Hydrofluorocarbon, catalytic reduction, hydrodefluorination, bimetal, groundwater
Pollution from hydrofluorocarbons (HFC) poses a serious challenge to the environmental community. Released from industrial operations, they have contaminated both the atmosphere and groundwater and are considered persistent in both media.1 For over the past 20 years, the practice of synthesizing hydrofluorocarbons as alternatives to chlorofluorocarbons (CFC) has been conducted in an effort to reverse the effects of stratospheric ozone layer depletion. 2,3 However, in doing so these new fluorinated compounds exhibited an unexpected property as a potent global warming greenhouse gas (GHG) with radiative forcing potentials in the range of 100 to 10,000 equivalents greater than carbon dioxide.4 Conversely, HFCs exhibit desirable properties as precision cleaning solvents due to their low surface energy but that use has lead to releases contaminating groundwater resulting in recalcitrant pollution in the form of dense nonaqueous phase liquids (DNAPL).5 The Environmental Protection Agency (EPA) has recently requested studies on the environmental impact of HFCs with respect to a number of petitions received from various environmental action groups imploring the use of the Montreal Protocol as the vehicle by which to achieve elimination of the compounds from industrial operations.6,7 Additionally, results from studies requested by the international community have shown HFCs to exhibit developmental and neurological damage in animal life along with their impact to humans remaining not completely understood.8,9,10 Therefore, the potential hazards of HFCs to human health and the environment necessitates the development of an effective and environmentally responsible technology for their remediation from groundwater. The National Aeronautics and Space Administration (NASA) has employed the use of various halogenated solvents in its spacecraft cleaning operations at its facilities for many years iv and in that time experienced accidental releases which eventually resulted in environmental contamination.11,12,13 Many of the organic solvents employed in these operations consisted of halogenated compounds with most being partially chlorinated and fluorinated hydrocarbons. Through normal use and operation, releases of these materials found their way into the environs of atmosphere, soil and groundwater. Remediation of fluorinated compounds has not followed the successful path laid by clean-up technologies developed for their chlorinated counterparts.14,15,16,17 Fluorinated compounds are resistant however to those methods due to their unreactive nature stemming from the properties of the strong carbon-fluorine bond. 18 This unique bonding property also ensures that their environmental persistence endures. 19 One particular fluorinated groundwater contaminant, the HFC 1,1,1,2,2,3,4,5,5,5-decafluoropentane (DFP), which serves as an excellent cleaning agent and has been used by NASA since the late 1990’s and still remains in use today, was selected as the focus of this study. 20 For this study, various reductive metal systems were evaluated for their capability towards effective degradation of DFP. These included the metals: iron, magnesium, aluminum and zinc and several bimetallic alloys as well as attempts employing some on carbon support. Variations in protic solvent reaction media and acidic metal activation were also explored. The bimetallic reductive catalytic alloy, magnesium with palladium on carbon support Mg(Pd/C) in aqueous media, proved to be the successful candidate with 100% conversion to simple hydrocarbons. Mechanistic evaluation for degradation is proposed via a series of stepwise catalytic reduction reactions. Kinetic studies revealed degradation to obey second order reaction kinetics. Further study should be conducted optimizing an in situ groundwater delivery method for field application. Additionally, the developed technology should be assessed against other v groundwater fluorocarbon pollutants; either as a method for remediating multiple fluorinated polluted sites or as a polishing agent where all other non-fluorinated halogen pollutants have been abated.
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Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dissertations, Academic -- Sciences, Sciences -- Dissertations, Academic
Tomlin, Douglas, "A Study Of The Degradative Capabilities Of The Bimetallic System: Mg(pd/c) As Applied In The Destruction Of Decafluoropentane, An Environmental Contaminant" (2012). Electronic Theses and Dissertations, 2004-2019. 2306.