Doan Pham
email: doan.phamminh@mines-albi.fr

Sébastien Rio
email: sebastien.rio@mines-albi.fr

Patrick Sharrock
email: patrick.sharrock@iut-tlse3.fr

Haroun Sebei
email: hsebei@hotmail.fr

Nathalie Lyczko
email: lyczko@mines-albi.fr

Ngoc Dung Tran
email: ngoc-dung.tran@mines-albi.fr

Mohamed Raii
email: mohamed.raii@yahoo.fr

Ange Nzihou
email: ange.nzihou@mines-albi.fr

Minh • Se ´bastien Rio

Ngoc Dung

Á S Rio

Á P Sharrock

Á H Sebei

Á N Lyczko

Á N D Tran

Á M Raii

Á A Nzihou

Hydroxyapatite starting from calcium carbonate and orthophosphoric acid: synthesis, characterization, and applications

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ], labeled thereafter Ca-HA, is well known as a multi-function/multi-utilization material in different fields including biomaterials and food industry [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF][START_REF] Morgan | Preparation and characterisation of monoclinic hydroxyapatite and its precipitated carbonate apatite intermediate[END_REF][START_REF] Murakami | Hydrothermal synthesis of porous hydroxyapatite ceramics composed of rod-shaped particles and evaluation of their fracture behavior[END_REF][4]. As the main component of animal bone and defects, Ca-HA-based materials are largely used for reconstruction or replacement of human bone defects [START_REF] Morgan | Preparation and characterisation of monoclinic hydroxyapatite and its precipitated carbonate apatite intermediate[END_REF][START_REF] Rodriguez-Lorenzo | Fabrication of hydroxyapatite bodies by uniaxial pressing from a precipitated powder[END_REF]. Ca-HA is also widely used as nutrient supplements in drinks, meats, or animal foods [4]. During the last years, research has been strongly focused on the use of Ca-HA in the fields of heterogeneous catalysis and toxic metal fixation. Catalytic use of Ca-HA relates to its intrinsic acidbasic properties, possible high-specific surface area, thermal stability, and capacity to support different active phases including metals, metal salts, and oxides etc. [START_REF] Matsumura | Catalytic oxidative coupling of methane over hydroxyapatite modified with lead[END_REF][START_REF] Tsuchida | Reaction of over hydroxyapatite affected by Ca/P ratio of catalyst[END_REF][START_REF] Verwilghen | Preparation of high specific surface area hydroxyapatite for environmental applications[END_REF][START_REF] Venugopal | Hydroxyapatite as a novel support for gold and ruthenium catalysts behaviour in the water gas shift reaction[END_REF][START_REF] Kanai | Selective photoepoxidation of propylene over hydroxyapatite-silica composites[END_REF]. The performance of Ca-HA for the removal of toxic metal from an aqueous solution has been also widely demonstrated at laboratory scale. Ca-HA has strong affinity for the fixation of different metals including lead, cadmium, copper, zinc, strontium, mercury, cobalt, etc. [START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: equilibria and kinetic processes[END_REF][START_REF] Fernane | Sorption of cadmium and copper ions on natural and synthetic hydroxylapatite particles[END_REF][START_REF] Nzihou | Role of phosphate in the remediation and reuse of heavy metal polluted wastes and sites[END_REF][14][START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF][START_REF] Sharrock | Wastewater sorption on HA: old recipes for new tastes[END_REF]. However, to the best of our knowledge, there is no application of Ca-HA at industrial scale for the treatment of wastewaters, despite a large number of laboratory reports with very promising results. In fact, for the viability of a given process, in particular for environmental purposes, costs must be minimized.

Various processes have been developed for the synthesis of Ca-HA. We now review the principal methods for Ca-HA synthesis, prior to communicating our synthesis strategy for obtaining Ca-HA from low-cost starting materials.

Double decomposition is the most common method for the synthesis of Ca-HA. It consists in the decomposition of both calcium and phosphate sources, for example Ca(NO 3 ) 2 and (NH 4 ) 2 HPO 4 [START_REF] Jarcho | Hydroxylapatite synthesis and characterization in dense polycrystalline form[END_REF] or NH 4 H 2 PO 4 [START_REF] Tsuchida | Reaction of over hydroxyapatite affected by Ca/P ratio of catalyst[END_REF], in an aqueous solution to form Ca-HA precipitate. Ammonium hydroxide is usually used for the control of pH. At room temperature, the reaction time was found to be crucial for the formation of a pure apatitic phase, as described previously [START_REF] Hayek | Pentacalcium monohydroxyorthophosphate (Hydroxylapatite)[END_REF][START_REF] Eanes | Intermediate states in the precipitation of hydroxyapatite[END_REF]. Ca-HA of high-specific surface area (up to about 150 m 2 g -1 ) was obtained [START_REF] Tsuchida | Reaction of over hydroxyapatite affected by Ca/P ratio of catalyst[END_REF]. But the product needs to be filtered and washed several times to eliminate all counter-ions, i.e., NO [START_REF] Murakami | Hydrothermal synthesis of porous hydroxyapatite ceramics composed of rod-shaped particles and evaluation of their fracture behavior[END_REF] -and NH 4 ? [START_REF] Jarcho | Hydroxylapatite synthesis and characterization in dense polycrystalline form[END_REF]. This step is usually arduous since fine Ca-HA particles are formed by precipitation. Considering the theoretical equation for the formation of Ca-HA from Ca(NO 3 ) 2 and NH 4 H 2 PO 4 [START_REF] Verwilghen | Preparation of high specific surface area hydroxyapatite for environmental applications[END_REF], (NH 4 ) 2 HPO 4 [START_REF] Jarcho | Hydroxylapatite synthesis and characterization in dense polycrystalline form[END_REF], or (NH 4 ) 3 PO 4 [START_REF] Hayek | Pentacalcium monohydroxyorthophosphate (Hydroxylapatite)[END_REF], the quantity of byproducts generated by this way is higher than the mass of Ca-HA (Eqs. [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF][START_REF] Morgan | Preparation and characterisation of monoclinic hydroxyapatite and its precipitated carbonate apatite intermediate[END_REF][START_REF] Murakami | Hydrothermal synthesis of porous hydroxyapatite ceramics composed of rod-shaped particles and evaluation of their fracture behavior[END_REF]. The difficulty of filtering and washing steps, the generation of by-products, and the high costs of the starting materials are the main drawbacks of this process.

10Ca NO 3 ð Þ 2 þ 6NH 4 H 2 PO 4 þ 14NH 4 OH ! Ca 10 PO 4 ð Þ 6 OH ð Þ 2 þ 20NH 4 NO 3 þ 12H 2 O ð1Þ
10Ca NO 3 ð Þ 2 þ 6 NH 4 ð Þ 2 HPO 4 þ 8NH 4 OH ! Ca 10 PO 4 ð Þ 6 OH ð Þ 2 þ 20NH 4 NO 3 þ 6H 2 O ð2Þ
10Ca NO 3 ð Þ 2 þ 6 NH 4 ð Þ 3 PO 4 þ 2NH 4 OH ! Ca 10 PO 4 ð Þ 6 OH ð Þ 2 þ 20NH 4 NO 3 ð3Þ
Sol-gel technique is also commonly used for the synthesis of a large number of solid materials, including Ca-HA [START_REF] Bezzi | A novel sol-gel technique for hydroxyapatite preparation[END_REF][START_REF] Anee | A novel technique to synthesize hydroxyapatite at low temperature[END_REF][START_REF] Tredwin | Hydroxyapatite, fluor-hydroxyapatite and fluorapatite produced via the sol-gel method. Optimisation, characterisation and rheology[END_REF][START_REF] Bakan | A novel low temperature solgel synthesis process for thermally stable nano crystalline hydroxyapatite[END_REF]. Bezzi et al. [START_REF] Bezzi | A novel sol-gel technique for hydroxyapatite preparation[END_REF], Anee et al. [START_REF] Anee | A novel technique to synthesize hydroxyapatite at low temperature[END_REF], and Bakan et al. [START_REF] Bakan | A novel low temperature solgel synthesis process for thermally stable nano crystalline hydroxyapatite[END_REF] detailed the synthesis of Ca-HA by this method starting from Ca(NO 3 ) 2 , (NH 4 ) 2 HPO 4 , and NH 4 H 2 PO 4 as starting reactants. Ammonium donor agents such as urea and ammonium hydroxide, gelling agents such as ethylenediamminetetraacetic acid (EDTA), and organic solvent such as ethanol were also used in this method. However, this approach calls for a multi-step synthesis, as illustrated in the flow sheet by Bezzi et al. [START_REF] Bezzi | A novel sol-gel technique for hydroxyapatite preparation[END_REF], with the generation of a very large quantity of by-products. For example, the production of 100 g of Ca-HA required 260 g of Ca(NO 3 ) 2 Á4H 2 O, 80 g of (NH 4 ) 2 HPO 4 , 360 g of EDTA, and 90 g of urea [START_REF] Bezzi | A novel sol-gel technique for hydroxyapatite preparation[END_REF]. This does not conform to the actual tendency of green chemistry.

Ca-HA nanoparticles with controlled morphologies could be obtained by reverse microemulsion technique using Ca(NO 3 ) 2 and H 3 PO 4 as starting materials and different organic solvents and surfactants [START_REF] Saha | Synthesis of nanocrystalline hydroxyapatite using surfactant template systems: role of templates in controlling morphology[END_REF]. These last ones are water-immiscible and form a reverse micelle with the aqueous phase. Ca-HA with different morphologies and specific surface areas could be obtained under the effect of various parameters such as ratios of inorganic cation to anion, ratios of water to oil, the nature of surfactant, etc. However, this procedure is not environmentally friendly because all organic solvents and surfactants were evaporated and burned for the recovery of Ca-HA powder. Gopi et al. [START_REF] Gopi | Synthesis of hydroxyapatite nanoparticles by a novel ultrasonic assisted with mixed hollow sphere template method[END_REF] investigated the Ca-HA nanoparticles in an aqueous solution using glycine-acrylic acid as templates. Ca-HA particles of about one hundred nm were formed but a large amount of templates was required (20 mol of templates for 5 mol of Ca(NO 3 ) 2 Á4H 2 O and 3 mol of (NH 4 ) 2 HPO 4 ) and which were also found as by-products of the synthesis process after filtration and washing steps.

Mechanochemical route via solid-solid reaction was also described for the synthesis of Ca-HA particles [START_REF] Silva | Structural properties of hydroxyapatite obtained by mechanosynthesis[END_REF][START_REF] Nasiri-Tabrizi | A comparative study of hydroxyapatite nanostructures produced under different milling conditions and thermal treatment of bovine bone[END_REF]. It consists in the milling of a solid mixture of calcium and phosphate sources. This technique can prevent the use of a solvent. However, the reaction may be incomplete despite long reaction times under rigorous conditions (high-rotation rate, high-weight ratio of milling balls to solid reactants) [START_REF] Silva | Structural properties of hydroxyapatite obtained by mechanosynthesis[END_REF].

In order to reduce or eliminate the generation of byproducts, a synthesis process using calcium hydroxide and orthophosphoric acid as starting materials was developed [START_REF] Tsuchida | Reaction of over hydroxyapatite affected by Ca/P ratio of catalyst[END_REF][START_REF] Conn | Process for producing hydroxyapatite[END_REF][START_REF] Lazic | The effect of temperature on the properties of hydroxyapatite precipitated from calcium hydroxide and phosphoric acid[END_REF]. In general, calcium hydroxide suspension could be progressively neutralized by orthophosphoric acid. In fact, these two reactants contain only H ? and OH -as counter-ions leading to the absence of any by-product other than water (Eq. 4). So, no further purification step is required for this process. Ca-HA of food and pharmaceutical grade is actually produced by this way at the industrial scale [4,[START_REF] Conn | Process for producing hydroxyapatite[END_REF].

10Ca OH ð Þ 2 þ 6H 3 PO 4 ! Ca 10 PO 4 ð Þ 6 OH ð Þ 2 þ 8H 2 O ð4Þ
According to Eq. (4), the use of Ca(OH) 2 and H 3 PO 4 should generate Ca-HA and avoid the generation of byproducts. But the energetic and economic balance of the process can still be improved. Indeed, Ca(OH) 2 is obtained by calcination followed by hydration of CaCO 3 . This last one is abundantly available in nature as the cheapest calcium source. From this point of view, we focused our latest research on the use of CaCO 3 and H 3 PO 4 as starting reactants for the synthesis of Ca-HA under moderate conditions. The only by-product of the reaction is carbonic gas, which automatically leaves the reaction mixture. The expected reaction is written as follows:

10CaCO 3 þ 6H 3 PO 4 ! Ca 10 PO 4 ð Þ 6 OH ð Þ 2 þ10CO 2 þ 8H 2 O ð5Þ
In this paper, we present the one-step synthesis of Ca-HA from CaCO 3 and H 3 PO 4 . The reactivity of the resulting Ca-HA in different applications is also described, in particular for the treatment of synthetic and real wastewaters containing toxic metals and organic pollutants.

Materials and methods

Chemical products

Calcium carbonate powder (98 %, Fisher Scientific) and orthophosphoric acid (85 wt% in water, Merck) were used as received. All other chemical products such as lead(II) nitrate, lead(II) chloride, etc. were also purchased from commercial suppliers including Fisher Scientific, Merck, Labosi, and Prolabo.

Synthesis of Ca-HA at atmospheric pressure

This synthesis was carried out in an open glass U-form reactor (i.d.: 10 cm, length: 30 cm, volume: 2.3 L) with a vertical stainless-steel stirrer. For a given synthesis, 800 mL of water and 200 g of CaCO 3 (2 mol) were introduced in the reactor under stirring (400 rpm). When the pH of the suspension was stable, 80 mL of H 3 PO 4 (1.2 mol) was pumped into the suspension of CaCO 3 at the rate of 2 mL min -1 . The reaction was kept at constant temperature (80 °C) for different reaction times. Aliquots withdrawn from the reaction mixture were filtered on a 0.45 lm filter paper to separate liquid and solid phases at different reaction times. The solid was washed and dried overnight prior to further analyses and characterizations. The liquid phase was acidified with nitric acid to avoid any further precipitation from calcium cations and orthophosphate species present in the liquid phase.

Physico-chemical characterizations X-ray diffraction (XRD) of powder products was carried out on a Phillips Panalytical X'pert Pro MPD diffractometer.

Simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC) analysis was carried out in a TA Instruments SDTQ600 analyzer with a heating rate of 5 °C min -1 .

Scanning electron microscopy coupled with energydispersive X-ray spectroscopy (SEM-EDX) measurement was performed on a Philips XL30 ESEM apparatus.

Infra-red (IR) spectroscopy measurement was carried out on a Shimadzu FTIR 8400S spectrometer.

The rheological behavior of Ca-HA gel was investigated using a 135 Rheostress (HAAKE RS 150 rheometer). Dynamic viscosity was measured in the shear rate range of 0-500 s -1 at 20 °C.

The elemental analysis of phosphorus and calcium in the liquid phase was carried out with inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a HORIBA Jobin-Yvon Ultima 2.

Results

Ca-HA synthesis

The initial suspension of CaCO 3 had a basic pH of 8.8 at 80 °C. When the first drops of H 3 PO 4 were added, carbonic gas was strongly emitted from the reaction mixture. The pH of the suspension decreased rapidly to about 4. Then, it increased slowly to about 6.5 after 24 h of reaction at 80 °C. After cooling to room temperature, a stable gel containing fine particles (SEM analysis section) was obtained. This gel contained about 70 wt% of water, determined by weight loss measurement at 105 °C. As shown in Fig. 1, the dynamic viscosity of the gel decreased with the increase of shear rate, signifying that this gel behaved as a thixotropic fluid. The critical value of the dynamic viscosity (g ? ) reached 0.14 Pa s when the shear rate was at 500 s -1 . Above this point, the liquid flowed. When the shear stress was stopped, the liquid recovered its structure.

Elemental analysis of the filtrate (liquid phase) by ICP-AES shows that soluble calcium and phosphate species were present at very low concentration, which were all smaller than 1 mmol L -1 . This means that all initial calcium and phosphate were transformed and/or remained in the solid state. The analysis of the solids obtained at 3 and 24 h of reaction shows also that the molar ratio of Ca to P was close to 1.67, which was the initial value of the reactant mixture introduced into the reactor.

TG analyses of the solid samples withdrawn at different reaction times from the reactor are present in Fig. 2. The first weight loss below 100 °C corresponded to the evaporation of surface moisture which was similar for both solids obtained at 3 and 24 h of reaction. The next weight loss at 112 °C could be attributed to the dehydration of monocalcium phosphate monohydrate (MCPM, Ca(H 2- PO 4 ) 2 Á2H 2 O) [START_REF] Elliott | Studies in inorganic chemistry 18: structure and chemistry of the apatites and other calcium orthophosphates[END_REF]. The slight next weight losses in the temperature range of 175-360 °C may be attributed to the dehydration/condensation of DCPD (dicalcium phosphate dehydrate, or brushite, CaHPO 4 Á2H 2 O) and DCPA (dicalcium phosphate anhydride, or monetite, CaHPO 4 ) [START_REF] Elliott | Studies in inorganic chemistry 18: structure and chemistry of the apatites and other calcium orthophosphates[END_REF]. Residual calcium carbonate was thermally decomposed around 610 °C, followed by the decarbonation of carbonated apatite (CAP) around 720 °C [START_REF] Elliott | Studies in inorganic chemistry 18: structure and chemistry of the apatites and other calcium orthophosphates[END_REF][START_REF] Pham Minh | Carbonate-containing apatite (CAP) synthesis under moderate conditions starting from calcium carbonate and orthophosphoric acid[END_REF]. In parallel with these apparent weight losses, a continuous weight loss was observed with the increase of the temperature up to 1310 °C, which was assigned to the partial transformation of OH -anions of Ca-HA to molecular water and bivalent oxygen. This transformation was nearly completed when the temperature reached 1310 °C and oxyapatite (OAP, Ca 10 (PO 4 ) 6 O) was formed [START_REF] Liao | Thermal decomposition and reconstitution of hydroxyapatite in air atmosphere[END_REF]. All these weight losses were endothermic (DSC results not presented).

From TG curves, the content of residual calcium carbonate could be calculated, which was 4.0 and 0.6 wt% for the solids withdrawn at 3 and 24 h of reaction, respectively. So, the reaction at 80 °C for 24 h allowed converting nearly completely the initial calcium carbonate into calcium phosphates. This temperature of 80 °C was found as the critical value, because below 80 °C high-residual quantities of calcium carbonate remained in solid products despite a long reaction time of 72 h (results not presented).

In order to identify the crystalline phase of the resulting solid calcium phosphates, XRD was performed, and Fig. 3 presents XRD patterns of the solids obtained at 3 and 24 h of reaction.

As expected, only small amounts of calcium carbonate (2h of 29.400°) remained in the solid product obtained after 3 h of reaction, and trace quantities were found in the solid product obtained after 24 h of reaction. This confirmed again the results of TG analyses above for the content of residual calcium carbonate. All other peaks corresponded to the pattern of Ca-HA, as the main crystalline calcium phosphate formed. Some traces of CAP were also detected as observed by TG analysis.

In summary, the one-step reaction of calcium carbonate powder with orthophosphoric acid in an aqueous solution (weight ratio of water to calcium carbonate equal to 4, and molar ratio of calcium to phosphorus equal to 1.67) at 80 °C for 24 h led to (i) a nearly-complete decomposition of calcium carbonate; (ii) the formation of Ca-HA as the main crystalline phase of the solid product; (iii) the formation of a stable gel containing fine Ca-HA particles; and (iv) the absence of soluble counter-ions in the Ca-HA gel. The advantages of the present synthesis process compared to the literature data are given in Table 1. No residue generated the use of Ca(OH) 2 and H 3 PO 4 as starting reactants. However, Ca(OH) 2 costs more than CaCO 3 , since Ca(OH) 2 is industrially obtained from CaCO 3 by calcination and hydration steps. We will see in the next section that the additional benefit in the use of CaCO 3 compared to Ca(OH) 2 , when synthesized Ca-HA is used for the fixation of toxic metals. Compared to other soluble calcium and orthophosphate sources, the present process has as advantages to be in one-step at moderate conditions; to generate CO 2 as the only by-product; and to use the cheapest initial reactants.

Reactivity of the as-synthesized Ca-HA in the removal of lead(II) from an aqueous solution

We were successful in the preparation of Ca-HA from CaCO 3 and H 3 PO 4 by a one-step synthesis process. Two types of products were obtained: (i) Ca-HA gel which was the as-synthesized product; and (ii) Ca-HA powder which was obtained from Ca-HA gel after classical filtering and drying steps. It is interesting now to test their reactivity/ property in some selected applications. Ca-HA gel and powder (filtered and dried overnight at 105 °C) were first tested in the removal of lead(II) as a model toxic metal. An aqueous solution containing 6000 mg L -1 of Pb 2? ions was prepared from lead nitrate and distilled water. This concentration was chosen for the evaluation of sorption capacity of the sorbents. For the lead(II) removal experiment, 300 mL of the prepared lead nitrate solution was introduced into a 700 mL glass reactor. Then 2.4 g of Ca-HA powder or an equivalent quantity of Ca-HA gel was added into the reactor at the stirring speed of 350 rpm and at ambient temperature, ca. 25 °C. The concentrations of lead, calcium, and phosphorus in solution during the run were determined using the ICP-AES technique.

Figure 4 shows the abatement of lead(II) when Ca-HA gel and Ca-HA powder were used in the same experimental conditions. Ca-HA gel shows faster lead(II) removal kinetics compared to that of Ca-HA powder. For example, at 180 min of contact, the removal of lead(II) reached 73 and 36 % for Ca-HA gel and Ca-HA powder, respectively. The filtration and drying steps may reduce partially the lability of the active species on the surface of Ca-HA particles [START_REF] Rey | Nanocrystalline apatites in biological systems: characterisation, structure and properties[END_REF][START_REF] Banu | Mise en forme d'apatites nanocristallines : ce ´ramiques et ciments[END_REF]. For both sorbents, lead(II) was nearly totally removed from the solution, at different contact times. This means that the sorption capacity (Q e ) of these two sorbents can reach at least 750 mg Pb g sorbent -1

. This Q e is much higher than those found in the literature for the sorption of lead(II) by Ca-HA synthesized from soluble calcium salt such as calcium nitrate, which did not exceed 450 mg Pb g sorbent -1 [START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: equilibria and kinetic processes[END_REF]. This higher performance of Ca-HA synthesized from CaCO 3 as calcium source can be related Multi-step, great quantity of by-products, calcinations at 1000 °C High cost of the initial reactants [START_REF] Jillavenkatesa | Sol-gel processing of hydroxyapatite[END_REF] to the insertion of carbonate anions in the apatitic structure, as shown previously by Miyake et al. [START_REF] Miyake | Synthetic carbonate apatites as inorganic cation exchangers: exchange characteristics for toxic ions[END_REF]. The replacement of a phosphate anion by a carbonate anion leads to a destabilization of the apatitic structure because of the change in charge balance. Carbonate anions can be attacked by the acidity of lead nitrate solution (initial pH of about 5). This promotes the fixation of lead(II) on Ca-HA particles. XRD analysis of the solid recovered after lead(II) removal showed the formation of lead hydroxyapatite (Pb 10 (PO 4 ) 6 (OH) 2 , JCPDS standard No. 01-087-2477) as the main lead-containing crystalline phase (results not presented). SEM observation highlighted also the fixation of lead on the surface of Ca-HA particles (Fig. 5). Ca-HA powder dried at 105 °C contained micrometric particles of various sizes (Fig. 5a). Layers of lead-containing compounds were observed on the surface of the solid recovered after lead(II) removal experiment (Fig. 5b). No evidence of lead(II) migration inside the solid particles was observed, when SEM images were focused on polished sections of particles, initially immobilized in a resin matrix (Fig. 5c). The presence of lead occurred only inside porous particles, with lead coating the inner surfaces of the pores. There was no evidence for the formation of a mixed calcium and lead phosphate or solid solution of lead and calcium phosphate.

Reactivity of the as-synthesized Ca-HA in the removal of lead(II) from a flue gas Work has been done on the fixation of several toxic metals in aqueous solutions on Ca-HA powder. However, there is practically no study on the fixation of metals in gas phase using Ca-HA-based materials. This part communicates first results on the fixation of lead(II) from a flue gas onto Ca-HA powder.

A 5 wt% PbCl 2 /CaCO 3 was prepared from commercial PbCl 2 and CaCO 3 powder by impregnation, which was then used as the source of vaporized lead(II) by heating at 600-850 °C. Detailed kinetic study on the vaporization of PbCl 2 -supported CaCO 3 was shown previously [START_REF] Rio | Heavy metal vaporization and abatement during thermal treatment of modified wastes[END_REF]. The abatement of vaporized lead(II) was carried out in a horizontal tubular quartz tube which was heated electrically at 600-850 °C (Fig. 6). When the reactor reached the desired temperature, a weighed quantity of 5 wt% PbCl 2 /CaCO 3 was rapidly pushed inside the reactor, followed by the setup of a filter at the outlet of the reactor. The filter was composed of a quartz tube, filled with Ca-HA powder as reactive phase. Then, an air flow rate was applied in order to create a flue gas containing vaporized lead(II) through the Ca-HA filter. The analysis of Pb content introduced into the reactor before the reaction and remaining after the reaction allowed to determine the vaporized lead(II) fraction. This vaporized fraction may be condensed on the reactor wall, fixed on Ca-HA filter, or trapped in the final nitric acid solution. Elemental analyses of lead(II) present in the filter and in nitric acid solution allowed to deduce the gaseous lead(II) lost elsewhere in condensed form and complete the mass balance.

Figure 7 presents the reactivity of Ca-HA powder in the removal of lead(II) from a synthetic flue gas at different temperatures. When the temperature increased, the vaporized fraction of lead(II) increased. Since the Ca-HA filter was set at the outlet of the reactor, about one-third of vaporized lead(II) was condensed. In contact with Ca-HA filter, most vaporized lead(II) was fixed on the filter and only traces of lead(II) were found in HNO 3 solution. This demonstrated that Ca-HA powder also shows remarkable affinity for vaporized lead(II) in the gas phase. Further studies will be done in order to determinate the sorption capacity of Ca-HA and to understand the mechanism of lead fixation in gas phase.

Synthesis of CAP and its bioactivity

Under atmospheric pressure, CO 2 formed from the decomposition of CaCO 3 left automatically the reaction mixture. But a small amount of the initial carbonate was incorporated in the apatitic structure of the final Ca-HA particles, as shown by TG (Fig. 2, peak at 720 °C) to form CAP. There is great interest in the synthesis of this CAP, since it is well known to be more effective than Ca-HA for bone and dental tissue reconstitution [START_REF] Rodrigues | Characterization of a bovine collagen-hydroxyapatite composite scaffold for bone tissue engineering[END_REF][START_REF] Thian | The role of electrosprayed apatite nanocrystals in guiding osteoblast behavior[END_REF].

A modification of the present synthesis process was implemented in order to increase the carbonate content in Ca-HA structure which needs to keep CO 2 in contact with the reaction mixture. The objective was to obtain Ca-HA containing at least 4 wt% of carbonate in its molecular structure. This carbonate content is the classical value in the mineral fraction of human bone. For this synthesis, 10 g CaCO 3 and 45 mL H 2 O were initially introduced in a 250 mL close stainless-steel reactor. After heating to 80 °C, 6.9 g of H 3 PO 4 was quickly injected into the reactor by an injection valve and the reaction started by adjustment of the stirring rate at 800 rpm. The final pressure in the reactor was found to be about 13 bar, due to the formation of CO 2 and the water vapor pressure at 80 °C in the closed reactor. After 48 h of reaction, the powder was recovered by filtration and drying steps. Details of the synthesis process were reported [START_REF] Pham Minh | Carbonate-containing apatite (CAP) synthesis under moderate conditions starting from calcium carbonate and orthophosphoric acid[END_REF].

Figure 8 compares IR spectra of two solids obtained under atmospheric pressure and under 13 bar. Peaks of phosphate groups were found at 1310-900 and 620-500 cm -1 . The low-intensity peak at 630 cm -1 was attributed to hydroxyl groups. As expected, characteristic peaks of carbonate groups inserted in the apatitic structure were found at 1545, 1450, 1415, 880, and 870 cm -1 [START_REF] Pham Minh | Carbonate-containing apatite (CAP) synthesis under moderate conditions starting from calcium carbonate and orthophosphoric acid[END_REF]. The intensity of the carbonate bands of the product synthesized at 13 bar was much higher than that of the product synthesized at atmospheric pressure. To quantify this difference, TG analysis was performed on the product synthesized at 13 bar and the result is presented in Fig. 9. Only a trace amount of remaining unreacted CaCO 3 , which was decomposed at about 610 °C, was observed and was confirmed by XRD characterization. From the decarbonation of CAP in the temperature range of 740-1250 °C, the carbonate content of the product was determined to reach 4.8 wt%. This was higher than the carbonate content of the product synthesized at 80 °C under atmospheric pressure for 24 h of reaction (Fig. 2), which was only 2.5 %. So, the contact of the reaction mixture with pressurized CO 2 atmosphere increased the carbonate content in Ca-HA structure as could be expected. The bioactivity test was then carried out for CAP synthesized under high-CO 2 pressure. Details of this test were described elsewhere [START_REF] Pham Minh | Carbonate-containing apatite (CAP) synthesis under moderate conditions starting from calcium carbonate and orthophosphoric acid[END_REF]. As shown in Fig. 10, after incubation in Tris-SBF-27 mM, the surface of CAP was covered by new calcium phosphate deposits resembling those described in earlier observations [START_REF] Jalota | Using a synthetic body fluid (SBF) solution of 27 mM HCO 3 to make bone substitutes more osteointegrative[END_REF]. This coating related to good bioactivity of synthesized CAP, appropriate for bone and dental tissue reconstitution.

Conclusions

Ca-HA was successfully synthesized from CaCO 3 and H 3 PO 4 as convenient and low-cost starting materials by a one-step synthesis process operated at moderate reaction conditions (80 °C, 24 h, and atmospheric pressure). CaCO 3 powder reactant could be completely decomposed and both initial products could be totally precipitated into Ca-HA. This synthesis process demonstrated several advantages compared to other methods starting from soluble calcium salts. Ca-HA could be obtained in gel or powder forms which were found to be active in the bioactivity test, and to be very efficient in the fixation of lead as a toxic model metal in liquid or gas phases. This opens new approaches in enlarging the use of Ca-HA for environmental purposes and for biomaterial development, when Ca-HA can be easily obtained from economical starting materials using simple one-step synthesis process.

Fig. 1

 1 Fig. 1 Dynamic viscosity against shear rate for the Ca-HA gel synthesized at 80 °C for 24 h

Fig. 2 Fig. 3

 23 Fig. 2 TG curves of the solid samples withdrawn from the reactor at different reaction times

Fig. 4

 4 Fig.4Removal of lead(II) as a function of contact time using Ca-HA gel and Ca-HA powder as sorbents

Fig. 5 Fig. 6 Fig. 7 Fig.

 567 Fig. 5 SEM images of a Ca-HA powder dried at 105 °C, b lead(II)loaded Ca-HA powder, recovered after lead(II) removal, c lead(II)loaded Ca-HA powder after immobilization in a resin and polishing to look inside particles

CAP → CO 2 Fig. 9

 29 Fig.9TG analysis of the product synthesized at 80 °C under 13 bar CO 2 pressure; the decarbonation of CAP in the temperature range of 740-1250 °C was confirmed by mass spectroscopy analysis of the gas from the outlet of TG analyzer, as shown elsewhere[START_REF] Silva | Structural properties of hydroxyapatite obtained by mechanosynthesis[END_REF]

Fig. 10

 10 Fig. 10 SEM images of Ca-HA synthesized at 80 °C under CO 2 pressure before (a) and after (b) incubation in Tris-SBF-27 mM

Table 1

 1 Summary of different processes for the synthesis of Ca-HA

	Calcium	Phosphate	Surfactant,	Synthesis	Main advantages	Main drawbacks	Ref
	source	source	template	conditions
	CaCO 3	H 3 PO 4	No	80 °C, 24 h One-step synthesis	Only CO 2 as by-product	This
					Low-cost reactants		work
					Moderate
					conditions
	Ca(OH) 2	H 3 PO 4	No	80-90 °C,	One-step synthesis	Calcination of CaCO 3 into Ca(OH) 2	[8]
				24 h	Without residues
					Moderate
					conditions
	Ca(NO 3 ) 2	NH 4 H 2 PO 4	No	25-80 °C,	Good homogeneity	Great quantity of by-products in both	[8, 18,
		(NH 4 ) 2 HPO 4		up to	of the reaction	liquid and gas phases High cost of the	19]
		(NH 4 ) 3 PO 4		120 h	mixture	initial reactants
	CaCl 2	Na 3 PO 4	No	Microwave	Good homogeneity	Na ? and Cl -as by-products High cost of	[34]
				assisted	of the reaction	the initial reactants
				synthesis	mixture
	CaCl 2	(NH 4 ) 2 HPO 4	Petroleum ether	Sol-gel	Good homogeneity	Great quantity of by-products (inorganic	[35]
			as oil phase,		of the reaction	and organic) in both liquid and gas
			KB6ZA as		mixture	phases High cost of the initial reactants
			surfactant
	Ca(NO 3 ) 2	H 3 PO 4	Glycine, acrylic,	Sol-gel	Good homogeneity	Great quantity of by-products (inorganic	[25,
		(NH 4 ) 2 HPO 4	C6, C12, C20,		of the reaction	and organic) in both liquid and gas	26]
			polymers		mixture	phases High cost of the initial reactants
	(Ca(C 2 H 3 O 2 ) 2 (PO(OC 2 H 5 ) 3	Alcohols	Sol-gel	Good homogeneity
					of the reaction
					mixture

Acknowledgements The authors thank gratefully colleagues at RAPSODEE Center, Christine Rolland, Ce ´line Boachon, Bernard Auduc, Denis Marty, for their technical help.


Doan Pham
email: doan.phamminh@mines-albi.fr

Abdoul Razac Sane

Nawal Semlal

Patrick Sharrock

Ange Nzihou

Kh

Po

P Kh

Alkali polyphosphates as new potential materials for thermal energy storage

Keywords:

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Energy consumption constantly increases worldwide. This is observed for all types of primary energy including coal, oil, natural gas, nuclear energy, hydroelectricity, and renewable energy. Fossil fuel (coal, oil and natural gas) still remains the world's dominant energy. However, the reserve of fossil fuel is limited with irregular distribution over the world. In addition, the consumption of fossil fuel leads to the emission of carbon dioxide in the atmosphere. Carbon dioxide is the main greenhouse gas (GHG) to cause global warming. Global warming causes serious problems to ecosystems, animal and humans: melting of glaciers leading to the rise in sea level, irregular weather patterns, massive crop failure, animal extinction, frequent wildfires… [START_REF] Anderson | CO 2 , the greenhouse effect and global warming: from the pioneering work of Arrhenius and Callendar to today's earth system models[END_REF][START_REF] Arora | Role of Indian Ocean SST variability on the recent global warming hiatus[END_REF][START_REF] Punzón | Response of a temperate demersal fish community to global warming[END_REF]. So, the development of renewable energy is highly recommended for the 21 st century and is part of the global strategy for limiting the emission of GHG. The most common renewable energies include solar, biomass, wind, geothermal power or hydropower. Among them, solar energy has the highest potential with the development of photovoltaic (PV) and concentrated solar power (CSP) systems. Nevertheless, solar energy presents also various limits which relate to the intermittence of sunlight, the seasonal character, the irregular distribution between different countries and the dephasing between energy production and consumption. For CSP systems, the development of efficient thermal energy storage (TES) technology is a decisive factor to overcome the limits mentioned above.

TES is generally classified into 3 categories: sensible heat storage, latent heat storage, and thermochemical energy storage [START_REF] Veerakumar | Phase change material based cold thermal energy storage: Materials, techniques and applicationsa review[END_REF][START_REF] Weinstein | Concentrating solar power[END_REF]. Sensible heat storage consists in the heating of storage media without chemical reaction or phase change. Latent heat includes a phase change which allows a higher energy storage density compared to sensible heat storage. Thermochemical energy storage is based on a reversible reaction with endothermal and exothermal phases. Up-to-date, only sensible heat storage is developed at large industrial scale, and has a high growth rate recently. According to the International Energy Agency (IEA) (2014a), the cumulative CSP capacity worldwide reached around 0.5, 1. 6 and 3.9 GW in 20096 and 3.9 GW in , 20126 and 3.9 GW in and 2014, respectively, respectively. IEA studies also predicted, in their BLUE Map scenario, that in 2050, CSP plants would provide 5% of the annual global electricity production (compared to 6% by solar PV), which shows the potential and interest in solar energy (IEA, 2010(IEA, , 2014b)).

Different materials can be used as media for sensible heat storage. They can be either liquid or solid during the charging and discharging phases. Liquid materials include mineral oil, synthetic oil, silicone oil, liquid sodium, carbonate salts, nitrate and nitrite salts [START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1 -Concepts, materials and modellization[END_REF]. The most common commercial liquid material is known as Solar Salt containing 60 wt% NaNO 3 and 40 wt% KNO 3 [START_REF] Fernández | 3 -Using molten salts and other liquid sensible storage media in thermal energy storage (TES) systems[END_REF][START_REF] Peiró | Experimental analysis of charging and discharging processes, with parallel and counter flow arrangements, in a molten salts high temperature pilot plant scale setup[END_REF]. It is currently used in different CSP plants such as ANDASOL [START_REF] Dintera | Operability, reliability and economic benefits of CSP with thermal energy storage: first year of operation of ANDASOL 3[END_REF][START_REF] Millenium | The Parabolic Trough Power Plants Andasol 1 to 3. www. solarmillennium[END_REF][START_REF] Trabelsi | Techno-economic performance of concentrating solar power plants under the climatic conditions of the southern region of Tunisia[END_REF], or GEMASOLAR [START_REF] Rellosoa | Tower technology cost reduction approach after Gemasolar experience[END_REF]. Nitrate salts including Solar Salt have several advantages: (i) they are in liquid state at atmospheric pressure (and high temperature), which facilitates their use in CSP plants as TES material or heat transfer fluid (HTF); (ii) they have a working temperature range (282-386 °C for ANDASOL plants [START_REF] Dintera | Operability, reliability and economic benefits of CSP with thermal energy storage: first year of operation of ANDASOL 3[END_REF][START_REF] Trabelsi | Techno-economic performance of concentrating solar power plants under the climatic conditions of the southern region of Tunisia[END_REF] which allows generating vapor for steam turbine; (iii) they are less toxic than organic oils [START_REF] Fernández | 3 -Using molten salts and other liquid sensible storage media in thermal energy storage (TES) systems[END_REF]. In addition, they have low viscosity and low vapor pressure. However, nitrate salts have also various disadvantages and drawbacks. Their operational temperature is limited at 500 °C to avoid thermal decomposition of nitrate salts [START_REF] Bauer | Overview of molten salt storage systems and material development for solar thermal power plants[END_REF]. They are also used as fertilizer so its massive utilization in CSP plants can affect and compete with agricultural activities. When they are used as TES material, nitrate salts of high quality are recommended to avoid corrosion of tank and other equipment, which usually increases cost of CSP plant [START_REF] Kuravi | Thermal energy storage technologies and systems for concentrating solar power plants[END_REF]. Thus, numerous works have recently been devoted to the improvement of Solar Salt or the development of new materials for sensible heat energy storage [START_REF] Fernandez | Selection of materials with potential in sensible thermal energy storage[END_REF][START_REF] Fernández | 3 -Using molten salts and other liquid sensible storage media in thermal energy storage (TES) systems[END_REF][START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1 -Concepts, materials and modellization[END_REF][START_REF] John | Concrete as a thermal energy storage medium for thermocline solar energy storage systems[END_REF][START_REF] Kuravi | Thermal energy storage technologies and systems for concentrating solar power plants[END_REF][START_REF] Martins | New concentrating solar power facility for testing high temperature concrete thermal energy storage[END_REF].

This work focused on the development of several alkali polyphosphates ((MPO 3 ) n ), with M = Na, K, or Li) as liquid media for sensible heat storage. In the literature, much work was reported on the investigation of nitrate salts, halogenated salts, carbonate salts, sulfates salts, metals and metal oxides [START_REF] Hoshi | Screening of high melting point phase change materials (PCM) in solar thermal concentrating technology based on CLFR[END_REF]. However, to the best our knowledge, there is no work reported yet on liquid (poly)phosphate-based products as TES materials. (Poly)phosphate-based products constitute a large family of inorganic salts with interesting physico-chemical and thermal properties, in particularly their low corrosion and high thermal stability compared to other inorganic salts.

Materials and methods

Table 1 summaries the chemicals used in this work. They were all provided by Aldrich, with high purity for R & D purpose. All these chemicals were in fine powder form. They were used as received without further modification.

TG-DTA analysis was performed with a TA Instruments SDTQ600 analyzer with the heating rate of 5 °C min -1 under air flux (100 mL min -1 ). Platinum crucibles of 3 mm of diameter and 3 mm of height were used. For each analysis, around exactly 10-15 mg of solid was needed. In general, the instrument used can measure masse change and heat flow of the analysis (W/g). However, it was only used as TG-DTA analyzer in this work. In fact, heating alkali phosphates (MH 2 PO 4 with M = Li, Na, or K) leads to the formation of alkali polyphosphates ((MPO 3 ) n ) as shown thereafter in the Results section. When alkali polyphosphate melts, it exists in liquid state which can affect the weight signal. In addition, liquid polyphosphate could overflow the platinum crucible and then stick it on the microbalance of the TG-DTA apparatus. In this case, the microbalance has to be changed. To solve this problem, fine alumina powder was set on the surface of the microbalance to avoid the direct contact of the platinum crucible with the microbalance. Thus, the platinum crucible could be easily removed after TG-DTA test, despite eventual overflow of alkali polyphosphates. But the presence of alumina power disturbs the heat change signal. Thus, only DTA curves could be obtained, instead of DSC curves.

FTIR spectra were recorded with a Shimadzu FTIR 8400 S spectrometer, using a sensitive pyroelectric detector with a l-alanine-doped deuterated triglycine sulfate (DLATGS) element (maximum resolution of 0.85 cm -1 , signal-to-noise ratio of 20,000: 1). The measurement was carried out directly with solid sample under powder form. Solid recovered from TG-DTA analysis was quickly analyzed by FTIR without further preparation. This compound decomposed by dehydrations as follows:

Results

Characterization of sodium polyphosphate, (NaPO 3 ) n

→ + NaH PO •2H O NaH PO •H O H O 2 4 2 2 4 2 2
(1)

→ + NaH PO •H O NaH PO H O 2 4 2 2 4 2
(2)

→ + NaH PO NaPO •1/2H O 1/2H O 2 4 3 2 2
(3)

→ + NaPO •1/2H O NaPO 1/2H O 3 2 3 2 (4)
The first two weight losses took place at the temperature range of 30-145 °C, and corresponded to the dehydration of crystalized water. The last two weight losses were due to the condensation of dihydrogenophosphate anions (H 2 PO 4 -), which happened in the temperature range of 190-350 °C. For each weight loss, the value recorded by TG-DTA apparatus was close to the theoretical value calculated for this compound (11.5 wt% for the first two weight losses, and 5.8 wt% for the last two weight losses). Sodium polyphosphate, (NaPO 3 ) n , was T m (628°C) Fig. 2. TG-DTA cycle analysis under air atmosphere with the heating rate of expected as the product of the condensation. This product was thermally stable below 628 °C without notable weight loss. Then, it melted to give the DTA peak at 628 °C. This phenomenon slightly disturbed the signal of the microbalance of TG measurement. Finally, sodium polyphosphate seemed to evaporate above 900 °C (Fig. 1(B)). In the temperature of 628-900 °C, no weight loss was observed. TG-DTA was repeated several times to confirm the repeatability of the analysis. The similarity of TG and DTA curves of Fig. 1(A) and (B) illustrates this repeatability. Fig. 2 shows TG-DTA curves of Na 3 P 3 O 9 and NaH 2 PO 4 •2H 2 O with 4, 17 or 20 consecutive heating-cooling cycles in the temperature range of 30-800 °C. For Na 3 P 3 O 9 (Fig. 2(A) and (B)), the first heating step led to some small weight losses (< 0.5 wt%) and the melting at 628 °C. For the next cooling and heating steps, there was no notable weight loss which demonstrated the thermal stability of the resulting product, (NaPO 3 ) n , within the temperature range studied. The results were repeated for both the analyses with 4 and 20 consecutive heating-cooling cycles. Further FTIR analysis shows that no significant change was observed inside its structure (Fig. 3). For NaH 2 PO 4 •2H 2 O (Fig. 2 (C) and (D)), the first heating step led to different dehydrations as mentioned previously, as well as the melting at 628 °C. Then, (NaPO 3 ) n formed from this step was thermally stable during 4 or 17 heating-cooling cycles. Fig. 3 shows FTIR spectra of Na 3 P 3 O 9 and NaH 2 PO 4 •2H 2 O before and after TG-DTA analyses. As mentioned above, the heating of Na 3 P 3 O 9 and NaH 2 PO 4 •2H 2 O above 628 °C led to the melting of these salts and the formation of sodium polyphosphate, (NaPO 3 ) n . The structure of (NaPO 3 ) n seemed to be similar after being heated to different temperatures of 700, 800 or 1000 °C. In fact, FTIR analysis shows that all the solids recovered from TG-DTA analysis had similar spectra. FTIR analysis illustrated also the structural change of Na 3 P 3 O 9 to more stable form ((NaPO 3 ) n ) after melting at 628 °C, due to the oligomerization or polymerization of NaPO 3 unit.

Characterization of lithium polyphosphate, (LiPO 3 ) n

Fig. 4 shows TG-DTA analysis of LiH 2 PO 4 under air atmosphere. LiH 2 PO 4 had a progressive dehydration in the temperature range of 200-415 °C (at the heating rate of 5 °C min -1 ). The maximum of DTA signal of this dehydration was found at 225.5 °C. The weight loss of this dehydration was recorded around 17 wt%, which corresponds to the following equation:

→ + LiH PO LiPO H O 2 4 3 2 (5)
LiPO 3 resulting from this dehydration melted at 657 °C. As previously observed with NaPO 3 , this melting caused a perturbation of TG signal. At liquid state, LiPO 3 was thermally stable up to around 900 °C before its evaporation, as observed in Fig. 4(B). Thus, consecutive heating-cooling cycles was then performed with LiH 2 PO 4 in the temperature range of 30-900 °C (Fig. 4(C)). Any notable weight loss was observed after the first heating step. Also, the maximum of DTA peak was constant at 657 °C indicating that there was probably no structural change of LiPO 3 after its formation by the first melting at 657 °C.

Fig. 5 shows FTIR spectra of the initial LiH 2 PO 4 powder and the solids recovered from TG-DTA analysis. LiH 2 PO 4 had several peaks at 1300-700 cm -1 , which must be due to P-O vibrations. For the solids recovered from TG-DTA analyses, all the spectra were similar to each other, and did not depend on the final temperature of TG-DTA analysis. In particular, as observed previously, TG-DTA analysis at 30-1100 °C showed the partial evaporation of (LiPO 3 ) n above 900 °C. We suppose that this evaporation did not cause any notable change in the structure of the remaining solid because its FTIR spectrum was also similar to those of the solids treated at lower temperatures.

Characterization of potassium polyphosphate, (KPO 3 ) n

TG-DTA analysis of KH 2 PO 4 is presented in Fig. 6. The dehydration of this compound took place in the temperature range of 210-350 °C. This dehydration seemed to happen in several consecutive steps as indicated by the presence of multiple DTA peaks. The global weight loss at 210-350 °C reached approximatively 13 wt% and corresponded to the following equation:

→ + KH PO KPO H O 2 4 3 2 (6)
Thus, KPO 3 was formed above 350 °C. This compound did not show any notable change up to 812 °C, where it melted. The disturbance of the melting step on TG signal was again observed. Liquid KPO 3 was thermally stable up to around 900 °C. Then, it evaporated above 900 °C as observed in Fig. 6 (B).

Fig. 6 (C) shows heating-cooling cycles of KH 2 PO 4 . The dehydration of KH 2 PO 4 was again confirmed during the first heating step. When KPO 3 was formed, it remained thermally stable during repeated heating-cooling cycles. Fig. 7 presents FTIR spectra of the initial KH 2 PO 4 and the solids recovered after TG-DTA analyses. For KH 2 PO 4 , peaks of P-O vibration were also observed at the wavelength of around 1300-700 cm -1 . For the solids recovered from TG-DTA analyses, their FTIR spectra were similar to each other, indicating the structural similarity of these melted solids. In other words, the formation of the same potassium polyphosphate, (KPO 3 ) n , was expected after its melting point at 812 °C and its structure was stable even above its evaporation temperature.

For a conventional CSP system using molten salt as TES material, the working temperature required is limited between around 281-386 °C, as previously mentioned. These nitrate-based salts are upto-date the main commercial products for sensible heat storage. There is no available solution for TES at higher temperatures yet. Current research is focused on the development of refractory concretes and ceramics as new TES materials [START_REF] Fernandez | Selection of materials with potential in sensible thermal energy storage[END_REF][START_REF] Fernández | 3 -Using molten salts and other liquid sensible storage media in thermal energy storage (TES) systems[END_REF][START_REF] Gil | State of the art on high temperature thermal energy storage for power generation. Part 1 -Concepts, materials and modellization[END_REF][START_REF] John | Concrete as a thermal energy storage medium for thermocline solar energy storage systems[END_REF][START_REF] Kuravi | Thermal energy storage technologies and systems for concentrating solar power plants[END_REF][START_REF] Martins | New concentrating solar power facility for testing high temperature concrete thermal energy storage[END_REF]. However, these materials still need improvements mostly concerning their low thermal conductivity, their difficulty for shaping, or their high cost. So, this work demonstrated that liquid alkali polyphosphates could be an interesting solution for sensible heat storage at high temperatures. Among three monoalkali polyphosphates studied, sodium polyphosphate ((NaPO 3 ) n ) seemed to be the most interesting product. It melts at 628 °C and could be stable up to around 900 °C. The melting point of lithium polyphosphate ((LiPO 3 ) n ) is close to that of sodium polyphosphate and this compound is also stable up to around 900 °C. But in general, lithium compounds are less available and more expensive than sodium compounds. As for potassium polyphosphate ((KPO 3 ) n ), it melts at 812 °C and evaporates above 900 °C. So this compound seems to be not convenient for sensible heat storage because of the narrow temperature range wherein it is stable in liquid state.

Conclusions

For the first time, alkali polyphosphates were investigated as liquid media for sensible heat storage at high temperature. Alkali polyphosphates ((MPO 3 ) n , with M = Li, Na or K) could be easily obtained from the dehydration of the corresponding monoalkali dihydrogenphosphate (MH 2 PO 4 , M = Li, Na, K). After their melting point, alkali polyphosphates were thermally stable up to around 900 °C. Above 900 °C, alkali polyphosphates evaporated, but no chemical decomposition took place. Sodium polyphosphate was the most promising product thanks to its large temperature range in the liquid state (628-900 °C), its availability and its low cost compared to lithium or potassium poly phosphates.

Further work should determine physico-chemical properties for these alkali polyphosphates in liquid state such as heat capacity, thermal conductivity, density, viscosity, corrosive property etc. These data would allow the design of a new solution for sensible heat storage at high temperature.

Fig. 1 .

 1 Fig. 1. TG-DTA under air atmosphere with the heating rate of 5 °C min -1 of NaH 2 PO 4 •2H 2 O up to 700 °C (A) and 1000 °C (B).

Fig. 1

 1 Fig. 1 shows TG-DTA curved of NaH 2 PO 4 •2H 2 O under air flux in the temperature range of 30-700 °C (A) or 30-1000 °C (B).This compound decomposed by dehydrations as follows:

Fig. 4 .

 4 Fig. 3. FTIR spectra of Na 3 P 3 O 9 and NaH 2 PO 4 •2H 2 O before and after TG-DTA analysis.

Fig. 5 .Fig. 6 .

 56 Fig. 5. FTIR spectra of synthesized LiPO 3 before and after TG-DTA analysis.

Table 1

 1 List of chemicals used in this work.

	Name	Chemical formula	Supplier
	Sodium trimetaphosphate	Na 3 P 3 O 9	Aldrich
	Sodium dihydrogenphosphate dihydrate	NaH 2 PO 4 •2H 2 O	Aldrich
	Lithium dihydrogenphosphate	LiH 2 PO 4	Aldrich
	Potassium dihydrogenphosphate	KH 2 PO 4	Aldrich

Acknowledgment

The authors would like to acknowledge the financial support from Prayon and OCP S.A Group, and ANRT (Association Nationale de la


Doan Pham
email: doan.phamminh@mines-albi.fr

Ngoc Dung Tran

Ange Nzihou

Patrick Sharrock

Carbonate-containing apatite (CAP) synthesis under moderate conditions starting from calcium carbonate and orthophosphoric acid

Apatite One-step synthesis Calcium carbonate Orthophosphoric acid

The synthesis of carbonate-containing apatite (CAP) from calcium carbonate and orthophosphoric acid under moderate conditions was investigated. In all cases, complete precipitation of orthophosphate species was observed. The reaction temperature influenced strongly the decomposition of calcium carbonate and therefore the composition of formed products. The reaction temperature of 80 °C was found to be effective for the complete decomposition of calcium carbonate particles after 48 h of reaction. Infra-red spectroscopy (IR), nuclear magnetic resonance (NMR), thermogravimetry/mass spectroscopy (TG-MS) coupling, and X-ray diffraction (XRD) characterizations allowed the identification of the composition of formed products. By increasing the reaction temperature from 20 °C to 80 °C, the content of A-type CAP increased and that of B-type CAP decreased, according to the favorable effect of temperature on the formation of A-type CAP. The total amount of carbonate content incorporated in CAP's structure, which was determined by TG-MS analysis, increased with the reaction temperature and reached up to 4.1% at 80 °C. At this temperature, the solid product was mainly composed of apatitic components and showed the typical flat-needle-like structure of CAP particles obtained in hydrothermal conditions. These results show an interesting one-step synthesis of CAP from calcium carbonate and orthophosphoric acid as low cost but high purity starting materials.

Introduction

The principal mineral composition of bones and teeth consists of apatite-based compounds, and among them calcium hydroxyapatite (Ca-HA, Ca 10 (PO 4 ) 6 (OH) 2 ) is considered as the main component. In addition, bones and teeth also contain substituted apatites with the presence of other elements including Na + , Mg 2+ , Cl -, F -and CO 3

2- [1]. Calcium carbonate-containing apatite (CAP) is considered as the major impurity, which is formed by the replacement of PO 4 3-and/or OH -groups in Ca-HA's structure by CO 3 2-groups [2,3]. Ca-HA has been considered as a prominent biomaterials for bone and dental tissue reconstitution for its excellent bioactivity with hard tissues, but recently, substituted Ca-HA such as CAP has been found to be more effective with a shorter post-operative rehabilitation program [4][5][6]. Thus, CAP base materials attract much attention and the awareness of the contribution of CAP to bone and dental enamel health is increasing [7].

Much work has been devoted to the preparation of CAP, which generally consists of the precipitation of orthophosphate anions by calcium cations in aqueous solution in the presence of CO 3 2-anions [8][9][10], or the heating of a calcium phosphate at high temperature under a flux of CO 2 [START_REF] Elliott | Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF][START_REF] Fleet | [END_REF]. By solution reactions, a mixture of water soluble salts is usually used to enhance the insertion of CO 3 2-anions in the apatitic structure. This leads to supplementary washing steps for the elimination of counter ions in order to purify final solid products. Washing steps may be arduous when nano-particles of calcium phosphates are formed, accompanied by the formation of waste by-products containing all counter ions. The heating route relates to a multi-step synthesis in rigorous conditions of temperature and pressure. By this way, a mixture of calcium phosphate (for example, tricalcium phosphate (Ca 3 (PO 4 ) 2 ), or calcium pyrophosphate (Ca 2 P 2 O 7 )), lime (CaO) and/or calcium carbonate (CaCO 3 ) is heated under CO 2 atmosphere [START_REF] Fleet | [END_REF][13][14].

The present study reports a simple one-step synthesis of CAP from calcium carbonate and orthophosphoric acid under moderate conditions of temperature and pressure. Calcium carbonate was chosen as the source of both calcium cations and carbonate anions, and orthophosphoric acid was chosen as the most effective orthophosphate for the dissolution of calcium carbonate powder particles. Their wide availability and low cost also constitute advantages for an eventual industrialization of the synthesis process.

Materials and methods

Calcite powder (CaCO 3 , 98%) with particles of volume-mean diameter of 23.3 μm from Fisher Scientific and orthophosphoric acid (H 3 PO 4 , 85 wt.% in water) from Merck were used as received, without further modification. The synthesis was carried out in a 250 mL stainless steel reactor (Top Industrial). The reactor was equipped with an electrical heating jacket and a magnetic stirrer. For each reaction, 10 g of CaCO 3 and 45 mL of water were fed into the reactor. After closing, 6.9 g of H 3 PO 4 was quickly injected into the reactor by an injection valve and the reaction started by adjustment of the stirring rate at 800 rpm. During the reaction, the reactor was thermostated at the selected temperatures in the range of 20-80 °C for 48 h. After reaction, solid phase was separated from liquid phase by filtration using 0.45 μm filter paper. Then the solid products were dried at 50 °C for 48 h before further characterizations.

Different analysis and characterization techniques were used for studying both solid and liquid phases. Elemental analysis was carried out using inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a HORIBA Jobin Yvon Ultima 2 apparatus. X-ray diffraction (XRD) of the solids was performed using a Philips PANalytical X'pert Pro MPD diffractometer with a Cu K α (1.543 Å) radiation source. Thermogravimetry (TG) was performed in a TA Instruments SDTQ600 analyzer with a heating rate of 5 °C min -1 under air flux (100 mL min -1 ). Exhaust gases from the TG analyzer outlet were analyzed by a Pfeiffer Vacuum OmniStar GSD 320 mass spectrometer which acquires mass spectra from 0 to 200 amu (atomic mass unit). Diffuse reflection infrared Fourier transform (DRIFT) spectra were recorded on a Shimadzu FTIR 8400S spectrometer on ground solid powders. 31 P and 1 H NMR spectra were registered on a Bruker Avance 400, operating at 10 kHz for both 31 P and 1 H NMR at room temperature (21 °C). The adsorption-desorption isotherm was determined with a MICROMETRICS ASAP 2010 using nitrogen as gas adsorbate with the data collection from relative pressure (P/P°) of 0.03 to 0.99. True density of the solid powders was measured by helium pycnometry using an Accupyc 1330 (Micromeritics). Scanning electron microscopy (SEM) was measured on a Philips XL30 ESEM apparatus (FEI Company). Particle size distribution was measured by laser scattering in a Mastersizer 2000 (Malvern Instruments Ltd., Malvern, UK) in the range from 0.020 to 2000 μm.

CAP obtained at 80 °C with the highest calcium carbonate decomposition was formed into cylindrical disks using a hydraulic press, with no additives. The disks were immersed in simulated body fluid (Tris-SBF-27 mM) made according to the method of Jalota et al. 00-004-0697); (◊) A-type CAP (JCPDS standard no. 00-035-0180); and (○) calcite (JCPDS standard no. 00-047-1743). [15]. Following seven days of incubation at 37 °C, the samples were rinsed with distilled water, dried at 37 °C and observed with SEM analysis technique in order to examine the growth of new calcium phosphate on the surface of CAP disks.

Results and discussion

Orthophosphate precipitation and calcium carbonate dissolution

When H 3 PO 4 was injected into the suspension of CaCO 3 , the reaction started immediately as observed by the evolution of the pressure in the batch reactor. In all cases, the pressure quickly increased for the first 10 min. Then it slowly increased and stabilized up to the end of the reaction. In the temperature range investigated, the partial pressure of water vapor was close to the atmospheric pressure (approximately 1 bar) so the increase of the pressure could be attributed to the formation of the carbonic gas as shown in Eq. (1).

CaCO 3 þ H 3 PO 4 →Calciumphosphates þ H 2 O þ CO 2 ð1Þ
Carbon dioxide from Eq. ( 1) could be in gas phase or dissolved in liquid phase. The quantity of carbon dioxide in gas phase could be estimated using the ideal gas law and taking into account the volume of the reactor occupied by the solid and liquid phases and the partial pressure of water vapor at the corresponding temperature (Fig. 1).

Both final pressure and quantity of carbon dioxide in gas phase linearly increased with the reaction temperature. At 80 °C, 88.9 mmol of carbon dioxide was released in gas phase which indicates that most initial calcium carbonate introduced in the reactor (100 mmol) was decomposed.

The filtrate recovered from the reaction mixture was acidified with concentrated nitric acid to avoid any further precipitation or transformation of calcium cations with orthophosphate and/or carbonate anions. Then, dissolved calcium and phosphorus contents in the filtrate were analyzed by ICP-AES technique. In all cases, there was less than 1% of the initial calcium and phosphorus introduced in the reactor which existed in soluble forms. This means that the precipitation of orthophosphoric acid into solid calcium phosphates was complete and all calcium cations released from Eq. ( 1) completely re-precipitated too. ICP-AES analysis of the bulk solids after mineralization with concentrated nitric acid showed that the molar ratio of Ca/P was about 1.8. This is slightly higher than the stoichiometric value of Ca-HA and will be discussed in more details in the NMR and IR characterizations section.

The content of residual calcium carbonate in the solid products was quantified by TG analysis (Section 3.4) and the obtained results are reported in Fig. 2. As expected, the decomposition of calcium carbonate increased with the increase of reaction temperature. At 80 °C, the disappearance of calcium carbonate was nearly complete after 48 h of reaction.

The solid products were then characterized by different physicochemical techniques to identify the intermediates present.

XRD characterization

XRD characterization was performed in order to investigate the crystalline phases present in the solid products using JCPDS standards. The results are presented in Fig. 3.

As shown in Fig. 3, unreacted calcium carbonate, having the main diffraction peak at 29.40°, was clearly observed in the solid products synthesized within the reaction temperature range of 20-60 °C. On the other hand, only trace amounts of this starting reactant was observed in the solid product obtained at 80 °C which confirmed the favorable effect of the reaction temperature on the decomposition of initial calcium carbonate particles. The yield of calcium carbonate decomposition strongly influenced the crystalline structure of the solid calcium phosphates formed. When the decomposition of calcium carbonate was nearly complete (80 °C), all diffraction peaks could be attributed to Ca-HA (JCPDS standard no. 01-072-1243) and calcium carbonate-containing apatites of both B-type (JCPDS standards nos. 00-019-0272 and 00-004-0697) and A-type (JCPDS standard no. 00-035-0180), labeled thereafter CAPs. However, it was difficult to distinguish these apatitic compounds because their main diffraction peaks are all in the 2 theta range of 31.5-31.8°. When the dissociation of calcium carbonate was incomplete (20-60 °C), different calcium phosphates co-existed. Octacalcium bis(hydrogenphosphate) tetrakis(phosphate)pentahydrate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 •5H 2 O), which has the main diffraction peak at 2 theta of 4.7°, was the main apatitic calcium orthophosphate. Its signal decreased with the increase of the reaction temperature indicating its transformation into more stable apatites. Ca-HA, which has the main diffraction peak at 2 theta of 31.7°, was also formed but with low crystallinity as shown by its weak broad peaks. Other calcium orthophosphates such as CAPs, dicalcium phosphate anhydrous (DCPA, JCPDS standard no. 00-003-0398) could be also present at much lower contents.

In all cases, the final pH of the reaction mixture was around 8, which was favorable for the formation of apatitic compounds.

NMR and IR characterizations

To enhance the results of XRD characterization, NMR and IR analyses of the solid products were investigated. Fig. 4(A) presents the 31 P NMR spectra of the dried solid products. The three solid products obtained at 20, 40 and 60 °C showed four characteristic chemical shifts of OCP [16]. On the other hand, the solid product synthesized at 80 °C showed only one chemical shift at 3.7 ppm, which was characteristic for Ca-HA based products [17].

Fig. 4(B) shows the 1 H NMR spectra of the dried solid products. The chemical shift at about 13 ppm was characteristic for the HPO 4 2-ions located in the apatitic layer of the OCP's structure [START_REF] Elliott | Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF]. This shift appeared only in the solid products synthesized at 20-60 °C with uncompleted dissociation of calcium carbonate, and was not found in the product synthesized at 80 °C with complete conversion of calcium carbonate. The shift at about -0.1 ppm was characteristic for OH - ions present in Ca-HA's structure [17]. This peak was much more intense in the product synthesized at 80 °C than in the other products. All these results confirmed those of XRD analysis, that OCP and Ca-HA were the main components of the solid products synthesized at 20-60 °C and 80 °C, respectively. Finally, the peak with chemical shift at about 5 ppm (Fig. 4(B)) could be attributed to adsorbed water, which was present in all four solid products [START_REF] Wilson | [END_REF]. Fig. 5 shows IR spectra of all four solid products, and most significant peaks in the wavelength range of 4000-500 cm -1 are summarized in Table 1. In the wavelength range of 4000-1700 cm -1 , there was only a weak broad peak at about 3600-3000 cm -1 (not presented), which was assigned to O\H stretching of molecular water.

According to the results of XRD analysis, the remaining calcium carbonate in the solid products was characterized by the peak at 711 cm -1 , which was not present in the solid obtained at 80 °C. This last one also lacked the peak at 1645 cm -1 , assigned to H\O\H bending of molecular water in the structure of OCP, which confirmed the absence of OCP as shown by XRD (Fig. 3). The weak absorption band at 633 cm -1 , assigned to the vibrational mode of OH -groups indicated the presence of stoichiometric Ca-HA. The bi-modal peak at 1450 cm -1 /1415 cm -1 and the peak at 870 cm -1 were characteristic for B-type CAPs (replacement of PO 4 3-groups by CO 3 2-groups). On the other hand, A-type CAPs (replacement of OH - groups by CO 3 2-groups) were characterized by the peaks at 1545 cm -1 and 880 cm -1 . Thus, B-type CAPs could be present in all four solid products while A-type CAPs could only be found in the solids obtained at 60 °C and 80 °C. So, the increase of reaction temperature was favorable for both the decomposition of calcium carbonate and the formation of CAPs, particularly A-type CAPs. The presence of B-type CAP explained why the molar ratio of Ca/P of the bulk solids was slightly higher the stoichiometric value of pure Ca-HA. Finally, the absorption bands in the wavelength ranges of 650-500 cm -1 and 1300-910 cm -1 were assigned to the vibrations of orthophosphate groups. Classical vibration bands of HPO 4 2-groups in OCP structure and PO 4

3-in Ca-HA or CAP structure could clearly be observed.

TG-MS coupling

The thermal behavior of the solid products was investigated using TG analysis. TG and DTG curves obtained in the temperature range of 25-1200 °C are shown in Fig. 6(A) and (B), and the formation of water and carbon dioxide during the thermal analysis, which was qualitatively measured by MS analysis, are presented in Fig. 6(C) and(D).

All changes in mass as a function of heating temperature could be attributed to the decarbonation and dehydration which were well confirmed by MS analysis. The thermal decomposition of OCP, which was the principal intermediate, is generally complex with different dehydrations in the temperature ranges of 30-80 °C; 80-170 °C and 200-300 °C [START_REF] Elliott | Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF]. These weight losses were not observed for the solid synthesized at 80 °C which was in coherence with the result of XRD, NMR and IR characterizations. The first weight loss at 30-80 °C was attributed to the elimination of physically absorbed surface moisture which should not be significant after drying. The weight loss at 170-200 °C, which was only observed for the solid synthesized at 20 °C, could be due to the dehydration of DCPD to form DCPA. This indicated that DCPD could also be an intermediate of the reaction as previously observed in the reaction of limestone with orthophosphoric acid [START_REF] Kim | [END_REF]. DCPA was then condensed into calcium pyrophosphate (Ca 2 P 2 O 7 ) at 400-500 °C. The clearest weight loss at 600-720 °C was due to the decarbonation of residual calcium carbonate. Its content decreased with the increase of reaction temperature, as discussed in Fig. 2. Two next weight losses at 740-850 °C and 850-1200 °C were attributed to the decarbonation and dehydroxylation of CAPs, as accompanied with the release of CO 2 and H 2 O, detected by MS analysis. According to the results of IR analysis, the decarbonation of B-type CAPs, which were present in all four solids, happened at 740-850 °C. On the other hand, the decarbonation of A-type CAPs, which were only formed at 60 °C and 80 °C, occurred at higher temperatures in the range of 850-1200 °C. The decarbonation of the solid synthesized at 80 °C was not finished at 1200 °C which indicated its high thermal stability.

All changes in mass in the temperature range investigated were endothermic as found by differential scanning calorimetry (DSC, not presented).

SEM and particle size distribution

In order to investigate the surface morphology of particles, SEM observation was performed. Fig. 7 presents SEM micrographs of the solid products at low magnification and starting calcium carbonate powder. This initial reactant had a high regularity in particle dimension (Fig. 7(A)) with a smooth and non-porous structure (Fig. 7(B)), which was in coherence with its low specific surface area (b2 m 2 g -1 ). On the other hand, all four solid products had a variation of particle sizes in large ranges from hundreds of nm to hundreds of μm, as usually observed previously [25][26][27]. Fig. 8 presents SEM micrographs of particles larger than 50 μm (L) and smaller than 5 μm (R) at the same magnification of 6400 times (5 μm scale). For all solid products, large particles seem to be a result of agglomeration of corresponding fine particles. This has been observed for the precipitation of calcium orthophosphates in aqueous solution [28][29][30]. Taking into account the rapid increase of pressure in the batch reactor during the first minute of reaction, it is assumed that the first step of the reaction must be the decomposition of calcium carbonate particles in acid medium, accompanied by the release of carbonic gas and the precipitation of calcium cations (Ca 2+ ) by orthophosphate species to form primary fine calcium phosphate particles at nanometric scale. Then, primary fine calcium phosphate particles agglomerated during the reaction time to form larger particles.

Change in morphology of particles as a function of reaction temperature can also be observed in Fig. 8. On the left-hand side, larger aggregates seem to have a porous structure. On the right-hand side, primary particles had a compact appearance and at higher magnification, no macro-pores could be observed (Supplementary data 1). At 20 °C, particles of sheet structure frequently appeared. They seemed to be more numerous at 40 °C and could be attributed to OCP particles [21,31] which were in coherence with XRD results (Fig. 3). Some needle-like particles also appeared at these temperatures. At 60 °C, the frequency of particles of sheet structure decreased and that of needle-like particles considerably increased. Particularly, the change was distinct at 80 °C where most particles were of flat-needle-like morphology (more details in Supplementary data 2). This morphology was characteristic for apatitic compounds formed under hydrothermal conditions, as observed in previous works [32][33][34]. Particle size distribution was also analyzed in order to better understand the formation of particles (Fig. 9).

According to the observation in Fig. 7(A), starting calcium carbonate had a narrow particle size distribution from 7 μm to 50 μm. On the other hand, solid calcium phosphate products showed large ranges of particle sizes from about 0.2 μm to about 270 μm. For the solid products synthesized at 20-60 °C, there was more than 93% of number of particles found in the range of 0.2 μm to 1 μm. On the other hand, the volumes corresponding to this fraction of particle size were found at less than 7%. For the solid synthesized at 80 °C, 97% of the number of particles was found in the range of 1.5 to 10 μm, which corresponded to only 3% of cumulative volume. This means that a deep agglomeration of fine particles took place during the synthesis under the conditions investigated.

Adsorption-desorption isotherm measurements and true density

SEM micrographs at high magnification showed that large solid particles could have macro-pores but this technique did not allow the observation of smaller pore sizes. To better understand the porosity of the obtained materials, adsorption-desorption isotherm measurements were carried out. Fig. 10 shows isotherm plots for the reaction products.

All four solids showed similar adsorption-desorption isotherms of type II solids according to the IUPAC classification of physisorption isotherms [35]. These results indicate that the solid products had only macro-porous structure which confirmed the results of SEM observation.

From the results of isotherm measurements, specific surface areas were obtained using BET method (S BET ). True density was measured also and the results are presented in Table 2.

The two solid products synthesized at 20 °C and 40 °C had the same specific surface area, in agreement with similar chemical composition with about 20-22% of the remaining calcium carbonate. At 60 °C, the decomposition of calcium carbonate was more advanced (Fig. 2) and the resulting solid had a higher specific surface area. At 80 °C, despite a complete decomposition of calcium carbonate, the specific surface area of the reaction product was smallest, which was due to the formation of larger particles by agglomeration (Fig. 9). In parallel with the increase of the reaction temperature, the reaction evolved to apatitic phases which were generally denser than the initial calcium carbonate [36]. This explains the increase of true density as a function of reaction temperature. Clogged porosity may be also formed during the precipitation of apatitic compounds since carbon dioxide was released from the decomposition of calcite. This porosity may be destroyed with the increase of the temperature leading to denser products.

Bioactivity test

SEM observations showed the CAP disks to be composed of flat-needle-like crystals of approximately one micrometer width and several micrometers length (Fig. 11). Following incubation in Tris-SBF-27 mM, the crystals were covered by new calcium phosphate deposits resembling those described in earlier works [15,37]. The underlying CAP crystals can hardly be recognized in Fig. 11(B). The non-compacted crystals are loosened in Fig. 11(D) as a result of water rinsing. Kokubo et Takadama [38] described the formation of such coatings as related to surface bioactivity appropriate for bone bonding activity. Possible improvements of the proposed bioactivity test have been proposed [39].

Discussion

In all our reactions using calcium carbonate and orthophosphoric acid as starting reactants, only traces of soluble calcium cations and orthophosphate species were observed to be present in the liquid phase. This indicates that orthophosphate species precipitated from solution into poorly soluble solid phases. The decomposition of calcium carbonate was also complete at 80 °C after 48 h of reaction. This resulted in the a From TG-MS coupling analysis in the temperature range of 25-1350 °C for a complete decarbonation of the product synthesized at 80 °C (Supplementary data 3).

Fig. 2 .

 2 Fig. 2. Content of residual calcium carbonate in the solid products after 48 h of reaction.

Fig. 1 .

 1 Fig.1. Quantity of the carbon dioxide at the end of the reaction estimated from gas phase using the ideal gas law and final pressure as a function of the reaction temperature.

Fig. 3 .

 3 Fig. 3. XRD patterns of the solid products; (•) OCP (JCPDS standard no. 00-026-1056); (*) Ca-HA (JCPDS standard no. 01-072-1243); (▪) B-type CAP (JCPDS standard no. 00-004-0697); (◊) A-type CAP (JCPDS standard no. 00-035-0180); and (○) calcite (JCPDS standard no. 00-047-1743).

Fig. 4 .

 4 Fig. 4. 31 P NMR (A) and 1 H NMR (B) spectra of the dried solid products.

Fig. 5 .

 5 Fig. 5. IR spectra of the solid products; dotted tick: peak position of C\O stretching of A-type CAPs (880 cm -1 ); and continuous tick: peak position of C\O stretching of B-type CAPs (870 cm -1 ).

Fig. 6 .

 6 Fig. 6. TG (A) and DTG (B) curves of the solid products dried at 55 °C; evolution of H 2 O (C) and CO 2 (D) in gas phase during TG analysis.

Fig. 7 .

 7 Fig. 7. SEM micrographs of solid calcium phosphate products at low magnification (20-80 °C) and the initial calcium carbonate (A and B).

Fig. 8 .

 8 Fig. 8. SEM micrographs of large particles (left-hand side, L) and smaller particles (right-hand side, R).

Fig. 9 .

 9 Fig. 9. Particle size distribution by number (N) or volume (V) of particles (particle size is in base 10 logarithmic scale).

Fig. 10 .

 10 Fig. 10. Adsorption-desorption isotherms of the solid products.

Fig. 11 .

 11 Fig. 11. SEM images of CAP synthesized at 80 °C: (A) disk just prepared; (B) disk after soaking in Tris-SBF-27 mM solution at 37 °C for 7 days; (C) dried CAP powder; and (D) CAP powder after soaking in Tris-SBF-27 mM solution at 37 °C for 7 days.

Table 1

 1 IR peak positions and corresponding assignment[START_REF] Wilson | [END_REF][20][21][22]; Y and N: with and without the presence of absorption band, respectively.

	Wavelength,	Solid				Assignment
	cm -1	20 °C	40 °C	60 °C	80	°C
	3600	Y	Y	Y	Y	O\H (water), ν1 mode
	1645	Y	Y	Y	N	H\O\H, ν3 mode
	1545	N	N	Y	Y	C\O, ν3 mode
	1450	N	N	Y	Y	C\O, ν3 mode
	1415	N	N	Y	Y	C\O, ν3 mode
	1296	Y	Y	Y	N	P\(OH) stretching
	1194	Y	Y	Y	N	P\(OH) stretching
	1120	Y	Y	N	N	PO 4 , ν3 mode
	1076	Y	Y	Y	N	PO 4 , ν3 mode
	1055	Y	Y	Y	N	PO 4 , ν3 mode
	1038	Y	Y	Y	N	PO 4 , ν3 mode
	1018	Y	Y	Y	Y	PO 4 , ν3 mode
	960	Y	Y	Y	Y	PO 4 , ν1 mode
	916	Y	Y	Y	N	HPO 4 , ν4 mode
	870	Y	Y	Y	Y	C\O, ν2 mode (B-type CAPs or
						calcite)
	880	N	N	Y	Y	C\O, ν2 mode (A-type CAPs)
	711	Y	Y	Y	N	C\O, ν4 mode
	633	Y	Y	Y	Y	O\H (Ca-HA), libration
						HPO 4 , ν4 mode
	603	Y	Y	Y	Y	PO 4 , ν4 mode
	559	Y	Y	Y	Y	PO 4 , ν4 mode
	530	Y	Y	Y	N	HPO 4 , ν4 mode

Table 2

 2 Specific surface area (S BET ) and true density (D) of the solid products.

	Product	S BET , m 2 g -1	D, g cm -3
	Initial CaCO 3	b2	2.675
	Product at 20 °C	15	2.670
	Product at 40 °C	14	2.677
	Product at 60 °C	25	2.784
	Product at 80 °C	12	2.991

Table 3

 3 Carbonate contents (CO 2 wt.%) determined from TG curves.

	Solid	Decarbonation (CO 2 wt.%)
		B-type CAP form	A-type CAP form	CaCO 3 form
		(740-850 °C)	(850-1250 °C)	(600-720 °C)
	20 °C	0.27	0	9.20
	40 °C	0.49	0	9.77
	60 °C	1.52	0.01	5.41
	80 °C	0.76	4.05 a	0.18

Acknowledgments

The authors gratefully acknowledge our colleagues, Nathalie Lyczko, Christine Rolland, Sylvie Delconfetto, Philippe Accart and Yannick Coppel, for the different characterization measurements.

final formation of "pure apatitic products" which did not require further washing for the elimination of soluble counter ions. This represents real advantages of the present synthesis process in comparison with other routes for CAP synthesis, discussed in the Introduction section [START_REF] Fleet | [END_REF][13][14], since water-rinsing is usually a limiting step for the purification of powder products.

From TG curves, the carbonate contents in apatitic structure or in CaCO 3 form could be both estimated quantitatively (Table 3). In parallel with the decomposition of the initial calcium carbonate, the carbonate content (CO 2 wt.%) of CAP increased. The formation of A-B-type CAP (replacement of both PO 43-and OH -groups by CO 3 2-groups) was favorable at 80 °C, where the reaction reached the complete decomposition of starting calcium carbonate.

Conclusions

One-step synthesis of carbonate-containing apatite (CAP) has been investigated under moderate conditions, using calcium carbonate and orthophosphoric acid as ubiquitous starting materials. Simple and well known starting materials are advantageous for pharmaceutical productions. The reaction temperature was found to be the key parameter for both the decomposition of initial calcium carbonate particles and the formation of CAP. At 80 °C, which was lower than most hydrothermal conditions, the decomposition of calcium carbonate was complete. At this temperature, the final solid product was composed of only apatitic phases including Ca-HA and A-B type CAP. At lower temperatures, other calcium phosphate phases were formed, in particular OCP. In all cases, only traces of soluble calcium cations and orthophosphate species were found, which shows the complete transformation of the starting materials into the desired product. Carbon dioxide, formed in gas phase, was the only by-product of the synthesis process. CAP obtained at 80 °C was found to be bioactive for the growth of calcium phosphate on its surface after soaking in a simulated body fluid (Tris-SBF-27 mM).

Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.msec.2013.03.023.