Geotech Geol Eng (2010) 28:211–221 DOI 10.1007/s10706-008-9235-y ORIGINAL PAPER Technical Challenges to In-situ Remediation of Polluted Sites Krishna R. Reddy Received: 17 January 2008 / Accepted: 22 June 2008 / Published online: 16 July 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Throughout the world, subsurface contam- such as irregular soil layers and/or lenses). Under such ination has become a widespread and pervasive heterogeneous conditions, integrated electrokinetic problem. Toxic chemicals such as heavy metals and remediation technology has great potential. As a safe organic compounds are commonly used in a myriad of and economical remedial option for so many contam- industries. However, largely through inadvertent or inated sites, the application of integrated electrokinetic accidental release, these chemicals are presently remediation offers enormous public health, environ- polluting sites across the United States. In order to mental, and financial benefits. protect public health and the environment, further pollution must be prevented and sites with existing Keywords Electrokinetic remediation Á contamination urgently need remediation. Unfortu- Soils Á Pollution Á Heavy metals Á Organic compounds nately, remediating subsurface contamination has proved to be a daunting task. Heavy metals and organic compounds often coexist and their distribution within the subsurface is highly dependent on particle 1 Introduction and macro-scale heterogeneities. Vast resources have been invested to develop efficient remediation tech- Polluted sites pose a serious hazard to public health and nologies, yet very few of these technologies have been the environment. The United States Environmental successful. In-situ remediation is often preferred due to Protection Agency (USEPA) estimates that over minimal site disturbance, safety, simplicity, and cost- 217,000 sites require urgent cleanup at an estimated effectiveness. The effectiveness of in-situ remediation cost of over $187 billion (USEPA 1997). Soil and technologies depends largely on the contaminant groundwater contamination has been a major problem chemistry and subsurface heterogeneities (including at the sites in question. The contaminants encountered particle-scale heterogeneities such as fine-grained at these sites include metals (such as lead, chromium, soils, soils with reactive minerals, and/or soils rich in nickel, strontium, and uranium), volatile organic organic matter as well as macro-scale heterogeneities compounds (such as benzene, toluene, and trichloro- ethylene), and semi-volatile organic compounds (such as polycyclic aromatic hydrocarbons (PAHs) and K. R. Reddy (&) polychlorinated biphenyls (PCBs)). Organic and metal Department of Civil and Materials Engineering, contaminants are found to coexist at many sites. University of Illinois at Chicago, 842, West Taylor Street, Chicago, IL 60607, USA Recently, environmental professionals have e-mail: [email protected] focused on risk-based approaches to remediating 123 212 Geotech Geol Eng (2010) 28:211–221 polluted sites. Risk assessment includes detailed site implemented at numerous polluted sites (USEPA characterization, human and ecological risk quantifi- 2000, 2006). Poor performance may be attributed to cation, and selection of remedial goals (Sharma and the following technical difficulties: (1) clayey soils Reddy 2004). The end-use of the site is also taken are difficult to remediate because of low permeability into consideration in the risk assessment process. If and complex composition (mineralogy and organic the risk posed by the contamination is unacceptable, a content); (2) many common technologies fail under remedial action must be chosen and implemented to heterogeneous subsurface conditions (e.g., clay lenses achieve the remedial goals in an efficient and cost- within sand formation); (3) hydrophobic organic effective manner. Remediation of polluted sites can contaminants such as PAHs and PCBs are difficult help preserve green lands (pristine land) from pollu- to treat; (4) very few technologies are available to tion caused by industrial development as well as cleanup sites contaminated with heavy metals; and provide opportunities for economic growth. (5) very few technologies exist that can remediate Several technologies have been developed to remedi- sites contaminated by mixed contaminants (e.g., ate contaminated sites (Sharma and Reddy 2004). These organic compounds combined with heavy metals technologies can be grouped as either ex-situ or in-situ and/or radionuclides). There is an urgent need to technologies. Ex-situ remediation technologies involve develop new cost-effective technologies that can removing the contaminated soil and/or groundwater from overcome these challenges. In-situ electrokinetic the subsurface and treating it on-site or off-site. Con- remediation technology has great potential to be versely, in-situ remediation technologies involve treating one of these technologies (Acar et al. 1995; Page and the contaminated soil and/or groundwater in place Page 2002). without removing it from the subsurface. Often, in-situ An electrokinetic remediation system for the in- remediation technologies are preferred because they situ treatment of contaminated sites consists of result in minimal site disturbance and expose workers and drilling wells or drains in which electrodes are the surrounding public to the lowest amount of contam- installed and then applying a very low direct current inants. In addition, in-situ technologies are often less electric potential. Pumping and conditioning systems costly due to simpler procedures. Common in-situ soil may be needed at the electrodes in some situations. A remediation technologies include soil vapor extraction, schematic of in-situ electrokinetic remediation sys- soil flushing, solidification and stabilization, thermal tem is shown in Fig. 1. Similarly, electrokinetic desorption, vitrification, bioremediation, and phytoreme- treatment may be accomplished ex-situ by using diation. Common in-situ groundwater remediation specially designed above-ground reactors. Generally, technologies include pump and treat, air/ozone sparging, the contaminants accumulated at the electrodes are flushing, permeable reactive barriers, immobilization, removed by either adsorption onto the electrodes or chemical oxidation, and bioremediation. All of these withdrawal followed by treatment. Electrokinetic remediation technologies are based on physico-chemical, remediation offers the following advantages as com- thermal, or biological processes that aim to remove the pared to conventional remediation methods: (1) contaminants from soils and groundwater or to immobi- simplicity—minimal equipment requirements; (2) lize and/or detoxify the contaminants within the soils and safety—neither personnel nor the public in the groundwater. vicinity are exposed to contaminants; (3) wide range This article summarizes the common technical of contaminated media—can be used for soils, challenges in remediating contaminated sites in-situ sludges, sediments, and groundwater (particularly and subsequently provides a description of integrated well suited for low-permeability clays and heteroge- electrokinetic remediation, an alternative technology neous soil deposits within the vadose zone where that has the potential to surmount such obstacles. conventional remedial methods have proven to be ineffective or expensive); (4) wide range of contam- inants—can be used for metals, organic compounds, 2 Technical Challenges radionuclides, or combinations of these contami- nants; (5) flexibility—can be used as an in-situ or The USEPA has documented the inadequate perfor- ex-situ remediation system and is easily integrated mance of various remediation technologies with other remediation technologies such as 123 Geotech Geol Eng (2010) 28:211–221 213 Fig. 1 Schematic of in-situ electrokinetic remediation system bioremediation; and (6) cost-effectiveness—requires two processes govern the overall contaminant migra- low electrical energy leading to lower overall cost, tion in compact soil systems; and the role of the other with costs ranging from $20 to $225 per cubic yard two processes, electrophoresis and diffusion, is often depending on site-specific conditions. negligible. However, electrophoresis may be a sig- The successful implementation of electrokinetic nificant contaminant transport process in sludge and remediation requires a thorough understanding of the sediments. transport mechanisms and physico-chemical pro- Research conducted to date shows that the elec- cesses that affect the transport and fate of trochemical processes are quite complex and are contaminants under an induced electric potential influenced by the site-specific geochemical environ- (Acar et al. 1995; Alshawabkeh et al. 1999). The ment. As a result of the induced electric potential, major contaminant transport mechanisms under an electrolysis of water occurs at the electrodes. The induced electric potential are: (1) Electroosmosis— electrolysis reactions generate H+ ions and oxygen bulk movement of pore fluid through the electrical gas at anode and OH- ions and hydrogen gas at double layer in clayey soils, generally occurring from cathode. The gases may be allowed to emit into the anode to cathode; (2) electromigration—transport of atmosphere, while the H+ ions migrate towards ions and ion complexes within the pore fluid towards cathode and OH- ions migrate towards anode. oppositely charged electrodes; (3) electrophoresis— Depending on