Page 25 - Materials Chemistry, Second Edition

8 Practical Design Calculations for Groundwater and Soil Remediation

i.e., 10 g benzene per gram of soil (1 μg/g), or 1 mg benzene per kilogram
−6
of soil (1 mg/kg).
For the liquid phase, 1 ppm benzene means that 1 μg of benzene is dis-
solved in 1 g of water, or 1 mg benzene per kilogram water. Since it is usually
more convenient to measure the liquid volume than its mass, and 1 kg of
water has a volume of approximately 1 L under ambient conditions, people
commonly use “1 ppm” for “1 mg/L compound concentration in liquid.” It
should be noted that 1 ppm (1 mg/kg or 1 mg/L) = 1,000 ppb (1,000 μg/kg
or 1,000 μg/L) = 1,000,000 ppt (1,000,000 nanogram/kg or 1,000,000 ng/L). In
addition, a concentration of 1% by weight (10,000 mg/kg) would be 10,000
ppm, since 1/100 = 10,000/1,000,000.
For the vapor phase, the story is totally different. The unit of ppm is on
a volume-per-volume basis (or a mole-per-mole basis per se). The unit of
ppm by volume (ppmV) is commonly used. One ppmV of benzene in the air
means one part volume of benzene in 1 million parts volume of air space. To
convert ppmV into mass concentration units, which is often needed in reme-
diation work, we can use the following formula:

MW
3
1 ppmV= [mg/m] at T =° 0C
22.4
MW
3
= [mg/m] at T = 20 °C (2.1)
24.05
MW
= [mg/m] at T = 25 °C
3
24.5
or

MW
3
1 ppmV= × 10 − 6 [lb/ft ]at T = 32°F
359
MW
3
= × 10 − 6 [lb/ft ]at T = 68°F (2.2)
385
MW
= × 10 − 6 [lb/ft ]at T = 77°F
3
392
where MW is the molecular weight of the compound, and the number in the
denominator of each equation above is the molar volume of an ideal gas at
that temperature and one atmosphere. For example, the volume of an ideal
gas is 22.4 L per gram-mole at 0°C, or 359 ft per pound-mole at 32°F and P
3
= 1 atm.
Let us determine the conversion factors between ppmV and mg/m or lb/
3
ft , using benzene (C H ) as an example (P = 1 atm). The MW of benzene is
3
6
6
78.1; therefore, 1 ppmV of benzene is the same as: