Envlron. Sci. Technol. 1993, 27,2373-2380
Investigation of the Effect of Moisture on the Sorption and Desorption of Chlorobenzene and Toluene from Soil Catherine Thibaud, Can Erkey, and Aydln Akgerman’ Chemical Engineering Department, Texas A&M University, College Station, Texas 77843-3 122
The sorption isotherms of chlorobenzene and toluene on a standard EPA soil at different relative humidities were measured using a dynamic technique based on frontal analysis chromatography. A t 0 % relative humidity, the isotherms were BET (Brunauer-Emmett-Teller) type I1 isotherms, indicating monolayer coverage of the surface followed by multilayer coverage. As the relative humidity increased, the shape of the isotherm became progressively a type I11isotherm, indicating weaker adsorbent/adsorbate interactions. The presence of water greatly reduced the organic uptake by soil, especially at low organic concentrations. The effect of humidity was attributed to adsorptive displacement of the organic by water. A mechanistic approach was considered to evaluate the contribution of the possible sorption mechanisms to the total sorption. I t was found that adsorption at the gasliquid interface was important and that dissolution in liquid water and partitioning into organic matter from the adsorbed water phase were negligible. The BET theory for adsorption of binary gas mixtures on solid surfaces successfully represented the data in the relative humidity range 0-50% and relative organic pressure range 0-3095. The effect of humidity on the desorption profile was investigated, and it was found that the efficiency of extraction was increased when using a water-saturated gas stream for desorption instead of a dry gas stream. Introduction Understanding and modeling of vapor-phase sorption of volatile organic contaminants (VOCs) on soil matrices in the presence of moisture are important for a wide variety of operations and calculations such as (a) design and development of in-situ or ex-situ remediation technologies (air-stripping, air-sparging, thermal desorption, bioventing), (b) development of analytical techniques for quantification of contamination levels in soil samples, (c) modeling of migration of VOCs in the subsurface environment, (d) quantification of soil contamination from soil gas analysis, and (e) calculation of emission rates of VOCs to the atmosphere. Numerous studies (1-5) have concentrated on VOCs adsorption on drysoils and have shown that the adsorption isotherms of VOCs on dry soils were Brunauer-EmmettTeller (BET) type I1 isotherms. It was also found that, in absence of moisture, the adsorption capacities at saturation were highly correlated to the adsorbent surface area as determined by nitrogen adsorption (BET method). Unfortunately, soils in the subsurface environment are not dry but contain a significant amount of water (6). Several authors (I-3,7,8)measured the effect of moisture on the sorption of various organics (chlorinated benzenes, xylenes) on soils and clay materials and found that the presence of moisture greatly reduced VOCs adsorption
* Author t o whom all correspondence should be addressed. 0013-936X/93/0927-2373$04.00/0
0 1993 American Chemical Society
capacities. In the presence of water, the organic sorption might occur through any of the following processes: (1) adsorption onto the mineral surface from the gas phase by competition with water molecules for adsorption sites (gas-solid interface), (2) adsorption onto the mineral surface through the adsorbed water phase (liquid-solid interface), (3) dissolution into organic matter from the gas phase, (4)dissolution into organic matter through the adsorbed water phase, (5) adsorption onto the surface of an adsorbed water film (gas-liquid interface), or (6) dissolution into adsorbed water. There are contradictory views in the literature as to which of these mechanisms dominates the sorption of VOCs on soils in the presence of moisture. Chiou and co-workers (2) investigated the sorption of various organics on soil and attributed the linearity of the isotherms obtained at high relative humidity (RH) to dissolution in soil organic matter. This conclusion was further supported by the linearity of the isotherms obtained for sorption on soil humic acid (9). However, when comparing experimental data on adsorption of VOCs and pesticides onto surface soils at high relative humidity with models based only on dissolution into organic matter, Valsaraj and Thibodeaux (IO) found out that the models consistently overpredicted the sorbed amounts. Rhue et al. ( 2 1 ) investigated the adsorption of alkylbenzene vapors on predominantly mineral surfaces at relative humidities up to 67 % . They obtained linear sorption isotherms in the organic relative vapor concentration range studied (0 I PIP0 I 0.48) and concluded that the linearity of the sorption isotherms was not necessarily due to partitioning into soil organic matter. Recently, Pennell et al. (I) also showed that adsorption at the gas-liquid interface for sorption of p-xylene on soil as a function of relative humidity could account for up to 50% of total organic sorption across the whole relative vapor concentration range of the organic. In this paper, we report on the sorption equilibria of toluene and chlorobenzene on an EPA standard soil in the presence of water. The experimental data were obtained using a dynamic response technique based on frontal analysis chromatography (5). Although the technique has been used in the past to determine binary adsorption of gas mixtures on solid adsorbents ( I 2 ) ,this is the first time that the technique is used for investigating sorption of gas-phase VOC and water mixtures on soil particles. In this technique, the response of an initially clean adsorbent soil column (i.e., free of adsorbates) to a step change in the adsorbate concentration at the inlet of the column is monitored to obtain a “breakthrough curve”. The shape of the breakthrough curve is determined by the type of the isotherm and is influenced by the individual transport processes in the bed and in the particle. Adsorption isotherms were obtained in the presence of moisture by simultaneously adsorbing water and the organic contaminant in the vapor phase. Whereas many authors studied the adsorption phenomena of organics on soil, the desorption process was Environ. Scl. Technol., Vol. 27, No. 12,1993 2373
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rarely investigated. Theliteratureavailableon this subject is very limited (5,13). One advantage of the experimental technique is that concentration histories can be obtained as a function of time for both adsorption and desorption. Using this technique enabled us to assess the effect of moisture on desorption rates by comparing the organic desorption from soils contaminated in a similar fashion employing dry (0% RH) and water-saturated (100% RH) gas streams. The reported desorption profiles constitute a novelty in the study of soil contamination. These data should be particularlyuseful in evaluation of the efficiency of air-sparging combined with soil-venting. In this new technique, air is sparged below the water table (instead of above), which enables the circulation of moist air through the contaminated area to remove the VOCs (14).
Methodology
A schematic diagram of the experimental setup used in this study is presented in Figure 1. It can be divided into three sections: the flow equilibration section, the adsorp tion/desorption section,and the analysis section. The gas stream from the helium cylinder was passed through a desiccator before entering the flow equilibration section. In this section, the main gas stream was modified such that the desired contaminant concentration, relative humidity, and total flow rate were obtained at the inlet of the soil column. Adjustment of the relative humidity was achieved by splitting the main stream into two secondary streams and diverting one of them to a water saturator. To obtnin dry isotherms, the water saturator was bypassed. The split ratio of the streams determined the final relative humidity and could be modified by adjusting two flowmeters. The same technique was used to obtain the desired contaminant concentration at the inlet ofthesoilcolumn. Theadaorption/desorptionsection consisted of a soil-packedcolumn equipped with a bypass line. The soil column was bypassed when adjusting flow rate and concentrations. To start the adsorption process, thestreamwasswitchedfrom thebypassto thesoilcolumn resultingin a step change in the contaminantconcentration at the inlet of the column. The effluent concentration from the column was then monitored until it reached a constant stable value equal to the inlet concentration, whichindicatedthesaturationofthesoilbedbytheorganic contaminant and the end of the adsorption process. The 2574 E m . Scl. Technol.. Vcd. 27. No. 12. 1883
Tabla I. Soil Properties
BET surface area, m V g 4 soil organic content, 9%
11.43 0.8
soil textural analysis sand, % ! clay, ?6 silt, 9%
56.4 28.9 14.7
desorption process was started by switching from the bypass to the soil column a gas stream free of contaminant. Thisstreamwaseitherpuredry heliumorwater-saturated helium. In both cases, the effluent concentration was monitored until it fell under detection limits, resulting in the desorption profile. Helium was used as carrier gas in the setup because of analysis requirements. Using a different carrier gas for the setup and the gas chromatograph would have necessitated separation of three compounds in the gas chromatograph (organic, water, setup carrier gas). As helium ensures better results for the gas chromatograph,helium was chosen as the stripping gas in the setup as well. It was verified that the choice of carrier gas for the setup did not affect the adsorption/desorption characteristics of the organic compounds. Theeffluent streamwasanal~edusingasamplingvalve (Valco)and a SRI Instruments gas chromatograph (Model 8610) equipped with a thermalconductivity detector (water analysis)anda flame ionization detector (organicanalysis). A chromatographic column (Poropaq T) was used for isothermal separation of the constituents. For more accurate measurement of the water isotherm, a gas chromatograph equipped with a more sensitive thermal conductivity detectorwasused (Gow-MacSeries550).The flow rate through the packed column was measured using a soap bubble meter. The soil used in this study was an EPA standard soil obtained from FW Enviresponse, crushed and sieved to the desired particle size (60/65mesh fraction corresponding to a particle diameter of 21&250 pm). The soil was oven-dried overnight at 150 O C prior to any experiment. The experiments were carried out at room temperature (24 i 1 "C). The soil characteristics are given in Table I. Chlorobenzene and toluene were obtained from Aldrich Co. and were used as received.
Results and Discussion (A) Breakthrough Curves at Different Relative Humidities. Typical adsorption breakthrough curves
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7000
Time (s) Figure 2. Adsorption breakthrough curves for chlorobenzene at different relative humidities and different inlet concentrations at 24
obtained for chlorobenzene at three different relative humidities are presented in Figure 2. The curves obtained at 0% and 38% R H present a similar shape: the effluent concentration increases until it reaches the inlet concentration and remains stable thereafter. In case of 38% RH, the breakthrough is faster although the organic relative vapor concentration is higher. A different behavior is observed for the breakthrough curve at 87 % RH: the effluent concentration takes on values greater than the inlet concentration. Similarly shaped curves have been reported in the literature for experiments where competitive adsorption of two or more substances occurs (15). It is usually referred to as the "rollup" effect. This effect is due to the different affinitiesof the two components (water and chlorobenzene or toluene) to the adsorbent (soil).The same behavior was observed for adsorption of toluene. The adsorption isotherms of water, chlorobenzene, and toluene on dry soil are shown respectively in Figures 3-5 (dashed curves). It should be noted that water is more strongly adsorbed than the organic compounds on dry soil since the uptake at saturation for water is around 40 mg/ gmil and only about 17 mg/g,,il for chlorobenzene and toluene. Consequently, when organics and water are simultaneously adsorbed, the water front travels slower than the organic front and water displaces the organic, leading to arise in the effluent concentration of the organic (weaker adsorbed species) above the inlet concentration. A quantitative analysis of each breakthrough curve yields one point on the corresponding sorption isotherm. The analysis assumes that at the end of each sorption experiment, there is equilibrium between gas and solid phases and that the organic concentration in the gas phase is equal to the organic concentration in the inlet stream. Then, the mass of contaminant sorbed by the soil is determined by a mass balance on the contaminant over the column during the sorption process (16):
- mout = maccumulated
(1)
OC.
I
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0,' 0
c
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0.2
4
0.4
0.6
0.8
1.0
Relative vapor concentration of water, Pip.
Figure 3. Adsorption isotherm of water on ovendried soil at 24 "C.
where min = [JochCidtlMu = CiMutfa mout= [ r C , ( t ) dtlMv
(3)
therefore
It followed that the mass sorbed is given by: (5) meorbed = maccumulated - EvciM where Ci is the inlet concentration, C,(t) is the effluent Envlron. Scl. Technol., VoI. 27, No. 12, 1993 2376
20
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0.2
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Relative vapor concentration of chlorobenzene, P/P.
Figure 4. Sorptlon isotherms of chlorobenzene on oven-dried soil at dlfferent relative humidities at 24 O C . 20
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is proportional to the area described by the breakthrough curve and the straight line corresponding to C,/Ci = 1 (16). It should be noted that the area corresponding to the rollup effect is a negative area and, therefore, corresponds to a mass desorbed. (B) Adsorption Isotherms at Different Relative Humidities. Analysis of the experiments conducted at various organic and water inlet concentrations by the above procedure enables the construction of the sorption isotherms at different relative humidities. The adsorption isotherms obtained for chlorobenzene and toluene at different relative humidities are presented in Figures 4 and 5. The adsorption isotherms at 0 % RH are BrunauerEmmett-Teller (BET)type I1adsorption isotherms, which indicate monolayer adsorption followed by vapor condensation to form multilayer adsorbates (17). Similar shaped isotherms were previously reported (3, 5, 7) for adsorption of various organic vapors on dry soil. This shape is also conserved for the sorption isotherm at low relative humidities. As the relative humidity increases, the shape of the isotherms changes from type I1 to type I11 isotherms, which means that the interactions between the adsorbent (soil) and the adsorbate (organic) are weakened (18). This finding is in accordance with previously reported studies (1, 2). Reduction of the adsorption by soil minerals, increasing importance of the uptake on soil organic matter as the relative humidity increases, and the adsorption at liquid-gas interface were hypothesized as possible mechanisms, but no conclusive evidence has been given yet. In order to assess the contribution of each possible sorption phenomena to the total sorption process, we have calculated the theoretical amounts of organic adsorbed at the gas-liquid interface, sorbed into soil by partitioning, and dissolved in liquid water independently. The results obtained for each toluene experiment are tabulated in Table 11. The amount of organic dissolved in liquid water was calculated using the solute's Henry's law constant and the amount of water adsorbed. The amount of organic partitioned into soil organic matter was calculated based on the value of the partition coefficient of the solute between soil organic matter and water, KO, estimated by Chiou et al. (9). The total partition coefficient, Kd, was then estimated by the relation: Kd
0 0.0
0.2
0.4
0.6
0.8
1.0
Relative vapor concentration of toluene, Pip.
Flgure 5. Sorption isotherms of toluene on oven-dried soil at different relatlve humidities at 24 O C .
concentration, u is the volumetric flow rate (constant during the experiment), Vis the volume of the soil column, M is the molecular weight of the contaminant, E is the combined particle and column porosity, and tfa is the time at the end of the sorption process. The correction term corresponding to the mass accumulated in the gas phase was usually negligible. The divisionof mwrbed by the weight of the soil in the column (determined gravimetrically) gives the organic uptake by soil or the amount of organic sorbed per gram of soil, X. Graphically, the mass accumulated 2376
Environ. Sci. Technol., Vol. 27, No. 12, 1993
= Komfom
where fom is the soil organic matter content from Table I. The estimated value of Kd obtained for chlorobenzene by eq 6 was 0.65, which compared well with the experimental value of 0.62 measured in our laboratory (19). Data in Table I1 show that, in all cases, the amount of organic dissolved in liquid water was negligible and that organic partitioning into soil organic matter accounted for only 2-10 % of the total sorption. Similar results were obtained for chlorobenzene regarding the importance of dissolution and partitioning. The theoretical amount adsorbed at the gas-liquid interface for toluene was calculated, at each condition, using the data obtained by Hauxwell et al. for adsorption of toluene vapors on water surface (20). We have used two different methods for the calculation, and the results of both methods are reported in Table 11. In method 1, the gas-liquid interface was assumed to be constant and to be equal to the BET surface area (column 3 in Table
Table 11. Predicted and Measured Toluene Sorption on Soil at 50%, 68% and 88% RH.
RH,%
P/Po
50 50 50 50 68 68 68 68 68 68 68 88 88 88 88 88 88
0.20 0.34 0.66 0.82 0.23 0.47 0.48 0.63 0.65 0.77 0.81 0.19 0.40 0.61 0.66 0.76 0.87
a
adsorption at gas-liquid interface, mg/g,bs method1 method 2 0.527 1.053 3.792 6.320 0.632 1.896 2.001 3.476 3.581 5.266 6.320 0.474 1.474 3.286 3.792 5.266 7.057
0.639 0.737 2.654 4.424 0.316 0.948 1.001 1.738 1.791 2.633 3.160 0.142 0.442 0.986 1.138 1.560 2.117
sorption by solid phase, mg/g,,il
dissolution into adsorbed water films, mg/g,ii
0.048 0.081 0.158 0.196 0.055 0.112 0.115 0.151 0.155 0.184 0.194 0.046 0.096 0.146 0.158 0.181 0.208
0.001 0.002 0.004 0.005 0.002 0.004 0.004 0.006 0.006 0.007 0.008 0.003 0.006 0.009 0.010 0.011 0.013
total sorption, mg/g predicted measured method 1 method 2 0.576 1.136 3.954 6.521 0.689 2.012 2.120 3.633 3.742 5.457 6.522 0.523 1.576 3.441 3.960 5.458 7.278
0.418 0.820 2.816 4.625 0.373 1.064 1.120 1.895 1.952 2.824 3.362 0.191 0.544 1.141 1.306 1.752 2.338
2.07 3.59 6.68 10.40 1.11 3.71 3.10 5.22 4.97 5.72 6.80 0.43 1.84 3.76 3.90 4.39 5.89
Partitioning coefficient, Kpm,from ref 9.
11). As mentioned by Pennell et al. ( I ) , this is only a first approximation, since an accurate calculation should take into account the reduction of the interface due to water adsorption and pore filling which would require accurate knowledge of pore size and shape and water-phase distribution. In method 2 (column 4 in Table 111, we used a simple model to account for the reduction of the interface area due to the increasing water content in the pore with increasing relative humidity. In this model, all the pores were assumed to be cylindrical with a uniform diameter equal to the mean pore diameter obtained by BET analysis as 38 A. The amount of water in the pores at each relative humidity was based on the water adsorption isotherm (Figure 31, and the corresponding number of layers of water molecules adsorbed was calculated on the basis of the monolayer coverage for pure water adsorption. Then, the interface area reduction was calculated using the formula:
s=sol( 2;d)
(7)
where S is the interface area (in m2/gsoil),SOis the BET surface area (in m2/gsoil),n is the number of layers of adsorbed water, d is the mean diameter of a water molecule (in 8) calculated by the hard sphere theory (21),and D is the mean pore diameter (in 8)(see Appendix). It was found that the surface reduction based on the BET surface - &/SO,was respectively 30 % ,50 % ,and 70 % at area, (SO the relative humidity levels of 50% RH, 68% RH, and 88% RH. We believe that the true value of the interface area is better approximated by method 2. As shown in Table 11,at 50% RH, the two methods gave consistent results and showed that a significant amount of sorption (about 70%)was not accounted for, which could mean that adsorption on mineral surface was important at this relative humidity. In addition, we used the BET theory for adsorption of gas mixtures developed by Hill (22, 23) and calculated a value of 2.57 mg/gsoilfor the amount adsorbed on soil at 50% RH and for a relative vapor concentration of toluene PIP0 = 0.20. This calculated value is comparable to the experimental value of 2.07 mg/gsoil(Figure 51, which also confirms the hypothesis of adsorption on solid surfaces directly from the gas phase, Le., competitive adsorption of VOC and water. Likewise, for chlorobenzene, at 38% RH and P/Po = 0.27 and 50%
RH and PIP0 = 0.07, the adsorbed amounts calculated using the BET model were respectively 4.49 and 1.04 mg/ gsoilwhich compare well with the experimental data of 4.69 and 1.50 mg/gsoil(Figure 4). However, the model fails at higher relative humidities and for higher organic partial pressures. At 68% and 88% RH, the results obtained by method 1 and ,method 2 were not consistent. Although both methods showed that the contribution of gas-liquid adsorption to the total sorption was significant, method 1 accounts for the whole sorption within experimental accuracy (comparison of columns 7 and 9) but method 2 (comparison of columns 8 and 9) did not (50% of the total sorption was not accounted for). We believe method 2 is a better approximation of the true phenomena, especially at higher RH where significant water adsorption resulting in significant gas-liquid interface reduction is expected. Therefore, we believe that organic adsorption on soil mineral surface does occur even at high relative humidities. Adsorption of chlorobenzene at the gas-liquid interface could not be estimated because no data are available on adsorption of chlorobenzene at water surfaces. (C) Desorption Profiles Obtained with Dry and Water-Saturated Gas. Figure 6 shows the desorption profiles obtained for the desorption of a similarly contamined dry soil by two different stripping gases: dry helium and water-saturated helium (100% RH). In the three experiments 6-8, the soil was initially saturated at the same inlet concentration of chlorobenzene corresponding to an organic relative vapor concentration of PIP0 = 0.30. The initial soil chlorobenzene contamination was about 5.39 mg/gsoii (5390 ppm) for each experiment. The gas stream used for desorption was dry helium in experiment 7 and water-saturated helium in experiments 6 and 8. It should be noted that experiments 6 and 8 show very good reproducibility. The three desorption profiles are similar for the first part of the desorption but, as clearly shown on the semilogarithmic plot (Figure 71, they are significantly different for low concentration values. At the end of the desorption, the “dry” profile exhibits a slow and continuous decrease in concentration but the “wet” profile exhibits, after a flat portion, a sharp and rapid increase followedby a drastic decrease. The sharp increase Environ. Sci. Technol., Vol. 27, No. 12, 1993 2877
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Volume Helium passed (cm3) Flgure 6, Adsorption breakthrough curves of chlorobenzene on oven-dried soil at relative vapor concentration PIPo = 0.17 and corresponding desorption profiles uslng dry helium or water-saturated helium as stripping gas. In the three experiments, the soil contamination before desorption is Identical. n
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Volume Helium passed (cm3) Figure 7. Desorption profiles from Figure 0 In a semilogarithmic scale.
was found to coincide with the breakthrough of water for all experiments. Figure 8 shows the evolution of percent organic removal as a function of the volume of gas passed over the soil for the three experiments. With wet helium as stripping gas, 100% removal is achieved with a volume of helium of about 15000 cm3 but with dry helium as stripping gas, passing 30000 cm3of gas achieves only 90 % removal. The improved desorption efficiency observed when using water-saturated gas (experiments 6 and 8) can be explained by the adsorptive displacement of the organic compound by water. We previously showed (5)that organic desorption from soil occurred in two steps: desorption of the multilayers 2378
Environ. Sci. Technol., Vol. 27, No. 12, 1993
followed by the desorption of the monolayer and that the desorption of the monolayer was the limiting step. The present findings show that moisture enhances this step. As can be seen from Figure 6, desorption profiles for the multilayer with humid or dry gas streams follow identical patterns; however, the desorption of the monolayer was accelerated in case of the humid gas stream. The effect of moisture on desorption could have been predicted by considering the behavior of the adsorption isotherms as a function of relative humidity (Figures 4 and 5 ) . The “dry” isotherms of chlorobenzene and toluene on soil are favorable to adsorption and, thus, unfavorable to desorption but, as the relative humidity increases, the
100
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Volume Helium passed (cm3) Figure 8. Percent removal of chlorobenzene from sol1 using dry helium and water-saturated helium as the stripping gas (same experiments as in Figures 6 and 7).
isotherm becomes more favorable to desorption. Such a feature might enable the use of soil vacuum extraction systems with air-sparging below the water table for remediation of soil contaminated withsemivolatile organic compounds. Conclusions The sorption capacities of chlorobenzene and toluene on soil were greatly reduced in the presence of water. The nature of the adsorption isotherm changed as the relative humidity increased. On dry soil, the adsorption isotherm was a BET type I1isotherm and, at high relative humidities, the adsorption isotherm became type I11 isotherm, characteristic of weaker adsorbent/adsorbate interactions. It was shown that adsorption at the gas-liquid interface played an important role but that dissolution into liquid water and partitioning into organic matter from the adsorbed water phase were negligible. However, these three mechanisms could not account for the totalsorption which suggests that the organic may be competing with water for available sites on the surface at high organic partial pressures. However, for conclusive evidence, there is a need to develop new experimental techniques which will enable the determination of fractional coverage of each compound on the surface at equilibrium, and further investigations are necessary before these effects can be incorporated into models. The organic's desorption from soil was enhanced when using a water-saturated gas stream as the stripping fluid. This last point might explain in part the improved VOCs removal observed at sites when using air-sparging technology and could allow the use of soil vacuum extraction/ air-sparging technologies to the remediation of soils contaminated with semivolatile organic compounds.
lecule Figure 9. Pore model for calculation of gas-liquid Interface reduction due to water adsorption.
stance Research Center. Their contribution is greatly appreciated. Appendix The pore model considered for the calculation of the gas-liquid interface reduction is shown in Figure 9. The pore is approximated by a cylinder of diameter D and length L. Then, the total surface area is
Acknowledgments
So = TLND (8) where N is the number of pores per gram of soil. If one layer of the water molecules is adsorbed as shown in Figure 9, the gas-liquid interface, approximated as a cylindrical surface, is
This project was funded through Grants 100TAM0202 and lllTAM2078 from the Gulf Coast Hazardous Sub-
S = rLN(D - 2d) where d is the diameter of a water molecule.
(9)
Environ. Sci. Technoi., Vol. 27, No. 12, 1993 2370
Identically, if n layers of water molecules are adsorbed, the gas-liquid interface is
S = ?rLN(D - 2nd)
(10)
or by combination with eq 8: (11)
Equation 11 is identical to eq 7 in the text.
Literature Cited (1) Pennell, K. D.; Rhue, R. D.; Rao, P. S. C.; Johnston, C. T.
Environ. Sci. Technol. 1992, 26 (4), 756. (2) Chiou, C. T., Shoup, T. D. Environ. Sci. Technol. 1985,19,
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