Rare Earth Element Complexation by POsub4/subsup3−/sup Ions in Aqueous Solution

Complexation of trivalent rare earths by PO ₄ ³⁻ ions has been assessed at t = 25° C by examining the influence of aqueous phosphate concentrations on the distributions of ¹⁴⁴ Ce and ¹⁵³ Gd between 0.68 molar NaClO ₄ and tributyl phosphate (TBP). Estimated Ce ³⁺ and Gd ³⁺ phosphate complexation constants appropriate to zero ionic strength are Ce ³⁺ + PO ₄ ³⁻ ↔ CePO ₄ ⁰ ; log _L β ₁ ⁰ ≤ 11.7 Gd ³⁺ + PO ₄ ³⁻ ↔ GdPO ₄ ⁰ ; log _L β ₁ ^o = 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 PO ₄ ³⁻ complexation. Linear free energy relationships relating the complexation of Ca ²⁺ and lanthanides (Ln ³⁺ ) by a variety of organic ligands, in conjunction with previous direct observations of CaPO ₄ ⁻ formation, are consistent with the experimentally derived CePO ₄ ⁰ and GdPO ₄ ⁰ formation constants reported in this work.

Gadolinium speciation calculations indicate that the PO ₄ ³⁻ ion can effectively compete with the CO ₃ ²⁻ ion for free Gd ³⁺ 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 .