In situ Analysis of Pore Scale Processes at Biogeochemical Interfaces

Christian Metz1, Natalia P. Ivleva1, Reinhard Niessner1, Thomas Baumann1
1 Institute of Hydrochemistry, Technische Universität München

O 3.2 in Von der Atmosphäre zum Grundwasser - die Hydrologie der vadosen Zone

29.05.2014, 10:20-10:40, H19, NW II

Biogeochemical interfaces (BGI) in soil control the fate of organic chemicals and the functioning of soil as a filter to protect groundwater resources. Biogeochemical interfaces are transient in space and time, thus rendering batch tests under equilibrium conditions and without spatial restrictions inadequate to predict the overall behavior. Instead, the concentration gradients of organic chemicals have to be measured and the spatial and temporal dynamics of the BGI themselves have to be monitored. Processes at BGI can be visualized and quantified using microfluidic structures mimicking the pore topology of the soil, so called micromodels. In combination with Raman micro­spectroscopy chemical information can be retrieved from a micromodel experiment with a spatial resolution on the order of 1 µm² and a temporal resolution in the s-range. To increase the sensitivity, silver nanoparticles have been added to the water phase flowing through the micromodel to make use of the amplifying surface-enhanced Raman effect. Currently chemical gradients of moderately lipophilic substances have been acquired with a limit of detection of 10e-8 mol/L. Challenges to overcome include the interactions between silver nanoparticles and target analytes which might alter the mass transfer rates, and the settling of nanoparticles in the channel. As high resolution acquisition comes with a limited field-of-view (FoV) and, e.g., the growth of a biofilm outside of the FoV alters the flow pattern, the flow velocity has to be monitored using fluorescent latex beads and single particle tracking. For a fast measurement of well-defined variables, like the pH-value or the oxygen concentration, thin film polymers with encapsulated sensor dyes are chosen. When looking at microbial growth in porous media, not only the development of a biofilm changes the flow paths and the accessibility to the microbes, but also the development of locally confined gas bubbles, as with P. denitrificans. Here, the growth rate is correlated with bacterial activity and the results indicate different bacterial densities in pore bodies and pore throats Imaging results at the interface and the development of the concentration gradient suggest, that the interface is highly dynamic in the beginning. Vortices with velocities in the upper µm/s range are reaching several dozens of µm from the interface into the solution. A diffusion controlled mass transfer is established at a later stage only. This observation might explain part of the first flush phenomenon.

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Letzte Änderung 01.11.2013