Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 120

Atmosphere

Cloud

Dry deposition
(particles)

Volatilization/absorption

Wet deposition
(particles + gas)

Water

Bubble ebullition

Sediment

Bedrock

FIGURE 4.1 Exchange of chemicals between the water and the atmosphere. Equilibrium is
important in many cases such as volatilization-absorption, wet deposition, dry deposition, and
gas bubble transport.

E XAMPLE 4.1 U SE OF A IR -W ATER P ARTITION C ONSTANT

Water from an oil-field waste pond contains hydrogen sulfide at a concentration of
500 mg/L at an ambient temperature of 298 K. What will be the maximum equilibrium
concentration of H 2 S above the solution?
At 298 K, K aw for H 2 S is 0.41 (Appendix 1). C i w = 500 mg/L = 0.5 / 34 =
0.0147 mol/L. Hence C i a = ( 0.41 )( 0.0147 ) = 0.006 mol/L. Partial pressure in air, P i =
C i a RT = 0.147 atm.

Many waste treatment separation processes also involve air-water exchange of
chemicals (Figure 4.2). Examples include wastewater aeration ponds and surface
impoundments in which surface and submerged aerators are used to transfer oxygen
to the water, and to remove volatile organic and inorganic compounds from water
using flotation. Volatile organics are removed using a tower where air and water
are passed countercurrent to one another through a packed bed. The operations are
nonequilibrium processes, but where, the tendency of the system to attain equilibrium
provides the driving force for separation.
The transfer of material (say i) from one phase to the other (e.g., water to air) is
due to a gradient in concentration of i between the phases (Figure 4.3). This flow of
material i from water to air is termed flux and is defined as the moles of i passing a
unit area of interface per unit time. The equation for flux is

K w C i w − C i w ,

N i = (4.3)