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VII. The Physicochemical Reactions of the Clay Minerals

VII. The Physicochemical Reactions of the Clay Minerals

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THE CLAY MINERALS IN SOILS



185



hydrogen atoms reside will carry a slight reeidual posit,ive charge and

the other two corners of the tetrahedron should carry an equally small

residual negative charge. Water molecules, therefore, tend to attract or

sorb each other. I n this way a molecule of water is subjected to less

strain if the residual positive side of the molecule has this posit.ive

charge neutralized by the negative side of a neighboring molecule. This

process, whereby residual positive hydrogen-rich spots of one molecule

neutralize residual negative spots on neighboring molecules, is called

hydrogen bonding. According to Bernal and Fowler, hydrogen bonding

does not stop with the union of only 2 molecules of water in the liquid

phase, but from x-ray diffract.ion patterns of water, Fowler and Bernal

(1933) have concluded that a few tens or hundreds of water molecules

tend to be hydrogen bonded. The x-ray data indicate that 4 water

molecules are grouped around a fifth molecule, thus forming a tetrahedron. These units are assumed to be propagated on and on until

broken by the kinetic nature of the water molecules. The edges of

oriented, organized, pseudocrystalline clumps of water molecules would

still not have all the residual charges neutralized, but there would be

both negative and positive spots remaining on the sides and edges of the

pseudocrystal.

a. Montmorillonitic Minerals. The residual positive and negative

spots on water molecules or clumps of molecules are important in the

clay mineral-water sorption relationships. The oxygen ions in the surface of the clay mineral crystals (Figs. 1, 2, 3 ) , like the oxygen atoms in

the water molecules, will also be weakly charged. These layers of oxygen

ions will direct most of their forces backward into the crystal towards

t,he positive silicon or metallic ions in the interstitial spaces of the clay

mineral crystal. Weak residual negative charges will result on the outside layer of oxygen atoms in the clay mineral crystals, which will be

free to take part as donors in hydrogen bonding. Each oxygen atom,

if free from other sorbed substances, should attract the hydrogenated or

positive side of a water molecule. Hendricks et al. (1940), however,

have suggested that water molecules on hydrated clay layers arrange

themselves in a hexagonal network in which the individual molecules are

three angstroms apart. Such an arrangement would allow each water

molecule to reside in the depression of the base of each tetrahedron (a

ratio of 1 water molecule to 1% oxygen ions), which is composed of 3

oxygen ions in the surface of the silica tetrahedral layers.

Hendricks (1941) gives four distinct steps for the hydration of t,he

surface layers of dry montmorillonite. They are: (1) hydration of the

exchangeable cations on the external and on the internal surfaces of the



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J . E. GIESEKING



expansible part of the montmorillonite crystal. According to Debye and

Falkenhagen (1928) water molecules orient between cations and anions,

(2) hydration of the remaining surface wit.h a hexagonal network of

water molecules as described above, (3) a t somewhat higher relative

humidities, a second hexagonal network of water molecules will be sorbed

on t,he first oriented layer due t o the propagation of polar attractive

forces by the preferentially oriented first layer of molecules, (4) when

the relative humidity approaches 100 per cent, water will condense on

the surfaces. As the water films become thicker and thicker, it is reasonable to assume that they gradually change to an organization in which

five molecules of water form a terahedron. Tetrahedral groups of this

type tend to propagate themeselves into a structure similar to that of

quartz, as shown by Bernal and Fowler (1933), for the liquid phase of

water.

The water molecules in the hexagonal network, as proposed by Hendricks et al. (1940), are not closely packed. Convincing support is

provided for the above theories of water sorption by montmorillonitic

clay minerals by data presented by Nitzsch (1940). He found that the

first water added to dried clay materials assumed a much greater volume

than ordinary liquid water and that as more and more water was added

to the clays, the specific gravity of the added water gradually rose to 1.

Hendricks (1941) and Grim et al. (1947) have shown that large organic cations interfere with the sorption of water by montmorillonitic

clays. Gieseking (1939) reported that montmorillonite clays lost their

tendency to swell by water sorption when saturated with a variety of

large organic cations. The author also has some unpublished data to

show that large organic cations sorbed on montmorillonitic clays decreases their water-holding capacity as measured by their moisture

equivalents. Other polar compounds or sorbed hydrated sesquioxides

have been observed to decrease the tendency of the montmorillonite clays

to form gels. These observations seem to indicate that positively charged

colloids and organic cations sorbed on the predominantly negatively

charged montmorillonitic crystals in the place of inorganic cations tend

to break up the network of forces responsible for the polar hydration of

these crystals and thereby decrease their hydration tendencies.

Edelman and Favejee (1940) have proposed a structure for the montmorillonites in which 4 hydroxyl ions replace 4 oxygen ions in the

montmorillonite structure of Hofmann e t al. (1933) (see Figs, 4 and 5 ) .

Two of these hydoxyl ions are in the octahedral layer and the other two,

according to the proposal, protrude from the tetrahedral layer. They

conclude that the protruding hydroxyl groups ionize to give replaceable

hydrogen ions and that they are centers for hydrogen bonding with water.



THE CLAY MINERALS IN SOILS



187



The author has some unpublished observations on the action of acetyl

chloride on Wyoming bentonite which supports the contention of Edelman and Favejee. Acetyl chloride is an extremely effective reagent for

the destruction of hydroxyl groups. Wyoming bentonite, a very lyophilic

substance, which forms gels in dilute suspensions, loses its tendency to

swell and form gels after complexing with acetyl chloride. This indicates

that destruction of the hydroxyl groups on the crystals of these montmorillonite crystals greatly reduces the tendency of these crystals to

sorb water. X-ray diffraction patterns of the acetylated montmorillonite

indicate that a t least part of the acetyl chloride is complexed within the

expansible portion of the crystal.

The pure montmorillonitic clay minerals form thixotropic gels (thixotxopic gels are gels that are solid when undisturbed but easily liquified

by mechanical disturbance) even in dilute suspensions. These gels form

as a result of a n organization of the clay mineral crystals in which a

continuous three-dimensional framework of crystals completely surrounds

and traps large quantities of preferentially oriented water molecules.

Suspensions containing as little as 2 per cent of a montmorillonitic clay

mineral with a high exchange capacity may form a thixotropic gel upon

standing a short time in an undisturbed condition. Fortunately, the

montmorillonitic clay minerals which have an extreme tendency to gel

do not exist long under most soil-forming conditions. Such clay minerals

are very active and as soon as they come in contact with basic nitrogenous organic substances or positively charged hydrated sesquioxides,

they sorb these substances and thereby lose much of their gel-forming

tendency. If this were not the case, small amounts of the montmorillonite

clay minerals would render soils practically impervious to water.

MacEwan (1946) and Bradley (1945b) have shown that there are

many organic molecules which mutually sorb clay mineral crystals by

hydrogen bonding. The carbohydrates, alcohols, and proteins contain

groups which enter into polar sorption reactions by this method. Myers

(1937) suggested that a chemical union resulted from combinations of

organic components of composts with a number of acid soil clays. The

proteins and other basic nitrogenous organic compounds can also be

sorbed by ionic exchange (see Section V I I - 2 4 ) . Compounds of this type

tend to interfere with the sorption of large quantities of water by the

clay mineral crystals. I n this way they reduce the tendency of the clay

minerals to form gels and while they may actually reduce the capacity

of the clay minerals to hold water, they may render a clayey soil more

permeable and more useful as a place for water storage.

The edges of the montmorillonite crystals have broken valencieb

which must be satisfied by ionic and polar sorption. Kelley et al. (1936)



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J . E. GIESEKING



have found broken bonds to be very effective sources of sorptive capacities in finely ground minerals which do not have layer lattices and

consequently have much of their surface area composed of broken bond

cleavages. On t,he other hand, their results with B variety of montmorillonites, which have only a small percentage of their surfaces in

positions of broken bond cleavage, showed that only about 5 to 15 per

cent of the polar xorptivc c:tp:icity of these minerals could be assigned

to broken bond edges of the cyvstal. This is to be expected in the sheetlike crystals of the montmorillonites since they have planar surfaces

completely populated with ions whose valencies are mostly satisfied

within the crystal.



b. The Zllitic Minerals. The illitic clay minerals sorb polar compounds by mechanisms similar to those operative in the montmorillonitic

clay minerals, except that the former have no expansible layer in which

sorption can take place. Consequently, the broken bond forces in the

illitic minerals, as has been shown by Kelley et al. (1936) for finely

ground biotite and muscovite, account for a higher percentage of the

total polar sorption than in the montmorillonitic minerals. Due to the

loss of internal surfaces, however, the illitic minerals have less total polar

sorptive capacities than the montmorillonitic minerals.



c. The Kaolinitic Minerals. The kaolinitic clay minerals usually exist

in relatively large crystals. The units in the kaolinite crystal are considered to be held together by an ideal system of hydrogen bonding in

which the bonding occurs between each out,side hydrogen atom of the

hydroxyl ions in the octahedral layer serving as the acceptors and each

outside oxygen ion of the silica tetrahedral layer acting as a donor.

The bonding energy from this ideal type of arrangement is too great to be

supplanted by polar molecules. Consequently, the kaolinitic clay

minerals exhibit still smaller sorptive capacities for polar compounds than

is exhibited by the illitic clay minerals.

2. Ionic Sorption Reactions



The clay minerals have the unique property of sorbing, through exchange reactions, both cations and anions. These reactions can provide

plant nutrient elements for maximum plant growth. Without these reactions, plants would be forced to feed on whatever nutrient elements

might become available from organic decomposition and the weathering

of minerals. During periods of little or no consumption of these released

products, they would be lost by leaching. During periods of peak plant

nutrient requirements, in many cases the plants would not have enough



THE CLAY MINERALS I N SOILS



189



of these elements for proper development. By cation exchange reactions,

however, nutrient elements released by weathering are sorbed on tshe

clay mineral crystals. These sorbed nutrient elements are available to

plants and during periods of peak requirements the sorbed supply of

plant nutrient elements serves as a reservoir which prevents starvation

of plants during the critical periods of their growth and reproductive

cycles. I n t.his way the clay minerals conserve plant nutrient elements

until they are needed in the various biological r p r l e c involved in the use

of soils as a medium for plant growth.

a. Base Exchange or Sorption of Cations. In Section 111-1 it has

been pointed out that substitutions in the tetrahedral and octahedral

layers of the clay mineral crystals may leave an overall excess of negative

charges on the surfaces of these crystals. The hydrogens in the hydroxyl

groups on the broken bonds a t the edges of the crystals may also ionize to

give negative spots, especially when the p H of the clays is high. These

negative charges are considered the source of the cation exchange

capacities of the clay minerals.

Each negative charge on the clay mineral crystals attracts a monovalent cation or two negative charges will share a divalent cation. The

hydrogen-saturated clay minerals are analogous to the inorganic acids.

When the hydrogen is replaced by other cations, the resulting combinations are analogous t o the corresponding inorganic salts. This analogy

is shown by the following series of reactions involving the common

exchangeable ions found in slightly acid, neutral, or alkaline soils:



+

+

+

+

+



2HC1+ Ca(0H)z + CaCls 2H20

2HC1+ Mg(OH), + MgCL 2H20

HCl + KOH --$ KCl HzO

HCl + NH,OH + NH4C1 HzO

HCI + NaOH --$ NaCl HBO



ZH-Clay

2H-Clay

H-Clay

H-Clay

H-Clay



+ Ca(0HL + Ca-Clay + 2Hz0

+ Mg(OH), + Mg-Clay + 2Hs0

+ KOH + K-Clay + HaO

+ N E O H + NHa-Clay + HzO

+ NaOH -+Na-Clay + HzO



A single clay mineral crystal has many sorbed exchangeable ions.

These ions most commonly are: C a + + , Mg ++ , H+, K+, NH4+, and

a trace of N a + except in alkali soils where N a + is a prominent sorbed

ion. The order of abundance of these ions in most productive soils will

usually be as given above. The amounts of these exchangeable ions in

different soils depend on the nature of the clay mineral components, the

nature of their parent material, and the history of their development.

The inorganic exchange capacity of soils may vary from practically no

exchange capacity to 60 milliequivalents per 100 g. of soil. The following

amount8 of exchangeable catione in 100 g. of productive soil can he

considered typical values:



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J. E. GIESEKING



Ca

Mg

H



K

NH4

Na



15 milli equivalents

5



5

0.25

trace

trace



Since the cation exchange capacity of soils is concentrated in the clay

mineral fraction, the capacity of some of the pure clay minerals must be

higher than the total of the typical values given above. Grim (1942)

gives the following exchange capacities for some of the common clay

minerals in soils:

Montmorillonite 60-100 milliequivalents per 100 g.

IlIite

20-40

Kaolinite

3-15

Most of the cation exchange reactions are equilibrium reactions.

This may be illustrated by considering the following reaction between

sorbed calcium and the potassium in potassium chloride:

Ca-Clay f2KC1# ZK-Clay f CaClp



When this reaction has reached equilibrium all of the KCI will not have

been sorbed nor will all of the calcium have been replaced. The point

of equilibrium can be influenced by changing the concentrations of the

soluble salts on either side of the equation. The addition of more KCl

or the removal of CaClz from the reaction will tend to make the reaction

go more and more to the right. The removal of KC1 or the addition of

CaC12 will make the reaction go more to the left. Most of the reactions

of soluble fertilizers with clays and the reactions involving the nutrition

of plants are equilibria exchange reactions of this type. They can be

represented by making appropriate substitutions in the above equilibrium.

The cation exchange reaction that results from the application of lime

to an acid soil clay is not an equilibrium reaction. This reaction may

be illustrated as follows:

ZH-Clay



+ CaCOs +Ca-Clay + HzO + COz



Water, a nonionized substance, is formed and C 0 2 is lost from the soil.

This amounts to the removal of two products from the right hand side

of the equation. Consequently, it goes to complet,ion and it is not an

equilibrium reaction.

Since the exchange of sorbed cations provides a source of plant

nutrient elements for plant nutrition, the ease of the release of these ions

has been widely st,udied. The early investigations of Jenny (1932) and



THE CLAY MINERALS IN SOILS



191



Gieseking and Jenny (1936) have dealt with the ease of release of a

single cation from clay mineral systems in which the ion to be studied

was artificially made to occupy essentially all of the cation exchange

spots on the mineral surfaces. B y various combinations with various

concentrations of replacing ions the ease of release of the agriculturally

important exchangeable cations has been found to decrease in the following order:

Na>K>NH4>Mg >Ca>H

More recent studies have dealt with the ease of release of cations

from the clay mineral surfaces when two or more cations occupy the

exchange spots on the mineral surfaces. Bray (1942) used a very low

concentration of hydrogen ions to exchange the naturally sorbed cations

from a group of soils containing montmorillonitic and illitic minerals.

He found the same order of ease of release in these heterionic systems

as was found by Gieseking and Jenny for homoionic systems, except that

the NH4+ ion was not included in Bray’s experiments.

Schachtschabel (1940) has demonstrated t.hat various clay minerals

differ in the ease with which they release sorbed cations. H e worked

with minerals which he considered pure clay minerals. He showed that

Ca+ + and Mg+ + ions are the most difficult of the plant nutrient cations

t.o release from montmorillonite and kaolinite and H + ions are somewhat

easier to release. He showed that ground micas hold H+, K + , and NHI+

ions very tenaciously and release C a + + and M g + + ions more easily.

He extended his work to studies on cation exchange reactions of the

humic acids and found that these acids held C a + + ions much more

tenaciously than NH4+ ions. Schachtschabel’s results emphasize the

importance of determining the amounts of the various components in soil

clays for the interpretation of their cation exchange relationships.

Wiklander (1946) has studied the interrelationships of competing

cations in exchange reactions. H e has found that the ease of release of

an ion depends not only on t.he nature of the ion itself but also upon

the nature of the complementary ions filling the remainder of the exchange spots and on the degree to which the replaced ion saturates the

exchange spots. Jenny and Ayers (1939) have obtained similar results.

Wiklander’s results show t.hat, as the amount of exchangeable calcium

on the clay mineral crystals becomes less, the calcium becomes more

and more difficult to release. Sodium, on the other hand, becomes easier

to release as the degree of saturation with sodium ions becomes less.

The magnesium and potassium are not affected by degree of saturation

to the extent that calcium and sodium are affected. Wiklander suggest8

this as an explanation for the fact that calcium generally occuoies most



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J . E. GIESEKING



of the exchange spots and sodium occupies the least number while exchangeable magnesium and potassium are intermediate in abundance on

the clay mineral crystals in nature. This distribution of exchangeable

ions usually holds irrespective of the nature or the composition of the

parent material from which the clay minerals are formed.

The effect of rontact exchange on surface migration of exchangeable

ions and upon the sorption of nutrient ions by plants has been investigated by Jenny e t al. (1939) and by Jenny and Overstreet (1939a,

1939b). According to the rontact exchange theory, since exchangeable

ions are subject to continuous thermal agitation, when neighboring

oscillations overlap, there should be opportunity for the exchangeable

ions in these neighboring spots to exchange without the aid of oppositely

charged ions in solution. This means that ions could migrate on exchange surfaces by jumping from spot to spot, provided there is another ion of like charge simultaneously jumping in the opposite direction.

Jenny and his coworkers have also shown by the use of radioactive

tracer ions that ions may exchange by contact of the oscillation volumes

of exchangeable ions on clay mineral surfaces with the oscillation

volumes of exchangeable ions on plant root surfaces. They have shown

that plants may sorb ions from exchangeable ions on clays or clays

may deplete plants of their sorbed ions, as was also suggested by Kelley

(1927), depending on concentrations of the ions in the two systems.

They consider that exchangeable ions may be moving from clays to

plant roots in one area, and the same kind of ions may be moving

from plant roots to clay in another area by contact exchange. These

investigators have shown in other experiments that radioactive rubidium

and sodium were more readily sorbed by decapitated barley plants from

clay surfaces than from bicarbonate solutions of these elements. These

experiments and the observations by Albrecht (1946b) show the many

possibilities whereby plants may feed on sorbed nutrient elements and

adjust themselves to unfavorable nutrient concentrations by mutual exchanges with the clay minerals.

There is evidence to show that clays may take ions from water suspensions of very insoluble substances and resistant minerals by means of

ionic sorption reactions. Lemberg (1876) showed that finely powdered

leucite, a potassium feldspar, could be changed to analcite, a sodium

aeolite, and vice versa, by Na and K exchange reactions in water suspensions. Bradfield (1932) found sodium-saturated clays were able to

take enough barium from barium sulfate (an extremely insoluble substance) to fill one-sixth to one-fifth of the cation exchange capacity of

the clay. Peech and Bradfield (1934), Graham (1941), and Albrecht

(1946a) show that clay minerals are effective in sorbing cations from



THE CLAY MINERALS IN SOILS



193



resistant minerals. These resistant. minerals in water suspensions are

in equilibrium with traces of ions which dissolve from their surfaces.

The clays destroy this equilibrium by sorbing the ions in solution. If the

equilibrium is maintained, ione mud, move from the resistant mineral into

solution and on to the clay until a new equilibrium is established which

involves the resistant mineral, the water solution, and the clay mineral.

The activity of the exchangeable ions, on the clay mineral crystals,

determines the extent to which the clay minerals enter into the many

dynamic soil processes. Marshall and McLean (1947) and their associates have developed clay membrane electrodes which permit the measurement of activities of single cations when sorbed on clays. McLean and

Marshall (1948) have extended these studies to clay systems containing

two cations. Their results on these more complex systems are encouraging, and it appears that these investigators may be able to realize their

goal, namely the measurement. of activities of sorbed ions in samples of

whole soil.

The clay minerals also enter into exchange reactions with the organic

cations. The most common organic cations contain basic amino groups.

These cations are ammonium ions in which one or more of the hydrogens

have been substituted by organic groups. These cations have been

found by Gieseking (1939), Ensminger and Gieseking (1939, 1941, 1942),

Hendricks (1941), Ensminger (1942), Bradley (1945b), Erickson (1948),

and by Allaway (1948) to be very strongly sorbed by the montmorillonitic clay minerals. Ensminger and Gieseking (1942) and Erickson

(1948) have found the proteins and amino acids to be more resistant to

enzymatic attack in the sorbed state than in the free state. As a rule

these large cations are difficult to replace by means of small cations, but

Gieseking (1939) has found them to be more easily replaced by other

large organic cations. It has been pointed out in Section VIII-1-a, that

the sorption of organic cations on the clay minerals interferes with the

tendency of these minerals to sorb large amounts of water. The organic

cation-clay complexes do not swell to form gelatinous impervious masses.

It appears that the organic cations destroy the undesirably high watersorbing capacity of the montniorillonitic clay minerals, thereby improving

water-clay-plant relationships in soils containing these minerals.

The Fixation of Cations in Difficultly Exchangeable Forms. Clay

minerals can be shown to sorb, in difficultly exchangeable form, many of

the exchangeable cations found in soils. While these difficultly exchangeable cations may have various degrees of availability to plants, i t is

generally accepted that the fraction of easily exchangeable cations are

readily available. A measure of these 2 fractions of exchangeable cations



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J . E. UIESEKING



in a soil is, therefore, by no means a perfect method of estimating their

availability to plants. A knowledge of the amounts of these fractions of

exchangeable cations in soils, however, and a knowledge of the tendency

of one fraction to change to the other is essential for the best management

of soils.

Because of the general economic importance of potash fertilization,

potassium has been by far the most widely studied cation with respect

to fixation in soils in a difficultly exchangeable form. Numerous conflicts

have appeared in the interpretation of results of the earlier experiments

on potassium fixation. Stanford (1947) has found that the various clay

minerals differ in their fixation of potassium with respect to amount of

fixation and conditions under which fixation occurs. H e has properly

attributed the early conflict,s on these problems to insufficient knowledge

of the clay mineral composition of the various soils studied. The investigations of Chaminade (1936) and Wiklander (1949) have shown bhat

high pH was conducive to the fixation of potassium in t,he clay minerals

with which they were working. Volk (1938), Page and Baver (1940),

Attoe and Truog (1946), Martin et al. (1946), Raney and Hoover (1946),

Attoe (1947), and Joffe and Levine (1947) have shown that fixation is

increased by drying. Hoover (1945) and Raney and Hoover (1946) have

shown that montmorillonitic soils fix much more potassium than kaolinitic soils. Wiklander and Gieseking (1949) have shown montmorillonitic

clays and illitic clays to be more effective in potassium fixation than

kaolinitic clays. Stanford (1947) has found illite to be effective in fixing

potassium in the moist state when the p H of the clay system was high

and that drying increased the fixation of potassium by illite and montmorillonite. Bray and DeTurk (1939) , Wood and DeTurk (1941), and

DeTurk et al. (1943) have shown that an equilibrium exists in soils

between the various fractions of exchangeable and nonexchangeable soil

potassium.

Since the ammonium ion is very similar to the potassium ion in its

properties with respect to easily exchangeable cations, it is interesting to

find that Chaminade (1940), Page and Baver (1940), and Stanford and

Pierre (1947) have demonstrated the fixation of the ammonium ion by

clays in difficultly exchangeable form. The latter workers have concluded

that the ammonium ion and the potassium ion are fixed by soils by the

same mechanism.



b. Anion Exchange. The investigations on the anion exchange reactions of soils and clay minerals have been associated almost entirely

with the sorption of the phosphate ions by these materials. It has been

shown by Mattson (1931) , Ravikovitch (1934), Scarseth (1935), Toth



THE CLAY MINERALS IN SOILS



195



(1937), Murphy (1939), Stout (1940), Nitzsch and Czeratzki (1940),

Bray and Dickman (1941), Dickman and Bray (1941), Kelly and

Midgley (1942), Coleman (1944), Kurtz et al. (1946), Sieling (1946),

Dean and Rubins (1947), Low and Black (1947), Ensminger (1948),

and Perkins (1948) that the minerals common to many soil clays exhibit

anion exchange reactions. Many of these investigators have shown conclusively that some of the hydroxyl groups in the hydrated sesquioxides can

be exchanged with phosphate ions by reactions analogous to the cation

exchange reactions. A number of these investigators have emphasized

the importance of the kaolinitic minerals in phosphate and other anion

exchange reactions. There has been considerable argument, however,

concerning the mechanism of anion exchange in the systems containing

kaolinite. Some investigators have concluded that the hydroxyl groups

of the kaolinitic minerals were replaced by phosphate ions in the same

manner that they are replaced from the hydrated sesquioxides. Other

investigators have concluded that the procedures used for demonstrating

anion exchange capacity have been drastic enough to decompose the

kaolinitic minerals forming one of the hydrated sesquioxides, namely

hydrated aluminum oxide. According to this latter argument, the hydrated aluminum oxide formed by the destruction of the kaolinite crystals

was then responsible for the phosphate exchange reactions.

The recent work of McAuliffe et al. (1947), however, has shown

conclusively that anion exchange reactions take place readily between

the surfaces of kaolinitic mineral surfaces and surrounding solutions as

well as bet.ween the surfaces of crystals of some of the hydrated iron

and aluminum oxides. They found that radioactive phosphate ions in

solutions replaced inactive phosphate ions from surfaces of minerals of

several soils. In additional experiments with hydrated iron and aluminum oxides and kaolinitic minerals, they found exchange between duterium tagged hydroxyl ions in solution and ordinary hydroxyl ions on

the hydrated oxide and clay mineral surfaces. These results explain some

of the uncertainties resulting from the inadequate procedures which the

earlier investigators on anion exchange were forced to use on clay

minerals.

An account of phosphate exchange and fixation by soils by L. A. Dean

will be found elsewhere in this volume (see p. 391).

3. Dispersion, Flocculation, and Gel Formation



When clays are completely dispersed in water, each clay mineral

crystal is entirely surrounded by water. Dispersed clay minerals move

with water movements. When they move, they tend to clog pores and

channels in the soil, and they cement neighboring soil aggregates to-



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VII. The Physicochemical Reactions of the Clay Minerals

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