1 Introduction

Biochar is a porous carbonaceous solid material produced from thermal decomposition of biomass including crop residues, biosolids, animal wastes and manures, etc. (Lehmann and Joseph 2015). The strategies for biochar production include slow and fast pyrolysis, flash carbonization (Uchimiya et al. 2015), gasification (You et al. 2018), hydrothermal carbonization (Kambo and Dutta 2015), and torrefaction (Kung et al. 2019). The most common methods used for biochar fabrication include slow pyrolysis and hydrothermal carbonization. Biochar presents specific structure and properties with high porosity, large specific surface area, and abundant oxygen-containing functional groups and minerals (Tan et al. 2015). Such characteristics allow biochar’s rising applications in climate change mitigation (Sohi 2012), renewable energy generation (Waqas et al. 2018), soil conditioner (Atkinson et al. 2010), wastewater treatment, and soil remediation (Ahmad et al. 2014).

Currently, engineered biochar is found to have improved sustainability in agriculture, environment, and energy. Engineering methods include physical (ball milling, microwave, steam/gas), chemical [acid and alkali, oxidizing agents, nanoparticles (NPs), etc.] and biological (anaerobic digestion, microorganism immobilization, bioaccumulation of target metals in biomass) modification (Wang et al. 2017, 2019b) (Fig. 1). For example, the improvement of porosity and specific surface area of biochar supported with metallic(oxyhydr)oxide NPs would facilitate the capture of greenhouse gas (Creamer et al. 2016; Xiao et al. 2018). The mineralization rates of soil organic carbon decreased after application of mineral-supported biochar with stronger stability, thus reducing greenhouse gases emissions (Li et al. 2014; Yang et al. 2018). Engineered biochar derived via physical or/and chemical modification showed excellent performance in adsorbing, degrading or immobilizing pollutants from wastewater and soil, reducing the leachability and bioavailability of pollutants (Chen et al. 2011; Trakal et al. 2018; Yang et al. 2016). However, engineered biochar synthesized via biological modification is less studied. Previous studies demonstrated that biochar produced from biomass through anaerobic digestion and then pyrolysis exhibited high-efficiency removal of heavy metals and cationic dyes (Inyang et al. 2011). Biochar derived from Mg-enriched tomato tissues had high adsorption capacity of P, which could be used as an effective slow-release P-fertilizer in agricultural fields (Yao et al. 2013). However, little information is available about other biological modification method (i.e., biochar immobilized with microorganism) before 2018. The microorganism immobilization technology based on biochar brings advantages of highly efficient removal of pollutants, cost-effective, without secondary pollution, etc. (Bharti et al. 2019; Lou et al. 2019). It is believed that this novel modification method would broaden biochar’s application in wastewater treatment and soil remediation. Whereas, its laboratory tests and practical applications have yet been summarized before 2018 (Wu et al. 2019b).

Fig. 1
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The modification methods for engineered biochar production and their effects on physicochemical properties of biochar

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Although the various benefits of biochar application in agriculture, environment and energy are verified in the past 20 years (1998–2018) (Wu et al. 2019b), the potential unintended consequences of biochar application are less well examined (He et al. 2019; Kookana et al. 2011). Polycyclic aromatic hydrocarbons (PAHs), heavy metals and harmful hydrophilic biodegradable substances in biochar can be released into environment, despite the low bioavailability and ecotoxicity reported recently (Wang et al. 2019d, f; Zhang et al. 2018). Moreover, the adverse effects of persistent free radicals (PFRs) within biochars generated from pyrolysis process are limitations for biochars’ large-scale application (Liao et al. 2014). To maximize the application of biochar, the concerns on the unintended environmental and health consequences of biochar need to be comprehensively examined in the future.

With the development of urbanization and industrialization, increasing types of feedstock can be utilized to produce biochar. Besides the commonly used biomass (mainly agricultural and forestry wastes) for biochar production, other solid biomass with beneficial properties may be suitable to produce biochar for agriculture, environment and energy demands (Gonzalez and Perez 2019; Xue et al. 2019; Yang et al. 2019) (Fig. 1). Moreover, the demand in agriculture, environment and energy with the application of biochar will change over time. Therefore, further summarization of research development and future trends of biochar in 2019 is necessary.

Therefore, the specific objectives of this study are as follows: (1) to address the current hot topics and trends of biochar research by a visual scientometrics analysis proceeded by CiteSpace; (2) to identify the future trend and unintended consequences of biochar application. This review may have significant importance for understanding the hot topics and future directions in biochar research and applications.

2 Data acquisition and methods

2.1 Data collection and processing

Web of Science (WoS) core collection was selected as the database to collect data for this paper due to the fact that the most relevant and influential articles are recorded in WoS core collection (Lin et al. 2019a). During data collection, the search topic was “biochar” and search time was limited within 2019. The articles including ‘biochar’ in the title, abstract or keywords were considered as valid data. The synonyms were merged after data collection, such as “cadmium” and “Cd”, “heavymetal” and “heavy metal”, “nitrogen dioxide” and “NO2”.

2.2 Scientometrics analysis tools

CiteSpace (version: 5.6.R2) was applied to analyze the data with high objectivity of the results in the study. CiteSpace is a quantitative bibliometric visualization tool based on JAVA developed by Chen et al. (2014). Based on the co-author, co-word, and clusters analysis functions, CiteSpace can figure out the relationship between authors and the correlation between keywords as well as point out the emerging trends, hot topics and gaps of biochar research. In the network maps, each node represents one item (e.g., keyword or author), and the size of node indicates the frequency or count of one item. The linkage between two nodes represents the correlation between these two nodes (Chen et al. 2014).

3 Results and discussion

3.1 Characteristics of publication outputs

2843 publications in biochar area were obtained from WoS core collection in 2019. The number of publications dramatically increased compared with previous years, indicating that biochar is still the research hotspot and deserves continuous attention of researchers. There are mainly nine types of publications, including article, review, proceeding paper, book chapter, meeting abstract, correction, editorial material, news item and letter. Such a complex composition of publications also reveals the importance of the biochar area.

3.2 Subject categories co-occurrence analysis

The results of top ten subject categories in biochar area during 2019 are presented in Table S1. Among all records, “Environmental Sciences & Ecology” received the most attention, followed by “Environmental Sciences”, “Engineering”, “Agriculture”, “Energy & Fuels”, “Chemistry”, “Engineering, Environmental”, “Engineering, Chemical”, “Soil Science” and “Science & Technology-Other topics” (Table S1). Slight changes of the research trend of the subjects in biochar area were observed in 2019 compared to previous 20 years (1998–2018) (Wu et al. 2019b). The interest in the subject “Soil science” decreased, indicating that the research on soil quality with biochar amendment faded. The subject “Chemistry” acquired the highest centrality, rather than “Energy & Fuels” previously reported (Wu et al. 2019b), which implied that “Chemistry” played a more critical role in biochar research.

3.3 Countries cooperation analysis

The top 15 most productive countries in biochar area during 2019 are depicted in Table S2. During 2019, China still published the most articles in biochar area and followed by “USA”, and “South Korea”. The percentage of publications from China among total publications from the top 15 most productive countries raised from 25.56% (1998–2018) to 34.90% (2019), indicating that China played an increasing role in biochar research.

3.4 Authors co-citation analysis

The collaboration network for productive authors based on CiteSpace, and the top 15 most productive authors and their publications in biochar area in 2019 are depicted in Fig. S1 and Table S3. Scientists from China occupied eight seats among the top 15 most productive authors (Table S3), which represents a major contribution of authors from China in biochar area. In addition, there are two authors from South Korea, two from Pakistan, one from USA, one from Germany and one from Sri Lanka (Table S3). Among all these authors, Yong Sik Ok is the most productive author with 55 publications. Furthermore, Yong Sik Ok, Daniel C.W.Tsang, Guangming Zeng, Bin Gao, Vithanage Meththika and Yunguo Liu are still in the list of the top 15 most productive authors in 2019 (Table S3) compared to previous research (Wu et al. 2019b).

3.5 Research hotspots

To further determine the hot topics and future trends of biochar research in 2019, a keywords analysis was conducted (Fig. 2 and Table 1). The nodes on the network map can be classified into four research topics, including “Biochar production”, “Organic pollutants removal”, “Heavy metal immobilization”, and “Bioremediation” (Fig. 2). It is worth noting that “Bioremediation” is a new topic listed in 2019, while the publication frequency covering “Biochar and global climate change” and “Soil quality and plant growth” dramatically decreased. This trend indicated that investigators have begun to pay more considerable attention to more efficient and eco-friendly remediation methods, such as bioremediation, during 2019. The details of each research topic will be further discussed below.

Fig. 2
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The network map of biochar research in 2019

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Table 1 The top 20 keywords related to biochar in 2019
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3.5.1 Biochar production

Biochar produced from green waste (mainly algae) and leaves waste deserves great attention. Algae with high lipid and nitrogen contents and significant mass is considered as an environmentally friendly, renewable and valuable biomass for bioenergy and biochar production in recent years. Direct and catalytic thermal pyrolysis of algae, especially microalgae for biofuels (bio-oil, biodiesel, syngas, etc.) and biochar production is a major research focus in 2019 (Anto et al. 2019; Ashokkumar et al. 2019; Cruce and Quinn 2019). Algae biofuels are renewable and represent a promising alternative to fossil fuels; its production could mitigate the issue of the excessive use of fossil fuels (Anto et al. 2019). Co-products from thermal pyrolysis of algae, biochar, could remediate heavy metal and organic pollutants pollution (Zou et al. 2019). Therefore, algal biorefinery for biofuels and biochar production through thermal pyrolysis can achieve energy and environmental sustainability. Microwave-assisted thermal pyrolysis of algae is a high-profile method as it holds faster heating rate and transfer (Cao et al. 2019; Chen et al. 2019c). Additionally, a novel application of microalgae biochar-derived materials for heavy metal sensing is proposed (Placido et al. 2019a, b).

With respect to leaves waste, especially fallen leaves, they are readily available and large-scale produced annually (Liang et al. 2019c). There is no suitable treatment for large amounts of leaf waste yet, becoming a heavy burden on agro- and urban eco-environments (Liang et al. 2019c; Wang et al. 2019k). Converting leaves to biochar through thermochemical processing is a novel way for organic waste management as well as circumventing greenhouse gas release (Gonzalez and Perez 2019). The waste biomass might provide the derived biochar with beneficial characteristics for pollutants removal and soil improvement, such as more functional groups, greater CEC, and higher inorganic minerals, etc. (Hu et al. 2019). For example, the Fe oxides NPs-supported biochar derived from leaves showed efficient removal ability for Cr(VI) and As(V) (Liang et al. 2019c; Sahu et al. 2019). Pristine or modified biochar derived from leaves also acted as an excellent adsorbent for organic pollutants (Hu et al. 2019; Ma et al. 2019; Vyavahare et al. 2019). Additionally, leaves-derived biochar could serve as a promising soil conditioner to increase agricultural productivity (Konaka et al. 2019).

The interaction between biochar and minerals could change biochar’s properties, thereby affecting its sorption capacities of pollutants. Wu et al. (2019c) indicated that dynamic interactions between amended biochar and γ-Al2O3 could significantly affect the immobilization pathways of Zn (Wu et al. 2019c) (Fig. 3a). Soil minerals (montmorillonite, kaolinite, and goethite) treatment enlarged the SSA, pore size, and electron donating capacity of biochar. The adsorption capacity of sulfamethoxazole by mineral-treated biochar was pyrolysis temperature dependent (Zhao and Zhou 2019) (Fig. 3b). Similarly, Zhao et al. (2019) have also reported that nano-sized silica addition enhanced biochar’s stability, aging resistance, and adsorption properties for tetracycline (Fig. 3c). These studies provided significance for better understanding the interaction between biochar and minerals and its environmental behavior.

Fig. 3
figure 3

Interactions between biochar and minerals in changing biochar properties and adsorption capacities of heavy metal and organic pollutants. Reprinted with permission from Wu et al. (2019c), Zhao et al. (2019), and Zhao and Zhou (2019)

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Research about the unintended environmental and health consequences of biochar application is increasingly explored in 2019. The toxicities of biochar to soil biota and aquatic organisms were heavily reported through the avoidance behavior bioassay, assessing survival rates, vertical distribution, weight changes, superoxide dismutase (SOD) activity and protein content for soil biota, and assessing inhibition rates and lethality percentages for aquatic organisms (Liang et al. 2019b; Prodana et al. 2019; Zhang et al. 2019b). A key insight into gene–environment interactions was also conducted to assess the toxicities of biochar to soil microbiota by metabolomics investigation (Hill et al. 2019). The toxicities of biochar were largely dependent on raw materials, pyrolysis temperature, and particle sizes of biochar (Liu et al. 2019; Prodana et al. 2019; Zhang et al. 2019b, c). The elevated pH, PAHs and heavy metals contents, reactive oxygen species (ROS) levels upon the incorporation of biochar were the main reasons for the toxicities of biochar to soil biota and aquatic organisms (Gruss et al. 2019; Liu et al. 2019; Zhang et al. 2019d). Hilber et al. (2019) investigated the desorption resistance of PAHs (as an indirect measure of their bioaccessibility) in biochar as feed additive in cow ruminal liquid, and their results indicated that biochar containing < 10 mg/kgdw PAHs would not pose an increased unintended consequence to ruminants, which provided a novel and relative realistic method to assess these unintended health consequences of biochar (Hilber et al. 2019). Therefore, challenges still exist in the ecological safety of biochar utilization in the future (Fig. 4).

Fig. 4
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Potential toxicities of biochar. PFRs persistent free radicals, PAHs polycyclic aromatic hydrocarbons, ROS reactive oxygen species, NPs nanoparticles, nZVI nanoscale zero-valent iron

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3.5.2 Organic pollutants removal

The research on the removal of organic contaminants still draws great attention in 2019; this phenomenon can be attributed to their extensive use in agriculture, printing and dyeing industry, energy field, etc. (Joseph et al. 2019; Zhou et al. 2019). The catalytic degradation of organic pollutants by biochar is one of the major research topics and trends in 2019, and most research papers were published in the “Chemical Engineering Journal”. Among the organic pollutants, antibiotics (especially sulfonamides) received extensive attention as they are increasingly used in the breeding industry, agriculture, and human medicine (Klein et al. 2018; Tian et al. 2019). Modified biochar was effective in activating persulfate/H2O2 for organic pollutants degradation through radical (SO·−4, HO·, O·−2) oxidation (Table 2). Non-radical 1O2 oxidation of antibiotics was also proposed, which provided a new, safe and efficient oxidation pathway for organic pollutants (Yin et al. 2019; Zou et al. 2019). Besides, biochar treated with KMnO4 could also enhance the degradation of sulfamethoxazole mediated by highly oxidative intermediate manganese species, which provides a new perspective for organic pollutants removal by biochar with the chemical/advanced oxidation (Tian et al. 2019). The possible degradation mechanisms of organic pollutants mediated by biochar are summarized in Fig. 5.

Table 2 The radical mediated degradation of antibiotics by biochar
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Fig. 5
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The possible degradation mechanisms of organic pollutants mediated by biochar

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The node of “dye” is remarkable in Fig. 2, meaning that dye is another kind of organic pollutant with frequent discussion in 2019. Every year, more than 280,000 tons of synthetic dyes produced from industrial activity were discharged into water bodies globally, posing a severe threat to the environment (Bharti et al. 2019; Yudha et al. 2019). Adsorption and photodegradation are the most commonly used methods for the treatment of dye wastewater. Various modified biochar or biochar produced from macroalgae could serve as viable adsorbent for the efficient removal of dye wastewater. Modification could change the structure of biochar with increased pore volume, specific surface area, functional groups, active sites and so on, which may be involved in the electrostatic attraction, and specific interaction with dyes (Lawal et al. 2019; Sewu et al. 2019; Streit et al. 2019). Macroalgae are rich in bioactive amines, sulfates, carboxyl, and hydroxyl compounds, properties that facilitate the binding of dye molecules on the surface of macroalgae-derived biochar (Ahmed et al. 2019; Gokulan et al. 2019). Another attractive method for the removal of dyes was photodegradation with complete mineralized advantage over adsorption. Metal oxides/biochar nanocomposites performed efficient photocatalytic activity towards dyes as metal oxides nanoparticles mediated ROS [photogenerated holes (h+), hydroxyl radicals (HO·) and superoxide anion radicals (O·−2)] production. The produced ROS as oxidizing agents could decompose dyes into a variety of intermediates, further decompose into CO2 and H2O under UV light irradiation (Chen et al. 2019e; Khataee et al. 2019; Sharma et al. 2019; Zhai et al. 2019).

3.5.3 Heavy metal immobilization

Remediation of heavy metals pollution is still a hotspot due to their non-degradable and high accumulative properties, which would pose a great threat to human health. Cd and Pb have received great concern (Fig. 2) because they are the primary pollutants in agricultural soils in many countries including China and Korea (Hamid et al. 2018). An increasing research interest focused on engineered biochars, supported by various methods including physical, chemical and biological modifications in 2019. Modification of biochar is an effective way to maximize its application in water decontamination and soil remediation of heavy metals. Phosphate modification is an effective way to improve the sorption performance of biochar for Cd and Pb, which has gained increasing attention. Phosphate-modified biochar and biochar produced from feedstocks that are rich in P such as animal carcass may remarkably reduce Cd and Pb bioavailability or bioaccessibility by forming stable precipitates [e.g., Cd3(PO4)2, Cd5(PO4)3Cl; Pb5(PO4)3Cl, Pb5(PO4)3OH, Pb3(PO4)2] (Chen et al. 2019b; Deng et al. 2019c; Penido et al. 2019). Kastury et al. (2019) also found that phosphate-rich biochar (bone meal biochar) was effective in reducing the exposure risk of Pb to mice, which provides an effective remediation strategy to mitigate the exposure risk of heavy metals to animals (Kastury et al. 2019). Iron (Fe) oxides-loaded biochars are still widely used in heavy metal remediation in 2019. Fe oxides-loaded biochars showed stronger heavy metal binding capacity. More importantly, Fe oxides introduced magnetic particles in biochar, making it easier for the separation of ‘spent biochar’ from aqueous solutions and showing regenerative properties (Chen et al. 2019d; Li et al. 2019d). The research about manganese (Mn)oxides-loaded biochars for heavy metal immobilization had also received extensive attention in 2019. Mn oxides are common in soils and sediments characterized by low isoelectric point, large specific surface area, powerful oxidizing activity, abundant vacancy sites, etc. (Post 1999; Simanova et al. 2015). These properties are responsible for the higher sorption capacities for some metal(loid)s than that of Fe oxides (Sun et al. 2019c). It is verified that Mn oxides-loaded biochar exhibited high sorption efficiency for heavy metals (An et al. 2019; Sun et al. 2019a). To combine the advantages of Fe and Mn oxides, ferromanganese binary oxide–biochar composites were fabricated and had excellent adsorption capacities for heavy metals (Gao et al. 2019b; Wang et al. 2019j).

It should be noted that the keyword “sewage sludge” had high frequency in publications relating to biochar. In China, wastewater treatment plants would indirectly produce approximately 60 million tons of sewage sludge each year, and its disposal can incur substantial costs. Converting sewage sludge into biochar (i.e., sewchar) would greatly decrease the risk of heavy metals in sewage sludge as well as reduce the disposal cost of sewage sludge (Kong et al. 2019; Melo et al. 2019; Wang et al. 2019g, h). Moreover, sewage sludge-derived biochars are high-efficiency adsorbents for heavy metals, and high pyrolysis temperature favors the sorption of heavy metals (de Figueiredo et al. 2019; Gao et al. 2019a). Therefore, sewage sludge-derived biochar had been widely used for heavy metal remediation in soil and wastewater in 2019. However, the concentrations and potential risk of heavy metals leaching from sewage sludge-derived biochars are highly pyrolysis temperature dependent. The leaching potential of heavy metals in sewage sludge-derived biochars into the environment generally decreased with increasing pyrolysis temperature (de Figueiredo et al. 2019; Wang et al. 2019g, h). While the leaching efficiencies of Mn, Zn, Cu significantly increased with sewage sludge biochar derived at ≥ 700 °C, even exceeding the amounts for pristine sewage sludge, presenting unintended consequence to the environment (Udayanga et al. 2019). Sewage sludge-derived biochar had also been reported to show toxic effects on wheat growth (Kong et al. 2019). Thus, it is important to assess the unintended consequences of sewage sludge-derived biochar before wide application.

Research on metal anions (i.e., metalloids, mainly As and Cr) removal is continuously popular because their speciation is highly sensitive to environmental changes such as redox potential, pH, O2 concentration (Lin et al. 2019b; Wei et al. 2019). It is demonstrated that the removal performances of electronegative forms of As(III, V) and Cr(V) by pristine biochar was not satisfactory. Engineered biochar is, thus, designed to remove As and Cr effectively, and to reduce their toxicity in water bodies and soils. Fe oxides-supported biochar is still commonly used to remove As in 2019. Fe oxides in biochar facilitated the removal of As mainly through inner-sphere complexing or precipitation with Fe oxides, complexing with oxygen-functional groups on biochar (Cui et al. 2019; Navarathna et al. 2019). The As(III) oxidation induced by Fe oxides-supported biochar was an important reaction for removing As through adsorption of oxidized As(V) species and decreasing As toxicity. Cui et al. (2019) demonstrated that adsorption of As(III) through electrostatic attraction, oxidation of As(III) to As(V), and then immobilization of As(III) and As(V) on biomass-derived magnetic nanocomposite were the three pathways for As(III) removal (Cui et al. 2019). The solid compounds in biochar after KOH and H2O2 modification also facilitated the oxidation of As(III) (Wongrod et al. 2019). It is worth noting that the redox-active moieties on biochar also promoted the oxidation of trace As(III); the oxidation process was dependent on solution pH and O2 concentration, which provided a new insight into the oxidation of As(III) with biochar (Zhong et al. 2019).

Considering the important roles of biochar in the removal of As, Fe oxides modification methods were improved currently to enhance As removal performances of biochar with high Fe utilization, fast adsorption kinetics towards As,and low toxicity to aquatic organisms. For example, Wei et al. (2019) fabricated Fe oxide nanoneedle array-decorated biochar fibers with high magnetism and reproducibility, which could maximize Fe utilization and increase As(III, V) removal kinetics and capacity (Wei et al. 2019). Kim et al. (2019) adopted the ionic gelation method to synthesize Fe-modified biochar bead and their results indicated that Fe-modified biochar bead was less toxic to Daphnia magna than Fe-modified biochar power; Fe-modified biochar bead is, thus, a safe and efficient adsorbent for As(III) (Kim et al. 2019). More importantly, the research about modified biochar for As removal was more focused on natural groundwater/wastewater and paddy soil than before, indicating that the engineering application of modified biochar is a research trend. Increasing research focused on the reduction of Cr(VI) with biochar supported nanoscale zero-valent iron (nZVI) (Fig. 2). Preparation process is not just to introduce nZVI; other materials (e.g., calcium alginate, chitosan) were combined with nZVI or silicon-rich biochar was used for nZVI carrier to improve the sorption and reduction of Cr(VI) (Chen et al. 2019g; Qian et al. 2019; Wan et al. 2019).

3.5.4 Bioremediation

The presence of the multiple branches of “bioremediation” in the network map in 2019 indicated the major focus on a promising remediation approach: bioremediation. Bioremediation included microbial remediation and phytoremediation, which was an effective and eco-friendly treatment of a variety of organic and inorganic pollutants, and biochar addition could improve its performance in environmental remediation. Based on all the papers published during 2019, it was evident that direct application of biochar into soil to improve the phytoremediation effect and immobilizing microorganisms on biochar to enhance their pollutant removal efficiency were the main methods of biochar-assisted bioremediation.

Biochar amendment could enhance the phytoremediation effects via promoting plant growth. Zheng et al. (2019) found that rice straw biochar could assist phytoextraction of As in a paddy soil by accelerating As uptake and translocation in As hyperaccumulator (Pteris vittata L.) (Zheng et al. 2019). Manure waste and tea waste biochars were also reported to facilitate the phytoremediation of heavy metals in contaminated soils or sediments by improving plant growth and mitigating the oxidative stress. Meanwhile, biochar addition provided benefits to the microbial community and enzyme activity, which favored the resilience of soil functions in heavy metals contaminated soils or sediments (Gasco et al. 2019; Gong et al. 2019; Wang et al. 2019b). Apart from heavy metals, biochar could also improve the phytoremediation of petroleum hydrocarbon-contaminated soil via enhancing the plant growth (Zhen et al. 2019). The augment of biomass by biochar application was attributed to the increased fertility and lower toxicity to plants induced by pollutants (Gasco et al. 2019; Gong et al. 2019; Zhen et al. 2019).

Microorganisms immobilized on biochar could directly or indirectly ameliorate environmental contamination of organic and inorganic pollutants. With a wide range of raw materials, high porosity, large surface area, abundant mineral components and surface functional groups, biochar is found to be a suitable carrier for microorganisms immobilization and reproduction by providing valuable habitats and minerals for microorganisms (Lou et al. 2019). Application of bacteria immobilized on biochar into soil could indirectly enhance the removal of Cd via promoting the growth of plants, thus promoting the phytoremediation effect (Chuaphasuk and Prapagdee 2019; Wu et al. 2019a). Phosphate-solubilizing bacteria immobilized on biochar were able to directly immobilize Pb by elevated phosphate release and thereby forming stable pyromorphite precipitation (Chen et al. 2019a). Moreover, Wang et al (2019b) stated that biochar-immobilized PAH-degrading bacterial strains and Cr(VI)-reducing bacterium was effective in simultaneous bioremediation pyrene and Cr(VI). Specific bacteria immobilized on biochar performed excellent functions of adsorption and biodegradation of organic compounds such as atrazine, nonylphenol and petroleum hydrocarbon, and the removal efficiencies had been apparently improved compared with biochar or bacteria alone (Lou et al. 2019; Tao et al. 2019; Zhang et al. 2019a). Apart from heavy metals and organic compounds, the removal of nitrogen, phosphorus and bioavailable carbon as quantified by chemical oxygen demand (COD) by microorganisms immobilized on biochar also received great attention. Heterotrophic nitrifying bacteria or photosynthetic bacteria immobilized on modified biochar presented efficient removal of ammonium, phosphate and COD from wastewater due to the combination of adsorption and biodegradation, with removal efficiency of ammonium, phosphate and COD reaching above 87.5%, 92.1% and 75.5%, respectively (He et al. 2017; Yu et al. 2019).

Using microorganisms as raw materials to produce biochar for treating heavy metal pollution was another bioremediation method. Due to the short cultivation time, availability in large quantities, and abundant organic ligands and functional groups of microorganisms, dead biomass-derived biochar could be regarded to as excellent biosorbents for heavy metals (Li et al. 2019a; Wang et al. 2019e). Surface precipitation with mineral, ion exchange, surface complexation, and cation–π interactions contributed to the removal of heavy metals (Li et al. 2019a; Wang et al. 2019e).

4 Perspectives

The hot topics and trends of biochar research in 2019 are reviewed by keyword cluster analysis with bibliometricsin this study, which will contribute to future research directions. Bioremediation is one of the future research trends in environmental remediation as it is cost-effective, without secondary pollution, and environmentally friendly (Fomina and Gadd 2014). The microorganism immobilization technology based on biochar was a promising technology for treating pollutants in wastewater and soil, especially for organic pollutants degradation (Abu Talha et al. 2018; Liu et al. 2012; Lou et al. 2019). However, little information is available about the degradation of antibiotics, methylene blue dye, PAHs and other macromolecular organic pollutants by microorganism immobilized on biochar. Such organic pollutants are mainly degraded by chemical methods mediated by free radical oxidation through activating persulfate/H2O2 or photocatalysis by biochar (Chen et al. 2019e; Deng et al. 2019a; Li et al. 2019c; Liang et al. 2019a). The generation of free radicals during chemical degradation of organic pollutants would cause ecotoxicological risk. Biodegradation, thus, plays a more critical role in degradation of organic pollutants, and it is necessary to further explore novel microorganism immobilization technology based on biochar for biodegradation of organic pollutants. It should be noticed that the presence of high concentrations of water-soluble organic compounds, PAHs and PFRs within biochar is likely to cause toxicity to microorganisms (Oleszczuk and Koltowski 2018; Smith et al. 2016; Zhang et al. 2019e), restricting the growth and reproduction of microorganisms immobilized on biochar and their role in biodegradation of organic pollutants. The contents of water-soluble organic compounds, PAHs and PFRs within biochar are largely dependent on feedstock type and pyrolysis temperature (Koltowski and Oleszczuk 2015; Stefaniuk et al. 2016; Sun et al. 2019b). The particle size of biochar may also affect biochar toxicity to microorganisms (Liu et al. 2019). To decrease the toxicities to microorganisms, it is required to select appropriate biochar as carriers for microorganism immobilization. The strong alkalinity of biochar caused by abundant minerals may also reduce microbial activity (Gruss et al. 2019). This toxic effect could be reduced by washing biochar with acids and water to remove the minerals in biochar or selecting feedstocks which can result in neutral biochar.

Given the benefits and wide application of engineered biochar in sustainable environment, agriculture, and energy development fields, its potential and unintended ecotoxicological consequences should be considered before its large-scale application. Nano-metal oxide/hydroxide–biochar composites are extensively fabricated due to their multitudinous superiority in remediation of environmental pollution over pristine biochar. However, metal oxides NPs (e.g., Fe3O4, ZnO, TiO2 NPs) loaded on biochar would be released into environment and affect terrestrial and aquatic ecosystems with the application of biochar. A large number of studies have highlighted the potential toxicities of metal oxides NPs themselves to soil biota and aquatic organisms (Gebara et al. 2019; Hu et al. 2010; Kunhikrishnan et al. 2015). The leaching of metal ions from metal oxides NPs-supported biochar would also contribute to its potential unintended consequences to organisms and human health (Miller et al. 2010; Wang et al. 2013). Additionally, the potential ecotoxicity of nano zero-valent iron (nZVI) has aroused great concerns (Lee et al. 2008; Zhou et al. 2017); thus, the potential ecotoxicity of modified biochar embedded with nZVI should be noticed. It has been stated that biochar-embedded nZVI produced negative effects on the growth of Escherichia coli with application of biochar by decreasing SOD activity of E. coli (Liang et al. 2019b). Therefore, the long-term and careful evaluations of unintended consequences are indispensable before the wide-spread application of engineered biochar in agriculture, environment and energy. Meanwhile, the expenses of the modifications limit the promotion and broad application of engineered biochar, thus more considerations should be paid to the low-cost and green methods for engineered biochar production in the future.

Apart from the unintended ecotoxicological consequences derived from toxic substances associated with pristine/engineered biochar, other potential unintended consequences have been identified upon biochar application into soil. For instance, the strong affinity of biochar for soil NH4–N may decrease the availability of NH4–N to plants (Takaya et al. 2016). Although biochar is effective in the immobilization of heavy metals in contaminated soils (Ahmad et al. 2014), it is likely to reduce the bioavailability of essential trace metals (e.g., Zn, Cu) in natural agricultural soils. Wu et al. (2018) have found that biochar amendment significantly decreased the bioavailability of Zn to rice and wheat. The concentration of Zn in wheat grains was even lower than the recommended concentration of 45 mg/kg at extremely high biochar application rates (124 and 270 t/ha) (Wu et al. 2018). The potential unintended consequences of essential trace metals deficiency in agricultural crops with long-term biochar application should also raise great concerns.