Abstract

Water pollution caused by refractory organics has attracted widespread concern in recent years. At this time peroxymonofulfate (PMS) has been widely used to generate sulfate radicals with high reactivity and potential. The direct reaction rate between PMS and organics is very low. However, the activated PMS has a strong oxidizing ability on organics due to its conversion into sulfate radicals. Recently, the free radicals generated by oxidant PMS and catalyst biochar have proven to be an effective species in dealing with refractory organics. In order to enable researchers to better understand the current research status of PMS/biochar, and to promote the development and application of PMS/biochar system, we have written this review. This review in detail described the mechanism of PMS activated by biochar materials, and summarized the influencing factors of refractory organics degradation in the PMS/biochar system. In addition, the active sites of PMS/biochar, the degradation mechanism of refractory organics, and the reusability of biochar catalysts were also discussed. Finally, the concluding remarks and perspectives were made for future research on the PMS/biochar system in the degradation of refractory organics.

HIGHLIGHTS

  • The mechanism of free radicals generated by PMS/biochar was elaborated.

  • The influencing factors for biochar activation of PMS were analyzed.

  • The mechanism and effect of PMS activated by biochar modified materials were reviewed.

  • The non-radical oxidation mechanism of biochar materials on refractory organics was discussed.

  • The existing problems and development direction of PMS/biochar research were proposed.

INTRODUCTION

In recent years, research on the removal methods for refractory organics in water has aroused the interest of researchers (Chen et al. 2020; Tufail et al. 2020). For example, organic chlorides, organophosphorus pesticides, aromatic polycyclic and other long-chain organic compounds are all difficult to degrade using some conventional oxidation methods. Therefore, the advanced oxidation processes (AOPs) have received extensive attention for the degradation of such refractory organics due to the use of free radicals with strong oxidizing abilities to oxidize refractory organics (Cheng et al. 2016; Yang et al. 2019; Giannakis et al. 2021). In the AOPs, highly reactive oxygen species (ROS) such as hydroxyl radicals (•OH), sulfate radicals ( ), superoxide radicals ( ), and single oxygen species (1O2) are generated by catalyzing or activating the oxidants ozone, hydrogen peroxide, persulfate (PS), or peroxymonosulfate (PMS) by ultraviolet light, electricity, heat, transition metals, or carbon materials (Wang & Wang 2018; Liu et al. 2020a, 2020b; Xiao et al. 2020; Yu et al. 2020). Among these free radicals, •OH and play important roles under different catalysis and environmental conditions in the AOPs. The hydroxyl radical is a chemical species that has an unpaired electron on the oxygen atom, and it is characterized by a lack of one electron in comparison with hydroxyl as a stable species (Cheng et al. 2016). The hydroxyl radical is a type of non-selective strong oxidant with the oxidation potential of 2.8 V, which can destroy the structure of organic compounds and even mineralize them to a certain extent (Zhang et al. 2020). Compared with •OH, has higher redox potential (2.5–3.1 V) based on activation methods (Liu et al. 2020b). On the one hand, due to the fact that is better at attacking organic compounds through electron-transfer reactions, while the •OH more likely acts through the hydrogen abstraction or addition reactions, therefore exhibits superior selectivity towards organic pollutants than •OH (Giannakis et al. 2021). On the other hand, has higher selectivity and a longer half-life than •OH in certain cases (Mahdi Ahmed et al. 2012). Therefore, could be expected to show a better ability to degrade refractory organics.

Sulfate radical-based AOPs have been extensively studied in recent years, as reflected in published papers on the activation and application of PS and PMS (Wei et al. 2016; Karimian et al. 2020; Li et al. 2020b; Qiu et al. 2020). PS and PMS are strong oxidizers, with a redox potential of 2.01 V and 1.82 V, respectively (Wang & Wang 2018; Yang 2019; Yang et al. 2019). However, they react directly with organics at a low reaction rate. In order to generate stronger oxidizers, and •OH, appropriate activation is requisite for PS and PMS (Wang & Wang 2018). In addition, PMS usually can be decomposed by various catalysts more easily than PS due to the asymmetric structure of PMS. Therefore, there is a certain amount of research on activating PMS to degrade organic matter. PMS can be activated by a variety of methods, such as heat, UV, alkaline, metal ions and carbonaceous materials (Duan et al. 2016b; Yang et al. 2019; Karimian et al. 2020; Yu et al. 2020). Among these activation methods, metal oxide activation, especially the activation by iron oxides and cobalt oxides, has been widely studied. Although metal oxides can effectively activate PMS to produce for the degradation of refractory organics, they will cause secondary pollution such as toxic metal leaching. In recent years, more and more attention has been paid to the activation of PMS by carbonaceous materials (Fang et al. 2015; Wang & Wang 2019a; Huong et al. 2020; Shan et al. 2020). Compared with metal oxides, carbonaceous materials as promising metal-free catalysts have the advantages of no toxic pollution (Cherifi et al. 2019). Therefore, the PMS/carbonaceous materials system is a promising alternative process compared with the traditional methods based on •OH oxidation for refractory organics degradation. Biochar, as one of carbon materials, contains rich functional groups, which could activate PMS, and some research has been conducted to investigate the PMS activation by biochar for the removal of refractory organics. The reviews on using PMS or biochar in removing organics have been published, however, the review of PMS/biochar in the degradation of refractory organics has not been reported yet (Ghanbari & Moradi 2017; Wang & Wang 2018; Ruan et al. 2019; Wang & Wang 2019b; Wang et al. 2019; Selvaraj et al. 2020). In this paper, we explained the mechanism of biochar activation of PMS in detail, discussed the influencing factors of PMS/biochar reaction, inferred the active sites of biochar and biochar-based catalysts, clarified the degradation mechanism of organics by free radicals, evaluated the reusability of biochar catalysts, and proposed future research directions.

ACTIVATION MECHANISM OF PMS BY BIOCHAR AND BIOCHAR-BASED MATERIALS

Carbon materials have the advantages of relative large specific surface area, large pore volume, and low cost, thus they have been used as common water treatment materials. As a carbon-based material, activated carbon is widely used as adsorbents and catalysts in water and wastewater treatment systems. Biochar is another carbonaceous material which can activate PMS to degrade organics. Compared with activated carbon, biochar and biochar-based materials have their own advantages, such as rich carbon content, high cation exchange capacity, large surface area, stability structure and environmental friendliness (Lyu et al. 2020). In the last decade, more and more articles have been written about the application of biochar in water treatment, which proves that biochar is a potential water treatment material (Wang & Wang 2019b; Wang et al. 2019; Krasucka et al. 2021). Biochar has a saturated adsorption capacity when used as a pollutant adsorbent. Adsorption is the main mechanism of removing heavy metals and organic pollutants from biochar. The adsorption capacity of biochar is directly related to its physical and chemical properties such as surface area, pore size distribution, functional groups and cation exchange capacity, while its physical and chemical properties change with different preparation conditions (Wang & Wang 2019a; Shan et al. 2020; Pan et al. 2021). However, adsorption is a nondestructive process in which pollutants are transferred from one phase to another phase, and it is difficult to completely remove pollutants. By adding effective oxidant PMS, biochar can catalyze the formation of ROS which have been used to degrade refractory organics (Wang & Wang 2019b). At this time, efforts have been made to prepare biochar and biochar-based catalysts with enhanced catalytic properties to expand their potential applications. As shown in Table 1, for the removal of the organic pollutants, different performances were observed when different biochar materials were used as catalysts.

Table 1

PMS activated by biochar and biochar-based materials for the degradation of refractory organics

PollutantPollutant concentrationPMS concentrationBiochar styleBiochar dosage (g/L)pHTime (min)Removal (%)Reference
Bisphenol A 20 mg/L 5.0 mM Rice husk biochar 4.0 3.0 60 98.6 Huong et al. (2020)  
Tetracycline 20 mg/L 5.0 mM Rice husk biochar 4.0 3.0 60 94.1 Huong et al. (2020)  
Bisphenol A 20 mg/L 0.4 g/L Nitrogen-doped biochar nanosheets 0.4 7.0 20 100 Xie et al. (2020)  
Tetracycline 20 mg/L 1.0 mM N-doped graphitic biochar fiber 0.1 7.0 150 96.5 Ye et al. (2020)  
Bisphenol A 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 100 Li et al. (2020a)  
Tartrazine 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 87.0 Li et al. (2020a)  
p-Hydroxybenzoic acid 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 72.0 Li et al. (2020a)  
Sulfadiazine 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 98.0 Li et al. (2020a)  
Phenol 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 75.0 Li et al. (2020a)  
Methylene blue 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 99.7 Li et al. (2020c)  
Acid orange 7 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 100 Li et al. (2020c)  
Rhodamine B 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 100 Li et al. (2020c)  
Atrazine 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 100 Li et al. (2020c)  
Ciprofloxacin 0.03 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 78.3 Li et al. (2020c)  
Ofloxacin 50 μM 0.5 mM Co3O4 nanoparticles and biochar 0.2 7.0 10 90.7 Chen et al. (2018b)  
Metolachlor 10 mg/L 0.5 mM Nitrogen-doped biochar supported CoFe2O4 composite 0.2 9.0 40 100 Liu et al. (2019)  
Orange II 20 mg/L 0.5 g/L Graphitized hierarchical porous biochar and MnFe2O4 magnetic composites 0.05 5.8 95.0 Fu et al. (2019a)  
Triclosan 10 mg/L 0.8 mM Sludge-derived biochar 1.0 7.2 240 99.2 Wang & Wang (2019b)  
Reactive brilliant red X-3B 1 g/L 1.0 mM Food waste digestate 0.5 3.8 10 >99 Huang et al. (2020)  
Chloramphen-icol 30 mg/L 10 mM Biochar-supported Co3O4 composite 0.2 7.0 10 98.8 Xu et al. (2020a)  
p-Hydroxybenzoic acid 10 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 30 100 Fu et al. (2019b)  
Bisphenol A 10 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 100 98.0 Fu et al. (2019b)  
Tartrazine 50 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 100 95.0 Fu et al. (2019b)  
Ciprofloxacin 20 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 80 100 Fu et al. (2019b)  
Orange II 50 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 40 100 Fu et al. (2019b)  
1,4-Dioxane 20 μM 8.0 mM Pine needle biochar 3.0 6.5 250 100 Ouyang et al. (2019)  
Bisphenol A 10 mg/L 2.0 mM Nitrogen-doped biochar 0.5 6.3 100 Xu et al. (2020b)  
PollutantPollutant concentrationPMS concentrationBiochar styleBiochar dosage (g/L)pHTime (min)Removal (%)Reference
Bisphenol A 20 mg/L 5.0 mM Rice husk biochar 4.0 3.0 60 98.6 Huong et al. (2020)  
Tetracycline 20 mg/L 5.0 mM Rice husk biochar 4.0 3.0 60 94.1 Huong et al. (2020)  
Bisphenol A 20 mg/L 0.4 g/L Nitrogen-doped biochar nanosheets 0.4 7.0 20 100 Xie et al. (2020)  
Tetracycline 20 mg/L 1.0 mM N-doped graphitic biochar fiber 0.1 7.0 150 96.5 Ye et al. (2020)  
Bisphenol A 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 100 Li et al. (2020a)  
Tartrazine 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 87.0 Li et al. (2020a)  
p-Hydroxybenzoic acid 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 72.0 Li et al. (2020a)  
Sulfadiazine 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 98.0 Li et al. (2020a)  
Phenol 10 mg/L 0.5 g/L Biochar loaded with CoFe2O4 nanoparticles 0.05 7.4 10 75.0 Li et al. (2020a)  
Methylene blue 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 99.7 Li et al. (2020c)  
Acid orange 7 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 100 Li et al. (2020c)  
Rhodamine B 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 100 Li et al. (2020c)  
Atrazine 0.1 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 100 Li et al. (2020c)  
Ciprofloxacin 0.03 mM 2.0 mM Biochar-supported copper oxide composite 0.2 7.0 30 78.3 Li et al. (2020c)  
Ofloxacin 50 μM 0.5 mM Co3O4 nanoparticles and biochar 0.2 7.0 10 90.7 Chen et al. (2018b)  
Metolachlor 10 mg/L 0.5 mM Nitrogen-doped biochar supported CoFe2O4 composite 0.2 9.0 40 100 Liu et al. (2019)  
Orange II 20 mg/L 0.5 g/L Graphitized hierarchical porous biochar and MnFe2O4 magnetic composites 0.05 5.8 95.0 Fu et al. (2019a)  
Triclosan 10 mg/L 0.8 mM Sludge-derived biochar 1.0 7.2 240 99.2 Wang & Wang (2019b)  
Reactive brilliant red X-3B 1 g/L 1.0 mM Food waste digestate 0.5 3.8 10 >99 Huang et al. (2020)  
Chloramphen-icol 30 mg/L 10 mM Biochar-supported Co3O4 composite 0.2 7.0 10 98.8 Xu et al. (2020a)  
p-Hydroxybenzoic acid 10 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 30 100 Fu et al. (2019b)  
Bisphenol A 10 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 100 98.0 Fu et al. (2019b)  
Tartrazine 50 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 100 95.0 Fu et al. (2019b)  
Ciprofloxacin 20 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 80 100 Fu et al. (2019b)  
Orange II 50 mg/L 1.0 g/L Fe3O4 and porous biochar 0.2 2.8 40 100 Fu et al. (2019b)  
1,4-Dioxane 20 μM 8.0 mM Pine needle biochar 3.0 6.5 250 100 Ouyang et al. (2019)  
Bisphenol A 10 mg/L 2.0 mM Nitrogen-doped biochar 0.5 6.3 100 Xu et al. (2020b)  

PMS/biochar

Some organic wastes can be used as feedstock to produce biochars, such as municipal solid waste and agricultural wastes. Sludge is generated during the wastewater treatment process, and is a solid waste required to be treated and disposed of. However, it is a promising feedstock for biochar production because it contains rich carbon and nutrients (Wang & Wang 2019b). Rice husk, straw, wheat and pine needles all can be used to made biochars under the oxygen limited pyrolysis and the hydrothermal carbonization (Ahmad et al. 2012; Yavari et al. 2016; Ouyang et al. 2019; Huong et al. 2020). These different sources of biochars all can be used as activators of PMS.

It has been reported that the persistent free radicals (PFRs) on the surface of biochar could react with O2 to generate •OH without the addition of oxidants (Fang et al. 2015). With the addition of oxidant PMS, biochar catalysts could activate them to produce ROS. ROS are a key factor affecting the catalytic activation of PMS by biochar. Biochar can activate PMS to produce ROS •OH or which was reported to be effective in degrading organic pollutants such as sulfamethoxazole, 1,4-dioxane and bisphenol A (Liu et al. 2020a). Duan et al. (2016b) found that a higher pyrolysis temperature leads to an increase in the defective structure of biochar, and the activation ability of biochar to PMS also increases with its pyrolysis temperature. As the pyrolysis temperature of biochar increases, the sp3 carbon will be modulated and transformed, resulting in the collapse of the carbon skeleton and more defective structures (Duan et al. 2016b). Therefore, the PMS activation capacity of biochar was improved with the increase in charring temperatures. Initially, approached and adsorbed onto the surface of biochar. Then, the defect structures of biochars, such as edge defects, curvatures and vacancies can generate overhanging σ bonds so that its electrons are not limited by edge carbon, and the electrons of biochar can be donated to PMS to produce and •OH as in Equations (1) and (2). In addition, with lower oxidation potential could be generated from Equations (3) and (4) according to a previous study (Ghanbari & Moradi 2017), and could further form PS through a self-reaction shown in Equation (5):
formula
(1)
formula
(2)
formula
(3)
formula
(4)
formula
(5)

Huong et al. (2020) investigated that activation of PMS by biochar derived from rice husk (RBC) toward oxidation of tetracycline (TC) and bisphenol A (BPA) in wastewater. They found that TC and BPA could be effectively removed more than 90% at 0.2 g dose and 5 mM PMS within 60 min of reaction. The degradation of TC and BPA also showed excellent stability, and the loss of the removal efficiency was less than 15% after five cycles. The activation mechanism was explained that the PMS adsorb on the surface of RBC, then the electron from RBC move to PMS and generate •OH and as oxidizing species. In the next step, •OH and attacked these pollutants and degraded them into smaller intermediates that eventually become CO2 and H2O (Ghanbari & Moradi 2017). In addition, the study found that biochar derived from sludge can effective active PMS to produce (Wang & Wang 2019b). Wang & Wang (2019b) found that triclosan could be effectively degraded by the biochar activation of PMS. The results showed that the removal rate of 10 mg/L triclosan was close to 100% within 4 h under the conditions of pH 7.2 with 1.0 g/L biochar and 0.8 mM PMS, and found that dechlorination and hydroxylation were the main degradation pathways of triclosan. Under the effects of tertiary butanol, ethanol and sodium azide, the removal rate of triclosan decreased by 44.1%, 52.8% and 17.4%, respectively. The quenching experiment results showed that •OH, and 1O2 contributed to the degradation of triclosan. Noteworthily, the results showed that sludge-derived biochar can effectively activate PMS to degrade organic pollutants. The conversion of wastewater sludge to biochar could be a good choice for the treatment of wastewater sludge. At present, the biochar/PMS processes are only used for the efficient degradation of one kind of organic pollutant in the laboratory. There are often a variety of refractory organics in the actual wastewater. In the future, further studies are needed to investigate the performance of biochar for activation of PMS when some refractory organics exist simultaneously.

PMS/biochar-based materials

In order to regulate the properties of biochar, many methods have been used to modify biochar. The commonly used methods are chemical modification and physical modification. Chemical modification is the most widely used method. It mainly includes acid modification, alkalinity modification, oxidant modification, metal salt modification and carbonaceous material modification. Recently, activation of PMS by modified biochar for the removal of organic pollutants has received extensive attention. The commonly used modification methods are nitrogen doping and metal oxide doping which are used to activate PMS.

Nitrogen doping biochar

Nitrogen doping presents one of the most effective doping strategies to bestow active sites on biochar for PMS activation. The catalytic activity of nitrogen-doped biochar largely depends on the type of nitrogen source and the calcination temperature. A variety of N species conducive to redox reaction, namely pyrrolic N, pyridinic N and graphitic N can be injected into the biochar and adjusted to promote the PMS activation reaction (Wang et al. 2012). Xie et al. (2020) reported that molten salt induced nitrogen-doped biochar nanosheets (NCS-X) as highly efficient PMS catalyst for organic pollutant degradation. The NCS-6/PMS system has shown the removal effect of 100% in degradation of bisphenol A (BPA) within 6 minutes under rapid reaction kinetics (k = 1.36 min−1). Xie et al. (2020) proposed the removal of BPA in NCS-6/PMS system is attributed to the collaboration of non-radical pathways and free-radical pathways, and further confirmed that 1O2 is a dominant ROS while and •OH play secondary role for in BPA removing. For free-radical pathway, the sp2 carbon mesh and graphitization N of hybrid graphene-like nanosheets provide abundant free-flowing and unpaired electrons for NCS-6 which can activate PMS to produce a certain amount of and •OH (Li et al. 2017). Moreover, the pyridine N at the edge of the sheet is also responsible for the formation of and •OH as a Lewis alkaline site during redox processes (Guo et al. 2016). For the non-radical pathway, the neighbor carbonyl carbon atoms have strong binding affinity to the nucleophilic addition of PMS and promote the self-dissection of PMS molecules to generate 1O2 (Duan et al. 2018). Ye et al. (2020) fabricated a novel nitrogen-doped biochar for promoting PMS activation to degradation tetracycline (TC). In this study, a metal-free biochar-based catalyst derived from biomass fiber was prepared by graphitization and nitrogen incorporation (PGBF-N). The non-radical pathways were elucidated as the predominant pathways for TC degradation, instead of the dominant role of radical pathway in pristine biochar. Both and •OH participated in the reaction for certain degrees of degradation in this process. The heterogeneous catalysis of PMS was triggered by PGBF-N with a degradation rate seven times higher than that of pristine biochar. The high catalytic efficiency was attributed to the accelerated electron transfer originated from the high degree of graphitization and nitrogen functionalization of PGBF-N, in which the non-radical pathways contained carbon-bridge and singlet oxygen-mediated oxidation. Besides the PMS molecules obtaining electrons to generate free radicals, the positive charge on the carbon near the graphite nitrogen causes the PMS molecules to lose electrons through the nucleophilic reaction to form 1O2. Based on the catalyst characterization results before and after the reaction, it is proposed that the electron-rich ketonic functional group, graphitic N and defect sites on carbon network are possible active sites to promote the TC degradation. The ketonic C=O as Lewis basic sites with lone-pair electrons was more likely to effectively increase the electron density of the neighboring carbon ring and initiate the redox reaction. Graphitic N accelerates the transfer of electrons from adjacent carbon atoms and effectively destroys the inertness of the conjugated graphitic carbon network, and the positive charge increases of carbon atoms are favorable for PMS molecules to lose electrons to form 1O2 through nucleophilic reaction.

In general, nitrogen doping is considered to be one of the most promising dopings due to (1) the production of modified functional groups and defective sites, (2) the activation of π-electrons due to increased electron mobility in sp2 carbon conjugation (Chen et al. 2018c), and (3) the modification of the electron density N dopants of locally adjacent carbon atoms. However, due to the complexity of the components in the pristine biochar and the disorder of the structure amorphous carbon, the directional transfer of electrons cannot be effectively realized. Nitrogen doping only increases the disorder and uncontrollable structure of the resulting biochar. A more regular carbon configuration (sp2-hybridized carbon) with a high degree of graphitization is of great significant. Biochar fibers have abundant free-flowing π-electrons on graphitic structures and can act as an excellent electron-bridge to accelerate electron transfer. Doped nitrogen has lone-pair electrons and high electronegativity, which can further activate the electron flow and electron reconstruction of PMS molecules adsorbed. In addition, the reusability of metal-free catalysts is poor (Wang et al. 2017). The degradation of metal-free carbon-based catalysts depends on their adsorption capacity to PMS. Once the degradation product or intermediate occupies the surface site, the interaction between PMS and the catalyst is greatly reduced. Therefore, the stability of non-metallic carbon catalysts was generally not comparable with that of metal oxides (Sun et al. 2013).

Metal oxides-biochar

The metal oxides are deemed as efficient heterogeneous catalysts for PMS activation, such as Co3O4, Fe3O4, MnO2 and CuO. However, these metal oxides tend to aggregate or release toxic metal ions into water, which limits their application (Orooji et al. 2019). Therefore, some materials with stable structure and chemical properties, such as graphene, activated carbon and biochar have been used as a matrix to support metal oxides. In particular, CuO and CuO-based catalysts have attracted extensive attention as efficient, economical and low toxicity catalysts (Ji et al. 2014). Recently, Li et al. (2020c) investigated that activation of PMS by biochar-supported CuO composites (BC-CuO/PMS) toward degradation of organic contaminants in highly saline wastewater. In the BC-CuO/PMS system, the decomposition rate of methylene blue (MB) reached 99.68%, while the decomposition rates of single PMS,BC/PMS and CuO/PMS was 2.38%, 32.28% and 58.37%, respectively. The results showed that the BC-CuO catalyst had good PMS activation ability under highly salt conditions. In order to clarify the main ROS in the catalytic oxidation reaction, quenching tests were carried out. The results indicated that •OH, , and 1O2 were produced during BC-CuO activating PMS and 1O2 was the dominant ROS contributing to MB degradation in the salt system. The possible mechanism of BC-CuO activation in PMS degradation of pollutants was proposed, which included two pathways. Firstly, 1O2 produced by a non-radical pathway. On the one hand, the functional groups in biochars, such as C-OH and COOH can directly activate PMS to produce 1O2 through Equations (6) and (7). On the other hand, BC-semiquinone•− can transfer electrons to molecular oxygen to generate and quinone groups (BC-quinone) from Equations (8) and (9) (Fang et al. 2015). The second was a free-radical pathway to produce 1O2. The BC-CuO surface hydroxylated in the aqueous phase to form the active sites of Cu2+-OH that react with HSO5 to produce Cu+-OOSO3 which subsequently decomposed to generate 1O2 through Equations (10) and (11) (Xu et al. 2016). In addition, PMS self-decomposition can produce 1O2 slowly from Equation (12) (Yang et al. 2018a). Moreover, Cu2+ sites could react with to produce , which generated oxygen and from Equations (13) and (14) (Xu et al. 2016). The production of oxygen further promotes 1O2 production through accelerated BC-semiquinone•− transfer electron to produce :
formula
(6)
formula
(7)
formula
(8)
formula
(9)
formula
(10)
formula
(11)
formula
(12)
formula
(13)
formula
(14)

Chen et al. (2018b) reported that BC-Co3O4/PMS systems exhibited high ofloxacin (OFX) degradation ability. The derived BC-Co3O4 has a large specific surface area and small pore diameter. Therefore, more oxygen species were adsorbed on the BC-Co3O4, which was conducive to the formation of Co-OH groups. For the heterogeneous activation process by cobalt oxides, the surface active Co2+ and •OH played a key role (Ren et al. 2015). The metal ions firstly acted as sites and bonded with H2O to form •OH on the surface of BC-Co3O4. Then, the was combined with •OH through a hydrogen bond to form . The •OH could be moderately produced through the reaction between and OH. Both and •OH contributed to the degradation process.

Furthermore, the addition of adventitious nitrogen atoms to biochar can improve its catalytic activity. The doped nitrogen cannot only increase the surface alkalinity which is beneficial to the adsorption of PMS, but also promote the electron-transfer reaction with PMS by activating the neighboring sp2 carbon atoms. Liu et al. (2019) developed magnetic nitrogen-doped biochar (MNBC) catalysts prepared from agricultural biomass waste (rice straw). Efficient degradation of metolachlor (MET) by MNBC catalysts coupling with PMS was achieved. They found that the biochar modification could significantly promote the activity of Co3O4 due to the alteration of the surface chemical compositions (Chen et al. 2018b). Electron Spin Resonance (EPR) analysis revealed that , •OH and 1O2 were involved in the degradation process and that was the major contributor. In general, there are two pathways that occur during degradation, namely the free radical pathway and the non-free-radical pathway. Firstly, on the surface of CoFe2O4 nanoparticles, was generated through the redox reactions between Co2+ and PMS from Equations (15) and (16) (Yang et al. 2018b). The hydroxyl groups on the surface of biochar can be used as electron donors to accelerate Co3+ reduction to Co2+. The non-radical pathway was attributed to the graphitic nitrogen in the carbon framework, which produces the positively charged sites. Hence, the PMS are easily adsorbed and produce electron-transfer intermediates. The organic compounds were directly decomposed through electron-mechanism instead of by radicals (Chen et al. 2018c). Moreover, PMS self-decomposition can produce 1O2 slowly from Equation (17). It is worth noting that the reaction can be accelerated with the help of the MNBC catalyst:
formula
(15)
formula
(16)
formula
(17)

Noteworthily, compared with Fe and Mn, the application of a Co2+-containing catalyst is not the optimum choice because of its high toxicity and price. For cobalt-based catalysts, the secondary pollution caused by the released cobalt ions has attracted much attention. As a heavy metal, cobalt ions released into the ecosystem may be toxic and carcinogenic, leading to serious health problems such as asthma, pneumonia and cardiomyopathy (Shukla et al. 2010; Rivas et al. 2012). Fe and Mn elements with comparatively low toxicity are abundant in nature, so MnFe2O4 used for PMS activation have been widely studied in the degradation of organic pollutants (Ren et al. 2015; Huang et al. 2017). However, pure MnFe2O4 is easy to agglomerate in water, resulting in the decrease of the number of active sites and catalytic efficiency (Karami & Mousavi 2018). The performance of MnFe2O4 can be enhanced when it is supported on carbonaceous nanomaterials such as graphene, and carbon nanofiber (Liu et al. 2016). Fu et al. (2019a) prepared three novel graphitic hierarchical porous biochar (MX) and MnFe2O4 magnetic composites (MnFe2O4/MX) to degrade organic pollutants by PMS activation (Fu et al. 2019a). The quenching experiments and EPR characterization showed that the degradation of MnFe2O4/MX organic pollutants involves three pathways. On the one hand, a free-radical pathway ascribed to the delocalized π-electrons on graphene layers of MX to PMS was established generating and •OH (Duan et al. 2015b). On the other hand, a non-radical pathway ascribed to 1O2 generated by promoted self-decomposition of PMS was useds. Moreover, MX could also act as an electron-transfer mediator through graphitized structures. Such an electron transfer would be achieved by electron transfer from organic compounds as the electron donor to PMS as the electron acceptor.

In summary, modification of metal salts or metal oxides can alter the characteristics of adsorption, catalytic and magnetic properties. There are three reasons for the development of metal salt or metal oxide modified biochar. (1) To enhance the adsorption of targeted pollutants. Adsorption is the main mechanism for biochar to remove heavy metals and organic pollutants. Metal modification can change the surface properties of biochar, and further improve the adsorption capacity to heavy metals and organic pollutants. (2) To recycle biochar. Because of the small volume of biochar, it is difficult to recycle biochar from water when applying biochar to remove pollutants in water. Iron salt or iron metal oxide modification can improve the magnetic properties of biochar, which is beneficial to the recycling of biochar. (3) To enhance the catalytic characteristic of biochar. Due to its rich surface functional groups and good porous structure, various biochar-based functional materials have been designed. Generally, with biochar as a carrier, the supported catalytic materials can be dispersed and stabilized to improve their reactivity to the catalytic reaction.

INFLUENCING FACTORS OF PMS/BIOCHAR REACTION

Solution pH

It is well known that solution pH plays an important role in PMS decomposition (Wang & Wang 2018). In general, the pH value of the solution has a great influence on the degradation performance, because the pH value has a great effect on the generation process of free radicals and the morphology of organics. Therefore, the effect of solution pH on the catalytic reaction also investigated. Previous studies have shown that the near-neutral conditions are favorable for degradation compared with excessive acidic or alkaline conditions. Wang & Wang (2019b) conducted a research on the degradation of triclosan by sludge-derived biochar/PMS under different pH. The removal rate of triclosan increased with pH, and reached 99.2% when the pH increased to 7.2. But when the pH increased to 9.4, the removal rate of triclosan decreased to 49.6%. Chen et al. (2018b) synthesized biochar-supported Co3O4 composite (BC-Co3O4) as a catalyst and attempted to activate PMS to promote ofloxacin degradation. The study found that the degradation of ofloxacin was significantly inhibited with the increase of pH value.

Under acidic conditions, it has been demonstrated that was the dominant radical, and H2SO5 would be the primary substitute for because its second acid dissociation constant was 9.4 (Ren et al. 2015). Therefore, the generation of was hindered. It has been reported that under alkaline conditions, a large amount of •OH is produced and plays a major role in the degradation process (Equation (18)) (Nie et al. 2019). This finding was consistent with a previous study, in which generation became difficult in a MnO2/PMS system under alkaline conditions (Eslami et al. 2018). Furthermore, under alkaline conditions, the solution of metal ions also reacts easily with hydroxide ions. When the pH was above 10, a large amount of cobalt hydroxide complexes would be produced, resulting in the reduction of oxidation potential and catalytic activity (Chan & Chu 2009). Chen et al. (2018b) also reached similar conclusions on the degradation of ofloxacin(OFX) by using the BC-Co3O4/PMS system:
formula
(18)
The pH of the solution not only affected the free radicals in the solution but also determined the surface electrical properties of the solid catalyst (Deng et al. 2013). Huong et al. (2020) investigated the effect of pH on the oxidation capacity of rice husk biochar (RBC)/PMS systems (Huong et al. 2020). It was found that the degradation efficiencies for BPA and TC were 98.6 (pH = 3.0) and 95.7% (pH = 6.0), and 94.1 (pH = 3.0) and 92.9% (pH = 6.0), respectively. When the pH value is low (pH = 3.0), the surface charge of RBC shows a positive charge, so that the enhanced RBC adsorbs PMS and produces more , and •OH. When the pH of the solution increases (higher than pHpzc), the surface of RBC becomes negatively charged, leading to a weakening of the interaction between RBC and PMS. However, under the conditions of high pH solution (pH = 10), PMS became unstable, leading to less effect of the RBC/PMS system on the degradation of BPA and TC. Compared with the solution with low pH, the oxidative capability under high pH is weaker, resulting in lower degradation efficiency. Huang et al. (2020) found that the surface electronegativity of biochar was stronger under the conditions of higher pH value because large numbers of oxygen-containing functional groups are deprotonated at a higher pH value (Huang et al. 2009). In addition, the free radicals produced could be combined on the surface of biochar, X-3B (a representative azo dye pollutant) was degraded in stages on the surface, and the contaminant X-3B was an anion dye that could be easily attracted to the biochar surface at a lower pH value (Su et al. 2013).

There are controversial results that describe the effect of pH on the heterogeneous activation of PMS (Yin et al. 2018). This may be caused by the different dissociation constants of individual pollutant and the different isoelectric points of each catalyst. Therefore, the effect of solution pH on the catalytic activity is closely related to the type of pollutants and catalyst. Fu et al. (2019a) developed a new graphitized hierarchical porous biochar (MX) and MnFe2O4 magnetic composites (MnFe2O4/MX) as a catalyst for PMS. The catalytic performance of MnFe2O4/MS changed little over a wide pH range of 3–11(Fu et al. 2019a). Similarly, Xie et al.(2020) used nitrogen-doped biochar nanosheets (NCS-6) as a catalyst for PMS activation. The initial pH of the solution had a negligible effect on NCS-6. BPA could be effectively degraded within 6 minutes at a pH range from 3 to 9. Therefore, in order to achieve the best catalytic activity and pollutant removal, it is necessary to study the optimum pH value suitable for different catalysts.

The dosage of catalyst

As an activating tool for oxidant to generate free radicals, the catalyst biochar plays an important role in the biochar/PMS system. The dosage of biochar is closely related to the number of free radicals. A higher amount of biochar catalyst can provide more active adsorption sites, and then activate PMS to produce free radicals which can improve the removal efficiency of refractory organics. In contrast, low doses of biochar catalyst cannot provide sufficient active sites to activate PMS. For example, the removal efficiency of triclosan increased with the increase in the sludge-derived biochar dosage (Wang & Wang 2019b). When the dosage of the sludge-derived biochar was 0.1 g/L and 0.3 g/L, the final concentration of triclosan was 8.4 mg/L and 3.3 mg/L, respectively. When the sludge-derived biochar dosage was increased to 1 g/L, triclosan was completely removed within 120 min. The study showed that the concentration of the biochar-CuO is closely related to the removal efficiency of MB (Li et al. 2020c). When increasing the biochar-CuO concentration from 0.05 to 0.2 g/L, the MB removal efficiency increased from 90% to 100%.

However, some researchers believed that excess catalyst would inhibit the degradation process due to the quenching effect of the catalyst itself (Yan et al. 2011). With increase of catalyst dosage, more active sites appeared that can generated more and •OH on the surface of the catalyst, which may lead to the reduction of catalyst removal efficiency due to self-quenching (Equations (19)–(21)) (Li et al. 2018; Qin et al. 2018):
formula
(19)
formula
(20)
formula
(21)

Recent studies have shown that the degradation efficiency of organic pollutants increases with the increase of catalyst dose within a certain range. When the catalyst dosage exceeds a certain range, the degradation of efficiency will not increase, and may even decrease. With the increase of rice husk biochar (RBC) dosage from 0.05 g to 0.5 g, the degradation of organic pollutants was significantly enhanced (Huong et al. 2020). When the dosage of RBC was increased to 0.2 g within 60 minutes, the removal rate of BPA per unit weight of adsorbent was rapidly increased up 96.3. However, the removal rate of BPA was not further improved when the rice husk biochar dosage was above 0.2 g. Huang et al. (2020) found that when the dosage of food waste digestate-biochar was 0.5 g/L, more than 99% of reactive brilliant red X-3B could be removed, and most of X-3B could be decomposed within 1 minute. When the dose of food waste digestate-biochar was increased from 0.5 g/L to 1.0 g/L, the removal efficiency did not improve significantly. Xu et al. (2020a) synthesized the biochar-supported Co3O4 composite (Co3O4-BC) as a catalyst and attempted to activate PMS to promote chloramphenicol (CAP) degradation. The degradation efficiency of CAP was dependent on the load of Co3O4 and the dosage of Co3O4-BC. When the Co3O4 load was 10 wt% and 20 wt%, the removal efficiency was 97.6 ± 1.2% and 96.8 ± 0.1%, respectively. Increasing the load of Co3O4 has little effect on the final CAP removal level. In general, high nanoparticle loads lead to Co3O4 aggregation, which may be detrimental to the catalytic activity of Co3O4-BC. When the catalyst was further increased to 0.8 g/L, the removal rate decreased from 95.4 ± 3.0% to 84.6 ± 2.1% because of ineffective PMS consumption with excess catalysts (Guan et al. 2013). This finding was consistent with a previous study, in which excessive MnFe2O4 inhibits production efficiency in the MnFe2O4/PMS system, resulting in a decrease in pollution degradation efficiency (Xu et al. 2019). The amount of biochar catalyst is closely related to the economic cost. Xu et al. (2020b) and Li et al. (2020a) were able to completely degrade BPA at a concentration of 10 mg/L in a very short period of time. The former used 0.5 g/L nitrogen-doped biochar, while the latter used 0.05 g/L CoFe2O4 nanoparticles biochar. The amount of catalyst used in the former is 10 times that of the latter. Moreover, the production cost of the biochar catalyst must also be taken into account. Therefore, it is necessary to find that efficient and economical biochar materials.

The concentration of PMS

The concentration of PMS is another important factor influencing catalytic reactions. Generally, raising the concentration of PMS will increase the catalytic oxidation rate. However, excessive dosage of PMS may reduce the catalytic activity by scavenging already produced radicals. This is mainly due to the high level of self-quenching reaction of PMS, which leads to the conversion of and •OH to with the weaker oxidation capacity, thus reducing the degradation rate (Equations (22) and (23)). Similar results have been reported in a previous study (Ma et al. 2018):
formula
(22)
formula
(23)

Li et al. (2020c) synthesized that biochar-supported copper oxide composite (BC-CuO) and attempted to activate PMS for the treatment of organic contaminants in highly saline wastewater. The study found that with the increase in PMS concentration from 0.5 to 2 mM, the MB removal rate increased from 84.76% to 99.68%. In contrast, when PMS concentration was increased to 3 and 4 mM, the removal rate of MB decreased slightly. Fu et al. (2019b) evaluated that the effect of PMS dose on p-hydroxybenzoic acid (HBA) degradation. When the concentration of PMS was 0.5 g/L, 1.0 g/L and 2.0 g/L, The Kobs of the HBA degradation was 0.113, 0.171 and 0.272 min−1 respectively. When the PMS dose was further increased to 3.0 g/L, Kobs was reduced to 0.231 min−1. In addition, Wang & Wang (2019b) also found that the removal efficiency of triclosan increased with the increase in PMS concentration until to 0.8 mM. However, when PMS concentration was 1.2 mM, triclosan removal efficiency decreased sharply to 64.9%. These experiments confirmed that excessive PMS leads to quenching of sulfate radicals. Moreover, excessive PMS is not welcome by the water treatment system because it releases large amounts of sulfate ions. High concentrations of sulfate ions can corrode equipment and increase processing costs (Huang et al. 2018). Thus, according to the actual environmental situation and cost perspectives, it is necessary to reduce PMS to the minimum to obtain the best catalytic activity and pollutant removal.

Inorganic anions and humic acid

Inorganic anions widely exist in the real aquatic systems, and they can react with ROS and impact the performance of catalysts. It is necessary to study the effects of different inorganic anions (Cl, , and H2PO4) on the degradation efficiency of refractory organic pollutants.

Generally, low concentration of Cl could inhibit the degradation of organic pollutants. The inhibition impact of Cl is due to the fact that anions could eliminate free radicals and form weak radical species (Equations (24)–(27)) (Golshan et al. 2018). In contrast, at high concentration, Cl could be easily oxidized by PMS to form HOCl and Cl2 with strong oxidizing capacity (Yang et al. 2018a), which can further accelerate the degradation of organic pollutants (Equations (28) and (29)). Li et al. (2020c) testified that a large number of Cl2 and HOCl could better decolorize MB and facilitate the reaction, which kept the removal rate of MB at a high level:
formula
(24)
formula
(25)
formula
(26)
formula
(27)
formula
(28)
formula
(29)
The inhibition effect at low concentrations of was due to the fact that consumes and •OH to produce CO3•−, and its oxidation potential is lower than and •OH, thus inhibiting the oxidation reaction (Equations (30) and (31)) (Guo et al. 2018). However, when high concentrations of were present in the solution, the system would be adjusted to be an alkaline buffer (pH 8.5) (Duan et al. 2016a). This condition could promote the dissociation of to SO52− (Equation (32)), which was more easily activated in the heterogeneous reaction system (Duan et al. 2016a). All the above results were consistent with the previous report by Fu et al.(2019b):
formula
(30)
formula
(31)
formula
(32)
and H2PO4 can quench producing less oxidizing NO3 and H2PO4, which severely limited the removal rate (Equations (33) and (34)) (Guo et al. 2018). In addition, H2PO4 has a strong affinity for the active sites on the catalyst surface, which deactivates the catalyst surface and leads to reduced catalytic activity. Ouyang et al. (2019) showed that 1,4-dioxane degradation efficiency within the biochar/PMS system was decreased from 85.1% to 70.4% in the presence of 10.0 mM after a 240 minutes reaction. In addition, the study also found that H2PO4 severely inhibited the degradation efficiency of orange II by MnFe2O4/MS systems (Fu et al. 2019a):
formula
(33)
formula
(34)

It is well known that humic acid (HA) as a typical natural organic matter widely exists in the environment. Low concentrations of HA can promote the oxidation of catalyst and facilitate the degradation of organic matter. At low concentrations, the semiquinone groups in HA can activate PMS through a self-redox cycle to produce , thus accelerate the degradation of organic pollutants (Guan et al. 2013). When the concentration of HA increases to a certain level, HA will compete with the target contaminants for free radicals in the solution. In contrast, at high concentrations, HA could be deposited on the catalyst surface by strong π–π stacking, resulting in deactivation of active sites (Chen et al. 2018a). Fu et al. (2019b) found that the removal efficiency of the p-hydroxybenzoic acid (HBA) decreased significantly with the increase in HA concentration. When the HA concentration was set at 0, 1, 5 and 10 mg/L, the kobs of the HBA degradation was 0.171, 0.121, 0.084, and 0.067 min−1, respectively. A previous study has found similar results (Fu et al. 2019b). Therefore, the effect of HA on the catalytic degradation of organic pollutants in PMS should be comprehensively considered in practical application according to the concentration of HA.

It is worthwhile to state that the existence of various concentrations of inorganic anions in actual industrial wastewater, and whether it will seriously affect the treatment effect of organic matter, needs further study. In order to achieve the optimal degradation conditions, we need to carry out additional treatment in the actual wastewater treatment. Conversely, biochar/PMS processes can be combined with other AOPs to achieve a more efficient degradation effect.

ACTIVE SITES OF BIOCHAR AND BIOCHAR-BASED CATALYSTS

It is well known that biochar has a defective structure containing functional groups such as carbonyl (C=O), carboxylic (COOH) and phenol (OH) (Liao et al. 2014). Studies have shown that the C=O structure was a significant active site for the activation of PMS in biochar. The removal of organics through the free-radical pathway mainly relies on and •OH in PMS/biochar system. Some and •OH radicals were generated by the actions of C=O groups of the catalyst in the system. The ketonic C=O as the Lewis basic sites with lone-pair electrons has a strong affinity to bond with PMS, and breaks the O − O bond by adjusting electron transfer to produce and •OH. Recently, Huang et al. (2020) found that fresh and used food waste digestate-biochar (FWDB) had different effects on activating PMS. Compared with fresh FWDB, the content sp2 hybridized carbon of used FWDB decreased, and the content sp3 hybridized carbon increased. The disorder and defect reduction of biochar materials indicated that part of the sp2 hybridized carbon was converted into oxygen containing sp3 carbon atoms. This showed that graphitized carbon was involved in catalytic degradation. Compared with fresh FWDB, the C=O content in used FWDB decreased from 16.07% to 11.47%. This result indicated that the C=O structure may be the active site for the formation of 1O2. Previous study also found that the C=O functional group could promote the self-decomposition of PMS to produce 1O2 (Sun et al. 2017). In most instances, graphitized carbon was considered to be a good catalytic reaction to produce and •OH due to its excellent electron-transfer ability, and the basic mechanism of PMS activation is as an electron acceptor. These results showed that graphitized carbon and C=O structures were important active sites for the activation of PMS in biochar.

N-carbon can be used as important catalytic active sites in the catalytic reaction induced by PMS. It has been reported that only a minor amount of N doping (0.8%) can significantly enhance the decomposition ability of phenol and markedly change the oxidation path from free-radical to non-radical process (Duan et al. 2015c). Nitrogen element in nitrogen-doped biochar could play an important role in its catalytic performance. The study has shown that nitrogen atoms have a smaller atomic radius and are more electronegative than carbon atoms (Long et al. 2019). The properties of N can induce electrons to transfer from adjacent carbon atoms to nitrogen atoms, thus forming an N-doped structure with catalytic activity in biochar (Duan et al. 2015c).

Graphitic N and pyridinic N were considered as the catalytic sites in redox reactions. Pyridinic N at the edge of carbon structure with Lewis alkalinity could activate PMS to produce and •OH. Graphitic N could oxidize contaminant directly in the PMS system by electron transfer (Long et al. 2019). In addition, graphitic N and pyridinic N have higher electron-transfer ability. Graphitic N and pyridinic N could regulate charge distribution of adjacent carbon due to the higher electronegativity of nitrogen than carbon, and mainly generate positively charged C(+), which shows a strong affinity to bond with peroxide and then produce ROS or act as an catalytic active site (C-PMS) (Duan et al. 2018). Duan et al. (2018) also reported that graphitic N acted as a main active site to produce , which adsorbs PMS onto the adjacent positively charged C(+) and then breaks the O–O bond (Duan et al. 2015a). Guo et al. (2016) found changes of pyridinic N and activation of adjacent C in the acidic solution. This phenomenon clearly showed the involvement of pyridinic N in PMS oxidation processes, while the active sites were activated carbon atoms near to pyridinic N rather than the pyridinic N themselves. Recently, Xu et al. (2020b) found that nitrogen-doped biochar had a higher BPA degradation efficiency than the pristine biochar. This is because the strength of , •OH and 1O2 on nitrogen-doped biochar were much higher than those of , •OH and 1O2 on the pristine biochar. On the one hand, nitrogen doping makes biochar-rich defects and large numbers of pyridinic N and graphitic N, which is conducive to the formation of reactive oxidation species. On the other hand, it has been reported that pyridinic N and graphitic N are the active sites that activate PMS through free-radical and non-radical pathways, respectively (Chen et al. 2018c). Due to the synergistic effect of these two aspects, N-biochar can effectively activate PMS, thus leading to enhance the degradation of organic species through N-biochar/PMS system.

In general, although biochar has many possible active sites, the activation processes are not independent. Graphitic N and pyridinic N can provide a pair of p-electrons for graphite carbon, thus improving its activation performance. Nitrogen doping could decrease the electrostatic repulsion between biochar and PMS, which increases the possibility of collision between PMS and biochar, facilitating the transfer of electrons to PMS through the C=O structure, and producing more reactive oxygen species. Through the analysis of oxidation mechanisms, it was found that biochar has multiple active sites and a great potential for the activation of PMS. The specific contributions of pyridinic N and graphitic N need to be further studied, because other nitrogen species also increase with the increase in total nitrogen content. Moreover, the adsorption strength of different N on organic components is different, which also affects the whole catalytic process.

MECHANISM OF REFRACTORY ORGANICS DEGRADATION

In general, both non-radical processes and free-radical processes synergistically performed organic pollutants degradation and mineralization in the biochar/PMS system (Figure 1). The non-radical process mainly refers to the process generating singlet oxygen, and the free-radical process includes hydroxyl radicals and sulfate radicals acting on organic matter. These ROS have powerful oxidizing powers to attack organic pollutants, then degrade the organic pollutants into smaller intermediates and eventually mineralize into CO2 and H2O (Equation (35)):
formula
(35)
Figure 1

The mechanism of organic pollutant degradation using biochar/PMS.

Figure 1

The mechanism of organic pollutant degradation using biochar/PMS.

The sulfate radical is good at attacking organic compounds through electron-transfer reactions, while the hydroxyl radicals more likely do that through hydrogen abstraction or addition reactions. There are three main ways of radical attack on organic pollutants. (1) The role of hydrogen abstraction: BPA forms phenols by breaking the C–C bond between the two benzene rings because of sulfate radical attack (Li et al. 2020a). Then, some intermediates were further degraded by dehydrogenation oxidation to generate anthraquinones and cyclic ketones. These intermediates with large molecular weights continue to be degraded by ring-opening reaction. In addition, these intermediates were decomposed into alkanes, which are easy to degrade and less toxic than BPA (Olmez-Hanci et al. 2013). Hydrogen abstraction is the main reaction process between sulfate radical and alkanes, alcohols, organic acids, ethers, esters and other organic compounds. (2) The role of electron transfer: the reaction of sulfate radical with benzene ring and aromatic organics are mainly based on electron transfer. Electron transfer can form carbon center radicals on the benzene ring, which results in hydroxylation of the benzene ring. Furthermore, electron transfer caused by the sulfate radicals could also lead to dechlorination. It is reported that dechlorination and hydroxylation were the main degradation pathways of triclosan (Wang & Wang 2019b). (3) The role of addition reaction: for unsaturated alkenes, alkynes or compounds containing C=C double bonds, the main reaction process of sulfate radical with them is the addition reaction. The single electron of sulfate radical can actively attack unsaturated bonds and break them. Then, the addition reaction occurs between the intermediate and the sulfate radical, by which means the sulfate radical electron forms the electron pair with the broken electron. The degradation of aromatic pollutants usually involves three mechanisms containing radical addition, hydrogen atom abstraction and single electron transfer.

REUSABILITY OF BIOCHAR CATALYSTS

Reusability is an important consideration when evaluating the availability and application costs of catalysts. Therefore, the recyclability of catalysts has attracted much attention in practical applications. In general, the used biochar catalysts were recovered by centrifugation, and reused repeatedly after cleaning with deionized water and anhydrous ethanol (Li et al. (2020c). In addition, the biochar loaded with CoFe2O4 nanoparticles endows it with excellent magnetic properties, which is advantageous for the recyclability of the biochar (Gan et al. 2019). Hence, the biochar can be separated from the solution by an external magnet. However, these recycling methods were only used for the experimental phase. The novel recycling methods be used need further research for large-scale applications in industry. If the catalyst is not appropriately separated from the water body after the pollutant degradation process, the catalyst itself will cause secondary pollution to the water since some catalysts contain heavy metal elements.

It is well known that the degradation effect of catalysts exhibited slight decreases after several cycles. Similar results have been reported in previous studies with biochar catalysts (Ye et al. 2020; Zaeni et al. 2020). The decrease in performance is attributed to two factors including adsorption of intermediate products on the catalyst, and minor change in surface chemistry due to surface oxidation. The adsorption of intermediate products on the surface of the catalyst could lead to the decrease in active site and specific surface area, which blocked the active site for activating PMS (Xu et al. 2017). In addition, surface chemistry of the catalyst is important because it determines the active sites available for PMS activation. The oxidation of functional groups and carbon structure of biochar during the catalytic process will result in the decrease of removal performance. Therefore, the catalyst properties might be recovered to a certain extent through re-pyrolysis. A previous study has shown that the removal performance of the recycled biochar after secondary pyrolysis treatment was almost identical to the fresh biochar (Huang et al. 2020). Hence, pyrolysis treatment was considered to be a priority means of regeneration.

CONCLUSION AND PROSPECTIVE

In summary, we provide a recent review of the current development of refractory organics removal by activating PMS through biochar and biochar-based catalysts. Sulfate radical-based AOPs have promising potentials as effective treatment methods for refractory organics in water. Sulfate radicals have many advantages such as being more selective, high oxidation potential, and have longer lifetime than hydroxyl radicals. PMS is an effective and promising oxidant for the generation of sulfate radicals or hydroxyl radicals at different pH. PMS can be activated by biochar and biochar-based catalysts to produce many ROS. These ROS have powerful oxidizing powers to attack refractory organics, then degrade these organics into smaller intermediates and eventually mineralize into CO2 and H2O. There are many active sites on biochar and biochar-based catalysts, such as ketonic functional groups, graphitic N, pyridinic N, and the defect sites in the carbon network may be the active sites to promote organic matter degradation. The purpose of biochar modification is to obtain more active sites. At present, the commonly used modification methods are nitrogen doping, and metal oxide doping. The dosage of biochar catalyst, and the concentration of PMS and HA, and pH value, all have great influences on the degradation efficiency of refractory organics. In addition, there are large numbers of inorganic anions in real water body, such as Cl, , , and H2PO4, which react with reactive oxygen species and affect the activity of catalysts and the degradation efficiency of organic pollutants to a different degree. This review briefly describes the recovery methods of biochar catalysts, including centrifugation and magnetic separation. From the practical point of view, the stability and reusability of heterogeneous solid catalysts are very important. The reasons for the degradation of the catalytic performance of the used biochar catalysts were analyzed, and re-pyrolysis was the preferred method to recover the catalytic performance of biochar. It is worth noting that biochar may lose its activation capacity after a period of use due to surface deactivation. Therefore, it is necessary to investigate the catalytic stability of biochar.

In order to facilitate the application of biochar or biochar-based materials in activating PMS for the degradation of refractory organics in water, several specific questions need to be investigated:

  • (1)

    To develop economical biochar precursors. Biochar can be prepared using a variety of biomass sources, such as sludge, wood residue, wheat straw, rice straw and other crop straw. These have good catalytic activity and reusability for environment remediation. However, the economic performance of these materials as catalysts should be evaluated in the future.

  • (2)

    To further elucidate the activation mechanism of PMS by biochar. Biochar has complex functional groups and defect structures. The relationship between the structure of biochar and PMS activation needs to be further investigated. The understanding of the catalytic mechanism and functions of catalytically active sites will be of scientific significance in the biochar/PMS process.

  • (3)

    To develop highly efficient biochar and biochar-based materials as activators for PMS. The catalytic capacities of biochar and biochar-based catalysts forcefully depend on synthesis processes, including metal, metal oxide and nonmetal element doping. The catalytic activity of biochar activated PMS can be improved by modification. The synergistic effect of metal oxides and nitrogen-doped maybe further enhance the activation of PMS.

  • (4)

    To solve the problem caused by the existence of sulfate ions in water. The water treated by PMS contains a large number of sulfate ions. More efforts should be made to solve the problems caused by the existence of sulfate ions. On the one hand, a subsequent process after the PMS process to remove high concentrations of sulfate ions should be set up. On the other hand, the PMS dosage should be better optimized to avoid excessive PMS thereby reducing the number of sulfate ions in water.

  • (5)

    To investigate the performance of the biochar/PMS process in the actual wastewater treatment. The actual environment is more complex than the laboratory environment, which leads to the certain decrease of degradation efficiency. Therefore, more experiments are needed to elucidate the impact under true environmental conditions on degradation efficiency prior to biochar/PMS processes being applied.

ACKNOWLEDGEMENT

The research group acknowledges the financial support provided by Liaoning Province Education Department of China (No. JZL202015406).

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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