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Synergistic effect between Fe and Bi2O3 on enhanced mechanochemical treatment of decabromodiphenyl ether
Accepted Manuscript Title: Synergistic effect between Fe and Bi2 O3 on enhanced mechanochemical treatment of decabromodiphenyl ether Author: Zhimin Zhang Nan Wang Lihua Zhu Hanqing Lv Xuelin Dong Huijuan Chai Heqing Tang PII: DOI: Reference:
S2213-3437(17)30007-6 http://dx.doi.org/doi:10.1016/j.jece.2017.01.008 JECE 1423
To appear in: Received date: Revised date: Accepted date:
21-7-2016 5-1-2017 7-1-2017
Please cite this article as: Zhimin Zhang, Nan Wang, Lihua Zhu, Hanqing Lv, Xuelin Dong, Huijuan Chai, Heqing Tang, Synergistic effect between Fe and Bi2O3 on enhanced mechanochemical treatment of decabromodiphenyl ether, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2017.01.008
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Synergistic effect between Fe and Bi2O3 on enhanced mechanochemical treatment of decabromodiphenyl ether Zhimin Zhanga, Nan Wanga,b,*, Lihua Zhua, Hanqing Lva,b, Xuelin Donga, Huijuan Chaia,b, Heqing Tangc,* a
Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong
University of Science and Technology), Ministry of Education，School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 430074, Wuhan, China b
Shenzhen Institute of Huazhong University of Science and Technology, 518000, Shenzhen,
China c
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission
and Ministry of Education, College of Resources and Environmental Science, South-Central University for Nationalities, Wuhan 430074, PR China *Corresponding author. Tel: +86 27 87543432; Fax: +86 27 87543632; E-mail: [email protected]; [email protected].
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ABSTRACT The present work investigated the mechanochemical (MC) treatment of powdery decabromodiphenyl ether (BDE209) with Bi2O3 and/or Fe powders as co-milling reagents. The simultaneous use of Bi2O3 and Fe with a Bi:Fe:Br molar ratio of 1:1:1 led to a BDE209 degradation of 96.6% within 2 h of ball milling at rotation speed of 400 rpm, whereas only 66.0% and 24.0% of BDE209 was removed in the MC-Bi2O3 and MC-Fe system, respectively. It was demonstrated that the MC process activated the lattice oxygen (O2−) of Bi2O3 and promoted its reaction with BDE209, whereas Fe initiated the reduction of BDE209. The MC treatment of BDE209 through a single reductive in the MC-Fe system or oxidative process in the MC-Bi2O3 system was not efficient, due to that BDE209 is rather difficultly oxidized and the debrominated intermediates are resistant to reduction. When combining Bi2O3 with Fe, the reductive debromination of BDE209 over Fe and the subsequent oxidation of debrominated products over Bi2O3 were integrated into a concerted process. The rapid reduction of BDE209 over Fe not only released Br ions, which acted as the dopant to improve the activity of O2− in Bi2O3, but also generated less brominated intermediates, which were more susceptible to the O2−-participating reaction over Bi2O3. Thus, a strong synergistic effect between Bi2O3 and Fe was observed on the MC degradation of BDE209. Keywords: Decabromodiphenyl ether; Mechanochemical degradation; Synergistic effect; Bi2O3, Iron
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1. Introduction Mechanochemical (MC) method based on grinding reactants in solid phase has recently attracted much attention as a simple route for treating toxic organo-halogen compounds [1, 2]. In the MC process, solid-to-solid reactions are activated by collisions with milling bodies in the milling devices. Compared with combustion techniques, the MC method is favorable for simplified operations on a large amount of toxic compounds by eliminating the use of organic solvent, and avoiding the formation of toxic dioxins, which was possibly released in the incineration treatment of organo-halogen pollutants [3]. Many earlier studies focused on the MC destruction of chlorinated contaminants like chlorobenzenes,
dichlorodiphenyltrichloroethane,
polychlorinated
biphenyls
and
polychlorinated dibenzodioxins [4-7]. Recently, some attention has been paid to the fluorinated
and
brominated
pollutants
such
as
perfluorooctane
sulfonyl,
fluoridehexabromobenzene, tetrabromobisphenol A (TBBPA) and polybrominated diphenyl ethers (PBDEs) [8-14]. These brominated flame-retardants are another class of halogenated molecules that are recognized to cause adverse effects to ecosystems and human health [15]. Among them, decabromodiphenyl ether (BDE209) is widely used in textiles, plastics, and electronic products, to prevent fire. As non-reactive additives, BDE209 easily migrates from products and spread into the environment, and is becoming a new class of global contaminants. Shintani et al. first applied MC treatment for the decomposition of BDE209 by using a planetary ball mill and CaO as the co-milling agent [11]. When the molar ratio of CaO to BDE209 was 343:1 and the rotation speed was 700 rpm, more than 99% of BDE209 was degraded after 1 h milling. In this case, both much excessive CaO (343 fold molar of BDE209) and a high rotation speed (700 rpm) were required to ensure high reaction rate and complete destruction. Our group proposed an oxidative method to the decomposition of powdery 4
BDE209 by using MC activation of persulfate (PS), which achieved a complete debromination and mineralization of BDE209 within 3 h of milling with the considerable lower PS-BDE209 molar ratio of 50:1 at a rotation speed of 400 rpm [12]. Zhang et al. reported that Bi2O3 could be used in the exactly stoichiometric amount (i.e., Bi2O3-BDE209 molar ratio of 5:1) for the destruction of BDE209 [13], during which a new compound BiOBr was formed. Both the carbonization to inorganic carbon and the decomposition to CO2 were observed in this system, although the underlying mechanism of the Bi2O3-involved reaction process remains unknown. In addition, this system required still a high rotation speed of 700 rpm to promote the MC destruction of BDE209. It was worth noting that the generation of CO2 and CO during the MC degradation of BDE209 with Bi2O3 indicated the destruction of aromatic structure of BDE209 [13]. As decreasing the number of Br atoms in the PBDEs molecule, the benzene ring of PBDEs will be more easily broken [16, 17]. Therefore, we anticipate that the combination of Bi2O3 with a proper reducing agent may improve the MC destruction of BDE209 at a considerable lower rotation speed. Zero-valent iron (Fe) powder merits our attention, because of its low cost, availability and especially excellent reductive capability for the reductive debromination BDE209 in liquid solvents [18, 19] and the treatment of halogenated organic compounds in the MC process [10, 20-22]. In this study, we utilized the combination of Bi2O3 and Fe to treat BDE209 under ball milling, and found that the addition of a little amount of iron powders significantly enhanced the MC degradation of BDE209 with Bi2O3 at 400 rpm. 2. Experimental section 2.1. Materials BDE209 (purity>98%) was purchased from Aldrich Chemical Co. (Milwaukee, WI). Bismuth(III) oxide (Bi2O3, 99% in purity) and iron powders (Fe, 99% in purity) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used without further 5
treatments. The solvents for gas chromatography-mass spectrometer (GC-MS) and high performance liquid chromatography (HPLC) analysis were Absolv grade and HPLC grade, respectively. All the other chemical reagents were analytic grade and used as received. Milli-Q water (18.2 MΩ cm) was used in the present work. 2.2. MC treatments The MC reactor consisted of a planetary ball mill (PM100, Retsch, Germany) and a stainless steel pots (250 cm3). In a typical run, 0.278 g BDE209 powders were finely mixed with 0.682 g of Bi2O3 and 0.164 g of Fe in a mortar. Then, the mixture powders were transferred into the stainless steel pot. Because we previously found that the simultaneous use of balls with 10 mm- and 20 mm-diameter at a number ratio of 4:1 obtained higher destruction efficiency than that by using singe sized balls, the present work employed 20 balls with 10 mm-diameter and 4 balls with 20 mm-diameter to grind samples. After the pot was sealed tightly at ambient pressure, the planetary ball mill was operated at 400 rpm with automatic rotation direction changing per 15 min. 2.3. Analysis and characterization For determining BDE209 residues and its degradation intermediates, 0.020 g of the milled sample were taken and extracted using 10 mL tetrahydrofuran (THF) under ultrasonic treatment for 10 min. After removing the powder by centrifugation, the collected solution was filtered with a 0.22-m membrane and then divided into two parts. One part was subjected to HPLC analysis on Agilent 1260 HPLC system equipped with a diode array detector (DAD) and a SB-C18 column (4.6×150 mm). If needed, some samples were diluted prior to HPLC treatments. The mobile phase was 97% acetonitrile and 3% water at 1.0 mL min−1, and the detection wavelength was set at 240 nm. The other part was used for the identification of
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organic bromine compounds by GC-MS with the same instrumental conditions as described in our previous study [12, 16]. When using Rhodamine B (RhB, C28H31ClN2O3) as the model pollutant, the RhB residues were extracted with ethanol under the same ultrasonic conditions, and determined on a UV-Vis spectrophotometer at the wavelength of 564 nm (Cary 60, Agilent). To compare the chemical changes during milling, the samples were analyzed by X-ray photoelectron spectroscopy (XPS) on a VG Multilab 2000 spectrometer (Thermo Electron Corporation) with Al Kα radiation as the exciting source (300 W). X-Ray diffraction (XRD) was performed on an X-ray diffractometer (Bruker D8 Advance) with monochromatized Cu Kα radiation. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. Thermogravimetric analyses were performed on a NETZSCH TG 209 F3 thermal analyzer. Samples were analyzed at a heating rate of 10 °C min-1 to 950 °C under an atmosphere of air or N2. 3. Result and discussions 3.1. Enhanced MC degradation of BDE209 with the combination of Bi2O3 and Fe Figure 1 illustrated the MC degradation of BDE209 by using Bi2O3, Fe powder, and their mixture. When using Bi2O3 alone as the co-milling reagent at the Bi/Br molar ratio of 1:1, the removal of BDE209 was negligible in the first 15 min of ball milling, but slowly reached to 66.0% after a 2 h of MC treatment. When using Fe alone as an additive with the Fe/Br molar ratio of 1:1, 24.0% of the added BDE209 was rapid degraded after 30 min of ball milling, and then the decay became very slow as prolonging milling time to 2 h. As an efficient reducing agent, Fe powder is capable of reducing BDE209. As the reaction proceeded, Fe powders were transformed to iron oxides, resulting in a full consumption of the reducing agent and no further increase of the degradation of BDE209 after 30 min. However, the combination of Bi2O3 and Fe powders with the Bi/Fe/Br molar ratio of 1:1:1 led to an efficient treatment of BDE209 7
with 96.6% of BDE209 removal after 2 h of ball milling. If further prolonging ball milling to 2.5 h, a complete degradation of BDE209 was observed (data not shown). This suggests that the simultaneous addition of Bi2O3 and Fe exhibits synergistic effect on the MC treatment of BDE209. 100 Bi2O3+Fe Bi2O3 Fe
BDE209 removal /%
80 60 40 20 0 0
30
60 90 Milling time /min
120
Figure 1. MC degradation of BDE209 (0.2776 g) with Fe, Bi2O3 or mixture of them (Bi2O3 + Fe) as the co-milling agents. Unless otherwise stated, the basic reaction conditions were as follows: the dosage of co-milling agents was fixed at the molar ratio of Bi/Fe/Br of 1:1:1; ball-to-powder mass ratio, 185:1; 20 of the 24 used balls were of a diameter (d) of 10 mm, while the remaining 4 balls with d of 20 mm; rotation speed, 400 rpm. 3.2 Effects of treatment conditions The effects of the molar ratio of Bi2O3 to Fe, ball-to-powder mass ratio (mb/mp), and the ball rotation speed were further studied. At a given initial amount of BDE209 (0.278 g), the influence of the Fe/Bi molar ratio on the MC degradation of BDE209 was studied by fixing the total amount of Bi2O3 and Fe at 0.846 g. As shown in Figure 2a, as the molar ratio of Fe/Bi was increased from 0 to 1, the removal of BDE209 within 1 h of milling was significantly
8
increased from 47.7% to 90.5%, and then decreased to 37.2% when further increasing the Fe/Bi molar ratio to 5. In the combination system, Fe makes a great contribution to initiating the reductive degradation of BDE209, while Bi2O3 plays a key role in the subsequent oxidation of the less brominated products (see section 3.4 for more detailed discussion). At the low Fe/Bi molar ratio range, the increasing of the reducing agents (Fe) enhanced its ability to initiate the reductive debromination of BDE209. At the high Fe/Bi molar ratio range, the excessive Fe would react with O2 to form iron oxides, leading to that the surface of Fe powders is passivated and consequently the reductive degradation of BDE209 becomes slower. Figure 2b illustrated the effect of the rotation speed on the degradation removal of BDE209 with both Bi2O3 and/or Fe within 1 h of ball milling. In the cases of Bi2O3 or Fe alone, the removal efficiency of BDE209 was almost increased linearly with increasing the rotation speed from 300 to 450 rpm. This is in good agreement with the observation of Zhang et al., who found that more than 95% of BDE209 could be removed after 1 h milling at 700 rpm [13]. An increase of rotation speed increases the collision velocity and the number of collisions per unit time, both of which are favorable to the reaction of co-milling reagents with BDE209 [23]. In contrast, by increasing the rotation speed from 300 to 450 rpm in the combination system of Bi2O3 and Fe, the removal efficiency of BDE209 was initially increased and reached a maximum at 400 rpm, and then is kept almost constant at higher rotation speeds. This means that in this coupled system, it is not necessary to use a high rotation speed, which will save energy consumption. It was noted that at any given rotation speed, the removal efficiency of BDE209 in the combination system containing both Bi2O3 and Fe was larger than in their single system. As shown in Figure 2c, as the mb/mp increased, the degradation removal of BDE209 within 1 h milling was continually increased, and achieves a maximum value at mb/mp of 185 in the 9
case of Bi2O3 + Fe. At any tested mb/mp, the removal efficiency of BDE209 in different systems increased in the order of Fe < Bi2O3< Bi2O3 + Fe, implying that the simultaneous introduction of Bi2O3 and Fe greatly promoted the MC treatment of BDE209.
BDE209 removal /%
100 80
Bi2O3+Fe
Bi2O3+Fe
Bi2O3
Bi2O3
Fe
Fe
60 40 20
(a)
0 0
1
2
3
4
5
nFe/nBi
(c)
(b) 300
350
400
450
50
Ratotion speed /rpm
100
150
200
250
mb/mp
Figure 2. Effect of (a) the Fe/Bi molar ratio (nFe/nBi) in mixture co-milling agents of Bi2O3 and Fe, (b) rotation speed, (c) ball-to-powder mass ratio (mb/mp) on the MC destruction of BDE209 within 1 h of ball milling. 3.3 Conversions of elements Br, Bi and Fe during the ball milling Because the atomic number and the position of bromine atom in the benzene ring greatly influenced the toxicity and stability of PBDEs, the transformation of bromine atoms was examined. Like in the previous reported MC-Bi2O3 system [13], it was not found any obvious brominated organic intermediates, water-dissolved or gaseous bromine species in the Bi2O3-Fe combined MC system. XPS analysis is used to measure the chemical state of Br as well as Fe, Bi and O in the samples before and after ball milling. Figure 3a showed the high resolution XPS spectra of Br 3d region. For the original powders, two types of bromine could be identified: the binding energies of Br 3d5/2 and Br 3d3/2 located at 69.9 eV and 71.0 eV were ascribed to the covalently bonded bromine atoms in BDE209 having the calculated natural bond orbital (NBO) 10
charge of (0.1540.001) e, and those at 69.6 eV and 70.6 eV was assigned to Br 3d5/2 and Br 3d3/2 of the covalently bonded bromine atoms with the NBO charge of (0.1440.003) e, respectively (Figure 3a). After 0.5 h of ball milling, the Br signals turned out to be comparatively broad as well as appeared new peaks at 68.2 and 69.2 eV, which was attributed to Br 3d5/2 and Br 3d3/2 of Br− ions, respectively (curve 2 in Figure 3a). As prolonging the ball milling time to 2 h, the ratio of covalently bonded bromine atoms to the total bromine was decreased to 15%, and that of Br− ions was increased to 85% (Figure 3a) [24, 25]. This suggests that the released bromine element from BDE209 was converted to Br− ions. Figure 3b illustrated the XPS profiles of Bi 4f. The characteristic peaks of Bi 4f5/2 at 163.9 eV and Bi 4f7/2 at 158.6 eV indicated a trivalent oxidation state for Bi–O bonding in the sample before ball milling [26, 27], and the very weak peak of Bi 4f5/2 (4f3/2) at 165.7 (160.4) eV suggested the presence of a small amount of Bi5+ (ratio to the total Bi, 5.4%), due to the intrinsic oxygen deficient in Bi2O3 [28]. After 0.5 h of MC treatment, the signals of both Bi3+ as the Bi2O3 form and Bi5+ were significantly decreased, and the new peaks at 164.6 eV (Bi 4f5/2) and 159.3 eV (Bi 4f7/2) appeared, suggesting the generation of Bi3+ as the form of BiOBr or BixOyBrz [24, 25]. After 2 h of MC treatment, the ratio of Bi as BiOBr and Bi5+ was increased to 64.3% and 19.2%, respectively, and that as Bi2O3 was decreased to 16.5%, suggesting that Bi2O3 was gradually converted to BiOBr as well as the formation of more oxygen deficiencies. Due to the greater ionic radius of Br− ions, the introduction of Br− turned to change the structure of Bi2O3 containing Bi3+, Bi5+ and oxygen deficient. Thus, the binding energies of Bi3+, Bi5+ as well as Br− was slightly changed. The high resolution XPS profile of O 1s showed dominant peaks at 529.4 eV, 531.2 eV and 532.8 eV for the sample before reaction (Figure 3c), which are the characteristic of structural oxygen, the O2− ions locating in an imperfect lattice with oxygen deficiencies and physically adsorbed oxygen on Bi2O3, respectively [26, 27]. After the MC reaction, the relative intensity of the above three peaks was significantly decreased, and all of 11
them moved to a higher binding energy (Figure 3c). According to the previous reports [24, 25], the O 1s peaks at 530.1 eV, 531.8 eV and 533.8 eV was attributed to the lattice oxygen, oxygen vacancies and surface adsorbed oxygen on bismuth oxybromides, respectively. From these results, it can be concluded that the MC treatment with both Bi2O3 and Fe led to the conversion of covalently bonded bromines in BDE209 to Br− as the form of BiOBr or other substoichiometric BixOyBrz. In addition, the binding energies of Fe 2p at 706.8 eV and 720.0 eV in the sample without ball milling are characteristic of Fe0 [29], while the peaks at the binding energy of 709.9 and 723.4 eV in the milled samples are characteristic of Fe3O4 or Fe2O3 (Figure 3d) [24, 30]. This further confirmed Fe was converted to iron oxides during the MC process. In other word, the role of Fe as the reducing reagent during the MC treatment of BDE209.
(b) Bi 4f 0h
0h
0.5 h
0.5 h
Intensity
Intensity
(a) Br 3d
1h
72
70 Binding energy /eV
2h
9000
2h
6000
74
1h
167
68
165
163
161
Binding energy /eV
12
159
157
(c) O 1s 0h
0h
0.5 h
0.5 h Intensity
Intensity
(d) Fe 2p
1h
2h
1h
2500
2h
6000
536
534
532 530 Binding energy /eV
730
528
723
716 709 Binding energy /eV
702
Figure 3. High resolution XPS spectra of (a) Br 3d, (b) Bi 4f, (c) O 1s, and (d) Fe 2p for the mixture of BDE209, Bi2O3 and Fe before and after being milled for 0.5 h, 1 h and 2 h. XRD patterns of powdery BDE209, Fe, Bi2O3 and their mixtures were recorded before and after ball milling. As shown in Figure 4a−4c, only the diffraction peaks of Bi2O3 (JCPDS Card no. 01-076-1730), Fe (JCPDS Card no. 01-087-0721) and/or BDE209 were observed clearly in the XRD spectra of the mixtures before reaction. After 2 h of milling, the patterns of Bi2O3 in the Bi2O3-BDE209 system became weaker, and the dominant peaks at 2θ of 27.0o, 27.5o and 28.1o were broadened, e.g., the peak width at half-maximum intensity (FWHM) of these three peaks was increased from 0.150o, 0.147o, and 0.148 o to 0.339o, 0.309o and 0.183o, respectively (Figure 4a). The peak broadening at 2θ of 28.1o were less serious than that for the other two peaks. This indicated that the broadening arose from the crystal imperfections and distortion of Bi2O3 instead of the crystallite size decreasing [31]. Moreover, new diffraction peaks appeared at 2θ of 29.5o and 31.8o, which were assigned to Bi4O5Br2 (JCPDS Card no. 00-037-0699) (Figure 4a). This further supported that the lattice distortion of Bi2O3 was due to the Br-doping. It seems that this result is different from the report of Zhang et al., who pointed out that Bi 2O3 was completely changed into BiOBr during the MC treatment of BDE209 [13]. However, it 13
was a false difference, because we noted that a similar peak corresponding to Bi4O5Br2 also appeared in their work, but they did not consider other forms of BiXOYBrZ except BiOBr. For the mixture of BDE209 and Fe after 2 h milling (Figure 4b), the patterns of Fe disappeared, and the new peaks at 2θ of 30.0o, 35.4o, 43.0o, and 56.9o corresponding to Fe3O4 (JCPDS Card no. 01-089-0688) appeared. The oxidation of Fe0 to Fe2+/3+ suggests that Fe acted as the reducing agent to reduce BDE209. In the Bi2O3-Fe-BDE209 system, the XRD patterns of Bi2O3 were decreased gradually as prolonging MC reaction time, and the dominant peaks at 2θ of 27.0o, 27.5o and 28.1o were also broadened (Figure 4c). However, the FWHM was firstly increased as the reaction time was increased from 0 to 0.5 h, and then decreased if further prolonging the reaction time to 2 h. For example, the FWHM at 2θ of 27.0o (27.5o) was changed from 0.144o (0.130o) at 0 h to 0.239o (0.200o), 0.204o (0.184o), and 0.181o (0.169o) at 0.5 h, 1 h, and 2 h, respectively. This suggested that the crystalline structure of Bi2O3 in the Bi2O3-Fe-BDE209 system became disordered after the reaction, but it was described in different terms of substitution disorder from that in the Bi2O3-BDE209 system. Accordingly, new different peaks were observed after ball milling and could be assigned to BiOBr (JCPDS Card no. 00-001-1004) instead of Bi4O5Br2 (Figure 4c). In addition, it was observed the oxidation of Fe to Fe3O4 after the reaction. Both the formation of BiOBr and Fe3O4 were increased as prolonging the reaction time. These results are in agreement with the above XPS results, and further confirmed that the organic bromines of BDE209 were transformed into inorganic form as BiOBr. Furthermore, relative to the product Bi4O5Br2 in the Bi2O3-BDE209 system, the formation of BiOBr implies a higher conversion ratio of organic bromine in the Bi2O3-Fe-BDE209 system. In other word, the combination of Bi2O3 and Fe in the MC process enhanced the debromination of BDE209.
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(b)
(a)
Bi2O3(01-076-1730)
Fe3O4(01-089-0688)
0h
0h
40 2θ /degree
50
20
60
Bi2O3(01-076-1730)
40 2θ /degree
50
60
0.5h 1h
30
40 2θ /degree
50
Fe (01-087-0721)
Fe3O4(01-089-0688)
Intensity
Intensity
30
Fe2O3(01-073-0603)
BiOBr(00-001-1004) Fe3O4(01-089-0688)
20
2h
(d)
2h
BDE209
(c)
0h
2h
30
20
Intensity
Intensity
Bi4O5Br2(00-037-0699)
Fe(01-087-0721)
BiOBr(00-001-1004) Bi4O5Br2(00-037-0699) Bi2O3(01-076-1730)
20
60
30
40 2θ /degree
50
60
Figure 4. XRD patterns of the samples before and after ball milling: (a) Bi2O3 and BDE209, (b) Fe and BDE209, and (c) Bi2O3, Fe and BDE209. (d) Standard diffraction patterns of the related compounds (vertical lines were referred to JCPDS Cards). The XRD patterns of pure BDE209 powders were given as a reference in Figure b. that no signals of BDE209 were observed at 0 h is consistent with the complete adsorption of BDE209 on the TiO2 particle surface. The conversion of BDE209 and co-milling reagents was also studied by formation of thermogravimetric (TG) analysis. When operating TG analysis in air atmosphere (Figure 5a), the mixture of Bi2O3, Fe, and BDE209 before the MC reaction (curve 1) showed a mass loss of 22.6% between 300−400°C (stage I) and a mass increase of 5.5% between 480−660 °C (stage 15
II), corresponding to the thermal decomposition of BDE209 (theoretic value, 24.7% in total mass), and the oxidation of iron by O2 (theoretic value, 14.6% (Fe in total mass) × 38.1% = 5.6%) (eq.1), respectively. If operating at N2 atmosphere, a similar mass loss (24.8%, stage I in Figure 5b) was observed in the sample without milling, but no mass was increased, due to the lack of O2 to reaction with Fe. For the 1h milled sample, the weight loss exhibited a much wider temperature range in air (curve 2 in Figure 5a). The nearly 5% loss between 300−400 °C (stage III in Figure 5a) exceed the weight ratio of residual BDE209, which was calculated to be 2.4% from the HPLC result in Figure 1. In addition, no significant weight change owing to the residual BDE209 was observed in N2 at the same temperature range (curve 2 in Figure 5b). This suggests that another component was formed, and could be assigned to the amorphous carbon (theoretic value, 2.7%), which is particularly reactive to oxygen at elevated temperatures [32]. No weight was increased for the 1 h milled sample in air (curve 2 in Figure 5a), which agreed with both XRD and XPS results, due to the transformation of Fe to iron oxide. Compared to the air atmosphere, the use of N2 atmosphere resulted in a larger mass loss in the temperature range of 390−610 °C (stage II in Figure 5b). It was attributed to the thermolysis of BiOBr, which can undergo different thermolysis routes in N2 and air (eqs. 2−4). The mass loss of 9.5% in air (stage IV in Figure 5a) corresponded to the decomposition of BiOBr into solid Bi24O31Br10 and Br2 gas (eq. 2), whereas the mass loss of 39.1% in N2 (stage II in Figure 5b) corresponded to the decomposition of BiOBr into Bi2O3 and BiBr3 gas (eq. 4). When prolonging the reaction to 2 h, the mass loss in air atmosphere at 300−400 °C decreased from 5% to 2.3% (stage V in Figure 5a), indicating all the residual BDE209 was further converted to amorphous carbon and BiOBr. Accordingly, a larger mass loss owing to BiOBr occurred in N2 atmosphere at 390−610 °C (stage III in Figure 5b). These results again demonstrate that the MC treatment of BDE209 with the Bi2O3 and Fe led to the conversion of organic bromine in BDE209 to BiOBr. 16
3Fe + 2O2
(1)
Fe3O4
24BiOBr + 3.5O2
Bi24O31Br10 + 7Br2
(2)
12Bi2O3 + 5Br2
(3)
Bi24O31Br10 + 2.5O2 3BiOBr
100
VI
3 2 1
Weight change /%
IV
I Weight change /%
100
V III
II
80
60
200
I 80
II III 1 2 3
60
(b) In N2
(a) In air
40
40 50
(4)
Bi2O3 + BiBr3
350 500 650 Temperature /oC
800
50
200
350 500 Temperature /oC
650
800
Figure 5. TG curves of the mixture of Bi2O3, Fe and BDE209 in the atmosphere of (a) air and (b) N2 before (1) and after being milled for 1 h (2), and 2 h (3). 3.4 Reaction mechanism of MC degradation of BDE209 by Bi2O3 and Fe The combination of Bi2O3 and Fe showed a synergistic effect on the MC treatment of BDE209 (Figure 1 and 2). BiOBr and Fe3O4 were observed accompanying the rapid disappearance of BDE209 (Figure 3−5). The oxidation of Fe indicated that it acted as the reducing reagent and provided electrons for the reductive debromination of BDE209. The conversion of BiOBr suggested the structure of Bi2O3 was destroyed and the lattice oxygen of Bi2O3 was replaced by bromine elements. In earlier reports, Bi2O3 is an important metal oxide semiconductor catalyst and has been identified as photocatalysts and sonocatalysts in the degradation of organic pollutants [33−35]. 17
In these systems, the heat and light energies can excite Bi2O3 particles to form indirectly various active species like photo-generated holes and •OH radicals, which are responsible for the oxidative degradation of organic pollutants. However, this mechanism could not explain well the present results, because the release of dissolved halide ions was observed in Bi2O3-partipantited photocatalytic and sonocatalytic systems [32-34], but the formation of bismuth oxyhalides was confirmed in the present MC system. It is worth noting that Bi2O3 is also a well-known fluorite-type oxide ion electrolytes at high temperature that transport O2− ions through intrinsic oxygen vacancies to the anode where the fuel reacts with the O2− ions to oxidation products [36]. The XPS analysis in Figure 3c showed that 47.9% of oxygen locating in the imperfect lattice with the oxygen deficiencies, which was much higher than that of the lattice oxygen of Bi2O3 (23.0%). The high-energy impact during ball milling produces transient temperature as high as 1000 °C, which is high enough to activate Bi2O3 to exhibit excellent ionic conductivity for the O2− migration and the subsequent O2−-involved oxidation process. Moreover, Lou et al. demonstrated that the doping of Bi2O3 promoted the oxygen mobility of Co3O4 and enhanced the reaction CO with the lattice oxygen to produce CO2 [37]. Based on the above discussion, we proposed a tentative explanation to the MC promoted reaction of BDE209 with Bi2O3 and Fe (and their synergistic effect) on the degradation of BDE209, as expressed by eqs. 5−12. 2 2 OO(Bi Os(Bi 2O3 ) 2O3 )
(5)
2 R - Br Os(Bi oxidation products(e.g., CO 2 ) VO(Bi 2e 2O3 ) 2O3 )
(6)
2 O2 2e VO(Bi OO(Bi 2 O3 ) 2 O3 )
(7)
R - Br e VO(Bi H2O RH Br OH 2 O3 )
(8)
2Br VO(Bi 2Br(Bi Bi X BrY O Z 2 O3 ) 2 O3 )
(9)
2 2 O O(Bi O s(Bi X BrY O Z ) X BrY O Z )
(10) 18
2 R - Br Os(Bi oxidation products VO(Bi 2e X BrY O Z ) X BrY O Z )
(11)
R - Br Fe H2O RH Br 0.5Fe2 OH
(12)
2 Upon the ball milling, the lattice oxygen of Bi2O3 ( OO(Bi ) becomes highly mobile and is 2 O3 )
2 diffused from the bulk to the surface (eq. 5), where the formed surface O2− ions ( Os(Bi ) 2O3 ) reacts with the adsorbed BDE209 to produce oxidation product, oxygen vacancy ( VO(Bi ) 2 O3 )
and electrons (eq. 6). Generally, the oxygen vacancy is replenished by O2 to form the new active oxygen species and complete the redox cycle (eq. 7). However, since BDE209 is very easily reduced, another adjacent BDE209 molecule may trap electrons, leading to the formation of Br− ions (eq. 8). Because of the strong interaction of Br− and Bi, the generated Br− ions enters the lattice of Bi2O3 (eq. 9), leading to that Bi2O3 was gradually transformed to substoichiometric forms of bismuth oxybromides, and finally to BiOBr if there is enough Br− source. Accompanying the degradation of BDE209, the surface O2− ions of Bi2O3 especially locating near the oxygen deficiencies were greatly consumed (Figure 3c), and it was also observed the formation of BiOBr (Figure 4c). Moreover, the relative distribution of O2− ions locating on the lattice oxygen defects of BiOBr was increased as prolonging the ball milling time from 0.5 h to 2 h (Figure 3c). The structural defect and lattice distortion arising from the Br− doping in turn improves the activation ability and mobility of O2− for the oxidation of BDE209 (eqs. 10 and 11). This is why the degradation of BDE209 became faster after a short time period of 15 min in the Bi2O3-BDE209 system (Figure 1). To validate the above presumption, an organic dye pollutant, Rhodamine B (RhB) without bromine atom was chosen as another model pollutant. Under the same MC treatment conditions, the degradation of RhB was nearly as fast as that of BDE209 within the first 15 min, but became slower than that of BDE209 in the following reaction stage in the MC-Bi2O3 system (Figure 6a). Wang et al. also observed a slow reaction rate in the first hour of milling 19
during the MC degradation of dechlorane plus with aluminum and quartz sand, due to that the repeated physical impact facilitates surface reaction activation [23, 38]. However, this explanation could not describe well the present results, because Bi2O3 showed different results towards the degradation of RhB and BDE209. Moreover, if using the common oxidation MC system with persulfate, the oxidation of RhB is much faster than BDE209 (Figure 6a), being different from those on Bi2O3. Relative to refractory BDE209, RhB is more easily to be oxidized by in situ produced SO4• radicals from the MC activation of persulfate. This suggests that the Bi2O3-involved reaction mechanism is also distinct from the general advanced oxidation systems. After adding a small amount of KBr, the MC degradation of RhB with Bi2O3 is greatly accelerated (Figure 6b). Meanwhile, the loss of dissolved free Br ions in the Bi2O3-KBr-RhB system is faster than that in the Bi2O3-KBr system without RhB (Figure 6c). Accordingly, the XRD analysis in Figure 6d shows that more Bi4O5Br2 is formed in the Bi2O3-KBr-RhB system than that in the binary system containing KBr and Bi2O3. For another control binary system composed of Bi2O3 and RhB, no any new diffraction peaks are observed except those of Bi2O3 during ball milling (Figure 6d). In addition, neither the loss of free Br ions nor the formation of Bi4O5Br2 was observed in the systems of Bi2O3-KBr and Bi2O3-KBr-RhB without ball milling. These results clearly confirm that the MC process promoted the doping of Bi2O3 by ex-situ added Br ions. As a result, the doping with Br ions improved the migration and activity of the lattice oxygen in Bi2O3 for the oxidation of RhB. In turn, the consumption of lattice oxygen by RhB also accelerated the substitution of oxygen vacancy by Br ions. Because of the strong electronegativity of bromine atoms, the oxidative debromination of BDE209 over Bi2O3 was quite slow, which also limited the formation of highly active Br-doped Bi2O3. In contrast, BDE209 tended to the reduction reaction over Fe powders. When Fe was added to the Bi2O3-BDE209 system, the rapid reductive debromination of BDE209 over Fe produced Br ions and less brominated intermediates (eq. 12). The in-situ 20
released Br ions acted as the dopant of Bi2O3 to improve the mobility and activity of lattice oxygen(eq. 9), while the debrominated intermediates became more susceptible to the O2−-participating oxidation reaction. In addition, the reaction of debrominated intermediates with Bi2O3 could save the reducing agent so that a larger amount of Fe was used to initiate the BDE209 reduction relative to that by using Fe as the milling agent alone. Therefore, it was observed a great promoting effect of Fe on the MC treatment of BDE209 with Bi2O3.
1.0
(a)
1.0 Bi2O3-RhB
0.8
Bi2O3-RhB
Bi2O3-BDE209 0.6
0.8
c/c0
c/c0
(b)
0.4
PS-BDE209
0.2
PS-RhB
0.6 Bi2O3-KBr-RhB
0.0
0
1.0
20 40 Milling time /min
0.4
60
0
(c)
20 40 Milling time /min
(d)
Bi2O3 Bi4O5Br2
Bi2O3-KBr
Bi2O3-KBr-RhB
0.6
Bi2O3-RhB
Intensity
c/c0
0.8
60
KBr-Bi2O3-RhB
KBr-Bi2O3
0.4 0
20 40 Milling time /min
20
60
30
40 2degree
50
60
Figure 6. (a) MC degradation of BDE209 or RhB with PS (nPS/nsubstrate = 50:1) or Bi2O3 (nBi2O3/nsubstrate = 5:1) under the same optimum conditions as either the previous MS-PS system [12] or the present MS-Bi2O3 system. (b) The removal of RhB and (c) water dissolved Br in MC systems composed of Bi2O3 (0.93 g), KBr (0.12 g), and/or RhB (0.38 g) under basic 21
conditions. (d) XRD patterns of the samples after 2 h of ball milling in Figure b and c. 4. Conclusions The MC treatment of BDE209 was studied by the simultaneous use of Bi2O3 and Fe as the co-milling reagents. A beneficial behavior between Bi2O3 and Fe was dependent on the molar ratio of Bi2O3 to Fe, the mass ratio of milling ball to the reaction mixtures, and the rotation speed. Under optimum conditions, a BDE209 degradation of 96.6% achieved in the Bi2O3 and Fe combined system after 2 h of ball milling, which was 1.5 and 4.0 times of that in the system of Bi2O3-BDE209 and Fe-BDE209, respectively. XPS, XRD and TG analysis confirmed that the MC process activated the lattice oxygen of Bi2O3 for oxidizing BDE209, and Fe acted as the reducing agent to initiate the reduction of BDE209. The significantly enhanced degradation in the Bi2O3 and Fe combined system was owing to an integration of the rapid reductive debromination of BDE209 over Fe and the subsequent oxidation of its debrominated intermediates over Bi2O3. Moreover, the fast in-situ released Br ions via the reduction process could enter the lattice of Bi2O3 and improve the mobility and activity of lattice oxygen. Because BDE209 is easily reduced and its less brominated intermediates are susceptible to oxidation, the combined system was much superior to the single reductive/oxidative process. The strategy of the concerted reduction of parent pollutant and subsequent oxidation of its daughter compounds will provide an alternative approach to the MC treatment of toxic halogenated pollutants. Acknowledgements The authors acknowledge the financial supports from the National Natural Science Foundation (Grants No. 21477043), Shenzhen Knowledge Innovation Plan of Shenzhen City Technology Innovation Committee (No. JCYJ20150616144425374), and Fundamental Research Funds for the Central Universities in China (2015YGYL024). 22
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S2213-3437(17)30007-6 http://dx.doi.org/doi:10.1016/j.jece.2017.01.008 JECE 1423
To appear in: Received date: Revised date: Accepted date:
21-7-2016 5-1-2017 7-1-2017
Please cite this article as: Zhimin Zhang, Nan Wang, Lihua Zhu, Hanqing Lv, Xuelin Dong, Huijuan Chai, Heqing Tang, Synergistic effect between Fe and Bi2O3 on enhanced mechanochemical treatment of decabromodiphenyl ether, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2017.01.008
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Synergistic effect between Fe and Bi2O3 on enhanced mechanochemical treatment of decabromodiphenyl ether Zhimin Zhanga, Nan Wanga,b,*, Lihua Zhua, Hanqing Lva,b, Xuelin Donga, Huijuan Chaia,b, Heqing Tangc,* a
Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong
University of Science and Technology), Ministry of Education，School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 430074, Wuhan, China b
Shenzhen Institute of Huazhong University of Science and Technology, 518000, Shenzhen,
China c
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission
and Ministry of Education, College of Resources and Environmental Science, South-Central University for Nationalities, Wuhan 430074, PR China *Corresponding author. Tel: +86 27 87543432; Fax: +86 27 87543632; E-mail: [email protected]; [email protected].
1
2
ABSTRACT The present work investigated the mechanochemical (MC) treatment of powdery decabromodiphenyl ether (BDE209) with Bi2O3 and/or Fe powders as co-milling reagents. The simultaneous use of Bi2O3 and Fe with a Bi:Fe:Br molar ratio of 1:1:1 led to a BDE209 degradation of 96.6% within 2 h of ball milling at rotation speed of 400 rpm, whereas only 66.0% and 24.0% of BDE209 was removed in the MC-Bi2O3 and MC-Fe system, respectively. It was demonstrated that the MC process activated the lattice oxygen (O2−) of Bi2O3 and promoted its reaction with BDE209, whereas Fe initiated the reduction of BDE209. The MC treatment of BDE209 through a single reductive in the MC-Fe system or oxidative process in the MC-Bi2O3 system was not efficient, due to that BDE209 is rather difficultly oxidized and the debrominated intermediates are resistant to reduction. When combining Bi2O3 with Fe, the reductive debromination of BDE209 over Fe and the subsequent oxidation of debrominated products over Bi2O3 were integrated into a concerted process. The rapid reduction of BDE209 over Fe not only released Br ions, which acted as the dopant to improve the activity of O2− in Bi2O3, but also generated less brominated intermediates, which were more susceptible to the O2−-participating reaction over Bi2O3. Thus, a strong synergistic effect between Bi2O3 and Fe was observed on the MC degradation of BDE209. Keywords: Decabromodiphenyl ether; Mechanochemical degradation; Synergistic effect; Bi2O3, Iron
3
1. Introduction Mechanochemical (MC) method based on grinding reactants in solid phase has recently attracted much attention as a simple route for treating toxic organo-halogen compounds [1, 2]. In the MC process, solid-to-solid reactions are activated by collisions with milling bodies in the milling devices. Compared with combustion techniques, the MC method is favorable for simplified operations on a large amount of toxic compounds by eliminating the use of organic solvent, and avoiding the formation of toxic dioxins, which was possibly released in the incineration treatment of organo-halogen pollutants [3]. Many earlier studies focused on the MC destruction of chlorinated contaminants like chlorobenzenes,
dichlorodiphenyltrichloroethane,
polychlorinated
biphenyls
and
polychlorinated dibenzodioxins [4-7]. Recently, some attention has been paid to the fluorinated
and
brominated
pollutants
such
as
perfluorooctane
sulfonyl,
fluoridehexabromobenzene, tetrabromobisphenol A (TBBPA) and polybrominated diphenyl ethers (PBDEs) [8-14]. These brominated flame-retardants are another class of halogenated molecules that are recognized to cause adverse effects to ecosystems and human health [15]. Among them, decabromodiphenyl ether (BDE209) is widely used in textiles, plastics, and electronic products, to prevent fire. As non-reactive additives, BDE209 easily migrates from products and spread into the environment, and is becoming a new class of global contaminants. Shintani et al. first applied MC treatment for the decomposition of BDE209 by using a planetary ball mill and CaO as the co-milling agent [11]. When the molar ratio of CaO to BDE209 was 343:1 and the rotation speed was 700 rpm, more than 99% of BDE209 was degraded after 1 h milling. In this case, both much excessive CaO (343 fold molar of BDE209) and a high rotation speed (700 rpm) were required to ensure high reaction rate and complete destruction. Our group proposed an oxidative method to the decomposition of powdery 4
BDE209 by using MC activation of persulfate (PS), which achieved a complete debromination and mineralization of BDE209 within 3 h of milling with the considerable lower PS-BDE209 molar ratio of 50:1 at a rotation speed of 400 rpm [12]. Zhang et al. reported that Bi2O3 could be used in the exactly stoichiometric amount (i.e., Bi2O3-BDE209 molar ratio of 5:1) for the destruction of BDE209 [13], during which a new compound BiOBr was formed. Both the carbonization to inorganic carbon and the decomposition to CO2 were observed in this system, although the underlying mechanism of the Bi2O3-involved reaction process remains unknown. In addition, this system required still a high rotation speed of 700 rpm to promote the MC destruction of BDE209. It was worth noting that the generation of CO2 and CO during the MC degradation of BDE209 with Bi2O3 indicated the destruction of aromatic structure of BDE209 [13]. As decreasing the number of Br atoms in the PBDEs molecule, the benzene ring of PBDEs will be more easily broken [16, 17]. Therefore, we anticipate that the combination of Bi2O3 with a proper reducing agent may improve the MC destruction of BDE209 at a considerable lower rotation speed. Zero-valent iron (Fe) powder merits our attention, because of its low cost, availability and especially excellent reductive capability for the reductive debromination BDE209 in liquid solvents [18, 19] and the treatment of halogenated organic compounds in the MC process [10, 20-22]. In this study, we utilized the combination of Bi2O3 and Fe to treat BDE209 under ball milling, and found that the addition of a little amount of iron powders significantly enhanced the MC degradation of BDE209 with Bi2O3 at 400 rpm. 2. Experimental section 2.1. Materials BDE209 (purity>98%) was purchased from Aldrich Chemical Co. (Milwaukee, WI). Bismuth(III) oxide (Bi2O3, 99% in purity) and iron powders (Fe, 99% in purity) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used without further 5
treatments. The solvents for gas chromatography-mass spectrometer (GC-MS) and high performance liquid chromatography (HPLC) analysis were Absolv grade and HPLC grade, respectively. All the other chemical reagents were analytic grade and used as received. Milli-Q water (18.2 MΩ cm) was used in the present work. 2.2. MC treatments The MC reactor consisted of a planetary ball mill (PM100, Retsch, Germany) and a stainless steel pots (250 cm3). In a typical run, 0.278 g BDE209 powders were finely mixed with 0.682 g of Bi2O3 and 0.164 g of Fe in a mortar. Then, the mixture powders were transferred into the stainless steel pot. Because we previously found that the simultaneous use of balls with 10 mm- and 20 mm-diameter at a number ratio of 4:1 obtained higher destruction efficiency than that by using singe sized balls, the present work employed 20 balls with 10 mm-diameter and 4 balls with 20 mm-diameter to grind samples. After the pot was sealed tightly at ambient pressure, the planetary ball mill was operated at 400 rpm with automatic rotation direction changing per 15 min. 2.3. Analysis and characterization For determining BDE209 residues and its degradation intermediates, 0.020 g of the milled sample were taken and extracted using 10 mL tetrahydrofuran (THF) under ultrasonic treatment for 10 min. After removing the powder by centrifugation, the collected solution was filtered with a 0.22-m membrane and then divided into two parts. One part was subjected to HPLC analysis on Agilent 1260 HPLC system equipped with a diode array detector (DAD) and a SB-C18 column (4.6×150 mm). If needed, some samples were diluted prior to HPLC treatments. The mobile phase was 97% acetonitrile and 3% water at 1.0 mL min−1, and the detection wavelength was set at 240 nm. The other part was used for the identification of
6
organic bromine compounds by GC-MS with the same instrumental conditions as described in our previous study [12, 16]. When using Rhodamine B (RhB, C28H31ClN2O3) as the model pollutant, the RhB residues were extracted with ethanol under the same ultrasonic conditions, and determined on a UV-Vis spectrophotometer at the wavelength of 564 nm (Cary 60, Agilent). To compare the chemical changes during milling, the samples were analyzed by X-ray photoelectron spectroscopy (XPS) on a VG Multilab 2000 spectrometer (Thermo Electron Corporation) with Al Kα radiation as the exciting source (300 W). X-Ray diffraction (XRD) was performed on an X-ray diffractometer (Bruker D8 Advance) with monochromatized Cu Kα radiation. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. Thermogravimetric analyses were performed on a NETZSCH TG 209 F3 thermal analyzer. Samples were analyzed at a heating rate of 10 °C min-1 to 950 °C under an atmosphere of air or N2. 3. Result and discussions 3.1. Enhanced MC degradation of BDE209 with the combination of Bi2O3 and Fe Figure 1 illustrated the MC degradation of BDE209 by using Bi2O3, Fe powder, and their mixture. When using Bi2O3 alone as the co-milling reagent at the Bi/Br molar ratio of 1:1, the removal of BDE209 was negligible in the first 15 min of ball milling, but slowly reached to 66.0% after a 2 h of MC treatment. When using Fe alone as an additive with the Fe/Br molar ratio of 1:1, 24.0% of the added BDE209 was rapid degraded after 30 min of ball milling, and then the decay became very slow as prolonging milling time to 2 h. As an efficient reducing agent, Fe powder is capable of reducing BDE209. As the reaction proceeded, Fe powders were transformed to iron oxides, resulting in a full consumption of the reducing agent and no further increase of the degradation of BDE209 after 30 min. However, the combination of Bi2O3 and Fe powders with the Bi/Fe/Br molar ratio of 1:1:1 led to an efficient treatment of BDE209 7
with 96.6% of BDE209 removal after 2 h of ball milling. If further prolonging ball milling to 2.5 h, a complete degradation of BDE209 was observed (data not shown). This suggests that the simultaneous addition of Bi2O3 and Fe exhibits synergistic effect on the MC treatment of BDE209. 100 Bi2O3+Fe Bi2O3 Fe
BDE209 removal /%
80 60 40 20 0 0
30
60 90 Milling time /min
120
Figure 1. MC degradation of BDE209 (0.2776 g) with Fe, Bi2O3 or mixture of them (Bi2O3 + Fe) as the co-milling agents. Unless otherwise stated, the basic reaction conditions were as follows: the dosage of co-milling agents was fixed at the molar ratio of Bi/Fe/Br of 1:1:1; ball-to-powder mass ratio, 185:1; 20 of the 24 used balls were of a diameter (d) of 10 mm, while the remaining 4 balls with d of 20 mm; rotation speed, 400 rpm. 3.2 Effects of treatment conditions The effects of the molar ratio of Bi2O3 to Fe, ball-to-powder mass ratio (mb/mp), and the ball rotation speed were further studied. At a given initial amount of BDE209 (0.278 g), the influence of the Fe/Bi molar ratio on the MC degradation of BDE209 was studied by fixing the total amount of Bi2O3 and Fe at 0.846 g. As shown in Figure 2a, as the molar ratio of Fe/Bi was increased from 0 to 1, the removal of BDE209 within 1 h of milling was significantly
8
increased from 47.7% to 90.5%, and then decreased to 37.2% when further increasing the Fe/Bi molar ratio to 5. In the combination system, Fe makes a great contribution to initiating the reductive degradation of BDE209, while Bi2O3 plays a key role in the subsequent oxidation of the less brominated products (see section 3.4 for more detailed discussion). At the low Fe/Bi molar ratio range, the increasing of the reducing agents (Fe) enhanced its ability to initiate the reductive debromination of BDE209. At the high Fe/Bi molar ratio range, the excessive Fe would react with O2 to form iron oxides, leading to that the surface of Fe powders is passivated and consequently the reductive degradation of BDE209 becomes slower. Figure 2b illustrated the effect of the rotation speed on the degradation removal of BDE209 with both Bi2O3 and/or Fe within 1 h of ball milling. In the cases of Bi2O3 or Fe alone, the removal efficiency of BDE209 was almost increased linearly with increasing the rotation speed from 300 to 450 rpm. This is in good agreement with the observation of Zhang et al., who found that more than 95% of BDE209 could be removed after 1 h milling at 700 rpm [13]. An increase of rotation speed increases the collision velocity and the number of collisions per unit time, both of which are favorable to the reaction of co-milling reagents with BDE209 [23]. In contrast, by increasing the rotation speed from 300 to 450 rpm in the combination system of Bi2O3 and Fe, the removal efficiency of BDE209 was initially increased and reached a maximum at 400 rpm, and then is kept almost constant at higher rotation speeds. This means that in this coupled system, it is not necessary to use a high rotation speed, which will save energy consumption. It was noted that at any given rotation speed, the removal efficiency of BDE209 in the combination system containing both Bi2O3 and Fe was larger than in their single system. As shown in Figure 2c, as the mb/mp increased, the degradation removal of BDE209 within 1 h milling was continually increased, and achieves a maximum value at mb/mp of 185 in the 9
case of Bi2O3 + Fe. At any tested mb/mp, the removal efficiency of BDE209 in different systems increased in the order of Fe < Bi2O3< Bi2O3 + Fe, implying that the simultaneous introduction of Bi2O3 and Fe greatly promoted the MC treatment of BDE209.
BDE209 removal /%
100 80
Bi2O3+Fe
Bi2O3+Fe
Bi2O3
Bi2O3
Fe
Fe
60 40 20
(a)
0 0
1
2
3
4
5
nFe/nBi
(c)
(b) 300
350
400
450
50
Ratotion speed /rpm
100
150
200
250
mb/mp
Figure 2. Effect of (a) the Fe/Bi molar ratio (nFe/nBi) in mixture co-milling agents of Bi2O3 and Fe, (b) rotation speed, (c) ball-to-powder mass ratio (mb/mp) on the MC destruction of BDE209 within 1 h of ball milling. 3.3 Conversions of elements Br, Bi and Fe during the ball milling Because the atomic number and the position of bromine atom in the benzene ring greatly influenced the toxicity and stability of PBDEs, the transformation of bromine atoms was examined. Like in the previous reported MC-Bi2O3 system [13], it was not found any obvious brominated organic intermediates, water-dissolved or gaseous bromine species in the Bi2O3-Fe combined MC system. XPS analysis is used to measure the chemical state of Br as well as Fe, Bi and O in the samples before and after ball milling. Figure 3a showed the high resolution XPS spectra of Br 3d region. For the original powders, two types of bromine could be identified: the binding energies of Br 3d5/2 and Br 3d3/2 located at 69.9 eV and 71.0 eV were ascribed to the covalently bonded bromine atoms in BDE209 having the calculated natural bond orbital (NBO) 10
charge of (0.1540.001) e, and those at 69.6 eV and 70.6 eV was assigned to Br 3d5/2 and Br 3d3/2 of the covalently bonded bromine atoms with the NBO charge of (0.1440.003) e, respectively (Figure 3a). After 0.5 h of ball milling, the Br signals turned out to be comparatively broad as well as appeared new peaks at 68.2 and 69.2 eV, which was attributed to Br 3d5/2 and Br 3d3/2 of Br− ions, respectively (curve 2 in Figure 3a). As prolonging the ball milling time to 2 h, the ratio of covalently bonded bromine atoms to the total bromine was decreased to 15%, and that of Br− ions was increased to 85% (Figure 3a) [24, 25]. This suggests that the released bromine element from BDE209 was converted to Br− ions. Figure 3b illustrated the XPS profiles of Bi 4f. The characteristic peaks of Bi 4f5/2 at 163.9 eV and Bi 4f7/2 at 158.6 eV indicated a trivalent oxidation state for Bi–O bonding in the sample before ball milling [26, 27], and the very weak peak of Bi 4f5/2 (4f3/2) at 165.7 (160.4) eV suggested the presence of a small amount of Bi5+ (ratio to the total Bi, 5.4%), due to the intrinsic oxygen deficient in Bi2O3 [28]. After 0.5 h of MC treatment, the signals of both Bi3+ as the Bi2O3 form and Bi5+ were significantly decreased, and the new peaks at 164.6 eV (Bi 4f5/2) and 159.3 eV (Bi 4f7/2) appeared, suggesting the generation of Bi3+ as the form of BiOBr or BixOyBrz [24, 25]. After 2 h of MC treatment, the ratio of Bi as BiOBr and Bi5+ was increased to 64.3% and 19.2%, respectively, and that as Bi2O3 was decreased to 16.5%, suggesting that Bi2O3 was gradually converted to BiOBr as well as the formation of more oxygen deficiencies. Due to the greater ionic radius of Br− ions, the introduction of Br− turned to change the structure of Bi2O3 containing Bi3+, Bi5+ and oxygen deficient. Thus, the binding energies of Bi3+, Bi5+ as well as Br− was slightly changed. The high resolution XPS profile of O 1s showed dominant peaks at 529.4 eV, 531.2 eV and 532.8 eV for the sample before reaction (Figure 3c), which are the characteristic of structural oxygen, the O2− ions locating in an imperfect lattice with oxygen deficiencies and physically adsorbed oxygen on Bi2O3, respectively [26, 27]. After the MC reaction, the relative intensity of the above three peaks was significantly decreased, and all of 11
them moved to a higher binding energy (Figure 3c). According to the previous reports [24, 25], the O 1s peaks at 530.1 eV, 531.8 eV and 533.8 eV was attributed to the lattice oxygen, oxygen vacancies and surface adsorbed oxygen on bismuth oxybromides, respectively. From these results, it can be concluded that the MC treatment with both Bi2O3 and Fe led to the conversion of covalently bonded bromines in BDE209 to Br− as the form of BiOBr or other substoichiometric BixOyBrz. In addition, the binding energies of Fe 2p at 706.8 eV and 720.0 eV in the sample without ball milling are characteristic of Fe0 [29], while the peaks at the binding energy of 709.9 and 723.4 eV in the milled samples are characteristic of Fe3O4 or Fe2O3 (Figure 3d) [24, 30]. This further confirmed Fe was converted to iron oxides during the MC process. In other word, the role of Fe as the reducing reagent during the MC treatment of BDE209.
(b) Bi 4f 0h
0h
0.5 h
0.5 h
Intensity
Intensity
(a) Br 3d
1h
72
70 Binding energy /eV
2h
9000
2h
6000
74
1h
167
68
165
163
161
Binding energy /eV
12
159
157
(c) O 1s 0h
0h
0.5 h
0.5 h Intensity
Intensity
(d) Fe 2p
1h
2h
1h
2500
2h
6000
536
534
532 530 Binding energy /eV
730
528
723
716 709 Binding energy /eV
702
Figure 3. High resolution XPS spectra of (a) Br 3d, (b) Bi 4f, (c) O 1s, and (d) Fe 2p for the mixture of BDE209, Bi2O3 and Fe before and after being milled for 0.5 h, 1 h and 2 h. XRD patterns of powdery BDE209, Fe, Bi2O3 and their mixtures were recorded before and after ball milling. As shown in Figure 4a−4c, only the diffraction peaks of Bi2O3 (JCPDS Card no. 01-076-1730), Fe (JCPDS Card no. 01-087-0721) and/or BDE209 were observed clearly in the XRD spectra of the mixtures before reaction. After 2 h of milling, the patterns of Bi2O3 in the Bi2O3-BDE209 system became weaker, and the dominant peaks at 2θ of 27.0o, 27.5o and 28.1o were broadened, e.g., the peak width at half-maximum intensity (FWHM) of these three peaks was increased from 0.150o, 0.147o, and 0.148 o to 0.339o, 0.309o and 0.183o, respectively (Figure 4a). The peak broadening at 2θ of 28.1o were less serious than that for the other two peaks. This indicated that the broadening arose from the crystal imperfections and distortion of Bi2O3 instead of the crystallite size decreasing [31]. Moreover, new diffraction peaks appeared at 2θ of 29.5o and 31.8o, which were assigned to Bi4O5Br2 (JCPDS Card no. 00-037-0699) (Figure 4a). This further supported that the lattice distortion of Bi2O3 was due to the Br-doping. It seems that this result is different from the report of Zhang et al., who pointed out that Bi 2O3 was completely changed into BiOBr during the MC treatment of BDE209 [13]. However, it 13
was a false difference, because we noted that a similar peak corresponding to Bi4O5Br2 also appeared in their work, but they did not consider other forms of BiXOYBrZ except BiOBr. For the mixture of BDE209 and Fe after 2 h milling (Figure 4b), the patterns of Fe disappeared, and the new peaks at 2θ of 30.0o, 35.4o, 43.0o, and 56.9o corresponding to Fe3O4 (JCPDS Card no. 01-089-0688) appeared. The oxidation of Fe0 to Fe2+/3+ suggests that Fe acted as the reducing agent to reduce BDE209. In the Bi2O3-Fe-BDE209 system, the XRD patterns of Bi2O3 were decreased gradually as prolonging MC reaction time, and the dominant peaks at 2θ of 27.0o, 27.5o and 28.1o were also broadened (Figure 4c). However, the FWHM was firstly increased as the reaction time was increased from 0 to 0.5 h, and then decreased if further prolonging the reaction time to 2 h. For example, the FWHM at 2θ of 27.0o (27.5o) was changed from 0.144o (0.130o) at 0 h to 0.239o (0.200o), 0.204o (0.184o), and 0.181o (0.169o) at 0.5 h, 1 h, and 2 h, respectively. This suggested that the crystalline structure of Bi2O3 in the Bi2O3-Fe-BDE209 system became disordered after the reaction, but it was described in different terms of substitution disorder from that in the Bi2O3-BDE209 system. Accordingly, new different peaks were observed after ball milling and could be assigned to BiOBr (JCPDS Card no. 00-001-1004) instead of Bi4O5Br2 (Figure 4c). In addition, it was observed the oxidation of Fe to Fe3O4 after the reaction. Both the formation of BiOBr and Fe3O4 were increased as prolonging the reaction time. These results are in agreement with the above XPS results, and further confirmed that the organic bromines of BDE209 were transformed into inorganic form as BiOBr. Furthermore, relative to the product Bi4O5Br2 in the Bi2O3-BDE209 system, the formation of BiOBr implies a higher conversion ratio of organic bromine in the Bi2O3-Fe-BDE209 system. In other word, the combination of Bi2O3 and Fe in the MC process enhanced the debromination of BDE209.
14
(b)
(a)
Bi2O3(01-076-1730)
Fe3O4(01-089-0688)
0h
0h
40 2θ /degree
50
20
60
Bi2O3(01-076-1730)
40 2θ /degree
50
60
0.5h 1h
30
40 2θ /degree
50
Fe (01-087-0721)
Fe3O4(01-089-0688)
Intensity
Intensity
30
Fe2O3(01-073-0603)
BiOBr(00-001-1004) Fe3O4(01-089-0688)
20
2h
(d)
2h
BDE209
(c)
0h
2h
30
20
Intensity
Intensity
Bi4O5Br2(00-037-0699)
Fe(01-087-0721)
BiOBr(00-001-1004) Bi4O5Br2(00-037-0699) Bi2O3(01-076-1730)
20
60
30
40 2θ /degree
50
60
Figure 4. XRD patterns of the samples before and after ball milling: (a) Bi2O3 and BDE209, (b) Fe and BDE209, and (c) Bi2O3, Fe and BDE209. (d) Standard diffraction patterns of the related compounds (vertical lines were referred to JCPDS Cards). The XRD patterns of pure BDE209 powders were given as a reference in Figure b. that no signals of BDE209 were observed at 0 h is consistent with the complete adsorption of BDE209 on the TiO2 particle surface. The conversion of BDE209 and co-milling reagents was also studied by formation of thermogravimetric (TG) analysis. When operating TG analysis in air atmosphere (Figure 5a), the mixture of Bi2O3, Fe, and BDE209 before the MC reaction (curve 1) showed a mass loss of 22.6% between 300−400°C (stage I) and a mass increase of 5.5% between 480−660 °C (stage 15
II), corresponding to the thermal decomposition of BDE209 (theoretic value, 24.7% in total mass), and the oxidation of iron by O2 (theoretic value, 14.6% (Fe in total mass) × 38.1% = 5.6%) (eq.1), respectively. If operating at N2 atmosphere, a similar mass loss (24.8%, stage I in Figure 5b) was observed in the sample without milling, but no mass was increased, due to the lack of O2 to reaction with Fe. For the 1h milled sample, the weight loss exhibited a much wider temperature range in air (curve 2 in Figure 5a). The nearly 5% loss between 300−400 °C (stage III in Figure 5a) exceed the weight ratio of residual BDE209, which was calculated to be 2.4% from the HPLC result in Figure 1. In addition, no significant weight change owing to the residual BDE209 was observed in N2 at the same temperature range (curve 2 in Figure 5b). This suggests that another component was formed, and could be assigned to the amorphous carbon (theoretic value, 2.7%), which is particularly reactive to oxygen at elevated temperatures [32]. No weight was increased for the 1 h milled sample in air (curve 2 in Figure 5a), which agreed with both XRD and XPS results, due to the transformation of Fe to iron oxide. Compared to the air atmosphere, the use of N2 atmosphere resulted in a larger mass loss in the temperature range of 390−610 °C (stage II in Figure 5b). It was attributed to the thermolysis of BiOBr, which can undergo different thermolysis routes in N2 and air (eqs. 2−4). The mass loss of 9.5% in air (stage IV in Figure 5a) corresponded to the decomposition of BiOBr into solid Bi24O31Br10 and Br2 gas (eq. 2), whereas the mass loss of 39.1% in N2 (stage II in Figure 5b) corresponded to the decomposition of BiOBr into Bi2O3 and BiBr3 gas (eq. 4). When prolonging the reaction to 2 h, the mass loss in air atmosphere at 300−400 °C decreased from 5% to 2.3% (stage V in Figure 5a), indicating all the residual BDE209 was further converted to amorphous carbon and BiOBr. Accordingly, a larger mass loss owing to BiOBr occurred in N2 atmosphere at 390−610 °C (stage III in Figure 5b). These results again demonstrate that the MC treatment of BDE209 with the Bi2O3 and Fe led to the conversion of organic bromine in BDE209 to BiOBr. 16
3Fe + 2O2
(1)
Fe3O4
24BiOBr + 3.5O2
Bi24O31Br10 + 7Br2
(2)
12Bi2O3 + 5Br2
(3)
Bi24O31Br10 + 2.5O2 3BiOBr
100
VI
3 2 1
Weight change /%
IV
I Weight change /%
100
V III
II
80
60
200
I 80
II III 1 2 3
60
(b) In N2
(a) In air
40
40 50
(4)
Bi2O3 + BiBr3
350 500 650 Temperature /oC
800
50
200
350 500 Temperature /oC
650
800
Figure 5. TG curves of the mixture of Bi2O3, Fe and BDE209 in the atmosphere of (a) air and (b) N2 before (1) and after being milled for 1 h (2), and 2 h (3). 3.4 Reaction mechanism of MC degradation of BDE209 by Bi2O3 and Fe The combination of Bi2O3 and Fe showed a synergistic effect on the MC treatment of BDE209 (Figure 1 and 2). BiOBr and Fe3O4 were observed accompanying the rapid disappearance of BDE209 (Figure 3−5). The oxidation of Fe indicated that it acted as the reducing reagent and provided electrons for the reductive debromination of BDE209. The conversion of BiOBr suggested the structure of Bi2O3 was destroyed and the lattice oxygen of Bi2O3 was replaced by bromine elements. In earlier reports, Bi2O3 is an important metal oxide semiconductor catalyst and has been identified as photocatalysts and sonocatalysts in the degradation of organic pollutants [33−35]. 17
In these systems, the heat and light energies can excite Bi2O3 particles to form indirectly various active species like photo-generated holes and •OH radicals, which are responsible for the oxidative degradation of organic pollutants. However, this mechanism could not explain well the present results, because the release of dissolved halide ions was observed in Bi2O3-partipantited photocatalytic and sonocatalytic systems [32-34], but the formation of bismuth oxyhalides was confirmed in the present MC system. It is worth noting that Bi2O3 is also a well-known fluorite-type oxide ion electrolytes at high temperature that transport O2− ions through intrinsic oxygen vacancies to the anode where the fuel reacts with the O2− ions to oxidation products [36]. The XPS analysis in Figure 3c showed that 47.9% of oxygen locating in the imperfect lattice with the oxygen deficiencies, which was much higher than that of the lattice oxygen of Bi2O3 (23.0%). The high-energy impact during ball milling produces transient temperature as high as 1000 °C, which is high enough to activate Bi2O3 to exhibit excellent ionic conductivity for the O2− migration and the subsequent O2−-involved oxidation process. Moreover, Lou et al. demonstrated that the doping of Bi2O3 promoted the oxygen mobility of Co3O4 and enhanced the reaction CO with the lattice oxygen to produce CO2 [37]. Based on the above discussion, we proposed a tentative explanation to the MC promoted reaction of BDE209 with Bi2O3 and Fe (and their synergistic effect) on the degradation of BDE209, as expressed by eqs. 5−12. 2 2 OO(Bi Os(Bi 2O3 ) 2O3 )
(5)
2 R - Br Os(Bi oxidation products(e.g., CO 2 ) VO(Bi 2e 2O3 ) 2O3 )
(6)
2 O2 2e VO(Bi OO(Bi 2 O3 ) 2 O3 )
(7)
R - Br e VO(Bi H2O RH Br OH 2 O3 )
(8)
2Br VO(Bi 2Br(Bi Bi X BrY O Z 2 O3 ) 2 O3 )
(9)
2 2 O O(Bi O s(Bi X BrY O Z ) X BrY O Z )
(10) 18
2 R - Br Os(Bi oxidation products VO(Bi 2e X BrY O Z ) X BrY O Z )
(11)
R - Br Fe H2O RH Br 0.5Fe2 OH
(12)
2 Upon the ball milling, the lattice oxygen of Bi2O3 ( OO(Bi ) becomes highly mobile and is 2 O3 )
2 diffused from the bulk to the surface (eq. 5), where the formed surface O2− ions ( Os(Bi ) 2O3 ) reacts with the adsorbed BDE209 to produce oxidation product, oxygen vacancy ( VO(Bi ) 2 O3 )
and electrons (eq. 6). Generally, the oxygen vacancy is replenished by O2 to form the new active oxygen species and complete the redox cycle (eq. 7). However, since BDE209 is very easily reduced, another adjacent BDE209 molecule may trap electrons, leading to the formation of Br− ions (eq. 8). Because of the strong interaction of Br− and Bi, the generated Br− ions enters the lattice of Bi2O3 (eq. 9), leading to that Bi2O3 was gradually transformed to substoichiometric forms of bismuth oxybromides, and finally to BiOBr if there is enough Br− source. Accompanying the degradation of BDE209, the surface O2− ions of Bi2O3 especially locating near the oxygen deficiencies were greatly consumed (Figure 3c), and it was also observed the formation of BiOBr (Figure 4c). Moreover, the relative distribution of O2− ions locating on the lattice oxygen defects of BiOBr was increased as prolonging the ball milling time from 0.5 h to 2 h (Figure 3c). The structural defect and lattice distortion arising from the Br− doping in turn improves the activation ability and mobility of O2− for the oxidation of BDE209 (eqs. 10 and 11). This is why the degradation of BDE209 became faster after a short time period of 15 min in the Bi2O3-BDE209 system (Figure 1). To validate the above presumption, an organic dye pollutant, Rhodamine B (RhB) without bromine atom was chosen as another model pollutant. Under the same MC treatment conditions, the degradation of RhB was nearly as fast as that of BDE209 within the first 15 min, but became slower than that of BDE209 in the following reaction stage in the MC-Bi2O3 system (Figure 6a). Wang et al. also observed a slow reaction rate in the first hour of milling 19
during the MC degradation of dechlorane plus with aluminum and quartz sand, due to that the repeated physical impact facilitates surface reaction activation [23, 38]. However, this explanation could not describe well the present results, because Bi2O3 showed different results towards the degradation of RhB and BDE209. Moreover, if using the common oxidation MC system with persulfate, the oxidation of RhB is much faster than BDE209 (Figure 6a), being different from those on Bi2O3. Relative to refractory BDE209, RhB is more easily to be oxidized by in situ produced SO4• radicals from the MC activation of persulfate. This suggests that the Bi2O3-involved reaction mechanism is also distinct from the general advanced oxidation systems. After adding a small amount of KBr, the MC degradation of RhB with Bi2O3 is greatly accelerated (Figure 6b). Meanwhile, the loss of dissolved free Br ions in the Bi2O3-KBr-RhB system is faster than that in the Bi2O3-KBr system without RhB (Figure 6c). Accordingly, the XRD analysis in Figure 6d shows that more Bi4O5Br2 is formed in the Bi2O3-KBr-RhB system than that in the binary system containing KBr and Bi2O3. For another control binary system composed of Bi2O3 and RhB, no any new diffraction peaks are observed except those of Bi2O3 during ball milling (Figure 6d). In addition, neither the loss of free Br ions nor the formation of Bi4O5Br2 was observed in the systems of Bi2O3-KBr and Bi2O3-KBr-RhB without ball milling. These results clearly confirm that the MC process promoted the doping of Bi2O3 by ex-situ added Br ions. As a result, the doping with Br ions improved the migration and activity of the lattice oxygen in Bi2O3 for the oxidation of RhB. In turn, the consumption of lattice oxygen by RhB also accelerated the substitution of oxygen vacancy by Br ions. Because of the strong electronegativity of bromine atoms, the oxidative debromination of BDE209 over Bi2O3 was quite slow, which also limited the formation of highly active Br-doped Bi2O3. In contrast, BDE209 tended to the reduction reaction over Fe powders. When Fe was added to the Bi2O3-BDE209 system, the rapid reductive debromination of BDE209 over Fe produced Br ions and less brominated intermediates (eq. 12). The in-situ 20
released Br ions acted as the dopant of Bi2O3 to improve the mobility and activity of lattice oxygen(eq. 9), while the debrominated intermediates became more susceptible to the O2−-participating oxidation reaction. In addition, the reaction of debrominated intermediates with Bi2O3 could save the reducing agent so that a larger amount of Fe was used to initiate the BDE209 reduction relative to that by using Fe as the milling agent alone. Therefore, it was observed a great promoting effect of Fe on the MC treatment of BDE209 with Bi2O3.
1.0
(a)
1.0 Bi2O3-RhB
0.8
Bi2O3-RhB
Bi2O3-BDE209 0.6
0.8
c/c0
c/c0
(b)
0.4
PS-BDE209
0.2
PS-RhB
0.6 Bi2O3-KBr-RhB
0.0
0
1.0
20 40 Milling time /min
0.4
60
0
(c)
20 40 Milling time /min
(d)
Bi2O3 Bi4O5Br2
Bi2O3-KBr
Bi2O3-KBr-RhB
0.6
Bi2O3-RhB
Intensity
c/c0
0.8
60
KBr-Bi2O3-RhB
KBr-Bi2O3
0.4 0
20 40 Milling time /min
20
60
30
40 2degree
50
60
Figure 6. (a) MC degradation of BDE209 or RhB with PS (nPS/nsubstrate = 50:1) or Bi2O3 (nBi2O3/nsubstrate = 5:1) under the same optimum conditions as either the previous MS-PS system [12] or the present MS-Bi2O3 system. (b) The removal of RhB and (c) water dissolved Br in MC systems composed of Bi2O3 (0.93 g), KBr (0.12 g), and/or RhB (0.38 g) under basic 21
conditions. (d) XRD patterns of the samples after 2 h of ball milling in Figure b and c. 4. Conclusions The MC treatment of BDE209 was studied by the simultaneous use of Bi2O3 and Fe as the co-milling reagents. A beneficial behavior between Bi2O3 and Fe was dependent on the molar ratio of Bi2O3 to Fe, the mass ratio of milling ball to the reaction mixtures, and the rotation speed. Under optimum conditions, a BDE209 degradation of 96.6% achieved in the Bi2O3 and Fe combined system after 2 h of ball milling, which was 1.5 and 4.0 times of that in the system of Bi2O3-BDE209 and Fe-BDE209, respectively. XPS, XRD and TG analysis confirmed that the MC process activated the lattice oxygen of Bi2O3 for oxidizing BDE209, and Fe acted as the reducing agent to initiate the reduction of BDE209. The significantly enhanced degradation in the Bi2O3 and Fe combined system was owing to an integration of the rapid reductive debromination of BDE209 over Fe and the subsequent oxidation of its debrominated intermediates over Bi2O3. Moreover, the fast in-situ released Br ions via the reduction process could enter the lattice of Bi2O3 and improve the mobility and activity of lattice oxygen. Because BDE209 is easily reduced and its less brominated intermediates are susceptible to oxidation, the combined system was much superior to the single reductive/oxidative process. The strategy of the concerted reduction of parent pollutant and subsequent oxidation of its daughter compounds will provide an alternative approach to the MC treatment of toxic halogenated pollutants. Acknowledgements The authors acknowledge the financial supports from the National Natural Science Foundation (Grants No. 21477043), Shenzhen Knowledge Innovation Plan of Shenzhen City Technology Innovation Committee (No. JCYJ20150616144425374), and Fundamental Research Funds for the Central Universities in China (2015YGYL024). 22
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