Gum tragacanth/graphene quantum dots porous nanocomposite hydrogel microspheres for high adsorption of crystal violet

Porous GT-GQDs hydrogel microspheres via one-step green ionic crosslinking. • GQDs incorporation enriches anionic sites and opens the network texture. • High crystal violet adsorption capacity of 2030.91 mg/g at 25 °C. Porous spherical nanocomposite hydrogel (GT-GQDs) microspheres composed of gum tragacanth (GT) and graphene quantum dots (GQDs) were fabricated for the efficient adsorption of crystal violet (CV) from aqueous solution. Unlike chemically cross-linked hydrogel adsorbents that require hazardous reagents and post-synthesis fragmentation, these microspheres form in a single green ionic-crosslinking step, in which the anionic polysaccharide network of GT is synergistically combined with the abundant functional groups and opened texture contributed by GQDs. GQDs were prepared via controlled carbonization of citric acid, and the microspheres were obtained by rapid Ca 2+ crosslinking of GT and GQDs, yielding an interconnected porous network that was thoroughly characterized by FTIR, XRD, TGA, BET, XPS, SEM, and HRTEM. Systematic batch experiments were conducted to evaluate the effects of adsorbent dosage, initial CV concentration, solution pH, and contact time. The optimized GT-GQDs-4 microspheres delivered a high maximum adsorption capacity of 2030.91 mg/g (25 °C, an initial CV concentration of 1200 mg/L, 20 mg of adsorbent in 50 mL of solution, near-neutral pH), more than twice that of the GQDs-free control (956.96 mg/g), demonstrating the remarkable contribution of GQDs doping to the adsorption performance. Kinetic analysis followed the pseudo-first-order model, consistent with film-diffusion-influenced physical uptake, while equilibrium data were best described by the Freundlich isotherm, revealing favorable uptake on heterogeneous binding sites. Thermodynamic analysis confirmed a spontaneous and exothermic process, and the microspheres exhibited pronounced selectivity for CV over methyl orange in binary dye systems. This work demonstrates a green and facile route to high-capacity polysaccharide-based adsorbent microspheres for the removal of cationic dyes from water. Download: Download high-res image (135KB) Download: Download full-size image