Fe Hydroxides Research Articles

Silicate coprecipitation reduces green rust crystal size and limits dissolution-precipitation during air oxidation

Green rusts (GR) are mixed-valence iron (Fe) hydroxides which form in reducing redox environments like riparian and wetland soils and shallow groundwater. In these environments, silicon (Si) can influence Fe oxides’ chemical and physical properties but its role in GR formation and subsequent oxidative transformation have not been studied starting at initial nucleation. Green rust sulfate [GR(SO4)] and green rust carbonate [GR(CO3)] were both coprecipitated from salts by base titration in increasing % mol Si (0, 1, 10, and 50). The minerals were characterized before and after rapid (24 h) aqueous air-oxidation by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fe extended x-ray absorption fine structure spectroscopy (EXAFS), and N2-BET surface area. Results showed that only GR(SO4) or GR(CO3) was formed at every tested Si concentration. Increasing % mol Si caused decreased plate size and increased surface area in GR(CO3) but not GR(SO4). GR plate basal thickness was not changed at any condition indicating a lack of Si interlayering. Air oxidation of GR(SO4) at all % mol Si contents transformed by dissolution and reprecipitation into lepidocrocite and goethite, favoring ferrihydrite with higher % Si content. Air oxidation of GR(CO3) transformed into magnetite and goethite but increasing Si caused GR to oxidize while retaining its hexagonal plate structure via solid-state oxidation. Our results indicate that Si has the potential to cause GR to form in smaller particles and upon air oxidation, Si can either stabilize the plate structure or alter transformation to ferrihydrite.Graphical

Speciation Controls the Kinetics of Iron Hydroxide Precipitation and Transformation at Alkaline pH.

The formation of energetically favorable and metastable mineral phases within the Fe-H2O system controls the long-term mobility of iron complexes in natural aquifers and other environmentally and industrially relevant systems. The fundamental mechanism controlling the formation of these phases has remained enigmatic. We develop a general partial equilibrium model, leveraging recent synchrotron-based data on the time evolution of solid Fe(III) hydroxides along with aqueous complexes. We combine thermodynamic considerations and particle-morphology-dependent kinetic rate equations under full consideration of the aqueous phase in disequilibrium with one or more of the forming minerals. The new model predicts the rate of amorphous 2-line ferrihydrite precipitation, dissolution, and overall transformation to crystalline goethite. It is found that the precipitation of goethite (i) occurs from solution and (ii) is limited by the comparatively slow dissolution of the first forming amorphous phase 2-line ferrihydrite. A generalized transformation mechanism further illustrates that differences in the kinetics of Fe(III) precipitation are controlled by the coordination environment of the predominant Fe(III) hydrolysis product. The framework allows modeling of other iron(bearing) phases across a broad range of aqueous phase compositions.

Microplastics–biofilm in aquatic ecosystem: Formation, pollutants complexation, greenhouse gas emission and ecotoxicology

The omnipresent microplastics (MPs) have gradually become a significant environmental problem due to its adverse consequences for ecological systems. MPs serve as substrates for biofilms colonization, which enhances adsorption of harmful contaminants on MPs surface in the aquatic ecosystem. The present study provides a critical discussion on the mechanism involved in MPs–biofilm formation, microbial colonization and the robust factors influencing the process in the aquatic ecosystem. Subsequently, the impact of MPs–biofilm on adsorption of inorganic and organic contaminants is explored. The ecological significance of MPs–biofilm associated pollutant complex for promoting greenhouse gases (GHGs) emissions from aquatic ecosystem is extensively discussed for understanding the climatic risk. Furthermore, the discussion is extended over ecotoxicological impact of MPs–biofilm on aquatic biodiversity and humans. The protective extracellular polymeric substances secreted by colonised bacteria over MPs during biofilm formation creates sticky MPs surface for heteroaggregates formation with swift adsorption of chemical compounds and microorganisms. MPs with functional aromatic groups facilitate the bacterial adhesion on the surface, but affect formation of biofilm. Alternatively, MPs–biofilm promotes the Mn and Fe hydrous oxides formation that can co–precipitate with heavy metal ions and facilitate in remediation measures. However, MPs biodegradation generates GHGs emission per unit mass, comparably more from freshwater than marine ecosystem. Considering the toxicity, MPs–biofilm induces the oxidative response in fishes, causing painful death and thus, destroys aquatic biodiversity. This study will be useful to address MPs–biofilm associated pollution scenario via trace, test and treat strategy involving future engineering research framework for ecological restoration.

Iron as a precursor of aggregation and vector of organic carbon to sediments in a boreal lake

AbstractWhile organic matter (OM) interactions in the water column prevent iron (Fe) precipitation and sedimentation, Fe also acts as a precursor of aggregation and a vector of OM to sediments. This study aims to characterize Fe–OM interactions to understand the role of Fe in promoting aggregation and transport of OM. Samples of Fe and OM were collected from water, settling material, and sediment along a gradient starting from the inlet and continuing offshore within a boreal lake. Fe speciation was determined using X-ray absorption spectroscopy (XAS), and the chemical composition of OM was assessed using Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT IR) and Nuclear magnetic resonance spectroscopy (NMR). The results show a decrease in Fe and OM concentrations in the water column with increasing distance from the inlet. Winter sampling revealed a shift in Fe speciation from dominance of organically complexed Fe to an increase in Fe(oxy)hydroxide, accompanied by a loss of aromatic and carboxylate function of OM. Summer sampling revealed no significant changes along the gradient, with Fe(oxy)hydroxide and carbohydrates dominating the water phase. Interestingly, settling particles and surface sediments were dominated by Fe(oxy)hydroxides and aliphatic OM. We propose that phototransformation may be an important process that influences the interaction between Fe and OM and, as a consequence, their fate along the spatial gradient. Our study suggests a photochemically induced loss of carboxylate groups, reflected by an increased carbohydrate-to-carboxylate ratio along the gradient, particularly in winter, and generally lower levels during summer. Loss of carboxylate function promotes the formation of Fe(oxy)hydroxides, which in turn, facilitates the aggregation and sinking of OM, particularly aliphatic components. These insights contribute to a broader understanding of carbon cycling and storage in lakes. Future studies should assess the significance of photochemical processes to OM burial and it how may change given trends in Fe and OM in northern regions.

Effects of varying pH on the passive film formation in high-strength anti-seismic rebar

ABSTRACT In this paper, electrochemical measurements (CV, PDP, M-S and EIS) and surface characterisation techniques (SEM, EDS, AFM, TEM and XPS) were used to study the passivation phenomenon in high-strength anti-seismic rebar under varying pH conditions. The results show that a passive film with a polycrystalline structure with a certain roughness is formed on the surface of the rebar. The passive film is mainly composed of Fe oxides and hydroxides, and there are also oxides of Nb, Mn, Si and Cr. The existence of these oxides builds a dense and stable passive film. The passive film is basically stable at 7d, but the early growth of the passive film at higher alkalinity show instability. After 10 days of passivation, with the decrease of pH, the passive film gradually densifies, the pitting potential increased by about 0.03–0.14 V, the carrier density decreased by 30% on average, the phase angle increased slightly, and the ratios of Fe2+/Fe3+ and O2−/OH− increased, resulting in a passive film thickness of 6.2 nm.

Molybdenum isotope behavior during subduction zone metamorphism

The molybdenum (Mo) isotope composition (defined as δ98Mo measured per mil relative to NIST-3134) of many modern arc systems and the upper continental crust is heavier than the mantle and most subducting slab lithologies. This observation has led to a model whereby fluids leaving the slab transfer isotopically heavy Mo preferentially to the mantle wedge, leaving the residual slab isotopically lighter. We explore this model via an Mo isotope study of the metasedimentary and mélange lithologies of the Catalina Schist in California. These rocks record subduction zone metamorphism over a wide range of high-pressure/low-to-medium temperature conditions.Mo isotope compositions of the metasedimentary rocks decrease with increasing metamorphic grade, from a δ98Momean of −0.04 ± 0.13 ‰ (1σ, n = 3) for the lawsonite albite facies to −0.38 ± 0.07 ‰ (1σ, n = 5) for the epidote–amphibolite facies. The highest grade [amphibolite facies] samples show a slight uptick in δ98Mo values, with a mean of −0.19± 0.14 ‰ (1σ, n = 4). When coupled with major and trace element variations, the changes in δ98Mo appear to reflect metamorphic effects rather than sedimentary source rock heterogeneity. The positive relationship between δ98Mo and [Mo] and negative relationship with Ce/Mo argue for progressive loss of Mo with isotope fractionation during increasing P-T conditions; the reversal of this trend, seen in the increase in δ98Mo between the epidote amphibolite and amphibolite facies may reflect a subsequent partial re-fertilization by fluids carrying isotopically heavy Mo. δ98Mo values in the mélange across all grades are highly variable, ranging from −1.75 to −0.19, consistent with large scale mobilization of Mo with isotope fractionation. A metasomatized amphibolite block and reaction rind show similar light isotope compositions and signs of Mo depletion and transport. Together these three whole-rock data sets demonstrate pervasive open system behavior and mobility of Mo in the Catalina Schist. LA-ICP-MS Mo measurement of minerals in the differing metasedimentary metamorphic grades indicates that Fe oxides and/or hydroxides, titanite, rutile, and epidote are important reservoirs of Mo but when comparing mineral Mo content and modal abundance to whole-rock Mo concentrations, several samples were found to have “missing Mo”, something that has been observed in other Mo inventory studies. We attribute this to sample heterogeneity between the thin section and whole-rock powder scales.

Nickel-based hydroxide composite as electrocatalyst for hydrogen evolution reaction

The development of earth-abundant transition-metal-based electrocatalysts for hydrogen evolution reaction (HER) is commercial. In this work, Fe (Co) and Ni hydroxide nanocomposites on Ni foam were synthesized as HER catalyst, FeNi-LDH/NF has the overpotentials of 291 mV at 50 mA cm−2 and 369 mV at 100 mA cm−2. It is found that FeNi-LDH/NF and CoNi-LDH/NF have large Cdl and ECSA, low Rct. Therefore, this simple method to prepare FeNi-LDH/NF and CoNi-LDH/NF can enhance the properties of electrocatalysts, which make FeNi-LDH/NF and CoNi-LDH/NF a promising material to replace noble metal-based catalysts.

Chemical speciation and rice uptake of soil molybdenum-Investigation with X-ray absorption spectroscopy and isotope fractionation

Molybdenum (Mo) contamination of farmland soils poses health risks due to Mo accumulation in crops like rice. However, the mechanisms regulating soil availability and plant uptake of Mo remain poorly understood. This study investigated Mo uptake by rice plants, focusing on Mo speciation and isotope fractionation in soil and rice plants. Soil Mo species were identified as sorbed Mo(VI) and Fe-Mo(VI) using X-ray absorption spectroscopy (XAS). Soil submergence during rice cultivation led to the reductive dissolution of Fe-associated Mo(VI) while increasing sorbed Mo(VI) and Ca-Mo(VI). Soil Mo release to soil solution was a dynamic process involving continuous dissolution/desorption and re-precipitation/sorption. Mo isotope analysis showed soil solution was consistently enriched in heavier isotopes during rice growth, attributed to re-sorption of released Mo and the uptake of Mo by rice plants. Mo was significantly associated with Fe in rice rhizosphere as sorbed Mo(VI) and Fe-Mo(VI), and around 60 % of Mo accumulated in rice roots was sequestrated by Fe plaque of the roots. The desorption of Mo from Fe hydroxides to soil solution and its subsequent diffusion to the root surface were the key rhizosphere processes regulating root Mo uptake. Once absorbed by roots, Mo was efficiently transported to shoots and then to grains, resulting in heavier isotope fractionation during the translocation within plants. Although Mo translocation to rice grains was relatively limited, human exposure via rice consumption remains a health concern. This study provides insights into the temporal dynamics of Mo speciation in submerged paddy soil and the uptake mechanisms of Mo by rice plants.

Modes of occurrence of tungsten in coals: a review

Relevance. The necessity to know the conditions and forms of W concentration in coals for solving a number of scientific and engineering problems at complex development of coal deposits. Aim. Complex estimation of W modes of occurrence in coal for development of measures for rational ecologically safe use of coal. Methods. Correlation analysis, scanning electron microscopy, infrared spectroscopy, coal group analysis, inductively coupled plasma mass spectrometry, instrumental neutron activation analysis. Results and conclusions. The modes of occurrence of W in coal were studied by a complex of methods. In the majority of W-enriched coal deposits, a negative significant correlation of its content in coal and coal ash with ash yield was found, indicating its association with organic matter. Using the method of coal group composition analysis, it was found that the main carrier and concentrator of W in the studied lignite deposits is organic matter. The contribution of the mineral phase in general in W-rich coals and in coals with its normal content does not exceed 20%, usually less than 5%. These conclusions are also confirmed by infrared spectroscopy data, according to which no more than 15% of the metal in the samples studied is associated with mineral phases. The association of W with high molecular humic acids predominates. In anomalously W-enriched lignites, the humic acid phase represents 76 to 88% of the gross metal content. The role of bitumen and low-molecular-weight humic acids in the balance of W is marginal at their different levels in the coals. Mineral phases of W are not characteristic of coal. Nano-microinclusions of scheelite, wolframite, hubnerite and ferberite were recorded as isolated occurrences. The main mineral phases are associated with Fe and Mn hydroxides, in which W is presented as a trace element in the amount of 1–5% together with other elements (Ge, As, etc.). In more metamorphosed bituminous and anthracite coals, authigenic mineral formations were found, represented by tungstite, phyllotungstite, native W and complex Fe-Mn-Ca-W-O mineral phases.

Enhanced mixing of north Pacific deep western-boundary currents after Panama seaway closure

The near closure of the Panama Seaway during the late Miocene resulted in the modification of deep-ocean currents and the formation of modern oceanic circulation. However, it is unclear how this closure impacted the north Pacific western-boundary deep circulation since the late Miocene. Here, we combine Nd isotope records in the north Pacific and a newly developed water-mass proxy (amorphous Fe hydroxide content, AFHs) from the western Pacific to investigate deep-current evolution since the late Miocene. These proxies consistently indicate a significant deep water-mass change in the north Pacific at ∼5 Ma, reflecting shoaling of the Panama seaway that curtailed intermediate and deep-water exchange between the East Equator Pacific and Atlantic. Although intensification of Antarctic bottom water since ∼3.4 Ma (Northern Hemisphere Glaciation) has been widely documented, a decrease in western-boundary currents has occurred since ∼3 Ma. We suggest that this phenomenon is likely due to the increased mixing of south-ward and north-ward deep western-boundary currents in the north Pacific. This water mass mixing may have a significant impact on the western Pacific biogeochemical cycling.

Effect of Temperature on Passive Film Characteristics of LPBF (Laser Powder-Bed Fusion) Processing on UNS-S31603.

The effect of temperature on the localized corrosion resistance and passive film characteristics of laser powder-bed fusion (LPBF) 316L (UNS S31603) was studied in a buffered 3.5 wt% NaCl solution at 25, 50, and 75 °C. DC techniques such as cyclic potentiodynamic polarization showed lower passive current densities, high breakdown potentials, and a higher resistance to initial breakdown compared with wrought 316L samples at all temperatures. However, LPBF 316L was more susceptible to metastable pitting at potentials before film breakdown and higher damage accumulation post film breakdown. AC techniques, such as Mott-Schottky analysis and electrochemical impedance spectroscopy, showed that the formed passive film was more robust on the LPBF 316L samples at all temperatures, accounting for the higher initial resistance to pitting. However, with increasing temperatures, the film formed had an increasing concentration of defect density. Passive compositions at the various test temperatures studied using X-ray photoelectron spectroscopy (XPS) showed that the LPBF samples showed higher amounts of Cr and Fe oxides and hydroxides compared with the wrought samples, which made the passive films on the LPBF samples more compact and protective. Investigation of the pits formed on the LPBF showed the preferential regions of attack were the melt-pool boundaries and cell interiors due to their being depleted of Cr and Mo when compared with the boundaries and matrix.

Enhanced Oxygen Evolution Reaction Performance in Co–Fe Hydroxides through Boron Doping

Among hydrogen production methods, water electrolysis stands out, but its efficiency is hampered by the substantial energy barrier of the oxygen evolution reaction (OER). To address this, incorporating electron‐deficient boron (B) into Co–Fe hydroxide (CoFeOxHy) promotes higher oxidation states of involved metals, greatly enhancing OER activity and charge transfer capabilities. Herein, the synthesis of a range of amorphous CoFeB nanoparticles with varying Fe to (Co+Fe) atomic ratios achieved through a simple chemical reduction method using CoFe‐Prussian blue analogs as precursors and employing Mössbauer spectroscopy to observe structural characteristics before and after transformation is reported. Among these nanoparticles, the CoFe0.25B variant, exhibiting favorable electrochemical properties, is chosen and subsequently subjected to hydrolysis to yield CoFe0.25BOH nanoparticles, serving as an active catalyst for OER. At a current density of 10 mA cm−2, the overpotentials for CoFe0.25OxHy and CoFe0.25BOH are 362 and 310 mV, respectively, with Tafel slopes decreasing from 393 to 93 mV dec−1. Furthermore, the i–t test reveals no significant loss of electrochemical performance within 24 h, substantiating the efficacy of enhancing the electrocatalytic performance of CoFeOxHy through the introduction of electron‐deficient elements. This research offers novel insights into the development of efficient and stable water electrolysis catalysts.

Exploring Structural Evolution Behaviors of Ligand‐Defect‐Rich Ferrocene‐Based Metal‐Organic Frameworks for Electrochemical Oxygen Evolution via Operando X‐Ray Absorption Spectroscopy

AbstractMetal‐organic frameworks (MOFs) have exhibited encouraging catalytic activity for the oxygen evolution reaction (OER), a crucial process for water electrolysis to produce green hydrogen. Nonetheless, distinguishing the source of catalytic activity and establishing the structure‐composition‐property relationships of MOFs during OER processes remain challenging. Here, for the first time, operando X‐ray absorption spectroscopy (XAS) is utilized to monitor the structural evolution and identify the active components of ferrocene‐based MOFs (Ni‐Fc) for OER. Ligand‐defect‐rich Ni‐Fc is synthesized via the co‐deposition method. After electrochemical activation, Ni‐Fc exhibits superior electrocatalytic activity (228 mV at 10 mA cm−2 in 0.1 m KOH), which is highly competitive compared with state‐of‐the‐art electrocatalysts. Operando XAS analysis and ex‐situ characterizations reveal the structural reconstruction of Ni‐Fc into amorphous NiFe‐catalysts (a‐NiFe) during the activation process, and further into real catalytic phases (a‐NiFe‐C) under catalytic potential greater than 1.45 V (vs RHE). In catalytic phases, in‐situ formed deprotonated and oxygen‐defected Ni oxyhydroxide analogues act as catalytic sites, while Fe hydroxide analogues derived from ligands optimize the electronic structure of Ni sites for improving OER activity. Density functional theory (DFT) analysis indicates a reduced energy barrier in a‐NiFe‐C compared to pristine MOFs, supporting the improved catalytic activity of the latter.

Ultraselective enrichment of trace elements in seawater by Co-rich ferromanganese nodules

Ferromanganese nodules are huge metal resources and windows into Earth processes, being widely distributed in vast deep-sea basins covered by sediments. Co-rich ferromanganese nodules are typical hydrogenetic deposits that can effectively scavenge and enrich multiple trace elements in seawater. However, uncertainty still exists regarding the enrichment process of hydrogenetic nodules and their interactions with the distribution of trace elements in seawater. Herein, we analyzed up to 73 elements in Co-rich ferromanganese nodules from the western Pacific and found that high-field strength and redox-sensitive elements are selectively distributed between the Fe and Mn hydroxide phases. These elements are highly enriched in ferromanganese nodules over seawater and upper continental crustal values. The enormous amounts of ferromanganese deposits make them the major, even exclusive, budget for Te, Mn, Co, Ce, Pb, Bi, Pt, Ru, Rh, Ni, and Mo. The distributions of trace elements in seawater are both the cause and result of scavenging by ferromanganese deposits and of biogeochemical cycling. In particular, ferromanganese deposition is responsible for the distributions of scavenged-type elements such as Mn, Co, Ce, Pb, Bi, and Te in seawater. Based on the distributions of elements in seawater and at the water–sediment interface, we propose a new two-stage model for nodule metallogenesis. Stage I is the initial enrichment of trace elements by the sinking of ferromanganese hydroxide colloids, which regulate the distributions of scavenged-type elements in the water column. Stage II is the top-down migration of trace elements dominated by bioparticle cycling, which promotes the re-enrichment of trace elements by ferromanganese hydroxides at the water–sediment interface.

Evaluation of coal mine drainage and associated precipitates for radium and rare earth element concentrations

Coal mine drainage (CMD) and associated metal-rich precipitates have recently been proposed as unconventional sources of rare earth elements (REEs). However, the potential occurrence of radium (Ra), a known carcinogen, with the REE-bearing phases has not been investigated. We hypothesized that Ra may occur in solids that are precipitated from CMD as a “radiobarite” solid solution ((Ba,Sr,Ra)SO4) and/or adsorbed with hydrous metal oxides. REEs have been documented to sorb or co-precipitate with iron (Fe), manganese (Mn), and aluminum (Al) oxyhydroxide in CMD solids. Likewise, Ra has been documented to sorb to hydrous Fe and Mn oxides especially where sulfate (SO4) and/or barium (Ba) concentrations are insufficient to precipitate radiobarite. Thus, we conducted the first-ever survey of Ra concentrations in corresponding CMD water and solid samples in the United States. Samples were analyzed from 4 untreated and 9 treated CMD sites in both the bituminous and anthracite coal regions of Pennsylvania across a range of pH and SO4 concentrations. The dissolved Ra in CMD was relatively low (<0.5 Bq/L), consistent with radiobarite solubility; however, CMD solids were largely composed of amorphous Fe, Al, and Mn oxyhydroxide and silicate minerals. Ra was associated with Mn-enriched CMD solids, upwards of 875 Bq/kg. Total REE + yttrium (Y) content in the CMD solids was enriched upwards of 3600 mg/kg and was significantly correlated with Al content. These preliminary results suggest that REE extraction may target Al-rich solids to avoid Ra in Mn-rich solids.

Compound database for gaseous metal hydroxides and oxyhydroxides

In this study computational methods are used to derive thermochemical data for Sc, Fe, Co, Ni, and Hf hydroxides and oxyhydroxides. As done previously, molecular geometries and vibrational modes were derived with DFT methods; for the enthalpies of formation more computationally intensive coupled cluster methods were necessary. For each species ΔfHo(298), So(298), and Cp in the form A + BT + CT2 + D/T + E/T2 with A, B, C, D, and E fitted constants are presented. These are combined with previously reported calculations for Al, Cr, Si, Ta, Al, Zr, Y, Yb, Gd, and Mn to build a compound database for metal hydroxides and oxyhydroxides. Sample calculations for applications where high temperature water vapor is encountered are shown. The majority of the database was generated from ab initio calculations; however, experiments were critical benchmarks for many of the species.

Enhancing the oxygen evolution reaction activity of Iron (oxy)hydroxide by increasing reactive sites with morphology modulation

Nonprecious transition metal (oxy)hydroxides play a vital role in accelerating the kinetics of sluggish oxygen evolution reaction (OER). The fast electron transfer from the electrocatalyst material surface to the electrode is key to obtaining improved OER activity in terms of improved reaction kinetics and reduced overpotential. Therefore, it is highly desirable to design electrocatalysts with efficient electron transport properties. Here, an approach of modulation of the morphology of Iron (oxy)hydroxide through the incorporation of Cr is used to obtain high electrocatalytic activity. The incorporation of Cr resulted in the modified morphology of Iron (oxy)hydroxide with the formation of a porous pit-like structure which offers large reactive sites to effectively adsorb the reactants and allow an efficient electron transfer pathway from electrocatalyst to circuit through Ni foam electrode. A 50 % Cr incorporated electrocatalyst (Fe5.0Cr5.0 (oxy)hydroxide) exhibits excellent OER activity with an overpotential of 237 mV and 297 mV at current densities of 10 and 100 mA cm─2, respectively owing to fast electron transport from electrocatalyst to substrate due to formation of porous pit-like structure. Furthermore, Fe5.0Cr5.0 electrocatalyst shows high durability of over 100 h at 10 mA cm─2. The enhanced OER activity is attributed to the effect of Cr incorporation and morphology modulation which helps to modulate the electronic structure as well as electrocatalytic active sites of Fe (oxy)hydroxide. The systematic incorporation and modulation of morphology pave the way for designing future electrocatalysts for various electrochemical activities.

Systematic evaluation of methods for iron-impregnation of biochar and effects on arsenic in flooded soils.

There is a need for innovative strategies to decrease the mobility of metal(loids) including arsenic (As) and cadmium (Cd) in agricultural soils, including rice paddies, so as to minimize dietary exposure to these toxic elements. Iron (Fe)-modified biochars (FBCs) are used to immobilize As and Cd in soil-water systems, but there is a lack of clarity on optimal methods for preparing FBCs because there are only limited studies that directly compare BCs impregnated with Fe under different conditions. There is also a lack of information on the long-term performance of FBCs in flooded soil environments, where reductive dissolution of Fe (oxy)hydroxide phases loaded onto biochar surfaces may decrease the effectiveness of FBCs. This study uses material characterization methods including FTIR, SEM-EDX, BET, and adsorption isotherm experiments to investigate the effects of Fe-impregnation methods (pH, pyrolysis sequence, and sonication) on the morphology and mineralogy of Fe loaded onto the biochar surface, and to FBC adsorbent properties for arsenate (As(V)), arsenite (As(III)), and Cd. Acidic impregnation conditions favored the adsorption of As(III) onto amorphous Fe phases that were evenly distributed on the biochar surface, including within the biochar pore structure. The combination of sonication with acidic Fe-impregnation conditions led to the best adsorption capacities for As(V) and As(III) (4830 and 11,166 μg As g-1 biochar, respectively). Alkaline Fe-impregnation conditions led to the highest Cd adsorption capacity of 3054 μg Cd g-1 biochar, but had poor effectiveness as an As adsorbent. Amending soil with 5% (w/w) of an acid-impregnated and sonicated FBC was more effective than an alkaline-impregnated FBC or ferrihydrite in decreasing porewater As concentrations. The acid-impregnated FBC also had greater longevity, decreasing As by 54% and 56% in two flooded phases, probably due to the greater stability of Fe(III) within the biochar pore structure that may have a direct chemical bond to the biochar surface. This study demonstrates that FBCs can be designed with selectivity towards different As species or Cd and that they can maintain their effectiveness under anaerobic soil conditions. This is the first study to systematically test how impregnation conditions affect the stability of FBCs in soils under multiple drying-rewetting cycles.

A comparative analysis of corrosion assessment techniques for steel in reinforced concrete exposed to brine water environments

Abstract Corrosion assessment of steel-reinforced concrete specimens submerged in synthetic brine water with various chloride concentrations for 1–16 weeks was performed. Mass loss measurements combined with electrochemical techniques – half-cell potential, linear polarization (LP), and electrochemical impedance spectroscopy (EIS) – were employed. The results obtained from all corrosion assessments – on-site testing (half-cell potential measurements), laboratory scale (LP and EIS measurements), and destructive testing (mass loss or immersion measurements) – exhibited remarkable consistency, complementarity, and mutual supportiveness. Corrosion rate (CR) values from mass loss were close to those obtained from LP and EIS. The corrosion resistance decreased with increasing chloride concentration and immersion time, as indicated by the highest CR, Ca2+, and Fe2+ concentrations, and the lowest half-cell potential and polarization resistance. X-ray photoelectron spectroscopy investigation on the corroded steel surface revealed Fe(III) oxides and hydroxides and Fe(III) (FeCl3), corresponding to the reduction in polarization resistance in the LP and EIS results.

State of the Art on Fe Precipitation in Porous Media: Hydrogeochemical Processes and Evolving Parameters

The mixing of terrestrial groundwater and seawater creates dynamic reaction zones in intertidal areas, where land-derived Fe(II) is oxidized to Fe(III) and then precipitates as Fe hydroxides at the groundwater–seawater interface. These hydrogeochemical processes contribute to the formation of iron bands at the saltwater wedge (SW) and beneath the upper saline plume (USP). This study provides a comprehensive review of physical and geochemical processes at field scale in coastal areas, explores the impact of mineral precipitation on pore structure at pore scale, and synthesizes reactive transport modeling (RTM) approaches for illustrating continuum-scale soil physio-chemical parameters during the evolution of porous media. Upon this review, knowledge gaps and research needs are identified. Additionally, challenges and opportunities are presented. Therefore, we reach the conclusion that the incorporation of observational data into a comprehensive physico-mathematical model becomes imperative for capturing the pore-scale processes in porous media and their influence on groundwater flow and solute transport at large scales. Additionally, a synergistic approach, integrating pore-scale modeling and non-invasive imaging, is equally essential for providing detailed insights into intricate fluid–pore–solid interactions for future studies, as well as facilitating the development of regional engineering-scale models and physio-chemical coupled models with diverse applications in marine science and engineering.