Phosphate Shifted Oxygen Reduction Pathway on Fe@Fe2O3 Core

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Phosphate Shifted Oxygen Reduction Pathway on Fe@Fe2O3 Core-Shell Nanowires for Enhanced Reactive Oxygen Species Generation and Aerobic 4-Chlorophenol Degradation Yi Mu, Zhihui Ai, and Lizhi Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01896 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017

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Environmental Science & Technology

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Phosphate Shifted Oxygen Reduction Pathway on Fe@Fe2O3 Core-Shell Nanowires for Enhanced Reactive Oxygen Species Generation and Aerobic 4-Chlorophenol Degradation

5

Yi Mu, Zhihui Ai, and Lizhi Zhang*

6

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of

7

Environmental & Applied Chemistry, Central China Normal University, Wuhan 430079, P. R. China

8

* To whom correspondence should be addressed. E-mail: [email protected]. Phone/Fax:

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+86-27-6786 7535

1 2 3

10

11 12

1

ACS Paragon Plus Environment

Environmental Science & Technology 13

ABSTRACT Phosphate ions widely exist in the environment. Previous studies revealed that the

14

adsorption of phosphate ions on nanoscale zero-valent iron would generate a passivating oxide shell

15

to block reactive sites and thus decrease the direct pollutant reduction reactivity of zero-valent iron.

16

Given that molecular oxygen activation process is different from direct pollutant reduction with

17

nanoscale zero-valent iron, it is still unclear how phosphate ions will affect molecular oxygen

18

activation and reactive oxygen species generation with nanoscale zero-valent iron. In this study, we

19

systematically studied the effect of phosphate ions on molecular oxygen activation with Fe@Fe2O3

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nanowires, a special nanoscale zero-valent iron, taking advantages of rotating ring disk

21

electrochemical analysis. It was interesting to find that the oxygen reduction pathway on Fe@Fe2O3

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nanowires was gradually shifted from a four-electron reduction pathway to a sequential one-electron

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reduction one, along with increasing the phosphate ions concentration from 0 to 10 mmol•L-1. This

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oxygen reduction pathway change greatly enhanced the molecular oxygen activation and reactive

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oxygen species generation performances of Fe@Fe2O3 nanowires, and thus increased their aerobic

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4-chlorophenol degradation rate by 10 times. These findings shed insight into the possible roles of

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widely existed phosphate ions in molecular oxygen activation and organic pollutants degradation

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with nanoscale zero-valent iron.

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Keywords: Fe@Fe2O3 nanowires; Phosphate ions; Molecular oxygen activation; Oxygen reduction

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pathway; 4-chlorophenol degradation

32 33

Introduction

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Nanoscale zero-valent iron (nZVI) can be used for environmental remediation because of its low

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cost, environmental friendliness, large surface area, and low standard redox potential (E0h = -0.44

36

V).1-4 Although the reaction of nZVI and dissolved oxygen is harmful for direct reductive 2


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remediation applications, as oxygen could competitively consume electrons and thus disfavor the

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direct electron transfer from nZVI to pollutants, molecular oxygen activation of nZVI could produce

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reactive oxygen species (ROS, i.e. H2O2, •OH) to oxidize and mineralize refractory organic

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pollutants.5, 6 For instance, Cheng’s group first reported the complete destruction of 4-chlorophenol

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and pentachlorophenol in the presence of iron particles and air.7 Waite’s group found that the

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degradation rates of carbothiolate herbicide and molinate became higher through an oxidative

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pathway with nZVI after the introduction of air.8 Therefore, molecular oxygen activation by nZVI is

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regarded as an alternative pathway to degrade organic pollutants.

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The mechanisms of molecular oxygen activation with nZVI have been explored previously.

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Generally, nZVI induced molecular oxygen activation process take place as follows. First, Fe0(s)

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reacts with O2 to generate H2O2 via a two-electron molecular oxygen reduction, accompanying with

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the Fe2+ release (Eq. 1). Subsequently, the released Fe2+ also reacts with O2 to generate H2O2 through

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a sequential single-electron molecular oxygen activation pathway (Eqs. 2- 3).8-12 The in situ

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generated H2O2 then reacts with Fe2+ to produce •OH (Eq. 4). However, the ROS yields in respect to

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the consumed iron (i.e., ∆(Oxidants)/∆(Fe0)) were very low in the pH range of 2-11, because most of

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O2 was reduced via a four-electron transfer pathway on the nZVI surface without generating any

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ROS (Eq. 5).6 Thus, molecular oxygen activation of nZVI is highly dependent on the oxygen

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reduction pathway. Meanwhile, the widely exited anions (Cl-, NO3-, SO42-, and HPO42-) can be

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adsorbed on amorphous iron (oxy)hydroxide shell of nZVI to generate a passivating oxide layer and

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thus block reactive sites of nZVI,13 and then definitely affect oxygen reduction pathway and

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molecular oxygen activation of nZVI, which is never investigated.

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O2 + Fe0 + 2H+ → Fe2+ + H2O2

(1)

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O2 + Fe2+ → •O2- + Fe3+

(2)

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•O2- + Fe2+ + 2H+ → H2O2 + Fe3+

(3) 3



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Fe2+ + H2O2 → Fe3+ + •OH + OH- (pH< 5)

(4)

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O2 + 2Fe0 + 4H+ → 2Fe2+ + 2H2O

(5)

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Phosphate ions widely exist in the municipal wastewater and agricultural drainage with total p

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concentration in a range of 4 ‒ 14 mg•L-1 (about 0.13 ‒ 0.45 mmol•L-1), and the concentration of

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phosphate ions can reach as high as 1000 mg•L-1 in some pre-treated industrial wastewater.14-16 In

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comparison with other anions (such as Cl-, SO42-, HCO3-), phosphate ions showed much higher

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affinity toward iron oxide shell of nZVI and formed sparingly soluble vivianite (Fe3(PO4)2•(H2O)8)

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with Fe(III) species due to the low KSP (KSP = 10-36),13, 17 so the phosphate uptake capacity of nZVI

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could reach 246 mg•g-1.16 Previous study suggested that phosphate ions could act as a pendant

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proton relay to tune the oxygen reduction pathway on metal oxides. For instance, Chen et al.

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reported that the photocatalytic oxygen reduction pathway on TiO2 surface was switched from the

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sequential single-electron processes to the concerted 4e-/4H+ reduction to H2O by simply adding 4

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mmol•L-1 of phosphate ions.18 However, the influences of phosphate ions on oxygen reduction

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pathway and molecular oxygen activation of nZVI are still unclear, but of great importance to

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predict the reactivity of nZVI for the aerobic degradation of organic pollutants in the presence of

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phosphate ions.

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In this study, we first systematically study the influence of phosphate ions on oxygen reduction

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pathway and molecular oxygen activation of Fe@Fe2O3 nanowires, a special nanoscale zero-valent

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iron, taking advantages of rotating ring disk electrochemical analysis. The ROS generation and the

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subsequent aerobic 4-chlorophenol degradation by Fe@Fe2O3 nanowires in the presence of

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phosphate ions are subsequently studied. Meanwhile, the influences of phosphate ions on the

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electron transfer activity and the structure changes of Fe@Fe2O3 nanowires are also checked, aiming

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to clarify the effects of phosphate ions on the aerobic degradation of 4-chlorophenol with Fe@Fe2O3

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nanowires. 4


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Experiment Section

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Chemicals. Sodium borohydride (NaBH4), 4-chlorophenol, Ferric chloride hexa-hydrate

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(FeCl3•6H2O),

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potassium dihydrogen phosphate

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iso-propanol were bought from National Medicines Corporation Ltd., China. Superoxide dismutase

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(SOD) and catalase (CAT) were purchased from Sigma-Aldrich. All chemicals and agents were of

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analytical grade. Fe@Fe2O3 nanowires were synthesized as the method reported before,19 and the

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detailed information about synthesis process was showed in the Supporting Information. Core-shell

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structure of Fe@Fe2O3 nanowires was confirmed by transmission electron microscopy (TEM) (SI

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Figure S1).

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Rotating Disc Electrode (RDE) Experiments. The electron transfer numbers (n) of oxygen

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reduction reaction (ORR) on Fe@Fe2O3 nanowires surface were estimated by the rotating disc

98

electrode (RDE) experiment using a computer-controlled CHI-660B electrochemical workstation

99

(CHI-660B, CH Instrument, China). A saturated calomel electrode (SCE) and a platinum electrode

100

were used as the reference electrode and the counter electrode, respectively. Glassy carbon disc

101

electrode purchased from Pine Research Instrumentation, was used as the working electrode. The

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glassy carbon disc electrode was composed of a shaft and a tip, and the tip featured a 5 mm in

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diameter disk glassy carbon electrode, which was shrouded with a 12 mm in diameter Teflon. To

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prepare the Fe@Fe2O3 nanowires film working electrode, 5 mg of Fe@Fe2O3 nanowires was

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dispersed in 1 mL of diluted nafion solution and sonicated for 10 min to form uniform slurry. A 2.5

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µL aliquot of uniform slurry was then dropped onto the glassy carbon surface to form a thin slurry

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layer. After drying, Fe@Fe2O3 nanowires was uniformly coated on the glassy carbon disk electrode.

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0.1 mol•L−1 Na2SO4 solutions (pH = 4) was used as the electrolyte. The electrolyte solution has been

benzoic

acid,

sulfuric (KH2PO4),

acid

(H2SO4),

sodium

sulfate

5


sodium (Na2SO4),

hydroxide

(NaOH),

benzoquinone

and


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bubbled with O2 (> 99%) for 30 min to obtain the O2-saturated aqueous environment before each test.

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Oxygen deficiency during the measurements could be ignored because of the continuous oxygen

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aeration on the headspace of the electrolyte. The linear sweep voltammetry of rotating Fe@Fe2O3

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nanowires film disc electrode at different rotating rates was performed in the absence or presence of

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phosphate ions. Current values were recorded in a proper potential window with a scan rate of 50

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mV•s−1.

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Degradation Experiments. During a typical aerobic degradation process, 0.03 g of Fe@Fe2O3

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nanowires was added into 50 mL of solution containing 20 mg•L-1 4-chlorophenol and 0-10

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mmol•L-1 phosphate ions in a conical glass flask. Air was then pumped into the solution (1.5 L•min-1)

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to trigger the reaction. The temperature was kept at 25 °C during the process, and the initial pH was

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adjusted to 4 with using 1 mol•L-1 NaOH and HCl solutions. About 1 mL reaction solution was

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withdrawn at regular intervals and filtered through 0.22µm filter membranes immediately for the

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4-chlorophenol

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4-chlorophenol was performed by bubbling nitrogen gas. After the reaction, the suspension was

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transferred from the conical glass flask to a centrifuge tube and then centrifuged to separate the

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Fe@Fe2O3 nanowires from the suspension. The obtained Fe@Fe2O3 dried under anaerobic condition

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was used for further characterization.

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Analysis Method. The H2O2 concentration was determined by fluorimetric method.20 To quantify

127

the yield of •OH, benzoic acid was employed as a probe compound to capture •OH. The initial

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concentration of benzoic acid was 1200 mg•L-1, which was sufficient to capture all the generated

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•OH. Benzoic acid could react with •OH to produce three isomers of hydroxyl benzoic acid. The

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isomers accounted for 95% of the possible products, and the para, ortho, and meta isomers were

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occurred in the ratio of 1.2:1.7:2.3.21 Total hydroxybenzoic acid yield was calculated from this ratio

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by measuring para-hydroxybenzoic acid (p-HBA).22 The concentrations of benzoic acid, p-HBA,

concentration

measurement.

For

comparison,

6


anaerobic

degradation

of

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4-chlorophenol were determined by high pressure liquid chromatograph (HPLC, LC-20A, Shimadzu,

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Japan) with a TC-C18 reverse phase column (4.6 × 150 mm, 5 µm, Agilent, USA) and a ultraviolet

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detector (wavelength: 256 nm). The mobile phase was composed of dilute phosphoric acid and

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acetonitrile with the volume ratio of 60%:40%. Samples were analyzed at a flow rate of 1.0

137

mL•min-1 and the injection volume was set as 10 µL. The Fe(II) concentration was quantified with

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the 1,10-phenanthroline method as Fe(II) could complex with 1,10-penanthroline to form

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red-colored Fe(II)-orthophenanthroline complex, which could be measured at 510 nm with a UV-vis

140

spectrophotometer (UV-2550, Shimadzu, Japan). The concentration of phosphate ions was

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determined by an ion chromatographer with Ion Pac AS14A Anion-Exchange Column (Dionex

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ICS-900, Thermo, USA). The mobile phase was of 1.0 mmol•L-1 NaHCO3 and 8.0 mmol•L-1

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Na2CO3.

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Tafel Scan Experiments. Tafel scans were carried out to determine the free corrosion potentials of

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Fe@Fe2O3 nanowires. The Pt counter electrode, the work electrode, and the calomel reference

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electrode were placed in 50 mL of Na2SO4 aqueous solution (50 mmol•L-1) in a 100 mL beaker. To

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determine the initial corrosion potential of Fe@Fe2O3 nanowires, Tafel scans were carried out

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immediately after the work electrodes were immersed into the electrolyte solution. To clarify the

149

effect of phosphate ions on the Fe@Fe2O3 nanowires corrosion potentials, Tafel scan experiments

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were conducted in the electrolyte solution containing 10 mmol•L-1 phosphate ions for comparison.

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The other information about Tafel scan experiments could be found in the Supporting Information.

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Characterizations. X-ray diffraction (XRD) patterns were collected with a Bruker D8 Advance

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X-ray diffractometer (Cu Ka radiation, λ = 1.54178 Å). High-resolution X-ray photoelectron

154

spectroscopy (HR-XPS) was recorded by a Kratos ASIS-HS X-ray photoelectron spectroscope (15

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kV, 10 mA), and HR-XPS depth profiles were determined by using an argon sputtering gun with etch

156

time from 0 to 1200 s. The binding energies obtained were referenced to the C 1s line at 284.5 eV. 5, 7



5-dimethyl-1-pyrroline-N-oxide (DMPO) and α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN)

158

were employed as the •OH spin trapper and the •H spin trapper, respectively. Electron paramagnetic

159

resonance (EPR) spectra of DMPO−•OH and POBN−•H were obtained with a Bruker A300 EPR

160

spectrometer

161

Results and Discussion

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In order to figure out the influences of phosphate ions on the oxygen reduction pathway of

163

Fe@Fe2O3 nanowires, rotating disc electrode experiment was performed to estimate the electron

164

transfer number of ORR on the surface of Fe@Fe2O3 nanowires. The linear sweep voltammetries

165

(LSV) of rotating Fe@Fe2O3 nanowires film glassy carbon disc electrode in O2-saturated 0.1 mol•L-1

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Na2SO4 solutions (pH = 4) in the absence or presence of phosphate ions (Figure 1a-e) were

167

performed at different rotating rates of 400, 700, 1000, 1300 and 1600 rpm. The plateau currents

168

were measured at a potential of -1.2 V. We did not observe any distinct plateau currents from -0.8 to

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-1.4 V in N2-saturated solution, and thus concluded that the reduction of H2O did not occur on the

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Fe@Fe2O3 nanowires electrode surface in this applied potential range (SI Figure S2).

171

Koutecky-Levich equation was then employed to estimate the overall electron transfer number of

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ORR at plateau currents. According to Koutecky-Levich model, the current density (i) were obtained

173

from the plateau current, which consisted of a kinetic part (ikin) and a diffusion part (idiff), as shown in

174

Eq. (6).

175

176

=

+

=

∙√ω

(6)

in which B factor is given by

177

+

B = 0.62 ∙ n ∙ F ∙ D ∙ υ ∙ C (7)

178

Where n is the electron transfer number during the reaction, F is the Faraday constant (F = 96485

179

C•mol-1), D and C are the diffusivity and the concentration of oxygen (D = 1.15×10-5 8


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cm2•s-1, C = 1.26×10-5 mol•L-1), υ is the kinematic viscosity of 0.1 mol•L-1 Na2SO4 electrolyte

181

(υ = 0.1 cm2•s-1), ω is the angular velocity of the disc. The other data related to the calculation

182

process was provided in supporting information (SI Table S1-S6). The electron transfer number of

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ORR was calculated to be 3.69 for Fe@Fe2O3 nanowires in the absence of phosphate ions (Figure

184

1f). This data suggested that the four-electron pathway was the main reduction pathway of O2 on the

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surface of Fe@Fe2O3, accompanying with the direct conversion of O2 to H2O, and also accounting

186

for the low efficient ROS generation in the nZVI/air system. Interestingly, the electron transfer

187

number gradually decreased with increasing the phosphate ions concentration, and finally declined

188

to 1.13 when the phosphate ions concentration was 10.0 mmol•L-1, indicative of a one-electron

189

pathway of ORR. Therefore, phosphate ions could gradually shift the oxygen reduction pathway

190

from the four-electron pathway to the one-electron pathway on the surface of Fe@Fe2O3 nanowires

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with increasing the concentration of phosphate ions.

192

The effect of phosphate ions on the aerobic degradation of 4-chlorophenol with Fe@Fe2O3

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nanowires was then investigated at room temperature (Figure 2a). Within 4 h, about 10% of

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4-chlorophenol could be aerobically degraded with Fe@Fe2O3 nanowires in the absence of

195

phosphate ions. It is interesting to find that the aerobic degradation percentages of 4-chlorophenol in

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the Fe@Fe2O3/phosphate/air systems were significantly higher than that in the Fe@Fe2O3/air system.

197

All the aerobic 4-chlorophenol degradation curves were found to obey pseudo first-order reaction

198

kinetic equations, and the apparent aerobic 4-chlorophenol degradation rate constants (k) were 0.037,

199

0.047, 0.071, 0.149 and 0.365 min-1 when the initial concentrations of phosphate ions were 0, 2.5,

200

5.0, 7.5, 10.0 mmol•L-1, respectively (Figures 2b). Therefore, the 4-chlorophenol degradation rate

201

gradually increased with increasing the initial phosphate ions concentration. The addition of

202

phosphate ions (10.0 mmol•L-1) could improve the aerobic 4-chlorophenol degradation rate of

203

Fe@Fe2O3 nanowires by 10 times. Moreover, we found that phosphate ions of concentration as low 9



as 0.5 mmol•L-1 could also enhance the aerobic degradation of 4-chlorophenol with Fe@Fe2O3

205

nanowires (SI Figrue S3). Typical coexisted ions, such as NH4+, Mg2+, Ca2+, Cl-, and NO3-, did not

206

affect the ROS generation and the aerobic organic contaminants degradation of Fe@Fe2O3 in the

207

presence of phosphate ions (SI Figure S4 and Figure S5). Therefore, the promotion effect of

208

phosphate ions on the aerobic degradation of 4-chlorophenol with Fe@Fe2O3 nanowires might be

209

applicable to all the waters contaminated with phosphate ions of different concentrations, such as

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industrial wastewater, as well as municipal wastewater and agricultural drainage. A control

211

experiment under deaerated conditions ruled out the direct reductive degradation of 4-chlorophenol

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(SI Figure S6). Meanwhile, the phosphate ions removal efficiency of Fe@Fe2O3 nanowires was also

213

examined. It was found that phosphate ions uptake capacity of Fe@Fe2O3 nanowires could reach

214

206 mg•g-1 during the aerobic 4-chlorophenol degradation process (SI Figure S7), suggesting the

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dual roles of Fe@Fe2O3 nanowires.

216

Given that the ROS generation and the aerobic organic contaminants degradation of nZVI were

217

strongly dependent on the pH of solution,6 and the addition of phosphate ions might change the

218

solution pH values to influence the aerobic degradation of 4-chlorophenol with Fe@Fe2O3 nanowires,

219

we adjusted all the initial pH values of reaction solutions to 4 with using 1 mol•L-1 HCl and NaOH

220

solution before initiating the aerobic 4-chlorophenol degradation. We also measured the pH value

221

changes of reaction solutions along with the degradation time and found that the pH values slightly

222

increased from 4.0 to 4.1 (SI Figure S8). Obviously, these tiny pH fluctuations would not

223

significantly alter the rate of 4-chlorophenol degradation, excluding the possibility of contribution of

224

pH change caused by phosphate ions to the enhanced aerobic degradation of 4-chlorophenol.

225

To understand why phosphate ions could dramatically enhance the aerobic degradation of

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4-chlorophenol with Fe@Fe2O3 nanowires, we first investigated the effect of phosphate ions on the

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ROS generation by Fe@Fe2O3 nanowires via molecular oxygen activation. It was found that more 10


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H2O2 was accumulated in the Fe@Fe2O3/phosphate/air system than that in the Fe@Fe2O3/air system

229

after 60 min of reaction (Figure 2c, blue line; SI Figure S9). As the amount of accumulated H2O2 was

230

dependent on both its formation (Eq. 1 - Eq. 3) and decomposition (Eq. 4, Eq. 5) under aerobic

231

conditions, we compared the H2O2 ((H2O2)initial = 200 µmol•L-1) decomposition rates over Fe@Fe2O3

232

under Ar atmosphere in the Fe@Fe2O3/phosphate/Ar or Fe@Fe2O3/Ar systems (SI Figure S10), and

233

found that the decomposition rate of H2O2 in the Fe@Fe2O3/phosphate/Ar system was higher than

234

that in the Fe@Fe2O3/Ar system (Figure 2c, black line), suggesting that phosphate ions could

235

accelerate the decomposition of H2O2 on Fe@Fe2O3. Regarding faster H2O2 decomposition in the

236

Fe@Fe2O3/phosphate/Ar

237

Fe@Fe2O3/phosphate/air system, it could be concluded that phosphate ions could greatly promote

238

the H2O2 generation by Fe@Fe2O3 via molecular oxygen activation. Subsequently, we monitored the

239

production of •OH in the Fe@Fe2O3/phosphate/air systems (Figure 2d), and realized that the •OH

240

formation rates continued to increase along with increasing the initial phosphate ions concentration

241

up to 10 mmol•L-1. Therefore, the presence of phosphate ions could favor the •OH generation with

242

Fe@Fe2O3 nanowires, which was further confirmed by stronger DMPO-•OH EPR signal in the

243

Fe@Fe2O3/phosphate/air system than that in the Fe@Fe2O3/air system (SI Figure S11).

system

and

more

amount

of

accumulated

H2O2

in

the

244

As stated above, phosphate ions suppressed the four-electron pathway of ORR, but promoted the

245

one-electron pathway, it was reasonable for us to propose that phosphate ions could decrease the

246

electron transfer activity of Fe@Fe2O3 nanowires and facilitate the sequential single-electron

247

molecular oxygen activation (Eqs. 2-3). Previous study reported that phosphate ions could form

248

Fe-phosphate complexes layer on the nZVI surface to regulate the electron transfer rate from the

249

metallic particle core to the species at the particle-water interface.23 This electron transfer was

250

related to the oxidation of iron core, being reflected by its free corrosion potential. For example, a

251

more negative free corrosion potential value corresponds to a higher electron transfer rate.24, 25 11



Subsequently, we investigated the influence of phosphate ions on the electron transfer activity of

253

Fe@Fe2O3 nanowires by measuring their free corrosion potential with Tafel polarization diagrams.

254

Tafel diagrams showed that the free corrosion potential of Fe@Fe2O3 nanowires shifted from -0.73

255

to -0.66 V after the addition of phosphate ions (Figure 3a), suggesting that phosphate ions could

256

inhibit the electron transfer from the Fe0 core to the oxide surface and slow down the corrosion of

257

Fe@Fe2O3 nanowires. This result was further confirmed by electrochemical impedance spectroscopy

258

(EIS), as the Nyquist plot diameter of Fe@Fe2O3 in the presence of 10 mmol•L-1 phosphate was

259

much larger than that in the absence of phosphate ions (SI Figure S12).

260

XRD analysis, HR-XPS depth profiles, and SEM images were then used to probe the morphology

261

and phase changes of Fe@Fe2O3 nanowires in the Fe@Fe2O3/phosphate/air and Fe@Fe2O3/air

262

systems. The XRD patterns of the used and freshly synthesized Fe@Fe2O3 nanowires were

263

compared in Figure 3b. An obvious diffraction peak at 44.9° arisen from Fe0 was observed in the

264

freshly synthesized Fe@Fe2O3 nanowires. The Fe0 diffraction peak was still observed in the used

265

Fe@Fe2O3 nanowires in the Fe@Fe2O3/phosphate/air system, but became much weaker in the

266

Fe@Fe2O3/air system, accompanying with the emergence of new peaks at 25.49°, 30.21°, 35.61°,

267

43.31°, 53.74°, 57.32° and 62.69°. These new diffraction peaks could be ascribed to γ-Fe2O3 (JCPDS,

268

file No. 2-1047), suggesting much more serious corrosion of Fe@Fe2O3 nanowires in the absence of

269

phosphate ions. The HR-XPS depth profiles analysis was used to further evaluate the corrosion of

270

Fe@Fe2O3 nanowires in the Fe@Fe2O3/phosphate/air and Fe@Fe2O3/air systems. Fe0/Fetotal molar

271

ratio was determined by calculating relative integrated intensities of metallic and oxidized iron (SI

272

Figure S13), and the ratio changes were monitored as a function of etch time. As showed in Figure

273

3c, the Fe0/Fetotal molar ratios increased with increase in etching time, implying that Fe0 mainly

274

existed in the inner part of used Fe@Fe2O3 nanowires. Moreover, the Fe0/Fetotal ratio of used

275

Fe@Fe2O3 nanowires in the Fe@Fe2O3/phosphate/air system was significantly higher than that in 12


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the Fe@Fe2O3/air system. Correspondingly, SEM images revealed that wire-like structure was

277

broken up and plenty of nanosheets were formed in the used Fe@Fe2O3 of Fe@Fe2O3/air system

278

(Figure 3d-e). However, TEM images revealed that the used Fe@Fe2O3 nanowires of

279

Fe@Fe2O3/phosphate/air system still possessed some wire-like structures and the core-shell

280

structures were also preserved (SI Figure S14), except some irregular oxide layer formed on the

281

surface (Figure 3f). These results suggested that phosphate ions could slow the corrosion of

282

Fe@Fe2O3 nanowires due to their passivating effect, which could benefit the molecular oxygen

283

activation process.

284

In our previous studies, we demonstrated that molecular oxygen activation induced by Fe@Fe2O3

285

nanowires involved the two-electron molecular oxygen reduction pathway through the outward

286

electron transfer from the Fe0 core and the sequential single-electron molecular oxygen reduction by

287

the surface bound Fe(II),26 while the contribution of sequential single-electron molecular oxygen

288

activation to the overall ROS production was more than 60%, higher than that of two-electron

289

molecular oxygen activation pathway.26,

290

sequential single-electron molecular oxygen reduction by Fe(II), we first compared the

291

concentrations of dissolved Fe2+ and surface bond Fe(II) on Fe@Fe2O3 nanowires in the

292

Fe@Fe2O3/Ar and Fe@Fe2O3/Ar/phosphate systems (SI Figure S15).28 Considering the

293

concentrations of dissolved Fe2+ were negligible in the solutions in the absence of oxygen, we

294

measured the concentration of surface bond Fe(II) by directly adding 1, 10-phenanthroline into the

295

reaction

296

Fe@Fe2O3/Ar/phosphate system was close to that in the Fe@Fe2O3/Ar system. Therefore, the

297

addition of phosphate ions did not significantly enhance the amount of surface bond Fe(II) on

298

Fe@Fe2O3 nanowires.

299

systems,

and

found

that

27

To examine the influence of phosphate ions on the

the

surface

bond

Fe(II)

concentration

in

the

In order to further estimate the relative contribution of sequential single-electron molecular 13



Page 14 of 26

300

oxygen reduction induced by Fe(II) to ROS generation, a number of ROS trapping experiments were

301

subsequently conducted in the Fe@Fe2O3/phosphate/air system after the addition of different

302

scavengers (SOD for •O2-, catalase for H2O2 and iso-propanol for •OH). As shown in Figure 4a, the

303

aerobic 4-chlorophenol degradation rate constant (k) was 0.365 h−1 in the absence of scavengers,

304

while the addition of SOD, catalase and iso-propanol decreased the 4-chlorophenol degradation rate

305

constants (ks) to 0.355, 0.06 and 0.01 h-1, respectively. We thus estimated the inhibitory efficiency (η)

306

of different scavengers during the aerobic degradation of 4-chlorophenol in Fe@Fe2O3/phosphate/air

307

system through Eq. 8. η = ((k – ks)/k) × 100%

308

(8)

309

The inhibitory efficiencies of SOD, catalase and iso-propanol were found to be 2.8%, 83.5% and

310

97.3%, respectively. As iso-propanol completely inhibited the degradation of 4-chlorophenol, •OH

311

was thought to be responsible for the oxidation of 4-chlorophenol.29 The 4-chlorophenol degradation

312

was depressed by 83.5% after addition of catalase, indicating that H2O2 was the major intermediate

313

for •OH generation. While the 4-chlorophenol degradation was merely depressed by 2.8% after the

314

addition of SOD, suggesting that this sequential single-electron molecular oxygen reduction induced

315

by Fe(II) did not contribute to the ROS generation so much as we expected initially. Therefore,

316

molecular oxygen activation in Fe@Fe2O3/phosphate/air system might be attributed to another

317

single-electron transfer pathway, not the sequential single-electron molecular oxygen reduction

318

induced by Fe(II).

319

As aforementioned, phosphate ions could provide pendant proton relays to increase surface H+

320

concentration when they were adsorbed on metal oxide surface.18 Because the standard potential of

321

Fe0/Fe2+ (-0.44 V vs. NHE) is more negative than the reduction potential of •H/H+ (0 V vs. NHE),

322

the surface H+ is capable of trapping electron to generate •H (Eq. 9), which might happen on the

323

Fe@Fe2O3 nanowires surface in the presence of phosphate ions. Thus, we employed EPR 14


Page 15 of 26


324

spectroscopy to check the production of •H with using POBN as a •H spin trapper. As expected, a

325

strong EPR signal was observed in the Fe@Fe2O3/phosphate/Ar system after 5 min of reaction

326

(Figure 4b). The shape and the parameters of the observed signal were consistent with those of

327

POBN-•H reported previously.30 However, POBN-•H signal was not observed in the Fe@Fe2O3/Ar

328

system. More interestingly, the POBN-•H signal in the Fe@Fe2O3/phosphate/Ar system disappeared

329

immediately after the introduction of oxygen, as the in situ generated •H could be quickly scavenged

330

by O2 to produce •HO2 radical (Eq. 11).31 Subsequently, •HO2 was further reduced to H2O2 (Eq. 12),

331

or disproportionate into H2O2 and O2 (Eq. 13).32, 33 Therefore, the formation of •HO2 from •H and O2

332

could be regarded as a novel one-electron transfer process of oxygen reduction, well accounting for

333

the one-electron transfer of oxygen reduction and the subsequent production of H2O2 and •OH in the

334

Fe@Fe2O3/phosphate/air system. Benzoquinone (Q) is a widely used electron acceptor, which could

335

scavenge •HO2 to generate benzoquinone radicals (•Q-).34 As expected, the addition of benzoquinone

336

(10 mmol•L-1) completely inhibited the aerobic degradation of 4-chlorophenol in the

337

Fe@Fe2O3/phosphate/air system (SI Figure S16), confirming that the ROS generation was

338

contributed to the sequential single-electron transfer route induced by •H radical (O2 •HO2

339

H2O2).

340

Fe0 + 2H+ → 2•H + Fe2+

341

•H + •H → H2

k10 = 1.7 × 109 L•mol-1•s-1

(10)

342

•H + O2 → •HO2

k11 = 2 × 1010 L•mol-1•s-1

(11)

343

•H + •HO2 → H2O2

k12 = 9.7 × 107 L•mol-1•s-1

(12)

344

2•HO2 → H2O2 + O2

k13 = (8.3 ± 0.7) × 105 L•mol-1•s-1

(13)

(9)

345

As the surface H+ concentration would decrease at higher pH values, suppressing the

346

single-electron molecular oxygen activation via Eq. 11, and then, we investigated the effect of initial

347

pH on the aerobic decomposition of 4-chlorophenol in the Fe@Fe2O3/phosphate/air system. As 15



expected, the degradation rate of 4-chlorophenol significantly decreased at higher pH values (SI

349

Figure S17), further supporting the single-electron molecular oxygen reduction via the reaction

350

between •H and O2 in the Fe@Fe2O3/phosphate/air system.

351

According to the above discussion, a possible mechanism was proposed to account for the

352

enhanced ROS generation with Fe@Fe2O3 nanowires in the presence of phosphate ions (Scheme 1).

353

In the absence of phosphate ions, the electron transfer number in oxygen reduction reaction (ORR)

354

on Fe@Fe2O3 surface was 3.63, indicative of four-electron pathway in ORR and direct conversion

355

O2 to H2O, accounting for the low ROS generation of the Fe@Fe2O3/air system. Interestingly, the

356

phosphate ions lowered the electron transfer number of oxygen reduction reaction on Fe@Fe2O3

357

nanowires surface from 3.63 to 1.13 along with the concentration of phosphate ions increase from 0

358

mmol•L-1 to 10 mmol•L-1, shifting to the one-electron pathway of oxygen reduction reaction in the

359

Fe@Fe2O3/phosphate/air system, accompanying with the decreased electron transfer ability and the

360

suppressed corrosion of Fe@Fe2O3 nanowires. As phosphate ions could provide pendant proton

361

relays and increase the surface H+ concentration after they were adsorbed on metal oxide surface,18,

362

35

363

The in situ generated •H could react with O2 to produce •HO2 radical and H2O2. Thus, this phosphate

364

ions induced •H generation pathway could also facilitate the sequential single-electron pathway of

365

molecular oxygen reduction and the ROS generation, similar to the sequential single-electron

366

molecular oxygen reduction induced by Fe(II).

367

Environmental Implications. Phosphate ions widely exist in the natural waters. Although adsorbed

368

phosphate ions on nZVI would generate a passivating oxide layer to decrease the reactivity of nZVI

369

for direct reduction of organic pollutants, we demonstrated that phosphate ions could gradually shift

370

the oxygen reduction pathway on the Fe@Fe2O3 nanowires surface from a four-electron pathway (O2

371

H2O) to a sequential single-electron pathway (O2 •HO2 H2O2) along with the concentration

the high concentration of surface H+ would favor the •H generation with Fe@Fe2O3 nanowires.

16


Page 16 of 26

Page 17 of 26


372

of phosphate ions increase from 0 mmol•L-1 to 10 mmol•L-1, which could significantly enhance

373

molecular oxygen activation and ROS generation. These findings could help us to predict the

374

efficiency and reactivity of nZVI for the aerobic degradation of organic pollutants in the presence of

375

phosphate ions. Meanwhile, environmental scientists previously attributed the low yield of ROS

376

with nZVI to iron precipitation, high surface reactivity of nZVI toward H2O2, and slow rates of

377

Fenton reaction,36 but usually neglected the importance of oxygen reduction pathway on molecular

378

oxygen activation and ROS generation, which is disadvantage for the wide application of nZVI. This

379

study suggests that tuning the oxygen reduction pathway of nZVI is a promising strategy to improve

380

the yield of ROS for aerobic organic pollutant removal.

381 382

AUTHOR INFORMATION

383

Corresponding Author

384

*Phone/Fax: +86-27-6786 7535; e-mail: [email protected].

385

Notes

386

The authors declare no competing financial interest.

387 388

Acknowledgements

389

This work was supported by Natural Science Funds for Distinguished Young Scholars (Grant

390

21425728), National Key Research and Development Program of China (Grant 2016YFA0203002),

391

the 111 Project (Grant B17019), Self-Determined Research Funds of CCNU from the Colleges’

392

Basic Research and Operation of MOE (Grant CCNU14Z01001 and CCNU16A02029), Excellent

393

Doctorial Dissertation Cultivation Grant from Central China Normal University (2015YBZD024 and

394

2016YBZZ031), and the CAS Interdisciplinary Innovation Team of the Chinese Academy of

395

Sciences. 17



ASSOCIATED CONTENT

397

Supporting Information

398

Synthesis and characterization of Fe@Fe2O3 nanowires; Tafel scan experiments and Nyquist plots of

399

Fe@Fe2O3 nanowires; effects of low concentration phosphate ions or coexisting ions on aerobic

400

degradation of 4-CP; phosphate ions removal with Fe@Fe2O3 nanowires; TEM images and

401

high-resolution XPS depth profiles of freshly prepared and used Fe@Fe2O3 nanowires; LSV of

402

rotating Fe@Fe2O3 film disc electrode in N2-saturated solutions; temporal pH values of the solution

403

as a function of reaction time; reactive oxygen species measurements.

404 405

References

406

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407

using supported, nanoscale zero-valent iron. Environ. Sci. Technol. 2000, 34 (12), 2564-2569.

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(2) Sayles, G. D.; You, G.; Wang, M.; Kupferle, M. J. DDT, DDD, and DDE dechlorination by

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zero-valent iron. Environ. Sci. Technol. 1997, 31 (12), 3448-3454. (3) Agrawal, A.; Tratnyek, P. G. Reduction of nitro aromatic compounds by zero-valent iron metal. Environ. Sci. Technol. 1996, 30 (1), 153-160.

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(4) Mu, Y.; Ai, Z. H.; Zhang, L. Z.; Song, F. F. Insight into core-shell dependent anoxic Cr(VI)

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removal with Fe@Fe2O3 nanowires: indispensable role of surface bound Fe(II). ACS Appl.

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Mater. Interfaces. 2015, 7 (3), 1997-2005.

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(5) Mackenzie, P. D.; Horney, D. P.; Sivavec, T. M. Mineral precipitation and porosity losses in granular iron columns. J. Hazard. Mater. 1999, 68 (1-2), 1-17.

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nanoparticulate zero-valent iron and oxygen. Environ. Sci. Technol. 2008, 42 (4), 1262-1267.

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Cheng, I. F. Destruction of chlorinated phenols by dioxygen activation under aqueous room

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temperature and pressure conditions. Ind. Eng. Chem. Res. 2003, 42 (21), 5024-5030.

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(8) Joo, S. H.; Feitz, A. J.; Waite, T. D. Oxidative degradation of the carbothioate herbicide,

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molinate, using nanoscale zero-valent iron. Environ. Sci. Technol. 2004, 38 (7), 2242-2247.

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zero-valent iron and oxygen. Environ. Sci. Technol. 2008, 42 (18), 6936-6941. (10) 10. Joo, S. H.; Feitz, A. J.; Sedlak, D. L.; Waite, T. D. Quantification of the oxidizing capacity of nanoparticulate zero-valent iron. Environ. Sci. Technol. 2005, 39 (5), 1263-1268. (11) Noradoun, C. E.; Cheng, I. F. EDTA degradation induced by oxygen activation in a zero-valent iron/air/water system. Environ. Sci. Technol. 2005, 39 (18), 7158-7163.

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(12) Englehardt, J. D.; Meeroff, D. E.; Echegoyen, L.; Deng, Y.; Raymo, F. M.; Shibata, T. Oxidation

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of aqueous EDTA and associated organics and coprecipitation of inorganics by ambient

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iron-mediated aeration. Environ. Sci. Technol. 2007, 41 (1), 270-276.

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(13) Liu, Y.; Phenrat, T.; Lowry, G. V. Effect of TCE concentration and dissolved groundwater

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solutes on nZVI-promoted TCE dechlorination and H2 evolution. Environ. Sci. Technol. 2007,

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41 (22), 7881-7887.

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(19) Lu, L. R.; Ai, Z. H.; Li, J. P.; Zheng, Z.; Li, Q.; Zhang, L. Z. Synthesis and characterization of

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Fe-Fe2O3 core-shell nanowires and nanonecklaces. Cryst. Growth Des. 2007, 7 (2), 459-464.

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benzoic acid. Isomer distribution in the radical intermediates. J. Phys. Chem. 1975, 79 (17),

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Linehan, J. C.; Matson, D. W.; Penn, R. L.; Driessen, M. D. Characterization and properties of 19



metallic iron nanoparticles: spectroscopy, electrochemistry, and kinetics. Environ. Sci. Technol.

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2005, 39 (5), 1221-1230.

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acid solution by triazole derivatives: Part I. Polarization and EIS studies. Electrochim. Acta 2007,

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(26) Ai, Z. H.; Gao, Z. T.; Zhang, L. Z.; He, W. W.; Yin, J. J. Core-shell structure dependent

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reactivity of Fe@Fe2O3 nanowires on aerobic degradation of 4-Chlorophenol. Environ. Sci.

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Technol. 2013, 47 (10), 5344-5352.

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degradation with Fe@Fe2O3 core-shell nanowires. Appl. Catal., B Environ. 2014, 150-151, 1-11.

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(28) Harvey, A. E.; Smart, J. A.; Amis, E. S. Simultaneous spectrophotometric determination of

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iron(II) and total iron with 1,10-Phenanthroline. Anal. Chem. 1955, 27 (1), 26-29.

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(29) Katsoyiannis, I. A.; Ruettimann, T.; Hug, S. J. pH dependence of fenton reagent generation and

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As(III) oxidation and removal by corrosion of zero valent iron in aerated water. Environ. Sci.

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Technol. 2008, 42 (19), 7424-7430.

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(30) Mišík, V.; Riesz, P. Effect of Cd2+ on the •H atom yield in the sonolysis of water. evidence against the formation of hydrated electrons. J. Phys. Chem. A 1997, 101 (8), 1441-1444. (31) Rao, P. S.; Hayon, E. Redox potentials of free radicals. IV. Superoxide and hydroperoxy radicals •O2- and •HO2. J. Phys. Chem. 1975, 79 (4), 397-402.

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hydroperoxides and peroxyl and alkoxyl radicals from isoflurane in aqueous solution. J. Phys.

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(33) Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B. Reactivity of •HO2/•O2- radicals in aqueous solution. J. Phys. Chem. Ref. Data 1985, 14 (4), 1041-1100.

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(34) Jiang, C.; Garg, S.; Waite, T. D. Hydroquinone-mediated redox cycling of iron and concomitant

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oxidation of hydroquinone in oxic waters under acidic conditions: comparison with iron-natural

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organic matter interactions. Environ. Sci. Technol. 2015, 49 (24), 14076-14084.

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(35) Fecenko, C. J.; Meyer, T. J.; Thorp, H. H. Electrocatalytic oxidation of tyrosine by parallel

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(36) Lee, C. Oxidation of organic contaminants in water by iron-induced oxygen activation: A short review. Environ. Eng. Res. 2015, 20 (3), 205-211.

491 492

21



Figure Captions

494 495

Figure 1. The linear scanning voltammetry (LSV) of rotating Fe@Fe2O3 film disc electrode in

496

O2-saturated 0.1 mol•L-1 Na2SO4 solutions (pH = 4) in the absence (a) and presence of phosphate

497

ions (b. 2.5; c. 5.0; d. 7.5; e. 10 mmol•L-1), at rotating rates of 400, 700, 1000, 1300 and 1600 rpm. (f)

498

Koutecky-Levich plots of Fe@Fe2O3 nanowires film immersed in different concentrations of

499

phosphate at plateau currents. The dashed lines show the standard slopes for n = 1 and n = 4.

500

22


Page 22 of 26

Page 23 of 26


501 502

Figure 2. (a) Time profiles of the aerobic 4-chlorophenol degradation in Fe@Fe2O3/air system with

503

different concentration of phosphate ions. (b) Plots of ln(C/C0) versus time for the aerobic

504

4-chlorophenol degradation. The concentration of 4-chlorophenol, Fe@Fe2O3 were 20 mg•L-1 and

505

0.6 g•L-1, respectively. The initial pH values were 4. (c) Blue line: Influences of phosphate ions on

506

the accumulation of H2O2 during aerobic 4-chlorophenol degradation. Black line: The temporal

507

concentration of H2O2 as function of time in the Fe@Fe2O3/phosphate/Ar and Fe@Fe2O3/Ar systems,

508

the initial concentration of H2O2 was 200 µmol•L-1. (d) Detection of hydroxyl radical (•OH)

509

generated in the Fe@Fe2O3/phosphate/air system. The concentration of phosphate ions ranged from

510

0 to 10 mmol•L-1. The concentration of benzoic acid, Fe@Fe2O3 were 1200 mg•L-1 and 0.6 g•L-1,

511

respectively. The initial pH values were 4.

512

23



Page 24 of 26

513 514

Figure 3. (a) Tafel scans of Fe@Fe2O3 nanowires made working electrode in the presence (red line)

515

or absence (red line) of phosphate ions under open circuit conditions. (b) The XRD patterns of fresh

516

prepared

517

Fe@Fe2O3/phosphate/air system (blue line) and Fe@Fe2O3/air system (pink line). (c) Ratio

518

Fe0/Fetotal as a function of etch time (0~1200 s) in the high-resolution XPS depth profiles analysis.

519

SEM of the Fe@Fe2O3 nanowires before and after the aerobic 4-chlorophenol degradation: (d) the

520

as-prepared Fe@Fe2O3 nanowires; (e) after reaction in Fe@Fe2O3/air system; (f) after reaction in

521

Fe@Fe2O3/phosphate/air system.

Fe@Fe2O3

nanowires

(black

line)

and

the

used

522 523 524 525 526

24


Fe@Fe2O3

obtained

from

Page 25 of 26


527 528

Figure 4. (a) The aerobic 4-chlorophenol degradation rate constant in the Fe@Fe2O3/phosphate/air

529

system with adding different scavengers (SOD for •O2-, CAT for H2O2, iso-propanol for •OH). (b)

530

EPR spectra of POBN spin-trapping •H radical obtained from Fe@Fe2O3/phosphate/Ar (red line)

531

and Fe@Fe2O3/Ar (black line) systems. The concentration of POBN was 25 mmol•L-1. The spectra

532

were collected at 5 min after mixing under anaerobic condition.

533 534

Scheme 1. Schematic illustration for enhanced H2O2 generation with Fe@Fe2O3 core-shell

535

nanowires in the presence of phosphate. 25


Environmental Science & Technology 536 537

TOC Art Figure

538

26


Page 26 of 26

Phosphate Shifted Oxygen Reduction Pathway on Fe@Fe2O3 Core

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