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4.5.2 Immobilisation & solidification (CT)

Immobilization, in contrast to physical containment, is intended to treat the contaminated material itself. The objective of immobilization is to change the contaminant form into one that is less susceptible to migration. Two basic options can be distinguished: in-situ and ex-situ treatment [IAEA-2004b].

In-situ immobilisation treats contaminants without the contaminated material being removed. Three major methods to effect in-situ immobilization can be distinguished, based on chemical, bio-chemical or thermal treatments:

  • Chemical immobilization is based on the injection of a variety of grouts or on changing pH and/or redox conditions in the groundwater, for example [IAEA-2004b]. These grouts can be based inter alia on ordinary Portland cement (OPC), water glass (sodium silicate), gypsum or organic polymers, for example acrylic or epoxy resins. The suitability of immobilizing agents via injection depends largely on the hydraulic properties of the contaminated material. Ordinary Portland cement and epoxy resins typically have a high viscosity, while water glass and gypsum solutions, or acrylic acid suspensions, can be made up with viscosities equal to that of water. The long term stability of the polymer stabilized material has to be carefully assessed. Breakdown products containing functional groups, such as carboxylic or phenolic groups, may actually act as a vehicle to facilitate transport of radionuclides.
    Injection of chemical reductants, including calcium polysulphide, has been used to promote contaminant reduction and precipitation within aquifers. Contaminants that are well suited to remediation using this approach include metals with a lower solubility under reduced conditions. Injection techniques have been used to treat Cr (VI), through reduction to Cr(III) and precipitation of Cr(III) hydroxides. In-situ redox manipulation (ISRM) [IAEA-2004b] is a variation on a chemical injection system, see Figure 4.11. When using in-situ redox manipulation, a strong reductant is pumped into the aquifer, converting oxidized Fe(III) bearing minerals to Fe(II) bearing minerals. These reduced phases remain stationary, and react with oxidized dissolved contaminants that migrate through the treated zone in the aquifer. This approach has been demonstrated on a pilot scale to treat groundwater contaminated by Cr(VI) at the Hanford Site in south-eastern Washington State, USA [IAEA-2004b].
Figure 4.11 In-situ redox manipulation
Figure 4.11 In-situ redox manipulation

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  • Bio-chemical or biological immobilisation methods are based on the introduction or stimulation of micro-organisms that change the chemical environment [IAEA-2004b]. Depending on the circumstances and intentions, a (enzymatic) reductive or oxidative precipitation of radionuclides can be effected. The application would be similar to creating a bio-wall, discussed in Section 4.5.5.1.
  • Thermal immobilisation treatments use heat processes to immobilize the contaminant. Thermal treatment, however, generally is not economically efficient for dispersed radioactive contamination [IAEA-2004b].

Ex-situ treatments are carried out in some sort of plant, either on or off the site. After treatment, the material is either returned or disposed of in an engineered repository. A number of treatment techniques can be used for both in-situ and ex-situ treatments, the method of application varying in each case.

Organic polymers and water glass are also used to immobilize surface contamination. The main effect is to enhance the cohesive properties of topsoils, thus preventing wind and water erosion, see Figure 4.12. Depending on the formulation, infiltration of rainwater may also be impeded and thus the downward migration of radionuclides retarded.
Over the years consultants and contractors have developed a wide range of proprietary engineering applications based on the fundamental processes outlined above [IAEA-2004b].

Figure 4.12 Binding of soil particles and entrapment of contaminants using organic polymers
Figure 4.12 Binding of soil particles and entrapment of contaminants using organic polymers

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The advantage of immobilisation techniques employing inorganic agents is that these techniques typically need little follow-up and monitoring once their functionality has been verified. In the case of organic solidification agents, the possibility of bio-degradation has to be taken into consideration and some monitoring may be needed.
However, there are some factors that may limit the applicability and effectiveness of in-situ immobilization [IAEA-2006b]:

  • The depth of contaminants may limit some types of application techniques.
  • Certain contaminants are incompatible with solidification agents.
  • Reagent delivery and effective mixing are more difficult than for ex-situ applications.
  • Future use of the site may be limited after treatment.
  • Treatment of contamination below the water table may require prior dewatering.