Toxicity of lanthanide coagulants assessed using four in vitro bioassays

Rare earth element (REE) coagulants are prime contenders in wastewater treatment plants to remove phosphorus; unlike typical coagulants, they are not affected by pH. However, the use of REEs in wastewater treatment could mean increased human exposure to lanthanides (Ln) through wastewater effluent discharge to the environment or through water reuse. Information on the toxicity of lanthanides is scarce and, where available, there are conflicting views. Using in vitro bioassays, we assessed lanthanide toxicity by evaluating four relevant endpoints: the change in mitochondrial membrane potential (Δψm), intracellular adenosine triphosphate (I-ATP), genotoxicity, and cell viability. At less than 5000 μmol-Ln³⁺/L, lanthanides increased the Δψm, while above 5000 μmol-Ln³⁺/L, the Δψm level plummeted. The measure of I-ATP indicated constant levels of ATP up to 250 μmol-Ln³⁺/L, above which the I-ATP decreased steadily; the concentration of La, Ce, Gd, and Lu that triggered half of the cells to become ATP-inactive is 794, 1505, 1488, 1115 μmol-Ln³⁺/L, respectively. Although La and Lu accelerated cell death in shorter studies (24 h), chronic studies using three cell growth cycles showed cell recovery. Lanthanides exhibited antagonistic toxicity at less than 1000 μmol-Ln³⁺/L. However, the introduction of heavy REEs in a solution amplified lanthanide toxicity. Tested lanthanides appear to pose little risk, which could pave the way for lanthanide application in wastewater treatment.