Domination of methylene blue over rhodamine B during simultaneous photocatalytic degradation by TiO₂ nanoparticles in an aqueous binary solution under UV irradiation

Simultaneous photocatalytic degradation of an aqueous mixture of methylene blue (MB) and rhodamine B (RhB) by anatase TiO₂ catalyst under UV light irradiation was investigated. In the mixed solution, degradation rate and efficiency of MB (0.151 ± 0.013 min^‒1, 92.3 ± 1.2%) was found to be much higher than for RhB (0.025 ± 0.003 min^‒1, 20.5 ± 0.6%). This was in contrast to the degradation rate and efficiency of MB and RhB when they were in solution alone (0.173 ± 0.019 min^‒1, 93.1 ± 1.1% and 0.115 ± 0.005 min^‒1, 96.1 ± 1.2%). The domination of MB over RhB in the simultaneous photocatalysis was further confirmed by adding different molar fractions of each dye and using H₂O₂-assisted methods. These results can be explained by considering that the planar MB molecules have steric interactions inhibiting Coulombic attraction for RhB to the catalyst surface and suppressing its photocatalytic degradation efficiency. This paper provides an important insight into the governing factors during simultaneous photocatalytic removal of synthetic dyes during the treatment of multicomponent wastewater.