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Biostimulated sulfate reduction is a promising strategy for remediating groundwater arsenic contamination. But biostimulation often induces pH changes alongside biogeochemical transformations of iron, sulfur, and arsenic, which all affect arsenic mobility and the efficacy of arsenic sequestration. This study tests whether ethanol-based biostimulation is a viable solution to arsenic contamination in iron-rich freshwater aquifers. We incubated the sediments from a volcaniclastic aquifer in laboratory microcosms and stimulated microbial sulfate reduction by adding ethanol and sulfate. The ethanol amendment also enhanced syntrophic ethanol oxidation, microbial iron reduction, and acetoclastic methanogenesis. These microbial processes changed pH, generated sulfide, ferrous iron, and bicarbonate, and removed arsenic – but only temporarily. Specifically, during the first 13 days of the experiments, pH fell by 2 units and, at the same time, dissolved arsenic levels first increased and then decreased by 90%. Afterwards, both pH and total arsenic levels increased steadily. We attribute the temporary arsenic removal to the adsorption on freshly-precipitated mackinawite, and the later accumulation to the desorption by the pH increase and the accumulation of ferrous iron and bicarbonate. These results support the potential of biostimulated sulfate reduction for remediating groundwater arsenic, and highlight the challenges brought by concurrent pH variations and biogeochemical processes.

Arsenic (As) immobilization in aqueous environments is controlled by sorption and/or coprecipitation with iron (Fe) oxyhydroxides under oxic conditions and with ferrous iron (Fe(II)) and sulfide (S(-II)) minerals under anoxic conditions. Shifts to reducing conditions trigger the release of dissolved As by reductive dissolution of Fe oxyhydroxides. More stringent reducing conditions in sulfur (S) rich environments form solid S(-II) phases that immobilize As. This work studied the association of arsenite and arsenate with freshly-formed disordered mackinawite (FeS), a model Fe and reduced S mineral of environmental relevance. The association of As to preformed FeS (Set P experiments) was contrasted to the case when As is present during FeS precipitation (Set C experiments). Most abiotic studies of As reactivity under reducing conditions have been done under Set P setting, despite that redox shifts are likely to induce environments analogous to Set C experiments. Dissolved As removal was initially higher for Set C compared to Set P experiments at least by a factor of two, except for arsenate with low initial redox potential. Set C experiments had a rapid initial As sorption, followed by some As desorption. When stringent reducing conditions ensue, the extent and rate of As removal by FeS is misrepresented by sorption experiments to preformed FeS. A continuous release of As concurrent with the formation of thioarsenates - a dissolved form of As with weaker binding to Fe minerals - was observed in Set C experiments with arsenite. The formation of thioarsenates occurred preferably in Set C experiments at low redox potential, while it was negligible for arsenate experiments. Remarkably, the occurrence of thioarsenates disputes a widespread notion that their formation is inhibited by Fe(II)-S(-II) affinity and precipitation as FeS. This study shows the potential of FeS to scavenge arsenite and to decrease its mobility in reduced sediments and groundwater, and highlights the role of thioarsenates in As mobility in systems with changing redox conditions where the reactive solid forms in presence of the contaminant.

The iron sulfide mineral greigite, Fe 3 S 4 , has previously been identified in the surface layers of the intertidal zone of a partially remediated acid sulfate soil wetland, representing an end-member in an iron sulfide mineralization pathway distinct from that of pyrite. The persistence of greigite is important for the stability of the remediated landscape, but the response of greigite to periods of oxygenation (for example, during a neap tide) is poorly understood. In this study, we employ X-ray absorption spectroscopy to identify the Fe and S speciation and mineralogy resulting from the oxidative disintegration of synthetic greigite under physiochemical solution conditions consistent with a partially remediated acid sulfate soil wetland. Results indicate divergent Fe and S mineralization pathways culminating in elemental sulfur and iron (hyr)oxide minerals. No sulfate-containing minerals were identified, and under all conditions tested, residual greigite remains. The oxidation products, and the presence of sulfur reducing bacteria, provide the right chemical environment for the reformation of greigite during the sub-oxic conditions of the rest of the tidal cycle. This likely explains the persistence of greigite in the intertidal zone, and implies that the oxidation of greigite is not detrimental to the long term stability of the acid sulfate soil remediation process.

The uptake of aqueous Ni(II) by synthetic mackinawite (FeS) was examined in anaerobic batch experiments at near-neutral pH (5.2 to 8.4). Initial molar ratios of Ni(II) to FeS ranged from 0.008 to 0.83 and maximum Ni concentrations in mackinawite, expressed as the cation mol fraction, were as high as XNi = 0.56 (Fe1 - x Ni x S; 0 ≤ x ≤ 1). Greater than 99% Ni removal from solution occurred when Ni loading remained below 0.13 ± 0.03 (1σ) mol Ni per mol FeS due to sorption of Ni at the mackinawite surface. Characterization of experimental solids using X-ray diffraction and Raman spectroscopy showed patterns characteristic of nanocrystalline mackinawite; no evidence of nickel monosulfide (α-NiS or millerite), polydymite (Ni3S4), or godlevskite [(Ni,Fe)9S8] formation was indicated regardless of the amount of Ni loading. Slight expansion of the c-axis correlated with increasing Ni content in synthetic mackinawite, from c = 5.07 ± 0.01 A at XNi = 0.02 to c = 5.10 ± 0.01 A at XNi = 0.38. Ni K-edge extended X-ray absorption fine structure (EXAFS) spectra of synthetic Ni-bearing mackinawite are similar in phase and amplitude to the Fe K-edge EXAFS spectrum of Ni-free mackinawite, indicating that the molecular environment of Ni2+ in Ni-bearing mackinawite is similar to that of Fe2+ in Ni-free mackinawite. EXAFS data fitting of Ni-bearing mackinawite with XNi = 0.42 indicated a coordination number of 4.04 ± 0.30 and an average Ni_S bond distance of 2.28 A, in good agreement with the Fe_S bond distance of 2.26 A in mackinawite, tetrahedral Fe coordination, and slight lattice expansion along the c-axis. At lower Ni loadings (XNi = 0.05-0.11), EXAFS analysis showed a decrease in Ni_S coordination towards CN = 3, which reflects the influence of sorbed Ni. Continued Ni uptake, past the maximum amount of sorption, was accompanied by proportional molar release of Fe to solution. Interstitial occupancy of Ni within the mackinawite interlayer may be transitional to structural substitution of Fe. The Ni-mackinawite solid-solution is described by a one-site binary mixing model: LnKd=lnKe-(W/RT)(1-2XNi) where Kd is the distribution coefficient, Ke is the ratio of equilibrium constants for Ni-mackinawite and mackinawite (14.4 ± 1.3), W is an ion interaction parameter, and XNi is the mole fraction of end-member NiS in the solid solution. The experimentally determined value of W is 17.74 ± 1.15 kJ/mol and indicates significant non-ideality of the solid solution. Transformation processes were evaluated by aging Ni-mackinawite with polysulfides and solutions saturated with air. Reaction of Ni-mackinawite with polysulfides led to the formation of pyrite (FeS2) and Ni retention in the solid phase. When Ni-mackinawite was aged in the presence of dissolved oxygen, transformation to goethite (FeOOH) and violarite (FeNi2S4) was observed.

Water samples were collected for arsenic (As) and molybdenum (Mo) analysis from different depths in Lakes Hoare and Fryxell, both of which are located in the Taylor Valley within the McMurdo Dry Valleys of Antarctica. Sampling depths within each lake were chosen to capture variations in As and Mo concentrations and As speciation in the oxic mixolimnia and anoxic monimolimnia of these meromictic lakes. Arsenic concentrations ranged from 0.67 nmol kg − 1 to 3.54 nmol kg − 1 in Lake Hoare and from 1.69 nmol kg − 1 to 17.5 nmol kg − 1 in Lake Fryxell. Molybdenum concentrations varied between 5.05 nmol kg − 1 and 43 nmol kg − 1 in Lake Hoare, and between 3.52 nmol kg − 1 and 25.5 nmol kg − 1 in Lake Fryxell. Concentrations of As and Mo generally increased with depth in the mixolimnion of each lake, consistent with uptake near the ice–water interface by organic particles and/or Fe/Mn oxides/oxyhydroxides, followed by gravitational settling and regeneration/remineralization at depth in the vicinity of the redoxcline. Arsenic concentrations either remained constant (Hoare) or increased with depth (Fryxell) in the anoxic monimolimnia, whereas Mo exhibited dramatic decreases in concentrations across the redoxcline in both lakes. Geochemical modeling predicts that As and Mo occur as thioanions in the anoxic bottom waters of Lakes Hoare and Fryxell, and further that the contrasting behavior of both trace elements reflects the respective reactivity of their thioanions towards Fe-sulfide minerals such as mackinawite (FeS) and/or pyrite (FeS 2 ). More specifically, the geochemical model suggests that Fe-sulfide mineral precipitation in the anoxic monimolimnia of both lakes regulates dissolved sulfide concentrations at levels that are too low for As-sulfide minerals (e.g., orpiment, realgar) to precipitate, whereas mackinawite and/or pyrite react(s) with particle reactive thiomolybdate anions, possibly forming an Fe–Mo–S mineral that precipitates and, hence, leads to Mo removal from solution.

The effects of environmental factors, such as dissolved metal species and their concentrations, on microbial sulfide formation and U(VI) reduction processes are important. During microbial sulfate reduction by Desulfovibrio desulfuricans , the initial aqueous uranium(VI) in our system decreased and later increased as a liquid uranium(IV) suspension. The change in the uranium phase was strongly associated with a biogenic FeS solid (mackinawite), the mineralogical growth of which was affected by the coexisting metal cations (e.g., Ni and Cu). Using laser-induced fluorescence and capillary cell spectroscopy, zeta potentiometry, and ultrafiltration techniques, the produced U(IV) suspension was investigated in situ at a very low level (≤ 10 − 6 M) and was identified to be uranium dioxides (UO 2 ) in the form of nm-sized colloids. In addition to the spectroscopic techniques, the direct examination by high-resolution transmission electron microscopy (HRTEM) revealed that the U(IV) phase evidently occurs from the FeS surface in the form of discrete UO 2 nanocrystals. Our results indicate that the metal sulfide produced by sulfate-reducing bacteria (SRB) may be an important agent for dispersing the aqueous U(VI) in a colloidal U(IV) form, which would be mobile in porous media such as soils and sediments due to its nanometer-sizes. This study contributes to the understanding of how the FeS metallic conductor abiotically affects the behavior of redox-sensitive uranium by transforming its properties at the mineral surface in sulfate reducing environments.

Manganese (Mn) enrichments in sediments of the deep basins of the Baltic Sea are believed to consist of Ca-Mn-carbonates that form from Mn oxides following periodic inflows of oxygen-rich North Sea waters. However, a range of Mn-bearing mineral phases, that besides Mn-carbonates (e. g. Ca-rhodochrosite), can include Mn-sulfides are known to be present in marine sediments, with formation mechanisms that are sensitive to redox conditions. In this study, we use high resolution synchrotron EXAFS and XANES combined with LA-ICP-MS and micro-XRF, to investigate the nature of the Mn enrichments in sediments from the Holocene Thermal Maximum (approx. 8000-4000 cal. yr BP) at a site in the northern Gotland Basin. Analyses were performed on epoxy-embedded sediment sequences. We specifically address the role of changes in redox conditions in bottom waters, as inferred from sediment molybdenum(Mo) contents, for Mn sequestration. We find that an up-core increase in Mo, indicating a transition into more anoxic and sulfidic (euxinic) bottom water conditions, is accompanied by a decline in total sediment Mn contents. While Mn-carbonates dominate the Mn mineral fraction in the low-Mo interval, in the more sulfidic, high Mo interval, Mn is associated with framboidal pyrite. Mn/Fe ratios in the sulfidic interval vary between the investigated sequences but reach values of up to 7.7 mol%. This exceeds ratios previously reported for sedimentary pyrite and EXAFS spectra indicate that sulfide bound Mn is predominately tetrahedrally coordinated when Mn/Fe ratios are high. This suggests that Mn is incorporated in other Fe-S phases such as mackinawite besides pyrite. There is also evidence for the presence of a low concentration of Mn associated with detrital aluminosilicates throughout the investigated intervals. Our results suggest that increased bottom water euxinia inhibited the formation of Mn-carbonate at this site during the Holocene Thermal Maximum, possibly due to a more rapid reduction of Mn oxides in more sulfidic bottom waters. (C) 2014 Elsevier B.V. All rights reserved. (Less)

Microbial sulfate reduction is an important terminal electron accepting process in arsenic-contaminated subsurface environments. Humic acids are ubiquitous in such environments, yet their impact on arsenic mobility under sulfate-reducing conditions is poorly understood. In this study, we examined the effects of microbial sulfate reduction and humic acid on arsenic mobilization via a series of advective-flow column experiments. The initial solid-phase in these experiments comprised quartz sand that was coated with As(III)-sorbed goethite (α-FeOOH). The effect of humic acid was assessed by comparing columns that received artificial groundwater in which humic acid was either absent or present at 100 mg L− 1, whilst the effect of microbial sulfate reduction was investigated by comparing columns that were inoculated with the sulfate-reducer Desulfovibrio vulgaris (ATCC strain 7757) versus abiotic control columns. The presence of high concentrations of humic acid alone did not enhance the overall extent of arsenic release from either the abiotic or the inoculated (sulfate reducing) columns. This is consistent with similar arsenic concentrations in porewaters filtered to both 40% of aqueous arsenic even at very low porewater sulfide concentrations (i.e.

Jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ) is an important host-phase for As in acid mine drainage (AMD) environments and coastal acid sulfate soils (CASS). In AMD and CASS wetlands, jarosite may encounter S(− II) produced by sulfate reducing bacteria. Here, we examine abiotic sulfidization of As(V)-bearing K-jarosite at pH 4.0, 5.0, 6.5 and 8.0. We quantify the mobilization and speciation of As and identify corresponding Fe mineral transformations. Sulfide-promoted dissolution of jarosite caused release of co-precipitated As and the majority of mobilized As was re-partitioned to a readily exchangeable surface complex (As Ex ). In general, maximum As mobilization occurred in the highly sulfidized end-members of all treatments and was greatest at low pH, following the order pH 5.0 ≈ 4.0 > 8.5 > 6.5. X-ray absorption spectroscopy revealed that most solid-phase As remained as oxygen-coordinated As(V) when pH values were > 5.0 — even during latter stages of sulfidization and the presence of ≥ 100 μM dissolved S(− II). In contrast at pH 4.0, As transitioned from oxygen-coordinated As(V) to a sulfur-coordinated orpiment-like phase. This transition coincided with a marked decrease in As Ex , attenuation of As (aq) and TEM-EDX spectra indicate concurrent formation of nano-scale zones variably enriched in As (~ 1–15%). Although discordant with geochemical modeling, the formation of an orpiment-like precipitate appears to be a primary control on As mobility during the late stages of complete jarosite sulfidization under acidic conditions (pH 4.0). Mackinawite was the main Fe-mineral end product in all pH treatments. However, at pH 8.0, jarosite rapidly ( (aq) , the transformation process itself was incongruent with electron transfer to Fe(III). Further investigation is required to determine whether the electron donor triggering this transformation was direct via S(− II), or indirect via surface complexed Fe(II) and hence akin to the widely-known Fe(II)-catalyzed transformation of Fe(III) minerals. The results demonstrate that abiotic sulfidization of As(V)-co-precipitated jarosite can mobilize substantial As and that pH exerts a major control on the subsequent As solid-phase speciation, electron transfer kinetics and Fe mineralization pathways and products. The findings are particularly relevant to heterogeneous sediments in which As-bearing jarosite encounters dissolved sulfide under a range of pH conditions.

The mobility and availability of arsenite, As(III), in anoxic environments is largely controlled by adsorption onto iron sulfides and/or precipitation of arsenic in solid phases. The interaction of As(III) with synthetic mackinawite (FeS m ) in pH 5 and 9 suspensions was investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), STEM elemental mapping, high resolution TEM, and X-ray photoelectron spectroscopy (XPS). At pH 5, arsenic sulfide phases precipitate among the FeS m particles as discrete particles that are an amorphous hydrous phase of arsenic sulfide. The oxidation state of As in the surface layers of the arsenic sulfide precipitates is ‘realgar-like’ based on XPS results showing that > 75% of the As 3d peak area is due to As with oxidation states between 0 and 2+. Discrete, arsenic sulfide precipitates are absent at pH 9, but elemental mapping in STEM-EDX mode shows that arsenic is uniformly distributed on the FeS m , suggesting that uptake is caused by the sorption of As(III) oxyanions and/or the precipitation of highly dispersed arsenic sulfides on FeS m . XPS also revealed that the FeS m that equilibrated without As(III) has a more oxidized surface composition than the sample at pH 9, as indicated by the higher concentration of O (∼ three times greater than that at pH 9) and the larger fraction of Fe(III) species making up the total Fe (2p 3/2 ) peak. These findings provide a better understanding of redox processes and phase transitions upon As(III) adsorption on iron sulfide substrates.

Pyrite is precipitated within the cells of higher plants and algae at ambient temperatures by diffusion of aqueous Fe(II) followed by reaction with microbial or inorganic S(− II). The absence of any additional oxidizing agent in sufficient concentrations to account for the mass of pyrite formed in the experimental systems suggests that the reaction involves aqueous H2S. Pyrite is formed from undersaturation relative to mackinawite, FeSm, since FeSm is not observed in the cells and it is seen to dissolve in the experimental systems. The overall pyrite forming reaction, starting with Fe(II) as a reactant, appears to be Fe(II) + H2S → FeS0 + H2S → FeS2p where FeS0 is the monomeric representation of an aqueous FexSx cluster or reaction intermediary. Although a series of balanced reactions could be written, none of these provides a unique description of the process and reaction mechanisms cannot be determined from this type of empirical experimentation. Pyrite formation is determined by the rate of H2S diffusion into the cells. An apparent diffusion coefficient for H2S of 10− 8 cm2.s− 1 is obtained, which includes the effects of tortuosity and effective porosity. The supersaturation limit for pyrite formation in these cells approaches 1011, which is lower than the value found for pyrite nucleation on pyrite seeds. Biological surfaces, such as plant cells, appear to have a greater surface activity than pyrite seeds. However, the supersaturation limit is affected by transport through the diffusion boundary layer, which is a surface-independent common rate-determining process. The rate of pyrite formation in the cells is relatively rapid, with over 50 vol.% pyrite being formed within the parenchyma cells of Apium sp. within 2 weeks. The rapid formation of pyrite within plant cells contributes to the preservation of cells during fossilization. There is no evidence for particulate iron sulfides penetrating the cells and the nature of the cell wall would preclude this. Greigite, Fe3S4 and mackinawite, FeSm, are therefore not necessarily direct precursors to pyrite formation, which is consistent with their observed limited abundance in sedimentary systems. The direct involvement of stoichiometric quantities of other inorganic components, such as dioxygen and elemental sulfur, was precluded in the experimental design, through the chemistry of the plant substrate and through the exclusion of particulates by the cell wall system. The intracellular chemical environment is complex, however, and trace oxidants may play a role in initiating pyrite nucleation.

The primary FeS precipitate formed from the reaction between Fe(II) and S(-II) in aqueous solutions at ambient temperatures and pressures is nanoparticulate stoichiometric mackinawite, Fe1.00 ± 0.01S. The material is not hydrated. It is probable that sedimentary mackinawite has this composition. Previous reports of mackinawite compositions of Fe1±xS are incorrect. Mackinawite dissolves in mineral acids to produce rhombic sulfur, the amount of sulfur formed being related to the acid concentration used. The formation of rhombic sulfur, which is not readily soluble in simple mineral acids, leads to poor total recoveries of Fe and S as well as to significant uncertainties in the precision of the analyses. The problem can be overcome by including a reducing agent, such as Ti(III) citrate, in the digestion procedure. Evaporative drying of the material leads to the formation of other contaminants which may be removed by intensive washing and re-suspension of the precipitate or long-term storage in low H2O anoxic environments. Analyses of S(-II) and Fe sulfide species in sediments and natural waters containing FeS using digestion by mineral acids in the absence of a reducing agent are likely to be affected by these results.

The formation pathways of pyrite are controversial. Time resolved experiments show that in reduced sulphur solutions at low temperature, the iron monosulphide mackinawite is stable for up to 4 months. Below 100°C, the rate of pyrite formation from a precursor mackinawite is insignificant in solutions equilibrated solely with H2S(aq). Mackinawite serves as a precursor to pyrite formation only in more oxidised solutions. Controlled, intentional oxidation experiments below 100°C and over a wide range of pH (3.3–12) confirm that the mackinawite to pyrite transformation occurs in slightly oxidising environments. The conversion to pyrite is a multi-step reaction process involving changes in aqueous sulphur species causing solid state transformation of mackinawite to pyrite via the intermediate monosulphide greigite. Oxidised surfaces of precursors or of pyrite seeds speed up the transformation reaction. Solution compositions from the ageing experiments were used to derive stability constants for mackinawite from 25°C to 95°C for the reaction: FeS(s) +2 H + ⇔ Fe 2+ + H 2 S The values of the equilibrium constant, logKFeS, varied from 3.1 at 25°C to 1.2 at 95°C and fit a linear, temperature-dependent equation: logKFeS=2848.779/T−6.347, with T in Kelvin. From these constants, the thermodynamic functions were derived. These are the first high temperature data for the solubility of mackinawite, where Fe2+ is the dominant aqueous ferrous species in reduced, weakly acidic to acidic solutions.

Pyrite formation under hydrothermal conditions has been studied using in situ time-resolved X-ray diffraction (XRD). This study employed two different synchrotron X-ray sources (National Synchrotron Light Source (NSLS), Advanced Photon Source (APS); Brookhaven and Argonne National Laboratories, USA, respectively) and two types of reaction cells (capillary and hydrothermal autoclave type) to examine reactions in the Fe–S system under both anoxic and controlled oxic conditions. Starting materials were mackinawite slurries equilibrated in reduced, H 2 S (aq) solutions and heated to a maximum of 190°C. The results show that under fully anoxic conditions, mackinawite persists to at least 120°C, while aerated (oxic) slurries transform rapidly to greigite, pyrite, magnetite and goethite. Possible reaction mechanisms are discussed in light of these results. These experiments are the first application of previously described in situ reaction cells to anoxic and oxic reactions in the Fe–S system. Finally, the capabilities of each cell type and their applicability to other systems are discussed.

This study characterizes the surface chemistry and morphology of poorly crystalline iron sulfides precipitated in a chemically defined growth media for sulfate-reducing bacteria. The precipitates were analyzed by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Based on XRD results, the precipitates exhibit only incipient crystallization with a single broad diffraction peak at about 5 A, characteristic of disordered mackinawite. The iron sulfides generally exhibit a platy habit with particles 100 to 300 nm in diameter; these particles appear often in 1 to 2 μm spherical aggregates. The XPS results indicate that the Fe(2p3/2) spectrum for poorly crystalline iron sulfide can be fitted with Fe(II) and Fe(III) components, both corresponding to Fe–S bond types. The absence of oxide oxygen in the O(1s) spectrum and Fe(III)–O bond types in the Fe(2p3/2) spectrum supports the conclusion that the poorly crystalline iron sulfides are composed of both ferric and ferrous iron coordinated with monosulfide, with lesser amounts of disulfide and polysulfides also present. These results suggest that the precipitates possess a surface composition similar to greigite, with the remaining mineral mass composed of disordered mackinawite.

Assumptions commonly made during analysis of the amount of monosulfides [acid-volatile sulfides (AVS)] and disulfides in modern sediments, may not be valid for ancient sedimentary rocks. It is known that ferric iron can oxidize H2S during AVS analysis unless a reducing agent such as stannous chloride is added to the treatment. In addition, some monosulfides such as greigite and pyrrhotite require heat during the AVS analysis in order to dissolve completely. However, the use of heat and/or stannous chloride in the AVS treatment may partially dissolve disulfides and it is generally recommended that stannous chloride not be used in the AVS treatment for modern sediments. Most of the monosulfides are assumed to be recovered as AVS without the addition of stannous chloride. This study investigates the recovery of monosulfides during sulfur speciation analysis with application to ancient sedimentary rocks. Sulfur in samples containing naturally occurring greigite and mackinawite or pyrite was measured using variations of a common sulfur-speciation scheme. The sulfur-speciation scheme analyzes for monosulfide sulfur, disulfide sulfur, elemental sulfur, inorganic sulfate and organically bound sulfur. The effects of heat, stannous chloride and ferric iron on the amounts of acid-volatile sulfide and disulfide recovered during treatment for AVS were investigated. Isotopic compositions of the recovered sulfur species along with yields from an extended sulfur-speciation scheme were used to quantify the effects. Hot 6 N HCl AVS treatment recovers > 60% of the monosulfides as AVS in samples containing pure greigite and mackinawite. The remaining monosulfide sulfur is recovered in a subsequent elemental sulfur extraction. Hot 6 N HCl plus stannous chloride recovers 100% of the monosulfides as AVS. The addition of ferric iron to pure greigite and mackinawite samples during AVS treatment without stannous chloride decreased the amount of monosulfides recovered as AVS and, if present in great enough concentration, oxidized some of the AVS to a form not recovered in later treatments. The hot stannous chloride AVS treatments dissolve

In low-temperature aqueous solutions ( 2 ) does not form through direct precipitation from solution. The pathway is exclusively via precursor iron sulphides and dissolved Cu salts. The reaction of dissolved Cu (II) salts with natural hexagonal pyrrhotite (Fe 0.9 S) is diffusion controlled. The initial stage has an apparent activation energy of 11.4 ± 1.8 kJ mol −1 and the rate (in units of mol dm −3 s −1 cm −2 ) is independent of the solid reactant surface area. The reaction proceeds through a series of metastable Cu-Fe-sulphide intermediaries. These phases form a series of ephemeral layers penetrating into the pyrrhotite surface. The first phase formed has the stoichiometry Cu 0.1 Fe 0.9 S. No Fe is released into the solution during its formation and this, together with the extremely low apparent activation energy and the stoichiometry, suggest that it is formed by stuffing of electron holes in the pyrrhotite structure with Cu ions. The transformation from the hexagonal close-packed arrangement of the pyrrhotite structure to the essentially cubic packing in chalcopyrite proceeds through a series of intermediaries, approximating in composition to members of the cubanite group. The rate of formation of these phases is controlled by the coupled diffusion of Fe (II), Fe (III), Cu (I) and Cu (II) species through the surface reaction zone, although the process as a whole can be approximated by steady-state diffusion of total Cu into a semi-infinite medium. Experiments with metastable precursor iron monosulphide phases, including amorphous FeS and synthetic mackinawite indicate similar reaction pathways. The results suggest that chalcopyrite formation in low-temperature natural systems may be significantly constrained by kinetic factors. Chalcopyrite is, at least, a diagenetic mineral since its formation requires the prior formation of iron sulphides. However, at ambient temperatures its formation is probably limited to very early diagenesis.

Sediment samples contaminated with metals arising from mine tailings drainage were obtained from Lower Moose Lake in the Onaping region near Sudbury, Ontario, Canada. The samples were examined by electron microscopy, selected-area electron diffraction and energy-dispersive X-ray spectroscopy. Individual bacterial cells and their remains were prominent as nucleation sites for both metal sulfides and a complex polymorphic (Fe,Al)-silicate. The principal metal sulfide species associated with the bacteria were amorphous mackinawite (FeS 1 - x ) and microcrystalline millerite (NiS). Trace amounts of Cu and Zu were also detected in some of the sulfide precipitates. At least two structural forms of the (Fe,Al)-silicate were present, and energy-dispersive X-ray spectroscopy point analyses revealed corresponding differences in chemical composition. Poorly ordered limonitic clay-type phases had a granular morphology and contained less Fe than well-developed crystalline material which generated hexagonal diffraction patterns with reflections ( d = 4.60 and 2.55 Ȧ) characteristic of an interstratified chamositic clay.

The first precipitate formed through the reaction between aqueous Fe(II) salts and dissolved sulphide at ambient temperatures and pH < 9, appears to be a highly disordered gel approaching the composition Fe(HS)2 on a water-free basis. After ∼ 0.4 s this precipitate loses sulphide and amorphous FeS begins to appear. The mackinawite structure begins to develop after several hours. The rate of formation of the initial precipitate can be approximated by a pseudo first-order reaction, directly dependent on total sulphide concentration and with an apparent pseudo first-order rate constant of 48 ± 9 s−1. Dissolved Fe concentration does not appear to be rate limiting. The estimated solubility of the initial phase is variable but consistently one to two orders of magnitude greater than the measured solubilities for amorphous FeS. In natural systems this may lead to variable Fe(II) solubilities in sulphidic environments. This initial material may play a more central role in iron sulphide reaction pathways than either mackinawite or amorphous FeS.