ZnO nanoparticle-enabled PFNA immobilization in soil: Plant uptake suppression with measurable biological costs

This study investigated the effects of zinc oxide nanoparticles (ZnO NPs) on the fate, mobility, and biological implications of perfluorononanoic acid (PFNA) in a soil-plant system of alfalfa ( Medicago sativa L.). ZnO NPs, along with ionic zinc (Zn2 +), were applied at 10, 50, 70, and 100 mg kg −1 to PFNA (1 mg kg −1 )-contaminated soil to evaluate their effects on PFNA uptake, plant antioxidant defense responses, and soil biochemical indicators. Zinc amendments reduced PFNA accumulation in alfalfa tissues by over 80% for Zn²⁺ and approximately 60–70% for ZnO nanoparticles, demonstrating strong speciation-dependent immobilization efficiency. While Zn²⁺ achieved the highest PFNA suppression, high-dose applications (≥70 mg kg⁻¹) led to measurable biological costs, including declines in microbial biomass and elevated antioxidant enzyme activity. Antioxidant enzymes, including catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), and superoxide dismutase (SOD), exhibited zinc-dependent activation, with Zn 2+ inducing the most pronounced responses, reflecting its higher bioavailability. ZnO NP treatments produced comparable but more moderate enzyme responses, consistent with gradual zinc release. Soil microbial biomass and soil organic carbon (SOC) remained relatively stable at moderate zinc levels but declined under high Zn 2+ application, indicating increased biological cost at elevated ionic zinc concentrations. Overall, zinc amendments effectively reduced PFNA bioavailability; however, excessive zinc, particularly in ionic form, imposed greater biological disturbance. These findings highlight a critical trade-off between PFNA immobilization efficiency and ecological compatibility, suggesting that optimized zinc form and loading are essential for sustainable PFAS remediation in agricultural soils.