Lanthanum and Gadolinium availability in aquatic mediums: New insights to ecotoxicology and environmental studies.

Studies dealing with Rare Earth Elements (REE) ecotoxicological behavior are scattered and with potential conflicting results. Climate change impacts on aquatic biota and is known to modify contaminants toxicokinetic. Nevertheless, the current knowledge on the potential interactions between climate change and REE is virtually non-existent. Therefore, we focus our research on La and Gd as representatives of Light and Heavy REE that also are of great environmental concern. Experiments on different mediums (fresh-, brackish- and seawater) were designed to run at present-day and near-future conditions (T°=+4 °C, pH=△-0.4). Sampling was taken at different time scales from minutes to hours for one day. The main challenge was to evaluate the availability of La and Gd under environmental conditions closely related to climate changes scenarios. Furthermore, this study will contribute to the baseline knowledge by which future research towards understanding REE patterns and toxicity will build upon. Lanthanum and Gd behave differently with salinity. Temperature also affects the availability of dissolved La in freshwater. On the other hand, pH reduction causes the decrease of Gd in freshwater. In this medium, concentrations reduce sharply, presumably due to sorption processes or precipitates. In the brackish water experiment only the dissolved La levels in the Warming (T°=+4 °C) and Warming & Acidification (T°=+4 °C, pH=△0.4) diminished significantly through time. Dissolved La and Gd levels in seawater were relatively constant with time. The speciation of both elements is also of great relevance for ecotoxicological experiments. The trivalent free ions (La and Gd) were the most common species in the trials. However, as ionic strength increases, the availability of other complexes rose, which should be subject of great attention for upcoming ecotoxicological studies.