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 (ε_C) for dimethyl phthalate (DMP) and diethyl phthalate (DEP) degradation in UV/H₂O₂ (•OH-mediated) and UV/K₂S₂O₈ (SO₄⁻•-mediated) systems. First, radical-specific ε_C 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 ε_C values remained statistically invariant in deionized water, establishing the robustness of CSIA for mixed degradation. Finally, the ε_C 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 ε_C variability. Since the calculation of degradation extent relies on deionized-water ε_C, absolute error introduced by matrix-specific ε_C reaches approximately 20±2.3%. Accordingly, in-situ natural water conditions should be prioritized to determine site-specific ε_C 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.