Anaerobic Microbial Dechlorination of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Contaminated Kymijoki River Sediments

Highly toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) accumulate in the environment in sediments and soils. PCDD/Fs may pose considerable environmental and health risks because of their persistence in the environment. PCDD/Fs can be degraded slowly by microbial reductive dechlorination under anaerobic conditions which produces less chlorinated PCDD/Fs. Sediments of the Kymijoki River located in South-Eastern Finland are highly contaminated with PCDD/Fs and other chlorinated compounds. PCDD/Fs in sediments of the river mainly originate as unwanted byproducts from the production of the chlorophenol wood preservative Ky-5 between 1939 and 1984. The total volume of the most toxic 2,3,7,8-substituted PCDD/Fs contaminated sediments in the Kymijoki River, from Kuusankoski to the Baltic Sea, are estimated to be five million cubic meters of wet sediment. The Kymijoki River is the largest known source of PCDD/Fs to the Baltic Sea. The objective of this thesis was to gain knowledge on the sediment PCDD/Fs and the dechlorinating bacterial community at the Kymijoki River. We aimed to determine the potential for anaerobic microbial dechlorination of weathered PCDD/Fs in river sediments. Sediment samples were collected from the Kymijoki River located at 0, 20, 30, and 60 km downstream from the main source of contamination, and at a reference site. An anaerobic microcosm study was conducted to determine dechlorination potential and the effect of temperature on the reductive dechlorination process for spiked 1,2,3,4-tetrachlorodibenzofuran (1,2,3,4-TeCDF). Concentrations of weathered PCDD/Fs and activity of the indigenous dechlorinating microbial community were determined in sediment samples. The measured mean total concentrations for 2,3,7,8-PCDD/Fs were high in sediments at all study sites of the river. The predominant PCDD/F congeners were 1,2,3,4,6,7,8-HpCDF, 1,2,3,4,6,8,9-HpCDF, and OCDF at all sites. The sediment PCDD/F congener profiles were similar to that of the Ky-5 wood preservative. Dechlorination of spiked 1,2,3,4-TeCDF to less chlorinated dibenzofurans was detected after 10 and 29 months in sediment microcosms from the all studied sites. The pathway of 1,2,3,4-TeCDF dechlorination was mainly via 1,3,4-TrCDF to 1,3-DiCDF. The low temperature decreased the dechlorination rate of spiked TeCDF which suggests that dechlorination of weathered PCDD/Fs is limited at the in situ temperature of the river sediments. A native dechlorinating Dehalococcoides-like Chloroflexi community was present in sediment samples at all studied sites. The dechlorination pathway of 1,2,3,4-TeCDF suggests that dechlorination of aged PCDD/F congeners could generate non-2,3,7,8-substituted and hence less toxic PCDD/Fs. However, concentrations of 2,3,7,8-substituted PCDD/Fs have remained at approximately the same levels over the last few decades at each sampling location of the Kymijoki River. Furthermore, no substantial changes in the relative abundances of weathered PCDD/Fs were observed over 2.5 years in laboratory microcosm studies, indicating that anaerobic dechlorination of weathered PCDD/Fs was limited over the course of the experiment. Results of this study indicates that no or only minimal biodegradation of PCDD/F congeners has occurred in the river sediments over the last few decades since the contamination events. Therefore, concentrations of weathered PCDD/Fs in the sediments of the Kymijoki River are expected to remain at the same level for decades or centuries with further migration towards the Baltic Sea.

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2018

Author dc:creator

• Mäntynen, Sanna

Statement dc:rights

• Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
• This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
• Publikationen är skyddad av upphovsrätten. Den får läsas och skrivas ut för personligt bruk. Användning i kommersiellt syfte är förbjuden.

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/236425

OAI record oai:identifier

https://helda.helsinki.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Ahelda.helsinki.fi%3A10138%2F236425