Soil washing uses water to remove contaminants from soils. The process works by either dissolving or suspending contaminants in the wash solution. It is often used in conjunction with other physical separation techniques. (See the description of Separation).
Soil washing separates soil by particle size. Most organic and inorganic contaminants tend to bind and sorb to clay, silt, and organic soil particles. Most silt and clay are stuck to larger particles (i.e., sand and gravel). Washing separates the small particles from the large particles by breaking adhesive bonds. The separated material is smaller in volume and is more easily disposed.
Limitations and Concerns
Soil washing is a technique of concentrating contaminants through separation. It does not destroy or immobilize the contaminants. Consequently, the resulting concentrated soil must be disposed of carefully.
The ÒcleanÓ portion of separated soil must be analyzed for residual contamination before it is disposed of as clean material. Sites using soil washing often have an on-site capability to test samples of treated soil before it is released as clean. For example, see the descriptions of X-Ray Fluorescence and Laser-Induced Fluorescence characterization technologies.
Soil contaminated with both metals and organic compounds make formulating a single suitable washing solution difficult. In this case, sequential washing, using different wash formulations may be required.
Some washing formulation may have a tendency to change the mobility of certain metals, and thus fully characterizing and understanding the site is of utmost importance.
High organic content of the soil may require pretreatment.
Wash water requires treatment before it can be discharged, as it is usually not completely free of smaller particles or organic particles.
Measuring contaminant concentration in air particulates during excavation and treatment is an important step to ensure that a wider population is not being unduly exposed.
Soil washing systems are used on soils contaminated with semi-volatile organic compounds (SVOCs), fuels, and heavy metals, including radionuclides. The technology can be used on selected VOCs and pesticides.
Technology Development Status
Soil washing is used extensively in Europe. Commercialization in the United States is not as extensive.
See the technology description of Separation.