Carbon and hydrogen isotopic fractionation during abiotic hydrolysis and aerobic biodegradation of phthalate esters

We systematically investigated the changes of carbon and hydrogen isotope signatures of three phthalate esters (PAEs) during (i) abiotic hydrolysis over the pH range of 2, 7 and 10, and (ii) aerobic biodegradation initiated by hydrolysis by Rhodococcus opacus strain DSM 43250. Significant carbon isotopic fractionation was exhibited under all investigated conditions. Hydrogen isotopic fractionation was observed in some experiments and is hypothesized to be a secondary isotope effect due to the absence of a hydrogen bond cleavage during hydrolysis. Dual stable isotope analysis (Lambda = Delta delta H-2/Delta delta C-13) resulting from abiotic hydrolysis and aerobic biodegradation showed similar magnitudes for dimethyl phthalate (DMP) and diethyl phthalate (DEP). The calculated carbon apparent kinetic isotope effects (AKIE(C)) for the hydrolytic pathway (C-O bond cleavage) of PAEs fall within an expected range of 1.03-1.09, with the exception of lower AKIE(C) values for dibutyl phthalate (DBP) during hydrolysis at pH 2 and aerobic biodegradation. The lower AKIE(C) of DBP at pH 2 and aerobic biodegradation is likely related to a transition state from reactant-like to tetrahedral intermediate-like structure. Abiotic and biotic hydrolysis of PAEs resulted in similar AKIE(C) and Lambda values due to the C-O bond cleavage pathway, indicating the potential of dual isotope analysis to detect and quantify hydrolytic processes of PAEs in the environment. The pronounced primary carbon and typically low secondary or absent hydrogen isotopic fractionation might form a typical pattern to identify the hydrolytic reaction of PAEs in the environment. The characteristic Lambda values of the hydrolytic reaction were different from Lambda values of chemical oxidation of PAEs and showed diagnostic potential of dual H-C isotope analysis to analyze reactions. (C) 2019 Elsevier B.V. All rights reserved.