Tracking microplastic-derived dissolved organic matter in the adsorption of its mixtures with natural organic matter via end-member mixing analysis.

Microplastic-derived dissolved organic matter (MP-DOM) has emerged as an environmental concern due to its co-existence with, and compositional similarities to, aquatic natural organic matter (NOM), potentially influencing adsorption-based environmental processes. This study evaluated the use of fluorescence-based indices to discriminate MP-DOM from NOM in mixtures during adsorption onto kaolinite and granular activated carbon (GAC), and to quantify MP-DOM contributions using end-member mixing analysis (EMMA). Individually, aquatic NOM exhibited higher adsorption (53.5 ± 0.2 %) than MP-DOM (30.1 ± 0.5 % for PE-DOM and 35.8 ± 1.0 % for PLA-DOM) on kaolinite. Similar trends were observed on GAC, with NOM adsorption reaching 70.3 ± 1.2 % compared to 51.7 ± 1.0 % (PE-DOM) and 60.2 ± 1.5 % (PLA-DOM). In mixtures, non-linear adsorption patterns suggested steric or hindrance effects influencing NOM adsorption. Among the tested optical indices, the log-transformed (H + P)/L fluorescence ratio was the most effective for MP-DOM discrimination during adsorption and was applied to quantify MP-DOM contribution via EMMA. Results revealed non-ideal mixing behaviors, driven by synergistic effects, that varied depending on adsorbent type and mixing ratio. Kaolinite exhibited enhanced MP-DOM adsorption at 20-50 % MP-DOM content, likely due to surface functionalization by NOM, which promoted hydrogen bonding and ligand exchange. In contrast, MP-DOM adsorption on GAC declined with increasing MP-DOM content, dropping to 12.1 ± 0.5 % (PE-DOM) and 28.2 ± 0.7 % (PLA-DOM) at 75 % MP-DOM, likely due to pore blockage by preferentially adsorbed NOM. These findings validate the use of fluorescence-based tracer as cost-effective and reliable tools for MP-DOM tracking in adsorption studies and highlight distinct competitive interactions across adsorbents. Further research is recommended to validate these findings under varied environmental conditions and with alternative source-tracking tools, such as stable isotope-labeled DOM.