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4.5.7 Enhanced recovery methods (SRT)

Contents
4.5.7.1 Introduction
4.5.7.2 Enhanced recovery chemical agent methods
4.5.7.2.1 Displacement by inert electrolytes
4.5.7.2.2 Co-solvent solubilization
4.5.7.2.3 Surfactants and micro-emulsions
4.5.7.3 Enhanced recovery physical methods
4.5.7.3.1 Hydraulic and pneumatic fracturing
4.5.7.3.2 Air sparging and venting
4.5.7.3.3 In-well aeration

4.5.7.1 Introduction

In order to facilitate or accelerate the recovery of radionuclides or to lower residual concentrations in pump and treat scenarios, it may be desirable to chemically treat aquifers. Such methods are often termed ‘soil flushing’. After removal of the contaminant and before being re-injected, the pumped water is dosed with lixiviants, for example acid, surfactants, complexing agents such as ethylenediaminetetraacetic acid (EDTA) and other macro-molecules, or inert electrolytes to replace sorbed radionuclides. However, unwanted side effects, such as dissolution of the rock matrix, may be difficult to predict. Some of the available extraction methods are used in hydrometallurgy to enhance metal value recovery. Figure 4.21 shows the principal layout for the treatment of an aquifer, while Figure 4.22 shows the arrangement for treating the unsaturated zone above an aquifer [IAEA-2004b].
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Figure 4.21 Sketch of an in-situ leaching or enhanced recovery arrangement
Figure 4.21 Sketch of an in-situ leaching or enhanced recovery arrangement

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Figure 4.22 Sprinkling of contaminated soil in the vadose zone to remove contamination
Figure 4.22 Sprinkling of contaminated soil in the vadose zone to remove contamination

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Electrochemical methods for enhancing recovery of radionuclides in aqueous solutions have been proposed [IAEA-2004b]. If an electric field is applied to a solution, inorganic and organic ions migrate according to their charges to the respective electrodes (Figure 4.23). Two primary mechanisms transport contaminants through the soil towards one or the other electrode: electro-migration and electro-osmosis. In electro-migration, charged particles are transported through the substrate. Electrolysis arrangements concentrate metal ions on the cathode and can aid the oxidation of organic contaminants. In contrast, electro-osmosis is the movement of liquid containing ions relative to a stationary charged surface. The direction and rate of movement of an ionic species will depend on its charge, both in magnitude and polarity, as well as on the magnitude of the electro-osmosis induced flow velocity. Non-ionic species, both inorganic and organic, will also be transported along with the electro-osmosis induced water flow.

Different types of electrode material have been tested to improve performance, including porous ceramics and the rather novel carbon aerogels that increase the effective surface area. Electro-osmosis may be combined with other techniques to remove contaminants from low permeability geo-matrices such as clays. LASAGNA is a technology demonstration project designed to evaluate a combination of techniques [IAEA-2004b].
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Figure 4.23 Generic layout for remediation by electrolysis and electro-osmosis
Figure 4.23 Generic layout for remediation by electrolysis and electro-osmosis

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Other chemical methods are intended to increase the solubilities of contaminants by changing the redox conditions, by introducing complexing agents, solvents or surfactants. They are described in detail in Section 4.5.7.1.

In addition to the chemical methods, different methods to improve on the recovery rates have been developed:

  • Physical methods, e.g., hydraulic and pneumatic fracturing, air sparging and venting and in-well aeration. They are described in detail in Section 4.5.7.2.
  • Thermal methods are also applied to enhance the recovery of organic compounds and can be achieved by steam injection heating. They are described in detail in Section 4.5.8.1.
  • Biological methods, e.g., biological in-situ leaching appears to be especially suitable for large scale locations, such as former industrial sites. As compared with flushing with inorganic acid (Figure 4.21), biological leaching has the advantage of a higher removal efficiency and/or less damage to the soil matrix. Biological leaching either aims at lowering the pore water pH without adding acid and/or changing the redox conditions due to the biological activity, thus increasing the solubility of inorganic contaminants. A more detailed discussion of biological methods in general is found in Section 4.5.8.2.

4.5.7.2 Enhanced recovery chemical agent methods

Following are some enhanced recovery techniques based on chemical methods:

  • Displacement by inert electrolytes
  • Co-solvent solubilization
  • Surfactants and micro-emulsions
4.5.7.2.1 Displacement by inert electrolytes

If the retention of the contaminant is primarily controlled by adsorption processes, a reactive agent can be chosen to compete for the adsorption sites. The aquifer may be swamped with an inert electrolyte to replace contaminants from sorption sites on the geomatrix. The effectiveness of these methods depends very much on the nature of the contaminants and the geomatrix. Competition is usually most effective for ionic solutes and least effective in displacing neutral organic molecules partitioned into soil organic matter. In general, competition will be significant only when the adsorption sites are near saturation or when the affinity of the displacing ion for the sorption sites is significantly higher than that of the contaminant. The most effective cat-ion to replace sorbed radionuclides would be protons, as indeed are used in in-situ mining, but these would also affect acid dissolution of some matrix minerals, namely carbonates and oxy-hydroxides. Such dissolution of the matrix may be rather undesirable, because it affects the structural and hydrodynamic properties of the rock and consumes large quantities of acid. An inert, toxicologically acceptable and cheap cat-ion is the sodium ion administered in the form of NaCl (rock salt).

4.5.7.2.2 Co-solvent solubilization

The rate of removal of hydrophobic organic contaminants is often limited by their relatively low solubility in water. However, the solubilities of many of these contaminants are much greater in other solvents. Co-solvents are chemical compounds that are miscible in water and also have a certain affinity for non-aqueous phase liquids (NAPL). These co-solvents promote non-aqueous phase liquid removal through a number of complementary mechanisms, including: reduction of interfacial tension between the aqueous and non-aqueous phase liquid phases; enhanced solubility of the chemical contaminants (non-aqueous phase liquid components) in the aqueous phase; swelling of the non-aqueous phase liquid phase relative to the aqueous phase; and, under certain conditions, complete miscibility of the aqueous and non-aqueous phase liquid phases. The relative importance of these different mechanisms depends on the ternary (water, co-solvent and non-aqueous phase liquid) phase behaviour of the specific system [IAEA-2006b]. Co-solvents that preferentially partition into the non-aqueous phase liquid phase are capable of mobilizing the non-aqueous phase liquid as a separate phase due to swelling of the non-aqueous phase liquid and reduction of interfacial tension. In cases where the co-solvent strongly partitions into the non-aqueous phase liquid phase, the non-aqueous phase liquid is effectively removed with about one pore volume of injected fluid. Co-solvents that preferentially stay with the aqueous phase can dramatically increase the solubility of non-aqueous phase liquid in the aqueous phase, and removal occurs by enhanced dissolution rather than in a separate phase.

Given a sufficiently high initial co-solvent concentration in the aqueous phase (the flooding fluid), large amounts of co-solvent will partition into the non-aqueous phase liquid. As a result of this partitioning, the non-aqueous phase liquid phase expands, and formerly discontinuous non-aqueous phase liquid ganglia can become continuous, and hence mobile. This expanding non-aqueous phase liquid phase behaviour, along with large interfacial tension reductions, allows the non-aqueous phase liquid phase to concentrate at the leading edge of the co-solvent slug, thereby increasing the mobility of the non-aqueous phase liquid. Under certain conditions, a highly efficient piston-like displacement of the non-aqueous phase liquid is possible. Because the co-solvent also has the effect of increasing the non-aqueous phase liquid solubility in the aqueous phase, small fractions of the non-aqueous phase liquid that are not mobilized by the above mechanism will be dissolved by the co-solvent slug.

Examples of co-solvents that preferentially partition into the non-aqueous phase liquid include higher molecular weight miscible alcohols, such as isopropyl and tertbutyl alcohol. Alcohols with a limited aqueous solubility, such as butanol, pentanol, hexanol and heptanol, can be blended with water miscible alcohols to improve their overall phase behaviour.

In field applications the co-solvent mixture is injected uphill of the contaminated area. The solvent with the dissolved contaminants is extracted downhill of the contaminated area and treated above ground. Physical barriers may be installed to prevent uncontrolled migration of solvent and contaminants.

Co-solvents that are used as substrates by microbes may have the added advantage of promoting co-metabolism of primary contaminants. Small amounts of bio-degradable co-solvent that are difficult to remove from the subsurface will be of less concern because of their eventual transformation. Thus, co-solvents, such as alcohols, are potentially effective reactive agents for chemical enhancement for pump and treat of hydrophobic organic compounds.

Order of magnitude decreases in adsorbed contaminants are generally achieved with co-solvent concentrations greater than 20 %. Fluids containing this amount of co-solvent will have densities and viscosities that differ substantially from the groundwater. Thus, the transport behaviour of these fluids is more complex and more difficult to predict than that for fluids with homogeneous properties.

Co-solvent interaction with clays in the aquifer matrix may either increase or decrease the permeability of the soil. The formation of such high permeability pathways may be particularly troublesome at sites where dense non-aqueous phase liquids (DNAPL) are present. Co-solvents such as methanol can serve as a substrate for subsurface microbes, resulting in bio-fouling of the aquifer. Bio-transformation may substantially alter the geochemistry of the aquifer and promote the reductive dissolution of iron and manganese oxides. These metals can create problems with well clogging and interfere with surface treatment.

4.5.7.2.3 Surfactants and micro-emulsions

Surfactants are molecules that have both hydrophilic and lipophilic moieties. The amphophilic nature of surfactant molecules causes them to accumulate at interfaces, such as air-water, oil-water and water-solid, and significantly reduce the interfacial tension [IAEA-2006b]. Because of this property, surfactants are useful in enhanced oil recovery and may also be applied to remediation of non-aqueous phase liquid contaminated sites. Surfactants are classified by the nature of their head group. The different types are: cat-ionic, an-ionic, non-ionic and zwitterionic (both cat-ionic and anionic groups). Different types of surfactant can be more or less effective depending on the particular contaminant involved.

The surfactant must be chosen to be compatible with the solvent under the conditions of use. Inadequate surfactant formulations may result in high viscosity macro-emulsions that are difficult to remove. The surfactant can alter the wetting properties of the soil matrix and cause the non-aqueous phase liquid to become the wetting phase. The non-aqueous phase liquid would then occupy the smaller pores of the soil matrix, thereby exacerbating clean-up efforts.

Introducing alien substances, such as surfactants, into an aquifer is always a concern and may meet with resistance from regulatory authorities. It has to be shown that they are non-toxic and, if possible, bio-degradable; otherwise, the surfactant itself will have to be removed from the treated zone.

There are two main mechanisms by which surfactant can affect recovery of subsurface non-aqueous phase liquids: micellar solubilization and mobilization of the non-aqueous phase liquid due to reduced interfacial tension.

Micellar solubilization
A unique characteristic of surfactant molecules is their ability to self-assemble into dynamic aggregates known as micelles [IAEA-2006b]. The surfactant concentration at which micelle formation commences is known as the critical micelle concentration (CMC). Micelle formation generally distinguishes surfactants from amphophilic molecules (e.g., alcohols) that exhibit a much lower degree of surface activity and do not form micelles.
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Figure 4.24 The principle of micelle formation
Figure 4.24 The principle of micelle formation

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Figure 4.24 shows an example of a micelle. The presence of micelles increases the apparent solubility of the contaminant in water. This in turn improves the mass removal per pore volume. To determine the appropriate amounts of surfactant to add to the systems, batch or column experiments are usually performed. Such experiments have determined that surfactant additions are often rate limited. As the surfactant concentration increases, additional micelles are formed and the contaminant solubility continues to increase [IAEA-2006b].

Winsor Type I micelles have a hydrophilic exterior (the hydrophilic heads are oriented towards 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. Winsor Type II surfactants are soluble in oil, i.e., they have a low hydrophile-lipophile balance, 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 Winsor Type I micelle systems and Winsor Type II micelle 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 micro-emulsion (Winsor Type III system). The middle phase system is known to coincide with ultralow interfacial tensions; thus, middle phase systems will result in bulk extraction of organic compounds from residual saturation.

Micro-emulsions are a special class of Winsor Type I system in which the droplet diameter of the dispersed phase is very small and uniform. Droplet diameters of oil-in-water micro-emulsions generally range between 0.01 and 0.1 mm. These micro-emulsions are single phase, optically transparent, low viscosity, thermodynamically stable systems that form spontaneously on contact with an oil or non-aqueous phase liquid phase. A properly designed micro-emulsion system can be diluted with water and transported through porous media by miscible displacement. This is in contrast to surfactant based technologies that utilize Winsor Type III middle phase micro-emulsions that depend on an immiscible displacement process to transport the non-aqueous phase liquid phase.

Micro-emulsions are usually stabilized by a surfactant and a co-surfactant. A mixture of water, surfactant and co-surfactant form the micro-emulsion ‘precursor’ and should also be a stable single phase, low viscosity system. Low molecular weight alcohols (propanol, butanol, pentanol, hexanol, etc.), organic acids and amines are all suitable as co-surfactants. There are many surfactants that will form oil-in-water micro-emulsions in the presence of alcohol co-surfactants. Some of these surfactants have been given direct food additive status, for example by the United States Food and Drug Administration, are non-toxic and are readily bio-degradable so that there is little concern over their release into the environment.

However, it is important in applications that surfactant losses due to sorption, precipitation, co-acervate formation or phase changes are minimal, and that environmental acceptance and bio-degradability are assured. Co-solvents can be used to stabilize the system and avoid macro-molecule formation. Recovery and reuse of surfactants will improve the cost effectiveness of a remedial system. Designing a system to recover and reuse the system requires trade-offs based on ease of recovery versus efficiency of the remedial system.

Mobilization
The second mechanism utilized in surfactant treatment is non-aqueous phase liquid mobilization due to a decrease in interfacial tension. The interfacial tension between the groundwater and the non-aqueous phase liquid produces large capillary forces that retain the non-aqueous phase liquid. This is the reason that conventional pump and treat operations cannot remove the majority of non-aqueous phase liquid at a given site [IAEA-2006b]. As the interfacial tension diminishes, the phase becomes virtually miscible. This results in direct mobilization of the non-aqueous phase liquid. Caution must be exercised, however, because the surfactant could cause the contamination to spread too easily and too quickly. This is particularly true with dense non-aqueous phase liquids, which can quickly spread to underlying uncontaminated zones.

In the pump and treat scenario, dilute surfactant solutions are injected into the contaminated aquifer and withdrawn together with the solubilised dense non-aqueous phase liquids. Vertical circulation wells (VCWs) are an alternative application under consideration. The surfactant is injected from one screened section of the well and the contaminant plus the surfactant is extracted from another screened section. The possible advantages of using vertical circulation wells over the multi-well system are:

  1. Reduced cost;
  2. Effective hydraulic control over limited volumes of the formation;
  3. Ability to capture non-aqueous phase liquids that might sink when mobilized;
  4. Application to both light non-aqueous phase liquids and dense non-aqueous phase liquids;
  5. Minimal loss of surfactants;
  6. Reduced volume of fluid requiring treatment;
  7. Induced mounding, which can remediate portions of the contaminated vadose zone around the well.

4.5.7.3 Enhanced recovery physical methods

Following are some enhanced recovery techniques based on physical methods:

  • Hydraulic and pneumatic fracturing
  • Air sparging and venting
  • In-well aeration
4.5.7.3.1 Hydraulic and pneumatic fracturing

These mechanical methods to enhance recovery typically strive to improve the hydrodynamics of the system as a whole or of individual contaminants. Insufficient permeability or hydraulic connectivity can be overcome by hydro-fracturing techniques. These technologies are borrowed from the oil industry, where they were developed in the 1970s for deep wells, and it has recently been shown that the yield of wells for recovering contaminating liquids and vapours from low permeable media at shallow depths can be stimulated [IAEA-2006b].

The fracturing process begins with the injection of water into a sealed borehole until the pressure of water exceeds the natural in-situ pressures present in the soil or rock (e.g., overburden pressure and cohesive stresses) and at flow rates exceeding the natural permeability of the subsurface. A slurry of coarse grained sand and guar gel or similar mixture is then injected. As bedding planes and fractures open up in hard rocks, the sand helps to keep open fractures propagating away from the injection point. Fracture propagation distances of 10 – 20 m are common in hard rock, while unconsolidated materials, such as silts and clays, typically exhibit fracture propagation distances of 5 – 15 m. The oil industry also uses high strength solids, such as zirconia spheres, at greater depths, where higher lithostatic pressures have to be counteracted. The hydro-fracturing increases the effective surface area and the radius of influence of the abstraction wells and promotes a more uniform delivery of treatment fluids and accelerated extraction of mobilized contaminants.

The increased permeability and hydraulic connectivity may be of benefit not only in pump and treat systems but also for in-situ bio-remediation, oxidation/reduction de-chlorination and soil vapour extraction (SVE) applications. Delivery of liquid substrates and nutrients would be facilitated.

Alternatively, gases (air) may be used as a fracturing medium. Pneumatic fracturing allows treatment of the vadose zone for enhanced recovery of volatile contaminants. A comparative field demonstration of hydraulic fracturing to enhance mass recovery or emplace reactive barriers was conducted from the autumn of 1996 to the spring of 1998 at the Portsmouth Gaseous Diffusion Plant, Ohio. Hydraulic fracturing demonstrations showed that mass recovery increased from 2.8 to 50 times and radius of influence from 25 to 30 times for pneumatic fracturing at Tinker Air Force Base, Oklahoma. This demonstration treated chlorinated solvents (specifically tri-chloro-ethylene (TCE)) in both the vadose and saturated zones within low permeability silt and clay deposits and was shown to double the hydraulic conductivity and increase the radius of influence by 33 % [IAEA-2006b].

Cohesive or hard low permeability geological media with distinct bedding planes or a pre-existing network of fractures, such as clays, shales or sandstones, are the most appropriate for hydraulic fracturing.

The baseline against which hydraulic fracturing plus an in-situ remediation technology in low permeability media can be compared is excavation and ex-situ treatment. The advantages of hydraulic fracturing include:

  • Improved accessibility to contaminants and delivery of reagents (steam, oxidant, etc.) due to increasing permeability and hydraulic connectivity (e.g., improved mass transfer rates);
  • Limited site disruption minimizing adverse effects on surface features as fewer wells can be installed.

Hydraulic fracturing is applicable to a wide range of contaminant groups with no particular target group. Factors that may limit the applicability and effectiveness of the process include:

  • The technique should not be used in bedrock susceptible to seismic activity.
  • Investigation of underground utilities, structures or trapped free-phase contaminant is required.
  • A potential to open new pathways exists, leading to the unwanted spread of contaminants.
  • Pockets of low permeability may remain after using this technology.
  • It is almost impossible to control the final location and size of the fractures created.
  • Fractures are anticipated to collapse due to overburden pressure if not reached by the stabilizing media.
4.5.7.3.2 Air sparging and venting

In the unsaturated zone, volatile organic compounds (VOC) can exist in gaseous, aqueous, sorbed and liquid-organic phases. A venting system consists basically of wells, or ‘extraction’ vents, completed above the water table in zones of contamination, very similar to a pump and treat system below the water table. A pump is used to apply a vacuum that induces a subsurface gas flow pattern converging on the extraction vents. Prior to venting operations, the soil gas concentrations are in equilibrium with the existing contamination. The induced gas flow displaces the equilibrated soil gas with fresh air, resulting in mass transfer from the aqueous, sorbed and liquid-organic phases to the sweeping gas phase. Continuous subsurface flushing of fresh air leads towards an almost complete removal of the volatile organic compounds. Fresh air can be either injected through vents or allowed to seep in through the ground surface. The extracted contaminant vapours are collected from the extraction vents and treated as required.

Air sparging systems are designed to inject air below the water table through sparge wells. This process is analogous to above ground air stripping treatment of water. The process is based on increasing the gas exchange surface area and a steep distribution gradient into the clean air bubbles. As the injected gas rises through the saturated zone and contacts contaminated water or liquid-organic phase, volatile organic compounds transfer to the gas phase. The contaminated vapours emerge into the unsaturated zone, where the gas is collected.

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. The permeability of the gas bubbles is a limitation. An unwanted side effect could also be the oxidation of iron bearing groundwater, leading to voluminous oxidation products clogging the pore space. However, the iron oxy-hydrates that form may also provide a substrate for sorption and thus increase retention, if such is desired, for radionuclides and heavy metals.

4.5.7.3.3 In-well aeration

The in-well aeration technology is also known as a ‘vacuum vaporizer well’. This technology was developed in Germany and has been used at several sites [IAEA-2006b]. The conceptual basis of this technology is to use air to strip volatile contaminants from water inside a well casing. The essential design of the system involves two screened intervals and a pump to generate vertical recirculation of water within the saturated zone. Depending on type and distribution of contaminants, water flow is either upwards or downwards. Air from the surface is introduced into the well to serve as the stripping agent. A slight vacuum is imposed on the well to collect the contaminated vapour, which can be treated at the surface. The goal is to remove volatile contaminants from the water before they are pumped back into the aquifer. Operation of the system continues until all volatile contaminant mass has been removed from the swept volume of the aquifer (aqueous, sorbed and immiscible liquid phases).

One potential advantage of the in-well sparging system in comparison with ‘normal’ air sparging involves vapour transport in vertically stratified porous media. In normal air sparging, the contaminant is recovered by use of soil vapour 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 airstream, 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 strata are advantageous in in-well aeration systems because they increase the swept volume affected by each well.