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Clays and Clay Minerals, Vol. 40, No. 3, 355-358, 1992.

NOTES

THE EFFECT OF CLAY DISPERSION ON THE SORPTION OF ACETONITRILE

Key Words--Clay dispersion, Organic sorption, Partition, Sorption mechanism.

The partition or distribution of a solute between two immiscible solvents was first treated by Nernst (Brom- berg, 1980). Generally the partition coefficient, which is the ratio of the concentrations of the solute in the two solvents, is a constant. The partition mechanism has been extended to describe the sorption of solutes on clay minerals and soil materials for systems in which no specific bonding was involved. It should be noted that two types of partition mechanisms have been pro- posed. Earlier, Greenland et al. (1962, 1965), Theng (1972), and Theng and Scharpenseel (1976) proposed that the linear sorption isotherm observed with clay minerals can be characterized by a partition of a solute between the bulk and intercrystalline water. Later, Chiou et al. (1979, 1983) proposed that the sorption of neutral organic chemicals is essentially a partition process between the soil organic matter and soil water. More recently, Lee et al. (1989), Smith et al. (1990), and Jaynes and Boyd ( 199 l) attributed this mechanism to the enhanced sorption of neutral organic molecules on soils and clays exchanged with organic cations with one or more long alkyl groups. Because clay minerals saturated with metal cations contain negligible amounts of organic matter, the sorption of neutral organic mol- ecules on these clays only involves the partition mech- anism proposed by the earlier investigators (Greenland et al . , 1962, 1965; Theng, 1972; Theng and Scharpen- seel, 1976).

Recently, Zhang et al. (1990a, 1990b, 1990c) have studied the sorption of several neutral organic mole- cules on montmorillonites saturated with different metal cations. Based on the partition mechanism, they have derived the following equation (Zhang et aL, 1990b),

C2saV = KC2botV, (1)

in which C2 s is the equilibrium concentration of the organic solute in the interracial phase, C2 b is the con- centration in the bulk solution phase, K is the partition coefficient, a is the fraction of the volume that is oc- cupied by the interfacial phase, and V is the volume of solution per unit mass of solid adsorbent. It should be noted that the left-hand side of Eq. (1), to a first approximation, is the amount of solute adsorbed in the interracial phase. Therefore, one can write

Copyright �9 1992, The Clay Minerals Society

P2~t)S = KC2baV, (2)

where re (t) is the relative surface excess of the solute per unit surface area of solid adsorbent and S is the specific surface area of the solid adsorbent, and con- sequently, F2(~)S is the relative surface excess of the solute per unit mass of solid adsorbent. It is clear from Eq. (2) that the amount of adsorption is not only related to K and C2 b, but is also related to a. A linear isotherm would result only if both K and ~ remain constant over the entire concentration range of the isotherm.

It was shown earlier that for sorption of neutral or- ganic molecules on clays, K increases as the organic concentration increases (Zhang et al . , 1990b). This is due to the increase in the organophilic nature of the interfacial phase as more organic molecules accumu- late on the clay surfaces. The effect of a on the sorption has not been addressed, except by Zhang et al. (1990b, 1990c), who noticed that downward curvature of the sorption isotherms of acetonitrile and acrylonitrile on K-montmori l lonite could be attributed to a decrease in a.

Although the values of a are not precisely defined for expanded clay layers, it can be generally accepted that a is directly related to the dispersion status of the clays. Therefore, we shall examine the effect of clay dispersion on the sorption of neutral organic mole- cules. This research is based on the premise that if the measured sorption of neutral organic molecules is re- lated to the dispersion status of the clay as predicted by Eq. (2), the general validity of the partition mech- anism, on which Eq. (2) was based, can also be further demonstrated.

EXPERIMENTAL

Homoionic Na- and K-montmoril lonite were pre- pared using Wyoming montmoril lonite (SWy-1) and C1 salts (Zhang et aL, 1990b). The acetonitrile was obtained from Aldrich Chemical Company and the ~4C labeled acetonitrile was from Sigma Chemical Com- pany. They were used without further purification.

Batch experiments were conducted to determine the sorption isotherms ofacetonitrile on Na- and K-mont-

355

356 Zhang, Sparks, and Scrivner Clays and Clay Minerals

Figure 1. The dispersion status of Na-montmorillonite pre- pared using two methods when the initial concentration of acetonitrile is 1.0 M.

morillonite at 25~ Two methods were used to control the dispersion status of the clays.

Method 1

A series of acetonitrile solutions ranging from 0.05 to 2.0 M (mol L -t) was prepared. The ~4C labeled ace- tonitrile solutions were prepared by adding t4C labeled acetonitrile to each of the solutions in the above series to yield a specific activity of ~400 Bq mL -t (~23,000 cpm mL ~, assuming a counting efficiency of 95%). Approximately 0.5 g K-montmoril lonite (or 0.3 g Na- montmorillonite) was weighed in a series of centrifuge tubes and 10 mL of each of the t4C labeled solutions were added to the tubes. The resulting clay suspensions were shaken on a reciprocating shaker for 24 h, cen- trifuged, and two, 1 mL aliquots of the supernatant solution from each tube were removed and added to separate vials containing 12 mL of aqueous counting scintillant. The radioactivities of these supernatant so- lutions were measured in a Beckman LS 5000 TA scin- tillation counter (Beckman Instruments, Fullerton, CA) with a counting error setting at 2a = 0.5%. The radioac- tivities of reference solutions (no clay added) were also determined. The values ofF2")S were determined using the procedure outlined by Zhang et al. (1990b).

Method 2

A second series ofacetonitri le solutions ranging from 0.1 to 4.0 M was prepared. The 14C labeled acetonitrile solutions were prepared by adding laC labeled aceto- nitrile to each of the solutions in the above series to yield a specific activity of ~800 Bq mL i. Approxi- mately 0.5 g K-montmori l loni te (or 0.3 g Na-mont- morillonite) was weighed in a series of centrifuge tubes and 5 mL deionized water was added to pre-wet the clays overnight. Then 5 mL of each of the 14C labeled solutions were added to these tubes. The resulting clay suspensions were equilibrated on a reciprocating shak- er for 24 h. Then the clay suspensions were centrifuged, the radioactivities of the supernatant solutions and ref- erence solutions were measured, and the values ofF2")S were determined.

Figure 2. The dispersion status of K-montmorillonite pre- pared using two methods when the initial concentration of acetonitrile is 1.0 M.

RESULTS A N D DISCUSSION

The dispersion status of Na- and K-montmori l loni te samples for the two methods is shown in Figures 1 and 2, respectively. One can see that the dispersion status o f Na-montmori l loni te was not affected by acetonitrile solution. Both methods yielded fully expanded clay layers with c-axis spacings greater than 10 nm. In con- trast to Na-montmoril lonite, the dispersion status of K-montmori l loni te was affected by acetonitrile solu- tion. The layers of K-montmori l loni te remained col- lapsed when acetonitrile solutions with concentrations above 0.6 M were added to dry clay (Method 1). On the other hand, the layers of K-montmori l loni te were

Vol. 40, No. 3, 1992 Sorption as affected by clay dispersion 357

o 4.0 o Method 1 o //P

,.-,. a Method 2 ^ / e I t : ~ 3 . 0

O

2 . 0

N o ~'~ 1 .0

nt o r i l l on i t e

0 .0 f v= I , I ~ I = I

0.0 0.5 1 .0 1 .5 2.0

m 2 ( too l kg - 1 )

Figure 3. The relative surface excess ofacetonitrile per unit mass of clay, F2")S, measured by the two methods, as related to the equilibrium concentration of acetonitrile, mz, for Na- montmorillonite.

e x p a n d e d w h e n the clay was p re -we t t ed wi th wa te r ( M e t h o d 2). It can be seen f rom Figure 2 t ha t w h e n M e t h o d 1 was used, K - m o n t m o r i l l o n i t e par t ic les re- t a i n e d the i r shape a n d the s u p e r n a t a n t so lu t ion re- m a i n e d clear at an in i t ia l ace ton i t r i l e c o n c e n t r a t i o n o f 1 . 0 M . X - r a y di f f rac t ion s h o w e d t h a t the c-axis spac ing o f the co l lapsed K - m o n t m o r i l l o n i t e layers was 1.23 n m (Zhang et el . , 1990b).

F igures 3 a n d 4 show the so rp t ion da ta for aceto- n i t r i le on Na - a n d K - m o n t m o r i l l o n i t e t h a t were ob - t a i n e d us ing the two m e t h o d s desc r ibed above . It ap- pea red t ha t va lues o f I~2(t)S for N a - m o n t m o r i l l o n i t e were no t affected by the m e t h o d s used (Figure 3), whi le va lues o f P2")S for K - m o n t m o r i l l o n i t e were affected (Figure 4). T o e x a m i n e i f i n d e e d the va lues o f F2(])S were affected by the m e t h o d s used, we e m p l o y e d the fo l lowing s ta t is t ical test. W e no t i ced t ha t Fz(~)S was a p p r o x i m a t e l y p r o p o r t i o n a l to m2, the e q u i l i b r i u m c o n c e n t r a t i o n o f ace toni t r i le , hence , it was r easonab le to a s s u m e tha t the F2t~)S/m2 ra t io fol lows a n o r m a l d i s t r ibu t ion . Because the va lues o f th i s ra t io were still s o m e w h a t d e p e n d e n t on the concen t r a t i on , we t r ea ted these va lues for the two m e t h o d s as d e p e n d e n t samples . By d e p e n d e n t , we m e a n t h a t the c o r r e s p o n d i n g va lues o f the P2"}S/m2 ra t io for the two m e t h o d s are paired. W e ave raged the repl ica te va lues o f the F2(l)S/m2 ra t io for Na - a n d K - m o n t m o r i l l o n i t e at each ini t ia l concen- t r a t i on o f ace toni t r i le , C2 i, for M e t h o d s 1 a n d 2, re- spect ively , a n d d e t e r m i n e d the differences be t w een t h e m . T h e resu l t ing da ta are p r e sen t ed in Tab le 1, a n d were e x a m i n e d by the fo l lowing t tes t (Kvanl i , 1988):

td - s j V / n , (3)

whe re n is the n u m b e r o f pa i rs o f obse rva t i ons , d is the m e a n a n d Sd is the s t a n d a r d d e v i a t i o n o f the n

I

"6 E

v

r

3.0 o Method 1 ~ "

a Method 2

2.0

, - ~

0 .0 ~ i I i I i I i I

o.o o.s 1.o 1.s 2.0

rn 2 ( rno l kg - 1 )

Figure 4. The relative surface excess of acetonitrile per unit mass of clay, P2(~)S, measured by the two methods, as related to the equilibrium concentration of acetonitrile, m2, for K-montmorillonite.

differences, respect ively. T h e degree o f f r eedom, d f f o r td i s n -- 1.

F r o m the da t a p re sen ted in Tab le 1, we f o u n d t h a t ta = 0.281 for N a - m o n t m o r i U o n i t e . T o conc lude t h a t the va lues o f P2(~)S/m2 d e t e r m i n e d by the two m e t h o d s are different at the 95% conf idence in te rva l , t he re- q u i r e d va lue o f t d for d f = 9 is 2.262. W e were led to conc lude t ha t va lues o f the r2(l)S/m2 rat io, a n d the reby va lues o f P2(1)S for N a - m o n t m o r i l l o n i t e , were inde- p e n d e n t o f the m e t h o d s used. Th i s resul t is expec ted in a c c o r d a n c e wi th Eq. (2). I f t he d i spe r s ion s ta tus a n d the a va lues o f a clay were the s ame for the two m e t h - ods, one would expect the a m o u n t s o f so rp t i on to be the same, p r o v i d e d the va lues o f K were no t affected by the m e t h o d s . Converse ly , i f the d i spe r s ion s ta tus a n d the a va lues o f a clay were no t the s a m e for the two m e t h o d s , one wou ld expect the a m o u n t s o f sorp- t ion to be different. W h e n the clay layers were col-

Table I. The average values of the F2~=)S/mz ratio obtained using Methods I and 2, and the difference between them, d, at different initial concentrations of acetonitrile, C2 ~ (tool L- ~), for Na- and K-montmorillonite.

Na-montmori l loni te K-montmoriUonite

r , ' JS/ rn : F2"'S/m,

Method Method Method Method C, ~ 1 2 d 1 2 d

0.1 -- -- -- 1.111 0.716 -0 .394 0.2 0.722 0.928 0.206 1.283 0.643 -0 .640 0.4 0.933 0.905 -0 .028 1.316 0.844 -0 .472 0.6 1.143 0.967 -0 .176 1.421 1.099 -0 .322 0.8 1.264 1.317 0.053 1.153 1.018 -0 .045 1.0 1.395 1.355 -0 .040 1.201 1.127 -0 .074 1.2 1.396 1.874 0.478 1.039 1.372 0.333 1.4 1.760 1.818 0.058 i.159 1.386 0.227 1.6 1.715 1.736 0.021 1.081 1.346 0.265 1.8 1.909 1.749 - 0 . 1 6 0 1.226 1.383 0.158 2.0 1.877 1.740 -0 .137 1.253 1.422 0.169

358 Zhang, Sparks, and Scrivner Clays and Clay Minerals

lapsed, the a value decreased, and the a m o u n t o f sorp- t ion should be reduced.

We have observed that the layers o f K - m o n t m o r i l - lonite r ema ined collapsed for Me thod 1 when C2 i > 0.6 M, whereas the layers were expanded for Me thod 2 (Figure 2). Based on the above analysis, we expected that the a m o u n t o f sorpt ion on K-mon tmor i l l on i t e should be dependen t on the me thods used when C2 i > 0.6 M. To val idate our analysis, we per formed the above t test for the differences in I '2o)S/m 2 for C2 i above 0.6 M. To conclude that the values P2~S/m2 measured by M e t h o d 2 are higher than those by Method 1 at the 95% confidence interval , the required value o f t d for d f = 6 is 2.447. The resulting td value is 2.544. Hence, we conc luded that the a m o u n t o f sorpt ion was indeed related to the dispersion status o f the clay. Namely , the amoun t s o f so rp t ion were at least qual i ta t ively in agree- m e n t with Eq. (2) at higher acetonitr i le concentrat ions.

At lower concentrat ions, however , the amount s o f sorpt ion appeared to be reduced when the clay was pre-wet ted with water (Method 2). When the differ- ences in F2o)S/m2 at lower C2 ~ (0.1 to 0.6 M) were analyzed by the same t test, it was found that td = - - 6.712, indicat ing a significant decrease in the a m o u n t o f sorpt ion for Me thod 2. In this regard, one should note that the sorpt ion f rom solut ion is essentially a d isp lacement process, i.e., the sorpt ion o f the solute molecules is coupled with the desorpt ion o f solvent molecules. The reduced amoun t s o f so rp t i on when the clay was prewetted with water suggest that at lower concent ra t ions the acetonitr i le molecules cannot effec- t ively compe te with water molecules adsorbed on the surfaces o f K-montmor i l lon i t e . Thus, the measured ef- fect o f clay dispersion on sorpt ion using sorpt ion dif- ferences for the two methods could be lower than the true effect itself. The enhanced sorption, as a result o f clay dispersion, was part ial ly neutral ized by the inef- fect ive compet i t ion o f acetonitr i le molecules over wa- ter molecules on K-mon tmor i l l on i t e when Me thod 2 was used. Conceivably , the reduced sorpt ion for Meth- od 2 in the concent ra t ion range o f 0.8 to 1.0 mol kg was due to this ineffective compet i t ion . Hence, the real effect o f clay dispersion on sorpt ion would be even more p rofound than we stated earlier.

In summary , exper iments were conducted to ex- amine the effect o f clay dispersion on the sorpt ion o f acetonitr i le. The exper imenta l data were used to test an equat ion, de r ived f rom the par t i t ion mechanism, which relates the a m o u n t o f so rp t i on o f neutral organic molecules on clay surfaces to the part i t ion coefficient, concent ra t ion o f the organic solute in the bulk solution, and the fraction o f the v o l u m e that is occupied by the interfacial phase. The results were at least qual i ta t ively

in agreement with the equat ion, further demons t ra t ing the general val idi ty o f the par t i t ion mechanism.

D e p a r t m e n t o f P lan t a n d Z . Z . Zhang So i l Sciences D . L . Sparks

Universi ty o f De laware Newark , Delaware 1 9 7 1 7 - 1 3 0 3

Du Pont Eng ineer ing N. C. Scr ivner D u P o n t Newark , De laware 19714

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Zhang, Z. Z., Low, P. F., Cushman, J. H., and Roth, C. B. (1990a) Adsorption of organic compounds on montmo- rillonite from aqueous solutions and their heats of adsorp- tion: Soil. Sci. Soc. Amer. J. 54, 59-66.

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(Received 21 August 1991; accepted 5 February 1992; Ms. 2132)