Ange Nzihou

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

Á P Sharrock

Role of Phosphate in the Remediation and Reuse of Heavy Metal Polluted Wastes and Sites

Keywords: Phosphates, Apatites, Heavy metals, Remediation, Waste, Sites

Heavy metal pollution is a major environmental concern because of the toxicity to humans and plants. This toxicity is lethal even in trace quantities and metals have a great tendency to bioaccumulate. The efficiency of phosphate and apatites M 10 (PO 4 ) 6 (OH) 2 , M: Metal, in particular, in removing and immobilizing heavy metals from wastewater, groundwater (as permeable reactive barriers for in situ site remediation), fly ash, dredged sludges and contaminated soil has led to various studies to understand and explain the mechanisms involved. This paper will address the use of apatite as sorbent and stabilizing agents for the removal of heavy metals from various media. Efficient physico-chemical immobilization of heavy metals brings new perspectives for reuse of polluted waste water, soils and land treated by selected phosphates.

Introduction

The presence of heavy metals in our environment is increasingly due to industrial pollution of the atmosphere, waterways, soils and sediments. Contrary to organic molecules which can be metabolized or destroyed, metals can only be transformed from labile toxic forms to more inert insoluble and less bio-available forms to minimize their harmful effects on living organisms.

Soluble phosphate was widely investigated as a means of chemical treatment for the conversion of heavy metals into insoluble compounds. Phosphoric acid, ammonium, sodium and potassium phosphates will react with multivalent cations to form insoluble orthophosphates. Several hydrogen or dihydrogen phosphates have been investigated also with the aim of scavenging soluble heavy metals. The end products of the reactions of soluble phosphates with metal ions are insoluble metal phosphate salts akin to naturally occurring minerals which are stable in geological environments.

Hydrargyrite, Pb 5 (PO 4 ) 3 Cl, is a typical example of a lead phosphate with very low solubility in a wide pH range [START_REF] Nriagu | Formation and stability of base metal phosphates in soils and sediments[END_REF]. Another frequently encountered example is the apatite family, with hydroxylapatite, Ca 5 (PO 4 ) 3 (OH), (HA), the lead member of a large class of substituted compounds of similar structures. Solid solutions may be formed with a large range of concentrations of divalent or trivalent metals replacing calcium, and with fluorine or chlorine substituting for hydroxyl ions and carbonate, vanadate or arsenate found in lieu of phosphate.

Fish bones and the skeleton of mammals are composed of calcium phosphate closely resembling HA. Bones, bone char and synthetic HA have been examined for their capacity to retain heavy metals by absorption from aqueous solutions. HA can effectively be used for the treatment of heavy metal polluted waste materials.

In this review we will examine recent developments in the treatment with calcium phosphate of solid and liquid streams contaminated with heavy metals. We will begin with a review of the physicochemical properties of HA and its capacity to sorb heavy metal ions. Sorption is the loss of a chemical species from an aqueous solution to a continuous solid phase. Adsorption to form a monolayer is the two-dimensional accumulation of matter at the mineralwater interface at specific sites. Multiple layers may also form at an interface by adsorption processes. Precipitation is the three-dimensional growth of a solid phase.

The use of natural or synthetic HA will be presented with comments on the mechanisms proposed for heavy metal adsorption. Treatment of heavy metal polluted water will be considered as well as the formation of barriers for remediation of contaminated soils or sediments.

The Apatite Family

Hydroxylapatite, Ca 5 (PO 4 ) 3 (OH), (HA) has a low solubility product [START_REF] Jaynes | Solubility and ion activity products of calcium phosphate minerals[END_REF] of K sp = 5 9 10 -119 . Carbonated HA constitutes the inorganic phase present in the bones of mammals and is very stable under normal environmental conditions. It does however dissolve in acidic solution to generate calcium cations and hydrogenophosphate anions, but remains stable under basic pH conditions. In pure water (with no added phosphate or calcium) the point of zero charge is precisely at the neutral pH value of pH 7 and defines the point where negative and positive charges are found in equal amounts on the surface of solid particles. The surface of HA particles are hydrated when present in aqueous media [START_REF] Amrah-Bouali | Surface modifications of hydroxyapatite ceramics in aqueous media[END_REF] and the surface phosphate groups are protonated near pH 7. Calcium and phosphate sites are not present in equal amounts: the theoretical values are 3.1 and 5.1 atom nm -2 for calcium and phosphate respectively. Figure 1 shows the equilibria involved. Under basic pH conditions the outer surface of HA solids will be negatively charged whereas under acidic conditions it will be positively charged.

Substitutions are ubiquitous in apatite chemistry and a wide variety of ions can be incorporated into the apatite structure. Fluorine can replace hydroxide ion, carbonate can be found in lieu of phosphate, and divalent metal ions can substitute for calcium ions in large proportions. The easy formation of mixed metal phosphates can explain the capacity of HA to remove heavy metals from aqueous solutions and retain them in the solid phase. Figure 2 illustrates the crystal structure of HA pointing out all the exchangeable groups, including the two crystallographically different calcium atoms.

Water Purification with Apatites

The first reports on the use of HA for metal ion capture from aqueous solutions were generated by Suzuki and collaborators in 1981 [START_REF] Suzuki | Synthetic hydroxyapatites employed as inorganic cation-exchangers[END_REF]. They discovered that HA collects heavy metal ions such as lead, copper and cadmium ions by exchange with calcium in its main structure. The group published data on single heavy metal sorption on HA as well as multi-metal sorption experiments showing that lead ions are preferentially picked up by HA, and can even replace other metals previously sorbed [START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 2[END_REF][START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 3. Exchange characteristics of lead ions (Pb 2? )[END_REF]. Binary and ternary equilibria were discussed, selectivity coefficients and values of mass transfer coefficients were determined. The rate of mass transfer was obtained for various ions and discussed in terms of film diffusion and an exchange process of ions at specific sites of the apatite structure.

The first basic conclusion was that the phenomenon taking place was a simple ion exchange between a part of the calcium ions in the apatite structure and the heavy metal ions in the aqueous phase, provided no exchange of calcium took place with protons. Indeed, the authors recognized that HA ''decomposes'' in acidic solutions. However, even under slightly acidic conditions (pH near 5), examination of breakthrough curves suggested that a very small amount of HA actually dissolved in the aqueous phase at the same time as ion exchange. When the pH was smaller than 3, HA crystals were observed to dissolve gradually by neutralization of the protons. Experiments conducted in the region of pH 5-6 showed rectangular isotherms, or irreversible isotherms, for all the systems investigated. Under these conditions it was found that the exchange reaction did not have a serious effect on the apatite structure. In any case, the HA structure was maintained and the divalent heavy metal ions were held in the structure. The rate of metal exchange was found to be particularly rapid for lead ions and it was concluded that the fast reaction seems to take place at the surface of the apatite particles.

The ranking of the ions according to the amount exchanged revealed that transition metals were more strongly sorbed than alkaline earth ions. To explain the differences observed between the various metal ions, a dehydration mechanism was proposed to be a rate determining step. Because dissolved heavy metal ions are associated with bound water molecules acting as ligands, the water to metal coordination bonds first have to be broken before exchange can take place with calcium ions at the surface of the HA. Lead ions have a lower enthalpy of hydration than calcium ions, and cadmium ions have a higher enthalpy of hydration, providing an explanation why lead ions are collected in a larger amount than other ions which form strong aqua complexes. Electronegativity and ionic radius mismatch were also invoked to compare the various ions and their capacity to exchange for calcium ions. It was also found that lead ions exchange very quickly with calcium ions, but that a slower process continues to remove lead from solution with time. It was recognized that the kinetics of metal sorption showed the coexistence of two processes. The fast initial mass transfer was related to the surface ion exchange reaction while the slower process was assumed to correspond to mass transfer resistance, described as governed by solid diffusion rather than by pore diffusion or surface diffusion. Thus the predominant resistance to mass transfer for heavy metal ions lied in intra-particle diffusion.

Mechanisms of Heavy Metals Removal from Aqueous Phase

In general, ion conducting channels and exchangeable ions are located in flexible frameworks that can adapt to its filling and chemistry by changing their lattice constants. HA has calcium ions located on external crystal faces and also along the tunnels that penetrate its structure. Members of the apatite family are thus ideally suited for extensive ion exchange reactions. The HA structure is tolerant to many substitutions and even complete replacement of calcium ions with other ions as well as the formation of solid solutions [START_REF] Bailliez | Removal of lead (Pb) by hydroxyapatite sorbent[END_REF]. However, intra-particle diffusion is difficult and slow at room temperature. Mixtures of apatites form a one phase solid solution when heated near sintering temperatures [START_REF] Marchat | Cadmium fixation by synthetic hydroxyapatite in aqueous solution-thermal behavior[END_REF]. The formation of secondary phases during exposure of HA to metal ions results in the formation of precipitates with low solubility on the HA surfaces effectively clogging the possible tunnel pathways for metal uptake inside the polycrystalline particles. Diffusion is then overwhelmed by accumulation of amorphous products causing mass transfer resistance. The common observation of the dual kinetics of heavy metal ion sorption on HA with the rapid first step limited in exchange capacity and related to specific surface area led authors to propose a dissolution-precipitation mechanism for the second slower step. Indeed, under acidic pH conditions when HA will dissolve more readily, HA shows increased capacity for heavy metal ion removal [START_REF] Chen | Effects of pH on heavy metal sorption on mineral apatite[END_REF]. This has led to some confusion concerning the actual mechanism responsible for the global exchange capacities and exchange rates [START_REF] Mavropoulos | Studies on the mechanisms of lead immobilization by hydroxyapatite[END_REF].

The two mechanisms most often cited for heavy metal capture by HA are the ion-exchange process and the dissolution-precipitation process [START_REF] Suzuki | Uptake of a few divalent heavy metal ionic species by a fixed bed of hydroxyapatite particles[END_REF]. In the first case, the metal attaches to the HA surface or diffuses inside and provokes calcium release by pushing out the more soluble endogenous calcium ions. The smaller solubility of the heavy metal phosphate pushes the exchange reaction towards release of calcium ions. In the second case, calcium is dissolved first, together with accompanying phosphate ions, and the heavy metal ions precipitate by homogeneous (in solution) or heterogeneous (on the remaining solid HA) nucleation [START_REF] Manecki | Kinetics of aqueous Pb reaction with apatites[END_REF][START_REF] Manecki | Uptake of aqueous Pb by Cl -, F -, and OHapatites: mineralogic evidence for nucleation mechanisms[END_REF]. Both mechanisms can account for the fact that the sorption reaction slows down considerably after the initial HA surfaces are covered by less soluble phosphates. Following ion exchange, the new less soluble metal phosphate forms a coating which prevents subsequent dissolution of the underlying calcium ions and blocks the exchange reaction. This is similar to the effect of heavy metal phosphate coating of HA particles caused by heterogeneous precipitation [START_REF] Lower | Aqueous Pb sorption by hydroxylapatite: applications of atomic force microscopy to dissolution, nucleation, and growth studies[END_REF]. In any case the final result is the removal of a certain quantity of heavy metal ions depending on the HA crystallinity [START_REF] Sto ¨tzel | Ion adsorption behaviour of hydroxyapatite with different crystallinities[END_REF], density, specific surface area, composition and other factors such as pH, metal concentrations, rate of addition, temperature and presence of ligands [START_REF] Takeuchi | Hironori Removal of coexisting Pb 2? , Cu 2? and Cd 2? ions from water by addition of hydroxyapatite powder[END_REF][START_REF] Hashimoto | Removal of aqueous lead by poorly-crystalline hydroxyapatites[END_REF][START_REF] Xu | Lead immobilization by hydroxyapatite in aqueous solutions[END_REF][START_REF] Sugiyama | Lead immobilization by non-apatite-type calcium phosphates in aqueous solutions[END_REF][START_REF] Lusvardi | Removal of cadmium ion by means of synthetic hydroxyapatite[END_REF]. The literature on this subject has been reviewed [START_REF] Monteil-Rivera | Sorption of inorganic species on apatites from aqueous solutions[END_REF].

Another important observation reported on examination of lead ions sorption on HA is the pH drop correlated with the initial fast metal uptake. This phenomenon can be explained by the formation of a metal-phosphate bond replacing the protonated surface phosphate groups. This also explains the lag in calcium release during the initial periods of lead fixation in column flow-through experiments. Recent evidence points to surface complexes formed by substituting surface protons or calcium ions by incoming metal ions [START_REF] Corami | Removal of Cu from aqueous solutions by synthetic hydroxyapatite: EXAFS investigation[END_REF][START_REF] Corami | Cadmium removal from single-and multi-metal (Cd ? Pb ? Zn ? Cu) solutions by sorption on hydroxyapatite[END_REF]. In the case of cupric ions, the large differences in radii between exchanging ionic species inhibits migration into the inner part of the HA structure. EXAFS study concludes that the fixation of copper in the calcium sites in the interior of the HA crystal is unlikely. The same applies to uranium [START_REF] Krestou | Mechanism of aqueous uranium(VI) uptake by hydroxyapatite[END_REF][START_REF] Fuller | Molecular-scale characterization of uranium sorption by bone apatite materials for a permeable reactive barrier demonstration[END_REF][START_REF] Fuller | Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation[END_REF].

The Various HA Forms Synthetic HA is not the only form of calcium phosphate used in metal removal investigations [START_REF] Shashkova | Removal of heavy metal ions from aqueous solutions by alkaline-earth metal phosphates[END_REF][START_REF] Sugiyama | Immobilization of aqueous heavy metal cations with phosphates and sulfates[END_REF]. Fish bones resulting from fishing waste have been studied as well as bone char and meat and bone meal calcination residues [START_REF] Deydier | Evaluation of meat and bone meal combustion residue as lead immobilizing material for in situ remediation of polluted aqueous solutions and soils: ''Chemical and ecotoxicological studies[END_REF][START_REF] Cheung | Removal of Cu(II) and Zn(II) ions by sorption onto bone char using batch agitation[END_REF]. Fishbones, a natural apatite, was found to be comparable to hydroxyapatite on a Ca content basis for removal of various aqueous heavy metal ions [START_REF] Aoki | Removal of Pb 2? and Cd 2? Ions by hydroxyapatite separated from scales of fish Zairyo[END_REF][START_REF] Giammar | Immobilization of lead with nanocrystalline carbonated apatite present in fish bone[END_REF][START_REF] Ozawa | Removal of aqueous lead by fish-bone waste hydroxyapatite powder[END_REF][START_REF] Hettiarachchi | In situ stabilization of soil lead using phosphorus[END_REF]. It compares favorably to apatite ores because of its purity and lower substitution of undesirable metals. When the fishbones were dried, crushed and fired in air at 900°C the immobilization property of fishbone apatite was improved. Thomson et al. [START_REF] Thomson | Removal of metals and radionuclides using apatite and other natural sorbents[END_REF] compared several natural and synthetic apatites in batch studies and column tests and reported the synthetic products had higher capacities than the natural products. On the other hand, Dybowska et al. [START_REF] Dybowska | An evaluation of the reactivity of synthetic and natural apatites in the presence of aqueous metals[END_REF] reported that meat and bone meal sterilized according to EU standards (containing 25% HA and 75% organic matrix) and then incinerated at 725 or 850°C performed as well as synthetic apatite and better than natural rock phosphate. The reactivity of apatites for all tested metals (Pb, Cd, Cu and Zn) followed the general order: synthetic [ biological [ mineral. Cheung et al. studied the sorption of metal ions from aqueous solution using bone char [START_REF] Cheung | Removal of Cu(II) and Zn(II) ions by sorption onto bone char using batch agitation[END_REF] composed mostly of HA but containing 9-11 wt% carbon contributing 50% of the overall 100 m 2 g -1 specific surface area. They found that the sorption mechanisms can be attributed to at least three pathways:

1. Ion exchange of divalent metal ions from hydroxyapatite. 2. Sorption of metal ions from the hydroxyapatite surface lattice. 3. Sorption of metal ions from the carbon surface.

Meat and bone meal combustion residues were found to contain over 56% of phosphate which could capture large quantities of aqueous Pb 2? ions in a few minutes with a 275 mg/g capacity. The ashes presented two beneficial actions in ecotoxicological studies: they not only neutralize lead toxicity but also act as a fertilizer.

Dried animal bones can also be used for Zn 2? removal from water and it was found that 10 mM sulfuric acid can release more than 90% of the metal by desorption [START_REF] Banat | Batch zinc removal from aqueous solution using dried animal bones[END_REF].

A Tunisian phosphate rock was activated by base and acid washings and found to have adequate heavy metal retention properties of between 12 and 15 mg/g for various metal cations [START_REF] Elouear | Use of phosphate rock for the removal of Ni 2? from aqueous solutions: Kinetic and thermodynamics studies[END_REF][START_REF] Elouear | Heavy metal removal from aqueous solutions by activated phosphate rock[END_REF]. The activated raw material is expected to have a competitive edge over other commercial species. Low grade phosphorite deposits of sedimentary origin located in India are considered as waste because of their low phosphate values of 12.5% [START_REF] Prasad | Multi-component sorption of Pb(II), Cu(II) and Zn(II) onto low-cost mineral adsorbent[END_REF][START_REF] Saxena | Heavy metal pollution abatement using rock phosphate mineral[END_REF]. They have been evaluated as eco-friendly and cost-effective sorbents for heavy metals. The used sorbents have been recycled in frit materials for glaze production in a zero waste concept.

The Role of Organic Molecules on Heavy Metal Exchange

Recent work has gained specific insight on the influence of organic molecules with metal complexing ability on the effectiveness of metal sequestration by HA [START_REF] Hadioui | Silica gel and hydroxyapatite composites with ormosils for heavy metal removal from water[END_REF]. It was shown that strong complexing ligands such as EDTA effectively stabilize the soluble form of metals and thereby decrease their capture by HA in the solid phase. This effect contrasts with the observation that various inorganic anions have no effect on heavy metal removal by adsorption on HA [START_REF] Ma | Factors influencing the effectiveness and stability of aqueous lead immobilization by hydroxyapatite[END_REF][START_REF] Ma | Effects of NO 3 -, Cl -, F -, SO 4 2-, and CO 3 2on Pb 2? immobilization by hydroxyapatite[END_REF].

Recent developments have led to the elaboration of composite materials wherein HA is embedded in a polymer matrix. An HA and polyurethane composite material [START_REF] Jang | Removal of lead ions in aqueous solution by hydroxyapatite/ polyurethane composite foams[END_REF] was made with 20% or 50% HA dispersed in the polymer foam to study lead ion uptake. It was found that higher HA contents had higher removal capability but slower removal rate. Also increased initial lead concentrations caused slower uptake rates. The dissolution of HA and precipitation of hydroxypyromorphite was the dominant mechanism at lower pH values of 2-3 for the incoming solution. The adsorption of Pb 2? ions on the HA/polyurethane composite surface and ion exchange reaction between Ca 2? of HA and Pb 2? in aqueous solution was dominant at higher pH values between 5 and 6. Both mechanisms were found to compete at pH 4. An HA/ cellulose composite was also studied for its exchange capacity and it was found that the cellulose matrix did not interfere with the HA-heavy metal reaction time or exchange capacity but helped in facilitating the watersorbent handling [START_REF] Choi | The removal of heavy metals in aqueous solution by hydroxyapatite/cellulose composite[END_REF]. Up to 70 wt% HA could be incorporated in a polyacrylamide gel causing lower swelling in water, but also slower removal rate of Pb 2? ions. The removed amount of Pb 2? ion was nearly same, regardless of pH values of 2-5 [START_REF] Jang | Preparation and lead ion removal property of hydroxyapatite/ polyacrylamide composite hydrogels[END_REF].

In a study on porous HA granules which could remove aqueous lead ions at a rate of 0.5 mg g -1 min -1 it was observed that small lead phosphate aggregates were released from the HA sorbent together with calcium ions under some hydrodynamic flow conditions. It was reported that organic ligands tend to interfere with the water purification procedure [START_REF] Hadioui | Reaction of lead ions with hydroxylapatite granules[END_REF].

In a study of the sorption of lead ions from simulated industrial wastewater onto Jordanian low-grade phosphate, it was found that increasing the concentration ratio of ligand to metal ion decreased the percentage removal of lead ions significantly for several organic carboxylic ligands [START_REF] Kandah | The potential use of low-grade phosphate rocks as adsorbent[END_REF]. Nevertheless, lead ions and lead-ligand complexes were observed to sorb on the phosphate ore [START_REF] Sarioglu | Removal of copper from aqueous solutions by phosphate rock[END_REF][START_REF] Mouflih | Removal of lead from aqueous solutions by activated phosphate[END_REF].

Wang et al. studied the effects of low molecular weight organic acids on the kinetics and adsorption isotherms of Cu 2? onto HA [START_REF] Wang | Efects of low molecular weight organic acids on Cu(II) adsorption onto hydroxyapatite nanoparticles[END_REF]. The maximal adsorption quantity decreased with increase in the logarithm of cumulative formation constants of Cu 2? and organic acids. Investigating the various species formed as a function of solution pH, it was found that the effects of ligands on the uptake of copper depended on the stabilities and charges of the complex formed. The stronger the organic acid coordinated to copper, the more the metal quantity adsorbed on HA decreased.

Remediation of Heavy Metal Contaminated Soils

There are many examples of remediated soils that were contaminated with organic pollutants but there are few cases where heavy-metal polluted soils were treated, despite the fact that in most polluted sites in industrialized countries a variety of heavy metals are present. Toxic elements that remain untreated in soils and sediments represent a threat to surface and groundwater reserves. The persistent presence of mobile heavy metals in soils is a continuing challenge for authorities responsible for environmental protection [START_REF] Dermont | Metal-contaminated soils: remediation practices and treatment technologies[END_REF][START_REF] Manning | Phosphate minerals, environmental pollution and sustainable agriculture[END_REF].

Usual remediation techniques involve ex-situ treatment of excavated material followed by land-filling and longterm monitoring following solidification/stabilization. This is not ecologically neutral to the landscapes and ecosystems involved. Alternative non destructive in situ treatment technologies are clean up options which can decrease the environmental disturbance and risk of exposure to living organisms. In situ stabilization is operated when leachable and bioavailable metals are transferred to less soluble forms that remain inert for long time periods. Chemical immobilization technologies can be combined with plant seeding and growth as a form of phytostabilization to attenuate contaminant dispersion via wind and hydrological processes. However, metal pollutions are also toxic to plants and the treatments used must specifically aim at reducing toxic metal mobility while maintaining needed mineral availability for plants in particular magnesium, iron and other vital oligo-elements [START_REF] Chen | Evaluation of different phosphate amendments on availability of metals in contaminated soil[END_REF][START_REF] Keller | Reduction of cadmium availability to tobacco (Nicotiana tabacum) plants using soil amendments in low cadmium-contaminated agricultural soils: a pot experiment[END_REF][START_REF] Boisson | Evaluation of hydroxyapatite as a metal immobilizing soil additive for the remediation of polluted soils. Part 1. Influence of hydroxyapatite on metal exchangeability in soil, plant growth and plant metal accumulation[END_REF].

The use of inexpensive minerals in remediation techniques is based on the well recognized role of stable species across a wide range of geological conditions. Clays, zeolites and apatites are attractive sequestering agents for in situ immobilization of toxic elements and reduction of risk associated with migration of potentially harmful species. Phosphates and particularly HA is known to be very effective in redistributing lead and other heavy metal ions into compounds more strongly withheld by soils. Reduced phytotoxicity goes with greater plant growth and lower concentrations of toxic elements in plant tissues. We will examine how HA and other phosphate chemical treatments leading to the formation of metal phosphates promote soil stabilization.

The Role of HA in Immobilizing Soil Heavy Metals

Contaminated soil has been dry mixed with HA with interesting results [START_REF] Montinaro | Immobilization of heavy metals in contaminated soils through ball milling with and without additives[END_REF], but most studies rely on water present in soils for transfer of reactants or heavy metals to the more residual fractions of soils [START_REF] Ciardelli | Role of Fe(II), phosphate, silicate, sulfate, and carbonate in arsenic uptake by coprecipitation in synthetic and natural groundwater[END_REF]. Sequential extraction procedures have demonstrated that exchangeable fraction of Cu and Zn in contaminated soils can be transformed into unavailable forms after chemical remediation [START_REF] Fawzy | Soil remediation using in situ immobilization techniques[END_REF]. The incorporation of heavy metals in apatites and the formation of silicate apatites have been compared to the formation of mineral reservoirs which can retain metals in their crystal structure following thermal treatment [START_REF] Po ¨llmann | Immobilisation of toxic elements in mineral reservoirs[END_REF]. Biogenic apatite (fish bones) was compared to other phosphate sources and found to be less contaminated initially with heavy metals than processed or mined phosphate rock or fertilizers, but was found to be apparently more soluble, enhancing metal capture and providing a slow release of phosphate for continued long-term treatment [START_REF] Knox | Phosphate sources and their suitability for remediation of contaminated soils[END_REF].

Aqueous Pb 2? in lead contaminated soils was reduced after reaction with HA [START_REF] Ma | In situ lead immobilization by apatite[END_REF]. The hypothesis set forth to explain this result was that the HA dissolved and precipitated hydroxypyromorphite [Pb 10 (PO 4 ) 6 (OH) 2 ] to effectively remove lead from the aqueous phase. More generally, it was found that lead chemical immobilization by any phosphate source could be related to the formation of pyromorphite Pb 5 (PO 4 ) 3 X where X = F, Cl, Br, OH, the most stable environmental Pb compounds under a wide range of pH values [START_REF] Miretzky | Phosphates for Pb immobilization in soils: a review[END_REF]. Two morphological forms of pyromorphite Pb 5 (PO 4 ) 3 Cl were observed in the presence of Cl -, phosphate, lead ions and goethite (a-Fe(OH) 3 ) present in soils: incrustations, indicating direct reaction of Pb 2? and Cl -ions with PO 4

3-adsorbed on goethite surface, and aggregates of pyromorphite crystals indicating that the reaction with PO 4

3-ions took place in the volume of the solution [START_REF] Manecki | Decrease of Pb bioavailability in soils by addition of phosphate ions[END_REF]. The insoluble lead sulfide galena (PbS) could be successfully treated by reacting with HA [START_REF] Zhang | Formation of chloropyromorphite from Galena (PbS) in the presence of hydroxyapatite[END_REF]. When galena, which is frequently found in mining wastes, is exposed to air atmosphere it becomes unstable causing it to become a labile source of Pb in soils. Transformation of unstable lead forms into sparingly soluble pyromorphite was confirmed in the presence of HA. Conversion of soil Pb to chloropyromorphite [Pb 5 (PO 4 ) 3 Cl] was observed in a contaminated soil collected adjacent to a historical smelter when HA was added. [START_REF] Ryan | Formation of chloropyromorphite in a lead-contaminated soil amended with hydroxyapatite[END_REF]. Addition of hydroxyapatite to the soil caused a decrease in each of the first four more bio-available fractions of sequential extractable Pb and an increase in the recalcitrant extraction residue. Extended X-ray absorption fine structure (EXAFS) spectroscopy provided additional evidence that the change of soil Pb to pyromorphite was possible by simple amendments of hydroxyapatite to soil.

The EXAFS approach illustrated that organically-complexed Pb was persistent in all HA amended soils, suggesting that an enriched soil organic carbon may be an inhibitory factor for pyromorphite transformations [START_REF] Hashimoto | Incomplete transformations of Pb to pyromorphite by phosphate-induced immobilization investigated by X-ray absorption fine structure (XAFS) spectroscopy[END_REF]. EXAFS was also used to provide evidence for the formation of several species during the immobilization of zinc ions by phosphate addition to contaminated dredged sediments. [START_REF] Ndiba | Heavy metal immobilization through phosphate and thermal treatment of dredged sediments[END_REF]. Both phosphate addition and calcination reduced leachability of heavy metals with the combined treatment achieving up to an 89% reduction. Due to the complexity of any soil matrix, the exact immobilization mechanisms following amendments have not been clarified, which could include precipitation, chemical adsorption and ion exchange, surface precipitation, formation of stable complexes with organic ligands, and redox reaction [START_REF] Qun | In situ immobilization remediation of heavy metalscontaminated soils: a review[END_REF]. Special attention has been devoted to immobilization of radioactive metal ions with emphasis on the long-term performance of HA as reactive material for the removal of uranium in passive groundwater remediation systems [START_REF] Simon | Behaviour of uranium in hydroxyapatite-bearing permeable reactive barriers: Investigation using 237 U as a radioindicator[END_REF][START_REF] Raicevic | Theoretical stability assessment of uranyl phosphates, apatites: selection of amendments for in situ remediation of uranium[END_REF]. The formation of autunite Ca(UO 2 ) 2 (PO 4 ) 2 or chernikovite H 2 (UO 2 )2(PO 4 ) 2 was proposed as the low solubility uranium salts formed by reaction with HA [START_REF] Simon | Groundwater remediation using permeable reactive barriers[END_REF]. EXAFS confirmed that uranium was phosphate bound to HA surfaces as an inner-sphere complex for relatively low U(VI) concentrations. Above 7000 ppm, chernikovite and autunite formations were detected by X-ray diffraction and fluorescence spectroscopy [START_REF] Fuller | Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation[END_REF]. HA was found to be a strong sorbent for 90 Sr, in soil for retardation of radionuclide migration. [START_REF] Moore | Formation of hydroxyapatite in soils using calcium citrate and sodium phosphate for control of strontium migration[END_REF]. The effectiveness of HA, alone or with clay, was tested for U retention by equilibrating with CaCl 2 and subjecting to the Toxicity Characteristic Leaching Procedure (TCLP). HA and mixtures of HA with illite or zeolite, were highly effective in reducing U extractability in both batch and leaching tests [START_REF] Seaman | Immobilization of cesium-137 and uranium in contaminated sediments using soil amendments[END_REF].

Poorly crystalline HA synthesized from gypsum waste or from incinerated poultry litter was described as advantageous when remediation targets vast amounts of contaminated soils. [START_REF] Hashimoto | Sorption of dissolved lead from shooting range soils using hydroxyapatite amendments synthesized from industrial byproducts as affected by varying pH conditions[END_REF] Numerous natural and synthetic phosphates materials have been used for soil remediation including HA, biological apatite, rock phosphate, soluble phosphate fertilizers such as monoammonium phosphate, diammonium phosphate, phosphoric acid, phosphatic clay and mixtures [START_REF] Miretzky | Phosphates for Pb immobilization in soils: a review[END_REF][START_REF] Al-Rub | Sorption of lead ions from simulated industrial wastewater onto Jordanian low-grade phosphate[END_REF]. Most of the studies on heavy metal stabilization in soils were performed using various phosphorus-containing amendments, which reduce the metal mobility by precipitating insoluble mineral species [START_REF] Kumpiene | Stabilization of As, Cr, Cu, Pb and Zn in soil using amendments-a review[END_REF][START_REF] Thawornchaisit | Evaluation of phosphate fertilizers for the stabilization of cadmium in highly contaminated soils[END_REF][START_REF] Cao | Immobilization of Zn, Cu, and Pb in contaminated soils using phosphate rock and phosphoric acid[END_REF]. All the observed results are compatible with available or predicted thermochemical data for the formation and stability of base metal phosphates in soils and sediments [START_REF] Nriagu | Formation and stability of base metal phosphates in soils and sediments[END_REF].

HA has been used on a large scale (90 tonnes) to make PRBs to successfully remediate groundwater containing several heavy metals [START_REF] Conca | An Apatite II permeable reactive barrier to remediate groundwater containing Zn, Pb and Cd[END_REF].

Role of HA as Permeable Reactive Barriers

Principles and Concept

As an alternative to traditional pump-and-treat and dig-and treat methods for the remediation of contaminated groundwater, permeable reactive barrier is a relatively new in situ technology, and is attracting increased attention [START_REF] Wilkin | Performance of a zerovalent iron reactive barrier for the treatment of arsenic in groundwater: Part 1. Hydrogeochemical studies[END_REF][START_REF] O'hannesin | Long-term performance of an in situ''ironwall'' for remediation of VOCs[END_REF][START_REF] Blowes | Treatment of inorganic contaminants using permeable reactive barriers[END_REF] Permeable reactive barrier (PRB) technology has gained acceptance as an effective passive remediation strategy for the treatment of a variety of chlorinated organic and inorganic contaminants in groundwater by combinations of chemical, physical and biological treatments [START_REF] Lee | Directed site exploration for permeable reactive barrier design[END_REF][START_REF] Gavaskar | Design and construction technique for permeable reactive barriers[END_REF].

In the subsurface the flow of the water is intercepted by a perpendicular ''wall'' of reactive materials that intercepts a plume of contaminated groundwater moving under the natural gradient and transforms the contaminant into an environmentally acceptable form. The goal is to have contaminant levels below target concentration at compliance points down gradient of the barrier. The barriers also prevent groundwater contaminants from migrating to uncontaminated aquifers, which may be difficult to locate and remedy [START_REF]National Research Council: Alternatives for Ground Water Cleanup[END_REF][START_REF]Environmental Protection Agency: Permeable Reactive Barrier Technologies for Contaminant Remediation[END_REF][START_REF] Kamolpornwijit | Preferential flow path development and its influence on long-term PRB performance: column study[END_REF][START_REF] Korte | Zero-valent Iron Permeable Reactive Barriers: A Review of Performance[END_REF][START_REF]Capstone report on the application, monitoring, and performance of permeable reactive barriers for ground-water remediation[END_REF][START_REF] Liang | Influence of hydrochemical processes on zero-valent iron reactive barrier performance: a field investigation[END_REF].

The principle of the barrier is described in Fig. 3. A benefit of its application is the large number of pollutants that can be treated often bringing their concentration below the detection limit. Moreover the site on the surface above the treated aquifer is available for other activities and can be economically re-used once the installation is concluded and during the remediation process. There is the possibility to treat waste plumes that are heterogeneous in composition and concentration and a reduction of costs (*50%) and energy compared to Pump and treat methodology could be achieve. In addition PRB has lower impact to the rate of groundwater flow compared to Pump and treat.

PRB requires minimal maintenance because it operates in a totally passive manner. However, the four following characteristics need to be evaluated for the implementation a PRB: (1) hydrogeology, (2) contaminant loading, (3) geochemistry, and (4) microbiology. Hydrogeologic characterization must be done accurately as it determines groundwater flowpatterns and the distribution of the contaminant plume.

Performance, maintenance of PRB as well as the orientation of the efforts in research over the past decade have generated number of contributions in the literature [START_REF] Li | Modeling porosity reductions caused by mineral fouling in continuous-wall permeable reactive barriers[END_REF][START_REF] Harrup | Polymer/silicate composites: new materials for subsurface permeable reactive barriers[END_REF][95][START_REF] Benner | A full-scale porous reactive wall for the prevention of acid mine drainage[END_REF].

Material Used for the Design of PRBs Numerous studies have been performed in PRB. The PRB could be specific for dedicated contaminants. Several reactive media are used.

Zero-valent Iron (Fe 0 ), ZVI: currently is the most widespread material used for remediation of groundwater. The ZVI wall functions to remove chlorinated organic contaminants from the subsurface by reductively dechlorinating these species as contaminated ground water flows through the wall. While this is an extremely effective remediation technology for halogenated organic contaminants, ZVI walls are not able to sequester metal ions. Other media such as Calcite (CaCO 3 ) PRB are used for the removal of fluoride [START_REF] Turner | The CO2 enhanced removal of fluoride using a calcite permeable reactive barrier[END_REF][START_REF] Turner | A calcite permeable reactive barrier for the remediation of Fluoride from spent potliner (SPL) contaminated groundwater[END_REF], Polymer and silicate composites and Zeolite, periclase (MgO), pecan shells, compost, peat moss, cottonseed meal, and lime, polysulfides, granulated activated charcoal (GAC), cottonseed meal and compost have been reported [START_REF] Turner | The CO2 enhanced removal of fluoride using a calcite permeable reactive barrier[END_REF][START_REF] Turner | A calcite permeable reactive barrier for the remediation of Fluoride from spent potliner (SPL) contaminated groundwater[END_REF][99][100][101][102][103][104][105]. In the various cases investigated, Microbial activities have been pointed out to have a significant effect on the interaction between the barriers and the pollutants containing the plume, or microbial activity [START_REF] Conca | An Apatite II permeable reactive barrier to remediate groundwater containing Zn, Pb and Cd[END_REF]105,106].

The following paragraph will be focused on the use of HA as PRB material.

A number of literature references have reported the positive role of hydroxyapatite (Apatite) PRBs in the removal of a range of metals ions. The results of previous and ongoing work demonstrate that stabilization of contaminated soils and groundwater by apatite-group minerals has the potential to be a successful and widely applicable remediation strategy for metals and radionuclides [107-120].

Apatite-group minerals form naturally and are stable across a wide range of geological conditions for hundreds of millions of years [START_REF] Manecki | Kinetics of aqueous Pb reaction with apatites[END_REF][121][122][123][124][125][126][127]. Work by Wright and others [122-124] investigated the trace-element composition of fossil apatite (teeth and bones) and sedimentary phosphorite deposits of various geological ages.

Metal ions are locked into the apatite structure with no subsequent desorption, leaching, or exchange, even in the face of diagenetic changes in the pore-water chemistry and pH, high temperatures, and tectonic disruptions. More than 300 apatite-type minerals exist, with elements from the entire periodic table replacing Ca, P, and OH in the fundamental apatite crystal structure [START_REF] Conca | An Apatite II permeable reactive barrier to remediate groundwater containing Zn, Pb and Cd[END_REF]. Their solubility products (K sp ) are very low and vary from about 10 -20 to 10 -150 .

Mechanisms [START_REF] Manecki | Kinetics of aqueous Pb reaction with apatites[END_REF]109,[125][126][127][128][129][130][131][132] Two types of apatite are particularly concerned in PRB. Calcium hydroxyapatite with the formula Ca 10 ðPO 4 Þ 6 ðOH) 2 and Ca 10Àx Na x ðPO 4 Þ 6Àx ðCO 3 Þ x ðOH) 2 so called Apatite (II), a particular form of apatite generated as a waste product from fish industry. The mechanism described for the uptake and stabilization of metal and radionuclides is dissolution and precipitation illustrated as follows with uranium and lead:

-Stabilisation of uranium:

Ca 5 ðPO 4 Þ 3 OH , 5Ca 2þ þ 3PO 3À 4 þ OH À ð1Þ
2UO 2þ 2 þ Ca 2þ þ 2PO 3À 4 , CaðUO 2 Þ 2 ðPO 4 Þ 2 ð2Þ
Ca(UO 2 ) 2 (PO 4 ) 2 has a low solubility product, log K sp = -45.5 -Stabilisation of lead:

Ca 10Àx Na x ðPO 4 Þ 6Àx ðCO 3 Þ x ðOHÞ 2 þ 14H þ ! ð10 À xÞCa 2þ þ xNa þ þ ð6 À xÞ½H 2 ðPO 4 Þ À þ xH 2 CO 3 þ 2H 2 O ð3Þ
10Pb 2þ þ 6H 2 ðPO 4 Þ À þ 2OH À ! Pb 10 ðPO 4 Þ 6 ðOHÞ 2 þ 12H þ ð4Þ
This mechanism is the same identified in solution as described in previous sections. The degree of protonation of the phosphate and carbonate in the intermediate reactions depends upon the pH. The above example is for the typical range of acid mine drainage, i.e., pH between 3 and 6. Reactions (1) and (3) do not necessarily lead to reaction (2) and (4). However, because the apatite provides a small but constant supply of phosphate to solution, if Pb 2? is in solutions contacting the apatite, then reactions (3) and (4) will take place. Apatite II dissolution is a strong pH buffering reaction, buffering around neutral (6.5-7) through its PO 3À 4 , OH À and substituted CO 2À 3 groups. Other mechanisms such as ion exchange and the formation of amorphous solids, surface complexation or chemisorption are also reported in the literature to explain the process of removal of the transition metals in particular, through its uncompensated phosphate and hydroxyl surface groups.

The metal-phosphate phase is very stable over geological time because of its low solubility products (K sp ). These metal-phosphate products are thermodynamically stable over a broad range of environmental conditions, and precipitate rapidly, thus ensuring long term immobilization of metal pollutants. Metals and apatite react very quickly at the molecular scale, but at a macroscopic dimension, limiting parameters like grain size, porosity, flow rate, barrier design, influence the efficiency with which dissolved metals come into contact with the surface of the reactive media.

The use of Apatite has several advantages. In fact, Apatite can sequester most heavy metals and radionuclides which then precipitate as mineral phases in which the metals are not bioavailable. Apatite is one of the best nonspecific surface sorbent for species that do not precipitate as a separate phase and is the most cost-effective reactive media for most metals and radionuclides. Apatite can be mixed into contaminated soil or waste, emplaced in PRB or used as a disposal liner and can be combined with most other reactive media e.g., grout, calcite, zeolite, polymers, bentonite, zero valent iron, and other remediation technologies, e.g., bioremediation, etc.

Conclusion

Phosphate chemistry can be used to develop environmentally friendly processes for pollution clean-up and valorization of brownfields and soils. Treated sediments with heavy metals locked in a stable insoluble structure could be valorized and work is in progress in this direction. Water purification and recycling is becoming an important issue and apatite chemistry will contribute to the valorization effort in the near future. From the scientific results gathered by much laboratory work it is becoming clear that processes with phosphates, apatite formation and use combined with thermal treatments are ready for industrial pilot developments.

Fig. 1

 1 Fig. 1 Illustration of the charged groups present on HA surfaces

Fig. 2

 2 Fig. 2 Hydroxylapatite crystal structure seen down the hexagonal axis. The central hydroxyl group lies in a tunnel surrounded by Ca(1) calcium atoms bound to phosphate groups

Fig. 3

 3 Fig. 3 Principle of the Permeable Reactive Barrier


Ange Nzihou

H El Feki

M Gargouri
email: gargouri1412@yahoo.fr.

C Chtara

P Charrock

Synthesis and Physicochemical Characterization of Pure Diammonium Phosphate from Industrial Fertilizer

INTRODUCTION

Diammonium phosphate (DAP) is an important commercial fertilizer. The important water-soluble P fertilizers include monoammonium phosphate (MAP) and diammonium phosphate (DAP). [START_REF] Duraisamy | Role of inorganic and organic soil amendments[END_REF] The fertilizer is produced industrially in large quantities via the reaction between gaseous ammonia and green phosphoric acid. The reaction is exothermic; the equation for the neutralization reaction between ammonia and phosphoric acid is as follows: 2

2NH 3 þ H 3 PO 4 f ðNH 4 Þ 2 HPO 4
When the molar ratio (MR) of ammonia to phosphoric acid is 2, DAP is formed. [START_REF] Campbell | Ammonium phosphate slurry rheology and particle properties The influence of Fe(III) and Al(III) impurities, solid concentration, and degree of neutralization[END_REF] The phosphoric acid used in producing DAP can be crude unpurified acid obtained by dissolving phosphate rock with sulfuric acid. In this case, crude DAP results, which may be adequate for some applications. However phosphate rock may contain heavy metals and these can be transferred to the fertilizer, and further to crops.

Two options are available to produce purified DAP: (1) use purified phosphoric acid; (2) purify the DAP. For producing industrial purified diammonium phosphate from wet process phosphoric acid, it is necessary to purify the phosphoric acid in advance according to a process of extracting phosphoric acid from wet process phosphoric acid according to a solvent process, followed by reacting ammonia with the resulting extract to obtain diammonium phosphate containing a small amount of impurities. However, this extraction process cannot be regarded as an industrially advantageous process owing to its complicated steps. [START_REF] Akiyama | Synthesis and physicochemical characterization of pure monoammonium phosphate (MAP) from industrial fertilizer[END_REF] Our work began with the purification of monoammonium phosphate (MAP) from industrial fertilizer, and the results are very encouraging. 4 This prompted us to pursue this study on another product of the same family, DAP.

In the present work, we are interested in eliminating impurities from industrial DAP. We present here the physicochemical characterization of DAP fertilizer upstream and downstream of the purification process, with an aim to determine the performance of recrystallization. We also compare the recrystallized DAP to the commercial DAP (Fisher).

EXPERIMENTAL SECTION

2.1. Recrystallization. Crystallization experiments of DAP (fertilizer synthesized in industry from Tunisia) were carried between 273 and 400 K and at 400 rpm stirring rate. The suspension was dissolved in mixed solvent (70% waterÀ30% alcohol). After that we add a quantity of charcoal in the solution. The suspension was filtered, and the solution was cooled to 273 K, whereupon crystals appeared. We recrystallized the DAP using a mixture of solvent, water, and alcohol. Impurities were analyzed by inductively coupled plasma-optical emission spectroscopy (ICPOES). a Dash (-) = trace. 2.2. Analytical and Testing Methods. P 2 O 5 content in the solution is determined gravimetrically. The molar rapport of ammonia to phosphoric acid (MR) was obtained by titration with standard 0.5 M NaOH and 0.5 M HCl solutions. The amount of HCl and NaOH added was used to calculate the MR that is equal to 2 (volume of NaOH/volume of HCl), according to the method described in ref 5. A pH Meter was employed to measure the pH values of slurries. N content in the solutions was determined by the Kejdahl method. Melting points were determined with a METTLER FP62 that uses a capillary tube. The curve solubility was determined following a standard procedure described by Mullin. 6 IR spectra were determined by a FTIR system spectrum BX Perkin-Elmer spectrometer. The analysis by scanning electron microscope (SEM) is determined with a MEB Philips s erie XL 30. Data collection is realized by using KappaCCD Server Software (Nonius, 2004); program(s) used to solve structure are SHELXS97. The spectra of X-ray powder diffraction (XRPD) is determined by a Seifert 3000. T.T. spectrometer.

RESULTS AND DISCUSSION

The principal chemical compositions of the DAP upstream and downstream from the recrystallization are presented in Table 1. After purification, the % P 2 O 5 and % N were increased while keeping the molar ratio (MR) range between 1.8 and 2.0. Also, after purification the percentage of H 2 O and value of pH are slightly changed but in the various ranges stated. There is 25% of impurities by weight in the crude industrial DAP.

Diammonium phosphate granules contain various compounds (Ca, Mg)(NH 4 )(Fe, Al)(PO 4 )(F, OH)H 2 O that are insoluble in water and can comprise up to 10% of the total P content of fertilizers. [START_REF] Gilkes | Poorly soluble iron-aluminium phosphates in ammonium phosphate fertilizers: Their nature and availability to plants[END_REF] The DAP contains toxic elements such as (Cd, Zn, Cu, and Ni) which are toxic for plants and have the potential for adverse effects on human health due to the transfer of metals from soils to food corps. Thus, it is important to eliminate or reduce these toxic elements in DAP. The analysis of the DAP after purification is given in Table 2 and shows that the quantity of impurity decreases. The results are comparable to those of the commercial DAP (Fisher).

After recrystallization, the melting point was found to be 194 °C, close to the one of laboratory grade DAP (196 °C).

The curve solubility of recrystallized DAP, that was determined following a standard procedure described by Mullin, 6 is shown in Figure1. The solubility of a solute is most conveniently stated as the parts by weight per part (or 100 parts) by weight of solvent.

The analysis by means of IR of the DAP before and after recrystallization is presented in Figures 2 and3. The characteristic wavelengths of the vibration modes of DAP are illustrated in Table 3, which shows the presence of the four modes of vibrations ascribed to PO 4

3-observed at about 444, 539, 917, and 1110 cm À1 , the vibrations of ammonia observed about 1400 cm À1 , and a wide band attributed to H 2 O and NH 4 þ stretching modes observed around 3137 cm À1 .

In addition, the analysis by sweeping electron microscope (MEB) of the industrial DAP after recrystallization, showed that the elements of aluminum, magnesium, and copper are eliminated, and the quantity of sulfur was decreased (Figure 4).

The crystal structure of the recrystallized diammonium phosphate (NH 4 ) 2 HPO 4 has been determined by X-ray single crystal analysis at room temperature. The space group is P21/c 10 with lattice parameter a = 10.781(2) Å, b = 6.771(2) Å, c = 8.047(2) Å, and Z = 4. The refinement converged to R 1 = 0.0231 and R W2 = 0.0660. The analysis by X-ray diffraction showed that the purification of the DAP did not change the crystalline system (monoclinic). Crystal data details and structure refinement are given in Table 4 and show that the crystallographic parameters of recrystallized DAP are close to the pure DAP (Fisher), which justifies the performance of the recrystallization.

h À13 < h < 13 À12< h < 12 k À1 < k < 8 À7 < k < 7 l À10 < l < 7 À9 < l < 9
In addition, the analysis by X-ray powder of industrial DAP downstream of the purification (Figure 6) shows an elimination of the impurities that were found upstream of the purification (Figure 5), and we got a good crystalline product that resembles the pure commercial DAP (Fisher) (Figure 7).

CONCLUSION

The long-continued application of impure DAP can redistribute and elevate heavy metal concentrations in soils. The redistribution of heavy metals can adversely affect water resources and endanger the health of surrounding ecosystems and human populations. Thus, it is important to eliminate the different toxic elements present in the fertilizer in order to prevent transfer to plants and humans, in this case with the use of purified fertilizers in biological agriculture. On the other hand, you can also use the purified product in the pharmaceutics and cosmetic industries.

After purification, the % P 2 O 5 and % N increased, while the molar ratio (MR) remained between 1.8 and 2.00 and the products obtained contained 1.5À2% H 2 O and had a pH value of 7À8. The analysis by X-ray diffraction showed that the purification of the plant DAP did not change the crystalline system (monoclinic).

The physicochemical characterization of the industrial DAP downstream of the purification, through spectroscopic analyses and chemical analyses, showed that we obtained purified DAP whose physicochemical properties are comparable with those of the pure commercial DAP (Fisher).

Figure 1 .

 1 Figure 1. Curve solubility of recrystallized DAP.

Figure 2 .

 2 Figure 2. The analysis by means of IR of the industrial DAP upstream from the recrystallization.

Figure 3 .

 3 Figure 3. The analysis by means of IR of the industrial DAP downstream from the recrystallization and pure commercial DAP (Fisher).

Figure 4 .

 4 Figure 4. The analysis by MEB of the industrial DAP upstream and downstream from the recrystallization.

III

  a R values are defined asR 1 = Σ||F o | À |F c ||/Σ|F o | and Rw 2 = [Σw(F o 2 À F c 2 ) 2 /Σw(F o 2 ) 2 ] 1/2 where w À1 = [σ 2 (F o 2 ) þ (0.0442P) 2 þ 2.4726P] and P = (F o 2 þ 2F c 2 )/3.

Figure 5 .

 5 Figure 5. The analysis by XRPD of the industrial DAP.

Figure 6 .

 6 Figure 6. The analysis by XRPD of the industrial DAP downstream from the recystallization.

Figure 7 .

 7 Figure 7. The analysis by XRPD of the pure commercial DAP (Fisher).

Table 1 .

 1 Chemical Compositions of the DAP Upstream and Downstream from the Recrystallization

		P 2 O 5	N	H 2 O	MR
	samples	(wt) %	(wt)%	(wt) %	(N/P)	pH
	plant DAP	46.05	17.65	1.50	1.94	7.6
	plant DAP recrystallized	49	18	1.68	1.86	7.9

Table 2 .

 2 Determination the Impurity Present in the DAP Upstream and Downstream from the Recrystallization a

	sample (ppm)	Fe	Al	Mg Ag As	Co	Pb Hg	Si	Sn Ti Cr	Zn	Cd Cu Ni Mn	V
	plant DAP	6769 4273 4907 6	26 5419 22 3	150 382 93 525 1203 34 59 25 65	1341
	plant DAP recrystallized (water-alcohol) 24	37	14	-	3	3	7	-	70	-	2	27	41	3	4	17 1	47
	commercial DAP (Fisher)	15	22	9	-	3	2	7	-	38	-	-	25	11	3	2	17 -	9

Table 4 .

 4 Crystal Data Details and Structure Refinement of the Plant DAP Downstream from the Recrystallization and Commercially Pure DAP

		DAP	pure commercial
		recrystallized	DAP (Fisher)
		I. Crystal Data
	formula	(NH 4 ) 2 HPO 4	(NH 4 ) 2 HPO 4
	formula weight (g/mol)	132.06	132.06
	system	monoclinic	monoclinic
	space group	P21/c	P 21/c
	a (Å)	10.781(2)	10.729(5)
	b (Å)	6.771(2)	6.682(5)
	c (Å)	8.047(2)	8.009(5)
	0	109.72(2)	109.670(5)
	V (Å 3 )	448.1 (2)	540.7(6)
	Z	4	4
	F cal (g cm À3 )	1.600	1.622
	F (000)	280	332
	μ (Mo KR) (mm À1 )	0.427	0.433
	crystal color	colorless	colorless
		II. Intensity Measurements
	temperature (K)	293 (2)	293 (2)
	wavelength Mo KR (Å)	0.71073	0.71073
	measurement area:

' ACKNOWLEDGMENT

The authors thank Dr. A. Driss from the University of Tunis, Tunisia, for his valuable collaboration.


S Bailliez

A Nzihou
email: nzihou@enstimac.fr

The kinetics of surface area reduction during isothermal sintering of hydroxyapatite adsorbent

Keywords: Hydroxyapatite, Calcination, Sintering, Kinetics, Diffusion

The behaviour of non-stoichiometric hydroxyapatite (HA) during the calcination in a solid bed was investigated. The structural properties are described in terms of the specific surface area. Calcination led to a significant decrease of the specific surface area by particle coalescence and densification. Hydroxyapatite begins to shrink near 780 • C and reaches 97% theoretical density at 1100 • C. The specific surface area and density variations are caused both by sintering and chemical reaction. Sintering data from these solids were correlated as a function of time and temperature. The rate of sintering is assumed to obey an Arrhenius equation. These results are compared with a number of literature models describing the mechanism of sintering kinetics using the specific surface area, and a good agreement is observed. The kinetic equation used is based on sintering driven by the curvature gradient in the interparticle neck region associated with initial stage sintering. Then, the decline in specific surface area is accurately described by the empirical equation of the form dS/dt = -B(T)k b . The changing value of b, also known as the "order" of the reaction, suggests that the diffusion mechanism for loss of surface area may be a function of the temperature.

Introduction

Immobilisation is a promising technology for cleaning up contaminated wastes and soils. Phosphate minerals would appear to have the potential to immobilise heavy metal contaminants in wastes and soils due to the low solubility of many orthophosphates of divalent metal ions [START_REF] Iretskaya | Metal leaching from MSW fly ash before and after chemical and thermal treatments[END_REF][START_REF] Nzihou | Calcium phosphate stabilization of fly ash with chloride extraction[END_REF][3][START_REF] Takeuchi | A study of equilibrium and mass transfer in processes for removal of heavy-metal ions by hydroxyapatite[END_REF][START_REF] Da Rocha | Cadmium uptake by hydroxyapatite synthesized in different conditions and submitted to thermal treatment[END_REF][START_REF] Mavropoulos | Studies on the mechanisms of lead immobilization by hydroxyapatite[END_REF]. Hydroxyapatite (HA) which is an insoluble calcium phosphate mineral of composition Ca 10 (PO 4 ) 6 (OH) 2 has been used as a cation exchanger and adsorbent in wastewater treatment and has a very high capacity for removing divalent heavy metal ions from water and liquid waste [START_REF] Ma | In situ lead immobilisation by apatite[END_REF][START_REF] Suzuki | Synthetic hydroxyapatites as inorganic cation exchangers. Part 2[END_REF][START_REF] Takeuchi | Removal of coexisting Pb 2+ , Cu 2+ and Cd 2+ ions from water by addition of hydroxyapatite powder[END_REF][START_REF] Ma | Effect of aqueous Al, Cd, Cu, Fe(II), Ni and Zn on Pb imobilization by hydroxyapatite[END_REF][START_REF] Ma | Effect of NO 3 -, Cl -, F -, SO 4 2and CO 3 2on Pb 2+ immobilization by hydroxyapatite[END_REF]. The exact mechanisms providing the retention of heavy metals by HA are not clear. Three types of reactions may control the retention of heavy metals by the HA matrix: surface adsorption, cation exchange (substitution) or precipitation. Some of the mechanisms described above are reversible depending on the pH conditions, and the trapped metal can be leached and cause pollution when the residues are landfilled or reused. The insolubilisation of hydroxyapatite absorbents containing heavy metal is obtained by calcination. This leads to significant textural changes, mainly concerning specific surface area, porosity, particle size and density. Other phenomena observed during the calcination are the structural change due to crystallisation of the matrices and the formation of the neo-adsorbent when heavy metals are incorporated [START_REF] Bailliez | Kinetic of sintering of hydroxyapatite powders and adsorption of lead in hydroxyapatite matrices[END_REF].

Before reacting a heavy metal contaminant the waste with a hydroxyapatite matrix, it seems useful to first understand the mechanisms that occur when only the pure hydroxyapatite is calcinated.

The results presented in this paper deal with the calcination of two hydroxyapatites: the first contains tricalcium phosphates and the second contains some carbonates. This first step is necessary to compare the behaviour of hydroxyapatite matrix doped and free of metal ions during the sintering process, and then understand the mechanism of the stabilisation of the heavy metal in a hydroxyapatite matrix. The parameters monitored are the crystallisation process, the specific surface area, the density and the weight variations.

The structural changes due to the combined effects of sintering and the reaction of hydroxyapatite formation are used to express the kinetics of the sintering of hydroxyapatite. This paper presents a review of the literature on sintering,

Greek letters α exponent ρ density (kg m -3 )
and a detailed experimental study of the change in structural properties caused by sintering. The aim is to understand and compare the structural change, i.e. specific surface area and density of two non-stoichiometric hydroxyapatites during calcination. The two hydroxyapatites have different initial specific surface area and different compositions in terms of their Ca/P ratios which are of 1.6004 for the first and 1.7275 for the second. In comparison, the Ca/P ratio of stoichiometric is 1.6667. The results are compared with literature models describing the sintering process by specific surface area reduction.

Theoretical approach of the surface reduction phenomenon during sintering

Structural property changes, although quite complex, can be grouped into two major categories. The first consists of changes which occur as the result of sintering while the second category is associated with chemical reaction and occurs when molar volumes of the solid reactant and the solid product are different. Most models have concentrated upon the first category addressing the reaction sintering problem [START_REF] Caillet | Structural property variations in the MnO-MnS system[END_REF][START_REF] Kim | Diffusion in nickel oxide pellets-effects of sintering and reduction[END_REF][START_REF] Ranade | The variable property grain model applied to the zinc oxide-hydrogen sulfide reaction[END_REF].

Sintering is broadly defined as the consolidation on heating of a loose mass of particles to a denser mass. Sintering causes both the specific surface area and the porosity to decrease and the density to increase. The Tammann temperature of the solid (0.4-0.5 of the melting point), is considered to be the temperature below which sintering does not occur to any appreciable extent.

Sintering occurs in three stages shown in Fig. 1. In the initial stage, the areas of contact between adjacent particles form and grow. In the intermediate stage, growing necks merge and the large number of small particles are replaced by a smaller number of large particles. This stage produces interparticle porosity whose surface may be inaccessible both to reactant gas during the reaction and to the nitrogen used to measure the specific surface area. In the final stage of sintering, the pore spaces become broken up with isolated closed pores remaining which shrink in size as densification proceeds [START_REF] White | Sintering and Related Phenomena[END_REF].

A model of the interparticle neck shape during sintering was proposed [START_REF] German | Surface area reduction during isothermal sintering[END_REF][START_REF] German | Sintering and catalysis[END_REF][START_REF] German | Sintering and catalysis[END_REF][START_REF] German | Identification of the initial stage sintering mechanism using aligned wires[END_REF][START_REF] Kuzynski | Self-diffusion in sintering of metallic particles[END_REF][START_REF] Gidaspow | Recent Developments in Separation Science[END_REF]. Through this model, various geometric parameters have been interrelated precisely. An important result of this approach is that the kinetic dependence for the rate of surface area reduction on the operative sintering mechanism has been established. Thus, the mass transport process can be identified via the kinetics of the specific surface area reduction. The proposed identification technique is applied to a variety of sintering data and discussed in terms of the underlying significance of the model. Several neck-growth kinetic equations have been provided based on the format of Kuczynski's initial derivation. The basic model is amenable to an alternative analysis aimed at extracting a time-dependence for surface area reduction. The neck-growth models generally take form

x a n = Bt (1)
where x/a is the ratio of interparticle neck radius to the particle radius, B a constant (includes particle size, temperature, and geometric term), t the sintering time, and n the mechanism-characteristic exponent which is dependent on the mass transport process. In the configuration of Fig. 1, the interparticle neck shape is assumed to be formed by a torus of radius r (Fig. 2). For small neck sizes such an approximate neck shape has little influence on the result. The significant point of Eq. ( 1) represents sintering driven by the curvature gradient in the neck region. Thus initial sintering does not result from excess surface energy but rather from the chemical potential gradient produced by the varying mean curvature.

Eq. ( 1) can be recast to give surface area reduction for the case of a small neck where surface area loss with neckgrowth can be approximated as

S = S 0 -S ≈ πr 2 (2)
where S is the surface area at a neck size of x and S 0 the initial surface area. For compact of spherical powder, the initial surface area per interparticle contact is given by

S 0 = 4πa N c 2 (3)
where N c is the particle packing coordination number which is related to the green density. Eq. ( 1) is generalized by the following relationship [START_REF] Caillet | Structural property variations in the MnO-MnS system[END_REF][START_REF] German | Surface area reduction during isothermal sintering[END_REF]:

S 0 -S S 0 α = K 1 (T)t (4)
The exponent α is related to the sintering mechanism through particle coordination for monodispersed spheres [START_REF] German | Sintering and catalysis[END_REF][START_REF] German | Sintering and catalysis[END_REF]. The values of α have been derived from the accepted time-dependencies of various neck-growth mechanisms.

Model limitations

Several results show errors emerging from the assumed functional independence of the surface area reduction on neck size [START_REF] Caillet | Structural property variations in the MnO-MnS system[END_REF][START_REF] German | Surface area reduction during isothermal sintering[END_REF]. Identification of the rate-controlling sin-tering process through the kinetics of specific surface area reduction involves a number of assumptions. For the model used, the particles are assumed to be monodispersed spheres. Large deviations in either particles shape or size distribution produce anomalous effects not accounted for by the model. Because the original calculations were for isolated necks, the point at which neighbouring necks impinge on each other, approximately at ((S 0 -S)/S 0 ) ∼ = 0.5, determines the limit of the model [START_REF] German | Surface area reduction during isothermal sintering[END_REF].

Shrinkage provides an important check for consistency in applying this approach to sintering studies; bulk and surface transport mechanisms can be distinguished by shrinkage checks. In general, such determinations can result from a few simple density or length measurements during the experiment. The original surface area (S 0 ) must be determined accurately. Uncertainties in S 0 will greatly influence S and thus, produce variable a values. Optimal analysis by the kinetics of surface area reduction should, therefore, include an accurate measurement of S 0 followed by multiple surface area measurements over a wide range of times up to ((S 0 -S)/S 0 ) ∼ = 0.5. Such an approach minimizes the potential error associated with measuring the slope of a log-log data plot. As in most sintering studies, rapid specimen heating and cooling is presumed. However, the simultaneous action of two mechanisms can lead to an improper analysis by using only this technique.

A few studies have examined the effect of sintering on solid surface area. Nicholson described the relationship between surface area, time, and temperature as follows:

dS dt = -K 2 (T)(S -S f ) (5)
This relationship was verified with the data of sintering of magnesium oxide and iron oxide. Caillet and Harrisson [START_REF] Caillet | Structural property variations in the MnO-MnS system[END_REF] used this equation to correlate their ZnO-ZnS sintering data. The temperature dependence k(T) was expressed by an Arrhenius equation and the equilibrium surface area at any temperature, S f was determined experimentally.

Nicholson [START_REF] Nicholson | Variation of surface area during the thermal decomposition of solids[END_REF] has shown that the specific surface area of a non-reacting, sintering solid varies with time according to the equation:

S(t) -S e S 0 -S e = e -k j t (6)
Both the sintering rate constant, k j and the equilibrium surface area, S e , are functions of temperature and must be determined experimentally. When and Ishida [24], Ranade and Harrison [START_REF] Ranade | The grain model applied to porous solids with varying structural properties[END_REF] related specific surface area and grain radius by assuming that all the grain are non-porous spheres having equal radius r, defined as follows:

r = 3 Sρ (7)
German [START_REF] German | Surface area reduction kinetics during intermediate stage sintering[END_REF] postulated that, at the intermediate stage of sintering the surface area reduction kinetics followed a power law equation

dS dt = -B ′ (T)S b (8)
The functional relationship of B ′ (T) with temperature was not discussed; the exponent b was reported to be between 2.6 and 3.3 for various materials. Schaffler et al. [START_REF] Schaffler | Aging of silica-alumina cracking catalyst. I. Kinetics of structural changes by heat and steam[END_REF] and Caillet and Harrisson [START_REF] Caillet | Structural property variations in the MnO-MnS system[END_REF] also used a power law equation to describe the surface area variation of silica-alumina catalysts in steam at temperatures from 478 to 863 • C. Reported values of the exponent, b, ranged from 9.1 at the lower temperature to 3.8 at the higher.

Compared to neck size and shrinkage measurements, surface area offers several advantages as a sintering-process monitor, because the surface area reductions accompany all sintering mechanisms, and it is an easily measurable quantity [START_REF] German | Surface area reduction kinetics during intermediate stage sintering[END_REF]. Also, the surface area is affected very little by the rearrangement events of the initial stage of sintering. Such measurements average many neck-growth events without tedious neck size measurements. Surface area as a sintering parameter is most appropriate to fine particles, a size region where it is difficult to measure neck diameter.

The basis for using surface area monitoring in the intermediate stage is the statistical sintering model of Kuczynski [START_REF] Kuczynski | The mechanism of densification during sintering of metallic particles[END_REF]. In this model the pore-elimination and grain growth processes are coupled to explain the intermediate and final stages of sintering (for crystalline materials).

This phenomenon of bonding of the two particles will take place in order to decrease the total surface area, even though the temperature is lower than the melting point (Tamman temperature) [START_REF] Kuczynski | The mechanism of densification during sintering of metallic particles[END_REF].

Materials and methods

Synthesis of hydroxyapatite (HAP) powder

The experiments were carried out with two hydroxyapatites synthesized at room temperature by precipitation in solution [START_REF] Nzihou | Mechanisms of sintering of macroporous hydroxyapatite adsorbents[END_REF]. The first one, HAP TCP , was obtained according to the reaction (9) between calcium chloride (CaCl 2 •2H 2 O from Aldrich, 98% minimum), and phosphoric acid (H 3 PO 4 from Prolabo, 85% minimum). The pH was adjusted with caustic soda solution. The temperature of the solution was maintained at 298 K. The second hydroxyapatite, HAP CaO , was prepared by mixing calcium nitrate (Ca(NO 3 ) 2 from Norskhydro) and ammonium phosphate ((NH 4 ) 2 HPO 4 from Norskhydro) at 100 • C and the pH was adjusted to 7-8 with a 20% ammonia solution according to reaction (10):

10CaCl 2 + 6H 3 PO 4 + 20NaOH → Ca 10 (PO 4 ) 6 (OH) 2 + 20NaCl + 18H 2 O (9) 10Ca(NO 3 ) 2 + 6(NH 4 ) 2 HPO 4 + 8NH 4 OH → Ca 10 (PO 4 ) 6 (OH) 2 + 20NH 4 NO 3 + 6H 2 O (10)

Characterisation of materials

The structure and the composition of the dried particles was identified and quantified by X-ray powder diffractometry (Siemens D5000). XRD measurements were carried out with Cu K␣ radiation generated at 40 kV and 40 mA, in the 3 • < 2θ < 60 • range at a scan speed of 2 • C min -1 . The phases present were determined by comparing the patterns with JCPDS standards.

Calcium and phosphorus contents in the obtained solids were determined using atomic absorption spectrometry (Varian spectrAA-400 plus).

The specific surface area of the particles was determined by nitrogen adsorption using a BET method (Micromeretics Gemini Vacprep 061). The value obtained for the dried particles was 28 m 2 g -1 for HAP TCP and 104 m 2 g -1 for HAP CaO .

The bulk density of the powder was measured by helium pycnometry (Micromeritics, Accupyc 1330) and the results obtained are: d HAP TCP = 3.1 g cm -3 , and d HAP CaO = 2.79 g cm -3 .

The particle size distribution of the powder was determined with a Malvern laser mastersizer Hydro 2000. The particles were placed in an ethanol suspension shaken by ultrasound. The mean sizes of 11 m for HAP TCP and of 18 m for HAP CaO were obtained.

Thermogravimetric and differential thermal analysis (TG-TDA) was executed under the following conditions: 30 mg of samples, platinum crucible, an air flow of 100 cm 3 min -1 , Fig. 3. XRD pattern of HAP CaO vs. the calcination temperature. and heating rate of 10 • C min -1 from 25 to 1200 • C using the thermobalance (Seteram TG 92).

Sintering procedure

The shrinkage of particles was followed by thermomechanical analyser (Setaram Setsys 16/20) with 5 g constant load on the powder sample. Thermal shrinkage was defined as L/L 0 , where L 0 is the diameter or thickness of the green specimen. The relative density of sintered hydroxyapatite was expressed using the bulk and theoretical densities (d HAP TCP = 3.1 g cm -3 , and d HAP CaO = 2.79 g cm -3 ). Two types of experiments were carried out: for the first, a constant heating of 10 • C min -1 from 30 to 1200 • C to determine the sintering temperature of the powders and the second, a heating rate of 10 • C min -1 followed by a plateau of 5 h at the isotherm temperature.

In parallel, calcination experiments were carried out on a 2 l furnace. The heating program was as follows: heating rate of 10 • C min -1 from 30 • C to the chosen isotherm temperature and the isothermal stages varying from 0 to 120 min. The samples were placed in alumina crucible and heated at different rates up to the selected temperature under flowing air, then held for the same time and cooled at the same rate with a maximum heating temperature of 1600 • C.

Results and discussion

Effect of thermal conditions on the structure of amorphous calcium phosphate (ACP)

The XDR patterns of Fig. 3 show the presence of an amorphous phase at temperatures below 500 • C. The sample heated at temperatures greater than 600 • C exhibited broad peaks on a apatite phase. When the temperature was increased, the apatitic peaks became sharper, because of crystal growth. Each pattern has hydroxyapatite as the major phase component. However, trace and significant amounts of other phases were detected. These phases are essentially composed by intermediate calcium phosphates between ACP and HAP.

The XRD references were obtained according the AFNOR standard [31]. Then, the two hydroxyapatites were calcined during 15 h at 1000 • C. The calcination allows the crystallization of the particles by comparison with the dried particles as it is shown in Fig. 4a for HAP TCP and in Fig. 4b for HAP CaO . The evolution of the cell parameters (a, b, c) and the crystallite mean size of HAP particles are not discussed in this paper. Nevertheless, approximately 39 wt.% of tricalcium phosphate (Ca 3 (PO 4 ) 2 ) was present in HAP TCP and about 2 wt.% of CaO was detected in HAP CaO . These XRD patterns of the calcined particles help to conclude that HAP TCP is a ACP with a Ca/P ratio of 1.6004 which becomes a mixture of hydroxyapatite and tricalcium phosphate (TCP) at high temperature and HAP CaO is a ACP with a Ca/P ratio of 1.7275 which becomes a mixture of lime (CaO) and hydroxyapatite at high temperature. Since the Ca/P ratio of stoechiometric hydroxyapatite is 1.6667, the first, HAP TCP , is a calcium deficient hydroxyapatite and the second, HAP CaO , is an hydroxyapatite with a calcium excess.

Sintering and shrinkage of hydroxyapatite matrix

The linear shrinkage and relative density of ACP and derived HAP are plotted in Fig. 5. The two hydroxyapatite powders have been studied by the thermomechanical analyzer in dynamic mode and in isotherm mode from 100 to 1200 • C. The results presented in Fig. 5a show the shrinkage of HAP TCP which begins at 750 • C stops at 1200 • C. This sintering temperature is similar to that published in the literature for stoichiometric hydroxyapatite [START_REF] Layrolle | Sol-gel synthesis of amorphous phosphate and sintering into microporous hydroxyapatite bioceramics[END_REF][START_REF] Raynaud | Calcium phosphate apatites with variable Ca/P atomic ratio. II. Calcination and sintering[END_REF]. In isotherm curves of Figs. 5 and 6 no significant shrinkage was measured at the temperature less than 700 • C. For the HAP TCP the final shrinkage percentage is 20%. For HAP CaO (Fig. 5b), the shrinkage begins at 600 • C, this temperature corresponds to the decomposition of lime Ca(OH) 2 to CaO. The reduction of length observed can be explained by the combined effect of shrinkage and the evaporation of water. At 900 • C, the sample height does not change any more: in this case, the final shrinkage is 30%.

The evolution observed occurs simultaneously with the reduction of the specific surface area of the particle. In fact, a significant surface reduction is observed both for HAP TCP and HAP CaO .

For HAP TCP , below 500 • C, the shrinkage is lower than 1% (Fig. 6). The specific surface area (Fig. 7) is constant and equal to that of the initial particles (28 m 2 g -1 ). In this domain, the structure of the particles does not change significantly.

For the temperatures above 650 • C, the shrinkage percentage increases linearly with the temperature like it is shown in Fig. 6. At 1000 • C, we can notice a break in the line that corresponds to a mechanism change. Because the slope of the line is more important, it may represent the beginning of a new reaction. This result is confirmed by the thermogravimetric curve of Fig. 8a reduced to 3 m 2 g -1 from 550 to 1000 • C. Above 1000 • C no significant specific surface area reduction is observed.

For HAP CaO , the shrinkage is effective between 500 and 900 • C. Under 500 • C, the shrinkage percentage (Fig. 5a) is less than 1% and the specific surface area is constant (Fig. 7). Above 900 • C, all the dilatometric curves are superposed (Fig. 5b) and the specific surface area is constant showing that the structure does not change any more.

Between 500 and 900 • C, the shrinkage percentage increases linearly with the temperature like it is shown in Fig. 6. This shrinkage corresponds to the decreases of the specific surface area (Fig. 7) from 104 to 2 or 3 m 2 g -1 . The in the temperature range investigated. So the density is calculated by the following formula:

ρ = m 0 (1 -weight loss percentage) πrh 0 (1 -shrinkage percentage) (11)
Fig. 8a andb shows that under 500 • C, the density of both HAP TCP and HAP CaO are constant, but after it increases confirming the sintering.

A curve with an inflexion point at ∼1100 • C was then observed. The shrinkage reached a maximum of -20% at 1100 • C, when the HAP structure became unstable. The ACP sample shrank at a considerably lower temperature than the HAP samples. The curves showed a step (-6.5%) at 600 • C that coincided with the crystallization of ACP. At higher temperatures, the curve stabilised to a plateau up to 850 • C. Finally, the relative density increased at 1000 • C and attained a value of 91.4% at 1100 • C.

Sintering kinetic study

All samples were analyzed by the method in order to determine the specific surface area as a function of time and temperature for the two hydroxyapatites investigated (Fig. 7). Three models of the literature (Table 1) were used to correlate the experimental data showing the decrease of specific surface area. The principles of these models were related in Section 1 of this paper.

The equations of Table 1 were solved using Matlab software. The required initial condition is the initial specific surface area and the temperature range. The results obtained are compared in Fig. 9a for HAP TCP and in Fig. 9b for HAP CaO .

Fig. 9a andb shows that the Nicholson equation does not fit the experimental data well for the two hydroxyapatites. The value derived for the α and b exponent parameters were used to chose the best model. Indeed, y values of more than 50 were found for the two hydroxyapatites which is not realistic. For the Schaffler model, the value of the constant b of Eq. ( 8) which vary from 3 to 10 with changing temperature for the two hydroxyapatites are in good agreement with those found in the literature. The value of b decreases regularly from 10 to 3 from 500 to 1200 • C. The changing value of b, which may be termed the "order" of the reaction, suggests that the mechanism of surface area reduction which was identified as a diffusion mechanism may be changing with temperature [START_REF] Caillet | Structural property variations in the MnO-MnS system[END_REF][START_REF] German | Surface area reduction during isothermal sintering[END_REF][START_REF] Schaffler | Aging of silica-alumina cracking catalyst. I. Kinetics of structural changes by heat and steam[END_REF][START_REF] Kuczynski | Self-diffusion in sintering of metallic particles[END_REF][START_REF] Layrolle | Sol-gel synthesis of amorphous phosphate and sintering into microporous hydroxyapatite bioceramics[END_REF]. The Schaffler model (Eq. ( 8)) was chosen to fit the specific surface area data. This model appears to have considerable merit in determining the dominant mass transport mechanism during the sintering. The assumption that the surface energy provides the driving force for sintering has led to proposals for kinetic laws represented by Eq. ( 8). This empirical model is in agreement with the specific surface area changes for many systems over time periods during initial, intermediate and final sintering stages.

In the Schlaffler model, the constant rate B(T) expressed the temperature dependence of the specific surface area. This dependence was determined using the Arrhenius equation indicated by Eq. [START_REF] Bailliez | Kinetic of sintering of hydroxyapatite powders and adsorption of lead in hydroxyapatite matrices[END_REF].

B(T) = k 0 e -E a /RT (12)
The activation energy E a of sintering was then deduced (Fig. 10). The values of 385 kJ mol -1 for HAP TCP and 371 kJ mol -1 for HAP CaO are in good agreement with the value of 400-500 kJ mol -1 reported by Raynaud et al. [START_REF] Raynaud | Calcium phosphate apatites with variable Ca/P atomic ratio. II. Calcination and sintering[END_REF] for stoichiometric hydroxyapatites [START_REF] Ababou | Etude experimentale et théorique du préfrittage et du frittage de l'hydroxyapatite Ca[END_REF].

Conclusions and perspectives

The behaviour during the calcination of two non-stoichiometric hydroxyapatites (calcium deficient and calcium excess) were investigated. In both case, a significant decrease of the specific surface area and an increase of the density was observed versus the temperature and the duration of the calcination. The hydroxyapatite begins to shrink near 780 • C and reaches 97% theoretical density at 1100 • C. The kinetics of this sintering process was investigated and compared with the results of the literature which takes into account the structural change of the particles on calcination and a good agreement is observed. The changing value of b, which may be termed the "order" of the reaction, suggests that the mechanism for loss of surface area which was identified as a diffusion mechanisms may be changing with temperature [START_REF] Caillet | Structural property variations in the MnO-MnS system[END_REF][START_REF] German | Surface area reduction during isothermal sintering[END_REF][START_REF] Schaffler | Aging of silica-alumina cracking catalyst. I. Kinetics of structural changes by heat and steam[END_REF][START_REF] Kuczynski | Self-diffusion in sintering of metallic particles[END_REF][START_REF] Layrolle | Sol-gel synthesis of amorphous phosphate and sintering into microporous hydroxyapatite bioceramics[END_REF]. The Schaffler model appears to help understand the dominant mass transport mechanism during sintering. This study provides evidence that the sintering of ACP powder occurs during the calcination. After heating at 600 • C, the ACP particles crystallize to a HAP phase. Thermal crystallization is accompanied by active surface diffusion within the initial particles, which constitutes the driving force for densification. The present research is focussed on the measurements of the thermal conductivity changes due to the change in porosity of hydroxyapatite particles during the calcination. A grain model which combines the effects of sintering and density changes observed during the calcination of hydroxyapatites powders in fixed bed will be used to help understand the effect of the temperature gradient on the sintering observed from 700 to 1100 • C. The model will also be used to further and further understand the experimental results of the sintering of hydroxyapatite doped with heavy metals.

Fig. 1 .

 1 Fig. 1. Sintering phenomenon [13].

Fig. 2 .

 2 Fig. 2. Interparticle neck shape: (1) diffusion in gas phase; (2) surface diffusion; (3) volume diffusion.

Fig. 4 .

 4 Fig. 4. (a) XRD pattern of (a) HAP TCP and (b) HAP CaO not calcined and calcined at 1000 • C during 15 h.

Fig. 5 .

 5 Fig. 5. Dilatometric curves of (a) HAP TCP and (b) HAP CaO during isothermal runs.

Fig. 6 .

 6 Fig. 6. Final shrinkage (%) vs. the calcination temperature.

Fig. 7 .Fig. 8 .

 78 Fig. 7. Specific surface area curves of HAP TCP and HAP CaO during the isothermal runs.

8 )Fig. 9 .

 89 Fig. 9. Modelling of experimental data of surface reduction for (a) HAP TCP and (b) HAP CaO .

Fig. 10 .

 10 Fig. 10. Activation energy of the sintering phenomenon for HAP TCP and HAP CaO .

Table 1

 1 Specific surface area models used

	German [26]	Nicholson [23]	Schaffler et al. [27]

Acknowledgements

The authors would like to thank Dr. Didier Bernache-Assolant, Dr. Patrick Sharrock and Dr. Eric Champion for stimulating discussion and for their interest in this study.