Page 280 - Soil and water contamination, 2nd edition

Gas exchange 267

3
3
he PCE concentration of 150 mg m corresponds to 150/165.83 = 0.90 mmol m
-1
-6
= 0.90·10 mol l . The partial pressure is calculated using the ideal gas law (Equation
2.15):
n i P i P RT n i
V RT i V
-1
-1
where R = the gas constant (= 0.0821 l atm mol K ). Thus, the partial pressure is
5
P i . 0 0821 293 . 15 . 0 90 10 6 . 2 17 10 atm
-3
The PCE concentration is thus 2.17·10 volume percent, which is considerably lower
than 5 volume percent. Therefore, Equation (14.25) can be used to estimate the gas flux
across the soil–air interface:
7 150 6 -2 -1 -2 -1
J 7 . 2 10 1 . 8 10 mg m s = 0.70 mg m d
5

EXERCISES

1. Explain why if the water film controls the gas exchange, an increase in the water
turbulence increases the gas exchange between water and atmosphere (think of stirring),
and why this does not affect the gas exchange when the air film controls the gas exchange.

2. What is the distance needed for a stream 60 cm deep to reduce the oxygen deficit by 50
-1
-1
percent of the initial value at flow velocities of respectively 10 cm s and 20 cm s ?
3. A lake 2 m deep is situated at 1500 m above sea level. The water temperature is 12 °C
and the dissolved oxygen saturation is 80 percent. The wind speed at 10 m above the
-1
-1
water surface is 9 km h and it is raining at an intensity of 20 mm h .
a. What contributes more to reaeration: wind or rain?
b. Calculate the time needed to increase the dissolved oxygen saturation to 90 percent.
4. Estimate the gas exchange coefficient at 20 °C for toluene (C H CH ) for a 2 m deep
6
5
3
-1
river with an average flow velocity of 1.5 m s .
+
-1
5. The total concentration of NH and NH in a small pond is 0.5 mg N l . The
3 4
temperature of the air and water is 20 °C and the wind speed at 10 m above the water
-1
surface is 4 m s . The following parameters are given:
-9
2 -1
Molecular diffusion coefficient of NH in water D = 1.76 10 m s ;
3 NH3
-1
Henry’s law constant K = 0.02 atm∙l mol ;
H
-5
+
-
Base constant K for the reaction NH + H O ↔ NH + OH = 1.75 10 .
b 3 2 4
a. Calculate the NH concentration in the pond at pH = 7 and pH = 8.
3
b. Calculate the volatilisation rate of NH from the pond at both pH values (hints:
3
evaluate both air-side controlled and water-side controlled NH exchange).
3

10/1/2013 6:45:20 PM
Soil and Water.indd 279 10/1/2013 6:45:20 PM
Soil and Water.indd 279