Reduced Graphene Oxide Aerogel as

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Remediation and Control Technologies

Self-Assembled Nano-FeO(OH)/Reduced Graphene Oxide Aerogel as a Reusable Catalyst for Photo-Fenton Degradation of Phenolic Organics Renlan Liu, Yiming Xu, and Baoliang Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01043 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Environmental Science & Technology

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Self-Assembled Nano-FeO(OH)/Reduced Graphene Oxide Aerogel as a

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Reusable Catalyst for Photo-Fenton Degradation of Phenolic Organics

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Renlan Liu1,2, Yiming Xu3,4, and Baoliang Chen1,2,*

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(1.Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310058, China; 2.

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Zhejiang Provincial Key Laboratory of Organic Pollutant Process and Control, Zhejiang University,

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Hangzhou Zhejiang 310058, China. 3. Department of Chemistry, Zhejiang University, Hangzhou,

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Zhejiang 310027, China. 4. State Key Laboratory of Silica Materials, Zhejiang University, Hangzhou

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310027, China.)

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* Corresponding author: phone & Fax: 0086-571-88982587

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Email: [email protected]

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ACS Paragon Plus Environment


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Abstract

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Fabrication of visible-light-responsive, macroscopic photo-Fenton catalysts is crucial for

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wastewater treatment. Here, we report a facile fabrication method for nano-FeO(OH)/reduced

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graphene oxide aerogels (FeO(OH)-rGA) equipped with a stable macrostructure and a high

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efficiency for catalytic degradation of phenolic organics. The structure of FeO(OH)/rGA was

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characterized by SEM, TEM, XPS, Raman analysis. The FeO(OH) is the main constituent of

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ferrihydrite, which dispersed in the graphene aerogel with a particle size of ~3 nm can

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efficiently activate H2O2 to generate abundant •OH. The excellent performance of the

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FeO(OH)/rGO aerogel was specifically exhibited by the outstanding catalyst activity,

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sustained mineralization and eminent reaction rate for phenolic organics. A synergy effect

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between FeO(OH) and graphene aerogel was observed, which came from the extensive

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electron transfer channels and active sites of the 3D graphene aerogel and the

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visible-light-activated FeO(OH) and H2O2 consistently producing •OH. The FeO(OH)/rGA

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could be reused for 10 cycles without a reduction in the catalytic activity and had less iron

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leaching, which guarantees that the active ingredient remains in the gel. Moreover, the

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FeO(OH)/rGA induced photo-Fenton degradation of 4-chlorophenol under near neutral pH

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conditions because the tight connection of FeO(OH) with the rGO aerogel results in less iron

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leaching and prevents the generation of Fe(OH)3. The 4-chlorophenol was completely

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removed in 80 min with a 0.074 min-1 rate constant in the FeO(OH)-rGA/H2O2 photo-Fenton

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system under visible-light irradiation, and mineralization rate was up to 80% after six hours.

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Oxidative •OH can continuously attack 4-chlorophenol, 2,4,6-trichlorophenol and bisphenol A

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without selectivity. These results lay a foundation for highly effective and durable

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photo-Fenton degradation of phenolic organics at near neutral pH and sufficient activation of

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H2O2 for future applications.

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TABLE OF CONTENTS

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Fe-rGA 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 3



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1. Introduction

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The traditional Fenton reaction is an advanced oxidation technology used for wastewater

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treatment, and the reaction involves a mixture of hydrogen peroxide (H2O2) and ferrous iron

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(Fe2+) in an acidic solution to generate hydroxyl radicals (·OH) for a subsequent attack of

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tartaric acid.1 Despite its use, the homogeneous Fenton reaction has several disadvantages,

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such as an acidic pH (pH 420 nm) in an XPA-7 type photochemical reactor (Xujiang

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Electromechanical Plant, Nanjing, China). The temperature was maintained at 30±2ºC by

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circulating cooling water. For the recycling, the organic compounds were added with H2O2

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after each cycle.

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2.5. Analytical Methods

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The residual concentrations of 4-CP and 2,4,6-TCP were detected by high-performance liquid

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chromatography (HPLC, Agilent 1200 Series) equipped with an ultraviolet detector with an

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SB-C18 (4.6×150 mm) column. The optimum mobile phase for 4-CP was a mixture of

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methanol, water and phosphoric acid (70/20/10 by volume) at a flow rate of 1.0 mL min-1 with

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an analytical wavelength of 279 nm. For 2,4,6-TCP detection, a mobile phase containing

189

methanol and 0.02 mol/L phosphoric acid (80:20 v/v) was used at a wavelength of 285 nm.

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BPA was detected by an HPLC equipped with a fluorescence detector with an SB-C18

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(4.6×150 mm) column. The mobile phase was a mixture of acetonitrile and 0.01 M

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trifluoroacetic acid (70:30, v/v), and a wavelength of 280 nm was used. The total organic

193

carbon (TOC) was measured using a TOC-V CPH TOC analyzer (SHIMADZU). The total

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dissolved iron in the reactive solution was detected using atomic absorption spectrometry

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(AAS, Perkin Elmer AAnalyst 700). For the different H2O2 concentrations, the potassium

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titanium (IV) oxalate spectrophotometric method was used at a wavelength of 385 nm.33 A

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pseudo first-order model was applied to describe the kinetics of phenolic organics

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photo-degration by Fe-rGA.

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The first-order constant k (min-1) was determined according 8


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to the following equation: ln(C0/Ct)=kt, where Ct is the concentration of phenolic organics at

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time t, C0 is the initial concentration of phenolic organics, k is the degradation rate constant,

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and t is the irradiation time. Hydroxyl radicals (•OH) were measured by adding methanol as

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scavenger and obtaining electron paramagnetic resonance (EPR) spectra with DMPO forming

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DMPO-•OH on a Bruker A300 EPR (modulation amplitude 1.00G; microwave power 20 mW;

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modulation frequency 9.866 GH). The intermediate products were determined by liquid

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chromatography-mass spectrometry (LC/MS) equipped with a linear ion trap interface with an

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HPLC and quaternary pump. More details regarding the analytical methods are available in

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the supporting information (SI).

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3. Results and discussion

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3.1. Characterization of the FeO(OH)-rGA aerogels and rGA

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A schematic illustration of the fabrication of FeO(OH)-rGA is presented in Figure S1. Initially,

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Fe3+ from FeCl3 can react via electrostatic interactions, cation-π interactions or metal

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coordination with the oxygen-containing functional groups on the GO nanosheets. Second,

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with the addition of VcNA for the chemical reduction at 95°C, the GO will transform into a

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reduced graphene oxide hydrogel (rGH), resulting in uniform deposition of the FeO(OH)

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nanometer structure on rGH layers. Finally, the black FeO(OH)-rGA was obtained after the

217

freeze-drying process. The simple rGA was prepared by the same procedure without the

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addition of Fe3+. The measured density of FeO(OH)-rGA is 14.5 mg cm-3 and can be seen in

219

Figure S2a. Meanwhile, FeO(OH)-rGA can bear weight 1,800 times more than its own weight

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without any mechanical deformation, as shown in Figure S2b. The excellent physical and

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mechanical performances are beneficial for its practical application. Nevertheless, excessive

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iron loading has an adverse impact on the aerogel structure stability. 25FeO(OH)-rGA

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presented an incomplete aerogel structure (Figure S2c). 9



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Figure 1. SEM image (a), TEM image (inset SAED image) (b), HRTEM image (c) and elemental mapping

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(d) of FeO(OH)-rGA.

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SEM and TEM techniques were used to observe the lamella and pore morphology.

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Figure 1a displays an abundant internal pore network. FeO(OH)-rGA possesses numerous,

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more uniform and smaller pore networks than rGA (Figure S3a), which can provide an

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extensive transportation network for electron transfer and effective reactions with H2O2.

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Figure 1b shows the abundant tiny particles (~3 nm) on the reduced graphene oxide (rGO)

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layers in FeO(OH)-rGA compared with that of pure rGA (Figure S3b), and the inset SAED

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shows the crystal diffraction with a crystal ring of FeO(OH)-rGA, implying that lattice

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striation of the particles and suggesting the generation of smaller and uniform crystals on the

236

rGO layers. By calculation, the d-spacing values were equal to 0.223 nm and 0.127 nm, which

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were assigned to the (112) and (220) lattice planes of ferrihydrite (FeO(OH)) (JCPDS No.

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46-1315). Ferrihydrite can enhance the decomposition of H2O2 and the oxidation of atrazine 10


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via the Fenton reaction in solution.36 Simultaneously, the elemental mapping showed C, O,

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and Fe elements in FeO(OH)-rGA (Figure 1d).

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To further confirm the oxygen functional groups and iron composition in the

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FeO(OH)-rGA aerogel, XPS was employed to analyze the FeO(OH)-rGA and rGA aerogels.

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We analyzed the C 1s spectra of the FeO(OH)-rGA and rGA aerogels and found that the

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functional groups, such as C-C/C=C (284.3 eV), C-O (285.5 eV), C=O (287.8 eV), and

245

O-C=C (289.7 eV), were not significantly different, as shown in Figure 2a and Figure S3(c,

246

d).37 The Fe 2p XPS spectra are shown in Figure 2b. The peaks at 711.8 eV and 725.6 eV are

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assigned to Fe(III)2p3/2 and Fe(III)2p1/2, respectively,38 which demonstrated that the iron ions

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were successfully loaded on the rGO layers and that a possible composition was FeO(OH).

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These XPS results demonstrated that the addition of iron did not influence the oxygen

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functional group contents in the rGA aerogel but increased the oxygen percentage via

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FeO(OH) generation (Figure S3 (e, f)). Meanwhile, the EDX analysis showed that the Fe

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mass percentage was 1.63%.

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Raman spectroscopy is an effective tool to observe ordered and disordered defects in

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graphene. In the Raman spectra (Figure 2c), two prominent peaks were observed at 1346 cm-1

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(D band) and 1582 cm-1 (G band), which correspond to sp3 carbon vibrational stretching and

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sp2 carbon vibrational stretching, respectively. The intensity ratios of the D to G bands (ID/IG)

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for the 15FeO(OH)-rGA and rGA aerogels are 1.09 and 1.24, respectively. The decrease of

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ID/IG ratio indicates that the FeO(OH) generation can reduce the disorder and defects of

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graphene during the synthesis process. It is possible that the iron ions easily combine with the

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–OH and –COOH groups of GO, and thus, small FeO(OH) crystals, which can prevent defect

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(e.g., holes) generation, form in the presence of VcNa at 95°C and are derived from the direct

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removal of oxygen functional groups.39 The Raman spectra are consistent with the SEM

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morphology, which also revealed that the pore network of FeO(OH)-rGA was more uniform,

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smaller and more plentiful than that of rGA. 11



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Figure 2. XPS spectra of C 1s (a) and Fe 2p (b) for FeO(OH)-rGA. Raman spectra of FeO(OH)-rGA and

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rGA (c). The optical contact angle of FeO(OH)-rGA (d).

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The contact angle values of FeO(OH)-rGA (Figure 2d) and rGA (Figure S4) were

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approximately 45° and 42°, which indicated hydrophilic surfaces. These surfaces are

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favorable for adsorbing dissolved phenolic compounds. We chose VcNa as the reducing

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reagent for a moderate reduction process compared to that with other reducing reagents (such

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as hydrazine hydrate) to retain a portion of the oxygen functional groups in the aerogel, as can

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be observed in the C 1s spectra of FeO(OH)-rGA and rGA from the XPS analysis. The

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moderate chemical reduction process of the synthesized aerogel restores the sp2 correlation of

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the π structure of the rGO layers and forms a loose porous structure, which provides more

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reactive reaction sites and electron transfer channels for photo-Fenton catalysis. 12


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The thermograms (TG) of 15FeO(OH)-rGA and rGA in air are shown in Figure S5. From

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room temperature to 500ºC, rGA and 15FeO(OH)-rGA exhibit a two-stage decomposition

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pattern. The first decomposition below 120 ºC corresponds to the loss of adsorbed water. The

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second decomposition in the range of 120-500ºC originates from the removal of labile

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oxygen-containing functional groups as CO2, CO, and H2O vapor. 15FeO(OH)-rGA and rGA

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have different weight losses because crystalline FeO(OH) gradually transforms into highly

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crystalline Fe2O3, which occupies 4.175%.

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3.2. Photo-Fenton catalytic activity of FeO(OH)-rGA aerogel

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Control experiments were conducted to compare the removal efficiency of 4-CP by the

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15FeO(OH)-rGA/H2O2 system at pH 3.0 with an initial 4-CP concentration of 10 mg/L and 20

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mM H2O2. As shown in Figure 3, 15FeO(OH)-rGA can dramatically degrade 4-CP within 80

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min under visible-light irradiation in the 15FeO(OH)-rGA/H2O2/4-CP system, whereas

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15FeO(OH)-rGA could not remove 4-CP, which demonstrated that 15FeO(OH)-rGA did not

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have catalytic activity without H2O2. In addition, H2O2 could not reduce 4-CP because the

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H2O2 direct photolysis rate constant is only 1.4×10-11 M-1 s-1.40 Most importantly, the

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visible-light adsorption of FeO(OH)-rGO after 2 h of heating presented a dramatic increase

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(Figure S6). This was mainly due to electron transfer between the iron center and rGO layers,

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which mobilized the FeO(OH)-producing Fe(II) in rGO. Consequently, 4-CP could not

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degrade without visible-light irradiation because the reaction rate constant of ferric iron with

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H2O2 is 2×10-3 M-1 s-1,41 which falls far below that of the ferrous ion reaction with H2O2 (76

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M-1s-1).42 Consequently, H2O2 and visible light were all essential conditions for 4-CP

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degradation. Furthermore, the reaction rate in the initial 30 min was comparatively slow

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because the concentration of the quinone intermediate the organics generated was limited, as

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shown in the latter HPLC analysis. According to previous reports, the coupling of visible

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irradiation and hydroquinone resulted in strong and universal driving forces in the Fenton

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reaction for organic pollutant degradation and mineralization.43 As the reaction proceeded, the 13



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4-CP removal remarkably accelerated, and the pseudo-first-order kinetic constant (k) was

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0.0695 min-1. Compared with other Fenton-like reactions previously reported (listed in Table

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S1),44-50 15FeO(OH)-rGA has a high degradation activity and efficiently utilizes H2O2 with

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less iron loading under visible-light irradiation.

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Figure 3. Time evolution of 4-CP removal under different conditions (reaction conditions: C0(4-CP): 10

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mg/L, C0(H2O2): 20 mM, pH=3, 500 W xenon lamp with a UV cutoff filter (λ > 420 nm)).

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3.3. Influence of solution conditions on degradation of phenolic organics

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The presence of iron in aerogels plays a vital role in their reaction activity. The

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performance of rGA changed significantly for 4-CP in the presence of iron χFeO(OH)-rGA

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(χ=0, 4, 8, 15) (Figure 4a) (C0(H2O2): 10 mM, pH=3). 15FeO(OH)-rGA completely removed

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4-CP within 110 min and was more efficient than the other iron-doped rGA aerogels. The

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removal of 4-CP increased as the iron doping increased because of the importance of iron in

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most of the catalytic components in the photo-Fenton reaction. Consequently, the addition of

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iron resulted in the activation of H2O2 to generate •OH radicals. Nevertheless, excessive iron

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loading will have an adverse impact on the aerogel structure stability. The probable reason is

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that Fe3+ could generate stable coordination compounds with GO.51 In the synthesis process,

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the excessive iron ions turn into FeO(OH) among the rGO layers, which can greatly hinder 14


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π-π stacking between the rGO layers in the aerogel, resulting in aerogel structural collapse, as

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shown in Figure S2c. Therefore, we chose 15FeO(OH)-rGA as the optimal aerogel ratio for

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further study.

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Figure 4. Influence of the initial iron content in FeO(OH)-rGA (a), influence of the initial solution pH (b),

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influence of the H2O2 concentration(c) on 4-CP degradation. Effectiveness for the degradation of various

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phenolic organics (d).

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The initial pH affects not only the photocatalytic performance but also the extent of iron

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leaching and the stability of the catalyst. In consideration of the significant influence of the

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pH on the photo-Fenton reaction, we studied the effect of the initial pH on the degradation of

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4-CP, as shown in Figure 4b (Catalyst: 15FeO(OH)-rGA, C0(H2O2): 10 mM). Although the

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degradation rate of 4-CP slightly declined as the pH increased from 3 to 6, approximately 15



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100% of 4-CP was removed within 150 min at pH=3, 4, 5, and 6, and this removal rate is

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evidently higher than that of other reported heterogeneous Fenton-like catalysts.52 One reason

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for the good degradation of 4-CP at near neutral pH is that the pH of the solution changed

338

after the reaction because of the generation of low molecular weight organic acids, which

339

were detected in this study, and mineralization to form CO2, which can also transform into

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carbonic acid, resulting in a decrease in the pH. Another reason is that FeO(OH) is

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encapsulated by graphene layers via strong π-π interactions, which could restrain ferric iron

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from leaching and transforming into less active Fe(OH)3. As shown in Figure S7, the color of

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the Fe3+ solution (16.4 mg/L, pH=3) changed from colorless to yellow after heating for 2 h

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without sediment formation, but after the 1st photo-Fenton reaction, an obvious red-brown

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precipitation was generated after 1 h, which directly proved the homogenous Fenton-like

346

reaction is not suitable for practical applications. The special internal network structure of the

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graphene aerogels alters the state of iron in the catalyst, and the catalyst can exist at near

348

neutral pH values and displays an excellent degradation performance that is completely

349

different from that obtained with a homogeneous Fenton system. In addition, FeO(OH)-rGA

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has outstanding photo-Fenton performance at near neutral pH, which makes FeO(OH)-rGA a

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good candidate for the preparation of stable catalysts for industrial wastewater treatment

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applications without drastically adjusting the pH. Moreover, the macroscopic aerogel is easier

353

to recycle, prevents sediment production, resists corrosion and possesses longer-term

354

prospects.

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In photo-Fenton catalytic reactions, the oxidant concentration is a crucial factor that can

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significantly influence organic matter degradation. Figure 4c shows the 4-CP degradation was

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accelerated as the H2O2 dosage increased from 0 to 20 mM (Catalyst: 15FeO(OH)-rGA,

358

pH=3), and the degradation rate constant (k) increased from 0.040 min under 5mM H2O2, to

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0.069 min-1 under 10mM H2O2, and to 0.074 min-1 15mM H2O2. This is because H2O2 is the 16


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precursor in the reaction to produce •OH. However, with a higher dosage (50 mM), the k

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value obviously decreased to 0.06 min-1, which was probably caused by the excess H2O2

362

scavenging •OH, which suppressed the 4-CP degradation. Consequently, the dosage of H2O2

363

should exists in an optimum concentration range as much as possible. The demand for H2O2 is

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always high in the traditional Fenton reaction; hence, the consumption of H2O2 is commonly

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high in the reaction and a supply of fresh H2O2 is constantly required. As seen in Figure S8,

366

we detected the variation in the H2O2 concentration in the photo-Fenton reaction and found

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that the concentration of H2O2 slightly decreased as the 4-CP degraded, which was mainly

368

ascribed to the nanometer, well-dispersed FeO(OH) with a small size (~3 nm) in rGA

369

effectively activating H2O2. Comparatively, the mass percentage of transition metal iron in

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15FeO(OH)-rGA merely was approximately 5%, which was far below that of synthetic iron

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oxide minerals. This further confirmed that the 3D macroscopic structures can disperse

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FeO(OH) on the graphene layers to provide sufficient catalytic sites and electron transfer

373

channels for the photo-Fenton reaction.

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The photo-Fenton catalytic efficiency was different based on the type of organic

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compound. We selected three phenolic organics, 4-CP, 2,4,6-TCP and BPA. These organics

376

are widely used as fungicides, herbicides, and plastic materials; considered toxic organic

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pollutants that are harmful to human health and aquatic organisms; and regarded as priority

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organic pollutants by the US Environmental Protection Agency (USEPA). In this study, when

379

the amount of added catalyst and H2O2 were determined, the content of •OH generated was

380

simultaneously confirmed. Consequently, the difference in the degradation kinetics among

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4-CP, 2,4,6-TCP and BPA, which is shown in Figure 4d, was ascribed to the different

382

molecular structures. The structural formulas of 4-CP, 2,4,6-TCP and BPA are shown in

383

Figure S9. 2,4,6-TCP has three chlorine substituent connections on one benzene ring, and

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BPA has two benzene rings connected to each other in contrast with 4-CP. The photo-Fenton 17



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degradation kinetics of 4-CP, 2,4,6-TCP and BPA are presented in Figure 4d (Catalyst:

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15FeO(OH)-rGA, C0(H2O2): 20 mM, pH=3). 4-CP was completely removed within 80 min,

387

and the degradation rate constant was 0.074 min-1, which was higher than that of 2,4,6-TCP

388

and BPA. 2,4,6-TCP and BPA were completely degraded after 150 min, and the degradation

389

rate constants were 0.029 min-1 and 0.031 min-1, respectively. Based on the above results, we

390

concluded that as the number of chlorine substituents on one benzene ring and the number of

391

benzene rings increase the rate of the photo-Fenton reaction decreases. Moreover, the

392

influence of the number of benzene rings was even more significant because the structure of

393

the benzene ring is very stable and not easily oxidized into an open loop. Nonetheless, •OH

394

has a strong oxidant activity (oxidation potential=2.8 V vs NHE) and is capable of oxidizing a

395

wide range of organics in wastewater. In this study, FeO(OH)-rGA, an outstanding

396

photo-Fenton catalyst, could absolutely and rapidly degrade phenolic organics. In Figure 4,

397

degradation rate of phenolic organics at the beginning was slow, which was probably because

398

the FeO(OH)-rGA has limited sorption capacity of phenolic organics. But as the quinone

399

organics produced in the reaction may be excited under visible light, and react with H2O2 to

400

generate •OH as well. This may explain the fast degradation of 4-CP, 2,4,6-TCP and BPA after

401

a short period of irradiation time.

402

3.4. Reusability and stability of the FeO(OH)-rGA aerogel

403

The reusability and stability of photo-Fenton catalysts are crucial for practical

404

applications. FeO(OH)-rGA was stored in a reaction solution for 10 cycles, and 4-CP and

405

H2O2 were added repeatedly after each cycle under visible-light irradiation. The reusability

406

performance of FeO(OH)-rGA is shown in Figure 5a. It can be seen that 4-CP was completely

407

degraded (100%) after each cycle, and the degradation continued until the 10th cycle.

408

Moreover, the morphology and internal network of FeO(OH)-rGA after 10 cycles of

409

regeneration did not show any significant changes, as shown in Figure S10. The reusability of 18


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FeO(OH)-rGA ensures its efficient performance and stability for several cycles without losing

411

its catalytic activity; thus, it is a superior candidate for the photo-Fenton process, particularly

412

for the removal of organic contaminants.

413

The stability of the photo-Fenton catalyst was also observed by examining the iron

414

leaching, which directly indicated the stability of the catalytically active component. The total

415

iron leaching after 10 cycles is displayed in Figure 5b. The total iron concentration was 16.4

416

mg/L in FeO(OH)-rGA. The range of the total iron leaching after each cycle was

417

approximately 0.4-0.6 mg/L, which accounted for only ~2.5% of 16.4 mg/L. Moreover, the

418

total iron content in solution after each cycle did not change significantly after the first cycle,

419

as shown in the inset picture (Figure 5b). We did not detect ferrous ions in the reaction

420

solution using the 1,10-phenanthroline method at a wavelength of 510 nm with a Shimadzu

421

UV-2550 spectrophotometer.53 Comparing the EDX results of 15FeO(OH)-rGA after 10

422

cycles (Figure S10d) with that of the initial 15FeO(OH)-rGA (Figure S3f), the 0.1% drop

423

suggests that the iron loss was practically negligible. Therefore, FeO(OH)-rGA is an excellent

424

photo-Fenton catalyst for practical applications in the future.

425 426

Figure 5. The reusability (a) and stability (inset of Fe(T) variation in the first cycle) (b) of the

427

FeO(OH)-rGA aerogel after 10 cycles in the FeO(OH)-rGA/H2O2/4-CP system under visible-light

428

irradiation. (Reaction conditions: C0(4-CP): 10 mg/L, C0(H2O2): 10 mM, pH=3, 500 W xenon lamp with a

429

UV cutoff filter (λ > 420 nm)). 19



430

3.5. Photocatalytic degradation mechanisms

431

The analysis of free radical quenching is effective for real-time identification of reaction

432

species in photo-Fenton system. Therefore, the reactive species were further distinguished by

433

adding methanol as scavenger for the •OH generated in the reaction solution.54 Methanol has

434

a high reaction rate constant with •OH, up to k=9.6 ×109 M-1 s-1.55 The results achieved after

435

the methanol addition were compared to those obtained without methanol addition and are

436

shown in Figure 6a. The results clearly revealed no degradation of 4-CP after the methanol

437

addition, which suggested that the 4-CP degradation was driven by •OH derived from

438

FeO(OH)-rGA-activated H2O2 under visible-light irradiation. To further identify the radical

439

species in the reaction, EPR with DMPO as a spin-trapping agent was used to detect •OH

440

radicals

441

•OH with an intensity ratio of 1:2:2:1 were obtained in the EPR spectra of

442

FeO(OH)-rGA/H2O2, which clearly indicated •OH was generated in the FeO(OH)-rGA/H2O2

443

system under visible-light irradiation.

48,56

after 5 min of visible-light irradiation. The typical four characteristic peaks of

444 445

Figure 6. Effect of radical scavengers on the degradation of 4-CP (a). EPR spectra of DMPO-•OH adducts

446

that formed over time with visible-light irradiation of a suspension of 15FeO(OH)-rGA/H2O2 (b).

447 448

20


Page 20 of 32

Page 21 of 32


449

The HPLC analysis is displayed in Figure 7a and shows the visual changes in the 4-CP

450

degradation. It can be concluded that A and D are 4-CP and H2O2, respectively, at t=0, and the

451

degree of 4-CP mineralization is shown in Figure 7b. The TOC removal gradually increased

452

to 80% after 6 h of reaction. The initial TOC removal was slow within 120 min, which was

453

mainly because of the intermediate product generation, i.e., quinone organics (C and D) with

454

a higher toxicity potential.57 The molecular structure of the intermediate products was

455

determined by HPLC-MS (Figure S11). These intermediate products retained the benzene ring

456

structure, which resulted in a solution with a relatively higher TOC at the beginning of the

457

reaction. The TOC removal dramatically accelerated once the reaction continued because the

458

benzene ring of the intermediate products was persistently attacked by •OH and eventually

459

transformed into low molecular weight organic acids with toxicity decreasing, which are more

460

susceptible to oxidative degradation. The possible reaction pathway for 4-CP degradation is

461

presented in Figure S12.

462 463

Figure 7. The intermediate products generation and mineralization of 4-CP. HPLC analysis of 4-CP

464

degradation within 150 min (a). TOC removal of 4-CP within 6 h of reaction (b). (Reaction conditions:

465

C0(4-CP): 10 mg/L, C0(H2O2): 10 mM, pH=3, 500 W xenon lamp with UV cutoff filter (λ > 420 nm)).

466 467 21



468

Based on the above analysis, a possible mechanism for phenolic organic degradation by

469

FeO(OH)-rGA with H2O2 under visible-light irradiation was proposed (Figure 8). Ferric ions

470

on the GO interface can create catalyst precursors with even and uniform distributions. Once

471

sodium ascorbate was added as a reducing agent, GO gradually transforms into rGO along

472

with recovery of the sp2 regions, which are beneficial for electron transfer between the iron

473

center and H2O2. Moreover, the ferric ions turn into nanometer FeO(OH) that is firmly fixed

474

in rGA via strong π-π interaction between the rGO layers, preventing iron leaching. In

475

addition, the nanoparticles (average size ~ 3 nm) are distributed evenly on the rGO layers,

476

improving the catalytic activity of the FeO(OH)-rGA aerogel compared with that of pure

477

Fe2O3, which has a slow reaction efficiency with H2O2 even under UV-light irradiation.58

478

FeO(OH) has a special optical activity that can easily be activated by visible light and

479

transforms to Fe(II) to activate H2O2 to produce •OH. This process is than pure H2O2 direct

480

photolysis, and an extraordinary catalytic activity is displayed for phenolic organic

481

degradation along with Fe(III)/Fe(II) catalytic cycles. Furthermore, FeO(OH)-rGA can

482

generate •OH with H2O2 under visible-light irradiation due to coupling the visible irradiation

483

and intermediate quinone organics, which have strong and universal driving forces in the

484

Fenton reaction for phenolic organic degradation and mineralization. The special

485

FeO(OH)-rGA structure allows superior degradation of phenolic organics at near neutral pH

486

values and maintains the stability of the catalyst, avoiding a negative influence from catalyst

487

aggregation.

22


Page 22 of 32

Page 23 of 32


488 489

Figure 8. Schematic illustration of the possible hydroxyl radical generation mechanism and phenolic

490

organic mineralization in the FeO(OH)-rGA/H2O2 system under visible-light irradiation.

491 492

In this study, we employed a facile synthetic method for iron-doped-rGO aerogels as an

493

effective and original photo-Fenton catalyst. The obtained aerogel not only had a stable

494

macrostructure and lighter density but also a prominent catalytic activity for phenolic organic

495

degradation at near neutral pH values with less iron leaching after 10 cycles of consecutive

496

uses. The minimal iron leaching was mainly because the smaller FeO(OH) crystals (average

497

size ~3 nm) were immobilized and dispersed on the rGO layers, and these iron crystals

498

possess a higher catalytic activity than pure iron oxides. The sp2 regions of rGO were

499

recovered by sodium ascorbate reduction to ensure fewer defects after iron doping.

500

Consequently, this FeO(OH)-rGA aerogel is an ideal photo-Fenton catalyst that overcomes the

501

disadvantages of conventional Fenton reactions and can be applied to practical applications

502

for effective removal of organic pollutants.

503 23



504

 ASSOCIATED CONTENT

505

Supporting Information

506

LC-MS analysis and comparison of the catalytic properties of different photo-Fenton catalysts

507

(Table S-1) and Figures S1-S12 are presented. This information is available free of charge via

508

the Internet at http://pubs.acs.org.

509 510

 AUTHOR INFORMATION

511

Corresponding Author

512

*Phone: 0086-574-87609508; e-mail: [email protected]

513

Notes

514

The authors declare no competing ﬁnancial interest.

515 516

 ACKNOWLEDGMENTS

517

This project was supported by the National Natural Science Foundation of China (Grant

518

21425730, 21537005, and 21621005), the National Basic Research Program of China (Grant

519

2014CB441106), and the National Key Technology Support Program of China (No.

520

2015BAC02B01).

521 522

References

523

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