Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 155

mol −1 for the
water-air system. Above this threshold value, chemical volatilization is liquid-film
controlled, and below it, it is often gas film controlled. 131 However, in actual cases, an
intermediate (mixed) stage usually occurs.
Because H generally increases with increasing vapor pressure and generally
decreases with increasing solubility of a compound, highly volatile low solubility
compounds are more likely to exhibit mass transfer limitations on the water side,
whereas relatively nonvolatile, high solubility compounds are more likely to exhibit
mass transfer limitations on the air side. 78

approximately 0.1 for the dimensionless H or about 2.2 × 10 −3 atm-m 3 ⋅

4.2.3.2 Ionization

Ionization is the dissociation of a chemical into multiple charged species. In an
aquatic environment, some chemicals occur only in their neutral form, while others
react with water molecules to form positively (cationic) or negatively (anionic)
charged ions. These reactions are rapid and are generally assumed to be at (local)
equilibrium. At equilibrium, the pH and temperature of water along with the ion-
ization constant control the distribution of a chemical's form between its neutral and
ionized species.
Ionization is important because of the different toxicological and chemical
properties of the neutral and ionized species. For example, in some cases, only the
neutral form of the chemical can transform or be transported through biotic mem-
branes, resulting in toxicity. As a result, it is often necessary to compute the distri-
bution of a chemical's form among its ionic and neutral forms as well as to allow
each species to react, transform, or sorb at different rates. 78
Several studies have shown that there is a relationship among salinity, pH,
temperature, and the concentration of organic compound on the toxicity of heavy
metals. The ionic forms of heavy metals are more toxic, because these forms can
easily permeate the cell membranes of aquatic organisms. Observed changes in the
biological availability of metals with pH, temperature, and alkalinity are the result
of several factors. In order to predict heavy metal effects, it is clear that pH,
temperature, and alkalinity of the water must be known. 139,140 It is also noted that
the presence of the organic complexes invariably decreases the concentration of the
ionic forms of heavy metals. 141 For example, there is a negative effect of increased
salinity on the uptake of cadmium by aquatic organisms. The increased availability
of cadmium at low salinity is actually due to increased free cadmium ion level. 142

4.2.3.3 Sorption

Soluble organic materials in natural waters can sorb onto particulate suspended
material or bed sediment. 131 Sorption is a process whereby a dissolved substance is
transferred to and becomes associated with solid material. It includes both the
accumulation of dissolved substances on the surface of solids (adsorption) and the
interpenetrating or intermingling of substances with solids (absorption). The sub-
stance that is sorbed is called the sorbate, and the solid is called the sorbent.
Desorption is the process whereby a sorbed substance is released from a particle. 45