Chemistry Reference
In-Depth Information
Acute and chronic toxicity of inorganic arsenic
to fi sh and aquatic invertebrates typically occur at
concentrations of 1 mg/L (Mance
et al
., 1984c).
In some parts of the world, the soil naturally con-
tains such high concentrations of arsenic that mobi-
lization of inorganic arsenic constitutes a human
health problem when groundwater is used to supply
drinking water.
Because of the erosion from rainwater in the upper
layers of the soil, small amounts of cadmium are con-
stantly transported to freshwater systems. Because of
the differences in the cadmium concentration in dif-
ferent types of soils, the natural background concen-
tration of cadmium in freshwater may vary from one
place to another; cadmium concentrations in uncon-
taminated freshwater range between 10 and 100 ng/L.
Much higher concentrations may occur in freshwa-
ter systems because of emissions with surface runoff,
drainage, and sewage (spillover or incomplete treat-
ment) from industrial and municipal areas, especially
high concentrations occur near mines with polymetal-
lic sulfi de deposits, up to 1 mg/L (Boyle and Jonasson,
1979).
It is estimated that 69% of the cadmium transported
in the rivers of the world precipitate in estuaries (Salo-
mons and Förstner, 1984), and an average sediment
enrichment factor of 5.3 in coastal areas and lakes has
been reported (Salomons and Förstner, 1984).
The biogeochemical cycle of Cd in freshwater follows
nutrients like phosphate, silicates, and nitrates. In fresh-
water, the bioavailability and toxicity Cd has been dem-
onstrated to decrease with lower pH (Andersson and
Borg, 1988; Lawrence
et al
., 1989; Yan
et al
., 1990) and
with increasing hardness. In general, suspended organic
material scavenges Cd and makes Cd less bioavailable
and transfers Cd to the sediment. Cadmium is easily
bound to clay and organic particles in the freshwater,
and the fraction of the total cadmium in freshwater that
is found in the particulate form increases with the parti-
cle content of the water. The dissolved cadmium will to
a smaller or larger degree—depending on the pH value
and the hardness of the water—form a CdCO
3
complex,
and some of the dissolved cadmium will also be bound
to organic material dissolved in the water. This means
that the free cadmium ions (Cd
++
) in many instances only
will constitute a small percentage of the total amount of
cadmium present in the freshwater.
In full-strength seawater, dissolved cadmium is
mainly present as chloride complexes (CdCl
2
0
, CdCl
+
,
CdCl
3
−
), and Cd
++
constitutes only between 2 and 3%
of the total dissolved cadmium (Mantoura
et al
., 1978;
Sunda
et al
., 1978). At reduced salinities in estuaries
and coastal areas, Cd
++
constitutes a higher proportion
of the dissolved cadmium.
8.3 Cadmium
8.3.1 Background Levels and Emissions
Cadmium has been called “the dissipated element,”
because its uses are most often as minor components
(surface treatment, pigments, plastics, solders, fertiliz-
ers), and recycling is, therefore, diffi cult. Coal burning,
scrap smelting, and garbage incineration are sources
for air pollution with Cd. The anthropogenic mobili-
zation index for cadmium is high. Because of poten-
tial toxicological consequences of high anthropogenic
emission of cadmium, this element has become substi-
tuted from many uses.
Cadmium is a natural component of rocks, soils, and
sediments. Average concentrations in the earth's crust
and in the upper lithosphere have been estimated to
0.11 mg/kg (Bowen, 1979) and 0.5 mg/kg
1
(Goldsmith,
1958), respectively. Cadmium concentrations in igne-
ous rocks are normally well below 1 mg/kg, whereas
sedimentary rocks as bituminous and carbonaceous
shales can contain very high Cd concentrations, up to
more than 200 mg/kg (Thornton, 1992) and thereby pro-
vide extensive exposure of plants and animals because
of weathering. A survey of Cd concentrations in surface
soils from many parts of the world reported average
concentrations between 0.07 and 1.1 mg/kg, and the
authors concluded that concentrations >0.5 mg/kg
refl ect anthropogenic input (Kataba-Pendias and
Pendias, 1984).
In the open areas of the oceans, the concentration
of total cadmium ranges between 5 and 25 ng/L in the
surface water, whereas the concentration in the deep
sea typically is approximately 50-100 ng/L (Bruland,
1980; Bruland
et al
., 1978; Yeats
et al
., 1995), somewhat
higher in the Pacifi c and South Atlantic than in the
North Atlantic. The relatively low surface concentra-
tions are likely because of scavenging of Cd by micro-
plankton where it may replace zinc (Price and Morel,
1990). In Arctic and Antarctic waters, with much lower
organic concentrations, the difference in concentra-
tion between surface and deep waters is much smaller
(Macdonald and Sprague, 1988). This may explain the
very high cadmium body burdens reported in Arctic
and Antarctic vertebrates (see later).
8.3.2 Uptake in Organisms
The unicellular algae that are the fi rst link in the
food chain of the open ocean take up cadmium directly
from the seawater so that the concentration in the algae
may be up to 1000 times higher than in the surrounding
water.
