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Intercalation of Nanosized Fe3C in Iron/Carbon to Construct Multifunctional Interface with Reduction, Catalysis, Corrosion Resistance and Immobilization Capabilities Jianfei Li, Huachun Lan, Huijuan Liu, Gong Zhang, Xiaoqiang An, Ruiping Liu, and Jiuhui Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03409 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Intercalation of Nanosized Fe3C in Iron/Carbon to Construct Multifunctional Interface with Reduction, Catalysis, Corrosion Resistance and Immobilization Capabilities

Jianfei Li1,3, Huachun Lan*2, Huijuan Liu2, Gong Zhang2, Xiaoqiang An2, Ruiping Liu1,3, Jiuhui Qu1,2,3

1. State Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 2. Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China 3. University of Chinese Academy of Sciences, Beijing 100039, China

*Corresponding author Tel.: +86 10 62790105; fax: +86 10 62790105 E-mail address: [email protected] (H. C. Lan)

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: As a robust reducing system in industrial wastewater treatment, iron/carbon (Fe/C) micro-electrolysis suffers from surface passivation and low utilization efficiency. Herein, we introduced Fe3C into the Fe/C system to develop a core-shell Fe0/Fe3C/C nanorod with a multifunctional interface (Fe3C/C) providing reduction, catalysis, adsorption and corrosion resistance. The results proved that the fabricated Fe0/Fe3C/C possesses 5.6 times higher reduction capacity (220 mg/g) for Cr(VI) reduction, but relatively lower Fe leakage (2.7 mg/L), than that of Fe/C. Based on the results of electrochemical characterization (Tafel polarization curves & EIS), the corrosion-resistant Fe3C/C shell can significantly prevent surface passivation of the Fe0 core, while Fe3C efficiently catalyzes electron transfer from the inner Fe0 to the external carbon surface. Moreover, the reductive species involved in Cr(VI) removal were identified as hydrogen atoms, adsorbed Fe(II) ions and electrons tunneling from Fe0. STEM, XPS and Mössbauer spectroscopy were further adopted to characterize the interface reaction of Fe0/Fe3C/C during the Cr(VI) removal process. Finally, the reaction mechanism for Cr(VI) reduction over Fe0/Fe3C/C was proposed, and the distribution of active sites was inferred. KEYWORDS: micro-electrolysis, core-shell structure, electron efficiency, surface passivation, interface reaction


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 INTRODUCTION Zero-valent iron/carbon (Fe/C) micro-electrolysis has attracted increasing attention for the removal of various contaminants in industrial wastewater due to its ease of operation and wide availability.1, 2 The dispersed carbon particles serve as cathodes to produce a reducing environment when coupled with iron by generating local galvanic cells, which help to create an electric current from the internal Fe0 to the surface carbon and accelerate iron corrosion.3, 4 This will spontaneously initiate a series of chain reactions including reduction, coagulation, adsorption and precipitation, leading to efficient decontamination.5 However, it must be noted that the treatment efficiency of traditional Fe/C systems can be greatly inhibited due to the in situ generation of passive films composed of iron oxides and hydroxide precipitates.6,

7

This surface

coating layer can hinder electron transfer from the iron anode to the carbon cathode, resulting in the reduction reaction being impeded because of the limited migration of electrons. At present, several strategies have been proposed to accelerate the electron transfer process and thereby enhance the utilization efficiency of zero-valent iron. For example, the introduction of a second precious metal, such as Ag or Pd, can efficiently inhibit the iron corrosion by decreasing the formation of the iron oxide layer.8, 9 However, it is widely accepted that Ag and Pd must be replaced by cheaper materials because of their high price and very low earth abundance. Hence, considering potential future application of the Fe/C technique, catalysts made of cheap and earth-abundant elements are especially important. Actually, the 3 ACS Paragon Plus Environment


surface-bound Fe(II) generated due to the electron donor effect from Fe0 to oxidizing agents, including Fe3O4, FeOOH and Fe2O3, has been reported to be an effective reductant, because multiple Fe(II) atoms in close proximity on the surface may promote multiple-electron-transfer reactions.10 Compared with pure ZVI, Fe0@Fe3O4 and Fe@Fe2O3 were found to exhibit much higher activity for H2O2 activation as Fenton catalysts as well as greater chromate reduction capacity as a result of an interfacial electron-transfer process.11, 12 Thus, designing a multifunctional interface is the key issue for continued optimization of the traditional Fe/C system, where non-productive Fe0 corrosion can be resisted while the interfacial electron-transfer efficiency is greatly enhanced. Recently, the excellent catalytic activity, high conductivity and corrosion resistance of Fe3C/carbonaceous matrix hybrid nanostructures (Fe3C/C) has drawn increasing attention in the catalytic field.13-15 Additionally, their well-developed porosities and high specific surface areas facilitate fast kinetics that help capture exogenous contaminants on the surface.16, 17 In our previous study, a Fe3C/N-doped carbon fiber composite (FeCx/NCNFs) was used for the first time as a catalyst for dimethylarsinate (DMA) degradation and as an adsorbent for inorganic arsenic (As (V)), with degradation and adsorption occurring simultaneously, in an electro-Fenton process.18 Hence, the concept of a Fe3C/C material with multiple advantages inspired our synthesis of Fe0/Fe3C/C composites with a core-shell structure. The Fe3C/C multifunctional interface is anticipated to not only provide adequate conductive channels for electron transfer, but also serve as a protective shell to prevent the


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interior iron cores from surface passivation by reaction with surrounding chemicals. Herein, Fe0/Fe3C/C nanocomposites were prepared in this work. A variety of techniques such as XRD, electron microscopy, Raman spectroscopy and BET analysis were employed to characterize the unique core-shell structure. The enhanced performance of Fe0/Fe3C/C compared with Fe/C was evaluated by Cr(VI) reduction because Cr(VI) is a highly toxic pollutant in wastewater from the electro-plating, pigment and chromate industries. It can cause great harm to the environment, ecology and human health.19, 20 Fe0/Fe3C/C exhibited much higher efficiency toward Cr(VI) reduction than Fe/C, and the formed Cr(III) was further immobilized in the process. Moreover, the internal Fe0 corrosion was greatly inhibited due to the connection of anodic iron and cathodic carbon by the Fe3C layer. Finally, the positive role of Fe3C in the enhancement of Fe/C performance was investigated in detail via electrochemical measurements, X-ray photoelectron spectroscopy and Mössbauer spectroscopy.

 MATERIALS AND METHODS Chemicals and Reagents. Cyanamide (>95%) was provided by J&K Scientific Ltd (China). FeCl3H2O (>98%), K2Cr2O7 (>99.9%) and Na2SO4 (>99.9%) were supplied by Sinopharm Chemical Reagent Co. Ltd (China). Nano zero-valent iron (nZVI, >99.9%) was obtained from Shanghai Macklin Biochemical Co. Ltd (China) with a mean diameter of ~100 nm and BET specific surface area of 9.012 m2/g. Fe/C composites (99%) were obtained from Jinzhou Haixin Metal Materials Co. Ltd (China) with BET specific surface area of 89.174 m2/g. (SEM images of nZVI and Fe/C 5 ACS Paragon Plus Environment


composites are presented in Figure S1). Tert-butyl alcohol (C4H10O, >99.9%), 5,5-dimethylpyrroline-oxide (DMPO, >99.0%) and 1,10-Phenanthroline monohydrate (C12H8N2, >99.9%) were purchased from Sigma-Aldrich, USA. All chemicals used herein were used as received without any further purification. Pressurized nitrogen and oxygen gas (99.5%) were adopted to saturate the solutions. All solutions were prepared with ultra-pure water obtained from a Millipore AutoPure WT101UV system (Millipore SAS, Molsheim, France) with a resistivity of 18.2 MΩ cm at 25 °C. Sample Preparation. Fe0/Fe3C/C was synthesized via a novel sol-gel procedure by classical carbothermal reduction.21 10 ml 50% NH2CN solution was added to 15 ml 0.8 M FeCl3 aqueous solution under stirring at 80 °C. The heated sol-gel process allowed evaporation of solvent from the precursor solution, and a tawny slurry was then formed. The slurry was then dried in a vacuum drying oven at 80 °C for 48 h to obtain a brown solid. After grinding, the powder was carbonized in a tube furnace that was heated to 800 °C at a rate of 4 °C/min and held at the set-point temperature for 3 h under N2 flow of 20 mL/min. The furnace chamber was then cooled to room temperature, resulting in a fine dark-grey powder, Fe0/Fe3C/C. Experimental Procedures. First, Cr(VI) stock solution was prepared by dissolving a known amount of K2Cr2O7 in ultrapure water. The targeted Cr(VI) concentration was set at 10 mg/L, and the initial pH value was adjusted to 4 in our study. In order to eliminate the undesirable effect of dissolved oxygen, the Cr(VI) solution was saturated with N2 (500 mL/min) for 60 min before use. Typically, Fe0/Fe3C/C (100 mg/L, Fe0 loading 45 wt%) was added to the Cr(VI) solution, and the


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equivalent amount of Fe0 was adopted for Fe0 and Fe/C systems. To ensure the comparability of these three materials, the removal capacity was referred to the removal amount per gram of Fe0. The mixture was stirred at room temperature for 120 min in a 100 mL conical flask containing 50 mL Cr(VI) solution, sealed with a rubber plug. During the experiment, a 1 mL sample of the solution was periodically withdrawn from the reactor with a syringe and then filtered through a 0.22 μm Millipore membrane for subsequent analysis. Characterization and Analysis. X-ray diffraction (XRD) was carried out by a Bruker D8 Advance X-ray diffractometer with a Cu Ka radiation source and λ at 1.5418 Å in the 2θ range of 10-90o. The Raman spectrum was obtained on a Via-Reflex Raman spectrometer with an argon ion laser (532 nm). The specific surface area was determined by N2 adsorption/desorption experiments using a surface area and porosity analyzer (ASAP2020 HD88). The morphology of Fe0/Fe3C/C was investigated

by

scanning

electron

microscopy

(SEM,

Hitachi

S-3000N).

High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectrometry; (TEM/EDS) images were obtained with a TEM H-7500 instrument (Hitachi, Japan) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi with Al-Kα X-ray radiation.

57Fe

Mössbauer spectroscopy (at 298 K) was used to both identify and

quantify the different iron species. Potentiodynamic polarization measurements were performed at a scan rate of 50 mV/s between -0.8 V and 0.3 V in a standard three-electrode configuration using a CHI 830 electrochemical analyzer (CHI, Inc.,



USA), with a platinum wire and SCE (saturated calomel electrode) as auxiliary and reference electrodes, respectively. The working electrode was obtained by dispersing the test samples on a glassy carbon electrode (3 mm in diameter) with a loading of 1.1 mg/cm2, and dried at ambient temperature. A N2-saturated 0.1 M Na2SO4 solution with pH of 4 was used as the electrolyte. Oxidation reduction potential and pH were detected by a portable water quality meter. The concentrations of Cr(VI), Cr(III) and dissolved Fe ions were quantified using an Optima 8300 ICP-OES spectrometer (PerkinElmer).

 RESULTS AND DISCUSSION Characterization of the Core-Shell Structure of Fe0/Fe3C/C. As shown in Figure 1A, the XRD diffraction peaks at 2θ = 44.67º, 65.02º, and 82.33º indicated the presence of large amounts of α-Fe in Fe0/Fe3C/C (JCPDS 06-0696). The peaks at 2θ = 42.88º, 43.74º, 44.99º, and 45.86º can be assigned to the 211, 102, 031 and 112 reflections of Fe3C (JCPDS 35-0772); simultaneously, a faint graphite peak due to the 012 reflection was also detected at 2θ = 26.38º (JCPDS 26-1080). The results indicated the coexistence of Fe, Fe3C and C phases within Fe0/Fe3C/C. According to the SEM image in Figure 1B, the prepared Fe0/Fe3C/C has a nanorod morphology with a length of up to 20 μm and diameter in the range of 100-200 nm. STEM images in Figure 1G also confirmed the rod-like morphology of Fe0/Fe3C/C. However, the EDS elemental map showed that the diameter of the Fe distribution in the rod is obviously lower than that of carbon, which indicated that Fe may be coated by C. The HRTEM images in Figure 1C (longitudinal section) and Figure 1D (cross section) 8 ACS Paragon Plus Environment

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provide further evidence for the core-shell structure of Fe0/Fe3C/C, where the inner Fe0 is encapsulated in the carbon shell with an average thickness in the range of tens of nanometers. As for the carbon shell, an inner crystalline graphite layer adjacent to Fe and outer disordered graphite layer were found as shown in Figure 1E. Besides, the well-defined lattice distances of 0.176 and 0.203 nm corresponded to the 122 and 220 planes of Fe3C, respectively (Figure 1F), which agreed well with the selected-area electron diffraction (SAED) pattern. Raman spectra in Figure 2A demonstrate the bonding between iron carbide and the carbon layer in Fe0/Fe3C/C. It has been reported that the three peaks at 1349, 1580 and 2700 cm-1 can be ascribed to amorphous sp2-bonded carbon (D band), the graphitic lattice vibration mode with E2g symmetry (G band), and highly ordered graphitic microcrystal (G’ band).22 The carbon D-band was similar to the expected position of 1350 cm-1, while the G-band shifted from 1600 to 1580 cm-1, suggesting that the graphitic layers were strongly affected by the electronic coupling from the adjacent Fe3C.23 Besides, the ratio of the D band to G band peak intensities (ID/IG) was 0.82, which implied a transformation process from amorphous carbon to a crystalline form via catalysis by the encased iron. As reported, carbon atoms can diffuse into bulk iron during the carbothermic reduction process until saturation. The formed Fe3C will then serve as a barrier to retard carbon diffusion due to its low diffusion coefficient.24,

25

By control of the annealing

temperature, a core-shell structure of Fe0/Fe3C/C with Fe central core, Fe3C transition layer and graphite carbon shell can be generated. The as-prepared Fe0/Fe3C/C was then treated with HNO3 to remove the internal Fe0.



The difference in surface area compared to untreated Fe0/Fe3C/C was characterized by nitrogen adsorption-desorption measurements. Type IV curves and rapid nitrogen uptake (P/P0 > 0.9) and distinct hysteresis loops in the medium pressure region were found for both treated and untreated Fe0/Fe3C/C, as shown in Figure S2. This indicated the coexistence of micropores and mesopores, which is beneficial to the enrichment of contaminants.26 However, the specific surface areas for Fe0/Fe3C/C and Fe3C/C (with HNO3 treatment) were 59.44 and 103.27 m2/g, respectively. The difference in surface area of 43.83 m2/g between Fe0/Fe3C/C and Fe3C/C can be attributed to the interface between the internal Fe0 and external C. The XPS technique was further adopted to clarify the spatial relationship of Fe0/Fe3C/C, and results are provided in Figure 2B. The binding energies of 284.7 eV of C-C and 285.0 eV for C=C due to the carbon layer were observed. The peak at 283.2 eV was due to the presence of Fe-C bonds in Fe3C, and a marked shift of 0.4 eV to lower binding energy was found compared with the standard position (283.6 eV). This indicated that Fe3C was introduced between Fe0 and the external carbon layer.27 Hence, the heterogeneous interface serves to protect the Fe0 from corrosion, and the electron utilization efficiency from Fe0 to carbon can be also increased due to the high conductivity of Fe3C.28 Therefore, a multifunctional interface was established in Fe0/Fe3C/C, which was expected to possess reduction, adsorption and corrosion resistance properties. Higher Efficiency Cr(VI) Removal over Fe0/Fe3C/C than Traditional Fe/C. The performance of Fe0/Fe3C/C and the positive role of the multifunctional interface were evaluated by Cr(VI) removal. As depicted in Figure 3, Cr(VI) was completely


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removed within 60 min by Fe0/Fe3C/C, with a maximum capacity of 220 mg/g. Figure S3 shows the variation of pH and ORP during the reaction in detail. In comparison, only 17.6% and 14.8% of Cr(VI) was abated over Fe/C and Fe0 (nZVI) even at 120 min. Their Cr(VI) removal capacities were 39.1 mg/g and 32.9 mg/g, respectively. Clearly, Fe0/Fe3C/C has much higher activity than that of Fe/C and Fe0. To confirm that the external Fe3C/C layer was capable of rapid enrichment of Cr products from the liquid phase, the dissolved Cr(VI) and Cr(III) obtained via sample separation were quantified by ICP according to their different solubility product constants (Ksp), respectively. In order to obtain accurate concentrations of the chromium in the adsorbed state, the samples were treated with acid digestion after magnetic separation. The inset of Figure 3 illustrates the mass balance of Cr species during the reaction process. For the Fe0/Fe3C/C system, dissolved Cr(III) was not detectable at the end, which indicates that most Cr species were effectively immobilized on the tested samples, as confirmed by EDS (Figure S4). By further XPS analysis, the adsorbed Cr within Fe0/Fe3C/C was identified as Cr(III), suggesting efficient conversion of Cr(VI) into Cr(III) via surface reduction and subsequent adsorption on Fe0/Fe3C/C (Figure S5). It was worthy of note that the Cr(III) generated from Cr(VI) reduction was totally adsorbed by Fe0/Fe3C/C, whereas it was not completely immobilized by Fe/C. This was due to the excellent adsorption potential of the carbon layer in Fe0/Fe3C/C. Moreover, it was found that the removal equilibrium was quickly achieved at 30 min in the Fe/C system, which implied that the iron surface was inactivated and the electron transfer to Cr(VI) was retarded.



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The key roles of the carbon layer and the introduction of Fe3C in the excellent performance of Fe0/Fe3C/C in Cr(VI) removal were investigated, as shown in Figure 4. A standard three-electrode system was used to characterize the electrochemical properties of Fe0/Fe3C/C and Fe/C. As shown in Figure 4A, in the cathodic region, Fe0/Fe3C/C exhibited a lower reaction rate, as evidenced by the larger Tafel slope compared with Fe/C (10.18 mV/dec vs. 9.09 mV/dec). This suggests a decrease in hydrogen evolution in the Fe0/Fe3C/C system, corresponding to a higher utilization efficiency of electrons than that of Fe/C. In comparison with Fe/C, a distinct cathodic shift of the free corrosion potential (-200 mV) was observed for Fe0/Fe3C/C. Thus, tunneling of electrons from the core to the surface is more favorable in the Fe0/Fe3C/C system. In the anodic region, a much lower Tafel slope for Fe0/Fe3C/C of 1.453 was found than that for Fe/C, which was 4.829 mV/dec (Figure 4B). Besides, the rapid electron exchange and improved reaction rate were also corroborated by measurement of

the

diffusion-controlled

current

density,

which

was

an

order

of

magnitude higher than that of Fe/C. The high electrochemical performance may result from the effect of Fe3C introduction on the electronic structure of the graphitic layers. Compared with the pristine graphite, the DOS (density of states) at the Fermi level of the C atoms adjacent to Fe3C clusters was prominently increased, and the interaction between the encased Fe3C and the neighboring C would lead to a decreased work function in this region.27, 29, 30 As a result, Fe3C modification made the C shell more conducive to electron-transfer mediation from Fe0 to the targeted Cr(VI). Most importantly, the shift of the active-passive transition to a more noble potential for


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Fe0/Fe3C/C (+101 mV vs. Fe/C) indicated that the Fe0/Fe3C/C was less vulnerable to inactivation and that the corrosion-resistant Fe3C/C shell can protect the internal Fe0 from surface passivation.31 Electrochemical impedance spectroscopy was further conducted to compare the conduction and anti-corrosion behaviors of Fe0/Fe3C/C and Fe/C. As shown in Figure 4C, the fitting parameters of the charge transfer resistance (RCT) and surface-passivating resistance (Rf) of Fe0/Fe3C/C were both lower than those of Fe/C (6.94 Ω and 34.84 Ω vs. 11.31 Ω and 1316 Ω, respectively).32 This provides direct proof that Fe3C/C not only favors rapid electron migration, but also significantly inhibits the surface passivation of Fe0/Fe3C/C. In addition, Fe ion leaching from Fe0/Fe3C/C and Fe/C during the Cr(VI) reduction process was monitored, and the results are shown in Figure 4D. The results show that the dissolved Fe ion concentration in the Fe0/Fe3C/C system was much lower than that of the Fe/C system, with equilibrium concentrations of 2.5 mg/L and 5.4 mg/L, respectively. Fe0/Fe3C/C had relatively lower Fe ion leaching but much higher Cr(VI) removal efficiency, which meant that the utilization efficiency of electrons from Fe0 was greatly enhanced compared to the Fe/C system. Interestingly, the concentration of Fe ion increased with reaction time and then achieved a plateau at 90 min for Fe0/Fe3C/C, but this process only required 20 min for Fe/C, which agreed well with the results in Figure 3. The results also demonstrated the excellent electro-conductivity and corrosion resistance afforded by the introduction of Fe3C in the multifunctional interface. The reaction conditions for Cr(VI) reduction over Fe0/Fe3C/C were then optimized,



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including the initial solution pH, dosage, Cr(VI) concentration and effect of sulfate concentration. Figure S6 shows that the Cr(VI) reduction efficiency decreased with the increase of solution pH. Cr(VI) reduction by Fe0/Fe3C/C was highly pH dependent, and acidic conditions such as pH 2.5 and 4.0 favored the occurrence of the reduction reaction. In comparison, the Cr(VI) removal efficiency increased with the dosage of Fe0/Fe3C/C and then achieved a steady state, while decreasing with increasing initial concentration of Cr(VI). The effect of sulfate electrolyte concentration was not significant, and Cr(VI) could always be completely removed in the range of 0-1.0 mmol/L. Therefore, the optimum reaction conditions were chosen as follows: [Cr(VI)]0=10 mg/L, pH 4.0, [Fe0/Fe3C/C]0=100 mg/L, without Na2SO4. Discussion on Reaction Mechanism for Efficient Cr(VI) Reduction by Fe0/Fe3C/C. Firstly, the effect of oxygen on the Cr(VI) reduction by Fe0/Fe3C/C was investigated under different gas atmospheres. Figure 5A shows that the Cr(VI) removal efficiency and removal rate decreased significantly in the order N2 > Air > O2. On the one hand, oxygen can compete with Cr(VI) in consuming electrons from Fe0Fe3CC.33 On the other hand, the O2 molecules may occupy the active sites of Fe0/Fe3C/C due to their preferential adsorption on the ferromagnetic iron core. Hence, an anaerobic atmosphere is beneficial to Cr(VI) reduction and Fe0/Fe3C/C is more suitable for pollution control in groundwater. It has been considered that both iron species and hydrogen atoms (H*) are the reductive species in the Fe/C micro-electrolysis system. Therefore, it is necessary to examine their contribution to Cr(VI)

reduction

in

the

Fe0/Fe3C/C

system.


Tert-butyl

alcohol

and

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1,10-Phenanthroline were used as scavengers to quench the hydrogen atoms and Fe(II) ions, respectively.34-36 Figure 5B shows that the Cr(VI) reduction was inhibited by 15% and 52% due to the addition of Tert-butyl alcohol and 1,10-Phenanthroline. According to the electron stoichiometry, the contributions of hydrogen atoms (f H*,Cr(VI)),

adsorbed Fe(II) (f

Fe(II), Cr(VI))

and direct Fe0 reduction (f

Fe , Cr(VI)) 0

to Cr(VI)

removal were calculated as about 15%, 52% and 33%, respectively (eq 1). In the initial stage, the excess of hydrogen ions led to hydrogen evolution, during which the solution pH was increased significantly (Figure S3). It should be noted that the iron species Fe0 and Fe(II), as double electron donors, were involved throughout the Cr(VI) reduction process. Co-functioning of these two active species was mainly due to the fact that the production of adsorbed Fe(II) depends entirely on the self-corrosion of Fe0 (eq 2 and 3).37 Therefore, the synergic effects of hydrogen atoms, Fe(II) and Fe0 can provide a rational explanation for the reductive mechanism of Fe0/Fe3C/C. f

0

Fe , Cr(VI)

= 1 – f

Fe(II), Cr(VI)

– f

H*,Cr(VI)

(1)

HCrO4– + 7H+ + Fe0  Cr3+ + 4H2O + 3Fe(III)(ads)

(2)

Fe0 + 2Fe(III)(ads)  Fe(II)(ads)

(3)

Secondly, the used Fe0/Fe3C/C after reaction was characterized by STEM analysis with EDS elemental mapping. As depicted in Figure 6, Fe0/Fe3C/C still maintained a nanorod morphology and core-shell structure after reaction with Cr(VI). However, a distinct signal of oxygen was observed in the iron-based core compared with the original Fe0/Fe3C/C material, which indicated that iron oxide was generated within the 15 ACS Paragon Plus Environment


Fe0/Fe3C/C structure. The XPS technique was then used to explore the changes undergone by iron during the process, including valence states and oxide species. As Figure 7A shows, by comparison with the fresh Fe0/Fe3C/C, the peak at B.E. 707.3-707.4 eV could be assigned to residual Fe0 after reaction, indicating that the internal Fe0 was well protected by the outside Fe3C/C layer. However, the peak intensity of Fe3C increased to some extent and a significant shift of 0.35 eV to the higher B.E region occurred, which may be attributed to the newly formed oxide on the surface of Fe3C particles. Moreover, a new peak at B.E 711.2 eV was observed due to the formation of Fe3O4, and the signal of FeO at B.E. 710.2 eV, which may be generated from the partial oxidation of Fe0, was also enhanced after reaction. In contrast, the characteristic peak of Fe0 at 706.5 eV showed no observable change after reaction with Fe/C, indicating that Fe0 was tightly wrapped in a passive film. Characteristic peaks of the products in the range of 709-712 eV were further confirmed as ferric dichromate, iron oxides, or oxyhydroxides (Figure S7). Hence, the unique core-shell architecture of Fe0/Fe3C/C played a very important role protecting Fe0 from passivation and maintained excellent stability during Cr(VI) reduction. This assumption was further confirmed by

57Fe

Mössbauer spectroscopy, as depicted in

Figure 7B. The spectrum was composed of two magnetically split sextets corresponding to α-Fe and Fe3C, and a paramagnetic quadrupole-split doublet in the center of the spectrum coinciding with the spectrum of the wustite FexO phase.38, 39 For Fe0/Fe3C/C before and after reaction, the phase ratios of Fe0: Fe3C: FexO were 45%: 52%: 3% and 17%: 49%: 34%, respectively. Clearly, the increased FexO should


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come from the oxidation of Fe0, as the Fe3C/C layer mediated efficient electron transfer from the Fe0 core to aqueous Cr(VI). Due to the significant performance difference between Fe0/Fe3C/C and Fe/C, the high activity potentially originated from the change in electronic structure caused by Fe3C introduction. This led to a high charge-density region (inferred as active sites) in contact with Fe3C nanoparticles, with higher activity than the pristine graphite without modification. The explanation was in accord with the XPS results (C 1s), where a decrease of 0.4 eV in binding energy was observed for the Fe-C bonding. Based on all the experimental results above, a reaction mechanism for efficient Cr(VI) reduction by Fe0/Fe3C/C was proposed, as shown in Figure 8. Firstly, the internal Fe0 reacted with H+ as an electron donor in acidic initial conditions, and the formed hydrogen atoms and hydrogen led to ~15% Cr(VI) reduction. Of course, Fe0 can also be oxidized by releasing electrons, and FexO was then generated in situ inside the iron-based core, whose contribution to Cr(VI) removal was ~33%. In the meantime, Fe3C as a catalyst increased the electron tunneling efficiency between Fe0 and the graphitic layer, and the Fe3C/C also served as a protector that could alleviate the surface passivation of internal Fe0 to a great extent. Secondly, the Fe(II) formed during the above two reactions is highly reducing, and ~52% of Cr(VI) was efficiently reduced by Fe(II). Finally, Cr(III) generated from Cr(VI) reduction was efficiently immobilized on the outer amorphous carbon surface of Fe0/Fe3C/C. Therefore, Cr(VI) contamination can be remediated with transformation to non-toxic immobilized Cr(III). Overall, the core-shell architecture of Fe0/Fe3C/C, with a multifunctional



interface providing reduction, catalysis, adsorption, and corrosion resistance, is very important to its application in practical water decontamination.

 CONCLUSIONS In summary, we developed a core-shell Fe0/Fe3C/C nanorod by a carbothermic reduction method to improve the utilization efficiency of traditional Fe/C micro-electrolysis. The intercalation of nanosized Fe3C endowed these engineered composites with changes in the electronic structure and porosity of the surface carbon. By comparison with Fe/C, the new material showed improved conductivity and corrosion resistance because of its larger diffusion-limited current density, lower anodic slope (-5mV/dec vs Fe/C) and more noble passivation potential (+101 mV vs Fe/C) in the Tafel polarization curves. In particular, efficient Cr(VI) reduction (220 mg/g) within 60 min and complete Cr(III) immobilization was found in the Fe0/Fe3C/C system, which was superior to that of Fe/C (39.1 mg/g). Stability testing indicated that Fe0/Fe3C/C maintained structural integrity throughout the Cr(VI) treatment, and the reduction mechanism could be attributed to the synergistic effects of direct charge transfer from Fe0, adsorbed Fe(II) and atom H*. This multifunctional interface (Fe3C/C) with reduction, catalysis, corrosion resistance and immobilization capabilities could open a new avenue for application of Fe/C micro-electrolysis in water treatment.

 ASSOCIATED CONTENT Supporting Information 18 ACS Paragon Plus Environment

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Additional experimental details and data are presented in the Supporting Information section.

 AUTHOR INFORMATATION Corresponding author Tel.: +86 10 62790105; fax: +86 10 62790105 E-mail address: [email protected] (H. C. Lan) Notes The authors declare no competing financial interests.

 ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51722811, 51438011, 51738013) and Special Funding for Guangxi Bagui Scholar Construction Projects.

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Figure 1.

(A) XRD pattern, (B) SEM image, TEM images of a longitudinal- (C)

and cross-section (D) of Fe0/Fe3C/C; HR-TEM images of core (E) and shell (F) of individual Fe0/Fe3C/C nanorod; (G) STEM with EDS elemental map of an individual Fe0/Fe3C/C nanorod.


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Figure 2.

(A) Raman spectra of Fe0/Fe3C/C powders; (B) C 1s XPS spectra of

Fe0/Fe3C/C.



Figure 3.

Comparison of the reduction capacities of Fe0/Fe3C/C, Fe0 and Fe/C for

Cr(VI) removal (initial Cr(VI)

concentration = 10 mg/L, Fe0/Fe3C/C dosage = 100

mg/L, initial pH = 4.0, N2 saturated), inset: mass balance of Cr species during Cr(VI) reduction.


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Figure 4.

(A) Potentiodynamic polarization curves of Fe0/Fe3C/C and Fe/C at

initial pH = 4; (B) Tafel slopes of Fe0/Fe3C/C and Fe/C in cathodic and anodic region; (C) Nyquist plots of Fe0/Fe3C/C and Fe/C electrodes obtained by applying a sine wave potential with an amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz; (D) The leaching of Fe ions from Fe0/Fe3C/C in the reduction process.



Figure 5.

(A) Influence of aeration atmosphere on reduction efficiency of

Fe0/Fe3C/C; (B) Quenching effect of reducing species (H*/Fe2+) on Cr(VI) removal.


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Figure 6.

STEM with EDS elemental map of an individual Fe0/Fe3C/C nanorod

after reaction.



Figure 7.

(A) XPS spectra of Fe0/Fe3C/C before and after reaction with Fe2p; (B)

Typical Mössbauer spectra for samples of Fe0/Fe3C/C.


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Figure 8.

The proposed reaction mechanism of Fe0/Fe3C/C in the Cr(VI) reduction

process.



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