Oxidation Of FeS Research Articles

Fabrication and characterization of foamed ceramics with a hierarchical pore structure from pyrite‐rich cyanide tailing

AbstractIn this study, foamed ceramics with hierarchical closed‐cell pore structures (HPS‐FC) were successfully prepared using pyrite‐rich cyanide tailing (CT) and fly ash. The formation mechanism of the hierarchical pore structure was analyzed by investigating the influence of the heating rate on both the pore structure and the physical properties of foamed ceramics. The results demonstrate that CT is suitable for the preparation of foamed ceramics due to its low softening temperature and self‐expansion capability at high temperatures. Interestingly, the heating rate significantly affects the pore structure of foamed ceramics prepared from CT. As the heating rate increases from 1 to 4°C/min, a hierarchical pore structure emerges, characterized by gradual alterations in bulk density, compressive strength, and thermal conductivity within the foamed ceramics. Whereas, excessive heating rates (≥6°C/min) result in the generation of abnormally large pores, which deteriorate the mechanical properties of foamed ceramics. The oxidation of FeS2 from CT consumes a large amount of oxygen during heating, which is the primary reason for the formation of the hierarchical pore structure. Therefore, HPS‐FC can be produced from CT by adjusting the heating rate, which combines high strength with good thermal insulation and demonstrates promising applications in prefabricated buildings.

Suppressing Surface Oxidation of Pyrite FeS2 by Cobalt Doping in Lithium Sulfur Batteries.

Lithium-sulfur batteries have emerged as a promising energy storage device due to ultra-high theoretical capacity, but the slow kinetics of sulfur and polysulfide shuttle hinder the batteries' further development. Here, the 10% cobalt-doped pyrite iron disulfide electrocatalyst deposited on acetylene black as a separator coating in lithium-sulfur batteries is reported. The adsorption rate to the intermediate Li2S6 is significantly improved while surface oxidation of FeS2 is inhibited: iron oxide and sulfate, thus avoiding FeS2 electrocatalyst deactivation. The electrocatalytic activity has been evaluated in terms of electronic resistivity, lithium-ion diffusion, liquid-liquid, and liquid-solid conversion kinetics. The coin batteries exhibit ultra-long cycle life at 1C with an initial capacity of 854.7 mAh g-1 and maintained at 440.8 mAh g-1 after 920 cycles. Furthermore, the separator is applied to a laminated pouch battery with a sulfur mass of 326 mg (3.7 mg cm-2) and retained the capacity of 590 mAh g-1 at 0.1 C after 50 cycles. This work demonstrates that FeS2 electrocatalytic activity can be improved when Co-doped FeS2 suppresses surface oxidation and provides a reference for low-cost separator coating design in pouch batteries.

Oxic corrosion model for KAERI Reference disposal system via O2 consumption reactions and mixed-potential theory

The durability of copper (Cu) canisters against corrosion is critical for the licensing of deep geological repositories. Assessing oxic corrosion, a primary degradation mechanism, is essential for ensuring the reliability of such repositories. Due to the complex interactions influencing oxic corrosion, a comprehensive model is necessary for evaluating Cu canister corrosion. This study develops a model for the KAERI Reference Disposal System (KRS), incorporating mixed-potential theory with key O2 consumption reactions, including Cu corrosion, Cu(I) oxidation, FeS2 oxidation, aerobic microbial activity, and O2 dissolution and consumption. Simulation of 11 scenarios revealed that the representative KRS case would experience a maximum corrosion depth of 9.3 μm on the Cu canister after 2.3 years due to oxic corrosion, under conditions that are unfavorable for the initiation of pitting corrosion. These results suggest that oxic corrosion is not a threat to the isolation of spent nuclear fuels in KRS.

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

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

Hydrogeochemical process and coal mining-motivated effect on the hydrochemistry for the groundwater system in mining area of Western China

The Jurassic coalfield in the mining area of western China exhibits a multi-layered groundwater system. However, it is subject to an arid and semi-arid climate with limited water resources. Consequently, the ecological environment is highly vulnerable, and the chemistry and quality of groundwater may be influenced by multiple factors. This study systematically the coal mining-motivated effect on the hydrochemistry and water quality of the groundwater system, using the Caojiatan Coal Mine as a case study. The analysis incorporates a combination of the self-organizing maps (SOM), entropy-weighted water quality index (EWQI), and traditional hydrochemical analysis methods. After coal mining, there was an increase in the proportion of HCO3-Ca and HCO3-Mg in the groundwater samples of J2z, J2y4, and J2y5. The groundwater is controlled by cation exchange as a whole. The J2y4 and above groundwater is influenced by both the dissolution of carbonate and silicate rocks before coal mining. After coal mining, the Quaternary and J2a groundwater in the western wing is primarily influenced by the dissolution of carbonate rocks; the J2z, J2y4 and J2y5 groundwater is primarily governed by the dissolution of silicate rocks and the oxidation of FeS2; the J2y3 and below groundwater is primarily controlled by the dissolution of evaporate rocks. The resulting dilution effect after coal mining and the implementation of measures for the discharge of treated mine water make the groundwater quality of the J2y4 and higher aquifers tend to be better. The research findings serve as a valuable reference for promoting the sustainable development and protection of groundwater resources not only in the study area but also in other coal mines.

Hydrology and oxygen addition drive nutrients, metals, and methylmercury cycling in a hypereutrophic water supply reservoir

Impaired water quality in Mediterranean climate reservoirs is mainly associated with eutrophication and internal nutrient loading. To improve water quality in hypereutrophic Hodges Reservoir, California, United States, a hypolimnetic oxygenation system (HOS), using pure oxygen gas, was implemented in 2020. This study encompasses 3 years of pre-oxygenation data (2017–2019) and 2 years of post-oxygenation data (2020–2021) to understand the cycling of nutrients, metals, and mercury in the reservoir. During the wet year of 2017, mildly reduced conditions lasted until mid-summer in the enlarged reservoir. Nutrients and metals were seen in the hypolimnion including ammonia (~2 mg-N/L), manganese (~0.5 mg/L), phosphate (~0.5 mg-P/L), and sulfide (~10 mg/L). Production of methylmercury (MeHg), an important bioaccumulative toxin, was favored from April to June with a hypolimnetic accumulation rate of around 200 ng/m2·d. In contrast, the dry year of 2018 exhibited higher hypolimnetic concentrations of ammonia (~4 mg-N/L), manganese (~1 mg/L), phosphate (>0.5 mg-P/L), and sulfide (>15 mg/L). The rapid onset of highly reduced conditions in 2018 corresponded with low MeHg hypolimnetic accumulation (~50 ng/m2·d). It seems that mildly reduced conditions were associated with higher MeHg accumulation, while sulfidic, reduced conditions impaired inorganic mercury bioavailability for MeHg production and/or promoted microbial demethylation. Sulfide also appeared to act as a sink for iron via FeS precipitation, and potentially for manganese via MnS precipitation or manganese coprecipitation with FeS. Mass flux estimates for 2017–2019 indicate that much of the nutrients that accumulated in the hypolimnion moved via turbulent diffusion into the epilimnion at loading rates far exceeding thresholds predicting eutrophic conditions. After oxygenation in 2020–2021, the reservoir water column was highly oxidized but showed a lack of thermal stratification, suggesting reservoir operations in combination with HOS implementation inadvertently mixed the water column in this relatively shallow reservoir. Post-oxygenation, concentrations of ammonia, phosphate, manganese, and mercury in bottom waters all decreased, likely in response to oxidized conditions. Oxygenated bottom waters exhibited elevated nitrate, a byproduct of ammonia nitrification, and iron, a byproduct of FeS oxidation, indicating a lake-wide response to oxygenation.

An effective method for recovering ultrafine SnO2, MgSn(OH)6, and Zn from complex iron tailings

The generation of complex tailings from iron mining is continuously increasing, reaching a reserve of 500 million tons. The iron tailings mainly contain 0.10–0.50 wt% ultrafine SnO2 (−9.6 μm, >50 %), acid-solule Sn (MgSn(OH)6) and 0.20–1.50 wt% Zn and are recognized as an important secondary Sn resource. Unfortunately, traditional mineral processing techniques have a low Sn recovery ratio. In this study, a novel method is proposed for effectively recovering Sn and Zn from complex iron tailings through reduction-sulfurization roasting, and their transformation behavior is determined. The results reveal that MgSn(OH)6 is first transformed into MgSnO3 at 1073 K, and SnO2 is reduced to SnO(g) and then sulfurized to SnS(g). COS acts as the main curing agent during the roasting process, which originates from the oxidation of FeS. As the temperature increases from 1173 to 1273 K, part of SnO2 can combine with CaO to form Ca2SnO4, and the generated CaSnO3 and MgSnO3 are further reduced to Sn(l) and Fe–Sn alloy. Meanwhile, ZnO and ZnS can be efficiently converted to Zn(g). Interestingly, the Fe–Sn alloy cannot be removed due to low Sn activity (a[Sn]). However, the Sn recovery ratio increases to 90 % due to low SnS partial pressure (PSnS) at 1373 K. Under optimal conditions, 90.10 wt% Sn and 94.90 wt% Zn can be extracted in the dust, which can be further processed into higher value-added Sn concentrate. Compared with current beneficiation process, the recovery ratio of SnO2 increased by 20 %. More importantly, MgSn(OH)6 can be efficiently recovered. This is of great significance for sustainable metallurgy of Sn.

Application of sodium alginate/iron sulfide/biochar beads as electron donors to enhance nitrate removal from carbon-limited wastewater

Efficient nitrate-nitrogen (NO3-N) removal through solid organic carbon-based heterotrophic denitrification is challenging because of the unstable organic carbon release and low electron utilization rate. A functional biocarrier (alginate/iron sulfide/biochar beads (SA/FeS/BC)), integrating heterotrophic and autotrophic denitrification, was prepared using the cross-linking method. SA/FeS/BC achieved high stability and efficiency in reducing NO3-N (98.48 ± 0.56 %) and maintained an average chemical oxygen demand (COD) and a sulfate (SO42–) concentration of 50.39 ± 36.30 and 146.45 ± 1.09 mg/L, respectively, meeting the Chinese Quality Standard for Ground Water (GB/T 14848–2017). Microbial community analysis indicated that FeS and BC induced a change in the dominant genera from Flavobacterium to Thiobacillus, and enriched genes involved in electron transfer, FeS oxidation, and denitrification. This indicated that SA/FeS/BC strengthened autotrophic denitrification and enhanced electron transport activity for NO3-N removal. Furthermore, this study provided new insights into the strengthening mechanisms for NO3-N removal in the SA/FeS/BC-based mixotrophic denitrification system. The findings of this study indicated that SA/FeS/BC with integrated electron donors could enhance NO3-N removal efficiency and electron utilization rate, offering a potential application for effective carbon-limited wastewater treatment.

Dramatically enhanced phenol degradation upon FeS oxygenation by low-molecular-weight organic acids

Oxidizing potential of FeS for organic contaminants degradation due to hydroxyl radicals (•OH) production has been recently documented, but the oxidizing efficiency was limited. Here, we revealed that low-molecular-weight organic acids (LMWOAs) can immensely enhance phenol degradation during FeS oxygenation due to increased utilization efficiency of FeS electron for •OH production. Upon oxygenation of 0.5 g/L FeS, phenol degradation boosted from 7.1% without LMWOAs to 91.5%, 84.6% and 95.0% with the addition of 1 mM oxalate, citrate and EDTA, respectively. Electron utilization efficiency of Fe(II) for •OH production dramatically rose from 0.3% with FeS alone to respective 2.0%, 2.5% and 2.7% in the LMWOAs systems. An increase in oxalate concentrations benefited •OH formation and phenol degradation. Coexisting oxalate led to an additional •OH production pathway from Fe(II)-oxalate oxidation, which expanded the O2 reduction to H2O2 from a two- to one-electron transfer process. Meanwhile, electron transfer from FeS to dissolved Fe(III)-oxalate promoted the redox cycling of Fe(III)/Fe(II), thus supplying the Fe(II) oxidation for •OH production. Moreover, the presence of oxalate decreased the crystallinity and particles size of lepidocrocite generated from FeS oxidation. Consequently, this study shed lights on the LMWOAs-enhanced contaminant degradation in either natural or engineered FeS oxidation systems.

Effect of Gas-Phase Passivation Technology on Spontaneous Combustion Characteristic Stage and Reaction Kinetics of FeS During Particle Size Fragmentation

ABSTRACT To investigate the mechanism of inhibition of spontaneous combustion of reactive FeS by gas-phase passivation, this paper used liquid-phase synthesis to prepare reactive FeS which were divided into five particle size ranges by crushing and grinding and then passivated those by low-oxygen gas. This paper analyzed the effect of gas-phase passivation on the characteristic parameters of the characteristic stages of crushed FeS from the perspective of thermodynamics, and used the KAS and FWO methods to calculate the apparent activation energy of the different characteristic stages of the FeS oxidation heating process. Inhibition mechanism of FeS spontaneous combustion reaction kinetics during particle size fragmentation by gas-phase passivation was revealed. The results indicate that in the spontaneous combustion stage at room temperature (RT), the exothermic heat of passivated FeS was significantly reduced to below 5 J/g, and the apparent activation energy rose up to 116 kJ/mol, which was significantly increased by about 50% compared to the activated FeS. The essence of gas-phase passivation is that the low oxygen concentration gas reacts with FeS to produce a passivation protective film, blocking the reaction and pushing the oxidation process backwards as a whole. In the weight gain stage of oxygen absorption, the weight gain of passivation FeS is reduced to less than 2.2%, and the heat release is reduced to 2.8J/g. The particle size of the passivated FeS decreased, and the apparent activation energy increased by 20 kJ/mol in the decomposition and combustion stage. With the decrease of FeS particle size, the gas-phase passivation is more effective and the inhibition effect of spontaneous combustion of FeS is more obvious.

The effect of FeS on the fate of Cr(VI) in the presence of organic matters under dynamic anoxic/oxic conditions.

The reduction of Cr(VI) by FeS under anoxic conditions has been studied extensively. However, when the redox environments alternate from anoxic to oxic, the effect of FeS on the fate of Cr(VI) in the presence of organic matters still remains unknown. Therefore, this study investigated the effect of FeS coupled with humic acids (HA) and alga on the transformation of Cr(VI) under dynamic anoxic/oxic conditions. HA could promote the reduction of Cr(VI) from 86.6% to 100% under anoxic conditions due to the enhancement of the dissolution and dispersibility of FeS particles by HA. However, the strong complexing and oxidizing properties of alga inhibited the reduction of FeS. Reactive oxygen species (ROS) generated from FeS oxidation under oxic conditions could oxidize 3.80μM of Cr(III) to aqueous Cr(VI) at pH 5.0, while aqueous Cr(VI) reached to 4.83μM in the presence of HA, which was ascribed to the increasing amount of free radicals. In addition, acidic conditions and excess FeS would increase strong reducing Fe(II) and S(-II) species, and improve the efficiency of Fenton reaction. The findings provided new insights into the fate of Cr(VI) in aquatic systems containing FeS and organic matters under dynamic anoxic/oxic conditions.

Sulfur oxidation process: A neglected contributor to minimize P release during sediment microbial fuel cell operation

• SMFC was an effective approach to minimize P release to overlying water. • SMFC was conducted to study the effect of S oxidation on P release from sediment. • FeS of sediment was decreased and SO 4 2− of pore water was increased in SMFC group. • Sulfur-oxidizing bacteria were dominant in the SMFC group. • S oxidation process was a neglected contributor to minimizing P release with SMFC. In this study, long-term operation (300 days) of sediment microbial fuel cell (SMFC) was conducted to investigate the effect of sulfur oxidation on phosphorus (P) release from sediment. The result described that the released P to the water column of SMFC group was much lower during the whole operated time, suggesting SMFC was an effective approach to inhibit endogenous P release from sediment. FeS in the sediment was decreased in the vicinity of anode and SO 4 2− concentration was accumulated in the pore water from day 160 onward, suggesting the occurrence of FeS oxidation process. At the same time, part of dissolved Fe 2+ was upward-diffused and oxidized to Fe(III) oxides, followed by adsorption P, rendering higher BD-P buildup in the surface sediment (612.50 ± 32.25 μg/g). Further, the potential of sulfur-oxidizing bacteria, Tumebacillus (7.31%) and family Desulfobulbaceae (3.06%), were dominant, confirming bio-sulfur oxidation process in the SMFC group. In conclusion, sulfur oxidation process was a significant contributor to minimize P release from sediment in SMFC.

Microbiologically Influenced Corrosion of Q235 Carbon Steel by Ectothiorhodospira sp.

The biological sulfur cycle is closely related to iron corrosion in the natural environment. The effect of the sulfur-oxidising bacterium Ectothiorhodospira sp., named PHS-Q, on the metal corrosion behaviour rarely has been investigated. In this study, the corrosion mechanism of Q235 carbon steel in a PHS-Q-inoculated medium is discussed via the characterization of the morphology and the composition of the corrosion products, the measurement of local corrosion and the investigation of its electrochemical behaviour. The results suggested that, initially, PHS-Q assimilates sulfate to produce H2S directly or indirectly in the medium without sulfide. H2S reacts with Fe2+ to form an inert film on the coupon surface. Then, in localised areas, bacteria adhere to the reaction product and use the oxidation of FeS as a hydrogen donor. This process leads to a large cathode and a small anode, which incurs pitting corrosion. Consequently, the effect of PHS-Q on carbon steel corrosion behaviour is crucial in an anaerobic environment.

Multi-catalysis of glow discharge plasma coupled with FeS2 for synergistic removal of antibiotic

Fe-based composites improved the energy utilization efficiency of plasma for removing contaminants through multi-catalysis have received much attention. However, the energy efficiency and catalytic activity are compromised by the slow transformation from Fe (Ⅲ) to Fe (Ⅱ). Here, given the electron-donating ability of reducing sulfur species, as well as the acidic environment generated by FeS2, single FeS2 was introduced into the glow discharge plasma (GDP) reactor for the removal of tylosin (TYL). The results showed that a significant synergistic effect between FeS2 and GDP improved the energy efficiency of plasma and the removal efficiency of TYL (99.7%). FeS2 boosted the generation of radicals (·OH, ·O2−) and nonradicals (h+, e−) rather than H2O2 and O3, which played an important role in TYL abatement. Moreover, the electrons donating sulfur and iron species from FeS2 can accelerate the conversion of Fe(III) to Fe(II), which was conducive to the generation of radicals. Besides, acid solution self-adjustment resulted from the oxidation of FeS2 improved heterogeneous Fenton reaction, the oxidation potential of ·OH and adsorption of positive charged TYL. The plausible degradation pathways of TYL were proposed in GDP/FeS2 system. In summary, enhanced removal of TYL was mainly attributed to the catalytic pathway altered by FeS2 through high-energy electrons, photocatalysis, heterogeneous Fenton and O3 catalysis in the GDP system simultaneously. The strategy of integrating GDP with FeS2 proposed in this work is expected to offer a feasible and potential technique for organic wastewater treatment.

Secondary Mineral Formation and Carbon Dynamics during FeS Oxidation in the Presence of Dissolved Organic Matter.

Iron (Fe) minerals constitute a major control on organic carbon (OC) storage in soils and sediments. While previous research has mainly targeted Fe (oxyhydr)oxides, the impact of Fe sulfides and their subsequent oxidation on OC dynamics remains unresolved in redox-fluctuating environments. Here, we investigated the impact of dissolved organic matter (DOM) on FeS oxidation and how FeS and its oxidation may alter the retention and nature of DOM. After the anoxic reaction of DOM with FeS, FeS preferentially removed high-molecular-weight and nitrogen-rich compounds and promoted the formation of aqueous sulfurized organic molecules, according to Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis. When exposed to O2, FeS oxidized to nanocrystalline lepidocrocite and additional aqueous sulfurized organic compounds were generated. The presence of DOM decreased the particle size of the resulting nano-lepidocrocite based on Mössbauer spectroscopy. Following FeS oxidation, most solid-phase OC remained associated with the newly formed lepidocrocite via a monodentate chelating mechanism (based on FTIR analysis), and FeS oxidation caused only a slight increase in the solubilization of solid-phase OC. Collectively, this work highlights the under-appreciated role of Fe sulfides and their oxidation in driving OC transformation and preservation.

Electrolysis-integrated constructed wetland with pyrite filler for simultaneous enhanced phosphorus and nitrogen removal

Constructed wetlands (CWs) have been widely applied for tail water treatment. However, conventional CWs usually encounter poor phosphorus and nitrogen removal performance due to limited adsorption capacity of fillers and insufficient electron donors. In this study, an electrolysis-integrated CW using pyrite as filler surrounding electrodes (e-PCW) was developed. After around 140 days’ operation with various voltages (5, 10 and 15 V), synergistic effects of pyrite filler and electrolysis process were observed. The optimal removal efficiencies of NH4+-N, TN and TP achieved at 5 V were 42.6 ± 3.5%, 68.2 ± 1.8% and 75.8 ± 2.5%, respectively. O2 generated at anode in e-PCW promoted oxidation of FeS2 in pyrite to release Fe3+, which could form FePO4 for phosphorus removal. At cathode, the produced H2 together with FeS2 could serve as electron donors, driving autotrophic denitrification to improve nitrogen removal. The results also showed that inputting 10 and 15 V voltages caused acidification in anodic area due to water electrolysis. Although minerals like CaMg(CO3)2 in pyrite could partially neutralize acidification, low pH (5–6) still inhibited nitrification process and led to desorption of phosphorus from fillers. Based on microbial community analysis, main functional bacteria were Desulfomicrobium (13.43%) and Thauera (4.66%) in cathodic area of e-PCW, which could reduce SO42− to S2− and afterwards drive sulfur-based autotrophic denitrification respectively. This work provided a novel and competitive technology for advanced wastewater treatment.

Free Radicals Produced from the Oxidation of Ferrous Sulfides Promote the Remobilization of Cadmium in Paddy Soils During Drainage.

Most of the cadmium (Cd) accumulated in rice grains is derived from its remobilization in soils during the grain filling period when paddy water is drained. The factors affecting Cd remobilization upon drainage remain poorly understood. Here, we show that the free radical effect produced from the oxidation of ferrous sulfides is an important mechanism affecting the oxidative remobilization of Cd during soil drainage. When soils were flooded, microbial sulfate reduction results in the formation of various metal sulfides including CdS and FeS. Upon soil drainage, the oxidation of FeS produced considerable amounts of hydroxyl free radicals (OH•), which could oxidize CdS directly and thereby promote the oxidative dissolution of CdS and increase Cd mobilization in soils. FeS and CdS could also form a within-sulfide voltaic cell, with FeS protecting the oxidative dissolution of CdS due to the lower electrochemical potential of the former. However, this voltaic effect was short-lived and was surpassed by the free radical effect. The amounts and composition of metal sulfides formed during soil flooding vary with soils, and the oxidative dissolution of CdS is affected by both the free radical and voltaic effects offered by different metal sulfides. These effects are also applicable to the biogeochemistry of other chalcophile trace elements coupled with sulfur and iron redox cycles during the anoxic-oxic transition in many environments.

Using Isotopic and Hydrochemical Indicators to Identify Sources of Sulfate in Karst Groundwater of the Niangziguan Spring Field, China

Karst groundwater in the Niangziguan spring fields is the main source to supply domestic and industrial water demands in Yangquan City, China. However, the safety of water supply in this region has recently suffered from deteriorating quality levels. Therefore, identifying pollution sources and causes is crucial for maintaining a reliable water supply. In this study, a systematic sample collection for the karst groundwater in the Niangziguan spring fields was implemented to identify hydrochemical characteristics of the karst groundwater through comprehensive analyses of hydrochemistry (piper diagram, and ion ratios,) and stable isotopes (S and H-O). The results show that the karst groundwater in the Niangziguan spring fields was categorized as SO4·HCO3-Ca·Mg, HCO3·SO4-Ca·Mg, and SO4-Ca types. K+, Cl-, and Na+ are mainly sourced from urban sewage and coal mine drainage. In addition, SO42− was mainly supplied by the dissolution of gypsum and the oxidation of FeS2 in coal-bearing strata. It is noteworthy that, based on H-O and S isotopes, 75% of the karst groundwater was contaminated by acidic water in coal mines at different degrees. In the groundwater of the Niangziguan spring field, the proportions of SO42− derived from FeS2 oxidation were 60.6% (N50, Chengxi spring), 30.3% (N51, Wulong spring), and 26.0% (N52, Four springs mixed with water). Acid mine drainage directly recharges and pollutes karst groundwater through faults or abandoned boreholes, or discharges to rivers, and indirectly pollutes karst groundwater through river infiltration in carbonate exposed areas. The main source of rapid increase of sulfate in karst groundwater is acid water from abandoned coal mines.

Investigation of the mechanism for fireside corrosion in coal-fired boilers in South Africa

SYNOPSIS Furnace wall tubes from 600 MW subcritical boilers at three coal-fired power stations were sampled and the fireside deposits examined to determine the mechanism of fireside corrosion. This involved an in-depth investigation into the morphology and composition of the fireside deposits and the conditions of the furnace that enable this type of attack. SEM-EDS analysis revealed high concentrations of oxygen, iron, and sulphur, QEMSCAN and XRD analyses identified the presence of Fe3O4, Fe2O3, FeS, and FeS2. Differential thermal analysis showed thermal activities at temperatures of 500-600°C, 900-1100°C, and 1100-1250°C, which are associated, respectively, with FeS2 oxidation to FeS and Fe2O3, at 475-525°C, formation of aluminosilicates at 925-1100°C, and melting of FeS around 1190°C. The absence of sodium and potassium eliminates the contribution of molten alkali sulphates to the corrosion. The consistent coexistence of iron sulphide and iron oxide is indicative of the substoichiometric conditions in the furnace, while the detection of pyrite suggests that the coal is not completely combusted, which points to a poor combustion process. These observations were affirmed by gas analysis at one of the stations, where very high levels of carbon monoxide were measured at the furnace wall (> 14 000 ppm) and furnace exit (> 3500 ppm). The high CO concentrations are indicative of limited combustion caused by limited O2. These reducing conditions promote the formation of FeS-rich deposit, which is the corrosive species responsible for degradation. Keywords: fireside corrosion, sulphidation, coal-fired boiler, furnace wall tubes.

Electrochemically induced metal- vs. ligand-based redox changes in mackinawite: identification of a Fe3+- and polysulfide-containing intermediate.

Under anaerobic conditions, ferrous iron reacts with sulfide producing FeS, which can then undergo a temperature, redox potential, and pH dependent maturation process resulting in the formation of oxidized mineral phases, such as greigite or pyrite. A greater understanding of this maturation process holds promise for the development of iron-sulfide catalysts, which are known to promote diverse chemical reactions, such as H+, CO2 and NO3- reduction processes. Hampering the full realization of the catalytic potential of FeS, however, is an incomplete knowledge of the molecular and redox processess ocurring between mineral and nanoparticulate phases. Here, we investigated the chemical properties of iron-sulfide by cyclic voltammetry, Raman and X-ray absorption spectroscopic techniques. Tracing oxidative maturation pathways by varying electrode potential, nanoparticulate n(Fe2+S2-)(s) was found to oxidize to a Fe3+ containing FeS phase at -0.5 V vs. Ag/AgCl (pH = 7). In a subsequent oxidation, polysulfides are proposed to give a material that is composed of Fe2+, Fe3+, S2- and polysulfide (Sn2-) species, with its composition described as Fe2+1-3xFe3+2xS2-1-y(Sn2-)y. Thermodynamic properties of model compounds calculated by density functional theory indicate that ligand oxidation occurs in conjunction with structural rearrangements, whereas metal oxidation may occur prior to structural rearrangement. These findings together point to the existence of a metastable FeS phase located at the junction of a metal-based oxidation path between FeS and greigite (Fe2+Fe3+2S2-4) and a ligand-based oxidation path between FeS and pyrite (Fe2+(S2)2-).