Dissolution-reprecipitation Mechanism Research Articles

New insights into selective leaching and ferritization in 15-15Ti austenitic steel in lead-bismuth eutectic through parent phase reconstruction

Selective leaching accompanied by ferritization is widely observed in the dissolution corrosion of austenitic steels exposed to lead-bismuth eutectic. This study proposes a refined mechanism for selective leaching involving solid-state diffusion followed by dissolution-reprecipitation processes. Pole figure analysis reveals that ferritization follows the Nishiyama-Wassermann orientation relationship, independent of corrosion environment or corrosion behavior. Moreover, austenite reconstruction analysis shows that the growth of ferrite is hindered by the misfits at random high-angle grain boundaries. Hence the above results suggest that ferritization is initially driven by solid-state diffusion. As LBE channels open within the grains, further growth of ferrite is assisted by the dissolution-reprecipitation mechanism. Additionally, the formation of dual-oriented ferrite grains at Σ3 twin boundary is observed, and the selectivity of this formation with respect to grain boundary and ferrite orientation is discussed.

Synergy between Ca2+ and high ionic field‐strength cations during the corrosion of alkali aluminoborosilicate glasses in hyper‐alkaline media

AbstractOne major factor impeding the design of nuclear waste glasses with enhanced waste loadings is our insufficient understanding of their composition–structure–durability relationships, specifically in the environments the waste form is expected to encounter in a geological repository. In particular, the high field‐strength cations (HFSCs) are an integral component of most waste streams. However, their impact on the long‐term performance of the glassy waste form remains mostly undeciphered. In this context, the present study aims to understand the impact of some HFSCs (i.e., Nb5+, Zr4+, Ti4+, and La3+) on the dissolution behavior of alkali/alkaline‐earth aluminoborosilicate‐based model nuclear waste glasses in hyper‐alkaline media. At pH = 13, the studied glasses dissolve through the dissolution–reprecipitation mechanism, with Ca precipitation being the most vital step to passivation. In Ca‐free glasses, although the HFSCs slow down the forward rate, they do not seem to impact the residual rate behavior of glasses. The presence of Ca2+, however, initiates the rapid precipitation of network polymerizing HFSCs (i.e., Nb5+, Zr4+, and Ti4+) into a Ca2+/HFSCs‐based passivating layer, thus suggesting a synergy between Ca2+ and HFSCs that contributes to the enhanced long‐term durability of the glasses. Such synergy is not strongly evident for La3+, but instead, a potential La/Si affinity is observed upon the formation of the alteration layer.

Molecular insight into the initial hydration of tricalcium aluminate

Portland cement (PC) is ubiquitously used in construction for centuries, yet the elucidation of its early-age hydration remains a challenge. Understanding the initial hydration progress of tricalcium aluminate (C3A) at molecular scale is thus crucial for tackling this challenge as it exhibits a proclivity for early-stage hydration and plays a pivotal role in structural build-up of cement colloids. Herein, we implement a series of ab-initio calculations to probe the intricate molecular interactions of C3A during its initial hydration process. The C3A surface exhibits remarkable chemical activity in promoting water dissociation, which in turn facilitates the gradual desorption of Ca ions through a metal-proton exchange reaction. The dissolution pathways and free energies of these Ca ions follow the ligand-exchange mechanism with multiple sequential reactions to form the ultimate products where Ca ions adopt fivefold or sixfold coordination. Finally, these Ca complexes reprecipitate on the remaining Al-rich layer through the interface-coupled dissolution-reprecipitation mechanism, demonstrating dynamically stable inner-sphere adsorption states. The above results are helpful in unmasking the early-age hydration of PC and advancing the rational design of cement-based materials through the bottom-up approach.

The role of magmatic-to-carbohydrothermal processes in rare earth mineralization in Hogenakkal carbonatites, India

The Paleoproterozoic Hogenakkal complex (India) consists of silicate-rich-, silicate-poor calcite carbonatite (carbonatite-I and II) and clinopyroxenite emplaced in granulite country rocks. Carbonatite-I contains abundant clinopyroxenite and K-feldspar pegmatite xenoliths. Silicate minerals are the primary governing factors in the formation of a distinctive lithology-specific REE mineralization. Carbonatite-I consists of magmatic Mg-Fe-Sr-bearing calcite (Cal-1), and REE-poor apatite (Ap-1) together with minor REE-rich apatite (Ap-2). The interaction of the carbonatite-forming magma with the clinopyroxenite and K-feldspar pegmatite xenoliths releases K, Al, and Si, forming phlogopite and pargasite-edenite. The late-magmatic carbonatite-II consists of Sr-rich calcite (Cal-2), Ap-2 and minor Ap-1. The carbohydrothermal REE-mineralization in both the carbonatites is induced by a magma-derived fluid dominated by REE, OH−, CO32−, and SO42−. The high modal abundance of silicate minerals and the continuous leaching of Si and Al from clinopyroxene, K-feldspar xenocrysts, and phlogopite by REE-enriched fluid facilitated the formation of allanite-(Ce) and actinolite in carbonatite-I. In contrast, in the silicate-poor carbonatite-II, the same fluid with low Si-saturation resulted in monazite-(Ce) + celestine-baryte±strontianite precipitation. The carbohydrothermal fluid is also responsible for the alteration of apatite to monazite-(Ce) by a dissolution-reprecipitation mechanism releasing fluorine. Consequently, localized crystallization of F-rich hydroxylbastnäsite-(Ce) is common in carbonatite-II. Complex intra-grain compositional and textural variations of monazite-(Ce) and hydroxylbastnäsite-(Ce) suggest further interaction of these minerals with evolving fluid. The waning phase of the carbohydrothermal activity is marked by an assemblage consisting of pure calcite (Cal-3) + quartz±magnetite in carbonatite-I. The Ba-Sr-REE mineralization and the overall mineral assemblages of the Hogenakkal carbonatites represent a transition from a magmatic to a carbohydrothermal stage.

Interaction Mechanism between Slags and Alkali Silicate Activators: An Approach Based on the Al Phases.

In this study, we examined the early-stage interaction of three types of slag and six activators with different chemical compositions. To determine the degree of hydration (DOH) and hydrate assemblage in alkali-activated slag (AAS), we employed EDX, XRD, and NMR analyses. We found that with increasing silicate concentration in the activator, the DOH in the AAS varied, whereas the proportion of C-(N)-A-S-H increased and the other Al-containing phase decreased. When examining the impact of the activator on glass dissolution, it is apparent that an index based on the degree of depolymerization of the glass structure correlates with the DOH and the proportion of hydrotalcite in the AAS. Coupled with the activator's modulus, this index can be utilised to elucidate the dissolution-reprecipitation mechanism that governs the interaction between the activator and slag.

Effect of particles size of TiC on oxidation resistance of in-situ TiC/Ni composite

The oxidation resistance of TiC/Ni composites is crucial for its application in high-temperature oxidation environment. The in-situ TiC/Ni composites are fabricated by reactive sintering method, and the influence of TiC particle size on oxidation resistance of composite is studied. The particle size of TiC increases from 1.54 μm to 2.40 μm as the sintering holding time prolongs from 2 h to 6 h, due to the dissolution-reprecipitation mechanism. The oxidation kinetics of in-situ TiC/Ni composite with different TiC particle size oxidized at 800 °C for 100 h obeys parabolic kinetics. The oxidation mass gain of composite increases from 7.471 mg•cm−2 to 8.454 mg•cm−2, and the oxide scale on composites becomes thicker, as the particle size of TiC increases from 1.54 μm to 2.40 μm. The reduction of TiC particle size facilitates the formation of a dense and continuous oxide scale on composite, helpful to restrict the diffusion of O, Ti and Ni atoms during oxidation. Therefore, the reduction of TiC particle size is contributed to the optimization of oxidation resistance of in-situ TiC/Ni composites.

Chemistry and Dynamics of Supercritical Carbon Dioxide and Methane in the Slit Pores of Layered Silicates.

ConspectusIn the mid 2010s, high-pressure diffraction and spectroscopic tools opened a window into the molecular-scale behavior of fluids under the conditions of many CO2 sequestration and shale/tight gas reservoirs, conditions where CO2 and CH4 are present as variably wet supercritical fluids. Integrating high-pressure spectroscopy and diffraction with molecular modeling has revealed much about the ways that supercritical CO2 and CH4 behave in reservoir components, particularly in the slit-shaped micro- and mesopores of layered silicates (phyllosilicates) abundant in caprocks and shales. This Account summarizes how supercritical CO2 and CH4 behave in the slit pores of swelling phyllosilicates as functions of the H2O activity, framework structural features, and charge-balancing cation properties at 90 bar and 323 K, conditions similar to a reservoir at ∼1 km depth. Slit pores containing cations with large radii, low hydration energy, and large polarizability readily interact with CO2, allowing CO2 and H2O to adsorb and coexist in these interlayer pores over a wide range of fluid humidities. In contrast, cations with small radii, high hydration energy, and low polarizability weakly interact with CO2, leading to reduced CO2 uptake and a tendency to exclude CO2 from interlayers when H2O is abundant. The reorientation dynamics of confined CO2 depends on the interlayer pore height, which is strongly influenced by the cation properties, framework properties, and fluid humidity. The silicate structural framework also influences CO2 uptake and behavior; for example, smectites with increasing F-for-OH substitution in the framework take up greater quantities of CO2. Reactions that trap CO2 in carbonate phases have been observed in thin H2O films near smectite surfaces, including a dissolution-reprecipitation mechanism when the edge surface area is large and an ion exchange-precipitation mechanism when the interlayer cation can form a highly insoluble carbonate. In contrast, supercritical CH4 does not readily associate with cations, does not react with smectites, and is only incorporated into interlayer slit mesopores when (i) the pore has a z-dimension large enough to accommodate CH4, (ii) the smectite has low charge, and (iii) the H2O activity is low. The adsorption and displacement of CH4 by CO2 and vice versa have been studied on the molecular scale in one shale, but opportunities remain to examine behavioral details in this more complicated, slit-pore inclusive system.

Formation of solid solution structures in (Ti,W,Ta,Mo)(C,N) cermet via spark plasma sintering

In this paper, a (Ti,W,Ta,Mo)(C,N) cermet material was prepared by spark plasma sintering. The formation of (Ti,W,Ta,Mo)(C,N) solid solution was characterized, and the effects of the sintering behavior and the binder content on the microstructure and mechanical properties of (Ti,W,Ta,Mo)(C,N) cermet materials were investigated. The results shown that a core-shell microstructure with cyclic gradient distribution was formed through the dissolution-reprecipitation mechanism, and the sintering behavior was improved and the relative density was increased. The optimized binder content can promote the dissolution and diffusion of Ti(C,N) particles. At the same time, the crack deflection and crack bridging caused by the core-shell microstructure improved the fracture toughness of the cermets, and the lattice distortion caused by fine grain and solid solution enhanced the flexural strength. When the binder content is 9 wt%, the mechanical properties of the cermet is better, the Vickers hardness, the flexural strength and the fracture toughness reach 18.17 ± 0.14 GPa, 1510.5 ± 47 MPa and 8.16 ± 0.40 MPa·m1/2, respectively.

Revisiting deformation anisotropy of ω-variants via microcompression of β oriented Ti85Nb15 alloy

The disperse-precipitated ω has a significant strengthening effect on metastable β-titanium (Ti) alloys. The (211)β[1¯11]β/(2¯11)β[111]β slip systems are massively activated during plastic deformation. However, the interaction of the four-equivalent crystallographic ω variants with these slip systems has not been investigated. Here, we revisit the evolvement of the internal ω-precipitate via systematically exploring the compressive mechanical behavior of <110>β-oriented Ti85Nb15 micropillars. Firstly, the four ω-variants show the strong deformation anisotropies. Secondly, this deformation anisotropy is pertinent to the interaction with dislocations. Dislocations originating from the β-matrix can continuously across the ω1-variant, but have no ability to across the ω2 variant. The accumulation of dislocations in the interface of ω2/β leads to severe local stress concentrations that ultimately prompts the dismemberment of ω2. Finally, we confirmed by first-principles calculations that the proliferation of ω1-variant owing to the reorientation of ω2-variant, that is, ω2→ω1, rather than a dissolution-reprecipitation mechanism, ω2→β→ω1. These findings are beneficial for a comprehensive understanding of the deformation mechanism of the ω-variants in metastable β-Ti alloys.

Morphology Control of Transition Metal Oxides by Liquid-Phase Process and Their Material Development

It is well known that the functionality of inorganic materials strongly depends on the chemical composition, morphology, particle size, crystal facet, etc., which are strongly influenced by the synthesis process. The precise control of the synthesis process is expected to lead to the discovery of new functionality and improvement of the functionality of materials. For example, in a high-temperature solid-phase reaction, it is difficult to control the morphology of nanocrystals. On the other hand, synthesizing functional materials using solution processes, such as hydrothermal and solvothermal reactions, makes it possible to control the morphology and particle size precisely. Usually, the solution process is strongly related to the dissolution reprecipitation mechanism. Therefore, the material composition can be strictly controlled and is suitable for forming fine particles with high crystallinity. In this review paper, the role of the solvent in the solution process, its effect on particle size and morphology of the transition metal oxide, and the related functional improvement will be focused. Furthermore, the direct formation of functional thin films by the solution process and the morphology control by non-oxide materials by the topotactic reaction will also be introduced.

A sustainable one-pot method to transform seashell waste calcium carbonate to osteoinductive hydroxyapatite micro-nanoparticles.

We have developed a straightforward, one-pot, low-temperature hydrothermal method to transform oyster shell waste particles (bCCP) from the species Crassostrea gigas (Mg-calcite, 5 wt% Mg) into hydroxyapatite (HA) micro/nanoparticles. The influence of the P reagents (H3PO4, KH2PO4, and K2HPO4), P/bCCP molar ratios (0.24, 0.6, and 0.96), digestion temperatures (25-200 °C), and digestion times (1 week-2 months) on the transformation process was thoroughly investigated. At 1 week, the minimum temperature to yield the full transformation significantly reduced from 160 °C to 120 °C when using K2HPO4 instead of KH2PO4 at a P/bCCP ratio of 0.6, and even to 80 °C at a P/bCCP ratio of 0.96. The transformation took place via a dissolution-reprecipitation mechanism driven by the favorable balance between HA precipitation and bCCP dissolution, due to the lower solubility product of HA than that of calcite at any of the tested temperatures. Both the bCCP and the derived HA particles were cytocompatible for MG-63 human osteosarcoma cells and m17.ASC murine mesenchymal stem cells, and additionally, they promoted the osteogenic differentiation of m17.ASC, especially the HA particles. Because of their physicochemical features and biological compatibility, both particles could be useful osteoinductive platforms for translational applications in bone tissue engineering.

Locking Active Li Metal through Localized Redistribution of Fluoride Enabling Stable Li-Metal Batteries.

The creation of fluorinated interphase has emerged as an effective strategy for improving Li-metal anodes for rechargeable high-energy batteries. In contrast to the introduction of fluorine-containing species through widely adopted electrolyte engineering, a Li-metal composite design is reported in which LiF can locally redistribute on the Li-metal surface in liquid electrolytes via a dissolution-reprecipitation mechanism, and enable the formation of a high-fluorine-content solid electrolyte interphase (SEI). For validation, a Li/Li22 Sn5 /LiF ternary composite is investigated, where the as-formed LiF-rich SEI locks the active Li metal from corrosive electrolyte. The Li/Li22 Sn5 /LiF anode displays an impressive average Coulombic efficiency (ACE, ≈99.2%) at 1mA cm-2 and 1 mAh cm-2 in a carbonate electrolyte and a remarkable cycling life of over 1600h at 1mA cm-2 and 2 mAh cm-2 . Applied to a LiCoO2 full cell with a high cathode areal capacity of 4.0 mAh cm-2 , a high capacity retention of ≈91.1% is realized for 100 cycles at 0.5 C between 2.8 to 4.5V with a low negative/positive (N/P) ratio of 2:1. This design is conceptually different from the design employing the widely used fluorine-containing electrolyte additive and provides an alternative approach to realize reliable Li-metal batteries.

Nb-Ta oxides as recorders of hydrothermal activity in the Shihuiyao Rb-Nb-Ta-(Be-Li) deposit, Inner Mongolia, NE China

The Nb-Ta ore body in the Shihuiyao Rb-Nb-Ta-(Be-Li) deposit comprises muscovite-albite granite (MAG) and greisen (GR), with the dominant Nb-Ta minerals of columbite-group minerals (CGM) and ixiolite. The petrographic and chemical data suggest that the Nb-Ta oxides well record the magmatic-hydrothermal evolution process. The magmatic CGM are generally homogenous crystals or with oscillatory zoning patterns, while the CGM formed during the magmatic-hydrothermal transition stage overprints the early CGM crystals in the MAG or shows a normal zoning pattern (decreasing Nb/Ta ratios from the core to rim) in the GR, and possesses higher contents of Sc and Ti. The hydrothermal Nb-Ta oxides in both the MAG and GR share the same origin, i.e., the hydrothermal fluids exsolved from the magma at the late stage of magmatic evolution. However, distinct chemical differences between them indicate that a single fluid activity did not pervade the entire system; instead, two different hydrothermal processes were present: fluid metasomatism and water–rock reaction. The hydrothermal CGM generally occurs in the GR and metasomatizes the primary crystals by a coupled dissolution-reprecipitation mechanism. The chemical compositions of these CGM grains were significantly modified during this process, which was manifested by the increasing Sn and W contents in the reprecipitated phases. The ixiolite is most likely formed by another hydrothermal mechanism, i.e., water–rock reaction, during which the hydrothermal fluids can extract a large amount of Sn, Sc, U, and REE from the wall rocks or earlier formed magmatic phases and finally controlled the formation and chemical distribution of the ixiolite.

Acid resistance of alkali-activated materials: recent advances and research needs

Cementitious materials are frequently applied in environments in which they are exposed to acid attack, e.g., in sewer systems, biogas plants, and agricultural/food-related industries. Alkali-activated materials (AAMs) have repeatedly been shown to exhibit a remarkably high resistance against attack by organic and inorganic acids and, thus, are promising candidates for the construction and the repair of acid-exposed structures. However, the reaction mechanisms and processes affecting the acid resistance of AAMs have just recently begun to be understood in more detail. The present contribution synthesises these advances and outlines potentially fruitful avenues of research. The interaction between AAMs and acids proceeds in a multistep process wherein different aspects of deterioration extend to different depths, complicating the overall determination of acid resistance. Partly due to this indistinct definition of the ‘depth of corrosion’, the effects of the composition of AAMs on their acid resistance cannot be unambiguously identified to date. Important parallels exist between the deterioration of low-Ca AAMs and the weathering/corrosion of minerals and glasses (dissolution-reprecipitation mechanism). Additional research requirements relate to the deterioration mechanism of high-Ca AAMs; how the character of the corroded layer influences the rate of deterioration; the effects of shrinkage and the bond between AAMs and substrates.

Interaction of high-entropy rare-earth monosilicate environmental barrier coatings subjected to corrosion by calcium-magnesium-alumino-silicate melts

The high-entropy design of rare-earth silicates has been expected to improve CMAS corrosion resistance, which is essential to the development of EBCs for aero-engines. In this work, CMAS corrosion behaviors of plasma-sprayed high-entropy monoslicates ((Lu0.25Yb0.25Er0.25Y0.25)2SiO5 and (Lu0.2Yb0.2Er0.2Ho0.2Y0.2)2SiO5) and Y2SiO5 coatings at 1350 °C for 25–50 h were investigated and compared. The corrosion products included apatite and garnet based on the dissolution-reprecipitation mechanism. The high-entropy coatings possessed improved corrosion resistance compared with the Y2SiO5 coating, which was mainly due to the doped RE elements with smaller ionic radius could improve the stability of crystal structure and form garnet phase more easily.

Structure, composition, and formation of ferromanganese micronodules at the eastern southwest Indian ridge

This study evaluated the structure, mineralogy, and chemical composition of ferromanganese micronodules in gravity core GC01 obtained from the eastern southwest Indian ridge using micro-Raman spectroscopy, WITec Alpha 300R confocal Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, TESCAN integrated mineral analysis and electron microprobe analysis. The results revealed vernadite as the major crystalline phase in the ferromanganese micronodules. In addition, the micronodules contained small amounts of Fe oxyhydroxides and silicate minerals. All micronodules had low Mn/Fe ratios (~1), and their internal structures included concentric, clastic, dendritic, and foraminifera-like structures, among which the concentric and clastic structures were predominant. These characteristics suggest that micronodules are formed in an oxidizing environment via hydrogenetic processes. Volcanic materials and biological debris may act as “seeds” for micronodules, among which volcanic materials may serve as material sources. During water-rock interaction, volcanic clastics and biological debris react with seawater, and some cations (such as Ca2+, Na+, Mg2+) are leached out; Fe2+ and Mn2+ remain and generate Fe oxides or oxyhydroxides and Mn oxides which forms ferromanganese micronodules. Conclusively, the formation of ferromanganese micronodules in this study was controlled by a coupled dissolution-reprecipitation mechanism.

Geology, mineralogy and pyrite trace elements constraints on gold mineralization mechanism at the giant Dayingezhuang gold deposit, Jiaodong Peninsula, China

Identifying the mechanism of gold deposition is important to reveal ore-forming processes and genesis. The giant Dayingezhuang gold deposit, with 283 tons of proven gold resources, is located in the northwestern of the Jiaodong gold province, China. Pyrite is the dominant sulfide minerals and visible gold dominates Au resources. Five types of pyrite were identified at the Dayingezhuang gold deposit: (1) Py-1a is euhedral grain with Co– and Ni-rich content; (2) Py-1b is subhedral to anhedral grain with intermediate rich in Co, Ni, As and minor Te; (3) Py-2a is rich in As and crystalizes with a mass of mineral inclusions; (4) Py-2b is also rich in As but glossy; and (5) Py-3 is mainly euhedral grain and crystalizes with much more galena.Laser ablation spectra commonly shows Au and Ag-Bi-Pb spikes in Py-1b, Py-2a, Py-2b and Py-3, reflecting the occurrence of nanoparticles in pyrite. LA-ICP-MS mapping analysis shows elemental zoning of Co, Ni and As, which is considered as the element depletion (Co and Ni) of pre-existing Py-1a and enrichment (As) from ore-forming fluid via coupled dissolution-reprecipitation (CDR) mechanism. Au in pyrite is totally below 1 ppm (average 0.154 ppm and median 0.264 ppm) and without arresting variation in different pyrites, and mass balance indecates that invisible gold mobilized from pyrite may not interpret the formation of world-class gold deposit in the Jiaodong gold province. Most of the gold grains distribute in the microfractures of pyrites which is caused by brittle deformation. Fracture of rocks and minerals significantly improve the efficiency of water–rock interaction and result in sudden drop of fluid pressure. A probable model is that a fluid with extremely different physicochemical conditions and high As-Au-Cu-Pb-Bi-Te content give rise to Au mineralization, and visible gold precipitate in a gold-supersaturated hydrothermal fluid which is attributed to sulfidation and fluid phase separation via hydrofracturing. In addition, similar occurrences of gold and silver (individual minerals in microfractures of pyrites and spikes on laser ablation spectra of pyrite) and paragenetic sequence indicate that gold and silver mineralization belong to single geological event.

Tunable corrosion resistance of rare-earth monosilicate to molten calcia-magnesia-aluminosilicate glass by RE-doping strategy

In this work, CMAS corrosion behavior of hot-pressed (Y1-xYbx)2SiO5 ceramics at 1573 K within 32 h has been investigated. The corrosion products contain non-stoichiometric apatite and garnet, which form according to the dissolution-reprecipitation mechanism. With abundant calcia-magnesia-aluminosilicate, the growth rate of apatite layer increases rapidly with prolonging corrosion time, which results in the rapid corrosion especially after 4 h. Furthermore, the measured thickness of apatite layer in (Y1-xYbx)2SiO5 is much lower than the predicted value based on the optical basicity (OB) theory. This work provides a new insight of solid-solutionizing strategy in the resistance of rare-earth monosilicate to CMAS corrosion.

Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides.

Transformation of metastable Fe(III) oxyhydroxides is a prominent process in natural environments and can be significantly accelerated by the coexisting aqueous Fe(II) (Fe(II)aq). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III)labile) as the primary intermediate species of general importance. However, a mechanistic aspect that remains unclear is the dependence of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor. Here, we compared the coupled evolution of Fe(II) species, solid phases, and Fe(III)labile throughout the Fe(II)-catalyzed transformation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2-1.0 mM Fe(II)aq at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution-reprecipitation mechanism mediated by Fe(III)labile that seeds the nucleation of products. Though for Fh we observed a transformation to goethite (Gt), accompanied by the transient emergence and decline of Lp, for initial Lp we observed magnetite (Mt) as the main product. A linear correlation between the formation rate of Mt and the effective supersaturation in terms of Fe(III)labile concentration shows that Fe(II)-induced transformation of Lp into Mt is governed by the classical nucleation theory. When Lp is replaced by equimolar Gt, Mt formation is suppressed by opening a lower barrier pathway to Gt by heterogeneous nucleation and growth on the added Gt seeds. The collective findings add to the mechanistic understanding of factors governing phase selections that impact iron bioavailability, system redox potential, and the fate and transport of coupled elements.

Efficient, scalable, closed-loop synthesis of highly crystalline pure phase MgAl-layered double hydroxides intercalated with hydroxyl anions.

Layered double hydroxides (LDHs) can play an important role in various areas, but conventional LDHs synthesis often causes product agglomeration and generates plenty of high-salt wastewater, and requires a time-consuming aging process to reach the desired purity and crystalline state. Herein, we report the synthesis of MgAl-LDH, a representative of these kinds of ionic lamellar inorganic solids, with a novel method involving the reaction of magnesium oxide (MgO) with aluminate ions (Al(OH)4-) in a strongly alkaline environment. The formation of MgAl-LDH follows a mechanism of interfacial dissolution-reprecipitation (IDR), i.e., Mg2+ ions released at the interface of dissolved MgO react immediately with Al(OH)4- ions to reprecipitate as MgAl-LDH. The obtained MgAl-LDH has no impurity phases and shows high crystallinity, high specific surface area, and a narrow particle size distribution. Moreover, MgAl-LDH is intercalated with OH- anions, so it can be directly used as a Brønsted base catalyst and ion exchanger. The novel method requires no time-consuming aging process and is highly scalable. It is also shown that a closed-loop synthesis of MgAl-LDH without waste discharge can be achieved with an appropriate Al source, e.g., Al(OH)3, and a recycled NaOH solution.