Key Takeaways & Executive Findings
- •• • Aging of PP microplastics increased maximum adsorption capacity for both SMX and Cr(VI) by 2–3 times, with aged PP showing ~30-fold higher adsorption for SMX than Cr(VI), indicating a strong affinity difference that must be considered in risk assessments. • • Mechanistic analysis revealed that SMX adsorption on aged PP is governed by hydrogen bonding and electrostatic interactions (intermolecular forces), whereas Cr(VI) adsorption relies on electrostatic interactions and pore-filling; the stronger intermolecular forces for SMX explain its higher adsorption capacity. • • Adsorption of both pollutants decreased with increasing pH due to electrostatic repulsion; high concentrations of Na+ and Mg2+ enhanced Cr(VI) adsorption via charge shielding but inhibited SMX adsorption by competing for active sites, demonstrating ion-specific effects. • • The presence of humic acid (organic matter) did not significantly affect Cr(VI) adsorption but reduced SMX adsorption, likely due to complexation between SMX and organic matter, highlighting the need to consider natural organic matter in predicting MP-facilitated transport.
Abstract
Microplastics (MPs) act as vectors for co-migrating antibiotics and heavy metals, forming complex pollution systems with potential joint toxicity. However, the differential adsorption behaviors and underlying mechanisms of MPs toward organic versus inorganic pollutants remain insufficiently understood. This study selected polypropylene (PP) microplastics, a major component of agricultural plastic films, and investigated the adsorption of sulfamethoxazole (SMX) and Cr(VI) onto aged PP under varying environmental conditions. Results demonstrated that aging increased the maximum adsorption capacity by 2–3 times for both pollutants. Notably, aged PP exhibited approximately 30 times higher adsorption capacity for SMX than for Cr(VI). Characterization revealed that aging introduced oxygen-containing functional groups (e.g., carbonyl) on the MP surface, enhancing adsorption. Mechanistic analysis indicated that hydrogen bonding and electrostatic interactions dominated SMX adsorption, while Cr(VI) adsorption was primarily governed by electrostatic interactions and pore-filling. The stronger intermolecular forces for SMX compared to reversible pore-filling for Cr(VI) explained the observed differences. Increasing pH induced electrostatic repulsion, reducing adsorption of both pollutants. High concentrations of Na+ and Mg2+ caused charge shielding, potentially enhancing Cr(VI) adsorption but inhibiting SMX adsorption due to competition for active sites. The presence of organic matter (humic acid) had negligible effects on Cr(VI) adsorption but reduced SMX adsorption, likely due to complexation. These findings elucidate distinct molecular-level pathways for organic versus inorganic pollutant adsorption on aged MPs, highlighting the roles of hydrogen bonding and pore-filling in driving differential behaviors.
1. Introduction
The pervasive presence of microplastics (MPs) in terrestrial and aquatic ecosystems has raised concerns about their role as vectors for co-contaminants, particularly antibiotics and heavy metals. Agricultural plastic films, predominantly composed of polypropylene (PP), undergo environmental weathering, leading to the formation of aged MPs with altered surface properties. While pristine MPs exhibit limited adsorption capacities, aging processes introduce oxygen-containing functional groups, enhancing their affinity for pollutants. However, the differential adsorption behaviors of aged MPs toward organic versus inorganic contaminants remain poorly characterized, hindering accurate risk assessment of complex pollution scenarios.
This study addresses this knowledge gap by systematically comparing the adsorption of sulfamethoxazole (SMX), a widely used antibiotic, and hexavalent chromium (Cr(VI)), a toxic heavy metal, onto aged PP microplastics. By employing controlled laboratory experiments and spectroscopic characterization, we elucidate the dominant adsorption mechanisms—hydrogen bonding and electrostatic interactions for SMX versus pore-filling and electrostatic interactions for Cr(VI)—and quantify the influence of environmental factors such as pH, ionic strength, and organic matter. These findings provide critical insights into the carrier effects of aged MPs, informing future research on the environmental fate and transport of co-contaminants.
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CHEN Lin, ZHOU Ying, LIU Lin, LIU Yonglin, ZHAO Erling, LI Debao, ZHANG Yaru, HAN Xiaoyan, WANG Weiliang (2026). Adsorption Pathways and Differential Mechanisms of Typical Organic/Inorganic Pollutants on Microplastics: A Case Study of Sulfamethoxazole and Cr(VI) on Aged Polypropylene. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025030403
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Frequently Asked Questions
What is the quantitative impact of aging on the adsorption capacity of PP microplastics for SMX and Cr(VI)?
Aging increased the maximum adsorption capacity by 2–3 times for both pollutants. Specifically, aged PP exhibited a maximum adsorption capacity for SMX that was approximately 30 times higher than that for Cr(VI), indicating a strong selectivity for organic pollutants.
What are the dominant adsorption mechanisms for SMX versus Cr(VI) on aged PP, and how do they explain the observed differences?
SMX adsorption is primarily governed by hydrogen bonding and electrostatic interactions, which are stronger intermolecular forces. In contrast, Cr(VI) adsorption relies on electrostatic interactions and pore-filling, which are reversible and weaker. This mechanistic difference accounts for the significantly higher adsorption of SMX compared to Cr(VI).
How do environmental factors such as pH, ionic strength, and organic matter affect the adsorption of SMX and Cr(VI) on aged PP?
Increasing pH reduces adsorption of both pollutants due to electrostatic repulsion. High concentrations of Na+ and Mg2+ enhance Cr(VI) adsorption via charge shielding but inhibit SMX adsorption by competing for active sites. The presence of humic acid does not significantly affect Cr(VI) adsorption but reduces SMX adsorption, likely due to complexation.
What are the implications of these findings for assessing the environmental risks of microplastics as pollutant carriers?
The differential adsorption behaviors imply that aged MPs may preferentially transport organic antibiotics over heavy metals, potentially increasing the bioavailability and toxicity of antibiotics in aquatic environments. Risk assessments should account for the type of pollutant and the aging state of MPs to accurately predict their carrier effects.
What future research directions are suggested by this study?
Future research should focus on nano-sized MPs, which may exhibit different adsorption behaviors due to higher surface area and reactivity. Additionally, studies should investigate the effects of natural aging processes (e.g., biofilm formation, mineral deposition) on MP surface properties, and explore the cross-media transport of MP-pollutant complexes through food chains.
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