The increasing contamination of water by organic dyes causes water pollution in the enviroment. Factories discharge untreated effluents into nearby water courses adding to the existing water pollution; this poses a significant environmental challenge. Hence there is a pressing demand to develop efficient technology for wastewater treatment, and photocatalysis has emerged as an advanced oxidation process with a green chemical approach for such treatment. This study aims to synthesize montmorillonite/TiO2 (Mnt/TiO2) photocatalysts and clarify the effect of montmorillonite content on the photodegradation of the organic dye rhodamine B (RhB). Mnt/TiO2 was prepared by a chemical method with various mass ratios of mMnt:mTiO2 based on the cation exchange capacity (CEC) of Mnt. The physicochemical properties of the samples prepared were determined by the following methods: energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The photocatalytic degradation efficiency of the RhB solution of Mnt/TiO2 was investigated by UV-Vis spectroscopy under UVC irradiation. Liquid chromatography-mass spectrometry (LCMS) was used to identify the photocatalytic by-products. The results showed that the structure of the nanocomposites has a ‘house-of-cards’ form with TiO2 nanoparticles randomly distributed on the surface and sheets of clay minerals. The best mass ratio of mMnt:mTiO2 is 10:1, corresponding to a 10 ppm RhB solution decolorization efficiency of 91.5% in 210 min. In this study, Mnt/TiO2 successfully cleaved the dye chromophore structure and broke the RhB rings into small and broken-ring compounds.

Organic dyes such as methylene blue (MB) are often used in the characterization of clays and related minerals, but details of the adsorption mechanisms of such dyes are only partially understood from spectroscopic data, which indicate the presence of monomers, dimers, and higher aggregates for varying mineral surfaces. A combination of quantum (density functional theory) and classical molecular simulation methods was used to provide molecular detail of such adsorption processes, specifically the adsorption of MB onto kaolinite basal surfaces. Slab models with vacuum-terminated surfaces were used to obtain detailed structural properties and binding energies at both levels of theory, while classical molecular dynamics simulations of aqueous pores were used to characterize MB adsorption at infinite dilution and at higher concentration in which MB dimers and one-dimensional chains formed. Results for the neutral MB molecules are compared with those for the corresponding cation. Simulations of the aqueous pore indicate preferred adsorption on the hydrophobic siloxane surface, while charge-balancing chloride ions adsorb at the aluminol surface. At infinite dilution and in the gas-phase models, MB adsorbs with its primary molecular plane parallel to the siloxane surface to enhance hydrophobic interactions. Sandwiched dimers and chains are oriented perpendicular to the surface to facilitate the strong hydrophobic intermolecular interactions. Compared with quantum results, the hybrid force field predicts a weaker MB adsorption energy but a stronger dimerization energy. The structure and energetics of adsorbed MB at infinite dilution are consistent with the gas-phase binding results, which indicate that monomer adsorption is driven by strong interfacial forces rather than by the hydration properties of the dye. These results inform spectroscopic studies of MB adsorption on mineral surfaces while also revealing critical areas for development of improved hybrid force fields.

Zwitterionic porphyrin (tetrakis {4-(2-carboxyethyl)pyridinio}porphyrin (TPyCP)) with four pyridinium and carboxyl groups in the molecule was synthesized. The adsorption behaviour of TPyCP on the montmorillonite clay surface was examined in an aqueous colloidal solution. While the adsorption maximum λmax of TPyCP was 424 nm in water, it shifted to longer wavelengths on complex formation with clay. The λmax on the exfoliated clay surface was 450 nm, and that in the stacked clay sheets was 471 nm, respectively. Under alkaline conditions ([NaOH] = 2 × 10–4 M), the stacking behaviour of the clay was completely suppressed. It emerged that anionic parts in the porphyrin molecule can suppress the stacking of clay sheets. Judging from the quantitative analysis, the maximum adsorption amount of TPyCP was 100% vs. cation exchange capacity (CEC) of the clay. As the adsorption density of TPyCP increased, λmax shifted slightly to longer wavelengths due to the interactions between adjacent porphyrins. When the loading level of TPyCP was 200% vs. CEC, the stacking of TPyCP was indicated. The average stacking layer number was calculated to be 1.48. On the other hand, it is known that tetra cationic porphyrins without anionic parts do not form such stacking structures. Thus, it seems that zwitterionic porphyrin has the potential to form a three dimensional structure through electrostatic interactions between porphyrins on the clay surface.