1 Introduction

Biochar is a porous carbonaceous material that is obtained from the pyrolysis (pyrochar) or hydrothermal carbonization (hydrochar) of raw biomass under oxygen-limited or -free conditions (Xiao et al. 2018). Rich biomass resources including agricultural waste, forestry waste, garden waste, biosolids, and industrial residues devote increasing attention to biochar for carbon sequestration, climate change mitigation, waste management, energy production, soil improvement, and environmental remediation (Alam et al. 2018; Manyà 2012; Woolf et al. 2010).

Biochar features low cost, C-richness, high porosity, large specific surface area (SSA), abundant oxygen-containing functional groups (OFGs), inorganic minerals, and redox properties (Klüpfel et al. 2014; Leng et al. 2021; Wang et al. 2020e). The performance of biochar in multiple applications and potential risks are highly dependent on its inherent properties. For example, the content and form of carbon within biochar are considered to be highly correlated with the long-term storage and stability of biochar in soil (Manyà 2012; Rechberger et al. 2017). The porosity and OFGs of biochar are key aspects for determining the availability of active sites in biochar (Weber and Quicker 2018). The redox properties of biochar play an important role in biogeochemically relevant electron transfer reactions (Klüpfel et al. 2014; Yuan et al. 2017). The properties of biochar are highly influenced by pyrolysis conditions including residence time, heat treatment temperature (HTT), and biomass feedstock (Li et al. 2019b; Xiao et al. 2018). Broadly speaking, the carbon and mineral contents, alkalinity, aromaticity, SSA, zeta potential, and persistent-free radical (PFRs) increase with increasing HTT (Wu et al. 2019; Xiao et al. 2018; Zhang et al. 2020e). In contrast, the product yield, cation-exchange capacity (CEC), OFGs, aliphaticity/polarity, heavy metal bioavailability, and polycyclic aromatic hydrocarbon (PAHs) contents decrease with increasing HTT (Wu et al. 2019, 2020a). Biochar obtained from crop residues presents higher product yield, carbon content, and ash content than that of wood-derived biochar (Wang et al. 2013). Considering the remarkable heterogeneity in physicochemical properties of biochar, it is necessary to provide an in-depth overview of the development of biochar regarding its multifunctional applications and unintended potential risks.

Currently, there have been numerous developments in improving the properties of pristine biochar by various engineering strategies including physical, chemical, and biological modification (Wang et al. 2017; Wu et al. 2020a). For example, Fe-impregnated biochar exhibited enhanced sorption of heavy metals and catalytic degradation of organic pollutants (Xu et al. 2020a; Yi et al. 2020a). Furthermore, the magnetic property allowed the recovery and reuse of spent biochar (Li et al. 2020b). Fe–Mn binary oxide-modified biochar has also been widely used as an engineering material to enhance the removal capacity toward different contaminants and to benefit the separation of biochar (Lai et al. 2019; Li et al. 2019a; Zhou et al. 2018). The enhanced capability or functionality of metal-based biochar arises from metallic(oxyhydr)oxides incorporated in the carbon matrix, which introduce more active sites (Huang et al. 2020a; Li et al. 2019a). It is important to note that microbial immobilization technology with biochar as a carrier is another engineering strategy for biochar modification (Tao et al. 2019). Biochar immobilized with functional microbes combines the advantages of highly efficient sorption performance of biochar and microbial metabolisms, resulting in higher removal of contaminants (Chen et al. 2019; Liu et al. 2012; Wang et al. 2018).

Although a number of reviewers have systematically discussed biochar application in agriculture, environment, and energy, few have investigated the evolution and development of biochar synthesis, research, and applications (Wu et al. 2019). Summarizing the research from biochar can help us to identify the current research frontiers and hot spots (Wu et al. 2019; Wu et al. 2020a). As new applications for biochar continue to emerge, the main progresses and insights of this field will change over time. Thus, further summarization of the main progresses and insights of biochar in 2020 is essential. Citespace, a bibliometric analysis software, has gained increasing attention and application in many fields due to its computing and visual analytic functions (Chen 2017; Chen et al. 2014).

In this study, a combination of bibliometric analysis and critical appraisal of the publications was used to evaluate the current progress, research hotspots, and trends of biochar in 2020. The specific objectives of the present review are to: (1) determine the main research subjects, authors, countries, and significant keywords of biochar research; (2) describe the current research status and hot topics associated with biochar; (3) discuss and summarize the research trends of biochar.

2 Data acquisition and analytical methods

2.1 Data collection and processing

All data used in this review for measurements and statistical analysis were retrieved from the Web of Science (WoS) core collection as it records the most representative and relevant articles. During the data collection process, “Biochar” was used as the keyword and the data collection period was limited within “2020” during the data screening. The data with “Biochar” appearing in the title, keywords, and abstracts of the English documents were collected. To guarantee the validity of data processed, the biochar-associated synonyms such as “PAHs” and “polycyclic aromatic hydrocarbon”, “Cadmium” and “Cd”, and “CO2” and “carbon dioxide” were manually merged.

2.2 Scientometrics analysis tools

Citespace is a widely used statistical analysis tool for bibliometric analysis and visualization (Li et al. 2020c). It is based on Java environment developed by Professor Chaomei Chen and his team in early 2004 (Chen 2004). Based on the critical analysis of cooperation networks including authors, countries, institutions, clusters, categories, and keywords, it can grasp hotspots and explore frontiers and emerging trends of biochar research. In the visualized maps, each node represents one item (e.g., author, keyword, or institution), and the nodes are denoted in different sizes and linked by lines. The node thickness indicates frequencies of the items. The thickness of line represents the correlation strength between two nodes.

3 Results and discussion

3.1 Publications analysis

A total of 3671 publications were collected from WoS core collection during 2020, which is much higher than that in 2019 (Fig. 1), indicating that the focus on biochar research is sustainably increasing. The publications are mainly categorized into ten types, including research article, review, proceedings paper, correction, editorial material, meeting abstract, book review, retraction, letter, and news item. Among the publications gathered, research articles accounted for the highest proportion with 92% of all the publications (Table S1 and Fig. S1). The diverse composition of biochar publications indicates that as biochar research becomes more in-depth, studies on biochar applications in agriculture, environment, and energy have become increasingly diverse, detailed, and systematic.

Fig. 1
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The number of published documents on biochar research

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3.2 Categories of co-occurrence analysis

The top ten subject categories of publications on the topic of biochar during 2020 are listed in Table S2. The most important field of research subject categories in biochar area were “Environmental Sciences & Ecology”, followed by “Environmental Sciences”, “Engineering”, “Engineering, Environmental”, “Agriculture”, “Energy & Fuels”, “Engineering, Chemical”, “Chemistry”, “Science & Technology—Other topics”, and “Green & Sustainable Science & Technology” (Table S2). Major changes in the subject categories of biochar research were observed in 2020 compared from those in 2019. The subject “Soil Science” has missed out from the top ten subject categories. It is worth noting that the subject “Green & Sustainable Science & Technology” deserves great attention to researchers. The development of sustainable fuels by converting lignocellulosic biomass into low-cost biofuels has created broad interests. Additionally, the synthesis of engineered biochar for green and sustainable environmental applications has been the research hotspot recently.

3.3 Contributing countries analysis

The top ten most contributing countries in biochar area during 2020 were recognized from the analysis of WoS core collection database. As shown in Table S3, China is still the highest contributing country in biochar area, indicating the critical role of China in this field. USA and India occupied the next places among the top ten contributing countries (Table S3).

3.4 Authors’ co-citation analysis

The cooperative network of authors and the top ten prolific authors and their publications on biochar research are presented in Fig. 2 and Table S4. It is worth noting that several new prolific scholars appeared in 2020 compared to previous 21 years (1998–2019) (Table S4) (Wu et al. 2019, 2020a). This suggests that biochar research has received increasing attention from emerging scholars. Yong Sik Ok and Daniel C. W. Tsang were the most prolific authors in terms of the number of biochar-related publications. Majority of the most productive authors come from China, indicating that China plays an indispensable role in biochar research. Researchers from South Korea, Pakistan, USA, Germany, Finland, and India also made great contributions in this field (Table S4).

Fig. 2
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The co-author map of biochar research in 2020

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3.5 Keywords’ analysis

To determine the frontiers, hotspots, and future trends of biochar research, the dissection analysis of keywords was further conducted (Table 1 and Fig. 3). In the network map, one node represents one keyword, and the size of nodes is positively related to the frequency of keyword. The keyword co-occurrence map in 2020 is more complicated and diversified compared to that in 2019 (Fig. 3), implying that studies on biochar have become increasingly detailed, diverse, and in-depth. The nodes on the network can be classified into six research topics in biochar applications, including “bioenergy production”, “global climate change mitigation”, “salinity and drought stress amelioration”, “Organic pollutants degradation”, “Heavy metal immobilization”, and “Bioremediation”. The emerging topic “Bioremediation” in 2019 analyzed by Citespace was still a hot topic in 2020 (Fig. 3). The degradation of organic pollutants rather than sorption by biochar played a dominant role in the removal of organic pollutants. It is important to note that “salinity and drought stress amelioration” was the emerging hot spot in biochar application in 2020 (Fig. 3).

Table 1 The top 20 keywords related to biochar in 2020
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Fig. 3
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The network map of biochar research in 2020

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3.5.1 Bioenergy production

Thermochemical conversion of biomass for biofuels (e.g., bioethanol, biodiesel, and biogas) production was a growing and active research topic in 2020 (Fig. 4). Thermochemical conversion of microalgae using wet torrefaction was one of the novel and promising techniques for bioethanol production. A high bioethanol yield of 7.61% and a solid biochar yield of 74.6% were achieved by this method (Yu et al. 2020a, b). Microbial immobilized biochar-based biocatalysts were developed for efficient bioethanol production using Saccharomyces cerevisiae immobilized on biochar. Biochar could effectively adsorb harmful contaminants and toxic microbial metabolites and preclude the washout of cells (Lu et al. 2020). Therefore, the immobilized cells had high resistance to environmental disturbance and achieved stable performance for repeatability, resulting in high bioethanol production and low operating costs (Kyriakou et al. 2020). Biochar is frequently used as a catalyst for biodiesel production by promoting the transesterification of oil and methanol as it possesses economic feasibility, high stability in acid/base media, and tunable surface functionality (Stamenković et al. 2020). A novel sulfonated and magnetic biochar catalyst showed high transesterification efficiency, with the biodiesel yield up to 90.2% at optimum conditions (65 °C, 102 min, 3.66 wt% catalyst, 13:1 methanol-to-oil ratio) (Quah et al. 2020). Similarly, CaO nanoparticle-supported biochar could efficiently promote biodiesel yield. Furthermore, the engineered biochar was easily recoverable and reusable with high catalyst activity (di Bitonto et al. 2020). Biochar-based catalyst pretreated with sulfonic acid exhibited high catalyst activity toward biodiesel production due to its increased acid density. A high biodiesel yield of 94.91% with 59.73% fatty acid methyl esters was achieved by transesterification of algae under optimal conditions (65 °C, 4 h, 5 wt% catalyst, 20:1 methanol-to-oil ratio) with the modified biochar as acid catalyst (Behera et al. 2020).

Fig. 4
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Biochar applications in bioenergy production

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The catalytic efficiency of biochar was mainly regulated by the nature of biomass and HTT (Behera et al. 2020; Wang et al. 2020g). The maximum biodiesel yield was highly sensitive to the lignin content of biomass rather than the contents of cellulose and hemicellulose in biomass (Bhatia et al. 2020). In general, the biodiesel yield was negatively correlated with the lignin content of biomass (Bhatia et al. 2020).

The use of biochar as an additive or accelerant to improve anaerobic digestion (AD) process of biowastes and methane yield was an interesting topic for researchers in 2020. Different biomass wastes for AD included food wastes, sewage sludge, and other biodegradable residues (Giwa et al. 2020). The addition of biochar during AD process could mitigate ammonium (NH4+) inhibition effects and enrich the microbial community of methanogenic archaea (i.e., Methanosaeta and Methanosarcina) (Zhang and Wang 2020). The redox activate moieties (e.g., quinones, phenolics, and phenazines) in biochar supported the direct interspecies electron transfer (DIET) between methanogenic and syntrophic bacteria (Li et al. 2020e; Wang et al. 2020f; Zhang and Wang 2020). Additionally, biochar was conducive to the immobilization of methanogenic archaea and syntrophic bacteria, which stimulated the growth and reproduction of bacteria and DIET mechanism (Yang et al. 2020b). Biochar addition also helped AD process stability by increasing buffering capacity (Cimon et al. 2020; Wei et al. 2020). The positive effects of biochar in AD system contributed to increasing methane yield. The minerals in biochar played an important role in methanogenic digestion. It was found that Fe–Mn-modified biochar enhanced the biodegradation of volatile fatty acids (VFAs), accompanied by accelerating DIET process. More importantly, biochar modification with Fe–Mn binary oxides was more beneficial to immobilize heavy metals in sewage sludge, thus reducing the bioavailability risk of heavy metals in sewage sludge (Zhang and Wang 2020). Therefore, biochar modification with metal oxides nanoparticles has great potential in improving the AD efficiency and digestate quality. It was noted that excessive biochar addition had harmful effects during AD process. It is probable that excessive Mn2+ leached from Fe–Mn-modified biochar had detrimental effects on the population and diversity of the microbial community involved in AD process (Zhang and Wang 2020). The sorption of VFAs on excessive biochar may also inhibit the methane production (Cimon et al. 2020). Therefore, an appropriate application rate of biochar is essential during AD process.

3.5.2 Global climate change mitigation

Biochar applications in carbon sequestration and mitigating greenhouse gases (e.g., CO2, CH4, and N2O) emissions were well documented and deserved increasing attention in 2020 (Fig. 5). Biochar amendment is considered to be a potential strategy to reduce CO2 emissions from soils. The reduction was ascribed to the adsorption of CO2 on the abundant functional groups present on biochar surface (Ashiq et al. 2020). The reduction in the content of labile C in soil amended with biochar was also responsible for the reduced emissions of CO2 (Ashiq et al. 2020). Biochar addition was also found to enhance microbial carbon use efficiency as well as reduce microbial mass-specific respiration, which favored greenhouse gas mitigation (Li et al. 2020d). On the other hand, biochar is proposed as a potential sustainable and cost-effective material for CO2 capture, thereby contributing to carbon sequestration. Moreover, the sorption affinity of CO2 by biochar could be comparable to that of other proposed CO2 adsorbents such as zeolite, activated carbon, and carbon nanomaterials (Dissanayake et al. 2020b). Engineered biochar with chemical modification exhibited excellent performance for CO2 capture (Dissanayake et al. 2020b). For instance, Dissanayake et al. (2020a) found that CO2 adsorption capacity of biochar increased from 1.60 to 2.92 mol/kg after activation with KOH due to the increased surface area and porosity. Similarly, the activation of biochar with KOH + CO2 dramatically increased CO2 adsorption capacity by 204% (Igalavithana et al. 2020). The introduction of N-rich functional groups was favorable for CO2 uptake as the N-functionalities provided binding sites for CO2 (Dissanayake et al. 2020b; Leng et al. 2020). Since the amine modification could improve biochar surface chemistry and increase CO2 adsorption through Lewis acid–base interactions (Qiao et al. 2020; Yuan et al. 2020b), biochar derived from amine-rich biomass such as human solid waste, Entada Rheedii herb shell, and chicken manure may have greater potential in CO2 capture (Krounbi et al. 2020; Mallesh et al. 2020).

Fig. 5
figure 5

Biochar applications in global climate change mitigation

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The extensive utilization of N fertilizers and urine deposition are considered as the main source of anthropogenic N2O emissions in agriculture (Gu et al. 2020). Biochar addition played an important role in regulating soil N2O emissions and N cycling (Zhang et al. 2019). N2O emissions naturally occur through microbial nitrification and denitrification processes (Feng et al. 2020b). Biochar amendment could mitigate N2O emissions from agricultural soils (Dong et al. 2020; Zhang et al. 2019). The suppressing mechanisms of N2O emissions included: (1) biochar liming effect increased nosZ gene abundances, leading to the accelerated reduction of N2O to N2 (Dong et al. 2020; Ji et al. 2020b); (2) biochar increased the adsorption of NO3− and/or NH4+, decreasing the availability of inorganic nitrogen substrate for nitrification and denitrification (Liu et al. 2020d); (3) inhibited nitrification or denitrification processes by releasing toxic compounds such as PAHs into soils (Ji et al. 2020a); (4) improved soil aeration and O2 content, restraining denitrification process (Liu et al. 2020d; Xu et al. 2020b). The pyrolysis temperature was an important factor affecting N2O emissions (Ji et al. 2020a). It was demonstrated that the SSA and the number of micropores of biochar increased with increasing pyrolysis temperature, which contributed to the mitigation of N2O by more adsorption of NO3− and NH4+ through microporous electrostatic action (Ye et al. 2020). Additionally, the H:C ratio decreased with increasing pyrolysis temperature, and a lower H:C ratio was associated with a larger reduction in N2O emissions. The lower H:C ratio implied a higher proportion of fused aromatic carbon rings, which favored the redox activity and sorption properties, leading to the accelerated reduction of N2O to N2 and the increased sorption to NO3− (Dong et al. 2020).

3.5.3 Biochar for salinity and drought stress amelioration

Soil salinization has become a threat for soil degradation worldwide (Mavi et al. 2020). It is estimated that approximately 30% of arable croplands worldwide are affected by salinization (Mavi et al. 2020). The increasing salinity problem has attracted considerable attention in recent years. Soil salinization has detrimental effects on soil physicochemical and biological properties, such as soil structural deterioration, nutrients (e.g., N, K, and P) and micronutrients (e.g., Cu, Zn, Fe, and Mn) bioavailability reduction, and soil enzymatic activities’ inhibition (Sahab et al. 2020). Moreover, high levels of salinity will pose adverse effects on the abundance and activities of soil microorganisms and soil-dwelling organisms (Sharif et al. 2016). The changes induced by soil salinization will decrease soil health and quality, leading to the reduction of crops production.

Biochar application as an effective strategy for soil quality and crop productivity improvement has attracted considerable attention recently. It was found that biochar application could alleviate negative effects of salinity stress on plants and improve plant growth by the release of essential nutrients (e.g., N, P, K, Ca, and Mg) to overcome nutrient deficiency in salt-affected soils (Abd El-Mageed et al. 2020; Farooq et al. 2020). Many indirect benefits for the improvement of crop productivity after biochar application in salt-affected soils have also been reported. For example, Phuong et al. (2020) indicated that the total porosity and saturated hydraulic conductivity increased, but the availability and uptake of Na decreased after biochar amendment. The improvement in soil properties and reduction in the bioavailability of toxic salts would benefit for increasing rice yield in saline soils (Phuong et al. 2020). Similarly, Hafez et al. (2020) found that the application of vermicompost–biochar mixture caused improvement in water holding capacity, stomatal conductance, and efficiency of photosynthesis in saline sodic soil. Biochar application also increased seed germination and decreased oxidation stress through the reduction in malondialdehyde (MDA), superoxide radical (O2·−), and hydrogen peroxide (H2O2) contents (Farooq et al. 2020; Ghassemi-Golezani et al. 2020). Consequently, the indirect benefits arised from biochar amendment contributed to plant growth and productivity under salt stress.

Ammonia (NH3) volatilization is one of the major pathways of nitrogen loss from soil, especially under alkaline conditions in salt-affected soils (Yu et al. 2020c). It was found that biochar amendment substantially decreased NH3 volatilization and increased nitrogen use efficiency (Ding et al. 2020; Liu et al. 2020e; Xiao et al. 2020b). The abundant OFGs and developed pore structure of biochar played an important role in NH4+ retention due to ion-exchange capacity and surface adsorption to NH4+ (Liu et al. 2020e; Yu et al. 2020c). Drought is considered as another abiotic stress limiting crop productivity (Guo et al. 2020). Biochar alone or in conjunction with other materials (e.g., silicon nutrition and plant growth promoting rhizobacteria) could ameliorate the drought stress through increasing photosynthetic rate, chlorophyll contents, and water use efficiency (Danish et al. 2020a, b; Sattar et al. 2020; Yang et al. 2020a).

It is important to note that the controversial effects of biochar application on the remediation efficacy of salt- and drought-affected soils were found in many studies. It was found that high application rates of biochar containing high levels of Na may lead to a marked enhancement of salinity, which would pose adverse effects on plant growth (Li et al. 2021; Saifullah et al. 2018). High application rates of biochar may also increase NH3 volatilization in salt-affected soils due to the constrained adsorption of NH3/NH4+ and aggravated inhibition of nitrification (Zhu et al. 2020a). Moreover, unsuitable application of biochar in salt- and drought-affected soils may have a minimal effect on soil quality and plant growth (Cui et al. 2021). The impacts of biochar on soil quality and plant growth depend on soil types and biochar types as well as biochar application dosage in salt- and drought-affected soils.

3.5.4 Organic pollutants’ degradation

The utilization of biochar as a heterogeneous catalyst in advanced oxidation processes (AOPs) gained extensive research attention in 2020. The catalytic degradation of organic pollutants generally occurs in persulfate (PS), peroxymonosulfate (PMS), Fenton-like, and photocatalysis systems (Fig. 6).

Fig. 6
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The possible degradation mechanisms of organic pollutants mediated by biochar (Wu et al. 2020a)

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Biochar derived from sewage sludge is widely used as a catalyst in the catalytic degradation of organic pollutants (Chen et al. 2020d). The thermochemical treatment of sewage sludge provides an economical and promising approach for the waste management of sewage sludge (Chen et al. 2020d). The derived functional biochar could effectively activate PS, PMS, and H2O2 to degrade a variety of organic pollutants (Gan et al. 2020; Huang et al. 2020b; Wang et al. 2020a). The abundant OFGs (e.g., hydroxyl and carboxyl groups) in biochar could transfer electrons to PS/PMS, thereby generating SO4·− (Liu et al. 2020c). The persistent free radicals (PFRs) (e.g., semiquinone-type and oxygen-centered radicals) played an important role in activating PS/PMS to directly or indirectly generate SO4·− and HO· (He et al. 2020b). The defect structure and graphitic carbon present in biochar also participated in catalytic decomposition of PS/PMS to produce SO4·− and HO· (Liu et al. 2020c; Wang et al. 2020d). The generated radicals were effective in attacking and then degrading organic pollutants. The non-radical degradation pathway was also proposed in many biochar-based PS and PMS activation systems. It is well documented that singlet oxygen (1O2) was generated during PS and PMS activation processes, which was the dominant reactive oxygen species (ROS) for organic pollutants oxidative degradation process (Li et al. 2020f; Sun et al. 2020; Wang et al. 2020a; Xie et al. 2020). The OFGs (e.g., carbonyl and ketone groups), oxygen vacancy, and graphitic carbon contributed to the formation of 1O2 (Meng et al. 2020; Wang et al. 2020d). Furthermore, pyridinic N and graphitic N of N-doped biochar promoted the generation of 1O2 (Mian et al. 2020; Zhu et al. 2020b). Another non-radical pathway for catalytic degradation of organic pollutants was electron transfer/shuttle between PS/PMS and organic pollutants (Chen et al. 2020c; Meng et al. 2020; Sun et al. 2020). The graphite structure of biochar acted as a bridge to accelerate the electron transport from organic pollutants to PS/PMS (Wang et al. 2020d). Biochar loaded with nanoscale zero-valent iron (nZVI) or Fe oxides’ nanoparticles (NPs) could effectively enhance the catalytic degradation performance of organic pollutants (He et al. 2020b; Jiang et al. 2020). The nZVI on biochar could directly activate PS/PMS to form SO4·−. The generated Fe2+ or amorphous Fe(II) during the activation process could further activate PS/PMS to produce SO4·− (Jiang et al. 2020; Wang et al. 2020c). The Fe2+ slowly released from Fe oxides NPs loaded on biochar was regarded as an effective activator for PS/PMS (He et al. 2020b). Particularly, Fe–Mn binary oxide-supported biochar exhibited greater potential in activating PS for promoting organic pollutants degradation (Chen et al. 2020b; Hao et al. 2020; Huang et al. 2020a). The synergistic mechanisms for greater catalytic degradation of organic pollutants by Fe–Mn binary oxide-supported biochar could be attributed to the increased OFGs, defect degree, and faster electron transfer (Hao et al. 2020; Huang et al. 2020a). Cobalt-impregnated biochar- or cobalt-impregnated magnetic biochar-based catalysts have also gained great attention due to the excellent catalytic performance of cobalt (Liu et al. 2020a; Luo et al. 2020). The contribution of these reactive oxygen species (SO4·−, HO·, 1O2) and electron transfer pathway in the degradation of organic matter are different. For example, Liu et al. (2020a) found that SO4·− played a dominant role in the degradation of atrazine in PMS activation process, to a lesser extent in the case of HO·. The degradation of micropollutants such as dyes, estrogens, and sulfonamides was achieved by a non-radical pathway via electron transfer in PS activation process (Chen et al. 2020c). The different roles of free-radical and non-radical pathways involved in the degradation of organic pollutants during PS/PMS activation process are summarized in Table 2.

Table 2 The application of biochar in advanced oxidation processes (AOPs)
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Biochar produced from Fe-rich sewage sludge or modified with Fe oxide NPs also had good performance in Fenton-like systems (Gan et al. 2020; Yi et al. 2020b). The PFRs and OFGs participated in H2O2 activation to generate HO· (Liu et al. 2020b; Zhang et al. 2020e). The Fe2+ leached from biochar also facilitated the activation of H2O2 (Gan et al. 2020). The introduction of light can greatly improve the degradation efficiency of organic pollutants during the heterogeneous Fenton process. UV–Vis light irradiation could directly activate H2O2 to produce HO·. Additionally, the UV–Vis light irradiation promoted the reduction of Fe(III) and accelerated the shuttle between Fe(III) and Fe(II) (He et al. 2020a; Li et al. 2020a).

Biochar-based composites involving metal oxide NPs, such as Zn, Fe, Co, and Ti oxides’ NPs, supported on biochar matrix enhanced the degradation efficiency of contaminants by the synergistic effects of adsorption and photocatalysis (Fazal et al. 2020; Leichtweis et al. 2020; Zhai et al. 2020). Biochar loaded with Zn–Co-layered double hydroxide (Zn–Co-LDH) also had high degradation efficiency due to the high photocatalytic activity of Zn–Co-LDH (Gholami et al. 2020). Ball milling improved photocatalytic performance of biochar by increasing OFGs (Xiao et al. 2020c). Biochar-based nanocomposites enhanced the separation of photogenerated charge carriers and electron–hole pairs as well as light absorption (Fazal et al. 2020; Zhai et al. 2020). The generated ROS (holes (h+), HO·, O2·−) enhanced the degradation of organic contaminants (Leichtweis et al. 2020; Xiao et al. 2020c). It is important to note that dissolved organic matter (DOM) extracted from biochar promoted photochemical formation of ROS (e.g., 1O2 and O2·−), thus accelerating photodegradation of pharmaceutically active compounds. In addition, the degradation promotion effect of biochar-based DOM was more effective than that of natural organic matters (NOMs) (Wang et al. 2020b).

3.5.5 Heavy metal immobilization

The application of engineered biochar on the removal of heavy metals from terrestrial and aquatic environments still drew great attention in 2020. Among the heavy metals, Cd has received great concern due to its highly mobile, toxic, and bioaccumulative properties (Table 1) (Liang et al. 2020b).

Biochar engineering via chemical approach is considered to be an effective method for heavy metal removal from wastewater and soil. For instance, Zhang et al. (2020c) indicated that the pore structure of biochar significantly increased after hydrochloric acid and hydrofluoric acid treatment, which was conducive to the physical adsorption of Pb on biochar. Chemicals such as 2-thiouracil, N, and MgCl2 impregnation into biochar also significantly increased Cu and Pb sorption capacities (Liatsou et al. 2020; Zhang et al. 2020b). Specially, 2-thiouracil-modified biochar exhibited extraordinary high affinity for Cu even under low pH (2–4), indicating that the engineered biochar had high selectivity for Cu (Liatsou et al. 2020). Fe-modified biochar received most attention as magnetic biochar not only enhanced heavy metal sorption performance, but also exhibited recovery properties of the spent biochar. Yuan et al. (2020a) found that the sorption capacity of Cd on biochar modification with Fe2+/Fe3+ and NaOH up to 406.46 mg/g, which was 16-fold of original biochar. Fe–Mn binary oxide-biochar nanocomposites was regarded as a promising and high-efficient adsorbent for heavy metals, which was ascribed to the increased surface area, pore volume, oxygen-functional, and sorption active sites (Xiao et al. 2020a; Yin et al. 2020).

Currently, a novel biochar-based hydrogel has triggered extensive concerns in the field of heavy metal removal from wastewater. Hydrogel features cross-linked three-dimensional (3D) network structure (Weerasundara et al. 2020). It has high flexibility and can be functionalized with a number of active groups (Mohamed and Mahmoud 2020). Encapsulation of biochar into hydrogel could increase its stability and mechanical strength as well as adsorption capacity. The nanocomposite hydrogel beads by introducing biochar into the network enhanced Cu(II) removal (Zhang et al. 2020d). MnOx-loaded biochar-based porous hydrogel had superior sorption performance over MnOx-loaded biochar as the 3D porous structure of the hydrogel provided more active sites. More importantly, the porous hydrogel possessed excellent thermostability and reusability with high sorption capacity after five reuse cycles (Wu et al. 2020b). A novel nanosorbent prepared by encapsulating biochar with starch hydrogel could effectively remove Cr(VI), with the sorption capacity as high as 420 mg/g (Mohamed and Mahmoud 2020). The removal mechanisms of heavy metals by biochar included ion exchange, cation–π interaction, electrostatic attraction precipitation, and surface and inner-sphere complexation (Fig. 7) (Liatsou et al. 2020; Yuan et al. 2020a; Zhang et al. 2020c).

Fig. 7
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Schematic diagram showing the various mechanisms of biochar for toxic metals’ removal

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Field or laboratory studies involving the application of biochar for heavy metal immobilization have shown promising results. The application of biochar modified with metal oxide NPs (e.g., Fe and Ca oxide NPs) or functional materials (e.g., graphene oxide, nZVI and thiol) to contaminated soils reduced the fraction of soluble and exchangeable heavy metals, while increasing the fraction of less bioavailable and stable forms of heavy metals (Fan et al. 2020; Mandal et al. 2020; Wan et al. 2020; Wu et al. 2020b). Sulfur or phosphate modification was an effective way to decrease Cd bioavailability by the formation of CdS or Cd(OH)PO3·H2O/Cd(PO3)2 precipitates (Chen et al. 2020a; Zhang et al. 2020a).

3.5.6 Bioremediation

Bioremediation using microorganism immobilization technology with biochar as a carrier gained growing interest in 2020. The immobilization technology has been widely applied in the removal of heavy metals and organic pollutants through biosorption or/and biodegradation process (Harindintwali et al. 2020; Liu et al. 2020c).

Tu et al. (2020) found that biochar immobilized with a heavy metal-tolerant strain (Pseudomonas sp. NT-2) enhanced the stability of Cd and Cu in contaminated soils by transforming the exchangeable and carbonate bound Cu and Cd into residual Cu and Cd. The OFGs and minerals present in biochar as well as the abundant functional groups (e.g., carboxyl, hydroxyl, amine, and phosphate) on microbial cells were responsible for the stabilization of Cu and Cd (Tu et al. 2020). Biochar immobilized with phosphate solubilizing bacteria (PSB) enhanced Pb immobilization by forming stable insoluble crystalline compounds (e.g., Pb5(PO4)3OH, Pb5(PO4)3Cl, and Pb10(PO4)6(OH)2) on the surface of biochar (Teng et al. 2020). The biochar immobilized with Mn(II)-oxidizing bacterium (Streptomyces violarus strain SBP1) was found to be effective in the removal of Mn(II), which was attributed to the synergistic effects of adsorption by biochar and biological oxidation of Mn(II) to Mn(IV) by cells (Youngwilai et al. 2020).

Biodegradation using biochar immobilized with organic pollutants-degrading bacteria provides an eco-friendly and sustainable method to remove organic pollutants by simultaneous adsorption of biochar and biodegradation of bacteria (Fig. 8) (Liu et al. 2020c). The phenol-degrading bacteria (Pseudomonas citronellolis) entrapped in biochar–calcium-alginate beads efficiently promoted the biodegradation efficiencies of phenol. The removal efficiencies of phenol increased from below 46% to over 60% at high initial phenol concentrations of 400–1200 mg/L (Zhao et al. 2020). Similarly, the atrazine-degrading bacteria (Arthrobacter sp. ZXY-2) immobilized on biochar were able to completely remove atrazine (50 mg/L) from water within 1 h, which was 61% higher than that achieved using free bacterial cells (Yu et al. 2020d). The addition of biochar immobilized with dibutyl phthalate (DBP)-degrading bacteria (Bacillus siamensis strain T7) into soil not only accelerated the rapid and complete degradation of DBP, but also reduced DBP uptake by leafy vegetables (Feng et al. 2020a). The beads of sulfuric acid modified biochar were beneficial to the attachment of Acinetobacter indicus screened from petroleum contaminated soil and the subsequent degradation of gaseous methyl tert-butyl ether (Pongkua et al. 2020).

Fig. 8
figure 8

The removal mechanisms of organic pollutants by cells immobilized on biochar

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4 Conclusions and perspectives

This review was undertaken to offer a systematic review regarding the research efforts and developments in biochar area based on 3671 publications retrieved from WoS core collection in 2020 by means of scientometric analysis. The cooperation network of keywords in 2020 was more complex with diverse research nodes compared to 2019. According to the keywords clustering analysis, biochar applications for “global climate change mitigation”, “salinity and drought stress amelioration”, “organic pollutants degradation”, “heavy metals immobilization”, and “bioremediation” were important research hotspots in 2020. Considering that soil salinization and drought are becoming major agricultural problems, especially under climate scenario, biochar for salinity and drought stress amelioration deserves increasing interests due to its great mitigation effects. Organic pollutants degradation via AOPs represents a sustainable growing topic. Biochar-mediated radical and non-radical degradation mechanisms were summarized for AOPs. The emerging topic “Bioremediation” in 2019 was still a biochar research hotspot in 2020. Bioremediation using functional bacteria immobilized on biochar showed excellent performance in removing various contaminants by combining the advantages of highly efficient physiochemical sorption of biochar and microbial metabolisms. To summarize, the present review provides a systematic critical analysis of the research hotspots and future trends in the field biochar.

Although the numerous benefits of biochar application in the field of agriculture, environment, and energy, great concerns have raised on the harmful substances in biochar and their potential unintended toxicity to organisms. The potential environmental risks of biochar application associated with heavy metals, polycyclic aromatic hydrocarbons (PAHs), PFRs, water-soluble organic compounds, and volatile organic compounds (VOCs) have been well documented (Buss et al. 2015; Oleszczuk and Koltowski 2018; Smith et al. 2016). The contents of potential hazardous compounds in biochar depend on the types of biochar, especially HTT and feedstock used. Since biochar derived from sewage sludge is widely used for environmental remediation, an elevated content of heavy metals in biochar may lead to negative impacts on organisms (Singh et al. 2020; Udayanga et al. 2019). The increase in HTT would decrease the bioavailability of heavy metals in sewage sludge-derived biochar (Jin et al. 2016). Therefore, selecting proper functional-specific biochar is extremely important. Besides, biochar applications at excessively high rates may also lead to negative impacts on organisms (Intani et al. 2018). Quantifying optimum rate of biochar application is essential before large-scale multifunctional utilization of biochar.