Date of Award

5-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Engineering

Advisor

Murdoch, Lawrence C

Committee Member

Falta , Ronald W

Committee Member

Freedman , David L

Committee Member

Looney , Brian B

Committee Member

Molz , Fred J

Abstract

Remediation of contaminated vadose zones is often hindered by an inability to effectively distribute liquid- or solid-phase amendments. Many amendment-based approaches, such as bioremediation, chemical oxidation, and reactive barriers have been successful in saturated formations. However, these remedial approaches have seen limited application in unsaturated materials, largely because of difficulties delivering amendments.
Aerosol delivery is a promising approach for distributing amendments in contaminated vadose zones. The amendments are aerosolized, creating a cloud of micron to sub-micron-scale liquid droplets held suspended in a gas by Brownian motion. During injection into porous media, the aerosol particles are transported along with the gas until they are deposited on the surfaces of soil grains. The process is continued until appropriate amendment concentrations are achieved, ideally resulting in a radially and vertically broad distribution. Such a distribution could not be achieved by injecting pure liquid-phase solutions.
The objectives of this work were A) to characterize transport and deposition of aerosols in unsaturated porous media, B) to develop capabilities for predicting results of aerosol injection scenarios at the field scale, and C) to evaluate biodegradation of trichloroethene (TCE) under partially saturated conditions when amendments and/or microbes are delivered as aerosols. Aerosol transport and deposition processes were investigated by conducting lab-scale injection experiments. These experiments involved injection of aerosols through sand in columns or in a wedge-shaped apparatus that creates a radial flow geometry. A particle-size analyzer was used to measure aerosol particle size distributions along gas flow paths through the sand-filled geometries with time, and sand samples were taken following injection for amendment content analysis. Predictive capabilities were obtained by developing a numerical model capable of simulating aerosol transport and deposition in porous media. Stimulation of TCE biodegradation using aerosolized amendments (electron donor, nutrients, and bioaugmentation culture) was investigated by constructing anaerobic microcosms in 160 ml serum bottles. Headspace samples were analyzed for TCE, cis-dichloroethene (cDCE), vinyl chloride (VC), and ethene to determine the rates and extent of biodegradation within each bottle. Multiple sets of microcosms were created to determine differences based on water saturation, electron donor, and amendment delivery method.
According to particle analysis, aqueous aerosols used during experimentation tended to occur over a 0 to 2 micron particle size range, whereas soybean oil and salt water aerosols tended to occur over a 2 to 6 micron range. Results of aerosol injection tests show that aerosol transport and deposition depend on the liquid used. Results from tests involving soybean oil aerosols show that oil saturations greater than 0.5 g oil/kg sand could be achieved throughout the sand-filled laboratory cells. Lab-scale tests conducted with aqueous (fresh water) aerosols show that liquid accumulation only occurs near the point of injection. Tests conducted using 200 g/L salt water (NaCl) confirm that changes in water saturation were small and limited to the vicinity of the injection region. However, aerosol particles were measured, and salt was deposited throughout the lab-scale apparatuses, even though liquid accumulation was negligible. Apparently, solid particles of salt were created and transported as the water evaporated.
A numerical model was developed based on a fully coupled transient analysis of gas and liquid in a porous medium. Aerosols were assumed to be advected in the gas and deposited as an irreversible sorption process governed by a collector efficiency. Changes in saturation resulting from aerosol deposition altered the relative permeability and mobility of the liquid and gas phases in the fully coupled system. The numerical model was calibrated using results from the laboratory test, and then used to evaluate aerosol injection at the field-scale. Modeling results suggest that gas injection rates and aerosol particle size are the most important factors in determining amendment distribution. Liquid saturations from field-scale simulations suggest that effective radii of influence on the scale of 3-4 meters around a well screen could be achieved in partially saturated sand.
Microcosm results show that anaerobic reductive dechlorination of TCE can occur in unsaturated systems. Significant differences in degradation activity were not observed based on whether amendments in aqueous solution were added directly or as an aerosol. Addition of a chloroethene bioaugmentation as aerosols resulted in complete conversion of TCE to ethene, however, initial reaction rates were typically slower than when culture was added directly. Delivery of culture in an oil-based mixture was tested as an alternative to overcome potential limitations with water aerosol delivery. Direct inoculation of microcosms with the mixture resulted in dechlorination of TCE, however, aerosol delivery of the same mixture resulted in little activity.
The aerosol delivery process appears to be capable of distributing oil amendments over considerable volumes of formation at concentrations appropriate for remediation purposes. Evaporation of water limited liquid accumulation when using aqueous aerosols. However, results from salt water experiments suggest that solid-phase aerosols created during the evaporation process can effectively distribute water soluble amendments (electron donor, pH buffer, oxidants, etc.). Successful implementation at the field-scale will require site-specific optimization of injection parameters and aerosolizers that are specifically designed to produce preferentially small particles. Utilization of aerosol delivery could considerably expand treatment options for contaminated vadose zones at a wide variety of sites.

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