Carbon isotope fractionation of di-(2-ethylhexyl)-phthalate during photosensitized degradation by •OH and [formula omitted] for characterization of reaction mechanisms

Abstract

Compound specific isotope analysis (CSIA) has been used to explore the degradation pathways of di-(2-ethylhexyl)-phthalate (DEHP) in photosensitization experiments with H2O2, TiO2, Fe(III) and PMS. The reaction kinetics could be described by first order rate constants [k (h−1)]. The highest degradation rates of DEHP were observed in UV/TiO2 catalyzed reactions.Carbon isotope fractionation factors (εC) and metabolite patterns were examined to analyze the radical reaction. Similar carbon isotope fractionation factors have been found in experiments dominated by OH radical reactions (UV/H2O2, −1.2 ± 0.3‰; UV/TiO2, −1.2 ± 0.5‰). An inverse carbon isotope fractionation has been found for UV/Fe(III) (0.8 ± 0.2‰) and SO4− (0.6 ± 0.2‰ at pH 3) catalyzed reactions indicating that these reactions preferentially taking place at the H atom bond at tert-butyl carbon of the side chain. This hypothesis is further supported by an experiment with isobutyl acetate (εC = 1.8 ± 0.2‰ for UV/Fe(III) at pH 7; εC = -1.1 ± 0.2‰ for UV/PMS at pH 6 and εC = 0.9 ± 0.3‰ for UV/PMS at pH 3).Quenching experiments in the UV/PMS catalyzed reaction indicated a coexistence of SO4− and OH at pH 7. SO4− was the predominant radical at pH 3. OH was found to be the predominant radical in UV/H2O2 and UV/TiO2 reactions. CSIA indicated that the addition of OH to the aromatic ring of DEHP by the RAF pathway is the main reaction mechanism in UV/H2O2 and UV/TiO2 experiments. The SO4− attacks preferentially the C-H bond at the tert-butyl carbon position of the side chain of DEHP which causes an inverse carbon isotope effect. Thus, carbon isotope enrichment factors can be used to distinguish different radical reaction mechanisms with DEHP as a substrate.