The photosensitized transformation of organic chemicals is an important degradation mechanism in natural surface waters, aerosols, and water films on surfaces. Dissolved organic matter including humic-like substances (HS), acting as photosensitizers that participate in electron transfer reactions, can generate a variety of reactive species, such as OH radicals and excited triplet-state HS (3HS*), which promote the degradation of organic compounds. We use phthalate esters, which are important contaminants found in wastewaters, landfills, soils, rivers, lakes, groundwaters, and mine tailings. We use phthalate esters as probes to study the reactivity of HS irradiated with artificial sunlight. Phthalate esters with different side-chain lengths were used as probes for elucidation of reaction mechanisms using 2H and 13C isotope fractionation. Reference experiments with the artificial photosensitizers 4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein (Rose Bengal), 3-methoxy-acetophenone (3-MAP), and 4-methoxybenzaldehyde (4-MBA) yielded characteristic fractionation factors (-4 ± 1, -4 ± 2, and -4 ± 1‰ for 2H; 0.7 ± 0.2, 1.0 ± 0.4, and 0.8 ± 0.2‰ for 13C), allowing interpretation of reaction mechanisms of humic substances with phthalate esters. The correlation of 2H and 13C fractions can be used diagnostically to determine photosensitized reactions in the environment and to differentiate among biodegradation, hydrolysis, and photosensitized HS reaction.
Keywords: humic substances; isotope fractionation; photodegradation; photosensitization; phthalate esters; radical reactions.