Abstract:

Neodymium (Nd), Praseodymium (Pr), and Yttrium (Y) are three rare earth elements (REEs) that occur in the mineral ore bastnaesite, the primary ore of Canada’s first REE mine on Thor Lake, Nechalacho, NT. Usage of these elements is an expanding market as they are key components in various modern technologies, including for example, green energy equipment such as wind turbines & electric vehicle batteries. As a result, there is a growing concern for the potential environmental risk due to anthropogenic contamination. Minimal data is currently available regarding toxicity of individual REEs and even less data is available on mixtures. The objective of this study was to investigate the bioaccumulation of Nd, Pr, and Y in Daphnia magna, alone and as ternary mixtures. Daphnids were initially exposed to sublethal concentrations in an artificial soft water medium with a hardness of 50 mg CaCO3/L and a pH of 6.8. Surviving daphnids were sampled after 24-h. At doubling concentrations of Pr (100, 200, 400, 800 & 1600 µg/L Pr), the accumulation of Pr was saturable and similar results were observed for Nd an Y. Bmax values for Nd were 50% of those for Pr and Y. Mixture exposures were designed using a toxic unit (TU) approach, based on converting the EC50 concentrations from previously conducted acute toxicity tests to TUs and applying a matrix isobologram approach. For example, at 0.2 TUs of each element (40 µg/L Nd, 133 µg/L Pr & 190 µg/L Y), similar levels of REE accumulation were observed whether it was in single metal exposures or in mixture exposures indicating that accumulation was not influenced by other REEs. A similar independent accumulation of REEs was seen in a bioaccumulation test conducted using Thor Lake water although the degree of accumulation was only 10% of lab tests. This research is supported via an NSERC Alliance Grant with additional funding from Environment and Climate Change Canada, Stantec Inc and Cheetah Resources.