Page 203 - Materials Chemistry, Second Edition

186 Practical Design Calculations for Groundwater and Soil Remediation

with bound COCs into the washing fluid. The COCs are readily washed off
from sand and gravel, which often account for a large portion of the soil
matrix. Separation of the sand and gravel from the heavily impacted clay
and silt particles greatly reduces the volume of impacted soil. Soil washing
makes further treatment or disposal easier.
Various chemicals can be added to the aqueous solution to enhance
desorption or dissolution of the COCs. For example, an acidic solution is
often used to extract heavy metals from impacted soil. Addition of chelating
agents can help the dissolution of heavy metals into the aqueous solution,
while addition of surface-active agents can help the dissolution of organ-
ics. Solvent extraction is similar to soil washing, except that solvents rather
than aqueous solutions are employed to extract organic COCs from soil.
Commonly used solvents include alcohol, liquefied propane and butane,
and supercritical fluids.
Soil flushing differs from soil washing or solvent extraction in that it is an
in situ process in which water or solvent flushes the impacted zone to desorb
or dissolve the COCs. The elutriate is then collected from the wells or drains
for further treatment.

5.3.2 Design of a Soil-Washing System

A mass-balance equation can be written to relate the COC concentrations in
the soil before and after washing with the COC concentration in the spent
washing fluid (assuming that the fresh washing fluid does not contain any
COCs) as:

M s,wet = M s,dry +
X initial S initial C initial V soilmoisture
(5.22)
= M s,dry + V l +
S final C final C final V soilmoisture
where
X inital = measured COC concentration of the soil sample before
washing (mg/kg)
M s,wet = mass of soil plus moisture before washing (kg)
M s,dry = mass of dry soil (kg)
S initial = initial COC concentration on the surface of the soil before
washing (mg/kg)
S final = final COC concentration on the surface of the washed soil
(mg/kg)
C initial = COC concentration in the soil moisture before washing (mg/L)
C final = COC concentration in the spent washing fluid (mg/L)
V = volume of the soil moisture before washing (L)
soil moisture
V = volume of the washing fluid used (L)
l