Degradation of Water Pollutants by Biochar Combined with Advanced Oxidation: A Systematic Review

Abstract

Recently, biochar has emerged as a promising option for environmentally friendly remediation due to its cost-effectiveness, extensive surface area, porosity, and exceptional electrical conductivity. Biochar-based advanced oxidation procedures (BC-AOPs) have gained popularity as an effective approach to breaking down organic pollutants in aqueous environments. It is commonly recognized that the main reactive locations within BC-AOPs consist of functional groups found on biochar, which encompass oxygen-containing groups (OCGs), imperfections, and persistent free radicals (PFRs). Additionally, the existence of metallic components supported on biochar and foreign atoms doped into it profoundly impacts the catalytic mechanism. These components not only modify the fundamental qualities of biochar but also serve as reactive sites. Consequently, this paper offers a comprehensive review of the raw materials, preparation techniques, modification approaches, and composite catalyst preparation within the biochar catalytic system. Special attention is given to explaining the modifications in biochar properties and their impacts on catalytic activity. This paper highlights degradation mechanisms, specifically pathways that include radical and non-radical processes. Additionally, it thoroughly examines the importance of active sites as catalysts and the basic catalytic mechanism of BC-AOPs. Finally, the potential and future directions of environmental remediation using biochar catalysts and advanced oxidation processes (AOPs) are discussed. Moreover, suggestions for future advancements in BC-AOPs are provided to facilitate further development.

1. Introduction

In the past few years, there has been a notable increase in global studies concentrating on the treatment of wastewater containing persistent organic pollutants. These compounds present difficulties because of their resistance to degradation [1]. Recently, AOPs have garnered considerable attention for their exceptional potential and distinct benefits, including thorough oxidation, rapid reaction rates, efficient treatment, and a minimal environmental impact [2,3]. AOPs involve the generation of potent oxidizing agents, such as superoxide radicals (•O₂⁻), hydroxyl radicals (•OH), sulfate radicals (SO₄⁻•), and singlet oxygen (¹O₂), through diverse mechanisms like electrical stimulation, acoustic waves, photolysis, and catalytic reactions [4].

This leads to the direct mineralization of recalcitrant substances in sewage or the oxidative degradation of large molecules into more readily biodegradable compounds, thereby enhancing the biodegradability of wastewater [5,6]. In contrast to conventional water treatment methods, AOPs exhibit distinctive features: (1) AOPs relies on the continuous improvement of free radical generation efficiency, and do not produce secondary pollution. This allows sewage treatment to achieve the goal of zero environmental pollution and zero waste discharge. (2) The generated free radicals all exhibit non-selective reactivity, providing a broad spectrum of and a non-discriminatory method for pollutant degradation, ensuring their ability to oxidize most organic pollutants [7]. (3) Free radicals have the advantages of a fast reaction speed, a short time of reaction time, mild reaction conditions, and easy control of operating conditions [8]. (4) AOPs can operate either autonomously or in combination with other processing methods. For example, the combined application of the UV-Fenton process can produce better treatment results, while the Fenton technology alone can effectively address challenging wastewater scenarios, providing cost-effective solutions and improving efficiency [9].

Specifically, AOPs typically employ reactive oxygen species (ROSs) with potent REDOX capabilities to achieve the complete breakdown of recalcitrant contaminants in water systems. ROSs mainly include •OH, SO₄⁻•, •O₂⁻, and ¹O₂. The main ROSs generation systems include Fenton oxidation, photocatalysis, persulfate (PS), ozone oxidation, ultrasonic oxidation, and electrocatalysis. Photocatalysis typically entails a semiconductor being exposed to light containing photon energy greater than the band gap energy of the material. As a result, photogenerated holes and photogenerated electrons are formed, leading to the generation of •OH, •O₂⁻, and ¹O₂ when dissolved oxygen, protons (H⁺), or water (H₂O) are present (Equations (1)−(5) in Table 1) [10]. The Fenton system is a traditional advanced oxidation process mode, and the classic Fenton reaction is performed by the •OH produced by the reaction of Fe²⁺ to H₂O₂ (Equation (6)). Due to the lower cost of Fe³⁺, it is also used as a catalyst, which is called a Fenton-like reaction (Equation (7)). In addition, other complex reactions may occur simultaneously in solution, such as the use of other varivalent metals like Mⁿ⁺, dual-cathode systems, etc., (Equations (8)–(15), Equations (2) and (5)) [2]. The primary mechanism for the electrocatalytic degradation of antibiotics involves both direct and indirect oxidation. Normally, reactions for direct oxidation occur at the anode surface, where electrons transfer directly from the electrode to the antibiotic. The indirect oxidation process primarily includes the generation of active species on either the anode or cathode surface. The most widely used systems are the electrocatalytic system (EC) and photocatalytic system (PEC). For example, Wang et al. [11] stimulate C-O units on the anode with PMS to generate C=O and SO₄⁻• (Equation (16)). Following this, the C=O units are diminished on the light-collecting anode to renew C-O (Equation (17)). SO₄⁻• has the capability to convert OH⁻ to •OH and ¹O₂ (Equations (18) and (19)) [12,13]. In aqueous solutions, ozone reacts with various components in two ways: through the direct oxidation of molecular ozone, which involves a selective reaction; or an indirect mechanism by which •OH radicals are produced through ozonolysis (Equation (20)), which are non-selective and highly reactive [14]. Ozone and •OH free radicals are powerful chemical oxidants involved in the disinfection and oxidation of pollutants. Ultrasonic (US) ( ))) ) treatment is an important advanced oxidation technology which can be used alone or in combination with other advanced oxidation technologies such as O₃/US, H₂ O₂/US, UV/US, etc. The ultrasonic treatment of water can form free radicals [15,16] (Equations (21)–(23) and Equation (9)). Sound waves react with oxygen in the gas phase to form oxygen atoms, which in turn react with water to produce additional free radicals (Equations (24)–(27)) [17].

•OH and SO₄⁻• are both highly reactive free radicals. Compared to the hydroxyl radical •OH, the sulfate radical SO₄⁻• has a longer half-life of 30–40 μs and a higher REDOX potential ranging from 2.5 to 3.1 V. Additionally, it demonstrates pH-independent reactivity. The reaction rate of •OH is 10⁶~10¹⁰ mol/L.s, and that of SO₄⁻• is (3.39 ± 0.22) 10⁹ mol/L.s. The way to distinguish these two types of free radicals is mainly through their production mode and reaction characteristics. •OH is usually produced by processes such as REDOX reactions or photolysis, while SO₄⁻• requires the decomposition of persulfate (PS) to produce it. Activated PS-AOPs have gained significant interest for their ability to efficiently degrade pollutants, attributed to their stability, efficacy, and lack of secondary contamination [18]. The central idea focuses on generating active radicals, such as SO₄⁻• or •OH. These free radicals drive the oxidation of pollutants, converting them to H₂O and CO₂ with excellent selectivity and reactivity. Common persulfate activators include persulfate bisulfate (PDS) and peroxybisulfate (PMS), which have different structures and exhibit different catalytic activities. The mechanisms for activating PDS and PMS vary, resulting in differing production of free radicals and REDOX potentials [19].

The bond energy of PS, which measures 140 kJ·mol⁻¹, is too high for direct interaction with pollutants. Usually, activation is required to break the -o-o- bond and produce SO₄⁻•. In the presence of light, the persulfate system uses energy from ultraviolet or visible light to break the peroxygen bond in the persulfate ion, resulting in the creation of sulfate radicals (Equations (1)–(3), (28), and (29)) [20]. The same principle applies to the thermal activation of PS (Equations (30)–(32)) [21]. Research has shown that ultrasound (US) treatment triggers the conversion of persulfates into sulfate anion radicals through the assistance of PS in collapsing and fragmenting bubbles (Equation (33)) [22]. Moreover, the role of •OH in the generation of SO₄⁻• is brought about by the ultrasound-induced formation of •OH from water (Equations (21) and (34) [23]. Within the electrocatalytic PS system, PS is catalyzed by sulfate ion formation following e-receipt at the cathode (Equation (35)) [24]. The activation of persulfate through alkali also serves as a popular technique, yielding •O₂⁻, SO₄⁻•, and •OH (Equations (36) and (37)) [25,26]. In addition, the transition metal activation of PS can produce SO₄⁻• (Equation (38)). PMS and PDA can self-decompose to generate ¹O₂ (Equations (39)–(42)) [27].

Table 1. Classification table of different free radical sources.

Method	Free Radical	Mechanism		Reference
Photocatalysis	•OH, •O2−, 1O2	Photocatalyst + UV/vis → h+ + e−	(1)	[10]
		h+ + H2O → •OH + H+	(2)
		e− + O2 → •O2−	(3)
		•O2− + H+ + e− → H2O2	(4)
		H2O2 + e− → •OH + OH−	(5)
Fenton process	•OH	Fe2+ + H2O2 → Fe3+ + •OH + OH−	(6)	[2]
		Fe3+ + H2O2 → Fe2+ + OOH• + H+	(7)
		Fe2+ + •OH → Fe3+ + OH−	(8)
		•OH + •OH → H2O2	(9)
		H2O2 + •OH → H2O + OOH•	(10)
		•OH + OOH• → H2O + O2	(11)
		Mn+ + H2O2 → M(n+1)+ + OH− + •OH	(12)
		Fe(OH)2+ + hv → Fe2+ + •OH	(13)
		e− + Fe3+ → Fe2+	(14)
		h+ + H2O → H+ + •OH	(2)
		O2 + 2H+ + 2e− → H2O2	(15)
		H2O2 + e− → •OH + OH−	(5)
Electrocatalysis	•OH, 1O2, SO4−•	C–O + PMS → C=O + SO4−•	(16)	[11,12,13]
		C=O + e− → C–O	(17)
		SO4−• + H2O → •OH + H+ + SO42−	(18)
		SO4−• + H2O → 1O2 + H+ + SO42−	(19)
Ozonation	•OH,	3O3 + OH− + H+ → •OH + 4O2	(20)	[14]
Ultrasonic	•OH	))) + H2O → •OH + •H	(21)	[15,16,17]
		•OH + •H → H2O	(22)
		•OH + •OH → H2O2	(9)
		•H + •H → H2	(23)
		))) + O2 → O + O	(24)
		O + H2O → •OH + •OH	(25)
		•H + O2 → •OOH	(26)
		•OOH + •OOH → H2O2 + O2	(27)
PS	•OH, •O2−, 1O2, SO4−•	S2O82− + e− + UV/vis → SO4−• + SO42−	(28)	[20,21,22,23,24,25,26,27]
		S2O82− + •O2− + UV/vis → SO4−• + SO42− + O2	(29)
		HSO5− + heat → SO4−• + •OH	(30)
		S2O82− + heat → 2SO4−•	(31)
		SO4−• + H2O → SO42−+ •OH + H+	(32)
		S2O82− + ))) → 2SO4−•	(33)
		S2O82− + •OH → HSO4− + SO4−• + 1/2O2	(34)
		S2O82− + e− → SO42− + SO4−•	(35)
		S2O82− + H2O → SO42− + HO2− + 2H+	(36)
		S2O82− + HO2− → SO42− + SO4−• + H+ + •O2−	(37)
		S2O82− + Mn+ → M(n+1)+ + SO4−• + SO42−	(38)
		HSO5− + SO52− → HSO4−+ SO42− + 1O2	(39)
		S2O82− → 2SO4−•	(40)
		SO4−• + H2O → •HSO42−+ •OH	(41)
		2•OH → H2O + 1/21O2	(42)

Table 1. Classification table of different free radical sources.

Method	Free Radical	Mechanism		Reference
Photocatalysis	•OH, •O2−, 1O2	Photocatalyst + UV/vis → h+ + e−	(1)	[10]
		h+ + H2O → •OH + H+	(2)
		e− + O2 → •O2−	(3)
		•O2− + H+ + e− → H2O2	(4)
		H2O2 + e− → •OH + OH−	(5)
Fenton process	•OH	Fe2+ + H2O2 → Fe3+ + •OH + OH−	(6)	[2]
		Fe3+ + H2O2 → Fe2+ + OOH• + H+	(7)
		Fe2+ + •OH → Fe3+ + OH−	(8)
		•OH + •OH → H2O2	(9)
		H2O2 + •OH → H2O + OOH•	(10)
		•OH + OOH• → H2O + O2	(11)
		Mn+ + H2O2 → M(n+1)+ + OH− + •OH	(12)
		Fe(OH)2+ + hv → Fe2+ + •OH	(13)
		e− + Fe3+ → Fe2+	(14)
		h+ + H2O → H+ + •OH	(2)
		O2 + 2H+ + 2e− → H2O2	(15)
		H2O2 + e− → •OH + OH−	(5)
Electrocatalysis	•OH, 1O2, SO4−•	C–O + PMS → C=O + SO4−•	(16)	[11,12,13]
		C=O + e− → C–O	(17)
		SO4−• + H2O → •OH + H+ + SO42−	(18)
		SO4−• + H2O → 1O2 + H+ + SO42−	(19)
Ozonation	•OH,	3O3 + OH− + H+ → •OH + 4O2	(20)	[14]
Ultrasonic	•OH	))) + H2O → •OH + •H	(21)	[15,16,17]
		•OH + •H → H2O	(22)
		•OH + •OH → H2O2	(9)
		•H + •H → H2	(23)
		))) + O2 → O + O	(24)
		O + H2O → •OH + •OH	(25)
		•H + O2 → •OOH	(26)
		•OOH + •OOH → H2O2 + O2	(27)
PS	•OH, •O2−, 1O2, SO4−•	S2O82− + e− + UV/vis → SO4−• + SO42−	(28)	[20,21,22,23,24,25,26,27]
		S2O82− + •O2− + UV/vis → SO4−• + SO42− + O2	(29)
		HSO5− + heat → SO4−• + •OH	(30)
		S2O82− + heat → 2SO4−•	(31)
		SO4−• + H2O → SO42−+ •OH + H+	(32)
		S2O82− + ))) → 2SO4−•	(33)
		S2O82− + •OH → HSO4− + SO4−• + 1/2O2	(34)
		S2O82− + e− → SO42− + SO4−•	(35)
		S2O82− + H2O → SO42− + HO2− + 2H+	(36)
		S2O82− + HO2− → SO42− + SO4−• + H+ + •O2−	(37)
		S2O82− + Mn+ → M(n+1)+ + SO4−• + SO42−	(38)
		HSO5− + SO52− → HSO4−+ SO42− + 1O2	(39)
		S2O82− → 2SO4−•	(40)
		SO4−• + H2O → •HSO42−+ •OH	(41)
		2•OH → H2O + 1/21O2	(42)

Biochar-derived catalysts, resembling a representative catalytic material, have been suggested as catalysts for cutting-edge oxidation technologies owing to their remarkable performance, expansive surface area, and various methods of preparation [28,29,30]. Furthermore, as a cutting-edge method in oxidation technologies, the activation of PS entails the implementation of unique catalysts derived from biochar to activate PS. The main mechanism of pollutant degradation is free radical attack. This mechanism, when compared to non-free radical pathways, exhibits lower effectiveness [31]. Therefore, modification of the original biochar-based catalyst is necessary to enhance its performance. Two crucial factors for improving catalyst performance are the introduction of a porous structure and modification with metal doping and abundant heteroatoms [32]. Enhancing the surface area of biochar offers the benefit of boosting the degradation efficiency of ROPs by increasing the absorption of pollutant molecules and exposing surface active sites more effectively. Additionally, modifying the initial carbon network through the introduction of metal doping or heteroatoms can activate the catalyst and redistribute the charge density of nearby carbon atoms, leading to the creation of numerous active sites [33]. As a result, the analysis of catalysts in this research primarily concentrates on elucidating the adjustments made to biochar properties and their impact on catalytic performance.

2. Preparation of Biochar

2.1. Feedstock

The specific conditions during the production of biochar can impact its properties, which are also influenced by the composition of the biomass source [34,35]. The pivotal role is played by the heterogeneity of the feedstock, which includes various organic components like proteins, lignocellulose, lipids, etc., as well as inorganic substances like mineral composition, moisture, and volatile fraction. Carbon fixation and the construction of aromatic structures are greatly influenced by lignocellulose among these constituents. Based on research conducted by Das et al. [36], the investigation revealed that an increase in the level of lignin within the feedstock led to an escalation in the production of biochar. Commonly used feedstocks include terrestrial plant sources, aquatic plant sources, animal manure, and sludge biogenic sources. In most cases, a higher specific surface area (SSA) and lower ash content are superior biochar, and biochar extracted from plants is more eligible than biochar obtained from manure [37]. The variances in the feedstock are evident in the biochar’s morphology and structure. Specifically, aquatic plant sources, such as algae or water hyacinth, typically have higher inorganic and nitrogen content due to minerals and proteins in the feedstock [38].

Compared to biochar derived from plants on land, aquatic biomass biochar generally exhibits cation exchange capacity (CEC) and lower SSA, as well as higher ash content [39,40]. The effectiveness of different sources of biochar depends on the ash content. For example, biochar obtained from animal manure demonstrates reduced carbon content, CEC, SSA, and porosity, as well as dissolved organic carbon (DOC), while displaying elevated ash content [41]. However, it is relatively rich in nitrogen compared to other categories [42]. The nature of sludge biochar is variable and is associated with changes in sludge source over time and space. When utilizing biochar made from sludge, it is crucial to acknowledge the potential hazards linked to the presence of high levels of metallic elements. Recent studies have provided evidence of the importance of these metallic elements in the creation of PFRs [43]. Wang et al. effectively degraded 2,4-dichlorophenol (2,4-DCP) using a biomimetic catalyst composed of iron-coated biochar (referred to as biomimetic Fe⨀BC). Their approach involved utilizing Siberian iris, an aquatic plant source with a considerable ash content and specific surface area [44]. The research is characterized by the possibility of reusing waste materials, in particular “biochar” obtained from iron-rich aquatic plants through direct pyrolysis.

2.2. Biochar Preparation Method

2.2.1. Pyrolysis

Pyrolysis is the process of thermal and chemical decomposition of biomass or living organisms at temperatures above 400 °C in environments with low oxygen levels or inert gases. The process leads to the creation of three primary products: biochar, a solid byproduct; bio-oil, also known as pyrolytic oil, a liquid substance; and finally, syngas, a blend of non-condensable gases. Syngas mostly contains CO, CO₂, H₂, and CH₄. The non-condensable gases generated during the pyrolysis process are commonly known as synthesis gases. There are two main stages. In the initial phase, heat is conveyed to the surface of the particle through radiation or convection, and subsequently to the particle. During pre- and first pyrolysis, a continuous temperature increase results in water removal from biomass pellets. During the duration of the process, the generated volatiles and syngas permeate the particle’s pores, facilitating heat transfer over time [38]. The solid pore expansion is initiated during the second phase, commencing with the conversion of biomass into gas [45]. During the pyrolysis of hot biomass [46], the interaction between volatile gases and syngas experiences significant enhancement due to the extensive enlargement of pores.

Different categories of pyrolysis processes can be classified based on their heating rates. These categories are divided by pyrolysis rate, including slow, fast, and flash. Among these, the most commonly utilized method is slow pyrolysis. In intermittent or continuous systems, the crucial role is played by the size of the particles and the moisture content, especially in continuous systems. During the pre-pyrolysis process, the temperature range typically falls between 200 °C and 246 °C. This marks the initial stage of biomass decomposition. This stage is characterized by multiple transformations, including internal reorganizations, the disruption of chemical bonds, the removal of water molecules, the appearance of unpaired electrons, and the creation of carboxyl groups, carbonyl groups, and hydroperoxides. The primary pyrolysis processes occur rapidly in the second stage, while the char undergoes slow decomposition in the third stage. As a consequence, a solid residue abundant in carbon content, known as biochar, is generated. Typically, the slow pyrolysis process produces a ratio of biochar, bio-oil, and syngas of 0.35:0.3:0.35. Higher heating rates facilitate biomass decomposition, producing more volatiles and less biochar.

Biomass pyrolysis degradation includes cellulose, hemicellulose, lignin, and other trace organic matter. The degradation process is affected by factors such as pyrolysis temperature, heating rate, pressure, biomass particle size, and reactor structure. Hemicellulose decomposition initiates between 240 and 270 °C, cellulose decomposition occurs between 260 and 320 °C, and lignin decomposition starts between 290 and 400 °C. The pyrolysis reaction consists of two basic reactions: one is cellulose pyrolysis, which decomposes and cokes at a lower heating rate and temperature; the other is cellulose pyrolysis, which rapidly volatilizes and produces levoglucan at higher heating rates and temperatures.

In addition to the above three substances, the pyrolysis mechanism of biomass is also significantly affected by a variety of inorganic compounds. In the course of biomass pyrolysis, a high mobility of K and Cl is observed, which allows them to volatilize at lower temperatures. Conversely, high temperatures are necessary for the evaporation of Ca and Mg as they are chemically bonded to organic molecules, either ionically or covalently. On the other hand, organic compounds in biomass and plant cells will decompose at lower temperatures during the pyrolysis process. During this procedure, particular inorganic elements like alkali metals and alkaline earth metals are crucial catalysts. For instance, compounds with K, Ca, and Mg serve as catalysts in the subsequent breakdown of unstable compounds produced during pyrolysis. The unstable compounds primarily consist of gaseous elements like CO, H₂, CH₄, C₂H₄, and CO₂.

2.2.2. Co-Precipitation

The process of co-precipitation is a vital technique used in the creation of biochar, which includes the concurrent precipitation of multiple substances from a solution. At the same time, it is also possible to combine the co-precipitation method with the carrier, and it becomes feasible to fix metals, metal oxides, or composite materials on the biochar. And, in biochar preparation, coprecipitation is often combined with other techniques such as pyrolysis and calcination. Thanks to the extensive use of this basic yet essential technique, scientists have been able to investigate the production of magnetic biochar and active biochar from different types of biomass. Do et al. [47] synthesized magnetic biochar through co-precipitation with the Fe²⁺ and Fe³⁺ solution, and examined the adsorption capabilities of magnetic biochar on crocin in a water-based solution. Another crucial feature of coprecipitation is that it involves the precipitation of a solute from a solution with the help of a carrier. This method causes the solute to adhere to the carrier rather than dissolve in the water. By employing this technique, it becomes feasible to immobilize metals, metal oxides, or composites onto biochar. Researchers have been able to explore the generation of magnetic biochar and activated biochar from various biomasses due to extensive utilization of this uncomplicated yet essential method. In their research, Saravanan and colleagues [48] investigated the use of the biological macromolecule gulan agglutinative (GK). The preparation of magnetic biochar GK involved co-precipitation of Fe²⁺ and Fe³⁺ ions on GK with the presence of biomacromolecules in GK. This was achieved by using an ammonia solution in a 2:1 ratio. This well-crystallized iron-containing biochar demonstrated thermal stability compared to raw GK containing iron oxide particles in a spherical aggregate structure. Additionally, Asgharzadeh et al. [49] followed a similar approach, where they produced biochar (MgBC) from rice bran in alkaline conditions through the co-precipitation method. Following this, a composite of TiO₂/MgBC was created utilizing the sol-gel approach. These composites displayed a 97% degradation rate of tetracycline, showcasing impressive durability. A recent study used co-precipitation to prepare MnFe₂O₄/biochar composites, which were then used to activate H₂O₂ to degrade tetracycline (TC). Throughout the entire experimental process, it was discovered that the catalyst could be reused and there was minimal leaching of metal ions [50]. Pi et al. [51] utilized a comparable approach to fabricate biochar with magnetite loading for triggering PS to break down TC. Furthermore, the produced catalyst exhibited consistency over three rounds, with leaching reported to be below 3 mg·L⁻¹.

2.2.3. Hydrothermal

The hydrothermal treatment process involves adding the necessary ingredients to purified water and then transferring the resulting mixture to a tightly sealed autoclave lined with PTFE. This ensures that the catalyst can be uniformly prepared under specific reaction conditions. The temperature of the autoclave is then heated and maintained to the set temperature. It is essential to conduct experiments at various time and temperature parameters to determine the optimal conditions for transforming the original substance into the final product, as these conditions could vary based on the characteristics of the original material. Zhang and colleagues [52] extensively investigated the development of magnetic biochar using iron sludge and bio-sludge for H₂O₂ activation. To maximize the efficiency of degradation, experimental trials were implemented to ascertain the ideal proportions, temperature, and duration of the reaction. The hydrothermal method offers the advantage of a one-step process for preparing biochar-loaded metal oxide catalysts, enabling the formation of metal oxides with diverse structures [53]. Ma et al. [54] utilized corn cobs, an abundant agricultural waste, to produce biochar within 6 h under the existence of FeCl₃·6H₂O. The procedure ensures that the iron oxide particles are securely attached to the corn cob surface. By hydrothermal method, Fe³⁺ was introduced to form a mixed structure of large and medium pores in the overall morphology. In a similar vein, Gao et al. [55] conducted a study where they synthesized biochar from pine cones utilizing FeCl₃·6H₂O. The prepared Fe₃O₄/P biochars exhibited good crystallization, a porous structure, and close adherence to iron tetraoxide particles.

Taken together, these technologies can make use of different categories of agricultural residues, each with its own advantages and disadvantages. It is important to consider these factors when choosing a biochar preparation method. Pyrolysis is convenient. When produced at high temperatures, it exhibits special porosity and extensive surface area and can be diversified via preparation of different metal forms of catalysts, but an atmosphere of inertia is required. Co-precipitation is the most common method for preparing supported catalysts and mixed-metal catalysts, but the process lacks control and precision. The temperature and pressure conditions required by the hydrothermal method are mild, but the biochar catalyst composites have a relatively low specific surface area. The biomass, characteristics, and degradation effects of biochar-based catalysts prepared by the above three methods are shown in

Table 2. Application of biochar-based catalysts prepared by different methods in water treatment.

Method	Biomass	Catalyst	Characteristics and Advantages	Removal Property	Reference
Pyrolysis	Wheat straw	CeO2	Higher BET, average pore size and pore volume.	The removal rate was high (95.8%), and the removal rate was still 87% after 5 cycles of experiment.	[56]
	Wood	Graphene oxide	Higher BET, porous structure and thermal stability.	More effcient removal of organic pollutants through π-π EDA interaction and the maximum adsorption capacity was 30.78 mg·g−1.	[57]
	Waste walnut shell	TiO2	Strong interaction between BC and TiO2 effectively promotes the transfer of e− in TiO2.	The decolorization rate reached 96.88%, and the activity was still high after 5 catalytic experiments.	[58]
	Corn stover	ZnO/ZnS	Higher BET, porous structure and total pore volume.	The maximum adsorption capacities were 135.8, 91.2 and 24.5 mg·g−1.	[59]
	Rice husk	Fe3O4	presents a superior magnetic response.	Displayed a preeminent adsorption performance for U(VI).	[60]
Co-precipitation	Corncob	MgCl2 and CaCl2	High surface area, mesoporous structure.	The adsorption of P reached 326.63 mg·g−1.	[61]
	Kans grass straw (Saccharum spontaneum)	FeSO4•7H2O and FeCl3•6H2O	MGKB has ferromagnetism, high thermal stability, higher surface porosity, high surface area, and large total pore volume.	When pH = 13.5, the adsorption capacities of As(III) and As(V) are 2.004 mg·g−1 and 3.132 mg·g−1. The adsorption effect is best at this time.	[62]
	Sugarcane baggase (SCB)	FeCl3 and FeSO4	It has a porous structure with a large number of carboxyl groups and negative charges on the surface.	The adsorption capacities of Pb2+ and Cd2+ were 1.2 and 1.1 mmol·g−1, respectively. When C(Pb): C(Cd) is greater than or equal to 4:1, the magnetic adsorbent can selectively adsorb Pb2+.	[63]
	Peeled pine wood (Pinus massoniana)	Hydrous-manganese oxide (HMO)	High BET and porosity. The surface hydroxyl group increases and the pHPZC(zero charge point pH) of the carbon decreases.	When pH = 5.00, the removal efficiency of lead (II) increased from 6.4 to 98.9%.	[64]
	Oak wood and oak bark	Fe2(SO4)3•nH2O (n: 6–9) and FeSO4	High surface area, large porosity, high MS value.	The adsorption capacities of MOWBC and MOBBC were Pb2+ 10.13 and 2.87 mg·g−1 and Pb2+: 30.2 and Cd2+: 7.4 mg·g−1.	[65]
Hydrothermal	Fresh olive waste	/	Carboxyl and carbonyl groups increased by about 300%.	Removes approximately 100% of methylene blue and Congo red. The three adsorption cycles have a repeat utilization rate of about 80%.	[66]
	Pine wood	/	The optimum carboxylic acid content of hydrocarbon surface was obtained by oxyketone treatment.	The maximum adsorption capacity of MB was 86.7 mg·g−1 and Pb(II) was 46.7 mg·g−1.	[67]
	wheat straw	Fe	Iron-modified hydrocarbons have higher voids, roughness and specific surface area.	Under the conditions of initial pH 6, concentration of RhB 5 mg·L−1, concentration of RhB 1 g·L−1 and adsorption time 90 min, the optimal adsorption efficiency of fe modified hydrocarbons for RHB is 91%.	[68]
	Mg-doped grape pomace Mg-doped corn cob Mg-doped Miscanthus × giganteus	Mg	Hydrogen bonding, π-π interactions, electrostatic interactions, and surface complexation all play a role	Their adsorbability is 289.65 mg·g−1, 262.30 mg·g−1, and 232.48 mg·g−1.	[69]
	sugarcane bagasse	Ni, Fe	High BET and contains a large number of carboxyl and metal carboxyl groups, hydrogen bonds, π-π or π-eda interactions, surface complexation, and acid-base interactions.	Qmax = 395.9 mg·g−1 for CV dye and 568.1 mg·g−1 for TCQmax. After 4 regeneration cycles, it has good recyclability.	[70]

.

2.3. Modification of Biochar

An important step in preparing biochar is modification. Reasonable modification can change its physical and chemical properties to improve its adsorption and activation properties. This process is crucial in order to optimize its effectiveness. Various methods can be utilized to categorize biochar modification, including physical, chemical, and biological modifications. Changes induced by external energy and matter manifest in the modification of basic structure, alterations in surface functional groups, and variations in loading characteristics. Lee and Shin conducted a comparative study on five different modification techniques, namely acid impregnation, alkali impregnation, oxidation impregnation, MnO_x impregnation, and FeO_x impregnation [71]. The findings revealed significant alterations in biochar structure, with a 7.3–61.3-fold rise in specific surface area, resulting in an improved capacity for heavy metal adsorption. Specifically, the acid modification technique eliminates impurities and introduces acidic functional groups [72]. Modifying the alkalinity method [73], conversely, elevates the surface area of biochar while introducing OCGs, predominantly -OH and -COOH. Alternatively, physical alterations like ball milling or mechanochemical adjustments aim to break down non-structural components and provoking structural modifications. Ball milling technology is a technology that can reduce the size of a material while increasing the specific surface area. Ball milling technology can also introduce new elements into the material, promoting the exposure and formation of functional groups in the material. If treated with ultraviolet light, oxygen-containing functional groups can also be introduced. Zhang et al. [74] and Peng et al. [75] found that the surface C=O and C-H intensifies after the biochar is treated with ultraviolet light, which improves the polarity and aromatization of the biochar. Steam treatment has the potential to improve the pore arrangement of biochar. It is crucial to acknowledge that every adjustment technique demonstrates its individual restrictions, encompassing energy usage and the likelihood of subsequent contamination. Despite receiving less focus, biological alteration presents a more harmonized ecological strategy. According to Yuvaraj’s findings [76], earthworm-manipulated biochar, coupled with enzymes like metallothionein, β-glucosidase, carboxylesterase, and humic acid, holds promise as a material for combatting pollution.

3. Metals Loading

The addition of metal into biochar has been acknowledged as a successful approach to boost its catalytic potential. There are many studies on the activation of PS by transition metals. Transition metals, with a lone electron in their electronic setup, play a crucial role in the activation of PS. This activation leads to the creation of SO₄⁻•, which assists in both gaining and getting rid of electrons. Adding metal to biochar can enhance the catalytic reaction efficiency, increase its utilization effectiveness, and decrease metal contamination [77,78]. PS-AOP systems have received thorough examination in the realm of single-metal, binary, and multi-metallic catalysts. The synthesis of biochar-based materials utilizes metal salts or metal oxides, wherein the metal-loaded biochar is derived in situ from biomass-containing metals [79,80]. Predominantly, metals like Co and Fe are utilized, and various transition metal ions (Mn²⁺, Co²⁺, Cu²⁺, and Fe²⁺) have exhibited catalytic abilities in activated persulfate systems. After the loading of transition metals, biochar usually generates free radicals for degradation. Mⁿ⁺ metal exhibits the capacity to facilitate REDOX reactions by supplying the essential electrons for free radical generation. Consequently, PS disrupts the O-O linkage, generating SO₄⁻•, which then interacts with H₂O to produce •OH. The cyclic process is completed by the reaction of Mⁿ⁺¹ with PMS, resulting in the regeneration of Mⁿ⁺ (Equations (43)–(45) and (18)). In a thorough investigation conducted by Tan et al. [81], emphasis was placed on producing nanocomposites from biomass with the aid of biochar. Their main approach involved treating biomass before and after processing biochar (Figure 1). Comparable methods have been employed to develop biochar-based substances for activating PS.

Mⁿ⁺ + HSO₅⁻ → Mⁿ⁺¹ + SO₄⁻• + OH⁻

(43)

Mⁿ⁺ + HSO₅⁻ → Mⁿ⁺¹ + •OH + SO₄²⁻

(44)

Mⁿ⁺¹ + HSO₅⁻ − → Mⁿ⁺ + SO₅⁻• + H⁺

(45)

3.1. Co-Based Catalysts

The activation of PS has garnered significant attention, particularly in relation to the use of catalysts like cobalt oxide and biochar. One particularly effective method of PS activation is the utilization of cobalt-loaded biochar, which shows remarkable performance under acidic conditions. The enhanced activity observed in cobalt ions is due to the creation of multiple SO₄⁻• species. These species are generated when divalent and trivalent cobalt ions undergo transformation. However, concerns arise regarding undesired cobalt leaching in the catalytic process, particularly under acidic conditions [82,83,84]. The research analyzed cobalt levels in water subjected to post-PMS catalytic activation reactions under acidic and neutral conditions. The results show that the leaching concentration of cobalt under neutral conditions is reduced by 0.70 mg·L⁻¹ compared with that under acidic conditions. These findings demonstrate that the pH level impacts the cobalt leaching concentration in the catalytic activation process [82]. To tackle the cobalt leaching problem, various carriers for Co₃O₄ have been investigated by researchers. These include inert materials in one, two, or three dimensions [85,86], metal oxides [87], waste from industrial processes [88], magnetic particles [89], molecular sieves [90], and adsorbents [91]. Adding Co₃O₄ to the biochar catalyst can notably boost its catalytic performance compared to using Co₃O₄ alone. This enhancement is likely due to the biochar’s ability to enhance the dispersal of Co₃O₄, decrease Co leaching, and facilitate the catalyst’s separation/recovery from the treated water. Furthermore, the catalyst has also been designed with additional functionalities, specifically photocatalysis. However, challenges persist, including the material’s monolithic chemical composition, limited acid resistance, and pronounced issues of metal ion leaching. The stability of this material in practical applications, particularly concerning the leaching of metal ions, requires further improvement. Liu reported the degradation of atrazine using cobalt-containing biochar [92]. Chen et al. [93] achieved the degradation of antibiotics through PMS activation using biochar-supported Co₃O₄. According to the research findings, the promotion of electron transfer between HSO₅⁻ and Co species leads to the emergence of a synergistic effect between the loaded Co₃O₄ and biochar. Consequently, this synergy facilitates the redox cycle of Co³⁺/Co²⁺ [94].

3.2. Cu-Based Catalysts

Compared to other metal-biochar materials, copper-biochar materials are cost-effective, making the exploration of their catalytic activation properties economically significant. When examining the comparative performance of two highly stable copper oxide catalysts, namely CuO and Cu₂O, it is evident that CuO displays a catalytic activity that is approximately 1.5–2.0 times greater than that of Cu₂O [95,96]. Nevertheless, an optimal performance can solely be attained with a heightened CuO load. To illustrate, when subjecting 50 mg·L⁻¹ phenol to a combination of copper oxide and mM Oxone^®, superior results are achieved [97]. In addition, by integrating CuO into ZSM₅, a carrier characterized by its expansive surface area, exceptional stability, and potent affinity for pollutants, the activity of CuO can be further amplified. This approach not only curtails CuO aggregation but also stimulates the augmentation of the catalytic reaction’s specific surface area [98,99]. CuO, apart from its ability to activate PMS, also demonstrates the capability to activate PS, thereby facilitating the breakdown of specific organic contaminants without generating SO₄⁻• [100]. In a study conducted by Li [101], an investigation into the activation of PS through the implementation of copper oxide-loaded biochar was performed. The outcome resulted in the effective breakdown of ROPs. In research by Luo et al. [102], the effectiveness of various metal-supported biochar nanocomposites (CuO/BC, Fe₃O₄/BC, ZnO/BC) was examined in catalyzing the degradation of Bisphenol A (BPA) (Figure 2a). The reaction pathway, primarily dominated by non-degradation, was elucidated based on the intermediates involved in electron transfer within the catalytic system (Figure 2b).

3.3. Fe-Based Catalysts

Due to their cost-efficiency, environmental friendliness, and superior performance, iron-based materials are widely used in various catalytic applications. Common Fe-based activators for PMS include zero-valent iron (ZVI), ferric oxide, and ferrous oxide. Aggregation poses a significant challenge for nanoscale particles such as nano ZVI, owing to their heightened surface energy and inherent magnetic interactions. If the nanoparticles accumulate in the biochar, the BET and the reactivity of the catalytic reaction will be reduced [103]. In addition, the oxidation of ZVI further reduces its reactivity [104]. The preparation of biochar loaded with ZVI can not only prevent the aggregation behavior, but also improve the effect of pollutant degradation [105]. Previous research has documented the capabilities of biochar/ZVI nanocomposites in eliminating substances such as nonylphenol [104], decabromodiphenyl ether [106], and bisphenol A [107]. Luo et al. found that [108] biochar/ZVI nanocomposites are characterized by oxidation resistance, reusability, and stability. The process of activating PMS with ZVI, as illustrated in Equations (46)–(49), has the capability to produce •OH and SO₄⁻• [109,110]:

Fe⁰ + HSO₅⁻ + 2H⁺ → Fe²⁺ + H₂O + HSO₄⁻

(46)

Fe²⁺ + HSO₅⁻ → Fe³⁺ + SO₄⁻• + OH⁻

(47)

Fe³⁺ + HSO₅⁻ → Fe²⁺ + SO₅⁻• + H⁺

(48)

Fe⁰ + HSO₅⁻ → Fe³⁺ + •OH + SO₄²⁻

(49)

Catalyst separation and recovery can be facilitated by loading biochar with Fe₃O₄ [111], γ-Fe₂O₃ [112], or CoFe₂O₄ [113], thus achieving magnetization. The ease of separation and high catalytic activity for persulfate activation are enhanced by the pyrolysis-prepared magnetic biochar, which contains encapsulated iron oxide nanoparticles. Rong et al. [112] synthesized Fe₂O₃ biochar (γ-Fe₂O₃-BC) using banana peel as a precursor material by employing hydrothermal technology, activated PS, and degraded BPA. In a similar study, Dong et al. [114] developed a composite material by loading Fe₃O₄ particles onto biochar. Furthermore, Ouyang et al. [115] synthesized a nanomagnetite biochar composite material, also incorporating biochar loaded with Fe₃O₄ particles.

Surface iron sites on biochar play a crucial role in Fenton-like systems. Biochar derived from sludge, as prepared by Li et al. [116], features abundant Fe₂O₃ and Fe²⁺ sites. At pH = 4.0, the synergistic effect of adsorption and the H₂O₂ activation process allowed the removal rate of ciprofloxacin to reach 90%. Furthermore, diverse iron sites like Fe⁰, Fe_0.95C_0.05, FeAl₂O₄, and Fe₃O₄ possess the capacity to induce both homogeneous and heterogeneous Fenton-like reactions concurrently, thereby promoting the decomposition of contaminants. Gan et al. [117] prepared an iron-enriched biochar with Fe⁰ and Fe_0.95C_0.05 sites on its surface, which induced Fenton reactions by means of leached Fe²⁺. Meanwhile, the FeAl₂O₄ and Fe₃O₄ sites directly engaged in H₂O₂ activation through the Fe²⁺ present on the surface. Additionally, iron sites enhance the stability of sludge-derived biochar. Zhang et al. [52] prepared magnetic biochar composites (MBCs) by mixing bio-sludge with iron-based sludge. The reinforced chemical bonding between biochar and Fe₃O₄ significantly enhanced the stability and reusability of MBCs, achieving a 98% removal rate for methylene blue after five cycles.

The Fenton-like reaction mechanism of iron-rich biochar can be delineated as follows: (1) Surface multivalent iron sites directly activate H₂O₂ to produce •OH or, under acidic conditions, leached Fe²⁺ on the surface activates H₂O₂ to generate •OH, as shown in Equations (50)–(53) and (7); (2) H₂O₂ can be activated by the single-electron transport mechanism of PFRs to produce •OH; (3) within a photo-Fenton system, the illumination facilitates the degradation of H₂O₂ and the subsequent production of •OH, concurrently promoting the regeneration of Fe²⁺, as presented in Equations (54) and (55); (4) Fenton-like systems mainly utilize •OH as the main free radical-degradation pollutant. Furthermore, biochar made from sludge possesses a porous composition and expansive surface area, enabling it to efficiently absorb various pollutants.

Fe⁰ + 2H⁺ → Fe²⁺ + H₂

(50)

Fe²⁺ + H₂O₂ + H⁺ → Fe³⁺ + H₂O + •OH

(51)

Fe⁰ + 2Fe³⁺ → 3Fe²⁺

(52)

Fe³⁺ + H₂O₂ → Fe²⁺ + HO₂• + H⁺

(7)

Fe³⁺ + HO₂• → Fe²⁺ + O₂ + H⁺

(53)

H₂O₂ + hν → 2•OH

(54)

Fe³⁺ + H₂O₂ + hν → Fe²⁺ + HO₂• + H⁺

(55)

3.4. Mixed-Metal Catalysts

Remarkable focus has been placed on the synergistic mechanism of multi-metal biochar due to its exceptional physicochemical characteristics, remarkable stability, and minimal metal leaching, surpassing that of biochars with only one metal. In multi-metal-loaded biochar, the catalytic efficiency is enhanced through electron transfer between metals. Recently, the development of bimetallic oxide biochars, such as AXB₃-XO₄-type materials (e.g., CuFe₂O₄, CoFe₂O₄, and CoMn₂O₄), has been explored for advanced applications in persulfate oxidation due to their stable spinel structures, synergistic effects, and stability [118]. Biochar impregnated with ferrite (MFe₂O₄, where M denotes a transition metal) possesses the capability to ameliorate the downsides associated with metal depletion and particle clustering. Moreover, it effectively tackles the concern of catalyst segregation [119]. Studies have demonstrated the effectiveness of composite materials like CoFe₂O₄/biochar and CuFe₂O₄/biochar as activators for the persulfate degradation of BPA [113], isoproterenol [120], and 4-nitrochlorobenzene [121].

Hao et al. [122] discovered a synergistic interaction between iron and manganese ions in Fe-Mn oxide biochar. Initially, Mn primarily governs the reaction, while Fe later assumes the forefront in activation processes. In studying the degradation of tetracycline, the literature has also delved into the application of Mn-fortified magnetic biochar [123] for PDS activation. Additionally, Co₃O₄-SnO₂/BC was used as catalyst to activate PS to degrade thiazole sulfonate [124].

Bimetallic biochar catalysts have been utilized in Fenton-like reactions, exhibiting collaborative impacts on catalyzing hydrogen peroxide (H₂O₂) [125]. Taking iron-copper-modified chili straw biochar (CuFeO₂/BC) as an example, the catalytic degradation of tetracycline by CuFeO₂/BC using H₂O₂ was investigated. The catalysis of •OH production from H₂O₂ can be facilitated by Fe²⁺ and Cu²⁺. Cu²⁺ undergoes reduction when it reacts with H₂O₂ and •O₂⁻, resulting in the production of Cu⁺. This process of reduction is intriguing because the standard potential reduction of Cu²⁺/Cu⁺ is less than that of Fe³⁺/Fe²⁺. Consequently, this allows for the regeneration of Fe²⁺ via the reaction between Fe³⁺ and Cu⁺ [126]. The reaction rate can be accelerated by constructing a suitable bimetal-modified biochar catalyst, which facilitates the cyclic interaction between metals of high and low valence and promotes the production of •OH.

Various transition metals, with a specific emphasis on cobalt, are incorporated into the biochar structure to enhance its catalytic efficiency. The incorporation of multiple metals is a highly promising approach for modifying biochar, offering various benefits including enhanced synergistic effects, minimal metal leaching, and excellent stability. Studies have demonstrated that incorporating metal nanoparticles into biochar can greatly enhance its catalytic performance. Nevertheless, attaining accurate regulation of the metal content during the preparation process remains a major obstacle. Additionally, there is a need to strengthen research on the mechanisms of synergistic effects among different metals.

4. Heteroatomic Doping

The introduction of heteroatoms into biochar is a feasible method to improve its catalytic efficiency. By introducing C, N, P, S, and other heteroatoms into the carbon lattice to replace the carbon atoms on biochar, the properties of biochar are changed [127,128,129]. The presence of heteroatoms causes shifts in the electron density, boosts electron transportability, raises the number of defect edges, and introduces fresh active locations. As a result, catalytic electron transfer reactions can be expedited. There are typically two doping methods: in situ doping and diffusion doping. The process of in situ doping includes the direct carbonization of biomass precursor materials that contain heteroatoms, resulting in the introduction of heteroatoms into the carbon material [130,131]. On the contrary, diffusion doping involves exposing carbon materials to gases or liquids containing heteroatoms in high-temperature or high-pressure environments, allowing heteroatoms to diffuse into the carbon matrix, thus creating carbon materials with heteroatom doping [132,133].

4.1. Nitrogen Doping

The manipulation of charge distribution and spin density via nitrogen (N) incorporation improves the method in enhancing the catalytic performance of biochar that is initially unreactive [134]. Various external methods for nitrogen doping have been reported, involving different nitrogen sources such as urea [135], melamine [136], dicyandiamide [137], thiourea [138], ethylenediamine [139], polyacrylamide [140], and nitrogen-containing compounds like ammonium nitrate [141], ammonium chloride [142], and ammonium phosphate [143]. Xu et al. [136] conducted an experiment to fabricate biochar with nitrogen doping through a one-step calcination process by utilizing various nitrogen sources. The experimental findings indicated that the nitrogen levels in the initial material significantly impacted the catalytic efficiency of the resulting biochar. Liu et al. [120] applied melamine as the nitrogen supplier and implemented a liquid-phase doping strategy to produce magnetically retrievable biochar doped with nitrogen. After being calcined at a temperature of 800 °C, the biochar displayed an outstanding surface area and graphitization performance, leading to its exceptional catalytic activity and stability. Solid-phase doping is a method that does not rely on solvents and instead involves grinding nitrogen sources and carbon materials together. The resulting nitrogen-doped biochar can be obtained through subsequent calcination under inert atmospheres. Ye et al. [144] created nitrogen-doped biochar by grinding biochar and impregnating it with K₂FeO₄ and urea. Figure 3 provides an illustration of their methodology. A structural analysis through XRD, Raman, and XPS spectra confirmed the graphite carbon structure and successful nitrogen doping into biochar.

An effective strategy for in situ nitrogen doping, which does not require the use of nitrogen sources, entails directly pyrolyzing precursors containing abundant nitrogen. Natural nitrogen precursors, such as proteins found in residue from spirulina, can be utilized for this purpose. A separate investigation carried out by Xie et al. [145] centered on the synthesis of biochar with N-doping, utilizing Fusarium, which is abundant in nitrogen, as the precursor. Moreover, achieving the accurate management of N-doped biochar characteristics by means of in situ N-doping poses a noteworthy obstacle that necessitates more thorough examination and study.

Biochar containing nitrogen, which has been prepared in advance, showcases a porous structure, abundant defects, a substantial number of nitrogen-containing substances, and a large surface area. These characteristics contribute to its exceptional catalytic performance in degrading ROPs. The size resemblance between carbon atoms and graphene enables the effortless integration of nitrogen atoms into the graphene sheet structure. Furthermore, due to nitrogen’s higher electronegativity compared to carbon, carbon atoms situated next to nitrogen atoms could potentially act as Lewis basic sites [146]. The electronegativity difference between nitrogen and the surrounding carbon accelerates electron transfer and contributes to the formation of defect structures [8,134,147].

Biochar contains dominant nitrogen functional groups, namely pyridine-N and pyrrole-N, structurally resembling nitrogen found in graphene. In a research project led by Yuan et al. [148], the advantageous impacts of graphene-N and pyridine-N, enhancing the electrocatalytic performance of graphene, were investigated. Wang et al. [134] used biochar obtained from different combinations of corncob and urea as a means of PDS activation to decompose sulfadiazine. The researchers found that the nitrogen-doped adaptive biochar was four times more catalytic than before doping. This progress can be attributed to the binding of different sites of active nitrogen (such as pyrrolo-N, pyridine-N, and graphite-N) or the nitrogen functionalization of biochar (including N oxides and amine functional groups), which enhance its catalytic capacity. In the subsequent year, this research group utilized sludge as a raw material to produce sulfurized sludge biochar (SSB) at a temperature of 700 °C and conducted a comparative examination of the activation of PDS and PMS. According to reports [144,149], carbon atoms have a larger radius than graphite-N, with lower electronegativity, allowing electrons to transfer from less electronegative carbon to more electronegative nitrogen. At this juncture, carbon turns electron-deficient and acquires electrons from PMS. The extraction of electrons from PMS results in the formation of SO₅⁻•, and subsequently, it can undergo a reaction with water, leading to the production of ¹O₂ (Equations (56) and (67)).

HSO₅⁻ →SO₅⁻• + H⁺ + e⁻

(56)

2SO₅⁻• + H₂O → 1/2 ¹O₂ + 2HSO₄⁻

(57)

Moreover, due to the larger ionic radius of nitrogen (N) compared to oxygen (O), nitrogen atoms are superior to other atoms, and the beneficial effect of nitrogen atoms is most pronounced in improving visible-light catalytic performance [150,151,152,153]. Li et al. [154] used phenolic resin microspheres as a template and employed a straightforward hydrothermal technique to fabricate nitrogen-doped TiO₂ hollow microspheres containing oxygen vacancies. The experimental findings exhibited the significance of oxygen voids and demonstrated how the existence of these voids within hollow microspheres can augment the photocatalytic fixation of nitrogen.

4.2. Sulfur Doping

Sulfur (S) is abundant, and sulfides exhibit diverse structures, providing excellent conditions for the structural design of biomass-based carbon materials. The electronegativity value of sulfur (2.58) is comparable to that of carbon (2.55). Consequently, the inclusion of sulfur atoms into carbon materials does not influence the distribution of electric charge within the atoms. Nevertheless, as sulfur atoms possess a larger atomic radius, the process of doping introduces a higher number of imperfections into the carbon structure, ultimately leading to the reorganization of electron spins [155]. The inclusion of sulfur in the carbon structure causes a disruption in the arrangement of carbon atoms. This disruption leads to the creation of patterns known as thienyl sulfur “-C-S-C-”. Additionally, the presence of sulfur atoms modifies the sp² conjugation of biochar, thereby ameliorating the electron transfer characteristics of the biochar substance.

The primary method for activating PS in carbon materials doped with sulfur involves using an in situ sulfur-doping strategy. At elevated temperatures, the pyrolysis technique is employed to convert polymers containing sulfur into S-doped carbon materials. Wang et al. [80] illustrated the remarkable ability of sulfurized biochar to effectively eliminate bisphenol A by activating PS. The S-doped biochar produces sulfur atoms at the edges of the biochar, which activates the SP²-hybrid graphene structure, and produces Lewis acid and Lewis base sites within the S-doped biochar. Yaglikci et al. [156] utilized tea as the primary ingredient and sodium thiosulfate pentahydrate as the sulfur dopant in the production of S-doped biochar. Their findings indicate a significant enhancement in the specific capacitance of the biochar post-S modification, showing a substantial increase in comparison to the baseline. This amplification can be ascribed to the incorporation of sulfur atoms, serving as electron donors within the framework and subsequently enhancing the conductivity. Consequently, the introduction of sulfur through the doping process exhibits a favorable impact on the electrochemical capabilities of supercapacitors. Ding and colleagues [138] showcased the positive impact of N-doping on the degradation of isoproturon by activating PMS, whereas the inclusion of S had a negative influence. Additionally, the research revealed that N atoms could substitute sp² carbon atoms within the carbon framework, whereas S atoms were prone to replacing groups containing oxygen in the carbon structure. The differences observed in sulfur-doped biochar may stem from the varying synthesis methods employed. Furthermore, further studies are necessary to examine the production of biochar doped with sulfur and understand its catalytic mechanisms.

4.3. Boron Doping

Boron is a non-metal found in the third main group of the periodic table. It has three valence electrons and can form bonds with carbon and oxygen. Because of its low electronegativity and similar size to C atoms, boron is considered one of the most popular modified elements in carbon materials. When boron enters the carbon lattice, it forms not only B-C bonds, but also B-O bonds. This modification boosts the transfer of e⁻ from the surface of carbon substances, potentially enhancing the electrical conductivity and characteristics of carbon-derived materials [157]. The addition of boron introduces oxygen onto the surface of carbon, enhancing the electronic plane and improving the chemical stability of carbon. In contrast to N-doped carbon, B-doped biochar exhibits prolonged endurance owing to the remarkable stability of boron sites. Studies have shown that B-doped biochar can activate PDS to eliminate sulfamethoxazole [158]. Liu et al. used boric acid and wheat straw to synthesize B-doped biochar by pyrolysis at 900 °C, and evaluated the improvement in the SMX degradation rate [158]. The inclusion of B in biochar enhances its surface affinity for PDS, making it more effective as a Lewis acid site. In addition, B also impacts the electron structure of biochar, leading to an increased electron transfer rate and improved catalytic performance. Both experimental and theoretical calculations confirm these findings.

4.4. Other Atoms Doping

Introducing multiple heteroatoms into biochar showcases the potential synergistic impacts of heteroatoms on improving the PMS activation performance. The investigation of co-doped carbon materials predominantly revolves around incorporating an additional heteroatom onto nitrogen-doped materials. Given that both P and N belong to Group V and are located in the third period, it is observed that the P atom demonstrates a greater atomic radius along with enhanced electron-donating capabilities compared to N. Consequently, this imparts benefits for the modification of the carbon network. Phosphorus, being widely abundant in nature, exhibits promising prospects for non-metallic doping. Consequently, potential elements for co-doping carbon materials encompass nitrogen-sulfur, nitrogen-boron, and nitrogen-phosphorus, among other options.

Currently, the field of PS-AOPs primarily depends on a co-doping method to alter oxidized carbon nanotubes and graphene. In contrast, the exploration of biochar modification remains comparatively restricted. Co-doped biochar exhibits superior catalytic activity compared to biochar doped solely with N or S [138]. In their study, Wang and colleagues [159] conducted an investigation into the synthesis of biochar co-doped with cobalt, sulfur, and N. Through their research, they made a significant observation that the reaction rate of biochar co-doped with sulfur and nitrogen exhibited a remarkable 4.5-fold increase compared to biochar solely doped with nitrogen.

4.5. Metal and Non-Metal Based Biochar

In a recent investigation by Li et al. [160], research was carried out on the domain of PMS activation, wherein the authors introduced the production of biochar co-doped with nitrogen and iron. This research utilized wheat straw, ferrous sulfate, and urea as starting materials to supply Fe and N. The findings indicated that the introduction of iron greatly enhanced the activation efficiency of PMS, resulting in the catalyst becoming magnetic and facilitating its easy separation and retrieval. In the realm of photocatalysis, solely relying on non-metal doping fails to produce remarkable outcomes. Therefore, researchers have attempted to modify TiO₂ with different non-metal elements and reducing agents to enhance its response to visible light. Wang’s team investigated the impact of hydrogen reduction on TiO₂ photocatalytic materials doped with nitrogen and fluorine for a visible light response [161]. Atomic spin resonance spectroscopy revealed that the Ti³⁺ and oxygen hole positions in the lattice produced by hydrogen reduction were the same as the substitution positions of nitrogen and fluorine elements, indicating that hydrogen reduced the nitrogen and fluorine in the lattice. The improved photocatalytic effectiveness of the TiO₂ with reduced visibility can be credited to the defects formed within the substance following reduction, resulting in disrupted energy bands and chaotic formations. Besides Ti, copper (Cu) can also be co-doped with non-metals. Zhong et al. [162] prepared nitrogen-doped Cu-biochar (N-Cu-BC) composites. The findings revealed that N-Cu-BC showcased a superior electron transfer capability when compared to Cu-biochar (Cu-BC), implying that the introduction of nitrogen can augment catalytic activity through the enhancement of the catalyst’s electron transfer capability. Exploring further non-metal modifications in the Cu-BC system is a highly promising direction for future research.

Nitrogen-doped biochar is the most extensively studied among the various types of heteroatom-doped biochars. While research in this area is still developing, the co-doping approach demonstrates promise in increasing the efficiency of AOPs’ activation. The successful manipulation of biochar’s composition through the addition of diverse atoms is an effective method for regulating it, thereby providing benefits for the degradation of ROPs without the need for radical involvement. Moreover, by incorporating heteroatoms into biochar, the risk of introducing harmful heavy metals that may harm water quality is effectively avoided. Nevertheless, the accurate creation of heteroatom-doped biochar presents considerable difficulties, and our precise understanding of the catalytic mechanisms involved is still uncertain and occasionally contradictory.

5. Catalytic Mechanisms

A large number of scholars have studied the use of biochar-based catalysts to degrade various persistent ROPs through AOPs. However, due to variations in experimental conditions, instruments, and operations, direct comparisons of their effects are not rigorous and reliable. Nevertheless, the findings confirm the potential for biochar-based materials to catalyze the degradation of various ROPs in AOPs. At the same time, many studies have made great progress in the in-depth exploration of the catalytic mechanism of BC-AOPs [163,164,165]. The decomposition of BC-AOPs can be categorized into two pathways: one involving free radicals and the other involving non-free radicals. The free radical pathway initiates charge transfer to generate polar oxidizing entities, activating specific sites such as surface functional groups, PFRs, π-π bonds, hydrogen bonds, defective sites, and transition metals [166,167]. In contrast, there are approximately three non-radical pathways, mainly including electron transfer, ¹O₂ generation, and surface activation [168,169].

5.1. Free Radical Pathway

5.1.1. Surface Functional Groups

The adsorption capacity of BC towards aromatic hydrocarbons is significantly influenced by the distribution and properties of surface functional groups. These functional groups include not only OCGs, but also N-containing and S-containing functional groups [170], and can vary in terms of their polarity, hydrophobicity, and distribution. The presence of positive/negative charges, as indicated by Tan et al. [171], can serve as a valuable indicator of the electron-donating/accepting properties of BC. OCGs are particularly abundant and have been highlighted as crucial structural features of biochar, influencing its adsorption and catalytic abilities [172,173]. During the adsorption process, various interactions occur between adsorbates and surface functional groups, especially when it comes to aromatic compounds and ionized aromatic pollutants such as herbicides and antibiotics [174]. The presence of surface charge and hydrogen bonding can improve the adsorption capabilities of BC, leading to a decrease in pollutant levels.

According to several studies [175,176], OCGs are the main active site of activated PS on biochar’s surface. Yan et al. [177] pointed out that biochar with OCGs like -OH and -COOH had the potential to serve as a platform for electron transfer and the stimulation of PS. Huong et al. [178] also found that OCGs containing C=O, -COOH, and phenol groups promoted the formation of free radicals. It is suggested that OCGs may accelerate the direct decomposition of PS and enhance the activation of PS through metal mediation. Additionally, Meng et al. [179] prepared OCG-containing biochar using orange peel as a precursor system. They found a direct relationship between the pyrolysis temperature and the degree of graphitization, the BET, the mesopore ratio, and the content of C=O. The enhancement discussed in this passage is related to improving the adsorption capacity and activation, which consequently results in a higher efficiency in catalyzing PMS. Quinone and hydroquinone groups, present on the biochar surface, operate as redox pairs. They facilitate the exchange of electrons between external oxidants and the reductant [180]. Moreover, OCGs can serve as electron mediators to transmit electrons to H₂O₂, producing extremely reactive •OH, which assists in the breakdown of organic contaminants.

In addition, the process through which ROSs are generated by OCGs resembles that of dye-sensitized semiconductor photocatalytic systems. The surface of biochar contains OCGs, which can function as a photosensitizer, while biochar particles act as electron shuttles. When OCGs are excited by light, electrons are excited and subsequently transferred from OCGs to an electron acceptor (like O₂), resulting in the production of •O₂⁻. Similar to the •O₂⁻ generated previously, it can further accept electrons or absorb H⁺ to form H₂O₂, leading to the production of •OH [181]. The primary photochemical reactions are as follows (Equations (58)–(61) and (54)):

BC-OCG + hv → BC-OCG*

(58)

BC-OCG* + O₂ → e⁻ + BC-OCG⁺

(59)

2•O₂⁻ +2H⁺ → 2HO₂• → H₂O₂

(60)

HO₂• → e⁻ + HO₂⁻ +H⁺ → H₂O₂

(61)

H₂O₂ + hv → •OH

(54)

Functional groups in biochar significantly boost its polarity, improving the interaction between pollutants and polar adsorbent materials [182]. Moreover, these functional groups have the potential to catalyze redox reactions and activate PMS. Table 3 shows the N-containing functional groups on the surface of biochar post-pyrolysis, predominantly consisting of pyridine N, N-amine, and potential pyridine n-oxides [183]. The catalytic activity of the oxygen reduction reaction is related to specific nitrogen configurations, namely pyridine N, graphite N, and pyrrole N, and the presence of amine N can adsorb heavy metals and CO₂ [184]. Additionally, incorporating and aligning nitrogen functional groups within biochar can elevate its alkalinity, ultimately augmenting CO₂ capturing through Lewis acid–base interactions [185]. These interactions play a vital role in adsorbing acidic gases [186]. The presence of pyridinic N has the ability to transform nearby carbon atoms into Lewis base sites, which promotes reactions with oxygen. This, in turn, improves the electrocatalytic redox capabilities of biochar. On the other hand, by combining graphite N and pyridine N, the electrocatalytic performance is further improved since the carbon atom adjacent to graphite N also acts as the active site for the reaction with oxygen. In terms of catalytic PMS activation, SO₄⁻• has been identified as an efficient oxidant for the effective removal of ROPs. They exhibit a longer lifespan and better selectivity compared to •OH [7,8]. By utilizing nitrogen-containing biochar catalysis, the efficiency of the removal of pollutants such as Orange G [141], sulfamethoxazole [134], hyaluronic acid [187], and tetracycline [188] can be significantly enhanced. Pyridine N, pyrrole N, graphite N, and N oxide are recognized as N-containing active groups and have shown remarkable efficacy in electrocatalysis. For instance, the improved elimination ability of sulfamethoxazole is a result of the heightened levels of pyridine N and pyridine N within the catalyst, whereas graphite N exhibits an enhanced catalytic performance when eliminating orange G [141]. Similarly, the removal efficiency of tetracycline is associated with pyridinic N and graphitic N [8]. Extensive research must be conducted on the effective nitrogen functional groups in biochar to enhance its catalytic performance. The mechanisms of action of these functional groups need to be understood in order to achieve this improvement. Biochar contains various sulfur-containing functional groups, such as organic sulfides, sulfoxides, sulfides, thiophenes, sulfones [189,190,191], C-SO_x-C [192], S=O [193], C=S, -S-S-, C-S, and -SH [194]. By altering the BET and porosity of biochar, these functional groups can improve its adsorption capacity [195]. Additionally, biochar that contains sulfur-containing functional groups has the capability to chemically adsorb mercury and transform it into mercuric sulfide [190]. On the other hand, low-sulfur or sulfur-free biochar primarily relies on physical adsorption for mercury removal as it lacks chemically active sites that can effectively accommodate mercury [196]. Biochar that contains sulfur effectively absorbs Hg²⁺ in the soil, thanks to the presence of bacteria that reduce sulfate. For example, the adsorption of Hg²⁺ by biochar doped with elemental sulfur in soil reached an astonishing 67.11 mg·g⁻¹ [197]. Under anaerobic conditions, the adsorbed Hg²⁺ can be converted into HgS. In the soil, sulfate-reducing bacteria efficiently absorb the HgS that is generated. Subsequently, this HgS undergoes methylation, effectively separating Hg²⁺ from the soil [198]. The adsorption performance of biochar for Cd is enhanced by functional groups like C-S and C-S, as well as -SO₃H [199], sulfates, sulfides [200,201], organic sulfides, and sulfite [202]. When Cd removal from soil is targeted using sulfur-containing biochar, both biochar and sulfur-containing functional groups work synergistically in adsorbing Cd [203]. Sulfur-containing biochar not only adsorbs Hg²⁺ and Cd but also facilitates the adsorption of nickel ions. Biochar adsorbents were prepared by Higashikawa et al. using four different biomasses, namely rice husk, chicken manure, sawdust, and sugarcane bagasse. The adsorption capacities of these biochar adsorbents for Ni²⁺ were subsequently tested at different temperatures. The results showed that the maximum Ni²⁺ removal range was from 0.20 to 10.9 mg·g⁻¹. Of the biochar varieties examined, chicken manure biochar exhibited the most effective pyrolysis capacity for Ni²⁺ removal at 650 °C, achieving a rate of 10.9 mg·g⁻¹ [204]. The positive role of S-O and -SOH in Ni²⁺ adsorption is notable. This phenomenon may be attributed to the interaction between weak acids (such as heavy metals) and weak bases (S), leading to an enhanced affinity of S towards heavy metals [205]. Consequently, the formation of metal sulfides such as PbS, Ag₂S, Cu₂S, CuS, and ZnS becomes more facile.

Sulfur-doped biochar has the ability to generate sulfur-containing functional groups. These groups of functions can not only function as sites for adsorption, but can also act as active catalytic sites for activating PS. Specifically, the active site on the surface of the catalyst binds to PMS, leading to the transfer of e⁻ from the catalyst to PMS. This results in the breakdown of PMS to generate hydroxyl radicals or sulfate radicals. Huang et al. [206] prepared the electron-donating thiophene group (SACx) by ball milling by activating PMS during the decomposition of diethyl phthalate. The findings indicate that S has been effectively infused into the sp² hybrid biochar framework through the creation of electron-rich thiophene groups (C-S-C), which is recognized as the primary active location for PMS stimulation. Additional research verifies that •OH radicals are generated rather than SO₄⁻•, and the quantity of •OH radicals is directly linked to the quantity of C-S-C linkages. The proposed theory suggests that, as a result of ball milling, the amalgamation of biochar and sulfur forms C-S-C, which in turn reacts with reactive O atoms in the O-O connection of PMS, resulting in the formation of free radicals (Equations (62) and (63)). Additionally, C-S-C [207] can cycle to generate SO₄⁻• (Equation (64)) through the interaction between SACx⁺ and another PMS, similar to the Fenton reaction. This causes the contaminant to be degraded by the generated •OH and SO₄⁻•, and the C-S-C groups can compete with the contaminant for these active substances. In addition to the C-S-C bond, the S-thiophene group located on the catalyst surface can also activate PS. Jin et al. [208] synthesized S-doped biochar rich in iron FeS₂-BC by a combination of ball milling and pyrolysis. When PS is activated, the activation is facilitated by the collaboration of surface ketone groups, Fe(II), and highly reactive S-thiophene groups. The electron-donating S-thiophene groups, as well as S²⁻/S_n²⁻ groups and ketone groups, assist in the regeneration of Fe(II) in both the lattice and solution by serving as electron donors in direct or indirect electron transfer processes.

SACx + HSO₅⁻ → SACx⁺ + •OH + SO₄²⁻

(62)

SACx + HSO₅⁻ → SACx⁺ + OH⁻ + SO₄⁻•

(63)

SACx⁺ + HSO₅⁻ → SACx + H⁺ + SO₅⁻•

(64)

Table 3. Biochar with nitrogen functional groups.

Biomass	Pyrolysis Temperature (°C)	N-Functional Groups	Refs.
Chlorella vulgaris	600–900	Pyrrolic-N, quaternary-N, pyridinic-N	[209]
Phragmites australis	450	Pyridinic-N, pyrrolic-N	[142]
Nannochloropsis sp. Spirulina platensis Enteromorpha prolifera	400–800	Pyridinic-N, pyrrolic-N, quaternary-N	[210]
Bamboo	600	Pyridinic-N, pyrrolic-N, quaternary-N,	[211]
Wheat straw	300–800	Pyridinic-N, amine-N, pyrrolic-N, quaternary-N, NH4-N	[212]
Corn straw	600	Pyridinic-N, pyrrolic/pyridonic-N, graphitic-N,	[213]
Corn straw	600–800	Pyridinic-N pyrrolic-N pyridonic-N, graphitic-N	[214]
Straw	300–400	Nitrile-N, pyridinic-N pyrrolic-N, amine-N NH4-N, NO3-N NO2 -N	[215]
A Mixture of sewage, cattle manure and eucalyptus wood chips	250–550	NH4-N, amine-N	[216]

Table 3. Biochar with nitrogen functional groups.

Biomass	Pyrolysis Temperature (°C)	N-Functional Groups	Refs.
Chlorella vulgaris	600–900	Pyrrolic-N, quaternary-N, pyridinic-N	[209]
Phragmites australis	450	Pyridinic-N, pyrrolic-N	[142]
Nannochloropsis sp. Spirulina platensis Enteromorpha prolifera	400–800	Pyridinic-N, pyrrolic-N, quaternary-N	[210]
Bamboo	600	Pyridinic-N, pyrrolic-N, quaternary-N,	[211]
Wheat straw	300–800	Pyridinic-N, amine-N, pyrrolic-N, quaternary-N, NH4-N	[212]
Corn straw	600	Pyridinic-N, pyrrolic/pyridonic-N, graphitic-N,	[213]
Corn straw	600–800	Pyridinic-N pyrrolic-N pyridonic-N, graphitic-N	[214]
Straw	300–400	Nitrile-N, pyridinic-N pyrrolic-N, amine-N NH4-N, NO3-N NO2 -N	[215]
A Mixture of sewage, cattle manure and eucalyptus wood chips	250–550	NH4-N, amine-N	[216]

5.1.2. Role of PFRs

PFRs are proposed as a contrast to short-lived free radicals (transient radicals). Transition metals facilitate the formation of these PFRs by accepting electrons transferred from organic compounds such as phenols and quinones [217,218]. Previous research indicates that the organic composition and metal elements present in biochar have an impact on the types of PFRs that are formed. Metal elements can affect the formation of PFRs [219,220]. There are three main types of PFRs in biochar: carbon center PFRs, carbon-centered radicals with adjacent oxygen atoms, and oxygen-centered PFRs [221]. The presence of PFRs in biochar is affected by many aspects, including the biomass type, sample size during pyrolysis, heating rate, residence time, etc. [222,223].

According to research reports, it has been observed that the inclusion of PFRs in biochar can effectively generate electron sources, thereby stimulating the activation of different compounds including H₂O₂, O₂ [224], and PS [225]. Then, •OH, O₂⁻•, ¹O₂, and SO₄⁻• are generated. The efficiency of ROS production is influenced by both the concentration and type of free radicals, as well as the presence of PFRs. In Fenton-like systems, PFRs with carbon-centered structures are considered active centers. Yu et al. made a significant discovery regarding carbon-centered PFRs found in biochar. They observed that these PFRs have the capability to transfer electrons to oxygen molecules that are adsorbed, consequently leading to the creation of O₂⁻• (Figure 4b) [226]. It is speculated that PFRs with oxygen-centered structures may serve as active centers for •OH generation, while carbon-centered PFRs could be responsible for the production of O₂⁻•. In addition, apart from their involvement in the e-transfer procedures for producing PFRs, PFRs also have the ability to directly interact with contaminants. In wheat straw-derived biochar, PFRs with a carbon center have the ability to serve as donors of electrons, facilitating the transfer of e⁻ between the surface of the biochar and Fe³⁺, thus activating electrons in persulfate and producing Fe²⁺ [227].

Furthermore, the preparation parameters greatly impact the amount and composition of PFRs, encompassing variables including the temperature and duration of pyrolysis [224,225,228]. Due to the presence of biochar-PFRs, hydrochar can also generate ROSs. Chen et al. [181] systematically evaluated the photochemical characteristics of hydrochar and pyrolysis char. Their findings revealed that both forms of biochar predominantly consist of PFRs centered around oxygen, yet pyrolysis char exhibited a greater abundance of PFRs. The pig manure biochar SBC600 at moderate temperature exhibited a higher PFR content, thus showcasing a more pronounced ability to activate H₂O₂, where oxygen-centered PFRs played a dominant role in electron transfer [220]. Regarding residence time, Wang et al. [229] emphasized that the peak proportion of PFRs was detected following a residence duration of 4 h.

Various scientific investigations have extensively recorded that the production of PFRs is regulated by the existence of phenolic compounds and transition metals in the feedstock [225]. The amount of PFRs is dictated by the phenolic compounds’ composition, whereas the development of PFRs is considerably impacted by transition metals. Remarkably, PFRs with the central presence of oxygen demonstrated superior catalytic efficacy in the PS activation mechanism when contrasted with PFRs centered on carbon [224]. Adding Fe³⁺ as a dopant boosts the generation of oxygen-centered PFRs, like semi-quinone radicals, and adjacent carbon-centered oxygen-centered PFRs, like phenoxyl radicals. Additionally, Deng and colleagues discovered that only oxygen-centered PFRs act as sites for generating hydroxyl radicals (Figure 4a). Chen Zhang [220] chose pig manure with a higher content of metal elements and organic matter as a raw material to prepare biochar, and activated hydrogen peroxide to degrade SMT. The resulting biochar contained a higher concentration of PFRs, with oxygen-centered PFRs exhibiting a higher catalytic activity.

The electron transfer capabilities of PFRs are determined by their type. Compared with carbon-centered PFRs with oxygen atoms close together, carbon-centered PFRs have stronger electron donating capacity [230]. On the other hand, PFRs centered on oxygen demonstrate a superior catalytic activity in comparison to carbon-centered PFRs with neighboring oxygen atoms as well as carbon-centered PFRs [231,232]. Hence, it becomes essential to regulate the elemental composition of biochar catalysts to optimize their catalytic performance.

5.1.3. π-π Bonds and Hydrogen Bonds

Non-covalent interactions, known as π-π interactions, pertain to the bonding phenomenon observed between planar functional groups and aromatic molecules. These interactions elucidate the mechanism underlying the binding of biochar to ROPs, specifically those featuring benzene rings or C-C functional groups (Figure 5) [171]. Meanwhile, biochar with aromatic rings exhibits a strong electron-donating capability, forming cation-π interactions with TC [233].

The interaction between electron acceptors and donors (EDA) enhances the adsorption of positively charged compounds on the surface of biochar. Moreover, the spatial structure of biochar can be further improved by π-π interactions, increasing its efficiency in removing ROPs [234]. Tang et al. conducted research involving the use of biochar as an adsorbent to remove tetrabromobisphenol A (TBBPA). Following the adsorption process, changes in the C-C functional group were observed in the absorption peak at 1618 cm⁻¹. This alteration indicates the existence of π-π electron interactions between TBBPA and biochar [235]. Zhou et al. further studied alkali acid-modified magnetic biochar. According to their observations, the strong adsorption of TC depends on the π-π stacking mechanism derived from the graphite structure. In addition, ketone and amino functional groups on biochar significantly enhance the electron acceptance ability of TC [236]. Carbon materials with an sp² hybrid structure have abundant π electrons and are catalytic active sites in biochar [237]. Zhu and colleagues [232] discovered that the percentage decrease in π-π interaction was 1.22% both pre- and post-catalysis. This led to the conclusion that the conversion of π-π interaction from the aromatic ring to PDS results in the generation of SO₄⁻•. In addition, according to the observation of Xu et al. [238], the presence of extended electron pairs in pyridine N of sp²-C significantly enhances the transfer of π-π interaction to PMS. The primary process involves π-π interaction with the aromatic ring of ROPs, and the arrangement of ROPs and biochar structures will influence the adsorption capacity of the π-π bond. It should be noted that different organic pollutants may have different spatial structures under different environmental conditions. As such, external conditions like pH levels, temperature variations, and the presence of additional anions can modify the spatial arrangement of ROPs, thereby impacting the adsorption effectiveness of biochar towards ROPs. Hence, it is essential to conduct further research on the structure of ROPs and the correlation between the spatial configuration of biochar and its adsorption capabilities.

In addition to a π-π bond, intermolecular hydrogen bonds can also carry out an adsorption process. For instance, pre-treated biochar containing iron (SDBC) exhibits abundant OCGs, which establishes hydrogen bond interactions with the hydroxyl and amino groups of TC and doxycycline (DOX) during degradation [239]. Moreover, the phenol group of TC can also develop hydrogen bonds with OCGs, facilitating the adsorption process. Various environmental conditions may influence the hydrogen bond strength, with pH being particularly significant in altering the biochar surface potential and antibiotic structure. When both the catalyst surface and antibiotic bear negative charges, this negativity can boost adsorption via hydrogen bonding [240]. Furthermore, polar functional groups in biochar can engage in hydrogen bonding with pollutants as well [241]. Zhou et al. indicated that the interaction between OCGs (such as C-O, Fe-O, and Co-O) and hydrogen bonds on GO/Co-Fe₂O₄-SDBC can adsorb Imidacloprid (IMI). Fe and Co are responsible for connecting GO/Co-FFe₂O₄-SDBC and IMI [242].

Figure 5. Mechanisms governing the adsorption of tetrabromobisphenol A (TBBPA) utilizing SDBCs [243].

Figure 5. Mechanisms governing the adsorption of tetrabromobisphenol A (TBBPA) utilizing SDBCs [243].

5.1.4. Defect Sites

The uniformity of the carbon lattice structure in carbon materials can be disrupted by defect sites, resulting in the presence of a high chemical potential and unpaired electrons. This imparting of a catalytic capability to defect sites has been observed [144]. The presence of defects, such as zigzag and armchair edges, results in the dispersion of π electrons that play a crucial role in the adsorption process and breaking of O-O bonds during PS activation [244]. To enhance the transformation of OCGs into graphite and defect sites, an ideal pyrolysis temperature must be maintained [245]. Research has indicated that increasing the pyrolysis temperature can cause the conversion of amorphous carbon (sp³-C) to graphitic carbon (sp²-C), which leads to the breakdown of the carbon structure and the formation of additional imperfections [246]. Nevertheless, an abundance of flaws in the structure can hinder both the mechanical integrity and catalytic function of biochar [123]. Defective structures in biochar, which have a high distribution of electron density, can boost electron transport and play a role in triggering the activation of PMS. A study by Ouyang et al. [247] found that imperfect structures like edge defects, curvations, or vacancies create suspended σ bonds. These bonds enable π electrons to move freely from biological carbon to PMS, ultimately leading to the formation of free radicals. According to Kim and Ko, the activation of persulfate depends primarily on the stratification of graphitized structures formed in biochar, defects, and the presence of delocalized electrons in basic groups [166]. Defect positions on the graphitized nitrogen and carbon network are also considered potential active sites for promoting pollutant degradation [144].

5.2. Non-Radical Pathways

Experts in scientific research have increasingly focused on non-free radical pathways, with particular emphasis on the integration of biochar and PS-AOPs. Non-radical oxidation, a surface reaction different from the predominant free radical oxidation in natural solutions, has garnered significant interest in recent years. The non-free radical pathway has several significant benefits: (i) the oxidation capacity of fully activated PS; (ii) self-quenching behavior involves the elimination of free radicals; (iii) minimizing interference from both inorganic and organic substances; (iv) wider operating pH levels; (v) potential selectivity for the degradation of specific contaminants [248].

5.2.1. Electronic Transfer

Biochar’s electrical conductivity (EC) is directly linked to both its graphitization degree and mesopore degree [249]. The presence of delocalized π electrons within the graphite layer leads to the creation of overhanging bond states at edges and defects, which in turn promotes rapid electron transfer [250]. Given that electron transfer is fundamental to the reaction process, biochar’s EC plays a crucial role in determining its catalytic activity. Serving as a unique “transporter for electrons”, biochar possesses a high EC, enabling the rapid transfer of electrons to facilitate the swift generation of ROSs and leading to the degradation of ROPs. Moreover, research has illustrated that the elevated EC of biochar supports the process of photoinduced electron transfer while impeding the recombination of electron-hole pairs (e⁻-h⁺) [245]. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) can directly identify the presence of non-radical electron transfer pathways. Research conducted by EIS [251] demonstrated an enhancement in the electron transmission capability of biochar substrates after exposure to KOH and CaCl₂ treatment. Through the mediation of electron transfer within the graphite framework, electrons are conveyed from natural substances to PMS, resulting in the degradation of ROPs via non-radical pathways [119].

Biochar surfaces can form metastable complexes (biochar/PS*) that can transfer electrons from ROPs to persulfate (PS). In turn, this process leads to the oxidation of ROPs without the production of ROSs [252]. Wang et al. [79] discovered that biochar plays a key role in enhancing electron transfer to photosystem (PS) at electron-abundant locations, thereby promoting the oxidation of sulfamethazine in adjacent electron-deficient sites. To ensure effective electron transfer, it is crucial that the activation potential of the biochar/PS* system is lower than the oxidation potential of the recalcitrant organic pollutants (ROPs). Stratified porous biochar prepared from shrimp shells by Yu et al. [246] has been used for persulfate activation to remove 2,4-DCP. The degradation mechanism of 2,4-DCP involves a direct double-electron transfer, with 2,4-DCP transferring electrons to either PS or O₂ to generate SO₄⁻• and •O₂⁻.

5.2.2. Single Line State Oxygen

The first excited electronic state of O₂, known as ¹O₂, is highly reactive. When in its excited form, ¹O₂ can return to its stable state of O₂ without engaging in chemical interactions or electron transfer [253]. Understanding the complex mechanism behind the generation of ¹O₂ has been the focus of the recent literature. Certain researchers have suggested that the generation of ¹O₂ occurs through the activation of PMS with the presence of oxygen vacancies located on the biochar surface [254]. Furthermore, carbonyl groups and various metallic elements could potentially act as the sites responsible for the production of ¹O₂. The transformation of O₂ to •O₂⁻ takes place at active sites like heterogeneous iron [255], soluble Fe²⁺ [256], and phenol-OH [257,258], followed by subsequent conversion to ¹O₂. The exact process of ¹O₂ formation is unknown, but it may be influenced by OCGs, the defect structure, and variable valence metal ions. Particularly, different structures of oxidants may have different effects on the formation mechanism of ¹O₂, such as the different structures of PMS and PDS, which produce ¹O₂ in different ways. Moreover, under certain conditions, free radicals may also be transformed into ¹O₂ [259]. In the case of persulfate activation, there are four possible pathways for ¹O₂ generation. First, applying the C-O of biochar can produce ¹O₂, as shown in Figure 6. The Lewis active center of C-O facilitates electron transfer, enhancing the electron density in adjacent carbon rings. This transfer aids in the cleavage of PS’s O-O bond to generate ¹O₂ [260]. Meng et al. [179] verified through density functional theory (DFT) that the catalytic active center for activating PMS and generating ¹O₂ is C-O located at the edge. Second, •O₂⁻, as a precursor, can also generate ¹O₂. •O₂⁻ reacts with H⁺ or H₂O to form ¹O₂. It can also combine with SO₄⁻• or self-combine to yield ¹O₂ (Equations (65)–(68)) [261,262]. Thirdly, PS has the ability to transfer electrons to the positively charged carbon atom found in biochar. This process triggers the creation of the superoxide radical anion SO₅⁻•. Subsequently, SO₅⁻• reacts with H₂O to generate ¹O₂ (Equations (69) and (70)) [80,263]. Fourth, PMS has the ability of self-decomposition to ¹O₂ [260]. Additionally, Li et al. [264] indicated that iron-doped biochar can make H₂O₂ to produce ¹O₂ to degrade tetracycline. And •O₂⁻ is a precursor to ¹O₂ production (Equations (71)–(73)).

•O₂⁻ + H⁺ → ¹O₂ + H₂O₂

(65)

2•O₂⁻ + 2H₂O → ¹O₂ + H₂O₂ + 2HO⁻

(66)

•O₂⁻ + SO₄⁻•→ ¹O₂ + SO₄²⁻

(67)

•O₂⁻ + •O₂⁻ → ¹O₂ + H₂O₂

(68)

HSO₅⁻ + SO₅⁻•→ H⁺ + e⁻

(69)

SO₅⁻• + H₂O → HSO₄⁻ + ¹O₂

(70)

•O₂⁻ + H₂O₂ → ¹O₂ +OH⁻ + •OH

(71)

2H₂O₂ → ¹O₂ + 2H₂O

(72)

2•O₂⁻ + 2H⁺ → ¹O₂ + H₂O₂

(73)

Some studies have shown that ¹O₂ is the main pathway of ROPs’ degradation in PS and Fenton-like processes. The predominant oxidation pathway observed in PDS systems to eliminate SMX involves non-radical oxidation, wherein the main species is ¹O₂. The 8N atoms of SMX and the benzene ring are more attractive to ¹O₂ oxidation, leading to the decomposition of SMX through the non-radical oxidation pathway [265]. Xu et al. observed that, when using nitrogen-doped biochar, •OH, SO₄⁻•, and ¹O₂ are generated to activate PMS, with ¹O₂ making a more significant contribution to organic degradation in N-biochar-activated PMS. The main cause of the biochar activation of PDS is the presence of OCGs, especially quinones, and transition metal ions such as iron [136]. Several studies also propose that the composition of ¹O₂ serves a distinct role in pollutant degradation through different ROS functions in SDBC. The generation of ¹O₂ is intricately linked to the existence of oxygen that is dissolved within water. Consequently, in degradation procedures led by ¹O₂, modifying the quantity of dissolved oxygen might have a more substantial influence on pollutant degradation.

Figure 6. Elucidation of the mechanisms involved in both radical and non-radical processes during phenol degradation, along with the progression of singlet oxygen evolution [266].

Figure 6. Elucidation of the mechanisms involved in both radical and non-radical processes during phenol degradation, along with the progression of singlet oxygen evolution [266].

5.2.3. Surface Activation

One crucial method by which the non-radical pathway operates is via its surface charge, a crucial factor in the interaction between biochar and ROPs or oxidants. The surface charge of biochar is significantly influenced by the pH of the surrounding solution. If the solution’s pH is below the biochar’s pHpzc (zero charge point), the biochar’s surface becomes positively charged [267]. When the biochar’s surface charge is contrary to that of the ROPs or oxidizer, molecules are drawn to the biochar’s surface by electrostatic forces. This phenomenon increases the likelihood that the reactive oxygen species will come into contact with the molecules, and then the activation process will occur at the catalytic active site. Additionally, the scientific literature has highlighted the significance of surface charge deprotonation in facilitating the electron transfer of PFRs located on the biochar surface [226].

Surface activation is a non-free radical process mechanism and a feasible way to degrade refractory ROPs. The degradation process is achieved by forming a surface complex, which is established by a strong electrostatic binding between positively charged C and PS [249]. Wang et al. [134] highlighted the important role played by surface-bound active complexes in N-doped biochar/PDS systems. The observation of a substantial rise in current upon the introduction of PDS suggests a confirmation of its interaction with biochar to create a metastable reaction complex [141]. The catalytic system predominantly relies on surface-bound and reactive PS complexes, along with the generation of ¹O₂, as the primary non-radical mechanisms. In addition, the biochar surface can bind free radicals to form another metastable substance present on the catalyst surface, which will not be quenched by conventional free radical quenchers, such as p-benzoquinone. It can only be quenched by KI, a quencher specifically designed to quench surfacing bound free radicals [118,119]. Xing and colleagues [268] developed a N and S co-doped biochar for the activation of PS in the degradation of methyl orange (MO). The activation of PS primarily involves the oxidation of surface-bound free radicals, such as SO₄⁻• and •OH. The degradation efficiency of MO reached 99% under gentle conditions. However, upon the addition of KI as a quencher, the degradation efficiency of MO decreased to 64% within the first 30 min.

6. Conclusions

This research presents a broad viewpoint for industry professionals on the implementation of biochar in AOPs. The manufacturing of catalysts based on biochar and the most recent advancements in mechanistic studies within the realm of AOPs are thoroughly explained. The synthesis methods and mechanisms employed by biochar-based catalysts are summarized, with a discussion on their roles in adsorption, persulfate activation, and Fenton-like processes. The key findings are as follows:

(1)

The choice of raw materials for biochar, the process of preparation, and the modification of biochar all have a substantial impact on the structure and characteristics of the material. Pyrolysis stands out as the most frequently employed method in biochar production, and the elevated temperature during this process significantly affects the overall performance of the biochar. Metal-doped catalysts, especially mixed-metal-doped catalysts, have a variety of applications such as a high REDOX activity of persulfate activation, versatility, ferromagnetism, and photocatalytic and antibacterial properties. OCGs present on biochar (e.g., -OH and -COOH) and the metals loaded on them react with PS to form free radicals. Therefore, they are suitable for use in heterogeneous catalysts or mixed systems consisting of different AOPs. In catalyst doped with heteroatoms, the heteroatoms not only promote the transfer of electrons, but also generate new active sites. During the degradation of pollutants by AOPs, it is important to consider that the process primarily involves the use of free radicals (such as SO₄⁻•, •OH, and •O₂⁻) as well as non-free radical pathways (like ¹O₂). The selection of a specific pathway is heavily influenced by the active sites found on the surface of biochar;

(2)

The mechanism of biochar combined with advanced oxidation to degrade AOPs involves both radical and non-radical pathways. Active sites that produce ¹O₂ consist of OCGs and defects, with OCGs helping to enhance the electron transfer process. Electron transfer from PFRs can lead to the generation of •OH, SO₄⁻•, and •O₂⁻. Furthermore, •OH is associated with -OH and oxygen-centered PFRs, while •O₂⁻ is linked to quinone structures and carbon-centered PFRs. The adsorption mechanism of biochar for AOPs is driven by hydrogen bonds and π-π interactions. The graphitization degree, pore structure, and specific surface area of biochar impact both the electron transfer and adsorption capabilities of the catalyst, thus playing a crucial role in AOPs’ activation.

As the domain of catalysis progresses, there arises a need for materials that are not only inexpensive but that also exhibit excellent performance. This has led to the emergence of biochar-derived materials, such as activated carbon, which hold great potential as viable substitutes. Despite this progress, achieving practical applications still requires substantial efforts. For example, there is a requirement for extensive investigations into the development of oxygen functional groups (OFGs) on the biochar surface across diverse conditions. This will facilitate the customized conception of OFGs on biochar surfaces. Additional examination is crucial to unravel the characteristics and influential variables of catalysts based on biochar in various reaction systems. Investigating doping methods and functional groups, along with understanding how specific functional groups contribute to the overall catalytic activity, is essential. Additionally, unraveling the complex interactions between different components in real-world applications and how they cross-influence the reaction mechanisms of catalysts is crucial. Numerous obstacles and prospective paths were underscored, encompassing the necessity for thorough examinations concerning catalyst deactivation or toxicity evaluations amidst and subsequent to AOP treatments predicated on biochar. Comprehensive research on functional groups, mechanisms, and toxicity is required to draw specific conclusions. Currently, there is an insufficient number of comprehensive studies on the deactivation of catalysts or evaluations of toxicity throughout and following AOP treatments utilizing biochar. Future studies should focus on addressing these knowledge gaps to advance the understanding and application of BC-AOPs.