Effects of rare earth elements in the aquatic environment: Implications for ecotoxicological testing

Rare earth elements (REE) are recognized as emerging pollutants due to their widespread use in modern society (e.g., in the production of electronics, renewable energy technologies, and advanced medical devices) which leads to anthropogenically elevated concentrations in the environment with potential consequences for ecosystem health. This article critically reviews the current scientific knowledge on aquatic bioavailability and toxicity of REE and focuses on pitfalls that could influence the outcome of ecotoxicity tests. After passing our quality criteria, we reviewed 38 papers on the ecotoxicity of REE in depth. Most studies focused on freshwater environments, indicating a need for more research on marine ecosystems, particularly on marine vertebrates. The results showed that heavy REE tend to be more toxic than light REE to aquatic organisms. Critical aspects for biotesting REE include complexation with ions such as phosphates (nutrient in algae tests) and carbonates. Carbonate complexation decreases potentially bioavailable aqueous REE species and may lower toxicity at increasing water hardness, although this may also be caused by competition of REE³⁺ and Ca²⁺ for the same binding sites in organisms. REE have a high tendency to adsorb to glass and it is recommended to use vessels made of polyethylene terephthalate or polycarbonate instead. More research is needed on chemical speciation and the interaction of REE with various organisms, also in multi-species mesocosm studies. A robust aquatic risk assessment on REE requires information on nominal and measured concentrations in both acute and chronic ecotoxicological bioassays as well as a thorough characterization of exposure.