Indian Journal of ChemistryVol. 32A, June 1993, pp. 525-530

A new method of preparation of solidsolutions of calcium-cadmium-Iead

hydroxylapatite and their characterisationby X-ray, electronmicrography and IR

spectra

P P Mahapatra" & D S SarangiPost Graduate Department of Chemistry, Gangadhar Meher

Autonomous College, Sambalpur 768 001, Indiaand

Bagmi MishraR D Women's College, Bhubaneswar

Received 12 May 1992; revised and accepted 5 October 1992

Solid solutions of calcium-cadmium-lead hydroxy-lapatites have been prepared in aqueous media at pH -12. The samples have been characterised by analysis,X-ray, electronmicroscopy and infra red spectra. Thesite occupancy of the cations in the apatite lattice and or-der-disorder phenomena has been discussed. Existanceof hydrogen bond mechanism originating from OHstretching frequency in the IR spectra of the apatites isdiscussed.Tt is observed that for high values of substitu-tion of cations, the OH group frequency becomes veryweak due to the progressive decrease in the polarisationpower of calcium, cadmium and lead. The point group ofthe tetrahedral PO~- ion is found to depend on the pro-portion of the cation substituted in calcium hydroxylapa-tite.

Calcium hydroxylapatite, CalO(P04)6 (OH}z, CaHAexists in nature as mineral hydroxylapatite. It is simi-lar if not identical to bone mineral and is a hospit-able one allowing various ion exchange reactions init. Lead and cadmium, the toxic heavy metal pollut·ants, are absorbed in the body 1.2 and are incorporat-ed into the bones through Ca2+ -. M2+(M2+ =Cd2+and Pb2 +) exchange reaction consequent upon theproximity of their ionic radii' (Ca2+ = 0.99A,Cd2+ =0.96A, Pb2+ = 1.20A), leading to painfulbone diseases'? and accelerating the oesteoporaticprocess", Besides, incorporation of cadmium andlead in tooth enamel results in dental caries 7.

Cadmium hydroxylapatite, CdlO(P04)tt(OHjz,CdHA andlead hydroxylapatite, PblO(P04)6(OHjz.PbHA, result by the complete substitution of Ca-"by Cd2+ and Pb2+ 1n CaHA. The earlier studies'" '?

on CaHA, CdHA and PbHA and their X-ray inves-tigations indicated that they are isomorphous. Apossibility therefore exists that solid solution

amongst end members can be formed consequentupon coupled incorporation of both cadmium andlead in CaHA resulting in the formation of solid so-lution of general composition Caw - x- yCdxPby(P04)6(OH}z. The ultimate site ofthe incorporat-ed coupled cations in the apatite structure is, there-fore, of great interest. Since no investigation hasbeen made in this direction, as a part of more specif-ic programme of research in this field, we reporthere a new method of preparation of solid solu-tions of calcium-cadmium-lead hydroxylapatite,CaCdPbHA in basic aqueous media and their phy-sico-chemical characterisation by analysis, X-ray,IR and electronmicroscopic technique.

ExperimentalThe samples were synthesised according to the

following equation:(lO-x-y)Ca2+ + x Cd2+ +y Pb2+ +6 PO~-+ 2 OH- -'C~IO-x_y)CdxPblP04)6(OHhSolutions of diammonium hydrogen phosphate andthe mixed cation solution in the form of their ni-trates prepared in CO2-free conductivity 'water asdesired by the above stoichiometry were maintainedat pH"'" 12 with ethylenediamine and were simul-taneously added dropwise (1 rnI.min - I) to 2 litres ofboiling conductivity water maintained at pH - 12 ina 4 litre flask. Nitrogen gas was bubbled to maintainan inert atmosphere. The precipitate was aged for 2hr under reflux, left overnight to improve the homo-geneity and sample size, centrifuged and washedrepeatedly with a 2% solution of EDTA followed byconductivity water and dried at 200°C for 24 hr.

The samples were analysed complexometrical-lyl3-'5. From the results of the estimation the weightper cent errors were found to be: Ca: ± 1%; Cd:± 0.5%; Pb: ± 0.5%; and.P: ± 1%.

The X-ray diffractograms of the samples were re-corded using Phillips X-ray diffraction camera withNi-filtered Cu - K u radiation. The IR spectra of thesamples heated at 600"C were recorded on a Perkin-Elmer 137E spectrophotometer using KBr pellettechnique. The electronmicrographic patterns wereobtained on a Akashi ISI-DS-130 scanning elec-tronmicroscope.

Resultsand discussionThe results of chemical analysis of the samples

synthesised are given in Table 1. The

526 INDIAN] CHEM, SEC A, JUNE 1993

Table I-Chemical analyses of samples of solid solutions of calcium-cadmium-Iead-bydroxylapatitiesMolecular formulae Wt% (Expt/Theo) (Ca +Cd +Pb }/I;'

g.atom ratio

CaO CdO PbO P20S

CaHCd.Pb,(P04)6(OHh 36.44 10.42 17.62 34.14 1.68(36.06) (10.32) {17.94} (34.22)

Ca7Cd .Pb2(P04)6(OHh 28.59 8.98 31.24 30.05 1.69(28.06) (9.09) (31.63) (30.17)

Ca6Cd.PbJ(P04)6(OH)2 21.44 8.02 42.39 26.89 1.67(21.31 ) (8.13) (42;42) (26.97)

Ca5Cd.Pb4(P04)6(OH)2 16.25 7.31 50.88 24.42 1.67(16.06) (7.35) (51.14) (24.38)

Ca4Cd .Pb5(P04)6(OH)2 12.31 6.40 58.01 22.45 1.67( 11.72) (6.71) (58.35) (22.26)

Ca,Cd.Pb6(P04)6(OH), 1l.41 5.77 64.71 20.38 1.68(8.0S) (6.17) (64.39) (20.47)

Ca ,Cd. Pb7( PO 4),,( OH), 5.04 5.64 69.73 18.97 1.67(4.99) (5.71) (69.53) (18.95)

Ca.Cd .Pb,(P04),,(OH), 2.24 5.13 74.{)1l 17.49 1.67(2.32) (5.31 ) (73.94) (17.61}

Ca,Cd,Pbl(P04),,(OH), 311.79 19.25 15.61 32.62 1.67(29.81) (19.41}) (16.1}5) (32.32)

Ca"Cd,Pbl(P04J,,(OH I: 25.23 211.23 14.50 30.73 1.(1)(24.22) (27.73) (16.07 ) (311.66)

Ca4Cd,Pb,( PO 4 ;,,( OH I: 13.44 21.111 31).05 24.53 1.(1)( 13.01 ) (22.34) (311.S4) \24.71 )

Ca,Cd4Pbl (P04)6(OH)2 11).34 34.1)1l 15.00 21).04 1.07(11}.19) (35.15) (15.27) (21).14)

Ca:Cd4Ph4( P04),,(OHJ: 6.16 20.31 44.63 21.1)3 1.00(5.96) (26.16) (45.411) (21.61})

Ca4Cd,Ph1IP04),,(OHI, 14.57 41.71 14.511 27.M 1.07(14.62) (41.115) (14.55) (27.76)

Ca,Cd,,PhIIP04),,(OHJ: 111.51 4ll.15 13.1}5 26.50 1.07\111.47) (47.1}7) ( I3.I}O) \26.52)

CaICd,,Pb,(P04),,(OH), 3.112 31).71) 34.35 21.1)2 1.67(2.S7) (31).61}) (34.50j (21.1}6)

(ia,Cd7Phl( P04J,,(OH), o.llo 53.1)1 12.113 25.27 1.611(6.611) (53.54) (13.29) (25.36)

CaICd,Ph1lP04),,(OH), 3.36 51).04 12.04 24.20 1.67(3.20) (511.66) (12.74) (24.31 )

(Ca + Cd + Pb )/P g atom ratio in all the cases isfound to be close to the theoretical value of 1.667.The precipitation of tertiary phosphates of heavymetals in aqueous media was complicated due to co-precipitation of their hydroxides. Although PbHAcould be precipitated by complexing lead ions withtartarate ions 10, this method was found unsuitablefor preparation of solid solutions of CaCdPbHA,since calcium and cadmium tartarates are found tobe sparingly soluble in alkaline media. Lead andcadmium are known to form well defined com-plexes ofthe type Pben , and Cden. with ethylenedi-amine( en) having instability constants 3 x ]() - 'i and6.33 x \0 - :I at ordinary temperature respect-

ively'":", which, therefore prevents the formation ofhydroxides of lead and cadmium at the pH of thepreparation. However the simultaneous addition ofthe solutions to a large excess of boiling water bringsabout dissociation of the complexes making thecations available for the reaction.

The X-ray diffractograms of the samples (Fig. I)exhibited peaks characteristic of apatites belongingto P"l,!ill space group. Absence of additional diffrac-tion peaks in the pattern indicated the samples to besingle phased. The lattice constants, 'a' and 'c'(Table 2) determined from a least square fitting ofseveral peak positions indicated a contraction and,an expansion as well, with increase in proportions of

)

Fig. l-X-ray diffraction patterns of samples of (I)CaKCdIPbl(P04)6(OHh, (3)' Ca5CdIPb4(PO.)6(OH)2, (5)CaJCdI Pb6(P04)6(OHli, (7) Ca6CdJPbl(P04)6(OHh, (9)

Ca5Cd4Pbl(P04)6(OHh, (12) CalCdHPb I(PO.)6(OHh

cadmium and lead in the samples respectively. Thecia ratio decreased in samples containing cadmiumfrom 10 to XO mol percent. Figure 2, showed thatthe samples did not strictly obey Vcgard's law. Al-though, this did not reflect on the homogeneity inthe composition of the samples prepared over theentire compositional range it indicated a partial or-dering phenomena of the cations in the triangular,617 and column, 4(sites of the apatite lattice. The or-dering phenomena is reflected in the changes in therelative intensity of the reflections and their posi-tions in XRD spectra consequent upon the alter-

NOTES

H

H

•• ••"3.~••"0

••

010 JO 50 70MOlE .,. (Cd + Pb)

527

.~7.2 u

Fie. 2-Dependence of lattice constants versus compositions inthe samples

ations in the atomic distance. The changes in the linewidths were due to the induced structural disor-dered occupancy of the cations during coupled sub-stitution of lead and cadmium in CaHA. The cia va-lues in Table 2 decreased up to 60 mol°/..incorpora-tion of lead into CaHA and then increased. This in-dicated that during substitution partial ordering oc-curs, namely, the Cd ' + and Ph~+ ions get statistical-ly ordered over the 6h and then the 4fsites.

The smaller Cd that substitutes for Ca. probably,locates at the 6h cation sites since this site is small-cr'" of the two, 6h and 4fsites. Besides Ph has apreference for 6h than 4f, therefore, for the total 60mol% substitution of (Cd + Pb), will occupy the 611and the rest will then have a preferential occupancyat the 4f Hence while cia ratio decreased from sam-ple 1 to 5, it increased thereafter. With higher weightpercent of Cd in the samples (Cd I to Cd.}, however,the cia ratio in general deereased when comparedwith the end members.

The e1ectronmieroseopie patterns of the sampleswere found to be needle shaped, characteristic ofapatites. Absence of extraneous phases in the elec-tronmicrographs indicated that the samples weresingle-phased, pure and homogeneous.

The infra-red spectra of the samples of apatitesmay he interpreted in terms of site symmetry withthe spatial group P6, II" There are nine IR activehands for the tetrahedral ~hosphate internal vibra-tion-" nable .3). Due to variations in the composi-tional parameters of the samples the phosphatevibrations and the proportion of symmetry, besidesthe spectral positions, are altered. For substitutionof Cd and Pb in the ratio 1:1 in CaHA, three v,modes, three V~ modes, one VI mode and two V2modes are detected in the IR spectra. The PO~ - ion

52H INDIAN J CHEM, SEC A, JUNE 1993

Table 2-Lattice parameters 'a' and 'c', cell volume, density and OH ....O distance (d) of samples of CaHA, CdHA, PbHA and solidsolutions of CaCdPbHA

Molecular formulae Lattice constants cia Unit cell d-valuesvolume (N)

c(N) a(N) (N)J

Ca.Cd IPbl(PO.).(OH)l 6.84 9.435 0.724 527.30 3.104Ca.Cd IPbJ(P04).(OH)1 6.825 9.485 0.719 531.73 3.072Ca,Cd IPb.(PO.)b(OH), 6.797 9.537 0.712 535.37 3.072Ca.Cd IPb,(P04)b(OHh 6.77 9.595 0.705 539.75 2.965cs.ca !.~b.(PO4).(OH), 6.775 9.623 0.701 541.70 2.965CaICdIPbg(P04).(O~)1 7.07 9.717 0.727 578.09 3.11CabCd.lPbl(P04).(OH), 6.7R2 9.415 0.720 520.61 3.072Ca.CdJPbJ(PO.).(OHh 6.80 9.455 0.719 526.44 3.043Ca,Cd.Pbl(PO.).(OH)l 6.75 9.412 0.717 517.82 2.989Ca2Cd4Pb4 (P04)6(OHh 6.72 9.475 0.709 522.45 3.043Ca,Cd6PbJ(P04MOHh 6.70 9.416 0.711 514.43 2.965CaICdHPbl(P04).(OH), 6.oR 9.39 0.711 510.06 3.07Caw(PO.).(OH), 6.86 9.42 0.728 517.17 3.068Cdl\l(PO.).(OH), 6.ol~ <J.O 1 0.734 464.99 2.955Pbl\l(PO.).(OH), 7.141 9.~71 0.733 611.01 3.061

will belong either to C, or C2v punctual group de-pending on the number of detectable v2 modes,either 2 or I respectively. Since in this case two v2modes are .observed, the sample evidently belongsto C, symmetry group. For sample with Cd:Pb in theratio I :3. the relative intensities of the V3 vibration at1035 ern -I and v4 at 598 cm-1 have become veryweak. This can be interpreted by considering that agiven proportion of PO~- ions increases theirsymmetry from C, to C."" though the statistical aver-age still gives a higher proportion of C" It is how-ever observed that the symmetry group of the PO~-ion is maintained at C3vover the entire composition-al range thereafter.

The different symmetries of P04 group may berepresented by:PO-l = [k(PO-l)c. + 1(PO-lk, + m(PO-l)c.,l/6where k + 1+ m =6 and k, I, mare the' number ofPO-l groups with C" Czvand C3,.symmetry respect-ively. Thus in the case of all the samples I is almostequal to zero and for Cd:Pb = 1, k = 6; while forsamples with Cd:Pb other than 1:3, m = 6.

Considering the reference values of the OHstretching frequency" as 3644 em-I, the OH .....Odistances (d) of solid solutions of the mixed apatitesystem, CalO_x_yCdxPbiP04)6(OHh was calculat-ed22 from logd= log 3.35 - 1/6 log (~vs/50) where~ vs is the difference between the OH reference frc-quencyand vs the hydroxyl stretching frequency ofthe samples (Table 3). The equation relates to theshift of the stretching frequency with 0 ..... 0 dis-tance. d, in hydrogen bonds. It is observed that d ve-

lues ranged within 2.955 to 3.11 A for all the sam-ples (Table 2) including CaHA, CdHA and PbHA.This is, therefore, an evidence of hydrogen bondingmechanism in samples of CaCdPbHA prepared.Since in the lattice the probability of formation oflinear hydrogen bonding between OH group andeach of the three surrounding PO 4 groups is thesame, it could be inferred that the OH group in thesamples is situated parallel to the c-axis.

The intensity of the hydroxyl group stretching fre-quency decreased with increase in the proportion ofCd and Pb in the samples. The progressive decreasein the cation polarisation power in the orderCa < Cd < Pb correlates with and may contribute tothe possible OH ....OPOJ vibration and in turn tothe decrease in the hydroxyl band intensities.

The absorption between 220-250 ern - 1 in the IRspectra are assigned to the lattice vibrations. Thegroup of bands around 265-355 cm -I may be attri-buted to the M - 0 stretching (Ca - 0, Cd - 0,Pb - 0) vibrations. The oxygen atom of M - 0 bondis that of the OH group. The frequency in ern -·1 ofthe cation-oxygen vibrations and lattice vibrationbands of the samples are given in Table 4. Figure 3shows the region in whieh Ca - 0, Cd - 0 andPb - 0 bands appeared for samples 1, 2, 3, 6 and 11(as in Table 4). In most of the cases we have detectedbands within 265-355 ern" 1 for different propor-tions of Cd and Pb in the samples. The reducedmasses of Ca - 0, Cd - 0 and Pb - 0 increases inthe order !1-ca-O < !1-Cd--O < !1-Ph-O and hence theirlattice vibrations are expected to follow the reverse

NOTES 529

Table 3- Frequency in em - I and assignments of the bands in the IR spectra of samples of solid solutions of calcium-cadmium-leadhydroxylapatities

Molecular formulae P04(cm-l) Hydroxyl (ern - ')

V, v2 ·v.• v4 v. v,CaHCd,Pb.(P04).(OH)2 955 420 1020 555 3565

460 1045 5601085 590

Ca.Cd ,Pb)(PO .).( OHh 940 395 1010 545415 1035 585 3560

1075 598

CasCd,Pb4(P04MOHh 940 400 1025 545440 1040 560 3560 665

600

Ca.Cd,Pbs(P04).(OHh 940 410 990 565 3540 655420 1020 590465 1060

Ca)Cd ,Pb.(P04).(OHb 940 395 1005 540 3540 655436 1065 580

Ca,Cd ,PbH(P04l.(OHh 950 375 980 525 3566 655460. 1030 560

Ca.Cd)Pb,(P04J.(OH)2 945 445 1020 545 35601080 580

Ca4Cd)Pb)(P04J.(OHh 940 440 1020 542 3555 6501085 585

CasCd4Pb,(P04l.(OH)2 955 400 1010 55i 3545 6551040 5901080 600

Ca2Cd.Pb4(P04l.(OHh 380 990 535 3555 660405 1060 570

Ca,Cd.Pb .•(PO.J.(OH)2 950 400 1020 545 3540410 1080 570

CaICdKPb,(PO.l.(OHh 930 380 1025 550 3566 660405 1065 580

Table 4-Frequency in em - I of the cation-oxygen vibration and lattice bands of solid solutions of calcium-cadmium-lead hydroxyl-apatities

Molecular formulae Cation- PO, Metal-oxygen vibrations (M .... OH)lattice vibrations

Ca-O Cd-O Pb-OCaxCdIPb,(PO,l.(OHh 220.230.250 295 340 270Ca.Cd,Pb,(PO.l.(OH)2 220,230,250 355 260. 270CasCd,Pb4(P04).(OHh 210,230,242 300,320 355 260,21Wcs.ca ,Pb,(PO,).(OHl2 205,230.245 300.320 325 265.285Ca,CdI Pb,;(PO,J.(OHh 205,225.250 295,320 275CaICd,PbK(PO,).(OH)2 220,240,- 320 335 265Ca"Cd.1Pb,(PO,),,(OHh -,240,- 295 270Ca,Cd.lPb.l(PO,l.(OH)2 210,226.240 290 271Ca,Cd.Pb,(PO,).(OHh 205,21(), 232 282 350 275Ca2Cd.Pb,(PO,).(OH), 230, i35, 242 305 335 275Ca,Cd"Pb,(PO,J.(OHh 220,235,242 290,302 330 268,272CaICdliPb,(P04MOHh 220,230,245 290,31() 335 265

530 INDIAN J CHEM, SEC A, JUNE 1993

Fig. 3-Carion-oxygen stretching bands for sample No. I, 2, 3,6 and II of Table 4

order. But the theoretical Cd - 0 frequency (334cm - ') is found to he greater than Ca - 0 (322cm - ') and Pb - 0 (262 em - , ) consequent upon thegreater force constant value of Cd - 0 (9.22 X 10-1dynes/em) in the solid solutions, and hence is thebasis of our indexing the vibrations in the IR spect-ra.

The coupled cation substitution in CaHA affectedthe shape and shifted the IR vibration to lower ener-gy side. This is due to the contraction in the distancebetween the P04 groups in the lattice and the intro-duction of covalent character consequent upon thegreater polarizability of Cd and Pb than Ca in theapatite lattice. In all systems the V3 vibrations be-come broader. The various crystal fields acting onthe stretching vibrations of the phosphates bringabout a mixture of the bands and hence the trans-itions become less clearly defined. In the mixed apa-tite system the appearance of this spectra is mostprobably due to the stronger distortion of thephosphate groups caused by the ordered substitu-tion.

AcknowledgementThe authors thank the Head, Materials group,

Vikram Sarabhai Space Centre, Trivandrum andDr. A.K. Rath, Scientist, for their help in providingthe instrumental facilities.

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