Europium Silicate Complexation at 25 °C and 0.7 Molar Ionic Strength

Authors

Alan L. Soli, Eckerd College
Robert H. Byrne, University of South FloridaFollow

Document Type

Article

Publication Date

2017

Keywords

Europium, Silicate, Formation constants, Sea water, Speciation

Digital Object Identifier (DOI)

https://doi.org/10.1016/j.marchem.2017.02.006

Abstract

Previous determinations of rare earth element (REE) complexation by silicate have been confined to observations of Eu(SiO(OH)₃)^{2 +} formation. Critical assessment of these observations reveals order-of-magnitude ambiguities in the stability constants derived from a variety of experimental designs. A salient feature of prior Eu(SiO(OH)₃)^{2 +} stability constant assessments is that the largest silicate stability constants were obtained in the presence of substantial silica polymerization. In the present work, Eu(SiO(OH)₃)^{2 +} formation is described under conditions where polymerization should be negligible. Stability constants were obtained via observations of the following equilibrium: Eu^{3 +} + Si(OH)₄⁰ ⇌ Eu(SiO(OH)₃)^{2 +} + H⁺ with total silicon concentrations on the order of 0.5 mM or less. The formation constant for this equilibrium at 25 °C and 0.7 M ionic strength is

logβSi∗1=EuOSiOH32+H+Eu3+SiOH40=−3.27±0.18">logβSi∗1=EuOSiOH32+H+Eu3+SiOH40=−3.27±0.18

This stability constant coupled with the first ionization constant of Si(OH)₄⁰ yields log_Siβ₁ = 6.18 ± 0.18 for the equilibrium Eu^{3 +} + OSi(OH)₃⁻ ⇌ Eu(SiO(OH)₃)^{2 +}. Both of these constants are more than an order of magnitude smaller than results obtained under conditions conducive to silica polymerization (i.e., total silica concentrations between 2 and 20 mM). These new stability constants indicate that Eu(SiO(OH)₃)^{2 +} is a significant fraction of the total dissolved europium in deep waters of the Atlantic and Pacific but a very minor species in surface waters. In contrast to predictions obtained via the recent speciation model of Akagi (2013), concentrations of REE–silicate complexes throughout the oceans are much smaller than those of REE–carbonate complexes.

Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

Marine Chemistry, v. 195, p. 138-142

Scholar Commons Citation

Soli, Alan L. and Byrne, Robert H., "Europium Silicate Complexation at 25 °C and 0.7 Molar Ionic Strength" (2017). Marine Science Faculty Publications. 1789.
https://digitalcommons.usf.edu/msc_facpub/1789