Deep groundwater irrigation altered microbial community and increased anammox and methane oxidation in paddy wetlands of Sanjiang Plain, China.

The over-utilizing of nitrogen fertilizers in paddy wetlands potentially threatens to the surrounding waterbody, and a deep understanding of the community and function of microorganisms is crucial for paddy non-point source pollution control. In this study, top soil samples (0-15 cm) of paddy wetlands under groundwater's irrigation at different depths (H1: 6.8 m, H2: 13.7 m, H3: 14.8 m, H4: 15.6 m, H5: 17.0 m, and H6: 17.8 m) were collected to investigate microbial community and function differences and their interrelation with soil properties. Results suggested some soil factor differences for groundwater's irrigation at different depths. Deep-groundwater's irrigation (H2-H6) was beneficial to the accumulation of various electron acceptors. Nitrifying-bacteria Ellin6067 had high abundance under deep groundwater irrigation, which was consistent with its diverse metabolic capacity. Meanwhile, denitrifying bacteria had diverse distribution patterns. Iron-reducing bacteria Geobacter was abundant in H1, and Anaeromyxobacter was abundant under deep groundwater irrigation; both species could participate in Fe-anammox. Furthermore, Geobacter could perform dissimilatory nitrate reduction to ammonia using divalent iron and provide substrate supply for anammox. Intrasporangium and norank_f_Gemmatimonadacea had good chromium- and vanadium-reducting potentials and could promote the occurrence of anammox. Low abundances of methanotrophs Methylocystis and norank_f_Methyloligellaceae were associated with the relatively anoxic environment of paddy wetlands, and the presence of aerobic methane oxidation was favorable for in-situ methane abatement. Moisture, pH, and TP had crucial effects on microbial community under phylum- and genus-levels. Microorganisms under shallow groundwater irrigation were highly sensitive to environmental changes, and Fe-anammox, nitrification, and methane oxidation were favorable under deep groundwater irrigation. This study highlights the importance of comprehensively revealing the microbial community and function of paddy wetlands under groundwater's irrigation and reveals the underlying function of indigenous microorganisms in agricultural non-point pollution control and greenhouse gas abatement.

Effects of chromate on nitrogen removal and microbial community in two-stage vertical-flow constructed wetlands.

Nitrogen and chromium (Cr(VI)) pollution in waterbodies pose great threats to human health, and a cost-effective alternative with Cr(VI) and nitrogen simultaneous removal is still needed. This study investigated the influence of Cr(VI) on nitrogen removal in the two-stage vertical-flow constructed wetlands (TS-VFCWs) along with iron ore and woodchip, and explored relationship between Cr(VI) and nitrogen removal. The results showed that efficient Cr(VI) and nitrogen removal were simultaneously achieved in TS-VFCWs together with iron-ore and woodchip under 2 mg/L-Cr(VI), whereas 10 mg/L-Cr(VI) gave significant and recoverable inhibition of nitrogen removal. Cr(VI) supplementation promoted the beneficiation of Cr(VI)-reducing/resistant bacteria IMCC26207 and Bryobacter on iron-ore. Woodchip enriched Cr(VI)-reducing bacteria Streptomyces and Thiobacillus. XRD and XPS showed that abundant bound-Cr existed in the surface of iron ore and woodchip, and Cr(III) precipitation/oxide was the major product. High abundances of nitrifying and autotrophic/heterotrophic denitrifying bacteria ensured good nitrogen removal at Cr(VI) stress.

Co-metabolic biodegradation of chlorinated ethene in an oxygen- and ethane-based membrane biofilm reactor.

Groundwater contamination by chlorinated ethenes is an urgent concern worldwide. One approach for detoxifying chlorinated ethenes is aerobic co-metabilims using ethane (C2H6) as the primary substrate. This study evaluated long-term continuous biodegradation of three chlorinated alkenes in a membrane biofilm reactor (MBfR) that delivered C2H6 and O2 via gas-transfer membranes. During 133 days of continuous operation, removals of dichloroethane (DCE), trichloroethene (TCE), and tetrachloroethene (PCE) were as high as 94 % and with effluent concentrations below 5 µM. In situ batch tests showed that the co-metabolic kinetics were faster with more chlorination. C2H6-oxidizing Comamonadaceae and "others," such as Methylococcaceae, oxidized C2H6 via monooxyenation reactions. The abundant non-ethane monooxygenases, particularly propane monooxygenase, appears to have been responsible for C2H6 aerobic metabolism and co-metabolism of chlorinated ethenes. This work proves that the C2H6 + O2 MBfR is a platform for ex-situ bioremediation of chlorinated ethenes, and the generalized action of the monooxygenases may make it applicable for other chlorinated organic contaminants.

Enhanced nitrate removal using in situ reactive zone with reduced graphene oxide supported nanoscale zero-valent iron.

Nitrate pollution in groundwater is becoming more serious, which is harmful to human health. The reduced graphene oxide supported nanoscale zero-valent iron (nZVI/rGO) composite prepared in this paper can effectively remove nitrate in groundwater. In situ remediation of nitrate-contaminated aquifer was also studied. The results showed that NH4+-N was the main product of NO3--N reduction, and N2 and NH3 were also produced. When the dosage of rGO/nZVI was more than 0.2 g/L, there was no accumulation of intermediate NO2--N during the reaction process. NO3--N was removed by rGO/nZVI mainly through physical adsorption and reduction process with the maximum adsorbing ability of 37.44 mg NO3--N/g. After the slurry of rGO/nZVI was injected into the aquifer, a stable reaction zone could be formed. NO3--N could be removed continuously within 96 h at the simulated tank, and NH4+-N and NO2--N were as the main reduction products. Moreover, the concentration of TFe near the injection well increased rapidly after rGO/nZVI injection, and could be detected at the downstream end, indicating that the reaction range was large enough for NO3--N removal.

Highly efficient removal of Se(IV) using reduced graphene oxide-supported nanoscale zero-valent iron (nZVI/rGO): selenium removal mechanism.

Se(IV) removal using nanoscale zero-valent iron (nZVI) has been extensively studied. Still, the synergistic removal of Se(IV) by reduced graphene oxide-supported nanoscale zero-valent iron (nZVI/rGO) has not been reported. In this study, nZVI/rGO was successfully synthesized for Se(IV) removal from wastewater. The effects of different environmental conditions (load ratio, dosage, initial pH) on Se(IV) removal by nZVI/rGO were investigated. When the load ratio is 10%, the dosage is 0.3 g/L, the initial pH is 3, and the removal rate is 99%. The adsorption isotherm and kinetics accorded with the Langmuir isotherm and first-order kinetics models (R2 > 0.99). The fitted maximum adsorption capacity reached up to 173.53 mg/g. NZVI/rGo and Se(IV) is a spontaneous endothermic reaction (△G < 0, △H > 0) and is characterized by EDS, XRD, and XPS before and after the reaction, to further clarify the reaction mechanism. The XPS narrow spectrum analysis suggested that Se(IV) was reduced to elemental selenium (Se(0)), while the intermediate Fe(II) was oxidized to form hydroxide precipitation. Therefore, nZVI/rGO was favored for Se-contaminated wastewater remediation.

Graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) for antimony removal: The role of active oxygen species and reaction mechanism.

Sulfidated nano zero-valent iron (S-nZVI) was used to remove various pollutants from wastewater. However, the instability, poor dispersibility, and low electron transfer efficiency of S-nZVI limit its application. Herein, graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) was successfully synthesized using graphene oxide (GO) as a carrier. The properties of S-nZVI@GO were characterized by scanning electron microscopy coupled to X-ray photoelectron spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) concerning the surface morphology, crystalline structure, and elemental components. S-nZVI@GO displayed an excellent capacity for antimony (Sb) removal under aerobic conditions (96.7%), with a high adsorption capacity (Qmax = 311.75 mg/g). It maintained a high removal rate (over 90%) during a wide pH range (3-9). More importantly, S-nZVI@GO activated the molecular oxygen in water via a single-electron pathway to produce •O2－ and H2O2, and then oxidized trivalent antimony (Sb(III)) to pentavalent antimony (Sb(V)) and further separated it by synergistic adsorption and co-precipitation. Therefore, S-nZVI@GO shows excellent potential for Sb contamination remediation.

Unveiling microbial community and function involved in anammox in paddy vadose under groundwater irrigation.

The extensive application of nitrogen fertilizer in intensive irrigation areas poses a potential threat to groundwater. Given that the vadose zone acts as a buffer zone for the underground entry of surface pollutants, an in-depth understanding of its microbial community structure and function was crucial for controlling groundwater nitrogen pollution. In this study, soil samples from paddy vadose under groundwater irrigation with different depths (G1: 6.8 m, G2: 13.7 m, G3: 15.6 m, and G4: 17.8 m) were collected to unravel the differences in microbial community structure and function at different vadose depths (0-250 cm), as well as their relationship with soil properties. Results showed some differences among soil physicochemical factors under groundwater irrigation with different depths and that some electron acceptors were more abundant than others under deep groundwater irrigation (G2-G4). Remarkable differences in microbial communities under shallow- and deep-groundwater irrigation were found. The high abundances of anammox bacteria Candidatus_Brocadia in G2 and G3 indicated that deep groundwater irrigation was beneficial to its enrichment. Iron-reducing bacteria Anaeromyxobacter and sulfate-reducing bacteria Desulfovibrio were widely distributed in vadose zone and possessed the potential for anammox coupled with Fe(III)/sulfate reduction. Norank_f_Gemmatimonadaceae had nitrate- and vanadium-reducing abilities and could participate in anammox in vadose zone. Dissimilatory nitrate reduction to ammonia (DNRA) bacteria Geobacter facilitated Fe(II)-driven DNRA and thus provided electron donors and acceptors to anammox bacteria. Soil nutrients and electron donors/acceptors played important roles in shaping microbial community structure at phylum and genus levels. Microorganisms in vadose zone under groundwater irrigation showed good material/energy metabolism levels. Deep groundwater irrigation was conducive to the occurrence of anammox coupled with multi-electron acceptors. Our findings highlight the importance of understanding the structure and function of microbial communities in paddy vadose under groundwater irrigation and reveal the potential role of indigenous microorganisms in in-situ nitrogen removal.

Insight into SO4(-II)- dependent anaerobic methane oxidation in landfill: Dual-substrates dynamics model, microbial community, function and metabolic pathway.

In anaerobic landfill, SO42- could serve as electron receptor for methane oxidation. In theory, concentrations of both methane and SO42- should be related to methane oxidation rate. However, the dynamics process has yet to be discovered, and the understanding of metabolic pathways of the sulfate-dependent anaerobic methane oxidation (S-DAMO) process in landfill remains limited. In this study, S-DAMO dynamics was investigated by observing the CH4 oxidation rates under different CH4/ SO42-counter-gradients. The CH4-SO42- dual-substrate model based on MichaeliseMenten equation was got (maximum substrate degradation rate Vmax [22.9 ± 1.31] µmol/[kg·d], half-saturation constants [Formula: see text] , and [Formula: see text] ). High-throughput sequencing analysis indicated Methanobacterials, Methanosarcinales, and Soil Crenarchaeotic were the main functional microorganisms for S-DAMO in landfill. The metabolic pathway of S-DAMO was speculated as the reverse methanogenesis pathway through Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUST) analysis, while methanogenesis was the methyl nutrition way based on methanol. The enzymes related to the carbon and sulfur cycles and their relative abundances in the microcosms were analyzed to graph the methane metabolic pathway and the sulfur metabolic pathway. The findings provide important parameters for CH4 mitigation in landfills, and give a new insight for understanding S-DAMO metabolic pathway in landfill.

Effects of pollution load and salinity shock on nitrogen removal and bacterial community in two-stage vertical flow constructed wetlands.

To understand the denitrification performance and microbial community of two-stage vertical flow constructed wetlands (TS-VFCWs) with iron ore/manganese ore and wood chips, COD and nitrogen removal were investigated under pollution load and salinity shock. High removal of COD (87%), NH4+-N (97%), and NO3--N (98%) were achieved with increasing load, but the high pollutant load inhibited the denitrification performance in TS-VFCW with iron ore and wood chips. TS-VFCW with iron ore and wood chips showed good recovery potential with decreasing load. High NH4+-N removal was observed in TS-VFCW with manganese ore and wood chips. Treatment with 3% salinity decreased COD and NH4+-N removal but improved NO3--N removal, maintaining relatively good nitrogen removal. The addition of iron ore and manganese ore enriched nitrifying bacteria Flavobacterium and autotrophic denitrifying bacteria, while wood chips promoted heterotrophic denitrification and organic degradation. In addition, ubiquitous denitrifying bacteria under salinity ensured excellent denitrification performance.

An in-situ bio-remediation of nitrobenzene in stimulated aquifer using emulsified vegetable oil.

Widespread nitrobenzene (NB) contamination in groundwater requires an economical and effective remediation technology. In situ microbial reactive zone enhanced by injecting emulsified vegetable oil (EVO) is an effective method for remediating NB-contaminated groundwater, which can be reduced to aniline (AN) effectively in the reactive zone. However, the bio-mechanism of NB remediation in a real contaminated site is still unclear. Thus, a 3-D tank was established to conduct a pilot-scale experiment and the bacterial communities in the tank were analyzed by 16S rDNA high-throughput sequencing. The results suggested that the injection of EVO can stimulate some certain microorganisms to grow, and reduce NB though biological and biochemical processes. There were three degradation pathways of NB: (1) direct oxidation by Pseudomonas; (2) direct mineralization by Clostridium sensu stricto; and (3) coupled reduction of NB through microbial dissimilatory iron reduction by Geobacter and Arthrobacter. Among these pathways, the coupled reduction process is the main degradation pathway.

Indigenous bacterial community and function in phenanthrene-polluted coastal wetlands: Potential for phenanthrene degradation and relation with soil properties.

The Yellow River Delta, adjacent to Shengli Oilfield, has a potential risk of petroleum pollution. In this study, soil samples were collected from phenanthrene (PHE)-polluted (adjacent to abandoned oil well, Zone D) and non-polluted (far away from abandoned oil well, Zone E) coastal wetlands. The influence of PHE pollution on indigenous bacterial community and function, and their relationship with soil characteristics were investigated. The levels of PHE, salinity and NH4+-N were higher in Zone D than in Zone E. PHE-degrading bacteria Achromobacter and Acinetobacter were mainly distributed in Zone E, whereas Halomonas, Marinobacter, and Roseovarius were highly abundant in Zone D. Halomonas and Marinobacter had the potential for denitrification and could achieve PHE degradation through mutual cooperation. PHE pollution could increase the abundance of functional bacteria but reduce the diversity of microbial community. PHE and salinity played key roles in shaping microbial community structure and function. High PHE level inhibited microbial metabolism but stimulated self-protection potential. PHE aerobic degradation associated with the catechol and phthalic acid pathways was found in Zone D, whereas the catechol pathway dominated in Zone E. Interestingly, PHE anaerobic degradation with nitrate reduction also dominated in Zone D, whereas the process coupled with multiple electron acceptors co-existed in Zone E, which was associated with tidal seawater carrying nutrients. This study illustrated the importance of comprehensive consideration of microbial community structure and function under PHE pollution, suggesting indigenous microorganisms as potential microbial consortium for bioremediation in coastal wetlands.

Unraveling bacterial community structure and function and their links with natural salinity gradient in the Yellow River Delta.

Salinization can change the soil environment and affect microbial processes. In this study, soil samples were collected from Zone A (Phragmites australis wetlands), Zone B (P. australis and Suaeda salsa wetlands), and Zone C (Spartina alterniflora wetlands) in the Yellow River Delta. The microbial community and functional potential along the natural salinity gradient were investigated. Total nitrogen, ammonia nitrogen, and soil organic matter presented a downward trend, and salinity first increased and then decreased from Zone A to Zone C. Nitrospira and norank_f_Nitrosomonadaceae were widely distributed throughout the zones. Denitrifying bacteria Alcanivorax, Marinobacterter, and Marinobacterium were abundant in Zone B and preferred high salinity levels. However, denitrifying bacteria Azoarcus, Flavobacterium, and Pseudomonas were mainly distributed in low-salinity Zones A and C, suggesting their high sensitivity to salinity. Dissimilatory nitrate reduction to ammonia (DNRA) bacteria Aeromonas and Geobacter dominated Zone C, whereas Caldithrix performed DNRA in Zone B. Interestingly, DNRA with organic matter as the electron donor (C-DNRA) occurred in Zone A; DNRA coupled with sulfide oxidation (S-DNRA) was dominant in Zone B; and C-DNRA and DNRA with divalent iron as electron donor and S-DNRA occurred simultaneously in Zone C. Salinity was the key factor distinguishing low and high salinity zones, and total nitrogen and total phosphorus had important effects at the phylum and genus levels. The abundance of genes encoding cell growth and death was relatively stable, indicating that the microbial community had good environmental adaptability. The genes related to the biodegradation of xenobiotics and the metabolism of terpenoids and polyketides were abundant in Zone B, revealing high metabolic potential for exogenous refractory substances. The microorganisms under low-salinity Zones A and C were more sensitive to environmental changes than those under Zone B. These results suggest that salinity plays important roles in microbial processes and shapes specific functional zones in coastal wetlands.

Soil organic matter and salinity as critical factors affecting the bacterial community and function of Phragmites australis dominated riparian and coastal wetlands.

Soil salinization poses a great threat to the natural ecosystem and interferes with the structure and function of the biological community, resulting in different vegetation distributions. However, little attention is paid to the changes in microbial community in different wetland types with the same vegetation. In this study, the Yellow River Delta was used as a model because of its typical and extensive distribution of Phragmites australis-dominated saltwater and freshwater wetlands. We investigated the differences in the structure and function of bacterial communities, as well as their relationships with soil properties in coastal (Zone A) and riparian (Zone B) wetlands. Results showed higher salinity and pH in Zone A than Zone B (p < 0.05), whereas TN (p < 0.05) and SOM were lower than those in Zone B. Significant differences existed in microbial community composition between Zones A and B. The nitrifying-bacteria Nitrospira and norank_f_Nitrosomonadaceae had high abundance in Zones A and B. Alcanivorax, Halomonas, and Marinobacter were extensively distributed in Zone A, whereas Flavobacterium and Pseudomonas were dominant in Zone B, indicating the diversity characteristics of denitrifying bacteria. Conversely, methane-oxidizing bacteria Methylophaga were significantly higher in Zone A than in Zone B (p < 0.05), indicating that high salinity was conducive to aerobic methane oxidation and that the genetic diversity at strain level endowed it with a certain denitrification potential. Salinity and SOM played important roles in shaping microbial community at phylum and genus levels. The gene abundances related to xenobiotics metabolism and repair were high in Zone A, whereas the genes encoding energy metabolism and signal transduction were relatively high in Zone B. Denitrification was more favored for the low-salinity Zone B, whereas methane oxidation was enriched in the high-salinity Zone A. Therefore, our study emphasized the importance of an in-depth understanding of the microbial-community structure and function in Phragmites australis-dominated saltwater and freshwater wetlands.

Influence of the structure and composition of Fe-Mn binary oxides on rGO on As(III) removal from aquifers.

Fe-Mn binary oxide (FMBO) possesses high efficiency for As(III) abatement based on the good adsorption affinity of iron oxide and the oxidizing capacity of Mn(IV), and the composition and structure of FMBO play important roles in this process. To compare the removal performance and determine the optimum formula for FMBO, magnetic graphene oxide (MRGO)-FMBO and MRGO-MnO2 were synthesized with MRGO as a carrier to improve the dispersity of the adsorbents in aquifers and achieve magnetic recycling. Results indicated that MRGO-FMBO had higher As(III) removal than that of MRGO-MnO2, although the ratios of Fe and Mn were similar, because the binary oxide of Fe and Mn facilitated electron transfer from Mn(IV) to As(III), while the separation of Mn and Fe on MRGO-MnO2 restricted the process. The optimal stoichiometry x for MRGO-FMBO (MnxFe3-xO4) was 0.46, and an extraordinary adsorption capacity of 24.38 mg/g for As(III) was achieved. MRGO-FMBO showed stable dispersive properties in aquifers, and exhibited excellent practicability and reusability, with a saturation magnetization of 7.6 emu/g and high conservation of magnetic properties after 5 cycles of regeneration and reuse. In addition, the presence of coexisting ions would not restrict the practical application of MRGO-FMBO in groundwater remediation. The redox reactions of As(III) and Mn(IV) on MRGO-FMBO were also described. The deprotonated aqueous As(III) on the surface of MRGO-FMBO transferred electrons to Mn(IV), and the formed As(V) oxyanions were bound to ferric oxide as inner-sphere complexes by coordinating their "-OH" groups with Mn(IV) oxides at the surface of MRGO-FMBO. This work could provide new insights into high-performance removal of As(III) in aquifers.

Bacterial community structure and function in soils from tidal freshwater wetlands in a Chinese delta: Potential impacts of salinity and nutrient.

Microorganisms in tidal freshwater wetlands affect biogeochemical cycling of nutrients, but the structures and functions of the wetland communities change due to natural and anthropogenic stresses. Soil samples were collected along a 350-m sampling belt in typical tidal freshwater wetlands of Yellow River Delta to investigate nutrient distributions, bacterial community structures and potential metabolic functions under tide and runoff stress by high-throughput sequencing and PICRUSt analysis. The total nitrogen (TN) contents varied greatly while total phosphorous (TP) contents were relatively stable. The bacterial community structures and predicted functions varied along a 350-m sampling belt. Some sulfate-reducing bacteria, nitrifying bacteria, Marmoricola, unclassified_f_Salinisphaeraceae and Oceanococcus exhibited a decreased trend with increasing distances far away from the river bank (B-0m). However, Salinisphaera was more dominant far away from the river bank (B-350m), indicating the stronger tolerance degree under salt stress. Marinobacterium and Marinobacter could be widely detected from B-0m to B-350m, demonstrating that those bacteria could tolerate a broad range of salinity and have its exceptional adaptation capacities. Redundancy analysis (RDA) indicated that nutrient and salinity played an important role in shaping bacterial community composition. NH4+-N and AP were the key factors in explaining the variance of the genus level. Predicted by PICRUSt analysis, nitrogen fixation (NF), nitrogen mineralization (NM), denitrification and dissimilatory nitrate reduction to ammonium (DNRA) might be the dominant processes of nitrogen metabolism and related genes abundance were abundant in tidal freshwater wetland soils. These findings could provide new insights into the prevention and control of potential nutrient pollution in tidal freshwater wetlands under the dual stress of tide and runoff.

Long-term impacts of graphene oxide and Ag nanoparticles on anammox process: Performance, microbial community and toxic mechanism.

The increasing application of engineered nanoparticles (NPs) has posed an emerging challenge to constructed wetland wastewater treatment. The performance, microbial community and toxic mechanism of anammox-based unplanted subsurface-flow constructed wetlands (USFCWs) were investigated under the long-term exposure of different graphene oxides (GOs) and Ag NP concentrations. Results showed that the addition of GO could promote TN removal, manifesting as function anammox bacteria C. Anammoxoglobus having a relative high abundance, for GO did not cause significant damage to the cell integrity though there was an increase in ROS concentrations. TN removal would not be obviously affected under exposure of 1 mg/L Ag NPs, for the function gene related to cell biogenesis and repair was up-regulated; while the addition of 10 mg/L Ag NPs would have an inhibiting effect on TN removal in the USFCWs, for the disappearance of some species having anammox ability. Key enzymes of anammox process (NIR and HDH) decreased to some extent under GO and Ag NP exposure, and function gene of defense mechanisms had an increase trend in samples.

Successful start-up of the anammox process in constructed wetland microcosms: influence of the electron acceptors on performance, microbial community, and functional genes.

Nitrogen removal by anammox process has been recognized as efficient, cost-effective, and low-energy alternative removal. The longer start-up periods of anammox process hindered the widespread application of anammox-based technology. In this study, four identical unplanted subsurface-flow constructed wetlands (USFCWs) were built up to investigate the effects of electron acceptors (Fe3+, Mn4+, SO42-) on the start-up of anammox process. Results indicated that the start-up time of anammox process was shortened to 105 days in R1 (with Fe3+ addition) and 110 days in R2 (with Mn4+ addition) with nitrogen removal efficiencies of above 75%, compared with 148 days in R0 (control). The addition of SO42- had no significant effect on start-up process. High-throughput sequencing results demonstrated that Shannon index increased significantly from 2.87 (R0) to 5.08 (R1) and 5.00 (R2), and the relative abundance of Candidatus Anammoxoglobus rose from 3.6 to 5.3% in R1. Denitratisoma increased significantly in R2 under addition of Mn4+, which was beneficial for the occurrence of anammox process. The functional genes that related to signal transduction mechanisms and secondary metabolite biosynthesis, transport, and catabolism were upregulated after addition of electron acceptors. These results demonstrated that adding electron acceptors Fe3+ or Mn4+ could be an effective way to accelerate the start-up of anammox process. Graphical abstract ᅟ.

Reduced graphene oxide-nano zero value iron (rGO-nZVI) micro-electrolysis accelerating Cr(VI) removal in aquifer.

Nanoscale zero-valent iron (nZVI) assembled on graphene oxide (GO) (rGO-nZVI) composites were synthesized by reduction of GO and ferrous ions with potassium borohydride, for use in Cr(VI) removal from aqueous solution. The results showed that the two-dimensional structure of GO could provide a skeleton support for Fe0, thus overcoming the bottleneck of aggregation for nZVI. Also, rGO-nZVI would form a ferric-carbon micro-electrolysis system in Cr(VI)-contaminated aquifers, enhancing and accelerating electron transfer, exhibiting high rate and capacity for Cr(VI) removal. The optimum dosage of the applied rGO-nZVI was linearly correlated with the initial Cr(VI) concentration. Characterization of rGO-nZVI before and after reaction with Cr(VI) revealed the process of Cr(VI) removal: rGO-nZVI firstly transferred electrons from Fe0 cores via their Fe(II)/Fe(III) shells to the GO sheet; there, negatively charged Cr(VI) received electrons and changed into positively charged Cr(III), which was adsorbed by the negatively charged GO sheet, avoiding the capping and passivating of nZVI. rGO-nZVI formed a good electrically conductive network, and thus had long-term electron releasing properties, which was important for groundwater remediation.

Insight into the impact of Fe3O4 nanoparticles on anammox process of subsurface-flow constructed wetlands under long-term exposure.

The increasing use of Fe3O4 nanoparticles (NPs) had posed an emerging challenge to wastewater treatment processes, and their potential impact on anaerobic ammonium oxidation (anammox) process of unplanted subsurface-flow constructed wetlands (USFCWs) was investigated firstly under the long-term exposure of different Fe3O4 NP concentrations. It was found that Fe3O4 NP exposure could improve total nitrogen (TN) removal. The abundance of Candidatus Anammoxoglobus increased significantly at 10 mg/L Fe3O4 NPs, while decreased under 1 mg/L Fe3O4 NP exposure. Desulfosporosinus and Exiguobacterium increased to some extent at 1 mg/L Fe3O4 NPs, suggesting that Fe-anammox played an important role in TN removal. The ROS production increased with the increase of Fe3O4 NP concentration, and the integrity of cell membrane was good under Fe3O4 NP exposure. The functional genes that related to inorganic ion transport and metabolism and lipid transport and metabolism were upregulated, and cell motility decreased after long-term exposure of 1 mg/L Fe3O4 NPs. Graphical abstract ᅟ.