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> ⇌ 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 °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"> logβSi∗1=EuOSiOH32+H+Eu3+SiOH40=−3.27±0.18">logβSi∗1=EuOSiOH32+H+Eu3+SiOH40=−3.27±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> β <sub> 1 </sub> = 6.18 ± 0.18 for the equilibrium Eu <sup> 3 + </sup> + OSi(OH) <sub> 3 </sub> <sup> − </sup> ⇌ 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–silicate complexes throughout the oceans are much smaller than those of REE–carbonate complexes. </p> </p> </p></p>
Original language | American English |
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Journal | Marine Chemistry |
Volume | 195 |
DOIs | |
State | Published - Jan 1 2017 |
Keywords
- Europium
- Silicate
- Formation constants
- Sea water
- Speciation
Disciplines
- Life Sciences