Surface chemistry of disordered mackinawite (FeS)

Abstract

Abstract-Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by K s app ϭ {Fe 2ϩ } · {H 2 S(aq)} · {H ϩ } Ϫ2 ϭ 10 ϩ4.87Ϯ0.27 . Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ϳ7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic monocoordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH Ͻ PZC and has a concentration of 1.2 ϫ 10 Ϫ3 mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 ϫ 10 Ϫ3 mol/g FeS, determines surface charge changes. Total site density is 4 sites nm Ϫ2 . The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.

Pertechnetate immobilization with amorphous iron sulfide

“…2), indicating the pH pzc of this synthetic amorphous FeS is pH 7.4 (±0.3, n = 3). This value is much higher than that reported by Widler (pH pzc = 2.9) [58], but consistent with the values reported by Wolthers (pH pzc = 7.5) [38]. Thus at pH >∼ 7.4, the surface becomes increasingly negatively charged, approaching saturation at high pH, while at pH <∼ 7.4, the surface becomes increasingly positively charged until the surface approaches saturation at pH 6.…”

“…A stock amorphous iron sulfide (FeS) suspension was prepared by mixing equal volumes (20 mL) of 1.0 M Fe(NH 4 ) 2 (SO 4 ) 2 and 1.0 M Na 2 S solutions at a constant rate of 0.5 L/min into a 2.0 L reaction vessel while stirring at 800 rpm under Ar [38]. The suspension was filtered using a 0.2 µm nylon filter to isolate the solid, which was then freeze-dried in a lyophilizer over 3-4 days.…”

“…Potentiometric titrations were used to measure the point of zero charge (pH pzc ) of the FeS [38,39]. Due to the sensitivity of the sulfide surface to oxidation by oxygen [40,41], all potentiometric titrations were done manually under anaerobic environments.…”

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides. 3. Influence of Chemical Speciation and Bioavailability on Contaminants Immobilization/Mobilization Bio-processes

“…A direct precipitation mechanism between iron and sulfide species is also encountered in hydrothermal sulfide deposits associated with volcanic activity. Mackinawite is the first crystalline iron sulfide phase that is formed in aqueous systems (Wolthers et al 2005) and is a precursor in sedimentary pyrite (FeS 2 ) formation (Wang & Morse 1996). Recent studies have demonstrated that mackinawite is an important natural reductant in soils and sediments for organic and inorganic contaminants (Moyes et al 2000;Moyes et al 2002;Lee & Batchelor 2003;Arias and Tabo 2003;Livens et al 2004;Bruggeman et al 2005).…”

Modification of Mackinawite with L-Cysteine: Synthesis, Characterization, and Implications to Mercury Immobilization in Sediment

“…Mackinawite was reported as an effective isolating material for use as a sediment cap (Liu et al, 2009). Although pure mackinawite can immobilize mercury, it has a low stability when exposed to oxidized conditions (Burton et al, 2009;Liu et al, 2008;Wolthers et al, 2005). Under field conditions, the capping material has to be in contact with the oxigenated water column during the capping building.…”