Photocatalytic mechanisms of 2,4-dinitroanisole degradation in water deciphered by C and N dual-element isotope fractionation.

In surface water environments, photodegradation may be an important process for the natural attenuation of 2,4-dinitroanisole (DNAN). Understanding the photolysis and photocatalysis mechanisms of DNAN is difficult because the photosensitivity of nitro groups and the behavior of DNAN as a potential photosensitizer are unclear in aqueous solutions. Here, we investigate the degradation mechanisms of DNAN under UV-A (λ ~ 350 nm) and UV-C (λ ~ 254 nm) irradiation in a photolysis reactor where aqueous solution was continuously recycled through a UV-irradiated volume from a non-irradiated external reservoir. By tracking C and N isotopic fractionation in DNAN and its reaction products, we observed normal 13C fractionation (εC = -3.34‰) and inverse 15N fractionation (εN = +12.30‰) under UV-A (λ ~ 350 nm) irradiation, in contrast to inverse 13C fractionation (εC = +1.45‰) and normal 15N fractionation (εN = -3.79‰) under UV-C (λ ~ 254 nm) irradiation. These results indicate that DNAN can act as a photosensitizer and may follow a product-to-parent reversion mechanism in surface water environments. The data also indicate that photocatalytic degradation of DNAN in aqueous systems can be monitored via C and N stable isotope analysis.