United States Environmental Protection Agency Office of Solid Waste and Emergency Response (5102G) EPA 542-F-98-002 October 1996 www.ciu-m.com v>EPA Recent Developments for In Situ Treatment of Metal Contaminated Soils Fact Sheet and Order Information This report (EPA 542-R-97-004) provides status information on four in situ technologies for treating soil contaminated with metals: Electrokinetic remediation Phytoremediation Soil flushing Solidification/stabilization The report discusses different techniques currently in practice or under development; identifies vendors and summarizes performance data; and discusses technology attributes that should be considered during early screening of potential remedies. SUMMARY Metals are prevalent at most Superfund sites. Almost three-quarters of these sites with signed Records of Decision (ROD) are contaminated with metals as the sole contaminant group (approximately 16%) or in combination with other contaminant groups such as volatile or semi- volatile organic compounds. In situ remedies should be capable of providing cost savings, when compared with traditional disposal methods, and are being selected more frequently at Superfund sites. However, relatively few alternatives exist for the in situ treatment of metals. This report provides hazardous waste cleanup professionals with a status update on available and promising technologies for in situ remediation of soil contaminated with heavy metals. The report is intended to assist in screening new technologies early in the remedy evaluation and selection process. Electrokinetics Electrokinetic remediation relies on the application of low density direct current between electrodes placed in the soil. Contaminants are mobilized in the form of charged species, particles, and ions. Several organizations are source of contaflnrnation ,-- 'plume' of contaminants. direction of grouhdwater flow current lines Contaminated Plume Stopped by Electrokinetic Fence developing technologies for the enhanced removal of metals by transporting contaminants to the electrodes where they are removed and subsequently treated above ground. Experience with this technology is limited to bench and pilot scales, with the notable exception of a metal removal process that has been commercially operated by a single vendor in Europe and recently licensed in the United States. Limited performance data from this vendor illustrate the potential for achieving removals greater than 90 percent for some contaminants. A broad range of metals is potentially treated by this technology. The commercial applications in Europe treated copper, lead, zinc, arsenic, cadmium, chromium, and nickel. There is also potential applicability for radionuclides and some types of organic compounds. Electrokinetics also can be used to slow or prevent migration of contaminants by configuring cathodes and anodes in a manner that causes contaminants to flow toward the center of a contaminated area of soil. The practice has been named "electrokinetic fencing." Phytoremediation This technology is in the early stage of commercialization, and in the future may provide a low cost option under specific circumstances. At the current stage of development for treatment of metals, this process is best suited for sites with widely dispersed contamination at low concentrations where only treatment of soils at the surface (that is, within depth of the root zone) is required. Two basic approaches for metals remediation include phytoextraction and phytostabilization. Phytoextraction relies on the uptake of contaminants from the soil and their translocation into aboveground plant tissue, which is harvested and treated. Although hyperaccumulating trees, shrubs, herbs, grasses, and crops have potential, crops seem to be most promising because of their greater biomass ------- production. Nickel and zinc appear to be the most easily absorbed, although preliminary tests with copper and cadmium are encouraging. Significant uptake of lead, a commonly occurring contaminant, has not been demonstrated in any of the plants tested thus far. However, one researcher is experimenting with soil amendments that make lead more available for uptake by plants, Soil Flushing Soil flushing involves extraction of contaminants from soil using water or other suitable aqueous solutions. Although additives such as acids and chelating agents have had limited commercial use for full-scale ex situ soil washing projects, they have not been demonstrated as feasible for in situ applications. Soil flushing has been selected at seven Superfund sites with metals present; however, at six of those sites, organic contaminants are the primary targets. Soil flushing is most effective in removing hexavalent chrome, Spray Application **" ,. "** /""*" Pump Graundwaler Treitment Pump ~ Groundwater Treatment Contaminated Area Lcachate Collection 1 Low Permeability Zone due to its solubility in water. Two soil flushing remedies are currently ongoing at Superfund sites, with some preliminary data available from a hexavalent chrome application. Solidification/stabilization This process (also referred to as immobilization) changes physical characteristics of the waste in order to immobilize contaminants. Metals are commonly remediated by ex situ solidification with pozzolans and possibly other additives. This technology also can be adapted to an in situ technique through the use of various proprietary- augers that provide mechanisms for reagent delivery and mixing. In situ treatment likely will have a cost advantage over ex situ applications for larger volumes and for depths greater than 10 feet. However, in situ immobilization is rarely selected for Superfund use, largely because of concerns with thoroughness of mixing and long-term Typical Soil Flushing System (Surface Sprinklers) A second in situ solidification technique involves vitrification where an electrical current is passed between electrodes to melt soil and incorporate metals into a vitrified product. This technology is commercially available and has been successfully used at two Superfund sites. Both processes are broadly applicable to a range of metals. Vitrification uses a hood to capture mercury and other volatile metals, such as lead and arsenic, which may be partially vaporized during operations. Vitrification is best suited for wastes that are difficult to treat, such as mixtures of organics and metals. ORDER INFORMATION This report (EPA 542-R-97-004) is available free from the EPA National Center for Environmental Publication and Information (NCEPI) at 800-490-9198 or 513-489-8190, or fax 513-489-8695. It is also viewable and downloadable from the Internet at http://www.clu-in.com :_.«,; -:#*;$?--ttz "*:$::7X;-^**:'.&?%*."?'', f&*^ffi'',^&&?&--^3^ ?!&:*, * ' ^ .^\-*^-:>&.-'-''.';::':-'f-'&:*'-1 :j!i ,:* ,* ±ar,->'f? , "'»»'. .' - »". ,<-. - .- -''; »; '^-'J^.:-'.,£-. r«^*, ,*.... ,.^*raT., " ,.: .::-: ^^ '-.' ,.*$,, «".. ** W-->*m-w~^'-.-' 'ttf? «. .-J?_,l »a 1 BX - J-^f,:,^-::.... 1 ISBfej,»_,.'.:i..«U"' ». .wi.ilT'i.-rfSJt,:'-- - ------- |