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

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 (LnHCO32+, LnCO3+ and Ln(CO3)2−) and phosphate (LnH2PO42+, LnHPO4+, Ln(HPO4)2− and LnPO40) 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., Ln3+). Our speciation calculations indicate that REE-carbonate complexes dominate and typically account for more than 99% of each REE in solution. Moreover, carbonato complexes (LnCO3+) were predicted to be the dominant species for the light REEs and dicarbonato complexes (Ln(CO3)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 ∑PO43− concentrations (< 0.3–1.58 μmol/kg), the much lower concentrations of the free phosphate ion (i.e., [PO43−]F = 10−9−10−6 μmol/kg), due to ion pair formation with Ca2+ and Mg2+, and the much higher free carbonate ion concentrations ([CO32−]F = 0.32−87 μmol/kg).