This paper describes the results of the first field-scale demonstration conducted to evaluate the performance of nanoscale emulsified zero-valent iron (EZVI) injected into the saturated zone to enhance in situ dehalogenation of dense, nonaqueous phase liquids (DNAPLs) containing trichloroethene (TCE). EZVI is an innovative and emerging remediation technology. EZVI is a surfactant-stabilized, biodegradable emulsion that forms emulsion droplets consisting of an oil−liquid membrane surrounding zero-valent iron (ZVI) particles in water. EZVI was injected over a five day period into eight wells in a demonstration test area within a larger DNAPL source area at NASA's Launch Complex 34 (LC34) using a pressure pulse injection method. Soil and groundwater samples were collected before and after treatment and analyzed for volatile organic compounds (VOCs) to evaluate the changes in VOC mass, concentration and mass flux. Significant reductions in TCE soil concentrations (>80%) were observed at four of the six soil sampling locations within 90 days of EZVI injection. Somewhat lower reductions were observed at the other two soil sampling locations where visual observations suggest that most of the EZVI migrated up above the target treatment depth. Significant reductions in TCE groundwater concentrations (57 to 100%) were observed at all depths targeted with EZVI. Groundwater samples from the treatment area also showed significant increases in the concentrations of cis-1,2-dichloroethene (cDCE), vinyl chloride (VC) and ethene. The decrease in concentrations of TCE in soil and groundwater samples following treatment with EZVI is believed to be due to abiotic degradation associated with the ZVI as well as biodegradation enhanced by the presence of the oil and surfactant in the EZVI emulsion.
The explosive triacetone triperoxide (TATP) has been analyzed by gas chromatography/mass spectrometry (GC/MS) and sub-nanogram detection limits are reported by ammonia positive ion chemical ionization (PICI), electron ionization (EI) and methane negative ion chemical ionization (NICI). Analysis by methane PICI and ammonia NICI gave detection limits in the low nanogram range. Analyses were carried out on (linear) quadrupole and ion trap instruments. Analysis of TATP by PICI using ammonia reagent gas is the preferred analytical method, producing low limits of detection as well as an abundant (greater than 60% of base peak) diagnostic adduct ion at m/z 240 corresponding to [TATP + NH4]+. Isolation of the [TATP + NH4]+ ion with subsequent collision-induced dissociation (CID) produces extremely low abundance product ions at m/z values greater than 60, and the m/z 223 ion corresponding to [TATP + H]+ was not observed. Density functional theory (DFT) calculations at the B88LYP/DVZP level indicate that dissociation of the complex to form NH4+ and TATP occurs at energies lower than peroxide bond dissociation, while protonation of TATP leads to cleavage of the ring structure. These results provide a method for pico-gram detection levels of TATP using commercial instrumentation commonly available in forensic laboratories. As a point of comparison, a detection limit of 15 ng was obtained by flame ionization detection.
Emulsified zero-valent iron (EZVI) is a surfactant-stabilized, biodegradable emulsion that forms droplets consisting of a liquid-oil membrane surrounding zero-valent iron (ZVI) particles in water. INTRODUCTIONSignificant laboratory and field research has demonstrated that zero-valent metals will reductively dechlorinate dissolved chlorinated solvents such as tetrachloroethene (PCE) and trichloroethene (TCE) to ethene in groundwater (Gillham & O'Hannesin, 1994;Powell et al., 1998). Permeable reactive barriers (PRBs) containing zero-valent iron (ZVI) as the reactive material have been shown to be effective in treating plumes of dissolved chlorinated solvents (O'Hannesin & Gillham, 1998;Vogan et al., 1999). PRB technology is passive and requires no energy; however, it still relies on dense nonaqueous phase liquid (DNAPL) dissolution and transport of dissolved chlorinated solvents to the PRB for treatment, and, therefore, PRBs do little to reduce the cleanup time for sites where DNAPL is present. Nanoscale ZVI (NZVI) particles, either in a water slurry or as particles contained within an oil emulsion droplet (EZVI), have advantages over the conventional PRB applications since they may be injected deeper in the subsurface than is practical for conventional PRBs, and can be injected directly into DNAPL source areas to treat these areas directly. Nanoscale ZVI particles also have a much greater surface area than microscale or granular ZVI and, therefore, degrade contaminants at a faster rate. , 1997). Nanoscale ZVI is also more reactive than microscale ZVI or iron powders because the smaller particle size gives the NZVI a larger surface area per unit mass.The degradation of chlorinated solvents by ZVI regardless of particle size is believed to occur via both -elimination and reductive dechlorination at the iron surface and require excess electrons produced from the corrosion of the ZVI in water (Arnold & Roberts, 2000).The -elimination pathway involves the conversion of TCE, for example, to chloroacetylene, which is further dechlorinated to acetylene. Acetylene is subsequently degraded to ethene and ethane. Some degradation may also occur via sequential dechlorination where the target chemicals undergo sequential dechlorination steps, resulting in the formation of nonchlorinated hydrocarbon products (e.g., ethene and ethane).As a result of their high reactivity, NZVI particles are quickly surrounded by a passivating layer-such as a shell of oxide, which limits the ZVI corrosion rate (Nurmi et al., 2005). Zhang (2003) demonstrated that NZVI particles could remain reactive for six to eight weeks in a water suspension in the laboratory. However, these highly reactive nanoscale particles change over time, with handling, during storage (in a slurry of water), and with exposure to natural environments where constituents in groundwater will decrease the reactivity of the particle surface.Due to their very small size, NZVI particles may remain in suspension in groundwater and migrate downgradient of an injection point with ...
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