Dr. Stefan Bossman
Environmental Analysis and Technology Department
University of Karlsruhe, Germany

EMSI Visiting Scholar
April 16 - 20, 2001

Seminar: "A Chemist's Approach to Iron III Induced Photooxidation Reactions in Rivers, Lakes and Seawater - How Do They Really Work and What Effects Do They Have on the Environment?"
Tuesday, April 17, 2001, 2:00pm, Interschool Lab, 7th Floor CEPSR

The photochemistry as well as the thermal chemistry of the naturally occurring matter containing complexed iron(III) and iron(II) in the environment play a distinct role in the abiotic metabolism of many toxic compounds of xenobiotic origin. Our experimental approach toward the deeper understanding of the prevailing reaction mechanisms and finally a molecular insight consists of several steps:

1. Investigation of photooxidation vs. photoreduction mechanisms of naturally occurring humic substances, depending on their iron-content.
2. Investigation of oxidative vs. reductive reaction pathways of iron(II/III)-containing humic substances using a model xenobiotic (2,4-xylidine).
3. Investigation of the thermal and photochemistry of mono- and dimolecular iron(II/III) complexes using 2,4-xylidine and 2,4-dichlorophenoxy acetic acid as model xenobiotics.

Our research is focused on the important mechanistic question, whether the decomposition of xenobiotics in natural ecosystems (as well as in model reaction systems for the environment) proceeds via the formation of hydroxyl radicals as highly reactive intermediates or whether electron-transfer must be regarded as main reaction pathway. Whereas the selectivity of hydroxyl radicals is very close to zero and therefore many intermediates are formed during their decomposition, generate electron transfer mechanisms considerably fewer intermediates during degradation. Especially the hydroxylation of aromatic hydrocarbons, which always happens, when the hydroxyl radical is the main reactive intermediate, creates many derivatives of the "mother xenobiotic" and thus raised much greater health concerns.

Seminar: "Removing Selected Pollutants from the Environment by Using Molecular Imprinted Photocatalysts"
Thursday, April 19, 2001, 11:00 pm, Interschool Lab, 414 Schapiro CEPSR

After two decades of intense research, it is well known that the efficiency of heterogeneous photocatalysts under solar irradiation is severely limited due to several factors:

1. The light absorption by classic photocatalysts, such as titanium dioxide (TiO2), is limited to the UV- and only a small fraction of the visible spectral domain.
2. Whereas numerous approaches to spectral sensitization of TiO2 exist for the generation of highly efficient solar cells, this concept could not be realized convincingly in the field of heterogeneous photocatalysis as of yet.
3. When "real waste waters" are to be treated, the most toxic components (for instance many chlorinated substances) are among the last oxidized by the heterogeneous photocatalyst. Therefore the detoxification of "real waste water" by means of heterogeneous photocatalysis is very time consuming. Obviously toxic wastewater cannot be further treated by bacteriological methods before the toxic compounds are sufficiently detoxified.

One possible solution to this dilemma consists of the synthesis of molecular imprinted titanium dioxide photocatalysts. Molecular recognition sites are generated during the sol-gel synthesis of TiO2. These molecular recognition sites have the ability to recognize aromatic acids and to bind them preferentially. This strategy offers the unique possibility to degrade xenobiotic compounds, which have an aromatic acid function, much faster than competing compounds of natural origin.