Negligible particle-specific antibacterial activity of silver nanoparticles.

For nearly a decade, researchers have debated the mechanisms by which AgNPs exert toxicity to bacteria and other organisms. The most elusive question has been whether the AgNPs exert direct "particle-specific" effects beyond the known antimicrobial activity of released silver ions (Ag(+)). Here, we infer that Ag(+) is the definitive molecular toxicant. We rule out direct particle-specific biological effects by showing the lack of toxicity of AgNPs when synthesized and tested under strictly anaerobic conditions that preclude Ag(0) oxidation and Ag(+) release. Furthermore, we demonstrate that the toxicity of various AgNPs (PEG- or PVP- coated, of three different sizes each) accurately follows the dose-response pattern of E. coli exposed to Ag(+) (added as AgNO(3)). Surprisingly, E. coli survival was stimulated by relatively low (sublethal) concentration of all tested AgNPs and AgNO(3) (at 3-8 µg/L Ag(+), or 12-31% of the minimum lethal concentration (MLC)), suggesting a hormetic response that would be counterproductive to antimicrobial applications. Overall, this work suggests that AgNP morphological properties known to affect antimicrobial activity are indirect effectors that primarily influence Ag(+) release. Accordingly, antibacterial activity could be controlled (and environmental impacts could be mitigated) by modulating Ag(+) release, possibly through manipulation of oxygen availability, particle size, shape, and/or type of coating.

Sulfidated microscale zero-valent iron/reduced graphene oxide composite (S-mZVI/rGO) for enhanced degradation of trichloroethylene: The role of hydrogen spillover.

Atomic hydrogen (H*) has long been thought to play an important role in the dechlorination of trichloroethylene (TCE) by carbon-supported zero-valent iron (ZVI), which offers an alternative pathway for TCE dechlorination. Herein, we demonstrate that the reductive dechlorination of TCE by sulfidated microscale ZVI (S-mZVI) can be further enhanced by promoting the formation of H* through the introduction of reduced graphene oxide (rGO). The completely degradation of 10 mg/L TCE can be achieved by S-mZVI/rGO within 24 h, which was 3.3 times faster than that of S-mZVI. The change in the distribution of TCE degradation products over time suggests that the introduction of rGO leads to a change in the dechlorination pathway. The percentage of ethane in the final products of TCE degradation by S-mZVI/rGO was 34.3 %, while that of S-mZVI was only 21.9 %. The electrochemical tests confirmed the occurrence of hydrogen spillover in the S-mZVI/rGO composite, which promoted the reductive dechlorination of TCE by H*. Although the S-mZVI/rGO composite had stronger hydrogen evolution propensity than S-mZVI, the S-mZVI/rGO composite still exhibited higher electron utilization efficiency than S-mZVI thanks to the increased utilization of hydrogen.

Reduction of nitrate by bimetallic Fe/Ni nanoparticles.

Bimetallic Fe/Ni nanoparticles were synthesized and their nitrate reduction capacity was studied. Nitrate (354 mg L(-1), equal to 5.71 mmol L(-1)) reduction was performed using Fe/Ni nanoparticles with various Ni contents (1.0, 5.0, 10 and 20%) in an unbuffered condition. Optimum nitrate reduction rate (1.03 +/- 0.087 x 10(-4) mol x min(-1) x greduc(-1)) was obtained with 5.0% nano-scale Fe/Ni, while only 25% nitrate (1.05 +/- 0.091 x 10(-5) mol x min(-1) x greduc(-1)) was transformed by nano-scale Fe(0) within the same reaction time, which means that these bimetallic nanoparticles are obviously more reactive than monometallic nano-scale Fe(0). For this bimetallic system a near-neutral initial pH (6.5) is more favourable than an acidic condition (2.0 and 4.0). Relatively air-stable nano-scale Fe/Ni particles were developed by slowly aging them for 22 h and exhibited similar reactivity to freshly synthesized nano-scale Fe(0). Although undesirable transformation of nitrate (91.0 +/- 0.37%) to ammonium was observed in this study, Fe/Ni particles showed a much higher nitrate reduction rate and an optimum reduction rate at near-neutral pH, which may have important implications for nitrate-contaminated site remediation.

Differential effect of common ligands and molecular oxygen on antimicrobial activity of silver nanoparticles versus silver ions.

The antibacterial activity of silver nanoparticles (AgNPs) is partially due to the release of Ag(+), although discerning the contribution of AgNPs vs Ag(+) is challenging due to their common co-occurrence. We discerned the toxicity of Ag(+) versus a commercially available AgNP (35.4 ± 5.1 nm, coated with amorphous carbon) by conducting antibacterial assays under anaerobic conditions that preclude Ag0 oxidation, which is a prerequisite for Ag(+) release. These AgNPs were 20× less toxic to E. coli than Ag(+) (EC50: 2.04 ± 0.07 vs 0.10 ± 0.01 mg/L), and their toxicity increased 2.3-fold after exposure to air for 0.5 h (EC50: 0.87 ± 0.03 mg/L) which promoted Ag(+) release. No significant difference in Ag(+) toxicity was observed between anaerobic and aerobic conditions, which rules out oxidative stress by ROS as an important antibacterial mechanism for Ag(+). The toxicity of Ag(+) (2.94 µmol/L) was eliminated by equivalent cysteine or sulfide; the latter exceeded the solubility product equilibrium constant (K(sp)), which is conducive to silver precipitation. Equivalent chloride and phosphate concentrations also reduced Ag(+) toxicity without exceeding K(sp). Thus, some common ligands can hinder the bioavailability and mitigate the toxicity of Ag(+) at relatively low concentrations that do not induce silver precipitation. Furthermore, low concentrations of chloride (0.1 mg/L) mitigated the toxicity of Ag(+) but not that of AgNPs, suggesting that previous reports of higher AgNPs toxicity than their equivalent Ag(+) concentration might be due to the presence of common ligands that preferentially decrease the bioavailability and toxicity of Ag(+). Overall, these results show that the presence of O2 or common ligands can differentially affect the toxicity of AgNPs vs Ag(+), and underscore the importance of water chemistry in the mode of action of AgNPs.

Speciation, controlling steps and pathways of silver release from the sludge generated from coagulation of wastewater spiked with silver nanoparticles.

Sludge generated in wastewater treatment facilities is an integral part for the introduction of silver nanoparticles (AgNPs) to the terrestrial environment, which would cause some adverse ecosystem responses. The understanding of silver release process from the sludge is important to evaluate their risks. In this study, the amount and speciation of the released silver were investigated by taking the sludge generated by wastewater coagulation with AgNPs added (denoted as sludgeC-AgNPs) an example, and kinetic analysis and density functional theory (DFT) calculations were first used to explore the controlling steps and pathways about the silver release. The results showed that sludgeC-AgNPs could release the dissolved silver and the colloidal silver. Beside Cl-, Ca2+ in the leaching solution could enhance the silver release of sludgeC-AgNPs, especially for the colloidal silver. The released colloidal silver restricted in size from 40 nm to 100 nm with irregular shape. Although the oxidative dissolution of Ag0 was the origin of the silver release pathways from the sludgeC-AgNPs, the silver diffusion was the controlling step due to the spontaneous binding between silver and the hydrolysates of polyaluminium chloride in sludgeC-AgNPs. However, Ca2+ in the leaching solution could occupy the binding site of silver on sludgeC-AgNPs, which would increase the diffusion rate of silver over the oxidative rate of Ag0. With this condition, the controlling step of silver release from sludgeC-AgNPs turned to the oxidative dissolution of Ag0. Our findings are important to assess the fate of AgNPs in wastewater treatment as well as sludge applications.

Effect of bare and coated nanoscale zerovalent iron on tceA and vcrA gene expression in Dehalococcoides spp.

Nanoscale zerovalent iron (NZVI) can be used to dechlorinate trichloroethylene (TCE) in contaminated aquifers. Dehalococcoides spp. is the only microbial genus known to dechlorinate TCE to ethene as a respiratory process. However, little is known about how NZVI affects the expression of genes coding for reductive dechlorination. We examined a high-rate TCE-dechlorinating mixed culture which contains organisms similar to known Dehalococcoides to study the effects of NZVI on the expression of two model genes coding for reductive dehalogenases (tceA and vcrA). A novel pretreatment approach, relying on magnetic separation of NZVI prior to reverse transcription qPCR (to avoid RNA adsorption by NZVI), was developed and used with relative quantification (relative to 16S rRNA as endogenous housekeeping gene) to quantify reductive dehalogenase gene expression. Both tceA and vcrA were significantly down-regulated (97- and 137-fold, respectively) relative to baseline (time 0) conditions after 72-h exposure to chlorinated ethenes (0.12 ± 0.03 mg/L cis-DCE, 0.69 ± 0.11 mg/L t-DCE, and 0.54 ± 0.16 mg/L VC) and bare-NZVI (1 g-NZVI/L). However, coating NZVI with an olefin maleic acid copolymer (a common approach to enhance its mobility in aquifers) overcame this significant inhibitory effect, and both tceA and vcrA were up-regulated (3.0- and 3.5-fold, respectively) after 48-h exposure. Thus, NZVI coating might enhance the expression of dechlorinating genes and the concurrent or sequential participation of Dehalococcoides spp. in the remediation process.

Reduction and immobilization of chromium(VI) by nano-scale Fe0 particles supported on reproducible PAA/PVDF membrane.

A new class of nano-scale Fe0 particles (NZVI) supported on a PAA/PVDF membrane (NZVI-PAA/PVDF) were synthesized and the feasibility of using NZVI-PAA/PVDF for reductive immobilization of Cr(VI) in water was investigated through laboratory batch tests. The results showed that the Cr(VI) removal capacity of NZVI-PAA/PVDF was 181 mg Cr/g Fe at an initial Cr(VI) concentration of 20 mg L(-1) under pH 6.5 +/- 0.1. XPS results showed that Cr(VI) was converted to nontoxic Cr(III). Interfering ions exerted various degrees of impact on NZVI-PAA/PVDF's Cr(VI) removal capacity. Specifically, Ca2+ alone showed the mildest impact while the presence of ions (Mg2+ and HCO3-) exerted the greatest impact. An advantage of NZVI-PAA/PVDF is that the nano-scale Fe0 and resultant particles were combined within a PAA/PVDF membrane, which prevents secondary pollution. Moreover, a piece of PAA/PVDF membrane (4.7 cm diameter) can still support 6.51 mg of nano-scale Fe0 particles after being renewed.

Elucidating the genetic basis for Escherichia coli defense against silver toxicity using mutant arrays.

Bacterial adaptation and defense mechanisms against silver are poorly understood at the genetic level. A library of Escherichia coli gene-deletion mutants was used to show that clones lacking sodB (coding for oxidative stress protection), lon (protein damage repair), or cusR (metal efflux pump) are quite sensitive to silver (with 7.3 ± 9.1%, 5.3 ± 1.8%, and 0.4 ± 0.1% of cells surviving, respectively, compared with 90.1 ± 5.4% survival for wild-type E. coli, after 6-h exposure to 8 mg/L AgNO(3)), suggesting the importance of the coded functions as defense mechanisms. Mutants lacking pgaB or wcaD, which code for production of extracellular polymeric substances (EPS), also showed significant (p < 0.05) sensitivity to silver exposure (23.4 ± 16.2% and 23.1 ± 32.6% survival, respectively). Transmission electron microscopy (TEM) with scanning TEM/energy-dispersive X-ray spectroscopy analysis showed accumulation of silver nanoparticles within EPS, suggesting that EPS serve as a protective barrier that also immobilizes dissolved silver as silver nanoparticles.

Impacts of silver nanoparticles on cellular and transcriptional activity of nitrogen-cycling bacteria.

The widespread use of silver nanoparticles (AgNPs) raises the potential for environmental releases that could impact microbial ecosystem services. In the present study, the authors address how the AgNPs and Ag(+) that they release may impact nitrogen-cycling bacteria. The authors studied the cellular and transcriptional response of the denitrifier Pseudomonas stutzeri, the nitrogen fixer Azotobacter vinelandii, and the nitrifier Nitrosomonas europaea exposed to 35 nm (carbon-coated) AgNPs or to Ag(+) (added as AgNO3 ). Based on minimum inhibitory concentrations (MICs), Ag(+) was 20 times to 48 times more toxic to the tested strains than AgNPs (including Ag(+) released during exposure). Exposure to sublethal concentrations of AgNPs or Ag(+) (representing 10% of the respective MIC for AgNO3 ) resulted in no significant effect on the expression of the denitrifying genes narG, napB, nirH, and norB in P. stutzeri or the nitrogen-fixing genes nifD, nifH, vnfD, and anfD in A. vinelandii, whereas nitrifying genes (amoA1 and amoC2) in N. europaea were upregulated (2.1- to 3.3-fold). This stimulatory effect disappeared at higher silver concentrations (60% of the Ag(+) MIC), and toxicity was exerted at concentrations higher than 60% of the Ag(+) MIC. The MIC for N. europaea was 8 times to 24 times lower than for the other strains, indicating higher susceptibility to AgNPs. This was corroborated by the lower half-lethal concentration for N. europaea (87 µg/L) compared with P. stutzeri (124 µg/L) and A. vinelandii (>250 µg/L) when cells were exposed with Ag(+) for 24 h in 1 mM bicarbonate buffer. This suggests that ammonia oxidation would be the most vulnerable nitrogen-cycling process in wastewater treatment plants receiving AgNPs and in agricultural soils amended with biosolids that concentrate them.

Adaptive microbial population shifts in response to a continuous ethanol blend release increases biodegradation potential.

The fate of fuel releases largely depends on the poorly-understood response in microbial community structure and function. Here, we evaluate the impacts to the microbial community resulting from a pilot-scale continuous release (10 months) of a 10% v:v ethanol solution mixed with benzene and toluene (50 mg/L each). Microbial population shifts were characterized by pyrosequencing-based 16S rRNA analysis and by quantitative PCR targeting Bacteria, Archaea, and functional genes for methanogenesis (mcrA), acetogenesis (fhs) and aerobic degradation of aromatic hydrocarbons (PHE), which could occur in hypoxic micro-environments. The release stimulated microbial growth, increased species richness and diversity, and selected for genotypes involved in fermentative degradation (the relative abundance of mcrA and fhs increased 18- and 6-fold, respectively). The growth of putative hydrocarbon degraders and commensal anaerobes, and increases in microbial diversity and in degradation rates suggest an adaptive response that increases the potential for natural attenuation of ethanol blend releases.

Analysis of biodegradation by-products of nitrobenzene and aniline mixture by a cold-tolerant microbial consortium.

A cold-tolerant microbial consortium, which can use nitrobenzene (NB) and aniline (AN) as sole carbon, nitrogen and energy sources, was isolated from an NB and AN contaminated site. Pilot 454 pyrosequencing analysis of the consortium showed that it was mainly made up of Pseudomonas spp. (98%). At 10 °C, the consortium degraded the mixture of 50mg/L NB and 50mg/L AN at a similar rate as those achieved at 20 °C and 30 °C. The biodegradation by-products with different initial NB and AN concentrations at 10 °C were analyzed. Azobenzene, azoxybenzene and acetanilide were observed in NB and AN mixtures degradation. These by-products are generated by the reaction between different intermediates resulting from the NB and AN degradation as well as the parent compounds. To the best of our knowledge, this is the first report confirming the by-products of NB and AN mixture biodegradation by a cold-tolerant microbial consortium.

Perfluorooctanoic acid degradation in the presence of Fe(III) under natural sunlight.

Due to the high bond dissociation energy (BDE) of CF bonds (116 kcal/mol), perfluorooctanoic acid (PFOA) is a highly recalcitrant pollutant. Herein, we demonstrate a novel method to decompose PFOA in the presence of sunlight and ferric iron (Fe(III)). Under such conditions, 97.8 ± 1.7% of 50 µM PFOA decomposed within 28 days into shorter-chain intermediates and fluoride (F(-)), with an overall defluorination extent of 12.7 ± 0.5%. No PFOA was removed under visible light, indicating that UV radiation is required for PFOA decomposition. Spectroscopic analysis indicates that the decomposition reaction is likely initiated by electron-transfer from PFOA to Fe(III), forming Fe(II) and an unstable organic carboxyl radical. An alternative mechanism for the formation of this organic radical involves hydroxyl radicals, detected by electron paramagnetic resonance (EPR). The observation that PFOA can be degraded by Fe(III) under solar irradiation provides mechanistic insight into a possibly overlooked natural attenuation process. Because Fe(III) is abundant in natural waters and sunlight is essentially free, this work represents a potentially important step toward the development of simple and inexpensive remediation strategies for PFOA-contaminated water.

Photocatalytic generation of multiple ROS types using low-temperature crystallized anodic TiO2 nanotube arrays.

This study investigates the photocatalytic efficiency, type of reactive oxygen species (ROS) produced, and potential for structural and morphological modification of anodic TiO2 nanotubes (NTs) synthesized using a novel, energy efficient, low temperature crystallization process. These TiO2 NTs show greater photocatalytic efficiency than traditional high-temperature sintered NTs or supported Degussa P25 TiO2, as measured by degradation of methyl orange, a model organic dye pollutant. EPR analysis shows that low-temperature crystallized TiO2 NTs generate both hydroxyl radicals and singlet oxygen, while high-temperature sintered TiO2 NTs generate primarily hydroxyl radicals but no singlet oxygen. This "cocktail" of reactive oxygen species, combined with an increased surface area, contributes to the increased efficiency of this photocatalytic material. Furthermore, variation of the NT crystallization parameters enables control of structural and morphological properties so that TiO2-NTs can be optimized for scale-up and for specific treatment scenarios.

One-step synthesis and characterization of core-shell Fe@SiO2 nanocomposite for Cr (VI) reduction.

A facile one-step method was developed to fabricate mono-dispersed Fe nanoparticles (Fe NPs) coated with SiO(2) shell by aqueous reduction method combined with modified Stöber method. Borohydride was acted not only as a reductant for iron salt but also as a catalyst for hydrolysis and polycondensation reaction of tetraethylorthosilicate (TEOS), and more importantly, there was no need to use surface primer for the generation of Fe NPs and catalyst NH(4)OH for SiO(2). Both the Fe NPs agglomeration and SiO(2) shell thickness can be controlled through the synthetic conditions. Lower potassium borohydride (KBH(4)) injection speed was preferable to assemble Fe NPs. The SiO(2) shell thickness increased gradually with the increase of TEOS amount. Under the condition of TEOS amount of 0.1mL and KBH(4) injection speed of 5mL/min, 25nm single Fe NP was coated with SiO(2) shell with thickness of about 9nm. The resulting nanoporous SiO(2) shell was proved to allow reactant to reach the Fe NPs while at the same time protect them from aggregation. The reactivity characterization of the SiO(2)-coated Fe nanoparticles (Fe@SiO(2)) showed that both TEOS concentration and KBH(4) injection speed had effect on Cr (VI) degradation ability. The highest removal capacity of Fe@SiO(2) can reach 467mgCr/gFe at an initial Cr (VI) concentration of 70mg/L under pH 6.0±0.1. XPS and TEM results showed that Cr (VI) was converted to nontoxic Cr (III) and the reaction product was completely adsorbed to SiO(2) shell.

Acute toxicity, biochemical and gene expression responses of the earthworm Eisenia fetida exposed to polycyclic musks.

AHTN (Tonalide) and HHCB (galaxolide) are recognized as ubiquitous contaminants in soil and have potential adverse impacts on soil organisms. The aim of this study is to investigate the effects of AHTN and HHCB on the earthworm (Eisenia fetida) as an important soil animal with attention to the acute toxicity, biochemical and transcriptional changes of representative antioxidant enzymatic (SOD, CAT) and stress-response gene (Hsp70). The 48 h-LC(50) value was 20.76 µg cm(-2) for AHTN and 11.87 µg cm(-2) for HHCB respectively in the acute lethal studies. The time-dependent elevation in the level of malondialdehyde (MDA) suggests that reactive oxygen species (ROS)-induced cellular oxidative injury of E. fetida might be one of the main toxic effects of AHTN and HHCB. SOD and CAT were both up-regulated at low exposure dose (0.6 µg cm(-2) AHTN and 0.3 µg cm(-2) HHCB) during 48 h testing period, which protected earthworms from oxidative stresses. However, the down-regulation of SOD and CAT after 48 h exposure to high dose contaminants might be caused by the extreme oxidative stress levels (maximum up-regulation 1.70-fold and 1.40-fold for MDA levels at 6.0 µg cm(-2) AHTN and 3.0 µg cm(-2) HHCB compared to the controls, respectively). The Hsp70 gene expression did not show variation during 48 h, except that it had a significant down-regulation (P<0.05) after 48 h of exposure to high doses of contaminants. These results showed that the dermal contact of AHTN and HHCB could result in pronounced biochemical and physiological responses to earthworms, and the transcriptional level changes in antioxidant genes could be potential molecular biomarkers for the stress of the pollutants.

Effect of natural organic matter on toxicity and reactivity of nano-scale zero-valent iron.

Nano-scale zero-valent iron (NZVI) particles are increasingly used to remediate aquifers contaminated with hazardous oxidized pollutants such as trichloroethylene (TCE). However, the high reduction potential of NZVI can result in toxicity to indigenous bacteria and hinder their participation in the cleanup process. Here, we report on the mitigation of the bactericidal activity of NZVI towards gram-negative Escherichia coli and gram-positive Bacillus subtilis in the presence of Suwannee River humic acids (SRHA), which were used as a model for natural organic matter (NOM). B. subtilis was more tolerant to NZVI (1 g/L) than E. coli in aerobic bicarbonate-buffered medium. SRHA (10 mg/L) significantly mitigated toxicity, and survival rates after 4 h exposure increased to similar levels observed for controls not exposed to NZVI. TEM images showed that the surface of NZVI and E. coli was surrounded by a visible floccus. This decreased the zeta potential of NZVI from -30 to -45 mV and apparently exerted electrosteric hindrance to minimize direct contact with bacteria, which mitigated toxicity. H(2) production during anaerobic NZVI corrosion was not significantly hindered by SRHA (p > 0.05), However, NZVI reactivity towards TCE (20 mg/L), assessed by the first-order dechlorination rate coefficient, decreased by 23%. Overall, these results suggest that the presence of NOM offers a tradeoff for NZVI-based remediation, with higher potential for concurrent or sequential bioremediation at the expense of partially inhibited abiotic reactivity with the target contaminant (TCE).

Effects of nano-scale zero-valent iron particles on a mixed culture dechlorinating trichloroethylene.

Nano-scale zero-valent iron particles (NZVI) are increasingly being used to treat sites contaminated with chlorinated solvents. This study investigated the effect of NZVI on dechlorinating microorganisms that participate in the anaerobic bioremediation of such sites. NZVI can have a biostimulatory effect associated with water-derived cathodic H(2) production during its anaerobic corrosion (730+/-30 micromol H(2) was produced in 166 h in abiotic controls with 1 g/L NZVI) or an inhibitory effect upon contact with cell surfaces (assessed by transmission electron microscopy). Methanogens, which are known to compete for H(2) with dechlorinators, were significantly biostimulated by NZVI and methane production increased relative to NZVI-free controls from 58+/-5 to 275+/-2 micromol. In contrast, bacteria dechlorinating TCE were inhibited by NZVI, and the first-order degradation rate coefficient decreased from 0.115+/-0.005 h(-1) (R(2)=0.99) for controls to 0.053+/-0.003 h(-1) (R(2)=0.98) for treatments with 1 g/L NZVI. Ethene production from TCE was initially inhibited by NZVI, but after 331 h increased to levels observed for an NZVI-free system (7.6+/-0.3 micromol ethene produced in 502 h compared to 11.6+/-0.5 mmol in the NZVI-free system and 3.8+/-0.3 micromol ethene for NZVI alone). Apparently, cathodic H(2) was utilized as electron donor by dechlorinating bacteria, which recovered following the partial oxidation and presumably passivation of the NZVI. Overall, these results suggest that reductive treatment of chlorinated solvent sites with NZVI might be enhanced by the concurrent or subsequent participation of bacteria that exploit cathodic depolarization and reductive dechlorination as metabolic niches.

[Microbial reductive dechlorination of TCE with nano iron serving as electron donor].

A trichloroethylene (TCE) dechlorinating enrichment (Dehalococcoides spp.), which was isolated from soil of chlorinated ethene contaminated site, was used to investigate whether nano-scale zero valent iron (NZVI) could serve as electron donor for this consortium via cathodic H2 production during anaerobic corrosion. The results show that in the presence of methanol serving as electron donor, dechlorinating culture of 25 fold dilution [(2.0 +/- 0.44) x 10(5) cell/mL] degraded 20 mg/L TCE completely in 96 h, which was accompanied by the production of 2.706 micromol ethene in 190 h. Methanol-free control caused partial degradation of TCE to primarily cis-DCE in 96 h, with only 0.159 micromol ethene produced in 190 h. This indicates bacteria cannot reduce TCE to ethene without electron donor. But when 4 g/L NZVI was added as sole electron donor, this dechlorinating culture degraded 20 mg/L TCE into ethene and vinyl chloride (VC) in 131 h at a speed higher than that by NZVI alone. Compared to 2.706 micromol ethene produced by Dehalococcoides spp. with methanol added as the electron donor, there was only 1.187 micromol ethene produced by bacteria with NZVI serving as the electron donor, which means NZVI has a potential toxicity on Dehalococcoides spp.. At the meantime, 0.109 micromol acetylene was produced in 190 h, which was relatively lower than 0.161 micromol produced by NZVI alone, indicating bacteria competed with NZVI under electron deficient condition. In conclusion, NZVI could serve as electron donor and support dechlorination activity for Dehalococcoides spp. which could enhance the application of NZVI and usage of dechlorinating culture as a polishing strategy in future ground water remediation.