Key Takeaways & Executive Findings
- •• • Aging PE microplastics for 600 h increased SMX photodegradation from 34% (virgin PE) to 77%, a 2.26-fold enhancement, demonstrating that aging-induced surface changes significantly boost indirect photolysis efficiency. • • The pseudo-first-order rate constant (kobs) for SMX photodegradation rose from 0.066 h−1 to 0.224 h−1 in the presence of aged PE, a 3.4-fold increase, indicating faster removal kinetics critical for water treatment design. • • Radical quenching and EPR confirmed that aged MPs generate ROS (·OH, 1O2, O2·−), which are the primary drivers of enhanced SMX photolysis, highlighting the role of surface reactive sites in pollutant degradation. • • DFT and LC-MS identified the benzene ring, five-membered heterocycle, and sulfonyl group as main attack sites, with p-aminobenzenesulfonamide as a key product, providing molecular-level understanding for predicting degradation pathways and toxicity.
Abstract
Sulfamethoxazole (SMX) and microplastics (MPs) are ubiquitous co-existing pollutants in aquatic environments. This study investigated the effects of polyethylene (PE), polypropylene (PP), and polystyrene (PS) MPs with varying aging degrees on the photodegradation of SMX. In the absence of MPs, SMX photodegradation was only 28%, while the presence of virgin PE increased it to 34%. Aging PE for 200, 400, and 600 h further enhanced degradation to 46%, 56%, and 77%, respectively. Pseudo-first-order kinetics showed that the rate constant (kobs) increased from 0.066 h−1 to 0.224 h−1 with aged PE. Aged MPs generated more reactive oxygen species (ROS) under irradiation, including hydroxyl radicals (·OH), singlet oxygen (1O2), and superoxide anions (O2·−), as confirmed by radical quenching and EPR analysis. Density functional theory identified the benzene ring, five-membered heterocycle, and sulfonyl group as primary ·OH attack sites. LC-MS analysis revealed degradation products such as p-aminobenzenesulfonamide, indicating both direct and indirect photolysis pathways. This work provides mechanistic insights into antibiotic-MP interactions and informs strategies for managing co-existing pollutants.
1. Introduction
Microplastics (MPs) and antibiotics like sulfamethoxazole (SMX) are pervasive co-contaminants in aquatic systems, yet their interactive photochemical behavior remains poorly understood. Conventional water treatment processes often fail to completely remove these micropollutants, and the role of MPs in altering antibiotic fate is frequently overlooked. Existing studies have focused on pristine MPs, but environmental weathering inevitably changes their surface chemistry and reactivity. This study addresses the critical gap by systematically investigating how aging of PE, PP, and PS MPs influences SMX photodegradation, providing mechanistic insights into ROS-mediated pathways.
The experimental protocol quantifies the enhancement of SMX photolysis by aged MPs, linking surface oxidation to increased ROS generation. By combining kinetic analysis, radical identification, and computational chemistry, the research identifies key reactive sites and degradation products. These findings are essential for predicting the environmental persistence of antibiotics in MP-contaminated waters and for developing advanced oxidation processes that leverage natural sunlight. The results also underscore the need to consider MP aging in environmental risk assessments and remediation strategies.
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WANG Qiongjie, WANG Shurui, HAO Zijing, WANG Jiaming, HU Yi (2026). Effect of Aged Microplastics on Photodegradation Behavior of Sulfamethoxazole. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2024112603
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Frequently Asked Questions
What is the quantitative impact of PE aging duration on SMX photodegradation rate?
Aging PE for 200, 400, and 600 h increased SMX photodegradation from 34% (virgin) to 46%, 56%, and 77%, respectively. The pseudo-first-order rate constant (kobs) rose from 0.066 h−1 to 0.224 h−1, indicating a 3.4-fold acceleration after 600 h aging.
Which reactive oxygen species are responsible for the enhanced photolysis, and how were they identified?
Radical quenching experiments and EPR analysis identified hydroxyl radicals (·OH), singlet oxygen (1O2), and superoxide anions (O2·−) as key ROS. These species were generated on aged MP surfaces under light, with their roles confirmed by selective quenchers and EPR signals.
What are the primary molecular sites on SMX attacked by hydroxyl radicals?
Density functional theory (DFT) calculations indicated that ·OH radicals preferentially attack the benzene ring, the five-membered heterocycle, and the sulfonyl (—SO2—) group of SMX. These sites are electron-rich and susceptible to radical addition, leading to bond cleavage and formation of products like p-aminobenzenesulfonamide.
How do aged microplastics compare to virgin ones in promoting SMX degradation?
Aged PE microplastics significantly outperform virgin PE. While virgin PE increased SMX photodegradation from 28% (no MPs) to 34%, aged PE (600 h) achieved 77% degradation. This enhancement is attributed to increased surface roughness, oxygen-containing functional groups, and higher ROS generation capacity.
What are the implications for real-world water treatment and environmental risk assessment?
The findings suggest that aged MPs can accelerate antibiotic photodegradation, potentially reducing their persistence in sunlit surface waters. However, the generation of ROS and transformation products may pose unknown ecological risks. Water treatment systems should account for MP aging and co-contaminant interactions to optimize removal efficiency and assess toxicity.
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