Chapter 13
Groundwater Remediation of Chromium Using Zero-Valent Iron in a Permeable Reactive Barrier 1
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Robert W. Puls , Robert M. Powell , Cynthia J. Paul , and David Blowes
Downloaded by STANFORD UNIV GREEN LIBR on September 24, 2012 | http://pubs.acs.org Publication Date: August 5, 1999 | doi: 10.1021/bk-1999-0725.ch013
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National Risk Management Research Laboratory, Robert S. Kerr Environmental Research Center, U.S. Environmental Protection Agency, Ada,OK74820 Powell & Associates Science Services, 8310 Lodge Haven, Las Vegas,NV89123 Department of Earth Sciences, University of Waterloo, Ontario N2L 3G1, Canada 2
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A series of laboratory experiments were performed to elucidate the chromium transformation and precipitation reactions caused by the corrosion ofzero-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 In Innovative Subsurface Remediation; Brusseau, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
Downloaded by STANFORD UNIV GREEN LIBR on September 24, 2012 | http://pubs.acs.org Publication Date: August 5, 1999 | doi: 10.1021/bk-1999-0725.ch013
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determine whether the use of a subsurface permeable reactive barrier (PRB) of zerovalent 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 οΠ 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 (Cr ,Fc, )(OH) (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 Fe The objectives of thefieldstudies included: Confirmation of effectiveness of Fe° to remediate chromiumcontaminated 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-scalefieldstudies at the Elizabeth City site. x
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Study Site Thefieldsite 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
