Chemical Characteristics of Zirconium Chloride and Zirconium Oxide Nanoparticles Driving Toxicity on Lemna minor

The increasing global production and utilization of zirconium (Zr) compounds, including zirconium chloride (ZrCl4) and zirconium oxide nanoparticles (NPs-ZrO2), raises concerns about their potential environmental impact. This study investigated the toxicity mechanisms of ZrCl4 and NPs-ZrO2 on the aquatic plant Lemna minor. The physicochemical properties of NPs-ZrO2 in the test medium were characterized, revealing concentration-dependent changes in the hydrodynamic diameter, zeta potential, and solubility over time. The analysis of Zr speciation showed the predominance of Zr(OH)4(aq) species from ZrCl4. Plants of L. minor exposed to ZrCl4 and NPs-ZrO2 exhibited differential Zr bioaccumulation, growth inhibition, oxidative stress, and antioxidant responses. ZrCl4 induced a higher toxicity than NPs-ZrO2, with bioaccumulation strongly correlating with adverse effects. The differential toxicity impact between these two Zr-compounds was also determined by the lowest observed-effect doses for growth and biochemical parameters. The scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy confirmed internalization of NPs-ZrO2 and Zr uptake in the L. minor plant. Therefore, these findings highlighted the importance of chemical speciation, environmental transformations, and biological responses in assessing the ecological impact of Zr-compounds for effective risk assessment and management strategies for protecting aquatic ecosystems.