HFPO-DA is a more potent soil arylsulfatase inhibitor than PFOA: Mechanistic insights from catalytic kinetics to molecular binding

Background Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are persistent environmental contaminants of global concern. However, the soil ecotoxicity of emerging alternatives, such as hexafluoropropylene oxide dimer acid (HFPO-DA, a PFOA substitute), remains poorly understood. Objectives This study evaluated the ecotoxicological effects of a typical long-chain PFAS, perfluorooctanoic acid (PFOA), and its short-chain substitute, HFPO-DA, on diverse arylsulfatase (ARS) pools across eight soil types. The inhibitory mechanisms were further elucidated using integrated enzyme kinetics and molecular simulation approaches. Results The results showed that intracellular ARS (intra-ARS) dominated the total enzyme pool (54.44%–77.44%) and was positively correlated with microbial biomass carbon ( p  < 0.05). The catalytic efficiency of intra-ARS was the most sensitive to PFAS exposure, with HFPO-DA exhibiting higher toxicity, as reflected by lower ED 10 values (the concentration causing a 10% reduction in intra-ARS catalytic efficiency), ranging from 2.06 to 36.33 mg kg −1 . Enzyme kinetics analyses indicated that both PFAS primarily caused competitive inhibition. Notably, HFPO-DA displayed a lower inhibition constant ( K ic ) than PFOA, suggesting a stronger binding affinity to the enzyme. Molecular docking further supported this finding, with binding energies of −5.8 and − 6.6 kcal mol −1 for PFOA and HFPO-DA, respectively, indicating that HFPO-DA forms a more stable complex with ARS. Principal Component Analysis (PCA) revealed that variability in the toxicity threshold (ED 10 ) was mainly driven by soil organic matter, total nitrogen, clay content, and pH. Based on these factors, a multivariate regression model was developed that accurately predicted toxicity thresholds (adjusted R 2  = 0.914 and 0.842), demonstrating strong predictive capability. Conclusion These findings provide a critical theoretical basis from the perspective of soil enzymatic function for establishing scientifically sound safety thresholds for PFAS across different soil types and for optimizing remediation strategies for contaminated soils.