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.

中文翻译：

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老化阳离子天然有机物在水生微塑料聚集-沉淀中被忽视的作用

微塑料 （MP） 的环境行为受到天然有机物 （NOM） 的严重调节，但壳聚糖 （CTS） 等老化阳离子生物聚合物的作用仍然知之甚少。在此，我们证明了老化 CTS 驱动两种 MP 聚集-沉降的无与伦比的能力：传统聚苯乙烯 （PS） 和可生物降解的聚甲基丙烯酸甲酯 （PMMA），挑战了 NOM 作为无处不在的稳定剂的范式。CTS 对两种 MPS 都表现出强烈的聚集沉降，而金属阳离子未能诱导 PMMA 聚集。值得注意的是，最佳的老化 CTS 实现了更高的 MPs 聚集能力，达到 5.0 g/g，而不会提高总有机碳或浸出生态毒性副产物。老化的 CTS 和 MPS 的结合主要归因于静电相互作用、范德华力和氢键。老化 CTS 表面的 -NH₂ 基团可以质子化为 -NH₃⁺，从而加强静电相互作用。这些基团通过氢键 （-0.05 a.u. < δ_g < -0.02 a.u.） 对 PMMA 及其水解衍生物的 O=C-O 基团表现出选择性亲和力，这超过了在 PS-CTS 中观察到的 π-氢键效应。虽然在纯悬浮液中实现了超过 90% 的 MP 去除，但真实水基质中的复杂生物膜不可避免地降低了老化的 CTS 絮凝效率，凸显了 NOM 介导的修复所面临的环境依赖性挑战。这项工作重新定义了老年 NOM 在 MP 环境命运中的作用，提出老年 CTS 作为一种受自然启发的凝结剂，用于可持续的 MP 管理，以减少 MP 的威胁。