Source-specific quantification of microplastic-derived dissolved organic matter in sludge liquor using δ¹³C-labeled sludge and isotopic fingerprinting under varying conditioning treatments

Microplastics (MPs) have emerged as persistent pollutants in wastewater treatment plants (WWTPs), accumulating in sludge and releasing microplastic-derived dissolved organic matter (MP-DOM) during sludge processing. This study presents the first source-specific quantification of MP-DOM in sludge liquor using ¹³C-labeled sludge and a stable carbon isotope tracing approach. Aerobic digestion experiments with ¹³C-labeled activated sludge, with and without polyethylene (PE) MPs, were conducted, followed by sludge conditioning treatments (Fenton oxidation, hydrothermal, and KMnO₄ oxidation). MP addition significantly increased dissolved organic carbon (DOC) release and shifted DOM quality toward more labile, protein-like fractions. Stable isotope analysis showed slower ¹³C depletion in the biomass and delayed ¹³C enrichment in the liquid phase of MP-amended systems compared to control, suggesting a possible retarding effect of MPs on microbial carbon turnover. Using a two-end-member isotope mixing model, MP-DOM was found to contribute 11.5% of DOC in untreated sludge liquor, 19.4% under Fenton oxidation, 13.5% under KMnO₄ oxidation, and 5.6% under hydrothermal treatment. These results suggest that oxidative treatments, particularly Fenton, promote MP-DOM mobilization, while hydrothermal treatment primarily enhances biomass-derived DOM release. Conventional techniques using optical and molecular weight measurements captured overall DOM transformation but lacked source specificity. This study demonstrates the utility of isotope-based source apportionment for identifying hidden contributions of MPs to sludge liquor DOM and underscores the implications for treatment performance, carbon cycling, and sludge reuse strategies in WWTPs.