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Journal of Hazardous Materials 324 (2017) 653–664

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal ho me pa ge: www.elsev ier .com/ locate / jhazmat

egradation of sulfamethazine using Fe3O4-Mn3O4/reduced graphenexide hybrid as Fenton-like catalyst

hong Wana, Jianlong Wanga,b,c,∗

Collaborative Innovation Center for Advanced Nuclear Energy Technology, INET, Tsinghua University, Beijing 100084, PR ChinaState Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, PR ChinaBeijing Key Laboratory of Radioactive Waste Treatment, Tsinghua University, Beijing 100084, PR China

i g h l i g h t s

Fe3O4-Mn3O4/reduced grapheneoxide (RGO) hybrid was prepared.It was an efficient heterogeneousFenton catalyst for SMT degradation.The removal efficiency of SMT wasabout 98% at optimal conditions.The possible catalytic mechanismwas proposed.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 1 September 2016eceived in revised form0 November 2016ccepted 14 November 2016vailable online 14 November 2016

a b s t r a c t

In this paper, Fe3O4–Mn3O4/reduced graphene oxide (RGO) hybrid was synthesized through polyolprocess and impregnation method and used as heterogeneous Fenton-like catalyst for degradation of sul-famethazine (SMT) in aqueous solution. The hybrid catalyst had higher catalytic efficiency compared withFe3O4–Mn3O4 and Mn3O4 as catalyst for degradation of SMT. The effects of pH value, H2O2 concentration,catalyst dosage, initial SMT concentration and temperature on SMT degradation were investigated. Theremoval efficiency of SMT was about 98% at following optimal conditions: pH = 3, T = 35 ◦C, Fe O /Mn O -

eywords:enton-like processydroxyl radicalsulfamethazine

3 4 3 4

RGO composites = 0.5 g/L, H2O2 = 6 mM. The inhibitor experiments indicated that the main active specieswas hydroxyl radicals (·OH) on catalyst surface. At last, the possible catalytic mechanism was proposed.

© 2016 Elsevier B.V. All rights reserved.

educed graphene oxideatalyst

∗ Corresponding author at: Energy Science Building, Tsinghua University, Beijing00084, PR China.

E-mail addresses: dzlyqlzq@163.com (Z. Wan), wangjl@tsinghua.edu.cn,angjl@mail.tsinghua.edu.cn (J. Wang).

ttp://dx.doi.org/10.1016/j.jhazmat.2016.11.039304-3894/© 2016 Elsevier B.V. All rights reserved.

1. Introduction

PhACs (pharmaceutically active compounds) are kinds of traceamount organic pollutants, which are widely existence in aquaticenvironment [1]. The conventional wastewater biological treat-

dx.doi.org/10.1016/j.jhazmat.2016.11.039http://www.sciencedirect.com/science/journal/03043894http://www.elsevier.com/locate/jhazmathttp://crossmark.crossref.org/dialog/?doi=10.1016/j.jhazmat.2016.11.039&domain=pdfmailto:dzlyqlzq@163.commailto:wangjl@tsinghua.edu.cnmailto:wangjl@mail.tsinghua.edu.cndx.doi.org/10.1016/j.jhazmat.2016.11.039

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54 Z. Wan, J. Wang / Journal of Haz

ent processes are not effective for removing these pollutants.hus, these kinds of pollutants are not only harmful to the aquaticrganism but also decrease the water quality [2]. At the same time,hey can accumulate in the animal and human body, leading to haz-rdous effects on ecological system [3]. Especially, the antibioticesistance is one of serious problems caused by PhACs [4]. Vari-us methods have been used to remove these kinds of pollutants,ncluding physical and chemical methods, such as activated carbondsorption [5], membrane filtration [6], and advanced oxidationrocesses [7,8].

Traditional Fenton process is one of advanced oxidation pro-esses based on the generation of hydroxyl radicals (•OH) througheaction between catalysts (Fe2+/Fe3+) and oxidant (H2O2) as reac-ions (1)–(3). Hydroxyl radical is a non-selective oxidant, which caneact with most of organics. However, in traditional Fenton processron sludge produced is too much to proceed in the following steps,o more alkali is required to add to remove the iron. This is a seriousroblem, which is needed to be addressed to.

e2+ + H2O2 → Fe3+ + OH− + •OH (1)e3+ + H2O2 → Fe2+ + H+ + HO2• (2)O2• + H2O2 → H2O + •OH + O2 (3)

Heterogeneous Fenton-like process can overcome these advan-ages in some extent due to the catalysts can easily be separatedith the properties of magnetism from the solution. Many

esearches have been carried out on the degradation of organic pol-utants with catalyst based on iron, including Fe0 [9], Fe0/CeO2 [10],e3O4 [11], FeOOH [12], Fe2O3 [13], Fe3O4-TiO2 and so on.

Fe3O4 has a structure of inverse spinel and electrons can moveast between Fe2+ and Fe3+ in octahedron. Nano sized Fe3O4 par-icles have small size effect [14], quantum size effect [15], surfaceffect [16], which lead to its good properties of electrical conduc-ivity with metallic character [17], super-paramagnetism and highatalytic activity [18]. Fe3O4 is semiconductor with a narrow bandap, which is important for electron transport. The magnetic prop-rties of Fe3O4 used as heterogeneous catalyst made it easy to be

eparated from the solution. Mn3O4 has a structure of spine andhe chemical character is very active. Manganese oxides are widelysed in field of catalytic [19,20], electrochemical [21], magnetics22] and energy storage [23]. The use of Fe3O4/Mn3O4 as catalysts in

Fig. 1. XRD patterns of Fe3O4/Mn3O4 – R

Materials 324 (2017) 653–664

heterogeneous Fenton process was studied in our group previously[24]. The results indicated that Fe3O4/Mn3O4 had a good cat-alytic activity, with SMT removal efficiency of 99% in 50 min whenpH = 3, T = 45 ◦C, H2O2 = 6 mM, Fe3O4-Mn3O4 dosage = 0.5 g/L andSMT = 0.07 mg/L.

Graphene is a new developed material with large surface area,good electrical conductivity and mechanical stability [25,26]. It isa perfect material used for carrier. The combination of grapheneand transition metal oxides can prevent the agglomeration ofgraphene and enhance the nucleation of nanoparticles. With largesurface area, graphene can supply more active sites for nanoparti-cles. Besides, the three-dimensional network structure can enhancethe conduction of electron [25,26]. In previous studies, manyresearchers have studied the metal oxides loaded on graphene,such as, Fe3O4/Mn3O4-RGO [27–29], graphene-BiFeO3 [29,30] andZnFe2O4–RGO [29]. However, to our best knowledge, this is noreport about using Fe3O4/Mn3O4-RGO as catalyst in heterogeneousFenton-like process.

In this study, we synthesized the nanoparticles with polyol pro-cess and impregnation method and used sulfamethazine (SMT)which is one of pharmaceutically active compounds as target pol-lutants. The prepared catalysts were characterized by means ofX-ray diffraction (XRD), transmission electron microscopy (TEM),BET, VSM, FTIR, Raman spectrum and the degradation efficiencyof sulfamethazine (SMT) with Fe3O4/Mn3O4-RGO as catalyst wereinvestigated.

2. Materials and methods

2.1. Chemicals

Iron (II) sulfate heptahydrate (FeSO4·7H2O), fer-ric sulfate (Fe2(SO4)3), manganous acetate tetrahydrate(Mn(CH3COO)2·4H2O), chromatography grade methanol anddiethyleneglycol (DEG) are purchased from Sinopharm Chemical

Reagent Co., Ltd. Ammonium hydroxide (25%) and A.R. gradehydrogen peroxide (30%, w/w) are purchased from Beijing chem-ical works. Sulfamethazine (>99%) purchased from Alfa Aesar. Allchemicals were used without further purification.

GO nanoparticles before reaction.

Z. Wan, J. Wang / Journal of Hazardous

F×

2

spMpwFnfg

in Fig. 1, indicating the Fe O -Mn O /RGO nanoparticles before

ig. 2. HRTEM micrographs of Fe3O4/Mn3O4-RGO composites before reaction (a)4000; (b) ×600000; (c) EDX of Fe3O4-Mn3O4/RGO composites.

.2. Synthesis of catalysts

The Fe3O4/Mn3O4-RGO composite was prepared in twoteps: Firstly, Mn3O4 nanoparticles were synthesized througholyol process [19]. A certain amount of diethyleneglycol andn(CH3COO)2·4H2O were under vigorously stirring with tem-

erature of 99–100 ◦C. Then the particles were centrifuged andashed using ethanol and water with ultrasonic wave. Secondly,

e O /Mn O -RGO nanoparticles were prepared through impreg-

3 4 3 4ation method and hydration hydrazine reduction method in a

our-necked flask with condenser device. A certain amount ofraphene oxide was dispersed with ultra-sonication for 2 h with

Materials 324 (2017) 653–664 655

300 W power into homogeneous solution. The weight ratio ofgraphene oxide and Fe3O4/Mn3O4 was 1:10. A certain amountof FeSO4·7H2O and Fe2(SO4)3 (mole ratio of 1:2) of 100 mL wereadded into the flask. At same time, the graphene oxide solutionand some Mn3O4 (weight ratio of Mn3O4:Fe3O4 = 1:1) were addedin it. After 1 h vigorous stirring, the Fe2+, Fe3+, Mn3O4 and grapheneoxide combined together. Then 25% ammonia water was added intothe flask dropwise until the pH reached 10–11. The whole processwas under room temperature and protection of argon atmosphere.After 1.5 h, the system was heated up to 90 ◦C and small amountof hydrazine hydrate were added, stirring for 4 h, Fe3O4/Mn3O4-RGO were obtained. The composites were washed using mentholand oxygen-free water for several times, and dried in a vacuumfreezing dryer.

2.3. Characterization of catalyst

X-ray diffraction (X-ray power diffractometer) measurementwas taken with Cu K� radiation (D8-Advance, Bruker, 40 kV and 40Ma) at room temperature with 1◦/min over range (2�) of 10◦−90◦.High resolution transmission electron microscopy (HRTEM, JEM2100 and JEOL) equipped with energy disperse X-ray spectrometeroperated at 200 kV were used to observe microscopic morphologyof catalyst loaded in carbon film. A nitrogen adsorption–desorptionapparatus (NOVA 3200e sorptometer) was employed to determinethe Brunauer-Emmett-Teller (BET) specific surface area and Barret-Joyner-Halenda (BJH) pore size distribution at 77 K and degassingat 393 K. X-ray photoelectron spectroscopy (XPS) analysis (ThermoScientific ESCALAB 250Xi) was performed by using an Al K� X-ray(1486.6 eV) source for excitation. Raman spectra were obtainedusing Raman system of LabRAM HR Evolution of HOEIBA JobinYvon Company with a regular model laser operated at wavelengthof 633 nm. The laser power was 0.8 mW. The Fourier transforminfrared (FTIR) was recorded with Nicolet 6700 spectrometer madeby Thermo Fisher Scientific. The powder pressing method in KBrpellet was used to prepare samples at room temperature. Vibratingsample magnetometer (SQUID-VSM (MPMS-3)) made by QuantumDesign Company was used to investigate the magnetic propertiesof the composites.

2.4. Experimental procedures

Catalytic degradation experiments were carried out in sealedserum bottles (100 mL) with 40 mL of SMT solution in a shakingincubator with 160 rmp at a certain temperature. The pH of initialsolution was adjusted with HCl (0.1 M). A known amount of H2O2and catalysts were added to initiate the reaction. The experimentswere carried out twice, and the results were the average value. Thesamples were taken at predetermined time intervals and filteredwith 0.22 �m film after quenching with n-butanol. The concentra-tion of sulfamethazine (SMT) was measured by high-performanceliquid chromatography (HPLC Agilent 1200) equipped with a diodearray detector (DAD) and an XDB-C18 (4.6 × 150 mm) column withdetection wavelength of 275 nm. The mobile phase for sulfamet-hazine was a mixture of distilled water and ethanol (55:45 (v/v)).The flow rate was 1 mL/min and the column temperature was 30 ◦C.

3. Results and discussion

3.1. Characterization of Fe3O4-Mn3O4/RGO nanoparticles

The results of X-ray diffraction (XRD) measurement are shown

3 4 3 4

reaction. The XRD patterns of Mn3O4 has a space group of Fd–3 m(227) [JCPSD 13-0162]. The diffraction peaks (18.2◦), (30.0◦),(35.3◦), (43.0◦), (53.3◦), (56.8◦), (62.3◦) and (73.7◦) corresponded to

656 Z. Wan, J. Wang / Journal of Hazardous

Fo

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ig. 3. (a) Nitrogen adsorption-desorption isotherm; (b) BJH pore size distributionf Fe3O4-Mn3O4/RGO composites.

he planar of (111), (220), (311), (400), (422), (511), (440) and (533),espectively. The peaks from (111), (220), (311), (400), (422), (511),440) and (533) corresponding the panes at (18.3◦), (30.1◦), (35.5◦),43.1◦), (53.5◦), (57.0◦), (62.6◦) and (74.0◦) respectively, which is inood agreement with Fe3O4 nanoparticles with a structure of cubicpinel: Fd–3 m (227) [JCPSD 65-3107]. From the inset image we canee the magnification of peak, peak at 35.3◦ and 35.5◦ correspond-ng to Mn3O4 and Fe3O4, respectively, indicating the coexistence ofe3O4 and Mn3O4 in the composites.

TEM image in Fig. 2 shows the morphology of synthesized ofanoparticles. From Fig. 2(a and b), we can see that Fe3O4/Mn3O4ere evenly distributed on graphene. The diameter of particlesas about 15–20 nm. The graphene was a sheet-like structure with

ome wrinkles on it. The lattice of fringe spacing of composites of.48 nm and 0.25 nm were corresponding to (111) and (311) reflec-ion of Mn3O4 and Fe3O4, respectively. From the HRTEM images, wean see the presence of intimate interaction between Mn3O4, Fe3O4nd graphene, which can enhance the electron transport amonghem, increasing the catalyst activity. EDX analysis shown in Fig. 2cndicated the coexistence of Fe, Mn, C and O.

Fig. 3 shows the nitrogen adsorption-desorption isotherms andhe pore size distributions of Fe3O4/Mn3O4-RGO composites. We

an see that the Fe3O4/Mn3O4-RGO belong to type IV with H3 typeysteresis loops, indicating this kind of nanoparticles are meso-orous materials [31]. The specific surface areas (SBET), pore volumend pore size were 176.6 m2/g, 0.174 cm3/g and 4.59 nm, respec-

Materials 324 (2017) 653–664

tively. The pore size distribution in Fig. 3(b) can illustrate themesoporous structure, ranging between 2 and 50 nm.

This result of SBET was similar with that of Fe3O4/hierarchial-Mn3O4/graphene oxide (180.8 m2/g) [27].

XPS analyses were used to characterize the chemical bondingstates change in Fe3O4/Mn3O4 – RGO composites before and afterreaction. The results were shown in Fig. 4. The bonding energy at284 eV, 711 eV, 528 eV and 640 eV responded to the C 1s, Fe 2p, O1 s and Mn 2p, repectively.

Fig. 4(b) shows the O 1 s sepctrum in the composities. Fromthe specrum before reaction, we can see that binding energy at533.4 eV, 533.0 eV, 531.5 eV, 532.0 eV and 530.1 eV corresponded toH2O, C O, C O, M OH and M O respectively [32–35]. After reac-tion, the area ratio of C O bond increased, which can be ascribed tothe byproduct of ring −opening reaction of SMT. The area ratio ofM OH component increased and M O component increased dueto the hydroxulation during reaction.

Fig. 4(c) shows the deconvoluted C1 s peak. The peaks at284.8 eV, 285.3 eV, 286.9 eV, 288.3 eV and 289.2 eV, which are cor-responding to peaks C C(sp2), C C(sp3), C OH and or C O C, C Oand O C O, respectively [18,36], which is in good agreement withspectrum of O 1s. Compared with specturm before reaction, thereis no obvious change of RGO after reaction.

Fig. 4(d) shows the Mn 2p high resolution spectrum. The peakscenterd at 641.8 ev and 653.3 eV are corrsponding to Mn 2p3/2 andMn 2p1/2 [37]. It was worthy to note that the electron transfersbetween Mn (II) and Mn (III) is obvious, i.e., from the peaks at641.8 eV and 653.5 eV transferred to 641.4 eV and 653.1 eV, whichis indicative of coordination between of Mn(II) and Mn(III). Theenergy separation of 11.7 eV is well in accordance with spectrumof Mn3O4 [38].

Fig. 4(e) shows the Fe 2p high resolution spectra before and afterreaction. The binding energy at 711 eV and 725 eV can be ascribedto Fe 2p3/2 and Fe 2p1/2 according to previous study [35,39]. Thereis almost no change in Fe 2p spectrum before and after reaction.The area ratio of Fe (III) to Fe (II) is 1.6 and 1.2, inditacting the littlehydroxulated during the degradation reaction and the stability ofthe catalyst.

The magnetic hysteresis of Fe3O4/Mn3O4-RGO compositesbefore and after reaction in the range of −10 kOe < H < 10 kOewere characterized through vibrating sample magnetometer andshown in Fig. 5. The saturated magnetization (Ms) of Fe3O4/Mn3O4– RGO composites are 36 A m2/kg and 35.8 A m2/kg before and afterreaction, respectively. It has properties of superparamagnetic withcoercivity and remanence near to zero. The saturated magnetiza-tion was lower than Fe3O4/RGO composites of 61.4 A m2/kg beforereaction, which can be ascribed to the addition of Mn3O4. With largesaturated magnetization, the composites can be easily separatedthrough external magnetic field.

Raman spectrum of GO and Fe3O4/Mn3O4-RGO composites areshown in Fig. 6. The G band at around 1590 cm−1 and D band ataround 1340 cm−1 can be observed in GO and Fe3O4/Mn3O4-RGOcomposites. The intensity of G band represents well-defined sp2

in structure and the intensity of D band represents the defectsin hexagonal graphitic structure. ID/IG represents the defectsand disorder of graphitized structure [36]. Compared spectrumFe3O4/Mn3O4-RGO with GO, we can see that ID/IG increased from0.844 to 2.59, indicating that GO has been reduced into RGO. WhenGO was reduced, the defect on the surface increased, leading to thatID is larger than IG. Besides, there is an obvious peak at 611 cm−1

in Fe3O4/Mn3O4-RGO composites, which ascribed to Fe O in mag-netite [40].

The FTIR spectrum of Fe3O4/Mn3O4-RGO composites was usedto identify the chemical structure change before and after reac-tion, and the results are shown in Fig. 7. The peak at 3419 cm−1,1619 cm−1, 1391 cm−1 and 1035 cm−1 corresponded to stretches of

Z. Wan, J. Wang / Journal of Hazardous Materials 324 (2017) 653–664 657

F high r

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ig. 4. (a) XPS spectra of survey scan; (b) high resolution scan of O 1s region; (c)esolution scan of Fe 2p region.

H, aromatic C C, bending vibration of O H in water and alkoxy O, respectively. The results are similar to previous ones [27,41].here are no stretches around in 1737 cm−1 and 1200 cm−1, whichepresent the stretches of C O and epoxy C O. This illustratedhat graphene oxide was reduced. The peaks centered at 570 cm−1

nd 417 cm−1 ascribed to the interaction of Fe O and Mn O ine3O4/Mn3O4-RGO composites [33,42].

We can see that there is almost no change in IR spectrum beforend after reaction, indicating the stability of the catalysts, which isonsistent with the characterization of XPS.

.2. Catalytic degradation of SMT

We also carried out the experiments on degradation sulfamet-azine (SMT) under various conditions, including catalysts type, pH,

resolution scan of C 1s region; (d) high resolution scan of Mn 2p region; (e) high

H2O2 concentration, initial concentration of SMT, catalyst dosageand temperature. The results are shown in Fig. 8. The prelimi-nary experiments showed that 98% SMT can be degraded in 80 minwhen pH = 3, T = 35 ◦C, H2O2 = 8.4 mM, Fe3O4/Mn3O4-RGO compos-ites = 0.5 g/L, SMT = 0.07 mmol/L.

3.2.1. Effect of different catalystsTo examine the efficiency of the catalyst, the experiments

with different catalyst types, including H2O2, Mn3O4 + H2O2,Fe3O4/Mn3O4 + H2O2, Fe3O4 + H2O2, Fe3O4/Mn3O4-RGO alone andFe3O4/Mn3O4-RGO + H2O2, were carried out. The possibility of SMT

degradation using H2O2 without catalyst was also evaluated. Theresults showed that the degradation of SMT can be negligible withH2O2 only. When there was only Fe3O4/Mn3O4-RGO in the solu-tion without H2O2, about 20% SMT can be eliminated, which can be

658 Z. Wan, J. Wang / Journal of Hazardous Materials 324 (2017) 653–664

Fig. 5. Magnetic hysteresis curve of Fe3O4-Mn3O4/RGO composites before and after reaction.

Fig. 6. Raman spectra of Fe3O4-Mn3O4/RGO composites and GO.

awodat(ctcTdpg

ate reacts also quickly with hydrogen peroxide (k = 1 × 10 M s )

scribed to adsorption. Fig. 8(a) shows that in the existence of H2O2,hen using Mn3O4 as catalyst, the degradation experiments cannot

ccur. When using Fe3O4/Mn3O4-RGO as catalyst, highest degra-ation efficiency of achieved (98%) compared with Fe3O4/Mn3O4s catalyst (73%) and Fe3O4 as catalyst (22%). This attributed tohe following reasons. Firstly, with large specific surface areasSBET) (176.6 m2/g) and more active sites on RGO, Fe3O4/Mn3O4an be uniformly distributed with the addition of RGO. Secondly,he three-dimensional network structure of RGO can enhance theonduction of electrons, which can enhance the catalyst efficiency.hirdly, the addition of Mn3O4 also played important role in degra-ation process (see detail in Section 3.3). Fourthly, the hydrophobic

roperty of reduced graphene and graphene is better than oxideraphene, which is favorable to adsorption of SMT.

Fig. 7. FTIR spectra of Fe3O4-Mn3O4/RGO composites before and after reaction.

3.2.2. Effect of pHThe effect of pH on SMT degradation is shown in Fig. 8(b), indi-

cating that the optimal pH was about 3.0. The value of pH played animportant role in the whole process. Firstly, when pH < 2, Fe (II) wasin the form of hexaaquo ion Fe(H2O)62+ (abbreviated as Fe2+). AspH increased (3.0–3.5), H2O was mainly replaced by HO− as com-plexes [Fe(OH)(H2O)5]+ and Fe(OH)2(H2O)4 (abbreviated as FeOH+

and Fe(OH)2). Reactions (1)–(2) are more effective to motivate H2O2to produce •OH when pH ranged from 3.0–3.5 [43,44]. This canbe attributed to that Fe(OH)2 reacts more quickly than Fe2+ withhydrogen peroxide (kFe2+ = 40–80 M−1s−1; kFe(OH)2 = 586 M−1s−1)[44,45]. Besides, the oxalate complex, FeII(C2O42−) as intermedi-

4 −1 −1

[46]. However, the rate of reaction reach a maximum at pH = 3. Thiscan be attributed to that Fe(III) is easily to precipitate when pH > 3

Z. Wan, J. Wang / Journal of Hazardous Materials 324 (2017) 653–664 659

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sitcTpirpstg

ig. 8. Degradation of SMT at different conditions (a) Catalyst type; (b) pH value; (c)s the effect of SMT adsorption with different dosage of catalyst of 0.2 g/L, 0.5 g/L an H2O2 = 8.4 mM, Fe3O4-Mn3O4/RGO composites = 0.5 g/L, pH = 3, 40 mL SMT = 0.07

nd Fe(II) reach a maximum around pH = 3 [47]. If the generationf Fe(II) is inhibited, the Fenton process will slow down.

Secondly, it is worth to note that the effects of Fe3O4-n3O4/RGO are better than Fe3O4-Mn3O4. It ascribed to the

dsorption of RGO. The adsorption mechanisms of graphene mate-ials mainly include electrostatic interaction, hydrophobic effectnd �–� bond influenced by pH values.

The potential of zero charge (pHpzc) of Fe3O4-Mn3O4/RGO mea-ured by zeta potentiostat was 4.2. The adsorption of SMT wasnfluenced largely by pH of the solution, pH(pzc) and pKa of pollu-ant. When the pH ≥ pH(pzc) and pH ≤ pH(pzc), there were negativeharge and positive charge on the catalyst surface, respectively.he pKa of SMT molecule is pKa1 = 2.28, pKa1 = 7.42 [48]. WhenH > 7.42, pH < 2.28 and 2.28 < pH < 7.42, the SMT molecules exist

n anionic form, cationic form and amphoteric molecules form,espectively [49]. Thus, when the pH of the solution pH > 7.42 or

H < 2.28, the adsorption function will be inhibited for the electro-tatic repulsion. The pHpzc of the catalyst ranged in 2.28 and 7.42,hus, the catalyst can combine with the anionic group and cationicroup in SMT molecule. In addition, the hydrophobic performance

concentration; (d) Initial concentration of SMT; (e) Catalyst dosage, the inset graph/L; (f) Temperature. Except for the investigated parameter, other parameters fixedl/L, T = 35 ◦C.

of deprotonated SMT molecule (pH > 7.42) is weaker than proto-nated amphoteric SMT ions, leading to the decrease of �–� EDAbetween SMT molecule and carbon material, which is not favorableto adsorption [54].

3.2.3. Effect of H2O2 concentrationThe effect of H2O2 concentration is shown in Fig. 8(c). The role

of H2O2 in the process of degradation of SMT is dual. With insuf-ficient H2O2, the amount of •OH (reaction (6)–(7)) is not enoughto degrade high concentration of SMT, leading to low SMT removalefficiency. And a minor increase of removal efficiency was observedwhen H2O2 increased from 3.6 mM to 6 mM. However, the removalefficiency decreased in the presence of excess of H2O2, when H2O2concentration increased from 6 mM to 8.4 mM. This ascribed to thatexcess H2O2 can be a scavenger of hydroxyl radical (reaction 8). Inaddition, it also can compete with SMT for the adsorption sites on

catalyst surface. Thus, there is an optimal concentration of H2O2(6 mM) for the maximum removal efficiency.

Parra et al. [50] reported the similar phenomenon in a heteroge-neous Fenton process with Fe-Histidine-Nafion as catalyst. When

6 ardous Materials 324 (2017) 653–664

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60 Z. Wan, J. Wang / Journal of Haz

2O2 concentration of 5 mM was added, the Orange II discolorationas possible and the discoloration attained a limit with H2O2

oncentration increased to 10 mM. However, the discoloration ofrange II didn’t become faster since scavenging of •OH by excess2O2 beyond of 10 mM.

e2+surf + H2O2 + H+ → Fe3+surf + H2O + •OHsurf (6)n2+surf + H2O2 + H+ → Mn3+surf + H2O + •OHsurf (7)

OH + H2O2 → HO2• + H2O (8)

.2.4. Effect of SMT concentrationIn order to investigate the influence of SMT initial concentration,

arious concentrations were used in this study, i.e., 0.087 mmol/L25 mg/L), 0.07 mmol/L (20 mg/L), 0.053 mmol/L (15 mg/L) and.036 mmol/L (10 mg/L), the results are illustrated in Fig. 8(d).lthough they are higher than their concentrations (ng-�g/L)etected in aquatic environments [51], we chose 15–25 mg/L as theoncentration of model pollutant based on the following reasons:1) in order to clearly illustrate the influence of different factorsn degradation of SMT with obvious degradation rate; (2) what’sore, we have referred many studies before, for example, Niu et al.

52] and Dias et al. [53], who used the concentration of target phar-aceuticals in level of mg/L as well.Fig. 8(d), we can see that higher initial concentration with

.087 mmol/L decreased the degradation rate compared with

.036 mmol/L and 0.07 mmol/L. because the pollutants could com-ete the active sites with H2O2 on the catalyst surface. The similaresults were also observed by other researchers [55–57]. For exam-le, Guo et al. [55] studied the degradation of phenol with differentoncentrations using Fe-Al pillared as catalyst. Their results showedhat higher initial phenol concentration led to a lower conversion,hich indicated that this kind of catalysts can be possibly applied inigh concentrated solution [55]. On the other hand, higher dosagef catalysts can enhance the removal of SMT with high concentra-ion.

.2.5. Effect of catalyst dosageThe catalyst dosage can remarkably influence the degradation of

MT in aqueous solution as shown in Fig. 8(e). The degradation ratencreased from 43% to 98% when catalyst dosage increased from.2 g/L to 1 g/L. When the catalyst dosage increased, there woulde more active sites on catalyst surface as well as hydroxyl radicalsenerated. Besides, more catalyst would increase the adsorptionf pollutants due to the large catalyst surface. The inset graph inig. 8(e) shows the adsorption of SMT with different dosage ofatalyst (0.2 g/L, 0.5 g/L and 1 g/L), indicating that with increase ofatalysts, the adsorption of SMT increased.

.2.6. Effect of temperatureThe effect of temperature was also investigated and the results

re shown in Fig. 8(f). The general trend of temperature is that theigher of temperature will lead to higher degradation rate. Theegradation rate increased from 40% to 100% when temperature

ncreased from 15 ◦C to 45 ◦C, indicating that SMT degradation reac-ion was endothermic. High temperature can promote to produce

ore active hydroxyl radicals and enhance the degradation rate.

.2.7. The catalyst stability and influence of homogeneous FentonTo evaluate the stability of the catalyst, five successive

xperiments were carried out under the same conditionH2O2 = 8.4 mM, Fe3O4/Mn3O4-RGO composites = 0.5 g/L, pH = 3,

0 mL SMT = 0.07 mmol/L and T = 35 ◦C). The catalyst was washedsing water after every time. The removal efficiency of SMT was8%, 92.5%, 90%, 86%, and 82%, respectively. The removal efficiencyid not decrease too much, which indicated that little inactivation

Fig. 9. Degradation of SMT with different inhibitor (H2O2 = 8.4 mM, Fe3O4-Mn3O4/RGO composites = 0.5 g/L, pH = 3, 40 mL SMT = 0.07 mmol/L, T = 35 ◦C).

of catalytic sites. Interestingly, we found that the removal effi-ciency did not change too much after the fifth-time reuse, and theremoval efficiency in the sixth-time and seventh-time was 81.5%and 80%, respectively. Thus, we supposed the catalyst was stableafter the fifth-time reuse. The initial relative large loss of activityascribed to the possible reasons as follows. Firstly, the leached ionsand sulfur as the by-products, leading to the decay of active sites[55,58]. Secondly, the aggregation of catalysts can decrease theiractivity. Thirdly, the rinsed function during discarding the super-natants [59]. The leaching concentration of iron and manganesewas 0.8 mg/L and 0.9 mg/L after first use. This also be ascribed tothe stability of the catalyst structure.

1 mg/L Fe (II) and 1 mg/L Mn (II) were used to evaluated thehomogeneous Fenton process under condition of H2O2 concen-tration ranging from 0 mM, 1.8 mM, 3.6 mM and 6 mM, pH = 3,SMT = 0.07 mmol/L and T = 35 ◦C. The results showed that the degra-dation efficiency was far less than in heterogeneous system. Thus,homogeneous Fenton process only played a little contribution toSMT degradation.

3.3. Possible catalytic mechanisms

The main active species in Fenton-like process is hydroxyl rad-ical (·OH) according to previous study [60]. Hydroxyl radicals canattack organic pollutants. In this study, n-butanol was used as ascavenger of ·OH both on the catalyst surface and in the bulk solu-tion. KI was used as a scavenger of •OH only on the catalyst surface.The results are shown in Fig. 9. The degradation ratio decreasedfrom 98% to 19% with addition of n-butanol compared to reactionwithout inhibitor. When excess KI was added, the degradation ofSMT decreased to 23%. Fig. 9 indicated that the removal efficiency ofSMT mainly resulted from •OH on the catalyst surface, that is to say,the surface-bound hydroxyl radicals played an important role in thereaction, which substantiate the results of homogeneous reaction(Section 3.2.7).

Fig. 9 The surface activity of iron oxides with H2O2 is differentfor different iron oxide [61]. Thus, many possible mechanisms ofmetal oxide/H2O2 system for oxidation of contaminants have beproposed, which are as follows.

Mechanism (I): Haber-Weiss mechanisms for the catalyst con-taining Fe(II) as activator. The reactions are listed in Table 1 [62,63].

Mechanism (II): Proposed mechanisms in Fe(III)/H2O2 system(e.g. goethite) [64]. The possible reactions are listed in Table 2.

The reactions are initiated by the formation of a precursor sur-face complex of H2O2 with Fe3+surf OH (reaction 2.1). Then the

Z. Wan, J. Wang / Journal of Hazardous

Table 1Haber-Weiss mechanisms in metal oxide/H2O2 system.

Fe3+ + H2O2 → Fe2+ + HO2• (•O2−) + H+(2H+) (1.1)Fe2+ + H2O2 → Fe3+ + OH− + •OH (1.2)Fe3+ + •O2− (HO2•) → Fe2+ + O2 (+H+) (1.3)Fe2+ + HO• → Fe3+ + OH− (1.4)•OH + H2O2 → H2O + HO2• (1.5)HO2• ↔ H+ + •O2− (1.6)

Table 2Proposed mechanisms in Fe(III)/H2O2 system (e.g. goethite).

Fe3+surf − OH + H2O2 → (H2O2)S (2.1)(H2O2)S↔Fe2+surf − •O2H + H2O (2.2)Fe2+surf − •O2H↔ Fe2+surf + HO2• (2.3)(H2O2)S → Fe2+surf + H2O + HO2• (2.4)Fe2+surf + H2O2 → Fe3+surf − OH + •OH (2.5)HO2• ↔ H+ + •O2− (2.6)Fe3+surf − OH + HO2•/•O2− → Fe2+surf + H2O/OH− + O2 (2.7)

Table 3Proposed mechanisms with high-valent iron species (FeIV) in metal oxide/H2O2system.

Fe3+surf + H2O2 → Fe2+surf + HO2• + H+ (3.1)Fe2+ surf + H2O2 → FeIVsurf + 2OH− (3.2)FeIVsurf + H2O2 → Fe2+surf + O2 + 2H+ (3.3)

rtdir

sooswa

pno

ctteiaa(ar(

ttc[rFwps

FeIVsurf + Fe2+surf → 2Fe3+surf (3.4)Fe3+surf + HO2•(•O2−) → Fe2+surf + O2(+H+) (3.5)

eversible electron transfers in surface complex (reaction 2.2) andhen the electronically excited state can be deactivated throughissociation of the peroxide radicals (reaction 2.3). The reduced

ron can react with H2O2 (reaction 2.5). The peroxide and hydroxyladicals can react with Fe(III) (reaction 2.7).

Mechanism (III): proposed mechanisms with high-valent ironpecies (FeIV) in metal oxide/H2O2 system. Except for the oxidantsf hydroxyl radical, activation of H2O2 by iron oxides could producexidants such as high-valent iron species (FeIVsurf) [65,66]. But theolution phase of FeIV are less reactive than ·OH and do not reactith aromatic compounds [67]. The recycle between Fe2+ and Fe3+

re listed in Table 3.Mechanism (IV): Some investigators proposed that the decom-

osition of H2O2 on the surface of iron oxides are mainly throughon-radical mechanism (reaction 3.2–3.4) without the productionf •OH [65,68].

In this experiment, the catalysts displayed stability in severalycles for the low leakage rate, and the catalysts were also charac-erized by XPS and FTIR. According to these results, we supposedhat H2O2 did not affect the structure of the catalysts. The inhibitorxperiments indicated that the main oxidants is •OH. High-valentron species FeIV played little role in the whole process for it do notttack aromatic compounds. Therefore, the whole process of cat-lytic degradation of SMT can be possibly divided into two parts:1) Fe2+ and Mn2+ react with H2O2 to generate surface-bound ·OH,s reactions (6–7), Fe3+ and Mn3+ produced at same time. (2) theeaction initiated by a precursor complex of H2O2 and Fe3+surf OHreaction 9–10).

The cycle of Fe3+/Fe2+ and Mn2+/Mn3+ can be achievedhrough reaction (11–14). On the other hand, the redox poten-ial of E0 (Fe3+/Fe2+) = 0.771 V and E0 (Mn3+/Mn2) = 1.51 V, Mn3+

an be reduced to Mn2+ thermodynamically as reaction (15)60]. The efficient regeneration of Mn2+ could contribute to theemarkable increase of SMT decomposition activity. In summary,e3O4/Mn3O4-RGO composite is a kind of stable material and the

hole degradation of SMT was catalytic process. At last, the organicollutants can be degraded on the catalyst surface or in the bulkolution.

Materials 324 (2017) 653–664 661

The mainly intermediates [24,69,70] identified by GC–MSincluded oxidation of SMT, which are shown in Fig. 10(a). Thedegradation process included: (1) the cleavage of N C bond toform 4-amino-N-carbamimidoyl-benzenesulfanamide (Fig. 10(a)-A), two kinds of intermediates in bracket were deduced, as shownin Fig. 10(a)-A; (2), the cleavage of N-S bond in pyrimidine toproduce sulfanilic acid, the cleavage of azo bond to form sulfanil-amide and 2-amino-4,6-dimethylpyrimidine, and the cleavageof S-C bond to form aniline (Fig. 10(a)-B). During this process,H-abstract and hydroxyl radical oxidation occurred. These resultsevidenced the degradation of SMT can be dominantly ascribedto the results of ·OH attack, which are similar to previous stud-ies [71,72]. With continuing ·OH oxidation, the intermediateswould be oxidized into NH4+, NO3−, CO2 and H2O. Besides, manystudies have been done to investigate the intermediates of SMTdegradation. The other possible degradation pathways (C–E) areshown in Fig. 10(b), including HO•-mediated oxidation of SMT toproduce hydroxylated products (Hydroxylated sulfamethazine, N-(4,6-dimethylpyrimidin-2-yl)-4-hydroxybenzenesulfonamideand 4-amino-N-(4,6-dimethylpyrimidin-2-yl)-3-hydroxybenzenesulfonamide), as shown in pathway C and E[73–76] and SO2 extrusion (pathway D) with 4-(2-imino-4,6-dimethylpyrimidin-1(2H)-yl) (aniline Smiles-type rearrangement[74,77]) and N-(4-aminophenyl)-4,6-dimethyl-2- pyrimidinamine[78] generated.

Fe2+surf + H2O2 + H+ → Fe3+surf + H2O + •OHsurf (6)

Mn2+surf + H2O2 + H+ → Mn3+surf + H2O + •OHsurf (7)

Fe3+surf − OH + H2O2 → (H2O2)S (8)

(H2O2)S ↔ Fe2+surf − •O2H + H2O (9)

Fe2+surf − •O2H ↔ Fe2+surf + HO2• (10a)

(H2O2)S → Fe2+surf + H2O + HO2• (10b)

Fe3+surf + HO2• → Fe2+surf + H+ + O2 (11)

Fe3+surf + H2O2 → Fe2+surf +H+ (12)

Mn3+surf + HO2• → Mn2+surf + H+ + O2 (13)

Mn3+surf + H2O2 → Mn2 surf + HO2• + H+ (14)

Mn3+surf + Fe2 surf → Mn2+surf + Fe3 surf (15)

4. Conclusions

Fe3O4/Mn3O4 – RGO composites were successfully synthe-sized with polyol process and impregnation method, which hadgood catalytic activity in degradation of SMT in heterogeneousFenton-like process. The TEM observations showed that theFe3O4/Mn3O4 are anchored on reduced graphene oxide. The char-acterization with XRD, XPS, VSM, Raman and FTIR showed thegood stability of the catalysts before and after reaction. SMT degra-dation experiments under different conditions showed that 98%degradation rate was achieved under optimal conditions (pH = 3,T = 35 ◦C, Fe3O4/Mn3O4 – RGO composites = 0.5 g/L, H2O2 = 6 mM).The mainly active species was hydroxyl radicals (•OH) on cata-lyst surface. More detailed studies are necessary to investigate thecatalytic mechanism.

Acknowledgments

The research was supported by the National Natural ScienceFoundation of China (51338005) and the Program for ChangjiangScholars and Innovative Research Team in University (IRT-13026).

662 Z. Wan, J. Wang / Journal of Hazardous Materials 324 (2017) 653–664

Fig. 10. Possible pathways of SMT degradation.

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