Formatted Title
Low-Temperature Haloalkane Treatment Using In Situ Thermal Hydrolysis
Background/Objectives
Halogenated alkanes (haloalkanes) are an important class of chemical compounds with a broad and diverse range of uses in manufacturing, agriculture, pharmaceuticals, refrigeration, and fire protection. Although the chemical properties of haloalkanes differ greatly depending on degree and nature of halogen substitution, nearly every compound within this functional class are persistent and recalcitrant contaminants. In situ thermal treatment (ISTT) is a demonstrated tool in the restoration of soil and groundwater contaminated with haloalkanes due to their high vapor pressures. Haloalkane degradation by chemical mechanisms including hydrolysis is also known to occur at temperatures well below conventional ISTT applications where vaporization is favored for contaminant mass recovery. Hydrolysis reactions are strongly influenced by temperature and many haloalkanes exhibit significant half-life reduction (years to days) with only modest temperature increase. Unlike typical ISTT applications where contaminants are removed during heating, many hydrolysis reactions are destructive and the need for concurrent fluid extraction and treatment systems can be eliminated simplifying design and reducing total remedy cost. Despite these advantages, relatively few studies have attempted to evaluate and quantify the benefits of low temperature heating to promote in situ hydrolysis for haloalkane destruction.
Approach/Activities
A controlled study to evaluate the thermal hydrolysis of an ethylene dibromide (EDB) and ethylene dichloride (EDC) source zone was recently completed. The study was performed in parallel with a conventional ISTT application implemented at an active chemical manufacturing facility. Using a controlled volume of site soil and groundwater placed in sealed reaction chambers, a novel approach was developed to evaluate the effects of temperature on EDB and EDC transformation. During ISTT operations, the reaction chambers were incubated in a subsurface water bath constructed from a solid section of well casing. The approach provided an isolated environment which duplicated the actual subsurface temperatures within the treatment zone. Individual chambers were selectively removed during ISTT operations at set times and subsurface temperatures for laboratory analyses. Laboratory results were used to document contaminant removal as a function of time and in situ temperature and to support analysis of subsurface soil samples collected following ISTT system shutdown.
Results/Lessons Learned
Hydrolysis study findings were directly applied to the full-scale design and implementation of two low-temperature treatment applications for in situ haloalkane remediation. The projects were both implemented within active manufacturing facilities using different commercial variations of electrical resistance heating (ERH) designed to reach a target temperature of 70 degrees centigrade. Both ERH systems were constructed and operated without vapor extraction and treatment which provided significant savings to total project costs. One project focused solely on EDC treatment; the second was applied for treatment of a haloalkane mixture composed primarily of EDB, and EDC. This presentation will summarize performance observations of two low-temperature ISTT applications targeting haloalkane hydrolysis. Specific discussion topics will include technology applicability, economic advantages of low-temperature treatment and considerations for the design, construction, and operation of ISTT equipment for haloalkane remediation.