Complexation of Yttrium and the Rare Earth Elements with Silicate at Seawater Ionic Strength

Aqueous speciation of Y and the rare earth elements (YREE) is among the best characterized for any group of metals, yet YREE–silicate complexes have received comparatively little attention and data exist for Eu only. This is related in part to silicate (Sil) polymerization as well as poor YREE solubility at conditions needed to form free Sil. Interest in new experiments has been spurred by a contention, based on older, less reliable data, that YREE speciation in seawater is dominated by Sil.

We determined the stability of Eu–Sil complexes by potentiometric titration of Eu in millimolar Sil solutions at seawater ionic strength (0.7 M NaClO₄) and pH 3–10. To prevent Eu hydroxide precipitation, titrations were conducted in the presence of the strong siderophore ligand desferrioxamine B (DFOB). Non-linear regressions were performed using FITEQL, with Eu–Sil stability constants, the DFOB concentration, and the initial proton balance as the only adjustable parameters. The Eu concentration was measured by ICP-MS. The Sil concentration was measured by ICP-AES (total Sil) and spectrophotometry of the blue molybdate complex (monomer and dimer only), to confirm the absence of polymeric species. To verify regression results and account for formation of the Eu(Sil)₂ complex, titrations were done at different Sil/Eu ratios, while keeping the DFOB/Eu ratio well above unity to minimize free Eu³⁺. Our log β₁ is quite similar to values lately derived from static potentiometry, a linear free-energy relation with Fe³⁺, and reanalysis of older literature data. Our log β₂ is also in agreement with published values. This supports recent model calculations showing that YREE–Sil complexes are a very minor species in seawater, except possibly in the deep Pacific Ocean.

Measurements of other YREE–Sil stability constants are ongoing, but more challenging due to lower HREE solubility and lower stability of LREE–DFOB complexes.