Reductive dechlorination

Event Date: 
Wednesday, June 26, 2013 - 18:00 - 18:15
Institution: 
UNSW
Title: 

Towards a hexachlorobenzene bioreactor

Abstract: 

Hexachlorobenzene (HCB) is highly persistent environmental pollutant due to its chemical stability. It has been used in the production of rubber, as wood preserving agent and as pesticide and it is considered a possible human carcinogen.  HCB is particularly relevant in Australia, since it holds the largest HCB stockpile in the world (Botany Bay Industrial Park, NSW). So far only physic-chemical technologies have been applied for the destruction of HCB; however, these methods do not ensure full destruction and may lead to the generation of more harmful compounds, such as dioxins.  On the other hand, it is well known that obligate anaerobic bacteria are able to reductively dechlorinate HCB to less chlorinated congeners.  Therefore, a biological approach seems to be a more suitable and environmental friendly solution.

In this study we present a microbial community, taken from a site contaminated with chlorinated solvents, capable of reductively dechlorinating HCB and 1,2,4,5- Tetrachlorobenzene (TeCB). Cultures were established using acetate and H2 or lactate as carbon source and electron donor, respectively. 1,3- and 1,4- dichlorobenzenes were the main breakdown products in the cultures supplied  with 1, 2, 4, 5- TeCB, monochlorobenzene was also observed in a lower extent. Cultures with HCB only showed  1, 3, 5-Trichlorobenzene as breakdown product. Quantitative PCR, targeting Dehalococcoides´ 16S (a well-known dechlorinating bacterium) showed high abundance of this species in the cultures. 

Event Date: 
Wednesday, May 29, 2013 - 19:00 - 19:45
Institution: 
CSIRO
Title: 

Sediment Biobarriers for Chlorinated Aliphatic Hydrocarbons in Groundwater Reaching Surface Water

Abstract: 

 
This study explored the potential of eutrophic river sediments to attenuate the infiltration of chlorinated aliphatic hydrocarbon (CAH)-polluted groundwater discharging into the Zenne River near Brussels, Belgium. Active biotic reductive dechlorination of CAHs in the riverbed was suggested by a high dechlorination activity in batch- and column biodegradation tests performed with sediment samples, and by the detection of dechlorination products in sediment pore water. Halorespiring Dehalococcoides spp. were present in large numbers in the riverbed as shown by quantification of their 16S rRNA and reductive dehalogenase genes. By using DGGE-fingerprint analysis of relevant nucleic acid markers, it was shown that the Zenne River sediments were inhabited by a metabolically diverse bacterial community. A large diversity of sulfate-reducing bacteria, Geobacteraceae and methanogens, which potentially compete with halorespiring bacteria for electron resources, was identified. The high organic carbon level in the top of the riverbed, originating from organic matter deposition from the eutrophic surface water, resulted in a homogeneous microbial community structure that differed from the microbial community structure of the sediment underneath this layer. Monitoring of CAH concentrations and stable isotope ratios of the CAHs (δ13C) and the water (δ2H and δ18O), allowed to identify different biotic and abiotic CAH attenuation processes and to delineate their spatial distribution in the riverbed. Reductive dechlorination of the CAHs was the most widespread attenuation process, followed by dilution by unpolluted groundwater discharge and by surface water-mixing. During a 21-month period, the extent of reductive dechlorination ranged from 27 to 89% and differed spatially but was remarkably stable over time, whereas the extent of abiotic CAH attenuation ranged from 6 to 94%, showed large temporal variations, and was often the main process contributing to the reduction of CAH discharge into the river. Although CAHs were never detected in the surface water, CAHs were not completely removed from the discharging groundwater at specific locations in the riverbed with high groundwater influx rates. Therefore, it was concluded that an increase in the extent of biotransformation in the riverbed is needed for acceptance of the Zenne biobarrier as a viable remedial option for attenuation of discharging CAH-polluted groundwater.

 
Prepared by Valentina Wong (UNSW PhD student)
On a cold Tuesday night, Adrian Low from University of New South Wales warmed the JAMS audience with his passion on bioremediation of organochlorine contaminated groundwater. Adrian described the discovery of Australia’s first 1,2-dichloroethane (DCA) degrading consortium, AusDCA. His work in the field demonstrated the efficacy and sustainability of using organochlorine respiring bacteria to remediate organochlorine contaminants in situ. He plans to isolate the bacterial species responsible for performing this unique task.

Event Date: 
Tuesday, July 24, 2012 - 18:00 - 18:15
Institution: 
University of NSW
Title: 

Bioremediation of Mixed Chlorinated Solvents by Combining Two Biogeochemical Processes

Abstract: 

Chloroethenes are a class of chlorinated solvents which cause extensive soil and groundwater contamination worldwide. They can be detoxified by anaerobic dehalogenating bacteria, in the process of reductive dechlorination.  However, chloroethenes are often found mixed with chloromethanes, a class of solvents which inhibit the enzymatic detoxification of chloroethenes by dehalogenating strains.  Iron sulfides are powerful chemical reductants for the dechlorination of chloromethanes, and can be generated through the metabolism of iron- and sulfate-reducing bacteria. In this study, a sulfate reducing bacterium was used to produce iron sulfide in the presence of moderate levels of tetrachloroethene and carbon tetrachloride to examine the ability of a sulfate reducing organism to drive reduction of a chloromethane in the presence of chloroethene.

 

Cultures of the sulfate-reducer Desulfovibrio vulgaris were established in the presence of 100 µM each of tetrachloroethene and carbon tetrachloride. Growth, sulfide formation and chlorinated solvents and their dechlorinated products were monitored. The effects of amorphous iron oxide and cyanocobalamin on the fate of chlorinated solvents compared with unamended control cultures were investigated. 

Following growth and sulfide formation, carbon tetrachloride was dechlorinated mostly to carbon disulfide while tetrachloroethene was dechlorinated to trichloroethene and acetylene.  Dechlorination rates were enhanced both by the presence of iron and cyanocobalamin separately, and significantly increased when both were present.

This study illustrates the potential to use sulfate reducing bacteria in zones of mixed chlorinated solvent groundwater pollution in order to produce iron sulfide minerals. Their cyanocobalamin-catalyzed action on chloromethanes, coupled with that of dehalogenating strains on chloroethenes is a promising strategy for the bioremediation of such contaminated areas."

How many microbiologists does it take to change a light bulb? None, as it turns out - the dozens of attendees at June's Joint Academic Microbiology Seminars (JAMS) at the Australian Museum waited patiently through a short blackout for drinks, snacks, and three servings of fresh scientific discussion.

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