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David J Vaughan - One of the best experts on this subject based on the ideXlab platform.
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X-ray absorption spectroscopy studies of reactions of technetium, uranium and neptunium with Mackinawite.
Journal of environmental radioactivity, 2004Co-Authors: Francis R. Livens, David J Vaughan, John M Charnock, Mark J Jones, Amanda J Hynes, J Fred W Mosselmans, Christoph Hennig, Helen Steele, David Collison, Richard A D PattrickAbstract:Technetium, uranium and neptunium may all occur in the environment in more than one oxidation state (IV or VII, IV or VI and IV or V respectively). The surface of Mackinawite, the first-formed iron sulfide phase in anoxic conditions, can promote redox changes so a series of laboratory experiments were carried out to explore the interactions of Tc, U and Np with this mineral. The products of reaction were characterised using X-ray absorption spectroscopy. Technetium, added as TcO4(-), is reduced to oxidation state IV and forms a TcS(2)-like species. On oxidation of the Mackinawite in air to form goethite, Tc remains in oxidation state IV but in an oxide, rather than a sulfide environment. At low concentrations, uranium forms uranyl surface complexes on oxidised regions of the Mackinawite surface but at higher concentrations, the uranium promotes surface oxidation and forms a mixed oxidation state oxide phase. Neptunium is reduced to oxidation IV and forms a surface complex with surface sulfide ions. The remainder of the Np coordination sphere is filled with water molecules or hydroxide ions.
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mechanisms of arsenic uptake from aqueous solution by interaction with goethite lepidocrocite Mackinawite and pyrite an x ray absorption spectroscopy study
Environmental Science & Technology, 2002Co-Authors: Morag L Farquhar, Francis R. Livens, John M Charnock, David J VaughanAbstract:The mechanisms whereby As(III) and As(V) in aqueous solution (pH 5.5−6.5) interact with the surfaces of goethite, lepidocrocite, Mackinawite, and pyrite have been investigated using As K-edge EXAFS and XANES spectroscopy. Arsenic species retain original oxidation states and occupy similar environments on the oxyhydroxide substrates, with first-shell coordination to four oxygens at 1.78 A for As(III) and 1.69 A for As(V). In agreement with other workers, we find that inner sphere complexes form, apparently involving bidentate (bridging) arsenate or arsenite. Interaction of As(III) and As(V) with the sulfide surfaces shows primary coordination to four oxygens (As−O: 1.69−1.76 A) with further sulfur (∼3.1 A) and iron (3.4−3.5 A) shells suggesting outer sphere complexation. Arsenic species were also coprecipitated with Mackinawite (pH 4.0), and these samples were further studied following oxidation. At high As(III) or As(V) concentrations, arsenate or arsenite species form, probably as sorption complexes, al...
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uranium uptake from aqueous solution by interaction with goethite lepidocrocite muscovite and Mackinawite an x ray absorption spectroscopy study
Environmental Science & Technology, 2000Co-Authors: Lesley N Moyes, Richard H Parkman, Francis R. Livens, David J Vaughan, Colin R. Hughes, John M Charnock, A BraithwaiteAbstract:The retention of radionuclides by interaction with mineral phases has significant consequences for the planning of their short- and long-term disposal to geological systems. An understanding of binding mechanisms is important in determining the ultimate fate of radionuclides following release into natural systems and will give increased confidence in predictive models. X-ray absorption spectroscopy (XAS) has been used to study the local environment of uranium taken up from aqueous solution by the surfaces of goethite, lepidocrocite, muscovite, and Mackinawite. On both iron hydroxides uranium uptake occurs by surface complexation and ceases when the surface is saturated. The muscovite surface does not become saturated and uptake increases linearly suggesting formation of a uranium phase on the surface. Uranium uptake on Mackinawite also suggests a replacement or precipitation process. XAS indicates that bidentate inner-sphere surface complexes are formed on the iron hydroxides by coordination of two surfac...
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reactions of copper and cadmium ions in aqueous solution with goethite lepidocrocite Mackinawite and pyrite
American Mineralogist, 1999Co-Authors: Richard H Parkman, Francis R. Livens, John M Charnock, N D Bryan, David J VaughanAbstract:The uptake of Cu and Cd in aqueous solution by interaction with the surfaces of carefully synthesized finely sized particles of goethite, lepidocrocite, Mackinawite, and pyrite was measured as a function of initial metal concentration in solution. The results show how reactions between metal ions in solution and mineral surfaces depend in a subtle way on the nature of the surfaces and, in certain cases, on the initial concentration of metal in solution. The uptake curves fall into two groups; type I in which the efficiency of uptake decreases with increasing concentration (Cu and Cd on goethite and lepidocrocite, Cd on pyrite), and type II in which it remains constant (Cu on Mackinawite and pyrite, Cd on Mackinawite). The total uptake is an order of magnitude greater for the latter group. X-ray absorption spectroscopies (XANES and EXAFS) were used to define the local environments of the metals taken up at the mineral surfaces. All examples showing the type I behavior yield information on local environments consistent with their being bound to the surfaces by an inner sphere complex formation mechanism. Thus, Cu on goethite appeared to form a Jahn-Teller distorted octahedral complex (four O atoms at 1.94 Aa; two O atoms at 2.41 Aa) but with evidence for interaction with two further Fe (or Cu) at 2.92 Aa. On lepidocrocite, the first coordination sphere was essentially identical to that on goethite but with a second Cu or Fe shell at 3.04 Aa and, for the highest Cu loadings, a third shell (2 Cu or Fe) at 3.67 Aa. The Cd on goethite showed best fits for sixfold coordination to O (at 2.26 Aa) but with evidence for a second shell of Fe atoms at 3.75 Aa. On lepidocrocite, the first shell was essentially the same as for goethite, but a second shell of Fe atoms appeared to occur at the shorter distance of 3.31 Aa. For Cd-loaded pyrite, best fits were given by a single shell of six O atoms at 2.27 to 2.28 Aa and with no evidence for a second shell of metal atoms. The systems exhibiting the type II behavior yielded spectra consistent with the formation of new phases on the surfaces, either by precipitation or replacement reactions. In the case of Cu interaction with Mackinawite, a chalcopyrite phase appeared to form, whereas interaction with pyrite seemed to produce binary Cu sulfides--covellite at lower loadings and chalcocite at higher loadings. Cd interaction with Mackinawite seemed to produce a CdS phase.
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Transformation of Mackinawite to greigite: An in situ X-ray powder diffraction and transmission electron microscope study
American Mineralogist, 1997Co-Authors: Alistair R. Lennie, Simon A. T. Redfern, Pamela E. Champness, Chris P. Stoddart, Paul F. Schofield, David J VaughanAbstract:Synthetic Mackinawite (tetragonal FeS) has been found to transform rapidly to greigite (Fe3S4) above ;373 K during heating experiments, as observed by in situ X-ray diffraction. Using monochromatic synchrotron radiation ( l5 0.60233 A ˚), we measured the unit-cell parameters of both synthetic Mackinawite between 293 and 453 K and of greigite formed from this Mackinawite between 293 and 593 K. The coefficients of thermal expansion for Mackinawite are a1 5a 2 5(1.36 6 0.11) 3 10 25 , a3 5 (2.98 6 0.12) 3 10 25 , and avol 5 (5.67 6 0.19) 3 10 25 between 293 and 453 K. The coefficients of thermal expansion for greigite are a1 5a 2 5a 3 5(1.63 6 0.15) 3 10 25 , and avol 5 (4.86 6 0.25) 3 10 25 between 293 and 593 K. On further heating in situ, we observed the reaction greigite → pyrrhotite 1 magnetite. Partial transformation of Mackinawite to greigite was also observed using transmission electron microscopy (TEM) following in situ heating. Electron diffraction patterns show that (001) of Mackinawite is parallel to (001) of greigite, and [110] of Mackinawite is parallel to [100] of greigite. This orientation relationship confirms that the cubic closepacked S array in Mackinawite is retained in greigite and implies that oxidation of some Fe 21 in Mackinawite drives rearrangement of Fe to form the new phase. Small regions of
D Rickard - One of the best experts on this subject based on the ideXlab platform.
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Arsenic mobility in the ambient sulfidic environment: Sorption of arsenic(V) and arsenic(III) onto disordered Mackinawite
Geochimica et Cosmochimica Acta, 2005Co-Authors: D RickardAbstract:Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered Mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of Mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of Mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The Kd (L g−1) values obtained from linear fits to the isotherm data are ∼9 for As(V) and ∼2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered Mackinawite controls arsenic mobility.
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Arsenic mobility in the ambient sulfidic environment: Sorption of arsenic(V) and arsenic(III) onto disordered Mackinawite.
Geochimica et Cosmochimica Acta, 2005Co-Authors: M. Wolthers, Laurent Charlet, C.h. Van Der Weijden, P.r. Van Der Linde, D RickardAbstract:Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered Mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of Mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of Mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The Kd (L g−1) values obtained from linear fits to the isotherm data are not, vert, similar9 for As(V) and not, vert, similar2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered Mackinawite controls arsenic mobility.
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Surface chemistry of disordered Mackinawite (FeS).
Geochimica et Cosmochimica Acta, 2005Co-Authors: Mariëtte Wolthers, D Rickard, Laurent Charlet, P.r. Van Der Linde, C.h. Van Der WeijdenAbstract:Disordered Mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by Ksapp = {Fe2+} · {H2S(aq)} · {H+}−2 = 10+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered Mackinawite lies at pH not, vert, similar7.5. The hydrated disordered Mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10−3 mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10−3 mol/g FeS, determines surface charge changes. Total site density is 4 sites nm−2. The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.
Laurent Charlet - One of the best experts on this subject based on the ideXlab platform.
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Oxidation state and local structure of plutonium reacted with magnetite, Mackinawite, and chukanovite.
Environmental science & technology, 2011Co-Authors: Regina Kirsch, Laurent Charlet, Marcus Altmaier, D. Fellhauer, Volker Neck, André Rossberg, Thomas Fanghänel, Andreas C ScheinostAbstract:Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-LIII-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous 242Pu(III) and 242Pu(V) reacted with magnetite, Mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO2 phases formed in the presence of Mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalen...
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Selenite reduction by Mackinawite, magnetite and siderite: XAS characterization of nanosized redox products.
Environmental science & technology, 2008Co-Authors: Andreas C Scheinost, Laurent CharletAbstract:Suboxic soils and sediments often contain the Fe(II)-bearing minerals Mackinawite (FeS), siderite (FeCO3) or magnetite (Fe3O4), which should be able to reduce aqueous selenite, thereby forming solids of low solubility. While the reduction of selenate or selenite to Se(0) by green rust, pyrite and by Fe2+ sorbed to montmorillonite is a slow (weeks), kinetically limited redox reaction as demonstrated earlier, we show here that selenite is rapidly reduced within one day by nanoparticulate Mackinawite and magnetite, while only one third of selenite is reduced by micrometer-sized siderite. Depending on Fe(II)-bearing phase and pH, we observed four different reaction products, red and gray elemental Se, and two iron selenides with structures similar to Fe7Se8 and FeSe. The thermodynamically most stable iron selenide, ferroselite (FeSe2), was not observed. The local structures of the reaction products suggest formation of nanoscale clusters, which may be prone to colloid-facilitated transport, and may have a hig...
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Arsenic mobility in the ambient sulfidic environment: Sorption of arsenic(V) and arsenic(III) onto disordered Mackinawite.
Geochimica et Cosmochimica Acta, 2005Co-Authors: M. Wolthers, Laurent Charlet, C.h. Van Der Weijden, P.r. Van Der Linde, D RickardAbstract:Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered Mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of Mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of Mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The Kd (L g−1) values obtained from linear fits to the isotherm data are not, vert, similar9 for As(V) and not, vert, similar2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered Mackinawite controls arsenic mobility.
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Surface chemistry of disordered Mackinawite (FeS)
Geochimica et Cosmochimica Acta, 2005Co-Authors: Mariëtte Wolthers, David Rickard, Laurent Charlet, P.r. Van Der Linde, C.h. Van Der WeijdenAbstract:Disordered Mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by Ksapp = {Fe2+} · {H2S(aq)} · {H+}−2 = 10+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered Mackinawite lies at pH ∼7.5. The hydrated disordered Mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10−3 mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10−3 mol/g FeS, determines surface charge changes. Total site density is 4 sites nm−2. The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.
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Surface chemistry of disordered Mackinawite (FeS).
Geochimica et Cosmochimica Acta, 2005Co-Authors: Mariëtte Wolthers, D Rickard, Laurent Charlet, P.r. Van Der Linde, C.h. Van Der WeijdenAbstract:Disordered Mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by Ksapp = {Fe2+} · {H2S(aq)} · {H+}−2 = 10+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered Mackinawite lies at pH not, vert, similar7.5. The hydrated disordered Mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10−3 mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10−3 mol/g FeS, determines surface charge changes. Total site density is 4 sites nm−2. The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.
Kim F Hayes - One of the best experts on this subject based on the ideXlab platform.
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Abiotic reductive dechlorination of cis -DCE by ferrous monosulfide Mackinawite
Environmental science and pollution research international, 2015Co-Authors: Sung Pil Hyun, Kim F HayesAbstract:Cis-1,2,-dichloroethylene (cis-DCE) is a toxic, persistent contaminant occurring mainly as a daughter product of incomplete degradation of perchloroethylene (PCE) and trichloroethylene (TCE). This paper reports on abiotic reductive dechlorination of cis-DCE by Mackinawite (FeS1-x), a ferrous monosulfide, under variable geochemical conditions. To assess in situ abiotic cis-DCE dechlorination by Mackinawite in the field, Mackinawite suspensions prepared in a field groundwater sample collected from a cis-DCE contaminated field site were used for dechlorination experiments. The effects of geochemical variables on the dechlorination rates were monitored. A set of dechlorination experiments were also carried out in the presence of aquifer sediment from the site over a range of pH conditions to better simulate the actual field situations. The results showed that the suspensions of freshly prepared Mackinawite reductively transformed cis-DCE to acetylene, whereas the conventionally prepared powder form of Mackinawite had practically no reactivity with cis-DCE under the same experimental conditions. Significant cis-DCE degradation by Mackinawite has not been reported prior to this study, although Mackinawite has been shown to reductively transform PCE and TCE. This study suggests feasibility of using Mackinawite for in situ remediation of cis-DCE-contaminated sites with high S levels such as estuaries under naturally achieved or stimulated sulfate-reducing conditions.
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uranium vi reduction by iron ii monosulfide Mackinawite
Environmental Science & Technology, 2012Co-Authors: Sung Pil Hyun, James A Davis, Kai Sun, Kim F HayesAbstract:Reaction of aqueous uranium(VI) with iron(II) monosulfide Mackinawite in an O(2) and CO(2) free model system was studied by batch uptake measurements, equilibrium modeling, and L(III) 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 O(2) 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 O(2). 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.
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Uranium(VI) reduction by iron(II) monosulfide Mackinawite.
Environmental science & technology, 2012Co-Authors: Sung Pil Hyun, James A Davis, Kai Sun, Kim F HayesAbstract: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 LIII edge U X-ray absorp...
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Model predictions of realgar precipitation by reaction of As(III) with synthetic Mackinawite under anoxic conditions.
Environmental science & technology, 2008Co-Authors: Tanya J. Gallegos, Young Soo Han, Kim F HayesAbstract:This study investigates the removal of As(III) from solution using Mackinawite, a nanoparticulate reduced iron sulfide. Mackinawite suspensions (0.1−40 g/L) effectively lower initial concentrations of 1.3 × 10−5 M As(III) from pH 5−10, with maximum removal occurring under acidic conditions. Based on Eh measurements, it was found that the redox state of the system depended on the Mackinawite solids concentration and pH. Higher initial Mackinawite concentrations and alkaline pH resulted in a more reducing redox condition. Given this, the pH edge data were modeled thermodynamically using pe (−log[e−]) as a fitting parameter and linear pe−pH relationships within the range of measured Eh values as a function of pH and Mackinawite concentration. The model predicts removal of As(III) from solution by precipitation of realgar with the formation of secondary oxidation products, greigite or a mixed-valence iron oxide phase, depending on pH. This study demonstrates that Mackinawite is an effective sequestration agen...
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characterization of synthetic nanocrystalline Mackinawite crystal structure particle size and specific surface area
Geochimica et Cosmochimica Acta, 2008Co-Authors: Hoon Young Jeong, Kim F HayesAbstract:Abstract Iron sulfide was synthesized by reacting aqueous solutions of sodium sulfide and ferrous chloride for 3 days. By X-ray powder diffraction (XRPD), the resultant phase was determined to be primarily nanocrystalline Mackinawite (space group: P4/ nmm) with unit cell parameters a = b = 3.67 A and c = 5.20 A. Iron K-edge XAS analysis also indicated the dominance of Mackinawite. Lattice expansion of synthetic Mackinawite was observed along the c-axis relative to well-crystalline Mackinawite. Compared with relatively short-aged phase, the Mackinawite prepared here was composed of larger crystallites with less elongated lattice spacings. The direct observation of lattice fringes by HR-TEM verified the applicability of Bragg diffraction in determining the lattice parameters of nanocrystalline Mackinawite from XRPD patterns. Estimated particle size and external specific surface area (SSAext) of nanocrystalline Mackinawite varied significantly with the methods used. The use of Scherrer equation for measuring crystallite size based on XRPD patterns is limited by uncertainty of the Scherrer constant (K) due to the presence of polydisperse particles. The presence of polycrystalline particles may also lead to inaccurate particle size estimation by Scherrer equation, given that crystallite and particle sizes are not equivalent. The TEM observation yielded the smallest SSAext of 103 m2/g. This measurement was not representative of dispersed particles due to particle aggregation from drying during sample preparation. In contrast, EGME method and PCS measurement yielded higher SSAext (276–345 m2/g by EGME and 424 ± 130 m2/g by PCS). These were in reasonable agreement with those previously measured by the methods insensitive to particle aggregation.
N. H. De Leeuw - One of the best experts on this subject based on the ideXlab platform.
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Activation and dissociation of CO2 on the (001), (011), and (111) surfaces of Mackinawite (FeS): A dispersion-corrected DFT study
The Journal of chemical physics, 2015Co-Authors: Nelson Y. Dzade, Alberto Roldan, N. H. De LeeuwAbstract:Iron sulfide minerals, including Mackinawite (FeS), are relevant in origin of life theories, due to their potential catalytic activity towards the reduction and conversion of carbon dioxide (CO2) to organic molecules, which may be applicable to the production of liquid fuels and commodity chemicals. However, the fundamental understanding of CO2 adsorption, activation, and dissociation on FeS surfaces remains incomplete. Here, we have used density functional theory calculations, corrected for long-range dispersion interactions (DFT-D2), to explore various adsorption sites and configurations for CO2 on the low-index Mackinawite (001), (110), and (111) surfaces. We found that the CO2 molecule physisorbs weakly on the energetically most stable (001) surface but adsorbs relatively strongly on the (011) and (111) FeS surfaces, preferentially at Fe sites. The adsorption of the CO2 on the (011) and (111) surfaces is shown to be characterized by significant charge transfer from surface Fe species to the CO2 molecule, which causes a large structural transformation in the molecule (i.e., forming a negatively charged bent CO2 (-δ) species, with weaker C-O confirmed via vibrational frequency analyses). We have also analyzed the pathways for CO2 reduction to CO and O on the Mackinawite (011) and (111) surfaces. CO2 dissociation is calculated to be slightly endothermic relative to the associatively adsorbed states, with relatively large activation energy barriers of 1.25 eV and 0.72 eV on the (011) and (111) surfaces, respectively.
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The surface chemistry of NOx on Mackinawite (FeS) surfaces: a DFT-D2 study
Physical chemistry chemical physics : PCCP, 2014Co-Authors: Nelson Y. Dzade, Alberto Roldan, N. H. De LeeuwAbstract:We present density functional theory calculations with a correction for the long-range interactions (DFT-D2) of the bulk and surfaces of Mackinawite (FeS), and subsequent adsorption and dissociation of NOx gases (nitrogen monoxide (NO) and nitrogen dioxide (NO2)). Our results show that these environmentally important molecules interact very weakly with the energetically most stable (001) surface, but adsorb relatively strongly onto the FeS(011), (100) and (111) surfaces, preferentially at Fe sites via charge donation from these surface species. The NOx species exhibit a variety of adsorption geometries, with the most favourable for NO being the monodentate Fe–NO configuration, whereas NO2 is calculated to form a bidentate Fe–NOO–Fe configuration. From our calculated thermochemical energy and activation energy barriers for the direct dissociation of NO and NO2 on the FeS surfaces, we show that NO prefers molecular adsorption, while dissociative adsorption, i.e. NO2 (ads) → [NO(ads) + O(ads)] is preferred over molecular adsorption for NO2 onto the Mackinawite surfaces. However, the calculated high activation barriers for the further dissociation of the second N–O bond to produce either [N(ads) and 2O(ads)] or [N(ads) and O2(ads)] suggest that complete dissociation of NO2 is unlikely to occur on the Mackinawite surfaces.
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Adsorption of methylamine on Mackinawite (FES) surfaces: A density functional theory study
The Journal of chemical physics, 2013Co-Authors: Nelson Y. Dzade, Alberto Roldan, N. H. De LeeuwAbstract:We have used density functional theory calculations to investigate the interaction between methylamine (CH3NH2) and the dominant surfaces of Mackinawite (FeS), where the surface and adsorption properties of Mackinawite have been characterized using the DFT-D2 method of Grimme. Our calculations show that while the CH3NH2 molecule only interacts weakly with the most stable FeS(001), it adsorbs relatively strongly on the FeS(011) and FeS(100) surfaces releasing energies of 1.26 eV and 1.51 eV, respectively. Analysis of the nature of the bonding reveals that the CH3NH2 molecule interacts with the Mackinawite surfaces through the lone-pair of electrons located on the N atom. The electron density built up in the bonding region between N and Fe is very much what one would expect of covalent type of bonding. We observe no significant adsorption-induced changes of the FeS surface structures, suggesting that amine capping agents would not distort the FeS nanoparticle surfaces required for active heterogeneous catalytic reactions. The vibrational frequencies and the infrared spectra of adsorbed methylamine have been calculated and assignments for vibrational modes are used to propose a kinetic model for the desorption process, yielding a simulated temperature programmed desorption with a relative desorption temperature of
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adsorption of methylamine on Mackinawite fes surfaces a density functional theory study
Journal of Chemical Physics, 2013Co-Authors: Nelson Y. Dzade, Alberto Roldan, N. H. De LeeuwAbstract:We have used density functional theory calculations to investigate the interaction between methylamine (CH3NH2) and the dominant surfaces of Mackinawite (FeS), where the surface and adsorption properties of Mackinawite have been characterized using the DFT-D2 method of Grimme. Our calculations show that while the CH3NH2 molecule only interacts weakly with the most stable FeS(001), it adsorbs relatively strongly on the FeS(011) and FeS(100) surfaces releasing energies of 1.26 eV and 1.51 eV, respectively. Analysis of the nature of the bonding reveals that the CH3NH2 molecule interacts with the Mackinawite surfaces through the lone-pair of electrons located on the N atom. The electron density built up in the bonding region between N and Fe is very much what one would expect of covalent type of bonding. We observe no significant adsorption-induced changes of the FeS surface structures, suggesting that amine capping agents would not distort the FeS nanoparticle surfaces required for active heterogeneous catalytic reactions. The vibrational frequencies and the infrared spectra of adsorbed methylamine have been calculated and assignments for vibrational modes are used to propose a kinetic model for the desorption process, yielding a simulated temperature programmed desorption with a relative desorption temperature of <140 K at the FeS(011) surface and <170 K at FeS(100) surface.
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Combined Density Functional Theory and Interatomic Potential Study of the Bulk and Surface Structures and Properties of the Iron Sulfide Mackinawite (FeS)
The Journal of Physical Chemistry C, 2008Co-Authors: A. J. Devey, Ricardo Grau-crespo, N. H. De LeeuwAbstract:The iron sulfide Mackinawite (FeS) is modeled by density functional theory calculations, the results of which are used to derive a set of interatomic potentials for this material. We have investigated the effect of adding a Hubbard Ueff term to the gradient-corrected (GGA) functional, but whereas good agreement is shown with experimental data when pure GGA (Ueff = 0) is employed, for values of Ueff greater than zero the solution becomes unphysical, an indication that the Fe valence electrons within Mackinawite are delocalized. This property is further evidenced by the density of states, which confirms a metallic nature from delocalization of the Fe d orbital at all Ueff values. Mackinawite is calculated to be a nonmagnetic material, in accord with experiment. We have also derived a set of interatomic potentials by fitting to observables including geometry, phonon frequencies, and elastic constants, which were calculated for isolated Mackinawite layers using DFT. The derived potential model is used to calc...
