Photochemical reactions in sunlit surface waters

Photochemical processes are important pathways for the transformation of biologically refractory organic compounds, including harmful pollutants, in surface waters. They include the direct photolysis of sunlight-absorbing molecules, the transformation photosensitised by the triplet states of chromophoric dissolved organic matter (CDOM), and the reaction with photochemically generated radical transients. Differently from the direct photolysis, the other processes (often indicated as indirect photochemistry) can also induce the phototransformation of compounds that do not absorb sunlight. The excited triplet states of CDOM, 3CDOM*, play a very important role in surface-water photoprocesses, both directly and as sources of singlet oxygen (1O2). The most important reactive radical species in surface waters are the hydroxyl radical ·OH, the carbonate radical CO3-·, and various peroxy radicals that can be produced upon degradation of dissolved organic matter (DOM), either chromophoric or not. Further radical species such as ·NO2, Cl2-· and Br2-· can be involved in the generation of harmful degradation intermediates such as aromatic nitro, chloro, and bromoderivatives.