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) and superoxide radicals (O2−) 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 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-reacted efficiency, it was found that the OH-reacted concentration shows a satisfactory linear correlation (r = 0.924) with AO7 degradation. The OH-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-based advanced oxidation processes.