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 C-labeled sludge and a stable carbon isotope tracing approach. Aerobic digestion experiments with C-labeled activated sludge, with and without polyethylene (PE) MPs, were conducted, followed by sludge conditioning treatments (Fenton oxidation, hydrothermal, and KMnO 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 C depletion in the biomass and delayed C 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 KMnO 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.