Chemistry Reference
In-Depth Information
2.5 Metals in Sediments
Sediments are important sinks for pollutants, includ-
ing metals. Metals bind both to the organic content of
sediment, to clay minerals, and form mixed complexes
with available oxyanions. Even at high sediment
concentrations of, for example, cadmium, the bind-
ing capacity of sediment is not saturated. Compared
with organic chemicals, metal transfer between sedi-
ment and water depends on highly complex processes.
Although organic pollutants, in principle, partition
between the water phase, the sediment phase, and
biota in aquatic systems mainly on the basis of their
lipophilic character (i.e., affi nity for the organic fraction
in sediment and lipid fractions in biota), the partition-
ing of metals depends much more on water chemis-
try and sediment chemistry, mainly relative amounts
of sulfur and iron compounds, organic content, and
redox status, and also on metal characteristics, mainly
hard and soft character. Metals interact with particles
by means of physical-chemical sorption (i.e., nonspe-
cifi c attraction [van der Waals forces]) acting between
surface charges on particles and the charges of metals/
metal compounds. Chemical processes include precipi-
tation and coprecipitation, ion exchange, isomorphous
substitution, and solid-state diffusion.
Sediments are most often already anoxic a few mil-
limeters below surface, and in this reducing environ-
ment, alternative terminal electron acceptors include
sulfate, nitrate, iron, and, in some cases, other met-
als, and organic matter (methanogenesis). The redox
status of sediments extensively affects the speciation
and solubility and thereby toxicity of sediment asso-
ciated metals. The toxicity of metals associated with
sediments depends on the bioavailability of the met-
als, mainly governed by the water concentration (pore
water and overlying water) and speciation (see later).
For sediment eating and suspension feeding inverte-
brates as, for example, polychaetes and mussels, direct
exposure to metals during digestion of sediment and
suspended matter is an important exposure route.
Four chemical processes are considered to increase
the transfer of metals from sediment to water: eleva-
tion of salinity, changes in sediment redox status,
reduced pH, and the presence of organic complexing
compounds. The fi rst three processes release “free”
metal in the form of mixed complexes with small inor-
ganic ions and water, the fourth releases organically
bound metal with another bioavailability than that of
the inorganic metal complexes.
Metal ions form highly insoluble hydroxides and
sulfi des, and for some metals, also phosphates, whereas
chlorides, carbonates, and sulfates are more soluble.
Also, metals are bound strongly to various oxygen-
containing iron compounds. Iron oxide hydroxide
(FeOOH) and other iron oxides form a lattice where
phosphate ions can exchange with OH, thereby locally
increased pH may occur from liberated OH−, and
also, a range of metal ions can bind to FeOOH. The
metal sequestering capacity of FeOOH is especially
high when some of the hydroxy groups are exchanged
by phosphate. For transition metals, the ions with the
lowest oxidation step typically form more stable com-
plexes, leading to metal mobilization in case of sedi-
ment oxidation. The pool of sulfi de available for metal
binding (acid volatile sulfi de) is an important determi-
nant of the toxicity of sediment-associated metals. This
pool is smaller in oxidized sediment, leading to metal
release. On the other hand, FeOOH is formed only at
oxic conditions. Accordingly, the effect of changing the
redox status of sediment on the bioavailability of sedi-
ment-associated metals depends on the chemical com-
position of the sediment.
3 UPTAKE AND ACCUMULATION OF
METALS
3.1 Bioavailability, Uptake, Accumulation,
and Elimination
The term bioavailable refers to the fraction of total
metal that can be taken up into an organism (Sanders
and Riedel, 1998).
Terrestrial animals absorb metals in their food over
their digestive epithelium, volatile metal compounds
through the lungs, and for the smaller invertebrates,
over the body surface. Aquatic organisms absorb toxic
metals from water over the gill epithelium and over
the entire body surface, besides from food and imbibed
water over digestive epithelium.
The bioavailability of metals varies extensively
among metals and organisms and also depends on the
speciation of the metals. Accordingly, changes in the
physicochemical characteristics of exposure media,
especially water, infl uence the proportions of metal
species that can be absorbed.
In principle, only lipophilic metal complexes (e.g.,
with carbamate pesticides) or organometals (e.g., mer-
cury and tin compounds) can diffuse over biological
membranes, whereas hydrophilic metal complexes
including free hydrated ions must pass cell membranes
by other mechanisms than free diffusion.
Metals can be taken up by endocytosis (e.g., FeOH
3
),
by diffusion (e.g., Hg(CH
3
)
2
), by facilitated diffu-
sion (mainly soft class B ions with affi nity for S- or
N- containing ligands in membrane carrier proteins),
or by active ATP-dependent uptake via cation pumps
