Gas Chromatographic Studies of Sorptive Interactions of Normal and Halogenated Hydrocarbons with Water-Modified Soil, Silica, and Chromosorb W Judy P. Okamura and Donald T. Sawyer‘ Department of Chemistry, University of California, Riverside, Calif. 92502 Gas chromatography with wetted soil columns has been used to determine the retention mechanism of pesticides and halogenated methanes (used as soil fumigants). The principal modes of interaction with the wetted soils include adsorption on the water surface and absorption by the water layer. The equilibrium constants for adsorption and absorption in model systems, water on chromosorb W and on porous silica beads, have been evaluated and interpreted in terms of molecular parameters. These models illustrate an approach for ascertaining the state of water on soil and for the prediction of retention volumes on this complex material. Data from the model systems allow calculation of both the amount of water on the soil surface and the surface area of the aqueous layer.
THE PERSISTENCE AND MOVEMENT of pesticides in soils is a matter of great interest. Edwards ( I ) has reviewed the mechanisms of disappearance of pesticides from soil and has concluded that volatilization, codistillation, leaching, oxidation, hydrolysis, and microbial activity all contribute to the disappearance of insecticides from soil. Volatilization, however, is the major pathway for movement of fumigants, both through and out of the soil. Therefore, the controlling factor is the sorption of the compound by the soil. The diffusion of fumigants in soils has been studied by many different methods, including reduction of oat-seed germination as an index of the relative degree of diffusion ( 2 ) and the analysis for 1 IC-labeled fumigant in diffusion half-cells (3). When sorption constants are measured directly using static experiments, gas chromatography is a common technique for the determination of the sample-material concentration in various phases. However, the use of elution gas chromatography with the fumigant as the sorbate and the soil as the chromatographic sorbent has been rare ( 4 ) . The use of wetted soils as chromatography columns offers several distinct advantages over other methods for determining the sorption characteristics of soils : 1) Extremely small sample sizes 2) Retention measurements determine sorption constants ; consequently, quantitative recovery of the sample is not necessary 3) Because of the short time of retention, chemical or microbial degradation of the sample compounds is not significant 4) Use of the molecular parameter approach (5)allows prediction of the sorption for compounds not previously tested. Author to whom correspondence should be addressed (1) C . A. Edwards, Residue REG.,13,83 (1966). (2) C . R. Youngson, R. G. Baker, and C . A. I. Goring, J . Agr. FoodChem., 10,21(1962). (3) W. Ehlers, W. J. Farmer, W. F. Spencer, and J. Letey, Soil Sei. SOC.Amer. Proc., 33,595 (1969). (4) B.Berck, J. Agr. Food Cliem., 13,248 (1965). ( 5 ) J. P. Okamura and D. T. Sawyer, ANAL.CHEM., 43, 1730 (1971). 80
Soils, however, are extremely complicated materials. The soils that have been used for the present study have extremely low organic contents (less than 1 but significant variation in their structures. Clay content and composition have significant effects upon the characteristics of wetted soils. For example, montmorillonite has layers which can be separated by variable amounts of water. A good review of the structures and properties of soils has been presented by Marshall (6). To obtain accurate adsorption and absorption equilibrium constants, model systems of water on porous silica beads and Chromosorb W have been studied. These are well characterized supports for which the quantity of water present on the surface can be determined.
z),
EXPERIMEh’TAL
The porous silica glass columns were prepared by packing. respectively, Porasil B and Porasil C (Waters Associates, Framingham, Mass.), 100-150 mesh, into 3-ft X l/s-in. 0.d. stainless steel tubing. This tubing (and all other tubing in the system) had been rinsed first with hexane, acetone, and water, and then dried. The Chromosorb W column was prepared in the same manner using untreated 60-80 mesh Chromosorb W (Wilkens Instrument and Research, Walnut Creek, Calif.). The soils, Rosita Very Fine Sandy Loam and Gila Silt Loam, were obtained from W. J. Farmer of the Soil Science Department, University of California, Riverside, Calif. They were ground and sieved to
b-m
z
-____
_____
(8) S. P. Wasik and W. Tsang, J. Plzys. Cliem., 74,2970 (1970). (9) F. M. Nelsen and F. T. Eggertsen, ANAL.CHEM.,30, 1387 (1 958). ( I O ) K . B. Wiberg, “Computer Programming for Chemists,” W. A.
Benjamin, New York, N.Y., 1965. pp 44-47. ( 1 1) E. S. Keeping, “Introduction to Statistical Inference,” D. Van Nostrand Company, Princeton, N.J., 1962, p 331. (12) “Handbook of Chemistry and Physics,” C . D. Hodgman, Ed., 40th ed., The Chemical Rubber Publishing Co., Cleveland, Ohio, 1958, pp 2530-2537.
0.0
0.04 wt/wt,
0,08 H,O/dry
0.12
0.16
0.20
G i l a Si l t Loom
Figure 2. Specific retention volumes of halomethanes as a function of the water content of Gila Silt Loam RESULTS AND DISCUSSION
Figures 1 and 2 illustrate retention volumes as a function of water content for several organic compounds on Gila Silt Loam. Similar curves are observed for the Rosita Very Fine Sandy Loam. The retention of normal alkanes (Figure 1) decreases with a n increase in water content. Hence, absorption by the water layer is not the dominant retention mechanism. In contrast, the retention of the halomethane fumigants on Gila Silt Loam (Figure 2) decreases with decreasing water contents except a t the lowest levels. The retention volumes of the different compounds, however, d o not decrease at the same rate and one compound, CCI,, exhibits characteristics that are similar to those for normal alkanes. (13) “Handbook of Chemistry and Physics,” R. C. Weast. Ed., 48th ed., The Chemical Rubber Co.. Cleveland, Ohio, 1967, p E-159. (14) “Selected Values of Electric Dipole Moments in the Gas Phase,” National Standard Reference Data Series-National Bureau of Standards 10, U. S. Govt. Printing Office, Washington, D.C.. 1967.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
81
60
04
02
wt/wt,
06
08
IO
"
Porosil C
H,O/dry
0.0
Figure 3. Specific retention volumes of organic compounds as a function of the water content of Porasil C
wt/wt,
H20/dry
Porosil C
Figure 4. Logarithms of the specific retention volumes for organic compounds as a function of the water content of Porasil C
The sorption behavior that is summarized in Figures 1 and 2 is typical of compounds with mixed retention mechanisms. Such systems, which involve adsorption on a liquid surface as well as absorption by the liquid, were first identified by Martin (15) in a series of gas chromatographic experiments. The behavior of even more complex sets of retention mechanisms has been discussed since then by Conder, Locke, and Purnell (16) and Urone, Takahashi, and Kennedy (17). When adsorption on the liquid surface and absorption by the liquid are the predominant interactions, the retention volume is governed by the relation VosT = KA X A L
+ KL X V L
(2)
where K A is the equilibrium constant for adsorption on the liquid surface (with units of ml/m2),AL the surface area of the (15) R. L. Martin, ANAL.CHEW, 33, 347 (1961). (16) J. R. Conder, D. C. Locke, and J. H. Purnell, J . Plzys. Cliern., 73,700(1969). (17) P. Urone, Y . Takahashi. and G. H. Kennedy, ibid., 74, 2326 (1970).
82
0.4 wt/wt,
HO ,
0.8 1*2 dry Chromosorb W
I .6
Figure 5. Specific retention volumes of organic compounds as a function of the water content of Chromosorb W
liquid (m2/g dry support), KL the equilibrium constant for absorption by the liquid layer (ml/ml of liquid), and V L the volume of the liquid phase (ml of liquid/g dry support). Because soils are complex materials which can contain water held between clay layers as well as on the surface, wetted porous silica beads and wetted Chromosorb W are useful as model systems. Karger (18,19) has shown that Equation 2 is valid for organic compounds sorbed by water adsorbed on Porasil D (0.05 to 0.40 wtjwt) and by water adsorbed on Chromosorb P (0.07 to 0.27 wtjwt). Typical curves of retention volume as a function of water content for organic compounds on wetted Porasil C are illustrated in Figure 3. All compounds show an increase in retention with decreasing water content, but the order of retention of the compounds changes drastically with water content. Figure 4 illustrates the same VosTvalues plotted as their logarithm. This method of data treatment is informative because compounds with similar retention mechanisms (i.e., compounds with similar KL/KA ratios) exhibit parallel lines for similar water contents. All of the alkanes give such behavior, as d o CHsC1 and CH2Cl2. The retention volume curves as a function of water content for organic compounds on Chromosorb W are illustrated by Figure 5. These are the same compounds that are illustrated in Figure 3. In comparison with the Porasil data, all of the compounds except the alkanes exhibit a decrease in retention with decreasing water content. The retention order of the halomethanes does not change until the water content is less than 0.1 wt water/wt dry support. Using experimentally determined values for VosT, AL, and VL,a least squares fit has been used to determine K A and KL for compounds sorbed by water adsorbed on Porasil C in the region of 0.363 to 0.755 wt water/wt dry Porasil C. This is the region for Porasil C which meets the criteria given in the experimental section for accurate estimation of the surface area. The data, which are summarized in Table IA, have standard deviation averages of 2 for the KA values, 5 for (18) B. L. Karger, P. A. Sewell, R. C. Castells, and A. Hartkopf, J . Colloidltlrerfuce Sci., 35,328 (1971). (19) B. L. Karger, R. C. Castells, P. A. Sewell, and A. Hartkopf, J . Pliys. Clzem.,75, 3870 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
Table I. Adsorption and Absorption Equilibrium Constants for Compounds Sorbed by Water on Porasil C and on Chromosorb W at 25.0 “C A. Porasil C
Compound CHFI CHIC1 CHiBr CHiCla CHC13 CCl, C, C5 c 6
c 7
CS
KA 0.0234 0.0708 0.114 0.332 0.580 0.280 0.0420 0.0936 0.216 0.501 1.17
cc1,
... ...
HzO/dry soil 0.202 0.190 0.170 0.166 0.126 0.096 0.072 0.056 0.036
KL 0.28 3.1 4.8 12.7 9.9 2.1 0.27 0.73 1.8 4.2
VLI
AL,
ml/g (dry) 0.135 0.128 0.124 0.104 0.092 0.053 0.040 0.035 0.021
m2/g(dry) 0.18 0.22 0.20 0.17 0.24 0.45 0.50 0.63 0.96
Table 111. Contributions of Molecular Parameters to the Logarithms of the Adsorption and Absorption Equilibrium Constants A. Parameter Contributions
B. Chromosorb W
CHaCl CH3Br CH2C12 CHCl,
Table 11. Computed Water Surface Volumes, VL,and Areas, A L , for Gila Silt Loam at Various wt/wt HzO/Dry Soil Contents
2.50 3.65 9.58 5.79 0.467
Equilibrium constant Porasil C KA KL
the KL values for the halomethanes, and 30z for the KL values for the normal alkanes. These values agree within experimental error with the values obtained on Porasil B wetted with 0.40 to 0.57 wt HzO/wt dry Porasil B. Table IB contains the KL values that are obtained by using a least squares fit of the VgsTdata for Chromosorb W in the region from 0.101 to 1.40 wt HaO/wt dry Chromosorb W. Because the liquid surface area is too small to be measured directly, it is presumed to be proportional to the retention of the normal alkanes. The KL values have standard deviations which average less than 2% of their magnitude. Using the KA values of alkanes on Porasil C to define the surface area yields the K A values of the halomethanes on Chromosorb W. However, because the effect is so small (the surface area ranges from 0.4 to 0.9 mz/g), the standard deviations average more than 5 0 z of the K A values, and therefore the latter are not reported. Prior to the interpretation of the experimental results, several possible retention mechanisms other than water surface adsorption have been considered. These include adsorption by non-wetted surface (i.e., dry islands of support), adsorption by solid surface which has been covered by water (perhaps by displacing a water molecule), and adsorption by Urone’s modified layer (17) (surface modified by a monolayer of water molecules to become a new surface). The significance of these mechanisms cannot be established by the present experimental results. However, the “dry island” model seems least probable for two reasons. First, one would expect this condition to be difficult to achieve reproducibly and yet the results are reproducible. However, when adsorption of water is used to achieve specified water contents rather than the desorption approach, the results are much less reproducible and may indicate the existence of “dry islands.” Second, one would expect the KA values for a dry silica surface to be much greater, and again that appears to be the case when the adsorption approach is used to achieve specified water contents. The other two alternative mechanisms are extremely difficult to test. However, the proposed water-surface adsorption model is consistent with the experimental data (Equation 2 ) and has been found to apply to several other systems ( I 5-1 9).
Log
Log
KIP
Log K/Cl
0 pt.
0.077 0.111
0.45 1.40
0.18 0.29
-2.94 -3.65
KIR
Chromosorb W KL 0.085 2.22 0.72 -5.40 B. Molecular Parameters for Model Compounds Compound R P Cl 6.98 I .65 0 CHFa 11.2 1.87 1 CHsC1 CH3Br 14.6 1.81 1 CHIC12 16.3 1.60 2 CHC13 21.2 1.01 3 cc14 25.9 0 3(K.4)4 ( K L ) 20.2 0 0 C4 C5
c 6
c 7
CS
0 0
24.3 29.9 34.6 39.2
0 0 0 0
0
0
The differences in the KL values for compounds absorbed by the water-Porasil C system compared to those found using the water-Chromosorb W system are too large to be explained by experimental error, especially for CHC13 and CCl,. The difference implies that the water on Porasil B and C is modified by the adsorbent. The mechanism of this modification has not been established, but there are two reasonable interpretations. Either the water is being oriented by the silica surface or the silica surface is being partially dissolved by the water. The latter has been proposed to explain the formation of anomalous water (20). The orientation argument is favored by the fact that the modified KL values appear to occur at surface areas greater than 1 m2/g, Le., where sufficient surface energy exists to cause the water surface to conform to the support surface. That the dissolution of silica also is a possibility is confirmed by the fact that the surface area of the dry Porasils (as measured by the BET method) changes by as much as 3 0 x after being treated with water. The extent of the change depends on the amount and length of treatment. CONCLUSIONS
The one definite conclusion from a comparison of the data for Porasil with that for Chromosorb W is that extreme care ~~
~~
~
(20) M. Prigogine and J. J. Fripiat, Chem. Phys. Lett., 12, 107 (1971).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
83
Table IV. Retention Volumes Calculated by the Molecular Parameter Approach Compared to Experimental Values A. 0.36 wt HgO/wt Dry Porasil C V,,T
Compound
R
CHFCIg CFzC1z
16.5 16.6
P 1.29 0.51
CFCI,
20.2
0.45
c1 1.5 0 KA 1 KL
1.5
Log K A -0.82 -1.43
KA 0.15 0.037
Log K L 0.43 -0.80
-0.92
0.12
-0.34
VL 0.36 0.36
Calcd
Exp
2.7 0.16
AL 35 35
6.2 1.4
8.1 1.1
0.46
35
0.36
4.4
3.3
9.6
0.46
0.14
1.48
4.5
KL
B. 0.2 wt HpO/wt Dry Gila Silt Loam
CH2Br2
20.7
1.43
2
-0.34
in the selection of the support material is necessary if a liquid is to be characterized as a bulk liquid. Indeed Guillemin er al. (21) consider liquid coatings on Spherosil (the European name for Porasil) to be best characterized as a form of modified gas-solid chromatography. They also note that the thermal stability of the modified supports is much higher than that normally associated with the coating liquid, which, they contend, proves that the bonding between the liquid layer and the silica surface is particularly strong. The water on Chromosorb W, however, does appear to be a good model for water on soil at all but very low water conon Gila Silt Loam, tents. For water contents below either water-modification or adsorption by exposed surface appears to occur. The KA values for Porasil C (the only accurate set of values available) have been used as the model for adsorption by the water surface. These agree within experimental error with the K A values determined on Chromosorb W. Furthermore, the water surface should be affected to a limited degree by any change in the solid support. Equation 2 therefore has been applied to the Gila Silt Loam through the use of the Porasil C K A values and the Chromosorb W KL values. A least squares fit of the haloalkanes yields the data in Table I1 for the volume of water on the surface of the Gila Silt Loam and the surface area of that water. The average standard deviation is 6 for the volume of surface water and 13 for the water surface areas. With this information, the problem of predicting the retention of other compounds on wetted Gila Silt Loam can be considered. The approach used to predict such retention volumes is a molecular parameter treatment which is similar to the one used to predict thermodynamic adsorption values in gassolid chromatography (5). Because enthalpies and entropies of adsorption and absorption have not been determined for the present systems, the molecular parameter approach has been applied directly to the logarithms of the absorption and adsorption equilibrium constants for Porasil C and Chromosorb W. The parameters that have been used are molar refraction (R), dipole moment ( p ) , and the number of chlorines which are available for hydrogen bonding. For this treatment, a bromine atom is assumed to have the same hydrogen bonding ability as a chlorine atom; compounds that contain both fluorine and chlorine atoms are assumed to have reduced hydrogen bonding ability due to the electron withdrawing effect of fluorine. The parameters and the results are summarized in Table 111. The data in Table 111 provide information about the liquid phase and its surface, and make it possible to predict the retention volumes of compounds. consideration of the data
3z
z
(21) C. L. Guillemin, M. Deleuil, S. Cirendini, and J. Vermont, ANAL.CHEM., 43,2015 (1971). 84
0.98
0.46
indicates that the water absorbed on Porasil C is not as polar as the water on Chromosorb W, nor are the hydrogens as available for hydrogen bonding to the halogens. The 0 pt. contributions to KL represent absorption for conditions of zero contribution from molecular parameters. Hence, the more positive value for Porasil C can be interpreted as a greater ordering of water on this surface than on Chromosorb W. The chief utility of the data contained in Table I11 is the prediction of retention volumes. Retention volumes are calculated by first evaluating the log KA value from the relation log KA
R X log KAIR f
F X
log
No. of C1 X log KA/cl
KA/p
+
+ 0 pt. contribution
(3)
The log KL value is calculated in a similar manner. The antilogs are taken to obtain VBsTfor any set of A, and V , values by substitution in Equation 2. Table IV summarizes several calculated and experimental retention volume values for compounds on Porasil C and Gila Silt Loam. The agreement is not good, but the correct order of retention is predicted. The problem encountered here is that the specific chlorine effect, which is assumed to result from hydrogen bonding to the chlorine atoms is extremely high. Because good data do not exist for the strength of a hydrogen bond to a carbon-bonded bromine atom (or an even more subtle case, the strength of hydrogen bonding to a chlorine atom in methyl chloride as compared to a chlorine atom in carbon tetrachloride), intuitive guesses are required which can affect the predicted retention volume of a compound by an order of magnitude. Despite the large errors, this method is superior to more simplistic interpretations of retention on columns containing water. Examples of such interpretations include those based on boiling point or solubility, which fail to give even the correct order of retention. The results of the present investigation establish that gas chromatography can provide new insights to the nature of water on soils. Thus, Equation 2 can be applied to any soil which has high water contents and low organic content such that the soil is wetted by water. In addition, it offers an improvement in the ability to predict the retention of a compound by the soil for any known surface water volume and water surface area, at all but very low water contents. The results for the model systems illustrate how gas chromatography can be used to study the modification of polar liquid phases by polar, relatively narrow pore adsorbents. RECEIVED for review June 23, 1972. Accepted September 11, 1972. We are grateful for the support of a US.Public Health Service Environmental Sciences Traineeship to J. P. 0.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
