Further Streaming Potential Studies on Apatite in Inorganic

Electrolytes

by P. Somasundaran and G. E. Agar

The role of calcium, phosphate, and fluoride species in determining the zeta potentialof apatite in aqueous solutions was investigated in this study. The zeta potential ofapatite was determined by streaming potential measurements over a wide range ofconcentrations of potassium nitrate, calcium nitrate, potassium dihydrogen phos-phate, and potassium fluoride solutions at various pH values. While the addition ofphosphate made the apatite surface mOTe negative at all pH values, calcium caU3ed asignificant increase of the zeta potential only when the zeta potential was negative.Fluoride addition had only a minOT effect on the zeta potential, but both negativeand positive values were increased.

Even though the soluble salt type minerals constitutea major portion of the minerals beneficiated u$;ing pro-cesses based on interfacial phenomena;'their surfaceproperties have been the subject of only a few studies.'-'Apatite is a mineral of considerable importance bothcommercially and biologically which belongs in thegroup of slightly soluble minerals that have not beenstudied extensively by researchers. The few results thatare available in the literature are often found to be con-tradictory.Our earlier work with apatite' gave a valueof pH 7 for the point of zero charge* in aqueous solu-tions, with an initial value of pH 4, shifting with timetowards pH 7. The previous work also indicated a de-pendence of the zeta potential on calcium, phosphate,and fluoride species. The purpose of this study was toelucidate the role of these species in determining thezeta potential of apatite by conducting experiments withapatite over a wide range of concentrations of saltscontaining calcium, phosphate, or fluoride. This paperreports the results obtained in the experiments and theanalysis of the role of the constituent ions of apatite indetermining its zeta potential.

Materials and Methods

Fluorapatite (Ca.(PO.).(F, OH» particles of macro-crystalline nature containing about 5% calcite werecrushed and sized, and the 35/65 mesh particles ob-tained were cleaned with dilute nitric acid, then washedwith conductivity water until the washings were free ofnitrate ions. All the chemicals used were analytical re-agent grade and the water used for the experiments hada conductivity of less than 7 X 10-1 ohm-'cm-l. In addi-

. Wher~as the lsoelectric point flEPI de5Crib~s that condition o( thesystem at which the zeta potential, as determined by electrokineticmethods, is zero. the point of zero charge (PZCI descrtbes that atwhich the net surface charge. commonly denoted by the symbol cr iszero. The species which determine the surface potential are calledpotential-det£rmining ions and the species which determine the dif-ference b~tween the surface potential and the zeta potential called('ounterous or co-Ions depending on whether their charge is oppoglteor the same as that of the surface charge.

P. SOMASUNDARAN, Member AIME, is Associate Professor of Min-erai Engineering, Henry Krumb School of Mines, Columbia University,New York; and G. E. AGAR, Member AIME, is with J. Roy GordonRsch. Laboratory, The International Nickel Co. of Canada, ltd. Clark-son, Ont., Canada. TP 718210, AI ME Annual Meeting, Washington,D.C., February 1969. Manuscript, April 3, 1971. Discussion of thispaper, submitted in duplicate prior to Dec. 15, 1972, will appear inSME Transoctions, March 1973, and in AIME Transactions, 1973,Vol. 254.

tion to measuring the zeta potential of apatite, the ex-periments included the measuring of the pH,°the calciumand the phosphate concentrations of the test solutions.The zeta potential was determined by the streamingpotential technique described in earlier papers-" 8-»

One hundred grams of the mineral was used formaking the plug to be used in the experiments. Thesolution (800 ml) was streamed through the plug untilthere was no difference between suCcessive readings.The zeta potential values reported in this paper arethose that were calculated from the final streaming po-tential values thus obtained. Mter the streaming wascompleted, the apatite particles were allowed to settleand the pH and calcium and phosphate concentrationsof the supernatant solutions were determined.

Previous experiments'" have shown that the valuesobtained in the foregoing manner do not necessarilyrepresent the equilibrium values. We h.ave observedthat the zeta potential, solution pH, isoelectric point,and the point of zero charge undergo a continuous shiftfor several weeks after mixing the solution with the freshmineral. Nevertheless, it was considered to be sufficientto do the experimentation in the manner described herefor the present purpose of determining the relative roleof the various ions mentioned previously. The concen-trations of calcium and phosphate in solution were de-termine-a br atomic absorption and optical spectropho-tometry of the vanadophospho-molybdate complex, re-spectivelY. Potassium nitrate solutions of known con-centrations adjusted to different pH values were used todetermine the zeta pOtential of apatite as a function ofpH. Aqueous solutions containing various amount., ofcalcium nitrate, potassium dihydrogen phosphate, andpotassium fluoride were used to examine the role of cal-cium, phosphate, and fluoride ions.

Results and DiscussionThe zeta potential of apatite is shown in Fig. 1 as a

function of pH at different concentrations of potassiumnitrate. It can be seen that an isoelectric point of 5.6 to5.7 was obtained under the conditions used in the pres-ent experiments. This is comparable to the results re-ported earlier for the isoelectric point of apatite.

'The nature of the chemical species that is obt'iinedwhen apatite is contacted with aqueous solutions hasbeen discussed earlier along with the mechanism bywhich hydrogen or hydroxyl ions determines the zetapotential of apatite. Some of the major reactions thatoccur are given for the purpose of later (1 scussions on

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Socicty of Mining (ft9illee". A'ME

Fig. I-Zeta potential of apatite as a function of pH at vari.ous concentrations of potassium nitrate.

Fig. 2-Zeta potential of apatite as a function of pH at vari.ous concentrations of potassium dihydrogen phosphate.

with those obtained in KNO. solutions to determine therelative effects at different pH values. Results obtainedwith calcium nitrate solutions are given in Fig. 3. It canbe seen that calcium affects the zeta potential signifi-cantly at higher pH values, but at lower pH values itsimply reduces the zeta potential. It should be noted

the inftuence of calcium, phosphate,and fluoride ions onthe zeta potential

(1) Ca-- + OH- ~ CaOH- K, = 10'" (11)-(2) CaOH- + OH- ~ Ca(OH) , K. = 10'.87 (8)(3)' Ca(OH) 0'..' ~ Ca(OH)...) Ka = 10". (8)(4) HaPO. ~ H- + H.PO.- K. = 10-0... (12)(5) H.PO.- ~ H- + HPO,-- K. = 10-T.o (12)(6) HPO.-- ~ H. + PO,-- K. = 10-w.o (12)(7) HF ~ H' + F- KT = 10-&." (13)(8) Ca'- + HPO.- ~ CaHPO.,q. K.. = 10'.T (14)(9) CaHPO.,q, ~ CaHPO..., K. = 10"& t(10) Ca'. + HaPO.- ~ CaHaPO.+,.., K'8 = 10L. (14)(11) Ca-- + 2F ~ CaF K.. = lOW" (15)(12) CaF+ ~ Ca-- + F- Ku;: 10-' (12)(13) CaHPO..., ~ Ca-- + HPO.-- Klo;: 10-4.. (14)

. Nwnbers refer to the references at the end of the paper.t Calculated from equilibrium constants for reactions 8 and

CaH(PO.I,., ~ Ca.- + HPO.-- = Ku = 10-7. (121

It should be noted from Fig. 1 that the isoelectric pointof apatite is independent of the concentration of potas-sium nitrate in solution and even though this is not asufficient condition, it may be assumed that neitherpotassium nor nitrate ions specifically absorb on apatite.We can then identify the role of the chemical constitu-ents of apatite by conducting streaming potential exper-iments in the presence of an added salt, one ion of whichis calcium, phosphate, or fluoride and the other is eitherpotassium or nitrate and by comparing the data withthat obtained using potassium nitrate only.

Towards this purpose, the zeta potential of apatitewas determined as a function of pH at various concen-trations of calcium nitrate, potassium dihydrogen phos-phate, and potassium flouride. The results obtainedwith KHsPO. are shown in Fie-. 2. The zeta potential ofapatite is given in this figure as a function of pH at10-1, 10'., 10-1, and 10-0 molar phosphate concentrations.It may be noted that phosphate has made the zeta po-tential of apatite more negative in the complete pHrange studied. These values are subsequently compared

Fig. 3-Zeta potential of apatite as a function of pH at Yari-ous concentratians of calcium nitrate.

SEPTEMBER 1972 - )49TRANSACTIONS - YOLo 252 Society of Mini"9 Engineers, AIME

that this effect is quite different than that observed withphosphate. Zeta potential data obtained with potassiumfluoride solutions is shown in Fig. 4. It can be seen thatthe effect of fluoride is in general smaller than that ofcalcium or phosphate. To facilitate discussion of the roleof the different ions in determining t~e zeta potential ofapatite, plots of the zeta potential were made at two pHvalues, one which is on the positive side of the PZC andone which is on the negative side of the PZC, as a func-tion of the ionic strength of various reagents. These plotsare shown as Fig. 5 and 6, respectively.

i

~ ~

CaIHO!'!

+40l . t ~ - I

10"5 10-4 10-' 10-1 10-1

IONIC STRENGTH DUE TO ADDED SALT, N

fig. 6-Zeta potential of opatite at pH 7 (above the isoelectricpoint) as a function of the ionic strength change due to additions ofKNO., KH"PO" Ca (NO,>.. and Kf.

('.

pH

Fig. 4-Zeta potential of apatite as 0 function of pH at yari.ous concentrations of potassium fluoride.

We shall now compare the curves obtained with thosefor potassium nitrate. The decrease in the zeta potentialwith KNo. concentration in solution is caused by com-pression of the diffuse layer due to the increase in ionicstrength. On the basis of the dissociation constantsgiven earlier for various complexes, most of the phos-phate would be present at pH 4 in the form of H.PO.-ions. It can be seen that at pH 4. the effect of phosphateis considerably larger than what could be attributed toan increase in ionic stren~ only, and it can be seenfrom Fig. 6 that on the negative side of the PZC thephosphate still has a considerable effect on the zetapotential Phosphate thus has made the mineral morenegative when the original zeta potential of the mineralis negative as well as when it is positive. The resultantshift in the isoelectric point of apatite can be seenclearly in Fig. 2. These results suggest that the phos-phate species might be considered as potential-deter-mining ions for apatite.

The~cium nitrate data must be compared with thepotassium nitrate data at the same ionic strengths todistinguish between specific adsorption and simple dif-fuse layer compression. The curves for calcium nitratein Figs. 5 and 6 are' based on ionic strength. At pH 4where the original zeta potential is positive, the curvefor calcium nitrate is slightly different from the potas-sium nitrate curve at least up to 10-- M per l However,where the original zeta potential is negative as at pH 7,calcium ions cause a relatively larger shift in the zetapotential to more positive values (see Fig. 6). This ~ug-rests the possibility that calcium ions might be specifi-cally adsorbed on apatite.

Potassium fluoride addition to the solution causes rel-atively lesser minor change in the zeta potential. At pHvalues where the zeta potential is negative, the fluorideion shifts the zeta potential to even more negativevalues; whereas, in the region where the zeta potentialis normally positive, the fluoride ion appears to causethe zeta potential to become slightly more positive. Atlow concentrations the general effect of fluoride ion issimi1ar to that of phosphate except that the magnitudeis not nearly as great. .

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Examination of the data as represented in Figs. 5and 6 reveals that increasing the concentration of po-tassium fluoride does not have much effect. In fact, at10-'),1 per 1 concentration, comparison of data obtainedwith potassium fluoride and nitrate solutions shows thatthe fluoride salt does not even reduce the zeta potentialas much as it is normally reduced due to the increase inionic strength of the solution by potassium nitrate. Theexplanation for this somewhat unexpected result maybe found in the distribution of fluoride species in solu-tion. Hydrofluoric acid is a weak acid with a pH =3.14.'" At pH 4 in a 1O-"M per I KF solution the ratio offluoride ion to undissociated hydrogen fluoride is 7.1and therefore the fluoride ions concentration is 8.8 X 10-'M per L Hence, the reduced effect cannot be due to thehydrolysis of fluoride to hydrogen fluoride. The forma-tion of the undissociated hydrogen fluoride should resultin a sli&,ht decrease in solution conductivity and thiswas indeed observed. Some of the observations areshown in Table 1. Calcium fluoride is a relatively insol-uble compound with a solubility product of 3.95 X10-11.," In acidic solutions the calcium ion concentrationwould be expected to be low in equilibrium with apatiteand the results with calcium nitrate bears this out. Thus,the addition of a soluble fluoride could bring out theprecipitation of calcium fluoride which would have theeftect of exchanging potassium ions for calcium ions insolution. In order to compensate for the decreased cal-

cium concentration, more apatite would dissolve untilthe equilibrium constant for apatite, which includescalcium, phosphate, and fluoride, was satisfied. Theoverall result would be an increase in the potassiumdihydrogen phosphate concentration. It would appearthat the tendency for the increased phosphate concen-tration to make the zeta potential more negative andfor the increased calcium concentration to make the zetapotential more positive are approximately equal so thatthe zeta potential is essentially independent of the p0-tassium fluoride addition.

The role of the various ions discussed previously canbe easily reviewed by examining a plot of the zeta po-tential of apatite in various salt solutions as a functionof pH. Such a plot, at 10'" molar concentration is shownin Fig. 7. Acain a comparison of the curve for phosphatewith that for nitrate suggests that phosphate is possiblysurface potential determining since on both sides of theisoelectric point the phosphate addition tends to makethe apatite more negative. An increase in solution pHshifts the equilibrium between phosphate species to-wards more of the divalent HPO.- - ion and trivalentPO.- - - ion, and this makes the mineral more negative

as the pH is increased.Addition of calcium nitrate makes the mineral sur-

face more positive and the magnitude of this effect in-creases with increase in pH. Even though the amount ofdivalent calcium ions would increase with a decrease of

.eI."'" l~ . LpH c dI.uy9.33 '7.9'7 X 10"'7.01 6.88 X 10"6.62 6.1 X 10-84.39 10.58 X 10-82.0 D8.1xl0-4-9.a 5.45 X 10-4'7.13 loa X 10-86.0 11.81 X 10-42.'7 118.1 X 10"

10.11 9.55 X 10-8'7'" 5.43 X 10-4'1.01 5.. X 10-88.S8 U X 10-81.0'7 5.32 X 10-86.89 5.11 X 10-84.59 5.84 X 10-8... 98.19 X 10"1.85 144.4xl0-8- - I." 11'1" X 10-8 '" 1a.II x 10-4 - 1.81 U X 10"

1.14 11..1 X 1... 8.11 u.8f x 10-8 LM 3.1 x 1...:r .,..1 115.1 X 1... 'I'" 1." x 1... .,.18 1.85 x 10-4

5.. 115.0 X 1... 5.5" 11.. x 10-'- Ul 3.'1 X 1...3.88 30.81 x 10-0 3.81 11.1'7 X 10"

. - - ~ tMt duU- - ~tacted with about 108 . of 8pattte 1ft the PiII8 durtD8 the m Ia8 e~t.

1 LC eU9itJ

--'811 1. $8IItiH c..-tiwitr . .. ,. C8 If , ,at 1 * It ~ ~ .. at Stna-iII PltMtiII EJp..~

1'-- . .- L 1... . .- L.. C UYl&~ ..c IaYl&~ -- -

8.n 138.3 X 10-8 21.38 X 1...8.J3 l-.s X 1..- 17.44 X 1..-3." 182.5 X 1..- 11.21 X 10-82.48 358.8 X 1..- 22.7 X 1..-

2271 X 1008 C c. 'I; i'i ,a.87 215.8 X 10-8 25 X 10-8 -, :I.n ~ x 1o-e 27.08 X 10-8 ,.. f:2.. X 10-8 51'»' X 1... . .' ~

18.8 X 10-8 ~ '"-,

1.58'1.31e.2e4.el2.43

8.41e.M3.452.55

KNo.

~~

.8.a'J.1a...12.83

...",..,...S.M...3.1a

3.51 x 10--2.01 x 1..3.01 x 1G-A

10.11 X 10--87.01 X 10--

--4.12 x 10--1.33 x 10--G.8x10--1.16 X 1..3.88 X 10--

3'7.88 x 1"

KHtPO.

J83.9xl~214.0 x 1~232xl~213.1 x 1~318.3 x 10'"

1.807.215.804.211.84

10.02'7.10'7.005.885.854.532.85

2'7.65 x 10--28.15 x lG-428.53 x 10--28.'74 X lG-428.28 X 10-028.8 X 10-1

149.4 X 10'"

Ca(N~).

Ta~le 2. Total Ca and P Concentrations in Slme of tile Test Solltius If Ta~le 1 at tile bd If tM Strl38il' Pltutial u,eriaem

Solution. per Lc..ee.traUon.. per L

1 per Lc..eeatrail.a.

.perL --

1.-0 . .er LC..eeatraU..8

.perL.....nLa

c-S.S x 1...1.81 X 10-0

..8..-1.883.U

10...,a.-

Ca

1.5 x 10--3.5 x 10-&LOS X 10-41.. X 10-&

I" X 10-41 X 1 LOS X 1&-8

1 X 10-&

.. Oa8.81 5 x IG-81.'7 1.18 X 10--

I'

x 1~8.04 x 10-1

... x 10-0

..05 X 10-01 X 1~1.74 X 1~

<1.8 X 10-4<1.8 X 10--<1.8 X 10-8<L8 X 10-4

XBaPOc

.La.

-- --~

8.13 x 10"

8.32 x 10-8

...8..

.S )( 10-4

.8 )( 10"

10.21 8.5 x 1~.,.. 1.27 x 10-48.0'1 l.es X 10-4a.85 1.41 X 1004

"'1 2.2 X 10'-.,.. 3.3 X lo.-Ut U X 1..-U8 a.. X 10-4

:1.8 x 1...:1.8 x 1...:1.8 x 1...9.04 x 10--'

:1.8 X 1...:1.6 x 1~:1.8 X 1~8.85 X 1...

C8~.

1.5 x 10-8 :1.8 X 1'"

8...T."

... x 10.-'1.8 x 10-8

:1.8 x 10.--1.8 x 10"RoO

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more negative. as it should if it were potential deter-mining, but it does not even reduce the zeta potential asa normal electrolyte would by compression of the dif-fuse layer. As mentioned earlier, this could possibly bedue to removal of part of the added fluoride as calciumfluoride precipitates. Several studies, 18-. particularly -;adsorption studies by those doing research in dentistry,have shown that fluoride specifically adsorbs On apatite.Fluoride ions are known to promote the icelike struc-ture of water rather strongly, and specific adsorption ofit in the iceberg layer near the surface might also be

expected.=t

SummaryZeta potential values were obtained for apatite in

aqueous solutions of potassium nitrate, calcium nitrate,potassium dihydrogen phosphate, and potassium fluorideand the following conclusions can be made on the basisof these results.

I) Isoelectric point of apatite is about pH 5.6.2) Phosphate species, hydrogen, and hydroxyl ions are

surface potential determining for apatite.3) In the pH range studied calcium isspeciftcally ad-

sorbed but does not appear to be potential determining.4) Fluoride does make the mineral more negative

above its isoelectric point, but the effect is smaller thanthat of phosphate species or hydroxyl ions. Below theisoelectric point it has signficant effect, possibly due toits removal as calcium fluoride.

Fig. 7-Zeta potential of apatite as a function of pH in waterand in 10-0 molar solutions of KNO.. KH"PO,. Co (NO.)" andKF.

AcknowledgmentThe authors wish to thank National Science Founda-

tion and International Minerals and Chemical Corp. forsupport of this research work.

References\ Somasundaran, P.. Journal of Colloid Interface Science, Vol. 27,

1968, p. 659.. Somasundaran, P., "Pretreatment of Mineral Surfaces and ItsEftect on Their Properties." Clean Surfaces: Their PreparotW" aMCh4..acterizatiD1l for Interfaci41 Studies. Marcel Dekker, New York,1970.. DobIas. B., Spumy. J., and SkU, Ivan D., RudV (Pracuel Vol. a.No. 12, 1960. p. 407.t Borisov, V.M., Khim. Prom.. 19 (41, 213 (1954); 19 (61, 336 (19541.

;; Mattson, S.. et al., Kungliga La1ltbrulcsho-G8koians Ann., Vol. 18.1951, p. 128.. Klien. H.J., Jourftal of Dental Resea..ch, Vol. 12, 1932, p. 95.

1 Patras. B.C.. M.A. Thesis, Indiana University, 1955.. Sornasundaran, P., and Agar, G.E., Jounlal of Colloid Science,

Vol. 24, 1967, p. 433.'So~undaran. P., Healy, T.W., and Fuerstenau, D.W., Jou..nal ofPhvsic41 Chemiatr1/, Vol. 68. 1964, p. 3562.

10 Fuerstenau. D. W., Mining Engineeri"g, VoL 8, 1958, p. 834.\, Ben, R.P.. and Georce, J.H.B., Transactions, Faraday Society,

VoL 46, 1953, p. 619.,. Hedberg, D.D., "Sar«ent Chart of Equilibrium Contents of

lnor«anlc Compounds." E.H. Sarcent and Co.. Chira"o, 1963.10 Hodgman, C.D., Weast, RoC., and Selby, S.M., Ha1&4book of

Chemistrv a1&4 Phvsic8. Und ed., The Chemical Rubber PubUahingCo., 1961.

't Bjerrum, J., Schwarzenbach, G.. and SllIen, LoG., "StabilityConstants of Metal ion Complexes. with Solubility Products ofrnorganlc Substances," Part U. Special Publication No.7. TheChemIcal Society. London, 1958.U Lange. N.A., ed., Handbook of Chemistrv, lOtb ed., McGraw-Hill, New York. 1961.,. Falkenhelm, M., and Hodge, H.C., Journal of Dental Ruea..ch,

Vol. 26. 1947, p. 241.11 Volker, J.F., et aI., Journal of Biological Chemutr1/, Vol. 134,194.0, p. 54.3.

IS Posner. A.S., Norelco Reporter, Vol. 2, 1955, p. 126... Englander, H.R., and Keyes, P.H., Joumal of American Dental

A88OCiation, Vol. 73, 1966, p. 134.2.. Fremlin, J.B., Hardwick. J.L., and Mathieson, J., Commun.Concr. "Organism Europe Cord. Researches." P'tour Proplva"ic Carie.Brussels, Vol. 5,1958, p. 177.~ Berube. Y.G., and de Bruyn. P.L., Journal of Colloid 4. IlOterfaceScience. Vol. 28. 1968, p. 92.

solution pH, the mineral surface is not actually becom-ing more positive with such a pH decrease. Thereforeas mentioned earlier calcium ions probably are not po-tential determining for apatite.

The increase in the positive nature of the apatitesurface with increasing pH in the presence of calciumnitrate solutions suggests that there is specific adsorp-tion of divalent calcium ions. The number of negativesurface sites increases with an increase in solution pHand therefore the specific adsorption of divalent calciumions also increases, the net effect being an increase inthe positive value of the zeta potential.

It is admittedly difficult to see calcium ions acting ascounter ions for apatite instead of potential deter-mining. Such a behaviour of the calcium ions can be dueto a difference between the nature of the calcium ions,such as that due to hydration while in solution and thatwhile in the lattice. Increases in zeta potential with anincrease in solution pH can, on the other hand, be due tothe increase in concentration of CaOH' and CaH.PO.species and any effects that these ions may have as po-tential determining. It might, however, be noted thatconcentrations of these species, at say pH 7, is rathersmall to account for the positive values that were ob-tained for the zeta potential of apatite in calcium ni-trate solution.

The effect of fluoride on the zeta potential is peculiarin that it not only does not make the mineral surface

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