Isolation and characterization of pure cultures for metabolizing 1,4-dioxane in oligotrophic environments.

1,4-Dioxane concentration in most contaminated water is much less than 1 mg/L, which cannot sustain the growth of most reported 1,4-dioxane-metabolizing pure cultures. These pure cultures were isolated following enrichment of mixed cultures at high concentrations (20 to 1,000 mg/L). This study is based on a different strategy: 1,4-dioxane-metabolizing mixed cultures were enriched by periodically spiking 1,4-dioxane at low concentrations (≤1 mg/L). Five 1,4-dioxane-metabolizing pure strains LCD6B, LCD6D, WC10G, WCD6H, and WD4H were isolated and characterized. The partial 16S rRNA gene sequencing showed that the five bacterial strains were related to Dokdonella sp. (98.3%), Acinetobacter sp. (99.0%), Afipia sp. (99.2%), Nitrobacter sp. (97.9%), and Pseudonocardia sp. (99.4%), respectively. Nitrobacter sp. WCD6H is the first reported 1,4-dioxane-metabolizing bacterium in the genus of Nitrobacter. The net specific growth rates of these five cultures are consistently higher than those reported in the literature at 1,4-dioxane concentrations <0.5 mg/L. Compared to the literature, our newly discovered strains have lower half-maximum-rate concentrations (1.8 to 8.2 mg-dioxane/L), lower maximum specific 1,4-dioxane utilization rates (0.24 to 0.47 mg-dioxane/(mg-protein ⋅ d)), higher biomass yields (0.29 to 0.38 mg-protein/mg-dioxane), and lower decay coefficients (0.01 to 0.02 d-1). These are characteristics of microorganisms living in oligotrophic environments.

Simultaneous biodegradation kinetics of 1,4-dioxane and ethane.

Biodegradation of 1,4-Dioxane at environmentally relevant concentrations usually requires the addition of a primary electron-donor substrate to sustain biomass growth. Ethane is a promising substrate, since it is available as a degradation product of 1,4-Dioxane's common co-contaminants. This study reports kinetic parameters for ethane biodegradation and co-oxidations of ethane and 1,4-Dioxane. Based on experiments combined with mathematical modeling, we found that ethane promoted 1,4-Dioxane biodegradation when the initial mass ratio of ethane:1,4-Dioxane was < 9:1 mg COD/mg COD, while it inhibited 1,4-Dioxane degradation when the ratio was > 9:1. A model-independent estimator was used for kinetic-parameter estimation, and all parameter values for 1,4-Dioxane were consistent with literature-reported ranges. Estimated parameters support competitive inhibition between ethane as the primary substrate and 1,4-Dioxane as the secondary substrate. The results also support that bacteria that co-oxidize ethane and 1,4-Dioxane had a competitive advantage over bacteria that can use only one of the two substrates. The minimum concentration of ethane to sustain ethane-oxidizing bacteria and ethane and 1,4-Dioxane-co-oxidizing bacteria was 0.09 mg COD/L, which is approximately 20-fold lower than the minimum concentration reported for propane, another common substrate used to promote 1,4-Dioxane biodegradation. The minimum 1,4-Dioxane concentration required to sustain steady-state biomass with 1,4-Dioxane as the sole primary substrate was 1.3 mg COD/L. As 1,4-Dioxane concentrations at most groundwater sites are less than 0.18 mg COD/L, providing ethane as a primary substrate is vital to support biomass growth and consequently enable 1,4-Dioxane bioremediation.

Measurement of fluorotelomer alcohols based on solid phase microextraction followed by gas chromatography-mass spectrometry and its application in solid waste study.

This work developed a method based on solid phase microextraction followed by gas chromatography/mass spectrometry (SPME-GC/MS) for the measurement of fluorotelomer alcohols (FTOHs) in gas samples. The method quantification limit (MQL) is 6-7 ng/L for 6:2 fluorotelomer alcohols (6:2 FTOH) and 8:2 fluorotelomer alcohols (8:2 FTOH). In contrast to common methods such as thermal desorption combined with GC-MS, it needs neither pre-concentration equipment nor large sample volume. The extraction-evaporation-GC/MS is commonly used in literature for FTOHs measurement in solids samples. We developed a method to measure FTOHs in solid samples by adding solvent extraction prior to headspace SPME-GC/MS. The extraction-headspace SPME-GC/MS method has a quantification limit of 40-43 ng per gram for 6:2 FTOH and 8:2 FTOH in solid samples. This is comparable to the MQLs for the extraction-evaporation-GC/MS method. Removing the solvent evaporation step decreased the risk of contamination and loss of analytes. The developed methods were successfully used in three examples of solid waste study: 1) measuring 6:2 FTOH and 8:2 FTOH above the MQL in gas emissions from a closed landfill, 2) finding 6:2 FTOH above MQL in 9 of 31 solid consumer products, and 3) finding that the release of 6:2 FTOH in simulated landfills containing popcorn bags was linear at a rate of 3.15 ng/g popcorn bags-day and that partial 6:2 FTOH was from the hydrolysis of precursors.

Goethite Formed in the Periplasmic Space of Pseudomonas sp. JM-7 during Fe Cycling Enhances Its Denitrification in Water.

Denitrification-driven Fe(II) oxidation is an important microbial metabolism that connects iron and nitrogen cycling in the environment. The formation of Fe(III) minerals in the periplasmic space has a significant effect on microbial metabolism and electron transfer, but direct evidence of iron ions entering the periplasm and resulting in periplasmic mineral precipitation and electron conduction properties has yet to be conclusively determined. Here, we investigated the pathways and amounts of iron, with different valence states and morphologies, entering the periplasmic space of the denitrifier Pseudomonas sp. JM-7 (P. JM-7), and the possible effects on the electron transfer and the denitrifying ability. When consistently provided with Fe(II) ions (from siderite (FeCO3)), the dissolved Fe(II) ions entered the periplasmic space and were oxidized to Fe(III), leading to the formation of a 25 nm thick crystalline goethite crust, which functioned as a semiconductor, accelerating the transfer of electrons from the intracellular to the extracellular matrix. This consequently doubled the denitrification rate and increased the electron transport capacity by 4-30 times (0.015-0.04 µA). However, as the Fe(II) concentration further increased to above 4 mM, the Fe(II) ions tended to preferentially nucleate, oxidize, and crystallize on the outer surface of P. JM-7, leading to the formation of a densely crystallized goethite layer, which significantly slowed down the metabolism of P. JM-7. In contrast to the Fe(II) conditions, regardless of the initial concentration of Fe(III), it was challenging for Fe(III) ions to form goethite in the periplasmic space. This work has shed light on the likely effects of iron on environmental microorganisms, improved our understanding of globally significant iron and nitrogen geochemical cycles in water, and expanded our ability to study and control these important processes.

Degradation of per- and polyfluoroalkyl substances in landfill leachate by a thin-water-film nonthermal plasma reactor.

Per- and polyfluoroalkyl substances (PFAS) are present in landfill leachate, posing potential challenges to leachate disposal and treatment. This work represents the first study of a thin-water-film nonthermal plasma reactor for PFAS degradation in landfill leachate. Of the 30 PFAS measured in three raw leachates, 21 were above the detection limits. The removal percentage depended on the category of PFAS. For example, perfluorooctanoic acid PFOA (C8) had the highest removal percentage (77% as an average of the three leachates) of the perfluoroalkyl carboxylic acids (PFCAs) category. The removal percentage decreased when the carbon number increased from 8 to 11 and decreased from 8 to 4. The effects of various landfill leachate components, including sodium chloride, acetate, humic acids, pH, and surfactants, had no or minor impacts (<30%) on PFOA mineralization in synthetic samples. This might be explained by the plasma-generation and PFAS-degradation mainly occurring at the gas/liquid interface. Shorter-chain PFCAs were produced as intermediates of PFOA degradation, and shorter-chain PFCAs and perfluorosulfonic acids (PFSAs) were produced as intermediates of perfluorooctanesulfonic acid (PFOS). The concentrations of the intermediates decreased with decreasing carbon number, suggesting a stepwise removal of difluoromethylene (CF2) in the degradation pathway. Potential PFAS species in the raw and treated leachates were identified at the molecular level through non-targeted Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The intermediates did not show accurate toxicity per Microtox bioassay.

A Review of Challenges and Opportunities for Microbially Removing 1,4-Dioxane to Meet Drinking-Water and Groundwater Guidelines.

1,4-Dioxane is an emerging contaminant in drinking-water sources and contaminated sites. Microbial removal of 1,4-dioxane has attracted a lot of attention, but faces a challenge: being not able to continuously metabolize 1,4-dioxane to below most drinking-water and groundwater guidelines. The 1,4-dioxane concentrations in most drinking-water sources and contaminated sites are too low to sustain biomass growth. This minireview discusses strategies that may potentially address the challenge. The strategies include: 1) finding oligotrophs for which the minimum 1,4-dioxane concentrations to sustain biomass are low, 2) determining conditions that maximize 1,4-dioxane co-metabolism or co-oxidation, 3) creating novel materials as biomass carriers and contaminant concentrators, and 4) lowering the life-cycle costs of technologies that combine biodegradation with (electro)chemical oxidation or phytoremediation.

Evidence and Mechanisms of Selenate Reduction to Extracellular Elemental Selenium Nanoparticles on the Biocathode.

Intracellular selenium nanoparticles (SeNPs) production is a roadblock to the recovery of selenium from biological water treatment processes because it is energy intensive to break microbial cells and then separate SeNPs. This study provided evidence of significantly more extracellular SeNP production on the biocathode (97-99%) compared to the conventional reactors (1-90%) using transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy. The cathodic microbial community analysis showed that relative abundance of Azospira oryzae, Desulfovibrio, Stenotrophomonas, and Rhodocyclaceae was <1% in the inoculum but enriched to 10-21% for each group when the bioelectrochemical reactor reached a steady state. These four groups of microorganisms simultaneously produce intracellular and extracellular SeNPs in conventional biofilm reactors per literature review but prefer to produce extracellular SeNPs on the cathode. This observation may be explained by the cellular energetics: by producing extracellular SeNPs on the biocathode, microbes do not need to transfer selenate and the electrons from the cathode into the cells, thereby saving energy. Extracellular SeNP production on the biocathode is feasible since we found high concentrations of C-type cytochrome, which is well known for its ability to transfer electrons from electrodes to microbial cells and reduce selenate to SeNPs on the cell membrane.

The role of medium molecular weight organics on reducing disinfection by-products and fouling prevention in nanofiltration.

Nanofiltration (NF) is utilized in water treatment for controlling disinfection by-products formation potential (DBPFP) and disinfection by-products (DBPs). Attention regarding NF-based technology has been paid on membrane fouling of NF and the rejection efficiency of contaminants by NF membranes. Natural organic matter (NOM) presenting in surface waters is one main removal target in drinking water treatment by NF-based technology, and is thereby a contributor to the membrane fouling of NF. In application, pretreatments of other membrane filtration (e.g., microfiltration (MF) and ultrafiltration (UF)) has been taken prior to NF, resulting in the separation of NOM of specific molecular weight. Meanwhile, it is well known that NOM is composed of organic compounds of different molecular weights. However, the effect of NOM of specific molecular weight has been seldom investigated from the aspects of membrane fouling and the resulting DBPFP after membrane filtration. By using combinations of MF and UF (molecular weight cut-off of 100K or 20K) as pretreatment prior to NF, the NOM of various molecular weight on DBPFP and DBPs in the NF-treated water were investigated. The experiments were conducted with two real-world surface water samples and one tap water sample. It was found that medium molecular weight NOM, defined as NOM that passed UF100K but did not pass UF20K in this study, reduced fouling of the NF membrane. This is supported by the excitation and emission matrix (EEM) fluorescence spectra, size exclusion chromatography (SEC) and flux analysis. In addition, the medium molecular weight NOM also reduced the DBPFP in the NF treated water and eventually the DBPs by participating in forming a protective layer on the NF surface, blocking the transfer of small molecular weight NOM into the NF filtrate, thereby reducing the DBPFP of the NF filtrate since small molecular weight NOM was the major contributor to DBPFP in this study.

Limiting factors of heavy metals removal during anaerobic biological pretreatment of municipal solid waste landfill leachate.

An anaerobic biofilm reactor was used to pretreat a typical municipal solid waste landfill leachate. It was challenging to remove Fe, Pb, and Ni to meet the discharge-to-sewer standards at a hydraulic retention time (HRT) typically used in previous studies. This work further systematically studied the factors that limited the metal removal. The HRT limited metal removal because the required metal sulfides precipitation time was more than 3.5 times of the HRT. Sulfide availability only slightly limited the metal removal since adding sulfate above the stoichiometric requirement improved the metal removal by only 5-11%. Via experiments combined with modeling, it was found that metal bisulfide was the dominant complex that limited Fe removal, but humic acids-metal complex was the dominant complex that limited the removal of Pb and Ni. When the total dissolved sulfide concentration is <18 mg/L, humic substances are more limiting the removal of the three metals than bisulfide. On the other hand, when the total dissolved sulfide concentration is >250 mg/L, bisulfide is more limiting than humic substances.

A Synergistic Platform for Continuous Co-removal of 1,1,1-Trichloroethane, Trichloroethene, and 1,4-Dioxane via Catalytic Dechlorination Followed by Biodegradation.

Groundwater co-contaminated with 1,4-dioxane, 1,1,1-trichloroethane (TCA), and trichloroethene (TCE) is among the most urgent environmental concerns of the U.S. Department of Defense (DoD), U.S. Environmental Protection Agency (EPA), and industries related to chlorinated solvents. Inspired by the pressing need to remove all three contaminants at many sites, we tested a synergistic platform: catalytic reduction of 1,1,1-TCA and TCE to ethane in a H2-based membrane palladium-film reactor (H2-MPfR), followed by aerobic biodegradation of ethane and 1,4-dioxane in an O2-based membrane biofilm reactor (O2-MBfR). During 130 days of continuous operation, 1,1,1-TCA and TCE were 95-98% reductively dechlorinated to ethane in the H2-MPfR, and ethane served as the endogenous primary electron donor for promoting 98.5% aerobic biodegradation of 1,4-dioxane in the O2-MBfR. In addition, the small concentrations of the chlorinated intermediate from the H2-MPfR, dichloroethane (DCA) and monochloroethane (MCA), were fully biodegraded through aerobic biodegradation in the O2-MBfR. The biofilms in the O2-MBfR were enriched in phylotypes closely related to the genera Pseudonocardia known to biodegrade 1,4-dioxane.

Long-Term Continuous Co-reduction of 1,1,1-Trichloroethane and Trichloroethene over Palladium Nanoparticles Spontaneously Deposited on H2-Transfer Membranes.

1,1,1-Trichloroethane (1,1,1-TCA) and trichloroethene (TCE) are common recalcitrant contaminants that coexist in groundwater. H2-induced reduction over precious-metal catalysts has proven advantageous, but its application to long-term continuous treatment has been limited due to poor H2-transfer efficiency and catalyst loss. Furthermore, catalytic reductions of aqueous 1,1,1-TCA alone or concomitant with TCE catalytic co-reductions are unstudied. Here, we investigated 1,1,1-TCA and TCE co-reduction using palladium nanoparticle (PdNP) catalysts spontaneously deposited on H2-transfer membranes that allow efficient H2 supply on demand in a bubble-free form. The catalytic activities for 1,1,1-TCA and TCE reductions reached 9.9 and 11 L/g-Pd/min, values significantly greater than that reported for other immobilized-PdNP systems. During 90 day continuous operation, removals were up to 95% for 1,1,1-TCA and 99% for TCE. The highest steady-state removal fluxes were 1.5 g/m2/day for 1,1,1-TCA and 1.7 g/m2/day for TCE. The major product was nontoxic ethane (94% selectivity). Only 4% of the originally deposited PdNPs were lost over 90 days of continuous operation. Documenting long-term continuous Pd-catalyzed dechlorination at high surface loading with minimal loss of the catalyst mass or activity, this work expands understanding of and provides a foundation for sustainable catalytic removal of co-existing chlorinated solvents.

Investigating promising substrates for promoting 1,4-dioxane biodegradation: effects of ethane and tetrahydrofuran on microbial consortia.

Cometabolic biodegradation of 1,4-dioxane (dioxane) in the presence of primary substrates is a promising strategy for treating dioxane at environmentally relevant concentrations. Seven aqueous amendments (i.e., tetrahydrofuran (THF), butanone, acetone, 1-butanol, 2-butanol, phenol and acetate) and five gaseous amendments (i.e., C1-C4 alkanes and ethylene) were evaluated as the primary substrates for dioxane degradation by mixed microbial consortia. The aqueous amendments were tested in microcosm bottles and the gaseous amendments were tested in a continuous-flow membrane biofilm reactor with hollow fibers pressurized by the gaseous amendments. Ethane was found to be the most effective gaseous substrate and THF was the only aqueous substrate that promoted dioxane degradation. A diverse microbial community consisting of several putative dioxane degraders-Mycobacterium, Flavobacterium and Bradyrhizobiaceae-were enriched in the presence of ethane. This is the first study showing that ethane was the most effective substrate among the short-chain alkanes and it promoted dioxane degradation by enriching dioxane-degraders that did not harbor the well-known dioxane/tetrahydrofuran monooxygenase.

Speciation and conversion of carbon and nitrogen in young landfill leachate during anaerobic biological pretreatment.

There is an increasing need for landfill leachate pretreatment prior to discharge to wastewater treatment plants due to increasingly stringent sewer discharge limits. Lab-scale tests have shown that the anaerobic biological processes can effectively remove chemical oxygen demand and dissolved organic carbon from landfill leachate. Our work expands the knowledge in anaerobic leachate pretreatment by systematically studying the conversion of carbon and nitrogen species, particularly their recalcitrant fractions in a submerged anaerobic biofilm reactor using real-world leachate from a typical young municipal solid waste landfill. After reaching steady state, the reactor removed 41.7% of the fulvic acids (i.e., 1290 mg C/L). While compounds with a low degree of oxidation (O:C < 0.2) and compounds with a low degree of saturation (H:C < 1) were removed, compounds that were more oxidized (O:C > 0.2) and more saturated (H:C > 1) were produced. At steady state, 98% of recalcitrant dissolved organic nitrogen (i.e., rDON = 222 mg N/L) was removed. Compared to the DON in the raw leachate, the produced DON in the pre-treated leachate were more oxidized (O:C > 0.35) and more bioavailable (N:C > 0.07). The submerged anaerobic biofilm reactor may be an efficient leachate pretreatment method if rDON removal is needed.

Model-based assessment of chromate reduction and nitrate effect in a methane-based membrane biofilm reactor.

Chromate contamination can pose a high risk to both the environment and public health. Previous studies have shown that CH4-based membrane biofilm reactor (MBfR) is a promising method for chromate removal. In this study, we developed a multispecies biofilm model to study chromate reduction and its interaction with nitrate reduction in a CH4-based MBfR. The model-simulated results were consistent with the experimental data reported in the literature. The model showed that the presence of nitrate in the influent promoted the growth of heterotrophs, while suppressing methanotrophs and chromate reducers. Moreover, it indicated that a biofilm thickness of 150 µm and an influent dissolved oxygen concentration of 0.5 mg O2/L could improve the reactor performance by increasing the chromate removal efficiency under the simulated conditions.

Cadmium-Bacteria Complexation and Subsequent Bacteria-Facilitated Cadmium Transport in Saturated Porous Media.

Cadmium (Cd) contamination is becoming a significant environmental concern due to its persistency in soil. In the subsurface, the fate and transport of Cd are significantly affected by the presence of organic carriers, including bacteria, which are ubiquitous. In this research, facilitated Cd transport by four different bacterial strains in zeolite was investigated by column experiments under three different Cd introduction scenarios (i.e., a Cd and bacteria mixture, Cd and bacteria introduced separately but simultaneously, and Cd added to the column, followed by bacterial flushing), through which Cd-bacteria complexes formed. In turn, Cd-bacteria complexation affected bacterial transport. Bacteria were least retarded when Cd was pre-deposited, with mass recovery>90% for all strains. More Cd was recovered when introduced as a mixture with bacteria (i.e., Cd mass recovery ranging from 16 to 25%). Obtained bacteria and Cd breakthrough curves were simulated by the attachment-detachment model in Hydrus-1D. Damkohler number and reversibility were both found to be suitable to control the mass recovery of Cd and bacteria in all investigated scenarios.

Kinetics of anaerobic methane oxidation coupled to denitrification in the membrane biofilm reactor.

Anaerobic oxidation of methane coupled to denitrification (AOM-D) in a membrane biofilm reactor (MBfR), a platform used for efficiently coupling gas delivery and biofilm development, has attracted attention in recent years due to the low cost and high availability of methane. However, experimental studies have shown that the nitrate-removal flux in the CH4 -based MBfR (<1.0 g N/m2 -day) is about one order of magnitude smaller than that in the H2 -based MBfR (1.1-6.7 g N/m2 -day). A one-dimensional multispecies biofilm model predicts that the nitrate-removal flux in the CH4 -based MBfR is limited to <1.7 g N/m2 -day, consistent with the experimental studies reported in the literature. The model also determines the two major limiting factors for the nitrate-removal flux: The methane half-maximum-rate concentration (K2 ) and the specific maximum methane utilization rate of the AOM-D syntrophic consortium (kmax2 ), with kmax2 being more important. Model simulations show that increasing kmax2 to >3 g chemical oxygen demand (COD)/g cell-day (from its current 1.8 g COD/g cell-day) and developing a new membrane with doubled methane-delivery capacity (Dm ) could bring the nitrate-removal flux to ≥4.0 g N/m2 -day, which is close to the nitrate-removal flux for the H2 -based MBfR. Further increase of the maximum nitrate-removal flux can be achieved when Dm and kmax2 increase together.

Microbial Community Analysis Provides Insights into the Effects of Tetrahydrofuran on 1,4-Dioxane Biodegradation.

Tetrahydrofuran (THF) is known to induce the biodegradation of 1,4-dioxane (dioxane), an emerging contaminant, but the mechanisms by which THF affects dioxane biodegradation in microbial communities are not well understood. To fill this knowledge gap, changes in the microbial community structure in microcosm experiments with synthetic medium and landfill leachate were examined over time using 16S rRNA gene amplicon sequencing and functional gene quantitative PCR assays. The overarching hypothesis being tested was that THF promoted dioxane biodegradation by increasing the abundance of dioxane-degrading bacteria in the consortium. The data revealed that in experiments with synthetic medium, the addition of THF significantly increased the abundance of Pseudonocardia, a genus with several representatives that can grow on both dioxane and THF, and of Rhodococcus ruber, a species that can use THF as the primary growth substrate while cometabolizing dioxane. However, in similar experiments with landfill leachate, only R. ruber was significantly enriched. When the THF concentration was higher than the dioxane concentration, THF competitively inhibited dioxane degradation since dioxane degradation was negligible, while the dioxane-degrading bacteria and the corresponding THF/dioxane monooxygenase gene copies increased by a few orders of magnitude.IMPORTANCE Widespread in groundwater and carcinogenic to humans, 1,4-dioxane (dioxane) is attracting significant attention in recent years. Advanced oxidation processes can effectively remove dioxane but require high energy consumption and operation costs. Biological removal of dioxane is of particular interest due to the ability of some bacteria to mineralize dioxane at a low energy cost. Although dioxane is generally considered recalcitrant to biodegradation, more than 20 types of bacteria can degrade dioxane as the sole electron donor substrate or the secondary electron donor substrate. In the latter case, tetrahydrofuran (THF) is commonly studied as the primary electron donor substrate. Previous work has shown that THF promotes dioxane degradation at a low THF concentration but inhibits dioxane degradation at a high THF concentration. Our work expanded on the previous work by mechanically examining the effects of THF on dioxane degradation in a microbial community context.

Chemical-Free Recovery of Elemental Selenium from Selenate-Contaminated Water by a System Combining a Biological Reactor, a Bacterium-Nanoparticle Separator, and a Tangential Flow Filter.

Biological selenate (SeO42-) reduction to elemental selenium nanoparticles (SeNPs) has been intensively studied but little practiced because of the additional cost associated with separation of SeNPs from water. Recovery of the SeNPs as a valuable resource has been researched to make the approach more competitive. Separation of the intracellular SeNPs from the biomass usually requires the addition of chemicals. In this research, a novel approach that combined a biological reactor, a bacterium-SeNP separator, and a tangential flow ultrafiltration module (TFU) was investigated to biologically reduce selenate and separate the SeNPs, biomass, and water from each other. This approach efficiently removed and recovered selenium while eliminating the use of chemicals for separation. The three units in the approach worked in synergism to achieve the separation and recovery. The TFU module retained the biomass in the system, which increased the biomass retention time and allowed for more biomass decay through which intracellular SeNPs could be released and recovered. SeNP aggregates were separated from bacterial aggregates due to their different interactions with a tilted polyethylene sheet in the bacterium-SeNP separator. SeNP aggregates stayed on the polyethylene sheet while bacterial aggregates settled down to the bottom of the separator.

Direct solid-state evidence of H2 -induced partial U(VI) reduction concomitant with adsorption by extracellular polymeric substances (EPS).

Adsorption of hexavalent uranium (U(VI)) by extracellular polymeric substances (EPS) has been studied, but the possibility of simultaneous U(VI) reduction mediated by EPS has not had experimental confirmation, as the reduction products have not yet been directly proven. Here, we reported the first direct evidence of lower-valent products of U(VI) immobilization by loosely associated EPS (laEPS) isolated from a fermenter strain of Klebsiella sp. J1 when the laEPS was exposed to H2 . During the 120-min tests for similarly 86% adsorption under O2 , N2 , and H2 , 8% more U was immobilized through a non-adsorptive pathway by the EPS for H2 than for N2 and O2 . A set of solid-state characterization tools (FT-IR, XPS, EELS, and TEM-EDX) confirmed partial reduction of U(VI) to lower-valence U, with the main reduced form being uraninite (UIV O2 ) nanoparticles, and the results reinforced the role of the reduction in accelerating U immobilization and shaping the characteristics of immobilized U in terms of valency, size, and crystallization. The laEPS, mostly comprised of carbohydrate and protein, contained non-cytochrome enzymes and electron carriers that could be responsible for electron transfer to U(VI). Taken together, our results directly confirm that EPS was able to mediate partial U(VI) reduction in the presence of H2 through non-cytochrome catalysis and that reduction enhanced overall U immobilization. Our study fills in some gaps of the microbe-mediated U cycle and will be useful to understand and control U removal in engineered reactors and in-situ bioremediation.

The effect of electron competition on chromate reduction using methane as electron donor.

We studied the effect of electron competition on chromate (Cr(VI)) reduction in a methane (CH4)-based membrane biofilm reactor (MBfR), since the reduction rate was usually limited by electron supply. A low surface loading of SO42- promoted Cr(VI) reduction. The Cr(VI) removal percentage increased from 60 to 70% when the SO42- loading increased from 0 to 4.7 mg SO42-/m2-d. After the SO42- loading decreased back to zero, the Cr(VI) removal further increased to 90%, suggesting that some sulfate-reducing bacteria (SRB) stayed in the reactor to reduce Cr(VI). However, a high surface loading of SO42- (26.6 mg SO42-/m2-d) significantly slowed down the Cr(VI) reduction to 40% removal, which was probably due to competition between Cr(VI) and SO42- reduction. Similarly, when 0.5 mg/L of Se(VI) was introduced into the MBfR, Cr(VI) removal percentage slightly decreased to 60% and then increased to 80% when input Se(VI) was removed again. The microbial community strongly depended on the loadings of Cr(VI) and SO42-. In the sulfate effect experiment, three genera were dominant. Based on the correlation between the abundances of the three genera and the loadings of Cr(VI) and SO42-, we conclude that Methylocystis, a type II methanotroph, reduced both Cr(VI) and sulfate, Meiothermus only reduced Cr(VI), and Ferruginibacter only reduced SO42-.