Nanospecific Phytotoxicity of CuO Nanoparticles in Soils Disappeared When Bioavailability Factors Were Considered

Bioavailability-modifying factors such as soil type and aging have only rarely been considered in assessing toxicity of metal-containing nanoparticles in soil. Here, we examined the toxicity to barley (Hordeum vulgare) of CuO nanoparticles (CuO-NPs) relative to CuO bulk particles (CuO-BPs) and Cu acetate (Cu(OAc)₂) in six different soils with or without aging. The set up allows identifying whether or not NPs-derived colloidal Cu in soil porewater contributes to toxicity. Ultrafiltration (50 kDa) was performed together with geochemical modeling to determine {Cu²⁺} (free Cu²⁺ activity in soil porewater). Based on total soil Cu concentration, toxicity measured with seedling root elongation ranked Cu(OAc)₂ > CuO-NPs > CuO-BPs in freshly spiked soils. The differences in toxicity among the three toxicants became smaller in soils aged for 90 days. When expressing toxicity as {Cu²⁺}, there was no indication that nanoparticulate or colloidal Cu enhanced toxicity. A calibrated bioavailability-based model based on {Cu²⁺} and pH successfully explained (R² = 0.78, n = 215) toxicity of all Cu forms in different soils with and without aging. Our results suggest that toxicity predictions and risk assessment of CuO-NPs can be carried out properly using the bioavailability-based approaches that are used already for their non-nano counterparts in soil.