Size and coating impacts on copper oxide nanoparticle transformations and toxicity to earthworms

Introduction: A key concern for the risk assessment of metallic engineered nanomaterials in the terrestrial environment is the degree to which particle size and functional coatings may alter particle dissolution transformations and subsequent toxicity and bioaccumulation effects. To address this question, we integrated nanoparticle characterization, surface transformation analyses, Cu2+ activity measurements, and chronic toxicity within a single standardized soil study to determine whether engineered particle properties remain predictive of biological responses following environmental transformation. Materials and methods: This study investigated the effects of nanoscale copper (II) oxide (CuO) particle size (10 and 25–55 nm, nominal) and coatings (polyvinylpyrrolidone (PVP), steric acid (SA), uncoated) on Cu bioaccumulation and toxicity (survival, reproduction) to the earthworm Eisenia andrei in a sandy loam soil, using the standardized Environment and Climate Change Canada test method STB 1/RM/43. Transformations to particle surfaces and coatings were analyzed by thermogravimetric analysis, while Cu2+ activity in soil extracts, determined by a Cu2+ ion-selective electrode, was used to assess the degree of particle dissolution over time. Results: The EC50 estimates did not indicate a consistent size effect on toxicity and only indicated a coating effect for the 10 nm substances, for which uncoated CuO showed the greatest toxicity to reproduction with an EC50 (lower–upper 95% confidence intervals) of 60 (43–85) μg Cu g−1 soil, compared to the 10 nm PVP- and SA-coated materials which showed the least toxicity of all the materials tested, with EC50s of 135 (86–211) and 258 (161–413) μg g−1, respectively. The 10 nm uncoated CuO also showed the greatest Cu bioaccumulation (BSAF = 1.9) compared to all other substances for which BSAFs ranged from 0.48 to 0.75. Particle surface transformations in soil porewater revealed that the uncoated materials accumulated organic matter (OM) on their surfaces (up to 3.9% mass loss) and showed the largest pools of Cu2+ activity in soils after 24 h. In contrast, the SA coating prevented OM deposition on particle surfaces and showed 4- to 60-fold less Cu2+ activity in soil exposures. Conclusions: Although particle size and surface coating influenced the early transformation and dissolution behaviour of nCuO in soil, neither property produced consistent effects on earthworm response across treatments, and extractable Cu2+ activity was not a sufficient predictor of toxicity. The rapid surface transformations observed suggest that pristine particle characteristics may have limited predictive value as exposure progresses. Hazard assessment of partially soluble metallic nanomaterials may therefore benefit from descriptors that better reflect their environmentally transformed state and the combined contributions of particulate and dissolved toxicant forms.