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  • Chapter 13

    Groundwater Remediation of Chromium Using Zero-Valent Iron in a Permeable Reactive Barrier Robert W. Puls1, Robert M. Powell2, Cynthia J. Paul1, and David Blowes3

    1National Risk Management Research Laboratory, Robert S. Kerr Environmental Research Center, U.S. Environmental Protection Agency,

    Ada, OK 74820 2Powell & Associates Science Services, 8310 Lodge Haven,

    Las Vegas, NV 89123 3Department of Earth Sciences, University of Waterloo,

    Ontario N2L 3G1, Canada

    A series of laboratory experiments were performed to elucidate the chromium transformation and precipitation reactions caused by the corrosion of zero-valent iron in water-based systems. Reaction rates were determined for chromate reduction in the presence of different types of iron and in systems with iron mixed with aquifer materials. Various geochemical parameters were measured to confirm the proposed reactions. Laboratory experiments were scaled up to pilot and full-scale field demonstrations. Intensive geochemical sampling in the field tests corroborate laboratory results and successfully demonstrate the effectiveness of this innovative in situ approach to remediate chromate contaminated ground water using a permeable reactive barrier composed of zero-valent iron.

    A great deal of recent interest has been focused on in situ techniques for treating contaminated ground water. One of the most promising of these techniques is the use of zero-valent metals in permeable reactive subsurface barriers for intercepting and remediating contaminant plumes. It has been shown that zero-valent iron (Fe) is effective for reductively dehalogenating halogenated hydrocarbons, such as trichloroethene (TCE). It can also reduce chromium from Cr(VI) to Cr(III), causing it to precipitate as an immobile and non-toxic hydroxide solid phase under most environmental conditions. Although these processes have been attributed to corrosion of the iron and cathodic reduction of the contaminants, there was little understanding of the reduction and precipitation transformation mechanisms. This research was implemented to develop an understanding of these processes and effects.

    Purpose and Objectives

    This research had a number of specific objectives, but its overall purpose was to

    182 1999 American Chemical Society

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    In Innovative Subsurface Remediation; Brusseau, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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    determine whether the use of a subsurface permeable reactive barrier (PRB) of zero-valent iron (Fe) would remediate a chromate plume at the U.S. Coast Guard (USCG) Air Support Center in Elizabeth City, North Carolina. Although not a major focus ofthis study, il was also expected that the iron PRB would reduce or eliminate the overlapping plume CE that would intercept the wall (/).

    It is known that in aqueous systems the distribution of chromium forms is strongly dependent on pH and redox potential (Eh) (2, J). It occurs in two stable oxidation states in the subsurface environment, Cr(Vl), or chromate, and Cr(IKI) ('/). The eliminate forms arc of greatest concern due to their toxic and carcinogenic properties and their greatly increased subsurface mobility compared to the relatively immobile and nontoxic Cr(Ill) and its species. The U.S. EPA has set the drinking water MCL (maximum contaminant level) for chromium at 100 /wg/L.

    When reduced from Cr(VI) to Cr(tll), Cr is removed from solution as Cr(OII),, a precipitated phase or, when dissolved iron is present, potentially as a chromium-iron hydroxide solid solution (Crx,Fc, x)(OH)3(ss). The formation ofthis solid solution is desirable for remediation, because precipitated Cr is less soluble in this form (5). In addition, the removal of Cr from solution had been demonstrated to occur in the presence of zero-valent iron (6).

    The specific objectives of the laboratory studies were: Determination of the rates of reactions for chromate reduction and precipitation by Fe

    * Developing an understanding of the reaction mechanisms Understanding the effects of native geochemistry on these rates and

    mechanisms * Optimizing remediation of the Elizabeth City site with respect to

    the type(s) of Fen The objectives of the field studies included:

    Confirmation of effectiveness of Fe to remediate chromium-contaminated ground water in situ using permeable reactive barriers and

    Evaluation of the effectiveness of barrier installation methods. To accomplish these goals, a number of laboratory experiments were carried out,

    along with pilot-scale and full-scale field studies at the Elizabeth City site.

    Study Site

    The field site is located at the USCO Air Support Center near Elizabeth City, North Carolina, about 100 km south of Norfolk, Virginia and 60 km inland from the Outer Banks region of North Carolina. The base is located on the southern bank of the Pasquotank River, about 5 km southeast of Elizabeth City. Hangar 7

  • 184

    sequences of suificial sands, silts and clays. Ground-water llow velocity is extremely variable with depth, with a highly conductive layer at roughly 4.5 to 6.5 meters below ground surface. This layer coincides with the highest aqueous concentrations of chromate and chlorinated organic compounds. The ground water table ranges from 1.5 to 2.0 m below ground surface.

    Materials and Methods

    Chromate Laboratory Studies. The experimental designs, materials setups and evaluations of the chromate laboratory studies have been extensively described elsewhere (

  • 185

    Aller allowing a period of approximately 48 hours for the systems (solution or solution plus aquifer material) to approach equilibrium with respect to pH and Eh, 8 grams of the Al&M Fe tilings were added to each reaction vessel. After a second equilibration period of approximately 60 hours, additions were made from a stock K2Cr04 solution, resulting in initial aqueous Cr042" concentrations in the eactor systems ranging from 136 to 156 mg/L. Baseline samples were withdrawn from the SBRs prior to the chromate addition as well as periodic withdrawals after the addition. All samples were immediately filtered through pre-weighed 0.2 pore diameter Gel man nylon membrane filters. An aliquot of the filtrate was removed for sulfate and chloride analysis by capillary electrophoresis. The remaining filtrate was acidified to pH < 2.0 with ultra-pure nitric acid and analyzed by inductively-coupled plasma emission spectroscopy (ICP). The solids and filters were dried at 70C and submitted for complete digestion and 1CP analysis.

    Shaken Batch Experiments. To evaluate effects of the variables aquifer material type, solid:solution ratio, iron mass and type, and S042~ concentration, shaken batch bottle (SBB) experiments were used. Such batch experiments are useful in differentiating effects from multiple variables in a relatively efficient experi mental fashion. A pseudo-factorial design was employed to reduce the number of replicates and repeated variables. These experiments were done in shaken Oak Ridge polyallomer centrifuge tubes in a N2-filled glovebox. They were set up with 5 concentrations or masses of each variable, in duplicate.

    Tubes of SAW and aquifer material were secured horizontally on a Thermolyne Maxi-Mix III rotating shaker at 300 RPM and equilibrated for 24 hours. Eh and pH were then measured in the glovebox, Fe metal added, and the samples returned to the shaker. After 48 hours, Eh and pll were again measured and CrO/" added (t,, = 0) to give a concentration of 126 mg Cr/L. The tubes were sampled in the glovebox at t = 20 minutes, 2, 7,24,48, and 96 hours. Samples were immediately filtered and analyzed as given previously for the SBRs.

    Pilot-Scale Field Test. A pilot-scale field test was initiated at the USCG site in November 1994 by researchers from the National Risk Management Research Laboratory of the USEPA. It was the first such test to evaluate the use of zero-valent iron to remediate a chromium-contaminated aquifer.

    Reactive Mixture. Based on results of the laboratory studies, zero-valent iron was mixed with aquifer material and a coarse sand to evaluate the effectiveness of the iron to remediate dissolved chromate at the Elizabeth City site. Two sources of iron were mixed and used in the field test. Low-grade steel waste stock, obtained from Ada Iron and Metal (Al&M, Ada, OK), was turned on a lathe (without use of cutting oils) using diamond bits to produce 200 L of turnings. The other iron material was obtained from Master Builder's Supply (MBS, Streetsboro, Ohio). The latter material was primarily in the 0.1 to 2 mm size range while the former was primarily in the 1 to 10 mm size range. The MBS iron exhibited greater total sulfur and carbon (as graphite) content than the AI&M iron.

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    In Innovative Subsurface Remediation; Brusseau, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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    Mixture Components %Vol % Wt.

    Particle Siz