Christoph Schüth

Hydrodechlorination and hydrogenation of aromatic compounds over palladium on alumina in hydrogen-saturated water

The catalytic transformations of 1,2-dichlorobenzene, chlorobenzene, 4-chlorobiphenyl, γ-hexachlorocyclohexane (Lindane), naphthalene and phenanthrene were studied over palladium on alumina in hydrogen-saturated water (Pd/Al2O3/H2) at room temperature and ambient pressure. The chlorinated benzenes were rapidly hydrodechlorinated and Lindane was dehydrochlorinated to benzene. Partial or complete hydrogenation was observed for biphenyl and the polycyclic aromatic hydrocarbons. The phenanthrene ring was cleaved at the 9,10-position. In general dechlorination reactions were faster than hydrogenation reactions.

Critical review of Pd-based catalytic treatment of priority contaminants in water

Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.

Carbon and hydrogen isotope effects during sorption of organic contaminants on carbonaceous materials.

Stable carbon and hydrogen isotopes can be an efficient means to validate biodegradation of organic contaminants in groundwater since it results in an isotopic fractionation. A prerequisite in applying this method in the field is the proof that other processes decreasing the contaminant concentration are conservative with respect to isotope effects. In this paper we show for carbon isotopes of halogenated hydrocarbon compounds [trichloroethene (TCE), cis-dichloroethene (c-DCE), vinylchloride (VC)] and carbon and hydrogen isotopes of BTEX compounds (benzene, toluene, p-xylene) that no significant fractionation occurs during equilibrium sorption onto activated carbon, lignite coke and lignite. In general, effects were in the range of the reproducibility limit of the analytical instrument (0.5 per thousand for delta13C, and 8 per thousand for delta2H). This observation was made for fractions sorbed of less than 5% to more than 95%. Also for rate-limited sorption of TCE onto activated carbon, no significant fractionation in carbon isotopes could be observed. These findings support the assumption that for these classes of compounds, sorption processes in aquifer systems are conservative with respect to isotope effects.

Carbon Isotope Fractionation during Aerobic Biodegradation of Trichloroethene by Burkholderia cepacia G4: a Tool To Map Degradation Mechanisms

ABSTRACT The strain Burkholderia cepacia G4 aerobically mineralized trichloroethene (TCE) to CO2 over a time period of ∼20 h. Three biodegradation experiments were conducted with different bacterial optical densities at 540 nm (OD540s) in order to test whether isotope fractionation was consistent. The resulting TCE degradation was 93, 83.8, and 57.2% (i.e., 7.0, 16.2, and 42.8% TCE remaining) at OD540s of 2.0, 1.1, and 0.6, respectively. ODs also correlated linearly with zero-order degradation rates (1.99, 1.11, and 0.64 μmol h−1). While initial nonequilibrium mass losses of TCE produced only minor carbon isotope shifts (expressed in per mille δ13CVPDB), they were 57.2, 39.6, and 17.0‰ between the initial and final TCE levels for the three experiments, in decreasing order of their OD540s. Despite these strong isotope shifts, we found a largely uniform isotope fractionation. The latter is expressed with a Rayleigh enrichment factor, ε, and was −18.2 when all experiments were grouped to a common point of 42.8% TCE remaining. Although, decreases of ε to −20.7 were observed near complete degradation, our enrichment factors were significantly more negative than those reported for anaerobic dehalogenation of TCE. This indicates typical isotope fractionation for specific enzymatic mechanisms that can help to differentiate between degradation pathways.

Carbon isotope fractionation during abiotic reductive dehalogenation of trichloroethene (TCE)

Dehalogenation of trichloroethene (TCE) in the aqueous phase, either on palladium catalysts with hydrogen as the reductant or on metallic iron, was associated with strong changes in delta13C. In general, the delta13C of product phases were more negative than those of the parent compound and were enriched with time and fraction of TCE remaining. For dehalogenation with iron, the delta13C of TCE and products varied from -42/1000 to +5/1000. For the palladium experiments, the final product, ethane, reached the initial delta13C of TCE at completion of the dehalogenation reaction. During dehalogenation, the carbon isotope fractionation between TCE and product phases was not constant. The variation in delta13C of TCE and products offers a new monitoring tool that operates independently of the initial concentration of pollutants for abiotic degradation processes of TCE in the subsurface, and may be useful for evaluation of remediation efficiency.

Modelling of geochemical and isotopic changes in a column experiment for degradation of TCE by zero-valent iron

Zero-valent iron (ZVI) permeable-reactive barriers have become an increasingly used remediation option for the in situ removal of various organic and inorganic chemicals from contaminated groundwater. In the present study a process-based numerical model for the transport and reactions of chlorinated hydrocarbon in the presence of ZVI has been developed and applied to analyse a comprehensive data set from laboratory-scale flow-through experiments. The model formulation includes a reaction network for the individual sequential and/or parallel transformation of chlorinated hydrocarbons by ZVI, for the resulting geochemical changes such as mineral precipitation, and for the carbon isotope fractionation that occurs during each of the transformation reactions of the organic compounds. The isotopic fractionation was modelled by formulating separate reaction networks for lighter ((12)C) and heavier ((13)C) isotopes. The simulation of a column experiment involving the parallel degradation of TCE by hydrogenolysis and beta-elimination can conclusively reproduce the observed concentration profiles of all collected organic and inorganic data as well as the observed carbon isotope ratios of TCE and its daughter products.

Carbon isotope fractionation during reductive dechlorination of TCE in batch experiments with iron samples from reactive barriers

Reductive dechlorination of trichloroethene (TCE) by zero-valent iron produces a systematic enrichment of 13C in the remaining substrate that can be described using a Rayleigh model. In this study, fractionation factors for TCE dechlorination with iron samples from two permeable reactive barriers (PRBs) were established in batch experiments. Samples included original unused iron as well as material from a barrier in Belfast after almost 4 years of operation. Despite the variety of samples, carbon isotope fractionations of TCE were remarkably similar and seemed to be independent of iron origin, reaction rate, and formation of precipitates on the iron surfaces. The average enrichment factor for all experiments was -10.1 per thousand (+/- 0.4 per thousand). These results indicate that the enrichment factor provides a powerful tool to monitor the reaction progress, and thus the performance, of an iron-reactive barrier over time. The strong fractionation observed may also serve as a tool to distinguish between insufficient residence time in the wall and a possible bypassing of the wall by the plume, which should result in an unchanged isotopic signature of the TCE. Although further work is necessary to apply this stable isotope method in the field, it has potential to serve as a unique monitoring tool for PRBs based on zero-valent iron.

Soil column experiments to quantify vadose zone water fluxes in arid settings

For the determination of groundwater recharge processes in arid environments, vadose zone water fluxes and water storage should be considered. To better understand and quantify vadose zone processes influencing groundwater recharge, a soil column experimental setup has been developed that mimics arid atmospheric conditions and measures water and temperature fluxes in high temporal and spatial resolution. The focus of the experiment was on the determination of water infiltration, redistribution, evaporation and percolation under non-isothermal conditions. TDR rod sensors and a specific TDR “Taupe” cable sensor were used for water content measurements and allowed the infiltration fronts to be traced over the whole column length. Applying single irrigations of different amount and intensity showed the applicability of the experimental setup for the measurement of water movement in the unsaturated soil column.

Application of GALDIT index to assess the intrinsic vulnerability to seawater intrusion of coastal granular aquifers

Abstract Groundwater within coastal aquifers is considered a very sensitive natural resource since its quantity and quality is threatened by anthropogenic activities as well as global and climate changes. The major environmental hazard with respect to coastal aquifers is the phenomenon of seawater intrusion, and is more pronounced in the Mediterranean basin. In the light of smart management of such hydrogeological systems, GALDIT index was introduced in the form of a numerical ranking system that aims to assess the vulnerability of such aquifers to seawater intrusion. This paper provides an application of GALDIT vulnerability index for a typical Mediterranean alluvial aquifer under variable hydrogeological conditions in different research periods (1992 and 2004). The results proved the significance of the piezometric conditions of the aquifer with respect to its vulnerability to seawater intrusion.

Simulation of Seawater Intrusion in Coastal Aquifers: Forty Five-Years Exploitation in an Eastern Coast Aquifer in NE Tunisia

Management of groundwater resources involves the allocation of groundwater supplies and water quality to competing water demands and uses. The resource allocation problem is characterized by conflicting objectives and complex hydrologic and environmental constraints, especially in coastal aquifers. The development of mathematical simulation models provides groundwater planners with quantitative techniques for analyzing alternatives groundwater resources management. The gathered knowledge was then used to build and to calibrate a 3D transient model of 45 years time period (1963-2008). The numerical model was used further to discuss some issues on the management of the regional groundwater resources. The calibration of the 3D model has been achieved in a time dependent, iterative and semi- automatic fashion. This paper describes a 3-D seawater intrusion modeling process for a specified study area in Eastern Coast aquifer, in NE Tunisia. The simulation results presented in this paper are based on the density-dependent miscible flow and transport modeling approach for simulation of seawater intrusion in coastal aquifers. Response evaluations con- sider the effects of vertical recharge on seawater intrusion, effects of boundary conditions, and effects of spatially varying pumping from the aquifer.