Photodegradation Of Organic Dyes By Diketones:Behavior And Mechanism Study

Dye effluents have posed great threat toward ecological system and human health due to their large emission and highly refractory nature. Hydroxyl radical based advanced oxidation processes have found wide applications in the treatment of dye wastewater. However, due to the non-selective nature of hydroxyl radicals, the co-existing organic and inorganic matters in wastewater will impair the decoloration efficiency of dyes. More efficient and selective pretreatment methods of dye wastewater are highly needed.Utilizing organic activators in dye degradation is another avenue which is different from the inorganic oxidizer and photocatalyst based photo-oxidation processes. In the present study, three diketones, acetylacetone, biacetyl and acetonylacetone, were investigated as activators for the degradation of organic dyes under UV light irradiation.All three diketones were very efficient in the degradation of acid orange 7 (AO7). The decoloration of AO7 in diketone systems was much faster than in H2O2 system with the same activator dose. For the degradation of methyl orange, congo red, methylene blue and rhodamine B, acetylacetone and acetonylacetone were more effective than biacetyl. The azo, benzene and naphthalene moieties in AO7 were destructed in diketone systems according to UV-Vis spectra, indicating that AO7 dye was not only decolorized but also degraded. The biodegradability of dyes was enhanced remarkably after irradiated with diketones. The main photodegradation products of diketones were formic acid and acetic acid, which can be the carbon source for microbial growth. Thus, the UV/diketone process has the potential to serve as the pretreatment step for biological treatment process to achieve dye wastewater disposal at low cost. Phytotoxicity study was carried out to evaluate the feasibility to use the UV/diketone process as the pretreatment process before biological treatment. Seeds of rice, radish and rape were investigated in terms of seedling growth and root elongation in dye solutions before and after degradation, and the results showed that different seeds reacted differently toward the tested dye solutions, suggesting that more toxicity assays need to be carried out to fully understand the toxicity of degraded dyes.Degradation mechanisms were investigated both experimentally and theoretically to understand the high efficiency of dye degradation by diketones. Hydroxyl radical and superoxide radial were ruled out as the reactive species in dye degradation. Singlet oxygen contributed to the degradation of dyes, but was not the determinative species. Dissolved oxygen can take part in the degradation of dyes, but dyes could be effectively degraded under N2 bubbling condition. Using LC-MS2, several degradation products of acid orange 7 and methyl orange were identified, including p-aminobenzene sulfonic acid, p-hydroxybenzenesulfonic acid and the adduct of dye and acetylacetone. With the aid of density functional calculations, several possible reaction mechanisms were proposed, including energy transfer from the triplet state of diketone to dye, electron transfer between the excited states of diketone and dye, carbon-centered radicals induced dye degradation and hydrogen transfer between diketone and dye followed by the formation and degradation of molecular adduct of diketone and dye. The formation and degradation of the adduct of diketone and dye is a uniqueness of the diketone/UV process, and may be the reason for the high efficiency of dye degradation.