Assessment of toxicity of heavy metals accumulated in lake sediments
Vladimir
Dauvalter
Institute of the North Industrial
Ecology Problems (INEP), Kola Science Centre, Russian Academy of Sciences, 14
Fersman St., 184200 Apatity, Murmansk region, Russia
Phone: +7 81555 79786, Fax: +7 81555
79772, E-mail: vladimir@inep.ksc.ru
Sediments
are habitat and energy supply for many freshwater organisms, which are used by
man in food. The polluting substances are accumulated by sediments and then
enter in water organisms as a result of live activity. Bioaccumulation and
transition of polluting substances by food chain are the complex processes, on
which geochemical, environment, physiological and biochemical features of
organism influence. It is very difficult to give the exact prognosis of
influence of the polluting substances accumulated in sediments on live
organisms. It is difficult also to determine a degree of biological influence
of polluting substances, particularly heavy metals, first, because different
organisms in a various degree are subject to their influence, and, secondly,
owing to complex behaviour of metals in reservoirs. The metals can be
considered as biologically available chemical substances, when they are
consumed by organisms and participate in their metabolism.
The
aim of the investigation is the assessment of toxicity of heavy metals (Ni, Cu,
Zn, Mn, Fe) accumulated in lake sediments by ammonium acetate extraction
technique. This extraction is offered as one of techniques to define an
influence of the toxic forms of metals accumulated in sediment for the period
of activity of the industrial enterprises on aquatic plants and animals and
water ecosystems.
Sediment samples were
collected at 18 stations of various zones of Imandra Lake (the largest
reservoir of Murmansk Region) polluted by air emissions and wastewaters of
mining and metallurgical enterprises.
Metal concentrations were determined by flame atomic absorption
spectrophotometry (Model Perkin-Elmer 460 and 560) using the standard addition
technique. For total metal analyses ~ 0.4 g (dry wt.) was
taken and digested in a Teflon “bomb” for 4 h at 140°C with 4 ml concentrated
nitric acid.
To determine the metal contents in the mobile form the sediment samples
of superficial (0-1 cm) and background (between 20 and 30 cm) layers were
digested by 1M solution of NH4OAc (pH=4.65) in weight ratio 1:10
during ~15 h with subsequent agitation during 2 h. After centrifugation (at
6000 rev/min.) the metal concentrations by flame atomic absorption spectrophotometry
were determined in the solution.
From a wide spectrum of investigated
elements the heavy metals essential to live of water organisms of Imandra Lake
have been chosen. They are priority pollutants – Ni,
Cu, Zn; Mn and Fe, i.e. metals, oxides and hydrooxides of which are perfect
adsorbents of heavy metals and behaviour of which depends on physical and
chemical environment conditions (pH, Eh).
Concentrations of the mobile
forms of Ni (Nimob.) in sediment of Imandra Lake in superficial and
background layers were 1-69 % (in average 23%) of total concentrations (Nitot.).
The maximal value of Nimob. (10940 mg/g dry wt.) was found in most polluted
water area of Imandra Lake – Monche Bay, where the wastewaters of Ni-Cu
metallurgical Company “Severonickel” enter. The Nimob. value was 69%
of Nitot. (15940 mg/g dry wt.). The part of the mobile forms of Ni in superficial layer
was in 2.5 times more that in background. Therefore, increasing total
concentrations in the superficial polluted sediment amplify the part of the
mobile forms of Ni, i.e. alongside with increasing Nitot. in
sediments of the polluted zones of Imandra Lake the percentage of the contents
of the mobile forms was also raising. Therefore, the ecological risk for
aquatic plants and animals and for whole lake ecosystem is increasing.
The same appropriateness, as
for distribution of Ni, is characteristic for distribution of the mobile forms
of Cu in sediments of Imandra Lake. However, the part of the mobile forms of Cu
in total concentrations is lower. Concentrations of Cumob. in
sediments of Imandra Lake were 4-39% (in average 13%) of Cutot.. The
part of Cumob. in superficial sediment layers, as well as Nimob.,
in average in 2.5 times is more, than in background layers. Therefore, as well
as for Ni, with increasing Cutot. in sediments in the polluted zones
of Imandra Lake the part of Cumob. was also raising, i.e. the
ecological risk of Cu pollution for aquatic biota increased.
Significant difference in
distribution of Znmob. in superficial and background sediments of
Imandra Lake was not noticed.
The research of the mobile
forms of Mn and Fe in sediments is very important, because Mn and Fe play one
of the central roles in migration of elements, including heavy metals. Mn is
able to be accumulated in water environment basically owing to reduction of Eh
and pH; versus other heavy metals, for it the low involving in complex
compounds are characteristic; Mn has the less ability to associate with organic
compounds.
The appropriateness of
distribution of Mnmob. in sediments of Imandra Lake is different
from that for Ni and Cu. Concentrations of Mnmob. in superficial and
background sediment layers of Imandra Lake were 4-95% (in average 48%) of Mntot..
Of all investigated metals Mn has the largest part of the mobile forms. Versus
Ni and Cu, with increasing Mntot. in superficial sediments of
Imandra Lake the part of the mobile forms reduced. In background sediment
layers the part of Mnmob. increased. To illustrate, distribution of
Mn in sediments of Kurenga Bay, where during a half of the century the
wastewaters of Olenegorsk ore dressing plant, which extracts and enriches of
iron ores (magnetite and haematite quartzites) with a high content of Mn,
enter. In superficial 0-1 cm layer Mnmob.=24% Mntot.
(5500 and 22900 mg/g dry wt., respectively). In 19-20-cm background layer Mnmob.=83%
Mntot. (3300 and 4000 mg/g dry wt., respectively). It is possible to explain this phenomenon,
that in anoxic conditions at the lower depth of sediments in reduced conditions
the significant part of Mn is in the mobile form, and in superficial layer
owing to contact of sediment with superstratum water saturated with oxygen, Mn
oxidized and passed in the insoluble forms.
The appropriateness of
distribution of Femob. in sediments of Imandra Lake was similar to
that revealed for Mn. However, the part of the mobile form of Fe was much
lower - 0.3-9% (in average 3.6%). As
well as for Mn, it was found, that with increasing total concentration of Fe in
superficial sediments of Imandra Lake the percentage of the mobile forms
reduced in comparison with background layers. For example, in superficial 0-1
cm layer of the Kurenga Bay Femob. was 2.6% of Fetot.
(1078 and 41635 mg/g dry wt., respectively). In 19-20-cm background layer Femob.
was 6.0% of Feвал. (2030 and 34100 mg/g
dry wt., respectively). The similar appropriateness, as well as for Mn, was
noticed at all stations of Imandra Lake.
Therefore, for assessment of
toxicity of metals accumulated in sediments the definition of the mobile forms by ammonium acetate extraction
technique is offered. In sediments of Imandra Lake
the increase of the part of the mobile form of Ni and Cu was noticed as
increase of total concentration in the superficial polluted sediment, so the
ecological risk for aquatic biota and for lake ecosystem raised. With
increasing total concentrations of Fe and Mn in superficial sediments the part
of the mobile forms reduced.