Chemistry Reference
In-Depth Information
The fi lter feeders among the zooplankton retain
only a minor fraction of the cadmium ingested with the
food; the rest is excreted with the feces of the animals.
Zooplankton organisms also take up cadmium directly
from the seawater—as well as through the gills as on
the external surface. The sum of these routes of uptake
gives zooplankton organisms (both on 2nd, 3rd, and
maybe the 4th level in the food chain) cadmium con-
centrations that are generally 500-2000 times higher
than the cadmium concentrations in the surrounding
seawater. The bioaccumulation of cadmium does thus
not lead to biomagnifi cation.
Cadmium is taken up by aquatic plants and may
reach extensive levels in polluted lakes. In macrophytes
from Lago Maggiore (Guillizoni
et al
., 1989) measured
1.6 mg/kg as mean of means, whereas some plants had
higher levels: 6.7 mg/kg in
Egeria densa
and 9.6 mg/kg in
Ceratophyllum demersum
, an effective accumulator species
for several metals. One of the most effi cient Cd accumu-
lators, the water hyacinth
Eichhornia crassipes
that pro-
duces MT-like proteins in root tissue has been suggested
for remediation of Cd-polluted waste water and fresh-
water systems because of rapid growth and high bio-
mass production (Nir
et al
., 1990). Several experimental
studies with this and other metal hyperaccumulators for
remediation of, for example, water ponds in old mining
areas are under way with cadmium and other metals.
Both in mammals and birds, cadmium is depos-
ited mainly in the kidneys. In penguins from the Ant-
arctic, an area not polluted by cadmium because of
human activities, kidney concentrations between 50
and 80 mg/kg wet weight have been reported in sev-
eral studies, leading to renal tubular necrotic changes
(Elinder, 1992). Also Arctic and Antarctic whales and
seals have very high renal cadmium levels; Greenland
harbour seals have much higher Cd levels than seals
from the North Sea, which receives cadmium dis-
charges because of human activities. The explanation is
likely because of extensive uptake of Cd by Arctic and
Antarctic amphipods (including krill), oysters, mus-
sels, and squid, which are important food resources for
many marine vertebrate species.
It has been found for a number of different marine
species that the cadmium concentration in the tissues
increases throughout the lifespan of the organism, so
that old organisms contain higher cadmium concentra-
tions than young organisms.
chemical industries emit cadmium to the aquatic envi-
ronment, and cadmium is spread with phosphate fer-
tilizers over agricultural areas. The largest input of
anthropogenic cadmium to marine areas comes from
atmospheric cadmium. The atmospheric input is, how-
ever, distributed over so large an area that it probably
only has a minor impact on the cadmium concentra-
tions in the marine organisms.
8.3.4 Cadmium's Toxicity in Water
Cadmium bound in a complex form is apparently
not available for uptake in organisms and thereby not
toxic. The free, dissolved cadmium ions (Cd
++
), on the
contrary, are toxic. Because a large amount of the cad-
mium present in seawater forms chloro complexes,
cadmium (in the same dissolved concentration) will be
far more toxic in freshwater than in seawater, because
the concentration of Cd
++
is higher in the freshwater.
Experiments have shown that the lowest concentration
of cadmium that has adverse effects on the marine fl ora
and fauna is in the range of 5000 ng Cd/L. There are no
well-documented examples that contamination of the
sea with cadmium has led to adverse effects in marine
ecosystems. On the other hand, contamination of the
sea with cadmium can give rise to considerations con-
cerning the use of seafood for human consumption,
because blue mussels even from uncontaminated areas
contain relatively high amounts of cadmium.
Because calcium reduces the uptake, as well as the
toxicity, of dissolved cadmium, the calcium content of
the water (hardness) plays an important role for cad-
mium's toxicity in freshwater. Daphnia are very sensi-
tive to cadmium and in soft water; the reproduction
in our most common species of daphnia is affected at
340 ng Cd/L, and the species composition in the zoo-
plankton community is changed at exposure to 200 ng
Cd/L (Marshall and Mellinger, 1980).
8.3.5. Cadmium in Agricultural Soil and Uptake of
Cadmium into Plants
The uptake of cadmium in a specifi c plant species
is proportional to the cadmium content of the soil. The
uptake is, however, somewhat dependent on the pH
value of the soil and the content of organic material,
clay content, and agricultural practices. The variability
in the cadmium uptake between different plant species
may be very large. Because most humans have their
most important exposure to cadmium from the plant
material in our basic food (especially potatoes, cereal
products, and rice), the increase in the cadmium con-
centration of the agricultural soil and thereby in the
cadmium content of the crops has caused considerable
concern in many countries.
8.3.3 Contamination with Cadmium
Cadmium is emitted into the atmosphere by metal
smelting and incineration of coal, oil, and different
types of garbage. Runoff from gutters and discharge
from galvanization industries and other types of
