Dissolution Of FeS Research Articles

Reaction of FeS with Fe(III)-bearing acidic solutions

The reaction of FeS with Fe(III)-bearing acidic solutions (Fe(III)BAS) was probed at 25°C and pH between 2 and 3. Initial dissolved Fe3+ ([Fe3+]init) was varied from 0.1mM to 1mM, and the length of the experiments was 240min. Except for the experiment at initial pH 2, total dissolved iron ([Fe]total) decreased immediately (within 1min) after contact of FeS and Fe(III). Afterwards, [Fe]total increased smoothly. A progressive increase in pH values and an Eh decrease within 240min of reaction time were also observed. The reaction order of FeS dissolution in Fe(III)BAS with respect to [H+] is estimated to 0.65 at initial pH 3.0, and increases up to 1.0 with decreasing initial pH, indicating that [H+] is an important parameter of FeS dissolution in Fe(III)BAS. In contrast, changes in [Fe3+]init have only a limited effect on the rate of FeS dissolution in Fe(III)BAS. Raman spectra of initial and reacted FeS samples reveal the accumulation of α-S8-like material on FeS surface. These results support a mechanism of FeS dissolution in Fe(III)BAS starting with the protonation of mineral surface and Fe3+(aq) adsorption. Adsorbed protons subsequently accelerate Fe2+ release from FeS matrix into solution. The adsorbed Fe3+ may oxidize sulfur moieties and generate insoluble species, presumably polysulfide and elemental sulfur. The subsequent migration of Fe2+ into solution is controlled by the formed sulfur rich layer.

Rhenium–osmium isotope and platinum-group elements in the Xinjie layered intrusion, SW China: Implications for source mantle composition, mantle evolution, PGE fractionation and mineralization

The Xinjie mafic–ultramafic layered intrusion in the Emeishan large igneous province (ELIP) hosts Cu–Ni–platinum group element (PGE) sulfide ore layers within the lower part and Fe–Ti–V oxide-bearing horizons within the middle part. The major magmatic Cu–Ni–PGE sulfide ores and spatially associated cumulate rocks are examined for their PGE contents and Re–Os isotopic systematics. The samples yielded a Re–Os isochron with an age of 262 ± 27 Ma and an initial 187Os/ 188Os of 0.12460 ± 0.00011 ( γ Os( t) = −0.5 ± 0.1). The age is in good agreement with the previously reported U–Pb zircon age, indicating that the Re–Os system remained closed for most samples since the intrusion emplacement. They have near-chondritic γ Os( t) values ranging from −0.7 to −0.2, similar to those of the Lijiang picrites and Song Da komatiites. Exceptionally, two samples from the roof zone and one from upper sequence exhibit radiogenic γ Os( t) values (+0.6 to +8.6), showing minor contamination by the overlying Emeishan basalts. The PGE-rich ores contain relatively high PGE and small amounts of sulfides (generally less than 2%) and the abundance of Cu and PGE correlate well with S, implying that the distribution of these elements is controlled by the segregation and accumulation of a sulfide liquid. Some ore samples are poor in S (mostly <800 ppm), which may due to late-stage S loss caused by the dissolution of FeS from pre-existing sulfides through their interaction with sulfide-unsaturated flowing magma. The combined study shows that the Xinjie intrusion may be derived from ferropicritic magmas. The sharp reversals in Mg#, Cr/FeO T and Cr/TiO 2 ratios immediately below Units 2–4, together with high Cu/Zr ratios decreasing from each PGE ore layer within these cyclic units, are consistent with multiple magma replenishment episodes. The sulfides in the cumulate rocks show little evidence of PGE depletion with height and thus appear to have segregated from successive inputs of fertile magma. This suggests that the Xinjie intrusion crystallized from in an open magma system, e.g., a magma conduit. The compositions of the disseminated sulfides in most samples can be modeled by applying an R factor (silicate–sulfide mass ratio) of between 1000 and 8000, indicating the segregation of only small amounts of sulfide liquid in the parental ferropicritic magmas. Thus, continuous mixing between primitive ferropicritic magma and differentiated resident magma could lead to crystallization of chromite, Cr-bearing magnetite and subsequently abundant Fe–Ti oxides, thereby the segregation of PGE-rich Cu-sulfide. When considered in the light of previous studies on plume-derived komatiites and picrites worldwide, the close-to-chondritic Os isotopic composition for most Xinjie samples, Lijiang picrites and Song Da komatiites suggest that the ferropicritic magma in the ELIP were generated from a plume. This comprised recycled Neoproterozic oceanic lithosphere, including depleted peridotite mantle embedded with geochemically enriched domains. The ascending magmas thereafter interacted with minor (possibly <10%) subducted/altered oceanic crust. This comparison suggests that the komatiitic melts in the ELIP originated from a greater-than normal degree of melting of incompatible trace element depleted, refractory mantle components in the plume source.

Aerobic oxidation of mackinawite (FeS) and its environmental implication for arsenic mobilization

Abstract Oxidation of mackinawite (FeS) and concurrent mobilization of arsenic were investigated as a function of pH under oxidizing conditions. At acidic pH, FeS oxidation is mainly initiated by the proton-promoted dissolution, which results in the release of Fe(II) and sulfide in the solution. While most of dissolved sulfide is volatilized before being oxidized, dissolved Fe(II) is oxidized into green rust-like precipitates and goethite ( α -FeOOH). At basic pH, the development of Fe(III) (oxyhydr)oxide coating on the FeS surface inhibits the solution-phase oxidation following FeS dissolution. Instead, FeS is mostly oxidized into lepidocrocite ( γ -FeOOH) via the surface-mediated oxidation without dissolution. At neutral pH, FeS is oxidized via both the solution-phase oxidation following FeS dissolution and the surface-mediated oxidation mechanisms. The mobilization of arsenic during FeS oxidation is strongly affected by FeS oxidation mechanisms. At acidic pH (and to some extent at neutral pH), the rapid FeS dissolution and the slow precipitation of Fe (oxyhydr)oxides results in arsenic accumulation in water. In contrast, the surface-mediated oxidation of FeS at basic pH leads to the direct formation of Fe (oxyhydr)oxides, which provides effective adsorbents for As under oxic conditions. At acidic and neutral pH, the solution-phase oxidation of dissolved Fe(II) accelerates the oxidation of the less adsorbing As(III) to the more adsorbing As(V). This study reveals that the oxidative mobilization of As may be a significant pathway for arsenic enrichment of porewaters in sulfidic sediments.

Oxidation of FeS by oxygen-bearing acidic solutions

Oxidation of FeS in oxygen-bearing acidic solutions was investigated at different temperatures (25 to 45 °C) and pH (2.75 to 3.45). The rate of the oxidative dissolution of FeS is strongly dependent on pH. The reaction order with respect to hydrogen ions has been found to be 1.03 ± 0.02 at 25 °C, and the apparent activation energy ( E a ) is 41.6 ± 10.7 kJ mol −1 at initial pH 3.00, suggesting that the FeS oxidative dissolution is controlled by the diffusion of oxidant species across a sulfur-rich layer (SRL) that undergoes chemical transformations leading to an increase in the mean number of sulfur atoms in polysulfide chains and the rearrangement of these chains. Fourier transform infrared spectroscopy and X-ray diffraction results obtained for the FeS samples reacted for 72 h at 25 °C and pH between 2.75 and 3.45 indicate the formation of goethite, of lepidocrocite, and of poorly ordered solid phases (assigned as SRL) on initial surfaces. The experimental data suggest a mechanism based on the protonation of FeS surfaces followed by oxidation of FeS by dissolved oxygen to produce Fe 2+, S 0, and S 2− n . Fe 2+ is unstable under oxidative conditions and transforms into Fe(OH) 3(s), goethite and lepidocrocite.

Sulfide oxidation and sulfate reduction in a shallow groundwater system (Oderbruch Aquifer, Germany)

Detailed groundwater monitoring was carried out over a period of two years in an anoxic, river recharged aquifer of the Oderbruch polder, north-eastern Germany. Isotope data from wells located in a 5 km transect along the flow direction was used to determine sources and sinks of SO 4 2− in the aquifer. The SO 4 2− originates from river water infiltration and from oxidative dissolution of FeS 2 within the alluvial loam covering the aquifer sands. A change of confined hydraulic conditions near the river to unconfined conditions in the central polder effects the hydrochemistry of the aquifer. The confined areas are dominated by sulfate reduction. Increasing δ 34S −SO 4 values suggest continuous but slow ( t 1/2=50 years) sulfate reduction from the beginning of inflow onwards with δ 34S −SO 4 values ranging from +1.8 to +44.7‰ versus CDT and an enrichment factor of −33‰. A zone with a strong sulfate depletion ( δ 34S −SO 4 of up to +85.7‰) exists in a shallow microenvironment rich in solid-phase organic carbon between river and levee. In the unconfined areas of the central polder, a SO 4 2− plume with concentrations exceeding the original river water content indicates FeS 2 oxidation by O 2 and/or NO 3 − within the alluvial loam. The lowered δ 34S −SO 4 value reflects the input of the isotopically lighter SO 4 2− from the sulfide.

Kinetic and mechanistic study of FeS dissolution in aqueous sulfur dioxide solution

FeS leaching was carried out in aqueous sulfur dioxide solution at low temperature and pressure ( T = 60° CC; P SO 2 = 51.7 kPa) using both a constant-pressure gas-uptake apparatus and conventional glass-leaching equipment. In the initial stages of the leaching reaction the ratio [SO 2]/[Fe 2+] was found equal to 1.3. This suggests a stoichiometry given by the following sequence of equations FeS + SO 2·H 2O ⇌ FeS + H + FeS · HSO 3 − + H + → Fe 2+ + H 2S + SO 3 2− ▪ As the reactions progressed a build up of various oxy-sulfur species was observed. The leaching process appears to proceed by two concurrent dissolution paths, namely acid dissolution and aquated sulfur dioxide dissolution. In both cases the rate determining step (r.d.s.) involves a surface desorption reaction. In the constant leaching region, the kinetics can be represented by the following rate expression: ▪ At low [H+] relative to [SO 2-H 2O], path I predominates. This can be represented by: | sFeS +SO 2 · H 2O ⇌ | 2FeS SO 2 · H 2O | sFeS SO 2 · H 2O ⇌ | s FeS HSO 3 − + H + | s FeS HSO 3 − → Fe 2+ (aq) r.d.s. At high [H +] relative to [SO 2·H 2O], path II predominates. This can be represented by: | sFeS + H + ⇌ | s FeS H + | sFeS H + → Fe 2+ (aq) r.d.s. The apparent activation energy was found to be 41.6 kJ per mole.

Sphalerite geobarometry of deformed sulphide ores from the C. S. A. Mine, Cobar, Australia

The C. S. A. Mine is located near Cobar, central New South Wales. The copper-zinc-lead ores occur in Early Devonian rocks of the Cobar Super-Group. Lower greenschist (“slate-grade”) metamorphism has developed elongate lenticular ore systems parallel to the extension (down-dip) lineation in cleavage. FeS contents of sphalerites coexisting with pyrite and pyrrhotite outside and inside pressure shadows indicate much higher pressures (7.7 to 9.0 kbar) than those inferred from stratigraphic reasoning and the low metamorphic grade. The homogeneous distribution of Fe in sphalerites suggests equilibration with pyrite-pyrrhotite; and concentrations of Co and Ni in iron sulphides, and Mn, Cd and Cu in sphalerite are too low to have influenced phase relations in the FeS-ZnS pseudobinary system. The anomalously high pressures are therefore ascribed to reequilibration of sphalerite compositions with a monoclinic pyrrhotite-pyrite buffer. The FeS contents of the reequilibrated sphalerites apparently reflect the differing mean stress domains that exist outside and inside pressure shadows. This suggests that reequilibration occurred under the same stress distribution as produced the original pressure shadows, and implies FeS dissolution during the decay of the cleavage-producing structuro-metamorphic event. The commonly observed scatter of sphalerite compositions in low grade assemblages appears to record micro-scale mean stress domains, and thereby testifies to the pressure sensitivity of the mole percent FeS contents.

Dissolution of iron sulfide (troilite) in aqueous sulfuric acid

The dissolution of iron sulfide (troilite) was studied in dilute sulfuric acid, sodium hydroxide, and H/sub 2/S saturated solutions of different pH. The dissolution kinetics of the reaction FeS + 2H/sup +/ reversible Fe/sup 2/ + H/sub 2/S (g) was studied by following the depletion of H/sup +/ from the bulk solution in which disks holding FeS pellets were rotated. These studies showed that the rate of reaction is not controlled by transport of the reactants or the products. The activation energy for the dissolution of FeS is 60 +- 7 kJ mole/sup -1/. The forward reaction is directly proportional to the acid concentration and the reverse reaction is one-half order with respect to both the concentration of Fe/sup 2 +/ and H/sub 2/S pressure. Thermodynamic quantities for the dissolution reaction are reported.