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4.3 Mobilization of heavy metals
Sediments are both carriers and potential sources of contaminants in aquatic systems.
Natural and man-induced processes (e.g. flood, sediment erosion, dredging) may
change physicochemical conditions in sediments, and consequently lead to the release
of the contaminants. The large amounts of contaminated sediments in the Neckar
River and its tributaries pose problems for this aquatic system. For instance, the
resuspension and transport of sediments by flood occur often in the Neckar river. This
may lead to a release of toxic substances from the sediments. Information about the
mobilization of heavy metals in this area is necessary to evaluate whether heavy
metals may be released into the overlying water and be bioavailabte. This knowledge
is essential for water management and sediment cleanup plans.
In surface waters, stable phases of heavy metals are adsorbed species. Fe and Mn
oxides are the main career for Cd, Zn, and Ni, while the organic fraction is
predominant for Cu and Pb (Tessier et al. 1985; Salomons et al. 1987). After
deposition, the mineralization of organic matter may influence the main carriers. In
this process, oxidation of organic matter, reduction of sulfate and Fe/Mn oxides are
Important. As reaction products, sulfide and organic colloids are released into the
porewater, resulting in the formation of highly insoluble metal sulfide compounds
and dissolved metal-organic complexes. There is a competition between metal-sulfide
precipitation, metal-organic-complex dissolution, and Fe/Mn oxides adsorption. This
competition can mainly control the mobility and bioavailability of heavy metals in
sediments.
4.3.1 Metal-sulfide precipitation/dissolution and mobility of Cd, Zn, Pb, and Cu
On the basis of the solubility calculations for metal-sulfide compounds, heavy metals
such as Cd, Zn, Pb, and Cu are supersaturated and remain fixed in the sediments as
sulfide. Variations of redox potentials in sediments lead to changes in the binding
forms of heavy metals on particles, which controls their chermcal behavior in the
sediments. If the sediments are removed from the reducing environment and have
contact with 02 in the overlying water, heavy metals bound to sulfides would be
released by oxidation (Moore et al. 1988), In the suspension of anoxic sediments from
the Hamburg harbor, pH values decreased from neutral (7.0) to strongly acidic (3.4)
(Calmano et al. 1992). This may be due to the oxidation of sulfide:
FeS + 9/4 O2 + 5/2 H20 ~ Fe(OH)3 + SO42" + 2 H §
Therefore, a low buffer capacity in the sediment samples cannot not prevent pH
decrease. A significant release of heavy metals (Cd, Zn, Pb, and Cu) during the
oxidation of anoxic sediments occurs only if the pH sinks below 4.5. Above this pH
value, heavy metals dissolved due to the oxidation of anoxic sediments, may be
adsorbed onto freshly formed Fe and Mn oxides surfaces. The pH is then a key factor
4.3 Mobilization of heavy metals
Sediments are both carriers and potential sources of contaminants in aquatic systems.
Natural and man-induced processes (e.g. flood, sediment erosion, dredging) may
change physicochemical conditions in sediments, and consequently lead to the release
of the contaminants. The large amounts of contaminated sediments in the Neckar
River and its tributaries pose problems for this aquatic system. For instance, the
resuspension and transport of sediments by flood occur often in the Neckar river. This
may lead to a release of toxic substances from the sediments. Information about the
mobilization of heavy metals in this area is necessary to evaluate whether heavy
metals may be released into the overlying water and be bioavailabte. This knowledge
is essential for water management and sediment cleanup plans.
In surface waters, stable phases of heavy metals are adsorbed species. Fe and Mn
oxides are the main career for Cd, Zn, and Ni, while the organic fraction is
predominant for Cu and Pb (Tessier et al. 1985; Salomons et al. 1987). After
deposition, the mineralization of organic matter may influence the main carriers. In
this process, oxidation of organic matter, reduction of sulfate and Fe/Mn oxides are
Important. As reaction products, sulfide and organic colloids are released into the
porewater, resulting in the formation of highly insoluble metal sulfide compounds
and dissolved metal-organic complexes. There is a competition between metal-sulfide
precipitation, metal-organic-complex dissolution, and Fe/Mn oxides adsorption. This
competition can mainly control the mobility and bioavailability of heavy metals in
sediments.
4.3.1 Metal-sulfide precipitation/dissolution and mobility of Cd, Zn, Pb, and Cu
On the basis of the solubility calculations for metal-sulfide compounds, heavy metals
such as Cd, Zn, Pb, and Cu are supersaturated and remain fixed in the sediments as
sulfide. Variations of redox potentials in sediments lead to changes in the binding
forms of heavy metals on particles, which controls their chermcal behavior in the
sediments. If the sediments are removed from the reducing environment and have
contact with 02 in the overlying water, heavy metals bound to sulfides would be
released by oxidation (Moore et al. 1988), In the suspension of anoxic sediments from
the Hamburg harbor, pH values decreased from neutral (7.0) to strongly acidic (3.4)
(Calmano et al. 1992). This may be due to the oxidation of sulfide:
FeS + 9/4 O2 + 5/2 H20 ~ Fe(OH)3 + SO42" + 2 H §
Therefore, a low buffer capacity in the sediment samples cannot not prevent pH
decrease. A significant release of heavy metals (Cd, Zn, Pb, and Cu) during the
oxidation of anoxic sediments occurs only if the pH sinks below 4.5. Above this pH
value, heavy metals dissolved due to the oxidation of anoxic sediments, may be
adsorbed onto freshly formed Fe and Mn oxides surfaces. The pH is then a key factor