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

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.