Gupta, G. K.

Kumar, Dinesh

Singhal, J. P.

English

NOPR

Indian Journal of ChemistryVoL 14A, December 197&, pp. 919-932Studies on the Thermodynamics of Exchange in Clays:Part II - Zinc Exchange on Na- & Ca-IllitesJ. P. SINGHAL, DINESH KUl\L\R & G. K. GUPTADepartment of Chemistry, Faculty of Engineering & Technology, Aligarh Muslim University, AligarhReceived 10 October 1975; raised 23 March 1976; accepted 18 May 1976Zn-Na and Zn-Ca exchanges on Na- and Ca-illites have been studied with the help of adsorp-tion isotherms and thermodynamic parameters. The isotherms and free energy changes areindicative of a lower preference of Zn ions for the illites as compared to the Na or Ca ions, i.e.the exchange is not spontaneous. Enthalpy effects point to a stronger binding of Na and aweaker binding of Ca as compared to Zn on the illite surface. These results find support fromthe entropy changes. A greater randomness and disorder seems to occur during Zn adsorptionon Na-illite probably as a result of depletion of the hydration shell around Zn ions. A reverseeffect is noticed during Zn adsorption on Ca-illite.CLA Y minerals occur widely as the colloidalconstituent of soils. Inspire of their b:illgthe most reactive fractions of soils, rclat ivrlvlittle work has been done all the mechanism O'fexchange of cations ill. solution and the surfacephase of these minerals. III view of the importanceof zinc exchange in soib1•2, it was considered use-fulto study the interaction of Z!I(Il) with tho Na-ill it«and Ca-illit« Hsing ('xchange i-ot lurm:', thermo-dynamic parame-te-rs ai«] surface phase activitycoefficients. The basis of the' trvatrn. lit havebeen the thermodynamic formulations of previousworkers>". It lias bl't'II fe-lt that such a st udy willthrow considerable light 011 th« mechallism of zincadsorption and release by base saturated illites.Materials and MethodsIllite used was an API sample obtained fromMorris, Illinois, USA. It was dispersed afte-r re-moval of organic matter in a. normal NaCI solutionand then centrifuged to givI' <2 n.•.icron suspension.The NaCl treatment wa-. repeated and the suspen-sion freed from chloride ions by n'pca1{'cl \\ ashingswith deionized water. A part of the Na-illitesuspension was converted into Ca-illite by saturatingit with a normal solution of CaCl2 and then washingit as above to remove the excess of the salt. Theconcentrations of the Na- and Ca-i llite suspensionswere 17·0 and 14·0 g pn litre respectively.For the exchange experiments thv pH of thesuspensions was adjusted to 4 with <IiI. HN03 toprovide a buffeting action and to eliminate hydroxideprecipitation, if ally. Ten ml «ach of the Na- andCa-illite suspensions wrr« taken in several tda"sstoppered tubes and treated with different amountsof O·03N ZnS04 solution and the volume of themixture adjusted to 25 ml with deionized water ineach case, The mixtures were shaken for 6 hr at30° ± 0·1 ° in the first set of experiments and at 60°± 0·1 ° ill the second set of experiments. The sus-pensions were then centrifuged and Na, Ca and Zncontents determined in the supernatant liquids.Sodium was determined with a flame photometerand Ca and Zn by atomic absorptiometer. Theammonium acetate method of Jackson was followedto yield a value of 23 mcqj100 g for t lu- base exchangecapacity (BEC) of illites. The correspondingconcentration of Na and Ca ill the clay phase wasobtained by difference (BEe minus the concontra-t ion of the cation in the supernatant liquid) andthat for ZII from Zn added minus Zn in the super-natant liquid.Results and DiscussionThe reversible interaction be-tween the zinc ionsin solution and Na and Ca ions on the illite surfacemay be represented by Eqs. (1) and (2):C~a+CZn~CZn+CNa ... (1)C:Ca+CZn~Czn+CCa ..• (2)where C represents the equivalent concentrationsof the ions concerned in the illite phase and CNa,eCa and CZn, the electrolyte concentration in thesolution. The equivalent ionic fractions of Zn , Caand Na in the clay phase and in the solution werecalculated from the expressions- CZn CZn X CNaXzn= c ,XZn = -C·-' Ka = C etc.where C' is the total electrolyte concentration inthe Clay phase and C that in the solution (C=Czn+CKa and C = CZn+CNa, ctc.].The values obtained for the equivalent ionicfractions at 30° and 60° are f.'ivE'n in Tables 1 and 2.The exchange isotherms are shown in Fig. 1 (curves1-4), The deviation of the isotherms from thediagoLal indicated a lower preference by illite forZn ions as compared to Na or Ca ions in the entirerange of concentration studied. It was also apparentthat the preference of Ca-illi te for Zn at 30° washigher than at 60°. No such correlation could be929INDIAN J. CHEM., VOL. 14A, DECEMBER 1976TABLE 1 - VALUES OF EQUIVALENTIONIC FRACTIONSANDSELECTIVITYQUOTIENTSAT 30° AND 60° FOR THE TABLE 2 - VALUES OF EQUIVALENTIONIC FRACTIONSANDZn EXCHANGEON Na-ILLITE SELECTIVITYQUOTIENTSAT 30° AND 60° FOR THElog x; Zn EXCHANGEON Ca-ILLITEXZn XZ" X:--;a XNa KcEXPERIMENTSAT 30° XZn x.; _YCa XCa Kc log Ki;0·112 0·305 0·808 0·694 0·2246 -0'64860·159 0·524 0·841 0·467 0·0969 -1,0137 EXPERIMENTS AT 30°0·204 0'623 0·796 0·376 0·0728 -1·13790·242 0'695 0·757 0·305 0·0565 -1'2480 0-073 0·524 0·927 0-476 0-0715 -1-14570-246 0·746 0-754 0-254 0-0371 -1-4306 0·191 0-595 0·809 0-405 0-1607 -0-79410-249 0·779 0·751 0-221 0·0277 -1·5575 0·313 0-583 0-686 0-417 0'3263 -0-48640·399 0·803 0·601 0-197 0-0536 -1-2708 0-422 0'576 0-578 0-424 0·5374 -0,26970·427 0·866 0'573 0-134 0-0270 -1'5686 0-482 0688 0-518 0·312 0-4219 -0,37480-439 0·910 0-561 0-089 0-0122 -1-9136 0-554 0-735 0-446 0-264 0-4461 -0-35600-482 0·924 0-518 0-075 0-0116 - 1-9355 0'628 0-807 0372 0·193 0-4037 -- 0-39390'604 0·931 0·396 0-069 0-0206 - 1-6861 0·737 0-822 0-262 0·177 0-6057 -0-21770-600 0·946 0-400 0-054 0-0118 -1,9882 0-784 0-847 0-215 0·153 0-6586 -0-18140·646 0·951 0·354 0·049 0·0108 -1-9666 0-820 0-789 0-180 0·121 0-6968 -0-1569EXPERIMENTSAT 60°0-066 0·324 0-934 0-676 0-1067 -0-9718 EXPERIME!,;TSAT 60'0·133 0-446 0-867 0·553 0-1213 -0-91620·133 0-573 0-867 0-426 0-0558 -1-2534 0-076 0-468 0-924 0'532 00934 -1·02970-235 0-617 0-765 0-382 0-0950 -1,0223 0-157 0-588 0-843 0·412 0·1304 -0-88480-319 0·646 0-680 0-354 0-1336 -0·8742 0-251 0-620 0-749 0-380 0-2053 -0-68980-329 0·686 0-670 0-314 0-1046 -0-9805 0·396 0-600 0-604 (J-400 0-4370 -0-35950-402 0-738 0-598 0·262 0·1049 -0-9793 0-474 0-692 0-526 0-307 0-3997 --0-39820-424 0·825 0-576 0-174 0-0464 -1-3335 0-471 0-754 (J-529 0-246 0-2904 -0,53740-458 0-893 (J-542 0-107 0-0191 -1-7190 0-609 0-797 0-390 0-203 0-3977 -0-40040-480 0·901 0-520 0·098 0-0197 -1-7055 0-748 0-807 0-252 0·192 0-7062 -0-15110·528 0·911 0-471 0·088 0·0208 -1-6819 0-819 0-828 0·181 0·171 0·9344 -0-02950·530 0-939 0·470 0·061 0-0102 -1'9914 0·864 0-846 0·136 0-154 1-1564 -0-06300-590 0-945 0-410 0·055 0-0111 -1-95471.00.9e e.e••0;0.'" 0.7~'-.su 0.6.sN'0c 0.5.s-u0.!:.~ 0.4e0C••"0 0.3.~:::JCT-ILl0.20.109300- -0 No- illite at 300e•...• Na-illiteat600eC>---O CO-illite at 300e.-. Co- illite at 60°CIIII///'"."9,A0.6 0.7 0.601 0_2 0.3 0.4 0.5Equivalent ionic fraction 01 zinc in solution (Xzn)Fig. 1 -- Plots of equi-valent ionic fractions ofzinc in illites against thatin solution0.9 1.0SINGHAL et al.: THER:.\IODYNAMICS OF EXCHANGE IN eLAYSobserved in the case of Na-illite, From theisotherms it was also inferred that the illite clayhad a higher preference for Na ions as comparedwith Ca ions at 30°. However, the effect at 60°was variable.To examine the interaction in the liquid andillite phases the selectivity coefficients at 30° and60° for different surface compositions of Zn weredetermined from the expressiolls9,10 (3 a-id 4)taking the ratio of activity coefficients a" nnityllin the dilute raugc studiedKc =~~~~Y;;~:for the Na-Zn exchange .. (3)s; = iZI! XCa for the Ca-Zn exchange ... (4)Ca XZnThe values for Kc (Tables 1 and 2) at differenttemperatures were plotted against XZI! (Fig. 2). Anexamination of the plots revealed that while theselectivity coefficients during the Ca-Zn exchangeincreased with an increase in the value of Xzn therewas a continuous decrease with a small rise inbetween, in the values of selectivity coefficientsduring Na-Zn exchange. The pattern of behaviourwas the same at both the temperatures ill the twocases. Such a variation was in accordance with0·10.0-0.1-0.2-0.3-0.4-0.5~ - 0·6••>C01 -0.7s-0.8C.!, -09-0:>0- -'.0>->-1.1-u -, .2.!•." -'.3-1.4-1.5-1.6-'.7-1.8·1.9-Z .00'0\\\\ ...•.,'" "'" ,~ '" .\ ,J \\ I \/~ \, '" \ I•..•. ,0 I, /.,.o.~\ " \ \C? " \ \I' \ \0../ 9 "I \I \, -,\ \\ \, \o 'Q ''"0.2 0.30.1the preference shown by Zn for the Ca-clay ascompared to Na-illite (Fig. 1).For a further study of the affinity the thermo-dynamic equilibrium constants K were calculatedfrom the relationship proposed by Gaines andThomas:'.1In K = (Za-ZbH- J In J{cdXzn ... (5)uwhere Za and Zb represent the charges on thecompeting ions. The integrals were evaluated fromthe areas under the curve" (Fig. 2) using the trape-zoidal rulc!-. Some uncertainty was involved inthis evaluation. The isotherms were extrapolatedat both the ends. Although justification12,13 forthe extrapolation was available from an examinationof the linearity ar.d finite slope of the isotherms atlarge and small values of Xzn, the values of Kobtained could not be classed as exact. In thediscussion that follows, the values for the equili-brium constant and other thermodynamic parametersgiven in Table 3 have, therefore, been consideredquali tatively only.The values of the thermodynamic equilibriumconstant (Table 3) were found to be lower at 30°0.4 0.5•...• No- illite ot 60°C•.....• Co-illiteot 60°C0·6 0.7 0·8 0.9 1.0Equivalent ionic fraction of zinc in illites (Xzn)Fig. 2 - Plots of selectivity quotient (log Kc) against equivalent ionic fractions of zinc in illites931INDIAN J. CHEM., VOL. 14A, DECE:'IBER 1976ThermodynamicparameterNa-Zn systemTABLE 3 - THERMODYNAMIC PARAMETERS FOR THE Zn EXCHANGE ON Na- AND Ca-ILLlTE AT 30° AND 60°Ca-Zn system30"KAGo [cal/rnole)AHo (calrrnole)ASo [calrrnole/deg)0·0498 0·0672+182085 +1775·95+2275611·500·4404 0·4232+494·40 +569·90-267'72-2·51than at 60° ill the Na-Zn system and higher at 30°than at 60° in the Ca-Zn system. These ",'ere indi-cative of the fact that the affinity of Na-illitc forzinc increased with a rise in temperature while thatof Ca-illitc decreased. Also the higher values ofK ill the Ca-Zn system as compared to the Na-Znsystem indicated that the affinity of Ca-illite for Znwas somewhat greater than that of Na-illitc for Zn.The positive !lG values (!lGo = -RT In 1\,)(Table 3) for the interactions indicated that bothat 30° and 60°, Na anr] Ca had a higher preferencefor illite than Zn and their replacement from illiteby Zn could not be spontaneous.The standard enthalpy changes !lHo were calcu-lated from the Van't Hoff [sochorc (6)In (KTZ) = _~Ho (~_.~_) ... (6)l\.Tl R r, r,The values are given in Table 3. It may be men-tioned that the values of ell thalpy calculated abovewere not the en thalpies of exchange alone. Theyincluded the en thai pie;; of hydration, di lutiou, mix; I'.gand, xchange. Also the method of (kkrmillationby measuring «qnilibrium cnustauts at L\\'O differenttemperatures involved an approximation to theextent of + 1 to 2 kcal, although whc-r« a smalltemperature range was involved Eq. 6, gaVl:reasonably satisfactory values.A positive ellthalpy effect during the Na-Znexchange pointed to a stronger binding' of Na ascompared to Zn on the illite surface. A smallnegative, enthalpy change during Ca-Zn ('xcl.\an.g(·,however, pointed to a somewhat stronger biudiruzof Zu as compared to Ca 01\ the illite surface. whichapp, arcd to be contrary to the assumptrous drawnfrom the adsorption isotherms.The standard entropy changes ss: were calculatedby the equation !lGu = I::!,.Ho-TASo and the valuesare given in Table 3.The observation based Oil enthalpy foundsupport from the entropy changes during theexchanges. The entropy changes arose from therandomness and disorder ill the exchanger andsolution phases as well as the hydration and dehyd-ration 14 of ions taking part in the exchange.Although a positive entropy effect dllril~g the932Na-Zn exchange may be ill agreement with a higherpreference on Na ions on the illite surface, yet itindicated a diffused and disordered arrangementof Zn ions in tho Gouy layer and a more orderedarraugcmcut of Na ions ill the Stern layer. Thegn'ater disorder in t lu- system appeared tobe due io the depletion of the hydration shellaround Zn iOllS caused hy Na ions during thel'xc\lal·.gc r.-actioi.s. 011 the other hand a l'.egativcentropy effect clnring the Ca-ZIl exchange indicatedthat the hydration shell around Zn ions remainedintact with Zn being bound at fixed, specific sitesOil the illite surface producing a greater order in thesystem. Valence, electrostatic considerations, theion sizes and the nature of the diffused electricaldouble layer in the two exchanges justified theseassumptions.Acknowledgementall,' of the authors (D.l<.) is grateful to the UGC,New Delhi, for providing a junior fellowship.References1. BDi(~lHM, F. T .. PACE, A. L. & SDIS, J. R., Soil Sci.S(I(. Alii. Prot ., 28 (1%4), 35\.2. DHILLO;>;. S. R., SI;>;I!A. :'II. K. & HANDHAWA, N. S.,.f. Indian Soc. Soil Sci., 23 (1975), 38.3. GAI;>;ES. (;. r.. <'I: THOMAS, H. C, J. phys. Chem., 21(\953). 714.4. Y.\NS.\KT, E. F. 0: U\·TTF.RHOEVEN, J. B., Clays andClay M inerots, 20 (1972), 47.5. SI;>;GIl.-\L. J. P. & Sl;>;GH, R. P .. J. soil su.. 24 (1973),2716. SI;>;GHAL. J.P., SlKGH. E. P., SINGH, C. P. & GUPTA,G. K, J. soil sn.. 27 (1976), 42.7. SIKGlIAL, J. P., SI:O:GH, R. 1'. & Kl;MAR, D., Colloid.Polv. su.. 253 (1975), 139.8. ] ,\CKSO:-'-. xt. L.. Soil chemical analysis (Prentice &Hall, Xew Jersey), lQ58. 57.9. ::'ofARI;>;SKY, J. .\., [Oil cxchange, a series of advances,vei. 1 (Xlarccl Dekker Inc., New York). 1966, 227.10. :\~!PHLETT. C. B., Inorganic ion exchangers (Elsevier,Amsterdam), 1964. 15.11. RORINSO:O<, R A. & STOKES, R H., Electrolyte solutions(Butterworths, London), 1959, 481. .12. EL-S,\YED, :'1. H .. BURAt', R. G. & BABCOOK, K. L.,Soil Sci. Soc . ..1111.PfOC., 34 (1970), 397.13. DUST. ]. & TALlHUDEEN. 0., J. soil Sci., 18 (1967),138.14. ]URINAK. J. J. & BAUER, N., Soil sa. Soc. Am. Proc.,20 (1956). 466

Studies on the Thermodynamics of Exchange in Clays: Part II - Zinc Exchange on Na- & Ca-Illites

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Studies on the Thermodynamics of Exchange in Clays: Part II - Zinc Exchange on Na- & Ca-Illites

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