Ion-modulated graphene oxide and GO@MXene hydrogels: enhanced adsorption performance and stability for methylene blue removal†

Ion cross-linking is often used to prepare graphene hydrogels, but the effects of different ion introductions on the properties of graphene hydrogels need to be further explored. In this study, graphene oxide (GO) and GO@MXene non-spherical hydrogels were prepared by introducing different ions, and their adsorption effects on methylene blue (MB) were discussed to determine their feasibility in water treatment. The adsorption efficiencies were significantly variable because the introduction of different ions had different effects on the internal structure and molding process of the hydrogels. The GO and GO@MXene hydrogels crosslinked with Ca²⁺ demonstrated superior MB adsorption performance compared to those prepared with K⁺ and Al³⁺, achieving 85.2% and 85.8% MB removal efficiencies within 9 hours, respectively. Interestingly, the morphology of the hydrogel can be changed by adjusting the drop height, which in turn affects the MB adsorption. The results showed that hydrogels had faster MB removal by preparing them from a higher height (3.5 cm). The results demonstrated that hydrogels prepared from a higher droplet fall height (3.5 cm) exhibited accelerated MB removal, with a 10–20% enhancement in removal efficiency. In addition, the effects of pH and contact time on the adsorption performance of the hydrogels were investigated. The results showed that the best removal effect was achieved under neutral conditions, and the adsorption process was consistent with the pseudo-quadratic kinetic model (R² > 0.97). In addition, we found that the introduction of MXene enhanced the water stability of the hydrogels, which increased with the metal ion valence number. Furthermore, our study demonstrated that Ca²⁺-crosslinked GO and GO@MXene hydrogels exhibit excellent selective adsorption of cationic dyes, achieving MB removal efficiencies of 96.6% and 98.3% in mixed dye systems, respectively. This study not only confirms that the introduction of ions can affect the properties of hydrogels by modulating their morphology, but also provides potential candidates for MB adsorption.