The transformation of mercury (Hg) into the more toxic and bioaccumulative form methylmercury (MeHg) in soils and sediments can lead to the biomagnification of MeHg through the food chain, which poses ecological and health risks. In the last decade, biochar application, an in situ remediation technique, has been shown to be effective in mitigating the risks from Hg in soils and sediments. However, uncertainties associated with biochar use and its underlying mechanisms remain. Here, we summarize recent studies on the effects and advantages of biochar amendment related to Hg biogeochemistry and its bioavailability in soils and sediments and systematically analyze the progress made in understanding the underlying mechanisms responsible for reductions in Hg bioaccumulation. The existing literature indicates (1) that biochar application decreases the mobility of inorganic Hg in soils and sediments and (2) that biochar can reduce the bioavailability of MeHg and its accumulation in crops but has a complex effect on net MeHg production. In this review, two main mechanisms, a direct mechanism (e.g., Hg-biochar binding) and an indirect mechanism (e.g., biochar-impacted sulfur cycling and thus Hg-soil binding), that explain the reduction in Hg bioavailability by biochar amendment based on the interactions among biochar, soil and Hg under redox conditions are highlighted. Furthermore, the existing problems with the use of biochar to treat Hg-contaminated soils and sediments, such as the appropriate dose and the long-term effectiveness of biochar, are discussed. Further research involving laboratory tests and field applications is necessary to obtain a mechanistic understanding of the role of biochar in reducing Hg bioavailability in diverse soil types under varying redox conditions and to develop completely green and sustainable biochar-based functional materials for mitigating Hg-related health risks.
Mercury (Hg) is a highly toxic pollutant worldwide (Chen et al. 2018; Nascimento and Edmar 2003) that is released into the environment mainly through natural processes (such as forest fires, volcanic and geothermal activities, and re-emission in soils and seas) and human activities (such as metal mining and refining, fossil fuel combustion, garbage incineration and other industrial activities) (Collado et al. 2015; Li et al. 2009; Pirrone et al. 2010; Beckers and Rinklebe 2017; O’Connor et al. 2019). Inorganic Hg (IHg) species, such as Hg2+ and Hg0 (Hu et al. 2013), neutral Hg sulfides (Drott et al. 2007) and low molecular mass Hg thiols (Schaefer and Morel 2009; Schaefer et al. 2011; Lyu et al. 2019), can be converted into methylmercury (MeHg) primarily through a group of anaerobic microorganisms in sediments and soils (Ullrich et al. 2001; Beckers and Rinklebe 2017). Since Minamata Bay disease was first reported in the 1950s, the world has been concerned about the accumulation of Hg (especially MeHg) in food and the dietary exposure to MeHg. Research has shown that MeHg can accumulate in fish, shellfish and other aquatic products (García-Hernández et al. 2018) and has a high concentration in rice (Cui et al. 2017; Feng et al. 2008; Zhang et al. 2010), thus threatening food safety.
Mercury-contaminated soils and sediments have been recognized as ‘hotspots’ of MeHg production and the main sources of MeHg in crops (e.g., rice) (Feng et al. 2008; Frohne et al. 2012; Meng et al. 2014; Wang et al. 2018b) and aquatic organisms (e.g., deposit feeders) (Lawrence et al. 1999; Ullrich et al. 2001). The adverse effects of Hg-contaminated soils and sediments on human health have been reported in the literature (Feng et al. 2008; Zhang et al. 2010; Bank 2020; Natasha et al. 2020; Jiang et al. 2021). The consumption of Hg-contaminated food such as rice, vegetables, and meat is one pathway for human exposure to Hg, although fish consumption is considered to be the primary pathway for human MeHg exposure (Mergler et al. 2007). Therefore, a number of studies have been devoted to the development of technologies for the remediation of Hg-contaminated soils and sediments (Wang et al. 2012). Compared with ex situ remediation technologies (e.g., thermal destruction and foreign soil replacement), in situ strategies (e.g., Hg immobilization and phytoremediation) have the following advantages (Pavel and Gavrilescu 2008): (1) generally low cost (due to the avoidance of infrastructure construction and long-distance transportation, among other considerations), (2) environmental friendliness, (3) easy operation and maintenance, (4) simple equipment operation and (5) a small degree of damage to the soil structure. Biochar is recognized as a functional material for in situ remediation of Hg-contaminated sites due to its advantageous properties (e.g., easy operation, little environmental degradation and high adsorption efficiency) and has also received considerable attention for the remediation of contamination by other metals (Chen et al. 2019; He et al. 2019).
Biochar is a solid and stable high-carbon material that is produced through thermal decomposition of organic material (biomass such as wood, manure or leaves) in the absence of oxygen or under oxygen-limited conditions at relatively low temperatures (< 700 °C) (Liu et al. 2015; Xiao et al. 2018). Studies have indicated that biochar has a relatively high porosity and surface area and presents a large number of functional groups and adsorption sites on the surface (Ahmad et al. 2014; Xiao et al. 2018; Tang et al. 2019). The properties of biochar are significantly influenced by many parameters, including pyrolysis temperature, residence time, and biomass feedstocks (Liu et al. 2015; Rajapaksha et al. 2016; Li et al. 2017a; Liu et al. 2016). Owing to the above-mentioned advantages of biochar, its adsorption of Hg is believed to be the major mechanism for reducing the mobility and bioavailability of Hg (Cao et al. 2011; Inyang et al. 2016; Li et al. 2017a). For example, studies have demonstrated that biochar can reduce the IHg concentration in soil leachate (O’Connor et al. 2018) and the bioavailability of IHg in sediments (Gomez-Eyles et al. 2013; Bundschuh et al. 2015). In addition, biochar can remove MeHg from solution (Gomez-Eyles et al. 2013; Liu et al. 2017a; Wang et al. 2019b), and biochar amendment of Hg-contaminated soil can also reduce the bioavailability of MeHg to Indian mustard (Brassica juncea) (Shu et al. 2016a) and rice plants (Shu et al. 2016a; Wang et al. 2019b). These pioneering studies have provided initial evidence of a reduction in Hg risk upon using biochar and suggest that biochar has great potential for the in situ remediation of Hg-contaminated agricultural soils and environmental sediments.
However, it is worth noting that studies suggest that the effect of biochar on the mobility and bioavailability of Hg (IHg and MeHg) is complex, considering the varying characteristics of biochars derived from different materials and differences in environmental conditions and application rates. For example, biochar has been reported to increase the MeHg content in soil (Shu et al. 2016b), and biochar addition can alter the physical and chemical properties of soil/sediment (Beesley et al. 2011; Xiao et al. 2018) and thus affect the biogeochemical processes of nutrient elements in soils or sediments (Beckers et al. 2019). These changes could indirectly influence Hg mobility and bioavailability. However, the underlying mechanisms are far from clear. These knowledge gaps hinder the comprehensive understanding of biochar’s effect on Hg in environmental media, as well as the application of biochar for environmental remediation.
Here, we summarize recent investigations on the influences of biochar amendments on Hg mobility and bioavailability in soils/sediments and discuss the potential mechanisms responsible for these influences. Furthermore, potential problems arising from biochar amendments to soils/sediments are briefly discussed. Finally, future development directions for exploring biochar-impacted Hg bioavailability are proposed. This review highlights the possible mechanisms of the interactions among biochar, soil, and Hg under redox conditions and suggests future avenues for developing effective in situ Hg remediation strategies and mitigating the risk of MeHg production and exposure. These strategies will expand the practical application of biochar to the remediation of other heavy metals. We hope this review will provide a reference for researchers, teachers, students, and soil remediation practitioners for the in situ remediation of Hg-contaminated soils and sediments.
Effects of biochar addition on the mobility/bioavailability of Hg in soils and sediments
A large number of studies on biochar applications for removing Hg (IHg and MeHg) indicate that biochar has a strong sorption affinity for IHg and MeHg in water/solution (Kong et al. 2011; Dong et al. 2013; Gomez-Eyles et al. 2013; Boutsika et al. 2014; Feng et al. 2020; Lyu et al. 2019). However, the effects of biochar on the mobility and bioavailability of Hg in soils/sediments compared to those in water/solution could be complex. The relationships between the physicochemical properties of biochar (e.g., porosity, surface area, pH, surface charge, functional groups, and mineral contents) (Yuan et al. 2017) and Hg sorption in water have been well summarized in a published review paper (Li et al. 2017b). Here, we focus on the physical and chemical characteristics of biochar that could affect the mobility and bioavailability of Hg (IHg and MeHg) and influence Hg biogeochemical processes in soils/sediments, especially under redox conditions.
Biochar reduces the mobility/bioavailability of IHg
Although biochar cannot decrease the total Hg concentration in soils/sediments, it can effectively reduce the mobility and bioavailability of IHg and consequently decrease Hg accumulation and toxicity to animals and plants. In the last decade, biochar has been proven to be effective in mitigating the risks of Hg in soils and sediments due to the sorption of IHg and MeHg by biochar (Table 1). For example, using Hg isotope tracer methodology, Bussan et al. (2016) found that biochar (an amendment with 5% pinewood-derived biochar) significantly reduced the IHg methylation rate in wetland sediment by 88% without having much impact on the demethylation rate. This result suggests that biochar may decrease the bioavailability of IHg for methylating microorganisms. Bundschuh et al. (2015) found in a field experiment that Hg bioaccumulation in Hyalella azteca decreased when the sediments were mixed with two different biochars but that the efficiency of the biochar depended on the initial particle size and contact time.
Recent studies suggest that biochar could decrease the bioaccumulation of total Hg (THg) in rice grain. For example, a pot experiment showed that dissolved THg in soil pore water decreased by 34–44% throughout the rice-growing season, and consequently, the polished rice THg content decreased by 58–70% with 24–72 t/ha rice shell biochar amendment (Xing et al. 2019). Another pot experiment indicated that THg in rice grain decreased by 81.9% with 5% w/w sewage sludge biochar amendment despite a promotion of Hg methylation in this acidic soil (Zhang et al. 2019). In addition, the Hg levels in soil leachate decreased by more than 94% with rice husk-derived biochar amendment (1–5% w/w), similar to the results of activated carbon amendment (THg reduction by 99.9% with 3% w/w amendment) (O’Connor et al. 2018). These results show that biochar could be a potential green environmental sorbent for the in situ remediation of Hg-contaminated soils/sediments.
Biochar affects net MeHg production and MeHg mobility/bioavailability
Given the sorption of IHg to biochar, biochar amendments can decrease the microbial methylation of IHg, and reduce net MeHg production. For example, Bussan et al. (2016) used Hg stable isotope tracers (202Hg) to explore the effect of biochar on Hg methylation potential in sediments. The results showed that biochar addition reduced the Hg methylation rate by 88% compared with that in the control group (without biochar addition). Gilmour et al. (2018) used biochar to perform in situ mercury remediation in the Penobscot River salt marsh. The results showed that biochar could reduce the MeHg content in porewater. Wang et al. (2019b) reported that bamboo-derived biochar decreased net MeHg production by ∼ 70% at a 5% addition rate in paddy soils. However, Shu et al. (2016a, b) found that rice straw-derived biochar could significantly increase the concentration of MeHg in paddy soils. In addition, a long-term microcosm incubation study showed that two peaks in MeHg occurred during incubation, although the aqueous and soil solid concentrations of MeHg were generally lower in the amended systems than in the controls (Liu et al. 2018a).
With the development of more in-depth research, biochar studies have shifted from initial research work in the laboratory to practical applications. Recently, additional studies have confirmed that biochar can reduce the bioavailability of MeHg and its accumulation in crops. For example, Shu et al. (2016b) reported that straw biochar amendment could substantially reduce MeHg levels in rice plants (reduced by, rice grain 49–92%, straw 28–83%, root 29–61%), although biochar enhanced net MeHg production. One possible explanation for this phenomenon may be the decreased phytoavailability of soil MeHg (defined as the “MeHg immobilization effect”), as reflected by decreased extraction rates of MeHg by (NH4)2S2O3. Alternatively, an increase in rice yield could partly contribute to reducing the rice MeHg concentration (defined as the “biological dilution effect”). To verify the effect of biochar on bioaccumulation and bioavailability, Zhang et al. (2018) studied the effects of the co-application of biochar and sodium nitrate on MeHg bioavailability and found that the content of MeHg in rice grain was significantly reduced following the co-application of biochar and sodium nitrate. Moreover, Wang et al. (2019b) reported that biochar amendments (0.5% w/w) further reduced MeHg accumulation (by 82–87%) in rice grains grown in selenium-amended paddy soil. The results provide new insights into the combined effects of biochar and other materials on reducing the bioavailability of MeHg in Hg-contaminated soils.
The interaction mechanisms of Hg with biochar and soils/sediments
Relative to aqueous solutions, the various substances in the soil/sediment environment biogeochemical cycle are extremely complex; therefore, the influence of biochar on the transformation and bioavailability of Hg is complex. According to reports, two main mechanisms have been proposed, as shown in Fig. 1. One mechanism is the sorption of Hg to biochar, which directly reduces the mobility and bioavailability of Hg. The other is that biochar indirectly affects the mobility and bioavailability of Hg associated with biochar–soil interactions under different conditions.
The direct interactions between Hg and biochar are governed by the structure and surface chemistry of the biochar. The porosity and surface area are critical components of the structure of biochar, and the surface chemistry of biochar is dominated by surface functional groups and element contents (Ahmad et al. 2014; Liu et al. 2015; Tan et al. 2016; Xiao et al. 2018). The various mechanisms proposed for the interactions of biochar with Hg in aqueous solutions, including complexation, precipitation, ion exchange, electrostatic interaction (chemisorption), and physical sorption, have been well summarized in published review papers (Li et al. 2017a; Deng et al. 2020). The major mechanisms of the direct interactions between Hg and biochar in soil solution based on these published papers are summarized in Fig. 2. Here, we focus on the mechanisms of complexation and precipitation that affect the biogeochemical processes of Hg in soils/sediments because these mechanisms may be critical for reducing Hg bioavailability and for remediation applications.
The different types of functional groups on the surface of biochars are critical for the complexation of Hg with biochar, as shown in Fig. 2. For example, the sorption of Hg(II) by bagasse-derived biochar was mainly attributed to the complexation of Hg(II) with phenolic hydroxyl (COH) and carboxylic (COOH) groups and the formation of (–O)2HgII and (–COO)2HgII (Xu et al. 2016), while the interactions of Hg(II) with C=C and C=O to form Hg–π bonds could be mainly responsible for Hg(II) sorption by hickory chips or wood-derived biochar (Xu et al. 2016; Park et al. 2019). Dong et al. (2013) suggested that Hg was irreversibly sorbed via complexation with phenolic hydroxyl and carboxylic groups in low-temperature biochars (Brazilian pepper biochar, 400 and 500 ℃) and by graphite-like structures in high-temperature biochar (600 ℃). In addition, sulfur-containing functional groups in biochars have been identified by advanced techniques, such as X-ray absorption near edge structure (XANES) (Cheah et al. 2014; Liu et al. 2016; Wang et al. 2019b) and X-ray emission spectroscopy (XES) (Holden et al. 2018), and are regarded as a key factor controlling the complexation of Hg with biochar. For example, Hg extended X-ray absorption fine structure (EXAFS) revealed that Hg(II) is bound to S in biochar with a high S content and to O and Cl in biochar with a low S content, indicating that binding of Hg(II) to reduced S is usually favored over binding to other functional groups, e.g., hydroxyl and carboxyl groups (Liu et al. 2016). Moreover, sulfurized biochars produced with reduced sulfur-containing chemical reagents (e.g., calcium polysulfide, dimercapto compounds, 3-mercaptopropyltrimethoxysilane (3-MPTS) and sodium sulfide) exhibited enhanced Hg(II) removal efficiency due to the binding of Hg(II) to reduced S, such as that in polysulfur-like structures (Liu et al. 2018a), thiophenic groups (Park et al. 2019), thiols (Huang et al. 2019) and sulfides (Tan et al. 2016; Feng et al. 2020), in biochars prepared via different modification methods.
Although a previous study suggested that IHg and MeHg sorption to biochars involves different sorption mechanisms (Gomez-Eyles et al. 2013), the mechanisms of MeHg binding to biochars could be similar to the mechanisms of IHg binding to biochars. Several studies have investigated the mechanisms of complexation between biochar and MeHg, and the interaction mechanism remains unclear. Studies based on microcosm anoxic incubation experiments showed that rice straw- or bamboo-derived biochar can significantly decrease the MeHg concentration in overlying water during soil incubation (Shu et al. 2016a, b; Wang et al. 2019b). Interestingly, the fraction of extractable MeHg (% of total) obtained with (NH4)2S2O3 presented a decreasing trend with increasing biochar dose in soils with or without selenium addition (Wang et al. 2019b) and showed a negative relation with MeHg log Kd values (Fig. 3). These results suggest that the dissolved MeHg is partitioned into the biochar, which is most likely a result of adsorption and complexation of MeHg by organosulfur groups in the biochar (Wang et al. 2019b). Furthermore, Huang et al. (2019) found that the active sites (–SH) on modified biochar surfaces play an important role in Hg(II) and MeHg scavenging from aqueous solution by surface complexation with –SH.
Another important direct interaction between Hg and biochar is precipitation, whereby IHg and MeHg are immobilized in soils/sediments. One study proposed that Hg(II) could be reduced via Hg2Cl2 or Hg(OH)2 precipitation on the biochar surface (Kong et al. 2011). Furthermore, Tan et al. (2016) reported that sodium sulfide impregnation of corn straw biochar was an efficient way to enhance Hg(II) removal from aqueous solution due to the formation of HgS precipitates. A long-term (1030 days) anaerobic microcosm experiment indicated that biochar amendment could stabilize the unstable fraction of Hg (e.g., dissolvable Hg, HgO, colloidal Hg, nano Hg, etc.) in sediment as less soluble Hg-sulfide phases on the surface or within biochar particles (Liu et al. 2018a).
The physical, chemical, and biological properties of soils can be altered following biochar amendment (Joseph et al. 2015; Lian and Xing 2017; Bandara et al. 2019), and such changes can impact the biogeochemical cycling of Hg and other elements related to Hg cycling.
Biochar-driven alterations in biogeochemical redox processes that impact the mobility and bioavailability of Hg in soils/sediments have barely been investigated, apart from a few studies. The published literature focuses on biochar-induced biogeochemical processes involving the redox elements S and Fe that are associated with the mobility and bioavailability of Hg. Positive effects of biochar amendment on sulfate-reducing bacteria (SRB) activities have been reported (Eastona et al. 2015; Sande 2016). Thus, biochar-induced changes in S cycling may indirectly affect Hg mobility/bioavailability. For example, Shu et al. (2016b) found that rice straw-derived biochar contained high levels of sulfate, which elevated sulfate concentrations in soil and could subsequently enhance microbial production of MeHg under anoxic conditions. Liu et al. (2018a) found that changes in MeHg concentration in the aqueous phase could be attributed to the activity of fermenters, iron-reducing bacteria (FeRB), and SRB in the early stages of microcosm incubation and to the activity of methanogens in later stages. Community shifts induced by biochar amendment may be correlated with changes in the concentrations of carbon sources and organic acids as electron donors and electron acceptors (NO3−, Fe, and SO42−) (Liu et al. 2018b). In addition, Xing et al. (2020) detected a significant amount of thiol compounds (e.g., cysteine) in the biochar-treated paddy soil compared to the control. These thiol compounds might complex with MeHg to form MeHg–thiol complexes, thereby immobilizing MeHg in the soil. Similarly, Wang et al. (2020a) found that nanoactivated carbon as a soil amendment significantly reduced Hg uptake by rice plants and induced a change in Hg binding from organic matter to nano-HgS in the soil. XANES of S and Hg and transmission electron microscopy linked with energy-dispersive X-ray (TEM–EDX) spectroscopy revealed that Hg speciation transformation might be coupled to the reduction of sulfoxide to reduced sulfur species (S0) by nanoactivated carbon. On the other hand, the precipitation of FeS under anoxic conditions could be induced by biochar due to the redox properties of biochar (Klüpfel et al. 2014; Joseph et al. 2015; Prévoteau et al. 2016; Yuan et al. 2017). Positive effects of biochar amendment on the activities of sulfate-reducing bacteria (SRB) may enhance the production of FeS, as reported by a recent study (Wang et al. 2020c). The formed FeS can react with Hg(II) and MeHg to precipitate as metacinnabar (β-HgS(s)) and consequently immobilize Hg (Jeong et al. 2010; Jonsson et al. 2016). In summary, we propose that biochar-induced biogeochemical processes involving the redox elements S and Fe and resulting in the formation of inorganic and organic S species could play a key role in impacting the biogeochemical redox processes of Hg (Fig. 1).
Advantages of biochar amendments for the remediation of Hg-contaminated soils and sediments
Over the past 10 years (from 2011 to 2020), the number of references related to the use of biochar to remediate Hg-contaminated soils and sediments has increased by 99% (as shown in Fig. 4), and the number of citations for articles has increased from 3 to 1103. Among these articles, more studies have been published on biochar used to treat mercury-contaminated soils than mercury-contaminated sediments. Table 2 shows the published in situ remediation technologies (stabilization/solidification and immobilization) used to reduce MeHg accumulation in rice grain in pot experiment and field studies. The results show that biochar can decrease THg and MeHg in rice grain by 30–82 and 45–88% (% total content of THg and MeHg in rice grain), respectively. All the results indicate that biochar is a promising material for the in situ remediation of Hg-contaminated soils and sediments due to its advantageous properties.
Existing problems with the use of biochar to treat Hg-contaminated soils and sediments
Although biochar has potential advantages, it has not yet been applied on a large scale for the in situ remediation of Hg-contaminated soils/sediments, which may be related to the following problems.
Dosage and price
Table 2 lists the amendment dose and the reduction rate of MeHg in rice grain for the application of different biochars to Hg-contaminated soils/sediments. From the results, the reduction rate of Hg was higher when biochar was applied at a higher rate. However, most of the studies on soil/sediment remediation with biochar are based on pot experiments, and the amendment dose is high (1–5%, w/w). High cost may be a problem in practical applications with a high application rate of biochar.
The mean price of biochar was US$ 2650/t (ranging from US$ 90/t in the Philippines to US$ 8850/t in the US) (Zhang et al. 2017). According to a published report, the current break-even price for biochar is 246 US$/t, which is approximately 1/6th the price of commercially available activated carbon (1500 US$/t) (Maroušek et al. 2017). However, the economic costs of biochar cover feedstock collection, hauling, storage, and processing. The operating expenses include production, maintenance, labor costs, and transport and distribution costs (Galinato et al. 2011; Kung et al. 2013; Dickinson et al. 2015; Ahmed et al. 2016). Although the price of the raw materials (e.g., livestock manure, crop straw, and wood) is low, transportation contributes the most to the total cost (Zhang et al. 2017). Furthermore, maintenance, labor, fuel, and staff expenses may also restrict the application of biochar.
The long-term effectiveness of biochar amendments is an important theoretical basis for discussing the environmental risks of biochar. In field experiments, many factors during biochar aging can affect biochar stability, and the adsorption capacity of biochar for Hg is still unclear. The effects of the raw materials, pyrolysis conditions, and soil properties on biochar stability should be taken into account. For example, Cui et al. (2012) and Jones et al. (2012) found that during the biochar aging process, the surface of the biochar is oxidized and many oxygen-containing functional groups (such as COH and COOH groups) are formed again, thus resulting in more negative charges and a higher ion exchange efficiency on the biochar surface. Furthermore, biochar has a highly aromatic structure and a high degree of chemical and biological stability after pyrolysis, and thus, it is very resistant to degradation. However, biochar persistence in the environment does not mean that it remains unchanged in a soil or sediment environment (Schmidt and Noack 2000). When biochar enters soil or sediment, with changes in environmental conditions (temperature, wind, moisture, rainfall, and soil type) (Zama et al. 2018) and the passage of time, the composition and surface chemical properties of the biochar change greatly, which may affect the passivation stability of biochar for Hg. For example, Bundschuh et al. (2015) found that although biochar can reduce the bioavailability and bioaccumulation of total mercury in sediments, the extent of the reduction decreased over time, resulting in the re-release of Hg into soils, sediments or water body and a subsequent increase in Hg bioavailability and bioaccumulation. In addition, changes in environmental conditions (e.g., flooded or unflooded) and/or the microorganism community may promote the biochar aging process and thus decrease the adsorption capacity (Wang et al. 2018a). For example, micropores can become blocked during aging, thereby decreasing the surface area of the biochar.
Outlook of using biochar for the remediation of Hg-contaminated soils and sediments
Biochar is a promising platform for the synthesis of many other functional materials due to its easily tuned surface functionality and porosity (Liu et al. 2015; Rajapaksha et al. 2016). Therefore, the development of biochar-based functional materials and modified biochars is important (Liu et al. 2015; Ahmed et al. 2016). For example, Feng et al. (2020) conducted an experiment to improve the surface characteristics of biochar and found incorporated Fe, S, and Cl species in Fe-modified biochar, which makes the modified biochar a prospective material for Hg(II) removal (Gong et al. 2019). Lyu et al. (2019) reported that thiol-modified biochar can achieve higher removal rates of Hg(II) and MeHg (320.1 and 104.9 mg/g) than unmodified biochar. In addition to using engineered biochar alone, the multiple applications of biochar and other chemical agents (e.g., sulfur or selenate/selenite) may have great potential to improve the effectiveness of remediation. For example, Wang et al. (2019b) suggested that multiple applications of selenium and biochar could be a novel remediation strategy to mitigate MeHg accumulation in rice. Moreover, Hg bioavailability, rather than the total concentration, should be the focus of risk management regarding Hg-contaminated soils and sediments because biochar amendment cannot decrease the total concentration of Hg in soils/sediments. Thus, the development of methods for Hg bioavailability risk assessment under biochar amendment is required for quantitative risk assessment and to meet remediation objectives.
To achieve a more effective mitigation of the risks from Hg in soils and sediments, more efforts should be made to investigate the long-term effects of biochar amendments on reducing Hg bioavailability apart from the above-mentioned technological applications. The major influencing factors that should be considered include the application rate, cost reductions, large-scale commercial availability and sustainability, and possible ecological environmental risks. Most importantly, minimizing the transfer of MeHg from soils/sediments to the food chain is a primary goal of the remediation of Hg-contaminated sites. Thus, the long-term benefit of combining biochar-based mitigation strategies with several other methods, including the minimization of Hg inputs, adoption of appropriate water management and changes in land use to grow low-accumulation crops, should be investigated.
Biochar amendment could be a practical and effective solution to mitigate the risk of Hg transfer from the environment to the biosphere. This review provides an overview of the current state of the development of biochar for the in situ remediation of Hg-contaminated soils and sediments and highlights the proposed mechanisms involved in the interactions between biochar, Hg and soils/sediments. Although the positive effects of biochar on the in situ remediation of Hg-contaminated soils and sediments have been identified, some of the remaining challenges and goals associated with biochar application remain to be investigated, including smart biochar design, multiple applications of biochar with other materials, long-term effectiveness measurements, low-dose applications, and multitier risk assessments. Implementing these strategies will further improve our ability to mitigate the ecological risks from Hg in the environment and to expand the practical application of biochar to the remediation of other heavy metals.
Ahmad M, Rajapaksha AU, Lim JE, Ming Z, Bolan N, Mohan D, Vithanage M, Sang SL, Yong SO (2014) Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99(3):19–33. https://doi.org/10.1016/j.chemosphere.2013.10.071
Ahmed MB, Zhou JL, Ngo HH, Guo WS (2016) Insight into biochar properties and its cost analysis. Biomass Bioenerg 84:76–86. https://doi.org/10.1016/j.biombioe.2015.11.002
Bandara T, Franks A, Xu JM, Bolan N, Wang HL, Tang CX (2019) Chemical and biological immobilization mechanisms of potentially toxic elements in biochar-amended soils. Crit Rev Environ Sci Technol 50(9):903–978. https://doi.org/10.1080/10643389.2019.1642832
Bank MS (2020) The mercury science-policy interface: history, evolution and progress of the Minamata Convention. Sci Total Environ 722:137832. https://doi.org/10.1016/j.scitotenv.2020.137832
Beckers F, Rinklebe J (2017) Cycling of mercury in the environment: sources, fate, and human health implications—a review. Crit Rev Environ Sci Technol 47:693–794. https://doi.org/10.1080/10643389.2017.1326277
Beckers F, Awad YM, Beiyuan JZ, Abrigata J, Mothes S, Tsang DCW, Ok YS, Rinklebe J (2019) Impact of biochar on mobilization, methylation, and ethylation of mercury under dynamic redox conditions in a contaminated floodplain soil. Environ Int 127:276–290. https://doi.org/10.1016/j.envint.2019.03.040
Beesley L, Moreno-Jiménez E, Gomez-Eyles JL, Harris E, Robinson B, Sizmur T (2011) A review of biochars’ potential role in the remediation, revegetation and restoration of contaminated soils. Environ Pollut 159(12):3269–3282. https://doi.org/10.1016/j.envpol.2011.07.023
Boutsika LG, Karapanagioti HK, Manariotis ID (2014) Aqueous mercury sorption by biochar from malt spent rootlets. Water Air Soil Pollut 225:1805. https://doi.org/10.1007/s11270-013-1805-9
Bundschuh M, Zubrod JP, Seitz F, Newman MC (2015) Effects of two sorbents applied to mercury-contaminated river sediments on bioaccumulation in and detrital processing by Hyalella azteca. J Soils Sediments 15(5):1265–1274. https://doi.org/10.1007/s11368-015-1100-z
Bussan DD, Sessums RF, Cizdziel JV (2016) Activated carbon and biochar reduce mercury methylation potentials in aquatic sediments. Bull Environ Contam Toxicol 96(4):536–539. https://doi.org/10.1007/s00128-016-1734-6
Cao XD, Ma L, Liang Y, Gao B, Harris W (2011) Simultaneous immobilization of lead and atrazine in contaminated soils using dairy-manure biochar. Environ Sci Technol 45:4884–4889. https://doi.org/10.1021/es103752u
Cheah S, Malone SC, Feik CJ (2014) Speciation of sulfur in biochar produced from pyrolysis and gasification of oak and corn stover. Environ Sci Technol 48(15):8474–8480. https://doi.org/10.1021/es500073r
Chen JQ, Dong J, Chang JJ, Guo TT, Yang QC, Jia W, Shen SL (2018) Characterization of an Hg(II)-volatilizing Pseudomonas sp. strain, DC-B1, and its potential for soil remediation when combined with biochar amendment. Ecotoxicol Environ Saf 163:172–179. https://doi.org/10.1016/j.ecoenv.2018.07.071
Chen HB, Yang X, Gielen G, Mandal S, Xu S, Guo J, Shaheen SM, Rinklebe J, Che L, Wang HL (2019) Effect of biochars on the bioavailability of cadmium and di-(2-ethylhexyl) phthalate to Brassica chinensis L. in contaminated soils. Sci Total Environ 678:43–52. https://doi.org/10.1016/j.scitotenv.2019.04.417
Collado S, Rosas I, González C, Diaz M (2015) Biodegradation of p-hydroxybenzoic acid by Pseudomonas putida. Desalin Water Treat 57(32):1–11. https://doi.org/10.1080/19443994.2015.1072584
Cui LQ, Pan GX, Li LQ, Yan JL, Zhang A, Bian RJ, Chang A (2012) The reduction of wheat Cd uptake in contaminated soil via biochar amendment: a two-year field experiment. BioResources 7(4):5666–5676. https://doi.org/10.15376/biores.7.4.5666-5676
Cui WB, Liu GL, Bezerra M, Lagos DA, Li YB, Cai Y (2017) Occurrence of MeHg in rice-based infant cereals and estimation of daily dietary intake of MeHg for infants. J Agric Food Chem 65(44):9569–9578. https://doi.org/10.1021/acs.jafc.7b03236
Deng R, Huang DL, Wan J, Xue WJ, Wen XF, Liu XG, Chen S, Lei L, Zhang Q (2020) Recent advances of biochar materials for typical potentially toxic elements management in aquatic environments: a review. J Clean Prod 255:119523. https://doi.org/10.1016/j.jclepro.2019.119523
Dickinson D, Balduccio L, Buysse J, Ronsse F, Gv H, Prins W (2015) Cost-benefit analysis of using biochar to improve cereals agriculture. Glob Change Biol Bioenergy 7(4):850–864. https://doi.org/10.1111/gcbb.12180
Dong X, Ma LQ, Zhu YJ, Li Y, Gu B (2013) Mechanistic investigation of mercury sorption by Brazilian pepper biochars of different pyrolytic temperatures based on X-ray photoelectron spectroscopy and flow calorimetry. Environ Sci Technol 47(21):12156–12164. https://doi.org/10.1021/es4017816
Drott A, Lambertsson L, Bjorn E, Skyllberg U (2007) Importance of dissolved neutral mercury sulfides for methyl mercury production in contaminated sediments. Environ Sci Technol 41(7):2270–2276. https://doi.org/10.1021/es061724z
Eastona ZM, Rogers M, Davis M, Wade J, Eick M, Bock E (2015) Mitigation of sulfate reduction and nitrous oxide emission in denitrifying environments with amorphous iron oxide and biochar. Ecol Eng 82:605–613. https://doi.org/10.1016/j.ecoleng.2015.05.008
Feng X, Li P, Qiu G, Wang S, Li G, Shang L, Meng B, Jiang H, Bai W, Li Z, Fu X (2008) Human exposure to methylmercury through rice intake in mining areas, Guizhou Province, China. Environ Sci Technol 42(1):326–332. https://doi.org/10.1021/es071948x
Feng Y, Liu P, Wang YX, Liu WF, Liu YY, Finfrock YZ (2020) Mechanistic investigation of mercury removal by unmodified and Fe-modified biochars based on synchrotron-based methods. Sci Total Environ 719:137435. https://doi.org/10.1016/j.scitotenv.2020.137435
Frohne T, Rinklebe J, Langer U, Du Laing G, Mothes S, Wennrich R (2012) Biogeochemical factors affecting mercury methylation rate in two contaminated floodplain soils. Biogeosciences 9:493–507. https://doi.org/10.5194/bg-9-493-2012
Galinato SP, Yoder JK, Granatstein D (2011) The economic value of biochar in crop production and carbon sequestration. Energy Policy 39(10):6344–6350. https://doi.org/10.1016/j.enpol.2011.07.035
García-Hernández J, Ortega-Vélez MI, Contreras-Paniagua AD, Aguilera-Márquez D, Leyva-García G, Torre J (2018) Mercury concentrations in seafood and the associated risk in women with high fish consumption from coastal villages of Sonora, Mexico. Food Chem Toxicol 120:367–377. https://doi.org/10.1016/j.fct.2018.07.029
Gilmour C, Bell T, Soren A, Riedel G, Riedel G, Kopec D, Bodaly D, Ghosh U (2018) Activated carbon thin-layer placement as an in situ mercury remediation tool in a Penobscot River salt marsh. Sci Total Environ 621:839–848. https://doi.org/10.1016/j.scitotenv.2017.11.050
Gomez-Eyles JL, Yupanqui C, Beckingham B, Riedel G, Gilmour C, Ghosh U (2013) Evaluation of biochars and activated carbons for in situ remediation of sediments impacted with organics, mercury, and methylmercury. Environ Sci Technol 47(23):13721–13729. https://doi.org/10.1021/es403712q
Gong YY, Huang Y, Wang MX, Liu FF, Zhang T (2019) Application of iron-based materials for remediation of mercury in water and soil. Bull Environ Contam Toxicol 102:721–729. https://doi.org/10.1007/s00128-019-02559-4
He LZ, Zhong H, Liu GX, Dai ZM, Brookes PC, Xu JM (2019) Remediation of heavy metal contaminated soils by biochar: mechanisms, potential risks and applications in China. Envion Pollut 252:846–855. https://doi.org/10.1016/j.envpol.2019.05.151
Holden WM, Seidler GT, Cheah S (2018) Sulfur speciation in biochars by very high resolution benchtop Kα X-ray emission spectroscopy. J Phys Chem A 122(23):5153–5161. https://doi.org/10.1021/acs.jpca.8b02816
Hu HY, Lin H, Zheng W, Tomanicek Stephen J, Alexander J, Feng XB, Elias Dwayne A, Liang L, Gu BH (2013) Oxidation and methylation of dissolved elemental mercury by anaerobic bacteria. Nat Geosci 6:751–754. https://doi.org/10.1038/NGEO1894
Huang Y, Xia SY, Lyu JJ, Tang JC (2019) Highly efficient removal of aqueous Hg2+ and CH3Hg+ by selective modification of biochar with 3-mercaptopropyltrimethoxysilane. Chem Eng J 360:1646–1655. https://doi.org/10.1016/j.cej.2018.10.231
Inyang MI, Gao B, Yao Y, Xue YW, Zimmerman A, Mosa A, Pullammanappallil P, Ok YS, Cao XD (2016) A review of biochar as a low-cost adsorbent for aqueous heavy metal removal. Crit Rev Environ Sci Technol 46:406–433. https://doi.org/10.1080/10643389.2015.1096880
Jeong HY, Sun K, Hayes KF (2010) Microscopic and spectroscopic characterization of Hg (II) immobilization by mackinawite (FeS). Environ Sci Technol 44:7476–7483. https://doi.org/10.1021/es100808y
Jiang L, Zhang RH, Zhang LN, Zheng R, Zhong MS (2021) Improving the regulatory health risk assessment of mercury-contaminated sites. J Hard Mater 402:123493. https://doi.org/10.1016/j.jhazmat.2020.123493
Jones DL, Rousk J, Edwards-Jones G, DeLuca TH, Murphy DV (2012) Biochar-mediated changes in soil quality and plant growth in a three year field trial. Soil Biol Biochem 45:113–124. https://doi.org/10.1016/j.soilbio.2011.10.012
Jonsson S, Mazrui NM, Mason RP (2016) Dimethylmercury formation mediated by inorganic and organic reduced sulfur surfaces. Sci Rep 6:27958. https://doi.org/10.1038/srep27958
Joseph S, Husson O, Graber ER, Lv Z, Taherymoosavi S, Thomas T, Nielsen S, Ye J, Pan GX, Chia C, Munroe P, Allen J, Lin Y, Fan XR, Donne S (2015) The electrochemical properties of biochars and how they affect soil redox properties and processes. Agronomy 5:322–340. https://doi.org/10.3390/agronomy5030322
Klüpfel L, Keiluweit M, Kleber M, Sander M (2014) Redox properties of plant biomass-derived black carbon (biochar). Environ Sci Technol 48:5601–5611. https://doi.org/10.1021/es500906d
Kong H, He J, Gao Y, Wu H, Zhu X (2011) Cosorption of phenanthrene and mercury(II) from aqueous solution by soybean stalk-based biochar. J Agric Food Chem 59(22):12116–12123. https://doi.org/10.1021/jf202924a
Kung CC, McCarl BA, Cao XY (2013) Economics of pyrolysis-based energy production and biochar utilization: a case study in Taiwan. Energy Policy 60:317–323. https://doi.org/10.1016/j.enpol.2013.05.029
Lawrence AL, McAloon KM, Mason RP, Mayer LM (1999) Intestinal solubilization of particle-associated organic and inorganic mercury as a measure of bioavailability to benthic invertebrates. Environ Sci Technol 33(11):1871–1876. https://doi.org/10.1021/es981328j
Li P, Feng XB, Qiu GL, Shang LH, Li ZG (2009) Mercury pollution in Asia: a review of the contaminated sites. J Hard Mater 168(2):591–601. https://doi.org/10.1016/j.jhazmat.2009.03.031
Li H, Dong X, da Silva EB, de Oliveira LM, Chen Y, Ma LQ (2017a) Mechanisms of metal sorption by biochars: biochar characteristics and modifications. Chemosphere 178:466–478. https://doi.org/10.1016/j.chemosphere.2017.03.072
Li Y, Zhao J, Guo J, Liu M, Xu Q, Li H, Li Y, Zheng L, Zhang Z, Gao Y (2017b) Influence of sulfur on the accumulation of mercury in rice plant (Oryza sativa L.) growing in mercury contaminated soils. Chemosphere 182:293–300. https://doi.org/10.1016/j.chemosphere.2017.04.129
Li Y, Li H, Yu Y, Zhao J, Wang Y, Hu C, Li H, Wang G, Li Y, Gao Y (2018) Thiosulfate amendment reduces mercury accumulation in rice (Oryza sativa L.). Plant Soil 430(1–2): 413-422. https://doi.org/10.1007/s11104-018-3726-2
Lian F, Xing BS (2017) Black carbon (biochar) in water/soil environments: molecular structure, sorption, stability, and potential risk. Environ Sci Technol 51(23):13517–13532. https://doi.org/10.1021/acs.est.7b02528
Liu WJ, Jiang H, Yu HQ (2015) Development of biochar-based functional materials: toward a sustainable platform carbon material. Chem Rev 115:12251–12285. https://doi.org/10.1021/acs.chemrev.5b00195
Liu P, Ptacek CJ, Blowes DW, Landis RC (2016) Mechanisms of mercury removal by biochars produced from different feedstocks determined using X-ray absorption spectroscopy. J Hazard Mater 308:233–242. https://doi.org/10.1016/j.jhazmat.2016.01.007
Liu P, Ptacek CJ, Blowes DW, Finfrock YZ, Gordon RA (2017a) Stabilization of mercury in sediment by using biochars under reducing conditions. J Hazard Mater 325:120–128. https://doi.org/10.1016/j.jhazmat.2016.11.033
Liu WJ, Li WW, Jiang H, Yu HQ (2017b) Fates of chemical elements in biomass during its pyrolysis. Chem Rev 117(9):6367–6398. https://doi.org/10.1021/acs.chemrev.6b00647
Liu P, Ptacek CJ, Blowes DW, Gould WD (2018a) Control of mercury and MeHg in contaminated sediments using biochars: a long-term microcosm study. Appl Geochem 92:30–44. https://doi.org/10.1016/j.apgeochem.2018.02.004
Liu P, Ptacek CJ, Elena KMA, Blowes David W, Douglas GW, Zou FY, Wang Alana O, Landis Richard C (2018b) Evaluation of mercury stabilization mechanisms by sulfurized biochars determined using X-ray absorption spectroscopy. J Hazard Mater 347:114–122. https://doi.org/10.1016/j.jhazmat.2017.12.051
Liu M, Zhang Q, Cheng M, He Y, Chen L, Zhang H, Cao H, Shen H, Zhang W, Tao S, Wang X (2019a) Rice life cycle-based global mercury biotransport and human methylmercury exposure. Nat Commun 10:5164. https://doi.org/10.1038/s41467-019-13221-2
Liu P, Ptacek CJ, Blowes DW (2019b) Mercury complexation with dissolved organic matter released from thirty-six types of biochar. Bull Environ Contam Toxicol 103(1):175–180. https://doi.org/10.1007/s00128-018-2397-2
Lyu HH, Xia SY, Tang JC, Zhang YR, Gao B, Shen BX (2019) Thiol-modified biochar synthesized by a facile ball-milling method for enhanced sorption of inorganic Hg2+ and organic CH3Hg+. J Hazard Mater 384:121357. https://doi.org/10.1016/j.jhazmat.2019.121357
Maroušek J, Vochozka M, Plachý J, Žák J (2017) Glory and misery of biochar. Clean Technol Environ Policy 19(2):311–317. https://doi.org/10.1007/s10098-016-1284-y
Meng M, Li B, Shao JJ, Wang T, He B, Shi JB, Ye ZH, Jiang GB (2014) Accumulation of total mercury and MeHg in rice plants collected from different mining areas in China. Environ Pollut 184:179–186. https://doi.org/10.1016/j.envpol.2013.08.030
Mergler D, Anderson AH, Chan HM, Mahaffey RK, Murray M, Sakamoto M (2007) Methylmercury exposure and health effects in humans: a worldwide concern. Ambio 36:3–11. https://doi.org/10.1579/0044-7447(2007)36[3:MEAHEI]2.0.CO;2
Nascimento AMA, Edmar CS (2003) Operon mer: bacterial resistance to mercury and potential for bioremediation of contaminated environments. Genet Mol Res 2(1):92–101. https://doi.org/10.1007/s10765-005-8109-2
Natasha MS, Sana K, Irshad B, Jochen B, Nabeel KN, Camille D (2020) A critical review of mercury speciation, bioavailability, toxicity and detoxification in soil-plant environment: ecotoxicology and health risk assessment. Sci Total Environ 711:134749. https://doi.org/10.1016/j.scitotenv.2019.134749
O’Connor D, Peng TY, Li GH, Wang SX, Duan L, Mulder J, Cornelissen G, Cheng ZL, Yang SM, Hou DY (2018) Sulfur-modified rice husk biochar: a green method for the remediation of mercury contaminated soil. Sci Total Environ 621:819–826. https://doi.org/10.1016/j.scitotenv.2017.11.213
O’Connor D, Hou DY, Ok YS, Mulder J, Duan L, Wu QR, Wang SX, Tack FMG, Rinklebe J (2019) Mercury speciation, transformation, and transportation in soils, atmospheric flux, and implications for risk management: a critical review. Environ Int 126:747–761. https://doi.org/10.1016/j.envint.2019.03.019
Park JH, Wang JJ, Zhou B, Mikhael Joseph ER, DeLaune RD (2019) Removing mercury from aqueous solution using sulfurized biochar and associated mechanisms. Environ Pollut 244:627–635. https://doi.org/10.1016/j.envpol.2018.10.069
Pavel LV, Gavrilescu M (2008) Overview of ex situ decontamination techniques for soil cleanup. Environ Eng Manag J 7(6):815–834. https://doi.org/10.30638/eemj.2008.109
Pirrone N, Cinnirella S, Feng X, Finkelman RB, Friedli HR, Leaner J, Mason R, Mukherjee AB, Stracher GB, Streets DG, Telmer K (2010) Global mercury emissions to the atmosphere from anthropogenic and natural sources. Atmos Chem Phys 10:5951–5964. https://doi.org/10.5194/acp-10-5951-2010
Prévoteau A, Frederik R, Inés C, Boeckx P, Korneel R (2016) The electron donating capacity of biochar is dramatically underestimated. Sci Rep 6:32870. https://doi.org/10.1038/srep32870
Rajapaksha AU, Chen SS, Tsang DCW, Zhang M, Vithanage M, Mandal S, Gao B, Bolan NS, Ok YS (2016) Engineered/designer biochar for contaminant removal/immobilization from soil and water: potential and implication of biochar modification. Chemosphere 148:276–291. https://doi.org/10.1016/j.chemosphere.2016.01.043
Sande K (2016) Biochar metal sorption and effect on microbial sulfate reduction. Master of Science Dissertation, University of Minnesota, Twin Cities, MN
Schaefer JK, Morel FMM (2009) High methylation rates of mercury bound to cysteine by Geobacter sulfurreducens. Nat Geosci 2(2):123–126. https://doi.org/10.1038/NGEO412
Schaefer JK, Rocks SS, Zheng W, Liang LY, Gu BH, Morel FMM (2011) Active transport, substrate specificity, and methylation of Hg(II) in anaerobic bacteria. Proc Natl Acad Sci 108(21):8714–8719. https://doi.org/10.1073/pnas.1105781108
Schmidt MWI, Noack AG (2000) Black carbon in soils and sediments: analysis, distribution, implications, and current challenges. Glob Biogeochem Cycles 14(3):777–793. https://doi.org/10.1029/1999GB001208
Selin H, Keane SE, Wang S, Selin NE, Davis K, Bally D (2018) Linking science and policy to support the implementation of the Minamata Convention on Mercury. Ambio 47:198–215. https://doi.org/10.1007/s13280-017-1003-x
Shu R, Dang F, Zhong H (2016a) Effects of incorporating differently-treated rice straw on phytoavailability of MeHg in soil. Chemosphere 145:457–463. https://doi.org/10.1016/j.chemosphere.2015.11.037
Shu R, Wang YJ, Zhong H (2016b) Biochar amendment reduced methylmercury accumulation in rice plants. J Hazard Mater 313:1–8. https://doi.org/10.1016/j.jhazmat.2016.03.080
Tan GC, Sun WL, Xu YR, Wang HY, Xu N (2016) Sorption of mercury (II) and atrazine by biochar, modified biochars and biochar based activated carbon in aqueous solution. Bioresour Technol 211:727–735. https://doi.org/10.1016/j.biortech.2016.03.147
Tang WL, Su Y, Gao YX, Zhong H (2019) Effects of farming activities on the biogeochemistry of mercury in rice-paddy soil systems. Bull Environ Contam Toxicol 102:635–642. https://doi.org/10.1007/s00128-019-02627-9
Ullrich SM, Tanton TW, Abdrashitova SA (2001) Mercury in the aquatic environment: a review of factors affecting methylation. Crit Rev Environ Sci Technol 31(3):241–293. https://doi.org/10.1080/20016491089226
Wang JX, Feng XB, Anderson Christopher WN, Xing Y, Shang LH (2012) Remediation of mercury contaminated sites—a review. J Hazard Mater 221–222:1–18. https://doi.org/10.1016/j.jhazmat.2012.04.035
Wang X, Tam NF, Fu S, Ametkhan A, Ouyang Y, Ye Z (2014) Selenium addition alters mercury uptake, bioavailability in the rhizosphere and root anatomy of rice (Oryza sativa). Ann Bot 114(2):271–278
Wang YJ, Dang F, Evans RD, Zhong H, Zhao JT, Zhou DM (2016) Mechanistic understanding of MeHg-Se antagonism in soil-rice systems: the key role of antagonism in soil. Sci Rep 6:19477. https://doi.org/10.1038/srep19477
Wang M, Zhu Y, Cheng L, Andserson B, Zhao X, Wang D, Ding A (2018a) Review on utilization of biochar for metal-contaminated soil and sediment remediation. J Environ Sci 63:156–173. https://doi.org/10.1016/j.jes.2017.08.004
Wang Z, Sun T, Driscoll CT, Yin Y, Zhang X (2018b) Mechanism of accumulation of methylmercury in rice (Oryza sativa L.) in a mercury mining area. Environ Sci Technol 52(17):9749–9757. https://doi.org/10.1021/acs.est.8b01783
Wang AO, Ptacek CJ, Blowes DW, Gibson BD, Landis RC, Dyer JA, Ma J (2019a) Application of hardwood biochar as a reactive capping mat to stabilize mercury derived from contaminated floodplain soil and riverbank sediments. Sci Total Environ 652:549–561. https://doi.org/10.1016/j.scitotenv.2018.10.213
Wang YJ, Dang F, Zheng XM, Zhong H (2019b) Biochar amendment to further reduce methylmercury accumulation in rice grown in selenium-amended paddy soil. J Hazard Mater 365:590–596. https://doi.org/10.1016/j.jhazmat.2018.11.052
Wang JX, Shaheen Sabry M, Anderson Christopher WN, Xing Y, Liu SR, Xia JC, Feng XB, Rinklebe J (2020a) Nanoactivated carbon reduces mercury mobility and uptake by Oryza sativa L: mechanistic investigation using spectroscopic and microscopic techniques. Environ Sci Technol 54:2698–2706. https://doi.org/10.1021/acs.est.9b05685
Wang LW, Hou DY, Cao YN, Ok YS, Tack FMG, Rinklebe J, O’Connor D (2020b) Remediation of mercury contaminated soil, water, and air: a review of emerging materials and innovative technologies. Environ Int 134:105281. https://doi.org/10.1016/j.envint.2019.105281
Wang YJ, Zhang Y, Ok YS, Jiang T, Liu P, Shu R, Wang DY, Cao XD, Zhong H (2020c) Biochar-impacted sulfur cycling affects methylmercury phytoavailability in soils under different redox conditions. J Hazard Mater. https://doi.org/10.1016/j.jhazmat.2020.124397 (Available online 29 Oct 2020)
Xiao X, Chen BL, Chen ZM, Zhu LZ, Schnoor JL (2018) Insight into multiple and multilevel structures of biochars and their potential environmental applications: a critical review. Environ Sci Technol 52:5027–5047. https://doi.org/10.1021/acs.est.7b06487
Xing Y, Wang JX, Xia JC (2019) A pilot study on using biochars as sustainable amendments to inhibit rice uptake of Hg from a historically polluted soil in a Karst region of China. Ecotoxicol Environ Saf 170:18–24. https://doi.org/10.1016/j.ecoenv.2018.11.111
Xing Y, Wang JX, Shaheen SM, Feng XB, Chen Z, Zhang H, Rinklebe J (2020) Mitigation of mercury accumulation in rice using rice hull-derived biochar as soil amendment: a field investigation. J Hazard Mater 388:121747. https://doi.org/10.1016/j.jhazmat.2019.121747
Xu X, Schierz A, Xu N, Cao X (2016) Comparison of the characteristics and mechanisms of Hg(II) sorption by biochars and activated carbon. J Colloid Interface Sci 463:55–60. https://doi.org/10.1016/j.jcis.2015.10.003
Xu M, Wu J, Luo L, Yang G, Zhang XH, Peng H, Yu X, Wang L (2018) The factors affecting biochar application in restoring heavy metal-polluted soil and its potential applications. Chem Ecol 34(2):177–197. https://doi.org/10.1080/02757540.2017.1404992
Xu X, Yan M, Liang L, Lu Q, Han J, Liu L, Feng X, Guo J, Wang Y, Qiu G (2019) Impacts of selenium supplementation on soil mercury speciation, and inorganic mercury and methylmercury uptake in rice (Oryza sativa L.). Environ Pollut 249:647–654. https://doi.org/10.1016/j.envpol.2019.03.095
Yuan Y, Bolan N, Prévoteau A, Vithanage M, Biswas J, Ok Y, Wang H (2017) Applications of biochar in redox-mediated reactions. Bioresour Technol 246:271–281. https://doi.org/10.1016/j.biortech.2017.06.154
Zama EF, Reid Brian J, Arp Hans Peter H, Sun G, Yuan H, Zhu Y (2018) Advances in research on the use of biochar in soil for remediation: a review. J Soils Sediments 18:2433–2450. https://doi.org/10.1007/s11368-018-2000-9
Zhang H, Feng XB, Larssen T, Qiu G, Vogt RD (2010) In inland China, rice, rather than fish, is the major pathway for methylmercury exposure. Environ Health Perspect 118(9):1183–1188. https://doi.org/10.1289/ehp.1001915
Zhang T, Liang F, Hu W, Yang X, Xiang H, Wang G, Fei B, Liu Z (2017) Economic analysis of a hypothetical bamboo-biochar plant in Zhejiang province. China Waste Manag Res. https://doi.org/10.1177/0734242X17736945
Zhang Y, Liu Y, Lei P, Wang Y, Zhong H (2018) Biochar and nitrate reduce risk of methylmercury in soils under straw amendment. Sci Total Environ 619–620:384–390. https://doi.org/10.1016/j.scitotenv.2017.11.106
Zhang J, Wu SC, Xu ZT, Wang MY, Man YB, Christie P, Liang P, Shan SD, Wong MH (2019) The role of sewage sludge biochar in methylmercury formation and accumulation in rice. Chemosphere 218:527–533. https://doi.org/10.1016/j.chemosphere.2018.11.090
Financial support was provided to Huan Zhong by the National Natural Science Foundation of China (41673075).
Conflict of interest
No conflicts of interest exist in the submission of this manuscript, and the manuscript is approved by all authors for publication.
About this article
Cite this article
Yang, Q., Wang, Y. & Zhong, H. Remediation of mercury-contaminated soils and sediments using biochar: a critical review. Biochar 3, 23–35 (2021). https://doi.org/10.1007/s42773-021-00087-1