1 Introduction

Nowadays, with the development of industrialization and urbanization, various contaminants, including heavy metals, antibiotics, polychlorinated biphenyls, pesticides, endocrine disruptors, volatile organic compounds, and polycyclic aromatic hydrocarbons are released into the air, water and soil environment (Kwon et al. 2020; Lu et al. 2020). These pollutants could pose a serious threat to the ecosystem and public health. Various green remediation technologies have been developed for the removal of these pollutants from the environment. Based on the concept of green restoration, biochar (BC), as a promising multifunctional porous carbon material, has gathered much more attention (Wang et al. 2019a). The abundant feedstocks, huge specific surface area, abundant functional groups, porous structure, and low cost endow biochar with enormous potential for application in the field of pollutant remediation (Issaka et al. 2022; Liang et al. 2021), energy shortage (Bolan et al. 2022; Chen et al. 2019b) and climate change reversal (Tam et al. 2020).

Biochar is a porous carbonaceous product obtained from the thermochemical decomposition of biomass under an oxygen-limited atmosphere (Aller 2016). Almost all biomass, including organic municipal solid wastes, food wastes, agricultural residues, sewage sludges, manures, algas, wood chips and so on, can be used to produce biochars (Lopes and Astruc 2021; Shaheen et al. 2019). Besides feedstock, the physicochemical properties of biochars are largely determined by thermochemical decomposition conditions including temperature, residence time and heating rate (Xiang et al. 2020; Zhao et al. 2018). Whereas the adsorption process could not completely decontaminate contaminants, compared to adsorption, catalysis technology can eliminate contamination by redox reaction. Recently, increasing attention has been paid to the applications of biochars in the field of pollutant catalytic oxidation–reduction (Fang et al. 2017; Lyu et al. 2020; Wang et al. 2019a). Easily tunable functional groups, chemical stability and electrical conductivity also make biochars act as a platform for supporting various catalytic nanoparticles (Ahmaruzzaman 2021; Kwon et al. 2018). The biochars with excellent electron-conductivity contribute to electron transfer, which reduce the recombination of the e‒/h+ pair of semiconductor photocatalysts (Zhai et al. 2020). Moreover, the incorporation of biochar can modify the weak visible light sensitivity, low adsorption capacity and easy agglomeration of the photocatalysts (Ye et al. 2019). Besides photocatalytic system, many researchers used biochar-based catalysts for peroxide activation to induce the generation of reactive oxygen species (ROS) (Gholami et al. 2020b; Liu et al. 2020c; Luo et al. 2021; Wang et al. 2022). However, some defects such as light dissolution and harmful metal ions release limited the larger-scale application of these catalysts. Based on the special surface structure and redox-active moieties (Chen et al. 2015, 2017; Fang et al. 2015, 2017; Li et al. 2020; Liu et al. 2019b), such as oxygen functional groups (OFGs), mineral components, dissolved organic matter (DOM) and persistent free radicals (PFRs), attentions have been paid to direct generation of ROS for the degradation of contaminants by using biochar-based catalysts (El-Naggar et al. 2020; Wu et al. 2021).

Few reviews have systematically reviewed the advance of biochar-based catalysts for the removal of contaminants. Consequently, the overarching objective of this work is to present a comprehensive review on the applications of biochar catalytic technology in pollutant control. Besides the detailed description of conventional preparation methods of biochar, the review supplements new technologies on the preparation of biochar and biochar-based catalysts, and the correlation of biochar properties with catalysis. Comparatively, the two parts of the content are important but were overlooked in the previous review. After that, this review digests current applications of biochar-based catalysts in redox, Fenton-like, sonocatalytic, and photocatalytic systems. Most importantly, the advantages/shortcomings, catalysis mechanisms, as well as the pollutant removal ability of different types of biochar-based catalysts are discussed. Moreover, as an environmental function material, eco-friendly is an extremely important property to evaluate, hence the environmental risks of the catalyst applications are also elaborated. Finally, the perspectives and future research directions of biochar-based catalysts are discussed.

2 Recent progress of biochar production technologies

Biochar, is the precursor of biochar-based catalysts, and its synthesis and properties greatly affect the catalysis performance of catalysts. Nowadays, conventional pyrolysis methods are widely used to prepare biochars, and emerging advanced pyrolysis techniques are developing boomingly. The yield, surface area, pH, ash, and fixed carbon, volatile matter, polarity and aromaticity with their according pyrolysis temperatures are important to design useful biochar catalysts as they indicate the success of the pyrolysis process. Therefore, the relationship between preparation conditions and performance of materials should be taken into consideration. Table 1 summarizes different technologies for the preparation of biochars with advantages and shortcomings.

Table 1 Technologies for production of biochars with advantages and shortcomings
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2.1 Conventional production technologies

The biochar conventional production technologies include pyrolysis, gasification and hydrothermal carbonization. Recently, the conventional pyrolysis and hydrothermal carbonization are still the most widely used methods to prepare biochars.

2.1.1 Pyrolysis

In general, pyrolysis proceeds under oxygen-free conditions at temperatures ranging from 300 to 900 °C. Figure 1 shows the effects of temperature on properties of biochars. The most rapid decrease in yield occurs at 200–400 °C, and yields at least 30% could be obtained under 800 °C. It could be found that the pH increased with increasing temperature due to the detachment of the acidic functional groups, which led to an increase in alkalinity. Additionally, the relative content of the ash and the surface area follow the same rule (Weber and Quicker 2018), but it is not possible to estimate the surface area based on the temperature alone, and the surface area may decrease after a further increase of temperature (Weber and Quicker 2018). Figure 1e shows the decrease rate of O/C ratios is higher than that ofH/C ratios with the increase in temperature, indicating decreased polarity and increased aromaticity. Obviously, the decrease in atomic ratios does not occur linearly with the pyrolysis temperature. Figure 1d shows that the biochar transformed from volatile matter to fixed C as temperature increased. Besides, a longer residence time can increase biochar yield and fixed carbon content due to the repolymerization of char constituents and decrease the volatile matter content, at the same pyrolysis temperature (Zhao et al. 2018). The surface area and morphology are more significantly influenced by residence time, which is often overlooked in previous literature (Zhao et al. 2018).

Fig. 1
figure 1

Percent yields (a), pH (b), Van Krevelen diagram (e), surface area (f) of biochar from different feedstocks at different pyrolysis temperatures; Biochar total carbon content (% w/w) changes with peak pyrolysis temperature and ash content (c); Biochar ash, volatile matter, and fixed carbon proportions expressed as weight % in a ternary graph for each biochar type (d). [Data are from reference (Cha et al. 2016; Chandra and Bhattacharya 2019; Das et al. 2021; Liu et al. 2017b; Sun et al. 2017; Tomczyk et al. 2020; Xie et al. 2015; Zhao et al. 2018)]

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2.1.2 Gasification

Gasification could produce syngas, tars and char via the gaseous media (nitrogen, air, oxygen, steam etc.) to partially oxidize the feedstock, hence, it requires high temperatures ranging from 700 to 900 °C (Tam et al. 2020). The low O/C ratio of feedstock used in this process usually leads to efficient gasification (Kim et al. 2020). Similarly, the product depends on the reaction temperature, pressure, and temperature rising rate. The temperature is the most important parameter directly affecting the gasification reactions, and a higher temperature can increase the yield of H2 and CO, carbon conversion and cold gas efficiency, while reducing the contents of CO2, CH4, hydrocarbons and tar contents (Cha et al. 2016; Han and Kim 2008; Taba et al. 2012). In addition, the types of gasification agents significantly affect the characteristics of the biochar. It was reported that the ratio of the aromatic ring structure of the aromatic compounds composing the char was lower and the contents of alkaline metals, such as Mg and Ca, were higher when H2O was used, the presence of steam could accumulate radicals on the char surface, forming some additional structures (such as hydroxides or oxides), which prohibited the formation of carbonates or their agglomerates (Tay et al. 2013).

2.1.3 Hydrothermal carbonization

Hydrothermal carbonization (HTC) based on the reaction medium of water at relatively lower temperatures and higher pressures can convert biomass with moisture content into biochar with higher carbon content (Gale et al. 2021). In HTC process, the biomass needs not to achieve the drying step, and the product yield is higher (up to 88%) than that from pyrolysis and gasification (less than 60%), hence HTC is more energy efficient to obtain the same quality biochars (Cha et al. 2016; Meyer et al. 2011; Parmar et al. 2014). The effects of the temperature on properties of hydrochar are shown in Fig. 2. In HTC process, the hydrochar products are usually generated below 250 °C (Fernandez-Sanroman et al. 2021), and temperature is a vital factor, which initiates an ionic reaction in the subcritical regions (Khan et al. 2019). In terms of pH, even with an increasing trend along with temperature, hydrochars exhibited slightly acidic or mildly basic with pH-values ranging between 6 and 8 due to the formation of organic acids during the hydrothermal process, which is different significantly from pyrolysis biochars (Vassilev et al. 2010). The proximate analysis indicated hydrochars have higher volatile matter content while lower ash content than pyrolysis biochars. Carbon content (C) correlates well with the ash content (A) and pyrolysis temperature (T) (R2 = 0.9447, Fig. 2c), so it is possible to accurately prepare hydrochars with desirable carbon and ash content by adjusting the hydrothermal temperature. Figure 2e indicates that an increase in hydrothermal temperature instigates a reduction in H/C and O/C ratios, resulting from the degree of the condensation reaction (Sevilla and Fuertes 2009). Note that the decrease in atomic ratios occurs linearly with pyrolysis temperature (R2 = 0.9022, Fig. 2e), proposing an estimation of atomic ratios based on the temperature alone could be possible.

Fig. 2
figure 2

Percent yields (a), pH (b), Van Krevelen diagram (e) of hydrochar from different feedstocks at different pyrolysis temperatures; Hydrochar total carbon content (% w/w) changes with peak pyrolysis temperature and ash content (c); Hydrochar ash, volatile matter, and fixed carbon proportions expressed as weight % in a ternary graph for each biochar type (d). [Data are from reference (Ghanim et al. 2016; Liu et al. 2017b; Raheem et al. 2022; Saqib et al. 2018; Wang et al. 2018c; Yao et al. 2016; Zhang et al. 2020b; Zhou et al. 2019; Zhu et al. 2015)]

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2.2 Advanced pyrolysis techniques

Conventional pyrolysis may not be promising for the development of multiple properties of biochars. Based on the shortcomings of conventional techniques, advanced pyrolysis methods are developed to improve the yields, qualities, characteristics and properties of products. Currently, advanced pyrolysis techniques involve several approaches, including catalytic pyrolysis, co-pyrolysis, microwave-assisted pyrolysis and combination of multiple technologies.

2.2.1 Catalytic pyrolysis

Catalytic pyrolysis is usually used to prepare high value-added chemical energy products. The use of proper catalysts can improve the pyrolysis process and characteristics of the products. Biochar materials can be used as promising catalysts. For example, a previous study showed that the co-production of bio-oil and MgO-impregnated biochar from catalytic pyrolysis of Mg-loaded biomass favored the thermal degradation process due to the weakened hydrogen-bonding networks and disrupted the crystalline structures. The relative contents of ketones and furans increased, and the relative contents of phenols and sugars decreased for Mg-loaded samples (Zhang et al. 2020c). Therefore, the use of catalysts is relatively promising due to lower energy requirement, pyrolysis temperature and enhancement of product selectivity (Lee et al. 2020). Because some catalysts may be expensive, poisoned or ineffective, a proper selection of catalysts is extremely necessary during the process of catalytic pyrolysis (Gholizadeh et al. 2021; Kabir and Hameed 2017; Zheng et al. 2017). It was noted that although the derived cobalt-impregnated biochar exerted excellent catalytic activity to activate peroxymonosulfate (PMS) for atrazine degradation, all transformation by-products and parent atrazine were still classified as very toxic compounds to aquatic organisms (Liu et al. 2020a). Fredriksson used olivine as an active bed material for catalytic cracking of tars during gasification of biomass, the carbon deposition observed after exposing olivine to reducing conditions can result in significant catalyst deactivation (Fredriksson et al. 2013).

2.2.2 Co-pyrolysis

Compared to conventional pyrolysis, co-pyrolysis is a process that entails the thermal degradation of a mixture of two or more biomass feedstocks through the generation of synergistic effects, hence it could improve the pyrolysis method for the development of multiple properties of biochar used in applications (Ahmed and Hameed 2020). Depending on the synergistic effects and interactions of multifarious biomass materials, co-pyrolysis can significantly improve the quality of the product. Many studies indicated that the yield of biochar, pore structure, ash content, and heating value would have an improvement by co-pyrolysis (Fakayode et al. 2020; Li et al. 2021). It demonstrated that the surface area and higher heating value of biochar derived from co-pyrolysis of rice husk-pine sawdust and seaweed are 3.83 and 1.16 times higher than those of seaweed-derived biochar (De Bhowmick et al. 2018). Some studies also indicated that co-pyrolysis could enhance adsorption performance toward a variety of pollutants. Kwon reported that Cu(II) adsorption capacity of co-pyrolysis biochar derived from chitin and oyster shell had a promotion and synergistic effects enhanced the properties of biochar (Kwon et al. 2022). A review summarized that merging lignocellulosic and macroalgae biomass feedstocks can be successfully employed for biochar production in the energy biorefinery (Fakayode et al. 2020). While co-pyrolysis is a developing technology that is being keenly explored at present, more considerable attention, such as temperature, heating rates and feedstock types and blending ratio, pretreatment and economic analysis should be given to it considering the various benefits derived from its utilization (Fakayode et al. 2020).

2.2.3 Microwave-assisted pyrolysis

Microwave-assisted pyrolysis has boomed over the past 5 years (Lee et al. 2020). In microwave pyrolysis of biomass, the heat transfer is more efficient as the heat is generated within the core of material by the interaction of microwaves with biomass (Foong et al. 2020), hence leading to energy saving. The microwave power and interaction of heating time and microwave power positively affected the maximum yield of biochar (Sahoo and Remya 2020). The pyrolysis heating rate greatly depends on the temperature, power of the microwave, microwave absorb ability and pyrolysis feedstock (Lee et al. 2020). The induction of additives, such as the absorber or catalysts, can further promote microwave-assisted pyrolysis, and the properties of biochars obtained through microwave pyrolysis are significantly different from those of conventional pyrolysis. Biochar is also an excellent additive for biomass pyrolysis due to its high microwave absorbability (Wang et al. 2020a). In a previous study (Haeldermans et al. 2019), an increasing microwave power without the use of absorbers led to a higher degree of aromaticity and a more open surface of biochar (Jiao et al. 2021). The microwave-assisted pyrolysis can be considered as a new form of development, however, the lack of excellent reactor design can be a main obstacle that impedes the commercialization of biomass recycling (Foong et al. 2020).

2.3 Cost factor

The cost is a non-negligible factor when preparing biochars. Pyrolysis is a so important technology in the biomass energy industry development with simplicity in design and operation. More importantly, it allows to input almost all types of biomasses at various sizes (Low and Yee 2021). Many researches have investigated the cost of pyrolysis. Jenny found that the conventional pyrolysis of biochar and methanol yields the minimum carbon price of $642.40 per Mg, while fast pyrolysis biochars achieve the minimum carbon price of $61.38 per Mg (Meyer et al. 2011; Raheem et al. 2022), but researchers should make a promotion on the property and yield of biochar by pyrolysis to improve this method. Catalytic pyrolysis could reduce the production cost by reducing energy input. The introduction of catalysts could obtain the product with higher yields and better quality at lower temperatures. Nevertheless, the cost of catalysts should be taken into consideration and most of the studies regarding catalytic pyrolysis are still in the lab scale, and attention should be paid to this aspect in future studies (Yang et al. 2022). Gasification is often performed at large scales thus producing several tons of biochar per day as a co-product. The cost of $380 per ton and large volumes of gas make gasification an incidental method to produce biochars, and interestingly, it was found a higher heating value of 18.9 MJ for gasification biochars (Low and Yee 2021; Meyer et al. 2011; Norgate and Langberg 2009). However, the cost will increase greatly if biomass moisture content is high. HTC is a suitable method that could convert biomass with any moisture content into biochars with higher carbon content, and it was reported that the yield could reach 20 kilotons per year with $190.15 per ton production cost by HTC (Nizamuddin et al. 2017; Pacheco Antero et al. 2020). Microwave-assisted pyrolysis is another method to handle biomass with high moisture content, and the water in biomass has been shown to increase the rate in a microwave pyrolysis reactor, thereby increasing the heating efficiency (Hadiya et al. 2022). Therefore, the microwave pyrolysis of biomass is profitable when biochar pyrolysis oil and non-condensable gas are considered. Co-pyrolysis is similar to pyrolysis on economic feasibility, and the difference lies in the selection of raw materials, but rare references investigated the co-pyrolysis costs. Note that the high value applications of the remaining co-pyrolysis products will greatly improve the economics of biochar production (Fakayode et al. 2020).

3 Recent progress of biochar-based catalyst synthesis

Several approaches have been developed for biochar-based catalyst synthesis from different feedstocks. Among them, calcination, hydrothermal treatment, sol–gel, and impregnation are by far the most important technologies (Table 2).

Table 2 Synthesis parameters and characteristics of various biochar-based catalysts
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3.1 Calcination

Calcination usually proceeds under N2 or air conditions at temperatures ranging from 300 to 700 °C, which is similar to pyrolysis. Calcination is one of the most common methods for preparing biochar based-catalysts, by which two or multifarious solid phase matters are closely bound together to achieve composite multi-functionalization. Compared with pyrolysis, calcination could increase the hardness of materials, prevent their breakage, and change material textural and mineralogical characteristics (Baig et al. 2016). Just as its name implies, the temperature, gas phase composition, and thermal stability of the compound significantly affect the calcination process. A series of TiO2/pBC composites were prepared by calcination (Zhang et al. 2017). The composites have a lower SBET (specific surface area) (102.16 m2·g−1) than that of raw biochar (125.04 m2·g−1). This phenomenon may be attributed to the reason that semiconductors cover a proportion of the biochar's surface, hence resulting in a decrease in surface area. Moreover, a higher calcination temperature of 500 °C can destroy the structure of biochar, hence resulting in significantly decreased SBET (0.4035 m2·g−1) of the composite. In contrast, the BET surface area of the Lotus-Leaf-Derived Activated-Carbon-Supported Nano-CdS increased from 734.6 m2·g−1 to 1245 m2·g−1 with calcination temperatures that varied from 600 °C to 800 °C (Huang et al. 2018b). It could be assumed that a higher calcination temperature is favorable for woody biomass rather than herbaceous biomass feedstock when preparing catalysts. Consequently, it is critical to reasonably select the feedstock and calcination temperature during the process of catalyst preparation by the calcination method. Moreover, thermal polycondensation is usually used to produce g-C3N4 based catalysts, which is actually similar to calcination. For example, biochar@C3N4 composites were obtained by a thermal polycondensation process (Pi et al. 2015). Typically, melamine was firstly mixed with biochar and ground for 1 h, the mixture was then heated to 300 °C for 2 h, and the further condensation treatment was performed at 520 °C for 2 h.

3.2 Hydrothermal treatment

Compared with pyrolysis chars, hydrochars possess more OFGs, making it more suitable for being used as an excellent catalyst (Wang et al. 2020d). The hydrothermal treatment process for the synthesis of biochar-based catalysts usually proceeds at the temperature ranging from 90 to 220 °C and a retention time of 2 to 24 h. A ZnO-biochar nanocomposite with high SBET (119.15 m2·g−1) and Vt (pore volume) (0.16 m3·g−1) was synthesized by the hydrothermal method at 90 °C for a duration of 2 h (Gholami et al. 2019a). The material with a nanosized structure can be easily prepared via hydrothermal techniques. The biochar-incorporated Zn-Co-layered double hydroxide (LDH) nanocomposite exhibited good incorporation of Zn-Co-LDH nanostructures into the biochar lattice, and the integration of Zn-Co-LDH nanoflakes with biochar matrix resulted in the formation of a regular-shaped nanocomposite with an improved SBET (95.76 m2·g−1) compared with the pristine biochar (68.19 m2·g−1) (Gholami et al. 2020b). Compared with calcination, hydrothermal treatment is more cost-efficient due to milder safety conditions and simple processes without the process of desiccation. The solvothermal method is usually used to prepare TiO2 or BiOX based catalysts, and it is similar to the hydrothermal method. A biochar@ZnFe2O4/BiOBr heterojunction was prepared via the simple solvothermal method at 160 °C for 12 h (Chen et al. 2019a). The heterojunction has a lower SBET and Vt of 27.16 m2·g−1, 0.069 cm3·g−1 and 30.28 m2·g−1, 0.056 cm3·g−1 than pure BiOBr, due to the BiOBr microspheres grown on the surface of biochar causing pore blockage.

3.3 Sol–gel

Sol–gel means that the colloidal suspension or sol is formed from a precursor, such as inorganic metal salts or metal–organic compounds, followed by hydrolysis and polymerization of the precursor (Cui et al. 2021; Silvestri and Goncalves 2019). Tailored properties of the material are advantages of the sol–gel method by controlling the morphology and distribution of the generated inorganic nanomaterials within the polymer matrix. The sol–gel method is mainly used to synthesize biochar-decorated TiO2 catalysts (Liu et al. 2019c). In TiO2-decorated biochar-based catalyst, the TiO2 nanoparticles are agglomerated on the surface of biochar, and the crystal phase structure, average size, and dispersion of TiO2 upon the biochar greatly depend on the sol–gel decomposition temperature (Mian and Liu 2018). After the solution reaction step, it is easy to uniformly and quantitatively incorporate some elements to improve catalyst performance. It is generally believed that the diffusion of the components in the sol–gel system is in the nanometer range, and the reaction is easier to proceed and the temperature is low compared with the solid phase calcination reaction. For example, Xie (Xie et al. 2019) used absolute ethanol and acetic acid as the solvent to prepare Zn doping TiO2/BC catalyst with SBET of 169.1579 m2·g−1 by simple sol–gel method at room temperature. The result indicated that the doping of zinc elements via the sol–gel method may make TiO2 easier to load on biochar, and effectively reduce the agglomeration of TiO2 and crystal size. However, sol–gel is just a critical step to prepare catalysts, and it needs to conduct a further calcination after the process of depositing catalytic nanoparticles on the biochar surface (Zhang et al. 2017). Moreover, there are some shortcomings involved with the sol–gel method. For example, the cost of raw metal alkoxide is relatively high, and organic solvents might be toxic, the entire sol–gel process usually takes a long time, and gas and organic matter will escape and shrinkage during the drying process.

3.4 Impregnation

3.4.1 Metal (oxides)

In this method, active metallic species are incorporated into biochar structures by mixing feedstock with metal precursors, forming active interfaces and binding sites (Tam et al. 2020). This method could have a great enhancement for biochar-based catalysts on the adsorption capacity, catalytic performance and magnetism (Pan et al. 2021). The magnetic nitrogen doped biochar catalysts were prepared through impregnating biochar with mixtures of Co(NO3)2 and FeSO4 under an oxygen-limited condition, followed by calcination at a desired temperature (400–800 °C) for 4 h (Liu et al. 2019a). The composite calcined at a higher temperature had higher SBET (150.7 m2·g−1) and Vt (0.081 cm3·g−1) than that calcined at comparatively low temperatures, which is not consistent with the previous study (Zhang et al. 2017), despite this identical biomass feedstock. This phenomenon can be attributed to the formation of a large number of CoFe2O4 crystals on the surface of biochar by impregnation, which could contribute to the stability of the biochar pore structure. Moreover, a higher calcination temperature can obtain a high graphitization degree structure, which is conductive to the charge transfer process and helpful for promoting electron transfer (Wang et al. 2018a). A TiO2/Fe/Fe3C-biochar composite was synthesized by impregnating sewage sludge and different ratios of nanoparticles with chitosan, subsequently thermal decomposition at 800 °C (Mian and Liu 2019). NPs (nanoparticles) ratio and chitosan support have significant influence on the properties and catalytic activity of catalysts,and high content of Fe impregnation reduces surface area and active site due to Fe0 agglomeration. Nevertheless, impregnation often produces composites rather than carbonaceous biochar, demanding scrutiny comparison among bare biochar, impregnated composites, and the exhausted catalysts (Tam et al. 2020). The precipitation method can be classified as one of the impregnation methods, and the  difference is that precipitates are formed and deposited on the biochar surface when caring out the impregnation process. The biochar@CoFe2O4/Ag3PO4 was synthesized by a facile in-situ precipitation process via using ethylene glycol as the solvent, followed by impregnating biochar@CoFe2O4 with mixtures of AgNO3 and Na2HPO4 (Zhai et al. 2020). Similarly, the composite exhibited a lower SBET (21.85 m2·g−1) and Vt (0.059 cm3·g−1) than raw magnetic biochar, the introduction of magnetic biochar had little effect on the crystal structure of Ag3PO4 by impregnation process, and the intensity of crystal characteristic peaks of the photocatalysts varies with the change of loading ratio.

3.4.2 Non-metallic

Attention has been increasingly paid to incorporating non metallic elements (such as N, S, P, B) into biochar-based catalysts for the catalytic activity recently (Wan et al. 2022). Due to the different atomic radiuses, atomic orbitals and electronegativity of these elements, this method could promote the electron density, reactive-active moieties, defective sites and reusability by chemically modifying and modulating the inherent carbon configuration (Tam et al. 2020; Wan et al. 2020; Yu et al. 2020), and it shows the superiority over many other methods. N-doped biochar-based catalysts could form pyridinic-N, pyrrolic-N, quaternary-N, and pyridone-N-oxide through the polymerization reaction with oxygen- containing functional groups on the surface of biochars, which increase the surface sorption sites, basicity and positive charges (Chen et al. 2018; Huang et al. 2022; Kasera et al. 2022). It was reported that the CO2 uptake of the N-doped hickory biochar was 31.6%-55.2% higher than that of  the corresponding pristine and ball-milled biochars. Chen found that N-doped biochars show a synergetic effect on the formation of phenols by combining the advantages of activated carbon, active functional groups, and NH3, and it also inhibits the generation of O-species and acetic acid while promotes the formation of aromatics (Chen et al. 2020). Biochar-based catalysts doping S could improve the persulfate activation capacity of sludge biochar-based catalysts via enhancing the electron shuttling capacity in nonradical pathways (Yun et al. 2018). Wang prepared sewage sludge biochars incorporating zigzag-edge sulfur (SSB), and it effectively activated the sp2-hybridized graphene lattice, which generated more Lewis acid and basic sites. 93.6% of bisphenol A (BPA) removal was achieved by SSB/PMS within 60 min (Wang et al. 2020b). The doping of P could introduce functionality onto biochars to realize chemical activation based on the low electronegativity and the large atomic radius of P (Suo et al. 2019; Zhou et al. 2022a). Zhou prepared the P-doped biochar composites with high specific surface area, mesoporous ratio and abundant P-containing functional groups, and P-O-Caromatic and Caromatic-PO3/C2aromatic-PO2 groups are the active sites for chemical adsorption of methylene blue (Zhou et al. 2022b). B dopants could modulate the electron distribution and physicochemical properties of sp2-hybridized carbons, and it was found that the biochar materials introduced B, which acts as the Lewis acid site, enhanced the surface affinity towards peroxydisulfate (PDS) and modulated the electronic structure of catalysts, and evidently resulted in superior electron transfer rate long-term durability and catalytic capacity owing to the high stability boron sites (Liu et al. 2020b; Sun et al. 2018).

4 Biochar-based catalyst applications in pollution control

Biochar-based catalysts have been widely used for the removal of contaminants in water and soil. Moreover, biochar-based catalysts have been widely used in different systems, including redox systems, Fenton-like systems, sonocatalytic systems and photocatalytic systems (Fig. 3) (Table 3).

Fig. 3
figure 3

A proposed mechanism for pollutant degradation by the pure biochar at light radiation (a); biochar-based catalysts by Fenton-like reaction (b); biochar-based catalysts by photocatalysis reaction (c); biochar-based catalysts by sonocatalysis reaction (d)

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Table 3 Applications of various biochar-based catalysts in pollutant control
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4.1 Biochar composite for redox systems

Organic contaminants can be directly degraded by biochar without the addition of oxidants (Lyu et al. 2020). PFRs in biochar can transfer electrons to O2 to produce the superoxide radical anion and hydrogen peroxide, which react further with PFRs to produce ·OH (Fang et al. 2015). Biochar carbonization temperature is a critical factor affecting the generation of PFRs, which further affects the degradation of organic pollutants. In a previous study, the rice husk-derived biochars exhibited the degradation rate for 1,3-dichloropropene initially decreased from 0.00152 h−1 to 0.00073 h−1 and subsequently increased to 0.00305 h−1 with an increasing pyrolysis temperature range from 300 to 700 °C (Qin et al. 2017). A certain high temperature promotes the formation of PFRs, but excessively high temperatures may cause the cracking and reorganization of the organic structure, resulting in the elimination of PFRs (Fang et al. 2015). Besides, DOM with abundant PFRs derived from a sugar beet tailing and Brazilian pepper biochars, serving as both electron donor and electron acceptor, exhibited 100% reduction of Cr(VI) (Dong et al. 2014). Moreover, the introduction of metal could significantly improve the oxidation–reduction activity of pure biochars. For instance, biochar-coated Fe(0) composite exhibited more than 90% removal of 2,4,6-Trinitrotoluene in 5 h, and the biochar of the composite may act as an electron transfer mediator (Oh et al. 2017). An Ag nanoparticle-embedded biochar hybrid material can completely catalytically reduce Cr(VI) using formic acid as a reducing agent, and the particle size of Ag plays an important role in this process (Liu et al. 2016). Obviously, pure biochar or biochar-coated metal composites can efficiently achieve an almost complete reduction of Cr(VI). The properties of pure biochar need to be adjusted to enhance its catalysisreaction. The cost and risks of heavy metal release of the biochar-coated metal composite may be high, due to the introduction of metal. Biochar catalyst selection for the pollutant oxidation and reduction should be based on the biochar characteristics, cost, and environmental risks.

4.2 Biochar-based catalysts for Fenton-like systems

4.2.1 Pure biochar

The biochars can effectively catalyze H2O2 to form ROS, resulting in the degradation of pollutants. The pyrolysis temperature of biochar influenced •OH generation in the presence of H2O2. It was reported that wheat-derived biochars at higher temperatures led to a higher yield of •OH during H2O2 activation, and the degradation efficiency of sulfamethazine (13.7 μM) went up from 93.4% to 100% with the increase of pyrolysis temperature, and the drawback is that reactivity was inhibited by the adsorption of sulfamethazine on their surface (Huang et al. 2016).

The peroxide could be effectively catalyzed by OFGs and PFRs of biochars to form ROS or sulfate radicals for pollutant degradation (Xia et al. 2020). The sewage sludge biochar was proven to be an efficient persulfate activator for pollutant degradation, a high mineralization efficiency of ∼ 80% for BPA could be achieved by PMS at a biochar dosage of 0.2 g·L−1 within 30 min, and 1O2 was the main reactive species (Huang et al. 2018a). The pyrolysis temperature significantly affects the activity of the activation reaction. In a study, N-doped graphitic biochars (N-BCs) were used as catalysts for PDS activation. In particular, N-BC derived at 900 °C (N-BC900) exhibited the highest degradation rate, and it could completely degrade phenol, sulfamethoxazole (SMX) and BPA in 20 min, and Orange G in 60 min at 20 ppm, which was 39-fold of that on N-BC400 (Zhu et al. 2018). Different from the radical-based oxidation in N-BC400/PDS via the PFRs, 1O2 and nonradical pathways dominate the process. Besides, the contributions made by organic matter, acid-soluble substance (ASS), carbon matrix (CM), and basal part (BSP) in a magnetic nitrogen-doped sludge-derived biochar (MS) for activation reaction were significantly different, and it  was reported that ASS was the most important contributor for MS-400/600 while CM took the absolute dominance for MS-800, ASS and CM took the responsibility for the generation of SO4·− and ·OH in MS-800/PDS system (Yu et al. 2019).

4.2.2 Transition metals-based biochar catalysts

Transition metals, such as iron, copper, cobalt and so on, can efficiently activate oxidants to generate ROS. Due to the excellent performance and low toxicity, iron-based catalysts have attracted extensive attention. Nanoscale zero valent iron (nZVI) and magnetite (Fe3O4), as two common potential alternative sources of Fe2+, can be used to activate oxidants for pollutant degradation. More than 70% of ciprofloxacin was removed, using BC/nZVI activating H2O2 (20 mM) in acidic conditions, and ·OH oxidation was the primary pathway in the process (Mao et al. 2019). Similarly, the pyrolysis temperature significantly affects the catalytic properties of magnetic biochar by affecting the size distribution of iron oxides, especially Fe (II) content in these composites. In a study, metronidazole was rapidly and completely degraded by the magnetic biochar at the temperature of 400 °C (SMBC400) activating H2O2, and the rate of which was higher than that of SMBC300 and SMBC500 (Yi et al. 2020). Recently, a ·SO4‒ mediated photocatalysis associated with a Fenton-like system was used for bacterial inactivation. In this study, Fe3O4-based magnetic hydrochar (MHC) was used for persulfate (PS) activation under visible light. Complete treatment of 8.0 log E. coli cells was reached within 40 min in visible light /PS/MHC system, suggesting higher removal than that of PS/MHC system (Wang et al. 2020d). As a result, photo-Fenton-like systems may be a promising water disinfection technology. The Cu and Co-based biochar catalysts were also used to activate peroxide or hydrogen peroxide for organic pollutant degradation. A solid digestate-biochar-Cu NP composite (0.5 g·L−1) exhibited a tetracycline (TC) (200 mg·L−1) degradation of 97.8% in the presence of H2O2 within 6 h of reaction time, which was mainly attributed to Cu(II)/Cu(I) redox reaction and the electron-transfer process of FRs in biochars (Fu et al. 2017). The degradation capacity for TC is well above that of almost all other photocatalysts. It was reported that nearly complete removal of chloramphenicol (30 mg·L−1) was achieved in a Co3O4-BC/PMS system within 10 min, the outstanding performance of which mainly stems from the synergistic effect of the conducting BC and the Co3+/Co2+ redox reaction (Xu et al. 2020). In addition, non-metal, such as S and N doping can further promote the activity performance of Co-based biochar catalysts. It was reported that the dinotefuran decomposition of S-doped Co3O4 composite increased by 1.71 times as compared to the S-free sample, which was attributed to the enhanced generation of 1O2 and various radicals by S incorporation (Du et al. 2020). Similarly, a higher pyrolysis temperature of biochar could improve PDS activation reaction. A N-doped graphitic biochar (SDBC-900) showed a faster degradation with complete SMX removal compared with 33.0% and 84.9% on the biochar pyrolyzed at 400 and 700 °C in 45 min (Ho et al. 2019). During this process, the SDBC 900-acid played versatile roles in PDS activation. While S doping is not always positive for PMS activation, according to a previous study, a heteroatom (N and S) doping biochar was used for the catalytic degradation of metolachlor through activating PMS, N-doping positively whilst S-doping negatively influenced the degradation process (Ding et al. 2020). In conclusion, the biochar-based catalysts exhibited better degradation performance in the Fenton-like system than that of photocatalysis system for refractory emerging organic pollutants, such as TC, SMX, ciprofloxacin and so on. However, pollutants degraded by biochar-based catalysts in the Fenton-like system might generate high-cost and ecotoxicity risks, due to the induction of peroxide (Li et al. 2022). Based on the cost-effectiveness, photo-Fenton-like systems may be a promising development direction in degradation-resistant wastewater treatment.

4.3 Biochar-based catalysts for sonocatalytic systems

The application of ultrasound as a primary source to stimulate chemical reactions has received increasing attention. However, there are some disadvantages, including long treatment time and huge consumption of energy, biochar-based materials can act as suitable sonocatalysts to counter these deficiencies (Wang et al. 2019a). As the name implies, the ultrasonic frequency had a substantial effect on the sonocatalytic reaction (Gholami et al. 2019b). Recently, a series of biochar-based composite catalysts, including pure biochar, transition metals, lanthanide metal and semiconductor-based biochar composites were synthesized.

Biochar, as a sonocatalyst, is more useful in low-frequency ultrasound systems for organic pollutant degradation. In a previous study, a peat-moss derived biochar exhibited higher removal efficiency for rhodamine B (100 mg·L−1) at 40 kHz (51.8%) than that at 300 kHz (26.2%) after 1 h, due to less dispersion of the biochar in the solution at 300 kHz, and stronger sonophysical effects at 40 kHz, which significantly enhanced the sonochemical oxidation reactions on the surface of the biochar (Kim et al. 2018). Similarly, Fe, Cu, Mn and Zn, as common transition metals, are usually used to prepare catalysts owing to the characters of high efficiency, non-toxicity, low cost and environmental friendliness. For instance, Fe-Cu layered double hydroxide/biochar exhibited a high sonocatalytic efficiency of 97.6% for cefazolin sodium (0.1 mM) at an ultrasonic power of 300 W within 80 min (Gholami et al. 2020a). In a study, two types of hierarchically structured manganese dioxide/biochar nanocomposites (MnO2/BCs) were prepared for BPA degradation in an ultrasound-assisted heterogeneous Fenton-like system. In this process, the crystalline properties of the nanocomposites affected their catalytic activity, and the α-MnO2/biochar exhibited complete removal of BPA (100 μM) in the presence of H2O2 (10 mM) by using an ultrasonic power of 130 W within 20 min (Jung et al. 2019). Cerium oxide (CeO2) and ZrO2, as oxides of the lanthanide metal, have been widely used to couple with biochars as heterogeneous catalysts. The prepared CeO2-H@BC nanocomposites exhibited the best Reactive Red 84 degradation efficiency, which was enhanced with the increase in ultrasonic power. In this process, a 98.5% degradation was obtained with an ultrasonic power of 450 W and an initial Reactive Red 84 concentration of 10 mg·L−1 (Khataee et al. 2018). A ZrO2-biochar exhibited 96.8% degradation efficiency with an initial Reactive Yellow 39 concentration of 20 mg·L−1 and ultrasonic power of 300 W (Khataee et al. 2017b). The high sonocatalytic activity could be explained by sonoluminescence and hot spot mechanisms. Interestingly, the TiO2-biochar nanocomposites could be used as sonocatalyst for dye degradation, the mechanisms of which are similar to those of ZrO2-biochar, and degradation efficiency of 97.5% was observed at initial Reactive Blue 69 concentration of 20 mg·L−1 and ultrasonic power of 300 W after 80 min (Khataee et al. 2017a). In conclusion, biochar-based composite catalysts could efficiently degrade (almost 100%) cationic dyes, and the performance increased with the increase of ultrasonic power, whereas the tendency was opposite to pure biochar. To develop new biochar-based composite and ultrasound-assisted heterogeneous Fenton-like system for the removal of emerging persistent organic pollutants may be currently an important development direction.

4.4 Biochar-based catalysts for photocatalytic systems

4.4.1 Pure biochar

Pure biochar can directly generate ROS without oxidants for organic pollutant degradation under UV light irradiation. It was reported that 100%, 95% and 89% of 5.0 mg·L−1 diethyl phthalate were degraded in pine needles, wheat and maize straw biochar (pyrolysis at 300 °C) suspensions in the presence of O2, respectively, with the reaction time of 24 h (Fang et al. 2015). In this process, Free radicals (FRs) in biochar transferred electrons to O2 to produce the ·O2‒ and H2O2, which reacted further with FRs to produce ·OH. Biochar carbon matrix (BCM) and DOM can induce the generation of ROS under solar irradiation. In a previous study, the photogeneration of ROS from biochar suspension was investigated, and the results indicated that BCM accounted for 63.6–74.6% of ·OH and 10–44.7% of 1O2 formation, while DOM derived from biochar generated 46.7–86.3% of 1O2 and 3.7–12.5% of •OH, and almost 59–72% diethyl phthalate degradations were achieved within 120 min under UV light by pine needle and wheat straw biochars (Fang et al. 2017). Moreover, it was reported that DOM strongly influences the photodegradation of organic pollutants, which varies depending on the structure of DOM. After exposure to UV light, the aromaticity and molecular weight of the DOM declined due to photo transformation, and significant enhancement was observed in imidacloprid photodegradation (almost 90% in 90 min) in the presence of biochar-derived DOM (Zhang et al. 2020a). It is noted that the biochars exhibited different performance, due to differences in biochar pyrolysis temperature. Besides, the pyrolysis temperature affects the photocatalytic activity of dissolved state biochar (DSB). It was found that the photocatalytic efficiency of DSB increased with the decrease of pyrolysis temperature, and DSB300 had the greatest gradation of 60% for atorvastatin (10 μM) in 5 h under sunlight. The greatest performance could be attributed to the dual role of DSB, the mineral and organic carbon components of which can act as heterogeneous photocatalysts and photosensitizers to synergistically enhance electron–hole separation, and contribute to the formation of 1O2 and triplet-excited state (3DSB*) (Shi et al. 2020). Consequently, FRs, BCM, DOM, and DSB play a critical part in enabling biochars to participate in photocatalytic reactions. To manufacture biochars with targeted photocatalytic performance, it is essential to define the correlation of photochemical reactivity with the structure and moieties of biochars.

4.4.2 Titanium dioxide

Titanium dioxide, as the most commonly used photocatalyst, provides a set of beneficial redox properties (Hu et al. 2020). However, the photocatalytic activity is hampered by limited light absorption and the high recombination of charge carriers (Naldoni et al. 2019). To mitigate this limit, different approaches have been explored for the material’s assembly, such as nanocrystals’ shape and facet engineering, heterojunction construction, and metal co-catalyst deposition (Chen et al. 2010). The previous researches had indicated that biochars can serve as a support for TiO2 to remedy the above shortcomings. The TiO2/biochar composite exhibited a higher decolorization efficiency of 96.88% than that of single TiO2 for methyl orange (Lu et al. 2019). An obvious enhancement of absorption in the visible light band was obtained owing to the presence of biochar, which can transfer electrons or serve as acceptor, and the absorbance in the visible light band with the increase of the ratio of biochar to TiO2. In addition, early work on doped TiO2 employed transition metals (e.g., Cr, Mn, Fe, Zn and Cu) introduced as substitutional atoms inside the crystalline habit to generate 3d electronic states lying in the range 0.5–1.5 eV below the conduction band (CB) of TiO2, thus providing visible light absorption, for example, the TiO2/Fe/Fe3C-biochar obtained a rising adsorption in the visible-light region due to the charge transfer of Ti3+, Si, Al, and Fe, and the methylene blue removal capacity evaluated as 376.9 mg·L−1 (Mian and Liu 2019). However, metal doping may show an unfavorable trade-off between absorption and photocatalytic activity, resulting in increased charge recombination via newly formed deep electronic levels (Naldoni et al. 2019). For instance, Zn-TiO2/biochar exhibited a lower degradation (81.21%) (Xie et al. 2019) than that of TiO2/biochar (91.27%) (Zhang et al. 2017) for SMX in 3 h. In contrast, the nonmetal (e.g., N, C, and S) doping can form 2p electronic states above the valence band (VB) capable of producing efficient charge transfer electronic transition to the 3d CB of TiO2 and thus providing high photocatalytic activities (Naldoni et al. 2019). Among these, nitrogen and sulfur have provided better and similar band gap narrowing, but the large ionic radius of sulfur requires much higher formation energy, hence nitrogen doping has proved to be more promising for visible-light-driven photocatalysis (Kumar et al. 2019a). For example, in the previous study, the incorporation of nitrogen into the crystalline TiO2 lattice may modify the electronic band structure of TiO2, then a new substitutional N 2p band was formed above the O 2p valance band, which narrowed the TiO2 band gap, and shifted the optical absorption to the visible light region, resulting in a better performance of N-TiO2@SiO2@Fe3O4 photocatalyst (Kumar et al. 2019a). The results showed that 93% of benzophenone-3 within 5 h and 71% of carbamazepine within 9 h were degraded under visible light. A Ti-coupled N-embedded chicken feather biochar also presented a 90.91% degradation rate of Rhodamine B under visible light after 240 min (Li et al. 2018). The direct contact of Fe3O4 and TiO2 may enhance the visible light response a little, whereas it brings an unfavorable heterojunction, resulting in lower photocatalytic performance. Obviously, the oxidation rate for Rhodamine B degradation (75% in 1 h) using TiO2-olive pits@Fe3O4 is faster than that of Ti-coupled N-embedded chicken feather biochar, due to the presence of olive pit biomass, which can enhance adsorption capacity and serve as  a charge acceptor via Ti–O–C bridge and inhibit the transfer of electron/hole charges from TiO2 to Fe3O4 (Djellabi et al. 2019a).

4.4.3 Graphite carbon nitride

Graphitic C3N4, as a nonmetal photocatalyst, possesses a comparatively narrower bandgap of ca. 2.7 eV, good thermal chemical stability and cheaper cost than TiO2. Unfortunately, bulk g‐C3N4 exhibits poor electronic properties, small specific surface area and low quantum yield due to structural defects of stacking of the layers, and tremendous efforts have been made to improve the photocatalytic efficiency through design strategies, including band-gap engineering, defect control, pore texture tailoring, dimensionality tuning, surface sensitization, heterojunction construction, co-catalyst and nanocarbon loading (Wen et al. 2017). Similarly, biochar can play an important role in enhancing the photocatalytic activity of g‐C3N4. A simple biochar@g-C3N4 composite initially was used to degrade cationic dye, exhibiting a more desirable decolorization rate of 91% for methylene blue than that of C3N4 in 3 h, due to enhanced methylene blue adsorption (Pi et al. 2015). It can be found that the cationic dye methylene blue decolorization capacity of biochar@g-C3N4 is lower than that of TiO2-Lignocellulosic Biomass@Fe3O4 (Djellabi et al. 2019a) and TiO2/Fe/Fe3C-biochar (Mian and Liu 2019). However,  biochar@g-C3N4 catalyst may be more practical in the application of cationic dye decolorization, due to better visible light absorption. Recently, the antibiotic TC degradation is still a challenge, due to its recalcitrance. It was reported the biochar spheres acted both as a reservoir and a sensitizer in biochar@g-C3N4 core–shell photocatalyst to enhance light absorption in nearly the entire wavelength range due to the narrower gap of the sp2 carbon cluster and large amounts of carbon clusters (Wang et al. 2018b). The core–shell non-metallic photocatalyst exhibited a degradation rate of 55% for TC (10 mg·L−1) in 80 min under visible light irradiation, which was 4 times as high as that of the g-C3N4. The degradation of TC using biochar@g-C3N4 in photocatalysis system is far below that of biochar-based catalyst using in Fenton-like system. An oxidation biochar supported magnetite particles (OBC-Fe3O4) exhibited better catalytic performance for PS activation due to its abundant oxygen-containing groups on the surface and Fe3O4 particles, which resulted in 92.3% of TC (20 mg·L−1) removal within 2 h (Pi et al. 2019). Even so, the degradation performance of TC is not good enough, using either the biochar@g-C3N4 composite in catalysis system or biochar-based catalysts in the Fenton-like system. To design a photo-Fenton system via taking advantage of photocatalysis and Fenton-like reactions may be a promising way to further enhance the degradation of TC.

4.4.4 Bismuth oxyhalide

As a common semiconductor with a special layered structure, BiOX is composed of [Bi2O2] slabs interleaved with double halogen atoms labs, which can effectively separate electrons and holes (Li et al. 2016). However, the inherent large band gaps of BiOCl (∼ 3.4 eV) and BiOBr (∼ 2.6 eV) still restricted their applications in solar conversion, various strategies such as heterojunction formation, doping, interface modification, enhancement in Bi content, inner coupling between different BiOX  photocatalysts, use of sensitizers and creation of oxygen vacancies can improve photocatalytic performance (Sharma et al. 2019). Li developed biochar/BiOBr composite photocatalysts, and the visible-light absorption of BiOX catalysts can be exceedingly enhanced by biochar, which mainly stems from the profoundly improved charge separation and delivery efficiency (Li et al. 2016). And the catalysis ability is affected by the mass ratio between biochar and BiOX,  and for10% biochar/BiOBr, the degradation efficiency of methyl orange reaches 82% under visible-light irradiation within 2.5 h, which is much higher than that of pure BiOBr. Obviously, the biochar/BiOBr exhibited slightly lower degradation efficiency of methyl orange than that of TiO2/biochar (Lu et al. 2019), while biochar/BiOBr had a better practical application prospect in decolorization of cationic dyes, due to the wide visible-light adsorption.

4.4.5 Other photocatalysts

Other photocatalysts were also used to couple with biochars. Liu prepared Ag/Ag3PO4 support amino-modified biochar composite, which exhibited greatly enhanced photocatalytic performance, and almost 80% amoxicillin was decomposed in 120 min, whereas only 65% with pure Ag3PO4 (Liu et al. 2017a). The CdS load activated biomass carbon composite displayed excellent photocatalytic ability toward methyl orange and methylene blue degradation with the efficiency of 97.8% at 60 min and 96.3% at 150 min, respectively (Huang et al. 2018b). CuWO4, a n-type semiconductor, was supported on biochar to construct a Biochar-CuWO4 composite with degradation up to 97% of ciprofloxacin after 90 min of visible light irradiation (Thiruppathi et al. 2020). Besides, the nanocomposites exhibited complete photodegradation of methylene blue and Rhodamine B solutions after 80 min of light irradiation. In a work, an N-modified biochar, with great conductivity and a special 2D sheet platform structure, was used as a platform for supporting Bi2WO6, causing a narrower band gap of the composites than that of Bi2WO6 (Wang et al. 2020c). The composites exhibited excellent photocatalytic activity for the degradation of 10 mg·L−1 Rhodamine B (99.1%, 45 min) and reduction of Cr(VI) (96.7%, 30 min) under visible light irradiation. In conclusion, it could be found that the new biochar-based CdS, CuWO4, Bi2WO6 composites can very efficiently (nearly 100% within 2.5 h) degrade cationic dyes, such as Rhodamine B, methyl orange, methylene blue and so on, under visible light, the performance of which is better than that of biochar/BiOX. Compared to conventional TiO2 or g-C3N4 based catalysts, the new catalysts, such as g-MoS2, Bi4Ti3O12 based catalysts and so on, have better performance for TC degradation. A porous graphite biochar self-assembled with g-MoS2 nanosheets is observed to have enhanced visible light harvesting, accelerated charge transfer, efficient carrier’s separation, and excellent performance for TC degradation, and more than 40% of the total organic carbon (TOC) could be removed after 60 min under visible light irradiation (Ye et al. 2019). Similarly, the metal doping can further improve photocatalytic performance. Wang reported Ag can be applied as excited electron–hole pairs in biochar quantum-dot/Bi4Ti3O12 nanosheets by transferring the plasmonic energy from the metal to the semiconductor, and the 2D Ag/carbon quantum dots/bismuth titanate exhibited a high degradation efficiency of almost 90% for TC (10 mg·g−1) under visible-light irradiation after 90 min (Wang et al. 2019b). The surface plasmon resonance of Ag can promote the electron transfer properties of the biochar quantum-dots, meanwhile, it can act as an electron buffer to decrease the recombination rate of the electron hole, and provide a chemical group to enhance the electron transfer and connection.

4.4.6 Heterojunctions

Among these strategies, the construction of heterojunctions has seen a recent explosion of interest due to further improvement in photocatalytic efficiency of biochar coupled single photocatalysts. According to the heterojunction energy band theory, when different semiconductors with the difference in energy band structure are combined, the semiconductors can absorb the radiant energy and be excited to generate electron–hole pairs, and with a heterojunction, the band gap energy can be significantly reduced and therefore light absorption as well as charge transfer will be increased (Wen et al. 2017). Recently, the biochar coupled heterojunction composites were prepared, and exhibited superior photocatalytic performance, due to the narrow band gap, high surface area, broader solar spectrum response and reduced charge carrier recombination. Among oxide-based semiconductors, Cu2O and CuO as the p-type semiconducting materials have a relatively narrow band gap of 1.8–2.2 eV and 1.2–1.8 eV, respectively. The potential difference in the VB or CB of Cu2O and CuO could lead to the transference of the electrons on the CuO and Cu2O CB and the hole from CuO to the VB of Cu2O, hence 94.12% degradation efficiency of Reactive Orange 29 was obtained by the composite at the initial concentration of 20 mg·L−1 after 90 min under UV-B light irradiation (Khataee et al. 2019). By the utilization of g-C3N4, polyaniline (PANI), reduced graphene oxide (RGO) and biochar, a Z-scheme photocatalyst g-C3N4/ACN/RGO@biochar was synthesized and exhibited a high degradation of 99.7% and 98.4% for 2,4-Dichlorophenoxy acetic acid and ibuprofen (20 mg·L−1) under visible light exposure in 50 min (Kumar et al. 2019b). Similarly, Ag3PO4 is a visible light-responsive photocatalyst with a higher quantum efficiency (90%) but susceptible to photo corrosion caused by Ag+ reduction, which could be solved by constructing Z-scheme heterojunctions with CoFe2O4, hence a biochar@CoFe2O4/Ag3PO4 Z-scheme composite was prepared, and exhibited the high photocatalytic and mineralization ability of 91.12% and 80.23% for bisphenol under visible-light within 60 min (Zhai et al. 2020). In conclusion, the heterojunctions, especially Z-scheme composite, exhibited higher photocatalytic efficiency than that of biochar coupled single photocatalyst, and the mass ratio between biochar and catalysts significantly affected the performance of the composite. However, the cost of heterojunction photocatalysts is comparatively higher, due to the multistep synthetic process and induction of some precious metals, such as Ag. Consequently, as illustrated in previous studies, the heterojunction photocatalysts may be more applicable to the degradation of toxic and refractory organic contaminants, such as methyl paraben, bisphenol, chlorophenol, ciprofloxacin and so on.

5 Environmental concern of biochar-based catalysts

The excellent performance of biochar-based catalysts applied to water and soil pollution control had accepted widely attention, but potential environmental effects remain largely unexplored and poorly understood. Generally speaking, the potential risk of biochar-based catalysts toward the environment mainly stems from the release of toxic contaminants during the production and application in environmental modifications.

5.1 Impacts of biochar-matrix on environment

The pyrolysis of biomass can generate some toxic contaminants (such as aromatic hydrocarbons, aldehydes and ketone), which are usually highly toxic, with high volatility and low water solubility, particularly high environmental mobility (Raclavska et al. 2018; Shaheen et al. 2019). In general, the pyrolysis at a low temperature can produce biochar with high toxicity owing to the inadequate combustion (Lyu et al. 2016). Polycyclic aromatic hydrocarbons (PAHs), as a highly toxic persistent organic pollutant, are generated as temperatures approach 1000 °C (Nakajima et al. 2007). PAHs might be toxic to organisms, such as crustaceans, while there is no evidence of the existence of a correlation between PAHs content and the ecotoxicity of biochar to terrestrial flora and fauna (Oleszczuk et al. 2013). This phenomenon might be explained by the synergetic impacts of various contaminants, and the interactions between biochars and contaminants (Liu et al. 2018). The persistent free radicals (PFRs), generated during biomass pyrolysis, could induce the formation of reactive oxygen species, which pose huge agro-environmental and human health risks, due to their half-lives and persistence in both biochar residues and the atmosphere (Odinga et al. 2020). In addition, biochar matrixes may release heavy metals when applied to environmental remediation. For example, the most toxic chemicals dioxins (Hale et al. 2012) and potentially toxic elements, such as Pb, Cd, Cr, Cu, Ni, Zn, and As, have also been detected in biochar (Freddo et al. 2012), which might be released into the environment. A study showed that corn cob biochars inhibited urease activity due to the release of heavy metals and polyaromatic hydrocarbons (Liu et al. 2018). Carbon nanoparticles are difficult to degrade under natural conditions and are harmful to living organisms, and the toxicity mechanism includes oxidative stress response, mechanical damage and effects on biological enzymes (Peng et al. 2020). Moreover, the ·OH stemming from biochar and biochar-based catalysts in the aqueous phase may cause significant inhibitory effects on seed germination, seedling root and shoot growth, and the damage of plasma membranes in corn, wheat, and rice (Liao et al. 2014). However, some studies reported that carbon nanoparticles can act as a plant growth inducer causing enhanced plant dry biomass and root/shoot lengths, hence are transpiring as a nano-carbon fertilizer in soil (Vithanage et al. 2017). And the accumulation of carbon nanoparticles in the soil can further modify the balance among plant-toxic metals in soil, thereby enhancing the translocation of heavy metals (loids) into the plant system (Vithanage et al. 2017).

5.2 Impacts of metal nanoparticles of biochar-based catalysts in environment

The preparation process of biochar-based catalysts involves acid/alkali/oxidant, hence might be a pollutant source (Wang et al. 2018e), the toxic elements or metal nanoparticles in biochar-based materials are also non-negligible risks. When applied in environmental modification, these metal compositions may be released into the environment, leading to secondary pollution. Semiconductors such as TiO2, CuO and ZnO are usually deemed as toxic (Zhang et al. 2018), but iron oxide particles including Fe3O4 and Fe2O3 are nontoxic or low-toxic matter (Lei et al. 2018). For example, nano-TiO2 has various negative impacts on the health of a variety of vertebrate animals, and it could cause damage to the cardiac and reproductive systems, and pose a significant threat to the recruitment of broadcast spawning invertebrates (Han et al. 2019). Particle size and shape are important factors affecting toxicity (Suresh et al. 2013). Generally, toxicity increases with the decrease of particle size, and the particles with sharp edges and corners are chemically and biologically reactive, and small spherical particles are more toxic than rod-like particles because they can damage the cell membrane more easily (Zhuang et al. 2017, 2019). Due to the complexity of environmental factors, metal oxide nanoparticles may undergo structural changes, which may further strengthen the toxicity of nanoparticles (Zhang et al. 2018). For example, in the presence of phosphate, ZnO nanoparticles change from uniform spherical particles into larger anomalous porous particles that are more toxic to mammalian cells than the initial nanospheres (Lv et al. 2012). Most of the toxicity studies only focused on the raw nanoparticles, which overlooked the potential harm of nanoparticles under complex natural environmental conditions (Zhuang et al. 2019). In future researches, it is necessary to understand the potential toxicity effects of particles relating to their possible nanostructure changes. The fabrication process of the biochar-based materials should be optimized to improve their stability before and after use, so as to minimize or eliminate the potential release of these toxic contaminants to the environment (Lu et al. 2020).

5.3 The toxicity of reactive intermediates

The intermediates with higher biochemical toxicity than that of the original pollutants might be generated when using biochar-based catalysts to degrade contaminants (Wang et al. 2018d). In most cases, organic pollutants are not completely mineralized by UV-AOPs but are partially oxidized into transformation products (TPs), which could increase the complexity of the water remediation. Although present at low concentrations, TPs might threaten the health of humans, living organisms and eco-systems (Wang et al. 2018d). Fortunately, in most cases, the results are optimistic, the degradation usually causes the reduction of contaminant toxicity (Liang et al. 2021). For example, the toxicity of Cr (III) obtained by the photocatalytic reduction of Cr (VI) is so far below that of Cr (VI). During the process of TC photocatalysis degradation, the acute toxicity of intermediates was lower than that of TC. Worryingly, the bio-accumulative of the TC intermediates increased significantly (Wang et al. 2020e).

6 Conclusions and future perspective

This work reviews recent advances in biochar-based catalyst preparation, application in contaminant removal, and environmental risks. The physicochemical properties of biochar are significantly affected by temperature, and hydrochar with desirable carbon and ash content could be accurately prepared by adjusting the temperature. The choice of the preparation method for catalysts is based on raw materials and desirable catalysts. The enhanced performance of biochar catalysts is mainly attributed to the unique physicochemical properties of biochars, which could be used both as support and electron reservoir, resulting in the increase in adsorption capacity, catalytic active sites and charge separation of catalysts. Nearly complete removal of cationic dyes and hexavalent chromium could be achieved by simple biochar catalysts in photocatalysis, sonocatalytic and redox systems. Biochar heterojunction catalysts are especially suitable for the removal of refractory emerging organic pollutants, due to the better performance but significantly complex preparation process. Emerging organic pollutants could be more efficiently removed by biochar catalysts in a Fenton-like system than those in  the other three systems. The toxic contaminants of biochar-matrix, metal nanoparticles of catalysts and intermediates stemming from the reaction process of pollutants are the main risks when using biochar-based catalysts.

Despite considerable progress made in preparation, and application in contaminant removal of biochar-based catalysts, further research still needs to be done due to the complex surface chemistry and heterogeneity of catalysts. In order to choose and design biochar-based catalysts, a deeper understanding is expected on the quantitative correlation and interaction or synergy between feedstock, synthetic process, structure, properties and catalysis performance of catalysts. More importantly, the underlying mechanism is worth studying deeply. Replacement of rare or noble metals with free metals is a promising research area to realize cost efficiency and environmental friendliness. Besides the aforementioned environmental remediation applications, future work should also be devoted to expanding new energy applications of biochar, and biochar-based materials may be a favorable alternative to the expensive carbon materials. We hope that this review not only provides the reader with a good overview of the current advances in biochar-based catalysts for environmental remediation, but also highlights some perspectives remaining in future research.