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 (Λ = ΔδH/ΔδC) 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) for the hydrolytic pathway (CO bond cleavage) of PAEs fall within an expected range of 1.03-1.09, with the exception of lower AKIE values for dibutyl phthalate (DBP) during hydrolysis at pH 2 and aerobic biodegradation. The lower AKIE 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 and Λ values due to the CO 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 Λ values of the hydrolytic reaction were different from Λ values of chemical oxidation of PAEs and showed diagnostic potential of dual HC isotope analysis to analyze reactions.