Efficient U(VI) Reduction and Sequestration by Ti₂CT_<i>x</i> MXene

Abstract

Although reduction of highly mobile U(VI) to less soluble U(IV) has been long considered an effective approach to in situ environmental remediation of uranium, candidate reducing agents are largely limited to Fe-based materials and microbials. The importance of titanium-containing compounds in natural uranium ore deposits suggests a role for titanium in uranium migration. Herein, for the first time, a two-dimensional transition metal carbide, Ti2CTx, is shown to efficiently remove uranium via a sorption-reduction strategy. Batch experiments demonstrate that TiC2Tx exhibits excellent U(VI) removal over a wide pH range, with an uptake capacity of 470 mg g–1 at pH 3.0. The mechanism for U(VI) to U(IV) reduction by Ti2CTx was deciphered by X-ray absorption spectroscopy and diffraction and photoelectron spectroscopy. The reduced U(IV) species at low pH is identified as mononuclear with bidendate binding to the MXene substrate. At near-neutral pH, nanoparticles of the UO2+x phase adsorb to the substrate with some Ti2CTx transformed to amorphous TiO2. A subsequent in-depth study suggests Ti2CTx materials may be potential candidates for permeable reactive barriers in the treatment of wastewaters from uranium mining. This work highlights reduction-induced immobilization of U(VI) by Ti2CTx MXene including a pH-dependent reduction mechanism that might promote applications of titanium-based materials in the elimination of other oxidized contaminants.