Haroun Sebei
email: haroun.sebei@mines-albi.fr

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

Ange Nzihou

Patrick Sharrock

Sorption behavior of Zn(II) ions on synthetic apatitic calcium phosphates

Keywords:

The synthesis, characterization and the reactivity of apatitic calcium phosphates (Ca-HA, chemical formula Ca 10 (PO 4 ) 6 (OH) 2 ) is reported. Calcium carbonate (CaCO 3 ) and potassium dihydrogen orthophosphate (KH 2 PO 4 ) were selected as economical starting materials for the synthesis of Ca-HA under atmospheric conditions. Monocalcium phosphate monohydrate (MCPM), dicalcium phosphate dihydrate (DCPD), and octacalcium phosphate pentahydrate (OCP) were identified as the main intermediates of the synthesis reaction. The product obtained after 48 h of reaction contains mainly low-crystalline Ca-HA and small amounts of other calcium phosphates such as octacalcium phosphate (OCP), B-type carbonate apatite (CAP), as well as unreacted calcium carbonate. This Ca-HA was found to be active for the removal of Zn 2+ from an aqueous solution. Its sorption capacity reached up to 120 mg of Zn 2+ per g of Ca-HA powder after 24 h of reaction. The monitoring of soluble Zn, Ca and P during the sorption experiment allowed characterizing the mechanism of Zn uptake. Dissolution-precipitation, ionic exchange and surface complexation are the three main mechanisms involved in the sorption processes. The contribution of these mechanisms is discussed in detail.

Introduction

The low solubility of apatite minerals that are stable over geologic time associated to the ability of these materials to bind various metal ions promotes their use for environmental purpose. Thus, calcium hydroxyapatite (Ca-HA, Ca 10 (PO 4 ) 6 (OH) 2 ) is used for the removal of heavy metals from contaminated soils, wastewater or fly ashes [START_REF] Chen | Evaluation of heavy metal remediation using mineral apatite[END_REF][START_REF] Nzihou | Role of phosphate in the remediation and reuse of heavy metal polluted wastes and sites[END_REF][START_REF] Nzihou | Calcium phosphate stabilization of fly ash with chloride extraction[END_REF][START_REF] Bostick | Use of Apatite for Chemical Stabilization of Subsurface Contaminants[END_REF]. Many studies of the sorption of metals by Ca-HA have been undertaken using Ca-HA particles in aqueous solutions contaminated with divalent metals like Zn 2+ [START_REF] Sheha | Sorption behavior of Zn(II) ions on synthesized hydroxyapatites[END_REF][START_REF] Ma | Effects of aqueous Al, Cd, Cu, Fe(II), Ni, and Zn on Pb immobilization by hydroxyapatite[END_REF], Cd 2+ [START_REF] Ma | Effects of aqueous Al, Cd, Cu, Fe(II), Ni, and Zn on Pb immobilization by hydroxyapatite[END_REF][START_REF] Fedoroff | Sorption kinetics and diffusion of cadmium in calcium hydroxyapatites[END_REF] and Pb 2+ [START_REF] Ma | Effects of aqueous Al, Cd, Cu, Fe(II), Ni, and Zn on Pb immobilization by hydroxyapatite[END_REF][START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: equilibria and kinetic processes[END_REF]. These studies observed the ion exchange between Ca on Ca-HA surface and metal ion during metal fixation, but suggested mechanisms including surface complexation, dissolution-precipitation and co-precipitation can also be considered. The difficulty remains the relative contribution of each process in removing metals. The implication of these mechanisms in the case of Zn sorption is discussed in this research.

From a technico-economical point of view, the minimization of costs (sorbent production, operational conditions, etc.) plays a crucial role for the viability of a given environmental process. The use of inexpensive reagents, and associated with soft reaction conditions for producing sorbents is a solution. The reuse of industrial wastes as economical starting materials seems to match well with this goal [START_REF] Sebei | Valorization of carbonate waste as hydroxyapatite for heavy metals remediation[END_REF][START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF][START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF][START_REF] Hashimoto | Removal of aqueous lead by poorly-crystalline hydroxyapatites[END_REF].

In the present work, the synthesis of Ca-HA solids has been carried out by neutralization method at room temperature under atmospheric pressure, using calcium carbonate (calcite, CaCO 3 ) powder and an aqueous solution of potassium dihydrogen orthophosphate (KH 2 PO 4 ). CaCO 3 is naturally available as the cheapest calcium source. The Ca-HA based solid powder obtained from this synthesis was thoroughly characterized by various methods, in particular for determining the Ca/P ratio at different surface levels. Ca-HA powder was then used for the removal of Zn 2+ from an aqueous solution. Zn was chosen as model trace metal because it is one of the most concentrated and recurrent metals which can be present in wastewater [START_REF] Pomiès | Devenir des métaux au sein des stations d'épuration -Analyse d'une base de données spécifique[END_REF]. Zn plays an essential role in biological function but can be toxic at high concentrations [START_REF] Barrea | Distribution and incorporation of zinc in biological calcium phosphates[END_REF]. The use of pure starting materials for the synthesis of sorbent facilitates the analysis and characterization of the final powder product, which allows correlating the relationship between its composition and its sorption performance. The kinetics of Zn sorption was studied and the mechanisms for Zn fixation were unraveled.

Materials and methods

Ca-HA synthesis

Calcium carbonate powder (CaCO 3 , 98%, Fisher Scientific) and potassium dihydrogen orthophosphate (KH 2 PO 4 , 99%, Fisher Scientific) were used for the synthesis of Ca-HA powder.

The reaction was carried out in a 1 L glass reactor thermostated at 25 • C. An aqueous solution of KH 2 PO 4 at a concentration of 1.20 mol L -1 was prepared. The calcite powder was progressively added into the solution of KH 2 PO 4 . The final theoretical molar ratio of Ca to P was 1.67, which is characteristic for the stoichiometric phosphocalcic hydroxyapatite. This mixture was stirred for 48 h at a speed of 350 rpm with a helical propeller. During the reaction, the pH of the suspension was continuously measured with a Mettler Toledo MPC pH-meter and recorded using LabView software. Samples withdrawn from the reaction mixture and final powder were filtered on a 0.45 m hydrophilic filter to separate liquid and solid phases. The solid was washed several times with permuted water, dried overnight at 105 • C and then ground before further analyses. The liquid phase was acidified with nitric acid to avoid all secondary precipitation from calcium cations and orthophosphate species present in the liquid phase [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF].

Characterization methods

The analysis by X-ray diffraction (XRD) was conducted using a Philips X'Pert diffractometer with copper anti-cathode ( = 1.540560 Å). The identification of crystalline phases was determined by comparing the patterns with JCPDS standards. Thermal analysis was performed using SDT Q600 equipment (TA instruments). About 20 mg of solid were heated under air flux at 20 • C/min in an alumina crucible in the temperature range of 30-1000 • C. Fourier transformed infra-red spectroscopy (FTIR) was performed using a SHIMADZU -8400S spectrophotometer scanning between 4000 and 500 cm -1 in the reflexion mode. Scanning electron microscopy (SEM) was performed with a Philips XL30 ESEM FEG equipped with an EDAX analyzer. X-ray photoelectron spectroscopy (XPS) analysis was conducted with a VG ESCALB MK-II electron spectrometer using an Mg K␣ X-ray source (1253.6 eV). The energy scale was internally calibrated by referencing the binding energy of the C1s peak at 284.6 eV for contaminated carbon. The elemental analysis of phosphorus, calcium and potassium in the liquid phase was carried out with inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a HORIBA JobinYvonUltima 2.

The true density of the powder was measured by helium pycnometry (MICROMERITICS Accupyc 1330). The particles size distribution of Ca-HA powder was determined with a MALVERN Laser Mastersizer Hydro 2000 in liquid medium. The specific surface area of the particles was determined by nitrogen adsorption using the BET method (MICROMERITICS Gemini Vacprep 061). The adsorption-desorption isotherm was determined with a MICRO-METRICS ASAP 2010 using nitrogen as gas adsorbate with the data collection from relative pressure (P/P 0 ) of 0.03-0.99.

Sorption study

Aqueous solution of zinc nitrate was prepared by dissolving zinc nitrate (p.a. grade from Fisher Scientific) in permuted water to obtain a 1500 ppm Zn(II) solution. The experiments of sorption kinetics were carried out in a stirred glass reactor containing 1000 mL of the prepared zinc nitrate solution. The experiment started by addition of 8 g of Ca-HA powder sorbent into the reactor under constant stirring (350 rpm) at room temperature (ca. 25 • C). The contact time chosen for all sorption experiments was 24 h, which was considered to be sufficient to get apparent equilibrium, as well as according to the literature [START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: equilibria and kinetic processes[END_REF][START_REF] Peld | Sorption and desorption of Cd 2+ and Zn 2+ ions in apatite-aqueous systems[END_REF]. Each sorption experiment was followed by filtration of the suspension through hydrophilic filter (0.45 m), then the solid was washed, dried at 105 • C and then ground for further characterizations. The concentration of released Ca, P, K and remaining Zn ions in the equilibrium solution was quantified using the ICP-AES technique. During the sorption, the pH of the solution was continuously measured.

In order to calculate the amounts of metal ions sorbed by Ca-HA per mass unit, the following expression was used:

Q = (C o -C e ) × V M ,
where Q is the amount of Zn 2+ removed from the solution (mg of Zn per g of sorbent), C o and C e are the concentrations (mg L -1 ) of Zn 2+ in the initial zinc nitrate solution and at the sorption equilibrium, respectively, V (L) is the volume of the solution and M (g) the amount of Ca-HA used.

Results and discussion

pH evolution during synthesis

The initial orthophosphate solution had acid pH value of 4.2. When calcium carbonate was added, it reacted immediately with the acid solution, liberating carbon dioxide gas. Fig. 1 shows the evolution of recorded pH value during the first hour of reaction where three zones could be distinguished:

-Zone A:

CaCO 3 + 2H + → Ca 2+ + H 2 O + CO 2 (1)
2H 2 PO 4 2-+ 2H + + CaCO 3 → Ca(H 2 PO 4 ) 2 •H 2 O + CO 2 (2)
The fast pH increase during the first minutes was due to the acid attack on the calcium carbonate powder (Eq. ( 1)) where free hydrated calcium cations (Ca 2+ ) were formed and protons were consumed. Then the first calcium phosphate compounds were formed by reaction between orthophosphate species and the calcium ions. At this early stage, monocalcium phosphate monohydrate (MCPM, Ca(H 2 PO 4 ) 2 •H 2 O) was formed as the main calcium phosphate (Eq. ( 2)), because H 2 PO 4 2-is the main species at the pH of the solution in the zone A [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF].

-Zone B: The next evolution has been identified as the transformation of MCPM into dicalcium phosphate dihydrate (DCPD, CaHPO 4 •2H 2 O), as described by Eq. (3) [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. This leads to the pH stabilization during few minutes (Zone B, Fig. 1). This is explained by the equilibrium between the consumption of protons (Eqs. ( 1) and ( 2)) and protons release (Eq. ( 3)).

Ca(H 2 PO 4 ) 2 •H 2 O + Ca 2+ + 3H 2 O → 2[CaHPO 4 •2H 2 O] + 2H + (3)
-Zone C:

The pH increased slightly indicating the slow evolution of MCPM (Ca(H 2 PO 4 ) 2 •H 2 O) and DCPD (CaHPO 4 •2H 2 O) into other calcium orthophosphates [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. At 48 h of reaction, the pH reached a value of 8.

ICP-AES analysis

The liquid fraction (filtrate) from the filtration of samples withdrawn from the medium during the synthesis was also analyzed by ICP-AES for the determination of soluble calcium and phosphate contents. Table 1 summarizes the results obtained.

For all the samples, low concentration of soluble calcium was observed. So, most of calcium can be found in solid form, either as calcium phosphates (CaP) or as undissolved calcium carbonate. For phosphate species, the conversion of phosphorus (X P ) was stagnated at about 72%, after 24-48 h of reaction. This limit may be due to the driving force induced by the presence of potassium ions (K + ) in solution, to counteract the ionic charge in aqueous phase [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF].

Table 2 gives the results of elemental analysis of the solid product obtained after 48 h of reaction. The bulk molar ratio of Ca to P of the solid after 48 h of reaction was 2.38. But TG analysis (in the next section) shows that this solid product contains about 30 wt.% of CaCO 3 (Table 3). This means that among 355 mg g -1 of calcium determined by ICP-AES, there were 120 mg g -1 of calcium under remaining in calcite form. So there were maximum 235 mg g -1 of calcium under calcium phosphates forms. Thus, the calculated molar ratio of Ca to P of this solid was 1.58 for calcium phosphates. This suggests the formation of calcium-deficient apatites, such as octacalcium phosphate (OCP, Ca 8 H 2 (PO 4 ) 6 •5H 2 O), as confirmed by Counts, a. u. XRD analysis (Section 3.4). Despite the washing of the solid several times with water, small amounts of potassium still remain in the final solid product. The potassium might exist in carbonate form (KHCO 3 ), or might be inserted into the apatitic structure [START_REF] Neuman | Synthetic hydroxyapatite crystals I. Sodium potassium fixation[END_REF].

theta

TG analysis

Fig. 2 shows TG results obtained for the initial calcium carbonate and Ca-HA solids recovered at different reaction times. The weight loss at the temperature below 100 • C is attributed to the elimination of physically absorbed surface moisture. Between 110 and 150 • C, only the sample after 2 h of reaction shows a significant weight loss. This one is attributed to the dehydration of MCPM

Ca(H 2 PO 4 ) 2 •H 2 O → Ca(H 2 PO 4 ) 2 + H 2 O (4) CaHPO 4 •2H 2 O → CaHPO 4 + 2H 2 O ( 5
)
The monetite is then thermally transformed into calcium pyrophosphate (Ca 2 P 2 O 7 ) between 365 and 510 • C (≈ 2%) following Eq. ( 6).

2CaHPO 4 → Ca 2 P 2 O 7 + H 2 O (6)
For the samples after 24 h and 48 h of reaction, these intermediates were not significantly detected by TG analysis, indicating that the synthesis reached equilibrium after 24-48 h of reaction.

For all the samples, the main weight loss observed between 550 and 830 • C corresponds to the release of carbon dioxide gas (Eq. ( 7)) as the result of the decarbonation of remaining calcium carbonate into quicklime (CaO). CaCO 3 → CaO + CO 2 [START_REF] Fedoroff | Sorption kinetics and diffusion of cadmium in calcium hydroxyapatites[END_REF] Regarding the samples after 24 h and 48 h of reaction, a small weight loss between 800 and 900 • C can be observed. This could be attributed to the decarbonation of B-type carbonate apatite (CAP) [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF][START_REF] Elliott | Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF].

From the TG results above, we could calculate the calcium carbonate content remaining in the solid products corresponding to its thermal decomposition between 550 and 830 • C (Table 3).

After 2 h of reaction, a half of CaCO 3 was converted into calcium phosphates. This is explained by a strong acid attack of calcium carbonate particles by phosphate solution at the beginning of the reaction. Then the decomposition of CaCO 3 is slowed down because of the increase of the pH to a neutral value (Fig. 1). A stable value of CaCO 3 content about 30% is obtained after 24-48 h of reaction under the chosen operating conditions. In addition, the precipitation of calcium phosphates during the reaction probably limits the contact between the particles of calcium carbonate and soluble phosphates, because of the formation of core-shell structure wherein remaining calcium carbonate was found in the core [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF].

From the results in Tables 1 and3, it can be observed that the conversions of both the reagents used (KH 2 PO 4 and CaCO 3 ) were fairly close to each other. Other physico-chemical characterizations will focus on the solid product obtained after 48 h of reaction in order to evidence its different components.

XRD characterization

The diffractogram of Ca-HA powder obtained after 48 h of reaction and dried at 105 • C is presented in Fig. 3. Remaining calcite could be clearly observed with its fine and intense peak at 29.4 • . This calcite is the crystal form of calcium carbonate (CaCO 3 ) used for Ca-HA synthesis. Different calcium phosphates were also observed.

Low crystallinity Ca-HA appeared as the main calcium phosphate with its principal diffraction peak at 32 • . This last one is a spread out peak, which is typical of a poorly crystalline phase. The low crystallinity of calcium phosphates based components is due to the moderate reaction temperature of synthesis, ca. 25 • C. Higher temperatures are required for the formation of well-crystallized Ca-HA [START_REF] Yoshimura | Hydrothermal conversion of calcite crystals to hydroxyapatite[END_REF][START_REF] Jinawath | Hydrothermal synthesis of monetite and hydroxyapatite from monocalcium phosphate monohydrate[END_REF]. To highlight the presence of Ca-HA as the main calcium phosphate, the synthesized solid was calcined at 1000 • C for 2 h. The XRD pattern of this calcined solid is presented in Fig. 4. Stoechiometric hydroxyapatite was clearly observed. Lime (CaO) and portlandite (Ca(OH) 2 ) were present as the results of the decarbonation of the remaining calcite at 1000 • C.

In addition to Ca-HA, B-type calcium-carbonate apatite (B-type CAP, formed by the substitution of PO 4

3-groups by CO 3 2-groups) seemed to be detected but at much lower amounts. The formation of carbon dioxide from the dissolution of calcite, associated with the increase of pH favored this substitution. This feature has been highlighted by IR analysis in a previous study [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. The presence of CAP is in agreement with the TG analysis presented above. Octacalcium phosphate (OCP, Ca 8 H 2 (PO 4 ) 6 •5H 2 O), was also formed in small amounts, evidenced by its main peak at 4.7 • . OCP is known as an intermediate state during the formation of Ca-HA in an aqueous solution [START_REF] Elliott | Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF].

SEM-EDX analysis

The Ca-HA powder obtained after 48 h of reaction was examined by SEM and some examples are presented in Fig. 5. In parallel, EDX analysis was also performed. For spot mode of analysis, a volume of about 1 m 3 was analyzed (1 m × 1 m × 1 m). In all cases, EDX analysis showed the presence of Ca, P, O, K and C (Table 4). Potassium was only found in trace amounts. Carbon came from the remaining calcium carbonate, carbonic gas in air and CAP formed during reaction. Ca, P, and O were the main elements of calcium phosphates and/or calcium carbonate.

Different morphologies were observed (Fig. 5a). The first population includes large spherical-like agglomerates consisting of needle-like assembled particles (>30 m), which seemed to be the result of the agglomeration of smaller particles, as highlighted in Fig. 5b andc. These morphologies would be characteristic of hydroxyapatite phase [START_REF] Legeros | Calcium phosphates in oral biology and medicine[END_REF][START_REF] Panda | SEM and solid state NMR investigations of carbonate-containing hydroxyapatite nano-particles synthesized by hydroxide-gel technique[END_REF]. EDX analysis of these large particles, repeated on more than 15 particles, showed the atomic ratio of Ca to P equal to 1.7. This ratio is close to that of stoichiometric hydroxyapatite (Table 4). The calculated standard deviation was low (Table 4), which suggests that the surface of large particles have a good homogeneity (for the depth of EDX analysis of 1 m).

The second population consists in spherical-like particles smaller than 30 m. EDX analysis was also performed on these particles (Table 4). The result showed an average atomic ratio of Ca to P of 2.9, with a high standard deviation. So the surface of these particles was more heterogeneous in terms of chemical composition, compared to large particles. EDX beam might reach the core of small particles, where are located the remaining calcium carbonates [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF]. Consequently, the atomic ratio of Ca to P is higher due to the presence of CaCO 3 . The average atomic ratio of Ca to P of both populations was equal to 2.1.

XPS analysis

The XPS analysis was conducted on an elliptical surface of 400 m × 200 m with the depth of 10 nm. Taking into account the size of the particles scanning by electron microscopy (average mean diameter of particles, d 50 of 24.2 m, Table 6 in the next section), the atomic percentages calculated by semi-quantitative XPS analysis corresponds to the average obtained on a powder bed. Therefore, it is not possible to discriminate the composition of different morphologies of particles as done with SEM-EDX. Fig. 6 shows the XPS spectrum of Ca-HA powder obtained after 48 h of reaction. Oxygen (O1s), calcium (Ca2p), phosphorus (P2p) and carbon (C1s) were detected as the major elements. The atomic content of these elements, determined by XPS analysis, were 13.1, 20.3, 50.5 and 16.1% for P2p, Ca2p, O1s and C1s, respectively. The peak of the carbon C1s at 284.6 eV could be attributed to the remaining calcium carbonate, to CAP, and to surface contamination. For Ca2p, peak deconvolution between 344 and 354 eV shows the major contribution of apatitic calcium phosphate (maximum of peak at 347.5 and 351 eV) for the first layers of surface (Fig. 7). Peaks corresponding to calcium carbonate (maximum of peak at 346.8 and 350.4 eV) had smaller intensity. On the other hand, the distinction of different phosphocalcic phases present in the sample, i.e. Ca-HA, OCP, DCPD,CAP, remains difficult [START_REF] Demri | XPS study of some calcium compounds[END_REF]. Table 5 compares the atomic ratio of Ca to P determined by different methods. A decrease of the atomic ratio of Ca to P from the core to the surface of the solid particles was observed. In fact, the remaining calcium carbonate must be found in the core of particles, as previously evidenced by Pham Minh et al. [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. In addition, apatitic calcium phosphate contains usually hydrogenophosphate groups (HPO 4 2-) on its surface, when it was in contact with the humidity in air. This surface would be similar to a mixture of phosphocalcic species including DCPD, DCPA and OCP, which have the molar ratio of Ca to P smaller than 1.67 [START_REF] Bengtsson | Solubility and Surface Complexation Studies of Apatites[END_REF].

Physical characterizations

Physical characterizations of the solid powder obtained after 48 h of reaction are presented in Table 6. Despite the incomplete conversion of the initial calcium carbonate, this solid powder had a high specific surface area (S BET ) of 148 m 2 g -1 . This specific surface area could be explained by the presence of mesopores with the average mean diameter of pores of 11 nm, obtained by nitrogen adsorption. The average mean diameter of particles obtained by laser scattering was 24.2 m, confirming the results of SEM analysis. Finally, this solid had the true density of 2.74 g cm -3 , usually found for a given calcium phosphate synthesized at moderate temperature.

Reactivity of the solid powder during Zn 2+ removal

This section communicates the results on the removal of Zn 2+ from an aqueous solution using the solid powder obtained after 48 h of reaction. As mentioned above, the solid sorbent was filtered, washed, dried at 105 • C and then ground before sorption experiment. The initial solid sorbent is thereafter called Ca-HA-P before sorption experiment. The solid recovered after sorption experiment is called Ca-HA-P-Zn.

Kinetic study

Fig. 8 shows the evolution of the concentration of the different elements present in solution as well as pH, for an initial concentration of Zn of 1500 mg L -1 . The initial pH of zinc nitrate solution was close to 4.9. It rapidly increased to 6 after only 2 min of contact with calcium phosphate powder. This increase was probably induced by the buffering capacity of apatitic calcium phosphates [START_REF] Smičiklas | The point of zero charge and sorption of cadmium (II) and strontium (II) ions on synthetic hydroxyapatite[END_REF], and also of calcium carbonate remaining in the solid sorbent. Then, the pH evolved slowly to the final value of 6.7 after 24 h of reaction.

For the evolution of the concentration of the soluble elements in the sorption medium, we observe that the kinetic of calcium release to the liquid was equivalent to that of Zn removal from the liquid. A rapid decrease of Zn concentration in the first minutes of sorption was observed. 30 and 43% of Zn were removed after respectively 1 and 10 min of reaction. Then the sorption yield slowed down strongly after 5 h of reaction time. This kinetic profile is typical for the sorption of a given metal using Ca-HA based sorbent in batch reactor [START_REF] Fedoroff | Sorption kinetics and diffusion of cadmium in calcium hydroxyapatites[END_REF][START_REF] Corami | Copper and zinc decontamination from single-and binary-metal solutions using hydroxyapatite[END_REF]. After 24 h of reaction, the concentration of Zn remaining in the solution was around 500 ppm, corresponding to the removal yield of 66% of the Zn present and the sorption capacity of 118 mg of Zn per g of sorbent. In parallel with Zn removal, calcium was quickly released to the liquid phase during the first minutes of reaction. Its concentration was 700 ppm after 180 min of contact time and finally reached 800 ppm after 24 h of reaction.

Soluble phosphorus and potassium were found at much lower concentrations. Soluble phosphorus did not exceed 1 ppm along the sorption experiment. The low concentration of soluble phosphate species is explained by the abundance of divalent cations (Ca 2+ and Zn 2+ ) in the reaction medium. The concentration of soluble potassium varied between 3 and 6 ppm.

In order to evaluate the sorption rate of Zn 2+ , a pseudo-second order kinetic model was applied to the results in Fig. 8 [START_REF] Ho | A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents[END_REF]. This model is described as follows:

t Q t = 1 h + t × 1 Q e (8)
where Q t and Q e is the amount of Zn sorbed on Ca-HA at any time t and at equilibrium respectively (mg g -1 ), t is the contact time (min) and h is the constant of sorption rate (g mg -1 min -1 ). The plot of t/Q t against the contact time t allows determining the values of Q e and h (Table 7). The value of R 2 close to 1 indicates that the experimental data of Zn sorption matches well with the pseudo-second order kinetic model. The value of Q e obtained from the model (123 mg g -1 ) is close to that from the sorption experiment (118 mg g -1 ).

Table 8 compares the performance of the Ca-HA based sorbent in this work with other Ca-HA reported in the literature for the removal of Zn 2+ . Starting from CaCO 3 and KH 2 PO 4 , Ca-HA sorbent Table 7 Sorption capacity (Qe), rate constant (h) and correlation coefficients (R 2 ) of synthesized Ca-HA calculated from the pseudo-second order sorption kinetic model.

Qe, mg g

-1 h, g mg -1 min -1 R 2
Ca-HA-P 123 6.2 × 10 -4 0.9993

Table 8

Comparative sorption capacity (Qe) of the synthesized Ca-HA sorbent in this work with other Ca-HA sorbents reported in the literature in the sorption of zinc(II) from aqueous solution.

Ca-HA, g L -1 Zinc, mg L -1 Qe, mg g [START_REF] Elouear | Heavy metal removal from aqueous solutions by activated phosphate rock[END_REF] in this work had higher Q e in comparison with Ca-HA made from soluble calcium salt like Ca(NO 3 ) 2 [START_REF] Sheha | Sorption behavior of Zn(II) ions on synthesized hydroxyapatites[END_REF][START_REF] Peld | Sorption and desorption of Cd 2+ and Zn 2+ ions in apatite-aqueous systems[END_REF][START_REF] Corami | Copper and zinc decontamination from single-and binary-metal solutions using hydroxyapatite[END_REF][START_REF] Smičiklas | Factors influencing the removal of divalent cations by hydroxyapatite[END_REF], as well as from phosphate rocks [START_REF] Elouear | Heavy metal removal from aqueous solutions by activated phosphate rock[END_REF][START_REF] Cao | Mechanisms of lead, copper, and zinc retention by phosphate rock[END_REF]. This may be explained by the low crystallinity of the Ca-HA synthesized in this work, which improves the physico-chemical properties of the sorbent (Table 6), i.e. high specific surface area, high porosity and small particle size. The low crystallinity of Ca-HA was due to the soft synthesis conditions of the sorbent (at 25 • C under atmospheric pressure), as well as the use of calcium carbonate as the non-conventional calcium source. This last one promotes the presence of gaps and impurities (CO 3 2-ion insertion in Ca-HA structure), improving the sorption properties of Ca-HA [START_REF] Miyake | Synthetic carbonate apatites as inorganic cation exchangers. Exchange characteristics for toxic ions[END_REF].

Discussion on sorption mechanism

According to the literature, several sorption mechanisms were proposed to occur simultaneously and/or successively. Ionic exchange, surface complexation reaction and dissolution-precipitation are the three main mechanisms invoked in the sorption process of Zn by Ca-HA based materials [START_REF] Peld | Sorption and desorption of Cd 2+ and Zn 2+ ions in apatite-aqueous systems[END_REF][START_REF] Xu | Sorption of Zn 2+ and Cd 2+ on hydroxyapatite surfaces[END_REF][START_REF] Lee | Sorption mechanisms of zinc on hydroxyapatite: systematic uptake studies and EXAFS spectroscopy analysis[END_REF].

The dissolution-precipitation mechanism consists in two successive steps, the first one is the dissolution of Ca-HA in an acid medium (Eq. ( 9)), followed by the precipitation of Zn phosphate (Eq. ( 10)). H 2 PO 4

-is expected as the main soluble phosphate species in the solution, taking into account the pH profile of the reaction medium (4.9-6.7) [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. However, the low concentration of the soluble phosphate species (Fig. 8) suggests that this mechanism is only marginal, as usually observed in the literature [START_REF] Corami | Copper and zinc decontamination from single-and binary-metal solutions using hydroxyapatite[END_REF]. In addition, XRD analysis did not show the formation of apatitic calcium-zinc-phosphate solid solution (data not shown).

In the case of Zn removal at high initial concentration, the formation of hopeite (hydrated Zn phosphate, Zn 3 (PO 4 ) 2 •4H 2 O) might also take place, as illustrated in Eq. ( 11) [START_REF] Lee | Sorption mechanisms of zinc on hydroxyapatite: systematic uptake studies and EXAFS spectroscopy analysis[END_REF][START_REF] Stötzel | Ion adsorption behaviour of hydroxyapatite with different crystallinities[END_REF][START_REF] Chen | Effects of pH on heavy metal sorption on mineral apatite[END_REF]. TG analysis of the solid recovered after sorption experiment (Ca-HA-P-Zn) evidenced the presence of hopeite (Fig. 9). The weight loss between 125 and 200 • C (around 1 wt.%) was attributed to the first dehydration of hopeite, followed by its second dehydration between 200 and 340 • C (around 2 wt.%) to form anhydrous hopeite [START_REF] Herschke | Polymer Controlled Mineralization of Zinc Phosphate Hydrates and Applications in Corrosion Protection, Catalysis and Biomedicine[END_REF]. Taking into account the pH profile of the sorption medium, we suppose that the dissolution-precipitation mechanism might have occurred at the beginning of the sorption experiment, when the pH was lowest, which is favorable for the dissolution step. also the participation of CAP to the removal of Zn 2+ . The decarbonation of B-type CAP at 800-900 • C disappeared from the TG curve of Ca-HA-P-Zn. In fact, the insertion of carbonate groups into the apatite structure increases the solubility of Ca-HA and promotes the presence of gaps in the apatitic structure [START_REF] Elliott | Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF]. Thus, a preferential dissolution of these carbonates groups may be assumed, releasing phosphate groups to the benefit of zinc phosphate precipitation. B-type CAP has been found to be more active for metal removal compared to Ca-HA [START_REF] Miyake | Synthetic carbonate apatites as inorganic cation exchangers. Exchange characteristics for toxic ions[END_REF].

3Zn 2+ + 2H 2 PO 4 -+ 4H 2 O ↔ Zn 3 (PO 4 ) 2 •4H 2 O + 4H + (11)
The ion-exchange mechanism can be described by Eq. ( 12) [START_REF] Xu | Sorption of Zn 2+ and Cd 2+ on hydroxyapatite surfaces[END_REF]. The equimolar ion exchange is expected, and there is no release of protons in solution. However, this mechanism could not occur alone, because of the difference in the concentration of the released calcium and that of the removed Zn (Fig. 10). In Fig. 10, we observe that the molar concentration of the released calcium was always higher than that of the removed Zn. In addition, ion-exchange mechanism did not involve the release or consumption of protons, but the pH profile of the sorption medium changed along the sorption experiment (Fig. 8).

xZn 2+ + Ca 10 (PO 4 ) 6 (OH) 2 ↔ (10 -x)Ca 2+ + (Zn x , Ca 10-x )(PO 4 ) 6 (OH) 2 [START_REF] Hashimoto | Removal of aqueous lead by poorly-crystalline hydroxyapatites[END_REF] The surface complexation mechanism has been proposed by several authors in the literature [START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: equilibria and kinetic processes[END_REF][START_REF] Mavropoulos | Studies on the mechanisms of lead immobilization by hydroxyapatite[END_REF]. According to Wu et al. [START_REF] Wu | Surface complexation of calcium minerals in aqueous solution: 1. Surface protonation at fluorapatite-water interfaces[END_REF], for a pH between 4.8 and 6, the main surface groups of an apatitic calcium phosphate are POH and CaOH 2 + . So the surface complexation mechanism can be described by Eqs. ( 13) and ( 14). As shown in Fig. 11,a 13) and ( 14)).

POH + Zn 2+ ↔ POZn + + H + (13)

CaOH 2 + + Zn 2+ ↔ CaOHZn 2+ + H + (14)
In summary, all three mechanisms discussed above might take place during Zn 2+ removal by Ca-HA starting with acid aqueous solutions. However, the pH profile and the concentration of soluble phosphate species allow confirming that dissolution-precipitation mechanism could happen marginally at the beginning of the sorption, wherein the pH was still sufficiently acid. The surface complexation must also start at the beginning of the reaction, leading to the decrease of pH during the first seconds of reaction. The surface complexation explains the non-equimolar exchange between Zn 2+ and Ca 2+ , observed in Fig. 10, according to Eqs. ( 13) and [START_REF] Barrea | Distribution and incorporation of zinc in biological calcium phosphates[END_REF]. Of course, the eventual acid attack on the remaining calcite particles, leading to the release of Ca 2+ to the solution, explains also the higher concentration of the released Ca 2+ compared to the concentration of the removed Zn 2+ . The ion-exchange mechanism might also occur, in parallel with other processes because of the driving force of the sorption medium. The driving force is the difference in solubility between Ca-HA and Zn-doped Ca-HA newly formed (Ca-HA-P-Zn) [START_REF] Elliott | Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF][START_REF] Nriagu | Lead orthophosphates-II. Stability of cholopyromophite at 25 • C[END_REF][START_REF] Takeushi | A study of equilibrium and mass transfer in processes for removal of heavy-metal ions by hydroxyapatite[END_REF]. In our case, the solubility product K s of the initial Ca-HA and of the final solid recovered from the sorption experiment (Ca-HA-P-Zn) was measured. Ca-HA-P-Zn had lower solubility (K s : 5.43 × 10 -99 ) than that of the initial Ca-HA (K s : 1.78 × 10 -94 ). This favors the ion-exchange between Zn 2+ from the solution with Ca 2+ from apatitic matrix. This ion-exchange is expected for a slow sorption kinetic. However, we cannot conclude that a solid solution was formed, as no differences in cell parameters were detected. An amorphous surface deposit of zinc phosphate could explain the result of ion exchange. About the formation of hopeite, the acid pH might the local dissolution of the phosphocalcic hydroxyapatite (Ca 2+ , PO 4 3-, OH -). This promotes the formation of heterogeneous precipitate, i.e. hopeite, under acid pH and with high initial concentration of Zn.

Regarding now the localization of Zn, fixed by the Ca-HA sorbent. Fig. 12 shows the imaging of the main elements present in the used sorbent. As expected, calcium was well located in the core of particles (Fig. 12A), probably in form of CaCO 3 . In fact, phosphorus was usually not present in the core of particles (Fig. 12B). Oxygen was present in the entire particles because oxygen is the component of both the remaining calcium carbonate and the calcium/zinc phosphates. Finally, Zn was only found on the surface of particles. This last confirms that the fixation of Zn by Ca-HA based particles was governed by surface processes, as discussed above.

Conclusions

Ca-HA based sorbent has been synthesized from calcium carbonate (calcite, CaCO 3 ) and potassium dihydrogen orthophosphate (KH 2 PO 4 ) at moderate conditions. Apatitic Ca-HA based sorbent of low crystallinity was formed. The intermediates of the synthesis reaction have been identified as mainly MCPM, DCPD and OCP. The characterization of the final product showed the presence of apatitic compounds including Ca-HA, B-type CAP, OCP, and unreacted calcium carbonate as the main components. Remaining calcium carbonate was found in the core of particles. The elemental analysis of the sorbent by three different methods (ICP-AES, SEM-EDX, and XPS) evidenced the decrease of the molar ratio of Ca to P from the core to the surface of particles. The average size of the final solid sorbent was 24.2 m. Two populations of particles were observed. The first one was composed of particles smaller than 30 m. The second one included big agglomerates of tens of m. Under soft synthesis conditions, the final solid sorbent had high specific surface area (148 m 2 g -1 ), associated with mesopores (average mean diameter of 11 nm).

This solid sorbent had high reactivity for the removal of Zn 2+ from an aqueous solution, with the sorption capacity (Q e ) of 118 mg of Zn 2+ per g of sorbent. This high performance is explained by the high specific surface area and porosity, the low crystallinity of the sorbent, and the presence of gaps and impurities (CO 3 2-ions) in the apatitic structure of the sorbent. The mechanisms of Zn uptake were also discussed, which were surface processes.

This study demonstrated a simple route for the production of Ca-HA based sorbent from economical starting materials. The high performance of this synthesized sorbent in the sorption of Zn opens new prospects on the valorization of carbonate and phosphate industrial wastes for environmental purposes and for wastewater remediation in particular.

Fig. 1 .

 1 Fig.1. pH evolution during the first 60 min of the reaction where calcium carbonate powder was progressively added into the potassium dihydrogen orthophosphate solution.

Fig. 3 .

 3 Fig. 3. XRD patterns of Ca-HA powder after 48 h of reaction, dried at 105 • C. Principal diffractions of CaCO3 (⃝); Ca-HA ( ); OCP ().

Fig. 4 .

 4 Fig. 4. XRD patterns of Ca-HA powder after 48 h of reaction, which was calcined at 1000 • C for 2 h. Principal diffractions of Ca-HA ( ); CaO (⃝); Ca(OH)2 (+).

Fig. 5 .

 5 Fig. 5. SEM micrographs of Ca-HA powder after 48 h of reaction and drying at 105 • C, (a) general morphology, (b) and (c) detailed morphology.

Fig. 6 .

 6 Fig. 6. XPS spectrum of Ca-HA powder obtained after 48 h of reaction.

Fig. 7 .

 7 Fig. 7. Peak deconvolution for the signal of Ca2p; 1: apatitic calcium phosphate; 2: calcium carbonate.

Fig. 8 .

 8 Fig.8. Evolution of pH and concentration of the soluble Zn, Ca, P, K in solution as a function of contact time during batch sorption experiment. Sorption conditions: 1 L of 1500 ppm of Zn 2+ , 8 g of Ca-HA-P, room temperature.

Fig. 9 Fig. 9 .Fig. 10 .

 9910 Fig. 9. Derivatives thermogravimetric (DTG) curves of Ca-HA-P and Ca-HA-P-Zn.

Fig. 11 .

 11 Fig. 11. pH evolution during the first 5 min of sorption experiment. Sorption conditions: 1 L of 1500 ppm of Zn 2+ , 8 g of Ca-HA-P, room temperature.

Fig. 12 .

 12 Fig. 12. Imaging of Ca-HA-P-Zn powder drying at 105 • C obtained by SEM-EDX analysis; (a) calcium, (b) phosphorus, (c) oxygen, (d) zinc. (Electronic version is required for a good observation of these images.)

Table 1

 1 Analysis of soluble phosphorus and calcium in the filtrate; XP: conversion of initial phosphorus into solid calcium phosphate products.

	Reaction time, h	[P], mmol/L	XP, %	[Ca], mmol/L
	0	1484	0	0
	2	757	49	1.7
	24	428	71	1.7
	48	421	72	0.3

Table 2

 2 Elemental composition of the solid powder obtained after 48 h of reaction.

	Element	K,	Ca, mg g -1 Ca,	P,	Molar ratio
		mmol g -1		mmol g -1	mmol g -1	of Ca to P
	Content by ICP-AES	0.46 ± 0.01 355 ± 2	8.87 ± 0.04 3.72 ± 0.02 2.38 ± 0.01

Table 3

 3 Evolution of calcium carbonate content remaining in the solid product calculated from the results of TG analysis. Derivatives thermogravimetrics (DTG) curves of pure CaCO3 and Ca-HA powders after 2 h, 24 h and 48 h of reaction.

	Reaction	CO2, wt%	Content of	CaCO3
	time, h		remaining	conversion
			CaCO3, %	(XCaCO 3 ), %
	0	43	100	0
	2	21	49	51
	24	13	30	70
	48	13	30	70

Table 4

 4 Elemental

	Atomic %
	C	O	P	K	Ca	Molar ratio of Ca to P

analysis of Ca-HA powder obtained after 48 h of reaction by EDX analysis (depth of EDX analysis of 1 m). Large particles (>30 m) 29.5 ± 3.7 44.0 ± 7.0 9.6 ± 1.3 0.7 ± 0.3 16.3 ± 2.7 1.7 ± 0.1 Average Ca/P 2.1 ± 0.7 Small particles (<30 m) 38.4 ± 6.7 43.9 ± 3.8 4.5 ± 0.5 0.2 ± 0 13.0 ± 3 2.9 ± 0.5

(Ca(H 2 PO 4 ) 2 •H 2 O), which was still present after 2 h of reaction (Eq. (

4

)). DCPD (or brushite, CaHPO 4 •2H 2 O) was the most important intermediate after 2 h of reaction (Eq. (

5

)), as shown by its dehydration between 155 and 230 • C (≈ 4%) to form monetite (DCPA, CaHPO 4 ):

Table 5

 5 Comparison of atomic ratio of Ca to P obtained by different methods for the solid powder after 24 h of reaction.

	Method	Depth of analysis	Molar ratio of
			Ca to P
	ICP-AES	Bulk analysis	2.4
	SEM-EDX	Around 1000 nm	2.1
	XPS	Around 10 nm	1.5

Table 6

 6 Physical properties of the solid powder obtained after 48 h of reaction.

	SBET, m 2 g -1	d50, m	True density, g cm -3	dpores, nm	Porosity, %
	148	24.2	2.74	11	61

  Ca 10 (PO 4 ) 6 (OH) 2 + 14H +

	↔ 10Ca 2+ + 6H 2 PO 4	-+ Ca 2+ + 2H 2 O	(9)
	xZn 2+ + 10Ca 2+ + 6H 2 PO 4	-+ 2H 2 O
	↔ (Zn x , Ca 10-x )(PO 4 ) 6 (OH) 2 + xCa 2+ + 8H +

Acknowledgments

The authors gratefully acknowledge the scientific and financial support of Solvay Group (Belgium) and colleagues at RAPSODEE Centre, Christine Rolland, Céline Boachon, Jean-Marie Sabathier, for their technical help.


Doan Pham

Ngoc Dung Tran

Ange Nzihou

Patrick Sharrock

One-Step Synthesis of Calcium Hydroxyapatite from Calcium Carbonate and Orthophosphoric Acid under Moderate Conditions

Calcium hydroxyapatite (Ca-HA) is a versatile material which can be used in several applications. This study aimed to investigate the one-step synthesis of Ca-HA using solely calcium carbonate as-is and orthophosphoric acid as widely available and low cost initial reactants. The stirring rate and quantity of water had only some slight effects on the reaction rate. On the other hand, the reaction temperature influenced strongly the decomposition of the initial calcium carbonate particles and therefore the composition of the solids obtained. At 25-60 °C, high contents of calcium carbonate remained in the solid products even after a long reaction time of 168 h. At 80 °C, the decomposition of calcium carbonate was nearly complete after 24 h of reaction. Brushite (CaHPO 4 •2H 2 O) and octacalcium phosphate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 •5H 2 O) were the main intermediates of the reaction. When the decomposition of calcium carbonate was complete (80 °C, 24 h), stoichiometric Ca-HA was obtained, with some little amounts of carbonate-containing apatites of both A-type and B-type. Carbon dioxide was the only by-product of the reaction, and no water rinsing was required for the purification of products.

INTRODUCTION

Calcium hydroxyapatite (Ca-HA, Ca 10 (PO 4 ) 6 (OH) 2 ) continues to attract much attention because of its numerous applications in the fields of biomaterial, [START_REF] Zamoume | Macroporous calcium phosphate ceramic implants for sustained drug delivery[END_REF][START_REF] Nathanael | Large scale synthesis of hydroxyapatite nanospheres by high gravity method[END_REF][START_REF] Rossi | Ultrastructure of regenerated bone mineral surrounding hydroxyapatite-alginate composite and sintered hydroxyapatite[END_REF] heterogeneous catalysis, [START_REF] Solhy | Efficient synthesis of chalcone derivatives catalyzed by reusable hydroxyapatite[END_REF][START_REF] Mondelli | Ruthenium at work in Ruhydroxyapatite during the aerobic oxidation of benzyl alcohol: An in situ ATR-IR spectroscopy study[END_REF][START_REF] Park | Oxidative coupling of methane using nonstoichiometric lead hydroxyapatite catalyst mixtures[END_REF][START_REF] Jemal | NO reduction with NH 3 under oxidizing atmosphere on copper loaded hydroxyapatite[END_REF] gas sensors, [START_REF] Mene | Surface modified hydroxyapatite thick films for CO 2 gas sensing application: Effect of swift heavy ion irradiation[END_REF][START_REF] Mene | Enhancement in CO gas sensing properties of hydroxyapatite thick films: Effect of swift heavy ion irradiation[END_REF] and wastewater treatment, etc. [START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: Equilibria and kinetic processes[END_REF][START_REF] Bhatnagar | Fluoride removal from water by adsorption-A review[END_REF] Thus, much work has been done on the synthesis of Ca-HA using different methods such as solid-solid reaction, [START_REF] Nakamura | Hydroxyapatite nano sol prepared via a mechanochemical route[END_REF][START_REF] Silva | Structural properties of hydroxyapatite obtained by mechanosynthesis[END_REF] ultrasonic spray freeze-drying, [START_REF] Itatani | Preparation of various calcium-phosphate powders by ultrasonic spray freeze-drying technique[END_REF] or spray-drying techniques generating Ca-HA powder of controlled morphology, [START_REF] Luo | Preparing hydroxyapatite powders with controlled morphology[END_REF] sol-gel technique, [START_REF] Anee | A novel technique to synthesize hydroxyapatite at low temperature[END_REF][START_REF] Bezzi | A novel sol-gel technique for hydroxyapatite preparation[END_REF][START_REF] Salimi | Effect of processing conditions on the formation of hydroxyapatite nanoparticles[END_REF] precipitation in aqueous solution, [START_REF] Jarcho | Hydroxylapatite synthesis and characterization in dense polycrystalline form[END_REF][START_REF] Rodriguez-Lorenzo | Fabrication of hydroxyapatite bodies by uniaxial pressing from a precipitated powder[END_REF][START_REF] Bailliez | The kinetics of surface area reduction during isothermal sintering of hydroxyapatite adsorbent[END_REF][START_REF] Swain | Synthesis and dispersion of hydroxyapatite nanopowders[END_REF] emulsion route, [START_REF] Okada | Solvent-free formation of hydroxyapatite coated biodegradable particles via nanoparticle-stabilized emulsion route[END_REF][START_REF] Saha | Synthesis of nanocrystalline hydroxyapatite using surfactant template systems: Role of templates in controlling morphology[END_REF] microwaveassisted synthesis, [START_REF] Ipekoglu | Silver substituted nanosized calcium deficient hydroxyapatite[END_REF] synthesis under hydrothermal conditions, [START_REF] Vasile | Physico-chemical and biocompatible properties of hydroxyapatite based composites prepared by an innovative synthesis route[END_REF]27 or synthesis using templates, etc. [START_REF] Son | Effect of alginate as polymer matrix on the characteristics of hydroxyapatite nanoparticles[END_REF][START_REF] Gopi | Synthesis of hydroxyapatite nanoparticles by a novel ultrasonic assisted with mixed hollow sphere template method[END_REF] The precipitation of orthophosphate species by a calcium source in an aqueous solution seems to be the most general technique to prepare Ca-HA powder particles. By this route, water-soluble calcium sources such as calcium nitrate, calcium chloride, or calcium acetate are preferred because of the resulting homogeneity of the reaction medium. Consequently, stoichiometric Ca-HA of high purity can be obtained for high value-added materials, for example in medicine [START_REF] Rouhani | Rapid growth of hydroxyapatite nanoparticles using ultrasonic irradiation[END_REF] or chromatography column making. [START_REF] Shepard | Discoloration of ceramic hydroxyapatite used for protein chromatography[END_REF][START_REF] Mccue | Use of an alternative scale-down approach to predict and extend hydroxyapatite column lifetimes[END_REF] Nevertheless, these soluble calcium salts present some drawbacks concerning their relative high costs in comparison with less watersoluble calcium sources, such as calcium hydroxide or calcium carbonate; the arduous washing steps when nanoparticles of Ca-HA are formed; and the treatment of waste by-product from counterions. In addition, when Ca-HA materials are used as sorbents in environmental applications, they must be of low-cost. Thus, calcium carbonate would appear as the most interesting starting material for the calcium source.

In our previous work, a comparative study on the synthesis of Ca-HA from calcium carbonate and different orthophosphate sources was demonstrated. [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: A comparative study[END_REF] Orthophosphoric acid was found to represent the best orthophosphate source because of a high dissolution of calcium carbonate and a complete precipitation of the orthophosphate species. However, the initial calcium carbonate could not be completely decomposed at room temperature, despite a long reaction time of 48 h. [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: A comparative study[END_REF] This work aims to find the proper conditions to synthesize Ca-HA starting from calcium carbonate and orthophosphoric acid under moderate conditions, which are comparable to those of the competing industrial process using calcium hydroxide and orthophosphoric acid. Different parameters of the reaction were investigated in order to define a one-step synthesis process of Ca-HA with complete conversion of calcium carbonate. The usual parameters examined for promotion of biphasic reaction are stirring speed, liquid to solid ratio (L/S), reaction time, and reaction temperature. All these are expected to influence reaction outcomes.

MATERIALS AND METHODS

Calcium carbonate powder (98%, Fisher Scientific) and orthophosphoric acid (85 wt % in water, Merck) were used as received. The preparation of Ca-HA was carried out in a 2.3 L glass U-form reactor (i.d., 10 cm; length, 30 cm) equipped with a vertical stainless steel two-blade mixing rotor. For each synthesis, 100 g of calcium carbonate powder and 260-530 g of permuted water, which corresponded to different weight ratios of water to calcium carbonate (L/S), were introduced in the reactor under stirring (200-600 rpm). Then 69.2 g of orthophosphoric acid were pumped into the reactor at the rate of 1 mL min -1 . This corresponded to a molar ratio of calcium to phosphorus (Ca/P) of 1.67 which is the stoichiometric Ca/P of Ca-HA. During the reaction, the reaction temperature was kept constant at 25-80 °C. Powder products were obtained by filtration of the suspension on a 0.45 μm filter paper followed by a drying overnight at 105 °C. Table 1 summarizes the synthesis conditions used.

Elemental analysis was carried out using inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a HORIBA Jobin Yvon Ultima 2. Thermogravimetry analysis (TGA) was carried out with a TA Instruments SDTQ600 analyzer (air flow rate at 100 mL min -1 ; heating rate of 20 °C min -1 ). Exhaust gases from the outlet of TG analyzer were analyzed by Pfeiffer Vacuum OmniStar GSD 320 mass spectrometer which acquires mass spectra from 0 to 60 amu (atomic mass unit). Infrared spectroscopy (IR) was performed on a Shimadzu FTIR 8400S spectrometer in the wavenumber range 4000-500 cm -1 . X-ray diffraction (XRD) measurements were carried out using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu Kα (1.543 Å) radiation source. Specific surface area was determined on a Micrometrics Gemini Vacprep 061 apparatus using the BET method (S BET ) by nitrogen adsorption. Scanning electron microscopy (SEM) measurement was performed 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. The median diameters (d 50 ) were obtained from particle number size distributions.

RESULTS

3.1. Determination of the Contents of Brushite and Remaining Calcium Carbonate. To evaluate the advancement of the reaction, TG analysis was performed in order to determine the contents of remaining calcium carbonate, which was the starting reactant, and brushite (CaHPO 4 •2H 2 O), which was a principal intermediate of the reaction. [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: A comparative study[END_REF] As an example, Figure 1 illustrates TG (A) and DTG (B) curves of the solid product obtained after 3 h of reaction at 25 °C, 600 rpm and L/S ratio of 4.5 (Syn-7, Table 1).

Under the analysis conditions used, the remaining calcium carbonate decomposed in the temperature range of 600-710 °C with the DTG max (the maximum of the DTG signal) of 688 °C (eq 1). This led to a weight loss by the release of CO 2 in gas phase. From this weight loss, the content of remaining calcium carbonate could easily be calculated, according to eq 1. As for brushite, its dehydration took place between 170 and 199 °C, with the DTG max of 186 °C. The content of brushite was calculated according to eq 2.

→ + CaCO CaO CO 3 2
(1)

• → + CaHPO 2H O CaHPO 2H O 4 2 4 2
(2)

To confirm the results mentioned above, qualitative analysis of gas from the outlet of TG analyzer was also performed using the MS technique. Figure 2 shows the evolutions of H 2 O (18 amu) and CO 2 (44 amu) as a function of temperature corresponding to TG analysis in Figure 1. The weight losses in the temperature ranges of 170-199 °C and 600-710 °C evidently corresponded to the dehydration of brushite and the decarbonation of remaining calcium carbonate, respectively. These results were consistently found for all TG analyses presented in the next sections.

Effect of Stirring Rate.

To improve the decomposition of calcium carbonate, the influence of different parameters such as the stirring rate, the weight ratio of water to calcium carbonate, the reaction temperature, and the reaction time, were investigated.

Figure 3A presents the DTG curves and Figure 3B compares the contents of brushite and the remaining calcium carbonate of the solid products obtained at different stirring rates. Different weight losses were observed in the temperature range of 25-1000 °C, which were already discussed in our previous work. [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: A comparative study[END_REF] By increasing the stirring rate from 200 to 400 rpm, the content of remaining calcium carbonate decreased from 21.1 to 18.4%. A strong decrease of the content of brushite from 39.6 to 20.1% was also observed. The increase of the stirring rate favored the contact of calcium carbonate particles with orthophosphate species, and therefore favored its decomposition. However, the increase of the stirring rate to 600 rpm did not lead to any further significant improvement, indicating that other parameters need to be optimized.

3.3. Effect of L/S ratio. The amount of water used as solvent of the reaction could also influence the advancement of the reaction. Thus, the effect of the L/S ratio was investigated in the range of 2.6-5.3 and the results are presented in Figure 4.

Under similar reaction conditions, the best L/S ratio was found to be 4.5, leading to the lowest content of remaining calcium carbonate, which was 18.4%. A lower L/S ratio might lead to a heterogeneity of the reaction mixture and reduced contact of the calcium carbonate particles with the orthophosphate species. A higher L/S ratio increased the homogeneity of the reaction mixture, but decreased the reaction rate because of the dilution of the initial reactants. As for brushite, its content decreased continuously with the increase of the L/S ratio. The increase of the L/S ratio induced a better solubility of brushite, formed during the reaction as a first intermediate, and therefore favored its transformation into other intermediates such as dicalcium phosphate anhydrous (DCPA, CaHPO 4 ) or octacalcium phosphate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 •5H 2 O).

3.4. Effect of Reaction Time and Reaction Temperature. In the ranges of stirring rate and L/S ratio investigated, the content of remaining calcium carbonate was still appreciable. Thus, the influence of the reaction time and reaction temperature were studied. Figure 5 presents the results of TG analysis of the solid products synthesized at 25 °C for different reaction times. The contents of both brushite and remaining calcium carbonate 1).

decreased with the reaction time, but with slow rates. After 168 h (or 7 days), the content of remaining calcium carbonate was still 12.3%. The reaction mixture had and maintained the appearance of a gel throughout this long reaction time.

To accelerate the reaction, a higher reaction temperature was required. Figure 6 shows the results of TG analysis of the solid products synthesized at 60 °C. Brushite appeared also at high content at the beginning of the reaction, and then its content decreased rapidly. Only trace amounts of brushite were found after 96-168 h of reaction, indicating the influence of reaction temperature on the advancement of the reaction, in comparison with the results in Figure 5. The content of remaining calcium carbonate also decreased with the reaction time, but was still 5.9% after 168 h of reaction. Therefore, another synthesis was carried out at 80 °C, and the results of TG analysis are presented in Figure 7.

Figure 7 illustrates the crucial effect of the reaction temperature on the advancement of the reaction. At 80 °C, the decomposition of calcium carbonate reached nearly completion after only 24 h (>99.5%). The amount of brushite was also very small. ICP-AES analysis of the liquid phase after 24 h of reaction showed only trace amounts of soluble calcium cations and orthophosphates species. Thus, the transformation of both initial reactants (calcium carbonate and orthophosphoric acid) into solid calcium phosphates was complete. The TG analyses confirmed the hypothesis and provided simple and sensitive results. However, the identification of these calcium phosphates needs other physico-chemical characterizations.

3.5. Physico-chemical Characterizations of the Solid Products. TG analyses allowed the quantification of the remaining calcium carbonate present in the solid products, and therefore the advancement of the reaction could be evaluated. It is interesting to analyze in more details the most significant solid products using other physico-chemical characterizations. The following products were selected for further studies: solids obtained at 25 °C/168 h; 60 °C/168 h; 80 °C/3 h, and 80 °C/24 h; which are labeled thereafter P25-168; P60-168; P80-003; P80-024, respectively.

3.5.1. XRD. XRD characterization was carried out in order to investigate crystalline phases of the selected solid products. The results are presented in Figure 8 wherein most principal diffraction peaks are designated. The remaining calcium carbonate, having the main diffraction peak at 2 theta of 29.40 deg, was clearly observed for the P25-168 and P60-168 samples. This peak was much less intense for P80-003 solid and was nearly negligible for P80-024 solid. Brushite with the main diffraction peak at 2 theta of 23.40 deg appeared only in P25-168 sample. These results confirmed again those of TG analysis (Figures 5,6,7).

The second crystalline calcium phosphate detected by XRD was OCP. Its content was highest in the P25-168 sample, where 1).

the content of remaining calcium carbonate was highest. As a function of the decomposition of the initial calcium carbonate, the content of OCP considerably decreased and was only in trace amounts in the P80-003 and P80-024 samples, taking into account the instability of the XRD background for the 2 theta lower than 7 degrees. [START_REF] Horvathova | In vitro transformation of OCP into carbonated HA under physiological conditions[END_REF] For the synthesis at 80 °C, the reaction was stopped at 24 h because the decomposition of calcite was already quite complete. Stoichiometric Ca-HA appeared in all four solid products as stable calcium phosphate. Its content increased with the decomposition of the initial calcium carbonate and the disappearance of OCP. In all our syntheses, weak broad peaks of Ca-HA were observed, which were characteristic for the formation of poorly crystallized Ca-HA by precipitation in aqueous solution. [START_REF] Bailliez | The kinetics of surface area reduction during isothermal sintering of hydroxyapatite adsorbent[END_REF][START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: A comparative study[END_REF][START_REF] Yao | Carbonate and fluoride incorporation in synthetic apatites: Comparative effect on physico-chemical properties and in vitro bioactivity in fetal bovine serum[END_REF] Carbonate-containing apatites (CAP) might also be present, but at low contents, as confirmed by IR results in the next section.

3.5.2. Infrared. IR spectra of the selected solid products and the initial calcium carbonate are shown in Figure 9. For the initial calcium carbonate, three net peaks at 711 cm -1 , 872 cm -1 , and 1389 cm -1 were observed in the wavenumbers from 1700 to 1). 500 cm -1 , which are the characteristic peaks of calcite. [START_REF] Yogȗrtcuoglu | Surface modification of calcite by wet-stirred ball milling and its properties[END_REF] For the synthesized solids, there was only a weak broad peak at 3570 cm -1 in the wavenumber range of 4000-1700 cm -1 , which was attributed to hydroxyl groups (not presented).

In the wavenumber of 1700-500 cm -1 , a weak peak at 1389 cm -1 was observed for the sample P25-168, despite its high content of remaining calcium carbonate (12.3%). In fact, previous work revealed that the remaining calcium carbonate particles were usually covered by calcium phosphate layers [START_REF] Pham Minh | Apatitic calcium phosphates: Synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF] and therefore it was only partially detected by IR. This peak nearly disappeared in three other selected solid products, due to the decrease of the content of remaining calcium carbonate. On the other hand, peaks of carbonate groups incorporated in the structure of Ca-HA were observed, and their intensity increased with the reaction temperature, or the decomposition of calcium carbonate (Figure 10). The replacement of both OH -groups (A-type CAP, characterized by the bimodal peak at 1450 cm -1 / 1415 cm -1 and the peak at 873 cm -1 ) and PO 4

3-groups (B-type CAP, characterized by the peaks at 1545 and 880 cm -1 ) [START_REF] Wilson | Rietveld structure refinement of precipitated carbonate apatite using neutron diffraction data[END_REF][START_REF] Antonakos | Micro-Raman and FTIR studies of synthetic and natural apatites[END_REF] was observed for both products synthesized at 80 °C (P80-003 and P80-024). On the other hand, the products synthesized at 25 and 60 °C had only the signals of B-type CAP.

3.5.3. SEM, Static Laser Light Scattering and Specific Surface Area. The surface morphology of the particles was investigated using the SEM method. Micrographs of the initial calcium carbonate are presented in Figure 11. Particles of regular dimensions with a compact smooth surface were observed, which had the median diameter (d 50 ) of 13.20 μm (Table 2). This seemed to be in agreement with its low specific surface area (S BET , < 2 m 2 g -1 , Table 2) indicating its nonporous structure.

SEM micrographs at different magnifications of the selected solid products are illustrated in Figure 12. The product synthesized at 25 °C (P25-168) showed a high heterogeneity in particles sizes, varying from some hundreds of nanometers up to 450 μm, which was confirmed by static laser light scattering. The median diameter (d 50 ) of this sample was only 0.53 μm, explained by the presence of numerous particles smaller than 1 μm in this sample. The micrograph at the 2 μm scale showed that larger particles were formed by the agglomeration of smaller ones. These small particles had a sheet structure, which could be attributed to the presence of OCP and/or brushite in this sample. [START_REF] Horvathova | In vitro transformation of OCP into carbonated HA under physiological conditions[END_REF][START_REF] Cama | Tailoring brushite for in situ setting bone cements[END_REF] The frequency of these particles decreased or disappeared in the other selected solids, which must be due to their evolution into Ca-HA or CAPs phases.

The solid synthesized at 60 °C (P60-168) also had a high heterogeneity in particle sizes. Large particles having plate-like morphology appeared at high frequency, which led to a higher median diameter (d 50 of 0.70 μm) than that of the solid synthesized at 25 °C. The change was distinct for the products synthesized at 80 °C (P80-003 and P80-024). Regular spherical particles were formed, and both solid products had the same median diameter of 0.55 μm.

Specific surface areas of solids can be attributed to both the external surface of particles and the internal surface of pores. Taking into account the median diameters in Table 2, the synthesized solid products must be porous, as observed previously for the solids obtained under similar synthesis conditions. [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: A comparative study[END_REF] The increase of the reaction temperature from 25 to 60 °C favored the decomposition of the initial calcium carbonate and therefore S BET increased. But the increase of the reaction temperature to 80 °C led to a slight reduction of S BET . This might be due to the unfavorable effect of the reaction temperature on the specific surface area of Ca-HA precipitated in aqueous solution, as observed previously. [START_REF] Kothapalli | Influence of temperature and concentration on the sintering behavior and mechanical properties of hydroxyapatite[END_REF] 3.6. Discussion. As discussed in the Introduction section, the precipitation of a water-soluble calcium salt with a orthophosphate source in an aqueous solution is the most common and classical route for the preparation of Ca-HA powder particles. However, it is accompanied by several drawbacks, mostly a high cost of the initial reactant and waste of the counterions. When Ca-HA-based materials of high purity are required, for example, for pharmaceutical or food applications, an arduous water-rinsing step of the powder suspension to eliminate all counter-ions is also necessary. Calcium hydroxide (Ca(OH) 2 ) and orthophosphoric acid seem to be the best alternatives for calcium and orthophosphate sources at the industrial scale. [START_REF] Conn | Process for producing hydroxyapatite[END_REF][43][START_REF]Innophos[END_REF] Using these reactants, Ca-HA was obtained under moderate reaction conditions of temperature (35-90 °C). [START_REF] Conn | Process for producing hydroxyapatite[END_REF] However, calcium hydroxide is naturally obtained by the calcination of calcium carbonate (CaCO 3 ), which is an endothermic process, followed by a hydration of resulting calcium oxide (Scheme 1).

Under similar synthesis conditions of temperature (<90 °C), reaction time (<24 h) and stirring (<600 rpm), the proposed synthesis process using solely calcium carbonate as-is and orthophosphoric acid has some energetic advantages compared to the competing industrial process using calcium hydroxide and orthophosphoric acid (Scheme 1), thanks to cost savings for calcination and hydration steps. Mixing a suspension of CaCO 3 and H 3 PO 4 at 80 °C for 24 h consumes less energy than heating calcium carbonate to at least 600 °C, followed by a hydration step to form Ca(OH) 2 and finally a neutralization of Ca(OH) 2 with H 3 PO 4 at the similar synthesis temperature (90 °C). Further study on global costs of the synthesis with respect to reaction conditions such as temperature, reaction time, stirring, etc. may be considered in order to quantify the effectiveness of the proposed synthesis route.

ICP-AES analysis showed only the presence of trace amounts of soluble calcium and orthophosphate ions in the liquid phase, indicating that Ca-HA of high purity up to 99% was obtained without water rinsing step, since all possible counter-ions were practically not present. For the P-80-024 sample, the overall yield of the synthesis in the solid product was 98%, taking into account the complete conversion of calcite and orthophosphoric acid used, the high selectivity in the formation of apatitic compounds (Ca-HA and CAP), and the weight of the solid product recovered after filtration and drying at 105 °C.

CONCLUSIONS

A one-step synthesis of Ca-HA from calcium carbonate and orthophosphoric acid was investigated under moderate reaction conditions. The stirring rate and the weight ratio of water to calcium carbonate (L/S) had slight effect on the advancement of the reaction. On the other hand, the reaction temperature and the reaction time were found to be crucial for the decomposition of calcium carbonate, which was the key step for the formation of stoichiometric Ca-HA. The essential reaction conditions for a total conversion of the initial reactants (CaCO 3 and H 3 PO 4 ) into desired products (Ca-HA) were determinate. Calcium carbonate decomposed nearly completely at 80 °C after 24 h of reaction and stoichiometric Ca-HA was obtained together with small amounts of carbonate-containing apatites (CAP). Carbon dioxide was the only by-product of the reaction. No water rinsing step was required thanks to the absence of other unneeded ions or molecules, and thanks to the complete transformation of the initial reactants into apatitic solids. Future work may further investigate the optimization of the one-step synthesis of Ca-HA. Kinetic modeling of calcite decomposition and Ca-HA crystal growth may be also considered. The investigation of the resulting Ca-HA as acid-base catalysts and supports for the dispersion of metal nanoparticles is planned, as is the use of Ca-HA gel for water and gas purification processes.

■ AUTHOR INFORMATION

Corresponding Author *Tel.: +33 563493258. Fax: +33 563493043. E-mail: doan. phamminh@mines-albi.fr.

Notes

The authors declare no competing financial interest.

Figure 1 .

 1 Figure 1. Example of TG (A) and DTG (B) curves and the determination of DTG max and the contents of brushite and remaining calcium carbonate.

Figure 2 .

 2 Figure 2. Example of qualitative analysis of H 2 O and CO 2 formed during TG analysis using the MS technique.

Figure 3 .

 3 Figure 3. DTG curves (A) and contents of brushite and remaining calcium carbonate (B) of the filtered solid products at different stirring rates. Synthesis conditions: reaction temperature of 25 °C, L/S ratio of 4.5, reaction time of 24 h (Syn-1, Syn-2, and Syn-3 in Table1).

Figure 4 .

 4 Figure 4. DTG curves (A) and contents of brushite and remaining calcium carbonate (B) of the filtered solid products at different weight ratios of water to calcium carbonate (L/S). Synthesis conditions: reaction temperature of 25 °C, stirring rate of 400 rpm, reaction time of 24 h (Syn-2, Syn-4, Syn-5, and Syn-6 in Table1).

Figure 5 .

 5 Figure 5. DTG curves (A) and contents of brushite and remaining calcium carbonate (B) of the filtered solid products at different reaction times. Synthesis conditions: reaction temperature of 25 °C, stirring rate of 600 rpm, L/S ratio of 4.5 (Syn-7 in Table1).

Figure 6 .

 6 Figure 6. DTG curves (A) and contents of brushite and remaining calcium carbonate (B) of the filtered solid products at different reaction times. Synthesis conditions: reaction temperature of 60 °C, stirring rate of 600 rpm, L/S ratio of 4.5 (Syn-8 in Table1).

Figure 8 .

 8 Figure 8. XRD patterns of the selected solid products: (▲) OCP (JCPDS standard No. 00-026-1056); ( * ) Ca-HA (JCPDS standard No. 01-072-1243); ( ■ ) calcite (JCPDS standard No. 00-047-1743); ( □ ) brushite (JCPDS standard No. 00-0411-0293).

Figure 7 .

 7 Figure 7. DTG curves (A) and contents of brushite and remaining calcium carbonate (B) of the filtered solid products at different reaction times. Synthesis conditions: reaction temperature of 80 °C, stirring rate of 600 rpm, L/S ratio of 4.5 (Syn-9 in Table1).

Figure 9 .

 9 Figure 9. IR spectra of the selected solid products and initial calcium carbonate.

Figure 10 .

 10 Figure 10. IR spectra of the selected solid products: illustration of A-type CAP (bimodal peak at 1450 cm -1 /1415 cm -1 and peak at 873 cm -1 ) and B-type CAP (peaks at 1545 and 880 cm -1 ).

Figure 11 .

 11 Figure 11. SEM micrographs of the initial calcium carbonate particles.

Scheme 1 .

 1 Scheme 1. Illustration of Industrial and Laboratory Syntheses of Ca-HA

Table 1 .

 1 Reaction Conditions Used in the Synthesis of Ca-HA

	synthesis	L/S ratio	stirring (rpm)	reaction temperature (°C)	reaction time (h)
	Syn-1	4.5	200	25	24
	Syn-2	4.5	400	25	24
	Syn-3	4.5	600	25	24
	Syn-4	2.6	400	25	24
	Syn-5	3.9	400	25	24
	Syn-6	5.3	400	25	24
	Syn-7	4.5	600	25	3-24-96-168
	Syn-8	4.5	600	60	3-24-96-168
	Syn-9	4.5	600	80	3-24

Table 2 .

 2 Specific Surface Area (S BET ) and Median Diameter (d 50 , by Static Laser Light Scattering) of the Selected Solid Products

	product	S BET (m 2 g -1 )	d 50 (μm)
	initial CaCO 3	<2	13.20
	P25-168	26	0.53
	P60-168	54	0.70
	P80-003	44	0.55
	P80-024	44	0.55

Figure 12

. SEM micrographs of the selected solid products.

■ ACKNOWLEDGMENTS

We thank our colleagues, Nathalie Lyczko, Christine Rolland, Philippe Accart, and Denis Marty for their technical help.