Evaluating the effect of natural organic matter quality on the bioavailability of a rare earth element (La)

Rare earth elements (REE) are in high demand due to their applications in the medical, green energy and high-tech industries. As a result, new REE mines are being developed with the potential of mobilizing REE and increasing their concentrations in the environment, including in landfills, wastewaters and natural waters. Unfortunately, the concentrations of trivalent REE cations are generally poorly predictive of biological effects, especially when attempting to understand the role of REE complexation within bioavailability models, such as the Biotic Ligand Model (BLM). We investigated the bioavailability of a representative REE, lanthanum (La), by measuring biouptake fluxes for Chlamydomonas reinhardtii. In the presence of a small amount (0.5 mg C L^-1) of natural organic matter (NOM), for a calculated reduction in La³⁺ of >90%, La biouptake decreased between 43 and 87%. Furthermore, the concentration of dissolved organic carbon was not sufficient to predict the effect of NOM on La biouptake, with the effect varying greatly as a function of NOM origin. Eleven NOM samples were extensively characterised in order to relate NOM quality to its effect on biouptake. The most aromatic and allochthonous NOM had the greatest effect on reducing La biouptake. For example, for an NOM concentration of 10 mg C L^-1, the most allochthonous NOM tested (Suwannee River Humic Acid, fluorescence index = 1.2) reduced La biouptake by 95% as compared to a reduction of 67% for the most autochthonous NOM (Cameroun River, fluorescence index = 1.9). The study is useful for improving our understanding of the important factors affecting REE bioavailability in natural systems.