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

Field and laboratory data are presented that show a soluble FeS species(FeS_aq) exists in sulfidic seawater solutions, and is observedwhen the IAP exceeds the K_sp of amorphous FeS. TheFeS_aq 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 FeS_aq occurs at -1.1 V whereas the peakpotential of Fe occurs at-1.45 V; the positive shift for Fe²⁺ reduction inFeS_aq 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⁺ andFeS_aq. Molecular orbital energy diagrams, further indicatethat Fe(II) does change from octahedral to tetrahedral geometry in solution.First, Fe(II) exists as octahedralFe 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: Soluble FeS species are important asreactants in the formation of iron-sulfide minerals including pyrite.