Abstract
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.
Highlights
Biochar is recognized as a functional material for in situ remediation of Hg-contaminated sites due to its advantageous properties and has received considerable attention for the remediation of contamination by other metals (Chen et al 2019; He et al 2019)
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
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
Summary
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). Biochar can remove MeHg from solution (Gomez-Eyles et al 2013; Liu et al 2017a; Wang et al 2019b), and biochar amendment of Hgcontaminated soil can 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. 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 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
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