Page 26 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 9

78.1
1ppmVbenzene = = 3.25 mg/m 3 at 20 °C
24.05
78.1
= = 3.19 mg/m 3 at 25 °C
24.5
(2.3)
78.1
−6
= = 0.203 10 lb/ft 3 at68F °
×
° (20C)
385
78.1
= = 0.199 × 10 lb/ft 3 at77F °
−6
° (25C)
392
For comparison, the mass concentration of 1 ppmV perchloroethylene
(PCE, C Cl , MW = 165.8) at 20°C and P = 1 atm is equal to:
4
2
165.8
°
1 ppmVPCE = = 6.89 mg/m 3 at 20 C
24.05
(2.4)
165.8
= = 0.431 10 lb/ft× − 6 3 at68F (20C)° °
385
From this practice, we learn that the conversion factors are different among
compounds because of the differences in molecular weight. The mass con-
centration of 1 ppmV PCE is twice as large as that of 1 ppmV benzene (6.89
vs. 3.25 mg/m at 20°C). In addition, the conversion factor for a compound is
3
temperature dependent because its molar volume varies with temperature.
The higher the temperature is, the smaller the mass concentration would be
for the same ppmV value.
In remediation design, it is often necessary to determine the mass of a
COC present in a medium. It can be found from the COC concentration and
the amount of the medium containing the COC. The procedure for such cal-
culations is simple, but slightly different for liquid, soil, and air phases. The
differences mainly come from the units of concentration.
Let us start with the simplest case in which a liquid is impacted with a
dissolved COC. Dissolved COC concentration in the liquid (C) is typically
expressed in mass of COC/volume of liquid, such as mg/L; therefore, mass
of the COC dissolved in liquid can be obtained by multiplying the concentra-
tion with the volume of liquid (V):
l
Mass of COC in liquid = (liquid volume) × (liquid concentration) = (V)(C)
l
(2.5)

When a soil sample is sent to a laboratory for analysis, it usually contains
some moisture. The sample would be weighed first. The measured value
includes the weight of the dry soil and that of the associated moisture. If the
soil is impacted, the COCs should be present on the surface of the soil grains