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Determination of the Electrochemical Properties of a Soluble Aqueous FeS Species Present in Sulfidic Solutions

Determination of the Electrochemical Properties of a Soluble Aqueous FeS Species Present in... Field and laboratory data are presented that show a soluble FeS species(FeSaq) exists in sulfidic seawater solutions, and is observedwhen the IAP exceeds the Ksp of amorphous FeS. TheFeSaq yields a discrete signal (double peak) using square-wavevoltammetry and two one-electron waves in sampled DC polarographyexperiments at the Hg electrode. The aqueous FeS species reacts irreversiblyat the electrode as a single FeS subunit and not as a polymeric entity. Thepeak potential of FeSaq occurs at -1.1 V whereas the peakpotential of Fe $$(H_2 O)_6^{2 + } $$ occurs at-1.45 V; the positive shift for Fe2+ reduction inFeSaq indicates a change in geometry for Fe2+from octahedral to tetrahedral. The kinetics of electron transfer at theelectrode are determined to be similar for both Fe2+ andFeSaq. Molecular orbital energy diagrams, further indicatethat Fe(II) does change from octahedral to tetrahedral geometry in solution.First, Fe(II) exists as octahedralFe $$(H_2 O)_6^{2 + } $$ in solution whichundergoes a substitution reaction of bisulfide for water. The resultingcomplex, Fe(H2O)5(HS)+, thentransforms to a tetrahedral complex on further addition of sulfide. Thisgeometry change is consistent with the formation of amorphous FeS thatconverts to mackinawite which has tetrahedral Fe(II). The process is entropydriven because of the water loss that occurs. The overall sequence can berepresented as: $$\begin{gathered} 3Fe(H_2 O)_6^{2 + } + 3HS^ - \to 3Fe(H_2 O)_5 (HS)^ + + 3H_2 O \hfill \\ 3Fe(H_2 O)_5 (HS)^ + + 3HS^ - \to Fe_3 S_3 (H_2 O)_6 + 3H_2 S + 9H_2 O \hfill \\ \end{gathered} $$ Soluble FeS species are important asreactants in the formation of iron-sulfide minerals including pyrite. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Aquatic Geochemistry Springer Journals

Determination of the Electrochemical Properties of a Soluble Aqueous FeS Species Present in Sulfidic Solutions

Aquatic Geochemistry , Volume 3 (3) – Sep 20, 2004

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References (32)

Publisher
Springer Journals
Copyright
Copyright © 1997 by Kluwer Academic Publishers
Subject
Earth Sciences; Geochemistry; Hydrology/Water Resources; Hydrogeology; Water Quality/Water Pollution
ISSN
1380-6165
eISSN
1573-1421
DOI
10.1023/A:1009648026806
Publisher site
See Article on Publisher Site

Abstract

Field and laboratory data are presented that show a soluble FeS species(FeSaq) exists in sulfidic seawater solutions, and is observedwhen the IAP exceeds the Ksp of amorphous FeS. TheFeSaq yields a discrete signal (double peak) using square-wavevoltammetry and two one-electron waves in sampled DC polarographyexperiments at the Hg electrode. The aqueous FeS species reacts irreversiblyat the electrode as a single FeS subunit and not as a polymeric entity. Thepeak potential of FeSaq occurs at -1.1 V whereas the peakpotential of Fe $$(H_2 O)_6^{2 + } $$ occurs at-1.45 V; the positive shift for Fe2+ reduction inFeSaq indicates a change in geometry for Fe2+from octahedral to tetrahedral. The kinetics of electron transfer at theelectrode are determined to be similar for both Fe2+ andFeSaq. Molecular orbital energy diagrams, further indicatethat Fe(II) does change from octahedral to tetrahedral geometry in solution.First, Fe(II) exists as octahedralFe $$(H_2 O)_6^{2 + } $$ in solution whichundergoes a substitution reaction of bisulfide for water. The resultingcomplex, Fe(H2O)5(HS)+, thentransforms to a tetrahedral complex on further addition of sulfide. Thisgeometry change is consistent with the formation of amorphous FeS thatconverts to mackinawite which has tetrahedral Fe(II). The process is entropydriven because of the water loss that occurs. The overall sequence can berepresented as: $$\begin{gathered} 3Fe(H_2 O)_6^{2 + } + 3HS^ - \to 3Fe(H_2 O)_5 (HS)^ + + 3H_2 O \hfill \\ 3Fe(H_2 O)_5 (HS)^ + + 3HS^ - \to Fe_3 S_3 (H_2 O)_6 + 3H_2 S + 9H_2 O \hfill \\ \end{gathered} $$ Soluble FeS species are important asreactants in the formation of iron-sulfide minerals including pyrite.

Journal

Aquatic GeochemistrySpringer Journals

Published: Sep 20, 2004

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