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The regional gravel aquifer (RGA) at the U.S. Department of Energy's Paducah Gaseous Diffusion Plant (Paducah, Kentucky) is contaminated with several plumes of trichloroethylene (TCE) and technetium-99 (99Tc) that are migrating off-site, towards the Ohio River. One of the technologies under consideration for treatment is a permeable treatment zone (PTZ). To test the feasibility of this approach, a 30.5-m test section of a reactive material will be installed. Because of the depth of the groundwater in the RGA (18-38 m below the surface), the reactive material will be delivered in a guar gum suspension by hydraulic fracturing. The purpose of this laboratory study was to evaluate three reactive materials, alone and in combination, for possible use in the PTZ: medium cut zero valent iron (Master Builders), surfactant modified zeolite (SMZ), and granular Humate (Sundine Enterprises). It was hypothesized that a mixture of reactive material might be required to accomplish removal of TCE and 99Tc. Batch adsorption and kinetic tests as well as continuous flow column tests were conducted with groundwater and soil from the site, containing approximately 940 μg/L of TCE and 840 pCi/L of 99Tc. Feo alone was effective at removing both TCE and 99Tc, and hydrolyzed guar gum did not appear to interfere with Feo performance. Adsorption was initially the principal mechanism of TCE removal by Fe°, although the column results demonstrated that adsorption alone was inadequate to explain long-term performance. Ethene, ethane and acetylene were the predominant dechlorination products; the highest mass balance observed in the columns was 38% (mol products/mol TCE). It is not yet known how 99Tc is removed by Fe°, although reduction of soluble pertechnate (TcO4 ) to insoluble TcO2 is one possibility. SMZ showed a very high adsorptive capacity for 99Tc, but a very low capacity for TCE. Humate removed TCE and 99Tc by adsorption, but its capacity was low in relation to other organic adsorbents such as activated carbon. It was recommended that Fe° alone be used in PTZ. Since 3.6 mg/L of dissolved oxygen in the groundwater represents an even greater demand on the Fe° than the TCE plus 99Tc, a reduction in oxygen upgradient of the PTZ (e.g., with sulfide or an organic amendment) will help to extend the life of the Fe°.

Document Type: Research Article


Publication date: 2001-01-01

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