Silver Nanoparticles Reinforced Hing Gum-Based Hydrogel Nanocomposite for Enhanced Hexavalent Chromium Adsorption: Green Synthesis, Characterization, and Mechanistic Insights

In this work, a Hing gum-based hydrogel and its silver nanoparticles (AgNPs) reinforced nanocomposite were fabricated and investigated for the removal of hexavalent chromium [Cr(VI)] ions from wastewater. The pure hydrogel was prepared via free-radical polymerization, while the AgNPs-reinforced nanocomposite was prepared via ex situ microwave irradiation. AgNPs incorporated into the bare hydrogel matrix were prepared using an environmentally friendly green synthesis route. Detailed physicochemical characterization of the prepared samples was performed using XRD, FE-SEM, EDS, XPS, TEM, and TGA, providing insight into the structural, morphological, elemental composition, thermal stability, and adsorption-induced changes in the hydrogel and nanocomposite. The adsorption behavior of Cr(VI) ions for both hydrogel and nanocomposite was examined by varying solution pH, adsorbent dosage, contact time, temperature, AgNPs loading, and stirring rate. The prepared nanocomposite exhibited significantly improved adsorption behavior, showing nearly a 20% improvement in Cr(VI) removal efficiency compared to the bare hydrogel. Equilibrium and kinetic analyses revealed that the adsorption data for both samples were best described by the Langmuir isotherm and the pseudo-second-order kinetic model, suggesting monolayer adsorption driven primarily by chemisorption. The maximum adsorption capacity increased from 88.73 mg/g for the pristine hydrogel to 3043.5 mg/g for the nanocomposite, representing one of the highest reported values of Cr(VI) adsorption using a natural polysaccharides-based hydrogel. Thermodynamic parameters confirmed the spontaneous nature of the adsorption process. Overall, the obtained results demonstrate that both Hing gum-based hydrogel and its nanocomposite with AgNPs are efficient and promising adsorbents for the removal of toxic heavy metal ions from wastewater, with the nanocomposite showing better results.