Thermodynamic analysis on calcium dissolution in the system of Ca(II)-NH3-NH4Cl-H2O

Based on the principles of ionic balance, electro-neutrality and the existing thermodynamic data, the thermodynamics equilibrium on calcium dissolution in the system of Ca(II)-NH₃-NH₄Cl-H₂O was theoretically calculated. The results show that the equilibrium pH values in the system vary from 11.43 to 12.59, and the solubility of Ca(II) is significant in the system of NH₄Cl-NH₃-H₂O. The c(NH₄OH) has little influence on c(Ca²⁺)_T and when the ammonia concentration is kept constant, the total calcium concentration (c(Ca²⁺)_T) with a maximum concentration of 2.03 mol/L increases linearly with the increase of concentration of c(NH₄Cl). Calcium mainly presents in the form of calcium-bearing complexes instead of calcium ion in the system. The reaction degrees of Ca²⁺ with ammonium hydroxide and H₂O are less in the system. The absolute average error of calcium equilibrium concentration between the experimental and theoretical calculated values is 7.07% in confirmatory experiments. The results are helpful to clarify the leaching behaviour of calcium ions in the system of Ca(II)-NH₃-NH₄Cl-H₂O and provide the theoretical foundation to improve the calcium yield and remove impurities in the leaching stage of the indirect aqueous CO₂ sequestration process.     

Synthesis of single phase aragonite precipitated calcium carbonate in Ca(OH)2-Na2CO3-NaOH reaction system

The formation behavior of precipitated calcium carbonate polymorphs was investigated in three different supersaturation levels. Because the most easily adjustable and influential variable determining supersaturation is the ion concentration of the major reactants — Ca²⁺ and CO3 ₃ ²⁻ — the supersaturation can be adjusted by changing the ion concentration of these two ions. At high supersaturation, free energy is necessary for a decrease in nucleation, promoting the formation of a sphere-shaped vaterite, while aragonite and calcite were seen to co-exist at medium supersaturation. At low supersaturation, aragonite was mainly formed by mixing with some calcite. Hence, we considered that lower supersaturation was necessary to obtain a single phase aragonite. Furthermore, we found that the solubility of Ca(OH)₂ was decreased with the addition of NaOH by a common ion effect. Thus, it is possible to perform an experiment at a lower Ca²⁺ concentration. The aragonite was synthesized by adding the Na₂CO₃ solution to the Ca(OH)₂ slurry containing several concentrations of NaOH solution at 75°C and under the addition rate of Na₂CO₃ at 3 ml/min. The formation yield of calcite decreased when the NaOH concentration was increased. In conclusion, in the case of the reaction of the 2.5 M NaOH solution over 210 minutes, single-phase aragonite with an aspect ratio of 20 was obtained.     

IFCC recommendation on sampling,transport and storage for the determination of the concentration of ionized calcium in whole blood,plasma and serum

The substance concentration of ionized calcium (c _Ca²⁺) in blood, plasma or serum preanalytically may be affected by pH changes of the sample, calcium binding by heparin, and dilution by the anticoagulant solution.pH changes in whole blood can be minimized by anaerobic sampling to avoid loss of Co₂, by measuring as soon as possible, or by storing the sample in iced water to avoid lactic acid formation. c_Ca ²⁺ and pH should be determined simultaneously.Plasma or serum: If centrifuged in a closed tube, and measured immediately, the pH of the sample will be close to the original value. If a delay has occurred between centrifugation and the measurement, causing substantial loss of Co₂, equilibration of the sample with a gas mixture corresponding to pCO2= 5.3 kPa prior to the measurement is recommended. Conversion of the measured values to c_Ca ²⁺ (7.4) is only valid if the pH is in the range 7.2-7.6.Ca²⁺ binding by heparin can be minimized by using either of the following:(1) A final concentration of sodium or lithium heparinate of 15 IU/ml blood or less(2) Calcium titrated heparin with a final concentration of less than 50 IU/ml blood.Dilution effect can be avoided by use of dry heparin in capillaries or syringes. When heparin solutions are used, errors due to dilution or calcium binding can be reduced by using syringes with a heparin solution containing free calcium ions corresponding to the mean concentration of ionized calcium in normal plasma.Conditions for blood collection, storage, and transport to avoid preanalytical errors are described in this paper.     

Ozonation of Phenol-Containing Wastewater Using O3/Ca(OH)2 System in a Micro Bubble Gas-Liquid Reactor

The degradation of phenol in aqueous solution was investigated in an integrated process consisting of O₃/Ca(OH)₂ system and a newly developed micro bubble gas-liquid reactor. The effects of operating parameters such as Ca(OH)₂ dosage, reactor pressure, liquid phase temperature, initial phenol concentration and inlet ozone concentration on degradation and mineralization (TOC removal) were studied in order to know the ozonation performance of this new integrated process. It is demonstrated that the degradation and TOC removal efficiency increased with increasing inlet ozone concentration and increasing Ca(OH)₂ dosage before 2 g/L, as well as decreasing initial phenol concentration. The optimum Ca(OH)₂ dosage should exceed Ca(OH)₂ solubility in liquid phase. The reactor pressure and liquid phase temperature have little effects on the removal and TOC removal efficiency. When Ca(OH)₂ dosage exceeded 3 g/L, the degradation and TOC removal of phenol almost reached 100% at 30 and 55 min, respectively. The intensification mechanism of Ca(OH)₂ assisted ozonation was explored through analysis of the precipitated substances. The mechanism for Ca(OH)₂ intensified mineralization of phenol solution is the simultaneous removal of CO₃^2- ions, as hydroxyl radical scavengers, due to the presence of Ca²⁺ ions. Results indicated that the proposed new integrated process is a highly efficient ozonation process for persistent organic wastewater treatment.     

Growth inhibition in calcium sulfate crystal using a copolymer in oil fields: theoretical study and experimental evaluations

A poly(itaconic acid-co-sodium vinylsulphonate) (PIASVS) was theoretically studied and experimentally evaluated as an inhibitory agent against the growth of calcium sulfate (CaSO₄) crystals, in both non-saline and saline solutions. Density functional theory revealed that the CaSO₄ crystal precipitation could be precluded through the effective pairing of Ca²⁺ and SO₄^2? ions by carboxylic group polymer heads and that, moreover, the Na⁺ cations of the sulphonate polymer heads could be easily replaced by Ca²⁺. With PIASVS concentration of 50 ppm, lower than what is required in oil recovery processes, the polymer inhibited 33% in CaSO₄ crystals growth in non-saline solution, but the salt increased the inhibitory performance of PIASVS up to 54%. Thermogravimetric analysis, scanning electron microscopy and X-ray diffractometry techniques showed that PIASVS changed the CaSO₄ crystal morphology from a bassanite phase in non-saline solution to a bassanite/gypsum mix. The crystal morphology observations along with the conductivity measurements confirmed the pairing of ions from dissolved CaSO₄ by NaCl and PIASVS. Dynamic light scattering revealed that, the PIASVS cluster size increased in non-saline solution but decreased in saline solutions, suggesting that NaCl increases the PIASVS solubility in aqueous solution. The performance of PIASVS as anti-scaling agent was found to be suitable for the conditions found in the Mexican oil reservoirs.     

Boron removal by means of chemical precipitation with calcium hydroxide and calcium borate formation

Boron removal was investigated by chemical precipitation from aqueous solutions containing boron using calcium hydroxide. pH, initial boron concentration, amount of Ca(OH)₂, stirring speed and solution temperature were selected as operational parameters in a batch system. The highest boron removal efficiency was reached at pH 1.0. Increasing initial boron concentration and amount of calcium hydroxide raised to boron removal efficiency. Boron removal efficiency was highest at a stirring speed of 150 rpm. The most important parameter affecting boron removal efficiency was solution temperature. Increasing solution temperature increased importantly boron removal. XRD analysis showed that CaB₃O₃(OH)₅·4H₂O, which is a borate mineral called inyoite, occurred between Ca(OH)₂ and borate ions. As a result of the obtained experimental data, when the optimum operational conditions were selected, over 96% of boron removal efficiency was reached by this method.     

From C–S–H to C–A–S–H: Experimental study and thermodynamic modelling

It has long been known that the stoichiometry of C–S–H varies with the calcium hydroxide concentration in solution. However, this issue is still far from understood. We revisit it in both experimental and modelling aspects. A careful analysis of the solubility confirms the existence of three different C–S–H phases, defined as Ca₄H₄Si₅O₁₆, Ca₂H₂Si₂O₇ and Ca₆(HSi₂O₇)₂(OH)₂, respectively. The variation of the Ca/Si ratio of the three phases has been described by surface reactions: the increase of the Si content is accounted for by silicate bridging, the increase of calcium content and the surface charge are accounted for by reactions involving silanol groups via deprotonation and complexation with calcium. In the presence of Al in solution, the uptake of Al by C–S–H is experimentally observed. The Al content increases with Al concentration. C–A–S–H formation is modelled by the competition between silicate and aluminate tetrahedra for the bridging of the dimeric silicates in C–S–H.     

Treatment of aqueous solutions containing nickel using crandallite‐type compounds

The removal of nickel from aqueous solutions streams has been investigated using an artificial amorphous crandallite‐type compound, CaAl₃(OH)₆(HPO₄)(PO₄) (Ca‐crandallite), synthesized in our laboratory. Equilibrium ion‐exchange isotherms in an aqueous medium of Ca²⁺/Ni²⁺ at different pH values at 293 K have been determined. The experimental equilibrium data were satisfactorily correlated using a Langmuir‐type empirical equation. At low pH values, the hydrogen ion competes with the heavy metal cation and the percentage removal of metal declines. It was found that the operating capacity of Ca‐crandallite with respect to the metal ion increased with the pH of the solution, in accordance with a second‐degree polynomial equation. However, the pH should not be allowed to rise to levels at which chemical precipitation as nickel hydroxide would occur, with 7.00 the highest value tested. Taking into account the variation of operating capacity with pH, the system exhibited a unique separation factor, namely all the experimental points can be described by a unique isotherm in a dimensionless form. The Ca‐crandallite showed a high capacity, 2.176 meq g^?1, for the exchange of Ni(II) from nickel nitrate solutions and the rate of exchange of metal increases with increasing solution temperature due to the enhancement of effective intraparticle diffusivity. Copyright © 2005 Society of Chemical Industry     

Experimental study of Si-Al substitution in calcium-silicate-hydrate (C-S-H) prepared under equilibrium conditions

C-A-S-H of varying Al/Si and Ca/(Al + Si) ratios have been prepared introducing C-S-H (Ca/Si = 0.66 and 0.95) at different weight concentrations in a solution coming from the hydration of tricalcium aluminate (Ca₃Al₂O₆) in water. XRD and EDX (TEM) analyses show that using this typical synthesise procedure, pure C-A-S-H is obtained only for calcium hydroxide concentrations below 4.5 mmol L^− 1. Otherwise, calcium carboaluminate or strätlingite is also present beside C-A-S-H. The tobermorite-like structure is maintained for C-A-S-H. A kinetic study has shown that the formation of C-A-S-H is a fast reaction, typically less than a few hours. The Ca/(Al + Si) ratio of C-A-S-H matches the Ca/Si ratio of the initial C-S-H, in the ionic concentration range studied i.e., less than 4.5 and 3 mmol L^− 1 of calcium hydroxide and aluminium hydroxide respectively. The Al/Si ratio increases with the aluminium concentration in the solution and reaches a maximum value of 0.19.     

Evaluation of citric acid added cleaning solution for removal of metallic contaminants on Si wafer surface

We have investigated cleaning solutions based on citric acid (CA) to remove metallic contaminants from the silicon wafer surface. Silicon wafers were intentionally contaminated with Fe, Ca, Zn, Na, Al and Cu standard solution by spin coating method and cleaned in various CA-added cleaning solutions. The concentration of metallic contaminants on the silicon wafer surface before and after cleaning was analyzed by vapor phase decomposition/inductively coupled plasma-mass spectrometry (VPD/ICP-MS). And the surface micro-roughness was also measured by atomic force microscopy (AFM) to evaluate the effect of cleaning solutions. It was found that acidic CA/H₂O solution has the ability to remove metallic contaminants from silicon surfaces. Fe, Ca, Zn and Na on silicon surface were decreased from the order of 10¹² atoms/cm² to the order of 10⁹ atoms/cm² even at low CA concentration, low temperature of CA solution and with short immersion time. CA was also effective in alkali cleaning solution. Fe, Ca, Zn, Na and Cu were reduced down to the order of 10⁹ atoms/cm² in CA added with NH₄OH/H₂O₂/H₂O solution without degradation of surface micro-roughness.     

Use of new methods of separation of chitosan sulfate to simplify its industrial production

The optimum conditions of separation of chitosan sulfate from the reaction medium by precipitation of CS in an organic precipitator with subsequent precipitation of impurity sulfate ions with a solution of Ba(OH)₂ were determined. This method of separation of CS makes it possible to obtain preparations whose structural characteristics and biological properties satisfy the requirements set for substances for drugs. The dependence of the solubility of CS-Ba on the concentration of polymer in the solution was demonstrated. The effect of the Ba²⁺ cation on the viscosity of dilute solutions, caused by intramolecular interaction of the polymer sulfate groups with Ba²⁺ ions and consequently compacting of the macromolecules, was established.Translated from Khimicheskie Volokna, No. 5, pp. 31–34, September–October, 1995.     

The effects of lithium hydroxide solution on alkali silica reaction gels created with opal

The reaction of Nevada opal with calcium hydroxide, potassium hydroxide and lithium hydroxide solutions was investigated. In addition, opal was exposed to a combined solution of these three hydroxides. The progress of the three reactions was followed using X-ray diffraction (XRD), ²⁹Si nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). The XRD results indicated the presence of a low-angle peak exclusive to the lithium-based reactions. The NMR results suggested a change in the silicate structure in the presence of lithium. These techniques indicated that the reaction of the alkali with the opal starting material is inhibited and perhaps stopped in the presence of lithium hydroxide. SEM revealed that the morphology of the reaction products on the surface of the reacted opal grains is markedly different invariably. It was concluded that evidence to support the theory of a protective layer exists and that the nature of the layer varies with ion type.     

Ca3(PO4)2 coating of spherical Ni(OH)2 cathode materials for Ni-MH batteries at elevated temperature

Calcium phosphate was used for surface modification of spherical nickel hydroxide to improve its high temperature performance at the first time due to its low cost. The Ca₃(PO₄)₂ and Co(OH)₂ coated nickel hydroxides were prepared by precipitation of Ca₃(PO₄)₂ on the surface of spherical nickel hydroxide, followed by precipitation of Co(OH)₂ on its surface. The optimum coating content of calcium was around 2% (atomic concentration) to obtain high discharge capacity both at 25 and 60 °C. It was shown that the discharge capacity of nickel hydroxide at higher temperatures was improved by coating of Ca₃(PO₄)₂ and cobalt hydroxide. The high temperature performances of the sealed AA-sized nickel-metal hydride (Ni-MH) batteries using Ca/Co coated nickel hydroxide as positive electrodes were carried out, showing much better than those using uncoated or only Co(OH)₂ coated nickel hydroxide electrodes. The charge acceptance of the battery using 2% Ca and 2% Co coated nickel hydroxide reached 81% at 60 °C, where the charge acceptances for uncoated and only Co(OH)₂ coated nickel hydroxide were only 42 and 48%, respectively. It has shown that the Ca/Co coating is an effective way to improve the high temperature performance of nickel hydroxide for nickel-metal hydride batteries. It is a promising cathode material of Ni-MH batteries for EV applications due to the cost.     

Alkali-silica reaction, pessimum effects and pozzolanic effect

The pessimum proportion and pessimum size effects for alkali-silica reaction-induced deterioration of concrete (ASR) and the pozzolanic effect of fine siliceous admixtures in concrete have been explained based on the proposed ASR model [T. Ichikawa, M. Miura, Modified model of alkali-silica reaction, Cem. Concr. Res. 37 (2007) 1291-1297.]. The attack of alkali hydroxide to aggregate particles composed of ASR-reactive minerals generates the layer of hydrated mature alkali silicate and the layer of less hydrated immature alkali silicate under the mature layer. The mature alkali silicate preferentially reacts with Ca²⁺ ions to convert to fragmental calcium alkali silicate, because the reaction accompanies a significant volume contraction. The immature alkali silicate gradually reacts with Ca²⁺ ions to cover the surface of the reactive minerals with tight layers of calcium alkali silicate called reaction rims. The reaction rims allow the penetration of alkaline solution but prevents the leakage of viscous alkali silicate generated afterward, so that the alkali silicate is accumulated inside the rims to give an expansive pressure enough for cracking the aggregate and the surrounding concrete. Due to the absorption of Ca²⁺ ions by mature alkali silicate, too much increase of the proportion of reactive aggregate causes the deficiency of Ca²⁺ ions for the formation of reaction rims, so that the ASR expansion decreases after passing the pessimum proportion. Very fine reactive aggregate and admixtures with the grain size less than ~ 50 µm preferentially react with alkali hydroxide to convert to mature alkali silicate without leaving any reactive minerals. Homogeneous mixing of the sufficient amount of very fine siliceous admixtures in concrete therefore inhibits the ASR by absorbing Ca²⁺ ions for the rim formation. The resultant fragmental calcium silicate fills the pores in concrete to increase the strength and the durability of the concrete. The admixtures thus act as pozzolanic materials.     

Formation of ASR gel and the roles of C-S-H and portlandite

Experimental investigations of the reactions between silica, alkali hydroxide solution, and calcium hydroxide show that alkali-silicate-hydrate gel (A-S-H) comparable to that formed by the alkali-silica reaction (ASR) in concrete does not form when portlandite or the Ca-rich, Si-poor C-S-H of ordinary portland cement (OPC) paste is available to react with the silica. Under these conditions, we observe either the formation of additional C-S-H by reaction of Ca(OH)₂ with the dissolving silica or the progressive polymerization of C-S-H. The A-S-H dominated by Q³ polymerization forms only after portlandite has been consumed and the C-S-H polymerized. These conclusions are consistent with previously published results and indicate that the ASR gel of concrete forms only in chemical environments in which the pore solution is much lower in Ca and higher in Si than bulk pore solution of OPC paste. These results highlight the similarity between ASR and the pozzolanic reaction and are supported by data for mortar bar specimens.     

Chemical Sequence and Kinetics of Alkali‐Silica Reaction Part I. Experiments

The deterioration induced by alkali‐silica reaction (ASR) is initiated by complicated heterogeneous chemical reactions. This study describes the experimental results obtained from the model reactant experiments focused on the kinetics of physical and chemical changes in the reactive aggregate‐simulated pore solution system undergoing ASR. Specifically, the study investigated the products formed by exposing reactive silica mineral (α‐cristobalite) to two alkali solutions in the presence of solid calcium hydroxide [Ca(OH)₂]. The experimental results showed that, as long as the Ca(OH)₂ remains in the system, the dissolution of the silica mineral proceeds at a constant rate and the only reaction product formed is the tobermorite‐type C–S–H. However, once the supply of Ca(OH)₂ in the system is exhausted, the level of dissolved silica ions starts to increase. At the same time, the previously formed C–S–H changes in composition by incorporating silicon and alkali ions from the solution. Continuous increase in the concentration of silica leads to formation of the ASR gel as a result of interaction between silica and alkali ions.     

Synthesis and characterization of cassava starch graft poly(acrylic acid) and poly[(acrylic acid)‐co‐acrylamide] and polymer flocculants for wastewater treatment

Starch‐g‐poly(acrylic acid) and poly[(acrylic acid)‐co‐acrylamide] synthesized via chemically crosslinking polymerization were then each mixed with inorganic coagulants of aluminum sulfate hydrate [Al₂(SO₄)₃·18H₂O], calcium hydroxide [Ca(OH)₂], and ferric sulfate [Fe₂(SO₄)₃] in a proper ratio to form complex polymeric flocculants (CPFs). All CPFs exhibited low water absorbency than those of the uncomplexed superabsorbent copolymers. The color reduction by the CPFs was tested with both synthetic wastewater and selected wastewater samples from textile industries. The synthetic wastewater was prepared from a direct dye in a concentration of 50 mg dm^?3 at pH 7. The CPFs of poly[(acrylic acid)‐co‐acrylamide] with calcium hydroxide at a ratio of 1:2 is the most effective CPF for the wastewater color reduction. The CPF concentration of 500 mg dm^?3 could reduce the color of the synthetic wastewater containing the direct dye solution by 95.4% and that of the industrial wastewater by 76%. Starch‐g‐poly(acrylic acid)/Ca(OH)₂ CPF can reduce the synthetic direct dye and the industrial wastewater by 74% and 18%, respectively. Chemical oxygen demand, residual metal ion concentrations, pHs, turbidity of the wastewater were also investigated and the potential use of the complex polymer flocculants for textile wastewater treatment was indicated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2915–2928, 2006     

On the role of calcium in the alkali-aggregate reaction

Model systems for the alkali-aggregate reaction, using sodium hydroxide, calcium hydroxide and silica gel, have been studied. In one series of experiments, powdered silica gel was agitated with the alkaline solutions, and the changes in solution concentrations with time were followed; some of the solid products were analyzed. C-S-H forms so long as appreciable quantities of calcium ion remain in solution; thereafter the system behaves in the same way as model systems with no calcium introduced. Data are presented to show how [Ca²⁺] and [SiO₂] in solutions in contact with C-S-H vary with pH. In a second series of experiments, the behaviour of lumps of reactive silica gel was observed; in some cases growths similar to those in a ‘silicate garden’ were noted, and the cause of this phenomenon is discussed. Analysis of the residual solid gel shows that the amount of calcium that penetrates into the interior is negligible.     

Potassium fixation and the separation from sodium through the formation of K-alunite using activated aluminum hydroxide

This report introduces a novel method of separating potassium from sodium by synthesizing K-alunite using activated aluminum hydroxide (Al(OH)₃). A two-step operation, consisting of the mechanochemical activation of Al(OH)₃ and subsequent hydrothermal processing at a low temperature of 80°C with Al₂(SO₄)₃ · 18H₂O and K₂SO4 or Na₂SO4, was conducted. K-alunite but not Na-alunite at this low temperature was successfully formed, leading to the possible separation of potassium sulfate from sodium sulfate more easily. The process may serve the purpose of recovering potassium from some aqueous alkali sources for providing potash fertilizer in agricultural application.     

Exploration of As(III)/As(V) Uptake from Aqueous Solution by Synthesized Calcium Sulfate Whisker

Although common calcium-containing minerals such as calcite and gypsum may fix arsenic, the interaction between modified calcic minerals and arsenic has seldom been reported. The uptake behavior of As(III)/As(V) from aqueous solutions by calcium sulfate whisker (CSW, dihydrate or anhydrite) synthesized through a cooling recrystallization method was explored. A series of batch experiments were conducted to examine the effect of pH, reaction time, whisker dosage, and initial As concentration. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the samples prepared. The results showed that pH of the aqueous solution was an important parameter for As(III)/As(V) uptake, and an excellent removal efficiency could be achieved under strongly alkaline condition. The data from batch experiments for reaction of As(V) with calcium sulfate dihydrate whisker (CSDW) and calcium sulfate anhydrous whisker (CSAW) were well described with extended Langmuir EXT1 model, from which theoretic maximum adsorption capacity of 46.57 mg As(V)·(g CSDW)^− 1 and 39.18 mg As(V)·(g CSAW)^− 1 were obtained. Some calcium arsenate solids products, such as CaAsO₃(OH) (weilite, syn), Ca₃(AsO₄)₂ (calcium arsenate), CaO–As₂O₅, Ca–As–O, Ca₅(AsO₄)₃OH·xH₂O (calcium arsenate hydroxide hydrate), and CaH(AsO₄)·2H₂O (hydrogen calcium arsenic oxide hydrate), were detected at pH = 12.5 through XRD analysis. This indicates that the interaction mechanism between As(V) and CSW is a complex adsorption process combined with surface dissolution and chemical precipitation.