Advances in Activation of Persulfate by Novel Carbon-Based Materials: Degradation of Emerging Contaminants, Mechanisms, and Perspectives

Abstract

Global industrialization has intensified the emission of emerging contaminants (ECs), posing a serious threat to the environment and human health. Persulfate-based advanced oxidation processes (PS-AOPs) have become a research hotspot due to their efficient degradation capability and environmentally friendly features; carbon-based materials are ideal catalysts for activating persulfate (PS) due to their tunable electronic structure, abundant active sites, and low cost. This study summarizes the application of carbon-based materials (graphene, single-atom catalysts (SACs), etc.) in PS-AOPs, and provides insights into the degradation mechanisms of radicals (e.g., sulfate radical (SO₄^−·), hydroxyl radical (·OH)) and non-radicals (e.g., ¹O₂(singlet oxygen), electron transfer). The removal efficacy of carbon-based catalysts for antibiotics, phenols, and dyes was compared, and the key degradation pathways were elucidated. In addition, the activation of PS can be accelerated, and catalytic efficiency can be improved by synergizing with ancillary technologies (e.g., light, electricity). Despite the great potential of carbon-based catalysts, their large-scale application is limited by the complexity of the catalyst preparation process and the lack of selectivity for complex water qualities. Future studies can accelerate the practical application of PS-AOPs in wastewater treatment through the precise design of SACs and the construction of multi-mechanism synergistic activation systems.

1. Introduction

With the continuous growth of the global population and the advancement of industrialization, the extensive utilization of organic chemicals has resulted in severe contamination of water resources, thereby posing significant threats to human health. These contaminants are “emerging contaminants” [1] (ECs), and they include pharmaceuticals and personal products (PPCPs), endocrine-disrupting chemicals (EDCs), and persistent organic pollutants (POPs) [2]. Their ubiquitous presence in water, even at low concentrations, may pose a threat to human health and ecosystems [3,4,5,6]. Wastewater with different concentrations of ECs can be detected in polluted surface water such as rivers, lakes, and groundwater [7,8,9]. As demonstrated in Figure 1, the presence of certain antibiotics in groundwater can lead to further contamination, either directly or indirectly. The study revealed a 65% surge in global antibiotic consumption between 2000 and 2015, accompanied by a 39% escalation in antibiotic consumption rates [10]. This rapid growth is closely associated with countries characterized by high population density, such as China, Brazil, and India, among others [11]. It is projected that global consumption of antibiotics will increase by 15% by the year 2030, under the condition that antibiotics are used continuously [12]. Concurrently, the rise in antibiotic resistance is concomitant with the exacerbation of the greenhouse effect [13]. In addition, phenolic pollutants are of great concern because of their high biotoxicity and environmental persistence, and the presence of their benzene ring structure and hydroxyl functional groups leads to the low efficiency of traditional biological treatment, while dye pollutants have strong chromatic interference and bioaccumulation risks due to the presence of complex aromatic conjugation systems and anodyne chromophore groups.

In the present context, a range of methodologies has been the subject of international study with a view to resolving the issue of ECs, including the following: biodegradation [15,16,17,18], adsorption [19], and advanced oxidation processes (AOPs) [20,21,22,23]. In comparison with alternative removal technologies, AOPs boast several advantages, including rapid reaction rates, the absence of secondary pollution, and the capacity to treat a broad spectrum of pollutants. Persulfate-based advanced oxidation possesses (PS-AOPs) the advantages of higher oxidation capacity [24], a wider pH application range [25], and higher stability [26]. Under the excitation of external energies such as light, high temperature and catalysts, the O-O bonds in PS are broken, which in turn generates reactive oxygen species (ROS) through an activation process. These strong oxidizing substances can effectively degrade ECs and eventually achieve the complete mineralization of pollutants into CO₂ and H₂O. PSs include peroxomonosulfate (PMS) and peroxydisulfate (PDS).

PS activation technology can be categorized into two distinct systems: homogeneous and non-homogeneous. Homogeneous activation primarily depends on the external provision of energy (e.g., elevated temperatures, UV light, etc., as illustrated in Figure 2a–c), which is effective in the degradation of pollutants. However, this approach is associated with limitations, including high energy consumption and constrained operational costs [27,28,29]. Conversely, non-homogeneous activation employs solid catalysts (including metal-based, non-metal-based, and their composite catalysts, as depicted in Figure 3a–c) to activate PS, exhibiting reduced energy consumption, cost-effectiveness, and enhanced stability [30,31]. Carbonaceous materials have been identified as optimal green catalysts for the degradation of organic pollutants. As demonstrated in Figure 4a–c, this is attributable to their stable structure, abundant surface functional groups, and diverse PS activation mechanisms, which are primarily attributed to the exceptional catalytic properties of these materials [32,33,34].

At present, a significant proportion of reviews on PS-AOPs carried out by carbon-based materials concentrate on the degradation mechanism and catalyst performance, with comparatively less attention paid to novel pollutants [35,36]. For example, Xiao et al. have reported the effectiveness of a carbon material-activated PS system for the removal of phthalate esters [37]. However, a plethora of novel pollutants have the potential to instigate a diverse array of reactions during AOPs. Furthermore, the majority of extant reviews introduce only single-carbon materials (e.g., graphene (GO) [38], carbon nanotubes (CNTs) [32], and biochar (BC) [39]), and lack a comprehensive description of various carbon-based catalysts, especially single-atom catalysts (SACs), which do not focus on the removal of novel pollutants. For instance, Gao et al. reported on the degradation of antibiotic contaminants by means of PS activation with different carbon materials [40].

This paper presents a systematic review of the degradation mechanism of antibiotics by carbon-based-material-activated PS-AOPs, encompassing both radical and non-radical pathways. Secondly, the optimal activation mode of PS and the degradation of emerging pollutants were revealed by analyzing the structural properties and active sites of different carbon materials. This study further elucidated the key degradation pathways and intermediate product conversion mechanisms by comparing and analyzing the pollutant removal efficiencies of typical carbon-based catalysts. Finally, the synergistic effect of the carbon activation system with auxiliary technologies, such as ultraviolet radiation and electrochemistry, was investigated.

Figure 2. Homogeneous activation of PS. (a) Schematic of thermally activated PS degradation of aromatic compounds [41]. (b) Schematic diagram of the light-activated PS mechanism [42]. (c) Schematic diagram of the Fe²⁺ activation of PS [43].

Figure 2. Homogeneous activation of PS. (a) Schematic of thermally activated PS degradation of aromatic compounds [41]. (b) Schematic diagram of the light-activated PS mechanism [42]. (c) Schematic diagram of the Fe²⁺ activation of PS [43].

Figure 3. Non-homogeneous phase activation of PS. (a) Schematic of PMS activation with BC catalyst (Fe-N₃O-C) [44]. (b) Schematic diagram of magnetic S-type ZnFe₂O₄/A-MoS₂ activation of PMS to degrade PPCPs [26]. (c) Mechanism diagram of MOF activation of PS [45].

Figure 3. Non-homogeneous phase activation of PS. (a) Schematic of PMS activation with BC catalyst (Fe-N₃O-C) [44]. (b) Schematic diagram of magnetic S-type ZnFe₂O₄/A-MoS₂ activation of PMS to degrade PPCPs [26]. (c) Mechanism diagram of MOF activation of PS [45].

Figure 4. (a) Structural characteristics of carbonaceous materials [46]. (b) Presentation of acidic and basic oxygen functional groups on carbon surfaces [47]. (c) Mechanism of degradation of antibiotic contaminants by carbon materials and their major interactions [40].

Figure 4. (a) Structural characteristics of carbonaceous materials [46]. (b) Presentation of acidic and basic oxygen functional groups on carbon surfaces [47]. (c) Mechanism of degradation of antibiotic contaminants by carbon materials and their major interactions [40].

2. Carbon-Based Material Catalysts

Carbon-based materials have shown significant advantages in the field of persulfate activation catalysis due to their high specific surface area, excellent electrical conductivity and tunable surface chemistry. Typical carbon sources include GO, CNTs, C₃N₄, and BC, etc. BC, prepared by the pyrolysis of biomass waste in an inert gas atmosphere, is characterized by its rich surface functional groups but relatively low specific surface area. In contrast, CNTs exhibit excellent catalytic performance in persulfate-activated systems due to their high specific surface area, large aspect ratio and unique sp² hybridized carbon structure. GO, on the other hand, combines the advantages of high specific surface area and outstanding electrical conductivity, acting as an ideal carrier for electron transfer. The surfaces of these carbon materials contain key functional groups such as hydroxyl (-OH), carboxyl (-COOH), carbonyl (C=O), and epoxide (O-C-O), which significantly enhance the pollutant adsorption capacity through multiple mechanisms (including electrostatic attraction, hydrogen bonding, ligand complexation, and π–π interactions), and at the same time effectively contribute to the activation of peroxynitrite and the efficiency of the degradation of ECs.

2.1. Graphene and Graphene Oxide

Figure 5a illustrates the structural nature of GO and graphene oxide (rGO), a prototypical two-dimensional carbon-based nanomaterial, which consists of a two-dimensional honeycomb lattice structure composed of sp² hybridized carbon atoms. In order to enhance catalytic activity, GO is thermally reduced to reduced rGO, with the reduction of GO altering the number and distribution of oxygen-containing functional groups on the surface of the material, which in turn affects the material’s catalytic activity and adsorption capacity. It has been established that the adsorption capacity is generally enhanced with the increase in GO reduction by introducing electron-donating functional groups, which favors the oxidation process occurring on rGO [48]. Furthermore, the doping of various elements into rGO by means of conventional high-temperature pyrolysis has been demonstrated to enhance the activity and stability of the catalysts [49]. Meanwhile, as illustrated in Figure 5b, WO₃ and rGO were combined through a hydrothermal method. rGO is structurally stable, and the doping of nitrogen and metal ions into it is regarded as an effective method to enhance its catalytic activity [50]. As demonstrated in Figure 5c, the SACs, synthesized through the incorporation of N and Co with rGO, preserve the initial structure while augmenting the specific surface area. An analysis of the structural aspect reveals that the SACs exhibit high stability and activity, accompanied by a substantial intermolecular electron transfer efficiency. The similarity of the chemical properties of the carbon and nitrogen elements means that doping nitrogen atoms into the graphene skeleton can effectively adjust the electronic structure and increase the number of active sites, which in turn enhances the activation efficiency of PS. Sun et al. pioneered the field by preparing nitrogen–sulfur co-doped graphene for the activation of PMS; the N-doped graphene activation of PS is demonstrated in Figure 5d. Moreover, metal-doped graphene has been demonstrated to effectively activate perovskite, thereby mitigating the risk of the secondary leaching of metal ions. As demonstrated in Figure 5e, the interaction force and charge density of Zn/Co-MOF@rGO-600 with PMS were calculated by density-functional theory (DFT). The results demonstrate that the sulfonic acid groups in PMS are chemically linked to monoatomic cobalt and zinc through covalent bonding, facilitating the conversion of electrons from metal atoms to -SO₃-, thus activating the PMS. Duan et al.’s computational study, based on DFT, demonstrated that the structural defects and oxygen-containing functional groups on the graphene surface could efficiently activate PMS. The defect sites and oxygen functional groups significantly reduced the energy barrier for O-O bond breaking in PMS molecules, directly accelerating the dissociation process of PMS. Secondly, the vacancies and defective edges of graphene led to the prolongation of the bond length of the O-O bond within the PMS molecule (ca. 0.128 Å), which optimized the charge distribution and enhanced the interfacial electron transfer efficiency, thus promoting the decomposition of PMS [51]. Furthermore, Kuang et al. discovered that this catalytic material could effectively extend the O-O bond length by incorporating N and P heteroatoms into graphene edge nanoribbons. Additionally, they found that the material could completely degrade carbamazepine (CBZ) within 15 min through an electron transfer mechanism, as determined by DFT calculations [52]. While graphene derivatives are effective for the activation of PS, the high cost of graphene hinders its large-scale utilization.

2.2. Carbon Nanotubes

CNTs are a class of one-dimensional tubular nanomaterials that are constructed based on sp² hybridized orbitals. These nanotubes are structurally characterized as coaxial cylinders formed by single- or multilayered graphene lamellae convoluted by chiral vectors (Figure 6a). This distinctive structural design renders them particularly well suited to function as effective metal catalyst carriers. In this capacity, the metal particles, confined within the lumen of the carbon nanotube, have been shown to exhibit a substantial enhancement in perovskite activation efficiency, a phenomenon attributed to the spatial confinement effect. Concurrently, the carbonaceous tube wall has been demonstrated to play a pivotal role in the prevention of metal catalyst deactivation and the loss of active components resulting from sintering. As illustrated in Figure 6b, the research team successfully achieved the uniform solidification of cobalt nanoparticles on the surface of CNTs through a high-temperature polymerization strategy. It is noteworthy that this method can be extended to the synthesis of SACs by precisely tuning the precursor ratios. To thoroughly examine the microstructural characteristics of the catalysts, advanced characterization techniques, including transmission electron microscopy (TEM), X-ray diffraction (XRD), and specific surface area analysis (BET), were employed for systematic investigation. As illustrated in Figure 6c, the Fe-N-CNT₅ material exhibits sub-nanoscale bright spots of a uniform particle size on the surface. These spots, in conjunction with the undetected characteristic diffraction peaks of metallic iron in the XRD spectra, confirm the successful construction of the iron monoatomic catalyst. It is noteworthy that the SACs constructed based on carbon nanotube carriers exhibit a highly specific surface area, which is a significant enhancement over traditional carrier materials. This provides abundant active sites and efficient mass transfer channels for the catalytic reaction. Modified CNTs show excellent potential for application in the PS-AOP. As illustrated in Figure 6d, N-doped single-walled CNTs have been shown to be effective in the removal of contaminants. Furthermore, due to the unique structure of CNTs, it offers a more extensive array of theoretical methods for activating PS. As illustrated in Figure 6e, CNTs exhibit four distinct mechanisms for the adsorption of PMS. Notably, Cui et al. successfully developed an efficient catalytic system by confining Co in the internal structure of CNT through an innovative design. This design utilized the unique interfacial interaction between Co and CNT to significantly enhance the activity and stability of the catalyst. It was shown that the combination of Co and CNT not only optimized the electronic structure of the active center but also improved the efficiency of the catalytic reaction through the interfacial synergy effect. In practical applications, the catalyst demonstrated excellent performance: 2-chlorophenol was efficiently removed in only 40 s with less consumption of PMS, showing its great potential for the rapid degradation of organic pollutants [58].

2.3. Biochar

As a natural carbon material derived from biomass (corn stover [63], sludge [64], reed [65], etc.), the surface properties and functions of BC can be modulated by the preparation process. As illustrated in Figure 7a, the surfaces of sludge and sugarcane BC exhibit a rich array of functional group structures, wherein the pristine BC was directly prepared by means of anaerobic pyrolysis. In contrast, the nitrogen and cobalt co-modified BC (Figure 7b) necessitate the synchronous introduction of nitrogen-containing precursors and cobalt salts during pyrolysis, with surface functionalization modification being achieved through in situ doping. Systematic characterization by XRD, X-ray photoelectron spectroscopy (XPS), and Scanning Electron Microscopy (SEM) (Figure 7c) revealed that the Fe-S co-modified BC has a complete crystalline structure and stable three-dimensional morphology. Furthermore, the XPS and Fourier transform infrared (FTIR) analyses confirm that the surface is rich in oxygen-containing functional groups, such as C=O, O-C=O, and O-Fe. This suggests that the material is both structurally stable and surface-reactive. As demonstrated in Figure 7d, the presence of a multitude of oxygen-containing functional groups has been shown to facilitate electron transfer during PS activation, thereby enhancing the activity of the catalyst. Furthermore, to enhance the performance of BC-activated PS, pristine BC is typically modified through elemental doping. In a related study, Tan et al. prepared nitrogen-doped BC for the activation of PDS in the removal of bisphenol A (BPA), taking advantage of the abundant functional groups on the carbon material’s surface and the impact of metal active sites [66]. As demonstrated in Figure 7e, the adsorption energies of pristine and N-modified BC on PS were calculated using DFT. The calculations demonstrated that the enhanced interfacial interactions between the modified BC and peroxynitrite molecules not only augmented the adsorption affinity of peroxynitrite but also expedited the electron transfer kinetics between the activated peroxynitrite and the target pollutant. Furthermore, structural defects and π–π electronic interactions within the carbon matrix have been identified as additional activation mechanisms that contribute to the efficiency of PS activation [67].

2.4. Other Carbon Materials

Other activated carbon materials have also been extensively studied. These include activated carbon [72], graphitic-phase carbon nitride [73], and mesoporous carbon [74]. These carbon materials have many applications in PS activation due to their large specific surface area and high photostability [75,76]. The study demonstrates that the aforementioned carbon-based materials are capable of efficiently degrading emerging organic pollutants through the photocatalytic activation of the PS system. Their distinctive electron transfer characteristics and surface synergy mechanism indicate significant potential for application in the domain of environmental remediation [77,78]. For example, Chen et al. successfully achieved the efficient removal of rhodamine B (RhB) through a PMS activation system constructed by Fe₃O₄-loaded carbon quantum dots [79]. A substantial body of research has been dedicated to the removal of antibiotics. For instance, He et al. developed N, S co-doped magnetic mesoporous carbon nanosheets, which achieved a 91% degradation rate of tetracycline (TC) within 10 min by activating PMS [80]. Furthermore, modified C₃N₄ has been shown to be capable of effectively degrading phenolic compounds, including phenol [81], BPA [82], and p-chlorophenol [83], under the condition of being exposed to visible light.

2.5. Single-Atom Catalysts

SACs have been demonstrated to exhibit excellent catalytic activity and stability, attributable to their unique active center structure. In the case of carbon-based SACs, their advantageous properties are attributable to the highly dispersed active sites and high atom utilization, which demonstrate significant advantages in AOPs [84,85]. In general, SACs can be prepared by pyrolysis, as illustrated in Figure 8a. The formation of clusters is often observed due to the difficulty in controlling the amount of metal. Yang et al. synthesized monoatomic iron–carbon-based catalysts using polyethylene glycol as a carbon source and urea as a nitrogen source [86]. Furthermore, Zhou et al. prepared Fe-SAC based on BC, and the Fe-N bond was smoothly transplanted into the BC skeleton and regenerated to form a monoatomic Fe-N₄ functional group [87]. This considerably improved the removal efficiency of organic pollutants. Furthermore, Zhang et al. successfully synthesized an asymmetric bimetallic SAC (Fe-Co-N@C), which exhibited a catalytic efficiency increase of approximately 1.41 times in comparison with that of the SAC-Fe [88]. As demonstrated in Figure 8b, the single-atom catalyst CuSA-X@C was analyzed by XRD, XPS, and other characterization methods. The results showed that the XRD spectrum did not display the characteristic diffraction peaks of metal Cu, thereby confirming that Cu was uniformly dispersed on the carbon skeleton in the form of a single atom. Furthermore, the XPS analysis demonstrated the existence of a coordination interaction between the elemental N and the single atoms of Cu, resulting in the formation of a stable N–Cu coordination structure. The TEM results further confirmed the uniform doping of Cu onto the carbon skeleton. Moreover, single-atom carbon-based catalysts have been shown to offer significant advantages with regard to the activation of PMS for the removal of emerging pollutants. In a study by Deng et al., Ru SACs were prepared for the purpose of activating PMS for the removal of organic dyes [89]. As demonstrated in Figure 8c, the schematic representation illustrates the synergistic degradation of BPA via both non-radical and radical pathways by SA-Mn-NC-activated PMS. As demonstrated in Figure 8d, the multi-active site property of PMS enables it to form O–M bonding interactions with the metal center of the SACs. This specific interaction not only enhances the chemisorption capacity of the catalyst for PMS but also significantly improves the activation efficiency of PMS by optimizing the electron transfer pathway. Consequently, single-atom carbon-based catalysts exhibit superior performance in comparison to other carbon-based catalysts. In recent years, the modulation of the coordination environment of SACs has emerged as a prominent area of research interest. Liu et al. conducted a study in which they investigated and revealed the alterations in the local coordination environment of Fe SACs (SA-FeN₆/CN) prior to and following optimization through X-ray absorption fine structure (XAFS) analysis. The findings revealed that the optimization of the coordination environment led to a decrease in the rate constant of atrazine (ATZ) removal by SA-FeN₆/CN, with a value of 0.034 min⁻¹ in five cycle experiments. This result indicates excellent stability and resistance to deactivation. After this finding, combined DFT calculations revealed a charge transfer of 0.88 e between SA-FeN₆/CN and PMS, with an adsorption energy of −2.73 eV. This outcome indicates that rapid electron transfer and strong interactions between the two catalysts occurred, leading to a substantial acceleration in the decomposition process of the pollutants. The optimized catalyst not only enhanced the catalytic efficiency but also exhibited the potential for reuse in practical applications [90].

3. Degradation Mechanism

The activated PMS produced by the carbon material has been shown to be effective in the degradation of emerging pollutants through the generation of ROS. The mechanism of action of activated PS is believed to consist of two types of pathways: radicals and non-radicals. The radical pathway is dominated by the hydroxyl radical (·OH) and sulfate radical SO₄^−·, while the non-radical pathway involves direct electron transfer, surface-mediated oxidation reactions, and the generation of singlet oxygen (¹O₂). Furthermore, the molecular structure of the pollutants may lead to a synergistic interaction between radical and non-radical mechanisms in the degradation process. The dominant pathways are regulated by the characteristics of the pore structure of the carbon materials, the modification of the surface functional groups, and the distribution of the active sites. Differences in contaminant structure may lead to the coexistence of radical and non-radical pathways, with the dominant pathway determined by the structure of the carbon material and surface properties [94,95]. It has been established that the presence of various carbon doping types has a substantial impact on the degradation mechanism of their activated PMS [96,97].

3.1. Radical Pathway

The utilization of carbon-based materials has been demonstrated to be an effective method for the degradation of emerging pollutants. This process involves the activation of PMS, which results in the generation of SO₄^−·, ·OH, and superoxide anion radicals (O₂^−·). Among these, ·OH is notable for its high reactivity, which enables a non-selective attack on organics. In comparison, SO₄^−· exhibits enhanced stability and higher redox potential in acidic to neutral environments, thus ensuring its applicability in a broader range of scenarios. It has been demonstrated that the defective sites present on the surface of carbon materials can function as active centers, thereby promoting PMS cleavage and directionally regulating the radical generation pathway through electron transfer. As demonstrated in Figure 9a, in order to clarify the contribution mechanism of radicals in the reaction system, quenching experiments combined with the electron paramagnetic resonance (EPR) technique were used for validation. This involved the selective quenching of ·OH with tertiary butyl alcohol (TBA), simultaneous quenching of ·OH and SO₄^−· with methanol and targeted trapping of O₂^−·. The precise identification of radical species was achieved based on the characteristic signal peaks of DMPO-captured radicals in the EPR spectra. As illustrated in Equations (1)–(6), PMS can be adsorbed by C surface active sites to generate specific radicals. Concurrently, the functional groups present on the surface of the carbon-based material can undergo a gradual decomposition of PMS, as illustrated in Figure 9b.

$$C-\pi +HS{O}_5^{-}\to C-\pi +O{H}^{-}+S{O}_4^{-·}$$

(1)

$$C-\pi +HS{O}_5^{-}\to C-\pi +H^{+}+S{O}_5^{-·}$$

(2)

$$C=C=O+HS{O}_5^{-}\to S{O}_4^{-·}+C=C=O+O{H}^{-}$$

(3)

$$C=C=O+HS{O}_5^{-}\to S{O}_5^{-·}+C=C=O+H^{+}$$

(4)

$$Carbo{n}_{surface}-OOH+S_2{O}_8{}^{2-}\to Carbo{n}_{surface}-OO·+{S{O}_4}^{-·}+HS{O}_4{}^{-}$$

(5)

$$Carbo{n}_{surface}-OH+S_2{O}_8{}^{2-} \to Carbo{n}_{surface}-O·+{S{O}_4}^{-·}+HS{O}_4{}^{-}$$

(6)

Moreover, O₂^−· typically functions as an intermediary for other reactive substances, thereby not directly participating in the oxidation reaction [98]. The generation of O₂^−· is demonstrated in Equation (7) through to (12).

$${HS{O}_5}^{-}+H_{2}O\to {HS{O}_4}^{-}+H_2{O}_2$$

(7)

$$H_2{O}_2\to 2·OH$$

(8)

$$H_2{O}_2\to H^{+}+H{O_2}^{-}$$

(9)

$$H_2{O}_2+·OH\to H{O}_2^{·}+H_{2}O$$

(10)

$$H{O_2}^{·}\to H^{+}+{O_2}^{-·}$$

(11)

$$S_2{O}_8{}^{2-}+H{O_2}^{-·}\to {S{O}_4}^{-·}+H^{+}+{O_2}^{-·}+{S{O}_4}^{2-}$$

(12)

As demonstrated in Figure 9c, the radicals produced by BC through the activation of PMS have been shown to be highly effective in the degradation of antibiotics. While atomic doping (e.g., N, P) typically promotes radical generation by reconfiguring the electronic structure, precursor differences in carbon materials may trigger competing effects in non-radical pathways. For instance, the N/P co-doped BC (N/P-FBC) prepared from fish scales contributed 76% of the TC removal efficiency of (¹O₂). This suggests that precursor properties may dominate the reaction pathway, shifting from radical to non-radical mechanisms [99].In contrast, SO₄^−· plays a major role in TC degradation in the iron-sludge-based magnetic camphor leaf BC (Fe-SLBC)/PS system developed by Zeng’s team [100]. Furthermore, the S/N co-doped iron-modified BC (S, N-Fe@BC) synergistic activation system, as constructed by Liu et al., has been demonstrated to achieve efficient removal of TC through the synergistic effect of ¹O₂ and O₂^−· [100]. It is imperative to acknowledge that the pathways and products of radical reactions are predominantly influenced by a multitude of environmental factors, including temperature, pH, and the type and concentration of anions present within the water column. These factors not only determine the rate of generation and the stability of radicals but may also significantly alter their redox capacity, thereby affecting the final outcome of the reaction. For instance, the presence of chloride ions (Cl⁻) in the water column has been observed to facilitate the reaction of radicals (e.g., ^·OH) with the formation of the less oxidizing chlorine radical (Cl·). This reaction typically occurs under neutral or alkaline conditions and accelerates with increasing chloride ion concentration. Conversely, under acidic conditions, the reaction path of the radicals with Cl⁻ may undergo a substantial change, resulting in the formation of a more oxidizing hypochlorous acid (HClO) [101].

Figure 9. (a) Experimental methods for detecting different free radicals, (1)–(3) quenching experiments, and (4)–(5) EPR tests [102]. (b) Schematic of ROS generation by C=C- and C=O-activated PMS system [20]. (c) Schematic of BC removes antibiotics via non-radical pathways [103].

Figure 9. (a) Experimental methods for detecting different free radicals, (1)–(3) quenching experiments, and (4)–(5) EPR tests [102]. (b) Schematic of ROS generation by C=C- and C=O-activated PMS system [20]. (c) Schematic of BC removes antibiotics via non-radical pathways [103].

3.2. Non-Radical Pathways

In comparison with the radical pathway, the non-radical pathway has been shown to possess the capacity to accurately identify and target pollutants containing specific functional groups (e.g., a benzene ring and an amino group) for degradation through ¹O₂-directed oxidation or interfacial electron transfer [104]. Concurrently, it has been demonstrated to maintain high efficiency and stability under conditions of complex water quality and within a wide pH range (3–11) [105]. Notably, for free radical-resistant pollutants, the non-radical pathway overcomes the constraints imposed by traditional radical chain reactions, achieving deep mineralization of pollutants through direct electron transfer at the interface or surface-limited domain oxidation [106].

3.2.1. Singlet Oxygen

In comparison with conventional radicals, the aqueous environment demonstrates significant advantages for the utilization of ¹O₂. Its longer half-life confers higher environmental stability, its selective oxidative properties enable the precise degradation of pollutants, and it is more resistant to substrate interference. Furthermore, the presence of this radical can be substantiated through EPR and quenching experiments, as demonstrated in Figure 10a. Based on the characteristic signal peaks of radicals captured by TEMP in the EPR spectra (1:1:1), the precise identification of radical species was realized [107]. Furthermore, the results of the selective quenching experiments demonstrated a substantial decline in the efficiency of pollutant degradation when specific bursting agents, such as furfuryl alcohol (FFA), were incorporated. This observation further confirms that the reaction system is a function of ¹O₂. In the Fe_xO_y/NG activation of PMS, with respect to the degradation of TC experiments, Jiang et al. verified that ¹O₂ can be generated by the self-decomposition of O₂^−·, as in Equation (13) through (15) [108]. In addition, as shown in Figure 10b, MBC can also activate PDS to produce ¹O₂ in different ways. Furthermore, PMS has been observed to undergo self-decomposition, resulting in the production of ¹O₂, as depicted in Equation (16). Additionally, the presence of SO₄^−· has been shown to catalyze the formation of ¹O₂, as evidenced by Equation (17) through to (20) [109].

$$S_2{O}_8{}^{2-}+2H_{2}O\to H{O_2}^{-·}+2S{O_4}^{2-}+3H^{+}$$

(13)

$$S_2{O}_8{}^{2-}+H{O_2}^{-·}+\to S{O_4}^{2-}+S{O_4}^{-·}+H^{+}+{O_2}^{-·}$$

(14)

$$2O_2{}^{-·}+2H_{2}O{\to }^1{O}_2{+H}_2{O}_2{+2OH}^{-}$$

(15)

$$HS{O}_5^{-}+S{O_5}^{2-}\to HS{O}_4^{-}+S{O}_4^{2-}{+}^1{O}_2$$

(16)

$$S{O_5}^{-·}+S{O_5}^{-·}\to S_2{O}_8{}^{2-}+^1{O}_2$$

(17)

$$S{O}_5{}^{-·}+S{O}_5{}^{-·}\to S{O}_4{}^{2-}+^1{O}_2$$

(18)

$$2S{O}_5{}^{-·}+H_{2}O\to 2HS{O}_4{}^{-}+{1.5}^1{O}_2$$

(19)

$$HS{O}_5{}^{-}+S{O}_4{}^{-·}\to S{O_5}^{-·}+S{O_4}^{2-}+H^{+}$$

(20)

It is noteworthy that the highly dispersed, isolated metal sites present in the SAC-Co can markedly reduce the PMS adsorption activation energy and expedite its decomposition by optimizing the metal–carrier electronic interactions, as illustrated in Figure 10c.

3.2.2. Direct Oxidation

It has been demonstrated that the PS system not only degrades pollutants via the radical pathway but also possesses direct oxidizing ability itself, which can effectively remove a wide range of emerging organic pollutants [112]. Zhang et al. found that PMS could effectively remove oxytetracycline (OTC) and methylene blue (MB) [113]. Yin’s team found that PMS could achieve structural damage by selectively attacking the N and S heteroatoms on the benzene ring of sulfonamide antibiotics [114]. Wang et al. demonstrated the degradation of levofloxacin by PMS alone [115]. It is important to note that the direct oxidation of PMS does not occur independently; it often exhibits a synergistic effect with non-radical mechanisms, such as radical pathways and electron transfer, during the actual reaction.

3.2.3. Electronic Transfer

In the non-radical activation mechanism based on direct electron transfer, the carbon material forms a directional electron transfer channel via the surface adsorption of PMS in a process that does not depend on the generation of radicals. Electrochemical characterization results demonstrated a substantial enhancement of the current response in the linear scanning voltammetry (LSV) curves of boron (B)-doped carbon nitride, as can be seen in Figure 11a. This finding, in conjunction with the reduction in impedance values obtained via electrochemical impedance spectroscopy (EIS), substantiates that the integration of B atoms optimizes the electronic structure of the carbon skeleton and facilitates efficient electron migration from the pollutant to the PMS. Concurrently, the ternary electron transfer system of pollutant–carbon catalyst–PMS (Figure 11b) elucidated the system electron transfer mode. Furthermore, DFT calculations (Figure 11c) demonstrated that the substantial charge density disparity at the interface between the carbon material and the PMS indicated considerable electron redistribution at the surface, which was instrumental in facilitating electron transfer.

The phenomenon of multi-mechanism synergism has been observed to be prevalent in the removal of emerging pollutants by carbon-based materials. It has been demonstrated that there are substantial discrepancies in the mechanisms of action of diverse catalytic systems. For instance, nitrogen-doped BC has been observed to degrade phenol through dual pathways, including electron transfer and direct oxidation [117]. The Fe-Cu bimetallic-loaded BC developed by Shang’s team employs a synergistic strategy that utilizes both radical and non-radical pathways for the degradation of tetracycline during the activation of PMS [118]. In contrast, the removal of sulfamethoxazole by the SA-Cu/rGO composite catalyst constructed by Chen et al. involves a triple mechanism of SO₄^−·, ·OH, and ¹O₂ [119]. Consequently, for particular catalyst–pollutant combination systems, it is imperative to elucidate the predominant action pathways through systematic studies.

4. Carbon-Based Catalyst Activation of PS for Degradation of ECs

4.1. Antibiotics

In response to the problem of water environment pollution caused by the overuse of antibiotics by human beings, studies have shown that antibiotics such as sulfonamides (containing benzene ring, sulfonamide group (-SO₂-NH₂), and para-amino acid (-NH₂)) have persistent residual properties in the natural environment due to their high structural stability and high water solubility. Tetracyclines (a planar polycyclic structure consisting of four hydrocarbon rings) and quinolones have been shown to possess persistent residual properties in the natural environment due to their high structural stability and high water solubility [120]. The physicochemical properties of antibiotics significantly influence their removal efficiency. Figure 12a systematically compares the molecular configuration differences among various sulfonamide antibiotics. Figure 12b provides the chemical structure information of four tetracycline antibiotics. For a given class of antibiotics, the carbon materials can be removed through different pathways in a synergistic manner. As illustrated in Figure 12c, the CNT/PMS system eliminates sulfamethoxazole (SMX) through the synergistic action of the radical pathway and the non-radical pathway, which primarily involves S-N bond breaking, hydroxylation, and nitroxylation. The efficacy of these novel carbon-based materials in removing these antibiotics is demonstrated in

Table 1. Studies on the degradation of antibiotic contaminants by different types of carbon-based catalysts.

Catalysts	Specific Surface Area	Antibiotics	Removal	Activator	Refer
CNT = 0.1 g·L−1	297 m2·g−1	SMX = 0.15 mM	99% in 30 min	PMS = 0.5 mM	[121]
Co3O4@NPC/rGO = 15 mg·L−1	40.6 m2·g−1	SMX = 25 mg·L−1	100% in 30 min	PMS = 0.2 mM	[122]
Fe-N/BC = 0.05 g·L−1	292.87 m2·g−1	TC = 30 mg·L−1	89.9% in 60 min	PMS = 0.5 g·L−1	[123]
FS-BC = 0.40 g·L−1	\	TC = 5 mg·L−1	99.71% in 180 min	PMS = 0.40 g·L−1	[99]
MOF-N/C = 0.3 g·L−1	264 m2·g−1	TCH = 96.2 mg·L−1	91% in 10 min	PS = 1 mM	[124]
NG = 0.1 g·L−1	580.7 m2·g−1	TC = 35 mg·L−1	100% in 120 min	PS = 0.75 mM	[125]
CuO/N-rGO = 0.2g·L−1	\	TC = 20 mg·L−1	97% in 120 min	PS = 1 mM	[96]
Fe3C@CNT = 0.1 g·L−1	374.03 m2·g−1	SMX = 5 mg·L−1	91% in 90 min	PS = 1 mM	[126]
Mn0.85Fe2.15O4-CNTs = 0.4 g·L−1	116.9 m2·g−1	TC = 20 mg·L−1	95.8% in 60 min	PMS = 0.8 mM	[127]
Sludge-derived BC = 2 g·L−1	\	SMX = 0.15 mM	94.6% in 180 min	PDS = 1.5 mM	[128]

.

The utilization of carbon-based materials, such as PS-AOP, in the removal of other antibiotics has also been a subject of study. For instance, Yang et al. developed cobalt-doped carbon nitride (Co@N-C) catalysts for the efficient degradation of the quinolone antibiotic norfloxacin by activation over PMS [129]. Pang et al. studied natural-wood-derived charcoal embedded in bimetallic iron/cobalt sites and found that it promoted ciprofloxacin degradation [130]. The degradation of such antibiotics occurs through the breakage of characteristic groups in their molecular structure (e.g., piperazine ring, cyclopropyl, etc.) by successive oxidations, which are eventually converted into CO₂ and H₂O. The degradation of other antibiotics by AOPs has also been reported in the literature, including amoxicillin [131], penicillin G [132], and β-lactam antibiotics [133], among others. However, there is a paucity of systematic studies on the synergistic mechanism of carbon-based materials. The relationship between the molecular structure and the degradation pathways of different antibiotics must be analyzed in depth.

4.2. Phenolic Compounds

Phenolic compounds originate mainly from industrial emissions, agricultural activities, and medical waste. Phenolic compounds are difficult to degrade, mainly because of the stabilizing structure of the benzene ring and the presence of hydroxyl groups that provide some water solubility. Notably, substituent species (e.g., -Cl, -NO₂) significantly affect the toxicity. Recent studies have shown that carbon-based catalytic materials in PS-AOP synergistically contribute to the removal of phenolic compounds via radical and non-radical pathways [134].

Table 2. Studies on the degradation of phenolic pollutants by different types of carbon-based catalysts.

Catalysts	Specific Surface Area	Phenolic	Removal	Activator	Refer
Co-N-C = 0.0005 g·L−1	\	2,4-DCP = 5 μM	80% in 30 min	PMS = 5 μM	[136]
Biochar = 0.2 g·L−1	594 m2·g−1	2,4-DCP = 0.1 g·L−1	98% in 120 min	PDS = 0.5 g·L−1	[137]
CNT = 0.1 g·L−1	497.7m2·g−1	Phenol = 0.1 mM	100% in 60 min	PDS/PMS = 1 mM	[138]
CoO–N-C = 0.3 g·L−1	453.75m2·g−1	4-CP = 50 mg·L−1	100% in 30 min	PMS = 1.5 mM	[139]
GBC = 0.15 g·L−1	307.73 m2·g−1	Phenol = 5 mg·L−1	100% in 30 min	PS = 2 mM	[140]
Fe/N-CNT = 0.05 g·L−1	220.68 m2·g−1	ACT = 10 mg·L−1	98% in 30 min	PS = 0.08 mM	[141]
CNT = 0.3 g·L−1	939.38 m2·g−1	ACT = 10 mg·L−1	100% in 10 min	PS = 0.21 mM	[142]
NS-CNT = 0.1 g·L−1	228 m2·g−1	BPA = 20 mg·L−1	100%in 30 min	PDS = 1.5 mM	[143]
Cu-rGO = 0.25 g·L−1	148.69 m2·g−1	BPA = 0.09 mM	99% in 40 min	PMS = 3 mM	[144]
N-biochar = 0.5 g·L−1	\	BPA = 10 mg·L−1	100% in 5 min	PMS = 2.0 mM	[145]
PSBC = 0.4 g·L−1	106.8 m2·g−1	4-CP = 0.1 mM	100% in 10 min	PMS = 1.0 mM	[146]

summarizes studies on the removal of phenolic compounds such as acetaminophen (ACT), 2,4 dichlorophenol (2,4-DCP), and BPA by different carbon-based materials. In addition, further studies have shown that phenol degradation follows a sequential oxidation pathway: first, hydroxylation to catechol or p-benzoquinone, followed by benzene ring breaking to form short-chain carboxylic acids such as maleic acid and oxalic acid, and finally mineralization to CO₂ and H₂O [135].

4.3. Dyes

Dye compounds exhibit high chemical stability due to the complex structure of benzene rings, azo groups (−N = N−), and sulfonic acid groups. However, they are susceptible to long-term residues in water and soil [147]. Common dyes include MB, RhB, and acid orange 7 (AO7). These organic dyes possess high coloration intensity even at low concentrations, and their relatively large molecular size hinders effective removal. Consequently, it has been demonstrated that adsorption can effectively remove dye-like substances, a process that does not necessitate the use of AOPs [148].

Table 3. Studies on the degradation of dye-based pollutants by different types of carbon-based catalysts.

Catalysts	Specific Surface Area	Dyes	Removal	Activator	Refer
Fe@NCNT-BC-80 = 0.05 g·L−1	225.4 m2·g−1	RhB = 20 mg·L−1	100% in 10 min	PS = 5 mM	[151]
CNT = 0.2 g·L−1	104 m2·g−1	RhB = 20 mg·L−1	100% in 210 min	PMS = 119.0 mg·L−1	[152]
CoO@meso-CN = 0.2 g·L−1	569 m2·g−1	MB = 0.1 mM	99% in 15 min	PS = 20 mM	[153]
EC-PN = 0.2 g·L−1	783.4 m2·g−1	MB = 100 mg·L−1	100% in 60 min	PMS = 0.15 mM	[154]
CNTs = 0.1 g·L−1	\	AO7 = 0.057 mM	100% in 60 min	PMS = 1.14 mM	[155]
MnFe2O4-SAC = 0.2 g·L−1	\	OG = 20 m g·L−1	100% in 30 min	PS = 0.5 g·L−1	[156]

systematically summarizes the relationship of functionalized carbon-based materials in dye degradation. In addition, the conventional carbon-based catalyst PS-AOP for RhB degradation has not been studied much [149]. Further studies found that the introduction of exogenous energy fields (e.g., UV light, thermal activation, or ultrasonic radiation) was able to significantly enhance the activation efficiency of PMS, which accelerated the dye mineralization process by reinforcing the synergistic interaction of SO₄^−·, ·OH, and ¹O₂ pathways. Taking the typical azo dye AO7 as an example, auxiliary means such as thermal activation can promote its further decomposition [150]. Table 3 systematically summarizes the relationship of functionalized carbon-based materials in dye degradation.

5. Auxiliary Activation Methods

Conventional carbon-based catalysts have been demonstrated to exhibit favorable degradation performance for ECs in PS-AOP; however, they are generally characterized by inherent limitations, including low electron transfer efficiency, an inadequate radical generation rate, and a limited lifetime of active species. To address this situation, the introduction of external field energy-assisted activation has been demonstrated to be an effective strategy to overcome reaction kinetic limitations. Currently, the primary developments in the field include heat activation, photochemical activation, electrochemical activation, and other synergistic enhancement pathways [157]. The assisted activation has been shown to facilitate electron-hole formation in carbon-based materials and expedite O-O breakage in PS. This bifunctional synergistic mechanism has been demonstrated to enhance the efficiency of the oxidation system to a significant extent. Cheng et al. constructed a Vis-GO-Fe/PMS photocatalytic system that demonstrated universal advantages in the degradation of typical pollutants, such as BPA, MB, RhB, etc., which confirmed the applicability of the synergistic action of the system [158]. The ultrasound-assisted effect has garnered significant interest, as evidenced by its notable enhancement within the rGO/PMS system. Experimental findings have demonstrated that the removal rate of RhB in the rGO/PMS+ultrasound system can reach 100% within 20 min, which is approximately seven times higher than the 15% removal rate observed in the no-ultrasound condition [159]. Furthermore, research has been conducted on electrically assisted PS-AOPs. Furthermore, the electrochemically assisted strategy has been demonstrated to achieve targeted modulation of activation efficacy through the construction of carbon-based functional electrodes [160]. The FeNi-NC bifunctional electrode developed by Yao’s team represents a significant innovation in the field, integrating the mechanisms of electro-activation and catalytic activation. This integration results in remarkable catalytic performance, evident in the effective simultaneous degradation of TC, RhB, and NOR [161].

Although these auxiliary activation approaches can accelerate radical generation to significantly enhance the degradation kinetics of pollutants, their practical applications remain constrained by the undefined quantitative relationship between energy input intensity and treatment efficiency. For instance, photoactivation requires precise matching between the light wavelength and catalyst bandgap, while electrochemical activation demonstrates high sensitivity to the current magnitude and catalyst surface properties. Therefore, more systematic investigations are imperative to optimize auxiliary activation conditions.

6. Prospects and Outlook

Emerging studies demonstrate that carbon-based materials exhibit exceptional catalytic potential in PS-AOPs. Their surface-abundant defect sites and π–π conjugation systems enable the targeted activation of PS to generate ROS while concurrently accelerating pollutant oxidation through both radical and non-radical pathways, presenting novel opportunities for domestic wastewater remediation. However, practical implementation of carbon-based catalysts faces multifaceted constraints: (1) in synthesis processes, the complexity of doping modification techniques coupled with energy-intensive high-temperature procedures substantially elevates production costs, posing economic viability challenges in industrialization; (2) regarding catalytic performance, inadequate selectivity becomes particularly pronounced in complex organic or inorganic coexisting systems, while structural instability under elevated temperatures or extreme pH conditions severely compromises material durability.

The precise engineering of SACs offers an innovative paradigm for enhanced environmental remediation. By constructing highly stable SAC systems through cost-effective carrier materials and metal precursors, this approach not only dramatically reduces synthesis costs but also enables the directional optimization of catalytic pathway selectivity and reactivity via coordination engineering-mediated electronic structure modulation of metal active centers. Notably, establishing synergistic activation systems through the integration of SACs/carbon-based catalysts with multi-modal activation strategies has been demonstrated to transcend the performance limitations of conventional single activation modes. This hybrid configuration leverages multi-mechanistic synergies to comprehensively activate PS for diversified ROS generation while triggering pollutant-specific degradation pathways tailored to environmental conditions and contaminant characteristics, achieving enhanced mineralization efficiency with reduced catalyst dosage. Collectively, the strategic coupling of precisely designed SACs with carbon-based catalysts and complementary activation modalities provides an economically efficient and technically superior approach for aquatic environment purification.

7. Conclusions

This paper presents a comprehensive review of the extant research on carbon-based materials in PS-AOPs, with a focus on their application in the degradation of emerging pollutants. The review encompasses a range of topics, including the degradation mechanisms (covering both radical and non-radical pathways), the structural properties of different carbon materials, the active sites for the adsorption of PS, the removal efficacies of pollutants by typical carbon-based catalysts, and the synergistic effects with ancillary technologies. The paper emphasizes the excellent catalytic potential of carbon-based materials in PS-AOPs, which is mainly attributed to their stable chemical properties, abundant active sites, and environmental friendliness. However, there are still some challenges in practical applications, such as high preparation costs, insufficient selectivity, and structural destabilization. In the future, the precise design of SACs and the construction of synergistic activation systems are expected to optimize their performance and provide more effective solutions for the purification of the water environment.