Environ. Sci. Technol. 2005, 39, 3762-3769
Evaluating Factors Affecting the Permeability of Emulsions Used To Stabilize Radioactive Contamination from a Radiological Dispersal Device GAREY A. FOX* Department of Civil Engineering, University of Mississippi, 208 Carrier Hall, Post Office Box 1848, University, Mississippi 38677-1848 VICTOR F. MEDINA Engineer Research & Development Center, U.S. Army Corps of Engineers, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180
Present strategies for alleviating radioactive contamination from a radiological dispersal device (RDD) or dirty bomb involve either demolishing and removing radioactive surfaces or abandoning portions of the area near the release point. In both cases, it is imperative to eliminate or reduce migration of the radioisotopes until the cleanup is complete or until the radiation has decayed back to acceptable levels. This research investigated an alternative strategy of using emulsions to stabilize radioactive particulate contamination. Emergency response personnel would coat surfaces with emulsions consisting of asphalt or tall oil pitch to prevent migration of contamination. The site can then be evaluated and cleaned up as needed. In order for this approach to be effective, the treatment must eliminate migration of the radioactive agents in the terror device. Water application is an environmental condition that could promote migration into the external environment. This research investigated the potential for water, and correspondingly contaminant, migration through two emulsions consisting of Topein, a resinous byproduct during paper manufacture. Topein C is an asphaltic-based emulsion and Topein S is a tall oil pitch, nonionic emulsion. Experiments included water adsorption/ mobilization studies, filtration tests, and image analysis of photomicrographs from an environmental scanning electron microscope (ESEM) and a stereomicroscope. Both emulsions were effective at reducing water migration. Conductivity estimates were on the order of 10-8 cm s-1 for Topein C and 10-7 cm s-1 for Topein S. Water mobility depended on emulsion flocculation and coalescence time. Photomicrographs indicate that Topein S consisted of greater and more interconnected porosity. Dilute foams of isolated spherical gas cells formed when emulsions were applied to basic surfaces. Gas cells rose to the surface and ruptured, leaving void spaces that penetrated throughout the emulsion. These experiments indicate that emulsions may be a viable means for containing RDD * Corresponding author phone: (662)915-5618; fax: (662)915-5523; e-mail:
[email protected]. 3762
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 10, 2005
residuals; however, improvements are needed for optimal performance.
Introduction A radiological dispersal device (RDD), or dirty bomb, disperses particulate radioactive contamination by the use of a conventional explosive (1, 2). In most cases, the amount of radioactive material present in a dirty bomb would likely not result in fatalities or cause radiation sickness. The main objective of a dirty bomb is to throw an area into a state of fear and panic, disrupting the daily life of millions of people. The most probable radioactive dirty bomb contains cesium137 due to its availability, dispersivity in the environment, and ease of handling (2, 5). Radiation may be carried several city blocks depending on environmental conditions (1), eventually settling on surfaces such as buildings, streets, grass, and soil. Without cleanup, the deposited radioactive contamination would render an area around the release point unusable (3-6). Current strategies for alleviating the contamination are limited. One approach is to remove contaminated materials from the site such that the remaining radiation levels meet public acceptance of the associated risk. The other alternative is to simply abandon areas near the release point until radioactivity decays to acceptable levels (3, 6). In both cases, there is a need to stabilize the affected areas in order to prevent or reduce migration of radioactive agents. In realizing the inherent limitations of these strategies, researchers are beginning to propose and develop alternative means for dealing with the radioactive contamination. On the basis of a review of publicly available refereed literature, the vast effort in cleanup of radioisotopes is focused on environmental applications (7), such as the cleanup of weapons production sites or uranium mines (8). Some of these methods could be useful, with some adaptation, to cleanup of dirty bomb attack sites. However, most environmental remediation approaches would not be directly applicable to dirty bomb cleanup operations due to the time required for the cleanup, their design for soil or groundwater applications (9, 10) as opposed to building surfaces, or their design for concentrated waste such as at nuclear waste facilities (8, 11-15). More useful would be products and techniques designed for cleanup of laboratory spills and for decontamination of radioactive fallout. Laboratory methods include various solvents and surfactants designed to wash glassware without scrubbing, thereby reducing exposure to radioisotopes, and absorbents to pick up small doses of spilled radiochemicals from surfaces (15, 16). Similar materials exist for decontamination of military equipment for combat in a nuclear battlefield. Spray-on foams are designed to wash away radioisotopes without scrubbing (17). Chemical foams and tents have also been developed that aim to suppress an explosive blast and trap radioactive particles thrown into it by the detonation. However, these chemical foams and tents require locating a terrorist bomb prior to detonation (18). As a proposed alternative to existing technologies for cleanup of building material surfaces and urban environments with peelable coatings based on poly(vinyl alcohol) and lignin (15) or clay suspensions (16), this research proposes a strategy for postdenotation decontamination involving the use of emulsions, asphalt or tall oil pitch, to stabilize the radioactive contamination released by a RDD. The proposed emulsions are widely available and commonly used in both 10.1021/es048710i CCC: $30.25
2005 American Chemical Society Published on Web 04/08/2005
asphalt pavement and soil stabilization practices. Emulsions would be sprayed as a thin film on surfaces within the contamination area and, upon setting, prevent migration of the contaminant. Emulsions are currently used as agents to stabilize heavy metals and hydrocarbons in soil (19-23). Anionic slow-setting aqueous-asphalt emulsions have been used to solidify a contaminated soil matrix. The asphalt emulsion successfully limited leaching of heavy metals and hydrocarbons in both water leaching tests and the toxicity characteristic leaching procedure (TCLP). Research is needed on the effectiveness of such emulsion methods for controlling radioactive contamination released by a RDD. One evaluation of effectiveness is the potential for water movement into, and possibly through, the emulsions as functions of coalescence and flocculation time and water evaporation during curing. Literature on water-emulsion interactions is limited. The primary research in emulsionwater interactions involves asphalt pavement. Williams and Miknis (24) used an environmental scanning electron microscope (ESEM) to visually determine liquid water penetration into asphalt. Void spaces within the asphalt were not connected, suggesting the asphalt was porous but not permeable. Robertson (25), on the other hand, determined that water could be transmitted through voids in asphalt. However, the porosity of such emulsion-based materials continues to be the subject of considerable debate.
Materials and Methods We considered two emulsions for use as RDD contaminant stabilizers: Topein C and Topein S formulated by Alon Inc. (Bakersfield, CA) (26). These emulsions are organic dispersion emulsions consisting of Topein, a resinous byproduct during paper manufacture (specific gravity of 0.99-1.01 at 25 °C, 42-48% solids content, and pH between 6 and 8.0). Topein C is a homogeneous emulsion of Topein and selected asphaltene resins. This emulsion is commonly used for extending the life of asphalt pavements and asphalt pavement surface treatments (23). Topein C consists of approximately 80-90% asphalt, 5-15% tall oil, 5% soap (emulsifier),
