Supersaturation, calcium phosphate solutions

It has been experimentally confirmed that hydroxyapatite cannot be precipitated directly from a supersaturated calcium phosphate solution (for example, Nancollas and Wu, 2000). Instead, it is thought that precursor compounds such as octacalcium phosphate (OCP) (Brown, 1966) and/or ACP (for example Eanes, Gillessen and Posner, 1965 Termine and Posner, 1966) are required as intermediates, the thermodynamic properties of which act towards lowering the Gibbs energy of formation of hydroxyapatite. OCP shows some structural similarity to hydroxyapatite and forms with HAp epitaxially controlled interlayered single... 

Addition of compounds with appropriate functionality to serve as nucleation sites for calcium phosphate growth to polymers can potentially improve the biocompatibility of the latter and thus the long-term stability of implant devices (Drelich and Field, 2007). Zinc stearate was added to poly(ethylene) to form poly(ethylene)-stearate blends with increased surface porosity potentially able to improve mechanical stability of the implant through enhanced osseointegration, improved rates and quality of bone-implant fusion and enhanced soft tissue wound healing via stimulation of angiogenesis. While immersion of these blends in supersaturated calcium phosphate solutions triggered deposition of a porous layer, the deposition rate was very slow, around 100 nm/day. 

In discussions of the precipitation of calciinn phosphate, the phase which is usually emphasized is the thermodynamically most stable, HAP. However, most calcium phosphate solutions in which precipitation experiments are made, are initially supersaturated with respect to four additional phases. The solubility isotherms are shown in Figure 1 as a function of pH. Thus, at a pH of 7.0, in order of increasing solubilities, it is necessary to take into account tricalcium phosphate (Ca3(PO )2 hereafter TCP), octacalcium phosphate (Ca H(PO ). 2 1/2 H2O, hereafter OCP), anhydrous dicalcium phosphate (CaHPO, hereafter DCPA), and dicalciiam phosphate dihydrate (CaHPO. 2H2O, hereafter DCPD). The corresponding thermodynamic solubility products. 

Sodium carboxymethyl chitin and phosphoryl chitin had most evident influences on the crystallization of calcium phosphate from supersaturated solutions. They potently inhibited the growth of hydroxyapatite and retarded the rate of spontaneous calcium phosphate precipitation. These chitin derivatives were incorporated into the precipitate and influenced both the phase and morphology of the calcium phosphate formed (flaky precipitate resembling octacalcium phosphate instead of spherical clusters in the absence of polysaccharide) [175]. 

As the precursor, e.g., an amorphous phase, precipitates and brings down the supersaturation of the solution, the more stable phase to be precipitated is using the precursor phase as a substrate for its own precipitation (Steefel and Van Cappellen, 1990). A classical example that documents this principle is the precipitation of calcium phosphates, where a metastable calcium phosphate precursor phase is nucleated initially and is then replaced, in some instances via an intermediate phase, by apatite. (Nancollas, 1990 Steefel and Van Cappellen, 1990). 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Despite the importance of the precipitation of calcium phosphates, there is still considerable uncertainty as to the nature of the phases formed in the early stages of the precipitation reactions under differing conditions of supersaturation, pH, and temperature. Although thermodynamic considerations yield the driving force for the precipitation, the course of the reaction is frequently mediated by kinetic factors. Whether dicalcium phosphate dihydrate (CaHPO HoO, DCPD), octacalcium phosphate (Ca HfPO, 2.5 H20, OCP), hydroxyapatite (Cag (PO fOH), HAP), amorphous calcium phosphate (ACP), or a defect apatite form from aqueous solution depends both upon the driving force for the precipitation and upon the initiating surface phase. Thermodynamically, the relative supersaturation, o, is given by... 

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... 

To cope with these problems, we have developed phosphate removal process using crystallization, which can minimize the amount of sludge and recover phosphate. Mechanism of this process is crystallization of calcium phosphate on the surface of phosphate rocks by contacting supersaturated solution with them. In case of application to wastewater containing 1-3 mg/jg phosphate as P, we proposed fixed bed type process, which has demonstrated excellent performance in the sewage treatment. 

When calcium phosphate is precipitated from aqueous solutions of high supersaturation and pH values above 7, the solid phase appearing initially is an ACP with the formula Ca9(P04)61U u2 - If this amorphous precipitate is allowed to remain in contact with the solution, it transforms to crystalline HA through a process of dissolution, nucleation and crystal growth77, unless stabilized in some manner. 

Phelan and Mattigod (1987) studied calcium phosphate precipitation from stable supersaturated solutions using pH/Ca-stat and pH-stat. The pH and Ca2+ activity of the titrand were kept constant utilizing ion-specific electrodes attached to automatic titrators. A schematic diagram of the apparatus used by Phelan and Mattigod is shown in Fig. 3.2. 

A saturated solution contains the maximum amount of a solute, as defined by its solubility. No more solute will dissolve in a solution saturated with that solute. If the solution is not saturated, more solute will dissolve in that solution. Sometimes, a solution will become supersaturated with a solute. A supersaturated solution contains more solute than allowed by the solubility of the solute. This is not a stable system, because there is more solute dissolved in the sample than the solvent can accommodate. In this case, the excess solute will come out of solution crystallizing as a solid, separating as a liquid, or bubbling out as a gas. For example, when blood or urine in the kidneys becomes supersaturated with calcium oxalate or calcium phosphate, a kidney stone can form. If the solute is a gas in liquid solvent, you would see bubbles forming in the solution. Perhaps you ve seen this phenomenon when you open a bottle of beer or soda pop. 

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. 

As the pH is increased (see figure 1), other calcium phosphate phases may be stabilized as precursors to the formation of apatite and the growth of HAP seed crystals in stable supersaturated solutions is much more complex (, 1, 49). ... 

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). 

The above observations may be explained on the basis of supersaturation of the solutions with respect to calcium citrate and calcium phosphate. Calcium citrate and calcium phosphate can be precipitated from a supersaturated solution. Ciavatta et al. (39) have reported that the solubility of calcium citrate in water at 25 °C is 1.5 mM, which corresponds to an overall calcium concentration of 4.5 mM and a citrate concentration of 3.0 mM. Since the concentration of calcium ion in a fresh SBF solution is 2.5 mM, and the total citrate ion concentration varies between 0.15 mM and 0.05 mM during weeks 2 through 9 for immersion in SBF, it is not surprising that calcium citrate does not precipitate during this period. 

The reactions (5.3a) and (5.3b) lead to local ionic supersaturation that causes precipitation of calcium phosphate phases with low solubility product and high thermodynamic stability (Drevet and Benhayoune, 2012). The type of calcium phosphate precipitated is a function of the pH of the solution adjacent to the cathode. At a pH value below about 6.5 brushite (calcium diphosphate dihydrate) is stable according to... 

Boistelle R, Astier JP (1988) Crystallization mechanisms in solution. J Cryst Growth 90 14-30 Boskey AL, Posner AS (1973) Conversion of amorphous calcium phosphate to microcrystalline hydroxylapatite. ApH-dependent, solution-mediated, solid-solid conversion. JPhys Chem77 2313-2317 Boskey AL, Posner AS (1976) Formation of hydroxylapatite at low supersaturation. J Phys Chem 80 40-45 Boudreau AE, Mathez EA, Mccallum IS (1986) Halogen geochemistry of the Stillwater and Bushveld complexes—Evidence for transport of the platinum-group elements by Cl-rich fluids. J Petrol 27 967-986... 

Feenstra TP, Brayn PLD (1979) Formation of calcium phosphates in moderately supersaturated solutions. JPhys Chem 83 475-479... 

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