Concentration-dependent transfer and phytotoxicity of iodine and rhenium in lettuce (Lactuca sativa L.) with implications for nuclear waste risk assessment

The long-term geological disposal of radioactive waste necessitates robust risk assessment models to accurately predict radionuclide transfer from contaminated growing media into the human food chain. Iodine-129 ( 129 I) and the rhenium isotopes 186 Re and 188 Re are of significant regulatory concern due to their anionic mobility, environmental persistence, and high bioavailability. However, current risk assessment frameworks typically rely on the assumption of constant, concentration-independent transfer factors (TFs), a foundational premise that remains largely untested across gradients spanning trace levels to acute phytotoxicity. Through controlled greenhouse experiments, we quantified the concentration-dependent uptake and physiological responses of iodine (0, 50, 100, 200 mg·kg -1 ) and rhenium (0, 5, 10, 20 mg·kg -1 ) in lettuce ( Lactuca sativa L.). Iodine reached a lethal threshold at 200 mg·kg -1 (EC 50 : 58.9 mg·kg -1 ), following a classical monotonic response. Conversely, rhenium exhibited significantly higher phytotoxic potency (EC 50 : 19.7 mg·kg -1 ) and a distinct biphasic hormetic response. Concentrations of 5-10 mg·kg -1 stimulated shoot biomass and canopy density, whereas 20 mg·kg -1 induced severe growth suppression. Crucially, TFs for both elements were concentration-dependent, indicating that the constant-TF assumption does not hold under the present conditions. Rhenium demonstrated exceptional mobility with shoot-to-root ratios of 22-52, indicating rapid translocation to edible tissues. In addition, a scenario-based radiological assessment using literature-reported 129 I soil activities indicated low screening-level ingestion doses under the modeled conditions. These findings provide vital empirical parameters for refining predictive environmental models and establishing evidence-based soil screening levels supporting human health and radiological risk assessment of radionuclide transfer through crop systems.