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The composition of a mineral is a defining characteristic. The various compositions listed for mackinawite in current mineralogical databases and reference books, such as Fe(Ni)S and Fe1+xS, are both wrong and misleading. Statistical analyses of over 100 mackinawite compositions reported over the last 50 years show a mean composition of Me1.0S where Me = Fe + Co + Ni + Cu. Mackinawite is stoichiometric FeS. As with many sulfide minerals, Ni-, Co-, and, possibly, Cu-rich varieties occur in addition to the simple iron monosulfide. These varieties are best referred to as nickelian mackinawite, cobaltian mackinawite, and cupriferous mackinawite. The results confirm that these metals substitute for Fe in the mackinawite structure rather than being contained in the interstices between the Fe-S layers. Most compositional data on mackinawites derive from electron probe microanalyses of small grains in magmatic/hydrothermal associations. The result means that there is no dichotomy between the composition of ambient temperature synthetic mackinawite (which is supposed to be equivalent to sedimentary mackinawite) and mackinawites from higher temperature associations. The correct representation of the composition of mackinawite has implications for a wide swathe of fundamental science, including the origin of life, the genesis of magmatic ore deposits, the provenance of meteorites as well as industrial applications such as water treatment and steel corrosion. The stoichiometric formulation permits the mackinawite formula to be balanced electronically using conventional Fe and S ionic species. It also enables simple, balanced chemical equations involving mackinawite. [ABSTRACT FROM AUTHOR]

Rhenium (Re) is an extremely rare element, with a crustal abundance of approximately 0.4 parts per billion (ppb) and a sea water concentration of 8.3 parts per trillion (ppt). However, Re concentrations in anoxic marine sediments range from 2 to 184 ppb, which is attributed to reduction of the highly soluble perrhenate ion (Re(VII)O4−) to insoluble Re(IV) species. Anoxic sediments typically contain Fe(II) and sulfide species, which could potentially reduce Re(VII) to Re(IV). In this study, we examined the interactions of KReO4 with magnetite (Fe3O4), siderite (FeCO3), vivianite (Fe3(PO4)2•8H2O), green rust (mixed Fe(II)/Fe(III) layered double hydroxide), mackinawite (FeS), and chemically reduced nontronite (NAu-1) using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy to determine the valence state and speciation of Re. Uptake of Re by green rust was rapid, with ~50% associated with the solids within 2 days. In contrast, there was <10% uptake by the other Fe(II) phases over 48 days. Reduction of Re(VII) to Re(IV) was only observed in the presence of green rust, producing clusters of bidentate-coordinated Re(IV)O6 octahedra.. These results suggest that except for green rust, the potential for other Fe(II)-bearing minerals to act as reductants for ReO4− in sedimentary environments requires further investigation. [ABSTRACT FROM AUTHOR]

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.

Abstract Mackinawite (FeS) was investigated for cadmium ion (Cd(II)) sorption under anoxic conditions. At the surface loading of Cd(II) (i.e., [Cd(II)]0/[FeS]0) ≤ 5 mmol/g, FeS quantitatively immobilized Cd(II). Adsorption and CdS precipitation were responsible for Cd(II) uptake, with their relative importance depending on [Cd(II)]0/[FeS]0. At pH 5.5–6.0, adsorption was more important when [Cd(II)]0/[FeS]0 ≤ 0.05 mmol/g. According to energy dispersive X-ray spectroscopy, Cd(II) exhibited strong spatial correlations with S and Cl. While Cd-S correlations corresponded to CdS precipitation and/or the surface complexation of Cd(II) with sulfhydryl functional sites, Cd–Cl correlations indicated the presence of chloride-complexed Cd(II). Given the strong correlations of both pairs, the adsorbed Cd(II) was likely present in chlorosulfide forms (e.g., ≡FeS–Cd(II)–Cl). When [Cd(II)]0/[FeS]0 exceeded 0.05 mmol/g, CdS precipitation became more important. X-ray diffraction, transmission electron microscopy, and selected area electron diffraction revealed the formation of hawleyite (cubic CdS) at higher surface loadings. The Fe(II) species liberated during CdS precipitation were resorbed through adsorption at acidic pH and the formation of Fe (oxyhydr)oxides at neutral to basic pH. Given the greater stability of CdS than adsorbed Cd(II), the prevalence of the former suggests that FeS can serve as an effective reagent to remedy Cd(II) contamination under anoxic conditions. Due to its ubiquitous presence, FeS may also control the environmental fate and mobility of Cd(II) in sulfidic sediments.

In this paper, FeSm/BC composites were prepared using coconut shell biochar (BC) as the carrier. FeSm on FeSm/BC is preferentially oriented along the (001) crystal plane and exhibits a uniform honeycomb morphology. It has a high specific surface area and a small pore size distribution, which is more suitable as a heavy metal ion removal material. The results show that Cr(VI) can be almost completely removed within 120 min under acidic conditions with an initial concentration of 10 mg/L of Cr(VI), a temperature of 20 °C, and a dosage of 0.6 g/L of adsorbent. Where pH was the main factor influencing the removal of Cr(VI):pH affects Cr(VI) removal by changing the form of Cr(VI) in the solution, adsorption free energy, redox potential, charged species on FeSm/BC surface, and the reduction properties of Fe2+ and S2−, low pH value is more favorable for Cr(VI) removal. Coexisting anions in solution inhibit Cr(VI) removal due to competitive adsorption, but coexisting cations have no effect. The adsorption process of Cr(VI) by FeSm/BC follows the pseudo-second-order kinetics and Langmuir adsorption isotherm model, indicating that the adsorption process is a monolayer surface adsorption controlled by the chemical adsorption mechanism and involves electron sharing or exchange between adsorbent and adsorbate. Thermodynamic study shows that the adsorption process is a spontaneous entropy-increasing process, and raise the temperature is beneficial to the adsorption. The removal of Cr(VI) by FeSm/BC is the synergistic effect of adsorption, reduction, and co-precipitation. [ABSTRACT FROM AUTHOR]

API 5L X65 microalloyed steels are extensively used for oil and gas transportation. In many cases, reserves of such resources contain H2S, constituting what is called a sour medium, at which hydrogen-induced failures are critical. Even though essential for improving mechanical resistance, the addition of high contents of manganese (Mn) may increase the tendency for segregation and the precipitation of MnS inclusions, increasing the susceptibility to hydrogen-induced failures. In this study, electrochemical techniques were employed to compare the corrosion resistance in the B solution of ASTM D1141−98 (synthetic seawater) of three API 5L X65 microalloyed steels with low Mn content, designed for severe sour service applications. Experiments were performed in naturally aerated (pH 8.0), deaerated (pH 8.0 and 5.0), and H2S-saturated (sour pH 5.0) media. Although the steels exhibit very similar chemical compositions and mechanical properties, it was possible to rank their corrosion resistance. Microstructural characterization allowed associating corrosion susceptibility with the presence of pearlite grains in a ferritic matrix. The results also demonstrated the superior aggressiveness of the sour medium, and that the precipitation of a poorly adherent layer of Mackinawite slightly increases the corrosion resistance with immersion time.

Mackinawite (FeS) was investigated for cadmium ion (Cd(II)) sorption under anoxic conditions. At the surface loading of Cd(II) (i.e., [Cd(II)]0/[FeS]0) ≤ 5 mmol/g, FeS quantitatively immobilized Cd(II). Adsorption and CdS precipitation were responsible for Cd(II) uptake, with their relative importance depending on [Cd(II)]0/[FeS]0. At pH 5.5–6.0, adsorption was more important when [Cd(II)]0/[FeS]0 ≤ 0.05 mmol/g. According to energy dispersive X-ray spectroscopy, Cd(II) exhibited strong spatial correlations with S and Cl. While Cd-S correlations corresponded to CdS precipitation and/or the surface complexation of Cd(II) with sulfhydryl functional sites, Cd–Cl correlations indicated the presence of chloride-complexed Cd(II). Given the strong correlations of both pairs, the adsorbed Cd(II) was likely present in chlorosulfide forms (e.g., ≡FeS–Cd(II)–Cl). When [Cd(II)]0/[FeS]0 exceeded 0.05 mmol/g, CdS precipitation became more important. X-ray diffraction, transmission electron microscopy, and selected area electron diffraction revealed the formation of hawleyite (cubic CdS) at higher surface loadings. The Fe(II) species liberated during CdS precipitation were resorbed through adsorption at acidic pH and the formation of Fe (oxyhydr)oxides at neutral to basic pH. Given the greater stability of CdS than adsorbed Cd(II), the prevalence of the former suggests that FeS can serve as an effective reagent to remedy Cd(II) contamination under anoxic conditions. Due to its ubiquitous presence, FeS may also control the environmental fate and mobility of Cd(II) in sulfidic sediments. [ABSTRACT FROM AUTHOR]

Understanding the formation of iron sulfides at low temperatures is essential for interpretation of geochemical signatures recorded in sedimentary sulfide minerals related to the evolution of the Earth's surface environment. Observed variations in the precipitation of nanoparticulate metastable phases prior to the formation of the stable-phase pyrite have been discussed from a kinetic perspective only. Here, using the ab-initio thermodynamic scheme to integrate surface thermodynamics with solid–aqueous equilibria, we demonstrate that stability relations among iron sulfides are dependent on particle size. The lower surface energies of metastable mackinawite and greigite than pyrite can drive reversals in relative thermodynamic stability at the nanoparticle scale and also lead to faster nucleation rates of the metastable phases, which are dependent on aqueous Eh-pH and Fe(II)/H 2 S conditions. This study emphasizes the role of surface energy in the formation pathways of iron sulfides and their particle-size dependent redox equilibria in anoxic aqueous solutions. [ABSTRACT FROM AUTHOR]

Sulphide stress corrosion (SSC) is an abundant type of corrosion in petroleum refineries. Steel pipes in petroleum refineries are influenced by the synergistic effect of tensile stress/pressure and high H2S concentrations in different environments. This paper focuses on the prediction of the propagation of depth of SSC in API 5L Grade B steel as a function of pH value, applied load, and time duration. The model has been established based on the experimental values of the depth of SSC under different predetermined test environmental conditions kept within the pH value of 2.7-3.5, an applied load of 400-800 N, and a time duration of 15-45 days. Further, the model was validated by another data set obtained for the depth of corrosion within the same aforementioned ranges of parameters. All the laboratory experiments were conducted in accordance with ANSI/NACE TM0177-2016 standard test methods. In addition, this paper discusses the microstructural observations of specimens kept under the aforementioned test environmental conditions. The scanning electron microscope (SEM) images of cross-sections of corroded specimens showed that crack initiation occurred after 15 days under all different test environmental conditions. Further, crack propagation occurred transversely, developing branches through the cross section until 30 days of time duration and the behaviour of the propagation of the crack was completely changed at 45 days of time duration. [ABSTRACT FROM AUTHOR]

Marine sediments represent the most important sink for natural organic matter (NOM) across geological time spans. Strong associations between minerals, including iron oxides, and organic matter reaching the seafloor play a fundamental role in this preservation and have been known for some decades. Despite the importance of this protective mechanism in the balances of the global carbon budget, very little is known about the affinity of NOM for reduced iron species such as mackinawite (FeS) in the anoxic layers of sediment. In this study, equilibrium partition coefficients (K d) for three types of NOM (soil extract, corn leaf extract and plankton extract) on FeS were determined through batch sorption experiments. Models were then built via multiple linear regression (MLR) and partial least squares regression (PLSR) in order to predict sorption K d 's based on the chemical characteristics of the NOM. Investigation of the PLSR regression coefficients indicates that functional groups characteristic of polysaccharides are the greatest positive predictors of NOM sorption onto FeS at marine sediment porewater pH. This conclusion was independently verified by analyses of NOM FTIR spectra pre- and post-sorption. PLSR outperformed MLR with a lower root mean square error of prediction (RMSEP) of 53.4 L/kg compared to 64.0 L/kg. To our knowledge, this research presents a novel machine-learning approach to the quantitative modelling of NOM sorption to minerals found in anoxic marine environments. [ABSTRACT FROM AUTHOR]

Pyrite (FeS 2) is the most abundant iron sulfide mineral in Earth's crust. Until recently, FeS 2 has been considered a sink for iron (Fe) and sulfur (S) at low temperature in the absence of oxygen or oxidative weathering, making these elements unavailable to biology. However, anaerobic methanogens can transfer electrons extracellularly to reduce FeS 2 via direct contact with the mineral. Reduction of FeS 2 occurs through a multistep process that generates aqueous sulfide (HS–) and FeS 2 -associated pyrrhotite (Fe 1-x S). Subsequent dissolution of Fe 1-x S provides Fe(II) (aq) , but not HS–, that rapidly complexes with HS– (aq) generated from FeS 2 reduction to form soluble iron sulfur clusters [ n FeS (aq) ]. Cells assimilate n FeS (aq) to meet Fe/S nutritional demands by mobilizing and hyperaccumulating Fe and S from FeS 2. As such, reductive dissolution of FeS 2 by methanogens has important implications for element cycling in anoxic habitats, both today and in the geologic past. Methanogens can reductively dissolve pyrite (FeS 2) and mobilize iron (Fe) and sulfur (S) under anaerobic conditions. Biological reduction of FeS 2 takes place via direct extracellular electron transfer (EET) that likely involves electrically conductive components of the S-layer or extracellular polymeric matrix. Fe(II) solubilized from Fe 1-x S that precipitates on the surface of FeS 2 during reduction combines with HS– released during FeS 2 reduction to form n FeS (aq) clusters that are likely assimilated to meet Fe and S nutritional demands. Growth with FeS 2 causes cells to perceive Fe limitation due to transport of n FeS (aq) rather than ionic Fe(II), thereby leading to hyperaccumulation of Fe in cells as putative thioferrate-like particles. Reduction of FeS 2 , assimilation of n FeS (aq) , and transformation into functional biocatalysts in methanogens provides a potential path to explore the transition from abiotic, mineral-based catalysis to biocatalysis on early Earth. [ABSTRACT FROM AUTHOR]

The biogeochemical cycling of arsenic (As) is often intertwined with iron (Fe) and sulfur (S) cycles, wherein Fe(III)- and sulfate-reducing bacteria (SRB) play a crucial role. Here, we isolated strain DS-1, a strictly anaerobic Fe(III)- and sulfate-reducing bacterium, from As-contaminated paddy soil. Using 16S rRNA gene sequence analysis, strain DS-1 was identified as a member of the genus Desulfovibrio. Strain DS-1 utilized energy derived from ferrihydrite reduction to support its cellular growth. Under anoxic sulfate-reducing conditions, the presence of strain DS-1 significantly increased As mobilization compared to sulfate-free conditions. Mechanistically, SRB-produced sulfide reacts with Fe(III) to form FeS, which disrupts Fe(III) minerals, thereby enhancing As release. These findings highlight the critical role of redox disequilibrium in As mobilization and suggest that SRB-produced sulfide may permeate to the rice rhizosphere, increasing As mobilization through Fe(III) reduction., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Mackinawite has unique structural properties and reactivities when compared to other iron sulfides. Herein we provide evidence for the mackinawite‐supported reduction of KCN into various reduced compounds under primordial conditions. We proposed a reaction mechanism based on the nucleophilic attack by the deprotonated mackinawite ‐SH surface groups at the carbon atom of HCN. The initial binding of the substrate and the subsequent reduction events are supported by DFT calculations and further experiments using other substrates, such as KSCN, KOCN and CS2. Until now, conversion of CN− into CH4 and NH3 has been limited to nitrogenase cofactors or molecular Fe‐CN complexes. Our study provides evidence for mackinawite‐supported cleavage of the C−N bond under ambient conditions, which opens new avenues for investigation of other substrates for mackinawite‐supported reactions while shedding light on the relevance of this type of reaction to the origin of life on Earth. [ABSTRACT FROM AUTHOR]

pH influences the reactivity of iron (II) minerals towards halogenated pollutants like hexachlorocyclohexanes (HCHs). To explore these incompletely understood interactions, we investigated the carbon isotope fractionation of the δ-HCH isomer during dehalogenation by iron sulfide at pHs spanning a pH range across slightly acidic to alkaline domains (5.8–9.6). The δ-1,3,4,5,6-pentachlorocyclohex-1-ene (δ-PCCH) was the intermediate degradation product, while benzene, monochlorobenzene (MCB), but especially 1,2-dichlorobenzene (1,2-DCB), and 1,2,4-trichlorobenzene (1,2,4-TCB), were the main degradation products of δ-HCH. These degradation products suggested dehydrochlorination as the main degradation pathway of δ-HCH by iron sulfide. Different kinetic experiments indicate that the rate constants (k a) during dechlorination of δ-HCH by iron sulfide rose with pH: 0.003 d−1 (pH 5.8) < 0.034 d−1 (pH 8) < 0.085 (pH 9.3) < 0.286 d−1 (pH 9.6). Upon Rayleigh model calculations, an enrichment factor (εC) of −7.8 ± 1.0 ‰ was calculated for δ-HCH dehalogenation by FeS at pH 8.0. This suggests an apparent kinetic isotope effect (AKIE C) value of 1.049 ± 0.006 for dehydrohalogenation. The magnitude of the isotope effect from this paper furthermore supports dehydrohalogenation and opens the possibility to study the degradation of HCHs by iron (II) minerals containing FeS as mackinawite in oxygen-deprived environments. [Display omitted] • Dehydrohalogenation of δ-HCH occurred during degradation with FeS. • δ-pentachlorocyclohexene was the intermediate degradation product. • The rate constants (k a) during dechlorination of δ-HCH by FeS rose with pH. • Dehydrohalogenation of δ-HCH induced a significant isotope fractionation. [ABSTRACT FROM AUTHOR]

Organic carbon sequestration in sedimentary environments controls oxygen and carbon dioxide concentrations in the atmosphere. While minerals play an important role in the preservation of organic carbon, there is a lack of understanding about the formation and stability of organo-mineral interactions in anoxic environments, especially those involving authigenic iron sulfide minerals. In this study, we quantified organic carbon and nitrogen sequestered in biogenic iron sulfide minerals co-precipitated with sulfate-reducing bacteria (SRB) in freshwater and marine conditions in long-term laboratory experiments. The amounts of C and N associated with biogenic iron sulfide minerals increased with increasing cell biomass concentrations available in the media. C and N levels stabilized over the first 2 months of incubation and remained stable for up to 1 year. Crystalline mackinawite (FeS) formed in all experimental conditions and transformed to greigite only in some experimental conditions. We did not find evidence that this mineral transformation affected C and N levels, neither could we identify the factors that controlled greigite formation. Pyrite did not form in our experimental conditions. While C concentrations in minerals correlated with concentrations of reduced sulfate in both the freshwater and marine media, removal of OC by iron sulfide minerals was more efficient in freshwater than marine conditions. Removal of OC by iron sulfide minerals was also more efficient when cells were present (SRB biomass) in comparison with abiotic incubations with organic mixtures (e.g., tryptone, yeast extract, and casamino acids). Our study highlights the potential for biogenic iron sulfide minerals to quantitatively contribute to organic carbon preservation in anoxic environments. [ABSTRACT FROM AUTHOR]

Organic carbon sequestration in sedimentary environments controls oxygen and carbon dioxide concentrations in the atmosphere. While minerals play an important role in the preservation of organic carbon, there is a lack of understanding about the formation and stability of organo-mineral interactions in anoxic environments, especially those involving authigenic iron sulfide minerals. In this study, we quantified organic carbon and nitrogen sequestered in biogenic iron sulfide minerals co-precipitated with sulfate-reducing bacteria (SRB) in freshwater and marine conditions in long-term laboratory experiments. The amounts of C and N associated with biogenic iron sulfide minerals increased with increasing cell biomass concentrations available in the media. C and N levels stabilized over the first 2 months of incubation and remained stable for up to 1 year. Crystalline mackinawite (FeS) formed in all experimental conditions and transformed to greigite only in some experimental conditions. We did not find evidence that this mineral transformation affected C and N levels, neither could we identify the factors that controlled greigite formation. Pyrite did not form in our experimental conditions. While C concentrations in minerals correlated with concentrations of reduced sulfate in both the freshwater and marine media, removal of OC by iron sulfide minerals was more efficient in freshwater than marine conditions. Removal of OC by iron sulfide minerals was also more efficient when cells were present (SRB biomass) in comparison with abiotic incubations with organic mixtures (e.g., tryptone, yeast extract, and casamino acids). Our study highlights the potential for biogenic iron sulfide minerals to quantitatively contribute to organic carbon preservation in anoxic environments.

Mackinawite was biologically synthetized by immersing carbon steel coupons in artificial seawater containing Sulphate-Reducing Bacteria (SRB) for 6 months. These coupons were removed from the culture medium solution and some were stored in air while others were placed in SRB-free culture medium solution. In operando Raman spectroscopy was used to analyse all these coupons immediately after extraction from the incubation medium and nanocrystalline, well-crystallized and partially oxidized mackinawite was detected. The evolution of these corrosion products was also monitored as a function of ageing time with this technique. Nanocrystalline and well-crystallised mackinawite transformed into partially oxidised mackinawite, greigite and sulphate green rust for an ageing time between 4 and 72 h. After 120 h, maghemite, magnetite, lepidocrocite, goethite appeared on the coupons placed in SRB-free culture medium solution as opposed to those stored in air atmosphere. Greigite and sulphate green rust were not observed for Raman measurements performed in air. [ABSTRACT FROM AUTHOR]

The adsorption and dissociation of water on mackinawite (layered FeS) surfaces were studied using dispersion-corrected density functional theory (DFT-D2) calculations. The catalytically active sites for H2O and its dissociated products on the FeS {001}, {011}, {100}, and {111} surfaces were determined, and the reaction energetics and kinetics of water dissociation were calculated using the climbing image nudged elastic band technique. Water and its dissociation products are shown to adsorb more strongly onto the least stable FeS{111} surface, which presents low-coordinated cations in the surface, and weakest onto the most stable FeS{001} surface. The adsorption energies decrease in the order FeS{111} > FeS{100} > FeS{011} > FeS{001}. Consistent with the superior reactivity of the FeS{111} surface towards water and its dissociation products, our calculated thermochemical energies and activation barriers suggest that the water dissociation reaction will take place preferentially on the FeS nanoparticle surface with the {111} orientation. These findings improve our understanding of how the different FeS surface structures and the relative stabilities dictate their reactivity towards water adsorption and dissociation. [ABSTRACT FROM AUTHOR]

We present a molecular dynamics investigation of the properties of water at the interface with the mackinawite (001) surface.We find water in the first layer to be characterised by structural properties which are reminiscent of hydrophobic substrates, with the bulk behaviour being recovered beyond the second layer. In addition, we show that the mineral surface reduces the mobility of interfacial water compared to the bulk. Finally, we discuss the important differences introduced by simulating water under conditions of high temperature and pressure, a scenario relevant to geochemistry. [ABSTRACT FROM AUTHOR]

Iron sulfide minerals, including mackinawite (FeS), are relevant in origin of life theories, due to their potential catalytic activity towards the reduction and conversion of carbon dioxide (CO2) to organic molecules, which may be applicable to the production of liquid fuels and commodity chemicals. However, the fundamental understanding of CO2 adsorption, activation, and dissociation on FeS surfaces remains incomplete. Here, we have used density functional theory calculations, corrected for long-range dispersion interactions (DFT-D2), to explore various adsorption sites and configurations for CO2 on the low-index mackinawite (001), (110), and (111) surfaces. We found that the CO2 molecule physisorbs weakly on the energetically most stable (001) surface but adsorbs relatively strongly on the (011) and (111) FeS surfaces, preferentially at Fe sites. The adsorption of the CO2 on the (011) and (111) surfaces is shown to be characterized by significant charge transfer from surface Fe species to the CO2 molecule, which causes a large structural transformation in the molecule (i.e., forming a negatively charged bent CO2-δ species, with weaker C--O confirmed via vibrational frequency analyses). We have also analyzed the pathways for CO2 reduction to CO and O on the mackinawite (011) and (111) surfaces. CO2 dissociation is calculated to be slightly endothermic relative to the associatively adsorbed states, with relatively large activation energy barriers of 1.25 eV and 0.72 eV on the (011) and (111) surfaces, respectively. [ABSTRACT FROM AUTHOR]

Canada's deep geological repository (DGR) design includes an engineered barrier system where highly compacted bentonite (HCB) surrounds the copper-coated used fuel containers (UFCs). Microbial-influenced corrosion is a potential threat to long-term integrity of UFC as bisulfide (HS−) may be produced by microbial activities under anaerobic conditions and transported via diffusion through the HCB to reach the UFC surface, resulting in corrosion of copper. Therefore, understanding HS− transport mechanisms through HCB is critical for accurate prediction of copper corrosion allowance. This study investigated HS− transport behaviour through MX-80 bentonite at dry densities 1070–1615 kg m−3 by performing through-diffusion experiments. Following HS− diffusion, bromide (Br−) diffusion and Raman spectroscopy analyses were performed to explore possible physical or mineralogical alterations of bentonite caused by interacting with HS−. In addition, accessible porosity ε was estimated using extended Archie's law. Effective diffusion coefficient of HS− was found 2.5 × 10−12 m2 s−1 and 5.0× 10−12 m2 s−1 for dry densities 1330 and 1070 kg m−3, respectively. No HS− breakthrough was observed for highly compacted bentonite (1535–1615 kg m−3) over the experimental timeframe (170 days). Raman spectroscopy results revealed that HS− reacted with iron in bentonite and precipitated as mackinawite and, therefore, it was immobilized. Finally, results of this study imply that HS− transport towards UFC will be highly controlled by the available iron content and dry density of the buffer material. [Display omitted] • Bisulfide diffusion coefficient decreased with increasing bentonite dry density. • Immobilization of HS− by geochemical reactions inhibited its diffusive transport. • Raman anawlysis revealed immobilization occurred via formation and precipitation of mackinawite. • Transport of HS− had negligible effects on bentonite pore structure. • Density and iron content of bentonite will control HS− transport towards UFC. [ABSTRACT FROM AUTHOR]

Pyrite formation at low temperature during early diagenesis in (sub-)surface sediments is an essential step of Fe and S biogeochemical cycles and the presence of this ubiquitous mineral of surface environments is often used as an indicator of paleo-redox conditions. Pathways of pyrite formation are usually discussed in environmental settings by involving a variety of nanosized Fe-S mineralogical precursors as a function of the local geochemical conditions. However, the influence of trace element impurities such as Ni and As in the solution at the time of pyrite formation has been poorly studied, whereas specific chemical signatures of trace elements are commonly observed in sedimentary pyrites. A better understanding of the impact of Ni and As incorporation at trace levels on pyrite formation is essential to help refining the use of these elements as paleo-redox indicators and to evaluate the role of pyrite as a sink regulating the biogeochemical cycle of potentially toxic trace elements. In this study, we have performed syntheses of pyrite at low temperature via the polysulfide pathway using aqueous Fe(III) and H 2 S in the presence of trace amounts of Ni(II) (0.001 mol%Fe) and As(III) (0.001 mol%Fe). Analysis of the solids collected at different time steps over the course of the experiments using X-Ray absorption spectroscopy at both the Fe and S K- edges shows that pyrite starts to precipitate within 5 days in presence of Ni(II) and within 32 days in presence of As(III), while the control experiment showed an intermediate precipitation rate of 14 days. Shell-by-shell analysis of Fe K- edge EXAFS data shows that the initial mineralogical precursors are the same in all the experiments and correspond to poorly-crystalline FeS (3.0 ± 0.1 Fe-S@2.25 Å; 1.7 ± 0.2 Fe-Fe@2.67 Å). In addition, XANES qualitative analysis suggests the incorporation of small amounts of Fe(III) within these FeS precursors. Synchrotron-based XRD and WAXS-PDF analysis of the starting solids show that in addition to S(0), the FeS precursors correspond to a continuum of FeS particles that ranges from tetragonal nanocrystalline FeS (a = 3.70(2) Å, c = 5.24(7) Å, MCD ab = 41 ± 4 Å MCD c = 21 ± 2 Å) to cluster-type FeS (MCD abc < 8.4 ± 4.3 Å). We propose that Ni(II) and As(III) have a different type of interaction with these FeS precursors, resulting respectively in an increase and a decrease in the rate of pyrite nucleation. While Ni(II) would incorporate within the structure of the FeS precursors, As would interact with (poly)sulfides in solution to form thio-As, possibly binding or precipitating onto FeS surfaces and thus slowing FeS transformation to FeS 2. Given that both Ni and As were introduced at trace levels in our experiments, these results suggest that the occurrence of trace amounts of impurities could have a strong influence on pyrite precipitation kinetics in natural settings such as pore-scale microenvironments. In addition to emphasizing the importance of trace elements such as Ni or As on the persistence of mobile colloidal FeS species in anoxic conditions, the results of the present study also point to the importance of considering the actual nature of the impurities when using pyrite composition for ancient environments and past climates reconstruction. [ABSTRACT FROM AUTHOR]

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 L III -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 FeS m (mackinawite) and FeS 2 (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 FeS m 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 FeS m may serve as a precursor for the formation of a more crystalline (less soluble) β- HgS(s) phase than present in environments devoid of FeS m. Support for this was provided by comparing our results with previously reported thermodynamic modelling results of Hg(II) and MeHg solubility in organic soils devoid of FeS m. In more general terms, we suggest the presence or absence of FeS m , through its influence on the chemical speciation of Hg and MeHg, may be a key factor behind the variability in rates of Hg(II) and MeHg transformation processes, such as methylation, reduction and demethylation, reported for different environmental settings. [ABSTRACT FROM AUTHOR]

We have used density functional theory calculations to investigate the interaction between methylamine (CH3NH2) and the dominant surfaces of mackinawite (FeS), where the surface and adsorption properties of mackinawite have been characterized using the DFT-D2 method of Grimme. Our calculations show that while the CH3NH2 molecule only interacts weakly with the most stable FeS(001), it adsorbs relatively strongly on the FeS(011) and FeS(100) surfaces releasing energies of 1.26 eV and 1.51 eV, respectively. Analysis of the nature of the bonding reveals that the CH3NH2 molecule interacts with the mackinawite surfaces through the lone-pair of electrons located on the N atom. The electron density built up in the bonding region between N and Fe is very much what one would expect of covalent type of bonding. We observe no significant adsorption-induced changes of the FeS surface structures, suggesting that amine capping agents would not distort the FeS nanoparticle surfaces required for active heterogeneous catalytic reactions. The vibrational frequencies and the infrared spectra of adsorbed methylamine have been calculated and assignments for vibrational modes are used to propose a kinetic model for the desorption process, yielding a simulated temperature programmed desorption with a relative desorption temperature of <140 K at the FeS(011) surface and <170 K at FeS(100) surface. [ABSTRACT FROM AUTHOR]

Wet iron-sulfur minerals have been shown to be ideal environments to allow for simple chemical reactions to occur in nature, for instance, in the framework of prebiotic chemistry. Yet, not much is known about such water/mineral interfaces beyond those involving pyrite, FeS2, which is, however, chemically rather inert. In contrast, mackinawite is chemically reactive and consists of a layered crystal structure comprising FeS sheets that can be easily cleaved. Here, the properties of water confined between such sheets in lamella-like setups is investigated in the spirit of surface science model systems. The properties of this intercalated water are found to depend significantly on the interlayer distance and change from 'arrested water' (in the limit of small interlayer distances) to liquid-like behavior. [ABSTRACT FROM AUTHOR]

• A novel layered iron sulfide phase has been discovered. • The new phase has a larger interlayer spacing of 8.03 Å than the conventional FeS of 5.31 Å. • The new phase exhibits a higher propensity to generate hydroxyl radicals (•OH). • The new phase exhibits significantly enhanced degradation efficiency towards typical pollutants. The hydroxyl radical (·OH) stands as one of the most potent oxidizing agents, capable of engaging in non-selective and instantaneous reactions with contaminants in water. Herein, we present a novel iron sulfide phase (S-FeS) characterized by an unprecedented structure, accompanied by its remarkable hydroxyl radical generation capability and contaminant degradation efficiency surpassing that of the conventional metastable iron sulfide phase, namely, the Mackinawite (FeS). In comparison to FeS, S-FeS exhibits enhanced degradation kinetics and higher efficacy in the removal of methylene blue, ciprofloxacin, and trivalent arsenic. Utilizing density functional theory (DFT) calculations, we postulate the mechanism for the exceptional contaminant degradation performance of S-FeS to be attributed to the increased exposure of the highly reactive (110) crystal facets. This research uncovers a new metastable phase that expands the polymorphisms within the iron sulfide family and showcases its capability for driving the oxygen reduction reaction. [Display omitted] [ABSTRACT FROM AUTHOR]

As iron sulfide mineral phases are important sedimentary sinks for naturally occurring or contaminant metals, it is important to know the fate of metals during the diagenetic transformation of primary sulfide minerals into more stable phases, such as pyrite (FeS2). Furthermore, the trace metal content of pyrite has been proposed as a marine paleoredox proxy. Given the diverse low-temperature diagenetic formation pathways for pyrite, this use of pyrite requires validation. We, therefore, studied nickel (Ni) and cobalt (Co) incorporation into freshly precipitated mackinawite (FeSm), and after experimental diagenesis to pyrite (FeS2) using S0 as an oxidant at 65 °C. Metal incorporation was quantified on bulk digests using ICP-OES or ICP-AES. Bulk mineralogy was characterized with micro-X‑ray diffraction (micro-XRD), documenting the transformation of mackinawite to pyrite. Epoxy grain mounts were made anoxically of mackinawite and pyrite grains. We used synchrotron-based micro-X‑ray fluorescence (µXRF) to map the distribution of Co and Ni, as well as to collect multiple energy maps throughout the sulfur (S) K-edge. Iron (Fe) and S K-edge micro-X‑ray absorption near edge spectroscopy (µXANES) was used to identify the oxidation state and mineralogy within the experimentally synthesized and diagenetically transformed minerals, and map end-member solid phases within the grain mounts using the multiple energy maps. Metal-free FeSm transformed to pyrite, with residual FeSm detectable. Co- and Ni-containing FeSm also transformed to pyrite, but with multiple techniques detecting FeSm as well as S0, implying less complete transformation to pyrite as compared to metal-free FeSm. These results indicate that Co and Ni may inhibit transformation for FeSm to pyrite, or slow it down. Cobalt concentrations in the solid diminished by 30% during pyrite transformation, indicating that pyrite Co may be a conservative tracer of seawater or porewater Co concentrations. Nickel concentrations increased several-fold after pyrite formation, suggesting that pyrite may have scavenged Ni from the dissolution of primary FeSm grains. Nickel in pyrites thus may not be a reliable proxy for seawater or porewater metal concentrations. [ABSTRACT FROM AUTHOR]

Abstract Iron(III) chloride deoxygenated alkaline aqueous colloidal solutions in the presence of 2-propanol and dextran sulfate were γ -irradiated with doses of 36 and 130 kGy. The dose rate was ∼31 kGy h−1. The samples isolated by washing with water mainly consisted of γ-FeOOH (lepidocrocite), δ-FeOOH and schwertmannite. The samples isolated by washing with ethanol contained mainly α-FeOOH (goethite), δ-FeOOH (feroxyhyte) and iron(III) hydroxide sulfate. At lower dose (36 kGy) magnetite (Fe 3 O 4) was formed. The samples isolated by admixing glycerol consisted of the intermediate phases Fe(OH) 2 , GR(SO 4 2−) and non-stoichiometric FeS 1-x (mineral name mackinawite). The amounts of Fe(II) in glycerol-isolated solid samples were 60.5% and 82.6% as determined by Mössbauer spectroscopy at doses of 36 and 130 kGy, respectively. The amount of Fe2+ in the acidified solutions containing dissolved γ -irradiation products was determined using potassium permanganate titration. The amounts of Fe2+ in acidified solutions were 72.0% and 92.0% at doses of 36 and 130 kGy, respectively. Thus, although the conventionally isolated powders consisted exclusively of Fe(III) products, the high reducing conditions upon γ -irradiation were confirmed by capturing the Fe(II) intermediate phases and by quantitative determinations of ferrous ions in the solid samples and solutions containing dissolved γ -irradiation products. Graphical abstract Image 1 Highlights • Dextran sulfate plays active role in radiolytic synthesis of magnetic iron oxides. • γ-FeOOH, δ-FeOOH and schwertmannite were isolated by washing with water. • α-FeOOH, δ-FeOOH and iron(III) hydroxide sulfate were isolated with ethanol rinsing. • Intermediate phases Fe(OH) 2 , GR(SO 4 2−) and FeS 1-x were isolated by glycerol. • Fe(II) intermediate phases oxidized to Fe(III) products by conventional isolation. [ABSTRACT FROM AUTHOR]

We report on a systematic study on the physicochemical attributes of synthetic Fe-Cu-sulfide nanoparticles (NPs) precipitated under conditions similar to the anoxic, low-temperature aqueous, sedimentary, soil, and subsurface environments where these NPs have been repeatedly identified. Characterizing the basic attributes of these NPs is the first step in understanding their behaviors in various processes including in the bio-availability of essential and toxic metals, environmental remediation, and resource recovery. Abiotic experiments are compared to biotic experiments in the presence of the sulfate-reducer Desulfovibrio vulgaris to elucidate biological controls on NP formation. First, the single-metal end-member NPs are determined by precipitation in a solution containing either aqueous Fe(II) or Cu(II). Limited differences are observed between biogenic and abiogenic precipitates aged for up to one month; the Fe-only experiments resulted in 4 -- 10 nm mackinawite (FeS) NPs that aggregate to form nanosheets up to ~1000 nm in size, while the Cu-only experiments resulted in mixtures of covellite (CuS) NPs comprised of <10 nm fine nanocrystals, 20 -- 40 × 6 -- 9 nm nanorods, and ~30 nm nanoplates. The crystal sizes of biogenic mackinawite and covellite are, respectively, larger and smaller than their abiogenic counterparts, indicating a mineral-specific response to biological presence. Structural defects are observable in the fine nanocrystals and nanorods of covellite in both biogenic and abiogenic experiments, indicative of intrinsic NP instability and formation mechanism via particle attachment. In contrast, covellite nanoplates are defect free, indicating high stability and potentially rapid recrystallization following particle attachment. Next, mixed-metal sulfide NPs are precipitated at variable initial aqueous Fe-to-Cu ratios (2:1, 1:1, and 1:5). With an increasing ratio of Fe-to-Cu, Fe-rich covellite, nukundamite (Cu5.5FeS6.5), chalcopyrite (CuFeS2), and Cu-rich mackinawite are formed. The Fe-rich covellite NPs are larger (100 -- 200 nm) than covellite precipitated in the absence of Fe, indicating a role for Fe in promoting crystal growth. Chalcopyrite and nukundamite are formed through the incorporation of Fe into precursor covellite NPs while retaining the original crystal morphology, as confirmed by doping a covellite suspension with aqueous Fe(II), resulting in the formation of chalcopyrite and nukundamite within days. Additionally, in the biological systems, we observe the recrystallization of mackinawite to greigite (Fe3S4) after six months of incubation in the absence of Cu and the selective formation of chalcopyrite and nukundamite at lower initial Fe-to-Cu ratios compared to abiotic systems. These observations are consistent with NP precipitation that are influenced by the distinct (sub)micro-environments around bacterial cells compared to the bulk solution. Comparative TEM analyses indicate that the synthetic NPs are morphologically similar to NPs identified in natural environments, opening ways to studying behaviors of natural NPs using experimental approaches. [ABSTRACT FROM AUTHOR]

Oil reservoir souring and associated materials integrity challenges are of great concern to the petroleum industry. The bioengineering strategy of nitrate injection has proven successful for controlling souring in some cases, but recent reports indicate increased corrosion in nitrate-treated produced water re-injection facilities. Sulfide-oxidizing nitrate reducing bacteria (soNRB) have been suggested to be the cause of such corrosion. Using the model oil field soNRB Sulfurimonas sp. strain CVO we conducted a detailed analysis of soNRB-induced corrosion at initial nitrate to sulfide (N/S) ratios relevant to oil field operations. The activity of strain CVO caused severe corrosion rates of up to 0.27 mm y-1 and up to 60 µm deep pitting within only nine days. The highest corrosion during growth of strain CVO was associated with production of zero-valent sulfur during sulfide oxidation, and accumulation of nitrite, when initial N/S ratios were high. Abiotic corrosion tests with individual metabolites confirmed biogenic zero-valent sulfur and nitrite as the main causes of corrosion under the experimental conditions. Mackinawite (FeS) deposited on carbon steel surfaces accelerated abiotic reduction of both sulfur and nitrite, exacerbating corrosion. Based on these results a conceptual model for nitrate-mediated corrosion by soNRB is proposed. [ABSTRACT FROM AUTHOR]

Abstract We developed a novel solidification and stabilization process using a nanoscale zerovalent iron (NZVI)-cement system for reductive immobilization of hexavalent uranium (U(VI)) in a soil–cement matrix. The NZVI suspension without cement demonstrated high removal efficiency (100% in 2 h) and fast removal kinetics (53.7 Lm−2d−1), which surpassed those of other Fe-containing minerals (i.e., green rust, mackinawite, magnetite, and pyrite). Significant removal of aqueous U(VI) was observed in NZVI-cement slurries and minimal adsorbed U was desorbed by a bicarbonate/carbonate (CARB) solution. Surface analysis using scanning electron microscopy and X-ray photoelectron spectroscopy revealed U distributed homogeneously on the surface of the NZVI-cement and transformed considerably from U(VI) to reduced U species by coupled oxidation of Fe(0)/Fe(II) to Fe(III). Furthermore, the increase in pH and NZVI concentration, and presence of humic acid resulted in the enhanced U(VI) reduction in NZVI-cement slurries. The NZVI-cement system was tested with a soil matrix, resulting in successful immobilization of aqueous U(VI) in both batch and column experiments. Moreover, the U(VI) removed in the NZVI-cement system was not leached out by the CARB solution during long-term experiments. The results suggest an NZVI-cement system could represent a promising remediation alternative for effective and stable immobilization of U(VI) in contaminated sites. Graphical abstract Image Highlights • A permeable NZVI-cement matrix is developed for solidification/stabilization of U(VI). • NZVI showed higher U(VI) removal than other Fe(II)-containing minerals in the matrix. • U(VI) was reductively immobilized in the NZVI-cement slurries system. • NZVI concentration, pH, and humic acid significantly influenced the U(VI) reduction. • Effective U(VI) removal was observed in a NZVI-cement-soil column. [ABSTRACT FROM AUTHOR]

Organic acid is prevalent in underground environments and, against the backdrop of biogeochemical cycles on Earth, holds significant importance in the degradation of contaminants by redox-active minerals. While earlier studies on the role of organic acid in the generation of reactive oxygen species (ROS) primarily concentrated on electron shuttle or ligand effects, this study delves into the combined impacts of organic acid decomposition and Mackinawite (FeS) oxidation in contaminant transformation under dark aerobic conditions. Using bisphenol A (BPA) as a model, our findings showed that oxalic acid (OA) notably outperforms other acids in enhancing BPA removal, attaining a rate constant of 0.69 h -1 . Mass spectrometry characterizations, coupled with anaerobic treatments, advocate for molecule-O 2 activation as the principal mechanism behind pollutant transformation. Comprehensive results unveiled that carbon center radicals, initiated by hydroxyl radical ( • OH) attack, serve as the primary agents in pollutant oxidation, accounting for at least 93.6% of the total • OH generation. This dynamic, driven by the decomposition of organic acids and the concurrent formation of carbon-centered radicals, ensures a steady supply of electrons for ROS generation. The obtained information highlights the importance of OA decomposition in the natural attenuation of pollutants and offers innovative strategies for FeS and organic acid-coupled decontamination.

Mackinawite (FeS) has gained increasing interest due to its potential application in contaminant removal by either reduction or oxidation processes. This study further demonstrated the efficiency of FeS in degrading nitrobenzene (ArNO 2 ) via a sequential two-step reduction and oxidation process under neutral conditions. In the reduction stage, FeS rapidly reduced ArNO 2 to aniline (ArNH 2 ), with nitrosobenzene (ArNO) and phenylhydroxylamine (ArNHOH) serving as the intermediates. X-ray photoelectron spectroscopy (XPS) analysis indicated that both Fe(II) and S(II) in FeS contributed electrons to the reduction of ArNO 2 . In the subsequent oxidation stage with oxygen, by addition of 0.5 mM tripolyphosphate (TPP), ArNH 2 generated in the reduction process could be effectively oxidized to aminophenols by hydroxyl radicals ( • OH), which would undergo eventual mineralization via ring-cleavage reactions. TPP exerted a favorable role in enhancing • OH production for ArNH 2 degradation by promoting the formation of the dissolved Fe(II)-TPP complex, thus enhancing the homogeneous Fenton reaction. Additionally, TPP adsorption inhibited the surface oxidation reactivity of FeS due to the change of Fe(II) coordination. Finally, the effective degradation of ArNO 2 by FeS in actual groundwater was demonstrated by using this sequential reduction and oxidation approach. These research findings provide a theoretical basis for a new FeS-based remediation approach, offering an alternative way for comprehensive removal of ArNO 2 .

Highlights • HPLC-AFS technique reveals that As(V) species is reduced into As(III) a low valence species. • Ratio of mackinawite and solution affects dissolved arsenic species. • Solid phases contain As(0), As(+1), As(+3) and As(+5). Abstract Biologically mackinawite was synthesized by FRB and SRB culture solutions, which had been shown a crucial role in removal arsenic under anaerobic environment. To investigate the changes of arsenic species in aqueous by As(V) sorption onto biologically FeS, As(III/V) in aqueous were detected by HPLC-AFS. And the solid phase was characterized by XRD, XPS, and BET. The adsorption envelope of As(T) shown the high uptake occurred at pH 5. The adsorption kinetics followed the pseudo- second- order model and the isotherm followed by Langmuir equilibrium for different solid/solution ratios. The maximum sorption capacities were 237.45 mg/g (0.05 g/L FeS), 160.45 mg/g (0.1 g/L FeS), and 56.71 mg/g (0.2 g/L FeS), respectively. The results showed that sorption for As(V) sorption by FeS decreased with increased in solid/solution ratios. Our results showed that As(V) was reduced to As(III) or lower valences in As(V) sorption experiments. The results showed that the As(others) dominated in the batch experiments and the As(V) was the main state in the column experiments. The XPS date suggested that the FeS particles were effective for reduce As(V) and the As(others) were possibly As-S and As-As complexes. [ABSTRACT FROM AUTHOR]

Biomineralization means mineral formation under the influence of organisms. Sulphate-reducing bacteria (SRB) constitute an essential role of iron sulphide minerals precipitation. Their composition involves amorphous, non-stoichiometric or crystalline iron sulphides, weakly or strongly magnetic. Variation in environmental conditions can alter the reactive iron species within the mineral, potentially modifying their magnetic properties. Biogenic iron sulphide minerals can be used as heavy metals and toxic ions adsorbents in soil or water remediation. For these reasons, a series of laboratory-scale iron sulphide synthesis experiments with the aim to study the chemical composition, mineralogy and magnetic properties of iron sulphide precipitates were carried out using SRB under various cultivation mode and nutrient medium composition. Energy-dispersive X-ray analysis (EDX) showed formation of iron sulphides in all biogenic samples and iron phosphates in abiotic controls. Results of X-ray diffraction analysis (XRD) in biomineralized samples confirmed nanocrystalline greigite, mackinawite and sulphur alpha. Magnetic measurements showed that sample prepared by static cultivation without addition of fresh nutrient medium was the most magnetic, magnetic hysteresis of sample formed under semicontinuous mode without any nutrient supply was the lowest. Abiotic samples contained only vivianite and they did not prove any significant response to magnetic field. [ABSTRACT FROM AUTHOR]

Graphical abstract Schematic diagram of controlling factor the SCC process for E690 steel in different solution. (The HE and AD/pitting effect lines represent the relative contribution of these two factor, rather than the real values.) Highlights • Corrosion of E690 steel in the local confined environment in seawater is studied. • Acidification of seawater and addition of thiosulfate increase the cathodic current. • Pitting corrosion occurs in the thiosulfate-containing, acidified seawater. • Hydrogen embrittlement dominates the cracking at low content of thiosulfate. • Occurrence of corrosion pits dominates the cracking at high content of thiosulfate. Abstract Electrochemical corrosion and stress corrosion cracking (SCC) behavior of E690 steel in the aerated, deaerated, acidified and thiosulfate-containing artificial seawater are investigated. Corrosion product is transformed from a single iron oxyhydroxide layer to a stratified structure with an additional outer mackinawite layer after long-time immersion with addition of thiosulfate. The SCC process at low thiosulfate concentration (10−4 M) is mainly dominated by hydrogen embrittlement because of the thiosulfate-induced promotion of hydrogen permeation. At high thiosulfate concentration (10−2 M), occurrence of severe pitting corrosion leads to the highest SCC susceptibility despite that the formation of continuous mackinawite inhibits hydrogen permeation. [ABSTRACT FROM AUTHOR]

Abstract To enhance the degradation of N, N-dimethylacetamide (DMAC) in aqueous solution, the natural mackinawite (NM) is introduced for catalytic ozonation in this study as it is an environmentally friendly catalyst with low cost and easy availability. The properties of the NM were initially characterized via scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Then, impact factors including NM dosage, ozone gas concentration and initial pH were investigated and the optimal conditions (i.e., NM dosage = 3.5 g/L, ozone gas concentration = 300 L/min, initial pH = 6.8) were obtained in NM/O3 process. Besides, the superiority of the NM/O 3 process was confirmed by the experiments that the degradation efficiency of DMAC in the NM/O 3 process (i.e., 95.4%) was much higher than that in the zero-valent iron (ZVI)/O 3 process (i.e., 46.1%) and the synthetic FeS/O 3 process (i.e., 68.6%). Furthermore, the intermediate and possible degradation pathway of DMAC were proposed, and the biological toxicity of the intermediate was subsequently evaluated by the activated sludge. Finally, the mechanism of the NM/O3 process was proposed in this study based on control experiment and radical scavenging experiment. The extraordinary efficiency for DMAC degradation was found to be mainly caused by HO• of the reactive oxygen species (ROS) (i.e., HO•, O 2 •- and H 2 O 2) generated in the NM/O 3 process. Therefore, this study confirmed that NM was a high efficient catalyst for degradation the toxic and refractory pollutants in catalytic ozonation system. Highlights • NM was first used as a catalyst for ozonation process. • DMAC degradation efficiency was remarkably enhanced by NM catalytic ozonation. • Degradation pathway of DMAC was proposed by NM/O 3 system. • The generations of ROS (i.e., HO.•, H 2 O 2 and O 2 •−) were proved in NM/O 3 system. [ABSTRACT FROM AUTHOR]

Highlights • FeS-coated sand completely reduces Cr(VI) into Cr(III) at all pH conditions. • The increased Cr(VI) removal due to FeS coating is evident at neutral to basic pH. • Cr(VI) removal mechanisms by both coated and uncoated sands are pH-dependent. • FeS-coated sand is the potential material in PRBs to treat Cr(VI) contamination. Abstract This study investigates the feasibility of mackinawite (FeS)-coated sand in permeable reactive barrier applications to treat Cr(VI)-contaminated groundwater under anoxic conditions. For this, Cr(VI) sorption experiments were conducted using both coated and uncoated sands. Solution-phase Cr speciation and Cr K-edge X-ray absorption near-edge structure (XANES) analysis indicated the complete reduction of Cr(VI) to Cr(III) by coated sand. At pH 4.7, substantial amounts of Cr(III) remained in solution due to its unfavorable cationic adsorption at acidic pH. At pH 7.1 and 9.8, it was quantitatively immobilized by forming Cr(III)-bearing precipitates. In contrast, uncoated sand showed the decreasing Cr(VI) sorption with pH. In uncoated sand, magnetite impurities would mediate the partial reduction of Cr(VI). Thus, the pH-dependent sorption by uncoated sand was due to both unfavorable anionic Cr(VI) adsorption and its lesser reduction to Cr(III) with pH. Compared to uncoated sand, coated sand showed significantly increased Cr(VI) sorption at neutral to basic pH. By Fe K-edge XANES analysis, FeS was mainly responsible for Cr(VI) reduction by coated sand, with a green rust-like phase being the major Fe product. Since Fe(OH) 3 is not thermodynamically stable under the redox conditions favoring formation of green rust, Fe(III)-substituted Cr(OH) 3 likely represents a Cr(III)-bearing phase. [ABSTRACT FROM AUTHOR]

Mackinawite FeS nanoparticles were synthesized by wet chemical reduction technique. The chemical composition of the nanoparticles determined by the energy dispersive analysis of X-ray (EDAX) showed them to be near stoichiometric and without any impurity. The X-ray diffraction (XRD) analysis showed the synthesized nanoparticles to be of single phase FeS and possess tetragonal unit cell structure with lattice parameters as a  = 3.68 Å, c  = 5.03 Å. The transmission electron microscopy showed the synthesized nanoparticles to be spherical in shape. The optical absorbance study revealed that the nanoparticles possess direct optical band gap of 2.39 eV. The thermogravimetric analysis exhibited two steps decomposition which was substantiated by the corresponding differential thermogravimetric peaks. The simultaneously recorded differential thermal analysis showed three regions of decomposition. The endothermic peak around 587 K is due to the solid state phase transformation of FeS structure to the orthorhombic MnP-type structure with space group Pnma. The kinetic parameters were evaluated by employing non-linear Kissinger-Akahira-Sunose method. The vibrating sample magnetometer study showed the nanoparticles samples to behave as super paramagnetic particles. All the obtained results are discussed in detail. [ABSTRACT FROM AUTHOR]

Susceptibility to hydrogen-induced cracking of API 5L B and X52MS low-carbon steels in NACE 177-A, 177-B, and 284-B solutions has been investigated by the present work. A metallographic analysis of these steels was performed before and after NACE TM0284 standard testing. Corrosion products were characterized by scanning electron microscopy and X-ray dispersive energy spectrometry, which were subsequently identified by X-ray diffraction. Thus it was found that pH directly affects the solubility of corrosion products and hydrogen permeation. Both steels showed generalized corrosion in solution 177-A, and a discontinuous film was formed on their surfaces in solution 177-B; however, only the API 5L B steel failed the HIC test and exhibited greater crack length ratio in solution 177-A. In solution 284-B whose pH is higher, the steels exhibited thick mackinawite films with no internal cracking. [ABSTRACT FROM AUTHOR]

During X‐ray absorption spectroscopy (XAS) measurements of arsenic (As), beam‐induced redox transformation is often observed. In this study, the As species immobilized by poorly crystallized mackinawite (FeS) was assessed for the susceptibility to beam‐induced redox reactions as a function of sample properties including the redox state of FeS and the solid‐phase As speciation. The beam‐induced oxidation of reduced As species was found to be mediated by the atmospheric O2 and the oxidation products of FeS [e.g. Fe(III) (oxyhydr)oxides and intermediate sulfurs]. Regardless of the redox state of FeS, both arsenic sulfide and surface‐complexed As(III) readily underwent the photo‐oxidation upon exposure to the atmospheric O2 during XAS measurements. With strict O2 exclusion, however, both As(0) and arsenic sulfide were less prone to the photo‐oxidation by Fe(III) (oxyhydr)oxides than NaAsO2 and/or surface‐complexed As(III). In case of unaerated As(V)‐reacted FeS samples, surface‐complexed As(V) was photocatalytically reduced during XAS measurements, but arsenic sulfide did not undergo the photo‐reduction. [ABSTRACT FROM AUTHOR]

Among the numerous iron-based materials for persulfate (PS) activation, mackinawite (FeS) particles have gained considerable interest as a ubiquitous natural mineral due to their high reactivity. However, the iron and sulfur co-mediated reaction mechanism of PS activation by FeS remains ambiguous. In this study, FeS was applied as a catalyst to activate PS for p -chloroaniline (PCA) degradation and mineralization over a wide initial pH range (3.0–11.0). The reaction was found to follow pseudo-first-order kinetics, with rate constants ranging from 0.0044 to 0.0144 min −1 . The reaction mechanism was elucidated by electron spin resonance (ESR) and quenching studies. A heterogeneous activation mechanism, in which surface Fe(II) species activated PS to produce OH ads and SO 4 − ads , controlled by surface reaction and diffusion was proposed, whereas OH free and SO 4 − free diffusing from the FeS surface were mainly responsible for PCA degradation. The sulfur-mediated cycling of iron species was investigated by comparing PS activation by zero-valent iron (ZVI) and FeS, exogenous Fe(III) addition and X-ray photoelectron spectroscopy (XPS). The results suggested that Fe(II) and S(-II) experienced independent oxidations and that S(-II) species could regenerate Fe(II) from Fe(III) at the FeS surface. Therefore, the S(-II)-promoted Fe(II)/Fe(III) cycle resulted in less PS decomposition but a higher PCA mineralization efficiency. The findings of this study elucidated the novel surface activation mechanism of PS by FeS and provided useful information for utilizing FeS to remediate contaminated water. [ABSTRACT FROM AUTHOR]

Sedimentary iron sulfide minerals play a key role in maintaining the oxygenation of Earth’s atmosphere over geological timescales; they also record critical geochemical information that can be used to reconstruct paleo-environments. On modern Earth, sedimentary iron sulfide mineral formation takes places in low-temperature environments and requires the production of free sulfide by sulfate-reducing microorganisms (SRM) under anoxic conditions. Yet, most of our knowledge on the properties and formation pathways of iron sulfide minerals, including pyrite, derives from experimental studies performed in abiotic conditions, and as such the role of biotic processes in the formation of sedimentary iron sulfide minerals is poorly understood. Here we investigate the role of SRM in the nucleation and growth of iron sulfide minerals in laboratory experiments. We set out to test the hypothesis that SRM can influence Fe-S mineralization in ways other than providing sulfide through the comparison of the physical properties of iron sulfide minerals precipitated in the presence and in the absence of the sulfate-reducing bacterium Desulfovibrio hydrothermalis AM13 under well-controlled conditions. X-ray diffraction and microscopy analyses reveal that iron sulfide minerals produced in the presence of SRM exhibit unique morphology and aggregate differently than abiotic minerals formed in media without cells. Specifically, mackinawite growth is favored in the presence of both live and dead SRM, when compared to the abiotic treatments tested. The cell surface of live and dead SRM, and the extracellular polymers produced by live cells, provide templates for the nucleation of mackinawite and favor mineral growth. The morphology of minerals is however different when live and dead cells are provided. The transformation of greigite from mackinawite occurred after several months of incubation only in the presence of live SRM, suggesting that SRM might accelerate the kinetics of greigite formation under strict anoxic conditions. Pyrite formation was not observed in any experiments. While SRM provide nearly all the sulfide to the Fe-S system at low temperatures, we also posit that SRM play an additional formative role in the size, morphology and potentially the mineralogy of iron sulfide minerals in sedimentary environments, therefore potentially influencing their reactivity. Attempting to reconstruct modern and ancient biogeochemical cycles based on the geochemistry of iron sulfide minerals formed under purely abiotic conditions should be therefore done with caution. [ABSTRACT FROM AUTHOR]

Iron sulphide nanocrystalline (FeS NCs) may have significant potential for applications in many areas. Ultrasonic spray pyrolysis (USP) method is used to deposit mackinawite FeS NCs film on glass substrate at 335 oC in this study. The film was characterized by vibrational (FTIR and Raman) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis spectra. The observed diffraction peaks at 2θ angles 28.12° and 50.45° correspond to the lattice planes (103) and (112) respectively. The crystallite size was calculated to be around 10 nm by using the Scherrer equation with the peak corresponding to the (112) plane. Raman spectrum revealed four peaks placed in the region of 200-750 cm-1 due to mackinawite FeS phase. FTIR spectrum revealed two peaks at 649 cm-1 and 827 cm-1 that are attributed to stretching frequency of FeS. Optical band gap of the FeS film was determined to be 2.52 eV. Refractive index, extinction coefficient and optical conductivity of this film were determined using transmittance and reflectance spectra. [ABSTRACT FROM AUTHOR]