1 Introduction

With the quick growth and development of agriculture and industry, the life quality has been improved and thereby requires higher standard level of environmental ecosystem and daily supply. However, large amounts of various pollutants such as metal ions, persistent organic pollutants (POPs), antibiotic, cosmetics and dyes etc. are inevitably released into the natural environment such as soils, water, rivers, lakes etc. (Li et al. 2021a; Liu et al. 2021a, b; Zhang et al. 2021a). The pollutants in soils, lakes, rivers and groundwater etc. could be accumulated and at last enter into human body through food chain. Thereby, the efficient elimination of the pollutants from aqueous solutions is crucial for environmental protection and human health. Although different kinds of techniques such as (co)precipitation, membrane, adsorption, oxidation–reduction, degradation, ion exchange, transformation etc. have been applied to remove the pollutants from large volume of solutions or in-situ solidification/immobilization on solid phases (Chen et al. 2022a; Hao et al. 2021; Wang et al. 2022; Yu et al. 2021a, 2022), it is still a worldwide challenge for the efficient decontamination of pollutants from solid phases such as soils, sediments etc., especial at low concentrations (Liu et al. 2022a). The photocatalytic degradation of POPs or reduction of metal ions is one effective strategy for the in-situ decontamination, to reduce the organic contaminant concentration or immobilize the heavy metal ions on solid particles (Cai et al. 2022a; Cheng et al. 2021; Li et al. 2021b; Yang et al. 2021a; Zhang et al. 2022a).

Biochar is a kind of low cost and environmentally friendly material with high efficiency and facile production. It has attracted increase attention in wastewater treatment. The high surface areas, high porosities, enough functional surface groups, excellent ion exchange ability and high stability make biochar a suitable material for pollution management. More importantly, the cooperation of biochar with other kinds of metals or metal oxides could enhance the adsorption capacity, increase visible light absorption and separation of photogenerated electrons and holes, reduce the band gap and thereby enhance the photocatalytic performance of biochar-based catalysts. The application of biochar and biochar-based materials has been extensively investigated in the decontamination of environmental pollutants.

Sutar et al. (2022) summarized the recent works about the application of biochar for the adsorption of textile dyes from wastewater, and also described the photocatalytic degradation of dyes using modified biochar simply. The biochar could remove dyes efficiently from aqueous solutions. However, the mechanism for the photocatalytic degradation of dyes was not described in detail, especially the relationship between the structures of catalysts and the degradation abilities for dyes, and the theoretical calculation/simulation of organic dye degradation mechanism and processes. Chandra et al. (2021) reviewed the preparation of TiO2/biochar photocatalysts and their application in the photodegradation of sulfamethoxazole. The possible mechanism for the enhanced photocatalytic capacity of the composites was discussed. The ·OH free radical was the main active species for sulfamethoxazole degradation and the increase of the composite photocatalytic ability was ascribed to the photocatalytic response in UV–visible range (200–700 nm) and enhanced adsorption of sulfamethoxazole on the TiO2/biochar composites. Gasim et al. (2022) summarized the methods for the functionality of biochar active sites and the application of the biochar for the removal of organic pollutants (OPs). The oxygen-containing functional groups, hybridized carbon and heteroatoms such S, N, P etc. could promote the organic molecule adsorption on the functionalized biochar through H-bonding, π–π interactions, ion exchange, and Coulombic attraction etc. The decontamination of OPs by the functionalized biochar could be achieved through physical adsorption, chemical interaction and redox reactions. The ·O2−, ·OH, and 1O2 free active species generated in the advanced oxidation processes, and the SO4·− species produced by the activation of PS such as peroxydisulfate (PDS) and peroxymonosulfate (PMS), could degrade the OPs efficiently. The –OH or –NH groups could serve as H-donors, whereas the N or O atoms in benzene rings could serve as H-acceptors. The surface –OH or –COOH groups could promote the hydrophobic reaction with the hydrophobic organic molecules. The graphitic N in biochar can improve the photodegradation of OPs (Sun et al. 2019). Zhao et al. (2021) reviewed the modification of biochar with nanometal oxides to improve the surface area and to increase functional groups. The effects of experimental conditions (i.e., solution pH, temperature, ionic strength, etc.) on the removal of environmental contaminants to nanometal oxide modified biochar were compared, and the removal mechanism of pollutants was summarized. Surface precipitation, ion exchange, electrostatic interaction, H-bonding, π–π interactions etc. are the main mechanisms for the adsorption of organic or inorganic pollutants. Feng et al. (2021) summarized the synthesis of magnetic biochar through impregnation pyrolysis, chemical coprecipitation, reductive coprecipitation, solvothermal, oxidative hydrolysis, ball milling, cross-linking, physical mixing, and combined methods. The degradation of OPs using the different magnetic biochar was reviewed in PDS, PMS, Fenton-like, NaBH4, and photocatalysis conditions. The relative contribution of different sections and active species in the magnetic biochar in the degradation of OPs were discussed and the authors concluded that the graphite structure, doped metals or supported metal oxides, oxygen-containing functional groups, and persistent free radicals had catalytic properties and promoted the reactive species to improve the OPs degradation. Li et al. (2020) summarized the preparation of magnetic biochar through coprecipitation, calcination and pyrolysis, and compared the advantages and disadvantages of the three methods, i.e., (1) coprecipitation produces the products with high purity in short time and simple reaction conditions; (2) calcination is simple to prepare magnetic biochar; and (3) pyrolysis method requires heavy mechanical system and inert gas to produce magnetic biochar. The adsorption of metal ions and POPs by magnetic biochar was reviewed and the adsorption mechanism varied with pollutants and was influenced by the functional surface groups, pore structures and chemical bonds on magnetic biochar surfaces. The authors gave the perspective that the photocatalytic degradation of organic contaminants by magnetic biochar-based catalysts may be the research hotspot in future. Minh et al. (2020) summarized the synthesis and functionalization of biochar and the application as catalysts for the degradation and mineralization of emerging OPs. The functional active moieties, free active radicals, charge transfer, hierarchical porosity, graphitic degree and redox potential on the degradation of OPs were evaluated and discussed in detail. The high photocatalytic, electrocatalytic and chemo-catalytic properties are attributed to the excellent chemical and electrical configuration of biochar.

The application of biochar for the remediation of organic contaminants showed that the adsorption properties were related to the pyrolyzing temperature (Zhang et al. 2020). Zheng et al. (2019) modified biochar with g-C3N4 containing N-functional groups, which showed better thermal stability than pristine biochar. The composites showed good adsorption ability and photocatalytic degradation activity to reactive red 120 dye. Lu et al. (2020) summarized the synthesis of magnetic biochar, 2D biochar-based membrane, and 3D macrostructure biochar, and the relationship between the adsorption abilities and structural properties. The relationship between adsorption, catalysis and redox properties and structure of biochar is discussed, pointing out that structure-functionalization-reactivity is important for understanding the interaction mechanism of pollutants with biochar, which is also crucial for the synthesis and application of biochar. Our research group summarized the recent works about the synthesis of biochar and biochar-based materials and their application in the removal of organic and inorganic pollutants from aqueous solutions and immobilization in soils (Huang et al. 2019; Liang et al. 2021; Qiu et al. 2022). The preferential interactions of organic molecules with biochar were mainly attributed to π-π interactions, hydrophobic effect and H-bonding, whereas the adsorption of metal ions was mainly dominated by ion exchange, surface complexation, (co)precipitation and adsorption-reduction on biochar. The adsorption mechanism was discussed from batch results and spectroscopy analysis. However, the photocatalytic degradation/reduction of contaminants was not discussed in detail, especially the contribution of different free active species on the degradation/reduction of contaminants. The theoretical calculations of pollutant molecule with biochar were simulated using density functional theory (DFT) and molecular dynamics (MD) (Hu et al. 2020). To our best knowledge, this is the first paper to discuss the computational calculation of pollutants’ interaction with biochar although the authors simplified the structure of biochar. The authors gave the reason for the complicated structures, which is impossible to carry out DFT or MD simulation exactly. The computational model of biochar was based on the aromatic carbon ring cluster model because biochar is composed of carbonaceous skeleton and functional groups. Till now, no DFT or MD calculation about the photocatalytic degradation or interaction of pollutants with real biochar or biochar-based materials was available. Qiu et al. (2021) summarized the main challenges and main problems for the application of biochar-based composites as photocatalysts for the photocatalytic degradation of OPs, and considered that the photocatalytic process is still “black-box”, the intermediate products’ real-time on-line analysis, efficient photogeneration and separation of e−-h+ pairs, effective visible light absorption and control of final products are the main challenges in future. They pointed out that no computational simulation of organic photocatalytic degradation is available, which may be attributed to the unclear complicated structures of biochar.

From the abovementioned references, one can see that biochar has been extensively studied for the removal of OPs and heavy metal ions. The adsorption mechanism has been discussed in detail. The photocatalytic degradation of OPs is an  effective method to decontaminate OPs using biochar-based catalysts, and the relative contribution of photogenerated active species on the photocatalytic degradation of OPs can be evaluated from quenching and ESR tests. However, the relationship between the structures of biochar-based catalysts and photocatalytic ability, the contribution of relevant components in the catalyst to photocatalytic performance, especially the photocatalytic reduction-solidification of heavy metal ions and the mechanism discussion from advanced spectroscopic analysis and computational calculation are still scarce. As the preparation of biochar and biochar-based composites has been mentioned in detail in the aforementioned references, we did not describe the methods and techniques for the preparation of biochar and biochar-based composites in this review. Herein we mainly summarized and discussed the newly recent works about the application of biochar-based photocatalysts in the adsorption-photocatalytic degradation of OPs and adsorption-(photo)catalytic reduction-solidification of heavy metal ions, especially in last 5 years. The removal of OPs and heavy metal ions through adsorption technique is not described in detail in this review. The contribution of metals or metal oxides to the photocatalytic properties of biochar-based catalysts, and the contribution of free active radicals to the photo-degradation/reduction of pollutants are mainly reviewed and compared, and the mechanisms are discussed in this review. The challenge and perspective in future are described in conclusion section.

2 Heavy metal ions

The elimination of heavy metal ions from aqueous solutions using different techniques such as ion exchange, adsorption, (co)precipitation, membrane separation, biology preconcentration, electrocatalytic/photocatalytic reduction-precipitation, and oxidation–reduction etc. has been extensively studied (Cai et al. 2022b; Hao et al. 2022a; Wang et al. 2019a). Herein, we mainly summarized the recent works for the elimination/immobilization of metal ions using the adsorption-photocatalytic/electrocatalytic reduction strategy. In this section, we separately summarized the recent works for metal anions and cations.

2.1 Metal anions

2.1.1 Zn-based biochar

Cr(VI) is the main metal anion in the natural environment and has been studied extensively because of its high toxicity and mobility. Yu et al. (2018) prepared ZnO-biochar composites and applied them for the removal of Cr(VI). 95% Cr(VI) removal was achieved with 30% ZnO-loaded biochar and precipitation of Cr(OH)3 on ZnO occurred as ZnO generated electrons to reduce Cr(VI) to Cr(III) under light irradiation. Yu et al. (2020) synthesized biochar from corn stalks and modified it  with FeCl3 and ZnCl2. The modified biochar showed an adsorption capacity of 139 mg/g for Cr(VI) at pH 2, which was mainly attributed to electrostatic interaction of the negatively charged Cr(VI) to the positively charged surface of biochar. The B-H group and ZnFe2O4 in biochar could provide electrons for the photoreduction of Cr(VI) and formed ZnFeCrO4 and FeCr2O4 precipitates on biochar. Zn-based biochar could efficiently eliminate Cr(VI) from aqueous solutions, and formed precipitates on solid particles.

2.1.2 Bi-based biochar

Geng et al. (2020) prepared BiOBr/biochar composites for the photoreduction of Cr(VI). The BiOBr could enhance the generation and separation of photogenerated e−-h+ pairs. The photogenerated electrons (e−) could reduce Cr(VI) to Cr(III) through a reduction process and the BiOBr provided more active sites for the photoreduction of Cr(VI). Wang et al. (2020) synthesized Bi2WO6-loaded N-doped biochar (BW/N-B) and applied it for the removal and photoreduction of Cr(VI) from wastewater. The band gap of BW/N-B was narrowed from 2.52 eV to 2.28 eV (value of Bi2WO6), which facilitated the separation and migration of photogenerated e−-h+ pairs and thereby enhanced the photocatalytic property. 30% Cr(VI) was adsorbed by the composites while 97% Cr(VI) was reduced to Cr(III) in 30 min under visible light irradiation, suggesting the high efficient elimination and solidification of Cr(VI) on the composites. The BiOBr/biochar catalyst showed high photocatalysis performance in the removal of inorganic and organic pollutants in wastewater treatment.

Mao et al. (2021) synthesized S, N-codoped Bi2WO6/biochar for the removal of Cr(VI) from solutions, and found that 99% Cr(VI) (10 mg/L) was removed in 75 min with high selectivity in the presence of coexisting ions such as Cl−, SO42−, Ca2+ and CO32− in wide pH range (3–9) under visible light irradiation. The narrow band gap favors the visible light absorption and photogenerated electron transfer, which promotes the photocatalytic performance. The generated ·O2−, ·OH, O2 could reduce Cr(VI) to Cr(III) (Fig. 1a). However, the function of the doped N and S atoms in the photocatalytic reduction was not described and discussed in detail. The contribution of codoped N and S was not studied and still unclear. The doping of metal and nonmetal ions could change band gap, increase the light absorption, efficiently separate the electron–hole pairs etc., thereby enhancing the photocatalysis performance of the catalysts.

Fig. 1
figure 1

Photocatalytic degradation of CIP and reduction of Cr(VI) by S,N-codoped Bi2WO6/biochar (a) (Mao et al. 2021); Possible mechanism of Cr(VI) interaction with biochar and photoreduction (b) (Mandal et al. 2017); Photocatalytic degradation of Cr(VI) by C3N4@CA/B-PET (c) (Chen et al. 2019a); Photooxidation and reduction of organic pollutants and Cr(VI) under UV light (d) (Jeon et al. 2017)

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2.1.3 Fe-based biochar

Mandal et al. (2017) incorporated chitosan and zero valent iron (ZVI) to biochar and applied it to immobilize Cr(VI) in soil. The functional oxygen-containing groups such as carboxyl, phenolic and carbonyl acted as proton donors for Cr(VI) reduction to Cr(III). The amine groups of chitosan could bind Cr(VI) strongly and ZVI could reduce Cr(VI) to Cr(III) efficiently. The addition of chitosan and ZVI-modified biochar to soil could reduce Cr(VI) to Cr(III) obviously and then adsorbed on biochar or immobilized in soil through strong surface complexation and precipitation reactions (Fig. 1b). Liang et al. (2019) synthesized magnetic biochar for the removal of Cr(VI) from wastewater and achieved an adsorption capacity of 55 mg/g. The biochar provided active sites for Cr(VI) binding and electron-donor groups for Cr(VI) reduction to Cr(III), whereas Fe3O4 immobilized the reduced Cr(III) to form Cr(III)-Fe(III) hydroxides. Cho et al. (2017) prepared magnetic biochar from spent coffee ground and applied it for the removal of As(V) from wastewater. The Fe3C and Fe3O4 phases in the composites contributed to the uptake of As(V) rather than the porosity and functional groups of biochar. However, it is well known that As(III) is more toxicity than As(V) in the natural environment. Dong et al. (2014) applied biochar for the reduction of Cr(VI) and oxidation of As(III) in the presence of organic matter, and found that the dissolved organic matter (DOM) could enhance As(III) oxidation and Cr(VI) reduction. More importantly, the reduction of Cr(VI) was coupled with the oxidation of As(III) in the absence of DOM. The adsorption of As(III) by biochar-based materials is still rarely explored and the interaction mechanism especially the structure information of As(III)/As(V) such as bonding coordination with different groups or atoms has not been intensively studied. Fan et al. (2020) synthesized nZVI-based biochar and applied it  for the immobilization of As(V) in soil. The results showed that the adsorbed As(V) was reduced to As(III) by nZVI, which could immobilize As(III) on biochar efficiently. Tan et al. (2019) applied ZVI-based biochar for the removal of Se(IV) and Se(VI) from solutions, and achieved an adsorption capacity of 62.5 mg/g for Se(IV) and 35.4 mg/g for Se(VI), which was mainly due to the ZVI and positive surface charge of the composites. Se(VI)/Se(IV) were reduced to Se(− II)/Se(0), which were less toxicity and solubility. The presence of other metal ions such as Ca2+, K+ and Mg2+ could enhance the removal of Se(VI)/(Se(IV) slightly, indicating the efficient elimination of Se(VI)/Se(IV) from complicated systems.

2.1.4 C3N4-based biochar

Chen et al. (2019a) synthesized C3N4@CA/B-PET and applied it  for the photoreduction of Cr(VI) from wastewater. The hexavalent Cr(VI) could be reduced to trivalent Cr(III) in the coexisting sulfaquinoxaline sodium under sunlight irradiation. The composites could simultaneously eliminate Cr(VI) and antibiotics with high stability and reusability (Fig. 1c). Jeon et al. (2017) prepared biochar/C3N4 composites and applied them  for the removal of Cr(VI) and Orange G from aqueous solutions. Orange G was oxidized and mineralized to form organic acid and CO2, whereas Cr(VI) was photo-reduced to Cr(III) under UV light conditions. The addition of persulfate enhanced the oxidation rate, which was attributed to the effective activation of persulfate to sulfate radicals by biochar. The schematic diagram of Cr(VI) reduction and Orange G oxidation under UV light irradiation is shown in Fig. 1d. Li et al. (2019a) synthesized biochar-coupled C3N4 nanosheets (biochar-C3N4) and applied them for the removal of Cr(VI) from wastewater by adsorption combined with photoreduction strategy. The surface adsorbed Cr(VI) was reduced by photogenerated e− species and then stabilized on biochar-C3N4 composites. The Cr(VI) was firstly adsorbed on the composites, and the photogenerated electrons generated by C3N4 were transferred to the composites. Then the adsorbed Cr(VI) was reduced to Cr(III) and stably fixed by electrostatic interaction with the C-H surface groups. The detailed process and the possible reactions for the reduction of Cr(VI) to Cr(III) are shown in Fig. 2. Rajapaksha et al. (2018) found that the adsorption of Cr(VI) on biochar was high at low pH values (2–5), and then decreased with the increase of pH, which was in good agreement with the surface charge of biochar and Cr(VI) species. The X-ray absorption fine structure (XAFS) analysis suggested the reduction of Cr(VI) to form Cr(OH)3 precipitate.

Fig. 2
figure 2

Fabrication of biochar-coupled C3N4 nanosheets and possible reactions for the reduction of Cr(VI) to Cr(III) under different experimental conditions (Li et al. 2019a)

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2.1.5 Others

Zhao et al. (2018) prepared biochar from corn straw and applied it for the selective removal of Cr(VI) in the presence of high salts. Cr(VI) could be reduced to Cr(III) at pH ~ 7, which was different with most other studies that Cr(VI) was only reduced to Cr(III) at low pH values (i.e., pH = 2 ~ 4). The ESR and free radical quenching analysis indicated that the persistent free radicals on biochar played important roles in the reduction of Cr(VI) to Cr(III) by forming Cr(OH)3. Zhang et al. (2018a) prepared biochar from tobacco petiole under different pyrolysis temperatures and applied it for the removal of Cr(VI) from solution. The results showed that the adsorption of Cr(VI) was affected by the surface functional groups rather than surface areas. The adsorption capacity increased with the increase of pyrolysis temperature, and the surface carboxyl groups could reduce Cr(VI) to Cr(III). Zhang et al. (2018b) applied biochar for the removal of Cr(VI) from aqueous solutions, and achieved 99.8% removal efficiency at pH = 3, dosage of 5 g/L and Cr(VI) initial concentration of 10 mg/L. The high removal was attributed to the adsorption-reduction-adsorption steps, i.e., Cr(VI) was adsorbed on biochar and then reduced to Cr(III), which was further adsorbed on biochar with an adsorption capacity of 25 mg/g. The main mechanisms for the photocatalytic reduction of Cr(VI) using different biochar-based catalysts are shown in Table 1. Zheng et al. (2021) summarized the synthesis and modification of different biochars. The advantages/disadvantages of different methods for the production of biochar were compared. The adsorption, electron shuttle, reduction and photoreduction of Cr(VI) by biochar under different conditions were reviewed, and the possible interaction mechanisms such as ion exchange, surface complexation, precipitation, electrostatic interaction, physisorption, chemisorption etc. were discussed for different conditions, and the authors concluded that adsorption-reduction-adsorption and reduction-adsorption were the two mainly pathways for the immobilization of Cr(VI). Xiao et al. (2018) prepared MgO-coated biochar and applied it for the removal of Cr(VI) from solutions. The Cr(VI) ions were directly adsorbed by chemical interaction of Cr(VI) with MgO, and then reduced to Cr(III) by electron-donor groups.

Table 1 Summary of biochar-based catalysts in the photocatalytic reduction of heavy metal ions
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From the abovementioned results, it is clear that the works about the adsorption-photoreduction elimination of heavy metal ions mainly focused on adsorption-photoreduction of Cr(VI) to Cr(III), and only few works focused on the adsorption-oxidation of As(III) to As(V), and Se(VI)/Se(IV) reduction to Se(0)/Se(− II). This is mainly attributed to the high toxicity of Cr(VI) and the possible photocatalytic reduction of Cr(VI) to Cr(III) by biochar-based catalysts under visible light irradiation. It is still relative difficult to achieve the photocatalytic reduction of Se(IV)/Se(VI) to Se(− II)/Se(0) under visible light irradiation.

2.2 Metal cations

As a kind of important nuclear fuel, uranium [U(VI)] also causes serious environmental pollution problems in nuclear energy utilization processes. The efficient extraction of U(VI) from wastewater or seawater is not only crucial to fission-based nuclear reactors, but also to environmental pollution management (Hao et al. 2022b). The photocatalytic reduction of U(VI) is an efficient method to extract or immobilize U(VI) from aqueous solutions. Chen et al. (2022b) summarized the recent works about the visible light-driven photocatalytic strategy for the selective U(VI) extraction, and concluded that the stability, the structure, surface functional groups, the band gap and visible light absorption properties of catalysts are crucial for U(VI) adsorption-photocatalytic reduction. Alam et al. (2018) studied the adsorption of U(VI) on biochar and found that the proton-active carboxyl group (–COOH) and phenolic hydroxy group (–OH) contributed mainly to U(VI) uptake, suggesting the importance of the functional groups on U(VI) adsorption.

2.2.1 Fe-based biochar

Chen et al. (2021a) synthesized chitosan-FeS@biochar and applied it for the removal of U(VI) from solutions. The result showed that chitosan could prevent FeS NPs aggregation and thereby enhanced the stability and reduction of U(VI) to U(IV). Liu et al. (2021c) applied SFeS@biochar for the adsorption of U(VI) and found that the removal of U(VI) was mainly dominated by electrostatic attraction, precipitation, surface complexation and reduction of U(VI) to U(IV), which was mainly attributed to iron sulfide nanoparticles. Pang et al. (2019) synthesized biochar supported sulfide NZVI and used it for U(VI) removal from solutions. The adsorption capacity achieved 428 mg/g at pH 5, and the spectroscopy analysis revealed the adsorption and reduction of U(VI) by sulfide NZVI. Hu et al. (2018) applied magnetic biochar for the removal of U(VI) from aqueous solution, and characterized the adsorbed materials using XPS and EXAFS techniques. The magnetic biochar could efficiently remove U(VI) from large volumes of aqueous solutions with good stability at pH = 3. The spectroscopy analysis showed the formation of U-Fe and U-U shells, suggesting the reduction and co-precipitation of U(VI) to U(IV). The adsorption of U(VI) by FeS@biochar showed that the presence of FeS significantly enhanced U(VI) removal and FeS could reduce U(VI) to U(IV) with high efficiency and chemical stability (Chen et al. 2021b). Baig et al. (2014) applied magnetic biochar for the removal of As(III, V) from aqueous solutions, and achieved an adsorption capacity of 2.0 mg/g for As(III) and 3.1 mg/g for As(V). The magnetic biochar could efficiently remove As(III, V) from solutions although the presence of PO43− inhibited the adsorption of As(III, V). The doping of iron or iron oxides could enhance the reduction ability and adsorption capacity of biochar, and thereby increase the elimination of metal ions from solutions.

2.2.2 Zn-based Biochar

Guo et al. (2022) synthesized 3D ZnO modified biochar and applied it for U(VI) removal. The adsorption capacity reached 239 mg/g at pH = 5 with high adsorption–desorption cycles and high stability. The mechanism was ion exchange with Zn2+ and complexation with –OH and –COOH surface groups. Yan et al. (2015) applied magnetic ZnS@biochar for Pb(II) removal from aqueous solutions, and achieved an adsorption capacity of 368 mg/g at pH = 6, much higher than other reported biochar materials. Li et al. (2018) prepared nano ZnO/ZnS modified biochar for the removal of Pb(II) and Cu(II) ions, which achieved an adsorption capacity of 136 mg/g for Pb(II) and 91 mg/g for Cu(II), much higher than the common biochar. The high adsorption of Pb(II) and Cu(II) was mainly attributed to the hydroxyl groups and the porous structures of Zn salt in the pyrolysis process. Xia et al. (2016) applied ZnCl2-activated biochar for the removal of As(III) from solutions, and found that the hydroxyl groups in Zn-OH could form Zn–O–As(III) complexes. The adsorption could achieve equilibrium in 90 min with the adsorption capacity of 27.7 mg/g at pH = 7. Tho et al. (2021) applied ZnO modified biochar for the simultaneous removal of As(III), Pb(II) and Cd(II) from solutions, and achieved the adsorption tendency of Pb(II) > Cd(II) > As(III) at pH 6–7, with the adsorption capacities of 44.3 mg/g for Pb(II), 42.1 mg/g for Cd(II) and 39.5 mg/g for As(III). The cation exchange and surface precipitation are the main mechanisms for the adsorption of the metal ions. The modification of biochar with zinc salt could efficiently increase the adsorption capacity of biochar.

2.2.3 Others

Besides the abovementioned biochar composites, other kinds of biochar materials were also applied to extract metal cations from complicated systems. The adsorption-electrocatalytic strategy was applied to extract U(VI) from seawater using In-N single atom-doped carbon and achieved 6.35 mg/g in 24 h. The in-situ Raman, XPS and EXFA analysis showed that U(VI) could be electro-reduced to unstable U(V) intermediate, and then reduced to U(IV) and re-oxidized to U(VI) to form insoluble Na2O(UO3·H2O)x through the reversible single electron transfer on In-N sites (Liu et al. 2022b). Yang et al. (2021a) synthesized Fe–N-doped carbon for the selective extraction of U(VI) through adsorption-electroreduction technique from seawater, and the results showed that U(VI) could form Na2O(UO3·H2O)x precipitates, which could be easily collected from seawater. The material had good hydrophilicity with a water contact angle of 40 ° (Fig. 3a), which was helpful to improve the interaction of U(VI) on material surfaces. The adsorption capacity achieved ~ 90 mg/g in the spiked seawater at U(VI) initial concentration of 20 mg/g (Fig. 3b) with fast adsorption kinetics (150 min to achieve adsorption equilibrium, Fig. 3c). The XAFS analysis indicated the formation of U(VI)O22+, and each U atom was coordinated with two axial O atoms, two O atoms from coordinated water molecules, two O atoms and N atoms from amidoxime groups (Fig. 3d and e). The cyclic voltammetry tests showed the sharp peak at − 0.77 V (vs SCE), suggesting the reduction of U(VI) to U(V), and the peak at − 0.43 V (vs SCE) suggested the oxidation of U(V) to U(VI) (Fig. 3f). The material showed the maximum adsorption capacity of 128 mg/g at 10 mg/g spiked seawater, suggesting the high extraction efficiency (Fig. 3g). More importantly, the yellow floc was formed on surface of carbon electrode, suggesting the U(VI) was transformed from seawater to electrode, forming precipitates and collecting on the electrode (Fig. 3h). From the results, one can see that the adsorption-electrocatalytic reduction of U(VI) is one of the most efficient methods to selective extraction of U(VI) from complicated systems. In near future, the co-doping the single metal ions on biochar for adsorption-electrocatalytic extraction of U(VI) or other kinds of heavy metal ions will attract extensively attraction.

Fig. 3
figure 3

a Contact angles for deionized water on pressed pellets of Fe − Nx − C (left) and Fe − Nx − C − R (right). b Equilibrium adsorption isotherms for uranyl ion adsorption on different materials at a fixed material—to—solution ratio of 0.1 mg·mL−1 in uranyl—spiked seawater (from 0 to 20 ppm). c Uranyl ion adsorption kinetics on different materials at an initial UO22+ concentration of 10 ppm in uranyl − spiked seawater. d U LIII − edge XANES spectra for Fe − Nx − C − R after adsorption of uranyl, and UO2(NO3)2·6H2O. e U LIII − edge EXAFS R − space and corresponding fitting curves for Fe − Nx − C − R after adsorption of uranyl. f Cyclic voltammograms for uranyl—spiked seawater and natural seawater. g Uranium extraction from spiked seawater with initial uranium concentrations of ~ 10 ppm, using Fe − Nx − C − R as an adsorbent—electrocatalyst. h Photographs of the Fe − Nx − C − R electrode in spiked seawater (initial uranium concentration of ~ 1000 ppm) during electrocatalytic extraction (Yang et al. 2021a)

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The adsorption-reduction-solidification/precipitation of high valent metal ions to low valent metal ions is most efficient to immobilize the mobile metal ions in the natural environment, especially in soil, river or lake pollution treatment. It is really difficult to effectively extract the heavy metal ions from the complicated systems under natural conditions. The best method to reduce the toxicity and to prevent the transfer of heavy metal ions in the environment through food-chain is the in-situ solidification of metal ions in solid phases. The reduction of high valent metal ions to low valent metal ions could precipitate the metal ions efficiently, and thereby reduce the potential pollution of metal ions in natural environments.

The doping of metal or metal oxides could narrow the band gap, enhance the generation of e−-h+ pairs and other free active species such as ·O2− and ·OH radicals, promote the separation of e−-h+ pairs and transfer the free active radicals, which increase the photocatalytic performance of the biochar-based catalysts (Fang et al. 2021). The high stability, reusability and environmentally friendly properties suggest that biochar and biochar-based composites are promising materials in the removal or in-situ solidification/immobilization of heavy metal ions from aqueous solutions or in solid phases.

3 Organic pollutants

Photocatalytic degradation of OPs is one of the most efficient methods to decontaminate the organic pollution, especially at extra low concentrations (Yao et al. 2021; Zou et al. 2022). Ndoun et al. (2021) prepared biochar from guayule bagasse and cotton gin waste, and applied it for the adsorption of pharmaceuticals from solutions. The results showed that hydrophobic partitioning, H-bonding and π–π electron donor–acceptor interaction were the three mechanisms for pharmaceuticals’ adsorption. With the increase of pyrolysis temperature, the surface hydrophobicity, zero-point charge value and surface area increased, which facilitated the binding the organic molecules on biochar. Ouyang et al. (2019) studied the defect structures of biochar on the photocatalytic performance. The biochar was synthesized under oxygen limited pyrolysis temperatures, the pyrolysis temperature had significant influence on biochar potency to active peroxymonosulfate. The degradation of 1,4-dioxane increased from 4 to 84% when the pyrolysis temperature of biochar increased from 300 to 800 °C. The ESR, XPS, FTIR and Raman analyses indicated that the biochar defect contributed to the generation of OH and SO4and O-containing groups were eliminated at high pyrolysis temperature, generating more defects and enhancing the biochar catalytic performance. The biochar activating peroxymonosulfate is a promising technique for the efficient remediation of organic contaminated water. Fang et al. (2017) studied the photochemical reduction of diethyl phthalate in biochar suspension under UV light irradiation. The singlet oxygen (1O2) and hydroxyl radicals (·OH) were the dominant reactive oxygen species for the reduction of diethyl phthalate. The electron paramagnetic resonance and free radical quenching analysis showed that biochar carbon matrix (BCM) generated about 64–74% ·OH and 10–45% 1O2, whereas DOM derived from biochar accounted for 47–86% 1O2 and 3.7–12% ·OH. The DOM derived from biochar contributed to the formation of 1O2 and ·OH, the BCM formed excited triplet states and generated 1O2 and superoxide radicals (·O2−) and further yielded H2O2. The photo-Fenton reaction and BCM-bound persistent free radicals’ activation generated ·OH radicals. The abovementioned possible pathways contributing to the degradation of diethyl phthalate are shown in Fig. 4a.

Fig. 4
figure 4

Possible pathways for the degradation of diethyl phthalate photodegradation by biochar (a) (Fang et al. 2017); Photodegradation mechanism of MO by TiO2/biochar (b) (Lu et al. 2019); Photocatalytic degradation mechanism of BPA by biochar@CoFe2O4/Ag3PO4 composites (c) (Zhai et al. 2020); Photocatalytic degradation of p-nitrophenol by Co(II)-doped BiOCl@biochar (d) (Cao et al. 2021); Photodegradation mechanism of TC by MoS2/biochar under sunlight (e) (Ye et al. 2019); Photodegradation of RhB by CdS/biochar under visible light irradiation (f) (Kang et al. 2022)

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3.1 Ti-based biochar

Hu (2022) prepared N-doped TiO2/biochar composites and compared the photocatalytic degradation of cephalosporin antibiotics with TiO2 and TiO2/biochar composites, and the results showed much higher photocatalytic degradation ability than the other two materials. The photoelectrochemical catalytic degradation is much superior to photocatalysis and electrocatalysis degradation of pollutants. Yang et al. (2021b) prepared phosphoric acid-modified biochar@TiO2 (PMBC@TiO2) composites and applied them for the removal of sulfadiazine from wastewater. The adsorption of sulfadiazine was spontaneous and endothermic, which was dominated by microporous binding, H-bonding, π–π interaction and electrostatic attraction. The surface adsorbed sulfadiazine could be quickly photocatalytic degraded under visible and UV light irradiation. The conjugated structure and O-containing groups promoted the efficient separation of e−-h+ pairs and the generation of free ·O2− and ·OH free radicals, which improved the photocatalytic capacity of the composites. Lu et al. (2019) prepared TiO2/biochar for the photodegradation of methyl orange (MO), and found that the TiO2/biochar catalyst exhibited much higher catalytic activity than TiO2 with high stability and reusability. The decolorization and mineralization efficiencies of the TiO2/biochar increased 21% and 51%, respectively, as compared to TiO2. The photocatalytic degradation of MO is shown in Fig. 4b. Fazal et al. (2020) synthesized biochar-TiO2 composite and applied it for photocatalytic degradation of simulated textile wastewater. The composite showed high separation of e−-h+ pairs, slow recombination of e−-h+ pairs, high visible light absorption and narrow band gap, which resulted in 99% degradation efficiency, much higher than those of biochar (85%) and TiO2 (43%). The generated free ·O2− and/or ·OH free radicals reacted with MB molecules and broke MB molecules to less toxic molecules or other products such as CO2 and H2O. The biochar acted as MB molecule adsorbent and provided efficient electron pathway through Ti–O–C bonds for photoexcited electrons to react with O2 to form ·O2−. Cai et al. (2018) synthesized TiO2/biochar composite and applied it for the photodegradation of safranine T (ST) under visible light irradiation. The binding of ST to the composite was mainly attributed to H-bond, π-π interaction and electrostatic attraction, which is similar to most published papers. The photodegradation of ST was the free active species of ·O2− and ·OH, also similar to the results of Fazal et al. (2020). The photocatalytic degradation of MB by TiO2/biochar was mainly dominated by hydroxyl radicals (·OH) (Silvestri et al. 2020), a little difference with the abovementioned TiO2/biochar catalyst for MB and ST. Nevertheless, the photocatalytic degradation of OPs is mainly attributed to the active species of ·O2− and ·OH, and the free electrons and holes. The TiO2/biochar composites showed much higher sulfamethoxazole removal and mineralization than TiO2 and biochar. The enhancement of sulfamethoxazole removal was due to hydrophobic interaction between sulfamethoxazole and biochar, whereas the enhanced photocatalytic degradation was attributed to the low recombination of e−-h+ pairs, efficient attachment of e−-h+ pairs to TiO2 and high adsorption of sulfamethoxazole to biochar (Kim and Kan 2016). Zhang et al. (2017) synthesized TiO2/biochar catalyst and applied it for the removal of sulfamethazine from wastewater. The catalyst showed high photocatalytic capacity after five cycles, and the biochar could promote the separation of e−-h+ pairs. The hydroxylation, cleavage of N-S bond and opening of isoxazole rings contributed to the photocatalytic degradation of sulfamethazine.

3.2 Fe-based biochar

Zhai et al. (2020) prepared biochar@CoFe2O4/Ag3PO4 composites and applied them for the photocatalytic degradation of bisphenol A (BPA) under visible light irradiation and showed high catalytic activity and stability. The intermediates and final products were measured by GC–MS and the photocatalytic degradation mechanism was proposed in Fig. 4c. Zhang et al. (2021b) prepared FeOOH/Fe3O4/biochar with high magnetic and photocatalytic activities, and applied it for the removal of MO from aqueous solutions. The composites showed 2.03 times of MO photodegradation activity higher than biochar although the surface area of the composite was much lower than that of biochar. The ESR and free radical quenching analysis showed that ·OH radicals were the primary species for the photocatalytic degradation of MO. More importantly, the catalysts showed high stability and reusability with at least 98% photocatalytic activity after 5 cycles, and can be easily separated from solution by magnetic separation technique. Huang et al. (2022) prepared nZVI/biochar and used it to activate PMS for sulfamethazine degradation. The catalyst had rich pores, defective structures and high graphitization, which resulted in high catalysis performance. The catalyst could degrade ~ 100% sulfamethazine in 20 min with high catalytic capacity and reusability. The EPR, quenching and XPS analyses showed that nonradical and radical pathways acted in sulfamethazine degradation, and the nZVI, pyridinic N, –COH and –COOH groups were the main active sites for the binding of sulfamethazine.

3.3 Bi-based biochar

Kumari et al. (2021) synthesized bismuth-modified biochar for photodegradation of MB and showed 90% degradation of MB, which was attributed to high adsorption of MB and efficient separation of e−-h+ pairs. However, the authors did not discuss the results in detail and the relative contribution of reactive free species was even not identified. Co(II)-doped BiOCl@biochar composite showed 99% photocatalytic degradation of p-nitrophenol in 90 min with high stability and recycle reusability, and 5% Co(II) doping in 30% biochar loading had the highest photocatalytic ability. The addition of BiOCl in biochar narrowed the band gap, enhanced light absorption, increased the separation of photogenerated e−-h+ pairs and thereby increased the photocatalytic performance. The photocatalytic degradation mechanism is shown in Fig. 4d (Cao et al. 2021).

3.4 Zn/Co-based biochar

Goncalves et al. (2020) synthesized ZnO/biochar under different temperatures and applied them for the degradation of MO and sulfamethoxazole antibiotic (SMX), and compared the degradation capacity of the samples. The results showed that the photocatalytic activity is dependent on the pyrolysis temperature, which affected the band gap energy and visible light absorption efficiency. Yu et al. (2021b) synthesized ZnO/biochar by ball milling method. The biochar was broken in ball milling process, and the meso- and macropores of the composites were increased, which enhanced the adsorption of MB on the composites. 95% MB was removed under visible light conditions through adsorption and photocatalytic degradation. The electrons were generated and then reacted with O2 to form ·O2− free radicals, which facilitated the degradation of MB. Welter et al. (2022) prepared ZnFe2O4/biochar composites and applied them for the degradation of RhB using heterogeneous photo-Fenton processes. Siara et al. (2022) applied ZnAl2O4-biochar for the degradation of ibuprofen, and found that 100% ibuprofen (20 mg/L) was eliminated in 120 min, which was attributed to the high surface area, pore volume and mesoporous size of ZnAl2O4-biochar. The free ·O2− and ·OH radicals contributed to the photocatalytic degradation of ibuprofen into by-products with lower molecular weight. Gholami et al. (2020) incorporated Zn-Co-LDH with biochar and used it for gemifloxacin photodegradation. 93% gemifloxacin was degraded in 130 min at the initial concentration of 15–35 mg/L. The free radical scavenging experiments showed the contribution of the radical species to gemifloxacin degradation in the sequence of ·OH > h+ > O2−. Yang et al. (2021c) applied N-doped biochar for the photodegradation of sulfamethazine and found that 1O2 was the main active species for sulfamethazine degradation from the ESR and quenching measurements. Further DFT calculation indicated that the pyridinic N-doping decreased the intersystem crossing energy and thereby benefited 1O2 generation by energy transfer and electron transfer.

3.5 Mo/Cd-based biochar

Ye et al. (2019) prepared porous graphite biochar assembled with MoS2 nanosheets, which have high graphitization degree and porosity, suitable band gap for visible light absorption, efficient separation of e−-h+ pairs and charge transfer acceleration. The batch experiments showed that adsorption of tetracycline hydrochloride (TC) was dominated by H-bonding, π–π interaction, electrostatic attraction and pore filling, whereas the photocatalytic degradation was mainly dominated by the hydroxy radicals (·OH) and holes (h+) (Fig. 4e). The composites showed much higher adsorption capacity and photocatalytic ability than biochar and MoS2, and the high stability and reusability indicated that the composites could be used as potential catalysts in the efficient degradation of OPs.

Kang et al. (2022) prepared sulphur-modified biochar/CdS composites and applied them for the adsorption-photocatalytic degradation of RhB under visible light irradiation. The composites showed high adsorption of RhB because of the large surface area and abundant active sites, and promoted the separation and utilization of photogenerated e−-h+ pairs. The electrostatic interaction and H-bonding were the main mechanisms for the uptake of RhB, whereas the superoxide radicals (·O2−) contributed mainly to the photodegradation of RhB from the quenching tests and EPR analysis. The mechanism for the generation of active radicals for RhB degradation was illustrated in Fig. 4f.

3.6 Ag-based biochar

Wang et al. (2019b) synthesized Ag nanoparticle-doped biochar/Bi4Ti3O12 to improve the photocatalytic performance. The doped Ag could promote the separation of electrons and holes, and the biochar could provide chemical groups to improve electron transfer. The surface doping of metal nanoparticles is an efficient strategy to improve the photocatalytic capacity. Wei et al. (2021) synthesized Ag3PO4/biochar catalyst. The Ag3PO4 nanoparticles were well dispersed in the porous structure of biochar and Ag0 nano-species were generated in the catalyst. The Ag0 nano-species could act as a bridge to transfer the photogenerated electrons quickly from Ag3PO4 to biochar, and then reacted with the adsorbed O2 to produce ·OH active radicals. The Ag3PO4/biochar showed high photocatalytic capacity in the degradation of MB, and 96% MB was efficiently removed in 60 min under visible light conditions. From the abovementioned results, one can see that the doping of metal nanoparticles could enhance the photocatalytic performance of biochar-based composites because of the high generation and separation of free active radicals, electrons and holes, and high adsorption ability of pollutant molecules on biochar.

3.7 C3N4-based biochar

Xiao et al. (2021) prepared C3N4/biochar and applied for for the removal of enrofloxacin through adsorption and photocatalysis strategy. The composite showed much higher adsorption and photodegradation capacities to enrofloxacin with higher stability and reusability than pure C3N4 and biochar. The ·O2− and h+ were the main species for the photocatalytic degradation of enrofloxacin. The g-C3N4/biochar showed much higher photodegradation of formaldehyde than g-C3N4 under visible light conditions, which was attributed to increased specific surface area and separation of e−-h+ pairs by chrysanthemum g-C3N4 and visible light absorption by biochar skeleton (Li et al. 2019b). Karpuraranjith et al. (2021) synthesized magnetic biochar-based C3N4 catalyst, and applied for the photodegradation of RhB dye under visible light conditions. The composite showed much higher photocatalytic performance than biochar and C3N4 with outstanding recyclability, stability and eco-friendliness. The efficient separation of photogenerated e−-h+ pairs and effective suppress of e−-h+ pairs recombination enhanced the photocatalytic capacity. The reactive ·O2− and ·OH free radicals contributed to the RhB degradation. The biochar/C3N4 composites also showed much higher adsorption capacity to cationic MB dye and high photocatalytic degradation of MB, which was attributed to the oxidative species of superoxide anion radicals and photogenerated e−-h+ pairs (Pi et al. 2015). Kumar et al. (2017) prepared g-C3N4/FeVO4/Fe@NH2-Biochar and applied it for the removal of methyl paraben and 2-cholrophenol through adsorption, photodegradation and photo-ozonation strategy. The catalyst showed high surface area, porous nanosheet structure, high stability, broad solar light absorption and good separation of e−-h+ pairs. The FeVO4 and Fe were favorable for higher light absorption and thereby increased the photocatalytic activity. The elimination of the two organic contaminants was carried out under four conditions, i.e., photodegradation, adsorption followed by photodegradation; simultaneous adsorption and photodegradation (SAP); and SAP plus O3 photo-ozonation. The results showed that adsorption and photodegradation was a good method for high elimination of organic contaminants, whereas the adsorption, photodegradation plus photo-ozonation showed a little higher efficiency than the other three methods. The photo-ozonation could accelerate the degradation rate and shorten the reaction time.

3.8 Mixed metal-based biochar

Azalok et al. (2021) synthesized MnFe-LDO-biochar and applied it to remove tetracycline from solutions. Under dark condition, 40% tetracycline (20 mg/L) was removed in 30 min, whereas 98% tetracycline was eliminated in 240 min under UV light irradiation. In the presence of K2S2O8 and H2O2, 98% tetracycline could be removed in 60 min. The photodegradation of tetracycline was mainly attributed to the ·OH and ·SO4− active species, whereas the mineralization was dominated by ·O2− and h+. The biochar/ZnFe2O4/BiOBr showed much higher photocatalytic degradation of ciprofloxacin than ZnFe2O4 and BiOBr under visible light irradiation. The ZnFe2O4 and BiOBr could promote the separation of photogenerated e−-h+ pairs as ZnFe2O4 and BiOBr could serve as heterojunctions. The high adsorption ability of biochar, efficient separation of e−-h+ pairs and prolonged recombination time of e−-h+ pairs resulted in the high photocatalytic degradation ability of biochar/ZnFe2O4/BiOBr with high stability and reusability (Chen et al. 2019b). Peng et al. (2021) synthesized ZnFe2O4/B,N-codoped biochar and applied it for enhancing adsorption-photodegradation of tetracycline hydrochloride (TCH) from solutions. The results showed that 98% TCH (150 mg/L) was removed from solutions in 120 min with the adsorption capacity of 244 mg/g. The B and N doping mass affected the adsorption and photocatalytic degradation efficiency of the composites. The relative contribution of the ·OH, e−, h+ and ·O2− active species to the degradation of TCH was evaluated by free radical quenching tests. The e− and ·O2− were vital for the photodegradation of TCH, which was also evidenced in the ESR spectra where the intensity of ·O2− was much stronger than that of ·OH. The photocatalytic degradation efficiency decreased from 98 to 89% after seven cycles, suggesting the relative stability and recyclability of the composites in the efficient elimination of TCH from aqueous solutions. Talukdar et al. (2020) synthesized Ag3PO4/Fe3O4-biochar with high stability and magnetic property. The photocatalytic degradation of bisphenol A (BPA) by the catalyst in peroxydisulfate system was carried out under visible light condition, and 96% BPA was degraded after 60 min, which was mainly attributed to ·O2−, ·SO4−, and ·OH active species.

Khataee et al. (2019) prepared Cu2O-CuO@biochar and applied it for the photodegradation of reactive orange 29 (RO29). 94% RO29 (20 mg/L) was degraded at pH 8.9 in 90 min. The by-products were measured by GC–MS and the by-products were finally mineralized to H2O, CO2 and inorganic ions. Navarathna et al. (2020) synthesized magnetic MOF/biochar catalyst and applied it for the photodegradation of RhB in the absence and presence of Cr(VI). The adsorption of RhB to the catalyst was mainly dominated by electrostatic attraction and π–π interaction. The presence of Cr(VI) increased the photocatalytic degradation of RhB, and Cr(VI) was also photo-reduced to Cr(III). However, the relative contribution of the free active radicals to the photodegradation of RhB was not discussed in detail, especially from ESR and quenching analysis. The photocatalytic degradation of different OPs using different biochar-based catalysts was summarized in Table 2. From Table 2, one can see that the main active species for the degradation of different OPs are quite different, even for similar biochar-based catalyst for the photodegradation of same pollutant molecules. Nevertheless, the photocatalytic degradation of organic molecules was mainly dominated by the photogenerated electron–hole pairs, free ·O2−, and ·OH active radicals.

Table 2 Summary of biochar-based catalysts in the photocatalytic degradation of organic pollutants
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Till now, the photocatalytic degradation of different OPs has been extensively studied using different kinds of biochar-based catalysts. The sources for the synthesis of biochar, the pyrolysis temperature and the conditions for the preparation of biochar affect the structures, surface functional groups, band gap, surface areas and inner-pore properties, which thereby influence the photocatalysis performance of biochar and biochar-based materials. The doping of metal or metal oxides is the main strategy to improve the photocatalytic performance of biochar-based catalysts. The doping of metal or metal oxides could narrow band gap, enhance the generation and separation of photogenerated electron–hole pairs, increase the absorption of visible light etc. It is necessary to note that the amount of metal or metal oxides in the composites, the types of metals, single or multi-doping of metals also influence the photocatalytic properties of biochar-based catalysts. The catalysts could simultaneously remove OPs through adsorption-photodegradation processes.

For the photodegradation of different OPs, the relative contributions of different free active species are a little different, which is mainly attributed to the generation of the free active radicals by different composites. One can speculate the degradation mechanism of OPs from ESR, quenching tests and other advanced spectroscopy analysis. However, the analysis of intermediate products and the exactly contribution of each active radicals to the photodegradation is still a challenge, which is helpful to construct the catalysts.

4 Mechanism discussion

4.1 Advanced spectroscopy analysis

The interaction mechanism of pollutants with biochar or biochar-based catalysts is dependent on the properties of materials (i.e., structures, surface functional groups, band gap, light absorption ability, (photo)catalytic performance, stability etc.) and the properties of pollutants (i.e., oxidation–reduction reactions, species, bond strength, coordination properties, structures of pollutants etc.). The interaction of OPs with biochar is generally dominated by electrostatic attraction, hydrophobic effect, H-bonding and π–π interaction etc., whereas the interaction of inorganic pollutants is generally attributed to ion exchange, surface complexation, (co)precipitation etc. The surface adsorbed organic molecules could be photocatalytic degraded by the photogenerated e−-h+ pairs, free ·O2− or ·OH active free radicals, and the metal ions could be reduced from high valence to low valence, which could be in-situ solidified on solid particles. Different kinds of spectroscopy techniques such as XPS, FTIR, Raman and XAFS have been applied to understand the interaction of pollutants. In the FTIR or XPS spectra, from the appearance of new peaks, the disappearance of peaks, change of peak intensities, the red/blue shift of peaks, one can achieve some useful information about the interaction of pollutants with the functional sites/groups, although such information is semi-qualitatively. From the XAFS analysis, one can get some information about the structures of metal ions such as the coordination number and bond distance with the atoms around the metal ions at molecular level, the oxidation–reduction state of the metal ions etc. The in-situ analysis could provide the dynamic interaction process information, such as the change of species with the change of interaction time. Liu et al. ( 2022b) applied in-situ electrochemical Raman system (Fig. 5a) to measure the U(VI) species and the formation of U(VI) precipitates as a function of reaction time in the adsorption-electrocatalytic reduction process. The Raman peak at 489 cm−1 suggested the presence of adsorbed U(VI) on the electrode (Liu et al. 2017). With the increase of reaction time, the peak at 489 cm−1 disappeared whereas a signal at 810 cm−1 appeared, suggesting the reduction of U(VI) to U(V) (Pointurier and Marie 2013; Stefaniak et al. 2008). New Raman peak at 374 cm−1 appeared in the Raman spectrum at 240 s, indicating the formation of unstable U(V) to stable U(VI) on electrode, forming Na2O(UO3·H2O)x precipitate (Fig. 5b). The in-situ Raman analysis clearly showed the change of U(VI) species as a function of contact time. The XAFS analysis showed that a U atom was coordinated with 2 axial O atoms and 4 square planar O atoms with the U–O bond distances of ~ 1.8 Å and ~ 2.2 Å, respectively, which is in good agreement with the results of in-situ Raman analysis. The good agreement of different spectroscopy characterization could confirm the interaction mechanism more exactly rather than one spectroscopy analysis.

Fig. 5
figure 5

a Photographs showing the in situ electrochemical—Raman microscope system. b In situ Raman spectra collected from the In − Nx − C − R/carbon felt working electrode in uranium—spiked seawater during the adsorption—electrocatalysis process (Liu et al. 2022b)

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For organic molecular photocatalytic degradation, the relative contributions of active free radicals to the degradation processes are usually determined from the electron paramagnetic resonance (EPR) analysis. The light absorption can be measured by UV–vis diffuse reflection spectroscopy (UV–vis DRS) directly, which is helpful to evaluate the electronic structures of catalysts. Generally, there are four main steps in organic molecule degradation, i.e., (1) light absorption to generate e−-h+ pairs; (2) efficient separation of e−-h+ pairs; (3) to transfer the holes and electrons to catalyst surface and to avoid the e−-h+ recombination; and (4) the degradation of organic molecules by the generated active radicals (Qiu et al. 2021). The free active species such as ·O2−, 1O2, ·OH, h+ etc. can be evaluated through the addition of radical scavengers. Quenching tests are generally carried out to evaluate the relative contribution of active free radicals. The degradation of different OPs by biochar-based catalysts through the addition of different radical scavengers is shown in Fig. 6a (Talukdar et al. 2020). One can see that the photocatalytic degradation of BPA was obviously inhabited after the addition of TBA, MeOH or BQ. The addition of BQ could scavenge ·O2− radicals, whereas the addition of TBA or MeOH could scavenge ·OH or ·SO4−/·OH, respectively. The effect of the radical trapping scavengers was in the order of TBA < MeOH < BQ, suggesting the contribution of ·O2− radicals was much stronger than that of ·OH or ·SO4− radicals. However, the total contribution of the free active species on the photocatalytic degradation of BPA is > 100%, suggesting the contribution of the free active radicals is dependent on each other and the quantitative contribution of each free radicals is not calculated accurately. The EPR spectroscopy is generally used to measure the generation of reactive free radicals. No signal was found for dark conditions, and the signals of ·O2− radicals were found for BC300 (biochar heated at 300 °C) and BM300 (BC300 was treated by ball milling) samples (Fig. 6b) (Xiao et al. 2020). The signal intensity of BC300 was weaker than that of BM300, suggesting more generation of ·O2− radicals by BM300, which is in good agreement with the results that the photocatalytic performance of BM300 was stronger than that of BC300.

Fig. 6
figure 6

a Effect of radical scavengers on photocatalytic degradation of BPA in the Vis/Ag–Fe@BAB/PDS system (Talukdar et al. 2020); b DMPO-·O2− concentration of BC300 and BM300 in methanol dispersion under dark and light conditions (Xiao et al. 2020); Schematic diagrams for photogenerated electron migration in c OPCN (Zhang et al. 2019a) and (d) DMCN (Zhang et al. 2019b) photocatalysts under visible light irradiation

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The influences of photogenerated electron migration on the degradation should also be considered. Once excited by light, photogenerated electrons (e−) and holes (h+) are generated in the desired photocatalysts, which directly participate in surface redox reactions through different routes. The separation and transfer of charge carriers, especially for photogenerated e−, play a vital role during photocatalysis. The rational regulation of photogenerated e− migration not only benefits for the separation of charge carriers, but also prolongs their lifetimes for photoreactions (Zhang et al. 2019a). For example, after the doping of strongly electronegative O atoms (Fig. 6c), more electrons and holes were separately gathered onto the aromatic ring planes of g-C3N4 by forming the e− and h+-related delocalized systems. As a result, more reactive species were yielded forthe removal of refractory organic pollutants. In addition, the surface functionalization can induce the directional transfer of photogenerated e− from the bulk to reaction centers, which shortens their transmission distances and reduces mass transfer losses. Thus, the utilization of photogenerated e− and optical quantum efficiency will be significantly improved. As previously reported, hydroxyl groups (–OH) and cyano groups (–C≡N) as electron-withdrawing groups were engineered into mesoporous g-C3N4 (Fig. 6d), which served as reaction sites to capture and directionally transport electrons from photocatalysts to persulfates (electron acceptors) (Zhang et al. 2019b). Benefiting from the unidirectional transfer, plenty of strong oxidized SO4·− radicals were produced from efficient photoreduction of S2O82− anions and then attacked organic pollutant molecules till their complete degradation. Synchronously, much more photogenerated holes remained on the valence band of photocatalysts for direct oxidation during reactions. In brief, the effective migration of photogenerated electrons will induce efficient separation of charge carriers and improve the utilization of solar energy.

For most literatures, the analysis of products was not carried out although the products could be measured from the liquid chromatography coupled to mass spectrometer (LC MS), and the intermediates can also be identified from LC MS analysis, which is helpful to propose the degradation mechanism (Siara et al. 2022). Nevertheless, the photocatalytic degradation mechanism of organic molecules is difficult to be confirmed. One can speculate the degradation routes from the analysis of intermediates and final products, from the measurements of free active radicals, and computational simulation to understand the possible degradation mechanism.

4.2 Computational calculation

As the structures of biochar are not well defined and the intermediates are difficult to be analyzed directly, computational simulation can be applied to simulate the possible degradation pathway. The reaction energies and intermediates can be simulated in DFT calculations, which is helpful to evaluate the photocatalytic degradation mechanism. The interaction between organic molecules and biochar-based catalysts can be simulated. The photocatalytic degradation of organic pollutants can be predicted and evaluated by computational calculation using different models (Wang et al. 2021). To our best knowledge, the DFT calculation about the photocatalytic degradation of organic molecules by biochar-based catalysts is still scarce, which may be attributed to the complicated structures of biochar. The possible degradation of BPA, the structures of intermediates, transition states and interaction energies are simulated and the results are shown in Fig. 7a and b (Zhang et al. 2019a). In the DFT simulation, the possible intermediates with reaction energies and bond distances with different atoms such as C, N or O atoms are calculated, which is helpful to draw the possible photocatalytic degradation pathway. The impossible intermediates with very high reaction energies can also be calculated. The photoexcited state is also important to understand the degradation of organic molecules. The generated electrons and holes with charge distribution and density are calculated and shown in Fig. 7c. The free e− transfer occurs to O atoms of Mo–O cluster, and the h+ transfers reversely to C atoms, resulting in efficient separation of photogenerated e--h+ pairs and thereby improves the photocatalytic performance. The possible surface and interlayer adsorption of BPA and 4-CP are simulated (Fig. 7d). The structures and bond distances are also calculated, and the BPA is preferably adsorbed in the interlayer with –OH groups. The adsorption of 4-CP demonstrates that 4-CP tends to be adsorbed on the inserted Mo–O clusters by –OH groups (Zhang et al. 2022b). The DFT calculation proved that the O atoms in Mo–O clusters served as the adsorption sites, which is useful to understand the photocatalytic degradation of BPA, 4-CP or other kinds of organic molecules. Nevertheless, the DFT calculation could provide qualitative information about the interaction of organic molecules with catalysts. Although computational calculation of OPs with biochar or biochar-based catalysts is still scarce, the DFT calculation could be carried out to simplify the complicated structures of biochar. In near future, the DFT simulation of organic molecules interaction with biochar and biochar-based catalysts and the degradation pathway evaluation will attract extensively interest.

Fig. 7
figure 7

DFT calculated structures of reactants, intermediates and transition state for the degradation of BPA attacked by OPCN catalysts with N atoms (a) or O atoms (b) as reactive sites (white, red, gray and blue balls represented H, O, C and N elements, respectively) (Zhang et al. 2019a); c Computed charge distributions on [Mo7O24]6−-CN once being excited by visible light. Isodensity representation of the electron and hole were given in cyan and mauve, respectively; d The optimized structures for the adsorption of BPA and 4-CP on [Mo7O24]6−-CN. Bond lengths were in Å. (The blue, gray, white, red, pale pink and green balls represented N, C, Mo, O, H, and Cl atoms, respectively) (Zhang et al. 2022b)

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5 Conclusion and perspective

In this review article, we mainly summarized the recent works, especially in last five years, about the adsorption-photocatalytic degradation of organic contaminants and adsorption-(photo)catalytic reduction of heavy metal ions using biochar and biochar-based catalysts. Although biochar itself has high adsorption capacity, the low photocatalytic ability restricts the application of biochar as photocatalyst in real works. The heteroatom (i.e., S, N, P, B etc.) doping of biochar has fruitful effect to improve the photocatalytic degradation of biochar. The in-situ synthesis or post-modification strategies could achieve the heteroatom doping. The doping of metal or metal oxides on biochar is an effective strategy to narrow the band gap of the catalyst, to enhance the visible light absorption, to increase the generation and separation of photogenerated electron and hole pairs, and to produce more free active radicals, thereby increasing the photocatalytic properties of biochar-based catalysts. The environmentally friendly properties of biochar make it a suitable material in the photocatalytic degradation of organic contaminants or the (photo)reduction of metal ions and then immobilization of the metal ions to decrease the transport of metal ions in the natural environment.

In the photocatalytic degradation of organic contaminants or reduction of heavy metal ions, there are still many challenges in future, for example: (1) the relative contribution of active species is generally evaluated from ESR and quenching tests, which are helpful to understand the qualitative analysis of different free active radicals on the degradation/reduction of the environmental contaminants; (2) the quantitative analysis of the active species’ contribution is still unclear. One can see that the sum of each free active radicals is much higher than 100%, suggesting the independent contribution of the active radicals. The relative qualitative contribution of free active radicals could be evaluated, but the quantitative contribution of each free radicals is not calculated accurately; (3) the whole degradation processes are still “black-box” as the intermediate products are difficult to be measured exactly. Computational calculation is a very useful technique to simulate the different degradation pathways, and to achieve the structures and transition state of reactants and intermediates. However, the structure of biochar is very complicated and it is really very difficult to construct the computational calculation mode. Till now, computational simulation of photocatalytic degradation/reduction of contaminants by biochar or biochar-based catalysts is still not available in large scale; (4) the toxicity of the catalyst should also be considered although biochar is environmentally friendly. The release of metals or metal oxides from the biochar-based catalyst is inevitably, which are generally toxic to environmental ecosystem. The stability of the catalyst should be evaluated, especially for long aging time in complicated systems; (5) The final products should be analyzed exactly, especially the toxicity of the products; (6) the photocatalytic reduction of other redox-active metal ions and direct measurement of remobilization of re-oxidation of metal ions adsorbed on biochar should be studied, especially for long-term conditions. The biochar itself is an environmentally friendly material, which can improve the soil nutrients and decontaminate the soil pollution. With the development of technology, the real application of biochar and biochar-based materials in environmental pollution will attract extensively attention.