Rare Earth Element Complexation by PO43− Ions in Aqueous Solution

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Complexation of trivalent rare earths by PO43− ions has been assessed at t = 25°C by examining the influence of aqueous phosphate concentrations on the distributions of 144Ce and 153Gd between 0.68 molar NaClO4 and tributyl phosphate (TBP). Estimated Ce3+ and Gd3+ phosphate complexation constants appropriate to zero ionic strength are Ce3+ + PO43−CePO40; logLβ10 ≤ 11.7Gd3+ + PO43−GdPO40; logLβ1o = 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 PO43− 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 CePO40 and GdPO40 formation constants reported in this work.

Gadolinium speciation calculations indicate that the PO43− ion can effectively compete with the CO32− 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 [HPO42−][HCO3−]">[HPO42−][HCO3−] is greater than approximately 1 × 10−3.

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Geochimica et Cosmochimica Acta, v. 55, issue 10, p. 2729-2735