Trace metals in the aquatic environment are generally concentrated
on the surface of solid geochemical phases which eventually become incorporated
into estuarine and marine sediments. The mechanism of trace
metal concentration is believed to be adsorption with various geochemical
phases such as hydrous metal oxides, clays, and organic matter.
Metals in estuarine or marine sediments can thus be expected to be partitioned
between different phases, depending on the concentration of the
phase and the strength of the adsorption bond.
The bioavailability of sediment-bound metals to deposit-feeding
organisms will depend on trace metal partitioning and the kinetics of
biological metal uptake from each geochemical phase. The present study
was undertaken to develop models for trace metal partitioning and bioavailability
in marine sediments.
An equilibrium adsorption model was developed that can be used to
predict the partitioning of trace metals between different geochemical
phases in aquatic sediments from laboratory studies. The model uses
conditional equilibrium constants determined from the linear portion
of an adsorption isotherm. Conditional equilibrium constants deterit
mined for the adsorption of Cu and Cd on bentonite clay, Fe(OH)₃,
Mn0₂, and humic acid in seawater show that the model is applicable for
trace metal concentrations existing in the natural environment. Based
on the laboratory results, the model predicts that the clay fraction
may be a major sink for Cu and Cd in marine sediments.
A kinetic bioavailability model was then developed which can be
used to estimate the relative bioavailability of trace metals from both
different sediment phases and seawater under short-term laboratory conditions.
This model was used to determine the bioavailability of Cu and
Cd from several sediment phases (bentonite clay, humic acid, Fe(OH)₃)
and seawater to the deposit-feeding polychaete worm, Abarenicola
pacifica. The results suggest that, under natural conditions similar
to those used in this study, the bioavailability of sediment-bound Cu
and Cd to A. pacifica can be much more significant than that of seawater