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Excess phosphorus abundance often drives eutrophication and affects surface water quality. Formation of vivianite (Fe3(PO4)2 • 8H2O) in aquatic sediments acts as a significant sink for phosphate (P), crucial for resorting surface waters. Authigenic vivianite formation, however, can be limited by other ferrous iron containing phases, in particular iron sulfides. Although thermodynamically feasible under suitable conditions, the formation of vivianite from mackinawite has been widely disregarded for authigenic phosphate mineral formation. Here we investigated the formation of vivianite from mackinawite (FeS) in batch experiments in which dissolved sulfide was continuously removed, at P levels between 0 – 5 mM in a pH of 6 to 8. Solid characterizations by electron microscopy, X-ray diffraction as well as Mössbauer and X-ray absorption spectroscopy demonstrates that vivianite was formed at all pH values in P amended experiments. The temporal evolution of dissolved Fe(II) concentrations indicates that the transformation proceeds via the release of the dissolved Fe(II) by FeS dissolution and subsequent vivianite precipitation, over time scales of days. The kinetics of the transformation are controlled by the dissolution rates of FeS. Aging and transformation of FeS, however, compete with vivianite formation. Aging is more pronounced at higher pH but is inhibited by P adsorption. Hence, the effect of pH and P concentration on aging is the main reason for the influence on these parameters on the rates and extent of vivianite formation. Our findings demonstrate that FeS can be an effective iron source for vivianite formation in aquatic sediments when sulfide concentrations decrease due to, for example, changes in external forcing or microbial sulfide oxidation. Formation of vivianite from FeS as an Fe source can also open new perspectives in P recovery in water treatment, for example when Fe is added to digesters to bind H2S.

Metal sulphides constitute cheap, naturally abundant, and environmentally friendly materials for energy storage applications and chemistry. In particular, iron (II) monosulphide (FeS, mackinawite) is a material of relevance in theories of the origin of life and for heterogenous catalytic applications in the conversion of carbon dioxide (CO2) towards small organic molecules. In natural mackinawite, Fe is often substituted by other metals, however, little is known about how such substitutions alter the chemical activity of the material. Herein, the effect of Ni doping on the structural, electronic, and catalytic properties of FeS surfaces is explored via dispersion-corrected density functional theory simulations. Substitutional Ni dopants, introduced on the Fe site, are readily incorporated into the pristine matrix of FeS, in good agreement with experimental measurements. The CO2 molecule was found to undergo deactivation and partial desorption from the doped surfaces, mainly at the Ni site when compared to undoped FeS surfaces. This behaviour is attributed to the energetically lowered d-band centre position of the doped surface, as a consequence of the increased number of paired electrons originating from the Ni dopant. The reaction and activation energies of CO2 dissociation atop the doped surfaces were found to be increased when compared to pristine surfaces, thus helping to further elucidate the role Ni could have played in the reactivity of FeS. It is expected that Ni doping in other Fe-sulphides may have a similar effect, limiting the catalytic activity of these phases when this dopant is present at their surfaces.

The chemical speciation of mercury (Hg), methyl mercury (MeHg), sulfur and iron was investigated in the sediment and porewater of Lake Ängessjön, a boreal, shallow (maximum depth 2.5 m), oligo-/dystrophic lake in northern Sweden. The lake receives terrestrial stream runoff from surrounding coniferous forest soils and peatlands having a low pH (4.6) and high concentrations of dissolved organic matter (DOM, annual average: 45 mg L−1), Fe (60 µM), sulfate (105 µM), inorganic Hg (1200 pM) and MeHg (250 pM). Sulfur K-edge XANES and Hg LIII-edge EXAFS spectroscopic measurements were used to characterize and quantify the sulfur speciation in the lake sediment at nine occasions, covering different seasons in the years of 2007 and 2009. In the surface sediment (0–3 cm) sulfate is reduced to zero-valent S and inorganic sulfide, that in turn reacts with Fe to form FeSm (mackinawite) and FeS2 (framboidal pyrite). The latter mineral becomes increasingly dominant by depth in the sediment. Thermodynamic modeling successfully predicted measured porewater concentrations of Hg in the sediment. Metacinnabar (β-HgS) and Hg(NOM-RS)2 complexes (the latter formed as a reaction between Hg(II) and thiol groups associated with natural organic matter, NOM-RSH) were the dominant forms of Hg(II) in the solid phase of sediments and Hg(II)-polysulfides (aq) dominated in the porewater. We argue that FeSm is a key component that indirectly controlled the Hg(II) speciation in the sediment by keeping the aqueous phase concentration of inorganic sulfide in the 0.5–2 µM range throughout the year. Besides providing a pool of readily soluble inorganic sulfide for formation of β-HgS(s), as demonstrated by previously reported EXAFS experiments, we further suggest FeSm may serve as a precursor for the formation of a more crystalline (less soluble) β-HgS(s) phase than present in environments devoid of FeSm. Support for this was provided by comparing our results with previously reported thermodynamic modellin

The precipitation of nickel with sulfide is an important process governing the bioavailability of Ni in natural waters, and this process has the potential to effectively remove aqueous Ni contaminants in near-surface environments. In this study, we use experimental approaches to investigate the diversity of Ni-hosting phases precipitated in sulfidic environments across a range of aqueous Ni-to-Fe ratios ([Ni](aq)/[Fe](aq)) and in the presence or absence of the sulfate-reducing bacteria (SRB), Desulfovibrio vulgaris. In the absence of Fe(II), the initial precipitates in abiotic experiments are found to consist primarily of polyphasic Ni-sulfides (average sizes <20 nm) with millerite (trigonal NiS) cores and amorphous shells. The precipitates' crystallinity is enhanced noticeably over a period of similar to 6 days, forming larger-sized hexagonal alpha-NiS, and observations of defects such as twinning and stacking faults implicate a formation pathway via reassembly of fine nanoparticulate precursors. By comparison, in the presence of SRB and in the absence of Fe, more crystalline phases such as polydymite (Ni3S4 ) and vaesite (NiS2) are also precipitated in addition to the monosulfide phases. The observed difference suggests that the presence of SRB enables the transformation of polyphasic precursors to more crystalline structures through the combined effects of bacterial metabolites and localized precipitation within a low pH micro-environment around the cell walls. The addition of Fe(II) (i.e., [Ni](aq)/[Fe](aq) = 5:1) leads to formation of less crystalline Ni-sulfides in both biotic and abiotic systems, indicating crystal structure distortion caused by substitution of Ni with Fe. With decreasing [Ni](aq)/[Fe](aq), Ni-sulfides become rarer, mixed Ni-Fe phases start to appear, and finally Ni-rich mackinawite (FeS) becomes the primary Ni-hosting phase at the lowest ratio tested ([Ni](aq)/[Fe](aq) = 1:5). We propose that whether aqueous Ni forms discrete Ni-S phases or

The mechanisms for the reduction and uptake of Tc by magnetite (Fe3O4) and mackinawite (FeS) are investigated using XAS techniques (XANES, EXAFS), in combination with thermodynamic calculations of the Tc/Fe systems and accurate characterization of the solution properties (pHm, pe, [Tc]). Batch sorption experiments were performed in the presence of freshly prepared magnetite and mackinawite in 0.1 M NaCl solutions with varying [Tc(VII)]0 (2·10–5 – 2·10–4 M) and loading of Tc (400–900 ppm ). XANES confirms the complete reduction of Tc(VII) to Tc(IV) in all investigated systems, as predicted by experimental (pHm + pe) measurements and thermodynamic calculations. Two Tc pure endmember species are identified by EXAFS in the magnetite system, corresponding to fully incorporated Tc in the magnetite structure and (likely) to a Tc-Tc dimeric structure with triple-bonding to the magnetite {111} faces. The latter endmember is favoured at higher [Tc], whereas incorporation prevails at low [Tc] and less alkaline pH conditions. The key role of pH in the uptake mechanism is interpreted in terms of magnetite solubility, with higher [Fe] and greater recrystallization rates occurring with decreasing pH values. A TcS2-like phase prevails in all investigated mackinawite systems, although the contribution of up to  20% of TcO2xH2O(s) (likely as surface precipitate) is observed for the highest investigated loadings (900 ppm). These results provide key inputs for an accurate mechanistic interpretation of the Tc uptake by magnetite and mackinawite, so far controversially discussed in the literature, and represent a very relevant contribution in the context of nuclear waste disposal.

The interaction of SnII with metastable, highly reactive mackinawite is a complex process due to transient changes of the mackinawite surface in the sorption process. In this work, we show that tin redox state and local structure as investigated by Sn-K X-ray absorption spectroscopy (XAS) change with pH. We observe at pH <7 that SnII forms two short (2.38 Å) Sn-S bonds, the S atoms being part of the S-terminated surface of mackinawite. Two longer Sn-S bonds of 2.59 Å point most likely towards the solution phase, completing the tetragonal SnS4 innersphere sorption complex, while precipitation of SnS or formation of a solid solution with mackinawite could be excluded. At pH > 9, SnII is completely oxidized by an FeII/FeIII (hydr)oxide, most likely green rust, forming on the surface of mackinawite. Six O atoms at 2.04 Å and 6 Fe atoms at 3.29 Å demonstrate a structural incorporation by green rust, where SnIV substitutes for Fe in the crystal structure. The transition between SnII and SnIV and between sulfur and oxygen coordination takes place between pH 7 and 8, in accordance with the transition from the mackinawite stability field to more oxidized Fe-bearing minerals. The uptake processes of SnII by mackinawite are largely in line with the uptake processes of divalent cations of other soft Lewis-acid metals like Cd, Hg and Pb.

During subsurface bioremediation of uranium-contaminated sites, indigenous metal and sulfate-reducing bacteria may utilize a variety of electron acceptors, including ferric iron and sulfate that could lead to the formation of various biogenic minerals in-situ. Sulfides, as well as structural and adsorbed Fe(II) associated with biogenic Fe(II)-sulfide phases, can potentially catalyze abiotic U6+ reduction via direct electron transfer processes. In the present work, the propensity of biogenic mackinawite (Fe1+xS, x = 0 to 0.11) to reduce U6+ abiotically was investigated. The biogenic mackinawite produced by Shewanella putrefaciens strain CN32 was characterized by employing a suite of analytical techniques including TEM, SEM, XAS and Mössbauer analyses. Nanoscale and bulk analyses (microscopic and spectroscopic techniques, respectively) of biogenic mackinawite after exposure to U6+ indicate the formation of nanoparticulate UO2. This study suggests the relevance of Fe(II) and sulfide bearing biogenic minerals in mediating abiotic U6+ reduction, an alternative pathway in addition to direct enzymatic U6+ reduction.

Biogenic mackinawite (FeS) produced by Shewanella putrefaciens CN32 is submicron in size but nanosized in one dimension (film-like morphology) Biogenic FeS is very reactive evident by its propensity to reduced U(VI) and rapid oxidation to form lepidocrocite upon exposure to air. U(VI) reduction is observed in the pasteurized control indicative of a non-enzymatic redox process. XAS and nanodiffraction techniques (bulk and nanoscale analyses) confirm the formation of UO2. UO2 particles are nanoparticulate measuring c.a. 2.5 nm in size. The present work suggests that a remediation strategy could potentially incorporate subsurface abiotic redox interactions between biogenic Fe(II)-bearing minerals such as mackinawite & contaminant U(VI) to immobilize U as UO2 – an in-situ waste form.

The environmental fate of plutonium, the major transuranium actinide in nuclear waste, is largely impacted by its sorption onto and redox reactions with iron oxide, carbonate or sulfide minerals that form as corrosion products of steel in the "near field" and occur widely in sediments. To obtain information on oxidation state and local structure of Pu in the presence of Fe(II) bearing minerals, electrolytically prepared Pu(V) or Pu(III) (242Pu, 1-3·10-5 M) were, under anoxic conditions, reacted with magnetite (FeIIFeIII2O4) (pH 6-8.5), chukanovite (Fe2(CO3)(OH)2) (pH 8.5) and mackinawite (FeS) (pH 6-8.5). Pu-LIII-edge X-ray absorption spectra (XAS) were collected after 40 d and 8 months of reaction. In all 14 samples, more than 98 % of Pu was associated with the solid phase and its redox speciation thus accessible by XAS. With magnetite, only the sample prepared at the highest pH and highest Pu loading contained Pu(IV)O2 while in all others Pu was solely present as a tridentate Pu(III) surface complex [1]. The three chukanovite samples all contained both Pu(III) (15 to 40 %) and PuO2. With mackinawite at pH 6 only Pu(III) was present, while all samples prepared at pH7 and higher contained mostly PuO2 and up to approx. 10 % Pu(III). Through comparison of the different types of minerals (oxide, carbonate, sulfide), reaction pH and Pu/mineral ratios, it becomes apparent that the type of surface complexation (e.g. inner-sphere on magnetite vs. outer-sphere on mackinawite) and total mineral surface area are key parameters in controlling concentrations of dissolved Pu and in determining whether a PuO2 solid phase precipitates. While PuO2 provides an upper limit for concentrations of dissolved Pu, the available mineral surface area and sorption complex stability control what percentage of Pu is present in surface complexes. Under reducing conditions as established through the Fe(II) bearing minerals used here, this mineral surface associated Pu was found to be trivalent. S

Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-LIII-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous 242Pu(III) and 242Pu(V) reacted with magnetite, mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO2 phases formed in the presence of mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalent PuO2 for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.

Sb exists in nature in a wide range of oxidation states and can be a potential hazardous contaminant depending on its speciation and reactivity. In this study we employed cryogenic-XPS and XAS techniques in order to understand the reduction of SbV at the surface of mackinawite (FeS) as a function of pH and surface loading. Experimental Conditions Nanoparticulate, Mackinawite samples were prepared and stored as suspensions under strictly anoxic conditions (~ 1 ppmv O2) in a Jacomex glovebox. Sorption experiments at pH 5 and pH 8 with increasing Sb concentrations (0.1, 0.3, 0.6, 0.8 mM) were also conducted in the same glovebox under anoxic conditions [1, 2]. X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) measurements were conducted on frozen wet pastes of reacted mackinawite samples. Results and Discussion Sb3d XPS spectra of reacted sample series at pH 5 revealed presence of increasing amounts of SbIII at the surface suggesting that the rate of reduction of SbV was directly proportional to surface loading. Corresponding Fe 2p ans S 2p spectra of the same samples showed significant increase in the proportion of FeIII species and presence of S0 at the surface. However, the proportion of S0 remained constant suggesting that Fe is the dominant redox partner in this sytem. At pH 8, presence of SbIII was only observed at higher surface loadings indicating that adsorption and redox processes both occur at much slower rate compared to that at pH 5. In addition, increased contribution of surface monosulfide group at pH 5 suggest that Sb is most likely bound to S atoms at the surface, which was confirmed by Sb K-edge EXAFS spectra. In contrast, at pH 8, such increase in surface monosulfide contributions are absent, suggesting Sb is most likely bound to O atoms at the surface.

Probing redox reactions at the mineral/water interface by X-ray absorption spectroscopy: Reduction of Se, Sb and Pu by Fe(II)-bearing minerals

Plutonium is the major transuranium actinide in nuclear waste and a highly toxic environmental contaminant. As iron(II) minerals are ubiquitous and known to reduce a range of metal(loid)s, including As, Cr, Se, Pu and Np, through surface-mediated redox reactions, we investigated here the reactivity of magnetite (Fe3O4) and mackinawite (FeS) towards Pu(III) and (V) using XAFS to analyse the oxidation state and molecular structure of the reaction products. Experimental Conditions Mineral syntheses and reactions were carried out under anoxic conditions in N2 or Ar glove-boxes. Fe3O4 and FeS were reacted for 40 days in 0.1 M NaCl with Pu (1x10-5M) at approx. pH 8. 242Pu was added as electrolytically prepared Pu(V) to Fe3O4 and FeS, and as Pu(III) to Fe3O4. Results and Discussion The spectra obtained from tri- and pentavalent Pu reacted with magnetite are nearly identical; shell fitting yielded a Pu-O distance of 2.50 Å, indicative of Pu(III). Using Monte Carlo modeling of the EXAFS spectra (MC) [1], we were able to unequivocally identify a distinct Pu(III) surface complex wherein Pu(III) is connected via three oxygen atoms to three edge-sharing FeO6-octahedra of the Fe3O4 {111} face. This result is in disagreement with [2], where in the absence of spectroscopic methods Pu(IV) had been determined as the oxidation state resulting from reduction of Pu(V) by magnetite under anoxic conditions. After reaction of Pu(V) with mackinawite, Pu(IV) was identified as the prevalent Pu oxidation state by both XANES and EXAFS shell fitting. The EXAFS spectrum closely ressembles that of PuO2 solids and colloids. Our study demonstrates that under reducing conditions in geological environments both Pu(IV) and Pu(III) species may be relevant and migration as eigen-colloids or adsorbed species may play an important role in controlling Pu mobility.

Speciation of Sb is strongly influenced by its oxidation state (V, III, 0, -III). Redox processes under anerobic groundwater conditions may therefore greatly alter the environmental behavior of Sb. Employing X-ray absorption and photon spectroscopy, we show here that Sb(V) is reduced to Sb(III) by magnetite and mackinawite, two ubiquitous Fe(II)-containing minerals, while Sb(III) is not further reduced. At the surface of magnetite, Sb(III) forms a highly symmetrical sorption complex at the position else occupied by tetrahedral Fe(III). The Sb(V) reduction increases with pH, at values above pH 6.5 Sb(V) is completely reduced to Sb(III) within 30 days. In contrast, Sb(V) is completely reduced at the mackinawite surface across a wide pH range and within 15 days. Sb(V) reduction proceeds solely by oxidation of surface Fe(II), while the oxidation state of sulfide is conserved. Independent of whether Sb(V) or Sb(III) was added, an amorphous or nanoparticulate SbS3-like solid forms.

Antimony finds a wide range of industrial applications, e.g. in flame retardants, brake pads and as a lead-alloy in storage batteries and ammunition and is widely distributed in the environment. Sb may occur in several oxidations states (-III, 0, III, V). Under oxic conditions, Sb0 oxidizes prevalently to SbV, forming the anionic species Sb(OH)6- which is strongly sorbed by Fe oxides[1]. In contrast, SbIII forms an uncharged complex Sb(OH)3(aq), which is more mobile. Under anoxic conditions, SbV and SbIII may be reduced by FeII-bearing minerals. Magnetite (FeIIFeIII2O4) and mackinawite (FeS) have been shown to reduce e.g. Se[2], As[3], and Pu[4]). We therefore investigated the reaction of SbIII and SbV with these two minerals at <1ppm O2 (v/v) using Sb-K XAS. When SbIII was reacted with magnetite at pH 4.7 to 7.6 during 1 h to 67 d, the oxidation state was stable and only one Sb species was identified by EXAFS. SbIII is coordinated with 4 to 5 iron atoms at a distance of 3.6 Å. FEFF Monte Carlo simulations revealed formation of a highly ordered surface complex on the {111} faces of magnetite. The trigonal pyramidal SbO3 units occupy positions of FeIII tetrahedra, that would be ideally coordinated to six Fe06 octahedra via corner-sharing. The experimental Fe coordination numbers below six suggest that Sb occupies positions near edges of the {111} faces. When SbV was reacted with magnetite, reduction to SbIII increased linearly between pH 4.5 and 6.5, with little influence of reaction time. The SbIII produced by the surface reaction formed the same surface complex as after direct addition of SbIII. In the presence of mackinawite, SbV was completely reduced to SbIII within 30 d and in the pH range 4.3 - 8.4. The local structure shows SbIII surrounded by three sulfur atoms at a distance of 2.5 Å as in Sb2S3. The lack of more distant atomic shells suggests a highly dissordered structure. Again the resulting surface complex is the same as after direct addition of SbIII. C

Poorly characterized phases (PCP's) may constitute up to 30 volume percent of some C2M carbonaceous chondrite matrices [1] and are an important key to an understanding of matrix evolution. PCPs are usually fine-grained (

Ultramafic bodies occur in the Archaean granitoids and greenstone belts and in the Svecokarelian schists. Most have been altered to serpentinites, metaperidotites or carbonate-bearing ultramafics and/or have undergone metamorphism. The textures and composition of the ore minerals are considered. The sulphide minerals have been altered and recrystallised and the assemblage includes pentlandite, awaruite, heazlewoodite, millerite and mackinawite instead of the primary pyrrhotite-pentlandite assemblage. Accessory nickel arsenides characterise the carbonatised ultramafics. Oxide minerals are chromite, magnetite and ilmenite., Ultramafic bodies occur in the Archaean granitoids and greenstone belts and in the Svecokarelian schists. Most have been altered to serpentinites, metaperidotites or carbonate-bearing ultramafics and/or have undergone metamorphism. The textures and composition of the ore minerals are considered. The sulphide minerals have been altered and recrystallised and the assemblage includes pentlandite, awaruite, heazlewoodite, millerite and mackinawite instead of the primary pyrrhotite-pentlandite assemblage. Accessory nickel arsenides characterise the carbonatised ultramafics. Oxide minerals are chromite, magnetite and ilmenite.

The incomplete ophiolite, part of the western Greek ophiolitic zone of the Mesozoic Tethyan orogenic belt, includes an upper unit of mantle peridotites and basaltic lavas. Serpentinised harzburgites and lherzolites contain pyrrhotite, pentlandite, awaruite, chalcopyrite and native copper while olivine gabbros and basalts contain pyrrhotite, pentlandite, bornite, chalcopyrite, covellite and mackinawite. The sulphides are disseminated and occur as small grains interstitial to/as inclusions in silicate minerals, suggesting a magmatic origin. Possible evolution of the magma is discussed., The incomplete ophiolite, part of the western Greek ophiolitic zone of the Mesozoic Tethyan orogenic belt, includes an upper unit of mantle peridotites and basaltic lavas. Serpentinised harzburgites and lherzolites contain pyrrhotite, pentlandite, awaruite, chalcopyrite and native copper while olivine gabbros and basalts contain pyrrhotite, pentlandite, bornite, chalcopyrite, covellite and mackinawite. The sulphides are disseminated and occur as small grains interstitial to/as inclusions in silicate minerals, suggesting a magmatic origin. Possible evolution of the magma is discussed.

The serpentinite massifs of Hitura are located in the municipality of Nivala, central Finland. The most important sulphide minerals in the ore are pentlandite, pyrrhotite and chalcopyrite. The ore body consists of amphibolite rocks in contact with serpentinite against mica gneiss. The mean Ni grade of the ore is about 0.7% and the highest Ni grades have been encountered in close contact. In general, the inner parts of the ore are strongly serpentinised ultramafite and some sulphides have been converted to mackinawite and valleriite. The degree of alteration varies considerably within the ore body and, correspondingly, the grain size of the sulphides varies as well. In this study, flotation tests were done with samples from different zones of the ore body. The flotation test results on the bench scale and plant scale are discussed in terms of the mineralogical features of the ore. Ores in the North Hitura zone differ from those in the South Hitura zone., The serpentinite massifs of Hitura are located in the municipality of Nivala, central Finland. The most important sulphide minerals in the ore are pentlandite, pyrrhotite and chalcopyrite. The ore body consists of amphibolite rocks in contact with serpentinite against mica gneiss. The mean Ni grade of the ore is about 0.7% and the highest Ni grades have been encountered in close contact. In general, the inner parts of the ore are strongly serpentinised ultramafite and some sulphides have been converted to mackinawite and valleriite. The degree of alteration varies considerably within the ore body and, correspondingly, the grain size of the sulphides varies as well. In this study, flotation tests were done with samples from different zones of the ore body. The flotation test results on the bench scale and plant scale are discussed in terms of the mineralogical features of the ore. Ores in the North Hitura zone differ from those in the South Hitura zone.