Adsorptive Removal of Cationic Dyes from Wastewater Using a Novel Sodium Alginate-Based Iron Oxide Nanocomposite Hydrogel: DFT and Advanced Statistical Physics Modelling

Abstract

A novel Sodium alginate-based hydrogel reinforced with varying Fe2O3 nanoparticles loading (NCH) was synthesized to remove cationic dyes methylene blue (MB) and crystal violet (CV). The NCH, with 5 % Fe2O3 loading, demonstrated optimal removal efficiency for both dyes. Detailed spectroscopic, microscopic, and spectrometric characterization techniques verified the nanocomposite structure and dye adsorption. Batch experiments revealed that at pH: 9, with a 20 mg 100 mL- 1 adsorbent dose and times of 60 min for MB and 40 min for CV, with an initial concentration of 500 mg L- 1, maximum adsorption capacities were achieved. MB and CV adsorption onto NCH showed distinct mechanisms: MB via physical interaction, CV via both physical and chemical interactions, including electrostatic forces and hydrogen bonding. Despite differences, the LangmuirFreundlich model best fit the equilibrium isotherms, suggesting monolayer adsorption dominated, even with heterogeneous surface sites on NCH. A dual-site M2 statistical physics model revealed that MB and CV adsorption occurred via a multi-molecular vertical configuration on the binding sites, with heterogeneous adsorption sites contributing to dye uptake. Density functional theory demonstrated higher adsorption affinity for MB over CV, primarily due to hydrogen bonding and electrostatic interactions. NCH exhibited 80 % dye removal efficiency after five cycles, highlighting its potential for industrial effluent treatment.