Grantee Research Project Results
2001 Progress Report: The Effect of Clay on DNAPL Behavior During Alcohol Flushing
EPA Grant Number: R827120Title: The Effect of Clay on DNAPL Behavior During Alcohol Flushing
Investigators: Hayden, Nancy J.
Institution: University of Vermont
EPA Project Officer: Aja, Hayley
Project Period: December 1, 1998 through November 11, 2001 (Extended to June 21, 2003)
Project Period Covered by this Report: December 1, 2000 through November 11, 2001
Project Amount: $375,240
RFA: Exploratory Research - Environmental Engineering (1998) RFA Text | Recipients Lists
Research Category: Sustainable and Healthy Communities , Land and Waste Management , Safer Chemicals
Objective:
The overall objective of this research project is to investigate the effect of clay and clay colloids on the distribution of dense nonaqueous phase liquids (DNAPLs) in porous media and the subsequent effect on DNAPL distribution, mass transfer, and interfacial phenomena during alcohol flushing. An improved understanding of DNAPL behavior is critical for improving restoration attempts using in situ methods such as alcohol flushing.
A series of experiments and analyses are being performed at the pore and column scales to meet the project objectives. Kaolinite and Ca-montmorillonite clays are being used for the majority of experiments. Isopropyl alcohol (IPA) and tetrachloroethylene (PCE) are being used as the alcohol and DNAPL, respectively.
Most of the batch and column studies were completed during Year 3 of this research. A new micromodel technique was developed to observe pore-scale phenomenon. The scope of work was expanded to include numerical modeling of the pore space.
Progress Summary:
Steady progress was made in Year 3. All pressure-saturation (Pc-S) experiments with 0 and 10 percent clay porous media have been completed. The data were fitted using the Brooks-Corey model to characterize the porous media with respect to entry pressure, pore-size distribution, and DNAPL saturation at 300 mbar. A new micromodel technique incorporating real sediments viewed under a microscope was developed as a better alternative to E-SEM work for pore visualization before and after DNAPL entrapment. Pore-scale images taken using micromodels were used to better understand the macroscale phenomena observed in the Pc-S experiments. Numerical modeling of the pore-size distribution and DNAPL distribution in sand-clay mixtures was added to this research to better understand the porosity and effect of the clay distribution in DNAPL movement and distribution in clay-containing porous media. Batch studies investigating the effect of clay colloids on PCE-aqueous phase interfacial tension were conducted, where a 0.01M CaCl2 buffer solution was used as the aqueous phase. A no-cost extension has been granted to complete the added numerical modeling and the pressure-saturation and mobilization experiments at clay contents other than 0 and 10 percent. These experiments were delayed primarily due to an initial late start in the first year of the project.
Pressure-Saturation Column Studies and Brooks-Corey Curve Fitting
Capillary pressure-saturation relationships were determined for the various media to quantify and compare the differences in PCE saturation, entry pressure, and pore-size distribution in media of the same total porosity. Overall, there is considerable difference in the pressure-saturation behavior for the different media. This was exhibited as differences in the shape of the curves, in the residual water saturation, and in the pressure needed to start drainage of water from the column, the entry pressure, Pe. As expected, the presence of the clay and especially the clay type had a significant effect on these parameters. The Brooks-Corey fitting procedure was used to analyze the Pc-S relationships. Values for entry pressure, PCE saturation at Pc=300 mbar, and the pore-size distribution index, , were determined for each curve. The mean and standard error for each treatment are shown in Table 1.
Media (No. of Columns) | Pe (mbar) | SPCE (%) | Brooks-Corey |
Sand (4) | 15.9 ± 1.0 | 92.6 ± 2.0 | 4.4 ± 0.4 |
10% Kalinite (5) | 19.1 ± 1.4 | 68.9 ± 3.7 | 2.6 ± 0.3 |
10% Montmorillonite (4) | 69.5 ± 3.9 | 39.1 ± 1.3 | 1.0 ± 0.2 |
Pore-Scale Observations Using Micromodels
Micromodels were used to observe pore-scale phenomena in sandy porous media with and without clay. The micromodels consisted of two glass microscope slides (2.5 x 7.6 cm) filled with porous media and the edges sealed with an epoxy. PCE was flushed through buffer-saturated models and later displaced by water to bring the media to residual PCE conditions. Random locations on the top slide were circled so that the same location could be viewed dry, wet, and with DNAPL. A Zeiss Axioscop2 microscope with AxioCam color camera was used to observe the micromodels.
Figures 1 and 2 show the pores of clay-containing media under dry, water-saturated, and PCE-saturated conditions. All four images in each figure represent a single location in the micromodel under the various conditions. Some slight reorientation of sand grains is noticeable, but overall, it is possible to view the same grains and pores under the different conditions. These images are very useful in understanding the clay distribution and behavior at the pore scale. The wet kaolinite aggregates seem to coat the grains, and concentrate in the small pores and narrow wedges of large pores. Many large pores are still visible, and they appear similar to those observed in the sand micromodel (not shown). The wet montmorillonite behaves differently than the kaolinite and forms a gel-like layer that spreads over the sand grains in a more continuous fashion. This is undoubtedly due to the swelling nature of this clay. The montmorillonite appears to cover or fill entire pores. This can dramatically affect fluid movement and the effective porosity. The movement of DNAPL into pore spaces (Figures 1d and 2d) is clearly influenced by the presence of the clay. In the kaolinite containing media (Figure 1d), the DNAPL can move into areas where the pores are relatively large and open. The dark red of the two large central pores shows that the PCE has penetrated the pores. In other areas, where the clay is filling most of the pore, the PCE has not penetrated. In the montmorillonite-containing media, PCE was observed to fill the larger pores (Figure 2d); however, the large coating of montmorillonite in the central portion of the figure appeared to exclude the DNAPL.
Figure 1. 10 percent kaolinite in sand micromodel showing: (a) dry, (b) water saturated, (c) clay/grain surface enlarged, and (d) PCE in the pore space.
Modeling the Pore Space in Clay Containing Media
The pore-scale spatial distribution of two immiscible fluid phases in clay containing media is quantitatively analyzed according to the ideal soil model. The basic pore space geometry is described using a "unit cell" approach. The objectives for this part are to: (1) analyze the effects of packing angle and grain size on pore shape, pore-size distribution, NAPL distribution, entry pressure, and NAPL saturations for "only sand" unit cell; and (2) analyze the effects of 10 percent clay content in a "sand-clay" unit cell. Here as well, pore shape, pore-size distribution, NAPL distribution, entry pressure, and NAPL saturations are studied. A MATLAB model was written for analysis of the "only sand" and the "sand-10 percent clay" unit cells. The model assumes that the "well rounded Ottawa sand" grains are ideal spheres.
Modeling a Sand Unit Cell
A packing angle, , of 64° was compared to the traditional packing of spheres with =90°. A packing of =64 was chosen because it produces porosity and density suitable to the experimental columns. The spheres, the unit cells, and some representative cross section ns through the cells are shown in Figure 3. The NAPL distributions in the various cross sections also are shown. It is seen that NAPL forms one flow channel in the =90 cell, while the NAPL flow channel splits to two fingers in the =64 cell. It is evident that NAPL distribution is more complicated for denser packing arrangements. Pore-size distributions and NAPL saturations in the unit cells were determined using the model.
Figure 2. 10 percent montmorillonite in sand micromodel showing: (a) dry, (b) water saturated, (c) clay/grain surface enlarged, and(d) PCE in the pore space.
Modeling a Sand-10 percent Kaolinite Unit Cell
A packing angle, =70, was used for the sand grains of the sand-kaolinite media. The kaolinite was added to the pores as a coating on the grains. The unit cell, the NAPL distribution, the pore-size distribution, and NAPL saturation in this unit cell are shown in Figure 4. It is apparent that introduction of kaolinite to the porous media widened pore-size distribution and changed the NAPL distribution in the pore space. The NAPL saturation profiles in the cells are interestingly complicated.
The unit cell analysis of an ideal soil contributes insights regarding pore shape, pore size, and pore-size distribution. NAPL distribution and saturation varies largely for different packing arrangements. Pore-scale analysis is essential for better understanding two-phase processes, especially if clays are present in the pores.
PCE-0.01M CaCl2 Solution IFT
Because it was suspected that the charged clay colloids have an impact on IFT, PCE/aqueous phase IFT measurements were made with either montmorillonite or kaolinite in the aqueous phase. Both distilled water (DI) and the 0.01M CaCl2 buffer solutions were tested. Statistic analysis was used to compare all fluid pairs to the PCE-DI case. Statistically significant differences in IFT at the 0.05 level were found between PCE-DI measurement and all other fluid pairs, except in the case of PCE-(DI + kaolinite).
Figure 3. (a) The packing of spheres and the unit cells for =90 and =64, (b) the sections of the unit cell, and (c) NAPL distribution in the cross-sections.
Figure 4. (a) The unit cell and NAPL distribution in the cross-sections, (b) pore-size distribution, and (c) NAPL saturation as a function of the location in the unit cell.
Precision and accuracy criteria, representativeness, completeness, and comparability criteria are being ensured by taking multiple samples from the same batch or column samples where applicable, performing replicates within treatment and replicate analyses, and establishing well-characterized and consistent initial and ongoing conditions with batch and column experiments and performing statistical analyses on the results.
Daily calibration curves for GC/FID analysis and other analytical instrumentation are used. For GC analysis, typically a 5-8 point calibration curve from multiple stocks is run. Check standards are run after every 10 samples.
Future Activities:
There are three main activities that will be completed during the extension: pressure-saturation column studies with 5 and 20 percent clay, DNAPL mobilization studies, and the numerical modeling of the pore size and DNAPL distributions. To investigate the effect of different clay fractions on the pressure-saturation relationship in clay-containing porous media, columns containing 5 and 20 percent clay will be packed. The three porous media will be brought to a residual DNAPL saturation and flushed with varying concentrations of IPA to measure the onset of mobilization. The next stage of the modeling work is to analyze the effect of 10 percent montmorillonite in the sandy media and relate the numerical results to the experimental results.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 8 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Matmon D, Hayden NJ. Pore space analysis of NAPL distribution in sand-clay media. Advances in Water Resources 2003;26(7):773-785. |
R827120 (2001) R827120 (2002) R827120 (Final) |
not available |
Supplemental Keywords:
cosolvent flushing, soil colloids, DNAPL, PCE., RFA, Scientific Discipline, Air, Toxics, Waste, Remediation, Environmental Chemistry, HAPS, chemical mixtures, Hazardous Waste, Engineering, Hazardous, Electron Microscopy, Engineering, Chemistry, & Physics, hazardous waste treatment, DNAPL, alcohol flushing, infrared spectroscopy sensor, interfacial phenomena, mass transfer, electrophoretic studies, hazardous chemicals, restoration, clayProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.