Fe Corrosion Research Articles

Effect of pyrite on enhancement of zero-valent iron corrosion for arsenic removal in water: A mechanistic study

In this study, the enhanced effect of pyrite (FeS2) on zero-valent iron (Fe0) corrosion for arsenic (As) removal was investigated in a combined-Fe0/FeS2 system. The effects of different Fe0/FeS2 composition, dosage and initial pH were evaluated by batch experiments. Results showed that the best combination ratio of Fe0:FeS2 (w/w) was 1:1 and the optimal dosage of mixture was 2.0 g/L. The combination of Fe0 and FeS2 in a system significantly enhanced the reactivity of Fe0 for effective As removal within a broad pH range (3.0–9.0). The effective As removal in the combined-Fe0/FeS2 system was primarily ascribed to being enhanced corrosion of Fe0 by addition of FeS2. SEM and XRD characterizations strongly verified this point. Specifically, the mechanism study (the releases of Fe2+ and total Fe ion, variations of pH values as well as XPS characterization) suggested that FeS2 in the combined-Fe0/FeS2 system could alleviate the passivation of Fe0 (pHini 3.0–5.0) and accelerate the dissolution of pristine oxide film that coated on Fe0 surface (pHini 6.8–9.0). Besides, FeS2 in combined-Fe0/FeS2 system could also accelerate the reactions between Fe0 to O2 at pHini 3.0–9.0. These phenomena were well explained by a galvanic couple between Fe0 and FeS2, where FeS2 was a cathode and Fe0 was an anode. Consequently, electrons released from Fe0 that mediated by FeS2 to oxide film, passivation layer and O2 were accelerated in combined-Fe0/FeS2 system and thereby enhanced the corrosion of Fe0 for efficient As removal. Our findings suggest that utilizing FeS2 to enhance the corrosion of Fe0 would be a promising technology for remediation of As-contaminated water.

Porous iron material for TcO4- and ReO4- sequestration from groundwater under ambient oxic conditions.

Technetium-99 (99Tc) is a major contaminant at nuclear power plants and several US Department of Energy sites. Its most common aqueous species, pertechnetate (TcO4-), is very mobile in the environment, and currently there are no effective technologies for its sequestration. In this work, a porous iron (pFe) material was investigated for TcO4- and perrhenate (ReO4-) sequestration from artificial groundwater. The pFe was significantly more effective than granular iron for both TcO4- and ReO4- sequestration under oxic conditions. The Tc removal capacity was 27.5 mg Tc/g pFe at pH ˜6.8, while the Re removal capacity was 23.9 mg Re/g pFe at pH ˜10.6. Tc K-edge XANES and EXAFS analyses indicated that the removed Tc species was 70-80% Tc(IV) that was likely incorporated into Fe corrosion products (i.e., Fe(OOH), Fe3O4) and 20-30% unreduced TcO4-. In contrast, the removed Re species was ReO4- only, without detectable Re(IV). In addition, the sequestered ReO4- was not extracted (<3%) by 0.1 M Na2SO4 and 1 M KI solution, which indicated that ReO4- and by chemical analogy, unreduced TcO4-, was likely incorporated into Fe corrosion products. This inexpensive pFe material may be applied to the sequestration and stabilization of 99TcO4- from contaminated environments and nuclear waste streams.

Soft X‐ray spectromicroscopy studies of pitting corrosion of reinforcing steel bar

Pitting corrosion of reinforcing steel bar (rebar) imbedded in concrete by chloride ions can cause concrete degradation. It is thus necessary to develop methods to mitigate concrete corrosion which could include using a protective polymer rebar coating. Corrosion studies of polyvinyl butyral‐carbon black polymer‐coated rebar using soft X‐ray fluorescence (XRF) microprobe and micro‐X‐ray absorption near edge spectroscopy (μ‐XANES) are reported in this study. After removal of the polyvinyl butyral‐carbon black polymer coating, Fe Lα1, Mn Lα1, and O Kα1 XRF maps were collected as well as Fe and Mn L3‐edge μ‐XANES spectra from different regions across the whole rebar surface by collection of total electron yield (TEY) and partial XRF yield (PFY) spectra. The distribution of metallic Fe and Fe corrosion products was determined by analysis of the Fe XRF map. The μ‐XANES spectra indicated a higher fraction of Fe (III) phases on the corroded rebar surface, while Fe (II/III) phases were the major corrosion products beneath the surface region. In addition, Mn (II) and Mn (III) were determined as the valence states of the manganese corrosion products.

Development of conversion coatings on iron via corrosion in LiPF6 solution

The corrosion process of iron in 1M LiPF6 solution using an ethylene carbonate/diethyl carbonate (EC/DEC) mixture solvent is studied using gravimetric and electrochemical methods. The electrochemical technique employed is electrochemical impedance spectroscopy, EIS, and a special electrode arrangement is used in a pouch cell. The electrochemical and gravimetrical data match reasonably well, indicating a decreasing corrosion rate with time of exposure. The EIS data are analysed using a transmission line model that accounts for the shifts observed in the high frequency limit of the impedance. The data presented suggest that PF6− corrodes Fe to Fe2+ that further evolves to Fe3+, forming porous structures based on FeF3. The formation of Fe3+ has also been confirmed by X-ray photoelectron spectroscopy, XPS analysis. In the presence of non-oxidising salts (e.g., LiBOB, LiBF4), the Fe corrosion rate decreases.

Simultaneous removal of aniline, antimony and chromium by ZVI coupled with H2O2: Implication for textile wastewater treatment.

Aniline, antimony (Sb) and chromium (Cr) are typical and regulated co-contaminants in textile wastewater, but their removal was often investigated individually. In this work, simultaneous removal of aniline, Sb and Cr by ZVI coupled with H2O2 was studied. With the dosage of 0.5 g L-1 ZVI and 2 mM H2O2, aniline, Sb and Cr can be removed completely at pH 3. Experiment with iso-propanol as the radical scavenger confirmed that OH derived from Fenton reaction accounts for aniline degradation, but not for Sb and Cr removal. H2O2 accelerated Fe(0) corrosion and generated the nanoscale iron(hydro)oxides. Aniline was degraded by OH first and then the degradation products were removed by iron(hydro)oxides via adsorption and co-precipitation. Both Fe(0) and iron(hydro)oxides were responsible for Sb and Cr removal, yet iron(hydro)oxides were identified as the major contributor. X-ray photoelectron spectroscopy analysis demonstrated that Sb and Cr were removed mainly as the states of Sb(III) and Cr(III). The real textile wastewater investigation confirmed that ZVI coupled with H2O2 can eliminate aniline, Sb and Cr effectively, which has important implications for the advanced treatment of textile wastewater.

An Acetobacterium strain isolated with metallic iron as electron donor enhances iron corrosion by a similar mechanism as Sporomusa sphaeroides.

Sporomusa sphaeroides related strains are to date the only homoacetogens known to increase metallic iron corrosion. The goal of this work was to isolate additional homoacetogenic bacteria capable of using Fe(0) as electron donor and to explore their extracellular electron transfer mechanism. Enrichments were started from anoxic corrosion products and yielded Acetobacterium as main homoacetogenic genus. Isolations were performed with a new procedure using plates with a Fe(0) powder top layer. An Acetobacterium strain, closely related to A. malicum and A. wieringae, was isolated, in addition to a S. sphaeroides strain. The Acetobacterium isolate significantly increased Fe(0) corrosion ((1.44 ± 0.16)-fold) compared to abiotic controls. The increase of corrosion by type strains ranged from (1.28 ± 0.13)-fold for A. woodii to (2.03 ± 0.22)-fold for S. sphaeroides. Hydrogen mediated the electron uptake from Fe(0) by the acetogenic isolates and tested type strains. Exchange of the medium and SEM imaging suggested that cells were attached to Fe(0). The corrosion enhancement mechanism is for all tested strains likely related to free extracellular components catalyzing hydrogen formation on the Fe(0) surface, or to the maintenance of low hydrogen concentrations on the Fe(0) surface by attached cells thereby thermodynamically favoring hydrogen formation.

A Novel Shewanella Isolate Enhances Corrosion by Using Metallic Iron as the Electron Donor with Fumarate as the Electron Acceptor.

The involvement of Shewanella spp. in biocorrosion is often attributed to their Fe(III)-reducing properties, but they could also affect corrosion by using metallic iron as an electron donor. Previously, we isolated Shewanella strain 4t3-1-2LB from an acetogenic community enriched with Fe(0) as the sole electron donor. Here, we investigated its use of Fe(0) as an electron donor with fumarate as an electron acceptor and explored its corrosion-enhancing mechanism. Without Fe(0), strain 4t3-1-2LB fermented fumarate to succinate and CO2, as was shown by the reaction stoichiometry and pH. With Fe(0), strain 4t3-1-2LB completely reduced fumarate to succinate and increased the Fe(0) corrosion rate (7.0 ± 0.6)-fold in comparison to that of abiotic controls (based on the succinate-versus-abiotic hydrogen formation rate). Fumarate reduction by strain 4t3-1-2LB was, at least in part, supported by chemical hydrogen formation on Fe(0). Filter-sterilized spent medium increased the hydrogen generation rate only 1.5-fold, and thus extracellular hydrogenase enzymes appear to be insufficient to explain the enhanced corrosion rate. Electrochemical measurements suggested that strain 4t3-1-2LB did not excrete dissolved redox mediators. Exchanging the medium and scanning electron microscopy (SEM) imaging indicated that cells were attached to Fe(0). It is possible that strain 4t3-1-2LB used a direct mechanism to withdraw electrons from Fe(0) or favored chemical hydrogen formation on Fe(0) through maintaining low hydrogen concentrations. In coculture with an Acetobacterium strain, strain 4t3-1-2LB did not enhance acetogenesis from Fe(0). This work describes a strong corrosion enhancement by a Shewanella strain through its use of Fe(0) as an electron donor and provides insights into its corrosion-enhancing mechanism.IMPORTANCEShewanella spp. are frequently found on corroded metal structures. Their role in microbial influenced corrosion has been attributed mainly to their Fe(III)-reducing properties and, therefore, has been studied with the addition of an electron donor (lactate). Shewanella spp., however, can also use solid electron donors, such as cathodes and potentially Fe(0). In this work, we show that the electron acceptor fumarate supported the use of Fe(0) as the electron donor by Shewanella strain 4t3-1-2LB, which caused a (7.0 ± 0.6)-fold increase of the corrosion rate. The corrosion-enhancing mechanism likely involved cell surface-associated components in direct contact with the Fe(0) surface or maintenance of low hydrogen levels by attached cells, thereby favoring chemical hydrogen formation by Fe(0). This work sheds new light on the role of Shewanella spp. in biocorrosion, while the insights into the corrosion-enhancing mechanism contribute to the understanding of extracellular electron uptake processes.

Degradation of diclofenac by H2O2 activated with pre-magnetization Fe0: Influencing factors and degradation pathways

Diclofenac sodium (DCF) is frequently detected as a non-steroidal pharmaceutical in the aquatic environment. In this study, the degradation of DCF in two heterogeneous systems, pre-magnetization Fe0/H2O2 (Pre-Fe0/H2O2) and Fe0/H2O2 system, was comparably studied. Our findings proved that Pre-Fe0 could significantly improve the degradation and dechlorination of DCF due to the change of Fe0 characteristics after pre-magnetization. Compared with Fe0/H2O2 process, Pre-Fe0/H2O2 process has 2.1–7.0 times higher rate constant for DCF degradation at different H2O2 dosages (0.25–2.0 mM), initial pH (3.0–6.0) and Fe0 dosages (0.25–1.5 mM). The characterizations by X-ray Photoelectron Spectroscopy and Electron Paramagnetic Resonance confirmed that the enhancement attributed to the increase of Fe0 corrosion and fast generation of OH. In addition, preliminary degradation mechanism was elucidated by major products identification using UPLC-MS, through which the degradation intermediates, such as 4-hydroxy-diclofenac or 5-hydroxydiclofenac, 2,6-dichloroaniline, phenylacetic acid, 1,3-dichlorobenzene and 2-aminophenylacetic acid were identified. Hydroxylation, decarboxylation, CN bond cleavage and ring-opening involving the attack of OH or other substances, were the main degradation mechanism. Therefore, Pre-Fe0/H2O2 process, which does not need extra energy and costly reagents, is an efficient and environmental-friendly process to degrade DCF.

X-ray microprobe characterization of corrosion at the buried polymer-steel interface

The simultaneous collection of x-ray fluorescence (XRF) intensity maps for several elements was combined with x-ray absorption near-edge spectroscopy (XANES) to perform a non-destructive study of polymer-steel interfaces exposed to high pH potash brine. Analysis of iron, manganese, potassium and chlorine XRF maps allowed the identification of anodic pits, disbonded areas containing precipitated corrosion products and several cathodic regions. Iron XANES was used to determine the phases of Fe corrosion products present. Our results confirm previous models of corrosion under polymer films where osmotic blisters are causally related to the spread of corrosion at the polymer-metal interface.

Quantitative analysis of the polarization behavior of iron in an aerated acidic solution using SECM

The apparent corrosion current of pure iron in aerated 5 mM HClO4 + 0.1 M NaClO4 solution was separated into Fe oxidation, proton reduction and oxygen reduction currents using SECM in a modified tip generation/substrate collection (TG/SC) mode. The oxidation current density of Fe was 1.11 × 10−3 A/cm2, equal to the sum of proton reduction and oxygen reduction current density, around 5.26 × 10−4 and 5.84 × 10−4 A/cm2, respectively, implying that oxygen reduction and proton reduction are equally important at open circuit potential (OCP). Moreover, oxygen reduction was suppressed on the Fe electrode in the potential range between −0.7 and −1.1 V vs Ag/AgCl because of the inhibitory effect of the proton reduction reaction and a potential-dependent surface effect, then the suppression is weakened and oxygen reduction is enhanced. Finally, the Fe corrosion current was dominated by oxygen reduction and Fe oxidation at potentials more positive than −0.6 V.

Weak magnetic field for enhanced oxidation of sulfamethoxazole by Fe0/H2O2 and Fe0/persulfate: Performance, mechanisms, and degradation pathways

In this study, effects of the nonuniform weak magnetic field (WMF) on sulfamethoxazole (SMX) degradation rate by H2O2 and persulfate (PS) coupled with different zero-valent iron (Fe0) samples were investigated. The kobs value (pseudo-first-order rate constant) of SMX degradation by Fe0/H2O2 or Fe0/PS was accelerated by WMF irradiation. Meanwhile, the release rate of dissolved iron was increased in the presence of WMF which indicated WMF irradiation could promote Fe0 corrosion in Fe0/H2O2 and Fe0/PS systems. Effects of WMF on SMX degradation kinetics by Fe0/H2O2 or Fe0/PS in the initial system pH (pHini) range of 3.0–7.0 were studied. Although WMF accelerated SMX removal by Fe0/H2O2 at pHini 3.0 and 4.0, WMF did not exhibit advantages at higher pHini. In Fe0/PS systems, WMF could promote SMX removal during the investigated pH range of 3.0–7.0, and kobs of SMX removal depended on the form of aquatic SMX which suggested that SO4− could selectively degrade dissociated or protonated SMX. Although SO42−, Cl−, and NO3− showed different effects on SMX degradation by Fe0/H2O2 or Fe0/PS, WMF could accelerate SMX degradation in the presence of these common anions. Chemical quenching experiments and electron paramagnetic resonance (EPR) studies identified that main reactive oxygen species (ROS) generated in Fe0/H2O2/WMF and Fe0/PS/WMF systems were OH and SO4−, respectively. Meanwhile, the detection of ROS proved that WMF did not change the type of ROS in Fe0/H2O2 or Fe0/PS systems. Finally, degradation pathways of SMX by Fe0/H2O2/WMF and Fe0/PS/WMF were proposed based on transformation products.

Separation and kinetic study of iron corrosion in acidic solution via a modified tip generation/substrate collection mode by SECM

Anodic dissolution and cathodic hydrogen evolution behavior of pure Fe in 5 mM HClO4 solution was investigated via a modified tip generation/substrate collection mode using scanning electrochemical microscopy. The modified TG/SC mode of SECM could instantaneously divide the apparent current of Fe corrosion process into Fe oxidation and proton reduction process, and standard rate constants and transfer coefficients of hydrogen evolution and Fe oxidation are 9.2 × 10−6 cm/s and 0.27, 2.5 × 10−6 cm/s and 0.7, respectively, based on COMSOL simulation and Tafel linear fitting. Comparing with traditional polarization curve and electrochemical noise, the more accuracy corrosion current can be obtained in time.

Selenite removal from groundwater by zero-valent iron (ZVI) in combination with oxidants

We recently demonstrated common oxidants simply coupled with ZVI to continuously drive the accelerated Fe0 corrosion and hence achieve fast and very efficient removal of heavy metals and metalloids from groundwater. In this study, we aimed first to answer a basic question of the oxidant dosage theoretically required to sequester a certain amount of selenite. The specific ratio of oxidant dosage to Se(IV) removal, which reflected the theoretically minimal oxidant dosage required to sequester one mole of Se(IV), was almost independent of the initial Se(IV) concentration but significantly affected by the difference in oxidant species. To sequester one mole of Se(IV), the minimum dosage of the required oxidant was calculated to be 3.94–4.09 for NaClO, 3.90–4.33 for H2O2, and 3.29–3.54 for KMnO4, respectively. Simultaneous aeration increased the removal efficiency of Se(IV) and substantially reduced the required dosage of oxidants. To form a strong contrast with very limited Se(IV) removal by ZVI alone, the coupling of NaClO and H2O2 into the ZVI system remarkably enhanced the performance of Se(IV) removal during the long term fixed-bed experiments with a real groundwater background. The ZVI columns coupled by NaClO and H2O2 steadily treated 10,000 volumes (BV) of real groundwater. X-ray adsorption near edge structure (XANES) demonstrated that more than 85% of the selenium was reduced to Se0 and Se2−, with Se0 as the dominant selenium species sequestered in the solid phase. Synchrotron-based scanning transmission soft X-ray microscopy (STXM) explicitly revealed the ubiquitous spatial distribution of selenium in the corrosion products and hence demonstrated a high accessibility of these corrosion products to selenite.

Iron-foam as a heterogeneous catalyst in the presence of tripolyphosphate electrolyte for improving electro-Fenton oxidation capability

Fe-Foam (Fe-F) was used as a catalyst in the presence of tripolyphosphate (TPP) electrolyte for electro-Fenton (EF) at circum-neutral pH. Characteristics of Fe-F catalyst were studied by scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) and x-ray photoelectron spectroscopy (XPS). Operational conditions (pH and current) influencing H2O2 electrogeneration in TPP electrolyte at a three-dimensional printed gas diffusion electrode (GDE) were systematically investigated. Moreover, linear sweep voltammetry was used to evaluate the system. Results showed that Fe-F, a three-dimensional porous structure catalyst, was covered with Fe2+, Fe3+ and Fe0. The Fe-F/TPP/EF process allowed an 8.55-fold increase in the rate of phenol degradation (used as a model pollutant), higher pseudo-first-order rate constants (6.22 × 10−2 min−1 against 7.30 × 10−3 min−1), a significantly higher mineralization yield (88.69%% against 30% after 8 h electrolysis) and lower energy consumption (0.10 kWhg−1 against 0.98 kWhg−1), as compared to conventional EF process. The high efficiency in Fe-F/TPP/EF was attributed to the following benefits: i) Fe-F was capable of providing Fe2+ ions by chemical corrosion to form Fe2+-TPP complex, which induced the one-electron transfer activation of O2 leading to formation of OH, ii) chemical Fe2+ regeneration combined with Fe0 corrosion provided an additional source of Fe2+ and compensated the low capacity of Fe3+ reduction at GDE, and iii) both surface-mediated and homogeneous reactions contributed to pollutant degradation. The new proposed Fe-F/TPP/EF process displayed promising properties for wastewater treatment applications.

Mediator-free enzymatic electrosynthesis of formate by the Methanococcus maripaludis heterodisulfide reductase supercomplex

Electrosynthesis of formate is a promising technology to convert CO2 and electricity from renewable sources into a biocompatible, soluble, non-flammable, and easily storable compound. In the model methanogen Methanococcus maripaludis, uptake of cathodic electrons was shown to proceed indirectly via formation of formate or H2 by undefined, cell-derived enzymes. Here, we identified that the multi-enzyme heterodisulfide reductase supercomplex (Hdr-SC) of M. maripaludis is capable of direct electron uptake and catalyzes rapid H2 and formate formation in electrochemical reactors (−800 mV vs Ag/AgCl) and in Fe(0) corrosion assays. In Fe(0) corrosion assays and electrochemical reactors, purified Hdr-SC primarily catalyzed CO2 reduction to formate with a coulombic efficiency of 90% in the electrochemical cells for 5 days. Thus, this report identified the first enzyme that stably catalyzes the mediator-free electrochemical reduction of CO2 to formate, which can serve as the basis of an enzyme electrode for sustained electrochemical production of formate.

Hydroxyl radical dominated degradation of aquatic sulfamethoxazole by Fe0/bisulfite/O2: Kinetics, mechanisms, and pathways

In this study, batch experiments were carried out to investigate the key factors on sulfamethoxazole (SMX) removal kinetics in a new AOPs based on the combination of zero valent iron (Fe0) and bisulfite (S(IV)). With the increase of Fe0 from 0.25 mM to 5 mM, the removal rate of SMX was linearly increased in the Fe0/S(IV)/O2 system by accelerating the activation of S(IV) and Fe0 corrosion to accelerate. In the first 10 min of reaction, the increasing concentration of S(IV) inhibited SMX removal after since the high S(IV) concentration quenched reactive oxidative species (ROS). Then SMX removal rate was accelerated with the increase of S(IV) concentration after S(IV) were consumed up. The optimal ratio of S(IV) concentrations to Fe0 concentration for SMX removal in the Fe0/S(IV)/O2 system was 1:1. With SMX concentrations increasing from 1 to 50 μM, SMX removal rate was inhibited for the limitation of ROS yields. Although the presence of SO4− and OH was confirmed by electron paramagnetic resonance (EPR) spectrum, OH was identified as the dominant ROS in the Fe0/S(IV)/O2 system by chemical quenching experiments. Besides, strong inhibitive effects of 1,10-phenanthroline on SMX degradation kinetics by Fe0/S(IV)/O2 proved that the generation of ROS was rely on the release of Fe(II) and Fe(III). The generation of SO4− was ascribed to the activation of S(IV) by Fe(II)/Fe(III) recycling and the activation of HSO5− by Fe(II). And OH was simultaneously transformed from SO4− and generated by Fe0/O2. Density functional theory (DFT) calculation was conducted to reveal special reactive sites on SMX for radicals attacking and predicted intermediates. Finally, four possible SMX degradation pathways were accordingly proposed in the Fe0/S(IV)/O2 system based on experimental methods and DFT calculation.

Particles and porous tablets based on Fe0/ZSM-5 composites prepared by ball-milling for heavy metals removal: Dissolved Fe2 +, pH, and mechanism

Novel, low-cost Fe0/ZSM-5-based particles and porous tablets were prepared by a ball-milling method and used for the removal of Pb2+ in solution. Solid-phase characterization by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) and transmission electron microscopy (TEM) revealed that the Fe0 microparticles were evenly loaded and tightly immobilized on the surface of ZSM-5 because of the extrusion/welding impact during ball-milling. For different Pb2+ concentrations, batch experiments indicated that the removal of Pb2+ increased with the decline of dissolved Fe2+ and pH value in the solution for particles; opposite results were obtained for the tablets. The differences in the contact between both materials and Pb2+ were the main factor controlling Pb2+ removal in the solution. Investigation into the effect of initial pH value revealed that high pH reduced the number of electrons released from Fe corrosion. Consequently, low levels of removed Pb2+ and dissolved Fe2+ were synchronously observed. Also, simulated electroplating wastewater was treated using the prepared particles and porous tablets, and the removal order of Pb2+>Cr6+>Cu2+≈Cd2+ was observed. The Fe0/ZSM-5 particles and tablets prepared through ball-milling show potential as materials for treatment of Pb2+ and other toxic metals.

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.

Aging effects on chemical transformation and metal(loid) removal by entrapped nanoscale zero-valent iron for hydraulic fracturing wastewater treatment

In this study, alginate and polyvinyl alcohol (PVA)-alginate entrapped nanoscale zero-valent iron (nZVI) was tested for structural evolution, chemical transformation, and metals/metalloids removal (Cu(II), Cr(VI), Zn(II), and As(V)) after 1–2month passivation in model saline wastewaters from hydraulic fracturing. X-ray diffraction analysis confirmed successful prevention of Fe0 corrosion by polymeric entrapment. Increasing ionic strength (I) from 0 to 4.10M (deionized water to Day-90 fracturing wastewater (FWW)) with prolonged aging time induced chemical instability of alginate due to dissociation of carboxyl groups and competition for hydrogen bonding with nZVI, which caused high Na (7.17%) and total organic carbon (24.6%) dissolution from PVA-alginate entrapped nZVI after 2-month immersion in Day-90 FWW. Compared to freshly-made beads, 2-month aging of PVA-alginate entrapped nZVI in Day-90 FWW promoted Cu(II) and Cr(VI) uptake in terms of the highest removal efficiency (84.2% and 70.8%), pseudo-second-order surface area-normalized rate coefficient ksa (2.09×10−1Lm−2h−1 and 1.84×10−1Lm−2h−1), and Fe dissolution after 8-h reaction (13.9% and 8.45%). However, the same conditions inhibited Zn(II) and As(V) sequestration in terms of the lowest removal efficiency (31.2% and 39.8%) by PVA-alginate nZVI and ksa (4.74×10−2Lm−2h−1 and 6.15×10−2Lm−2h−1) by alginate nZVI. The X-ray spectroscopic analysis and chemical speciation modelling demonstrated that the difference in metals/metalloids removal by entrapped nZVI after aging was attributed to distinctive removal mechanisms: (i) enhanced Cu(II) and Cr(VI) removal by nZVI reduction with accelerated electron transfer after pronounced dissolution of non-conductive polymeric immobilization matrix; (ii) suppressed Zn(II) and As(V) removal by nZVI adsorption due to restrained mass transfer after blockage of surface-active micropores. Entrapped nZVI was chemically fragile and should be properly stored and regularly replaced for good performance.

Acceleration of Fe Corrosion in Cement Paste and Mortar By Enhancing Oxygen Supply

Generally, in concrete, alkalinity of the environment is maintained and reinforcing steels exhibit superior corrosion resistance owing to passive film. Therefore, it is difficult to demonstrate Fe corrosion in concrete in the laboratory in a short time. A new method for acceleration of Fe corrosion is necessary. Recently, some researchers tried to accelerate the Fe corrosion in concrete. They accelerated Fe corrosion by anodic polarization and cyclic corrosion test. In this study, an accelerated corrosion test method with enhancing oxygen supply was developed to enhance oxygen reduction on Fe surface and Fe corrosion in concrete. The effect of amount of oxygen supply on Fe corrosion in cement paste and mortar will be discussed in our presentation. The sample was 99.5% Fe sheet with 7×7 mm2and 1 mm thickness. The Fe sample was embedded in cement paste or mortar with 5 mm cover thickness and cured for 28 days. In order to prompt breakdown of passive film on the Fe sample, 2.06 M NaCl solution was mixed to the cement paste and mortar. Fig. 1 shows the schematic image of Hyperbaric-oxygen accelerated corrosion test we developed. In the gas pressure chamber, Fe in cement and Fe in mortar were immersed in 2.06 M NaCl solution under the pressured oxygen of 0.5 or 2.0 MPa for 14 days. For comparison, corrosion of Fe in cement and Fe in mortar immersed in the NaCl solution under ambient air was also discussed. After finishing the immersion under pressured oxygen or ambient air, surface and cross section of Fe sample were observed using an optical microscope and SEM. In addition, composition of the rust on Fe sample was analyzed using Raman spectroscopy measurement. Corrosion of the Fe in cement and Fe in mortar was obviously accelerated under the pressured oxygen of 0.5 MPa, and the corrosion of Fe in mortar was further enhanced than that of Fe in cement. Composition of the rust formed under the pressured oxygen of 0.5 MPa was similar to those formed in the real environment. It is thus demonstrated that the enhancement of oxygen supply is beneficial and effective to validly accelerate Fe corrosion in concrete. Corrosion of the Fe in cement and Fe in mortar was suppressed under the pressured oxygen of 2.0 MPa compared to corrosion under the pressured oxygen of 0.5 MPa. It is considered that corrosion of Fe in cement and Fe in mortar is dominated by oxygen reduction reaction under the pressured oxygen of 0.5 MPa, while the excess oxygen makes passive films more protective and corrosion of Fe is suppressed under the pressured oxygen of 2.0 MPa. Figure 1