Mechanisms of the Removal of U(VI) from Aqueous Solution Using Biochar: A Combined Spectroscopic and Modeling...

Biochar has been touted as a promising sorbent for the removal of inorganic contaminants, such as uranium (U), from water. However, the molecular-scale mechanisms of aqueous U(VI) species adsorption to biochar remain poorly understood. In this study, two approaches, grounded in equilibrium thermodynamics, were employed to investigate the U(VI) adsorption mechanisms: (1) batch U(VI) adsorption experiments coupled to surface complexation modeling (SCM) and (2) isothermal titration calorimetry (ITC), supported by synchrotron-based X-ray absorption spectroscopy (XAS) analyses. The biochars tested have considerable proton buffering capacity and most strongly adsorb U(VI) between approximately pH 4 and 6. FT-IR and XPS studies, along with XAS analyses, show that U(VI) adsorption occurs primarily at the proton-active carboxyl (-COOH) and phenolic hydroxyl (-OH) functional groups on the biochar surface. The SCM approach is able to predict U(VI) adsorption behavior across a wide range of pH and at varying initial U(VI) and biochar concentrations, and U adsorption is strongly influenced by aqueous U(VI) speciation. Supporting ITC measurements indicate that the calculated enthalpies of protonation reactions of the studied biochar, as well as the adsorption of U(VI), are consistent with anionic oxygen ligands and are indicative of both inner- and outer-sphere complexation. Our results provide new insights into the modes of U(VI) adsorption by biochar and more generally improve our understanding of its potential to remove radionuclides from contaminated waters.