Aged nZVI Research Articles

Effects and mechanisms of nZVI on CO2 and CH4 emissions in uncontaminated and pentachlorophenol-contaminated soils

With the increasing application of nanoscale zero-valent iron (nZVI) for in situ soil remediation, its effects on soil functionality and ecosystem need to be thoroughly evaluated. Herein, we investigated the effects of nZVI on CO2 and CH4 emissions from uncontaminated and pentachlorophenol (PCP)-contaminated soils and the underlying microbial mechanisms by designing a 68-day anaerobic soil culture experiment; thereafter, the effects of above aged nZVI on soil CO2 and CH4 emissions in the following 20 days were further studied. In the uncontaminated soil, 1–10 g/kg nZVI treatments reduced soil CO2 emission by 17.4–82.6% and increased soil CH4 emission by 10.8%–119.7%, but these effects disappeared after the nZVI was aged. The emissions of soil CO2 and CH4 were significantly inhibited by the PCP contamination (100 mg/kg) mainly due to the toxicity to related soil microorganisms. The applications of 1–10 g/kg nZVI significantly reduced CO2 emissions from the PCP-contaminated soil by 24.0–86.7%, while 10 g/kg nZVI markedly increased soil CH4 emission by 1875.4% and restored the methanogenic activity to the control level after the nZVI was aged. The 10 g/kg nZVI treatment enriched hydrogenotrophic methanogen (Methanobacterium) and organics-degrading bacteria by releasing H2, increasing soil pH, and decreasing soil Eh; the abundance of genes encoding key enzymes (Mcr, Mtr, Hdr, Mta, and Mtb) in all methanogenic pathways significantly increased after the nZVI treatment, indicating that nZVI could have a broad promoting effects on soil methanogenic processes. The findings demonstrate that the addition of nZVI for in situ remediation of organochlorines-contaminated soils will affect soil greenhouse gas emissions and provide basic data for safe nZVI applications.

Effect of aged nanoscale zero-valent iron amendment and capping on phosphorus mobilization in sediment

Nanoscale zero-valent iron (nZVI) has been extensively investigated as remediation materials. Understanding the influence of nZVI on sedimentary phosphorus mobilization is vital to its application in sediment remediation. In this study, the effects of aged nZVI (A-nZVI) amendment and capping on the mobilization of phosphorus from sediment to overlying water (O-water) were studied. The results showed that under high A-nZVI dosage condition, A-nZVI addition effectively reduced the efflux of dissolved reactive phosphorus (DRP) from sediment to O-water under oxygen deficit condition, and induced the transformation of reductant-soluble phosphorus (BD-P) to metal oxide-bound inorganic phosphorus (NaOH-IP) and residual phosphorus (Res-P) in sediment, making the inorganic phosphorus more stable than the control. Under low A-nZVI dosage condition, A-nZVI amendment still had the ability to diminish the risk of DRP released from sediment into O-water. Furthermore, A-nZVI capping effectively hindered DRP release from sediment into O-water, and its controlling efficiency was higher than A-nZVI amendment under oxygen deficit condition. Under anoxic condition, the A-nZVI amendment and capping both can decrease the concentrations of DGT (diffusive gradient in thin-films)-measured labile phosphorus (PDGT) in the profile of O-water and uppermost sediment, but they both had ineffective reduction efficiencies of PDGT in the lower sediment profile. The A-nZVI amendment and capping increased the microbial diversity and richness of surface sediment and stimulated the growth of some Fe-reducing bacteria in the surface sediment. The uptake of DRP by the A-nZVI capping layer located on the sediment–water interface was an important reason for intercepting DRP diffusion from sediment into O-water by the A-nZVI capping. Overall, A-nZVI can be used as an in-situ active capping material for controlling DRP release from sediment to O-water.

Rapid reactivation of aged NZVI/GO by Shewanella CN32 for efficient removal of tetrabromobisphenol A and associated reaction mechanisms

Tetrabromobisphenol A (TBBPA) is one of the most widely applied brominated flame retardants, which has consequently resulted in severe environmental contamination. Graphene oxide-loaded nanoscale zero-valent iron (NZVI/GO) has previously shown excellent performance for potential TBBPA removal. However, the aging of NZVI/GO resulted in reduced reaction rates. In this study, Shewanella CN32 was used to reactivate the performance of aged NZVI/GO. The results demonstrated that CN32 significantly reactivated aged NZVI/GO (27%) and consequently improved the removal of TBBPA by 240% in aged NZVI/GO+CN32 (92%). Compared to aged NZVI (0.07 mM), CN32 rapidly decomposed the iron oxide passivation layer of aged NZVI/GO, with a subsequent increase in the release of Fe2+ ions (0.12 mM). Moreover, Fe(II) vivianite (FeII3(PO4)2·8H2O) and SO42− type green rust (Fe3.6Fe0.9(O,OH,SO4)9) formed at the surface of aged NZVI/GO+CN32. GO accelerated the reduction of dissimilatory iron (α-Fe2O3) and the formation of vivianite crystals. Vivianite further improved the debromination of TBBPA. Furthermore, the semiquinone radicals (C=O) in the interface of the aged NZVI/GO effectively mediated electron transfer and reactivated the performance of aged NZVI/GO by CN32. It could therefore be concluded that CN32-induced iron oxide reduction and NZVI exposure, C=O mediated electron transfer, and vivianite-induced debromination were the main removal mechanisms of TBBPA in the system. It is a potential technology for convenient remediation of soil, groundwater, and lake.

Effects of Nano-Scale Zero Valent Iron Fresh and Aged Particles on Environmental Microbes

Currently, nano-scale zero valent iron particles (nZVI) are being increasingly used in many types of environmental remediation. Due to their usage, nZVI can be left in the environment and may cause toxic effects to living beings, especially surrounding microorganisms. Environmental bacteria in soil and water are some of the main factors affecting plant productivity and other microorganisms in ecosystems. This study evaluated the toxicological effects of nZVI and aged nZVI on bacteria commonly found in the environment. Bacterial, namely E. coli, P. aeruginosa, S. aureus, B. subtilis, and Rhodococcus sp., were treated with different concentrations of nZVI at different times of exposure in in vitro conditions, and bacterial cell viability was determined in order to analyze the toxic effects of nZVI over the course of treatments. The data revealed that at the highest nZVI concentration (1,000 mg/L), B. subtilis and Rhodococcus sp. had the highest resistance to nZVI (49.35% and 48.31% viability) and less resistance in P. aeruginosa (2.26%) and E. coli (0.50%) was observed. The growth of microorganisms significantly increased after exposure with seven and 14-day aged nZVI particles. Therefore, the toxicity of aged nZVI to microbial organisms was reduced. Hence, this study demonstrated the toxic effects of nZVI and aged nZVI particles on several species of bacteria in vitro. Less toxicity to bacteria was observed in aged nZVI. These findings provide more understanding in the toxic effect of nZVI to microorganisms.

Transformation and implication of nanoparticulate zero valent iron in soils

Knowledge of nanoparticulate zero-valent iron (nZVI) transformation in soils and its relationship with the potential impacts on soil properties are crucial to evaluate the environmental implication and application of nZVI. This study investigated nZVI transformation and the effects on soil properties in eight soils with various ageing time and soil moisture content (SMC). Spherical nZVI was gradually oxidized, collapsed, and adhered to clay minerals, and crystalline maghemite and magnetite were the primary oxidation products. Compared with the flooded condition, nZVI oxidation was accelerated under 70% SMC but was limited under 30% SMC. Acidic soil with lower content of dissolved aromatic carbon was advantage to nZVI oxidation under the flooded condition, while carboxymethylcellulose coating and iron oxides on nZVI surface limited nZVI oxidation. The aged nZVI existed mainly in the form of association with soil mineral or organic matter rather than in ion-exchangeable or carbonate form. nZVI treatment promoted soil aromatic carbon sequestration and decreased soil redox potential, and the impacts of nZVI on soil pH, electrical conductivity, ζ-potential, dissolved organic carbon, and catalase and urease activities were dependent on soil type and SMC. The findings are of significance for the evaluation of the environmental risk and proper application of nZVI.

Interactions of iron-based nanoparticles with soil dissolved organic matter: adsorption, aging, and effects on hexavalent chromium removal

The interactions and mechanisms between soil dissolved organic matter (DOM) and three types of iron-based nanoparticles (NPs), i.e., nanoscale zero-valent iron (nZVI) particles, Fe2O3 NPs, and Fe3O4 NPs, were investigated in short-term exposure experiments. The adsorption results showed that soil DOM was rapidly adsorbed on the surface of the iron-based NPs with the adsorption rate varying according to Fe3O4 > Fe2O3 > nZVI. Spectral analysis results revealed that aromatic DOM fractions with high-molecular-weights were preferentially adsorbed. The binding mechanism was determined as hydrogen bonding and ligand exchange via Fourier transform infrared spectroscopy (FT-IR) analysis. Scanning electron microscopy, FT-IR, X-ray photoelectron spectroscopy, and X-ray diffraction were used to identify the corrosion products of the three iron-based NPs at the adsorption equilibrium. The results suggest that Fe3O4 and/or γ-Fe2O3 and α-FeOOH were the main corrosion products of nZVIs and α-FeOOH was obtained as an aged product of Fe3O4 NPs. Results of Cr(VI) removal tests suggest that the aged nZVI achieved 79.87% of Cr(VI) removal and the Cr(VI) removal efficiency was significantly improved by coating DOM onto Fe2O3 NPs. The overall data indicate the fate and transformation of iron-based NPs and the enhancement for Cr(VI) removal after interactions between DOM and NPs.

Correlating surface chemistry and hydrophobicity of sulfidized nanoscale zerovalent iron with its reactivity and selectivity for denitration and dechlorination

Sulfidized nanoscale zerovalent iron (S-nZVI) has been reported to be highly reactive towards lots of contaminants. However, the effects of surface chemistry evolution (e.g. surface S/Fe) and hydrophobicity on its reactivity and selectivity towards the reduction of different functional groups on contaminants are unclear. Here, the reactivity and electron selectivity of aged nZVI and S-nZVI in water for the denitration of chloramphenicol (CAP) and dechlorination of florfenicol (FF) were investigated. The reactivity of nZVI for the denitration of CAP was greatly inhibited after sulfidation, and the kobs, CAP by nZVI decreased along with the aging time. However, the reactivity of nZVI for the dechlorination of FF was significantly improved after sulfidation, and the kobs, FF by S-nZVI firstly increased then decreased with the aging time, which was correlated well with the evolution of S/Fe molar ratio on the surface. S-nZVI with higher hydrophobicity was proved to be more reactive for the dechlorination of FF but less reactive for the denitration of CAP. Interestingly, the electron efficiencies of CAP denitration by nZVI and FF dechlorination by S-nZVI were both maintained >99% no matter the materials were fresh or aged. These results revealed that the sulfidation would change the physicochemical properties and reactivity of nZVI, and it is of great importance to measure the surface chemical composition and hydrophobicity of S-nZVI and apply the materials with appropriate target contaminants to maximum the electron utilization.

Simultaneous removal of NO and SO2 from flue gas using vaporized H2O2 catalyzed by nanoscale zero-valent iron.

To remove NO and SO2 from flue gas simultaneously, a heterogeneous catalytic oxidation system was developed with the nanoscale zero-valent iron (nZVI), vaporized H2O2, and sodium humate (HA-Na) acting as the catalyst, oxidant, and absorbent, respectively. The experimental results indicated that the desulfurization was mainly influenced by the absorption, and the denitrification was significantly affected by the catalytic oxidation parameters. Under the optimal conditions, the simultaneous removal efficiencies of SO2 and NO were 100 and 88.4%, respectively. The presence of ·OH during the removal process was proved by the scavenger tests, and the production of ·OH with and without nZVI was indirectly evaluated by the electron paramagnetic resonance (EPR) and methylene blue experiments. Moreover, the fresh and aged nZVI were characterized by a series of techniques and the results suggested that the redox pair Fe2+/Fe3+ released by nZVI could react with H2O2 to provide the sustainable ·OH, which was important for the oxidation from NO and SO2 to NO3- and SO42-. The removal mechanism was proposed preliminarily based on the correlative experiments, characterizations, and references.

Effect of bicarbonate on aging and reactivity of nanoscale zerovalent iron (nZVI) toward uranium removal

Bicarbonate, ubiquitous in natural and waste waters is an important factor regulating the rate and efficiency of pollutant separation and transformation. For example, it can form complexes with U(VI) in the aqueous phase and at the solid-water interface. In this work, we investigated the effect of bicarbonate on the aging of nanoscale zero-valent (nZVI) in the context of U(VI) reduction and removal from wastewater. For fresh nZVI, over 99% aqueous uranium was separated in less than 10 min, of which 83% was reduced from U(VI) to U(IV). When nZVI was aged in water, its activity for U(VI) sequestration and reduction was significantly reduced. Batch experiments showed that for nZVI aged in the presence of 10 mM bicarbonate, only 20.3% uranium was reduced to U(IV) after 6 h reactions. Characterizations of the iron nanoparticles with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) suggest that in fresh nZVI, uranium was concentrated at the nanoparticle center; whereas in nZVI aged in bicarbonate, uranium was largely deposited on the outer surface of the nanoparticles. Furthermore, aged nZVI without bicarbonate contained more lepidocrocite (γ-FeOOH) while aged nZVI in the presence of bicarbonate had more magnetite/maghemite (Fe3O4/γ-Fe2O3). This could be attributed to the formation of carbonate green rust and pH buffer effect of ▪. Primary mechanisms for U(VI) removal with nZVI include reduction, sorption and/or precipitation. Results demonstrate that bicarbonate alter the aging products of nZVI, and reduces the separation efficiency and reduction capability for uranium removal.

Factors influencing degradation of trichloroethylene by sulfide-modified nanoscale zero-valent iron in aqueous solution

Sulfide-modified nanoscale zero-valent iron (S/NZVI) has been considered as an efficient material to degrade trichloroethylene (TCE) in groundwater. However, some critical factors influencing the dechlorination of TCE by S/NZVI have not been investigated clearly. In this study, the effects of Fe/S molar ratio, initial pH, dissolved oxygen and particle aging on TCE dechlorination by S/NZVI (using dithionite as sulfidation reagent) were studied. Besides, the feasibility of reactivation of the aged-NZVI by sulfidation treatment was looked into. The results show that the Fe/S molar ratio and initial pH significantly influenced the TCE dechlorination, and a higher TCE dechlorination was observed at Fe/S molar ratio of ∼60 under alkaline condition. Spectroscopic analyses demonstrate that the enhanced TCE dechlorination was associated with the presence of FeS on the surface of S/NZVI. Dissolved oxygen had little effect on TCE dechlorination by S/NZVI, revealing that the FeS layer could be able to alleviate the surface passivation of NZVI caused by oxidation. Aging of S/NZVI up to 10–20 d only slightly decreased the dechlorination efficiency of TCE. Although an obvious drop in dechorination efficiency was observed for the S/NZVI aged for 30 d, it still exhibited a higher reactivity than the bare NZVI. This indicates that sulfidation of NZVI did prolong its lifetime. Additionally, sulfidation treatment was used to reactivate the aged NZVI, and the results show that the reactivated NZVI even had higher reactivity than the fresh NZVI, suggesting that sulfidation treatment would be a promising method to reactivate the aged NZVI.

Physicochemical transformation of carboxymethyl cellulose-coated zero-valent iron nanoparticles (nZVI) in simulated groundwater under anaerobic conditions

This study investigated the long-term compositional and structural evolution of bare nZVI (nanoscale zero-valent iron) and carboxymethyl cellulose (CMC) coated nZVI (CMC-nZVI) in simulated groundwater under anaerobic conditions. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman and Fourier transform infrared spectroscopy (FTIR) were used to characterize the corrosion products of nZVI and CMC-nZVI. Results show that both structure and chemical composition of the aging products of nZVI/CMC-nZVI transformed over time. The SEM images show that the original spherical particles transformed into the sheet-like and needle-like materials for the aged nZVI, but formed bulk aggregates with some flaky-shaped structures for the aged CMC-nZVI. Compare with the aging products of nZVI and CMC-nZVI (i.e., magnetite/lepidocrocite) in the deionized water under anaerobic conditions, the formed secondary products in the simulated groundwater were complicated. The composition analysis confirm that magnetite, lepidocrocite, feroxyhyte and iron carbonate hydroxide hydrate/iron oxide hydroxide chloride were the main corrosion products for both bare nZVI and CMC-nZVI, except that the amount of each secondary mineral varied for different particles. Besides, small amount of calcium carbonate was also identified in the aging products of nZVI. This indicates that the groundwater ions could be involved in the formation of secondary minerals during nZVI corrosion.

Residual impact of aged nZVI on heavy metal-polluted soils

In the present study, the residual toxicity and impact of aged nZVI after a leaching experiment on heavy metal (Pb, Zn) polluted soils was evaluated. No negative effects on physico-chemical soil properties were observed after aged nZVI exposure. The application of nZVI to soil produced a significant increase in Fe availability. The impact on soil biodiversity was assessed by fluorescence in situ hybridization (FISH). A significant effect of nZVI application on microbial structure has been recorded in the Pb-polluted soil nZVI-treated. Soil bacteria molecular response, evaluated by RT-qPCR using exposure biomarkers (pykA, katB) showed a decrease in the cellular activity (pykA) due to enhanced intracellular oxidative stress (katB). Moreover, ecotoxicological standardised test on Caenorhabditis elegans (C. elegans) showed a decrease in the growth endpoint in the Pb-polluted soil, and particularly in the nZVI-treated. A different pattern has been observed in Zn-polluted soils: no changes in soil biodiversity, an increase in biological activity and a significant decrease of Zn toxicity on C. elegans growth were observed after aged nZVI exposure. The results reported indicated that the pollutant and its nZVI interaction should be considered to design soil nanoremediation strategies to immobilise heavy metals.

Rapid point-of-use water purification using nanoscale zero valent iron (nZVI) particles

Nanoscale zero-valent iron (nZVI) particles are increasingly being investigated in removing aqueous contaminants. Here, we have demonstrated its inactivation and magnetic removal of bacteria and endotoxins from environmental wastewater samples. Varying dosages (10–1,000 μL) of 0–6 days aged nZVI with a concentration of 5 mg/mL for 2 mL wastewater samples were tested, and relevant removal efficiencies were determined using culturing method for bacteria and limulus amebocyte lysate (LAL) for endotoxins. The supernatants of wastewater samples after reacting with nZVI and subsequent magnetic separations were subjected to spectroscopic, qPCR and DGGE analysis. Overall, high magnetic bacterial removal efficiencies were observed up to 3–4 logs for 1 mL nZVI, while the removal efficiencies decreased sharply down to 0.5 log for 10 μL nZVI. qPCR and DGGE results revealed that higher dosages of nZVI caused severe bacterial cell membrane ruptures, releasing significant amounts of DNA up to 107–108 gene copies/mL when 1 mL nZVI was used. Richer DGGE patterns were observed for higher nZVI dosages. In addition, regardless of the dosages (10–1,000 μL) we have observed more than 90 % removal of endotoxins from the wastewater samples. The described technology has great promise to be used as a point-of-use water purification solution for various purposes.