On the formation and structure of rare-earth element complexes in aqueous solutions under hydrothermal conditions with new data on gadolinium aqua and chloro complexes

Abstract

Synchrotron X-ray spectroscopy experiments were made on the Gd(III) aqua and chloro complexes in low pH aqueous solutions at temperatures ranging from 25 to 500 °C and at pressures up to 480 MPa using a hydrothermal diamond anvil cell. Analysis of fluorescence Gd L 3-edge X-ray absorption fine structure (XAFS) spectra measured from a 0.006 m Gd/0.16 m HNO 3 aqueous solution at temperatures up to 500 °C and at pressures up to 260 MPa shows that the Gd–O distance of the Gd 3+ aqua ion decreases steadily at a rate of ∼ 0.007 Å/100 °C whereas the number of coordinated H 2O molecules decreases from 9.0 ± 0.5 to 7.0 ± 0.4. The loss of water molecules in the Gd 3+ aqua ion inner hydration shell over this temperature range (a 22% reduction) is smaller than exhibited by the Yb 3+ aqua ion (42% reduction) indicating that the former is significantly more stable than the later. We conjecture that the anomalous enrichment of Gd reported from measurement of REE concentrations in ocean waters may be attributed to the enhanced stability of the Gd 3+ aqua ion relative to other REEs. Gd L 3-edge XAFS measurements of 0.006 m and 0.1 m GdCl 3 aqueous solutions at temperatures up to 500 °C and pressures up to 480 MPa reveal that the onset of significant Gd 3+–Cl − association occurs around 300 °C. Partially-hydrated stepwise inner-sphere complexes most likely of the type Gd(H 2O) δ− n Cl n +3− n occur in the chloride solutions at higher temperatures, where δ ≈ 8 at 300 °C decreasing slightly to an intermediate value between 7 and 8 upon approaching 500 °C. This is the first direct evidence for the occurrence of partially-hydrated REE Gd (this study) and Yb [Mayanovic, R.A., Jayanetti, S., Anderson, A.J., Bassett, W.A., Chou, I-M., 2002a. The structure of Yb 3+ aquo ion and chloro complexes in aqueous solutions at up to 500 °C and 270 MPa. J. Phys. Chem. A 106, 6591–6599.] chloro complexes in hydrothermal solutions. The number of chlorides ( n) of the partially-hydrated Gd(III) chloro complexes increases steadily with temperature from 0.4 ± 0.2 to 1.7 ± 0.3 in the 0.006 m chloride solution and from 0.9 ± 0.7 to 1.8 ± 0.7 in the 0.1 m GdCl 3 aqueous solution in the 300–500 °C range. Conversely, the number of H 2O ligands of Gd(H 2O) δ− n Cl n +3− n complexes decreases steadily from 8.9 ± 0.4 to 5.8 ± 0.7 in the 0.006 m GdCl 3 aqueous solution and from 9.0 ± 0.5 to 5.3 ± 1.0 in the 0.1 m GdCl 3 aqueous solution at temperatures from 25 to 500 °C. Analysis of our results shows that the chloride ions partially displace the inner-shell water molecules during Gd(III) complex formation under hydrothermal conditions. The Gd–OH 2 bond of the partially-hydrated Gd(III) chloro complexes exhibits slightly smaller rates of length contraction (∼ 0.005 Å/100 °C) for both solutions. The structural aspects of chloride speciation of Gd(III) as measured from this study and of Yb(III) as measured from our previous experiments are consistent with the solubility of these and other REE in deep-sea hydrothermal fluids.