S t& ilin y . iS . J ie tu L 'u c J ii
DIV ISIO N O F SO IL A N D FERTILIZER INVESTIGATIONS, U. S. DEPARTMENT O F AGRICULTURE, BELTSVILLE, M D.
Concepts fundamental for an understanding of base exchange by silicates are presented. Necessary conditions for cation exchange in zeolite structures are the presence of negative portions in the lattice frameworks and of multiconnected channels large enough for ionic migration. The ways in which negative portions of lattices arise in zeolites and clay minerals are discussed in detail. Termina
tion of an ionic lattice at a surface often involves incomplete balanc
ing of charge with a corresponding requirement for presence of external ions. Base exchange capacity of kaolin minerals is due to this factor . . . Strip mining of bentonite in eastern W yoming is shown in the above picture (courtesy, American Colloid Company);
montmorillonite is the essential mineral constituent of bentonites.
f V A S E exchange of silicates a n d organic m aterials in soils is the W m o s t p rom inent factor in m aintaining n u trie n t supply to p lan ts. I t s discovery by W ay (18) alm ost a cen tu ry ago w as due to questions arising from th e introduction of soluble s a lts as artificial m anures. W ay m ade a second essential step in show ing t h a t exchange of cations is particu larly exhibited by alum inum silicates and he prepared th e first p erm u tit. In recent some insight has been gained in to th e m echanism of base ex ch an g e an d th e purpose of th is p ap er is to describe th e process fo r silicates.
An elem entary exam ple will illu strate th e im portance of base exchange to agriculture. F rom an ionic p oint of view, p lants produce an d exchange hydrogen ions for cation n u trie n ts. T he released hydrogen ions are in p a rt held by th e base exchange com
pounds, an d th e soil becomes m ore acid. T h e acidity is some
tim es reduced by w eathering of silicates to supply free bases or requires application of a lim ing m aterial. T hese factors, which are n o t the only ones contributing to equilibrium , are schem ati
cally show n as follows (SO):
P lan ts H +
i
C a +»,
t
K + K +, C a +2
1
B ase Exchange Com pounds in Soils C a +1, K ° . n«+» K
H +
Ca-*
W eathering of F eldspars a n d M icas
H + Liming M aterials
K + C a + 2
Fertilizers, P la n t Residues Base exchange compounds are heterogeneous electrolytes, and cation replacem ent is sim ply th e process of exchanging an ion from th e environm ent of a solution to th a t of a solid in contact w ith solution. T he following w ork deals w ith th e n a tu re of the 625
626 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
solids, which includes dem onstrations of the causes for base exchange. T hree distinct types of base exchange compounds are ob
served in crystalline m aterials in a classifi
cation, depending upon the n a tu re of th e ex
change site:
1. W ithin th e stru c tu ra l framework:
zeolites, ultram arines, noselite 2. E xternal to th e stru c tu ra l framework
а. U pon an inner surface accessible by swelling: m ontm orillonite-type clay minerals, graphitic acid б . A t the lim iting surfaces of the crys
tals: micas including glauconite (greensand), illite (present in m any shales and soils)
c. On the side faces of ta b u lar crystals:
kaolin m inerals, gibbsite, etc.
3. Upon negative organic groups in close proxim ity to positive groups, pro
teins
Amorphous m aterials will n ot be considered;
since the discussion is lim ited to silicates, the last group, although the best understood of all, will n o t be m entioned again.
S tru ctu ra l features of ionic solids are too well known to require detailed restatem ent here, b u t some factors of p articular im por
tance in silicates should be m entioned. In the sim plest term s, silicate stru ctu res are de
term ined by th e ratios of the positive to the negative ions, which are usually oxygen, the ratios of the ionic radii which determ ine the coordination figures of the positive ions, and a general “principle of microscopic n e u tra lity ” . T his principle, one of th e m ost
fundam ental in chem istry, was first recognized by P auling (IS) as th e “electrostatic valence principle” . I t can simply be stated as follows: “Ionic system s are statistically n eu tral on the sm allest possible scale” . (This restatem en t is m ade w ith Linus Pauling’s approval.)
C ations a t exchange positions conform to th e requirem ents of microscopic n eu trality in th a t they are opposite potentially negative positions of a lattice and still have a p athw ay for reach
ing a contact solution. T he negative position in a lattice is usually b rought about by presence of one cation in place of an
other of g reater charge having a sim ilar ionic radius, such as A l+3 for S i o r M g +2 for A1+3. Ions th a t occupy equivalent positions and are particularly involved in th e discussion of cation exchange follow:
Ionic R ad iu s
No. of fo r O bsvd.
Oxygen I m p o r t a n t C o o rd in a tio n
N eig h b o rs C atio n w ith Oxygen
4 Si +<, A1+*, B e « 0 . 5 5 A. .
6 A l « , M g « , F e +2, Fe+*. L i+ i 0 . 5 5 t o 0 . 8 0 A .
> 6 K +, N a +, C a +!, Ba*> 0 . 9 5 A.
SODALITE NaeAI6Si6024CI2
Figure 1. Portion of Aluminum Silicate Framework in the Structural Unit of Sodalite That Is Present in a Distorted Form in Chabazite
Connections from one void to another are indicated b y arrows.
neutral. A necessary condition for base exchange, entirely over
looked in th e p ast, is for th e volum es enclosed by th e fram ew ork to be m ulticonnected b y channels sufficiently large for cation m igration. C om pounds such as beryl, Al2(Be3Si6)O i8, having bi
connected voids ( th a t is, unconnected channels which often con
ta in appreciable num bers of alkali ions) do n o t show base ex
change.
Ze o l i t e s. Base exchange com pounds having fram ew ork stru ctu res (4, 16) are well illu strated by zeolites. T h ere are a t least four distinct stru c tu ra l types of crystalline zeolites, differing in framework, shape of enclosed volum e, and m u ltiplicity of volume connections, as follows:
T y p e of C o o rd in a tio n
P o sitio n T e tr a h e d r a l O c ta h e d ra l G r e a te r t h a n
o cta h e d ra l
Zeolite Form ula
A naleite [(AlSiîJOs] -1N a + l . H 20 C habazite [(Al2Si<)Oi2] _2C a +2,N a2 +1. 6 H 20 N atro lite [(Al2Si3)Oio]~2N a2 + 1.2HaO H eulandite [(Al2Si7)O u] _2C a + 2.6 H 20
M u lti
connection of Voids
4 86 U nknow n
T he im portance of th e general principles will become ap p aren t w ith detailed discussion of each ty p e of exchange m aterial.
EX C H A N G E SITE WITHIN FRAM EW ORK
Ionic compounds in which th e exchange sites are inside the lattice generally have rigid framework structures, in which all the oxygen ions are shared by two cations having tetra h e d ra l coordi
natio n b u t w ith an average charge less th an + 4 . T he framework th u s can have compositions such as [ (AlSi3) Oa]— [(Al2Si3) Oio]— 2, [(Al2Si7) 0 18] - 2, etc., an d it is like a house in including large vol
umes. O ther cations required to balance th e charge an d w ater m olecules w ith which th ey are associated are located in th e open spaces in such a w ay as to m ake th e stru ctu re microscopically
T he first two approach cubic sym m etry which perm its approxi
m ately isotropic diffusion of an exchanging cation th ro u g h th e m ulticonnected voids. N atro lite is ty p ic a l of th e fibrous zeolites which are tetrag o n al or pseudotetragonal in sym m etry, an d in which diffusion is still possible in th e principal directions th a t are n o t equivalent. H eulandite, th e stru c tu re of w hich is unknow n b u t which probably is a fram ew ork of lim ited thickness, is m ono
clinic and ap p aren tly th e voids are connected only p arallel to (010) and n o t along th e b axis; th u s ionic diffusion is restricted to nonequivalent directions in parallel planes (17).
S tru c tu ra l features of th e zeolites are here illu stra te d b y th e fram ew ork u n it of th e ultram arines, sodalite an d helvite, w hich is present in a d istorted form in chabazite (21). F igure 1 shows the actual u n it for sodalite, an d directions of possible ionic m igration are indicated. I n th is an d o th er figures th e usual convention
July, 1 9 4 5 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y NEUTRAL SURFACE
6 2 7
OH/
NEUTRAL SURFACE KAOLINITE AI2 S¡2 0 5 »0H>4
Figure 2. Plan and Elevation of Hexagonal Network of Silica Groups Present in Kaolins, Pyrophyllite, and Structurally Related Silicates Its combination with lorn, coordinated about A +* to form the kaollnite structure, 1« ihown on the right.
is followed of indicating ionic centers b y sm all circles. Fram e
works of a nalcite a n d th e fibrous zeolites are illu strated in stan d ard books on stru c tu re s of solids (1).
Ions w ithin th e voids of zeolites can be completely exchanged w ith vary in g ease, depending upon th e p o ten tial barrier betw een sites. A large external cation, such as tetra m e th y l ammonium N (C H |)«+1, can n o t en te r th e connecting channels, and exchange does n o t ta k e place (Iff). T h is is also tru e for replacem ent of w ater b y larger molecules such as benzene (15). T h e form ulas as show n are only typical, an d th e re can be some v ariatio n due to th e presence of A1+ 3 in place of S i+ 4 in th e framework. I n this way th e n um ber of exchangeable cations in heulandite can v ary from 1.8 to 3.0 p er u n it of 36 oxygen ions, th e form ulas (21) v ary ing betw een a b o u t [(AU.eSin.OCMCai.s. 12H20 an d [(Al^Si^)036]- Caj.oNa. 12HjO. T h e ex te n t to w hich A l+ 3 is p resen t in th e fram ew ork appears to be lim ited by th e a tta in m e n t of microscopic n e u tra lity th ro u g h th e arran g em en t an d num bers of positive ions an d w ater molecules t h a t can be situ ate d in th e voids. T his lim iting am o u n t of A l+ 3 in chabazite is such as to give an A l+3:
S i+‘ ra tio of ab o u t 1:2, while it can be 1 :1 in th e stru ctu rally related noselite a n d sodalite where additional negative groups are present in th e voids in stead of w ater molecules.
E X C H A N G E SITE EXTERNAL T O FRAM EW ORK
Silicate base exchange com pounds of th e second ty p e have sheetlike stru c tu re s in which th e d eterm inative stru c tu ra l elem ent is th e hexagonal netw ork show n in F igure 2 (13). T his netw ork is b u ilt up from tetra h ed ra l groups of oxygen ions around A1+3 or S i+t cations. M any silicates contain th is netw ork, joined through the unshared oxygen ion of each tetra h ed ra l grouping to groups of oxygen an d O H - ions octahedrally coordinated ab o u t A l+3, M g +i, L i+1, etc. Com posite stru c tu ra l sheets of kaolinite, A l^ C W O H )« , an d pyrophyllite, Al2Si4Oio(OH) 2 (Figures 2 and
3), show th e tw o principal types of stru c tu res to be discussed.
Tw o striking features influencing cation exchange of th e sheet
like silicate stru ctu res are to be emphasized. F irst, th e sheets are too com pactly filled to allow ionic m igration through them . Sec
ond one th ird of th e octahedral coordination positions are v acan t in th e pyrophyllite sheet. T h i s second feature and th e presence of A l+a in place of S i + * in th e netw ork of F igure 2 play th e m ost
i m p o r t a n t p a rt in determ ining th e ex ten t of base exchange in the
i m p o r t a n t class of clay m inerals related t o m ontm orillonite, which are industrially known as bentonites.
Mo n t m o k i l l o n i t e s. Silicates of th e m ontm orillonite group have stru c tu ra l sheets sim ilar to th a t shown in F igure 3 for pyro
phyllite (6). T he sheets, however, have an excess negative charge which is balanced by external and exchangeable cations.
Form ulas for mem bers of th e group derived from analyses of m any pure m inerals (14) can be expressed in th e following m anner:
Ions in Ions in
octahedral tetrah ed ral
Sheet coordination coordination Oio(OH)2
Exchange cations external to sheet held because of a deficit in th e positive charge in octahedral or tetrah ed ral positions
P a rticu lar form ulas of pyrophyllite, talc, m uscovite, and phlogo- pite micas, and some m em bers of th e m ontm orillonite group, indicated by italics, are:
P yrophyllite (Al2)(Su)Oio(OH) 2
E x t
M ontmorillonite (Al1.67Mgo.33) (Sit) Oio(OH)»
Ex"o.33
Beidellite (AI2.17) (Al^.asSis.n) Oio(OH) 2
Ex "0.33
M uscovite (Al2)(AlSi3.o)Oio(OH) 2 K! + Talc ( M |3.o)(Su.o)0 1o(OH)2
E x t
Hectorite (M g2.67Lio.33) (Si4.o)Oio(OH)2 E x t. 33
Saponite (M g3.0) (Alo.33Sis.j7)Oxo(OH) 2 E x t.3 3
Phlogopite (Mgs.o) (Ali.oSi3.o)Oio(OH) 2
K +
H ectorite and saponite illu stra te lim iting cases where the presence of external cations is due to deficits of positive charge in octa
hedral and tetrah ed ral coordination, respectively, w ithin the stru c tu ra l sheets. Analyses of m inerals of th e m ontm orillonite group show th a t these exchangeable ions are restricted to one
628 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
EX C H A N G E C A T IO N S O N L Y O N EXTERNAL SURFACES O F M IC A
K A Ig C A IS i^ C ^ tO H ^
LAYER IN PYROPHYLLITE
V . 4 ° |0 OH)2
EFFECTIVE CHARGE
NH
C6 H5 NH3
EXCHANGE CATIONS BETWEEN LAYERS IN MONTMORILLONITE
AI1 6 1 ^ 3 3 Î '4 °IO(OH^
o
Figure 3. Structural Layer of Pyrophyllite and Its Modifications in the Micas and Montmorillonites Various types of positions for exchange cations are indicated.
th ird of an equivalent per Oi0(OH) 2 u nit. T hey occupy positions, betw een th e layers, th a t are completely filled in th e micas. In th e micas, however, th e K + ions bring ab o u t microscopic n eu tral
ity for excess negative charge due to th e presence of A l+S in place of S i+1 in the im m ediate region in th e layer above an d below.
T h ey th u s serve to bind th e layers together and p rev en t ready exchange w ith o th er cations. In m inerals of th e m ontm orillonite group an exchangeable cation seems to be required for microscopic n e u tra lity b y only one of th e neighboring sheets, even though all sheets carry th e sam e average charge. T he forces betw een the layers th u s are sufficiently sm all to p erm it h y d ratio n of th e sur
face and w andering of th e ions.
An excellent illustration of th e principle of microscopic n eu tra l
ity is afforded by beidellite. In it th e am o u n t of A1+3 in te tra h edral coordination approaches th a t required for a mica. T he n um ber of ions external to th e stru c tu ra l sheet, however, is re
duced by increase of th e A l+3 ions in o ctahedral coordination, microscopic n e u tra lity th u s being a tta in e d by in te rn al compensa
tion. H ere conditions w ithin th e fram ew ork keep th e equiva
lence of th e external ions constant, which is in co n tra st to the behavior of th e zeolites.
Mi c a s. W hile cations betw een th e stru c tu ra l sheets are dif
ficult to exchange in th e m icas and m icalike m inerals, such as
m uscovite, phlogopite, glauconite, and illite, th ey are n o t re
stricted a t th e lim iting surfaces of the crystals. B ase exchange is, apparently, due chiefly to these ions a t th e surfaces, an d th e equivalence is determ ined by th e ex ten t of th e surface. I n a m ica having a cleavage surface of 60 M 2 per gram , a value a tta in e d in m any soils, th ere is 0.1 m illiequivalent per gram of exchangeable cations on th e cleavage surfaces. A dditional places for cation exchange occur a t th e late ral surfaces, as will be discussed in de
tail for kaolin minerals, and the equivalence of this exchange m ay be as g reat as th a t a t the cleavage surfaces. I n th e m ore finely divided micaceous minerals, such as glauconite of greensands and illite of soils an d shales, th ere is probably some a vailability of ex
change sites betw een stru c tu ra l sheets, near th e edges. T he to ta l exchange capacity of these m aterials is in th e order of 0.25 milli
equivalent per gram , which is approxim ately one th ird to one fo u rth t h a t of m ontm orillonite group m inerals.
Ka o l i n s. S tru ctu ral sheets of kaolin m inerals (Figures 2 and 4) are form ed from th e hexagonal netw ork found in th e micas an d th e corresponding lay er of ions w ith octahedral coordination.
T he sheets, however, are term in ated by (OH) ~ ions on one surface an d are n eutral. Slight d ep artu re from n e u trality , due to the presence of A l+3 in place of S i+1 in te tra h e d ra l coordination, is com pensated by increase in th e num ber of ions in o ctahedral
Iulï. 1S4S I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 629
EXCESS CHARGE
ATOM
- 2
o
DISTANCE FROMPROJECTION PLANE
S i + 4
0 - 2
OH_l
A . + 3
OH-1
+ 0.5 A.
0.0
- 1.6
- 1.6
-2 .7
') -3 .7
Figure 4 . Cation Exchange Positions Due to Lateral Termination of Kaolinite Crystals (Plan and Elevation) Attainment o f microscopic neutrality about A l +*, O ' 1, and O H -1 Is shown In detail.
coordination in th e sam e w ay as for beidellite. W hile th e stru c
tu ral sheets in kao lin ite a re n eu tral, th ere is considerable a ttra c tion betw een th em due to hydrogen bonding of th e (O H )- ions on the b o tto m of one lay er to oxygen ions in th e to p of th e neigh
boring lay er (3). R elativ ely large crystals are b u ilt u p in this way w ith n e u tra l in stead of charged cleavage surfaces.
D ependence of cation exchange in k aolinite on th e exposed sur
face is show n in Figure 5 (7). E arlier evidence h ad been obtained from changes in base exchange capacities b rought ab o u t b y grind
ing (8 9) Surface areas of kaolinite se p arated from soil are as g reat as 8 0 A f 2 per gram , of which a b o u t 2 0 % is due to lateral faces {11). W ith a base exchange capacity of 0 .1 2 m illiequiva- len t per gram , an
a rea of a b o u t 20 sq.
A.
w ould be available for each un iv a
le n t cation. T his w o u l d c o r r e s p o n d to ab o u t two uni
v a le n t cations or each laterally te r
minating stru ctu ral s h e e t , w' h l c h . 1®
a b o u t t h e e x p e c t e d v a l u e ( F i g u r e 4 ).
0 10 2 0 3 0 4 0
SPECIFIC SURFACE-SQ.M.PER GM.
Figure 5. Variation of Cation Exchange of Kaolinite with Partide Size (7)
Se p i o l i t e s. AH silicates and other ionic structures would be expected to show ion exchange due to lattice term ination, but the am o u n t m ight be very sm all on account of large particle size.
A nother group of silicate minerals, th e sepiolites, having consider
able cation exchange capacity should be m entioned. A ttapulgite, th e principal co n stitu en t of F lorida fuller’s earth, is of this type.
T he group as a whole cannot be discussed fu rth er since little in
form ation is available ab o u t th eir stru ctu res and cation exchange behavior.
EX C H A N G E IN NETWORK TYPES
C ation exchange of th e netw ork ty p e stru ctu res in th e clays differ in several im p o rtan t dynam ic w ays from th a t of the zeolites, which possibly will be a p p aren t w ithout detailed explanation.
E xchange reactions are extrem ely rap id in the clays a t room tem perature, while elevated tem peratures and relatively long times are required in zeolites because of th e lim itation of diffusion. In sta b ility of the fram ew ork has prevented form ation of hydrogen zeolites w hereas hydrogen clays are ra th e r stable. Large cations, as previously m entioned, cannot en ter th e zeolites, while clays exhibit g reater affinity for these ions th a n for L i+, N a +, etc.
R elative distributions of cations a t equivalent concentrations betw een a solution and an exchange m aterial have been deter
m ined as a m easure of cation affinities a t exchange sites. T he orders too often have been discussed in vague term s of ionic
hydra-SU M M A R Y
cations depends upon th e presence of m ulticonnected voids. C lay m inerals an d o th er m icaceous silicates have sheetlike stru ctu res in which external cations are necessary for microscopic n eu tra lity . These cations are principally on th e cleavage surfaces of th e m ont- m orillonite m inerals an d o r , ¿he late ral faces of kaolin m inerals.
B o th types of positions co n trib u te prom inently to th e cation ex
change of th e m icalike m inerals, glauconite and illite. O rders of cation replacem ents are determ ined b y relativ e values of in terac
tio n forces betw een th e cation and th e adjoining lattice.
LITERATURE CITED
(1) Bragg, W . H ., “ A tom ic S tru c tu re of M inerals” , p. 255 et seq., Ith a c a , Cornell U niv. Press, 1937.
(2) H endricks, S. B., J . P hys. Chem., 45, 65 (1941).
(3) H endricks, S. B „ Z. K rist.,100, 509 (1939).
(4) H ey, M . H ., an d B annister, F . A., M in in g M ag., 23, 51, 243 (1932).
(5) H offm ann, U ., and Bilke, W ., Kolloid Z ., 77, 238 (1936).
(6) Jenny, H ., and Reitem eier, R . F ., J . Phys. Chem., 39, 593 (1935).
(7) Johnson, A. L., and Law rence, W . G., J . A m . Ceram. Soc., 25, 344 (1942).
(8) K elley, W . P ., Dore, W . H ., and Brow n, S. M ., Soil Sci., 31, 25 (1931).
(9) K elley, W . P ., an d Je n n y , H ., Ibid., 41, 367 (1936).
(10) M arshall, C. E ., and K rinbill, C. A., J . Phys. Chem., 46, 1077 (1942).
(11) Nelson, R . A., and H endricks, S. B., Soil Sci., 56, 285 (1943).
(12) Pauling, L., J . A m . Chem. Soc., 51, 1010 (1929).
(13) Pauling, L., Proc. N atl. Acad. Sci. U. S., 16, 123 (1930).
(14) Ross, C. S., an d H endricks, S. B.. U. S. Geol. S urvey, Profes
sional Paper, in press.
(15) Schm idt, O., Z. physik. Chem., 133, 263 (1928).
(16) T aylor, W . H ., Proc. Roy. Soc. (London), A145, 80 (1934).
(17) Tiselius, A., J . Phys. Chem., 40, 223 (1936).
(18) W ay, J. T ., J . Roy. Agr. Soc. Eng., 11, 13 (1850).
(19) W iegner, G ., Trans. Soc. Chem. In d ., 50, 657 (1931).
(20) W ood, L. K ., and D eT u rk , E . E ., Soil Sci. Soc. A m . Proc., 5, 152 (1940).
(21) W y art, J., B ull. soc. franç., minéral., 56, 81 (1933).
Ph e s e n t e d before th e E le v e n th A n n u al C h em ical E n g in eerin g S ym posium (A dsorption a n d Io n E x c h an g e) h eld u n d e r t h e auspices of t h e D ivision of In d u s tria l a n d E n g in eerin g C h em istry , Am e b i c a n Ch e m i c a l So c i e t y, a t
Ph e s e n t e d before th e E le v e n th A n n u al C h em ical E n g in eerin g S ym posium (A dsorption a n d Io n E x c h an g e) h eld u n d e r t h e auspices of t h e D ivision of In d u s tria l a n d E n g in eerin g C h em istry , Am e b i c a n Ch e m i c a l So c i e t y, a t