Initial Stages Of Dissolution Research Articles

The basic characteristics of paste backfill materials based on highly active mineral admixtures: Part I. Preliminary study on flow, mechanics, hydration and microscopic properties

The cost performance of backfill materials in coal mines has long limited the widespread use of backfill technology. Therefore, developing backfill materials that are both cost-effective and high-performing is a pressing scientific issue that requires immediate attention. This study proposes replacing ordinary Portland cement (OPC) with highly active mineral admixtures, such as mineral powder and limestone powder, to prepare a green, low-cost, and high-performance coal mine backfill material. The use of mineral admixtures can improve the performance of the backfill material while reducing its environmental impact. The proposed approach is cost-effective and environmentally friendly. The study employed experimental tests and microscopic characterization methods, such as the slump test, hydration heat test, uniaxial compressive strength test, and microstructure test, to clarify the synergistic reaction mechanism of highly active mineral admixture-based paste backfill material (HAM-PBM). This clarification is helpful in regulating the macroscopic properties of hardened paste. The results show that: (1) The rheological properties of fresh HAM-PBM slurry are consistent with the Herschel-Bulkley model, and its fluidity meets the technical requirements for coal mine backfilling. The slump of fresh HAM-PBM slurry ranges from 140 to 145 mm, and the yield stress ranges from 3.188 to 12.819 Pa. (2) Highly active mineral admixtures significantly affect the early hydration process of HAM-PBM. The parameters of the hydration characteristics during the initial dissolution stage, acceleration stage, deceleration stage, and stabilization stage gradually decrease, while the duration of the induction stage gradually increases. This confirms that the early hydration activity of mineral powder (MP) and limestone powder (LF) is lower than that of OPC. (3) The uniaxial compressive strength (UCS) of the MP series exhibits an "anti-hook" change pattern, while the UCS of the LF series shows a decreasing change pattern. The microstructure, including the number of hydration products, microscopic density, and other characteristics, follows the same change pattern as the UCS. (4) Based on the synergistic reaction mechanism of hardened slurry, it is recommended that MP replace a maximum of 10 % of OPC and LF replace a maximum of 5 % of OPC. The need for composite replacement of OPC with MP and LF is clarified, resulting in a cost-effective coal mine backfill material. This study has contributed to the national "double carbon" policy and the low-cost backfilling application of coal mines.

Contrasting the major characteristics of pinewood and Amazon hardwoods to provide high-quality cement-bonded particleboards

The combined effect of wood chemical composition, anatomical traits, and density on cement curing, matrix-reinforcement interface, and final performance of cement-bonded particleboards (CBPBs) needs deeper investigation. Besides, many uncharacterized Amazon hardwoods challenge softwoods' often-reported superiority for producing CBPBs. Certified Cordia goeldiana, Brosimum parinarioides, Parkia gigantocarpa, and Pinus oocarpa woods were characterized by histology, cell biometry, basic density, and chemical composition. The hydration kinetics of the Portland cement pastes was evaluated by isothermal heat conduction. The CBPBs production parameters were wood:cement ratio of 1:2.75, water:cement ratio of 1:2.5, 4% of CaCl2 additive (based on cement mass), and target density of 1.25 g cm−3. They were submitted to physical and mechanical tests, and the polished surfaces were analyzed by scanning electron microscopy. The cement kinetics showed that wood's higher contents of extractives and hemicelluloses harmed the initial dissolution stage of Portland cement curing but not the essential stages of acceleration and deacceleration. C. goeldiana wood showed the highest compatibility indexes with cement, and all hardwoods surpassed P. oocarpa. Thin-walled fibers, big-diameter fibers, and big-diameter pores favored cement impregnation. Debonding at wood-cement interfaces of CBPBs reinforced with higher-density woods was critical for crack propagation throughout the matrix. Less critical inter-particle cracks occurred in CBPBs reinforced with lower-density woods. The better interface and the highest volume of wood particles of the lowest-density P. gigantocarpa wood provided the CBPBs with the best physical and mechanical performance overcoming the chemical incompatibility relevant only in the initial dissolution stage of cement curing.

Molecular Dynamics Investigation of the Effect of the Interface Orientation on the Intensity of Titanium Dissolution in Crystalline and Amorphous Aluminum

The influence of the interface orientation on the intensity of dissolution of titanium in crystalline and amorphous aluminum is studied by molecular dynamics simulation. The following four orientations of the Ti–Al interface with respect to the Ti (hcp) and Al (fcc) lattices are considered: (1) (0001):(111), (2) (0001):(001), (3) (101¯0101¯0):(111), and (4) (101¯1101¯1):(001). The interface orientation is found to influence the intensity of dissolution of titanium in aluminum, which increases for the accepted designations in the order 1–2–3–4. An important phenomenon in this case turns out to be the formation of a thin (2–3 atomic planes thick) crystalline layer in aluminum, which repeats the crystal lattice of titanium, at the initial stage of dissolution. At a temperature below the melting point of aluminum, a grain boundary parallel to the interface forms behind this layer. At temperatures above the melting point of aluminum, this crystalline layer is preserved, but its thickness decreases gradually as the temperature increases. For aluminum in an amorphous state at temperatures below its melting point, the dissolution of titanium occurs at almost the same intensity as in the crystalline state of aluminum, which is explained by the formation of a similar crystalline layer in aluminum at the interface in all cases.

Study on the Microstructure Evolution and Strength Damage Mechanism of Dolomite under Dissolution Condition

Dolomite is a common type of natural soluble rock. The strength of rock decreases under the action of corrosion, which has a significant impact on the self-stability and long-term safety of the tunnel surrounding the rock. To reveal the microscopic structure evolution and strength-damage law of carbonate rock caused by chemical corrosion, a series of tests such as rock chemical corrosion test, rock uniaxial compression test and electron microscope scanning test are conducted at different pH values on the dolomite of the Doushantuo Formation. The rock dissolution at different pH values exhibits four typical stages: the initial dissolution stage, secondary dissolution acceleration stage, stable dissolution rate stage and dissolution attenuation stage. During the dissolution process, the initial dissolution rate is 25.91 times that of the stable stage, and the maximum strength attenuation is 76.2% after 21 days of dissolution. For macroscopic failure, the rock is developed from 1 to 2 external fractures to multiple internal and external fractures and penetrated, and the specimen transforms from brittle to flexible. For microstructure, the sample exhibits corrosion characteristics along the joint surface, intensified corrosion at the edge, etc. The porosity increase rate is 0.6%/d; however, the length–width ratio of the pores is maintained at 1.7–1.85, indicating that the development rate of pores in different directions is similar. The results of this study have enriched the study of the dolomite dissolution mechanism and, in addition, have important reference value for the stability evaluation of tunnel surrounding rock in karst environments.

Diamond-destructive mantle metasomatism: Evidence from the internal and external textures of diamonds and their nitrogen defects

Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds.Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions.Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate‑carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.

The Effect of Food Thickeners on the Bitterness and Dissolution of Amlodipine Besilate Loaded Oral Disintegration Tablets: Assessment of Potential Suitability for Patients with Dysphagia

The purpose of this research was to evaluate the effect of starch- and xanthan gum-based food thickeners on the bitterness and dissolution of amlodipine besilate (AMPB) loaded orally disintegrating tablets (ODT) for potential use with patients with dysphagia. A conventional dissolution test simulating the oral cavity was performed and the taste sensor output of the dissolved sample was evaluated over a 60-seconds period. When four types of AMPB loaded ODTs were tested alone, at 60 seconds, branded product (A) was the least bitter, followed by generic product (B)/generic product (C) which were equal, and finally generic product (D) which was the most bitter. Inhibition of bitterness of AMPB loaded ODTs mixed thickeners, 1.0 (w/v) % xanthan gum-based food thickener solution was significantly strong. The 7.1 (w/v) % and 4.7 (w/v) % starch-based food thickeners solution also effective in bitterness inhibition compared to the 2.4 (w/v) % starch-based food thickener solution. The dissolution test under pH 1.2 in related to 7.1 (w/v) % and 4.7 (w/v) % starch-based thickener contained each of AMPB loaded ODTs were associated with an almost complete amlodipine (AMP) dissolution (almost 90% at 10 minutes), whereas the 1.0, 2.0, 3.0 (w/v) % xanthan gum-based food thickener solution containing AMPB loaded ODTs did not show complete AMP dissolution and there were large variations in the initial dissolution stage. This suggests that a mixture of xanthan gum-based thickener and AMPB loaded ODT poses a risk of reduction of bioavailability. In conclusion, a mixture of 4.7 (w/v) % or 7.1 (w/v) % starch-based thickener with ODTs provides complete release of AMP and superior bitterness inhibition, so is the best choice for administration to patients with dysphagia.

The dissolution of simulant UK Ca/Zn-modified nuclear waste glass: the effect of increased waste loading

The influence of increasing the waste loading of Ca- and Zn-containing MW high-level-waste glass, from 20 to 35 wt%, on the chemical durability was investigated using 112-day PCT-B tests at 90 °C in ultra-high-quality water. During the initial stage of dissolution, from 0 to 3 days, increased waste loadings corresponded to reduced boron dissolution rates. By 112 days, however, there was no difference in the normalised mass loss of boron as a function of waste loading, within experimental uncertainty. In comparison to literature values obtained for traditional MW high-level-waste glasses, containing no Ca and Zn, the boron dissolution rates of CaZn MW glasses in the residual stage of dissolution, > 28 days, were greater by at least an order of magnitude. Analysis of secondary phases, combined with geochemical modelling of the solutions, indicate that the dual formation of Mg- and Zn-smectite phyllosilicate clays, potentially saponite (Ca0.2Mg3(Si,Al)4O10(OH)2·4H2O) and sauconite (Na0.3Zn3(Si, Al)4O10(OH)2·nH2O), respectively, may be the cause of this enhanced dissolution.

Investigation of dissolution behavior of laves phase in inconel 718 fabricated by laser directed energy deposition

The mechanical properties of Inconel 718 are closely related to the morphology and size of the Laves phase, which must be quantitatively controlled to change the effect of the Laves phase from deleterious to beneficial. In this study, post-heat treatment was used to regulate the morphology and size of the Laves phase in Inconel 718 fabricated using laser directed energy deposition, and the dissolution behavior of the Laves phase during solution heat treatment was investigated. The results indicated that the sharp corners and grooves of the Laves phase preferentially dissolved, causing the morphology of the Laves phase to change from a long-striped to granular shape during dissolution. The dissolution kinetics of the Laves phase were also investigated using the Johnson–Mehl–Avrami–Kolmogorov and Singh–Flemings models. The initial stage of dissolution was controlled by both the long-range diffusion of Nb and the interfacial reaction. However, with decreasing degree of Nb segregation, the interfacial reaction became dominant.

Diffusive processes in aqueous glass dissolution

High level nuclear waste is often immobilised in a borosilicate glass for disposal. However, this glass corrodes in contact with aqueous solutions. To predict radionuclide releases from wasteforms, their dissolution mechanisms must be understood. Understanding glass dissolution mechanisms presents a challenge across numerous other disciplines and many glass dissolution models still remain conflicted. Here we show that diffusion was a significant process during the later stages of dissolution of a simplified waste glass but was not evidenced during the initial stages of dissolution. The absence of measurable isotopic fractionation in solution initially supports models of congruent dissolution. However, the solution becoming isotopically lighter at later times evidences diffusive isotopic fractionation and opposes models that exclude diffusive transport as a significant mechanism. The periodically sampled isotopic methodologies outlined here provide an additional dimension with which to understand glass dissolution mechanisms beyond the usual measurement of solution concentrations and, post-process, nano-scale analysis of the altered glass.

Carbon dioxide dissolution and ammonia losses in bubble columns for precipitated calcium carbonate (PCC) production

The slag2PCC concept aims at transforming steelmaking slag into precipitated calcium carbonate (PCC) with market value. This paper reports on R&D work on two features that impact the overall performance and costs of slag2PCC as a carbon capture and utilisation (CCU) technology. Operating near ambient conditions, calcium is selectively leached from steelmaking slag using aqueous ammonium salt solvent, followed by carbonation using a CO2-containing stream. Separators for removing spent slag and PCC connect two reactors for extraction and carbonation, respectively, between which the solvent solution is cycling. One requirement is effective conversion of the CO2 fed to the system: while the dissolution of CO2 is the rate-liming step it is essential to minimise release of unreacted CO2. Mixing the solutions enhances mass transfer. High-speed video recordings were made around mixers located at various heights in a bubble column, analysing CO2 bubble swarm dissolution. A second feature studied are losses of ammonia (NH3) from the solution. An outlet for unreacted gas presents a risk of NH3 vapour release, which lowers solution alkalinity while adding costs. Multicomponent mixture mass transfer analysis showed that diffusion of NH3 into CO2 bubbles may be significant at least during initial stages of dissolution. Experimental findings were verified.

Elucidation of the Controlled-Release Behavior of Metoprolol Succinate from Directly Compressed Xanthan Gum/Chitosan Polymers: Computational and Experimental Studies.

The development and evaluation of a controlled-release (CR) pharmaceutical solid dosage form comprising xanthan gum (XG), low molecular weight chitosan (LCS), and metoprolol succinate (MS) are reported. The research is, partly, based upon the utilization of computational tools: in this case, molecular dynamics simulations (MDs) and the response surface method (RSM) in order to underpin the design/prediction and to minimize the experimental work required to achieve the desired pharmaceutical outcomes. The capability of the system to control the release of MS was studied as a function of LCS (% w/w) and total polymer (LCS and xanthan gum (XG)) to drug ratio (P/D) at different tablet tensile strengths. MDs trajectories, obtained by using different ratios of XG/LCS as well as XG and high molecular weight chitosan (HCS), showed that the driving force for the interaction between XG and LCS is electrostatic in nature, the most favorable complex is formed when LCS is used at 15% (w/w) and, importantly, the interaction between XG and LCS is more favorable than that between XG and HCS. RSM outputs revealed that the release of the drug from the LCS/XG matrix is highly dependent on both the % LCS and the P/D ratio and that the required CR effect can be achieved when using weight fractions of LCS ≤ 20% and P/D ratios ≥2.6:1. Results obtained from in vitro drug release and swelling studies on the prepared tablets showed that using LCS at the weight fractions suggested by MDs and RSM data plays a major role in overcoming the high sensitivity of the controlled drug release effect of XG on ionic strength and pH changes of the dissolution media. In addition, it was found that polymer relaxation is the major contributor to the release of MS from LCS/XG tablets. Using Raman spectroscopy, MS was shown to be localized more in the core of the tablets at the initial stages of dissolution due to film formation between LCS and XG on the tablet surface, which prevents excess water penetration into the matrix. In the later stages of the dissolution process, the film starts to dissolve/erode, allowing full tablet hydration and a uniform drug distribution in the swollen tablet.

Dissolution of heat activated serpentine for CO2 sequestration: The effect of silica precipitation at different temperature and pH values

The dissolution of magnesium silicate minerals such as serpentine in aqueous solutions saturated or near saturated with carbon dioxide (CO2) enables its subsequent reaction to form magnesium carbonate, a process called aqueous mineral carbonation. The dissolution rate of magnesium ions (Mg2+) from thermally activated serpentine and the factors influencing the rate and extent of dissolution have been studied in our research group. The current contribution focuses on the effect of temperature and pH on the dissolution of heat activated lizardite (a polymorph of serpentine). The extent of dissolution of thermally activated lizardite was measured experimentally as a function of temperature (25 °C ≤ T ≤ 75 °C) and pH (1.2 ≤ pH ≤ 9.8). It was found that at higher temperatures the level of Mg extraction is greater during the initial stage of dissolution but is then hindered by the re-precipitation of amorphous silica. Thermodynamic modelling was used to assess the susceptibility of solid phase formation and confirmed the likelihood of re-precipitation of amorphous silica from the solutions. For the first time, in this work, the crackling core model (CCM) was used to model experimental data at different pH values.

Effect of dissolution of magnesium alloy AZ31 on the rheological properties of Phosphate Buffer Saline.

The issue of long-term incompatible interactions associated with the permanent implants can be eliminated by using various biodegradable metal implants. The recent research is focusing on the use of degradable stents to restore most of the hindrances of capillaries, and coronary arteries by supplying instant blood flow with constant mechanical and structural support. However, internal endothelialization and infection due to the corrosion of implanted stents are not easy to diagnose in the long run. In the recent past, magnesium (Mg) has been widely investigated for the cardiovascular stent applications. Here we made an attempt to understand the biodegradation process of Mg alloy stent by studying the degradation of Mg alloy AZ31 (3 wt% Aluminum, 1 wt% Zn) powder at various time-intervals in simulated blood fluid using the Rheological methods. The degradability of the Mg stent in the arteries affects the stress-strain properties of blood plasma and the subsequent flow conditions. Blood and plasma viscosities alter due to the degradation of Mg resulting from the stress-strain experienced in the blood vessels, in which the stent is inserted. Here our objective was to explore the influence of Mg degradation on the blood plasma viscosity by studying the viscoelastic properties. In this work, the effect of dissolution of Mg alloy AZ31 on the rheological properties of Phosphate Buffer Saline (PBS) at various time intervals have been investigated. The viscosity of the PBS-AZ31 solution increased with the dissolution of both slurries and percolated clear solution. The only exception was day-7 of the percolated clear solution, where viscosity was decreased showing a reduction in viscosity at initial stages of dissolution. The frequency sweep showed the tendency of the PBS-AZ31 gelation up to 100 rad/s frequency.

Growth Behavior of ZnO during Discharge of Zinc Negative Electrode

Zinc is promising negative electrode for secondary batteries due to high energy density and low cost. However, morphological changes of the electrode during charge-discharge cycles result in poor cyclability. Especially, formation of ZnO during discharge (anodic dissolution) is important process because precipitation of ZnO strongly influences cyclability and discharge capacity of the electrode [1]. In addition, ZnO formation mechanism is characteristic since ZnO is not directly formed by electron transfer but by dynamic concentration change of ions near electrode surface as described in Eq. (1) and (2). Zn + 4OH− = Zn(OH)4 2− + 2e− (1) Zn(OH)4 2− = ZnO + 2OH− + H2O (2) After formation of zincate ion by oxidation (Eq. (1)), ZnO precipitates when local concentration of zincate ion exceeds solubility limit (Eq. (2)). In previous study, ZnO morphologies were qualitatively classified based on dissolution conditions [2]. However, nucleation and growth behavior of ZnO, which is coupled with dynamic concentration change near electrode surface, is still unclear. In this study, in order to shed light on formation process of ZnO accompanying anodic dissolution, morphological and structural evolution of ZnO were analyzed in detail. Anodic dissolution was performed in galvanostatic conditions (+20 and +40 mA cm−2) in electrolyte containing 2.0 – 6.0 M KOH and 0.050 M ZnO. Effects of Li+addition were investigated by adding 0.50 M LiOH to electrolyte. Zinc working electrode was horizontally upward faced to counter electrode in order to avoid natural convection effect [3]. Counter electrode was zinc wire and reference electrode was Hg/HgO. Crystal structure of precipitated ZnO was analyzed by X-ray diffraction (XRD) and Raman spectroscopy. Scanning electron microscopy (SEM) was used to analyze morphology of the precipitates. First, to understand the formation process of ZnO, morphology and structure of precipitates were analyzed in time-resolved manner; dissolution was stopped at several oxidation time (200, 400, 600, 750 s) in 2.0 M KOH at +20 mA cm−2 and precipitates were characterized. At 750 s, electrode potential drastically changed to positive value. This potential increase was considered as passivation of electrode by precipitated ZnO [1]. From XRD measurements, ZnO peaks first appeared at 600 s. Simultaneously, SEM observations showed ZnO crystals with a few μm length. On the other hand, Raman scattering showed peak of ZnO E1 mode, which was assigned to defective ZnO (Zn excess ZnO), even at 200 s and 400 s where no ZnO peaks appeared in XRD. At 600 s, ZnO E2 mode (crystalline ZnO) also appeared and became dominant over E1 mode at passivated condition (750 s). These results suggested that the formation process of ZnO transiently changed; low crystalline ZnO formed at initial stages of dissolution and crystal growth of ZnO became dominant as dissolution proceeded. We hypothesized that this transient behavior was originated from dynamic change in nucleation and growth behavior of ZnO as follows. First, at initial stages of dissolution, nucleation by zincate condensation in supersaturated solution was favored. Then, crystal growth became dominant by decomposition of zincate species on precipitated ZnO surface. This assumption was testified by Li+ addition; in homogeneous precipitation of ZnO from zincate solutions, Li+ was known to prevent decomposition of zincate on ZnO surface and following crystal growth of ZnO particles [4]. Accordingly, the crystal growth observed at the latter stages of dissolution would be influenced by Li+ when this process was originated from zincate decomposition on the surface. As a result, acicular morphology of ZnO was changed to particulate by Li+ addition. Intensity of ZnO peaks decreased in XRD profiles. Thus, crystal growth was mitigated by Li+addition and this results supported the hypothesis above. From these results, ZnO growth behavior accompanying anodic dissolution of zinc dynamically changed from nucleation to crystal growth. Dominant growth behavior observed at the latter stages of dissolution was possibly caused by decomposition of zincate species on ZnO surface. This study was supported by JSPS KAKENHI Grant Number 25249104. T. O. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University from MEXT, Japan. [1] A. Nakata, H. Arai, T. Yamane, T. Hirai, Z. Ogumi, J. Electrochem. Soc., 163, A50 (2016). [2] R. W. Powers, M. W. Breiter, J. Electrochem. Soc., 116, 719 (1969). [3] I. Arise, Y. Fukunaka, F. R. McLarnon, J. Electrochem. Soc., 153, A69 (2006). [4] N. Uekawa, R. Yamashita, Y. J. Wu, K. Kakegawa, Phys. Chem. Chem. Phys., 6, 442 (2004).

Electrolyte effects on surface chemistry of basaltic glass in the initial stages of dissolution

For an understanding of the effect of solution composition on the dissolution rate of basaltic glass detailed knowledge of surface chemistry is important. Here the zeta potential (ζ) as a characteristic parameter of the magnitude of surface charge at the solid–liquid interface was used to determine ionic effects on surface chemistry in initial stages of basaltic glass dissolution. In a systematic approach powdered synthetic basaltic glass was dispersed in solutions of different cations (NO3− salts of Na+, K+, Mg2+, Ca2+, Ba2+, Zn2+, and Al3+) and anions (Na+ salts of F−, Cl−, I−, NO3−, SO42−, C2O42−, HPO42−), each in concentrations of 0.1, 0.5, 1.0, 2.5, and 5.0mmol/L. ζ was traced in time sequences up to 12,000h at ideally circumneutral pH. Ion affinities to glass surfaces were characterized by sorption isotherms. A change of glass chemical composition by the formation of altered layers was determined by depth profiling using secondary neutral mass spectrometry (SNMS). The dissolution of the glass was quantified by the amount of Si released after 4000h.A marked decrease of ζ in deionized water within the first 3h reaction time is assigned to the desorption of alkali and alkaline earth metal cations from the glass surface and formation of negatively charged SiO− sites. The addition of anions resulted in stronger negative initial ζ values in comparison with the experiment in deionized H2O indicating marked anion adsorption on surface sites, most obvious for F−, C2O42− and HPO42−. The initial ζ was increased upon the addition of divalent cations indicating neutralization of negatively charged surface sites. Over time a striking shift from negative to positive ζ was obtained, most markedly for Ca2+ and Zn2+. The addition of trivalent Al3+ resulted directly in positive ζ indicating a strong adsorption on glass surfaces. With the progress of the experiment the sign of ζ reversed to negative values again. The reason for charge reversal is not fully understood and might be related with cation adsorption exceeding the negative surface charge and a concentration of Fe oxides at the glass surface. After an ~2000h reaction time ζ adjusted for most electrolyte additions to slightly negative ζ until the end of the experiment, indicating that a final state in the composition of surface sites was reached. The presence of monovalent Na+ and K+ in solution suppressed Si release from the glass, whereas it is accelerated by bivalent cations. It appears that the neutralization of deprotonated SiO− sites by monovalent cations – their preferential binding is also indicated by chemical analysis – favors polymerization resulting in slower Si release. Upon the addition of Al3+ it is likely that SiOAlOSi bonds are formed, which can suppress Si release. The presence of F−, C2O42−, and HPO42− clearly enhances glass dissolution, most probably by increasing the coordination of network forming cations, hereby weakening bonds. The observed generation of positive ζ on basaltic glass surfaces is remarkable, and can improve in natural systems the adsorption capability of the basaltic glass surface for negatively charged compounds from pore solution, anions, dissolved organic matter and also bacterial cell walls.

Dissolution of ZrO2 based pyrochlores in the acid pH range: A macroscopic and electron microscopy study

The dissolution kinetics of ZrO2–Nd2O3 polycrystalline pyrochlore and defect fluorite ceramic powders under acidic conditions were observed following a combined macroscopic and electron microscopic (SEM) approach. Dynamic dissolution experiments were carried out with a variation of temperature and pH as well as the chemical composition within the ZrO2–Nd2O3 system. SEM observations indicate a preferential leaching at the grain boundaries for all experiments. A preferential release of Nd during the initial stages of dissolution, which is several orders of magnitude higher than the Zr release, was measured by ICP-MS. At steady state, the normalised Nd-rate approaches the Zr based dissolution rate within one order of magnitude, becoming congruent for most experiments. Zr-based BET surface area normalised steady state rates at c(H+)=0.1N and 90°C are in the range between 4×10−7 and 3×10−6gm−2d−1, indicating no significant influence of the transition pyrochlore to defect fluorite and the chemical composition on the macroscopic dissolution rate. Based on the different numbers of grain boundaries per surface area in the pyrochlore compared to the defect fluorite, a slightly higher “true” dissolution rate could be assumed for the defect fluorite. The influence of pH and temperature variations to the dissolution rates of defect fluorite and pyrochlore are similar and in the range observed for other multioxide materials.

Imprints of multiple fluid regimes on episyenites in the Bohus granite, Sweden

A number of episyenite occurrences within the Neoproterozoic Bohus granite, southwestern Sweden, were investigated in order to elucidate their genesis and past associated fluid regimes. Fluid inclusion microthermometry and O–H isotope systematics integrated with information from detailed mineralogy reveal a paragenetic succession within individual episyenites, including: (1) albitization of plagioclase, accompanied by dissolution of quartz, with a vuggy feldspathic reservoir rock as a result; (2) resumed infilling and replacement of igneous Fe–Ti-phases by various phyllosilicates (muscovite, chlorite and/or illite)±hematite assemblages and quartz, due to rift-related reactivation and (3) sporadic argillitic alteration and precipitation of fracture calcite. Although sparse owing to later overprinting, there are a few remnants that provide arguments for the involvement of meteoric water, shifted to a more 18O-rich composition, during the initial stage of dissolution and albitization. The main vug-filling phase is proposed to be associated with the ingress of surface-derived fluids with negative δ18O values, during the regional thermal anomaly accompanying the development of the nearby Permo-Carboniferous Oslo Rift at ~250Ma. The fluids involved during this event show consistently a marked, but variable 18O-depletion and resemble the modern-day shield brines, with modal salinities of 11–24 wt.% NaCl eq., low NaCl/(NaCl+CaCl2) weight ratios and no discernible CO2. Even though no P–T path can be defined for this phase, the fluid pressure can be restricted to 0.3–0.8kbar and temperature to 160–280°C, assuming a sedimentary overburden of ~3km. An overall variability in the δ18O and δD data can mainly be ascribed to (1) precipitation from fluids affected by isotopic exchange with the host rock and/or sedimentary overburden under highly variable water/rock ratios, and to some extent with (2) varying degrees of partial re-equilibration during subsequent hydrothermal interaction at lower temperatures, as emphasized by the presence of fluid inclusions of suspected secondary origin in quartz.

Leaching behaviour of high surface area synthetic boehmite in NaOH solution

This study pertains to alkali leaching of a high surface area synthetic boehmite prepared by thermal decomposition of gibbsite. Synthesised boehmite particles are found to be agglomerates of platelets. These particles are characterised by a monomodal size distribution with a median size of 110μm and high specific surface area (~264m2/g). Leaching studies, in the temperature range 70 to 90°C, have been carried out in NaOH solutions of Na2O concentration, CNa2O=100 and 180g/l. Significant dissolution of this boehmite is seen to occur in NaOH solutions, unlike in many other studies, owing to the high surface area. Both alkali concentration and temperature have influenced the dissolution. Reduced time plot analysis indicates that the shrinking core with surface reaction model (R3) describes the initial stages of the dissolution process reasonably well. Particle size analysis data complemented the results of the reduced time analysis. The value of apparent activation energy for the initial stages of dissolution is evaluated to be around 65kJ/mol.Deviation from shrinking core model R3 is observed in the later stages. Besides the shrinking of size due to dissolution, particle size reduction is also identified due to the detachment of platelets from the agglomerates of particles. Detailed characterisation of the leach residues involving morphological and particle size analyses has established such detachment of platelets and ensuing particle size reduction. Deviation from shrinking core model in the later stages of leaching has been attributed to particle fragmentation arising from platelet separation.

Real-Time Observation of High Temperature Interface between SiC Substrate and Solution during Dissolution of SiC

Precise morphological control of the interface between SiC and solution during the solution growth of SiC is crucial for obtaining high quality crystals with fewer defects and less step bunching. In this paper, a new technique for real-time observation of the high temperature interface between SiC and solution through the back surface of SiC was developed by focusing on the “wide” bandgap of SiC. Real-time observation of the interface during dissolution of SiC into an Fe-Si solvent alloy was carried out using a digital microscope, and the submicron-height structure of the solid-liquid interface was clearly observed at up to 1773 K. Interface morphologies, such as numerous hexagonal pits which were present at the initial stage of dissolution, followed by preferential dissolution in the lateral direction, were observed.

Apparent Inverse Gibbs-Thomson Effect in Dealloyed Nanoporous Nanoparticles

The Gibbs-Thomson effect (the reduction of local chemical potential due to nanoscale curvature) predicts that nanoparticles of radius r dissolve at lower electrochemical potentials than bulk materials, decreasing as 1/r. However, we show here that if the particle is an alloy--susceptible to selective dissolution (dealloying) and nanoporosity evolution--then complete selective electrochemical dissolution and porosity evolution require a higher electrochemical potential than the comparable bulk planar material, increasing empirically as 1/r. This is a kinetic effect, which we demonstrate via kinetic Monte Carlo simulation. Our model shows that in the initial stages of dissolution, the less noble particle component is easily stripped from the nanoparticle surface, but owing to an increased mobility of the more noble atoms, the surface of the particle quickly passivates. At a fixed electrochemical potential, porosity and complete dealloying can only evolve if fluctuations in the surface passivation layer are sufficiently long-lived to allow dissolution from percolating networks of the less-noble component that penetrate through the bulk of the particle.