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

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.