Concentration and partitioning of heavy metals in the Scheldt estuary
Van Alsenoy, V. (1993). Concentration and partitioning of heavy metals in the Scheldt estuary. PhD Thesis. Universiteit Antwerpen: Antwerpen. 290 pp.
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Document type: Dissertation
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| Abstract |
The continuous growth of technology has lead to an (uncontrolled) introduction of contaminants into the biosphere. The efforts of removing man-made pollutants from the natural environment have been unable to cope with the increasing amounts of waste materials and growing population. This work studies one group of substances which have a particular lasting effect on the natural balance in aquatic systems; the heavy metals. Trace metals are present in all the abiotic reservoirs of the aquatic systems; i.e. the water, the suspended material and the sediments and its pore waters. Complexation of the dissolved metals with organic and inorganic ligands, sorption and biological processes are the main processes determining the distribution of the investigated metals (Cd, Cu, Hg, Pb and Zn) between the dissolved and particulate phase under oxic conditions. Under anoxic conditions, sulphide precipitation controls the behaviour of the investigated metals. Chemical processes occurring at the solid-water interface greatly influence the geochemical cycles of many elements, as well as the composition of natural waters and the flux of material through the hydrosphere. A theoretical description based on the surface complexation model, the competitive adsorption model and sulphide precipitation, was used to model the behaviour of the investigated heavy metals in the Scheldt estuary. Different analytical techniques: AAS, XRF, ASV, were optimized and used for the experimental determination of the heavy metals in the different abiotic reservoirs and their partitioning between the dissolved and particulate phase. An intercomparison exercise was carried out to investigate the possibility of determining elemental concentrations in very small amounts (1-5 mg) of natural suspended material after acid digestion. Results show that the precision and accuracy of the analytical procedure are mainly limited by inhomogeneities inherent to natural suspended material. Great difficulties were encountered for the determination of Cu in estuarine and marine water. The digestion of the organic material by UV and micro wave radiation improved the accuracy of the measurement, but the problem could not be solved entirely. For the determination of Hg in sediments, investigation of the extraction techniques and of the possible interferences during measurement, showed that Hg could be determined with satisfactory accuracy and precision using cold vapour AAS. The heavy metals content of the sediments, the suspended material and the water of the Scheldt estuary and part of the North Sea were investigated. Several normalization methods showed that the sediments of the Belgian North Sea and the Scheldt estuary are polluted with the heavy metals Zn, Cd, Pb and Hg, with average enrichment factors between 2 and 10. This pollution becomes extreme in the industrial part of the Scheldt where enrichment factors for a single sample can reach up to e.g. 130 for Cd. These maxima occur for each of the four elements. Cu and Ni do not show such an enrichment in most of the sediments of the area. The pollution level of the North Sea and harbour sediments seems to be controlled by the same factors; these sediments form a population for which grain size is the more important factor controlling the heavy metal content. This was proven by the obtained regression lines, correlation analysis and the analysis of the fraction <63 pm. For a second population, the sediments of the industrial part of the Scheldt, grain-size becomes a minor factor and the pollution is very variable with time and location. Industry and discharge of urban and domestic waste water are the most likely causes. The newly obtained data have been compared with previous data to establish the trends with time. Comparison of the slopes of the regression curves suggests that little changes in the metal contamination have occurred during the last 15 years. Sequential extraction analysis shows that the Fe/Mn oxide fractions are important for the sorption of Cd, Zn and Pb in the sediments of the Scheldt estuary. For Cu this fraction seems to be of no importance. Probably the organic/sulphide fraction is important for the sorption of Cu. The investigation of the heavy metals in the different phases during a tidal cycle revealed no trends as a function of salinity, current velocity, DO or any other investigated parameters. The distribution coefficient of Cd, Cu, Zn and Pb considering desorption processes and the adsorption of 109Cd were investigated using natural particulate material from the Scheldt estuary .The desorption of Cu from polluted sediments is very slow. Only at high salinities, minor desorption was detected. At persistently low pH (< 4), desorption was quantitative. Also Pb desorption is very limited and only strong acidification of the solution will desorb considerable fractions of the element. The desorption of Cd is significant as a function of all the parameters. The desorption process is fast and up to 60 % of the adsorbed Cd can be released into natural water. Both salinity and pH can change the distribution between the solid and the dissolved phase considerably. The extent of the Zn desorption is intermediate between Cd and Pb. Both salinity and pH are important parameters to be considered for the sorption processes in the natural environment. Temporary changes in the pH change the desorption of Zn and Pb irreversibly, but have only a minor influence on the desorption of Cu and Cd. The character of metal-sediment binding, which is involved in the desorption process, is not changed by drying the sediment. The adsorption of 109Cd on natural particulate material from the Scheldt estuary changes as a function of temperature, salinity and composition of the particulate material. The temperature has only a minor influence on the Kd-salinity relation, while the composition of the suspended matter appeared to be very important. Even though the number of samples used was very low, preliminary results do show that the organic content of the sediments may be the most important parameter for the Cd adsorption. The similar relation between the Kd of the adsorption process, measured using 109Cd and the desorption process as a function of salinity, indicates that character of the metal-sediment binding of the adsorbed 109Cd may be identical to that of the naturally bound Cd which is released during the desorption process. The magnitude of the Kd, however, needs to be determined taking into account the different environmental parameters. Comparison of the calculated and measured dissolved Cd, Hg and Pb concentrations in the Scheldt estuary under anoxic conditions shows that these concentrations cannot be described by sulphide precipitation and complexation reactions. The conditions of anoxicity in the estuary cannot be compared to the conditions present in a permanently anoxic basin. The dissolved Cu concentrations can be described using sulphide precipitation and complexation if Cu+ is used as the dominant Cu-species. The model calculations show that the dissolved Zn concentration could be reasonably well explained by sulphide precipitation and complexation. During oxic conditions, the use of average input parameters gives a reasonably good agreement between the measured Kd values in the field and the calculated values for Cd and Zn. The calculated values for Cu, Hg and Pb are generally too high compared to the measured ones, which implies that the use of the Ki and N may be unrealistic. To validate the model properly it is of primary importance to construct a database containing all the experimentally determined input parameters, so that the use of average values can be avoided. The model explains well the observed trends and there is generally little difference in the calculated values when the surface complexation concepts were or were not used. Both models result in similar relations and magnitudes for the Kd values. It is possible that the used dissolved phase for the laboratory experiments does not contain the organic ligands determined by Van den Berg et al. (1987) since these solutions were obtained by mixing open sea water with riverine water. Van den Berg et al. (1987) investigated real estuarine water, while our solutions only approximate the concentrations of the components which behave conservatively during estuarine mixing. No conservative behaviour of the dissolved organic ligands has so far been reported for the Scheldt estuary. When no organic ligands were included the model explains well the obtained Kd-salinity relation for Cu and Zn for the desorption experiments. |
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