Phytotoxicity of copper oxide nanoparticles in soil with and without biosolid amendment

Copper-engineered nanomaterials (ENM) are being used in many applications, some of which are expected to release these materials to the wastewater stream, potentially resulting in their build-up in biosolids destined for agricultural fields. Using a sandy loam field soil, we investigated the phytotoxicity of nano-CuO (< 50 nm, nominal size) to Trifolium pratense and Elymus lanceolatus in two exposure scenarios (added directly to soil, added via biosolids amendment to soil) compared to similar exposures prepared with the readily soluble Cu form, CuSO4. Copper free ion activity (Cu2+) was measured in KNO3 extracts using a Cu2+ selective electrode at three time points: beginning of plant bioassays, and ends of T. pratense (14 d) and E. lanceolatus (21 d) tests. Extractable Cu2+ was generally lower in the biosolids-amended soils, regardless of Cu form, at the beginning of tests. By the end of the tests (d 14 or 21), Cu2+ activity remained consistent or increased in nCuO exposures, whereas Cu2+ activity of soluble Cu increased overtime in biosolids-amended soils, but decreased in the soil only exposures. Based on median effect levels (EC50's), exposures with nano-CuO were equally, or less, toxic than CuSO4 exposures, regardless of plant species, with root elongation being the most sensitive endpoint. Expressing toxicity in terms of Cu2+ generally resulted in no difference in toxicity between biosolids-amended and non-amended soils. In total, these data contribute to the mounting evidence that the assessment of nano-CuO toxicity in soils fits into current risk assessment models that utilize bioavailable Cu parameters (e.g., Cu2+).