In recent years, the use of Rare Earth Elements (REE) has rapidly increased, resulting in numerous potential anthropogenic inputs to the environment. As a result, these metals are emerging as microcontaminants and pose a potential threat to aquatic life. However, the toxicity of REE are largely unknown due, in part, to the limited information on their chemical speciation. The purpose of this project was to gain an understanding of REE precipitate formation and solubility as the foundation for the development of the chemical equilibrium component of toxicity prediction models. Solubility experiments were conducted with Samarium, (Sm), a light REE, and Dysprosium, (Dy), a heavy REE and measured using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). Water chemistries of varying pH (6, 7, 8 and 9), total metal (1 mM, 10 mM and 100 mM) and total carbonate (atmospheric CO2, 1 mM and 10 mM) were used to study the kinetics of precipitate formation over a 120-h period. Experimental results indicate that data obtained at atmospheric CO2 and low total metal concentrations was unreliable, likely due to the difficulty in measuring solubility limits near the ICP-OES detection limit. Furthermore, most of the water chemistries explored appeared to achieve steady-state conditions within 24-h for both Sm and Dy, indicating the suitability of 24-h renewal processes used in acute toxicity tests. However, measured dissolved metal concentrations did not approach the predicted equilibrium concentrations, indicating that while steady-state conditions were achieved, equilibrium was not reached. For Sm, geochemical models over predicted the amount of precipitation for most water chemistries, with the exception of at low pH where no precipitation was predicted. The opposite trend was predicted for Dy, with over predicted precipitation at pH 8 and 9. Very little precipitation was observed under atmospheric conditions, while data for 1 mM and 10 mM total carbonate concentrations agreed strongly with one another for all total Sm and Dy concentrations. This was an indication that the Sm or Dy available for complexation, and therefore precipitation, was in much lower concentration than carbonate. This was especially true at high pH values. All metal precipitated at pH 9 for most water chemistries. Under these conditions, the system was saturated with both hydroxides and carbonates, which provided a greater opportunity for precipitation. Future work is required to investigate the formation of multi-ligand precipitates with Sm and Dy, as well as the investigation of the role that DOM plays in precipitate formation
