Effects of water chemistry on decolorization in three photochemical processes: Pro and cons of the UV/AA process

The poor selectivity of hydroxyl radicals is a major restriction in the practical application of the UV/H₂O₂ process for dyeing wastewater treatment. As an alternative, the target-selective UV/acetylacetone (AA) process was found highly efficient for dye decolorization. For the proper selection and application of the two photochemical processes, the effects of water matrices, including common inorganic anions (Cl^-, SO₄^2-, NO₃^-, HCO₃^-), natural organic matter, metal cations (Mg²⁺, Mn²⁺, Cu²⁺, Fe³⁺, Cr³⁺), and temperature, on the photo-degradation of an azo dye, Acid Orange 7 (AO7), were systematically investigated. The experimental results demonstrate that the UV/AA process was more sensitive to inner filter effect. NO₃^-, Cu²⁺, and Fe³⁺ were all detrimental to the UV/AA process, whereas at certain concentrations they were beneficial to the UV/H₂O₂ process. However, even with severe inhibitory effects, the decolorization efficiency of the UV/AA process was still several times higher than that of the UV/H₂O₂ process. The results are helpful for us to better understand the mechanisms behind the UV/AA process and may shed light on the application of UV-based advanced oxidation processes for wastewater treatment.