Reductive Dechlorination Pathways Research Articles

Tourmaline guiding the electric field and dechlorination pathway of 2,3-dichlorophenol by Desulfitobacterium hafniense

The dehalogenation of organohalides has been a research hotspot in bioremediation field; however, the influence of tourmaline, a natural ore that can generate spontaneous electric field, on organohalide-respiring bacteria (OHRB) and their dechlorination process is not well known. In this study, the effect and mechanism of tourmaline on the reductive dechlorination of 2,3-dichlorophenol (2,3-DCP) by Desulfitobacterium hafniense DCB-2T were explored. The characterization results confirmed that tourmaline had good stability and the optimal dosage of tourmaline was 2.5 g/L, which shortened the total time required for dechlorination reaction to 72 hr. Besides, tourmaline amendment also increased the proportion of 2-chlorophenol (2-CP) from 18% to 30% of end products, while that of 3-CP decreased correspondingly. The theoretical calculations showed that the bond charge of the ortho-substituted chlorine declined from -0.179 to -0.067, and that of meta-substituted chlorine increased from -0.111 to -0.129, which indicated that the spontaneous electric field of tourmaline affected the charge distribution of 2,3-DCP and was more conducive to the generation of 2-CP. Overall, tourmaline with the spontaneous electric field affected the reductive dechlorination pathway of Desulfitobacterium,and the tourmaline-OHRB combining system might serve as a novel strategy for the bioremediation of environments polluted with chlorinated phenols.

Microbial reductive dechlorination of polychlorinated dibenzo-p-dioxins: Pathways and features unravelled via electron density

Microbial reductive dechlorination provides a promising approach for remediating sites contaminated with polychlorinated dibenzo-p-dioxins (PCDDs). Nonetheless, the overall dechlorination pathways and features remain elusive. Herein, we address these issues by quantum chemical calculations, considering the calibrations of reductive dechlorination of 15 PCDDs mediated by three Dehalococcoides strains. Chlorine substituents with lower electron density are prone to be microbially abstracted, which differentiates 72 microbe-active PCDDs from 3 nonactive analogues with a success rate of 100%. For all 256 transformation routes of 75 PCDDs, electron density differences of chlorines pinpoint 105 viable and 125 unviable pathways, corresponding a success rate of 90%. The feasibility of 26 reductive dechlorination pathways are uncertain because of the limited available experimental data. 98% (251/256) of microbial chlorine abstraction follows an order of ClO,Cl>ClCl,Cl>ClH,O>ClH,Cl>ClH,H=0. PCDDs solely containing chlorines at C1, C4, C6, and/or C9 can be completely dechlorinated to non-chlorinated dioxin; while PCDDs housing chlorines at C2, C3, C7, and/or C8 can be dechlorinated to 2-MCDD or 2,7/8-DCDD as final products. These findings also support reductive dechlorination of PCDDs in mixed cultures and sediments (> 98% and 83%). These findings would promote the application of dechlorinating bacteria in targeted remediation and facilitate the respective studies on other POPs.

A comparison study of Fenton-like and Fenton reactions in dichloromethane removal

ABSTRACT Dichloromethane (DCM), as a low-chlorinated organic compound, is hardly to be degraded through the reductive dechlorination pathway. In this study, the removal of DCM in Fenton-like system, using activated carbon fibres-supported zero-valence Fe/Ni nanoparticles (ACF-Fe/Ni) as catalysts, was investigated and compared with that of a traditional Fenton system (Fe2+/H2O2). The influence of vital parameters, including initial solution pH, DCM concentration, catalyst and H2O2 dosages, temperature and cosolute on the removal of DCM, was systematically studied. The results showed that 94.2% of DCM with an initial concentration of 5 mg/L could be removed in the Fenton-like reaction under the optimum condition: initial pH of 2.0, 0.4 g/L of ACF-Fe/Ni, 10 mM of H2O2 and a temperature of 30°C. In comparison, the removal of DCM in the Fenton-like system was faster than that of the Fenton system and the corresponding activation energies were 39.69 and 33.82 kJ/mol, respectively. The coexistence of solute was adverse to the removal of DCM in both Fenton-like and Fenton systems. Moreover, the active species for DCM removal in the Fenton-like system was confirmed as hydroxyl radical (·OH) via the quenching experiment and electron paramagnetic resonance measurement. The incomplete mineralisation (41.7%) of DCM after reaction indicated that the Fenton-like technology had the potential to realise DCM’s non-toxic and harmless conversion and organic intermediates formed needed to take longer time to be decomposed.

Sequential Reductive/oxidative Bioelectrochemical Process for Groundwater Perchloroethylene Removal

Chlorinated aliphatic hydrocarbons (CAHs) are common groundwater contaminants, microbial communities naturally present in groundwater can reduce CAHs as perchloroethylene (PCE) and trichloroethylene (TCE) to ethylene through reductive dechlorination (RD) reaction while low chlorinated CAHs like cis-dichloroethylene (cis DCE) and vinyl chloride (VC) can be oxidized by aerobic pathways. A combination of reductive and oxidative dechlorination results an effective strategy for the complete mineralization of CAHs. Bioelectrochemical systems (BES) are innovative processes which can be adopted to stimulate both reductive and oxidative dechlorination biomass through polarized electrodes. The present study describes the performances of a an oxidative bioelectrochemical reactor composed by a membrane-less microbial electrolysis cell (MEC) equipped with an internal graphite counterelectrode. In the oxidative reactor the oxygen provided by a mixed metal oxides (MMO) anode stimulated the oxidative dechlorination of the cisDCE contained in synthetic groundwater. Throughout the experimental period, both reductive and oxidative dechlorination pathways were identified due to presence of an internal counter electrode that acted as electron donor. Reductive and oxidative bioelectrochemical reactions, including anions reduction were determined and their relative contribution to the overall flowing current has been quantified in terms of oxidative and reductive coulombic efficiencies.

Enhanced alteration of poly(vinyl chloride) microplastics by hydrated electrons derived from indole-3-acetic acid assisted by a common cationic surfactant

In this study, a new photo-irradiated reductive dechlorination pathway and the underlying transformation mechanism are described for poly(vinyl chloride) microplastics (PVC-MPs). PVC-MPs underwent photo-reductive dechlorination process with the release of chloride ions. This reaction could be facilitated in the presence of indole-3-acetic acid (IAA) and hexadecyltrimethylammonium bromide (CTAB) under neutral pH and simulated sunlight irradiation conditions. Electrostatic interaction between IAA and CTAB produced neutral IAA/CTAB complex, which might account for the enhanced adsorption of IAA on PVC powders. Upon photo-irradiation, the adsorbed IAA was excited to generate hydrated electrons (eaq−), which could pass through a shorter distance to PVC-MP surface than that derived from homogeneous IAA molecules in aqueous solution. Transient spectra of laser flash photolysis provided direct evidence for the generation of eaq−, which supported the proposed dechlorination mechanism. Based on the results of attenuated total reflectance/Fourier transform infrared (ATR/FTIR) and Raman spectra, C-Cl bond cleavage and polyene formation were involved in the structural transformation of PVC-MPs. Due to the hydrophobic effects and π-π interactions between aromatic rings and polyene structures in PVC-MP surface, the PVC-MP powders irradiated in the presence of IAA/CTAB showed an enhanced sorption for both hydrophobic and hydrophilic aromatic chemicals.

Reduction of chlorendic acid by zero-valent iron: Kinetics, products, and pathways

Chlorendic acid (CA) is a recalcitrant groundwater contaminant for which an effective treatment technology does not currently exist. In this study, a series of batch experiments were conducted to investigate the treatment of CA by zero-valent iron (ZVI) under various water chemistry conditions. It was observed that CA was removed by ZVI via both adsorption and degradation, with the degradation rate being proportional to the fraction of CA adsorbed onto ZVI. The rate of CA degradation decreased as pH increased, presumably due to the passivation of ZVI and diminishing CA adsorption. Chloride (Cl−) did not appreciably affect CA adsorption and degradation, while sulfate (SO42−) significantly inhibited both processes because SO42− competed with CA for ZVI adsorptive sites. The rate of CA degradation was significantly accelerated by ZVI-associated Fe(II). Nine byproducts of CA transformation were identified by high-resolution mass spectrometry. The formation and subsequent degradation of these products revealed that the transformation of CA by ZVI occurred via a step-wise reductive dechlorination pathway. Overall, this study suggests that ZVI may be effective at remediating CA-contaminated sites.

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.

Geochemical Parameters and Reductive Dechlorination Determine Aerobic Cometabolic vs Aerobic Metabolic Vinyl Chloride Biodegradation at Oxic/Anoxic Interface of Hyporheic Zones.

Hyporheic zones mediate vinyl chloride (VC) biodegradation in groundwater discharging into surface waters. At the oxic/anoxic interface (OAI) of hyporheic zones subjected to redox oscillations, VC is degraded via coexisting aerobic ethenotrophic and anaerobic reductive dechlorination pathways. However, the identity of aerobic VC degradation pathways (cometabolic vs metabolic) and their interactions with reductive dechlorination in relation to riverbed sediment geochemistry remain ill-defined. We addressed this using microcosms containing OAI sediments incubated under fluctuating oxic/anoxic atmosphere. Under oxic atmosphere, aerobic metabolic VC oxidation was absent in sediments with high total organic carbon (TOC) and VC was reductively dechlorinated to ethene. Ethene was oxidized by ethenotrophs that can degrade VC cometabolically. Contrastingly, VC was metabolically oxidized by ethenotrophs in low-TOC sediments with low reductive dechlorination potential. Accordingly, enrichment and isolation of metabolic VC-oxidizing ethenotrophs was successful only from the low-TOC sediment. Sequence analysis of etnE genes from the microcosms as well phylogenetic typing of the isolates showed that ethenotrophs in the sediments were facultative anaerobic Proteobacteria capable of coping with OAI-associated redox fluctuations. Our results suggest that local sediment heterogeneity supports/selects divergent VC degradation processes at the OAI and that high reductive dechlorination potential suppresses development of aerobic metabolic VC oxidation potential.

Simultaneous degradation of toxic refractory organic pesticide and bioelectricity generation using a soil microbial fuel cell

In this study, the soil microbial fuel cells (MFCs) were constructed in the topsoil contaminated with toxic refractory organic pesticide, hexachlorobenzene (HCB). The performance of electricity generation and HCB degradation in the soil-MFCs were investigated. The HCB degradation pathway was analyzed based on the determination of degradation products and intermediates. Experimental results showed that the HCB removal efficiencies in the three groups (soil MFCs group, open circuit control group and no adding anaerobic sludge blank group) were 71.15%, 52.49% and 38.92%, respectively. The highest detected power density was 77.5mW/m2 at the external resistance of 1000Ω. HCB was degraded via the reductive dechlorination pathway in the soil MFC under anaerobic condition. The existence of the anode promoted electrogenic bacteria to provide more electrons to increase the metabolic reactions rates of anaerobic bacteria was the main way which could promote the removal efficiencies of HCB in soil MFC.

Dechlorination of PCBs in the rhizosphere of switchgrass and poplar

Polychlorinated biphenyl (PCB) congeners (PCB 52, 77, and 153) singly and in mixture were spiked and aged in soil microcosms and subsequently planted with switchgrass (Panicum virgatum) or poplar (Populus deltoids x nigra DN34). The planted reactors showed significantly greater reductions in PCB parent compounds when compared to unplanted systems after 32 weeks. There was evidence of reductive dechlorination in both planted and unplanted systems, but the planted microcosms with fully developed roots and rhizospheres showed greater biotransformation than the unplanted reactors. These dechlorination products accounted for approximately all of the molar mass of parent compound lost. Based on the transformation products, reductive dechlorination pathways are proposed for rhizospheric biotransformation of PCB 52, 77, and 153. This is the first report of rhizosphere biotransformation pathways for reductive dechlorination in marginally aerobic, intermittently flooded soil as evidenced by a mass balance on transformation products.

Degradation of carbon tetrachloride in modified Fenton reaction

We showed that the dechlorination of carbon tetrachloride (CT) can be significantly enhanced at nearneutral pH by modified Fenton reaction in the presence of Fe(II) chelated by cross-linked chitosan (CS) with glutaraldehyde (GLA). CT dechlorination was verified by monitoring the release of chloride and detection of intermediates such as trichloromethane and dichloromethane in the modified Fenton system with Fe(II) chelated by cross-linked CS with GLA (Fe(II)-CS/GLA). Measured chlorine mass balance of each sample was greater than 91% of total chlorine mass corresponding to initial CT concentration throughout the reaction. Addition of hydroxyl radical scavenger (2-propanol) enhanced the CT degradation in 5 h at near-neutral pH (removal efficiency from 57.2% to 92.4%), while the addition significantly inhibited trichloroethylene (TCE) degradation at the same condition (74.7% to 19.9%). This implies that, in contrast to the dechlorination of TCE, that of CT did not follow an oxidative dechlorination pathway but a reductive dechlorination pathway in the modified Fenton system with Fe(II)-CS/GLA. Dechlorination kinetics of CT in the modified Fenton system was affected by the concentrations of H2O2, Fe(II), and CT. The formation of surface Fe(II)-CS/GLA complex and its valence change from Fe(II) to Fe(III) observed during the modified Fenton reaction gave a clue to identify the proposed reaction mechanism properly.

Dechlorination pathways of diverse chlorinated aromatic pollutants conducted by Dehalococcoides sp. strain CBDB1

Dechlorination of chlorinated aromatic pollutants (CAPs) has become a major issue in recent decades. This paper reported a theoretical indicator for predicting the reductive dechlorination pathways of polychlorinated dibenzo- p-dioxins (PCDDs), chlorobenzenes and chlorophenols transformed by Dehalococcoides sp. strain CBDB1. Density functional theory (DFT) calculations were carried out at the B3LYP/6-31G(d) level for all related CAPs and Mulliken atomic charges on chlorine atoms ( Q Cl( n) ) were adopted as the probe of the dechlorination reaction activity. Q Cl( n) can consistently indicate the main dechlorination daughter products of PCDDs, chlorobenzenes and chlorophenols conducted by strain CBDB1. The dechlorination reaction favors elimination of the chlorine atoms having greater Q Cl( n) values. The chlorine atom with the greatest Q Cl( n) value tends preferentially to be eliminated, whereas the chlorine atom with the smallest Q Cl( n ) value tends unlikely to be eliminated or does not react at all. For a series of compounds having similar structure, the maximal Q Cl( n) of each molecular can be used to predict the possibility of its daughter product(s). In addition, the difference (∆ Q Cl( n) ) between the maximal Q Cl( n) and the next maximal Q Cl( n) of the same molecule can be used to assess the possibility of formation of multiple dechlorination products.

Rules of thumb for assessing reductive dechlorination pathways of PCDDs in specific systems

This paper reports a theoretical validation and proposition of the reductive dechlorination pathways for polychlorinated dibenzo- p-dioxin (PCDD) congeners. Density functional theory (DFT) calculations were carried out at the B3LYP/6-31G(d) level for all PCDDs and Mulliken atomic charges on chlorine atoms were adopted as the probe of the dechlorination reaction activity. The experimentally substantiated dechlorination pathways of 1,2,3,4-tetrachlorodibenzo- p-dioxin (1,2,3,4-TCDD) and its daughter products in the presence of zero-valent zinc were validated and the complete pathway of dechlorination of octachlorodibenzo- p-dioxin (OCDD) was proposed. The proposed pathways were found to be consistent with anaerobic biotransformation of several PCDD congeners. Four rules of thumb arrived from this study include (1) the chlorine atoms in the longitudinal (1,4,6,9) positions are removed in preference to the chlorine atoms on lateral (2,3,7,8) positions; (2) the chlorine atom that has more neighboring chlorine atoms at ortho-, meta- and para-positions is to be eliminated; (3) reductive dechlorination prefers to take place on the benzene ring having more chlorine substitutions; and (4) a chlorine atom on the side of the longitudinal symmetry axis containing more chlorine atoms is preferentially eliminated. These rules of thumb can be conveniently used for rapidly predicting the major dechlorination pathway for a given PCDD in specific systems.

Theoretical investigation of the sequential reductive dechlorination pathways of chlorobenzenes and chloroanilines

The distribution of product isomers during the sequential reductive dechlorination of pentachloroaniline (PCA) and pentachlorobenzene (PeCB) was examined based on calculated thermodynamic, chromatographic, and electronic properties and then compared to the product distribution achieved by enrichment cultures. The dechlorination pathway analysis based on free energy considerations matched 78% and 67% of the experimental results for the sequential reductive dechlorination of chlorobenzenes (CBs) and chloroanilines (CAs), respectively. Chromatographic properties of CBs and CAs were able to explain some but not all of the reactions in the observed dechlorination pathways. Correlations between the observed dechlorination pattern and electronic properties of the parent compounds were able to explain most of the formation of the observed products. Experimentally observed sequential reductive dechlorination of CBs and CAs were similar to predicted dechlorination pathways based on the charge differential values calculated for the carbon-chloride bonds. Chlorine atoms were removed from the carbon atom that has the highest charge differential or the second highest charge differential. However, although thermodynamic, electronic as well as chromatographic properties of the CBs and CAs are certainly important factors, they may not be sufficient to completely describe the sequential microbial reductive dechlorination. Enzymatic specificity, as well as other factors (i.e., culture acclimation, environmental factors) should be considered for the interpretation of observed sequential reductive dehalogenation pathways of haloorganic compounds. This work provides the most comprehensive analysis to date of theoretical factors that control the sequential reductive chlorination of two homologous series of single-ring chloroaromatic species.

Electrically induced reduction of trichloroethene in clay

Chlorinated compounds such as trichloroethene (TCE) are recalcitrant contaminants commonly detected in soil and groundwater. Contemporary remedies such as electron donor amendment tend to be less or ineffective in treating chlorinated compounds in matrix of lower permeability, such as clay. In this study, electrically induced reduction (EIR) was tested by inserting electrodes in saturated clay containing 122.49-125.43 mg TCE kg(-1). Weak electric potentials (E) of 6, 9, and 12 V m(-1) were applied, and up to 97% of TCE were depleted during the study period. Corresponding increases in chloride concentrations was observed during TCE depletion, indicating a reductive dechlorination pathway. No migration of TCE was observed between the two electrodes, neither were intermediate compounds such as dichloroethene (DCE) or vinyl chloride (VC). Results were also tested against a mathematical equation we previously established for field applications. Electrically induced reduction may offer a novel method for in situ degradation of chlorinated contaminants, especially in low-permeable media such as clay.

Electrolytic Methanogenic−Methanotrophic Coupling for Tetrachloroethylene Bioremediation: Proof of Concept

Coupling of methanogenic and methanotrophic catabolisms was performed in a single-stage technology equipped with a water electrolysis cell placed in the effluent recirculation loop. The electrolysis-generated hydrogen served as an electron donor for both bicarbonate reduction into CH4 and reductive dechlorination, while the O2 and CH4, supported the cometabolic oxidation of chlorinated intermediates left over by the tetrachloroethylene (PCE) transformation. The electrolytical methanogenic/methanotrophic coupled (eMaMoC) process was tested in a laboratory-scale setup at PCE loads ranging from 5 to 50 micromol/L(rx) x d (inlet concentrations from 4 to 11 mg/L), and at various hydraulic residence times (HRT). Degradation followed essentially a reductive dechlorination pathway from PCE to cis-1,2-dichloroethene (DCE), and an oxidative pathway from DCE to CO2. PCE reductive dechlorination to DCE was consistently over 98% while a maximum oxidative DCE mineralization of 89% was obtained at a load of 4.3 micromol PCE/ L(rx) x d and an HRT of 6 days. Controlling dissolved oxygen concentrations within a relatively low range (2-3 mg/L) seemed instrumental to sustain the overall degradation capacity. Degradation kinetics were further evaluated: the apparent half-saturation constant (K(s)) had to be set relatively high (29 microM) for the simulated data to best fit the experimental ones. In spite of such kinetic limitations, the eMaMoC system, while fueled by water electrolysis, was effective in building and sustaining a functional methanogenic/methanotrophic consortium capable of significant PCE mineralization in a single-stage process. Hence, degradation standards are within reach so long as the methanotrophic DCE-oxidizing potential, including substrate affinity, are optimized and HRT accordingly adjusted.

Reductive Dechlorination Pathways of Tetrachloroethylene and Trichloroethylene and Subsequent Transformation of Their Dechlorination Products by Mackinawite (FeS) in the Presence of Metals

Because of frequent co-occurrence of metals with chlorinated organic pollutants, Fe(II), Co(II), Ni(II), and Hg(II) were evaluated for their impact on the dechlorination pathways of PCE and TCE and the subsequent transformation of the initial dechlorination products by FeS. PCE transforms to acetylene via beta-elimination, TCE via hydrogenolysis, and 1,1-DCE via alpha-elimination, while TCE transforms to acetylene via beta-elimination and cis-DCE and 1,1-DCE via hydrogenolysis. Acetylene subsequently transforms in FeS batches, but little transformation of cis-DCE and 1,1-DCE was observed. Branching ratio calculations indicate that the added metals decrease the reductive transformation of PCE and TCE via beta-elimination relative to hydrogenolysis, resulting in a higher production of the toxic DCE byproducts. Nonetheless, acetylene is generally the dominant product. Production of highly water-soluble compound(s) is suspected as a significant source for incomplete mass recoveries. In the transformation of PCE and TCE, the formation of unidentified product(s) is most significant in Co(II)-added FeS batches. Although nearly complete mass recoveries were observed in the other FeS batches, the subsequent transformation of acetylene would lead to the formation of unidentified product(s) over long time periods.

Anaerobic treatment of 2,4,6-trichlorophenol in an expanded granular sludge bed-anaerobic filter (EGSB-AF) bioreactor at 15 °C

Expanded granular sludge bed-anaerobic filter (EGSB-AF) bioreactors were operated at 15 °C for the treatment of 2,4,6-trichlorophenol (TCP)-containing volatile fatty acid (VFA)-based wastewaters. The seed sludge used as inoculum for the control (no TCP) and test reactor was unexposed to chlorophenols (CPs) prior to the 425-day trial. TCP supplementation to the feed at 50 mg TCP l −1 partially inhibited the anaerobic degradation of the VFA feed measured as COD removal efficiency. However, the withdrawal and subsequent application of stepwise increments to the TCP loading resulted in steady COD removal. Terminal restriction fragment length polymorphism analysis showed Methanosaeta-like Archaea in the control reactor over the experimental period. Different methanogenic populations were detected in the test reactor and responded to the changes in feed composition. Bacterial community analyses indicated changes in the community structure over time and suggested the presence of Campylobacter-like, Acidimicrobium-like and Heliophilum-like organisms in the samples. TCP mineralisation was by a reductive dechlorination pathway through 2,4-dichlorophenol (DCP) and 4-chlorophenol (4-CP) or 2-chlorophenol (2-CP). CP degradation rates in sludge granules from the lower chamber of the hybrid EGSB-AF reactor was in the order TCP > DCP > 4-CP > 2-CP. However, a biodegradability order of lower CPs > TCP was observed in fixed-film biomass taken from the upper reactor chamber, thus reflecting the role of this reactor section in the metabolism of residual lower CPs from the lower sludge-bed stage of operation.

Hydrogen thresholds as indicators of dehalorespiration in constructed treatment wetlands.

Anaerobic degradation of cis-1,2-dichloroethene (cis-1,2-DCE) and 1,2-dichloroethane (1,2-DCA) was studied in microcosms derived from a laboratory-scale upflow treatment wetland system used to biodegrade chlorinated compounds present in groundwater from a Superfund site. Dechlorination kinetics of cis-1,2-DCE (0.94-1.57 d(-1)) and 1,2-DCA (0.15-0.71 d(-1)) were rapid, and degradation proceeded to completion with ethene or ethane as terminal dechlorination products. Hydrogen concentrations, measured simultaneously during dechlorination, were significantly different for the two compounds, approximately 2.5 nM for cis-1,2-DCE and 38 nM for 1,2-DCA. Methanogenesis proceeded during the degradation of 1,2-DCA when H2 concentrations were high but not during the dechlorination of cis-1,2-DCE when H2 concentrations were below published thresholds for methanogenesis. A 16S rRNA gene-based approach indicates that microorganisms closely related to Dehalococcoides ethenogenes were present and that they were distributed throughout the bottom, middle, and top of the upflow treatment wetland system. These results coupled with consideration of hydrogen thresholds, degradation kinetics, daughter products, and measurements of methanogenesis strongly suggest that halorespirers were responsible for dechlorination of cis-1,2-DCE and that 1,2-DCA dechlorination was co-metabolic, likely mediated by acetogens or methanogens. Rapid dechlorination potential was distributed throughout the wetland bed, both within and below the rhizosphere, indicating that reductive dechlorination pathways can be active in anaerobic environments located in close spatial proximity to aerobic environments and plants in treatment wetland systems.

Kinetics of tetrachloroethylene-reductive dechlorination catalyzed by vitamin B12

Reductive dechlorination kinetics of tetrachloroethylene (PCE) to ethylene catalyzed by vitamin B12 using Ti[III] citrate as the bulk reductant was examined in a vapor/water batch system. A kinetic model incorporating substrate–B12 electron-transfer complex formation and subsequent product release was developed. The model also accounted for the primary reductive dechlorination pathways (hydrogenolysis and reductive β elimination) and vapor/water-phase partitioning. Reaction rate constants were sequentially determined by fitting the model to experimental kinetic data while moving upward through consecutive reaction pathways. The release of product from the complex was found to be second order with respect to substrate concentration for both PCE and acetylene; all other substrates appeared to release by first order. Reductive β elimination was found to be a significant reaction pathway for trichloroethylene (TCE), and chloroacetylene was observed as a reactive intermediate. Acetylene production appears to be primarily due to the reduction of chloroacetylene derived from TCE. The reduction of cis-dichloroethylene (cis-DCE), the primary DCE isomer formed, was extremely slow, leading to a significant buildup of cis-DCE. The kinetics of acetylene and vinyl chloride reduction appeared to be limited by the formation of relatively stable substrate–B12 complexes. The relatively simple model examined appears to adequately represent the main features of the experimental data.