A magnetite-driven cryptic iron cycle

Abstract. The bulk magnetic mineral record from Lake Ohrid, spanning the past 637 kyr, reflects large-scale shifts in hydrological conditions, and, superimposed, a strong signal of environmental conditions on glacial–interglacial and millennial timescales. A shift in the formation of early diagenetic ferrimagnetic iron sulfides to siderites is observed around 320 ka. This change is probably associated with variable availability of sulfide in the pore water. We propose that sulfate concentrations were significantly higher before ∼ 320 ka, due to either a higher sulfate flux or lower dilution of lake sulfate due to a smaller water volume. Diagenetic iron minerals appear more abundant during glacials, which are generally characterized by higher Fe / Ca ratios in the sediments. While in the lower part of the core the ferrimagnetic sulfide signal overprints the primary detrital magnetic signal, the upper part of the core is dominated by variable proportions of high- to low-coercivity iron oxides. Glacial sediments are characterized by high concentration of high-coercivity magnetic minerals (hematite, goethite), which relate to enhanced erosion of soils that had formed during preceding interglacials. Superimposed on the glacial–interglacial behavior are millennial-scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Like the processes on glacial–interglacial timescales, low summer insolation and a retreat in vegetation resulted in enhanced erosion of soil material. Our study highlights that rock-magnetic studies, in concert with geochemical and sedimentological investigations, provide a multi-level contribution to environmental reconstructions, since the magnetic properties can mirror both environmental conditions on land and intra-lake processes.

Porewater dynamics and the formation of iron sulfides were studied in the Brazilian mangrove. Porewater samples were collected during tidal cycles using in situ equipment and physical–chemical and chemical data were obtained. The advective transport of porewater by tidal currents was found to be more effective between 0 and 5 cm depth than below 15 cm. The diffusive input of atmospheric oxygen during low tides resulted in oxidation of porewater dissolved sulfides. Thermodynamic considerations identified pyrite as the main iron sulfide forming at this depth, and the amorphous iron hydroxides are probably the main iron source. Below 15 cm depth, irregular root distribution underground and benthic faunal bioturbation caused considerable spatial variation in a small scale (20 cm). The correlation between alkalinity and total dissolved sulfide suggests that the main pathway for organic matter decomposition is sulfate reduction. Formed in the highly sulfidic diagenetic zone, the dissolved iron went through iron sulfide formation process. Crystalline iron oxides like hematite and magnetite may constitute an important iron source below 15 cm, whereas pyrite is still the most probably occurring iron sulfide, according to thermodynamic considerations.

Description of Paper This paper discusses various mechanisms that can lead to the formation of iron sulfide scale downhole, techniques that can be used to prevent its formation and methods to remove it. Iron sulfide scale is present in oil and gas producing wells, water injection and supply wells. There are various mechanisms that can lead to the formation of iron sulfide. However, all of these mechanisms require sources of hydrogen sulfide and iron. Hydrogen sulfide can result from sulfate reducing bacteria, thermal decomposition of sulfate, or being introduced into the well as in gas lift operations. Iron can be produced from the formation, especially sandstone reservoirs and is also present downhole as a result of various corrosion processes. Combination of hydrogen sulfide and iron will cause formation of various iron sulfide species. The ratio of iron to sulfide in these species depends on temperature, pressure, pH, and hydrogen sulfide concentration. This ratio plays a key role in determining the best method to remove iron sulfide scales. Hydrochloric acid can be used to dissolve iron sulfide species that contain iron and sulfur at a molar ratio close to unity. Non-acid formulae can be used to remove iron sulfide scale, however, their ability to dissolve iron sulfide depends on the molar ratio of iron to sulfide. To prevent the formation of iron sulfide, squeeze treatments to the formation were found to be very effective. This paper discusses various mechanisms that can lead to the formation of iron sulfide, chemical and mechanical methods to remove it and chemical squeeze treatments to prevent its formation and/or deposition. Results, Conclusions Extensive field work was conducted to identify the type of iron sulfide scale present, and the mechanisms that lead to its formation. Iron sulfide species were present in gas, oil and water supply wells. The chemical and physical characteristics of iron sulfide scale were found to be a function of temperature, pressure, pH and the age of the scale. Other properties of the scale, density and thickness, were found to vary with the scale depth and age. Various mechanical and chemical treatments to remove iron sulfide scale were examined in detail. Advantages and disadvantages of each method were identified. The best method to deal with iron sulfide scale is to avoid its formation in the first place. Chemical squeeze treatments were found to be effective in this regard. Once iron sulfide scale is formed, then it is recommended to remove the scale using acid washes with appropriate additives. Mechanical means are recommended for old iron sulfide scale, which has low acid solubility. Area of Interest Iron sulfide scale is present in sour oil and gas wells and injectors that are contaminated with sulfate reducing bacteria (SRB). It enhances the corrosion rate of the downhole tubulars, and adversely affects the performance of various wells. It reduces the efficiency of oil-water separation in various GOSPs. Removing iron sulfide scale is a complex process, especially at downhole conditions. Optimizing this process will require full understanding of various chemical interactions.

Formation of iron sulfide at faecal pellets and other microniches within suboxic surface sediment

Diagenetic dissolution, maghemitization and sulphidization of magnetic minerals in rapidly deposited gas hydrate bearing sediments from the Bay of Bengal

Magnetic properties of gas hydrate-bearing sediments and their association with iron geochemistry in the Sea of Marmara, Turkey

Pyrite, or iron disulfide, is the most common sulfide mineral on the 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 microscopy, X-ray diffraction, and synchrotron-based spectroscopy to evaluate the formation of iron sulfides by the 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 bacterial 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⋅8H2O) 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 of nanometers 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 sulfate-reducing bacteria strongly depends on the source of iron, which has strong implications for the interpretation of the presence of pyrite and vivianite in natural environments.

Sulfide–iron interactions in domestic wastewater from a gravity sewer

Inorganic sulfur cycles in sediments of the Pearl River Estuary: Processes, mechanisms, and isotopic indicators

Usefulness of In-Situ Synchrotron Study on Scale Formation during CO 2 Corrosion of Mild Steel: A Review

This paper presents the results of complex lithological, mineralogical, and geochemical studies of bottom sediments of deep-water basins of the Caspian Sea (Derbent and South Caspian Basins) in areas contaminated by hydrogen sulfide. In the course of complex studies, numerous manifestations of authigenic mineral formation associated with the stage of early diagenesis have been established. Authigenic minerals belonging to the groups of sulfates (gypsum, barite), chlorides (halite), carbonates (calcite, low Mg-calcite; kutnohorite), and sulfides (framboidal pyrite), as well as their forms and composition, have been identified by a complex of analytical methods (X-ray diffractometry (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS); atomic absorption spectroscopy (AAS); coulometric titration (CT)); the nature of their distribution in bottom sediments has been assessed. Carbonates and sulfates are predominant authigenic minerals in the deep-water basins of the Caspian Sea. As a part of the study, differences have been established in the composition and distribution of associations of authigenic minerals in the bottom sediments in the deep-water basins. These are mineral associations characteristic of the uppermost part of the sediments (interval 0–3 cm) and underlying sediments. In the Derbent Basin, in sediments of the interval 3–46 cm, an authigenic association is formed from gypsum, calcite, magnesian calcite, siderite, and framboidal pyrite. An association of such authigenic minerals as gypsum and calcite is formed in sediments of the 0–3 cm interval. In the South Caspian Basin, in sediments of the interval 3–35 cm, an association of such authigenic minerals as gypsum, halite, calcite, magnesian calcite, and framboidal pyrite is formed. The association of such authigenic minerals as gypsum, halite, calcite, magnesian calcite, kutnohorite, and framboidal pyrite is characteristic of sediments of the 0–3 cm interval. We consider the aridity of the climate in the South Caspian region to be the main factor that determines the appearance of such differences in the uppermost layer of sediments of the basins. Judging by the change in the composition of authigenic associations, the aridity of the South Caspian increased sharply by the time of the accumulation of the upper layer of sediments (interval 0–3 cm). Taking into account lithological, mineralogical and geochemical data, the features of the processes of authigenic mineral formation in the deep-water basins of the Caspian Sea under conditions of hydrogen sulfide contamination have been determined. Analysis of the results obtained and published data on the conditions of sedimentation in the Caspian Sea showed that hydrogen sulfide contamination recorded in the bottom layer of the water column of the deep-water basins of the Caspian Sea may affect the formation of authigenic sulfides (framboidal pyrite), sulfates (gypsum), and carbonates (calcite and kutnohorite) associated with the activity of sulfate-reducing bacteria in reducing conditions.

The formation of iron(II) sulfides in aqueous solutions

Iron sulfide, as one of the main products of sour corrosion in oil and gas production systems, has become a focal point for flow assurance research. The formation of iron sulfide can cause many production problems such as the malfunction of downhole devices which can lead to a significant decline in oil production. Once iron sulfide forms in the production system, it is difficult or impossible to remove chemically and costly to remove physically. Accurate prediction models for iron sulfide formation at reservoir conditions are currently lacking in the industry and are necessary to help control scale and improve flow assurance. Solubility product (Ksp) of iron sulfide is the key parameter to make accurate scale predictions. However, research towards iron sulfide including precipitation, dissolution, inhibition, and removal are notoriously difficult not only due to the complexity of iron sulfide phases and their transitions but also due to the involvement of hydrogen sulfide in the gas phase. Tomson Technologies has developed new technologies to simulate realistic field downhole conditions for scale research. A reliable flow-through apparatus has been customized to perform mineral solubility studies under xHPHT (up to 1720 bar and 250 °C). In order to simulate the strictly anoxic environment and prevent dissolved ferrous iron from oxidizing, dissolved oxygen in the test solutions has been reduced to far less than 1 ppb. This paper is the first to examine the solubility of iron sulfide under these realistic downhole conditions with temperature up to 250 °C, pressure up to 1720 bar in 1M and 3M ionic strength solutions, under a strictly anoxic environment (<< 1 ppb dissolved oxygen). Under the HPHT and high salinity conditions studied, iron sulfide tends to form pyrrhotite (Fe1-xS) and troilite (FeSt) phases instead of mackinawite, the metastable phase (FeSm), which is most common at lower temperatures. Phase transition between pyrrhotite and troilite at elevated temperatures was observed during the solubility experiments. Solubility of iron sulfide decreases with increasing temperature and increases with increasing pressure which is consistent to previous reported results (Kharaka, et al., 1988). Experimental details and major findings from this research will be discussed.

The protective effect of H2S on steel corrosion arises from the formation of iron sulfide (FeS) passive films on steel surfaces. Various iron sulfides with different crystal structures, including mackinawite, cubic ferrous sulfide, pyrrhotite, and greigite, can develop as corrosion products for steel. The effect of those scales on corrosion damage is intricately linked to their physicochemical attributes and morphology. As multiphase streams move through pipelines, interplaying physical and chemical factors can lead to localized pitting attacks, especially where the iron sulfide film shows a weak crystalline structure. In this study, corrosion tests were performed under different H2S field environments. Scanning electron microscopy (SEM) and atomistic modeling were employed to understand the formation and disruption of protective scales at the atomic level.

We propose that iron sulfides can be formed in the first bowl of a commercial wool scour. Under the low redox conditions recorded, ferric iron, a ubiquitous component of minerals on the fleece, is dissolved reductively to the ferrous form, which reacts with sulfide present either as residual depilatory on slipe wool or from degraded wool protein. The black iron sulfide may deposit on the scoured wool, making it dull and gray. The observation that some scoured wools become brighter and more yellow with time is consistent with the expected behavior of deposited iron sulfides, which slowly oxidize in air.