Determination of the Electrochemical Properties of a Soluble Aqueous FeS Species Present in Sulfidic Solutions
Authors: Theberge, S.M.; Luther III, G.W.
Source: Aquatic Geochemistry, Volume 3, Number 3, 1997 , pp. 191-211(21)
Abstract:Field and laboratory data are presented that show a soluble FeS species (FeS_aq) exists in sulfidic seawater solutions, and is observed when the IAP exceeds the K_sp of amorphous FeS. The FeS_aq yields a discrete signal (double peak) using square-wave voltammetry and two one-electron waves in sampled DC polarography experiments at the Hg electrode. The aqueous FeS species reacts irreversibly at the electrode as a single FeS subunit and not as a polymeric entity. The peak potential of FeS_aq occurs at -1.1 V whereas the peak potential of Fe(H_2O)_6^2+ occurs at -1.45 V; the positive shift for Fe^2+ reduction in FeS_aq indicates a change in geometry for Fe^2+from octahedral to tetrahedral. The kinetics of electron transfer at the electrode are determined to be similar for both Fe^2+ and FeS_aq. Molecular orbital energy diagrams, further indicate that Fe(II) does change from octahedral to tetrahedral geometry in solution. First, Fe(II) exists as octahedral Fe(H_2O)_6^2+ in solution which undergoes a substitution reaction of bisulfide for water. The resulting complex, Fe(H_2O)_5(HS)^+, then transforms to a tetrahedral complex on further addition of sulfide. This geometry change is consistent with the formation of amorphous FeS that converts to mackinawite which has tetrahedral Fe(II). The process is entropy driven because of the water loss that occurs. The overall sequence can be represented as: 3Fe(H_2O)_6^2+ + 3HS^- →3Fe(H_2O)_5(HS)^+ + 3H_2O 3Fe(H_2O)_5(HS)^++ 3HS^- →Fe_3S_3(H_2O)_6 + 3H_2S + 9H_2O Soluble FeS species are important as reactants in the formation of iron-sulfide minerals including pyrite.
Document Type: Regular Paper
Affiliations: Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716; theberge@Udel.edu
Publication date: 1997-01-01