Unraveling the adsorption behaviors of uranium and thorium on the hydroxylated titanium carbide MXene

Abstract

Hydroxylated titanium carbide Ti3C2(OH)2, a representative of MXenes, is employed to investigate adsorption behaviors for uranium and thorium using density functional theory based simulation methods. The structural analysis of the complexes compares very well to existing computational and experimental literatures. A closer look at the adsorption configurations and energies indicates that the main adsorption sites are deprotonated Os atoms of the OH groups terminated on the Ti3C2(OH)2 MXene surface. The most stable models for U(VI) and Th(IV) adsorbed on the Ti3C2(OH)2 MXene in the gas phase are bidentate and tridentate configurations, respectively. The adsorption ability of Th(IV) on the Ti3C2(OH)2 MXene is stronger than that of U(VI). More importantly, the aqueous solution has a remarkable effect on the coordinated environment of uranyl and Th4+ ions in the binding configurations. Uranyl and Th4+ ions adsorbed on the Ti3C2(OH)2 MXene in the aqueous environment form pentavalent coordinated structures. Extra OH– ligands from water molecules are found to interact with U and Th atoms compared with the adsorption configurations in the gas phase. Moreover, U-Oax double bonds in the uranyl break to form U-OH bonds in the adsorption model. For all the stable adsorption configurations, the coordinating interaction is the dominant factor, and Th-Os bonds present more covalent nature than U-Os bonds due to the larger charge transfers between Th and Os atoms. This work gives supplement to the experimental observations and will provide deeper insights into the physical chemistry behind the removal of uranium and thorium by using MXenes.