Sediments contaminated with weathered polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are problematic around the world. River Kymijoki is highly contaminated with PCDFs mainly originating from the production of the wood preservative Ky-5 until the 1980s. Limited information is available on the in situ microbial reductive dechlorination of weathered PCDD/Fs. The overall objectives of my work were to assess the potential for anaerobic microbial dechlorination of weathered PCDFs and to gain information on the application in bioremediation and detoxification of contaminated sediment using Kymijoki River sediments as a case model. Experiments in mesocosms (30 L), microcosms (200 mL) and enrichment cultures (40 mL) demonstrated the potential for dechlorination of weathered PCDFs as well as spiked PCDD/Fs by the indigenous microorganisms of Kymijoki River sediment. The relative decrease of highly chlorinated dibenzofurans (CDFs) was accompanied with an increase of tetra- and penta-CDFs over a 7-year period in mesocosms. Amendment with halogenated co-substrates, tetrachlorobenzene (TeCB) and pentachloronitrobenzene (PCNB), and bioaugmentation with Dehalococcoides ethenogenes strain 195 selectively stimulated dechlorination of weathered PCDFs after 18 months in microcosms. Dechlorinating enrichment cultures were established from the microcosms and spiked with 1,2,3,4-tetra-CDD/F and octaCDF with PCNB as a co-substrate. 1,2,3,4-tetra-CDD was dechlorinated mainly via 1,2,3-triCDD to 2,3-di-CDD and 2-mono-CDD over 13 months incubation. Dechlorination of 1,2,3,4-tetra-CDF was slow and less extensive compared to that of 1,2,3,4-tetra-CDD, with 2,4-di-CDF as the most abundant dechlorination product accumulating after 13 months. More extensive dechlorination of 1,2,3,4-tetra-CDD was observed in the presence of PCNB, which demonstrated that PCNB was capable of enhancing the dechlorinating potential of the indigenous microorganisms in Kymijoki sediments. Dechlorination of spiked octa-CDF was observed with the production of hepta-, hexa-, penta- and tetra-CDFs over 5.5 months incubation. DGGE analysis revealed diverse Chloroflexi community in mesocosms and a highly selected Chloroflexi community in the enrichment cultures. One of the stimulated Chloroflexi populations clustered closely with the Pinellas subgroup of Dehalococcoides. Our results suggest that dechlorination of weathered PCDD/Fs contaminants may be mediated by indigenous microbial populations as a means for in situ bioremediation of PCDD/F contaminated sediments.

The introduction of synthetic organic chemicals into the environment during the last few decades has given rise to major concern about the ecotoxicological effects and ultimate fate of these compounds. The pollutants that are considered to be most hazardous because of their intrinsic toxicity, high exposure level, or recalcitrant behavior in the environment have been placed on blacklists and other policy priority lists. The fate of synthetic compounds that enter the environment is mainly determined by their rate of biodegradation, which therefore also has a major effect on the degree of bioaccumulation and the risk of ecotoxicological effects. The degree and rate of biodegradation is also of critical importance for the feasibility of biological techniques to clean up contaminated sites and waste streams. The biodegradation of xenobiotics has thus been the subject of numerous studies, which resulted in thousands of publications in scientific journals, books, and conference proceedings. These studies led to a deeper understanding of the diversity of biodegradation processes. As a result, it has become possible to enhance the rate of degradation of recalcitrant pollutants during biological treatment and to design completely new treatment processes. At present, much work is being done to expand the range of pollutants to which biodegradation can be applied, and to make treatment techniques less expensive and better applicable for waste streams which are difficult to handle.