Abstract
The binding forms of iron and trace elements (Ni, Co, As) in overburden dump sediments of the Rheinland lignite mining district in Germany were investigated by means of a newly developed sequential extraction procedure in columns (see part 1: Development and description of a new method). The extraction tests aimed to prove the long-term immobilization of iron and heavy metals due to the addition of crushed limestone to the sediment in an effort to reduce the effects of pyrite oxidation reactions on the groundwater quality in lignite overburden dumps.
Because of pyrite oxidation reactions the groundwater in the lignite overburden dumps will contain high concentrations of iron, heavy metals and sulfate at low pH values after the recovery of the groundwater level without taking measures against the acidification. In order to neutralize the acid and to limit the iron, sulfate and heavy metal mobility, crushed limestone is added to the sediment. The investigations aimed to determine the differences between the iron and trace metal binding forms in sediment samples with and without the addition of alkaline substances. The experimental set-up described in part 1 could ensure anaerobic conditions as in the dumps and enabled the determination even of very small amounts of freshly precipitated mineral phases, which was necessary to identify the metal immobilizing processes under realistic conditions.
In investigating the untreated overburden material, iron and heavy metals were mainly extracted from the water-soluble and exchangeable fraction, which means that they are highly mobile and will dissolve in the groundwater in high concentrations. The addition of crushed limestone causes the precipitation of iron carbonate and iron hydroxide mineral phases and the coprecipitation of the trace elements. Under the hydrochemical conditions that are present in the overburden dumps after the limestone addition, these phases are stable and act as an effective and long-term iron and trace element sink.
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Acknowledgements
The results presented were obtained in two comprehensive projects funded by RWE Rheinbraun AG (Cologne), Niederrheinische Versorgung und Verkehr AG (Mönchengladbach), Niederrheinwerke Viersen AG (Viersen) and the Environmental Protection Agency/NRW (LUA/NRW, Essen).
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Wisotzky, F., Cremer, N. Sequential extraction procedure in columns—part 2: application of a new method. Env Geol 44, 805–810 (2003). https://doi.org/10.1007/s00254-003-0845-7
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DOI: https://doi.org/10.1007/s00254-003-0845-7