Overlooked role of aged cationic natural organic matter in aquatic microplastics aggregation-sedimentation

Environmental behavior of microplastics (MPs) is critically modulated by natural organic matter (NOM), yet the role of aged cationic biopolymers like chitosan (CTS) remains less understood. Herein, we demonstrate the unparalleled ability of aged-CTS to drive aggregation-sedimentation of two types of MPs: conventional polystyrene (PS) and biodegradable polymethyl methacrylate (PMMA), challenging the paradigm of NOM as omnipresent stabilizers. CTS exhibited strong aggregation-sedimentation for both MPs while metal cations failed to induce PMMA aggregation. Remarkably, the optimal aged-CTS achieved a greater MPs aggregation capacity of 5.0 g/g without elevating total organic carbon or leaching ecotoxic by-products. The binding of aged-CTS and MPs was primarily attributed to electrostatic interactions, van der Waals forces, and hydrogen bonding. −NH₂ group on the surface of aged-CTS can be protonated to −NH₃⁺, thereby strengthening electrostatic interaction. These groups exhibit selective affinity toward the O=C−O group of both PMMA and its hydrolyzed derivatives through hydrogen bonding (-0.05 a.u. < δ_g < -0.02 a.u.), which surpasses the π-hydrogen bonding effect observed in PS-CTS. While over 90 % MPs removal was achieved in pure suspensions, the complex biofilm in real water matrices inevitably reduced aged-CTS flocculation efficiency, highlighting the environment-dependent challenges of NOM-mediated remediation. This work redefines aged NOM's role in MPs environmental fate, proposing aged CTS as a nature-inspired coagulant for sustainable MPs management that could reduce MPs’ threat.