1,4-Dioxane (not to be confused with dioxin…although we also can certainly thermally treat that! See our updates from the Danang Vietnam project) is an industrial chemical widely used as a chemical stabilizer for chlorinated solvents. It is of environmental concern due to its classification as a probable human carcinogen coupled with its unique chemical properties such as high water solubility (and therefore high subsurface mobility in groundwater), low octanol-water partition coefficient (log Kow), and relative resistance to biodegradation . Due to these properties, 1,4-dioxane is often present on the leading edge of a contaminant plume in groundwater and presents unique remediation challenges.
TerraTherm recently had the opportunity to perform a comprehensive bench scale thermal treatability study on soils contaminated with a mixture of volatile and semi-volatile organic compounds, including 1,4-dioxane. Three soil materials (two different sandy materials and one clay material) were subjected to various thermal treatment techniques including Thermal Conductive Heating (TCH), otherwise known as In Situ Thermal Desorption or ISTD) at 100°C, Steam Enhanced Extraction (SEE), and two lower temperature hot water flushing applications at 50°C and 75°C. In addition, a static degradation test was performed where site soils and groundwater were slowly heated in a closed system to explore the effects of heat alone with no physical flushing.
SEE bench scale testing was performed, and as expected proved to be the most effective remedy for both of the sandy site materials, removing over 99.99% of the 1,4-dioxane (which had initial untreated concentrations around 10,000 mg/kg, present as non-aqueous phase liquid [NAPL] spiked into the site materials). Hot water flushing of the sandy materials at two different temperatures (50°C and 75°C) also proved effective to a slightly lesser degree. Interestingly, TCH treatment (heating to 100°C, and removing soil moisture equivalent to 50% of the soil pore volume [PV]) of the sandy site materials, as well as the clay material also proved moderately effective in the removal of 1,4-dioxane. Figure 1 displays the percent removal of 1,4-dioxane from the site materials with the different thermal treatment techniques.
Degradation tests were performed on the two sandy materials by heating the materials (equal masses of soil and site groundwater inside the test chambers) slowly up from approximately 60°C to 95°C (just below the boiling point of water) over the course of 10 days in a closed system. At the end of 10 days, the soil, groundwater and vapor phases of the test chamber were tested for 1,4-dioxane, as well as a full suite of volatile and semi-volatile organic compounds, as well as tentatively identified compounds (identified by gas chromatography/mass spectrometry). Chloride and pH measurements were collected in the degradation test chambers over the course of the 10 days to understand the chemical behavior and mechanisms at play.
These tests illustrate what can be learned from performing relatively inexpensive laboratory-scale tests while contemplating various field-scale remediation strategies. Refining the potential treatment options ahead of moving to pilot or full-scale design can save resources and time, and help set realistic expectations for what can be accomplished during remediation.
In this case, bench-scale testing indicates how effective thermal treatment of 1,4-dioxane can be, and which approaches may be more effective. Of the three thermal techniques tested, SEE was the only technique to fully remove (>99.99%) 1,4-dioxane NAPL in the sandy materials. Heating to lower temperatures or heating without substantial pore volume exchanges was shown to result in lesser removals in the range of 94-97%. The data also confirmed that moving from the bench scale to a pilot or full scale, a robust liquid and vapor treatment system will need to be designed to handle 1,4-dioxane. 1,4-Dioxane is known for being resistant to many typical water treatment options such as air stripping and adsorption onto typical media such as granular activated carbon.
 EPA Technical Fact Sheet- 1,4-Dioxane (January 2014). EPA Office of Solid Waste and Emergency Response, EPA 505-F-14-011. Available at http://www2.epa.gov/sites/production/files/2014-03/documents/ffrro_factsheet_contaminant_14-dioxane_january2014_final.pdf