Photochemical degradation of the aromatic amine 4-(N,N-dimethylamino)benzonitrile: Triplet sensitisation, back reduction, and (quasi-)global modelling

Dissolved organic matter (DOM) acts as an inducer as well as an inhibitor in the photodegradation of 4-(N,N-dimethylamino)benzonitrile (DMABN), which undergoes both photodegradation by the excited triplet states of chromophoric DOM (3CDOM*), and inhibition upon back reduction by the antioxidant moieties of DOM itself. On average, water with dissolved organic carbon (DOC) > 2.3 mgC L−1 would inhibit DMABN photodegradation by over 50 %. From the available data of [3CDOM*] steady-state concentrations and the levels of DOC in lake water on a quasi-global scale (60°S-60°N latitude range), we were able to numerically assess the photochemical lifetimes of DMABN, here reported as year averages. Lifetimes of DMABN in lake water would amount to a couple of months or longer, and they would be the shortest in the tropical belt. Results of numerical calculations were interpolated using a suitable model function, with which an analytical equation was derived that relates DMABN lifetimes with lake depth, DOC, and latitude. For the first time to our knowledge, a general equation is here proposed to assess the photodegradation kinetics of a pollutant on a (quasi–)global scale.