Facile and green synthesis of highly dispersed tar-based heterogeneous Fenton catalytic nanoparticles for the degradation of methylene blue

Journal of Cleaner Production xxx (xxxx) xxx

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Facile and green synthesis of highly dispersed tar-based heterogeneous Fenton catalytic nanoparticles for the degradation of methylene blue Dong Li a, b, Tianxue Yang c, Yi Li d, Zhengang Liu a, b, *, Wentao Jiao a, b, ** a

Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, Beijing, 100085, China University of Chinese Academy of Sciences, Beijing, 100049, China State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100012, China d State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 August 2019 Received in revised form 29 September 2019 Accepted 24 October 2019 Available online xxx

Thermochemical conversion of biomass produces tar, as an inevitable byproduct, which was a potentially precursor for carbon material preparation. This study provided an approach of preparation of tar-based Fenton-like catalytic nanoparticles from waste biomass tar via green solvent-free approach. The monoand bimetallic nanocatalysts were synthesized from biomass tar and metal salts by ball milling combined with calcination. The synthesized catalysts were utilized for H2O2 decomposition and subsequent methylene blue (MB) degradation in aqueous solution. The results showed that the presence of tar resulted in the porous structure and defective graphite layers on the surface of resultant catalysts. The bimetallic nanocatalysts had higher H2O2 decomposition rate than monometallic nanocatalysts and among all the catalysts, the bimetal catalyst prepared at 300
C (FeNi/C-300) showed the highest rate constant (10.55  103 min1) for the decomposition of H2O2. This high catalytic activity of FeNi/C-300 was attributed to the promotion of Fe3þ reducing to Fe2þ due to accelerated electron exchange by the introduction of nickel and the formation of defective graphite layers. For MB degradation, the monometal nanocatalyst prepared at 300
C (Fe/C-300) and FeNi/C-300 showed high removal efficient in the degradation of methylene blue, and the highest removal efficiency reached 100% and 98%, respectively. This study offered a promising approach for the preparation of heterogeneous Fenton catalytic nanoparticles for organic pollutant removal using waste biomass tar and by green synthesis method. Most importantly, this approach has the potential to be extended for other metal based Fenton catalytic nanoparticles synthesis. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: M.T. Moreira Keywords: Nanoparticles Biomass tar Fenton catalyst Hydrogen peroxide Metallic oxide

1. Introduction With the continuous development of science and society, the limited resources on earth are being consumed constantly, and utilization of waste materials has been attracting increasing attention. The waste resources are always with complex composition and it is necessary to develop utilization approach according to the characteristics of different wastes. Pyrolysis tar is a by-product generated during thermochemical conversion (such as pyrolysis and gasification) of biomass with the yield up to more than 40%

* Corresponding author. 18 Shuangqing Road, Beijing, 100085, China. ** Corresponding author. E-mail addresses: [email protected] (Z. Liu), [email protected] (W. Jiao).

dependent on the conversion conditions and the type of biomass (Neves et al., 2011; Worasuwannarak et al., 2007). The presence of pyrolysis tar could foul and clog facilities, and cause serious pollution to the environmental. Therefore, extensive research have been conducted focused on reducing its yield (such as catalytic cracking and thermal cracking) (Anis and Zainal, 2011; Liu et al., 2018; Sun et al., 2017). Pyrolysis tar is a kind of complex, multicompound and condensable organics rich of aromatic compounds and featured by high carbon content, low ash content and thermoplastic. The conjugated aromatic structure in the tar can induce the interaction of van der Waals, hydrogen bonds and charge transfer to form porous aggregates (Jalilov et al., 2015). In addition, pyrolytic tar can be used as stabilizer, template agent, etc., due to the presence of organic functional groups and the wide range of

https://doi.org/10.1016/j.jclepro.2019.119033 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

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molecular weight (Filiciotto et al., 2017). For all this, the existing reports have provided the feasibility of preparing low-cost carbon materials by treating pyrolysis tar as a carbon precursor. Advanced oxidation processes (AOPs), such as photocatalysis and Fenton reactions, can provide highly reactive and non-selective oxidants like hydroxyl radicals (OH) (Hama Aziz, 2019) and as effective and promising organic wastewater treatment processes, AOPs have the advantages of high oxidation efficiency and environmental friendliness (Hien et al., 2020). Fenton process has been widely adopted due to its low cost, wide applicability and simple operation. It is the most common method that using Fe2þ as Fenton reagents to catalyze the decomposition of hydrogen peroxide (H2O2) to produce highly reactive hydroxyl and hydroperoxyl radicals and then oxidize organic pollutants from wastewater (Fu et al., 2014; Ghasemi et al., 2015; Ouyang et al., 2019). However, this homogeneous catalyst requires a highly acidic environment and is difficult to separate the dissolved iron from the reaction solution (Georgi et al., 2007; Tartaj et al., 2011). To overcome these drawbacks and improve catalytic activity, iron oxides, as heterogeneous Fenton-like reagents, have been studied extensively (Farshchi et al., 2018; Qin et al., 2018; Xiao et al., 2018). The studies show that the properties of catalysts were closely related to the structure of materials, including particle size, porosity, crystallinity and specific surface area. The soft and hard template were necessary to prepare active catalysts with expected structure (He et al., 2013). Generally, the morphology of materials such as particle size or crystal structure is subject to change during the calcination process. The synthesis of iron oxide based catalysts is difficult to control, especially the formation of nano-sized or micron-sized porous iron oxides. Therefore, it is necessary to find a simple and environmentally friendly route to realize the preparation of iron oxide with desirable morphology. The synthesis of bimetallic Fenton-like catalysts with high catalytic activity has been attracting wide interest due to the improved the performance by the synergistic effect between two metals (oxides) (Wang et al., 2014; Xu and Wang, 2012). Several studies have reported the properties and mechanism of composite nickel and iron materials. For example, Xie et al. demonstrated that doping Ni0 in Fe3O4 promoted the reduction of Fe3þ to Fe2þ, and subsequently accelerated the decomposition of H2O2 to remove DDT (Xie et al., 2017). Li et al. obtained a bimetallic catalyst with uniform size and homogeneous structure by combining nickel and iron oxide, and NiO exhibited positive contribution on high catalytic activity (Li et al., 2015). Zhang et al. proposed that NiFe2O4 has excellent catalytic performance in Fenton process due to the rapid electron exchange between Ni2þ and Fe3þ (Zhang et al., 2017). These reports proved that the addition of nickel could improve the catalytic performance of Fenton-like catalysts. The preparation methods in these reports all required the aid of solvents, and there is no reports of solvent-free preparation of bimetallic Fenton-like catalysts to date. Herein, the main purpose of this study was to demonstrate the feasibility of synthesizing tar-based mono- and bimetallic catalyst by mechanochemical synthesis method under green solvent-free conditions. In addition, the effect of different calcination temperatures on the structure of the catalyst was investigated. Moreover, the efficiency of catalyst was determined by catalytic decomposition of H2O2 to remove methylene blue in aqueous solution. This study was helpful to achieve green utilization of waste carbon materials towards environmental application. 2. Experimental 2.1. Materials Biomass tar was supplied by a biomass pyrolysis plant located in

Pingyao, Shanxi Province, China. Ferric nitrate nonahydrate (Fe(NO3)3$9H2O, AR, 98.5%), nickel nitrate hexahydrate (Ni(NO3)2$6H2O, AR, 98%) and hydrogen peroxide (H2O2, AR, 30%) were purchased from Sinopharm Chemical Reagent, China. Methylene blue (MB) was purchased from Macklin Reagent Company. All chemicals were used without further purification. 2.2. Catalyst preparation For mono Fe-based catalyst preparation, the biomass tar was crushed and then mixed with Fe(NO3)3$9H2O in a planetary ball mill (F-P2000, FOCUCY) at a mass ratio of 5:2. The mixtures were milled at 400 rpm (ran for 10 min, stopped for 5 min for 6 times) to ensure that the heat from milling could softened the tar and promoted the Fe(NO3)3 evenly dispersed and coated by the carbonaceous material. The samples were dried overnight at 105
C and followed by heating (5
C/min) to desired temperatures (300, 400 and 500
C) for 4 h. The final materials were named as Fe/C-300, Fe/ C-400 and Fe/C-500 corresponding to the calcination temperature, respectively. For the bimetallic catalyst, the biomass tar, Fe(NO3)3$9H2O and Ni(NO3)2$6H2O were mixed with a mass ratio of 5:1:1. The mixture was treated by identical conditions with the monometal catalyst. The resultant composite were named as FeNi/C-300, FeNi/C-400 and FeNi/C-500 corresponding to the calcination temperature, respectively. For comparison, the catalysts were synthesized from metal salt precursors at same ratio (without tar) under identical procedure. These samples were named as Fe-300, Fe-400, Fe-500, FeNi-300, FeNi-400 and FeNi-500 corresponding to the calcination temperature, respectively. 2.3. Catalyst characterization Nitrogen adsorption/desorption isotherms at 196
C were analyzed by using a Quadrasorb SI automatic specific surface area/ pore size distribution analyzer. Prior to the analysis, all the samples were degassing at 120
C for 3 h. The specific surface area was calculated by BrunauereEmmetteTeller model, whereas the pore volume was determination by N2 desorption branch. The pore size distribution was calculated by the DFT method, and the micropore surface area and micropore volume were determined by t-plot method. The Bruker Advance D8 diffractometer with CuKa radiation (l ¼ 0.15406 nm) was employed for X-ray diffraction (XRD) experiments, and the XRD patterns were introduced in the range of 10
 2q  90
with a step size of 0.02
and a scanning rate of 5
/ min. The phase composition was obtained by using the X’Pert HighScorePlus software and the JCPDS-PDF-4 database. The average crystallite size was obtained from Debye-Scherrer method (Tanhaei et al., 2015). Scanning electron micrographs (SEM) were carried out on a JEOL JSM-7500F microscope equipped with an Energy Dispersive Spectrometer (EDS) at 15 kV. High-resolution transmission electron micrographs (HR-TEM) were recorded on a JEOL JEM 2100 instrument at 200 kV. Prior to the analysis, the powder samples were dispersed in ethanol (treated in sonicator bath for 5 min) and dried on a holey-carbon-coated copper grid at room temperature. Raman spectra were recorded on a Horiba LabRam HR800 instrument at room temperature. Thermogravimetric analysis (TG) of the biomass tar was performed on a NETZSCH TG 209 thermo-gravimetric analyzer in the range of 40e800
C at the heating rate of 10
C/min under air flow. The inductively coupled plasma-optical emission spectroscopy (ICPOES) was performed on PerkinElmer 3000DV. The mineralization degree was investigated by the total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan).

Please cite this article as: Li, D et al., Facile and green synthesis of highly dispersed tar-based heterogeneous Fenton catalytic nanoparticles for the degradation of methylene blue, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119033

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2.4. Catalytic testing The performance of as-prepared catalysts were evaluated by H2O2 decomposition experiment, and the experiments were carried out in 250-ml conical flasks at 25
C. The mixtures of 20 mM aqueous H2O2 solution (pH z 6.5) and 1 g/L as-prepared catalyst powders were placed in a thermostatic shaker to ensure the dispersion of catalyst particles. The samples were extracted by syringes at preset time interval and the suspensions were filtered by 0.45 mm filter membranes (polyether sulfone). The concentration of H2O2 was determined by permanganate titration after filtration of

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the solution samples extracted at different time. The decomposition of H2O2 catalyzed by iron-based catalysts follows the firstorder rate kinetics law: ln([H2O2]/[H2O2]0) ¼ kt, where [H2O2] and [H2O2]0 are the concentrations of H2O2 at time t and time zero, and k is the first-order rate constant (Hermanek et al., 2007). The catalytic activity of as-prepared catalysts for degradation of organic dye was assessed, and the catalysis experiments were carried out in 150-mL conical flasks with stopper. In a typical experiment, the reactor was filled with 50 mL of MB solution (MB concentration: 30 mg/L) and 50 mg of the catalyst. After mixing the MB solution with catalyst powder, a certain amount of H2O2 was

Fig. 1. XRD patterns of (a) Fe/C-series and Fe-series; (b) FeNi/C-series and FeNi-series.

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added to ensure the concentration was 0.1 M and the initial pH was adjusted to 7. The mixture was placed in a thermostatic shaker of 25 and 45
C, respectively. Subsequently, 2 mL of the mixture was accurately extracted every 10 min and was filtered through 0.45 mm filter membranes (polyether sulfone). The filtrate concentration was determined by Shimadzu UV-1800 at 663 nm. To evaluate the effect of initial pH on degradation of MB, the initial pH value was adjusted to 3, 5, 7, 9 and 11 by 50 mM phosphate buffer solutions at 25
C and the rest of experimental conditions were identical as mentioned above. To explore the effect of the H2O2 concentration, the concentration of H2O2 was changed from 0.1 M to 0.01 M, and other conditions were not changed. In addition, the blank test of H2O2 decomposition and MB adsorption was carried out for comparison. The stability of the catalyst is of great significance to its performance, the catalysts (Fe/C-300 and FeNi/C-300) were re-used 5 times at 25
C. In addition, the effects of pH on the stability of catalysts were evaluated at pH 3 and 7, respectively. After each run, the used catalyst was washed with deionized water three times. After washing, the catalyst was dried at 105
C for 24 h. The solution concentration at 60 min was taken as the result for evaluation.

3. Results and discussion 3.1. Characterization of prepared catalysts During calcination process, biomass tar will lose mass due to the oxidation of its active components. The TG curve of biomass tar was shown in Fig. A1. In this study, the biomass tar lost about 20% of mass weight before 300
C, and the mass loss was significantly increased to 80% at 400
C. With further increased temperature, almost all biomass tar was decomposed and oxidized. The change of mass loss with temperature indicated that the tar had high reactivity (mainly ascribed to high oxygen content) and 300
C was appropriate carbonization temperature for preparation of tar-based materials. Fig. 1 showed XRD analysis patterns of the synthesized catalyst materials and Table 1 showed the crystallite sizes of all the samples calculated by Debye-Scherrer method. As shown in Fig. 1a, the intense and narrow peak indicated high degree of crystallinity for all the samples. The peaks around 24.13
, 33.14
, 35.59
, 40.83
, 49.42
, 54.04
, 57.39
, 62.38
and 63.93
in XRD patterns for Fe/Cseries corresponded to (012), (104), (110), (113), (024), (116), (122), (214) and (300) planes of a-Fe2O3 (PDF card: # 89e2810), respectively. The peak strength was increased gradually by increasing the calcination temperature from 300 to 500
C, indicating the increased crystallinity and the nanoparticle size of a-Fe2O3. The Table 1 The crystallite sizes of the prepared catalysts and the rate constants for the decomposition of H2O2 (Reaction condition: [H2O2]0 ¼ 20 mM, [catalyst] ¼ 1 g/L, pH ¼ 6, T ¼ 25
C.). Catalyst

Crystallite size (nm)

Rate constant (min1)

Fe/C-300 Fe/C-400 Fe/C-500 Fe-300 Fe-400 Fe-500 FeNi/C-300 FeNi/C-400 FeNi/C-500 FeNi-300 FeNi-400 FeNi-500

18.8 nm 33.6 nm 50.8 nm 28.5 nm 32.5 nm 40.4 nm 11.3 nm 28.0 nm 37.5 nm 16.7 nm 18.6 nm 22.4 nm

9.71 ± 0.26  103 3.85 ± 0.08  103 2.23 ± 0.13  103 2.04 ± 0.13  103 1.70 ± 0.12  103 1.14 ± 0.11  103 10.55 ± 0.06  103 5.53 ± 0.08  103 2.72 ± 0.04  103 5.62 ± 0.08  103 2.70 ± 0.08  103 2.11 ± 0.16  103

diffraction peak at 26.60
in the XRD pattern was originated from the characteristic (002) plane of the graphite carbon. The unexpected presence of graphite at these low temperatures was mainly ascribed to vital important catalytic role of Fe in the graphitization reaction of the tar (Yan et al., 2019). The catalytically active metals can lower the energy barrier for the solid-state transformation from amorphous carbon to graphite carbon. Therefore, the graphite layer was nucleated and grew on the metal surface. The gradual decrease of graphite peak with the increased temperature was caused by the increasing oxidation of carbon during calcining process. The calculated crystallite sizes of Fe/C-series and Fe-series ranged in 18.8e50.8 nm and 28.5e40.4 nm, respectively. By comparing XRD patterns of Fe/C-series and Fe-series, it was observed that the presence of biomass tar enhanced the influence of temperature on crystallinity of iron oxide. At low calcining temperature of 300
C, the coating of tar inhibited the growth of iron oxide particles and the resultant nanoparticles showed a low crystallinity, resulting in the smallest particle size (18.8 nm). At calcining temperature of 400
C, the tar was removed by oxidization to release the space, and the small particles of iron oxide were aggregated and thermally recrystallized, resulting in a high crystallinity. The sample with the largest particle size (50.8 nm) was obtained at calcination temperature of 500
C. In Fig. 1b, the peaks around 37.22
, 43.25
and 62.83
corresponded to (111), (200) and (220) planes of NiO (PDF card: # 73e1523), respectively. The peaks around 33.14
and 49.42
corresponded to (104) and (024) planes of a-Fe2O3 (PDF card: # 89e2810), respectively. In addition, the diffraction peak at 30.31
, 35.70
, 53.81
and 57.39
in the XRD patterns for bimetallic nanocatalysts were caused by the characteristic (220), (311), (422) and (511) planes of the NiFe2O4 (PDF card: # 54e0964), respectively. The graphite phase, a-Fe2O3 phase, NiO phase and NiFe2O4 were all observed in bimetallic nanoparticles. The crystallinity of bimetallic oxides was also increased with the increase of calcination temperature. The crystallite sizes of FeNi/C-series and FeNi-series were around 11.3e37.5 nm and 16.7e22.4 nm, respectively. This calculated result indicated that the addition of tar had significant influenced on the nanoparticle growth of bimetallic catalyst at 300e500
C. Therefore, it was safely concluded that particle size of metal oxide could be accurately manipulated by adjusting calcination temperature and addition of the tar. TEM images (Fig. 2) showed that metallic oxide nanoparticles were well dispersed on as-prepared catalysts. The observation was mainly caused by the homogeneous distribution metal precursor within carbonaceous materials by softening the tar by the heat generated during ball-milling process. The nanoparticle sizes observed in Fig. 2 were in accordance with the calculated crystallite sizes shown in Table 1. The size of nanoparticles in Fe/C-300 mainly fall in the range of 4e9 nm. With the increase of the temperature, the sizes of nanoparticles in Fe/C-400 and Fe/C-500 were increased to 12e25 and 30e50 nm range, respectively. As for the FeNi/Cseries, the nanoparticle size showed similar trend as Fe/C series and the sizes of the nanoparticles was increased from range of 5e15 to 20e36 nm with the increase of temperature from 300 to 500
C. In Fig. 2c and d, HR-TEM observations of Fe/C-300 and FeNi/C-300 showed that nanoparticle surface was covered with the curved graphitic layers with amorphous carbon regions. The translucent layer at the edge indicated the presence of carbon on the surface of the catalyst and these observations was in accordance with the XRD analysis. In addition, it can be clearly observed from HR-TEM images that the interplanar spacing distance between adjacent lattice fringes in Fe/C-300 and FeNi/C-300 were 0.27 nm and 0.25 nm, respectively. The values corresponded to the body-centered cubic lattice in the (104) plane of Fe2O3 and face-centered cubic lattice in the (311) plane of NiFe2O4, respectively.

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Fig. 2. TEM micrographs of the synthesized catalysts (a, b, e, f, g and h) and HRTEM images of Fe/C-300 (c) and FeNi/C-300 (d).

The microstructure of Fe/C-300 and FeNi/C-300 were displayed in Fig. 3. The morphology showed that the nanoparticles with different sizes were clearly present on the surface of the catalysts. The irregular carbon surfaces in Fe/C-300 and FeNi/C-300 were ascribed to incomplete oxidation of the tar at 300
C. The unique pore structure was formed by the spontaneous foaming characteristic of tar (Li et al., 2019). Because of the high contents of hydrogen and oxygen, certain amount of gas were generated and emitted through dehydrogenation and deoxygenation of biomass tar (X. Hu et al., 2018). The tiny particles on these porous surfaces showed that the iron precursor was exposed by the oxidization and decomposition of carbon, and then iron oxide was formed.

Therefore, it could be concluded the coating of tar on metal precursor inhibited the growth of metal oxide during heat treatment. Similar to the trend shown by TEM images, the crystal size was increased with the increase of temperature. For FeNi/C-400, the particles sizes were 30e100 nm, while the maximum particle size was increased to 250 nm in FeNi/C-500. It should be noted that the surface structures of the catalysts were complex due to the oxidation of tar and the growth and agglomeration of metal oxides. The incomplete oxidation of the tar led to the most complex surface structure prepared at 300
C. In summary, calcination temperature and presence of biomass tar significantly affected the nanoparticle size of the catalysts. In addition, it is clearly observed that the cubic

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Fig. 3. SEM micrographs of the synthesized catalysts (Fe/C-series: a-c; FeNi/C-series: d-f).

structure from the FeNi/C-500 SEM image (shown in Fig. 3f), indicating that the bimetallic oxides were fully exposed from the catalyst surface. The results of elemental mappings showed that the element oxygen, iron and nickel distribution well matched up with the shape of the corresponding catalyst, indicating the extremely homogeneous catalysts. The elemental mappings of Fe/C-300 and FeNi/C-300 were conducted to determine the elemental distribution and they were shown in Fig. A2 and A.3, respectively. Fig. 4 showed nitrogen adsorptionedesorption isotherms of Fe/ C-300 and FeNi/C-300, respectively and they could be interpreted as a combination of type I (microporous) and type IV (mesoporous) (Datta et al., 2015). The step-down characteristic signature appeared at the low relative pressure (p/p0) near to 0.4 implied H4type hysteresis loop. These results indicated that the catalysts obtained at 300
C had micro-mesoporous structure. Table 2 listed the textural parameters of the synthesized carbon-based catalysts. As expected, due to the growth of metal oxides and the oxidation of carbon, the specific surface area was gradually decreased with the increase of calcining temperature, and the average pore size showed an opposite trend. Noticeably, the specific surface area of both samples calcined at 300
C was much higher (185.2 and 195.6 m2/g for Fe/C-300 and FeNi/C-300, respectively) than those calcined at 400
C or 500
C. The high specific surface area was caused by the incomplete oxidation of tar at 300
C. When calcining temperature reached 400
C or 500
C, the tar was further oxidized and decomposed, resulting in reduced specific surface area (13.24 and 20.32 m2/g for Fe/C-500 and FeNi/C-500, respectively) and enlarged average pore diameter (38.33 and 29.94 nm for Fe/C-500 and FeNi/C-500, respectively). Interestingly, the total pore volume of the samples calcined at 400
C was the highest, probably resulting from the additional space provided by the oxidation of carbon. At calcination temperature of 500
C, the samples exhibited the lowest specific surface area and total pore volume were due to the vanishment of pore structure and the growth of the metal oxides. Moreover, compared to Fe/C catalyst, bimetallic nanocatalysts always had high specific surface area and total pore volume under identical conditions. In Fig. 5, Raman spectra of Fe/C-300 and FeNi/C-300 showed clear D and G bands, which further confirmed the existence of

graphite structure. The calculated ID/IG values of Fe/C-300 and FeNi/ C-300 were 0.94 and 0.91, respectively, which indicated that the existence of defective graphite structure. It was hypothesized that the combination of shearing force, friction and potential energy during the milling resulted in highly defects on the surface of materials, leading to the formation of defective graphite (Filiciotto et al., 2017). The defective graphite structure was favorable for surface electron conduction and excellent catalytic performance was expected of the catalyst (Yoo et al., 2017). It should be noted that neither D nor G bands in the Raman spectra were detected in other samples obtained at the temperature higher than 300
C. 3.2. H2O2 decomposition by prepared catalysts The first-order rate constants for the decomposition of H2O2 were given in Table 1. The FeNi/C-300 had the highest rate constant (10.55  103 min1) among all the catalysts. The rate constants of each series of samples were decreased with the increase of calcination temperature, which indicated that the smaller the catalyst size, the higher the catalytic efficiency. By comparison, the rate constants of Fe/C-series and FeNi/C-series were generally higher than those of Fe-series and FeNi-series, respectively. The improved catalytic efficiency was associated with the accelerated electron exchange by the defective graphitic layers on the catalyst surface. In addition, it was reported that the presence of nickel ions promoted the reduction of Fe3þ to Fe2þ (de Souza et al., 2010). Therefore, the bimetallic catalysts had higher rate constants than those monometallic catalysts prepared under identical conditions. 3.3. Catalytic degradation of MB by prepared catalysts Fig. 6 showed the effect of initial pH on removal of MB by Fe/C300 and FeNi/C-300, respectively. In general, due to the strong hydrolysis and precipitation of iron ions, pH value was considered to be the dominant factor in the traditional Fenton system (Duesterberg et al., 2008). Iron oxide as a substitute for soluble iron ions and the Fenton-like reaction could be used under neutral condition. As shown in Fig. 6, Fe/C-300 and FeNi/C-300 showed the highest MB removal efficiency of 98% and 94% at pH 3, respectively.

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Fig. 4. N2 adsorption-desorption isotherms and pore size distributions of (a) Fe/C-300 and (b) FeNi/C-300.

Table 2 Textural parameters of the synthesized carbon-supported catalysts. Sample

SBET (m2/g)

Smic (m2/g)

Vtot (cm3/g)

Vmic (cm3/g)

Dap (nm)

Fe/C-300 Fe/C-400 Fe/C-500 FeNi/C-300 FeNi/C-400 FeNi/C-500

185.2 22.31 13.24 195.6 37.04 20.32

124.3 4.794 6.863 118.4 3.917 7.090

0.161 0.192 0.036 0.195 0.198 0.132

0.061 0.002 0.003 0.058 0.002 0.003

3.742 16.80 38.33 3.766 12.22 29.94

In particular, the removal of MB by Fe/C-300 and FeNi/C-300 showed similar results about 70 and 75% at pH 5e9, respectively. This was a significant improvement in contrast to soluble Fe2þ catalysts, which only work at pH < 4 and caused serious secondary pollution of iron sludge. The high catalytic performance of Fe/C-300 and FeNi/C-300 over a wide range of pH values was probably ascribed to the small size of nanocatalysts, which provided more active sites and improved catalytic activity (Lai et al., 2019). In

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Fig. 5. Raman spectra of Fe/C-300 and FeNi/C-300.

Fig. 6. Effect of initial pH on removal of MB by prepared catalysts (Reaction condition: [MB] ¼ 30 mg/L, [H2O2]0 ¼ 0.1 M, [catalyst] ¼ 1 g/L, T ¼ 25
C).

addition, the presence of defect graphite structure also contributed to the high catalytic performance by promoting the reduction of Fe3þ to Fe2þ (Bach and Semiat, 2011; Yoo et al., 2017). However, the removal efficiency was decreased obviously at pH 11 and this was mainly ascribed to the inhibition of OH production and OH oxidation ability (Shi et al., 2014; Xie et al., 2017). The degradation of MB was tested with Fe/C-300 (Fig. 7a and b) and FeNi/C-300 (Fig. 7c and d). In all tests, the effect of adding H2O2 on the concentration of MB was negligible, which indicated that MB removal was not affected by the thermal decomposition of H2O2. FeNi/C-300 had higher adsorption capacity of MB than Fe/C-300 and by increasing solution temperature from 25 to 45
C. MB concentration was decreased from 50% to 32%e37% and 17% onto Fe/C-

300 and FeNi/C-300 in 60 min, respectively. Compared with the adsorption curve, the addition of H2O2 obviously increased the removal rate of MB. By adding 0.01 M H2O2, the concentration of MB was decreased from 50, 32, 37 and 17% to 45, 16, 30 and 12% (in the order of Fig. 7a, b, c and d) in 60 min due to the oxidation of reactive free radicals formed by H2O2. By increasing concentration of H2O2 to 0.1 M, increased MB was decomposed, and the concentration was significantly reduced from 45, 16, 30 and 12% to 24, 0, 20 and 2% (in the order of Fig. 7a, b, c and d) in 60 min, respectively. The MB removal rate of both catalysts was significantly increased by increasing solution temperature and this was ascribed to the accelerated contact between MB and free radicals. In this study, H2O2 decomposition rate of FeNi/C-300 (10.55  103 min1) was higher than that of Fe/C-300 (9.71  103 min1). However, it was worth noting that by adding 0.1 M H2O2, total MB removed was achieved for Fe/C-300 within 40 min at 45
C, while 2% of MB was remained at 60 min at 45
C for FeNi/C-300. For the removal of MB by Fenton-like reaction, the concentrations of iron ions and H2O2 were vital important (Qin et al., 2018; Yoo et al., 2017). The high iron content in the catalyst and the high H2O2 concentration in the solution were conducive to the formation of OH, thus improving the removal efficiency of MB. In addition, the increase of solution temperature was conducive to the improved diffusion of H2O2 and desorption of MB in the porous structure of the catalyst. Therefore, the Fe/C-300 had the higher decomposition efficiency of MB due to the higher iron content than FeNi/C-300. In addition, the mineralization degree of target pollutant is vital to evaluate wastewater treatment system. As shown in Fig.A5, the TOC removal rate of MB degradation reached 62.79 and 59.21% by Fe/C-300 and FeNi/C-300 in 60 min, respectively, indicating that the synthesized nanoparticle effectively catalyzed mineralization of MB. 3.4. Stability tests The stability results are shown in Fig. 8. It can be seen that after five times runs under acidic conditions, the catalytic removal

Please cite this article as: Li, D et al., Facile and green synthesis of highly dispersed tar-based heterogeneous Fenton catalytic nanoparticles for the degradation of methylene blue, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119033

D. Li et al. / Journal of Cleaner Production xxx (xxxx) xxx

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Fig. 7. The catalytic degradation of MB by Fe/C-300 (a, b) and FeNi/C-300 (c, d) (Reaction condition: [MB] ¼ 30 mg/L, [H2O2]0 ¼ 0.1 M, [catalyst] ¼ 1 g/L, pH ¼ 7, T ¼ 25
C).

leaching rate of Fe/C-300 and FeNi/C-300 was 12.6% and 14.2% after five times runs, respectively. Under neutral conditions, the removal efficiency of MB by Fe/C-300 and FeNi/C-300 decreased from 98.75% to 98.38%e86.68% and 81.93%, respectively. The iron ions leaching rate of Fe/C-300 and FeNi/C-300 under neutral condition was 5.9% and 7.8%, respectively. The results indicated that both catalysts exhibited sufficient stability under neutral conditions. The higher iron ions leaching rate of FeNi/C-300 resulted in its poor stability in repeated tests. The nickel ions leaching rate of FeNi/C300 under acidic and neutral condition was 13.8% and 7.1%, respectively. As a result, both catalysts retained over 90% metal load amount after five times runs under neutral conditions, indicating that metallic nanoparticles could be well immobilized on the surface of biomass tar. The above results further proved that the heterogeneous Fenton catalysts prepared in this study exhibited good catalytic ability and stability under neutral conditions.

3.5. Mechanism of H2O2 decomposition by FeNi/C-300 Fig. 8. MB degradation in the stability tests of Fe/C-300 and FeNi/C-300 (Reaction condition: [MB] ¼ 15 mg/L, [H2O2]0 ¼ 0.1 M, [catalyst] ¼ 1 g/L, T ¼ 25
C).

efficiency of MB by Fe/C-300 and FeNi/C-300 were decreased from 99.24% to 98.52%e70.48% and 65.01%, respectively. The iron oxide was dissolved from the surface of the catalysts under acidic conditions, resulting in a significant decrease of catalytic activity after stability test. The ICP measurement indicated that the iron ions

In traditional Fenton reaction, H2O2 is activated by iron ions and then decomposed to generate OH. The specific steps are given by the following equations (Duesterberg and Waite, 2007; Munoz et al., 2015):

Fe2þ þ H2 O2 /Fe3þ þ OH þ OH k1 ¼ 55 M1 s1

(1)

Please cite this article as: Li, D et al., Facile and green synthesis of highly dispersed tar-based heterogeneous Fenton catalytic nanoparticles for the degradation of methylene blue, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119033

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D. Li et al. / Journal of Cleaner Production xxx (xxxx) xxx

Fe3þ þ H2 O2 /Fe2þ þ HO2 þ Hþ

k2

¼ 2:00  103 M1 s1

(2)

Fe3þ þ  HO2 /Fe2þ þ O2 þ Hþ k3 ¼ 7:82  105 M1 s1 (3) 3þ

2þ

The reduction of Fe to Fe (Eq. (2)) with the lowest rate constant is the rate limiting step of the catalytic cycle for OH generation. Therefore, the acceleration of the Fe3þ reduction is an important factor to improve the degradation efficiency. In the Fenton-like process catalyzed by nickel-iron oxide, the rapid electron exchange between Ni2þ and Fe3þ ions could accelerate formation of Fe2þ (Kulkarni et al., 2014; Zhang et al., 2017). In addition, the presence of carbonaceous materials could provide delocalized p-electrons from the hexagonal carbon ring conjugated system to Fe3þ, thus facilitating the reduction of Fe3þ to Fe2þ (Bach and Semiat, 2011). It has been reported that NiFe2O4 had photo Fenton degradation performance and was stimulated to enhance the separation of electron-hole pairs to promote the decomposition of hydrogen peroxide (Huang et al., 2018; Singh et al., 2016). Moreover, the dispersion of catalyst particles on the surface of carbonaceous materials was also conducive to the decomposition of H2O2. In addition, homogeneous and heterogeneous catalyst on MB degradation were compared to clarify catalytic degradation mechanism of synthesized catalysts. As shown in Fig. A6, homogeneous Fenton experiments were carried out on the basis of the maximum leaching concentration of Fe and Ni ions. Under neutral pH, MB removal rates by Fe3þ homogeneous catalytic system and Fe3þ, Ni2þ homogeneous catalytic system were only 18 and 12% after 60 min, respectively. This observation confirmed that the MB degradation was mainly caused by the catalysts through producing OH through heterogeneous reaction (Xiao et al., 2018). Moreover, the lower degradation efficiency of Fe3þ, Ni2þ homogeneous catalytic system indicated that the existence of carbon promoted the synergistic effect between Fe3þand Ni2þ. Therefore, based on the above results and previously reported studies, the mechanism of H2O2 decomposition by FeNi/C-300 catalyst is proposed (shown in Fig. 9). The surface of catalyst was covered with defective graphitic layers and the active metal oxide particles were exposed and reacted with H2O2. The defective graphitic layers and NiO on the surface of catalyst promoted the transformation from Fe3þ to Fe2þ by accelerating electron

exchange. Therefore, the FeNi/C-300 rapidly reacted with the H2O2 to induce Fenton-like reaction. The NiFe2O4 provided electrons by separating electron-hole pairs, accelerated the rate of electron exchange in the catalytic process. In addition, the leached Fe3þ could react with the H2O2 to form Fe2þ, and further reacted with the H2O2 to induce the Fenton reaction. Therefore, FeNi/C-300 had high decomposition rate constant of H2O2 due to its unique structure including the present and well dispersion of bimetallic oxides in carbon matrix, porous structure, tiny particle size and the defective graphite layers. 4. Conclusions In this study, heterogeneous Fenton catalysts were synthesized with tar as precursor via solvent-free direct method and utilized for MB removal from aqueous solution. The results confirmed that the possibility of synthesizing catalytic nanocomposites by calcining after ball milling. The characterization results showed the porous structure and defective graphite layers on the surface of synthesized catalysts, which were caused by the addition of tar. FeNi/C300 showed the highest rate constant for the decomposition of H2O2. This is because the presence of nickel and defective graphite layers accelerate electron exchange, thus facilitating the reduction of Fe3þ to Fe2þ. Moreover, the synthesized heterogeneous Fenton catalysts were highly active and had potential applications in organic pollutant removal. The Fe/C-300 and FeNi/C-300 showed fairly good stability and reusability under neutral conditions. This study proved a promising approach to synthesize catalytic active composite materials from renewable tar for catalytic removal of organic pollutants. It is worth noting that the addition of tar in the catalyst synthesis process can expand the changing range of particle size with temperature, which can be used to accurately control the particle size of catalysts by changing temperature in the future. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge financial support for Zhengang Liu from the “100 Talents” Program of the Chinese Academy of Sciences (Project No. Y5N41I1C01). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.119033. References

Fig. 9. Mechanism of H2O2 catalytic decomposition by FeNi/C-300.

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Please cite this article as: Li, D et al., Facile and green synthesis of highly dispersed tar-based heterogeneous Fenton catalytic nanoparticles for the degradation of methylene blue, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119033