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VI. Kinetics of Ionic Exchange in Clay Minerals

VI. Kinetics of Ionic Exchange in Clay Minerals

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259



KINETICS OF IONIC REACTIONS



-



o KAOLlNlTE



?



0, 25.64



MONTMORlLLONlTE

A VERMICULITE

0



X



7m

Y



20.56



6 t

15.39



z

0

F 10.26

a



cc



cl



5.13



Q

Y



0



20



40



60



80



100 120 140

TIME (rnin)



160 180 200



220



FIG.8. Potassium adsorption versus time on clay minerals. (After Sparks and Jardine, 1984.)



surface sites are available for ionic exchange. With montmorillonite, the inner

peripheral space is not held together by hydrogen bonds, but instead it is able

to swell with adequate hydration and thus allow for rapid passage of ions into

the interlayer. Malcolm and Kennedy (1969) found that the rate of bariumpotassium exchange on kaolinite and montmorillonite was rapid, with 75 %

of the total exchange occurring in 3 sec. Rapid kinetics were also noted by

Komareni (1978) for cesium sorption on kaolinite and by Sawhney (1966) for

cesium sorption on illite and montmorillonite. Regression analysis showed

that the sorption curve of calcium-illite had a significant but small negative

slope, while the slope for calcium-montmorillonite was not significantly

different from zero. Kinetic expressions have also been developed by Hague

and Sexton (1968) for 2,4-D adsorption on kaolinite and montmorillonite.

Tamers and Thomas (1960), investigating the ion exchange properties of

kaolinite slurries, found that the approach to equilibrium was slow. This was

ascribed to the change in state of aggregation of the clay particles during

slurry agitation.

B. RATE OF EXCHANGE

ON VERMICULITE AND MICACEOUS

CLAYS



Rates of exchange involving ions such as K + (Fig. 8) and Cs+ on

vermiculitic and micaceous minerals tend to be extremely slow. Both are 2: 1

lattice clays with peripheral spaces which impede many ion exchange

reactions. Micaceous minerals typically have a more restrictive interlayer

space than vermiculite since the area between layer silicates of the former is

selective for certain types of cations (viz., K + , Cs+, and others). Sawhney



260



DONALD L. SPARKS



(1966) found that cesium sorption on a calcium-vermiculite did not quickly

reach an equilibrium. Instead, calcium-vermiculite continued to sorb cesium

and equilibrium was not attained, even after 500 h. Keay and Wild (1961)

investigated the exchange of numerous divalent cations with sodium on a

vermiculite and found that diffusion-controlled exchange to the external

surface of the crystal was the rate-limiting process. Reed and Scott (1962)

investigated the kinetics of potassium release from biotite and muscovite, and

they also concluded that intraparticle diffusion was the rate-limiting step. In a

later publication, Ismail and Scott (1972) found that the rate of potassium

release from the peripheral spaces of muscovite, biotite, and phlogopite

increased with increasing temperature. In 1963 Bolt et al. theorized the

existence of three types of binding sites for potassium exchange on a hydrous

mica.

The authors hypothesized that slow reactions were due to interlattice

exchange sites, rapid reactions to external planar sites, and intermediate

reactions to readily exposed edge sites. Sawhney (1966) noted that two

distinct reactions existed for cesium sorption on calcium-vermiculite. He

proposed that the first reaction resulted from a rapid exchange of cesium with

the cations on the external planar surfaces and the interlattice edges followed

by a second, slow reaction in which cesium diffuses into the interlayers. A

further indication of these two reactions was found in comparing sorption of

cesium by 0.10 .and 0.15 g samples of vermiculite. When the sample size

increased, the initial sorption also increased. However, the slope of the line

representing the second, slow diffusion reaction remained the same. Since the

external planar and the edge surfaces together made up about 4% of the total

surface area of the clay, the surface sites to which the first fast reaction was

attributed did not greatly exceed the cesium added. Therefore, Sawhney

(1966) concluded that an increase in sample size increased the surface sites

and thus the cesium sorbed initially also increased. Similarly, an increase in

the sample size of montmorillonite increased cesium sorption, again representing the initial fast reaction. In contrast to the surface sites, the interlayer

sites constituted >90% of the exchange sites and greatly exceeded the

quantity of cesium added, and hence the slope representing the rate of

diffusion of cesium into vermiculitic interlayers did not change.



c. RATE OF REACTIONON OTHER PURE SURFACES

Many other surfaces have been used to study reaction rates, including

hematite, gibbsite, calcite, and others. Kuo and Lotse (1972) investigated the

rate of phosphorus adsorption by calcium-kaolinite and by CaCO,. The

authors observed very slow rates near the end of the reaction, which could



KINETICS OF IONIC REACTIONS



26 1



indicate diffusion. Griffin and Jurinak (1974) investigated the kinetics of

phosphate interactions with calcite and found that two simultaneous reactions described the phosphate sorption. The first reaction was second order

and was attributed to adsorption of phosphate on the calcite surface, while

the second reaction was first order and was ascribed to phosphate complexation with the calcite mineral. Desorption kinetics were described by two

simultaneous first-order reactions and were attributed to the release of

calcium phosphate precipitates and to ordinary phosphate molecules, respectively.



VII. KINETICS OF IONIC REACTIONS IN HETEROGENEOUS

SOIL SYSTEMS

A voluminous amount of research has been performed on various aspects

of ionic exchange in soil systems, but a meager amount has appeared in the

literature on the kinetics of ion exchange in these systems. Soil systems are

typically composed of complex mixtures of clay minerals, noncrystalline

components, oxides, hydroxides, and organic matter. The inevitable interaction of these various components creates a heterogeneous exchange complex

which makes the kinetics of ionic exchange in these systems difficult to study.



A. POTASSIUM

KINETICS



Sparks et a/. (1980a,b) studied potassium exchange in two Dothan soils

from Virginia. A low rate of potassium exchange in these soils was ascribed to

intraparticle transport and to diffusion processes, which reflected the relatively large quantities of vermiculitic material present in the soils. The

adsorption rate coefficients k, in the Dothan soils ranged from 1 to 20 hr-'

compared to values of 81 to 216 h- calculated for Florida soils (Selim et al.,

1976). The difference in the k, values can be explained on the basis of the

predominance of kaolinite in the Florida soils as compared with vermiculitic

minerals in the Virginia soils.

Sparks and Jardine (1981) investigated the thermodynamics of potassium

exchange in a Delaware silt loam using a kinetics approach. A diffusive

process was noted and found to be slower for potassium desorption than for

potassium adsorption. This would be expected due to the difficulty in

desorbing potassium from partially collapsed interlayer positions of the

vermiculitic clay minerals. Once potassium is adsorbed into this peripheral

space, the coulombic attraction between K + ions and the silicate layers

would be greater than the hydration forces between the individual K + ions;



262



DONALD L. SPARKS



thus, a partial layer collapse would occur (Sawhney, 1966). The observation

of slower desorption than adsorption suggests that potassium reactions are

nonsingular and that hysteresis could be occurring (Ardakani and McLaren,

1977;Rao and Davidson, 1978; Sparks et al., 1980b). Slow rates of potassium

desorption have also been noted by others (Talibudeen and Dey, 1968;

Feigenbaum and Levy, 1977).

B. PHOSPHORUS

KINETICS

Kinetic studies in soil systems have been investigated extensively with

phosphorus (Olsen and Khasawneh, 1980; Sharpley et al., 1981a,b). Amer et

al. (1955) studied the rate of phosphorus desorption from a soil suspension

using an anion exchange resin. They concluded that initial phosphorus

desorption increased very rapidly, then leveled off with time. Evans and

Jurinak (1976), using an anion exchange resin to simulate plant uptake of

released phosphorus, found that phosphate kinetics were characterized by

three simultaneous first-order rate expressions. Phosphate sorption interactions in a sandy soil were investigated by Fiskell et al. (1979). Phosphate

sorption was described by both a rapid and a slow reversible reaction which

occurred simultaneously at two separate types of sorption sites.

C. NITROGEN

KINETICS

An extensive amount of literature is present on the kinetics of nitrogen

reactions in soils. This includes work on nitrification (Stevens and Reuss,

1975; Feigenbaum and Hadas, 1980; Tabatabai and Al-Khafaji, 1980),

denitrification (Stanford et al., 1975a; Kohl et al., 1976), and volatilization of

NH3 (Fenn et al., 1981, 1982).

Feigenbaum and Hadas (1980) found that the nitrification rate constant in

a sandy soil was slightly higher than that in a clay soil. This was ascribed to

the more favorable aeration conditions in the sandy soil. A decrease in

NH4-15N recovery in the soils with time during the first 20 days after

fertilization was exponential, indicating first-order kinetics of the nitrification

process. The rate constant was 0.13 day-' and the half-life of NH: in

the field was 5 days, which the authors noted was considerably higher than

reported in the literature for laboratory incubation experiments at similar

temperatures.

Many researchers have noted that the rate of denitrification in soils

conforms to zero-order kinetics (Patrick, 1960; Broadbent and Clark, 1965;

Keeney, 1973). Other workers have shown denitrification to follow first-order

kinetics (Bowman and Focht, 1974; Stanford et al., 1975a,b; Kohl et al.,



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