Novel CaxMnOy/TiO2 composites for efficient photocatalytic degradation of methylene blue and the herbicide imazapyr in aqueous solution under visible light irradiation

CaxMnOy-TiO2 composites with different manganese oxide birnessite fractions (1-50 wt.%) have been synthesized based on the precipitation of birnessite and mixing with commercial titanium dioxide TiO2 by mechanical grinding. The CaxMnOy-TiO2 composites exhibited high photoactivity for the decomposition of imazapyr herbicide and methylene blue under UV-vis illumination. The results suggested that a correlation exists between the calcium manganese oxide content, oxidation state and the changes in the UV-vis absorption properties. The composite with 5% Ca(0.1)MnOy/TiO2 possessed the highest photocatalytic performance among all the as-obtained catalysts, the enhanced photocatalytic activity was attributed to the reduction of the band gap from 3.18 eV to 2.89 eV for pure TiO2 and 5% Ca(0.1)MnOy/TiO2 composite respectively. In addition, an efficient inhibition of electron-holes recombination rates due to the deposition of manganese oxide mainly on the surface of titanium dioxide producing an efficient charge separation, and transfer between the materials. Furthermore, the photocatalytic rate constant was found to increase from 0.124 min(-1) to 0.542 min(-1) and from 0.155 min(-1) to 0.425 min(-1) for methylene blue dye and imazapyr, in the presence of commercial titanium dioxide TiO2 and 5% Ca0.1MnOy/TiO2 composite, respectively.