Removal of methyl violet by Moroccan clays: kinetics study and equilibrium mechanism

In this study, the removal of methyl violet by natural clays was investigated. Rhassoul-clay, A-clay, E-clay, R-clay, and Y-clay were sampled from different sites in Morocco. Their surface properties were investigated by XRD, XRF, and SEM, as well as zeta potential measurements. Furthermore, the hexamine cobalt and BET methods were employed to estimate the CEC and SSA, respectively. In addition, parameters such as pH, time, temperature, and initial concentration were investigated to evaluate their effect on adsorption. Adsorption kinetics were thoroughly investigated by several mathematic models. The pseudo-second-order model and intraparticle diffusion were found to be the most suitable to describe adsorption. The increase in pH was found to promote the adsorption capacity of methyl violet, except in the case of Rhassoul-clay, which exhibited a non-significant pH dependence. Furthermore, the Langmuir, Freundlich, Harkin-Jura, Halsey, and Temkin models were employed to investigate the adsorption mechanism. The adsorption of methyl violet obeyed the Langmuir model at high concentration, and Temkin model at low concentration. The maximal monolayer adsorption capacity decreased in the order of Rhassoul-clay > A-clay > E-clay > R-clay > Y-clay. Thermodynamic studies indicated that the increase in temperature improved the adsorption capacity of basic dye on A-clay and Rhassoul-clay and that adsorption processes were endothermic.