Quantitative characteristics and multiple probabilistic risk assessment of small-sized microplastics in the middle and lower reaches of the Hanjiang River, China.

Microplastics (MPs) are widely distributed in inland river systems and pose significant threats to aquatic organisms. Small-sized MPs <500 μm may pose greater environmental risks to freshwater ecosystems, but are often overlooked or underestimated in previous surveys. This study employed laser direct infrared (LDIR) imaging technology to focus on the pollution characteristics and potential ecological risks of small-sized MPs (20-500 μm) in the surface waters of the middle and lower reaches of the Hanjiang River (MLHR). The results of LDIR analysis showed that the average number concentration of small-sized MPs in this region was as high as 7278 particles/L (1,900-16,200 particles/L), with an estimated average mass concentration of 2.867 mg/L (0.691-8.554 mg/L), exceeding traditional detection methods by 2-3 orders of magnitude. Morphologically, MPs in the 20-50 μm size group (averaging 64.7 %) and film-form MPs (averaging 60.7 %) overwhelmingly dominated, with polyamide (PA), rubber, and polyurethane (PU) being the most prevalent polymer types. Various correlation analyses confirmed that water temperature, total phosphorus, and conductivity had significant impacts on MPs abundance and polymer types, respectively. Combined with the environmental concentration of MPs and the collected acute and chronic toxicity data, multiple probabilistic analyses revealed that MPs posed a high chronic and acute ecological risk in the surface water of the MLHR, particularly when assessed based on mass concentrations. This study firstly realized multiple probabilistic ecological risk assessment based on the number and mass concentrations of MPs, providing a novel perspective for evaluating the pollution risks of small-sized MPs in aquatic environments.