Nanoscale Zerovalent Iron Surface Research Articles

Insights into the mechanisms of aqueous Cd(II) reduction and adsorption by nanoscale zerovalent iron under different atmosphere conditions

Nanoscale zero-valent iron (NZVI) can effectively remove and recover Cd(II) from aqueous solutions. However, the oxygen effects on Cd(II) removal by NZVI have been overlooked and not well studied. In this research, the Cd MNN auger lines obtained by X-ray photoelectron spectroscopy (XPS) revealed that Cd(II) adsorbed on the NZVI surface could be reduced to Cd(0) by the Fe(0) core under anaerobic conditions. With coexisting oxygen, the Cd(II) removal efficiency declined significantly, and Cd(II) reduction was inhibited by the thickened surface γ-FeOOH layer. Furthermore, the post-oxygen intrusion corroded the generated Cd(0) and led to the dramatic leaching of Cd(II) ions. According to the density functional theory (DFT) simulation, the adsorbed Cd(II) was preferably coordinated via a monodentate model on the surface of Fe3O4 and γ-FeOOH, which are the dominant surface species of NZVI under anaerobic and aerobic conditions, respectively. Thus, γ-FeOOH with doubly coordinated hydroxyl groups provided fewer adsorption sites than Fe3O4 for Cd(II) ions. Overall, the atmospheric conditions of subsurface remediation and wastewater treatment should be considered when applying NZVI for Cd(II) removal. Favorable atmospheric conditions would improve the efficiency and cost-effectiveness of NZVI-based technologies for the practical remediation of Cd(II) pollution.

Controllable shell corrosion of coated nanoscale zero valent iron induces long-term potentiation of its reactivity for uranium removal

Coating a soluble shell material on the surface of nanoscale zero-valent iron (nZVI) particles to eliminate the problems of agglomeration and rapid aqueous corrosion of Fe(0) in groundwater is an effective strategy for long-term maintaining uranium removal efficiency. However, the precise role of controllable shell corrosion on uranium removal mechanism remains unclear. In this study, a homogeneously dispersed Mg(OH)2 shell coated on nZVI (nZVI@Mg(OH)2) prepared by one-step surface precipitation was exploited to investigate the relationship between shell corrosion and uranium removal mechanism. The results confirm that nZVI with uniform coating shell (Mg(OH)2/Fe(0) = 1:2, mass ratio) was synthesized by simply mixing magnesium hydroxide with Fe2+ and NaBH4 solution under the flowing stream of N2 (25 ± 1 °C). The longevity and reactivity release rate of coated nZVI depended upon the initial pH value of the aqueous solution. At initial pH = 5, coated nZVI showed a high reduction activity, its ORP measured was 1.3 times that measured at initial pH = 3, and the duration could be prolonged to more than 4.5 times. The exposed Fe(0) core rapidly elimination U(VI) by synergistic attributed to the adsorptive (44.1%) and reductive (55.9%) processes after the passivation shell was eroded by reaction with H+. The results provide an efficient approach to improve the long-distance transport capacity and controllable release of nZVI, which can maintain its long-term reactivity in groundwater remediation.

Confining polyacrylic acid on the surface of nanoscale zero-valent iron by aluminum hydroxide for in-situ anti-passivation

Aggregation and surface passivation of nanoscale zero-valent iron (NZVI) particles have limited their reactivity and application for environmental remediation. In this study, an aluminum hydroxide/polyacrylic acid (Al(OH)3/PAA) hybrid shell was homogeneously coated on the NZVI surface to overcome the limitations. PAA molecules were confined onto the NZVI surface by hydration of Al(III) cations. The Al(OH)3/PAA coating shell provided more electrostatic repulsion forces between NZVI particles to hinder the particle aggregation and preserve the NZVI reactivity. On the other hand, the surface-anchored PAA provided a thickened reactive layer for Cr(VI) reduction. Besides, XPS and TEM results showed that the surface carboxylic groups bound produced Cr(III) and Fe(III) cations and inhibited the precipitation of hydroxides on the NZVI surface. The reduced passivation layer increased the longevity of NZVI for surface reactions. As a result, the 24-h Cr(VI) reduction capacity of NZVI particles was improved from 49.4 to 92.6 mg/g with a 2 wt% (Al/Fe) Al(OH)3/PAA coating shell. Overall, this study presented a promising strategy to effectively tune the surface properties of nanoparticles and improve the feasibility of NZVI for environmental remediation.

Insights into kinetics, isotherms and thermodynamics of phosphorus sorption onto nanoscale zero-valent iron

The main aim of this study is to investigate the suitability of several adsorption isotherms and kinetic models for describing the removal of phosphorus (P) from aqueous solutions by nanoscale zero-valent iron (NZVI). Removal efficiency was assessed through batch experiments using initial P-concentration ranged from 5 to 100 mg/L. Statistical error analysis was conducted to examine the validity of the applied models to interpret the removal process. Results showed that NZVI dosage of 1 g/L could remove up to 76.8 mg-P with 100 mg/L of P-initial concentration. Moreover, Langmuir isotherm and pseudo 2nd order kinetic model were the best models to describe the experimental data of P-sorption onto NZVI, with the highest linear and non-linear correlation (R2). Results suggested that chemisorption is the main mechanism involved in P-removal by NZVI in addition to the possibility of physical-electrostatic deposition of P-species onto the surface of NZVI within the liquid-solid controlling step. Functional error analysis quantitatively confirmed the applicability of the proposed isotherm and kinetic models in describing the P-sorption data. Akaike Information Criterion (AIC) values confirmed the goodness-of-fit of the Langmuir isotherm model, linear pseudo 2nd and 1st order models, and non-linear pseudo 1st order model to the experimental data. Thermodynamic analysis, with high enthalpy value of 67.87 kJ/mol, confirmed endothermic nature in the NZVI/P system and the involvement of physio/chemisorption process. The present insights into sorption and reaction rate analysis of P by NZVI revealed that it could have subsequently occurred on two main steps of liquid-film and liquid-solid diffusions.

Graphene oxide-induced formation of a boron-doped iron oxide shell on the surface of NZVI for enhancing nitrate removal

The surface products have a significant influence on the reactivity of zero-valent iron-based materials. Although the enhancing effect of graphene on the reactivity of nanoscale zero-valent iron (NZVI)/graphene composites have been confirmed, the effect of graphene on the formation of surface products of NZVI is not well understood. In order to assess the effect of graphene on the structural of the outer iron oxide layers of NZVI, the NZVI was pre-oxidized by graphene oxide (ONZVI-GO). Compared with the NZVI oxidized by O2 (ONZVI-O2), ONZVI-GO was shown to be effective at NO3− removal with a high efficiency over a wide range of initial pH values. The results from characterization showed that GO could induce the formation of a tight iron oxide shell with dense spinel structures. The boron introduced during the preparation of NZVI was doped into iron oxides on the surface of ONZVI-GO. The B–O in adsorbed borate was transformed to B–B/B–Fe in the lattice structure of iron oxides, causing the formation of highly electron-deficient Lewis acid sites on the surface of ONZVI-GO, which could effectively gather NO3− and OH−, leading to the higher efficiency removal of NO3− than ONZVI-O2 over a wide range of initial pH values. This study provides new insight into the interaction between graphene and the surface species of NZVI.

Preparation of new materials by ethylene glycol modification and Al(OH)3 coating NZVI to remove sulfides in water

In this study, nanoscale zero-valent iron (NZVI) modified by ethylene glycol (EG), and then an aluminum hydroxide (Al(OH)3) film was wound on it to make a new material (EG-NZVI@Al(OH)3), it is used to remove sulfides in water and it has greatly improved the performance of sulfide removal. At different pH values, Al(OH)3 film can effectively improve the adsorption of sulfide by EG-NZVI @Al(OH)3. Al(OH)3 film can also enhance suspension stability and reduce NZVI corrosion in water. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) characterization methods were used to prove that the NZVI was successfully modified by EG and coated by Al(OH)3, achieved the role of protecting NZVI from being oxidized during preparation and drying, and enhanced suspension stability, chemical reactivity and longevity. The removal of sulfides in water by NZVI is mainly through the formation of surface complexes, iron mercapto oxide (FeOSH) and the precipitates of iron sulfide (FeS, FeS2, FeSn) adsorbed on the surface of NZVI. Al(OH)3 film is positively charged It will cause electrostatic adsorption and adsorption on sulfur ions. EG-NZVI@Al(OH)3 is used to remove sulfide from 2.5−50 mg/L aqueous solution. It shows the highest adsorption capacity is 175.5 mg/g. And the mechanism of adsorption is speculated.

Modeling arsenic removal by nanoscale zero-valent iron.

Arsenic removal by nanoscale zero-valent iron (NZVI) was modeled using the USGS geochemical program PHREEQC. The Dzombak and Morel adsorption model was used. The adsorption of As(V) onto NZVI was assumed to happen because of the hydrous ferric oxide (Hfo) which was the surface oxide for the model. The model predicted results were compared with the experimental data. While the experimental study reported that 99.57% arsenic removal by NZVI, the model predicted 99.82% removal which is about 0.25% variation. All the arsenic species have also been predicted to be significantly removed by adsorption onto NZVI surface. The effect of pH on As(V) removal efficiency was also evaluated using the model and it was found that above point-of-zero-charge (PZC), the adsorption of As(V) decreases with the increase of pH. The authors conclude that PHREEQC can be used to model contaminant adsorption by nanomaterials.

Investigating the design parameters for a permeable reactive barrier consisting of nanoscale zero-valent iron and bimetallic iron/copper for phosphate removal

There is a growing interest in deploying nanoscale zero valent iron (NZVI) in permeable reactive barriers (PRBs) for groundwater remediation. In the present study a series of packed-column experiments were conducted in order to investigate the effectiveness of phosphorus removal from groundwater using NZVI and bimetallic NZVI/Cu as reactive materials within PRBs. Seven sets of packed-column experiments were conducted in order to study the effect of different design parameters for PRB; including delivery approach of NZVI into porous media, PRB's configuration, coexisting groundwater ions and change in flowrate. Results implied that doping NZVI surface with copper had an anti-aggregation effect and enhanced its performance in terms of phosphorus removal 2.2 times higher than bare NZVI. Moreover, the lower flowrate (10 ml/min) demonstrated improved phosphorus removal by 22% compared with higher flowrate (60 ml/min). Additionally, groundwater ions barely interfered phosphorus removal process with only ±6%. Overall, geochemical properties and characteristics of the supporting materials were key parameters in the removal process of phosphorus by NZVI/Cu.

Improved longevity of nanoscale zero-valent iron with a magnesium hydroxide coating shell for the removal of Cr(VI) in sand columns

Nanoscale zero-valent iron (NZVI) has been engineered as an attractive tool for in-situ groundwater remediation. However, the poor mobility and aqueous corrosion of NZVI in the porous subsurface have hindered its practical applications. In this research, the NZVI surface was coated with a novel Mg(OH)2 shell (NZVI@Mg(OH)2) to improve the feasibility of NZVI for remediation. In the column tests for continuous removal of Cr(VI) from the flowing water, the Mg(OH)2 shell greatly improved the delivery of NZVI into the sand columns. Coating NZVI with Mg(OH)2 shell also showed considerably greater chemical stability than bare NZVI and thus greater resistance to aqueous corrosion. In addition, the dissolution of Mg(OH)2 allowed the reactivity to be gradually recovered along the sand column for Cr(VI) reduction. As a result, compared to bare NZVI in the columns, NZVI@Mg(OH)2 significantly prolonged the breakthrough period of Cr(VI) and hence increased the columns’ Cr(VI) removal capacity. Moreover, the Cr(III) produced was effectively immobilized by NZVI@Mg(OH)2, even under an acidic condition (pH 4.0). The results show that Mg(OH)2 coating is a promising technique to improve the longevity and capacity of NZVI for full-scale in-situ soil and groundwater remediation.

Modified tapioca starch for iron nanoparticle dispersion in aqueous media: potential uses for environmental remediation

Synthesis of cost-effective surface-modified nanoscale zero-valent iron (NZVI) for use in environmental remediation is a challenge. Herein, we report the synthesis of modified tapioca starch-coated NZVI (CNZVI) particles and their application for effective aqueous nitrate remediation. We used 2-octen-1-ylsuccinic anhydride (OSA) to modify the native starch and, thus introduced carboxyl and ester groups for effective binding of the starch to the NZVI surface. Coating NZVI (3 g L−1) with 35% OSA-modified tapioca starch (10 g L−1) significantly improved colloidal stability of CNZVI in deionized (DI) water compared to bare (unmodified) NZVI (p = 0.000). Approximately 66% of CNZVI remained suspended till 2 h as compared to 4% of bare NZVI. Nitrate removal studies (20, 40, and 60 mg $${\text{NO}}_{3}^{ - }$$ –N L−1 and 1 g L−1 of nanoparticles) showed CNZVI removed nitrate effectively (54–71% removal). The nitrate removal by CNZVI followed second-order reactions with surface normalized reaction rate constants ranging from 0.003 to 0.007 L m−2 h−1. Shelf life of the coated nanoparticles was found to be 3 months. High biochemical oxygen demand was expressed during respirometric biodegradation studies indicating ease of biodegradation of the CNZVI particles. Coating NZVI particles with OSA-modified tapioca starch is expected to have widespread scientific and industrial application potentials for ex situ as well as in situ contaminant remediation. The authors opine that the use of tapioca starch as the raw material for coated nanoparticle synthesis will be a value addition for the crop, and thus, nanotechnology will contribute to agricultural economy.

Hexavalent chromium removal by a new composite system of dissimilatory iron reduction bacteria Aeromonas hydrophila and nanoscale zero-valent iron

Core-shell Fe@Fe2O3 nanowire is one special nanoscale zero-valent iron (NZVI) with an iron core and an outside shell of iron oxide containing the surface bound ferrous ions, which has been used to remove various pollutants including Cr(VI). However, it can be easily oxidized to accumulate iron oxides on the shell, resulting in the loss of reactive activity during its long-term application. Therefore, the dissolution of iron oxides by reduction is required to maintain NZVI reactivity. In this study, we combine NZVI and a dissimilatory iron reduction bacteria (DIRB), Aeromonas hydrophila HS01, to establish a composite system for a sustainable anoxic reduction of Cr(VI). HS01 can stably coexist with NZVI, reduce the oxidized Fe on NZVI and generate more dissolved Fe(II) under anoxic conditions. This presence of HS01 can maintain the core-shell structured morphology and relatively more Fe0 and Fe(II) on NZVI surface, and enhance the reactivity in Cr(VI) removal. Our results indicated that a suitable DIRB can be used to prolong the reactivity of NZVI, and their combination is an efficient method for anoxic Cr(VI) removal.

Nanoscale zerovalent iron particles for magnet-assisted soil washing of cadmium-contaminated paddy soil: proof of concept

Environmental contextCadmium contamination in paddy soil can lead to elevated cadmium concentrations in rice, potentially affecting millions of rice consumers worldwide. This study used nanoscale zerovalent iron to sequester cadmium from soil before using magnetic separation to retrieve cadmium-sorbed particles from the soil slurry. This approach hypothetically resulted in a 97% reduction of cadmium levels in rice, rendering the rice safe for human consumption. AbstractCadmium (Cd) exposure causes serious health effects, including osteopenia, itai-itai disease, kidney disease and cancer. Millions of people are at risk of Cd-contaminated rice consumption resulting from Cd-contaminated paddy soil. While several soil restoration techniques, including phytoremediation (time-consuming) and soil washing using calcium chloride (generating Cd-contaminated wastewater requiring further treatment), face technical challenges, there is room for nanotechnology to offer a rapid and low-cost restoration technique. Here, we propose novel magnet-assisted (ex situ) soil washing using nanoscale zerovalent iron (NZVI) to remove Cd from paddy soil. Conceptually, Cd-contaminated paddy soil is mixed with water and NZVI to create a soil slurry. The NZVI promotes a reduction condition, which accelerates the Cd desorption from the paddy soil to the aqueous phase in the soil slurry. Subsequently, desorbed Cd in water is resorbed onto the NZVI surface, which is retrieved from the soil slurry through magnetic separation, leaving behind treated paddy soil and treated washing water. In our laboratory feasibility study with actual Cd-contaminated paddy soil (191.51±5.54mgkg−1), we found that, although magnet-assisted soil washing using NZVI cannot remove all the Cd from the soil to meet the EU and Thai soil standards (78% removal of total Cd), it effectively removes mobile Cd (exchangeable and carbonate fractions) from the soil (93%), which potentially results in a 97% reduction of cadmium in rice, which is safe for human consumption. The proposed technique has no unacceptable effects on the decline of macro- and micro-nutrients or the germination of rice seed.

Advances in Surface Passivation of Nanoscale Zerovalent Iron: A Critical Review

Nanoscale zerovalent iron (NZVI) is one of the most extensively studied nanomaterials in the fields of wastewater treatment and remediation of soil and groundwater. However, rapid oxidative transformations of NZVI can result in reduced NZVI reactivity. Indeed, the surface passivation of NZVI is considered one of the most challenging aspects in successfully applying NZVI to contaminant degradation. The oxidation of NZVI can lead to the formation of FeII-bearing phases (e.g., FeIIO, FeII(OH)2, FeIIFeIII2O4) on the NZVI surface or complete oxidation to ferric (oxyhydr)oxides (e.g., FeIIIOOH). This corrosion phenomenon is dependent upon various factors including the composition of NZVI itself, the type and concentration of aqueous species, reaction time and oxic/anoxic environments. As such, the coexistence of different Fe oxidation states on NZVI surfaces may also, in some instances, provide a unique reactive microenvironment to promote the adsorption of contaminants and their subsequent transformation via redox reactions. Thus, an understanding of passivation chemistry, and its related mechanisms, is essential not only for effective NZVI application but also for accurately assessing the positive and negative effects of NZVI surface passivation. The aim of this review is to discuss the nature of the passivation processes that occur and the passivation byproducts that form in various environments. In particular, the review presents: (i) the strengths and limitations of state-of-the-art techniques (e.g., electron microscopies and X-ray-based spectroscopies) to identify passivation byproducts; (ii) the passivation mechanisms proposed to occur in anoxic and oxic environments; and (iii) the effects arising from synthesis procedures and the presence of inorganics/organics on the nature of the passivation byproducts that form. In addition, several depassivation strategies that may assist in increasing and/or maintaining the reactivity of NZVI are considered, thereby enhancing the effectiveness of NZVI in contaminant degradation.

Enhanced removal of Ni(II) by nanoscale zero valent iron supported on Na-saturated bentonite

Nanoscale zero valent iron (NZVI) can remove Ni(II) from wastewater through surface adsorption and then reduction into lower-toxic Ni0, but the reduction is often blocked by the iron oxide shell of NZVI. In this study, the negatively charged Na-saturated bentonite (Na-bent) with high adsorption capacity to Ni(II) was used to support NZVI for improving the removal and reduction of Ni(II), and the functions of Na-bent were investigated by X-ray photoelectron micro-spectroscopy (XPS), transmission electron microscope (TEM) and Fe(II) determination. The results showed that Na-bent as a carrier could enrich Ni(II) on the reaction surface, protect the surface of NZVI from oxidation, prevent the aggregation of NZVI particles, and decrease the iron oxides products on NZVI surface by pH buffering. Therefore, NZVI/Na-bent not only showed much higher removal efficiency of Ni(II) (98.5%) than the sum (48.8%) of those by bare NZVI removal (41.9%) and by Na-bent adsorption (6.9%), but also greatly enhanced the reduction efficiency of Ni(II) into Ni0 by facilitating the electron transfer from Fe0 core to the surface-adsorbed Ni(II). In general, the unique property of bentonites will provide effective solutions to support NZVI for enhancing the removal and transformation of various environmental contaminants.

Removal of polybrominated diphenyl ethers by biomass carbon-supported nanoscale zerovalent iron particles: influencing factors, kinetics, and mechanism.

In this study, nanoscale zerovalent iron (NZVI) immobilized on biomass carbon was used for the high efficient removal of BDE 209. NZVI supported on biomass carbon minimized the aggregation of NZVI particles resulting in the increased reaction performance. The proposed removal mechanism included the adsorption of BDE 209 on the surface or interior of the biomass carbon NZVI (BC-NZVI) particles and the subsequent debromination of BDE 209 by NZVI while biomass carbon served as an electron shuttle. BC-NZVI particles and the interaction between BC-NZVI particles and BDE 209 were characterized by TEM, XRD, and XPS. The removal reaction followed a pseudo-first-order rate expression under different reaction conditions, and the k obs was higher than that of other NZVI-supported materials. The debromination of BDE 209 by BC-NZVI was a stepwise process from nona-BDE to DE. A proposed pathway suggested that supporting NZVI on biomass carbon has potential as a promising technique for in situ organic-contaminated groundwater remediation.

The influence of polyelectrolyte modification on nanoscale zero-valent iron (nZVI): Aggregation, sedimentation, and reactivity with Ni(II) in water

Polyelectrolyte modification on the surface of nanoscale zero-valent iron (nZVI) particles could affect colloidal stability, aggregation, mobility, oxidation as well as reactivity. In this article, two anionic polyelectrolytes with different molecular weight (MW), anionic polyacrylamide (APAM, MW=3million) and carboxymethylcellulose sodium (CMC, MW=300–800) were modified on the surface of nZVI. Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), and X-ray diffraction (XRD) were used to characterize the polyelectrolyte-modified nZVI. Results from UV–vis spectrophotometer (UV) and sedimentation tests showed that APAM modification led to the aggregation of nZVI in suspension, CMC modification made nZVI disperse well. XRD results indicated that polyelectrolyte modification made the slow oxidation of nZVI under ambient conditions. APAM-modified nZVI had much lower Ni2+ removal efficiency than that of bare nZVI while CMC-modified nZVI had higher Ni2+ removal efficiency after 1.5hours (h). The study provided the information on the modification effect of nZVI on the colloidal stability and aggregation, sedimentation, reduction and sorption of heavy metal ions in the environment.

Effects of Rhamnolipid and Carboxymethylcellulose Coatings on Reactivity of Palladium-Doped Nanoscale Zerovalent Iron Particles.

Nanoscale zerovalent iron (NZVI) particles are often coated with polymeric surface modifiers for improved colloidal stability and transport during remediation of contaminated aquifers. Doping the NZVI surface with palladium (Pd-NZVI) increases its reactivity to pollutants such as trichloroethylene (TCE). In this study, we investigate the effects of coating Pd-NZVI with two surface modifiers of very different molecular size: rhamnolipid (RL, anionic biosurfactant, M.W. 600 g mol(-1)) and carboxymethylcellulose (CMC, anionic polyelectrolyte, M.W. 700 000 g mol(-1)) on TCE degradation. RL loadings of 13-133 mg TOC/g NZVI inhibited deposition of Pd in a concentration-dependent manner, thus limiting the number of available Pd sites and decreasing the TCE degradation reaction rate constant from 0.191 h(-1) to 0.027 h(-1). Furthermore, the presence of RL in solution had an additional inhibitory effect on the reactivity of Pd-NZVI by interacting with the exposed Pd deposits after they were formed. In contrast, CMC had no effect on reactivity at loadings up to 167 mg TOC/g NZVI. There was a lack of correlation between Pd-NZVI aggregate sizes and TCE reaction rates, and is explained by cryo-transmission electron microscopy images that show open, porous aggregate structures where TCE would be able to easily access Pd sites.

Effect of promoter and noble metals and suspension pH on catalytic nitrate reduction by bimetallic nanoscale Fe0 catalysts

ABSTRACTExperiments were conducted to investigate the effect of experimental factors (types of promotor and noble metals, H2 injection, and suspension pH) on catalytic nitrate reduction by bimetallic catalysts supported by nanoscale zero-valent iron (NZVI). NZVI without H2 injection showed 71% of nitrate reduction in 1 h. Cu/NZVI showed the almost complete nitrate reduction (96%) in 1 h, while 67% of nitrate was reduced by Ni/NZVI. The presence of noble metals (Pd and Pt) on Cu/NZVI without H2 injection resulted in the decrease of removal efficiency to 89% and 84%, respectively, due probably to the electron loss of NZVI for formation of metallic Pd and Pt. H2 injection into Cu-Pd/NZVI suspension significantly improved both catalytic nitrate reduction (>97% in 30 min) and N2 selectivity (18%), indicating that adsorbed H on active Pd sites played an important role for the enhanced nitrate reduction and N2 selectivity. The rapid passivation of NZVI surface resulted in a dramatic decrease in nitrate reduction (79–28%) with an increase in N2 selectivity (8–66%) as the suspension pH increased from 8 to 10.

Effect of NaBH4 on properties of nanoscale zero-valent iron and its catalytic activity for reduction of p-nitrophenol

The reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) by nanoscale zero-valent iron (NZVI)/NaBH4 system in an oxygen environment was studied by means of liquid chromatography, spectroscopy (vibration and X-ray photoelectron), solid analyses (transmission electron microscopy and X-ray diffraction) and density functional theory (DFT) calculations. Addition of NaBH4 into NZVI suspension showed the disintegration of NZVI (60–100nm), resulting in the formation of much smaller particles (15–40nm) due to the chemical etching of outermost surfaces (i.e., magnetite). Interestingly, complete reduction of p-NP and high conversion efficiency of p-AP (>98%) were observed in NZVI/NaBH4 system even after four recycling which is quite comparable with widely used noble metallic catalysts. Surface analysis confirmed that NaBH4 can prevent the oxidation of NZVI surface, leading to the continuous reduction of p-NP in oxygen environments. Experimental results and DFT calculations suggested that not only the formation of smaller NZVI but also thermodynamic preferences for reduction of p-NP on outermost surfaces of NZVI (i.e., magnetite) may significantly affect the reduction process of p-NP in NZVI/NaBH4 system. These novel findings can promote the development of new NZVI technologies which can be used for wastewater reductive treatment in oxygen environments.

Short-chain organic acids increase the reactivity of zerovalent iron nanoparticles toward polychlorinated aromatic pollutants

The use of heavy metals or rare noble metal doping on nanoscale zerovalent iron (NZVI) generally enhances the treatment of hazardous chlorinated organic chemicals (COCs). To avoid the metal use, the effects of four short-chain organic acids (SCOAs) on the degradation of COCs by NZVI were evaluated. Pentachlorophenol (PCP) was the target COC because it has caused serious soil and groundwater contamination. Our results showed that PCP could not be removed by NZVI alone, and the addition of SCOAs promoted PCP degradation by NZVI in acid conditions. Oxalic acid (OA) enhanced the PCP degradation by NZVI more than the other three selected SCOAs. The formation of oxalate-iron complexes out of NZVI particles was observed, indicating that OA could strongly complex with ferrous ions produced during PCP dechlorination and decrease the formation of ferrous (hydro)oxides, which would precipitate on the NZVI surface. Consequently, less passivation layers and even fresh NZVI surfaces were obtained. A more complete PCP dechlorination and the degradation pathways were proposed according to the identification of degradation byproducts. OA led to the highest efficiency of PCP dechlorination around 89% by NZVI due to its strong complexating and pH buffering properties. This green treatment strategy, without doping secondary metals, can facilitate the decontamination of polychlorinated organic pollutants.