Add time:08/13/2019         Source:sciencedirect.com

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: Eu3 + + 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 islogβSi∗1=EuOSiOH32+H+Eu3+SiOH40=−3.27±0.18This stability constant coupled with the first ionization constant of Si(OH)40 yields logSiβ1 = 6.18 ± 0.18 for the equilibrium Eu3 + + 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.

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