RARE-EARTH ELEMENT COMPLEXATION BY PO4(3-) IONS IN AQUEOUS-SOLUTION

Complexation of trivalent rare earths by PO4(3-) ions has been assessed at t = 25-degrees-C by examining the influence of aqueous phosphate concentrations on the distributions of Ce-144 and Gd-153 between 0.68 molar NaClO4 and tributyl phosphate (TBP). Estimated Ce3+ and Gd3+ phosphate complexation constants appropriate to zero ionic strength are Ce3+ + PO4(3-) <--> CePO4(0); log L-beta-1(0) less-than-or-equal-to 11.7 Gd3+ + PO4(3-) <--> GdPO4(0); log L-beta-1(0) = 12.2. Our estimates of these formation constants at zero ionic strength are approximately seven to eight orders of magnitude lower than previously reported estimates for lanthanide and actinide PO4(3-) complexation. Linear free energy relationships relating the complexation of Ca2+ and lanthanides (Ln3+) by a variety of organic ligands, in conjunction with previous direct observations of CaPO4- formation, are consistent with the experimentally derived CePO4(0) and GdPO4(0) formation constants reported in this work. Gadolinium speciation calculations indicate that the PO4(3-) ion can effectively compete with the CO3(2-) ion for free Gd3+ in model groundwater at pH 7-9. Rare earth element phosphate complexation is a significant process in natural freshwater systems which are neutral to mildly basic when the concentration ratio [HPO4(2-)]/[HCO3-] is greater than approximately 1 x 10(-3).