Unraveling the photocatalytic degradation kinetics and efficiency of methylene blue, rhodamine B, and auramine O in their ternary mixture: diffusion and conformational insights

Heterogeneous photocatalytic degradation behavior of cationic methylene blue (MB), rhodamine B (RhB), and auramine O (AO) dyes in their ternary aqueous solution, as a model of multicomponent mixture closely imitating a real wastewater, was investigated in great detail. In this study, 100 nm anatase TiO₂ nanoparticles irradiated using 365 nm light were utilized to generate reactive oxygen species capable of oxidizing and degrading unselectively the dyes into small fragments of organic compounds. The underlying kinetics and mechanism of photocatalytic degradation of the dyes were elucidated based on the Langmuir–Hinshelwood kinetic, Weber–Morris intraparticle diffusion, and Smoluchowski diffusion-limited reaction models. The simultaneous photocatalytic degradation of the dyes in their ternary mixture at different irradiation times, catalyst dosages, initial concentrations, pHs of medium, and molarity ratios clearly suggested the dominance of MB in the photocatalytic degradation process due to its faster diffusion over RhB and AO. Increasing temperature or adding a small amount of hydrogen peroxide further highlighted the advantage of MB in the photocatalytic degradation. Overall results revealed a general concept that the molecular structure, especially planarity and electron donating power of attached groups, plays an important role in controlling diffusion dynamics, immobilization, and efficiency of photocatalytic degradation of dyes in multicomponent wastewater.

Graphical abstract