69
total and organic Cr in the porewater of Narragansett Bay sediments increase with
depth, suggesting the formation of organic-Cr-complexes.
4.2.6 Cobalt
In contrast to other heavy metals, the concentrations of particulate Co in the
sediments of the study area display no difference between the younger and the older
sediment layer (Table 4.7). The sediments are relatively unpolluted in respect to Co
(Fig. 4.32 and 4.33)) 9
Co in the surface water was below the detection limit (20 ruM) at all sites. Porewater
Co increased with depth below the sediment-water interface, which coincides with the
9
2+
2+ .
increase of Fe andMn m the porewater.
The strong affinity of Co for Fe/Mn oxides has been well described in the scientific
literature (Gendron et al. 1986; Shaw et al. 1990; Williams 1992; Wallmann 1992b),
Balistrieri et al. (1992) reported that temporal changes of dissolved Co in Lake
Sammamish, Washington are very similar to those of dissolved Mn. Based on a
laboratory experiment, Wallmarm (1990) demonstrated that dissolved Co is released
with the reduction of Fe/Mn oxides. Co in the solution decreased with the increase of
HS- concentrations, suggesting the formation of CoS. This is supported by the
saturation index of CoS in the solution:
Co 2+ + ItS" ~ COS,[,+ H +
The saturation calculation for CoS indicates that CoS was m equilibrium with the
porewater (Fig. 4.32 and 4.33) at all sites. It appears that Co cycling m the sediments
is related to Fe and Mn oxide reduction and CoS precipitation. In the bottom water,
Co is most likely associated with Fe/Mn oxides and is removed from the water by
settling particles. After the deposition, dissolved Co is released during the reduction
of Fe and Mn oxides and is controlled by the precipitation of CoS.
Table 4.7. Distribution of Co in the sediments (< 20 ~tm) of the Neckar River and its
tributaries
Depth
Average
Min.
Max.
cm
mg/kg
mg/kg
mg/kg
Lauffen
0-20
12
10
14
20-64
13
9.4
16
Kochendorf
0-22
7.8
5.8
12
22-60
9.5
8.7
10
Wieblingen
0-6
8.0
8.0
8.0
6-32
9.3
8.0
10
Elsenz
0-34
12
8.3
13
Enz
0-22
I0
9.3
I I
22-60
9.1
6.2
11
total and organic Cr in the porewater of Narragansett Bay sediments increase with
depth, suggesting the formation of organic-Cr-complexes.
4.2.6 Cobalt
In contrast to other heavy metals, the concentrations of particulate Co in the
sediments of the study area display no difference between the younger and the older
sediment layer (Table 4.7). The sediments are relatively unpolluted in respect to Co
(Fig. 4.32 and 4.33)) 9
Co in the surface water was below the detection limit (20 ruM) at all sites. Porewater
Co increased with depth below the sediment-water interface, which coincides with the
9
2+
2+ .
increase of Fe andMn m the porewater.
The strong affinity of Co for Fe/Mn oxides has been well described in the scientific
literature (Gendron et al. 1986; Shaw et al. 1990; Williams 1992; Wallmann 1992b),
Balistrieri et al. (1992) reported that temporal changes of dissolved Co in Lake
Sammamish, Washington are very similar to those of dissolved Mn. Based on a
laboratory experiment, Wallmarm (1990) demonstrated that dissolved Co is released
with the reduction of Fe/Mn oxides. Co in the solution decreased with the increase of
HS- concentrations, suggesting the formation of CoS. This is supported by the
saturation index of CoS in the solution:
Co 2+ + ItS" ~ COS,[,+ H +
The saturation calculation for CoS indicates that CoS was m equilibrium with the
porewater (Fig. 4.32 and 4.33) at all sites. It appears that Co cycling m the sediments
is related to Fe and Mn oxide reduction and CoS precipitation. In the bottom water,
Co is most likely associated with Fe/Mn oxides and is removed from the water by
settling particles. After the deposition, dissolved Co is released during the reduction
of Fe and Mn oxides and is controlled by the precipitation of CoS.
Table 4.7. Distribution of Co in the sediments (< 20 ~tm) of the Neckar River and its
tributaries
Depth
Average
Min.
Max.
cm
mg/kg
mg/kg
mg/kg
Lauffen
0-20
12
10
14
20-64
13
9.4
16
Kochendorf
0-22
7.8
5.8
12
22-60
9.5
8.7
10
Wieblingen
0-6
8.0
8.0
8.0
6-32
9.3
8.0
10
Elsenz
0-34
12
8.3
13
Enz
0-22
I0
9.3
I I
22-60
9.1
6.2
11