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1.
The generation of carbon dioxide from organic precursors in aqueous media in presence of calcium chloride and under conditions where calcium carbonate is formed was investigated. Photochemical decomposition of pyruvic acid, 2-nitro-phenylacetic acid and 4-benzoylphenyl acetic acid gave ill-defined morphologies of calcium carbonate contaminated by organic by-products. Alkaline hydrolysis of dimethyl- and diethyl-carbonate yielded amorphous calcium carbonate in form nanospheres of narrow distribution of radii if the precipitation occurred under quiescent conditions. Addition of a polymer surfactant gave spherical particles as well, however consisting of vaterite.The formation of spherical particles of amorphous calcium carbonate is rationalized in terms of a liquid–liquid phase separation followed by rapid gelation of the droplet phase of high concentration of calcium carbonate. A lower critical solution temperature is postulated and its value was estimated as 10 °C. 相似文献
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Donghui Zhao Yingchun Zhu Fang Li Shengmao Zhang Fangfang Xu 《Materials Research Bulletin》2010,45(1):80-6230
In this article, the polymorph selection of calcium carbonate has been successfully achieved in water-soluble carboxymethyl chitosan aqueous solution at different temperatures (25-95 °C). Vaterite is formed in carboxymethyl chitosan solution 25 °C accompanied with trace of calcite, whereas pure aragonite is obtained at 95 °C. Scanning electron microscopy and transmission electron microscopy analyses show that the products are formed from the recrystallization of nanometer crystallites. Thermodynamic and kinetic analyses reveal that the polymorph of calcium carbonate is controlled and selected by kinetics in various temperatures. As a heterogeneous nucleator and stabilizing agent, carboxymethyl chitosan changes the nucleation and growth of calcium carbonate from thermodynamic into kinetic control. Under kinetic limitation, the reaction rate of aragonite increases along with the elevating of temperature and surpasses the rate of vaterite above 327 K. 相似文献
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Normal Portland cement mortar discs containing precipitated or ground limestone at dosages of 0, 2.5, 5 and 15% were made at water/cement ratios of 0.42 or 0.60. They were hydrated in lime water or sea water solution for periods up to 1 year. The length and modulus of elasticity changes were monitored periodically. Some mortars containing CaCO3 exhibited more than three times the expansion of the reference in sea water. The expansions were greater in samples containing fine CaCO3. The modulus of elasticity was especially low for samples containing precipitated CaCO3 and exposed to sea water. 相似文献
5.
Size-controllable synthesis of calcium carbonate nanoparticles using aqueous foam films as templates
A new method for the synthesis of calcium carbonate nanoparticles using two types of foam which are separately stabilized by surfactants with opposite change has been described here. In detail, one type of foam is formed by the aqueous solution of CaCl2 which contains the anionic surfactant alkyl polyoxyethylene alcohol sulfate sodium (AES) and the other is formed by the aqueous solution of Na2CO3 and the cationic surfactant alkyl polyoxyethylene quaternary ammonium chloride (AEAC). Two types of foam contact each other in a specially designed apparatus after draining completely, then Ca2+ and CO32− entrapped by the surfactant layers at the thin borders between the foam bubbles react and result in the generation of CaCO3 nanoparticles. TEM determination showed that perfect mono-dispersed spherical nanoparticles were obtained and the size of particles can be conveniently controlled by changing the concentration of CaCl2 and Na2CO3. Also, the mechanism leading to the synthesis of CaCO3 nanoparticles was discussed in detail. 相似文献
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A new adsorbent system for removing methylene blue (MB) from aqueous solutions has been investigated. This new adsorbent is cereal chaff, an agriculture product in middle-west region in China. Variables of the system, including biosorption time, chaff dose, pH, salt concentration and initial MB concentration, were adopted to study their effects on MB removal. The results showed that as the dose of chaff increased, the percentage of MB sorption increased accordingly. There was no significant difference in the dye concentration remaining when the pH was increased from 4.0 to 11.0. The salt concentration has negative effect on MB removal. At the experimental range of MB concentration, the amount of MB adsorbed onto per unit mass of chaff (q(e)) is direct ratio to MB initial concentration (c(0)). The equilibrium data were analyzed using five equilibrium models, the Langmuir, the Freundlich, the Redlich-Peterson, the Koble-Corrigan and the Temkin isotherms. The results of non-linear regressive analysis are that the isotherms of Langmuir, Redlich-Peterson and Koble-Corrigan are better fit than the isotherms of Freundlich and Temkin at different temperatures according to the values of determined coefficients (R(2)) and Chi-square statistic (chi(2)). The maximum equilibrium capacities of chaff from Langmuir models are 20.3, 25.3 and 26.3 mg g(-1) at 298, 318 and 333K, respectively. Using the equilibrium concentration constants obtained at different temperatures, various thermodynamic parameters, such as DeltaG(0), DeltaH(0) and DeltaS(0), have been calculated. The thermodynamics parameters of MB/chaff system indicate spontaneous and endothermic process. It was concluded that an increase in temperature results in a bigger MB loading per unit weight of the chaff. 相似文献
7.
Hideo Watanabe Yoshiaki Mizuno Takeshi Endo Xiaowei Wang Masayoshi Fuji Minoru Takahashi 《Advanced Powder Technology》2009,20(1):89-93
Hollow calcium carbonate (CaCO3) particles are synthesized by continuous bubbling CO2 gas into CaCl2 aqueous solution. In order to study the formation kinetics of the hollow CaCO3 particles, the transmitted light strength of the CaCl2 solution during the reaction was measured automatically with an on-line monitoring. It was verified that the pH for the formation of the hollow particles depended on the reaction temperature. Higher initial pH of the CaCl2 solution resulted in the higher ratio of hollow particles as compared to that obtained at lower pH. 相似文献
8.
Chengyu Wang 《Materials Letters》2008,62(16):2377-2380
Two metastable calcium carbonate polymorphs, rod-like aragonite and spherical vaterite are selectively formed in this study. Aragonite rods were synthesized from a calcium acetate (Ca(AC)2) and urea (CO(NH2)2) solution under a given condition. In contrast, the addition of polyacrylamide (PAM) and oleic acid results in the formation of spherical vaterite. The morphology, size and crystal structure were characterized by means of Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), and X-ray diffraction (XRD). The results show that PAM and oleic acid can be used as additives to select the polymorph from aragonite and vaterite. The contact angle of the modified products reached 112.49°. We have succeeded in surface modification of particles in situ at the same time. 相似文献
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Ozonation of p-chlorophenol in aqueous solution. 总被引:1,自引:0,他引:1
The ozonation of p-chlorophenol (CHP) in aqueous solution has been studied in the pH range 2.0-8.0, in the presence of tert-butyl alcohol, which prevents the activation of the radical mechanism of oxidation. Results indicate that the pH influences the system reactivity and that only a partial chlorine release is observed for lengthy ozonation too, after the complete substrate disappearance. For adopted experimental conditions the oxidation process develops under a quasi-diffusional regime of absorption with reaction, a transition domain between kinetic and diffusional regimes in which ozone and dissolved substances react exclusively in the liquid film. A proper mathematical model has been developed and used to simulate the system behaviour 相似文献
11.
S Biradar V Goornavar A Periyakaruppan J Koehne R Jeffers JC Hall V Ramesh M Meyyappan GT Ramesh 《Nanotechnology》2012,23(37):375601
With the advent of nanotechnology, many methods of synthesis of nanoparticles have come into practice and the 'polymer mediated growth' technique is among them. In this route, ions of one of the reactants are allowed to diffuse from an external solution into a polymer matrix where the other reactant is complexed and bound. The exact role of ionic diffusion in the formation of nanoparticles was investigated in the current study by studying the patterns of kinetics of nanoparticle formation using UV vis spectroscopy. Typically, calcium carbonate nanoparticles were formed by the aforementioned technique using polyethylene glycol solution. The particle size was calculated using Scherrer's formula on x-ray diffraction plots and was reconfirmed with field emission scanning electron microscope and transmission electron microscope images. Energy-dispersive x-ray analysis was used to study the composition and purity of the nanoparticles formed. The reactant to polymer ratio, reaction temperature and molecular weight of polyethylene glycol affected the size of the particles formed. Through this knowledge we optimized these parameters to obtain particles as small as 20?nm and confirmed that this technique can be used to control the size of nanoparticles. 相似文献
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In this paper, sorption potentials of uranium ions were studied using alginate polymer beads in diluted aqueous solutions. The ability of alginate beads to adsorb uranium(VI) from aqueous solution has been studied at different optimized conditions of pH, U(VI) concentration, contact time, biomass dosage and temperature. In order to determine the adsorption characteristics, Langmuir, Freundlich, and Dubinin–Radushkevich adsorption isotherms were applied to the adsorption data. The thermodynamic parameters such as variations of enthalpy ΔH, entropy ΔS and variation of Gibbs free energy ΔG were calculated from the slope and intercept of ln Kd vs. 1/T plots. The results suggested that alginate beads could be suitable as a sorbent material for adsorption and removal of uranium ions from dilute aqueous solutions. 相似文献
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Matrix-assisted laser desorption/ionization reflector time-of-flight (MALDI-reTOF) and electrospray ionization (ESI) mass spectrometry (MS) have become essential tools for the characterization of peptides and proteins. Whereas ESI in combination with a triple quadrupole analyzer allows product ion, precursor ion, and neutral loss analyses, MALDI-reTOF instruments can only be used to record product ion spectra based on the in-source or postsource decay (PSD). We describe a new method to perform neutral loss analyses in MALDI-reTOF instruments in a manner that identifies posttranslationally modified peptides and furthermore retrieves sequence information from peptides. The method is based on the selection of ions in a small time interval to record only signals within the corresponding mass interval. By stepping the time interval through the complete mass range, we obtained a spectrum of stable ions by combining the signals of all individually recorded time intervals. This method furthermore permits PSD fragment ions to be identified, since they reach the detector earlier than the stable ions transmitted in the chosen time interval. The neutral loss analysis were calculated by correlating the PSD fragment ions to the corresponding parent ion detected in this time interval. Moreover, this MALDI-MS mode increased the number of detectable signals in complex peptide mixtures and the signal-to-noise ratio. 相似文献
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The kinetics of degradation of tobramycin (Ne-De-Ka) in aqueous solution was studied as a function of pH. Tobramycin hydrolyzes in acidic solution to yield kanosamine (Ka-OH) and nebramine (Ne-De-OH) with a pseudo first-order rate constant of 2.7 × 10-6 s-1 in 1 N HCl at 80°C. The activation energy for the acid catalyzed hydrolysis is 32 kcal mol-1. In basic solution, the hydrolysis products are deoxystreptamine (De-OH), nebramine (Ne-De-OH) and deoxystreptamine-kanosaminide (HO-De-Ka). The pseudo first-order rate constant for the hydrolysis in 1 N KOH is 1 × 10-8 s-1 at 80°C. The activation energy for the base catalyzed hydrolysis is 15 kcal mol-1. Tobramycin is very stable towards hydrolysis at neutral pH; however, it rapidly oxidizes giving several products including De-OH, Ne-De-OH, and HO-De-Ka. In pH 7 phosphate buffer (0.01 M), the t90 value is 70 hr at 80°C. 相似文献
15.
The rates of hydrolysis of valganciclovir to ganciclovir and L-valine and isomerization of the R and S diastereomers of valganciclovir in aqueous buffer solution from pH 3.8 to 11.5 were determined at 37 degrees C. The kinetics of hydrolysis were first order for at least two half-lives in neutral and basic solutions. In acidic solutions where less than 10% degradation occurred, the rate of hydrolysis was determined assuming a first-order loss in drug. At 37 degrees C and pH 7.08, the half life is 11 h. The maximum stability at the pH values studied occurred at pH 3.81 with a half life of 220 days. The kinetics of the approach to equilibrium for the isomerization were first order and the ratio of the R:S isomer at equilibrium was 52:48. Isomerization was approximately 10 fold faster than hydrolysis over the pH range studied with a half-life at pH 7.01 of 1 h. The maximum stability toward isomerization (t1/2>533 h) occurs at a pH below 3.8. The pH-rate profile for the hydrolysis and the isomerization reaction are best described by hydroxide ion catalyzed mechanisms. In acidic and neutral solutions, the hydroxide reacts with the protonated form of the drug, while in basic solutions, the hydroxide reacts with the neutral form of the drug. 相似文献
16.
《Drug development and industrial pharmacy》2013,39(13):1423-1436
AbstractThe kinetics of degradation of tobramycin (Ne-De-Ka) in aqueous solution was studied as a function of pH. Tobramycin hydrolyzes in acidic solution to yield kanosamine (Ka-OH) and nebramine (Ne-De-OH) with a pseudo first-order rate constant of 2.7 × 10?6 s?1 in 1 N HCl at 80°C. The activation energy for the acid catalyzed hydrolysis is 32 kcal mol?1. In basic solution, the hydrolysis products are deoxystreptamine (De-OH), nebramine (Ne-De-OH) and deoxystreptamine-kanosaminide (HO-De-Ka). The pseudo first-order rate constant for the hydrolysis in 1 N KOH is 1 × 10?8 s?1 at 80°C. The activation energy for the base catalyzed hydrolysis is 15 kcal mol?1. Tobramycin is very stable towards hydrolysis at neutral pH; however, it rapidly oxidizes giving several products including De-OH, Ne-De-OH, and HO-De-Ka. In pH 7 phosphate buffer (0.01 M), the t90 value is 70 hr at 80°C. 相似文献
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Kinetics of the dissolution of E-glass fibres in alkaline solutions was investigated. To allow an accurate determination of conversion, glass fibres were immersed individually in the corrosive medium and the diameter change was measured with the use of a scanning electron microscope. Few studies have been reported in the literature on the kinetics of E-glass fibre dissolution or the dissolution of individual fibres. Our experimental results fit well in the zero-order and shrinking cylinder models, suggesting either the diffusion of hydroxide ions through the solution or the glass fibre etching itself was rate-limiting step. The rate constant for the reaction of glass fibre with alkaline solution at 95 °C was found to be between 1.3 × 10?4 and 4.3 × 10?4 g/(m2 s). The reaction order (n) was determined as 0.31–0.49 with respect to the alkaline solution, and the activation energy was 58–79 kJ/mol. 相似文献
18.
Photo-degradation of chlorophenols in the aqueous solution 总被引:2,自引:0,他引:2
Czaplicka M 《Journal of hazardous materials》2006,134(1-3):45-59
The review presents the chlorophenols photo-degradation kinetics and mechanism in the aquatic environment under UV-vis in the presence of hydroxyl radicals and singlet oxygen. The influence of experimental parameters e.g. pH, dissociation degree, presence of oxidants in solution, number and position of Cl atoms on the quantum yield and reaction rate constant of chlorophenols are discussed. Mechanisms of photolysis, reaction with hydroxyl radicals, singlet oxygen and secondary reactions for mono-, di-, tri-, tetra- and pentachlorophenol are proposed. The pathways for intermediate reactions e.g. dechlorination, oxidation, dimerization for chlorophenols are also presented. 相似文献
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