Key Takeaways & Executive Findings
- •• • NOM type dictates aggregation: humic substances enhance electrostatic repulsion, while proteins and EPS can induce bridging flocculation, with EPS from marine diatom showing significant aggregation of nanoplastics (reference [34]). • • Ionic strength and ion type critically modulate NOM effects: for example, NO3− and SO4^2− influence aggregation kinetics of CeO2 nanoparticles (reference [30]), and specific ion effects alter colloidal stability, impacting MP transport in porous media. • • NOM accelerates photochemical aging of MPs, increasing surface oxidation and hydrophilicity, as demonstrated by enhanced aggregation and oxidation of nanoplastics under simulated sunlight (reference [36]). • • Protein corona formation on nanoplastics significantly alters their transport in seawater-saturated porous media (reference [31]), indicating that NOM components can either enhance or inhibit mobility depending on surface interactions.
Abstract
Microplastics (MPs) are persistent emerging contaminants ubiquitously distributed in soil-groundwater environments, where their aggregation and transport critically govern pollutant fate and ecological risks. Natural organic matter (NOM), a complex assemblage of organic compounds, interacts with MPs and porous media via hydrogen bonding, π-π interactions, hydrophobic effects, and electrostatic binding, thereby modulating MP surface properties and environmental behavior. This review systematically synthesizes the mechanisms by which NOM influences MP aggregation and transport, with emphasis on the distinct roles of humic substances, proteins, and extracellular polymeric substances (EPS), and their synergistic modulation with solution chemistry (pH, ionic strength, ion type). Additionally, NOM accelerates MP aging and alters surface characteristics, consequently impacting transport capacity. Current research limitations are identified, and future directions are proposed to inform MP pollution risk assessment and management strategies. Key findings indicate that NOM generally enhances MP stability and mobility at low ionic strengths, while high ionic strengths may induce aggregation depending on NOM type and ion valence. Humic substances predominantly increase electrostatic repulsion, whereas proteins and EPS can bridge particles, promoting aggregation. Aging processes, accelerated by NOM photochemical activity, increase surface oxygen functionality and hydrophilicity, further altering transport. The review underscores the need for systematic studies under environmentally relevant conditions to predict MP fate accurately.
1. Introduction
Microplastic contamination of soil and groundwater systems poses a persistent and escalating threat to ecosystem health and water security. The sheer volume of plastic waste entering terrestrial environments—via sewage irrigation, sludge amendment, agricultural film use, and atmospheric deposition—has led to the accumulation of microplastics (MPs) in soils, where they can migrate vertically and horizontally, contaminating aquifers and surface waters. The environmental fate of MPs is governed by their aggregation and transport behaviors, which are in turn modulated by interactions with natural organic matter (NOM), a ubiquitous and chemically heterogeneous mixture of humic substances, proteins, polysaccharides, and other biopolymers. Despite the recognized importance of NOM, existing commercial and regulatory frameworks for MP risk assessment largely overlook these interactions, relying on oversimplified models that treat MPs as inert particles. This oversight stems from a lack of mechanistic understanding of how NOM composition and solution chemistry jointly control MP colloidal stability and mobility, leading to inaccurate predictions of MP transport distances and retention in subsurface environments.
This review addresses this critical knowledge gap by systematically analyzing the current body of research on NOM–MP interactions, with a focus on the distinct roles of humic substances, proteins, and extracellular polymeric substances (EPS) in mediating aggregation and transport. We synthesize evidence from controlled laboratory studies that quantify the effects of pH, ionic strength, and ion type on these processes, and we examine how NOM-induced aging alters MP surface properties and fate. By integrating findings from studies such as those on protein corona formation (Dong et al., 2020) and EPS-mediated agglomeration (Summers et al., 2018), we provide a mechanistic framework that can inform more accurate predictive models and risk assessment protocols. This work is intended to bridge the gap between fundamental colloid science and practical environmental management, offering actionable insights for engineers and policymakers tasked with mitigating MP pollution in soil-groundwater systems.
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ZHAN Feiyu, XING Weiqin, LI Yanru, CHU Xianxian, DU Zhimin, LI Tiantian (2026). Mechanisms of Natural Organic Matter in Regulating Microplastic Aggregation and Transport in Soil-Groundwater Systems: A Review. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2024112806
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Frequently Asked Questions
What are the dominant mechanisms by which humic substances versus proteins influence microplastic aggregation under varying ionic strengths?
Humic substances typically adsorb onto microplastic surfaces, increasing electrostatic repulsion and steric hindrance, thereby enhancing stability and reducing aggregation at low ionic strengths. In contrast, proteins and extracellular polymeric substances (EPS) can form bridges between particles, promoting aggregation even at moderate ionic strengths. For example, EPS from marine diatoms significantly agglomerated nanoplastics (Summers et al., 2018). The effect is ion-specific: divalent cations like Ca2+ can cross-link negatively charged NOM, enhancing aggregation, while monovalent ions may have less pronounced effects.
How does NOM-induced photochemical aging alter microplastic surface properties and subsequent transport behavior?
NOM, particularly chromophoric dissolved organic matter, generates reactive oxygen species under sunlight, accelerating the oxidation of microplastic surfaces. This increases oxygen-containing functional groups (e.g., carboxyl, hydroxyl), enhancing hydrophilicity and negative surface charge. As a result, aged microplastics exhibit greater colloidal stability and mobility in porous media, as observed in studies where dissolved organic matter enhanced aggregation and oxidation of nanoplastics under simulated sunlight (Zhang et al., 2022). This aging effect can significantly increase the transport distance of microplastics in soil and groundwater.
What is the role of protein corona formation on nanoplastics in seawater-saturated porous media, and how does it affect transport?
Protein corona formation on nanoplastics significantly alters their surface properties, including charge and hydrophobicity, which in turn influences their aggregation and deposition in porous media. Dong et al. (2020) demonstrated that protein corona-mediated transport of nanoplastics in seawater-saturated columns resulted in enhanced mobility compared to pristine particles, likely due to increased electrostatic repulsion and steric stabilization. This suggests that in organic-rich environments, nanoplastics may travel farther, posing a greater risk to groundwater.
How do specific anions such as NO3− and SO4^2− affect the aggregation kinetics of nanoparticles in the presence of NOM?
Specific anions can influence aggregation kinetics by altering the electric double layer and ion-specific effects. Song et al. (2020) studied the role of NO3− and SO4^2− on CeO2 nanoparticle aggregation and found that these anions, in combination with extracellular polymeric substances, modulated aggregation rates. Generally, divalent anions like SO4^2− can compress the double layer more effectively than monovalent NO3−, potentially reducing electrostatic repulsion and promoting aggregation, but the presence of NOM can mitigate this by providing steric hindrance. The net effect depends on the balance between ionic strength and NOM concentration.
What are the key limitations in current research on NOM-microplastic interactions that hinder accurate risk assessment?
Current research often uses idealized conditions (e.g., single NOM types, synthetic microplastics, controlled solution chemistry) that do not reflect the complexity of natural soil-groundwater systems. There is a lack of studies using environmentally relevant concentrations and mixtures of NOM, as well as aged microplastics. Additionally, most studies focus on short-term aggregation or transport experiments, neglecting long-term transformations and the influence of biofilm formation. These limitations lead to uncertainties in predicting microplastic fate and transport, underscoring the need for more realistic experimental designs and field validation.
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