Vertically aligned MXene@PB electrode by directional freeze-casting for simultaneous capacitive deionization of U(VI) and Cs(I) with enhanced performance

Abstract

The elimination of radioactive contaminants by the capacitive deionization strategy in aqueous solutions has received significant attention. However, existing functional electrodes prepared by the traditional dip-coating method have high tortuosity, and numerous active sites are inaccessible, resulting in poor removal efficiency. Herein, a vertically aligned MXene-Prussian blue (PB) composite (MXene@PB) electrode was prepared by directional freeze-casting, which achieves simultaneously efficient removal of uranium(VI) and cesium(I). The adsorption experiments reveal that the adsorption equilibrium of the MXene@PB electrode for uranium(VI) and cesium(I) can be reached within 5 min at pH 7 under an applied potential of 1.2 V, which is much faster than those of the non-vertically aligned electrode according to the kinetic constants. The vertically aligned MXene@PB electrode exhibited maximum removal capacities of 476.2 and 378.8 mg/g for uranium(VI) and cesium(I), respectively, significantly higher than those of the non-vertically aligned MXene@PB electrode (365.0 and 307.7 mg/g). Kinetics and isotherm analyses show that the adsorption data fit the pseudo-second-order kinetic and Langmuir isotherm models. Most importantly, the electrode shows remarkable selectivity for both uranium and cesium compared to numerous competing ions, with distribution coefficients of 4.43 × 10⁴ mL/g for uranium and 3.56 × 10⁴ mL/g for cesium. XPS studies confirmed that the oxygen-containing functional groups of MXene and amino groups of chitosan interact strongly with uranium(VI), and a portion of uranium(VI) was reduced to uranium(IV) by TiO₂ generated from the oxidation of Ti₃C₂T_x, while Cs⁺ ions were mainly trapped in the Prussian blue (PB) lattice and bound to cyano-groups. This work demonstrates the great potential of the vertically aligned MXene@PB electrode for the removal of uranium and cesium, providing a reference for eliminating multiple pollutants simultaneously through rational electrode design.