Influence of Network Architecture on Adsorption Efficiency of Flexible and Rigid Cross-Linked Guar Gum Xerogels for Selective Cationic Dye Removal

This study investigates how cross-linking architecture influences the selective adsorption of methylene blue (MB) from mixed dye solutions using guar gum (GG) based xerogels. Flexible networks were prepared with N,N-methylene bis(acrylamide) (MBA) and rigid networks with glutaraldehyde (GLU). The MBA-cross-linked xerogel (GMBA2) exhibited higher swelling (280%) and porosity than the rigid GGLU7. Under optimal circumstances (pH 8, 330 K, 60 min), GMBA2 removed 98.1% of methylene blue with a maximum adsorption capacity of 235.44 mg·g–1, while GGLU7 removed 82.1% of the dye. Adsorption kinetics were consistent with the pseudo-second-order model, but equilibrium results were best represented by the Langmuir isotherm, indicating monolayer adsorption on homogeneous active sites. Thermodynamic study showed the adsorption process as spontaneous and favorable. Furthermore, GMBA2 maintained 96.7% of its adsorption effectiveness after five adsorption–desorption cycles, indicating high reusability. DFT calculations show a favorable binding free energy of −85.63 kJ·mol–1 between MB and the flexible polymer network. Molecular dynamics simulations confirm stable hydrogen-bonding interactions between MB molecules and hydroxyl groups within the xerogel matrix. Overall, the findings show that flexible cross-linking designs considerably improve swelling, intermolecular interactions, and selective cationic dye adsorption, providing an efficient and sustainable method for wastewater treatment applications.