Tom N. P. Bosma

Stimulation of reductive dechlorination for in situ bioremediation of a soil contaminated with chlorinated ethenes

A soil from a former chemical redistribution company, contaminated with mainly chlorinated aliphatics, was studied for bioremediation purposes. Groundwater analyses revealed that the original pollutants, i.e. tetrachloroethene (PCE) and trichloroethene (TCE), were present at levels ranging from 2.3 to 122 mg/L. Dichloroethene (DCE), vinylchloride (VC), ethene and ethane were also detected at significant concentrations although they had never been introduced to the soil. Relatively high concentrations of cis -DCE as compared to trans -DCE and 1,1-DCE indicated that a slow in situ biodegradation had taken place by reductive dechlorination. Laboratory experiments with flow-through soil columns were performed to determine the optimal conditions for the enhancement of reductive dechlorination by the indigenous dechlorinating population. The addition of single electron donors to artificial groundwater resulted in the dechlorination of PCE to TCE and cis -DCE, whereas complete dechlorination to ethene was solely achieved with compost extract added to native groundwater.

Versatility of soil column experiments to study biodegradation of halogenated compounds under environmental conditions

Soil column experiments were performed to obtain insight in the different biological and physico-chemical processes affecting biodegradation of halogenated compounds under natural conditions in a water infiltration site. Lower chlorinated aromatic compounds could be degraded under aerobic conditions, whereas highly chlorinated compounds and chlorinated aliphatic compounds were mainly transformed under anaerobic conditions. Microorganisms which derive energy from reductive dechlorination were enriched and characterized. It was found that microbes could adapt to using chlorinated benzenes by evolution of new enzyme specificities and by exchange of genetic material. For halogenated pollutants, which are generally hydrophobic, sorption processes control the concentration available for biodegradation. The effects of very low concentrations of halogenated compounds on their biodegradability are described. The use of isolated bacterial strains to enhance biodegradation was evaluated with respect to their temperature-related activity and to their adhesion properties.

Simulation model for biotransformation of xenobiotics and chemotaxis in soil columns

In this paper a model is presented which can be used to simulate the behaviour of xenobiotic chemicals in soil columns with respect to their physical and chemical properties. Terms describing biological transformation of xenobiotics are also included in the model. It incorporates microbial growth following Monod kinetics and a chemotactic response of the transforming bacteria towards the xenobiotic substrate. The model appeared to yield good simulations of an experiment by Van der Meer et al. (1987) who investigated the degradation of 1,2-dichlorobenzene in soil columns inoculated with Pseudomonas sp. strain P51. The behaviour and the fate of 1,4-dichlorobenzene as found by Kuhn et al. (1985) can also be simulated using this model but their results were also adequately simulated using a simple second order model. The results generated by the model correspond to kinetic parameters obtained in other studies. It is concluded that the model is a useful tool for the investigation of the activity of bacteria degrading xenobiotics in soil columns, provided that the microbial parameters can be determined in independent experiments, and that the active microbial mass in the soil can be measured.

Biodegradation of Xenobiotics in Environment and Technosphere

Microorganisms play an important role in the removal of synthetic organic compounds from the environment. This chapter gives an overview of the evolution of biodegradation pathways and describes the strategies that microorganisms have evolved to transform important molecular structures. The actual effectiveness of biodegradation in the environment is determined by the bioavailability of the compounds. As a general rule, one could state that the release rates of synthetic compounds should not exceed the environment’s ability to degrade them.

Microbial Aspects of the Behaviour of Chlorinated Compounds During Soil Passage

Microbial transformation of chlorinated organic contaminants in columns, packed with sediment from a dune infiltration area and from River Rhine, has been investigated under different redox conditions. Low redox potentials favour reductive dechlorination of chlorinated ethylenes and benzenes to lower or non-chlorinated compounds. Under aerobic conditions transformation of 1,2- and 1,4 dichlorobenzene, but not of 1,3-dichlorobenzene took place. Biotransformation of chlorinated organic substances under denitrifying conditions was hardly observed. Only a slight removal of 1,2dichloro,4-nitrobenzene could be detected. Results indicate that biotransformations may also occur under conditions which approach a field situation (lowered temperature, presence of extra carbon source).

Fundamentals: Reactions, Transport and Modeling

The discussion in the session was based on the statement papers of the participants. The most important conclusions from the papers and the discussion are summarized below.