The International Information Center for Geotechnical Engineers

# Soil Washing

Theoretical Background

Soil washing theory is based upon the physiochemical processes that occur between the solid particles of soil and the solution in which they are dispersed (Sharma and Reddy 2004). In general, soil washing is based on the principle that contaminants are likely to adhere to the fine grained soils, which, in turn, are likely to adhere to the coarse grained soils (through adhesion and compaction). Washing with water and possibly additives allows the coarse grained soils to separate from the contaminated fines. Several physiochemical processes such as desorption, complexation, dissolution/solubilization, and oxidation reduction are involved in soil washing (Sharma and Reddy 2004). Desorption occurs in soil washing when the washwater (and associated additives) is mixed with the soil. During this stage, the contaminants are desorbed from the soil particle surface. Dissolution or solubilization of the contaminants can occur due to pH changes that result from acid-base reactions of the washwater. The washwater may also cause the formation of complexes with the contaminants (which may be soluble). Oxidation-reduction reactions may also be initiated by the washwater, resulting in desorption or solubilization of the contaminants. The goal of soil washing is to effectively separate the coarse grained soils from the contaminated fine grained soils, and to remove the contaminants from the fines into the washing solution (Sharma and Reddy 2004). The contaminants may not be completely removed from the soil though. An equation that relates the contaminant concentration in soil washing is:

Eq. (1)

where Csi is the initial concentration of the contaminant in the soil (mg/kg), Ms is the total dry mass of the soil (kg), Csf is the final concentration of the contaminant in the soil after washing (mg/kg), Vl is the total volume of the washing solution (L), and Cl is the concentration of the contaminant in the solution (mg/L). After adequate washing time, the system will reach equilibrium and the distribution coefficient of the contaminant between the soil and the washing solution (Kd) is applicable (Sharma and Reddy 2004):

Eq. (2)

Equation 2 can be substituted into Equation 1 to find the removal efficiency, which is given by:

Eq. (3)

Or

Eq. (4)

These equations are used to determine the effectiveness of soil washing at a site (Sharma and Reddy 2004).