Mixed Ca Research Articles

Production of mixed carbonate phases using ammonium carbonate-metal acetate reactions

Materials such as bone, nacre, and fish scales have unique structures that give the material strength as well as ductility. The ultimate goal of our research is to develop new composite materials that have improved strength and ductility compared to conventional materials. The US Army Engineer Research and Development Center is incorporating these design principles to develop hierarchical materials for structural applications. In the present study, the mineralogy and morphology of calcium-containing carbonates precipitated by reacting (NH4)2CO3 with mixed Ca, Sr, Mg, and Mn-acetates was investigated. As the proportion of the non-Ca component increased, the products shifted toward double carbonates and mixtures of double carbonates with single carbonates. Characterization by scanning electron microscopy and X-ray diffraction to determine crystal sizes, morphology, and structure of precipitated phases indicated a potential for re-crystallizing the products to form new composite materials. Ongoing research efforts are focused on using information obtained in the present study to develop composites by hydrothermal recrystallization of metastable phases.

Impact of pit-toilet leachate on groundwater chemistry and role of vadose zone in removal of nitrate and E. coli pollutants in Kolar District, Karnataka, India

Assessment of chemistry of groundwater infiltrated by pit-toilet leachate and contaminant removal by vadose zone form the focus of this study. The study area is Mulbagal Town in Karnataka State, India. Groundwater level measurements and estimation of unsaturated permeability indicated that the leachate recharged the groundwater inside the town at the rate of 1 m/day. The average nitrate concentration of groundwater inside the town (148 mg/L) was three times larger than the permissible limit (45 mg/L), while the average nitrate concentration of groundwater outside the town (30 mg/L) was below the permissible limit. The groundwater inside the town exhibited E. coli contamination, while groundwater outside the town was free of pathogen contamination. Infiltration of alkalis (Na+, K+) and strong acids (Cl−, SO4 2−) caused the mixed Ca–Mg–Cl type (60 %) and Na–Cl type (28 %) facies to predominate groundwater inside the town, while, Ca–HCO3 (35 %), mixed Ca–Mg–Cl type (35 %) and mixed Ca–Na–HCO3 type (28 %) facies predominated groundwater outside/periphery of town. Reductions in E. coli and nitrate concentrations with vadose zone thickness indicated its participation in contaminant removal. A 4-m thickness of unsaturated sand + soft, disintegrated weathered rock deposit facilitates the removal of 1 log of E. coli pathogen. The anoxic conditions prevailing in the deeper layers of the vadose zone (>19 m thickness) favor denitrification resulting in lower nitrate concentrations (28–96 mg/L) in deeper water tables (located at depths of −29 to −39 m).

Study of mixed Ca–Zn hydroxyapatite surface modified by lactic acid

The new hybrid inorganic–organic composites, Ca(10−x)Znx(PO4)6(OH)2-lactic acid, at different amounts of zinc and lactic acid were prepared by dissolution of the organic compound in an hydroxyapatite suspension. They were characterized by XRD, IR, MAS NMR (13C and 1H) and chemical analysis. The crystallinity was slightly affected by the presence of organic fragments. IR and (13C and 1H) MAS NMR measurements indicate that the carboxylic groups of the acid interact with calcium and zinc ions of hydroxyapatite surface. Chemical analysis displays that zinc promotes the acid grafting. A mechanism of surface modification is proposed based on the obtained results.

Mineralogical aspects of CO2 sequestration during hydrothermal basalt alteration — An experimental study at 75 to 250 °C and elevated pCO2

The alteration of basaltic glass was studied experimentally at elevated CO2 pressures (~10–25bar) and hydrothermal conditions (75–250°C) to determine the effects of temperature and extent of reaction (ξ) on the secondary mineral formation and compositional evolution of alteration assemblages resulting from CO2–water–basalt interaction. At <100°C, the alteration products consisted of concentric layers of ankerite and dolomite–ankerite solid solutions (Ca–Mg–Fe carbonates) and amorphous silica. At ≥150°C, mixed Ca–Mg–Fe smectites and chlorite, calcite, amorphous silica and zeolites formed instead of Ca–Mg–Fe carbonates. Competing reactions between carbonates and clays for major divalent cations (Ca, Mg and Fe) were affected by the extent of reaction with smectites formed initially and progressively being replaced by calcite and chlorite. The basaltic glass dissolution rate and mechanism were also affected by temperature and reaction time. At lower temperatures (≤150°C), a hydrated leached layer formed on the glass surface and the mass fluxes in the system were largely controlled by the dissolution rate and mechanism of the glass, whereas at higher temperature (250°C) the dissolution rate of the basaltic glass was fast with the reactions primarily driven by secondary mineral replacement and growth. We conclude that the driving force for carbonate mineralization in basaltic glass at elevated CO2 conditions is linked to the geochemical behavior and mobility of Ca, Mg and Fe together with the availability of Si for Al–Si-mineral formation. At <100°C, the divalent cations are available for Ca–Mg–Fe carbonate formation, whereas at ≥150°C abundant clays limit the availability of Mg and Fe resulting in calcite being the only carbonate formed. This implies a quantitatively more effective fixation of CO2 at <100°C associated with the formation of Ca–Mg–Fe carbonates.

Geochemical and statistical evaluation of groundwater in Imphal and Thoubal district of Manipur, India

In order to identify the hydrogeochemical process controlling the quality of groundwater, an extensive study was carried out in Imphal and Thoubal district of Manipur, India. The objectives of the studies were to delineate the spatial and temporal variability in groundwater quality and understand its suitability for human uses. In the study area groundwater samples from 45 location have been collected during the pre-monsoon, monsoon and post-monsoon seasons and analyzed for the major ions such as Ca2+, Mg2+, Na+, K+, HCO3-, Cl−, SO42-, NO3-, PO43-, F−, Fe and silica. The water quality of both districts was good for domestic and agricultural uses except for few samples. These saline samples were localized and were due to the inherent lithology of the study area. Three major hydrochemical facies (Ca–HCO3 type, mixed Ca–Na–HCO3 and Ca–Mg–Cl types in Imphal district and Ca–HCO3, mixed Ca–Mg–Cl and Ca–Cl type in Thoubal district) were identified. The mineral stability diagrams indicated that the groundwater is in equilibrium with kaolinite and Ca-montmorillonite whereas Gibbs plot showed that the chemical composition of ground water in both districts is controlled by the natural weathering processes irrespective of seasons. Among the chemical weathering processes, silicate weathering was dominant. The results were supported by Wilcox plot and USSL diagrams. The study reflected the overall suitability of groundwater for anthropogenic use.

Phase developments and dielectric responses of barium substituted four-layer CaBi4Ti4O15 Aurivillius

In this paper, mixed Ca–Ba oxide Ca1 − x Ba x Bi4Ti4O15 (CBBT) ceramics, fabricated by the improved traditional ceramics process were investigated by doping concentrations of Ba ion up to \(\emph{x} {=}\) 0·9 (in steps of 0·1). At room temperature, an orthorhombic crystal system was confirmed using XRD, and their parameter was obtained using the Rietveld method. Dielectric properties and phase transitions were studied and are explained in terms of lattice response of these ceramics. A shift in ferroelectric–paraelectric phase transition (\(\emph{T}_{C}\)) to lower temperatures and a corresponding decrease in permittivity peak with increasing concentration of Ba2 + are also observed. The ferroelectric–paraelectric phase transition of CBBT compounds is of normal type in nature, differing from the relaxor characteristic of BBT. The decrease of orthorhombicity in the lattice structure by the larger Ba2 + ion incorporation, indicating an approach of a and b, results in lower Curie temperature. Appearance of anomalous loss peaks of Ba-rich compounds at 530°C reveals a phase transition development trend from ferroelectric orthorhombic structure to the paraelectric orthorhombic structure. Relationship of polarization with lattice response is discussed.

Long-range volcanic ash transport and fallout during the 2008 eruption of Chaitén volcano, Chile

The May 2008 eruption of Chaitén volcano, Chile, provided a rare opportunity to measure the long-range transport of volcanic emissions and characteristics of a widely-dispersed terrestrial ash deposit. Airborne ash mass, quantified using thermal infrared satellite remote sensing, ranged between 0.2 and 0.4Tg during the period 3–7 May 2008. A high level of spatiotemporal correspondence was observed between cloud trajectories and changes in surface reflectivity, which was inferred to indicate ash deposition. The evolution of the deposit was mapped for the first time using satellite-based observations of surface reflectivity.The distal (>80km) ash deposit was poorly sorted and fine grained, and mean particle size varied very little beyond a distance >300km. There were three particle size subpopulations in fallout at distances >300km which mirror those identified in fallout from the 18 May 1980 eruption of Mount St. Helens, known to have a high propensity for aggregation. Discrete temporal sampling and characterisation of fallout demonstrated contributions from specific eruptive phases. Samples collected at the time of deposition were compared to bulk samples collected months after deposition and provided some evidence for winnowing.Experimentally-derived ash leachates had near-neutral pH values and charge balance which indicates minimal quantities of adsorbed acids. X-ray Photoelectron Spectroscopy (XPS) analyses revealed surface enrichments in Ca, Na and Fe and the presence of coatings of mixed Ca-, Na- and Fe-rich salts on ash particles prior to deposition. Low S:Cl ratios in leachates indicate that the eruption had a low S content, and high Cl:F ratios imply gas–ash interaction within a Cl-rich environment. We estimate that ash fallout had potential to scavenge ∼42% of total S released into the atmosphere prior to deposition. XPS analyses also revealed ash particle surfaces were strongly enriched in Fe (in contrast to the results from bulk leachate analyses), which suggests that Chaitén ash fallout over oceans had potential to influence productivity in high-nutrient, low-chlorophyl regions of the oceans. Therefore ash particle surface geochemical analysis should be applied to quantify Fe-modulated biologically-forced CO2 draw-down potential of volcanic ash fallout over oceans.

Study on Influencing Factors of Flow-Ability of Cement Asphalt Mortar

Flow-ability of CA mortar is characterized by fluidity, and the effects of the CA mortar fluidity according to water, time, and feeding sequence are studied. Studies have shown that the amount of the used of water affects the fluidity most, in which the suitable amount of water is between 0.23 and 0.28. With the time delay, the flow-ability of CA mortar is reduced. When mixed with SBR latex, the process of decay of the flow-ability is significantly reduced. Feeding sequence has great effect on fresh mixed CA mortar. In the mixing process, to avoid asphalt emulsion and cement dry material in direct contact, the speed of demulsification is reduced, which can effectively increase the flow-ability of CA mortar.

Novel method of generation of Ca(HCO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;)&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and CaCO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; aerosols and first determination of hygroscopic and cloud condensation nuclei activation properties

Abstract. Atmospheric mineral aerosols contain CaCO3 as a reactive component. A novel method to produce CaCO3 aerosol was developed by spraying Ca(HCO3)2 solution, which was generated from a CaCO3 suspension and CO2. By aerosol mass spectrometry the freshly sprayed and dried aerosol was characterized to consist of pure Ca(HCO3)2 which under annealing in a tube furnace transformed into CaCO3. Transmission Electron Microscopy demonstrated that the particles produced were spherical. The method was able to generate aerosol of sufficient concentration and proper size for the study of physiochemical properties and investigations of heterogeneous reactions of mineral aerosol. The dried Ca(HCO3)2 particles were somewhat more hygroscopic than CaCO3 particles. However, during humidification a restructuring took place and ∼2/3 of the Ca(HCO3)2 was transformed to CaCO3. The mixed Ca(HCO3)2/CaCO3(s) particles were insoluble with a growth factor of 1.03 at 95% (hygroscopicity parameter κ=0.011±0.007) relative humidity. This compares to a corresponding growth factor of 1.01 for CaCO3(s) (κ=0.0016±0.0004). Mass spectrometric composition analysis, restructuring, and insolubility of the mixed particles suggested that solid Ca(HCO3)2(s) was observed. This would be in contrast to the current belief that Ca(HCO3)2(s) is thermodynamically instable. The CCN activity of Ca(HCO3)2(s) aerosol (κ≈0.15) is remarkably higher than that of CaCO3 aerosol (κ=0.0019±0.0007) and less than that of Ca(NO3)2. The noticeable but limited solubility of Ca(HCO3)2 of ≈0.01 mol/l explains limited hygroscopic growth and good CCN activity. Experiments in the Large Jülich Aerosol Chamber indicated that Ca(HCO3)2(s) could exist for several hours under dry atmospheric conditions. However, it was likely buried in a protective layer of CaCO3(s). We conclude that Ca(HCO3)2 may be formed in the atmosphere in cloud droplets of activated mineral dust by reaction of CaCO3 with CO2 and H2O. The presence of Ca(HCO3)2 and as a consequence an enhanced CCN activity may alter the influence of mineral aerosol on global climate.

Hydrogeochemical assessment of groundwater in Neyveli Basin, Cuddalore District, South India

In the light of progressive depletion of groundwater reservoir and water quality deterioration of the Neyveli basin, an investigation on dissolved major constituents in 25 groundwater samples was performed. The main objective was detection of processes for the geochemical assessment throughout the area. Neyveli aquifer is intensively inhabited during the last decenniums, leading to expansion of the residential and agricultural area. Besides semi-aridity, rapid social and economic development stimulates greater demand for water, which is gradually fulfilled by groundwater extraction. Groundwaters of the study area are characterized by the dominance of Na + K over Ca + Mg. HCO3 was found to be the dominant anion followed by Cl and SO4. High positive correlation was obtained among the following ions: Ca–Mg, Cl–Ca,Mg, Na–K, HCO3–H4SiO4, and F–K. The hydrochemical types in the area can be divided into two major groups: the first group includes mixed Ca–Mg–Cl and Ca–Cl types. The second group comprises mixed Ca–Na–HCO3 and Ca–HCO3 types. Most of the groundwater samples are within the permissible limit of WHO standard. Interpretation of data suggests that weathering, ion exchange reactions, and evaporation to some extent are the dominant factors that determine the major ionic composition in the study area.

Impact of anthropogenic activities on the Densu River in Ghana

AbstractA detailed study has been carried out on the Densu River and its tributaries to identify the chemical characteristics of the water, examine the hydrological connections among the land cover types and evaluate the relative contributions of anthropogenic activities to the river's pollution. The results indicate that sites closer to the most urbanized, industrialized and agricultural areas are severely impaired. The river water was found to vary considerably in terms of chemical and physical properties. The water was slightly alkaline, brackish to fresh and of mixed Ca–Na–HCO3 and Ca–HCO3 types. Nutrient and organic matter were found to be the most frequent cause of pollution of the river. Pollution has reached such a proportion that it has destroyed the aquatic ecosystem in most exposed sections of the river.

Groundwater quality assessment from a hard rock terrain, Salem district of Tamilnadu, India

A total of 162 groundwater samples for three representative seasons were collected from Salem district of Tamilnadu, India to decipher hydrogeochemistry and groundwater quality for determining its suitability for drinking and agricultural proposes. The water is neutral to alkaline in nature with pH ranging from 6.6 to 8.6 with an average of 8.0. Higher electrical conductivity was observed during post-monsoon season. The abundance of major ions in the groundwater was in the order of \( {\text{Na} > \text{Ca} > \text{Mg} > \text{K} = \text{Cl} > \text{HC}}{{\text{O}}_3}\; > \;{\text{S}}{{\text{O}}_4}\; > \;{\text{N}}{{\text{O}}_3} \). Piper plot reveals the dominance of geochemical facies as mixed Ca–Mg–Cl, Na–Cl, Ca–HCO3, Ca–Na–HCO3, and Ca–Cl type. NO3, Cl, SO4, and F exceed the permissible limit during summer and post-monsoon seasons. Sodium adsorption ratio was higher during post-monsoon and southwest monsoon season indicating high and low salinity, satisfactory for plants having moderate salt tolerance on soils. Permeability index of water irrespective of season falls in class I and class II indicating water is moderate to good for irrigation purposes. As per the classification of water for irrigation purpose, water is fit for domestic and agricultural purposes with minor exceptions irrespective of seasons.

Anomalous Dielectric Behavior in CaCu3Ti4O12 at High Temperatures

Dielectric and structural behavior in perovskite-related CaCu 3 Ti 4 O 12 ceramics was studied at high temperatures up to 920 K. A new dielectric anomaly was found at T 1 = 732 K. With increasing temperature, the dielectric constant ϵ (2.6 × 10 4 at room temperature) shows a shoulder-like round anomaly (ϵ ∼ 7.6 × 10 4 ) around 630 K and a peak (ϵ peak ∼ 8.5 × 10 4 , f = 1 MHz.) at T 1 . The doping effect of Cd ions on the dielectric constant was studied in mixed Ca 1 − x Cd x Cu 3 Ti 4 O 12 ceramics. With adding a small amount of Cd-dopant, the peak value decreases to ϵ peak ∼ 5 × 10 3 for a mixed sample with x = 0.03. The mixed sample has still high dielectric constant and no drastic suppression was observed by Cd-doping. The T 1 decreases linearly with increasing Cd-dopant. At high temperatures over T 1 , several X-ray diffraction lines of CaCu 3 Ti 4 O 12 disappear. The dielectric and structural evidences indicate that CaCu 3 Ti 4 O 12 undergoes a structural phase transition at T 1 .

The effect of calcium source and particle size on the production performance and bone quality of laying hens

The efficacy of 3 local limestone sources as potential Ca sources for laying hens was studied. Limestone sources were assessed for in vitro solubility. Four Ca sources (control, A, B, or C) and 2 particle size combinations (ground, 100% ground; or mixed, 67% ground + 33% large particle) was used. The control consisted of a commercial ground limestone for the ground Ca source and oyster shell of the large particle Ca source. DeKalb laying hens were randomly placed in 32 battery cage units (n = 12/cage). At 19 wk of age, hens received 1 of 8 Ca source and particle size treatments (4 replicates/treatment) until 74 wk. Egg production, feed consumption, BW, and egg quality were measured throughout. Bone mineral density (by quantitative computed tomography), breaking strength, ash, and Ca were assessed at the end of lay. In vitro solubility was dependent upon Ca source and particle size (P < 0.0001) with oyster shell generally having a greater in vitro solubility than test limestone sources A, B, and C of similar particle sizes. Feed consumption (100 to 117 g/bird per day), BW (1,500 to 1,800 g), egg production (92% peak egg production), egg weight (55 to 67 g), and egg specific gravity (1.090 to 1.078) did not differ among hens fed the different Ca sources (P > 0.05). Tibia bone mineral density indicated the oyster shell treatment had a lower trabecular density than Ca source A; however, all other Ca sources had similar bone mineral density for all measures (P > 0.05). Hens fed the mixed Ca particle treatments consistently had greater feed consumption from 27 to 70 wk than those fed the 100% ground Ca source (P < 0.05). Bone mineralization was enhanced in hens that received the mixed Ca particle treatments (P < 0.05). Overall, the results of this study indicate that the local limestone sources A, B, and C would be suitable alternatives to current commercial sources of Ca for laying hens. In addition, large particle Ca did improve bone quality.

Nondestructive on‐site identification of ancient glasses: genuine artefacts, embellished pieces or forgeries?

AbstractA selection of 23 rare glass objects, mostly enameled, of various provenance and age, from the 5th century BC to the 19th century AD including the Western and Islamic Middle Ages but with a focus on 16th–18th century Venetian and French ‘façon de Venise’ artefacts, have been studied on‐site at the Sèvres museum or at the laboratory. The Raman signatures of the transparent or opacified glass matrix and of enameled decorations are discussed and compared to those previously recorded on ceramics and stained glasses. The Raman parameters allow discrimination between 2 groups (with some variations) of glass bodies, belonging to mixed Ca–Na and Ca‐containing Na‐rich silicates, with some exceptions. Most enamels are instead lead‐based glasses, but we also found enamels having a composition close to that of the glass body. Most of the pigment signatures are similar to those recorded on ceramic glazes, which proves the link between the two technologies. A particular emphasis was given to the identification of white opacification techniques. Very specific signatures could question the authenticity of some artefacts, and at least in two cases, arguments have been found to identify a fake or embellished artefact. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Preferential Affinity of Calcium Ions to Charged Phosphatidic Acid Surface from a Mixed Calcium/Barium Solution: X-ray Reflectivity and Fluorescence Studies

X-ray reflectivity and fluorescence near total reflection experiments were performed to examine the affinities of divalent ions (Ca(2+) and Ba(2+)) from aqueous solution to a charged phosphatidic acid (PA) surface. A phospholipid (1,2-dimyristoyl-sn-glycero-3-phosphate, DMPA), spread as a monolayer at the air/water interface, was used to form and control the charge density at the interface. We find that, for solutions of the pure salts (i.e., CaCl(2) and BaCl(2)), the number of bound ions per DMPA at the interface is saturated at concentrations that exceed 10(-3) M. For 1:1 Ca(2+)/Ba(2+) mixed solutions, we find that the bound Ca(2+)/Ba(2+) ratio at the interface is 4:1. If the only property determining charge accumulation near PA were the ionic charges, the concentration of mixed Ca(2+)/Ba(2+) at the interface would equal that of the bulk. Our results show a clear specific affinity of PA for Ca compared to Ba. We provide some discussion on this issue as well as some implications for biological systems. Although our results indicate an excess of counterion charge with respect to the surface charge, that is, charge inversion, the analysis of both reflectivity and fluorescence do not reveal an excess of co-ions (namely, Cl(-) or I(-)).

Investigation of the hygroscopic properties of Ca(NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;)&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and internally mixed Ca(NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;)&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;/CaCO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; particles by micro-Raman spectrometry

Abstract. To understand and predict the role of mineral aerosol particles processed by reactive nitrogen species in the atmosphere, the hygroscopic properties of both Ca(NO3)2 and Ca(NO3)2-containing mineral particles must be well understood. Using a micro-Raman system, the hygroscopic behaviors of micro-sized individual Ca(NO3)2 and internally mixed Ca(NO3)2/CaCO3 particles in both dehumidifying and humidifying processes were investigated systematically. In addition to accurate quantification of the dependence of water content on relative humidity (RH), significant new spectroscopic evidence related to chemical structure was also obtained to confirm the occurrence of amorphous solid state and to better understand the phase transition process. The Ca(NO3)2 particles exhibit reversible behavior in the dehumidifying and humidifying processes; they are in the state of solution droplets above 10% RH and amorphous hydrates below 7% RH, and phase transition occurs at 7–10% RH. The hygroscopic behavior of Ca(NO3)2/CaCO3 particles is identical to that of pure Ca(NO3)2 particles, suggesting a negligible effect of the slightly soluble CaCO3 inclusion on the hygroscopic behavior of a(NO3)2/CaCO3 particles.

Formation and stability of mixed Mg 2+/Ca 2+/phytate species in synthetic seawater media : Consequences on ligand speciation

The results of a potentiometric investigation (by ISE-H +, glass electrode) on the speciation of phytate ion (Phy 12−) in an ionic medium simulating the major components (Na +, K +, Ca 2+, Mg 2+, Cl − and SO 4 2−) of natural seawater, at different salinities and t = 25 °C, are reported. The work was particularly aimed at determining the possible formation of mixed Ca 2 +–Mg 2+–phytate ion pairs, and to establish how including the formation of these mixed species would affect the speciation modeling in seawater media. After testing various speciation models, that considering the formation of the MgCaH 3Phy 5−, MgCaH 4Phy 4−, Mg 2CaH 3Phy 3− and Mg 2CaH 4Phy 2− species was accepted, and corresponding stability constants were determined at two salinities ( S = 5, 10). A discussion is reported both on the choice of the experimental conditions and on the possibility to extend these results to those typical of real seawater. A detailed procedure is also described to demonstrate that the stability of these species is higher than that statistically predicted. As reported in literature, a parameter, namely log X, has been determined in order to quantify this extra stability for the formation of each mixed species at various salinities. For example, at S = 10, log X 113 = 2.67 and log X 114 = 1.37 for MgCaH 3Phy 5− and MgCaH 4Phy 4− (statistical value is log X stat = 0.60), and log X 213 = 6.11 and log X 214 = 2.15 for Mg 2CaH 3Phy 3− and Mg 2CaH 4Phy 2− (log X stat = 1.43), respectively. Results obtained also showed that the formation of these species may occur even in conditions of low salinity (i.e. low concentration of alkaline earth cations) and low pH (i.e., more protonated ligand).

Dynamic simulation of the effect of calcium-release activated calcium channel on cytoplasmic Ca 2+ oscillation

A mathematical model is proposed to illustrate the activation of STIM1 (stromal interaction molecule 1) protein, the assembly and activation of calcium-release activated calcium (CRAC) channels in T cells. In combination with De Young–Keizer–Li–Rinzel model, we successfully reproduce a sustained Ca 2+ oscillation in cytoplasm. Our results reveal that Ca 2+ oscillation dynamics in cytoplasm can be significantly affected by the way how the Orai1 CRAC channel are assembled and activated. A low sustained Ca 2+ influx is observed through the CRAC channels across the plasma membrane. In particular, our model shows that a tetrameric channel complex can effectively regulate the total quantity of the channels and the ratio of the active channels to the total channels, and a period of Ca 2+ oscillation about 29 s is in agreement with published experimental data. The bifurcation analyses illustrate the different dynamic properties between our mixed Ca 2+ feedback model and the single positive or negative feedback models.

NMR Confirmation for Formation of [B12H12]2- Complexes during Hydrogen Desorption from Metal Borohydrides

^(11)B NMR spectroscopy has been employed to identify the reaction intermediates and products formed in the amorphous phase during the thermal hydrogen desorption of metal tetrahydroborates (borohydrides) LiBH_4, Mg(BH_4)_2, LiSc(BH_4)_4, and the mixed Ca(AlH_4)_(2-)LiBH_4 system. The ^(11)B magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) spectral features of the amorphous intermediate species closely coincide with those of a model compound, closo-borane K2B_(12)H_(12) that contains the [B_(12)H_(12)]^(2-) anion. The presence of [B_(12)H_(12)]^(2-) in the partially decomposed borohydrides was further confirmed by high-resolution solution ^(11)B and ^1H NMR spectra after dissolution of the intermediate desorption powders in water. The formation of the closo-borane structure is observed as a major intermediate species in all of the metal borohydride systems we have examined.