Convective Transport and Removal of Vapors of Two Volatile

50015, Zaragoza, Spain. The convective transport and removal of vapors of a polar compound (methyl ethyl ketone) and a nonpolar compound (n-octane) in...
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Environ. Sci. Technol. 1999, 33, 3774-3780

Convective Transport and Removal of Vapors of Two Volatile Compounds in Sand Columns under Different Air Humidity Conditions J O A Q U IÄ N R U I Z , * R A F A E L B I L B A O , A N D M A R IÄ A B . M U R I L L O Chemical and Environmental Engineering Department, University of Zaragoza, C/Marı´a de Luna 3, 50015, Zaragoza, Spain

The convective transport and removal of vapors of a polar compound (methyl ethyl ketone) and a nonpolar compound (n-octane) in quartz sand has been studied. A sharp front in the transport of the organic vapors through the sand and a tailing process in the removal of the compounds as a consequence of nonlinear adsorption have been observed. Due to the great influence that water vapor can have on the sorption of organic compounds onto soils, several experiments have been designed and carried out under different air humidity and sand conditions. A competitive effect between the water and the organic molecules for the adsorption sites has been found as well as a substantial difference in behavior between the polar and the nonpolar compounds in the presence of water in the transport and in the removal of the organics from the sand. Simultaneously, a simple mathematical model has been developed to simulate the behavior of the two volatiles. This model takes into account the competitive sorption between organic and water molecules and considers that the adsorption takes place in equilibrium conditions, disregarding any kinetic effects. This model has been solved numerically and validated with the experimental results obtained, predicting quite accurately the behavior of the two organic compounds under very different conditions.

Introduction Great efforts have been made in recent years to increase the knowledge of the behavior of volatile organic compounds (VOC) in soils. Much of this effort has concentrated on the transport of VOC through soils in order to understand and predict the fate of such compounds in landfills and, in the case of accidental spills, to estimate their emission into the atmosphere or to evaluate the efficiency of remediation techniques such as soil venting. The complexity of the phenomena that can take place in soil (diffusion into aggregates, adsorption onto mineral surfaces or organic matter, absorption into water, biodegradation, etc.), and the variety of soils and types of VOC studied has originated a great deal of work, not only experimental but also theoretical (model developing) to characterize the influence of the different variables on the transport of VOC through soil. * Corresponding author e-mail: [email protected]; phone: 34-976761880; fax: 34-976761861. 3774

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Several authors have studied the transport of VOC through soil by diffusion, analyzing the effect of water content or organic matter on the effective diffusion coefficient (1-7). Although nonlinear adsorption has been found in several cases, some of these authors have simplified the study by considering a linear adsorption, and in these cases the diffusion coefficient might be overestimated. Whereas some authors have found a good correspondence between experimental and predicted values for the flow of volatiles (13), other works overestimate the diffusion flow, finding a greater retardation in the transport of the volatiles through soils than the predicted value (4-7). Other works have studied the convective transport of VOC by means of air flow, simulating the remediation techniques of soil vapor extraction and air sparging in laboratory columns (8-13). One of the main conclusions of some of these works is that intra-aggregate diffusion was found to be the dominant rate-limiting mechanism in the transport and removal of volatiles through the soil. At the same time, different models for the transport and removal of VOC have been developed: models in one dimension to simulate experiments carried out in soil columns and models in three dimensions to simulate the efficiency of a vapor extraction well in the removal of volatile compounds from contaminated soil. The earlier models considered that the transport and removal of VOC from soils occurred in conditions of equilibrium between all phases (solid, liquid, and gas) and were based on the homogeneity of the soil in physical and chemical properties (pneumatic permeability, sorption capacity, etc.) (14-16). However, some experimental results (4, 9, 17-19) have shown that the transport of VOC could be affected by kinetic effects such as mass transfer resistance, diffusion into aggregates, adsorption kinetics, etc. These nonequilibrium effects have been considered in more recent models (9, 20-22). Most of the models developed for the simulation of transport of contaminant vapors through soils use concepts such as the dual-porosity concept developed for the transport of compounds in water-saturated media (23-25). This concept represents the spatial heterogeneity of the soil by considering an advective and a nonadvective domain with a diffusional exchange between the two regions described by a first-order mass transfer equation. Another concept derived from the models for saturated media that is widely used in the modeling of the transport of vapors is the twosite concept (26, 27). This takes into account the chemical heterogeneity of the soil by considering two types of adsorption sites: one in equilibrium and the other with a first-order sorption process. When the adsorption isotherm is linear, both types of model (the dual-porosity and the twosite) have been shown to be mathematically equivalent by Knedi-Kizza et al. (28). Other ways to describe chemical heterogeneity are to consider two sites of different adsorption capacities (11, 29) or a continuous distribution of adsorption sites (30, 31). Although some analytical expressions have been deduced for particular situations (31-33), the majority of the models have to be solved by numerical methods. The complexity of some of them makes their use difficult or impractical because of the large number of parameters required. Much of the previous work in modeling assumes that VOC molecules do not adsorb onto the mineral surface of the soil but only adsorb onto the organic matter of the soil or dissolve into the water present in the soil. These interactions can be described by linear relationships. However, some 10.1021/es9811344 CCC: $18.00

 1999 American Chemical Society Published on Web 09/25/1999

experimental works (6, 13, 34, 35) have shown that at low water contents in the soil the VOC molecules interact strongly with the mineral surface and this interaction can be highly nonlinear. Below a water content of 4-5 molecular layers of water, Henry’s law is not valid to describe the retention of VOC by soils (34, 35). There is a lack of knowledge of the advective transport of VOC through soils at these low levels of water content, where water and VOC molecules may compete for the adsorption sites. In a previous work (36), the adsorption of several VOC onto common soil minerals was studied. A nonlinear adsorption and a substantial influence of air humidity on this adsorption were observed. The aim of this work, therefore, is to study the effect of this nonlinear adsorption of VOC and air humidity on the transport and removal of VOC by means of an air flow through a column of quartz sand. To compare the behavior of different types of VOC, the experiments have been done with a nonpolar compound (n-octane) and a polar compound (methyl ethyl ketone). Complementary to the experimental work, a simple mathematical model has been developed to simulate the transport and removal of VOC in different experimental situations.

and removal of VOC in sand. The transfer of VOC between the gas and the mineral particles occurs under equilibrium conditions.

Model Development The mathematical model developed for the simulations of the transport and removal of n-octane and methyl ethyl ketone in the sand column is based on several assumptions: (i) that there is a steady and one-dimensional air flow through the column; (ii) that the transport of VOC through the soil is achieved by convection and dispersion only, without any diffusion into aggregates; (iii) that the air behaves as a noncompressible fluid with no significant pressure loss; (iv) that the adsorption and desorption of VOC from the mineral take place so rapidly that they can be considered in equilibrium (v) that the adsorption isotherms can be described by the Freundlich equation (eq 3); (vi) that the monocomponent adsorption isotherm of water vapor is also valid in the presence of the organic compounds. On these assumptions and applying the mass balance for the vapor and adsorbed species, the governing equations are

Experimental System The experimental system has been described in a previous work (36). It basically consists of a preconditioning system for the inlet gas to reach the VOC and water concentrations desired, a metallic column where the soil sample is held at room temperature (298 K), and a gas analysis system composed of a gas chromatograph equipped with a flame ionization detector whose signal is continuously monitored by a computer and recorded every 60 s or if a variation in the signal greater than 1% is detected. The length of the column is 19 cm, and its inlet diameter is 3.1 cm. The amount of sand used was 182.5 g. The properties of the quartz sand are also described in the previously mentioned work (36). Several types of experiments were performed under different experimental conditions that will be described in each case. To characterize the pore volume of the sand column and the mechanical dispersion induced by the air flow and its dependence on the air velocity, experiments have been done with methane as a nonreactive tracer at different air flows analyzing the breakthrough curves of methane by the method of moments (20, 30). The pore volume calculated in this way is 75.7 cm3, giving a porosity of 0.53. The dispersion coefficient is related to the air velocity through (37):

Do + ωv D) τ

(1)

giving a tortuosity factor of 3.57 and a dispersivity of 1.96 × 10-3 m. Some preliminary experiments were carried out at three different air flows (10, 50, and 200 cm3/min) in order to determine the presence or absence of kinetic effects in the transport and removal of VOC through sand. The sand was previously conditioned by being heated inside the column up to 403 K and simultaneously having clean air (containing neither water nor VOC) passed through it to remove any adsorbed compound. During the experiment, the air introduced was free water (Hr ) 0%), and the relative VOC concentration (cgi/cgo) was 1.0. The VOC removal was carried out using air without VOC. Experiments were performed with n-octane and methyl ethyl ketone. The coincident results of outlet concentration (cgs/cgo) versus the number of pore volumes (N), used instead of time in order to normalize the results regardless of the air flow used, indicate the absence of kinetic effects in the transport

∂cg ∂ 2c g ∂cg ∂cg (1 - ) + Fsand ) -v +D 2 ∂t  ∂t ∂x ∂x

(2)

cs ) kcgn

(3)

These equations describe the one-dimensional convectivedispersive transport of a compound that exhibits a nonlinear adsorption. They will be applied to the organic compounds as well as to the water vapor in the cases where the two compounds coexist in the same experiment. Due to the nonlinearity of the system of equations, this must be solved using numerical methods rather than analytically. The differential equation has been approximated by finite differences, taking a forward difference for the time derivative and a backward finite difference for the spatial derivative and dropping the dispersive term of eq 2. Thus eqs 2 and 3 can be written as

(1 - ) Fsandcs(i,j + 1) )  (1 - ) ∆t Fsandcs(i,j) + v [cg(i - 1,j) - cg(i,j)] (4) cg(i,j) +  ∆x

cg(i,j + 1) +

cg(i,j + 1) ) kcg(i,j + 1)n

(5)

cg(x,t) ≡ cg(i∆x,j∆t) ≡ cg(i,j)

(6)

where

These equations have been applied for each of the compounds and have been solved iteratively by the numeric “bisection method” (38). This method causes a numerical dispersion that has been controlled by varying the step size (∆t, ∆x) to simulate the mechanical dispersion observed experimentally in the transport of the compound through the column. The use of this method justifies the dropping of the mechanical dispersion term in eq 4. The stability and accuracy of the numerical method has been tested by comparing its results with analytical solutions proposed by several authors (32, 33) for different specific situations. In the case of the simultaneous presence of VOC and water, the equations must be solved for each point and time for water in order to determine both the water adsorbed onto the sand and the water in the gas phase. The equations are resolved at the same point and time for the VOC. VOL. 33, NO. 21, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Values of the Freundlich Parameters at Different Relative Air Humidities Hr ) 0% k 10-3

6.47 × 1.88 × 10-3

n-octane methyl ethyl ketone

Hr ) 20%

Hr ) 50%

n

k

n

k

n

0.298 0.169

3.68 6.98 × 10-3

0.903 0.307

73.02 5.12 × 10-2

1.26 0.52

The parameter values of the Freundlich adsorption isotherms (eq 3) have been obtained by fitting the experimental sorption results obtained at 0%, 20%, and 50% relative humidity at 298 K, results that were shown in the previously mentioned work (36). In the case of water, these parameters are k ) 1.72 and n ) 0.7. The values corresponding to n-octane and methyl ethyl ketone at different relative humidities are shown in Table 1. The parameter values (constant and exponent) have been interpolated from the values at these humidities to cover any value of air humidity in the range of 0-50%. This interpolation has been done by fitting the parameters versus the relative humidity to second-order polynomials. The values of Do for n-octane and methyethyl ketone used are 6.6 × 10-6 and 9.2 × 10-6 m2/s, respectively, and the dispersion coefficients (D) for both compounds calculated with eq 1 are 5.9 × 10-6 and 6.6 × 10-6 m2/s, respectively. From the experimental sorption results and the BET surface area of sand (0.72 g/m2), the water content at 50% air humidity has been estimated to yield 3.4 molecular layers of water on the sand, i.e., conditions where direct interaction of VOC and the mineral surface can occur. To describe the goodness of the fit between experimental and model results, a quantification criterion (q) has been defined as Nt

∑|mass q)

i-experimental

- massi-predicted|

i)1

Nt

×

100 mass

(7)

where Nt is the total number of pore volumes in transport or removal; massi is the mass adsorbed or removed at a number i of pore volumes, with i ) 1, 2, ..., Nt; and mass is the total mass adsorbed or desorbed. This parameter can be interpreted as a percentage of deviation between theoretical and experimental results in respect to the total mass adsorbed or desorbed.

Experimental Results and Model Evaluation Experiments Using Dry Air. The effect of VOC concentration in the inlet air on the transport and removal of n-octane and methyl ethyl ketone through sand was studied. Water-free air (Hr ) 0%) with relative VOC concentrations (cgi/cgo) of 0.1, 0.5, and 1.0 was introduced into sand free of adsorbed compounds. The removal of the compound was carried out using air without VOC and water. Figure 1 shows the transport front and tailing removal for both volatile organics. Some differences can be observed between the transport through the clean mineral and the removal of the organic adsorbed onto the sand as the VOCfree air enters the column and eliminates the VOC retained in the sand. While the transport of VOC is represented as a sharp front, the desorption shows a continuous tailing in the outlet concentration. These differences in transport and removal (sharp front and tailing) are a consequence of nonlinear adsorption. Some authors (30) have indicated that the front shape and velocity remain constant independently of the column length due to the effect of nonlinear adsorption that inhibits front-spreading caused by dispersion. The 3776

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FIGURE 1. Comparison between theoretical and experimental results for the transport and removal of VOC at different concentrations: (a) n-octane and (b)methyl ethyl ketone. semilog plot used in this figure makes it easier to analyze the tailing process. It can be observed that the transport fronts show an increasing retardation as the VOC concentration decreases for both compounds, as would be expected with nonlinear isotherms with exponents less than unity. The experimental behavior is predicted by the model, although there is a slightly poorer correspondence at the lower concentration studied that can be attributed to the Freundlich isotherm used to describe the partition between sand and air. In the removal step for methyl ethyl ketone (Figure 1b), the predicted results show a good agreement with the experimental ones over the whole range of pore volumes. For n-octane (Figure 1a), the model overestimates the outlet concentrations at low concentrations of cgs/cgo, although it is interesting to observe that the cgs/cgo values are very low for high N values. The values of q for the transport of n-octane at relative concentrations 1.0, 0.5, and 0.1 are 9.5, 4.4, and 7.3% respectively, whereas for removal the values are 3.5, 2.3, and 6.5%. The corresponding values for methyl ethyl ketone are 13.1, 2.2 and 7.2% for transport and 5.4, 6.3 and 2.2% for removal. Influence of Air Humidity. As has been shown in several studies, the air humidity has a significant influence on the

FIGURE 2. Influence of air humidity on the transport and removal of VOC: (a) n-octane and (b) methyl ethyl ketone. adsorption of VOC onto soil minerals. In a previous paper (36), it was observed that for nonpolar compounds (aromatics and aliphatics), as the air humidity increases, the adsorption capacity decreases and a linearization in the adsorption isotherms was found. For the polar compound studied (methyl ethyl ketone), the reduction in adsorption capacity was only significant at low vapor concentrations, and there was no linearizing effect on the isotherms. The reduction in the adsorption capacity was attributed to the competition between organic and water molecules for the adsorption sites, this competition being stronger between the polar compound and water than for the nonpolar compounds and water. To know the effect of air humidity on the dynamic conditions of the transport and removal of VOC from sand, four different types of experiments have been designed and carried out. The experiments have been performed with n-octane and methyl ethyl ketone in order to analyze the differences between a nonpolar and a polar compound. The first type of experiment has been performed with the sand preconditioned with air at 50% relative humidity during 12 h prior to the input of VOC. Therefore, when the organic molecules enter the column, the adsorption sites are occupied by water molecules. The same air humidity is maintained during the transport step, when the VOC is introduced, until the outlet concentration of VOC reaches the same value as the inlet concentration. During the removal step, the adsorbed VOC is removed by passing air containing water (Hr ) 50%) but without the organic compound. The results of the transport and removal of the two volatiles at 50% relative humidity are shown in Figure 2. To compare the influence of the air humidity, the results of the experiments carried out at 0% humidity during transport and removal are presented in the same figure.

The n-octane results (Figure 2a) show a substantial reduction in the retardation of the transport of n-octane in the case of 50% air humidity as compared with the experiment of 0%, the outlet concentration front appearing at a much lower number of pore volumes. This difference can be explained by the reduction in adsorption of VOC by the soil when water is present. A rapid decrease in the outlet concentration of VOC as compared with its tailing in the experiment at 0% humidity can also be observed at the desorption stage. This is a result of the reduction in the adsorption of VOC onto the soil. Again, the simple model is able to predict these results satisfactorily showing a good correspondence for both the transport and removal steps, predicting the lower retardation in the transport front and the quick removal of the n-octane retained. The values of q are 12.3 and 0.9% for the transport and removal steps respectively at 50% humidity, and they are 4.4 and 2.3% at 0% humidity. The results obtained with methyl ethyl ketone (Figure 2b) show less difference between 0% and 50% air humidity than in the case of n-octane. The transport front is less retarded for 50% humidity, and the removal of the methyl ethyl ketone occurs in a tailing evolution of the outlet concentration similar to the case of 0% humidity. The model predicts a lower retention of the transport front at 50% humidity, but there are significant differences between model and experimental values with a value of q of 48.6%. For the removal step, again the model predictions for 50% humidity are not coincident with the experimental results as a consequence of the mass balance, since the outlet concentration decreases more quickly than predicted, and this translates into a high value of q of 22.9%. These results suggest that the adsorption of methyl ethyl ketone does not take place in equilibrium conditions in the presence of water, i.e., there could be a kinetic limitation in the process of displacement of one molecule by the other from the adsorption sites. The predictions for 0% humidity are more satisfactory; the values for q are 2.2 and 6.3% for transport and removal. The second type of experiment has been designed to test if the desorption rate of the VOC retained in the absence of water is modified by eluting with humidified air. First, the sand is saturated with VOC at a relative concentration (cgi/ cgo) of 0.5 and 0% relative humidity. The organic compound retained is then removed by passing through VOC-free air with a humidity of 50%. The experimental and predicted results for n-octane and methyl ethyl ketone are plotted in semilog scale in Figure 3. This figure also shows the results of the experiments carried out at 0% humidity. It can be seen that for both volatiles the concentrations are similar at the beginning of the removal step for both 50% and 0% humidities, followed in the former case by a period with higher VOC concentrations in the outlet air. This indicates that the water molecules displace the organic molecules from the adsorption sites, accelerating the process of removal as compared with the removal in absence of water. This displacement begins when the concentration levels are around 0.1 for n-octane whereas for methyl ethyl ketone they are about 0.02. Other differences in the behavior of the two compounds can be observed. The n-octane outlet concentration decreases abruptly after the period of high concentration levels and at 50% air humidity remains lower than that of the 0% humidity experiment over the whole range of pore volumes. The decrease in methyl ethyl ketone concentration is more progressive and over a wide range of pore volumes is higher at 50% air humidity than in the case of 0% humidity. The model predicts the increase in the outlet concentration, the sharp decrease of the n-octane, and the more progressive decrease for methyl ethyl ketone with a better correspondence for n-octane (q ) 2.0%) than for methyl ethyl ketone (q ) 5.1%). The differences between the VOL. 33, NO. 21, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Influence of air humidity on the removal of VOC: (a) n-octane and (b)methyl ethyl ketone.

FIGURE 4. Displacement of n-octane molecules from the adsorption sites by water molecules: (a) n-octane and (b) methyl ethyl ketone.

theoretical and experimental results are more marked at low concentration levels when the majority of the VOC has been removed. The small peak observed in Figure 3a at the end of the removal of n-octane might be attributed to the existence of a fraction of adsorption sites with stronger bonds with organic molecules. This would cause the n-octane molecules adsorbed in these sites to be displaced by water at the end of the removal step. A mass balance over this peak indicates that the amount of n-octane desorbed is about 3% of the total n-octane. The third type of experiment has been designed to check if water is able to displace the adsorbed VOC molecules from the adsorption sites in the presence of VOC in the air entering the column. In this experiment, the sand is presaturated in VOC by an input of air with a relative VOC concentration of cgi/cgo ) 0.5, free of water. Once the outlet VOC concentration is equal to the inlet concentration, the relative humidity of the air entering the soil column is changed from 0% to 50%, maintaining the same inlet VOC concentration. The outlet VOC concentration is monitored until it reaches a constant value. The results obtained for both volatiles are shown in Figure 4. In the case of n-octane, an increase in the outlet relative VOC concentration is observed when water is introduced into the sand. This outlet concentration reaches an almost constant value of approximately 0.65 over a pore volume number range of 150-200 and then decreases to the level of the inlet relative concentration, 0.5. This behavior is assumed to be due to the displacement of the VOC molecules from the adsorption sites as the water molecules enter the soil and adsorb onto these sites. The theoretical results predicted by the model correspond well with the experimental data,

reproducing the pattern of sharp increase, stability, and gradual decrease in concentration, giving a value of q of 3.3%. In the case of methyl ethyl ketone (Figure 4b), the results are slightly different. When the water vapor is introduced into the sand, the outlet VOC concentration remains at almost the same value, with only a very slight increase followed by a slight decrease. This indicates that the polar molecules of the methyl ethyl ketone have stronger bonds than n-octane with the adsorption sites and that water is not able to displace them. The results predicted by the model follow the experimental data approximately with a small increase in the value of the outlet concentration but with a q value of 15.7%. These results indicate the validity of the model for the simulation of the behavior of different compounds with different chemical characteristics. Finally, a fourth type of experiment has been developed to analyze the effect of air humidity. This experiment has been designed to examine the competition between VOC and water molecules for the adsorption sites. The sand was previously conditioned by being heated to 403 K to remove any adsorbed water or VOC. After this preconditioning, the water and the VOC were introduced into the sand. The air humidity was 50%, and the inlet relative VOC concentration was 0.5. The experimental and predicted results are shown in Figure 5. In this figure, the results corresponding to the transport step of the experiment carried out at 0% air humidity have also been plotted for comparison. The results for n-octane (Figure 5a) show less retardation in the transport of VOC through the soil at 50% humidity and a higher outlet relative concentration than the inlet, around 0.65. This level is maintained over a range of approximately 60 pore volumes and begins to decrease until it reaches the value of the inlet concentration. This “rollup” effect has also

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cgi cgo cgs cs D Do Hr k N n q Q t ∆t v x ∆x

VOC concentration in the inlet air to the column (g/cm3) saturation concentration of VOC in air at 273 K (g/cm3) VOC concentration in the outlet air of the column (g/cm3) adsorbed VOC concentration in sand (g of VOC/g of sand) dispersion coefficient (m2/s) molecular diffusion coefficient in air (m2/s) relative humidity of the air at 298 K (%) Freundlich equation constant number of pore volumes Freundlich equation exponent quantification criterion of fit between experimental and predicted results volumetric air flow (cm3/min) time (s) time increment for the numerical resolution of the model equations (s) average air velocity (m/s) distance (m) spatial increment for the numerical resolution of the model equations (m)

Greek Symbols

FIGURE 5. Influence of air humidity on the transport of VOC: a) n-octane, b) methyl ethyl ketone. been described by other authors (13) and is assumed to be due to the displacement of the VOC molecules from the adsorption sites by the water molecules as water and VOC travel through the soil column. As can be observed, the results obtained with the model correspond with the experimental ones quite satisfactorily, and the value for q is 23.9%. In the case of methyl ethyl ketone (Figure 5b), the results are quite different from those for n-octane. The two experiments at 0% and 50% air humidity give almost the same results, and there is no significant difference between the two situations. Again, the model predicts the behavior of the polar VOC (q ) 14.7%), indicating that water is not able to displace the VOC from the adsorption sites as the two compounds pass through the sand column. It can be concluded that a simple model assuming local equilibrium conditions and with experimentally determined adsorption isotherms is able to predict the behavior of two compounds of different chemical characteristics (a polar and a nonpolar VOC) in the transport and removal of VOC through a sandy soil under different air humidity conditions. The polar compounds can interact with the adsorption sites more strongly than the nonpolar compounds, and their removal is more difficult with a convective flow of air.

Acknowledgments The authors express their gratitude to the Diputacio´n General de Arago´n, Spain, for providing financial support for the work and for the predoctoral grant awarded to J.R.

Notation cg

concentration of each species in the gas phase (g/cm3)

 Fsand τ ω

porosity of the sand column density of the sand particles (g/cm3) tortuosity factor dispersivity (m)

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Received for review November 4, 1998. Revised manuscript received July 22, 1999. Accepted July 28, 1999. ES9811344