FeS Surface Research Articles

Phosphate-Induced Acidic Microenvironment for Improved Contaminant Removal during FeS Oxygenation.

The coexistence of mackinawite (FeS) and phosphate is widely observed in natural systems. However, the underlying mechanism regarding how phosphate influences the environmental behavior of FeS, especially during the FeS oxygenation in aquatic systems, remains in its fancy. This study for the first time reported that the presence of phosphate, even at a low concentration of 0.3 mM, significantly promoted the FeS-mediated O2 activation and thus the pollutant degradation. The enhancement was attributed to a substantial increase in the generation of •OH, as evidenced by the electron paramagnetic resonance tests and the identification of the probing products. A combination of experiments and theoretical calculations revealed that phosphate adsorbed onto the FeS surface via a monodentate mononuclear configuration, establishing an acidic microenvironment on the FeS surface. Such acidic microenvironment not only increased the utilization efficiency of Fe(II) toward H2O2 generation (i.e., ), but also prevented the subsequent side reaction of H2O2 self-decomposition (i.e., ). The results highlight the beneficial role of commonly encountered phosphate in FeS-based systems, which has profound implications for the degradation of waterborne contaminants.

N-nitrosodimethylamine removal by a novel silver/sulfur-coated nanoscale zero-valent iron/activated carbon composite: Adsorption kinetics, mechanisms, and degradation pathways

The removal of disinfection by-products from water is of critical importance to achieve the water purification. N-nitrosodimethylamine (NDMA), an emerging nitrogenous disinfection by-product produced by the chlorine-ammonia disinfection, remains significantly hazardous even at low concentrations. Herein, an activated carbon (AC)-supported nanoscale zero-valent iron (nZVI) coated with sulfur (S) and silver (Ag) composite (Ag@S-nZVI/AC) was synthesized, characterized via several techniques and evaluated for NDMA removal performance. The results demonstrated that crystalline Ag and mackinawite (FeS) coated amorphous nZVI spheres were successfully anchored on AC. The removal of NDMA at ultra-lower concentrations by the Ag@S-nZVI/AC composite was better fitted by pseudo-second-order (PSO) kinetic and Freundlich isotherm models. NDMA was chemically adsorbed in multiple layers on heterogeneous surface of the Ag@S-nZVI/AC composite via spontaneous endothermic process. The adsorption constant (KF) of NDMA removal by Ag@S-nZVI/AC at T=298 K was 9.89 times greater than that of NDMA removal by pristine AC. The van der Waals forces, hydrogen bonds and surface-mediated electron transfer were the main mechanisms responsible for the highly effective remediation of ultra-lower concentrations of NDMA from drinking water. The decomposition of NDMA was significantly enhanced by Ag@S-nZVI/AC, resulting in the generation of more dimethylamine (DMA), ammonium and the nitrogen-containing compounds. The decomposition process involved the oxidation of FeS, S and Fe0 surface as well as the hydrogenation of Ag. The post-leaching concentrations of Fe, Ag and S in treated drinking water were lower than the standard permissible limits.

Atomic-level into the microscopic mechanism of gold capture by FeS during gold collection in matte process

Understanding the microscopic mechanisms involved in gold capture by FeS at the atomic level is crucial for optimizing the matte process and improving the gold recovery from gold concentrate containing arsenic and antimony. Herein, the DFT method was employed to explore the adsorption structure and electronic characteristics of a gold (Au) atom on the FeS surface. Furthermore, the effects of Fe/S vacancy and As/Sb dopant on Au adsorption were also investigated. The results reveal that the FeS(100) with S termination exhibits the smallest surface energy (0.56 J/m2) and possesses the highest stability amongst the low-index surfaces. Furthermore, the Au atom exhibits a preference for adsorption on the Fe-bridge site of FeS(100) through chemisorption due to the formation of a strong Au-Fe metallic bond, large adsorption energy (−5.40 eV), and small adsorption distance. Moreover, the presence of S vacancy enhances the Au capture capability of FeS(100) with an adsorption energy of −6.35 eV, while the Fe vacancy slightly weakens it. Additionally, the doping of As and Sb enhances the adsorption of Au atoms on FeS(100), and the Sb doping showcases a more distinct enhancement in Au capture efficiency. The findings of this work contribute to the advancement of matte processing technology, enabling enhanced gold recovery and potentially opening new avenues for the treatment of refractory gold ores.

Nitrate Chemodenitrification by Iron Sulfides to Ammonium under Mild Conditions and Transformation Mechanism.

Autotrophic denitrification utilizing iron sulfides as electron donors has been well studied, but the occurrence and mechanism of abiotic nitrate (NO3-) chemodenitrification by iron sulfides have not yet been thoroughly investigated. In this study, NO3- chemodenitrification by three types of iron sulfides (FeS, FeS2, and pyrrhotite) at pH 6.37 and ambient temperature of 30 °C was investigated. FeS chemically reduced NO3- to ammonium (NH4+), with a high reduction efficiency of 97.5% and NH4+ formation selectivity of 82.6%, but FeS2 and pyrrhotite did not reduce NO3- abiotically. Electrochemical Tafel characterization confirmed that the electron release rate from FeS was higher than that from FeS2 and pyrrhotite. Quenching experiments and density functional theory calculations further elucidated the heterogeneous chemodenitrification mechanism of NO3- by FeS. Fe(II) on the FeS surface was the primary site for NO3- reduction. FeS possessing sulfur vacancies can selectively adsorb oxygen atoms from NO3- and water molecules and promote water dissociation to form adsorbed hydrogen, thereby forming NH4+. Collectively, these findings suggest that the NO3- chemodenitrification by iron sulfides cannot be ignored, which has great implications for the nitrogen, sulfur, and iron cycles in soil and water ecosystems.

New insights of FeS–Cr interaction in sequestration remediation: Model, mechanism, and DFT calculation

Chromium (Cr), especially hexavalent Cr, is a commonly found heavy metal in both water and soil. In this work, we synthesized FeS nanoparticles (NPs) stabilized per carboxymethyl cellulose (CMC) for reductive immobilization of Cr(VI) in both soil/water matrix. A CMC-to-FeS molar ratio of 0.0010 was determined to ensure effective immobilization (96.3%) and high soil mobility. The overall removal kinetic was rapid and the removal capacity reached up to 327.9 mg/g. Reduction of Cr(VI) to Cr(III) (accounting for 91.7%) was the major immobilization mechanism, rather than adsorption (only 4.5%). Fe(II) was the predominant reducing agents, but S species in FeS also served as electron donors for Cr(VI). Moreover, characterization results and theoretical calculations indicated that the Sulfur sites on the FeS surface played a crucial role in adsorbing CrO42− molecules and facilitating the rapid conversion of Cr(VI) to Cr(III) via the electron pathway Fe(II)–S=O–Cr(VI). The removal isotherm curve exhibited a “S” shape at low Ce concentrations and could be adequately fitted by a modified dual mode model (R2 = 0.995). The efficient immobilization of Cr(VI) in water could be achieved in pH range of 5–9. Humic at high concentration can strongly complex with Cr(VI) oxyanions leading to an inhibited removal efficiency. Both soil batch tests and column tests demonstrated the high mobility and reactivity of CMC-FeS with suppressed Cr release. This study gave new insights on the role of S in FeS–Cr(VI) interactions and theoretical support for nanoparticle injection to immobilize Cr in soil matrix.

Adsorption and dissociation of CH3SH on FeS surface: A DFT study

Methyl mercaptan (CH3SH) is a common sulfide in oil and natural gas, easily corroded in transportation pipes and storage containers. In this paper, the adsorption and dissociation processes of CH3SH on FeS (0 0 1), (0 1 1), (1 0 0) and (1 1 1) surfaces have been studied by the density functional theory (DFT). And the characteristics of the adsorption and dissociation processes were elucidated. The calculated surface energies show that the order of surface stability is (0 0 1) > (0 1 1) > (1 0 0) > (1 1 1). The stability of CH3SH adsorption is (1 1 1) > (1 0 0) > (0 1 1) > (0 0 1) by the adsorption energy, and the most stable adsorption configurations are obtained. Compared with the changes in electron density and the partial density of states (PDOS) before and after adsorption, there is an obvious electron orbital hybrid phenomenon between CH3SH and FeS (1 0 0), (1 1 1) surface, and its adsorption is stronger than that of the other two surfaces. It can be seen from the calculation of the transition state that the dissociation reaction of CH3SH is endothermic on the (0 0 1) and (0 1 1) surfaces, while the exothermic reaction on the (0 1 1) and (0 0 1) surfaces. CH3SH will dissociate preferentially on the FeS (1 0 0) surface because the reaction of Ea and ΔE on this surface is the smallest. Overall, the adsorption and dissociation processes are exothermic, the increase in temperature makes the crystal surface unstable and susceptible to spontaneous combustion, and the phenomenon occurs first on the FeS (1 0 0) and FeS (1 1 1) surfaces. These findings increase the understanding of the electronic structure and energy of the sulfides' action on the FeS surface and also have a reference value for flame retardant research.

Mechanism of gas-phase passivation inhibition during FeS self-ignition based on pore structure and fractal characteristics

Reactive FeS that spontaneously combusts was prepared under field conditions and subjected to gas-phase passivation using various atmospheres and durations. The goal was to uncover how gas-phase passivation inhibits the spontaneous combustion of reactive FeS. The surface characteristics and elemental composition of FeS were analyzed at different gas-phase passivation levels using scanning electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). The effect of gas-phase passivation on the evolution of the FeS pore structure was investigated using low-temperature liquid nitrogen adsorption, and the FHH fractal theory was employed to address changes in the fractal properties of the FeS surface. The results highlighted the intricate and densely dispersed pore structure of reactive FeS, along with its pronounced surface roughness. With increased levels of gas-phase passivation, the FeS surface became smoother and more regular, the number of oxidation-expanding pores increased, most sulfur elements were converted into sulfur dioxide gas, and iron trioxide passivation films formed on the FeS surface. Gas-phase passivation significantly reduced the specific surface area and cumulative pore volume of reactive FeS. Specifically, the cumulative pore volume decreased by approximately 20.9%, while the specific surface area decreased by 28.5% as gas-phase passivation deepened. The passivation process transformed more micropores into transition pores, with resulting iron-oxygen compounds adhering to the FeS surface. This reduced the number of reactive adsorption sites and substantially slowed down spontaneous combustion. As gas-phase passivation deepened, the volume proportion of FeS micropores decreased to 16.5%, and the micropore area decreased by around 6%. Gas-phase passivation had a significant impact on reducing the spatial fractal dimension and complexity of FeS, with a greater influence on spatial complexity than surface complexity. The passivation effect was initially enhanced by higher oxygen concentrations within the first 6-hours of passivation, but its effectiveness diminished thereafter.

Hybrid FES-exoskeleton control: Using MPC to distribute actuation for elbow and wrist movements.

Individuals who have suffered a cervical spinal cord injury prioritize the recovery of upper limb function for completing activities of daily living. Hybrid FES-exoskeleton systems have the potential to assist this population by providing a portable, powered, and wearable device; however, realization of this combination of technologies has been challenging. In particular, it has been difficult to show generalizability across motions, and to define optimal distribution of actuation, given the complex nature of the combined dynamic system. In this paper, we present a hybrid controller using a model predictive control (MPC) formulation that combines the actuation of both an exoskeleton and an FES system. The MPC cost function is designed to distribute actuation on a single degree of freedom to favor FES control effort, reducing exoskeleton power consumption, while ensuring smooth movements along different trajectories. Our controller was tested with nine able-bodied participants using FES surface stimulation paired with an upper limb powered exoskeleton. The hybrid controller was compared to an exoskeleton alone controller, and we measured trajectory error and torque while moving the participant through two elbow flexion/extension trajectories, and separately through two wrist flexion/extension trajectories. The MPC-based hybrid controller showed a reduction in sum of squared torques by an average of 48.7 and 57.9% on the elbow flexion/extension and wrist flexion/extension joints respectively, with only small differences in tracking accuracy compared to the exoskeleton alone. To realize practical implementation of hybrid FES-exoskeleton systems, the control strategy requires translation to multi-DOF movements, achieving more consistent improvement across participants, and balancing control to more fully leverage the muscles' capabilities.

First-principles insights into sulfur oxides (SO2 and SO3) adsorption and dissociation on layered iron sulfide (FeS) catalyst

The adsorption of sulfur oxides (SOx) represents the fundamental step towards their conversion to lower-risk sulfur-containing species. Herein, we investigate the adsorption and dissociation mechanism of sulfur dioxide (SO2) and sulfur trioxide (SO3) on layered iron sulfide (FeS) nanocatalyst (001), (011), and (111) surfaces using density functional theory methodology. Both SO2 and SO3 exhibit strong reactivity towards the (011) and (111) surfaces, with the most stable geometry for SO2 and SO3 on the (011) surface predicted to be a tridentate η23(S,O,O) and a bidentate η22(O,O) configuration, respectively, whereas on the (111) surface, they are predicted to be coordinated in a monodentate η21(S) and η21(O) geometry, respectively. Significant charge donation from the FeS surface to the SOx species is observed, which resulted in elongation of S−O bond lengths, confirmed by vibrational frequency analyses. Favourable reaction energy and activation barrier is predicted for SO2 dissociation at the (111) surface.

FeS combined ozonation to remove p-aminobenzenesulfonamide from water: Density functional theory insights into the mechanism

The applicability and performance of FeS in ozonation process to remove p-aminobenzenesulfonamide (SN) from water was assessed, and the working mechanism of FeS was comprehensively explored by both experimental means and density functional theory (DFT) simulation. FeS combined ozonation achieved 74% of SN removal in 60 min under the optimal condition, which was 37% higher than by ozonation alone, and 12% higher than FeO combined ozonation. Highly active species of •OH, •SO4−, 1O2 and •O2− were detected in the FeS combined ozonation system, the evolution pathway of the involved species was expounded with the aid of DFT calculation. The results revealed that •O2−, H2O2 and SO42− were originally formed via interface reactions on FeS surface, then gradually transformed into •OH, 1O2 and •SO4− through subsequent chain reactions. Moreover, FeS had a lower energy barrier of 0.16 eV than FeO with a value of 0.83 eV for the transformation of ozone to active atomic oxygen. The presented study provided a significant insight into the role of Fe-based materials in ozonation, and was of great importance to guide the route for ozonation process improvement.

Heterogeneous activation of persulfate by FeS – Surface influence on selectivity

Recently, several studies have been published on sulfate radicals as strong oxidants and methods of their generation. A shift from homogeneous to heterogeneous methods can be observed for persulfate activation as a means of generating sulfate radicals. However, to date, the influence of the surface on the radical chemistry has not been examined in detail.In the present study, homogeneous persulfate activation methods (elevated temperature and dissolved iron(II)) are compared with heterogeneous activation by FeS. The selectivity patterns for the oxidation of chlorinated ethenes, ethanes and benzenes differ notably depending on the persulfate activation method. The obtained kinetic data are in conformity with the hypothesis that sulfate radicals generated at the FeS surface have different selectivities than freely dissolved radicals. The assumption of different reactive species is further confirmed by the determination of hydrogen kinetic isotope effects (H-KIEs) for the oxidation of methanol isotopologues with sulfate radicals using the set of activation methods. The H-KIE obtained in the heterogeneous system (H-KIEFeS = 3.0 ± 0.3) is significantly higher than for the homogeneous system (H-KIEhom = 2.3 ± 0.1). This leads us to the conclusion that sulfate radicals generated on the FeS surface react as surface-associated radicals, which show a different selectivity pattern than freely dissolved radicals.Furthermore, the apparent second-order rate constants in homogenous solution at ambient temperature, ranging from 5 · 107 to 1.7 · 109 M−1 s−1, were determined for the reaction of sulfate radicals with various chlorinated hydrocarbons under study.

Slab model studies of H2S adsorption/dissociation and diffusion on pristine FeS(001) surfaces and FeS(001) surfaces with pre-adsorbed X atoms (X = H, O, and S)

Dispersion-corrected density functional theory (DFT-D2) was used to calculate the adsorption/dissociation mechanisms of H2S and diffusion mechanism of H atoms on a pristine FeS(001) surface and on FeS(001) surfaces with various pre-adsorbed atoms. The calculation results showed that the pre-adsorption of atoms was beneficial to H2S adsorption/dissociation, and the first-order dissociation energy barrier (Ea) of H2S on the FeS(001) + S surface is reduced from 2.06 eV on the pristine surface to 0.97 eV. Additionally, the pre-adsorption of an H atom was found to not only reduced the diffusion Ea of H atoms (0.62 eV), but also shortened the diffusion path (P3→P4). Finally, calculations indicated that the thermodynamic conditions on the pristine FeS(001) surface were conducive to the generation of H2 molecules (Ea = 0.08 eV; reaction heat (ΔE) ΔE = −1.54 eV). These results increase our understanding of H2S adsorption and dissociation and H atom diffusion on different FeS (001) surfaces, and provide a theoretical basis for corrosion prevention. This work also has reference value for the study of the mechanisms by which molecules/atoms in the environment influence related systems.

Effective degradation of Direct Red 81 using FeS-activated persulfate process

As a burgeoning advanced oxidation process (AOP), heterogeneous activation of persulfate (PS) for synthetic refractory contaminants decontamination has recently received much attention. In this study, FeS was selected as a heterogeneous PS activator to facilitate the degradation of a typical recalcitrant contaminant of diazo dye Direct Red 81 (DR 81). The results showed that approximately 95% of 0.03 mM DR 81 was removed within 60 min with FeS and PS doses of 1.5 × 10−3 M. The efficient decomposition of DR 81 by the FeS/PS system was assumed to be mainly attributed to the highly reactive SO4−• and •OH, which was related to PS cleavage by both dissolved Fe2+ leached from FeS and Fe2+ bound on the FeS surface. Except for strongly alkaline conditions, DR 81 decolorations by FeS/PS were insignificantly affected by operational parameters such as temperature, initial solution pH, and rotate speed. Meanwhile, the presence of five inorganic anions being studied had distinct impacts on DR 81 degradation and followed a strict order of NO3− < Cl− < SO42− < CO32− < PO43−. However, FeS/PS system was highly adaptable, and FeS, which is used as a PS activator was more stable. GC/MS and TOC data revealed that thorough mineralization of DR 81 by PS/FeS in an initial fast reaction phase to transform DR 81 to aromatic intermediates, followed by a slow reaction phase that mineralized these organic intermediates into carboxylic acids and carbon dioxide through further oxidation.

Effective removal of heavy metal ions by attapulgite supported sulfidized nanoscale zerovalent iron from aqueous solution

Sulfidation of nanoscale zero valent iron (nZVI) has attracted increasing interest for improving the reactivity and selectivity of nZVI towards heavy metal ions. In this study, attapulgite-loaded sulfide modified nZVI (APT/S-nZVI) was synthesized, and used for removal Cr(Ⅵ), Cu(Ⅱ) and Zn(Ⅱ) alone or coexisting. The morphology and structure characterizations indicated that sulfide-modified nZVI particles were well distributed and immobilized on the attapulgite surface. Batch experiments of Cr(Ⅵ), Cu(Ⅱ) and Zn(Ⅱ) removal were conducted at varying mass ratios, S/Fe ratios, pH and initial concentrations. Moreover, kinetic and thermodynamic analyses were used to study the removal process, and possible mechanisms of removal for Cr(Ⅵ), Cu(Ⅱ) and Zn(Ⅱ) were discussed. The results of removal performance tests demonstrated that APT/S-nZVI displayed high removal capacity and fast removal rate for three heavy metal ions compared to the nZVI, which derived from the synergistic effects of the strongly reducing characteristics of nZVI combined with high electronic conductivity of FeS and high surface area of APT. Dynamic studies revealed that the removal process was highly consistent with the pseudo-second-order model for three metal ions adsorption and reaction process. Additionally, combined the value of activation energy, it has been proved that the removal Cr(VI) and Cu(Ⅱ) were controlled by chemical surface-limiting step, and removal Zn(Ⅱ) was controlled by chemically diffusion step. The modification of S changes the removal mechanism between APT/S-nZVI and metal ions, which is closely related to the redox potential of metal ion and the solubility product constant (Ksp) of metal sulfides. Overall, the work suggests that APT/S-nZVI has a good potential for the elimination of micropollutants in the aquatic environment.

A comprehensive evaluation of factors affecting the reactivity of FeS towards hexabromocyclododecane diastereoisomers

Reactivity of iron sulfide (FeS) towards hexabromocyclododecane (HBCD) was explored under conditions of varying temperature, pH, inorganic ion and dissolved organic matter (DOM) in this study. Results show that the reduction of HBCD by FeS has an activation energy of 29.2 kJ mol−1, suggesting that the rate-limiting step in the reduction was a surface-mediated reaction. The reduction of HBCD by FeS was a highly pH-dependent process. The optimal rate for HBCD reduction by FeS was observed at a pH of 6.2. All the tested inorganic ions suppressed the reduction of HBCD by FeS. XPS analysis confirmed that both Fe(II)-S and bulk S(-II) on FeS surface could be impacted by solution pH and inorganic ions and were responsible for the regulation of HBCD reduction. Some DOMs (i.e., fulvic acid, humic acid, salicylic acid, catechol and sodium dodecyl sulfate) were found to hinder the reduction via competing with HBCD for active sites on FeS surface. However, the presence of 2,2′-bipyridine, triton X-100 and cetyltrimethyl ammonium bromide was able to significantly enhance the rate of HBCD reduction by 5.8, 9.0 and 20 times, respectively. Different factors could influence the reduction efficiency of HBCD diastereoisomers to different extent, but not the reaction orders of HBCD diastereoisomers (α-HBCD < γ-HBCD < β-HBCD). Moreover, FeS could completely remove HBCD diastereoisomers in sediments with multiple factors within 9 d reaction. Our results contribute to give a better understanding on the performance of FeS towards HBCD under real and complex conditions and facilitate the application of FeS in remediation sites.

New insights of the interaction of H2S with mackinawite FeS in a wet environment: An ab initio molecular dynamics study

Mackinawite FeS is the most common corrosion product in the early stage of steel in H2S environment and also has an effective catalysis for hydrodesulfurization, hydrogen release reactions and heavy metal ion removal since its high specific surface area and reactive surface like other van der Waals materials. In this paper, ab initio molecular dynamics (AIMD) is used to study the interactions between small molecules (H2S, H2O and H) and FeS at 300 K. The calculation results show that the primary dissociation of H2S only occurs on the FeS(111) surface and H2S is chemically adsorbed on the (011) and (100) surfaces but physically adsorbed on the (001) surface. It will dissociate and generate H atoms when different amounts H2S or H2O molecules appear in the interlayer of FeS, in which H2S is more prone to dissociation. The dissociated H atoms will be “captured” by Fe/S atoms in mackinawite FeS, leading to the shrink of layer. Moreover, H atoms could combine into H2, which suggests that layered FeS has great potential for hydrogen generation. The diffusion coefficient of H atoms in mackinawite FeS layer is estimated about 1.67 × 10−8 m2/s. These findings have significant meaning for application of mackinawite FeS in corrosion science, hydrogen generation, hydrogen storage and other fields.

Enhanced degradation of tetracycline by Cu(II) complexation in the FeS/sulfite system

This study applied a mineral material of FeS to activate sulfite for efficient degradation of TTC in the presence of Cu(II) based on the identified complexation mechanism through UV–Vis spectra, FTIR spectroscopy and DFT calculation. pH plays an important role in TTC degradation and the initial pH of 6 and 7 were the divide in the contributions of FeS/sulfite oxidation and complex-precipitation. TTC-Cu(II) exhibits a superior promoting effect on the TTC degradation in FeS/sulfite system due to the improvement of TTC electron transfer reactivity and Fe(II) dissolution from FeS. Moreover, the formation of Cu(I) improved the recycling of Fe(II) from Fe(III). Dissolved oxygen-dependent free radicals’ generation was confirmed, and TTC degradation was mainly attributed to SO4·− and ·OH. The characterization of FeS surface through XPS, XRD, SEM-EDS, Fe(II) deactivation tests, together with the comparison of pseudo-first-order rate constants for TTC degradation by FeS and ferrous ion supported the important role of surface and dissolved Fe(II) in sulfite activation. Furthermore, reasonable degradation pathways of TTC have been proposed according to the detected products by LC-MS. This work highlights the important role of pH, DO and Cu(II) complexation in sulfite activation and TTC degradation, furnishing theoretical support for further relevant studies.

Exploring the Evolution Mechanism of Sulfur Vacancies by Investigating the Role of Vacancy Defects in the Interaction between H2S and the FeS(001) Surface.

Vacancy defects are inherent point defects in materials. In this study, we investigate the role of Fe vacancy (VFe) and S vacancy (VS) in the interaction (adsorption, dissociation, and diffusion) between H2S and the FeS(001) surface using the dispersion-corrected density functional theory (DFT-D2) method. VFe promotes the dissociation of H2S but slightly hinders the dissociation of HS. Compared with the perfect surface (2.08 and 1.15 eV), the dissociation energy barrier of H2S is reduced to 1.56 eV, and HS is increased to 1.25 eV. Meanwhile, S vacancy (VS) significantly facilitates the adsorption and dissociation of H2S, which not only reduces the dissociation energy barriers of H2S and HS to 0.07 and 0.11 eV, respectively, but also changes the dissociation process of H2S from an endothermic process to a spontaneous exothermic one. Furthermore, VFe can promote the hydrogen (H) diffusion process from the surface into the matrix and reduce the energy barrier of the rate-limiting step from 1.12 to 0.26 eV. But it is very hard for H atoms gathered around VS to diffuse into the matrix, especially the energy barrier of the rate-limiting step increases to 1.89 eV. Finally, we propose that VS on the FeS(001) surface is intensely difficult to form and exist in the actual environment through the calculation results.

Iron(II) monosulfide (FeS) minerals reductively transform the insensitive munitions compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO)

As military applications of the insensitive munitions compounds (IMCs) 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO) increase, there is a growing need to understand their environmental fate and to develop remediation strategies to mitigate their impacts. Iron (II) monosulfide (FeS) minerals are abundant in freshwater and marine sediments, marshes, and hydrothermal environments. This study shows that FeS solids can reduce DNAN and NTO to their corresponding amines under anoxic ambient conditions. The reactions between IMCs and the FeS minerals were surface-mediated since they did not occur when only dissolved Fe2+(aq) and S2−(aq) were present. Mackinawite, a tetragonal FeS with a layered structure, reduced DNAN mainly to 2-methoxy-5-nitroaniline (MENA), which in turn was partially reduced to 2-4-diaminoanisole (DAAN). The layered structure of mackinawite provided intercalation sites likely responsible for partial adsorption of MENA and DAAN. Mackinawite entirely reduced NTO to 3-amino-1,2,4-triazol-5-one (ATO). The reduction of IMCs showed concurrent oxidation of mackinawite to goethite and elemental sulfur. A commercial FeS product, composed mainly of pyrrhotite and troilite, reduced DNAN to DAAN and NTO to ATO. At pH 6.5, DNAN and NTO transformation rates were 667 and 912 μmol h−1 m−2, respectively, on the mackinawite surface and 417 and 1344 μmol h−1 m−2, respectively, on the commercial FeS surface. This is the first report of the reduction of a nitro-heterocyclic compound (NTO) by FeS minerals. The evidence indicates that DNAN and NTO can be rapidly transformed to their succeeding amines in anoxic subsurface environments and aquatic sediments rich in FeS minerals.

Properties of lead-free copper matrix composites prepared through in situ Ni-coated FeS surface modification and mechanical alloying

Lead-free FeS/Cu-Bi (FCB) copper matrix composites were prepared by ball milling and in situ Ni-coated FeS surface modification, and their mechanical and tribological properties were studied. Results showed that the hardness of the prepared FCB material increases from 50.5 HRB to 64 HRB, and its density also increases from 5.82 g/cm3 to 6.96 g/cm3. Although FeS surface modification can improve the mechanical properties of the prepared materials, their impact toughness and crushing strength decrease after the mechanical alloying of the mixed powders. However, these two processes have a good synergistic effect on improving the tribological properties of the prepared materials. The friction coefficient and wear rate of the prepared FCB composites were reduced by 29% and 73.2%, respectively. The surface modification and uniform dispersion of the second phase particles in the matrix led to enhancement in the binding energy with the matrix and the mechanical and tribological properties of the materials.