The relative importance of these different mechanisms depends on the ternary (water, cosolvent, NAPL) phase behavior of the specific system (Falta, 1998). Laboratory experiments have shown that cosolvents which preferentially partition into the NAPL phase are capable of mobilizing the NAPL as a separate phase due to swelling of the NAPL and reduction of interfacial tension. In cases where the cosolvent strongly partitions into the NAPL phase, the NAPL is effectively removed with about one pore volume of injected fluid. On the other hand, cosolvents which preferentially stay with the aqueous phase can dramatically increase the solubility of NAPL components in the aqueous phase. For systems in which the solvent preferentially partitions into the aqueous phase, separate phase NAPL mobilization is not usually observed and the NAPL removal occurs by the mechanism of enhanced dissolution. Under solubilizing conditions, the NAPL removal rate is dependent on the increase in solubility of the NAPL in the cosolvent:water flushing fluid.
Examples of cosolvents which preferentially partition into the NAPL include higher molecular weight miscible alcohols such as isopropyl and tert-butyl alcohol. Alcohols with a limited aqueous solubility such as butanol, pentanol, hexanol and heptanol can be blended with the water miscible alcohols to improve the overall phase behavior. Given a sufficiently high initial cosolvent concentration in the aqueous phase (the flooding fluid), large amounts of cosolvent will partition into the NAPL. As a result of this partitioning, the NAPL phase expands, and formerly discontinuous NAPL ganglia can become continuous, and hence mobile. This expanding NAPL phase behavior, along with large interfacial tension reductions allows the NAPL phase to concentrate at the leading edge of the cosolvent slug, thereby increasing the mobility of the NAPL. Under certain conditions, a highly efficient piston-like displacement of the NAPL is possible. Because the cosolvent also has the effect of increasing the NAPL solubility in the aqueous phase, small fractions of the NAPL which are not mobilized by the above mechanism will be dissolved by the cosolvent slug.
Surfactant additions above the CMC result in formation of additional micelles; i.e., the extramicellar surfactant concentration (the aqueous surfactant activity) is constant above the CMC. Winsor Type I micelles have a hydrophilic exterior (the hydrophilic heads are oriented to the exterior of the aggregate) and a hydrophobic interior (the hydrophobic tails are oriented towards the interior of the aggregate). Thus, micelles are analogous to dispersed oil drops; the hydrophobic interior of the micelle acts as an oil sink into which hydrophobic contaminants can partition. The increased global scale "aqueous solubility" of organic compounds at supra-CMC surfactant concentrations is referred to as solubilization; as the surfactant concentration increases, additional micelles are formed and the contaminant solubility continues to increase.
Winsor Type II surfactants are oil soluble (have a low hydrophile-lipophile balance -- (HLB), will partition into the oil phase, and may form reverse micelles). Reverse micelles have hydrophilic interiors and lipophilic exteriors; the resulting phenomenon is analogous to dispersed water drops in the oil phase. Surfactant systems intermediate between micelles Winsor Type I systems and Winsor Type II systems can result in a third phase with properties (e.g., density) between oil and water. This third phase is referred to as a middle phase microemulsion (Winsor Type III system). The middle phase system is known to coincide with ultra-low interfacial tensions; thus, middle phase systems will result in bulk extraction of organics from residual saturation. Surfactant enhanced remediation via this approach is often referred to as mobilization. Mobilization has the potential to be significantly more effective than solubilization, but adequate control of the density and viscosity of the NAPL phase to permit extraction of the NAPL is essential. Phase control at a DNAPL site is particularly critical.
Microemulsions are a special class of a Winsor Type I system in which the droplet diameter of the dispersed phase is very small and uniform. Droplet diameters of oil-in-water microemulsions generally range between 0.01 and 0.10 mm. These microemulsions are single phase, optically transparent, low viscosity, thermodynamically stable systems that form spontaneously on contact with an oil or NAPL phase. A properly designed microemulsion system is dilutable with water and can be transported through porous media by miscible displacement. This is in contrast to surfactant-based technologies that utilize Winsor Type III middle-phase microemulsions which depend on mobilization to transport the NAPL phase as an immiscible displacement process.
Microemulsions are usually stabilized by a surfactant and a cosurfactant. A mixture of water, surfactant, and cosurfactant form the microemulsion "precursor" and should also be a stable single-phase, low viscosity system. When this precursor is injected into a porous medium containing residual NAPL, the NAPL is microemulsified and can be transported to an extraction well as a single phase, low viscosity fluid. Suitable cosurfactants are low-molecular-weight alcohols (propanol, butanol, pentanol, hexanol, etc.), organic acids, and amines. There are many surfactants that will form oil-in-water microemulsions in the presence of alcohol cosurfactants. Some of these surfactants have been given direct food additive status by the FDA, are non-toxic, and are readily biodegradable.
Any surfactant-based remediation technology must utilize surfactants with optimum efficiency (i.e., minimal losses to sorption, precipitation, coacervate formation, or phase changes), environmental acceptance, and biodegradability. Surfactants can be lost from a solution by adsorption onto aquifer solid phases and by precipitation with polyvalent cations dissolved in ground water or adsorbed onto cation exchange sites. Surfactants without cosolvents sometimes create viscous macromolecules or liquid crystals when they combine with the contaminants essentially blocking fluid flow. Cosolvents can be used to stabilize the system and avoid macromolecule formation. It has been suggested that chromatographic separation of surfactants and cosurfactants could reduce microemulsification efficiency. However, experimental observations on systems containing 10 to 15 percent residual saturation indicate that, if chromatographic separation occurred, its effect on microemulsification was negligible.
Recovery and reuse of surfactants will improve the cost effectiveness of a remedial system. Designing a system to recover and reuse the system requires tradeoffs based on ease of recovery versus efficiency of the remedial system. Significant additional research is needed before optimal systems are implemented. The objective of this study is the demonstration of feasibility for remediation. Optimization studies were beyond the scope of the research.
Laboratory research has also shown that the solubilization power of cyclodextrins is not affected by ionic strength or pH. Cyclodextrins have been shown to reduce the sorption and retardation of compounds like pyrene and trichlorobiphenyl where retardation was reduced from 162 and 828, respectively, to approximately 2 in a laboratory soil column (Brusseau et al., 1994). It was also shown that cyclodextrin exhibited no sorption and retardation, and that pore exclusion did not occur.
Given the fact that cyclodextrins are sugar molecules, they are non-toxic to both humans and microorganisms. Thus, there are no health-related concerns associated with injection of cyclodextrins into soil. An additional advantage of cyclodextrins is that they have the potential to enhance in-situ bioremediation. Wang et al., (1995) have shown that the presence of cyclodextrins enhanced the rate of biodegradation of phenanthrene. Thus, it is possible that cyclodextrins can enhance both in-situ solubilization and in-situ bioremediation. The objective of this study is to extract the NAPL from the subsurface. Most technologies will have some form of transformation taking place. Biological transformations are not detrimental to NAPL extraction unless they result in significant loss of the remediating chemical (increasing the cost of extraction) or reduced permeability of the formation.
The in-situ steam enhanced extraction process is designed to remove volatile and semi-volatile organic compounds from an area of contaminated soil without the need for excavation. In a conceptual cleanup, vertical steam injection wells are placed within the region of contamination and extraction wells are placed within and around this region. Steam can be injected both above and below the water table, assuming contamination exists in both zones. The steam injection pressure must be higher than the hydraulic pressure of the aquifer to enable injection. In the extraction wells, contaminated ground water and NAPL are removed and the highest practical vacuum is applied. This aids in directing the steam toward the extraction wells. Some of the contaminant is pushed ahead of the condensing steam front and into the extraction wells. In the steam zone, the residual contaminant is volatilized at the elevated temperature and swept toward the extraction well by the flowing steam. After the entire soil mass under treatment has reached the steam temperature, as determined by soil-temperature monitors, and steam breakthrough occurs at the extraction wells, the flow of steam continues only intermittently with a constant vacuum applied to the extraction wells. The vacuum extraction removes much of the remaining contamination. As the soil in the high permeability region cools, the steam remaining in the low permeability region evaporates the contaminants. For a compound such as trichloroethylene (TCE), with a boiling point less than water, remediation should be very rapid.
The injection of steam can enhance contaminant removal through several mechanisms. First, relatively high pressure gradients develop in the steam zone due to the high vapor velocities. These pressure gradients facilitate displacement of formation water and contaminants. Liquids which are pushed into the well are pumped until steam breakthrough occurs. Application of a vacuum to the recovery wells during the injection of the steam aids in directing the flow toward the well through the vadose zone and contaminant recovery is identical to that of vapor venting until steam breakthrough. After breakthrough, the steam vapor acts in a manner similar to air during soil venting, only now the soil is also at an elevated temperature. The vapor pressures of typical organic compounds increase by factors from 25 to 40 over those at ambient temperature and thus evaporation rates are greatly accelerated. The rate of diffusion from low permeability materials into high permeability flow paths is also dramatically increased with elevated temperatures. The boiling point of mixtures is generally less than the boiling point of the compounds individually. Therefore, it is generally not necessary to elevate the temperature to 100 oC to initiate boiling. If there is sufficient increase in temperature, the fluids in low permeability layers may change phase enhancing the vacuum extraction of the contaminants.
Air sparging systems are designed to inject air below the water table through sparge wells. This process is analogous to aboveground air stripping treatment of water. As the injected gas rises through the saturated zone and contacts contaminated water or liquid-organic phase, VOCs transfer to the gas phase. The contaminated vapors emerge into the unsaturated zone where the gas is collected by SVE.
While both technologies are limited to removing only volatile contaminants, they provide a means of encouraging biological degradation of organic pollutants by supplying an active source of oxygen to the subsurface. Depending on the types of contaminants and the air flow rates, a fraction of the volatile chemicals can also be degraded in-situ (Wojick, 1998).
One potential advantage of the in-well sparging system in comparison to "normal" air sparging involves vapor transport in vertically stratified porous media. For normal air sparging, the contaminant is recovered by use of soil vapor extraction. However, the presence of a water-saturated, low-permeability stratum between the point of air injection and the vadose zone may impede the vertical movement of the air stream, thereby reducing recovery. This may affect the efficiency and safety of air sparging. The use of in-well aeration eliminates this potential recovery problem. Low permeability stratums are advantageous in in-well aeration systems because they increase the swept volume affected by each well. In addition, effectiveness may be enhanced because water, rather than air, is the fluid moving through the low-permeability stratum.
Although air stripping is the basic "treatment" operation involved in this technology, additional modes of treatment are possible. One such mode is biodegradation. The combination of in-well aeration and water recirculation results in continual introduction of oxygenated water into the formation. This additional oxygen can enhance the rates of biodegradation of labile organic contaminants when background concentrations of oxygen are low. Another possible treatment alternative is to emplace a catalytic agent in the well such that contaminants in the water will be transformed upon contact with the catalyst.