Reactivity Of nZVI Research Articles

Activation process of air stable nanoscale zero-valent iron particles

Nanoscale Zero Valent Iron (nZVI) represents a promising material for subsurface water remediation technology. However, dry, bare nZVI particles are highly reactive, being pyrophoric when they are in contact with air. The current trends of nZVI manufacturing lead to the surface passivation of dry nZVI particles with a thin oxide layer, which entails a decrease in their reactivity. In this work an activation procedure to recover the reactivity of air-stable nZVI particles is presented. The method consists of exposing nZVI to water for 36h just before the reaction with the pollutants. To assess the increase in nZVI reactivity based on the activation procedure, three types of nZVI particles with different oxide shell thicknesses have been tested for Cr(VI) removal. The two types of air-stable nZVI particles with an oxide shell thickness of around 3.4 and 6.5nm increased their reactivity by a factor of 4.7 and 3.4 after activation, respectively. However, the pyrophoric nZVI particles displayed no significant improvement in reactivity. The improvement in reactivity is related mainly to the degradation of the oxide shell, which enhances electron transfer and leads secondarily to an increase in the specific surface area of the nZVI after the activation process. In order to validate the activation process, additional tests with selected chlorinated compounds demonstrated an increase in the degradation rate by activated nZVI particles.

Continuous preparation of nanoscale zero-valent iron using impinging stream-rotating packed bed reactor and their application in reduction of nitrobenzene

Nanoscale zero-valent iron (nZVI) was continuously prepared by high-gravity reaction precipitation through a novel impinging stream-rotating packed bed (IS-RPB). Reactant solutions of FeSO4 and NaBH4 were conducted into the IS-RPB with flow rates of 60 L/h and rotating speed of 1000 r/min for the preparation of nZVI. As-prepared nZVI obtained by IS-RPB were quasi-spherical morphology and almost uniformly distributed with a particle size of 10–20 nm. The reactivity of nZVI was estimated by the degradation of 100 ml nitrobenzene (NB) with initial concentration of 250 mg/L. The optimum dosage of nZVI obtained by IS-RPB was 4.0 g/L as the NB could be completely removed within 10 min, which reduced 20% compared with nZVI obtained by stirred tank reactor (STR). The reduction of NB and production of aniline (AN) followed pseudo-first-order kinetics, and the pseudo-first-order rate constants were 0.0147 and 0.0034 s−1, respectively. Furthermore, the as-prepared nZVI using IS-RPB reactor in this work can be used within a relatively wide range pH of 1–9.

Nanoparticulate zero-valent iron coupled with polyphosphate: the sequential redox treatment of organic compounds and its stability and bacterial toxicity

TPP enhances the reactivity and stability of nZVI, decreasing the bacterial toxicity.

Reductive Dechlorination of Trichloroethene by Zero-valent Iron Nanoparticles: Reactivity Enhancement through Sulfidation Treatment.

Zero-valent iron nanoparticles (nZVI) synthesized in the presence of reduced sulfur compounds have been shown to degrade trichloroethene (TCE) at significantly higher rates. However, the applicability of sulfidation as a general means to enhance nZVI reactivity under different particle preparation conditions and the underlying cause for this enhancement effect are not well understood. In this study, the effects of sulfidation reagent, time point of sulfidation, and sulfur loading on the resultant particles were assessed through TCE degradation experiments. Up to 60-fold increase in TCE reaction rates was observed upon sulfidation treatment, with products being fully dechlorinated hydrocarbons. While the reactivity of these sulfur-treated nZVI (S-nZVI) was relatively unaffected by the sulfidation reagent (viz., sodium sulfide, dithionite, or thiosulfate) or the sequence of sulfidation relative to iron reduction, TCE reaction rates were found to depend strongly on sulfur to iron ratio. At a low sulfur loading, TCE degradation was accelerated with increasing sulfur dose. The rate constant reached a limiting value, however, as the sulfur to iron mole ratio was greater than 0.025. Different from previous propositions that iron sulfidation leads to more efficient TCE or tetrachloroethene (PCE) degradation by enabling depassivation of iron surface, affording catalytic pathways, or facilitating electron transfer, we show that the role of sulfur in nZVI lies essentially in its ability to poison hydrogen recombination, which drives surface reactions to favor reduction by atomic hydrogen. This implies that the reactivity of S-nZVI is contaminant-specific and is selective against the background reaction of water reduction. As the effect of sulfur manifests through surface processes, sulfidation represents a broadly applicable surface modification approach to modulate or increase the reactivity of nZVI for treating TCE and other related contaminants.

New insights into the role of Ni loading on the surface structure and the reactivity of nZVI toward tetrabromo- and tetrachlorobisphenol A

Nanoscale zero-valent iron (nZVI) and nickel/nanoscale zero-valent iron (Ni/nZVI) bimetallic particles were explored for tetrabromobisphenol A (TBBPA) and tetrachlorobisphenol A (TCBPA) dehalogenation under ambient condition. The characteristics of the fresh and aged nanoparticles were analyzed by SEM, BET, XRD, XPS and FTIR, and found out that bimetal nanoparticles accelerated the corrosion of Fe0 in the synthesized particles during the reaction. The results shows that the addition of Ni to nZVI particles with low Ni loading (⩽0.5wt%) significantly enhanced dehalogenation of TBBPA and TCBPA compared to the pure nZVI, while high Ni loading (>0.5wt%) did less favor. The observed pseudo first-order rate constant (kobs) of 0.5%Ni/nZVI for TBBPA and TCBPA were 3.74 (±0.29)×10−2 and 0.95 (±0.04)×10−2min−1, which were about 27 and 20 times higher than that of nZVI, respectively, probably due to the formation of highly activated hydrogen atom (H*) and galvanic cells. Ni/nZVI with lower Ni loading (<0.5wt%) resulted in partial dehalogenation of TBBPA and TCBPA, while with higher Ni loading (⩾0.5wt%) curtailed the accumulation of intermediates and completely transformed them to bisphenol A (BPA). A dramatic decrease in oxidation–reduction potential (ORP) values from about +300 to −750mV after Ni/nZVI addition indicates that Ni/nZVI is an effective material to achieve highly reducing conditions. Both ORP and pH shows a correlative tendency with TBBPA and TCBPA removal efficiency. This suggests that ORP and pH can serve as indicators for the reductant reactivity.

The effect of aqueous corrosion on the structure and reactivity of zero-valent iron nanoparticles

Abstract The following work investigates the effect of aqueous corrosion on the structure of zero-valent iron nanoparticles (hereafter nZVI), and their effectiveness in removing heavy metal contaminants from solution. Our results indicate the corrosion of nZVI is characterised by three periods of activity: i) ( 3+ hydroxides with considerable adsorbed water, tentatively, ferrihydrite) with concurrent formation of magnetite; ii) ( 4 w) a stabilisation of surface stoichiometry and surface area, the loss of all zero-valent iron and the formation of goethite, which is ascribed as the likely final state of nZVI in geochemically simple solutions. This final transformation was continuing as the experiment concluded. All nZVI corrosion products identified in this study have also been identified as existing in natural environments at the same scale lengths. The reactivity of nZVI exhibited a significant reduction with increasing oxidation, although the reduction in reactivity varies according to the contaminant uptake mechanism. From this work, it is proposed that nZVI will have a considerably greater reactive longevity in the natural environment than previously thought, in the order of weeks or months rather than days as previously reported.

EDDS-assisted reduction of Cr(VI) by nanoscale zero-valent iron

In the reduction of Cr(VI) by nanoscale zero-valent iron (nZVI), the reactivity of nZVI drops overtime due to surface passivation, which severely inhibits its effective utilization. This study investigated the effect of EDDS (ethylene diamine disuccinic acid) on the Cr(VI) reduction by nZVI at different pH (5.6, 7.0 and 9.0) and the underlying mechanisms. The removal of Cr(VI) by nZVI increased from around 60–100% as the concentration of EDDS increased from 0 to 4–5 mM at pH 5.6–7.0. It was proposed that the EDDS chelated with the generated Cr(III)/Fe(III) and inhibited their co-precipitation onto nZVI surface. However, under alkaline condition (pH 9.0), the EDDS slightly decreased the Cr(VI) reduction, which was due to the fact that the EDDS facilitated the reaction between nZVI/Fe(II) with dissolved oxygen (DO) in the solution. In the absence of DO, a similar positive effect of EDDS on Cr(VI) reduction was observed at pH 9.0 as that at pH 5.6–7.0. Additionally, it was found that the EDDS could significantly increase the kinetics of nZVI corrosion in deionized water. These results demonstrate that EDDS could be used effectively as a chelating agent for inhibiting the surface passivation and increasing the effective utilization of nZVI.

Enhanced immobilization of ReO4− by nanoscale zerovalent iron supported on layered double hydroxide via an advanced XAFS approach: Implications for TcO4− sequestration

This paper was concentrated on a promising remediation strategy for the reductive immobilization of Tc(VII) by studying its chemical analogue Re(VII) to ease the experimental conditions. A novel composite namely nanoscale zero-valent iron supported on layered double hydroxide (NZVI/LDH) was synthesized and applied for the reductive immobilization of Re(VII). Compared to bare NZVI, the as-synthesized NZVI/LDH exhibited higher efficiency on Re(VII) immobilization due to the good synergistic effect between adsorption and reduction. The precise role of LDH was revealed in detail using an advanced approach. The phrase and morphology analysis indicates that NZVI was uniformly supported on LDH, which apparently reduces the aggregation of NZVI and increases its reactivity. 57Fe Mössbauer spectra suggest that NZVI/LDH displayed a good anti-oxidation performance, leading to the enhanced stability during reaction. X-ray absorption fine structure (XAFS) results show that LDH could promote complete reduction of Re(VII) into Re(IV) by NZVI, while sorption and reduction occurred simultaneously on bare NZVI. Moreover, LDH not only acts as a pH buffering agents but also as a scavenger for the coexisting anions and sparingly soluble products, which improves the reactivity of NZVI. The excellent immobilization performance of NZVI/LDH provides a promising strategy for decontamination of Tc(VII) from groundwater.

Chromate removal by surface-modified nanoscale zero-valent iron: Effect of different surface coatings and water chemistry

This study investigated the correlation between the colloidal stability and reactivity of surface-modified nano zero-valent iron (SM-nZVI) as affected by the surface coating (i.e., polyacrylic acid [PAA] and starch) under various geochemical conditions. Generally, the colloidal stability of nZVI was enhanced with increasing loading of surface coating, while there is an optimum loading for the most efficient Cr(VI) removal by SM-nZVI. At lower loadings than the optimum loading, the surface coating could enhance the particle stabilization, facilitating the Cr(VI) reduction by providing more available surface sites. However, the over-loaded surface coating on the surface of nZVI particles decreased the Cr(VI) reduction due to the occupation of the reactive sites and the inhibition of the mass transfer of Cr(VI) ions from water to the particle surface by providing the electrostatic or steric repulsion. The effects of Ca2+ ions or humic acid (HA) on the colloidal stability and reactivity of PAA-modified nZVI (P-nZVI) and starch-modified nZVI (S-nZVI) were examined. Differing stability behavior and reactivity were observed for different SM-nZVI. It was found that the presence of Ca2+ or HA altered surface chemistry of SM-nZVI, the particle-particle interaction and the particle-contaminant interaction, and hence influencing the stability behavior and reactivity of the particles.

Influence of fulvic acid on the colloidal stability and reactivity of nanoscale zero-valent iron

This study investigated the effect of fulvic acid (FA) on the colloidal stability and reactivity of nano zero-valent iron (nZVI) at pH 5, 7 and 9. The sedimentation behavior of nZVI differed at different pH. A biphasic model was used to describe the two time-dependent settling processes (i.e., a rapid settling followed by a slower settling) and the settling rates were calculated. Generally, the settling of nZVI was more significant at the point of zero charge (pHpzc), which could be varied in the presence of FA due to the adsorption of FA on the nZVI surface. More FA was adsorbed on the nZVI surface at pH 5–7 than pH 9, resulting in the varying sedimentation behavior of nZVI via influencing the electrostatic repulsion among particles. Moreover, it was found that there was a tradeoff between the stabilization and the reactivity of nZVI as affected by the presence of FA. When FA concentration was at a low level, the adsorption of FA on the nZVI surface could enhance the particle stabilization, and thus facilitating the Cr(VI) reduction by providing more available surface sites. However, when the FA concentrations were too high to occupy the active surface sites of nZVI, the Cr(VI) reduction could be decreased even though the FA enhanced the dispersion of nZVI particles. At pH 9, the FA improved the Cr(VI) reduction by nZVI. Given the adsorption of FA on the nZVI surface was insignificant and its effect on the settling behavior of nZVI particles was minimal, it was proposed that the FA formed soluble complexes with the produced Fe(III)/Cr(III) ions, and thus reducing the degree of passivation on the nZVI surface and facilitating the Cr(VI) reduction.

Research highlights: improved understanding of ecological impacts resulting from nanomaterial-based in situ remediation.

Nanomaterials are currently being used for in situ remediation of soils and groundwater. However, the continued use of currently implemented nanomaterials and the systematic development of more effective and ecologically benign materials require a more complete understanding of their ecological impact, which should include the transport through the subsurface, acute, chronic and long term effects of exposure, and the role of nanomaterial characteristics (e.g., composition, surface coating). In the current highlight, three articles that examine different aspects of nanoscale zero-valent iron (nZVI) transport, reactivity or exposure to model organisms are summarily reported, which advance the development of more sustainable remediation approaches. The first study examines the role of a model biofilm on the transport of different Pd-doped nZVI species through granulated media, and also the associated nanomaterial toxicity to the forming and sessile bacteria. The second study examines the multigenerational reproductive impacts of C. elegans resulting from nZVI exposure. Lastly, the resulting products of nZVI reactivity with U(vi) species at environmentally relevant molar ratios are examined, and a thorough analysis of the resulting products are reported, which provide valuable data for predicting the consequential role nZVI remediation will have on the ecosystem at and near contaminated sites.

Hexahydro-1,3,5-trinitro-1,3,5-triazine(RDX)의 환원적 분해를 위한나노영가철의 성능평가: 회분식 및 칼럼 실험

ABSTRACT Reductive degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by nanoscale zero-valent iron (nZVI) wasinvestigated to evaluate the feasibility of using it for in-situ groundwater remediation. Batch experiments were conductedto quantify the kinetics and efficiency of RDX removal by nZVI, and to determine the effects of pH, dissolved oxygen(DO), and ionic strength on this process. Experimental results showed that the reduction of RDX by nZVI followedpseudo-first order kinetics with the observed rate constant (k obs ) in the range of 0.0056-0.0192 min −1 . Column tests wereconducted to quantify the removal of RDX by nZVI under real groundwater conditions and evaluate the potential efficacyof nZVI for this purpose in real conditions. In column experiment, RDX removal capacity of nZVI was determined to be82,500 mg/kg nZVI. pH, oxidation-reduction potential (ORP), and DO concentration varied significantly during thecolumn experiments; the occurrence of these changes suggests that monitoring these quantities may be useful in evaluationof the reactivity of nZVI, because the most critical mechanisms for RDX removal are based on the chemical reductionreactions. These results revealed that nZVI can significantly degrade RDX and that use of nZVI could be an effectivemethod for in-situ remediation of RDX-contaminated groundwater.Key words : Nanoscale Zero-Valent Iron, RDX, Column, Groundwater, Reduction

Nanoscale zero-valent iron particles modified on reduced graphene oxides using a plasma technique for Cd(II) removal

Nanoscale zero-valent iron particles supported on reduced graphene oxides (NZVI/rGOs) from spent graphene oxide (GO)-bound iron ions were developed by using a plasma reduction method to improve the reactivity and stability of NZVI. The specific surface area (SSA) of NZVI/rGOs was 117.97 m2 g−1, while the SSA of NZVI particles was 59.59 m2 g−1. The adsorption kinetics was quite fast, which could be completed within 50 min. The adsorption capacity of NZVI/rGOs toward Cd(II) was 425.72 mg g−1, which was higher than other adsorbent materials. Moreover, the NZVI/rGOs could be regenerated by the plasma reduction technique and maintained high removal performance after four cycles. NZVI/rGOs also showed high removal capability toward Cd(II) in the simulated Cd(II)-bearing effluent. The above experimental results show that the NZVI/rGO can be efficient materials for the purification of Cd(II)-contaminated water in the environmental pollution management.

NZVI injection into variably saturated soils: Field and modeling study

Nano-scale zero valent iron (nZVI) has been used at a number of contaminated sites over the last decade. At most of these sites, significant decreases in contaminant concentrations have resulted from the application of nZVI. However, limited work has been completed investigating nZVI field-scale mobility. In this study, a field test was combined with numerical modeling to examine nZVI reactivity along with transport properties in variably saturated soils. The field test consisted of 142L of carboxymethyle cellulose (CMC) stabilized monometallic nZVI synthesized onsite and injected into a variably saturated zone. Periodic groundwater samples were collected from the injection well, as well as, from two monitoring wells to analyze for chlorinated solvents and other geochemistry indicators. This study showed that CMC stabilized monometallic nZVI was able to decrease tricholorethene (TCE) concentrations in groundwater by more than 99% from the historical TCE concentrations. A three dimensional, three phase, finite difference numerical simulator, (CompSim) was used to further investigate nZVI and polymer transport at the variably saturated site. The model was able to accurately predict the field observed head data without parameter fitting. In addition, the numerical simulator estimated the mass of nZVI delivered to the saturated and unsaturated zones and distinguished the nZVI phase (i.e. aqueous or attached). The simulation results showed that the injected slurry migrated radially outward from the injection well, and therefore nZVI transport was governed by injection velocity and viscosity of the injected solution. A suite of sensitivity analyses was performed to investigate the impact of different injection scenarios (e.g. different volume and injection rate) on nZVI migration. Simulation results showed that injection of a higher nZVI volume delivered more iron particles at a given distance; however, the travel distance was not proportional to the increase in volume. Moreover, simulation results showed that using a 1D transport equation to simulate nZVI migration in the subsurface may overestimate the travel distance. This is because the 1D transport equation assumes a constant velocity while pore water velocity radially decreases from the well during injection. This study suggests that on-site synthesized nZVI particles are mobile in the subsurface and that a numerical simulator can be a valuable tool for optimal design of nZVI field applications.

Graduated characterization method using a multi-well microplate for reducing reactivity of nanoscale zero valent iron materials

Even though nanoscale zero valent iron (nZVI) has been intensively studied for the treatment of a plethora of pollutants through reductive reaction, quantification of nZVI reactivity has not yet been standardized. Here, we adapted colorimetric assays for determining reductive activity of nZVI and its composites with other metals. The assay quantifies reduction products to avoid interfering reactions, such as sorption and volatilization. Three different reaction products, ammonium, phenol, and aniline, generated as the result of reduction of nitrate, p-halophenols, and nitrobenzene, respectively, could be quantified using the same reagent for all reactions. The colorimetric assays were further adapted to the 96-well microplate format, thus minimizing sample and reagent use, as well as lowering color development time to 2h. The substrates showed graduated reactivity, and thus, reduction potency and kinetics of different materials and reaction mechanism was distinguished. The applicability was successfully proven by determining the reactivity of a commercial nZVI sample, and investigating the effect of nickel content on dehalogenation. Therefore, the suggested reactivity test with different compounds, combined with the use of a multi-well microplate based color assay, promises to be a useful and simple tool in various nZVI related research topics.

Intensify chemical reduction to remove nitrate from groundwater via internal microelectrolysis existing in nano-zero valent iron/granular activated carbon composite

A composite combination of nano-zero valent iron (nZVI) and granular activated carbon (GAC) was introduced to achieve nitrate transformation into nitrogen with minimum byproducts via chemical reduction. The results showed that nitrate removal was significantly intensified and GAC was an excellent partner of nZVI chemical reduction. Both the dispersion and long-term reactivity of nZVI were improved, and more than 80% of nitrate was removed at 10 g/L nZVI/GAC composite within 90 min in steady state. The byproducts of nitrite and ammonium formed in reduction process were below 0.008 and 0.04 mg/L, respectively, and nitrogen was the main end product, which was confirmed by X-ray photoelectron spectroscopy analysis. Moreover, the kinetic model of nitrate reduction by nZVI/GAC composite can be described well by pseudo-first-order reaction kinetic (R2 = 0.984). The intensification is closely related to the dispersion effect of GAC matrix to nZVI and the promotion effect to electrons transfer between nZVI and nitrate pollutant of internal iron–carbon microelectrolysis existing in such a composite. Therefore, the findings are very beneficial to develop an efficient and low-cost chemical remediation method for nitrate-contaminated groundwater.

Nanoscale zero-valent iron particles supported on reduced graphene oxides by using a plasma technique and their application for removal of heavy-metal ions.

Nanoscale zero-valent iron particles supported on reduced graphene oxides (NZVI/rGOs) from spent graphene oxide (GO)-bound iron ions were developed by using a hydrogen/argon plasma reduction method to improve the reactivity and stability of NZVI. The NZVI/rGOs exhibited excellent water treatment performance with excellent removal capacities of 187.16 and 396.37 mg g(-1) for chromium and lead, respectively. Moreover, the NZVI/rGOs could be regenerated by plasma treatment and maintained high removal ability after four cycles. X-ray photoelectron spectroscopy analysis results implied that the removal mechanisms could be attributed to adsorption/precipitation, reduction, or both. Such multiple removal mechanisms by the NZVI/rGOs were attributed to the reduction ability of the NZVI particles and the role of dispersing and stabilizing abilities of the rGOs. The results indicated that the NZVI/rGOs prepared by a hydrogen/argon plasma reduction method might be an effective composite for heavy-metal-ion removal.

Optimizing synthesis conditions of nanoscale zero-valent iron (nZVI) through aqueous reactivity assessment

Nanoscale iron particles (nZVI) is one of the most important engineered nanomaterials applied to environmental pollution control and abatement. Although a multitude of synthesis approaches have been proposed, a facile method to screen the reactivity of candidate nZVI materials produced using different methods or under varying synthesis conditions has yet been established. In this study, four reaction parameters were adjusted in the preparation of borohydride-reduced nZVI. The reductive properties of the resultant nanoparticles were assayed independently using two model aqueous contaminants, Cu (II) and nitrate. The results confirm that the reductive reactivity of nZVI is most sensitive to the initial concentration of iron precursor, borohydride feed rate, and the loading ratio of borohydride to ferric ion during particle synthesis. Solution mixing speed, in contrast, carries a relative small weight on the reactivity of nZVI. The two probing reactions (i.e., Cu(II) and nitrate reduction) are able to generate consistent and quantitative inference about the mass-normalized surface activity of nZVI. However, the nitrate assay is valid in dilute aqueous solutions only (50 mg∙L–1 or lower) due to accelerated deactivation of iron surface at elevated nitrate concentrations. Additional insights including the structural and chemical makeup of nZVI can be garnered from Cu(II) reduction assessments. The reactivity assays investigated in this study can facilitate screening of candidate materials or optimization of nZVI production parameters, which complement some of the more sophisticated but less chemically specific material characterization methods used in the nZVI research.

Carbothermal synthesis of ordered mesoporous carbon-supported nano zero-valent iron with enhanced stability and activity for hexavalent chromium reduction

Composites of nano zero-valent iron (nZVI) and ordered mesoporous carbon (OMC) are prepared by using simultaneous carbothermal reduction methods. The reactivity and stability of nZVI are expected to be enhanced by embedding it in the ordered pore channels. The structure characteristics of nZVI/OMC and the removal pathway for hexavalent chromium (Cr(VI)) by nZVI/OMC are investigated. Results show that nZVI/OMC with a surface area of 715.16m2g−1 is obtained at 900°C. nZVI with particle sizes of 20–30nm is uniformly embedded in the OMC skeleton. The stability of nZVI is enhanced by surrounding it with a broad carbon layer and a little γ-Fe is derived from the passivation of α-Fe. Detection of ferric state (Fe 2p3/2, around 711.2eV) species confirms that part of the nZVI on the outer surface is inevitably oxidized by O2, even when unused. The removal efficiency of Cr(VI) (50mgL−1) by nZVI/OMC is near 99% within 10min through reduction (dominant mechanism) and adsorption. nZVI/OMC has the advantage in removal efficiency and reusability in comparison to nZVI/C, OMC and nZVI. This study suggests that nZVI/OMC has the potential for remediation of heavy metal pollution in water.

Zero-valent iron nanoparticles with sustained high reductive activity for carbon tetrachloride dechlorination

Fe oxide shell breakdown and new oxyhydroxide formation during CT dechlorination contribute to pH self-buffering and sustained nZVI reactivity.