Environ. Sci. Technol. 2000, 34, 764-770
Influence of Porous Media, Airflow Rate, and Air Channel Spacing on Benzene NAPL Removal during Air Sparging SHANE W. ROGERS AND SAY KEE ONG* Department of Civil and Construction Engineering, 490 Town Engineering Building, Iowa State University, Ames, Iowa 50011
To study the effects of porous media type, airflow rate, and air channel spacing on NAPL removal, air sparging of a benzene NAPL was performed in a lab-scale reactor with two isolated vertical air channels on either side of the NAPL. Experimental conditions included three discrete air channel distances, three types of saturated porous media, and five airflow rates. Benzene NAPL removal efficiency was shown to increase from 7.5% to 16.2% with increasing porous media mean particle size (from 0.168 to 0.305 mm, respectively) over the 168 h of operation. Initial change in the airflow rate had an effect on contaminant removal rate, but further change in the airflow rate had little effect. Benzene NAPL removal efficiency was shown to decrease with increasing channel spacing, but the mere presence of air channels was shown to suppress lateral contaminant migration. Benzene removal efficiency was shown to be highly correlated (r 2 ) 0.96) with the mean particle diameter, the square root of the uniformity coefficient, and the inverse of the square of the distance between the NAPL and the air channel.
Introduction Nonaqueous-phase liquids (NAPLs) are a common remedial challenge at sites contaminated with volatile organic compounds (VOCs). When introduced into the unsaturated subsurface, NAPLs travel downward and laterally toward the water table. Once at the water table, light NAPLs tend to pool on the phreatic surface due to buoyancy forces whereas dense NAPLs continue their downward and lateral migration through the saturated porous media. NAPLs can become trapped within the porous matrix in both the saturated and the unsaturated zone under capillary and surface tension forces becoming a long-term source of residual contamination. In-situ air sparging (IAS) coupled with soil vapor extraction (SVE) is becoming a popular remediation tool for the removal of NAPLs from the subsurface. IAS involves the injection of contaminant-free air into the saturated zone of the porous media in an attempt to volatilize contaminants and transport them to the vadose zone to be removed with SVE. Very limited research, however, is devoted specifically to NAPL removal during an IAS operation (1, 2). In field-scale studies, Boersma and co-workers (3) determined that rebounding of dissolvedphase contamination can occur at sites in which air sparging * Corresponding author telephone: (515)294-3927; fax: (515)2948216; e-mail:
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is used to remove NAPL contamination. On the laboratory scale, Braida and Ong (4) showed that the presence of airflow controlled the extent of diffusion from a NAPL glob as contaminants were volatilized and removed from the system. The influence of the physical properties of the contaminated system such as soil porosity as well as contaminant properties and air sparging system properties such as airflow rate and air channel density on NAPL removal is still unclear. Many researchers have studied the effects of sparged air on dissolved-phase contaminant removal. Due to the difficulty in quantifying field-scale variability of an air-sparged system (5), many of these researchers have performed studies in laboratory-scale reactors (6-9). These reactors typically consist of a cylindrical column or a large tank filled with saturated porous media into which air is injected in irregular and arbitrary patterns. Contaminant removal is typically measured and related to the operational system parameters and/or porous media parameters. Unfortunately, these experiments do not eliminate all variability because the exact number and location of air channels within the reactor cannot be determined. It is unclear to what degree changes in the airflow rate, porous media properties, and air channel density affect contaminant removal. For example, a change in porous media properties may result in a change in the air channel density and the average air channel diameter confounding the effects the porous media properties had on contaminant removal. In summary, not much research has been performed in an attempt to describe the fundamental properties affecting NAPL removal from saturated porous media during IAS. Furthermore, conducting air sparging experiments with soil columns or on the pilot scale do not allow for the separation of the physical properties of the soil and air sparging system from the chemical properties of the soil and contaminant that affect air channel density, air channel diameter, contaminant migration, and contaminant removal. The objective of this work was to measure the influence of two discrete air channels at predetermined spacing through saturated porous media on the removal of a glob of benzene NAPL. This approach allowed the quantification of the effects of airflow rate, porous media properties, and channel spacing separately from each other in order to obtain an understanding of the important properties of an air sparged system.
Materials and Methods Three experimental setups with two discrete air channels within the saturated porous media were used to perform visualization studies on the dissolution patterns of a benzene NAPL and its subsequent volatilization under air sparging conditions. Each experimental setup consisted of a Plexiglas box with the dimensions of 10 cm wide by 18.5 cm high by 4 cm deep with a lid that kept the saturated media from fluidizing (see Figure 1). Twenty sampling points with Teflonlined septa were located in front of each reactor and were used for aqueous-phase sampling. This allowed for a visualization of the NAPL dissolution and spreading within the reactor under varying air sparging conditions. Two discrete vertical channels of air were forced through each setup with a spacing of 30, 45, and 60 mm, respectively. The air channels were made by inserting two 3 mm diameter rods into the reactor before packing it with saturated porous media. With a 3-cm layer of water in the reactor bottom, presaturated porous media was added to a depth of 2 cm and packed with 20 blows of a blunt-ended rod. The process continued until the reactor was filled with saturated porous media at which point the lid was placed on the reactor and 10.1021/es9901112 CCC: $19.00
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FIGURE 1. Experimental setup showing the sampling port layout in the packed bed. clamped down tight. Upon commencement of airflow, the rods were pulled out of the reactor. The airflow through the air channels was then allowed to reach steady state. Once equilibrated, a glob of benzene was introduced into the reactor by slowly injecting 50 µL of HPLC grade benzene (43.7 mg) into the porous media halfway between the air channels. The air used was filtered in-house compressed air, and the inlet airflow rate was measured with a Gilmont 250 mL/ min ball flow meter (Barrington, IL). The air was separated into two air lines in which the airflow rate was adjusted using two Gilmont needle valves. The air was humidified by passing through a 250-mL gas washing bottle filled with water. Stainless steel screens were attached to the holes in the bottom of the reactor to prevent loss of porous media. As air left the reactor, it passed through two 125-mL gas-sampling ampules (one for each air channel) where the gas-phase concentration was sampled (see Figure 2). The exhaust airflow rate in each channel was monitored with a Hewlett-Packard 100-mL soap film flow meter. All connectors were made of Teflon, and all connections were wrapped in PTFE tape and
secured with hose clamps to prevent leakage of air in all experimental runs. The exhaust airflow rates were equal to the inlet airflow rates. Air sparging air channels are typically conceptualized as a zone of air that flows through the saturated porous media around sand grains toward the water table (10). For the purposes of simulating natural air channels while maintaining straight air channels that could be measured, the air was forced to pass through the saturated porous media as an empty pipe of air. At the end of the experiment (168 h), the lid was removed, and the air channel diameters were measured at the top of the reactor and at 0.5-cm intervals into the packed sand by carefully scraping off layers of sand until the sand could not be scraped without collapsing the air channels (typically at a depth of between 4 and 6 cm from the bottom of the reactor). The resulting average air channel diameters were determined to be between 0.8 and 2 mm and were within the 10- 20 pore sizes as observed by Ji and co-workers (11) and Braida and Ong (4). Initial experiments to ensure that there were two discrete air channels were conducted with the reactor filled with a slightly alkaline bromothymol blue solution while passing air saturated with hydrochloric acid gas through the reactor. The blue sand in the reactor turned yellow when it came in contact with the acid gas, indicating that two distinct air channels were formed in the reactor that exclusively followed the air paths forced through the reactor (12). No crossover or bending of the channels was observed. Furthermore, adjusting the needle valves placed before the reactor showed that decreasing the flow in one air channel increased the flow in the other channel by the same increment and vice versa. This further indicated the existence of two discrete air channels within the reactor as well as conservation of flow. The effects of porous media on benzene removal was studied using three porous media with an air channel spacing of 30 mm and an airflow rate in each channel of 45 mL/min. The properties of the three porous media are shown in Table 1. To study the effects of airflow rate, airflow rates in each air channel ranging from 45 to 125 mL/min were used with Ottawa sand and an air channel spacing of 30 mm. The effect of air channels spacing was studied with silica sand 35/50 (sand passing a USS no. 35 sieve and retained on a USS no. 50 sieve) at an airflow rate of 45 mL/min per air channel. The air channel spacings used were 30, 45, and 60 mm. A set of experimental runs without air channels (therefore without airflow) were conducted using the three porous media. The experimental setup was kept at a constant temperature of 20 ( 2 °C.
FIGURE 2. Schematic flow diagram of experimental setup. VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Air Sparging Experiment Results
TABLE 1. Properties of the Porous Media mean particle uniformity packed bed type of diameter, coeff, UC porosity, η porous media d50 (mm) (dimensionless) (dimensionless) silica sand 35/50 Ottawa sand silica sand 70/100
org C (%)
0.305
1.41
0.370
0.0066
0.190 0.168
2.16 1.64
0.377 0.400
0.0062 0.0063
Samples were taken with a syringe from 2 cm into the porous media for each of the 20 ports in the reactor at 0, 8, 24, 72, 120, and 168 h of the experimental run. Air samples were taken from the gas sampling ampules for the same time periods. The exhaust air concentrations were measured via direct injection of a 1-mL air sample with a Hewlett-Packard 5890 series II gas chromatograph (Avondale, PA) equipped with a HP-5 capillary column and flame ionization detector. Liquid samples were measured using the headspace technique. The headspace technique used consisted of sampling a 25-µL aqueous sample from the reactor and placing the aqueous sample in a 1.8-mL glass sampling vial. The vial was tightly secured with a PTFE-lined aluminum crimp cap and allowed to equilibrate. Once equilibrated, 200 µL of the headspace was analyzed with the gas chromatograph, and the aqueous concentration was estimated using the headspace concentration and Henry’s law constant.
Presence of Airflow versus Background Diffusion. The gasphase concentrations at each time step were used to calculate mass removal from the system while dissolved-phase contaminant migration was expressed qualitatively through the use of two-dimensional isoconcentration plots drawn using the Surfer 5.03 software package (Golden Software Inc., Golden, CO). Figure 3a-d shows the isoconcentration plots for benzene in Ottawa sand with 30 mm channel spacing and an airflow rate in each air channel of 45 mL/min at 24, 72, 120, and 168 h after benzene NAPL injection while Figure 3e-h shows the isoconcentration plots of benzene in Ottawa sand with no airflow for the same times. To show the ability of the Surfer 5.03 software package to accurately represent the contaminant plume, benzene concentrations (in mg/L) at various sampling points in the reactor are plotted in Figure 3. For clarity, not all the 20 sampling point concentrations are shown. The plots in Figure 3a-d show an initial rapid spread as the benzene NAPL dissolved (Figure 3a) followed by a period in which there was little spreading of the benzene laterally, but there was migration of the benzene vertically (Figure 3c). Without air channels, the dissolved-phase benzene migrated much farther laterally than when air channels were present (compare Figure 3h with Figure 3d, 168 h after NAPL injection). The vertical spread of the contaminant was much less without the influence of the airflow. The dissolved-phase contaminant plume was much more concentric as would be expected with diffusive
FIGURE 3. Isoconcentration plots of benzene migration with 30 mm air channel spacing and 45 mL/min airflow rate in Ottawa sand at (a) 24, (b) 72, (c) 120, and (d) 168 h and without air channels in Ottawa sand at (e) 24, (f) 72, (g) 120, and (h) 168 h. 766
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FIGURE 4. Effect of porous media on benzene NAPL removal with an airflow rate of 45 mL/min per air channel and 30 mm air channel spacing.
FIGURE 5. 120 h isoconcentration plots of benzene migration with 30 mm air channel spacing and 45 mL/min airflow rate in (a) silica sand 70/100, (b) Ottawa sand, and (c) silica sand 35/50. transport. In summary, the presence of air channels removed contaminant mass from the reactor and suppressed the diffusive spread of dissolved-phase contaminants laterally while moving the contaminants vertically toward the top of the reactor. Effect of Porous Media. Figure 4 is a plot of mass removal efficiency (mass of benzene removed divided by the total mass of benzene injected into the system) for three different media types at an airflow rate in each air channel equal to 45 mL/min and channel spacing of 30 mm. The mass removal efficiency for the silica sand 35/50 was 1.5 times larger than that for the finer grained Ottawa sand and 2.2 times greater than the even finer grained silica sand 70/100. A lag phase of benzene removal can be seen for all three porous media. Mass removal in each case appeared to increase with
increasing mean particle diameter (d50) of the porous media. One possible reason is that the larger grained porous media had larger air channels resulting in a larger interfacial area available for mass transfer at the air-water interface. The average measured air channel diameters at the end of each experimental run were 2 mm for the 35/50 sand, 1.2 mm for the Ottawa sand, and 0.8 mm for the 70/100 sand. Figure 5 shows isoconcentration plots of the three media types 120 h after benzene NAPL injection. The extent of the dissolved-phase contamination at or above 100 mg/L was much greater in the Ottawa sand than in either the silica sand 70/100 or the silica sand 35/50, suggesting that dissolution was higher in the Ottawa sand than in the silica sands. Since the dissolution rate is proportional to the surface area of the NAPL glob (13, 14), it is possible that injecting the VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Effect of airflow rate on benzene NAPL removal in Ottawa sand with 30 mm airchannel spacing.
FIGURE 7. 120 h isoconcentration plots of benzene migration with an airflow rate of 90 mL/min in silica sand 35/50 with an air channel spacing of (a) 30, (b) 45, and (c) 60 mm. benzene into the less uniform Ottawa sand formed a more irregularly shaped NAPL blob, resulting in a larger surface area to volume ratio. This may have caused a faster rate of dissolution than in the silica sands. The uniformity coefficient (UC) for the silica sand 35/50, silica sand 70/100, and Ottawa sand were 1.41, 1.64, and 2.16, respectively. These results seem to suggest that the uniformity of the porous media played a role in the dissolution of the benzene in saturated porous media. Effect of Airflow Rate. The effect of varying airflow rates can be seen in Figure 6, which shows the mass removal efficiency versus five different airflow rates in Ottawa sand with a 30 mm channel spacing. In Figure 6, the total mass removed for an airflow rate of 45 mL/min was lower than the other airflow rate experiments until approximately 150 h after benzene NAPL injection. The final mass removal efficiency for all experiments was in the range of 9.6-12.7%. Except for the 90 mL/min airflow rate experiment, the results seemed 768
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to suggest a slight increasing trend in mass removal efficiency with increasing airflow rate, but the magnitude of the differences may not be significantly different. Gas-phase concentrations for all experiments ranged from 1 to 7 µg/L and increased with decreasing airflow rate. These results appeared to be in agreement with the work of Braida and Ong (4) where airflow rate had insignificant impact on the total mass removed. Effect of Air Channel Spacing. Figure 7 shows the isoconcentration plots of three different channel spacings in 35/50 sand with an airflow rate of 45 mL/min at 120 h after benzene NAPL injection. The benzene plume size appeared to increase with larger air channel spacing. Furthermore, lateral migration of the contaminant became more evident, and contaminant migration approached the migration associated with no airflow as air channel spacing was increased (compare Figure 7a with Figures 7c and 3h). Notice that, even for an air channel spacing of 60 mm, lateral migration
FIGURE 8. Effect of air channel spacing on benzene NAPL removal in silica sand 35/50 with an airflow rate of 45 mL/min per air channel.
FIGURE 9. Effect of porous media properties and air channel spacing on benzene NAPL removal. of the contaminant was still suppressed by the presence of the air channels. Figure 8 is a graph of the mass removal efficiency versus time for the three different air channel spacings. At an air channel spacing of 30 mm, contaminant mass removal was much higher than that for an air channel spacing of 45 mm. The difference in mass removal between 45 and 60 mm channel spacing was much less evident. This shows that benzene removal was lower with larger air channel spacing. In the far field of a field-scale air sparging system where air channel density becomes increasingly lower with increasing distance from the air injection point or in heterogeneous regions of low air saturation (1, 7), air sparging will likely become much less effective.
Combined Effects of Air Channel Spacing, Airflow Rate, and Porous Media Type on NAPL Removal Combining the effects of the various soil and air sparging system parameters provides fundamental information on
how air sparging effects NAPL removal. Soil properties exert a strong influence on the dissolution of a NAPL glob. Dissolution of the benzene NAPL occurred more rapidly in the Ottawa sand than in the silica sand 35/50 or silica sand 70/100. This was probably due to the formation of a more irregular NAPL glob with a larger surface area to volume ratio in the less uniform Ottawa sand than either of the other sands (13, 14). Both soil and system properties affected the dissolved-phase benzene migration from the NAPL glob to the air channels. Migration of benzene from the aqueous phase to the air-water interface is dependent on the length of the tortuous path the benzene had to travel through the soil. Millington and Quirk (15) developed a commonly used model that positively correlates the tortuosity factor of a saturated porous media to its porosity. As a porous medium becomes larger in grain size, the porosity tends to be smaller and the pathway becomes less tortuous. Similarly, as a porous medium becomes less uniform (the UC becomes larger), the porosity becomes smaller as larger pore spaces are filled in VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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with smaller grained media. Considering this, the tortuosity of a porous media can be inversely related to both the mean particle diameter and the uniformity coefficient of the porous media. The presence of air channels in the saturated porous media suppressed lateral migration but increased the vertical migration of the dissolved-phase benzene between the air channels. Increasing the airflow rate initially increased the benzene removal rate, but further increases in airflow rate had little effect. This may be attributed to a larger air channel diameter for higher airflow rates but with no further increase in the air channel diameter beyond a given airflow rate. Similarly, the large pore spaces in the larger mean particle diameter porous media may have allowed lower air entry pressures and therefore the observed larger air channel diameters. These increases allowed for a larger interfacial area for mass transfer and, therefore, more volatilization. Figure 9 is a plot of a dimensionless parameter versus the mass removal efficiency for all experiments at all sampling times. The dimensionless parameter consists of the aqueousphase diffusion coefficient (D) of benzene (9.59 × 10-4 mm2/ s) multiplied by the square root of the uniformity coefficient, the normalized mean particle diameter (do ) d50/0.05 cm), and time (t) and divided by the square of the distance between the NAPL glob and the air channels (X2). The aqueous-phase diffusion coefficient and the normalized mean particle diameter were used to make the parameter dimensionless. Furthermore, the normalized mean particle diameter and uniformity coefficient were used in lieu of the tortuosity alone in order to account for both the effect of tortuosity as well as the interfacial area available for mass transfer at the airwater interface. The linear correlation (r 2 ) 0.96) for Figure 9 seemed to suggest that NAPL removal from the air-sparged system was limited by diffusive transport. In summary, the presence of air channels was shown to suppress lateral dissolved-phase benzene migration but, at the same time, promote vertical migration of the dissolvedphase benzene plume parallel to the air channels as compared to the no air channel experiments. Mass removal was shown to be directly proportional to the mean particle size of the porous media. In addition, mass removal was shown to be inversely proportional to the square of the distance between the NAPL glob and the air channel. The results of this work suggest that air sparging may be a useful remedial tool for the removal of VOCs in saturated porous media and may be used to slow contaminant migration within the air channel boundaries.
Notation
d60 diameter at which 60% of the porous media is smaller (mm) do
mean particle diameter (dimensionless)
D
aqueous diffusivity of benzene (mm2/s)
t
time (s)
UC uniformity coefficient of the porous media ()d60/d10) (dimensionless) X
distance between the NAPL globule and an air channel (mm)
Greeks η
porosity of the media (dimensionless)
τ
tortuosity factor of the porous media (dimensionless)
Literature Cited (1) McCray, J. E.; Falta, R. W. Ground Water 1997, 35 (1), 99-110. (2) Johnson, P. C.; Das, A.; Johnson, R. L.; McWhorter, D. in Proceedings of the Fourth International In-Situ and On-Site Bioremediation Symposium, New Orleans, LA, April 28-May 1, 1997; Vol. 1, pp 135-140. (3) Boersma, P. M.; Pointek, K. R.; Newman, P. A. B. In In Situ Aeration: Air Sparging, Bioventing, and Related Remediation Processes; Hinchee, R. E., Miller, R. N., Johnson, P. C., Eds.; Battelle Press: Columbus, OH, 1994; pp 39-46. (4) Braida, W. J.; Ong, S. K. Transp. Porous Media 2000, 38(1/2), 29-42. (5) Johnson, R. L.; Johnson, P. C.; McWhorter, D. B.; Hinchee, R. E.; Goodman, I. Ground Water Monit. Rem. 1993, Fall, 127135. (6) Ji, W.; Ph.D. Dissertation, University of Connecticut, Storrs, CT, 1994. (7) Hein, G. L.; Gierke, J. S.; Hutzler, N. J.; Falta, R. W. Ground Water Monit. Rem. 1997, Summer, 222-230. (8) Chao, K. P.; Ong S. K.; Protopapas, A. ASCE J. Environ. Eng. 1998, 124 (11), 1054-1060. (9) Reddy, K. R.; Kosgi, S.; Zhou, J. Hazard. Waste Hazard. Mater. 1995, 12 (2), 97-117. (10) Elder, C. R.; Benson, C. H. Ground Water Monit. Rem. 1999, Summer, 171-181. (11) Ji, W.; Dahmani, A.; Ahfeld, D. P.; Lin, J. D.; Hill, E., III. Ground Water Monit. Rem. 1993, Fall, 115-126. (12) Rogers, S. W. Master’s Thesis, Iowa State University, Ames, IA, 1999. (13) Powers, S. E.; Abriola, L. M.; Weber, W. J., Jr. Water Resour. Res. 1994, 30 (2), 321-332. (14) Rixey, W. G. Hazard. Waste Hazard. Mater. 1996, 13 (2), 197211. (15) Millington, R. J.; Quirk, J. M. Trans. Faraday Soc. 1961, 57, 12001207.
d10 diameter at which 10% of the porous media is smaller (mm)
Received for review February 1, 1999. Revised manuscript received August 16, 1999. Accepted December 7, 1999.
d50 mean particle diameter of the porous media (mm)
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