VOLTAMMETRIC CHARACTERIZATION OF IRON(II) SULFIDE COMPLEXES IN LABORATORY SOLUTIONS AND IN MARINE WATERS AND POREWATERS

We report on metal sulfide complexes in marine waters and porewaters using voltammetric methods and compare field data with standard solutions containing Fe(II) and sulfide. Field data indicate that sulfide is complexed and not free at the sulfidic/nonsulfidic interface when the Fe(II) content greater-than-or-equal-to 0.1 of the sulfide content of the sample. Acid-base titration data for field samples indicate that the forms of the sulfide complex are FeSH+ and [Fe2(SH)]3+ at seawater pH. From pH 7-10, the complexes FeS and Fe(H2S)2+ are not consistent with the data. Salt marsh creek waters obtained at low tide and Chesapeake Bay sulfidic waters contain strong Fe(II) sulfide complexes [log K(f) near 6.95 for Fe(SH)+]. This compares well with the conditional stability constants evaluated for Fe(SH)+ (log beta1 = 5.50 +/- 0.24) and [Fe2(SH)]3+ (log beta2 = 11.08 +/-0.25) by electrochemical methods with standard iron(II) sulfide solutions in seawater. The environmental complexes can be kinetically inert to dissociation of sulfide whereas the laboratory solutions are labile. Thus, organic chelates may stabilize the complexes. In salt marsh creek waters, strong sulfide complexes may be transported over reasonable distances. In porewaters and waters in enclosed basins, the complexes are likely important in pyrite synthesis.