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 less tha n or equal to pH less than or equal to 3.5), hypersaline groundwaters from Australia and from the Pale Duro Basin in Texas, USA, using a com bined specific ion interaction (Fitter model) and ion pairing model. T he free metal ion species (i.e. Ln(3+)) is the dominant form of dissol ved REEs in these systems, accounting for 40-70% of the dissolved meta l in groundwater from Lake Tyrrell (Victoria, Australia), 50-90% of th e REEs in groundwater from Lake Gilmore (Western Australia), and alway s > 90% of each REE in the Texas groundwaters. (Lakes Tyrrell and Gilm ore are actually dry lakes that act as groundwater discharge zones.) T he abundance of the free metal ion species increases in these waters w ith increasing ionic strength and with decreasing FH. The free metal i on species is followed in abundance by REE-sulfate and REE-chloride co mplexes that account for: 20-50% and 10-15%, respectively, of the diss olved REEs in the groundwaters from Lake Tyrrell; 15-30% and 5-20%, re spectively, in the groundwaters from Lake Gilmore; and < 3% and < 7%, respectively, in the Pale Duro Basin groundwaters.The groundwaters of the Lake Tyrrell system and the Pale Duro Basin are enriched in the mi ddle REEs (MREEs) compared to both the light REEs (LREEs) and the heav y REEs (HREEs). (Only Nd, Sm, and Dy were determined in the Lake Gilmo re groundwaters and, consequently, it is unclear whether these acid gr oundwaters are also enriched in the MREEs.) Previous investigators sug gested that Fe-rich organic flocs, REE-phosphate complexation, and sol id-liquid exchange reactions between terrestrial waters and MREE-enric hed surface Fe-Mn coatings, suspended particulates, or secondary miner al phases within aquifer materials, may promote the development of MRE E 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 sec ondary phases as well as sulfate complexation, more likely controls th e development of MREE enrichments in acidic natural terrestrial waters .