S─N Co-Doped MXene and Its High-Efficiency Adsorption for Methylene Blue

Organic water pollution poses a global challenge. The unique properties of transition metal carbides (MXenes) make them ideal candidate adsorbents for removing water contaminants. However, limitations in the types and quantities of surface functional groups on MXenes, coupled with their inherent instability, have restricted their application in adsorbing organic dyes such as methyl blue. This study prepared sulfur–nitrogen co-doped MXenes (S─N─MX) through thermal annealing co-doping of MXenes with thiourea. This method successfully introduced sulfur and nitrogen elements as functional groups into the MXene structure, increasing the variety and quantity of surface functional groups and significantly enhancing the number of active sites. This process effectively suppresses nanolayer stacking, enhances structural stability, and substantially increases the material's adsorption capacity. Langmuir adsorption isotherm measurements reveal a maximum adsorption capacity of 239.8 mg·g −1 at 298 K and pH = 9. The material achieves efficient adsorption of methylene blue through electrostatic interactions, Lewis acid–base interactions, and hydrogen bonding. The incorporation of sulfur and nitrogen elements provides a protective effect, ensuring the composite adsorbent maintains excellent stability during adsorption–desorption cycles. After five adsorption-desorption cycles, its adsorption capacity retained over 85%. Graphical S–N co-doped MXene is synthesized via thermal annealing with thiourea, which introduces abundant surface functional groups and enhances active sites. This adsorbent exhibits a high methylene blue adsorption capacity of 239.8 mg·g - 1 through electrostatic attraction, Lewis acid–base interaction, and hydrogen bonding. It also demonstrates excellent recyclability, retaining over 85% efficiency after five cycles.