Uranium and Fluoride Removal from Aqueous Solution Using Biochar: A Critical Review for Understanding the Role of Feedstock Types, Mechanisms, and Modification Methods

Abstract

Uranium (U) and fluoride (F?) are the major global geogenic contaminants in aquifers and pose serious health issues. Biochar, a potential adsorbent, has been widely applied to remediate geogenic and anthropogenic contaminants. However, there is a lack of research progress in understanding the role of different feedstock types, modifications, adsorption mechanisms on physico-chemical properties of biochar, and factors affecting the adsorption of U and F? from aqueous solution. To fill this lacuna, the present review gives insight into the U and F? removal from aqueous solution utilizing biochar from various feedstocks. Feedstock type, pyrolysis temperature, modifications, solution pH, surface area, and surface-charge-influenced biochar adsorption capacities have been discussed in detail. Major feedstock types that facilitated U and F? adsorption were crop residues/agricultural waste, softwood, grasses, and animal manure. Low-to-medium pyrolyzing temperature yielded better biochar properties for U and F? adsorption. Effective modification techniques were mainly acidic and magnetic for U adsorption, while metal oxides, hydroxides, alkali, and magnetic modification were favourable for F? adsorption. The major mechanisms of U adsorption were an electrostatic attraction and surface complexation, while for F? adsorption, the major mechanisms were ion exchange and electrostatic attraction. Lastly, the limitations and challenges of using biochar have also been discussed. � 2022 by the authors.