Biochar-Facilitated Soil Remediation: Mechanisms and Efficacy Variations

Abstract

Recent research suggests that biochar amendment is a promising approach to mitigate soil contamination via immobilizing heavy metals and organic pollutants. Through intensive literature review, this paper was aimed to better understand the processes, mechanisms, and effectiveness of biochar in immobilizing chemical contaminants in soil. The quality characteristics of biochar as a soil amendment varied greatly with the feedstock materials and the pyrolysis conditions. Biochar products from different sources demonstrated remarkably diversified capacities and efficiencies for stabilizing soil contaminants. Soil-incorporated biochar was able to stabilize Cd, Cu, Ni, Pb, and Zn and reduce their bioavailability through enhanced sorption (based on electrostatic attraction, ion exchange, and surface complexation) and chemical precipitation (incurred from soil pH elevation and ash addition of carbonates and phosphates). The stabilization efficacy was largely determined by cation exchange capacity, pH, and ash content of the biochar. Biochar amendment increased the mobility of anionic toxic elements [e.g., CrO 2− 4 , AsO − 3 , and Sb(OH) − 6 ] in soil. Soil-incorporated biochar was also able to adsorb non-polar organic compounds (through pore filling, partition, and hydrophobic effect) and polar organic compounds (via H-bonding, electrostatic attraction, specific interaction, and surface precipitation). The adsorption efficiency was controlled by the biochar surface properties specific surface area, microporosity, and hydrophobicity. Biochar may facilitate the mineralization of organic pollutants by enhancing soil microbial activities. The effectiveness of biochar-facilitated soil remediation was case specific, changing with the biochar source, amendment rate, placement, soil type, and pollutant species. More field studies are needed to evaluate the long-term effectiveness of biochar-facilitated soil remediation under practical circumstances.

Biochar remediation of soil: linking biochar production with function in heavy metal contaminated soils

“…The mechanisms involved in controlling the removal of HMs from contaminated soils by direct adsorption of biochar includes physical sorption, ion exchange, electrostatic interaction, precipitation, and complexation (Inyang et al 2016). The surface of biochar possesses various functional groups, including hydroxyls, carbonyls, and carboxyls (Tan et al 2015), and their abundance, as indicated by CEC, is the most important factor for regulating the sorption based HM stabilisation (Guo et al 2020). Biochar exhibits its electrical charges depending on the pH and dissociation (protonation) of these functional groups.…”

“…The introduction of additional alkalinity followed by an elevation of soil pH by biochar provides another key mechanism for the precipitation of soil HMs. The hydroxyls react with the HM cations and precipitate as metal hydroxides (Guo et al 2020). Additionally, the mineral components serving as supplementary adsorption sites, provide biochar with another property for controlling HM adsorption processes in the soil (Tan et al 2015).…”

“…The biochar properties controlling these mechanisms predominantly include ash and mineral content, aromaticity, surface structure, functional groups, and pH (Wang et al 2018b). Moreover, the soil type, biochar amendment rate and its placement in the soil controls the overall efficacy of biochar for HM remediation (O'Connor et al 2018b, Guo et al 2020). Biochar's preparation condition also influence its physicochemical properties, which indirectly also control its HM immobilisation effects (Wang et al 2021).…”

Sustainable Use of Biochar in Environmental Management

“…The greatest concern of organic contaminants such as pesticides, herbicides, polycyclic aromatic hydrocarbons, dyes, and antibiotics have been a concern due to its toxicity and accumulative properties [38]. In the soil medium, biochar has been used for heavy metal sequestration [39,40]. In this process, heavy metals are immobilised not removed and maybe converted into hydroxide, carbonate, and phosphate precipitates [40].…”

“…In the soil medium, biochar has been used for heavy metal sequestration [39,40]. In this process, heavy metals are immobilised not removed and maybe converted into hydroxide, carbonate, and phosphate precipitates [40]. Sequestration of pesticides [15] and carbon sequestration (climate change mitigation measure) have also been achieved in soils amended with biochar [15].…”