Introduction
The migration of solutes through fine-grained soils of low hydraulic conductivity (e.g., clays) is of considerable importance with respect to the design and performance of waste containment barriers (e.g., liners, cutoff walls) that are key components in landfills and other waste containment systems. Clay soils are used in such applications because a low hydraulic conductivity is necessary to minimize advective transport of contaminants through the barrier. In addition, research has demonstrated that significant membrane behavior is possible in clay soils containing a high content of bentonite, and that this behavior may result in enhanced waste containment performance in benonite-rich barriers such as geosynthetic clay liners (GCLs), sandbentonite liners, and soil-bentonite cutoff wall backfills (Kemper and Rollins 1966, Kemper and Quirk 1972, Malusis et al. 2001, Malusis and Shackelford 2002a, Malusis and Shackelford 2002b, Malusis et al. 2003). This enhanced waste containment performance is characterized by a reduction in solute flux through the barrier due to chemico-osmotic counter flow, solute exclusion at the interface between the soil and contaminant solution, and greater tortuosity of the pathways available for solute diffusion (e.g., see Mitchell and Soga 2005, Malusis and Shackelford 2002a, Malusis and Shackelford 2004).
There are two primary mechanisms that are responsible for solute exclusion due to membrane behavior in a clay soil. One mechanism is electrostatic repulsion of charged solutes (ions) by adjacent clay particles. This electrostatic repulsion is considered the primarily mechanism for enhanced containment of inorganic contaminants (e.g., heavy metals) by soil membranes, since such contaminants typically are present as cations in aqueous solution. The other mechanism for membrane behavior is a size restriction that occurs when solute molecules are larger than the pore size of the membrane. This latter mechanism, known as steric hindrance (see Grathwohl 1998), may contribute to exclusion of larger inorganic solutes and may be more significant for solutions containing organic solutes that are polyatomic and, thus, larger in size than inorganic solutes. Moreover, since steric hinderance represents a geometric restriction rather than an electrostatic restriction, exclusion of non-electro lye (uncharged) organic solutes is possible.
One commercially-available engineered waste containment barrier that has demonstrated the ability to restrict the migration of inorganic solutes due to membrane behavior is a geosynthetic clay liner (GCL) that consists of a layer of bentonite sandwiched between two geotextiles (e.g., Bentomat) (see Malusis and Shackelford 2002a). However, no testing has ever been performed to evaluate whether or not similar (or greater) membrane behavior occurs in a GCL for containment of organic solutes. Given that organic compounds such as chlorinated solvents (e.g., trichloroethylene), aromatics (e.g., phenol), polycyclic aromatic compounds (e.g., benzo(a)pyrene), pesticides (e.g., dieldrin) and plasticizers (e.g., phthalates) are recognized as common and widespread pollutants in soil and ground water (Grathwohl 1998), the potential for restricted transport of organic solutes through a GCL due to membrane behavior has significant practical implications for waste containment systems and warrants investigation. Thus, the objective of this research program is to investigate the existence of membrane behavior in a GCL exposed to organic solutes. The final outcome of this project is expected to be a refereedjoumal and/or conference paper. In addition, an on-campus presentation of the findings will be given during the annual Kalman Symposium in Spring 2007.
Process and Methods
Membrane (i.e., chemico-osmotic) tests will be carried out on specimens of a commerciallyavailable GCL (Bentomat; Colloid Environmental Technologies Company, Lovell, WY). The specimens will be prepared at a known void ratio (e) and pre-permeated to remove soluble salts and establish a baseline hydraulic conductivity (K). The chemico-osmotic tests will be performed in a constant-volume reservoir apparatus (see Figure I) using procedures similar to those described by Malusis et al. (200 I). Chemico-osmotic efficiency coefficients (ill) thatquantitatively describe the extent of membrane behavior will be determined by introducing contaminant solutions in the reservoir on one side of the GCL while maintaining distilled water in the other reservoir. The resulting difference in solute concentration across the membrane causes an induced differential pressure of a magnitude directly proportional to ill (see Malusis et al. 2001).
Two different types of organic solutes will be tested: (I) potassium acetate (KC2H3O2) and (2) phenol (C6HsOH). Use of the KC2H3O2 solution will provide comparison against the test results obtained using KCI (Malusis and Shackelford 2002a) and will facilitate evaluation of membrane efficiency for a compound containing a polyatomic organic anion (CH3CO2} Phenol (C6HsOH) has been chosen for this study, in part, because phenol is one of the top 200 hazardous substances on the 2005 CERCLA Priority List, as published by the Agency for Toxic Substances and Disease Registry ( Also, phenol exhibits a relatively high solubility in water such that phenol is expected to provide a conservative estimate of any membrane behavior that would be expected for a non-polar organic solute with a lower (i.e., more dilute) aqueous solubility. A total of four multi-stage chemico-osmotic tests (see Malusis et al. 2001) will be performed using four different solute concentrations and two different GCL thicknesses to determine the influence of solute concentration and GCL void ratio on membrane efficiency (see Table 1). During each test, the reservoir concentrations will be measured to determine that quantity of solute that migrates through the GCL.
Table 1 - Proposed Chemico-Osmotic Efficiency Tests.
Test No. / Organic Contaminant / Concentration / Specimen Thickness(mol/L) / (mm)
1 / KC2H3O2 / 0.004, 0.008, 0.02, 0.05 / 10
2 / KC2H3O2 / 0.004, 0.008, 0.02, 0.05 / 7.5
3 / C6HsOH / 0.004, 0.008, 0.02, 0.05 / 10
4 / C6HsOH / 0.004, 0.008, 0.02, 0.05 / 7.5
Jd= diffusive flux
delP = chemico-osmotic pressure difference
V R = volume of reservoir
A = cross-sectional area
L = specimen length
0 = 2-way valve
Figure 1 - Schematic of Constant-Volume Chemico-Osmotic Testing Cell.
Research Environment
All testing will be performed in the Bucknell geotechnical laboratory (Breakiron 63) and/or Professor Malusis' assigned project space (Breakiron 263). Necessary equipment for all experimentation is available on campus. Because Mentor-Student interaction is paramount for undergraduate research, extensive interaction will occur on essentially a daily basis. Professor Malusis will be available on campus throughout the summer on a full-time basis.
Faculty Endorsement
The proposed project represents an extension of my Ph.D. dissertation work conducted at Colorado State University (Malusis 2001) and is an integral component of my future scholarship plans at Bucknell. The project and this proposal have been developed by Mr. Scalia under my direct supervision. My experience with Mr. Scalia in CENG 350 (Fall 2005) indicates that he is a very capable and responsible student and works well in a laboratory environment.
Through our collaborative work associated with this project to date, I am confident that Mr. Scalia is capable of conducting the project successfully. The proposed research involves innovative and complex concepts, testing procedures, and equipment. As such, Mr. Scalia has taken the opportunity during his holiday break to review several papers on the topic and develop a solid understanding of the work involved with the project. In addition, Mr. Scalia is enrolled in my CENG 451 (Environmental Geotechnology) course in Spring 2006, a course that will further prepare him for this work. Lastly, I will be available on essentially a daily basis to provide oversight and assistance to Mr. Scalia for the duration of the project. In my opinion, Mr. Scalia will receive significant benefits from this project, including (1) potential co-authorship of a refereed publication, (2) hands-on experience in the laboratory that will better prepare him for future graduate research, and (3) an in-depth understanding of a state-of-the-art research area in geoenvironmental engineering.
References
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Kemper, W. D. and Rollins, J. B. (1966). Osmotic efficiency coefficients across compacted clays. Soil
Science Society of America, Proceedings, 30, 529-534.
Malusis, M. A. (2001). Membrane Behavior and Coupled Solute Transport through a Geosynthetic Clay
Line. PhD Dissertation, Colorado State University.
Malusis, M. A. and Shackelford, C. D. (2002a). Chemico-osmotic efficiency of a geosynthetic clay liner.
Journal of Geotechnical and Geoenvironmental Engineering, 128(2),97-106.
Malusis, M. A. and Shackelford, C. D. (2002b). Coupling effects during steady-state solute diffusion
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Malusis, M. A. and Shackelford, C. D. (2004). Explicit and implicit coupling during solute transport
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Malusis, M. A., Shackelford, C. D., and Olsen, H. W. (2001). A laboratory apparatus to measure
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Malusis, M. A., Shackelford, C. D., and Olsen, H. W. (2003). Flow and transport through clay membrane
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Mitchell, J. K., and Soga, K. (2005). Fundamentals of Soil Behavior, 3rd Ed., John Wiley and Sons, New
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