Determining the Mechanism and Efficiency of Industrial Dye Adsorption through Facile Structural Control of Organo-montmorillonite Adsorbents

The structural evolution of cost-effective organo-clays (montmorillonite modified with different loadings of CTAB (cetyltrimethylammonium bromide)) is investigated and linked to the adsorption uptake and mechanism of an important industrial dye (hydrolyzed Remazol Black B). Key organo-clay characteristics, such as the intergallery spacing and the average number of well-stacked layers per clay stack, are determined by low-angle X-ray diffraction, while differential thermogravimetric analysis is used to differentiate between surface-bound and intercalated CTAB. Insights into the dye adsorption mechanism are gained through the study of the adsorption kinetics and through the characterization of the organo-clay structure and surface charge after dye adsorption. It is shown that efficient adsorption of anionic industrial dyes is driven by three key parameters: (i) sufficiently large intergallery spacing to enable accommodation of the relatively large dye molecules, (ii) crystalline disorder in the stacking direction of the clay platelets to facilitate dye access, (iii) and positive surface charge to promote interaction with the anionic dyes. Specifically, it is shown that, at low modifier loadings (0.5 cation exchange capacity (0.5CEC)), CTAB molecules exclusively intercalate as a monolayer into the clay intergallery spaces, while, with increasing modifier loadings, the CTAB molecules adopt a bilayer arrangement and adsorb onto the exterior clay surface. Bilayer intercalation results in sufficiently large expansion of the intergallery spaces and significant disordering along the (001) stacking direction to enable high and relatively fast dye uptake via intraparticle diffusion. Poor and slow dye uptake is observed for the organo-clays with a monolayer structure, suggesting relatively inefficient dye adsorption at the clay edges. The optimized bilayer organo-clays (montmorillonite modified with 3CEC of CTAB) also show enhanced adsorption efficiencies for other important industrial dyes, highlighting the importance of structural control in organo-clays while also showing the adsorbents' great potential for use in industry where dye mixtures are encountered.