Mn Profiles Research Articles

Numerical modeling and experiment for isothermal selective external oxidation behavior of Fe-Mn binary alloys

This paper described work undertaken to the selective external oxidation of Fe-Mn binary alloys during isothermal recrystallization annealing. The effect of different peak temperature, soaking time, dew point conditions on external oxidation of Fe-Mn alloys were investigated. Based on an image segmentation technology, external oxidation behavior was evaluated by the equivalent diameters and coverage fraction of external oxides. A numerical kinetics model of the coupled selective internal-external oxidation during isothermal annealing of Fe-Mn binary alloys was proposed. The model allowed the case of internal and external oxidation simultaneously occurred. Based on the assumptions of oxidation reaction at the steel/atmosphere interface reached an instantaneously local thermodynamic equilibrium, the coupled relationship was described by two interactions. The surface dissolved oxygen content could be controlled by both the annealing atmosphere and the coverage fraction of external oxides on the surface, while the diffusion of oxidant and oxygen inside the steel were blocked by the precipitation behavior of internal oxides. Therefore, the model achieved predict quantitatively the coverage of oxides on the external surface of the steel and the mass fraction profiles of Mn after annealing. The correctness of the calculation results was verified experimentally.

The behaviour of aluminium, manganese, iron, cobalt, and lead in the subarctic Pacific Ocean: boundary scavenging and temporal changes

AbstractAluminium (Al), manganese (Mn), iron (Fe), cobalt (Co), and lead (Pb) are trace metals that exhibit significant scavenging tendencies in the oceans. This study presents the full-depth distributions of the dissolved (d) and labile particulate (lp) fractions of these five elements in the subarctic Pacific Ocean, obtained during the GEOTRACES Japan KH-17-3 cruise. Along the 145° W meridional transect, the d and lp species of Al, Mn, Fe, and Co reflected fluvial supply from Alaska and benthic input from the continental shelf. We estimated that the boundary-scavenging zone has a width of approximately 250 km off Alaska. Along the 47° N zonal transect (GEOTRACES GP02 Line), we found input of trace metals from the Okhotsk and Bering Seas in the west, contrasting to the limited input of trace metals due to boundary scavenging in the east. The hydrothermal activity of the Juan de Fuca Ridge influenced the distribution of deep-water trace metals at the easternmost station, CL-21. Temporal change in the vertical profiles of dPb in the middle of the subarctic gyre highlighted a decline in anthropogenic Pb emissions from 2005 to 2017. Temporal change of the vertical profiles of Al, Mn, and Fe at 47° N, 160° E from 2011 to 2017 indicates the influence of the Great East Japan Earthquake. In particular, lp trace metals were brought by tsunami and ocean circulation in 2011, and decreased over time through scavenging.

Confocal micro-X-ray fluorescence analysis for difference identification of ceramic samples.

A nondestructive analytical method for difference identification is required in the research fields such as forensic science or archeology. An X-ray fluorescence (XRF) analysis is one of feasible techniques for this purpose. Micro-XRF using an X-ray micro-beam gives elemental distributions by scanning the samples. A confocal micro-XRF (CM-XRF) technique is a unique analytical technique to analyze limited volume. CM-XRF enables elemental depth imaging and elemental profiling in depth nondestructively. Therefore, CM-XRF has been applied for various samples such as industrial samples, paintings, forensic samples, food materials, and human hairs. CM-XRF technique would be a suitable method for difference identification because the CM-XRF gives detailed information on elemental distribution not only on the surface of the sample but also in depth. We developed CM-XRF instrument in the laboratory and applied to two very similar ceramics samples. It showed differences in the intensity profiles of Fe and Mn for blue paintings on the ceramics. In addition, depth elemental analysis revealed different depth profiles especially of Co and Zn for both samples. These results suggest that CM-XRF provides useful information for the identification of ceramic samples.

Diffusion timescale conundrum in contact metamorphism: Oxygen isotope and trace element profiles in dolomite

Successful application of diffusion chronometry is based on detailed knowledge of diffusion parameters obtained from laboratory experiments, which also need to be upscaled to be applied to natural samples. While generally applicable, several examples of disagreement with other dating techniques and discrepancies with geological observations highlight the importance of validating diffusion parameters and their use to geochronologic application by testing them on samples from well-characterized geologic environments.The incomplete dissolution and reprecipitation reaction of dolomite associated with infiltration along grain boundaries of a fluid of magmatic origin in a contact metamorphic environment is the starting point of this study. The reaction produces dolomite rims, which are characterized by lower δ18O values (about 16‰) and slightly higher Fe (∼1000 ppm) and Mn (∼200 ppm) concentrations. The cores, in comparison, have no Fe and Mn and a δ18O value of about 30‰. This compositional and isotopic gradient led to chemical re-equilibration through diffusion at the reaction interface. We acquired oxygen isotope profiles using a secondary ion mass spectrometer (SIMS), with spot sizes as small as 3 µm, and iron and manganese profiles with a NanoSIMS and a beam diameter of about 240 nm. These profiles were then compared against cumulative theoretical diffusion distances calculated for a modeled time – temperature path of the contact aureole using the experimentally determined diffusion parameters for O, Fe and Mn in dolomite.The oxygen isotope profiles yielded diffusion distances of ca. 10 µm, corresponding to a formation temperature of about 550 °C, which is concordant with phase petrology and a timing for fluid infiltration at the onset of cooling of the contact aureole (i.e., non-isothermal evolution of the dolomite marbles). The Fe and Mn profiles were very sharp, with a diffusion length of about 150 nm. Such short diffusion length would be achieved at near-surface temperature and corresponding cooling timescale are several orders of magnitude smaller than those derived from the oxygen diffusion profiles. From the mineralogy and textures, a decoupling of the oxygen isotopes and Fe–Mn reaction interface can be ruled out, suggesting that the published diffusion parameters are not fully representative, either due to the experimental conditions or by the assumed diffusion mechanism of natural processes. This case study is not the first example in the literature where the diffusion rate of Fe and Mn in natural dolomite seems to be substantially slower than predicted by experiments. Thus, there is a need to re-assess the experimental diffusivities before Fe and Mn diffusion in dolomite can be reliably used to determine duration, cooling or heating paths of metamorphic processes.

Consistent garnet Lu–Hf and Sm–Nd ages indicate short‐lived high‐pressure metamorphism and rapid subduction in oceanic subduction belt: An example from the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

AbstractThe integration of garnet‐based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet‐bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh‐pressure metamorphic belt in southeast Tibet, in order to characterize tectono‐metamorphic events and determine the duration of Paleo‐Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P–T path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well‐preserved Rayleigh‐fractionation‐style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M‐shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high‐pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic‐type and continental‐type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.

Phase field simulation of segregation and precipitates formation during steel solidification

Multi-phase field modelling using MICRESS® has been used to perform simulations of the nucleation and evolution microstructure in a 2D domain as function of fundamental thermo-physical parameters, time, cooling rate and chemical composition during solidification. In order to have a better understanding of primary precipitates evolution during solidification in a case hardening steel and the precipites chosen were niobium-carbides (NbC) and manganese sulphides (MnS), simulations were done including the nucleation and growth of precipitates and coupled through thermodynamic databases. The results show a good comparison with the experimental microstructure measured with SEM-EDX measurements. The simulated time dependent segregation profiles of Nb and Mn in the interdendritic regions, and the precipitates composition, have a good similarity with the measurements. Even when the size and morphology of the NbC and MnS particles has a difference in the both simulated and experimental situations, this can be explained by the low resolution used in the simulation. This publication shows the modelling results and the comparison with the experimental measurements.

Chemical and mineralogical variability of sediment in a Quaternary aquifer from Huaihe River Basin, China: Implications for groundwater arsenic source and its mobilization

Arsenic (As) is a conspicuous contaminant, and exposure to this element through contaminated drinking groundwater poses a significant challenge to public health. Geogenic groundwater arsenic is associated with sedimentary setting. This work concentrates on the investigation of lithology, elemental abundance and mineralogical compositions about the arsenic profile and its effect to the groundwater from Huaihe River Basin, China. There are 90 sediment samples from the borehole at the field monitoring sites were collected and analyzed. The results reveal that sedimentary concentrations of As, Fe, Mn, S, Al, N, organic carbon and mineralogical compositions vary across the Quaternary aquifer. Arsenic abundance of sediments is 10.63 ± 0.56 mg/kg, and peak As concentrations occur between 59.0 m and 64.8 m in fine particle sediments. The specific higher As concentrations in sedimentary aquifer are concordant with arsenic-rich groundwater around the investigated borehole. Fe, Mn, and Al depth profiles follow similar tendency to those of As. Sedimentary As concentrations are significantly correlated to Fe, Al, and Mn concentrations, but are not correlated to organic carbon and S concentrations. Arsenic probably exists in the form of non-crystalline colloids, and Fe, Al minerals are potential host minerals for arsenic. Under alkaline conditions, groundwater arsenic is released and enriched within the Quaternary aquifer by reductive dissolution of As-hosting Fe and Al minerals.

Majority and Minority Carrier Traps in Manganese as‐Implanted and Postimplantation‐Annealed 4H‐SiC

Ion implantation of amphoteric impurities, like vanadium (V) or manganese (Mn), can be employed for the formation of resistive layers in 4H‐SiC. Such layers can then be used for device isolation, in order to reduce the parasitic capacitance. While V implantation can form thermally stable resistive layers, not much is known on Mn implantation. Herein, Mn ion‐implanted samples are electrically characterized by means of capacitance–voltage, deep‐level transient spectroscopy, and current–voltage measurements. Two ion implantation schedules are carried out: single‐energy and multiple‐energy ions so to have Gaussian and box‐shaped Mn and related ion damage profiles, respectively—the former with a maximum concentration in the low 1016 cm−3 and the latter with a Mn plateau of 1017 cm−3. It is found that several majority carrier traps, in the 0.4–1.7 eV range below the conduction band edge, and two minority carrier traps arise after implantation and after postimplantation annealing in the 1000–1800 °C temperature range. The detected traps, as well as previous reports in the literature, show that most of them can be associated with the intrinsic defects. Box‐profile implanted layers show resistivity values in the ≈106 Ω cm after heat treatments of, at least, 1600 °C.

Controllable Polymerization of N -Substituted β-Alanine N -Thiocarboxyanhydrides for Convenient Synthesis of Functional Poly(β-peptoid)s

Controllable Polymerization of <i>N</i> -Substituted β-Alanine <i>N</i> -Thiocarboxyanhydrides for Convenient Synthesis of Functional Poly(β-peptoid)s

Trace element variations across Middle–Upper Cambrian carbonates: Implications for the paleoenvironment of eastern Laurentia

The investigated Middle–Upper Cambrian carbonates span, from bottom to top, the uppermost Hawke Bay (40 m), the March Point (∼83 m thick), and the lowermost Petit Jardin (∼47 m) formations of the Port au Port Group (western Newfoundland, Canada). A multi-technique evaluation of the preservation of carbonates was applied by using several petrographic and chemical screening tools. The variations in several proxies of bioproductivity (e.g., P, Ni, and Cu), input of weathering products (Mn, Fe, Al, and ∑REE), and paleoredox (e.g., Th/U) have been utilized to study the paleoenvironmental conditions during the investigated time interval. Distinct positive shifts in the profiles of Al, ∑REE, Mn, and Fe have been associated with the negative shifts recorded by the δ13C profile and correlated with the DICE (Middle Cambrian) and base of the SPICE (lowermost Upper Cambrian) events. They reflect contributions from detrital weathered material during sealevel falls. Similar correlated positive shifts are also documented by the P, Ni, and Cu profiles, thus suggesting an increase in the riverine inputs of nutrients associated with the drop of sealevel and increase of weathering activities. In addition, the changes in the bioproductivity and weathering proxies were associated with relative rising in the Th/U ratios (0.1–8.3), which reflects variations in the redox state towards relatively more oxidizing conditions.

Microsegregation and solidification characteristics of an advanced high strength steel – part II: experimental validation

ABSTRACT Good knowledge of the redistribution characteristic of solute elements is essential for the design and the production of new alloying systems. Part I of this series demonstrated the deviations of microsegregation predictions on an advanced high strength steel (AHSS) caused by equilibrium partition coefficients based on the analytical microsegregation model. In this study, the aim is to conduct a feasible experiment to assess the microsegregation and solidification characteristics of the AHSS steel and to validate the effect of partition coefficients on microsegregation predictions. The SQ method (solidification followed by quenching) was adopted for validation in this study. Solid fraction evolution was obtained on the basis of quenched solidification microstructures to compare with predicted solidification paths. Subsequently, EPMA mappings were performed on quenched microstructures to generate the concentration profiles of Mn in the liquid phase, and the measured data were used to assess the predicted degree of microsegregation.

Multi-isotopic and trace element evidence against different formation pathways for oyster microstructures

Shells of oysters (Ostreidae) are predominantly composed of foliated and chalky calcite microstructures. The formation process of the more porous chalky structure is subject to debate, with some studies suggesting that it is not formed directly by the oyster but rather through microbial mineralization within the shell. Here, this hypothesis is tested in modern shells of the Pacific oyster (Crassostrea gigas) from coastal regions in France and the Netherlands. We combine measurements of stable carbon, oxygen, nitrogen, sulfur, and clumped isotope ratios with high-resolution spatially resolved element (Na, Mg, Cl, S, Mn and Sr) data and microscopic observations of chalky and foliated microstructures in the oyster shells. Our results show no isotopic differences between the different microstructures, arguing against formation of the chalky calcite by microorganisms. However, we observe a small difference in the oxygen isotope ratio (0.32‰) and clumped isotope composition (0.017‰) between the microstructures, which is likely caused by sampling biases due to seasonal differences in growth rate and the short timespan over which the chalky microstructure forms. We therefore recommend sampling profiles through the foliated microstructure to control for strong seasonal variability recorded in the shell which can bias environmental reconstructions. High-resolution (25–50 µm) Na, Mg, Cl, S, Mn and Sr profiles yield empirical distribution coefficients between seawater and shell calcite for these elements. Significant differences in element concentrations and distribution coefficients were confirmed between the two microstructures, likely reflecting differences in mineralization rates or inclusion of non-lattice-bound elements. Only Mg/Ca ratios in the foliated microstructure vary predictably with growth seasonality, and we show that these can be used to establish accurate oyster shell chronologies. The observed effect of mineralization rate on element incorporation into oyster shells should be considered while developing potential element proxies for paleoclimate reconstructions. Trace element proxies in oyster shells should be interpreted with caution, especially when element chemical properties were measured in different microstructures.

Trace‐element zoning patterns in porphyroblastic garnets in low‐T eclogites: Parameter optimization of the diffusion‐limited REE‐uptake model

AbstractCompositional zoning patterns of the major elements and REEs in prograde‐zoned garnets whose Mg/(Mg + Fe) atomic ratios increase rimward have been widely used to understand the metamorphic P–T–t trajectories, and the diffusion‐limited REE‐uptake model is a promising way to interpret their growth rates and the REE diffusion kinetics in the low‐temperature eclogite. In order to elucidate their growth kinetics with Skora et al.'s (2006) diffusion‐limited REE uptake model for prograde‐zoned garnets, we examine the trace‐element zoning patterns of two prograde‐zoned porphyroblastic garnets (~6 mm in size) in low‐temperature eclogites from two different localities. Core‐to‐rim trace‐element profiles in a garnet (prp5–9alm61–67sps1–3grs24–30) of a glaucophane‐bearing epidote eclogite of Syros (Cyclades, Greece) are characterized by the presence of Y + HREE peaks in the mantle, which might be attributed to a continuous breakdown of the titanite to form rutile during the garnet growth. In contrast, those in a garnet (prp4–7alm61–68sps3–10grs23–24) extracted from a lawsonite‐eclogite of the South Motagua Mélange (SMM) (Guatemala) have prominent central peaks of Y + HREEs. Although the REE profiles of both the garnets can be explained by the diffusion‐limited uptake, their Mn profiles suggest that their growth‐rate laws are different: i.e., diffusion‐controlled (Syros) and interface‐controlled (SMM). Prior to the model application, we optimize the number of the parameters as the garnet grows with the interface‐controlled processes based on the growth Péclet number. In particular, we propose the ratio of the REE diffusivity in the eclogitic matrix to the garnet growth rate as the new parameter. Visualizing the values of the new parameters allows to readily understand the relationship between the REE profiles and the REE‐diffusion/garnet‐growth kinetics in low‐T eclogite. Our model refinement leads to the simple quantitative characterization of core‐to‐rim REE profiles in garnet in low‐temperature eclogites.

Advances in Understanding Mobilization Processes of Trace Metals in Marine Sediments.

Different mobilization mechanisms control the metal distribution in surface sediments of the Belgium coastal zone (BCZ) and the anoxic Gotland basin (GB). This mobilization was studied using DGT (diffusive gradients in thin films): vertical one-dimensional (1D) profiles of Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn were measured at 5 mm intervals, while two-dimensional (2D) high-resolution (100 μm) images of smaller zones of the sediment profile were obtained on separate DGT probes. Removal of dissolved Cd, Cu, and Pb in BCZ sediments caused steep vertical gradients at the sediment-water interface that were well replicated in 1D profiles and 2D images. While 1D profiles showed apparent coincident maxima of Co, Mn, and Fe, 2D images revealed mutually exclusive Co and Fe mobilization. Correlation analysis supported this observation and showed a consistent linkage between Co and Mn. Sharp maxima of some metals in the vertical 1D profiles of GB sediment were attributed to localized mobilization in microniches. Examination of an ∼1 mm diameter Cu and Ni maximum in 2D, defined by ∼300 data points, showed that the metals were supplied from localized decomposition of reactive organic material, rather than from reductively dissolving Fe or Mn oxides, and that they were removed as their sulfides.

Arsenic behavior across soil-water interfaces in paddy soils: Coupling, decoupling and speciation

The sharp redox gradient at soil-water interfaces (SWI) plays a key role in controlling arsenic (As) translocation and transformation in paddy soils. When Eh drops, As is released to porewater from solid iron (Fe) and manganese (Mn) minerals and reduced to arsenite. However, the coupling or decoupling processes operating within the redox gradient at the SWI in flooded paddy soils remain poorly constrained due to the lack of direct evidence. In this paper, we reported the mm-scale mapping of Fe, As and other associated elements across the redox gradient in the SWI of five different paddy soils. The results showed a strong positive linear relationship between dissolved Fe, Mn, As, and phosphorus (P) in 4 out of the 5 paddy soils, indicating the general coupling of these elements. However, decoupling of Fe, Mn and As was observed in one of the paddy soils. In this soil, distinct releasing profiles of Mn, As and Fe were observed, and the releasing order followed the redox ladder. Further investigation of As species showed the ratio of arsenite to total As dropped from 100% to 75.5% and then kept stable along depth of the soil profile, which indicates a dynamic equilibrium between arsenite oxidization and arsenate reduction. This study provides direct evidence of multi-elements’ interaction along redox gradient of SWI in paddy soils.

Influence of initial microstructures on intercritical annealing behaviour in a medium Mn steel

ABSTRACTThe present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 900°C. The specimen austenitised at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitisation temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to a difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at an early stage of intercritical annealing but retards the transformation kinetics afterwards.This paper is part of a Thematic Issue on Medium Manganese Steels.

Palaeoenvironmental changes in slope carbonates across the Late Cambrian–Early Ordovician in western Newfoundland

The Martin Point section (western Newfoundland, Canada) spans the uppermost Cambrian Broom Point and Martin Point members of the Green Point Formation (upper Furongian). The investigated interval (~90 m) consists of rhythmites of thinly‐bedded marine carbonates (lime mudstones) alternating with green and black shale and thin conglomerate beds. Samples were extracted from the micritic carbonates, and their preservation was evaluated by multiple petrographic and geochemical examinations. The δ13Corg profile exhibits a positive shift (~2‰) associated with the globally well‐documented negative δ13Ccarb HERB (Hellnmaria–Red Tops boundary) excursion and correlated with similar distinct shifts in the Al, Si, and ∑REE components, thus suggesting contributions from detrital organic matter relatively depleted in 12C induced likely by a eustatic sea‐level drop and enhancement of terrigenous inputs. Similarly, the δ15Norg variations are consistent with the proposed sea‐level drop. This is also correlated with positive shifts on the Fe and Mn profiles reflecting the overprint of terrigenous inputs rather than redox conditions.

In-situ multi-sensor characterization of soil cores along an erosion-deposition gradient

Soil landscape research is faced with wide-ranging questions of soil erosion, precision farming, and agricultural risk management. Digital Soil Morphometrics is a powerful tool to provide respective answers or recommendations but requires soil data from the pedon-to-field scale with high horizontal and vertical resolutions, including the subsoil. We present an efficient sampling and measurement method for easily obtainable soil driving cores with low-destructive preparation. Elemental contents and soil organic and mineral matter composition were measured rapidly and in large numbers using a multi-sensor approach, i.e., visible and near-infrared (Vis-NIR), diffuse reflectance infrared Fourier transform (DRIFT), and X-ray fluorescence (XRF) spectroscopy. The suitability of the approach with respect to three-dimensional soil landscape models was tested using soils along a slope representing different stages of erosion and deposition in a hummocky landscape under arable land use (Calcaric Regosols, Calcic Luvisols, Luvic Stagnosols, Gleyic-Colluvic Regosols). The combination of soil core sampling, pedological description, and three spectroscopic techniques enabled rapid determination and interpretation of horizontal and vertical spatial distributions of soil organic carbon (SOC), soil organic and mineral matter composition, as well as CaCO3, Fe, and Mn contents. Depth profiles for SOC, CaCO3, and Fe contents were suitable indicators for site-specific degrees of erosion and matter transport processes at the pedon-to-field scale. Fe and Mn profiles helped identifying zones of reductive and oxic domains in subsoils (gleyzation). Further methodical developments should implement plant-availability of nutrients, characterization of Fe-oxides, and calibration of the spectroscopic techniques to field-moist samples.

Controlling reduced iron and manganese in a drinking water reservoir by hypolimnetic aeration and artificial destratification

The concentrations of iron (Fe) and manganese (Mn) in the water column have extremely important effects on the water quality of drinking water reservoirs; however, reservoirs often experience problematic Fe and Mn levels during seasonal stratification and rainfall events. Water-lifting aerators (WLAs) were deployed in the Jinpen Reservoir to control these issues with Fe and Mn at the source via bottom aeration and artificial destratification. In this study, variations of Fe and Mn concentrations in the water column, porewater, and sediments, were used to characterize behaviors of reduced Fe and Mn under the conditions of hypolimnetic aeration and artificial destratification during periods of hypolimnion hypoxia and rainfall events. The results showed that replenishing aquatic oxygen levels by aeration can effectively decrease the dissolved Fe and Mn in the water column thereby increasing the sedimentation rate and the diffusive flux of Fe and Mn at the sediment-water interface (SWI). The dissolved Fe was significantly chemically oxidized and the concentration remained relatively low in the water column during WLA operations, while dissolved Mn persistently accumulated in the near-sediment regions because of its complex kinetics. Our in situ profiles of labile Fe and Mn in the sediments demonstrated that the diffusive flux of Mn (JMn) was largely increased by the increased concentration gradient at the SWI, while the diffusive flux of Fe (JFe) decreased. The sediments were observed to rapidly become anoxic and release Fe and Mn after WLA deactivation; this emphasized the importance of appropriate operations linking the artificial and natural mixing periods to prevent SWI hypoxia and the release of reduced substances.

A cellular automaton model integrated with CALPHAD-based thermodynamic calculations for ferrite-austenite phase transformations in multicomponent alloys

A cellular automaton (CA) model, which is directly coupled with thermodynamic calculations based on the CALPHAD method, is developed for the simulation of ferrite (α)-austenite (γ) phase transformations involving the partitioning and long-range diffusion of both interstitial and substitutional elements in multicomponent alloys. A data management scheme is proposed, which renders the efficient coupling between the kinetic simulation using the CA approach and thermodynamic calculations. The present CA model is applied to simulate the cyclic phase transformations between the α- and γ-phases for a quaternary Fe-0.02C-0.2Mn-0.1Si (wt%) alloy in both one- and two-dimensions. The simulated curves of α-volume fraction varying with temperature clearly illustrate the different α-γ transformation stages. The transformation kinetics and concentration profiles of Mn and Si obtained by the 1-D CA simulation agree well with DICTRA predictions. The 2-D simulation of a single circular grain is found to have shorter stagnant stages and faster transformation kinetics than that of the 1-D planar case. This can be attributed to the different growth geometries of 1-D planar and 2-D circular, and the effect of substitutional element Mn on the migrating interface. The non-uniform distribution of C and residual spikes of Mn and Si can be clearly delineated in the 2-D multi-grain simulation. It is shown that the data management scheme proposed in the present work significantly reduces the simulation time for the required thermodynamic data by two orders of magnitude approximately, which makes it feasible to perform valid simulations of microstructural evolution and solute distributions during α-γ phase transformations in steels with more than three components.