Our research interests cover a wide range of areas in molecular environmental sciences including biogeochemical processes in soils and groundwater, reactions and phase transfer processes of pollutants and development and application of in situ methods to characterize and quantify processes in pristine and polluted subsurface environments. As surface mediated processes play a key role in determining the transport and transformation of natural and xenobiotic compounds in the subsurface we are interested in a process-based understanding of the factors that control the formation of reactive surfaces, in particular at minerals, and how such surfaces interact with natural and anthropogenic compounds. To this end our work interfaces aquatic and environmental chemistry with geomicrobiology and geochemistry. For our laboratory and field studies we apply a wide array of modern instrumentation and techniques, including compound specific stable isotope analysis (CSIA) as well as Mössbauer spectroscopy.
Sketch illustrating the significance of surface mediated processes for speciation, reactivity, bioavailability, and biodegradation of natural and anthropogenic organic compounds in the subsurface.
The overall objective of our research is to provide through an improved understanding of biogeochemical key processes the scientific basis for assessment, management and remediation options of soil and groundwater environments
We are active in the following fields of research:
In environmental sciences, compound specific stable isotope analysis (CSIA) is applied for source identification, detection and quantification of in situ biodegradation and identification of reaction mechanisms or other processes related to isotope fractionation of organic contaminants.
Nowadays online-coupling of chromatographic systems to isotope ratio mass spectrometers (GC-IRMS, LC-IRMS) is well established. This allows analyzing the compound-specific isotope composition of organic analytes with regard to carbon (13C/12C), hydrogen (2H/1H) and nitrogen (15N/14N). Recently developed approaches also enable measuring chlorine isotope ratios (37Cl/35Cl), - a breakthrough for evaluating degradation processes of chlorinated ethenes. The availability of δ37Cl- CSIA will enable us to determine chlorine isotope fractionation factors for microbial dehalogenation processes; where the database is still lacking. Most importantly it will open the enormous prospects of 2-dimensional isotope analysis for process identification for halogenated organic compounds. To evaluate a compound´s transformation processes the kinetic isotope effect(s) (KIE) accompanying (bio)chemical reactions can be used. In general, organic contaminants consist of a mixture of “heavy” and “light” molecules (isotopologues). During the breakdown, typically molecules exhibiting a heavy isotope (e.g. 13C) in the reactive position are discriminated from those exhibiting a light isotope, as a higher activation energy is needed to break the bonds with the heavy isotope. This leads to gradual enrichment of the lighter isotopologues in the product, while the heavier ones remain in the substrate.
Using cultivation-based approaches we investigate pure and mixed cultures of organohalide respiring organisms that utilizes organohalides (e.g. tetrachlorethene) as the terminal electron acceptor to gain energy. In collaboration with the Geomicrobiology - Microbial Ecology group, we use molecular techniques like qPCR, cDNA synthesis or Western blot analysis to investigate the response of the cells to defined growth conditions. These techniques enable us to monitor e.g. total cell numbers, the abundance and transcriptions of specific genes as well as the detection of processed enzymes like reductive dehalogenases (the key enzmyes in organohalide respiration where the carbon-chlorine bond is cleaved).
Combining molecular techniques with 2D isotope analysis will improve our understanding of microbe driven contaminant degradation and therefore the approach of engineered bioremediation using microorganisms to clean up contaminated sites.
Electron transfer reactions at iron mineral surfaces in the presence of organic sorbates
Dr. Lic Chem Silvia Orsetti
(Funding: DFG Research Group FOR 580 "Electron Transfer Processes in Anoxic Aquifers)
Redox reactions at iron mineral surfaces play an important role in determining the overall biogeochemical milieu in anoxic groundwater systems. Previous studies have shown that oxidation of sorbed ferrous iron at mineral phases may cause remodelling of the mineral- water interphase and thus may affect electron transfer processes in anoxic aquifers.
Up to date, process based studies on surface mediated transformation of redox active solutes in iron mineral systems have been conducted primarily in model systems devoid of natural organic matter. In natural systems, however, mineral surfaces are inevitably in contact with OM. Sorbed DOM is likely to affect heterogeneous electron transfer processes due to its interactions with iron both in aqueous solution and at the mineral surface. On one hand, DOM sorption at iron hydroxides may interfere with the formation of reactive Fe(II) surface sites. On the other hand, DOM contains redox active quinone moieties and may act as a mediator enhancing the electron-transfer across the mineral surface. In this project we propose to investigate the effects of various organic sorbates such as redox-active quinones, humic substances and DOM on electron transfer reactions at iron mineral surfaces. In order to achieve this aim, fluorescence properties of quinones and natural organic matter in different redox states will be analyzed, since it is known that fluorescence behavior of these substances depend on redox speciation. In particular, multivariate chemiometric tools will be applied, such as Parallel Factor Analysis (PARAFAC), to analyze the emission-excitation maps of the different species. Regarding the redox modification of the samples, a well-controlled electrochemical process will be applied, which allows following up both the electron and proton transfer.