A new bi-radical species formed during the photochemical degradation of synthetic musk tonalide in water: Study of in-situ laser flash photolysis and validation of synthesized standard sample

The ubiquitous presence of synthetic musks is causing serious concern due to the species produced from their transfor-mation and environmental impacts. In this study, tonalide was selected as a representative synthetic musk to evaluate the transformation mechanism and pathway in water under ultraviolet (UV) irradiation. The results showed that tonalide could undergo rapid photochemical degradation through a new pivotal bi-radical, which acts as the initial active species. The bi-radicals with a typical absorption peak at 340 nm was observed by in-situ laser flash photolysis technology, and the absolute decay rate constant was obtained as 3.61 +/- 0.01 x 109 M-1 s-1 with the life-time of 83.3 ns. The photochemical degradation by-products of tonalide were also identified by high-performance liquid chro-matography coupled with quadrupole time-of -flight mass spectrometry, and the precise structures of key by-products have been validated by our preparative synthesized standard samples confirmed by nuclear magnetic resonance. Thus, the mechanism of tonalide photochemical degradation, continuous photoenolization of the bi-radicals and followed cycloaddition reaction with O2, was proposed as the predominant pathway. The main degradation by-product, photoenol which has a higher bioconcentration than that of tonalide, was found to form from the bi-radicals photoenolization. This study is the first work to propose a new bi-radical as the photoenol precursors during photo-chemical degradation of tonalide in water.

Automated summary

The study investigates the transformation mechanism and pathway of tonalide, a representative synthetic musk, under ultraviolet (UV) irradiation in water. It is found that tonalide undergoes rapid photochemical degradation through a new pivotal bi-radical acting as the initial active species. The main degradation by-product, photoenol, with higher bioconcentration than tonalide, is formed through the photoenolization of the bi-radicals.