ReviewMechanisms of metal sorption by biochars: Biochar characteristics and modifications
Graphical abstract
Introduction
Heavy metals are ubiquitous in the environment, adversely impacting human health (Järup, 2003). However, various anthropogenic activities including mining, smelting, fertilizer and pesticide application, and electronic manufacturing discharge have increased the amount of metal-containing wastewater into the aquatic environment, leading to water contamination with metals. To deal with metal-contaminated water, different methods have been suggested to remove metals from aqueous solution including chemical precipitation, ion exchange, electrochemical treatment, and membrane technologies (Demirbas, 2008). Among the methods, biosorption technique is the most common and cost-effective. This is because biosorbents are environmentally friendly and readily available in large quantities, and one of the most popular biosorbents is biochar.
Biochar is a carbon-rich, fine-grained, and porous material. It is usually produced by thermal decomposition of biomass under oxygen-limited conditions at temperature <900 °C (Lehmann et al., 2006). It has received increasing attention due to its ability to store large amount of carbon, increase crop yield, reduce soil emission of greenhouse gases, improve soil quality, decrease nutrient leaching, and reduce irrigation and fertilizer requirements (Lehmann, 2007, Bird et al., 2008, Kimetu et al., 2008, Nguyen et al., 2009). More importantly, due to the presence of highly-porous structure and various functional groups (e.g., carboxyl, hydroxyl, and phenolic groups), biochar shows a great affinity for heavy metals (Mohan et al., 2007, Cao et al., 2009, Park et al., 2011). Much research has explored its ability for heavy metal removal from water (Ahmad et al., 2014, Mohan et al., 2014). Biochars are produced from various feedstocks (wood bark, dairy manure, sugar beet tailing, pinewood, and rice husk) at different pyrolysis conditions (temperature, heating transfer rate, and residence time) to sorb metals from water, including arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), and lead (Pb) (Qian et al., 2015, Xie et al., 2015, Inyanga et al., 2016). For simplicity, metalloid As is grouped with metals in this review.
Based on literatures, five mechanisms governing metal sorption from water by biochar have been proposed (Ahmad et al., 2014, Mohan et al., 2014, Nartey and Zhao, 2014, Qian et al., 2015, Tan et al., 2015, Xie et al., 2015, Inyanga et al., 2016). They include: (1) electrostatic interactions between metals and biochar surface; (2) cation exchange between metals and protons or alkaline metals on biochar surface; (3) metal complexation with functional groups and π electron rich domain on the aromatic structure of biochar; (4) metal precipitation to form insoluble compounds; and (5) reduction of metal species and subsequent sorption of the reduced metal species. The sorption mechanisms and capacity vary considerably with biochar properties and target metals. Recently, researchers reviewed biochar production technologies and metal removal performance (thermodynamics, kinetics, isotherms, capacity, and mechanisms) from water using biochar (Ahmad et al., 2014, Mohan et al., 2014, Nartey and Zhao, 2014, Qian et al., 2015, Tan et al., 2015, Xie et al., 2015, Inyanga et al., 2016). However, most reviews provided sorption mechanisms for metals as a group, lacking a comparison of the main mechanisms for removal of different metals. Since different metals show different species or valence states at different solution pH conditions, the main mechanisms for their sorption are different.
Compared to activated carbon, biochar is a promising adsorbent with lower cost for metal removal from water. Metal sorption capacities of biochar are 2.4–147, 19.2–33.4, 0.3–39.1, 3.0–123 mg g−1 for Pb, Ni, Cd, and Cr, respectively (Inyanga et al., 2016). However, they are generally lower than that of activated biochar, which are 255 and 91.4 mg g−1 for Pb and Cd (Wilson et al., 2006). Therefore, biochars have been modified to enhance their metal sorption capacity by loading biochar with minerals, organic functional groups, reductants, and nanoparticles, and by activating biochar with alkali solution (Mohan et al., 2014). However, so far, there is no review on recent progress on biochar modification except Mohan et al. (2014) who briefly discussed biochar modification by incorporating nanoparticles including magnetic particles and carbon nanotubes.
In this review, we aimed to: 1) review the characteristics of biochar to better understand its efficiency in metal sorption, 2) discuss the dominant sorption mechanisms of individual metals by biochar, 3) describe biochar modification to enhance its metal removal from aqueous solutions, and 4) identify research needs and suggest directions for future research. The novelty of the paper is to compare the main mechanisms for removal of different metals by biochar and to review recent progress on biochar modification to enhance metal sorption capacity. This review provides insight into biochar materials and their capability to sorb different heavy metals, which is useful for future research and biochar field application.
Section snippets
Characteristics of biochar
Physicochemical properties of biochar significantly influence its ability to sorb metals. Prior to exploring the mechanisms governing metal removal by biochar, its properties need to be well characterized, including surface area, porosity, pH, surface charge, functional groups, and mineral contents.
Mechanisms of metal sorption by biochar
Table 2 summarizes the sorption capacity and optimum solution pH for metal sorption by biochar. The metal sorption capacity of biochar varies by 1–3 orders of magnitude, ranging from 1 to 200 mg g−1 (Table 2). The pH for maximum metal sorption varies with metals, as solution pH significantly influences both metal speciation and surface charge of biochar. Change in solution pH impacts the complexation behavior of functional groups such as carboxyl, hydroxyl, and amino. For example, the
Modification of biochar to enhance metal sorption
Though biochar has ability to sorb metals from water, its capacity is usually lower compared to other common biosorbents such as activated C. Therefore, recent studies have modified biochar to enhance its metal sorption capacity. For example, efforts have been made to increase its surface area, porosity, pHPZC, and/or functional groups. Approaches to modify biochars include loading with minerals, reductants, organic functional groups, and nano-particles and activation with alkali solution.
Future research directions
Biochar has potential for metal sorption and has received increasing attention during the past decade. However, studies are mostly at a lab scale, focusing on sorption of single metal from spiked solution. In natural waters, different heavy metals may coexist with other pollutants, thereby there is competition for sorption sites on biochar surface between metals and other ions or organic pollutants. However, by far, few studies have assessed the competitive sorption of metals by biochar. Park
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