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 circumneutr al pH (7 less than or equal to pH less than or equal to 9) groundwater s from south-central Nevada and the Mojave Desert in eastern Californi a, U.S.A. The inorganic speciation of the REEs in these water were eva luated primarily to assess the relative importance of carbonate (LnHCO (3)(2+), LnCO(3)(+) and Ln(CO3)(2)(-)) and phosphate (LnH(2)PO(4)(2+), LnHPO(4)(+), Ln(HPO4)(2)(-) and LnPO(4)(0)) complexes. Other REE comp lexes with sulfate, hydroxyl, chloride, and fluoride tons were also as sessed, as was the significance of the free metal ion species (i.e., L n(3+)). Our speciation calculations indicate that REE-carbonate comple xes dominate and typically account for more than 99% of each REE in so lution. Moreover, carbonate complexes (LnCO(3)(+)) were predicted to b e the dominant species for the light REEs and dicarbonato complexes (L n(CO3)(2)(-)) were predicted to increase in importance with increasing atomic number. All other complexes were predicted to be negligible. F urthermore, the combined percentage of all REE-phosphate complexes nev er accounted for more than 0.1% of the dissolved REEs, and generally m uch less than 0.1%. Phosphate complexes can not compete with carbonate complexes in these groundwaters because of the low Sigma PO43- concen trations (< 0.3-1.58 mu mol/kg), the much lower concentrations of the free phosphate ion (i.e., [PO43-](F) = 10(-9)-10(-6) mu mol/kg), due t o ion pair formation with Ca2+ and Mg2+, and the much higher free carb onate ion concentrations ([CO32-](F) = 0.32-87 mu mol/kg).