Comparative Carbonate Complexation of Yttrium and Gadolinium at 25 °C and 0.7 Mol Dmsup−3/sup Ionic Strength

Stability constants for yttrium and gadolinium complexation by carbonate ions have been measured by solvent extraction procedures at 25 °C and 0.7 mol dm ⁻³ ionic strength. The results of six experiments indicate that log _{CO 3} β ₁ ′( Y ) = 5.71, log _{CO 3} β ₂ ′( Y ) = 10.34, log _{CO 3} β ₁ ′( Gd ) = 5.65 and log _{CO 3} β ₂ ′( Gd ) = 10.12 where _{CO 3} β _n ′( M ) = [ M ( CO ₃ ) _n ^{3 − 2 n} ][ M ³⁺ ] ⁻¹ [ CO ₃ ²⁻ ] _T ^{− n} , [ ] represents concentrations and [CO ₃ ²⁻ ] _T = [CO ₃ ²⁻ ] + [NaCO ₃ ⁻ ]. When compared with previous complexation results for Eu ³⁺ , Gd ³⁺ and Tb ³⁺ obtained using identical procedures, excellent agreement is observed for Gd ³⁺ complexation, and the solution complexation of Y ³⁺ in seawater is seen to closely resemble that of Tb ³⁺ . In spite of closely linked solution chemistries of Y ³⁺ and Tb ³⁺ , the shale normalized concentrations of these elements in seawater should be distinct due to their differing reactivities with organic ligands on particle surfaces. Model calculations indicate that while shale normalized Y ³⁺ concentrations in seawater may exceed the shale normalized concentrations of all rare earths, similarities in Y ³⁺ and Tb ³⁺ solution chemistries may produce substantial coherence in the oceanic distributions of these elements.