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

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

Reactive mineral-water interfaces exert control on the bioavailability of contaminant arsenic species in natural aqueous systems. However, the ability to accurately predict As surface complexation is limited by the lack of molecular-level understanding of As-water-mineral interactions. In the present study, we report the structures and properties of the adsorption complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained from density functional theory (DFT) calculations. The fundamental aspects of the adsorption, including the registries of the adsorption complexes, adsorption energies, and structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration, as is perhaps to be expected because the adsorbed water molecules stabilize the low-coordinated surface atoms. As(OH)3 adsorbs weakly at the water-FeS(001) interface through a network of hydrogen-bonded interactions with water molecules on the surface, with the lowest-energy structure calculated to be an As-up outer-sphere complex. Compared to the water-FeS(001) interface, stronger adsorption was calculated for As(OH)3 on the water-FeS(011) and water-FeS(111) interfaces, characterized by strong hybridization between the S-p and O-p states of As(OH)3 and the surface Fe-d states. The As(OH)3 molecule displayed a variety of chemisorption geometries on the water-FeS(011) and water-FeS(111) interfaces, where the most stable configuration at the water-FeS(011) interface is a bidentate Fe-AsO-Fe complex, but on the water-FeS(111) interface, a monodentate Fe-O-Fe complex was found. Detailed information regarding the adsorption mechanisms has been obtained via projected density of states (PDOS) and electron density difference iso-surface analyses and vibrational frequency assignments of the adsorbed As(OH)3 molecule. [ABSTRACT FROM AUTHOR]

Mackinawite (FeS) nanoparticles have been extensively tested for reducing contaminants under anoxic conditions, while the oxidizing impact induced by FeS under oxic conditions has been largely underestimated. In light of previous findings that hydroxyl radicals (∙OH) can be produced from oxygenation of sediment Fe(II), herein we revealed that ∙OH can be produced efficiently from FeS oxygenation at circumneutral conditions, yielding 84.7 µmol ∙OH per g FeS. Much more ∙OH was produced from the oxygenation of FeS compared with siderite, pyrite, and zerovalent iron nanoparticles under the same conditions. The oxidation of FeS was a surface-mediated process, in which O2 was transformed by the structural Fe(II) on FeS surface to ∙OH with the generation of H2O2 intermediate. A small proportion of Fe(II) was regenerated from the reduction of Fe(III) by FeS and S(-II), but this proportion did not significantly contribute to ∙OH production. We further validated that the ∙OH produced from FeS oxygenation considerably contributed to the oxidation of arsenic. As the change of redox conditions from anoxic to oxic is common in both natural and artificial processes, our findings suggest that the oxidizing impact induced by FeS at oxic conditions should be concerned due to ∙OH production. [ABSTRACT FROM AUTHOR]

Inthis study, we demonstrated the reductive substitution of aquo–cobalamin(III)(Cbl(III)–H2O) by sulfide–cobalamin(III)(Cbl(III)–S2–) in the presence of S2–. Cbl(III)–H2O was transformed to Cbl(III)–S2–via ligand substitution of H2O by S2–at its axial position, due to the weakness of thebond strength of cobalt with H2O. Cbl(III)–S2–was reduced to sulfide–cobalamin(II) and partiallyfurther to sulfide–cobalamin(I) as the concentration of S2–increased from 0.1 to 5.0 mM. In the presence ofnanomackinawite (nFeS), Cbl(III)–S2–specieswere fully adsorbed onto the nFeS surface, and its reactivity wasenhanced by the reactive surface chemical species. The dechlorinationkinetic rate constant of tetrachloroethene by Cbl(III)–S2–in nFeS suspension (knFeS–Cbl(III)–S2–= 0.008 ± 0.001 h–1)was 4 times greater than that by Cbl(III)–H2O, showinga potential role for ≡S2––Cbl(II)as a more effective electron transfer mediator than ≡Sn2––Cbl(II) and ≡OH–Cbl(II).The experimental results provide fundamental knowledge on potentialbiogeochemical reactions relevant to interactions between S2–and bacterial coenzyme in nFeS suspension, which can significantlyenhance reductive dechlorination of chlorinated compounds in contaminatedsites. [ABSTRACT FROM AUTHOR]

Uranium (U) and arsenic (As) often occur together naturally and, as a result, can be co-contaminants at sites of uranium mining and processing, yet few studies have examined the simultaneous redox dynamics of U and As. This study examines the influence of arsenate (As(V)) on the reduction of uranyl (U(VI)) by the redox-active mineral mackinawite (FeS). As(V) was added to systems containing 47 or 470 μM U(VI) at concentrations ranging from 0 to 640 μM. In the absence of As(V), U was completely removed from solution and fully reduced to nano-uraninite (nano-UO2). While the addition of As(V) did not reduce U uptake, at As(V) concentrations above 320 μM, the reduction of U(VI) was limited due to the formation of a trogerite-like uranyl arsenate precipitate. The presence of U also significantly inhibited As(V) reduction. While less U(VI) reduction to nano-UO2 may take place in systems with high As(V) concentrations, formation of trogerite-like mineral phases may be an acceptable reclamation end point due to their high stability under oxic conditions. [ABSTRACT FROM AUTHOR]

Iron sulfide minerals produced during in situ bioremediation of U can serve as an oxygen scavenger to retard uraninite (UO2) oxidation upon oxygen intrusion. Under persistent oxygen supply, however, iron sulfides become oxidized and depleted, giving rise to elevated dissolved oxygen (DO) levels and remobilization of U(IV). The present study investigated the mechanism that regulates UO2 oxidative dissolution rate in a flowthrough system when oxygen breakthrough occurred as a function of mackinawite (FeS) and carbonate concentrations. The formation and evolution of surface layers on UO2 were characterized using XAS and XPS. During FeS inhibition period, the continuous supply of carbonate and calcium in the influent effectively complexed and removed oxidized U(VI) to preserve an intermediate U2O9 surface. When the FeS became depleted by oxidization, a transient, rapid dissolution of UO2 was observed along with DO breakthrough in the reactor. This rate was greater than during the preceding FeS inhibition period and control experiments in the absence of FeS. With increasing DO, the rate slowed and the rate-limiting step shifted from surface oxidation to U(VI) detachment as U(VI) passivation layers developed. In contrast, increasing the carbonate concentrations facilitated detachment of surface-associated U(VI) complexes and impeded the formation of U(VI) passivation layer. This study demonstrates the critical role of U(VI) surface layer formation versus U(VI) detachment in controlling UO2 oxidative dissolution rate during periods of variable oxygen presence under simulated groundwater conditions. [ABSTRACT FROM AUTHOR]

Mercury (Hg) is a toxicant of global concern that accumulates in organisms as methyl Hg. The production of methyl Hg by anaerobic bacteria may be limited in anoxic sediments by the sequestration of divalent Hg [Hg(II)] into a solid phase or by the formation of elemental Hg [Hg(0)]. We tested the hypothesis that nanocrystalline mackinawite (tetragonal FeS), which is abundant in sediments where Hg is methylated, both sorbs and reduces Hg(ll). Mackinawite suspensions were equilibrated with dissolved Hg(ll) in batch reactors. Examination of the solid phase using Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Hg(ll) was indeed reduced in FeS suspensions. Measurement of purgeable Hg using cold vapor atomic fluorescence spectrometry (CVAFS) from FeS suspensions and control solutions corroborated the production of Hg(0) that was observed spectroscopically. However, a fraction of the Hg(ll) initially added to the suspensions remained in the divalent state, likely in the form of β-HgS-like clusters associated with the FeS surface or as a mixture of β HgS and surface-associated species. Complexation by dissolved S(-II) in anoxic sediments hinders Hg(0) formation, but, by contrast, Hg(Il)-S(-II) species are reduced in the presence of mackinawite, producing Hg(0) after only 1h of reaction time. The results of our work support the idea that Hg(0) accounts for a significant fraction of the total Hg in wetland and estuarine sediments. [ABSTRACT FROM AUTHOR]

Mackinawite, Fe(II)S, samples loaded with uranium (10-5, 10-4, and 10-3 mol U/g FeS) at pH 5, 7, and 9, were characterized using X-ray absorption spectroscopy and X-ray diffraction to determine the effects of pH, bicarbonate, and oxidation on uptake. Under anoxic conditions, a 5 g/L suspension of mackinawite lowered 5 x 10-5 M uranium(VI) to below 30 ppb (1.26 x 10-7 M) U. Between 82 and 88% of the uranium removed from solution by mackinawite was U(IV) and was nearly completely reduced to U(IV) when 0.012 M bicarbonate was added. Near-neighbor coordination consisting of uranium-oxygen and uranium-uranium distances indicates the formation of uraninite in the presence and absence of bicarbonate, suggesting reductive precipitation as the dominant removal mechanism. Following equilibration in air, mackinawite was oxidized to mainly goethite and sulfur and about 76% of U(IV) was reoxidized to U(VI) with coordination of uranium to axial and equatorial oxygen, similar to uranyl. Additionally, uranium-iron distances, typical of coprecipitation of uranium with iron oxides, and uranium-sulfur distances indicating bidentate coordination of U(VI) to sulfate were evident. The affinity of mackinawite and its oxidation products for U(VI) provides impetus for further study of mackinawite as a potential reactive medium for remediation of uranium-contaminated water. [ABSTRACT FROM AUTHOR]

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

The isotope 79Se may be of great concern with regard to the safe disposal of nuclear wastes in deep geological repositories due to its long half-life and potential mobility in the geosphere. The Se mobility is controlled by the oxidation state: the oxidized species (Se(IV)) and (Se(VI)) are highly mobile, whereas the reduced species (Se(0) and Se(-II)) form low soluble solids. The mobility of this trace pollutant can be greatly reduced by interacting with the various barriers of the repository. Numerous studies report on the oxidized species retention by mineral phases, but only very scarce studies report on the selenide (Se(-II)) retention. In the present study, the selenide retention by coprecipitation with and by adsorption on mackinawite (FeS) was investigated. XRD and SEM analyses of the samples reveal no significant influence of Se on the mackinawite precipitate morphology and structure. Samples from coprecipitation and from adsorption are characterized at the molecular scale by a multi-edge X-ray absorption spectroscopy (XAS) investigation. In the coprecipitation experiment, all elements (S, Fe, and Se) are in a low ionic oxidation state and the EXAFS data strongly point to selenium located in a mackinawite-like sulfide environment. By contacting selenide ions with FeS in suspension, part of Se is located in an environment similar to that found in the coprecipitation experiment. The explanation is a dynamical dissolution-recrystallization mechanism of the highly reactive mackinawite. This is the first experimental study to report on selenide incorporation in iron monosulfide by a multi-edge XAS approach. [ABSTRACT FROM AUTHOR]

Reaction of aqueous uranium(VI) with iron(II) monosulfide mackinawite in an O2 and CO2 free model system was studied by batch uptake measurements, equilibrium modeling, and LIIIIII edge U X-ray absorption spectroscopy (XAS). Batch uptake measurements showed that U(VI) removal was almost complete over the wide pH range between 5 and 11 at the initial U(VI) concentration of 5 × 10-5 M. Extraction by a carbonate/bicarbonate solution indicated that most of the U(VI) removed from solution was reduced to nonextractable U(IV). Equilibrium modeling using Visual MINTEQ suggested that U was in equilibrium with uraninite under the experimental conditions. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the U(IV) phase associated with mackinawite was uraninite. Oxidation experiments with dissolved O2 were performed by injecting air into the sealed reaction bottles containing mackinawite samples reacted with U(VI). Dissolved U measurement and XAS confirmed that the uraninite formed from the U(VI) reduction by mackinawite did not oxidize or dissolve under the experimental conditions. This study shows that redox reactions between U(VI) and mackinawite may occur to a significant extent, implying an important role of the ferrous sulfide mineral in the redox cycling of U under sulfate reducing conditions. This study also shows that the presence of mackinawite protects uraninite from oxidation by dissolved O2. The findings of this study suggest that uraninite formation by abiotic reduction by the iron sulfide mineral under low temperature conditions is an important process in the redistribution and sequestration of U in the subsurface environments at U contaminated sites. [ABSTRACT FROM AUTHOR]

A theoretical investigation of the relationship betweenchemicalcomposition and electronic structure was performed on the nonstoichiometriciron sulfide, mackinawite (Fe1), which is isostructuraland isoelectronic with the superconducting Fe1씧 and Fe1(Te1–ySey) phases. Even thoughFe1 has not been measured for superconductivity, theeffects of stoichiometry on transport properties and electronic structurein all of these iron-excess chalcogenide compounds has been largelyoverlooked. In mackinawite, the amount of Fe that has been reportedranges from a large excess, Fe1.15S, to nearly stoichiometric,Fe1.00(7)S. Here, we analyze, for the first time, the electronicstructure of Fe1 to justify these nonstoichiometricphases. First principles electronic structure calculations using supercellsof Fe1 yield a wide range of energeticallyfavorable compositions (0 < x< 0.30). Theincorporation of interstitial Fe atoms originates from a delicatebalance between the Madelung energy and the occupation of Fe–Sand Fe–Fe antibonding orbitals. A theoretical assessment ofvarious magnetic structures for “FeS” and Fe1.06S indicate that striped magnetic ordering along [110] is the lowestenergy structure and the interstitial Fe affects the values of momentsin the square planes as a function of distance. Moreover, the formationof the magnetic moment is dependent on the unit cell volume, thusrelating it to composition. Finally, changes in the composition causea modification of the Fermi surface and ultimately the loss of a nestedvector. [ABSTRACT FROM AUTHOR]

In cement-rich radioactive waste repositories, mackinawite (FeS) forms at the steel corrosion interface within reinforced concrete and potentially retards the transport of redox-sensitive radionuclides (e.g., 79 Se) in porous cement media. Redox interactions between selenite and mackinawite under hyperalkaline conditions remain unclear and require further investigations. Here, using comprehensive characterization on both aqueous and solid speciation, we successfully monitored the whole interaction process between selenite and mackinawite under hyperalkaline conditions. The results show similar chemical environments for SeO 3 2- and S 2- at the mackinawite-water interface, verifying an immediate reduction. After 192 h of reaction, SeO n 2- at the mackinawite-water interface, verifying an immediate reduction. After 192 h of reaction, SeO 3 2- was reduced to solid Se 0 and SeS 2 species, accompanied by the oxidation of S 2- and Fe(II) to Fe(III) in mackinawite. Aqueous speciation results showed that ∼99% of aqueous selenium was present as Se n 2- to S 2 was released into the solution, with mackinawite transforming into magnetite, Fe(OH) 3 2- and Fe(II) to Fe(III) in mackinawite. Aqueous speciation results showed that ∼99% of aqueous selenium was present as Se 4 S nanoparticles due to the dissolution of Se from the solid. In parallel, ∼62% of S 2- /S n 2- was released into the solution, with mackinawite transforming into magnetite, Fe(OH) 3 and FeS 2 O 3 + complexed to Cl - or OH - species, and magnetite subsequently dispersed in the solution. This study provides valuable data about the retardation mechanisms of redox-sensitive radionuclides by soluble iron sulfides, which is critical to advance our understanding of reactive concrete barriers used in nuclear waste disposal systems.

Iron pyrite (FeS 2 ) is a promising lithium-ion battery cathode material because of its low cost and ultrahigh energy density (1671 Wh kg -1 ). However, its reaction mechanisms are still controversial. In this work, we find that different from the conventional belief that an intermediate phase Li 2 FeS 2 is formed followed by Fe/Li 2 S composites at the initial discharge, it undergoes a one-step reaction (FeS 2 → Fe + Li 2 S) or a two-step reaction (FeS 2 → FeS + Li 2 S → Fe + Li 2 S), which depends on the current rate and temperature. In the charge process, it undergoes a two-step reaction: phase transition Fe + Li 2 S → FeS at about 1.74 V and generation of elemental sulfur (Li 2 S → S, 2.30 V). FeS is a mackinawite phase that is formed on the interface of Li 2 S via heteroepitaxial growth. Subsequent cycles involves a combination reaction of FeS and S. The reaction mechanism suggests that FeS 2 suffers from the demerits of both FeS and S, such as a large volume change, voltage hysteresis, and polysulfide dissolution. These findings would help us to understand the intrinsic capacity fading of FeS 2 and provide guidelines to improve its electrochemical performances.