Characterization of radical-mediated degradation of phthalate esters in natural waters using carbon isotope fractionation.

Compound-specific stable isotope analysis (CSIA) has emerged as a promising tool for tracking organic pollutant transformation, current approaches predominantly limited to individual compounds in deionized water systems, overlooking critical environmental complexities. This study investigated the characteristics of natural aqueous matrices on the variability of carbon isotopic enrichment factors (εC) for dimethyl phthalate (DMP) and diethyl phthalate (DEP) degradation in UV/H2O2 (•OH-mediated) and UV/K2S2O8 (SO4⁻•-mediated) systems. First, radical-specific εC values were quantified, revealing significant differences (p<0.001) between •OH- (-2.4±0.1‰ for DMP; -1.6±0.1‰ for DEP) and SO4⁻•-mediated degradation (-1.9±0.1‰ for DMP; -1.3±0.1‰ for DEP). Mechanistic analysis through apparent kinetic isotope effects and intermediate identification demonstrated preferential irreversible C-H bond cleavage at both aromatic rings and aliphatic chains. Furthermore, competitive UV absorption in the mixed systems reduced degradation rates to 2-3 fold by reducing the radical generation rate; however, the εC values remained statistically invariant in deionized water, establishing the robustness of CSIA for mixed degradation. Finally, the εC values varied distinctly in natural seawater matrices. This variation was closely linked to indigenous dissolved organic matter (DOM), which has been verified in matched datasets, indicating that DOM acts as a critical factor driving εC variability. Since the calculation of degradation extent relies on deionized-water εC, absolute error introduced by matrix-specific εC reaches approximately 20±2.3%. Accordingly, in-situ natural water conditions should be prioritized to determine site-specific εC and achieve accurate degradation extent assessment. This work provides novel insights into evaluating the reliability of extrapolating laboratory-derived ε values to complex field conditions.