Investigation of pore scale processes at biogeochemical interfaces using sensor micromodels and Raman microscopy
Zusammenfassung der Projektergebnisse
Biogeochemical interfaces in soil serve as the initial filter for contaminants. While inorganic interfaces cause limited and temporary retardation due to sorption, biogeochemical interfaces provide an active barrier for contaminants. Biogeochemical interfaces are considered as highly dynamic in space and time. Thus, limited spatial access to the interfaces and temporal changes of the pore topology through the interfaces are important to describe the processes in soil. This research project aimed at the visualization and quantification of biogeochemical interfaces using magnetic resonance imaging (MRI). While only some substances, like some heavy metals, fluorinated hydrocarbons, and humic substances, are directly accessible to MRI the visualization of other contaminants has to be done using MRI-labeled tracers. During the first part of this project an antibody against benzo[a]pyrene (B[a]P) was coupled to magnetite nanoparticles. The selectivity and unspecific binding of the anti-B[a]P-antibody were studied in column studies. It turned out, that the anti-B[a]P-antibody is not able to recognize B[a]P adsorbed directly to a silical gel surface. This is due to the planar adsorption of B[a]P. However, the antibody is able recognize B[a]P covalently linked to the silica gel surface. Here B[a]P sticks into the solution. This is most likely the case in the presence of humic substances. Unspecific binding was negligible in both cases. The coupling procedure can be extended to other metal oxides easily. On the other hand, antibodies against other organic contaminants are already available or can be produced with reasonable effort. Thus, the concept of tailored antibodies with MRI active labels seems to be versatile. Due to size and charge restrictions, not all of the pore spaces are accessible to the labeled antibodies. This, however, can be considered as a feature which mimics the accessibility of the pore space to bacteria. Column tests with NMR relaxometry show a small, but significant change of the relaxation times in places where the labeled antibody binds to B[a]P. The column tests also revealed, that even negligible unspecific filtration can lead to false positive results. Here, additional washing steps are needed to remove unspecific filtered nanoparticles. MRI studies showed, that a visualization of B[a]P is possible for larger spots of B[a]P. Again, washing steps are needed and the visualization of single interfaces seems to be several steps ahead. Due to the requirement of washing steps an online visualization is not feasible for the majority of soils.
