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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]

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]

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]

• 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]

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]

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]

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]

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]

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]

Greigite forms as an intermediate phase along the pyrite reaction pathway. Despite being considered metastable, it is observed in numerous shallow natural systems, suggesting it could be a unique proxy for diagenetic and environmental conditions. We use thermodynamic reaction pathway modelling in PHREEQC software, to understand the role of iron and sulphur ratios, pH and Eh, and temperature on the formation and retention of greigite in aqueous solutions. With newly available experimental thermodynamic properties, this work identifies the chemical boundary conditions for greigite formation in aqueous solutions. Greigite precipitation is likely favourable in anoxic and alkaline aqueous solutions at or below 25 °C. Our numerical experiments show that greigite is closer to saturation in iron-rich solutions with minor sulphur input. Greigite precipitation in strongly alkaline solutions suggest polysulfides and ferric iron-bearing minerals may be favourable reactants for its formation. Greigite precipitates at iron and sulphur concentrations that are over two orders of magnitude greater than iron sulphide-hosted natural porewaters. This disparity between model and field observations suggest microenvironments within bulk solutions may be important for greigite formation and retention. These constraints suggest greigite is more likely to form alongside pyrite in shallow, non-steady state aqueous solutions. • Greigite precipitates in iron-rich, alkaline and anoxic solutions. • Greigite precipitates provided saturation states are altered. • Greigite is more likely to precipitate alongside pyrite at lower temperatures. • Polysulfide reactions favour precursor iron sulphide formation in the models. • Model comparisons with natural systems suggest greigite forms in microenvironments. [ABSTRACT FROM AUTHOR]

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

Anodic polarization of carbon steel electrodes in deaerated 0.01 mol L −1 NaCl + 0.01 mol L −1 NaHCO 3 + 0.001/0.01 mol L −1 Na 2 S solutions (with pH ∼ 7.5) led to heterogeneous layers made of magnetite and mackinawite. A locally accelerated dissolution of the steel was observed in any case and the most corroded areas were covered with a layer enriched with mackinawite. This suggests that part of the electrode surface was blocked, covered with a magnetite rich layer where water reduction was favored. The effects of the galvanic coupling between a magnetite electrode and a bare carbon steel electrode covered with a ∼5 mm thick argillites layer were then studied in a 0.01 mol L −1 NaCl + 0.01 mol L −1 NaHCO 3 solution at room temperature and 80 °C. Electrochemical measurements demonstrated that a relatively high galvanic current initially flowed between the magnetite electrode (cathode) and the steel electrode (anode). However, this current decreased with time down to negligible values. Micro-Raman spectroscopy analysis revealed that a magnetite layer was formed on the steel electrode, thus suppressing the initial difference between anode and cathode. When the same experiment was performed with the steel electrode set under an argillites layer enriched with Na 2 S, the galvanic current density remained constant at a significant value (∼20 μA cm −2 ) during the 11 days of experiment. It led to the formation of a layer composed of mackinawite (and chukanovite) on the steel electrode. This result confirms that heterogeneous magnetite/mackinawite layers can favor galvanic corrosion even in anoxic conditions. [ABSTRACT FROM AUTHOR]

The thermodynamic properties of Fe-S compounds with different crystal structure are very different. In this study, the structural, elastic and thermodynamic properties of mackinawite (FeS) and pyrite (FeS 2 ) were investigated by first-principles calculations. Examination of the electronic density of states shows that mackinawite (FeS) is metallic and that pyrite (FeS 2 ) is a semiconductor with a band gap of E g = 1.02 eV. Using the stress–strain method, the elastic properties including the bulk modulus and shear modulus were derived from the elastic C ij data. Density functional perturbation theory (DFPT) calculations within the quasi-harmonic approximation (QHA) were used to calculate the thermodynamic properties, and the two Fe-S compounds are found to be dynamically stable. The isothermal bulk modulus, thermal expansion coefficient, heat capacities, Gibbs free energy and entropy of the Fe-S compounds are obtained by first-principles phonon calculations. Furthermore, the temperature of the mackinawite (FeS) ⟶ pyrite (FeS 2 ) phase transition at 0 GPa was predicted. Based on the calculation results, the model for prediction of Fe-S compounds in the Fe-H 2 S-H 2 O system was improved. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) can provide accurate, computationally-manageable, first-principles characterizations of mineral surface speciation at molecular scales. This is accomplished by solving the Kohn-Sham equation, a tractable method of applying quantum mechanics to multi-electron systems that was inspired by some visionary remarks of P. A. M. Dirac, one of the founders of quantum theory. Fifty years of advances in DFT and the development of massively-parallel computing platforms have paved the way for molecular-scale modeling of the mineral surface that is directly relevant to the study of natural nanoparticles. In fact, DFT has matured sufficiently to be applied to metal sorption by geomedia as a “computer experiment,” yielding quantitative molecular-scale information that fully complements insights gained from surface spectroscopy. This powerful approach is illustrated first through a series of structural simulations of chalcophanite group minerals and hetaerolite–hausmannite solid solutions, then through several recent studies by the authors and their collaborators focusing on transition and heavy metal sorption by layer-type Mn(IV) oxide and Fe(II) sulfide nanoparticles. Our results show how DFT simulations reveal the mechanistic basis of metal partitioning trends observed experimentally while creating useful models for resolving experimental surface speciation conundrums. [ABSTRACT FROM AUTHOR]

The uptake of aqueous Ni(II) by synthetic mackinawite (FeS) was examined in anaerobic batch experiments at near-neutral pH (5.2 to 8.4). Initial molar ratios of Ni(II) to FeS ranged from 0.008 to 0.83 and maximum Ni concentrations in mackinawite, expressed as the cation mol fraction, were as high as X Ni = 0.56 (Fe 1 − x Ni x S; 0 ≤ x ≤ 1). Greater than 99% Ni removal from solution occurred when Ni loading remained below 0.13 ± 0.03 (1 σ ) mol Ni per mol FeS due to sorption of Ni at the mackinawite surface. Characterization of experimental solids using X-ray diffraction and Raman spectroscopy showed patterns characteristic of nanocrystalline mackinawite; no evidence of nickel monosulfide (α-NiS or millerite), polydymite (Ni 3 S 4 ), or godlevskite [(Ni,Fe) 9 S 8 ] formation was indicated regardless of the amount of Ni loading. Slight expansion of the c -axis correlated with increasing Ni content in synthetic mackinawite, from c = 5.07 ± 0.01 Å at X Ni = 0.02 to c = 5.10 ± 0.01 Å at X Ni = 0.38. Ni K -edge extended X-ray absorption fine structure (EXAFS) spectra of synthetic Ni-bearing mackinawite are similar in phase and amplitude to the Fe K -edge EXAFS spectrum of Ni-free mackinawite, indicating that the molecular environment of Ni 2+ in Ni-bearing mackinawite is similar to that of Fe 2+ in Ni-free mackinawite. EXAFS data fitting of Ni-bearing mackinawite with X Ni = 0.42 indicated a coordination number of 4.04 ± 0.30 and an average Ni S bond distance of 2.28 Å, in good agreement with the Fe S bond distance of 2.26 Å in mackinawite, tetrahedral Fe coordination, and slight lattice expansion along the c -axis. At lower Ni loadings ( X Ni = 0.05–0.11), EXAFS analysis showed a decrease in Ni S coordination towards CN = 3, which reflects the influence of sorbed Ni. Continued Ni uptake, past the maximum amount of sorption, was accompanied by proportional molar release of Fe to solution. Interstitial occupancy of Ni within the mackinawite interlayer may be transitional to structural substitution of Fe. The Ni-mackinawite solid-solution is described by a one-site binary mixing model: ln K d = ln K e − W RT 1 − 2 X Ni where K d is the distribution coefficient, K e is the ratio of equilibrium constants for Ni-mackinawite and mackinawite (14.4 ± 1.3), W is an ion interaction parameter, and X Ni is the mole fraction of end-member NiS in the solid solution. The experimentally determined value of W is 17.74 ± 1.15 kJ/mol and indicates significant non-ideality of the solid solution. Transformation processes were evaluated by aging Ni-mackinawite with polysulfides and solutions saturated with air. Reaction of Ni-mackinawite with polysulfides led to the formation of pyrite (FeS 2 ) and Ni retention in the solid phase. When Ni-mackinawite was aged in the presence of dissolved oxygen, transformation to goethite (FeOOH) and violarite (FeNi 2 S 4 ) was observed. [ABSTRACT FROM AUTHOR]

In euxinic sediments, the reaction between iron and sulfides results in the formation of Fe(II)-sulfides, which are known to play a key role in trace metal sequestration. The present study investigates the sequestration of nickel during Fe(II)-sulfide formation mediated by the (thio)sulfate-reducing bacterium Desulfovibrio capillatus in the presence of soluble Fe(III)-citrate and thiosulfate as the terminal electron acceptor. XRD, HRTEM and Fe K-edge EXAFS data indicate that biogenic mackinawite (FeS) was the sole mineral formed in our experiments. These data also show that the kinetics of mackinawite crystal growth was significantly accelerated when nickel was present in the starting solution. In addition, the lack of detection of other Fe(II)-sulfides indicates inhibition of the mackinawite (FeS) to greigite (Fe 3 S 4 ) and/or pyrite (FeS 2 ) transformation that is likely related to ( i ) the efficiency of Desulfovibrio capillatus in reducing S(0) to H 2 S and ( ii ) the absence of O 2 during the experiments. Finally, chemical analyses show that 98% of the nickel is associated with biogenic mackinawite and no release of nickel from mackinawite was observed after up to 10 months of incubation under anoxic conditions. This finding is consistent with Ni K-edge EXAFS data which show that Ni(II) substitutes for Fe(II) in the structure of biogenic mackinawite. This study shows that (thio)sulfate-reducing bacteria can efficiently promote the formation of mackinawite in euxinic sedimentary environments and that these Fe(II)-sulfides can act as efficient and long-term trapping minerals for nickel in such settings. Considering the capacity of mackinawite at incorporating other trace metals such as Mn, Co, Cu and Zn, this iron sulfide could also serve as a host for these elements as well. This study suggests that mackinawite likely plays a more important role in the biogeochemical cycles of Fe, S, and associated trace metals than considered up to now. [ABSTRACT FROM AUTHOR]

Mackinawite (FeS), widespread in low temperature aquatic environments, is generally considered to be the first Fe sulfide formed in sedimentary environments which has shown effective immobilization of heavy metals and toxic oxyanions through various sorption reactions. The spectroscopic study researches on mackinawite formed by FRB and SRB and its environmental implication for in-situ remediation of acid mine drainage where contains large amounts of Fe 3 + and SO 4 2 − . The XRD result of biologically synthetic particles shows that these particles are mainly composed of mackinawite (FeS 0.9 ). The Raman peaks observed at 208, 256, 282, 298 cm − 1 are attributed to Fe S stretching vibrations of mackinawite. The Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) reveals that the diagnostic bands of low intensity for these FeS particles occur at 412–425 cm − 1 and 607–622 cm − 1 , which are assigned to the stretching vibrations of S S and Fe S bonds. The Raman and IR vibrations from organic components both confirm that these particles are biogenic origin. The IR spectra of biologically synthesized mackinawite for different aging times show that the nano-sized particles mackinwate will be completely oxidized within 10 h. All these findings have good implications for in-situ remediation of acid mine drainage. [ABSTRACT FROM AUTHOR]

In this work, the morphology, structure and ion-selectivity of surface sulfide films formed on an X65 pipeline steel in solutions with various pH values and H 2 S concentrations were characterized, and the effect of the film on hydrogen permeation was investigated. Results demonstrated that crystalline iron sulfide (FeS) and/or mackinawite forms on the steel surface during H 2 S corrosion, depending on the H 2 S concentration and the solution pH. The presence of sulfide films blocks the hydrogen permeation through the steel, and the blocking effect increases with the increasing H 2 S concentration and reducing pH value. Compared to the film containing crystalline FeS only, the mixed compositions of mackinawite and FeS in the film cause an increased effect to block hydrogen permeation. The sulfide films are an n -type semiconductor with the anion selectivity, which, combined with the film thickness, contributes to the blocking effect on hydrogen permeation into the filmed steel. [ABSTRACT FROM AUTHOR]

Although reductive dissolution of Fe(III) oxides has been well accepted for As mobilization in alluvial aquifers, the key factors controlling this process are poorly understood. Arsenic(V)-adsorbing ferrihydrite, goethite and hematite were used to examine in-situ mobilization and transformation of adsorbed As(V) and Fe(III) oxides. In the Hetao basin, seven wells with wide ranges of groundwater As were selected to host As(V)-Fe(III) oxides sand. During 80 d experiments, As was firstly desorbed and then released via reductive dissolution of iron oxide from ferrihydrite, while only desorption was observed from goethite/hematite sand. Desorbed As was predominantly controlled by groundwater HCO 3 − and DOC, while reductive dissolution-related As release was mainly regulated by ORP values, DOC and Fe(II) concentrations. Mineral transformation from ferrihydrite to lepidocrocite and goethite/or mackinawite would also contribute to As release. Arsenic species was transformed from As(V) to As(III) on ferrihydrite, but remained unchanged on goethite and hematite. Arsenic partition between As-Fe(III) oxide sand and real groundwater ranged between 0.012 and 0.102 L/g. K d-sand between As-goethite sand/As-hematite sand and groundwater fell within the ranges observed between sediments and groundwater. This study suggests that As desorption, reductive dissolution and mineral transformation of ferrihydrite would be the major processes controlling As mobility. [ABSTRACT FROM AUTHOR]

Nanoparticulate mackinawite (FeS) can be an important host-phase for arsenic (As) in sulfidic, subsurface environments. Although not previously investigated, phosphate (PO 4 3− ) may compete with As for available sorption sites on FeS, thereby enhancing As mobility in FeS-bearing soils, sediments and groundwater systems. In this study, we examine the effect of PO 4 3− on sorption of arsenate (As(V)) and arsenite (As(III)) to nanoparticulate FeS at pH 6, 7 and 9. Results show that PO 4 3− (at 0.01–1.0 mM P) did not significantly affect sorption of either As(V) or As(III) to nanoparticulate FeS at initial aqueous As concentrations ranging from 0.01 to 1.0 mM. At pH 9 and 7, sorption of both As(III) and As(V) to nanoparticulate FeS was similar, with distribution coefficient (K d ) values spanning 0.76–15 L g −1 (which corresponds to removal of 87–98% of initial aqueous As(III) and As(V) concentrations). Conversely, at pH 6, the sorption of As(III) was characterized by substantially higher K d values (6.3–93.4 L g −1 ) than those for As(V) (K d = 0.21–0.96 L g −1 ). Arsenic K-edge X-ray absorption near edge structure (XANES) spectroscopy indicated that up to 52% of the added As(V) was reduced to As(III) in As(V) sorption experiments, as well as the formation of minor amounts of an As 2 S 3 -like species. In As(III) sorption experiments, XANES spectroscopy also demonstrated the formation of an As 2 S 3 -like species and the partial oxidation of As(III) to As(V) (despite the strictly O 2 -free experimental conditions). Overall, the XANES data indicate that As sorption to nanoparticulate FeS involves several redox transformations and various sorbed species, which display a complex dependency on pH and As loading but that are not influenced by the co-occurrence of PO 4 3− . This study shows that nanoparticulate FeS can help to immobilize As(III) and As(V) in sulfidic subsurface environments where As co-exists with PO 4 3− . [ABSTRACT FROM AUTHOR]

Experiments were conducted to investigate the reductive dechlorination of tetrachloroethylene (PCE) by nano-Mackinawite (nFeS) with cyano-cobalamin (Cbl(III)) in cement slurries. Almost complete degradation of PCE by nFeS-Cbl(III) was observed in cement slurries in 5 h and its degradation kinetics (k obs-PCE = 0.57 h −1 ) was 6-times faster than that of nFeS-Cbl(III) without the cement slurries. PCE was finally transformed to non-chlorinated organic compounds such as ethylene, acetylene, and C3-C4 hydrocarbons by nFeS-Cbl(III) in cement slurries. X-ray photoelectron spectroscopy and PCE degradation by cement components (SiO 2 , Al 2 O 3 , and CaO) revealed that both the reduced Co species in Cbl(III) and the presence of Ca in cement played an important role for the enhanced reductive dechlorination of PCE. The increase in the concentration of Cbl(III) (0.005–0.1 mM), cement ratio (0.05–0.2), and suspension pH (11.5–13.5) accelerated the PCE degradation kinetics by providing more favorable environments for the production of reactive Ca species and reduction of Co species. We also observed that the degradation efficiency of PCE by nFeS-Cbl(III)-cement lasted even at high concentration of PCE. The experimental results obtained from this study could provide fundamental knowledge of redox interactions among nFeS, Cbl(III), and cement, which could significantly enhance reductive dechlorination of chlorinated organics in contaminated natural and engineered environments. [ABSTRACT FROM AUTHOR]

In this study, we investigated the interaction between selenite and either Fe (II) aq or S (− II) aq in solution, and the results were used to investigate the interaction between Se (IV) aq and FeS in suspension. The reaction products were characterized by a combination of methods (SEM, XRD and XAS) and the reaction mechanisms were identified. In a first experiment, Se (IV) aq was reduced to Se (0) by interaction with Fe (II) aq which was oxidized to Fe (III) , but the reaction was only partial. Subsequently, some Fe (III) produced akaganeite (β-FeOOH) and the release of proton during that reaction decreased the pH. The pH decrease changed the Se speciation in solution which hindered further Se (IV) reduction by Fe (II) aq . In a second experiment, Se (IV) aq was quantitatively reduced to Se (0) by S (− II) aq and the reaction was fast. Two sulfide species were needed to reduce one Se (IV) , and the observed pH increase was due to a proton consumption. For both experiments, experimental results are consistent with expectations based on the oxidation reduction potential of the various species. Upon interaction with FeS, Se (IV) aq was reduced to Se (0) and minute amounts of pyrite were detected, a consequence of partial mackinawite oxidation at surface sulfur sites. These results are of prime importance with respect to safe deep disposal of nuclear waste which contains the long-lived radionuclide 79 Se. This study shows that after release of 79 Se (IV) upon nuclear waste matrix corrosion, selenite can be reduced in the near field to low soluble Se (0) by interaction with Fe (II) aq and/or S (− II) aq species. Because the solubility of Se (0) species is significantly lower than that of Se (IV) , selenium will become much less (bio)available and its migration out of deep HLW repositories may be drastically hindered. [ABSTRACT FROM AUTHOR]

Stabilized FeSm(Mackinawite) nanocrystals have been synthesized by hydrothermal treatment of single precursor polymer metal complex. The polymer metal complex was prepared by the reaction of thiourea-formaldehyde resin with metal Fe(II) ion. The synthesized FeSmnanocrystals were characterized by X-ray spectrometry and electronic microscopy. The transmission electron microscopy micrographs showed needle shaped nanocrystals and the thickness and length of these nanocrystals were found to be 12 ± 3 and 120 ± 5 nm, respectively. The lattice fringes in the HRTEM images, withd-spacing 0.521 ± 0.005 nm very close to (0 0 1) plane of mackinawite nature. The interaction between aqueous Hg(II) and synthetic FeSmwas studied via batch sorption experiments. The maximum absorption capacity of FeSmhas been found with 0.001:0.08 M ratio of Hg(II) and FeSm,while the minimum absorption capacity was found with 0.001:0.005 M ratio of Hg(II) and FeSmand removed only 60% of Hg(II) from aqueous solution. Batch studies revealed that FeSmnanocrystals were effectively remove Hg(II) from aqueous solution along a broad pH range. [ABSTRACT FROM PUBLISHER]

[Display omitted] • Hg0 adsorption with C and S on the FeS surface has been studied via DFT-D2 method. • DOS and ICOHP demonstrated the interaction between the adsorbates and the surfaces. • Carbon can promote mercury physisorption on the pristine FeS(0 0 1)-S surface. • The HgS generation process occurs through Langmuir-Hinshelwood Mechanism. Elemental mercury (Hg0) adsorption behaviors and mechanisms on the low-Miller index mackinawite surface (FeS(0 0 1)-S surface), with the engagement of carbon or sulfur, were investigated via DFT-D2 theoretical calculations. The simulation results show that the most favorable adsorption site for the C and Hg atoms was the lower-S site on the pristine surface, while S atom preferred the upper-S site. C atom could cause charge transfer and inter-atomic charge redistribution during adsorption. The carbon atom performs as a linker to connect the substrate and the Hg atom in the most stable mercury adsorption geometry onto the carbon-doped surface. Hg0 is physically adsorbed over the sulfur-doped surface. The HgS molecule chemically attached to the surface at the upper-S site via the Hg atom to form a S-Hg-S structure, and its generation process follows Langmuir-Hinshelwood Mechanism. Firstly, the Hg and S atoms were pre-adsorbed on the FeS(0 0 1)-S surface. Then the system needed to overcome an activation energy barrier (E b) of 89.82 kJ·mol−1. It was an endothermic process in which two separate atoms generated HgS*. However, C atom will impede the reaction as occupying the adsorption sites of mercury and sulfur. [ABSTRACT FROM AUTHOR]

[Display omitted] • Activation of PS by FeS@BC and FeS 2 @BC was compared. • FeS@BC had better catalytic performance than FeS 2 @BC. • FeS@BC has larger specific surface areas, abundant micropores, higher defect degree. • Surface C = O, Fe(II), and S-thiophene groups of FeS@BC accounted for PS activation. • S2−/S n 2– species and graphene structure of FeS 2 @BC accounted for PS activation. In this work, two novel heterogeneous catalysts were prepared by loading natural mackinawite (FeS) and pyrite (FeS 2) particles into peanut shell-derived biochar (i.e., FeS@BC and FeS 2 @BC), and the persulfate (PS) activation processes by these two composites for sulfamethazine (SMT) degradation were systematically examined. After doping natural iron-sulfur ores, the catalytic removal performance of the above two composites were 37.2%–80.0% and 31.0%–35.2% higher than those of their respective original minerals and primitive biochar. Simultaneously, FeS@BC had better catalytic performance than FeS 2 @BC. The larger specific surface areas, abundant micropores, higher defect degree, better conductivity and FeS crystals favored the release of more iron, resulting in higher reaction stoichiometric efficiency (RSE) value, excellent catalytic activity, and enhanced SMT mineralization. In the FeS@BC/PS system, surface ketonic groups (C = O), Fe(II), and highly active S-thiophene groups all accounted for PS activation. In the FeS 2 @BC/PS system, C = O groups, Fe(II), highly reductive S2−/S n 2– species, and graphene structure participated in the catalytic reaction. The electron-rich S-thiophenic groups, S2−/S n 2– species, and ketonic moieties could act as electron donors, prompting the regeneration of Fe(II) in the lattice (≡Fe(II)) and solution (Fe2+) by direct or indirect electron transfer. In view of the higher catalytic activity of FeS@BC, we further investigated different influencing factors on SMT degradation, activation mechanism, and degradation pathway of SMT in the FeS@BC/system. Overall, this study would fill the gap in the research on the difference in the catalytic activity of catalysts prepared by loading different natural iron-sulfur minerals onto biochar for persulfate activation. [ABSTRACT FROM AUTHOR]

One model for the emergence of life posits that ancient, low temperature, submarine alkaline hydrothermal vents, partly composed of iron-sulfides, were capable of catalyzing the synthesis of prebiotic organic molecules from CO 2 , H 2 and CH 4 . Specifically, hydrothermal mackinawite (Fe II S) and greigite (Fe II Fe III 2 S 4 ) have been highlighted in previous studies as analogs of the active centers of hydrogenase, ferredoxin, acetyl coenzyme-A synthase and carbon monoxide dehydrogenase featured in the biochemistry of certain autotrophic prokaryotes that occupy the base of the evolutionary tree. Despite the proposed importance of iron sulfide minerals and clusters in the synthesis of abiotic organic molecules, the mechanisms for the formation of these sulfides from solution and their preservation under the anoxic and low temperature (below 100 °C) conditions expected in off-axis submarine alkaline vent systems is not well understood ( Bourdoiseau et al., 2011; Rickard and Luther, 2007 ). To rectify this, single hydrothermal chimneys were precipitated using a unique apparatus to simulate growth at hydrothermal vents of moderate temperature under supposed Hadean ocean-bottom conditions. Iron sulfide phases were observed through Raman spectroscopy at growth temperatures ranging from 40° to 80 °C. Fe(III)-containing mackinawite is confirmed to be present with mackinawite and greigite, supporting an Fe III -mackinawite intermediate mechanism for the transformation of mackinawite to greigite below 100 °C. Raman spectroscopy of the chimneys revealed a maximum yield of greigite at 75 °C. These results suggest abiotic production of catalytically active mackinawite and greigite are possible under early Earth hydrothermal conditions as well as on other wet, rocky worlds geochemically similar to the Earth. [ABSTRACT FROM AUTHOR]

Corrosion of Fe in H 2 S containing solutions is a general problem in oil production. Here, layers rich in the the tetragonal FeS modification mackinawite were grown by polarisation of iron 200 mV above the corrosion potential in H 2 S saturated acetic saline solution. These layers have no band gap above room temperature thermal energy, hence behave metallic. The sulfide layers are duplex layers, with an inner, oxide-rich and an outer, oxide-poor region. Strain induced by differences in oxygen content may be responsible for the poor adhesion between the two layers, which leads to eventual loss of the outer layer. [ABSTRACT FROM AUTHOR]

The interaction of Sn II 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 divalent Sn forms two short (2.38 Å) Sn–S bonds to the S-terminated surface of mackinawite, and two longer (2.59 Å) Sn–S bonds pointing most likely towards the solution phase, in line with a SnS 4 innersphere sorption complex. Precipitation of SnS or formation of a solid solution with mackinawite could be excluded. At pH > 9, Sn II is completely oxidized to Sn IV by an Fe II /Fe III (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 indicate a structural incorporation by green rust, with Sn IV substituting for Fe in the crystal structure. The transition between Sn II and Sn IV and between sulfur and oxygen coordination takes place at a pH of 7 to 8 and an Eh of − 250 mV, close to the thermodynamically predicted transitions from mackinawite to Fe (hydr)oxide and from sulfide to sulfate. The uptake processes of Sn II by mackinawite are largely in line with the uptake processes of divalent cations with soft Lewis-acid character like Cd, Hg and Pb, and lead to a strong retention of Sn with logR d values from 5 to 7 across the investigated pH range of 5 to 11. [ABSTRACT FROM AUTHOR]

The growth of monophasic iron sulfide thin films onto glass substrates has been achieved by chemical bath deposition at acidic values of pH. Powder X-ray diffraction (p-XRD) confirms the deposition of tetragonal FeS (mackinawite ) with preferred orientation along (001) plane. The crystallite size estimated by Scherrer equation was found to be 14 nm. Scanning electron microscopy (SEM) shows the formation of nanoflakes as base layer and nanoflowers as top layer. Energy Dispersive X-ray (EDX) analysis of the deposited iron sulfide thin films shows the iron to sulfur ratio close to 1:1 confirming the deposition of FeS. UV–vis absorption spectroscopy showed a blueshift due to the nanosize crystallites FeS with a band gap of 1.87 eV. [ABSTRACT FROM AUTHOR]

Electrochemical impedance spectroscopy (EIS) and polarization curve techniques were used to study the early stages of hydrogen sulfide (H 2 S) corrosion. Results revealed the formation of different iron sulfide compounds during the early stages of corrosion. Meanwhile, Warburg impedance characteristics were determined through EIS. Numerous flake-like mackinawites were observed after 15 h. The diffusion-controlled mechanism gradually disappeared during the process. After immersion for 15 h to 7 d in aqueous H 2 S, the flake-like mackinawites grew larger and adhered, which increased the thickness of corrosion product films. The EIS and polarization curve tests proved that the increase in thickness of corrosion product films enhances corrosion resistance. [ABSTRACT FROM AUTHOR]

Nitroguanidine (NQ) is an emerging contaminant being used by the military as a constituent of new insensitive munitions. NQ is also used in rocket propellants, smokeless pyrotechnics, and vehicle restraint systems. Its uncontrolled transformation in the environment can generate toxic and potentially mutagenic products, posing hazards that need to be remediated. NQ transformation has only been investigated to a limited extent. Thus, it is crucial to expand the narrow spectrum of NQ remediation strategies and understand its transformation pathways and end products. Iron-based reactive minerals should be investigated for NQ treatment because they are successfully used in existing technologies, such as permeable reactive barriers, for treating a wide range of organic pollutants. This study tested the ability of micron-sized zero-valent iron (m-ZVI), mackinawite, and commercial FeS, to transform NQ under anoxic conditions. NQ transformation followed pseudo-first-order kinetics. The reaction rate constants decreased as follows: commercial FeS > mackinawite > m-ZVI. For the assessed minerals, the NQ transformation started with the reduction of the nitro group forming nitrosoguanidine (NsoQ). Then, aminoguanidine (AQ) was accumulated during the reaction of NQ with m-ZVI, accounting for 86% of the nitrogen mass recovery. When NQ was reacted with commercial FeS, 45% and 20% of nitrogen were recovered as AQ and guanidine, respectively, after 24 h. Nonetheless, NsoQ persisted, contributing to the N-balance. When mackinawite was present, NsoQ disappeared, but AQ was not detected, and guanidine accounted for 11% of the nitrogen recovery. AQ was ultimately transformed into cyanamide, whose dimerization triggered the formation of cyanoguanidine. Alternatively, NsoQ was transformed into guanidine, which reacted with cyanamide to form biguanide. This is the first report systematically investigating the NQ transformation by different iron-based reactive minerals. The evidence indicates that these minerals are attractive alternatives for developing NQ remediation strategies. [Display omitted] • Iron-based reactive minerals effectively transform nitroguanidine (NQ). • Iron sulfide minerals reduce NQ faster than micron-sized zero-valent iron (m-ZVI). • NQ undergoes nitro-reduction, forming nitrosoguanidine. • Additional NQ transformation products are aminoguanidine, guanidine, and cyanamide. • Polymerization of NQ transformation products forms cyanoguanidine and biguanide. [ABSTRACT FROM AUTHOR]

The objective of this study was to study the transformation of powdered mackinawite (FeS) to greigite (Fe 3 S 4 ) and pyrite (FeS 2 ) under geochemical conditions similar to those in pristine or contaminated aquifers. To observe the transformation without altering or damaging the crystal structures, mackinawite particles were immobilized on poly(methyl methacrylate) (PMMA) supports, and mineralogical changes monitored by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDS). Powders of mackinawite immobilized on PMMA supports were placed in aqueous solutions containing polysulfides and mildly elevated temperatures (75–78 °C) to promote the transformation to greigite and pyrite. SEM–EDS results were consistent with previously reported X-ray diffraction analysis that showed the transformation of mackinawite to greigite and pyrite. The similarities in sizes between most crystals over the course of the transformation from mackinawite to pyrite, as well as the coexistence of solids with morphologies and elemental compositions characteristic of mackinawite, greigite, and pyrite, are consistent with a solid state transformation. In some cases, however, much smaller pyrite crystals were formed, which may have resulted from direct nucleation from solution. The results reported here extend the applicability PMMA supports for studying mineralogical transformations (Birkefeld et al., 2005) to particles with dimensions from 100 nm to 2 μm, and to elevated temperatures. Such supports can be used to monitor iron–sulfur mineralogical changes in pristine and contaminated environments. [ABSTRACT FROM AUTHOR]

The corrosion products of carbon steel in aqueous H2S environment are investigated. The products, which include mackinawite, cubic FeS, troilite, and pyrite, are characterized through their shapes, chemical compositions and crystal structures. Mackinawite appears with a flake shape. Cubic FeS has a perfect/truncated octahedral shape, and pyrite is framboid-shaped. Flower-shaped troilite is developed from beam- or hexagonal wire-shaped grains by electrostatic interactions along a certain lattice plane. The large single beam-shaped troilite has a growth pattern along the c axis. The corresponding crystal structure and micro-morphology of the corrosion products are provided, and the three-dimensional models of them are generated. [ABSTRACT FROM AUTHOR]

Abstract: We report conductivity measurements for mackinawite at atmospheric pressure and up to values of 1.64 GPa. As far as the authors are aware, this is the first low temperature transport measurements to be conducted on mackinawite. The sample shows semiconducting-like behaviour up to the maximum pressures applied but with a gradual flattening of the resistivity curve it is tentatively suggested that the system will undergo a semiconductor–metallic transition with a further application of pressure. Interestingly, this contradicts band structure and more recent density functional theory (DFT) calculations which predict a metallic system. However, the experimental data agrees with the original prediction of semiconducting behaviour made by Bertaut 1965. This research also shows that, at least in its parent form, mackinawite does not exhibit a superconductive state unlike its iso-structural counterpart tetragonal FeSe. This is also briefly discussed. [Copyright &y& Elsevier]

Highlights: [•] Extended incubation time to 16 days allowed significant FeS crystallization. [•] A weakly acidic pH greatly enhanced particle growth of mackinawite. [•] Microbial metabolism of different donors systematically altered the ambient pH. [•] Greater sulfide accumulation stimulated mackinawite transformation to greigite. [ABSTRACT FROM AUTHOR]

Abstract: This study investigated removal of Hg(II) from water using FeS(s) with batch and continuous contact filtration systems. For the batch system, kinetic experiments showed that removal of Hg(II) by FeS(s) was rapid at lower concentration (500 μM), but at higher concentration (1000 and 1250 μM), more time was required to achieve greater than 99% removal. The concentration of iron released to the solution remained relatively low, typically below 3 μM. This would theoretically present less than 1% of the Hg(II) removed. Thus, a simple exchange of Hg(II) for Fe(II) in the solid (FeS(s)) does not explain the results, but if the Fe(II) released could react to form another solids, low concentrations of Fe do not preclude a mechanism in which Hg(II) reacts to form HgS and release Fe(II). A continuous contact dead-end ultrafiltration (DE/UF) system was developed to treat water containing Hg(II) by applying a FeS(s) suspension with stirred or non-stirred modes. A major reason for applying stirring to the system was to investigate the role of “shear” flow in rejection of Hg(II)-contacted FeS(s) by a UF membrane and the stability of Hg on the FeS(s). The Hg(II)-contacted FeS(s) was completely rejected by the DE/UF system and mercury was strongly retained on the FeS(s) particles. Almost no release of Hg(II) (≈0 mM) from the FeS(s) solids was observed when they were contacted with 0.1M-thiosulfate, regardless of whether the system was operated in stirred or non-stirred mode. However, rapid oxidation of FeS(s) was observed in the stirred system but not in the non-stirred system. Determining the mechanism of oxidation requires further study, but it is important because oxidation reduces the ability of the solids to remove additional Hg(II). [Copyright &y& Elsevier]

Abstract: The effects of environmental factors, such as dissolved metal species and their concentrations, on microbial sulfide formation and U(VI) reduction processes are important. During microbial sulfate reduction by Desulfovibrio desulfuricans, the initial aqueous uranium(VI) in our system decreased and later increased as a liquid uranium(IV) suspension. The change in the uranium phase was strongly associated with a biogenic FeS solid (mackinawite), the mineralogical growth of which was affected by the coexisting metal cations (e.g., Ni and Cu). Using laser-induced fluorescence and capillary cell spectroscopy, zeta potentiometry, and ultrafiltration techniques, the produced U(IV) suspension was investigated in situ at a very low level (≤10−6 M) and was identified to be uranium dioxides (UO2) in the form of nm-sized colloids. In addition to the spectroscopic techniques, the direct examination by high-resolution transmission electron microscopy (HRTEM) revealed that the U(IV) phase evidently occurs from the FeS surface in the form of discrete UO2 nanocrystals. Our results indicate that the metal sulfide produced by sulfate-reducing bacteria (SRB) may be an important agent for dispersing the aqueous U(VI) in a colloidal U(IV) form, which would be mobile in porous media such as soils and sediments due to its nanometer-sizes. This study contributes to the understanding of how the FeS metallic conductor abiotically affects the behavior of redox-sensitive uranium by transforming its properties at the mineral surface in sulfate reducing environments. [Copyright &y& Elsevier]

Abstract: The pH impact on reductive dechlorination of cis-dichloroethylene (cis-DCE) was investigated using in situ Fe precipitates formed under iron-rich sulfate-reducing conditions. The dechlorination rate of cis-DCE increased with pH, which was attributed to changes in the solid-phase Fe concentration, the composition of Fe minerals, and the surface speciation of Fe minerals. With increasing pH, larger quantities of Fe minerals, having much greater reactivity than dissolved Fe(II), were produced. Fe–K edge X-ray absorption spectroscopy (XAS) analysis of Fe precipitates revealed the presence of multiple Fe phases with their composition varying with pH. Correlation analyses were performed to examine how the solid-phase Fe concentration, the composition of Fe minerals, and their surface speciation were linked with the cis-DCE dechlorination rate. Such analyses revealed that neither mackinawite (FeS) nor magnetite (Fe3O4) was reactive with cis-DCE dechlorination, but that Fe (oxyhydr)oxides including green rusts and Fe(OH)2 were reactive. Based on a proposed model of the surface acidity of Fe minerals, the increasing deprotonated surface Fe(II) groups with pH correlated well with the enhanced cis-DCE dechlorination. [Copyright &y& Elsevier]

Abstract: Given the numerous unknowns and uncertainties in sediment systems, understanding of the mechanisms of U(VI) reduction is still at the stage of improvement. Recent studies have shown that reductive immobilization of U(VI) in the subsurface is not caused by purely biological or purely abiotic reactions but rather a group of interconnected abiotic–biotic pathways (e.g. via mackinawite and biomass). These new findings necessitate an update of the existing mathematical models that make simplifications typically involving a single reducing agent (e.g. indigenous bacteria) for in situ bioremediation of uranium-contaminated groundwater. In this study, a comprehensive model was constructed based on new experimental observations, including mixed U(VI) reduction by chemical and enzymatic reactions. Thermodynamic analysis was done to predict the feasibility of the potential pathways that affect mackinawite formation under field conditions. Model simulations indicate that low concentrations of the reactant species make the reaction of homogeneous Fe(II) oxidation coupled to U(VI) reduction unfavorable in the field. Instead, FeS precipitation is an important Fe(II) sequestration reaction once sulfate metabolism dominates. The subsequent reduction of U(VI) by FeS (mackinawite) contributes to the total U(VI) removal under a variety of U(VI) concentrations encountered at field sites, which is in accordance with experimental observations. The model suggests the potential for both competition and coordination between chemical and biological pathways on the cell surface, providing a possible explanation as to why U(VI) can be efficiently reduced at either low or high sulfate concentration during the process of in situ bioremediation. Further increase in the resolution of the model (e.g. across multiple scales such as genome-, micron- and pore-sale) is necessary for better understanding of the interactions between biotic and abiotic pathways. [Copyright &y& Elsevier]

Abstract: Microbial sulfate reduction is an important terminal electron accepting process in arsenic-contaminated subsurface environments. Humic acids are ubiquitous in such environments, yet their impact on arsenic mobility under sulfate-reducing conditions is poorly understood. In this study, we examined the effects of microbial sulfate reduction and humic acid on arsenic mobilization via a series of advective-flow column experiments. The initial solid-phase in these experiments comprised quartz sand that was coated with As(III)-sorbed goethite (α-FeOOH). The effect of humic acid was assessed by comparing columns that received artificial groundwater in which humic acid was either absent or present at 100mgL−1, whilst the effect of microbial sulfate reduction was investigated by comparing columns that were inoculated with the sulfate-reducer Desulfovibrio vulgaris (ATCC strain 7757) versus abiotic control columns. The presence of high concentrations of humic acid alone did not enhance the overall extent of arsenic release from either the abiotic or the inoculated (sulfate reducing) columns. This is consistent with similar arsenic concentrations in porewaters filtered to both <0.45μm and <3kDa, demonstrating that aqueous arsenic did not form mobile colloidal humic acid complexes. In contrast, microbial sulfate reduction was found to mobilize substantial levels of arsenic relative to those observed in the corresponding abiotic control columns. Iron and sulfur K-edge X-ray absorption spectroscopy (XAS) showed that reaction between goethite and microbially-produced sulfide lead to accumulation of mackinawite (FeS) and elemental S. Microbial sulfate reduction also caused important changes in arsenic speciation, especially the formation of aqueous dithioarsenate and monothioarsenate. However, arsenic K-edge XAS showed that arsenic sulfide mineral phases (orpiment and realgar) did not form during the 60day advective-flow experiment. The formation of poorly-sorbing thioarsenate species appeared to contribute to the observed enhancement of arsenic mobilization from the inoculated columns. Dithioarsenate and monothioarsenate were relatively stable, and were found to make up>40% of aqueous arsenic even at very low porewater sulfide concentrations (i.e. <10μmolL−1). Accordingly, the formation, stability and sorption–desorption of thioarsenate species need to be considered when evaluating and predicting subsurface arsenic mobility. [Copyright &y& Elsevier]

Abstract: Iron sulfide (FeS) has been extensively assessed as a reactive medium to remove both metals and halogenated organics from groundwater. However, to address its suitability as a material for permeable reactive barriers (PRBs), its propensity for solids and gas production, which result in reduced permeability, must be evaluated. The reduction in permeability for sands coated with FeS (as mackinawite), under the anoxic conditions often encountered at contaminated groundwater sites, was examined through column experiments and geochemical modeling under conditions of high calcium and nitrate, which have been previously shown to cause significant permeability reduction in zero-valent iron (ZVI) systems. The column experiments showed negligible production of both solids and gases. The geochemical modeling predicted a maximum reduction in permeability of 1% due to solids and about 30% due to gas formation under conditions for which a complete loss of permeability was predicted for ZVI systems. This difference in permeability reduction is driven by the differences in thermodynamic stability of ZVI and FeS in aqueous solutions. The results suggest that geochemical conditions that result in high permeability losses for ZVI systems will likely not be problematic for FeS-based reactive materials. [Copyright &y& Elsevier]

Abstract: The long-term success of in situ reductive immobilization of uranium (U) depends on the stability of U(IV) precipitates (e.g., uraninite) in the presence of natural oxidants, such as oxygen, Fe(III) hydroxides, and nitrite. Field and laboratory studies have implicated iron sulfide minerals as redox buffers or oxidant scavengers that may slow oxidation of reduced U(IV) solid phases. Yet, the inhibition mechanism(s) and reaction rates of uraninite (UO2) oxidative dissolution by oxic species such as oxygen in FeS-bearing systems remain largely unresolved. To address this knowledge gap, abiotic batch experiments were conducted with synthetic UO2 in the presence and absence of synthetic mackinawite (FeS) under simulated groundwater conditions of pH=7, =0.02atm, and =0.05atm. The kinetic profiles of dissolved uranium indicate that FeS inhibited UO2 dissolution for about 51h by effectively scavenging oxygen and keeping dissolved oxygen (DO) low. During this time period, oxidation of structural Fe(II) and S(-II) of FeS were found to control the DO levels, leading to the formation of iron oxyhydroxides and elemental sulfur, respectively, as verified by X-ray diffraction (XRD), Mössbauer, and X-ray absorption spectroscopy (XAS). After FeS was depleted due to oxidation, DO levels increased and UO2 oxidative dissolution occurred at an initial rate of r m =1.2±0.4×10−8 molg−1 s−1, higher than r m =5.4±0.3×10−9 molg−1 s−1 in the control experiment where FeS was absent. XAS analysis confirmed that soluble U(VI)-carbonato complexes were adsorbed by iron oxyhydroxides (i.e., nanogoethite and lepidocrocite) formed from FeS oxidation, which provided a sink for U(VI) retention. This work reveals that both the oxygen scavenging by FeS and the adsorption of U(VI) to FeS oxidation products may be important in U reductive immobilization systems subject to redox cycling events. [Copyright &y& Elsevier]

Abstract: Benthic sediments in coastal acid sulfate soil (CASS) drains can contain high concentrations (∼1–5%) of acid volatile sulfide (AVS) as nano-particulate mackinawite. These sediments can sequester substantial quantities of trace metals. Because of their low elevation and the connectivity of drains to estuarine channels, these benthic sediments are vulnerable to rapid increases in ionic strength from seawater incursion by floodgate opening, floodgate failure, storm surge and seasonal migration of the estuarine salt wedge. This study examines the effect of increasing seawater concentration on trace metal mobilization from mackinawite-rich drain sediments (210–550 μmol g−1 AVS) collected along an estuarine salinity gradient. Linear combination fitting of S K-edge XANES indicated mackinawite comprised 88–96% of sediment-bound S. Anoxic sediment suspensions were conducted with seawater concentrations ranging from 0% to 100%. We found that mobilization of some metals increased markedly with increasing ionic strength (Cu, Fe, Mn, Ni) whereas Al mobilization decreased. The largest proportion of metals mobilized from the labile metal pool, operationally defined as ∑exchangeable + acid-extractable + organically-bound metals, occurred in sediments from relatively fresh upstream sites (up to 39% mobilized) compared to sediments sourced from brackish downstream sites (0–11% mobilized). The extent of relative trace metal desorption generally followed the sequence Mn > Ni ≈ Cu > Zn > Fe > Al. Trace metal mobilization from these mackinawite-rich sediments was attributed primarily to desorption of weakly-bound metals via competitive exchange with marine-derived cations and enhanced complexation with Cl− and dissolved organic ligands. These results have important implications for trace metal mobilization from these sediments at near-neutral pH under current predicted sea-level rise and climate change scenarios. [Copyright &y& Elsevier]