Applications of biochar in redox-mediated reactions

Highlights

• A first-time comprehensive overview on the role of biochar in redox applications.

• Biochar is chemically more reduced and reactive than the original feedstock.

• Graphite, functional groups and redox-active metals contribute to redox capacity.

• Biochar impacts microbial electron shuttling and solute transformation.

• Redox reactions of biochar can be manipulated to manage contaminants and nutrients.

Abstract

Biochar is chemically more reduced and reactive than the original feedstock biomass. Graphite regions, functional groups, and redox-active metals in biochar contribute to its redox characteristics. While the functional groups such as phenolic species in biochar are the main electron donating moieties (i.e., reducers), the quinones and polycondensed aromatic functional groups are the components accepting electrons (oxidants). The redox capacity of biochar depends on feedstock properties and pyrolysis conditions. This paper aims to review and summarize the various synthesis techniques for biochars and the methods for probing their redox characteristics. We review the abiotic and microbial applications of biochars as electron donors, electron acceptors, or electron shuttles for pollutant degradation, metal(loid)s (im)mobilization, nutrient transformation, and discuss the underlying mechanisms. Furthermore, knowledge gaps that exist in the exploration and differentiation of the electron transfer mechanisms involving biochars are also identified.

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.