RESEARCH ARTICLE

Enhanced degradation of sulfamethoxazole by Fe–Mn binary oxidesynergetic mediated radical reactions

Kang Wu1& Xiongyuan Si1 & Jin Jiang1

& Youbin Si1 & Kai Sun1& Amina Yousaf1

Received: 15 November 2018 /Accepted: 25 February 2019# Springer-Verlag GmbH Germany, part of Springer Nature 2019

AbstractIn this study, a novel Fe–Mn binary oxide (FMBO), which combined the oxidation capability of iron and manganese oxides, wasconstructed to remove sulfamethoxazole (SMX) effectively using the simultaneous co-precipitation and oxidation methods, andthe reaction products were probed by liquid chromatography-mass spectrometry (LC/MS). Particularly, FMBO-mediated trans-formation mechanisms of SMX were explored using radical scavengers and electron paramagnetic resonance (EPR). Resultsindicated that the best removal efficiency was obtained at a pH of 4.0, with the H2O2 of 6.0 mmol/L and the FMBO dosage of2.0 g/L, giving 97.6% removal of 10 mg/L SMX within 60 min. More importantly, we found that the hydroxyl (•OH) radicalsgenerated by FMBO through Fenton-like reaction were responsible for the SMX oxidation. EPR studies were confirmed that thepeak intensities of hydroxyl adduct decreased remarkably with increasing pH values. Moreover, the four SMX degradationintermediate products were detected by LC/MS and a reaction pathway for the possible mineralization of SMX, with •OHradicals as the main oxidant, was proposed. These findings provide a novel insight into the removal of SMX by FMBO-mediated radical reactions in aquatic environments. Moreover, this research suggested that FMBO can act as an efficient catalystto remove SMX in hospital wastewater.

Keywords Sulfamethoxazole . Fenton-like reaction . Fe–Mnbinary oxide . Radical mechanism

Introduction

Sulfamethoxazole (SMX), pertains to the sulfonamide class ofantibacterial compounds, is widely used in veterinary and hu-man medicine (Mookherjee et al. 2012). Recently, the SMXconcentration has been tested at levels from 30 to 480 ng/L innature water systems (Heberer et al. 2008; Luo et al. 2010) and2000 ng/L in many urban sewage treatment plants (Andreozziet al. 2010; Bueno et al. 2007). In addition, the SMXwas usedworldwide for treatment of urinary infection (Klamerth et al.

2010), and also reported to result in genetic mutations even inlow concentration (Zhang et al. 2010). The SMX exhibitsbiotoxicity for some algae and fish growth, and also has indi-rect effects on human health (Sanderson et al. 2004). So, thereis a need to develop effective methods to degrade SMX.

In recent years, various techniques such as Fenton-like cata-lytic degradation (Virender et al. 2006; Gao et al. 2016; Liu et al.2018), photo-Fenton degradation (García et al. 2016; Bian andZhang 2016), ozonation catalytic degradation (Gao et al. 2016),and adsorption (Wang et al. 2016) have been applied for theremoval of sulfa antibiotics from water and soil. As it is knownin the conventional Fenton system, Fe2+ ions are likely to formthe precipitates in the form of ferric hydroxide (Fe(OH)3) whichis difficult to dissolve and ultimately limits the usage of catalyst.To overcome this issue, heterogeneous Fenton-like processesusing solid catalysts have been reported in previous studies, suchas Zn–Fe–CNTs (Liu et al. 2018), Fe3O4/Mn3O4 (Wan andWang 2016) , Co–Fe LDH (Bai et a l . 2017) , α -Fe2O3@diatomite (He et al. 2018), Fe–Cu@MPSi (Zhenget al. 2017), and Fe/Cu/Al-pillared clays (Sesegma et al. 2017).

According to previous studies of Zhang and Qu, we havefound a novel Fe–Mn binary oxide (FMBO) catalyst (Zhang

Responsible editor: Vítor Pais Vilar

Electronic supplementary material The online version of this article(https://doi.org/10.1007/s11356-019-04710-4) contains supplementarymaterial, which is available to authorized users.

* Youbin Siyoubinsi@ahau.edu.cn

1 Anhui Province Key Laboratory of Farmland EcologicalConservation and Pollution Prevention, School of Resources andEnvironment, Anhui Agricultural University, Hefei 230036, China

Environmental Science and Pollution Researchhttps://doi.org/10.1007/s11356-019-04710-4

et al. 2007; Liu et al. 2012). They reported that because of thesynergistic effects between ferric oxide and manganese oxidewithin FMBO, it has high oxidative capacity toward tetracy-cline and As(V). Fe and Mn ions are often found together innatural water systems (Huang et al. 2012; Postawa and Hayes2013). Mn oxide minerals have the ability to dissolve humicsubstances (Guo et al. 2009) and to transform organics as wellas heavy metals (Carmichael et al. 2013). In an oxygenatedsystem, Fe(II) may be oxidized to Fe(III) oxide, which com-bines together with Mn oxide to form FMBO. Therefore, itcan be assumed that the combined oxidation abilities of ironoxide and manganese oxide in FMBO would be able to oxi-dize SMX. The prepared FMBO has been well function else-where (Zhang et al. 2009a). However, to our best knowledge,the catalytic behavior of FMBO in the Fenton-like system forSMX degradation has not yet been reported.

The objectives of the present research were (1) to evaluatethe potential of FMBO for the oxidation of SMX in water; (2)to examine the effect of initial solution pH, H2O2 concentra-tion, FMBO dosage on the SMX degradation, and to measurethe reusability of the FMBO for SMX degradation; (3) toidentify the reactive oxygen species (ROS) responsible forSMX degradation; and (4) to determine the intermediate prod-ucts by LC/MS and the degradation mechanism was proposedbased on the experiment results. The research mainly focusedon the effect of FMBO on the degradation of SMX through abatch equilibrium method. The research has obtained the bestremoval efficiency under the optimal degradation condition,indicating that FMBO exhibited satisfactory degradationeffects.

Materials and methods

Materials and chemicals

Sulfamethoxazole (SMX, purity > 98%) was purchased fromAladdin company, Corp. (CA, USA). Magnetite, hematite,goethite, and ferrihydrite (purity > 99%) were purchased fromEmperor Nano Material Ltd., Corp. (Nanjing, China).Analytical grade sulfuric acid and FeSO4•7H2O (purity >99%) were purchased from Aladdin Industrial Ltd., Corp.(Shanghai, China). KMnO4 (purity > 99.5%) was purchasedfrom Yasheng Chemical Ltd., Corp. (Wuxi, China). Aceticacid and acetonitrile were chromatographic grade and pur-chased from Tedia Company (Fairfield, USA). Milli-Q ultra-pure water (18.2 Ω) was used for making all the experimentalsolutions.

Preparation and characterization of FMBO

The FMBO was obtained according to the following steps:0.015 mol/L KMnO4 and 0.045 mol/L FeSO4•7H2O were

separately dissolved in 200 mL of Milli-Q ultrapure water.After that, the solution of FeSO4•7H2O was slowly added intothe KMnO4 solution under vigorous magnetic-stirring.Simultaneously, keep the solution pH in the range of 7.0–8.0, adjusted by 5.0 mol/L NaOH. The suspension was con-tinuously stirred for 1 h, aged for 12 h, and then washed threetimes with Milli-Q ultrapure water. The suspension wasfiltrated and dried at 105 °C for 4 h; then, the FMBO samplewas grinded and preserved in a glass container until use(Zhang et al. 2007).

The morphology of FMBO was characterized by fieldemission scanning electron microscope (SEM, S-4800,Hitachi, Japan). X-ray powder diffraction (XRD) measure-ment was implemented using a SmartLab 9 kW X-ray poly-crystalline diffractometer (Rigaku Co., Japan) at a tube volt-age of 45 kV and a tube current of 200 mA. X-ray photoelec-tron spectroscopy (XPS) was measured via an ESCALAB250Xiphotoelectron spectroscopy (Thermo Fisher Scientific) using AlKα X-ray beam (1486.6 eV). The point of zero charge (pHZPC)of FMBOwas characterized by zeta potentiometer (Nano-ZS90,Malvern, England). Thermogravimetric analysis (TGA) ofFMBO was carried out using a synchronous thermalanalyzer—special for thermogravimetry (STA 6000, Netzsch,Germany). The surface area and pore size distribution were mea-sured by nitrogen adsorption with ASAP 2460 specific surfacearea analyzer (Micromeritics Instrument Corp., USA).

Experimental procedure of SMX degradation

The Fenton-like degradation of SMX by FMBOwas conduct-ed in a 500-mL conical flask using a batch equilibrium meth-od. One hundred milliliters of 10.0–100.0 mg/L SMX,consisting of 3.0–24.0 mmol/L H2O2 and 0.5–5.0 g/LFMBO, was added into each flask and shaken at room tem-perature (25 ± 2 °C) on an incubator shaker 180 r/min.0.1 mol/L H2SO4 was used to adjust the solution pH of desiredvalue (3.0–7.0). At predetermined time intervals, 1.0 mL ofsupernatant was collected, filtered through 0.45-μm mem-brane filter and transferred into a sample vial for high-performance liquid chromatography (HPLC) analysis. The re-action was immediately quenched by adding 1.0 mL salicylicacid (0.72 mmol/L) into the collected sample. Initial solutionpH, H2O2 dosage, FMBO dosage, and the reusability ofFMBO factors were used to investigate their influence onSMX degradation.

To evaluate the radicals in the FMBO and H2O2 system, theradical scavengers test was employed to identify the occur-rence of •OH radicals from FMBO, and the intensities of rad-icals were identified by a JES-FA200 EPR spectrometer(JEOL, Japan). 5,5-dimethyl-1-pyrroline N-oxide (DMPO)was used as a spin trap which mainly with •OH radicals innatural water environment. In addition, the degradation

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mechanism was analyzed and intermediate products wereidentified.

Analytical methods

SMX was detected by Agilent 1220 HPLC equipped with aUV detector, using an Agilent HC-C18 (4.6 × 250 mm, 5 μm)column. Acetonitrile and 0.4% acetic acid solution (70:30)were used as the mobile phases, with flow rate 1 mL/min,column temperature 40 °C, detection wavelength 270 nm,and injection volume 20 μL.

The degradation intermediate products of SMX were de-tected by LC-MS (Agilent 1290 + G6460) with a C18 column(2.1 × 100 mm, 1.7 μm). Acetonitrile and 0.4% acetic acidsolution (70:30) were the mobile phases, and the flow ratewas 0.2 mL/min. The electronic sprayer ionizer was in thepositive mode, with capillary voltage 4.0 kV, pressure45 psi, gas flow 6.0 L/min, and gas temperature 325 °C.

Data analysis

The degradation efficiencies (X%) of the SMX were calculat-ed using the equation (Eq. (1)):

X% ¼ C0−Ctð Þ=C0 � 100 ð1ÞCt ¼ C0 þ Kobse−x=t ð2Þ

where C0 and Ct were the initial concentration and theconcentration at time t respectively (mg/L). The fitting expo-nential function equation is shown in Eq. (2).

Results and discussion

The performance of SMX degradation by FMBOand iron/manganese oxides

To appraise the catalytic ability of the FMBO, magnetite, he-matite, goethite, ferrihydrite, and MnO2, a controlled experi-ment was carried out. The degradation rates of SMX under sixcatalysts were shown in Fig. 1. It can be clearly seen fromFig. 1 that the SMX removal process could be divided intotwo processes. In the initial 30 min, the degradation efficiencywas fast but after 30 min, the degradation efficiency sloweddown. Within 60-min exposure to FMBO, magnetite, hema-tite, goethite, ferrihydrite, and MnO2, the degradation rates ofSMX were 97.6%, 80.4%, 70.6%, 75.2%, 85.8%, and 82.9%,respectively. Therefore, it is concluded that the efficient SMXremoval caused due to the contribution of the FMBO catalystsfunction (Jin et al. 2017). The results illustrated that the deg-radation efficiency of SMX by FMBO was better than theother five iron and manganese oxides.

Effect of initial SMX concentration on SMXdegradation by FMBO

The FMBO was selected as the catalyst to check the effect ofdifferent concentrations of SMX. The effect of the initial SMXconcentration on the degradation of SMX was shown inFig. 2. At a reaction time of 30 min, the degradation rate ofSMX increased rapidly, and then gradually slowed down.After 60-min reaction, with the initial SMX concentration of10.0 mg/L, the degradation rate of SMX reached 97.6%, butwith the increase in the initial SMX concentration to100.0 mg/L, the degradation rate was decreased to 57.5%. Itwas indicated that the increase of initial SMX concentrationhad a negative effect for the degradation efficiency. This phe-nomenon could be explained as the •OH radicals were gener-ated by the decomposition of H2O2 catalyzed by FMBO. Andwhen the initial concentration of H2O2 and FMBO dosageswere already optimized, then the generated amount of •OHradicals was unchanged. The results showed that the generated•OH radicals were sufficient to remove the SMX when theinitial SMX concentration was low, but insufficient in highinitial SMX concentration.

Effect of initial pH on SMX degradation by FMBO

The pH could change the interface properties of theFMBO and affect the degradation efficiency of catalyticreactions (Liu et al. 2017). The effect of pH on the deg-radation of SMX via FMBO was shown in Fig. 3. Whenthe pH increased from 3.0 to 4.0, the removal efficiencyof SMX at 10 min was 98.1% and 88.5%, respectively.The maximum degradation efficiency of SMX was 98.1%at the reaction time of 60 min at pH 3.0, and the degra-dation efficiency of SMX was lowered to 97.6%, 89.9%,78.6%, and 56.6% at pH of 4.0, 5.0, 6.0, and 7.0, respec-tively. As the removal was very quick at pH 3.0 and evenafter 10 min, it comes into an equilibrium state. Whereasat pH 4.0, the removal was gradually increased. From theabove results, optimum pH 4.0 was selected for all otherexperiments.

The solution pH mainly affected the concentration ofdissolved iron and manganese of the removal reaction,which had an impact on the production of •OH radicalsin H2O2 decomposition. Acidic conditions were condu-cive for H2O2 to be decomposed into •OH radicals inFenton-like reaction (Dutta et al. 2001; Panda et al.2011). SMX is a compound of easily ionized, mainly de-termined with the two acid dissociation constants (pKa =1.7 and 5.6). The pKa1 was used to dissociation the groupof the amino (–NH3

+), usually occurred in the low pH (pHranges from 3.0 to 4.0). The pKa2 acting on the sulfanil-amide groups protonation (–SO2NH–), usually occurredin pH (5.0–11.0) (Gao and Pedersen 2005). At different

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pH values, SMX might exist as cation (SMX+), zwitter-ions (SMX0), or negatively charged ions (SMX−) becauseit was an amphoteric molecule with multiple ionizablefunctional groups. At 1.7 < pH < 5.7, the mainly SMXspecies were SMX+ and SMX0. Both the FMBO surfaceand SMX species carried enough positive charge, thusdecreasing electrostatic repulsion and causing an increasein the SMX degradation. At pH > 5.7, the predominatingSMX species were SMX−, both SMX species and theFMBO surface carried more negative charge, causing anenhancement in electrostatic repulsion and decreasing thedegradation efficiency of SMX. The change of degrada-tion rate showed that pH is an important factor forSMX degradation.

Parameter optimization for SMX degradationby FMBO

H2O2 acted as the significant role in the Fenton catalytic reac-tion. The effect of H2O2 concentration on the degradation ofSMX was shown in Fig. 4a. The degradation efficiency ofSMX increased from 72.5 to 97.6% after 30 min with theH2O2 concentration increasing from 3.0 to 6.0 mmol/L. Thedeficiency of H2O2 might cause the •OH radicals decreasing,which would reduce the degradation efficiency (Wan et al.2016). When the H2O2 concentration was increased to18.0 mmol/L, the degradation of SMX was decreased due tothe scavenging reaction by H2O2 itself with •OH radicals gen-erate hydroperoxyl radicals (•OOH); thus, the •OH radicals

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decreased (Eq. (3)). The •OOH radicals could scavenge the•OH radicals and had lower oxidizing ability (Eq. (4)), there-fore decreased the degradation efficiency of SMX. Similarresults were reported by some other researchers (He et al.2018; Panda et al. 2011; Rafael et al. 2017; Ali et al. 2013).

H2O2 þ ⋅OH→H2Oþ ⋅OOH ð3Þ

⋅OOHþ ⋅OH→H2Oþ O2 ð4Þ

Based on the degradation and economic considerations,6.0 mmol/L was decided as the best H2O2 concentration inthe subsequent experiments.

As FMBO exhibited the enhanced catalytic activity forSMX degradation, it was important to further explore the

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Fig. 4 Effect of a H2O2 concentration, b FMBO dosages, c SMOAs, and d reusability of the FMBO on SMX degradation by FMBO

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Fig. 3 Effect of initial pH onSMX degradation by FMBO

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effects of FMBO dosages on SMX degradation. DifferentFMBO dosages on SMX removal were shown in Fig. 4b.As expected, the SMX removal in 30 min increased from84.4 to 97.6% with increase of the FMBO dosage from 0.5to 2.0 g/L. Whereas the FMBO dosage increased from 2.0 to5.0 g/L, the SMX degradation rate did not increase. It mightattribute to that an excess amount of Fe(II) would have anegative effect on SMX removal (Eq. (5)) (ElShafei et al.2010). The results were consistent with the study reportedby Zhang and Bai that the degradation rate was influencedby the dissolved iron and manganese in the reaction (Baiet al. 2017; Zhang et al. 2014).

Fe2þ þ ⋅OH→Fe3þ þ OH− ð5Þ

Small molecular organic acids (SMOAs) could remarkablyaffect the solubility, mobility, and fate of SMX in the naturalwater systems (White et al. 2003). The effects of differentSMOAs (oxalic acid, citric acid, gallic acid, and malic acid) onthe degradation of SMX were shown in Fig. 4c. It could beclearly seen that the four SMOAs would decrease the degrada-tion efficiency of SMX, and the oxalic acid had greater inhibitoryeffect. Through some researches, we have found that the promot-ing effect of the reactivity of SMOAs with •OH radicals wasmainly due to the electrostatic attraction between anionicSMOAs and FMBO surfaces, which could decrease the SMXdegradation. The results showed that the degradation of SMXwas caused mainly on the active sites of FMBO, while theseactive sites of FMBO occupied by SMOAs due to strong elec-trostatic attractions and the reactivity of SMOAs towards •OHradicals lead to the decreased degradation efficiency of SMX(Lin et al. 2017). In conclusion, we speculated that the low accessof H2O2 to the active sites of FMBO causing the inhibitory effectof SMOAs on SMX degradation.

The stability and reusability of FMBO were an importantindex to evaluate the performance of the catalyst ability. Asshown in Fig. 4d, the removal rates of SMX were respectively97.6%, 95.2%, 90.9%, 87.4%, and 84.7%within 30 min for fiverepeated runs, indicating the degradation rate of the catalyst wasrelatively stable. Many factors causing the activity loss of theFMBO, including the following: (1) iron and manganese ionsleaching from the FMBO surface lead to recession of activecatalytic sites (Xu and Wang 2012; Thi et al. 2011), (2) scarceremoval of the byproducts with ultrapure water wash causing theloss of catalytic function of FMBO (Thi et al. 2011; Guo and Al-Dahhan 2006; Zhang et al. 2009a), and (3) aggregation of cata-lysts (Zhang et al. 2009b). This indicated that FMBOwas a kindof stable and reusable catalyst in the SMX removal process.

The metal leaching of catalyst from solid to solutionis a key issue to investigate in the reactions. During theoxidation of SMX by H2O2 in the presence of FMBO,iron and manganese dissolved from the solid into the

solution. The iron- and manganese-dissolved concentra-tions with reaction time are shown in Fig. S1. The Fe3+

ion firstly increased in aqueous solution because Fe3+

ion was gradually dissolved from FMBO under theacidic condition (pH = 4.0), and the dissolved Fe3+ ini-tiated the production of •OH with the consumption ofH2O2, which subsequently oxidized SMX. The concen-trations of both total iron and ferrous iron in theFenton-like oxidation increased from 0 mg/L to thepeak values (12.12 mg/L total iron and 2.34 mg/L fer-rous ion) were obtained at the time of 20 min.Manganese concentration was increased from 0 to6.85 mg/L after 20 min of reaction possibly due tothe conversion of manganese by H2O2 and then bal-anced continuously until the end of reaction. The pos-sible free radical initiation reactions which may happenduring the oxidative degradation are given below.

Identification of free radical species on SMXdegradation by FMBO

In order to identify the reactive oxygen species (ROS)involved in the SMX degradation by FMBO, the radicalscavenger experiments were conducted. The •OH andO2

•− radicals produced by FMBO in the catalyst systemhad already been confirmed by trapping experimentsusing tertiary butyl alcohol (TBA) and vitamin C (AA)as radical scavengers (Liu et al. 2017; Kun and Ting2017). As revealed in Fig. 5a, compared with scaven-ger-free, the degradation rate of SMX was obviouslyinhibited in the presence of TBA and AA, and the in-hibition effect followed the order of TBA > AA. Theseresults indicated that the formation of •OH radicals wasa crucial factor for degradation of SMX by FMBO.Figure 5a showed that 97.6% of SMX was degradedin 30 min in the absence of TBA and AA. When20 mg/L TBA was added, the degradation efficiencywas significantly decreased to 51.5%; further, increasein the concentration of TBA to 40 mg/L, the removalefficiency of SMX was decreased to 20.2%. In addition,when 20 mg/L and 40 mg/L AA was added, the remov-al efficiency of SMX was decreased to 84.8% and80.7%, respectively. Therefore, the results show thatSMX was removed mainly through the •OH radicalpathway (Bai et al. 2017).

Electron paramagnetic resonance (EPR) analysis was con-ducted to further verify the occurrence of •OH radicals fromFMBO and their intensities, during the SMX degradation atpH 4.0, 5.0, 7.0, and 8.0. As shown in Fig. 5b, the EPR spectraexhibited a fourfold characteristic peak of DMPO-OH with anintensity ratio of 1:2:2:1 which were observed at pH values of

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4.0, 5.0, 7.0, and 8.0, indicating that •OH radicals were pro-duced in the FMBO/H2O2/SMX system under a wide range ofpH values (Hirakawa and Nosaka 2002). It is noted that thepeak intensities of DMPO-OH decreased obviously with in-creasing pH values, suggesting acidic conditions were morebeneficial to •OH radical formation. This study has indicatedthat the higher degradation efficiency of SMX by FMBO un-der acidic conditions because of the generation of more •OHradicals in comparison to basic conditions. Tian et al. (2017)also evaluated •OH radical concentration by varying the pHfrom 3.6 to 10 over a modified mesoporous iron oxide (Tianet al. 2017); Jin et al. (2017) suggested that the removal ofofloxacin by Cu substituted magnetic Fe3O4@FeOOH nano-composite occurred because of the generation of more •OHradicals (Jin et al. 2017).

Degradation intermediates and possible degradationmechanism

Due to the significant interaction among the iron ions andmanganese ions causes the remarkable enhancement incatalytic ability. Therefore, the possible mechanism couldbe proposed as follows:

Firstly, Fe2+/Mn2+ initiated the decomposition of H2O2

to •OH radicals (Eqs. (6) and (7)) (Hara 2017). In addi-tion, the elongated O–O bond of H2O2 due to moderateoxygen vacancies becomes activated and easier todecomposed into •OH and O2

•− radicals (Kumar et al.2010). Secondly, the Fe3+/Mn3+ was reduced to Fe2+/Mn2+, but the kinetic rate constants associated with thisstep were generally very small (Eqs. (8) and (9)). Thirdly,the addition of manganese ions can enhance the catalyticactivity, the spontaneous electron flow from Mn3+ to Fe3+,favored to the Fe3+ conversion to Fe2+ because of themultivalence of iron and manganese ions. In addition,since the potential of E0 (Mn2+/Mn3+) = 1.51 V, E0

(Fe2+/Fe3+) = 0.771 V, Mn3+ was reduced to Mn2+ byFe2+ (Eq. (10)) (Wan et al. 2016). Finally, the •OH

radicals attacked the SMX in the solution, and the Fe3+/Mn3+ was reduced to Fe2+/Mn2+ by intermediate productradicals (Eqs. (11) and (12)).

Fe2þ þ H2O2 þ Hþ→Fe3þ þ H2Oþ ⋅OH ð6ÞMn2þ þ H2O2→Mn3þ þ H2Oþ ⋅OH ð7ÞFe3þ þ H2O2→Fe2þ þ HO2⋅þ Hþ ð8ÞMn3þ þ H2O2→Mn2þ þ HO2⋅þ Hþ ð9ÞMn3þ þ Fe2þ↔Mn2þ þ Fe3þ ð10ÞRHþ ⋅OH=O2

⋅−=HO2⋅→H2Oþ R⋅ ð11ÞRHþ ⋅OH=O2⋅−=HO2⋅→H2Oþ R⋅ ð12Þ

Based on the mechanism above, Mn enhanced the removalefficiency by producing •OH radicals (Eq. (7)) which facili-tated the cycle of Fe2+/Fe3+ and Mn2+/Mn3+, thus enhancedthe electron transfer in the FMBO and H2O2 system (Wanet al. 2016).

To elucidate the degradation mechanism of SMX in theFMBO synergetic mediated radical reactions, total ionchromatograph (TIC) was used to identify the intermedi-ate products (Fig. S2). A possible degradation pathway ofSMX with •OH radicals as the main oxidant was shown inFig. 6. The degradation processes of the SMX could bedivided into four processes: firstly, different sites of theSMX molecule attacked by •OH radicals and form hy-droxylated derivatives. Secondly, as was shown, •OH rad-icals attacked the amino group on the benzene ring andoxidized it to a NH2O– group, then to form intermediateA (m/z = 270) (Niu et al. 2013). Meanwhile, the generated•OH radicals attacked the sulfonyl group to break the S–Nbond to form intermediate B (m/z = 99) and intermediateC (m/z = 174) (Chansik et al. 2015). Finally, due to thestrong oxidizing ability of FMBO, the generated •OH rad-icals continued to oxidize the para amino group and brokethe C–N bond to form intermediate D (m/z = 143)(Virender et al. 2006; Liu et al. 2018).

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Fig. 5 a Effect of radical scavengers on the degradation of SMX and b the EPR spectra of FMBO collected at different pH values

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The degradation of SMX in real wastewater by FMBO

The above experimental results have demonstrated thatFMBO can serve as a catalyst for chemical oxidant (H2O2)to remove SMX, and therefore could be used to treatantibiotic-contaminated wastewater. We detected that theSMX concentration of 36 ng/L in hospital wastewater, andthen SMX was added to the hospital wastewater making theSMX concentration of 0.1, 0.2, 0.4, 0.8, and 1.6 mg/L, respec-tively. Some quality parameters of real wastewater are shownin Table. S1. As shown in Fig. 7, with the initial SMX con-centration of 0.1 mg/L, the degradation rate of SMX reached100% after 10-min reaction; then, with the increase in theinitial SMX concentration to 1.6 mg/L, the degradation rate

was reached 92.8%. Hence, this research suggested thatFMBO can act as an efficient catalyst to remove SMX andtreat antibiotic pollution in wastewater.

Characterization of FMBO

The surface morphology of prepared FMBOwas examined bySEM was shown in Fig. 8. SEM image (× 3000 and × 30,000magnification) of the fresh FMBO was shown in Fig. 8a,which depicts the structure of the FMBO surface. The imageshows that many small particles aggregate and constitute theFMBO, which led to a porous structure and rough surface. Asshown in Fig. 8b, after reaction with SMX, the surface ofFMBO had a floccule-like membrane and a globular blurred

SMX; m/z = 254 Intermediate A; m/z = 270

Intermediate B; m/z = 99

Intermediate C; m/z = 174

Intermediate D; m/z = 143

Fig. 6 Proposed reaction pathways of Fenton-like reaction mechanism in the FMBO and H2O2 system

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0.6

0.8

1.0

0.1 mg/L 0.2 mg/L

0.4 mg/L 0.8 mg/L

1.6 mg/L

C/C

0

Time (min)

Fig. 7 The pseudo-first-orderkinetic equation fitting curves ofSMX degradation by FMBO inreal wastewater

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outline. These characteristics might be owing to the degrada-tion of SMX by FMBO on the surface. The point of zerocharge (pHpzc), reflecting acidic and basic groups on the sur-face of FMBO, was determined to be 6.52 before degradationand 5.06 after degradation (Fig. S3). The variation in pHpzc

indicated that the surface of FMBO carried more acidic groupsafter degradation. Thermogravimetric analysis (TGA) curvesof FMBO in different temperatures are shown in Fig. S4.According to Fig. S4, it could be seen that approximately16.26% and 19.29% (fresh and used) of overall weight losswithin temperature range from 30 to 990 °C. The N2

adsorption-desorption isotherms and pore size distributionsof FMBO are given in Fig. S5. The pore size distributionwas calculated using the Barrett-Joyner-Halenda (BJH) meth-od. The surface area (SBET), pore volume (VP), and averagepore size (Da) were 103.52 m2/g, 0.16 cm3/g, and 8.93 nm,respectively.

XRD employed to analyze the composition of freshlysynthesized FMBO sample and used FMBO for the deg-radation test and results were shown in Fig. 9. Asdepicted in Fig. 9 (fresh), the five characteristic strongpeaks of FMBO at 2θ = 21.2°, 35.4°, 43.3°, 56.9°, and

62.9°, corresponding to diffraction from the (110), (311),(400), (511), and (440) of the structure of FMBO.However, after the catalytic reaction, the FMBO catalystpeaks showed a differed pattern as shown in Fig. 9 (used);the intensities for five characteristic strong peaks ofFMBO were decreased and two other characteristic peaksof FMBO at 2θ = 24.9° (Mn2O3) and 36.6° (γ-Fe2O3)were further produced (Cui et al. 2014). Although thepeak intensity was weak after the use of FMBO, it meanta small amount of Fe(III) and Mn(III) formed on its sur-face. Since FMBO is a kind of reduction material, H2O2

might react with FMBO and resulted in the formation ofFe(III) and Mn(III) on the surface of FMBO by the trans-fer of •OH radicals from H2O2 to FMBO.

XPS employed to analyze the surface element distribu-tions and interfacial electronic interactions of FMBO andresults were shown in Fig. 10. Figure 10a displays thecomparison of the survey spectra of freshly synthesizedFMBO sample and used FMBO. In Fig. 10b (fresh), twobroad peaks around 711.8 eV and 725.5 eV assigned toFe2p3/2 and Fe2p1/2 were identified, indicating the exis-tence of Fe2+ and Fe3+ on the FMBO surface. In Fig. 10c

10 20 30 40 50 60 70 80

γ-F

e2O

3

Mn

2O

3

MnF

eO

4

MnF

eO

4MnF

eO

3

MnF

eO

3

Used

).u.

a(

ytis

net

nI

2 theta (degree)

Fresh

MnF

eO

3

Fig. 9 XRD pattern of FMBO

a b

10 µm 10 µm

1 µm 1 µm

Fig. 8 SEM image of FMBO (abefore degradation and b afterdegradation of SMX)

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(used), the intensity of Fe2+ peaks for used FMBO ismuch weaker than that of fresh FMBO, and the peakshape indicated that the change in oxidation state ofFe2+ to Fe3+. In Fig. 10d (fresh), two broad peaks around643.1 eV and 654.2 eV assigned to Mn2p3/2 and Mn2p1/2were detected, indicating the presence of Mn2+, Mn3+, and

Mn4+ on the surface of FMBO. In Fig. 10e (used), theintensity of Mn2+ peaks for used FMBO is much weakerthan that of fresh FMBO, and the peak shape indicatedthat the oxidation state of Mn changed from + II to + III,and the decrease of binding energy could be ascribed tothe increased proportion of reduced Mn2+ species on the

0 100 200 300 400 500 600 700 800 900 1000 1100

0

50000

100000

150000

200000

250000

300000

350000

400000

Fe-Mn compound

Used

O1s

Fe2pFe2p

Mn2p

Mn2p

N1s

O1s

C1s

Mn3s

Fe3p

s/st

nu

oC

Binding Energy (eV)

Fresh

a

700 705 710 715 720 725 730 735 740

20000

25000

30000

35000

40000

45000

50000

55000

60000

Fe( )

Fe( )

Fe( )

Fe2p1/2

Fe2p3/2

s/st

nu

oC

Binding Energy (eV)

Fresh

700 705 710 715 720 725 730 735 740

20000

25000

30000

35000

40000

45000

50000

55000

Fe( )

Fe( )

Fe2p3/2

s/st

nu

oC

Binding Energy (eV)

Fe( )

Fe2p1/2

Used

636 639 642 645 648 651 654 657 660

16000

18000

20000

22000

24000

26000

28000

30000

Mn( )Mn( ) / Mn( )

Mn( )

Mn( ) / Mn( )

Mn( )

Mn( )

Mn2p1/2

Mn2p3/2

s/st

nu

oC

Binding Energy (eV)

Fresh

636 639 642 645 648 651 654 657 660

16000

18000

20000

22000

24000

26000

28000

30000

Mn( ) / Mn( )

Mn( )

Mn( ) / Mn( )

Mn( )

Mn2p1/2

s/st

nu

oC

Binding Energy (eV)

Mn2p3/2

Mn( )

Mn( )

Used

b c

d e

Fig. 10 XPS analyses of FMBO: a full-survey, b Fe2p core-level spectra before degradation of SMX and c after degradation of SMX, dMn2p core-levelspectra before degradation of SMX and e after degradation of SMX

Environ Sci Pollut Res

surface of the FMBO (Zhang et al. 2007). The resultsshowed that the Fe and Mn elements in the fresh FMBOwere in the oxidation states + II.

Conclusions

In this study, a novel Fenton-like system was developed torapidly degrade SMX via the generated •OH radicals fromthe reaction of FMBO and H2O2. It was shown that pH value,SMX concentration, H2O2 concentration, FMBO dosage, andthe reusability of the FMBO effected the degradation rate ofSMX. The proposed degradation mechanism of SMX inFMBO and H2O2 systems was validated by examining theeffect of radical scavengers as well as EPR analysis involving•OH radicals. The results obtained in this study were essentialto further design and utilize FMBO for Fenton-like reactions.This research offers the feasibility of FMBO acted as a prom-ising functional material for SMX removal. The application ofnovel FMBO to treat antibiotics has high degradation efficien-cy in wastewater.

Funding information This work was financially supported by theNational Natural Science Foundation of China [grant number 41471405].

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