Apatitic Calcium Phosphate Research Articles

Characterization of β-tricalcium phosphate-clay mineral composite obtained by sintering powder of apatitic calcium phosphate and montmorillonite

Calcium phosphates and clay minerals have taken great attention by researchers for medicals applications. Recently, numerous researchers have been developed the calcium phosphate/ montmorillonite composites as a synthetic bone graft substitutes. In this study, β-tricalcium phosphate (Ca3(PO4)2,β-TCP)/Montmorillonite(MNa) composites were developed by sintering powder at 900 °C of apatitic tricalcium phosphate (Ca9(HPO4)(PO4)5(OH), TCPa) and unmodified MNa at various amounts (2, 5, and 10 wt.%). The composites were characterized by X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FT-IR) and thermal analysis (TG / DSC). XRD results show that the sintering of TCPa with and without MNa lead to the formation of a single crystallized β-TCP phase and that the addition of the MNa causes a change in his crystalline lattice. The results of XRD, FT-IR and the structural refinement indicate the insertion of the clay ions in the β-TCP structure. This ionic substitution leads to a decrease in lattice parameters and an increase in index distortion. The low percentages of MNa lead to an increase in crystallite size while the high values (10 wt.%) lead to an opposite effect. The change of the composition leads to the stabilization of the β-TCP phase, and the temperature of the allotropic transformation of β to α-TCP shifts toward the higher temperature.

Localization, Morphologic Features, and Chemical Composition of Calciphylaxis-Related Skin Deposits in Patients With Calcific Uremic Arteriolopathy

Calcific uremic arteriolopathy (CUA), a rare, potentially fatal, disease with calcium deposits in skin, mostly affects patients with end-stage renal disease who are receiving dialysis. Chemical composition and structure of CUA calcifications have been poorly described. To describe the localization and morphologic features and determine the precise chemical composition of CUA-related calcium deposits in skin, and identify any mortality-associated factors. A retrospective, multicenter cohort study was conducted at 7 French hospitals including consecutive adults diagnosed with CUA between January 1, 2006, and January 1, 2017, confirmed according to Hayashi clinical and histologic criteria. Patients with normal renal function were excluded. For comparison, 5 skin samples from patients with arteriolosclerosis and 5 others from the negative margins of skin-carcinoma resection specimens were also analyzed. Localization and morphologic features of the CUA-related cutaneous calcium deposits were assessed with optical microscopy and field-emission-scanning electron microscopy, and the chemical compositions of those deposits were evaluated with μ Fourier transform infrared spectroscopy, Raman spectroscopy, and energy dispersive radiographs. Thirty-six patients (median [range] age, 64 [33-89] years; 26 [72%] female) were included, and 29 cutaneous biopsies were analyzed. Calcific uremic arteriolopathy and arteriolosclerosis skin calcifications were composed of pure calcium-phosphate apatite. Calcific uremic arteriolopathy vascular calcifications were always circumferential, found in small to medium-sized vessels, with interstitial deposits in 22 (76%) of the samples. A thrombosis, most often in noncalcified capillary lumens in the superficial dermis, was seen in 5 samples from patients with CUA. Except for calcium deposits, the vessel structure of patients with CUA appeared normal, unlike thickened arteriolosclerotic vessel walls. Twelve (33%) patients died of CUA. Calcific uremic arteriolopathy-related skin calcifications were exclusively composed of pure calcium-phosphate apatite, localized circumferentially in small to medium-sized vessels and often associated with interstitial deposits, suggesting its pathogenesis differs from that of arteriolosclerosis. Although the chemical compositions of CUA and arteriolosclerosis calcifications were similar, the vessels' appearances and deposit localizations differed, suggesting different pathogenetic mechanisms.

The Addition of Poly(Vinyl Alcohol) Fibers to Apatitic Calcium Phosphate Cement Can Improve Its Toughness.

Calcium phosphate cements, and in particular hydroxyapatite cements, have been widely investigated for use as bone void fillers due to their chemical similarity to bone and related osteoconductivity. However, they are brittle, which limits their use to non-load-bearing applications. The aim of the current study was to improve the toughness of hydroxyapatite cements through fiber reinforcement. The effect of the addition of hydrophilic, poly(vinyl-alcohol) (PVA) fibers to hydroxyapatite cement was evaluated in terms of mechanical properties, including compressive strength, diametral tensile strength and toughness (work of fracture), as well as setting time, phase composition and cement morphology. The fiber reinforcement enhanced the fracture resistance of the hydroxyapatite cement, but also simultaneously reduced the compressive strength and setting time of the cements. However, cement with 5 wt % of fibers (of the powder component) could be considered a good compromise, with a compressive strength of 46.5 ± 4.6 MPa (compared to 62.3 ± 12.8 MPa of that without fibers), i.e., still much greater than that of human trabecular bone (0.1–14 MPa). A significantly higher diametral tensile strength (9.2 ± 0.4 MPa) was found for this cement compared to that without fibers (7.4 ± 1.5 MPa). The work of fracture increased four times to 9.1 ± 1.5 kJ/m2 in comparison to the pristine apatite. In summary, the hydroxyapatite cements could be reinforced by suitable amounts of PVA fibers, which resulted in enhancing the material’s structural integrity and ductility, and increased the material’s resistance to cracking.

In vivo resorption of injectable apatitic calcium phosphate cements: Critical role of the intergranular microstructure.

The in vivo resorption rate of two injectable apatitic calcium phosphate cements used in clinics (Graftys® HBS and NORIAN®) was compared, using a good laboratory practice (GLP) study based on an animal model of critical-sized bone defect. To rationalize the markedly different biological properties observed for both cements, key physical features were investigated, including permeability and water-accessible porosity, total porosity measured by mercury intrusion and gravimetry, and microstructure. Due to a different concept for creating porosity between the two cements investigated in this study, a markedly different microstructural arrangement of apatite crystals was observed in the intergranular space, which was found to significantly influence both the mechanical strength and in vivo degradation of the two calcium phosphate cements.

Hierarchical surface morphology on Ti6Al4V via patterning and hydrothermal treatment towards improving cellular response

Titanium implants are widely used in load bearing applications. Failure analysis for several decades revealed poor interfacial bonding at implant-tissue interface is the major cause. Diverse morphological and chemical modifications are carried out to overcome to this issue. In this study, we developed a simple, user friendly combinatorial approach of embossing followed by hydrothermal treatment for generating nano/micro/macro hierarchical architecture on Ti6Al4V surface with deposition of calcium phosphate minerals for improving osseointegration property. The hydrothermally treated groove/pillar patterned sample had uniformly distributed nanorods with 65 ± 3 nm diameter, surface roughness ~118 nm and ~532 nm height, respectively. Presence of phosphate and calcium were confirmed through EDS and XRD with Ca/P ratio ~1.66 which is similar to apatite like deposition. Calcium phosphate apatite formation was also confirmed on the surface by XPS. The hydrothermal treated samples demonstrated improved hMSC cell adhesion, migration, proliferation and differentiation as compared to untreated ones. Therefore, this method has potential to minimize interfacial failure through morphological fixation via tissue ingrowth as well as ongrowth.

Bioactive coating on a new Mg-2Zn-0.5Nd alloy: modulation of degradation rate and cellular response

ABSTRACTPure magnesium implants have a problem of faster degradation which leads to the collapse of the implant before the recovery of physiological organ. A new magnesium alloy viz Mg-2Zn-0.5Nd (ZN20) is designed using a well-known alloying method to decrease the degradation rate of pure magnesium metal and its implants so as to match the tissue recovery rate. MTT assay of the ZN20 alloy via a direct contact method for 2, 4 and 6 days using MC3T3-E1 cells showed cytotoxicity. Whereas, the MTT assay results of this alloy via ion-extract method showed non-cytotoxic behaviour. The degradation rate of ZN20 alloy in SBF was ~2.5% in 16 days which is comparable to the rate of regeneration and recovery of physiological tissue. The pH of the outer solution after degradation of ZN20 alloy for 3 days was around 9.31 ± 0.05 which was higher than the initial pH of SBF used (pH 7.4). The reason for the cytotoxic behaviour of this alloy could be the increase in pH of immersion solution with the degradation of the alloy. Interestingly, the scanning electron microscope images followed by EDX spectrum of ZN20 alloy showed deposition of white crystalline mineralisation product (apatite) after 3, 7, 11 and 16 days of incubation in SBF at 37°C. The FT-IR spectrum and X-ray chromatogram of the white substance confirmed the deposition of apatite (calcium phosphate) on their surfaces. These results concluded the bioactivity of ZN20 alloy and thus could be used as an implant material for regeneration of bones/tissues.

Setting Properties of Calcium Phosphate Bone Cement

Calcium phosphate cements (CPCs) are widely used in bone repair. CPCs show many advantages comparing to other calcium phosphate-based materials because of its chemical similarity to the mineral constituent of natural bone. This work describes a new apatitic calcium phosphate cement composition with a Ca/P ratio = 1.38, by mixing two components which were prepared by wet chemical synthesis methods. One component is chemically synthesized by mixing α-tricalcium phosphate (αTCP, Ca3(PO4)2) and HA (Ca10(PO4)6OH)2), while the second component is again a chemically synthesized single phase DCPD (CaHPO4.2H2O) powder. The sitting solution used is 3 wt% Na2HPO4. 2H2O dissolved in distilled water with a liquid to powder ratio L/P = 0.4. The cement was characterized in terms of setting time, compressive strength. XRD and SEM techniques were employed to evaluate the phase composition and surface morphology of the cement, the powder XRD data shows that the cement sample soaked in water has transformed into calcium deficient hydroxyapatite. The cement showed a fast setting time (14 min), this cement also showed a high compressive strength (35 MPa).

Redescription and molecular analysis of Neoechinorhynchus (Neoechinorhynchus) johnii Yamaguti, 1939 (Acanthocephala, Neoechinorhynchidae) from the Pacific Ocean off Vietnam.

Neoechinorhynchus (Neoechinorhynchus) johnii Yamaguti, 1939 is redescribed from Eleutheronema tetradactylum (Polynemidae), Johnius carouna (Sciaenidae), Johnius sp., and Otolithes ruber (Sciaenidae) along the north and south coasts of Vietnam. Our description completes missing and inadequate information in the original descriptions and line drawings from Johnius goma in Japan and from Pseudosciaena diacanthus in the Indian Ocean. We add new information documented by scanning electron microscopy (SEM) and photomicroscopy, and explore the wide morphological diversity attributed to host species. The redescription includes: worms cylindrical with round proboscis with prominent apical organ, and large anterior hooks distant from small middle and posterior hooks; neck longer than the proboscis, nucleated lemnisci subequal, and receptacle with large basal triangulate cephalic ganglion and attached para-receptacle structure (PRS); male reproductive system in posterior half of trunk; adult females with introvert genital vestibule; and eggs spherical or rectangular. Gallium cuts and X-ray scans of hooks show high concentrations of sulfur on edge layer aiding in forming hardened calcium phosphate apatite of that layer with calcium and phosphorus in higher concentration in central part of hook. Molecular results consistently yielded a strongly supported distinct clade for the Neoechinorhynchus species from Vietnam for both 18S gene and the ITS1-5.8S-ITS2 region of ribosomal RNA. Phylogenetic analysis demonstrated that N. johnii occupies a separate position in the trees, probably indicating an Asian origin of this species.

Physicochemical Characteristics of Nanocalcium Extract from Bones of Yellowfin Tuna (Thunnus Albacares)

Fish bone calcium is known to have good bioavailability and can be used as a natural calcium source. This study was aimed to compare the physicochemical characteristics of nano-calcium from yellowfin tuna (<em>Thunnus albacares</em>) obtained by three different extraction methods: milling, acid and alkaline. The research was divided into three stages: preparation of bone powder, production ofnano-calcium from fish bone and characterization of the physicochemical properties of bone powder and nano-calcium. Acid extracted nano-calcium was found to have better properties as compared to that extracted by milling and alkaline as indicated by the degree of whiteness 92.61%, moisture content 0.33%, ash 99.03%, protein 0.19%, fat 0.22%, particle size 259 nm, calcium content 83.25% and phosphorus 9.65%. The Fourier Transform Infrared (FTIR) spectra profiles indicated the presence of phosphate group (PO43-) suggesting the nano-calcium was in the form of calcium phosphate apatite.

Push-Out Bond Strength of Experimental Apatite Calcium Phosphate Based Coated Gutta-Percha.

Objectives To evaluate the push-out bond strength of experimental apatite calcium phosphate coated gutta-percha (HAGP) compared to different commercially available coated gutta-percha root obturation points. Methods Extracted teeth were selected and instrumented using ProTaper rotary files. The canals were assigned into five equal groups and obturated using matching single cone technique as follows: EndoREZ cones and EndoREZ sealer, Bioceramic Endosequence gutta-percha (BCGP) with Endosequence BC sealer, Active GP with Endosequence BC sealer (ActiV GP), conventional GP with Endosequence BC sealer, and HAGP with Endosequence BC sealer. Each root was sectioned transversally at the thickness of 1±0.1 mm to obtain 5 sections (n=25 per group). The specimens were subjected to push-out test using a Universal Test Machine at a loading speed of 0.5 mm/ min. Failure modes after push-out test was examined under stereomicroscope and the push-out data were analyzed using ANOVA and the post hoc Dunnett T3 test (p = 0.05). Results The highest mean bond strength was yielded by HAGP followed by BCGP, ActiV GP, conventional GP, and EndoREZ. There were significant differences between EndoREZ and all other groups (p<0.001). The prominent failure mode of HAGP was mixed mode, whereas EndoREZ exhibited adhesive failure mode. Conventional GP, ActiV GP, and BCGP showed cohesive failure mode. Conclusion HAGP showed promising results to be used as root canal filling material in combination with bioceramic sealer.

Nanocrystalline apatites: The fundamental role of water

Bone is a natural nanocomposite. Its mineral component is nanocrystalline calcium phosphate apatite, whose synthetic biomimetic analogs can be prepared by wet chemistry. The initially formed crystals, whether biological or synthetic, exhibit very peculiar physicochemical features. In particular, they are nanocrystalline, nonstoichiometric, and hydrated. The surface of the nanocrystals is covered by a non-apatitic hydrated layer containing mobile ions, which may explain their exceptional surface reactivity. For their precipitation in vivo or in vitro, for their evolution in solution, for the 3D organization of the nanocrystals, and for their consolidation to obtain bulk ceramic materials, water appears to be a central component that has not received much attention. In this mini-review, we explore these key roles of water on the basis of physicochemical and thermodynamic data obtained by complementary tools including FTIR, XRD, ion titrations, oxide melt solution calorimetry, and cryo-FEG-SEM. We also report new data obtained by DSC, aiming to explore the types of water molecules associated with the nanocrystals. These data support the existence of two main types of water molecules associated with the nanocrystals, with different characteristics and probably different roles and functions. These findings improve our understanding of the behavior of bioinspired apatite-based systems for biomedi- cine and also of biomineralization processes taking place in vivo, at present and in the geologic past. This paper is thus intended to give an overview of the specificities of apatite nanocrystals and their close relationship with water.

Influence of the Ca/P ratio and cooling rate on the allotropic α↔β-tricalcium phosphate phase transformations

Alpha-tricalcium phosphate (α-TCP) is a component of particular interest in the formulation of apatitic calcium phosphate based bone cements. During the setting reaction of this type of cements, a calcium deficient hydroxyapatite with composition similar to bone hydroxyapatite is formed. But the purity levels of the commercially available products based on α-TCP are generally low, and their prices are prohibitive. Such stringent conditions are pushing most of the researchers in the area of bone cements towards preparing α-TCP in their own laboratories. The low degree of purity of α-TCP based products is generally due to an incomplete β-TCP → α-TCP phase transformation and/or to a partial α-TCP → β-TCP reversal during cooling. Despite extensive research efforts, contradictory reports exist about the effects of composition of the starting material, cooling rates and the importance of quenching for maintaining α-TCP purity. The present work aims at shedding further light on this issue by controlling the relevant parameters of wet synthesis of the starting β-TCP powders and their heat treatment schedule in order to transform them in high purity α-TCP products. The key experimental variables investigated included the composition of the starting β-TCP powders, namely the Ca/P ratio, the maximum heat treatment temperatures, and the cooling rates.

Interfacial Mineral Fusion and Tubule Entanglement as a Means to Harden a Bone Augmentation Material.

A new bone augmenting material is reported, which is formed from calcium-loaded hydrogel-based spheres. On immersion of these spheres in a physiological medium, they become surrounded with a sheath of precipitate, which ruptures due to a build-up in osmotic pressure. This results in the formation of mineral tubes that protrude from the sphere surface. When brought into close contact with one another, these spheres become fused through the entanglement and subsequent interstitial mineralization of the mineral tubules. The tubular calcium phosphate induces the expression of osteogenic genes (runt-related transcription factor 2 (RUNX2), transcription factor SP7 (SP7), collagen type 1 alpha 1 (COL1A1), and bone gamma-carboxyglutamic acid-containing protein (BGLAP)) and promotes the formation of mineral nodules in preosteoblast cultures comparable to an apatitic calcium phosphate phase. Furthermore, alkaline phosphatase (ALP) is significantly upregulated in the presence of tubular materials after 10 d in culture compared with control groups (p < 0.001) and sintered apatite (p < 0.05). This is the first report of a bioceramic material that is formed in its entirety in situ and is therefore likely to provide a better proxy for biological mineral than other existing synthetic alternatives to bone grafts.

Design and properties of a novel radiopaque injectable apatitic calcium phosphate cement, suitable for image-guided implantation.

An injectable purely apatitic calcium phosphate cement (CPC) was successfully combined to a water-soluble radiopaque agent (i.e., Xenetix® ), to result in an optimized composition that was found to be as satisfactory as poly(methyl methacrylate) (PMMA) formulations used for vertebroplasty, in terms of radiopacity, texture and injectability. For that purpose, the Xenetix dosage in the cement paste was optimized by injection of the radiopaque CPC in human cadaveric vertebrae under classical PMMA vertebroplasty conditions, performed by interventional radiologists familiar with this surgical procedure. When present in the cement paste up to 70 mg I mL-1 , Xenetix did not influence the injectability, cohesion, and setting time of the resulting composite. After hardening of the material, the same observation was made regarding the microstructure, mechanical strength and alpha-tricalcium phosphate to calcium deficient apatite transformation rate. Upon implantation in bone in a small animal model (rat), the biocompatibility of the Xenetix-containing CPC was evidenced. Moreover, an almost quantitative release of the contrast agent was found to occur rapidly, on the basis of in vitro static and dynamic quantitative studies simulating in vivo implantation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2786-2795, 2018.

Standing transcutaneous surgical excision of a sialolith in an 11‐year‐old Thoroughbred mare

SummaryAn 11‐year‐old Thoroughbred mare was diagnosed with sialolithiasis of the right parotid salivary duct. A firm nonpainful subcutaneous mass was palpable in the right maxillary region adjacent to premolar 4 (tooth 108). Radiographic and ultrasonographic examinations identified a discrete mineralised ovoid mass that was hyperechoic and produced an acoustic shadow. Surgical excision of the mass was performed under standing sedation using a transcutaneous approach and the excised sialolith submitted for histopathological and mineral composition analyses. Histological examination found no evidence of a nidus at the sialolith's core. Mineral analysis of the sialolith revealed its composition to be 40% calcium phosphate (apatite) and 60% calcium carbonate. The mare recovered with no post‐operative complications and was clinically unremarkable 2 years later.

Composition of urinary calculi in infants: a report from an endemic country.

Pediatric urolithiasis remains endemic in low-resource countries where infants constitute 17-40% of all children with urolithiasis. This study reports socio-economic factors, medical history and chemical composition of urinary stones in 976 infants of up to 2years of age. Between 1992 and 2016, 976 infants presented to our institute with 1038 stones. Chemical composition of stones was analyzed by Fourier transformation infrared spectroscopy (FTIR). The mean age of infants was 19.5±5.74months with a M:F ratio of 5.5:1. Half (50%) of the infants were rural dwellers, 90% belonged to low socio-economic class and 70% were malnourished. A history of chronic diarrhoea was reported in 13% and urinary tract infections in 5%. The anatomic location was bladder in 487 (46.9%), kidney in 246 (23.6%), ureter in 142 (13.6%), urethra in 2 (0.2%) and 161 (15.5%) were passed spontaneously. Overall frequency of compounds in stones showed ammonium acid urate (AAU) in 772 (74.37%), calcium oxalate (CaOx) in 410 (39.5%). Uric acid (UA) in 119 (11.46%), calcium phosphate apatite (CaP) in 96 (9.25%), magnesium ammonium phosphate (Struvite) in 45 (4.34%), cystine in 12 (1.16%) and xanthine in 40 (3.85%). Frequency of compounds was similar in genders. Infants of age 0-6months had higher frequency of UA (28%), CaOx (50%) and low AAU (61%) as compared to 11, 39 and 75%, respectively, in 7-24months (p<0.049) (p<0.002) (p<0.001). Nucleus of stones showed pure AAU in 63 and 43% on the surface due to addition of CaOx, struvite and CaP. Our study has shown that AAU is a major component of stones in infants where the main risk factors are poverty, malnutrition, diarrheal diseases and dehydration.

A straightforward approach to enhance the textural, mechanical and biological properties of injectable calcium phosphate apatitic cements (CPCs): CPC/blood composites, a comprehensive study

A major challenge in the design of bone graft substitutes is the development of injectable, cohesive, resorbable and self-setting calcium phosphate cement (CPC) that enables rapid cell invasion with initial mechanical properties as close as bone ones. Thus, we describe specific conditions in CPC-blood composites where the formation of a 3D clot-like network can interact with the precipitated apatite crystals formed during the cement setting process. The resulting microstructure appears more ductile at short-term and more sensitive to biological degradation which finally promotes new bone formation. This important and original paper reports the design and in-depth chemical and physical characterization of this groundbreaking technology.

Free DNA precipitates calcium phosphate apatite crystals in the arterial wall in vivo

Background and aimsThe arterial wall calcium score and circulating free DNA levels are now used in clinical practice as biomarkers of cardiovascular risk. Calcium phosphate apatite retention in the arterial wall necessitates precipitation on an anionic platform. Here, we explore the role of tissue-free DNA as such a platform. MethodsThe first step consisted of histological observation of samples from human and rat calcified arteries. Various stains were used to evaluate colocalization of free DNA with calcified tissue (alizarin red, fluorescent Hoechst, DNA immunostaining and TUNEL assay). Sections were treated by EDTA to reveal calcification background. Secondly, a rat model of vascular calcifications induced by intra-aortic infusions of free DNA and elastase + free DNA was developed. Rat aortas underwent a micro-CT for calcium score calculation at 3 weeks. Rat and human calcifications were qualitatively characterized using μFourier Transform Infrared Spectroscopy (μFTIR) and Field Emission-Scanning Electron Microscopy (FE-SEM). ResultsOur histological study shows colocalization of calcified arterial plaques with free DNA. In the intra-aortic infusion model, free DNA was able to penetrate into the arterial wall and induce calcifications whereas no microscopic calcification was seen in control aortas. The calcification score in the elastase + free DNA group was significantly higher than in the control groups. Qualitative evaluation with μFTIR and FE-SEM demonstrated typical calcium phosphate retention in human and rat arterial specimens. ConclusionsThis translational study demonstrates that free DNA could be involved in arterial calcification formation by precipitating calcium phosphate apatite crystals in the vessel wall.

TREATMENT OF MUNICIPAL WASTEWATER WITH CALCIUM PHOSPHATE: A NEW PHYSICOCHEMICAL PURIFICATION STEP

An apatitic calcium phosphate gel (HaG) was elaborated and used to treat municipal wastewater in a physicochemical cycle prior to the standard biological degradation process. Preliminary laboratory batch experiments outlined the kinetics of heavy metal sorption by monitoring the concentrations found in filtered fractions as a function of treatment time. On site experiments were performed in a municipal wastewater treatment plant by derivation of real wastewater into an experimental tank. In this case, the sorption of organic and metal pollution was studied on the whole effluent. The solid and liquid phases were both characterized using elemental analysis, total and organic carbon, and thermal analysis of the solids collected. Results showed significant reductions in heavy metal contents of about 50 to 60% for main metals such as lead, aluminum and iron in the liquid phase. The concentrations went respectively from initial values of 0.07, 0.15 and 1.80 ppm to 0.04, 0.11 and 0.03 ppm in the treated waters for these elements. Organic matter also reacted with the HaG by adsorbing to the solid particles, with the contents of organic carbon increasing from zero to 80 g/Kg on the solids and decreasing from 130 ppm to 24 ppm in the liquid phase. This allows foreseeing improvements in the biological treatment process following this physicochemical purification step. The elemental analysis showed that up to 50 and 90 percent of the heavy metals could be removed from the effluent. The presence of the organic matter adsorbed on calcium phosphate was confirmed by thermal methods.

Solid solution in the apatite OH-Cl binary system: Compositional dependence of solid-solution mechanisms in calcium phosphate apatites along the Cl-OH binary

The method of accommodation of solid solution along the OH-Cl binary in calcium phosphate apatites is not fully understood; because of steric constraints in mixtures of OH and Cl anions in the apatite [0,0, z ] anion column, the positions of OH and Cl anions in the pure hydroxylapatite and chlorapatite end-members cannot coexist in the binary anion column. We have undertaken high-precision single-crystal X-ray structure studies of eight synthetic samples along the OH-Cl apatite binary ( R 1 ≈ 0.0159). We found that for all samples solid solution is attainable in space group P 6 3 / m , but the particular method of solid solution depends on composition. For samples with Cl > OH, three column anion sites (two for Cl, one for OH) provide allowable bond distances with the Ca2 atoms and allow a sequence of column anions that provides sufficient anion-anion distances and also effects reversal of the sense of ordering of the column anions relative to the mirror planes at z = ¼ and ¾. In a sample with OH > Cl, three sites exist in the anion column that also provide allowable bond distances to the triangle of Ca2 atoms or its disordered Ca2′ equivalent, and afford a sequence of atoms that permits reversal of the anion column and maintenance of P 6 3 / m symmetry. One of those sites is occupied by OH and provides acceptable Ca2-OH distances, and another accommodates Cl with ideal Ca2-Cl distances. A third column anion site is unique among the calcium phosphate apatites. That site, termed the ClOH site, accommodates both OH and Cl. The site has an ideal bond distance for OH to the Ca2 atoms in the Ca2 triangle and also has an ideal bond distance for a Cl occupant to disordered Ca2′ atoms; thus, because of the disordering of the Ca2-Ca2′ atoms, a single site can accommodate either anion with ideal, but disparate, bond distances to Ca. Finally, in OH-Cl apatites with OH ≈ Cl, also crystallizing in space group P 6 3 / m , four anion positions are occupied in the anion column, including the ClOH site that allows occupancy by both OH and Cl. In addition to that site and distinct OH and Cl sites, OH is found to occupy the site within the mirror plane at (0,0,¼), the site occupied by F in F-bearing apatite. Occupancy of that site is essential to reversing the sense of ordering of the anion column relative to the mirror planes and preserving P 6 3 / m symmetry. Thus, the methods of effecting solid solution along the OH-Cl are composition dependent and complex.