Removal of phosphate from solution by adsorption and precipitation of calcium phosphate onto monohydrocalcite

The sorption behavior and mechanism of phosphate on monohydrocalcite (CaCO₃?H₂O: MHC) were examined using batch sorption experiments as a function of phosphate concentrations, ionic strengths, temperatures, and reaction times. The mode of PO₄ sorption is divisible into three processes depending on the phosphate loading. At low phosphate concentrations, phosphate is removed by coprecipitation of phosphate during the transformation of MHC to calcite. The sorption mode at the low-to-moderate phosphate concentrations is most likely an adsorption process because the sorption isotherm at the conditions can be fitted reasonably with the Langmuir equation. The rapid sorption kinetics at the conditions is also consistent with the adsorption reaction. The adsorption of phosphate on MHC depends strongly on ionic strength, but slightly on temperature. The maximum adsorption capacities of MHC obtained from the regression of the experimental data to the Langmuir equation are higher than those reported for stable calcium carbonate (calcite or aragonite) in any conditions. At high phosphate concentrations, the amount of sorption deviates from the Langmuir isotherm, which can fit the low-to-moderate phosphate concentrations. Speciation–saturation analyses of the reacted solutions at the conditions indicated that the solution compositions which deviate from the Langmuir equation are supersaturated with respect to a certain calcium phosphate. The obtained calcium phosphate is most likely amorphous calcium phosphate (Ca₃(PO₄)₂?xH₂O). The formation of the calcium phosphate depends strongly on ionic strength, temperature, and reaction times. The solubility of MHC is higher than calcite and aragonite because of its metastability. Therefore, the higher solubility of MHC facilitates the formation of the calcium phosphates more than with calcite and aragonite.     

Ion-carrier controlled precipitation of calcium phosphate in giant ABA triblock copolymer vesicles

An ionophore assisted metal-ion transport across block copolymer membranes has been used to control the local Ca2+ concentration during precipitation of calcium phosphate in giant block copolymer vesicles.     

Formation of calcium phosphate having a hierarchically laminated architecture through periodic precipitation in organic gel

Hierarchically laminated calcium phosphate was produced through Liesegang periodic precipitation in a gel matrix of poly(acrylic acid) containing phosphate anions by diffusion of calcium cations.     

A radiometric investigation of the precipitation of calcium fluoride

Errors arising from solubility losses and co-precipitation in the gravimetric determination of fluoride by precipitation of calcium fluoride by classical and homogeneous methods have been investigated by radiometric techniques. It is shown that co-precipitation of chloride is the larger source of error and least in the homogeneous method. Distribution of chloride in the calcium fluoride precipitate is shown to follow the Doerner-Hoskins distribution law.     

Kinetics of calcium phosphate reduction by carbon

A fundamental study was made on the reduction of calcium phosphate by carbon. The mechanism of reduction was presented on applying different additions. Both silica and alumina increased the extent of reduction but with variable rates. The activation energies were calculated on the basis of first-order reactions. The phases formed during reduction were investigated by X-ray analysis.     

Thermoluminescence of bone equivalent calcium phosphate ceramics

TL-glow curves of calcium phosphate ceramic powders prepared either from natural or synthetic phosphates have been investigated. The TL-samples from synthetic phosphates, particularly those having Ca/P ratio=2.8, show higher TL-sensitivity, about 1/10 times that of LiF TLD-100 powders, with glow curves having maxima (peaks) around 110, 175 and 325 °C. The TL-response of all phosphate ceramic samples showed a dependence on -dose, well described by a power function in a range from about 1 to 10⁴ Gy, which is useful for therapeutic and radiation processing levels. For regeneration of irradiated TL-samples, annealing around 350 °C for 20 min was found suitable. A low Ca/P ratio can be recommended for high dose measurements, while higher ratio gives greater TL-sensitivity, hence allowing measurement of lower -doses.     

Termal decomposition of carbonated calcium phosphate apatites

The thermal stability of AB-type carbonated calcium phosphate apatites prepared by precipitation from aqueous media was studied. The behavior of powders was investigated using temperature programmed XRD, infrared spectroscopy and thermogravimetry. In N₂ atmosphere, two successive peaks of decarbonatation with maxima at about 700 and 950°C occurred. This behavior is explained by different substitution modes for carbonates in the apatite. The decarbonatation peaks were shifted to higher temperature under CO₂ (around 900 and 1150°C). The analysis of the thermal stability allowed further densification of carbonate apatite ceramics without important carbonate loss. This revised version was published online in July 2006 with corrections to the Cover Date.     

The nucleation of calcium phosphate from solution

Dicalcium phosphate was precipitated from homogeneous solution in the pH range 5–6 by using the hydrolysis of cyanate to control the precipitation process. Turbidimetric determination of the onset of precipitation enabled the critical supersaturation to be determined. In this region the average critical supersaturation ratio was found to be 2.3.     

A new ph electrode for gas-sensing probes

A pH-dependent electrode obtained by thermomoulding a mixture of powdered antimony and a thermoplastic polymer is shown to be highly reproducible and reliable. The electrodes are useful in gas-sensing probes for NH₃ , CO₂, and SO₃. Calibration curves, selectivity and response times are evaluated.     

Polarographic investigation of zinc phosphate precipitation process at trace level

Summary Polarography is attempted in the study, at trace level, of zinc phosphate precipitate which is not sparingly soluble. The choice of the supporting electrolyte is very important as it functions also as a coagulating agent. In contrast to conventional polarography, the supporting electrolyte concentration was found to be critical. The effect of p_H, temperature, volume, time, stirring, etc. was studied. Complete precipitation was obtained under proper conditions for 40g/ml of zinc solution.

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Zusammenfassung Das deutlich lösliche, in Spuren gefällte Zinkphosphat wurde polarographisch untersucht. Die Wahl des Trägerelektrolyten ist hierbei sehr ausschlaggebend, da er auch koagulationsfördernd wirkt. Im Gegensatz zur konventionellen Polarographie erwies sich die Konzentration des Trägerelektrolyten als kritisch. Der Einfluß des p_H, der Temperatur, des Volumens, der Zeit, des Rührens der Lösung usw. wurde untersucht. Unter geeigneten Bedingungen wurde die vollständige Fällung einer Lösung von 40g Zn/ml erreicht.

     

Cooperative calcium phosphate nucleation within collagen fibrils

Although "chaperone molecules" rich in negatively charged residues (i.e., glutamic and aspartic acid) are known to play important roles in the biomineralization process, the precise mechanism by which type I collagen acquires intrafibrillar mineral via these chaperone molecules remains unknown. This study demonstrates a mechanism of cooperative nucleation in which three key components (collagen, chaperone molecules, and Ca(2+) and PO(4)(3-)) interact simultaneously. The mineralization of collagen under conditions in which collagen was exposed to pAsp, Ca(2+), and PO(4)(3-) simultaneously or pretreated with the chaperone molecule (in this case, poly(aspartic acid)) before any exposure to the mineralizing solution was compared to deduce the mineralization mechanism. Depending on the exact conditions, intrafibrillar mineral formation could be reduced or even eliminated through pretreatment with the chaperone molecule. Through the use of a fluorescently tagged polymer, it was determined that the adsorption of the chaperone molecule to the collagen surface retarded further adsorption of subsequent molecules, explaining the reduced mineralization rate in pretreated samples. This finding is significant because it indicates that chaperone molecules must interact simultaneously with the ions in solution and collagen for biomimetic mineralization to occur and that the rate of mineralization is highly dependent upon the interaction of collagen with its environment.     

Association of calcium phosphate and fibroblast growth factor-2: a dynamic light scattering study

The size distributions of fibroblast growth factor-2 (FGF-2) in aqueous solutions with neutral pH were investigated with a dynamic light scattering technique. We found that the FGF-2 was distributed in dimer or trimer form at concentrations of 0.1-1.0 mg . mL(-1). An aggregate with a hydrodynamic radius of approximately 90 nm coexisted with this and its proportion increased with a decrease in concentration. At lower concentrations (less than 0.10 mg . mL(-1)) FGF-2 aggregates with an average radius of 80-100 nm were dominant and were stable for more than a day. These FGF-2 solutions were mixed with calcium phosphate solutions to produce a sub-micron sized compound of FGF-2 and hydroxyapatite, which could be used as a biological implant that possessed a pharmacological function for bone formation. By utilizing a transformation from amorphous calcium phosphate to hydroxyapatite, FGF-2 was effectively incorporated into polycrystals of hydroxyapatite.SEM photograph of a mixture of hydroxyapatite and FGF-2.