A review of biochar functionalized by thermal air oxidation

Biochar, the product of anaerobic pyrolysis of biomass, has attracted immense interest not only as an adsorbent in agricultural and environmental remediation applications but also as a carbonaceous redox catalyst in air/water purification research. Various chemical approaches have been developed to modify biochar; however, most of them are costly because they require additional chemicals and a series of treatment steps, such as the dewatering the so-treated biochar and post-treatment removal of oxidant products. Recently, researchers, including the author of this article, developed a convenient and inexpensive method for enhancing adsorption of organic and inorganic compounds by subjecting the biochar to a brief thermal air oxidation (AO) step. In this review, the author outlines the basic mechanisms of thermal AO and critically examines the property changes of biochar after the thermal AO treatment. This review aims to improve the understanding of biochar after it is exposed to hot air (e.g., wildfires), provide a detailed discussion of scientific evidence, and offer major directions for future research concerning thermal AO and its applications. A comprehensive review of relevant literature indicates that the important factors governing the resultant biochar after thermal AO include the heat treatment temperature (HTT) at which biochar is made and the feedstocks of biochar. Biochar made from lignin-rich feedstocks such as coconut shells and nutshells is preferable for thermal AO treatment. Thermal reactions between molecular oxygen and biochar (1) improve surface oxygen functionality more effectively for biochar made at HTTs than for high-HTT biochar, (2) increase the surface area and porosity especially for high-HTT biochar; and (3) create new adsorption sites and/or relieve steric restrictions of organic molecules to micropores, thereby enhancing the adsorptivity of biochar.