Potency of (doped) rare earth oxide particles and their constituent metals to inhibit algal growth and induce direct toxic effects

Use of rare earth elements (REEs) has increased rapidly in recent decades due to technological advances. It has been accompanied by recurring rare earth element anomalies in water bodies. In this work we (i) studied the effects of eight novel doped and one non-doped rare earth oxide (REO) particles (aimed to be used in solid oxide fuel cells and gas separation membranes) on algae, (ii) quantified the individual adverse effects of the elements that constitute the (doped) REO particles and (iii) attempted to find a discernible pattern to relate REO particle physicochemical characteristics to algal growth inhibitory properties. Green algae Raphidocelis subcapitata (formerly Pseudokirchneriella subcapitata) were used as a test species in two different formats: a standard OECD201 algal growth inhibition assay and the algal viability assay (a 'spot test') that avoids nutrient removal effects. In the 24 h 'spot' test that demonstrated direct toxicity, algae were not viable at REE concentrations above 1 mg metal/L. 72-hour algal growth inhibition EC₅₀ values for four REE salts (Ce, Gd, La, Pr) were between 1.2 and 1.4 mg/L, whereas the EC₅₀ for REO particles ranged from 1 to 98 mg/L. The growth inhibition of REEs was presumably the result of nutrient sequestration from the algal growth medium. The adverse effects of REO particles were at least in part due to the entrapment of algae within particle agglomerates. Adverse effects due to the dissolution of constituent elements from (doped) REO particles and the size or specific surface area of particles were excluded, except for La₂NiO₄. However, the structure of the particles and/or the varying effects of oxide composition might have played a role in the observed effects. As the production rates of these REO particles are negligible compared to other forms of REEs, there is presumably no acute risk for aquatic unicellular algae. - Highlights: • Rare earth elements (REE) are emerging contaminants in aquatic environments. • Toxicity of REEs and (doped) rare earth oxides (REO) to algae was evaluated. • (Doped) REOs entrapped algae into agglomerates. • REEs inhibited algal growth by removing phosphate from the algal medium (OECD201). • REE proved directly toxic to algae (a 'spot' test), 24 h MBC < 1 mg/L.