Soil Washing


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When soils are seriously contaminated with organic chemicals (e.g. toluene, trichloroethylene) or inorganic substances (metals), a process known as soil washing may be used to remediate them.  Soil washing may involve an expensive ex-situ (off site) cleaning process.  By definition this type of ex-situ remediation means that large quantities of soil will be transported to a remote facility.  In addition to the considerable expense in equipment and labor, fuel costs will almost certainly be extremely expensive.  

 

 

 

 

 

 

 

 

 

Given these excessive expenses, some soil remediation organizations have developed mobile washing systems that can be used on site.


 

In either case, the fundamental principle of soil washing is somewhat analogous to washing clothes.  In washing clothes we employ the force of water and the chemical action of detergent to separate dirt from clothing.  The dirt and dirty water can then be disposed of and treated separately.  Similarly, in soil washing, water, solvents ("detergent"), and mechanical manipulation are utilized to separate contaminates from soil and then dispose of them separately.  Once soil has been transported to a site where the washing will take place, a number of washing or cleansing techniques may be employed.

One of the key features of a number of washing processes is that they take advantage of a soil property we have discussed in lecture and lab: soil texture.  As we have learned in this class the high surface area and negative charge of finer particles (clay, silt, organic matter) means that many contaminants will be preferentially adsorbed to them instead of sand.  And so by mechanically separating fine particles from sand, we can begin to isolate contaminates for further treatment.   This particle separation may be accomplished by mechanical sieving, as seen in the diagram below.

 

If the larger soil particles separated in the initial process are certified to be uncontaminated, they can be returned to general use as a soil material.

The isolated, and now more concentrated soil material will require further remediation and/or disposal.   At this stage in the process, various detergents or solvents are used to extract the contaminants from the finer soil particles.  The choice of detergents will be made based on the specific mix of contaminants present. You will learn about a specific detergent application by reading the article: Remediating Soil and Groundwater Contamination.

The diagrams below illustrate two ways in which washed soil may be processed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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