60
removed from solution to particles. Furthermore, this removal occurred just below the
sediment-water interface. Comparing the Saturation Index of CdS (4.2 < SI < 4.8) and
the distribution of Cd between sediments and porewater, it seems that the depletion of
dissolved Cd in the anoxic sediments results from the precipitation of CdS or
coprecipitation with FeS:
Cd 2+ + HS" --> CdS$ + H +
Evidence of CdS formation in anoxic sediments was found by determination of
electron beam microprobe analysis in a heavy metal polluted harbor sediments from
Michigan City (Lee and Kittrick 1984b). They reported that 89 % of the particulate
Cd is associated with sulfur in the sample. X-ray dot pictures showed that Cd, like
Cu, is dispersed rather than concentrated along the edge of particles. It seems that Cd
has been coprecipitated with sulfur, instead of being adsorbed. In the sediments of the
Northwest Mediterranean Continental Shelf, Femex et al. (1986) found the existence
of polymetallic sulfides, with which Cd could be associated. The formation of CdS
was also confirmed by laboratory experiments. In a suspension experiment of the
sediment sample from the Elbe River, WaUmann (1992a) found that the
concentrations of dissolved Cd decreased when HS" was detected in the solution.
Thus, the precipitation of CdS is suggested.
However, some studies lead to different conclusions. In the porewater of the
Continental Margin of Central California, McCorkle and Klinkhammer (1990)
showed that dissolved Cd concentrations increased from 0.6 to 1.5 nM in the top 0.5
cm of the sediments, and then rapidly decreased to values of approximately 0.12 riM.
Because there was no significant SO42- reduction and no HS" was detected in the
sediments, the decrease of dissolved Cd in the sediments can be explained by
scavenging onto freshly formed iron oxide surfaces. Tessier et al. (1985) proposed
that trace metal concentrations are controlled by adsorption onto solid substances
such as Fe oxides above the SO42" reduction zone. In these cases, the low availability
of HS" in the sediments may limit the formation of metal-sulfides. The sharp
decreases of SO42" in the Neckar River (from 1.26 mM to 0.05 raM) and its
tributaries (from 0.66 mM to 0.01 mM) show a strong SO42" reduction and HS"
production within the sediments. This may lead to the precipitation of CdS and
coprecipitation of Cd with FeS. As a result, the high concentrations of Cd in the
sediments do not result in a release of Cd into the porewater. This interpretation is in
agreement with other observations (Carignan and Nriagu 1985; Carignan and Tessier
1985; Morfett et al. 1988; Dahmke et al. 1991; Williams 1992). Therefore, the
formation of metal sulfide may act as a sink for Cd in the heavily contaminated
sediments of the study area.
4.2.3 Lead
Particulate Pb concentrations in the sediments of the Neckar River are shown in Table
4.4. High concentrations of Pb in the older sediment layer are related to those of Cu
and Cd, indicating the influence of human activities (Fig. 4.27).
removed from solution to particles. Furthermore, this removal occurred just below the
sediment-water interface. Comparing the Saturation Index of CdS (4.2 < SI < 4.8) and
the distribution of Cd between sediments and porewater, it seems that the depletion of
dissolved Cd in the anoxic sediments results from the precipitation of CdS or
coprecipitation with FeS:
Cd 2+ + HS" --> CdS$ + H +
Evidence of CdS formation in anoxic sediments was found by determination of
electron beam microprobe analysis in a heavy metal polluted harbor sediments from
Michigan City (Lee and Kittrick 1984b). They reported that 89 % of the particulate
Cd is associated with sulfur in the sample. X-ray dot pictures showed that Cd, like
Cu, is dispersed rather than concentrated along the edge of particles. It seems that Cd
has been coprecipitated with sulfur, instead of being adsorbed. In the sediments of the
Northwest Mediterranean Continental Shelf, Femex et al. (1986) found the existence
of polymetallic sulfides, with which Cd could be associated. The formation of CdS
was also confirmed by laboratory experiments. In a suspension experiment of the
sediment sample from the Elbe River, WaUmann (1992a) found that the
concentrations of dissolved Cd decreased when HS" was detected in the solution.
Thus, the precipitation of CdS is suggested.
However, some studies lead to different conclusions. In the porewater of the
Continental Margin of Central California, McCorkle and Klinkhammer (1990)
showed that dissolved Cd concentrations increased from 0.6 to 1.5 nM in the top 0.5
cm of the sediments, and then rapidly decreased to values of approximately 0.12 riM.
Because there was no significant SO42- reduction and no HS" was detected in the
sediments, the decrease of dissolved Cd in the sediments can be explained by
scavenging onto freshly formed iron oxide surfaces. Tessier et al. (1985) proposed
that trace metal concentrations are controlled by adsorption onto solid substances
such as Fe oxides above the SO42" reduction zone. In these cases, the low availability
of HS" in the sediments may limit the formation of metal-sulfides. The sharp
decreases of SO42" in the Neckar River (from 1.26 mM to 0.05 raM) and its
tributaries (from 0.66 mM to 0.01 mM) show a strong SO42" reduction and HS"
production within the sediments. This may lead to the precipitation of CdS and
coprecipitation of Cd with FeS. As a result, the high concentrations of Cd in the
sediments do not result in a release of Cd into the porewater. This interpretation is in
agreement with other observations (Carignan and Nriagu 1985; Carignan and Tessier
1985; Morfett et al. 1988; Dahmke et al. 1991; Williams 1992). Therefore, the
formation of metal sulfide may act as a sink for Cd in the heavily contaminated
sediments of the study area.
4.2.3 Lead
Particulate Pb concentrations in the sediments of the Neckar River are shown in Table
4.4. High concentrations of Pb in the older sediment layer are related to those of Cu
and Cd, indicating the influence of human activities (Fig. 4.27).