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| Resource Library > Technology Transfer > Programs and Initiatives > Source Zone Treatment > Background > Cosolvent-Surfactant Extraction-Flushing |
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Cosolvent or Surfactant-Enhanced Remediation
Cosolvent or surfactant-enhanced remediation, also known as in-situ flooding or soil flushing, is intended to address the removal of residual NAPLs that have become trapped in the pore spaces of a water-bearing unit. The removal of NAPL contaminants frequently is inefficient and expensive using conventional technologies such as groundwater pump-and-treat, due to the low solubilities and rates of dissolution of NAPL contaminants. If it is not removed, NAPL will persist as a long-term source of contaminants to surrounding soils and groundwater, greatly prolonging the longevity of the associated groundwater plume.
Soil flushing is accomplished by passing the extraction fluid through in-place soils using an injection or infiltration process. Cosolvent or surfactant-enhanced remediation is designed to enhance the removal of NAPL from the subsurface by increasing the effective aqueous solubility of the NAPL and by reducing the interfacial tension between the water and NAPL phases. Extraction fluids must be recovered from the underlying aquifer and, when possible, they are recycled. The technology most often has been applied at sites contaminated by chlorinated solvents existing as DNAPLs. DNAPLs present a unique remediation challenge because they are denser than water and will migrate downward within a water-bearing unit until retarded by a low permeability layer. Therefore, DNAPL contaminants can move to depths that are not amenable to remediation by traditional methods such as excavation.
Cosolvent/surfactant flushing involves injecting a solvent or surfactant mixture (e.g., water plus a miscible organic solvent such as alcohol, or a special surfactant) into the vadose zone, the saturated zone, or both, to extract organic contaminants. Cosolvent/surfactant flushing can be applied to soils to dissolve either the source of contaminants or the contaminant plume emanating from it. The aqueous mixture normally is injected into, or upgradient of the contaminated zone, and the solvent/surfactant with dissolved contaminants is extracted simultaneously downgradient to maintain hydraulic control over the movement of the surfactant solution and the mobilized contaminants. Cosolvent/surfactant flooding is followed by water flooding to remove residual contaminants and injected chemicals. Conventional wastewater treatment technologies may be used to process the extracted effluent above-ground so long as cosolvent/surfactant foaming can be controlled, such as with the addition of an anti-foaming agent. To the maximum extent practical, recovered fluids should be re-used in the flushing process. The ability to separate cosolvents/surfactants from recovered flushing fluid, for re-use in the process, is a major factor in controlling the cost of soil flushing. Treatment of the recovered fluids results in process sludges and residual solids, such as spent carbon and spent ion exchange resin, which must be appropriately treated before disposal. Air emissions of volatile contaminants from recovered flushing fluids should be collected and treated, as appropriate, to meet applicable regulatory standards. Residual flushing additives remaining in soil may be a concern and should be evaluated on a site-specific basis.
Conceptual Representation of Surfactant-Enhanced NAPL Removal
Variations in aquifer permeability can reduce the performance of cosolvent/surfactant flushing. Because it is not possible to characterize all of the permeability heterogeneities in a system, it is advisable to use measures that will control the subsurface mobility of the cosolvent/surfactant solution when implementing this technology. By injecting polymer in the solution, or by creating a surfactant foam in the higher permeability zones, it is possible to avoid short-circuiting of the cosolvent/surfactant solution through higher permeability zones, thus improving the sweep of the flushing solution through lower permeability zones. The use of mobility control measures is subject to depth limitations because higher gradients are necessary to propagate a solution that contains polymer or to sustain foam through an aquifer. For sites having a permeability of 10-3 to 10-4 centimeters per second (cm/sec), a minimum depth of 20 to 25 feet to the underlying aquitard is necessary, while sites having lower permeabilities require a minimum depth of 50 feet to the aquitard. It may be cost prohibitive to apply cosolvent/surfactant flooding to remove NAPL from zones having permeabilities less than 10 -4 cm/sec, particularly when heterogeneities are present and mobility control measures cannot be implemented. Cosolvent/surfactant floods in low permeability systems must be carefully designed to avoid pore plugging.
Cosolvent/surfactant soil flushing can be used to treat VOCs, SVOCs, fuels, and pesticides, but it may be less cost-effective than alternative technologies for these contaminant groups. The majority of demonstrations for this emerging technology have been conducted at DNAPL-contaminated sites, and the technology has been applied to remove chlorinated solvents, creosote, and polychorinated biphenyl compounds (PCBs) from the subsurface. Recovery of other NAPLs, such as gasoline, jet fuel, and fuel oil also has been accomplished with the surfactant-enhanced recovery process.
The following factors may limit the applicability and effectiveness of the process:
- Subsurface heterogeneities can interfere with effective delivery and recovery of the surfactant solution and require the use of mobility control measures.
- Remediation technologies that rely on the addition of cosolvents/surfactants can suffer from inconsistent delivery and mixing of introduced cosolvents/surfactants with contaminants in the subsurface, and from limitations on recovering the chemical/contaminant solutions.
- Low-permeability soils are difficult to treat.
- Residual surfactants in the subsurface can have toxic effects.
- Regulatory approval to inject surfactants into an aquifer can be difficult to obtain.
- Surfactant flow may mobilize contaminants deeper into the aquifer or off-site if inadequate hydraulic control is maintained.
- Surfactant solution must be recovered and treated.
Duration of treatment is directly dependent on soil permeability. A full-scale application of SEAR can require from a few weeks to several months. Aquifer characterization activities recommended for design of cosolvent/surfactant systems may require periods of days to weeks. Any readily mobile free-phase NAPL in the system should be removed prior to surfactant injection.
Costs vary with soil permeability, heterogeneities in permeability, and scale of application. Cosolvent/surfactant costs can range from $200 per cubic yard of contaminated soil treated at a site where conditions are relatively homogeneous and the subsurface is permeable, to $1,400 per cubic yard of contaminated soil treated for a heterogeneous site having low-permeability conditions.
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