A novel sustainable CMC/XG/KAO biopolymer–clay composite hydrogel for efficient dye removal and water treatment

Abstract

A sustainable biopolymer clay composite hydrogel based on carboxymethyl cellulose (CMC), xanthan gum (XG), and kaolinite (KAO) was successfully synthesized via ionic crosslinking using Ca 2 ⁺ ions and evaluated for the removal of anionic Orange II (OII) and cationic Malachite Green (MG) dyes from aqueous solutions. Structural and morphological analysis by SEM, XRD, FTIR, and EDX improved the formation of a porous three-dimensional hydrogel network with strong polymer–clay interactions. The FTIR bands at 912, 756, 700 and 537 cm −1 confirmed the presence of kaolinite in the hydrogel matrix. The adsorption performance was investigated as a function of adsorbent dosage, contact time, initial dye concentration, and solution pH. The hydrogel exhibited significantly higher adsorption efficiency toward MG than toward OII, which was attributed to the strong electrostatic interactions between the negatively charged functional groups (–COO⁻ and –OH) in the hydrogel matrix and the cationic dye molecules. Maximum removal efficiencies of approximately 97.15% for MG at pH 12 and 85% for OII at pH 2 were observed. Adsorption equilibrium was attained within 180 min. Isotherm studies revealed that MG adsorption followed the Langmuir model, with a maximum adsorption capacity of 90.09 mg/g, whereas OII adsorption was better explained by the Freundlich model, suggesting heterogeneous multilayer adsorption. According to the kinetic analysis, the pseudo-first-order model better explained the adsorption mechanism for both dyes. Furthermore, the hydrogel demonstrated good reusability with minimal loss in performance after five adsorption–desorption cycles. The hydrogel surface becomes negatively charged at pH values above the pHPZC (≈6.0), resulting in strong electrostatic attraction toward the cationic MG molecules. These results demonstrate the potential of the CMC/XG/KAO hydrogel as an effective, affordable, and eco-friendly adsorbent for treating wastewater contaminated with dyes.