Dechlorination Of Trichloroethylene Research Articles

Use of carbon-14 labeled trichloroethene to assess degradation potential in rock core microcosms.

In fractured rock aquifers contaminated with trichloroethene (TCE), the extent of groundwater plumes is impacted by degradation occurring within the rock matrix. The objective of this study was to evaluate TCE degradation in rock samples from three sites where in situ conditions may favor natural or enhanced attenuation. Intact rock core microcosms (94 total) were used to assess in situ conditions and enhancement by addition of lactate or lactate + sulfate. A key advance for this study was inclusion of carbon-14 (14C) labeled TCE in the experimental design, which enables monitoring of 14C-labeled products in addition to more readily detectable compounds associated with TCE degradation (i.e., cis-1,2-dichloroethene (cDCE), vinyl chloride, acetylene, ethene, and ethane). 14C-labeled products comprised 35-95 % of the total degradation products recovered over 9-21 months of monitoring, indicating that inclusion of 14C-TCE was essential to capturing the full potential for abiotic and biotic degradation of TCE. Microcosms infused with TCE but not 14C-TCE exhibited enrichment in δ13C-TCE, and enrichment in δ13C-cDCE in microcosms that underwent reductive dechlorination of TCE to cDCE. The results demonstrate the advantages of using diffusion-transport microcosms and 14C-TCE to document degradation of chlorinated ethenes in fractured rock aquifers.

Kirkendall effect enhanced sustained-release Fe-C composites for long-term reductive dechlorination of trichloroethylene: Interfacial reactions and slow-release degradation

Rebounding and lagging during the process of remediating contaminated sites are challenging problems. A potential solution is developing slow-release materials that could assist in the long-term remediation of contaminated sites. This study synthesised a slow-release Fe-C composite (CSBC-nZVI@β-CD), which exhibits a synergistic effect of encapsulated layers and nanocracks, using a liquid-phase reduction method. Furthermore, the reduction and dechlorination mechanism of trichloroethene (TCE) was systematically investigated. The introduction of β-cyclodextrin (β-CD) intensified the Kirkendall effect, forming nano zero-valent iron (nZVI) with abundant radial nanocracks and a litchi-like appearance on the biochar surface. Additionally, β-CD created an encapsulation layer measuring 5–7 nm on the surface of nZVI, improving its electron transport capacity and hydrophobic properties. The CSBC-nZVI@β-CD nanocomposite successfully removed 99 % of TCE in solution within 2 h and accomplished a degradation efficiency of 98.9 % within 15 d. The synergistic effect of the β-CD encapsulating layer and nanocracks facilitated the reactivity and electron-retarding capacity, resulting in a broad pH tolerance and anti-interference ability. Density-functional theory (DFT) calculations and experimental methods were used to study the mechanism of TCE reduction dechlorination; the unique structure of β-CD tended to bind with Fe as well as adsorb TCE, thereby facilitating electron transfer and the enrichment of TCE at the reaction interface. In addition, the sustained-release Fe-C composites reduced the bonding energy required during the TCE degradation reaction and promoted the complete reaction of the product. Overall, CSBC-nZVI@β-CD exhibited significant potential for the long-term remediation of groundwater.

Impact of methanogenesis inhibitors on a mixed dechlorinating community in TCE contaminated groundwater

Trichloroethene (TCE) is a chlorinated solvent that can adversely affect human health. TCE-contaminated groundwater bioremediation relies on the presence of an active and diverse reductively dechlorinating microbial community. This process requires the presence of reduced conditions, which are also favorable to methanogens. Methanogens and dechlorinating bacteria can both use hydrogen for metabolism thus competition can arise. Previous studies have indicated that methanogens might also be beneficial for the dechlorinating microbial community since they can provide enzymes, cofactors and vitamins necessary for the metabolism. In this study, the methanogenesis inhibitors garlic oil (GO) and 2-chloroethanesulfonate (CES) were assessed for their effect on reductive dechlorination and methanogenesis in TCE contaminated groundwater. GO has been used in the field as a methanogenesis inhibitor but limited information is available on its effects on the microbial community. The results from this study indicated that GO did not impact the activity of the microbial community, while CES partially inhibited methanogenesis. At the assessed electron donor concentration (5 mM) neither of the compounds affected TCE dechlorination in the reactor microcosms. This information can be beneficial for decision-makers, when designing a TCE-bioremediation plan to assess the need for amendments to enhance long-term TCE dechlorination activity at the site.

Control of dissolved H2 concentration enhances electron generation, transport and TCE reduction by indigenous microbial community

Electrokinetic enhanced bioremediation (EK-Bio) is practical for trichloroethene (TCE) dechlorination because the cathode can produce a wide range of dissolved H2 (DH) concentrations of 1.3–0 mg/L from the electrode to the aquifer. In this study, TCE dechlorination was investigated under different DH concentrations. The mechanisms were discussed by analyzing the microbial community structure and abundance of organohalide-respiring bacteria (OHRB) using 16S rRNA, and the gene abundances of key enzymes in the TCE electron transport chain using metagenomic analysis. The results showed that the moderate DH concentration of 0.19–0.53 mg/L exhibited the most pronounced TCE dechlorination, even better than the higher DH concentrations, due to the optimal redox environment, the enrichments of OHRB, reductive dehalogenase (rdhA) genes and key enzyme genes in the electron generation and transport chain. More electrons were obtained from H2 metabolism by Dehalobacter by promoting the formation of [NiFe] hydrogenase (HupS/L/C) or from glycolysis by versatile OHRB by stimulating the formation of formate and enriching formate dehydrogenase (FDH) under moderate DH conditions. In addition, the enhanced amino acid metabolism improved the vitamin K cycle for electron transport and enriched the reductive dechlorinating enzyme (RDase) genes. This study identifies the optimal DH concentration that facilitates bioremediation efficiency, provides insights into microbial community shifts and key enzymatic pathways in EK-Bio remediation.

A slow-release reduction material of Escherichia sp. F1 coupled with micron iron powder achieves the remediation of trichloroethylene-contaminated soil

Trichloroethylene (TCE) is a prevalent organic pollutant found in soil. The oxide passivation layer on the surface of micron iron powder inhibits the release of its reducing components, leading to ineffective reduction and purification of TCE in soil. To enhance TCE degradation, a slow-release reduction material "Escherichia sp. F1-micron iron powder" was developed. A novel iron-reducing bacterium, Escherichia sp. F1, was isolated from soil contaminated with chlorinated hydrocarbons. This bacterium demonstrated a sustained iron reduction capability, achieving a reduction rate of 38.7% for Fe(Ⅲ) within 15 days. Genome sequencing revealed that strain F1 harbors 53 functional iron reduction genes and 2 dehalogenation genes. Single-factor experiments identified the optimal conditions for TCE degradation in soil using the coupling material: glucose concentration at 40 mmol/kg, soil water content at 50%, and bacterial inoculum at 1% (v:w). Under these optimal conditions, the coupled material achieved 86.86% degradation of TCE in soil within 28 days. Further analysis using X-ray photoelectron spectroscopy of micron iron powder, soil Fe(Ⅱ) concentration, and soil physicochemical properties demonstrated that the addition of strain F1 to the soil could disrupt the passivation layer of iron oxide on the surface of micron iron powder, promoting the exposure of its reactive sites and internal reducing active components. This resulted in an in situ self-actuated activation of passivated micron iron powder, leading to an improved removal rate and complete dechlorination of TCE in the soil. Soil microbial high-throughput sequencing revealed that the addition of strain F1 regulated the soil bacterial community, significantly enriching of Escherichia-Shigella species associated with iron-reducing functions. This enrichment facilitated the degradation of TCE in the soil through coupling materials. The functional material plays a crucial role in achieving green treatment and risk control of sites contaminated with chlorinated organic pollutants.

Synergistic biodegradation of trichloroethylene-contaminated soil using Typha angustifolia L. and an anaerobic degrading bacterial consortium

Plant-microorganism combined bioremediation is a highly efficient and environmentally sustainable method for the remediation of contaminated soils. Despite its potential, the synergistic effects of wetland plants and anaerobic microbial consortium on the removal of chlorinated hydrocarbons (CAHs) from soil remain inadequately understood. In this study, an anaerobic bacterial consortium, capable of completely dechlorinating trichloroethylene (TCE), was enriched and screened from long-term CAH-contaminated soil. Subsequently, the combined effects of the wetland plant Typha angustifolia L. and the degrading bacterial consortium on the removal of TCE from soil were investigated, along with the underlying microbial mechanisms. The results demonstrated that the bacterial consortium was able to completely convert 0.5 mM TCE to vinyl chloride (VC) within 7 days, and subsequently degrade VC to ethylene within 48 days. The integration of Typha angustifolia L. with the degrading bacterial consortium significantly enhanced both the removal and complete dechlorination of TCE from the soil compared to either treatment alone. Furthermore, this combination substantially increased the diversity and richness of soil bacterial communities, enriched dechlorinating microorganisms, and elevated the relative abundance of dehalogenating enzymes and peroxidases involved in pollutant degradation. In addition, the combination resulted in a more complex soil bacterial community, closer microbial interactions, and a more stable microbial co-occurrence network. This study extends the scope of plant-microorganism combined bioremediation for CAH- contaminated soils.

CO-driven electron and carbon flux fuels synergistic microbial reductive dechlorination

BackgroundCarbon monoxide (CO), hypothetically linked to prebiotic biosynthesis and possibly the origin of the life, emerges as a substantive growth substrate for numerous microorganisms. In anoxic environments, the coupling of CO oxidation with hydrogen (H2) production is an essential source of electrons, which can subsequently be utilized by hydrogenotrophic bacteria (e.g., organohalide-respring bacteria). While Dehalococcoides strains assume pivotal roles in the natural turnover of halogenated organics and the bioremediation of chlorinated ethenes, relying on external H2 as their electron donor and acetate as their carbon source, the synergistic dynamics within the anaerobic microbiome have received comparatively less scrutiny. This study delves into the intriguing prospect of CO serving as both the exclusive carbon source and electron donor, thereby supporting the reductive dechlorination of trichloroethene (TCE).ResultsThe metabolic pathway involved anaerobic CO oxidation, specifically the Wood-Ljungdahl pathway, which produced H2 and acetate as primary metabolic products. In an intricate microbial interplay, these H2 and acetate were subsequently utilized by Dehalococcoides, facilitating the dechlorination of TCE. Notably, Acetobacterium emerged as one of the pivotal collaborators for Dehalococcoides, furnishing not only a crucial carbon source essential for its growth and proliferation but also providing a defense against CO inhibition.ConclusionsThis research expands our understanding of CO’s versatility as a microbial energy and carbon source and unveils the intricate syntrophic dynamics underlying reductive dechlorination.Graphical 3ez2V3iuLHAKAp3AxS5o4qVideo

Enhancing reductive dechlorination of trichloroethylene in bioelectrochemical systems with conductive materials

The incorporation of conductive materials to enhance electron transfer in bioelectrochemical systems (BES) is considered a promising approach. However, the specific effects and mechanisms of these materials on trichloroethylene (TCE) reductive dechlorination in BES remains are not fully understood. This study investigated the use of magnetite nanoparticles (MNP) and biochars (BC) as coatings on biocathodes for TCE reduction. Results demonstrated that the average dechlorination rates of MNP-Biocathode (122.89 μM Cl·d−1) and BC-Biocathode (102.88 μM Cl·d−1) were greatly higher than that of Biocathode (78.17 μM Cl·d−1). Based on MATLAB calculation, the dechlorination rate exhibited a more significantly increase in TCE-to-DCE step than the other dechlorination steps. Microbial community analyses revealed an increase in the relative abundance of electroactive and dechlorinating populations (e.g., Pseudomonas, Geobacter, and Desulfovibrio) in MNP-Biocathode and BC-Biocathode. Functional gene analysis via RT-qPCR showed the expression of dehalogenase (RDase) and direct electron transfer (DET) related genes was upregulated with the addition of MNP and BC. These findings suggest that conductive materials might accelerate reductive dechlorination by enhancing DET. The difference of physicochemical characteristics (e.g. particle size and specific surface area), electron transfer enhancement mechanism between MNP and BC as well as the reduction of Fe(III) by hydrogen may explain the superior dechlorination rate observed with MNP-Biocathode.

Single-Atom Iron Can Steer Atomic Hydrogen toward Selective Reductive Dechlorination: Implications for Remediation of Chlorinated Solvents-Impacted Groundwater.

Atomic hydrogen (H*) is a powerful and versatile reductant and has tremendous potential in the degradation of oxidized pollutants (e.g., chlorinated solvents). However, its application for groundwater remediation is hindered by the scavenging side reaction of H2 evolution. Herein, we report that a composite material (Fe0@Fe-N4-C), consisting of zerovalent iron (Fe0) nanoparticles and nitrogen-coordinated single-atom Fe (Fe-N4), can effectively steer H* toward reductive dechlorination of trichloroethylene (TCE), a common groundwater contaminant and primary risk driver at many hazardous waste sites. The Fe-N4 structure strengthens the bond between surface Fe atoms and H*, inhibiting H2 evolution. Nonetheless, H* is available for dechlorination, as the adsorption of TCE weakens this bond. Interestingly, H* also enhances electron delocalization and transfer between adsorbed TCE and surface Fe atoms, increasing the reactivity of adsorbed TCE with H*. Consequently, Fe0@Fe-N4-C exhibits high electron selectivity (up to 86%) toward dechlorination, as well as a high TCE degradation kinetic constant. This material is resilient against water matrix interferences, achieving long-lasting performance for effective TCE removal. These findings shed light on the utilization of H* for the in situ remediation of groundwater contaminated with chlorinated solvents, by rational design of earth-abundant metal-based single-atom catalysts.

Influence of microbial inoculation site on trichloroethylene degradation in electrokinetic-enhanced bioremediation of low-permeability soils

The integration of electrokinetic and bioremediation (EK-BIO) represents an innovative approach for addressing trichloroethylene (TCE) contamination in low-permeability soil. However, there remains a knowledge gap in the impact of the inoculation approach on TCE dechlorination and the microbial response with the presence of co-existing substances. In this study, four 1-dimensional columns were constructed with different inoculation treatments. Monitoring the operation conditions revealed that a stabilization period (∼40 days) was required to reduce voltage fluctuation. The group with inoculation into the soil middle (Group B) exhibited the highest TCE dechlorination efficiency, achieving a TCE removal rate of 84%, which was 1.1–3.2 fold higher compared to the others. Among degraded products in Group B, 39% was ethylene. The physicochemical properties of the post-soil at different regions illustrated that dechlorination coincided with the Fe(III) and SO42− reduction, meaning that the EK-BIO system promoted the formation of a reducing environment. Microbial community analysis demonstrated that Dehalococcoides was only detected in the treatment of injection at soil middle or near the cathode, with abundance enriched by 2.1%–7.2%. The principal components analysis indicated that the inoculation approach significantly affected the evolution of functional bacteria. Quantitative polymerase chain reaction (qPCR) analysis demonstrated that Group B exhibited at least 2.8 and 4.2-fold higher copies of functional genes (tceA, vcrA) than those of other groups. In conclusion, this study contributes to the development of effective strategies for enhancing TCE biodechlorination in the EK-BIO system, which is particularly beneficial for the remediation of low-permeability soils.

Bioremediation of trichloroethylene-contaminated groundwater using green carbon-releasing substrate with pH control capability

In this research, a sustainable substrate, termed green and long-lasting substrate (GLS), featuring a blend of emulsified substrate (ES) and modified rice husk ash (m-RHA) was devised. The primary objective was to facilitate the bioremediation of groundwater contaminated with trichloroethylene (TCE) using innovative GLS for slow carbon release and pH control. The GLS was concocted by homogenizing a mixture of soybean oil, surfactants (Simple Green™ and soya lecithin), and m-RHA, ensuring a gradual release of carbon sources. The hydrothermal synthesis was applied for the production of m-RHA production. The analyses demonstrate that m-RHA were uniform sphere-shape granules with diameters in micro-scale ranges. Results from the microcosm study show that approximately 83% of TCE could be removed (initial TCE concentration = 7.6 mg/L) with GLS supplement after 60 days of operation. Compared to other substrates without RHA addition, higher TCE removal efficiency was obtained, and higher Dehalococcoides sp. (DHC) population and hydA gene (hydrogen-producing gene) copy number were also detected in microcosms with GLS addition. Higher hydrogen concentrations enhanced the DHC growth, which corresponded to the increased DHC populations. The addition of the GLS could provide alkalinity at the initial stage to neutralize the acidified groundwater caused by the produced organic acids after substrate biodegradation, which was advantageous to DHC growth and TCE dechlorination. The addition of m-RHA reached an increased TCE removal efficiency, which was due to the fact that the m-RHA had the zeolite-like structure with a higher surface area and lower granular diameter, and thus, it resulted in a more effective initial adsorption effect. Therefore, a significant amount of TCE could be adsorbed onto the surface of m-RHA, which caused a rapid TCE removal through adsorption. The carbon substrates released from m-RHA could then enhance the subsequent dechlorination. The developed GLS is an environmentally-friendly and green substrate.

Enhancing trichloroethylene dechlorination capacity by utilizing granular ceramic material derived from Fe0-carbon-kaolin nanocomposite as substrate filter

In this study, we present a novel approach to fabricate a granular ceramic material utilizing a composite of nano zero-valent iron (nZVI), carbon, and kaolin. The optimal mass ratio of the prepared material was kaolin, nZVI, activated carbon, and appropriate additives = 40%:40%:10%:10%. The resulting composite demonstrated microelectrolysis effects, enhancing its reactivity for pollutant removal. Activated carbon within the composite significantly increased the interlayer spacing of kaolin, and improved material performance. Furthermore, the addition of activated carbon proved effective in preventing the agglomeration of nZVI, and ensuring a stable and efficient pollutant removal process. The results of 40-day column experiments showed that nano iron-carbon-kaolin micro-electrolysis particles (NIMP) consistently demonstrated stable trichloroethane (TCE) removal with an average removal rate of 88.15 ± 3.24%. Batch experiments confirmed that the synthesized material can achieve complete dechlorination of TCE, which was consistent with density functional theory calculations. The developed material exhibited a robust soil structure and high reactivity for pollutant reduction, making it a promising substrate filler for efficiently degrading refractory organic pollutants. It holds great potential for applications in areas such as water treatment and soil remediation.

Mechanochemical nitridation of micron zero-valent iron for enhanced dechlorination of trichloroethylene: Mechanistic insights into nitrogen sources and ball milling conditions

The performance optimization of zero-valent iron (ZVI) based materials for reductive dechlorination remains challenging. After extensive research on sulfidation modification, the doping of heteroatom nitrogen (N) into ZVI for reductive dechlorination of chlorinated hydrocarbons (CHCs) becomes a new promising trend. In this study, surface modification of microscale ZVI (mZVI) was carried out by a facile ball milling method, and Fe–N(C) coated mZVI (abbreviated as N(C)-mZVIbm) synthesized from different N sources of dimethylimidazole, urea and melamine were characterized. The effects of nitrogen-to-iron ratios (N/Fe), ball milling time and speed on the reductive dechlorination of trichloroethylene (TCE) were further investigated. A series of characterization and mechanistic analysis showed that the high reactivity of N(C)-mZVIbm with melamine as nitrogen source (abbreviated as N(C)-mZVIbm(M)) was attributed to the properties and thickness of its particle passivation layer, and faster electron transfer from the reduced iron species to the surface layer. In addition, N(C)-mZVIbm(M) exhibited great performance for dechlorination at a wide pH range of 3 to 11, due to alleviated passivation of the Fe0 core under alkaline conditions by Fe–N(C). This study presents new ideas and evidences for the environmentally friendly synthesis of N-modified ZVI materials and their practical application in groundwater remediation.

Unravelling the bifunctional role of biochar in promoting nZVI/Ni towards complete dechlorination of trichloroethylene: Not only a carbonouces support

Trichloroethylene (TCE) is a widely recognized persistent groundwater pollutant, and the majority of dechlorination processes for its reduction yield hazardous by-products with low levels of chlorination. Herein, highly reactive catalysts (BCs@FN) were developed using bifunctional biochar (BC) loaded with nZVI/Ni (FN) nanoparticles for the complete dechlorination of trichloroethylene (TCE). BC plays bifunctional roles in promoting TCE dechlorination by facilitating FN dispersion and enhancing the conductivity of bimetallic FN, thereby promoting the production of dominant reactive atomic hydrogen. Moreover, both the reaction rate constant (kobs) of dechlorination and the amount of essential reduction species of atomic hydrogen exhibited a positive correlation with the conductivities of BCs@FN. Specifically, BC900@FN achieved complete dechlorination of 20 mg/L TCE within 3 h at a kobs value of 1.4 h−1. Analysis regarding carbon balance and Cl existence form disclosed all C-Cl bonds could be broken and 100 % TCE was transformed into ethylene (4.1 %) and ethane (95.9 %). Furthermore, the proposed BC900@FN catalyst demonstrates remarkable TCE degradation efficiencies ranging from 71.2 % to 91.8 % in tap water, municipal water and groundwater samples. These findings highlight the potential application of bifunctional BCs in synthesizing bimetallic catalysts and their practical use for remediating chlorinated contaminants.

Molybdenum disulfide modified microscale zero-valent iron (MoS-mZVI) for enhanced trichloroethylene dechlorination

The potential of microscale zero-valent iron (ZVI) for dechlorinating chlorinated hydrocarbons holds promise in pollution remediation. However, Fe0 passivation presents a challenge, leading to slow dechlorination and the buildup of hazardous incomplete dechlorinated intermediates. To address these concerns, we introduce molybdenum disulfide (MoS2)-modified microscale ZVI (MoS-mZVI). Our study demonstrates the significant enhancement of trichloroethylene (TCE) dechlorination by MoS-mZVI compared to standard mZVI. Remarkably, the optimal Fe/MoS2 molar ratio of 20 in MoS-mZVI achieves a remarkable 26.8-fold acceleration in TCE removal compared to mZVI. Acetylene emerges as the primary intermediate in TCE dechlorination by MoS-mZVI, peaking at 38%, accompanied by major end products including ethene (45%) and ethane (31%). Importantly, the MoS-mZVI-20 system avoids toxic byproducts, diverging from the dichloroethylene accumulation observed in the mZVI system. Notably, electron utilization efficiency shows significant improvement, with MoS-mZVI reaching 65.6% compared to mZVI's 7.2%. Furthermore, MoS2′s selective OH- adsorption triggers pH reduction, mitigating mZVI passivation and optimizing its reactivity. This innovative approach highlights the effectiveness of MoS2-modified microscale ZVI as an environmentally friendly solution for enhancing TCE dechlorination processes.

Relationship Between Sulfidated Nano Zero Valent Iron and a Reductive Dechlorinating Microbial Culture - Synergistic or Antagonistic?

Sulfidated nano zerovalent iron (S-nZVI) has garnered significant attention from researchers due to its potential for effective in-situ remediation applications. Compared to bare nZVI, sulfidation process enhances its reactivity towards chlorinated volatile organic compounds (cVOCs) and improves its longevity (Nunez Garcia et al. 2021). Stabilizing the particles with a polymer, like carboxymethyl cellulose (CMC), can further improve the performance of S-nZVI by imparting higher stability, less toxicity towards microbial cells, and a potential biostimulatory effect, making CMC-S-nZVI a promising in-situ remediation technology (Nunez Garcia et al. 2021). Recently, CMC-S-nZVI has also been applied for field-scale remediation (Nunez Garcia et al. 2020;Brumovský et al. 2021). The contaminated sites usually have multiple pollutants and not all can be degraded by CMC-S-nZVI, thus, leaving some recalcitrant cVOCs untreated (Zhang et al. 2021). Biodegradation of cVOCs by dechlorinating microbial cultures may generate highly toxic intermediates like vinyl chloride (Kocur et al. 2016). However, coupling the two treatments may be able to compensate for each other’s drawbacks, resulting in higher efficiency, longer effectiveness, non-accumulation of intermediates, and degradation of a wider range of target contaminants. However, interacting effects of CMC-S-nZVI on dechlorinating microbial cultures have not been studied yet. This research investigates the potential of combining CMC-S-nZVI and a reductive dechlorinating microbial culture (KB-1) to degrade trichloroethylene (TCE) and 1,2-dichloroethane (1,2-DCA). CMC-S-nZVI was synthesized by a two-step method: (1) CMC-nZVI was first synthesized by reducing ferrous sulfate-CMC solution with dropwise addition of sodium borohydride solution with continuous mixing and (2) then sodium dithionite solution was added as a sulfidation agent to the freshly-synthesized CMC-nZVI (Nunez Garcia et al. 2020). Effects of different sulfur-iron ratios (S/Fe), iron, and CMC concentrations on TCE degradation were studied to obtain an effective CMC-S-nZVI formulation. Results showed a successful TCE removal by the CMC-S-nZVI but 1,2-DCA was not degraded. TCE degradation by CMC-S-nZVI fitted the first-order kinetic model, with the highest degradation rate constant (0.35 h-1) achieved at S/Fe = 0.1 with iron and CMC concentrations of 1 gL-1 and 0.4 wt%, respectively. This CMC-S-nZVI formulation was further tested to examine its interaction with KB-1 in terms of cVOCs dechlorination and microbial population responses. A four-day aged CMC-S-nZVI was also tested to study the effect of aging. Degradation pathways for TCE and 1,2-DCA were proposed, based on the formation of degradation products. For the coupled treatment, an increase in microbial abundance was observed by quantifying DNA concentrations. This demonstrated a synergistic relationship between CMC-S-nZVI and KB-1. Unlike the CMC-S-nZVI only treatment, microcosms containing both CMC-S-nZVI and KB-1 were found to successfully degrade the 1,2-DCA. The coupled treatment degraded TCE and 1,2-DCA at faster rates and generated lesser amounts of vinyl chloride than the KB-1 only treatment, confirming the biostimulatory effect of CMC-S-nZVI. In the KB-1 only treatment with CMC as the sole carbon and energy source, TCE and 1,2-DCA were successfully dechlorinated. Transmission electron microscopy illustrated that CMC-S-nZVI particles were attached to the microbes but did not penetrate the bacterial cells. In summary, synergistic abiotic-biotic dechlorination of TCE and 1,2-DCA was achieved by the combined treatment of CMC-S-nZVI and KB-1, suggesting that multi-contaminant sites can benefit from this approach. Additionally, the four-day aged CMC-S-nZVI performed similar to the freshly-synthesized one, demonstrating that the field-scale remediation can have a more feasible time scale for the preparation and application of these amendments.

A multiple Kirkendall strategy for converting nanosized zero-valent iron to highly active Fenton-like catalyst for organics degradation

Nanosized zero-valent iron (nZVI) is a promising persulfate (PS) activator, however, its structurally dense oxide shell seriously inhibited electrons transfer for O-O bond cleavage of PS. Herein, we introduced sulfidation and phosphorus-doped biochar for breaking the pristine oxide shell with formation of FeS and FePO4-containing mixed shell. In this case, the faster diffusion rate of iron atoms compared to shell components triggered multiple Kirkendall effects, causing inward fluxion of vacancies with further coalescing into radial nanocracks. Exemplified by trichloroethylene (TCE) removal, such a unique "lemon-slice-like" nanocrack structure favored fast outward transfer of electrons and ferrous ions across the mixed shell to PS activation for high-efficient generation and utilization of reactive species, as evidenced by effective dechlorination (90.6%) and mineralization (85.4%) of TCE. [Formula: see text] contributed most to TCE decomposition, moreover, the SnZVI@PBC gradually became electron-deficient and thus extracted electrons from TCE with achieving nonradical-based degradation. Compared to nZVI/PS process, the SnZVI@PBC/PS system could significantly reduce catalyst dosage (87.5%) and PS amount (68.8%) to achieve nearly complete TCE degradation, and was anti-interference, stable, and pH-universal. This study advanced mechanistic understandings of multiple Kirkendall effects-triggered nanocrack formation on nZVI with corresponding rational design of Fenton-like catalysts for organics degradation.

Syntrophic Interactions Ameliorate Arsenic Inhibition of Solvent-Dechlorinating Dehalococcoides mccartyi.

Interactions and nutrient exchanges among members of microbial communities are important for understanding functional relationships in environmental microbiology. We can begin to elucidate the nature of these complex systems by taking a bottom-up approach utilizing simplified, but representative, community members. Here, we assess the effects of a toxic stress event, the addition of arsenite (As(III)), on a syntrophic co-culture containing lactate-fermenting Desulfovibrio vulgaris Hildenborough and solvent-dechlorinating Dehalococcoides mccartyi strain 195. Arsenic and trichloroethene (TCE) are two highly prevalent groundwater contaminants in the United States, and the presence of bioavailable arsenic is of particular concern at remediation sites in which reductive dechlorination has been employed. While we previously showed that low concentrations of arsenite (As(III)) inhibit the keystone TCE-reducing microorganism, D. mccartyi, this study reports the utilization of physiological analysis, transcriptomics, and metabolomics to assess the effects of arsenic on the metabolisms, gene expression, and nutrient exchanges in the described co-culture. It was found that the presence of D. vulgaris ameliorated arsenic stress on D. mccartyi, improving TCE dechlorination under arsenic-contaminated conditions. Nutrient and amino acid export by D. vulgaris may be a stress-ameliorating exchange in this syntrophic co-culture under arsenic stress, based on upregulation of transporters and increased extracellular nutrients like sarcosine and ornithine. These results broaden our knowledge of microbial community interactions and will support the further development and implementation of robust bioremediation strategies at multi-contaminant sites.

Reductive dechlorination of trichloroethene by sulfided microscale zero-valent iron in fresh and saline groundwater: Reactivity, pathways, and selectivity

S/mZVI is a promising material for groundwater remediation due to its excellent properties. However, the reactivity and electron selectivity toward target contaminant are critical. Thus, this study investigated the effect of complex groundwater chemistries (Milli-Q water, fresh groundwater and saline groundwater) on the reactivity of S/mZVI toward trichloroethylene (TCE), dechlorination pathway, hydrogen evolution kinetic, electron efficiency and aging behaviors. Results showed that sulfidation appreciably increased the reactivity and electron selectivity. The major degradation product of TCE dechlorination by S/mZVI was acetylene, which was consistent with TCE dechlorination by β-elimination. Moreover, reductive β-elimination was still the dominant dechlorination pathway for the application of S/mZVI in three groundwater conditions. However, the rates and the quantities of major products from TCE degradation varied significantly. S/mZVI in saline groundwater can maintain the reactivity towardTCE due to the protection of Fe0 by Fe3O4 deposited on the surface. Thus, the higher TCE removal efficiency and less hydrogen accumulation resulted in the greatest electron efficiency (4.3–79.2%). Overall, S/mZVI was more effective for the application in saline groundwater. This study proved insight into the comprehensive evaluation and implications for the application of S/mZVI based technologies in saline contaminated groundwater.

Soil organic matter and nutrient availability affect the applicability of low-carbon energy source in Dehalococcoides-augmented soil

Dehalococcoides is a functional microorganism that completely dechlorinates trichloroethene (TCE). Augmentation with pure Dehalococcoides is important for reducing environmental disturbances that accompany bioaugmentation. However, the applicability of Dehalococcoides-bioaugmentation to contaminated soils is unclear. In this study, seven low-carbon energy sources (methanol, formate, acetate, ethanol, lactate, citrate, and benzoate) were used as electron donors for Dehalococcides to evaluate its applicability in remediating TCE-contaminated soils. Soil microcosms supplemented with ethanol, formate, or lactate showed relatively high dechlorination activity within 111–180 days. The functional gene profiles predicted by PICRUSt2 from 16 S rRNA gene sequences were similar in the proportions of dehydrogenases, which initiate electron donor oxidation, in all soils and did not seem to reflect Dehalococcoides-bioaugmentation applicability. Soils with higher organic matter content (>3.2%; dry weight base) and protein concentration (>1.6 µg/mL) supported complete dechlorination. These results suggest that organic matter and nutrient availability mainly affect successful TCE dechlorination in Dehalococcoides-augmented soils. The study offers significant experimental support for comprehending the suitability of low-carbon energy sources in successful bioaugmentation, aiming to mitigate environmental disturbances associated with the process.