Kinetics of zero valent iron nanoparticle oxidation in oxygenated water

Corrosion behaviors and kinetics of nanoscale zero-valent iron in water: A review

Knowledge on corrosion behaviors and kinetics of nanoscale zero-valent iron (nZVI) in aquatic environment is particularly significant for understanding the reactivity, longevity and stability of nZVI, as well as providing theoretical guidance for developing a cost-effective nZVI-based technology and designing large-scale applications. Herein, this review gives a holistic overview on the corrosion behaviors and kinetics of nZVI in water. Firstly, Eh-pH diagram is introduced to predict the thermodynamics trend of iron corrosion. The morphological, structural, and compositional evolution of (modified-) nZVI under different environmental conditions, assisted with microscopic and spectroscopic evidence, is then summarized. Afterwards, common analytical methods and characterization technologies are categorized to establish time-resolved corrosion kinetics of nZVI in water. Specifically, stable models for calculating the corrosion rate constant of nZVI as well as electrochemical methods for monitoring the redox reaction are discussed, emphasizing their capabilities in studying the dynamic iron corrosion processes. Finally, in the future, more efforts are encouraged to study the corrosion behaviors of nZVI in long-term practical application and further build nanoparticles with precisely tailored properties. We expect that our work can deepen the understanding of the nZVI chemistry in aquatic environment.

Reactive membranes for groundwater remediation of chlorinated aliphatic hydrocarbons: competitive dechlorination and cost aspects

A nanocomposite membrane incorporating reactive Pd-Fe nanoparticles (NPs) was developed to remediate chlorinated aliphatic hydrocarbons (CAHs) from groundwater. Other than recapturing the produced Fen+ for in-situ regeneration, the functionalized polyanions prevented NPs agglomeration and resulting in a spherical Fe0 core (55 nm, O/Fe = 0.05) and an oxidized shell (4 nm, O/Fe = 1.38). The reactive membranes degraded 92% of target CAHs with a residence time of 1.7 seconds. After long-term treatment and regeneration, reusability was confirmed through recovered reactivity, recurrence of Fe0 in X-ray photoelectron spectroscopy, and >96% remaining of Fe and Pd. The total cost (adjusted present value for 20 years) was estimated to be 13.9% lower than the granular activated carbon system, following an EPA work breakdown structure-based cost model. However, non-target CAHs from groundwater can compete for active sites, leading to decreased surface-area normalized dechlorination rate ( k sa ) by 28.2-79.9%. A hybrid nanofiltration (NF)/reactive membrane was proposed to selectively intercept larger competitors, leading to 54% increased dechlorination efficiency and 1.3 to 1.9-fold enlarged k sa . Overall, the practical viability of the developed reactive membranes was demonstrated by the stability, reusability, and cost advantages, while the optional NF strategy could alleviate competitive degradation towards complex water chemistry.

Degradation mechanism of Bisphenol S via hydrogen peroxide/persulfate activated by sulfidated nanoscale zero valent iron

Fenton-like oxidation processes are widely used to degrade recalcitrant organic pollutants, but are limited by narrow application pH and low reaction efficiency. This study investigated the synchronous activation of H2O2 and persulfate (PDS) by sulfidated zero valent iron (S-nZVI) in ambient conditions for Fenton-like oxidation of bisphenol S (BPS), an estrogenic endocrine-disrupting chemical. The activation of S-nZVI induced H2O2 or PDS could be greatly enhanced with the assistance of PDS and H2O2, respectively, even across a wide range of pH value (3-11). The first-order rate constant of S-nZVI/H2O2/PDS, S-nZVI/PDS and S-nZVI/H2O2 systems was found to be 0.2766 min-1, 0.0436 min-1, and 0.0113 min-1, respectively. A significant synergy between H2O2 and PDS was achieved when the PDS-H2O2 molar ratio was above 1:1, and where sulfidation promoted iron corrosion and decreased solution pH were observed in the S-nZVI/H2O2/PDS system. Radical scavenging experiments and electron paramagnetic resonance (EPR) investigations suggest that both SO4•- and •OH were generated and that •OH played a crucial role in BPS removal. Furthermore, four BPS degradation intermediates were detected and three degradation pathways were proposed in line with the HPLC-Q-TOF-MS analysis. This study demonstrated that compared to the traditional Fenton-like system, the S-nZVI/H2O2/PDS system could be a more efficient, advanced oxidation technology capable of being used across a broad pH range for emerging pollutants' degradation.

Enhanced edible plant production using nano-manganese and nano-iron fertilizers: Current status, detection methods and risk assessment

BACKGROUND

Nanotechnology offers many benefits in the globally important field of food production and human nutrition, particularly by implementing agricultural nanoproducts. Of these, edible plant fertilizers enriched with nanosized forms of essential metals, Mn and Fe, are growing in importance with the advantages of enhanced action on plant roots.

SCOPE AND APPROACH

This review focuses on the importance of tracking the bioaccumulation and biodistribution of these pertinent nanofertilizers. An emphasis is given to the critical analysis of the state-of-the-art analytical strategies to examine the Mn and Fe nanoparticles in edible plant systems as well as to shedding light on the vast gap in the methodologies dedicated to the speciation, in vitro simulation, and safety testing of these promising nanomaterials. Also provided are guidances for the food chemists and technologists on the lights and shadows of particular analytical approaches as a matter of authors' expertise as analytical chemists.

KEY FINDINGS AND CONCLUSIONS

While the use of nanotechnology in agriculture seems to be growing increasingly, there is still a lack of analytical methodologies capable of investigating novel Mn- and Fe-based nanomaterials as potential fertilizers. Only the advent of reliable analytical tools in the field could bridge the gaps in our knowledge about processes in which those materials participate in the plant systems and their effects on crop production and quality of the produced food.

Performance Enhancement of Biogenetic Sulfidated Zero-Valent Iron for Trichloroethylene Degradation: Role of Extracellular Polymeric Substances

Chemical sulfidation has been considered as an effective strategy to improve the reactivity of zero-valent iron (S-ZVI). However, sulfidation is a widespread biogeochemical process in nature, which inspired us to explore the biogenetic sulfidation of ZVI (BS-ZVI) with sulfate-reducing bacteria (SRB). BS-ZVI could degrade 96.3% of trichloroethylene (TCE) to acetylene, ethene, ethane, and dichloroethene, comparable to S-ZVI (97.0%) with the same S/Fe ratio (i.e., 0.1). However, S-ZVI (0.21 d^-1) exhibited a faster degradation rate than BS-ZVI (0.17 d^-1) based on pseudo-first-order kinetic fitting due to extracellular polymeric substances (EPSs) excreted from SRB. Organic components of EPSs, including polysaccharides, humic acid-like substances, and proteins in BS-ZVI, were detected with 3D-EEM spectroscopy and FT-IR analysis. The hemiacetal groups and redox-activated protein in EPS did not affect TCE degradation, while the acetylation degree of EPS increased with the concentration of ZVI and S/Fe, thus inhibiting the TCE degradation. A low concentration of HA-like substances attached to BS-ZVI materials promoted electron transport. However, EPS formed a protective layer on the surface of BS-ZVI materials, reducing its TCE reaction rate. Overall, this study showed a comparable performance enhancement of ZVI toward TCE degradation through biogenetic sulfidation and provided a new alternative method for the sulfidation of ZVI.

Potential promotion of activated carbon supported nano zero-valent iron on anaerobic digestion of waste activated sludge

A large amount of waste activated sludge (WAS) harms the ecological environment, and anaerobic digestion (AD) is an effective method for WAS treatment. In this study, activated carbon (AC)/ nano zero-valent iron (NZVI) was synthesized by a liquid-phase reduction method, and was used to boost methane production. The associated mechanisms and effects of additives on AD during the addition and removal stage were investigated systematically. Compared to the blank group, the cumulative methane production was increased by 14.3%, 26.3% and 34.1% in the groups of AC, NZVI and AC/NZVI, respectively. The addition of AC/NZVI significantly increased the concentration of VFAs and promoted the hydrolysis and acidification of WAS. After the AD of the additives addition stage was finished, the additives were removed and the sludge was replenished in all groups, the methanogenesis performance of the experimental groups was significantly inhibited. The cumulative methane production in the AC and AC/NZVI groups was 21.7% and 13.5% lower than the blank group, respectively. The experimental results have a good correlation with curve fitting by the modified Gompertz model. The modified Gompertz model found that AC, NZVI and AC/NZVI increased the methanogenic potential and maximum methane production rate of WAS, but also prolonged the lag-phase time. AC/NZVI might play a role in coupling effects. It could not only maintain the original characteristics of NZVI and increase its stability, but also develop the advantages of AC promoting direct interspecies electron transfer. Microbial community analysis indicated that the abundance of hydrogenotrophic methanogens was enriched by AC/NZVI.

Unveiling the positive effect of mineral induced natural organic matter (NOM) on catalyst properties and catalytic dechlorination performance: An experiment and DFT study

Herein, we report the significant effects of natural organic matter contained in natural zeolite (Z-NOM) on the physicochemical characteristics of a Ni/Fe@natural zeolite (NF@NZ) catalyst and its decontamination performance toward the dechlorination of trichloroethylene (TCE). Z-NOM predominantly consists of humic-like substances and has demonstrable utility in the synthesis of bimetallic catalysts. Compared to NF@NZ_600C (devoid of Z-NOM), NF@NZ had increased dispersibility and mobility and showed significant enhancement in the catalytic dechlorination of TCE owing to the encapsulation of Ni⁰/Fe⁰ nanoparticles by Z-NOM. The results of corrosion experiments, spectroscopic analyses, and H₂ production experiments confirmed that Ni⁰ acted as an efficient cocatalyst with Fe⁰ to enhance the dechlorination of TCE to ethane, and Z-NOM-capped Ni⁰ showed improved adsorption of TCE and atomic hydrogen on their reactive sites and oxidation resistance. The density functional theory (DFT) studies have substantiated the improved adsorption of TCE due to the presence of NOM (especially by COOH structure) and the enhanced charge density at the Ni site in the Ni/Fe bimetal alloy for the stronger adsorption of hydrogen atoms that ultimately enhanced the TCE reduction reaction. These findings illustrate the efficiency of NOM containing natural minerals toward the synthesis of bimetallic catalysts for practical applications.

The role of nitrate in simultaneous removal of nitrate and trichloroethylene by sulfidated zero-valent Iron

Sulfidated zero-valent iron (S-ZVI) is commonly used to degrade trichloroethylene (TCE). The reactivity of S-ZVI is related to not only the properties of S-ZVI but also the geochemical conditions in groundwater, such as coexisted NO₃^-. Therefore, the effect of NO₃^- on TCE degradation by S-ZVI and its mechanism were systematically studied. 95.17% of TCE was degraded to acetylene, dichloroethene, ethene, ethane and multi‑carbon products via β-elimination by fresh S-ZVI that contained 85.31% Fe⁰ and 14.69% FeS in the presence of NO₃^-, demonstrating that NO₃^- did not affect the degradation pathway of TCE. While high concentration of NO₃^- (> 10 mg/L) competed for electrons at the Fe/FeO_x interface with degradation products, leading to a continuous rising of acetylene. Moreover, the rapid reduction of NO₃^- to NH₄⁺ (89.79%) at the Fe⁰ interface contributed to the release of 5.08 mM Fe²⁺ from S-ZVI, which promoted the formation of Fe₃O₄ with excellent electron conduction properties on the surface of S-ZVI. Accordingly, NO₃^- improved the degradation and electron selectivity of TCE by 51.07% and 2.79 fold, respectively. This study demonstrated that S-ZVI could remediate the contamination of NO₃^- and TCE simultaneously and the presence of NO₃^- could effectively enhance the degradation of TCE in groundwater.

Degradation of sulfamethazine in water by sulfite activated with zero-valent Fe-Cu bimetallic nanoparticles

In this work, zero-valent Fe-Cu bimetallic nanoparticles were synthesized using a facile method, and applied to activate sulfite for the degradation of sulfamethazine (SMT) from the aqueous solution. The key factors influencing SMT degradation were investigated, namely the theoretical loading of Cu, Fe-Cu catalyst dosage, sulfite concentration and initial solution pH. The experimental results showed that the Fe-Cu/sulfite system exhibited a much better performance in SMT degradation than the bare Fe⁰/sulfite system. The mechanism and possible degradation pathway of SMT in Fe-Cu/sulfite system were revealed. The reactive radicals that played a dominant role in the SMT degradation process were •OH and SO₄^•-, while the loading of Cu induced the synergistic effect between Fe and Cu. The redox cycle between Cu(I)/Cu(II) remarkably contributed to the conversion of Fe(III) to Fe(II), greatly enhancing the catalytic performance of Fe-Cu bimetal. In real groundwater applications, the Fe-Cu/sulfite system also exhibited satisfactory SMT degradation. The 30-day aging tests of Fe-Cu particles demonstrated that the aging of catalyst was not obviously affecting the removal of SMT. Furthermore, the reusability of catalyst was evidenced by the recycling experiments. This study provides a promising application of bimetal activated sulfite for enhanced contaminant degradation in groundwater.

Aminopolycarboxylic acids modified oxygen reduction by zero valent iron: Proton-coupled electron transfer, role of iron ion and reactive oxidant generation

The oxygen reduction reaction (ORR) activated by Fe⁰ in the presence of three aminopolycarboxylic acids (CAs), i.e. nitrilotriacetic acid (NTA), ethylenediamine-N,N'-disuccinic acid (EDDS) and ethylenediaminetetraacetic acid (EDTA), for the degradation of sulfamethazine (SMT) was investigated. At optimum conditions, Fe⁰/EDDS/O₂, Fe⁰/EDTA/O₂ and Fe⁰/NTA/O₂ systems presented SMT removal of 58.2%, 75.3% and 93.8%, respectively, being much higher than that in the Fe⁰/O₂ system (1.36%). The generation of surface-bound Fe²⁺ (Fe²⁺) and dissolved iron ion was enhanced by CAs. ORR through a two-electron transfer pathway was mainly responsible for H₂O₂ generation in NTA and EDTA systems, while a single-electron ORR was the major source for producing H₂O₂ in EDDS system. •OH produced by the homogeneous reaction of Fe²⁺ and H₂O₂ was the main species for SMT degradation. Fe⁰/EDDS/O₂ produced more ¹O₂ than Fe⁰/EDTA/O₂ and Fe⁰/NTA/O₂; however, the radical contributed negligibly to SMT removal. The caging effect of CAs might be a major factor influencing the reaction rate of Fe²⁺ and O₂. CAs provided protons to accelerate the electron transfer, the production of Fe²⁺ and thus the contaminant removal. This study is of great significance for revealing ORR mechanisms in the Fe⁰-chelate system.

Transformation of sulfamethoxazole by sulfidated nanoscale zerovalent iron activated persulfate: Mechanism and risk assessment using environmental metabolomics

The threat caused by the misuse of antibiotics to ecology and human health has been aroused an extensive attention. Developing cost-effective techniques for removing antibiotics needs to put on the agenda. In current research, the degradation mechanism of sulfamethoxazole (SMX) by sulfidated nanoscale zerovalent iron (S-nZVI) driven persulfate, together with the potential risk of intermediates were studied. The degradation of SMX followed a pseudo-first order kinetics reaction with k_obs at 0.1176 min^-1. Both SO₄^•- and •OH were responsible for the degradation of SMX, and SO₄^•- was the predominant free radical. XPS analysis demonstrated that reduced sulfide species promoted the conversion of Fe (III) to Fe (II), resulting in the higher transformation rate of SMX. Six intermediates products were generated through hydroxylation, dehydration condensation, nucleophilic reaction, and hydrolysis. The risk of intermediates products is subsequently assessed using E. coli as a model microorganism. After E.coli exposure to intermediates for 24 h, the upmetabolism of carbohydrate, nucleotide, citrate acid cycle and downmetabolism of glutathione, sphingolipid, galactose by metabolomics analysis identified that SMX was effectively detoxified by oxidation treatment. These findings not only clarified the superiority of S-nZVI/persulfate, but also generated a novel insight into the security of advanced oxidation processes.

Time dependent zero valent iron oxidation and the reductive removal of pertechnetate at variable pH

Elemental iron Fe⁰ is a promising reductant for removal of radioactive technetium-99 (Tc) from complex aqueous waste streams that contain sulfate, halides, and other inorganic anions generated during processing of legacy radioactive waste. The impact of sulfate on the kinetics of oxidation and reduction capacity of Fe⁰ in the presence of Tc has not been examined. We investigated the oxidative transformation of Fe⁰ and reductive removal of TcO₄^- in 0.1 M Na₂SO₄ as a function of initial pH (i.e., pH_i 4, 7, and 10) under aerobic conditions up to 30 days. Tc reduction was the fastest at pH_i 7 and slowest at pH_i 10 (Tc reduction rate pH_i 7 > 4 > 10). Aqueous fraction of Tc was measured at 0.4% at pH_i 7 within 6 h, whereas ≥ 97% of Tc was removed from solutions at pH_i of 4 and 10 within 24 h. Solid phase characterization showed that magnetite was the only oxidized crystalline phase for the first 6 h regardless of initial pH. Lepidocrocite was the most abundant oxidized product for pH_i 10 after 5 days, but was not observed at pH of 4 or 7.

Trace Key Mechanistic Features of the Arsenite Sequestration Reaction with Nanoscale Zerovalent Iron

Nanoscale zerovalent iron (nZVI) is considered as a highly efficient material for sequestrating arsenite, but the origin of its high efficacy as well as the chemical transformations of arsenite during reaction is not well understood. Here, we report an in situ X-ray absorption spectroscopy (XAS) study to investigate the complex mechanism of nZVI reaction with arsenite under anaerobic conditions at the time scale from seconds to days. The time-resolved XAS analysis revealed a gradual oxidation of As^III to As^V in the course of minutes to hours in both the solid and liquid phase for the high (above 0.5 g/L) nZVI dose system. When the reaction time increased up to 60 days, As^V became the dominant species. The quick-scanning extended X-ray absorption fine structure (QEAXFS) was introduced to discover the transient intermediate at the highly reactive stage, and a small red-shift in As K-edge absorption edge was observed. The QEAXFS combined with density functional theory (DFT) calculation suggested that the red-shift is likely due to the electron donation in a Fe-O-As complex and possible active sites of As sequestrations include Fe(OH)₄ and 4-Fe cluster. This is the first time that the transient reaction intermediate was identified in the As-nZVI sequestration system at the fast-reacting early stage. This study also demonstrated usefulness of in situ monitoring techniques in environmental water research.

Modified nanoscale zero-valent iron in persulfate activation for organic pollution remediation: a review

Under the action of different activators, persulfate can produce sulfate radicals (SO₄·^-) with strong oxidizing ability, which can destruct many organic compounds. Meanwhile, persulfate is widely used in groundwater and soil remediation because of its fast reaction and wide application. With the high specific surface area and reactivity of nanoscale zero-valent iron (nZVI), it can enhance the degradation efficiency of the persulfate system on organic pollutants in soil and water as a persulfate activator. However, nZVI is easy to get oxidized and has a tendency to aggregation. To solve these problems, a variety of nZVI modification methods have been put forward and put into to applications in the activation of persulfate. This article will give a systematic introduction of the background and problems of nZVI-activated persulfate in the remediation of organic pollution. In addition, the modification methods and mechanisms of nZVI are summarized, and the applications and progress of modified nZVI-activated persulfate are reviewed. The factors that affect the removal of organic compounds by the activation system are discussed as well. Worldwide, the field studies and full-scale remediation using modified nZVI in persulfate activation are yet limited. However, the already known cases reveal the good prospect of applying modified nZVI in persulfate activation to organic pollution remediation.

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.

Compositional evolution of nanoscale zero valent iron and 2,4-dichlorophenol during dechlorination by attapulgite supported Fe/Ni nanoparticles

Transformation of chloro-organic compounds by nFe(0) has been studied extensively, but limited study exists on the transformation and fate of nFe(0) during the dechlorination of chloro-organics even though such knowledge is important in predicting its surface chemistry, particularly, toxicity in the environment. In this study, the nFe(0) core became hollowed, collapsed and gradually corroded into poorly crystallized ferrihydrite (Fe₅O₃(OH)₉) at the pristine reaction time, which later gave rise to lath-like lepidocrocite (γ-FeOOH), acicular goethite (α-FeOOH) and cubic magnetite (Fe₃O₄) by the end of the reaction time (120 min). Also, dechlorination of 2,4-DCP into 2-CP, 4-CP and phenol was achieved within 120 min. The rapid dechlorination of 2,4-DCP and transformation of nFe(0) could not be achieved significantly without doping Ni on nFe(0) and supporting on attapulgite. The schematic representation of the transformation and compositional evolution of nFe(0) in A-nFe/Ni was proposed. These findings are critical in understanding the compositional evolution and the fate of nFe(0) upon reaction with chloro-organics and can provide guidance for more efficient uses of the nFe(0) reactivity towards the target contaminants in groundwater remediation.

Simultaneous Removal of Selenite and Selenate by Nanosized Zerovalent Iron in Anoxic Systems: The Overlooked Role of Selenite

The application of nanosized zerovalent iron (nZVI) for reductive immobilization of selenite (Se(IV)) or selenate (Se(VI)) alone has been extensively investigated. However, as the predominant species, Se(IV) and Se(VI) usually coexist in the environment. Thus, it is essential to remove both species simultaneously in the solution by nZVI. Negligible Se(VI) removal (∼7%) by nZVI was observed in the absence of Se(IV). In contrast, the Se(VI) was completely removed in the presence of Se(IV), and the removal rate and electron selectivity of Se(VI) increased from 0.12 ± 0.01 to 0.29 ± 0.02 h^-1 and from 1% to 4.5%, respectively, as the Se(IV) concentration increased from 0.05 to 0.20 mM. Se(IV) was rapidly removed by nZVI, and Se(VI) exerted minor influence on Se(IV) removal. Se(IV) promoted the generation of corrosion products that were mainly composed of magnetite (26%) and lepidocrocite (67%) based on the Fe K-edge XANES spectra and k³-weighted EXAFS analysis. Fe(II) released during the Se(IV) reduction was not the main reductant for Se(VI) but accelerated the transformation of F(0) to magnetite and lepidocrocite. The formation of lepidocrocite contributed to the enrichment of Se(VI) on the nZVI surface, and magnetite promoted electron transfer from Fe(0) to Se(VI). This study demonstrated that Se(IV) acted as an oxidant to activate nZVI, thus improving the reactivity of nZVI toward Se(VI), which displays a potential application of nZVI in the remediation of Se(IV)- and Se(VI)-containing water.

Montmorillonite-supported nanoscale zero-valent iron for thiamethoxam removal: response surface optimization and degradation pathway

As a highly efficient insecticide, thiamethoxam was widely used in the world. However, it was bioaccumulative and toxic to aquatic organisms that must be removed from water. In this work, nanoscale zero-valent iron particles loaded on montmorillonite (nZVI/Mt) were successfully synthesized for effective removal of thiamethoxam. The properties of nZVI/Mt for the removal of thiamethoxam were investigated, and the reaction conditions were optimized through response surface methodology. Furthermore, the degradation products were analyzed by liquid chromatography-mass spectrometry (LC/MS). The results demonstrated that the reaction activity of nZVI was enhanced because the agglomeration and oxidation of nZVI particles were effectively inhibited by using montmorillonite as a support. The significance of the effects of each factor on the removal of thiamethoxam was determined to be in the order of pH ˃ temperature ˃ reaction time ˃ nZVI/Mt dosage. The optimal conditions were as follows: a dosage of nZVI/Mt of 2 g/L, a reaction time of 2 h, a reaction temperature of 35 °C, and a solution pH of 3. The removal efficiency of thiamethoxam (C₀ = 20 mg/L) was observed to be as high as 94.29% under the optimal conditions, which was close to the value of 94.47% that was predicted using the mathematical model, indicating that the model could accurately predict the removal efficiency of thiamethoxam. The degradation mechanism involved the -NO₂ group on the thiamethoxam molecule was reduced and eliminated by nZVI/Mt.

Preparation of highly-conductive pyrogenic carbon-supported zero-valent iron for enhanced Cr(Ⅵ) reduction

In this work, electron transfer (ET) moiety of PC was ascertained in chromate (Cr(Ⅵ)) reduction by zero-valent iron supported by pyrogenic carbon (PC) (ZVI/PC) prepared by pyrolysis of hematite (α-Fe₂O₃)-treated pinewood. X-ray diffraction analysis suggested successive phase transformation of α-Fe₂O₃→magnetite (Fe₃O₄)→wustite (FeO)→ZVI (Fe^o). Raman spectra and Brunauer-Emmett-Teller analysis revealed that ZVI/PC is characterized with more ordered graphitic carbon and greater surface area than pristine PC. Maximal Cr(Ⅵ) removal capacity (pH = 3) as predicted by Langmuir isotherm model were 5.78, 36.12 and 8.39 g kg^-1 for PC, ZVI/PC and ZVI, respectively. ZVI/PC maintained significantly greater Cr(Ⅵ) removal capacity than ZVI and PC at pH 3-9, but Cr(Ⅵ) removal dropped rapidly to 6.78 g kg^-1 at pH 4 and above. X-ray photoelectron spectroscopy and successive desorption of Cr-laden ZVI/PC and ZVI showed trivalent Cr was the dominant species, suggesting reduction was an important mechanism for Cr(Ⅵ) detoxification. Electrochemical analysis demonstrated that ZVI/PC exhibited greater Tafel corrosion rate and ET quantity, with lower electrical resistance. Besides, Cr(Ⅵ) reduction showed reversal trend with electrical resistance of ZVI/PC. To conclude, ET capacity was closely associated with electrical conductivity of ZVI/PC due to intensified conductive graphitic carbon structure of PC at higher pyrogenic temperatures.

Reductive Degradation of CCl4 by Sulfidized Fe and Pd-Fe Nanoparticles: Kinetics, Longevity, and Morphology Aspects

In this study a systematic comparison in morphology, long-term degradation, regeneration and reuse were conducted between palladized and sulfidized nanoscale zero-valent iron (Pd-Fe and S-Fe). Pd-Fe and S-Fe were prepared, after the synthesis of precursor Fe0 nanoparticles (spherical, ~35 nm radius) for carbon tetrachloride (CTC) treatment. With HAADF-TEM-EDS characterization, dispersive Pd islets were found on the Fe core of Pd-Fe. However, the Fe core was covered by the FeSx shell of S-Fe (FeS/FeS2 = 0.47). With an excessive Pd dose (10 mol%), the Pd-Fe were dramatically deformed to dendritic structures which significantly decreased reactivity. For CTC degradation, Pd-Fe (0.3 atomic% Pd) increased the degradation rate by 20-fold (ksa= 0.580 Lm-2min-1) while S-Fe presented a greater life time. The major intermediate chloroform (CF) was further degraded and less than 5% CF was observed after 24 h using Pd-Fe or S-Fe while above 50% CF remained using Fe. During aging, the Fe core was converted to FeOOH and Fe3O4/γ-Fe2O3. The restoration of Fe0 was achieved using NaBH4, which regenerated Fe and Pd-Fe. However, the formed FeSx shell on S-Fe was disappeared. The results suggest that S-Fe extends longevity of Fe, but the loss of FeSx after aging makes S-Fe eventually perform like Fe in terms of CTC degradation.

Zerovalent nickel nanoparticles performance towards Cr(VI) adsorption in polluted water

Heavy metals are one of the most common types of pollutants in ground water due to their wide sources, non-degradability and high toxicity. Many traditional wastewater treatments were not capable of removing enough such contaminants in order to meet quality standards. Nanosized zerovalent transition metals have emerged as a great candidate for ground water remediation, due to their simplicity and low fabrication cost, furthermore they can comply with simple chemical synthesis. Here, we present the synthesis of nano zerovalent nickel (nZVN) by a simple grinding reduction method. The obtained nZVN was characterized with XRD, SEM, EDS and BET surface area. The results confirms the formation of nZVN and the active particle cluster size ranges from 100 to 200 nm. N₂ adsorption isotherms revealed that the formation mesoporous cluster of nZVN with good surface area. The adsorption of Cr(VI) using nZVN showed 96% removal efficiency for 10 ppm concentration, and even up to 98% when the temperature is slightly raised to 36 °C (309 K). The removal efficiencies of Cr by zerovalent nickel was well fitted by the Langmuir-Hinshelwood first order reaction kinetic model with deceptive rate constant values of 0.6699, 0.7956 and 1.0251 min^-1 at temperature 200, 303 and 309 K, respectively. In total, our studies suggest that nanoscale zerovalent iron is a capable material for Cr(VI) remediation from groundwater.

Aging of zero-valent iron-based nanoparticles in aqueous environment and the consequent effects on their reactivity and toxicity

A fundamental understanding of the long-term fate of nanoscale zero-valent iron (nZVI)-based particles in aqueous environment and the corresponding impacts on their reactivity and toxicity is essential for the responsible use and management of the nanoparticles in environmental applications. This paper comprehensively reviews the physicochemical transformations of nZVI-based particles and the consequent effects on the particle's reactivity and toxicity. The corrosions of nZVI in water under both anaerobic and aerobic conditions are summarized. The transformation of contaminant-bearing nZVI is also discussed. Besides, the factors influencing the transformation of nZVI (i.e., pH, typical anions and cations, natural organic matter, surface stabilizers, bimetal decoration, and sulfidation treatment) are summarized and discussed. In addition, the effects of particle aging on its reactivity and toxicity are discussed. Generally, the aging of nZVI-based particles would have negative impact on the removal of contaminants, especially for the degradation of organic pollutants. However, the aging process of nZVI-based particles would cause a significant reduction in their toxicity. It is suggested that the nZVI-based particles would finally transform to less toxic or benign materials (i.e., iron (oxyhydr)oxides) over time. Finally, future perspectives are proposed to better quantify and predict the transformation of nZVI-based particles in aqueous environment. PRACTITIONER POINTS: The corrosion rates and products of nZVI in water varied much under anaerobic and aerobic conditions. Typical anions and cations, natural organic matter, and iron types are critical factors influencing the physicochemical transformation of nZVI. The aging of nZVI would have negative impact its reactivity, especially for the degradation of organic pollutants. Although the fresh nZVI exhibits obvious toxicity, the aging process would cause a significant reduction in its toxicity.

Highly efficient degradation of 2,2',4,4'-tetrabromodiphenyl ether through combining surfactant-assisted Zn0 reduction with subsequent Fenton oxidation

2,2',4,4'-tetrabromodiphenyl ether (BDE47) was difficult to be rapidly degraded by common reductive debromination or oxidative decomposition. In this study, the debromination via surfactant-assisted zero valent zinc (Zn⁰) reduction and subsequent Fenton oxidation was combined to completely degrade BDE47. Firstly, Zn⁰ integrated with surfactants including cetyltrimethylammonium chloride (CTAC), polyethylene glycol dodecyl ether (Brij35), or 1-dodecanesulfonic acid sodium salt (SDS) were evaluated for their reactivity to debrominate BDE47. CTAC-assisted Zn⁰ system presented the highest removal efficiency of 98.6% for BDE47 (C₀ = 5 mg/L) under the optimized conditions including 0.3 g/L of Zn⁰ particles and 0.05 g/L of CTAC at 25 °C and pH 4.0 during 1-h reaction. Subsequently, the debromination products as low-brominated BDEs were attacked by hydroxyl radicals (•OH) from Fenton reagent, which were decomposed into short-chain carboxylic acids and even mineralized within 2-h oxidation. In addition, HPLC, GC-MS, LC-MS/MS, and IC were employed to detect intermediates during this reaction/oxidation process and the pathways of debromination and oxidation were proposed according to carbon and bromine balance. The above combination achieved the complete degradation of BDE47 via a relative low-cost method to rapidly remove PBDEs, which provide a new approach for the effective treatment of halogenated organic pollutants.

Comparative analysis of ZVI materials for reductive separation of 99Tc(VII) from aqueous waste streams

Technetium-99 (Tc) is a long-lived radioactive contaminant present in legacy nuclear waste streams and contaminated plumes of the nuclear waste storage sites worldwide that poses risks for human health and the environment. Pertechnetate (TcO₄^-), the most common chemical form of Tc under oxidative conditions, is of particular concern due to its high aqueous solubility and mobility in the subsurface. One approach to treatment and remediation of TcO₄^- is reduction of Tc⁷⁺ to less soluble and mobile Tc⁴⁺ and its removal from the contaminated streams such as liquid secondary waste generated during vitrification of the Hanford low activity tank waste. Zero valent iron (ZVI) is a common reactive agent for reductive treatment of environmental contaminants, including reducible heavy metal ions, which can offer a potential solution to this challenge. Here, we present a comparative study of eleven commercial ZVI materials manufactured by different methods that were evaluated for the reductive removal of TcO₄^- from an aqueous 80 mM NaCl solution at near neutral pH representing low activity waste off-gas condensate. Performance of ZVI materials was analyzed in relation to time-dependent Fe²⁺ dissolution as well as pH and ORP profiles of the contact solution. Large variability in the efficiency and kinetics of Tc⁷⁺ reduction by different ZVI materials was contingent on their origin. ZVI materials manufactured by electrolytic method exhibited superior performance, and the kinetics of the Tc⁷⁺ reduction correlated to particle size. ZVI materials manufactured by iron pentacarbonyl reduction with hydrogen were ineffective for Tc⁷⁺ reduction. In general, our results highlight the need for thorough performance analysis of commercial ZVI materials for any contaminant of interest.

Structural characterization of self-assembled chain like Fe-FeOx Core shell nanostructure

One of the big challenge of studying the core-shell iron nanostructures is to know the nature of oxide shell, i.e., whether it is γ-Fe₂O₃ (Maghemite), Fe₃O₄ (Magnetite), α-Fe₂O₃ (Hematite), or FeO (Wustite). By knowing the nature of iron oxide shell with zero valent iron core, one can determine the chemical or physical behavior of core-shell nanostructures. Fe core-shell nanochains (NCs) were prepared through the reduction of Fe³⁺ ions by sodium boro-hydride in aqueous solution at room atmosphere, and Fe NCs were further aged in water up to 240 min. XRD was used to study the structure of Fe NCs. Further analysis of core-shell nature of Fe NCs was done by TEM, results showed increase in thickness of oxide shell (from 2.5, 4, 6 to 10 nm) as water aging time increases (from 0 min, 120 min, 240 min to 360 min). The Raman spectroscopy was employed to study the oxide nature of Fe NCs. To further confirm the magnetite phase in Fe NCs, the Mössbauer spectroscopy was done on Fe NCs-0 and Fe NCs-6. Result shows the presence of magnetite in the sample before aging in water, and the sample after prolonged aging contains pure Hematite phase. It shows that prolonged water oxidation transforms the structure of shell of Fe NCs from mixture of Hematite and Magnetite in to pure hematite shell. The Magnetic properties of the Fe NCs were measured by VSM at 320 K. Because of high saturation magnetization (Ms) values, Fe NCs could be used as r₂ contrasts agents for magnetic resonance imaging (MRI) in near future.

Degradation of Tetrabromobisphenol A by Sulfidated Nanoscale Zerovalent Iron in a Dynamic Two-Step Anoxic/Oxic Process

A dynamic two-step anoxic/oxic process using sulfidated nanoscale zerovalent iron (S-nZVI) was employed to degrade tetrabromobisphenol A (TBBPA). In the anoxic stage, TBBPA followed a four-step sequential debromination pathway and was completely transformed to bisphenol A (BPA) with the optimal S/Fe molar ratio of 0.3. S-nZVI inhibited H₂ evolution and preserved the reducing capacity of Fe(0). Fe(0), rather than the formed FeS in S-nZVI, was responsible for TBBPA debromination. In the oxic stage, the product BPA was attacked by ^•OH, transformed to dihydroxybenzenes and benzoquinones, and eventually, achieved mineralization via ring-opening reactions. The sulfidation process facilitated ^•OH production through a two-electron transfer pathway by surface-bound Fe(II), in which structural Fe(II) in FeS and regenerated Fe(II) from Fe(III) reduction by Fe(0) played significant roles toward total BPA degradation. S-nZVI was transformed to S₈ and α-FeOOH after the oxic treatment. After these two steps, complete degradation of TBBPA was achieved. This study demonstrated the feasibility that refractory contaminants could be completely degraded in the dynamic two-step anoxic/oxic process, thus broadening the utility of S-nZVI for environmental applications in water treatment.

Biochar-supported nZVI (nZVI/BC) for contaminant removal from soil and water: A critical review

The promising characteristics of nanoscale zero-valent iron (nZVI) have not been fully exploited owing to intrinsic limitations. Carbon-enriched biochar (BC) has been widely used to overcome the limitations of nZVI and improve its reaction with environmental pollutants. This work reviews the preparation of nZVI/BC nanocomposites; the effects of BC as a supporting matrix on the nZVI crystallite size, dispersion, and oxidation and electron transfer capacity; and its interaction mechanisms with contaminants. The literature review suggests that the properties and preparation conditions of BC (e.g., pore structure, functional groups, feedstock composition, and pyrogenic temperature) play important roles in the manipulation of nZVI properties. This review discusses the interactions of nZVI/BC composites with heavy metals, nitrates, and organic compounds in soil and water. Overall, BC contributes to the removal of contaminants because it can attenuate contaminants on the surface of nZVI/BC; it also enhances electron transfer from nZVI to target contaminants owing to its good electrical conductivity and improves the crystallite size and dispersion of nZVI. This review is intended to provide insights into methods of optimizing nZVI/BC synthesis and maximizing the efficiency of nZVI in environmental cleanup.

Assessment of pollutant removal processes and kinetic modelling in vertical flow constructed wetlands at elevated pollutant loading

Constructed wetland (CW), an ecological wastewater treatment technology, is low cost and easily to operate. Vertical flow constructed wetland (VF-CW) systems have been used to treat various wastewaters across the world. The present work exhibits the detail study of five type's multi-layered vertically constructed wetlands operated at 24 h hydraulic retention time under semi-continuous vertical flow mode. Except N-NO₃^-, all the pollutants were sufficient removal in iron scraps constructed wetland (ISs-CW). The highest average pollutant removal efficiency achieved in ISs-CW was 85.04%, 77.57%, 85.99%, 62.01% and 88.91% for N-NH₄⁺, N-NO₂⁺, total nitrogen, total phosphate and sulphate respectively. The present CWs planted with Eichhornia crassipes is a promising system for municipal wastewater treatment. The first-order kinetic modelling was best suited for the removal rate since it presents higher R², rate constant (k) and B values.

Elucidating the dechlorination mechanism of hexachloroethane by Pd-doped zerovalent iron microparticles in dissolved lactic acid polymers using chromatography and indirect monitoring of iron corrosion

The degradation mechanism of the pollutant hexachloroethane (HCA) by a suspension of Pd-doped zerovalent iron microparticles (Pd-mZVI) in dissolved lactic acid polymers and oligomers (referred to as PLA) was investigated using gas chromatography and the indirect monitoring of iron corrosion by continuous measurements of pH, oxidation-reduction potential (ORP), and conductivity. The first experiments took place in the absence of HCA, to understand the evolution of the Pd-mZVI/PLA/H₂O system. This showed that the evolution of pH, ORP, and conductivity is related to changes in solution chemistry due to iron corrosion and that the system is initially cathodically controlled by H⁺ mass transport to Pd surfaces because of the presence of an extensive PLA layer. We then investigated the effects of Pd-mZVI particles, temperature, initial HCA concentration, and PLA content on the Pd-mZVI/PLA/HCA/H₂O system, to obtain a better understanding of the degradation mechanism. In all cases, HCA dechlorination first requires the production of atomic hydrogen H^*-involving the accumulation of tetrachloroethylene (PCE) as an intermediate-before its subsequent reduction to non-chlorinated C₂ and C₄ compounds. The ratio between Pd-mZVI dosage, initial HCA concentration, and PLA content affects the rate of H^* generation as well as the rate-determining step of the process. A pseudo-first-order equation can be applied when Pd-mZVI dosage is much higher than the theoretical stoichiometry (600 mg for [HCA]₀ = 5-20 mg L^-1). Our results indicate that the HCA degradation mechanism includes mass transfer, sorption, surface reaction with H^*, and desorption of the product.

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 Fe^II-bearing phases (e.g., Fe^IIO, Fe^II(OH)₂, Fe^IIFe^III₂O₄) on the NZVI surface or complete oxidation to ferric (oxyhydr)oxides (e.g., Fe^IIIOOH). 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.

Calcium hydroxide coating on highly reactive nanoscale zero-valent iron for in situ remediation application

Nano scale zero-valent iron (nZVI), a promising engineering technology for in situ remediation, has been greatly limited by quick self-corrosion and low mobility in porous media. Highly reactive nZVI particles produced from the borohydride reduction method were enclosed in a releasable Ca(OH)₂ layer by the chemical deposition method. The amount of Ca(OH)₂ coated on nZVI surface were well controlled by the precursor dosage. At moderate Ca(OH)₂ dosage (R_Ca/TFe = 0.25) condition, the increment of Fe⁰ content for the obtained nZVI/Ca-0.25 sample was observed. The interfacial reactions between the iron oxide shell and the Ca(OH)₂ saturated environment were delicately elucidated by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) spectrum. And the coverage of Ca(OH)₂ shell on spherical nZVI surface was found more complete and uniform for the nZVI/Ca sample obtained from the moderate precursor dosage condition (R_Ca/TFe = 0.25). The Ca(OH)₂ shell before dissolution was demonstrated owning the anti-corrosion capability to slow down the oxidation of Fe⁰ core in air, during ethanol storage and in aqueous environment. The mechanism of anti-corrosion capability for nZVI/Ca-0.25 particle was interestingly found to be attributed to the Ca(OH)₂ shell isolation and also be potentially due to the iron oxide shell phase transformation mediated by the outer Ca(OH)₂ shell. An improved trichloroethylene reduction performance was observed for nZVI/Ca-0.25 than bare nZVI. The mobility of nZVI/Ca particles in water-saturated porous media was moderately improved before shell dissolution.

Synthesis and characterisation of stable and efficient nano zero valent iron

Nano zero valent iron (nZVI) is an excellent adsorbent/reductant with wide applicability in remediation of persistent contaminants in soil, water and groundwater aquifers. There are concerns about its environmental fate, agglomeration, toxicity and stability in the air. Several modification methods have applied chistosan, green tea, carboxyl methyl cellulose and other coating substances to ensure production of nZVI with excellent air stability and effectiveness. The synthesis of a novel green nZVI (gNZVI) with Harpephyllum caffrum leaf extracts was successfully executed in the current study. Production of gNZVI involved the simultaneous addition of an optimum amount of the NaBH₄ and H. caffrum extract to FeCl₃ in an inert environment (Nitrogen). The solution was stirred for 30 min, washed with dilute ethanol (50%) and freeze dried. This procedure offered the best option for the synthesis of gNZVI in terms of nontoxic and inexpensive choice of stabiliser/reductant. Systematic characterisations using TGA, TEM, SEM, XRD, FT-IR and XPS confirmed the synthesis of crystalline, stable, reactive, well-dispersed and predominantly 50 nm diameter sized gNZVI compared to the conventionally synthesised nZVI which is 65 nm. The activity testing using Orange II sodium salt (OR2) confirmed the effectiveness of the synthesised gNZVI as an excellent Fenton catalyst with 65% degradation of 20 ppm OR2 dye in 1 h reaction time.

Nano Zero-Valent Iron Mediated Metal(loid) Uptake and Translocation by Arbuscular Mycorrhizal Symbioses

Nano zero-valent iron (nZVI) has great potential in the remediation of metal(loid)-contaminated soils, but its efficiency in metal(loid) stabilization in the plant-microbe continuum is unclear. This study investigated nZVI-mediated metal(loid) behavior in the arbuscular mycorrhizal (AM) fungal-maize ( Zea mays L.) plant association. Plants with AM fungal inoculation were grown in metal(loid)- (mainly Zn and Pb) contaminated soils (Litavka River, Czech Republic) amended with/without 0.5% (w/w) nZVI. The results showed that nZVI decreased plant metal(loid) uptake but inhibited AM development and its function in metal(loid) stabilization in the rhizosphere. AM fungal inoculation alleviated the physiological stresses caused by nZVI and restrained nZVI efficiency in reducing plant metal(loid) uptake. Micro proton-induced X-ray emission (μ-PIXE) analysis revealed the sequestration of Zn (possibly through binding to thiols) by fungal structures in the roots and the precipitation of Pb and Cu in the mycorrhizal root rhizodermis (possibly by Fe compounds originated from nZVI). XRD analyses further indicated that Pb/Fe mineral transformations in the rhizosphere were influenced by AM and nZVI treatments. The study revealed the counteractive effects of AM and nZVI on plant metal(loid) uptake and uncovered details of metal(loid) behavior in the AM fungal-root-nZVI system, calling into question about nZVI implementation in mycorrhizospheric systems.

Enhanced Oxidative and Adsorptive Removal of Diclofenac in Heterogeneous Fenton-like Reaction with Sulfide Modified Nanoscale Zerovalent Iron

Sulfidation of nanoscale zerovalent iron (nZVI) has shown some fundamental improvements on reactivity and selectivity toward pollutants in dissolved-oxygen (DO)-stimulated Fenton-like reaction systems (DO/S-nZVI system). However, the pristine microstructure of sulfide-modified nanoscale zerovalent iron (S-nZVI) remains uncovered. In addition, the relationship between pollutant removal and the oxidation of the S-nZVI is largely unknown. The present study confirms that sulfidation not only imparts sulfide and sulfate groups onto the surface of the nanoparticle (both on the oxide shell and on flake-like structures) but also introduces sulfur into the Fe(0) core region. Sulfidation greatly inhibits the four-electron transfer pathway between Fe(0) and oxygen but facilitates the electron transfer from Fe(0) to surface-bound Fe(III) and consecutive single-electron transfer for the generation of H₂O₂ and hydroxyl radical. In the DO/S-nZVI system, slight sulfidation (S/Fe molar ratio = 0.1) is able to nearly double the oxidative removal efficacy of diclofenac (DCF) (from 17.8 to 34.2%), whereas moderate degree of sulfidation (S/Fe molar ratio = 0.3) significantly enhances both oxidation and adsorption of DCF. Furthermore, on the basis of the oxidation model of S-nZVI, the DCF removal process can be divided into two steps, which are well modeled by parabolic and logarithmic law separately. This study bridges the knowledge gap between pollutant removal and the oxidation process of chemically modified iron-based nanomaterials.

Role of membrane pore polymerization conditions for pH responsive behavior, catalytic metal nanoparticle synthesis, and PCB degradation

This article describes the effects of changing monomer and cross-linker concentrations on the mass gain, water permeability, Pd-Fe nanoparticle (NP) loading, and the rate of degradation of 3,3',4,4',5-pentachlorobiphenyl (PCB 126) of pore functionalized polyvinylidene fluoride (PVDF) membranes. In this study, monomer (acrylic acid (AA)) and cross-linker (N, N'- methylene-bis (acrylamide)) concentrations were varied from 10 to 20 wt% of polymer solution and 0.5-2 mol% of monomer concentration, respectively. Results showed that responsive behavior of membrane could be tuned in terms of water permeability over a range of 270-1 L m-2 h-1 bar-1, which is a function of water pH. The NP size on the membrane surface was found in the range of 16-23 nm. With increasing cross-linker density the percentage of smaller NPs (< 10 nm) increases due to smaller mesh size formation during in-situ polymerization of membrane. NP loading was found to vary from 0.21 to 0.94 mg per cm2 of membrane area depending on the variation of available carboxyl groups in membrane pore domain. The NPs functionalized membranes were then tested for use as a platform for the degradation of PCB 126. The observed batch reaction rate (Kobs) for PCB 126 degradation for per mg of catalyst loading was found 0.08-0.1 h-1. Degradation study in convective flow mode shows 98.6% PCB 126 is degraded at a residence time of 46.2 s. The corresponding surface area normalized reaction rate (K sa ) is found about two times higher than K sa of batch degradation; suggesting elimination of the effect of diffusion resistance for degradation of PCB 126 in convective flow mode operation. These Pd-Fe-PAA-PVDF membranes and nanoparticles are characterized by TGA, contact angle measurement, surface zeta potential, XRD, SEM, XPS, FIB, TEM and other techniques reveal the details about the membrane surface, pores and nanoparticles size, shape and size-distribution. Statistical analysis based on experimental results allows us to depict responsive behavior of functionalized membrane. In our best knowledge this paper first time reports detail study on responsive behavior of pore functionalized membrane in terms of permeability, NPs size, metal loading and its effect on PCB 126 degradation in a quantified approach.

Degradation of 2,4-dichlorophenol by activating persulfate and peroxomonosulfate using micron or nanoscale zero-valent copper

The ability of persulfate (PS) and peroxymonosulfate (PMS) activated by micron or nanoscale zero-valent copper (ZVC or nZVC) to degrade 2,4-dichlorophenol (2,4-DCP) was quantified under various conditions. Mechanism investigation revealed that PS and PMS accelerated the corrosion of ZVC or nZVC to release Cu⁺ under acidic conditions. The in-situ generated Cu⁺ further decomposed PS or PMS to produce SO₄^- and OH, which then dramatically degraded 2,4-DCP. The k_obs for 2,4-DCP removal followed pseudo-first-order kinetics, k_obs of ZVC/PMS and nZVC/PMS systems were 10∼30 times greater than these in ZVC/PS and nZVC/PS systems. The nZVC/PMS system was most effective to remove 2,4-DCP which even did better than the nZVI/PMS system, with rate constant values ranging from 0.041 to 1.855min^-1. At higher pH ZVC is ineffective, but nZVC can activate PS and PMS to significantly degrade 2,4-DCP at pH up to 7.3. The 2,4-DCP degradation pathway was found to involve dechloridation, dehydrogenation, hydroxylation, ring open and mineralization. 56.7% and 45.3% of TOC removals were respectively obtained in the ZVC/PMS and nZVC/PMS systems within 120min. This study helps to comprehend the application of zero-valent metals in reactive radicals-based oxidation processes and the reactivity of Cu⁺ as an activator of PS and PMS.

Physicochemical transformation of Fe/Ni bimetallic nanoparticles during aging in simulated groundwater and the consequent effect on contaminant removal

To assess the fate and long-term reactivity of bimetallic nanoparticles used in groundwater remediation, it is important to trace the physicochemical transformation of nanoparticles during aging in water. This study investigated the short-term (within 5 d) and long-term (up to 90 d) aging process of Fe/Ni bimetallic nanoparticles (Fe/Ni BNPs) in simulated groundwater and the consequent effect on the particle reactivity. Results indicate that the morphological, compositional and structural transformation of Fe/Ni BNPs happened during the aging. In the 5-d short-term aging, Fe⁰ corrosion occurred rapidly and was transformed to ferrous ions which were adsorbed onto the surface of Fe/Ni BNPs, accompanied by the elevation of solution pH and the negative redox potential. In the long-term aging, scanning electron microscopy (SEM) images show that the particles transformed from spherical to rod-like and further to sheet-like and needle-like. X-ray diffraction (XRD) analysis reveals that the main aging product was magnetite (Fe₃O₄) and/or maghemite (γ-Fe₂O₃) after aging for 60-90 d. Energy dispersive spectrometer (EDS) analysis demonstrates that the mass ratio of Fe/Ni increased with aging, revealing that Ni were possibly gradually entrapped and covered by the iron oxides. Besides, the release of Ni into solution was also detected during the aging. The reactivity of the aged Fe/Ni BNPs was examined by studying its performance in tetracycline (TC) removal. The aged Fe/Ni BNPs within 2 d kept similar removal efficiency of TC as the fresh particles. However, the removal efficiency of TC by Fe/Ni BNPs aged for 5-15 d dropped by 20-50% due to aggregation and oxidation of particles, and the removal efficiency further decreased slowly with the prolongation of aging time up to 90 d. This reveals that Fe/Ni BNPs were vulnerable to passivation in water environments.

Advances in Sulfidation of Zerovalent Iron for Water Decontamination

Sulfidation has gained increasing interest in recent years for improving the sequestration of contaminants by zerovalent iron (ZVI). In view of the bright prospects of the sulfidated ZVI (S-ZVI), this review comprehensively summarized the latest developments in sulfidation of ZVI, particularly that of nanoscale ZVI (S-nZVI). The milestones in development of S-ZVI technology including its background, enlightenment, synthesis, characterization, water remediation and treatment, etc., are summarized. Under most circumstances, sulfidation can enhance the sequestration of various organic compounds and metal(loid)s by ZVI to various extents. In particular, the reactivity of S-ZVI toward contaminants is strongly dependent on S/Fe molar ratio, sulfidation method, and solution chemistry. Additionally, sulfidation can improve the selectivity of ZVI toward targeted contaminant over water under anaerobic conditions. The mechanisms of sulfidation-induced improvement in contaminants sequestration by ZVI are also summarized. Finally, this review identifies the current knowledge gaps and future research needs of S-ZVI for environmental application.

Evolution of nanoscale zero-valent iron (nZVI) in water: Microscopic and spectroscopic evidence on the formation of nano- and micro-structured iron oxides

Knowledge on the transformation of nanoscale zero-valent iron (nZVI) in water is essential to predict its surface chemistry including surface charge, colloidal stability and aggregation, reduction and sorption of organic contaminants, heavy metal ions and other pollutants in the environment. In this work, transmission electronic microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy are applied to study the compositional and structural evolution of nZVI under oxic and anoxic conditions. Under anoxic conditions, the core-shell structure of nZVI is well maintained even after 72h, and the corrosion products usually contain a mixture of wustite (FeO), goethite (α-FeOOH) and akaganeite (β-FeOOH). Under oxic conditions, the core-shell structure quickly collapses to flakes or acicular-shaped structures with crystalline lepidocrocite (γ-FeOOH) as the primary end product. This work provides detailed information and fills an important knowledge gap on the physicochemical characteristics and structural evolution of engineered nanomaterials in the environment.

Adsorptive removal of arsenate from aqueous solutions by biochar supported zero-valent iron nanocomposite: Batch and continuous flow tests

Arsenate (As(V)) removal ability by nanoscale zero-valent iron (nZVI) is compromised by aggregation of nZVI particles. In this work, pine derived biochar (PB) was used as a supporting material to stabilize nZVI for As(V) removal. The biochar supported nZVI (nZVI/BC) was synthesized by precipitating the nanoparticles on carbon surfaces. Experiments using batch and continuous flow, completely mixed reactors (CMRs) were carried out to investigate the removal of As(V) by the nZVI/BC from aqueous solutions. Batch experiments showed that nZVI/BC had high As(V) removal capacity in a wide range of pH (3-8). Kinetic data revealed that equilibrium was reached within 1h and the isotherm data showed that the Langmuir maximum adsorption capacity of the nZVI/BC for As(V) at pH 4.1 was 124.5gkg^-1. As(V) (100mgL^-1) adsorption in anoxic condition was about 8% more than in oxic conditions, where As(V) reduction was observed in anoxic condition. The performance of the nZVI/BC in flowing condition was evaluated in CMRs at influent As(V) concentrations of 2.1 and 5.5mgL^-1 and the adsorbent removed 100% and 90% of the As(V), respectively. Furthermore, the nZVI/BC composite is magnetic which facilitates collection from aqueous solutions.

Nanoscale zerovalent iron (nZVI) supported by natural and acid-activated sepiolites: the effect of the nZVI/support ratio on the composite properties and Cd2+ adsorption

Natural (SEP) and partially acid-activated (AAS) sepiolites were used to prepare composites with nanoscale zerovalent iron (nZVI) at different (SEP or AAS)/nZVI ratios in order to achieve the best nZVI dispersibility and the highest adsorption capacity for Cd²⁺. Despite the higher surface area and pore volume of AAS, better nZVI dispersibility was achieved by using SEP as the support. On the other hand, a lower oxidation degree was achieved during the synthesis using AAS. X-ray photoelectron spectroscopy (XPS) analysis of the composite with the best nZVI dispersibility, before and after Cd²⁺ adsorption, confirmed that the surface of the nZVI was composed of oxidized iron species. Metallic iron was not present on the surface, but it was detected in the subsurface region after sputtering. The content of zerovalent iron decreased after Cd²⁺ adsorption as a result of iron oxidation during Cd²⁺ adsorption. The XPS depth profile showed that cadmium was present not only at the surface of the composite but also in the subsurface region. The adsorption isotherms for Cd²⁺ confirmed that the presence of SEP and AAS decreased the agglomeration of the nZVI particles in comparison to the pure nZVI, which provided a higher adsorption capacity. The results showed that the prevention of both aggregation and oxidation during the synthesis was necessary for obtaining an SEP/AAS-nZVI composite with a high adsorption capacity, but oxidation during adsorption was beneficial for Cd²⁺ removal. The formation of strong bonds between Cd²⁺ and the adsorbents sites of different energy until monolayer formation was proposed according to modeling of the adsorption isotherms.