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Effect of cyanide-mineral interactions on granite weathering in a glacier forefield

Abstract:

In alpine regions weathering processes are of particular interest with respect to the effects of climate change. Large areas of bare rock will be exposed as glaciers retreat. Dissolution of most primary rock-forming minerals is limited by slow kinetics of the reactions at the mineral-water interfaces. Dissolution rates depends on extrinsic factors (T, pH, Eh, and exudates from microbes and plant roots) and on intrinsic factors (mineral surface properties and weathering state). Microorganisms can act as geological agents affecting biogeochemical processes through metal speciation as well as metal immobilization and solubilisation. As part of an interdisciplinary project for the investigation of a glacier forefield (Damma glacier in Central Switzerland), we examined the significance of cyanide as a ligand for the nutrient mobilization for microorganisms. Typically, cyanide is produced during bacterial growth for a short time period only (early stationary phase) as secondary metabolite. From a chemical point of view, it has been proposed that cyanide formed in the prebiotic soup acted as lixiviant on rocks. So, cyanide can be considered as the oldest complexing agent. However, until now cyanide has not been investigated with respect to mineral weathering and initial soil formation. Rock samples from the glacier forefield (granite) were crushed (63-250 µm) and treated (up to 24 h) in batch reactors purged with HCN gas (0.84 ± 0.1 mM HCN in solution) at different pH (5.5-7). The dissolved elements were analyzed with ICP-OES. A significant effect of cyanide on the dissolution rates of the granite was observed. Cyanide generally leads to a faster dissolution. However, for iron and manganese lower concentrations in the solutions were measured in the presence of cyanide (iron concentrations after 5 hours at pH 6: 6 ppm without cyanide but 0.3 ppm with cyanide). Modelling of chemical speciation has shown that the iron in the solution mainly occurs as ferrocyanide, and subsequently a ferrocyanide solid phase was precipitated (most likely Prussian blue). Cyanide seems to have two functions for the microorganisms: Increase of nutrients’ availability, and regulation of individual element concentrations in the solution.

Authors:

Gärtner, Dominique and Plötze, Michael and Furrer, Gerhard

Index Terms:

clastic sediments; ClayGroup; glacier; weathering; granite; cyanide; Damma; Furrer, Gerhard; Gartner, Dominique; Plotze, Michael

Further Information:

Date published: 2008