Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 153

• Ammonia is very soluble in water, but because the concentration of
ammonia in the atmosphere is small, volatilization of ammonia from water
to air will always take place. At neutral pH, most of the total ammonia
is in the form of ammonium ion; however, during summer in eutrophic
coastal environments, such as lagoons, a pH of 8.5 or even higher is
observed, producing a greater ratio of ammonia to ammonium ion. This
ratio can be calculated from the mass balance equation, and the influence
of salinity on this ratio can be accounted for by use of the concept of
ionic strength, I. 131 For example, at pH 9.3, about half of the total ammonia
is ammonia. Therefore, the volatilization of ammonia can be significant
in very eutrophic environments during the summer months.

• Volatilization is an important process for many toxic organic compounds

in the environment. When discharged into the water environment even in
small concentrations, they often have sufficient vapor pressure to give rise
to significant volatilization.

In many cases, volatilization is the most important removal process for toxic
substances from aquatic ecosystems, since other processes, including biodegrada-
tion, can be very slow.
Some of the factors that affect volatilization in the environment, apart from vapor
pressure, are climate, sorption, hydrolysis, and phototransformation. A chemical
with a low vapor pressure (VP), high adsorptive capacity, or high water solubility
is less likely to volatilize into the air. A chemical with a high VP, low sorptive
capacity, or very low water solubility is more likely to volatilize into the air. Chem-
icals that are gases at ambient temperatures will get into the air. 131,137
During volatilization, the dissolved concentration of the unionized molecule
attempts to equilibrate with the gas phase concentration. Equilibrium occurs when
the partial pressure exerted by the chemical in solution equals the partial pressure
of the chemical in the overlying atmosphere. The rate of exchange is proportional
to the gradient between the dissolved concentration and the concentration in the
overlying atmosphere, and to the conductivity across the interface between the liquid
and gas phase. The conductivity (mass transfer coefficient) is influenced both by chem-
ical properties, such as molecular weight and Henry's law constant, and by environ-
mental conditions at the air-water interface, e.g., turbulence-controlled by wind
speed, current velocity, and water depth. 78
The governing volatilization reaction equation is presented and discussed later
in Section 4.2.4. Liquid-gas transfer models are often based on the two-film theory,
as illustrated in Figure 4.7. The mass transfer rate is governed by molecular diffusion
through a stagnant liquid and gas film at the interface. Mass moves from areas of
high concentration to areas of low concentration. The transfer rate can be limited at
either the gas film or liquid film side of the interface. Oxygen, for example, is
controlled by the liquid film resistance. Nitrogen gas, although approximately four
times more abundant in the atmosphere than oxygen, still has a greater liquid film
resistance than oxygen. 131 Concentration differences serve as the driving force for
the water layer diffusion. Pressure differences drive the diffusion for the air layer.
From mass balance considerations, it is obvious that the same mass must pass