Oxygen-vacancy-rich MoO3-x nanostructures for ultrahigh cationic dye adsorption: Kinetics, isotherms, thermodynamics, and mechanism

Oxygen vacancies (OVs) govern pollutant adsorption, but quantifying their impact remains challenging. Herein, oxygen-vacancy-rich amorphous MoO 3-x nanostructures (A-MoO 3-x ) were fabricated through a simple one-step hydrothermal route. Compared with previously reported MoO 3 -based adsorbents, A-MoO 3-x exhibits higher adsorption capacity. The Liu model yields a maximum methylene-blue (MB) uptake of 1549.8 mg/g, kinetics obeys a pseudo-second-order model, and thermodynamic analysis confirms a spontaneous, exothermic process. Moreover, A-MoO 3-x selectively removes cationic dyes within a competitive six-dye system (four cationic and two anionic dyes). XPS, EPR and FTIR reveal that cationic dyes are captured via a dual mechanism of electrostatic attraction and electron transfer. When OVs density is deliberately reduced to 76 % of its original value, the adsorption capacity decreases by 66 %, directly evidencing OVs as the dominant active sites. This work elucidates the decisive role of OVs in modulating the adsorption performance of metal oxides.