Cationic Dyes Adsorption by Na-Montmorillonite Nano Clay: Experimental Study Combined with a Theoretical Investigation Using DFT-Based Descriptors and Molecular Dynamics Simulations

Abstract

In this work, adsorption kinetics, isotherms and mechanisms of two cationic dyes, Toluidine Blue (TB) and Crystal Violet (CV), on a Na-Montmorillonite Nano Clay (Na-MNC) have been experimentally and theoretically determined. The structure and the morphology of the Na-MNC were characterized by XRD, FTIR, SEM, EDAX and BET. High adsorption capacity was observed for both dyes (5.80 mmol/g for TB and 5.40 mmol/g for CV). Maximum removal of both dyes was observed at pH 11.0. Equilibrium was attained after the contact time of 30 min for CV dye and 50 min for TB dye. The adsorption isotherms were described by Langmuir isotherm model suggesting that the dyes were adsorbed homogenously over a monolayer surface of the Na-MNC. The adsorption kinetics was well fitted to pseudo-second-order kinetic model (R2 > 0.98). The theoretical investigation carried out on the CV and TB dyes structures using the quantum molecular descriptors derived from density functional theory (DFT) calculations have proved that the TB cations are more electrophilic, reactive and have the ability to accept electrons allow to adsorb strongly on the adsorbent surface compared to the CV ones. Molecular dynamic (MD) simulations were performed to obtain information on molecular arrangements and to predict interaction energies of studied dyes with the Na-MNC surface. These simulations show that the adsorption of the studied dyes can result in a variety of configurations, including single layers parallel to the basal surface, tilted and random piles. © 2019 Elsevier B.V.