Sodion Induced Interfacial Polarization and π‒π Synergy Enable Irreversible Dye Adsorption in g‒C3N4 Quantum Dot–Embedded Hydrogels

Hydrogel-based composites have attracted increasing attention for dye removal owing to their tunable porous networks and excellent recyclability. In this study, two quantum dot/polyacrylamid·e (PAM) hydrogels, g‒C ₃ N ₄ quantum dot (CNQD/PAM) and TiO ₂ quantum dot (TDQD/PAM), were synthesized and systematically compared in terms of their adsorption–desorption behavior toward methylene blue (MB). CNQD/PAM exhibited a significantly higher adsorption capacity, slower desorption rate, and stronger dye retention than TDQD/PAM. Kinetic fitting using a dual-site diffusion–adsorption model revealed that CNQD/PAM possesses a greater fraction of strong and irreversible adsorption sites, while both hydrogels exhibit comparable diffusion resistance. Spectroscopic and pH-dependent analyses further demonstrated that the superior performance of CNQD/PAM originates from the synergistic effects of π‒π stacking, hydrogen bonding, and Na ⁺ induced electrostatic attraction. These findings elucidate the mechanism by which alkali-metal-modified g‒C ₃ N ₄ enhances dye binding in hydrogel composites and provide valuable insights for designing durable, high-capacity, and recyclable adsorption materials.