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

Ngoc Dung Tran

Ange Nzihou

Patrick Sharrock

Calcium phosphate based materials starting from calcium carbonate and orthophosphoric acid for the removal of lead(II) from an aqueous solution

Keywords:

Calcium phosphates starting from calcium carbonate and orthophosphoric acid as unconventional and low cost initial reactants were investigated in the removal from aqueous solution of lead(II) as model heavy metal. Sorbent in gel form, which was directly obtained from the reaction of calcium carbonate powder and orthophosphoric acid, and sorbent in powder form, which was obtained by filtration and drying of the gel at room temperature showed the highest reactivity for the removal of lead(II). The sorption capacity could reach up to 1226 mg of lead(II) per gram of powder sorbent. On the other hand, the conventional drying overnight at 105 °C decreased the reactivity of the sorbent in powder form. The intermediates of the reaction between calcium carbonate and orthophosphoric acid, including brushite (CaHPO 4 Á2H 2 O), monetite (CaHPO 4 ) and MCPM (Ca(H 2 PO 4 ) 2 ÁH 2 O) were found to be more active than hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) which was the final stable product of the reaction between calcium carbonate and orthophosphoric acid. Lead hydroxyapatite (Pb 10 (PO 4 ) 6 (OH) 2 ) was the main crystalline phase formed from the contact of calcium phosphate based sorbents with lead(II) nitrate solution. It was only formed at the external surface or surface of open-pores of calcium phosphate particles, highlighted by SEM characterization. The results obtained open new prospects for the design of efficient orthophosphate-based sorbents for the removal of lead(II) and other heavy metals from liquid effluents.

Introduction

Lead (Pb), a typical heavy metal, is indeed toxic and environmental contamination with Pb causes serious injury to both the plant and animal kingdoms. Lead can interfere with the metabo-lism of other essential metals such as copper and zinc, and can indirectly cause other problems such as oxidative stress, inhibition of DNA repair, and cognitive deficits in children [1,[START_REF] Chen | Lead causes human fibroblasts to mis-sort arylsulfatase A[END_REF]. Despite its toxicity which limits its use, lead is widely used for different applications, in particular as electrodes in the lead-acid batteries [START_REF] Wagner | Large lead/acid batteries for frequency regulation, load levelling and solar power applications[END_REF][START_REF] Lailler | Comparative study for ''36 V'' vehicle applications: advantages of lead-acid batteries[END_REF], photovoltaic cells [START_REF] Garche | The influence of different operating conditions, especially over-discharge, on the lifetime and performance of lead/acid batteries for photovoltaic systems[END_REF], lead glasses [START_REF] Yao | Growth, structure, and morphology of lead titanate crystallites in the sol-gel derived glass-ceramics[END_REF], lead paint [START_REF] Turner | Concentrations and bioaccessibilities of metals in exterior urban paints[END_REF], etc. In parallel with its large use, lead leaches into the environment by erosion of contaminated soil sites, or from air as a consequence of lead-con-taining waste incineration. In order to eliminate or reduce environmental lead mobility, highly effective and low-cost processes for lead abatement are continuously sought after.

In liquid phase, the most common method for the treatment of heavy metals consists in separation processes, wherein soluble metal ions are transformed into insoluble forms (simple hydroxide precipitates when possible) or are fixed on a solid substrate (sorbent) and therefore are separated from the liquid phase. During the last decades, calcium phosphates, and particularly calcium hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , Ca-HA), attracted much interest because of their high potential application for the remediation of toxic metals in liquid wastes [START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 3. Exchange characteristics of lead ions (Pb 2+ )[END_REF][START_REF] Miyake | Synthetic carbonate apatites as inorganic cation exchangers[END_REF][START_REF] Ma | Effects of NO À 3 , Cl À , F À , SO 2À 4 , and CO 2À 3 on Pb 2+ immobilization by hydroxyapatite[END_REF][START_REF] Nzihou | Role of phosphate in the remediation and reuse of heavy metal polluted wastes and sites[END_REF][START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF]. For example, in the removal of lead, the sorption capacity (Q e ) of Ca-HA particles was reported to be in the typical range of 300-600 mg g À1 , which is much larger than capacities observed with activated carbon as conventional sorbent [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF]. In contact with metallic cations in aqueous solution, in particular with multivalent cations (M 2+ , M 3+ ), Ca-HA can incorporate these cations in its apatitic structure and release non-toxic Ca 2+ to the liquid phase by an ion exchange reaction. Furthermore, heterogeneous nucleation of metal phosphates can form and metal phosphates grow on Ca-HA surfaces. The hydrated and protonated surfaces of Ca-HA in aqueous media have been described at length [START_REF] Rey | Physico-chemical properties of nanocrystalline apatites: Implications for biominerals and biomaterials[END_REF].

Usually, calcium phosphate based ''sorbents'' are mostly used under dried powder form for the removal of toxic metals from aqueous effluents, mainly due to the ease of handling solid particles. However, our last study on lead(II) removal showed that apatitic calcium phosphate in as-synthesized gel form prior to filtration was more active than that in dried powder form [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF].

In this paper, we report a comparative study of the reactivity of different apatitic calcium phosphates, in both gel and powder form, obtained under different synthesis conditions, for the fixation of lead(II) as model toxic metal in an aqueous solution. Ca-HA is not a sorbent in the usual sense in that after contacting lead ions, the solid phase contains the initial calcium phosphate plus Pb 2+ and minus Ca 2+ . So, there occurs a reaction best described as ion exchange, different from adsorption-desorption phenomena in which the solid substrate (sorbent) remains intact. We will demonstrate that the reaction is not limited to surface phenomena, but continues as the surface-formed lead phosphate spalls of the sorbent by erosion to create new reactive sites. The influence of aging of the calcium phosphate particles in water, their drying temperature, and the effect of remaining CaCO 3 contents on the reactivity of Ca-HA is communicated.

Materials and methods

All chemical products used in this study had pure analytical grades, including CaCO 3 , H 3 PO 4 and Pb(NO 3 ) 2 . Apatitic calcium phosphates were synthesized using CaCO 3 and H 3 PO 4 as available starting materials. Details of the synthesis process were described elsewhere [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calciumccarbonate and orthophosphoric acid under moderate conditions[END_REF]. Briefly, H 3 PO 4 was added in an aqueous suspension of CaCO 3 particles, with theoretical Ca/P molar ratio of 1.67. The mixture was kept at 25-80 °C under stirring for 48 h. A stable gel was formed, which was composed of fine calcium phosphate particles and various amounts of unreacted calcium carbonate as a function of synthesis and drying temperatures (Table 1). Only traces of soluble calcium and phosphate species were present in the gel, as shown in our previous work [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF]. Solid powders could be obtained by filtration of the gel on a 0.45 lm filter paper followed by a drying stage at 25 or 105 °C. All gel or powder products were stored at room temperature in closed glass flasks. Table 1 lists all products investigated, obtained under different specified synthesis conditions.

Lead(II) sorption experiments were described elsewhere [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF]. Briefly, lead nitrate was dissolved in permuted water to prepare an aqueous solution containing 6000 mg L -1 of Pb 2+ (or 28.96 mmol L -1 of Pb 2+ ). For each sorption experiment, 300 mL of this solution was introduced in a 700 mL stirred glass reactor at ambient temperature, ca. 25 °C. Then 2.4 g of the sorbent powder or an equivalent quantity of the fresh gel containing also 2.4 g of dried matter was rapidly added to the reactor under stirring (350 rpm). During the experiment, small aliquots were withdrawn from the reactor and filtered on 0.45 lm filter paper. The liquid fraction was analyzed by inductive coupled plasma atomic emission spectroscopy (ICP-AES, HORIBA Jobin Yvon Ultima 2) for the determination of the concentration of soluble calcium, phosphorus, and lead. Fresh sorbents, conserved in sealed bottle at room conditions for less than 10 days were used. For the study on the influence of the conservation time, lead(II) sorption was carried out with both fresh and aged sorbent, this last one being kept for 9 months at room conditions. Solid powders were characterized by different physico-chemical techniques. X-ray diffraction (XRD) data were collected using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu Ka radiation source. Thermogravimetric analysis (TG) was carried out with a SDTQ600 analyzer (TA Instruments). Infra-red spectra (DRIFT) of solid samples were directly recorded on a Shimadzu FTIR 8400S spectrometer. Scanning electron microscopy (SEM) was measured on a Philips XL30 ESEM apparatus (FEI Company).

Results and discussions

Influence of the synthesis temperature on the reactivity of sorbents

The reactivity of the fresh gels synthesized at different temperatures (G25, G60 and G80, Table 1) for the removal of lead(II) is illustrated in Fig. 1. Unexpectedly, G25 containing the highest brushite and remaining CaCO 3 contents was found to be the most active material, translated by its fastest lead(II) fixation kinetics. Lead(II) was completely removed within the first minute of reaction by G25 (Fig. 1(a)). When the temperature of Ca-HA synthesis increased to 60 and 80 °C, the yield in Ca-HA increased and the unconverted CaCO 3 contents of G60 and G80 decreased. However, the removal of lead(II) was only complete between 120 and 180 min of contact, using these two sorbents. In parallel with the removal of lead(II), calcium(II) was released to the solution and its concentration is shown in Fig. 1(b). The molar concentration of released calcium during lead(II) fixation was close to that of removed lead(II), as observed previously for apatitic calcium phosphate sorbents [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF][START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: Equilibria and kinetic processes[END_REF].

Influence of the drying step

Standard drying is generally carried out at 105 °C under air atmosphere. However, this temperature may have significant effects on the evolution of physico-chemical properties and reactivity of the solid calcium phosphates since all sorbents were synthesized at temperatures below 105 °C. Thus, the solid fractions obtained from the filtration of G25 and G80 were dried at 25 and 105 °C, and were thereafter tested in the removal of lead(II). Fig. 2 compares the reactivity of G25 and G80 with the corresponding dried solid products.

In all cases, the trend is similar for the products synthesized at 25 (Fig. 2(a)) and 80 °C (Fig. 2(b)). Sorbents dried at 25 °C (P25_DRY25 and P80_DRY25) had similar reactivity for lead(II) removal compared to the corresponding gels (G25 and G80), indicating that the filtration and drying step at 25 °C did not change physico-chemical properties of calcium phosphate particles present in the gels. On the other hand, sorbents dried at 105 °C (P25_DRY105 and P80_DRY105) had slower lead(II) sorption kinetics, compared to the corresponding gels and powder sorbents dried at 25 °C. Previous work described the specificities of the surface of solid apatitic calcium phosphate, consisting of hydrated layers which have higher ionic exchange capacity than the apatite core [START_REF] Rey | Physico-chemical properties of nanocrystalline apatites: Implications for biominerals and biomaterials[END_REF][START_REF] Cazalbou | Minéralisations biologiques à base de phosphate de calcium[END_REF][START_REF] Rey | Bone mineral: update on chemical composition and structure[END_REF]. The drying at 105 °C may remove or modify the hydrated layers compared to those of fresh gel or powder dried at room temperature, and therefore reduces the lability of the species on the surface of apatitic particles. Hydrogenophosphate groups are present on the surfaces of Ca-HA in water near neutral pH values. The unfavorable effect of the thermal treatment step on the reactivity of apatitic calcium phosphates in the removal of heavy metals was previously observed [START_REF] Sugiyama | Effect of calcinations and sieving of calcium hydroxyapatite on ion-exchangeability with lead cation in the presence and absence of HCl[END_REF][START_REF] Hadioui | Reaction of lead ions with hydroxylapatite granules[END_REF].

Determination of the sorption capacity and influence of the conservation time

Further experiments were carried out with smaller amounts of G25 in order to determine the sorption capacity of this material, which had the best reactivity among the synthesized products. The objective was to determine the shelf life of the gel, and analyze any decrease in sorption performance as a function of aging time (conservation). Fig. 3 shows the evolution of the concentration of consumed lead(II) during the experiment, when a quantity of fresh G25 equivalent to 0.75 g powder sorbent was used. Under similar conditions, another experiment was carried out using G25 aged for 9 months in a hermetic flask at room temperature in order to investigate the effect of conservation time (aging) on the reactivity of this sorbent.

As expected for the fresh G25, lead(II) removal took place rapidly within the first minute of contact with fresh gel. The removal of lead(II) reached equilibrium after about 6 h of contact and the concentration of removed lead(II) leveled at 14.8 mmol L -1 . From this result, the sorption capacity (Q e ) of fresh G25 could be derived, and was found to represent 1226 mg of removed Pb 2+ per gram of equivalent powder sorbent.

The conservation of G25 for 9 months at room temperature in a hermetic flask decreased dramatically its reactivity. The removal of lead(II) seemed to be at equilibrium after 24 h of contact with AGED_G25 sorbent. From the concentration of removed lead(II) of 5.8 mmol L -1 , Q e of AGED_G25 was calculated at 480 mg of removed Pb 2+ per gram of equivalent powder sorbent. In all cases, the initial pH of lead(II) nitrate solution was 5.2. After 24 h of contact with calcium phosphate-based sorbent, the final pH of the reaction mixture was equilibrated at 6.2-7.5 as a function of sorbent used. XRD characterization in the next section reveals that only trace amounts of lead oxides were formed in these conditions. Some results on the evolution of pH during lead(II) removal experiment was previously reported in our earlier work [START_REF] Pham Minh | Hydroxyapatite gel for the improved removal of Pb 2+ ions from aqueous solution[END_REF].

In order to better understand the influence of different parameters on the reactivity of apatitic calcium phosphate sorbents, various physico-chemical characterizations were carried out for the powder sorbents before and after use, and the results are discussed in the next sections.

XRD characterization

Crystalline phases of powder materials were determined by XRD technique. Fig. 4 shows the XRD patterns of the powders synthesized at 25-80 °C, and dried at 25-105 °C before lead(II) removal. For P25_DRY25 (synthesized and dried at 25 °C), residual CaCO 3 (JCPDS standard No. 01-081-2027) and brushite (JCPDS standard No. 00-011-2093) were found as the two main crystalline phases. The only apatitic compound found in this powder was octacalcium phosphate (OCP, JCPDS standard No. 00-026-1056), existing at smaller content compared to those of residual CaCO 3 or brushite. For P25_DRY105, obtained when the drying was carried out at 105 °C, Ca-HA appeared as new crystalline phase. This phase seemed to be the result of the solid-solid reaction between brushite, OCP and residual CaCO 3 at 105 °C because the relative intensity of these phases decreased compared to the product dried at 25 °C. So the drying at 105 °C not only removed free water, but also favored the formation of Ca-HA, which is thermodynamically the most stable calcium phosphate.

For P80_25 and P80_105, as previously communicated [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calciumccarbonate and orthophosphoric acid under moderate conditions[END_REF], the synthesis temperature of 80 °C allowed the complete dissolution of CaCO 3 and promoted the formation of Ca-HA as the main crystalline phase. No significant effect of the drying temperature in the range of 25-105 °C was observed for these powders, and judging from the broad line widths of the diffraction lines, the solids were nanocrystalline or partly amorphous.

Fig. 5 presents XRD patterns of different solids after lead(II) removal, using dried powder or gel sorbents. They had similar XRD patterns to each other and in all cases, lead hydroxyapatite (JCPDS standard No. 01-087-2478) was found as the main crystalline phase. Lead oxides (JCPDS standard Nos. 01-078-1664 or 01-072-1551) seemed to be present but only at trace amounts. In addition to these phases, residual CaCO 3 present at low contents was also revealed for the sorbents synthesized at 25 °C and trace amounts of poorly-crystalline Ca-HA were detected.

TG analysis

Fig. 6 presents TG curves of solid powders before and after lead(II) removal. All solids had the first weight loss around 100 °C, which is attributed to the removal of surface moisture. For both solids synthesized at 80 °C and dried at 25 or 105 °C (P80_DRY25 and P80_DRY105), slight and continuous weight losses were observed up to about 650 °C, due to the dehydration of lattice molecular water [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calciumccarbonate and orthophosphoric acid under moderate conditions[END_REF][START_REF] Pham Minh | Hydroxyapatite gel for the improved removal of Pb 2+ ions from aqueous solution[END_REF]. No significant weight loss near 610 °C was observed, which confirms again that the synthesis temperature of 80 °C was sufficient for total decomposition of CaCO 3 during the Ca-HA synthesis. Above 650 °C, a small weight loss was observed, explained previously by the decarbonation of carbonated apatite (CAP) [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calciumccarbonate and orthophosphoric acid under moderate conditions[END_REF]. The presence of CAP was highlighted by FTIR analysis (Supporting information 1 and 2).

On the other hand, the drying had remarkable effect on the solids synthesized at 25 °C. Mono-calcium phosphate monohydrate (MCPM, Ca(H 2 PO 4 )2ÁH 2 O), which has a typical dehydration around 110 °C, was present in both solids dried at room temperature (P25_DRY25) and at 105 °C (P25_DRY105) [START_REF] Pham Minh | synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. The dehydration of brushite (CaHPO 4 Á2H 2 O) takes place around 180 °C [START_REF] Pham Minh | synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. From TG curves, its content could be calculated which was found at 55 and 7 wt.% for the product dried at 25 °C (P25_DRY25) and at 105 °C (P25_DRY105), respectively (Table 1). A similar phenomenon was observed for monetite (CaHPO 4 ) and remaining calcite. Their contents were higher in P25_DRY25 than in P25_DRY105.

So the drying at 105 °C pushed the reaction of remaining calcite with different acid calcium phosphates such as MCPM, brushite, and monetite to the formation of Ca-HA as the more stable calcium phosphate phase, as found previously by XRD results in Fig. 4.

From the results of lead(II) removal in Figs. 123, and the composition of the sorbents used for lead(II) removal (Table 1), we observe that the higher the contents of brushite, monetite and MCPM, the better was the reactivity of the sorbents. In the other word, brushite, monetite and MCPM were more active than Ca-HA. The reaction of solid calcium phosphate particles with free lead(II) ions in solution to form solid lead hydroxyapatite layers (hydroxypyromorphite) on the external surface of calcium phosphate particles and release free calcium(II) must be considered as a biphasic liquid-solid reaction. This reaction is favorable when acidic orthophosphate species are more available in the reaction medium. Thus, brushite, monetite and MCPM with higher solubility compared to Ca-HA [START_REF] Elliott | Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF] and solids with HPO 2À 4 surface groups are more active than dehydrated Ca-HA for the fixation of lead(II). So, the increase of the temperature for synthesis and/or drying of the sorbents was favorable for the conversion of brushite, monetite and MCPM to apatitic phases, and so decreased the reactivity of the sorbents. The similar phenomenon was observed when the conservation time of the sorbents increased. Table 1 shows that, a half of brushite in the fresh G25 was evolved into apatitic phase during 9 months of conservation at room conditions, which decreased dramatically its reactivity for lead(II) removal (Fig. 3). FTIR and SEM analyses of the fresh and aged sorbent confirmed the evolution of brushite into apatitic products (Supporting information 3 and 4).

TG curves of the solids recovered after contact with lead(II) solution are presented in Fig. 6(B). For the sorbents synthesized at 80 °C, a continuous weight loss within the studied temperature range was observed, due to the removal of surface moisture and to the dehydration of lattice molecular water. The sorbents synthesized at 25 °C had very similar TG curves, except for small decarbonations of remaining calcite around 610 °C. On the other hand, other calcium phosphates including MCPM, brushite and monetite were no longer present. According to the XRD results in Fig. 5, these calcium phosphates were converted to lead(II) hydroxyapatite, as shown previously [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF][START_REF] Pham Minh | Hydroxyapatite gel for the improved removal of Pb 2+ ions from aqueous solution[END_REF].

SEM characterization

SEM study was carried out in order to investigate the change due to the drying step and the fixation of lead(II) on calcium phosphate particles. Fig. 7 shows the surface morphology of the powders, synthesized at 25 °C. For P25_DRY25, particles of platelet structure were dominant, which can be explained by the abundance of brushite in this powder, as shown previously by TG analysis (Fig. 6) [START_REF] Pham Minh | synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. When the drying temperature rose to 105 °C, the particles of platelet structure disappeared corresponding to its transformation to apatitic phase. In both cases, particle sizes varied in a large range, from less than 1 lm up to hundreds of lm, resulting from agglomeration phenomena.

Fig. 8 presents SEM images of the external surface of particles recovered after the contact of P25_DRY25 (Fig. 10A, B, andC) and P25_DRY105 (Fig. 8D, E, andF) with lead(II) solution. The results were similar to each other, with the disappearance of particles of platelet structure. In parallel, a large number of particles smaller than about 1 lm appeared, in particular for the sample recovered from the removal of lead(II) by P25_DRY25 (Fig. 8A). At higher magnifications, particles of needle-like morphology were dominant. This morphology resulted from the fixation of lead(II) on the surface of calcium phosphate particles, was confirmed by EDX analysis (results not shown), and corresponds to the formation of hydroxypyromorphite [START_REF] Kamiishi | Nano-scale reaction processes at the interface between apatite and aqueous lead[END_REF].

The bi-phasic reaction between lead(II) ions in solution and solid calcium phosphate particles is a surface phenomenon, with the fixation of lead(II) on the surface of phosphate crystals. To highlight this, a SEM study of polished samples of resin embedded particles recovered after lead(II) sorption was performed in order to analyze qualitatively the composition inside particles. The samples were polished gently to ensure a plane surface where particles were cut without damaging their morphology.

As observed in Fig. 9, lead(II) was only immobilized on the surface of particles, for both small (Fig. 9A) and large (Fig. 9B) particles. In some case, the fixation of lead(II) on the surface of open-pores of particles can be observed (arrow, Fig. 9A). In fact, in contact with lead(II) solution, Ca-HA particles were quickly covered by layers of lead hydroxyapatite (Pb 10 (PO 4 ) 6 (OH) 2 , hydroxypyromorphite), as identified by XRD characterization. This inhibited further immediate contact of free lead(II) in solution with the inside of the calcium phosphate matrix, except in open pores, and caused the fixation of lead(II) to slow down. We relate the slower second lead uptake phase to the reaction occurring following lead phosphate spalling and regeneration of fresh calcium phosphate surfaces. Similar results were observed for other sorbents recovered after contact with lead(II) solution. Previous investigators described the initial rapid heavy metal uptake followed by a slower subsequent uptake [START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: Equilibria and kinetic processes[END_REF].

For the solids synthesized at 80 °C, before lead(II) removal, there was no significant difference on the external morphology of particles for the products dried at room temperature and 105 °C (Fig. 10A, B, C, andD). In both cases, regular spherical particles were formed, resulting also from the agglomeration of smaller particles. After contact with lead(II) solution, as mentioned above, the look inside cut and polished particles recovered highlighted the fixation of lead(II) on the surface of particles (Fig. 10E andF).

Pilot scale

Much research has focused on Ca-HA based materials for the removal of heavy metals from synthetic, laboratory-made aqueous effluents [START_REF] Baillez | Removal of aqueous lead ions by hydroxyapatites: Equilibria and kinetic processes[END_REF][START_REF] Hashimoto | Removal of aqueous lead by poorly-crystalline hydroxyapatites[END_REF][START_REF] Smiciklas | Removal of Co 2+ from aqueous solutions by hydroxyapatite[END_REF][START_REF] Mobasherpour | Comparative of the removal of Pb 2+ , Cd 2+ and Ni 2+ by nano crystallite hydroxyapatite from aqueous solutions: adsorption isotherm study[END_REF]. Results obtained are usually very satisfactory with good performance reported for the retention of different heavy metals by Ca-HA. However, to the best of our knowledge, no process development for the application of Ca-HA based materials in the treatment of wastewaters has been described at industrial scale. The reason is certainly due to the high cost of Ca-HA materials made by conventional methods. For environmental application, the minimization of global cost is crucial, as is the absence of byproducts such as ammonium nitrate during Ca-HA synthesis. So, Ca-HA has to be synthesized by the simplest method and from the cheapest starting materials. Scheme 1 compares our synthesis process and the most popular synthesis way used in the literature from water soluble calcium salts.

It is obvious that calcium carbonate and orthophosphoric acid are the cheapest calcium and orthophosphate sources, as starting materials for the synthesis of calcium phosphates. Since carbon dioxide is the only co-product of the reaction between calcium carbonate and orthophosphoric, which automatically leaves the mixture during the reaction, no purification (filtration, washing, drying) is required for the gel product formed. On the other hand, conventional Ca-HA powder is obtained by multi-step synthesis process which generates also a great quantity of wastewater containing all counter-ions such as nitrate, chloride, alkali, and ammonium. Finally, the results obtained in this work showed that fresh calcium orthophosphate gel had much higher reactivity compared to conventional Ca-HA powder.

From the results obtained on the synthesis of calcium orthophosphate gel and its performance in the removal of lead(II), we were also successful in the development of a pilot, being operated with several tons of calcium phosphate gel, for the treatment of real wastewater contaminated by heavy metals (Scheme 2). More details on the composition of the wastewater used for pilot test can be found elsewhere [START_REF] Sebei | Valorisation of carbonate waste into hydroxyapatite for the remediation of heavy metals[END_REF]. The results were also very satisfactory for the treatment of a mixture of domestic and industrial wastewaters containing both heavy metals and organic pollutants. Calcium orthophosphate gel was found to be efficient for the retention of both heavy metals and organic compounds. Benchmarking showed that the wastewater treatment based on the large-scale use of calcium orthophosphate gel can be exploited at industrial scale. This will be communicated in more details in a forthcoming paper.

Conclusions

Calcium phosphate based materials with various compositions could be obtained from calcium carbonate powder and orthophosphoric acid by the one-step synthesis process at moderate conditions. All calcium phosphates present in the solids formed participated to the immobilization of lead(II) in an aqueous solution. Among them, the intermediates of the synthesis process, including brushite, monetite and MCPM were more active than the final more stable Ca-HA phase. The highest sorption capacity was obtained with G25 and P25_DRY25, which were synthesized at 25 °C and contained up to 55 wt.% of brushite.

Future work will focus on the use of other particulate calcium phosphates having higher water solubility, such as MCPM, brushite or monetite; and the functionalization of more inert solid supports by surface modifications with labile orthophosphate species.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Consumed lead (a) and released calcium (b) during the sorption experiments using different gels obtained at different synthesis temperatures.

Fig. 3 .

 3 Fig. 3. Consumed lead(II) during the sorption experiment using fresh G25 (j) and AGED_G25 (N); Sorption conditions: ca. 25 °C; 350 rpm; 300 mL of 28.96 mmol L À1 of lead(II); mass of gel used equivalent to 0.75 g of powder dried at 105 °C.

Fig. 4 .

 4 Fig. 4. XRD patterns of the powders synthesized at 25-80 °C, and dried at 25-105 °C before lead(II) removal; calcite (I); brushite (j); octacalcium phosphate (Ç) Ca-HA (d).

Fig. 5 .

 5 Fig. 5. XRD patterns of the sorbents recovered after contact with lead(II) solution; lead hydroxyapatite ( Ã ).

Fig. 6 .

 6 Fig.[START_REF] Yao | Growth, structure, and morphology of lead titanate crystallites in the sol-gel derived glass-ceramics[END_REF]. TG analysis of the powder sorbents before (A) and after (B) lead(II) sorption.

Fig. 7 .

 7 Fig. 7. SEM images of P25_DRY25 (A and B), and P25_DRY105 (C and D). Images obtained with a detector for secondary electrons (SE).

Fig. 8 .

 8 Fig. 8. SEM images of the solids recovered after lead(II) sorption: P25_DRY25 (A, B, and C); and P25_DRY105 (D, E, and F). Images obtained with a detector for back-scattered electrons (BSE).

Fig. 9 .

 9 Fig. 9. SEM images of cut and polished particles recovered after contact of P25_DRY25 with lead(II) solution; lead(II) has the highest contrast compared to calcium, phosphorus and oxygen. Images obtained with a detector for back-scattered electrons (BSE).

Fig. 10 .

 10 Fig. 10. SEM images of P80_DRY25 (A and B), and P80_DRY105 (C and D) before lead(II) removal; Observation of external surface (E) and look inside cut and polished particles (F) of P80_DRY105 after contact with lead(II) solution; Lead(II) has the highest contrast compared to calcium, phosphorus and oxygen; (A, B, C, and D) images obtained with a detector for secondary electrons (SE); (E and F) images obtained with a detector for back-scattered electrons (BSE).

Table 1

 1

	Synthesis conditions of apatitic calcium phosphates from CaCO 3 and H 3 PO 4 as initial reactants; other conditions: stirring rate of 400 rpm; reaction time of 48 h; the contents of
	remaining CaCO 3 and brushite were obtained by TG analysis; n.a means not analyzed; ( Ã ) powder sample obtained after aging G25 for 9 months, filtering and drying at room
	temperature.
	Synthesis T, °C	Filtration (0.45 lm)	Drying T, °C	Remaining CaCO 3 content, wt.%	Brushite content, wt.%	Labeled product
	25	No	No	n.a	n.a	G25
		Yes	25	23	55	P25_DRY25
		Yes	105	13	7	P25_DRY105
		Yes	25	15	26	AGED_G25 ( Ã )
	60	No	No	n.a	n.a	G60
		Yes	25	6	3	P60_DRY25
		Yes	105	6	3	P60_DRY105
	80	No	No	ND	ND	G80
		Yes	25	1	1	P80_DRY25
		Yes	105	1	1	P80_DRY105

Acknowledgement

The authors gratefully acknowledge colleagues at RAPSODEE center, Christine Roland and Nathalie Lyczko, for technical help.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.01.032.


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

Ngoc Dung Tran

Ange Nzihou

Patrick Sharrock

Hydroxyapatite gel for the improved removal of Pb 2+ ions from aqueous solution

Keywords:

SEM images of Ca-HA particles: (A) Non-polished and before Pb 2+ sorption; (B) Polished and before Pb 2+ sorption; (C) Non-polished and after Pb 2+ sorption; (D) Polished and after Pb 2+ sorption.

Introduction

Heavy metals cause serious environmental concerns because of their sustained bio-toxicity. Heavy metals can be present in the environment from different anthropic sources as well as from natural geochemical processes. For example, the treatment of both municipal and industrial solid wastes by incineration technique is a major source of heavy metal emissions. Under high incineration temperatures, heavy metals present in wastes are partially or totally vaporized as a function of the nature of metals and combustion conditions. Gases emitted from incinerators require specific pollution control equipment adapted to high temperatures and complexity in terms of composition and physico-chemical properties [START_REF] Williams | Pollutants from incineration: an overview[END_REF][START_REF] Rio | Heavy metal vaporization and abatement during thermal treatment of modified wastes[END_REF]. In addition to anthropic sources, heavy metal contamination can also arise from natural geochemical processes, where significant heavy metal background levels can be found in large environment areas (soil, sediment, groundwater, etc.) [START_REF] Horst | Heavy metal distribution in sediments and ecological risk assessment: the role of diagenetic processes in reducing metal toxicity in bottom sediments[END_REF][START_REF] Miller | The role of fluvial geomorphic processes in the dispersal of heavy metals from mine sites[END_REF][START_REF] Malcolm | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF][START_REF] Navas | Geochemical speciation of heavy metals in semiarid soils of the central Ebro Valley (Spain)[END_REF][START_REF] Van Der Grift | Modelling assessment of regional groundwater contamination due to historic smelter emissions of heavy metals[END_REF][START_REF] Hashim | Remediation technologies for heavy metal contaminated groundwater[END_REF].

In the last decades, calcium hydroxyapatite (Ca-HA, chemical formula: Ca 10 (PO 4 ) 6 (OH) 2 ) was largely studied as an effective sorbent for the abatement of heavy metals under laboratory conditions, particularly in liquid phase. Because of its interesting physico-chemical properties such as low water solubility, possibility to high specific surface area, and its particular chemical structure [START_REF] Verwilghen | Preparation of high specific surface area hydroxyapatite for environmental applications[END_REF][START_REF] Kay | Crystal structure of hydroxyapatite[END_REF], Ca-HA can be used for the removal of several metallic cations from aqueous solutions, for example, Cu 2+ [START_REF] Sljivic | The influence of equilibration conditions and hydroxyapatite physico-chemical properties onto retention of Cu 2+ ions[END_REF][START_REF] Corami | Copper and zinc decontamination from single-and binary-metal solutions using hydroxyapatite[END_REF], Pb 2+ [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF], Zn 2+ [START_REF] Corami | Copper and zinc decontamination from single-and binary-metal solutions using hydroxyapatite[END_REF][START_REF] Sheha | Sorption behavior of Zn(II) ions on synthesized hydroxyapatites[END_REF], Co 2+ [START_REF] Smiciklas | Removal of Co 2+ from aqueous solutions by hydroxyapatite[END_REF], Cd 2+ and Ni 2+ [16] etc. For environmental purposes, Ca-HA of low cost is required for the large scale viability of treatment processes. However, the most common method for Ca-HA synthesis is the so-called double decomposition where a calcium salt (i.e. Ca(NO 3 ) 2 ) reacts with an orthophosphate salt (i.e. NH 4 H 2 PO 4 ) under controlled pH and temperature, followed by separation and eventually washing steps. In this case, large amounts of NH 4 NO 3 by-product are formed and its separation from well-dispersed Ca-HA particles is not easy. Other methods such as sol-gel synthesis [START_REF] Anee | A novel technique to synthesize hydroxyapatite at low temperature[END_REF], solid-solid reaction [START_REF] Silva | Structural properties of hydroxyapatite obtained by mechanosynthesis[END_REF], and ultrasonic spray freezeÀdrying [START_REF] Itatani | Preparation of various calciumphosphate powders by ultrasonic spray freeze-drying technique[END_REF] have been investigated but they also present drawbacks, namely high energy cost and/or the generation of aqueous wastes and difficulties for the separation and washing step.

Our previous work was devoted to the elaboration of Ca-HA from orthophosphoric acid (H 3 PO 4 ) and calcium carbonate (CaCO 3 ) as unconventional calcium source [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF]. The results showed that Ca-HA could be obtained by the addition of concentrated orthophosphoric acid into a suspension of calcium carbonate at 80 °C, under ambient pressure for about 24 h, conditions comparable to those used for the traditional synthesis of Ca-HA from soluble calcium salts. Carbon dioxide was the only by-product which left the reaction mixture in gas form. A stable gel of finely-divided Ca-HA particles in aqueous suspension was formed, which could be used as-is, or solid Ca-HA powder could be recovered by a simple filtration or centrifugation step, without any rinsing.

In this work, we investigated the removal of soluble lead(II) as a model heavy metal from a synthetic aqueous solution using the so-called green Ca-HA, obtained from the reaction of calcium carbonate and orthophosphoric acid, mentioned above. The results obtained will help to evaluate the performance of the synthesized Ca-HA compared to the literature data. The novelty of this study is found in the use of Ca-HA under gel form which constitutes a new approach for the design of wastewater treatment processes.

Materials and methods

Ca-HA was synthesized according to our previous study [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF]. Briefly, orthophosphoric acid (85 wt.%, p.a. grade from Merck) was pumped into an aqueous suspension of calcium carbonate (CaCO 3 , d 50 of 13.20 lm, >98% from Fisher Scientific) at 80 °C.

The reaction mixture was kept at atmospheric pressure under 350 rpm for 24 h. A white and stable gel of fine particles was formed which contained up to 97 wt.% of Ca-HA and some traces of carbonated apatite (CAP) [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF]. The gel generated by this synthetic procedure contained only trace amounts of soluble calcium and orthophosphate species [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF]. A third of this gel was kept at ambient temperature in a closed glass bottle for further sorption study. This part of the gel was designated ''Gel_25'' thereafter.

The second third of the gel was filtered on a 0.45 lm filter paper and the powder was dried overnight at 105 °C. This powder was designated ''Powder_105''. The last third of the gel was also filtered but the powder was dried freely at room temperature (about 25 °C) up to unchanged mass. Supplementary infra-red (FTIR) and thermogravimetry (TG) analyses were used to follow water evaporation at room temperature. The resulting powder was designated ''Powder_25''.

Aqueous solution of lead nitrate was prepared by dissolving lead nitrate (p.a. grade from Fisher Scientific) in permuted water for obtaining 6000 ppm (or 28.96 mmol L À1 ) lead(II) solution. Sorption experiments were carried out in a 700 mL stirred glass reactor containing 300 mL of the prepared lead nitrate solution. The experiment started by addition of 2.4 g of powder sorbent into the reactor under regular stirring (350 rpm) at room temperature (ca. 25 °C). When Gel_25 was used as sorbent, its water content was previously determined in order to calculate the equivalent quantity compared to powder sorbents. Samples were periodically withdrawn from the reactor for the determination of the concentration of soluble lead, calcium and phosphorus by inductively coupled plasma atomic emission spectroscopy (ICP-AES). During the reaction, the pH was continuously monitored using LabView program. At the end of the sorption experiment, the solid phase was recovered by filtration and dried overnight at 105 °C for further characterizations.

Elemental analysis was carried out by ICP-AES technique on a HORIBA Jobin Yvon Ultima 2. X-ray diffraction (XRD) data were collected on a Phillips Panalytical X'pert Pro MPD diffractometer. Simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC) analysis was performed in a TA Instruments SDTQ600 analyzer using an air flow rate at 100 mL min À1 and a heating rate of 5 °C min À1 . For each TG-DSC analysis, an exact weight of about 30 mg of powder was used. IR characterization was obtained on a Shimadzu FTIR 8400S spectrometer in the wavenumber range 4000-500 cm À1 . Specific surface area of the solid was determined by nitrogen adsorption using Micrometrics Gemini Vacprep 061 apparatus and BET equation (S BET ). 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 lm. Scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDX) was performed on a Philips XL30 ESEM apparatus (FEI Company). This apparatus is equipped with a secondary electron detector (SE mode), and a back-scattered electron (BSE) mode. SEM study was either directly carried out on the external surface of powder particles using only metallization as pre-treatment, or inside particles after embeding and mechanical polishing steps. In the last case, the immobilization of powder particles in a solid matrix of resin was carried out with a Mecaprex resin and triethylenetetramine (TETA) as hardening agent. Then, the samples were polished to fracture powder particles without damage in order to look inside.

Results and discussion

Lead(II) sorption

The concentration evolution of consumed lead(II) and released calcium(II) as a function of sorption time is shown in Fig. 1, where 2.4 g of Ca-HA powder or an amount of Gel_25 equivalent to 2.4 g of Ca-HA powder were set in contact with 300 mL of 6000 ppm lead(II). For these experiments, the sorbent concentration was 8.0 g L À1 . In all cases, lead(II) removal took place in two stages. The first stage occurred within about 60 min where about 4 mmol L À1 of lead(II) were removed. Lead(II) was then nearly totally removed in the second sorption stage. For Gel_25 and Pow-der_25 sorbents, the second stages finished before 300 min of sorption. For Powder_105 sample, a longer sorption time was required for the complete removal of lead(II) from the synthetic wastewater. In other words, the drying step at 105 °C led to a decrease in the reactivity of Ca-HA sorbent. For all sorbents, the concentration of released calcium(II) was close to that of consumed lead(II) throughout the reaction time. This suggested an equimolar ionic exchange between lead(II) and calcium(II) as observed previously with other Ca-HA materials [START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 3. Exchange characteristics of lead ions (Pb 2+ )[END_REF]22].

In order to better understand the fixation of lead(II) on Ca-HA sorbents, different physico-chemical characterizations were performed on Powder_25, Powder_105 as well as the solid recovered after sorption experiments, which are discussed in the next sections. On the other hand, Gel_25 was not characterized because the physico-chemical techniques used did not allow the study of a gel form.

XRD characterization

Fig. 2A presents XRD patterns of Powder_25 and Powder_105 which both were similar to each other. All peaks could be attributed to poorly crystallized Ca-HA phase (JCPDS standard No. 01-072-1243), as indicated by their weak broad peaks [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF].

Fig. 2B shows the XRD patterns of the solid recovered after reaction with lead(II). All solids had higher crystallinity than the initials, and showed similar patterns to each other. All peaks could be attributed to lead hydroxyapatite (Pb 10 (PO 4 ) 6 (OH) 2 , JCPDS standard No. 01-087-2477) and eventually some trace of lead oxide (PbO, JCPDS standard No. 01-078-1664 or 01-072-1551). Particularly, only trace amounts of poorly-crystalline Ca-HA were detected although about two-thirds of the initial Ca-HA sorbent might not participate in the sorption, taking into account the amount of removed lead(II). In fact, in these sorption experiments, the initial quantity of lead(II) in the synthetic wastewater was 8.69 mmol and the initial quantity of calcium(II) present in the sorbent of each sorption was about 24 mmol. Assuming equimolar ionic exchange as suggested by the evolution of consumed lead(II) and released calcium(II) in Fig. 1, only a third of the calcium(II) ions of the sorbents was replaced by lead(II). The decrease of visible diffractions of poorly-crystalline Ca-HA phase suggests that during sorption experiment, a destructuring of the crystalline network of the sorbents occurred. This was previously observed for the removal of lead nitrate by Ca-HA at acidic pH [START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 3. Exchange characteristics of lead ions (Pb 2+ )[END_REF], and was attributed to the changes of interatomic distances and bond angles by the insertion of lead(II) into the structure of Ca-HA [START_REF] Miyake | Structure refinements of Pb 2+ ion-exchanged apatites by X-ray powder pattern-fitting[END_REF]. It could also be due to the loss of the broad diffraction signal for poorly crystallized Ca-HA concealed under the strong lines of the more crystalline lead hydroxyapatite.

FTIR analysis

FTIR spectra of the initial sorbents are presented in Fig. 3A. Only a weak broad peak was observed at about 3570 cm À1 in the wavenumber range of 4000À1700 cm À1 , attributed to hydroxyl groups (not presented). No evidence of the principal peak of calcite at 1389 cm À1 was observed because of its complete decomposition during the Ca-HA synthesis. On the other hand, a bi-modal peak at 1450/1415 cm À1 and single peaks at 1545, 880 and 870 cm À1 were observed, which are characteristic for vibrations of carbonate groups, inserted in the apatitic structure [START_REF] Pham Minh | One-step synthesis of calcium hydroxyapatite from calcium carbonate and orthophosphoric acid under moderate conditions[END_REF]. Phosphate groups of Ca-HA structure were clearly observed at about 1040, 960, 601 and 570 cm À1 [START_REF] Elliott | Studies in Inorganic Chemistry 18: Structure and Chemistry of the Apatites and Other Calcium Orthophosphates[END_REF].

FTIR spectra of the solids after lead(II) sorption are shown in Fig. 3B. The reaction of lead(II) with the apatitic structure of Ca-HA led to some changes of IR vibrations of functional groups. A decrease in the intensity of carbonate vibrations was observed, which might be due to the partial decarbonation when Ca-HA sorbents were in contact with acidic solution of lead(II) nitrate (initial pH of 5.1 at room temperature). The bi-modal peak at 1020/ 970 cm À1 and peaks at 601 and 570 cm À1 were characterized as phosphate vibrations in lead(II) substituted Ca-HA [START_REF] Park | Oxidative coupling of methane using non-stoichiometric lead hydroxyapatite catalyst mixtures[END_REF]. This confirmed again the presence of lead(II) phosphate together with the apatitic structure of Ca-HA as shown above by XRD results.

TG-DSC analysis

Thermal behavior of sorbents before and after sorption experiment was investigated using TG-DSC technique. Both initial Pow-der_25 and Powder_105 had similar TG profiles to each other (Fig. 4A). The only difference between these two sorbents was found as a higher weight loss of Powder_25 in the temperature range of 25-105 °C, compared to that of Powder_105. This was attributed to the location of free water in porous Ca-HA matrix, which could not be removed by drying at room temperature. The next weight loss in the temperature range of 105-650 °C was due to the dehydration of lattice molecular water [START_REF] Suchanek | Mechanochemical-hydrothermal synthesis of carbonated apatite powders at room temperature[END_REF]. From 650 to 1280 °C, the weight loss was attributed to the decarbonation of CO 2À 3 groups present in Ca-HA structure [START_REF] Suchanek | Mechanochemical-hydrothermal synthesis of calcium phosphate powders with coupled magnesium and carbonate substitution[END_REF][START_REF] Lafon | Thermal decomposition of carbonated calcium phosphate apatites[END_REF]. Finally, a small net weight loss was observed at 1280-1315 °C and was attributed to the loss of OH À groups present in apatitic network and the formation of oxyhydroxyapatite [START_REF] Lafon | Thermal decomposition of carbonated calcium phosphate apatites[END_REF][START_REF] Liao | Thermal decomposition and reconstitution of hydroxyapatite in air atmosphere[END_REF].

The solids recovered after lead(II) sorption had practically the same thermal behavior. The weight losses corresponding to the removal of both surface moisture (25-105 °C) and lattice molecular water (105-650 °C), and the decarbonation of CAP (650-1000 °C) were proportionally smaller than those of the initial sorbents, because of the presence of lead(II), heavier than calcium(II), in the apatitic structure. Particularly, a strong weight loss starting at 850 °C was observed and it did not yet end when the temperature reached 1450 °C. At this temperature, the total weight loss was of about 44%, much higher compared to that of the initial sorbent. This could be due to evaporation of P 2 O 5 . This is of interest, for further study, to understand the thermal behavior of lead-doped Ca-HA, since it is used in high temperature processes, for example in the catalytic reforming of refractory molecules such as methane and butan-1-ol [START_REF] Park | Oxidative coupling of methane using non-stoichiometric lead hydroxyapatite catalyst mixtures[END_REF][START_REF] Park | Preparation of Pb-substituted hydroxyapatite catalyst and use in oxidative coupling of methane[END_REF][START_REF] Lee | Oxidative coupling of methane over transition-metalsubstituted strontium hydroxyapatite[END_REF][START_REF] Boukha | Comparative study of catalytic activity of Pd loaded hydroxyapatite and fluoroapatite in butan-2-ol conversion and methane oxidation[END_REF].

All weight losses mentioned above were endothermic (DSC results not presented).

SEM analysis

The morphology and external surface texture of Powder_105 is illustrated in Fig. 5. Particle sizes varied in a large range from hundreds of nm to more than 100 lm (Fig. 5A andB). At higher magnifications, it is evident that Ca-HA particles were formed by agglomeration of smaller ones (Fig. 5C). At the external surface, sheet or plate-like structure was then observed which is characteristic for Ca-HA particles [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF].

Fig. 6 presents SEM images of fractured Ca-HA particles. In most of cases, hollow Ca-HA particles were observed. In addition to the porosity due to this hollowness, pores of different sizes could be also observed on the border of these hollow Ca-HA particles. This porosity is favorable for the contact of lead(II) cations with the (Fig. 5A andB). However, the textural structure at the surface of particles was completely modified. Particles of sheet structure disappeared and new needle-like particles were abundantly formed (Fig. 7C andD). EDX analysis confirmed clearly the presence of lead on the surface of the sorbent (not shown).

In order to better observe the immobilization of lead, SEM study inside sorbent particles was carried out and the BSE images are illustrated in Fig. 8. With higher atomic number compared to those of other elements (calcium, phosphorus, oxygen), lead had the highest contrast and its presence could be distinctly observed on the surface of the sorbent, located either on the external surface of particles or surface of pores. The embeding and polishing techniques used for preparing SEM sample highlighted also the presence of both open macro-porosity (yellow arrows, Fig. 8A), where lead could get into and be immobilized, and closed macro-porosity (red arrows, Fig. 8B) where no trace of lead was observed. Several EDX analyses were repeated which confirmed the observation of BSE images, as illustrated in Fig. 8C.

Imaging of the main elements present in the used sorbent was also performed in order to have a global observation on the distribution of lead, calcium and phosphorus (Fig. 9). All images in Fig. 9 were set at the same scale. Fig. 9 A presents the BSE image from which imaging of lead, calcium and phosphorus was built by EDX analysis. Lead was present at high frequency on the external surface of particles and at lower frequency inside open-pores (Fig. 9B). On the other hand, calcium was principally observed in the underlying layers where lead could not get into (Fig. 9D). This indicated the substitution of calcium(II) by lead(II) at the accessible surface of Ca-HA particles. Phosphorus was generally present over entire sorbent particles (Fig. 9C). Image treatment shows clearly that lead distribution surrounds phosphorus (Fig. 9E) but calcium and phosphorus are superimposed (Fig. 9F). Similar SEM results were observed for Powder_25 before and after lead(II) sorption. SEM images show that particle sizes and visible macro-porosity of sorbents seemed unchanged after lead(II) sorption. To confirm this, particle size distribution and specific surface area were measured. On Fig. 10A, the initial powder dried at 25 °C (Powder_25) presents two populations of particles of 0.2-5 lm and 5-60 lm.

When the sorbent was dried at higher temperature (105 °C), the 3rd population appeared at 60-220 lm, which must be due to the agglomeration of smaller particles under thermal effect as shown in Fig. 5C. After lead(II) sorption, particle size distributions of Gaussian-type were observed for both sorbents, which were found in the particle size ranges of 6-85 lm and 7-65 lm for Powder_25 and Powder_105, respectively. This means that particles smaller than 5 lm or larger than 85 lm in the initial sorbents were not observed after reaction. In all cases, these results were in agreement with SEM results shown above.

Specific surface areas of sorbents before and after lead(II) sorption were also measured. There was no significant change of the specific surface area after sorption compared to the initial sorbents, which were all close to ca. 40 m 2 g À1 . From ICP-AES results, the amount of released calcium was practically equal to the amount of consumed lead during sorption. The fixation of lead seemed to not change particle sizes, as well as the visible macro-porosity of the sorbent, as illustrated in Fig. 7 and Fig. 10. Thus, specific surface area was unchanged after reaction.

Discussions

Sorption capacity and role of carbonate anions

Using an aqueous solution containing 6000 ppm of lead(II) and the concentration of sorbent of 8.0 g L À1 , lead(II) was completely immobilized on Ca-HA matrix as shown in Fig. 1. This means that the quantity of lead(II) potentially retained by one gram of sorbents can reach at least 750 mg. Table 1 compares this result with those previously reported in the literature for lead(II) sorption in liquid phase.

Most literature studies reported the use of Ca-HA obtained by a precipitation method with controlled pH from soluble calcium salts. Depending on the experimental conditions of the sorption process, in particular the pH, the sorbent concentration, the initial concentration of lead(II) and the sorption time, the sorption capacity (Q e ) reported varies in the range of 104-620 mg g À1 . Synthetic Ca-HA is generally found to be more active than commercial Ca-HA. Under similar sorption conditions, we found that the sorbent synthesized from CaCO 3 and H 3 PO 4 was much more active than those starting from calcium soluble salts reported in the literature. This higher activity may be due to the presence of carbonate anions remaining in the apatitic structure of our sorbent, as shown by IR results (Fig. 3). Supplementary TG-MS coupling showed that our sorbents contained approximately 2.6 wt.% of carbonate. In fact, the effect of carbonate anions on the reactivity of Ca-HA-based materials for the removal of heavy metals has been studied by Miyake et al. [37]. In this study, Ca-HA-based sorbents containing various contents of carbonate anions were synthesized from Ca(CH 3 COO) 2 and Na 2 HPO 4 in the presence of NaHCO 3 as carbonate source. Under the same experimental conditions, the removal of a given amount of lead(II) in an aqueous solution was completely group is required for the charge balance in the apatitic structure. This leads to a destabilization of apatitic structure which favors the immobilization of lead(II) [37]. Carbonated apatite (CAP) may also be less stable under acidic conditions and carbonate groups may be leached. When CAP containing high carbonate content (16 wt.%) was used, a small amount of lead carbonate could be detected by XRD and IR analysis [37].

About the mechanism for lead(II) fixation

Much work has been carried out on the fixation of lead(II) using apatitic sorbents and different mechanisms have been developed. The first one calls for the dissolution of Ca-HA followed by precipitation of lead hydroxyapatite or co-precipitation of lead-doped Ca-HA [22,[START_REF] Chen | Effects of pH on heavy metal sorption on mineral apatite[END_REF][START_REF] Xu | Kinetic research on the sorption of aqueous lead by synthetic carbonate hydroxyapatite[END_REF]. In our case, the precipitation of lead is supported by the formation of Pb 10 (PO 4 ) 6 (OH) 2 as shown by XRD results. However, no XRD evidence of lead-doped Ca-HA exists in the analysis of samples derived at room temperature. SEM images (Fig. 8 and9) enhanced this hypothesis. The layer thickness of lead-rich phase was estimated at lm scale, as a result of the precipitation step.

However, because the size and the visible macro-porosity and the specific surface area of sorbent particles were practically similar before and after lead(II) removal, complete dissolution of Ca-HA particles did not occur, as confirmed by BSE images. So, a dissolution-precipitation mechanism could occur partly at the surface of sorbent particles.

The second mechanism consists of the ionic exchange between calcium(II) and lead(II) [START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 3. Exchange characteristics of lead ions (Pb 2+ )[END_REF]34,37]. Most authors announced this mechanism based on the molar ratio of released calcium(II) to removed lead(II) close to unity. Suzuki et al. [START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 3. Exchange characteristics of lead ions (Pb 2+ )[END_REF] indicated a long exchange time under room conditions for the fixation of lead(II) by this way. Thus, the results in Fig. 1 demonstrate the possibility of ionic exchange. However, it should be emphasized that ionic exchange is not a sorption but an exchange reaction where lead displaces calcium and the end product is different from the starting one. Thus, it cannot contribute to a sorption isotherm.

In addition to these two mechanisms, lead(II) fixation at the beginning of the reaction may occur by a fast surface complexation of lead(II) on "POH sites [22,35,[START_REF] Deydier | Beneficial use of meat and bone meal combustion residue: ''an efficient low cost material to remove lead from aqueous effluent[END_REF][START_REF] Deydier | Physical and chemical characterisation of crude meat and bone meal combustion residue: ''waste or raw material?[END_REF]. This is supported by the continuous monitoring of pH during sorption (Fig. 11), wherein the pH decreased during the first 30 s according to Eq. ( 1). This pH decrease might also be explained by the presence of calcium phosphates having molar ratios of Ca/P smaller than 1.67, such as octocalcium phosphate pentahydrate (Ca 8 (HPO 4 ) 2 (PO 4 ) 4 Á5H 2 O), brushite (CaHPO 4 Á2H 2 O), monetite (CaHPO 4 ), or hydrated Ca-HA surfaces containing such hydrogenphosphate species, according to Eqs. ( 2

)-(4). 2½BPOH þ Pb 2þ ! ½BPO À 2 Pb þ 2H þ ð1Þ
Ca 8 ðHPO 4 Þ 2 ðPO 4 Þ 4 Á 5H 2 O þ 10Pb 2þ ! Pb 10 ðPO 4 Þ 6 ðOHÞ 2 þ 8Ca 2þ þ 2H þ þ 5H 2 O ð2Þ
6½CaHPO 4 Á 2H 2 O þ 10Pb 2þ ! Pb 10 ðPO 4 Þ 6 ðOHÞ 2 þ 6Ca 2þ þ 8H þ þ 10H 2 O ð3Þ
6CaHPO 4 þ 2H 2 O þ 10Pb 2þ ! Pb 10 ðPO 4 Þ 6 ðOHÞ 2 þ 6Ca 2þ þ 8H þ ð4Þ
After this first short decrease, the pH increased slightly up to about 10 min, then decreased strongly between 10 and 140-180 min of contact. This might correspond to the dissolution (pH increase)-precipitation (pH decrease) phenomenon, but also to a surface precipitation of lead(II). After the first min of contact, particles of sorbents were covered by some amounts of lead(II) at the surface, which might have some catalytic nucleating effect for the surface precipitation of free lead(II) cations, present in the reaction mixture at high concentration (Eq. ( 5)) [START_REF] Kwestroo | Influence of impurities on the formation of red and yellow PbO[END_REF]. This hypothesis was enhanced by the detection of lead oxide at small amounts by XRD analysis.

Pb

2þ þ 2OH À ! PbO þ H 2 O ð5Þ
Finally, the pH increased progressively up to 24 h of reaction, because of the buffering capacity of Ca-HA, and according to the surface property of lead-doped Ca-HA which has the point of zero charge (pHpzc) in the range of 5-6 [START_REF] Smiciklas | Factors influencing the removal of divalent cations by hydroxyapatite[END_REF].

In summary, different reactions are involved for the removal of lead(II) using such Ca-HA sorbents. The molar ratio of released calcium(II) to removed lead(II) close to unity and the decrease of pH at the beginning of the reaction suggest the preliminary fast surface complexation mechanism, leading to ionic proton-Pb 2+ exchange. However, the destruction of the crystalline structure of Ca-HA particles, the formation of a lead-rich layer including Pb 10 (PO 4 ) 6 (OH) 2 , and trace amounts of PbO on the surface of sorbent particles with layer thickness at lm scale suggest that dissolution-precipitation occurs simultaneously, particularly under acidic initial conditions when Ca-HA is not very stable as solid phase.

Effect of drying temperature

The sorption results in Fig. 1 show that the drying step at 105 °C led to lower reactivity of the sorbent compared to drying at 25 °C. Firstly, this effect could be attributed to the formation of large Ca-HA particles of 60-220 lm (Fig. 10A). The removal of water at 105 °C might also lead to a lower permeability of Ca-HA. These all can perturb the diffusion of lead(II) into pores and/or lattices of Ca-HA for fixation. The inhibition by thermal treatment of the reactivity of Ca-HA for lead(II) removal was also observed by Sugiyama et al. [36] and Hadioui et al. [START_REF] Hadioui | Reaction of lead ions with hydroxylapatite granules[END_REF].

Conclusions

Calcium hydroxyapatite (Ca-HA) obtained by the one-step synthesis method from calcium carbonate and orthophosphoric acid as economical starting materials at moderate conditions (80 °C, atmospheric pressure) was found to be very efficient for the removal of lead(II) as model heavy metal from an aqueous solution.

No counter-ions were present in the final suspension of Ca-HA which could be directly used in gel form for lead(II) removal. In addition, the presence of carbonate anions in the apatitic structure favored the removal of lead(II) which explains the higher sorption capacity of Ca-HA synthesized in this study compared to literature data. Ca-HA particles in suspension form or dried at ambient conditions were found to be more active than Ca-HA particles dried at 105 °C which reveals the importance of the non-stoichiometric hydrated surface composition. Lead(II) was mostly immobilized on the surface of sorbent particles (external surface and pore surface) as highlighted by SEM study.

Further study will focus on the improvement of the sorption capacity of Ca-HA. A reflux of carbonic gas formed from the decarbonation of calcium carbonate to the reaction mixture may increase the carbonate content in Ca-HA sorbent, since it favors the fixation of lead(II). Synthesis conditions will be improved for obtaining smaller particles, which should further improve the fixation of lead(II).

It is also interesting to investigate the thermal behavior of lead-doped hydroxyapatite since it can be used in different hightemperature processes, such as in heterogeneous catalysis.

Fig. 1 .

 1 Fig. 1. Consumed Pb (h) and released Ca (N) during the sorption of Pb using different Ca-HA sorbents: (A) Ca-HA gel; (B) Ca-HA powder dried at the ambient conditions; (C) Ca-HA powder dried at 105 °C; reaction time is in logarithm base 10 scale.

Fig. 2 .

 2 Fig. 2. XRD patterns of the solid sorbents before lead(II) sorption experiments (A); and solids recovered from lead(II) sorption experiments and dried overnight at 105 °C (B).

Fig. 3 .

 3 Fig. 3. FTIR spectra of the initial sorbents (A); and of the sorbents after lead(II) sorption, dried overnight at 105 °C (B).

Fig. 4 .

 4 Fig. 4. TG curves of the initial sorbents (A); and of the sorbents after lead(II) sorption, dried overnight at 105 °C (B).

Fig. 5 .

 5 Fig. 5. SEM observation on the external surface of Powder_105 before lead(II) sorption.

Fig. 6 .

 6 Fig. 6. SEM observation inside particles of Powder_105 before lead(II) sorption.

Fig. 7 .

 7 Fig. 7. SEM observation on the external surface of Powder_105 after lead(II) sorption.

Fig. 8 .

 8 Fig. 8. SEM observation inside particles of Powder_105 after lead(II) sorption; (A) yellow arrows: open macro-porosity; (B) red arrows: closed macro-porosity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9 .

 9 Fig. 9. Imaging of lead, calcium and phosphorus present in Powder_105 after lead(II) sorption; (A) BSE image; (B-D) imaging of lead, phosphorus and calcium, respectively; (E) image treatment of phosphorus (yellow) and lead (blue); (F) image treatment of phosphorus (yellow) and calcium (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10 .

 10 Fig. 10. Particle size distribution of the initial powder sorbents (A); and of the sorbents after lead(II) sorption, dried overnight at 105 °C (B).

Fig. 11 .

 11 Fig. 11. pH evolution during lead(II) sorption using different Ca-HA.

Table 1

 1 Comparison of the present study with literature data.

	Sorbent	Starting materials for the synthesis of sorbent	[Sorbent], g L À1	[Pb], mg L À1	Q e , mg g À1	Ref.
	Gel_25 Powder_25 Powder_105	CaCO 3 , H 3 PO 4 CaCO 3 , H 3 PO 4 CaCO 3 , H 3 PO 4	8.0 8.0 8.0	6000 6000 6000	750 a 750 a 750 a	This work This work This work
	Ca-HA Ca-HA	Ca(NO 3 ) 2 , NH 4 H 2 PO 4 Ca(OH) 2 , H 3 PO 4 b	8.0	5200	620	[34]

Acknowledgments

The authors thank gratefully Christine Rolland and Nathalie Lyczko for technical help.