nZVI Addition Research Articles

A novel strategy for improving ammonia resistance of acidogenesis using domesticated sludge combined with nZVI addition in an ambient anaerobic digestion system

A novel strategy for improving ammonia resistance of acidogenesis using domesticated sludge combined with nZVI addition in an ambient anaerobic digestion system

Highly effective removal of 4-chloroaniline in water by nano zero-valent iron cooperated with microbial degradation

The misuse of aromatic amines like 4-chloroaniline (4-CA) has led to severe environmental and health issues. However, it’s difficult to be utilized by microorganisms for degradation. Nano-zero-valent iron (nZVI) is a promising material for the remediation of chloroaniline pollution, however, the synergistic effect and mechanism of nZVI with microorganisms for the degradation of 4-CA are still unclear. This study investigated the potential of 4-CA removal by the synergistic system involving nZVI and 4-CA degrading microbial flora. The results indicate that the addition of nZVI significantly enhanced the bio-degradation rate of 4-CA from 43.13 % to 62.26 %. Under conditions involving 0.1 % nZVI addition at a 24-hour interval, pH maintained at 7, and glucose as an external carbon source, the microbial biomass, antioxidant enzymes, and dehydrogenase were significantly increased, and the optimal 4-CA degradation rate achieved 68.79 %. Additionally, gas chromatography-mass spectrometry (GC-MS) analysis of intermediates indicated that the addition of nZVI reduced compounds containing benzene rings and enhanced the dechlorination efficiency. The microbial community remained stable during the 4-CA degradation process. This study illustrates the potential of nZVI in co-microbial remediation of 4-CA compounds in the environment.

Remobilization of Cd caused by iron oxide phase transformation and Mn2+ competition after stabilization by nano zero valent iron

Stabilization techniques are vital in controlling Cd soil pollution. Nano zero valent iron (nZVI) has been extensively utilized for Cd remediation owing to its robust adsorption and reactivity. However, the environmental stress-induced stability of Cd after nZVI addition remains unclear. A pot experiment was conducted to evaluate the Cd bioavailability in continuously flooded (130 d) soil after stabilization with nZVI. The findings indicated that nZVI application did not result in a decline in Cd concentration in rice, as compared to the no-nZVI control. Additionally, nZVI simultaneously increased the available Cd concentration, iron-manganese oxide-bound (OX) Mn fraction, and relative abundance of Fe(III)-reducing bacteria, but it decreased OX-Cd and Mn availability in soil. Cadmium in rice tissues was positively correlated with the available Cd in soil. The results of subsequent adsorption tests demonstrated that CdO was the product of Cd adsorption by the nZVI aging products. Conversely, Mn2+ decreased the adsorption capacity of Cd-containing solutions. These results underscore the crucial role of both biotic and abiotic factors in undermining the stabilization of nZVI under continuous flooding conditions. This study offers novel insights into the regulation of nZVI-mediated Cd stabilization efficiency in conjunction with biological inhibitors and functional modification techniques.

The polyspecificity and adaptability of multidrug resistance shaped the antibiotic resistome in anaerobic digestion amended by nZVI: Evidence from efflux pumps and rRNA methyltransferase genes

Nano zero-valent iron (nZVI) assisted anaerobic digestion (AD) is considered an effective approach for mediating antibiotic resistance genes (ARGs) dissemination in activated sludge. However, little is known about the mechanisms that drive antibiotic resistomes evolution in relation with their physiological roles distinct from protection of bacteria against antimicrobials. Metagenomic sequencing showed that efflux pumps and rRNA methyltransferase genes were the most prevalent ARGs in anaerobes, being responsible for the variations of antibiotic resistome upon nZVI addition. Particularly, to withstand the oxidative stimuli derived from initially 9 days nZVI exposure, genes involved in the two component systems (PhoRB, BaeSR, EvgSA) and global regulator networks (MarA, SoxS) were increased, activating the chromosomally encoded intrinsic efflux pumps expression. Furthermore, to benefit for survival, the polyspecific efflux pumps were involved in the microbial physiological adaptations of enhanced siderophores, lipid moieties, and quorum sensing molecules exportation, which induced the adaptative efflux pumps expression. On the other hand, to maintain the protein synthesis under oxidative condition, genes encoding Rsm, Erm, and Rlu family methyltransferases were notably up-regulated via horizontal transfer, inducing the acquired rRNA methylation resistance. Importantly, after prolonged exposure to nZVI for 18 and 27 days, the antibiotic resistomes were measurably decreased, due to the recovered oxidative state and increased membrane rigidity. Overall, these findings provide a deep understanding on the necessity of antibiotic resistome evolution for anaerobes to adapt to nZVI exposure for survival, which can be introduced to improve the ARGs risks analysis.

Evaluation of nano-scaled zero valent iron (nZVI) effects on continuous syngas biomethanation under the thermophilic condition

The nano-scaled zero valent iron (nZVI) particles were applied to strengthen the syngas biomethanation under the thermophilic condition in a continuous bubble column reactor with gas circulation. The CH4 productivity was increased by 6.80% from 71.20 mmol∙Lr−1∙day−1 to the highest 76.04 mmol∙Lr−1∙day−1 at the nZVI concentration of 2.5 g/L. The measurement of iron concentration and the observation of the iron nanoparticles distribution indicate that nZVI can act as an electron conduit to enhance more efficient direct interspecies electron transfer by physically close contact between microorganisms, instead of biological corrosion. Further analysis of metabolic products shows that the nZVI addition can stimulate the EPS secretion, and the direct electron transfer relying on nZVI particles tends to replace other transfer modes. Microbial community analysis reveals that the Bacteria Bacteroidia and Firmicutes, and Archaea Methanothermobacter are the potential dominating enriched syntrophic partners. The expression of functional genes involved in methane production was also found to increase. On the other hand, the nZVI accumulation can lead to the albefaction and inactivation of partial sludge granules due to its toxicity. The negative effects of nZVI at high concentration are also more pronounced. This work shows the feasibility of improving continuous syngas biomethanation by strengthening interspecific association and accelerating electron transfer.

Comparison of eco-improvement on constructed wetlands with nano zero valent iron introduction under different levels of PFOA stress: Perspectives on plant, microbe, and PFOA removal

The ecological hazards of perfluoro octanoic acid (PFOA, a typical perfluoroalkyl substances) have been continually reported in constructed wetlands (CWs) for wastewater treatment. In present study, nano zero valent iron (nZVI) was adopted to alleviate PFOA stress at different levels (1 and 10 mg/L) in CWs. It was revealed that the effects of nZVI on specific ecological parameters varied at different PFOA dosages. PFOA influenced plant photosynthetic and antioxidant parameters with significant concentration-dependence. NZVI addition caused more obvious promotion of chlorophyll (25.30–31.84 %) and reduction of catalase (172.64 %) and malondialdehyde (83.01 %) with 10 mg/L PFOA exposure. For microbe, nZVI was prone to stimulate enzyme activities under 1 mg/L PFOA, in which the relative activity of dehydrogenase, urease, phosphatase, and four nitrogen cycling enzymes increased by 86.25–375.56 %, 43.10–71.16 %, 1.52–29.38 %, and 4.49–315.18 %. However, nZVI caused more abundant of functional bacteria (like nitrifying bacteria and phosphorus-accumulating organisms) and function genes (like amoA, hao, and ppx) with PFOA at 10 mg/L. On the whole, changes in bacterial community confirmed the enhancement potential of nZVI on ammonium and phosphorous removal. PFOA removal at 10 mg/L was higher compared to 1 mg/L, resulting from higher abundance of class Gammaproteobacteria, and nZVI addition further contributed to the highest removal efficiency (73.54 %). This study provided evidence on nZVI as a possible manner for optimizing eco-function in CWs with PFOA stress at different levels.

Attenuation of Pb and Sb in shooting range soils by Fe amendments

Lead (Pb) and antimony (Sb) contamination pose a major environmental risk at firing ranges and threaten land sustainability. Methods for the stabilization of metal (loid) contaminants are necessary to prevent off-site migration of metals in surface and ground water or from soil erosion. In the present study, two remediation treatments (ferric chloride/calcium carbonate and nanoscale zero-valent iron (nZVI)) were applied to flow-through soil columns containing four types of soils (sand, sandy loam, loamy sand, and silty loam) to study Pb and Sb behavior. Water runoff was continuously monitored for three months prior to amendment addition and for the following ten months. Soils were characterized before and after reaction. We found Sb was more mobile than Pb in all soil systems and was primarily present in the dissolved fraction whereas Pb was associated with both soil organic matter (SOM) and Fe colloids. Dominant Pb solid phase species were comprised of Pb0, PbO, PbCO3, and Pb sorbed to Fe(III) oxides while Sb was present as fully oxidized Sb(V) in soil and soil solution. The nZVI addition had minimal impact on Pb and Sb immobilization compared to control soil. The FeCl2 and CaCO3 amendment decreased pore water Sb concentrations by >80% for all soil types and >96% reduction in the fine- and coarse-grained soil types (silt loam and sand). Lead was initially mobilized coinciding with a decrease in pH from the hydrolysis of Fe(II) in solution. Additional soil treatments have the potential to be effective for system-wide immobilization with adequate additions of CaCO3 buffer. Though this study focused on bullet fragment weathering as a source of Pb and Sb the results have application to environmental monitoring and remediation efforts at mining or industrial runoff sites.

Enhanced nitrogen removal from low C/N municipal wastewater employing algal biochar supported nano zero-valent iron (ABC-nZVI) using A/A/O-MBR: Duration and rehabilitation

To bridge the organic-dependent barrier on nitrogen from low carbon/nitrogen (C/N) municipal wastewater, employing algal biochar supported nano zero-valent iron (ABC-nZVI) was investigated using A/A/O-MBR. Firstly, it can be seen that adequate carbon source is indispensable for the removal, since total nitrogen (TN) removal reached 77.89 % with the influent C/N of 7.8. Secondly, conducted in batch experiments with different doses of ABC-nZVI with/without active sludge, removal efficiency of total inorganic nitrogen (TIN) and the effective time achieved 84.94 % and 24 h with an ABC-nZVI dose of 300 mg/L, respectively. Thirdly, it was found that the duration of high-efficiency denitrification reached 9 h with the addition of 250 mg/L of ABC-nZVI to the anoxic tank of A/A/O-MBR, and the effluent ammonium nitrogen (NH4+-N) also meet the national discharge standard. Besides, biodiversity of both anoxic and aerobic sludge was apparently promoted with the addition of ABC-nZVI, while the lab-scale A/A/O-MBR could also be fully rehabilitated within 12 h. Finally, predicted through PICRUSt2, relevant abundance of functional genes involved in nitrogen metabolism could be enriched by nZVI addition. As an alternative supporting electron donor and mediator, ABC-nZVI can also be participated in the enhanced nitrogen removal in A/A/O-MBR at low C/N.

Impacts of nano-zero-valent iron on antibiotic removal by anaerobic membrane bioreactor for swine wastewater treatment

This study investigated the impact of nano-zero-valent iron (nZVI) on the removal of veterinary antibiotics by an anaerobic membrane bioreactor (AnMBR) for swine wastewater treatment. Ten veterinary antibiotics belonged to three commonly used groups, namely tetracyclines, fluoroquinolones, and sulfonamides, were evaluated. Results show that nZVI addition could improve AnMBR performance for the removal of both phosphorus and ammonium. Moreover, nZVI addition to AnMBR could improve the biodegradation of antibiotics to enhance their overall removal. Of the three groups of antibiotics, the enhancement was most obvious for sulfonamides with an increase in the removal rate from 32 – 62% to 64–97% when 2.6 g L−1 nZVI was added to AnMBR. Further microbial and redundancy analyses indicate that the removal of sulfonamides was closely and positively correlated to the relative abundance of the genera Lentimicrobium and Methanomethylovorans. By contrast, nZVI reduced the removal of tetracycline, ciprofloxacin, and norfloxacin by AnMBR. Microbial analysis showed that nZVI dosage slightly reduced methane yield by promoting the growth of Mesotoga but reducing the proliferation of Methanomassiliicoccus to potentially disrupt the hydrogenotrophic pathway. In addition, adding nZVI reduced the ratios of protein to polysaccharide in both extracellular polymeric substances and soluble microbial products, thereby mitigating membrane fouling.

Insights into the role of nanoscale zero-valent iron in Fenton oxidation and its application in naphthalene degradation from water and slurry systems.

Few researches have focused on the role of nanoscale zero-valent iron (nZVI) in Fenton-like process for polycyclic aromatic hydrocarbons (PAHs) removal. In this study, the naphthalene (NAP) degradation tests in ultrapure water showed that nZVI addition could enhance NAP degradation from 79.7% to 99.0% in hydrogen peroxide (H2O2)/Fe (II)/nZVI/NAP system at the molar ratio of 10/5/3/1, showing the excellent role of nZVI in promoting NAP removal. Multiple linear regression analysis found that the correlation coefficient between H2O2 consumption and NAP degradation was converted from -9.17 to 0.48 with nZVI and 1-mM H2O2, indicating that nZVI could decompose H2O2 more beneficially for NAP degradation. Multiple Fe (II)-dosing and iron leaching tests revealed that nZVI could gently liberate Fe (II) and promote Fe (II)/Fe (III) redox cycle to enhance the NAP degradation. When the H2O2/Fe (II)/nZVI/NAP molar ratios of 10/5/3/1 and 50/25/15/1 were applied in the simulated NAP contaminated actual groundwater and soil slurry, respectively, 75.0% and 82.9% of NAP removals were achieved. Based on the major degradation intermediates detected by GC/MS, such as 1,4-naphthalenedione, cinnamaldehyde, and o-phthalaldehyde, three possible NAP degradation pathways were proposed. This study provided the applicable potential of nZVI in Fenton process for PAHs contaminated groundwater and soil remediation. PRACTITIONER POINTS: nZVI enhanced the NAP degradation in Fenton-like process. Three schemes of NAP degradation pathway were proposed. nZVI performed well in the remediation of the simulated NAP contamination.

Enhanced biogas biological upgrading from kitchen wastewater by in-situ hydrogen supply through nano zero-valent iron corrosion

The in-situ hydrogen supply by nano zero-valent iron (nZVI, nFe0) corrosion provided a feasible way to improve the efficiency of biogas biological upgrading. This work studied the effects of nZVI at different dosages (0, 2, 4, 6, 8 and 10 g/L) on anaerobic digestion of kitchen wastewater by two buffer systems 2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid (HEPES) and sodium hydrogen carbonate (NaHCO3). The addition of nZVI improved the content of methane (CH4) and stability of anaerobic digestion process. In HEPES buffer system, the CH4 was all increased and the maximum reached 90.51% with 10 g/L nZVI, higher than 32.25% compared to the control. The maximum hydrogen enrichment (HE) was 113 ppb after nZVI addition, indicating the mass transfer efficiency of hydrogen (H2) was improved. Microbial community analysis showed that the total relative abundance of Methanobacterium and Methanolinea at 10 g/L nZVI was 53.72%, which was 1.62 times of the control group. However, in the NaHCO3 buffer system with 10 g/L nZVI addition, the content of CH4 and the loosely bound extracellular polymeric substances (LB-EPS) was lower than the control. The results indicated that the addition of nZVI was feasible for biogas upgrading, and the bidirectional effect of nZVI on the promotion or inhibition of bio-methanation might be related to the buffer system of the anaerobic process.

Enhanced antibiotic degradation and hydrogen production of deacetoxycephalosporin C fermentation residue by gamma radiation coupled with nano zero-valent iron

Antibiotic fermentation residue (AFR) has been categorized as hazardous waste in China. Anaerobic biohydrogen fermentation may be a promising technology for handling AFR, which could achieve dual goals of waste treatment and clean energy production at the same time. However, the low hydrogen yield and low removal efficiency of residual antibiotics are two major factors limiting the AFR biohydrogen fermentation process. This work firstly applied gamma radiation (50 kGy) to remove the residual antibiotic in AFR and improve the bioavailability of organic matters, then adding nano zero-valent iron (nZVI) (100–1000 mg/L) to further enhance the AFR biohydrogen fermentation performance. Results showed that residual deacetoxycephalosporin C in AFR was removed with a high efficiency of 98.6%, and hydrogen yield achieved 20.45 mL/g-VSadded with the combined approach of gamma radiation pretreatment and 500 mg/L nZVI addition, which was 139.2% higher compared to the control experimental result. The combined approach also promoted the biohydrogen production rate, decreased the lag phase of hydrogen production, and increased the organics utilization. Microbiological analysis revealed that highly efficient hydrogen-producing genera Clostridium sensu stricto were enriched in much higher abundance with the combined approach, which might be the fundamental mechanism for the enhanced AFR fermentation performance.

In-situ methane enrichment in continuous anaerobic digestion of pig slurry by zero-valent iron nanoparticles addition under mesophilic and thermophilic conditions

The effect of zero-valent iron nanoparticles (nZVI) addition on methane production during anaerobic digestion of pig slurry was assessed. Experiments were conducted using two experimental set-ups: batch and long-term continuous operation at a fixed nZVI dosage. Two different temperature operation ranges (mesophilic and thermophilic) were assessed. Biogas production and methane content were monitored, and the specific methanogenic activity of the biomass and nZVI oxidation state were evaluated at different times. The results of batch experiments at mesophilic temperature operation showed an inhibition of methane production at all tested dosages (42, 84, 168 and 254 mgnZVI g−1 VSS concentrations), while methane production was boosted with the lowest dosage in thermophilic temperature operation. In continuous operation, nZVI addition produced an increase in methane content of biogas, achieving values between 80 and 85% in both temperature ranges. The average methane production rate increased 165% and 94% with respect to the control in thermophilic and mesophilic temperature range, respectively. The oxidation state of nZVI showed a value of +3 almost immediately after contact with substrate and a slower progressive oxidation during the reactors operation. The obtained results indicate that nZVI addition in anaerobic digestion is an interesting strategy for in situ biogas upgrading.

Nanoscale zero-valent iron inhibits the horizontal gene transfer of antibiotic resistance genes in chicken manure compost

Livestock manure has been identified as a significant hotspot for antibiotic resistance genes (ARGs). However, the impact of nanoscale zero-valent iron (nZVI) on the fate of ARGs during livestock manure composting remains poorly understood. Here, we investigated the evolution of ARGs in chicken manure compost exposed to 100 and 600 mg kg−1 nZVI. The results showed that nZVI addition reduced the concentration of some antibiotics such as doxycycline and sulfamethoxazole. Furthermore, nZVI addition decreased the abundances of most ARGs at the end of composting, but nZVI dosage did not have any significant effect. The abundances of the dominant ARGs (sul1 and sul2) were significantly correlated to the class 1 integron-integrase gene (intI1). A network analysis revealed that the genera Bacteroides, Bacillus, Corynebacterium, Thiopseudomonas and Pseudomonas were the main potential hosts for multiple ARGs, and the decreased abundance of these bacteria contributed to the removal of ARGs. Structural equation models demonstrated that the reduction in intI1 played a predominant role in ARG removal. The nZVI also had direct effects on the intI1 abundance. These findings suggest that the addition of nZVI is a promising strategy to minimize ARG release in chicken manure compost.

Combination of immobilized TiO2 and zero valent iron for efficient arsenic removal in aqueous solutions

The photocatalytic removal of arsenic from aqueous solutions was investigated using titania (TiO2) immobilized on a glass support, both bare and combined with synthesized metallic iron nanoparticles (nZVI) or commercial microscale iron (ZVI). Three procedures, namely dip-coating, rotational coating, and sponge coating, were tested for achieving the immobilization of TiO2. The photocatalytic activity of the semiconductor films under UV-irradiation after cumulative coatings was evaluated for 10 mg L−1 aqueous As(III), which was entirely oxidized to As(V) with all settings within 90 and 180 min. Titania immobilized by dip-coating was found to be the most effective as it showed the faster kinetics. The reuse of immobilized TiO2 was also investigated, detecting no changes in the photocatalytic activity after five consecutive reactions. The addition of commercial ZVI particles to the immobilized TiO2 system did not bring about significant changes in the kinetics for As(III) oxidation at the three pH values investigated, i.e., 5, 7, and 9. By contrast, the addition of nZVI not only led to a faster depletion of As(III) compared to bare titania but also the removal of As(V) from the solution to concentrations below 10 μg L−1, the upper limit recommended by the World Health Organization for human consumption.The role of iron species in the arsenic removal process with both [ZVI + immobilized-TiO2] and [nZVI + immobilized-TiO2] systems was further investigated by performing adsorption and irradiation experiments without titania. It was inferred that within the pH range evaluated, the minor corrosion of the ZVI surface, even under UV irradiation, restricts the production of reactive oxidizing species and the generation of sites for arsenic species adsorption. By contrast, adsorption should be the main process responsible for the overall diminution of As(III) and As(V) species in solution attained upon nZVI addition, promoted by the increase of the external oxides/hydroxides layer on iron nanoparticles. Nevertheless, it might be also considered a certain contribution of UV-generated oxidant species formed in nZVI to the photocatalytic oxidation performance of titania.

Transcriptomics Uncovers the Response of Anammox Bacteria to Dissolved Oxygen Inhibition and the Subsequent Recovery Mechanism.

Understanding the recovery of anaerobic ammonium-oxidizing (anammox) bacteria after inhibition by dissolved oxygen (DO) is critical for the successful applications of anammox-based processes. Therefore, the effects of oxygen exposure (2 mg L-1 DO for 90 min) and subsequent recovery treatments [N2 purging or nano zero-valent iron (nZVI) addition] on the activity and gene expression in a Kuenenia stuttgartiensis enrichment culture were examined. Combining the self-organizing map clustering and enrichment analysis, we proposed the oxidative stress response of anammox bacteria based on the existing concepts of oxidative stress in microbes: the DO exposure triggered a stringent response in K. stuttgartiensis, which downregulated the transcription levels of genes involved in the central metabolism and diverted energy to a flagellar assembly and metal transport modules; these changes possibly promoted survival during the inhibition of anammox activity. According to the cotranscription with central catabolism genes, putative reactive oxygen species (ROS) scavenger genes (kat and sod) were presumed to detoxify the anammox intermediates rather than ROS. In addition, both activity and mRNA profiles with appropriate amount of nZVI addition (5 and 25 mg L-1) were close to that of control, which proved the effectiveness of nZVI addition in anammox recovery. These results would be relevant to the physio-biochemistry development of anammox bacteria and further enhancement of nitrogen removal in wastewater treatment.

Impact of different environmental particles on degradation of dibutyl phthalate in coastal sediments with and without Cylindrotheca closterium

This study investigated the impact of different environmental particles at different concentrations (0.2% and 2%, w/w) on biodegradation of dibutyl phthalate (DBP) in sediments with and without Cylindrotheca closterium, a marine benthic diatom. The particles included biochar pyrolyzed at 400 °C, multi-walled carbon nanotube (MWNT), nanoscale zero-valent iron (nZVI) and polyethylene microplastic. In treatments without C. closterium, inhibition effect of the particles on degradation percentage of DBP (up to 15.7% decrement except 1.7% increment for 0.2% nZVI) increased with the increase of particle sorption ability to DBP and particle concentration in general. The results of 16s rDNA sequencing showed that C. closterium was probably the most abundant DBP-degrader, accounting for 20.0–49.3% of the total taxon read numbers. In treatments with C. closterium, inoculation of C. closterium increased the degradation percentage of DBP in all treatments with particle addition by 0.0–11.3%, which increased with the increase of chlorophyll a content in general but decreased with the increase of particle concentration from 0.2% to 2%. The increment was the highest for treatment with 0.2% nZVI addition due to its highest promotion effect on algal growth. In contrast, the increment was the lowest for treatments with MWNT addition due to its strong sorption to DBP and strong inhibition on the growth of C. closterium. Our findings suggested that the environmental particles could influence bioavailability of DBP by sorption and biomass of C. closterium, and thus degradation of DBP in sediments.

Shift in Natural Groundwater Bacterial Community Structure Due to Zero-Valent Iron Nanoparticles (nZVI).

Toxic and persistent contaminants in groundwater are technologically difficult to remediate. Remediation techniques using nanoparticles (NPs) such as nZVI (Zero-Valent Iron) are applicable as in situ reduction or oxidation agents and give promising results for groundwater treatment. However, these NP may also represent an additional contamination in groundwater. The aims of this study are to assess the impact of nZVI on the nitrate-reducing potential, the abundance and the structure of a planktonic nitrate-reducing bacterial community sampled in groundwater from a multicontaminated site. An active nitrate-reducing bacterial community was obtained from groundwater samples, and inoculated into batch reactors containing a carbon substrate, nitrate and a range of nZVI concentrations (from 0 to 70.1 mg Fe.L-1). Physical (pH, redox potential), chemical ( concentrations) and biological (DNA, RNA) parameters were monitored during 1 week, as well as nZVI size distribution and mortality of bacteria. Nitrate-reducing activity was temporally stopped in the presence of nZVI at concentrations higher than 30 mg L-1, and bacterial molecular parameters all decreased before resuming to initial values 48 h after nZVI addition. Bacterial community composition was also modified in all cultures exposed to nZVI as shown by CE-SSCP fingerprints. Surprisingly, it appeared overall that bacteria viability was lower for lower nZVI concentrations. This is possibly due to the presence of larger, less reactive NP aggregates for higher nZVI concentrations, which inhibit bacterial activity but could limit cell mortality. After 1 week, the bacterial cultures were transplanted into fresh media without nZVI, to assess their resilience in terms of activity. A lag-phase, corresponding to an adaptation phase of the community, was observed during this step before nitrate reduction reiterated, demonstrating the community’s resilience. The induction by nZVI of modifications in the bacterial community composition and thus in its metabolic potentials, if also occurring on site, could affect groundwater functioning on the long term following nZVI application. Further work dedicated to the study of nZVI impact on bacterial community directly on site is needed to assess a potential impact on groundwater functioning following nZVI application.

Comparison of short-term dosing ferrous ion and nanoscale zero-valent iron for rapid recovery of anammox activity from dissolved oxygen inhibition

As obligate anaerobes, anammox bacteria are sensitive to oxygen, which might hinder the maximization of anammox activity. However, there are very few effective strategies to rapidly recover anammox activity after its deterioration under exposure of oxygen. In this study, the activity recovery of anammox bacteria encountering dissolved oxygen (DO) exposure (0.2 and 2.0 mg L−1) were compared by three strategies in short-term experiments, nZVI, Fe(II) dosing, and N2 purging. nZVI is more effective in recovering anammox activity with a high DO exposure (2 mg L−1), compared to a low DO exposure (0.2 mg L−1). After inhibiting by 2.0 mg L−1 DO, anammox activity recovery (normalized to the control) was ranked in the order of nZVI (5 mg L−1) addition (63 ± 8.2%) > Fe(II) (5 mg L−1) addition (41 ± 8.0%) >N2 purging (39 ± 4.0%). In contrast to Fe(II) ion additions, the shell structure of nZVI combined with the buffering effect of biomass-extracellular polysaccharide (EPS) prevented the sharp pH variation and excessive dissolved Fe(II)/Fe(III) in solution. Under such circumstances, nZVI addition (5 and 25 mg L−1) increased the intracellular reactive oxygen species (ROS) to a moderate level (<200%), which might be responsible for the better activity recovery of anammox than that of Fe(II) addition and N2 purging. Specifically, 5 mg L−1 nZVI dosage moderately enhanced the intracellular O2− production (∼150% of the control) after scavenging 2.0 mg L−1 DO, and the anammox activity recovered better than that of both 5 and 25 mg L−1 Fe(II) ions additions. However, high dosage nZVI (75 mg L−1) inhibited anammox activity in spite of low or high DO exposure. Our findings elucidate that appropriate amount of nZVI (short-term dosing) can rapidly recover anammox activity when anammox bacteria encountering oxygen exposure accidentally and could be useful in facilitating the robust operation of anammox-based processes.

New insights into the impact of nZVI on soil microbial biodiversity and functionality

Nanoscale zero-valent iron (nZVI) is a strong reducing agent used for in situ remediation of soil. The impacts of nZVI (5–10% w/w) on the soil microbial biodiversity and functionality of two soils (Lufa 2.2 and 2.4) were assessed. Illumina MiSeq technology was used to evaluate the structure of soil microbiomes after 21 days of exposure. Proteobacteria, Verrucomicrobia, Firmicutes and Actinobacteria were the most abundant phyla in both soils. However, the dynamics of bacterial community composition following nZVI addition differed. nZVI exposure induced pronounced shifts in the microbial composition of soil 2.4, but not in soil 2.2; an increase in Verrucomicrobia abundance was the unique common taxonomic pattern observed in both soils. The PICRUSt approach was applied to predict the functional composition of each metagenome. Environmental information processing function (membrane transport) was decreased in both nZVI-spiked soils, although soil 2.4 samples were enriched in functions involved in cellular processes and metabolism. The effects of nZVI on autochthonous bacterial communities clearly varied with the soil type assessed; changes at the phylogenetic level appeared to be more abundant than those observed at the functional level, and thus, the overall effort of the soil ecosystem might involve the maintenance of functionality following nZVI exposure.