Rare earth element complexation behavior in circumneutral pH groundwaters: Asessing the role of carbonate and phosphate ions

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Nevada, Mojave Desert, Ground Water, Rare Earths, Phosphate Ion, Carbonate Ion, Bicarbonate Ion, Elsevier: Nevada, Mojave Desert, Ground Water, Rare Earths, Phosphate Ion, Carbonate Ion, Bicarbonate Ion

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Rare earth element (REE) concentrations were determined in circumneutral pH (7 pH 9) groundwaters from south-central Nevada and the Mojave Desert in eastern California, U.S.A. The inorganic speciation of the REEs in these water were evaluated primarily to assess the relative importance of carbonate (LnHCO 2+ 3 , LnCO + 3 and Ln(CO 3 ) - 2 ) and phosphate (LnH 2 PO 2+ 4 , LnHPO + 4 , Ln(HPO 4 ) - 2 and LnPO 0 4 ) complexes. Other REE complexes with sulfate, hydroxyl, chloride, and fluoride ions were also assessed, as was the significance of the free metal ion species (i.e., Ln 3+ ). Our speciation calculations indicate that REE-carbonate complexes dominate and typically account for more than 99% of each REE in solution. Moreover, carbonato complexes (LnCO + 3 ) were predicted to be the dominant species for the light REEs and dicarbonato complexes (Ln(CO 3 ) - 2 ) were predicted to increase in importance with increasing atomic number. All other complexes were predicted to be negligible. Furthermore, the combined percentage of all REE-phosphate complexes never accounted for more than 0.1% of the dissolved REEs, and generally much less than 0.1%. Phosphate complexes can not compete with carbonate complexes in these groundwaters because of the low PO 3- 4 concentrations (< 0.3-1.58 mol/kg), the much lower concentrations of the free phosphate ion (i.e., [PO 3- 4 ] F = 10 -9 -10 -6 mol/kg), due to ion pair formation with Ca 2+ and Mg 2+ , and the much higher free carbonate ion concentrations ([CO 2- 3 ] F = 0.32-87 mol/kg).

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