Enhancing Cr(vi) removal performance of Ti3C2Tx through structural modification by using a spray freezing method

Abstract

Structural modification is expected to be a facile way to enhance the adsorption performance of MXene. In this work, the structural modification of Ti₃C₂T_x was carried out by a spray freezing method, and two kinds of nano-structure (spherical and flaky) of Ti₃C₂T_x were prepared by adjusting the solution concentration of Ti₃C₂T_x. Then the Cr(VI) adsorption capacity and removal efficiency of the spherical and flaky Ti₃C₂T_x was investigated, respectively. It is found that flaky Ti₃C₂T_x was produced with a Ti₃C₂T_x concentration of 3 mg mL⁻¹, while spherical Ti₃C₂T_x was obtained with a concentration of 6 mg mL⁻¹. The long diameter of flaky Ti₃C₂T_x is about 8–10 μm, and the specific surface area is 17.81 m² g⁻¹. While spherical Ti₃C₂T_x had a diameter of about 1–4 μm and a specific surface area of 17.07 m² g⁻¹. The optimized structure of flaky and spherical Ti₃C₂T_x improves the maximum adsorption capacity by 97% and 33%, respectively, compared with the few-layer Ti₃C₂T_x. The maximum adsorption capacity of flaky Ti₃C₂T_x was 928 mg g⁻¹, while that of spherical Ti₃C₂T_x was 626 mg g⁻¹. The adsorption capacity of both Ti₃C₂T_x structures decreased with the increase of pH, and reached the maximum value at pH = 2; meanwhile, the adsorption capacity of both Ti₃C₂T_x structures increased with the increase of Cr(VI) concentration. The adsorption of Cr(VI) on flaky Ti₃C₂T_x was very fast, reaching equilibrium in 3 min, while spherical Ti₃C₂T_x took 5 min. The adsorption of Cr(VI) on both Ti₃C₂T_x structures belonged to the monolayers, heat-absorbing chemical adsorption, and the diffusion process of Cr(VI) was regulated by the external diffusion and internal diffusion of particles. Its adsorption mechanism was the combination of reductive adsorption and electrostatic adsorption.