pH control for enhanced reductive bioremediation of chlorinated solvent source zones

Primary to post-depositional microbial controls on the stable and clumped isotope record of shoreline sediments at Fayetteville Green Lake

Lacustrine carbonates are a powerful archive of paleoenvironmental information but are susceptible to post-depositional alteration. Microbial metabolisms can drive such alteration by changing carbonate saturation in situ, thereby driving dissolution or precipitation. The net impact these microbial processes have on the primary δ18O, δ13C, and Δ47 values of lacustrine carbonate is not fully known. We studied the evolution of microbial community structure and the porewater and sediment geochemistry in the upper ~30 cm of sediment from two shoreline sites at Green Lake, Fayetteville, NY over 2 years of seasonal sampling. We linked seasonal and depth-based changes of porewater carbonate chemistry to microbial community composition, in situ carbon cycling (using δ13C values of carbonate, dissolved inorganic carbon (DIC), and organic matter), and dominant allochems and facies. We interpret that microbial processes are a dominant control on carbon cycling within the sediment, affecting porewater DIC, aqueous carbon chemistry, and carbonate carbon and clumped isotope geochemistry. Across all seasons and sites, microbial organic matter remineralization lowers the δ13C of the porewater DIC. Elevated carbonate saturation states in the sediment porewaters (Ω > 3) were attributed to microbes from groups capable of sulfate reduction, which were abundant in the sediment below 5 cm depth. The nearshore carbonate sediments at Green Lake are mainly composed of microbialite intraclasts/oncoids, charophytes, larger calcite crystals, and authigenic micrite-each with a different origin. Authigenic micrite is interpreted to have precipitated in situ from the supersaturated porewaters from microbial metabolism. The stable carbon isotope values (δ13Ccarb) and clumped isotope values (Δ47) of bulk carbonate sediments from the same depth horizons and site varied depending on both the sampling season and the specific location within a site, indicating localized (μm to mm) controls on carbon and clumped isotope values. Our results suggest that biological processes are a dominant control on carbon chemistry within the sedimentary subsurface of the shorelines of Green Lake, from actively forming microbialites to pore space organic matter remineralization and micrite authigenesis. A combination of biological activity, hydrologic balance, and allochem composition of the sediments set the stable carbon, oxygen, and clumped isotope signals preserved by the Green Lake carbonate sediments.

Acetylene Tunes Microbial Growth During Aerobic Cometabolism of Trichloroethene

Microbial aerobic cometabolism is a possible treatment approach for large, dilute trichloroethene (TCE) plumes at groundwater contaminated sites. Rapid microbial growth and bioclogging pose a persistent problem in bioremediation schemes. Bioclogging reduces soil porosity and permeability, which negatively affects substrate distribution and contaminant treatment efficacy while also increasing the operation and maintenance costs of bioremediation. In this study, we evaluated the ability of acetylene, an oxygenase enzyme-specific inhibitor, to decrease biomass production while maintaining aerobic TCE cometabolism capacity upon removal of acetylene. We first exposed propane-metabolizing cultures (pure and mixed) to 5% acetylene (v v-1) for 1, 2, 4, and 8 d and we then verified TCE aerobic cometabolic activity. Exposure to acetylene overall decreased biomass production and TCE degradation rates while retaining the TCE degradation capacity. In the mixed culture, exposure to acetylene for 1-8 d showed minimal effects on the composition and relative abundance of TCE cometabolizing bacterial taxa. TCE aerobic cometabolism and incubation conditions exerted more notable effects on microbial ecology than did acetylene. Acetylene appears to be a viable approach to control biomass production that may lessen the likelihood of bioclogging during TCE cometabolism. The findings from this study may lead to advancements in aerobic cometabolism remediation technologies for dilute plumes.

Efficient bioremediation of indigo-dye contaminated textile wastewater using native microorganisms and combined bioaugmentation-biostimulation techniques

In this work, the bioremediation of wastewater from the textile industry with indigo dye content was carried out using combined bioaugmentation, bioventilation, and biostimulation techniques. Initially, the inoculum was prepared by isolating the microorganisms from the textile wastewater in a 2 L bioreactor. Then, the respirometry technique was implemented to determine the affinity of the microorganisms and the substrate by measuring CO2 and allowed the formulation of an empirical mathematical model for the growth kinetics of the microorganism. Finally, the bioremediation was carried out in a 3 L bioreactor obtaining an indigo dye removal efficiency of 20.7 ± 1.2%, 24.0 ± 1.5%, and 29.7 ± 1.1% for equivalent wavelengths of 436 nm, 525 nm, and 620 nm. The chemical oxygen demand showed an average reduction of 88.9 ± 2.5%, going from 470.7 ± 15.6 to 52.3 ± 10.7 ppm after 30 days under constant agitation and aeration. A negative generalized exponential model was fitted to assess the affinity of the microorganism with the wastewater as a substrate by evaluating the production of CO2 during the bioremediation. Bioremediation techniques improve water discharge parameters compared to chemical treatments implemented in the industry, reducing the use of substances that can generate secondary pollution. Bioaugmentation, biostimulation, and bioventing of the textile wastewater in this study demonstrate the potential of these combined techniques to serve as an efficient alternative for indigo-contaminated wastewater in the textile industry.

Carbonate Minerals and Dissimilatory Iron-Reducing Organisms Trigger Synergistic Abiotic and Biotic Chain Reactions under Elevated CO2 Concentration

Increasing CO₂ emission has resulted in pressing climate and environmental issues. While abiotic and biotic processes mediating the fate of CO₂ have been studied separately, their interactions and combined effects have been poorly understood. To explore this knowledge gap, an iron-reducing organism, Orenia metallireducens, was cultured under 18 conditions that systematically varied in headspace CO₂ concentrations, ferric oxide loading, and dolomite (CaMg(CO₃)₂) availability. The results showed that abiotic and biotic processes interactively mediate CO₂ acidification and sequestration through "chain reactions", with pH being the dominant variable. Specifically, dolomite alleviated CO₂ stress on microbial activity, possibly via pH control that transforms the inhibitory CO₂ to the more benign bicarbonate species. The microbial iron reduction further impacted pH via the competition between proton (H⁺) consumption during iron reduction and H⁺ generation from oxidization of the organic substrate. Under Fe(III)-rich conditions, microbial iron reduction increased pH, driving dissolved CO₂ to form bicarbonate. Spectroscopic and microscopic analyses showed enhanced formation of siderite (FeCO₃) under elevated CO₂, supporting its incorporation into solids. The results of these CO₂-microbe-mineral experiments provide insights into the synergistic abiotic and biotic processes that alleviate CO₂ acidification and favor its sequestration, which can be instructive for practical applications (e.g., acidification remediation, CO₂ sequestration, and modeling of carbon flux).

The importance of proper pH adjustment and control to achieve complete in situ enhanced reductive dechlorination

In situ bioremediation of chlorinated compounds such as perchloroethylene (PCE) and trichloroethylene (TCE) through enhanced reductive dechlorination (ERD) requires appropriate growth conditions for organohalide-respiring bacteria (OHRB). One of the most important factors controlling OHRB metabolism is groundwater pH. Dehalococcoides spp. (DHC) growth may be inhibited when pH is lower than 6.0, which can lead to the accumulation of toxic daughter compounds including cis-dichloroethylene (cDCE) and vinyl chloride (VC). Aquifer pH may decline as HCl is released during reductive dechlorination and from substrate fermentation to fatty acids and carbonic acid. In this article, we demonstrate that using proper pH adjustment and control in situ is an appropriate strategy to achieve complete ERD (i.e., complete conversion of PCE and TCE to nontoxic ethylene) in remediation sites with inherently low pH values and/or low buffering capacity. To analyze the effectiveness of this approach, field monitoring results are presented for a challenging site containing high concentrations of PCE and TCE (>10 000 µg/L and >1000 µg/L, respectively) and low aquifer pH (~4.9). Addition of a bioaugmentation culture, emulsified vegetable oil (EVO), and a colloidal buffer (CoBupH^TM ) to increase pH, stimulated rapid conversion of PCE and TCE to cDCE and VC. However, further conversion of cDCE and VC was very limited. To stimulate complete conversion to ethylene, additional CoBupH^TM and nutrients were injected, resulting in a rapid increase in metabolic rates, and maintained the aquifer pH at ~6.5 for more than five years, thus demonstrating that complete ERD can be achieved in sites with similar characteristics. Proper pH adjustment and control is needed to limit the accumulation of toxic intermediates, maintaining in situ bioremediation as an efficient, affordable, and environmentally friendly option to treat chlorinated compounds. Integr Environ Assess Manag 2022;00:1-6. © 2022 SETAC.

Organic carbon metabolism is a main determinant of hydrogen demand and dynamics in anaerobic soils

Hydrogen (H₂) is a crucial electron donor for many processes in the environment including nitrate-, sulfate- and, iron-reduction, homoacetogenesis, and methanogenesis, and is a major determinant of microbial competition and metabolic pathways in groundwater, sediments, and soils. Despite the importance of H₂ for many microbial processes in the environment, the total H₂ consuming capacity (or H₂ demand) of soils is generally unknown. Using soil microcosms with added H₂, the aims of this study were 1) to measure the H₂ demand of geochemically diverse soils and 2) to define the processes leading to this demand. Study results documented a large range of H₂ demand in soil (0.034-1.2 millielectron equivalents H₂ g^-1 soil). The measured H₂ demand greatly exceeded the theoretical demand predicted based on measured concentrations of common electron acceptors initially present in a library of 15 soils. While methanogenesis accounted for the largest fraction of H₂ demand, humic acid reduction and acetogenesis were also significant contributing H₂-consuming processes. Much of the H₂ demand could be attributed to CO₂ produced during incubation from fermentation and/or acetoclastic methanogenesis. The soil initial total organic carbon showed the strongest correlation to H₂ demand. Besides external additions, H₂ was likely generated or cycled in the microcosms. Apart from fermentative H₂ production, carboxylate elongation to produce C4-C7 fatty acids may have accounted for additional H₂ production in these soils. Many of these processes, especially the organic carbon contribution is underestimated in microbial models for H₂ consumption in natural soil ecosystems or during bioremediation of contaminants in soils.

Assessment of the groundwater quality, physicochemical composition, and human and ecological health risks in a coastal metropolitan: A case study of a residential estate in Lagos, Nigeria

Water is essential for humankind's existence, providing food security, amongst others, as well as promoting industrial and economic development. The physicochemical composition, potentially toxic metals, water quality index, human health, and ecological risks of groundwater in a residential estate in Lagos, Nigeria, were studied to assess their possibility for drinking and domestic purposes. Groundwater samples were collected from twelve designated sites and analysed using standard methods. TDS and pH values of 5.4 ± 1.7-20.8 ± 0.84 and 5.08 ± 0.26-5.56 ± 0.57, respectively, were recorded for the groundwater samples whereas DO and BOD were detected at 4.2 ± 1.2-5.45 ± 0.52 mg O₂/L and 2.6 ± 1.2-24.6 ± 5.7 mg/L, respectively. Additionally, water conductivity ranged from zero to 0.60 ± 0.89 µS/cm. Of the seven PTMs (Pb, Cd, Zn, Cu, Cr, Ni, & Fe) analysed, Pb was present, over the standard limit, in all the samples whereas Fe was detected below the limit in over 83.3% of the groundwater sampled. The hazard index (HI) indicated that 83.3% and 100% of the samples were within the acceptable limits, with no health risks, for the children and adult groups, respectively. WQI showed that 75.0% of the samples was of good quality and suitable for domestic purposes, with low ecological and carcinogenic health risks. The results show that the groundwater samples are good for domestic purposes but require pre-treatment to make them safe for drinking.

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

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

Hydrogeochemical characteristics of a multi-layered coastal aquifer system in the Mekong Delta, Vietnam

Groundwater is a primary freshwater source for various domestic, industrial and agricultural purposes, especially in coastal regions where there are lacking surface water supply. However, groundwater quality in coastal regions is often threatened by seawater intrusion and contamination due to both anthropogenic activities and natural processes. Therefore, insights into groundwater geochemistry and occurrences are necessary for sustainable groundwater management in coastal regions. The main aim of this study is to investigate the hydrogeochemical characteristics and their influencing factors in a coastal area of the Mekong Delta, Vietnam (MD). A total of 286 groundwater samples were taken from shallow and deep aquifers for analyzing major ions and stable isotopes. The results show that deep groundwater is dominated by Ca-HCO[Formula: see text], Ca-Na-HCO[Formula: see text], Ca-Mg-Cl, and Na-HCO[Formula: see text] while shallow groundwater is dominated by the Na-Cl water type. In this region, the main geochemical processes controlling groundwater chemistry are ion exchanges, mineralization and evaporation. Groundwater salinization in coastal aquifers of the Mekong Delta is caused by (1) paleo-seawater intrusion and evaporation occurring in the Holocene and Pleistocene aquifers, (2) dissolution of salt sediment/rock and leakage of saline from upper to lower aquifers due to excessive groundwater exploitation and hydraulic connection. High nitrate concentrations in both shallow and deep aquifers are related to human activities. These results imply that groundwater extraction may exacerbate groundwater quality-related problems and suitable solutions for sustainable groundwater management in the coastal area of the Mekong Delta are needed.

Transport and release of electron donors and alkalinity during reductive dechlorination by combined emulsified vegetable oil and colloidal Mg(OH)2: Laboratory sand column and microcosm tests

Emulsified vegetable oil combined with colloidal Mg(OH)₂ (EVO-CM) can slowly release electron donors and OH^- into groundwater and is therefore regarded as a promising amendment for enhanced in situ treatment of chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE). However, its migration ability in different porous media and the simultaneous release of electron donor and pH buffer during enhanced reductive dechlorination (ERD) have never been evaluated in detail. In this study, EVO-CM with uniform drop size and desirable stability was prepared. Laboratory-scale column tests were conducted to investigate the transport and spatial distribution of the EVO-CM in different porous media. Batch microcosm experiments were carried out to study the dechlorination efficiency under different EVO:Mg(OH)₂ ratios. Experimental results indicate that prepared EVO-CM emulsions can be transported effectively through different porous media with grain size ranging from 0.1-1.0 mm. The emulsified vegetable oil (EVO) and colloidal Mg(OH)₂ showed synchronous movement through the porous media column systems, providing both electron donor and alkalinity at the same location. The retention degree of EVO-CM is greater in the finer grained media and decreases with increasing distance from the inlet. The injection of EVO-CM lead to a significant mobilization of TCE in the column. The reductive dechlorination of TCE in the microcosms was remarkably enhanced in the presence of EVO-CM. The coaddition of sufficient colloidal Mg(OH)₂ effectively limits the deleterious pH decline caused by acid release. The microcosm achieves an optimum dechlorination efficiency when the EVO:Mg(OH)₂ ratio is 1:1.

Monitoring well utility in a heterogeneous DNAPL source zone area: Insights from proximal multilevel sampler wells and sampling capture-zone modelling

Groundwater-quality assessment at contaminated sites often involves the use of short-screen (1.5 to 3 m) monitoring wells. However, even over these intervals considerable variation may occur in contaminant concentrations in groundwater adjacent to the well screen. This is especially true in heterogeneous dense non-aqueous phase liquid (DNAPL) source zones, where cm-scale contamination variability may call into question the effectiveness of monitoring wells to deliver representative data. The utility of monitoring wells in such settings is evaluated by reference to high-resolution multilevel sampler (MLS) wells located proximally to short-screen wells, together with sampling capture-zone modelling to explore controls upon well sample provenance and sensitivity to monitoring protocols. Field data are analysed from the highly instrumented SABRE research site that contained an old trichloroethene source zone within a shallow alluvial aquifer at a UK industrial facility. With increased purging, monitoring-well samples tend to a flow-weighted average concentration but may exhibit sensitivity to the implemented protocol and degree of purging. Formation heterogeneity adjacent to the well-screen particularly, alongside pump-intake position and water level, influence this sensitivity. Purging of low volumes is vulnerable to poor reproducibility arising from concentration variability predicted over the initial 1 to 2 screen volumes purged. Marked heterogeneity may also result in limited long-term sample concentration stabilization. Development of bespoke monitoring protocols, that consider screen volumes purged, alongside water-quality indicator parameter stabilization, is recommended to validate and reduce uncertainty when interpreting monitoring-well data within source zone areas. Generalised recommendations on monitoring well based protocols are also developed. A key monitoring well utility is their proportionately greater sample draw from permeable horizons constituting a significant contaminant flux pathway and hence representative fraction of source mass flux. Acquisition of complementary, high-resolution, site monitoring data, however, vitally underpins optimal interpretation of monitoring-well datasets and appropriate advancement of a site conceptual model and remedial implementation.

Enhanced reductive de-chlorination of a solvent contaminated aquifer through addition and apparent fermentation of cyclodextrin

In a field study, aqueous cyclodextrin (CD) was investigated for its ability to extract chlorinated volatile organic compounds (cVOC), such as trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and dichloroethene (DCE) through in-situ flushing of a sandy aquifer. After cessation of aquifer flushing, a plume of CD was left. Changes in CD, cVOC, and inorganic terminal electron acceptors (TEAs) (DO, nitrate, sulfate, iron) were monitored in four rounds of wellwater sampling (20, 210, 342, and 425days after cessation of active pumping). Post-CD flushing VOC levels rebounded (850% for TCE, 190% for TCA, and 53% for DCE) between the first two sampling rounds, apparently due to rate-limited desorption from aquifer media and dissolution from remaining NAPL. However, substantial reduction in the mass of TCE (6.3 to 0.11mol: 98%) and TCA (2.8 to 0.73mol: 74%) in groundwater was observed between 210 and 425days. DCE should primarily be produced from the degradation of TCE and is expected to subsequently degrade to chloroethene. Since DCE levels decreased only slightly (0.23 to 0.17mol: 26%), its degradation rate should be similar to that produced from the decaying TCE. Cyclodextrin was monitored starting from day 210. The mass of residual CD (as measured by Total Organic Carbon) decreased from 150mol (day 210) to 66 (day 425) (56% decrease). The naturally anaerobic zone within the aquifer where residual CD mass decreased coincided with a loss of other major potential TEAs: nitrate (97% loss), sulfate (31%) and iron (31%). In other studies, TCE and 1,1,1-TCA have been found to be more energetically favorable TEAs than sulfate and iron and their degradation via reductive dechlorination has been found to be enhanced by the fermentation of carbohydrates. Such processes can explain these observations, but more investigation is needed to evaluate whether residual levels of CD can facilitate the anaerobic degradation of chlorinated VOCs.

Bacterial Community Dynamics in Dichloromethane-Contaminated Groundwater Undergoing Natural Attenuation

The uncontrolled release of the industrial solvent methylene chloride, also known as dichloromethane (DCM), has resulted in widespread groundwater contamination in the United States. Here we investigate the role of groundwater bacterial communities in the natural attenuation of DCM at an undisclosed manufacturing site in New Jersey. This study investigates the bacterial community structure of groundwater samples differentially contaminated with DCM to better understand the biodegradation potential of these autochthonous bacterial communities. Bacterial community analysis was completed using high-throughput sequencing of the 16S rRNA gene of groundwater samples (n = 26) with DCM contamination ranging from 0.89 to 9,800,000 μg/L. Significant DCM concentration-driven shifts in overall bacterial community structure were identified between samples, including an increase in the abundance of Firmicutes within the most contaminated samples. Across all samples, a total of 6,134 unique operational taxonomic units (OTUs) were identified, with 16 taxa having strong correlations with increased DCM concentration. Putative DCM degraders such as Pseudomonas, Dehalobacterium and Desulfovibrio were present within groundwater across all levels of DCM contamination. Interestingly, each of these taxa dominated specific DCM contamination ranges respectively. Potential DCM degrading lineages yet to be cited specifically as a DCM degrading organisms, such as the Desulfosporosinus, thrived within the most heavily contaminated groundwater samples. Co-occurrence network analysis revealed aerobic and anaerobic bacterial taxa with DCM-degrading potential were present at the study site. Our 16S rRNA gene survey serves as the first in situ bacterial community assessment of contaminated groundwater harboring DCM concentrations ranging over seven orders of magnitude. Diversity analyses revealed known as well as potentially novel DCM degrading taxa within defined DCM concentration ranges, indicating niche-specific responses of these autochthonous populations. Altogether, our findings suggest that monitored natural attenuation is an appropriate remediation strategy for DCM contamination, and that high-throughput sequencing technologies are a robust method for assessing the potential role of biodegrading bacterial assemblages in the apparent reduction of DCM concentrations in environmental scenarios.

Organohalide Respiration with Chlorinated Ethenes under Low pH Conditions

Bioremediation at chlorinated solvent sites often leads to groundwater acidification due to electron donor fermentation and enhanced dechlorination activity. The microbial reductive dechlorination process is robust at circumneutral pH, but activity declines at groundwater pH values below 6.0. Consistent with this observation, the activity of tetrachloroethene (PCE) dechlorinating cultures declined at pH 6.0 and was not sustained in pH 5.5 medium, with one notable exception. Sulfurospirillum multivorans dechlorinated PCE to cis-1,2-dichloroethene (cDCE) in pH 5.5 medium and maintained this activity upon repeated transfers. Microcosms established with soil and aquifer materials from five distinct locations dechlorinated PCE-to-ethene at pH 5.5 and pH 7.2. Dechlorination to ethene was maintained following repeated transfers at pH 7.2, but no ethene was produced at pH 5.5, and only the transfer cultures derived from the Axton Cross Superfund (ACS) microcosms sustained PCE dechlorination to cDCE as a final product. 16S rRNA gene amplicon sequencing of pH 7.2 and pH 5.5 ACS enrichments revealed distinct microbial communities, with the dominant dechlorinator being Dehalococcoides in pH 7.2 and Sulfurospirillum in pH 5.5 cultures. PCE-to-trichloroethene- (TCE-) and PCE-to-cDCE-dechlorinating isolates obtained from the ACS pH 5.5 enrichment shared 98.6%, and 98.5% 16S rRNA gene sequence similarities to Sulfurospirillum multivorans. These findings imply that sustained Dehalococcoides activity cannot be expected in low pH (i.e., ≤ 5.5) groundwater, and organohalide-respiring Sulfurospirillum spp. are key contributors to in situ PCE reductive dechlorination under low pH conditions.

Microbial degradation of chloroethenes: a review

Contamination by chloroethenes has a severe negative effect on both the environment and human health. This has prompted intensive remediation activity in recent years, along with research into the efficacy of natural microbial communities for degrading toxic chloroethenes into less harmful compounds. Microbial degradation of chloroethenes can take place either through anaerobic organohalide respiration, where chloroethenes serve as electron acceptors; anaerobic and aerobic metabolic degradation, where chloroethenes are used as electron donors; or anaerobic and aerobic co-metabolic degradation, with chloroethene degradation occurring as a by-product during microbial metabolism of other growth substrates, without energy or carbon benefit. Recent research has focused on optimising these natural processes to serve as effective bioremediation technologies, with particular emphasis on (a) the diversity and role of bacterial groups involved in dechlorination microbial processes, and (b) detection of bacterial enzymes and genes connected with dehalogenation activity. In this review, we summarise the different mechanisms of chloroethene bacterial degradation suitable for bioremediation and provide a list of dechlorinating bacteria. We also provide an up-to-date summary of primers available for detecting functional genes in anaerobic and aerobic bacteria degrading chloroethenes metabolically or co-metabolically.

Survival of Vinyl Chloride Respiring Dehalococcoides mccartyi under Long-Term Electron Donor Limitation

In anoxic groundwater aquifers, the long-term survival of Dehalococcoides mccartyi populations expressing the gene vcrA (or bvcA) encoding reductive vinyl chloride dehalogenases are important to achieve complete dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) to nonchlorinated ethene. The absence or inactivity of vcrA-containing Dehalococcoides results in the accumulation of the harmful chlorinated intermediates dichloroethene (DCE) and vinyl chloride (VC). Although vcrA-containing Dehalococcoides subpopulations depend on synergistic interaction with other organohalide-respiring populations generating their metabolic electron acceptors (DCE and VC), their survival requires successful competition for electron donor within the entire organohalide-respiring microbial community. To understand this dualism of synergy and competition under growth conditions relevant in contaminated aquifers, we investigated Dehalococcoides-level population structure when subjected to a change in the ratio of electron donor to chlorinated electron acceptor in continuously stirred tank reactors (CSTRs) operated over 7 years. When the electron donor formate was supplied in stoichiometric excess to TCE, both tceA-containing and vcrA-containing Dehalococcoides populations persisted, and near-complete dechlorination to ethene was stably maintained. When the electron donor formate was supplied at substoichiometric concentrations, the interactions between tceA-containing and vcrA-containing populations shifted toward direct competition for the same limiting catabolic electron donor substrate with subsequent niche exclusion of the vcrA-containing population. After more than 2000 days of operation under electron donor limitation, increasing the electron donor to TCE ratio facilitated a recovery of the vcrA-containing Dehalococoides population to its original frequency. We demonstrate that electron donor scarcity alone, in the absence of competing metabolic processes or inhibitory dechlorination intermediate products, is sufficient to alter the Dehalococcoides population structure. These results underscore the importance of electron donor and chloroethene stoichiometry in maintaining balanced functional performance within consortia composed of multiple D. mccartyi subpopulations, even when other competing electron acceptor processes are absent.

A Data Mining Approach to Predict In Situ Detoxification Potential of Chlorinated Ethenes

Despite advances in physicochemical remediation technologies, in situ bioremediation treatment based on Dehalococcoides mccartyi (Dhc) reductive dechlorination activity remains a cornerstone approach to remedy sites impacted with chlorinated ethenes. Selecting the best remedial strategy is challenging due to uncertainties and complexity associated with biological and geochemical factors influencing Dhc activity. Guidelines based on measurable biogeochemical parameters have been proposed, but contemporary efforts fall short of meaningfully integrating the available information. Extensive groundwater monitoring data sets have been collected for decades, but have not been systematically analyzed and used for developing tools to guide decision-making. In the present study, geochemical and microbial data sets collected from 35 wells at five contaminated sites were used to demonstrate that a data mining prediction model using the classification and regression tree (CART) algorithm can provide improved predictive understanding of a site's reductive dechlorination potential. The CART model successfully predicted the 3-month-ahead reductive dechlorination potential with 75.8% and 69.5% true positive rate (i.e., sensitivity) for the training set and the test set, respectively. The machine learning algorithm ranked parameters by relative importance for assessing in situ reductive dechlorination potential. The abundance of Dhc 16S rRNA genes, CH4, Fe(2+), NO3(-), NO2(-), and SO4(2-) concentrations, total organic carbon (TOC) amounts, and oxidation-reduction potential (ORP) displayed significant correlations (p < 0.01) with dechlorination potential, with NO3(-), NO2(-), and Fe(2+) concentrations exhibiting precedence over other parameters. Contrary to prior efforts, the power of data mining approaches lies in the ability to discern synergetic effects between multiple parameters that affect reductive dechlorination activity. Overall, these findings demonstrate that data mining techniques (e.g., machine learning algorithms) effectively utilize groundwater monitoring data to derive predictive understanding of contaminant degradation, and thus have great potential for improving decision-making tools. A major need for realizing the predictive capabilities of data mining approaches is a curated, open-access, up-to-date and comprehensive collection of biogeochemical groundwater monitoring data.

Evidence of α-, β- and γ-HCH mixture aerobic degradation by the native actinobacteria Streptomyces sp. M7

The organochlorine insecticide γ-hexachlorocyclohexane (γ-HCH, lindane) and its non-insecticidal α- and β-isomers continue to pose serious environmental and health concerns, although their use has been restricted or completely banned for decades. In this study we report the first evidence of the growth ability of a Streptomyces strain in a mineral salt medium containing high doses of α- and β-HCH (16.6 mg l(-1)) as a carbon source. Degradation of HCH isomers by Streptomyces sp. M7 was investigated after 1, 4, and 7 days of incubation, determining chloride ion release, and residues in the supernatants by GC with µECD detection. The results show that both the α- and β-HCH isomers were effectively metabolized by Streptomyces sp. M7, with 80 and 78 % degradation respectively, after 7 days of incubation. Moreover, pentachlorocyclohexenes and tetrachlorocyclohexenes were detected as metabolites. In addition, the formation of possible persistent compounds such as chlorobenzenes and chlorophenols were studied by GC-MS, while no phenolic compounds were detected. In conclusion, we have demonstrated for the first time that Streptomyces sp. M7 can degrade α- and β-isomers individually or combined with γ-HCH and could be considered as a potential agent for bioremediation of environments contaminated by organochlorine isomers.

Heterogeneous hyporheic zone dechlorination of a TCE groundwater plume discharging to an urban river reach

The typically elevated natural attenuation capacity of riverbed-hyporheic zones is expected to decrease chlorinated hydrocarbon (CHC) groundwater plume discharges to river receptors through dechlorination reactions. The aim of this study was to assess physico-chemical processes controlling field-scale variation in riverbed-hyporheic zone dechlorination of a TCE groundwater plume discharge to an urban river reach. The 50-m long pool-riffle-glide reach of the River Tame in Birmingham (UK) studied is a heterogeneous high energy river environment. The shallow riverbed was instrumented with a detailed network of multilevel samplers. Freeze coring revealed a geologically heterogeneous and poorly sorted riverbed. A chlorine number reduction approach provided a quantitative indicator of CHC dechlorination. Three sub-reaches of contrasting behaviour were identified. Greatest dechlorination occurred in the riffle sub-reach that was characterised by hyporheic zone flows, moderate sulphate concentrations and pH, anaerobic conditions, low iron, but elevated manganese concentrations with evidence of sulphate reduction. Transient hyporheic zone flows allowing input to varying riverbed depths of organic matter are anticipated to be a key control. The glide sub-reach displayed negligible dechlorination attributed to the predominant groundwater baseflow discharge condition, absence of hyporheic zone, transition to more oxic conditions and elevated sulphate concentrations expected to locally inhibit dechlorination. The tail-of-pool-riffle sub-reach exhibited patchy dechlorination that was attributed to sub-reach complexities including significant flow bypass of a low permeability, high organic matter, silty unit of high dechlorination potential. A process-based conceptual model of reach-scale dechlorination variability was developed. Key findings of practitioner relevance were: riverbed-hyporheic zone CHC dechlorination may provide only a partial, somewhat patchy barrier to CHC groundwater plume discharges to a surface water receptor; and, monitoring requirements to assess the variability in CHC attenuation within a reach are expected to be onerous. Further research on transient hyporheic zone dechlorination is recommended.

Inhibition of iron (III) minerals and acidification on the reductive dechlorination of trichloroethylene

Reductive dechlorination of chlorinated ethenes is inhibited by acidification and by the presence of Fe (III) as a competitive electron acceptor. Synergism between both factors on dechlorination is predicted as reductive dissolution of Fe (III) minerals is facilitated by acidification. This study was set-up to assess this synergism for two common aquifer Fe (III) minerals, goethite and ferrihydrite. Anaerobic microbial dechlorination of trichloroethylene (TCE) by KB-1 culture and formate as electron donor was investigated in anaerobic batch containers at different solution pH values (6.2-7.2) in sand coated with these Fe minerals and a sand only as control. In the absence of Fe, lowering substrate pH from 7.2 to 6.2 increased the time for 90% TCE degradation from 14±1d to 42±4d. At pH 7.2, goethite did not affect TCE degradation time while ferrihydrite increased the degradation time to 19±1d compared to the no Fe control. At pH 6.2, 90% degradation was at 78±1 (ferrihydrite) or 131±1d (goethite). Ferrous iron production in ferrihydrite treatment increased between pH 7.2 and 6.5 but decreased by further lowering pH to 6.2, likely due to reduced microbial activity. This study confirms that TCE is increasingly inhibited by the combined effect of acidification and bioavailable Fe (III), however no evidence was found for synergistic inhibition since Fe reduction did not increase as pH decreases. To the best of our knowledge, this is the first study where effect of pH and Fe (III) reduction on TCE was simultaneously tested. Acid Fe-rich aquifers need sufficient buffering and alkalinity to ensure swift degradation of chlorinated ethenes.

Use of silicate minerals for long-term pH control during reductive dechlorination of high tetrachloroethene concentrations in continuous flow-through columns

The long-term buffering potential of three silicate minerals (diopside, fayalite and forsterite) present as fine particles in porous quartz sand medium was evaluated in flow-through column experiments over a period of 6.5 months. The columns were operated with PCE concentrations close to saturation and inoculated with the organohalide-respiring consortium SDC-9™, which is able to completely dechlorinate PCE to ethene at high concentrations. In the absence of pH buffering agents, fermentation and organohalide respiration drove the pH close to 6.1, leading to severe inhibition of PCE dechlorination. Forsterite and fayalite were able to maintain the pH close to 7.5 and 6.5, respectively, and to sustain the production of VC and ethene. Diopside gradually lost its buffering capacity during the first 84 days due to the formation of a low reactive leached layer but dechlorination to cis-DCE was still achieved. Among the three minerals tested, forsterite was identified as the best buffering agent. Its presence led to the best PCE removal performance and the highest relative abundance of Dehalococcoides. This study showed that forsterite and fayalite are promising sources of long-term pH buffering for in situ bioremediation of source-zone PCE.

Use of silicate minerals for pH control during reductive dechlorination of chloroethenes in batch cultures of different microbial consortia

In chloroethene-contaminated sites undergoing in situ bioremediation, groundwater acidification is a frequent problem in the source zone, and buffering strategies have to be implemented to maintain the pH in the neutral range. An alternative to conventional soluble buffers is silicate mineral particles as a long-term source of alkalinity. In previous studies, the buffering potentials of these minerals have been evaluated based on abiotic dissolution tests and geochemical modeling. In the present study, the buffering potentials of four silicate minerals (andradite, diopside, fayalite, and forsterite) were tested in batch cultures amended with tetrachloroethene (PCE) and inoculated with different organohalide-respiring consortia. Another objective of this study was to determine the influence of pH on the different steps of PCE dechlorination. The consortia showed significant differences in sensitivities toward acidic pH for the different dechlorination steps. Molecular analysis indicated that Dehalococcoides spp. that were present in all consortia were the most pH-sensitive organohalide-respiring guild members compared to Sulfurospirillum spp. and Dehalobacter spp. In batch cultures with silicate mineral particles as pH-buffering agents, all four minerals tested were able to maintain the pH in the appropriate range for reductive dechlorination of chloroethenes. However, complete dechlorination to ethene was observed only with forsterite, diopside, and fayalite. Dissolution of andradite increased the redox potential and did not allow dechlorination. With forsterite, diopside, and fayalite, dechlorination to ethene was observed but at much lower rates for the last two dechlorination steps than with the positive control. This indicated an inhibition effect of silicate minerals and/or their dissolution products on reductive dechlorination of cis-dichloroethene and vinyl chloride. Hence, despite the proven pH-buffering potential of silicate minerals, compatibility with the bacterial community involved in in situ bioremediation has to be carefully evaluated prior to their use for pH control at a specific site.

Impact of Groundwater Salinity on Bioremediation Enhanced by Micro-Nano Bubbles

Micro-nano bubbles (MNBs) technology has shown great potential in groundwater bioremediation because of their large specific surface area, negatively charged surface, long stagnation, high oxygen transfer efficiency, etc. Groundwater salinity, which varies from sites due to different geological and environmental conditions, has a strong impact on the bioremediation effect. However, the groundwater salinity effect on MNBs’ behavior has not been reported. In this study, the size distribution, oxygen transfer efficiency and zeta potential of MNBs was investigated in different salt concentrations. In addition, the permeability of MNBs’ water through sand in different salt concentrations was studied. The results showed that water salinity has no influence on bubble size distribution during MNBs generation. MNBs could greatly enhance the oxygen transfer efficiency from inner bubbles to outer water, which may greatly enhance aerobic bioremediation. However, the enhancement varied depending on salt concentration. 0.7 g/L was found to be the optimal salt concentration to transfer oxygen. Moreover, MNBs in water salinity of 0.7 g/L had the minimum zeta potential. The correlation of zeta potential and mass transfer was discussed. The hydraulic conductivities of sand were similar for MNBs water with different salt concentrations. The results suggested that salinity had a great influence on MNBs performance, and groundwater salinity should be taken into careful consideration in applying MNBs technology to the enhancement of bioremediation.

Use of emulsified vegetable oil to support bioremediation of TCE DNAPL in soil columns

The interaction between emulsified vegetable oil (EVO) and trichloroethylene (TCE) dense non-aqueous phase liquid (DNAPL) was observed using two soil columns and subsequent reductive dechlorination of TCE was monitored over a three year period. Dyed TCE DNAPL (~75 g) was emplaced in one column (DNAPL column), while the second was DNAPL-free (plume column). EVO was added to both columns and partitioning of the EVO into the TCE DNAPL was measured and quantified. TCE (1.9 mM) was added to the influent of the plume column to simulate conditions down gradient of a DNAPL source area and the columns were operated independently for more than one year, after which they were connected in series. Initially limited dechlorination of TCE to cDCE was observed in the DNAPL column, while the plume column supported complete reductive dechlorination of TCE to ethene. Upon connection and reamendment of the plume column with EVO, near saturation levels of TCE from the effluent of the DNAPL column were rapidly dechlorinated to c-DCE and VC in the plume column; however, this high rate dechlorination produced hydrochloric acid which overwhelmed the buffering capacity of the system and caused the pH to drop below 6.0. Dechlorination efficiency in the columns subsequently deteriorated, as measured by the chloride production and Dehalococcoides counts, but was restored by adding sodium bicarbonate buffer to the influent groundwater. Robust dechlorination was eventually observed in the DNAPL column, such that the TCE DNAPL was largely removed by the end of the study. Partitioning of the EVO into the DNAPL provided significant operational benefits to the remediation system both in terms of electron donor placement and longevity.

Inhibition of microbial trichloroethylene dechlorination [corrected] by Fe (III) reduction depends on Fe mineralogy: a batch study using the bioaugmentation culture KB-1

Microbial reductive dechlorination of trichloroethylene (TCE) in groundwater can be stimulated by adding of electron donors. However, side reactions such as Fe (III) reduction competes with this reaction. This study was set-up to relate the inhibition of microbial TCE dechlorination to the quantity and quality (mineralogy) of Fe (III) in the substrate and to calibrate a substrate extraction procedure for testing bioavailable Fe (III) in sediments. Batch experiments were set-up with identical inoculum (KB-1 culture) and liquid medium composition, and adding either 1) variable amounts of ferrihydrite or 2) 14 different Fe (III) minerals coated onto or mixed in with quartz sand (at constant total Fe) at a stoichiometric excess Fe (III) over electron donor. Increasing amounts of ferrihydrite significantly increased the time for complete TCE degradation from 8 days (control sand) to 28 days (excess Fe). Acid extractable Fe (II) increased and magnetite formed during incubation, confirming Fe (III) reduction. At constant total Fe in the sand, TCE dechlorination time varied with Fe mineralogy between 8 days (no Fe added) to >120 days (Fe-containing bentonite). In general, poorly crystalline Fe (III) minerals inhibited TCE dechlorination whereas crystalline Fe (III) minerals such as goethite or hematite had no effect. The TCE inhibition time was positively correlated to the Fe (II) determined after 122 days and to the surface area of the Fe (III) minerals. Only a fraction of total Fe (III) is reduced, likely because of solubility constraints and/or coating of Fe (III) minerals by Fe (II) minerals. Iron extraction tests based on Fe (III) reduction using NH2OH(.)HCl predict the competitive inhibition of TCE degradation in these model systems. This study shows that Fe mineralogy rather that total Fe content determines the competitive inhibition of TCE dechlorination.

Acidification due to microbial dechlorination near a trichloroethene DNAPL is overcome with pH buffer or formate as electron donor: experimental demonstration in diffusion-cells

Acidification due to microbial dechlorination of trichloroethene (TCE) can limit the bio-enhanced dissolution of TCE dense non-aqueous phase liquid (DNAPL). This study related the dissolution enhancement of a TCE DNAPL to the pH buffer capacity of the medium and the type of electron donor used. In batch systems, dechlorination was optimal at pH7.1-7.5, but was completely inhibited below pH6.2. In addition, dechlorination in batch systems led to a smaller pH decrease at an increasing pH buffer capacity or with the use of formate instead of lactate as electron donor. Subsequently, bio-enhanced TCE DNAPL dissolution was quantified in diffusion-cells with a 5.5 cm central sand layer, separating a TCE DNAPL layer from an aqueous top layer. Three different pH buffer capacities (2.9 mM-17.9 mM MOPS) and lactate or formate as electron donor were applied. In the lactate fed diffusion-cells, the DNAPL dissolution enhancement factor increased from 1.5 to 2.2 with an increase of the pH buffer capacity. In contrast, in the formate fed diffusion-cells, the DNAPL dissolution enhancement factor (2.4±0.3) was unaffected by the pH buffer capacity. Measurement of the pore water pH confirmed that the pH decreased less with an increased pH buffer capacity or with formate instead of lactate as electron donor. These results suggest that the significant impact of acidification on bio-enhanced DNAPL dissolution can be overcome by the amendment of a pH buffer or by applying a non acidifying electron donor like formate.

Assessing TCE source bioremediation by geostatistical analysis of a flux fence

Mass discharge across transect planes is increasingly used as a metric for performance assessment of in situ groundwater remediation systems. Mass discharge estimates using concentrations measured in multilevel transects are often made by assuming a uniform flow field, and uncertainty contributions from spatial concentration and flow field variability are often overlooked. We extend our recently developed geostatistical approach to estimate mass discharge using transect data of concentration and hydraulic conductivity, so accounting for the spatial variability of both datasets. The magnitude and uncertainty of mass discharge were quantified by conditional simulation. An important benefit of the approach is that uncertainty is quantified as an integral part of the mass discharge estimate. We use this approach for performance assessment of a bioremediation experiment of a trichloroethene (TCE) source zone. Analyses of dissolved parent and daughter compounds demonstrated that the engineered bioremediation has elevated the degradation rate of TCE, resulting in a two-thirds reduction in the TCE mass discharge from the source zone. The biologically enhanced dissolution of TCE was not significant (~5%), and was less than expected. However, the discharges of the daughter products cis-1,2, dichloroethene (cDCE) and vinyl chloride (VC) increased, probably because of the rapid transformation of TCE from the source zone to the measurement transect. This suggests that enhancing the biodegradation of cDCE and VC will be crucial to successful engineered bioremediation of TCE source zones.

Role of bicarbonate as a pH buffer and electron sink in microbial dechlorination of chloroethenes

BACKGROUND

Buffering to achieve pH control is crucial for successful trichloroethene (TCE) anaerobic bioremediation. Bicarbonate (HCO3-) is the natural buffer in groundwater and the buffer of choice in the laboratory and at contaminated sites undergoing biological treatment with organohalide respiring microorganisms. However, HCO3- also serves as the electron acceptor for hydrogenotrophic methanogens and hydrogenotrophic homoacetogens, two microbial groups competing with organohalide respirers for hydrogen (H2). We studied the effect of HCO3- as a buffering agent and the effect of HCO3--consuming reactions in a range of concentrations (2.5-30 mM) with an initial pH of 7.5 in H2-fed TCE reductively dechlorinating communities containing Dehalococcoides, hydrogenotrophic methanogens, and hydrogenotrophic homoacetogens.

RESULTS

Rate differences in TCE dechlorination were observed as a result of added varying HCO3- concentrations due to H2-fed electrons channeled towards methanogenesis and homoacetogenesis and pH increases (up to 8.7) from biological HCO3- consumption. Significantly faster dechlorination rates were noted at all HCO3- concentrations tested when the pH buffering was improved by providing 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as an additional buffer. Electron balances and quantitative PCR revealed that methanogenesis was the main electron sink when the initial HCO3- concentrations were 2.5 and 5 mM, while homoacetogenesis was the dominant process and sink when 10 and 30 mM HCO3- were provided initially.

CONCLUSIONS

Our study reveals that HCO3- is an important variable for bioremediation of chloroethenes as it has a prominent role as an electron acceptor for methanogenesis and homoacetogenesis. It also illustrates the changes in rates and extent of reductive dechlorination resulting from the combined effect of electron donor competition stimulated by HCO3- and the changes in pH exerted by methanogens and homoacetogens.

Use of statistical tools to evaluate the reductive dechlorination of high levels of TCE in microcosm studies

A large, multi-laboratory microcosm study was performed to select amendments for supporting reductive dechlorination of high levels of trichloroethylene (TCE) found at an industrial site in the United Kingdom (UK) containing dense non-aqueous phase liquid (DNAPL) TCE. The study was designed as a fractional factorial experiment involving 177 bottles distributed between four industrial laboratories and was used to assess the impact of six electron donors, bioaugmentation, addition of supplemental nutrients, and two TCE levels (0.57 and 1.90 mM or 75 and 250 mg/L in the aqueous phase) on TCE dechlorination. Performance was assessed based on the concentration changes of TCE and reductive dechlorination degradation products. The chemical data was evaluated using analysis of variance (ANOVA) and survival analysis techniques to determine both main effects and important interactions for all the experimental variables during the 203-day study. The statistically based design and analysis provided powerful tools that aided decision-making for field application of this technology. The analysis showed that emulsified vegetable oil (EVO), lactate, and methanol were the most effective electron donors, promoting rapid and complete dechlorination of TCE to ethene. Bioaugmentation and nutrient addition also had a statistically significant positive impact on TCE dechlorination. In addition, the microbial community was measured using phospholipid fatty acid analysis (PLFA) for quantification of total biomass and characterization of the community structure and quantitative polymerase chain reaction (qPCR) for enumeration of Dehalococcoides organisms (Dhc) and the vinyl chloride reductase (vcrA) gene. The highest increase in levels of total biomass and Dhc was observed in the EVO microcosms, which correlated well with the dechlorination results.

A remediation performance model for enhanced metabolic reductive dechlorination of chloroethenes in fractured clay till

A numerical model of metabolic reductive dechlorination is used to describe the performance of enhanced bioremediation in fractured clay till. The model is developed to simulate field observations of a full scale bioremediation scheme in a fractured clay till and thereby to assess remediation efficiency and timeframe. A relatively simple approach is used to link the fermentation of the electron donor soybean oil to the sequential dechlorination of trichloroethene (TCE) while considering redox conditions and the heterogeneous clay till system (clay till matrix, fractures and sand stringers). The model is tested on lab batch experiments and applied to describe sediment core samples from a TCE-contaminated site. Model simulations compare favorably to field observations and demonstrate that dechlorination may be limited to narrow bioactive zones in the clay matrix around fractures and sand stringers. Field scale simulations show that the injected donor is expected to be depleted after 5 years, and that without donor re-injection contaminant rebound will occur in the high permeability zones and the mass removal will stall at 18%. Long remediation timeframes, if dechlorination is limited to narrow bioactive zones, and the need for additional donor injections to maintain dechlorination activity may limit the efficiency of ERD in low-permeability media. Future work should address the dynamics of the bioactive zones, which is essential to understand for predictions of long term mass removal.

Reductive dechlorination of tetrachloroethene by two compost samples with different maturity

This study sets up microcosms using two types of compost samples, bagasse/manure compost, and yard-trimming compost with different maturity, to evaluate their capacity for reductive dechlorination of tetrachloroethene (PCE). The experimental results show that less matured compost samples could reduce 300 μM of PCE to ethene within 180 days of incubation. Decreasing initial PCE concentration and removing dissolved oxygen from the solution could enhance reducing efficiency. The solution remains near neutral pH throughout the experiment, and ethene emerged when the redox potential dropped to below -150 mV. Different microbial inhibition agents, such as 2-bromoethanesulfonic acid and sodium molybdate 2-hydrate, exhibit different effects on the dechlorination efficiency. The potential advantages of using compost to remove PCE are discussed. Overall, due to their high carbon content, diverse microbial activity, high buffer capacity, and complex physical structure, compost samples could serve as suitable media for dechlorinating PCE.

Natural and enhanced anaerobic degradation of 1,1,1-trichloroethane and its degradation products in the subsurface--a critical review

1,1,1-Trichloroethane (TCA) in groundwater is susceptible to a variety of natural degradation mechanisms. Evidence of intrinsic decay of TCA in aquifers is commonly observed; however, TCA remains a persistent pollutant at many sites and some of the daughter products that accumulate from intrinsic decay of TCA have been determined to be more toxic than the parent compound. Research advances from the past decade indicate that in situ enhanced reductive dechlorination (ERD) offers promise as a cost-effective solution toward the cleanup of groundwater contaminated with TCA and its transformation daughter products. Laboratory studies have demonstrated that pure or mixed cultures containing certain Dehalobacter (Dhb) bacteria can catalyze respiratory dechlorination of TCA and 1,1-dichloroethane (1,1-DCA) to monochloroethane (CA) in groundwater systems. 16S rRNA Dhb gene probes have been used as biomarkers in groundwater samples to both assess ERD potential and quantify growth of Dhb in ERD applications at TCA sites. Laboratory findings suggest that iron-bearing minerals and methanogenic bacteria that co-occur in reduced aquifers may synergistically affect dechlorination of TCA. Despite these advances, a number of significant challenges remain, including an inability of any known cultures to completely dechlorinate TCA to ethane. CA is commonly observed as a terminal product of the biological reductive dechlorination of TCA and 1,1-DCA. Also important is the lack of rigorous field studies demonstrating the utility of bioaugmentation with Dhb cultures for remediation of TCA in the field. In this paper we review the state-of-the-science of TCA degradation in aquifers, examining results from both laboratory experiments and twenty-two field case studies, focusing on the capabilities and limits of ERD technology, and identifying aspects of the technology that warrant further development.

Reductive dechlorination of 2-chlorophenol by Anaeromyxobacter dehalogenans with an electrode serving as the electron donor

Electrodes poised at potentials low enough to serve as an electron donor for microbial respiration, but high enough to avoid the production of hydrogen, have been proposed as an alternative to the use of soluble electron donors for stimulating the bioremediation of chlorinated contaminants and/or metals. However, this form of respiration using pure cultures of microorganisms has only been reported in Geobacter species. To further evaluate this bioremediation strategy studies were conducted with Anaeromyxobacter dehalogenans, which has previously been reported to reductively dechlorinate 2-chlorophenol to phenol with acetate as the electron donor. Anaeromyxobacter dehalogenans could oxidize acetate with electron transfer to a graphite electrode poised at a positive potential, demonstrating its ability to directly exchange electrons with electrodes. Anaeromyxobacter dehalogenans attached to electrodes poised at -300 mV versus standard hydrogen electrode reductively dechlorinated 2-chlorophenol to phenol. There was no dechlorination in the absence of A. dehalogenans and electrode-driven dechlorination stopped when the supply of electrons to the electrode was disrupted. The findings that microorganisms other than Geobacter species can accept electrons from electrodes for anaerobic respiration and that chlorinated aromatic compounds can be dechlorinated in this manner suggest that there may be substantial potential for treating a diversity of contaminants with microbe-electrode interactions.