Photoresponse of TiCT MXene promotes its adsorptive-reductive removal of Cr(VI) from water.

MXenes, such as TiCT, demonstrate tremendous potential as heavy metal adsorbents due to their abundant reaction sites, high hydrophilicity, controllable interlayer spacing, and inherent reduction ability. However, their structural dependent pollutant removal performances and the related mechanisms are far less studied. Therefore, the removing abilities of Cr(VI) from water on TiCT MXenes with different structures (multilayer (ML-) and delaminated (DL-) TiCT) synthesized via several etching techniques were evaluated. Focusing on the most effective ML- and DL-TiCT obtained by acid/fluoride salt etching, the impacts of structural variations on the Cr(VI) removal performances were explored. Both ML- and DL-TiCT demonstrate outstanding Cr(VI) adsorption and reduction capabilities, achieving equilibrium within 500 min with capacities of 92.7 and 205 mg/g, respectively. The differences in removal mechanisms stemed from the varying adsorption and reduction capacities of two MXenes. ML-TiCT, with lower surface area and porosity, had low adsorption capacity but superior reduction ability, efficiently converting most Cr(VI) to Cr(III) (66.8%). Conversely, DL-TiCT exhibited better removal efficiency but a lower capacity for reduction (45.7%). Notably, although the partial reduction of DL-TiCT to TiO results in its limited chemical reduction capacity, TiCT might serve as a co-catalyst for TiO, boosting the photoresponsiveness of DL-TiCT or TiO through TiCT/TiO heterojunctions, thereby facilitating photocatalysis to realize the reduction of Cr(VI). Both TiCT exhibited both excellent Cr(VI) removal capacity and detoxification capacity, demonstrating their high potential in treating heavy metal pollutants in wastewater.