Applying molecular oxygen for organic pollutant degradation: Strategies, mechanisms, and perspectives

Abstract

Molecular oxygen (O₂) is an environmentally friendly, cost-effective, and non-toxic oxidant. Activation of O₂ generates various highly oxidative reactive oxygen species (ROS), which efficiently degrade pollutants with minimal environmental impact. Despite extensive research on the application of O₂ activation in environmental remediation, a comprehensive review addressing this topic is currently lacking. This review provides an informative overview of recent advancements in O₂ activation, focusing on three primary strategies: photocatalytic activation, chemical activation, and electrochemical activation of O₂. We elucidate the respective mechanisms of these activation methods and discuss their advantages and disadvantages. Additionally, we thoroughly analyze the influence of oxygen supply, reactive temperature, and pH on the O₂ activation process. From electron transfer and energy transfer perspectives, we explore the pathways for ROS generation during O₂ activation. Finally, we address the challenges faced by researchers in this field and discuss future prospects for utilizing O₂ activation in pollution control applications. This detailed analysis enhances our understanding and provides valuable insights for the practical implementation of organic pollutant degradation.

Graphical abstract

Highlights

• •The generation of ¹O₂ involves both energy transfer and electron transfer.
• •Defect engineering plays an important role in O₂ activation.
• •Different activation strategies necessitate varying oxygen concentration levels.
• •The electrochemical activation achieves wastewater purification and energy storage.
• •O₂ activation mainly occurs in both acidic and slightly alkaline environments.

1. Introduction

The rapid advancement of industrial civilization has led to the proliferation of various organic contaminants, including antibiotics [1], pesticides [2], personal care products [3], and surfactants [4]. These substances pose significant threats to ecological systems and human health due to their high toxicity and extensive production. Their inherent structural stability makes them resistant to natural degradation processes [5]. Advanced oxidation processes (AOPs) have gained widespread recognition for their efficacy in degrading organic pollutants [6,7]. AOPs facilitate the abundant generation of reactive oxygen species (ROS) by harnessing the potential of highly reactive oxidants within optimal catalytic environments. These potent agents play a pivotal role in decomposing a wide range of organic pollutants, encompassing organic compounds in industrial wastewater [8], agricultural pollutants [9], and pharmaceutical residues in medical wastewater [10]. The strategic implementation of AOPs is crucial for improving water quality, preserving the ecological environment, and promoting sustainable development principles.

At present, AOPs encompass a variety of methodologies, including Fenton and Fenton-like processes [11], photocatalytic methods [12], and electrocatalytic techniques [13]. The ROS generated by these modalities possess distinct attributes that influence their effectiveness in contaminant removal. These ROS are produced through redox reactions. Traditionally, the majority of ROS are produced through electron acquisition. However, the pathway's sources for electron donors are diverse, primarily including metals, oxidants, photonic energy, and electric currents. For example, the classical Fenton reaction represents a reduction pathway for generating hydroxyl radicals (•OH), where Fe²⁺ acts as an electron donor to produce hydrogen peroxide (H₂O₂), resulting in •OH and Fe³⁺. Photo-generated electrons can activate peroxymonosulfate (PMS), leading to the production of •OH and •SO₄⁻ [14]. Additionally, PMS can acquire electrons from low-valence metals to generate •OH [15]. On the other hand, oxidation reactions also play a crucial role in ROS generation. High-value metals can directly oxidize prevalent oxidants into various ROS [16]. For example, Mn³⁺ can directly oxidize PMS and H₂O₂ to yield superoxide radicals (•O₂⁻) [17]. Mo⁶⁺ possesses the ability to transform •O₂⁻ into singlet oxygen (¹O₂) [18]. Within the photocatalytic process, the oxidation of •O₂⁻ by photoinduced holes represents one avenue for producing ¹O₂ [19]. Additionally, water oxidation is widely recognized as a precursor to ROS. Tungsten trioxide and zinc sulfide possess abundant valence band holes, facilitating the direct oxidation of hydroxide ions (OH⁻) or water molecules to generate •OH, which actively participates in the decomposition of tetracycline (TC) [20]. Various AOPs currently exist that can generate a significant amount of ROS, thereby aiding in eliminating pollutants. While these conventional methodologies have demonstrated their effectiveness, they often pose risks of causing secondary pollution. Therefore, there is an urgent need to explore novel technologies capable of efficiently degrading environmental contaminants.

Molecular oxygen (O₂), acclaimed for its environmental friendliness and cost-efficiency, is ubiquitously accessible in the atmosphere. However, the direct oxidation of numerous organic contaminants by O₂ is hindered due to spin-forbidden reactions [21]. In contrast, the activation of O₂ can generate various ROS, including •O₂⁻, ¹O₂, •OH, and H₂O₂. These ROS play a pivotal role as oxidizing agents in the decomposition processes of organic compounds.

Given the significance of activating O₂, this review provides a comprehensive analysis of research advancements in the degradation of organic contaminants through the activation of O₂. This paper extensively discusses primary strategies for activating O₂ to degrade organic molecules, including photocatalytic, chemical, and electrochemical methods. Moreover, it intricately dissects factors influencing O₂ activation and explores generative pathways of ROS considering electron transfer and energy transfer dimensions. This concludes with a discussion of prospects for novel degradation approaches through O₂ activation. This article aims to enhance understanding of O₂ activation methodologies and their role in environmentally removing organic pollutants.

2. Photocatalytic oxygen activation

The degradation of organic pollutants via the photochemical activation of O₂ involves the conversion of dissolved O₂ into various ROS. Based on the generation of ROS, this conversion mechanism can occur via two principal pathways: energy transfer or electron transfer [22]. The efficiency of ROS generation in this innovative oxidation process depends on several factors, including the performance characteristics of the semiconductor, the illumination mode, and the source of O₂.

2.1. Metal-based photocatalysts

Recently, various metal oxides, including titanium dioxide (TiO₂), zinc oxide (ZnO), copper(I) oxide (Cu₂O), and iron oxide (Fe₂O₃), have been applied to activate oxygen for the degradation of organic pollutants under light irradiation. Notably, bismuth-containing photocatalysts have emerged as crucial semiconductors in this field.

Zhao et al. [23] employed a solvent-thermal modification technique to synthesize BiOCl, wherein the exposure to a 300 W xenon lamp activated O₂, resulting in the prolific generation of •O₂⁻ and ¹O₂. With a modest quantity of H₂O₂, the rhodamine B (RhB), methyl orange (Mo), and phenol decomposition rates were amplified by 2.6, 3.1, and 2.4 times, respectively. ¹O₂ is generated as a result of the energy transfer or electron transfer. Wang et al. [24] ingeniously crafted an iodine-doped Bi₂WO₆ catalyst that created high-density oxygen vacancies (OVs) to enhance the photochemical activation of O₂. Their findings indicated that •O₂⁻ and ¹O₂ were the principal ROS implicated in the disintegration of sodium pentachlorophenate (NaPCP) [24]. Nd-doped BiO_2−x exhibited proficient near-infrared ray light-driven photocatalytic O₂ activation, where the abundant production of •O₂⁻ accelerated doxycycline degradation with a reaction rate constant reaching up to 134.4 × 10⁻⁴ min⁻¹ [25]. In the photochemical activation process, two-electron reduction of O₂ is the main pathway for H₂O₂ formation, which is an important ROS in pollutant degradation [25]. Sheng et al. [26] investigated the photocatalytic prowess of Bi₂WO₆ under visible light irradiation using phenol as a prototypical pollutant. Their findings revealed that post-irradiation treatment resulted in the liberation of photo-induced electrons from Bi₂WO₆ capable of converting O₂ into H₂O₂. In the presence of electrons, H₂O₂ undergoes decomposition to yield the highly oxidative •OH, thereby accelerating the efficient degradation of phenol. Liu et al. [27] engineered an iodine-doped Bi₂MoO₆ semiconductor and developed a light-stimulated H₂O₂ production system. This system demonstrated a 96.0% efficiency in the photodegradation of acetaminophen (APAP).

In addition to Bi-based photocatalysts, iron oxides have also been frequently mentioned for their effectiveness in the photocatalytic activation of O₂. Yue et al. [28] reported synthesizing a Z-type heterojunction 0Dα-Fe₂O₃/TiO₂. The Z-scheme charge transfer mechanism significantly accelerated the Fe³⁺/Fe²⁺ cycling rate, optimizing the utilization of H₂O₂ for •OH generation. This system demonstrated superior degradation efficacy against 2,4-dichlorophenol (2,4-DCP), achieving a degradation kinetic rate constant of 1.08 min⁻¹. Another advanced photocatalyst, N-GQDs/Bi₂Fe₄O₉, was designed to oxidize organic contaminants [29]. Upon light exposure, N-GQDs/Bi₂Fe₄O₉ exhibited remarkable H₂O₂ production (1.6 μM min⁻¹). Moreover, electrons congregating around N-GQDs enhanced the redox cycling of Fe³⁺/Fe²⁺ reactions, promoting •OH production and resulting in a 99.9% bisphenol A (BPA) elimination rate.

Metal sulfides, characterized by their narrow band gap, exhibit remarkable responsiveness to visible light and have thus been utilized in synthesizing photocatalysts for degrading organic pollutants by activating O₂ [30]. In scenarios where the ROS generated through photocatalytic O₂ activation is H₂O₂, this process is also called the photo-Fenton system [31]. Yang et al. [32] designed a CdS/Fe²⁺ photo-Fenton system to degrade sulfamethazine (SMT) under visible light, highlighting the capability of CdS nanorods in converting O₂ to H₂O₂ via a direct two-electron reduction pathway. Similarly, Jiang et al. [33] developed a CdS/rGO/Fe²⁺ photo-Fenton system capable of degrading phenol through in situ H₂O₂ generation under visible light irradiation. Their findings suggested that the in situ production of H₂O₂ in this system may follow a two-electron, one-step reduction mechanism.

The photo-Fenton process, which involves the reduction of dissolved O₂ under light irradiation, is regarded as an innovative strategy. Shi et al. [34] developed a CdS/reduced graphene (rGO)/ZnFe₂O₄ (ZFO) nanocomposite system that effectively degrades TC. This degradation occurs through the in situ generation of H₂O₂ via a single-electron, two-step reduction of O₂ under visible light irradiation. Yang et al. [35] established a Sv-MoS₂/BN heterojunction. Upon light exposure, the Sv-MoS₂/BN-5 achieved an H₂O₂ production rate of 428.17 μmol h⁻¹ g⁻¹ and completed in situ disinfection for Escherichia coli (E. coli). Moreover, ternary metal sulfides exhibit superior catalytic activity and a broader range of properties than binary metal sulfides. Du et al. [36] synthesized Zn₂In₂S₅ using a meticulous in situ hydrothermal approach. This system can generate abundant H₂O₂ under light irradiation and exhibits excellent performance in aromatic pollutant removal, with a phenol degradation efficiency of 97.6% and an impressive H₂O₂ yield of 1.3 mmol L⁻¹. As shown in Table 1, the photocatalytic activation of O₂ has demonstrated remarkable efficiency in degrading organic pollutants through the generation of ROS.

Table 1.

Representative studies on the degradation of organic pollutants by photocatalytic oxygen activation.

Catalysts	Mechanism	Contami-nants	Reaction conditions	O2 supply	ROS	Removal efficiency	TOC removal	Reference
OV-BOC	Energy transfer	RhB	[RhB]0 = 10.0 mg L−1, 300 W Xenon lamp, pH = 3.5	Dissolved oxygen	1O2, •O2−	99.8% (30.0 min)	-	[23]
BWO-0.18	Energy transfer	NaPCP	[NaPCP]0 = 30.0 × 10−6 mg L−1, 300 W Xenon lamp	Dissolved oxygen	1O2, •O2−	99.9% (1.0 h)	90.0%	[24]
Nd-BiO2−x	Electron transfer	Doxycycline	[Doxycycline]0 = 10.0 mg L−1, 300 W xenon lamp	Dissolved oxygen	•O2−	94.2% (1.0 h)	72.0%	[25]
BMO-3	Electron transfer	APAP	[APAP]0 = 20.0 μmol L−1, 300 W Xenon lamp, pH = 6.8	Dissolved oxygen	H2O2	96.0% (2.0 h)	61.0%	[27]
FT-200	Electron transfer	2,4-DCP	[2,4-DCP]0 = 20.0 μmol L−1, 300 W Xenon lamp, pH = 3.0	Dissolved oxygen	H2O2, •OH	100.0% (6.0 min)	91.5%	[28]
N-GQDs/Bi2Fe4O9	Electron transfer	BPA	[BPA]0 = 20.0 mg L−1, 300 W Xenon lamp, pH = 3.0	Dissolved oxygen	•OH	99.8% (90.0 min)	-	[29]
Sv-MoS2/BN-5	Electron transfer	E. coli	[E. coli]0 = 6.0 × 107 CFU mL−1, 300 W Xenon lamp, pH = 3.0	Dissolved oxygen	H2O2	100.0% (60.0 min)	-	[35]
CZS-5	Electron transfer	phenol	[phenol]0 = 20.0 mg L−1, 300 W Xenon lamp, pH = 7.0	O2 aeration	H2O2	97.6% (180.0 min)	87.0%	[36]
Cv-PCNNS	Electron transfer	MTZ	[MTZ]0 = 10.0 mmol L−1, 300 W Xenon lamp, pH = 7.0	O2 aeration	H2O2, •OH	90.7% (100.0 min)	62.0%	[47]
g-C3N4	Electron transfer	TC	[TC]0 = 50.0 ppm, 300 W Xenon lamp, pH = 7.0	Dissolved oxygen	H2O2, •OH	90.0% (300 min)	-	[48]
FeOOH/UPCN	Electron transfer	OTC	[OTC]0 = 20.0 mg L−1, 300 W Xenon lamp, pH = 7.0	Dissolved oxygen	H2O2, •OH	86.2% (120.0 min)	48.6%	[49]

Note: “Dissolved oxygen” refers to the oxygen that comes from the dissolved oxygen in the aqueous solution without any aeration; “Air aeration” refers to the main source of oxygen in the system from the air; “O₂ aeration” signifies that the primary source of oxygen in the system is pure oxygen. Abbreviations: OV-BOC, BiOCl with oxygen vacancy; BWO, Bi₂WO₆; Nd-BiO_2−x, Nd doped BiO_2−x; BMO-3, iodine-doped Bi₂MoO₆; FT-200, Fe₂O₃ loaded TiO₂ nanoparticles; N-GQDs/Bi₂Fe₄O₉, nitrogen-doped graphene quantum dots/Bi₂Fe₄O₉; Sv-MoS₂/BN-5, sulfur vacancy-rich MoS₂/boron nitride; CZS-5, N-doped hollow carbon spheres/Zn₂In₂S₅; Cv-PCNNS, porous g-C₃N₄ nanosheets with carbon vacancies; g-C₃N₄, graphitic carbon nitride; FeOOH/UPCN, iron-based oxyhydroxides/ultrathin porous g-C₃N₄; RhB, rhodamine B; NaPCP, sodium pentachlorophenate; APAP, acetaminophen; 2,4-DCP, 2,4-dichlorophenol; BPA, bisphenol A; E. coli, Escherichia coli; MTZ, metronidazole; TC, tetracycline; OTC, oxytetracycline.

2.2. Organic photocatalysts

The application of metal-based photocatalysts is typically hindered by their poor light responsiveness, biological toxicity, and instability [37]. In contrast, organic photocatalysts have demonstrated superior qualities such as ease of fabrication, appropriate visible light response band gap, high stability, and beneficial metal-free properties [38,39]. Currently recognized as the most prominent organic photocatalyst for activating O₂ and removing contaminants [40], graphite carbon nitride (g-C₃N₄) faces challenges due to inefficient light utilization and rapid recombination of photogenerated charge carriers that significantly impair O₂ activation [41]. To address these limitations in g-C₃N₄ photocatalytic efficiency under light irradiation, various strategies have been applied, including defect engineering [42], doping [43], the creation of ultrathin structures [44], and ion intercalation [45] (Fig. 1).

Fig. 1.

a, Proposed mechanism for the photocatalysis-self-Fenton system based on carbon defective Cv-PCNNS-550. Reproduced with permission from Ref. [47]. Copyright 2023, Elsevier. b, The photo-Fenton degradation mechanism proposed in the FeOOH/UPCN composite system with ultrathin material. Reproduced with permission from Ref. [49]. Copyright 2022, Elsevier. c, Mechanism for the photocatalysis-self-Fenton system of P doped g-C₃N₄. Reproduced with permission from Ref. [50]. Copyright 2020, Elsevier. d, The ionothermal synthesis of K⁺/I⁻ intercalated g-C₃N₄ (CN-KCl/KI). Reproduced with permission from Ref. [52]. Copyright 2023, NAS.

Defect engineering plays a crucial role in enhancing exciton dissociation, thereby influencing the activation of O₂. Zhou et al. [46] employed a gas reduction technique to introduce nitrogen vacancies in layered g-C₃N₄. The vacancies at the interface create a significant energy disparity, enabling the dissociation of singlet excitons into free electron-hole pairs and effectively activating O₂ to efficiently generate •OH for organic pollutant degradation. Moreover, defect engineering enhances charge separation and supports the reduction of O₂ to H₂O₂ during photocatalysis, substantially contributing to the degradation of organic pollutants. Li et al. [47] developed porous g-C₃N₄ nanosheets with carbon vacancies (Cv-PCNNS) via a two-step heat treatment approach (Fig. 1a). Their findings showed that these nanosheets exhibited remarkable capability in generating a substantial amount of H₂O₂ through a one-step dual-electron reduction reaction of O₂, achieving an impressive 984.8 μmol L⁻¹ h⁻¹. Activating O₂ via photocatalysis-self-Fenton reaction achieved notable degradation efficiency for metronidazole (MTZ) (90.7%) within 100 min.

Owing to its extensive specific surface area, ultrathin material offers significant advantages in activating O₂. Hu et al. [48] synthesized a series of catalysts consisting of ultrathin g-C₃N₄ nanosheets that demonstrated high-efficiency light-driven catalytic performance in the production of H₂O₂, achieving a rate of 170.0 μmol L⁻¹ h⁻¹. In the presence of this catalyst, the produced H₂O₂ can be further converted into •OH for TC degradation. Moreover, the ultrathin structure facilitates short migration distances for photo-generated carriers, enhancing O₂ reduction and H₂O₂ generation under light irradiation. Shi et al. [49] fabricated ultrathin porous g-C₃N₄ (UPCN) integrated with amorphous FeOOH quantum dots (Fig. 1b). This configuration enabled the production of 23.9 μmol L⁻¹ H₂O₂ within 120 min under visible light irradiation at natural pH levels. Notably, in the photo-Fenton system, this catalyst achieved effective degradation rates of oxytetracycline (OTC) (20.0 mg L⁻¹) of 86.2% and total organic carbon (TOC) removal efficiency of 48.6% within 120.0 min.

Doping also plays a significant role in enhancing oxygen activation. Phosphorus-doped g-C₃N₄ (P-doped g-C₃N₄) has demonstrated remarkable photocatalytic reactivity in the degradation of 2,4-DCP [50] (Fig. 1c). Comparative studies have revealed that the degradation activity of 2,4-DCP can be increased by a factor of 20.5 using P-doped g-C₃N₄ compared to bulk g-C₃N₄ photocatalysis. This increase is primarily attributed to the generation of •OH from H₂O₂ produced via the oxygen reduction. Inefficient carrier separation negatively affects O₂ activation, particularly during electron transfer processes. To address this issue, doping has emerged as an effective strategy for enhancing carrier separation and ensuring efficient O₂ activation. An oxygen-doped g-C₃N₄ (OCN) variant has been synthesized, resulting in a 3.5-fold increase in H₂O₂ production under monochromatic light compared to bulk g-C₃N₄ [51]. The improved carrier transfer efficiency in OCN facilitates the two-electron reduction of O₂ and consequently demonstrates superior degradation efficiencies for phenol and 2,4-DCP.

Intercalation engineering in carbon nitride significantly improves carrier separation and transfer, enhancing O₂ activation for H₂O₂ production. In a recent study, Liu et al. [52] modified g-C₃N₄ by the intercalation of potassium ions (K⁺) and iodide ions (I⁻) into its layered structure, resulting in the achievement of a H₂O₂ production rate of 13.1 mM g⁻¹ h⁻¹ and an apparent quantum yield of 23.6% at 400 nm for this ion-intercalated crystalline g-C₃N₄. This suggests that interstitial doping engineering can facilitate the creation of rapid charge diffusion channels and promote efficient two-electron oxygen reduction (Fig. 1d).

In summary, the photocatalytic activation of O₂ has been extensively investigated and applied in environmental purification. The protocol eliminates the need for additional oxidants during the process. Moreover, this procedure can generate various ROS, which are advantageous for the degrading and mineralizing organic pollutants. Photocatalytic reactions typically occur at room temperature, with simple operation and without requiring complex equipment or conditions. However, the efficiency of photocatalytic O₂ activation is limited by factors such as charge carrier recombination, low carrier migration rate, and insufficient activated O₂ adsorption sites. Although certain emerging catalysts can overcome these challenges, they often increase production costs. Additionally, prolonged usage leads to catalyst deactivation or breakdown, necessitating periodic replacement or regeneration, escalating costs and complicating operations. Current studies primarily employ artificial illumination for photocatalysis; however, this approach is energy-intensive. While the photocatalytic activation of O₂ to generate ROS holds promise for pollutant eradication, further investigation is needed to overcome existing obstacles and enhance its practicability and efficacy.

3. Chemical oxygen activation

The activation of O₂ through photochemistry requires strict conditions, including temperature, light radiation intensity, photoreactor design, and selection of photocatalytic materials [53,54]. These limitations may hinder the broader application of photocatalytic O₂ activation. Conversely, certain zero-valent metals (ZVMs), low-valent transition metals (LVTMs), and defect materials can activate O₂ without such demanding operating environments.

3.1. Zero valent metals

In recent years, the potential of ZVMs, such as Fe, Al, and Cu, to activate O₂ has gained increasing recognition due to their ability to generate various ROS for pollutant removal. Notably, zero-valent iron (ZVI, Fe⁰) has garnered significant attention owing to its safety and widespread availability [55]. When introduced into a solution containing dissolved oxygen, Fe⁰ undergoes a thermodynamically spontaneous corrosion reaction. This process involves electron transfer from Fe⁰ to O₂, leading to the oxidation of Fe⁰ into Fe²⁺. Concurrently, O₂ is reduced to H₂O₂. In acidic or neutral environments, Fe²⁺ further catalyzes H₂O₂ formation into •OH (equations (1), (2)).

$$\mathrm{F}{\mathrm{e}}^0+{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to \mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2$$ (1)

$$\mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{F}{\mathrm{e}}^{3+}+•\text{OH}+\mathrm{O}{\mathrm{H}}^{-}$$ (2)

Numerous studies have established that ROS generated from the activation of O₂ by Fe⁰ predominantly consists of H₂O₂, thus making this process analogous to a Fenton-like reaction. The Fe⁰/O₂ system has been used for treating refined cottonseed oil (CTO) wastewater (Fig. 2a) [56]. Experimental findings have demonstrated the effective degradation of CTO in wastewater using the Fe⁰/O₂ system with an observable enhancement in degradation efficiency due to increased oxygen concentration. To further enhance the degradation efficiency and prevent Fe⁰ aggregation within this system, researchers have developed composite materials by combining Fe⁰ with other substances. Carbon-coated nano ZVI (Fe@C), obtained by high-temperature calcination of complexes formed between Fe and organic compounds, exhibit small size, high content, and excellent dispersion properties, ensuring their heightened activity in Fenton-like reactions [57]. This catalyst activates O₂ to generate H₂O₂ and forms the Fe@C-H₂O₂ Fenton system, effectively degrading SMT. Yang et al. [58] encapsulated nano ZVI within a three-dimensional graphene network material (3D-GN@nZVI), successfully activating dissolved oxygen and facilitating sulfadiazine (SDZ) degradation. In addition, introducing foreign metals can accelerate the corrosion of Fe⁰ and enhance the efficiency of O₂ activation [59].

Fig. 2.

a, The degradation mechanisms of CTO by ZVI system. Reproduced with permission from Ref. [56]. Copyright 2022, Elsevier. b, The degradation mechanism of SMT in the Cu⁰/C-air system. Reproduced with permission from Ref. [62]. Copyright 2021, Elsevier. c, Mechanism for active species generation in the Zn⁰-CNTs-O₂ system. Reproduced with permission from Ref. [66]. Copyright 2017, Elsevier. d, Ascorbic acid enhances the activation of oxygen by the Fe²⁺/air process. Reproduced with permission from Ref. [71]. Copyright 2016, Elsevier. e, Mechanisms of activating O₂ by Cu(I) on Cu-Al/KIT-6. Reproduced with permission from Ref. [73]. Copyright 2020, Elsevier. f, Degradation mechanism of nitrobenzene (NB) in air/CuCoFe/Tar-LDH reaction systems. Reproduced with permission from Ref. [80]. Copyright 2022, ACS. g, The mechanism of ROS generation in the CoS_2–1.5 h system. Reproduced with permission from Ref. [85]. Copyright 2021, Wiley. h, The oxygen molecule activation by carbon defects. Reproduced with permission from Ref. [86]. Copyright 2023, Elsevier.

Similar to ZVI, zero valence copper (ZVC, Cu⁰) can also activate O₂, leading to the production of H₂O₂ and the formation of a Fenton-like system. Meanwhile, during the oxidation process, Cu⁰ generates Cu⁺ and Cu²⁺ ions, which subsequently react with in situ generated H₂O₂ to produce •OH and •O₂⁻ radicals (equations (3), (4), (5)).

$$2\mathrm{C}{\mathrm{u}}^0+{\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2\to 2\mathrm{C}{\mathrm{u}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2+2\mathrm{O}{\mathrm{H}}^{-}$$ (3)

$$\mathrm{C}{\mathrm{u}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2\to 2\mathrm{C}{\mathrm{u}}^{2+}+•\text{OH}+2\mathrm{O}{\mathrm{H}}^{-}$$ (4)

$$\mathrm{C}{\mathrm{u}}^{2+}+{\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{C}{\mathrm{u}}^{+}+•{\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}$$ (5)

The involvement of Cu⁺ and Cu²⁺ ions adds complexity to the process of Cu⁰ activating O₂. In an nZVC/air system, nZVC can activate O₂ to produce H₂O₂ while simultaneously releasing Cu ⁺ [60]. This activation further catalyzes the transformation of H₂O₂ into •OH and facilitates the destruction of -N=N- azo pollutants, forming carbon-centered radicals. Both radicals contribute to mineralizing low molecular weight organic acids under neutral conditions. Cu⁰ exhibits significant potential for activating O₂ and removing organic pollutants. However, Cu⁰ tends to aggregate and release a large amount of copper ions after use during its preparation process, leading to contamination in water environments and reducing its catalytic activity [61]. Additionally, factors such as dissolved oxygen concentration and mass transfer efficiency also influence the catalytic performance of Cu⁰. To address these issues, Yang et al. [62] developed a novel mesoporous carbon-hybridized zero-valent copper (Cu⁰/C) through the calcination of copper-based metal-organic frameworks (Cu₃(BTC)₂) in a tubular furnace (Fig. 2b). The carbon-based matrix not only adsorbs pollutants but also effectively inhibits the leaching of copper ions. Under neutral pH conditions, the Cu⁰/C-air system achieved a high SMT mineralization rate (73.1%). Table 2 lists ZVMs/O₂ systems for the degradation of contaminants.

Table 2.

The removal of pollutants based on ZVMs/O₂ systems.

ZVMs	Contaminants	Reaction conditions	O2 supply	ROS	Removal efficiency	TOC removal	Reference
ZVI	CTO	pH = 2.5–3.1	O2 aeration	H2O2, •OH	93.5% (3.0 h)	-	[56]
Fe@C	SMT	[SMT]0 = 20.0 mg L−1, pH = 4.0	Dissolved oxygen	H2O2, •OH	97.6% (300.0 min)	93.8%	[57]
3D-GN@nZVI	SDZ	[SDZ]0 = 10.0 mg L−1, pH = 3.0	Dissolved oxygen	H2O2, •OH	81.0% (120.0 min)	-	[58]
BM Fe/Cu	4-CP	[4-CP]0 = 20.0 mg L−1, pH = 3.0	Air aeration	•O2−, •OH	100.0% (120.0 min)	66.4%	[59]
nZVC	MO	[MO]0 = 20.0 mg L−1, pH = 6.6	Air aeration	H2O2, •OH	100.0% (4.0 h)	56.1%	[60]
ZVCMMT	ATZ	[ATZ]0 = 15.0 μM, pH = 3.0	Air aeration	•OH	90.0% (180.0 min)	-	[61]
Cu0/C	SMT	[SMT]0 = 20.0 m L−1, pH = 4.0	Air aeration	H2O2, •OH	96.7% (90.0 min)	68.8%	[62]
Zn0-CNTs-Fe3O4	OTC	[OTC]0 = 100.0 mg L−1, pH = 3.0	O2 aeration	H2O2, •OH	98.6% (60.0 min)	-	[67]
Zn-Fe-CNTs	SMX	[SMX]0 = 20.0 mg L−1, pH = 1.50	O2 aeration	H2O2, •OH	100.0% (120.0 min)	51.3%	[68]
ZVZ/Cu(II) system	MTZ	[MTZ]0 = 10.0 mg L−1, pH = 5.0	Dissolved oxygen	H2O2, •OH, •O2−	90.0% (10.0 min)	-	[69]

Abbreviations: ZVI, zero-valent iron; Fe@C, carbon-coated nanoscale zero-valent iron; 3D-GN@nZVI, nanoscale zero-valent iron encapsulated in three-dimensional graphene network; BM Fe/Cu, ball milling Fe/Cu; nZVC, nanoscale zero-valent copper; ZVCMMT, montmorillonite templated zero-valent copper; Cu⁰/C, mesoporous carbon hybrid loaded zero-valent copper; Zn⁰-CNTs-Fe₃O₄, Zn⁰-carbon nanotubes-Fe₃O₄; Zn-Fe-CNTs, Zn-Fe-carbon nanotubes; ZVZ/Cu(II) system, zero-valent zinc/Cu(II) system; CTO, cottonseed oil; SMT, sulfamethazine; SDZ, sulfadiazine; 4-CP, 4-chlorophenol; MO, methyl orange; ATZ, atrazine; OTC, oxytetracycline; SMX, sulfamethoxazole; MTZ, metronidazole.

Additionally, zero-valent zinc (ZVZ, Zn⁰) can facilitate the in situ generation of H₂O₂ through the activation of O₂. However, the concentration of H₂O₂ generated by the Zn⁰/air system is relatively low, necessitating further modifications to enhance its in situ generation [63]. According to electrochemical corrosion theory, Zn⁰ acts as an anode in a corrosion cell, releasing electrons that are transferred to the cathode. The received electrons on the cathode surface can react with O₂ to produce H₂O₂. Simultaneously, Zn⁰ undergoes oxidation, leading to the formation of precipitates such as ZnO or Zn(OH)₂, as depicted by equations (6), (7), (8), (9), (10).

$$2\text{Zn}+3{\mathrm{O}}_2+4{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2+2\text{ZnO}+\text{Zn}{\left(\text{OH}\right)}_2$$ (6)

$$\text{Anode}:\text{Zn}+2{\mathrm{e}}^{-}\to \mathrm{Z}{\mathrm{n}}^{2+}$$ (7)

$$\text{Cathode}:{\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ (8)

$$\text{Zn}+{\mathrm{H}}_2{\mathrm{O}}_2\to \text{ZnO}+{\mathrm{H}}_2\mathrm{O}$$ (9)

$$\mathrm{Z}{\mathrm{n}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{e}}^{-}\to \text{Zn}{\left(\text{OH}\right)}_2$$ (10)

Therefore, selecting suitable cathode materials is crucial for improving the efficiency of O₂ activation and increasing H₂O₂ production. Among various materials, carbon nanotubes (CNTs) have emerged as a standout choice due to their cost-effectiveness, high conductivity, and stability, making them widely utilized in electrochemical oxidation and fuel cell applications for in situ H₂O₂ generation [64,65]. Gong et al. [66] developed the Zn-CNTs-O₂ system wherein Zn and CNTs form a corrosion cell that reduces O₂ to H₂O₂ on CNTs’ surface (Fig. 2c). Simultaneously, dissolved Zn²⁺ precipitates as Zn(OH)₂ onto the CNTs. The formation of this corrosion cell is primarily responsible for increasing the yield of H₂O₂. To further enhance the operational efficiency of the Zn/CNTs corrosion battery, some researchers have incorporated iron oxide and iron ions into Zn-CNTs. Liu et al. [67] synthesized a catalyst named Zn⁰-CNTs-Fe₃O₄, which achieved a degradation rate of 98.6% toward OTC. Furthermore, combining zinc with iron yields superior outcomes; a Zn-Fe-CNTs catalyst based on Zn⁰-CNTs and Fe⁰ eliminated sulfamethoxazole (SMX) within just 10.0 min [68].

3.2. Low valence transition metals

In contrast to ZVMs, certain LVTMs also function as electron donors for the activation of O₂, resulting in the generation of ROS. Among these LVTMs, extensive research has been conducted on the activation of O₂ by Fe²⁺. Fe²⁺ can activate dissolved oxygen under aerobic conditions, leading to the formation of •O₂⁻, which subsequently accepts electrons and ultimately leads to the generation of H₂O₂. Moreover, Fe²⁺ plays a crucial role in the Fenton reaction with H₂O₂, facilitating the production of •OH [70]. This mechanism exhibits high efficacy in degrading organic pollutants, according to equations (11), (12), (13).

$$\mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{O}}_2\to \mathrm{F}{\mathrm{e}}^{2+}+•{\mathrm{O}}_2^{-}$$ (11)

$$\mathrm{F}{\mathrm{e}}^{2+}+•{\mathrm{O}}_2^{-}+2{\mathrm{H}}^{+}\to \mathrm{F}{\mathrm{e}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2$$ (12)

$$\mathrm{F}{\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{H}}^{+}\to \mathrm{F}{\mathrm{e}}^{3+}+•\text{OH}+{\mathrm{H}}_2\mathrm{O}$$ (13)

In a neutral environment, forming irreversible precipitates by Fe³⁺ impedes the activation of O₂ and the generation of ROS. Additionally, reducing Fe³⁺ to Fe²⁺ is a slow process mediated by •O₂⁻, resulting in low efficiency and limiting the removal of pollutants. Researchers have explored incorporating ligands into O₂ activation systems to enhance pollutant removal efficiency to facilitate ROS production. Hou et al. [71] established an AA/Fe(II)/air system. They discovered that ascorbic acid (AA) enhances the activation process of O₂ by Fe²⁺, unveiling an accelerated production rate of ROS through its pivotal role in promoting the Fe²⁺/Fe³⁺ cycle (Fig. 2d). Furthermore, Zong et al. [72] constructed a Fe(II)-tetrapolyphosphate (TPP)/air system, which demonstrated that TPP significantly enhances the activation of O₂ by Fe²⁺ and accelerates the reduction of Fe³⁺.

In recent studies, the activation of O₂ by Cu ⁺ has garnered significant attention. Wang et al. [73] synthesized Cu-Al/KIT-6 and utilized it for the activation of O₂ (Fig. 2e). The experimental findings demonstrated that Cu ⁺ effectively activated O₂ and generated ROS, thereby facilitating BPA removal, revealing its potential as an O₂ activator. However, the environmental instability of Cu ⁺ poses a challenge to activating O₂. Zheng et al. [74] synthesized a hydrothermal carbonaceous carbon material containing stable CuCl (CuCl-HTCC) to address this limitation. Their study revealed that CuCl-HTCC contained a substantial amount of stable Cu⁺, which exhibited significantly higher efficiency in activating O₂ than unstable homogeneous reactions involving only Cu⁺. Table 3 provides examples showcasing pollutant degradation by LVTMs through the activation of O₂.

Table 3.

The degradation of pollutants by LVTMs/O₂ systems.

LVTMs	Contaminants	Reaction conditions	O2 supply	ROS	Removal efficiency	TOC removal	Reference
AA/Fe(II)	RhB	[RhB]0 = 0.01 mmol L−1, pH = 6.0	Air aeration	H2O2, •OH	47.0% (6.0 h)	16.0%	[71]
Fe(II)-TPP complex	SMX	[SMX]0 = 1.3 mg L−1, pH = 8.0	Air aeration	H2O2, •OH, •O2−	75.7% (20.0 min)	-	[72]
Cu-Al/KIT-6	BPA	[BPA]0 = 25.0 mg L−1, pH = 5.0	O2 aeration	H2O2, •OH	99.0% (120.0 min)	46.0%	[73]
CuCl-HTCC	IBU	[IBU]0 = 10.0 mg L−1, pH = 4.6	Magnetic stirring	•O2−, •OH	Almost 100.0% (120.0 min)	-	[74]
P-Cu	IBU	[IBU]0 = 10.0 mg L−1, pH = 6.0	Air aeration	•OH	100.0% (120.0 min)	52.6%	[75]
Mo/Cu2+ system	IBU	[IBU]0 = 5.0 ppm, pH = 3.0	Air aeration	1O2, •O2−	67.2%	-	[76]
Fe3O4/O2/TPP system	PNP	[PNP]0 = 10.0 mg L−1, pH = 3.0	Dissolved oxygen	•OH	80.0% (480.0 min)	-	[77]

Abbreviations: AA/Fe(II), ascorbic acid/Fe(II); Fe(II)-TPP complex, Fe(II)-tetrapolyphosphate complex; Cu-Al/KIT-6, framework Cu doped Al₂O₃ dispersed on KIT-6; CuCl-HTCC, copper chloride loaded hydrothermal carbonaceous carbon; P-Cu, low valence copper on red P; Fe₃O₄/O₂/TPP system, Fe₃O₄/O₂/tripolyphosphate system; RhB, rhodamine B; SMX, sulfamethoxazole; BPA, bisphenol A; IBU, ibuprofen; PNP, p-nitrophenol.

3.3. Defect engineering

Defect engineering has become a prominent research focus due to its ability to directly activate O₂, making it an attractive area for investigation in water treatment applications. Defect engineering involves deliberately manipulating a catalyst's performance by introducing, regulating, and repairing internal defects such as lattice, vacancies, and surface flaws. This approach does not rely on external energy or sacrificial reagents [78].

The presence of OVs plays a crucial role in the activation of O₂, making them recognized for their exceptional capability and potential to revolutionize this field. Wu et al. [79] discovered that OVs are crucial in generating ROS by capturing electrons and forming •O₂⁻. Additionally, OVs without electrons can extract electrons from organic pollutants, degrading them and maintaining electrostatic equilibrium, resulting in a 32.0% degradation rate of BPA without requiring an additional oxidant PMS.

However, the rapid reversion of •O₂⁻ back to O₂ may explain the decreased degradation rates of pollutants. Addressing this phenomenon, Wang et al. [80] synthesized CuCoFe/Tar-LDH with OVs and provided an explanation for the process (Fig. 2f). They proposed that the process began with the adsorption of O₂ onto the OVs in CuCoFe/Tar-LDH, which is then converted into •O₂⁻ and subsequently transformed into •OH. The study by Wang et al. underscores the effectiveness of oxygen vacancy-mediated catalysts in directly activating O₂ to generate highly reactive oxidizing species. Based on these findings, it can be inferred that ROS production during the activation of O₂ through OVs is directly influenced by the dissolved oxygen concentration. Research conducted by Wang et al. [81] investigated the regulatory mechanism and transformation pathway of ROS during the process of O₂ activation. Their findings revealed that under low dissolved oxygen concentration conditions, O₂ predominantly leads to forming ¹O₂ and •O₂⁻. Conversely, as the concentration of dissolved oxygen increases, there is a more prominent conversion from O₂ to •OH. This study demonstrates that the potential for selective ROS production can be controlled by adjusting dissolved oxygen levels. Their research holds significant value in advancing wastewater treatment.

Moreover, this process also exhibits significant antimicrobial properties. For instance, a form of magnesium oxide with surface OVs can directly generate ¹O₂ and •O₂⁻ through the reaction between OVs and O₂, even in the absence of light. This catalyst has demonstrated outstanding antibacterial performance by effectively eliminating E. coli within 2 h [82]. Additionally, Prasanna et al. [83] found that zinc oxide enriched with OVs exhibited significant antibacterial properties due to the generation of ROS through the activation of surface OVs.

However, research on the activation of O₂ by sulfur vacancies is less prevalent than that on OVs. Ultrasonic treatment of CoS_2−x material led to numerous sulfur vacancies on its surface, which can reduce dissolved oxygen to •O₂⁻ (Fig. 2g) [85]. Consequently, continuous generation of ¹O₂ occurs, effectively degrading organic pollutants. OVs and sulfur vacancies are specific forms of anionic vacancies that arise due to the absence of oxygen or sulfur lattice sites in metal oxides and metal sulfides. These vacancies possess the ability to effectively promote O₂ activation. However, they are prone to instability and metal leaching, limiting their practical applicability. Therefore, non-metal carbon materials featuring carbon vacancies have attracted considerable interest. Liu et al. [86] conducted a study wherein they synthesized a type of defective porous carbon (DPC) capable of adsorbing dissolved oxygen and activating it into •O₂⁻ and ¹O₂ (Fig. 2h). The experimental findings suggest that the primary active center for chemisorption and direct conversion of dissolved oxygen into •O₂⁻ may be the non-diagonal cyclic structure of carbon vacancies. The activation of O₂ by defect engineering for degradation of organic contaminants is systematically summarized in Table 4.

Table 4.

The activation of O₂ by defect engineering for degradation of organic contaminants.

Catalyst	Defect engineering	Contaminants	Reaction conditions	O2 supply	ROS	Removal efficiency	TOC removal	Refe-rence
Fe-Co LDH	Oxygen Vacancy	BPA	[BPA]0 = 2.0 mg L−1, pH = 7.0	Dissolved oxygen	1O2, •O2−	32.0% (60.0 min)	49.0%	[79]
CuCoFe/Tar-LDH	Oxygen Vacancy	NB	[NB]0 = 150.0 mg L−1, pH = 7.2	Air aeration	•OH, •O2−	78.3% (240.0 min)	45.5%	[80]
CuCoFe-300	Oxygen Vacancy	CIP	[CIP]0 = 15.0 mg L−1, pH = 4.0	O2 aeration	•OH, •O2−	75.0% (240.0 min)	54.1%	[81]
MgO	Oxygen Vacancy	E. coli	[E. coli]0 = 0.05 mol L−1, pH = 8.0	O2 aeration	•O2−, 1O2	100.0% (2.0 h)	-	[82]
ZnO	Oxygen Vacancy	S. aureus	[S. aureus]0 = 3.0 × 1012 CFU mL−1, pH = 8.5	Dissolved oxygen	•OH, •O2−, H2O2	17.0% (120.0 min)	-	[83]
DR-MoS2	Sulfur vacancy	Black T	[Black T]0 = 10.0 mg L−1, pH = 3.0	Dissolved oxygen	•OH, H2O2	99.8% (25.0 min)	-	[84]
CoS2-x	Sulfur vacancy	RhB	[RhB]0 = 20.0 mg L−1, pH = 4.5	Dissolved oxygen	1O2	91.9% (120.0 min)	70.7%	[85]
DPC-800	Carbon vacancy	CIP	[CIP]0 = 20.0 mg L−1, pH = 6.6	Dissolved oxygen	1O2, •O2−	98.0% (60.0 min)	-	[86]

Abbreviations: Fe-Co LDH, Fe-Co layered double hydroxide; CuCoFe/Tar-LDH, tartrate-modified CuCoFe-layered double hydroxide; CuCoFe-300, calcined CuCoFe-layered double hydroxide with 300 °C; DR-MoS₂, defect-rich MoS₂; DPC-800, calcined defected porous carbon with 800 °C; BPA, bisphenol A; NB, nitrobenzene; CIP, ciprofloxacin; E. coli, Escherichia coli; S. aureus, Staphylococcus aureus; Black T, Eriochrome Black T; RhB, rhodamine B.

Compared to the photocatalytic activation of O₂, the chemical oxygen activation modality offers operational simplicity, reduced material preparation expenditures, and superior O₂ activation effects, resulting in effective pollutant removal. However, ZVMs and LVTMs are fundamentally expendable materials characterized by single-use functionality and minimal reusability. Moreover, the instability of these materials may lead to the liberation of metal ions, posing a risk of secondary contamination. Additionally, in alkaline environments, the catalytic activity of ZVMs and LVTMs may be curtailed, impacting the efficiency of ROS production. In contrast to ZVMs and LVTMs, defect structures can reduce reaction barriers, promote O₂ activation, and enhance pollutant degradation efficiency. The utilization of defect engineering for O₂ activation represents an emerging and cutting-edge technological approach. Nonetheless, a lack of dedicated research reports is currently elucidating the process and mechanism underlying O₂ activation via defect engineering. While defect engineering is indispensable for O₂ activation, the accumulation of reaction intermediates, water, and other catalysts on the molecular surface may fill up defects, diminishing adsorption capacity toward O₂ while hindering electron transfer. The design and synthesis of defect materials with proficient activation properties pose a significant challenge, requiring careful consideration of catalyst architecture, composition, and synthesis techniques, potentially extending the trajectory of research and development.

4. Electrochemical oxygen activation

The electrochemical activation of O₂ is a pivotal pathway for the in situ synthesis of H₂O₂, playing a significant role in various processes. At the cathode, O₂ undergoes a two-electron reduction reaction to produce H₂O₂. The presence of catalysts or electrons influences the catalytic decomposition of H₂O₂ into various ROS. Based on the mode of ROS formation, the electrochemical activation of oxygen can be further categorized into four processes: the electro-Fenton (EF) process, the photoelectro-Fenton (PEF) process, the photoelectrocatalytic (PEC) process, and the microbial fuel cell (MFC) process.

4.1. Electro-Fenton process

The EF process involves the introduction of O₂ or air at the cathode, facilitating the continuous in situ generation of H₂O₂ through 2e⁻ oxygen reduction reaction (ORR) (Fig. 3a). The incorporation of the Fe²⁺ catalyst within the Fenton reaction leads to the generation of •OH (equations (14), (15)). •OH indiscriminately degrades organic pollutants into smaller organic molecules, salts, and carbon dioxide via addition, substitution, and bond cleavage. Variations in reaction conditions, including pH, cathode material, and anode catalyst, can result in distinct mechanisms and efficiencies in the EF reaction.

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ (14)

$${\mathrm{O}}_2+3{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}\to •\text{OH}+{\mathrm{H}}_2\mathrm{O}$$ (15)

$${\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O}$$ (16)

Fig. 3.

The ROS generation pathway based on the activation of O₂ by electro-Fenton process (a), photoelectro-Fenton process (b), photoelectrocatalytic process (c), mcrobial fuel cell process (d).

Zhao et al. [87] documented a FeCuC-embedded carbon aerogel with ORR activity, demonstrating high efficiency in removing organic pollutants through the EF reaction. By rapidly oxidizing surface Fe⁰ to Fe²⁺, this material facilitated the generation of •OH from H₂O₂, resulting in a significant removal rate of TOC for various organic pollutants (88–99%). Moreover, other metals, such as Co²⁺ and Fe²⁺, were found to catalyze H₂O₂ effectively in an R-FPE system used for degrading 2,4-DCP [88]. The synergistic effect between Co²⁺ and Fe²⁺ led to the abundant production of •OH by transforming electrochemically generated H₂O₂. However, several challenges currently hinder the EF process, including the limited availability of selective and active catalysts, suboptimal H₂O₂ synthesis via the two-electron ORR pathway, the reduced efficacy of H₂O₂ decomposition through the Haber–Weiss cycle, and sluggish activation of catalytic metal sites [89]. Fortuitously, the emergence of single-atom catalyst (SAC) membranes has been shown to augment two-electron ORR, thereby facilitating the conversion of H₂O₂ into •OH via one-electron ORR. This advancement improves degradation efficiency and the system's stability [90]. Jin et al. [91] successfully developed a p-block BiN₃/MXene electrocatalytic membrane that proficiently executes a three-electron ORR to generate •OH from O₂. Without additional chemical reagents, this novel approach achieves a maximum production rate of 26.7 μmol L⁻¹ h⁻¹ cm⁻² for •OH and an observed rate constant (K_obs) for SMX degradation reaching 1.03 min⁻¹.

The cathodic surface can generate H₂O₂ through a two-electron reduction pathway or water via a direct four-electron pathway (equation (16)), depending on the type and intrinsic properties of the cathode material and other operating conditions. The selectivity of the cathode material for the ORR is crucial in achieving the in situ generation of H₂O₂ at the cathodic interface, particularly when compared to metallic substrates. Carbon-based materials are considered more suitable for the in situ synthesis of H₂O₂ due to their distinct features, including enhanced stability, minimal toxicity, affordability, corrosion resistance, and pronounced selectivity for the two-electron pathway in O₂ reduction. To activate O₂ efficiently, a large-pore graphene aerogel (GA) cathode with a substantial surface area has been utilized [92]. Abundant large pores served as reactive sites facilitating the efficient generation of •OH through the electrochemical decomposition of Fe²⁺, producing H₂O₂ (107.6 mg L⁻¹), resulting in a ciprofloxacin (CIP) TOC removal rate of 91.0%. Gu et al. [93] employed oxygen-doped CNTs (OCNT) as cathodes to catalyze O₂ activation for the degradation of organic pollutants.

Within O₂-activated EF systems, H₂O₂ and •OH are the primary ROS. However, •OH has a short lifespan and limited selectivity. In contrast, metastable ¹O₂, which possesses unoccupied π∗ orbitals, can selectively target electron-rich micropollutants through electrophilic addition. Therefore, investigating EF systems for generating ¹O₂ is of particular significance. Jin et al. [94] developed a fluidic single-atom electrode specifically designed for the selective electrocatalysis of O₂ into ¹O₂. This system achieved an impressive selectivity of over 98.0% for ¹O₂ production and a removal rate of SMX of 91.9%. Yang et al. [95] engineered a conductive electrocatalytic membrane with Fe(II)-modulated FeCo-layered double hydroxide nanosheets (Fe(II)-FeCo LDHs). These nanosheets effectively catalyzed H₂O₂ into ¹O₂, exhibiting high kinetic activity with a rate constant of 0.04 min⁻¹ and achieving a degradation efficiency of 93.2% for atrazine (ATZ). The selective production of ¹O₂ is crucial in precise chemical site targeting. Feng et al. [96] proposed a novel strategy to construct an oxidation–reduction interface on the electrode surface. They discovered that this strategy predominantly generated ¹O₂, effectively circumventing the competitive formation of other ROS. By leveraging a cutting-edge carbon-based electrocatalytic bilayer system, which takes advantage of the synergistic effect of dual active electrochemical membrane electrodes, they enhanced the production ¹O₂ [97]. The optimized production of ¹O₂ achieved a concentration of 2583.0 μmol L⁻¹ without additional chemical reagents. This system accomplished a propranolol (PRO) removal efficiency of 97.5%. Table 5 lists the recent studies on the degradation of contaminants in water or wastewater by the EF process.

Table 5.

The pollutants degradation based on the activation of O₂ by electro-Fenton process.

Cathodes	Anodes	Contaminants	Reaction conditions	O2 supply	ROS	Removal efficiency	TOC removal	Reference
FeCuC	BDD	BPA	[BPA]0 = 2.0 mg L−1, Na2SO4 = 50.0 μM, I = 30.0 mA, pH = 3.0	O2 aeration	H2O2, •OH	-	97.0%	[87]
GF-C	Pt	RhB	[RhB]0 = 60.0 mg L−1, Na2SO4 = 50.0 μM, j = 6.0 mA cm−2, pH = 3.0	O2 aeration	H2O2, •OH	99.0%	80.0%	[88]
Ti sheet	BiN3/MXene membrane	SMX	[SMX]0 = 10.0 mg L−1, Na2SO4 = 20.0 mmol L−1, I = 10.0 mA, pH = 3.0–9.5	O2 aeration	H2O2, •OH	>95.0%	60.0%	[91]
GA	Pt sheet	CIP	[CIP]0 = 50.0 mg L−1, Na2SO4 = 50.0 μM, j = 30.0 mA cm−2, pH = 3.0,	O2 aeration	H2O2, •OH	Almost 100.0%	91.0%	[92]
CrN4/MXene	Ti sheet	SMX	[SMX]0 = 10.0 mg L−1, pH = 6.8	O2 aeration	1O2	95.0%	61.2%	[94]
FE-ECM	Titanium plate	ATZ	[ATZ]0 = 20.0 μM, Na2SO4 = 100.0 mM, j = 2.0 mA cm−2, pH = 7.0	O2 aeration	H2O2, •OH, 1O2	93.1%	73.04%	[95]
Carbon aerogels	Pt foil	BPA	[BPA]0 = 10.0 mg L−1, Na2SO4 = 50.0 μM, pH = 3.0 and 9.0	O2 aeration	H2O2, 1O2, •O2−, •OH	98.0%	72.3%	[96]
MOF-SnO2@CF	Fe@CF membranes	PRO	[PRO]0 = 5.0 mg L−1, Na2SO4 = 10.0 μM, pH = 3.0	O2 aeration	H2O2, 1O2, •O2−, •OH	97.5%	83.0%	[97]

Abbreviations: FeCuC, iron-copper-embedded carbon aerogel; BDD, boron-doped diamond; GF-C, graphite felt-C; GA, graphene aerogel; FE-ECM, Fe(II)-modulated FeCo LDHs on a conductive OCNT-precoated membrane; MOF-SnO₂@CF, metal organic frameworks-SnO₂@ carbon fibers; Fe@CF, Fe@carbon fibers; BPA, bisphenol A; RhB, rhodamine B; SMX, sulfamethoxazole; CIP, ciprofloxacin; ATZ, atrazine; PRO, propranolol.

4.2. Photoelectro-Fenton process and photoelectrocatalytic process

The PEF process is an advanced oxidation method combining photocatalytic and electrochemical reactions. By integrating the properties of photocatalysis with electrochemical oxidation, the dual-functional PEF catalysts synergistically exploit photoelectric properties, effectively accelerating the recycling of Fe²⁺/Fe³⁺ in traditional electrochemical reactions, expanding the operational pH range, and simultaneously maintaining elevated degradation efficiencies (Fig. 3b). Bai et al. [98] developed a novel PEF-like system utilizing a bifunctional Z-scheme WO₃/g-C₃N₄ catalyst, which significantly accelerated the degradation of CIP. The PEF system exhibited superior efficacy in removing CIP compared to singular photocatalytic and EF systems, achieving complete elimination within just 2 h under visible light irradiation.

In the PEF process, various ROS can be generated through light and electric reactions. Wang et al. [99] developed an integrated system based on zinc-air batteries that incorporate polyterthiophene (pTTh) as a photo-enhanced electrocatalyst, achieving a high H₂O₂ production rate (34.8 mg L⁻¹ h⁻¹) and effectively degrading co-generated pollutants through efficient catalysis of the 2e⁻ ORR. Compared to the EF process, PEF generates •OH and H₂O₂ and produces active substances such as e⁻, h⁺, and •O₂⁻, resulting in a broader spectrum of ROS that facilitates the effective breakdown of stubborn organic pollutants. Du et al. [100] synergized UV irradiation and heterogeneous electro-Fenton (Hetero-EF) treatment by employing FeCu@PC porous carbon, adeptly reducing O₂ in the Hetero-PEF process with a TOC removal rate of 73.3% for SMT, which is 2.2 times higher than that achieved by the Hetero-EF process. Sunlight is the most cost-effective illumination compared to costly artificial light sources in PEF. Therefore, solar-powered PEF (SPEF) is a more viable alternative to conventional PEF. Ye et al. [101] discovered that SPEF outperformed electrooxidation (EO), EF, and UVA (ultraviolet with wavelength 320–400 nm) -assisted PEF in efficiently degrading bronopol under a consistent current density. After 360 min of operation, SPEF realized a mineralization rate of 94.0%. Pinheiro et al. [102] utilized Fe₃O₄ nanoparticles as heterogeneous catalysts to initiate SPEF, which exhibited a catalytic removal efficiency of 98.7% for fuel within just 90.0 min. Salmerón et al. [103] conducted an inaugural pilot-scale experiment of SPEF, meticulously engineering the experimental apparatus to optimize the electrocatalytic synthesis of H₂O₂. By fine-tuning the conditions, they achieved an H₂O₂ production rate of 64.9 mg min⁻¹ with a current efficiency of 89.3%. The SPEF process facilitated rapid degradation of over 50.0% of pesticide residues within a mere 5.0 min.

The PEC process combines the principles of photocatalysis and electrochemical oxidation, synergistically enhancing charge carrier segregation and reducing recombination events upon illumination (Fig. 3c). In contrast to the PEF paradigm, an electrical bias is strategically applied in the PEC system to mobilize photoelectrons for oxygen reduction at the cathode [104]. Simultaneously, photogenerated holes migrate to the anode's surface, generating highly oxidative and indiscriminate •OH, while electrons catalyze additional oxidants. Apart from •OH and hole, other oxidizing entities such as •O₂⁻ and perhydroxyl radicals are also present in the PEC system, as described by equations (17), (18), (19), (20), (21) [105,106].

$$\text{hv}+\text{semiconductor}\to {\mathrm{h}}^{+}+{\mathrm{e}}^{-}$$ (17)

$${\mathrm{h}}^{+}+{\mathrm{H}}_2\mathrm{O}\to •\text{OH}+{\mathrm{H}}^{+}$$ (18)

$${\mathrm{e}}^{-}+{\mathrm{O}}_2\to •{\mathrm{O}}_2^{-}$$ (19)

$$•{\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}\to •\mathrm{H}{\mathrm{O}}_2$$ (20)

$$2•\mathrm{H}{\mathrm{O}}_2\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$$ (21)

In the PEC-based WO₃/FPC system, an anodic potential was applied to the WO₃ photoanode, facilitating the segregation of photogenerated charges. Meanwhile, the F-doped porous carbon served as the cathode and was selectively designed to catalyze the reduction of O₂ to H₂O₂ [107]. This system measured an H₂O₂ generation rate of 0.87 mmol L⁻¹ h⁻¹ with a Faradaic efficiency (FE) of 75.0%. The degradation rates of phenol, BPA, and ATZ exceeded 90.0% within 60.0 min, achieving a mineralization rate of 40.0%. Kaushik et al. [108] pioneered the development of a Fe₃O₄@NiCO₂O₄ (FNCO) photoanode, which was subsequently integrated into a PEC system. The designed PEC water treatment system achieved nearly 60.0% TOC removal efficiency. Furthermore, compared with electrochemical oxidation processes that require higher cell potentials for organic compound mineralization [109,110], one notable advantage of PEC systems is their lower bias potential requirement. Liang et al. [111] proposed a novel separated PEC system utilizing black TiO₂ nanospheres (b-TiO_2−x) as the photoanode. This innovative system demonstrated exceptional water purification efficacy (98.3%) and H₂O₂ synthesis (6.8 μmol h⁻¹ cm⁻²), operating at an impressively low voltage of 0.5 V while maintaining minimal energy expenditure for wastewater treatment at only 0.03 kWh m⁻³. Zhang et al. [112] employed a magnetic Fe₃O₄@SiO₂@mesoporous TiO₂ (FST) photocatalyst-imbued electrode to implement a synergistic PEC/H₂O₂ process for the degradation of organic contaminants. In this FST PEC/H₂O₂ system, the removal rates of TOC within 8 min were found to be 77.9% (RhB), 80.2% (MO), and 65.5% (AMX). However, recent advancements in PEC technology have led to the development of a bias-free system capable of simultaneously synthesizing H₂O₂ and harvesting electrical energy [113]. Chen et al. [114] revealed that, upon visible light exposure, the WO₃/W photoanode induced a negative bias on the Pt/PVC photoelectrode while concurrently bestowing a positive bias upon the WO₃/W photoanode. The collaborative separation of hole-electron pairs across these dual photoelectrodes created an inherent bias. Jeon et al. [115] successfully synthesized pristine H₂O₂ solution through PEC by utilizing sunlight as the sole external energy input at zero external bias (0.0 V battery voltage) without any reduction in photocurrent. In PEC systems operating without external bias, water, oxygen, and sunlight alone are sufficient to achieve simultaneous wastewater purification and energy storage. This technology holds significant potential for advancements in both the environmental and energy sectors. Dong et al. [116] recently reported a groundbreaking study on a self-sustaining PEC system driven by solar energy without external biasing. The researchers successfully synthesized and utilized H₂O₂ in situ using synthetic leaf blades. Their findings indicated that by optimizing the coupling between the photoanode and cathode, they achieved an impressive unassisted production rate of H₂O₂ at 0.7 μmol min⁻¹ cm⁻². This achievement represents an efficient self-circulating light Fenton-like system that solely relies on oxygen, water, and sunlight to accomplish wastewater treatment and energy storage objectives. The activation of O₂ by the PEF and PEC processes for organic pollutant degradation are summarized in Table 6.

Table 6.

The activation of O₂ by photoelectro-Fenton process and photoelectrocatalytic process.

Cathodes	Anodes	Contaminants	Reaction conditions	O2 supply	ROS	Removal efficiency	TOC removal	Reference
Carbon felt	Pt	CIP	[CIP]0 = 50.0 mg L−1, pH = 3.0, Na2SO4 = 50.0 μM, 300 W xenon lamp	O2 aeration	H2O2, •O2−, •OH	100.0%	80.3%	[98]
CP@pTTh	Zn foil	RhB	[RhB]0 = 16.0 mg L−1, pH = 3.0, Na2SO4 = 50.0 μM, j = 195.0 mA cm−2, 300 W xenon lamp	O2 aeration	H2O2, •O2−, •OH	Almost 100.0%	-	[99]
FeCu@PC	RuO2	SMT	[SMT]0 = 80.0 mg L−1, pH = 3.0–7.0, Na2SO4 = 20.0 mmol L−1, I = 25.0 mA, 5 W UV lamp	O2 aeration	H2O2, •O2−, •OH	96.3%	73.3%	[100]
Co-free MWCNTs	BDD	Bronopol	[Bronopol]0 = 0.28 mM, pH = 3.0, Na2SO = 50.0 μM, Sunlight, j = 40.0 mA cm−2	O2 aeration	H2O2, •OH	94.0%	88.0%	[101]
GDE	Pt	SY dye	[SY dye]0 = 100.0 mg L−1, pH = 3.0, Na2SO4 = 100.0 mM, pH = 3.0, Solar irradiation, I = 100.0 mA	O2 aeration	H2O2, •OH	100.0%	71.0%	[102]
GDE	BDD thin film	MET	[MET]0 = 90.0 mg L−1, pH = 3.0, Na2SO4 = 50.0 mM, j = 100.0 mA cm−2	Air aeration	H2O2, •OH, 1O2	70.0%	32.0%	[103]
FPC	WO3	BPA	[BPA]0 = 10.0 mg L−1, pH = 3.0, Na2SO4 = 50.0 μM, 500 W xenon lamp, j = 1.0 mA cm−2	O2 aeration	H2O2, •OH	90.0%	40.0%	[107]
Carbon cloth	Fe3O4@NiCo2O4	RhB	[RhB]0 = 10.0 mg L−1, pH = 3.0, Na2SO4 = 0.1 M, j = 7.5 mA cm−2, 150 W SAIC visible light lamp	O2 aeration	H2O2, •OH	99.0%	60.0%	[108]
Pt	FST	SMT	[SMT]0 = 10.0 mg L−1, Acidic condition, Na2SO4 = 0.1 M, j = 7.7 mA cm−2, LED light	Air aeration	H2O2, •O2−, •OH	96.3%	73.3%	[112]
Mo-SACs/mrG	SnO2−x/BiVO4	MB	[MB]0 = 5.0 mg L−1, Acidic condition, Simulated solar light, j = 1.5 mA cm−2	O2 aeration	H2O2, •OH	99.5%	-	[116]

Abbreviations: CP@pTTh, carbon paper@polyterthiophene; FeCu@PC, FeCu@porous carbon; MWCNTs, multiwalled carbon nanotubes; BDD, boron-doped diamond; GDE, gas diffusion electrode; FPC, F-doped porous carbon; Mo-SACs/mrG, Mo single-atom catalysts/mildly reduced graphene; FST, Fe₃O₄@SiO₂@mesoporous TiO₂; CIP, ciprofloxacin; RhB, rhodamine B; SMT, sulfamethazine; SY, sunset yellow; MET, methomyl; BPA, bisphenol A; MB, methylene blue.

4.3. Microbial fuel cell process

The MFC represents an innovative device that utilizes a microbial metabolic process to convert organic waste and various biomass into electrical energy [117,118]. In this metabolic process, microorganisms facilitate the conversion of organic substrates into electrical energy. Traditionally, an MFC consists of two electrodes (anode and cathode), separated by an electrolytic medium (Fig. 3d). Microorganisms thrive on the anode's surface and engage in respiration utilizing organic compounds, generating electrons and protons. Electrons flow through an external circuit toward the cathode, where O₂ undergoes a two-electron reduction, leading to H₂O₂ formation. The anaerobic exoelectrogenic bacteria in the anodic chamber of the MFC are capable of extracellular electron transfer [119], commonly known as “exoelectrogens” or “electrogens,” including sulfate-reducing bacteria such as Pseudomonas aeruginosa, Shewanella oneidensis, and E. coli, among others [120]. These microorganisms significantly influence the oxygen reduction potential in an MFC.

An investigation focused on utilizing an MFC with graphite felt as an electrode and S. oneidensis MR-1 as the electroactive bacterium [121]. The MFC generated electricity in situ beneath the composite cathode, resulting in a peak H₂O₂ production of 135.9 mol L⁻¹, which subsequently reacted with leached Fe²⁺ to produce H₂O₂. At the same time, the system demonstrated a degradation efficiency of 78.3% toward triphenyltin chloride (TPTC). The presence of abundant microorganisms in the activated sludge effectively facilitated O₂ reduction. Soltani et al. [122] employed mixed active microorganisms collected from anaerobic digestion tanks in wastewater treatment plants as inoculum, using glucose as the carbon substrate. The findings revealed that after 8 h, the H₂O₂ concentration within the system reached 3.2 mg L⁻¹.

The electrode serves as a crucial site for microbial growth and adherence. The choice of electrode material significantly impacts the reduction of O₂ and the attenuation of pollutants. Wang et al. [123] developed an MFC utilizing acetylene black as the catalyst for H₂O₂ generation and Fe-Mn binary oxide as the catalyst for •OH generation. Compared to the EF process, the MFC exhibited a 38.0% increase in the CBZ elimination rate over 24 h. Rafaqat et al. [124] proposed a low-cost composite cathode composed of activated carbon/stainless steel (AC/SS) and graphite powder/stainless steel (GP/SS), where SS facilitated current distribution while AC/GP supported simultaneous H₂O₂ production. These dual-cathode variants achieved concurrent H₂O₂ generation at 1420.0 μM/2680.0 μM and an enhanced degradation rate for AR 114 of 96.0%/90.0%. Furthermore, the influence of factors such as proton exchange membranes, co-substrate diversity, MFC architecture, and hydraulic retention duration on in situ H₂O₂ synthesis within an MFC was uncovered. An MFC based on activated oxygen presents an economically viable and environmentally friendly technology for wastewater remediation and energy generation. However, the widespread implementation of MFCs faces challenges, including reduced current density, suboptimal conductivity, and inadequate H₂O₂ yield.

In summary, the newly developed methodologies for electrochemical oxygen activation exhibit significant advantages in pollutant remediation while also presenting technical and environmental challenges. The utilization of the Fenton reaction not only provides economic benefits but also reduces hazards associated with the handling, transportation, and containment of H₂O₂. Moreover, it effectively degrades pollutants by generating highly reactive •OH. However, practical implementation is hindered by high energy consumption and the requirement for supporting electrolytes. Additionally, the PEF process combines photocatalysis and electrocatalysis to enhance O₂ activation, resulting in increased production of ROS and improved capacity for pollutant removal. Nevertheless, this method necessitates simultaneous consumption of light and electrical energy and strict criteria for selecting the photocathode. The PEC process initiates O₂ activation through energy transfer and charge transfer processes. The efficiency of the entire process is determined by the choice of photoanode material, which presents a challenge in developing cost-effective and high-performing alternatives. MFC technology offers an economically and environmentally friendly approach by generating H₂O₂ within the cathode compartment, thereby reducing operational costs for wastewater treatment. MFCs utilize microorganisms to convert the chemical energy of organic compounds into electrical energy, representing a promising renewable energy technology. However, this technology faces limitations such as suboptimal H₂O₂ yield, diminished unit throughput, reduced current density, and inadequate conductivity. This investigation highlights that the efficacy of electrochemical activation processes heavily relies on acidic environments and access to pure oxygen (refer to Table 5, Table 6). Overcoming these challenges becomes crucial for advancing electrochemical oxygen activation.

5. Influencing factors of O₂ activation

The efficacy of O₂ activation methodologies relies on various factors, including the availability of O₂, reaction temperature, and pH level. Therefore, it is crucial to systematically optimize these parameters to enhance the efficiency of O₂ activation, improve pollutant removal capabilities, and establish a foundation for reducing operational expenditures.

5.1. Oxygen supply

Among the O₂ activation methods discussed in this paper, three types of O₂ supply are identified: air aeration, pure oxygen aeration, and dissolved oxygen in the solution itself. Based on the data presented in Tables 1 and it has been found that the primary source of O₂ during photocatalytic oxygen activation is the dissolved oxygen in the solution. These findings suggest these systems have demonstrated effective O₂ activation capabilities even under lower O₂ concentrations. This enhanced efficiency can be attributed to various modification strategies, such as defect engineering, doping, surface engineering, and ultrathin structures, which enhance the adsorption capacity of O₂.

Most studies in Table 2, Table 3 exhibit the use of air or pure oxygen aeration to activate O₂, indicating potential limitations in the efficacy of ZVMs and LVTMs for oxygen utilization. This may require high concentrations to activate O₂ and generate ROS, possibly due to the low adsorption capacity of ZVMs and LVTMs toward oxygen. The analysis in Table 4 reveals that dissolved oxygen is the primary source for activating O₂ through defect engineering, highlighting the crucial role of defect sites with a strong adsorptive propensity for O₂. Table 5, Table 6 further reveal that pure oxygen aeration is the principal method for the electrochemical activation of O₂, where purity significantly impacts H₂O₂ generation. However, the practical implementation of pollution control measures favors alternative approaches over pure oxygen aeration due to suboptimal oxygen utilization and substantial energy consumption losses, posing challenges for large-scale adoption.

During the aeration process, bubbles’ size plays a crucial role in activating O₂. Microbubbles (MBs) and nanobubbles (NBs), with their larger surface areas compared to large bubbles, facilitate faster diffusion rates of O₂ and longer hydraulic retention times [125,126]. Consequently, MBs and NBs can effectively provide water with a sustained oxygen supply. It is worth noting that MBs and NBs release a substantial amount of energy upon collapse, generating ROS [127]. For instance, Ghadimkhani et al. [128] elucidated that when NBs burst into molecular gas, they liberate considerable surface energy capable of converting O₂ into radicals.

In conclusion, air aeration, pure oxygen aeration, and dissolved oxygen within solutions are vital sources in activation systems that significantly contribute to the degradation of pollutants through ROS. Additionally, oxygenation efficiency and pollutant removal are profoundly influenced by the size of gas bubbles. Therefore, it is imperative to carefully select an appropriate method for supplying oxygen based on the specific requirements and specifications of the processing system.

5.2. Reaction temperature

As an important experimental parameter, reaction temperature significantly impacts O₂ activation. Typically, oxygen activation processes occur in ambient room temperature conditions. The reaction temperature has a bidirectional effect on both O₂ activation and the generation of ROS. Temperature increases can enhance pollutant mass transfer capacity and O₂ diffusion [129]. However, excessively high temperatures decrease O₂ solubility and accelerate the H₂O₂ self-decomposition rate [130].

5.3. pH

The pH level plays a crucial role in the activation of O₂. As shown in Table 1, the photochemical activation of O₂ is feasible under both acidic and marginally alkaline conditions. In contrast to acidic environments, alkaline settings can significantly reduce the oxidation efficiency of pollutants. Under strongly alkaline conditions, semiconductor material surfaces may become negatively charged, attracting holes and repelling electrons, thereby influencing their recombination at the surface. This phenomenon hinders the formation of ROS, ultimately resulting in a diminished capacity for pollutant degradation [131]. Table 1 demonstrates that mild alkaline conditions promote pollutant degradation, which may be attributed to the dissociation constant (pKa) of the pollutants. However, when the pH exceeds the pKa value of the pollutants, an excess of OH⁻ can induce electrostatic repulsion between pollutant anions and the negatively charged catalyst surface, impeding pollutant degradation efficiency [132].

The chemical activation of O₂ predominantly occurs in both acidic and marginally alkaline environments, providing informative background information (Table 2, Table 3, Table 4). It is well-known that acidic conditions are favorable for pollutant degradation due to the corrosion of zero-valent metals, which initiates the activation of O_2. Under acidic conditions, a lower pH facilitates the release of low-valent metal ions from zero-valent metals and the dissolution of oxide layers on their surfaces. However, it should be noted that excessively high pH levels impede the efficacy of pollutant degradation as metal ions quickly bind with OH⁻, forming adverse metal hydroxide precipitates that hinder ROS generation [71]. Moreover, compared to alkaline milieus, acidic conditions significantly elevate the redox potential of O₂/•O₂⁻, thereby increasing its propensity for conversion into •O₂⁻ in the presence of acid [133]. The importance of pH is further highlighted by its facilitation of the in situ synthesis of H₂O₂ during reduction processes involving proton participation. Conversely, alkaline conditions favor H₂O₂ disintegration and mitigate catalyst deactivation phenomena. Nonetheless, an overly diminished pH level within the solution may lead to alternative reactions and concurrently impact catalyst stability.

An acidic environment is essential for the optimal efficacy of the electrochemical activation process of O₂ (Table 5, Table 6). During this process, H₂O₂ serves as the main ROS generated. The production of H₂O₂ relies on the presence of •O₂⁻, a pair of protons, and a single electron. In an alkaline milieu, an abundance of OH⁻ persistently consumes protons, thereby hindering the formation of H₂O₂. However, excessively low pH levels impede the interaction between metal ions and H₂O₂, reducing diminished •OH production.

6. ROS generation path based on O₂ activation

O₂, known for its safety, environmental friendliness, and green properties, exists in the stable triplet oxygen (³O₂) with two unpaired electrons in its highest occupied molecular orbital (HOMO). The activation of O₂ involves the acquisition of electrons and energy, which weakens the oxygen–oxygen bond and leads to the generation of ROS [134]. This study elucidates three fundamental approaches for activating O₂: photocatalytic, chemical, and electrochemical methods. This article provides a comprehensive summary and analysis of each ROS production pathway, contributing to a better understanding of strategies for activating O₂.

•O₂⁻ is an important ROS involved in the mineralization of organic molecules and serves as an intermediate in the generation of other ROS. The predominant source of •O₂⁻ production is through the single-electron reduction of O₂, although the electron donor for this reduction varies depending on different activation strategies. In photochemical activation processes, O₂ is activated by photo-excited electrons in a semiconductor; however, the reduction of O₂ to •O₂⁻ via photo-generated electrons requires that the semiconductor's conduction band potential be lower than the standard redox potential of O₂/•O₂⁻ (E = −0.05 eV vs. normal hydrogen electrode (NHE)) [135]. Additionally, valence band holes can oxidize H₂O₂ and generate •O₂⁻. In chemical activation processes, electron donors such as ZVMs, LVTMs, and materials with defect engineering reduce O₂. ZVMs and LVTMs possess good reducibility because they can easily donate one or multiple electrons. Moreover, defect engineering involving OVs provides an abundance of electrons capable of reducing O₂, forming •O₂⁻. Unlike ZVMs and LVTMs, OVs can induce the adsorption and interaction of O₂ in their vicinity, influenced by localized charge effects, reduced surface energy, and other contributing factors. O₂ is sequestered at these OVs, leading to elongation of the O–O bond within the oxygen molecule toward its theoretical length of •O₂⁻ (1.26 Å) [80]. This elongation occurs as electrons are transferred from the OVs to the π2py∗ orbital of O₂, ultimately resulting in the formation of •O₂⁻. During electrochemical activation, O₂ can acquire electrons on the photoanode and cathode, thereby undergoing reduction to form •O₂⁻.

The in situ production of H₂O₂ plays a crucial role in the degradation process of organic pollutants. This is primarily achieved through oxygen reduction by photo-generated electrons, metals, defects, and the cathode surface. In photochemical processes, semiconductors can generate photoelectrons that reduce O₂ to produce H₂O₂. There are two distinct pathways for the reduction of O₂ to H₂O₂: the first entails a direct two-electron reduction (equation (22)), while the second involves a one-electron, two-stage indirect reduction sequence (equations (23), (24)). O₂ can directly gain two electrons and two protons to form H₂O₂. Additionally, O₂ initially undergoes a reaction to yield •O₂⁻ by accepting a single electron before assimilating an additional electron and two protons to form H₂O₂.

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ (22)

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to •{\mathrm{O}}_2^{-}$$ (23)

$$•{\mathrm{O}}_2^{-}+2{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ (24)

The band structure is critical to distinguishing between the two activation pathways. Considering that the standard potential for the two-electron transfer process is 0.68 V vs. NHE [136], O₂ activation can occur through a one-step two-electron transfer process if the catalyst's conduction band potential is below this threshold. Additionally, in the two-step single-electron transfer of O₂, •O₂⁻ acts as an intermediate species; therefore, introducing an •O₂⁻ scavenger can inhibit the single-electron transfer sequence. If introducing a scavenger leads to no formation of H₂O₂, it indicates that H₂O₂ synthesis exclusively occurs via the single-electron transfer pathway of O₂. However, ORRs do not always result in H₂O₂ formation. O₂ undergoes a four-electron reduction process at excessively high voltages, producing H₂O.

The •OH radical, known for its exceptional oxidative properties in eliminating organic pollutants, is formed through the reduction and hydrogenation of O₂ to •O₂⁻, followed by a subsequent conversion into H₂O₂ with a lower energy barrier. This electron reduction process facilitates a more favorable reaction pathway, leading to the subsequent reduction of H₂O₂ to •OH. Moreover, photo-induced holes in semiconductors can generate •OH through oxidation when the valence band position of the semiconductor aligns with the required standard potential (equation (25)).

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+}\to •\text{OH}+{\mathrm{H}}^{+}$$ (25)

O₂ exists in three electron configurations: two low-lying singlet excited states (O₂(1Δg) and O₂(1Σg⁺)) and a ground state triplet (O₂(3Σg⁻)) [137]. Since the energies of O₂(1Δg) (95.0 kJ mol⁻¹) and O₂(1Σg⁺) (158.0 kJ mol⁻¹) are higher than the ground state triplet, singlet excited states exhibit higher reactivity. When the spin restriction of ³O₂ is overcome, electrons join one of the orbitals to form “singlet oxygen delta” O₂(1Δg), a rare chemical species with high chemical reactivity [138]. The transition from the 1Δg state to the 3Σg⁻ state is spin-forbidden, making O₂(1Δg) a relatively long-lived species. O₂(1Σg⁺), on the other hand, has a shorter lifespan due to the allowed spin transition to the O₂(1Δg) state, so the term “singlet oxygen” mainly refers to O₂(1Δg) [139].

Based on the activation of O₂, the generation pathways of ¹O₂ can be broadly categorized into electron transfer and energy transfer (refer to Table 1, Table 6). In the electron transfer process, ¹O₂ can be produced through the redox reactions involving O₂ or radicals. Although H₂O₂ is capable of generating ¹O₂ through self-decomposition, this sequence occurs at a relatively slow rate. •O₂⁻ plays a crucial role as an intermediate species generating of ¹O₂. Potent oxidants possess the ability to directly convert •O₂⁻ into ¹O₂. Photo-generated holcan to interact with •O₂⁻ leading to the formation ¹O₂. The oxidative processes involving radicals such as •OH and •OOH further facilitate the conversion of •O₂⁻ into ¹O₂. Additionally, high-valence metal ions have a propensity for oxidizing •O₂⁻. These reactions can be observed in the photochemical, chemical, and electrochemical activation of O₂ (equations (26), (27), (28), (29)).

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to •{\mathrm{O}}_2^{-}$$ (26)

$$•{\mathrm{O}}_2^{-}+{\mathrm{h}}^{+}\to {}^1{\mathrm{O}}_2$$ (27)

$$•{\mathrm{O}}_2^{-}+2•\text{OH}+2{\mathrm{H}}^{+}\to {}^1{\mathrm{O}}_2+2\mathrm{O}{\mathrm{H}}^{-}$$ (28)

$$•{\mathrm{O}}_2^{-}+\mathrm{C}{\mathrm{o}}^{4+}/\mathrm{C}{\mathrm{o}}^{3+}\to {}^1{\mathrm{O}}_2+2\mathrm{C}{\mathrm{o}}^{2+}$$ (29)

The transformation of O₂ into ¹O₂ via energy transfer is currently limited to photochemical O₂ activation. In its ground state, the oxygen molecule exists as ³O₂, with the spins of its two unpaired electrons parallel [140]. When the spin states of these electrons change to become antiparallel, the oxygen molecule converts into ¹O₂. Under illumination, semiconductor photocatalysts generate excitons, electron-hole pairs induced by photon absorption. Before exciton–exciton annihilation (EEA), singlet excitons undergo intersystem crossing (ISC) and transition to energetically favorable triplet states [141]. This process involves a change in electron spin orientation, resulting in the formation of long-lived triplet excitons. The triplet excitons transfer energy to O₂, inducing a spin state change (spin flip) that generates ¹O₂ (Fig. 4). ¹O₂ is a high-energy form of oxygen with strong oxidizing power, enabling its involvement in diverse chemical reactions, including the oxidation of organic compounds.

Fig. 4.

The production mechanism of ¹O₂ by O₂ activation through energy transfer.

7. Conclusions and outlook

O₂, known for its environmentally friendly, non-toxic, and cost-effective properties, stands out as a superior oxidant in the sustainable degradation of contaminants. The activation of O₂ leads to the generation of ROS, which are instrumental in efficiently degrading and mineralizing various organic pollutants. This study presents a comprehensive analysis of recent advancements in utilizing oxygen as an oxidant for environmental remediation. Several strategies have been developed to activate O₂, including photochemical, chemical, and electrochemical methods. These approaches have shown remarkable outcomes in wastewater treatment. Notably, energy and electron transfer are the primary mechanisms underlying O₂ activation. Given its paramount importance in environmental remediation, there is significant interest among researchers regarding the activation of O₂. However, this field is still at an early stage, with numerous complexities and challenges that have yet to be addressed. Therefore, meticulous research is essential to provide valuable theoretical references for future investigations.

First, it is crucial to minimize energy consumption in applications to achieve energy-efficient and environmentally benign water purification technologies. The electrochemical activation process using pure oxygen aeration may have low oxygen utilization efficiency and high energy consumption. Therefore, exploring the use of ambient air or oxygen generated via water electrolysis as alternatives for the reaction becomes essential to address this research gap. Simultaneously, transitioning toward low-power light sources or solar energy should be considered wherever feasible for photochemical activation.

Second, intensifying research efforts on the selective generation of ¹O₂ is pivotal. ¹O₂ stands out due to its extended lifespan and selectivity as the sole ROS producible via both energy transfer and electron transfer mechanisms. Optimizing the activation process to enhance ¹O₂ yield is important while conducting comprehensive studies elucidating the underlying formation mechanisms during O₂ activation.

Moreover, comprehensive investigations are warranted to unravel the intricate activation mechanism of O₂ within complex aquatic ecosystems. Rigorous experimental studies are essential for unveiling the nuanced complexities of O₂ activation, aiming to achieve targeted pollutant removal and efficient resource utilization.

Ultimately, a versatile wastewater treatment system can be structured by synergizing diverse activation methods and leveraging their strengths. For instance, by synergistically integrating photocatalytic and electrocatalytic technologies, solar energy can be harnessed while concurrently regulating reactions via electricity, thereby amplifying the efficiency of the treatment process.

CRediT authorship contribution statement

Xiaohu Fan: Writing - Original Draft, Visualization, Software, Methodology, Investigation, Data Curation. Qiang Fu: Methodology, Investigation. Guorui Liu: Methodology, Investigation. Hongliang Jia: Methodology, Conceptualization. Xiaolong Dong: Visualization. Yi-Fan Li: Methodology, Formal Analysis. Song Cui: Writing - Review & Editing, Validation, Supervision, Resources, Methodology, Funding Acquisition, Conceptualization.

Declaration of competing interest

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