Behavior of stable carbon isotope of phthalate acid esters during photolysis under ultraviolet irradiation

The photolysis of three phthalic acid esters (PAEs) (dimethyl (DMP), di-n-butyl (DBP), and di-n-octyl (DOP) phthalates) under ultraviolet (UV) irradiation at 254 nm in laboratory experiments was investigated by gas chromatography coupled with isotope ratio mass spectrometry through a combustion interface (GC-C-IRMS). The degradation processes of DMP, DBP and DOP were well described by a first-order kinetic, with rate constants of 0.02636, 0.1005 and 0.958 h−1 for DMP, DBP and DOP, respectively, indicating that the photolysis rate of PAEs is related to the number of carbon atoms in molecule. The results of TOC analysis indicated that PAEs could not be completely mineralized under UV irradiation. Stable carbon isotope fractionation of the three PAEs produced during photolysis was evaluated with compound-specific isotope analysis (CSIA). Pronounced 13C-enrichment, with maximum δ13C shifts of Δδ13CDMP = 10.04 ± 0.13‰ (f = 0.09), Δδ13CDBP = 7.4 ± 0.19‰ (f = 0.06) and Δδ13CDOP = 2.9 ± 0.17‰ (f = 0.25) in the residual DMP, DBP and DOP, respectively, were clearly a direct evidence for photolysis of three PAEs. The order of stable carbon isotope fractionation of the three PAEs during photolysis, DMP > DBP > DOP, is an inverse function of the number of carbon atoms in molecule. The kinetic isotope effects (KIE) values, from 1.0018 to 1.0045 for the three PAEs, were consistent with the KIE values (1.00-1.03) of the C-O bond cleavage reported in literature.