Enrichment Of Cr Research Articles

The spontaneous passivation of (CoFeNi)(1-x)/3Crx alloys in sulfuric acid

The influence of at% Cr on the spontaneous passivation of (CoFeNi)(1-x)/3Crx alloys in 0.1 M H2SO4 was investigated. During spontaneous passivation, Cr enrichment occurs due to the selective dissolution of the matrix elements. The quantity of Cr enrichment is relatively constant with at% Cr while the total quantity of alloy dissolved decreases with increasing at% Cr. Element resolved polarization curves predict spontaneous passivation by comparing the critical dissolution rate with the cathodic kinetics near the corrosion potential. The stoichiometric relationships between the spontaneous dissolution rate, Cr enrichment, and the open circuit potential were determined and discussed.

Effects of standard heat treatment on micro-to nanostructure heterogeneities in a Rene 65 Ni-based superalloy billet

Advancements in the manufacturing of aeroengine components with superior mechanical properties rely on the microstructure homogeneity of industrially forged Ni-based superalloys. Billets often contain microstructural heterogeneities that can persist throughout the process, potentially compromising the integrity of the final product. Rene 65 is a relatively new superalloy with high-temperature mechanical properties superior to those of Alloy 718. However, systematic studies on its microstructure-property relationships are scarce. Here, we correlate the microstructural evolution with the hardness of a Rene 65 billet subjected to industrial processing and standard heat treatments. Multiscale microstructural heterogeneities, including grain size, geometrically necessary dislocations, twin boundary density, and the multimodal distribution of γʹ precipitates, are characterised. Grain growth occurs near the billet centre. Precipitation varies with radial position, showing up to a two-fold change in primary and secondary γʹ fractions. Tertiary γʹ precipitates exhibit variations in morphology and chemical composition, with Cr and Co enrichment near the billet surface. These heterogeneities arise from varying strain distributions and cooling rates during radial forging. Sub-solvus annealing and ageing are shown to mitigate these heterogeneities by promoting recrystallisation, recovery, and homogeneous precipitation of tertiary γʹ. A strengthening model elucidates the active mechanisms in as-received and heat-tread conditions, predicting hardness with a deviation of <8 %. Analyses of grain boundaries and γ/γʹ interfaces reveal strong segregation of Mo, W, Nb, C and B before and after heat treatment, with Mo and B showing the highest interfacial excess values. Insights into the microstructural evolution and its impact on properties could guide the development of next-generation aerospace parts.

Stability optimization of iron-based catalysts: Application of Cr-enriched alloy steel catalyst in ozonation pharmaceutical wastewater

Catalytic ozonation for advanced treatment of industrial wastewater has proved to be effective, and the stability of the catalyst is crucial for the long-term application of the ozonation process in practical engineering. In this study, the electrochemical characteristics of four iron (Fe) shavings: alloy steels (38CrMoAl and 40Cr) and carbon steels (A3 and 45#) were investigated, and a new citric acid-optimized H2O2 modification method was proposed. The low open circuit potential (OCP) of carbon steels indicated that they were more likely to be oxidized therefore required shorter modification times. The higher the OCP of alloy steels, the higher H2O2 concentration was required to enhance modification efficiency. By adding citric acid, the rapid modification of alloy steel was achieved, and the catalysts exhibited lower passivation current (Ip), indicating a lower rate of substrate loss and a longer lifetime of alloy steel catalysts. Furthermore, chromium (Cr) enrichment occurred in the oxide layer of alloy steel catalysts. The Cr/Fe (atomic ratio) in oxide layer for 38CrMoAl and 40Cr increased from 0.018 to 0.040 and from 0.010 to 0.037, respectively. The presence of CrOOH and Cr2O3 within the oxide layer reinforced the layer’s overall structural integrity and enhanced its protection for the internal substrate, making the catalysts more resistant to adapt to high-salinity wastewater. Through laboratory tests, it was found that among the four catalysts, 38CrMoAl showed the highest stability and good catalytic activity with low Fe loss concentration. Therefore, the pilot-scale application effect of 38CrMoAl catalyst was evaluated. In the advanced treatment of pharmaceutical wastewater, chemical oxygen demand (COD) and total organic carbon (TOC) removals were 56.3% and 32.4%, respectively. These results showed that the stability of catalyst can be improved by optimizing the selection of steel, and the introduction of citric acid achieved Cr enrichment effect and amplified the potential stability advantage of alloy steel. The research provides guidance on raw material selection for catalyst preparation to different industries' wastewater treatment, especially in high-salinity wastewater ozonation treatment, which can realize the long-term stable application of catalyst.

Effect of heat treatment on the microstructure, LBE corrosion resistance, and bonding strength of the FeCrAl‐based coatings

AbstractThe bonding strength and LBE corrosion resistance of the Fe15Cr11Al2Si, Fe15Cr11Al0.5Y, and Fe15Cr11Al2Si0.5Y coatings heat‐treated at 500–650°C for 500 h were investigated. The results showed that the as‐deposited Fe15Cr11Al0.5Y coating has the strongest bonding strength with the F/M steel cladding tube compared with the Fe15Cr11Al2Si and Fe15Cr11Al2Si0.5Y coatings. Heat treatment deteriorates the bonding performance of the coatings, and obvious enrichment of Cr and Al elements appeared. The consumed Al element inside the heat‐treated coatings promotes the formation of Fe3O4 on the surface of the coatings after the corrosion test. The Y element can inhibit the enrichment of elements and the formation of Fe3O4. The bonding strength of the heat‐treated coatings can be improved after the corrosion test. The underlying mechanism of the evolution of microstructure and properties of the coatings after heat treatment and corrosion test was discussed.

Phase-Field Simulation of Precipitation and Grain Boundary Segregation in Fe-Cr-Al Alloys under Irradiation.

A phase-field model for the precipitation of Fe-Cr-Al alloy is established incorporating grain boundary (GB) effects and irradiation-accelerated diffusion. The radiation source and grain boundary effect are incorporated to broaden the applicability of the Fe-Cr-Al precipitated phase-field model. The model is firstly employed to simulate the precipitation of the Cr-rich α' phase in a single-crystal alloy. The precipitation rate and the size distribution of the precipitated phase were analyzed. Subsequently, the model is utilized to simulate segregation at GBs in a double-crystal system, analyzing the enrichment of Cr and depletion of Al near these boundaries. The simulation results are consistent with experimental observations reported in the references. Finally, the model is applied to simulate the precipitation in a polycrystalline Fe-Cr-Al system. The simulated results revealed that the presence of GBs induces the formation of localized regions with enhanced Cr and Al content as well as depleted zones adjacent to these boundaries. GBs also diminish both the quantity and precipitation rate of the formed phase within the grains.

Solidification path and precipitation mechanism of a Ni-Cr-co-Mo based heat-resistant alloy

The solidification path and precipitation mechanism of the C-HRA-3 Ni-Cr-Co-Mo-based heat-resistant alloy are investigated by methods such as differential scanning calorimetry analysis, in-situ observations, quenching experiments, and scanning transmission electron microscopy. The results confirm that the primary solidification precipitates in the center of the C-HRA-3 alloy VAR ingot consist of Ti(C,N), M6C-M23C6 symbiotic phases, Ti(C,N)-M6C-M23C6 symbiotic phases and surrounding dispersed M6C and M23C6 carbides. The precipitation temperatures for Ti(C,N), M6C, and M23C6 are 1310 °C, 1280 °C, and 1230 °C, respectively. The C-HRA-3 alloy exhibits a wide solidification temperature range of 176 °C (1390 °C ∼ 1214 °C). Within 1390 °C ∼ 1355 °C, the γ phase grows vigorously, accompanied by a significant reduction in the fraction of liquid phase. At 1355 °C ∼ 1214 °C, the decrease in liquid phase fraction slows down, accompanied by significant enrichment of Mo, Cr, C and Ti elements. There is a clear functional relationship between the liquid phase percentage and temperature. M6C carbides undergo two heterogeneous nucleation mechanisms in the liquid phase. Due to the degradation of M6C carbides discharging Cr and C atoms, M23C6 carbides form at their edge. Dispersed M6C and M23C6 carbides nucleate at misfit dislocations around the symbiotic phases. Appropriately reducing the cooling rate during the stage above the precipitation temperature and increasing the cooling rate below the precipitation temperature can significantly reduce the volume percentage and size of the symbiotic phases.

Heavy metal(loid)s contamination and ecological-health risk assessment of coastal sediment from Salwa Bay, Saudi Arabia

Salwa Bay is an elongated, narrow bay which extends southwards from the Gulf of Bahrain down to the Saudi-Qatari border, separating the peninsula of Qatar from Saudi Arabia. The present study aims to investigate the degree of heavy metal(loid)s (HMs) contamination and the associated health risks in surface sediments of the Salwa Bay, Saudi Arabia. Thirty samples were collected for analysis of Ni, Cu, Cr, Pb, Zn, and As using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Several contamination and health risk indices, and multivariate analysis were applied. The following order was detected for average concentrations of metal(loid)s (μg/g dry weight): Cr (5.56) > Zn (4.63) > Ni (4.43) > As (2.34) > Pb (1.66) > Cu (1.51). The coastal sediments exhibit a significant enrichment in As, moderate enrichment in Pb, Zn, Cr, and deficiency to minimal enrichment for Ni, and Cu. Sediment quality guidelines indicated that HMs do not pose a risk to the benthic communities in Salwa Bay area. The average hazard index values ranged from 0.00002 to 0.0038 in adults and from 0.0002 to 0.0353 in children. This suggests that there is no significant possibility of non-carcinogenic effects to the people inhabiting the coastline of Salwa Bay. The results of the excess lifetime cancer risk (ELCR) for As, Cr, and Pb were below 1 × 10−4, indicating no carcinogenic health risk except for a few sediment samples showing elevated levels of Cr and As.

Characterization of Metal Elements in Atmospheric Fine Particulate Matter and Their Sources in Winter in the Southern Sichuan Urban Agglomeration

To investigate the concentration characteristics and sources of metal elements in PM2.5 during winter heavy pollution in the southern Sichuan urban agglomeration (Zigong, Luzhou, Neijiang, and Yibin), the metal elements in PM2.5 were measured using membrane sampling methods from December 30, 2018 to January 14, 2019, and the enrichment factor method (EF) and positive matrix factorization(PMF) were applied to investigate the sources of metal elements. The metal element observation data of Zigong in the same period of 2015 were also used to investigate the changes in metal element pollution and enrichment in Zigong in the middle and end of the implementation of China's Air Pollution Prevention and Control Action Plan. The main findings were as follows:① The concentrations and percentages of metal elements in particulate matter in different cities did not differ significantly. The elements with higher concentrations in the four cities showed similarities, with Al, Sb, and Fe at the top. From the comparison of different observation periods in Zigong, the concentrations of all elements except Tl changed. ② The results of the enrichment factor calculation showed that the enrichment of the elements Cr (Zigong and Yibin), Ni, Cu, As, Se, Ag, Cd, Sb, Tl, and Pb in the urban agglomeration was high. The comparison of the enrichment levels of elements in Zigong for different observation periods showed that the enrichment levels of all elements, except Cu, tended to decrease in the winter observation period of 2018. ③ The results of PMF source analysis showed that the metal elements in each city mainly originated from dust sources, coal-fired sources, industrial sources, and traffic sources, whereas there was a mixed contribution among the sources. The contribution of the main sources differed among cities, in which Zigong was dominated by traffic dust sources and mixed sources, Luzhou was dominated by industrial sources, Neijiang had a similar contribution from different sources, and Yibin was dominated by traffic sources.

OPTIMIZING VANADIUM CONVERTER SLAG UTILIZATION: TARGETED ENRICHMENT AND STABILIZATION OF VANADIUM THROUGH NON-EQUILIBRIUM SOLIDIFICATION

The objective of this research was to optimize the comprehensive utilization of vanadium converter slag through targeted enrichment and stabilization of heavy metal vanadium. Employing the non-equilibrium solidification theory and FactSage software, we investigated the potential of modifying vanadium converter slag. When the original slag failed to generate vanadium-rich spinel with usable V elements, introducing modifying agents Fe and Al proved effective. Fe facilitated the enrichment of Cr within spinel, while Al significantly promoted the V enrichment. Expanding on this, we systematically examined the influence of Fe2O3, Al2O3 and MgO contents on spinel phase precipitation during vanadium slag solidification. The addition of Al resulted in the precipitation of corundum, hematite, spinel, olivine and diopside phases. With an increase in the Fe2O3 content, the precipitation of FeV2O4 and MgV2O4 initially increased, peaking at 9.67 % before subsequently decreasing. Maintaining the Fe2O3 content within a range of 25–30 % proved optimal for enhancing vanadium precipitation and enrichment. In contrast, variations in the Al2O3 content had minor impacts on SP-V phase precipitation, with slight effects on FeV2O4 reaching 10.34 %. Furthermore, the incorporation of MgO facilitated the precipitation of MgV2O4 while concurrently suppressing the FeV2O4 precipitation. By judiciously controlling the MgO content at approximately 20 %, vanadium enrichment in the form of FeV2O4 and MgV2O4 spinel phases reached a remarkable 94.46 %.

Corundum in Sulfide Ore at the Thompson Mine, Manitoba, Canada: An Unusual Occurrence of Cr- and Ni-Bearing Corundum

Abstract A novel type of corundum host environment is described from the Thompson Mine, northern Manitoba, where corundum occurs within a high-grade metamorphic reaction zone between boudinaged metaperidotite bodies, sulfide ore, and Al-rich pelitic schist of the Pipe Formation (Ospwagan Group). Zoned corundum porphyroblasts, up to 20 mm across, contain coevally formed sulfide inclusions, zircon, rutile, abundant exsolved rutile needles, and secondary sulfide inclusions formed within fractures. Retrograde alteration products of corundum include greenish muscovite (in contact with the schist) and diaspore (in contact with sulfides). The median concentrations of trace elements in Thompson corundum are as follows (in µg/g): 119 Mg, 1720 Ti, 69.9 V, 390 Cr, 8.2 Mn, 1924 Fe, 9.1 Ni, and 106 Ga. The geochemical signature of Thompson corundum (enrichment in Fe, Cr, and Ni) resembles that of ruby in other metamorphic and metasomatic mafic-ultramafic-hosted deposits, and the Cr concentration is comparable to the low range of rubies. Thompson corundum contains high Ni concentrations relative to most Cr-enriched corundum occurrences, a result of formation in an exceptionally Ni-rich environment. The high abundance of exsolved rutile inclusions imparts a dominantly brownish-gray color to the corundum, which precludes it from being of gem quality. Chromium-bearing corundum has been suggested as a possible indicator mineral for metamorphosed massive sulfide deposits, however, it is an indirect indicator that risks yielding false positives. Further investigation into Ni concentrations in corundum, a potential indicator of formation in a Ni-rich environment, is recommended.

Influence of Furnace Pressure on the Growth Characteristic of Oxide Film on Piercing Plug Steel

Different oxidation processes are carried out by using 20Cr2Ni4 piercing plug steel to study the influence of furnace pressure on the growth characteristic of oxide film in high‐temperature C2H5OH–H2O atmosphere. The results show that oxide film under different processes shows clearly two layers. The inner film is porous with the main constituents of FeO and Fe3O4 while the outer film is dense with large bubbles. The elemental Ni aggregates as a dot‐like substance in the inner oxide, where the enrichment of Cr lags behind the frontier boundary of Ni distribution. It is found that the thickness of the oxide film shows a parabolic relationship with product of the square root of oxidation time and furnace pressure. High furnace pressure can keep the pressure balance of the bubbles to swell to sufficient size, but may cause cracking during the relief process of pressure.

Peritectic orthopyroxene entrainment during partial melting of garnet peridotite produced the Bushveld Complex chromite deposits

AbstractOne of the largest chromium deposits on Earth occurs in the Rustenburg Layered Suite (RLS) of the Bushveld Complex as laterally continuous chromitite layers. None of the hypotheses proposed for the origin of the chromitites can explain both the abundance of Cr in the RLS and the unusual enrichment in Cr and V over Ni, relative to typical depleted mantle values. This study investigates the possibility that the layering and chromitite formation are consequences of the entrainment of source components into the magmas that formed the RLS. Thermodynamic modelling results reveal a wedge-shaped domain in pressure-temperature space in the subcratonic mantle within which Cr-bearing orthopyroxene forms as a peritectic product of incongruent melting. Entrainment of this orthopyroxene produces magmas that crystallise peritectic olivine and chromite on ascent, due to the consumption of orthopyroxene by melt. The chromite- and olivine-bearing magmas intrude as sills and can produce chromite and dunite layers by density separation. This model, which interprets the RLS Sr-isotopic composition to reflect prior mantle metasomatism by crustal fluids (ideally ancient and of low volume), readily explains the formation of chromitite layers from relatively thin sills, as well as the very high ratios of Cr and V to other compatible elements relative to typical mantle compositions. The special circumstances required to produce the RLS chromitites do not relate to some oddity of repetitive crustal assimilation or magma compositions that allow chromite-only saturation. Rather, they relate to speed of melting and magma extraction which enabled peritectic orthopyroxene entrainment to the magmas.

Profile-wise enrichment of metallic pollutants (Pb, Cr and Cd) in various sewage and non-sewage irrigated soils of Prayagraj

The study examined the profile-wise enrichment of metallic pollutants (Pb, Cr and Cd) at different sites, i.e., Naini, Draupadi Ghat, Phaphamau and Beligaon in Prayagraj (U.P.), India. The soil samples were collected from different soil profile depths: 0-15, 15-30, 30-45 and 45-60 cm. It was observed that maximum enrichment of Pb, Cr and Cd was found at the (0-15 cm) depth of sewage and non-sewage irrigated soil (5.12±0.42, 4.08±038 and 3.58±031 mg kg-1 ) and (1.58±0.09, 1.08±0.09 and 1.02±0.08 mg kg-1 ) and minimum enrichment was found at the (45-60 cm) depth (2.18, 0.56 and 0.48 mg kg-1 ) and (0.69±0.04, 0.56±0.04 and 0.47±0.03 mg kg-1 ) in Naini, respectively. Maximum enrichment of Pb, Cr and Cd was found at the (0-15 cm) depth of sewage and non-sewage irrigated soil (2.44±0.22, 2.19±0.14 and 2.08±0.16 mg kg-1 ) and (1.09±0.07, 0.97±0.06 and 0.84±0.07 mg kg-1 ) and minimum enrichment was found at the (45-60 cm) depth (1.01±0.06, 0.98±0.05 and 0.71±0.06 mg kg-1 ) and (0.19±0.02, 0.15±0.01 and 0.11±0.01 mg kg-1 ) in Beligaon, respectively. To find the maximum enrichment of heavy metals in Naini and the minimum enrichment in Beligaon. The enrichment of metallic pollution decreased with increasing soil profile depth in both sewage and non-sewage irrigated soils. Metallic contamination in soil can persist for a long time and create serious problems; therefore, proper monitoring of sewage irrigated soils is required to reduce metallic pollution

Effects of deoxidation methods on the inclusion characteristics and corrosion behaviour of high-strength low-alloy steels in marine environments

The effects of deoxidation methods on the inclusion characteristics and corrosion behaviour of high-strength low-alloy steels were examined in marine environments using various microcharacterisation techniques and first-principles calculations. After Ca treatment, MnS–MgO–Al2O3 inclusions were modified to CaS–MgO–Al2O3 and microcrevices were existed at the interface between inclusions and Fe. After Zr–Ti-RE deoxidation, spherical CaS + TiN + ZrO2+MgAl2O4+(Ca, La) Al3O7 inclusions appeared. (Ca, La)Al3O7 in steel displayed similar mechanical properties to Fe, reducing the degree of lattice distortion and the formation of microcrevices around the inclusions. Micro-galvanic corrosion between Fe and MnS accelerated the localised corrosion rate and promoted the formation of large pits on the surface. However, the chemical activity of CaS resulted in dissolution of the inclusion, which relatively inhibited the localised corrosion propagation. The enrichment of Cr in the rust layer after Zr–Ti-RE deoxidation improved the compactness, contributed to a reduction in the localised corrosion rate and long-term corrosion rate. Finally, the underlying mechanism influencing corrosion behaviour was established.

Multiple sulfur isotopes evidence deep intra-slab transport of sulfate-rich fluids

Sub-arc mantle oxidation is often attributed to sulfates from seawater transported in aqueous fluids derived from slab dehydration, but sulfur speciation and origin(s) in high-pressure fluids remain weakly constrained. Here, we use the four sulfur stable isotopes (32S, 33S, 34S, 36S) to decipher processes, e.g. mixing of two end-member reservoirs and open-system reaction occurring during sulfur devolatilization, and deduce the nature and origin of sulfur species in aqueous slab-derived fluids. We analyzed high pressure metamorphic rocks from the Monviso massif Lower Shear Zone (LSZ; Western Alps), recording evidence of an interface-parallel, channelized fluid flow near peak burial metamorphic conditions (∼550 °C and 2.7 GPa). We sampled variably metasomatized Fe-Ti metagabbros, hybrid (talc) schists and serpentinites containing sulfides supposed to preserve the aqueous fluid composition, as well as other alpine metagabbro, metabasalt and serpentinites from greenschist to eclogite-facies devoid of the studied metasomatic overprint.The millimeter to centimeter-sized sulfide crystals found in abundance within LSZ lithologies are mainly pyrites containing inclusions of the peak burial mineral assemblage (e.g. garnet, omphacite, rutile) indicating their crystallization at peak conditions or during incipient exhumation. The sulfides in the non-metasomatic metagabbros and metabasalts cover a narrow range in δ’34S of 0.89 ± 0.63 ‰ and Δ’33S of 0.017 ± 0.01 (n = 6) that could be inherited from hydrothermal sulfides formed on the seafloor. In contrast, the LSZ sulfides from the Fe-Ti metagabbros, serpentinites and hybrid schists (n = 24) are enriched in 34S and 33S (bulk δ’34S from 4.91 to 21.32 ‰ and Δ’33S from 0.02 to 0.06 ‰) compared to the sulfides from the non-metasomatic rocks. Mixing between reduced sulfur species cannot explain all the data without requiring an end-member composition as high as δ34S ∼ 22 ‰ and Δ33S ∼ 0.06 ‰, i.e. values rarely recorded by sulfides in the literature. Instead, we propose that this 33S-34S-enrichment results from open-system sulfate reduction to sulfides in aqueous fluids at 550°C, with an initial δ34S ∼ 12 ± 3 ‰ and Δ33S ∼ 0.025 ± 0.010 ‰. Because this signature is different from seawater sulfate, we suggest that sulfate was produced by hydrothermal sulfide oxidation during antigorite breakdown at 650°C, in agreement with Mg, Ni and Cr enrichments in the metasomatized HP mafic lithologies. This model is consistent with relatively oxidized 34S-enriched arc lavas and can potentially explain recycled sulfur with positive δ34S signature recorded by ocean island basalts.

Historical construction, quantitative source identification and risk assessment of heavy metals contamination in sediments from the Pearl River Estuary, South China

Historical reconstruction of heavy metals (HMs) contamination in sediments is a key for understanding the effects of anthropogenic stresses on water bodies and predicting the variation trends of environmental state. In this work, eighteen sediment cores from the Pearl River Estuary (PRE) were collected to determine concentrations and geochemical fractions of HMs. Then, their potential sources and the relative contributions during different time periods were quantitatively identified by integrating lead-210 (210Pb) radioisotope dating technique into positive matrix factorisation (PMF) method. Pollution levels and potential ecological risks (PERs) caused by HMs were accurately assessed by enrichment factors (EF) based on establishment of their geochemical baselines (GCBs) and multiparameter evaluation index (MPE). HMs concentrations generally showed a particle size- and organic matter-dependent distribution pattern. During the period of 1958–1978, HMs concentrations remained at low levels with agricultural activities and natural processes being identified as the predominant sources and averagely contributing >60%. Since the reform and opening-up in 1978, industrial and traffic factors become the primary anthropogenic sources of HMs (such as Cu, Zn, Cd, Pb, Cr, and Ni), averagely increasing from 22.1% to 28.1% and from 11.6% to 23.4%, respectively. Conversely, the contributions of agricultural and natural factors decreased from 37.0% to 28.5% and from 29.3% to 20.0%, respectively. Subsequently, implementation of environmental preservation policies was mainly responsible for the declining trend of HMs after 2010. Little enrichment of sediment Cu, Zn, Pb, Cr and Ni with EFs (0.15–1.43) was found in the PRE, whereas EFs of Cd (1.16–2.70) demonstrated a slight to moderate enrichment. MPE indices of Cu (50.7–252), Pb (52.0–147), Zn (35.5–130), Ni (19.6–71.5), Cr (14.2–68.8) and Cd (0–9.90) highlighted their potential ecological hazards due to their non-residual fractions and anthropogenic sources.

Surface water oxygenation and low bioproductivity during deposition of iron formation of the Jacadigo Group (Brazil): Insights from combined cadmium – Chromium isotopes

The Banda Alta Formation (Urucum district, Mato Grosso do Sul, Brazil) comprises ∼600 Ma Fe and Mn deposits, which are among the world's youngest and largest Neoproterozoic sedimentary Fe and Mn formations (IF; MnF). These have been deposited in a redox-stratified, marine sub-basin (Jacadigo Basin), which was strongly influenced by glacial advance/retraction cycles with temporary influx of continental freshwater and upwelling of metal-enriched deep anoxic seawater. Cr and Cd isotopes measured on meticulously separated hematite mesobands from drill core samples are relatively homogenous throughout the ca. 325 m thick sequence sampled in the Banda Alta Fm., with average authigenic δ53Cr values of +0.93 ± 0.24 ‰ (2σ; n = 23) and δ114Cd values of −0.14 ± 0.14 ‰ (2σ; n = 15). The significant enrichment of Cr, in parallel with the strong enrichments of other redox sensitive elements (U, Mo), attests for effective and efficient reduction removal processes in the surface waters during cycles where upwelling Fe2+-rich waters reached the oxygenated surface layer exposed to the atmosphere during episodic glacier retreat stages. Assuming a similar quantitative and efficient removal pathway of dissolved Cd by iron oxyhydroxides, the so-inferred average δ114Cd signature of −0.14 ± 0.14 ‰ in the Jacadigo Basin surface water is significantly lower than signatures of modern ocean surface waters with a range of δ114Cd of ca. ∼0.4 to ∼1 ‰ and even lower than the signature of modern ocean deep waters with δ114Cd of ∼0.3 ‰. It possibly attests to reduced primary production levels and lower nutrient utilization rates during deposition of the Late Neoproterozoic Jacadigo Group, compared to today. This despite the inferred oxidized surface water layer that must have prevailed during this time, as implied by the strongly positively fractionated Cr isotope signatures and pronouncedly negative Ce-anomalies recorded in the seawater-like, shale-normalized Rare Earth Element and Yttrium (REY) patterns exhibited by the hematite mesobands. Data presented herein speak for: (1) a stable, isotopically heavy Cr input to the Jacadigo Basin at the time of deposition, implying high atmospheric O2 levels in the Late Neoproterozoic (2) likely quantitative, reductive incorporation / adsorption processes of dissolved Cr and Cd, respectively, into/onto precipitating iron oxyhydroxides, and (3) the prevalence of low nutrient concentrations and utilization rates in the Jacadigo Basin during glacier retreat cycles. Banded iron formations are considered suitable archives for reconstructing redox and bioproductivity levels in past marine depositional basin, including those prevalent in Neoproterozoic glacial conditions, via employing the CrCd isotope double tracer to iron-rich mesobands.

Variation of chemical ordering induced by grain boundaries in multi-principal element alloys

Cr segregation at grain boundary (GB) of Multi-principal element alloys (MPEAs) is extensively observed and holds significance in physical processes, e.g., oxidation, but its atomic-level mechanism is still debated. Here, correlations among atomic-level volume, electronegativity, charge transfer, and atomic stress, are systematically investigated in equiatomic CrFeCoNi alloys, both with and without GBs. The correlations slightly disturbed in GBs inadequately elucidate the discrepancy of Cr short range ordering (SRO) in the interior grain and at GBs. High excess free volume inherent in GBs, distinguishing from the interior, is a crucial factor, driving Cr enrichment by means of the mitigation of electron loss and the alleviation of very high tensile stress. The stress mechanism in conjunction with the electronegativity of element and the characteristics of GB structure, not only adapts to Cr segregation in CrFeCoNi, but also to other element of low electronegativity within considered alloys.

Removal and reduction mechanism of Cr (VI) in Leersia hexandra Swartz constructed wetland-microbial fuel cell coupling system

Cr (VI) is extremely harmful to both the environment and human health, and it can linger in the environment for a very long period. In this research, the Leersia hexandra Swartz constructed wetland-microbial fuel cell (CW-MFC) system was constructed to purify Cr (VI) wastewater. By comparing with the constructed wetland (CW) system, the system electricity generation, pollutants removal, Cr enrichment, and morphological transformation of the system were discussed. The results demonstrated that the L. hexandra CW-MFC system promoted removal of pollutants and production of electricity of the system. The maximum voltage of the system was 499 mV, the COD and Cr (VI) removal efficiency was 93.73% and 97.00%. At the same time, it enhanced the substrate and L. hexandra ability to absorb Cr and change it morphologically transformation. Additionally, the results of XPS and XANES showed that the majority of the Cr in the L. hexandra and substrate was present as Cr (III). In the L. hexandra CW-MFC system, Geobacter also functioned as the primary metal catabolic reducing and electrogenic bacteria. As a result, L. hexandra CW-MFC system possesses the added benefit of removing Cr (VI) while producing energy compared to the traditional CW system.

Essential role of Si in enhancing corrosion resistance of high strength low alloy steels in marine environment

High strength low alloy steels are extensively applied in marine engineering structures (cross-sea bridge, ocean platform, etc), which are usually subjected to severe corrosion in marine atmosphere. In this work, the corrosion behaviors of 550 MPa-grade high strength low alloy marine steels with varied Si content were studied in simulated marine atmosphere. Results show the increase of Si could significantly facilitate the formation of α-FeOOH and enrichment of Cr and Cu in inner rust layer, making the rust layer compact and cation-selective. Si and Cr can be co-enriched in inner rust layer to form mixed or complex oxides with amorphous network structure, which could greatly improve the compactness and stability of rust layer and thereby enhance the corrosion resistance of low alloy steel.