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

Alan L. Soli, Robert H. Byrne

Research output: Contribution to journalArticlepeer-review

Abstract

<p> <p id="x-x-sp0020"> Previous determinations of <a title="Learn more about Rare Earth Element from ScienceDirect's AI-generated Topic Pages"> rare earth element </a> (REE) <a title="Learn more about Complexation from ScienceDirect's AI-generated Topic Pages"> complexation </a> by <a title="Learn more about Silicate from ScienceDirect's AI-generated Topic Pages"> silicate </a> have been confined to observations of Eu(SiO(OH) <sub> 3 </sub> ) <sup> 2 + </sup> 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) <sub> 3 </sub> ) <sup> 2 + </sup> stability constant assessments is that the largest silicate stability constants were obtained in the presence of substantial silica <a title="Learn more about Polymerization from ScienceDirect's AI-generated Topic Pages"> polymerization </a> . In the present work, Eu(SiO(OH) <sub> 3 </sub> ) <sup> 2 + </sup> formation is described under conditions where polymerization should be negligible. Stability constants were obtained via observations of the following equilibrium: Eu <sup> 3 + </sup> + Si(OH) <sub> 4 </sub> <sup> 0 </sup> &rlhar; Eu(SiO(OH) <sub> 3 </sub> ) <sup> 2 + </sup> + H <sup> + </sup> with total silicon concentrations on the order of 0.5 mM or less. The formation constant for this equilibrium at 25 &deg;C and 0.7 M <a title="Learn more about Ionic Strength from ScienceDirect's AI-generated Topic Pages"> ionic strength </a> is <p id="x-x-sp0025"> log&beta;Si&lowast;1=EuOSiOH32+H+Eu3+SiOH40=&minus;3.27&plusmn;0.18"&gt;log&beta;Si&lowast;1=EuOSiOH32+H+Eu3+SiOH40=&minus;3.27&plusmn;0.18 <p id="x-x-sp0030"> This stability constant coupled with the first <a title="Learn more about Ionization Constant from ScienceDirect's AI-generated Topic Pages"> ionization constant </a> of Si(OH) <sub> 4 </sub> <sup> 0 </sup> yields log <sub> <em> Si </em> </sub> &beta; <sub> 1 </sub> = 6.18 &plusmn; 0.18 for the equilibrium Eu <sup> 3 + </sup> + OSi(OH) <sub> 3 </sub> <sup> &minus; </sup> &rlhar; Eu(SiO(OH) <sub> 3 </sub> ) <sup> 2 + </sup> . 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) <sub> 3 </sub> ) <sup> 2 + </sup> is a significant fraction of the total dissolved <a title="Learn more about Europium from ScienceDirect's AI-generated Topic Pages"> europium </a> 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&ndash;silicate complexes throughout the oceans are much smaller than those of REE&ndash;carbonate complexes. </p> </p> </p></p>
Original languageAmerican English
JournalMarine Chemistry
Volume195
DOIs
StatePublished - Jan 1 2017

Keywords

  • Europium
  • Silicate
  • Formation constants
  • Sea water
  • Speciation

Disciplines

  • Life Sciences

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