Reductive Dechlorination Rates Research Articles

Tetrachloroethane (TeCA) removal through sequential graphite-mixed metal oxide electrodes in a bioelectrochemical reactor

Electro-bioremediation offers a promising approach for eliminating persistent pollutants from groundwater since allows the stimulation of biological dechlorinating activity, utilizing renewable electricity for process operation and avoiding the injection of chemicals into aquifers. In this study, a two-chamber microbial electrolysis cell has been utilized to achieve both reductive and oxidative degradation of tetrachloroethane (TeCA). By polarizing the graphite granules cathodic chamber at −650 mV vs the standard hydrogen electrode and employing a mixed metal oxide (MMO) counter electrode for oxygen production, the reductive and oxidative environment necessary for TeCA removal has been established. Continuous experiments were conducted using two feeding solutions: an optimized mineral medium for dechlorinating microorganisms, and synthetic groundwater containing sulphate and nitrate anions to investigate potential side reactions. The bioelectrochemical process efficiently reduced TeCA to a mixture of trans-dichloroethylene, vinyl chloride, and ethylene, which were subsequently oxidized in the anodic chamber with removal efficiencies of 37 ± 2%, 100 ± 4%, and 100 ± 5%, respectively. The introduction of synthetic groundwater with nitrate and sulphate stimulated reductions in these ions in the cathodic chamber, leading to a 17% decrease in the reductive dechlorination rate and the appearance of other chlorinated by-products, including cis-dichloroethylene and 1,2-dichloroethane (1,2-DCA), in the cathode effluent. Notably, despite the lower reductive dechlorination rate during synthetic groundwater operation, aerobic dechlorinating microorganisms within the anodic chamber completely removed VC and 1,2-DCA. This study represents the first demonstration of a sequential reductive and oxidative bioelectrochemical process for TeCA mineralization in a synthetic solution simulating contaminated groundwater.

Hydroxyl groups bridge the electron transfer from Fe(II) to carbon tetrachloride

Reductive dechlorination of chlorinated organic pollutants (COPs) by Fe(II) occurs in natural environments and engineered systems. Fe(II) ions undergo hydroxylation in aqueous solutions to form Ferrous Hydroxyl Complex (FHC), which plays an essential role in Fe(II)-mediated reductive dechlorination. However, how hydroxyl groups of FHC bridge the electron transfer from Fe(II) to COPs is still not fully understood. This work shows that the rate of reductive dechlorination of carbon tetrachloride (CT) by FHC increased with increasing OH− dosage. XRD data shows the increase of OH− dosage transform FHC from Fe2(OH)3Cl to Fe(OH)2, which leads to increased reductive strength of FHC. More non-hydrogen bonded hydroxyl groups coordinate with Fe(II) in FHC with increasing the OH− dosage, which stabilizes the octahedral structure of Fe(II) as shown by Mössbauer data. Electrochemical analysis reveals that the increase of OH− dosage enhances the reductive activity of FHC, which is also confirmed by the decreased HOMO-LUMO gap. It was found that FHC dechlorinated CT to methane, which was attributed to the stabilization of trichlorocarbene anion(˸CCl3−) by [surface-O-Fe(II)-OH]+. This work deepens our understanding on the bridge effect of hydroxyl groups on the electron transfer from Fe(II) to COPs, and provides a theoretical foundation for the reductive dechlorination of COPs in both natural environments and engineered systems.

Evaluation of the Bioelectrochemical Approach and Different Electron Donors for Biological Trichloroethylene Reductive Dechlorination.

Trichloroethylene (TCE) and more in general chlorinated aliphatic hydrocarbons (CAHs) can be removed from a contaminated matrix thanks to microorganisms able to perform the reductive dechlorination reaction (RD). Due to the lack of electron donors in the contaminated matrix, CAHs’ reductive dechlorination can be stimulated by fermentable organic substrates, which slowly release molecular hydrogen through their fermentation. In this paper, three different electron donors constituted by lactate, hydrogen, and a biocathode of a bioelectrochemical cell have been studied in TCE dechlorination batch experiments. The batch reactors evaluated in terms of reductive dechlorination rate and utilization efficiency of the electron donor reported that the bio-electrochemical system (BES) showed a lower RD rate with respect of lactate reactor (51 ± 9 µeq/d compared to 98 ± 4 µeq/d), while the direct utilization of molecular hydrogen gave a significantly lower RD rate (19 ± 8 µeq/d), due to hydrogen low solubility in liquid media. The study also gives a comparative evaluation of the different electron donors showing the capability of the bioelectrochemical system to reach comparable efficiencies with a fermentable substrate without the use of other chemicals, 10.7 ± 3.3% for BES with respect of 3.5 ± 0.2% for the lactate-fed batch reactor. This study shows the BES capability of being an alternative at classic remediation approaches.

Quantitative Proteomics and Quantitative PCR as Predictors of cis-1,2-Dichlorethene and Vinyl Chloride Reductive Dechlorination Rates in Bioaugmented Aquifer Microcosms

Quantitative measurement of process-specific biomarker genes of Dehalococcoides mccartyi (Dhc) supports monitoring at chlorinated ethene contaminated sites. In this study, we varied Dhc cell abundances from ∼103 to 108 cells/mL in aquifer microcosms and correlated the corresponding reductive dehalogenase (RDase) gene and RDase protein abundances with measured reductive dechlorination (RD) rates of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). An additional set of microcosms tested the RD rate-predictive power of the regression analyses. These efforts revealed (1) that targeted proteomics quantifies Dhc biomarker proteins (e.g., TceA and VcrA, OmeA) over a relevant range of Dhc cell densities, and (2) that protein and gene abundances can predict RD rates. Protein detection limits translated to a rate coefficient of 10–4 day–1 (0.04 year–1) for both kcDCE and kvc, which is within the range observed at sites undergoing monitored natural attenuation (MNA) (i.e., without the implementation of enhanced bioremediation treatment). Rates predicted using a combination of quantitative biomarker gene and protein measurements generally resulted in the best match with experimentally determined rate constants. These new findings provide evidence that quantitative biomarker measurements may be useful predictors of in situ RD rates, which would constitute a major advance for the cost-effective management of contaminated sites.

Metagenomic Analysis Reveals Microbial Interactions at the Biocathode of a Bioelectrochemical System Capable of Simultaneous Trichloroethylene and Cr(VI) Reduction.

Bioelectrochemical systems (BES) are attractive and versatile options for the bioremediation of organic or inorganic pollutants, including trichloroethylene (TCE) and Cr(VI), often found as co-contaminants in the environment. The elucidation of the microbial players’ role in the bioelectroremediation processes for treating multicontaminated groundwater is still a research need that attracts scientific interest. In this study, 16S rRNA gene amplicon sequencing and whole shotgun metagenomics revealed the leading microbial players and the primary metabolic interactions occurring in the biofilm growing at the biocathode where TCE reductive dechlorination (RD), hydrogenotrophic methanogenesis, and Cr(VI) reduction occurred. The presence of Cr(VI) did not negatively affect the TCE degradation, as evidenced by the RD rates estimated during the reactor operation with TCE (111±2 μeq/Ld) and TCE/Cr(VI) (146±2 μeq/Ld). Accordingly, Dehalococcoides mccartyi, the primary biomarker of the RD process, was found on the biocathode treating both TCE (7.82E+04±2.9E+04 16S rRNA gene copies g−1 graphite) and TCE/Cr(VI) (3.2E+07±2.37E+0716S rRNA gene copies g−1 graphite) contamination. The metagenomic analysis revealed a selected microbial consortium on the TCE/Cr(VI) biocathode. D. mccartyi was the sole dechlorinating microbe with H2 uptake as the only electron supply mechanism, suggesting that electroactivity is not a property of this microorganism. Methanobrevibacter arboriphilus and Methanobacterium formicicum also colonized the biocathode as H2 consumers for the CH4 production and cofactor suppliers for D. mccartyi cobalamin biosynthesis. Interestingly, M. formicicum also harbors gene complexes involved in the Cr(VI) reduction through extracellular and intracellular mechanisms.

Microbial Chain Elongation and Subsequent Fermentation of Elongated Carboxylates as H2-Producing Processes for Sustained Reductive Dechlorination of Chlorinated Ethenes

In situ anaerobic groundwater bioremediation of trichloroethene (TCE) to nontoxic ethene is contingent on organohalide-respiring Dehalococcoidia, the most common strictly hydrogenotrophic Dehalococcoides mccartyi (D. mccartyi). The H2 requirement for D. mccartyi is fulfilled by adding various organic substrates (e.g., lactate, emulsified vegetable oil, and glucose/molasses), which require fermenting microorganisms to convert them to H2. The net flux of H2 is a crucial controlling parameter in the efficacy of bioremediation. H2 consumption by competing microorganisms (e.g., methanogens and homoacetogens) can diminish the rates of reductive dechlorination or stall the process altogether. Furthermore, some fermentation pathways do not produce H2 or having H2 as a product is not always thermodynamically favorable under environmental conditions. Here, we report on a novel application of microbial chain elongation as a H2-producing process for reductive dechlorination. In soil microcosms bioaugmented with dechlorinating and chain-elongating enrichment cultures, near stoichiometric conversion of TCE (0.07 ± 0.01, 0.60 ± 0.03, and 1.50 ± 0.20 mmol L-1 added sequentially) to ethene was achieved when initially stimulated by chain elongation of acetate and ethanol. Chain elongation initiated reductive dechlorination by liberating H2 in the conversion of acetate and ethanol to butyrate and caproate. Syntrophic fermentation of butyrate, a chain-elongation product, to H2 and acetate further sustained the reductive dechlorination activity. Methanogenesis was limited during TCE dechlorination in soil microcosms and absent in transfer cultures fed with chain-elongation substrates. This study provides critical fundamental knowledge toward the feasibility of chlorinated solvent bioremediation based on microbial chain elongation.

Increasing electron donor concentration does not accelerate complete microbial reductive dechlorination in contaminated sediment with native organic carbon.

Experiments with Fe(III)-rich, chloroethene-contaminated sediment demonstrated that trichloroethylene (TCE) and vinyl chloride (VC) were completely reduced to ethene regardless of whether electron donor(s) were added at 1 × stoichiometry or 10 × stoichiometry relative to all-electron acceptors. Unamended controls uniformly reduced TCE to ethene with a mean time to complete dechlorination (operationally defined as the presence of stoichiometric ethene production) of 79days. Adding 1 × and 10 × acetate hindered the rate and extent of TCE and VC reduction relative to unamended controls, with several only partially reduced when the experiments were terminated. Adding high molecular mass (soybean oil derivative) substrates did not increase microbial reductive dechlorination relative to unamended incubations, and in many cases, hindered microbial dechlorination in favor of methanogenesis. The mean time to complete dechlorination was comparable between low (× 1) and high (× 10) electron donor concentration for all lipid-based electron donors tested. Those tested included Newman Zone® Standard without sodium lactate (96 vs. 75days, respectively), CAP 18 ME (85 vs. 94days, respectively), EOS 598B42 (68 vs. 72days, respectively), and acetate (134 vs. 125days, respectively). These data suggest that the addition of an electron donor does not always increase the rate and extent of reductive dechlorination but will increase costs. In particular, increasing the concentration of electron donors higher than the stoichiometric demand only decreased complete microbial reductive dechlorination, which is the opposite of most standard "more time and more electrons" approaches. These data argue that site-specific electron donor demands must be evaluated, and in some cases, a monitored natural attenuation (MNA) approach is most favorable.

Correlations between maximum reductive dechlorination rates and specific biomass parameters in Dehalococcoides mccartyi consortia enriched on chloroethenes PCE, TCE and cis-1,2-DCE.

One of the challenges to implementing the modeling of the biological reductive dechlorination (RD) process is the evaluation of biological parameters that represent the abundance/activity levels of the microorganisms involved in the biodegradation of chloroethenes. Here we report a combined analysis of kinetic and specific biomass parameters conducted on three dechlorinating consortia enriched on PCE, TCE and cis-1,2-DCE. In these consortia, Dehalococcoides mccartyi (Dhc) represented ≥70% of the bacterial population identified via 16S rRNA gene amplicon sequencing. Quantitative biomolecular methods were used to generate specific biomass parameters targeting either the Dhc population (16S rRNA genes or cells) or specific genes encoding RD process-involved reductive dehalogenases. The correlation factor between the abundance of active Dhc cells or tceA gene copies and maximum RD rates allowed to predict an increment of 7E+09 of active Dhc cells or 5E+09 tceA gene copies/L under controlled conditions. Diversely, the utilization of gene transcripts as biomass parameters for RD modeling did not provide reliable correlations with kinetic performances. This study provides valuable insights for further modeling of the RD process through the utilization of specific biomass parameters.

Impact of Fixed Nitrogen Availability on Dehalococcoides mccartyi Reductive Dechlorination Activity.

Biostimulation to promote reductive dechlorination is widely practiced, but the value of adding an exogenous nitrogen (N) source (e.g., NH4+) during treatment is unclear. This study investigates the effect of NH4+ availability on organohalide-respiring Dehalococcoides mccartyi (Dhc) growth and reductive dechlorination in enrichment cultures derived from groundwater (PW4) and river sediment (TC) impacted with chlorinated ethenes. In PW4 cultures, the addition of NH4+ increased cis-1,2-dichloroethene (cDCE)-to-ethene dechlorination rates about 5-fold (20.6 ± 1.6 versus 3.8 ± 0.5 μM Cl- d-1), and the total number of Dhc 16S rRNA gene copies were about 43-fold higher in incubations with NH4+ ((1.8 ± 0.9) × 108 mL-1) compared to incubations without NH4+ ((4.1 ± 0.8) × 107 mL-1). In TC cultures, NH4+ also stimulated cDCE-to-ethene dechlorination and Dhc growth. Quantitative polymerase chain reaction (qPCR) revealed that Cornell-type Dhc capable of N2 fixation dominated PW4 cultures without NH4+, but their relative abundance decreased in cultures with NH4+ amendment (i.e., 99 versus 54% of total Dhc). Pinellas-type Dhc incapable of N2 fixation were responsible for cDCE dechlorination in TC cultures, and diazotrophic community members met their fixed N requirement in the medium without NH4+. Responses to NH4+ were apparent at the community level, and N2-fixing bacterial populations increased in incubations without NH4+. Quantitative assessment of Dhc nitrogenase genes, transcripts, and proteomics data linked Cornell-type Dhc nifD and nifK expression with fixed N limitation. NH4+ additions also demonstrated positive effects on Dhc in situ dechlorination activity in the vicinity of well PW4. These findings demonstrate that biostimulation with NH4+ can enhance Dhc reductive dechlorination rates; however, a "do nothing" approach that relies on indigenous diazotrophs can achieve similar dechlorination end points and avoids the potential for stalled dechlorination due to inhibitory levels of NH4+ or transformation products (i.e., nitrous oxide).

Accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol by weak electrical stimulation

Microbial reductive dechlorination of chlorinated aromatics frequently suffers from the long dechlorination period and the generation of toxic metabolites. Biocathode bioelectrochemical systems were verified to be effective in the degradation of various refractory pollutants. However, the electrochemical and microbial related working mechanisms for bio-dechlorination by electro-stimulation remain poorly understood. In this study, we reported the significantly improved 2,4,6-trichlorophenol dechlorination activity through the weak electro-stimulation (cathode potential of −0.36 V vs. SHE), as evidenced by the 3.1 times higher dechlorination rate and the complete dechlorination ability with phenol as the end dechlorination product. The high reductive dechlorination rate (20.8 μM/d) could be maintained by utilizing electrode as an effective electron donor (coulombic efficiency of 82.3 ± 4.8%). Cyclic voltammetry analysis of the cathodic biofilm gave the direct evidences of the cathodic respiration with the improved and positive-shifted reduction peaks of 2,4,6-TCP, 2,4-DCP and 4-CP. The optimal 2,4,6-TCP reductive dechlorination rate (24.2 μM/d) was obtained when a small amount of lactate (2 mM) was added, and the generation of H2 and CH4 were accompanied due to the biological fermentation and methanogenesis. The electrical stimulation significantly altered the cathodic biofilm structure and composition with some potential dechlorinators (like Acetobacterium) predominated. The microbial interactions in the ecological network of cathodic biofilm were more simplified than the planktonic community. However, some potential dechlorinators (Acetobacterium, Desulfovibrio, etc.) shared more positive interactions. The co-existence and possible cooperative relationships between potential dechlorinators and fermenters (Sedimentibacter, etc.) were revealed. Meanwhile, the competitive interrelations between potential dechlorinators and methanogens (Methanomassiliicoccus) were found. In the network of plankton, the fermenters and methanogens possessed the more positive interrelations. Electro-stimulation at the cathodic potential of −0.36 V selectively enhanced the dechlorination function, while it showed little influence on either fermentation or methanogenesis process. The study gave suggestions for the enhanced bioremediation of chlorinated aromatics, in views of the electro-stimulation capacity, efficiency and microbial interrelations related microbial mechanism.

Mechanisms for Abiotic Dechlorination of Trichloroethene by Ferrous Minerals under Oxic and Anoxic Conditions in Natural Sediments.

Bench-scale experiments were performed on natural sediments to assess abiotic dechlorination of trichloroethene (TCE) under both aerobic and anaerobic conditions. In the absence of oxygen (<26 μM), TCE dechlorination proceeded via a reductive pathway generating acetylene and/or ethene. Reductive dechlorination rate constants up to 3.1 × 10-5 d-1 were measured, after scaling to in situ solid:water ratios. In the presence of oxygen greater than 120 μM, TCE dechlorination proceeded via an oxidative pathway generating formic/glyoxylic and glycolic/acetic acids, and oxidative dechlorination rate constants (again scaled to in situ conditions) up to 7.4 × 10-3 d-1 were measured. These rates correspond to half-lives of 60 and 0.25 years for abiotic TCE dechlorination under anaerobic and aerobic conditions, respectively, indicating the potentially large impact of aerobic TCE oxidation in the field. For both reductive and oxidative TCE dechlorination pathways, measured first-order rate constants increased with increasing ferrous iron content, suggesting the role of iron oxidation. Hydroxyl radical formation was measured and increased with increasing oxygen and ferrous iron content. Rate constants associated with TCE oxidation products increased with increasing hydroxyl radical generation rates, and were zero in the presence of a hydroxyl radical scavenger, suggesting that oxidative TCE dechlorination is a hydroxyl radical driven process.

DDT and related compounds in pore water of shallow sediments on the Palos Verdes Shelf, California, USA

Abstract For nearly two and a half decades following World War II, production wastes from the world's largest manufacturer of technical DDT (1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene) were discharged into sewers of Los Angeles County. Following treatment, the wastes were released via a submarine outfall system to nearshore coastal waters where a portion accumulated in shallow sediments of the Palos Verdes Shelf (PVS). An investigation of the pore-water geochemistry of DDT-related compounds (DDX) was undertaken in an effort to understand factors controlling the rate of reductive dechlorination (RDC) of the major DDT degradate, 4,4′-DDE (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene). Equilibrium matrix-solid phase microextraction (matrix-SPMEeq) combined with automated thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) was used to determine freely dissolved concentrations of ten DDX analytes in sediment cores collected from three locations on the PVS (stations 3C, 6C, 8C, which are 7 km, 2 km, and 0 km, respectively, downcurrent from the outfall system). Pore-water concentrations (pM) of the principal DDX compounds involved in RDC were: 3C-DDE: 6.0–24, DDMU (1-chloro-4-[2-chloro-1-(4-chlorophenyl)ethenyl]benzene): 11–160, DDNU (1-chloro-4-[1-(4-chlorophenyl)ethenyl]benzene): 1.8–68; 6C-DDE: 5.6–170, DDMU: 5.6–177, DDNU: 1.7–87; 8C-DDE: 27–212, DDMU: 31–403, DDNU: 5.5–89. Variations in the spatial distribution of DDX analytes in pore water reflect several factors including proximity to the outfalls, RDC reaction rates, and natural variability in sedimentation and post-depositional transport processes. A comparison of pore-water data produced using matrix-SPMEeq/TD-GC/MS and whole-core squeezing/solvent extraction/liquid injection-GC/MS indicates that the majority of the DDE in the upper sediment column (≤about 10 cm) is associated with dissolved/colloidal organic matter. Below that depth, freely-dissolved DDE predominates. The principal organic geochemical phase controlling sorption of DDE in PVS sediments are residual hydrocarbons, the vast majority of which originated from petroleum refinery wastes. Organic carbon-normalized sediment-water distribution coefficients (KOC) were calculated from solid-phase and pore-water concentrations of DDX and organic carbon. Log KOC values (L/kg) were relatively invariant across the shelf and with depth in the sediment column. Shelf-wide compound-specific coefficients (log KOC) were: DDE: 7.5 ± 0.11, DDMU: 6.92 ± 0.13, DDNU: 6.37 ± 0.19. The spatial uniformity of KOC means that biological exposure and availability of the DDX compounds can, in principle, be estimated from solid-phase chemical measurements.

Biomarkers' Responses to Reductive Dechlorination Rates and Oxygen Stress in Bioaugmentation Culture KB-1TM.

Using mRNA transcript levels for key functional enzymes as proxies for the organohalide respiration (OHR) rate, is a promising approach for monitoring bioremediation populations in situ at chlorinated solvent-contaminated field sites. However, to date, no correlations have been empirically derived for chlorinated solvent respiring, Dehalococcoides mccartyi (DMC) containing, bioaugmentation cultures. In the current study, genome-wide transcriptome and proteome data were first used to confirm the most highly expressed OHR-related enzymes in the bioaugmentation culture, KB-1TM, including several reductive dehalogenases (RDases) and a Ni-Fe hydrogenase, Hup. Different KB-1™ DMC strains could be resolved at the RNA and protein level through differences in the sequence of a common RDase (DET1545-like homologs) and differences in expression of their vinyl chloride-respiring RDases. The dominant strain expresses VcrA, whereas the minor strain utilizes BvcA. We then used quantitative reverse-transcriptase PCR (qRT-PCR) as a targeted approach for quantifying transcript copies in the KB-1TM consortium operated under a range of TCE respiration rates in continuously-fed, pseudo-steady-state reactors. These candidate biomarkers from KB-1TM demonstrated a variety of trends in terms of transcript abundance as a function of respiration rate over the range: 7.7 × 10−12 to 5.9 × 10−10 microelectron equivalents per cell per hour (μeeq/cell∙h). Power law trends were observed between the respiration rate and transcript abundance for the main DMC RDase (VcrA) and the hydrogenase HupL (R2 = 0.83 and 0.88, respectively), but not transcripts for 16S rRNA or three other RDases examined: TceA, BvcA or the RDase DET1545 homologs in KB1TM. Overall, HupL transcripts appear to be the most robust activity biomarker across multiple DMC strains and in mixed communities including DMC co-cultures such as KB1TM. The addition of oxygen induced cell stress that caused respiration rates to decline immediately (>95% decline within one hour). Although transcript levels did decline, they did so more slowly than the respiration rate observed (transcript decay rates between 0.02 and 0.03 per hour). Data from strain-specific probes on the pangenome array strains suggest that a minor DMC strain in KB-1™ that harbors a bvcA homolog preferentially recovered following oxygen stress relative to the dominant, vcrA-containing strain.

Reductive dechlorination rates of 4,4′-DDE (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene) in sediments of the Palos Verdes Shelf, CA

Abstract Wastes from the world's largest manufacturer of DDT (1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene) were released into the Los Angeles County municipal sewer system from 1947 to 1971. Following primary treatment, the effluent was discharged through a submarine outfall system whereupon a portion of the DDT and associated degradation products were deposited in sediments of the Palos Verdes Shelf (PVS). Parent DDT is present only in trace amounts in the sediments today, the vast majority having been transformed to DDE (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene) shortly following deposition. Previously believed to be inert, DDE is slowly being converted to DDMU (1-chloro-4-[2-chloro-1-(4-chlorophenyl)ethenyl]benzene) and DDMU to DDNU (1-chloro-4-[1-(4-chlorophenyl)ethenyl]benzene) via microbially-mediated reductive dechlorination (RDC). Kinetic and compositional data suggest that this process began sometime in the mid- to late 1970s. Rates of DDE RDC in shelf sediments are spatially variable and have proven difficult to determine accurately. This limits our ability to understand the factors controlling RDC rates and to predict the course of natural recovery. In the present study, concentrations of ten DDT compounds and twelve PCB (polychlorinated biphenyl) congeners were determined in cores collected at two locations on the PVS (stations 3C, 6C, ~ 7 km and ~ 2 km downcurrent from the outfalls, respectively). DDE inventories, normalized to those of non-degrading PCB congeners having similar physico-chemical properties , were modeled to yield first-order RDC rates for the period 1981–2010. Average rates at stations 3C and 6C were 0.044 ± 0.004 and 0.008 ± 0.002 yr− 1, respectively, with depth-dependent RDC rates at station 3C (1992–2003) ranging from 0.0025 to 0.102 yr− 1. Comparison of RDC and total loss (i.e., RDC + physical loss) rates suggests that the average per cent loss of DDE due to RDC is ~ 90% at station 3C (1981–2010) and ~ 57% at station 6C (1992–2010). Trajectories of adjusted molar inventories of DDE, DDMU, and DDNU were forecast using a first-order multi-step reaction series (M-SRS) model. The results for DDE are consistent with the normalization procedure; RDC rates at stations 3C and 6C were 0.036 ± 0.002 yr− 1 and 0.010 ± 0.001 yr− 1, respectively. At station 6C, the DDE to DDMU transformation appears to be the rate limiting step in the reaction sequence, DDE k 1 → DDMU k 2 → DDNU k 3 → unidentified compound(s), whereas at station 3C RDC rates for DDE and DDMU are roughly equivalent. At both locations the transformation rate of DDNU is 7–20 times that of the other steps. Estimated half-lives of DDE at stations 3C and 6C based on the M-SRS model results are ~ 19 and 72 years, respectively.

High-throughput sequencing revealed novel Dehalococcoidia in dechlorinating microbial enrichments from PCB-contaminated marine sediments.

In this study, six PCE-to-ethene dechlorinating cultures, fed with a fermentable substrate (lactate) or hydrogen as electron donor, were obtained from PCB and PCE dechlorinating microcosms constructed with PCB-contaminated marine sediments. A novel Chloroflexi member (OTU-DIS1) affiliated to Dehalococcoidales Incertae Sedis, only distantly related to known dechlorinating bacteria, dominated the enrichment cultures (up to 86% of total OTUs). Sulfate-, thiosulfate- and sulfur-reducing bacteria affiliated to genera Desulfobacter, Dethiosulfatibacter and Desulfuromusa were also found to lesser extent. Remarkably, tceA, vcrA and the bifunctional PCE/PCB dehalogenase genes pcbA1, pcbA4 and pcbA5 were found in all dechlorinating microbial enrichments indicating the coexistence of different Dehalococcoides mccartyi strains. The reductive dechlorination rate in each culture remained unvaried over long-term operation (≈ 30 months) and ranged between 0.85 and 0.97 mmol Cl-1 released L-1 d-1 in the lactate-fed microbial enrichments and between 0.66 and 0.85 mmol Cl-1 released L-1 d-1 in the H2-fed microbial enrichments. Overall, this study highlights the presence of yet unexplored biodiversity in PCBs contaminated marine sediments and indicates these environments as promising sources of novel organohalide-respiring bacteria.

Effects of Goethite, Magnetite and Gypsum on the Anaerobic Degradation of 2,4-Dichlorophenol

2,4-dichlorophenol(2,4-DCP), a highly toxic and refractory organic compound, was commonly used in pesticide production and thus widely distributed in water and soil. Goethite, magnetite and gypsum were added into the anaerobic system which simulated the natural anaerobic process to evaluate their effects on the anaerobic degradation of 2,4-DCP. It indicated that goethite, magnetite and gypsum had no appreciable adsorption ability toward 2,4-DCP under anaerobic condition. Mass balance analysis showed that only the transformation of 2,4-DCP to 4-chlorophenol(4-CP) occurred in all experimental groups. The addition of sodium acetate doubled the reductive dechlorination rate of 2,4-DCP in comparison with the groups without exogenous carbon source. The reductive dechlorination rate of 2,4-DCP was enhanced by the addition of goethite and magnetite, which was caused by the improved metabolic activity of dechlorination bacteria that played an important role in the anaerobic degradation of 2,4-DCP. Gypsum greatly inhibited or even stopped the degradation process of 2,4-DCP through restraining the growth and activity of dechlorination bacteria. This study will shed light on the migration and degradation of refractory chlorinated organic contaminants in anaerobic sedimentary environment, and the treatment of such matters in environmental technology.

Reductive dechlorination of tetrachloroethene in marine sediments: Biodiversity and dehalorespiring capabilities of the indigenous microbes

Chlorinated compounds pose environmental concerns due to their toxicity and wide distribution in several matrices. Microorganisms specialized in leading anaerobic reductive dechlorination (RD) processes, including Dehalococcoides mccartyi (Dhc), are able to reduce chlorinated compounds to harmless products or to less toxic forms. Here we report the first detailed study dealing with the RD potential of heavy polluted marine sediment by evaluating the biodegradation kinetics together with the composition, dynamics and activity of indigenous microbial population.A microcosm study was conducted under strictly anaerobic conditions on marine sediment collected near the marine coast of Sarno river mouth, one of the most polluted river in Europe. Tetrachloroethene (PCE), used as model pollutant, was completely converted to ethene within 150days at reductive dechlorination rate equal to 0.016meqL−1d−1. Consecutive spikes of PCE allowed increasing the degradation kinetics up to 0.1meqL−1d−1 within 20days. Strictly anaerobiosis and repeated spikes of PCE stimulated the growth of indigenous Dhc cells (growth yield of ~7.0E+07DhccellsperμMCl−1 released). Dhc strains carrying the reductive dehalogenase genes tceA and vcrA were detected in the original marine sediment and their number increased during the treatment as demonstrated by the high level of tceA expression at the end of the microcosm study (2.41E+05tceAgenetranscriptsg−1). Notably, the structure of the microbial communities was fully described by Catalysed Reporter Deposition Fluorescence In Situ Hybridization (CARD-FISH) as wells as the dynamics of the dechlorinating bacteria during the microcosms operation. Interestingly, a direct role of Dhc cells was ascertained suggesting the existence of strains adapted at salinity conditions. Additionally, non-Dhc Chloroflexi were retrieved in the original sediment and were kept stable over time suggesting their likely flanking role of the RD process.

The corrinoid cofactor of reductive dehalogenases affects dechlorination rates and extents in organohalide-respiring Dehalococcoides mccartyi.

Corrinoid auxotrophic organohalide-respiring Dehalococcoides mccartyi (Dhc) strains are keystone bacteria for reductive dechlorination of toxic and carcinogenic chloroorganic contaminants. We demonstrate that the lower base attached to the essential corrinoid cofactor of reductive dehalogenase (RDase) enzyme systems modulates dechlorination activity and affects the vinyl chloride (VC) RDases BvcA and VcrA differently. Amendment of 5,6-dimethylbenzimidazolyl-cobamide (DMB-Cba) to Dhc strain BAV1 and strain GT cultures supported cis-1,2-dichloroethene-to-ethene reductive dechlorination at rates of 107.0 (±12.0) μM and 67.4 (±1.4) μM Cl(-) released per day, respectively. Strain BAV1, expressing the BvcA RDase, reductively dechlorinated VC to ethene, although at up to fivefold lower rates in cultures amended with cobamides carrying 5-methylbenzimidazole (5-MeBza), 5-methoxybenzimidazole (5-OMeBza) or benzimidazole (Bza) as the lower base. In contrast, strain GT harboring the VcrA RDase failed to grow and dechlorinate VC to ethene in medium amended with 5-OMeBza-Cba or Bza-Cba. The amendment with DMB to inactive strain GT cultures restored the VC-to-ethene-dechlorinating phenotype and intracellular DMB-Cba was produced, demonstrating cobamide uptake and remodeling. The distinct responses of Dhc strains with BvcA versus VcrA RDases to different cobamides implicate that the lower base exerts control over Dhc reductive dechlorination rates and extents (that is, detoxification), and therefore the dynamics of Dhc strains with discrete reductive dechlorination capabilities. These findings emphasize that the role of the corrinoid/lower base synthesizing community must be understood to predict strain-specific Dhc activity and achieve efficacious contaminated site cleanup.

Accelerated anaerobic dechlorination of DDT in slurry with Hydragric Acrisols using citric acid and anthraquinone-2,6-disulfonate (AQDS)

The application of electron donor and electron shuttle substances has a vital influence on electron transfer, thus may affect the reductive dechlorination of 1,1,1-trichoro-2,2-bis(p-chlorophenyl)ethane (DDT) in anaerobic reaction systems. To evaluate the roles of citric acid and anthraquinone-2,6-disulfonate (AQDS) in accelerating the reductive dechlorination of DDT in Hydragric Acrisols that contain abundant iron oxide, a batch anaerobic incubation experiment was conducted in a slurry system with four treatments of (1) control, (2) citric acid, (3) AQDS, and (4) citric acid+AQDS. Results showed that DDT residues decreased by 78.93%–92.11% of the initial quantities after 20days of incubation, and 1,1-dichloro-2,2-bis(4-chlorophenyl)-ethane (DDD) was the dominant metabolite. The application of citric acid accelerated DDT dechlorination slightly in the first 8days, while the methanogenesis rate increased quickly, and then the acceleration effect improved after the 8th day while the methanogenesis rate decreased. The amendment by AQDS decreased the Eh value of the reaction system and accelerated microbial reduction of Fe(III) oxides to generate Fe(II), which was an efficient electron donor, thus enhancing the reductive dechlorination rate of DDT. The addition of citric acid+AQDS was most efficient in stimulating DDT dechlorination, but no significant interaction between citric acid and AQDS on DDT dechlorination was observed. The results will be of great significance for developing an efficient in situ remediation strategy for DDT-contaminated sites.

Relative contribution of set cathode potential and external mass transport on TCE dechlorination in a continuous-flow bioelectrochemical reactor

Microbial bioelectrochemical systems, which use solid-state cathodes to drive the reductive degradation of contaminants such as the chlorinated hydrocarbons, are recently attracting considerable attention for bioremediation applications. So far, most of the published research has focused on analyzing the influence of key (bio)electrochemical factors influencing contaminant degradation, such as the cathode potential, whereas only few studies have examined the potential impact of mass transport phenomena on process performance. Here we analyzed the performance of a flow-through bioelectrochemical reactor, continuously fed with a synthetic groundwater containing trichloroethene at three different linear fluid velocities (from 0.3m d−1 to 1.7m d−1) and three different set cathode potentials (from −250mV to −450mV vs. the standard hydrogen electrode). The obtained results demonstrated that, in the range of fluid velocities which are characteristics for natural groundwater systems, mass transport phenomena may strongly influence the rate and extent of reductive dechlorination. Nonetheless, the relative importance of mass transport largely depends on the applied cathode potential which, in turn, controls the intrinsic kinetics of biological reactions and the underlying electron transfer mechanisms.