Redox-mediated biochar-contaminant interactions in soil

Biochars contain a large reserve of various carbon fractions which can act as a strong reducing agent, thereby impacting contaminant interactions in soil. Toxic metal(loid)s, including arsenic (As), chromium (Cr), manganese (Mn), mercury (Hg), and selenium (Se), are commonly subject to redox reactions in natural environments, thereby influencing their speciation and mobility. Similarly, nutrient elements such as nitrogen (N), sulfur (S), and iron (Fe) undergo redox reactions in soil that control their plant availability and losses through gaseous emission and leaching. The mobility and bioavailability of many of the organic contaminants, such as trichloroethane (TCE), are dictated by their redox reactions. In the chapter, the effect of biochar on the redox-mediated reactions of heavy metal(loid)s (e.g., As and Cr), nutrient element (e.g., N) and organic contaminant (e.g., TCE) in relation to their mobilization and bioavailability will be discussed. Biochar application is found to be beneficial in reducing Cr toxicity by converting Cr(VI) to Cr(III) and subsequent immobilization of Cr species. In contrast, the biochar-induced reduction of As(V) to As(III) species increases the toxicity of As, as As(III) is more toxic in soils. Biochar addition induces the reduction of nitrate nitrogen, thereby increasing nitrous oxide emission. Biochar addition enhances the reduction of TCE, thereby decreasing its bioavailability and toxicity. The presence of functional groups on biochar provide electrons, thereby promoting the redox reactions of contaminants.