Dissolved organic matter proxies for tracing microplastics and phthalates in urbanized rivers: Fluorescence and molecular insights

Widely distributed in aquatic environments, emerging contaminants including microplastics (MPs) and phthalates (PAEs) necessitate efficient tracing proxies. We propose dissolved organic matter (DOM) proxies as effective indicators of the occurrence and potential anthropogenic effects of MPs and PAEs in the Haihe River (China), with validation in multiple global urbanized rivers. Fluorescence subtraction revealed that wastewater discharges increased protein-like DOM (C2) while decreased humic-like DOM (C1), establishing C2/C1 ratio as a universal wastewater proxy. Principal component analysis demonstrated two major sources of DOM: terrestrial-sourced DOM (i.e., surface runoff) and anthropogenic-sourced DOM (i.e., wastewater discharge). DOM proxies (C2/C1, C2%, and BIX) were well correlated with MPs and PAEs. Molecular-level analyses showed elevated levels of sulfur-containing compounds/aliphatics and reduced carboxyl-rich alicyclic molecules (CRAM) downstream of wastewater outlets. MPs and low-molecular-weight PAEs were closely associated with environments rich in protein and aliphatic compounds and poor in humic-like substances (with high C2/C1 and low HIX), suggesting higher ecological risks. High-molecular-weight PAEs were associated with environments rich in humified compounds. Multivariate linear regression confirmed significantly positive correlations (p < 0.001) between C2/C1 and both MPs and low-molecular-weight PAEs, which were validated across multiple global urbanized rivers. Overall, this work identified DOM proxies (particularly C2/C1) as reliable and effective tracers of plastic-derived contaminants (MPs and PAEs), providing a low-cost screening tool for ecological risk assessment and targeted monitoring in urbanized rivers.