Page 32 - The Toxicology of Fishes

12 The Toxicology of Fishes

Chemical structure and properties dictate not only persistence in the aquatic environment but also the
primary compartments (e.g., water, sediment, suspended particles) with which a chemical is associated
and, by extension, routes through which a fish is likely to be exposed. One particularly important property
controlling the behavior of a chemical in the aquatic environment is its hydrophobicity. A measure of
relative hydrophobicity that is often used in aquatic toxicology is the octanol–water partition coefficient
(K ).* Chemicals with large log K values, in particular those with log K > 5, are considered
ow
ow
ow
hydrophobic and tend to be associated with organic materials in aquatic systems, especially sediments.
Hydrophobic chemicals of environmental concern in this class include most PAHs, PCBs, PCDFs,
PCDDs, and some organochlorine pesticides and organometals.
An important variable in chemical fate and bioavailability is the fraction of chemical that is freely
dissolved; for example, nonionic organic compounds or cationic metals complexed by dissolved organic
carbon (DOC)† are often far less bioavailable than their uncomplexed forms. Although this is an important
guiding principle in the area of bioavailability prediction, empirical demonstration of the concept has
proved challenging in that it is very difficult to reliably measure freely dissolved chemicals. As an
example, although a common operational definition of dissolved metal is the ability to pass through a
0.45-µm filter, it is well known that metals bound to some types of colloidal organic carbon also can
pass through this pore size. Analogous problems exist in defining fractions of organic chemicals that are
freely dissolved. Often, it is easier to predict than measure dissolved fractions of some chemicals using
partitioning and speciation models. As a consequence, assessment of contaminant bioavailability might
rely on predictive models that consider properties of both the chemical and the aqueous environment
under consideration, in conjunction with measurements of total chemical concentrations. An example of
this approach is provided in the case study below concerning 2,3,7,8-tetrachlorodibenzo-p-dioxin.
Another important property used to describe the behavior of many ionizable compounds is their degree
of dissociation at pH values typically found in aquatic systems. Charged molecules are more water
soluble, while neutral forms of the same chemical tend to be more hydrophobic and thus more associated
with organic carbon than water. The speciation of strong acids (or bases) will not be affected by pH
variations typical of most aquatic environments; however, the behavior of chemicals with pK ‡ values
a
in the range of approximately 5 to 9 can be greatly affected by system-specific variations in pH. Chemicals
whose pK values are important determinants of fate and bioavailability include weak acids, such as
a
phenolic compounds, some surfactants and resin acids, and weak bases such as ammonia.
The interaction of chemicals with biotic and abiotic ligands in the aquatic environment influences
their speciation and partitioning into different environmental compartments. A major partitioning phase
for many nonionic organic chemicals is DOC and particulate organic carbon (POC), both in the water
column and sediments. The interaction between these types of chemicals and organic carbon can be
modeled based on their K values. A predictable and generally linear relationship exists between the
ow
log K of a chemical and the log K ,§ so knowledge of K can be useful for deriving organic carbon
oc
ow
ow
partitioning relationships for predicting bioavailability. In the case of organic and inorganic ions, ionic
constituents of the water can affect chemical speciation; for example, certain types of DOC (i.e., humic
acids) as well as some inorganic anions can strongly complex cationic metals, thereby controlling
partitioning and subsequent bioavailability.
* The octanol–water partition coefficient (K ow ) is the equilibrium ratio of the concentration of a chemical in n-octanol to its
concentration in water, commonly expressed as its base 10 logarithm, log K ow .
† Dissolved organic carbon (DOC) is the carbon present in the complex mixture of organic molecules dissolved in water, con-
sisting largely of organic acids originating from the decomposition of plant material, including fulvic and humic acids. This
is in contrast to particulate organic carbon (POC), which refers to the carbon present in the complex mixture of particulate
material suspended in water, including resuspended sediment and detritis, soil particles, leaf litter, bacteria, phytoplankton,
and zooplankton. POC and DOC are often defined operationally as that portion of the total organic carbon (TOC) in water
retained and not retained, respectively, by a filter with a pore size of 1.0 µm or less (commonly 0.45 µm).
‡ The acid dissociation constant (K a ) is the equilibrium constant for the dissociation of an acid into hydrogen ion and its con-
jugate base. pK a = –log 10 (K a ) and is equal to the pH at which half of the chemical is ionized.
§ The organic carbon–water partition coefficient (K oc ) is the equilibrium ratio of the concentration of a chemical associated
with organic carbon (measured as the mass of chemical per mass of carbon) to its free concentration in water. The value for
this coefficient will depend on the composition of the organic carbon phase, which varies among aquatic systems, between
the water column and sediment, and between dissolved and particulate phases.