Nov., 1959
ACIDICPROPERTIES OF BENTONITE
1917
ACIDIC PROPERTIES OF BENTONITE1 BY ARTHURC. THOMPSON~ AND J. L. CULBERTSON Contribution from the Department of Chemistry, State College of Washington, Pullman, Washington and of the Department of Physical Sciences, University of Idaho, Moscow, Idaho Received M a y $8, 1969
An investigation of the acidic properties of dibasic acid Wyoming bentonite was undertaken to determine the effects of aging of suspensions in the hydrogen form and a t various degrees of saturation of H-bentonite with sodium ion. The effects of suspension concentration, neutral salt addition and the cation of the titrating base on the dibasic acid property are described. Polyelectrolyte theory in particular is applied to account for the weak acid function of bentonite titration curves. Methods of modifying the acidic character of the Wyoming bentonite by processing bentonite suspensions on ion-exchange resin columns are described.
Introduction The dibasic acid titration behavior of Wyoming H-bentonite has been reported by various investigators. Characteristic of these titration curves is a first inflection a t pH values 4.6-5.0 and a second inflection at pH 7.8-8.4. Where the pH is plotted as a function of the meq. ( M ) of base per 100 grams of H-bentonite oven-dried a t l l O o , the position of the first inflection, MI, is found around 45 meq. The “ill”value for the second inflection, M 2 , corresponds to the titrated base exchange capacity of the bentonite; it is often found a t about 90 meq. Slabaugh3 has stressed the importance of the aging effect on the titration properties of H-bentonite suspensions. On titration of H-bentonite that has been allowed to stand a few months, the first equivalence point appears farther t o the Ieft on the abcissa of the pH versus “.iW” plot, the more so the older the suspension in the hydrogen form before titration. This effect is thought to be due to an auto-degradation of the clay structure by the adsorbed hydrogen ions according to Low4 and Coleman and H a r ~ a r d . This ~ apparent degradative process is presumed to occur in the process of preparation by electrodialysis or acid leaching and continues slowly when hydrogen saturated suspensions are allowed to stand. Low4 and Coleman and Harward5 have prepared H-Al-bentonites in various H/A1 ratios and have correlated the proportion of dibasic titration character to these ratios. I n order to determine more precisely the conditions under which this autodegradation occurs, the effect of aging H-bentonite suspensions and of suspensions partially neutralized with sodium hydroxide and in the presence of varying amounts of added neutral salt were investigated. Also, titration curves on pure H-bentonite suspensions were determined after varying periods of aging. The appearance of some H-bentonite titration curves suggests that for the weak acid function the pH values are linearly related to the function, log a 2/(1 - a 2 ) , where a z is the degree of neutralization of the weak acid function. It is of interest to consider the titration curves from the point of view of polyelectrolyte theory. I n this connection it should be possible to relate the pH values of H(1) I n partial fulfillment of the requirements for the Ph.D. degree
at the State College of Washington, Pullman, Washington. (2) Nalco Chemical Co., 0216 W. 66th Place, Chicago 38,Ill. (3) W. H.Slabaugh, J . A m . Chem. Soc., 14, 4462 (1952). (4) P. F. Low, Soil S c i . Chem. Proc., 19, 135 (1955). (5) N. T. Coleman and M. E. Harward, J. Am. Ckem. Soc., 1 6 , 6045 (1953).
bentonite suspensions a t various degrees of neutralization and at various suspension concentrations. Also, in this connection, the effect of saturation of the bentonite with aluminum ion is of interest. Since the apparent dibasic acid character of Hbentopite titration curves is affected by aging, it is of interest to determine if the effect of aging could be reversed by substitution of hydrogen ion for exchange site substituted aluminum. I n this connection, Low4 has reported titration curves of Wyoming H-bentonite obtained by addition of sufficient hydrochloric acid to silver saturated bentonite to convert all the ion-exchange site substituted silver ion to silver chloride. The Hbentonite obtained by this method is essentially monobasic and strongly acidic. Experimental The Wyoming bentonite used in this investigation was supplied by the National Lead Company, Baroid Division. A fine particle bentonite was dispersed to a 301, suspension and allowed to settle for several days. The settled portion was rejected. H-bentonite was prepared by forcing the 1% solids bentonite suspension through a deionizing resin column by application of an aspirator vacuum. A monobed resin performed the function of electrodialysis by exchanging the small cations for hydrogen ion and the anions present for hydroxyl ion. The titrations were carried out with carbonate-free, approximately 0.1 N NaOH. The pH measurements were made with a model “G” Beckman pH meter. The bentonite suspension was stirred a t a constant rate during a given titration, using a magnetic stirrer. Frequently, a conductance titration was carried out simultaneously using a dipping conductance cell. The H-bentonite suspensions were titrated after various periods of standing a t room temperature. The pH values at various standing times were measured for a series of partially neutralized samples which were prepared by addition of the requisite volume of base to 25 ml. of a 1.27% solids suspension of H-bentonite already 60 days aged. Each sample was diluted to 40 ml., making the suspension concentration 0.79% for all samples. It is possible to saturate bentonite with a given cation by passing the sodium or hydrogen bentonite through the cation saturated resin several times. Aluminum bentonite, however, was prepared by shaking the H-bentonite with sulfonic acid resin particles saturated with aluminum ion. Slow centrifugation of this mixture, followed by washing of the settled resin with distilled water, was sufficient to separate the resin particles from the Al-bentonite.
Results and Discussion Conductance and pH titration curves were run on suspensions of H-bentonite prepared by sulfonic acid ion-exchange resin after aging periods of 3 days, 60 days, 5 months and 18 months. The pH titration curve (Fig. 1, curve E) of a freshly prepared suspension is similar to that of an approximately 1:1 mixture of a strong acid and a weak acid. The
ARTHURC. THOMPSON AND J. L. CULBERTSON
1918
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M E 4 OF BASE P E R 100 GRAMS OF H - B E N T O N I T E
Fig. 1.-Effect of aging Wyoming H-bentonite and of recycling aged sample on H-sulfonic and Na-carboxylic ion exchange resins: ,,(A) H-bentonite 1.270/,, aged 18 months; (B) same as “A, aged 60 days; (C) sample “A” after 10 Na-H resin recycles; (D) Sam le “A” after 24 Na-H resin recycles; (E) titration curve 0!0.76% H-bentonite suspension from monobed resin column.
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’ i i i i i i 10 20 3 0 40 50 60 70 80 80 MEQ OF EASE P E R 100 GRAMS OF H - B E N T O N I T E ~
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Fig. 2.-Conductance and pH titrations of aged and resin cycled H-bentonite: A and B, 1 27% solids, aged 18 months; C and D, titration of aged sample after many Na-Carboxylic and H-sulfonic resin cycles.
effect of aging of H-bentoni te suspensions for 60 days resulted in a decrease of only 4 meq. in the titrated exchange capacity, Mz. The position of the first inflection, M I , shifted from 46 to 37 meq. The effect of aging is much more apparent for Hbentonite suspensions that were aged 18 months (Fig. 1, curve A, Fig. 2, curves A and B); the pH rises sharply on the addition of a few meq. of base and M z is reduced from 88 to 57 meq. The conductance curve for H-bentonite suspensions aged 18 months, which was obtained simultaneously with the p H curve, shows a steep decline in the conductance on addition of a few meq. of base to a minimum, which corresponds to the position and pH of the first equivalence point. The conduct-
Vol. 63
ance at first slowly, and then abruptly, increases on titration to the second equivalence point. The titration curves of the H-bentonite suspension aged 5 months or more resemble the titration curves of electrodialyzed H-bentonite reported by Marshall6 and LOW.^ Electrodialysis requires considerable time; it may be presumed that this process contributes to the degradation of the clay on aluminum ion saturated bentonite rather than Hbentonite being formed. However, the titration curve of an H-bentonite obtained by the rapid ion exchange process (Fig. 1, curve E) is almost identical with the titration curve reported by Slabaugh and Culbertson7 for electrodialyzed H-bentonite. It is concluded that electrodialysis or ion-exchange resin treatment is not responsible for any special degradation of Wyoming bentonite. It seems likely that the weak acid property is inherent in the particular sample; electrodialysis or processing the bentonite suspensions on ion-exchange resins will not remove exchange site substituted aluminum ion. The effect of aging partially neutralized Hbentonite suspeneions is shown in Fig. 3. For samples neutralized up to the first inflection, there is a very slow pH rise on aging. The pH curve of the suspensions aged for various periods of time cross the p H curve of suspensions aged for 3 days a t the position of the first equivalence point. From this point to about 80% neutralization of the weak acid function, there is a p H drop of 0.25-0.35 pH unit. The p H drop for suspensions to which excess base was added was much greater. There is a tendency for all the higher pH values to drop down to pH values of 7.5-8.0. The exchange capacity obtained from the position of the second inflection is somewhat larger than obtained in the usual titration. Varying amounts of neutral salt were added to partially neutralized suspensions of H-bentonite. The effect of salt addition is to decrease the pH. If sufficient salt is added, it is difficult to identify the position of the first equivalence point; the position of the second equivalence point is more sharply defined. The effect of aging these partially neutralized suspensions, 0.002 and 0.032 N in NaCl is shown in Fig. 4. The p H changes are qualitatively similar to suspensions to which no salt is added. The curves for fresh and aged suspensions cross near the first equivalence point. If sufficient neutral salt is present as in the case of the suspensions in curves C-1 and C-2, autodegradation of the clay due to hydrogen ion probably does not occur to an appreciable extent. Most of the hydrogen ion associated with the clay micelle is dissociated into the “outside solution” and does not participate in the autodegradative process. The concentration of hydrogen ion a t the surface of the bentonite particles (“inside solution”) is much greater in those suspensions to which no neutral salt was added than in suspensions in curves C-1 and C-2. These results indicate that the marked decrease in osmotically active hydrogen ion as indicated by ( 6 ) C. E. Marshall, “The Colloidal Chemistry of Silicate Minerals,” Academic Press, Inc., New York, N. Y.,1949. (7) W. H. Slabsugh and J. L. Culbertson, THIBJOUBNAL,56, 705 (1950).
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ACIDICPROPERTIES OF BENTONITE
Nov., 1959
1919
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p H and conductance measurements continues in samples of H-bentonite not neutralized beyond the first equivalence point and this process can occur at fairly high solution pH (up to p H 4.5). These results again suggest the involvement of adsorbed aluminum ion in the weak acid segment of the titration curve. The weak acid function may be due to the hydrolysis of exchange site adsorbed aluminum. The strong acid function is possibly due to the hydrogen ion that balances the negative sites not covered by aluminum ion. The bentonite particle may be considered to be an assembly of negative charges distributed on its surface to which hydrogen ions are bound as counterions. The dissociation of hydrogen ion from a given charge site increases the net charge on the particle from i to i+l. An intrinsic dissociation constant can be assigned to each undissociated hydrogen ion. For the special case of no interaction between charged sites the intrinsic dissociation constant IC is the same regardless of the position of the particular ionization site with respect to a previously ionized state. Similarly, the dissociation constant for the ionization of the ith 1hydrogen ion is the same regardless of the particular position of the dissociating site with respect to neighboring dissociated or undissociated sites. The ionization constant k i differs from the ionization constant, ICi+ I, by statistical factors only. An analysis of the neutralization of such an acid according to this simple model, using the statistical method outlined by Pauli and Valkos and by VeissY leads to the derivation of a modified HendersonHasselbach equation such as has been proposed by Kernlo and Kachalsky. l1 For a given concentration of polyacid; where the pK, is called the ‘(mean”pK and
0
pH = pK,
- n log 1- cc
where 0: corresponds to the half neutralization pH a t SL given concentration of acid; the slope factor n is equal to unity for a polyacid having the properties previously described. Such an analysis was carried out on the titration curve for the specially prepared series in which the concentration is 0.7970 and the suspension already 60 days old. If the pH versus the function log a 1/ (1 - 0: 1) is plotted for the strong acid function, the slope approaches one and is equal to one for cc 2 1/2. The HendersonHasselbach plots are given for both neutralizations of a fresh 0.76y’ solids H-bentonite in Fig. 5. For the weak acid function the slope n2 is found to be unity within experimental error. This is found to be true for the titration of fresh samples of H-bentonite when the base is simply added to the pH measured after stirring. For aged H-bentonite, the slope n2 decreases to lower values. (nz = 0.85 for the instantaneous titration of the 60 day old sample). For samples aged 5 months or more, the Henderson-Hasselbach plot is linear only over a short range and the slope is less than one. Apparently, one of the effects of the aging is a modi( 8 ) W. Pauli and E. Valko, “Electrochem. Kolloide,” 114, 115, Julius Springer, Vienna, (1929). (9) A. Veiss, THIS JOURNAL, 67, 1G9 (1953). (10) W. Kern, Mucromol. Chem., 2, 299 (1948). (11) A. Katohalsky and J. Gillis, Rev. trau. cfiim., 68, 879 (1049).
10 20 30 40 50 60 70 80 90 MEQ OF B A S E D E R IO0 G R A M S OF H - B E N T O N I T E
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Fig. 3.-Effect of aging partially neutralized samples of H-bentonite: (A) 0.79y0 H-bentonite base, samples from (B) aged one year; (B) 0.79% H-bentonite, aged 60 days before addition of base; (C) titration of 1.27y0 H-bentonite, aged GO days.
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Fig. 4.-Effect of addition of NaCl and aging on the pH of H-bentonite suspensions, 0.76% solids, prepared samples neutralized to various extents: A-1, no salt, aged 3 days: A-2, aged 18 months: B-l,O.O02 N NaC1, aged 3 days: B-2, 0.002 N NaC1, aged 18 months; C-1, 0.032 N NaC1, aged 3 days; C-2, 0.032 N NaCl, aged 18 months; all samples prepared from 1.27% stock suspension of H-bentonite aged 60 days.
fication in the character of the weak acid function. The apparent pK value, the pH at a = ‘/zfor an approximately 0.570 suspension of Al-bentonite was 6.76, or 2 to 3 times as great as the apparent kz observed for any of the fresh or aged H-bentonite of comparable concentration. For the titration curve the slope n2was found to be unity. An empirical relationship between the pH values at a given degree of neutralization can be derived by plotting the pH versus the logarithm of concentration in arbitrary units, the concentration of the most concentrated suspension being equal to 16A and the concentration of the most dilute suspension being equal to A . For the neutralization of the H-bentonite suspension to the first equivalence point, a linear relationship is found = 0 and 0 = 1.25 a t with slope e = 0.85 a t 0:1 = 0.5. For the neutralization of the weak acid function the slope e is equal to 1.25 between de= 0.9. The grees of neutralization a 2 = 0. l to pH is thus related to the logarithm of the concentration according to
1920
ARTHURC. THOMPSON AND J. L. CULBERTSON
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Vol. 63
strong acid cation-exchange resin. It is evident from Fig. 3 that only a slight effect on the titration curve and exchange capacity of the H-bentonite resulted from such treatment. However, when an H-bentonite suspension aged 18 months was titrated with base and then regenerated to the hydrogen form by processing on a strong acid resin column, the position of the first equivalence point on the pH-M scale was shifted 5-6 meq. to the right. Then, this same suspension was processed successively on sodium saturated carboxylic resinI2 and sulfonic resin (H-IR 120) for 5, 10, 15 and 25 cycles. The effect of this treatment of the bentonite suspension after 10 and 25 recycles is shown in Fig. 1, curves C and D. By this method, an Hbentonite was produced which titrated very much like a strong monobasic acid. The processing of an H-bentonite suspension on the sodium saturated carboxylic resin was equivalent to titration to the second equivalence point. The pH of the eluate was 7.6, which is the pH of the second equivalence point in the H-bentonite titration. However, when an H-bentonite suspension was passed through a column of sodium saturated sulfonic acid resin, the pH of the eluate was only 5.2. Apparently, the weak acid sites on the bentonite were not saturated with hydrogen ion in €his case since the weak acid protons are not readily exchanged by the sodium ion of the sulfonic acid resin. The sulfonic acid groups of the strongly acidic resin have a lesser affinity for protons than the carboxylic anionic groups of the weak acid resin. The results for this series of titrations are shown in Fig. 1 and 2. In this process, the weak acid property is almost entirely removed; the exchange capacity of the titrated fresh sample is recovered. Apparently, the aging effect does not result in a permanent loss of titrated exchange capacity. It seems likely that the weak acid function, apparently due to exchange site adsorbed aluminum, of the H -bentonite prepared by electrodialysis, acid leaching, or ion-exchange column processes, does not necessarily result from an autodegradation during the preparation. This property is not removed by any simple application of these processes, though the acid-leaching process might be expected to remove some of the adsorbed aluminum ion. This work suggests that the weak acid property is dependent on the past history of the sample and can be modified by aging in the past history of the sample and by aging in the hydrogen state or, in the opposite way, by repeated recycling on ion-exchange resin as previously described. Acknowledgment.-The authors wish to acknowledge Dr. W. H. Cone and Mr. C. C. Cowin of the Department of Physical Sciences of the University of Idaho for their cooperation in providing materials for the experimental work. (12) CS-101 from the Chemical Proceaa Co., Redwood City, Calif.
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