Organo cationic dyes adsorption on charged nano-clay: Revealing dual mechanisms through GCS modeling and DFT molecular simulations

Abstract

This study investigates the adsorption mechanisms of organic cationic dyes, specifically malachite green (MG) and toluidine blue (TB), on sodium montmorillonite clay (Na-MMC). Using montmorillonite sourced from the Nador deposit in Northern Morocco, we aim to model cation exchange with the Gouy-Chapman-Stern (GCS) model. The adsorption capacities are found to be 192.5 meq/100 g for TB and 175.8 meq/100 g for MG at room temperature and uncontrolled pH. X-ray diffraction (XRD) analysis was employed to determine the interlayer spacing of the montmorillonite, contributing to the understanding of adsorption mechanisms. GCS modeling provided insight into the adsorption constants for both dyes, alongside parameters related to the solid/solution interface such as the amount adsorbed per layer, ion concentration in the solution, and clay surface potential. These findings confirmed a significant affinity between the dyes and the clay, aligning with the literature. Moreover, Density Functional Theory (DFT) simulations indicated a stronger affinity of toluidine blue for negatively charged surfaces compared to other dyes.