21
From: ADSORPTION ON AND SURFACE CHEMISTRY OF HYDROXYAPATITE Edi~ed by Dwarika N. Misra (Plenum Publ ishing Corpora~ion, 1984) SURFACE CBEKICAL CBARACTEl.ISTICS AND ADSORPTION PROPERTIES OF APAl'I'l'E P. Somasundaran and Y.H.C. Wang School of Engineering and Applied Science Columbia University New York. NY 10027 ABSTRACT In~erfac1al behavior of apa~i~es is governed to a large ex~ent by ~heir electrochemical properties which in turn are determined by pH. concen~ration of calcium. phospha~e and fluoride. Adsorption of surfactants and polymers on apatite is dependen~. among other fac- tors. on the interfacial potential of ~he apati~e. In this paper elec~rokinetic proper~ies of synthetic hydroxyapatite and natural ore apatite containing fluoride are repor~ed as a func~ion of the pH. KNO3. Ca(NO3) 2. K2HPO4 and K.F and mechani888 governing the surface charge genera~ion are reviewed. Electrokinetic effects obtained for apatite upon trea~t with concen~rated K.F solutions and calci~e supernatant are analyzed to determine possible chemical al~era~ions of its sur- face. Adsorption proper~ies of ionic surfactan~s and ionic and non- ionic polymers on apa~i~e a~ differen~ pH values are also discussed. INTRODUCTION Surface charge is an important property of a solid since it can determine as to what can adsorb. penetrate or adhere. Indeed. pro- cesses such as adsorption. particularly of surfactants or macromole- cules. can alter the interfacial behavior of the solids markedly. While considerable information is available on surface charge charac- teristics of oxides such aa alumina and silica. less is known about the behavior of sparingly soluble ~erals such as apatite. Surface charge properties of apatite type materials are affected by many more variables and the mechanisms governing the charge generation are much more involved. 129

P. Somasundaran Y.H.C. Wang - Columbia University in …ps24/PDFs/Surface Chemical...From: ADSORPTION ON AND SURFACE CHEMISTRY OF HYDROXYAPATITE Edi~ed by Dwarika N. Misra (Plenum

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Page 1: P. Somasundaran Y.H.C. Wang - Columbia University in …ps24/PDFs/Surface Chemical...From: ADSORPTION ON AND SURFACE CHEMISTRY OF HYDROXYAPATITE Edi~ed by Dwarika N. Misra (Plenum

From: ADSORPTION ON AND SURFACE CHEMISTRYOF HYDROXYAPATITE

Edi~ed by Dwarika N. Misra(Plenum Publ ishing Corpora~ion, 1984)

SURFACE CBEKICAL CBARACTEl.ISTICS AND

ADSORPTION PROPERTIES OF APAl'I'l'E

P. Somasundaran and Y.H.C. Wang

School of Engineering and Applied ScienceColumbia UniversityNew York. NY 10027

ABSTRACT

In~erfac1al behavior of apa~i~es is governed to a large ex~entby ~heir electrochemical properties which in turn are determined bypH. concen~ration of calcium. phospha~e and fluoride. Adsorptionof surfactants and polymers on apatite is dependen~. among other fac-tors. on the interfacial potential of ~he apati~e. In this paperelec~rokinetic proper~ies of synthetic hydroxyapatite and natural oreapatite containing fluoride are repor~ed as a func~ion of the pH. KNO3.Ca(NO3) 2. K2HPO4 and K.F and mechani888 governing the surface chargegenera~ion are reviewed. Electrokinetic effects obtained for apatiteupon trea~t with concen~rated K.F solutions and calci~e supernatantare analyzed to determine possible chemical al~era~ions of its sur-face. Adsorption proper~ies of ionic surfactan~s and ionic and non-ionic polymers on apa~i~e a~ differen~ pH values are also discussed.

INTRODUCTION

Surface charge is an important property of a solid since it candetermine as to what can adsorb. penetrate or adhere. Indeed. pro-cesses such as adsorption. particularly of surfactants or macromole-cules. can alter the interfacial behavior of the solids markedly.While considerable information is available on surface charge charac-teristics of oxides such aa alumina and silica. less is known aboutthe behavior of sparingly soluble ~erals such as apatite. Surfacecharge properties of apatite type materials are affected by many morevariables and the mechanisms governing the charge generation are muchmore involved.

129

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P. SOMASUNDARAN AND Y. H. C. WANG130

Properties of tooth apatite and its resistance to dental cavitieshave been known to be affected by the presence of che8ical speciesof fluoride (1-4). vaDadiu. (5.6). tin (7-9). 8Olybdenua (10.11).cbelating agents (12-14). and lana-chain surfactant. (15-18). Therole of the electroche81cal propertie8 of the 8urface in determiningthe uptake of the.. ch881cal speci.. by apatite is not. however.established. Further80re. electrokinetic studies on apatites haveproduced results that are often in conflict with each other. Syste-aatic work of Saleeb aDd de Bruyn (19) baa yielded values for thepoint of zero charge of hydroxyapatite and fluorapatite that are inagre...nt with those obtained by us for Datural are apatite (20-23)and synthetic apatite during this work. Bell et al.. (24) havealso attempted to determiDe point of zero charge of apatites usingtitration techniques; this technique is. however. applicable strictlyonly for in8oluble materials. Also. S088 of the prior work has beendone with sodiua salt solutiOD as the supporting electrolyte. Sincesodiua can substitute for calciua in apatite lattice and therebychange i~ properties. the r..u1ts obtained in sodiua salt solutioncaDDOt be considered to truly represent that of apatite.

Surface charge of a mineral is deter81Ded by the concentration of"potential determ1n1ng iona" in solution; the potential deterainingion. in the case of apatite can be the lattice iOO8 or their reactionproducts with water. Other inorganic specie.. surfactants as well aspol~ric reagents can also affect the interfacial charge and this inturn can be i8p0rtaDt in controllina tranaport of ions through the.urface layer. (25.26). but these secondary effects are controlledpr18arlly by the potential deteraining iOO8. It is important to iden-tify ~b88e ions since their concentrations do control the overall sur-face behavior. In this paper. our work to deter81De the role of var-ious ionic speci88 of Ca. PO4. OB-. and ,- and adsorption propertiesof selected surfactants and poly..rs of different charge characteris-tics are discussed.

EXPD.IMENTAL

Both natural ore apatite (CalO(PO4)6(F.OH)2) and synthetic hy-droxyapatite were used in this study. Zeta potential was measuredus1D& stre881Dg potential techD1que (27) in the case of natural apa-tite aDd using electrophoresis in the case of synthetic hydroxyapatite.The 88thod used for the study of the natural apatite essentiallyiDvolved measuring strea81ng potential of 35/65 mesh particles (cleanedusinl dilute nitric acid) in potassiua nitrate solutions adjustedto different pH values (20-22). In addition. the tests also includedthe pH 88&Sur888Dt aDd the aDaly.is of approz18ate fluoride contentusinl a fluoride electrode. Aqueous solutions containing varicus88OUnt8 of calciua nitrate. potas8iua dihydrol8D phosphate. and potaa-.iua fluoride were uaed to deter81ne the role of calciua. phosphate.and fluoride ions.

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131SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE

Syn~he~ic hydroxyapa~i~e vas prepared using precipi~a~ion ~ech-nique by mixing appropria~e amoUD~s of ~HPO4 in KOB solu~ion andCa(NO3) in wa~er and boiling. Se~~led and dried crys~als werefreeze aried and charac~er1zed using X-ray diffrac~ion and chemicalanalysis. Samples were s~oich1ome~ric (Ca/P . 1.67) and had hydroxy-apati~e s~ruc~ure. The ze~a poten~ial of ~he syn~he~ic samples wasde~era1ned using Ze~a-me~er af~er ~he elec~roly~e solu~ions adjus~ed~o cons~an~ ionic s~~eng~h values by adding sufficien~ KNO3 and con-taining apati~e were aged overn1gh~ and ~hen equilib~a~ed a~ ~he de-sired pH for 1 hour. Ze~a po~en~ial and superna~an~ pB we~e measu~edand fil~~a~es were analyzed fo~ calcium using ~i~ra~ion or a~o.1cabsorp~ion spec~~opho~ome~e~ (£UTA was added ~o elim1na~e ~he in~er-ference of phospha~e) and for ~ota1 phospha~e using a colorime~~ic

technique.

RESULTS AND DISCUSSION

30

zo

~~

>E.oJ- 10c~zw~ 00a.

c~ -10N

-8)

-30

-40 0 Z 4 .pH

Zeta potential of synthetic hydroxyapatite as a function~f pH at different ionic strengths.

128 10

FIGURE 1.

40 . ., , .

1. ..oa/al KNo,

-0-0

-0- 2.10-~

-0- 10-2.

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132 P. SOMASUNDARAN AND Y. H. C. WANG

Kt«>:s . ..11,.3

20

~~~z..,t-o..

~..,N

0

~

-20

-40' . . . . I .

0 Z 4 . 8 10 12pH

FIGURE 2. Zeta potential of natural apatite containing fluoridedetarmiaed .. a function of pH at different ionic-strengths (21)

aiDing ions for boch apacices. Ic is Co be noced chac KNO3 does DOCah1fc che isoeleccric po1nc. This ~ chac neiCher ~ nor NO) hasany surface pocencial decermiDing role and chaC KNO3 can be used as. reference eleccrolyce.

The 88Cban1aa by which pH determine. the .urface potential canbe developed by exA1IIining the reactioD8 that the apatite can undergo inwater.

Apatite Cheaic.a1 Equilibria in Water

Ca2+ 101.4+ CH- :: CaCli +

(1 (28)

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SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE 133

101.')7+

CaOH (29)

102.45 (29)

H~4 :: H+ 10-2.1'+ ~PO4 (:30)

10-7.2 ~)

10-12.3 ~}

102.7(7) (,32)

104.3*(8)

101.08(9) )2)

When the pH is increased, equations 1 through 6 will be driven towardsthe right hand side and this will result in a reduction in theactivities of the positive spec~e. aDd an increase in those of thenegat~ve species. The net result of these reactions will be an excessof negative potential determining ions in solution and this will makethe surface negatively charged. When the pH is decreased, the abovereactions will move in the reverse direct~ou and the surface willbec~ positively charged. If the above reactions are indeed responsi-ble for the surface charge generation, then for apatite, which can berepresented as KlO(PO4) Z where K can be calciua, barium, stront~umetc. aDd Z stands for b9dfoxyl, fluoride etc., addition of any of thespecies should change the surface potential in a manner dictated bythe following equat~ons:

. (y2+)lO (POt>6 (Z-)2

y2+ . Ca2+ Z- . (JI-K

sp

2+ )RT (Ca. - 1n , - 2+ ')~ (Ca PZQ.0

*Calc~t.d f~O8 equilibriua constants for reaction [7] and CaHPO4(s)* Ca2+ + HPO -. K - 10-7.

4 sp

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134 P. SOMASUNDARAN AND Y. H. C. WANG

.0

'0

It is to be noted at this po~nc chac since more than two types ofspecies can determine the surface potential of hydroxyapatite, chereis not just one point of zero charge as in the case of silica oralumina but many points of zero charge that will constitute a line ofzero charge on a calciU8-phoaphate-hydroxyl ternary diagram. Azero zeta potential curve can be constructed on such a diagram if thecondition of constant solubility product is met.

The role of calciua aDd phosphate as potential determining spe-cies for apatite can be tested by conducting tests as a function oftheir concentrations below as well aa above its isoelectric point.If a cation is potential determining. then its addition will makeit more positively charged above aDd below the isoelectric point, thatis. whether the mineral is origiDaIry negatively charged or positivelycharged. Similarly an anion. if potential determining, will make itmore negatively charged both above and below the isoelectric point.Strictly, variation of the surface potential itself with respect tothe potential determining ions should be dictated by the Nernstrelationship. In a syst- where the mineral dj.ssolves and the result-ant species undergo a complex set of steP-Wise reactions. it is,however. dj.fficult to estimate either the activity of relevant speciesor the surface potential. The indirect test on the effect of speciesUDder conditions when the 81neral is similarly charged is more usefulfor such systems. The effect of phosphate. calciua and fluoride atpH 10 (above the isoelectric point) is illustrated in Figure 3. Notethat the ionic strength is kept constant. It can be seen that cal-cita. as expected. makeS the mineral less negative and t!!~n even 3reverses the charge to make it positive at pH 10 with 10 kmol/mcalciua salt addj.tion. Importantly. phosphate makes the mineral morenegatively charged even though the mineral is already negat~ve; thisconfirms the potential determining role of the phosphate. It mightbe noted that fluoride does not produce any significant effect underthese conditions. The effect of these species at pH 5 (below theisoelectric point) is illustrated in Figure 4. Note that cal~3um has3an effect UDder these conditions also. but now ~y at 4 x 10 ~1/mand above essentially becsus!30f ~he !resence of significant 88Juntsof d~s8olved calciua (3 x 10 kmo1/.) . While phosphate has theexpected effect to make the mineral .ore negative at this pH, flouride,an anion is observed to aake the mineral aore positive.

The above species can ex1s~ ~ ~he !om of variQus complexesw1~h each o~her as well as w1~h R or OR. The effec~ on surface

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SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE 135

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P. SOMASUNOARAN AND Y. H. C. WANG136

potential can be either direct or due to complexation with potentialdetermining ions. The role of different ions and complexes can bemore easily estimated by plotting zeta potential in various elec~ro-lytes as a function of total concentration of the relevant species.Zeta potential is plotted in this manner as a function of total P andCa at pH S and 10 in Figures S to 8. It is seen from Figures S and7 that while calcium changes zeta potential at pH S without alteringthe phosphate concentration significantly. at pH 10. the zeta poten-tial change upon addition of calciua is accompanied by a reduc~ion inphosphate concentration. Higher effects ,of calciua in the alkaline pH

30

20

10

>e

~~z'"..

2

='"N

FIGURE 5.

010-4 10-3 10-2

TOTAL PHOSPHATE CONCENTRATION, _01/.3

Zeta potent~ of synthetic hydroxyapatite in Ca(N°.1)2.K BPO and KF solutions at pH 5 as a function of totaIp!osp!ate concentration. Ca. PO4 and F additions corre-~ to the concentrations shown in Figure 4.

FI GUB.E 6. Zeta potential of synthetic hydroxyapatite in Ca(NO3)2'K BPO 4 and KF solutions at pH 5 as a function of totaIcilciU8 concentration. Ca, PO4 and F additions corru-spODd to the concentration. in Figure 4.

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SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE 137

40

pH . 10I . 3.10-2 kmol/m3 KNO!

~.coT

--- -ADDED

>e 20A

..J:!...Z 0!AI...0Q.C...!AI -20N

"",,:::::~OEO P04-40" I I-~--- ~

10.8 10-4 10-3 1cf"f.TOTAL PHOSPHATE CONCENTRAT10N, t..v..3

FIGURE 7. Zeta potential of synthetic hydroxyapatite in Ca(NO3) 2.K2HPO4 and KF solutions at pH 10 as a function of totalphosphate concentration. Ca. PO ~ and F additions corre-spond to the concentrations in Figure 3.

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P. SOMASUNDARAN AND Y. H. C. WANG138

reg~on than in the acid~c range (see F~gure 9) can be attr~buted tot~ reduct~on in phosphate concentrat~on and pos8~ble alterat~onsin the lev~8 of various ca1c~ua phosphate complexes present. +It18 noted that ~re calc~ua will exist in the bydroxylated CaOH format pH 10 than at pH 5 and that the above effect cfP also b$+the resultof the greater potential deter81n1ng role of CaOH than Ca . Anadd1t~onal compl~cat~on in interpreting the results ar~ses fr08 thepresence of a larger amount of ca1C~U8 in ac~d~c solut~ons. thusreducing the effect of added CalC~U8 (see Table 1). From Figures6 and 8 it is seen that phosphate also acts in the same manner d~rect-ly at pH 5 and with SO8e reduct~on in calc~ua concentration at pH 10.

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SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE 139

Table 1. Total Ca and P Concentrations in Some of the Test Solutions

3 3pH Ca CODC. (kmol/m ) P conc. (kmol/a )

(I) TripI. distilled 11.2 3.75 x 10-; < 1.6 x 10-5water 10.3 < 2.5 x 10- < 1.6 x 10-~

7.1 1.5 x 10-5 < 1.6 x 10-6.2 1.24 x 10-4 1.13 x 10-45.57 1.17 x 10-3 4.87 x 10-;4.85 4.41 x 10-3 2.7 x 10-4.7 5.12 x 10-3 3.29 x 10-32.9 5.62 x 10-3 3.35 x 10-3

Solution

(II) 2 x lO-3kmol/.3

003

< 1.6 x 10-5< 1.6 x 10-5< 1.6 x 10-5

2.73 x 10-33.48 x 10-3

< 2.S x 10-S2.7S x 10-S4.38 x 10-S3.68 x.lo-3S.Sl x 10-3

11.10.7.S2.

(III) lO-~1/.3

11tO3

4.7S z 10--:

3.38 z 1°-41.27 z 1°-34.2 z 10

< 1.6 x 10-5< 1.6 x 10-5

8.06 x 10-52.69 x 10-3

11.29.857.85.8

Fluoride is seen to produce a marked reduction in calcium con-centra~ion a~ all pH values. The effec~ of fluoride addition onzeta potential is also peculiar in that it makes the mineral morepositive in the acidic region and sligh~ly more negative in the alka-line region. This can be ciearly'seen in Figure 9. This 1nforma~ioncan also be used to determine the surface chemical effects of fluorideIt has been suggested that fluoride in sufficient amounts can formfluorapatite at high pH values and fluorite at low pH values. Fluor-ite 18 reported to show a higher point of zero charge than hydroxy-apatite (32.33) and fluorapatite a lower poin~ of zero charge (19).Therefore. it requires ~he formation of fluorite on the surface tomake the mineral more positively charged and the formation of fluor-apatite to make it more negatively charged. The ze~a potentialchanges observed here in the present study with the addition of fluor-ide suggests the formation of fluorite at the ..surface of hydroxYapa-tite at low pH values and fluorapatite at high pH values. Solutionconditions also dictate the above 2~ssibiliJ! since the calcium con-cen~ration at pH 5 is about 3 x 10 k80l/- and Cal2 with a solu-bility product of 3.95 x 10-11 can be expected to form above abou~10-4 kmol/m3 fluoride (34). Formation of C&E2 is also indicated by

3534

5

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140 P. SOMASUNDARAN AND Y. H. C. WANG

the observed decrease in ~ciua concentration at this pH from 3 x 10-3to 8 x 10-6 kmol/.3 in 10- kmol/.3 fluoride !olution. At pH 10 also.with a total calcium concentration 3f 4 x 10- kmol/.3. fluorite canprecipitate above about 10-3 kmol/. fluoride concentration. In thepresent case. however. more of the calciua will be present in COR-plexed forms with hydroxyl and also with phosphate since the stabilityof ~4 (the predoainant species at pH 10) is higher than that ofCa2~a (the predO81nant species at pH 5). Precipitation of fluorite~ tnerefore require larger concentrations of fluoride at this pH.

In view of the above findings. the permanent nature of the aboveeffects was investigated by soaking natural apatite in 1. bol/.3 KFsolution at neutral pH and then determining the zeta potential afterwashing for almost 8 hours (35). The 1soelectric point of the naturalapatite was found to shift fr08 5.5 to about 7.6 (Figure 10). It isindeed likely that if the ~ur~t8 had been done after rinsingit for a few ainutes instead of 8 hours. a larger shift of the 1so-electric point would have resulted. In order to determine the

+50. . . . . . , J+40 a BEFORE AGING

A CONTROL+30 0 F TREATED

>E. +20..J~ +10

~ 0

0A. -10c

-20~N

-30

0

---0

-40

-~i

FIGURE 10.

. . . . . . ..3 4 5 6 7 8 9 10

pH

Diagraa illustrating the effect of prolonged fluoridecon~act on the electrocheaical properties of natural apa-tite. Zeta potentials in 2 x 10-3 kmol/.3 KNO solutionof freshly cleaned apatite t cleaned and aged a~atite t

and F-treated (using 1 kmol/m3 KF following cleaningand aging) apatite.

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SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE 141

permanence of this effect. zeta potential of three fluoride treatedsamples at icitial pH values of 4. 7 and 10 were .eaaured as a func-tion of t188.

Also pH. aDd F concentratiooa of the supernatants were ..aaured.Figure 11 sh0w8 the change in the par88eters at pH 7. It is notedthat the change in the zeta potential of fluoride treated s-.ple.4 mV. is abou~ one-third of the untreated sample. When the simulta-neoU8 change in pH is takan in~o accoun~. it becomes apparent thateven the 4 .V change is that which is expec~ed with the observedchange in pH. It is interesting that the fluoride concentration offluoride treated &yst.. did increase while that of the solution con-taining the untreated s.-ple did not change significantly~ Resultsobtained at pH 4 and pH 10 and given in Figures 12 and 13 also show the sametrend except for the slight increase observed for the untreated sa8plea~ pH 4.3 Treat8ent with concen~rated fluoride solutiooa (such asI kmol/. IF solution used) is thus found to 1B8ke the natural apatite~le .ore positively charged aad interfacial potential values .orestable. again suppor~ing the po8sibility of fluorite precipita~ion onthe apa~ite surface during contact with sufficiently concentra~edfluoride solutiooa. rapllcationa of these chang.. on the solubilityof the mineral as well as the rate of ~ranspor~ of charged chemicalspecies into apati~e are to be recognized.

+30 9 ..,4

b-o-.~o-.A, o o_-,~ +20

..J~ +10 8 -

pFpH00..c -101.-...N -20

7 --~

7

"CUD 11.

-301 . I , "6

0 100 ~ 300TIME, ",1ft

Chang88 in ze~. po~8D~ial. F cODCen~r.~ion in solu~ionand pH v1~h ~188 for fluoride ~rea~ed and un~re.~ed(con~rol) na~ural ap.~i~e/po~..eiua n1tra~e &yet... inthe neutral pH region; 0 z.~a -po~en~1al-F ~rea~ed.~ z.~e po~ent1al-untreat8d. C ,H-F trea~.d. 9 pH-untreated. -F - F treated. F - untreated.

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142 P. SOMASUNDARAN AND Y. H. C. WANG

+60. , ,s,s

...

4

"/

/+~

>e

..J +40c~

+30

II

II

e.o.., """"---

.~oH

4.8c+20...~N

+10 v

o~. . . '4 0 170 100 200 300 .

TIME, ...in

Changes in the zeta potential. P concentration in solu-tion. aDd pH with time for fluoride treated and untreatednatural apatite/potassium nitrate systems in the acidicpH range. () zeta potential - P treated. 6 zetapotential - untreated. C pH - P treated. Q pH - un-treated, P - F treated. -.-;- F - untreated.

FIGURE 12.

FIGURE 13.

-eo I I I I '10.8 .J 6.50 100 ZOO 300

TIME, Iftlft

Changes in the zeta poten~ial F concen~ra~ion in solu~ion,and pH with ~1me for fluoride trea~ed and un~rea~ednatural apatite/po~assiua n1~ra~e sys~ems in ~be alka-line pH rangeo 0 zeta po~en~la1 - F ~reated, 0 zetapo~en~ial - un~rea~ed, C pH - F ~reated, " pH-untrea~ed, - F - F treated, _0-0- F - untrea~edo

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SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE 143

It is important to DOte that the electrochemical properties ofthe apatite are very much dependent upon the method of preparationof the mineral. cleaning. storing etc. (22). Also other mineralspresent in the systea can alter the surface properties totally. Forexaaple. conditioning of synthetic hydroxyapatite in calcite super-natant (obtained by stirring calcite in water and removing all thesolid particles by centrifugation) is found to shift the isoelectricpoint to that of the calcite suggesting precipitation of calcite orother possible solids on the hydroxyapatite surface (Figure 14.(36».Solid~solution equilibria in mineral syste88 containing apatite canbe extreDely complex. leading to the possibility of precipitation ofdifferent minerals depending upon the solution conditions such aspH (37-40). The results suggest that in real systems containingmore than one mineral. behavior of the minerals can be totallydifferent from their behavior when present alone.

FIGURE 14. Diagraa illustrating the effect of calcite supernatantou the zeta potential of synthetic hydroxyapatite (36).

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P. SOMASUNDARAN AND Y. H. C. WANG144

As mentioned earlier. surface charge characteristics are ex-tremely important in determining adsorption of different species onhydroxyapatite. This is found to be the case with anionic surfac-tant. dodecylsulfonate (Figures 15 and 16). While adsorption of the a-nionic sulfonate is significant at pH 6.7. there is no adsorption at pH10.7. In contrast to the sulfonate. the dodecylamine is found toadsorb at both pH values. While zeta potential results of Miahraet al. (39). also suggest adsorption of dodecyltrimethylammoniumchloride and dodecyl~niU8 chloride under all pH conditions ata concentration of 10-3 kmol/m3. at lower concentrations ofdodecylamine significant shift in zeta potent~ is observed onlyabove the isoelectric point. Dodecylsulfonate. on the other hand.lowers the zeta potential at all levels in the reported pH range5 to 10. Oleate also lowers the zeta potential. shifting it towardsthat of Ca-oleate at high oleate to apatite ratios. Evidently. theprecipitation of Ca-surfactant salt can also be a major phenomenonin these systems.

1&

12

8

4

0"-

-a

l.,:..0z~z0~4-«0titQC

0

FI.GURE 15.

10-5 10-4 10-3RESIDUAL SURFACTANT CONCENTRATION, kmol/".3

Adsorption iso~herms of dodecylsulfonate and dodecylamineon syn~hetic hydro~apa~ite a~ pH 6.7 a~ an ionic strengthof 3 x 10-2 kmol/mJ NaCl.

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SURFACE CHEMISTRY OF AND ADSORPT10N ON APATITE 145

.16"0E~

>12...iiz'"QZ 80~A.~0Gl4Qc

0

FIGURE 16.

-o-S -0-4 -0-5RESIDUAL SURf'ACTANT CONCENTRATION, kmol/m3

Adsorption isotherms of dodecy1s~fonate and dodecy1aaineon synthetic hydro~apatit. at pH 10.7 at an ionicstrength of 3 x 10- kmo1/.3 MaCl.

Adsorption of polymers on hydroxyapatite are al80 found tobe influenced by their charge characteristics. Adsorption of selectednonionic. anionic and cationic polY88rs on hydroxyapatite at pH 11and 6.6 is shown in Figure. 17 aDd 18 respectively. ~e in thealkaline pH range where the 81D8ral is negatively charged. only thecationic polymer is found to adsorb. in the neutral pH range all thepoly.ers are found to adsorb. PolY88r adsorption on solids is con-sidered to result aaiDly froa hydrogen bonding. electroscstic bondingand covalent bonding depending on the mineral/polymer system (41).Evidently hydrogen bonding 8igbt be sufficiently active to causeadsorpCion of all the polymers at neutral pH values. It i8 notedthat there 1. no -..urable adsorption of even the nonionic poly-..r at pH 11.1. Tb18 1. attributed to the hydrolys1a of the poly-mer to the anionic fora in alkaline solutions (42) and the electro-static repulsion betWeen the re8ultant functional group on the poly--ric species and s1a1larly charged aineral particles. It is evi-dent that adsorption of macromolecules viII be a complex function ofnot only the properci.. of the solid and the poly.er but also the

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146 P. SOMASUNDARAN AND Y. H. C. WANG

~02fI) ~Q IC I

0 NONIONIC

~ ANIONIC

0 CATIONIC

pH 8 11.1 - 11.2

nGURE 17.

O-m.o '.' '611' ,0 100 ~ '300 400 ~ 800 900 \000

RESIQUAt. POLYMER CONCENTRATION, ",,/-9

Adsorption 1aothe~ of nonionic (polyacrylamide) anionic(polyacrylamide containing carboxyl group) and cationic(polyacrylamide containing a81ne functional group) poly-_rs on synthetic hydroxyapatite at pH 11.1 to U.2at aD ionic strength of 3 x 10-2 kmol/m3 NaCl.

16

14

0 NONIONIC

6 ANIONIC

a CATIONIC

pH 86.8-6.7

.~ 12e~~ 100z'"Q 8'z2

~ 8«0(I)

~ ..

2

FIGURE 18.

0- . . , , . .Om~300400500600700RESIDUAL POLYMER CONCENTRATION, .../~

Adsorption isotherms of nonionic (polyacrylamide). anionic(polyacrylamide containing carboxyl group) and cationic(polyacrylamide containing aaine group) polymers onsynthetic hydroxyapatite at3pH 6.6 - 6.7 at an ionicstrength of 3 x 10-2 kmol/. NaCl.

0

0e

>-"l-iiz

~

~j:A.

6

5

4

3

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SURFACE CHEMISTRY OF AND ADSORPTION ON APATITE 147

solution cODditiou. aDd po88ible alterati0D8 of both the solid andthe polymer in the 8olutioaa. It 18 important to noc. thac adsorp-cion of the poly.ers on solid can also depeDd on thac of the pre8enceof any 8urfactant in solution (43). Other incerfacla1 propercies ofche aineral 8uch a8 V8Ctabil1cy can also be affecCed 8arkedly bysuch poly.er-8urfaccanc interactiou. (49).

CONCLUSIONS

Electrocb8.1cal pr0p8rty of apat~te is a co.plex fUDct~on of notonly pH. but also concentrat~on of var1oU8 constituent species aswell &8 the type of pretreat8eDt of the solid. Hydrogen. hydroxyl.~ua aDd phosphate playa potential dete~g role eitherdirectly or by altering the concentration of other potential deter-m1DiD1 ions. While phosphate addition ..kas the mineral more nega-t~valy charged aDd calciua ..us it more positively charged underall pH coad~tions. fluoride is found to ..ka the 81neral ~repositively charged in ac~d~c solutions and more negstively charged1n alkaljnA 80lutions pos8ibly due to the formation of fluoriteaDd fluorapatite respect~valy. Prolonged contact with fluoride alsodoe. produce a significant 1Dcr...e 1n the 1soelectr~c point 8uggest-1ng soma fluorite precipitation on the surface. S181larly. contactof hydroxyapatite with calc~te superuatant produced a shift of theisoalactric point t~d. that of calcite. It is clear that inapatite .yst... cont-ioins other 8ineral8 alterations 1n surface caa-p08ition due to various precipitations are possible depending on thea1neral-solution equ1l.1bria at various pH valuea. The resultantelectroc:he81cal nature of the 8inaral particl.. is found to playagoverning role in determining the adsorption of surfactaots and poly-aars of varioU8 charge characteri.tics.

ACDOWL~S

The aut.hon viab to acknowledge the support of the NationalInstitute of Health (5-101-01 03460) aDd the Chemicsl aDd ProcessEngineering Div1a~on of the Nati~ Science Foundation. We alsothank S. phillip.. 1..0. r.ulkam1.. aDd K:.P. Anantbap-.t-..ft-bhan fortechnical. ...iatance and d1.cua.ion.

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P. SOMASUNDARAN AND Y. H. C. WANG148

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36.37.

39.

41.

42.

43.

44.