Field and laboratory data are presented that show a soluble FeS species(FeSaq) exists in sulfidic seawater solutions, and is observed when 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 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 FeSaq occurs at -1.1 V whereas the peak potential of Fe(ð»2ð‘‚)2+6 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 the electrode are determined to be similar for both Fe2+ and FeSaq. Molecular orbital energy diagrams, further indicate that Fe(II) does change from octahedral to tetrahedral geometry in solution.First, Fe(II) exists as octahedralFe(ð»2ð‘‚)2+6 in solution which undergoes a substitution reaction of bisulfide for water. The resulting complex, Fe(H2O)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 entropyd riven because of the water loss that occurs. The overall sequence can be represented as: 3ð¹ð‘’(ð»2ð‘‚)2+6+3ð»ð‘†(minus) creates 3ð¹ð‘’(ð»2ð‘‚)5(ð»ð‘†)++3ð»2ð‘‚
3ð¹ð‘’(ð»2ð‘‚)5(ð»ð‘†)++3ð»ð‘†(minus) creates ð¹ð‘’3ð‘†3(ð»2ð‘‚)6+3ð»2ð‘†+9ð»2ð‘‚
Soluble FeS species are important as reactants in the formation of iron-sulfide minerals including pyrite.
