Biodegradation Of Chlorinated Solvents Research Articles

Microbial ecology of chlorinated solvent biodegradation

This study focused on the microbial ecology of tetrachloroethene (PCE) degradation to trichloroethene, cis-1,2-dichloroethene and vinyl chloride to evaluate the relationship between the microbial community and the potential accumulation or degradation of these toxic metabolites. Multiple soil microcosms supplied with different organic substrates were artificially contaminated with PCE. A thymidine analogue, bromodeoxyuridine (BrdU), was added to the microcosms and incorporated into the DNA of actively replicating cells. We compared the total and active bacterial communities during the 50-day incubations by using phylogenic microarrays and 454 pyrosequencing to identify microorganisms and functional genes associated with PCE degradation to ethene. By use of this integrative approach, both the key community members and the ecological functions concomitant with complete PCE degradation could be determined, including the presence and activity of microbial community members responsible for producing hydrogen and acetate, which are critical for Dehalococcoides-mediated PCE degradation. In addition, by correlation of chemical data and phylogenic microarray data, we identified several bacteria that could potentially oxidize hydrogen. These results demonstrate that PCE degradation is dependent on some microbial community members for production of appropriate metabolites, while other members of the community compete for hydrogen in soil at low redox potentials.

Aerobic/anaerobic/aerobic sequenced biodegradation of a mixture of chlorinated ethenes, ethanes and methanes in batch bioreactors

A novel aerobic/anaerobic/aerobic treatment was implemented in batch reactors containing aquifer materials from a site contaminated by tetrachloroethylene (PCE), trichloroethylene (TCE), vinyl chloride (VC), 1,1,2-trichloroethane (1,1,2-TCA) and chloroform (CF). Consortia grown aerobically on methane, propane, n-pentane and n-hexane completely biodegraded the chlorinated solvent mixture, via aerobic cometabolism of VC, CF, TCE and 1,1,2-TCA, followed by PCE reductive dechlorination (RD) to 1,2-cis-dichlorothylene (cis-DCE) or TCE, and cis-DCE/TCE cometabolism in a further aerobic phase. n-Hexane was the best substrate. No electron donor was supplied for RD, which likely utilized cellular material produced during the aerobic phase. Chloride release was stoichiometric with chlorinated solvent biodegradation. According to the Lepidium sativum ecotoxicity test, a decreased toxicity was observed with propane, n-pentane and n-hexane, but not methane. A kinetic study of PCE RD allowed to estimate the PCE maximum specific rate (0.57±0.07mgmgprotein-1day-1) and half-saturation constant (6.7±1.5mgL-1).

Complementing approaches to demonstrate chlorinated solvent biodegradation in a complex pollution plume: Mass balance, PCR and compound-specific stable isotope analysis

This work describes the use of different complementing methods (mass balance, polymerase chain reaction assays and compound-specific stable isotope analysis) to demonstrate the existence and effectiveness of biodegradation of chlorinated solvents in an alluvial aquifer. The solvent-contaminated site is an old chemical factory located in an alluvial plain in France. As most of the chlorinated contaminants currently found in the groundwater at this site were produced by local industries at various times in the past, it is not enough to analyze chlorinated solvent concentrations along a flow path to convincingly demonstrate biodegradation. Moreover, only a few data were initially available to characterize the geochemical conditions at this site, which were apparently complex at the source zone due to (i) the presence of a steady oxygen supply to the groundwater by irrigation canal losses and river infiltration and (ii) an alkaline pH higher than 10 due to former underground lime disposal. A demonstration of the existence of biodegradation processes was however required by the regulatory authority within a timeframe that did not allow a full geochemical characterization of such a complex site. Thus a combination of different fast methods was used to obtain a proof of the biodegradation occurrence. First, a mass balance analysis was performed which revealed the existence of a strong natural attenuation process (biodegradation, volatilization or dilution), despite the huge uncertainty on these calculations. Second, a good agreement was found between carbon isotopic measurements and PCR assays (based on 16S RNA gene sequences and functional genes), which clearly indicated reductive dechlorination of different hydrocarbons (Tetrachloroethene—PCE-, Trichloroethene—TCE-, 1,2- cisDichloroethene— cis-1,2-DCE-, 1,2- transDichloroethene— trans-1,2-DCE-, 1,1-Dichloroethene—1,1-DCE-, and Vinyl Chloride—VC) to ethene. According to these carbon isotope measurements, although TCE biodegradation seems to occur only in the upgradient part of the studied zone, DCE and VC dechlorination (originating from the initial TCE dechlorination) occurs along the entire flowpath. TCE reductase was not detected among the Dehalococcoides bacteria identified by quantitative PCR (qPCR), while DCE and VC reductases were present in the majority of the population. Reverse transcriptase PCR assays (rt-PCR) also indicated that bacteria and their DCE and VC reductases were active. Mass balance calculations showed moreover that 1,1-DCE was the predominant DCE isomer produced by TCE dechlorination in the upgradient part of the site. Consequently, coupling rt-PCR assays with isotope measurements removes the uncertainties inherent in a simple mass balance approach, so that when the three methods are used jointly, they allow the identification and quantification of natural biodegradation, even under apparently complex geochemical and hydraulic conditions.

HiSpOD: probe design for functional DNA microarrays

The use of DNA microarrays allows the monitoring of the extreme microbial diversity encountered in complex samples like environmental ones as well as that of their functional capacities. However, no probe design software currently available is adapted to easily design efficient and explorative probes for functional gene arrays. We present a new efficient functional microarray probe design algorithm called HiSpOD (High Specific Oligo Design). This uses individual nucleic sequences or consensus sequences produced by multiple alignments to design highly specific probes. Indeed, to bypass crucial problem of cross-hybridizations, probe specificity is assessed by similarity search against a large formatted database dedicated to microbial communities containing about 10 million coding sequences (CDS). For experimental validation, a microarray targeting genes encoding enzymes involved in chlorinated solvent biodegradation was built. The results obtained from a contaminated environmental sample proved the specificity and the sensitivity of probes designed with the HiSpOD program. http://fc.isima.fr/~g2im/hispod/.

Methanotrophic Biodegradation of Trichloroethylene in a Hollow Fiber Membrane Bioreactor

Biodegradation of trichloroethlyene (TCE) in a hollow fiber membrane bioreactor was investigated using a mutant of the methanotrophic bacteria, Methylosinus trichosporium OB3b. Contaminated water flowed through the lumen (i.e., fiber interior), and the bacteria circulated through the shell side of the membrane module and an external growth reactor. In mass transfer studies with a radial cross-flow membrane module, 78.3-99.9% of the TCE was removed from the lumen at hydraulic residence times of 3-15 min in the lumen and the shell. In biodegradation experiments, 80-95% of the TCE was removed from the lumen at hydraulic residence times of 5-9 min in the lumen. The TCE transferred to the shell was rapidly biodegraded, with rate constants ranging from 0.16 to 0.9 L (mg of TSS) -1 day -1 . Radiochemical data showed that over 75% of the transferred TCE was biodegraded in the shell, with the byproducts being approximately equally divided between carbon dioxide and nonvolatiles. This study shows that a hollow fiber membrane bioreactor system coupled with the mutant strain PP358 of M. trichosporium OB3b is a very promising technology for chlorinated solvent biodegradation.

Chlorinated Solvent Biodegradation by Methanotrophs in Unsaturated Soils

Chlorinated aliphatic solvents are common contaminants of unsaturated soils. This research focuses on the feasibility of treatment by \Iin situ\N bioremediation with indigenous methanotrophs. Batch biodegradation experiments are conducted with three unsaturated soils in which indigenous methanotrophs are stimulated by exposure to a methane/air atmosphere. Of the three chlorinated solvents tested, 1,2-dichloroethane is degraded the fastest, followed by trichloroethylene, and finally chloroform. Zero-order rate constants for conversion to CO\d2 range from 0.05 to 1.4 μ\Ng solvent/g dry soil-day. Assuming contaminant concentrations on the order of 100 μ\Ng/soil, these rates imply that \Iin situ\N bioremediation would require several months to several years for completion. Depending upon the particular chlorinated solvent and soil, microbial activity produces, in addition to CO\d2, significant quantities of nonvolatile products. The products represent disappearance of the parent chlorinated solvent, but may or may not represent acceptable bioremediation depending upon the nature of the products and the treatment goals. Availability of nutrients also significantly affects degradation rates.