Geochemistry of Rare Earth Elements and Yttrium in Alkaline Lakes and Hotsprings from the East African Rift Valley

Rare Earth Elements and Yttrium (REY) are considered Technology-Critical Elements and contaminants of emerging concern due to their wide application but limited supply. The growing use of REY poses an environmental threat due to increasing entry into natural surface waters and the food web. Therefore, it is crucial to better understand speciation and complexation of REY under a wide variety of natural surface water conditions. Saline-alkaline lakes are present on all continents, cover up to 20 % of the area of all lakes worldwide¹ and up to 75 % in the East African Rift². Here we report REY from (saline) alkaline lakes and hotsprings from the Kenyan part of the East African Rift. The samples are characterized by alkaline pH values ranging between 7.24 and 9.91, which increases with increasing electrical conductivity as well as high DOC in lake waters (up to ~98 mg/L). Shale-normalized REY patterns show typical features of alkaline lake waters such as strong HREY enrichment over LREY. Preliminary inorganic speciation modelling suggests that carbonate is the dominant inorganic ligand. However, the alkaline rift valley lakes do not show the positive Ce anomaly that was previously reported for other alkaline lakes. The high DOC content of the lake waters (i.e. low molecular weight organic matter) might suppress typical Ce redox behavior. In marked contrast, alkaline hotsprings show similarly strong HREY over LREY enrichments, but positive Ce anomalies.

¹Wurtsbaugh et al. (2017) Nat. Geosci. 10 (11), 816.

²Butturini et al. (2020) Water Research, 173