Scheme of the equilibrium Environmental Compartments Model
Life-cycle of gaseous contaminants
Processes leading to atmospheric deposition
Dry and wet atmospheric deposition
Wet deposition
Rainout, washout and deposition of aerosols are oneway transport processes that transfer chemicals from the
air to water and soils. Most efficient for soluble gases and
for aerosols with diameters > 1 µm
Dry deposition
Airborne particles and dry gases
Processes
affecting the
compositio of
rain droplets
Deposition routes
of some acidforming gases and
ammonia
Solubility of gases in liquids
Gas solubility is an equilibrium process controlled by the
gas (partial) pressure. At constant temperature, the
number of molecules transferred from the gas phase to the
liquid phase (solution) is the same as the number of
molecules transferred the opposite way. Increasing
pressure leads to higher solubility:
Henry’s Law
Distribution of a gas between gas phase (air) and liquid phase (water)
is described by the air-water distribution coefficient
Kaw = C(air)/C(aq)
where C(air) is the concentration of a chemical in the air and C(aq) its
concentration in water. C(air) can be calculated from the equation of
state
C(air) = ni/V = pi/RT
ni is the number of moles of the chemical in volume V of air and pi is
partial pressure of the chemical:
pi = xip
where p is the (total atmospheric) pressure and xi molar fraction of the
chemical in the air. If this chemical is below its critical point and
aqueous solution is saturated, i.e. C(aq) = CS, partial pressure is
equal to the saturated vapor pressure – pS
More commonly applied is the Henry’s law constant H
H = pS / CS = ( pi/C(aq) ) = Kaw RT
Solubility of gases in liquids – example
Calculate the solubility of oxygen in water at 25°C (H = 76900 Pa m3 mol-1)
O2 in air= 20,95 % (molar percentage)
PO2 = 0,2095 patm
C[O2 aq] = pO2 / H
Normal pressure = 105 Pa = 1 bar = 1 atm = 760 mm Hg (torr)
(conversion to atm is not exact)
pO2 = 0,2095 105 = 2.095 ·104 Pa
C[O2 aq] = pO2 /H = 2,7 10–4 mol/l
Vapor pressure
Pressure in a closed system containing only pure liquid and
gas phases of a given compound is called its (saturated)
vapor pressure.
Vapor pressure depends only on temperature. Many
empirical correlation equations are used for this relation,
most often the
Antoine equation
where A, B, C are constants derived from experimental data
and valid only in some limited temperature interval. This
equation is usually applied for vapor pressures from 1 to
about 200 kPa.
B
log p  A 
T C
s
Measurements of vapor pressure – static method
H2O (l)
start
H2O (g)
equilibrium
Phase diagram of water
Factors affecting gas solubility
Pressure
According to Henry’s law, solubility of a gas is proportional to its partial
pressure in the air. Because atmospheric pressure is more or less constant,
it depends just on the concentration of this gas in the air
Temperature
Connected to gas expansivity, i.e. the equation of state: higher temperature
leads to more expanded gases and thus lower concentration. Even more
important is that Henry’s law constant of gases is growing with temperature.
Both effects lead to lower gas solubility
Salts
Gases are „salted-out“ from the solution: higher salinity leads to lower gas
solubility
Chemical reactions in water
If the gas is reacting with water, its solubility is increased
Oxygen in the air
Oxygen equilibrium is attained on one side by deoxygenation (aerobic processes of biochemical degradation
of organic compounds) and on the other side by re-aeration
(dissolution of oxygen from the air, if water is less than
saturated). Equilibrium amount of dissolved oxygen
depends on temperature.
Equilibrium concentration
temperature (mg/l)
of
aqueous
oxygen
t(°C)
0
10
15
20
25
30
O2(mg/l)
14.6
11.3
10.1
9.2
8.3
7.6
vs.
Concentrations below 4 mg/l are lethal to fish and other
water organisms
Non-equilibrium dynamics of gas dissolution: the oxygen curve
Re-aeration processes
The rate of oxygen dissolution depends on the water
surface quality:
Still water dissolves 1.4 mg O2 per m2 / day
Stirred water dissolves 5.5 mg O2 per m2 / day
Turbulent water dissolves 50 mg O2 per m2 / day
The rate of re-aeration exponentially depends on the
oxygen deficit – the above numbers refer to stationary state.
Oxygen deficit can be increased by sudden increase of
contamination and/or by increased temperature.