D. A
d
DIVISION OF SOIL AND FERTIUZER INVESTIGATIONS, U. S. DEPARTMENT OF AGRICULTURE, BELTSVILLE, MD.
An elementary example will illustrate the importance of base exchange to agriculture. From an ionic point of view, planta produce and exchange hydrogen ions for cation nutrients. The relertsed hydrogen ions are in part held by the base exchange compounds, and the soil becomes more acid. The acidity is sometimes reduced by weathering of silicatea to supply free bases or requires application of a liming material. These factors, which are not the only ones contributing to equilibrium, are schematically shown as follows (90):
Concepts fundamental for an understanding of base exchange b y silicates are presented. Necessary conditions for cation exchange in zeolite structures are the presence of negative portions i n the lattice frameworks and of mufticonnected channels large enough for ionic migration. The ways i n which negative portions of lattice8 arise in zeolites and clay minerals are discussed i n detail. Termination of an ionic lattice at a surface often involves incomplete blancing of charge with a corresponding requirement for presence of external ions. Base exchange capacity of kaolin minerals i s due to this factor Strip mining of bentonite In eastern Wyoming i s shown in the above picture (courtesy, American Colloid Company)) montmorillonite i s the essential minerel constituent of bentonites.
. ..
Plants
I
K+, Ca+*
Ca+*, I(+
$+Base Exchan e Compounds f in Soils
F
SE exchange of silicates and organic materialsin soils is the most prominent factor in maintaining nutrient supply to plants. Ita discovery by Way (18) almost a century ago waa due to questions arising from the introduction of soluble salts as artificial manures. Way made a second essential step in showing that exchange of cations is particularly exhibited by aluminum silicates and he prepared the first permutit. In recent years some insight has been gained into the mechanism of base exchange, and the purpose of this paper is to describe the process for silioates.
H+
I
I
Weathering of Feldspars and Micas
I
E+ I Liming Materials
I
Fertilizers, hlant Residues
Base exchange compounds are heterogeneous electrolytes, and cation replacement is simply the process of exchanging an ion from the environment of a solution to that of a solid in contact with solution. The following work deals with the nature of the 625
626
INDUSTRIAL A N D ENGINEERING CHEMISTRY
VOl. 37, No. 7
solids, which includes demonstrations of t , h t b causes for base exchange. Three distinct types of base exchange compounds are observed in crystalline materials in a classification, depending upon the nature of the exchange site: 1. Within the
structural framework: zeolites, ultramarines, noselite 2. External to the structural framework a. Upon an inner surface accessible by swelling: montmorillonite-type clay minerals, graphitic acid At the limiting surfaces of the crysb. tals: micas including glauconite (greensand), illite (present in many shales and soils) c. On thesidefacesof tabular crystals: kaolin minerals, gibbsite etc. 3. Upon negative organic groups ’in close proximity to positive groups, proteins
3
Amorphous materials will not be considered; since the discussion is limited to silicates, the lmt group, although the best understood of all, will not be mentioned again. Structural features of ionic solids are too well known to require detailed restatement here, but some factors of particular importance in silicates should be mentioned. In the simplest terms, silicate structures are determined by the ratios of the positive to the negative ions, which are usually oxygen, the Figure 1. Portion of Aluminum Silicate Framework in the Structural Unit of Sodalite ratios of the ionic radii which determine the That Is Present in a Distorted Form in Chabazite coordination figures of the positive ions, Connections from one void to another are Indicated by mow% and a general “principle of microscopic neutrality”. This principle, one of the most fundamental in chemistry, was first recognized by Pauling (1.8) neutral. A necessary condition for base exchange, entirely overas the “electrostatic valence principle”. It can simply be stated looked in the past, is for the volumes enclosed by the framework to be multiconnected by channels sufficiently large for cation as follows: “Ionic systems are statistically neutral on the smallest possible scale”. (This restatement is made with Linus migration. Compounds such &s beryl, Alz(Be&.) 0 1 8 , having biconnected voids (that is, unconnected channels which often conPauling’s approval.) Cations at exchange positions conform to the requirements of tain appreciable numbers of alkali ions) do not show base exmicroscopic neutrality in that they are opposite potentially change. negative positions of a lattice and still have a pathway for reachZEOLITES. Base exchange compounds having framework ing a contact solution. The negative position in a lattice is structures (4, 18) are well illustrated by zeolites. There are at usually brought about by presence of one cation in place of anleast four distinct structural types of crystalline zeolites, differing other of greater charge having a similar ionic radius, such as Al+a in framework, shape of enclosed volume, and multiplicity of volume connections, as follows: for Si +4 or Mg +2 for Alfa. Ions that occupy equivalent positions and are particularly involved in the discuseion of cation exchange Multiconnection follow: Zeolite Formula of Voids Ionic Radius Ty e of No. of for Obavd. Analcite [(AlSiz)0~]-*Na+~. HzO 4 CoorJnation Oxygen Important Coordmation Chabaaite [(Al~Si~)01z]-~Ca+z,Na~+~. 6HzO 8 Position Neighbors Cation with Oxygen Natrolite [(AlrSi~)O1o]-2Na~+~. 2H20 6 Tetrahedral 4 Si+d Al+s Be+Z 0.551L [(AIzSii)018]-2Ca’2.6HaO Unknown Heulandite Octahedral 6 A1 ti, Mg lz, F e +2, Be +’a, Li + I 0 . 5 5 to 0.80k. Greater than octahedral >6 K +, Na t, Ca tz, B a t * 0 958. The first two approach cubic symmetry which permits approximately isotropic diffusion of an exchanging cation through the The importance of the general principles will become apparent multiconnected voids. Natrolite is typical of the fibrous zeolites with detailed discussion of each type of exchange material. which are tetragonal or pseudotetragonal in symmetry, a i d in which diffusion is still possible in the principal directions that are E X C H A N G E SITE WITHIN F R A M E W O R K not equivalent. Heulandite, the structure of which is unknown but which probably is a framework of limited thickness, is monoIonic compounds in which the exchange sites are inside the lattice generally have rigid framework structures, in which all the clinic and apparently the voids are connected only parallel to oxygen ions are shared by two cations having tetrahedral coordi(010) and not along the b axis; thus ionic diffusion is restricted nation but with an average charge less than +4. The framework to nonequivalent directions in parallel planes (17’). Structural features of the zeolites are here illustrated by the [(A12Si3)Olo]-z, thus can have compositions such as [(AlSi,)0~]-~, framework unit of the ultramarines, sodalite and helvite, which is [(A12Si,)Ols]-2, etc., and it is like a house in including large volpresent in a distorted form in chabazite (21). Figure 1shows the umes. Other cations required to balance the charge and water actual unit for sodalite, and directions of possible ionic migration molecules with which they are associated are located in the open are indicated. I n this and other figures the usual convention spaces in such a way as to make the structure microscopically
July, 1945
627
INDUSTRIAL AND ENGINEERING CHEMISTRY
is followed of indicating ionic centers by small circles. Frameworks of analcite and the fibrous zeolites are illustrated in standard books on structures of solids (1). Ions within the voids of zeolites can be completely exchanged with varying ease, depending upon the potential barrier between sites. A large external cation, such as tetramethyl ammonium N(CHg)d+', cannot enter the connecting channels, and exchange does not take place (19). This is also true for replacement of water by larger molecules such as benzene (16). The formulas MI shown are only typical, and there can be some variation due to the presence of A1+8 in place of Si+*in the framework. I n this way the number of exchangeable cations in heulandite can vary from 1.8to 3.0 per unit of 36 oxygen ions, the formulas (21) varying between about [(Ala.sSi~c.c)Oas]Catl.a. 12H20 and [(Al&)Oae]Cs.ONa.12HzO. The extent to which A1+* is present in the framework appears to be limited by the attainment of microscopic neutrality through the arrangement and numbers of positive ions and water molecules that can be situated in the voids. This limiting amount of Al+a in chabazite is such as to give an Al+s: Si+' ratio of about 1:2, while it can be 1:l in the structurally related noselite and sodalite where additional negative groups are present in the voids instead of water molecules. EXCHANGE SITE EXTERNAL TO FRAMEWORK
Silicate base exchange compounds of the second type have sheetlike structures in which the determinative structural element is the hexagonal network shown in Figure 2 (13)). This network is built pp from tetrahedral groups of oxygen ions around A1 +a or Si+'cations. Many silicates contain this network, joined through the unshared oxygen ion of each tetrahedral grouping to groups of oxygen and OH- ions octahedrally coordinated about Al+S, Mg+P, Li+1, etc. Composite structural sheets of kaolinite, AhSiz06(OH)4, and pyrophyllite, Al$3ibOto(OH) 2 (Figures 2 and 3), show the two principal types of structures to be discussed. Two striking features influencing cation exchange of the sheetlike silicate structures are to be emphasized, First, the sheets are too compactly filled to allow ionic migration through them. Second, one third of the octahedral coordination positions are vacant in the pyrophyllite sheet. This second feature and the presence of A1+8 in place of Si+' in the network of Figure 2 play the most important part in determining the extent of base exchange in the important class of clay minerals related to montmorillonite, which are industrially known as bentonites.
MONTMORILLONITES. Silicates of the montmorillonite group have structural sheets similar to that shown in Figure 3 for pyrophyllite (6). The sheets, however, have an excess negative charge which is baltpced by external and exchangeable cations. Formulas for members of the group derived from analyses of many pure minerals (14) can be expressed in the following manner: Ions in
octahedral coordination
Sheet
+
Ions in tetrahedral coordination
Olo(0H)~
Exchange cations external to sheet held because of a deficit in the positive charge in octahedral or tetrahedral positions Particular formulas of pyrophyllite, talc, muscovite, and phlogopite micas, and some members of the montmorillonite group, indicated by italics, are: Pyrophyllite
Montmorillonite Beidellite Muscovite Talc
Hectorile Saponite Phlogopite
Hectorite and saponite illustrate limiting cases where the presence of external cations is due to deficits of positive charge in octahedral and tetrahedral coordination, respectively, within the structural sheets. Analyses of minerals of the montmorillonite group show that these exchangeable ions are restricted t o one
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
628
VOl. 37, No. 7
EXCHANGE CATIONS O N L Y ON EXTERNAL SURFACES OF M I C A K AkCAI Si3> q o C q
0 LAYER IN PYROPHYLLITE A12Si4 OloQHh
P
w
3z* E:
EFFECTIVE
I ICHARGE
EXCHANGE CATIONS BETWEEN LAYERS IN MONTMORILLONITE A '1.6?839i4 'I 0' OH
h
0 Figure 3. Structural Layer of Pyrophyllite and Its Modifications in the Micas and Montmorillonites Various type] of positions for exchange cations are indicated.
third of an equivalent per Olo(OH)z unit. They occupy positions, between the layers, that are completely filled in the micas. I n the micas, however, the K + ions bring about microscopic neutrality for excess negative charge due to the presence of A1+*in place of Si+4 in the immediate region in the layer above and below. They thus serve to bind the layers together and prevent ready exchange with other cations. In minerals of the montmorillonite group an exchangeable cation seems to be required for microscopic neutrality by only one of the neighboring sheets, even though all sheets carry the same average charge. The forces between the layers thus are sufficiently small to permit hydration of the surface and wandering of the ions. An excellent illustration of the principle of microscopic neutrality is afforded by beidellite. In it the amount of Al+a in tetrahedral coordination approaches that required for a mica. The number of ions external to the structural sheet, however, is re duced by increase of the Alia ions in octahedral coordination, microscopic neutrality thus being attained by internal compensation. Here conditions within the framework keep the equivalence of the external ions constant, which is in contrast to the behavior of the zeolites. MICAS. While cations between the structural sheets are difficult to exchange in the micas and micalike minerals, such as
muscovite, phlogopite, glauconite, and illite, they are not restricted a t the limiting surfaces of the crystals. Base exchange is, apparently, due chiefly to these ions a t the surfaces, and the equivalence is determined by the extent of the surface. In a mica having a cleavage surface of 60 M * per gram, a value attained in many soils, there is 0.1 milliequivalent per gram of exchangeable cations on the cleavage surfaces. Additional places for cation exchange occur a t the lateral surfaces, zw will be discussed in detail for kaolin minerals, and the equivalence of this exchange may be as great as that a t the cleavage surfaces. In the more finely divided micaceous minerals, such 89 glauconite of greensands and illite of soils and shales, there is probably some aSailability of exchange sites between structural sheets, near the edges. The total exchange capacity of these materials is in the order of 0.25 milliequivalent per gram, v:hich is approximately one third to one fourth that of montmorillonite group minerals. KAOLINS. Structural sheets of kaolin minerals (Figures 2 and 4) are formed from the hexagonal network found in the micas and the corresponding layer of ions with octahedral coordination. The sheets, however, are terminated by (OH)- ions on one surface and are neutral. Slight departure from neutrality, due to the presence of A1+3 in place of Si+4 in tetrahedral coordination, is compensated by increase in the number of ions in octahedral
-
Idy, 194s
INDUSTRIAL AND ENGINEERING CHEMISTRY EXCESS CHARGE
ATOM
c2
DISTANCE FROM PROJECTION PLANE
0
t 0.5;.
0.0
sit4
AI+^ OH''
Figure 4.
629
-2.7
/-., 1 -3.7
I
Cation Exchange Positions Due to Lateral Termination of Kaolinite Crystals (Plan and Elevation) Alblnment of microscopic neublity about AI*',
coordination in the same way aa for beidellite. While the structural sheets in kaolinite are neutral, there is considerable attraction between them due to hydrogen bonding of the (OH)-ions on the bottom of one layer to oxygen ions in the top of the neighboring layer @). Relatively large crystals are built up in this way with neutral instead of charged cleavage surfaces. Dependence of cation exchange in kaolinite on the exposed surface is shown in Figure 5 (7). Earlier evidence had been obtained from changes in base exchange capacities brought about by grinding (8, 9). Surface areas of kaolinite separated from soil are as great as 80 M2 per gram, of which about 20% is due to lateral faces (11). With a base exchange capacity of 0.12 milliequivalent per gram, an area of about 20 sq. A, would be available for each univaf,
0-1,
SEPIOLITEB. All silicates and other ionic structures would be expected to show ion exchange due to lattice termination, but the amount might be very small on account of large particle size. Another group of silicate minerals, the sepiolites, having considerable cation erichange capacity should be mentioned. Attapulgite, the principal constituent of Florida fuller's earth, is of this type. The group aa a whole cannot be discussed further since little information is available about their structures and cation exchange behavior. E X C H A N G E IN NETWORK TYPES
Cation exchange of the network type structures in the clays differ in several important dynamic ways from that of the zeolites, which possibly will be apparent without detailed explanstion. Exchange reactions are extremely rapid in the clays at room temperature, while elevated temperatures and relatively long times are required in zeolites because of the limitation of diffusion. Instability of the framework has prevented formation of hydrogen zeolites whereas hydrogen clays are rather stable. Large cations, as previously mentioned, cannot enter the zeolites, while clays exhibit greater affinity for these ions than for Li+, Na+, etc. Relative distributions of cations at equivalent concentrations between a solution and an exchange material have been determined as a measure of cation affinities at exchange sites. The orders too often have been discussed in vague terms of ionic hydra-
2 3 v i
to about would lent cation. correspond two This uni-
%lg
valent cations for each laterally terminating structural s h e e t , which is the expected value (Figure 4).
9 o2 gy
IO 20 30 40 SPECIFIC SURFACE- SQ.M.PER GM.
0
Figure 5. Variation of Cation Exchange of Kaolinite with Particle Size (7)
and OH-* Is shown in detail.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
a
D.Athaction
e2 - 31I 2 =f-r2 R In
a
Vol. 37, No. 1
cntions depcials upon the prescncc ofmulticonnected voids. Clay rriirrorala and ather micaceous silicates have sheetlike struelures in which external eations are necessary for micraseopio neutrality. These eations arc principally on the cleawgeuurfaees of the montmorillonite mineral8 and on the lateral faces of kaolin minerals. Both typrs oi positions contribute prominently to the eation exchange of the micalike miner&, glauconite and illite. Orders of mtion ropliieemcnts :ire detcnnined by relative values of interaction forces between tho cation and the adjoining lattice LIERAWRE UlED
(.1 ). Biiiur. .... W. 13.. "Aioinio S L W C L W ~of Minerals".. .D. 255 01 *eo.. Itliaos, C&xIl L'niv. Prosl. 1937. (2) Hondrieks, S.1% J . Phys. Chem.. 45, 65 (1941). ( 3 ) Heridricks, S. B . , Z.K+t., 100. 5WJ (1939). (4) H ~ M. ~ H.. , naiinistei,F. A,. ~ i n i n gM W . . a,51.2-28 11 932). ( 5 ) Hoffmaiin. U.. ned Hilke, W., Kolroid 2.. 77,238 (193F). (0) Jenny, H., and Reitcmeici. 11. F., J . P h w . Chem., 39,593 (1935) ( i j .loliiisun. A . L.. nnd Lawrence. W.G., J . Am. Ceiam. SOE..25,344
~."=",.
( 8 ) lielicy, W. P., Ihie. W. H., and Biown, S. M.,Soil Sci., 31, 25 (1931).
C
D *Attraction=
e2
-
'In
2 'a
-
Figure 6. Variation of Coulomb Forcer, per Equivalent, with Charge for a Linear Away of Charges (0 Dielochic Constant)
tion, and there are few values for ionic activities i n the prcreiice of an exchange material (10). A decreased cation nctivity must aeoompany increased attraotion of the ion to the cxchangr material. An order of oation sttrmtion to montnrorillonite type clays is: larga organic cation, brucine+ > Hi > La+' > Bat*> Cs+*> K + > Lit; this i8 also the wries for Eooculation (6). Partial re&sons for such an order ®raphically illustrated in Figures 3 and 6. The large organic oations m e hcld t u the Rat network suriaw by van der Wads forces hetween the nautral portions as well as by electrostatic interaction of the charged parts ( 8 ) . Since the hydrogen ion is very smsll, it can penetrate the lsttiee more than is possible for other positive ions (10, page 1089). If coulomb forces alone are oonsidered, the repulsion term decreases with increased charge an the cation, distributed in sn array as shown ior a linesr array oi points in Figure 6c. More detailed oonsiderstiom are necessary to account for the relatively greater attrwtion of tho larger ions of s given charge. The purpose has been to present here the iew concepts that are fundamental for an understanding of base
The prinoipsl types of erystdline silicatea showing considerable cation exchange capaoity have frsmework or sheetlike struotures. Zeolites belong to IL^
^I^^^
:-
,.
