Formation Of Iron Sulfides Research Articles

Iron sulphide formation and interaction with corrosion inhibitor in H2S-containing environments

Corrosion of carbon steel in H2S-containing environments poses serious challenges to operations in oilfield and geothermal applications. Carbon steels exposed to H2S-containing environments are often susceptible to pitting and uniform corrosion, and linked to the properties of iron sulphide (FeS) layers that forms at the corroding interface. This paper investigates the corrosion characteristics of carbon steel in H2S-containing environments focussing on the early stage kinetics of FeS formation and its interaction with corrosion inhibitors (CI). Experiments were conducted in 3.5 and 5.8 wt% NaCl solution at 80 °C in 10% H2S-90% CO2-containing environments. In-situ electrochemical responses were measured using the linear polarisation resistance (LPR) technique. This was combined with post-experiment surface analysis; scanning electron microscopy (SEM) and X-ray diffraction (XRD) to investigate the nature and morphology of formed FeS. 3D surface profilometry was used to investigate the pitting corrosion characteristics of carbon steel caused by FeS interaction with CI. The results show a fast kinetics of FeS formation by direct reduction of H2S(aq) with a distinct and easily identifiable morphology, and evidence of minimum level of ferrite dissolution. The early stages of FeS formation and its interaction with CI have a strong influence on the overall corrosion characteristics of carbon steel. A dose of 30 ppm of a commercial corrosion inhibitor (CI); salts of nitrogenous molecules with a fatty acid group, at the corrosion interface induced a competition for surface absorption between CI and emerging FeS. Over the 48 h duration of this test, the FeS formation process was delayed and the mechanisms of its formation quickly became the main driver of pitting corrosion. The amount of CI dosed in this study was not able to prevent FeS-induced pitting corrosion attack on the surface.

Retention and vertical distribution of heavy metals in mangrove sediments of the protected area swamp of Mallorquin, Colombian Caribbean

The accumulation, retention and vertical distribution of six heavy metals (Fe, Cu, Zn, Ni, Pb and Hg) in two contrasting sedimentary environments of the Swamp of Mallorquín Colombian Caribbean, one with mangrove plants (M) and another without plant covering (SM), were analyzed in order to establish the incidence that such plants may have on the distribution of those metals in their adjacent sediments. Total metal concentrations (μg g−1, Fe in %) in the sediment profile for the different sampling areas was: Fe (M: 3.4 ± 0.6; SM: 3.0 ± 1.2), Zn (M: 64.9 ± 3.7; SM: 60.7 ± 13.9), Cu (M: 42.6 ± 6.2; SM: 33.3 ± 8.9), Ni (M: 28.5 ± 3.6; SM: 24.3 ± 6.9), Pb (M: 3.5 ± 0.4; SM: 2.9 ± 0.7), Hg (M: 0.14 ± 0.03; SM: 0.11 ± 0.04). Correlation analysis suggest that distribution of Cu, Zn and Ni in the M zone would be dominated by the formation of iron sulfides and oxyhydroxides in sediments, while the organic matter would be influencing the distribution of Pb and Hg. On the other hand, SM showed correlation for most of the variables, making it difficult to establish the sedimentary phases influencing metal distribution, due to the codependency between the different diagenetic processes. Cu, Ni and Hg concentrations exceed prescribed sediment quality guidelines (SQGs) indicating potential adverse effect on biota. The results corroborate the suitability of mangrove sediments as metal sinks and the importance of mangrove ecosystem restoration and conservation in the Swamp of Mallorquín.

Sulphate reduction determines the long-term effect of iron amendments on phosphorus retention in lake sediments

PurposeThis field study aimed to guide the planning of iron amendments for phosphorus retention by investigating the long-term fate of iron added to two urban lakes (Plötzensee and Groß Glienicker See) in Berlin, Germany. The contributions of iron dosing to improve lake status as well as the relevance of competing processes for management success were evaluated.MethodsSediment stratigraphy, as well as occurrence of iron minerals, and fluxes between water and sediment were examined using geochemical analyses (i.e. element composition, sequential extraction, X-ray diffraction, and pore water analyses). A one-box lake model was used to relate these fluxes to monitoring data from the water column and to sediment inventories.ResultsIn both lakes, the added iron was preserved in the sediment. Whereas phosphorus retention increased following the addition of iron to Groß Glienicker See, sulphur was retained by the excess iron in Plötzensee. This contrasting effect is attributed to significantly different sulphate reduction rates in two lakes (Wilcoxon rank sum test: W = 25, p = 0.008). According to the one-box model, sulphate reduction explained both the decrease in measured sulphate concentrations after iron application as well as the observed increase in sulphur deposition in the sediments.ConclusionManagement interventions involving iron amendments to enhance phosphorus retention must consider the competing process of iron sulphide formation during the entire management plan period, and additional iron may need to be applied to account for this effect.

Gypsum utilization on selective reduction of limonitic laterite nickel

The world's nickel demand is increasing, especially with more countries starting to produce battery-based electric vehicles. Currently, nickel is the main element used for the fabrication of Nickel Manganese Cobalt (NMC) batteries. Nickel itself can be extracted from its ore through pyrometallurgical route. Selective reduction of nickel using a selective reducing agent is needed in order to produce nickel with appropriate grade. This study aims to study the possibility of gypsum as a selective reducing agent in the direct reduction process. In this process, laterite nickel ores with nickel content of 1.59% were crushed and classified until the feed particle size was smaller than 50 standard mesh. The crushed laterite nickel ores were then mixed with gypsum and a binder material to undergo briquetting and drying. A bed mix composed of limestone powder and coal was added into the crucible alongside the ore briquettes. Carbothermic reduction was done at 1400 °C for 6 h. The product was then crushed and separated through magnetic separation. The reduced product's chemical composition was analyzed using Scanning Electron Microscope-Energy Dispersive X-Ray and X-Ray Diffractometer. The results indicate that gypsum can provide a selective effect on the direct reduction process of laterite nickel ore. A selective effect that lowers the grade of iron can occur due to the formation of iron sulfides. The use of 12% gypsum was able to produce a product with Ni grade of 4.13%, Ni recovery of 91.81%, and selectivity factor of 5.72. Therefore, based on the results of this experiment, gypsum minerals can be used as a potential low-cost alternative as an additive for nickel selective reduction.

Enhanced weathering in the seabed: Rapid olivine dissolution and iron sulfide formation in submarine volcanic ash

Abstract In basaltic volcanic ash recovered from a seamount at 3000 m water depth, we discovered marcasite and pyrite precipitation within cavities that formed by partial to complete dissolution of olivine. In places, these cavities are reminiscent of negative crystal shapes; elsewhere they apparently continue along cracks. In strong contrast, adjacent volcanic glass shows little, if any, evidence for dissolution. The FeS2 precipitates were commonly found to be conjoined and planar aggregates, occurring in the center of the voids. Their maximum volume fraction in relation to the void space as determined by 2D and 3D imaging techniques corresponds to the amount of iron released by olivine dissolution. Almost all occurrences of FeS2 precipitation are related to Cr-spinel inclusions in the former olivine. We propose that rapid olivine dissolution was initiated by reduced H2S-bearing fluids at olivine grain boundaries or surfaces exposed by cracks. Many of these cracks are connected to spinel grains, where the iron liberated from olivine is mineralized as FeS2, initially facilitated by heterogeneous nucleation. Subsequent pyrite and/or marcasite precipitation occurred as overgrowths on existing FeS2 aggregates. The particular chemical environment of low-pH, hydrogen sulfide-bearing fluids may have enhanced olivine dissolution by (1) keeping Fe in solution and (2) sequestering important quantities of Fe as FeS2. The in situ oxidation of ferrous Fe and precipitation of ferric hydroxides at the olivine surface commonly observed in oxic environments were obviously impeded. It would have slowed down olivine dissolution to rates more similar to the dissolution of basaltic glass. We have no direct indication that the process of rapid olivine dissolution was aided by subseafloor life. However, the presence of fibrous structures with small sulfide particles could indicate late colonization of sulfate-reducing bacteria that may add an additional path of iron fixation.

On a possibility of replacing grey cast iron for manufacturing cast elements of electrolyzer gas collecting bell

The main process leading to the destruction of the cast elements of gas-collection bell of electrolyzer, made of grey cast iron, is the oxidation of iron by oxygen, SO2 gas and sulfur vapors to form magnetite, hematite and pyrrhotin. The simultaneous formation of iron oxides and sulfides does not prevent further corrosion, since scale is formed with a loose structure that does not have protective properties. Reducing the length of the interfacial boundaries inside the material of the cast enables to reduce the rate of corrosion destruction, which can be achieved by modifying the cast iron to change the shape of graphite inclusions, i.e. obtaining high-strength cast iron with a spherical shape of graphite inclusions. However, the obtaining spherical graphite in cast iron using magnesium modification does not exclude the access of aggressive gases to the surface of the products and the possibility of their diffusion along the grain boundaries. It was shown that alloying can be an alternative, which leads not only to the exclusion of lamellar secretions of graphite in the structure of cast iron, but also to the formation of surface oxide layers based on the alloying element preventing the corrosion. Alloying with chromium gives cast iron high abrasive resistance due to the presence of a carbide component in the structure, as well as corrosion resistance due to the alloying of the metal base, heat resistance due to increasing the electrochemical potential of the metal base and creating a strong neutral oxide film on the surface of the castings, heat resistance, etc. An experimental comparative analysis of the corrosion resistance of cast iron used for manufacturing of gas collecting bell of electrolyzers showed that chromic cast iron ЧХ3 has a higher corrosion resistance than high-strength cast iron with spherical graphite ВЧ50 and much higher than grey cast iron with lamellar graphite. However, chromic cast iron ЧХ3 has low casting properties, is very sensitive to the cooling rate and has a large heterogeneity in structure, which makes it difficult to use it for the manufacture of gas collecting bell of electrolyzers.

Rapid, concurrent formation of organic sulfur and iron sulfides during experimental sulfurization of sinking marine particles.

Organic matter (OM) sulfurization can enhance the preservation and sequestration of carbon in anoxic sediments, and it has been observed in sinking marine particles from marine O2-deficient zones. The magnitude of this effect on carbon burial remains unclear, however, because the transformations that occur when sinking particles encounter sulfidic conditions remain undescribed. Here, we briefly expose sinking marine particles from the eastern tropical North Pacific O2-deficient zone to environmentally relevant sulfidic conditions (20°C, 0.5 mM [poly]sulfide, two days) and then characterize the resulting solid-phase organic and inorganic products in detail. During these experiments, the abundance of organic sulfur in both hydrolyzable and hydrolysis-resistant solids roughly triples, indicating extensive OM sulfurization. Lipids also sulfurize on this timescale, albeit less extensively. In all three pools, OM sulfurization produces organic monosulfides, thiols, and disulfides. Hydrolyzable sulfurization products appear within ≤ 200-μm regions of relatively homogenous composition that are suggestive of sulfurized extracellular polymeric substances (EPS). Concurrently, reactions with particulate iron oxyhydroxides generate low and fairly uniform concentrations of iron sulfide (FeS) within these same EPS-like materials. Iron oxyhydroxides were not fully consumed during the experiment, which demonstrates that organic materials can be competitive with reactive iron for sulfide. These experiments support the hypothesis that sinking, OM- and EPS-rich particles in a sulfidic water mass can sulfurize within days, potentially contributing to enhanced sedimentary carbon sequestration. Additionally, sulfur-isotope and chemical records of organic S and iron sulfides in sediments have the potential to incorporate signals from water column processes.

Characterization of Iron Removal Process Products from Atmospheric Lateritic Ore Leaching Solution by using 20 and 25% of Calcium Carbonate

AbstractElectric vehicles become the alternative to solve the climate change and global warming problems by providing a more eco-friendly and sustainable source of energy. As the demand for sustainable vehicles increased, the functionality of batteries become crucial. One of the important aspects inside the batteries is nickel. Nickel plays a big role in lithium-ion batteries by delivering greater amounts of energy density with a higher storage capacity, which means it provides bigger efficiency to the batteries. Yet, the attempt of optimizing nickel extraction remains a challenge. Therfore, nickel extraction process of lateritic ore with high efficiency is investigated by using hydrometellurgy process, specifically the iron removal process in atmospheric condition in mixed hydroxide precipitates (MHP) route.The reagent solution of (20% w/w and 25% w/w) calcium carbonate (CaCO3) at pH (1, 2, 3) were utilized as additive in this process. The precipitates resulted from PLS were characterized by x-ray diffraction (XRD) and Scanning Electron Microscopy - Energy Dispersive X-Ray (SEM–EDS), while the filtrates were investigated by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). Analysis based on precipitates demonstrates that the acid neutralization process took place with a sufficient amount of iron in the precipitates with the least amount of nickel. In addition, all pH and concentration of precipitates qualitatively illustrate the same neutralization process involving calcium and sulfur. From the results of filtrate through ICP testing in this study, pH 1 for both 20% and 25% concentration provides the lowest recovery rate alongside the smallest ppm compare to pH 2 and 3; thus, the iron precipitates in the formation of iron sulfide and/or iron sulfate. Overall, the optimum parameter is 25% of calcium carbonate, pH 1, 90oC for 2 hours of agitation to reduce the amount of iron in the solution.

Formation of Iron Sulfides on Carbon Steel in a Specific Cement Grout Designed for Radioactive Waste Repository and Associated Corrosion Mechanisms.

Carbon steel coupons were buried in a specific low-pH cement grout designed for radioactive waste disposal and left 6 months in anoxic conditions at 80 °C. The corrosion product layers were analyzed by µ-Raman spectroscopy, XRD, and SEM. They proved to be mainly composed of iron sulfides, with magnetite as a minor phase, mixed with components of the grout. Average corrosion rates were estimated by weight loss measurements between 3 and 6 µm yr−1. Corrosion profiles revealed local degradations with a depth up to 10 µm. It is assumed that the heterogeneity of the corrosion product layer, mainly composed of conductive compounds (FeS, Fe3S4, and Fe3O4), promotes the persistence of corrosion cells that may lead to locally aggravated degradations of the metal. New cement grouts, characterized by a slightly higher pH and a lower sulfide concentration, should then be designed for the considered application.

Behavior of light elements in iron-silicate-water-sulfur system during early Earth\u2019s evolution

Hydrogen (H) is considered to be one of the candidates for light elements in the Earth’s core, but the amount and timing of delivery have been unknown. We investigated the effects of sulfur (S), another candidate element in the core, on deuteration of iron (Fe) in iron–silicate–water system up to 6–12 GPa, ~ 1200 K using in situ neutron diffraction measurements. The sample initially contained saturated water (D2O) as Mg(OD)2 in the ideal composition (Fe–MgSiO3–D2O) of the primitive Earth. In the existence of water and sulfur, phase transitions of Fe, dehydration of Mg(OD)2, and formation of iron sulfide (FeS) and silicates occurred with increasing temperature. The deuterium (D) solubility (x) in iron deuterides (FeDx) increased with temperature and pressure, resulting in a maximum of x = 0.33(4) for the hydrous sample without S at 11.2 GPa and 1067 K. FeS was hardly deuterated until Fe deuteration had completed. The lower D concentrations in the S-containing system do not exceed the miscibility gap (x < ~ 0.4). Both H and S can be incorporated into solid Fe and other light elements could have dissolved into molten iron hydride and/or FeS during the later process of Earth’s evolution.

Laboratory and initial field testing of the Min‐Trap™ for tracking reactive iron sulfide mineral formation during in situ remediation

AbstractThe Mineral Trap, or Min‐Trap™, is a monitoring well‐based sampler designed to collect direct physical evidence of reactive mineral formation in situ without collecting soil or rock core samples. The Min‐Trap consists of a nonreactive granular medium (e.g., silica sand) within water‐permeable mesh pillows that are supported inside a slotted polyvinyl chloride housing that is incubated within a conventional monitoring well. The primary objective of the Min‐Trap in this application is to collect reactive minerals that are forming in the aquifer in a retrievable format that can be submitted for laboratory analysis. To evaluate the capability of Min‐Traps to capture reactive iron minerals, both a laboratory tank test and a field test were conducted. Both tests confirmed that iron sulfide minerals form in the Min‐Trap under sulfate reducing conditions within several weeks. Analysis of the precipitated minerals via the AMIBA analysis suite showed that they almost entirely consisted of weak acid soluble (biogenic, microcrystalline) ferrous iron‐based minerals, and at least two thirds of the sulfur‐containing minerals were monosulfides (i.e., mackinawite) at the end of each test. Scanning electron microscopy confirmed the colocation of iron and sulfur in the mineral masses. The dominance of the ferrous iron and reduced sulfur verifies that little to no oxidation of the captured minerals occurred between sample collection and analysis. A subsurface soil core was collected during the field test next to the Min‐Trap‐containing well. AMIBA results were consistent between the native soil and the Min‐Trap except for much higher strong acid soluble (crystalline) ferric iron in the native soil when compared to the silica sand of the Min‐Trap, as expected. This work shows that Min‐Traps are useful for documenting the formation of reactive iron sulfides (FeSx) that can form during in situ anaerobic biostimulation and can drive complementary abiotic treatment of chlorinated volatile organic compounds. Mineralogical data obtained from Min‐Traps can be applied to assess remedial objectives at several stages of the remedial program, including initial characterization, alternatives evaluation, feasibility testing, remedy optimization, and transition from active treatment to passive remedial methods.

Small-Scale Geochemical Heterogeneities and Seasonal Variation of Iron and Sulfide in Salt Marshes Revealed by Two-Dimensional Sensors

Loss of tidal wetlands is a world-wide phenomenon. Many factors may contribute to such loss, but among them are geochemical stressors such as exposure of the marsh plants to elevated levels on hydrogen sulfide in the pore water of the marsh peat. Here we report the results of a study of the geochemistry of iron and sulfide at different seasons in unrestored (JoCo) and partially restored (Big Egg) salt marshes in Jamaica Bay, a highly urbanized estuary in New York City where the loss of salt marsh area has accelerated in recent years. The spatial and temporal 2-dimensional distribution patterns of dissolved Fe2+ and H2S in salt marshes were in situ mapped with high resolution planar sensors for the first time. The vertical profiles of Fe2+ and hydrogen sulfide, as well as related solutes and redox potentials in marsh were also evaluated by sampling the pore water at discrete depths. Sediment cores were collected at various seasons and the solid phase Fe, S, N, C, and chromium reducible sulfide in marsh peat at discrete depths were further investigated in order to study Fe and S cycles, and their relationship to the organic matter cycling at different seasons. Our results revealed that the redox sensitive elements Fe2+ and S2– showed significantly heterogeneous and complex three dimensional distribution patterns in salt marsh, over mm to cm scales, directly associated with the plant roots due to the oxygen leakage from roots and redox diagenetic reactions. We hypothesize that the oxic layers with low/undetected H2S and Fe2+ formed around roots help marsh plants to survive in the high levels of H2S by reducing sulfide absorption. The overall concentrations of Fe2+ and H2S and distribution patterns also seasonally varied with temperature change. H2S level in JoCo sampling site could change from &amp;lt;0.02 mM in spring to &amp;gt;5 mM in fall season, reflecting significantly seasonal variation in the rates of bacterial oxidation of organic matter at this marsh site. Solid phase Fe and S showed that very high fractions of the diagenetically reactive iron at JoCo and Big Egg were associated with pyrite that can persist for long periods in anoxic sediments. This implies that there is insufficient diagenetically reactive iron to buffer the pore water hydrogen sulfide through formation of iron sulfides at JoCo and Big Egg.

Tribofilm formation during dry sliding of graphite- and MoS2- based composites obtained by spark plasma sintering

Composites of Astaloy 85Mo with the addition of MoS2, graphite or a mixture of graphite/MoS2 were consolidated using Spark Plasma Sintering. Dry sliding wear tests were conducted at different normal loads (5, 10, and 20 N) using the sintered samples against an AISI 52100 ball. The effect of contact pressure on the extrusion of solid lubricant was investigated using scratch tests. Graphite addition to the matrix provided the formation of a tribofilm, which lead to the lowest values of friction and wear. For the MoS2-composites, the formation of iron sulfides and oxidative products resulted in an increase in the friction coefficient at the highest normal loads. For the MoS2/graphite-composite, the samples exhibited an intermediate friction coefficient and the highest hardness.

Preparation of highly stable and easily regenerated sulfuretted nZVI via one-pot fast pyrolysis method for the removal of diclofenac

Sulfide-modified nanoscale zero-valent iron (S-nZVI) is a commonly used reductive agent in pollutant removal. However, the aggregation of S-nZVI can lower its reduction capacity in the water, and its regeneration remains challenging. To overcome these limitations, one-pot fast pyrolysis of FeSO4-loaded lignocellulosic biomass was proposed to obtain highly stable biochar-supported S-nZVI (S-nZVI/BC) nanoparticles. During this process, Fe2+ were reduced and sulfuretted in situ to form S-nZVI and subsequently covered by the freshly formed carbon film. A series of S-nZVI/BCs pyrolyzed at different temperatures (500–1000 °C) were prepared, among which S-nZVI/BC pyrolyzed at 800 °C (S-nZVI/BC-800) exhibited a fast and stable removal of diclofenac (DCF) via adsorption and reduction. The DCF removal efficiency of S-nZVI/BC-800 reached 80.1% within 4 h, with a particle dosage of 0.5 g/L and Fe dosage of 1 mmol/g. The adsorption and reduction efficiencies of DCF by S-nZVI/BC-800 were 43.2% and 36.9%, respectively. The improved reactivity of S-nZVI/BC-800 can be attributed to the formation of iron sulfides (i.e., FeS and Fe1−xS) in ZVI particles, which increased the electrical conductivity and improved the material selectivity for the reduction of DCF in the water. S-nZVI/BC-800 could be easily regenerated through pyrolysis, during which the carbon film could act as the electron donor. This study provides a facile and skillful method to prepare a highly stable and refreshable S-nZVI, which could be a promising permeable reactive bed material for the remediation of halogenated organic pollutants in contaminated water.

H2S Dissociation on Defective or Strained Fe (110) and Subsequent Formation of Iron Sulfides: A Density Functional Theory Study

In this paper, spin-polarized periodic density functional theory-based simulations were performed to investigate H2S adsorption and dissociation on the defective and strained Fe (110) surfaces which exist in real oil exploitation and transport. It was found that the defective surface facilitates H2S adsorption and dissociation with respect to the perfect surface, while the homogeneous external stresses, giving rise to uniform lattice expansion, show negligible effects for the strain estimated from experiment. More interestingly, the simulations predicted a correct order of phase transition at the Fe (110) surface by using the chemical potential based thermodynamic model, that is, from the elementary Fe crystal to the FeS crystal to the FeS2 crystal to the elementary S crystal with the S coverage on the Fe (110) surface increasing.

Mechanisms of Pyrite Formation Promoted by Sulfate-Reducing Bacteria in Pure Culture

Pyrite, or iron disulfide, is the most common sulfide mineral at Earth’s surface and is widespread through the geological record. Because sulfides are mainly produced by sulfate-reducing bacteria (SRB) in modern sedimentary environments, microorganisms are assumed to drive the formation of iron sulfides, in particular pyrite. However, the exact role played by microorganisms in pyrite formation remains unclear and, to date, the precipitation of pyrite in microbial cultures has only rarely been achieved. The present work relies on chemical monitoring, electron microscopies, X-ray diffraction and synchrotron-based spectroscopy to evaluate the formation of iron sulfides by sulfate-reducing bacteria Desulfovibrio desulfuricans as a function of the source of iron, either provided as dissolved Fe2+ or as FeIII-phosphate nanoparticles. Dissolved ferrous iron led to the formation of increasingly crystalline mackinawite (FeS) with time, encrusting bacteria cell walls, hence preventing further sulfate reduction upon day 5 and any evolution of iron sulfides into more stable phases, e.g. pyrite. In contrast, ferric phosphate was transformed into a mixture of large flattened crystals of well crystallized vivianite (Fe3(PO4)2 . 8 H2O) and a biofilm-like thin film of poorly crystallized mackinawite. Although being hosted in the iron sulfide biofilm, most cells were not encrusted. Excess-sulfide delivered by the bacteria and oxidants (such as polysulfides) promoted the evolution of mackinawite into greigite (Fe3S4) and the nucleation of pyrite spherules. These spherules were several hundreds nanometer wide and occurred within the extracellular polymeric substance (EPS) of the biofilm after only 1 month. Altogether, the present study demonstrates that the mineral assemblage induced by the metabolic activity of SRB strongly depends on the source of iron, which has strong implications for the interpretation of the presence of pyrite and vivianite in natural environments.

Preparation of nickel-iron hydroxides by microorganism corrosion for efficient oxygen evolution

Nickel–iron composites are efficient in catalyzing oxygen evolution. Here, we develop a microorganism corrosion approach to construct nickel–iron hydroxides. The anaerobic sulfate-reducing bacteria, using sulfate as the electron acceptor, play a significant role in the formation of iron sulfide decorated nickel–iron hydroxides, which exhibit excellent electrocatalytic performance for oxygen evolution. Experimental and theoretical investigations suggest that the synergistic effect between oxyhydroxides and sulfide species accounts for the high activity. This microorganism corrosion strategy not only provides efficient candidate electrocatalysts but also bridges traditional corrosion engineering and emerging electrochemical energy technologies.

Investigation of Structural Phase Conversions of an Iron-Containing Catalyst by Mossbauer Spectroscopy (Part 1)

The active form of an iron-containing catalyst, an Fe(II, III) mixed oxide, was studied by Mossbauer spectroscopy in order to identify the mechanism of formation of catalysts used for upgrading of heavy oil from Ashalchinsk field. The iron-oxide phase participates in the formation process by breaking carbon–heteroatom bonds in high-molecular-mass components (resins and asphaltenes) of heavy oil and reduces their molecular mass. Thus, the disperse iron oxides are enriched in a sulfur-containing phase. Mathematical processing of Mossbauer spectra obtained during the experiment was used to study the quantitative composition of the products of thermocatalytic action at 300°C. Magnetite (Fe3O4), greigite (Fe3S4), pyrite (FeS2), and pyrrhotite (Fe1–xS) were mainly detected in the isolated catalysts. The magnetite content decreased toward formation of iron sulfides (pyrite and pyrrhotite). Hydrogen sulfide and hydrocarbons were formed as a result of cracking, hydrolysis, and hydrogenolysis reactions at 300°C.

The formation of iron sulphide on oxidised pyrite during sulphidisation and subsequent interactions with xanthate

Pyrite can be partially or completely oxidised during the stockpiling of pyritic ores, leading to a poor flotation performance. Sulphidisation with potential control has been commonly practised to convert the oxidised pyrite surface into a sulphide form before flotation. However, the lack of agreement on the suitable potential for the formation of iron sulphide species on the oxidised pyrite surface makes this practice genuinely uncertain. Using electrochemical techniques, this work identified that it was only at a potential below −300 mV that the sulphidisation with Na2S could generate iron sulphide species on the oxidised pyrite surface. In order to have the newly sulphidised pyrite surface to react with xanthate, the potential had to be elevated to 100 mV, but the newly formed iron sulphide species started to oxidise at a potential of around −50 mV. These findings provide a fundamental understanding of the nature of sulphidisation of oxidised sulphide minerals and a guideline to improve their flotation.

Hydrogen sulfide removal from geothermal fluids by Fe(III)-based additives

A new method to remove hydrogen sulfide from geothermal fluids during well operation was tested in situ at a geothermal site in Vienna (Austria). For this purpose, ferric iron was added either as granulated iron hydroxide or as FeCl3 solution into a reaction vessel containing the thermal water directly removed from the wells. From the container, the water would be pumped through a particle filter. Physicochemical parameters as well as sulfide were measured constantly over time before and after the filter. It was found that the sulfide was fully removed from the water by both iron additives. While the addition of FeCl3 led first to the formation of black iron(II) sulfide (FeS), which subsequently oxidized in presence of oxygen to Fe(III) hydroxide, no visible change of the granulated iron hydroxide was observed. The reaction time was longer when using the Fe(III) hydroxide additive as compared to the FeCl3 (completed in less than 20 min) but could be enhanced by increasing the amount of added particles. In all experiments the pH was constantly rising during the reaction from about 6.3 to 7.5, which was explained by loss of protons due to purging out of the gaseous H2S. The redox value, which was measured over time, remained rather constant after addition of granulated iron hydroxide (about −350 mV), but strongly increased from −350 mV to −50 mV after adding the FeCl3 suggesting a strong electron-consuming reaction. This can be explained by a two-step reaction: first, the Fe(III) was reduced to Fe(II) by oxidation of either sulfide or thiosulfate to sulfate. Afterward, the Fe(II) oxidized again by dissolved oxygen forming orange Fe(III) hydroxides. The application of the investigated method during operation of geothermal wells could prevent H2S-induced corrosion and would eliminate the toxic effects of this gas.