Experimental and theoretical insights into the adsorption mechanism of methylene blue on the (002) WO3 surface

Abstract

This work investigates the efficiency of green-synthesized WO₃ nanoflakes for the removal of methylene blue dye. The synthesis of WO₃ nanoflakes using Hyphaene thebaica fruit extract results in a material with a specific surface area of 13 m²/g and an average pore size of 19.3 nm. A combined theoretical and experimental study exhibits a complete understanding of the MB adsorption mechanism onto WO₃ nanoflakes. Adsorption studies revealed a maximum methylene blue adsorption capacity of 78.14 mg/g. The pseudo-second-order model was the best to describe the adsorption kinetics with a correlation coefficient (R²) of 0.99, suggesting chemisorption. The intra-particle diffusion study supported a two-stage process involving surface adsorption and intra-particle diffusion. Molecular dynamic simulations confirmes the electrostatic attraction mechanism between MB and the (002) WO₃ surface, with the most favorable adsorption energy calculated as -0.68 eV. The electrokinetic study confirmed that the WO₃ nanoflakes have a strongly negative zeta potential of -31.5 mV and a uniform particle size of around 510 nm. The analysis of adsorption isotherms exhibits a complex adsorption mechanism between WO₃ and MB, involving both electrostatic attraction and physical adsorption. The WO₃ nanoflakes maintained 90% of their adsorption efficiency after five cycles, according to the reusability tests.