The biogeochemistry of heavy metals in polluted lakes and streams at Flin Flon,Canada, and a proposed method for limiting heavy-metal pollution of natural waters |
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| Authors: | Togwell A Jackson |
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| Institution: | (1) Freshwater Institute, 501 University Crescent, R3T 2N6 Winnipeg, Manitoba, Canada |
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| Abstract: | The biogeochemistry of Zn, Cd, Cu, Hg, and Fe in lakes and streams polluted by mine and smelter wastes emitted at Flin Flon,
Canada, was investigated. In Schist Lake, a repository for both tailings-pond drainage and sewage, green algal blooms generated
by nutrients from sewage promote entrapment of metals in sediments by (1) accumulation of metals from solution by algal seston,
with preferential uptake of Zn, the most abundant metal, followed by sinking of the seston; and (2) production of H2S during decomposition of dead algae, resulting in sulfide precipitation. Metals are partially resolubilized from seston as
it decomposes while sinking. Preferential retention of Cu by sinking seston and by mud promotes Cu enrichment in the mud but
the Cu/Zn ratio of mud varies with the Cu/Zn ratio of surface water seston. In bottom muds, partitioning of a metal between
sulfide and organic matter is strongly dependent on the stability of the metal sulfide as measured by its standard entropy,
the proportion of sulfide-bound metal decreasing in the order Hg>Cd>Cu>Fe>Zn. When sulfide-rich muds were heated under helium,
x-ray diffraction revealed abundant well-crystallized ZnS (sphalerite) containing Cd, Hg, and Fe; only poorly crystallized
traces of the mineral were detected in unheated mud, however. Cu sulfide failed to crystallize, suggesting interference by
sorbed impurities. Metals were concentrated in H2S-rich muds and extraction of muds with various solvents and by electrodialysis showed that sulfide was much more effective
than organic matter in suppressing remobilization of metals. Remobilized Cu is probably bound to organic complexing agents.
Some extractable complexing agents bind Cu preferentially with respect to Zn and Cd but others preferentially bind Zn and
Cd; the complexes, being stable in the presence of free sulfide, may cause some release of metals from sulfide-rich muds in
nature.
These results indicate that introduction of sewage together with heavy-metal effluents into settling ponds could be an effective
and economic method for limiting heavy-metal pollution of natural waters. |
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