Utilizing cobalt-doped materials as heterogeneous catalysts to activate peroxymonosulfate for organic pollutant degradation: a critical review

Qiang Gao *, Guanshuai Wang , Yiren Chen , Bo Han , Kaisheng Xia and Chenggang Zhou *
Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China. E-mail: gaoqiang@cug.edu.cn; cgzhou@cug.edu.cn; Fax: +86 027 6788 3731; Tel: +86 027 6788 3731

First published on 31st May 2021

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Water impact A critical review of the recently published experimental parameters and results for the use of cobalt-doped catalysts for peroxymonosulfate (PMS) activation to degrade organic pollutants in wastewater is presented. The catalytic mechanisms involved in various cobalt-doped catalyst/PMS systems are discussed in depth, where the effect of substrate on the catalytic activity is emphasized. Knowledge gaps are identified and discussed, where despite the number of publications on this subject, more broad-reaching guidelines are needed for optimizing the applications of cobalt-doped catalyst/PMS systems in water treatment.

1. Introduction

With the rapid industrial development and population growth, water pollution caused by various organic chemicals (e.g., antibiotics and dyes) is increasingly becoming one of the most ubiquitous and severe threats to human health and the ecosystem.^1,2 Various technologies have emerged for treating organic pollutant-containing wastewaters, including adsorption, membrane separation, flocculation, extraction, biodegradation, chemical oxidation, and advanced oxidation processes (AOPs).^3–5 Among them, AOPs have drawn immense interest because they are characterized by the formation of powerful reactive oxygen species (ROS) such as hydroxyl (˙OH) and sulfate (SO₄˙⁻) radicals, which have proven to be extremely effective for removing organic pollutants from wastewater via degradation and/or mineralization pathways.^6–10 Besides, AOPs possess several unique advantages, which differentiate them from other technologies: 1) offering the possibility of organic pollutant removal without producing secondary pollution, 2) possessing the ability to tolerate harsh operational conditions, 3) low cost, and 4) capable of eliminating highly toxic and persistent organic pollutants.¹¹

Although there are a variety of methods in which AOPs generate ROS (e.g., photocatalysis, ultrasonication, microwave irradiation, and electrochemical processes),^12–15 the use of chemical oxidants is currently the most popular method. Hydrogen peroxide (H₂O₂), ozone (O₃), and persulfate (S₂O₈²⁻) have been intensively used as chemical oxidants to generate ROS for organic pollutant removal.^16–20 Recently, the new chemical oxidant peroxymonosulfate (PMS) was proposed by Dionysiou et al. for the degradation of organic pollutants.²¹ Commercially, PMS is available as a solid powder known by the trade name Oxone with the chemical composition of 2KHSO₅·KHSO₄·K₂SO₄ (Fig. 1). The structure of the PMS anion is HOOSO₃⁻, which can be regarded as a derivate of HOOH, where one H atom is replaced by an SO₃⁻ group. The basic physicochemical properties of PMS are summarized in Table 1. Since the pioneering work by Dionysiou et al., PMS-based AOPs have received increasing attention in the past few years, which is reflected by the increasing number of reports in the literature on this research topic, as depicted in Fig. 2. In general, PMS is a thermodynamically strong oxidant with a high redox potential of 1.82 V, but the direct reaction of PMS with most organic pollutants is very slow, and thus activation is usually required.²² A variety of activation methods, including the use of heat, UV irradiation, alkali, ultrasound, transition metal catalysis, and electrochemical process, have been applied for PMS activation.^23–29 Particularly, the use of transition metal ions (e.g., Co²⁺, Fe²⁺, Mn²⁺, Cr³⁺, and V³⁺) as activators of PMS has recently received significant interest due to its easy operation and high efficiency.³⁰ Besides, it requires no specific equipment, which is highly advantageous for large-scale applications. Among the available transition metal catalysts, cobalt (Co²⁺) has been proven to be the best, and the Co²⁺/PMS system shows an excellent degradation performance, which is even significantly superior to the classical Fenton system (Fe²⁺/H₂O₂).^30,31 Moreover, it should be noted that cobalt exhibits good cycling properties in PMS activation due to the higher standard potential (1.81 V) of Co³⁺ than that of other transition metal ions (e.g., Fe³⁺, Cu²⁺, Mn³⁺, and Mn⁴⁺). However, given that Co²⁺ is toxic and essentially a pathogenic metal ion, the homogeneous Co²⁺/PMS system has potential environmental problems.³²

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Thus, to overcome the drawbacks of the homogeneous Co²⁺/PMS system, numerous cobaltic oxides/hydroxides (e.g., Co₃O₄, CoO, Co₂O₃, and CoOOH),^37,38 cobalt-based binary oxides (e.g., Co₂MnO₄, CoFe₂O₄, and Co₂TiO₄),^39–41 and supported cobalt catalysts (e.g., Co/Al₂O₃, Co/TiO₂, and Co/SiO₂)⁴² have been developed as heterogeneous catalysts for PMS activation. Compared to the Co²⁺/PMS system, these heterogeneous cobalt-based catalysts have the significant advantages of low pollution and cost effectiveness due to their insolubility and good recyclability. Therefore, the use of heterogeneous cobalt-based catalysts represents a general trend in PMS-based AOPs. For a more detailed discussion on the use of the above-mentioned cobalt-containing materials for PMS activation, readers should refer to several recently published reviews.^43–46

Recently, many researchers have focused on the design of cobalt-doped materials as heterogeneous catalysts to activate PMS for organic pollutant removal. Similar to other heterogeneous cobalt-based catalysts, these insoluble cobalt-doped materials also possess the advantage of recyclability. More importantly, various solid materials can be chosen as substrates for cobalt doping, among which, metal oxides/hydroxides, carbon-based materials, and some specific solid substrates (e.g., metal organic frameworks (MOFs) and FePO₄) have been demonstrated as being capable of creating ideal microenvironments to promote the activity of cobalt sites. This means that the doped cobalt species may function as more favorable active sites for PMS activation due to the promotion or synergistic effect from the substrate, which can significantly reduce the dosage of the toxic Co²⁺ ion. Moreover, many substrates can well stabilize cobalt species to inhibit cobalt leaching, which is also fairly beneficial for practical applications. Therefore, the design and utilization of cobalt-doped catalysts for PMS activation provides a new important opportunity for the development of high-efficiency heterogeneous PMS-based AOP systems. However, to date, a comprehensive and systematic introduction to this subject is lacking, despite the considerable progress have achieved in recent years.

This critical review presents the classification of cobalt-doped catalysts based on the types of substrates for cobalt doping and summarizes their catalytic performances in the degradation of various organic pollutants in wastewater. The catalytic mechanisms involved in various cobalt-doped catalyst/PMS systems are discussed in depth, where the effect of the substrate on the catalytic activity is emphasized. Furthermore, the cobalt leaching in cobalt-doped catalysts and their durability are reviewed. It is worth noting that gauging the actual potential of this treatment technology as an effective substitute for the well-established processes in industry requires careful evaluation of multiple factors such as its operational cost, toxicological consequences, and engineering challenges. For instance, compared to some other redox-active metals (e.g., iron and manganese), cobalt is significantly more expensive,⁴⁷ which will lead to a high treatment cost for water purification. Moreover, given that cobalt is highly toxic,⁴⁸ the secondary contamination during catalytic activation should not be ignored. Although numerous reports state that the cobalt leaching amount is much less than the maximum permitted emission level (1 mg L⁻¹) within relatively short periods of time, the long-term operation of cobalt-based materials may result in the accumulation of toxic cobalt in water bodies. Besides, it should also be noted that cobalt-based materials sometimes cause an enhancement in the unwanted reaction pathways. For example, Co(III) as an oxidant is involved in the oxidative conversion of bromide into hypobromous acid or hypobromite as a key intermediate for the production of the potentially carcinogenic bromate.⁴⁹ Overall, considerable progress has been achieved in the development of cobalt-based material/PMS systems for water treatment, but there are still some limitations and barriers that need to be overcome to significantly promote their practical application.

2. Types of solid substrates for cobalt doping

To obtain a cobalt-doped material, the key is selecting a suitable substrate that can well accommodate the cobalt dopant. Thus far, many solid materials have been examined as substrates for cobalt doping, which can be roughly divided into three types, namely metal oxides/hydroxides, carbon-based materials, and other solid materials (e.g., MOF and FePO₄).

2.1 Metal oxides/hydroxides

Among the various metal oxides/hydroxides, titania, alumina, manganese oxide, ferric oxide, and some multicomponent oxides/hydroxides are preferable as substrates for cobalt dopants due to their stability, large surface area, and easy preparation. Doping cobalt into these solid substrates is normally achieved through the substitution of cations in the substrate lattice with Co²⁺ ions. The synthesis methods mainly include hydrothermal, sol–gel, coprecipitation, solution combustion, and ion exchange–calcination. It should be noted that the radius ratio of the host cation to the guest Co²⁺ ion has a significant effect on the distribution of the cobalt dopant. When Co²⁺ is smaller than the host cation, the guest Co²⁺ species will be preferentially located at the surface of the substrate,⁵⁰ which is beneficial for the Co²⁺ sites to improve their utilization. In some cases, cobalt doping can induce the generation of abundant oxygen vacancies, which may have a positive impact on the overall catalytic performance. More importantly, there is a common phenomenon that cannot be ignored, which is the cations of the substrate lattice may promote the redox properties of Co²⁺/Co³⁺ or directly participate in the process of PMS activation, resulting an enhanced catalytic performance. The catalytic mechanism of cobalt-doped metal oxides/hydroxides for PMS activation will be discussed in detail later. Table 2 summarizes the reported results in the literature on PMS activation by cobalt-doped metal oxide/hydroxide catalysts for the degradation of different contaminants.^50–63 It can be seen that the catalysts obtained by doping cobalt with different metal oxides/hydroxides can effectively activate PMS to degrade refractory organic pollutants such as antibiotics and dyes. The degradation efficiency is basically greater than 90% within relative short periods.

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2.2 Carbon-based materials

Carbon-based materials such as carbon aerogels,⁶⁴ N-doped porous carbon,⁶⁵ and N-doped graphene⁶⁶ are considered to be promising substrates for cobalt doping because of their unique merits, including chemical inertness, large specific surface area, excellent electrical conductivity, strong adsorption properties, and high availability. In general, the cobalt-doped carbon materials are prepared via the pyrolysis method using a cobalt salt and organic ligand as the cobalt source and carbon source, respectively. Unlike cobalt-doped metal oxides/hydroxides, cobalt-doped carbons usually contain Co(0) (some or even the total cobalt species) owing to the reduction of the cobalt ion by carbon at high temperatures. When cobalt-doped carbon is employed for PMS activation to degrade organic pollutants, there are commonly two advantages that other cobalt-doped materials lack. Firstly, the carbon substrates themselves have certain catalytic activity for PMS activation, and it is likely that there is a synergistic interaction between the cobalt species and carbon substrate to facilitate PMS activation. Thus, cobalt-doped carbons frequently exhibit a higher catalytic efficiency than normal cobalt-based catalysts. Secondly, carbon substrates usually have strong adsorption affinity toward various organic pollutants, which enables effective enrichment of organic pollutants on the surface of the catalyst, thereby leading to a superior removal efficiency. To further improve the catalytic performance of cobalt-doped carbon-based materials, there have been new attempts recently to introduce other heteroatoms (e.g., N, S, B, Si, and I) in the carbon substrate. It has been demonstrated that heteroatom-doped carbons seem to be better substrates than pure carbon due to the enhanced electronic transfer property and the possible synergism between Co and heteroatoms.⁶⁵Table 3 presents the key information about the PMS activation by cobalt-doped carbon-based catalysts for the degradation of refractory organic pollutants.^64–74 Obviously, it can be found that these cobalt-doped carbon-based catalysts show good catalytic performances, indicating their great potential in wastewater treatment.

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2.3 Other solid substrates

In addition to metal oxides/hydroxides and carbon-based materials, other solid materials including metal organic frameworks (MOFs),⁷⁵ metal phosphate (e.g., FePO₄),⁷⁶ hydroxyapatite (e.g., Ca₁₀(PO₄)₆(OH)₂),⁷⁷ metal sulfide (e.g., MoS₂),⁷⁸ carbon nitride (e.g., g-C₃N₄),^79–82 and mesoporous silica (e.g., MCM-41)⁸³ can be also used as effective substrates for cobalt doping to activate PMS. In most cases, cobalt doping in these solid substrates is implemented by cation substitution through the use of a small amount of cobalt salt and a large fraction of other metal salt (or silicate) as the precursors. For Co-doped g-C₃N₄, its synthesis is similar to that for Co-doped carbon materials, e.g., the typical hydrothermal–calcination method. Besides, some special methods have been reported for the synthesis of this type of cobalt-doped material. For example, Pang and coworkers synthesized Co-doped hydroxyapatite (Co-HAP) via a two-step method (i.e., ion exchange and calcination) to catalytically activate PMS for organic contaminant degradation.⁷⁷Table 4 summarizes the reported results in the literature on the PMS activation by this type of cobalt-doped material for organic pollutant degradation. Compared to the commonly used benchmark catalyst Co₃O₄, these catalysts basically exhibit better catalytic performances in PMS activation and organic pollutant degradation.

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3. Catalytic mechanism on PMS activation by cobalt-doped catalysts

Generally, the cobalt species in a cobalt-doped catalyst acts as the main active sites for PMS activation, while the substrate also plays a crucial role and largely affects the overall catalytic performance of the catalyst. Therefore, two factors (cobalt and substrate) should considered to better understand the catalytic mechanisms on PMS activation by various cobalt-doped catalysts. In the following sections, five possible catalytic mechanisms are reviewed and discussed. It is worth noting that the currently reported mechanisms are mainly concentrated on the generation of radicals and singlet oxygen, and other possible non-radical oxidation reaction pathways have not been considered. However, in some newly emerging homogeneous and heterogeneous cobalt-activated PMS activations, high-valent cobalt-oxo species and surface-bound reactive complexes are disclosed,^84–86 which contribute to high-performance organic pollutant degradation. For example, Zong et al. demonstrated that methyl phenyl sulfoxide (PMSO) could be readily oxidized to the corresponding sulfone (PMSO2) with a transformation ratio of up to ∼100% under acidic conditions, which is due to the generation of high-valent cobalt-oxo species [Co(IV)].⁸⁵ These investigations are significant, which may give insight into the design of new cobalt-doped materials for achieving superior catalytic performances.

3.1 Substrate-promoted cobalt redox for PMS activation

During the heterogeneous activation of PMS by cobalt species, the process roughly includes the following steps: PMS adsorption, electron transfer, desorption, and cobalt regeneration. Specifically, PMS activation is initiated by adsorbing PMS onto the surface of a cobalt-based catalyst, and then electron transfer from Co²⁺ to PMS will occur, leading to the generation of SO₄˙⁻ radicals, which are subsequently desorbed from the catalyst surface to the aqueous solution to attack organic pollutants. Simultaneously, the oxidized cobalt species can be regenerated by withdrawing electrons from PMS. During the PMS activation process, the electron transfer from Co²⁺ to PMS is the prime driving force for the generation of SO₄˙⁻ radicals. Thus, some studies focused on the selection of appropriate substrates to promote this electron transfer.

Pan's group utilized a Co-doped FePO₄ PMS activator to eliminate ATZ and proposed a reasonable mechanism in the catalytic process.⁷⁶ As shown in Fig. 3, the Co²⁺ dopant acts as the main active sites on the surface of Co-doped FePO₄ to induce the evolution of HSO₅⁻ to SO₄˙⁻. Particularly, the transformation of Co³⁺/Co²⁺ is facilitated through the charge redistribution between the Fe atom (from substrate) and Co atom, leading to an excellent catalytic performance for the degradation of ATZ. Our group reported the use of a novel hierarchical Co(II)-doped TiO₂ (hCTO) nanostructure for PMS activation.⁵⁰ It was confirmed that hCTO had significantly higher catalytic activity than the commercial Co₃O₄ (Fig. 4a and b, respectively). To understand the difference in activity between hCTO and Co₃O₄, cyclic voltammetry (CV) was conducted (Fig. 4c). It was revealed that the redox potential of hCTO (0.406 V) is lower than that of Co₃O₄ (0.435 V), suggesting that the oxidation process from Co²⁺ to Co³⁺ in hCTO is more likely to occur. Moreover, the CV size for hCTO (0.109 V) was significantly narrower than that of Co₃O₄ (0.136 V), indicating that the electron transfer was accelerated due to the presence of Ti in hCTO. Lim et al. employed cobalt-doped black TiO₂ nanotubes (Co-black TNT) for the activation of PMS.⁵² It was also demonstrated that there was a rapid redox cycle of Co²⁺/Co³⁺ due to the interaction between Co and TiO₂.

3.2 Two-site synergistic activation of PMS

Once the solid substrate for cobalt doping originally contains catalytically active elements (e.g., Mn or Fe), two-site synergistic catalysis will occur to achieve higher-efficiency PMS activation for organic pollutant degradation. For example, Luo et al. synthesized cobalt-doped BMO materials (Co-BMO) through a simple impregnation and calcination method, which showed very high catalytic activity for the activation of PMS.⁵⁴ The possible activation mechanism was proposed (eqn (1)–(10)), where the cobalt and manganese ions on the catalyst surface readily bind with water molecules and form Co–OH and Mn–OH intermediates, respectively. Then, the Co–OH and Mn–OH species combine with PMS through hydrogen bonding. Subsequently, the adsorbed PMS, as an electron acceptor, decomposes to release a strongly oxidative SO₄˙⁻ radical, accompanied by the oxidation of Co(II), Mn(II) and Mn(III) (eqn (1)–(3)). Meanwhile, PMS can also donate an electron to reduce Co(III), Mn(III), and Mn(IV) due to the low redox potential of HSO₅⁻/SO₅˙⁻ (E₀ = 1.1 V) (eqn (4)–(6)). Particularly, it is worth noting that the presence of Mn(III) and Mn(II) in Co-BMO can also contribute to the reduction of Co(III) due to the significantly lower redox potentials of Mn(III)/Mn(II) (E₀ = 1.51 V) and Mn(IV)/Mn(III) (E₀ = 0.15 V) than that of Co(III)/Co(II) (E₀ = 1.81 V) (eqn (7) and (8)), which further accelerates the PMS activation. Shi et al. reported a cobalt-doped manganese oxide octahedral molecular sieve catalyst (Co-OMS-2) for the degradation of diclofenac in the presence of PMS.⁵⁹ The results from the material characterization and degradation test suggested that the good catalytic efficiency should be meditated by the redox pair of Co(III)/Co(II) and Mn(IV)/Mn(III) in the catalyst. Wang et al. synthesized magnetic Co-doped Fe₃O₄@FeOOH nanocomposites for catalyzing PMS to eliminate methylene blue (MB).⁶⁰ The magnetic catalyst showed a high efficiency for MB removal in the presence of PMS, which was ascribed to the redox cycles of Fe³⁺/Fe²⁺ and Co³⁺/Co²⁺ in its structure. Moreover, it was also proposed that Co³⁺ can be easily regenerated through the reduction of Fe²⁺.

Co(II) + HSO5− → Co(III) + SO4˙− + OH−	(1)

Mn(II) + HSO5− → Mn(III) + SO4˙− + OH−	(2)

Mn(III) + HSO5− → Mn(IV) + SO4˙− + OH−	(3)

Co(III) + HSO5− → Co(II) + SO5˙− + H+	(4)

Mn(III) + HSO5− → Mn(II) + SO5˙− + H+	(5)

Mn(IV) + HSO5− → Mn(III) + SO5˙− + H+	(6)

Co(III) + Mn(II) → Mn(III) + Co(II)	(7)

Co(III) + Mn(III) → Mn(IV) + Co(II)	(8)

SO4˙− + H2O → ˙OH + SO42− + H+	(9)

SO4˙− + OH− → ˙OH + SO42−	(10)

3.3 Utilization of oxygen vacancies to activate PMS

It is well-known that oxygen vacancies can serve as important adsorption and active sites for heterogeneous catalysis, and thus strongly affect the reactivity of metal oxides.^87–89 Oxygen vacancies can be introduced deliberately via thermal treatment under a reducing or oxygen-depleted atmosphere, bombardment using high energy particles, and doping with metal or non-metal ions.^90–93 If there are abundant oxygen vacancies existing in a cobalt-doped catalyst, they may play a crucial role in the activation of PMS.⁹⁴ Lim et al. prepared Co-doped black TiO₂, which contained a large amount of oxygen vacancies, which could easily realize the chemical bonding of PMS.⁵² Oxygen vacancies are also effective oxygen ion conductors, which can help PMS easily realize Co²⁺/Co³⁺ redox cycles (eqn (11) and (12)).

	(11)

	(12)

where and O^×_O represent a doubly charged oxygen vacancy and the oxygen ion in an oxygen site on the Co-doped black TiO₂ surface, respectively. Li et al. reported the synthesis of Co-doped Bi₂₅FeO₄₀ and further utilized it as a PMS activator for acid orange 7 removal. Through the substitution of Bi⁵⁺ by Bi³⁺, oxygen vacancies were subsequently formed, which could be released to form active oxygen (O*) to generate ¹O₂ (one of the major reactive oxygen species in the degradation process).⁶² Zhang et al. synthesized a surface oxygen vacancy (VO)-rich cobalt-doped zinc ferrite (ZnFe_0.8Co_0.4O_2.4) nanocatalyst, which exhibited high activity for refractory pollutant degradation with PMS activation.⁶³ It was found that PMS could be adsorbed and trapped by the surface oxygen vacancies in the form of OI-Vo or OII-Vo, which facilitated the formation of SO₄˙⁻ or ˙OH radicals (Fig. 5).

3.4 Generation of singlet oxygen

Besides SO₄˙⁻ and ˙OH radicals, singlet oxygen (¹O₂) is usually involved in the degradation of organic matter in various PMS-based AOP systems via non-radical processes. As widely reported, the oxidation efficacy of SO₄˙⁻ or ˙OH radicals is significantly inhibited by the characteristics of water, such as the coexistence of halogen anions, HCO₃⁻ ions, and natural organic matter (NOM), due to the common radical trapping effect.⁹⁵ In comparison, ¹O₂ can maintain a relatively stable efficiency in the degradation of organic pollutants even in real wastewater, which seems to have a more promising application in water treatment.⁹⁶ The possible ways for the formation of ¹O₂ are as follows: a) ¹O₂ originating from dissolved oxygen activated by oxygen vacancies;⁹⁷ b) ¹O₂ derived from the conversion of O₂˙⁻;⁹⁸ and c) PMS can release ¹O₂via self-decomposition and this process can be accelerated by oxygen vacancies.⁹⁹ Following these ways, ¹O₂ has been found to be generated, and in some cobalt-doped materials/PMS-based AOP systems, it acts as the dominant reactive oxygen species toward the efficient degradation of organic pollutants. Pang's group used a Co-HAP/PMS system to treat rhodamine B (RhB)-containing wastewater and confirmed the important contribution of ¹O₂ to the degradation of RhB.⁷⁷ PMS can be decomposed to generate ¹O₂ with the assistance of the nucleophile (Nu) tetrahedral PO₄ on HAP. Then, the generated ¹O₂ can react with RhB, leading to the decomposition of the organic pollutant. Liu et al. used Co-MIL-53(Al)/PMS to remove tetracycline (TC) in water and revealed that the TC degradation was mainly ascribed to the generation of SO₄˙⁻ and ¹O₂.⁷⁵ Chen et al. employed a Co–MoS₂ NFs–PMS system to degrade ofloxacin (OFX).⁷⁸ In the quenching experiment, it was found that although the concentration of MeOH (200 mM) was approximately 80 times higher than that of PMS, the degradation rate still could reach 46.1%, indicating the contribution of ¹O₂ to the degradation of OFX. Yang et al. synthesized a cobalt-embedded waste diaper carbon (Co-WDC) to activate PMS for the degradation of bisphenol A (BPA).⁶⁹ It was demonstrated that the degradation proceeds through two pathways, namely the radical and non-radical pathways, where ¹O₂ is the dominant species in the second pathway. The reason for the generation of ¹O₂ is as follows. The carbon atoms around the Co–N coordination structure capture the electrons from the PMS adsorbed on the Co–N coordination structure, which induces PMS excitation to generate ¹O₂ (Fig. 6). Consequently, excellent BPA degradation was achieved by this oxidation system. Recently, Ye et al. fabricated ZIF-67-derived cobalt-doped carbon frameworks on a poly(vinylidene fluoride) (PVDF) membrane as a PMS activator for the degradation of BPA.¹⁰⁰ The scavenger experiments and EPR analysis verified that ¹O₂ participates in the degradation process. Sun et al. used an FeCo-MCM-41 catalyst for PMS activation and confirmed that ¹O₂ is the dominant reactive oxygen species in this oxidation system. Specifically, the chemisorbed oxygen (O*) transferred from the lattice oxygen can be converted into ¹O₂ in the presence of PMS.⁸⁷

For the ¹O₂-induced degradation of organic pollutants, the kinetic information (especially bimolecular rate constant k_¹O2 (M⁻¹ s⁻¹)) deserves special attention to better understand the catalytic mechanism. According to the literature,^101,102 the bimolecular rate constant for BPA oxidation by singlet oxygen is approximately 10^5.48 M⁻¹ s⁻¹ when BPA exists primarily in the form of phenol instead of the phenolate anion. Martinez et al. reported that the bimolecular rate constant for OFX by singlet oxygen was in the order of 10⁶ M⁻¹ s⁻¹.¹⁰³ Conversely, the observed rate constants (k_obs, min⁻¹ or s⁻¹) for the ¹O₂-dominated degradation of organic pollutants can be obtained experimentally. Importantly, there is a definite relationship between k_¹O2 and k_obs, which can be expressed as follows:¹⁰¹

kobs = k1O2[1O2]	(13)

where [¹O₂] represents the concentration of singlet oxygen (M⁻¹) in the degradation system. Therefore, by determining the concentration of ¹O₂ and further comparing the values of k_¹O2 and k_obs, one can easily estimate whether singlet oxygen is the dominant ROS or not in the degradation system.

However, in the reported works regarding cobalt-doped material-catalyzed PMS activation processes, no attempt has been made to accurately determine the ¹O₂ concentration and the ¹O₂ species generally coexisting with free radicals (SO₄˙⁻), thereby making it impossible to obtain the above-mentioned kinetic information to date. Thus, further efforts should be focused on conducting relevant investigations to gain profound insight into the ¹O₂-dominated catalytic degradation.

3.5 Carbon or carbon nitride-based substrate synergy with Co to activate PMS

Previous studies have shown that carbon-based materials (pure carbon or heteroatom (N, B, S, Si, I, etc.)-doped carbon) can be directly used as activators of PMS,^104–117 although they are less efficient than cobalt species. Carbocatalysis for PMS activation with the pristine carbon configuration, heteroatom functionality, defect degree (exposed edge sites and vacancies), and dimensional structure has been reviewed in the literature.^109–115 Thus, when carbon-based materials are used as substrates for cobalt doping, carbocatalysis should be considered. Furthermore, it should be also recognized that carbon-based substrates can act as efficient adsorbents for organic pollutants, which leads to the coupling of adsorption and catalysis, facilitating the degradation of organic pollutants. Thus, when cobalt-doped carbon materials are used for PMS activation, the cobalt species are usually the dominant active sites, but the carbon substrate also plays an important role in the activation process. For example, Zeng et al. constructed a cobalt-embedded N-doped carbon nanosheet for PMS activation.⁶⁸ It was confirmed that the carbon substrate with highly porosity and large surface area not only stabilized the cobalt nanoparticles but also greatly facilitated the accessibility of the substrate to the active sites through adsorption. Zhao et al. synthesized hierarchical cobalt/nitrogen-co-doped carbonaceous beads for PMS activation.⁷⁰ It was found that the abundant hierarchical pores of the carbon substrate made it possible to achieve the outstanding adsorption of reactants, leading to an acceleration in the combination of PMS and organic pollutants. Li et al. demonstrated single cobalt atoms anchored on porous N-doped graphene for PMS activation.⁶⁶ Their experiment and density functional theory (DFT) calculation results indicated that the CoN₄ site with a single Co atom acts as the active site for PMS activation, while the adjacent pyrrolic N site adsorbs organic molecules (Fig. 7). The dual reaction sites greatly reduce the migration distance of the active singlet oxygen produced from PMS activation, and thus improve the catalytic performance. In addition to carbon-based substrates, carbon nitride (e.g., g-C₃N₄) used for cobalt doping also severs as an efficient adsorbent of organic pollutants to promote their degradation. For example, Zhan's group used Co-doped g-C₃N₄ to activate PMS.⁷⁹ Obvious adsorption of organic molecules occurred on the substrate, which facilitated the degradation of organic pollutants.

When cobalt-doped carbon substrate is codoped with other heteroatoms, there is a possible synergism between Co and the heteroatom to achieve the enhanced activation of PMS. For example, Wang et al. constructed Co and N-codoped porous carbons (Co–N-PCs) for PMS activation. The experimental and theoretical results indicated the synergism between Co and N during catalysis. The N is mainly responsible for the production of the charged carbon sites for PMS adsorption, while the Co dopant facilitates the electron transfer from carbon to PMS for its decomposition to the SO₄˙⁻ radical.⁷¹ Du et al. synthesized the Co–S@NC nanomaterial as a catalyst for PMS activation.¹¹³ Their results indicated that the activation of PMS by the inherent N in the nitrogen-doped carbon (NC) was negligible, and the catalytic performance was mainly determined by the Co in the Co@NC catalyst. However, the Co–N moieties enhanced the adsorption toward PMS, which promoted the activation of PMS. Zhou et al. reported cobalt(0/II)-incorporated N-doped porous carbon as a heterogeneous PMS catalyst for the degradation of quinclorac (QNC).⁶⁵ The Co species were confirmed to be the dominant active sites, while the N-doped porous carbon enhanced the electron transfer and triggered a synergistic reaction between Co and the substrate. Recently, Gao et al. developed nitrogen-coordinated cobalt embedded in a hollow carbon polyhedron (NCoHCP) as a catalyst for PMS activation.¹¹⁴ It was confirmed that the formation of the Co–pyridinic N moiety in NCoHCP created Co atoms with a higher electron density and the C atom next to the pyridinic N with lower electron density as active sites, which increased the number of catalytic sites, and hence greatly enhanced the catalytic specific activity of NCoHCP for PMS activation toward organic pollutant degradation. In the case of carbon nitride as a substrate, the co-doping of cobalt and other heteroatoms can also possibly induce synergistic catalysis. For example, Wang et al. synthesized Fe–Co–O-codoped graphite carbon nitride for the degradation of sulfamethoxazole.¹¹⁵ This catalyst showed superior activity, which was ascribed to the synergistic effect of the metal oxides and O–g-C₃N₄. Specifically, O–g-C₃N₄ can act as a carrier, activator and electron mediator to promote the conversion of Fe(III) to Fe(II) and Co(III) to Co(II).

4. Durability of cobalt-doped materials as PMS activators

Besides catalytic activity, the durability of cobalt-doped materials as PMS activators is also crucial for their practical application.^42,74,116 When cobalt ions are doped in metal oxides/hydroxides, these cobalt ions mainly exist in the form of M–O–Co, where M represents the cation in the substrate. Many investigations showed that the binding interaction in the M–O–Co unit is stronger than that in the Co–O–Co unit, which makes cobalt-doped metal oxides/hydroxides have better durability and lower cobalt leaching than pure cobalt oxides/hydroxides. For example, Zhu et al. used cobalt-doped TiO₂ to activate PMS for the degradation of Red-3BS.⁵³ Compared to the Co₃O₄-immobilized TiO₂ catalyst, the cobalt-doped TiO₂ reduced half of the cobalt dosage but exhibited roughly equivalent catalytic activity; moreover, the cobalt leaching declined by about 60%. Our group also confirmed that the hierarchical Co-doped TiO₂ (hCTO), benefitting from the strong Ti–O–Co binding, was indeed an effective catalyst for improving the catalyst stability and inhibiting cobalt leaching.⁵⁰ Therefore, hCTO showed satisfactory reusability, retaining 95.3% of its initial activity after five cycles. Wang et al. employed Co-doped Al₂O₃ nanofibrous membranes for PMS activation, and the catalyst also showed low cobalt leaching due to the formation of strong Co–O–Al bonds.⁵⁶ Besides M–O–Co binding, the electrostatic interaction may also contribute to good durability. For example, Ding et al. synthesized Co-doped NaBiO₃ nanosheets for PMS activation.⁵⁷ Due to the strong electrostatic interaction, the released Co ions (if any) can be further captured by NaBiO₃ to minimize the cobalt leaching in the reaction solution.

For cobalt-doped carbon-based or carbon nitride-based materials, the substrate protection effect and/or strong cobalt-heteroatom interaction are two possible factors for their long-term durability. For example, Zhou et al. used cobalt(0/II)-incorporated N-doped porous carbon for PMS activation.⁶⁵ The embedded Co nanocrystals were protected by the carbon matrix, which led to significantly inhibited cobalt leaching. Yang et al. used Co-WDC to activate PMS for BPA degradation.⁶⁹ It was revealed that the carbon shell around metallic cobalt improved the stability of Co-WDC, leading to its satisfactory reusability. Lin's group developed a Co@CN catalyst for PMS activation to degrade amaranth (AMR).¹¹⁷ It was confirmed that Co nanoparticles were well embedded on the carbon nitride (CN) substrate, and more importantly, a thin shell was formed to wrap these Co nanoparticles. This embedding of Co nanoparticles and their thin shells were very beneficial to prevent leaching of the Co components, thereby resulting in stable recyclability. Wang et al. used Co–N-PCs for PMS activation and confirmed that the formation of Co–N coordination could effectively inhibit cobalt leaching due to its strong binding.⁷¹

5. Conclusion and outlook

The cobalt-catalyzed activation of PMS to generate sulfate free radicals is considered a promising method for water treatment. However, considering the toxicity of homogeneous cobalt ions, there is an urgent need for the utilization of efficient heterogeneous cobalt-based catalysts to activate PMS. Accordingly, cobalt-containing catalysts, including cobaltic oxides/hydroxides, cobalt-based binary oxides, supported cobalt catalysts, and cobalt-doped catalysts, have been developed. Among them, cobalt-doped materials as novel heterogeneous catalysts exhibit unique advantages. Firstly, doping cobalt in a solid substrate can effectively disperse the active sites, which helps enhance the utilization of the cobalt active sites, stabilize the cobalt species, and reduce the leaching of cobalt. Secondly, many solid substrates can provide a suitable chemical microenvironment for the cobalt element, and the catalytic activity is significantly improved through the synergistic effect between the solid substrate and the cobalt element. Thirdly, it can reduce the amount of cobalt while maintaining the catalytic activity, which not only reduces the cost but also meets the requirements of environmental protection, making the catalyst more suitable for practical applications. Nevertheless, more efforts should be made to promote the large-scale implementation of cobalt-doped catalyst/PMS systems in practical environmental cleanup.

a) Further research on the development of novel synthetic strategies and suitable solid substrates for the synthesis of cobalt doped catalysts is desirable. Especially, it is necessary to better understand the interaction between the solid substrate and the cobalt species and their cooperative behavior in PMS activation.

b) How to more effectively prevent cobalt leaching from cobalt-doped catalysts and how to further improve the efficiency of PMS are worthy of further investigation.

c) Heterogeneous catalysis for PMS activation still faces difficulties in separation and recovery. Thus, the development of new cobalt-doped catalysts and operating technologies such as magnetic-separable catalysts and monolithic catalysts is highly desirable.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the research grant provided by Natural Science Foundation of Zhejiang Province (No. LQY19B060001) and Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG170101).

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