New insights on crystal violet dye adsorption on montmorillonite: Kinetics and surface complexes studies

Abstract

Adsorption of crystal violet (CV) on montmorillonite (MMT) was investigated through kinetic analysis, microstructural characterization, surface charge and contact angle determination. Adsorption kinetic was studied in situ by attenuated total reflectance Fourier-transform infrared analysis (ATR-FTIR); MMT microstructural changes were determined by small angle X-ray scattering (SAXS) and N2 adsorption/desorption analysis; wettability and surface charge were determined by contact angle and a particle charge detection technique, respectively. Kinetic studies indicated that the analyzed total adsorption rate was the result of the combination of intraparticle diffusion and/or surface adsorption processes, where the contribution of each phenomenon depended on the initial concentration of CV. Obtained findings of characterization studies allowed proposing different molecular arrangements according to the concentration of adsorbed dye in water. For low CV concentration, the dye molecules entered in the MMT interlaminar space as a monolayer, with the aromatic rings parallel to the interlayer space surface. At medium CV content, the dye molecules entered the MMT interlaminar space either in a paraffin-like monomolecular arrangement with a tilt angle or in a bilayer configuration. In both cases, increment of the basal spacing, reduction of wettability, diminution of negative surface charge and the interaction of CV molecule with surface through its quaternary amine group were determined. At higher CV content, the wettability increases and surface charge shifted to positive values, indicating the presence of CV bilayer arrangements, with the positively charged region of the dye molecules facing away from the surface. Results of this work will help to develop efficient adsorbents and to achieve a deeper understanding on the fate of CV (and similar compounds) in water and sediments.