Biochar-supported nZVI (nZVI/BC) for contaminant removal from soil and water: A critical review

Highlights: • Aggregation and passivation of nZVI can be alleviated by surfactants and doping methods. • BC hinders corrosion and improves the dispersion and electron transfer of nZVI. • Properties of nZVI depend on those of the BC, feedstock and pyrogenic temperature. • BC enhances electron transfer from nZVI to the contaminants due to the presence of quinone and graphene moieties. • nZVI/BC shows strong ability to remove HMs, nitrates, and organic contaminants in soil and water. -- Abstract: The promising characteristics of nanoscale zero-valent iron (nZVI) have not been fully exploited owing to intrinsic limitations. Carbon-enriched biochar (BC) has been widely used to overcome the limitations of nZVI and improve its reaction with environmental pollutants. This work reviews the preparation of nZVI/BC nanocomposites; the effects of BC as a supporting matrix on the nZVI crystallite size, dispersion, and oxidation and electron transfer capacity; and its interaction mechanisms with contaminants. The literature review suggests that the properties and preparation conditions of BC (e.g., pore structure, functional groups, feedstock composition, and pyrogenic temperature) play important roles in the manipulation of nZVI properties. This review discusses the interactions of nZVI/BC composites with heavy metals, nitrates, and organic compounds in soil and water. Overall, BC contributes to the removal of contaminants because it can attenuate contaminants on the surface of nZVI/BC; it also enhances electron transfer from nZVI to target contaminants owing to its good electrical conductivity and improves the crystallite size and dispersion of nZVI. This review is intended to provide insights into methods of optimizing nZVI/BC synthesis and maximizing the efficiency of nZVI in environmental cleanup.