Biochar-Based Advanced Oxidation Processes for the Degradation of Organic Contaminants in Water

Fei Wu; Wei Ren; Cheng Cheng; Yan Wang; Heng Lin; Hui Zhang

doi:10.7536/PC210109

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (4) : 992-1010 DOI: 10.7536/PC210109

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Biochar-Based Advanced Oxidation Processes for the Degradation of Organic Contaminants in Water

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Abstract

Carbonaceous materials with superior catalytic activity, which could avoid drawbacks of heavy metal ion leaching for metal-based catalysts, are widely used in advanced oxidation processes (AOPs). Biochar, a carbon-rich material produced by pyrolysis of biomass under oxygen limited condition, is low-cost, widely available, and environmentally friendly. Biochar has been utilized to activate peroxides such as hydrogen peroxide, peroxymonosulfate and peroxydisulfate for the degradation of pollutants in water. In this paper, precursors and preparation methods of biochar as well as their influence on catalytic activity of biochar are discussed. The activation mechanisms of peroxides by biochar and the effects of water matrices on the degradation of pollutants are summarized. The progress in the modification of biochar is reviewed. The reusability of biochar is elucidated and the regeneration methods of biochar are provided. In the end, the problems and the prospects of the biochar-based AOPs are put forward.

Contents

1 Introduction

2 Biomass precursors

2.1 Lignocellulosic biomass precursors

2.2 Non-lignocellulosic biomass precursors

2.3 Effects of biomass precursors on catalytic performance of biochar

3 Preparation methods of biochar

4 Activating mechanism of peroxides by biochar

4.1 Radical mechanism

4.2 Non-radical mechanism

5 Effects of water matrices

5.1 Effect of pH

5.2 Effects of anions and NOM

6 Modification of biochar

6.1 Acid/base modification

6.2 Graphitizing modification

6.3 Doping modification

7 Reusability and regeneration of biochar

7.1 Change of physical properties

7.2 Change of chemical properties

7.3 Regeneration methods

8 Conclusion and prospection

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Keywords

biochar / advanced oxidation processes / peroxide / radical mechanism / non-radical mechanism

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Fei Wu, Wei Ren, Cheng Cheng, Yan Wang, Heng Lin, Hui Zhang. Biochar-Based Advanced Oxidation Processes for the Degradation of Organic Contaminants in Water. Progress in Chemistry, 2022, 34(4): 992-1010 DOI:10.7536/PC210109

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1 Introduction

With the rapid development of social economy, the current situation of water pollution is becoming more and more serious. Among the pollutants in water, endocrine disruptors, drugs, personal care products, dyes and other organic pollutants are persistent and difficult to be biodegraded, and traditional biochemical methods are difficult to meet the environmental protection needs of degrading such pollutants^[1]^[2]^[3]^[4]. Therefore, it is urgent to find an efficient treatment technology.

Advanced oxidation processes (AOPs) activate oxidants by adding energy or catalysts to produce active species with strong oxidation ability to degrade these pollutants, which has attracted more and more attention in the field of wastewater treatment^[5⇓~7]. At present, the commonly used oxidants mainly include peroxides such as hydrogen peroxide (H₂O₂), peroxymonosulfate (

HSO 5 -

, PMS), and peroxodisulfate (S₂O₈^2-,PDS). Various activation modes such as ultraviolet light, ultrasound, heat, and transition metals have been reported^[8]^[9]^[10]^[11]. Among them, transition metal catalysts have attracted the attention of researchers because of their simple operation and good catalytic activity^[12]. Dionysiou et al. Studied and compared nine transition metal ions (Ag⁺, Ce³⁺, Co²⁺, Fe³⁺,Fe²⁺ 、Mn²⁺ 、Ni²⁺ 、Ru³⁺ and V³⁺) to activate the effects of H₂O₂, PMS, and PDS^[13]. However, transition metal catalysts have disadvantages such as environmental toxicity (e.g., Co) and high price (e.g., Ru)^[14]^[15].

Biochar (BC) is a kind of carbon material prepared from agricultural and forestry wastes, municipal sludge, food waste and other waste biomass, which can avoid the precipitation of toxic metals in metal catalysts, and can show good catalytic activity and stability under acidic, alkaline and neutral conditions^[16]. The application of biochar in wastewater treatment has the advantages of low cost and environmental protection. At the same time, biochar has ideal pore structure, large specific surface area, abundant surface functional groups and mineral elements, and has good catalytic potential^[17]. Therefore, there are more and more research reports on the degradation of organic pollutants in water by biochar activated peroxide. In this paper, the research progress in this field in recent years was reviewed, the precursor types of biochar, the preparation methods and their effects on the activation ability of biochar were summarized, and the mechanism of biochar activation peroxide system was summarized.The influence of water quality conditions on the degradation of pollutants was analyzed, the modification methods of biochar were summarized, the reasons for the poor reusability of biochar were revealed, and the corresponding regeneration methods were introduced.Finally, the follow-up research was prospected.

2 Biomass precursor

2.1 Lignocellulose precursor

Traditional lignocellulose precursors mainly include agricultural wastes, forestry wastes (such as hardwood) and so on. The lignocellulosic feedstock consists of cellulose (25 wt% – 50 wt%), hemicellulose (15 wt% – 40 wt%), lignin (10 wt% – 40 wt%) and some minor inorganic minerals^[18]. During pyrolysis, lignin is the main biochar precursor, and its highly aromatic structure increases the yield of biochar, while the pyrolysis volatilization of cellulose can promote the formation of biochar pore structure. Lignocellulosic biomass is higher in oxygen, carbon, and hydrogen but lower in nitrogen than non-lignocellulosic precursors such as algae^[19].

2.2 Non-lignocellulosic precursor

In addition to traditional lignocellulosic precursors, municipal sludge, food waste, livestock manure, algae, and yeast have also been used as biochar precursors in recent years (Table 1)^{[20⇓⇓~23]}^[24⇓~26]^[27,28]^[29⇓~31]^[32,33].

Municipal sludge is a by-product of wastewater treatment containing a large amount of water^[34]. The carbon content of the dehydrated sludge is 25 wt% ~ 70 wt%, while the ash content can reach 15 wt% ~ 50 wt%, so the prepared sludge biochar usually has a low surface area due to ash clogging^[19]. At the same time, there are heavy metals such as Fe in the ash, which is conducive to the activation of peroxides, but the leaching of heavy metals will lead to secondary pollution of water^[35,36]. Therefore, inhibiting the leaching of heavy metal ions is one of the key problems to be solved in the practical application of sludge biochar. The research of Chen et al. Showed that the sludge biochar prepared at a pyrolysis temperature of 1000 ℃ had the lowest heavy metal dissolution during the activation of PDS, which was lower than the heavy metal discharge standard of China (GB 5085.3-2007)^[37].

Food waste is a major kind of municipal waste, accounting for 1/3 of the daily output of municipal waste^[38]. Food waste is rich in organic carbon. During pyrolysis, the vaporization of low-boiling organic matter and the decomposition of minerals can make biochar form developed pores and have a large specific surface area^[39]. Food waste contains less heavy metals than municipal sludge^[40]. The biochar prepared by Huang et al. From pig bone has a porous hierarchical structure, which can accelerate the electron migration in the carbon layer, and the rate of electron migration in the carbon layer is one of the important factors affecting the activation ability of biochar^[41].

Livestock manure emissions are increasing due to the growing demand for milk and meat. In 2017, the discharge of livestock manure in China has reached 3.8 billion tons. The use of antibiotics in the field of livestock breeding leads to their discharge into the environment with livestock manure, which will cause harm to the ecological environment if not properly disposed of^[42]. Livestock manure is usually disposed of by composting and anaerobic digestion, but neither of these methods can completely remove antibiotics and pathogens from livestock manure. Pyrolysis of livestock manure into biochar at high temperature can not only completely decompose antibiotics and completely kill pathogens, but also realize the reuse of waste resources^[43].

Algal biomass is mainly composed of protein, lipid, carbohydrate and ash, and is quite different from lignocellulosic biomass, for example, the nitrogen content in algal biomass accounts for 5 wt% ~ 10 wt%^[44].

Yeasts are microorganisms commonly used in the food industry. In the brewing industry, 30 kg of yeast biomass is produced as a by-product for the production of 1 m³ wine. Yeast residue is rich in carbon and suitable for the preparation of biochar^[45].

Some sludge, livestock manure, algae and yeast have high nitrogen content and can be used as precursors of nitrogen-doped biochar^{[44,46⇓~48]}. By introducing Lewis basic sites, the electrical structure of biochar is improved, and the activation ability of biochar is enhanced (nitrogen-doped biochar will be discussed in detail in Section 6.3.1).

2.3 Effect of Biomass Precursor on Catalytic Capacity of Biochar

The activation performance of biochar can be affected by the content and type of metal and phenolic compounds in the biomass precursor, the moisture content, and the content of lignin and cellulose.

Meng et al. Explored the effect of lignin and cellulose content in biomass on biochar activation capacity^[49]. They found that compared with the biochar prepared from lignin-containing orange epicarp, the biochar prepared from cellulose-containing orange mesocarp had flake-like and more developed pore structure, and the efficiency of activated PMS for phenol degradation was better than that of the former. Further study showed that the biochar prepared from biomass precursor with the mass ratio of lignin to cellulose of 1:4 had the best activation ability, while the biochar prepared from pure lignin or cellulose had the worst activation ability.

Fang et al. Used pine needles to adsorb metals (Persistent free radicals, Cu²⁺, Ni²⁺, or Zn²⁺) or phenolic compounds (hydroquinone, catechol, or phenol) to regulate the types and contents of Persistent free radicals (PFRs) in the prepared biochar^[50]. They found that the biochars prepared by adsorption of different metals or phenolic compounds had different types of PFRs, and the PFRs content of biochars prepared by adsorption of pine needles mixed with 0.1 mmol/L Ni²⁺ or 5 mmol/L catechol was the highest compared with other metals or phenolic compounds. Huang et al. Used extraction to regulate the content of metals or phenolic compounds in biomass, and found that the effect of metals in biomass on the content of PFRs in the prepared biochar was greater than that of phenolic compounds^[28].

Yu et al. Reported that impurities such as Ca, Na, Mg, P, and Cl contained in shrimp shells could be used as templates for biochar pore formation^[26]. These impurities are uniformly distributed in the biomass and can be removed by HCl washing after biochar preparation to form a pore structure. The sludge biochar prepared by Gu et Al. Contains elements such as Fe, Si, Al, etc. Except that the Fe₃O₄ formed by Fe during pyrolysis can be used as the active site of H₂O₂ and is magnetic for recycling, the hydrogen bond formed by O in Si — O — Si is easy to adsorb H₂O₂, which makes H₂O₂ gather near the surface of biochar, and Si — OH is deprotonated when pH is greater than pH_pzc(2.0~3.5), forming an acidic microenvironment on the surface, which is conducive to the formation of · OH^[51]. Al increases the basic site strength of the Fe-containing catalyst, and also acts as a Lewis acid to attract the electron density of the Fe³⁺, making it easier for the Fe³⁺ to react with H₂O₂ and reduce the Fe³⁺ to Fe²⁺.

Hu et al. used wet sludge as a precursor to prepare biochar by a combination of hydrothermal carbonization and pyrolysis, and found that the higher the moisture content, the higher the C = O content in the biochar, and C = O is an important active site for activating peroxides on the surface of biochar (the introduction of active sites will be described in detail in Section 4)^[22].

3 Method for preparing biochar

Biochar is mainly prepared by pyrolysis, hydrothermal carbonization, microwave pyrolysis and gasification pyrolysis, and the biochar prepared by pyrolysis is usually used to activate peroxides.

Pyrolysis is a process of thermal decomposition of organic matter under high temperature and limited oxygen or anoxic conditions, and the solid product formed by the decomposition of biomass during pyrolysis is biochar^[64]. Pyrolysis temperature and atmosphere have great influence on the activation ability of biochar.

Pyrolysis temperature affects the surface morphology and physicochemical properties of biochar, including pore structure, defects and functional groups. The higher the pyrolysis temperature, the stronger the activation ability of biochar. Huang et al. Used gradient pyrolysis temperature to control the morphology and surface chemical characteristics of wheat straw biochar. When the pyrolysis temperature increased from 300 ℃ to 800 ℃, the specific surface area, pore volume and surface alkaline functional group content of biochar increased, while the surface acidic functional group content decreased^[65]. The degradation of sulfamethazine by biochar activated H₂O₂ prepared at different temperatures was studied.It was found that the increase of specific surface area, pore volume and surface basic functional group content was beneficial to the biochar activated H₂O₂, while the increase of surface acidic functional group content was inhibitory. Yu et al. Changed the hybrid configuration of shrimp shell biochar by increasing the pyrolysis temperature (sp³ hybrid dominated changed to sp² hybrid dominated)^[26]. With the change of configuration, the mechanism of 2,4-dichlorophenol degradation by biochar activated PDS changed from free radical mechanism (pyrolysis temperature of 400 ℃) to electron transfer mechanism (pyrolysis temperature of 800 ℃), and the removal rate of 2,4-dichlorophenol increased by 28 times. Both Sun et al. And Ouyang et al. Found that the increase of pyrolysis temperature could promote the generation of biochar defects, and the defects were confirmed to be involved in the activation of PDS and PMS^[54]^[59]. When the pyrolysis temperature is 300 ~ 500 ℃, PFRs can be produced^[50]. When the pyrolysis temperature is increased to 500 ℃, PFRs begin to quench. The concentration of PFRs is greatly reduced above 700 ℃^[66]. PFRs are thought to be involved in the generation of free radicals from activated peroxides.

Pyrolysis atmosphere also affects the activation ability of biochar. Huang et al. Found that pyrolysis in H₂ or N₂H₄ atmosphere could increase the C = O content of sludge biochar, and the biochar prepared in both atmospheres could increase the degradation efficiency of BPA by activated PMS by about 20% compared with the original biochar^[23]. Sun et al. Showed that pyrolysis under CO₂ atmosphere could introduce a large number of micropores into lignin biochar, increase defects, C = O and specific surface area, and enhance surface reducibility, which was beneficial to the activation of PDS^[67].

4 Peroxide activation mechanism of biochar

The degradation of organic pollutants by biochar activated peroxide can be carried out according to two mechanisms of free radical and non-free radical (Figure 1), and the cases of pollutant removal by biochar activated peroxide are summarized in Table 1.

4.1 Free radical mechanism

Biochar activated peroxides cleave O — O bonds and produce strong oxidizing radicals, such as · OH (1.8 – 2.7 V vs NHE) and V vs NHE, which can oxidize and even mineralize organic pollutants^[68]. Biochar activation of H₂O₂ mainly produced · OH. Because of the different chemical structures of PDS and PMS, biochar activated PMS can directly produce ·SO₄^- or · OH, while activated PDS only directly produces ·SO₄^-, and the produced ·SO₄^- will react with H₂O or OH^- to produce · OH. Both ·SO₄^- and · OH can degrade pollutants through electron transfer, hydrogen abstraction or addition reaction, but · OH prefers to oxidize organic compounds through addition or hydrogen abstraction mechanism, while ·SO₄^- is electrophilic and prefers to selectively attack pollutants containing unsaturated bonds and aromatic structures through electron transfer mechanism^[69]^[70,71].

The active radicals produced by biochar activation of peroxides are distributed in the bulk of solution (free radicals) and on the surface of biochar (adsorbed radicals). In different activation systems, the two types of radicals have different contributions to the removal of pollutants. Zhu et al. Reported that in the system of PDS activation by sawdust biochar, the adsorbed free radicals on the surface of biochar were the main active species for the oxidation of golden orange II^[56]. Liang et al. Prepared peanut shell biochars (BC400 and BC700) at two pyrolysis temperatures of 400 ℃ and 700 ℃, and used them to activate PDS to degrade sulfamethoxazole. It was found that the removal of sulfamethoxazole by BC400 activated PDS was entirely due to the contribution of free radicals, while in BC700 activated PDS system, the contribution was reduced to 53.1%, and the contribution of adsorbed radicals was 46.9%^[72]. Ouyang et al. Found that free · OH played a major role in the degradation of 1,4-dioxane in the PMS system activated by pine needle biochar^[59].

The active site is where the surface of the biochar comes into contact with the peroxide and causes it to decompose into active species. In the biochar activated peroxide system, researchers usually use: 1) compare the change of intensity or content of a site on the biochar before and after the reaction to determine the active site, such as Fourier transform infrared spectroscopy.FT-IR was used to detect the intensity of chemical bond peaks in biochar before and after the reaction, X-ray photo Electron spectroscopy (XPS) was used to detect the content of functional groups on the surface of biochar before and after the reaction, and Electron paramagnetic resonance (EPR) was used to determine the content of PFRs after the reaction^[59]^[56]^[73]. 2) inferring the active site by fitting the quantitative relationship between the pollutant removal effect and the content of a certain site^[54]; 3) Combined with functional Density theory calculations (Density functional theory, DFT) to determine the active site^[31,74]. Recently, Zhou et al. Proposed a new way to explore the active site, that is, to introduce 2-Bromo-1-phenylethanone (BrPE), Phenylhydrazine (PH) and Benzoic anhydride (BA) to selectively inactivate — COOH, C = O and — OH on the surface of pig bone biochar to determine the active site^[62].

PFRs, oxygen-containing functional groups, defects, sp² hybrid carbon, delocalized π electrons, original metal Fe and nitrogen in biomass have been reported as active sites on biochar in the existing free radical mechanism studies^[50,67,73]^[53,55]^[59,67]^[72]^[56]^[51,75]^{[33,76⇓~78]}.

Strictly speaking, PFRs cannot be defined as "active sites", because PFRs are a class of free radicals formed by the interaction between metals in biomass and phenolics during pyrolysis (Fig. 2)^[50]. Fang et al. Studied the key role of PFRs in the activation of H₂O₂, and proposed a single electron transfer mechanism, that is, PFRs transfer a single electron to H₂O₂ to produce · OH^[73]. Fang et al. Further reported the mechanism of PFRs activating molecular oxygen to produce · OH (Fig. 3)^[79]. At the same time, PFRs can also directly oxidize pollutants^[60,80]. Deng et al. Also proposed an electron transfer mechanism for the activation of H₂O₂ by PFRs, and oxygen-centered PFRs were converted to carbon-centered PFRs while activating H₂O₂^[27]. Fang et al. Found that oxygen-centered PFRs had a stronger ability to activate PDS than carbon-centered PFRs, and the concentration and type of PFRs had an important effect on the ·SO₄^- of activated PDS^[50]. Similarly, activation of PMS by PFRs can also produce ·SO₄^-^[81]. Fig. 2 summarizes the formation process of PFRs and their pathway to activate PMS, H₂O₂, and PDS.

The surface of biochar contains abundant oxygen-containing functional groups, which can act as active sites for activated peroxides. Luo et al. Reported that C = O in PFRs is the main active site for the production of · OH from activated H₂O₂^[60]. Yan et al. Suggested that biochar surface functional groups (BC_surface-COOH and BC_surface-OH) could activate PDS as intermediates of electron transfer, as shown in reactions (1) and (2)^[82]. Zhu et al. Analyzed the XPS spectra of sawdust biochar before and after activation of PDS, and found that the content of — OH decreased from 11.91% to 8.20%, suggesting that — OH was the active site of activated PDS^[56]. Zhou et al. Used BrPE, PH and BA to selectively inactivate — COOH, C = O and — OH on the surface of pig bone biochar to determine the active site without destroying other structures of biochar^[62]. The results showed that — OH and — COOH were related to the production of · OH and ·SO₄^-, while C = O was related to the production of · OH in the PDS system activated by biochar.

(1)BC_surface-COOH + S₂O₈^2- → BC _surface-COO· + ·SO₄^- +HSO₄^-

(2)BC_surface-OH + S₂O₈^2- →BC _surface-O· + ·SO₄^- + HSO₄^-

There are defects on the surface of biochar, which can break the homogeneity of graphitic carbon network structure in carbon materials, induce high chemical potential and unpaired electrons, and thus have activation ability^[83]. Sun et al. Reported that the defects on sunflower stem biochar weakened the O — O bond in PDS, and the weakened O — O bond could be broken by thermal activation at 60 ℃ to produce ·SO₄^-^[54]. According to Ouyang et al., the π-electron at the defect, which is not restricted by the carbon atom, transfers to PMS to produce ·SO₄^- and · OH, as shown in reactions (3) and (4):^[84]

(3)BC_defects +

HSO 5 -

→ OH ^- + ·

SO 4 -

BC d e f e c t s +

(4)BC_defects +

HSO 5 -

→·OH +

SO 4 2 -

BC d e f e c t s +

The sp² hybridized carbon and delocalized π electrons on the biochar surface can also serve as active sites. Luo et al. Considered that the sp² hybrid carbon was the active site by comparing the XPS spectra of sludge biochar before and after activation of H₂O₂^[60]. In the report of Zhu et al., the comparison of XPS spectra before and after PDS activation of sawdust biochar showed that delocalized π electrons were the active sites^[55,56].

Doping nitrogen atoms is an effective means to enhance the activation ability of biochar. Graphite nitrogen, pyridine nitrogen, and pyrrole nitrogen can all act as Lewis basic sites to activate PMS to produce ·SO₄^-^{[33,77，78,85]}.

Sludge usually contains metal elements such as Fe, so the sludge biochar prepared by pyrolysis may contain metal active sites. Gu et al considered that the Fe₃O₄ contained in sludge biochar could be used as the active site for activating H₂O₂, while Wang et al reported that Fe (Ⅱ) on the surface of sludge biochar was the active site for activating PDS^[51]^[21].

4.2 Non-radical mechanism

In addition to free radical oxidation, pollutants can also be degraded by non-free radical pathways. At present, the non-radical mechanism has not been reported in the biochar activated H₂O₂ system, but mainly focused on the biochar activated persulfate system. Pollutants can be degraded by non-radical mechanisms such as singlet oxygen (¹O₂), electron transfer, and non-photogenerated hole oxidation.

4.2.1 ¹O₂ oxidation mechanism

¹O₂ is an electrophile and readily degrades electron-rich organics^[86]. Compared with · OH and ·

SO 4 -

, the oxidation ability of ¹O₂ is milder and less affected by environmental matrix^[87]. Researchers usually combine quenching experiments, solvent displacement and EPR to determine the production of ¹O₂. In the biochar-activated PMS system with ¹O₂ as the main active species, the degradation rate constant of pollutants showed a good linear relationship with the content of C = O in the biochar^[22,23]. Huang et al proposed that C = O is the active site of activated PMS, and the ¹O₂ is produced by reactions (5) ~ (8), and reaction (7) is the rate-limiting step^[23]. This mechanism has been confirmed by Meng et al. Using DFT calculations^[49]. In nitrogen-doped biochar, graphitic nitrogen can attract the electrons of carbon atoms around it, which makes the carbon in C = O more positively charged and more likely to nucleophilic addition with PMS to produce ¹O₂^[88]. Ding et al. Reported that sp² hybrid carbon has excellent conductivity and can also be used as an active site for activating PMS to produce ¹O₂^[88]. Wang et al. Found that both the doped sulfur atom and its surrounding carbon atoms in sulfur-doped biochar (SSB) were positively charged due to electron deficiency, and PMS could act as an electron donor to produce ¹O₂ through reactions (9) and (10)^[46]. Dissolved oxygen can accept electrons from biochar through reaction (11) to form

O 2 -

, and then form ¹O₂ through reactions (12) ~ (15)^[24]. At the same time, PMS can also produce ¹O₂ through the self-decomposition reactions (16) and (17), but the rate is slow (about 0.2 M^-1·s^-1)^[88].

(5)

(6)

(7)

(8)

(9)SSB +

HSO 5 -

→·

SO 5 -

+ H⁺

(10)2·

SO 5 -

+ H₂O →

2HSO 4 -

+ 1.5¹O₂

(11)BC + O₂ → BC ⁺ + ·

O 2 -

(12)2·

O 2 -

+ 2H₂O → ¹O₂+ H₂O₂ + OH^-

(13)·

O 2 -

+ H₂O →·OOH + OH ^-

(14)·

O 2 -

+·OOH →¹O₂ + HOO^-

(15)·OOH +·OOH →¹O₂ + H₂O₂

(16)

HSO 5 -

→ H ⁺+

SO 5 2 -

(17)

SO 5 -

HSO 5 -

→

SO 4 2 -

HSO 4 -

+¹O₂

PDS activated by biochar also produces ¹O₂ and degrades pollutants. Sun et al. Proposed that PDS can be activated to produce ¹O₂ through reactions (18) ~ (21)^[67].

(18)

(19)

(20)

(21)

4.2.2 Electron transfer mechanism

The electron transfer mechanism is a hot topic in the study of persulfate activation by carbon. Zhu et al. Reported that graphite nitrogen-doped reed biochar could form a metastable surface complex with PDS, which degraded pollutants by abstracting their electrons, and considered that the adsorption of pollutants was the rate-limiting step of the electron transfer oxidation process^[66]. Because the adsorption of organic pollutants is mainly related to their pK_a, the electron transfer mechanism depends on the pK_a of pollutants^[89]. Ye et al. Also found that the electron transfer mechanism was involved in the removal of tetracycline in the study of tetracycline degradation by Enteromorpha prolifera biochar activated PMS^[90].

Electrochemical methods, such as electrochemical impedance spectroscopy (Eis) to measure the conductivity of carbon materials, Linear-sweep voltammograms (LSV) and chronoamperometry, are usually used to study the mechanism of electron transfer between pollutants, biochar and persulfate^{[26,30,74,91]}^{[26,30,74,89,91]}^[37,74,91]. It should be noted that when LSV is used to study the electron transfer mechanism, the positive potential applied may cause oxidation of the surface of carbon materials, while electron-rich compounds are prone to oxidation at lower potentials to form intermediate products. If the intermediate can also promote the electron transfer process, it is difficult to distinguish whether the electron donor in the electron transfer process is the substrate itself or its intermediate from the LSV results^[92]. Our group used in situ electrochemical technique to reveal the removal process of phenolic pollutants by carbon nanotube (CNT) activated PDS to produce CNT-PDS * metastable complex, and found that the complex degraded phenolic pollutants by abstracting electrons from them^[92]. The lower the half-wave potential of phenolic compounds, the faster the removal rate, but when the half-wave potential of phenolic pollutants is higher than the open circuit potential of the complex, the electron transfer process between CNT-PDS * and phenolic pollutants will be inhibited. This method can provide a reference for the study of the electron transfer process of biochar activated persulfate degradation of pollutants. Although the oxidation potential of metastable surface complexes is lower than that of free radicals, they have the following unique advantages: 1) the complexes are more inclined to attack electron-rich pollutants and are more selective^[90,93]; 2) It is less affected by natural organic matter and inorganic ions in the environment; 3) The self-quenching reaction of free radicals can be avoided.

The definition of the active site in the electron transfer mechanism is still vague. Zhu et al. Reported that PDS combined with defects to form a metastable complex during the activation of PDS by nitrogen-doped reed biochar^[55]. Ho et al. Reported that when PDS was activated by biochar from spirulina residue, PDS was closely combined with positively charged oxygen atoms around the doped nitrogen atoms to form a complex^[30]. Wang et al. Reported that the specific rate constant (the ratio of apparent rate constant to specific surface area) of sulfadiazine removal by PDS activated by nitrogen-doped corncob biochar had a good linear positive correlation with the contents of pyridine nitrogen and pyrrole nitrogen on the biochar, respectively.Electrostatic potential distribution (ESP) showed that there were electron-rich regions around the pyridine nitrogen and pyrrole nitrogen in the biochar, which proved that they were the active sites of PDS^[87]. Qi et al. Considered that the graphitic nitrogen on Enteromorpha prolifera biochar was beneficial to the combination of PDS and nitrogen-doped biochar to form a complex, and calculated the binding energy of PDS with graphitic nitrogen, pyridinic nitrogen, pyrrolic nitrogen and sp² hybrid carbon, and the results showed that the binding energy of PDS with graphitive nitrogen was the highest, which meant that graphitive nitrogen was more likely to combine with PDS^[31]. Liu et al. Found that the boron species (—BC₃, —BCO₂, and —BC₂O) in the biochar of boron-doped wheat straw could be used as Lewis acidic sites to tightly bind to PDS to form a complex, and the binding energy calculation also confirmed that the binding energy of PDS with boron species was higher than that with carbon^[91]. Zaeni et al. Suggested that graphitic nitrogen on nitrogen-doped wood biochar could be tightly combined with PMS to form a complex^[78]. An accurate understanding and determination of the active site in the electron transfer mechanism is a concern for follow-up studies.

4.2.3 Cavitation mechanism

In the photocatalytic process, photogenerated holes usually act as active species for the degradation of pollutants^[94,95]. In the biochar activated peroxide system, there are few such reports. He et al. Used sawdust biochar to activate PDS to degrade golden orange II. When EDTA, a hole quencher with a concentration ratio of 30:1 to golden orange II, was added, the decolorization of golden orange II was almost completely inhibited^[58]. Based on the results of radical quenching experiment and EPR test, He et al. Considered that the hole was the key factor for the degradation of pollutants in this system. When Wu et al studied the degradation of aniline by PDS activated by rice straw biochar, they found that free radicals had little contribution to the removal of aniline from the results of free radical quenching experiment and EPR test, and then confirmed that the electron transfer mechanism did not participate in the degradation of aniline. Finally, they inferred that holes were the main active species from the result of EDTA inhibiting the removal of aniline^[57].

It should be pointed out that the degradation of pollutants by biochar activated peroxides is mostly due to the combined action of free radical and non-free radical oxidation.Active species are generally identified based on whether they contribute to the removal of pollutants. In fact, not all active species in the biochar/peroxide system contribute to the degradation of pollutants^[62]. Ouyang et al. Reported that there were two active species, · OH and ¹O₂, in the PMS activation system of pine needle biochar, but 1,4-dioxane was mainly oxidized by · OH, and ¹O₂ did not contribute to the degradation of pollutants^[91]. Liu et al. Determined by EPR that the PDS system activated by boron-doped wheat straw biochar produced ¹O₂, but the reaction rate constant of ¹O₂ with H₂O (2.5×10⁵M^-1·s^-1) was much larger than that with sulfamethoxazole (2×10⁴M^-1·s^-1), so it did not participate in the removal of sulfamethoxazole^[91].

5 Water quality impact

5.1 Effect of pH

For the free radical oxidation mechanism, pH mainly affects the removal of pollutants through the following aspects: 1) Solution pH affects the surface charge of biochar. Persulfate mostly exists in the form of anion (S₂O₈^2-,

HSO 5 -

), and when pH>pH_pzc, there is electrostatic repulsion between biochar and persulfate, which will hinder the contact between persulfate and biochar surface, thus inhibiting activation. However, most of the pK_a>11 of H₂O₂ exist in molecular state in solution, so the influence is small. 2) Some pollutants show molecular, cationic and anionic forms at different pH. If the biochar and the pollutant have opposite charges, the adsorption of the pollutant on the surface of the biochar is facilitated, and the contact probability between the pollutant and the active species is increased, thereby promoting the removal of the pollutant; If biochar and pollutants have the same charge, they will repel each other and inhibit the oxidation of pollutants. 3) ·

SO 4 -

can be converted to · OH under alkaline conditions. The half-life of · OH is only 20 ns, while the ·

SO 4 -

is 30 ~ 40 μs. In addition, the oxidizing power of · OH varies with pH. At pH = 0, the redox potential of V vs NHE is 2.59 V vs NHE, while at pH = 14, it is reduced to 1.64 V vs NHE^[96]. 4) PMS is unstable under strong alkaline conditions and decomposes into

SO 4 2 -

and O₂ (e.g. Reaction 22)^[97]. In addition, under strong alkaline conditions,

HSO 5 -

are converted to less reactive

SO 5 2 -

(e.g. Reaction 23)^[98]. Ouyang et al. Reported that the degradation efficiency of 1,4-dioxane by PMS activated by pine needle biochar decreased (from 67. 6% to 40. 2%) with the increase of pH (from 3 to 11), which was caused by the negative charge on the surface of biochar and the repulsion of PMS and the decomposition of PMS into O₂ under alkaline conditions^[59].

(22)

HSO 5 -

SO 5 2 -

→ H ⁺ + O₂ +

2SO 4 2 -

(23)

HSO 5 -

→

SO 5 2 -

+ H⁺ pK_a=9.4

The non-radical oxidation mechanism is less affected by pH and can generally degrade pollutants over a wide pH range^[21,24,66]. Ho et al. Reported that the oxidation of sulfamethoxazole by PDS activated by biochar of spirulina residue was dominated by electron transfer mechanism, and the removal rate of sulfamethoxazole was more than 95% in the pH range of 2. 0-10. 0^[30]. Wang et al. Reported that in the food waste biochar activated PDS system, the degradation rate of pollutant p-chlorophenol was more than 90% in the pH range of 2.0 ~ 9.4 under the action of the main active species ¹O₂^[24]. However, in the non-radical mechanism, pH still affects the degradation of pollutants by changing the electrical properties of pollutants and biochar surfaces. Sun et al. Reported that ¹O₂ and complexes participated in the oxidation of phenol in the PDS system activated by cellulose biochar, and phenol could be completely removed in the pH range of 2. 91 ~ 9. 21^[67]. When the pH increased to 10. 90, which was higher than the pH_pzc(7.67) of biochar and the pK_a(9.89) of phenol, the biochar surface was negatively charged and phenol was anionic, which inhibited the electron transfer process, and the removal rate of phenol dropped sharply to 33%.

5.2 Effects of anions and natural organic matter

Natural organic matter (NOM) and Natural organic matter such as

、HCO 3 -

NO 3 -

and H₂PO₄^- are widely present in real water and wastewater, which may affect the degradation of pollutants by biochar activated peroxide system.

Anions and NOM have a great influence on the radical oxidation process, because anions quench · OH and ·

SO 4 -

to produce active species with low oxidation ability, which reduces the removal rate of pollutants.

Cl^- quenches · OH and ·SO₄^- through reactions (24) ~ (27), producing Cl · and ·

Cl 2 -

, which are less active than · OH and ·SO₄^-, resulting in the inhibition of pollutant oxidation^[99]. Deng et al. Used pig manure biochar activated H₂O₂ to remove sulfamerazine, and found that 20 mM Cl^- could reduce the removal rate of sulfamerazine from 100% to about 80%^[27]. Our group found that 10 mM Cl^- had little effect on the oxidation of clofibric acid by PDS activated by sawdust biochar, because the reaction rate constant of clofibric acid with Cl ·, (9.76±0.15)×10¹⁰M^-1·s^-1, was higher than that with ·SO₄^-^[55]. However, Cl · is further transformed into ·

Cl 2 -

with lower activity, so the removal rate of clofibric acid is basically unchanged.

(24)·

SO 4 -

+ Cl^- → Cl· + S₄^2-

(25)·OH + Cl^- → Cl·OH ^-

(26)Cl·OH^- + H⁺ → Cl· + H ₂O

(27)Cl^- + Cl· → ·

Cl 2 -

HCO 3 -

can also quench free radicals through reactions (28) and (29), and the generated ·

HCO 3 -

and ·

CO 3 -

are less active than · OH and ·

SO 4 -

, so the removal of pollutants is inhibited^[100]. Ouyang et al. Reported that free radicals produced by PMS activation by pine needle biochar could completely remove 1,4-dioxane, while 10 mM

HCO 3 -

could reduce the removal rate of pollutants to 20.2%^[59]. Devi et al. Reported that in the degradation of 1-methylcyclohexane carboxylic acid by wheat straw biochar activated H₂O₂, the addition of 50 mg/L

HCO 3 -

in the system increased the pH of the system, and the removal rate of 1-methylcyclohexane carboxylic acid was inhibited by 50%^[52]. It was also found that in the system of cotton straw biochar activating H₂O₂ to degrade sulfonamide antibiotics,

HCO 3 -

could be indirectly used as the source of the main active species ·

CO 3 -

through reactions (30) ~ (31), and cotton straw biochar could not activate H₂O₂ in this process^[101].

(28)·

SO 4 -

HCO 3 -

→ H ⁺+·

CO 3 -

+ H₂O 或·

SO 4 -

HCO 3 -

→ ·

HCO 3 -

+ H₂O

(29)·OH +

HCO 3 -

→ ·

CO 3 -

SO 4 2 -

+ H₂O

(30)H₂O₂ +

HCO 3 -

→

HCO 4 -

+ H₂O

(31)

HCO 4 -

+ BC → ·

CO 3 -

+ OH^-+ BC⁺

NO₃^- and H₂PO₄ can also act as quenchers and inhibit the removal of pollutants by generating more weakly oxidizing NO₃· and H₂PO₄· through reactions (32) ~ (35)^[100]^[102]. NOM inhibits the degradation of target pollutants by covering the active sites on biochar or scavenging free radicals^[103,104].

(32)·

SO 4 -

NO 3 -

→

SO 4 2 -

+ NO₃·

(33)·OH +

NO 3 -

→ OH ^-+ NO₃·

(34)·

SO 4 -

HPO 4 2 -

→

SO 4 2 -

+ ·

HPO 4 -

(35)·OH +

HPO 4 2 -

→ OH ^- + ·HPO ₄

In relative terms, anions have less effect on the non-radical oxidation process because the lower oxidation potential of ¹O₂ and metastable surface complexes cannot oxidize these anions^[31].

Chen et al. Found that the removal of sulfathiazole by activated PDS from sludge biochar was dominated by electron transfer mechanism, and Cl^- had almost no effect on the removal of pollutants^[37]. Yin et al. Reported that the removal of sulfamethoxazole by sludge biochar activated PDS was dominated by ¹O₂ oxidation, and 10 mM Cl^- also had no effect on the removal rate of pollutants^[20]. Ye et al. Reported that the removal of tetracycline by activated PMS with graphitized nitrogen-doped ramie biochar was dominated by electron transfer and ¹O₂ oxidation, and 10 mM Cl^- and

NO 3 -

had little effect on the removal rate^[90].

Some anions can inhibit the production of active species by competing with persulfate adsorption, thereby reducing the removal of pollutants in non-radical systems. Sun et al. Reported that the removal of phenol by PDS activated by cellulose biochar was dominated by two mechanisms: ¹O₂ oxidation and electron transfer. 30 mM

HCO 3 -

would compete with PDS for adsorption on the surface of biochar, reducing the removal rate of phenol from nearly 100% to 55%^[67]. However, 10 mM H₂

PO 4 -

also competed with PDS for adsorption, resulting in a decrease of phenol removal to 55%; 30 mg/L humic acid (typical NOM) competed with PDS and phenol for adsorption, and competed with phenol for degradation, resulting in a decrease of phenol removal to 76%.

Compared with PDS, PMS has higher activity and can directly react with anions to affect the degradation of pollutants by biochar activated PMS. Luo et al. Found that under high salt environment, 500 mM Cl^- reacted with PMS to produce HOCl, which had a synergistic effect with ¹O₂ produced by PMS activation to promote the removal of BPA, and the time required for complete removal of BPA was reduced from the original 6 min to 0.5 min^[76]. Meng et al studied the degradation of phenol by PMS activated by biochar from orange endocarp, and found that biochar-PMS complex and ¹O₂ were involved in the removal of phenol^[49]. The presence of

HCO 3 -

inhibited the degradation of phenol, because

HCO 3 -

competed with biochar to activate PMS, resulting in the rapid consumption of PMS, but the efficiency of

HCO 3 -

to activate PMS to produce active species was low, resulting in the reduction of the overall degradation rate of phenol. Ye et al. Reported that the complex formed by Enteromorpha biochar activated PMS and ¹O₂ were involved in the removal of tetracycline, and the reaction pH was maintained at alkaline by adding 10 mM

HCO 3 -

, and the

HSO 5 -

in the system was decomposed into

SO 5 -

, which reacted to produce ¹O₂ to promote the degradation of tetracycline^[90].

6 Modification of biochar

6.1 Acid-base modification

Acid-base modification is a common way to prepare biochar, and acid-modified biochar is more used in advanced oxidation process than alkali-modified biochar. Generally, acid modification is performed after pyrolysis of biomass, which can remove ash to increase the specific surface area of biochar and expose more active sites. The catalytic performance of biochar can be improved by introducing different functional groups with different kinds of acids. HCl, HF, HNO₃, H₂SO₄, and H₃PO₄ have been reported for acid-modified biochar, with HCl being the most commonly used. Huang et al. Reported that Fe, Ca and Mg metal elements in sludge biochar were removed after modification by 50% HCl (V/V), the specific surface area increased from 32.42 m²/g to 117.05 m²/g, and the removal efficiency of sulfamethoxazole by activated PMS increased from 73.98% to 99.65%^[23]. HF modification can increase the C = O and phenolic hydroxyl content of rice husk biochar, thereby enhancing the ability of rice husk biochar to activate PDS^[105]. The modification with HNO₃ not only increased the specific surface area of sludge biochar, but also enhanced the content of C = O, pyridine nitrogen and graphite nitrogen as active sites, and increased oxygen-centered PFRs. The removal efficiency of ciprofloxacin by modified sludge biochar activated H₂O₂ was increased from 65. 20% to 85. 32%^[80]. Huang et al. Reported that modification with HNO₃ or H₂SO₄ would reduce the content of C = O as the active site of sludge biochar, thus weakening the efficiency of bisphenol A oxidation by sludge biochar/PMS system with ¹O₂ as the main active species^[23]. However, the modification of H₂SO₄ can introduce sulfonic acid groups to improve the surface acidity of sludge biochar, and the removal efficiency of ofloxacin by activated H₂O₂ increased from 27. 3% to 91. 5%^[106]. In contrast, the modification effect of H₃PO₄ was not dominant, and the ofloxacin removal efficiency of activated H₂O₂ of sludge biochar modified by H₃PO₄ was only increased from 27. 3% to 47. 8%, while the activation of H₂O₂ of cornstalk biochar modified by H₃PO₄ decreased^[106]^[107].

Alkali modification can also increase the specific surface area and functional group content of biochar, thereby improving the activation ability of biochar. Wang et al. Found that the specific surface area of sludge biochar increased from 32.42 m²/g to 117.05 m²/g after modification by NaOH, and the efficiency of activated PMS to mineralize triclosan reached 32.5% in 240 min^[61]. In addition to NaOH, KOH and NH₃OH were also used to modify the biochar. Wang et al. Investigated the effect of KOH concentration on alkali modification of different biomass substrate biochar, and found that KOH modification was only effective on cornstalk biochar, and the higher the alkali concentration, the better the modification effect^[107]. Mian et al. compared the effect of modification with 0.5 M KOH before and after pyrolysis of sludge biochar, and found that the modification before pyrolysis could greatly increase the specific surface area and oxygen content, while the modification after pyrolysis had little effect on the specific surface area and oxygen content^[108]. However, when 7 M NH₃OH was used to modify sludge biochar, there was no significant difference between pre-pyrolysis modification and post-pyrolysis modification, and both modification methods significantly increased the carboxyl content. The removal rate of golden orange Ⅱ was increased from 64. 2% to 88. 2% and 93. 5%, respectively, by the modified sludge biochar activated PMS^[108]. They further found that the effect of activated PMS on biochar modified by 7 M of NH₃OH (before pyrolysis) and 0.5 M of KOH (after pyrolysis) was significantly better than that modified by a single alkali^[108].

6.2 Porous graphitization modification

Porous graphitized carbon materials with high porosity and high degree of hybridization of sp² have been used in the preparation of electrodes, conductive plates, capacitors and other materials because of their excellent electronic conductivity, improved crystal structure and unique physical and chemical properties^[109]^[110]^[111]. Porous graphitization of carbon materials mainly includes catalytic graphitization (focusing on graphitization), chemical activation (focusing on pore-making) and catalytic graphitization combined with chemical activation^[112].

Biochar can be graphitized by pyrolysis alone, but the pyrolysis temperature is usually required to be more than 2500 ℃, which leads to a large amount of energy consumption, which is inconsistent with the concept of energy saving and emission reduction^[112]. At present, transition metal catalysts are mostly used to prepare graphitized biochar. Metal elements such as Fe, Co, Ni and Mn have been used to catalyze the preparation of graphitized biochar materials^[113⇓~115]^[114,115]^[113,116]^[114]. Demir et al. Compared the catalytic performance of Fe(NO₃)₂, Co(NO₃)₂ and Mn(NO₃)₂ for the graphitization of lignin biochar^[114]. Among them, Co and Mn can achieve a high degree of graphitization at 900 ℃, while Fe catalysis can be achieved at 1100 ℃. The graphitized biochar prepared by Mn catalysis has the highest content ratio of graphitic carbon to amorphous carbon (0. 6) and good conductivity, while the graphitized biochar prepared by Co catalysis has poor conductivity, and the graphitized biocarbon prepared by Fe catalysis has larger specific surface area. Sevilla et al. compared the graphitized biochar prepared by Fe and Ni catalysis, and the XRD results showed that the graphitized biochar prepared by Fe catalysis had a higher graphitization peak value, indicating that the graphitization ability of Fe was better than that of Ni^[113]. They further reported that the lignin-graphitized biochar prepared by Ni catalysis possessed a coil-like morphology, a high specific surface area (114 m²/g), and a graphitic carbon to amorphous carbon content ratio of 0.756^[116]. At present, only graphitized biochar materials prepared by metal catalysis such as Fe and Co are used in advanced oxidation processes. Du et al. Prepared graphitized sawdust biochar using ammonium ferric citrate as catalyst, and removed Fe from it by acid washing^[117]. The prepared graphitized biochar produced mesoporous structure with increased C = O content and total pore volume. The graphitized biochar could adsorb 64% of sulfamethoxazole within 120 min, and 98% of sulfamethoxazole could be removed by activated PMS. However, the ungraphitized biochar could hardly activate PMS to degrade sulfamethoxazole. The phenol removal efficiency of activated PMS from graphitized cornstalk biochar prepared by Li et al. Using Fe(NO₃)₃ and Co(NO₃)₂ catalysis combined with acid washing was increased by about 30% compared with that before modification^[118].

For chemical activation, KOH, ZnCl₂, and K₂CO₃ have been reported to be applied to prepare porous graphite-like biochar activated persulfate^[91]^[89]^[31]. Qi et al. Used K₂CO₃ as an activator and nitrogen-rich Enteromorpha prolifera as a precursor to prepare three-dimensional porous graphite-like nitrogen-doped biochar and applied it to activate PDS for sulfamethoxazole degradation^[31]. The specific surface area of the porous graphite-like nitrogen-doped biochar prepared by K₂CO₃ activation at 900 ℃ increased from 28.32 m²/g to 979.89 m²/g, and the pore volume increased from 0.42 cm³/g to 1.47 cm³/g compared with that before activation. SEM showed that the nanosheet structure appeared in the graphitized biochar. Only 55.4% of sulfamethoxazole was removed by activated PDS of ungraphitized N-doped biochar, while more than 95% of sulfamethoxazole was removed by activated PDS of graphitized N-doped biochar. Liu et al. Used the same mass of KOH as a pore-forming agent to activate wheat straw biochar, and found that the carbon layer structure was destroyed, the specific surface area increased from 63.29 m²/g to 1358.37 m²/g, the pore volume increased from 0.031 cm³/g to 0.65 cm³/g, more active sites were exposed, and the defects increased, and the removal efficiency of sulfamethoxazole applied to activated PDS increased from 8% to 64%^[91]. Xie et al. Studied the preparation of nitrogen-doped graphite-like biological nanosheets by inducing Candida with molten salt (NaCl/KCl, molar ratio of 1:1), and applied it to the removal of bisphenol A by activated PMS (Fig. 4)^[33]. The molten salt can induce graphite-like structure at about 700 ℃ and retain nitrogen element well. When the mass ratio of molten salt to yeast was 6:1, the prepared nanosheets (NCS-6) had the highest activity, and bisphenol A was completely removed within 6 min, while the ungraphitized biochar-activated PMS could hardly degrade bisphenol A.

Although chemical activation has a great contribution to the formation of pores, the effect of graphitized biochar is not outstanding, so the research on the preparation of porous graphitized biochar by metal catalysis combined with chemical activation is gradually rising^[112]. Zhu et al. Soaked lignin in a solution containing FeCl₃ and ZnCl₂. During pyrolysis, Fe catalyzed the formation of graphitized structure, and Zn acted as an activator to promote the production of a large amount of pore structure^[89]. The prepared graphite-like nanosheets (GNS) have a layered structure, abundant micropores, a high degree of graphitization, and a low defect content, and can remove 99% of orange G in 180 min when applied to activated PDS, while the ungraphitized biochar activated PDS can only remove 76. 5% of orange G. Ye et al. Prepared nitrogen-doped graphitized bio-carbon fiber (PGBF-N) activated by PMS to remove tetracycline using ramie as precursor, K₂FeO₄ as graphite pore-forming agent and urea as nitrogen precursor^[90]. Fe(OH)₃ and KOH produced by hydrolysis during the activation of K₂FeO₄ were used as graphitizing agent and activating agent, respectively, for the graphitization and pore-forming of biochar. PGBF-N has an average pore size of 3.953 nm, a total pore volume of 0.096 cm³/g, and a specific surface area of 81.243 m²/g, and this structure is considered to better balance mass transfer and electrical conductivity. PGBF-N activated PMS could remove 96. 5% of tetracycline in 90 min, while BF activated PMS could only remove 48. 3% of tetracycline, and the removal rate of the former was 7 times that of the latter.

6.3 Doping modification

Heteroatom doping is one of the main methods to modify carbon materials. Heteroatoms possess different atomic radii or orbitals, electron density and electronegativity from carbon atoms, which can disturb the carbon layer of sp² hybrid configuration and break its chemical inertness. At present, nitrogen, boron and sulfur are mainly used in doped biochar.

6.3.1 Nitrogen-doped modification

The electronegativity of nitrogen (χ_N) is higher than that of carbon (χ_N=3.04 vsχ_C=2.55), which easily attracts the electrons of the surrounding carbon, increases the degree of electron delocalization, and breaks the inertness of the sp² hybrid carbon structure. According to Ye et al., nitrogen with localized unpaired electrons can modify carbon materials as follows: 1) increase the electron density of adjacent carbon atoms^[90]; 2) increasing the delocalized π electrons of the sp² carbon structure in a conjugated manner; 3) Carbon materials produce more functional groups and defects. The results show that the adsorption energy of carbon atoms around nitrogen in carbon materials increases by doping nitrogen, which proves that PMS is more easily adsorbed to this site^[119]. Nitrogen in carbon-based materials mainly exists in the form of graphite nitrogen, pyridine nitrogen and pyrrole nitrogen, and the form of nitrogen added to biochar is related to the doping mode and pyrolysis temperature. Graphite nitrogen is more easily produced at high temperature due to its high thermal stability, which is converted from pyridine nitrogen and pyrrole nitrogen at high temperature^[30]^[31,66]. In-situ doping mainly introduces graphite nitrogen, and post-treatment doping usually introduces surface functionalized pyridine nitrogen and pyrrole nitrogen^[120]. Zaeni et al. Reported a "one-pot" process for the preparation of nitrogen-doped biochar using urea as the nitrogen source and sawdust as the biomass^[74]. As shown in Fig. 5, in the pyrolysis process, urea is first decomposed into ammonia, and sawdust is converted into biochar, then ammonia is combined with the surface of biochar and heated to form pyrrole nitrogen and pyridine nitrogen, and finally pyridine nitrogen and pyrrole nitrogen are converted into graphite nitrogen at higher temperatures.

Nitrogen sources such as ammonium nitrate, urea, thiourea, melamine, and dicyandiamide have been applied to modify biochar with nitrogen to activate persulfate^[66]^{[74,76,78,88]}^[121]^[121]^[121]. Xu et al. Explored the effect of different nitrogen sources on nitrogen doping^[121]. Among the four nitrogen sources of thiourea, urea, melamine and dicyandiamide, the nitrogen-doped biochar doped with dicyandiamide showed the best activity and the highest nitrogen content. The ratio of nitrogen source to biomass also affects the effect of nitrogen doping. Xu et al. Studied the effect of nitrogen doping when the mass ratio of dicyandiamide to sawdust was 1:1, 2:1, 3:1, 4:1 and 6:1 respectively, and found that the amount of nitrogen doping reached the highest when the mass ratio was 4:1. This may be due to the limited carbon material matrix^[121]. The mass ratio of dicyandiamide to sawdust was positively correlated with the amount of doped nitrogen and pyridine nitrogen. Wang et al. Found that the specific surface area increases with the increase of nitrogen content^[74]. However, Zaeni et al. found that when the mass ratio of urea to sawdust was 0:1, 25:1, 50:1 and 100:1 respectively, the nitrogen content of graphite increased and the specific surface area decreased when the doping ratio increased, which may be due to the blockage of pores by excessive doping^[78]. In addition to additional nitrogen source doping, there are also reports on the direct preparation of nitrogen-doped biochar from nitrogen-rich biomass, such as spirulina residue and Enteromorpha prolifera^[30]^[31].

Sun et al. Used urea as nitrogen source to synthesize nitrogen-doped biochar to activate PMS to degrade golden orange II, and the nitrogen mainly existed in the form of pyridine nitrogen and graphite nitrogen^[80]. Under the optimal reaction conditions, the removal of golden orange Ⅱ by biochar activated PMS system was dominated by · OH and ·

SO 4 -

, and the apparent rate constant was 9. 6 times higher than that of undoped biochar activated PMS system. Xu et al. Used dicyandiamide as nitrogen source to prepare sawdust nitrogen-doped biochar to activate PMS to remove bisphenol A. Nitrogen mainly existed in the form of graphite nitrogen and pyridine nitrogen, and the main active species produced included · OH, ·

SO 4 -

and ¹O₂, which almost completely removed bisphenol A within 30 min, while the removal rate of bisphenol A was only about 40% when undoped biochar was used^[121]. Wang et al. Used urea as a nitrogen source to mix with corn stalks and prepared nitrogen-doped biochar at 700 ℃, mainly introducing pyridine nitrogen and pyrrole nitrogen^[87]. The removal of sulfadiazine by activated PDS was dominated by the electron transfer mechanism, and the reaction rate constant was 4. 6 times higher than that of undoped biochar.

6.3.2 Other doping modification

Although nitrogen-doped carbon materials have good catalytic properties and low toxicity, their long-term performance is not ideal^[122]. Boron is an ideal doping element because of its low electronegativity and similar atomic size to carbon. Liu et al. Used rice straw as a precursor and mixed boric acid (boron source) and KOH (pore-forming agent) to prepare boron-doped graphite porous biochar (B-KBC) to activate PDS to remove sulfamethoxazole. B-KBC has a hierarchical pore structure dominated by micropores, abundant structural defects and a B-O-C structure. The removal rate of sulfamethoxazole reached 94% in 120 min, while the degradation rate of sulfamthoxazole in the biochar activated PDS system was less than 20%^[91]. The results of DFT calculations show that: 1) the introduced boron, as a Lewis active site, can enhance the attraction of the surface for PDS and promote electron transfer; 2) The substitution of carbon atoms by boron atoms and oxygen atoms bonded with boron can control the structure of carbon matrix and enhance the catalytic electron conductivity; 3) The C — B bond produced by boron doping has a high binding energy, which enhances the chemical and structural stability of biochar.

The electronegativity of sulfur is close to that of carbon, which can generate spin charges in the carbon lattice, and sulfur doping can enhance the electronic conductivity, catalytic properties and chemical stability of carbon materials^[123]. Wang et al. Reported that sulfur doping enhanced the ability of sludge biochar to activate PMS and PDS, and the doped sulfur atoms were located at the jagged edge of the carbon layer, and the electrons were rearranged.More Lewis acid sites and alkaline sites are generated by cooperating with graphite nitrogen and pyridine nitrogen directly converted from nitrogen elements in the sludge, so that the chemical potential is increased, and the catalytic activity of the biochar is improved^[46]^[46]. However, Ding et al. Found that when sludge biochar was doped with sulfur atoms, most of the sulfur tended to replace oxygen-containing functional groups in the form of thiophene sulfur, and sulfur doping inhibited the removal efficiency of metolachlor by biochar activated PMS^[88]. This is because: 1) the introduction of some acidic sulfur oxide functional groups by sulfur doping inhibits the adsorption of acidic metolachlor; 2) The ESP results show that there is no obvious electron flow between the doped sulfur atoms and the surrounding carbon atoms, which may be that the sulfur doping breaks the equilibrium of the covalent carbon electron system and disturbs the electron distribution.

7 Recycling and Regeneration of Biochar

As a catalyst, biochar activates peroxides, but it also interacts with peroxides, free radicals or pollutants, which changes the surface properties of biochar and affects the recycling of biochar. The recycling effect of biochar activated peroxide system is not ideal, and the decrease of its performance is mainly due to the change of physical and chemical properties of biochar^[78,88].

7.1 Change of physical property

In the process of pollutant removal, pollutants and their intermediate products cover the surface of biochar and block the pores, resulting in the decrease of specific surface area of biochar and the reduction of active sites, which weakens the cycle performance of biochar. Du et al. Reported that the removal efficiency of sulfamethoxazole by activated PDS decreased from 98% to 48% after one recycling of graphitized sawdust biochar, and the reason was that pollutants and their degradation intermediates covered the surface of biochar, which reduced the specific surface area from 297.5 m²/g to 94.74 m²/g^[117]. Yu et al. Have shown that shrimp shell biochar can be recycled twice.Its specific surface area (594 m²/g down to 169.9 m²/g),Both the microporous surface area (236 m²/g down to 4.7 m²/g) and the mesoporous surface area (357 m²/g down to 165.2 m²/g) are significantly reduced,The removal of 2,4-dichlorophenol by activated PDS was inhibited^[26]. The sp², hybrid carbon content and defects of biochar will also be reduced after use^[117]^[31,90]. sp² hybridization is the key structure of biochar electron conduction ability, while the defect is the active site, and the reduction of both will directly affect the activity of biochar.

7.2 Change in chemical properties

Similar to other carbon-based materials, the pH_pzc value of biochar also decreased in different degrees after activation of peroxide^[56,57,124]. This is because peroxide acts on the surface of biochar, which increases the content of acidic functional groups (mainly carboxyl, phenolic hydroxyl and lactone groups), resulting in the decrease of pH_pzc^[124]. The C = O as the active site will also be reduced, which is not conducive to activating the peroxide again^[33]. Gu et al. Reported that the content of surface basic sites decreased from 0.27 mM/G to 0.12 mM/G after biochar activation of H₂O₂, while the content of acidic sites increased from 0.32 to 0.63 mM/G^[51]. Sun et al. Reported that the oxygen content on the surface of lignin biochar increased from 7.8 at% to 17.4 at% after PDS activation, indicating an increase in oxygen-containing functional groups^[67].

For doped biochar, nitrogen doping can introduce nitrogen-containing active sites, and the loss of total nitrogen and the change of nitrogen type are the main reasons for the decrease of activity in the recycling process. Zaeni et al. found that the total amount of nitrogen on the surface of biochar decreased from 12. 1 at% to 9. 5 at%, and the ratio of pyrrole nitrogen to pyridine nitrogen increased from 1. 3 to 2. 2 by comparing the XPS spectra of nitrogen-doped sawdust biochar before and after the first activation of PMS^[78]. Xu et al. Reported that the graphite nitrogen content on the surface of nitrogen-doped sawdust biochar decreased from 48.28 at% to 44.60 at%, the pyridine nitrogen content decreased from 37.68 at% to 26.12 at%, and the pyrrole nitrogen content increased from 14.04 at% to 29.28 at% after PMS activation^[121]. Oh et al. Found that graphite nitrogen was converted to pyridine nitrogen after activation of PMS by nitrogen-doped coffee grounds biochar^[125].

Boron-doped rice straw biochar has excellent stability, the structure of biochar is stable before and after the reaction, and the ratio of carbon to boron is basically unchanged, which benefits from the high bond energy of C — B bond^[91].

7.3 Regeneration mode

There are two main ways to regenerate biochar: pyrolysis and organic solvent washing. Pyrolysis can remove the pollutants and their intermediates covered on the surface of biochar, so that the active sites can be regenerated. Its advantages are high regeneration efficiency, short time, no selectivity for removing pollutants and intermediate products covered on the surface of biochar, but high energy consumption. Organic solvent washing can restore the structure of biochar to a certain extent by extracting the pollutants and their intermediate products covered on the surface and pores of biochar through the solvent.

Sun et al. Pyrolyzed the used cellulose biochar at 700 ° C in an N₂ atmosphere to remove the oxygen-containing functional groups at non-active sites, and the phenol removal rate was increased from 10% to 94% when the regenerated biochar was used to activate PDS^[67]. Xie et al. Regenerated yeast biochar by methanol washing, and the removal efficiency of BPA by PDS activated by regenerated yeast biochar increased from 13.4% to 36%^[33]. Liu et al. Regenerated eggshell biochar by ethanol elution and pyrolysis respectively. After ethanol washing, the removal efficiency of 2,4-dichlorophenol by PDS activation of eggshell biochar was restored from 50% to 70%^[25]. After pyrolysis at 500 ℃, the removal rate of 2,4-dichlorophenol can be restored to 80%.

8 Summary and Prospect

Biochar can activate peroxides such as H₂O₂, PDS and PMS to remove pollutants from water through free radical or/and non-free radical mechanisms. Doping modification can change the physical and chemical properties of biochar, such as increasing the specific surface area, defects, and introducing active sites. By adjusting the pore and graphitization structure of biochar, the conductivity of biochar can be enhanced, and the electron migration of carbon layer can be accelerated. At the same time, biochar has great application potential in the field of advanced oxidation because of its wide source, simple preparation and environmentally friendly characteristics.

However, there are still many problems worth exploring in the current research on the removal of organic pollutants in water by biochar activated peroxide: 1) The physical and chemical properties of biochar prepared by different methods are different. At present, biochar activated by peroxides is mainly prepared by pyrolysis, and the effects of pyrolysis temperature and atmosphere on the catalytic ability of biochar are mainly discussed. However, the effect of different preparation methods and preparation parameters on the activation of biochar needs to be further studied. 2) The composition of biomass precursors is complex, and the current research focuses on comparing the activation ability of biochar prepared from different biomasses, while the effects of different components of biomass (such as lignin, cellulose, protein, mineral elements, etc.) On the activation ability of biochar have not been studied in depth, which is expected to provide a theoretical basis for directional regulation of biochar preparation in the future; 3) The mechanism of peroxide activation by biochar is complex, and the relationship between active sites and active species on the surface of biochar needs to be established; 4) The cavitation mechanism of peroxide activation by biochar has only been verified by quenching experiments, and more and more direct methods are needed to confirm it; 5) Compared with other carbon materials, there are few studies on boron or sulfur doping in the existing heteroatom doping studies, and the doping and activation mechanism of other elements need to be further studied; 6) At present, there are many reports on the removal of pollutants by advanced oxidation technology combined with light, electricity, microwave and ultrasound, but there is no related research report on the advanced oxidation technology based on biochar combined with light, electricity, microwave and ultrasound, which is a development direction in the field of advanced oxidation; 7) In terms of engineering application, although the cost of raw materials and preparation of biochar is low, its activated peroxide also shows good removal effect on pollutants. However, the recycling performance of biochar is not good, and the pore structure and surface properties are easily destroyed, so the preparation of high stability biochar and the research of efficient regeneration technology are the direction of future attention. At the same time, metals in the ash of biochar and the precipitation of polycyclic aromatic hydrocarbons produced during the preparation of biochar (pyrolysis) also need to be solved when activating peroxides to remove pollutants. The following measures can be taken to reduce the secondary hazards of metals and polycyclic aromatic hydrocarbons in the practical engineering application of biochar: 1) adding activators such as Fe in the pyrolysis process,This can not only graphitize biochar, but also reduce the production of polycyclic aromatic hydrocarbons. Higher pyrolysis temperature can also improve the activation ability of biochar and the stability of metals in biochar^[126]^[127,128]. 2) Metals and polycyclic aromatic hydrocarbons in biochar can be removed by washing before use^[129].

We believe that with the deepening and breakthrough of related research, biochar will play an increasingly important role in the field of advanced oxidation.

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Funding

National Natural Science Foundation of China(21806125)

Postdoctoral Science Foundation of China(2016M602365)

Fundamental Research Funds for Central Universities of China (awarded at Wuhan University)

Natural Science Foundation of Anhui Province(1808085MB49)

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1 Introduction

2 Biomass precursor

2.1 Lignocellulose precursor

2.2 Non-lignocellulosic precursor

2.3 Effect of Biomass Precursor on Catalytic Capacity of Biochar

3 Method for preparing biochar

4 Peroxide activation mechanism of biochar

4.1 Free radical mechanism

4.2 Non-radical mechanism

4.2.1 1O2 oxidation mechanism

4.2.2 Electron transfer mechanism

4.2.3 Cavitation mechanism

5 Water quality impact

5.1 Effect of pH

5.2 Effects of anions and natural organic matter

6 Modification of biochar

6.1 Acid-base modification

6.2 Porous graphitization modification

6.3 Doping modification

6.3.1 Nitrogen-doped modification

6.3.2 Other doping modification

7 Recycling and Regeneration of Biochar

7.1 Change of physical property

7.2 Change in chemical properties

7.3 Regeneration mode

8 Summary and Prospect

References

Funding

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4.2.1 ¹O₂ oxidation mechanism