Calculation and application of the selenium toxicity coefficient for potential ecological risk assessment.

To address the lack of a unified quantitative index for the ecological risk assessment of selenium (Se) in soils, the toxicity coefficient of Se was first determined based on Håkanson's principles, and the spatial distribution and potential ecological risk of Se in surface soils across China were systematically evaluated. The results showed that the toxicity coefficient of Se was determined as 2. Based on the corrected background reference value, approximately 69.9% of the sampling sites were classified as low to moderate ecological risk, 24.4% as considerable risk, and 5.7% as high or very high risk. The elevated risk sites were predominantly confined to localized areas underlain by Se-rich strata, such as Enshi in Hubei Province and Ziyang in Shaanxi Province. A substantial consistency was observed between the potential ecological risk factor (Eri) and the geoaccumulation index (Igeo), with a quadratic weighted Kappa coefficient (κw) of 0.729, indicating that the two classification systems yield comparable results in terms of risk ranking. This study provides a preliminary screening benchmark for identifying areas that warrant further site-specific investigation.