Question 5 of 11: In-Situ Vitrification

Question 5: In-Situ Vitrification (5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

In-situ vitrification (ISV) treats contaminated soil in place by converting it into a durable, glass-like solid, combining thermal destruction of organics with permanent immobilization of inorganics in a single process:

1. Electrode placement and starter path. An array of large graphite/molybdenum electrodes is inserted into the ground around the contaminated zone, and a conductive graphite/flake material is laid on the surface between electrodes as a starter path (native soil is not initially conductive).
2. Joule heating and melt initiation. A high electric current passed between electrode pairs resistively heats the starter path and the surrounding soil to extreme temperature (roughly 1600–2000°C), well above the melting point of most silicate soils, initiating a molten glass pool.
3. Melt growth. As soil melts, the molten (electrically conductive) zone itself becomes the current path, and the melt front grows downward and outward, progressively engulfing the contaminated volume.
4. Organic destruction. Organic contaminants within and ahead of the melt front are pyrolyzed/combusted at the extreme temperature; the resulting off-gas (a mixture of combustion products and volatilized organics) is captured under a negative-pressure hood and treated (thermal oxidizer, scrubber, particulate/HEPA filtration) before atmospheric release.
5. Inorganic immobilization and cooling. Heavy metals and radionuclides are incorporated directly into the silicate melt structure; once power is removed, the melt cools and solidifies over weeks to months into a chemically durable, extremely low-leachability vitrified (obsidian-like) block that remains in place, achieving a large volume reduction relative to the original soil.

ISV is chiefly used where excavation is undesirable or infeasible — deep contamination, mixed radioactive/chemical (hazardous) waste, or sites where disturbing the soil would itself create an unacceptable exposure — because it treats both the organic and inorganic contaminant fractions simultaneously without ever removing the soil from the ground.