Degradation of methylene blue and methyl orange by palladium-doped TiO2 photocatalysis for water reuse: Efficiency and degradation pathways.

Abstract This work prepared palladium-doped titanium dioxide (Pd-doped TiO 2) catalysts by a facile sol-gel method to enhance degradation of single and binary methylene blue and methyl orange by ultraviolet photocatalysis. The physicochemical properties of Pd-doped TiO 2 particles were analyzed by a X-ray powder diffractometer, X-ray photoelectron spectroscope, scanning electron microscope, transmission electron microscope, UV–visible diffuse reflectance spectroscope, sorptiometer, and transient photocurrent spectroscope. More efficient degradation was achieved in acidic and basic/neutral media for methyl orange and methylene blue, respectively. Also, methylene blue was always degraded faster than methyl orange. The highest degradation efficiency of both dyes in single and binary systems was obtained using 0.5 wt.% and 0.75 wt.% Pd TiO 2 , respectively. The degradation pathways of single methylene blue and methyl orange with Pd-doped TiO 2 and commercial P25 TiO 2 were proposed and compared by identifying reaction intermediates produced during photocatalysis. Recycling tests have confirmed excellent stability of Pd TiO 2 materials. Also, the prepared materials were easier to separate from the treated liquid using a simple sedimentation or filtration method than P25 TiO 2. In particular, the use of sol-gel method could readily synthesize a large number of catalysts, which was beneficial for scale up consideration. All these factors highlighted their potential and prospect in practical applications for water reuse. Graphical abstract In this work, methylene blue (MB, a thiazine dye) and methyl orange (MO, an azo dye), which have similar molecular weights but different structures, are selected as model pollutants. The Pd-doped TiO 2 catalysts are prepared using a facile sol-gel method for enhanced photocatalysis because this process achieves not only decolorization but also complete mineralization. The intermediates produced during photocatalysis of single dyes are identified by an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC®-MS/MS, Waters), and consequently the degradation pathways are proposed and discussed. Image 1 Highlights • Pd-doped TiO 2 particles with different Pd amount are synthesized and used as photocatalysts. • Degradation of single and mixed methylene blue (MB) and methyl orange (MO) are compared. • Initial concentration, pH, dose and Pd amount of photocatalyst affect degradation efficiency. • Mineralization of dyes highly depends on the intermediates produced during UV photocatalysis. • Degradation pathways of MB and MO are proposed based on UPLC-MS/MS measurements. [ABSTRACT FROM AUTHOR]