Master of Science (MSc)
Faculty of Science
Dr. James McGeer
Rare earth elements (REEs) are a group of similarly reactive metals that are divided into two groups; the light (LREEs) and heavy (HREEs) rare earths. These metals are used in a variety of industries and are critical to the function of modern society. The demand for REEs is expected to increase and as a result new mining projects are being developed with a potential for future mining in Canada. The toxicological understanding of these metals in the aquatic environment is very limited. The objective of this research is to compare the toxicological effect of REEs to the freshwater invertebrates Daphnia pulex and Hyalella azteca and in the more sensitive Hyalella to understand the toxicity modifying influence of cationic competition (Ca2+, Mg2+, Na+, H+) and dissolved organic matter (DOM) complexation. In this study cerium (Ce; a light REE) and dysprosium (Dy; a heavy REE) were chosen as representative of the two REE groups. Standard methods (Environment Canada) were followed for testing and culture; acute Ce tests were performed at low hardness (12.5 mg CaCO3 mg/L, pH 7.0, Ca 0.18, Na 0.14, Mg 0.14 (mM) for D. pulex and 32 mg CaCO3/L, pH 7.2, Ca 0.1, Na 0.101, Mg 0.03 (mM) for Hyalella at 21oC) while Dy tests were completed at intermediate hardness (62.5 mg CaCO3 mg/L) adjusted to pH 7.8 with Ca at 0.5, Na 0.5, Mg 0.125 (mM) and 23oC for both organisms. Acute toxicity tests were performed using <24 h old neonates for 48 h in the case of Daphnia and with 2-9 d old offspring for 96 h tests with Hyalella. Dissolved concentrations (0.45 µm filtered) for Ce and Dy were lower than total (unfiltered) which demonstrated precipitation over the course of the exposures. As a result, the LC50s based on end concentrations provided the most conservative estimates of toxicity. Daphnia was identified as the more resilient organism to REE exposures and thus further testing with toxicity modifying factors was performed using Hyalella. The protective effect of cationic competition was tested with Ca (0.1 to 2.0 mM), Na (0.1 to 2.0 mM) and Mg (0.03 to 0.5 mM) and it was determined that Ca and Na (in the case of Dy exposures) were protective of acute REE toxicity while Mg had no effect. Moreover, tests where pH (6.5 - 8.0) was altered showed a protective effect of increased H+ on acute Dy toxicity. The ability of dissolved organic matter (DOM) to complex Ce and Dy were tested with DOM additions (4 to 15 mg C/L) and showed a dose dependent decrease in LC50s. Biotic ligand based parameters (Log K values) were calculated based on free ion relationships using geochemical equilibrium software (WHAM ver. 7.02). The Log K values derived from Ce3+ and Dy3+ LC50 to Hyalella were calculated as 7.3 and 7.75, respectively. The protective influence of Ca2+ was determined for Ce3+ (Log K = 3.9) and Dy3+ (Log K = 3.95) and in the case of Na+ and Dy3+ (Log K = 4.10). This study contributes data to the development of site specific water quality guidelines and criteria for Ce and Dy in freshwater.
Vukov, Oliver, "DEVELOPING A SITE SPECIFIC UNDERSTANDING OF THE TOXICITY OF RARE EARTH ELEMENTS, CERIUM AND DYSPROSIUM, TO DAPHNIA PULEX AND HYALELLA AZTECA" (2015). Theses and Dissertations (Comprehensive). 1788.