ReviewApplications of biochar in redox-mediated reactions
Graphical abstract
Introduction
Biochar is produced by pyrolysis of biomass such as crop residue, manure, and solid wastes (Ok et al., 2015). Traditionally biochar is used for carbon sequestration due to its stability in soils. Biochar has been extensively studied to enhance nutrient availability in soils (Schulz and Glaser, 2012) and plant growth (Hussain et al., 2017). There is increasing interest in the potential of biochar in sorption of contaminants and their subsequent immobilization in relation to remediation of contaminated soil and groundwater (Ahmad et al., 2014, Rajapaksha et al., 2016). Biochar has high surface area and favourable pore architecture characteristics enabling high efficiency in the retention of contaminants, including metal(loid)s and organic pollutants in soils (Mohan et al., 2014, Ahmed et al., 2016, Li et al., 2017).
The mobility and bioavailability of contaminants and nutrients including heavy metal(loid)s and organic compounds in soils, sediments, and groundwater are affected by both adsorption and redox reactions in soil-biochar systems (Adriano, 2001, Beiyuan et al., 2017, Cho et al., 2017). Redox reactions influence the speciation and mobility of metal(loid)s. Metals are generally less soluble in their higher oxidation state, whereas the solubility and mobility of metalloids depend on both the oxidation state and the ionic form (Ross, 1994). Redox reactions play a key role in the transformation of toxic heavy metal(loid)s, especially arsenic (As), chromium (Cr), mercury (Hg), and selenium (Se), in soils and sediments (Gadd, 2010, Rajapaksha et al., 2013). Redox reactions also impact the bioavailability of plant nutrients, especially nitrogen, sulfur, iron, and manganese in soils. For example, reduction of nitrate resulted in the release of nitrous oxide, which is an important greenhouse gas (Saggar et al., 2015, Mandal et al., 2016). Similarly, the biodegradation of many organic contaminants (e.g., trichloroethane) is mediated through redox reactions (Graber et al., 2007). Thus, a deeper understanding of the redox reactions involving biochars will help in developing in situ bioremediation technologies that are environmentally compatible.
Redox reactions of contaminants and nutrients in soil, sediments, and groundwater are affected by the physicochemical characteristics of the medium (e.g., moisture content, pH, and temperature), solute characteristics (concentration and speciation), and biological factors (e.g., plant and microbial activity). The redox reactions can also be manipulated through the addition of organic and inorganic amendments such as composts, biosolids, and biochar (Park et al., 2011, Bolan et al., 2013, Beiyuan et al., 2017). For example, the addition of organic matter-rich soil (electron donor) enhances the reduction of metal(loid)s such as Cr and Se (Park et al., 2011, Rajapaksha et al., 2013). Biochar is chemically more reduced than the original feedstock and hence participates in redox reactions in soil and aquatic systems. The redox capacity of biochar depends on the nature of feedstock materials, pyrolysis conditions, and modification of the biochar.
Several comprehensive reviews describe the potential value of biochar in the immobilization of metal(loid)s and organic contaminants (Mohan et al., 2014, Ahmed et al., 2016, Rizwan et al., 2016). However, the impact of redox reactions of biochar on the mobility and bioavailability of environmental contaminants and nutrients has not been extensively studied yet (Cho et al., 2017). This review paper aims to: (i) identify the components of biochar contributing to its redox reactions; (ii) describe the factors affecting the redox reactions involving biochars; and (iii) provide case studies examining the influence of biochar-induced redox-mediated reactions on microbial electron shuttling, organic pollutant degradation, and inorganic contaminant (im)mobilization.
Section snippets
Synthesis of biochar
Biochar can be produced from various plants, woody biomass, crop residues, animal litter, and various solid wastes by using thermochemical processes including pyrolysis, slow pyrolysis, fast pyrolysis, torrefaction, microwave assisted pyrolysis, and hydrothermal carbonization (Van Poucke et al., 2016, Igalavithana et al., 2017). Pyrolysis is an inexpensive and robust technology, which can result in the thermochemical decomposition of organic matter into non-condensable syngases, condensable
Characteristics of biochar
Physical and chemical properties of biochar are affected by feedstock types and production conditions such as pyrolysis temperature (Supplementary Data).
Microbial electron shuttling
Van der Zee and Cervantes (2009) defined electron shuttles, also referred to as redox mediators, as organic molecules that can be reversibly oxidized and reduced, thereby conferring the capacity to serve as an electron carrier in multiple redox reactions. Similarly, some soil organic matters are redox-active and can act as terminal electron acceptors in anaerobic microbial respiration (Beiyuan et al., 2017). Their ability to re-oxidize during aeration of temporarily anoxic systems, such as
Future research directions
Two important issues need to be addressed when applying biochar-based redox reactions to manage contamination and control the bioavailability of nutrients. First, the implementation of bioremediation methods should be done with caution, because many sites contain multiple contaminants, including metal(loid)s and organic compounds, and organisms that affect the output of bioremediation approaches. Therefore, the remediation of contaminated sites usually requires a combination of many different
Conclusions
In this review we discussed the redox characteristics of biochars and the corresponding biogeochemical impacts with their surrounding environment. Biochar can be used as an environmentally-sustainable electron donor, acceptor, or mediator. It can enhance the reduction of oxidized contaminants and participate in elemental cycling in terrestrial, groundwater, or waste water ecosystems. We illustrated that it is possible to tailor the redox characteristics of the biochar by selecting specific
Acknowledgements
The authors would like to acknowledge Prof M. B. Kirkam at Kansas State University for thorough proof reading of this paper. Tim Lacoere from Ghent University is warmly thanked for the artwork in the graphical abstract and Fig. 1. Y.Y. is supported by the National Natural Science Foundation of China (No. 51678162) and Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2014A030306033); A.P. is supported by the European Research Council via the Starter Grant ELECTROTALK; A.P.
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