Aerobic biodegradation of cis-1,2-dichloroethene as sole carbon source: Stable carbon isotope fractionation and growth characteristics

The optimization of poly(vinyl)-alcohol-alginate beads with a slow-release compound for the aerobic cometabolism of chlorinated aliphatic hydrocarbons

Chlorinated aliphatic hydrocarbons (CAHs), such as cis-1,2-dichloroethylene (cDCE), are prevalent in groundwater at many locations throughout the United States. When immobilized in hydrogel beads with slow-release compounds, the bacteria strain Rhodococcus rhodochrous ATCC 21198 can be used for the in situ bioremediation of cDCE. These hydrogel beads must exhibit high mechanical strength and resist degradation to extend the lifetime of slow-release compounds and bioremediation. We engineered poly(vinyl)-alcohol - alginate (PVA-AG) beads to immobilize ATCC 21198 with the slow-release compound, tetrabutoxysilane (TBOS) that produces 1-butanol as a growth substrate, for high mechanical strength. We optimized three inputs (concentration of PVA, concentration of AG, and the crosslinking time) on two responses (compressive modulus and rate of oxygen utilization) for batch incubation experiments between 1 and 30 days using a design of experiments approach. The predictive models generated from design of experiments were then tested by measuring the compressive strength, oxygen utilization, and abiotic rates of hydrolysis for a predicted optimal bead formulation. The result of this study generated a hydrogel bead with immobilized R. rhodochrous ATCC 21198 and TBOS that exhibited a high compressive modulus on day 1 and day 30, which was accurately predicted by models. These hydrogel beads exhibited low metabolic activity based on oxygen rates on day 1 and day 30 but were not accurately predicted by the models. In addition, the ratio between oxygen utilization and abiotic rates of hydrolysis were observed to be roughly half of what was expected stoichiometrically. Lastly, we demonstrated the capability to use these beads as a bioremediation technology for cDCE as we found that, for all bead formulations, cDCE was significantly reduced after 30 days. Altogether, this work demonstrates the capability to capture and enhance the material properties of the complex hydrogel beads with predictive models yet signals the need for more robust methods to understand the metabolic activity that occurs in the hydrogel beads.

Aerobic co-metabolic cis-Dichloroethene degradation with Trichloroethene as primary substrate and effects of concentration ratios

Pollution with chloroethenes threaten groundwater resources worldwide. Cis-Dichloroethene (cDCE) and Trichloroethene (TCE) are widespread pollutants that often occur together at contaminated sites, either as primary discharges or as degradation products of anaerobic dechlorination. In this study, comprehensive microcosm experiments were conducted with groundwater samples of seven sites contaminated with chloroethenes. In total, twelve wells with different pollutant concentrations and chloroethene compositions were sampled, and aerobic microcosms including sterile controls were set up. The results revealed interactions as well as interferences between cDCE and TCE. First, co-metabolic cDCE degradation with TCE as growth substrate was detected for the first time in this work. Transformation yields Ty' (molar ratio of co-substrate degraded to primary substrate degraded) of the degradation process were determined and showed a linear relationship with the cDCE/TCE concentration ratio. At low cDCE/TCE ratio, aerobic metabolic TCE degradation can result in complete cDCE removal due to co-metabolic degradation. Secondly, interfering effects were detected at notable cDCE levels resulting in deceleration of TCE degradation and residual concentrations which were also correlating linearly with the cDCE/TCE concentration ratio. These findings are significant for investigating chloroethene contaminated sites and planning remediation strategies. In particular, the efficiency biological remediation methods in the presence of both pollutants can be evaluated more precisely through the knowledge of interactions and interferences. Our study emphasizes that co-contaminants and possible effects of contaminant mixtures on the degradation rates of individual substances should be considered in general.

Mobilization pilot test of PCE sources in the transition zone to aquitards by combining mZVI and biostimulation with lactic acid

The potential toxic and carcinogenic effects of chlorinated solvents in groundwater on human health and aquatic ecosystems require very effective remediation strategies of contaminated groundwater to achieve the low legal cleanup targets required. The transition zones between aquifers and bottom aquitards occur mainly in prograding alluvial fan geological contexts. Hence, they are very frequent from a hydrogeological point of view. The transition zone consists of numerous thin layers of fine to coarse-grained clastic fragments (e.g., medium sands and gravels), which alternate with fine-grained materials (clays and silts). When the transition zones are affected by DNAPL spills, free-phase pools accumulate on the less conductive layers. Owing to the low overall conductivity of this zone, the pools are very recalcitrant. Little field research has been done on transition zone remediation techniques. Injection of iron microparticles has the disadvantage of the limited accessibility of this reagent to reach the entire source of contamination. Biostimulation of indigenous microorganisms in the medium has the disadvantage that few of the microorganisms are capable of complete biodegradation to total mineralization of the parent contaminant and metabolites. A field pilot test was conducted at a site where a transition zone existed in which DNAPL pools of PCE had accumulated. In particular, the interface with the bottom aquitard was where PCE concentrations were the highest. In this pilot test, a combined strategy using ZVI in microparticles and biostimulation with lactate in the form of lactic acid was conducted. Throughout the test it was found that the interdependence of the coupled biotic and abiotic processes generated synergies between these processes. This resulted in a greater degradation of the PCE and its transformation products. With the combination of the two techniques, the mobilization of the contaminant source of PCE was extremely effective.

Intrinsic and bioaugmented aerobic trichloroethene degradation at seven sites

Trichloroethene (TCE) is one of the most prevalent contaminants in groundwater pollution worldwide. Aerobic-metabolic degradation of TCE has only recently been discovered at one field site. It has significant advantages over aerobic co-metabolism because no auxiliary substrates are required, and the oxygen demand is considerably lower. This study investigated the intrinsic degradation potential as well as the stimulation potential by bioaugmentation in microcosm experiments with groundwater from seven different sites contaminated with chloroethenes. An enrichment culture metabolizing TCE aerobically served as inoculum. The groundwater samples were inoculated with liquid culture in mineral salts medium as well as with immobilized culture on silica sand. Additionally, some samples were inoculated with groundwater from the site where the enrichment culture originated. The microcosms without inoculum proved the occurrence of aerobic TCE-metabolizing bacteria stimulated by the supply of oxygen in 54% of the groundwater samples. TCE degradation started in most cases after adaptation times of up to 92 d. The doubling time of 24 d indicated comparatively slow growth of the aerobic TCE degrading microorganisms. Bioaugmentation triggered or accelerated TCE-degradation in all microcosms with chlorothene concentrations below 100 mg L^-1. All inoculation strategies (liquid and immobilized enrichment culture or addition of groundwater from the active field site) were successful. Our study demonstrates that aerobic-metabolic TCE degradation can occur and be stimulated across a broad hydrogeologic spectrum and should be considered as a viable option for groundwater remediation at TCE-contaminated sites.

Natural Biodegradation of Vinyl Chloride and cis-Dichloroethene in Aerobic and Suboxic Conditions

Chlorinated ethene (CE) groundwater contamination is commonly treated through anaerobic biodegradation (i.e., reductive dechlorination) either as part of an engineered system or through natural attenuation. Aerobic biodegradation has also been recognized as a potentially significant pathway for the removal of the lower CEs cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). However, the role of aerobic biodegradation under low oxygen conditions typical of contaminated groundwater is unclear. Bacteria capable of aerobic VC biodegradation appear to be common in the environment, while aerobic biodegradation of cDCE is less common and little is known regarding the organisms responsible. In this study, we investigate the role of aerobic cDCE and VC biodegradation in a mixed contaminant plume (including CEs, BTEX, and ketones) at Naval Air Station North Island, Installation Restoration Site 9. Sediment and groundwater collected from the plume source area, mid-plume, and shoreline were used to prepare microcosms under fully aerobic (8 mg/L dissolved oxygen (DO)) and suboxic (< 1 mg/L DO) conditions. In the shoreline microcosms, VC and cDCE were rapidly degraded under suboxic conditions (100% and 77% removal in < 62 days). In the suboxic VC microcosms, biodegradation was associated with a > 5 order of magnitude increase in the abundance of functional gene etnE, part of the aerobic VC utilization pathway. VC and cDCE were degraded more slowly under fully aerobic conditions (74% and 30% removal) in 110 days. High-throughput 16S rRNA and etnE sequencing suggest the presence of novel VC- and cDCE-degrading bacteria. These results suggest that natural aerobic biodegradation of cDCE and VC is occurring at the site and provide new evidence that low (< 1 mg/L) DO levels play a significant role in natural attenuation of cDCE and VC.

Direct aerobic oxidation (DAO) of chlorinated aliphatic hydrocarbons: A review of key DAO bacteria, biometabolic pathways and in-situ bioremediation potential

Contamination of aquifers and vadose zones with chlorinated aliphatic hydrocarbons (CAH) is a world-wide issue. Unlike other reactions, direct aerobic oxidation (DAO) of CAHs does not require growth substrates and avoids the generation of toxic by-products. Here, we critically review the current understanding of chlorinated aliphatic hydrocarbons-DAO and its application in bioreactors and at the field scale. According to reports on chlorinated aliphatic hydrocarbons-DAO bacteria, isolates mainly consisted of Methylobacterium and Proteobacterium. Chlorinated aliphatic hydrocarbons-DAO bacteria are characterized by tolerance to a high concentration of CAHs and highly efficient removal of CAHs. Trans-1,2-dichloroethylene (t-DCE) is easily transformed biomass for bacteria, followed by 1,2-dichloroethane (1,2-DCA), dichloromethane (DCM), vinyl chloride (VC) and cis-1,2-dichloroethylene (c-DCE). Significant differences in the maximum specific growth rates were observed with different CAHs and biometabolic pathways for DCM, 1,2-DCA, VC and c-DCE degradation have been successfully parsed. Detection of the functional genes etnC and etnE is useful for the determination of active VC DAO bacteria. Additionally, DAO bacteria have been successfully applied to CAHs in new types of bioreactors with satisfactory results. To the best of the authors' knowledge, only one study on DAO-CAHs was conducted in-situ and resulted in 99% CAH removal. Lastly, we put forward future development prospect of chlorinated aliphatic hydrocarbons-DAO.

Examining aerobic degradation of chloroethenes mixture in consortium composed of Comamonas testosteroni RF2 and Mycobacterium aurum L1

An environmental isolate Comamonas testosteroni RF2 has been previously described to cometabolize trichloroethene (TCE), 1,2-cis-dichloroethene (cDCE), 1,2-trans-dichloroethene (tDCE), and 1,1-dichloroethene (1,1DCE) when grown on phenol and lactate sodium. In this study, three vinyl chloride (VC) degrading strains, Mycobacterium aurum L1, Pseudomonas putida PS, and Rhodococcus ruber Sm-1 were used to form consortia with the strain RF2 in terms to achieve the removal of VC along with above-mentioned chloroethenes. Degradation assays were performed for a binary mixture of cDCE and VC as well as for a mixture of TCE, all DCEs and VC. The consortium composed of C. testosteroni RF2 and M. aurum L1 showed to be the most efficient towards the removal of cDCE (6.01 mg L^-1) in the binary mixture with VC (10 mg L^-1) and was capable of efficiently removing chloroethenes in the mixture sample at the initial concentrations of 116 μg L^-1 for TCE, 662 μg L^-1 for cDCE, 42 μg L^-1 for tDCE, 16 μg L^-1 for 1,1DCE, and 7 mg L^-1 for VC with a removal efficiency of nearly 100% for all of the compounds. Although complete removal of VC took a significantly longer time than the removal of other chloroethenes, the consortium composed of C. testosteroni RF2 and M. aurum L1 displayed strong bioremediation potential for aquifers with downstream contamination characterized by the presence of less chlorinated ethenes.

Vapor Intrusion Investigations and Decision-Making: A Critical Review

At sites impacted by volatile organic compounds (VOCs), vapor intrusion (VI) is the pathway with the greatest potential to result in actual human exposure. Since sites with VI were first widely publicized in late 1990s, the scientific understanding of VI has evolved considerably. The VI conceptual model has been extended beyond relatively simple scenarios to include nuances, such as biological and hydrogeological factors that may limit the potential for VI and alternative pathways, such as preferential pathways and direct building contact/infiltration that may enhance VI in some cases. Regulatory guidance documents typically recommend initial concentration- or distance-based screening to evaluate whether VI may be a concern, followed by a multiple-lines-of-evidence (MLE) investigation approach for sites that do not screen out. These recommendations for detailed evaluation of VI currently focus on monitoring of VOC concentrations in groundwater, soil gas, and indoor air and can be supplemented by other lines of evidence. In this Critical Review, we summarize key elements important to VI site characterization, provide the status and current understanding, and highlight data interpretation challenges, as well as innovative tools developed to help overcome the challenges. Although there have been significant advances in the understanding of VI in the past 20 years, limitations and knowledge gaps in screening, investigation methods, and modeling approaches still exist. Potential areas for further research include improved initial screening methods that account for the site-specific role of barriers, improved understanding of preferential pathways, and systematic study of buildings and infrastructure other than single-family residences.

Aerobic metabolic trichloroethene biodegradation under field-relevant conditions

Chloroethenes belong to the most widely distributed groundwater contaminants. Since 2014, it has been known that trichloroethene (TCE) can be degraded aerobically and metabolically as growth substrate by a mixed bacterial enrichment culture (named SF culture). In this study, the degradation capabilities under a range of field-relevant conditions were investigated in fixed-bed reactors as well as in batch experiments. Aerobic metabolic TCE degradation was stable over the long term, with degradation optima at 22 °C and pH 7. Degradation of up to 400 μM TCE was observed. The longest starvation period after which degradation of TCE was regained was 112 days. The possible co-contaminants perchloroethene, trans-1,2-dichloroethene, and cis-1,2-dichloroethene did not inhibit TCE degradation, even though they were not degraded themselves. The presence of equimolar amounts of 1,1-dichloroethene and vinyl chloride inhibited TCE degradation. Experiments with groundwater from different chloroethene-contaminated field sites proved the potential of the SF culture for bioaugmentation. Thus, aerobic metabolic TCE degradation should be considered as a promising method for the bioremediation of field sites with TCE as the main contaminant.

Natural attenuation of chlorinated ethenes in hyporheic zones: A review of key biogeochemical processes and in-situ transformation potential

Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.

Recent advances in vapor intrusion site investigations

Our understanding of vapor intrusion has evolved rapidly since the discovery of the first high profile vapor intrusion sites in the late 1990s and early 2000s. Research efforts and field investigations have improved our understanding of vapor intrusion processes including the role of preferential pathways and natural barriers to vapor intrusion. This review paper addresses recent developments in the regulatory framework and conceptual model for vapor intrusion. In addition, a number of innovative investigation methods are discussed.

Isotopic effects of PCE induced by organohalide-respiring bacteria

Reductive dechlorination performed by organohalide-respiring bacteria (OHRB) enables the complete detoxification of certain emerging groundwater pollutants such as perchloroethene (PCE). Environmental samples from a contaminated site incubated in a lab-scale microcosm (MC) study enable documentation of such reductive dechlorination processes. As compound-specific isotope analysis is used to monitor PCE degradation processes, nucleic acid analysis-like 16S-rDNA analysis-can be used to determine the key OHRB that are present. This study applied both methods to laboratory MCs prepared from environmental samples to investigate OHRB-specific isotope enrichment at PCE dechlorination. This method linkage can enhance the understanding of isotope enrichment patterns of distinct OHRB, which further contribute to more accurate evaluation, characterisation and prospection of natural attenuation processes. Results identified three known OHRB genera (Dehalogenimonas, Desulfuromonas, Geobacter) in diverse abundance within MCs. One species of Dehalogenimonas was potentially involved in complete reductive dechlorination of PCE to ethene. Furthermore, the isotopic effects of PCE degradation were clustered and two isotope enrichment factors (ε) (- 11.6‰, - 1.7‰) were obtained. Notably, ε values were independent of degradation rates and kinetics, but did reflect the genera of the dechlorinating OHRB.

Secondary compound hypothesis revisited: Selected plant secondary metabolites promote bacterial degradation of cis-1,2-dichloroethylene (cDCE)

Cis-1,2-dichloroethylene (cDCE), which is a common hazardous compound, often accumulates during incomplete reductive dechlorination of higher chlorinated ethenes (CEs) at contaminated sites. Simple monoaromatics, such as toluene and phenol, have been proven to induce biotransformation of cDCE in microbial communities incapable of cDCE degradation in the absence of other carbon sources. The goal of this microcosm-based laboratory study was to discover non-toxic natural monoaromatic secondary plant metabolites (SPMEs) that could enhance cDCE degradation in a similar manner to toluene and phenol. Eight SPMEs were selected on the basis of their monoaromatic molecular structure and widespread occurrence in nature. The suitability of the SPMEs chosen to support bacterial growth and to promote cDCE degradation was evaluated in aerobic microbial cultures enriched from cDCE-contaminated soil in the presence of each SPME tested and cDCE. Significant cDCE depletions were achieved in cultures enriched on acetophenone, phenethyl alcohol, p-hydroxybenzoic acid and trans-cinnamic acid. 16S rRNA gene sequence analysis of each microbial community revealed ubiquitous enrichment of bacteria affiliated with the genera Cupriavidus, Rhodococcus, Burkholderia, Acinetobacter and Pseudomonas. Our results provide further confirmation of the previously stated secondary compound hypothesis that plant metabolites released into the rhizosphere can trigger biodegradation of environmental pollutants, including cDCE.

Bioelectrochemical approach for reductive and oxidative dechlorination of chlorinated aliphatic hydrocarbons (CAHs)

A sequential reductive-oxidative treatment was developed in this study in a continuous-flow bioelectrochemical reactor to address bioremediation of groundwater contaminated by trichloroethene (TCE) and less-chlorinated but still harmful intermediates, such as vinyl chloride. In order to optimize the anodic compartment, whereby the oxygen-driven microbial oxidation of TCE-daughter products occurs, abiotic batch experiments were performed with various anode materials poised at +1.20 V vs. SHE (i.e., graphite rods and titanium mesh anode coated with mixed metal oxides (MMO)) and setups (i.e., electrodes embedded within a bed of silica beads or graphite granule). The MMO anode displayed higher efficiency (>90%) for oxygen generation compared to the graphite electrodes. Additionally, the graphite bed presence adversely affects oxygen generation, likely due to the oxygen scavenging. This effect was completely eliminated by replacing the graphite granules with silica beads. The anodic setups were thereafter verified in a mentioned reactor at an applied TCE loading rate of approximately 20 μM d^-1 and a hydraulic retention time of 1.4 d in each compartment. The cathode consisted of a bed of graphite granules and was potentiostatically controlled at -0.65 V vs. SHE. The best reactor performance in terms of removal efficiency (i.e., >97%), removal rate (i.e., 121.8 ± 2.7 μeq L^-1 d^-1), and the residual concentration (i.e., 5.03 ± 0.63 μeq L^-1) of chlorinated contaminants was achieved with the MMO anode placed in a silica bed. Ecotoxicity tests performed with algae confirmed these results by showing progressive toxicity reduction from inlet to cathodic and anodic effluent using this reactor configuration.

Since 2015 the SinoGerman research project SIGN supports water quality improvement in the Taihu region, China

The Taihu (Tai lake) region is one of the most economically prospering areas of China. Due to its location within this district of high anthropogenic activities, Taihu represents a drastic example of water pollution with nutrients (nitrogen, phosphate), organic contaminants and heavy metals. High nutrient levels combined with very shallow water create large eutrophication problems, threatening the drinking water supply of the surrounding cities. Within the international research project SIGN (SinoGerman Water Supply Network, www.water-sign.de), funded by the German Federal Ministry of Education and Research (BMBF), a powerful consortium of fifteen German partners is working on the overall aim of assuring good water quality from the source to the tap by taking the whole water cycle into account: The diverse research topics range from future proof strategies for urban catchment, innovative monitoring and early warning approaches for lake and drinking water, control and use of biological degradation processes, efficient water treatment technologies, adapted water distribution up to promoting sector policy by good governance. The implementation in China is warranted, since the leading Chinese research institutes as well as the most important local stakeholders, e.g. water suppliers, are involved.

C, Cl and H compound-specific isotope analysis to assess natural versus Fe(0) barrier-induced degradation of chlorinated ethenes at a contaminated site

Compound-specific isotopic analysis of multiple elements (C, Cl, H) was tested to better assess the effect of a zero-valent iron-permeable reactive barrier (ZVI-PRB) installation at a site contaminated with tetrachloroethene (PCE) and trichloroethene (TCE). The focus was on (1) using (13)C to evaluate natural chlorinated ethene biodegradation and the ZVI-PRB efficiency; (2) using dual element (13)C-(37)Cl isotopic analysis to distinguish biotic from abiotic degradation of cis-dichloroethene (cis-DCE); and (3) using (13)C-(37)Cl-(2)H isotopic analysis of cis-DCE and TCE to elucidate different contaminant sources. Both biodegradation and degradation by ZVI-PRB were indicated by the metabolites that were detected and the (13)C data, with a quantitative estimate of the ZVI-PRB efficiency of less than 10% for PCE. Dual element (13)C-(37)Cl isotopic plots confirmed that biodegradation was the main process at the site including the ZVI-PRB area. Based on the carbon isotope data, approximately 45% and 71% of PCE and TCE, respectively, were estimated to be removed by biodegradation. (2)H combined with (13)C and (37)Cl seems to have identified two discrete sources contributing to the contaminant plume, indicating the potential of δ(2)H to discriminate whether a compound is of industrial origin, or whether a compound is formed as a daughter product during degradation.

Carbon isotopic fractionation during biodegradation of phthalate esters in anoxic condition

Here we evaluate the quantitative relationship between carbon isotopic fractionation and anoxic biodegradation of phthalate esters (PAEs), a kind of endocrine disruptors. The stable carbon isotope delta values (δ(13)C) of 4 PAEs, i.e. di-methyl phthalate (DMP), di-ethyl phthalate (DEP), di-n-butyl phthalate (DBP), and di-iso-butyl phthalate (DiBP), were analyzed during biodegradation by a pure bacteria strain isolated from the shallow aquifer sediment in anoxic condition. Results showed that the carbon isotopic fractionation in the initial degradation of PAEs was well-described by the Rayleigh equation model with R(2) from 0.8885 to 0.9821. The carbon isotopic fractionation (ε) for DMP and DEP were -4.6±0.4‰ and -2.9±0.1‰, respectively, while DBP and DiBP showed limited isotopic fractionation. A linear relationship between ε values and the total carbon atoms present in straight-carbon-chain PAE molecules with R(2) of 0.9918. The apparent kinetic isotope effects (AKIEs) were calculated for proposed 4 initial transformation pathways of PAEs. The high carbon AKIEs of 1.048 and 1.036 were obtained for single enzymatic hydrolysis of DMP and DEP, respectively, and fell in the expected KIE range of 1.03-1.09. However, the intrinsic carbon isotope effects for enzymatic hydrolysis of DBP and DiBP might be masked.

Biodegradation of chloro- and bromobenzoic acids: effect of milieu conditions and microbial community analysis

Monohalogenated benzoic acids often appear in industrial wastewaters where biodegradation can be hampered by complex mixtures of pollutants and prevailing extreme milieu conditions. In this study, the biodegradation of chlorinated and brominated benzoic acids was conducted at a pH range of 5.0-9.0, at elevated salt concentrations and with pollutant mixtures including fluorinated and iodinated compounds. In mixtures of the isomers, the degradation order was primarily 4-substituted followed by 3-substituted and then 2-substituted halogenated benzoic acids. If the pH and salt concentration were altered simultaneously, long adaptation periods were required. Community analyses were conducted in liquid batch cultures and after immobilization on sand columns. The Alphaproteobacteria represented an important fraction in all of the enrichment cultures. On the genus level, Afipia sp. was detected most frequently. In particular, Bacteroidetes were detected in high numbers with chlorinated benzoic acids.

In situ aerobic cometabolism of chlorinated solvents: a review

The possible approaches for in situ aerobic cometabolism of aquifers and vadose zones contaminated by chlorinated solvents are critically evaluated. Bioaugmentation of resting-cells previously grown in a fermenter and in-well addition of oxygen and growth substrate appear to be the most promising approaches for aquifer bioremediation. Other solutions involving the sparging of air lead to satisfactory pollutant removals, but must be integrated by the extraction and subsequent treatment of vapors to avoid the dispersion of volatile chlorinated solvents in the atmosphere. Cometabolic bioventing is the only possible approach for the aerobic cometabolic bioremediation of the vadose zone. The examined studies indicate that in situ aerobic cometabolism leads to the biodegradation of a wide range of chlorinated solvents within remediation times that vary between 1 and 17 months. Numerous studies include a simulation of the experimental field data. The modeling of the process attained a high reliability, and represents a crucial tool for the elaboration of field data obtained in pilot tests and for the design of the full-scale systems. Further research is needed to attain higher concentrations of chlorinated solvent degrading microbes and more reliable cost estimates. Lastly, a procedure for the design of full-scale in situ aerobic cometabolic bioremediation processes is proposed.

Aerobic biodegradation of trichloroethene without auxiliary substrates

Trichloroethene (TCE) represents a priority pollutant and is among the most frequently detected contaminants in groundwater. The current bioremediation measures have certain drawbacks like e.g. the need for auxiliary substrates. Here, the aerobic biodegradation of TCE as the sole growth substrate is demonstrated. This new process of metabolic TCE degradation was first detected in groundwater samples. TCE degradation was stable in an enriched mixed bacterial culture in mineral salts medium for over five years and repeated transfers of the culture resulting in a 10(10) times dilution of the original groundwater. Aerobic TCE degradation resulted in stoichiometric chloride formation. Stable carbon isotope fractionation was observed providing a reliable analytical tool to assess this new biodegradation process at field sites. The results suggest that aerobic biodegradation of TCE without auxiliary substrate could be considered as an option for natural attenuation or engineered bioremediation of contaminated sites.

Effects of varying growth conditions on stable carbon isotope fractionation of trichloroethene (TCE) by tceA-containing Dehalococcoides mccartyi strains

To quantify in situ bioremediation using compound specific isotope analysis (CSIA), isotope fractionation data obtained from the field is interpreted according to laboratory-derived enrichment factors. Although previous studies that have quantified dynamic isotopic shifts during the reductive dechlorination of trichloroethene (TCE) indicate that fractionation factors can be highly variable from culture-to-culture and site-to-site, the effects of growth condition on the isotope fractionation during reductive dechlorination have not been previously examined. Here, carbon isotope fractionation by Dehalococcoides mccartyi 195 (Dhc195) maintained under a variety of growth conditions was examined. Enrichment factors quantified when Dhc195 was subjected to four suboptimal growth conditions, including decreased temperature (-13.3 ± 0.9‰), trace vitamin B12 availability (-12.7 ± 1.0‰), limited fixed nitrogen (-14.4 ± 0.8‰), and elevated vinyl chloride exposure (-12.5 ± 0.4‰), indicate that the fractionation is similar across a range of tested conditions. The TCE enrichment factors for two syntrophic cocultures, Dhc195 with Desulfovibrio vulgaris Hildenborough (-13.0 ± 2.0‰) and Dhc195 with Syntrophomonas wolfei (-10.4 ± 1.2‰ and -13.3 ± 1.0‰), were also similar to a control experiment. In order to test the stability of enrichment factors in microbial communities, the isotope fractionation was quantified for Dhc-containing groundwater communities before and after two-year enrichment periods under different growth conditions. Although these enrichment factors (-8.9 ± 0.4‰, -6.8 ± 0.8‰, -8.7 ± 1.3‰, -9.4 ± 0.7‰, and -7.2 ± 0.3‰) were predominantly outside the range of values quantified for the isolate and cocultures, all tested enrichment conditions within the communities produced nearly similar fractionations. Enrichment factors were not significantly affected by changes in any of the tested growth conditions for the pure cultures, cocultures or the mixed communities, indicating that despite a variety of temperature, nutrient, and cofactor-limiting conditions, stable carbon isotope fractionations remain consistent for given Dehalococcoides cultures.

Dechlorination and organohalide-respiring bacteria dynamics in sediment samples of the Yangtze Three Gorges Reservoir

Several groups of bacteria such as Dehalococcoides spp., Dehalobacter spp., Desulfomonile spp., Desulfuromonas spp., or Desulfitobacterium spp. are able to dehalogenate chlorinated pollutants such as chloroethenes, chlorobenzenes, or polychlorinated biphenyls under anaerobic conditions. In order to assess the dechlorination potential in Yangtze sediment samples, the presence and activity of the reductively dechlorinating bacteria were studied in anaerobic batch tests. Eighteen sediment samples were taken in the Three Gorges Reservoir catchment area of the Yangtze River, including the tributaries Jialing River, Daning River, and Xiangxi River. Polymerase chain reaction analysis indicated the presence of dechlorinating bacteria in most samples, with varying dechlorinating microbial community compositions at different sampling locations. Subsequently, anaerobic reductive dechlorination of tetrachloroethene (PCE) was tested after the addition of electron donors. Most cultures dechlorinated PCE completely to ethene via cis-dichloroethene (cis-DCE) or trans-dichloroethene. Dehalogenating activity corresponded to increasing numbers of Dehalobacter spp., Desulfomonile spp., Desulfitobacterium spp., or Dehalococcoides spp. If no bacteria of the genus Dehalococcoides spp. were present in the sediment, reductive dechlorination stopped at cis-DCE. Our results demonstrate the presence of viable dechlorinating bacteria in Yangtze samples, indicating their relevance for pollutant turnover.

Cytochrome P450 initiates degradation of cis-dichloroethene by Polaromonas sp. strain JS666

Polaromonas sp. strain JS666 grows on cis-1,2-dichoroethene (cDCE) as the sole carbon and energy source under aerobic conditions, but the degradation mechanism and the enzymes involved are unknown. In this study, we established the complete pathway for cDCE degradation through heterologous gene expression, inhibition studies, enzyme assays, and analysis of intermediates. Several lines of evidence indicate that a cytochrome P450 monooxygenase catalyzes the initial step of cDCE degradation. Both the transient accumulation of dichloroacetaldehyde in cDCE-degrading cultures and dichloroacetaldehyde dehydrogenase activities in cell extracts of JS666 support a pathway for degradation of cDCE through dichloroacetaldehyde. The mechanism minimizes the formation of cDCE epoxide. The molecular phylogeny of the cytochrome P450 gene and the organization of neighboring genes suggest that the cDCE degradation pathway recently evolved in a progenitor capable of degrading 1,2-dichloroethane either by the recruitment of the cytochrome P450 monooxygenase gene from an alkane catabolic pathway or by selection for variants of the P450 in a preexisting 1,2-dichloroethane catabolic pathway. The results presented here add yet another role to the broad array of productive reactions catalyzed by cytochrome P450 enzymes.

Effect of chloroethene concentrations and granular activated carbon on reductive dechlorination rates and growth of Dehalococcoides spp

This study focused on the investigation of (i) the tetrachloroethene (PCE) toxicity threshold of a reductively dechlorinating mixed culture containing Dehalococcoides spp., (ii) the adsorption of PCE on different types of granular activated carbon (GAC), and (iii) the bioavailability and reductive dechlorination in the presence of GAC. The abundance of Dehalococcoides spp. detected by quantitative real-time polymerase chain reaction (qPCR) was found to increase by 2-4 orders of magnitude during degradation of PCE. No degradation occurred at dissolved concentrations beyond 420 μM (70 mg/L). Different adsorption isotherms were determined for thermally and chemically activated carbons. The addition of GAC to biological assays reduced the dissolved PCE concentration below the toxicity threshold. The combination of microbial reductive dechlorination with GAC adsorption proved to be a promising method for remediation of groundwater contaminated by high concentrations of chloroethenes.

Dechlorination of trichloroethene in a continuous-flow bioelectrochemical reactor: effect of cathode potential on rate, selectivity, and electron transfer mechanisms

The exciting discovery that dechlorinating bacteria can use polarized graphite cathodes as direct electron donors in the reductive dechlorination has prompted investigations on the development of novel bioelectrochemical remediation approaches. In this work, we investigated the performance of a bioelectrochemical reactor for the treatment of trichloroethene (TCE). The reactor was continuously operated for about 570 days, at different potentiostatically controlled cathode potentials, ranging from -250 mV to -750 mV vs standard hydrogen electrode. The rate and extent of TCE dechlorination, as well as the competition for the available electrons, were highly dependent on the set cathode potential. When the cathode was controlled at -250 mV, no abiotic hydrogen production occurred and TCE dechlorination (predominantly to cis-DCE and VC), most probably sustained via direct extracellular electron transfer, proceeded at an average rate of 15.5 ± 1.2 μmol e(-)/L d. At this cathode, potential methanogenesis was almost completely suppressed and dechlorination accounted for 94.7 ± 0.1% of the electric current (15.0 ± 0.8 μA) flowing in the system. A higher rate of TCE dechlorination (up to 64 ± 2 μmol e(-)/L d) was achieved at cathode potentials lower than -450 mV, though in the presence of a very active methanogenesis which accounted for over 60% of the electric current. Remarkably, the bioelectrochemical reactor displayed a stable and reproducible performance even without the supply of organic carbon sources with the feed, confirming long-term viability.

Sequential reductive and oxidative biodegradation of chloroethenes stimulated in a coupled bioelectro-process

This article for the first time demonstrates successful application of electrochemical processes to stimulate sequential reductive/oxidative microbial degradation of perchloroethene (PCE) in mineral medium and in contaminated groundwater. In a flow-through column system, hydrogen generation at the cathode supported reductive dechlorination of PCE to cis-dichloroethene (cDCE), vinyl chloride (VC), and ethene (ETH). Electrolytically generated oxygen at the anode allowed subsequent oxidative degradation of the lower chlorinated metabolites. Aerobic cometabolic degradation of cDCE proved to be the bottleneck for complete metabolite elimination. Total removal of chloroethenes was demonstrated for a PCE load of approximately 1.5 μmol/d. In mineral medium, long-term operation with stainless steel electrodes was demonstrated for more than 300 days. In contaminated groundwater, corrosion of the stainless steel anode occurred, whereas DSA (dimensionally stable anodes) proved to be stable. Precipitation of calcareous deposits was observed at the cathode, resulting in a higher voltage demand and reduced dechlorination activity. With DSA and groundwater from a contaminated site, complete degradation of chloroethenes in groundwater was obtained for two months thus demonstrating the feasibility of the sequential bioelectro-approach for field application.

Impacts of microbial community composition on isotope fractionation during reductive dechlorination of tetrachloroethylene

Isotope fractionation has been used with increasing frequency as a tool to quantify degradation of chlorinated aliphatic pollutants in the environment. The objective of this research was to determine if the electron donor present in enrichment cultures prepared from uncontaminated sediments influenced the extent of isotope fractionation of tetrachloroethylene (PCE), either directly, or through its influence on microbial community composition. Two PCE-degrading enrichment cultures were prepared from Duck Pond (DP) sediment and were incubated with formate (DPF) or H(2) (DPH) as electron donor. DPF and DPH were significantly different in both product distribution and extent of isotope fractionation. Chemical and isotope analyses indicated that electron donors did not directly affect the product distribution or the extent of isotope fractionation for PCE reductive dechlorination. Instead, restriction fragment length polymorphism (RFLP) and sequence analysis of the 16S rRNA clone libraries of DPF and DPH identified distinct microbial communities in each enrichment culture, suggesting that differences in microbial communities were responsible for distinct product distributions and isotope fractionation between the two cultures. A dominant species identified only in DPH was closely related to known dehalogenating species (Sulfurospirillum multivorans and Sulfurospirillum halorespirans) and may be responsible for PCE degradation in DPH. Our study suggests that different dechlorinators exist at the same site and can be preferentially stimulated by different electron donors, especially over the long-term (i.e., years), typical of in-situ ground water remediation.

Inhibition of aerobic metabolic cis-1,2-di-chloroethene biodegradation by other chloroethenes

The presence of other chloroethenes influences aerobic metabolic biodegradation of cis-1,2-dichloroethene (cDCE). A new metabolically cDCE degrading enrichment culture was identified as also being capable of degrading vinyl chloride (VC), but not 1,1-dichloroethene (1,1DCE), trans-1,2-dichloroethene (tDCE), trichloroethene (TCE), or tetrachloroethene (PCE). The fastest degradation of cDCE was observed in the absence of any other chloroethene. In the presence of a second chloroethene (40-90 microM), the rate of cDCE (60 microM) degradation decreased in the following order: cDCE (+PCE) > cDCE (+tDCE) > cDCE (+VC)>cDCE (+1,1DCE) approximately cDCE (+TCE). With increasing concentrations of VC, ranging from 10 to 110 microM, the rate of cDCE (60 microM) degradation decreased. This study demonstrates that the inhibiting effects of chloroethene mixtures have to be considered during laboratory studies and bioremediation approaches based on metabolic cDCE degradation.