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Masters Theses 1911 - February 2014
1970
Potassium-calcium cation exchange selectivity of some clay minerals.
Ayemou Desire Assa University of Massachusetts Amherst
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POTASSIUM-CALCIUM CATION EXCHANGE SELECTIVITY
OF SOME CLAY MINERALS
A Thesis
By
Ayemou Desire Assa
B.S. Kansas State University
Submitted to the Graduate School of the University of Massachusetts in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
University of Massachusetts
Amherst, Massachusetts
August 1970 ii
POTASSIUM-CLACIUM CATION EXCHANGE SELECTIVITY
OF SOME CLAY MINERALS
A Thesis
By
Ayemou Desire Assa
Approved as to style and content by:
Chairman of Committee
Member
August 1970 iii
ACKNOWLEDGEMENTS
The author wishes to express his appreciation to
Dr. John H. Baker for his unfailing help in providing the materials and for his ceaseless suggestions while pursuing this study. He also thanks Dr. Mack Drake for his interest
and encouragement during this investigation. The members of the Department of Plant and Soil Sciences are appreciated for their cooperation during the author's stay at the
University of Massachusetts.
Finally, the author wishes to express his sincere
appreciation to his wife, Marie Marcelle, for her patience
and understanding during the preparation of this thesis. iv
TABLE OF CONTENTS
Page ACCEPTANCE PAGE. ii
ACKNOWLEDGEMENTS. iii
LIST OF TABLES.. . vi
LIST OF FIGURES . . • viii
INTRODUCTION . 1
REVIEW OF LITERATURE. 3
!• Thermodynamical and Thermochemical study of K~Ca exchange equilibria . 3 II• The Double-Layer Equation . 6 III. Identification of Sites for K-Adsorption in Clays. 7 IV. K-Adsorption Sites and Their Selectivity. 11 V. Location of the Specific Sites for K- Adsorption .. 13 VI. Vciriability in K Selectivity among Clay Minerals. 19
THEORETICAL. 21
MATERIALS AND METHODS. 24
I. Materials. 24 II. Preparation of the Clays. 24 III. Analytical Methods .. 26
RESULTS AlTD DISCUSSION . 29
I. Preliminary Experiments . 29 A. Time of equilibration. 29 B. pH value. 30 II. Potassium Selectivity by Clay Minerals . 32 III. Specific Adsorption Sites for Potassium in Clays. 47 A. Illite. 48 B. Muscovite. 55 C. Biotite. 60 D. Vermiculite. 64 E. Montmorillonite ...... 70 93 V 78 adsorption. 84 equilibria • K- of the clay on properties 81 • • . and selectivity hydration size, ion between ion B, Relation 83 theory • • • of the C, Shortcomings of the K-Ca ionic D* Significemce and structural effect of charge A, The 75 . Theory , The Double-Layer 78 Discussion . . General > > 92 SUMMARY. LITERATURE CITED vi
LIST OF TABLES
Tabl© Page 1. Composition of solutions used to study K~Ca exchange equilibrium ...... 27
2. Effect of time of equilibration on the selectivity coefficient in clays ... 31
3. Effect of pH on the selectivity coefficient in clays. 31
4. Ionic equilibrium for K-Ca saturated Fithian illite ...... 33
5. Ionic equilibrium for K-Ca saturated muscovite ...... 34
6. Ionic equilibrium for K~Ca saturated biotite 35
7. Ionic equilibrium for K~Ca saturated vermiculite. 36
8. Ionic equilibrium for K-Ca saturated montmorillonite . 37
9. Gapon constants and cation exchange ca¬ pacities of specific and non-specific sites for K-adsorption on illite. 49
10. Experimental and calculated values of exchangeable K, Y^K, for illite . . . 54
11. Gapon constants and cation exchange ca¬ pacities of specific and non-specific sites for K-adsorption on muscovite . 56
12. Experimental and calculated values of exchangeable K, Vk, for muscovite 59
13. Gapon constants and cation exchange ca¬ pacities of specific and non-specific sites for K-adsorption on biotite 61
14. Experimental and calculated values of exchangeable K, V^K, for biotite . . . 63 Vll
Table Page 15. Gapon constants and cation exchange ca¬ pacities of specific and non-specific sites for K-adsorption on vermiculite . 66
16. Experimental and calculated values of ex- chcingeable K, f K, for vermiculite 69
17. Gapon constants and cation exchange ca¬ pacities of specific and non-specific sites for K-adsorption on montmorillonite, 71
18. Relation between the composition of the equilibrium solution and the ratio of the non-specifically adsorbed potassium to the non-specifically adsorbed calcium ( Tk / rca ) for K-Ca saturated illite, ns ns ' muscovite, biotite, vermiculite and montmorillonite . 76 Vlll
LIST OF FIGURES
Figure Page 1. Diagrammatic cross-section of illite showing the three types of potassium. 10
2. Diagram of a weathered mica, showing wedge- shaped zones vjhere ion selectivity according to hydrated size may take place. The open circles indicate water of hydration about Ca'^’^ ions (after Rich and Black, 1964). 14
3. Typical isotherm of Quantity-Intensity (Q/I) relations showing linear (Gapon) and curved (specific) parts (Beckett, 1964) .... 17
4. Flow sheet for the determination of CEC and cation exchange selectivity (CES) of clay minerals (Dolcater ^ 1968) .... 25
5. Relation between the selectivity coefficient, Kq, and the reduced ratio, R, for 5 K-Ca saturated clays.. 42
6. Relation between the cation exchange se¬ lectivity (J^K/^^Ca) and the percent K- saturation for K-Ca saturated clays ... 43
7. Semi-logarithmic plot showing the relation between the cation exchange selectivity (^l^a;:) and the percent K-saturation for K-Ca illite and muscovite at low K- saturation ...... 44
8. Relation between the exchangeable K cind the reduced ratio, R, for five K-Ca saturated clay minerals. 45
9. Relation between the exchangeable K and the product of the reduced ratio (R) X exchange¬ able Ca for K~Ca Fithian illite • . • • 50
10. Relation between the specifically adsorbed K and the product of the reduced ratio (R) X specifically adsorbed Ca for K-Ca illite from Fithian, Illinois ...... 52 ix
Figure Page 11. Relation between the exchangeable K and the product of the reduced ratio (R) X exchangeable Ca for K~Ca muscovite ... 57
12. Relation betv;een specifically adsorbed K and the product of the reduced ratio (R) X specifically adsorbed Ca for muscovite from Effingham, Ontcurio. 58
13. Relation between the exchangeable K and the product of the reduced ratio (R) X exchange¬ able Ca for K-Ca biotite .. 62
14. Relation between the exchangeable K and the product of the reduced ratio (R) X exchcinge- able Ca for K-Ca vermiculite. 65
15. Relation between specifically adsorbed K and the product of the reduced ratio (R) X specifically adsorbed Ca for K-Ca vermiculite. 67
16. Relation betv/een the exchangeable K and the product of the reduced ratio (R) X exchange¬ able Ca for K-Ca montmorillonite .... 12
17. Logarithmic plot of the cation exchange se¬ lectivity (k^X^al ) versus the reduced ratio, R, for 5 clays. 74
18. Logcirithmic plot, shov;ing the correspondence betv;een the double-layer theory and the ratio Vk / ^ Ca , of the cimounts of K and Ca aBiorbed^Sn the non-specific sites of some clays. 77 INTRODUCTION
Ion exchange studies have revealed that during the exchange process one of the involved species^ as a rule, is preferentially adsorbed. This preference that an ex¬ changer exhibits for one ion compared to another, is referred to as ion exchange selectivity. The preferential adsorption of K and Ca by soil colloids may be of great importance in soil fertility, controlling the relative concentration of the ions in the soil solution and consequently the relative movement of these ions by both mass-flow and -diffusion to plant roots. Many investigators have reported that K is held more strongly than Ca on clay minerals (9,11,12,
15,25,26,82,86),
The results of studies of K and Ca exchange equilibria in soils (9,91) and illite (15,86) have been interpreted by assuming the presence of sites with different affinities for K, Recent experiments have concentrated on the charac¬ terization of such exchange sites in illite and on measuring the binding strength of each type of sites for K ions.
Bolt et (15) and van Schovwenburg and Schuffelen
(86) in investigations of the potassium behaviour of illite found that experimental results confirmed theoretical assumptions of the presence of specific and non-specific sites for K, Unfortunately, no such test has been reported for other minerals that may be present in the clay fraction 2
of soils.
The general objective of this study was to test the ability of the theory, used so successfully with illite, to describe K-Ca exchange equilibria on muscovite, biotite, vermiculite and montmorillonite.
Specific Objectives
These are threefold: (1) To characterize the affinity of adsorption of K versus Ca on illite, muscovite, biotite, vermiculite and montmorillonite; (2) To investigate the effect of varying K/Vca ratios on the selectivity coefficient,
K^, in these clays; (3) To attempt to explain the mechcinism of the above selectivity as related to ion hydration, ion size and valence, as well as to structural and charge proper¬ ties of the same minerals. 3
REVIEW OF LITERATURE
I, Thermodynamical and Thermochemical Study
of K-Ca exchange equilibria
It is very probable that the study of ion exchange reactions was the very first attempt to apply the methods of physical chemistry to problems of soil fertility. The body of results available at the present time on ion exchange reactions in soils and clays allows us to draw only vague and incomplete general conclusions regarding ionic equilibria in soils. The reasons underlying this limited possibility of building generalizations on the available data are various, but in their essentials they can be summarized as follows:
A. Experimental technique is not published in suf¬ ficient detail to make clear the conditions under which the reaction attained equilibrium, or even if it was in fact attained,
B. The material being studied, soil or clay, is poorly or incompletely characterized, which clearly limits the possibility of extending the findings to other materials.
C. The results of exchange reactions have^ often been formulated according to certain empirical relationships, involving "constants" which are incapable of being given any fundsimental interpretation.
It is now q[uite easy to remedy the deficiencies mentioned under (A) and (B), It is also fitting to let 4
go the older formulations of ion exchange which still encumber the literature, and replace them by the rigorous
thermodynamic expression of the experimental results*
Several studies (9,12,15,24,25,51) have concentrated
on potassium in consideration of its great practical im¬ portance and the fact that all in all, little enough is known about its behaviour in ion exchange reactions, quite
apart from any question of "fixation" or, to put it another way, irreversibility of exchange reactions.
Gaines and Thomas (32), Beckett (9), Wild and Keay
(99) and more recently. Deist and Talibudeen (25,26) have used the thermodynamic approach to explain the K-Ca exchange
selectivity in clays using the Gibbs equation:
6f s= - tAs where AF is the standard free o o o o energy change, is the standard enthalpy change,
T = absolute temperature ( =298°K, corresponding to 25°C),
and As^ is the standard entropy change.
A decrease in free energy accompanying the formation
of K-clay indicates that K is strongly bound. Small enthalpy
and large entropy change when K-clay is formed, also indicate
a stronger bonding for K. Most studies have favored prefer¬
ence for K over Ca by the clays studied. However, Deist
and Talibudeen (26) have reported a study in which Ca prefer¬
ence was shown by an isotherm. These investigators indicated
"that this does not necessarily mean that Ca is more strongly 5
bound than K because the selectivity shown by the isotherm depends on electrolyte concentration. Thus Nye et (26) found that A1 was preferred to K at 0.01 N electrolyte concentration but at 1.0 N concentration, the isotherm showed K-preference, probably as a result of the "valence effect"•
Some research workers have questioned the accuracy of the results obtained by thermodynamical analysis of K-Ca ion exchange selectivity. Thomas (32) found that plotting the logarithm of the selectivity coefficient versus the equivalent fraction of potassium in solution is very difficult.
Furthermore, Deist and Talibudeen (26) have indicated that although the negative free energy suggests binding of the
K ions, that is not conclusive because free energy is determined by enthalpy and entropy, and binding strengths are normally defined in terms of enthalpy alone. Calculated enthalpy values (AH^) include the enthalpy of hydration of the ions, dilution, mixing and exchange. Because of the uncertainties about the energy of hydration of adsorbed ions, most of these terms cannot be calculated with reasonable accuracy, so that the enthalpy of exchange alone cannot be computed readily.
However, it is recognized that the thermodynamic method shows a great advantage because it allows us to obtain, without further experimental measurements, a large 6
number of thermodynamic exchange constants* Despite this apparent advantage, the formulation of mass-action-type ion exchange equations constitutes a useful approach to characterize the relation between ions adsorbed on the clay complex and those in solution*
II. The Double-Layer Equation
An equation based on the Poisson-Boltzman differential equation, showing the charge fraction of monovalent to total ions in the diffuse double-layer of a plate-like exchanger as a function of the concentrations of the mono¬ valent and divalent ions in the bulk solution, was derived by Eriksson (30)* The equation in a simplified form was given by Bolt (14):
r+/p = -2-r \fp~ sinh“^ R +^ -- (la) in which P is the surface charge density of the exchanger in m*e*/cm^; P+Zp fraction of the surface charge that is neutralized by the sum of the excess of the mono¬ valent cations and the deficit of monovalent anions near
the surface, R 2= c^\/~^ (mol./l*)^ where Cj^ and c^ are the bulk concentrations of the monovalent and divalent I 5 o P ss 1*05 X 10 cm/m mol. at 25 C,
and v^ e unity.
Bolt (14) has shown that the exchange equilibrium of 7
a Na-Ca illite in mixed solutions of NaCl and CaCl^ is satisfactorily described by this equation, which is referred to as the double-layer equation. Moreover, he showed that Gapon's equation for the above equilibrium closely approximates the double-layer equation over the exchangeable sodium percentage range 1 to 70. Bolt indicated that except for a small correction factor related to the ions in water, the relative adsorption of Na and Ca by soils can be explained as a result of the valency of the ions, and that variations in the values of the Gapon constant,
Kq, for soils are mainly related to differences in their surface charge density.
III. Identification of Sites for
K-Adsorption in Clays
Thus, the wide use of the Gapon equation (34) at the present time in the study of ion-exchange equilibria existing between electrolyte solutions and clay ion exchange media may be justified on a basis of theoretical considerations and experiment. Numerous workers have used the Gapon equation in different forms to fit their objectives. For equilibria on the exchange surface of soils or clay minerals between monovalent (K) or divalent (Ca) cations, the Gapon equation may be written as follows:
(lb) 8
where Yk and Vca refer to exchangeable K and Ca in m.e./lOOg of exchanger; CoK and CoCa refer to concentration of K and
Ca in solution in moles/liter, and K is the Capon constant.
CoK For simplicity, the ratio is written as R, and VcoCa" this relationship will hold in the entire paper. The above equation therefore becomes: Vk _ Vca " (Ic)
Theoretically, it is implicit that is independent of the overall concentration of the solution. However in some studies, has been found to vary to some degree with the composition and concentration of the solution (50).
Bolt (14) indicated that, if the only forces of attraction between the exchange surfaces and their surrounding cations are non-specific electrical field forces, the value of K_ should be independent of the cation concerned. However, if there is some form of specific interaction between a species of exchangecible ion and the exchange surfaces, then the value of K_ may be different from that in the ideal case, perhaps by a factor of five or so. Variation in can be used as a measure of the preference of the clay for a particular: ion relative to another, and therefore, it will be a measure of the cationic balance in the clay.
With illite, it appears that does not have a constant value (82,86). It shows .a distinct increase as the potassium concentration in solution falls; that is, at a constant 9
concentration of calcium in the solution, potassium in the solution diminishes more rapidly than does the exchange¬ able potassium content of the clay mineral, van Schouwenburg and Schuffelen (86) suggested that this phenomenon of increasing with decreasing K in the system may be explained either by assuming the existence of exchange sites with continuously increasing bonding energy for K or by assuming the existence of several types of exchange positions each having a specific exchange constant.
Bolt et (15) identified three types of exchange sites with an illite mineral. These are: planar sites, edge sites and interlayer sites. Potassium would be expected to be adsorbed on each type of sites with a different strength. On the planar sites, the K ions could be rapidly replaced by all cations (within 1 hour). On the edge sites,
K is held more strongly and exchange by other ions was found to be very difficult because of a very unfavorable equilibrium constant so that renewal of the extractant to remove liberated K is necessary for replacement to proceed.
On the interlayer sites, K is even more strongly held than on the edge sites, but the exchange is determined by a film-diffusion process. Figure 1 presents the three types of K in a cross-section of illite. 10
Interlayer Planar, K / I \ 7^ / / I '\ / ‘ \ / ! ' \ / » \ ,L i A. © t > 1 © \ ' V ^ i© # ''m .. ~m~w ~w —d \ \ ^ Edge K \ \ \ \ \ \ # # m [0) (0)"- \ / \ / \ ^a- jj-ar-# g m 'm~(6y q ^ 7 \ \ © © © @
Non-exchangeable K
© Exchangeable K
Fig. I - Diagrammatic cross-section of illite showing the three types of potassium. 11
IV, K-Adsorption Sites and Their Selectivity
In recent years, the important questions concerning
K and Ca or K and Na equilibria have been to detect experi¬ mentally the various types of K bound by clay minerals,
and to define to some extent the nature of the binding forces at the different sites and to gain an insight in the relative amounts of K as distributed on these sites.
van Schouwenburg and Schuffelen (86) investigated the potassium behaviour of illite and made a great con¬ tribution in this respect. These workers wrote the Gapon equation in the follov;ing way:
Vk K. Y ca R (2a) and analysed the so-obtained curve, after carrying out a number of experiments with either K-Ca or K-Mg systems.
The data obtained were used to calculate the amounts of potassium distributed on the planar sites, the edge sites and interlattice sites respectively.
For the K-Mg exchange they obtained for the respective values of the constant K_: G Kq planar a 2.21 + 0.059
K_ edge a 102.3 + 10.3 G K_ interlattice a infinite G CEC planar = 0.4258 + 0.004 m.e./g of clay
CEC edge + interlattice = 0,0248 + 0.00026 m.e./g of clay
CEC interlattice = 0.0028 + 0.00025 m.e./g of clay 12
The values found for the K-Ca exchange were:
Kq planar = 2.12 + 0.071
CEC planar *= 0.472 + 0.005 m.e./g of clay
CEC edge + interlattice = 0.0202 + 0.00075 m.e./g of clay
Thus there was a good agreement between those quite
independently obtained values although one must bear in
mind that they all refer to one and the same clay. This
fact limits the values of these results so long as there
is no concurring data available.
Analysis of the results of successive extractions
of illite with salt solution by Bolt et al. (15) also showed
that for the K-Ca system the K- planar should have an VJ approximate value of 2.0 while edge + interlattice for
K-Na system should be about 300-500. They also calculated
the CEC planar to about 94 to 96 per cent and a CEC edge
+ interlattice to 4 to 6 per^ cent of the total CEC. Thus,
Bolt et al. (15) and van Schouwenburg and Schuffelen (86) reached essentially the same conclusions. The interlattice sites showed the highest exchange constant, whereas the edge sites showed a very marked preference for K. Several in¬ vestigators (9,54,64,82,91,96) working either with soils or clays have generally confirmed the presence of sites with different affinities for K, but the difficulty was to show evidence for the location of sites with highest affinity for K 13
Tucker (96) in more recent work has agreed that difficulty exchangeable K in illite is held at interlayer sites with a strong preference for K ions in a K-Ca system.
However, he believes that the continuous change of the
Gapon constant with potassium content indicates that no clear-cut distinction between categories of external-exchange able and interlayer-difficulty-exchangeable potassium could be made on the basis of large differences of Gapon constant.
Tucker thinks that the potassium fractions to be allocated to each category depend on the displacing cation. Bolt et al. (15) also found that this is the case. Tucker also believes that exchangeable K ( ^K) and exchangocible Ca ( Yca) should refer to the Scirae array of sites, since to compare
when it is being adsorbed on interlayer sites with Yca on external as well as interlayer sites may seriously under¬ estimate K for these conditions. G
V. Location of the Specific Sites
for K-Adsorption
As mentioned earlier, the main difficulty in K-Ca exchange equilibria has been to determine the exact location of the sites with the highest affinity for K ions.
Rich and Black (82) studied the potassium exchange of soils and clays and proposed that small amounts of exchangeable K, such as that normally found in soils, are frequently located in wedge-shaped spaces in the interlayers 14
Non-exchangcable K - / /
jcig W' Ca 'szizwMinaartM
0 9 o 0 9
'Wedge-shaped zone
Hydroxy-Al or Fe ” islands '*
Fig. 2 - Diagram of a v/eathered mica, showing v/edge-shaped zones where ion selectivity according to hydrated size may take place. The open circles indicate water of hydration about Ca"^ ions. ( After Rich and Black, I96A). 15
of v/eathered micas. These wedge-shaped places occur at frayed edges or deeper within the weathered mica particles.
Weathering is probably initiated simultaneously from all sides and all layers are not stripped of K ions; there are probably many places where the stripped layers do not "match" in the interior, thus, forming a wedge. Because of their small hydration energy and small size when hydrated, K ions are selected at the apex of the v;edge. This would enable the silicate layer to collapse partially and make new wedges for more K ions. Ca ions would be excluded because of the high concentration of K ions in such places, but also because of the large hydrated size and hydration energy of Ca ions.
Pig. 2 shows the location of the wedge-shaped places in weathered micas.
Beckett (9) was able to ascertain the presence of specific adsorption sites for K in soils and clays. According to Beckett, there are a small number of special sites at which potassium can be strongly held; the special sites must be occupied before appreciable amounts of exchangeable potassium are held by the non-specific sites. Calcium ions are held by non-specific forces. Furthermore, the specific adsorption of K compared to Ca could be due to the fact that K ions have an incomplete hydration shell at these specific sites, Beckett proposed that the specific sites might be in intralattice positions at the edges of 16
the clay crystals but this needed further proof,
Beckett and Nafady (11,12) investigated the location of non-specific (Gapon) and specific exchange sites in clays on which the per cent saturation with K was less than
5-10 per cent. The results obtained by these workers con¬ firmed that the exchange sites with highest affinity for K are associated with the edges, rather than the faces, of stacks of clay plates. Their approach consisted in subjecting samples of a clay to treatments which are believed, on other grounds, to affect only the edge-faces of a stack of clay plates. If the treatments are observed to modify only the portion of the exchange isotherm associated with the specific sites, this would provide evidence to associate the specific sites with the edge-faces. The same reasoning would apply to treatments known to affect only the non¬ specific sites (planar sites). The data obtained showed that cetyl trimethyl bromide (CTAB), known to be adsorbed on the negatively charged faces of clay minerals, displaced exchangeable Ca from suspensions of clays, and affected the non-specific portion of the exchange curve. It has no effect on the specific portion of the curve.
On the other hand, decrease in pH, affected the specific portion of the curve but not the non-specific portion.
Addition of sodium hexaraetaphosphate, v/idely used to block positive edge charges, increased the amount of K adsorbed 17
Ak
Fig. 3 - Typical isotherm of Quantity-Intensity(Q/I) relations, showing linear(Gapon) and curved(Specific) parts. (Beckett, 1964).
i
I 18
on the specific sites at low pH, but had small effect at higher pH. Beckett and Nafady (12) concluded from their study that sites on the planar surfaces achieved equilibrium very rapidly as shown by Beckett (9), The specific sites associated with the edge-faces took up K from solution much more slowly. Such slow equilibration may be due to a high activation energy of exchange or may be the result of a relatively slov; diffusion-controlled exchange with ions held in the wedge-shaped interstices within the weathering edges. Beckett and Nafady admitted that their experiment could not discriminate betv;een the two alternatives but the latter suggestion (82) seems more probable.
Fig. 3 shows a typical isotherm obtained from K-Ca equilibria in soils or clays using the so-called quantity- intensity (Q/I) relations (8,10,13). The linear part conforms to the Gapon equation and is believed to be the portion for non-specific adsorption. The curved part is believed to describe exchange of K held at sites which show a more specific affinity for K (9,15,86). The differ¬ ence (X on the curve) between the linear and curved parts represents the exchange isotherm of the specific sites.
K therefore, is the amount of K adsorbed on the specific sites. 19
VI. Variability in K Selectivity
among Clay Minerals
Cation exchange selectivity has been found to be subject to variations cind these variations may be due to the effect of solution concentration, pH, temperature, ion size, ion valence, ion hydration as well as of structural and charge properties of the clay minerals,•
Newman (66) showed that the selectivity of the micas for K increased with the degree of K depletion. This decrease was particulcirly evident as the exchange progressed from the potassium on the outer surfaces to the interlayer potassium at the periphery of the particles. Hov/ever, the continuous nature of this relationship does not lead to a distinction of discrete fractions of potassium that are held v;ith different specificity.
Dolcater et (28) found that the cation exchange selectivity (CES) as defined by the ratio ^K/^Ca in which
'IJk and Yca refer to exchangeable K and Ca (in m.e./lOOg) respectively, increased in the order: montmorillonite ^ vermiculite4 biotite be responsible for the increased selectivity* The extent of swelling in a clay mineral affects the magnitude of the selectivity coefficient* The more swelling properties a clay shows, the smaller tlie magnitude of the selectivity coefficient 21 THEORETICAL Since the clays under investigation are 2:1 lattice type of clay minerals, it may be assumed that at least two types of sites for K exchange are likely to occur, that is, non-specific planar, and specific edge-interlayer sites. On this assumption, it is possible to divide the 0 exchange reaction into specific and non-specific portions, with the exchange reaction on each site described by a Capon equation, as shown by Bolt et (15) and Vcin Schouwenburg and Schuffelen (86). Follov7ing the derivation of van Schouv/enburg and Schuffelen (86) we may write: u s tr R (2) Vca s and (3) where the subscripts s and ns refer to specific and non¬ specific sites respectively, and are the Capon '^s ^ns constants for the specific and non-specific sites, respectively. _ ^ and R is the reduced ratio CoK/ VCoCa (moles/liter) . By combination and arrangement of equations (2) and (3) we obtain for conditions such that any specific sites are saturated with K: Y K « Kq • R • Y Ca + CEC^ (4) ns 22 where CEC is the cation exchange capacity of the specific s sites. For high values of exchangeable K, a plot of '{k against Yca»R should yield a straight lino, because specific sites should be saturated with K. The gradient of this line gives K and the intercept CEC , The cation vJ ns O exchange capacity of the non-specific sites is obtained as the difference between total CEC and CEC-, By rearranging 3 equation (3) we obtain: R Yk °ns ns (5) Yk + tea ^ K„ R + 1 'ns 'ns °ns where \K + Yca is the cation exchange capacity of the ns ns non-specific sites, Substraction of the calculated amounts of exchangecible cations on the non-specific sites from the total amounts determined experimentally gives and Yca , the exchange- 3 S able cations on the specific sites. These values are then used to calculate K_ : ^s Xk = K_ R /ca + intercept (6) S G _ S S Finally, by combination of equations (4), (5) and (6), the following equation relating Y K to the components due to specific and non-specific sites is obtained: K. R Kg R (CEC^ int 6) ns - + int 6 (7) 1 + K_ R 1 + R °ns 23 where "int 6" is the intercept of equation (6), The presence of an intercept may represent K that is extracted by Mg(0AC)2 but that is not in exchange equilibrium with K and Ca ions in solution. This equation may be used to test the validity of the separation of exchangeable K into specifically and non- specifically held potassium. 24 MATERIALS AND METHODS I. Materials The clay minerals used for this investigation were the following: montmorillonito from Polkville, Mississippi (American Petroleum Institute reference clay No. 21); verraiculite from Montana; illito from Fithian, Illinois I (American Petroleum Institute reference clay No. 35); muscovite from Effingham, Ontcirio; and biotite from Bancroft, Ontario. The minerals wore obtained from Wards Natural Science Establishment, Rochester, N.Y. II. Preparation of the Clays Flakes of the pure material were ground and suspended in water using a Waring blonder. Particles smaller than 2 ^ in diameter v/ore separated by decantation after larger particles had settled from the suspension, according to the method given by Jackson (44). The suspension was con¬ centrated with a filter candle. The percent clay of each suspension was determined by drying an aliquot in an oven at llO^C and v;eighing. The percent clay in the suspension of montmorillonite, vermiculite, illite, muscovite and biotite v/ere: 1.45, 1.70, 2.30, 2.0 and 2.0, respectively. Each clay was sodium-saturated v/ith NaCl. I c 25 Fig. 4 __ Flow sheet for the determination of CEC and cation exchange selectivity (CES) of clay minerals. ( Dolcater et al. 1968). 26 III, Analytical Methods Cation exchange capacity and cation exchange selectivity were determined according to the method given by Dolcater et al. (28), The flow sheet is presented in Fig, 4: In the determination of cation exchange selectivity (CES) a Na-saturated clay sample was equilibrated in tared centrifuge tubes with a series of solutions that varied in the ratio from 10 ^ to 10 when K and Ca in solution are expressed in moles/liter as shown in Table 1, Except for illite, the intermediate ratios of 0,3 and 5 were used. To each clay sample was added 40 ml of the mixed solutions and the suspension was shaken for 30 minutes on a wrist-action shaker. The suspension was then centrifuged and the supernatant discarded. The process was repeated seven times for each sample. After the last saturation, the centrifuge tubes were weighed again and the weight of excess KCl + CaCl^ solution was determined by the differ¬ ence in weights. The K-Ca saturated clays were then ex¬ tracted 4 times with 40 ml portions of 0.5 N Mg(OAC)2, pH 7.0, and the amounts of K and Ca extracted were measured using a Perkin-Elmer Atomic Absorption Spectrophotometer. The amounts of exchangeable K and Ca were calculated by sub¬ tracting the amounts of the ions in the excess KCl + CaCl2 solution from the total amount extracted by the Mg(0AC)2* As pointed out by Dolcater et al. (28), the exchange Table I - Composition of solutions used to study K-Ca exchange equilibrium in clays. KCl CaCl2 xk"^ R (mol./I.) (mol./I.) (mol./l.)^ 5 X 0.0025 0.99 X 10"^ 10”^ 5 X 10"^ 0.0025 0.99 X lO'^ lO"^ 5 X I0‘5 0.0025 0.99 X 10"^ 10"^ -4 5 X 10 0.0025 0.91 X 10“^ 10"^ 5 X 10"^ 0.0025 0.50 10"^ • 1.5 X 10"^ 0.0025 0.86 0.3 5 X 10"^ 0.0025 0.91 I 2.5 X 10“^ 0.0025 0.99 5 5 X 10"^ 0.0025 0.995 10 Note: XK"^ equivalent fraction of potassium in solution 28 selectivity thus determined is different from the sorption selectivity calculated from the difference between the composition of the solution added and that of the super¬ natant# In the latter method, fixed, collapse-inducing cations are included v^ith exchangeable cations. All determinations were run in duplicate and average values were used for the calculation of the .various parameters. pH is known to have a distinct effect on the measure¬ ment of cation exchamge selectivity in clay minerals (82,91). As a result, all the K-Ca solutions were adjusted to pH 7.0 with Ca(OH)2. Some precaution was taken to prevent a large drop in the pH value of the mixed solutions, by covering the beakers with aluminum foil. However, it was observed that before the seventh saturation, most of the solutions had a pH value close to 6.5, the drop resulting from the action of carbon dioxide. It is not believed that the rate of drop in pH was fast enough to affect the saturation process to a large extent. A sharp decrease in pH during the saturation procedure would increase potassium desorption. 29 RESULTS AND DISCUSSION I. Preliminary Experiments Preliminary experiments were performed to determine the effect of time of equilibration and pH on the selectivity coefficient, K^. A, Time of equilibration. In ionic equilibrium studies, it is necessary to assure equilibrium during the saturation process* However, deciding when equilibrium has been reached may be arbitrary. For soils, longer equilibration time is necessary due to their complex nature. For clay minerals, equilibration time may vary from 10 to 30 minutes. It is sometimes thought that in some clays, selectivity measurements should be made during the first 10 minutes of equilibration because the structure of these clays changes quite rapidly. Increasing shaking time may cause potassium release from the clays, the rate of release depending on the clay type. Insofeir as this release of K exposes new exchange sites, one would expect an increase in the slope of the exchange isotherm. Two possible explanations can be invoked to explain the release of K on prolonged equilibration. Firstly, the release might be due to a diffusion process whereby K moves from interlattice positions to the equilibrium solution. Such a release should be relatively independent of shaking 30 but should depend mainly on the diffusion path and gradient existing in the system. On the other hand when the clay is shaken, the possibility of abrasion taking place on the edges of the clay crystals cannot be overlooked. Any such abrasion of mica could be expected to release potassium, thus artificially increasing the potassium in solution, which in turn may affect the amount of potassium adsorbed, especially on the specific sites. Much evidence is available to support the former mechainism of K release from inter¬ lattice positions (60), This is particularly enhanced when large amounts of calcium are present in solution which tend to open up the lattice (49,50,82,86). In the present investigation, it is believed that equilibrium was reached after 30 minutes of shaking on the wrist-action shaker, because v;hen the time of contact was varied by letting the suspension stand overnight, followed by another 30 minutes of shaking the follov/ing morning, the data in Table 2 show that there was no real difference between the selectivity values, indicating that equilibrium was reached after 30 minutes. B. pH value. The results of a study of the effect of pH on the selectivity coefficient are presented in Table 3. It is clear from the data obtained that pH has a distinct effect on K values. A decrease in pH is followed by a G decrease in K- due to the high concentration of H ions G 31 Table 2 - Effect of time of equilibration on the selectivity coefficient in clays. Mineral R (mol./l.)^ 1 o M 10"^ I0"2 10“ I (1./mo1.)^ * Illite 415.0 67.9 10.8 3.4 Muscovite 580.0 70.0 30.1 9.8 Biotite 5600.0 690.0 71.0 6.9 Vermiculite 289.0 38.0 4.2 0.9 MontmorilIonite 40.0 8.0 1.9 0.4 Kg (l./mol.)^ ** Illite 402.3 65.6 12.1 3.2 Muscovite 572.8 68.9 28.9 10.5 Biotite 5589.0 680.0 69.2 5.9 Vermiculite 280.9 35.0 3.8 0.9 MontmorilIonite 40.0 , 6.8 1.5 0.3 * Samples equilibrated for 30 minutes Samples allowed to stand overnight plus 30 minutes equilibration. Table 3 - Effect of pK on the selectivity coefficient in clays. 32 in solution which compete with K on the exchange sites of the clays. II. Potassium Selectivity by Clay Minerals The K/Ca cation exchange selectivity (CES) values, expressed as the equivalent ratio of exchangeable K to Ca, according to the definition of Dolcater et (28) are shown in the sixth column of Tables 4, 5, 6, 7 and 8 for illite, muscovite, biotite, vermiculite and montmorillonite, respectively. For illite, the CES values ranged from 0.04 to 15.3 as the reduced ratio (R) varied from 10 to 10. Values for muscovite lay between 0.06 and 5.0. Biotite gave values between 0.4 and 8.0. For vermiculite, CES values ranged from 0.03 to 6; montmorillonite had CES values -5 between 0.004 and 2.0 as R was varied from 10 to 1.0. Montmorillonite did not adsorb any K at R=5,10 probably due to experimental error. At a particular reduced ratio, R, the exchangeable K/Ca equivalent ratio generally increased in the order: montmorillonite The order of the first three clay minerals might be expected. However, the results of most studies indicate that muscovite should have a higher CES value than biotite (28,82). The order of CES is believed to correspond to the order of increasing layer charge and surface charge density, and decreasing cation exchange capacity (28). These results 33 rJP* - 1 1 1 • 1 o O O M >d- O CO 1^ 1 « • • • VO M m : ^ o 1 o O M • • • 1 e 1 o o> M CM CM M 1 1 m 1 • 1 1 1 N-/ 1 1 1 f, 1 i 1 1 1 1 O 1 VO CM cn in vO CO r>» 1 U 1 CM 'd- cn CO CO VO 00 1 u 1 1 in CO CM O I*'* CO 1 M CM VO CTv 1 -4 1 1 1 1 1 1 1 1 cc5 1 Ml- M M Ml- in 00 1 in CTv Mf VO o^ CO iVo 1 o O O M CM o CO 1 1 • • • • • • • 1 1 o o o O O CM in - 1 M 4J 1 1T 1 rH 1 ■ 1 1 1 •H 1 d 1 CM 1 u 1 O CM Ml- vO o 1 o o CM CM CM Mf o =► 1 o o O CM M CM Mf u 1 1 • « • • • « • va 1 1 o o o O CM CO in M yi 1 * 3 1 1 . . • » 4J 1 t nJ 1 1 CO 1 cfl 1 (d 1 O 1 o 1 o o CO cr» o vO 1 |>o 1 CO VO Ml- C'' in M 1 • • • • ♦ • « 1 + 1 m in vO in vO m in 1 CM CM CM CM CM CM CM o 1 M-t !>» 1 1 E 1 1 1 1 •rH 1 1 1 oj ^ 60 1 1 O O 1 i-« 1 CtJ o 1 o m O m O >d- Mf 1 • O M 1 CM CM in CM CM in •H 1 1 • • • • • • • d 1 o • 1 Mt Mj- Ml- CM M 00 M cr 1 X ^ (U 1 CM CM CM CM CM Individual value used instead of average-value so that the data could be.plotted using, equation (4). 36 Table 8 - Ionic equilibrium for K-Ca saturated montmorillonite ><» .o Xvtt w O o u + CJ) u CtJ w ^ w ed CJ X ^ o td Ctf • O flj ^ • S'? 1 O 1 O 1 l-« 1 M 1 fcO 1 * o O o • • • • • CM mT Mt Ml- VO Ml- VO mT Ml- OV in o 00 rx o o o CO o o o O o M M O M • • • • • M I CM vO 1^ Ml- CJv o CT\ CM fx CM CO O rx CO CO M M M w M O M M O H M • • • • • o Ml- CM CM VO CM CO O Ml- mT VO uo CO CO O Ov Ml- in CO M M in M M CO m o M M M • • . • • - CM VO VO VO VO CM CTi crv O VO CO o o in O m O CO in OV O w o O o o H • • « • • M^ *4- CM CM rx CM CM m m as m OV m M 1 « • o M 17.85 17.85 178.50 1 37 38 showed some evidence of the effect of layer charge and surface charge density since the micas which have a higher layer charge than vermiculite and montmorillonite, showed higher CES values. The layer charge of illite is intermediate between the micas and vermiculite and montmorillonite which explains its intermediate CES values between the two groups of minerals. Dolcater e^ (28) used a reduced ratio of 0.1 and obtained CES values of 0.1, 0.23, 0.95 and 1.7 for montmorillonite, vermiculite, biotite and muscovite, respectively. In this study, CES values of 0.10, 0.29, 1.54 and 0.98 were measured for the above clay minerals, respectively, at the same reduced ratio of 0.1. Again, the two micas showed a reverse sequence. A possible cause of such reversal would be differences in size of the mica particles used in the two studies. There is also the possibility that some K was released during extraction, from interlayer positions in biotite since the composition of biotite is known to be more easily altered than that of muscovite. Sumner and Bolt (90) indicated that apparent increase in preference for K at low K~saturation would indicate that Mg(OAC)2 extraction probably also liberates interlattice K, the effect of which would increase pro¬ portionally with decreasing percentage surface exchangeable K. The pH of the Mg(OAC)2 extracting solution, however, was between 6.0 and 7.0, so that little or no interlayer K 39 should have been extracted. The selectivity for K as well as the apparent exchange constant increases as K-saturation decreases. In as much as the "exchange constant" is not truly constant but corre¬ sponds to a specific point on the exchange isotherm, and depends on experimental conditions (77), the term "selectivity coefficient" is subsequently used. The selectivity coef¬ ficient, defined by the Gapon equation is: (8) Selectivity coefficients calculated using equation (8) are reported in the eighth column of Tables 4, 5, 6, 7 and 8 for illite, muscovite, biotite, vermiculite amd montmorillonite, respectively. The selectivity coefficient, K^, measures, in fact, the relative difference in preference of the solution and exchanger phases for K and Ca, It represents the net resultant of all interactions in both exchanger and solution phase that gives rise to selectivity. The increase in selectivity coefficient with decreasing reduced ratio, R, found in this study, confirmed results reported in the literature (86,91,95,96), The order of increase in selectivity coefficient corresponds to that reported earlier, that is: montmorillonite vermiculite ^ illite muscovite ^ biotite, The micas showed higher se¬ lectivity coefficients than vermiculite and montmorillonite. 40 while illite had intermediate K value. G High values of selectivity coefficients ranged from 41,000 (l./mol.)^ for biotite to 400(montmorillonite)^ low values ranged from zero for montraorillonite to 1.5 for illite. Increasing the amount of K in solution readily affected the amount of Ca adsorbed on the various exchangers, as shown in the third column of Tables 4, 5, 6, 7 and 8. As the -5 U reduced ratio was increased from 10 to 10 (moles/liter)^, the amount of adsorbed calcium was decreased 94, 88, 82, 88 and 82 per cent for illite, biotite, muscovite, vermiculite and montmorillonite, respectively. K affected Ca adsorption more in illite than it did in the other clay minerals. On the other hand, increased K-saturation resulted, in general, in a decrease in the cation exchange capacity of the clays. The cation exchange capacity determined as the sum of the adsorbed K and Ca was preferred to the cation exchange capacity determined by a single cation saturation (calcium used by Dolcater e^ 1968) as it is generally done, because when the sum of the cations is used, the charge on the clay is divided over the cations concerned pro¬ portionally to the equilibrium concentrations. The decrease in CEC on saturation with K may be attributed to the "fixation" of K ions but it might also be caused by large hydrated Ca ions being trapped inside 41 the clay lattice if the peripheral layers of the clay minerals collapse when lattice-collapsing K ions are adsorbed. If some of the clays fixed potassium appreciably but much less than the decrease in CEC, this would suggest that the CEC decreased mainly because Ca ions were trapped and physically prevented from exchanging v/ith MgCOAC)^ (25). Fig. 5 shows the relationship between .the selectivity coefficient, and the reduced ratio, R. The main features of the curves were a slow increase in the selectivity coef- -2 ficient values with decreasing reduced ratio till about 10 (moles/liter) for most of the clays V7ith the exception of -4 montmorillonite which showed slow increase till about 10 (moles/literwhen the increase became steeper. This increase was accentuated even more above 10 (moles/liter) • The steeper portions of the curves have been attributed to the regions where K is held specifically by the clays, that is, where K is bound with the highest energy to the clays (9,12,83,86). This concept of specific adsorption is discussed in length in a separate section since it constitutes a major objective of this investigation. Fig. 6 is a plot of the cation exchange selectivity (CES) ratio, ^Ca against the per cent K-saturation of the clays, ^K/CEC. The main characteristic of these curves was an increase in the CES values as the K-saturation increased. The curves for the different clays appear to be very close 42 A--' A Illite Fig. 5 - Relation between the selectivity coefficient, K«, and the reduced ratio,R, for 5 K-Ca saturated clays. l Fig. ^ Relation between the cation exchange aelectivity and the percent K-saturation for K-Ca saturated Clays. yj(/CEC 7, CNj ■ o H Exch,K/Exch.Ca X 2C' 10» 3C. Fig. 7_Semi-logarithmicplot showingtherelationbetween K-saturation forK-Caillite and muscoviteatlowK-satu- ration. cation exchangeselectivity(^^i(t)al )andthepercent 0.5 I.O 1.5 log)fK/CEC * —*—■.. —- •-- Illite Muscovite 44 45 •— ^*111 ite ^ Muscovite -- V Biotite Fig. 8 46 to one another, and particularly at low K-saturations, At low K-saturations, the points were so close to one another that it was difficult to draw the curves. Also at very low K-saturations, the curves seemed to merge in a straight line, and this was especially true for illite, muscovite and vermiculite. Montroorillonite would probably show the same trend but its low values of per cent K,. made it diffi¬ cult to plot these points. A semi-logarithmic plot of CES values against low percent K-saturation would enable us to see more clearly how the curves end. Fig. 7 represents such a plot. When exchangeable K was plotted against the reduced ratio, R, a similcir series of curves to those in Fig. 4 was obtained. This is, in fact, a semi-logarithmic plot since R values increased by a factor of 10, except for the intermediate values of 0.3 and 5. Fig. 8 shows the relation betv/een exchangeable K and R. From the curves just discussed, it is proposed that with a low content of adsorbed K, the K-equilibrium obeyed a logarithmic function, and that the logarithmic section of the exchange function was predominantly determined by the process of exchange of the most firmly held fraction of potassium. This agrees with the results obtained by Ehlers et al. (54). Ill, Specific Adsorption Sites for Potassium in Clays It was found that the Capon constant, K_, tends to increase with a decrease in the reduced ratio^ R, Since the Capon constant is a measure of the preference of a clay for a particular ion relative to another (in this case K relative to Ca), the results indicate that K is specifically held by the clays. The increased K_ with G decreasing reduced ratio, may be explained either by postulating the existence of exchange sites with continuously increasing bonding energy for K, or several types of ex¬ change site, each having its own characteristic exchange constant. By substituting the data in equations (2) to (6), it was possible to calculate values for the Capon constant and cation exchange capacity of specific and non-specific sites, assuming that only two types of site exist. The results of such calculation are presented in Tables 9, 11, 13, 15 and 17, for illite, muscovite, biotite, vermiculite and montmorillonite, respectively. Of the five clays, illite is the one which has been studied to a greater extent with regard to the afore mentioned specific and non-specific adsorption sites for K. Thus it is possible to compare the results of this study with those reported by Bolt et al. (15) and van Schouwenburg and Schuffelen (86). 48 These workers identified three sites in illite, viz, planar sites on the outer surface of the lattice, edge sites and interlattice, edge sites and interlattice sites situated between the layers of the mineral. Fig, 1 shows the three types of site. In this study, only two sites are assumed: a non-specific site, possibly located on the planar surface and a specific or "interlattice site". This interlattice site, therefore, includes the K located at the edge in the interlayer region, Illite. Table 9 shows the Gapon constcints and cation exchange capacities of specific and non-specific sites for K-adsorption on illite. Using equation (4) the data are presented in a graphical form in Fig, 9. For exchangeable K, Yk, values greater than 3.18 m.e./lOOg the experimental points are situated along a straight line whereas for less than 3.18 m.e./lOOg, the points deviate from this course. In the theoretical consideration, we indicated that for high values of the specific sites are saturated with K. Thus for illite, the exchangeable K obtained for R greater than 10 refer to the high values in question. The slope of the straight line thus obtained, is the Gapon constant of the non-specific site, K , which 1 ' has a value of 1,35 (1,/mol,)^, The intercept of this line with the Yk -axis, gives the cation exchange capacity of the specific sites (CEC^) with a value of 2.80 m.e./lOOg. m o U-J•H O Cu<1> (0 i d o d ’O d o <}• CO CO O oj o CO VO CM CO vO (y\ CO CO VO M VO in O % • • • • « • •H CM CM CO CM M Ov M 4-« CM CM CM CM CM O 0) d. (0 CTV M vO VO Ov VO CO 'd- vO O CM M-l OV 00 VO r'. CO CM O O o CO H 00 CM CO • ‘ • • • • • • n M M CM CM CM Ml- CM Ti0) COU CU CO CO l-l c;^ Mf Mj- o O CO M m CO O CO o o CO o in •d- M o • o o o CO 00 o CO bO • • • « • « • d o o o o CM CO o CO M CM ,d u X o M OJ 1—1 CO H m CO CM M Calculated from equation (6) which gives an intercept of 1.05 m.e./IOOg I t I I I o o o o o O •je M M H M M M M X A o CO o CM O H exchangeable Ca for K-Ca Fithian illite. Fig. sr. _ Relation between the exchangeable K and product of reduced ratio (R) OS 51 These two values appear in columns 2 and 5 of Table 9, The value of 2.80 m.e./lOOg is therefore the cation exchange capacity attributed to the interlattice sites in accord with results reported in the literature (15,86). van Schouwenburg and Schuffelen (86) obtained 2.12 (l./mol.)'^ and 2.0 m.e./lOOg for K and CEC respectively for their ^ns ^ illite, which does not differ by much from the results of this study. The agreement is remarkable because the clay samples and experimental procedures were different in the tv^o studies. Appcxrently, a particular clay mineral tends to have a fixed number of specific sites for K. Cation exchange capacity of the non-specific sites (CEC ) were obtained as the difference between the total ns cation exchange capacity ( Yk + Yca) shown in column 5 of Table 4. and the CEC . CEC values appear in column 4 ' s ns of Table 9. CEC values varied with K-saturation due to ns the unlimited number of non-specific sites. K-saturation affects the total CEC of the clay and since the CEC is fixed, any variation in the total CEC should affect only the amount of cations adsorbed on the non-specific sites. Equation (5) was used to calculate the amount of K adsorbed on the non-specific site, which appears in the 6th column of Table 9. As would be expected, increased as the reduced ratio increased. Also K adsorbed on the specific site,YKg, generally increased as R increased. <1- o ratio(R) x specifically adsorbed Ca for K-Ca illite from Fithian,- Illinois. Fig. I0_ Relation between the specifically adsorbed K and product of reduced (3oOI/*9*“) psq-iospe XxiBOTjToads A Z5 53 Both the amount of Ca adsorbed on the non-specific ( Yca ) ns and specific sites ( Yca ) decreased vvith increase in R. s ' which is in accord v;ith the decrease in the total Ca adsorbed. At high reduced ratios^ r'^ 10 ^ there was no calcium adsorbed on the specific sites. This agrees with the theoretical consideration in which it was stated that at high reduced ratios, the specific sites are saturated in totality with potassium. Negative values obtained for Yca were interpreted s to mean that there was no Ca adsorbed on the specific sites, and therefore may be taken as zero. The Capon constant for the specific sites, was s calculated from equation (6) and is the slope of the line obtained by plotting Yk against the product RY^a , as s s shown in Fig, 10. The value of for illite, shown in s the third column of Table 9, was found to be 1,743, which means that the interlattice sites or the specific sites show very great preference for K, In comparison, van Schouwenburg and Schuffelen (86) obtained a value of 102 for the Capon constant of the edge sites, and a value greater than 2,000 for the interlattice sites. The results of this study, therefore, compare quite reasonably with those obtained by the above investigators. The intercept of equation (6) was found to be 1.05 m.e./lOOg. This value appears at the bottom of Table 9. This value may represent K that was extracted by Mg{OAC)2 but that was not in exchange 54 Table 10 - Experimental and calculated values of exchangeable K, YK, for illite ■ * R Expt, Calc. Vk (m.e./IOOg) -5 10 1.10 1.08 1 O M 1.39 1.32 10"^ 2.20 2.20 10“^ 3.18 3.01 10'^ ; 5.59 5.64 I 17.26 15.84 10 23.62 23.61 * Calculated from equation (7) 55 equilibrium with K and Ca ions in solution. VJhen the values in Table 9 were substituted in equation (7), it V7as possible to calculate the adsorbed potassium, Yk, as a function of reduced ratio, R, as a test of the validity of the separation of the exchangeable potassium into specifically and non-specifically held K. The calculated and experimental values of are presented in Table,10. These two values agreed reasonably. This supports the hypothesis that the specificity of K adsorption may be explained by the existence of two different types of exchange sites with greatly different exchange constants. B. Muscovite. Using the same approach as that for illite, the Gapon constants and cation exchange capacities of the specific and non-specific sites for K adsorption on muscovite were obtained. The results are presented in Table 11. The plot of Vk against R YCa is shown in Fig. 11. The data points are situated along a straight line forYK value greater than 8.34 m.e./lOOg, and values less than 8.34 m.e./lOOg deviate from this course. In Fig. 11, the data point for R = 0.1 is not consistent v^ith the other data and may therefore be in error. K , shown ns in column 2 of Table 11, was found to have a value of 0.35 from the slope of the linear portion of the curve in Fig. 11. The intercept for equation (6) plotted in Fig. 12, 56 ir» 'd- CO 1 1 0 M 00 m 0 1 CO t CM vO C'. 00 CM 00 u • d 1 CO ov 0 0 r>. 0 0 •H 1 • • • • • • 4-C • >• 1 VO vO m 0 0 0 o 0 Ou CO I c 0 1 CO 1 0 CJv CO vO CO CM in M d 1 d 1 Ml- H 0 Mf CO CSV CO m vO 1 d 1 0 CO M M vO m M 'O 1 • d • >» 1 M 0 M M crv C'* m d 1 1 CM CM CM CM M o m•t-I T-l o a> Cu 1 0 CJv CO vO CO CM in M Mf to 1 CO 1 CO CO CM M M M M 1 60 1 vO M VO CO CO 0 0 CM iw 1 >0 0 1 0 1 0 1 M M CM in CO r-^ 00 CO 1 M 1 M to 1 1 d 1 • 1 •H 1^ d 1 4-1 1 • 1 1 e 1 0 1 1 d 1 CO 1 0 * cu ' f 1 0 0 Ml- !'>■ CO in C^ VO d 1 0 0 0 CO CM in CO u • >-»* 1 0 0 0 0 vO 0 vO 0 0 • 1 • • • • • • • « • d 1 1 0 0 0 0 0 M CM CO CO C>0 1 1 M H d 1 1 d 1.- 1 x: • 1 CO 1 M 0 d 1 0 1 • 1 1 1 1 1 1 1 1 1 X 4J 1 m 1 CO d t4 1 0 1 > 1 1 d 0 >- 1 0 0 1 1 Ml- CM M CM CM CM mT CTV Ml- •H to 1 CO 1 0 CO M CM CO CO CM Mf CM 4-1 d 1 d 1 • • • « • • • • • d e 1 u 1 M 0 M M 0 0 0 0 CO 0 1 » 1 CM CM CM CM CM M M CM CM d O 'O o d rJf* (d d o Ht to to •H 4J ^ o d Ou 6 o d M CO ■u o M to CO d 'O o d u I d o CO cu u d m d o o o CO O 4-C e M M d rie* t-t XI d UO «cl- CO es H oi I I I CO Calculated from equation (6) which gives an intercept of 1.65 m.e./IOOg. O O o o o M H M m M O B 57 Wi Ui o O K (m.e./IOOg) Exchangeable • N> M ts> M o Fig. II - Relation between the exchangeable K and the product of the reduced ratio(R) X exchangeable Ca for K-Ca muscovite. 58 adsorbed K (m.e./IOOg) Specifically M o Fig, 12 Relation between specifically adsorbed K and the product of the reduced ratio (R) specifically adsorbed Ca for muscovite from Effingham, Ontario. 59 Table 12 - Experimental and calculated values of exchangeable K, for muscovite. R Expt. Calc.* (mol./l.)^ Yk ( m.e./IOOg ) 10"^ 1.68 1.65 10"^ I.7A 1.70 10"^ 2.13 2.39 - lO'^ 5.75 5.37 io“^ 14.0 8.32 • 0.3 8.34 8.96 I 10.73 10.71 5 21.05 21.12 10 25.29 26.17 * Calculated from equation(7) 60 was 1.65 m.e./lOOg; the slope of the line, , was 130, s These values were used in equation (7) to calculate |^K. The calculated and experimental values of K are presented in Table 12, Again these two values showed good agreement except at R = 10*"^ where, as mentioned above, may be in error. These results could not be compared to others because there has not been a similar study of muscovite reported in the literature. However, the results show the same trend as observed with illite. C, Biotite. Biotite behaved similarly to muscovite. The values for the Gapon constants and cation exchange capacities of the specific and non-specific sites were greater than those for muscovite. The calculations for biotite core presented in Table 13. A plot of equation (4) which yielded a value of 0.48 (l./mol,)*^ and a CEC^ ^ns value of 13.50 m.e./lOOg, is presented in Fig. 13. The K_ value was estimated to be about 31,000. This value was excessively greater than the value for muscovite or illite. As mentioned earlier, the possible explanations for the greater values obtained with biotite are that the size of the biotite particles may have been much less than that for muscovite, or that some kind of interlattice position was established during the extraction with Mg(OAC)2» If this latter alternative is true, then the treatment for Table 13 - Gapon constants and cation GKchangc capacities of specific and non-specific sites for K- adsorption on biotite. >- Je o tc O « o o w O W O pet CO e p CO c (0 c R CO CO o c (0 ^ 1 o o ^ 1 t-l 1 U) e o O 1 O 1 s « e O • 1 • 1 1 I 1 1 1 1 m CO O CM M •>3- in CO 00 M 00 O • m • »n I CM in o CM O CM <3- o CM in ON O M O O M <3" o CM C3N I o M 1 • 1 1 • • « I CM o Cv CM CN w CO o o 00 CM o CO CO O M O O o M O I CO o M o M 1 1 • 1 • • « ,o CO vO CM O CM vO vO vO CN St O M CM M CM (ON 00 I o H 1 • • • • CM vO m '3‘ vO M CM CO 00 M M -3- CM O M I in 00 O M 1 • • • • M VO CM CM Cv CM in vO *3- M M M I CO M CM CO CO CO M O M 1 • • • • CM 'd' CM Ov O CO rs. M 00 CO M 00 M M M CO 1 • • • • >3- 3- St vO O vO CO CM ON M M (ON ON 00 1 • • • • CM CM CM o m vO M CO CO CO M CM CM CO CO 00 00 l-l ON M in CO M M in 1 • • • • CO erv CN CO m CO CO CO o CO M CJN M CM o M M M o M 1 • • • • 61 Calculated from equation (6) which gives an intercept of 12.2 m.e./l60g 62 Exchangeable K (m.e./IOOg) M ro uj o o o o \ \ H* 00 M CO O >! (t> O »-• M p> cn P) rt D P* CIO O o P P) cr cr* (P D rt «: O (P P> a> 3 K» O ft 3* (P I (P CO O X O pj o 3 3 PJ H* P O OO rt fp H* P rt 3 P »-• • (P 5^ P P O- i Cn 3 P •O H O CL c o rt I O Hi rt 3 3 P O i-J P CU c o p cu p 3p 9 H* O !« K I k r t i 63 Table I4 - Experimental and calculated values of exchangeable K, Yk. for biotite. * R Expt. Calc. (mol./l.)^ )(k (ra.e./IOOg) 10“ 5 12.58 12.61 10-4 13.21 13.18 10'^ 14.68 13.47 lO"^ 18.54 13.62 « . 14.68 lO"^ • 22.34 0.3 14.91 14.98 I 16.36 16.42 5 22.42 22.78 10 . 30.52 30.64 * Calculated from equation (7) 64 biotite resulted in a lattice expansion and a release of K from the interlayer position. The pH of the extracting MgCOAC)^ solution was adjusted to value very close to 7, so that any effect of hydrogen ions may be excluded. As in the case of muscovite, the values of exchange¬ able K (Table 6) at reduced ratios of 10”^ and lO”^ (moles/ literwere assumed to be in error as they were not con¬ sistent v/ith the other data plotted in Figure 13. Conse¬ quently, the experimental and calculated shown in Table 14, did not agree at these R values. It should be admitted that the results obtained with the micas, and especially with biotite, did not appear as consistent as those obtained with illite, but generally, the results obtained for all three clay minerals showed evidence to support the apportionment ofexchangeable K, Yk, to specific and non-specific sites. The experimental data fit quite well the theoretical equation. D. Vermiculite. The results for vermiculite, shown in Table 7, could not be plotted readily using equation (4). Of the nine reduced ratios studied, only three fell on the -2 -1 linezir portion of the curve. These ratios were 10 , 10 and 0.3 (raol./l.)'^. Even at these selected ratios, a reason¬ able plot of YK (exchangeable K) against RYca was obtained only by using the single experimental value of 66.76 m.e./lOOg for exchangeable K, at R = 0.3, instead of the average value of 60.27 m.e./lOOg. At higher values of R, and especially 100 65 K (m.e./IOOg) Exchangeable o o o o CO ro o Fig,l4 - Relation between the exchangeable K and the product, of the reduced ratio(R) exchangeable Ca for K-Ca vermiculite. 66 m CO CM M I 1 1 1 1 1 1 o o O o O 1 to I M M M M M 1 Cd 1 O CM 00 1 o 1 X X X X X 00 in CM ‘>o 1 o m mT 1 M (TV VO CM « • 1 Pi 1 • • • • • • 00 1 1 r'- vO M o M CM CM M CO 1 1 ® 1 VO M o Ml- m CM u 1 1 (U 1 1 Cu 1 1 CO 1 1 i c 1 1 o 1 1 c 1 1 CM CM CM 'd- VO Mf 00 o r>. 1 1 CO 1 lO M o CO Ml- 00 M in 1 1 d 1 m (TV CM vO M O Mf m 1 , d 1 cd 1 1 nj •1 o 1 m o CO CM 00 O M CO Ml- 1 1 CO CO Ml- M Ov Ml- M 1 o 1 M M M M M 1 •H 1 1 1 60 y-i 1 1 1 O •H f a 60 1 1 O o 1 O 1 1 M ^ Mj- o M 1 B 1 M M 1^ VO 1 00 CO 1 1 M 1 CO . Ml- 1 0) 1 • 1 u 0) 1 o O O CM CO Mj- CO m o 1 bO 1 1 CM to 00 CO o 1 (U c • 1 1 M M 1 4J td cu 1 1 1 d rd 4J 1 1 •H o •H 1 CO 1 O X r-^ 1 O 1 « 1 1 1 1 1 1 1 1 1 1 d 1 O 1 • • • • • • • • • o 1 M 1 m O CO m M Ml- Ml- OV in d 1 O 1 CO CO Ml- Ml- MJ- CM •d- o no o 1 1 M M M M M M M M M d 1 1 cd d 1 ■ rJP* 1 o 1 * 1 CO •H 1 • 1 4-1 4J 1 CO T—J 1 CM d CU 1 O O 1 VO cd Ui 1 i4 B « • 1 1 1 1 1 1 1 1 4-1 o 1 cn to to 1 • 1 H d 1 r-l 1 CO o cd 1 1 o 1 1 I- 1 1 d 1 1 o 1 1 cu M 1 CO • 1 cd O 1 d 1 O 4-t 1 O O 1 M • 1 B 1 O 1 1 • 1 1 1 1 1 1 1 1 1 1 « 1 CM m 1 r-l 1 TJ M 1 1 O - ♦ Fig. 15 “ Relation between specifically adsorbed K and the product of the reduced ratio(R) X specifically adsorbed Ca for K-Ca vermiculite. 68 at R values of 5 and 10, the exchangeable K values did not fall on the line. The plot of %K versus R^Ca is presented in Fig, 14, The Gapon constants and cation exchange capacities of specific and non-specific sites estimated using the data that seemed to fit gave a value of 2,01 (1,/mol,)^ for the Gapon constant of the non-specific sites (K_ ); ®ns a value of 12,0 m,e,/100g for the cation exchange capacity of the specific sites (CEC^), as shown in Table 15, Equation (6) gave a value of 313,6 (1,/mol,)^ for , the Gapon constant for the specific sites, and the inter¬ cept for equation (6) was found to be 4,38 m,e,/100g. This value shov/n at the bottom of Table 15, was used to test the validity of apportionment of the exchangeable K to specific and non-specific sites, using equation (7), K_ value for vermiculite was greater than those °ns obtained for muscovite, biotite and illite. Its value was greater than that of muscovite but much less than the Kq for biotite and illite. The experimental values of the exchangeable K, Vk, and those calculated using equation (7) are presented in Table 16. The test for equation (7) showed good agreement between the experimental and calculated values of YK for R values up to 0,3, For R above 0,3, the agreement was not as good. However, it may be concluded that in general, 69 Table 16 - Experimental and calculated values of exchangeable K, Vk. for vermiculite. * R Expt. Calc. (mol./l.)^ J(k (m.e./lOOg) 10" 5 4.06 4.40 lO"^ 4.66 4.64 -3 , 10 6.16 6.47 10"^ 12.94 13.02 10”^ 35.04 35.51 0.3 66.76 66.22 I 100.76 95.37 5 142.06 147.76 10 100.24 112.44 *.Calculated from equation (7) 70 results for vermiculite could be treated with the theoretical assumption of specific and non-specific sites. The reduced ratio had a distinct effect on the K-adsorption in vermiculite. The deviation of K and Ca exchangeable from the theory at larger values of R might be due to segregation of K and Ca in the interlayers. Only X-ray analysis could have been used to test this possibility. E. Montmorillonite. The results of the experiment using montmorillonite are presented in Table 8. For this clay, the theory could be tested only up to a reduced ratio of 1.0. Above R = 1, probably because of experimental error, no exchangeable K was measured. Consequently, no values were shown in the sixth, seventh and eighth coluinns of Table 8, at R = 5 and R t= 10. The Capon constants and cation exchange capacities of specific and non-specific sites for K-adsroption for montmorillonite are presented in Table 17. K value 1 ^ns was found to be 1.92 (l./mol.)'^, from a plot of equation (4) which gave CEC value of 2.0 m.e./lOOg, as shown in Fig. 16. s As it appears in the sixth column of Tcible 17, the — 5 specifically adsorbed K ( had a value of 0.428 at R = 10 — 3 but decreased to 0.267 at R = 10 . Then it rose to 2.33 o -2 at R = 10“'^, followed by another decrease from R t= 10 to R t= 0.3, and finally it increased again at R = 1. Thus the change in Yk„ did not follow the general trend of a Table 17 - Gapon constants and cation exchange capacities of specific and non-specific sites for K- adsorption on raontmorillonite. o o o w o W O u Pi Pi CC CO d i CO d CO CO O CO CO d CO r-4 1 O 1 O 1 M 1 O 1 o s bO 1 O S t 6 • • 1 m.e 1 1 1 1 1 1 CM CM o o^ M • m m I I Ml- CM CM CM vO CM VO CM O O o oo O O CO O o O O o M M M M M o M X 1 • • • • • •cf I .X Ml- VO CM CM CJ\ VO o 00 o CO OV m 00 00 cyv 00 O O a\ CO M M M M M o M < • 1 • • • • CO CO I I CM CM vO VO O CO O 00 in o r** CO CO m cr» O 00 M M M o M X 1 • • • • . CM I CM VO CM CM CO CO in 00 CO 00 O 00 Fis* I6 “ Relation between the exchangeable K and the product of the reduced ratio (R) exchangeable Ca for K-Ca montmorillonite. 73 continuous increase in as R is increased, as observed in the previous clay minerals. As a result of this in¬ consistency, equation (6) could not be plotted and there¬ fore, the Gapon constant of the specific site, K_ , could not be obtained. The results for montmorillonite did not confirm the theoretical consideration for two sites with differing bonding for K. From the results obtained with the five clays, it is clear that the apportionment of exchangeable K to specific and non-specific showed an apparent success in illite, the micas and to a lesser extent in vermiculite. Montmorillonite did not follow the same trend as the other clays. In Fig. 17, which is a plot of Y^/Vca versus R on a log-log scale, we can see that there are tv;o distinct portions of the curves. This may give a picture of what takes place during the K-adsorption process. The lower portion of the curves corresponds to adsorption on the specific sites and the upper portion to non-specific sites. _3 The breaking point for the micas occurred at about R = 10 . For vermiculite and illite, the breaking point occurred at a higher reduced ratio. If the values of 0.43 for exchangeable K, as presented in column 2 of Table 8, were assumed to be very close to zero, then montmorillonite would lack a breaking point in its curve shown in Fig. 17, Pi •H 4J Cxi CD U bO O 'O -l > O >» 4J U Q) (U CO 0) c>o CM c I cd x: o X o; c o 1-t 4J cd n o I 0) CW O 4J O I •HO CO>, e cd x: •-« ■p o •H p m Cd CO} >-i o o m tJ cw I I fsi M tQ •r4 P4 ?3A/^ 2°^ PL 75 which might give evidence to support the presence of only one adsorption site for K, IV, The Double-Layer Theory The cation exchange equation based on double-layer theory ns formulated by Bolt (14) was applied to the results obtained in this study by postulating that, if K-adsorption at non-specific sites involves K-Ca exchange at plan£u: surfaces as suggested by van Schouwenburg and Schuffelen (86), then the portion of the exchange reaction attributed to such sites might be expected to obey the double-layer equation. Thus, the results for K-adsorption at non-specific sites were compared to predictions of equation (ICL), For this purpose, the ratio of potassium adsorbed on non-specific sites to calcium adsorbed on non-specific sites. Vk / Vca , was calculated and is presented in Table 18 for illite, muscovite, biotite, vermiculite eind montmorillonite. The double-layer equation was used to calculate the fraction of exchange capacity filled with monovalent and divalent cations, as well as the ratio of monovalent to divalent cations, assuming charge densities, P , of 1X10 , 2X10 , 3X10 and 3,5X10 m,e,/cm , The charge density of montmorillonite is cibout 7 2 ”7 1X10“ m,c./cm v;hile that for the micas is about 3.5/10 m,e,/cm^ so that the values of P used in the calculations include the range of values of P to be expected for the minerals studied. 76 Table 18 - Relation between the composition of the equilibrium solution and the ratio of the non-specifically adsorbed potassium to the non-specifically adsorbed calcium ( /Y^^ns ) K-Ca saturated illite, muscovite, biotite, vermiculite and montmorillonite. Clays R Illite Muscovite Biotite Vermiculite Montmorillonite h (mol./l.) lO'^ I.415 X lO"" 3.498x10"^ 4.960x10"^ 2.01 X lO”^ 1.92 X 10"^ 10-^ 1.445 X 10"^ 3.499 X 10"^ 4.80 X 10“^ 2.0 X 10"^ 1.92 X 10"^ .3 10"^ 1.354 X 10 ^ 3.501 X 10"^ 4.80 X 10'^ 2.0IX lO"^! 1.92 X 10”^ i -3 10-2 1.350 X 10-2 3.50 X 10 4.80 X 10“^ 2.0IX 10"2 1.92 X 10-2 -2 1.350 X 10"^ 3.50 X I0“2 5.33 X 10 I.97x 10"^ 1.92 X lO"^ 0.3 - 0.105 0.144 0.603 0.57 I 1.350 p.50 X I0"I 0.480 2.01 1.92 • 5 - E.75 2.400 10.05 1 10 13.518 3.50 4.800 21.II ! 77 Fig.18 - Logarithmic plot, showing the correspondence between the double-layer theory and the ratio, > of the amounts of K and Ca adsorbed on the non-specific sites of some clays. 78 Fig. 18 shows the curves obtained with the double-layer equation for P = 1X10*"*^ m.e./cm^ and P^ 3.5X10“^ m.e./cm^ respectively, as well as the data points obtained by plotting Vca^^ versus R, for montmorillonite, muscovite and biotite. The points for montmorillonite lay very close to the curve obtained with r= IXIO" m.e./cm , while the points for the micas reasonably fit the curve for P = 3.5X10""*^ 2 m.e./cm . The general slope of the curves obtained with the double-layer equation is the same as that of the curves obtained by plotting XK / Yca versus R. The results for this test, therefore, support the proposition that potassium adsorbed on the non-specific sites may be expected to be on the exposed planar surfaces where a diffuse double¬ layer may be expected to form. V. General Discussion Having established that K is specifically held, it is important to attempt to localize the sites or positions in the clays, which might account for this, and also to explain the factors involved in determining the differences in K-adsorption among the clay minerals investigated. These factors include charge density, ion size and hydration energy, as well as structural composition. A. The effect of charge and structural properties of the clay on K-adsorption. Layer charge and surface 79 charge density of the clay play an important role in determining the bonding energy for K. Wild and Keay (99) indicated that in vermiculite and illite the K ions fit into the hexagonal structural “holes" in the oxygen layers of two adjacent silica sheets and therefore sit closer to the seat of negative charges of the clay than the polyvalent ions which would be forced away from the clay surface by their hydration shell. The differ¬ ence in bonding energy for K between vermiculite and mont- morillonite may be attributed to the fact that, in addition to the larger charge density in vermiculite than montmorillonite, the distance between interlayer cation and charge site is much smaller in vermiculite than in montmorillonite. The charge in vermiculite originates in the tetrahedral layer. The origin of the charge in the dioctahedral montmorillonite on the other hand, is largely in the octahedral layer. Thus the charge in vermiculite is 3.66 A® closer to the positive charge on the interlayer K ion. As the electrostatic attraction between the charges is inversely proportional to the square of the distance between them, the attraction betv/een K ions and the vermiculite layers is several times more than that between K ions and the montmorillonite layers. Since in montmorillonite the negative charges are further from the surface, the effect of an ion being pushed away from the surface by hydration or entering a "hole" is 80 consequently smaller. Potassium would thus be less specifically adsorbed on montmorillonite than on vermiculite. Wild and Keay (99) also indicated that since about one-sixth of the tetrahedral sites are substituted by A1 for Si in illite, it might behave like vermiculite. The difference in specificity for K between the micas has been explained by relating their chemical composition and K exchange. Trioctahedral micas (biotite, for instance) usually exchange K much more easily than dioctahedral (muscovite) micas. Bassett (7) suggested that this is because the orientation of the OH groups differs in the two series. The proton on the adjacent OH group is repelled equally from each octahedral site when all the sites are occupied, that is, the OH in trioctahedral micas are oriented so that the OH dipole is perpendicular to the layer or basal plane of the structure', but when only two in three cation positions are filled (dioctahedral), the proton is attracted toward the empty site and the OH is oriented (in muscovite) at 74^ to the vertical. According to Bassett’s theory, when the oH groups are perpendicular to the basal plane, the protonic charge is close to the inter¬ layer K and repels it, weakening the binding of K in the structure, whereas in the tilted orientation the protonic charge is more remote from K and does not repel it. This may be the reason why muscovite generally holds K more 81 tightly than biotite (28,82,86). This study shov;ed a reverse sequence, which may be attributed to difference in the size of the particles of the two micas, as stated earlier. It should be recognized that measurement of selectivity in clay minerals such as vermiculite and pcirtially weathered micas presents several difficulities. These minerals are exchangers with phase transition; that is, the structure of the mineral may change while selectivity is being measured. Potassium or similar ions, may collapse the interlayer space, thus changing the exchange properties of the mineral. Reichenbach (76) showed that counter ions of the contracted portions of the interlayers are not in equilibrium with the solution phase. A further problem in the case of potassium is that added potassium adsorbed by the mica-vermiculite is difficult to measure because of the presence of native potassium. The vermiculite used in this study might have behaved in this way, which made it difficult to measure the ionic equilibrium adequately. Perhaps, as mentioned earlier, segregation of K and Ca in the interlayers at high values of the reduced ratio accounts for the deviation of K and Ca exchangeable from the theory. B. Relation between ion size, ion hydration and selectivity. Two factors widely used in the literature 82 to try to explain cation exchange selectivity in clays are the ionic size of the cations, and the hydration energy. Ionic size determines selectivity in the sense that, the smaller the ionic radius, the larger the hydrated radius and hydrated ion volume, and therefore the greater the distance from the exchanger surface. Thus, the smaller Ca ion (0.99 A°), is more largely hydrated (4.12 A^) than K ion which has cin ionic radius of 1.33 A® and an hydrated radius of 3.31 A^. Adsorbed Ca ions are a greater distance from the surface charge than the less hydrated K ions. Wells and Norrish (98) reported that cations of low hydration energy, such as K, Rb, Cs and NH have an inherent advantage for entry into the unexpanded lattice of clay minerals because they can shed their hydration shell more easily than ions of higher hydration energy such as Na, Mg or Ca. For this reason, K, Rb, Cs and are able to compete successfully with, for instance sodium even when its concentration is three or more orders greater. They enter the lattice sites so much more readily than the larger hydrated ions that they are able to prevent the large ions from reaching a concentration within the lattice that allows them to rehydrate and expand. In addition to the above explanation based on ion hydration, it is possible that the preference shown by micas for K may be due to the coordi¬ nation of K with surface oxygens of adjacent mica sheets. 83 The degree of ordering will be less, and the coordination different for the interlayer K in a vermiculite, for instance, than in micas, hence the preference for K should by this argument be much less evident in vermiculite than in micas. Rich and Black (82) proposed that exchange£ible K is located in wedge-shaped spaces in the interlayers of weathered micas. Because of low hydration energy and .small size, when dehydrated, K ions are selected at the apex of the wedge. This will enable the silicate layer to collapse pairtially and make new wedges for more K ions. Ca ions would be excluded because of larger hydration energy and large size of hydrated Ca ions. C, Shortcomings of the theory. Although the apportion¬ ment of exchangeable K, Yk, to specific and non-specific sites shows an apparent success with illite and the micas, a number of shortcomings have been suggested (91). For instance, the subdivision of K into '^^ns that the exchange behaviour of the two sites is described by the Capon equation and that , the Capon constant of ^ns the non-specific sites is much less than , the Capon constant of the specific sites. The former condition is not altogether valid, since, as certain workers (82,86) have shown, the distribution of YK between the two sites depends not only on the reduced ratio, R, as the Capon equation implies, but also on the total concentration of 84 the equilibrium solution. Nevertheless, the values presented Tables 9, 11, 13 and 15, although only indicative of an order of magnitude, offer an acceptable explanation of the exF>erinental data. D. Significance of the K/Ca ionic equilibria. To have a sound physico-chemical basis the K/\/Ca or the K/Vca+Mg ratio should be of the active masses of the ions. The concept of quantity-intensity (Q/I) (8,10) or the nutrient potential and capacity (4), as far as K and Ca are concerned is derived from this ratio. For neutral soils, the measure¬ ment is expressed as (K)/V(Ca+MgT where parentheses indicate activities, or as the logarithmic function: p(K) - —(Ca+Mg) At equilibrium, the ratio of the activities of the ions in solution may be considered equal to the ratio of the activities of the same ions on the solid phase. But it does not follov/ that a ratio is the best description of a soil solution when considering availability to plants. For example, if uptake of K by plauits is independent of the calcium and magnesium concentrations, the ratio might be misleading when comparing soils with different Ca and Kg contents. If two soils A and B have the same ratio (K)/yCa-i-V^ but the (Ca+Mg) in the solution of soil A is foiir tines as high as in the solution of soil B, (K) in ' A 85 soil A is twice that in soil B and the K uptake from these soils might be quite different. Beckett (8) has drawn attention to this point when he said that it may be prefer¬ able to consider the ratio as a comparative measure of the chemical potential of K only betv;een soils of comparable Ca status. Wild (100) found that the activity ratio is an un¬ suitable measure of the intensity (availability) of K in soil solution unless the ions in the denominator of the ratio, Ca and Mg, are present in all the soil solutions being examined. The relevant measure of intensity or potential of K in soil solution is the concentration or activity. Knowledge of the reaction of K with clay minerals may increase our basic understanding of the micaceous layer silicate minerals and the role of the K-ion in affecting their properties. This, in turn, may be of use in more satisfactorily evaluating the K-fixation phenomenon in soils, the fate of K fertilizer and the availability of K in plants. Before specimen mineral properties are extrapolated to soil systems,it should/^ established that the specimen properties are actually those of a nearly monomineralic sample and that the analogous soil minerals behave similarly. A knowledge of the properties of a particular clay mineral 86 is important in evaluating the chemistry and physics of soils that contain it in significant amounts. One should also show some caution because it is assumed in exchange studies, that the material obtained from the mineral suppliers is pure clay mineral, whereas a great number of samples actually contain a mixture of clay minerals. The proportions of the minerals veiry with the sample source,, sample prepa¬ ration, and the peirticle size employed in the study, A good example of such a mixture is vermiculite, hydrobiotite and biotite, which is sometimes taken simply as vermiculite. Assuming that results obtained through the study of ionic equilibria on clay specimens are applicable to soils that contain these specimens, it is clear that the study of cationic ratio is of importance. Indeed, one of the problems in fertilizing soils is to provide a desirable balance of cations in the exchange complex. Although some systems of soil testing took into account the exchange capacity of the soil in arriving at the amount of the exchangeable cations that should be present, they failed to consider the differences in intensities with which the cations are held in the exchange complexes of different kinds of soils. However, relatively recent studies have concentrated more on this problem through the use of the energy of exchange concept. Woodruff (102) used this concept and even developed the relationship: Af = 1364 log R 87 or Af = 1364 log ..— — v;here and are activities V ^Ca in moles/liter of K and Ca, which is the expression of the free energy of exchange of one chemical equivalent of K and Ca at 25^C, The change in free energy, Af, accompanying cationic exchange between a clay and the standard state may be evaluated for different ratios of cations in the clay, and the results plotted in the form of a graph so that they will portray the energy relationships over the entire range of cationic compositions of a clay, • Within certain limits these relationships may follow a regular pattern that would permit chciracterizing the behavior of the clay with a single constant. Such a constant may be viewed as a convenience rather than a necessity because a curve or a family of curves would portray the relation¬ ships completely. The plant in the final analysis, must be the criterion by which a medium is judged as a suitable substrate for growth. The relationships between plant nutrition and energies of exchange may be ascertained most readily by considering those soils for which an abundance of experimental data is already available. The intensity factor in cation exchange as exaraplified by energy of exchange is a fundamental measure, and if those soils for which correlations already exist cire evaluated in terms of energies of exchange, the results should provide the necessary guide posts for evaluating 88 the nutritional status of any kind of an exchange medium with respect to the K—Ca balance* The concept of energies of exchange is related to the concept of cation exchange selectivity in that the more negative the change in free energy when a cation is associated with an exchanger, the more strongly will that cation be bound to the exchanger, and the less available it will be to plants*. Woodruff (102) indicated that energies of exchange of the replacement of Ca by K should fall within a range of - 2,500 to - 3,000 calories for balanced nutrition of plants* Energies of exchange in excess of - 3,500 calories v;ere associated with K deficiency, whereas energies of exchange of ~ 2,000 or less may be expected to be associated with Ca deficiency, as a result of excessive amounts of K* The practical implications of most of the results obtained in this study are that under field conditions, a very large proportion of the exchangeable potassium is held at specific sites in illitic and micaceous soils* In situations where K is limiting, diffusion to the plant roots is the major factor determining the amount of K that will be available (60)* Annual crops such as corn probably require K at a faster rate than it can be supplied from the "fixed" forms by diffusion and, together with differential root patterns, a perennial crop v/ill obtain adequate K whereas annual crops will shov/ signs of potassium deficiency* 89 Another inference from this investigation is that the planar (non-specific) and the interlattice (specific) sites differ in bonding energy for K ions but for both sites a state of equilibrium is reached v/ithin a short period of time. The bonding energy of the interlattice sites is not only greater but this reaction needs a much longer period to achieve an equilibrium status. It is not yet clear whether all potassium present on the interlattice sites can be ultimately exchanged (v7hich reaction would consequently entail a breaking up of the lattice), or it is only a certain limited part of it which participates in these equilibria. Leaving out of consideration the effect of roots within the rhizosphere it may be postulated that the supply of potassium, or in other words, the potassium concentration of the solution will be determined by the number of K ions adsorbed on the planar sites and the edge-interlattice sites and by Ca or better (Ca+Mg) concentrations. This would indicate the maximum supply of potassium ions because a transition of interlattice into plcinar and edge potassium could only lead to restoring the quantity initially present. Besides it is not to be anticipated that a supply from this diffusion-governed process v;ould be able to keep pace with the quantity of potassium consumed by the plant. Considering the case of illite, probably the reaction 90 of the plant v/ill not differ appreciably in soils with an open illite (vermiculite) from that in soils with a closed illite. Practically it will be governed by the amount of potassium present on the planar and edge-inter¬ lattice sites. On the contrary, there will be a great difference betv/een soils containing open and those with closed illite in so far as the response of crops to potash dressings is concerned. Soils containing an open illite will fix a great part of the applied potassium on the interlattice sites. This will be practically withdrawn from the equilibrium. Soils with ordinary illite will adsorb potassium for the major part on the planar and edge sites. The lower the content of adsorbed potassium the more of applied potassium will land on the edge-interlattice sites and the lower will be its concentration in the soil solution. Therefore, a very strong response to potash fertilization will be found in cases when a considerable amount of the added potash is adsorbed on the planar sites. Rezk and Amer (77) have shown that the selectivity coefficient could be used to correct the exchangeable K for differences in selectivity among soils and the ratio of exchangeable K to selectivity coefficient^K/k, is more closely related to K supply than exchangeable K alono. 91 This defined ratio is equivalent to Ca. shown in the fifth column of Tables 4, 5, 6, 7 and 8, which means that the product R^Ca may be considered as an important index for K availability in soils. The higher the product, the more available K tends to become. An apparent consequence of the selectivity for K over Ca in some of the clays studied is that it v;ill generally be quite difficult to saturate those clays V7ith calcium. If this is the case, then, Ca-CEC determination in those may not be very accurate. Also, if at high cxchangecible K values the selectivity for calcium is very large, even small amounts of impurities of calcium in analytical grade KCl could compete effectively for exchangeable sites, in which case, the distinction between selectivity and speci¬ ficity for Ca ions becomes difficult to make. 92 SUMMARY The K/Ca cation exchange selectivity of illite, muscovite, biotite, vermiculite and montmorillonite was studied. The following results were obtained: 1) The micas (muscovite and biotite) showed strong preference for K as compared to illite, vermiculite and montmorillonite. 2) The selectivity coefficient, K^, for all five clays increased as K-saturation decreased, which confirms results reported in the literature. The strong preference for K by the micas is indicated by their large selectivity coefficient values, 3) The K-adsorption could be appoi'tioned to specific and non-specific sites on illite, muscovite, biotite and vermiculite but not on montmorillonite. The specific sites hold K more strongly than the non-specific sites. For a particular clay mineral, there is a fixed number of specific sites, as shown by the constancy of the cation exchange capacity of the specific sites. The cation exchange capacity of the non-specific varies v/ith the degree of K-saturation, 93 LITERATURE CITED !• Babock^ K* L. ejt ad. 1951, Ionic activities in ion exchange systems. Soil Sci. 72:213-260. 2, Barber, S, A, and Marshall, C, E, 1952. Ionization of soils and soil colloids. III. K-Ca relationship in illite, kaolinite and halloysite. Soil Sci. 73:403-413. 3. Barbier, G. and Trocme, S. 1963. The basis of potassic manuring-Potash Review Subject 16, 25th suite:1-13. 4. B^lrrow, N. 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The activity ratio as a measure of the intensity factor in K supply to plants. Soil Sci. 108:432-439. 101. Woodruff, C. M. 1955. Ionic equilibria between clay and dilute salt solutions. Soil Sci. Soc. Amer. Proc. 19:36. 102. Woodruff, C. M. 1955. The energies of replacement of Ca by K in soils. Soil Sci. Soc. Amer. Proc. 19:167-171 : ■; V ; ; . ■ • ■ • ■. ; . ' 1. • V '•i : ;■ ■■;'i - sir I: f ■■ y- h*" i t • ' ■••;..■ :•' ■■ ^ ’'t iri'ffi. ' '- y; hi*):' ■'! ;■/ ' i . • . • , ‘ ‘ \ \. • I *.« : * f » . V 5. t <■« 1 •■;. ' :■. I r , ;pa: l|r- m , '-yrJifjf'yH I f. ;k; ; ;./:i