was then washed thoroughly with dilute hydrochloric acid (0.1 M), deionized water, and ethanol in succession, and finally dried under vacuum at 105 °C to yield the FeCuC-SiO2 catalyst. For the preparation of the metal-free control catalyst (C-SiO2), an identical synthetic route was followed, with the sole exception that the addition of Fe(NO3)3·9H2O, Cu(NO3)2·3H2O, and hydrazine hydrate was omitted entirely.

3.3. Characterization and Analysis

Morphological features and elemental composition were characterized using a field-emission scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (FSEM-EDS, GeminiSEM 300, ZEISS, Oberkochen, Germany). Crystallographic information was acquired via X-ray diffraction analysis (XRD, D8 ADVANCE, Bruker, Karlsruhe, Germany), while surface chemical states and elemental quantification were determined through X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD, Shimadzu, Kyoto, Japan). Textural properties, encompassing specific surface area and pore size distribution, were evaluated by means of nitrogen physisorption measurements performed on a surface area and porosimetry system (ASAP2460, Micromeritics, Norcross, GA, USA). The identification of functional moieties present on the catalyst surface relied on Fourier-transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Fisher, Waltham, MA, USA).
Quantification of dimethyl carbonate (DMC) was carried out on a gas chromatograph (Agilent 7890B, Agilent, Santa Clara, CA, USA) fitted with a VF-WAXms capillary column; the relevant parameters can be referred to the method proposed by Terborg et al. [56]. The extent of Fe and Cu leaching during consecutive reuse cycles was monitored using inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7800MS, Agilent, Santa Clara, CA, USA). The degree of DMC mineralization post-reaction was assessed with a total organic carbon/total nitrogen analyzer (TOC-L TNM-L CSN, Shimadzu, Japan). Furthermore, the identity of the prevailing active species was elucidated via electron spin resonance spectroscopy (ESR5000, Bruker, Germany), employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidone (TEMP) as spin-trapping agents.

3.4. Experimental Procedures

The catalytic evaluation was conducted in a series of 250 mL flasks under constant mechanical agitation at 120 rpm. A systematic investigation was undertaken to elucidate the influence of several operational parameters on the reaction performance, namely the incorporation of sulfur (comparing FeCuC-SiO2 and C-SiO2), the loading of catalyst (varied at 0.05, 0.1, 0.2, and 0.3 g L−1), the concentration of PMS (adjusted to 0.5, 1, 1.5, and 2 mM), the initial pH value of the medium (3, 5, 7, and 9), and the reaction temperature (15, 25, 35, and 45 °C). To gauge the long-term robustness of the FeCuC-SiO2 material, five successive cyclic runs were performed, during which the extent of Fe and Cu ion dissolution was monitored alongside the degradation efficiency and mineralization yield of DMC. The resistance of the catalytic system to inorganic salts was further assessed by introducing a variety of background anionic species into the degradation matrix. In an effort to delineate the specific contributions of the transient active species generated during the catalytic oxidation, quenching assays were carried out employing graded concentrations of MeOH, TBA, FFA, and p-BQ. Upon completion of each individual reaction, the oxidative pathway was immediately arrested through the introduction of excess Na2S2O3. The residual DMC was subsequently recovered via solid-phase extraction using an HLB cartridge, followed by elution with CH2Cl2. The eluate was passed through a 0.22 μm organic syringe filter prior to injection into the gas chromatograph for quantitative analysis.

4. Conclusions

FeCuC-SiO2 and C-SiO2 were prepared for PMS activating to degrade DMC in wastewater from lithium battery recycling. The experimental study revealed that Fe and Cu were embedded on the surface of the C-SiO2 framework in the form of CuFeO2 crystal, thereby achieving catalytic activity. The degradation rate of DMC within the FeCuC-SiO2/PMS system reached 91.65% after 60 min, which is 4.44 times higher than the C-SiO2/PMS system (20.66%) without Fe and Cu doping. Moreover, at a temperature of 45 °C, it achieved the best performance with the DMC degradation rate of 90.57% within 10 min. The catalyst has good stability, which can maintain the high catalytic efficiency, high mineralization rate, and low leaching amount even after multiple uses. Moreover, it can still retain high catalytic activity even in the presence of impurity anions. Importantly, the dominant active species are OH•, SO4•−, and 1O2 in the FeCuC-SiO2/PMS system, which are involved in both free radical and non-free radical pathways and lead to excellent catalytic oxidation performance and environmental adaptability. In this study, a novel design for a Fenton-like catalyst was presented, providing a theoretical basis for the improvement of oxidation efficiency and the regulation of reaction pathways in Fenton-like reaction.

Author Contributions

S.H.: Conceptualization, Formal analysis, Data curation, Methodology, Writing—original draft. F.J. and Q.H.: Data curation, Investigation, Visualization, Software. L.W. and J.S.: Conceptualization, Funding acquisition, Supervision, Project administration, Writing—review and editing. Y.X.: Validation, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Basic Research Program of Jiangsu grant number BK20251636, National Natural Science Foundation of China grant number 52504320, Jiangsu Funding Program for Excellent Postdoctoral Talent grant number 2024ZB724, The Fundamental Research Funds for the Central Universities grant number 2025QN1107.

Data Availability Statement

Data is contained within the article. All the data have been presented in the main text and the figures.

Acknowledgments

This work was supported by Basic Research Program of Jiangsu (BK20251636), National Natural Science Foundation of China (Grant No. 52504320), Jiangsu Funding Program for Excellent Postdoctoral Talent (2024ZB724), and The Fundamental Research Funds for the Central Universities (2025QN1107). We are grateful to the editors and anonymous reviewers for their valuable comments and suggestions for our paper.

Conflicts of Interest

The authors declare no conflict of interest.

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