Photochemical Transformation Processes in Sunlit Surface Waters
Abstract
Photochemical reactions are major processes in the transformation of hardly biodegradable xenobiotics in surface waters. They are usually classified into direct photolysis and indirect or sensitised degradation. Direct photolysis requires xenobiotic compounds to absorb sunlight, and to get transformed as a consequence. Sensitised transformation involves reaction with transient species (e.g. °OH, CO3-°, 1O2 and triplet states of chromophoric dissolved organic matter, 3CDOM*), photogenerated by so-called photosensitisers (nitrate, nitrite and CDOM). CDOM is a major photosensitiser: is it on average the main source of °OH (and of CO3-° as a consequence, which is mainly produced upon oxidation by °OH of carbonate and bicarbonate) and the only important source of 1O2 and 3CDOM* [1, 2]. CDOM origin plays a key role in sensitised processes: allochthonous CDOM derived from soil runoff and rich in fulvic and humic substances is usually more photoactive than autochthonous CDOM (produced by in-water biological processes and mainly consisting of protein-like material) or of CDOM derived from atmospheric deposition. An interesting gradual evolution of CDOM origin and photochemistry can be found in mountain lakes across the treeline, which afford a gradual transition of allochthonous- autochtonous - atmopheric CDOM when passing from trees to alpine meadows to exposed rocks [3]. Another important issue is the sites of reactive species photoproduction in CDOM. While there is evidence that smaller molecular weight fractions are more photoactive, some studies have reported considerable 1O2 reactivity in CDOM hydrophobic sites and inside particles [4]. We have recently addressed the problem and found that dissolved species in standard humic acids (hydrodynamic diameter < 0.1 μm) account for the vast majority of 1O2 and triplet states photoproduction. In hydrophobic sites of particles, the formation rate of 1O2 is considerably lower than in the solution bulk [5], but the absence of water can significantly increase 1O2 half-life time (the main deactivation process of 1O2 in solution is collision with the solvent), thereby affording considerable reactivity toward hydrophobic solutes. The current knowledge in the field of natural photosensitizers in surface waters allows photoinduced transformation processes of organic pollutants to be assessed and modelled. For instance, it is possible to predict pollutant half-life times by knowing absorption spectrum, direct photolysis quantum yield and reaction rate constants with °OH, CO3-°, 1O2 and 3CDOM*, as a function of sunlight irradiance, water chemical composition (also affecting absorption) and column depth. Some examples of model application to real cases will be presented [6-8]. [1] Halladja et al., Environ Sci Technol 41, 6066 (2007) [2] Canonica et al., Environ Sci Technol 39, 9182 (2005) [3] De Laurentiis et al., Chemosphere 88, 1208 (2012) [4] Latch & McNeill, Science 311, 1743 (2006) [5] Minella et al., Chemosphere, accepted [6] Vione et al., Wat Res 45, 6725 (2011) [7] Sur et al., Sci Total Environ 426, 296 (2012) [8] De Laurentiis et al., Environ Sci Technol, DOI 10.1012/es3015887
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A34C..05V
- Keywords:
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- 1806 HYDROLOGY / Chemistry of fresh water;
- 1807 HYDROLOGY / Climate impacts;
- 1845 HYDROLOGY / Limnology;
- 1871 HYDROLOGY / Surface water quality