Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

Foam Injection for PFAS Washing? Main Properties to be Considered

Background/Objectives:

The treatment of PFAS is complex, but washing trials have shown interesting results. High concentrations of PFAS are often present in the unsaturated zone. The difficulty of washing in the unsaturated zone is that the water flows towards the water table by gravity. It has been established that PFAS have a particular affinity for air-water interfaces, which has led to the development of ex situ techniques using foam to extract them from extracted water. The use of foam in situ could therefore represent a treatment opportunity, bearing in mind that foam can retain and transport large quantities of PFAS, and could also facilitate horizontal transport in unsaturated zones.

Approach/Activities:

The theory of foam will be described in order to highlight the major parameters for field application. By definition, foam consists mainly of air and is therefore favorable to the transport of PFAS at the air-water interface. In the oil industry, foam is made from surfactants which are not always compatible with environmental standards in the soil. Therefore, the selection of the right surfactant is a crucial step of the project. Foam is a viscous fluid, so if it is injected with sufficient speed it creates a bubble around the well whose shape is not very sensitive to gravity. At a greater distance, the gas may tend to cause the foam bubble to rise up. It is therefore possible to push a foam front laterally to transport PFAS. The behavior of the foam is described by a parametric model calculating the apparent viscosity as a function of the type of surfactant and the characteristics of the medium. It is therefore necessary to carry out column tests to upscale to the field tests. A major limitation of the use of foam is the need for high injection pressures which cannot always be applied at shallow depths. However, recent research has shown that gas compressibility plays an important role in the foam behavior. The injection flow pattern can thus be specified to use the compressibility to enhance the radius of action.

Results/Lessons Learned:

As such we did not apply foam to transport PFAS. However, we have carried out various foam injection tests on real sites. The first test aimed to confine a source zone of chlorinated solvents located under a machine in a factory. This confinement was carried out using six wells surrounding the source zone, into which we injected foam for 96 hours at a pressure of 2 bar. Despite a radius of action which appeared to be less than 1 m, the flow of pollutant emanating from the source zone was divided by 4 to 5. The source is currently being treated using this mode of containment. On another site, foam injection was tested close to the surface in an environment polluted by hydrocarbons. The radius of action of the foam could be monitored using geophysical techniques and after 15 hours of injection it reached a distance of 2 m. Foam injection is therefore demonstrated but must be carried out with significant control before the field injection to adjust the quality of the foam in relation to the characteristics of the environment, as well as during the injection to identify the foam spatial distribution.

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Alexandre Vicard