1 Introduction

With the rapid improvement of urbanization and the living standards of people, environmental hazards caused by organic pollutants have attracted widespread attention. For example, antibiotics, endocrine disruptors, and pharmaceuticals and personal care products have been widely detected in water, posing a threat to human health and the ecological environment (Cheng et al. 2021b; Jiang et al. 2023b). At present, organic pollutants in wastewater can be removed through different technologies, including adsorption, electrochemistry, biodegradation, and advanced oxidation processes (AOPs) (Jiang et al. 2023a). However, these methods may have low mineralization levels, produce toxic intermediate products, and possibly generate secondary pollution. For example, adsorption, as a non-destructive physical process, only transfers organic pollutants from the liquid phase to the solid phase. Biodegradation through the metabolic activity of microorganisms and their adaptability to the environment to degrade pollutants, this method takes a relatively long time (Liu et al. 2022c). In addition, with the increase of organic wastewater discharge standards, AOPs are considered one of the most promising methods for treating organic wastewater, such as photocatalysis, Fenton oxidation, persulfate (PS) -based  AOPs, ozonation, and so on. Nevertheless, the toxic intermediates produced during the degradation process limit their large-scale application (Lu et al. 2022d). For example, 4-nitrosulfamethoxazole (Chaturvedi et al. 2022) and C13H11FN2O3 (Li et al. 2023c), as the degradation by-products, were more toxic than their parent compound, exhibiting carcinogenicity or mutagenicity. Therefore, it is necessary to explore new treatment technologies to promote the complete degradation of organic pollutants.

Up to now, many studies have found that the limitations of a single-AOPs or the low mineralization of pollutants can be solved by coupling different AOPs (Xiao et al. 2023; Xu et al. 2023). Meanwhile, to further improve the synergy produced by the coupling processes and reduce costs, the selection of catalysts has also been widely concerned. The metal-based catalysts currently used may not be optimal due to high preparation costs and metal leaching during the catalytic process (Liu and Dai 2016). On the contrary, carbonaceous materials (CMs) are considered potential substitutes for metal catalysts due to their excellent physicochemical properties, low production costs, and good application effects (Liu and Dai 2016). The common CMs include graphene, carbon nanotubes (CNTs), carbon quantum dots (CQDs), biochar, etc. (Duan et al. 2018). They have been successfully applied in various advanced oxidation coupling processes (C-AOPs) because of their excellent biocompatibility, metal-free leaching, and large specific surface area (SSA). Based on the reasons above, by searching for topics such as “advanced oxidation coupling”, “advanced oxidation coupling”, and “carbonaceous materials (graphene, CNTs, CQDs, biochar, etc.)” in “Web of Science”, it was found that CMs have received extensive research in C-AOPs in recent years, gradually attracting enthusiastic attention from researchers (Fig. 1).

Fig. 1
figure 1

Research trend of C-AOPs and C-AOPs based on CMs. The number of publications retrieved using “advanced oxidation coupling”, “advanced oxidation coupling”, and “carbonaceous materials (graphene, CNTs, CQDs, biochar, etc.)” as subjects (Data obtained from Web of Science)

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Previous studies mainly focused on single C-AOPs, studying the degradation and mineralization of organic pollutants. Meanwhile, some articles have been published on the removal and degradation mechanisms of organic pollutants by C-AOPs, but most of them are limited to the degradation and mineralization of pollutants by a single C-AOPs (Chen et al. 2021a; Liu et al. 2022c; Sukhatskiy et al. 2023; Van Aken et al. 2019; Vieira et al. 2021). For example, Zhang et al. (2022a) reviewed the mechanisms and practical applications of intimately coupled photocatalysis and biodegradation. Abdurahman et al. (2021) elaborated on the sonophotocatalytic coupling processes with their degradation mechanisms. However, the coupling processes based on CMs has not yet attracted people’s attention, and the synergistic effects and degradation mechanisms generated during the degradation of organic pollutants are still unclear. Therefore, it is necessary to systematically summarize the latest research progress to reveal the roles and mechanisms of CMs in C-AOPs for the degradation of organic pollutants in wastewater. This review first summarized the applications of different CMs in C-AOPs. Secondly, the synergistic mechanisms of C-AOPs based on CMs are discussed. Then, the toxicity assessment methods in C-AOPs based on CMs were analyzed. Finally, future research trends and suggestions for coupling processes were proposed. This review not only contributes to a comprehensive understanding of the synergistic mechanisms of C-AOPs based on CMs, but also provides green and efficient solutions for organic wastewater treatment.

2 Application of different CMs in C-AOPs

In the past, various metal-based catalysts have been used to account for a relatively large proportion of applications in AOPs (Liu and Dai 2016). However, due to their high cost and the biological toxicity of metal leaching, there is an urgent need to seek green catalyst material alternatives. In recent years, CMs with high porosity, large SSA, and strong stability have been considered good catalysts in AOPs. With the development of various C-AOPs, the rich active sites and good conductivity of CMs can promote the generation of rich reactive oxygen species (ROS) and the generation of different degradation pathways, to enhance the synergy of the coupling processes (Fig. 2). This section would introduce the application of several commonly used CMs in C-AOPs. Meanwhile, the applications of CMs in C-AOPs are listed in Table 1.

Fig. 2
figure 2

Characteristics of different CMs and their roles in the C-AOPs (CNTs-Carbon nanotubes; CQDs-Carbon quantum dots; GR-Graphene; BC-Biochar)

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Table 1 Application of different CMs in C-AOPs and degradation effects of organic pollutants
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2.1 Carbon nanotubes

Carbon nanotubes (CNTs) are co-axial circular pipes composed of single or multiple-layer graphene sheets. The carbon atoms in CNTs undergo sp2 hybridization. Compared to sp3 hybridization, the sp2 orbital composition in sp2 hybridization is relatively large, making the CNTs have high modulus and strength. According to the existence of layers, CNTs are divided into single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs). SWNTs have high chemical inertness and a relatively simple chemical structure. MWNTs possess higher stability, larger SSA, and more defect structures than SWNTs, which can inhibit e−-h+ recombination. Due to the unique properties of CNTs, they have been extensively studied as green and efficient catalysts. Research has found that during the catalytic ozonation process, CNTs were conducive to the sustained formation of the interphase “HO zone” and served as an initiator to convert \({\text{O}}_{3}^{-}\) into \(\cdot {\text{O}}_{2}^{-}\)(Zhang et al. 2017). In addition, when CNTs are used for PS activation, CNTs act as an electron shuttle, PS and pollutants act as electron acceptors and electron donors, respectively, and organic pollutants are degraded by non-free radical pathways without generating ROS (Jiang et al. 2020). On the other hand, CNTs contain an inner cavity structure and many unpaired electrons that can move freely, promoting catalytic reactions when used as a carrier (Bhuvaneswari et al. 2021).

Generally, composites, metal, nonmetal doping, and surface functionalization can affect the catalytic activity, magnetic properties, or structural properties of CNTs (Soares et al. 2016). For example, single or multiple heteroatom doping can adjust the chemical inertness, charge distribution, and density of CNTs, producing more active sites (Li et al. 2022b). Nawaz et al. (2020) studied the CNT-NiFe2O4 composite that could enhance the separation of e−-h+ and the generation of \(\cdot {\text{OH}}\) during the photo-Fenton reaction process. Moreover, researchers also loaded nanoparticles onto CNTs to prepare magnetic composite catalysts (Wang et al. 2014). The magnetized composite catalysts had good ion exchange capacity and a low e−-h+ recombination rate. Al Musawi et al. (2022) studied MWCNTs/CoFe2O4 magnetic nanocomposite as sono-photocatalysts. The results showed that the stability of the catalyst was improved, and the doping of MWCNTs increased the absorption range of CoFe2O4 for visible light. Although magnetized CNTs increase the ability of solid–liquid separation, they may cause the overflow of magnetic substances and result in secondary pollutants during recycling. Further research should focus on improving the separability and large-scale practical applications of CNTs in the future.

2.2 Graphene and its derivatives

Graphene is the lightest material in the world, with high optical transparency, large SSA, and good biocompatibility. Graphene has been applied as a catalyst or catalyst carrier in AOPs (Prasad et al. 2020). However, it cannot achieve a good purification purpose in wastewater treatment when used as a carrier due to the hydrophobicity of the original graphene (Chen et al. 2018). Graphene oxide (GO) can be formed by loading a large number of oxygen-containing functional groups (such as C–O, C=O, and O–H) on the monolayer graphene, which enhances the hydrophilicity and dispersion of GO. Kuntail et al. (2021) found that the strong interaction between H2O2 and the epoxide functional groups on the GO surface promoted the \(\cdot {\text{OH}}\) production in photo-Fenton oxidation. In addition, reduced graphite oxide (rGO) with greater stability is formed by removing the oxygen-containing functional groups on the GO surface (Desmecht et al. 2019). rGO has hydroxyl groups, Lewis acid sites, and electrons, thus enhancing its catalytic activity.

To explore the synergistic effects of GO/rGO in C-AOPs, researchers carried out a series of related experiments. For example, Wu et al. (2018) studied the catalytic activity of ZnO and ZnO-rGO catalysts in photocatalytic ozonation. Babu et al. (2019) studied the effects of CuO-TiO2/rGO composite catalyst on the sonophotocatalytic processes. Song et al. (2014) studied the role of GO-TiO2 composite catalyst in photocatalytic ozonation, etc. Compared to a single process, the degradation and mineralization efficiency of most combined processes are significantly improved. These are attributed to the following aspects: (1) GO/rGO as a photocatalyst carrier accelerated the transport of charge carriers photocatalysts (Prasad et al. 2020), (2) large SSA provided more catalytic active sites (Dayana Priyadharshini et al. 2022), and (3) the inhibited e−-h+ pairs recombination and enhanced absorption of visible light. In the past decade, researchers also doped graphene with other atoms/functional groups to improve the catalytic activity. In general, introducing heteroatoms into graphene can alter the sp2 hybrid structure of C atoms, affect the surface electron distribution of graphene, and lead to more defect structures in the graphene lattice (Xu et al. 2018). At the same time, under the action of the π–π bond, it could enhance the adsorption of pollutants and accelerate the electron transfer rate. The above research explores the removal mechanisms and the degradation effects of graphene-based catalysts on single pollutants, but the catalytic performance of graphene under the coexistence of multiple pollutants is still unclear. Therefore, research in this area needs further exploration.

2.3 Carbon quantum dots

Carbon quantum dots (CQDs) are composed of carbon nanoparticles with a particle size of less than 10 nm, and exhibit excellent tunable photoluminescence, high quantum yield, and excellent biocompatibility characteristics (Si et al. 2020). At present, it is used to synthesize new CQDs-based semiconductor materials, applied in medical imaging technology, environmental monitoring, and catalyst preparation (Ge et al. 2021). Due to the highly adjustable surface functional groups, excellent electron transfer/storage ability, and environmental friendliness of CQDs, it has shown great potential as a catalyst (Cheng et al. 2022). Especially in photocatalysis, CQDs were highly sensitive to visible light, and the catalytic activity was improved by acting as photosensitizers or spectral converters for photoelectron receptors and catalysts (Lu et al. 2016). In the photocatalysis coupled PS oxidation process, CQDs reduce the recombination of e−-h+ and the photocorrosion of nanomaterials, and tetracycline reached 100% degradation efficiency within 70 min (Amiri et al. 2022). CQDs could emit light with short wavelengths after absorbing visible light, inducing the generation of photogenerated e−-h+ (Zhao et al. 2018). In addition, the reduction of SSA and active sites caused by the aggregation of CQDs can be solved through surface functionalization/passivation or doping of CQDs (Syed et al. 2022).

After the modification by functional groups (–OH, –COOH, and C=O, etc.), it can cause changes in the emission color, wavelength, and surface defect structure of CQDs, resulting in good visible light absorption range and photoluminescence properties (Oh et al. 2022; Pirsaheb et al. 2018). The doped heteroatoms can induce the generation of electron delocalization and reduce the work function of CMs (Koe et al. 2020), enhancing the structural defects and fluorescence performance of the CQDs surface. For example, N-doped CQDs/g-C3N4 enhanced the expansion of electron delocalization and interface structure connection in aromatic wafers, thereby enhancing the absorption of visible light (Chen et al. 2021b). Besides, CQDs can also improve the catalytic activity of photocatalysts (TiO2, g-C3N4, etc.) when acting as carriers. They are considered excellent fixed electron media in Z-type nanomaterials and can improve the utilization of visible light, and reduce the recombination rate of photogenerated carriers (Low et al. 2017). Ding et al. (2022) studied the In2S3/CQDs/TiO2 heterojunction catalyst, which had good photoelectrochemical properties, redox ability, and stability. Although the above studies have discussed the excellent catalytic performance of CQDs, the application of CQDs in C-AOPs is still in its infancy stage regarding its mechanisms. Moreover, the CQDs doped with different heteroatoms possess different fluorescence characteristics, but it is still unclear whether they can affect the removal effect and mechanisms of pollutants. Therefore, more studies in this area need to be strengthened in the future.

2.4 Biochar

Biochar is a carbon-rich solid material obtained by pyrolysis of biomass feedstock in anaerobic environments (Lehmann 2007). Compared with the CMs above, biochar with low cost and high carbon content has been widely used in environmental remediation, energy, and agriculture fields (Jiang et al. 2023a; Li et al. 2022e; Rajput et al. 2022; Zhao et al. 2021b). At the same time, the easily modified functional group characteristics of biochar help to synthesize various biochar based catalysts and accelerate catalytic reactions (Shan et al. 2020; Wang et al. 2017). The persistent free radicals, oxygen-containing functional groups, and surface defects on the surface of biochar can be used as the active site of AOPs to induce ROS generation to degrade organic pollutants (Jiang et al. 2023a). In addition, the degree of graphitization of biochar also affects its catalytic performance (Qiu et al. 2021a; Zhao et al. 2021a). The graphitized structure can promote electron transfer, and facilitate the transfer of electrons from pollutants to PS through the graphite carbon lattice, accelerating the degradation of pollutants (Chen et al. 2020). It is reported that biochar prepared from different feedstocks has different physical and chemical properties. For example, plant-derived biochar often has richer surface functional groups and higher porosity (Cheng et al. 2021a), and the fundamental reaction pathway based on free radicals was the main mechanism during its activation process of PS (Song et al. 2022a). Zhang et al. (2022b) found that the acidic functional groups on the surface of coconut husk-derived biochar could enrich metal ions and act as active sites, promoting H2O2 to form \(\cdot {\text{OH}}\) or \(\cdot \text{HO}_{2}\). However, the active sites and catalytic activity of the original biochar are limited, so it is necessary to carry out heteroatom doping, metal and acid–base modification of biochar (Cheng et al. 2021b; Wang et al. 2021a; Zhao et al. 2021c). Mustafa and Hama Aziz (2023) investigated that persulfate activated by Fe-modified biochar could effectively degrade rhodamine B and diclofenac from water, and the stability of the catalyst was enhanced. Modifying biochar not only enhances its surface area and Lewis acid sites, but also improves its electron shuttle ability.

In the C-AOPs, biochar with high porosity can serve as a carrier material for catalysts, which not only protects the activity of the catalyst from being affected and evenly distributed, but also facilitates catalyst recovery and recycling (Yuan et al. 2023). For example, Tai et al. (2023) studied biochar as a carrier for S-NaTaO in photocatalysis coupled PS. The results indicate that biochar provides a large number of S-NTO loading sites, reduces the crystallinity of S-NaTaO, increases the contact reaction between catalyst and pollutant molecules, and promotes the degradation of pollutants. Meanwhile, the biochar composite catalyst changed the band gap of the photocatalyst, making the valence-band electrons more easily transferred to the conduction band under light irradiation (Cheng et al. 2023). In addition, Diao et al. (2020) studied sludge biochar-based catalysts in ultrasound coupled PS, and found that with the increase of biochar catalyst dosage, the active sites increased, thereby achieving a significant degradation effect of bisphenol A. According to previous studies, biochar application in C-AOPs was mostly concentrated on PS activation. Therefore, the synergistic mechanisms of biochar in other C-AOPs need to be explored.

2.5 Other CMs

In addition to the four CMs mentioned above, there are also fullerene, carbon fibers (CF), activated carbon (AC), and g-C3N4. Among them, fullerene is a zero-dimensional carbon nanomaterial with a closed cage spherical structure. Fullerene and its derivatives have photosensitivity, nucleophilicity, and a unique three-dimensional structure. They can be used as catalysts or dispersed on semiconductors as enhancers of other catalysts (Pan et al. 2020). Zhou et al. (2020a) found that C60 fullerenol not only affected the formation of \(\text{O}_{2}^{\cdot-}\) under visible light, but also acted as an electron donor and complexing agent to enhancing the degradation of diethyl phthalate by the F/Fe/PMS process. Filamentous CF prepared by carbonization and graphitization of organic fiber has high SSA and conductivity (Hiremath et al. 2017). It is often used as a catalyst carrier or metal-free catalyst in environmental remediation. Such as carbon fiber cloth, a derivative of CF, is used in photocatalysis as a TiO2 carrier. It can provide more contact sites, and promote the effective separation and migration of photogenerated carriers (Liu et al. 2022b). AC is a kind of porous CMs formed by pyrolysis and activation of some biomass containing raw materials. In AOPs, AC as the carrier of catalyst can solve the problem of the catalyst agglomeration, activate PS, and enhance the degradation of pollutants (Liu et al. 2021a). In addition, as a new type of CMs with high visible light-responsive and low cost, g-C3N4 has received widespread attention in recent years (Lu et al. 2022b). Research has shown that doping with heteroatoms can optimize the electronic structure of g-C3N4, improve its conductivity and electron migration, and thereby improve the catalytic activity of the original g-C3N4 (Li et al. 2022a).

To sum up, different CMs have been widely used in C-AOPs. When CMs are used as catalyst carrier, they can improve the SSA, porous structure, electron transfer speed, and stability of the catalyst. Among them, porous or layered structures can provide transport channels for reactants/intermediates, which is conducive to catalytic reactions. On the other hand, CMs as catalysts not only have high conductivity and no metal leaching, but also can regulate their catalytic activity through atomic doping to form more active substances. Therefore, CMs have corresponding advantages as both catalysts and catalyst carriers. Further exploration is needed on how to enhance the synergistic effect of CMs in C-AOPs in the future.

3 The synergistic mechanisms of C-AOPs based on CMs for the degradation of organic pollutants

Currently, the wastewater treated by a single AOPs still cannot fully mineralize and degrade organic pollutants, and the production of toxic intermediates has posed potential hazards to humans and the ecosystems. Therefore, researchers have gradually explored different C-AOPs for treating organic pollutants in wastewater and have shown good degradation effects. The following sections would provide a detailed introduction to the synergistic mechanisms of C-AOPs based on CMs for degrading organic pollutants.

3.1 AOPs coupled biodegradation based on CMs

The biodegradation of organic pollutants is relatively slow, and the degradation effect of low-concentration pollutants is generally better than that of high-concentration pollutants. The AOPs coupled biodegradation can enhance the degradation of high-concentration and recalcitrant organic pollutants. Meanwhile, the intermediate products generated during the degradation process of AOPs can also be further degraded and mineralized through the life activities of microorganisms, achieving a win–win effect. Currently, the AOPs coupled biodegradation mainly involves photocatalysis and ozonation (Fig. 3), which would be briefly introduced below.

Fig. 3
figure 3

Photocatalysis coupled biodegradation (ICPB) and ozonation coupled biodegradation (SCOB)

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3.1.1 Photocatalysis coupled biodegradation (ICPB)

Rittmann proposed a direct coupling method of photocatalysis biodegradation to remove pollutants in 2008, which gradually attracted enthusiastic attention. It mainly includes an independent sequence of photocatalysis and biodegradation, or ICPB. However, due to the accumulation of intermediates or toxic by-products during the photocatalytic oxidation process, may have adverse effects on subsequent bioreactors (Ma et al. 2018). Therefore, the ICPB system that integrates photocatalysis and biodegradation has been a widespread concern in recent years. At present, organic pollutants such as sulfamethoxazole, oxytetracycline, ciprofloxacin, and tetracycline have been proven to be effectively degraded in the ICPB system (Liu et al. 2022c). It is worth noting that to protect microorganisms from harm caused by ROS generated in photocatalysis, the selection of photocatalysts and the fixation sequence with microorganisms on the carrier are very important. For example, Li et al. (2021b) used a metal-free photocatalyst g-C3N4 to degrade ciprofloxacin. After being excited by visible light irradiation, g-C3N4 generated e−-h+ pairs, which are transferred to microorganisms. The microorganisms separate the e−-h+ pairs, delaying the recombination rate of the e−-h+ pairs, thereby increasing mineralization efficiency. In another study, Zhang et al. (2023a) used a metal-free and efficient biochar/g-C3N4 photocatalyst to degrade tetracycline in ICPB. The doping of biochar is beneficial for reducing the interlayer spacing of biochar/g-C3N4 composite materials, improving the visible light response ability of the composite materials, and also reducing the aggregation of g-C3N4 to obtain a higher SSA. Meanwhile, the addition of biochar is also beneficial for inhibiting e−-h+ recombination and enhancing electron transfer with microorganisms by enhancing the interaction between biochar and g-C3N4, thereby improving the removal and mineralization efficiency rate of organic pollutants.

In addition, Wang et al. (2023) found that CdS@SiO2@TRP was attached to the surface of graphene, forming the photo-controlled reversible photocatalytic system. Under visible light, the temperature was heated to the lowest co-solubility temperature of the catalyst through the photothermal conversion of graphene, effectively protecting microorganisms from the attack of ROS. Graphene, which also has high SSA, can promote the flow of electrons to denitrification microorganisms and improve electron transfer efficiency (Li et al. 2022c). Therefore, the current application of CMs in ICPB mainly improves the photocatalytic efficiency in the early stage, creates conditions for subsequent biodegradation, and is more effective in protecting microorganisms compared to metal catalysts. However, the high cost and poor biocompatibility currently used as carriers for ICPB systems hinder the growth of microorganisms, limiting the large-scale application of ICPB. Biochar, as a type of CMs with high porosity and biocompatibility, can accelerate mass transfer, provide more catalytic active sites, and protect microorganisms (Bolan et al. 2023). It is considered a potential material for ICPB carrier. The research in this area is still in the preliminary stage, and further exploration can be conducted in the future.

3.1.2 Ozonation coupled biodegradation (SCOB)

The pollutants in water can be directly oxidized by O3, or O3 reacts with water molecules under the action of a catalyst to \(\cdot {\text{OH}}\) to degrade pollutants. However, studies have reported that stable or toxic intermediate products may be formed after ozonation (Mecha and Chollom 2020). For example, fluoxetine is an antidepressant that produces carboxylic acids and toxic aldehydes that are much more toxic than their parent compound through ozonation (Zhao et al. 2017). Dias et al. (2023) found that the toxicity of dye wastewater degraded by single ozonation increased, while the SCOB process further mineralized the intermediates produced by ozonation, reducing its toxicity. Biodegradation can further mineralize intermediate products, thereby reducing the amount of toxic by-products. Compared with the ICPB system, the SCOB system has good cleanliness and light penetration. At present, the SCOB has been successfully applied to the degradation of naphthenic acid (Vaiopoulou et al. 2015), 2,4-dichlorophenol (Van Aken et al. 2017), tetracycline (Su et al. 2020b), and other pollutants. Generally, some electrophilic compounds with double bonds, aromatic structures, or amine groups are preferentially used as ozonation sites (Li et al. 2019b). They may be converted into easily degradable oxidation products through direct or indirect free radical reactions. For example, a neutral O3 addition reaction can occur in compounds containing C=C, and O3 addition reaction can occur to form cyclic trioxide, which can be decomposed into carbonyl and carboxyl compounds (Han et al. 2022). Compounds containing aromatic ring structures undergo a ring-opening process, where the hydroxyl groups on the ring are replaced to form carboxyl compounds (Solís et al. 2016). Then, microorganisms can further degrade intermediate products into low-toxicity or non-toxic small molecule compounds by secreting metabolites or enzymes (spore laccase, cytochrome P45, etc.).

Meanwhile, to improve the selectivity, reaction speed, and biodegradability of intermediate products of ozone, nano-catalyzed ozonation can be used to enhance its mineralization (Malik et al. 2019; Vaiopoulou et al. 2015). For example, Gonçalves et al. (2013) studied the degradation of benzoate by CNTs-catalyzed ozonation. The results showed that the removal rate of TOC from water by CNTs catalytic ozonation (60%) was significantly higher than that by single ozonation (30%). Meanwhile, CNTs promoted the production of \(\cdot \text{O}_{2}^{-}\), 1O2, and the initiators in the \({\text{OH}}\cdot\) chain reaction, enhancing the continuous production of \(\cdot {\text{OH}}\). Similarly, in the ozonation process catalysed by biochar, some biochar containing carboxylic acid and lactone functional groups also promotes the production of \(\cdot\)OH (Zhang et al. 2023c). In addition, structural control and surface modification based on CMs can alter the reaction pathway and the rate of catalytic ozonation (Liu et al. 2021b). A study found that fluorine-doped CNTs make the original CNTs more dispersed and exhibit strong affinity for ozone and oxalic acid, thereby improving catalytic activity (Wang et al. 2018). Other studies have reported that CMs may not promote the production of \(\cdot \text{O}_{2}^{-}\) and \(\cdot {\text{OH}}\) from O3 during the catalytic ozonation process, but rather promote the degradation of pollutants through non-free radical pathways through electron transfer (Sun et al. 2019). Although the above studies indicate that the degradation effect of ozonation can be changed through nano-catalysed ozonation based on CMs, the relatively high cost of preparing nanoparticles limits the application of this technology. In addition, ozone, as a high-energy oxidant, is another important factor limiting its development. Therefore, there is currently less research on SCOB compared to ICPB, and research should be strengthened on how to reduce ozone consumption in the future. The degradation mechanisms of organic pollutants by the SCOB process based on CMs also needs further exploration.

3.2 AOPs coupled persulfate based on CMs

The traditional AOPs eliminate the target pollutants through the highly active and non-selective \(\cdot {\text{OH}}\) generated in situ. However, achieving the ideal degradation effects is difficult due to the short lifespans and poor stability of \(\cdot {\text{OH}}\). Recently, a large number of studies have shown that AOPs coupled PS can produce more free radicals, a wider pH range, and higher reusability. Therefore, the following would introduce the synergistic effects produced by the photocatalysis coupled PS, and ozonation coupled PS (Fig. 4).

Fig. 4
figure 4

Photocatalysis coupled persulfate (a), ozonation coupled persulfate (b)

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3.2.1 Photocatalysis coupled persulfate (PCP)

Photocatalysis is usually constrained by problems such as low transparency, low light energy utilization rate, and difficulty in recovering photocatalysts. To solve these problems, extensive research has been conducted and confirmed that PCP can achieve low energy and efficient catalytic oxidation, resulting in a positive synergistic effect on pollutant removal (Chen et al. 2021a). In catalytic reactions, PS can exert its strong oxidation ability through catalyst and photoactivation (Wu et al. 2022). Meanwhile, PS can also capture e− through self-activation, produce \(\text{SO}_{4}^{\cdot-}\) and \(\cdot {\text{OH}}\), and inhibit the recombination of photogenerated carriers through electron consumption (Sun et al. 2021b). However, the dissociation of O–O bond in PS molecule can affect the quantity of \(\text{SO}_{4}^{\cdot-}\). Therefore, the selection of PS activator is very important. Previous extensive research has shown that CMs are highly favored as new activators for PS in this field. Generally, CMs are used as an activator for electron transfer to effectively activate PS. The oxygen-containing functional groups of CMs include nucleophilic ketone and quinolone groups that are electron-rich, which provide active sites for activating PS (Oyekunle et al. 2022). Tang et al. (2022) studied the activation of PS by biochar and graphene composites. The results showed that the graphene-like structure of biochar induced electron delocalization effects, which enhanced the formation of \(\text{SO}_{4}^{\cdot-}\), \(\cdot {\text{OH}}\), \(\cdot \text{O}_{2}^{-}\) and other free radicals. In addition, Meng et al. (2022) found that O-doped g-C3N4 could cause reflection of multiple light, and oxygen doping expanded the delocalised π-electron system, shortened the C-N bond length, and narrowed its band gap. The doped oxygen atoms also improved the conductivity of g-C3N4, accelerating the transfer of photo-generated charge carriers from the bulk to the surface.

The PCP makes the degradation of organic pollutants no longer only depend on a single degradation mechanism, such as active free radical oxidation or non-free radical (\(\text{SO}_{4}^{\cdot-}\)) oxidation, etc. (Ren et al. 2022). Wang et al. (2021b) studied that bifunctional polymeric carbon nitride/biochar hybrids activate PS to degrade phenol under visible light, involving multiple degradation pathways (mediated electron transfer, free radical oxidation, and hole oxidation). In the degradation process of 2,4-dinitrophenylhydrazine, there are also non-free radical and 1O2 oxidative degradation pathways (Zhang et al. 2022e). Lu et al. (2022c) also found that biochar improved the catalytic performance of g-C3N4 and the photocatalytic efficiency of PS through both free and non-free radical mechanisms. Among them, 1O2 and electron transfer are non-free radical pathways for CMs to activate PS (Ren et al. 2019). It is reported that PS in PCP can enhance the charge separation and transfer efficiency of catalysts by building bridges, inducing more \(\text{O}_{2}^{\cdot-}\) production (Yin et al. 2022). A study reported that PS could also combine with the surface of CMs catalysts to form reactive complexes, which then directly react with organic pollutants (Wang et al. 2019a). Similarly, PS also generates free radicals through self-activation, further enhancing the synergistic effect of the coupling processes. Due to PS being used as an oxidant, excessive use may lead to excessive \(\text{SO}_{4}^{\cdot-}\) in water and react to generate sulfate. It may pose potential harm to human health and the ecosystem. Therefore, exploring the optimal dosage of PS is beneficial to effectively solve the bottleneck of its practical application.

3.2.2 Ozonation coupled persulfate (OCP)

Traditional ozonation has problems with a low degradation rate of pollutants, low ozone solubility, and incomplete organic degradation, even generating BrO3− in the presence of Br− (Cong et al. 2015). Therefore, coupling ozonation with other processes is very important for the treatment of organic pollutants in the future. Among them, the OCP is a new sulfate-based AOPs proposed in 2015. O3 as a strong oxidant can activate PS to produce \(\cdot {\text{OH}}\) and \(\text{SO}_{4}^{\cdot-}\), two strong oxidizing radicals, by producing 1O2 or \(\cdot \text{O}_{2}^{-}\) during the reaction processes (Yang et al. 2015). The synergistic effects of OCP are mainly attributed to the direct oxidation of pollutants by O3 and PS. Another pathway is the reaction of O3 with H2O to generate \(\cdot {\text{OH}}\), and the activation of PS to produce \(\text{SO}_{4}^{\cdot-}\) to indirectly oxidize pollutants (Yang et al. 2022). Lu et al. (2022a) found that in an alkaline environment, O3 activated PS to generate \(\cdot {\text{OH}}\), which then reacted with O3 to generate \(\cdot {\text{OH}}\). Meanwhile, \({\text{OH}}^{-}\) and \(\text{SO}_{4}^{\cdot-}\) react to convert into \(\cdot {\text{OH}}\), generating more \(\cdot {\text{OH}}\) than under acidic conditions. When pH < 7.0, \(\text{SO}_{4}^{\cdot-}\) is the main reaction substance, while under neutral conditions, hydroxyl and sulfate radicals are equally involved in the reaction processes (Izadifard et al. 2017). Thus, many organic pollutants could be effectively degraded under the action of two strong oxidation free radicals.

OCP can lead to the formation of bromate when treating wastewater containing bromine, which is carcinogenic and genotoxic. However, some studies found that adding CMs to it can not only inhibit the formation of bromate, but also enhance the degradation of organic pollutants (Wen et al. 2020). The addition of CMs divides the ozone reaction into dissolution and surface stage. In the dissolution stage, the CMs acts as an initiator to generate active substances (Staehelin and Hoigne 1985). At the surface stage, –OH on the surface of CMs causes O3 decomposition to produce active free radicals (Ahn et al. 2017). Therefore, the utilization rate of O3 has been improved. Liu et al. (2022e) added CuCo2O4-GO catalyst to OCP. Graphene, as a type of CMs rich in oxygen functional groups, can be attacked by hydroxyl radicals and converted into carboxyl groups. Then, ozone can react with these converted functional groups to produce more active radicals. Meanwhile, the addition of low-dose graphene can effectively inhibit the further conversion of HBr/OBr− in the coupling processes, thereby reducing the formation of highly toxic bromide (Huang et al. 2017). It was attributed to the excellent surface defect structures, developed mesoporous structures, and more oxygen-containing functional groups of graphene. In addition, the biochar composite catalyst can increase the content of SP2 C, enhance the conductivity of the catalyst, and improve the mass transfer rate through adsorption, increasing the contact area between free radicals and pollutants (Li et al. 2021a). In summary, the addition of CMs not only promotes ozone reaction, but also improves the utilization rate of ozone. However, the large-scale application of OCP technology in the degradation of organic pollutants is limited by the mass transfer rate of ozone and the high energy consumption of ozone. The degradation mechanisms of organic pollutants in OCP systems with CMs catalysts are still not clear, thus many problems still need to be further addressed in the future.

3.3 AOPs coupled ultrasound based on CMs

Ultrasound has mass transfer effects and mechanical effects, which can continuously remove activated and oxidized residues in the reaction processes (Wang et al. 2019c). However, when using ultrasound alone to degrade pollutants, the degradation effects are not satisfactory due to the low energy utilization efficiency. Therefore, many scholars propose to couple ultrasound with AOPs to improve the utilization efficiency of ultrasound. At present, there are three main processes including ultrasound coupled PS, ultrasound coupled photocatalysis, and ultrasound coupled Fenton oxidation (Fig. 5).

Fig. 5
figure 5

Ultrasound coupled persulfate, photocatalysis, and Fenton oxidation

Full size image

3.3.1 Ultrasound coupled persulfate (UCS)

Ultrasound can form cavitation bubbles in liquid through acoustic cavitation or acoustic activation. After the formation, growth, and collapse of cavitation bubbles, a local high-temperature or high-pressure environment can be generated inside and around the cavitation bubbles (Wang et al. 2019b). PS is a non-selective anion that is stable in the environment and must be activated to achieve the degradation effects. Therefore, the high temperature and pressure environment generated near the collapse of bubbles can help PS to decompose into \(\text{SO}_{4}^{\cdot-}\), thereby improving the degradation and mineralization of organic pollutants. Wang et al. (2021c) found that ultrasound activation of PS not only induced the production of active species, but also promoted the production of \(\cdot {\text{OH}}\) and \(\text{SO}_{4}^{\cdot-}\). Research reports suggest that there may be two ways to activate PS by ultrasound. The first is to attack PS with hydroxyl radicals generated by the dissociation of water molecules in cavitation bubbles, producing \(\text{SO}_{4}^{\cdot-}\) and \({\text{SO}}_{4}^{2-}\) (Wei et al. 2017). The second is that the collapse of cavitation bubbles can produce a local high pressure and temperature environment, which is another mechanism for ultrasound to activate PS (Chen and Su 2012). In the degradation process of UCS, a series of free radicals (Eqs. 1–4) are produced, but \({\text{HO}}\cdot\) and \(\text{SO}_{4}^{\cdot-}\) are considered to play a major role (Yang et al. 2019). In addition to activation, the intense turbulence caused by ultrasound enhances the mass transfer effect in the aqueous, thus improving the reaction processes.

$${\text{S}}_{2} {\text{O}}_{8}^{2 - } + ))) \to 2{\text{SO}}_{4}^{ \cdot- }$$
(1)
$${\text{SO}}_{4}^{ \cdot- } + {\text{OH}}^ {-} \to {\text{SO}}_{4}^{2 - } + {\text{HO}}\cdot$$
(2)
$${\text{S}}_{2} {\text{O}}_{8}^{2 - } + 2{\text{H}}_{2} {\text{O}} \to {\text{HO}}_{2}^{ - } + 2{\text{SO}}_{4}^{2 - } + 3{\text{H}}^{ + }$$
(3)
$${\text{HO}}_{2} ^{- } + {\text{S}}_{2} {\text{O}}_{8}^{2 - } \to {\text{SO}}_{4}^{ \cdot- } + {\text{SO}}_{4}^{2 - } + {\text{H}}^{ + } + {\text{O}}_{2}^{ \cdot- }$$
(4)

To optimize the degradation efficiency of the UCS coupling system for pollutants and improve the catalytic activity of the CMs catalyst, some related studies have been carried out. Earlier, sludge biochar was added to the UCS system to study the positive synergistic effect produced by sonochemistry and catalytic chemistry (Diao et al. 2020). This study found that Fe2+ dissolved under acidic conditions is released from the biochar catalyst and reacts with PS to generate \({\text{HO}}\cdot\) and \({\text{SO}}_{4}^{ \cdot- }\). Ultrasound can continuously clean and refresh the Fe on the surface of biochar to improve the reactivity and mass transfer at the solid–liquid interface, and enhance the degradation of pollutants. Biochar (BC) can also adsorb pollutants to the surface, and the oxygen-containing functional groups on the surface act as active sites to activate PS form \({\text{SO}}_{4}^{ \cdot- }\), and oxidize pollutants (Eqs. 5–7) (Liu et al. 2022a). Meanwhile, Ouiriemmi et al. (2022) also found that biochar could be oxidized and regenerated in the UCS system, and the regeneration effect reached 79.2%. Therefore, the addition of biochar is conducive to the activation of PS and the generation of hydroxyl radicals under the action of ultrasound, and the improvement of mass transfer efficiency. However, the mass transfer coefficient, temperature, and ultrasound frequency in the actual environment cannot be well controlled as in the laboratory, which can affect the production of \({\text{SO}}_{4}^{ \cdot- }\) and prevent the ideal degradation effect from being achieved. To better adapt UCP technology to practical environments, further improvements and optimizations are needed.

$${\text{BCsurf}} - {\text{OH}} + {\text{S}}_{2} {\text{O}}_{8}^{2 - } \to {\text{BCsurf}} - {\text{O}}\cdot + {\text{SO}}_{4}^{ \cdot- } + {\text{HSO}}_{4}^{ - }$$
(5)
$${\text{SO}}_{4}^{ \cdot- } + {\text{H}}_{2} {\text{O}} \to {\text{HSO}}_{4}^{ - } + {\text{OH}}\cdot$$
(6)
$${\text{BCsurf}} - {\text{OH}} + {\text{OH}}\cdot \to {\text{BCsurf}} - {\text{OO}} + {\text{H}}^{ + }$$
(7)

3.3.2 Ultrasound coupled photocatalysis (UCP)

The UCP has been widely used in the degradation of organic pollutants. The UCP can significantly enhance photocatalytic efficiency, and the reaction rate constant of UCP is much greater than that of a single sonocatalytic or photocatalytic reaction rate constant (Meroni and Bianchi 2022). The synergistic effects of this coupling process benefit from the local high-pressure and temperature environment generated by acoustic cavitation. It induces the pyrolysis of water molecules into highly active free radicals (\(\cdot {\text{OH}}\), \({\text{H}}\cdot\), etc.), and promotes the mass transfer rate of the liquid phase and photocatalyst surface. At the same time, acoustic cavitation can remove chemical intermediates from the active catalyst site and improve the mineralization of pollutants. Yentür and Dükkancı (2021) studied the removal of carbamazepine by photocatalysis, sonocatalytic, and UCP, respectively. The results showed that the degradation effect of the coupling process was significantly higher than that of the two separate processes. The ultrasound frequency would affect the formation of cavitation bubbles, thus affecting the number of active free radicals. However, the selection of catalysts in most sonophotocatalytic processes also has a very intimate impact on the coupling process, for example, the degradation effect of N-doped TiO2 catalyst on ciprofloxacin in sonophotocatalytic (Karim and Shriwastav 2020). Due to the increase in dispersion and mass transfer rate of N-doped catalysts, as well as the increased in SSA and free radicals of the catalyst, it was beneficial for the formation of \(\cdot {\text{OH}}\) in ultrasound catalysis. In addition, CMs have also been well applied as a green non-metallic catalyst.

As one of the CMs, biochar has been applied to the degradation of rifampicin by UCP. Biochar enhanced the formation of cavitation bubbles in the processes of UCP, resulting in the generation of more \(\cdot {\text{OH}}\). On the other hand, the addition of biochar reduces the band gap value of catalysts and accelerates the generation of e−-h+ pairs under ultrasound irradiation and light source (Sadeghi Rad et al. 2022b). When CNTs are used to degrade methyl orange, the UCP catalytic efficiency is also enhanced (Wang et al. 2009). CNTs serve as catalyst carriers to transfer captured photoelectrons to the surface, promoting the separation of the photogenerated charge carriers and hindering the recombination of e−-h+ pairs. At the same time, due to the high conductivity and active site of CNTs, pollutants can be adsorbed on the surface, and photocatalytic activity can be enhanced through in-situ photoreaction (Jin et al. 2019). In addition, ultrasound can also remove pollutants or by-products on the surface of CNTs, and produce more active sites, and the coupling of ultrasound and CNTs enhances the cavitation phenomenon (Mohamed et al. 2019). The cavitation phenomenon is enhanced due to the additional atomic nucleus provided by CMs catalysts, which enhances the number of bubble collapses, thus improving the degradation effect of pollutants (Gholami et al. 2019). Therefore, CMs can enhance both photocatalysis and ultrasound oxidation in the coupling process. On the other hand, the supercritical water effect generated by ultrasound should also be considered during the degradation process (Dias et al. 2023). When the high-temperature and high-pressure environment caused by bubble collapse exceeds the critical temperature and pressure of water molecules, organic pollutants are prone to undergo supercritical water oxidation with water, generating more stable intermediate products. The UCP degradation has achieved good results in the laboratory, but its pilot application is limited due to the need to introduce additional equipment. In the future, efforts should be made to integrate ultrasound units with photocatalytic equipment to simplify operations.

3.3.3 Ultrasound coupled Fenton oxidation (UCF)

Fenton oxidation is an effective method for the degradation of refractory organic pollutants. This process usually uses Fe2+ as a catalyst to promote the conversion of H2O2 into strong oxidizing \(\cdot {\text{OH}}\) radicals under acidic conditions. However, the large amount of sludge produced in the reaction processes and the narrow pH application range have greatly hindered its practical application. Previous studies have proved that the UCF could enhance the oxidation performance of Fenton, thus accelerating the degradation and mineralization of organic pollutants (Hassan et al. 2019), for example, aureomycin (Pulicharla et al. 2017), decabromodiphenyl ether (Panda and Manickam 2019), sulfamethazine (Zhang et al. 2020b), etc. This is mainly due to the enhanced mass transfer effect of turbulence and the close contact between oxidants and pollutants. Meanwhile, ultrasound causes the breakage of some double bonds in the complex, as well as the regeneration of ferrous ions, which promotes the mineralization rate of pollutants (Gujar et al. 2021). In this reaction, Fe2+ and H2O2 first react to generate Fe3+, and finally react to generate Fe (OOH)2+, which is further dissociated into Fe2+ under the action of ultrasound. The final separated Fe2+ further reacts with H2O2 to produce \(\cdot {\text{OH}}\) (Eqs. 8–11) (Bagal and Gogate 2014). Therefore, the synergistic effects of the UCF promoted the continuous generation of hydroxyl radicals. In addition, the high concentration of Fenton reagent can reduce the contact between pollutants and active free radicals, affecting the degradation effect of pollutants (Kodavatiganti et al. 2021). Similarly, excessive ferrous compounds can cause the formation of iron hydroxide in the form of solid sludge.

$${\text{Fe}}^{2 + } + {\text{H}}_{2} {\text{O}}_{2} \to {\text{Fe}}^{3 + } + {\text{HO}}^{ - } + \cdot{\text{OH}}$$
(8)
$${\text{H}}_{2} {\text{O}}_{2} + {\text{Fe}}^{3 + } \to {\text{Fe}}({\text{OOH}})^{2 + } + {\text{H}}^{ + }$$
(9)
$${\text{Fe}}\left( {{\text{OOH}}} \right)^{ 2+ } + ))) \to {\text{Fe}}^{ 2+ } + {\text{HOO}}\cdot$$
(10)
$${\text{Fe}}^{ 3+ } + {\text{HOO}}\cdot \to {\text{Fe}}^{ 2+ } + {\text{H}}^{ + } + {\text{O}}_{2}$$
(11)

To solve the narrow pH range of traditional Fenton oxidation and the production of sludge, multiphase catalysts have been used to overcome its limitations. Therefore, CMs have also been applied to some extent. Biochar provides more active sites and generates more electrons to promote the conversion of Fe (III) to Fe (II) (Chu et al. 2021). For example, biochar used as a carrier for MnFe2O4 nanoparticles in UCF for methylene blue degradation can effectively prevent the aggregation of nanoparticles. On the surface of MnFe2O4/BC, Mn (III) and Fe (II) can also be converted into Mn (II) and Fe (III), providing more atomic nuclei for the formation of cavitation bubbles (Cheng et al. 2021c). There are also reports that different pyrolysis temperatures of biochar can affect the concentration of persistent free radicals and their activation effect on H2O2 (Liu et al. 2020a). Graphene is used as an electron-catching device in UCF to improve the catalytic efficiency of the whole reaction process. Under the action of ultrasound, CoFe2O4 can be well dispersed on the rGO nanosheet, promoting the reaction between the surface active sites and H2O2, and reducing the recombination rate of e−-h+ pairs on the nanocomposites (Hassani et al. 2018). Therefore, CMs are not only beneficial to improve the defects of traditional Fenton oxidation, but also enhance the synergistic effects of UCF. However, the actual composition of wastewater is complex, which may affect the cavitation effect of ultrasound, thereby affecting the degradation efficiency of organic pollutants. The reactors currently used are also small devices, so improving the ultrasound reactor and its supporting facilities is also an important aspect of future research.

4 Toxicity assessment methods of organic pollutants in C-AOPs

As an important method for wastewater treatment, AOPs may produce some intermediate products or toxic by-products during the degradation processes, posing a potential threat to the environment and human health. Therefore, conducting toxicity assessment on degraded organic pollutants helps us understand whether the treated wastewater can be reused. Additionally, some previous studies have shown that the by-products of AOPs may be more toxic than the parent compound. For instance, when carbamazepine (CBZ) is oxidized by ozone, it can be produced two by-products that are carbamazepine-10,11-epoxide and 10,11-dihydrocarbamazepine (Han et al. 2018; Pohl et al. 2019). The cytotoxicity and genotoxicity experiments of these two by-products showed highly toxic effects. In the processes of photocatalysis coupled PS or Fenton coupled biodegradation, CBZ can degrade rapidly through hydroxylation and carboxylation. Meanwhile, it can effectively avoid the interference of natural organic components in water (Lee et al. 2022; Meng et al. 2022). Besides, previous studies have reported that sulfamethoxazole (SMX) through AOPs may produce some highly toxic intermediates, such as 3-amino-5methylisoxazole and p-benzoquinone (Wang et al. 2019d). However, through some coupling systems, SMX may undergo hydroxylation, ammoxidation, and S–N bond breaking (Ao et al. 2018b; Isari et al. 2020a). It not only effectively degrades SMX in a short time, but also has weak toxicity prediction results for Ecological Structure Activity Relationships (ECOSAR).

The ubiquitous bisphenol A in the environment may also produce intermediate products such as 4-hydroxybenzaldehyde, 4’-(methylene) bisphenol, and 4-(2-hydroxypropyl-2-yl) phenol during the photocatalytic process. These intermediate products often have higher ecological risks than bisphenol A due to their relatively high toxicity, mutagenicity, and bioaccumulation (Zhou et al. 2020b). In addition, the ECOSAR model predicts that intermediates containing –NHC(O) produced by ciprofloxacin have ecological toxicity to aquatic organisms (Bai et al. 2020). However, the production of –NHC(O) compounds can be reduced through ICPB, and the intermediate products are biodegraded and mineralized into CO2 and H2O (Li et al. 2021b). The chlorinated organic compounds and aldehydes produced by ozonation degradation of chlorophenol compounds usually also lead to an increase in acute toxicity in the aquatic environment. Hama Aziz (2019) has confirmed that coupling ozonation with other AOPs can enhance the degradation of pollutants and achieve higher mineralization rates. In summary, a single AOPs may inevitably produce toxic intermediates when degrading organic pollutants. Table 2 also lists the applications of other C-AOPs based on CMs in the removal of organic pollutants. Therefore, integrating efficient, low energy consumption, and high mineralization rate C-AOPs to reduce the production of toxic intermediates is of great significance. Additional file 1: Table S1 lists the advantages and disadvantages of some other coupling processes in removing organic pollutants.

Table 2 Application of C-AOPs based on CMs
Full size table

In addition, toxicity assessment has only been applied sparingly in the degradation of organic wastewater using C-AOPs based on CMs. For example, Jia et al. (2023) used an acute toxicity test to detect the toxicity of biochar combined with ultrasound activated persulfate in the degradation of atrazine wastewater, and confirmed the green and safety of this coupled system. Al-Musawi et al. (2021) proved the safety and low toxicity of the UCP based on graphite-based catalysts through toxicity testing. In the future, attention should be paid to the application of toxicity assessment in C-AOPs based on CMs. On the other hand, it is also necessary to evaluate the toxicity of degraded organic pollutants for the safe reuse of water. At present, acute toxicity testing, genetic toxicity testing, and the ECOSAR model are commonly used to evaluate the toxicity of aqueous solutions before and after pollutant degradation (Table 3). The following would introduce the above toxicity assessment methods:

Table 3 Toxicity assessment methods of organic pollutants
Full size table

4.1 Acute toxicity testing

Toxicity assessment is necessary to determine whether secondary pollution or toxic effects would occur after the degradation of organic pollutants. Acute toxicity testing is used for the identification of hazardous substances or environmental risk assessment and is commonly used in industrial and agricultural chemicals. The tested organisms used for acute toxicity testing can be divided into fish, plankton, and microorganisms. Earlier, bacteria (such as Vibrio fischeri, P. subcapitata, Vibrio qinghaiensis, and so on) were used to evaluate the toxicity of organic wastewater degradation in the photo-Fenton coupling process (De Luna et al. 2014; Gonçalves et al. 2020). The above acute toxicity test results showed that the early toxicity increased briefly due to the toxic by-products produced during the degradation process, and then gradually decreased, reaching a non-toxic level. In addition, Orge et al. (2017) also confirmed through Vibrio fischeri that the TiO2@CNTs photocatalytic ozonation system reduced the biological toxicity of metolachlor aqueous solution. At the same time, when conducting toxicity assessment, comprehensive consideration should be given to the economy and operating conditions. For example, the acute toxicity testing of fish and plankton requires a long time and is not suitable for rapid detection of large amounts of water samples. The luminescent bacteria method has the advantages of simplicity, speed, and accuracy, making it widely used (Ding et al. 2015). However, acute toxicity testing is highly harmful to animals, and some tests may be fatal. Therefore, the future should move towards non-animal acute toxicity testing.

4.2 Genetic toxicity testing

Genetic toxicity refers to the damage caused by some pollutants to the DNA or chromosomes of organisms, thus causing the abnormal development or metabolic function dysfunction of organisms. This testing method applies to the evaluation of water environments with multiple mixed pollutants, used to determine whether they can induce human cancer or genetic mutations (Pellegri et al. 2014). Genetic toxicity testing methods mainly include comet, micronucleus, Ames, and endocrine interference tests. Among them, the Ames test is the most commonly used genetic toxicological method for detecting chemical-induced gene mutations in prokaryotic cells (Sun et al. 2020). The comet test is widely used in the genotoxic effect of air pollution exposure, while the micronucleus test is widely used in the genotoxic test of nanomaterials. Genetic toxicity testing is an important test to evaluate the potential toxicity of medical wastewater before and after degradation (Gupta et al. 2009). For example, when photocatalytic ozonation is used to degrade medical wastewater, chromosome aberration, micronucleus spontaneous rate, and mitosis index are used as indicators for genotoxicity evaluation (Kern et al. 2013). Ao et al. (2018a) proved that the genetic toxicity of ciprofloxacin was significantly reduced after degradation by PPS through genetic toxicity testing. However, due to the long duration of genetic toxicity testing and the correspondingly high cost of reagents and strains, there are few applications in C-AOPs at present.

4.3 ECOSAR model prediction

The ECOSAR model has been a commonly used toxicity assessment method in recent years. The model can predict six toxicity endpoints, including fish LC50, daphnia LC50, green algae EC50, fish chronic toxicity, daphnia chronic toxicity, and green algae chronic toxicity. With the help of a computer, the ECOSAR model can rapidly predict the potential toxicity of thousands of chemicals by using a structure–activity relationship (Li et al. 2023a). Compared with the above two toxicity assessment methods, its operation is simple, fast, low-cost, and it can simulate complex environmental conditions to a greater extent. Currently, it has also been widely used in different coupling processes. For example, after photocatalysis coupled PS to degrade bisphenol A, its intermediates were evaluated using the ECOSAR model (Wang et al. 2021b). The aquatic toxicity of ciprofloxacin and its nine intermediates was also predicted by the ECOSAR model in the UCP (Fan et al. 2022). In addition, He et al. (2023) evaluated the potential toxicity of antibiotic degradation in PPS systems of Co/Fe-CNT catalysts using the ECOSAR model. It is worth noting that when using the ECOSAR model for prediction, it is often combined with toxicity assessment software (TEST) to improve the accuracy of the prediction results.

To sum up, the current toxicity assessment has gradually become an important method for judging whether there is potential toxicity of organic wastewater after degradation. However, most of the toxicity assessment methods in practical applications usually adopt a single test, which may lead to low accuracy of prediction results (Dom et al. 2010). Therefore, two or more evaluation methods should be combined in the future for toxicity evaluation to enhance the accuracy of prediction results. In addition, the above toxicity assessment methods are still less applied in C-AOPs based on CMs, and most studies only calculate the mineralization rate of pollutants without conducting toxicity tests. In the future, this application should be strengthened.

5 Conclusions and perspectives

The characteristics of different CMs and the application of C-AOPs based on CMs in removing organic pollutants in wastewater are systematically reviewed. Compared with single AOPs, C-AOPs can greatly solve the problems of low efficiency, low mineralization, and potential ecological risks of a single process. In addition, the synergistic effects and degradation mechanisms generated by the coupling processes are elaborated. Among them, CMs provide more ROS, strong oxidizing free radicals, reactive active sites, etc. in the coupling processes, which is considered to be the main factor in enhancing the synergistic effects. Meanwhile, the stability of traditional metal catalysts and secondary pollution problems are also improved through CMs, enabling the development of green and environmentally friendly organic wastewater degradation processes. At present, most of the coupling processes based on CMs catalysts have shown good degradation effects for the removal of organic pollutants, highlighting their enormous application potential. However, there are still some issues that have not been resolved. For example, how does the coupling sequence of AOPs affect the degradation effect of organic pollutants? Can the expected degradation effect be achieved in the face of a complex actual water environment? The optimal input and modification methods of CMs catalysts also need further exploration. Therefore, to better develop cost-effective and green C-AOPs wastewater treatment technology and expand its practical application, the following suggestions and perspectives are proposed for future research directions:

  1. (1)

    At present, there are still certain knowledge gaps in exploring the synergistic mechanisms generated by CMs. For example, how CMs catalysts affect the degradation and mineralization of pollutants still needs further study. Meanwhile, the toxic effects caused by the decomposition of CMs during the degradation processes may affect the water environment and the mineralization of organic pollutants. Therefore, attention should be paid to the impacts of CMs catalysts decomposition when they are used in practical applications. More stable and green CMs catalysts need to be developed in the future.

  2. (2)

    Most of the current coupling processes only focus on the removal of single pollutants in water. However, there may be organic, inorganic co-pollution, or coexistence of multiple pollutants in actual polluted water. Therefore, the performance of coupling processes for the removal of composite pollutants under practical situations needs to be verified before large-scale application.

  3. (3)

    The degradation mechanisms of organic pollutants in the coupling processes cannot be explained merely by free radicals due to the complexity of organic pollutants. In the future, quantum mechanics calculations are necessary to analyze the structural properties of pollutants so that it can deeply explore their degradation mechanisms.

  4. (4)

    Different environmental factors (pH, initial concentration, temperature, ultrasound frequency, etc.) could significantly affect the degradation performance of organic pollutants. However, most coupling processes are still carried out in the laboratory stage. There are relatively few studies that comprehensively consider the impact of actual environmental conditions on degradation performance in practical applications. Future studies should focus on the pilot scale research of coupling processes, and comprehensively evaluate the impact of different environmental factors on coupling processes.

  5. (5)

    Cost, degradation performance, and environmental safety are important indicators for evaluating C-AOPs. Thus, economic evaluation of the coupling processes should be conducted from the aspects of materials, equipments, and degradation effects to determine the feasibility of its large-scale application. In addition, considering the importance of toxicity testing in evaluating wastewater safety reuse, it should be considered as a necessary work for evaluating the performance of C-AOPs.