This study investigates the dual chemical modification of xanthan gum (XG) through grafting with acrylonitrile, followed by amidoximation. The grafting conditions were systematically optimized by adjusting the concentrations of the initiator and monomer, as well as the reaction temperature and time, in order to maximize grafting efficiency. Structural characterization of xanthan gum-g-polyacrylonitrile (XG-g-PAN) and its amidoxime derivative (XG-amidoxime) was performed using FTIR, XRD, SEM, and BET analyses. Both materials were evaluated as adsorbents for the removal of methylene blue (MB) from aqueous solutions, demonstrating maximum removal efficiencies of 93.6% for XG-g-PAN and 95.4% for XG-amidoxime under optimal conditions (0.02 g adsorbent, pH 7, 180 min, room temperature). The adsorption behavior was well described by the Freundlich isotherm model, while the kinetic data were best fitted by the pseudo-second-order model. The novelty of this work lies in the strategic combination of dual-step chemical functionalization and advanced quantum chemical modeling to provide a comprehensive molecular-level understanding of the adsorption mechanism, which has not previously been reported for XG-based systems. Density functional theory (DFT) simulations confirmed strong binding affinities and selectivity of XG-g-PAN and XG-amidoxime toward MB, with lower adsorption energies in the aqueous phase due to solvation effects. Frontier molecular orbital (FMO) analysis and global reactivity parameters supported the stability of the adsorption complexes. The interactions between XG-g-PAN/XG-amidoxime and MB were further investigated using Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analyses. QTAIM results identified two primary types of interactions in both gas and aqueous phases: polar covalent (partially covalent and partially electrostatic) and purely electrostatic interactions. NBO analysis revealed significant charge transfer from the σ and n orbitals of C and O atoms in XG-g-PAN/XG-amidoxime to the n* orbitals of N and S atoms in MB, confirming strong donor–acceptor interactions. These findings provide fundamental insights into the electronic properties and adsorption behavior of these modified xanthan gum derivatives, contributing to the development of highly efficient adsorbents for dye removal in wastewater treatment applications.
Arafa et al. (Thu,) studied this question.