Chemical extraction is a process that separates contaminants from soils and thereby reduces the volume of the hazardous waste that must be treated. The process differs from soil washing, which generally uses water, and varies with contaminant and soil type. Often, physical separation is used before chemical extraction on the assumption that smaller particles contain most of the contamination. The two major chemical extraction processes are described below
Acid extraction uses hydrochloric acid to extract heavy metal contaminants from soils. In this process, hydrochloric acid is mixed with soil in a closed extraction unit. The residence time in the extraction unit generally ranges between 10 and 40 minutes. When extraction is complete, the soils are rinsed with water to remove entrained acid and metals. The extraction solution and rinse waters are regenerated using precipitation. The heavy metals are potentially suitable for recovery. The clean soils are dewatered and mixed with lime and fertilizer to neutralize any residual acid.
Solvent extraction uses an organic solvent, to remove metals and mixtures of metal and organic compounds. It is commonly used in combination with other technologies, such as solidification/stabilization, incineration, or soil washing. Soil is removed and treated.
Limitations and Concerns
Organically bound metals can be extracted along with the target pollutants, thereby creating residuals with special handling requirements.
The toxicity of the solvent is an important consideration as traces may remain in the treated soil.
If solvent extraction is used for PCBs and other chlorinated compounds, concentrations of these contaminants in the solvent must be kept very low if the resulting solvent is going to be burned. Burning may cause the formation and release of dioxin and furans.
After acid extraction, any residual acid in treated soil needs to be neutralized.
"Clean soil" must meet cleanup criteria. This may prove difficult and uneconomical.
In solvent extraction, impermeable membrane liners and covers should be used to reduce solvent evaporation and to protect against rain.
Some soil types and moisture content levels will adversely impact process performance. Higher clay content may reduce extraction efficiency and require longer contact times.
Technology Development Status
Commercial-scale units are in operation.
See description of Terra-Kleen.
See http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA342564&Location=U2&doc=GetTRDoc.pdf for description of solvent extraction to detect chemical weapons by-products.