76
4.3.2 Influence of organic complexes and Fe/Mn redox processes
Natural and synthetic organic substances have been recognized as important factors in
controlling transport and deposition of heavy metals in aquatic systems. The
coexistence of organic ligands (e.g. humic substances) and metal ions in aquatic
solutions often result in the formation of organic complexes. For example, humic
acids can rapidly decompose sulfides such as sphalerite (ZnS), galena (PbS), or pyrite
(Ftrsmer and Wittmann 1979). Park and Huang (1989) reported that the dissolution
of CdS is greatly enhanced in the presence of EDTA. As the mineralization of organic
matter leads to the release of organic complexing materials into the porewater,
dissolved organic carbon (DOC) increases with depth (Orem et al. 1986; Vuy-novich
1989; Chin and Gschwend 1991). This increase may lead to the formation of metalorganic-complexes and the mobilization of heavy metals. Gerringa (1990) found that
dissolved Cu concentrations are controlled by concentrations of organic ligands. The
Cu concentrations decreased proportionally with the decrease of the ligand
concentration. For Cd and Pb, no significant influence was found.
As organic complex formation enhances the mobility of heavy metals, it is
important to know whether metals in the porewater are controlled by precipitationdissolution reactions or by metal-organic-complex reactions. If complex formation is
the dominating process, the increase of DOC should cause an increase in the
concentrations of heavy metals. Indeed, dissolved Cd, Zn, Pb, and Cu did not increase
with depth in the study area. Since the sediments are able to provide HS" by the
reduction of SOa 2-, the precipitation of metal-sulfides, instead of the formation of
organic omplexes, may control the concentrations of dissolved Cd, Zn, Pb, and Cu.
Elderfield (198 I) reported that 80 % of the dissolved Cu, 40 % of Fe were associated
with organic material in the porewater from Narragansett Bay. The concentration of
DOC increased with depth, but the concentrations of organic-bound metals decreased
with depth. This can be interpreted that organic-bound metals are released from
organic complexes and formed insoluble metal-sulfides. Similar results were reported
by Douglas et al. (1986). It seems that the interaction of organics with metalz is not
sufficient to prevent the precipitation of metal-sulfides.
Another important factor influencing the mobility of heavy metals are Fe and Mn
oxides. With the reduction of Fe and Mn oxides, heavy metals adsorbed on these
oxides could be released into porewater. In a laboratory experiment, Francis and
Dodeg (1990) found that there was a significant release of Cd, Pb, Zn, Ni, and Cr
bound to Fe oxides during the anoxic degradation of organic matter.
It is then important to know whether dissolved metals are controlled by the
adsorption/ desorption on Fe/Mn oxides, or by precipitation/dissolution of" metalsulfides in the sediments of the study area. If the letter is the case, the eoncerxtrations
of heavy metals should be independent on the metal concentrations in the solid phase.
However, an increase in the concentrations of dissolved Cd, Zn, Pb, and Cu with
depth was not found in the study area. The probable explanation is that these metals
released from the Fe and Mn oxides are rapidly precipitated as metal-sulfides. This is
supported by the observation in the sediments of the Milltown Reservoir (Moore et al.
4.3.2 Influence of organic complexes and Fe/Mn redox processes
Natural and synthetic organic substances have been recognized as important factors in
controlling transport and deposition of heavy metals in aquatic systems. The
coexistence of organic ligands (e.g. humic substances) and metal ions in aquatic
solutions often result in the formation of organic complexes. For example, humic
acids can rapidly decompose sulfides such as sphalerite (ZnS), galena (PbS), or pyrite
(Ftrsmer and Wittmann 1979). Park and Huang (1989) reported that the dissolution
of CdS is greatly enhanced in the presence of EDTA. As the mineralization of organic
matter leads to the release of organic complexing materials into the porewater,
dissolved organic carbon (DOC) increases with depth (Orem et al. 1986; Vuy-novich
1989; Chin and Gschwend 1991). This increase may lead to the formation of metalorganic-complexes and the mobilization of heavy metals. Gerringa (1990) found that
dissolved Cu concentrations are controlled by concentrations of organic ligands. The
Cu concentrations decreased proportionally with the decrease of the ligand
concentration. For Cd and Pb, no significant influence was found.
As organic complex formation enhances the mobility of heavy metals, it is
important to know whether metals in the porewater are controlled by precipitationdissolution reactions or by metal-organic-complex reactions. If complex formation is
the dominating process, the increase of DOC should cause an increase in the
concentrations of heavy metals. Indeed, dissolved Cd, Zn, Pb, and Cu did not increase
with depth in the study area. Since the sediments are able to provide HS" by the
reduction of SOa 2-, the precipitation of metal-sulfides, instead of the formation of
organic omplexes, may control the concentrations of dissolved Cd, Zn, Pb, and Cu.
Elderfield (198 I) reported that 80 % of the dissolved Cu, 40 % of Fe were associated
with organic material in the porewater from Narragansett Bay. The concentration of
DOC increased with depth, but the concentrations of organic-bound metals decreased
with depth. This can be interpreted that organic-bound metals are released from
organic complexes and formed insoluble metal-sulfides. Similar results were reported
by Douglas et al. (1986). It seems that the interaction of organics with metalz is not
sufficient to prevent the precipitation of metal-sulfides.
Another important factor influencing the mobility of heavy metals are Fe and Mn
oxides. With the reduction of Fe and Mn oxides, heavy metals adsorbed on these
oxides could be released into porewater. In a laboratory experiment, Francis and
Dodeg (1990) found that there was a significant release of Cd, Pb, Zn, Ni, and Cr
bound to Fe oxides during the anoxic degradation of organic matter.
It is then important to know whether dissolved metals are controlled by the
adsorption/ desorption on Fe/Mn oxides, or by precipitation/dissolution of" metalsulfides in the sediments of the study area. If the letter is the case, the eoncerxtrations
of heavy metals should be independent on the metal concentrations in the solid phase.
However, an increase in the concentrations of dissolved Cd, Zn, Pb, and Cu with
depth was not found in the study area. The probable explanation is that these metals
released from the Fe and Mn oxides are rapidly precipitated as metal-sulfides. This is
supported by the observation in the sediments of the Milltown Reservoir (Moore et al.