Enhanced synergistic photocatalytic activity of TiO2/oxidant for azo dye degradation under simulated solar irradiation: A determination of product formation regularity by quantifying hydroxyl radical-reacted efficiency

Synergizing heterogeneous photocatalysis with strong oxidants is a simple and useful way towards the improvement of organic wastewater treatment. This study verified that the synergies of TiO2 and two oxidants, K2Cr2O7 (Cr(VI)) and KMnO4 (Mn(VII)), can effectively improve the degradation of an azo dye acid orange 7 (AO7) under simulated solar irradiation, which is based on a combination of the oxidizing ability of oxidants and the enhanced synergistic photocatalytic activity. Meanwhile, nearly 60 % of toxic Cr(VI) could be photocatalytically reduced into innocuous Cr(III) species, being free from secondary contamination. The measurements of hydroxyl radicals ((OH)-O-center dot) and superoxide radicals (O-2(center dot-)) by using electron spin resonance (ESR) and fluorescence probe techniques, indicated that the enhanced synergistic photocatalytic activity of TiO2/oxidant can be attributed to the facilitated separation of electron-hole pairs and the increased utilization of electrons, thus promoting overall (OH)-O-center dot formation. Analyzed by liquid chromatography-mass spectrometry (LC/MS), the intermediate product distributions of AO7 degradation were determined to further propose the possible degradation pathways. Through applying the quantification of (OH)-O-center dot-reacted efficiency, it was found that the (OH)-O-center dot-reacted concentration shows a satisfactory linear correlation (r = 0.924) with AO7 degradation. The (OH)-O-center dot-reacted concentration was thereby adopted as an indicator parameter to further establish a stoichiometry of 'OH which can be used to quantitatively describe the occurring mechanisms and product formation characteristics during the photocatalytic degradation of AO7. This study provides an experimental approach for revealing the regularity of product formation in the wastewater treatment by various (OH)-O-center dot-based advanced oxidation processes.

Automated summary

This study demonstrated that the synergistic effect of TiO2 and K2Cr2O7/KMnO4 can improve the degradation of organic pollutants in wastewater, while simultaneously reducing toxic Cr(VI) to Cr(III). By analyzing the linear correlation between the concentration of hydroxyl radicals and the degradation of pollutants, a stoichiometry of OH was established to quantitatively describe the mechanisms of photocatalytic degradation.