Dissolution-precipitation Process Research Articles

Mg isotopic systematics during early diagenetic aragonite-calcite transition: Insights from the Key Largo Limestone

The seawater magnesium (Mg) isotopic composition (δ26MgSW) can be applied to quantify marine Mg cycle in Earth's history. However, reconstruction of δ26MgSW by using marine carbonate has been challenged by the large range of variation in Mg isotopic composition of carbonate (δ26Mgcarb). Such variation, on the one hand, reflects, e.g. the mineralogy, temperature, and precipitation rate dependent isotopic fractionation during Ca‑carbonate precipitation. On the other hand, the pristine seawater record could be modified in carbonate diagenesis, during which less stable carbonate minerals would spontaneously convert to more stable mineral phases. In the modern ocean with high Mg/Ca (molar) ratio (~5), aragonite precipitation is favored. However, aragonite is thermodynamically less stable than low-Mg calcite at ambient surface environment, resulting in the spontaneous transition from aragonite to low-Mg calcite in the earliest stage of diagenesis (i.e. the aragonite-calcite transition). As a common process in the modern ocean, Mg isotopic fractionation in aragonite-calcite transition may provide valuable constraints on the variation of δ26Mgcarb. In this study, we measured Mg isotopic compositions of Pleistocene Key Largo Limestone (Florida), which has been experiencing the aragonite-calcite transition during early diagenesis. The Key Largo Limestone samples consist of a mixture of calcite and aragonite. δ26Mg of aragonite (δ26Mgarag) shows higher and more stable values, while calcite has lower yet more variable Mg isotopes (δ26Mgcal). We suggest a model conceptualizing a water-film attached to the mineral surface, in which the aragonite-calcite transition initiates with aragonite dissolution and the Mg exchange with external fluid can be variable, followed by Mg isotopes fractionate during low-Mg calcite precipitation. In this model, δ26Mgcal is controlled by (1) the chemical composition of water-film, which is dependent on the mineralogical composition of aragonite (i.e. Mg content and δ26Mgarag), the composition of external fluid (i.e. fresh water, seawater, or mixed fresh water and seawater), and the mixing ratio between dissolved aragonite and external fluid, and (2) the isotopic fractionation during calcite precipitation. Because the isotopic fractionation in calcite is sensitive to the rate of calcite precipitation that is dependent on the calcite saturation state of solution. Therefore, δ26Mgcal could be highly heterogeneous depending on the aragonite dissolution and calcite precipitation processes, further complicating the interpretation of carbonate data. Thus, the process-controlled Mg isotopic fractionation should be taken into consideration when reconstructing δ26MgSW by using carbonate sediments/rocks.

The Capo Castello Shear Zone (Eastern Elba Island): Deformation at the Contact between Oceanic and Continent Tectonic Units

Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.

A fractal model for the electrical conductivity of water-saturated porous media during mineral precipitation-dissolution processes

Precipitation and dissolution are prime processes in carbonate rocks and being able to monitor them is of major importance for aquifer and reservoir exploitation or environmental studies. Electrical conductivity is a physical property sensitive both to transport phenomena of porous media and to dissolution and precipitation processes. However, its quantitative use depends on the effectiveness of the petrophysical relationship to relate the electrical conductivity to hydrological properties of interest. In this work, we develop a new physically-based model to estimate the electrical conductivity by upscaling a microstructural description of water-saturated fractal porous media. This model is successfully compared to published data from both unconsolidated and consolidated samples, or during precipitation and dissolution numerical experiments. For the latter, we show that the permeability can be linked to the predicted electrical conductivity.

Graphitic carbon-based core-shell platinum electrocatalysts processed using nickel nanoparticle template for oxygen reduction reaction

Extensive efforts have aimed at securing stable Pt-based electrocatalysts having both the performance-efficiency and long-term durability particularly for proton-exchange membrane fuel cells. Herein, we introduce an unprecedented electrocatalyst architecture based on a core-shell phase-mediated Pt/carbon black (CB) support structure which was enabled by the graphitization process using a Ni nanoparticle template. A ~1.03 nm-thick shell layer of graphitic carbon (GC) formed around the surface of ~9 nm-scale Ni core-nanoparticles via the dissolution-precipitation process with the annealing step of Ni on CB. The final Pt/GC/Ni/CB catalyst demonstrated noticeably better electrocatalytic performance in oxygen reduction reaction than that of the catalyst without the GC layer. The accelerated durability test with 5,000 cycles confirmed the advantages of the GC shell layer by demonstrating less electrochemical degradations by ~4.4 times in electrochemical surface area and ~2.5 times in the half-wave potential, compared to those of the reference sample.

Direct imaging of coupled dissolution-precipitation and growth processes on calcite exposed to chromium-rich fluids

Recent increases in industrial activity have resulted in elevated chromium (Cr) pollution in the natural environment, from the degradation of concrete and the leakage of water from landfills or mine waste. The release of toxic chromate ions into the environment could have harmful consequences. Methods for Cr removal are therefore becoming increasingly important to control the release of this highly toxic metal into the environment. This study examines how chromate (CrO42−) can be incorporated into a new, stable solid phase at a dissolving calcite surface. To study the relationship between solutions containing chromate and calcite, we performed two series of time-lapse atomic force microscopy (AFM) experiments to identify calcite dissolution and growth rates in the presence of chromate. In addition, we conducted complementary experiments in a stirred flow-through reactor to determine the amount of Cr removal from solution. All experiments were performed at room temperature and under a constant initial pH in the range 6–12. During the AFM experiments, we observed calcite dissolution via etch pit formation and propagation. In the presence of chromate, nanoparticles of a new phase nucleated and grew at the calcite surface by a coupled dissolution-precipitation process. In experiments with concentrations above 10 ppm Cr or at pH 10 and higher, we observed many nanoparticle precipitates, while precipitates were rarely observed at lower concentrations of Cr. At pH 10.5, the precipitates covered the calcite surface. This partly passivated further calcite dissolution, an observation confirmed by the low amounts of Cr removal measured in the flow-through experiments. Scanning electron microscopy analysis demonstrated that Cr was associated with the precipitates, indicating that Cr could be captured from solution and trapped in solid nanoparticles in the presence of calcite. However, the passivation of the calcite surface by a new precipitate may eventually slow down Cr sequestration.

Carbonatación de diatomeas y génesis de calizas/dolomías diagenéticas en ambiente continental y marino (Mioceno de Tresjuncos, Cuenca y Níjar, Almería). Un ejemplo de secuestro de CO<sub>2</sub> en la Naturaleza

En el presente trabajo se estudian los procesos de carbonatación de frústulas de diatomeas y la consecuente formación de calizas/dolomías diagenéticas, en estratos de diatomitas marinas (Messiniense, sección de Níjar) y lacustres (Turoliense, sección de Tresjuncos). Se realiza un estudio mineralógico y petrológico para conocer el reemplazo de ópalo A por calcita o dolomita, considerando los factores que favorecen este reemplazo en ambos ambientes. Los datos obtenidos son relevantes para la investigación del uso de diatomitas en la captura y alma­cenamiento de dióxido de carbono a través de su carbonatación.
 Los procesos pseudomórficos de carbonatación de las valvas se llevaron a cabo mediante el reemplazo directo del ópalo A por calcita o dolomita y la cementación de la microporosidad. La perfecta reproducción de las valvas indica un mecanismo de reacción con acoplamiento de disolución-precipitación en la interfase de reacción. Se interpreta que la organización de las microesferas de ópalo A, dentro de las valvas, condicionaría la organización de complejos amor­fos de varios iones o nanopartículas de carbonatos amorfos, que posteriormente recristalizarían a calcita o dolomita.
 En las diatomitas lacustres, el proceso de carbonatación de las valvas fue favorecido por la materia orgánica proveniente de la gran cantidad de biota (anfibios, crustáceos, insectos, plantas... etc.) que incorporan. En este ambiente, los procesos de sulfato-reducción durante la degradación de la materia orgánica, generaron CO2, que junto con cambios de pH y salinidad facilitaron los reemplazos. Es posible que las carbonataciones de las valvas fueran coetáneas a la precipitación de calcita y dolomita que fosiliza la biota y por lo tanto, diagenéticas tempra­nas. En las diatomitas marinas, el proceso desencadenante del reemplazo de las valvas fue la formación de capas lenticulares y nódulos opalinos, ya que al disolverse los microfósiles calcáreos que las diatomitas incluían, se liberó CO2 y Ca. Pequeñas variaciones de pH en un entorno alrededor de 9, y la presencia de sulfatos facilitarían el reemplazado durante la diagénesis por enterramiento.

Densification kinetics and sintering behaviour of a Ti(C0.7N0.3)-WC-Mo2C–NbC-(Co,Ni) compact

In order to understand the densification kinetics of a Ti(C0.7N0.3)-WC-Mo2C–NbC-(Co, Ni) compact, the linear shrinkages were measured using a horizontal dilatometer at different heating rates (2, 4, and 6 °C/min) in an argon atmosphere. Densification is more efficient when it is sintered at a lower heating rate. A master sintering curve was constructed over a range from the relative density of approximately 0.658–0.816; the corresponding sintering activation energy was 630 kJ/mol. With the aid of an Arrhenius curve, the sintering behaviour of cermet was analysed at different temperatures. The sintering process changed from solid-phase to liquid-phase sintering at approximately 1385 °C. The sintering activation energy turned negative when the temperature exceeded 1385 °C at a heating rate of 2 °C/min due to the formation of a liquid phase, and the exothermic process of dissolution-precipitation.

Halloysite-smectite mixed-layered clay in fluvio-volcanic soils at the southern foot of Mount Kilimanjaro, Tanzania

The formation of kaolinite-smectite mixed-layered clays results from weathering of smectite in leached and silica-depleted environments. Halloysite-smectite mixed-layered clays have been proposed to form from weatherable minerals in volcanic ash soils through simultaneous formation of 1:1 and 2:1 units in silica-rich environments. Here, we report a mineralogical study of weathered B horizon material from soils developed on Late Quaternary fluvio-volcanic deposits of basaltic origin at the southern foot of Mount Kilimanjaro in northern Tanzania. This area is characterized by a strong seasonality: two months of highly humid conditions followed by a 10-month pronounced dry season during which mineral authigenesis takes place. The weak leaching conditions are reflected in the high base saturation of the soils and by their high content of amorphous silica. The soils contain no allophane. The clay fractions contain dioctahedral 1:1 layer silicates, trioctahedral mica, gibbsite and feldspar, as well as a substantial amount of 1:1/2:1 mixed-layered clays with predominance of 1:1 layers. The latter consist of dehydrated halloysite that only partially expands after formamide intercalation, because of its history of drying in field conditions. The interstratified phases are high-charge halloysite-smectite mixed-layered clays (CEC 30–72 cmol(+) kg−1 clay) that are predominantly very fine grained (<0.05 µm). This small crystal size is typical for minerals formed by dissolution-precipitation processes, rather than for minerals formed by solid-state transformation of inherited 2:1 phyllosilicates. Halloysite and smectite thus formed simultaneously as constituents of mixed-layered clays under soil conditions of moderate to weak weathering, weak leaching, high base status and high silica concentrations.

Dissolution and Precipitation Dynamics at Environmental Mineral Interfaces Imaged by In Situ Atomic Force Microscopy.

Chemical reactions at the mineral-solution interface control important interfacial processes, such as geochemical element cycling, nutrient recovery from eutrophicated waters, sequestration of toxic contaminants, and geological carbon storage by mineral carbonation. By time-resolved in situ imaging of nanoscale mineral interfacial reactions, it is possible to clarify the mechanisms governing mineral-fluid reactions.In this Account, we present a concise summary of this topic that addresses a current challenge at the frontier of understanding mineral interfaces and their importance to a wide range of mineral re-equilibration processes in the presence of a fluid aqueous phase. We have used real-time nanoscale imaging of liquid-cell atomic force microscopy (AFM) to observe the in situ coupling of the dissolution-precipitation process, whereby the dissolution of a parent mineral phase is coupled at mineral interfaces with the precipitation of another product phase, chemically different from the parent. These nanoscale observations allow for the identification of dissolution and growth rates through systematically investigating various minerals, including calcite (CaCO3), siderite (FeCO3), cerussite (PbCO3), magnesite (MgCO3), dolomite (CaMg(CO3)2), brushite (CaHPO4·2H2O), brucite (Mg(OH)2), portlandite (Ca(OH)2), and goethite (α-FeOOH), in various reacting aqueous fluids containing solution species, such as arsenic, phosphate, organo- or pyrophosphate, CO2, selenium, lead, cadmium, iron, chromium, and antimony. We detected the in situ replacement of these parent mineral phases by product phases, identified through a variety of analytical methods such as Raman spectroscopy, high-resolution transmission electron microscopy, and various X-ray techniques, as well as modeling by geochemical simulation using PHREEQC. As a consequence of the coupled processes, sequestration of toxic elements and hazardous species and inorganic and organic carbon, and limiting or promoting recovery of nutrients can be achieved at nano- and macroscopic scales.We also used in situ AFM to quantitatively measure the retreat rates of molecular steps and directly observe the morphology changes of dissolution etch pits on calcium phosphates in organic acid solutions present in most rhizosphere environments. By molecular modeling using density functional theory (DFT), we explain the origin of dissolution etch pit evolution through specific stereochemistry and molecular recognition and provide an energetic basis by calculating the binding energies of chemical functionalities on organic acids to direction-specific steps on mineral surfaces. In addition, we further quantified precipitation kinetics of calcium phosphates (Ca-P's) on typical mineral surfaces at the nanoscale in environmentally relevant solutions with various organic molecules, by measurements obtained from sequential images obtained by liquid-cell AFM. In situ dynamic force spectroscopy (DFS) was used to determine binding energies of single-molecules with different chemical functionalities found in natural organic matter at mineral-fluid interfaces. Quantifying molecular organo-mineral bonding or binding energies mechanistically explains phosphate precipitation and transformation. From DFS measurements, molecular-scale interactions of mineral-natural organic matter (DNA, proteins, and polysaccharides) associations were determined. With this powerful tool, single-molecule determinations of polysaccharide-amorphous iron oxide or hematite interactions provided the mechanistic origin of the phase- or facet-dependent adsorption. These systematic investigations and findings significantly contribute to a more quantitative prediction of the fate of nutrients and contaminants, chemical element cycling, and potential geological carbon capture and nuclear waste storage in aqueous environments.

High-pH/low pH ordinary Portland cement mortars impacts on compacted bentonite surfaces: Application to clay barriers performance

The design of engineered barrier systems for high-level radioactive waste isolation requires the use of cement-based materials. Small-scale surface reactivity interface experiments were designed to perform a reference laboratory study. It simulated granitic groundwater infiltration through cement mortar/bentonite (CB) binary columns. The tests ran duplicated over 6 and 18 months and included low-pH, CEM I and CEM II ordinary Portland cement mortars in contact with bentonite. After the experiments, the materials were analysed using scanning electron microscopy attached to energy dispersive energy X-ray analysis (EDX), surface mineralogy using grazing incidence X-ray diffraction configurations, specific surface area and effluent chemistry. EDX chemical profiles measured from CB interface contact reproduced, in 100 μm to 1 mm thickness, a characteristic geochemical Mg perturbation in the bentonite and calcite precipitation in cement materials. These processes are known to be developed in 10–15-years within <5 mm of the interface. The comparison of these long, large scale experiments with the small scale experiments showed that the chemical perturbation thickness development rate will decrease over time. The limestone addition (CEM II) leaves less room for calcite precipitation, and ettringite became stabilized, which limited the extent of dissolution-precipitation processes. Then, the CEM II mortar had better resistance to perturbation.

Graphitic nanorings for super-long lifespan lithium-ion capacitors

Porous graphitic carbon nanorings (PGCNs) are proposed by smart catalytic graphitization of nano-sized graphene quantum dots (GQDs). The as-prepared PGCNs show unique ring-like morphology with diameter around 10 nm, and demonstrate extraordinary mesoporous structure, controllable graphitization degree and highly defective nature. The mechanism from GQDs to PGCNs is proven to be a dissolution-precipitation process, undergoing the procedure of amorphous carbon, intermediate phase, graphitic carbon nanorings and graphitic carbon nanosheets. Further, the relationship between particles size of GQDs precursor and graphitization degree of PGCNs products is revealed. The unique microstructure implies PGCNs a broad prospect for energy storage application. When applied as negative electrode materials in dual-carbon lithium-ion capacitors, high energy density (77.6 Wh·kg−1) and super long lifespan (89.5% retention after 40,000 cycles at 5.0 A·g−1) are obtained. The energy density still maintains at 24.5 Wh·kg−1 even at the power density of 14.1 kW·kg−1, demonstrating excellent rate capability. The distinct microstructure of PGCNs together with the strategy for catalytic conversion from nanocarbon precursors to carbon nanorings opens a new window for carbon materials in electrochemical energy storage.

FT-NIR and DSC characterisation of water in opal

Opal is a hydrous silica (Si0 2 .nH 2 O) formed through a dissolution-precipitation process. The formation process incorporates water into the structure as bound silanol and molecular water. As the water is distributed in a range of states, multiple methods of characterisation are required to identify each state. This study reports the results of temperature dependent FT-NIR and DSC investigation on natural opal samples of the opal-A (amorphous) and opal-CT (poorly crystalline cristobalite with tridymitic stacking faults) types. Significant differences in the melting behaviour of crystallisable water as well as differences in the spectral characteristics of the non-crystallisable molecular water are observed. These differences are ascribed to the different microstructures of the opal types.

The solubility of monazite (LaPO4, PrPO4, NdPO4, and EuPO4) endmembers in aqueous solutions from 100 to 250 °C

Monazite is a light rare earth element (REE) phosphate found in REE mineral deposits, such as those formed in (per)alkaline and carbonatite magmatic-hydrothermal systems, where it occurs in association to the development of alteration zones and hydrothermal veins. Although it has been recognized that monazite may undergo replacement by coupled dissolution-precipitation processes, currently there is no model describing the compositional REE variations in monazite resulting from direct interaction with or precipitation from hydrothermal fluids. To develop such a model requires quantification of the thermodynamic properties of the aqueous REE species and the properties of the monazite endmembers and their solid solutions. The thermodynamic properties of monazite endmembers have been determined previously using calorimetric methods and low temperature solubility studies, but only a few solubility studies have been conducted at >100 °C. In this study, the solubility products (logKs0) of LaPO4, PrPO4, NdPO4, and EuPO4 monazite endmembers have been measured at temperatures between 100 and 250 °C and saturated water vapor pressure. The solubility products are reported with an uncertainty of ±0.2 (95% confidence) according to the reaction, REEPO4(s) = REE3+ + PO43−.▪The REE phosphates display a retrograde solubility, with the measured Ks0 values varying several orders of magnitude over the experimental temperature range. Discrepancies were observed between the experimental solubility products and the calculated values resulting from combining calorimetric data of monazite with the properties of the aqueous REE3+ and PO43− species available in the literature. The differences between the calculated and measured standard Gibbs energy of reaction (ΔrG0) for PrPO4, NdPO4, and EuPO4 increased with higher temperatures (up to 15 kJ mol−1 at 250 °C), whereas for LaPO4 these differences increased at lower temperatures (up to 8 kJ mol−1 at 100 °C). To reconcile these discrepancies, the standard enthalpy of formation (ΔfH0) of monazite was optimized by fitting the experimental solubility data and extrapolating these fits to reference conditions of 25 °C and 1 bar. The optimized thermodynamic data provide the first internally consistent dataset for the solubility of all the monazite endmembers, and can be used to model REE partitioning between monazite and hydrothermal fluids at >100 °C.

Effect of LaB6 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Si3N4 ceramics was hot-pressed with LaB6–MgO as sintering additives in the temperature range from 1550 °C to 1750 °C for 2 h under the external mechanical pressure of 60 MPa, followed by post-sintering treatment at 1900 °C for 2 h under the N2 pressure of 2 MPa. The effect of sintering temperature on the densification behavior, mechanical properties, thermal conductivity, phase composition and microstructure evolution were investigated in detail. Compared to the Si3N4 ceramics sintered with La2O3–MgO, the addition of LaB6–MgO was beneficial for the improvement of fracture toughness, Vickers' hardness and thermal conductivity of Si3N4 ceramics. The maximum Vickers’ hardness of 18.98 ± 0.63 GPa was achieved at 1550 °C. The bending strength of 1039.1 ± 70.1 MPa and fracture toughness of 8.46 ± 0.23 MPa m1/2 could be obtained at 1750 °C. After post-sintering treatment, the Si3N4 ceramics exhibited improved thermal conductivity of 75.63 W m−1 K−1 and fracture toughness of 8.72 ± 0.11 MPa m1/2 due to the repeated dissolution-precipitation process.

Elemental behavior during chlorite alteration: New insights from a combined EMPA and LA-ICPMS study in porphyry Cu systems

Chlorite is a common alteration product of fluid-rock interaction in hydrothermal deposits, and it's compositional variations provides useful information about the physicochemical conditions of formation, but element behavior during the chloritization process is still not well understood. Chlorite from two large porphyry Cu deposits, Atlas (Philippines) and Xiaokelehe (NE China) have been chosen to evaluate the geochemical variations with a focus on mass transfer during alteration of precursor minerals.Conversion of two biotite to one chlorite is the main mechanism for the chloritization of biotite. Iron, Mg, Al, and Ni are mostly retained in chlorite, whereas Co, Ga, Mn, and Zn are commonly transferred to chlorite from the hydrothermal fluid, and Sc, Sr, Si, V, Li, K, Nb, Ba, Rb, Ti, Cl, Na, Sn, and Cu from the biotite mostly do not enter chlorite during chloritization. Chlorite alteration of hornblende formed probably through the processes of dissolution-precipitation. Iron are mostly retained in the chlorite, whereas Li, Cu, Ni, Zn, Co, Al, and Ga in the chlorite are at least in part derived from hydrothermal fluid, and Mg, Mn, V, Si, Ca, Zr, Nb, Sn, Cl, REE, Y, Na, Ti, Sr, K, Sc and Ba originally in the hornblende mostly enter hydrothermal fluid, titanite or epidote, rather than chlorite during chloritization. The MgO, FeOT, MnO, Zn, Li, Sc, V, Co and Ni concentrations, and FeOT/MgO ratio of chlorite are strongly influenced by those of the precursor minerals. Moreover, although formed in the same porphyry system, the MnO, Zn, Li and Cu concentrations and FeOT/MgO ratios of chlorite formed from biotite and hornblende are markedly different.Given that Fe/Mg ratios of chlorite are controlled by the precursor minerals in wall rock, the Fe/(Fe + Mg) correction on the well-established chlorite thermometers during hydrothermal alteration may need to be reconsidered. Aluminum, Ga, and Ti in chlorite are not controlled by precursor minerals, but by formation temperature of chlorite, so we can use these elements of chlorite to map out the thermal structure of porphyry system which is related to individual mineralized porphyry intrusion, which can help to identify the centers of hydrothermal systems.

Effects of Magnetic Field on Uneven Dissolution of Iron in Sulfuric Acid Solution with Chlorides

The effects of magnetic field on anodic dissolution and passivation of iron in a sulfuric acid solution with chlorides are investigated by electrochemical measurements and surface observations. In the anodic potentiodynamic polarization curve, the potential for the drastic current drop is not significantly affected by the potential sweep rate under 0 T, which moves in the negative direction with increasing potential sweep rate under 0.4 T magnetic field that is parallel to the working electrode surface. The uneven surface produced during the potentiodynamic polarization hinders the transition from active dissolution to passivation. The area fraction of the locally accelerated dissolution increases with prolonging polarization time at high potentials where the surface film precipitation-dissolution process is the rate-determining step for metal dissolution. Pretreatment under potentiostatic polarization at 0.4 T magnetic field produces an uneven surface that would result in unrecoverable electrochemical states after switching from 0.4 T to 0 T, depending on the applied potential. The positive-feedback mechanism for the magnetic field effect and the surface morphological effect is proposed. The results demonstrate the direct magnetohydrodynamic effect and its resultant uneven surface on the anodic behavior of iron.

The application of the yield approach to study slurry migration in drill cuttings waste underground disposal

The underground disposal of drill cuttings waste is a common practice for the gas/oil industry to achieve zero-discharge sustainable development. In this study, a numerical modeling approach was developed to simulate the slurry flow for underground disposal of drill cuttings waste. The modeling approach was coupled with and implemented in the well-known general purpose subsurface multiphase flow simulator, TOUGH2. The new modeling approach treats the slurry flow behavior in subsurface systems as Bingham plastic liquid, with a linear relationship representing the yield stress and the concentration of the gelatinizer in the slurry. In addition, the precipitation-dissolution process was taken into account for solid-aqueous phase changes of the water-slurry mixture under and over the threshold pressure. The model has been verified by the analytical solution of a transient flow of single-phase Bingham fluid, and has further been tested by modeling field-scale injection of drill cutting wastes into a multi-layered geological formation in Texas. A hypothetical model has also been used to conduct sensitivity analysis of the impact of slurry density, injection depth and injection pattern on the storage formation performance. The results revealed that the effect of injection volume is greater than the mass on pressure buildup. In addition, a short period of intermittent reinjection can lead to an earlier formation breakdown due to particle sedimentation and reduce the storage capacity. The developed model can be used to evaluate the prediction of slurry transport, storage capacity, pressure distribution, and the formation breakdown time in a drill cuttings waste disposal project.

Synthesis of B-type carbonated hydroxyapatite by a new dissolution-precipitation method

Carbonated hydroxyapatite (C-HAp) is one of the most attractive biomaterials for bone tissue engineering due to its compositional and biological similarity to the mineral part of human bone. In the present work, we have used a two-step fabrication process to precipitate a poorly crystalline Carbonated Hydroxyapatite (C-HAp) with nano-size particles at room temperature (20 °C) through the dissolution-precipitation process. For the first time, stoichiometric hydroxyapatite (S-HAp) was used as a precursor. The obtained C-HAp was evaluated for its structure; chemical composition and particle size using numerous analytical tools, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), Thermogravimetric analysis (TGA), and Scanning Electron Microscopy (SEM). The chemical composition, carbonate content, and molar ratio (Ca/P) of the C-HAp phase was investigated by the corresponding X-ray energy dispersive spectrometer (EDX). Therefore, the dissolution-precipitation reaction was confirmed to be useful to transform S-HAp to C-HAp.

Influence of surface coating on the microstructures and dielectric properties of BaTiO3 ceramic via a cold sintering process.

We present herein a modified cold sintering process (CSP) for BaTiO3 ceramics using a surface coating at the particle surface which could enhance the relative density of BaTiO3 up to ∼93.5% at 220 °C and 500 MPa. The surface coating greatly enhances the ceramic density, mainly because it facilitates the dissolution–precipitation process during CSP. Ba vacancies form at the surface of the coated powders, so Ba(OH)2 solution is used to compensate Ba ions in the as-cold-sintered ceramics, which increases the dielectric permittivity. Post-annealing at 700 and 900 °C increases the relative density to 97%, and the resulting relative dielectric permittivities are 810 and 1550, respectively, at room temperature and 1 kHz. This technique may also be extended to materials with very small, incongruent solubility in water or volatile solutions that use the cold sintering process.

Organic carbon decomposition rates with depth and contribution of inorganic carbon to CO2emissions under a Mediterranean agroforestry system

AbstractAgroforestry systems have been much studied for their potential to store soil organic carbon (SOC). However, few data are available on their specific impact on potential SOC mineralization, especially at depth in subsoils. Moreover, many soils of the world, especially in arid and semiarid environments, also contain large stocks of soil inorganic carbon (SIC) as carbonates. Consequently, the organic carbon dynamics have been poorly investigated in these soils due to the complexity of measurements and of the processes involved. To assess mineralization rates of SOC with depth, we incubated soil samples from an 18‐year‐old agroforestry system (both tree row and alley) and an adjacent agricultural plot established on a calcareous soil in France. Soil samples were taken at four different depths: 0–10, 10–30, 70–100 and 160–180 cm. Total CO2emissions, the isotopic composition (δ13C, ‰) of the CO2and microbial biomass were measured. The SIC concentrations ranged from 48 to 63 g C kg−1soil and the SOC concentrations ranged from 4 to 17 g C kg−1soil. The contribution of SIC‐derived CO2represented about 20% in the topsoil and 60% in the subsoil of the total soil CO2emissions. The microbial biomass and the SOC‐derived CO2emissions were larger in the topsoil, but the decomposition rates (day−1) remained stable with depth, suggesting that only the size of the labile carbon pool was modified with depth. Subsoil organic carbon seems to be as prone to decomposition as surface organic carbon. No difference in CO2emissions was found between the agroforestry and the control plot, except in the tree row at 0–10 cm. Our results suggest that the measurement of soil respiration in calcareous soils could be overestimated if the isotopic signature of the CO2is not taken into account. It also advocates more in‐depth studies on carbonate dissolution–precipitation processes and their impact on CO2emissions.HighlightsWe measured SOC mineralization and inorganic carbon contribution to CO2emissions in agroforestrySubsoil organic carbon was as prone to decomposition as surface organic carbonInorganic carbon contribution to CO2emissions ranged from 20 to 60% depending on soil depthMeasurement of soil respiration in calcareous soils could be overestimated