Geochemistry of the rare-earth elements in hypersaline and dilute acidic natural terrestrial waters: Complexation behavior and middle rare-earth element enrichments

Rare-earth element (REE) speciation was modelled in acid (2.9 ≤ pH ≤ 3.5), hypersaline groundwaters from Australia and from the Palo Duro Basin in Texas, USA, using a combined specific ion interaction (Pitzer model) and ion pairing model. The free metal ion species (i.e. Ln³⁺) is the dominant form of dissolved REEs in these systems, accounting for 40-70% of the dissolved metal in groundwater from Lake Tyrrell (Victoria, Australia), 50-90% of the REEs in groundwater from Lake Gilmore (Western Australia), and always > 90% of each REE in the Texas groundwaters. (Lakes Tyrrell and Gilmore are actually dry lakes that act as groundwater discharge zones.) The abundance of the free metal ion species increases in these waters with increasing ionic strength and with decreasing pH. The free metal ion species is followed in abundance by REE-sulfate and REE-chloride complexes that account for: 20-50% and 10-15%, respectively, of the dissolved REEs in the groundwaters from Lake Tyrrell; 15-30% and 5-20%, respectively, in the groundwaters from Lake Gilmore; and < 3% and < 7%, respectively, in the Palo Duro Basin groundwaters. The groundwaters of the Lake Tyrrell system and the Palo Duro Basin are enriched in the middle REEs (MREEs) compared to both the light REEs (LREEs) and the heavy REEs (HREEs). (Only Nd, Sm, and Dy were determined in the Lake Gilmore groundwaters and, consequently, it is unclear whether these acid groundwaters are also enriched in the MREEs.) Previous investigators suggested that Fe-rich organic flocs, REE-phosphate complexation, and solid-liquid exchange reactions between terrestrial waters and MREE-enriched surface Fe-Mn coatings, suspended particulates, or secondary mineral phases within aquifer materials, may promote the development of MREE enrichments in natural waters. We propose that organic colloids and REE-phosphate complexes are insignificant in acidic natural waters in regards to MREE enrichments. Instead, solid-liquid exchange reactions or dissolution of surface coatings, suspended particulates, and/or secondary phases as well as sulfate complexation, more likely controls the development of MREE enrichments in acidic natural terrestrial waters.