Soluble iron sulfide species in natural waters: Reappraisal of their stoichiometry and stability constants

Abstract

A wide range of stoichiometries has been previously proposed for soluble iron sulfide species and there is no general agreement on their importance in natural waters. The solubility of Fe(II) in 0.1 M NaClO 4 equilibrated at 20 ± 0.1°C with various partial pressures of H 2 S (0.1, 0.001, 0.0001, 0.00001 MPa) was measured in the pH range 3.1-7.9. Equilibrium was established within 1 -6 h when amorphous FeS was the solid phase. The results could all be fitted using values for the solubility product constant (I = 0) of p*K s = 3.00 ± 0.12 and of the stability constant for a soluble Fe(HS) 2 species (I = 0) of pb 2 = -6.45 ± 0.12 where *K s = aFe 2+ · aHS -/aH + and b 2 = aFe(HS) 2 / aFe 2+

Surface chemistry of disordered mackinawite (FeS)

“…where ␤ 1 ϭ 10 ϩ2.05Ϯ0.5 (Luther et al, 1996), ␤ 2 ϭ 10 Ϫ3.43Ϯ0.1 (Davison et al, 1999). The way reactions 4, 5, and 1 are written simplifies further derivations; no assumptions about the actual reactive species or reaction mechanisms are made.…”

“…Aqueous iron-sulfide complexation plays an important role in this pH range (Rickard, 1989;Davison, 1991;Luther, 1991;Luther and Ferdelman, 1993;Zhang and Millero, 1994;Rickard, 1995;Luther et al, 1996;Davison et al, 1999). The two most simple complexation reactions reported are…”

“…The way reactions 4, 5, and 1 are written simplifies further derivations; no assumptions about the actual reactive species or reaction mechanisms are made. Davison et al (1999) determined a much lower value for ␤ 1 , that is, ␤ 1 ϭ 10 ϩ0.05Ϯ0.1 . There is still considerable disagreement on aqueous iron-sulfide complexation reactions and their thermodynamic constants, the latter should therefore be treated with caution (Davison et al, 1999).…”

“…K s app is the apparent solubility constant at zero ionic strength that is derived from the experimental pH and total dissolved Fe data and compared to the solubility K s * ϭ 10 ϩ3.98Ϯ0.12 from Davison et al (1999)…”

Chemical Speciation of Sulfur and Metals in Biogas Reactors : Implications for Cobalt and Nickel Bio-uptake Processes

“…Iron is also able to form Fe-sulfide complexes and clusters in the aqueous phase. Davison et al (1999) demonstrated that, in a solution saturated with respect to FeS(s), the formation of aqueous Fe(HS)2 and Fen(HS)2n (n ≥2) complexes may explain the solubility. Using an alternative approach, Rickard (2006) suggested that the dissolution of FeS(s) involves the formation of aqueous FeS nano-clusters (i.e.…”