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J. J. JURINAK
Vol. 65
THE EFFECT OF PRETREATMEKT O N THE ADSORPTION AKD DESORPTION OF WATER VAPOR BY LITHIUM AND CALCIUM KAOLINITE' BY J. J. JTJRINAK Department of Soils and Plant Nutrition, University of California, Dacis, California Received Mag 16. 1960
The adsorption and desorption of water vapor by lithium and calcium kaolinite degassed a t 33, 70, 100 and 205" was studied. Li-kaolinite degassed at 33 and 70' resulted in the reversible adsorption-desorption of water vapor. Hysteresis became prevalent at the higher degassing temperatures. Reversible dehydroxylation was considered initiated a t 205". Ca-kaolinite adsorbed more water and hysteresis was noted at all pretreatment levels. Factors considered important in the variability of water adsorption by Li- and Ca-kaolinite, in the degassing temperature range studied, are ionic hydration, particle coalescence and surface dehydroxylation.
Introduction Previous studies have indicated that, following the dehydration of a lithium-saturated kaolinite, the adsorbed lithium ions have no apparent effect on subsequent water adsorption.2 This property has been attributed to the small ionic size of lithium which permits the unhydrated ion to fit into the surface structure thus becoming sterically hindered from interaction with water vapor. Recent investigation^,^^^ however, have indicated that this view may be oversimplified. Despite the uncertain relation of the lithium ion with kaolinite the attraction of this system as a reference for adsorption studies increases the need for data concerning the availability and stability of Likaolinite with regard to the water molecule, particularly in the monolayer region. This paper presents the results of the effect of degassing temperature on the adsorption and desorption of water vapor by lithium kaolinite as compared with a similarly treated calcium kaolinite.
stopcock to the adsorption chamber is closed and the equilibrium vapor is compressed by filling the gas buret with mercury. The resultant pressure is read and the initial equilibrium pressure is calculated, using the ideal gas law. The compression coupled with the Pearson gauge yielded a magnification factor of better than 2000 to the equilibrium pressure with little uncertainty. Pressure as low as 7.6 X lo-* f 3.8 X mm. could be read. Approximately 2-3 grams of clay were placed in the removable adsorption chamber and the isotherms were determined volumetrically. An agreement of 2 4 % was obtained when calculations were checked gravimetrically by periodic weighing of the adsorption chamber plus sample. The degassing temperatures used in this study were 33, 70,100 and 205 i 2' and were maintained by use of a portable furnace. The weight loss associated with each degassing treatment was determined by removing the adsorption chamber from the system and weighing. Duplicate samples were reproducible within f 0.3 mg. Both the water and n-butane in this study were degaased thoroughly by repeated freezing and thawing under vacuum. Adsorption and desorption were carried out at 29.45 i 0.05".
Results Figures 1 and 2 (curve A) show the adsorption and desorption of water vapor by Li-kaolinite degassed for 48 hours at mm. pressure a t the Experimental Methods designated temperatures. Pretreatment a t 33 The kaolinite was from Dry Branch, Georgia, and was and 70 O resulted in identical adsorption-desorpdonated by the Georgia Kaolinite Company. X-Ray dif- tion isotherms with no hysteresis noted at either fraction analysis showed no visible montmorillonitic contamination which would indicate a minimum purity of about pretreatment level. As the degassing tempera95y0. The exchange capacity was determined as 1.5 ture was raised to looo, the adsorption of water meq./100 g. with regard to the lithium as well as the sodium decreased and hysteresis became prevalent. Inion which further suggested a minimal, if any, contamination creasing the temperature to 205" not only increased by montmorillonite. The surface area was determined by nthe adsorption of water over the 100" treatment but butane adsorption at 0.0" and was calculated as 15.0 m.e/g. The kaolinite was saturated with lithium by washing with also changed the nature of the hysteresis curve. normal lithium chloride a t pH 6.8 until the leachate con- The data shown a t each temperature are a composite tained only lithium ions and then washed free of chlorides of two or more independently run samples indicating with 95% ethyl alcohol. A similar method was used for calcium kaolinite, except the pH of the normal calcium the reproducibility of the system. Figure 3 shows chloride was 6.1. The clay was air-dried, crushed, and the the adsorption of water vapor by Ca-kaolinite 0.5-1.0 mm. aggregate fraction retained. This material which had been degassed in a manner similar to represented the initial state of clay samples used in this Li-kaolinite. Samples degassed a t all four temstudy. The adsorption apparatus has been described previously5 peratures produced hysteresis upon desorption. except that the volume ratio of the manometer to the capil- The data are not shown. At all pretreatment lary arm of the Pearson gauge has been modified to give a levels the adsorption of water by Ca-kaolinite was pressure magnification of better than 50 times. The low greater than Li-kaolinite. The effect of degaspressure equilibrium readings were obtained by the previously described compression method,6 whereby, after sing temperature on n-butane adsorption is shown equilibrium is reached between the sample and vapor, the in Fig. 4. The data are a composite of 4 adsorption-desorption cycles by Li-kaolinite which (1) Support of this work by a grant (NSF-G10228) from the Kahas been degassed a t 33, 70, 100 and 205". No tional Science Foundation is gratefully acknowledged. indication of pretreatment effect was noted. h (2) (a) A. G Keenan, R. W. Mooney and L. A . Wood, THISJOURsummary comparing the pretreatment effect on the SAL, 56, 1462 (1951); (b) R. T. Martin, "Clays and Clay Minerals," adsorption of water by Li- and Ca-kaolinite is monograph 82. Pergamon Press, New York. N. Y., 1959, p. 259. (3) G . H. Cashen, Faraday Sac. Trans., 65, 477 (1959). shown in Fig. 5. The primary mechanism con(4) R. Greene-Kelley, THISJOURNAL, 59, 1151 (1955). sidered responsible for the adsorption variability (5) F. A. Bettleheim, C. Sterling and D. H. Volman, J . Pclymer is designated in its appropriate temperature range. Rei., 22, 303 (1956). Table I gives the total volatile weight loss recorded (6) J. J. Jurinak and D. H. Volman, Soil Sci.. 88, 6 (1857).
ADSORPTION AKD DESORPTION OF WATERVAPORBY LI AND CA KAOLINITE
Jan., 1961
o
63
Adsorption Desorption
+--
1
2
3
4
5
6
7
8
P (mm Hgl,
Fig. 1.-Adsorption-desorption isotherms of water vapor a t 29.45' by lithium ka,olinite degassed at various temperatures.
"
1
2
3
4
5
6
;
P ( m m Hgl,
Fig. 3.-Adsorption of water vapor at 29.45' by calcium kaolinite degassed at various temperatures.
o Adsorption
Desorotion
"
,
._..L__,
"0
I
2
3
4
P
(mm
5
6
7
Hg).
Fig. 2.-Adsorption-desorption isotherm of water vapor a t 29.45' by lithium kaolinite degassed a t 205' (curve A). Curve B is the adsorption-desorption data of hydroxylated lithium kaolinite degassed at 100" as compared with the corresponding system in Fig. 1 (dash line).
at each degassing temperature and the pH of a 1 Fig. 4.-Composite adsorption-desorption isotherm of to 5 clay-water suspension after 24 hours of equili- n-butane a t 0.0" by lithium kaolinite degassed a t 33, 70, 100 and 205'. bration. TABLE I PRETREATMENT EFFECT O N SAMPLE WEIGHTLoss Degassing temp., OC.
Lithiam kaolinite Total w t . loss,' rug. pH
33 16.2 70 17.2 100 17.2 205 20.3 a Based on 2-g. sample.
6.80 6.80 6.45 5.35
AND
Calcium kaolinite Total wt. loss.0 mg. pH
16.8 18.9 19.9 22.8
6.13 6.12 6.37 6.63
pH
Discussion The data in this study are explained on the basis of the difference in the relation of the adsorbed cation with the surface, which in turn depends largely on the rglative ionic radii of the cation, ie., 0.60 and 0.99 A. for lithium and calcium, respectively. (7) L. Paulina. "The Nature of the Chemical Bond." Cornell Univ. Press, Ithaca, N. Y.,1948, p. 346.
J. J. JURINAK
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Vol. 65
3'5r---F77 Calcium
is explained either by competitive hydration of the calcium ion as additional water is removed or to the protrusion of the calcium ion on the surface which hinders the union of individual clay platelets. These results are in line with the data presented by Martin* which showed that prolonged aging of Li-kaolinite, under vacuum, resulted in particle coalescence and reduced water adsorption. The substitution of Cs for Li as the exchangeable ion produced only a slight coalescence. A degassing temperature of 205" resulted in greater water adsorption in both systems. The increase in adsorption by Li-kaolinite coupled with the hysteresis curve change (Fig, 2, curve A) and weight loss of the sample suggested that the 205" pretreatment initiated surface dehydroxylation and the addition of water resulted in hydroxylation of the surface. Evidence of this reaction and its reversibility was obtained by taking the hydroxylated sample of curve A, degassing it for 48 hours a t 100" then measuring an adsorption-desorption cycle of water. The data are shown in Fig. 2, curve B, and indicate the reversible nature of the proposed reaction. The dash line is the adsorption-desorption curve taken from Fig. 1. The question arose whether the surface variability as shown by water adsorption is reflected by the adsorption of n-butane. Figure 4 shows the inertness of n-butane adsorption to pretreatment and indicates that the surface of Li-kaolinite available to the n-butane molecule (44 is unchanged in the temperature range studied. The data of Gregg and step hen^,^ using oxygen as the adsorbate, showed a decrease in the surface area of kaolinite when the outgassing temperature was varied from 50 to 100". The area remained constant until the outgassing temperature reached 200" or greater, then a continual downward trend in area was noted. Nitrogen adsorption showed a more general continual decrease in the low outgassing temperature range. These data along with the results of this study suggest that the postulated coalescence of kaolinite manifests itself in the alteration of the microstructure of the system and hence is sensitive to the adsorbate's dimensions. Figure 5 summarizes the adsorption variability of Li- and Ca-kaolinite as well as the mechanisms involved as a function of degassing temperature. The over-all greater water adsorption by Cakaolinite is ascribed to ion hydration, since a t least in the dehydrated state the surface area of kaolinite is independent of exchangeable cation. A further indication of the difference in which lithium and calcium ion react with kaolinite is shown by the suspension pH given in Table I. Although this p H variation has been previously noted,a the definite cause remains to be studied. (8) R. T. Martin, "Clays and Clay Minerals," Nat. Acad. Soi. Nat. Res. Counoil Pub. 566, p. 23, 1958. (8) S. J. Gregg and M. J. Stephens, J . C h e n . Soc., 3951 (1953).
