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Repartitioning of River Water Trace Metals Following Addition of Minor Amounts of Ferrous Iron
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In earlier studies along a mining region recipient (the river Vormbäcken, northern Sweden), the fraction of certain trace metals recovered in the particulate phase has been found to increase with increasing concentrations of particulate iron. This study was conducted to see if the pattern could be reproduced by adding minor amounts of ferrous iron, simulating groundwater inputs. When the iron is oxidised in the near neutral river water, it will form iron oxyhydroxides and precipitate. Trace metals may be either co-precipitated along with the iron, or adsorbed onto the freshly formed precipitates. If this is the case, such a mechanism may be incorporated as a part of a final polishing step in a mine drainage treatment system based on natural attenuation processes. The experiments were performed at 0°C and 22°C, on authentic river waters collected during the winter and summer of 2002, respectively. In order to investigate the influence of particles and bacteria initially present, filtered river waters (<1 kDalton or <0.2 μm) were also included. Small volumes of a freshly prepared FeSO4 solution were added to the experimental beakers, increasing the iron content of the waters by 0 - 1.5 mg/L. The oxidation and precipitation processes were followed by means of filtration (0.2 μm) for up to seven days. In order to be able to compare the experimental results with the actual field situation, on both experimental occasions, surface water samples were collected also from three stations situated along the river. The samples collected were analysed for iron, calcium, and trace metals by AAS and ICP-MS. The iron oxyhydroxides formed in the experiment did not influence the partitioning of As, Cu, and Pb to the same extent as the particulate iron encountered along the river appeared to do. Cd and Zn were recovered mainly in the dissolved (<0.2 μm) fraction, both in the samples collected from the experiment, and in those collected from the field. Thus, the complementary mine drainage treatment system suggested above appears to be of limited practical value. More interestingly, a clear difference was noted between the rate of formation of the iron oxyhydroxides in March and June. Most likely, the more rapid precipitation in June was due to the higher temperature. Furthermore, presence of dissolved organic matter in the river water appears to have prevented a significant part of the iron added to precipitate.
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- Date created
- 2003
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- Type of Item
- Article (Published)
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