CATION-EXCHANQE BEHAVIOR OF C L U E
1429
THE CATION-EXCHANGE BEHAVIOR OF m T E R O I O N I C AND HOMOIONIC CLAYS OF SILICATE MINERALS A. K. GANGULY AND 5. K. MUXHERIEEL University College of Science and Technology, Cdcutkr, India Received July 17, 1060 INTRODUCTION
The present-day approach in the interpretation of the cation-exchange behavior of clay and other minerals is through their characteristic crystalline structures. This approach has been fruitful in many ways but complications still exist. Cation-exchange property, like any other property, may be characterized by a capacity factor and an intensity factor. Base-exchange capacity determines the capacity factor. In a recent paper (1) one of the authors (A. K. G.) has attempted to correlate some of the complicating factors regarding this capacity factor with the known features of a number of crystalline minerals. The intensity factor is usually expressed as the relative distribution of the interacting cations between the colloidal clays and electrolytes and is commonly represented by the well-known exchange isotherm. According to Jenny’s suggestion (5) it has been the practice to determine what he termed the symmetry value, which is but a single point on the exchange isotherm. The suggestion contains the fundamental assumption that all the exchange spots are identical. Deviations from the lyotrope series are not unusual. Complications also arise owing to the presence of hydrogen ions (5, 16). The presence of more than one kind of cation in the exchange substance introduces further difficulties. Wiegner (22) invoked the idea of metastructure involving intra- and extra-micellar exchange to explain the experimental data of Mitchell (10) and Renold (19) on ionic exchange in heteroionic permutites, kaolinites, bentonites, and chabasites. For instance, it was observed that two exchangeable cations, MI and Mn, showed a marked difference in their exchangeability depending on the history of their formation. Any one of these cations could be less readily exchanged if it were the first to be incorporated into the heteroionic system. The interaction between the colloidal clay acids and alkalis has necessitated the postulation of mobile and bound hydrogen ions, which constitute the electrical double layer (13, 14). I t has been further possible to distinguish kaolinites and micas, respectively, as dibasic (11) and tribasic (12) acids on the basis of their potentiometric titrations with alkalis and of measurements of their exchange capacities (1). The bonding energies of cations, observed with hydrogen ions, should also appear in the case of other cations. The present investigation is aimed a t an understanding of the differences in the binding of metallic ions to clays and other minerals in relation to their crystal chemistry. 1 Lecturer in Chemistry, Calcutta University (on leave); at present postdoctorate research worker in the University of Missouri, Columbia, Missouri.
1430
A . K. GANGULY AND S. K. MUKHERJEE
EXPERIYEKTAL MATERIALS
Cation exchange of heteroionic clays was studied with colloidal salts prepared from a bentonite constituted of montmorillonite alone. The exchange property of the heteroionic systems depends on the method of preparation, which is described later. In order to study the symmetry values and to make a detailed investigation of the exchange isotherms other minerals, such as a kaolinite, an illite, and a mica, were used in addition to the bentonite. The clay fraction (I2 was introduced
first and h l l second.
1432
A. K. QANQULY AND 8. IC. MUKHERTEE
explaining the difference in the exchangeability of either sodium or barium in the two bentonites. In order to explain this, one hss to assume that the exchange spots are not of equal value, that there are certain “privileged” positions which exert a much stronger binding force on the cation than the others, and that these are occupied by the cations which are first incorporated into the system. Again, the proportion of a total number of such “privileged” spots may be more accessible to divalent than to monovalent cations. The lower “critical” TABLE 3 Effect of varying cation ratios on exchange i n heteroionic bentonites
--__
NIL:BPIN~*Y
‘B N m U AYYOMM BSNTONI1E + CaClr “4 uch.ngal per c&
9.0 2.3 1.0 0.42 0.11
BP CIChrnged
pn C&
65 50
pn
CMll
70 56 44 23 10
30
8 17 22
Na exchanged
Ba exchanged
pn C r n l
pn cw
per Cenl
per Ccnl
9 16 26
88 72 59 33 18
9 17 24 31
79 67 55 33 18
40
BALIM CALCIUM BENIONlIE
.o
Bh eIChanged
CCRl
22 10
Ca:Ba IN m r
9.0 2.3 1 0.42 0.11
per
8 17 25 36
40
9.0 2.3 1.0 0.42 0.11
NH. exchanged
+ MgCh
Ca CXChnged
Ba exchanged
per cml
per ccml
36.6 29.9 22 .‘J 13.3 5.6
12.1 15.9 25.9 36.7
Ca exchanged per c&
42.2 32.8 2Q.6 13.3 6.1
’
Ba exchanged
pn cenl
11.0 16.8 21.7 30.7
ratios observed with monovalent cations compared to the divalent ones may perhaps be understood in this light. The study of exchange isotherms of homoionic clays discuased later on gives, however, a clearer view of this differentiation.
Homoionic clay salts As already mentioned, the homoionic colloidal salts were prepared by adding the requisite amounts of bases to the hydrogen clays. The final colloid contents
1433
CATION-EXCHANQE BEHAVIOR OF CLAYS
of the salts of the same mineral were kept the same, and the equilibrium WBB followed by means of periodical pH measurements of the suspensions. Most of the systems required a t least 3 days to attain a more or less constant pH. Urnally a slow interaction proceeded even after this period and therefore the systems were kept for about a week before measurements were made with them.
Symmetry values and th lyotrope series The symmetry values were determined as follows: To the clay salt was added an amount of cation in the form of a chloride which was equivalent to the baseexchange capacity of the clay itself. After the mixture was allowed, with occasional shaking for 10 days, to attain equilibrium, the clear supernatant liquid TABLE 4 Syniinetry values of bentonites Eouilibrium uH = 7.55
1
SYSTEM
Ammonium bentonite. . . . . . . . . . . . . . . . . . . . . . . . . Calcium bentonite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Barium bentonite.. . . . . . . . . . . . . . . . . . . . . . . . . .
NaCl
BaClr
82.4 8 7 . 8 71.6 73.8 78.8 40.4 44.3 16.1
60.0 78.6 17.7
-__
63.6 54.5 48.5 32.7 49.1 11.4 25.2 20.2 11.5 23.5 21.8
__
CaClr
MKlz
KCI
__
_ .
88.0
--
TABLE 5
8Yrn.Y
Sodium illite. . . . . . . . . . . . . . . . . . . . . . . . Potassium illite.. . . . . . . . . . . . . . . . . . . . Ammonium illite.. . . . . . . . . . . . . . . . . . . . Calcium illite.. . . . . . . . . . . . . . . . . . . . . . . Barium illite. . . . . . . . . . . . . . . . . . . . . . . .
NXCI
60.0 47.4 27.6 25.0
MgClr
'I !
41.7 69.0
cacll
BaCh
76.5
83.7 67.7 70.0 50.5
70.8
-L;
51.2
obtained either by centrifuging or by ultrafiltration was analyzed. The amount of cation released or taken up, expressed as percentage of the total quantities added, determined the symmetry value. Tables 4, 5, and 6 give the symmetry values of the different c l w salts of bentonite, illite, and mica, respectively, against various cations. If the symmetry values of a given clay salt against various cations are compared, the orders of replacement are as shown in tables 7, 8, and 9. It will be observed from the above results that the lyotrope series is generally followed. The following peculiarities are, however, noticed: ( 1 ) potassium ion has a greater replacing power in most of the mica systems than in the bentonites. Also, the symmetry values of potassium mica against ammonium ion and
1434
A. K . GANGULY AND S . K . MUKHERJEE
calcium ion are much lower than the symmetry values of potassium illite and potassium bentonite against those cations. That potassium ion is held comparatively fast on a mica surface is plausible, owing to its peculiar position in the crystal lattice of mica. A similar preferential adsorption of calcium ion by T.4l3LE 6 Symmetiu values of mica< Equilibrium pH = 8 1
1
SYSTEM
KCl
1
80.0
1,
71.3 17.7
Sodium mica. . . . . . . . . . . . . . . . . . . Potassium mica.. . . . . . . . . . . . . . . . .. .; Ammonium mica . . . . . . . . . . . . . . . Calcium mica . . . . . . . . . . . . . . Ihrium mica . . . . . . . . . . . . . . .
1
NHLI
I
CaClt
1
1
21.3 14.7
’
15 5 71.4
1
BaClr
91.3 77.3 83.5 82.6
8.7
~
TABLE T Order of symmetry vullies with bentonilea Sodium bentonite . . . . . . . . . . . . . . . . . . . . . . . . . . Potassium bentonite. . . . . . . . . . . . . . . . . . . . . Ammonium Ixntonite . . . . . . . . . . . . . . . . . . . . . . . Calcium bentonite.. .............................
Order
0.f
~
> M g > K > NH4 > S F 1 1 > Ka = Ca > hlg > K > K a
R a = Ca Mg
I Ba Da
>Mg2
li
> KH4 > N a
TABLE S summetry vulues with illitrs Ba > Ra > l3a = B:L > Ca >
Sodium illite.. . . . . . . . . . . . . . . . . . . . . . . . . . . Potassium illite . . . . . . . . . . . . . . . . . . . . . . . . . . . . rZmmonium illite.. Calcium illite . . . . . . . . . . . . . . . . . . . . . . . . . . Barium illite..
Ca > KH4 NIT4 2 RIg Ca > b l g \Ig Jlg
>> ”4 > KHC
-
‘ r A m L E9 Order of srlinmetry valiies with micas Sodium mica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I’otassium mica Ammonium mien . . . . . . . . . . . . . . Calcium mica.. r . . . . . . . . . . . . . . . . . . . . . . . . . . h r i u r n mibx..
i ’
Ba > K 1 XH4 Ba >> C a >> NHd K 2 Ba > Ca
K >SIlr
bentonites has been observed by Vanselow (21) and Kerr (€9,who postulated the hypothesis of “mixed crystals” in such cases. ( 2 ) Calcium ion has a greater effect than magnesium ion. Jenny (6) observed a stronger relative effect of magnesium ion than of calcium ion, whereas Wiegner (22) found the reverse. According to Wiegner, the greater effect of magnesium ion found by Jenny is
CATION-EXCHAKGE BEH.\VIOR O F CLAYS
1-135
to bc attributed to the “previous history” of the clay and its method of prepartition. A comparison of the symmetry values of various clay salts against a particular clcct idyte shows that the lyotrope serics is generally obeyed. Ammonium ion and potassium ion, and also barium ion and calcium ion, are found to interchange their positions. The lyotrope series first observed by Hofmeister (4) in the case of proteins has sincc been found to hold in numerous systems. The relative effects of the cations may vary with the systems, in which case we are to take into account the nature of the surfaces and the ions involved in the eschange reaction, but the gencral validity of the lyotrope series may be taken to demonstrate that the eschange phcnomenon in various systems is principally due to the same type of electrochemical forces. For particular systems the prevailing geometrical pcculiaritirs may, however, account better for the dcriation from lyot,ropic sequence. In general, the lyotrope effect is determined by the valency and mobility (13, 14) and the hydration (23) of the cations. Loeb (9), hoirevei., denied the existence of the Iyotrope series, at least in protein systems, ant1 he claimed to have shown that if the hydrogen-ion concentration were taken into consideration, the differences between the effects of various cations of the same valency disappeared. Gortner, Hoffman, and Sinclair (3) showed, on the other hand, that the hydrogen-ion concentration alone failed t o account for the lyotrope series observed by them in their \vork on the peptization of proteins by various electrolytes. In order to find out the influence of hydrogen-ion concentration on the eschange powers of different cations and hence on their lyotropic effects, symmetry values of sodium, potassium, and ammonium bentonites against various chlorides were measured at three pH values. Requisite quantities of hydrochloric acid were added to the clay salts and the equilibrium pH’s were adjusted at 3.0, 5.0, and G . G . The centrifugate was as usual analyzed and symmetry values were calculated as before. The results are presented in table 10 and shown graphically in figures 1 and 2 . The symmetry values against all the cations increase with diminishing pH. This is due to an increasingly greater share taken by the hydrogen ions in the eschange of cations as the pH decreases. From the curves in figures 1 and 2 it will be seen that the lyotropic order is gencrally retained at low pH but that the effects of the different cations become less and less marked as the pH decreases; possibly at a still l o w r pH the effects will almost disappear, owing to the overriding influence of the hydrogen ion These results confirm the earlier findings of Nukherjee and Mukherjee (15) as to the depcndence of symmetry values on hydrogen-ion concentration. If the amounts of the added cations adsorbed by the bentonites in competition with the hydrogen ions are estimated, the lyotrope effect is shown to persist even at the low pH of 3.0. This is illustrated by the results obtained with a magnesium clay3 and shown in table 11. J
.I kaolinite clay containing :i small amount of montmorillonite as impurity
1436
A. K. QANQULY AND 8. K. MUKHERJEE
The amounts of magnesium actually entering the clay have been estimated after displacing it with a third cation. These amounts (shown in column 7) are in agreement with the lyotrope series, though the symmetry values against all these cations are nearly the same. TABLE 10 Symmetry values of bentonites a f dejnite p H valrtea At pH 3 . 0 Sodium bentonite.. . . . . . . . . . . . . . . . . . . . . . Ammonium bentonite.. Potsasium bentonite. . . . . . . . . . . . . . . . . . . .
~
89.2 81.0
~
96.6 92.0
1
100.0 100.0 91.0
At pH 5.0
I 1 I 1
Sodium bentonite.. . . . . . . . . . . . . . . . . . . . . . . 76.5 Ammonium bentonite.. . . . . . . . . . . . . . . . . . . 65.4 76.5 Potsesium bentonite.. . . . . . . . . . . . . . . . . . . . 65.0
81.0
91.5 ~
~
80.0
~
78.0
86.4 92.4 85.0
~
91.5 97.6 90.7
At pH 6.6
Sodium bentonite.. . . . . . . . . . . . . . . . . . . . . . . i Ammonium bentonite.. . . . . . . . . . . . . . . . . . 51.0 65.2 I Potassium bentonite . . . . . . . . . . . . . . . . . . . 54.0 ~
21
0
:I I 3
~
40
20
~
ao
60
87.5 62.2
1 1 93‘2
90.0 98.0 64.2
100
$ Displaced at symmetry concn.
Fro. 1. Variation of symmetry values with pH
+ + +
potassium bentonite HCI sodium bentonite HCI potassium bentonite KaC1
1
!
~
+ +
+
Symmetry value and “hysteresis” in cation exchange The exchange reaction may be schematically represented as follows:
M1 clay
+M
a S MI clay
-
potassium bentonite MgClt potassium bentonite CaClt sodium bentonite BaCI,
+ MIX
1437
CATION-EXCHANGE BEHAVIOR OF CLAYS
The symmetry value of the M1 clay against Mz and that of the Mi clay againet M1determined separately should add up to 100.0. Vanselow (21),Schachtschabel (20),and Gieseking and Jenny (2) found that this is not always true and that it often falls short of 100.0.From analogy they called it a “hysteresis” effect. The results given in tables 12,13, and 14 show that the sum of the symmetry values of such pairs of *stems aa above is in most cases less than 100.0,irrespective of the nature of the cations. Since the experiments were performed at
2J-
40
20
0
60
80
100
$ Displaced a t symmetry Concn. FIG.2. Variation of symmetry values with pH
ammonium bentonite ammonium bentonite ammonium bentonite
+ HC1 + NaCl + KCl
ammonium bentonite ammonium bentonite ammohium bentonite
syyLLECTnOLYII
pH
YAGNLSIDY IN
slution
millie&o.
................................ CaCl;. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ”&I.
BaCl;. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.98 3.00 2.95 2.91
W D W CATION IN
YEITY
__----KCI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+ MgClr + CaClt + BaClt
65.9 62.4 63.9 63.4
0.1320 0.1258 0.1286 0.1276
clay
s~~ution ~ k y
millie~io.millicpuia. nillicquio.
0.0681 0.0755 0.0745 0.0741
0.1996 0.0025 0.1236 0.0783 0.1042 0.0976 0.0879 0.1145
1438
A. K . GANGULY AND S . K . MUKHERJEE
solutions in each of the pair of reactions will have deficit of the cation with which it w&s initially saturated; hence the sum of the symmetry values will fall short of 100.0. TABLE 12 “Hvsleresis” effect in bentonite syste~izs SYKMETBY VALUE
SYSTEM
+
SUM
Sodium bentonite KCI.. . . . . . . . . . . . . . . . . Potassiuni bentonite NaCl.. . . . . . . . . . . . .
+
97.6
Sodium bentonite NH4Cl. . . . . . . . . . . . . . . Ammonium bentonite NaCI. . . . . . . . . . . . .
85.2
+
+
+ CaCI, . . . . . . . . . . . . . . . . + NaC1 . . . . . . . . . . . . . Sodium bentonite + BaCl?. . . . . . . . . . . . . . . Barium bentonite + N a C l . . . . . . . . . . . . . . . Potassium bentonite + ”&I.. ........... .4mmonium bentonite + K C I . . . . . . . . . . . . . . Ammonium bentonite + CaCla.. . . . . . . . . . . . Calcium bentonite + NR4CI.. . . . . . . . . . . . . Ammonium bentonite + BaCI, . . . . . . . . . . . . Sodium bentonite Cdciuni bentonite
Barium bentonite
+ “&I.
87.8’ 11.4)
99.2
88.0 11.5)
99.5
48.5)
. . . . . . . . . . . . i.
9i.6
78.11 20.q
98.3
78.6) 22.33
100.9
04.1
TABLE 13 “Hysteresis” eflect i n illitc systems
___
’
SYSTEM
~_____
+
SYMMETRY VALUE
Calcium illite I3:trium illite
.i
~
+ BaCLr. . . . . . . . . . . . . . . . .
+ C:iCI,. +
~
. . . . . . . . . . . . I
Barium illite S H O .................. Amnionium illite 13uClp.. . . . . . . . . . . . .
+
__-___ SUM
-,-
C:Llciuni illite S H i C I . .. . . . . . . . . . . . . . . . . .kininonium illite CaCli . . . . . . . . . . . . . . . . .
+
1
I ~
-~
98.4
70.8 50.5) 51.2,
25,01 70.0
101.7 ~
05.0 ~
___
This state of affairs is markedly altered in the presence of even a small amount of hydrogen ions, The hydrogen ions in competition with the added cations will displace more of t,he adsorbed cations from both the systems; hence t,he sum will be greater than 100.0. This is illustrated by the results (table 15) obtained with
1439
CATION-EXCHANGE BEHAVIOR OF CLAYS
a pair of bentonite systems; a slight lowering of pH causes the sum of the s w metry values to increase well above 100.0.
Symmetry value and mineralogical composition of clays From the summary of results given in table 16 emerge the following points regarding the mineralogical make-up of the minerals and their exchange behavior: ( 1 ) Potassium ion displaces the largest amount of cations from all the TABLE 11 “Hysteresis” effect in mica systems
1
SYSTEY
SYMMETRY VALUE
+
I’ot:rssium mica BaCI2. . . . . . . . . . . . . . Barium mica KCI. . . . . . . . . . . . . . . . .
+
77.3 17.7)
~
+ + KCI I Potassium mica + CaCI?. . . . . . . . . . . . . . . . . I Calcium mica + KCI . . . . . . . . . . . . . . . . . . . . . i Ammonium micz + CaCI,. . . . . . . . . . . . . . . . .1 . Calcium mica + NHdCI. . . . . . . . . . . . . . . . . . . . . Ammonium mica + BaC12.. . . . . . . . . . . . . . . Barium mica + XH,CI.. . . . . . . . . . . . . I.
II
suy
95.0
I
Potassium mica Ammonium mica
102.5 ~
71.3 15.5] 71.41 16.0\ 83.5 R.71
86.8 88.3
~
I
92.2
TABLE 15 Influence of increasing hydrogtmion concentration on (he symnietru values of benloiiilc ~~
S Y K X E T R Y VALUE
+ BaCIJ + SaCl Ammonium bentonite + BaCI2.. . . . . . Barium bentonite + SHaCl. . . . . . ., __ Sodium bentonite Barium Iientonite
~
~
6.78 7.11
94.8 17.71 72.01 22.01
, ~
1
srialons -
SUM
“’2” 01.0
micas except barium mica; conversely also, ammonium ion and even calcium ion is able to replace only small proportions of potmsium ion from potassium mica, but barium ion does it to a large extent; (2) the monovalent cations are generally less easily replaceable from illite than from bentonite; (3) the bivalent cations are, on the other hand, somewhat less easily replaceable from bentonite than from illite. The smaller release of bivalent cations from montmorillonite systems is consistent with the “mixed crystal hypothesis” of Kerr (8) and Vanselow (21), if it is remembered that the bentonite sols were prepared from a deposit of the mineral originally saturated with calcium ions. The bivalent cations readily fit
1440
A. K. GANGULY AND 8. K. MUKHERJEE
into positions previously occupied by calcium ions, forming m it were “mixed orystals” with the original clay, and the closer fitting is responsible for their weaker release. The close fitting of potassium ion in micas may similarly be brought in to explain its peculiar behavior, but it fails to explain the comparatively weak binding in mica of ammonium ions which, having approximately the same si? as potassium ions, behave in the case of other minerals like potassium ions. Similarly, the difference in the relative adsorption of calcium ions and barium ions by mica cannot be met on the mixed crystal hypothesis. Geometrical TABLE 16
&nparison of the symmetry values of colloidal salts of bentonite, illi
1
and mica
S y y Y m Y VALUES
OBDE. or - LXCBANO~ABILIFY
Mica
(M)
~
+ K + . .. . . . . . . . . . ........ + NH:. . . . . . . . . ........ + Cn++.......................... + Ba++.......................... P o t y s i u m clay + NH:. . . . . . . . . . . . . . Potaseium clay + Ba++. ......................
Sodium clay Sodium clay Sodium clay Sddium clay
+ K+. . . . . . . . . . . . . . . . . . . . . . . . . . . + NH:. . . . . . . . . . . .. + Ba-. . . . . . . . . . . . . . . . . . . . . ._ Barium clay + K+.. . . .. Barium clay + NH:. . ... Barium clay + Ca-. . Calcium clay Calcium clay Calcium clay
63.6 54.5 87.8 88.0
80.0 21.3
M > B
91.3
M > B > I
47.4 67.7
14.7 77.3
hf>I>B
70.8 70.0
87.8 71.4 83.5
M > B B > M L I M > B > I
47.7
71.3 16.9 82.6
M > B
27.6 50.5
23.6 22.3 46.4
25.0 51.2
17.7 8.7
B > M I > B > h l
48.5
60.0
60.0 76.5 83.7
25.2
m.2
--
I > B > h l B > I
B > I > M
I > B > M M > B > I
I > B
factors cannot, therefore, be made solely responsible for such peculiarities of exchange reactions. EXCHANGE ISOTHERMS
General features
As pointed out earlier, the concept of the symmetry value characterizing the exchange reaction admittedly assumes equality of exchange spots. The nature of the exchange isotherms obtained by previous investigators is also in accord with this assumption. The differences in the exchangeability of cations have already been demonstrated by the experiments with heteroionic clays. It was, however, recognized that the smooth exchange isotherms were perhaps the result of comparatively large additions of electrolytes. In order, therefore, to b r i q out
1441
CATION-EXCEIANQE BEHAVIOR OF CLAYS
the finer details, if any, of the exchange reactions it was thought desirable to study them over a wider range of concentration than was previously done, starting in particular from exceedingly small concentrations of electrolytes. The exchange data represented graphically in figures 3, 4, 5, and 6 were obtained over such a wide range of concentration, varying from 1$ to 4 times the symmetry concentration. For purposes of convenience the initial, instead of the equilibrium, concentrations have been plotted against the symmetry values. Unlike the usual exchange isotherms these curves are characterized by well-defined inflections. The experimental points in the region of small concentrations of electrolytes
80
I?
3e
4 3g
60
110
d U
Bc
20
0
Symnetry Concentration
FIQ.3. Exchange isotherms of bentonite clays
1
+ +
potassium bentonite NaCl potssaium bentonite BaClt barium bentonite NH&l
+
+
calcium bentonite NH&l KCI ammonium bentonite
+
and near the inflections were checked in some cases by means of duplicate determinations. In this connection mention may be made of similar results with silver clays obtained by Mukherjee and Ghosh (17), who used more accurate electrometric methods to follow the exchange reactions. The isotherms with kaolinites (figure 6) sirhulate the nature of titration curves having a single inflection. They may also be looked upon &s two isotherms, one followed by the other, but differing in their parameters or their slopes. The mica systems viewed in the same manner show three such isotherms and behave similarly to the bentonite systems. The features of the curves are most markedly developed when a clay salt with a more strongly adsorbable cation is exchanged
1442
A. IC. QANGULY AND S. K. MUKHERJlBE
Symmetry Concentration FIQ.4. Exchange isotherms of bentonite clays: 0 , sodium bentonite ammonium bentonite CaClz; X , ammonium bentonite KCI.
+
-~
+
+ NH4CI;
FIG.,5. Exchange isotherms of mica clays
+
calcium mica NH&l potassium mica 4-"&I amnioniummica KC1
+
~1
I3 ~
+ + +
sodium mica SHlCl potasllium mica BaC12 hariummica KCI
1443
CATIOK-EXCHAXGE BEHAVIOR OF CLAYS
+
by a comparatively weak one; for instance, calcium or barium clay ammonium chloride or potassium clay sodium chloride. In the opposite cases, such as sodium bentonite ammonium chloride or potassium mica barium chloride, features are less marked. This is possibly due to the fact that the strongly adsorbable cation levels off the small differences in the bonding energies of the weakly adsorbable cation. The ammonium clays in general start with a relatively large release of cations even at Ion concentrations of the added electrolytes, owing perhaps to hydrolysis effects. This is very prominent with ammonium kaolinite and ammonium mica but less so with the ammonium bentonite. The exchange isotherms demonstrate in general the lyotropic effect of the cations. But peculiarities are also noticed which have perhaps a bearing on the
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2oL--OO
0.5
1Symmetry 1.5 Concentration 2 2.5
FIG.6. Exchange isotherms of kaolinite clays. 0 , ammonium kaolinite barium k:rolinite CaCl?; X , sodium kaolinite KCI.
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crystalline nature of the exchange material. For instance, calcium ion is more displaceable than potassium ion in mica systems against ammonium ion. Again, at low concentration of ammonium ion, both calcium ion and sodium ion are almost equally replaceable from mica. Often the interchange in positions between calcium and barium ions is observed. Exchange isotherms in relation to crystal chemistry of minerals From measurements of the eschange capacities of silicate minerals under a variety of conditions it was shown in a previous paper (1) that the eschange spots differ as regards intensity of bonding according to their location in the crystal lattice. For instance, in kaolinite the hydrogen ions of hydroxyl groups which are supposed to be responsible for its exchange capacity possess different
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