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Chapter 10
Treatability Study for a TCE Contaminated Area using Nanoscale- and MicroscaleZerovalent Iron Particles: Reactivity and Reactive Life Time Tanapon Phenrat1,2, Daniel Schoenfelder1, Mark Losi3, June Yi3, Steven A. Peck4, and Gregory V. Lowry1,2 1
2
Department of Civil and Environmental Engineering Center for Environmental Implications of Nanotechnology (CEINT) Carnegie Mellon University, Pittsburgh, PA, USA 3 Tetra Tech EC, Santa Ana, CA, USA 4 BRAC Program Management Office, San Diego, CA, USA
Nanoscale zerovalent iron (NZVI) is a potentially attractive tool for in situ source zone remediation of chlorinated solvents. Microscale zerovalent iron (MZVI) is already widely used as a reactive media within permeable reactive barriers (PRBs) for treating plumes of chlorinated organics (e.g. trichloroethylene) in groundwater. Several types of NZVI and MZVI are commercially available, each made by different processes, having different surface properties, and therefore likely to perform differently as remedial agents. Treatability studies are conducted at sites proposed for treatment to ensure the suitability of a specific type of NZVI for that site’s geochemical conditions. This laboratory study determined the rates of tricholoethylene (TCE) degradation, propensity to form chlorinated intermediates, the mass of TCE degraded per mass of ZVI added, and the effect of ZVI addition on the site geochemistry for five commercially available NZVI products and two MZVI products. The groundwater and aquifer materials used in this study were from a former naval air base in a marine location in California where ZVI products are proposed as remedial agents. The TCE dechlorination rates for the NZVI products were all faster than their MZVI © 2009 American Chemical Society
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In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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184 counterparts. TCE half-life times ranged from as little as 1.2 hours to as high as 89 hours for NZVI (2 g/L), and ranged from 200 hrs to 2000 hrs using MZVI (2 g/L). The reactive lifetimes of NZVI ranged from 3 days to more than 60 days. Generally, the fastest reacting material had the shortest reactive lifetime. Acetylene, ethene, and ethane were the dominant reaction products, but chlorinated intermediates persisted in reactors where NZVI had fully reacted, accounting for as much as 5 mol% of the TCE degraded in one case, but typically less than 1-2 mol%. The ratios of mass of TCE reduced to mass of iron added at the end of the NZVI particle reactive lifetime ranged from as much as 1:17 to as little as 1:120. The addition of NZVI or MZVI increased the pH from 7.3 to 7.8, but the high alkalinity of the site groundwater limited the pH increase. Differences in the TCE dechlorination rates and reactive life times between NZVI products were attributed to differences in their compositions (catalyzed vs. bare particles) and surface modifiers (polymeric modified vs. bare particles). Unfavorable geochemical conditions (high DO, neutral to acidic pH, and a high nitrate concentration) contributed to the relatively short reactive life times and low TCE dechlorination rates using NZVI products. Two NZVI products and two MZVI products were recommended as the most promising for application at this site.
Introduction Zerovalent iron (ZVI) in the form of bulk iron filings has been widely used as permeable reactive barrier (PRB) to treat groundwater plumes of chlorinated organics in situ for a decades (1-3). Recent advances in material science and nanotechnology gave rise to nanoscale zerovalent iron particles (NZVI), allowing greater utilization of Fe0 in the particles and providing higher reactivity compared to the bulk micron sized iron filings (MZVI) due to their small size and higher surface to volume ratio (4, 5). NZVI also provides the potential to install in situ reactive barriers deep in the subsurface using pneumatic injection (6), gravity feed, or geoprobes(4). Similarly, the small size of NZVI provides the potential to deliver it to the dense nonaqueous phase liquid (DNAPL) source area of a contaminated aquifer and accelerate the time to site closure (4, 7, 8). Several types of MZVI and NZVI are commercially available, each made by different processes and having different surface properties(4, 5, 9, 10). Some also have proprietary surface coatings to enhance their mobility in saturated porous media, making subsurface emplacement feasible (11). Dechlorination by ZVI is a surface meditated reaction; therefore, different ZVI products with different surface properties can yield different performance in term of dechlorination rate and by-product formation. Site geochemistry (e.g. dissolved solutes, pH, natural organic matter, and competing oxidants) can also affect the efficacy of both NZVI (12) and MZVI (13, 14) for trichloroethylene (TCE)
In Environmental Applications of Nanoscale and Microscale Reactive Metal Particles; Geiger, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
Downloaded by OHIO STATE UNIV LIBRARIES on June 28, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1027.ch010
185 degradation. However, the fundamental reasons for these effects are not fully understood so treatability studies are conducted at sites proposed for treatment to ensure the suitability of a specific type of ZVI for that site’s geochemical conditions. This laboratory study evaluates NZVI and MZVI products for treating a contaminated site at a former naval air base in a marine location in California. The groundwater and soil is contaminated with trichloroethylene (TCE) and some chlorinated daughter products including dichloroethene (DCE) and vinyl chloride (VC). The contaminated zone with the highest aqueous TCE concentration detected (2400 μg/L) is referred to as source reduction area (SRA) where cleaned up using NZVI is proposed, while MZVI is proposed to treat the plume in an area called the plume interception area (PIA). This study compares the effectiveness of five types of commercially available NZVI for source zone treatment and two types of commercially available MZVI for the treatment of the plume. NZVI particles were selected for the SRA because their small size (
