The solubility of FeS

The solubility of FeSm, synthetic nanoparticulate mackinawite, in aqueous solution was measured at 23 °C from pH 3–10 using an in situ precipitation and dissolution procedure and the solution species was investigated voltammetrically. The solubility is described by a pH-dependent reaction and a pH-independent reaction. The pH-dependent dissolution reaction can be described byFeSm+2H+→Fe2++H2SFeSm+2H+→Fe2++H2Sand logKsp,1∗=3.5±0.25(n=84). The pH-independent dissolution reaction involves the formation of the aqueous FeS cluster complex, FeSaq, and can be represented by the intrinsic solubilityFeSm→FeS0FeSm→FeS0where FeS0 is a monomeric representation of the aqueous cluster complex, FexSx, and log K0 (FeSm) = −5.7. The overall process is described by the relationshiplog∑[Fe(II)]=logK0(FeSm)+logKsp,1∗-log{H2S}-2pH,where ∑[Fe(II)] is the total dissolved Fe(II) concentration. The model closely describes the solubility of FeSm at 23 °C for pH 3–10 and total dissolved S(-II) concentrations, ∑[S(-II)] = 10−1 to 10−6 M. The results show that in neutral to alkaline environments with greater than micromolar ∑[S(-II)], the total solubility of Fe(II) in equilibrium with FeSm approaches 1 μM and the dominant species is FeSaq. Relative to oxic ocean water, Fe(II) is transportable in solution at quite significant concentrations in sulfidic sediments in the presence of FeSm. However, the availability of the hexaqua Fe(II) ion, which may be significant biologically, is correspondingly reduced in these environments although it dominates in all systems with <10−6 M ∑[S(-II)].