7 Chemical Associations of Heavy Metals in Polluted Sediments from the Lower Rhine
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River ULRICH FÖRSTNER and SAMBASIVA R.
1
PATCHINEELAM
Institut für Sedimentforschung, Universität Heidelberg Im Neuenheimer Feld 236, D-69 Heidelberg, West Germany
Differentiation of sedimentary metal phases was performed on grain size fractionated samples from the lower Rhine River by successive chemical leaching (review). Pollution affects the significant increase of nonresidual associations of chromium, copper, lead, and zinc. Except for manganese the metal contents in most of the extracted phases decrease as the grain size increases. Phase concentration factors (PCF; relative enrichment of metal content in major carrier substances) are high for chromium in moderately reducible phases (20-fold increase in clay-sized particles), for man ganese and zinc in the easily reducible sediment fraction (30- and 55-fold enrichment), and for copper and zinc in the carbonates (15 or 25 times compared with total sediment).
T h e availability of trace metals for metabolic processes is closely related to their chemical species both i n solution and i n particulate matter. The type of chemical association between metals and sediment constituents has therefore become of interest i n connection with prob lems arising from the recovery and disposal of contaminated dredged material. Chemical extraction procedures from soil studies ( I ) and sedimentary geochemistry (such as the differentiation of detrital and authigenic phases i n limestones ( 2 ) , shales (3), pelagic Mn/Fe-concrer
A
Present address: Instituto de Geosciências Universidade Federal da Bahia, 40.000-Salvador, Brazil. 1
0-8412-0499-3/80/33-189-177$05.00/0 © 1980 American Chemical Society
Kavanaugh and Leckie; Particulates in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1980.
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PARTICULATES
IN WATER
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tions (4), and recent deposits of less polluted aquatic environments (5, 6,7)) have been developed enabling the estimation of the toxicity potential of a sediment (8-13). The basic rationale of these studies is that the nonlithogenic fractions of the sediment, dredged material, or sewage substances constitute "the reservoir for potential subsequent release of contaminants into water columns and into new interstitial waters" (14), thus being predominantly available for biological uptake (15,16,17,18). Criteria to predict the pollution potential of these substances must reflect the sediment fraction that has a detrimental effect on water quality and the associated biota (19-23). In that respect, the differentiation of oxidizable, reducible, and residual phases on polluted sediments near a sewer outfall of the County of Los Angeles was made by Bruland et al. (24). Studies by Gupta and Chen (25) on sediment from the Los Angeles harbor and by Brannon et al. (26) on contaminated harbor sediments from Lake Erie, Mobile Bay, Alabama and Bridgeport, Connecticut, were performed by applying successive extraction techniques to include the determination of the metal contents i n interstitial water, and i n ion-exchangeable, easily reducible, and moderately reducible organic and residual sediment fractions. It has been established by these and other investigations (27,28) that the sur plus of metal contaminants introduced into the aquatic system from man's activities usually exists i n relatively unstable chemical associations and is, therefore, predominantly accessible for biological uptake (29). Direct investigations on the effect of metal partitioning i n different sediment components on metal availability to organisms were initially performed by Luoma and Jenne (19,20,21). Using deposit-feeding clams on various types of sedimentary substrates, which were labeled with heavy metal nuclides (for example, cadmium-109), these studies indicate that the bioavailability of the heavy metals was inversely related to the strength of metal-particulate binding i n the sediments. Regression analyses (availability indices) were evaluated from sediment fractions and metal concentrations i n related deposit-feeding bivalves (30), indicating, for example, that the uptake of zinc by Macoma baltica in San Francisco Bay is controlled by the competitive partitioning of zinc between extractable iron and manganese forms i n the substrate. Despite many unanswered questions, this new approach may be useful as "a statistical interface between more sophisticated chemical and biological models" (30). Further insight into the processes of metal accumulation i n aquatic sediments and estimations of these possible effects on biota could be expected from an analysis of the metal phases within different grain size intervals and from the correlation of metal associations with the per centages of their respective carrier substance i n the sediment samples.
Kavanaugh and Leckie; Particulates in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1980.
7.
FORSTNER A N D P A T C H I N E E L A M
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Associations of Heavy Metals
Results of studies on recent mud deposits of the heavily polluted lower Rhine River (31, 33) near the German/Dutch border (13) are compared with the data from a sediment study on Lake Constance (33), where the contamination by heavy metals is still relatively low (34).
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Extraction
Procedures
Grain size fractionation was performed by sieving, separating in settling tubes, and by centrifuging (laboratory centrifuge; 100-mL test tubes) in the range 63 /xm. The latter two procedures were performed with distilled water and each step was repeated 5-10 times. The solids were recovered by evaporation of the suspension in a por celain bowl at 70° C. Other than in the case of filtration no metal components are lost. However, it cannot be excluded that metal w i l l be stripped off from coarser grained particles, which would enable accumu lation in finer grained fractions. The finest grained centrifuge fraction (
