Effect of oxides and nitrates of lead on the sintering and densification of hydroxyapatite adsorbents

Ange Nzihou, Benu Adhikari

In tro d u c tio n

H ea vy m et a l pollu t ion is a m a jor en vir on m en t a l con cer n beca u se of t h e t oxicit y t o h u m a n a n d pla n t life. Th is t oxicit y is let h a l even in t r a ce qu a n t it ies a n d t h ese m et a ls h a ve a gr ea t t en den cy t o bioa ccu m u la t e. 1 The efficiency of ca lcium hydroxya pa tites (Ca HAP) in r em ovin g biva len t h ea vy m et a ls, especia lly lea d, zin c, and cadmium, from water, wastewater, municipal solid wa st e in cin er a t or (MSWI) a sh , a n d con t a m in a t ed soil h a s led t o va r iou s st u dies t o u n der st a n d a n d expla in t h e m ech a n ism s wit h wh ich it r em oves t h ese pollutants. 2-6 F u r t h er m or e, it s su it a bilit y in in sit u imm obiliza t ion of t h ese pollu t a n t s en h a n ced it s a pplicability. [7][8][9] It has been found that use of a mere 1% of HAP in con t a m in a t ed soil effect ively r em oves t h e lea d, zin c, copper , a n d ca dm iu m a n d t h a t t h e pla n t gr owt h wa s r est or ed, 8 and also that the use of 4 g of H AP per kg of soil sign ifica n t ly r edu ced t h e u pt a ke of Cd a n d P b by cr ops. 10 Th e effica cy of solid H AP in im m obilizin g lea d a nd decrea sing its ga strointestina l a bsorption wa s tested in vivo in albino rats, and a 60% decrease in lead accumulation was observed without any negative effect in t h e ca lciu m -phosphorus metabolism of the animal. 11 Th e pa r t ia l or com plet e su bst it u t ion of ca lciu m ion in Ca H AP by lea d ion lea ds t o t h e for m a t ion of h ydr oxypyr om or ph it e, t h e a cciden t a l pr esen ce of wh ich in dr in kin g wa t er does n ot pose a n y da n ger a s t h e pr ecipit a t ed lea d is n o lon ger bioa va ila ble. 11 Th e u se of Ca H AP in r em edia t ion of MSWI fly a sh es con t a in in g h ea vy m et a ls is becom in g a pr om isin g t echnology because the classical method of solidifying MSWI fly a sh es wit h P or t la n d cem en t a n d su bsequ en t la n dfillin g h a s som e sever e lim it a t ion s. Th ese lim it a t ion s in clu de difficu lt y in pr ot ect ion a ga in st h u m idit y a n d con t r ollin g t h e lea ch in g of t h e h ea vy m et a ls, 12 and also t h a t t h e r equ ir em en t of t h e bin der cem en t is r ela t ively h igh . Th e Ca H AP a dded ext er n a lly, or for m ed du r in g t h e r ea ct ion wh ile t h e MSWI fly a sh is ch em ica lly t r ea t ed wit h a m ixt u r e of ca lciu m h ydr oxide a n d ph os-phoric acid maintained at a certain ratio, has been found effect ive in im m obilizin g biva len t h ea vy m et a ls. 12,13 A sintering step is still required even when the in solu ble h ea vy m et a l-h ydr oxya pa t it e is for m ed. Th e thermal treatment process (sintering) effectively embeds t h e h ea vy m et a l in t h e h ydr oxya pa t it e m a t r ix t h r ou gh den sifica t ion a n d cr yst a lliza t ion a n d a lso a lm ost elim in a t es lea ch in g of t h e h ea vy m et a ls fr om t h e h ydr oxya pa t it e m a t r ix u pon su bsequ en t la n dfillin g. 12-14 Furthermore, the ion exchange process between the calcium ion in Ca H AP a n d t h e h ea vy m et a l ion s is ver y r a pid, a n d t h e ext er n a l r esist a n ce t o ion t r a n sfer (su ch a s ion diffusion) basically controls the process. [START_REF] Ta Keu Ch I | Rem ova l of coexist in g P b 2+ , Cu 2+ and Cd 2+ ion s fr om wa t er by a ddit ion of h ydr oxya pa t it e powder[END_REF] Similarly, the surfa ce a rea of the excha nge or the specific surfa ce a rea of t h e Ca H AP pla ys a n im por t a n t r ole in det er m in in g how fast the (exchange) reaction takes place. [START_REF] Su Giya M A | E ffect of ca lcin a t ion a n d sievin g of ca lciu m h ydr oxya pa t it e on ionexch a n gea bilit y wit h lea d ca t ion in t h e pr esen ce a n d a bsen ce of HCl[END_REF][START_REF] Da Roch A | Ca dm iu m u pt a ke by h ydr oxya pa t it e syn t h esized in differ en t con dit ion s a n d su bm it t ed to thermal treatment[END_REF] Hence, a com pa r a t ive st u dy of t h e sin t er in g a n d den sifica t ion pr ocess of t h e Ca H AP wit h t h e n it r a t es a n d oxides of lea d is im por t a n t for fu r t h er in g t h e u n der st a n din g of the immobilization behavior of the CaHAP with respect t o t h e h ea vy m et a ls. Alt h ou gh t h e sin t er in g a n d densifica t ion ph en om en a of t h e Ca H AP is bein g st u died quite extensively, [START_REF] Ru Ys | Sin t er in g effect s on t h e st r en gt h of h ydr oxya patite[END_REF][START_REF] ; H On | E ffect of ca lcin a t ion on sin t er in g of h ydr oxya pa t it e. B iom aterials[END_REF][START_REF] Ba Illiez | Mech a n ism s of sin t er in g of macroporous hydroxyapatite adsorbents[END_REF] the effect of oxide and nitrite salts of lea d on t h e sin t er in g a n d den sifica t ion of Ca H AP st ill a wa it s a com pr eh en sive st u dy.

Th is pa per a im s t o r epor t a com pr eh en sive st u dy on the sintering and densification behavior of CaHAP along wit h t h e effect of oxides a n d n it r a t es of lea d on it s sin t er in g beh a vior .

Ma te ria ls a n d Me th o d s

Lea d oxide (P bO 2 ) a n d lea d n it r a t e [P b(NO 3 ) 2 ] were r ea gen t gr a de wit h 99% pu r it y a n d wer e obt a in ed fr om Aldr ich a n d La bosie com pa n ies, r espect ively. Th ey wer e fin ely gr ou n d befor e u se. Ca lciu m h ydr oxya pa t it e (Ca H AP ) wa s pr epa r ed by m ixin g ca lciu m n it r it e [Ca(NO 3 ) 2 ] and ammonium hydrogen phosphate [(NH 4 ) 2 -HPO 4 ] bot h obt a in ed fr om Nor skh ydr o. Th e t em per at u r e wa s m a in t a in ed a t 25 °C a n d t h e pH wa s a dju st ed to 7-8 wit h 10% a m m on ia solu t ion a ccor din g t o r ea ct ion 1. Det a ils of t h e pr epa r a t ion a n d pu r ifica t ion pr ocedu r e of Ca H AP powder (t er m ed H AP h en cefor t h ) t h r ou gh t h e r ou t e of r ea ct ion 1 a r e given elsewh er e. [START_REF] Ba Illiez | Th e kin et ics of su r fa ce a r ea reduction during isothermal sintering of hydroxyapatite adsorbent[END_REF] Th e powder wa s finely ground using a la bora tory-sca le grinder (IKA Wor ks, In c.) t o obt a in fin e pa r t icles wit h n a r r ow size dist r ibu t ion . Th er m a l st a biliza t ion of t h e powder wa s ca r r ied ou t by su bject in g it t o 400 °C for 2 h . Th e st a bilized sa m ple exh ibit ed a gglom er a t ion a n d t h e pa rticle size wa s widely a ltered. Hence, the powder wa s gr ou n d a ga in t o obt a in fin er pa r t icles. Th is st a bilized H AP powder wa s u sed t h r ou gh ou t t h e st u dy.

Th e st r u ct u r e a n d com posit ion of t h e H AP pa r t icles was quantified by X-ray powder diffractometry (Siemens D5000). X-r a y diffr a ct ion m ea su r em en t s (XRD) wer e ca r r ied ou t wit h Cu KR r a dia t ion gen er a t ed a t 40 m A, 3°< 2θ < 60°r a n ge a n d a t a sca n n in g speed of 2°/m in . Th e ph a ses pr esen t wer e det er m in ed by com pa r in g t h e pa t t er n s wit h t h e J CP DS st a n da r ds. Th e ca lciu m / ph osph or u s (Ca /P ) r a t io of t h e H AP wa s det er m in ed u sin g a t om ic a bsor pt ion spect r om et r y (Va r ia n Sepct r -AA-400). Th is H AP wa s fou n d t o con t a in excess ca lciu m a n d t h e Ca /P r a t io wa s 1.75.

Th e specific su r fa ce a r ea of t h e sa m ples wa s det erm in ed u sin g n it r ogen a dsor pt ion wit h t h e BE T m et h od (Micr om et r ics Gem in i Va cpr ep 061). Th e solid den sit y of t h e H AP wa s det er m in ed by h eliu m pycn om et r y (Micr om et r ics Accu pyc 1330). Th e specific su r fa ce a r ea a n d solid den sit y of t h e H AP powder wer e 43.48 m 2 /g a n d 2987 kg/m 3 , r espect ively.

Th e pa r t icle size dist r ibu t ion of t h e H AP wa s det ermined using a Malvern mastersizer (HYDRO 2000). The pa r t icles wer e su spen ded in et h a n ol a n d sh a ken by u lt r a sou n d. Th e m ea n pa r t icle size [d (0.5)] of t h e H AP befor e t h e st a biliza t ion st ep wa s 5.05 µm, with the dist r ibu t ion bein g m on om oda l. H owever , wh en t h e st a biliza t ion st ep (ca lcin a t ion a t 400 °C for 2 h ) wa s a pplied, t h e m ea n pa r t icle size in cr ea sed. Alt h ou gh t h e st a bilized powder wa s cr u sh ed, t h e m ea n pa r t icle size [d (0.5)] r em a in ed a t 23.05 µm a n d t h e pa r t icle size dist r ibu t ion wa s bim oda l.

Th e m a ss loss h ist or y of t h e sa m ples wa s det er m in ed in dyn a m ic m ode u sin g a t h em ogr a vim et r ic a n a lyzer (TGA-DTA, Net zsch STA 409) a t a sca n n in g r a t e of 10 °C/m in fr om r oom t em per a t u r e t o 1200 °C.

Sh r in ka ge t est s of t h e sa m ples wer e ca r r ied ou t by a t h er m om ech a n ica l a n a lyzer (TMA, SE TARAM Set sys 16/20) wit h 5-g con st a n t loa d. Th e sh r in ka ge, r esu lt ed fr om t h e r ise in t em per a t u r e, is defin ed a s ∆L /L 0 , where L 0 is t h e in it ia l len gt h of t h e sa m ple a n d ∆L ) L 0 -L (t). L (t) is t h e len gt h of t h e sa m ple a s a fu n ct ion of t im e. Th e r ela t ive den sit y of t h e sin t er ed specim en is expressed as F b /F s , where F b and F s are the bulk and solid den sit y, r espect ively. Th e t h er m om ech a n ica l a n a lyses (dila t om et r y) wer e ca r r ied ou t in dyn a m ic m ode wit h a sca n n in g r a t e of 10 °C/m in fr om r oom t em per a t u r e t o 1200 °C.

Calcination experiments were carried out on a furnace (Au br y Com pa n y, F r a n ce). A h ea t in g r a t e of 10 °C/m in wa s im posed fr om r oom t em per a t u r e t o t h e design a t ed isot h er m t em per a t u r es. Th e isot h er m a l don a t ion s wer e m a in t a in ed for 2 t o 300 m in . Th e sa m ples wer e pla ced on a n a lu m in a cr u cible a n d su bject ed t o t h e a bove ca lcin a t ion r egim e u n der flowin g a ir . Th e sa m ples wer e cooled a t t h e r a t e of 10 °C/m in t o r oom t em per a t u r e. Th e h ea t in g (sca n n in g) r a t e of 10 °C/m in wa s im posed for TGA a n d TMA, a s well a s for t h e ca lcin a t ion pr ocess at the furnace, in order to facilitate the comparison. The su r fa ce ch a r a ct er ist ics of t h e sin t er ed a gglom er a t e a s well a s t h e u n sin t er ed pa r t icles/a ggr ega t es wa s st u died u sin g en vir on m en t a l sca n n in g elect r on m icr oscopy (XL 30 ESEM-FEG, Phillips). The ESEM micrographs when com bin ed wit h t h e m a ss loss (TGA) a n d t h e sh r in ka ge (TMA) ca n pr ovide excellen t in sigh t in t o t h e den sificat ion pr ocess.

Mo d e lin g o f S in te rin g a n d D e n s ific a tio n

Th e specific su r fa ce a r ea is on e of t h e m ost r elia ble in dica t or s of t h e sin t er in g even t beca u se it s kin et ics of r edu ct ion is lin ked t o t h e m ech a n ism of t h e sin t er in g, a n d a lso beca u se it is ea sily a n d r elia bly m ea su r ed. Va r iou s pr edict ive m odels a r e a va ila ble in t h e lit er ature [22][START_REF] Ger M A N | Su r fa ce a r ea r edu ct ion kin et ics du r in g in t er m edia t e st a ge sin t er in g[END_REF][START_REF] Nich Olson | Va r ia t ion of su r fa ce a r ea du r in g t h e t h er m a l decom posit ion of solids[END_REF][START_REF] Sch A Ffler | Agin g of silica-alumina cracking catalyst. I. Kinetics of structural changes by h ea t a n d st ea m[END_REF] for t h e qu a n t ifica t ion of t h e specific su r fa ce a r ea a s a fu n ct ion of sin t er in g t em per a t u r e a n d t im e. Recen t ly, Ba illiez a n d Nzih ou [START_REF] Ba Illiez | Th e kin et ics of su r fa ce a r ea reduction during isothermal sintering of hydroxyapatite adsorbent[END_REF] com pa r ed t h ese m odels a n d fou n d t h a t t h e Sch a ffler m odel (eq 2) bet t er pr edict ed t h e kin et ics of su r fa ce a r ea r edu ct ion of Ca H AP s. H en ce, t h e Sch a ffler m odel is u sed in t h is st u dy. S , K, and n a r e specific su r fa ce a r ea (m 2 /g), r ea ct ion con st a n t (1/s), a n d r ea ct ion or der (dim en sion less), r espect ively. E qu a t ion 2 is in t egr a t ed u sin g t h e specific su r fa ce a r ea (S o ) of the stabilized HAP as given by eq 3. E qu a t ion 3 wa s u sed t o det er m in e t h e K and n va lu es u sin g m in im ized fu n ct ion (f min ) represented by eq 4. S i exp and S i ca l a r e t h e exper im en t a l a n d ca lcu la t ed specific su r fa ce a r ea , r espect ively.

Th e evolu t ion of bu lk den sit y of t h e specim en a s a fu n ct ion of t em per a t u r e ca n be det er m in ed fr om t h e measured shrinkage history. Bulk density is an import a n t pa r a m et er in u n der st a n din g t h e evolu t ion of t h e voidn ess of t h e specim en . Th is qu a n t it y is a lso im port a n t for followin g t h e r ela t ive den sit y (F b /F s ) of t h e sa m ple. Th e bu lk den sit y (F b ) ca n be det er m in ed u sin g eq 5. [START_REF] Ba Illiez | Th e kin et ics of su r fa ce a r ea reduction during isothermal sintering of hydroxyapatite adsorbent[END_REF] m o and L o a r e t h e in it ia l m a ss (kg) a n d in it ia l len gt h (m ) of t h e sa m ple, a n d r is t h e in n er r a diu s (m ) of t h e cr u cible in t h e dila t om et er (TMA). E qu a t ion 5 wa s der ived by a ssu m in g t h a t t h e va r ia t ion in t h e sa m ple dim en sion is isot r opic a n d u n idir ect ion a l, a n d t h a t t h e weight of the mass remains constant at the temperature range of interest. The validity of these assumptions will be fu r t h er discu ssed.

Re s u lts a n d D is c u s s io n s

S in te rin g a n d D e n s ific a tio n o f HAP .

S (t) ) [S o (1-n ) + (n -1)Kt] 1/(1-n ) (3) f min ) ∑ i)1 n [S i exp -S i ca l ] 2 (4) F b ) m o [100 -weigh t losss (%)] πr 2 L o [100 -sh r in ka ge (% )] (5)
by u sin g t h e specific su r fa ce a r ea a s a n in dica t or . Th e kin et ics of su r fa ce a r ea r edu ct ion a t differ en t sin t er in g temperatures is presented in Figure 1. As shown in this figu r e, t h e su r fa ce a r ea is a st r on g fu n ct ion of t em pera t u r e a s well a s t im e. F or a given t em per a t u r e, t h e su r fa ce a r ea decr ea ses ver y r a pidly wit h in t h e fir st 10 t o 30 m in a n d st a bilizes t h er ea ft er . Th is figu r e fu r t h er sh ows t h a t t h e sin t er in g of H AP a t 400 °C for 5 h does n ot ch a n ge t h e specific su r fa ce a r ea m u ch . At 1200 °C, the surface area reduces by 97.8% and reaches 0.97 m 2 /g wit h in 30 m in . F u r t h er m or e, a t 1000 °C a n d beyon d t h e specific su r fa ce a r ea va lu es a t differ en t t im es a r e ver y close t o ea ch ot h er , sign ifyin g t h a t t h e sin t er in g complet es a lm ost in st a n t ly. Th e effect of sin t er in g t im e a n d t em per a t u r e on t h e solid den sit y of H AP powder a r e sh own in F igu r e 2. It ca n be seen fr om t h is figu r e t h a t for a given sin t er in g temperature the solid density becomes stable within 30 m in , wh ich is sim ila r t o t h e effect of sin t er in g t im e on t h e specific su r fa ce a r ea (F igu r e 1). On e im por t a n t fea t u r e of t h is figu r e is t h a t t h e solid den sit y st a r t s decreasing instead of increasing at 1200 °C after 10 min. Th is in dica t es t h a t t h e H AP h a s u n der gon e st r u ct u r a l ch a n ge wh ich is pr oba bly du e t o t h e pa r t ia l deh ydr ox-yla t ion of H AP wh ich lea ds t o t h e for m a t ion of t et r aca lciu m ph osph a t e [Ca 4 (P O 4 )O, TTCP ] a n d R-t r ica lciu m ph osph a t e [R-Ca 3 (P O 4 ) 2 , R-TCP ]. [START_REF] La Yvolle | Sol-gel Syn t h esis of Am or ph ou s ca lciu m ph osph a t e a n d sin t er in g in t o m icr opor ou s h ydr oxya pa t it e biocer a m ics[END_REF] Th is fea t u r e br in gs a bou t a n im por t a n t effect in t h e sin t er in g m ech a n ism as will be discussed shortly. Furthermore, the maximum solid density obtained (1000 °C, 5 h) is 3158 kg/m 3 which is merely 5.7% higher than the density of the unsintered sa m ple. It ca n be con clu ded t h a t t h e solid den sit y does n ot ch a n ge m u ch wit h sin t er in g t em per a t u r e a n d t im e, a n d h en ce, it is a poor in dica t or of t h e sin t er in g ph en om en on .

4.1.2 M ech an ism s of S in terin g. Figure 3 presents the exper im en t a l a n d pr edict ed va lu es of specific su r fa ce a r ea a s a fu n ct ion of t im e a n d t em per a t u r e. Th e pr edict ion s wer e ca r r ied ou t u sin g eq 3. Beca u se t h e su r fa ce a r ea does n ot begin t o ch a n ge a t 400 °C, t h e su r fa ce a r ea va lu es a t t h is t em per a t u r e a r e n ot u sed for pr edict ion . F igu r e 3 sh ows t h a t t h e pr edict ion s follow t h e exper im en t a l ou t com es qu it e well. Th e va lu es for K, wh ich is u su a lly kn own a s sin t er in g r a t e con st a n t or r ea ct ion r a t e con st a n t , r a n ge fr om 1.70 × 10 -14 s -1 at 700 °C to 1.3 × 10 -4 s -1 at 1200 °C which means that it in cr ea ses by 7.65 × 10 9 wit h in t h is t em per a t u r e r a n ge. On t h e ot h er h a n d, t h e n va lu e, wh ich is kn own Figu re 1. Kin et ics of specific su r fa ce a r ea r edu ct ion of H AP a s a fu n ct ion of t im e a n d t em per a t u r e.

Figu re 2. Va r ia t ion of solid den sit y a s a fu n ct ion of t im e a n d t em per a t u r e. as reaction order, decreases from 8 to 6. The magnitude of K va lu e in dica t es pa ce or speed of t h e su r fa ce a r ea r edu ct ion wh ile t h e n va lu e in dica t es t h e m or e im port a n t a spect , t h e m ech a n ism s in volved.

Figure 4 presents the prediction error while assuming a r ea ct ion or der (n va lu e). Th e or der of r ea ct ion va r ies fr om 5 t o 8 wh en t h e sin t er in g t em per a t u r e va r ies fr om 700 t o 1200 °C. As will be discu ssed in t h e sect ion dea lin g wit h den sifica t ion , t h e den sifica t ion pr ocess of H AP wou ld n ot com m en ce below 850 °C, h en ce t h e r ea ct ion or der below 850 °C is expect ed t o be sa m e a s t h e m ech a n ism of sin t er in g befor e t h e com m en cem en t of den sifica t ion ca n be, t o a r ea son a ble ext en t , expect ed t o be t h e sa m e. Th is is con fir m ed fr om F igu r e 4 t h a t n ) 8 gives the minimum error in prediction. Similarly, for sin t er in g t em per a t u r e a t a n d a bove 1200 °C, H AP u n der goes pa r t ia l deh ydr oxyla t ion a n d even t h e solid density gets altered (Figure 2). The rate of densification, a s will be discu ssed la t er , sh ows t h a t t h e m a xim u m den sifica t ion occu r s below 1200 °C. H en ce, for t h e ch em ica lly in t a ct H AP , it is expect ed t h a t sin t er in g a t 1200 °C a n d a bove sh ou ld be gover n ed by a differ en t m ech a n ism . Th is fa ct is con fir m ed by F igu r e 4 wh er e n ) 5 gives m in im u m er r or in pr edict ion . In fa ct , t h is va lu e (n ) 5) m a y n ot be t h e sin t er in g m ech a n ism for ch em ica lly in t a ct H AP , r a t h er , it m a y be t h e sin t er in g m ech a n ism of pa r t ia lly deh ydr oxyla t ed H AP , wh ich is expect ed t o h a ve differ en t ph ysicoch em ica l pr oper t ies due to the presence of tetracalcium phosphate monoxide (Ca 4 O(P O 4 ) 2 ) and R-t r ica lciu m ph osph a t e [Ca 3 (P O 4 ) 2 ]. Th e t em per a t u r e r a n ge bet ween 850 a n d 1200 °C is r ea son a bly expect ed t o be t h e t em per a t u r e r egion a t which the densification commences as well as completes. H en ce, sin t er in g even t s wit h in t h is t em per a t u r e r a n ge sh ou ld be gover n ed by a u n iqu e sin t er in g m ech a n ism . This fact is confirmed from Figure 4 in which n ) 7 gives m in im u m er r or in pr edict ion for sin t er in g t em per a t u r e of 900 a n d 1000 °C, a n d t h is va lu e of n is expect ed for 1100 °C a lso.

Th e m ech a n ism of sin t er in g of H AP , a s eviden ced by t h e r edu ct ion in specific su r fa ce a r ea , ca n be divided in t o t h r ee m ech a n ism s. F ir st , below 850 °C, t h e su r fa ce area reduces without densification (i.e., sintering withou t den sifica t ion ) a n d is r epr esen t ed by n ) 8. Secon d, a t t em per a t u r e r a n ge of 850 t o 1200 °C, t h e r edu ct ion in su r fa ce a r ea is a ccom pa n ied by den sifica t ion pr ocess wh ich is r epr esen t ed by n ) 7. Th ir d, a t t em per a t u r es at and above 1200 °C, the HAP undergoes some chemi-Figu re 3. Redu ct ion of specific su r fa ce a r ea wit h sin t er in g t im e a n d t em per a t u r e: exper im en t a l va lu es a n d pr edict ion s.

Figu re 4. Va r ia t ion of er r or in pr edict ion wit h differ en t r ea ct ion or der . ca l ch a n ges in st a n t ly (wit h in 10 m in ). Th e su dden decr ea se in t h e solid den sit y of H AP wh ile sin t er ed a t 1200 °C (F igu r e 2) in dica t es t h is ch em ica l ch a n ge. In fa ct t h e sin t er in g com plet es a t t h e t im e wh en H AP a t t a in s 1200 °C it self. It is expect ed t h a t n va r ies bet ween 5 a n d 6 in t h is t em per a t u r e r a n ge.

F or a ch em ica lly (st r u ct u r a lly) in t a ct H AP , it is r ea son a ble t o st a t e t h a t t h er e a r e t wo m ech a n ism s of sin t er in g: on e t h a t dr ives t h e r edu ct ion in t h e specific su r fa ce a r ea wit h ou t den sifica t ion a n d t h e ot h er t h a t dr ives t h e r edu ct ion wit h den sifica t ion . Th e "r ea ct ion or der " for t h e for m er is n ) 8 wh ile t h a t for t h e la t t er is n ) 7. Th e m a t er ia l t r a n spor t pr ocess in sin t er in g is con sider ed t o occu r du e t o va por ph a se t r a n spor t , su r fa ce m igr a t ion , volu m e diffu sion , a n d viscou s flow. [START_REF] Sch A Ffler | Agin g of silica-alumina cracking catalyst. I. Kinetics of structural changes by h ea t a n d st ea m[END_REF] Considering the sintering of HAP is solid phase and that the material transport by vapor phase can be neglected, t h e possible dr ivin g m ech a n ism s of t h e sin t er in g ca n be su r fa ce m igr a t ion a n d volu m e diffu sion . H en ce, t h e sin t er in g wit h ou t den sifica t ion ca n be con sider ed t o be su per ficia l diffu sion -dr iven pr ocess (n ) 8) a n d t h e sin t er in g wit h den sifica t ion ca n be con sider ed t o be dr iven by volu m e diffu sion (n ) 7).

4.1.3 Den sification . Th e ph en om en on of den sifica t ion of H AP h a s been illu st r a t ed in F igu r e 5. Th e m a ss loss of H AP du e t o loss of a bsor bed or a dsor bed wa t er (∼1%) u su a lly occu r s below 400 °C a n d is n ot sh own h er e. In the temperature range where densification occurs (>850 °C), t h e loss of m a ss is a lm ost n egligible (∼0.5%). Hence, the densification process is due to volumetric shrinkage a t a con st a n t m a ss. Th e sh r in ka ge com m en ces a t 850 °C a n d con t in u es u p t o t h e t em per a t u r e st u died (1200 °C) a ch ievin g 14% sh r in ka ge. Th is fa ct is a lso m a n ifest ed in en vir on m en t a l sca n n in g elect r on m icr oscope (E SE M) m icr ogr a ph s sh own in F igu r e 6. Th e su r fa ce of t h e pa r t icles is n ot m u ch com pa ct ed a t 400 °C a n d a t 800 °C. Th e su r fa ce a ppea r s t o be sligh t ly com pa ct ed a t 1000 °C, wh er ea s it is h igh ly com pa ct ed a n d fu sed a t 1200 °C. Th is cor r obor a t es well wit h F igu r e 5, wh ich sh ows t h a t t h e den sifica t ion pr ocess in t en sifies a ft er 1000 °C, a n d a t 1200 °C a com plet e den sifica t ion ca n be achieved. The rate of densification (rate of shrinkage) r evea ls t h a t t h e m a xim u m den sifica t ion occu r s below 1200 °C. Th e va r ia t ion of bu lk den sit y r esu lt in g fr om the densification was calculated using eq 5 and is shown in F igu r e 7. Th e bu lk den sit y fir st decr ea ses a n d t h en st a r t s in cr ea sin g a t a poin t wh ich cor r espon ds t o t h e st a r t of t h e den sifica t ion pr ocess. H owever , t h e in it ia l bulk density is achieved at about 1000 °C only. The bulk den sit y a t t a in ed a bou t 40% of t h e in it ia l solid den sit y (in it ia l solid or t h eor et ica l den sit y ) 2987 kg/m 3 ) at the m a xim u m t em per a t u r e st u died (1200 °C). Th is m ea n s t h a t t h er e is a con sider a ble a m ou n t of voids on t h e sin t er ed pa r t icle bed.

Effe ct of P bO 2 an d P b(NO 3 ) 2 on Sin te rin g an d De n sification of HAP .

Th e effect of a ddit ion of P bO 2 and Pb(NO 3 ) 2 on t h e sin t er in g a n d den sifica t ion pr ocess is discu ssed in t h is sect ion . Th e a im wa s t o u n der st a n d the effect of the lead, existing in different chemical forms (oxides a n d n it r a t es), on t h e sin t er in g a n d den sifica t ion of H AP . Th e specific su r fa ce a r ea of H AP wit h 2% a ddit ives wa s det er m in ed a t differ en t sin t er in g t emper a t u r es for 2 h . Also, a com pa r a t ive pr esen t a t ion of t h e den sifica t ion of H AP wit h 2% a ddit ives a lon g wit h t h e pu r e H AP wa s m a de. Th e m a ss loss of t h ese m ixt u r es wa s n ot ver y differ en t fr om t h e m a ss loss of t h e pu r e H AP , a n d t h e loss du r in g t h e den sifica t ion pr ocess wa s a lm ost n egligible (<1%, da t a n ot sh own ).

Com pa r ed t o H AP , t h e evolu t ion of specific su r fa ce a r ea of t h e m ixt u r es sh owed t wo dist in ct fea t u r es Figu re 5. P lot s of sh r in ka ge (%), r a t e of sh r in ka ge (%/m in ), a n d m a ss loss (%) for H AP .

Figu re 6. E n vir on m en t a l sca n n in g elect r on m icr ogr a ph s for H AP sin t er ed a t differ en t t em per a t u r es for 2 h .

(F igu r e 11). Below 700 °C, com pa r ed t o pu r e H AP , bot h t h e m ixt u r es h a d h igh er specific su r fa ce a r ea . Th e in cr ea se of t h e specific su r fa ce a r ea for H AP + 2% P bO 2 and HAP + 2% Pb(NO 3 ) 2 within this temperature range wa s 5.4% a n d 6%, r espect ively. As t h e m ea su r em en t er r or of t h e equ ipm en t wa s fou n d t o be of t h e or der of (4.5%, t h er e wa s n o sign ifica n t ch a n ge in t h e su r fa ce a r ea du e t o t h e a ddit ion of P bO 2 a n d P b(NO 3 ) 2 . The small increase in the surface area was probably brought a bou t by t h e sm a ller pa r t icle sizes of t h e P bO 2 and Pb-(NO 3 ) 2 . Th e specific su r fa ce a r ea s of t h e m ixt u r es a n d the HAP almost coincided at 700 °C but were lower than tha t of HAP itself. At 800 °C a nd a bove, the specific surface areas of the mixtures were much lower than that of H AP . Up t o t h e lim it of t h e m ea su r em en t by t h e equ ipm en t (∼1 m 2 /g), t h e a ver a ge (%) differ en ce for mixtures with PbO 2 and Pb(NO 3 ) 2 were 14.9 a nd 17.6%, respectivelyswhich is significant. These results showed t h a t t h e a ddit ion of t h e P bO 2 a n d P b(NO 3 ) 2 in H AP accelerates the reduction of specific surface area at and above 800 °C, a regime where sintering accompanies the den sifica t ion .

A com pa r a t ive pr esen t a t ion of t h e den sifica t ion of H AP a n d it s m ixt u r es wit h 2% P bO 2 a n d P b(NO 3 ) 2 is m a de in F igu r e 8. Th is figu r e sh ows t h a t t h e den sifica t ion of t h e bot h m ixt u r es st a r t s a t a bou t t h e sa m e t em per a t u r e (820 °C), wh ich is a bou t 30 °C lower t h a n the commencement of densification of pure HAP (Figure 7). Th e com m en cem en t of den sifica t ion is r epr esen t ed by com m en cem en t of sh r in ka ge, a poin t a t wh ich t h e sh r in ka ge ver su s t em per a t u r e plot begin s t o m ove t owa r d n ega t ive. Th ese a ddit ives h a ve a lso br ou gh t about an increment in the extent of shrinkage. The final sh r in ka ge (a t 1200 °C) is 15.6% a n d 17.1% for t h e mixture with PbO 2 a n d P b(NO 3 ) 2 , r espect ively, wh ich is 1.5% a n d 2.1% h igh er t h a n t h e sh r in ka ge of H AP . Th ese fa ct s a r e cor r obor a t ed by t h e E SE M m icr ogr a ph s (F igu r e 9) wh ich sh ow t h a t t h e su r fa ce of t h e pa r t icles h a s n ot u n der gon e m u ch den sifica t ion or sin t er in g u p t o 800 °C, wh er ea s t h e su r fa ce a ppea r s t o be qu a n t it a -Figu re 7. Va r ia t ion of bu lk den sit y (kg/m 3 ) du r in g den sifica t ion pr ocess.

Figu re 8. Va r ia t ion of sh r in ka ge (%) a n d bu lk den sit y of H AP a n d it s m ixt u r e wit h 2% P bO2 a n d P b(NO3)2 a s a fu n ct ion of sin t er in g temperature.

t ively a lt er ed a t 1000 °C. At 1200 °C t h e su r fa ce of t h e m ixt u r es a ppea r s t o be com plet ely den sified a n d fu sed, a nd a lso the surfa ces of these mixtures a ppea r to be sim ila r . Wh en F igu r e 9 is com pa r ed wit h F igu r e 6, wh ich pr esen t s t h e E SE M m icr ogr a ph s for t h e pu r e HAP, it can be seen that the surface of the samples with a ddit ives a ppea r s t o be m or e den sified a n d coa lesced. F igu r e 8 a lso m a kes a com pa r a t ive pr esen t a t ion of t h e in cr em en t in bu lk den sit ies of t h e m ixt u r es wit h t h a t of pu r e H AP . Th e bu lk den sit y of t h e sin t er ed m ixt u r es (1200 °C) wa s 40% com pa r ed t o t h e solid den sit ies of t h e u n sin t er ed m ixt u r es a t r oom t em per a t u r e. Th is is not significantly different from the bulk density of pure H AP (Sect ion 4.1) a t 1200 °C. H en ce, t h e den sifica t ion br ou gh t a bou t by a ddit ion of 2% P bO 2 a n d P b(NO 3 ) 2 does n ot a ppea r t o be sign ifica n t . H owever , t h e r a t e of den sifica t ion cou ld be differ en t . Th e r a t e of sh r in ka ge wa s det er m in ed by differ en t ia t in g t h e sh r in ka ge ver su s t im e plot wit h r espect t o t im e. Th is wa s t h en plot t ed a ga in st t em per a t u r e t o obt a in sh r in ka ge r a t e ver su s t em per a t u r e plot s. F igu r e 10 sh ows t h e t h e r a t e of den sifica t ion (sh r in ka ge) of H AP a n d it s m ixt u r e wit h 2% P bO 2 a n d P b(NO 3 ) 2 is qu it e differ en t . Com pa r ed t o H AP , t h e r a t e of sh r in ka ge of t h e m ixt u r es is m u ch fa st er fr om 800 t o 1050 °C, a ft er wh ich it declin es. Furthermore, there are two peaks in the shrinkage rates of t h e m ixt u r es wit h in 1050 a n d 1110 °C, wh er ea s t h e r a t e of pu r e H AP sh ows on ly on e pea k a t 1195 °C. Th e r a t e of sh r in ka ge of m ixt u r e wit h P b(NO 3 ) 2 is gr ea t er t h a n t h a t of t h e m ixt u r e wit h P bO 2 . Th is, t o som e ext en t , ca n be a t t r ibu t ed t o t h e eva por a t ion of NO 2 and gr ea t er m a ss loss (5.6% m a ss loss for P b(NO 3 ) 2 compared to 5.3% loss for PbO 2 at 1200 °C). The more porous su r fa ce of t h e P b(NO 3 ) 2 com pa r ed wit h t h a t of P bO 2 (E SE M m icr ogr a ph s a t 1200 °C, F igu r e 9) su ggest s t h e eva por a t ion of NO 2 . Th e r a t e of sh r in ka ge of t h e H AP , as well as its mixtures, declines rapidly at the proximity of 1200 °C, in dica t in g t h a t t h e sin t er in g even t com plet es there.

E ffect of Con cen tration . H ow do t h e h igh er concentra tions of PbO 2 and Pb(NO 3 ) 2 affect the sintering a n d den sifica t ion pr ocess of H AP ? Th is qu est ion is

Figu re 9. Environmental scanning electron micrographs for HAP with 2% PbO2 and Pb(NO3)2 sin t er ed a t differ en t t em per a t u r es for 2 h . Figu re 10. Va r ia t ion of t h e r a t e of sh r in ka ge (%/m in ) for H AP a n d it s m ixt u r e wit h 2% P bO2 a n d P b(NO3)2 a s a fu n ct ion of sin t er in g temperature.

a n swer ed h er e by m on it or in g t h e specific su r fa ce a r ea a n d sh r in ka ge by in cr ea sin g t h e a m ou n t of P bO 2 and P b(NO 3 ) 2 t o 10% (w/w).

Effect on S pecific S urface Area R eduction. Figure 11 pr esen t s t h e specific su r fa ce a r ea of H AP wit h 2%

a n d 10% of bot h P b(NO3) 2 and PbO 2 a s a fu n ct ion of t em per a t u r e. F r om t h is figu r e it ca n be seen t h a t , a t a given temperature, the specific surface areas of mixtures of 10% P b(NO 3 ) 2 and PbO 2 a r e a lwa ys lower t h a n t h ose of t h eir cor r espon din g 2% m ixt u r es. F u r t h er m or e t h e specific su r fa ce a r ea r edu ct ion is sign ifica n t com pa r ed t o t h a t of H AP a bove 800 °C wh er e t h e sin t er in g accompanies densification. The reduced specific surface a r ea s in t h ese m ixt u r es below 800 °C m a y be du e t o increased particle coalescence or surface reorganization. Com pa r ed t o t h e 2% m ixt u r es t h e specific su r fa ce a r ea s differ sign ifica n t ly on ly a t 800 °C. Th is fa ct ca n a lso be obser ved fr om t h e E SE M m icr ogr a ph s (F igu r e 15), which show that the particles of mixture containing 10% a ddit ives a r e gr own la r ger , pr oba bly du e t o su r fa ce diffu sion . Aft er t h e a dva n cem en t of t h e den sifica t ion , t h e specific su r fa ce a r ea s of m ixt u r es wit h 2% a n d 10% a ddit ives a r e close t o ea ch ot h er . Th is sign ifies t h a t t h e a ddit ion of m er ely 2% of t h ese a ddit ives br in gs a bou t close t o opt im u m r edu ct ion in su r fa ce a r ea of H AP .

4.2.3 E ffect on Den sification . Figure 12 presents the effect of con cen t r a t ion of P bO 2 on t h e m a ss loss a n d den sifica t ion of t h e H AP . Th is figu r e sh ows t h a t t h e den sifica t ion for t h e m ixt u r e con t a in in g 10% P bO 2 has st a r t ed a t 780 °C wh ich is 70 °C a n d 50 °C lower t h a n t h a t of H AP a n d H AP + 2% P bO 2 , r espect ively. Th is figu r e fu r t h er sh ows t h a t t h e r a t e of sh r in ka ge of H AP + 10% P bO 2 is m u ch fa st er t h a n t h a t of bot h t h e H AP a n d H AP + 2% P bO 2 u p t o 980 °C t h en it declin es su bst a n t ia lly. At 1200 °C t h e fin a l sh r in ka ge of H AP + 10% P bO 2 is a bou t 6.8% wh ich is m u ch less com pa r ed t o 15.6% of H AP + 2% P bO 2 a n d 14% t h a t of H AP . Th e m a ss loss of t h e H AP + 10% P bO 2 a bove 1050 °C h a s in cr ea sed su bst a n t ia lly (5%) ou t of t ot a l loss of 10.2% (a t 1200 °C). Con sider in g t h e fr ee or a dsor bed wa t er in H AP is less t h a n 1-2%, t h e r em a in in g 8% loss in m a ss might be due to partial dehydroxylation and evaporation du e t o ch em ica l r ea ct ion cou pled wit h t h e sin t er in g (ph ysica l r ea ct ion ) a s it wa s descr ibed in t h e lit er a -Figu re 11. E ffect of con cen t r a t ion of P bO2 a n d P b(NO3)2 on t h e su r fa ce a r ea of H AP (sin t er in g t im e ) 2 h ).

Figu re 12. E ffect of con cen t r a t ion of P bO2 on t h e den sifica t ion a n d m a ss loss of H AP . ture. [START_REF] La Yvolle | Sol-gel Syn t h esis of Am or ph ou s ca lciu m ph osph a t e a n d sin t er in g in t o m icr opor ou s h ydr oxya pa t it e biocer a m ics[END_REF][START_REF] Va N La | ce of h igh sin t er in g t em per a t u r es on t h e m ech a n ica l pr oper t ies of h ydr oxya pa t it es[END_REF][START_REF] Raynaud | Calcium phosphate apatites with variable Ca/P atomic ratio II. Calcination and sintering[END_REF][START_REF] Ra Yn A U D | Mech a n ica l pr oper t ies a n d degr a da t ion in solu t ion of h ot pr essed cer a m ics[END_REF] Th e E SE M m icr ogr a ph s pr esen t ed in F igu r e 15 a lso su ggest t h a t t h e pa r t icles of t h e m ixt u r e con t a in in g 10% P bO 2 a ppea r t o be gr own la r ger a t 800 °C com pa r ed t o t h ose in t h e 2% m ixt u r e. H owever , t h e E SE M m icr ogr a ph s sh ow t h a t t h e su r fa ce of m ixt u r e con t a in in g 10% P bO 2 is m or e coa lesced a n d fu sed (F igu r e 15) t h a n t h e 2% on e. Th is m a y be du e t o t h e fa ct t h a t h igh er a m ou n t s of a ddit ives ca n select ively deposit on t h e su r fa ce, beca u se of wh ich t h e E SE M m icr ogr a ph s a ppea r t o be su bst a n t ia lly coa lesced.

F igu r e 13 pr esen t s t h e effect of con cen t r a t ion of P b(NO 3 ) 2 on t h e m a ss loss a n d den sifica t ion of H AP . Th is figu r e sh ows t h a t t h e den sifica t ion for t h e m ixt u r e con t a in in g 10% P b(NO 3 ) 2 h a s st a r t ed a t 810 °C wh ich is sligh t ly lower t h a n t h a t of H AP a n d H AP + 2% P b(NO 3 ) 2 . Th is fa ct is cor r obor a t ed fr om t h e E SE M m icr ogr a ph s (F igu r e 15) t h a t t h e pa r t icles wit h 10% P b(NO 3 ) 2 a r e la r ger com pa r ed t o t h ose of 2% m ixt u r es. F igu r e 13 fu r t h er sh ows t h a t t h e r a t e of sh r in ka ge of HAP + 10% Pb(NO 3 ) 2 is slightly fa ster compa red to tha t of H AP + 2% Pb(NO 3 ) 2 up to 950 °C and then it declines su bst a n t ia lly. At 1200 °C t h e fin a l sh r in ka ge of H AP + 10% P bO 2 is a bou t 9.3% wh ich is m u ch less com pa r ed t o 17.1% of H AP + 2% P b(NO 3 ) 2 a n d 14% of H AP . Th is figu r e a lso sh ows t h a t t h e m a ss loss of t h e H AP + 10% Pb(NO 3 ) 2 is ver y su bst a n t ia l com pa r ed t o t h a t of t h e H AP a n d H AP + 2%P b(NO 3 ) 2 under the same experim en t a l con dit ion . It h a s lost 12.3% a n d 17.2% m a ss a t 800 a n d 1200 °C, r espect ively. It h a s t o be n ot ed t h a t t h is la r ge loss of m a ss h a s n ot su bst a n t ia t ed in t o a n in cr ea sed den sifica t ion of t h e sa m e ext en t . Th is ext en t of m a ss loss cou ld be a ccor ded t o t h e eva por a t ion of NO 2 a n d ot h er ch em ica l r ea ct ion s. It ca n a lso be seen t h a t the mass loss of HAP + 10% Pb(NO 3 ) 2 accelerates above 1050 °C wh ich in dica t es in cr ea se in t h e r a t e of ch em ica l r ea ct ion . It wa s expect ed t h a t t h e E SE M m icr ogr a ph s (F igu r e 15) of sa m ples con t a in in g 10% P b(NO 3 ) 2 wou ld be m or e por ou s t h a t t h ose of t h e sa m ples con t a in in g 2% Figu re 13. E ffect of con cen t r a t ion of P b(NO3)2 on t h e den sifica t ion a n d m a ss loss of H AP .

Figu re 14. E volu t ion of bu lk den sit y of H AP , H AP wit h 2% a n d 10% of P bO2, a n d P b(NO3)2 a s a fu n ct ion of t em per a t u r e. P b(NO 3 ) 2 t o cor r obor a t e wit h t h e m a ss loss kin et ics. H owever , it seem ed den ser a n d m or e coa lesced a n d fu sed com pa r ed wit h t h a t of H AP + 2% P b(NO 3 ) 2 . This m a y be du e t o t h e fa ct t h a t t h e P b(NO 3 ) 2 is n ot com plet ely exh a u st ed a n d t h a t t h e u n r ea ct ed P b(NO 3 ) 2 is cover in g t h e su r fa ce of t h e H AP pa r t icles, a n d a lso t h a t it h a s t en den cy t o select ively occu py t h e su r fa ce of t h e m ixt u r e a gglom er a t e.

F igu r e 14 m a kes a com pa r a t ive pr esen t a t ion of t h e evolu t ion of bu lk den sit y for H AP a n d H AP wit h 2% a s well as 10% PbO 2 and Pb(NO 3 ) 2 . This figure reveals that a t t h e en d of t h e sin t er in g pr ocess t h e bu lk den sit ies of H AP wit h 10% a ddit ives a r e lower t h a n t h ose of t h eir cor r espon din g 2% m ixt u r es. Th is is du e t o t h e fa ct t h a t m u ch h igh er m a ss loss a n d m u ch lower den sifica t ion wer e a ch ieved in t h e m ixt u r es wit h 10% a ddit ives. Th is figu r e fu r t h er sh ows t h a t t h e bu lk den sit y of H AP + 10% P bO 2 h a s declin ed a ft er 1000 °C, wh ich is r esu lt ed fr om fa st er r a t e of m a ss loss com pa r ed t o t h e r a t e of den sifica t ion . Th is m a y be in dica t ive of fa st er ch em ica l r ea ct ion occu r r in g t h er e. At t h e en d of sin t er in g, t h e bu lk den sit ies for H AP wit h 10% of a ddit ives a ppea r t o be ver y close t o t h eir r espect ive bu lk den sit ies a t 400 °C.

Co n c lu s io n s

Th e sin t er in g a n d den sifica t ion beh a vior of H AP a n d the effect of addition of PbO 2 and Pb(NO 3 ) 2 were studied u sin g su r fa ce a r ea r edu ct ion a n d sh r in ka ge (den sificat ion ) a s in dica t or s. Th e su r fa ce a r ea r edu ct ion of t h e pu r e H AP wa s fou n d t o be dr iven by su per ficia l a n d volu m et r ic diffu sion du r in g sin t er in g wit h ou t den sifica t ion a n d sin t er in g wit h den sifica t ion , r espect ively. Addit ion of 2% P bO 2 a n d P b(NO 3 ) 2 br ou gh t a bou t su bst a n t ia l su r fa ce a r ea r edu ct ion du r in g den sifica t ion pr ocess a n d a lso lower ed t h e t em per a t u r e a t wh ich sin t er in g wit h den sifica t ion com m en ced. Th e h igh er con cen t r a t ion of P bO 2 a n d P b(NO 3 ) 2 fa iled t o en h a n ce t h e degr ee of den sifica t ion pr ocess a lt h ou gh t h e effect on t h e su r fa ce a r ea r edu ct ion wa s sign ifica n t . H en ce, it wa s fou n d t h a t t h er e is n o st r a igh t for wa r d cor r ela t ion between surfa ce a rea reduction a nd the volume shrinka ge. Th e r a t e of den sifica t ion of pu r e H AP h a d on ly on e pea k, wh er ea s t h e m ixt u r e h a d t wo or m or e pea ks in dica t in g t h a t t h e a ddit ives br in g a bou t m u lt iple speeds in t h e den sifica t ion pr ocess. (2) Su zu ki, T.; H a t su sh ika , T.; H a ya ka wa , Y. Syn t h et ic h ydr oxya pa t it es em ployed a s in or ga n ic ca t ion -exch a n ger s. J . Ch em . S oc., Farad ay T ran s. 1 1981, 77, 1059.

Lite ra tu re Cite d

(3) Su zu ki, T.; H a t su sh ika , T.; Miya ke, M. Syn t h et ic h ydr oxya pa t it es a s in or ga n ic ca t ion exch a n ger s. P a r t 2. J . Ch em . S oc., Farad ay T ran s. 1 1982, 78, 3605.

(4) Su zu ki, T.; Ish iga ki, K.; Miya ke, M. Syn t h et ic h ydr oxya pa t it es a s in or ga n ic ca t ion exch a n ger s. P a r t 3. E xch a n ge ch a ra ct er ist ics of lea d ion s (P b 2+ ). J . Ch em . S oc., Farad ay T ran s. 1 1984, 80, 3157.

(5) Xu , Y.; Sch wa r t z, F . W.; Tr a ln a , S. J . Sor pt ion of Zn 2+ and Cd 2+ on h ydr oxya pa t it e su r fa ces. (8) Boisson , J .; Ru t t en s, A.; Men ch , M.; Va n gr on sveld, J . Evaluation of hydroxyapatite as a metal immobilizing soil additive for t h e r em edia t ion of pollu t ed soils. I. In flu en ce of h ydr oxya pa t it e on m et a l exch a n gea bilit y in soil, pla n t gr owt h a n d pla n t m et a l a ccu m u la t ion .

( 1 )

 1 Ma , Q. Y.; Tr a in a , S. J .; Loga n , T. J .; Rya n , J . A. E ffect s of a qu eou s Al, Cd, Cu , F e (II), Ni a n d Zn on P b im m obiliza t ion by h ydr oxya pa t it e. E n viron . S ci. T ech n ol. 1994, 28, 1219.

  E n viron . S ci. T ech n ol. 1994, 28, 1472. (6) Ma vr opou los, E .; Rossi, A. M.; Cost a , A. M.; P er ez, C. A.; Mor eir a , J . C.; Sa lda n h a , M. St u dies on t h e m ech a n ism s of lea d im m obiliza t ion by h ydr oxya pa t it es. E n viron . S ci. T ech n ol. 2002, 36, 1625. (7) Ma , Q. Y.; Tr a in a , S. J .; Loga n , T. J .; Rya n , J . A. In sit u lea d im m obiliza t ion by a pa t it e. E n viron . S ci. T ech n ol. 1993, 27, 1803.

  E n viron . Pollu t. 1999, 104, 225. (9) La per ch e, V.; Tr a in a , S. J .; Ga dda m , P .; Loga n , T. J . Ch em ica l a n d m in er a logica l ch a r a ct er iza t ion s of P b in a con t a m inated soil: Reactions with synthetic apatite. Environ. S ci. T echnol. 1996, 30, 3321. (10) Ch lopecka , A.; Adr ia n o, D. C. In flu en ce of zeolit e, a pa t it e a n d F e-oxide on Cd a n d P b u pt a ke by cr ops. S ci. T otal E n viron . 1997, 207, 195. (11) Ar n ich , N., L.; An h er s, M.-C.; La u r en sot , F .; P odor , R.; Mon t iel, A.; Bu r n el, D. In vit r o a n d in vivo st u dies of lea d immobilization by synthetic hydroxyapatite. Environ. Pollut. 2003, 124, 139. (12) Nzih ou , A.; Sh a r r ock, P . Ca lciu m ph osph a t e st a biliza t ion of fly a sh wit h ch lor ide ext r a ct ion . Waste M an age. 2002, 22, 235. (13) Ir et ska ya , S.; Nzih ou , A.; Za h r a ou i, C.; Sh a r r ock, P . Met a l lea ch in g fr om MSW fly a sh befor e a n d a ft er ch em ica l a n d t h er m a l treatments. E n viron . Prog. 1999, 18, 144.

Figu re 15 .

 15 Figu re 15. E n vir on m en t a l sca n n in g elect r on m icr ogr a ph s for H AP wit h 10% P bO2 a n d P b(NO3)2 sin t er ed a t differ en t t em per a t u r es for 2 h.

  4.1.1 S in terin g. Th e sin t er in g beh a vior of H AP wa s st u died 10 Ca (NO 3 ) 2 + 6 (NH 4 ) 2 HPO 4 + 8 NH 4 OH f Ca 10 (P O 4 ) 6 (OH ) 2 + 20 NH 4 NO 3 + 6 H 2 0 (1)

	dS dt	) -KS n	(2)


S Iretskaya

A Nzihou

C Zahraoui

P Sharro

Metal Leaching from MSW Fly Ash Before and After Chemical and Thermal Treatments

Metal leaching from MSW fly ash before and after chemical and thermal treatments

S Iretskaya, Ange Nzihou, C Zahraoui, P

Municipal Solid Waste (MSW) incineration fly ash was collected in the industrial facilities in the city of Toulouse, equipped with recent gas s c rubbing equipment which collects daily 15 t of lime treated fly ash. The fly ash contains a large pro p o rtion of water soluble chlorides, besides tra c e elements such as iron, zinc, titanium and copper. Pre s e n t l y, the fly ash is landfilled after a cement solidification process. Matrix stability and leaching behavior of heavy metals (Cd, Cu) from samples of doped MSW fly ash we re examined before and after chemical and thermal tre a t m e n t s . A new phosphate process leading to hyd roxylapatite formation is described. To evaluate it's perf o rmance as a stabilization process, cupric and cadmium ions we re added as typical pollutant tracers and their distribution was studied as a function of different treatment para m e t e r s . The hyd roxylapatite process used effectively re m oves chloride ions in the water extract and retains most of the added metal ions in the solid residues. Evaluation of this pro c e d u re as a cold process shows 50% reduction in leachable metal ions. Calcination of the solid residues at 900°C results in agglomerated particles with increased resistance to metal ion dissolution. This is attributed to heavy metal incorporation in the h yd roxylapatite matrix formed during the thermal treatment. Thus, combined chemical and thermal treatments are efficient for heavy metal stabilization in MSW fly ash.

I N T R O D U C T I O N
Management of municipal solid waste (MSW) re q u i res the con side ration an d u n derstand in g o f man y d ive rse e n v i ronmental and ecological impacts that result from the ultimate disp osal of these wastes. Landfilling is the most expedient, inexpensive disposal method for MSW; however, g roundwater pollution from landfill leachate is of concern, as a re gas emissions. Furtherm o re, any toxic products present in the original waste will remain in unknown, uncontrolled form in the landfill. Legislation in France calls for the ban of standard landfilling sites in the year 2002. Only non-recyclable waste, socalled "ultimate waste", will be admitted in authorized, wellequipped sites. To meet these re q u i rements, recyclable MSW will have to be removed in an efficient and cost-eff e c t i v e m a n n e r. O ne me thod of o rganic waste sep aratio n is combustion. This process recycles or converts waste to steam or energy. The non-combustible residues from this process are classifie d as bo ttom ash (w hich contains th e ferro u s , magnetically removed metals) and fly ash. Comparative studies of bottom and fly ash have shown that fly ash is enriched in m o re volatile metals such as mercury, lead and cadmium [1,2]. The bottom ashes are generally considered to be suitable for use as an additive in concrete, or for use as roadbase material. Contamination of fly ash with toxic heavy metals is contro l l e d by the volatility of these metals and their chlorides at high t e m p e r a t u re and their effective precipitation and retention in gas scrubbing equipment. Demanding atmospheric emission s t a n d a rds have led to efficient neutralization techniques for h y d rochloric acid and heavy metals contents. The pro c e d u re usually involves lime injection, with sodium carbonate as an a l t e rnative [3].

As an alternative method to the actually used cement solidification process prior to landfilling, we have developed a new process consisting in a phosphate chemical tre a t m e n t followed by a thermal treatment. The phosphate treatment is an aqueous sol-gel process which produces agglomerated fly ash particles and extracted chlorides. The thermal tre a t m e n t co nsists, after drying at 60°C, in calcin ing th e fly ash agglomerates in air at 900°C.

We have added small amounts of heavy metals (Cu, Cd) to generate doped fly ash in order to study matrix stability and leaching behavior of treated samples. Our main objective was to determine whether or not the chemical treatment, alone or combined with a thermal treatment, stabilizes the residues by d e c reasing the amounts of metal ions released in leaching tests carried out with water and complex forming solutions.

M ATERIA LS AND METHODS

1-Sample collection. The fly ash used in this study was collected as a homogeneous 10 kg sample from the municipal waste incinerator in Toulouse. This facility burns 450 tons of waste daily and produces 15 tons of fly ash. The stack scrubber was installed in 1994 and includes a lime injection unit followed by filters which collect the fly ash. Gases are then sent thro u g h a gas washing unit which collects residual hydrochloric acid and heavy metals. The process water is treated, and filter cake residues are produced. These cakes were not included in the p resent study.

2-Sample composition. The collected fly ash contains calcium, silica, alumina and chloride ion s as majo r co nstituents characterized by energy dispersive X-ray analysis (EDX).Major elements include, by weight : Ca (8.2%), Na (3.1%), and Cl (9.0%) ; the trace elements include Cu (0.08%) and Cd (0.02%).

3-Process description and experimental pro c e d u re . In a first series of experiments, 1 kg of fly ash and 2 l of distilled water were i n t roduced in a stirred tank reactor of 10 l capacity. 10 g of copper or cadmium was added in the form of a concentrated solution. To homogenize the mixture, the suspension was s t i r red 30 minutes at room temperature (20°C). The suspension was filtered and the cakes were air-dried in a Jouan oven at 60°C. The process waters were sampled for chemical analysis. The solid residues were sampled as 100 g aliquots and calcined in sintered alumina crucibles under air atmosphere at 900°C in a T h e rmolyne muffle furnace. In a second series of experiments, to a mixture containing1 kg of fly ash and 2 l of distilled water and 10 g of copper (or cadmium) was added 20 g of Ca(OH) 2 and 60 g of H 3 P O 4 . The suspension was stirred 30 minutes and t reated as described above. The various steps of the process are illustrated in F i g u re 1.

4-Qu a n t i t a t i ve chemical analysis. 2.0 g samples were digested 15 minutes with 30 ml concentrated nitric acid under reflux. The suspensions were cooled and filtered. The filtrates were diluted to 100 ml and examined for various elements. The chloride contents were analyzed by a silver nitrate titration method using potentiometric end point determ i n a t i o n .

5-Leaching tests. Experiments were carried out using water or 1% EDTA or tartaric acid solutions with a liquid to solid ratio of 200 ml of solution for 100 g of solids. The suspensions were s t i r red for half an hour and filtered on Buchner funnels to sep arate the solid and liquid phases. Leach liquors were analyzed for Cu, Cd, Ca, Na and Cl. The solids were analyzed for the same elements following nitric acid digestion.

6-Analytical techniques. Atomic absorption measurements were made with a Unicam model 919 spectrophotometer using airacetylene and multielement discharge lamps for calcium, co pp er and cad miu m. A Co rn ing mo del 410 emission s p e c t rophotometer was used with air-natural gas to determ i n e sodium concentrations. A Jeol scanning electron micro s c o p e was used to observe the samples, which were analyzed with no s p u t t e red coating by spreading small samples of powder on conductive tape. Identification and semi quantitative analysis w e re perf o rmed with a Link analytical EDX analyzer. A Sieff e r t XRD diffractometer was used to obtain powder X-ray diff r a c t i o n p a t t e rns for the phosphate treated fly ash and a commerc i a l h y d roxylapatite re f e rence sample.

R E S U LTS AND DISCUSSION

The analytical results concerning the fly ash used in our experiments show that copper and cadmium are present in negligible amounts. 1% of copper was added as a typical transition metal and 1% of cadmium as a re p resentative heavy metal [4]. The presence of these metals in easily quantified concentrations helps to study their leaching behavior.

Distribution of copper and cadmium: Table 1a presents the results obtained when fly ash spiked with 1% copper is tre a t e d with water. 100% of the copper remains in the solid phase. On the other hand, the majority of soluble chloride salts are extracted by water, as has been previously reported [5] The insolu bility of co pp er may simp ly be exp lain ed by the E D TA or tartaric acid, both adjusted to pH= 6 to accelerate the leaching process, removed partly cupric ions from the solid phase (from 41 to 66 % of the copper contents). Similar re s u l t s w e re observed at pH= 8, showing that dissolution is related to metal-ligand complex formation, as could occur with org a n i c material in the enviro n m e n t . Table 1b presents the distribution of cadmium ions in solid and liquid phases following analogous treatments. In this case, very little cadmium is soluble at high pH, which may be acco un ted for by the fo rmation o f sp aringly so lu b le h y d roxycadmiates and insoluble cadmium hydroxide. These results show that the distribution of metals depends on the n a t u re of the considered elements, its capacity to interact with o rganic ligands or hydroxide ions to form stable, soluble or insoluble complexes.

Calciu m p h osp hate treatme nt: Whe n calcium and orthophosphate ions are mixed in solution at neutral pH, a gel f o rms which may precipitate hydroxylapatite when the pro p e r calcium to phosphorous ratio is present. Reaction 1 occurs to completion because of the very low solubility product of h y d roxylapatite [6].

(1) By isomorphic substitution, metal ions may be incorporated in the solid matrix according to reaction 2.

(2) F i g u re 2 c o m p a res the powder diffraction pattern of a re f e rence hydroxylapatite to that obtained for the fly ash tre a t e d by the previously described phosphate process. Hydro x y l a p a t i t e is identified in the treated fly ash residues by the presence of the characteristic diffraction peaks for interplanar distances of d= 2.81; 2.78 and 2.71 Å and labeled * in figure 2.

When a calcium p hosphate gel was formed, no major change was observed with respect to water solubility as shown in Table 2a. On the other hand, a significant decrease in cupric or calcium ion extraction by complexing ligands was observed, reaching over 50%. Interestingly, some cadmium is re l e a s e d near neutral pH following calcium phosphate treatment, as shown by the results presented in Table 2b. This may be related to the high concentration of chloride ions observed under our experimental conditions, leading to the dissolution of cadmiu m h yd ro xid e in the form of water solu ble c h l o rocadmiates. Once again, the main effect of calcium phosphate treatment is to reduce the calcium and heavy metal dissolution in leaching tests.

Heat treatmen t an d SEM-EDX evaluation. SEM-EDX examination of fly ash shows a heterogeneous mixture of Ca 10 (PO 4 ) 6 (OH) 2( s) + xCd (aq) Table 3 p resents the effect of calcination on heavy metal dissolution by aqueous ligands. A further significant decrease in copper and cadmium leachability is observed. This effect may be related to the incorporation of metal ions in a more crystalline and insoluble calcium phosphate matrix. Substitution of calcium by other metals accounts for the d e c reased solubility of heavy metals in the hydro x y l a p a t i t e s t r u c t u res [7,8]. The cadmium spiked untreated fly ash has a low concentration of leacheable cadmium (2%), possibly due to the loss of cadmium as volatile chloride during the calcination step. The treated fly ash shows higher retention of cadmium in the phosphate matrix during calcination. This leads to some cadmium release during the EDTA leaching test (19%).

2 + → Ca (1-x ) Cd x (PO 4 ) 6 (OH) 2( s ) + xCa (aq) 2 + 10Ca (aq ) 2 + + 6H 3 PO 4( aq ) + 2OH ( aq ) - → Ca 10 (PO 4 ) 6 (OH) 2( s)

C O N C L U S I O N

Chemical treatment of MSW fly ash by a calcium phosphate gel addition effectively reduces heavy metal ion solubility, particularly under aggressive chelating condition s. This c o n f o rms to present knowledge on heavy metal removal fro m contaminated soils [9]. Thermal treatment also significantly a ffects the solubility of metal ions with respect to chelating ligands. The observed decreases in metal ion leach ate concentrations may be attributed to the incorporation of the metals in a calcium phosphate matrix. The thermal tre a t m e n t s under mild conditions (900°C) generally avoid excessive metal loss by evaporation [10], but contribute to apatite crystallization, as confirmed by SEM and X-ray diffraction analysis.

The chemical treatment method proposed presents the advantage of eliminating some soluble chlorides combined with the advantage o f lead ing to a low temperature therm a l t reatment for the solidification-stabilization step. In order to optimize the best combination of chemical and therm a l t reatments, several p arameters need to be exp lored and include: heavy metal concentration, particle size control, drying technique, and temperature and duration of calcination, nature and quantities of phosphate additives. In parallel with this experimental approach, thermodynamic equilibrium and mass transfer of metal ions in mineral matrixes are being examined to evaluate the distribution of pollutants. These combined eff o r t s should yield new processes [11,12] for municipal solid waste incinerator fly ash disposal or re c y c l i n g .

A C K N O W L E G M E N T

We thank Dr. J.V. Lake of the European Enviro n m e n t a l R e s e a rch Organization for providing a re s e a rch fellowship to

F

  I G U R E 1. Chemical treatment diagram showing wet pro c e s s followed by filtration and further heat tre a t m e n t .TA BLE 1a . Be ha vio r o f Fly A sh Sp ik e d w ith Co p p e r of cupric hydroxide at the pH resulting from water t reatment (pH= 11.5). Further water extraction of the solid residues at neutral pH was unable to dissolve cupric hydro x i d e .

FIGURE 2 .

 2 FIGURE 2. X-ray diffraction p atterns of tre ated fly ash (b ) sh o wing m atch ing p eaks (*).an d (a) re f e re n c e hydroxylapatite.

TA BLE 3 .FlyF

 3 Effe c t o f Ca lc ina tio n o n % Me ta l Extra c te d in Le a c hing Te sts I G U R E 3 . SEM pictures of fly ash (top), treated fly ash (middle) and treated and calcined fly ash (bottom). TA BLE 2b . Be ha vio r o f Ca lc ium Pho sp ha te Tre a te d Fly Ash with Ca d m ium

L I T E R ATURE CITED

1 . Ki r by, C.S.; Rimstidt J.D., "Mineralogy an d Su rf a c e P roperties of Municipal Solid Waste Ash," En v. Sci. Te c h n o l . , 2 7 , 652-660 (1993). 2 . Ei g h m y, T., Eusden, J., Krz a n owski, J., Domingo, D., St a m f l i , D., Ma rtin, J.


Cédric Verwilghen

Sébastien Rio

Ange Nzihou

Daniel Gauthier

Gilles Flamant

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

Cedric Verwilghen

A E Sebastien

Rio Ae

A E Daniel

Gauthier Ae

A E Patrick

Joel Sharrock

C Verwilghen

Á S Rio

Á A Nzihou

Á G Flamant

Preparation of high specific surface area hydroxyapatite for environmental applications

Introduction

There is much interest in the synthesis of hydroxyapatite (HA) for various purposes [1,[START_REF] Elliot | Structure and chemistry of the apatites and other orthophosphates[END_REF]. Synthetic HA is a calcium phosphate with close resemblance to the mineral phase of bone. Major efforts were devoted to the elaboration of bone substitutes based on artificially prepared calcium phosphates with high purity as required for medical purposes [3,4]. It has long been known that HA has a large capacity for element substitution and this has triggered research on the use of HA for water purification, for waste stabilization, contaminated soil remediation, and heavy metal adsorption in general [5][6][7]. For environmental uses, the phase purity, crystalline structure and elemental impurities are not as much of a concern as are the cost of production and the heavy metal exchange capacity. For widespread use, natural apatites available in large quantities such as phosphate rock and natural fish bones have been investigated and subjected to field trials [START_REF] Conca | Treatment of metal-contaminated leachates utilizing fish bones and fish hard parts[END_REF]. However, naturally occurring substances vary in nature and cannot have well tailored properties. This led us to seek a synthetic pathway for the production of HA which would be efficient, simple and yield a product with high reactivity towards heavy metals. In particular, our goal was to make a new adsorbent based on synthetic HA with large specific surface area for use in gas scrubber equipment. With this objective in mind we developed a low temperature precipitation reaction for HA in aqueous medium. This contrasts with the traditional methods of HA syntheses which always use heat, to warm the reaction solution or eliminate the nitrate by-products during calcinations [3,4]. Compared to the usual synthesis methods, the novelty resides in the use of open vessels under air atmosphere and room temperature, which allows convenient production of large quantities. We present here the characteristics of the resulting HA as a function of reaction conditions.

Materials and methods

Two HA (Ca 10 (PO 4 ) 6 (OH) 2 ) powders were prepared by wet methods using calcium nitrate [3,4] or lime [START_REF] Osaka | [END_REF] as source of calcium. In the first case, a monoammonium phosphate solution (NH 4 )H 2 PO 4 was mixed with a calcium nitrate solution, and the pH was maintained basic by addition of NH 4 OH.

In the second case, phosphoric acid was added to a Ca(OH) 2 solution. The H 3 PO 4 addition rate was controlled using a peristaltic pump. The suspensions were continuously stirred at room temperature. After complete addition, the suspensions were matured for defined time spans (from 12 h to 120 h) at various low temperatures (25, 40 and 75 °C). Finally, the suspensions were washed, filtered and the resulting solids air dried at low temperatures under different conditions. Powders were obtained by ball milling and sieving to collect fractions below and above 80 lm.

More specifically, a laboratory batch was made using 210.63 g of calcium nitrate in 500 mL of water. The pH was adjusted to 10.7 with aqueous ammonia and the solution diluted to 1 L volume. Similarly, 67.66 g of ammonium nitrate at pH 10.3 was dissolved in a total volume of 1.5 L, and added to the calcium solution with stirring. This reaction was also scaled up by a factor of 20 in a 100 L reactor.

Another laboratory batch was made as follows: 77.18 g of calcium hydroxide were suspended in 2.5 L of demineralized water. 40 mL of 85% phosphoric acid were introduced over a 15 mn period while stirring.

Powders were analyzed by X-ray diffraction (XRD) with a Philips Xpert working at 40 kV and using the CuKa radiation (1.5406 A ˚). XRD diagrams were recorded in the interval 20°< 2h < 55°.

The specific surface area was measured by BET method using nitrogen adsorption at 77 K with a Micromeritics Vacrep 061. Powders were previously degassed at 100 °C under reduced pressure (50-100 mbar).

A Malvern Mastersizer 2000 was used to determine particle sizes and a Helium Accupyc 1330 from Micromeritics used for density measurements.

Results and discussion

Hydroxyapatite synthesis

There are two simple routes for the synthesis of HA in aqueous media. The standard method consists in precipitating HA under basic conditions from soluble calcium and phosphate salts. This is conveniently realized with calcium nitrate and ammonium phosphate, and by adjusting the pH with base as summarized in Eq. ( 1). The second method uses an acid-base reaction between phosphoric acid and calcium hydroxide, as shown in Eq. [START_REF] Elliot | Structure and chemistry of the apatites and other orthophosphates[END_REF].

10 Ca NO 3 ð Þ 2 ,4H 2 O þ 6 NH 4 H 2 PO 4 þ 14 NH 4 OH ! Ca 10 PO 4 ð Þ 6 OH ð Þ 2 þ 20 NH 4 NO 3 þ 52 H 2 O ð1Þ
10 Ca OH ð Þ 2 þ 6 H 3 PO 4 ! Ca 10 PO 4 ð Þ 6 OH ð Þ 2 þ 18 H 2 O ð2Þ
The problem with Reaction (2) is the difficulty in dissolving the calcium hydroxide and the rate of introduction of phosphoric acid which must remain slow enough to prevent from acidifying the reaction solution, which would result in the formation of calcium hydrogenophosphate. The main advantage is the absence of by-products. On the other hand, the precipitation is well controlled by the rate of addition of base in Reaction ( 1), but large amounts of nitrate salts are formed which must be water-washed to yield an impurity-free product.

Both reaction schemes were studied in parallel to compare the physico-chemical characteristics of the HA produced. Small laboratory samples were made, as well as larger 5 kg lots using a 100 L reactor. In every case, with proper mixing, the reaction proceeded smoothly and the formation of a white gelatinous precipitate was observed. X-ray analysis of the collected, washed and dried precipitates show that hydroxyapatite was formed, with low crystallinity. Following sintering at high temperature, Xray diffraction confirms the presence of HA (JCPDS 9-432) with the presence of trace amounts of CaO (JCPDS 37-1497) (see Fig. 1).

SEM observations reveal the small grain sizes of the precipitates, less than 0.1 micrometer, while the sintered product shows larger sizes, near 0.5 micrometers as illustrated in Fig. 2.

Reaction conditions and specific surface areas

The effect of reaction times was investigated first because of its known influence on HA phase purity [10,11,12]. Results are presented in graphical form in Fig. 3. It appears that the calcium phosphate precipitates obtained have specific surface areas (SSA), which increase during the first two days of maturation and then stabilize between 120 m 2 / g and 160 m 2 /g. The effect of reaction temperature is reported in Table 1 for the 48 h reactions. No significant effect on resulting SSA is observed due to temperature, but the preparation starting from the soluble calcium nitrate reached slightly higher SSA.

The influence of mixing speed was investigated by varying stirring conditions from 100 to 200, 300, 400 and 500 rpm. The results are presented in Fig. 4 and show that under our experimental conditions, 300 rpm stirring yielded the highest SSA with a more pronounced effect for the calcium nitrate reaction.

One laboratory batch made with calcium nitrate was dried at various temperatures and the resulting SSA mea-sured. Results presented in Fig. 5 pertain to a temperature interval where sintering does not yet occur [13]. Clearly, heating decreases the SSA and low temperature drying favors large SSA. Longer drying times (up to 24 h depending on sample size) are needed at low temperatures and large amounts of water, reaching 77% by weight, are observed.

Following synthesis and maturation, the standard procedure is to dry and grind the solids to obtain fine powder. An alternative spray drying process was tried to evaluate its effect on SSA. The results are gathered in Table 2. Basically, similar SSA values were obtained for spray drying and for air drying. Spray drying is interesting because it produces small monomodal spherical particles rapidly [START_REF] Luo | Methods of synthesizing hydroxylapatite powders and bulk materials[END_REF]. All the particle densities measured were similar and near 2.70 ± 0.02 g/cm 3 . The sprayed particles had lower diameters than the ball-milled ones.

Two samples obtained with standard air drying were followed as a function of time, while maintained in sealed screw capped vials at room temperature. The graph of the SSA as a function of time shows a progressive decrease of the SSA, which tends to stabilize near 100 m 2 /g following a 21 months period (see Fig. 6). Identical SSA results were observed with the larger scale batches made.

Hydroxyapatite formation mechanism

Our results can be interpreted as showing that submicron sized particles precipitate out of solution when calcium ions are mixed with orthophosphate ions at basic pH. The colloidal particles are heavily impregnated with water and form a gel type material which dries slowly at room temperature. The composition of the kinetic product of the precipitation is in fact tricalcium phosphate (TCP) [10]. The thermodynamic product, HA, forms slowly thereafter during maturation according to the stoechiometry of Eq.

(3).

3 TCP þ Ca OH ð Þ 2 ! HA ð3Þ
The agglomeration of TCP precipitates eventually forms the HA particles, thus explaining the required maturation step. This can be accelerated by heating, but is not necessary. The formation of HA corresponds in fact to the incorporation of lime, and this can have an effect on suspension pH. Excess calcium hydroxide contributes to HA formation but may lead to non-stoechiometric HA with some calcium oxide present in the final product. Once the HA particles are formed, they will stick together to form larger crystallites, whether they are in suspension or in the solid state. Thus, too much stirring decreases the SSA, and heating also contributes to particle consolidation and SSA reduction. Low temperature synthesis and maturation is a viable route to high SSA HA which is much simpler than the emulsion based methods described in the literature for obtaining nanosized HA crystallites [START_REF] Wu | [END_REF]16].

Conclusion

Synthetic HA with SSA well above 100 m 2 /g may be obtained by purely aqueous precipitation route starting from widely available reactants. Best conditions include gentle stirring at room temperature, 48 h maturation, followed by

Fig. 1 X

 1 Fig. 1 X-ray diffractogram for HA powder calcined at 1,000 °C

Fig. 4 Fig. 5

 45 Fig. 4 Influence of stirring velocity on Specific Surface Area

Table 1

 1 Specific Surface Area as function of synthesis temperature

	(m 2 /g) ± 6%
		25 °C	4 0 °C	7 5	°C
	Ca(NO 3 ) 2	147	166	156
	Ca(OH) 2	124	132	110

Table 2

 2 Specific Surface Area as function of drying step (m 2 /g) temperature drying or rapid spray drying. There is a 20% loss in the SSA of HA powders during a 1 year period, but all evidence shows that the particles maintain sufficient reactivity for use in adsorption reactions, and particularly heavy metal ion exchange and stabilization. Further work on this subject is in progress and shows promising results.

	Drying	Humidity	Specific	Density	Median
		(%) ± 0.2	Surface	(g/cm 3 )	diameter
			Area (m 2 /g)	± 0.01	(lm) ±
			± 6%		0.5
	Ca(NO 3 ) 2 Atomization	4.8	128	2.68	2
	(3 bars)
	105 60 °C then °C	3.0	122	2.69	15
	105 25 °C then °C	3.0	147	2.69	7
	Ca(OH) 2 Atomization	12.8	130	2.69	4
	(3 bars)
	Atomization	11.9	118	2.70	4
	(4 bars)
	105 60 °C then °C	2.5	114	2.70	12
	105 25 °C then °C	3.0	124	2.72

low

Acknowledegments We gratefully acknowledge help from Solvay HSE (Belgium) in the form of a research stipend. Our gratitude goes to ADEME (French Environment Agency) for funding this research under the grant No 02-74-111.