Natural Apatite Research Articles

Crystallinity in apatites: how can a truly disordered fraction be distinguished from nanosize crystalline domains?

In the last decade synthetic apatites mimicking the human natural one have been widely prepared and characterized from the physico-chemical point of view; however a shading zone is still remaining related to the evaluation and distinction of the less crystalline part, almost amorphous, and the crystallographically well ordered, nano-sized part, inside the apatite itself. Actually natural apatite forming bone tissue can include both types of crystals whose prevalence is dependent from the specific bone evolution stage and the specialized tissue performance. The quantitative description of such a combination usually represents a puzzling problem, but the result can also clarify the definition of "crystallinity in apatite" that appears still controversial. Many different synthetic apatites, including those nucleated on organic templates, were analyzed with different techniques (X-ray diffraction, transmission electron microscopy, and so on) to clarify the true nature of the disordered part. The results, manipulated by the classical methodologies devised for substances with highly perturbed structural order, led to establish that only specifically prepared amorphous calcium phosphate is really a glass, while the distorted portion coexisting with more or less crystalline regions is simply nanocrystalline. Moreover, at the conceptual limit of crystallinity tending to zero, the two models surprisingly cease to be conflicting.

Rock Biofertilizers with Acidithiobacillus on Sugarcane Yield and Nutrient Uptake in a Brazilian Soil

The experiment was conducted with the aim to evaluate the effects of biofertilizers with phosphate and potash rocks and soluble fertilizers (Triple super phosphate and potassium chloride) in chemical attributes of a Brazilian tableland soil grown with sugarcane. The experiment was arranged in a completely randomized factorial design 2 × 4 × 3 + 1, with four replicates. Two varieties of sugarcane, three sources of P and K mixture (natural apatite + natural biotite; P + K biofertilizers with Acidithiobacillus and P + K chemical fertilizers) were applied in four levels. A control treatment with no P and K fertilization (P0K0) was added for comparative purposes. Significant differences between varieties were observed in all analyzed parameters, with better results when applied the recommended levels of biofertilizers and chemical fertilizers. Stalk fresh matter increased with fertilizers and biofertilizers applications, especially when applied in levels near recommendation. Total N, total P and total K in stalk dry matter increased significantly when biofertilizers were applied. The results indicate potential use of biofertilizers that may be used as P source; however, long-term studies are necessary due to soil pH reductions and its possible adverse effects.

Phosphorus removal from wastewater by mineral apatite

Natural apatite has emerged as potentially effective for phosphorus (P) removal from wastewater. The retention capacity of apatite is attributed to a lower activation energy barrier required to form hydroxyapatite (HAP) by crystallization. The aim of our study was to test the P removal potential of four apatites found in North America. Minerals were collected from two geologically different formations: sedimentary apatites from Florida and igneous apatites from Quebec. A granular size ranging from 2.5 to 10 mm to prevent clogging in wastewater applications was used. Isotherms (24 and 96 h) were drawn after batch tests using the Langmuir model which indicated that sedimentary apatites presented a higher P-affinity ( K L=0.009 L/g) than igneous apatites ( K L≈0.004 L/g). The higher density of igneous material probably explained this difference. P-retention capacities were determined to be around 0.3 mg P/g apatite (24 h). A 30 mg P/L synthetic effluent was fed during 39 days to four lab-scale columns. A mixture of sedimentary material (apatite and limestone 50–50%, w/w) showed a complete P-retention during 15 days which then declined to 65% until the end of the 39 days lab scale test period. A limitation in calcium may have limited nucleation processes. The same mixture used in a field scale test showed 60% P-retention from a secondary effluent (30 mg COD/L, 10 mg Pt/L) during 65 days without clogging.

Elaboration and properties of new ceramic microfiltration membranes from natural and synthesised apatite

In this work, we were interested in the development and the characterisation of new mineral microfiltration membranes coated on apatite macroporous supports which are prepared and characterised beforehand. The choice of material is based primarily on its low cost (considering its abundance in the Tunisian layers for natural apatite) and its thermal and chemical resistance. The active layers were prepared from lacunary hydroxyapatite synthesised (HA) and from natural apatite (AB). The membranes were deposited on the tubular support basing of natural apatite, already prepared by the “slip-casting” process. The thermal treatment consists of drying at room temperature for 24 h then a sintering at 600°C for the membrane with HA (MS) and at 750°C for the membrane with AB (MB). The surface and the cross-section morphologies observed through a scanning electron microscope (SEM) are homogeneous and do not present any macro defects (cracks, etc.). The average pore diameters of the active layer are approximately 0.25 μm and 0.2 μm for the MS and MB membrane respectively. The obtained membrane was used to treat cuttlefish effluents generated from the conditioning seawater product industry which consumes a great amount of water where the washing baths are discharged in the littoral of Sfax (Tunisia) (about 1000 m 3/d). Cross-flow microfiltration was performed then, in order to reduce the turbidity and chemical oxygen demand (COD). The result showed a high retention of turbidity (99%), conductivity and COD (85%).

Clinohydroxylapatite: a new apatite-group mineral from northwestern Ontario (Canada), and new data on the extent of Na-S substitution in natural apatites

A monoclinic analogue of the mineral hydroxylapatite was found in altered leucogabbro making up one of the intrusive units in the Mesoproterozoic Seagull pluton in northwestern Ontario, Canada. The mineral is part of a replacement assemblage developed metasomatically after igneous plagioclase. It occurs as masses of coalescent spherulites < 30 μm in diameter, and is paragenetically associated with prehnite, hibschite, titanite, rutile and an unidentified Ca silicate. The new mineral is white and chalky in appearance, has a Mohs hardness of 5 and a measured density of 3.07(2) g/cm3 ( D calc 3.13 g/cm3), and shows little deviation from a uniaxial optical behavior (α 1.632, γ 1.649). Compositionally, it is an intermediate member of the ternary system Ca5(PO4)3(OH) - Ca5(PO4)3Cl - Na3Ca2(SO4)3(OH). The average content of the latter two end-members is ca. 20 mol. % each. The combination of high Na and S contents (up to 0.63 and 0.71 apfu, respectively) is unparalleled by any previously reported natural composition, and indicates significant solubility of these elements in natural apatites even at low crystallization temperatures (< 400 °C). The monoclinic symmetry of this mineral, determined from microbeam X-ray diffraction patterns, distinguishes it from hydroxylapatite. By analogy with synthetic Ca5(PO4)3OH, its space group is P 21/ b [ a 9.445(2) A, b 18.853(4) A, c 6.8783(6) A, γ 120.00(2)°], and the deviation of symmetry from the archetypal (space group P 63/ m , a ∼9.4 A, c ∼6.9 A) probably results from ordering of (OH) anions in [00z] anionic columns and consequent doubling of periodicity along [010]. In keeping with the nomenclature of apatites, this new mineral was named clinohydroxylapatite.

Elaboration and characterisation of apatite based mineral supports for microfiltration and ultrafiltration membranes

The development and the characterisation of new supports for microfiltration and ultrafiltration membranes from apatite applied to filtration are presented. The choice of this material is based primarily on its low cost (considering its abundance in the Tunisian ores). The support, with tubular configuration, was prepared from natural apatite: apatite powders were crushed for 30 min and sieved to 200 μm. The resulting powders, mixed with organic additives and water, could be extruded to elaborate a porous structure. The firing temperature of the support is 1160 °C. The morphologies of the surface and the cross-section observed on scanning electron microscope (SEM) are homogeneous and do not present any macro defects (cracks, etc.). The mean pore diameter, measured by mercury porosimetry, is 6 μm and the pore volume is 48%.

Interatomic potential models for natural apatite crystals: Incorporating strontium and the lanthanides

A comprehensive set of interatomic potential parameters for modeling natural apatite crystals is presented. These potentials build on those previously used in research on apatites with new potentials fitted empirically to crystal structures and their properties using the GULP program. We demonstrate that the new potentials produce good models for the different compounds used for fitting, as well as for several natural apatites. Also presented are predicted enthalpies of mixing of strontium and calcium apatites and predicted cation site preferences in strontium calcium fluorapatite.

Experimental studies of REE fractionation during water–mineral interactions: REE release rates during apatite dissolution from pH 2.8 to 9.2

Rare earth elements (REE) release rates were measured during the open-system dissolution of natural apatite at 25 °C, 2.8 < pH < 9.2, and at ionic strengths ranging from 0.001 to 0.02 mol/kg. All inlet solutions were Ca, REE, and P free. In agreement with previous studies, apatite dissolution rates determined from outlet solution Ca concentrations decrease monotonically with increasing pH. Outlet solution molar REE / Ca ratios differ from that of the dissolving apatite during the experiments, this behaviour is interpreted to stem from dissolution or precipitation of minor quantities of rhabdophane (REE(PO 4)· nH 2O). This interpretation is supported by solute speciation calculations, which indicate that REE are retained in the solid phase when the outlet fluids are supersaturated with respect to Nd-rhabdophane ( K sp ≈ 10 − 24.5 ± 0.5 ), but preferentially released when the outlet fluids are undersaturated with respect to Nd-rhabdophane. The relative concentrations of the REE in the outlet solutions suggest that the secondary rhabdophane is slightly enriched in light REE (Ce to Eu). The results of this study suggest that rhabdophane dissolution and precipitation during apatite–fluid interaction plays a major role in controlling surface and ground water REE signatures.

Monitoring of the evolved gases in apatite-ammonium sulfate thermal reactions

Thermal reactions in natural fluorapatite or fluorcarbonate apatite and ammonium sulfate mixtures with mole ratio 1:4 at calcination up to 500°C were studied by simultaneous thermogravimetry and FTIR analysis of the evolved gases. The composition of natural apatite has little impact on the release of NH3. Upon the evolution of NH3 nitrous oxides were found in minor amounts. The release of SO2 at temperatures above 400°C is more intensive and occurs at lower temperatures in the case of fluorapatite than of carbonate containing apatites. Evolution of CO2 starts at 250°C with maximum at 350-360°C.

Theoretical stability assessment of uranyl phosphates and apatites: Selection of amendments for in situ remediation of uranium

Addition of an amendment or reagent to soil/sediment is a technique that can decrease mobility and reduce bioavailability of uranium (U) and other heavy metals in the contaminated site. According to data from literature and results obtained in field studies, the general mineral class of apatites was selected as a most promising amendment for in situ immobilization/remediation of U. In this work we presented theoretical assessment of stability of U(VI) in four apatite systems (hydroxyapatite (HAP), North Carolina Apatite (NCA), Lisina Apatite (LA), and Apatite II) in order to determine an optimal apatite soil amendment which could be used for in situ remediation of uranium. In this analysis we used a theoretical criterion which is based on calculation of the ion–ion interaction potential, representing the main term of the cohesive energy of the matrix/pollutant system. The presented results of this analysis indicate (i) that the mechanism of immobilization of U by natural apatites depends on their chemical composition and (ii) that all analyzed apatites represent, from the point of view of stability, promising materials which could be used in field remediation of U-contaminated sites.

Apatite as an interesting seed to remove phosphorus from wastewater in constructed wetlands

Intensive use of phosphates has resulted in high P levels in surface waters and therefore eutrophication problems. Over the last decade many studies have revealed the advantage of using specific materials with efficient phosphorus retention capacities. Recent studies state that Ca materials are of particular interest for long-term retention of P, but can induce negative effects. To improve P retention and avoid negative counter-effects we tested the potential of natural apatites. Apatite sorption was evaluated using batch and open reactor experiments. Batch experiments identify sorption mechanisms and the influence of the ionic characteristics of the solution; open reactor experiments evaluate sorption capacities in relation to the ionic composition of the solution and biomass development. In parallel, observation of the material by electron microscopy was used to give more precision information about the mechanisms involved. This work reveals the strong chemical affinity between apatites and phosphorus. Compared to other calcareous materials apatite is better able to maintain low outlet P levels. After more than 550 days feeding, sorption was still present and low P outlet levels were still being obtained when sufficient contact time and calcium content in the solution were ensured. This work demonstrates the advantages of using apatites for phosphorus removal in constructed wetlands. The behaviour of apatite in phosphorus retention is explained and its suitability for use in such extensive systems defined.

Tin plating

Remarkably thick nanoporous anodic films have been formed through simple DC anodization by adding hydrogen peroxide as an oxidant to a sulfuric acid electrolyte. This result was not possible when using the previously reported pulsed polarization technique. While this anodic film's growth was significantly promoted by DC anodization, having only sulfuric acid resulted in the preferential anodic dissolution of the substrate material. Transmission electron microscopy of the anodic film's cross-section confirmed that the film has numerous fine pores with a size distribution ranging from 5 nm to 20 nm. Energy-dispersive x-ray spectroscopic analysis of the anodized specimens revealed that the films were composed of a complex chromium-rich oxide. In addition, coating stainless steel with thick nanoporous oxide films through anodization in the optimized conditions confirmed that it could act as a highly suitable host for synthetic hydroxyapatite coating, resulting in the enhanced growth of natural apatite in a simulated body environment. The enhancement of the apatite deposition is attributed to the availability of the empty pore volume in the thick anodic porous films, formed on stainless steel surfaces. These findings suggest a novel functionality to stainless-steel surface modification.

Impact of anionic substitutions on apatite structure and properties

A review of the results of the authors on single and coupled substitutions of F for OH, and of CO 3 and SO 4 for PO 4 in synthetic and natural apatites, their influence on apatite structure and properties, studied by peak fitting FTIR, XRD, TG/DTA and TG/EGA methods, is presented. Calcination of carbonate and sulphate substituted apatites leads to the formation of stable stoichiometric apatite.

In situ stabilization of toxic metals in polluted soils using phosphates: theoretical prediction and experimental verification

The in situ (in place) immobilization of toxic metals, using inexpensive “reactive” amendments, is considered as a simple and cost-effective approach for the treatment of soils, contaminated by the presence of heavy metals, when these soils are difficult or costly to be removed and treated ex situ. Several application studies have demonstrated that the stabilization of contaminated soils and groundwaters by the addition of apatite minerals has the potential to be a successful and widely applicable remediation strategy for the case of Pb, Cd, as well as for other toxic metals, existing in polluted soils. On the other hand, the specific immobilization mechanism(s) of these toxic metals remains rather elusive. The present work involves an interdisciplinary theoretical and experimental approach, designed to gain at the fundamental (molecular) level the understanding of respective mechanisms, considering the immobilization of Pb and Cd by the addition of apatites. The theoretical analysis of stability, regarding the apatite/Pb or apatite/Cd systems and the relevant results of sorption experiments, pointed out two different mechanisms for the immobilization of Pb or Cd by the use of apatites. The possible practical consequences of these findings for the selection/application of natural apatites for the remediation of contaminated soils by the presence of heavy metals have been also discussed.

The compressibility of a natural apatite

In-situ synchrotron X-ray diffraction (XRD) experiments of a natural apatite with the formula of Ca5(PO4)3F0.94Cl0.06 were carried out using a diamond anvil cell and angle-dispersive technique at Photon Factory (PF), Japan. Pressure–volume data were collected up to 7.12 GPa at 300 K. The pressures were determined from the ruby fluorescence spectra shift. The unit-cell parameters and volume decreased systematically with increasing pressure, and a reliable isothermal bulk modulus and its pressure derivative were obtained in this study. The third-order Birch–Murnaghan equation of state yielded the isothermal bulk modulus of K T =91.5(38) GPa, its pressure derivative K T ′= 4.0(11), and the zero-pressure volume V0=524.2(3) A3.

Nucleation of biomimetic apatite in synthetic body fluids: dense and porous scaffold development

The effectiveness of synthetic body fluids (SBF) as biomimetic sources to synthesize carbonated hydroxyapatite (CHA) powder similar to the biological inorganic phase, in terms of composition and microstructure, was investigated. CHA apatite powders were prepared following two widely experimented routes: (1) calcium nitrate tetrahydrate and diammonium hydrogen phosphate and (2) calcium hydroxide and ortophosphoric acid, but using SBF as synthesis medium instead of pure water. The characteristics of the as-prepared powders were compared, also with the features of apatite powders synthesized via pure water-based classical methods. The powder thermal resistance and behaviour during densification were studied together with the mechanical properties of the dense samples. The sponge impregnation process was used to prepare porous samples having morphological and mechanical characteristics suitable for bone substitution. Using this novel synthesis was it possible to prepare nanosized (≈20 nm), pure, carbonate apatite powder containing Mg, Na, K ions, with morphological and compositional features mimicking natural apatite and with improved thermal properties. After sintering at 1250 °C the carbonate-free apatite porous samples showed a surprising, high compressive strength together with a biomimetic morphology.

CaO-P&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; Glass-Hydroxyapatite Thin Films Obtained by Laser Ablation: Characterisation and In Vitro Bioactivity Evaluation

Hydroxyapatite (HA) coatings have been applied to improve adhesion of non -cemented implants to host bone. Plasma spraying is the most common technique leading to thick calcium phosphate films (>120μm). Pulse laser deposition (PLD), is a possible alternative method to obtain thin (<10 μm), well adherent hydroxyapatite (HA) films. Similarly to synthetic HA, biological apatites contain Ca 2+ , PO4 3- and OH - , but also several trace ions, like Na + , Mg 2+ , K + and F - , which may be introduced by CaO-P2O5 glasses. In this study, calcium phosphate coatings based on HA and glass modified HA were applied by PLD onto Ti-6Al-4V, using deposition times of 3 hours. SBF immersion up to 1 week was used to test the films bioactivity. PLD thin films before and after SBF immersion were observed by SEM/EDS and analysed by XPS and XRD. PLD thin films presented columnar cross section structures, independently of the coatings' chemical composition. After SBF immersion, apatite films formed on PLD coatings, both of HA and HA+1.5% glass, did not present the usual morphology of immersion films, but appeared to replicate the previous films. The main difference between HA and modified HA coatings could be seen in the XPS analyses at short immersion periods. Natural apatite was calcium deficient with a Ca/P of ±1.2-1.3. The results seem to indicate that modified HA coatings with lower Ca/P ratio induced earlier formation of natural apatite.

Removal of Metals and Radionuclides Using Apatite and Other Natural Sorbents

Treatment of waste water for removal of low levels of actinides, particularly plutonium, uranium, and americium, and hazardous metals has been and continues to be a challenge. This study evaluated the use of apatite and other natural materials for use as sorbents to treat water streams containing As, Am, Pu, Se, Tc, and U. Nine sorbents were tested including natural and synthetic apatites, tri-calcium phosphate, bone char, and activated magnetite. The sorption properties were tested in batch studies and in column tests. There was moderate sorption of As, Se, Sr, Cs, and Tc, but very good removal of U and Pu by the phosphate adsorbents. The Am results were inconclusive. The synthetic apatites had higher capacities than the natural apatites from phosphate rock and fish bones.

Non-Henry’s Law behavior of REE partitioning between fluorapatite and CaF 2-rich melts: Controls of intrinsic vacancies and implications for natural apatites

The partitioning of rare-earth elements (REEs: Gd and multiple REEs), Sr, and Mn between fluorapatite and CaF 2-rich melts was investigated over a wide range of REE concentrations (i.e., from 0.8 ± 0.1 to 25,000 ± 2600 ppm Gd in fluorapatite) in two different sample assemblies (i.e., tightly covered Pt crucibles and sealed Pt capsules) at 1220 °C and atmospheric pressure. Attainment of equilibrium is indicated by selected reversal experiments. The partition coefficient D(Gd) decreases from ∼2 to ∼0.5 with increasing Gd in fluorapatite, hence a marked non-Henry’s Law behavior, but becomes independent of composition at and above ∼5000 and ∼1000 ppm Gd for experiments in Pt crucibles and Pt capsules, respectively. Non-Henry’s Law behavior is also observed in experiments involving multiple REEs. All REE patterns are convex upward in shape with maxima between Nd and Gd, and D(La)/D(Nd) and D(Nd)/D(Yb) decrease systematically with increasing total REEs in fluorapatite, suggesting that REE fractionations are partly related to non-Henry’s Law behavior. These experimental results and local structural data from previous electron paramagnetic resonance spectroscopic studies suggest that the non-Henry’s Law behavior of REE partitioning between fluorapatite and melt is controlled by intrinsic Ca 2+ vacancies in the c-axis channels. The D(Sr) and D(Mn) values are independent of composition and, therefore, do not deviate from the Henry’s Law in their respective compositional ranges investigated in this study. Nonstoichiometry, such as Ca 2+ and F − vacancies in the c-axis channels, is well known in natural apatites, particularly in biogenic apatites. Therefore, the observed non-Henry’s Law behavior of REE partitioning is expected to have important implications for REE geochemical modeling involving apatites and for the uptake of REEs by natural apatites. Particularly, the non-Henry’s Law behavior of REE partitioning is at least partly responsible for the commonly observed, bell-shaped REE patterns in fossil biogenic apatites.

Exchange of light rare earths for Ca in apatite

Abstract Diffusion experiments were carried out on natural apatite crystals that were immersed in molten light rare earth element (REE) chloride salt at temperatures between 900 and 1150 °C for periods up to 35 days. Electron microprobe analysis of the crystals showed that light REEs replaced Ca according to electronic balance, i.e. 2 REE 3+ for 3 Ca 2+ . These diffusion profiles indicate that a maximum amount of substitution in the structure occurs when two of the ten Ca ions in apatite are replaced by the REE diffusing elements. Anisotropic diffusion is observed between a and c crystallographic directions. Comparison of maximum distance indicates that the larger ions move more easily in the apatite structure. We conclude that the light REEs diffuse within the channel structures of the mineral, and that this diffusion is controlled by the substitution type of elements in the calcium sites. To cite this article: A. Iqdari et al., C. R. Geoscience 335 (2003).