Reductive Dechlorination Processes Research Articles

Analyzing Dehalochip: A functional DNA microarray for reductive dichlorination in chloroethene-contaminated sites

Interpreting high-throughput transcriptomic and metagenomic data from non-model microorganisms presents a challenge due to the significant number of genes with unknown functions and sequences. In this study, we applied an innovative microarray, Dehalochip, for detecting the expression of genes in various microorganisms, particularly focusing on genes involved in chloroethene degradation. Our results demonstrated that this approach can effectively identify dechlorination genes, such as 16S rRNA, tceA, bvcA, and vcrA, in Dehalococcoides mccartyi from samples of groundwater contaminated with chloroethene. Noticeably, the sensitivity and specificity of our Dehalochip are comparable to that of quantitative PCR. However, it stands out as a more viable option for in-situ applications due to its greater capacity to infer potential dechlorination genes. Consequently, we believe our dechlorination microarray offers valuable insights into the role of known microorganisms and their associated functional genes in chloroethene-contaminated environments. This contributes to a deeper understanding of the in-situ reductive dechlorination process.

Highly efficient removal of 1,1,1-trichloroethane from simulated groundwater by polydopamine-modified iron/polylactic acid/biochar composite coupling with Shewanella oneidensis MR-1

Biochar (BC) loaded with nanoscale zero-valent iron (nZVI) has been extensively used to remove chlorinated hydrocarbons from groundwater through biotic/abiotic reductive dechlorination. However, the rapid deactivation due to surface passivation, hydrophobicity, and inadequacy of carbon sources severely limits its practical application. Herein, a novel polydopamine (PDA)-modified iron/polylactic acid (PLA)/biochar composite (PDA@OBC-nZVI) was successfully synthesized to address this issue. Its effect and mechanism for 1,1,1-trichloroethane (1,1,1-TCA) removal from simulated groundwater coupling with Shewanella oneidensis MR-1 (MR-1) were investigated. Results showed that nZVI and PLA particles were uniformly dispersed on the BC surface, being smoother after PDA coating. The composite coupling with MR-1 exhibited efficient and long-term performances for 1,1,1-TCA removal. Under the experimental conditions comprising 10 % PLA mass fraction in the composite, 10 g·L−1 composite dosage and 20 mL·L−1 MR-1 inoculum volume, 83.14 % of 100 mg·L−1 1,1,1-TCA was removed by PDA@OBC-nZVI + MR-1 within 360 h, compared to only 48.12 % by PDA@OBC-nZVI alone. The primary removal pathways included the rapid chemical reductive dechlorination and the long-term microbial dissimilatory iron reduction. The synergistic dechlorination mechanism was manifested in the composite providing sustained carbon sources, accelerating electron transfer, raising sorbed Fe(II) level, and promoting cytochrome c secretion by MR-1. The novelty of this study is enhancing the hydrophilicity and redox activity of carbon-iron composites by PDA modification for the first time in 1,1,1-TCA removal. Overall, the results suggest that PDA@OBC-nZVI + MR-1 can effectively promote the reductive dechlorination process, being great potential for the remediation of chlorinated hydrocarbon-contaminated groundwater.

Synergy of atomic hydrogen reduction and reactive oxygen species oxidation over confined Mn bifunctional site for electrocatalytic deep mineralization

Traditional reduction or oxidation processes generating one−component free radicals face challenges in deep dechlorination and mineralization of chlorophenols from wastewater. Herein, an efficient electrocatalytic process has been developed, which couples atomic H* reduction with reactive oxidation species (•OH and 1O2) oxidation on a bifunctional cathode for 4 −chlorophenol (4 −CP) removal. The N − doped carbon nanotubes encapsulated manganese nanoparticles was fabricated as cathode, which could generate atomic H* , initiating nucleophilic hydrodechlorination in presence of confined MnO sites. Subsequently, electrophilic oxidation by generating mainly 1O2 on confined Mn7C3 sites and •OH on confined MnO sites, facilitating the oxidative processes. Experimental results and theory calculations demonstrated that reductive dechlorination and oxidative mineralization processes could mutually promote each other, resulting in an enhancement factor of 2.90. At pH 7, this process achieved 100 % removal for 4 −CP, 84 % dechlorination, 76 % total organic carbon (TOC) removal and low energy consumption (0.76 kWh g−1TOC) within 120 min. Notably, TOC for chlorophenols containing Cl substituents at different positions and real lake water containing 4 −CP could be almost completely removed. This research establishes confined non−noble bifunctional active sites that synergistically enhance reductive dechlorination and oxidative degradation processes, holding significant treatment potential for application in deep mineralization of organochlorine from water/wastewater.

Chemical-physical parameters and microbial community changes induced by electrodes polarization inhibit PCB dechlorination in a marine sediment

Microbial reductive dechlorination of organohalogenated pollutants is often limited by the scarcity of electron donors, that can be overcome with microbial electrochemical technologies (METs). In this study, polarized electrodes buried in marine sediment microcosms were investigated to stimulate PCB reductive dechlorination under potentiostatic (−0.7 V vs Ag/AgCl) and galvanostatic conditions (0.025 mA·cm−2-0.05 mA·cm−2), using graphite rod as cathode and iron plate as sacrificial anode. A single circuit and a novel two antiparallel circuits configuration (2AP) were investigated. Single circuit polarization impacted the sediment pH and redox potential (ORP) proportionally to the intensity of the electrical input and inhibited PCB reductive dechlorination. The effects on the sediment’s pH and ORP, along with the inhibition of PCB reductive dechlorination, were mitigated in the 2AP system. Electrodes polarization stimulated sulfate-reduction and promoted the enrichment of bacterial clades potentially involved in sulfate-reduction as well as in sulfur oxidation. This suggested the electrons provided were consumed by competitors of organohalide respiring bacteria and specifically sequestered by sulfur cycling, which may represent the main factor limiting the applicability of METs for stimulating PCB reductive dechlorination in marine sediments.

Impact of different zero valent iron-based particles on anaerobic microbial dechlorination of 2,4-dichlorophenol: Comparison of dechlorination performance and the underlying mechanism

The integration of iron-based materials and anaerobic microbial consortia has been extensively studied owing to its potential to enhance pollutant degradation. However, few studies have compared how different iron materials enhance the dechlorination of chlorophenols in coupled microbial systems. This study systematically compared the combined performances of microbial community (MC) and iron materials (Fe0/FeS2 +MC, S-nZVI+MC, n-ZVI+MC, and nFe/Ni+MC) for the dechlorination of 2,4-dichlorophenol (DCP) as one representative of chlorophenols. DCP dechlorination rate was significantly higher in Fe0/FeS2 +MC and S-nZVI+MC (1.92 and 1.67 times, with no significant difference between two groups) than in nZVI+MC and nFe/Ni+MC (1.29 and 1.25 times, with no significant difference between two groups). Fe0/FeS2 had better performance for the reductive dechlorination process as compared with other three iron-based materials via the consumption of any trace amount of oxygen in anoxic condition and accelerated electron transfer. On the other hand, nFe/Ni could induce different dechlorinating bacteria as compared to other iron materials. The enhanced microbial dechlorination was mainly due to some putative dechlorinating bacteria (Pseudomonas, Azotobacter, Propionibacterium), and due to improved electron transfer of sulfidated iron particles. Therefore, Fe0/FeS2 as a biocompatible as well as low-cost sulfidated material can be a good alternative for possible engineering applications in groundwater remediation.

Insights into the influence on 2,4,6-trichlorophenol microbial reductive dechlorination process by exposure to microplastics

As a newly emerging pollutant, the adverse effects of ubiquitous microplastic on ecosystems and living have attracted increasing attention, however, the microplastic intervention influencing anaerobic biotransformation behaviors of chlorinated aromatics remains poorly understood. Here, the significantly altered 2,4,6-trichlorophenol microbial reductive dechlorination activities were found by single- and multi-exposure to microplastics. Single contact with five different types of microplastics showed diverse dechlorination performance, that PET was the most promoted ( k T of 0.037 h -1 ) and PP was the largest inhibited ( k T of 0.01 h -1 ) compared to control ( k T of 0.023 h -1 ). The microplastic size was negatively correlated with dechlorination activities, with 1.8~1.9 times decreased k T as the particle size decreased from 1000μm to 75μm. The higher the concentration, the more promoted effects by PET and the more inhibited effects by PP on dechlorination performance, with the respective k T of 0.052 h -1 and 0.006 h -1 under 2.0 g/L condition. Bio-enrichment on hydrophilic surfaces of microplastics (e.g. PET) was speculated to be responsible for the favored dechlorination activity, while adsorption of chlorophenols posed little influence. However, the overall inhibited 2,4,6-TCP dechlorination performance was found under all conditions by extending exposure cycles. The obvious bacterial succession that the abundance of some absolute anaerobic fermentator ( Dysgonomonas ) or dechlorinator ( Desulfitobacterium ) was greatly decreased, while, some facultative anaerobic genera ( Comamonas , Pseudomonas ) that held microplastic degradation potential were enriched. The increased positive correlations among networks were found and the more simplified molecule network as the size of microplastics became larger. The study provided significant theoretical support for understanding anaerobic biotransformation process of CAs influenced by the microplastic intervention. • Altered microbial reductive dechlorination activities by microplastic intervention • PET mostly promoted and PP largest inhibited the dechlorination by single contact • Microplastic size was negatively correlated with the dechlorination activities • Serious inhibited dechlorination performance was found by multi-cycle exposure • Obvious population succession and increased positive correlations of eco-networks

A starch-based controlled-release targeted nutrient agent to stimulate the activity of volatile chlorinated hydrocarbon-degrading indigenous microflora present in groundwater

Volatile chlorinated hydrocarbons (VCHs) contaminated groundwater has a low indigenous microorganism population, and lack of nutrient substrates involved in degradation reactions, resulting in a weak natural remediation ability of groundwater ecosystems. In this study, based on the principle of degradation of VCHs by indigenous microorganisms in groundwater, and combined with biostimulation and controlled-release technology, we developed a starch-based encapsulated targeted bionutrient (YH-1) with easy uptake, good stability, controllable slow-release migration, and long timeliness for the remediation of groundwater contaminated by VCHs by indigenous microorganisms. The results showed that YH-1 is easily absorbed by microorganisms and can rapidly initiate itself to stimulate the microbial degradation of VCHs, and the degradation rate of various VCH components within 7 days was 82.38–92.38 %. The release rate of nutrient components in YH-1 increases with increasing VCH concentrations in groundwater; this could effectively prolong the action time of nutrient components, while also improving the degradation efficiency of pollutants with a sustained effect of more than 15 days. Simultaneously, owing to the fluidity, water solubility, and biodegradability of YH-1 in lithologic media, YH-1 injection did not cause blockage of the lithologic media in the aquifer. Through YH-1 stimulation, indigenous microorganisms grew rapidly in the underground environment, the diversity of microbial communities and the total number of species increased, and the correlation between genera strengthened. Simultaneously, YH-1 improved the ability of microbial community to convert inorganic electron donors/acceptors, thereby strengthening the co-metabolic mechanism between microorganisms. Additionally, there was a significant increase in the percentage of many microorganisms (e.g., Sphingomonas, Janthinobacterium, Duganella, etc.) that mediated the reductive dechlorination process and were redox inorganic electron donors/acceptors. This was conducive to the reductive dechlorination process of VCHs and achieved the efficient degradation of VCHs.

Generation of adsorbed atomic hydrogen by Pd(II) doped Fe(OH)2 enables ultra-fast reductive dechlorination of trichloroethylene

Trichloroethylene (TCE) is a notorious persistent pollutant in groundwater, while most reductive dechlorination processes result in the formation of toxic less-chlorinated by-products. In this work, adsorbed atomic hydrogen (H*) was produced by Pd(II) doped Fe(OH)2, which was used for ultra-fast reductive dechlorination of TCE without the formation of less-chlorinated by-products. When the Fe(OH)2 dosage is 0.5 mM and the Pd(II) dosage is 20 μM, TCE (42 μM) is completely removed in 60 min with the pseudo first-order reaction rate constant of 7.74 h−1. Radical quenching experiment and electrochemical analysis both confirmed that adsorbed H* rather than absorbed H* contribute to the reduction of TCE. Density function theory calculation demonstrated that small Pd clusters were more beneficial to TCE dechlorination than large Pd clusters. The pseudo first-order rate constant for TCE reduction in real groundwater was as high as 0.018 min–1, demonstrating the potential application of Pd/Fe(OH)2 in groundwater remediation.

Metabolome patterns identify active dechlorination in bioaugmentation consortium SDC-9™.

Ultra-high performance liquid chromatography-high-resolution mass spectrometry (UPHLC-HRMS) is used to discover and monitor single or sets of biomarkers informing about metabolic processes of interest. The technique can detect 1000's of molecules (i.e., metabolites) in a single instrument run and provide a measurement of the global metabolome, which could be a fingerprint of activity. Despite the power of this approach, technical challenges have hindered the effective use of metabolomics to interrogate microbial communities implicated in the removal of priority contaminants. Herein, our efforts to circumvent these challenges and apply this emerging systems biology technique to microbiomes relevant for contaminant biodegradation will be discussed. Chlorinated ethenes impact many contaminated sites, and detoxification can be achieved by organohalide-respiring bacteria, a process currently assessed by quantitative gene-centric tools (e.g., quantitative PCR). This laboratory study monitored the metabolome of the SDC-9™ bioaugmentation consortium during cis-1,2-dichloroethene (cDCE) conversion to vinyl chloride (VC) and nontoxic ethene. Untargeted metabolomics using an UHPLC-Orbitrap mass spectrometer and performed on SDC-9™ cultures at different stages of the reductive dechlorination process detected ~10,000 spectral features per sample arising from water-soluble molecules with both known and unknown structures. Multivariate statistical techniques including partial least squares-discriminate analysis (PLSDA) identified patterns of measurable spectral features (peak patterns) that correlated with dechlorination (in)activity, and ANOVA analyses identified 18 potential biomarkers for this process. Statistical clustering of samples with these 18 features identified dechlorination activity more reliably than clustering of samples based only on chlorinated ethene concentration and Dhc 16S rRNA gene abundance data, highlighting the potential value of metabolomic workflows as an innovative site assessment and bioremediation monitoring tool.

Improved understanding on biochar effect in electron supplied anaerobic soil as evidenced by dechlorination and methanogenesis processes

Research interest in biochar as an environmental remediation material has rapidly increased over the past few years. However, the effect of biochar on typical environmental processes in anaerobic soil environment has been insufficiently discussed. By regulating the electron donors with sodium acetate or pyruvate, the effects and underpinning chemical-microbiological coupling mechanisms of biochar under anaerobic conditions were disclosed. Unlike the electron limited condition, the addition of electron donors alleviated the competition for electrons among various reduction processes in the soil. The effect of biochar in regulating the electron transfer processes was lessened. But more than doubled methane emissions were resulted by the exogenous substances, especially with the synergic effect of biochar. Biochar addition increased soil environmental heterogeneity. It might indirectly affect the reductive transformation of γ-HCH via increasing the bioavailability of pollutants through adsorption and promoting the metabolism of some rare microorganisms. Anaerolineaceae, Peptococcaceae and Methanosarcina had coherent phylogenetic patterns and were likely to be the enablers for the reductive dechlorination process in flooded soil. Environmental implicationPrevious studies have widely reported the performance characteristics of biochar, but its effects under anaerobic environments are not systematically understood. By regulating the electron donors, the competition for electrons among various reduction processes in the soil might be alleviated, resulting in a lessened effect of biochar in regulating the electron transfer processes. The findings presented in this study highlight the role of biochar to the dynamic changes of reduction processes under anaerobic environments. The relevant soil conditions such as the electron donors and the functional microbial groups should be adequately considered for maximizing the all-around beneficial efficiency of biochar amendments.

Field application of glycerol to enhance reductive dechlorination of chlorinated ethenes and its impact on microbial community

Chlorinated ethenes (CEs) are common and persistent contaminants of soil and groundwater. Their degradation is mostly driven by a process of bacterial reductive dechlorination (also called organohalide respiration) in anaerobic conditions. This study summarizes the outcomes of the long-term in-situ application of glycerol for the enhanced reductive dechlorination of CEs on a highly contaminated site. Glycerol injection resulted in an almost immediate increase in the abundance of fermentative Firmicutes, which produce essential sources of carbon (acetate) and electrons (H2) for organohalide-respiring bacteria (OHRB) and change groundwater conditions to be suitable for OHRB growth. The decreased redox potential of groundwater promoted also the proliferation of sulfate-reducing bacteria, which compete for electron donors with OHRB but at the same time support their growth by producing essential corrinoids and acetate. A considerable increase in the abundance of OHRB Dehalococcoides, concurrently with vinyl chloride (VC) reductase gene levels, was revealed by real time polymerase chain reaction (qPCR) method. Consistent with the shifts in bacterial populations, the concentrations of pollutants tetrachloroethylene and trichloroethylene decreased during the monitoring period, with rising levels of cis-1,2-dichloroethylene, VC, and most importantly, the final CE degradation products: ethene and ethane. Our study implies the importance of syntrophic bacterial interactions for successful and complete CE degradation and evaluates glycerol as convenient substrate to enhance reductive dechlorination and as an effective source of electrons for OHRB.

The influence of anaerobic dechlorination on the aerobic degradation of PCBs in e-waste-contaminated soils in an anaerobic-aerobic two-stage treatment

The combination of microbial reductive dechlorination and aerobic oxidation (RD-AO) process was proposed to be a promising strategy for extensive bioremediation of highly chlorinated polychlorinated biphenyls (PCBs). Nonetheless, experimental evidence on the impact of the RD on subsequent AO in anaerobic-aerobic two-stage treatment remains scarce. The present study applied stable-isotope probing (SIP) to explore the RD-AO mediated degradation of PCBs in an e-waste-contaminated soil. The RD-AO treatment resulted in 37.1 % and 48.2 % degradation of PCB180 and PCB9, respectively, while the PCB9 degradation efficiency decreased compared to the sole AO (81.2 %). The inhibition of PCB aerobic degradation might be caused by the alteration of aerobic bacterial community, which was proved by a higher abundance of anaerobic bacteria and a lower abundance of aerobic bacteria being observed in the aerobic stage of RD-AO. Further evidence was obtained using DNA-SIP that the anaerobic stage altered the PCB degraders' community structures and changed three of the five degraders. There were four lineages (Arenimonas, Steroidobacter, Sulfurifustis, and Thermoanaerobacterales) identified as PCB degraders for the first time. Interestingly, three of them were found in RD-AO microcosm, suggesting that anaerobic-aerobic two-stage treatment can recruit novel bacteria involved in PCBs aerobic degradation. The present study provided novel insight into the synergistic integration of anaerobic and aerobic processes for extensive degradation of highly chlorinated PCBs.

Dehalogenation of Chlorinated Ethenes to Ethene by a Novel Isolate, "Candidatus Dehalogenimonas etheniformans".

Dehalococcoides mccartyi strains harboring vinyl chloride (VC) reductive dehalogenase (RDase) genes are keystone bacteria for VC detoxification in groundwater aquifers, and bioremediation monitoring regimens focus on D. mccartyi biomarkers. We isolated a novel anaerobic bacterium, "Candidatus Dehalogenimonas etheniformans" strain GP, capable of respiratory dechlorination of VC to ethene. This bacterium couples formate and hydrogen (H2) oxidation to the reduction of trichloro-ethene (TCE), all dichloroethene (DCE) isomers, and VC with acetate as the carbon source. Cultures that received formate and H2 consumed the two electron donors concomitantly at similar rates. A 16S rRNA gene-targeted quantitative PCR (qPCR) assay measured growth yields of (1.2 ± 0.2) × 108 and (1.9 ± 0.2) × 108 cells per μmol of VC dechlorinated in cultures with H2 or formate as electron donor, respectively. About 1.5-fold higher cell numbers were measured with qPCR targeting cerA, a single-copy gene encoding a putative VC RDase. A VC dechlorination rate of 215 ± 40 μmol L-1 day-1 was measured at 30°C, with about 25% of this activity occurring at 15°C. Increasing NaCl concentrations progressively impacted VC dechlorination rates, and dechlorination ceased at 15 g NaCl L-1. During growth with TCE, all DCE isomers were intermediates. Tetrachloroethene was not dechlorinated and inhibited dechlorination of other chlorinated ethenes. Carbon monoxide formed and accumulated as a metabolic by-product in dechlorinating cultures and impacted reductive dechlorination activity. The isolation of a new Dehalogenimonas species able to effectively dechlorinate toxic chlorinated ethenes to benign ethene expands our understanding of the reductive dechlorination process, with implications for bioremediation and environmental monitoring. IMPORTANCE Chlorinated ethenes are risk drivers at many contaminated sites, and current bioremediation efforts focus on organohalide-respiring Dehalococcoides mccartyi strains to achieve detoxification. We isolated and characterized the first non-Dehalococcoides bacterium, "Candidatus Dehalogenimonas etheniformans" strain GP, capable of metabolic reductive dechlorination of TCE, all DCE isomers, and VC to environmentally benign ethene. In addition to hydrogen, the new isolate utilizes formate as electron donor for reductive dechlorination, providing opportunities for more effective electron donor delivery to the contaminated subsurface. The discovery that a broader microbial diversity can achieve detoxification of toxic chlorinated ethenes in anoxic aquifers illustrates the potential of naturally occurring microbes for biotechnological applications.

Coupled Adsorption and Biodegradation of Trichloroethylene on Biochar from Pine Wood Wastes: A Combined Approach for a Sustainable Bioremediation Strategy.

Towards chlorinated solvents, the effectiveness of the remediation strategy can be improved by combining a biological approach (e.g., anaerobic reductive dechlorination) with chemical/physical treatments (e.g., adsorption). A coupled adsorption and biodegradation (CAB) process for trichloroethylene (TCE) removal is proposed in a biofilm–biochar reactor (BBR) to assess whether biochar from pine wood (PWB) can support a dechlorinating biofilm by combining the TCE (100 µM) adsorption. The BBR operated for eight months in parallel with a biofilm reactor (BR)—no PWB (biological process alone), and with an abiotic biochar reactor (ABR)—no dechlorinating biofilm (only an adsorption mechanism). Two flow rates were investigated. Compared to the BR, which resulted in a TCE removal of 86.9 ± 11.9% and 78.73 ± 19.79%, the BBR demonstrated that PWB effectively adsorbs TCE and slows down the release of its intermediates. The elimination of TCE was quantitative, with 99.61 ± 0.79% and 99.87 ± 0.51% TCE removal. Interestingly, the biomarker of the reductive dechlorination process, Dehalococcoides mccartyi, was found in the BRR (9.2 × 105 16S rRNA gene copies/g), together with the specific genes tceA, bvcA, and vcrA (8.16 × 106, 1.28 × 105, and 8.01 × 103 gene copies/g, respectively). This study suggests the feasibility of biochar to support the reductive dechlorination of D. mccartyi, opening new frontiers for field-scale applications.

Enhancing the biological reductive dechlorination of trichloroethylene with PHA from mixed microbial cultures (MMC)

The biological reductive dechlorination (BRD) process is often limited by the lack of suitable electron donors, requiring the supply of long-lasting substrates to enhance the biological metabolism. Among the others, polyhydroxyalkanoates (PHA) are a particularly interesting slow-release source of electron-donors, being entirely biodegradable polyesters. Generally, industrial PHA production processes are based on pure culture fermentation with high related costs. In recent years, innovative and cost-effective PHA production from mixed microbial cultures (MMC) and waste feedstocks is attracting considerable attention. This research, for the first time, investigated the effect of distinctive types of PHA on the BRD process of trichloroethylene (TCE), in a continuous-flow lab-scale system using two PHA materials with different purity grade produced from MMC at pilot scale in comparison with a commercial PHA produced from pure culture. Promising results have been obtained with non-extracted MMC-PHA, a material consisting of both PHA (56%, w/w) and microbial cells, with constant production of acids over 110 days, which stimulated a nearly complete TCE dechlorination with the non-toxic ethene as the main byproduct (approximately 92%). Dehalococcoides mccartyi was the main microorganism responsible for TCE dechlorination process, representing ≥ 54.05% of the bacterial population, mainly carrying the reductive dehalogenase genes tceA and vcrA (≥9.17E+08 and ≥2.01E+07 gene copies/g of PHA or sand, respectively). This finding suggests the possibility to directly use PHA-rich biomass deriving from MMC production process, which does not require any polluting and expensive extraction procedures, as a novel material in the field of groundwater remediation with noticeable economic and environmental advantages.

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.

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.

Photochemical reactions between superoxide ions and 2,4,6-trichlorophenol in atmospheric aqueous environments

The superoxide anion radical (O2•−) is an important reactive oxygen species (ROS), and participates in several chemical reactions and biological processes. In this report, O2•− was produced by irradiating riboflavin in an O2-saturated solution by ultraviolet light with a maximum emission at 365 nm. And the contribution of O2•− to 2, 4, 6-trichlorophenol (2, 4, 6-TCP) was investigated by a combination of laser flash photolysis (LFP) and UV light steady irradiation technique. The results of steady-state experiments showed that the photochemical decomposition efficiency of 2, 4, 6-TCP decreased with the increase of the initial concentration of TCP, while the increase of pH and riboflavin concentration promoted the photochemical reaction. The second-order rate constant of the reaction of the superoxide anion radical with 2, 4, 6-TCP phenoxyl radical (TCP•) was (9.9 ± 0.9) × 109 L mol−1 s−1 determined by laser flash photolysis techniques. The dechlorination efficiency was 61.5% after illuminating the mixed solution with UV light for 2 h. The conversion of 2, 4, 6-trichlorophenol was accompanied by the reductive dechlorination process induced by superoxide ions. The main steady products of the photochemical reaction of 2, 4, 6-TCP with O2•− were 2, 6-dichlorophenol (DCP), 2, 6-dichloro-1, 4-benzoquinone (DCQ) and 2, 6-dichlorohydroquinone (DCHQ). The addition process was the preferred process in the total reaction of superoxide ions with 2, 4, 6-TCP phenoxyl radical. These results indicated that the reaction of 2, 4, 6-TCP with O2•− was a potential conversion pathway and contribute to atmospheric aqueous phase chemistry.

Microbiome Reengineering by Heat Selection for Rapid Biodegradation of Trichloroethylene with Minimal Vinyl Chloride Formation

Trichloroethylene (TCE) was a widely used industrial solvent but is now regarded as a notorious groundwater contaminant. Both physicochemical and biological methods have been applied to remediate groundwater contaminated by TCE. For medium to low level of TCE contamination, bioremediation could be more cost-effective. However, bioremediation approaches suffer from slow degradation rates and accumulation of vinyl chloride (VC). In addition, bioaugmentation is often highly encouraged but may introduce foreign genes and increase the pace of microbial evolution. In this study, a microbiome reengineering strategy by heat selection is applied to solve these problems. Out of eight heat-treated mixed cultures, two showed a much-improved TCE degradation rate, more than 70 times higher than the untreated. The biodegradation half-life (t1/2) of TCE was 0.0627 d or shorter. No VC was detected by a gas chromatography equipped with flame ionization detector (GC-FID) and only a minimal amount by a GC-mass spectrometer (GC-MS). Ethene achieved a fairly good mass balance. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and next-generation sequencing (NGS) results showed that the heating process did not kill most bacteria, but Dehalococcoides were either not present or very scarce. Acetoanaerobium and Methanosarcina could be the most important species in this reductive dechlorination process. Kinetic study results showed that the maximum specific TCE degradation rate was approximately 1,271 nmole/min/mg cell protein, which are two orders of magnitude higher than that of the mixed cultures reported in literature. These results suggest that apart from biostimulation and bioaugmentation, microbiome reengineering could be a promising approach for rapid bioremediation of TCE-contaminated aquifers.

Molecular characterization of microbial communities in a peat-rich aquifer system contaminated with chlorinated aliphatic compounds.

In an aquifer-aquitard system in the subsoil of the city of Ferrara (Emilia-Romagna region, northern Italy) highly contaminated with chlorinated aliphatic toxic organics such as trichloroethylene (TCE) and tetrachloroethylene (PCE), a strong microbial-dependent dechlorination activity takes place during migration of contaminants through shallow organic-rich layers with peat intercalations. The in situ microbial degradation of chlorinated ethenes, formerly inferredby the utilization of contaminant concentration profiles and Compound-Specific Isotope Analysis (CSIA), was here assessed using Illumina sequencing of V4 hypervariable region of 16S rRNA gene and by clone library analysis of dehalogenase metabolic genes. Taxon-specific investigation of the microbial communities catalyzing the chlorination process revealed the presence of not only dehalogenating genera such as Dehalococcoides and Dehalobacter but also of numerous other groups of non-dehalogenating bacteria and archaea thriving on diverse metabolisms such as hydrolysis and fermentation of complex organic matter, acidogenesis, acetogenesis, and methanogenesis, which can indirectly support the reductive dechlorination process. Besides, the diversity of genes encoding some reductive dehalogenases was also analyzed. Geochemical and 16S rRNA and RDH gene analyses, as a whole, provided insights into the microbial community complexity and the distribution of potential dechlorinators. Based on the data obtained, a possible network of metabolic interactions has been hypothesized to obtain an effective reductive dechlorination process.