Geoökologisches Kolloquium WS 2006/07

Alle Termine

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Prof. Dr. Margot Isenbeck-Schroeter
Institut für Umwelt-Geochemie, Universität Heidelberg
Dienstag, 06.02.2007 16:15 H6

Arsen im Grundwasser: Versuche zum Verständnis des geochemischen Verhaltens

Redox processes control the transport behavior of arsenic in the aquatic environment, especially in the aquifer, because its mobility depends on the redox speciation. To predict concentrations of the toxic inorganic arsenic compounds in the water, redox processes and transport behavior have to be studied together and have to be coupled in a transport reaction model. Data can be obtained in transport experiments in columns or in field tests. The studies presented here show results of both, laboratory and field experiments with arsenic at varying geochemical conditions.

In spring and summer 2000, large scale transport experiments at the Cape Cod Site near Boston (Mass., USA) were performed in close cooperation with scientists from the USGS. The project was funded by the DFG (German Science Foundation) and supported by the USGS Toxic Substances Program. As(III) was injected into the oxic and suboxic zones using a pulse injection of high loads of As(III) in water and Br- as an ideal tracer. Features of transport and reaction behavior in the two redox environments differ significantly. Whereas the arsenic mass in the water decreases very quickly at oxic conditions, because the As is fixed at the solid material, transport rates for both species are significantly higher at suboxic conditions. As (V) reduction was observed during a permanent injection into the anoxic zone of the aquifer. The arsenic accumulated at iron surfaces due to iron oxidation during the injection period and was reduced as soon as anoxic conditions re-established.

Further process understanding was obtained by batch- and column experiments with arsenic with sandy material which was amended by iron hydroxide and manganese oxide, respectively. Results show very clearly the differences in redox and sorption behavior. Whereas As(III) is oxidised quickly at the MnO2 surfaces and transported as As(V) species, the reduced As(III) species was only partly transported in the iron amended column and the goethite lead to strong surface complexation. We could consider significant oxidation of As(III) to As(V) at the goethite surfaces using XANES and quantify oxidation rates. The different breakthrough curves could be simulated using PHREEQC. The reduction of As (V) could be shown in batch test using H2S as reductant.

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