Crystallization experiments, calcium phosphates

Zhang et al. used Langmuir monolayers to study the initial stage of nucleation and crystallization of calcium phosphate [160]. The monolayers used in their study consisted of dipalmitoylphosphatidylcholine, arachidic acid, and octadecylamine. The experiments show that the adsorption of calcium ions to the respective mono-layer is a prerequisite for subsequent nucleation. The authors also demonstrate that calcium phosphate forms through a multistage assembly process, in which first an amorphous calcium phosphate dihydrate layer forms, which then recrystallizes to form a crystalline hydroxyapatite layer (Fig. 12). This transformation of an amorphous phase to a crystalline phase provides direct evidence of a multistep crystallization process, which the authors claim is similar to the processes occurring in biomineralization. 

A disadvantage of the conventional precipitation method in which the supersaturation was allowed to decrease during the reactions, was that different calcium phosphate phases could form and subsequently dissolve during the course of the reactions. In the present work, the constant composition method was used to investigate the influence of sodium chloride, potassium chloride, and potassium nitrate, as background electrolyte upon the rate of crystallization of HAP in solutions supersaturated only with respect to this phase. These experiments were made in solutions containing totaj... 

Chemically pure reagents were used. Cadmium was added as its sulfate salt in concentrations of about 50 ppm. Lanthanides were added as nitrates. For the experiments with other metal ions so-called "black acid from a Nissan-H process was used. In this acid a large number of metal ions were present. To achieve calcium sulfate precipitation two solutions, one consisting of calcium phosphate in phosphoric acid and the other of a phosphoric acid/sulfuric acid mixture, were fed simultaneously in the 1 liter MSMPR crystallizer. The power input by the turbine stirrer was 1 kW/m. The solid content was about 10%. Each experiment was conducted for at least 8 residence times to obtain a steady state. During the experiments lic iid and solid samples were taken for analysis by ICP (Inductively Coupled Plasma spectrometry, based on atomic emission) and/or INAA (Instrumental Neutron Activation Analysis). The solid samples were washed with saturated gypsum solution (3x) and with acetone (3x), and subsequently dried at 30 C. The details of the continuous crystallization experiments are given in ref. [5]. 

Numerous kinetic studies have been made of the spontaneous precipitation of calcium phosphates from solutions containing concentrations of lattice ions considerably in excess of the solubility values (33, 34). Although attempts, are usually made to control the mixing of reagent solutions, it is difficult to obtain reproducible results from such experiments since chance nucleation of solid phases may take place on foreign particles in the solution. Many of these difficulties can be avoided by studying the crystal growth of well-characterized seed crystals in metastable supersaturated solutions of calcium phos.phate. 

In other studies, solutions of high supersaturation were employed which yielded complex, poorly defined solid phases. The multi-picity of complex crystal polymorphs and hydrates in the calcium phosphate system is now well recognized and must be considered in the interpretation of experiments involving the removal of phosphate from natural waters (]A, 15). 

Extensive seeded calclte growth experiments in the presence of phosphate ion indicate that the phosphate ion adsorbs onto the crystal surface as a monolayer. At a concentration of 10" M, phosphate ion can strongly inhibit calcite formation however, short term experiments show that this monolayer adsorption removes insignificant amounts of phosphorus from solution. In experiments lasting several days a further decrease in solution phosphate concentration occurs, presumably caused by nucleation of a surface calcium phosphate phase on the calcite seed. 

In addition to large B-form structural variations, [Fe(EDTA)] has been used to probe for subtle changes in the sequence-dependent helical twist of DNA (64). As can be seen in Fig. 10, by depositing the rodlike DNA on an inert surface (in this case calcium phosphate crystals), one can protect an entire face of the helical polymer. Exposed backbone on the other side is then cleaved with a periodicity that reflects the number of base pairs per turn of the helix. Experiments with [Fe(EDTA)] have therefore helped to illustrate that there are sequence-dependent deviations in the number of bases that can pack into a helical turn. 

Brown JL (1981) Calcium phosphate precipitation Effects of common and foreign ions on hydroxyapatite crystal growth. Soil Sci Soc J 45 482-486 Bulka GR, Vinokurov VM, Nizamutdinov NM, Hasanova NM (1980) Dissymmetrization of crystals Theory and experiment. Phys Chem Minerals 6 283-293 Bums RG (1970) Mineralogical Applications of Crystal Field Theory. Cambridge University Press, Cambridge... 

In a related study, a biomass material, bone char was investigated by Ma et al. (2008) for its feasibility as a cost-effective biosorbent for F removal from drinking water. The amorphous biosorbent powder, which is composed mainly of calcium phosphate and a small amount of carbon, was prepared by heating bone-biomass. The adsorption capacity of the hone char was shown to be better than that of activated aluminum and tourmaline (i.e., crystal horon sUicate mineral compounded with elements such as aluminum, iron, magnesium, sodium, hthium, or potassium). Removal of F was attributed to the processes of ion binding and ion exchange between bone char and F". The authors developed static and kinetic models which provided a satisfactory prediction of F concentration after adsorption. Experiments with fixed-bed columns... 

In this experiment, tap water with added phosphate was used as influent. Concentration of phosphate was adjusted to an adequate range from 2 to 23 mg/jg. Calcium chloride and sodium hydroxide solution were added to maintain calcium concentration from 70 to 100 mg/jg and pH of the effluent from 9.0 to 9.5. Using this equipment, we performed experiments to obtain efficiency of phosphate removal, relationship between phosphate concentration, and crystallization rate and factors affecting phoshate removal. 

In vitro experiments by Iijima and her colleagues demonstrate the advantage of separating the cation and the anion in terms of collagen mineralization [97]. The setup used involved diffusion of calcium and phosphate from opposite sides of collagen membranes produced from the Achilles tendon - a tissue that normally never mineralizes. Crystal formation, density, morphology and orientation were drastically influenced by the orientation of the fibers within the... 

Bone solubility studies suggest that the equilibrium between tissue fluid calcium and phosphate and bone mineral, the crystals of which possess a very large surface area (E5), is a physicochemical one. This is compatible with the classic experiments of Hastings and Huggins (H2), amplified by the elegant work of Copp (C12, C13) and Talmage (T2), who have shown that the removal of ionic calcium from extracellular fluid is followed by its rapid replacement from bone mineral stores whatever the level of parathyroid activity. The intimate relationship between blood and bone calcium has also been confirmed by the use of isotopes which demonstrate rapid ionic exchange between tissue fluid and bone mineral (LI). 

It now remains to explain the apparent paradox that the ionic product for calcium and monohydrogen phosphate ions in serum exceeds the solubility product of both hydroxyapatite and biological apatite and yet crystals of these substances are not formed in the bloodstream. Of the several reasons for this, the most basic one can be demonstrated by a simple experiment. 

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