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Fe(II)-bearing minerals (magnetite, siderite, green rust, etc.) are common products of microbial Fe(III) reduction, and they provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of redox active elements such as vanadium; which can exist as V(V), V(IV), and V(III) under conditions typical of near-surface aquatic and terrestrial environments. To better understand the redox behavior of V under ferrugenic/sulfidogenic conditions, we examined the interactions of V(V) (1 mM) in aqueous suspensions containing 50 mM Fe(II) as magnetite, siderite, vivianite, green rust, or mackinawite, using X-ray absorption spectroscopy at the V K-edge to determine the valence state of V. Two additional systems of increased complexity were also examined, containing either 60 mM Fe(II) as biogenic green rust (BioGR) or 40 mM Fe(II) as a mixture of biogenic siderite, mackinawite, and magnetite (BioSMM). Within 48 h, total solution-phase V concentrations decreased to <20 µM in all but the vivianite and the biogenic BiSMM systems; however, >99.5% of V was removed from solution in the BioSMM and vivianite systems within 7 and 20 months, respectively. The most rapid reduction was observed in the mackinawite system, where V(V) was reduced to V(III) within 48 h. Complete reduction of V(V) to V(III) occurred within 4 months in the green rust system, 7 months in the siderite system, and 20 months in the BioGR system. Vanadium(V) was only partially reduced in the magnetite, vivianite, and BioSMM systems, where within 7 months the average V valence state stabilized at 3.7, 3.7, and 3.4, respectively. The reduction of V(V) in soils and sediments has been largely attributed to microbial activity, presumably involving direct enzymatic reduction of V(V); however the reduction of V(V) by Fe(II)-bearing minerals suggests that abiotic or coupled biotic–abiotic processes may also play a critical role in V redox chemistry, and thus need to be considered in modeling the global biogeochemical cycling of V.

Background Iron sulfide nanomaterials have been successfully employed as therapeutic agents for bacterial infection therapy and catalytic-ferroptosis synergistic tumor therapy due to their unique structures, physiochemical properties, and biocompatibility. However, biomedical research and understanding of the biological functions of iron sulfides are insufficient, and how iron sulfide nanomaterials affect reactive oxygen species (ROS) in diseases remains unknown. Acute kidney injury (AKI) is associated with high levels of ROS, and therefore nanomedicine-mediated antioxidant therapy has emerged as a novel strategy for its alleviation. Results Here, mackinawite nanozymes were synthesized from glutathione (GSH) and iron ions (Fe 3+ ) (denoted as GFeSNs) using a hydrothermal method, and then evaluated as ROS scavengers for ROS-related AKI treatment. GFeSNs showed broad-spectrum ROS scavenging ability through synergistic interactions of multiple enzymes-like and hydrogen polysulfide-releasing properties. Furthermore, both in vitro and in vivo experiments demonstrated that GFeSNs exhibited outstanding cytoprotective effects against ROS-induced damage at extremely low doses and significantly improved treatment outcomes in AKI. Conclusions Given the synergetic antioxidant properties and high biocompatibility, GFeSNs exhibit great potential for the treatment of AKI and other ROS-associated diseases. Graphical Abstract

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.

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.

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.

Due to pollution discharge and climate change, hypoxia is becoming more prevalent in water bodies such as lakes, rivers, and oceans. It leads to black blooms in lakes, black and odors in rivers, and dead zones in the oceans. Iron sulfide, especially amorphous mackinawite, is the main blackening substance. In this paper, the physical characteristics and detection methods of mackinawite were used to determine how to detect it in hypoxic water bodies. The population and metabolism of key microorganisms during mackinawite generation were analyzed to provide a reference for studying related microorganisms during the blackening process. Finally, the influence of macromolecular organic matter and microorganisms on the stable suspension of mackinawite was determined, and the existing patterns of mackinawite during the blackening process of hypoxic waters were illustrated. The generation of amorphous mackinawite and its stable suspension in a supernatant are two indispensable factors during the blackening process. The clarification of substance characteristics and key action processes are conducive to the accurate and targeted control of the blackening process and promote the restoration of the sustainable self-purification ability of water bodies.

The current study is aimed at proposing a common mechanism of localized corrosion of mild steel in H2S containing aqueous environments, by utilizing experimental findings reported in a number of studies from the open literature. It is hypothesized that a discontinuity in an iron sulfide corrosion product layer due to poor formation or a disruption results in initiation of localized corrosion. Then a galvanic coupling between the underlying steel and the conductive iron sulfide corrosion product layer leads to propagation of localized corrosion at an enhanced rate. This hypothesis was tested by using five different cases where localized corrosion was observed, which were all in support of the proposed mechanism. These were Case 1: a poorly formed mackinawite layer Case 2: a partially dissolved pyrrhotite layer Case 3: a disrupted pyrrhotite layer due to pyrite formation Case 4: a disrupted pyrrhotite layer due to presence of sand, and, finally, Case 5: a disrupted mackinawite layer due to interference by pyrite.

The Fe(II) monosulfide mineral mackinawite (FeS) is an important phase in low-temperature iron and sulfur cycles, yet it is challenging to characterize since it often occurs in X-ray amorphous or nanoparticulate forms and is extremely sensitive to oxidation. Moreover, the electronic configuration of iron in mackinawite is still under debate. Mössbauer spectroscopy has the potential to distinguish mackinawite from other FeS phases and provide clarity on the electronic configuration, but conflicting results have been reported. We therefore conducted a Mössbauer study at 5 K of five samples of mackinawite synthesized through different pathways. Samples show two different Mössbauer patterns: a singlet that remains unsplit at all temperatures studied, and a sextet with a hyperfine magnetic field of 27(1) T at 5 K, or both. Our results suggest that the singlet corresponds to stoichiometric mackinawite (FeS), while the sextet corresponds to mackinawite with excess S (FeS1+x). Both phases show center shifts near 0.5 mm/s at 5 K. Coupled with observations from the literature, our data support non-zero magnetic moments on iron atoms in both phases, with strong itinerant spin fluctuations in stoichiometric FeS. Our results provide a clear approach for the identification of mackinawite in both laboratory and natural environments.

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

Iron monosulfides and neoformed pyrite below the sulfate–methane transition zone (SMTZ) of rapidly accumulating turbiditic sediments from the Gaoping submarine canyon off southwestern Taiwan were examined by SEM-EDS-EBSD, HRTEM, and HAADF STEM to investigate their microstructural characteristics and processes of formation and transformation. Within a few meters below the SMTZ, mackinawite (Mkw) is largely replaced by interlayered greigite-pyrrhotite (Grg-Po) with 111Grg//001Po and ⟨110⟩Grg//⟨110⟩Po, followed by pyrite neoformation in clusters of disseminated matrix grains consisting of coalescing pyrite microcrystals, arrays of polycrystalline interlayer pyrite grains between the cleavage planes of layer silicates, with each grain’s core having inclusions of interlayered Grg-Po locally containing relict Mkw, and amassed pyrite microcrystals on the surface of porous interlayered Grg-Po micronodules. In the deeper sediments, neoformed pyrite is absent and Mkw is largely preserved, with partial replacement by interlayered Grg-Po having an overall topotactic relationship of ⟨110⟩Grg//⟨110 ⟩Po//⟨100⟩Mkw and 111Grg//(001)Po//~011Mkw and a sharp reaction front without transitional profiles. The mineral grain boundaries and structural discontinuities with Mkw resulting from extensive interlayering between Grg 111 cubic close-packed segments and Po 001 hexagonal close-packed layers could serve as conduits for fluid flow and mass transport to drive the replacement reaction. The conversion of Mkw to metastable interlayered Grg-Po is inferred to occur through interface-coupled dissolution–reprecipitation processes associated with partial oxidation while the partial replacement of interlayered Grg-Po ± minor relict Mkw by pyrite microcrystals with irregular grain boundaries and orientations probably occurred via a dissolution–precipitation mechanism. Mkw could be initially formed by sulfate reduction driven by anaerobic oxidation of methane in reactive iron-rich sediments in paleo-SMTZs and subsequently transformed into interlayered Grg-Po followed by pyrite neoformation in the sulfidization front below the SMTZ or recent SMTZs in the Gaoping submarine canyon sediments.