Thermal behavior of bone and synthetic hydroxyapatites submitted to magnesium interaction in aqueous medium

Ion exchange synthesis of copper-based hydroxyapatite for the catalytic degradation of phenol

Hydroxyapatite (HAP) is a material renowned for its exceptional capabilities in adsorbing and exchanging heavy metal ions, making it a widely employed substance within the environmental domain. This study aims to present a novel material, namely copper-HAP (Cu-HAP), which was synthesized via an ion exchange method. The resulting material underwent comprehensive characterization using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and Brunauer-Emmett-Teller (BET) analysis. Subsequently, based on the principle of the Fenton-like oxidation reaction, the material was used for the degradation of phenol. The outcomes of the investigation revealed that the optimal preparation conditions for the catalyst were achieved at a temperature of 40 °C, a pH value of 5, and a relative dosage of Cu-HAP at 100 mg/g. Under the reaction conditions of a catalyst dosage of 2 g/L, a 30% hydrogen peroxide concentration of 30 mM, a phenol concentration of 20 mg/L, a pH value of 6, a temperature of 40 °C, and the degradation rate of phenol impressively reached 98.12%. Furthermore, the degradation rate remained at 42.31% even after five consecutive cycles, indicating the promising potential of Cu-HAP in the treatment of recalcitrant organic compounds within this field.

Nucleation, growth and evolution of calcium phosphate films on calcite

Marble, a stone composed of the mineral calcite, is subject to chemically induced weathering in nature due to its relatively high dissolution rate in acid rain. To protect monuments and sculpture from corrosion, we are investigating the application of thin layers of hydroxyapatite (HAP) onto marble. The motivation for using HAP is its low dissolution rate and crystal and lattice compatibility with calcite. A mild, wet chemical synthesis route, in which diammonium hydrogen phosphate salt was reacted with marble, alone and with cationic and anionic precursors under different reaction conditions, was used to produce inorganic HAP layers on marble. Nucleation and growth on the calcite substrate was studied, as well as metastable phase evolution, using scanning electron microscopy, grazing incidence X-ray diffraction, and atomic force microscopy. Film nucleation was enhanced by surface roughness. The rate of nucleation and the growth rate of the film increased with cationic (calcium) and anionic (carbonate) precursor additions. Calcium additions also influenced phase formation, introducing a metastable phase (octacalcium phosphate) and a different phase evolution sequence.

Synthesis and characterization of nanocrystalline apatites from eggshells at different Ca/P ratios

Nanocrystalline apatites with different Ca/P ratios were synthesized using eggshell as a calcium source by microwave processing. The apatites were found to have a minor amount of Mg, Sr, Si and Na ions inherited from the eggshells. The presence of several foreign ions results in a perturbed lattice structure indicated by an increase in lattice constants and shift in vibrational frequencies of the functional groups. The apatites were heat treated to investigate the influence of foreign ions on thermal stability. The minor amounts of ions do not affect the thermal stability. The differences in thermal behaviour of these apatites were due to the presence of HPO(2-)(4) ions only and not due to other ions because of their low content.

Biomimetic Mg-substituted hydroxyapatite: from synthesis to in vivo behaviour

The incorporation of magnesium ions (in the range 5-10 mol% in respect to Ca) into the hydroxyapatite structure, which is of great interest for the developing of artificial bone, was performed using magnesium chloride, calcium hydroxide and phosphoric acid, as reactants. Among the synthesized powders, the synthetic HA powder containing 5.7% Mg substituting for calcium was selected, due to its better chemico-physical features, and transformed into granules of 400-600 microm, for biocompatibility tests (genotoxicity, carcinogenicity, toxicity, in vitro cytotoxicity and in vivo skin irritation-sensitization tests). In vivo tests were carried out on New Zealand White rabbits using the granulate as filling for a femoral bone defect: osteoconductivity and resorption were found to be enhanced compared to commercial stoichiometric HA granulate, taken as control.

Preparation of magnesium-substituted hydroxyapatite powders by the mechanochemical–hydrothermal method

Magnesium-substituted hydroxyapatite (Mg-HAp) powders with different crystallinity levels were prepared at room temperature via a heterogeneous reaction between Mg(OH)(2)/Ca(OH)(2) powders and an (NH(4))(2)HPO(4) solution using the mechanochemical-hydrothermal route. The as-prepared products contained unreacted Mg(OH)(2) and therefore had to undergo purification in ammonium citrate aqueous solutions at room temperature. X-ray diffraction, infrared spectroscopy, thermogravimetric and chemical analyses were performed and it was determined that the purified powders were phase-pure Mg-HAp containing 0.24-28.4 wt% of Mg. The concentration of Mg was slightly lower near the surface than in the bulk of the HAp crystals as indicated by X-ray photoelectron spectroscopy. Dynamic light scattering revealed that the median particle size of the room temperature Mg-HAp powders was in the range of 102 nm-1.2 microm with a specific surface area between 91 and 269 m(2)/g. Scanning electron microscopy confirmed that the Mg-HAp powders consisted of submicron agglomerates of nanosized crystals, less than approximately 20 nm.

Chemical and structural characterization of the mineral phase from cortical and trabecular bone

X-ray diffraction, infrared spectroscopy and chemical investigations have been carried out on the inorganic phases from rat cortical and trabecular bone. Although both inorganic phases consist of poorly crystalline B carbonated apatite, several significant differences have been observed. In particular, trabecular bone apatite displays reduced crystallite sizes, Ca/P molar ratio, and carbonate content, and exhibits a greater extent of thermal conversion into beta-tricalcium phosphate than cortical bone apatite. These differences can be related to the different extents of collagen posttranslational modifications exhibited by the two types of bone, in agreement with their different biological functions.

Magnesium-containing carbonate apatites

Hydroxyapatites precipitated at pH 7.0 and 9.0 with and without carbonate and with different amounts of magnesium were studied. Mg uptake, Ca/P ratios, and lattice constant data indicate that Mg is incorporated into the apatite lattice. IR spectra demonstrate the formation of B-type carbonate apatites with carbonate substituting for phosphate. Decomposition of carbonate-containing apatites at elevated temperatures up to 1000 degrees C is more gradual for apatites prepared at pH 9.0 than for those prepared at pH 7.0 for which an abrupt loss of carbonate occurs after 600 degrees C. Compounds synthesized without added carbonate partially transform to beta Ca3(PO4)2 (TCP) at about 700 degrees C. Greater transformation to TCP occurs as the Mg incorporation is increased, indicating the insertion of Mg into TCP and consequent stabilization of the TCP. SEM micrographs show increases in the size of crystallites when apatites are precipitated with Mg (in the 0.2-1.5% range), providing further evidence for Mg incorporation into the apatite structure.

Age and temperature related changes to the ultrastructure and composition of human bone mineral

This X-ray diffraction (XRD) investigation of heat-treated human femoral bone showed that the main mineral phase of both unheated bone and bone heated to 600 degrees C resembled that of a poorly crystalline form of hydroxyapatite. The rod-shaped apatite crystals in unheated bone persisted in bone heated up to 400 degrees C. Recrystallization at approximately 600 degrees C, produced larger crystals, which either retained their original morphology or changed to tabular or equidimensional shapes. The size of the apatite crystals in unheated and heated bone specimens was dependent on both temperature and age. When heated above 600 degrees C the crystallinity of the bone mineral increased, and the XRD pattern more closely resembled that of hydroxyapatite. Partial decomposition of the hydroxyapatite phase to calcium oxide above 1000 degrees C, and beta-tricalcium phosphate, alpha-tricalcium phosphate, and calcium oxide phosphate between 1200 degrees C and 1400 degrees C, indicated that the original apatite phase was both calcium deficient and contained carbonate. The relative peak intensities of the thermal decomposition products were related to some extent to the age of the deceased person and reflected the compositional changes that occur during bone aging. Because the thermally induced changes to the composition and ultrastructure of bone mineral were influenced by the age of the individual, this investigation proposed that the heat treatment of bone tissue may offer an alternative way of studying bone aging.

The role of magnesium on the structure of biological apatites

X-ray diffraction, infrared absorption spectroscopy, and chemical investigation have been carried out on deproteinated samples of turkey leg tendon at different degrees of calcification. The inorganic phase consists of poorly crystalline B carbonated apatite. On increasing calcification, the apatite crystal size, as well as its thermal stability, increase while the relative magnesium content is reduced. On the other hand, synchrotron X-ray diffraction data clearly indicate that apatite lattice parameters do not change as the crystals get larger. At the last stage of calcification the crystal size, chemical composition, and thermal conversion of the apatite crystallites approximate those of bone samples, which have been examined for comparison. The results provide a quantitative relationship between relative magnesium content and extent of apatite conversion into B-tricalcium phosphate by heat treatment. Furthermore, they suggest that the smaller crystallites laid down inside the gap region of the collagen fibrils are richer in magnesium than the longer ones that fill the space between collagen fibrils.

Thermal decomposition of Lingula shell apatite

Lingula shell is composed of apatite with a preferred orientation. The shell apatites of Lingula unguis(Lu) and Lingula shantoungensis(Ls) were characterized and compared with apatite of human tooth enamel. Insight into the Lingula apatite was studied by following the change of lattice parameter, transformation to beta-tricalcium phosphate (beta-TCP), and the loss and change of CO3, OH, and H2O after heating up to 1,000 degrees C in air and N2 for 24 hours. The OH stretching band was not observed in unheated apatites and in apatites heated in dried N2. Lu and Ls apatite produced 26 and 17 wt% of beta-TCP at 700 degrees C, respectively. Fifty to 60% of H2O was lost at 200 degrees C, being accompanied by a drastic contraction of the a- and c-axis and a drastic decrease in the crystallinity. These results indicate that (1) Lu and Ls shell apatite is CO3 containing F + Cl-apatite, and (2) the structural H2O of the Lingula apatite is loosely bounded such that they are lost at lower temperature than tooth enamel.

Thermal decomposition of developing enamel

The decomposition of forming, maturing, and mature enamel was studied between room temperature and 1,000 degrees C by powder X-ray diffraction and infrared absorption methods. In mature dental enamel, carbonate decomposition proceeds relatively fast until 500 degrees C and at a slower rate beyond it. In forming and maturing enamel, decomposition is faster and is completed around 800 degrees C. The formation of beta-Ca3(PO4)2 is observed in dental enamel at 500 degrees C. At 1,000 degrees C, the apatite phase in forming and maturing enamel transforms almost completely to beta-Ca3(PO4)2, whereas in mature enamel, even at 1,000 degrees C, only partial decomposition occurs. Infrared results show the appearance in dental enamel of (1) A-type carbonate at room temperature and in the 500-900 degrees C range, in addition to the commonly observed B-type carbonate, and (2) intermediate CO2 molecules during carbonate decomposition (200-500 degrees C).

Structural and chemical characterization of inorganic deposits in calcified human mitral valve

X-ray diffraction, i.r. absorption, and chemical analyses have been carried out on the mineral deposits of calcified human mitral valves and glutaraldehyde-preserved porcine aortic grafts. The mineral deposits isolated from highly calcified mitral valves and porcine aortic grafts are constituted of type B-carbonate apatite. Magnesium substituted beta-tricalcium phosphate is present, together with an apatitic phase similar to dahllite, in the ashes of poorly calcified mitral valves. The contraction of the unit cell of beta-tricalcium phosphate due to magnesium incorporation is compared with the variation of the lattice constants of synthetic beta-tricalcium phosphate at different degree of magnesium substitution for calcium. The results reveal the important role of magnesium on the calcification of human valves. In fact, the apatitic phase deposited at the beginning of the calcification process, when there is a high magnesium content, converts completely into beta-tricalcium phosphate by heat treatment at 1,000 degrees C. On the other hand, when the calcification becomes massive, magnesium content appears highly reduced, and the deposited apatitic phase is characterized by a high thermal stability.