The solubility of FeS

• Published in: Geochimica et cosmochimica acta 2006-12, Vol.70 (23), p.5779-5789
• Main Author: Rickard, David
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2006.02.029

The solubility of FeS m, 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 by FeS m + 2 H + → Fe 2 + + H 2 S and log K sp, 1 ∗ = 3.5 ± 0.25 ( n = 84 ) . The pH-independent dissolution reaction involves the formation of the aqueous FeS cluster complex, FeS aq, and can be represented by the intrinsic solubility FeS m → FeS 0 where FeS 0 is a monomeric representation of the aqueous cluster complex, Fe x S x , and log K 0 (FeS m) = −5.7. The overall process is described by the relationship log ∑ [ Fe ( II ) ] = log K 0 ( FeS m ) + log K sp, 1 ∗ - log { H 2 S } - 2 pH, where ∑[Fe(II)] is the total dissolved Fe(II) concentration. The model closely describes the solubility of FeS m 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 FeS m approaches 1 μM and the dominant species is FeS aq. Relative to oxic ocean water, Fe(II) is transportable in solution at quite significant concentrations in sulfidic sediments in the presence of FeS m. 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