Substituent effect in self-sensitized degradation of Acid Orange 7 in solar/diketone processes

Self-sensitized degradation is a typical fate of azo dyes in aquatic systems but its kinetics is far too slow to meet the requirement for a thorough decolorization. Small diketones were found being able to enhance the degradation of dyes under UV irradiation and the introduction of an electron-denoting group at the & alpha;-position of the simplest & beta;-diketone (acetylacetone, AA) had a further enhancement effect on the degradation. However, it is unclear whether such enhancement effects exist in the solar-irradiated systems. To clarify this question, the effects of three AA derivatives were investigated with Acid Orange 7 (AO7) as a representative azo dye. Compared with the AA derivatives with a methyl or an ethyl group at the central carbon (AA-CH3 and AACH2CH3), 3,5-diacetylheptane-2,6-dione (AA-CH2-AA, which is a condensed combination of AA and AA-CH3) showed a >2-fold stronger enhancement effect. The nonstoichiometric enhancement effect of AA-CH2-AA on AO7 photodegradation disclosed a novel pathway for dye degradation: AA-CH2-AA could enhance the degradation of AO7 by reacting with the photo-sensitized 1O2, whereas AA led to an inhibitory effect on the self sensitized degradation of AO7 by quenching the excited state of the azo dye. These results demonstrate that small diketones could be designed to regulate the photodegradation of azo dyes in multiple pathways.