Assessment of Fe(III) and Eu(III) Complexation by Silicate in Aqueous Solutions

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Rare earth element, Silicate, Complexation, Speciation, Europium, Silicate

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Prior investigations of Eu3+ complexation by silicate have led to predictions that rare earth silicate complexes (REESiO(OH)32+)">(REESiO(OH)32+) are the dominant species of REEs in deep waters of the Atlantic and Pacific Oceans. The proposed importance of REE–silicate complexes has been used as a foundation to explain oceanic REE profiles. In the present work, we examine the significance of rare earth element complexation by silicate ions. As one fundamental means of assessing prior depictions of REE–silicate formation constant behavior, our work examines the comparative stability constant behavior of Eu(III) and Fe(III). Plots of Eu(III) complexation constants against Fe(III) formation constants, in conjunction with experimental determinations of FeSiO(OH)32+">FeSiO(OH)32+ formation constants, indicate that previously published EuSiO(OH)32+">EuSiO(OH)32+ formation constants are substantially overestimated. Assessment of prior EuSiO(OH)32+">EuSiO(OH)32+ formation constant determinations reveals that results obtained in the presence and absence of silicic acid polymerization are inconsistent. Much larger EuSiO(OH)32+">EuSiO(OH)32+ formation constants are obtained in the presence of polymeric silica. Reanalysis of complexation results obtained under conditions of minimal silicate polymerization leads to a EuSiO(OH)32+">EuSiO(OH)32+ formation constant that is smaller than previously published estimates by as much as a factor of ∼25. The dramatically reduced magnitude of Siβ1(Eu) relative to previously published results indicates that the role of silicate complexation in oceanic REE cycling is much less significant than previously proposed. The spectrophotometric investigations of FeSiO(OH)32+">FeSiO(OH)32+ formation in the present study yield the first characterization of FeSiO(OH)32+">FeSiO(OH)32+ formation constant behavior as a function of ionic strength: logsi*β1(Fe)=(-0.121±0.043)-2.036*I1/2/(1+1.5*I1/2)+(0.568±0.098)I">logsi*β1(Fe)=(-0.121±0.043)-2.036*I1/2/(1+1.5*I1/2)+(0.568±0.098)I where Si∗β1(Fe)=[FeSiO(OH)32+]">Si∗β1(Fe)=[FeSiO(OH)32+] [H+]/[Fe3+] [Si(OH)40]">[Si(OH)40] . This result can be used to show that ferric silicate complexes in seawater are present at much lower concentrations than ferric hydroxide complexes. The Siβ1(Eu) formation constant obtained in our reanalysis of prior work is given as Siβ1(Eu) = [EuSiO(OH)32+]/([Eu3+][SiO(OH)3-)]">[EuSiO(OH)32+]/([Eu3+][SiO(OH)3-)] = 106.03 ± 0.16 (25 °C and ∼0.7 molal ionic strength). This result indicates that europium silicate complexes in seawater are present at concentrations much lower than europium carbonate complexes.

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Geochimica et Cosmochimica Acta, v. 202, p. 361-373