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Fe3C nanoparticles into carbonized polydopamine nanospheres for catalytic degradation of tetracycline via persulfate activation
Accepted Manuscript Encapsulation of Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles into carbonized polydopamine nanospheres for catalytic degradation of tetracycline via persulfate activation Kairuo Zhu, Huan Xu, Changlun Chen, Xuemei Ren, Ahmed Alsaedi, Tasawar Hayat PII: DOI: Reference:
S1385-8947(19)30942-8 https://doi.org/10.1016/j.cej.2019.04.157 CEJ 21577
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
9 February 2019 22 April 2019 23 April 2019
Please cite this article as: K. Zhu, H. Xu, C. Chen, X. Ren, A. Alsaedi, T. Hayat, Encapsulation of Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles into carbonized polydopamine nanospheres for catalytic degradation of tetracycline via persulfate activation, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.04.157
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Encapsulation of Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles into carbonized polydopamine nanospheres for catalytic degradation of tetracycline via persulfate activation Kairuo Zhua,b,1, Huan Xua,1, Changlun Chena,c,d,*, Xuemei Rena, Ahmed Alsaedid, Tasawar Hayat d a
Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of
Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China b
University of Science and Technology of China, Hefei 230000, PR China
c
Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education
Institutions, Soochow University, Suzhou 215123, PR China d
NAAM Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia
1
These authors contribute equally to this work.
*Corresponding author: [email protected] (C.L. Chen); Phone: +86-551-65592788. Fax: 86-551-65591310.
1
Abstract Porous carbonized polydopamine nanospheres encapsulated with Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles (Fe@C-PDA) were feasibly fabricated by a one-step pyrolysis of the Fe-PDA polymer, which was employed to activate persulfate (PS) toward degrading tetracycline
(TC).
The
morphology,
structure,
chemical
compositions and textural properties of the resulting catalyst were characterized systematically, and effects of catalyst dosages, PS concentration, inorganic anions (HCO3- and Cl-), natural organic matter and practical water body were studied in detail. The N-doping derived from C-PDA not only enhanced TC adsorption capacity, but also worked as active sites for PS excitation. Moreover, Fe@C-PDA displayed an excellent efficiency and stability in the 5th cycle tests. Electron paramagnetic resonance, classical radical scavenging experiments and X-ray photoelectron spectroscopy analysis disclosed that the active species in the system were identified as sulfate radicals (SO4·−) and hydroxyl radicals (·OH), and the variable chemical valences of Fe3O4/Fe0/Fe3C nanoparticles as well as N-doping in the C-PDA nanospheres contributed to the outstanding catalytic activity. This work can provide new insight of making this kind of Fe-based materials as the promising catalysts for application in eliminating organic pollutants. Keywords: Fe3O4/Fe0/Fe3C nanoparticles; Catalysts; Tetracycline; Degradation
2
1. Introduction Water pollution arising from the abuse of multifarious aromatic hydrocarbon and its derivatives has raised remarkable environmental concerns because of their severe threat to human health and natural aquatic safety [1-3], among which, tetracycline (TC) is an antibiotic product widely used and is not biodegradable under natural conditions [4]. In the past decades, great efforts have been made to locate effective technologies for the disposal of tetracycline from aqueous media. Lately, persulfate (PS) activation was regarded as a potentially effective method degrading the various toxic organic contaminants by sulfate radicals (SO4·−) because of its higher standard redox potential (2.5-3.1 eV), long duration of time in solution and wide operative pH range [5, 6]. Transition metals (Fe, Co, Ni, and Mn, etc.) are commonly applied for PS activation. Among the various transition metals, Fe-based catalysts are widely implemented for persulfate activation due to its merits like the earth-abundance, eco-friendliness and cost-effectiveness [7-10]. However, the majority of them are prone to aggregate and subject to low catalytic activity [11]. Thus, increasing attention has been paid to the development of novel Fe-based materials for improving the activity and stability of catalyst. Recently, the encapsulation of transition metal into nitrogen doped carbon shells has been proposed to be a promising technique to synthesize the functional catalysts with high activity and stability, which not only prolongs their residence and but also promotes the catalytic reaction on a carbon surface owing to the electron transfer from encapsulated transition metal [12, 13]. For instance, Wen and his groups prepared the 3
Fe0/Fe3C-encapsulated in nitrogen doped carbon microcubes for the superior degradation of Bisphenol A [14]. Li et al. synthesized ordered mesoporous nitrogen doped carbon encapsulating Co nanoparticles via thermolysis [15], which exhibited high efficiency in HCOOH dehydrogenation. Wang et al. adopted one-pot pyrolysis route to synthesize the bamboo-like N-doped CNTs encapsulated with Ni nanoparticles [16], which showed the excellent catalytic stability toward removing sulfachloropyridazine. Of note, the direct carbonization of metal-coordinated polymers under an inert gas is seen as a straightforward route to prepare such catalysts [17], in which transition metal nanoparticles were confined in a carbonaceous matrix. Hence, the preparation of the high-performance catalyst based on the encased Fe-based
nanoparticles
and
nitrogen
doped
carbon
layer
derived
from
metal-coordinated polymers for organic contaminant degradation are amongst the ideal scenarios in this work. Herein, we synthesized porous carbonized polydopamine encapsulated with Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles (Fe@C-PDA) catalysts, in which Fe0 is generated by in-situ carbothermal reduction of the rationally designed Fe-PDA polymer precursor. The C-PDA shell not only prevented the Fe species from poisoning or leaching in the extremely harsh environment, but also enhanced catalytic property by its nitrogen-doping. The resulted Fe@C-PDA catalyst was employed to degrade TC by PS activation. The objectives of the present work were (i) to evaluate the catalytic performance of Fe@C-PDA toward TC degradation via PS activation; (ii) to investigate the impacts of some factors including initial TC concentration, different 4
catalyst dosage, solution’s pH, PS dosage, typical anions and practical water bodies on the TC degradation; (iii) to explore the mechanism of PS activation and TC degradation by electron paramagnetic resonance (EPR) and radical scavenging experiments.
2. Material and methods 2.1. Material synthesis. Fe@C-PDA hybrids were synthesized by a pyrolysis technique according to our previous report (seen in Supplementary Material) [18]. The possible preparation route of the materials was shown in Scheme 1. 2.2. Catalytic degradation of TC The TC catalytic degradation activities of Fe@C-PDA hybrids with PS were performed in a 250 mL beaker with continuous shaking under natural light condition. Briefly, Fe@C-PDA catalyst was dispersed into various concentrations of TC solution (150 mL) and then kept stirring for 1 h. The degradation reaction was initiated by introducing certain amounts of PS. At a regular time intervals, 3.0 mL of the suspension was taken out and then filtered (0.22 μm, PTFE) to remove the catalyst solids, following with immediately analyzing by UV-vis spectra (UV-2550, PerkinElmer) at 355 nm [19]. To detect reactive radicals, the catalysts and PS were immediately mixed with 5, 5-dimethyl-pyrroline-N-oxide (DMPO), filtered and analyzed using a Bruker EPR A200 spectrometer. For the recycle tests, the catalysts were magnetically separation and centrifugation, washed with Milli-Q water, dried and used in the following run with similar experimental conditions. Additionally, 5
HCO3-, Cl- or humic acid (HA) (5.0 mM) was added into the suspensions before initiating the reaction to ascertain their influences on TC degradation. Tap water and lake water were also collected to replace Milli-Q water and carried out the degradation experiment. To study the role of N doping, rGO was prepared by the pyrolysis of GO at 700 ℃ under Ar atmosphere and used to degrade TC. Reagents and characterization are also described in the Supplementary Material.
3. Results and discussion 3.1 Structural Characterizations The crystal structure and phase composition of Fe-PDA and Fe@C-PDA were performed by powder X-ray diffraction (XRD) (Fig. 1A). Typical diffraction peaks appeared in the Fe-PDA polymer confirmed that it was completely amorphous. After pyrolysis, the broad peak centered at 26.0° may be due to the successful incorporated nitrogen defects in the graphitic structure [20, 21]. Of note, Fe@C-PDA hybrids contained three Fe species, i.e. Fe0 (JCPDS no. 87-0722), Fe3O4 (JCPDS no. 79-0419) and Fe3C (JCPDS no. 85-1317) [18]. Among these, Fe0 showed the highest intensity, indicative of the maximum proportion of Fe0. 57
Fe Mössbauer spectroscopy was also employed to distinguish different Fe
species (Fig. 1B). The curve of Fe@C-PDA was fitted to four components: two sextet patterns shown in the figure in olive and magenta are assigned to the Fe3O4 phase [22]. The two remaining components-one doublets in blue and purple corresponded to the Fe0 and the Fe3C phases, respectively [23]. Obviously, the relative Fe amount in the Fe0 was higher than that of Fe3O4 and Fe3C (Fe0: 47.96 %; Fe3O4: 15.13 %; Fe3C: 6
36.91 %, Table S1), confirming that the high proportion of Fe0 in the Fe@C-PDA hybrid, in accordance with the XRD result. The scanning electron microscope (SEM) image (Fig. 2A and 2B) showed the structural change in the morphology from Fe -PDA to Fe@C-PDA. Typically, they all had a uniform spherical shape with a diameter of 100-150 nm. In contrast to the Fe-PDA with a smooth surface, the Fe@C-PDA nanosphere surface became rough and presented the obvious collapse of structure. The transmission electron microscope (TEM) images of Fe@C-PDA in Figs. 2C and 2D showed that the Fe3O4/Fe0/Fe3C nanoparticles appeared as black dots (5-20 nm) in the C-PDA nanospheres and were well encapsulated by graphitic carbon layers. The Fe3O4/Fe0/Fe3C nanoparticles were further characterized by high-resolution TEM (HRTEM) measurements. The lattice fringes with spacings of 0.21 nm corresponded to crystalline Fe0 [24, 25]. This geometric confinement of Fe@C (Fig. 2F) not only suppressed dissolution and agglomeration of the inner Fe3O4/Fe0/Fe3C nanoparticles in harsh conditions, but also enriched the electron density on the C-PDA surface, thus promoting the surface catalysis reaction [26]. Moreover, elemental mapping analysis of Fe@C-PDA (Fig. 2G) validated the strong Fe signals and showed that C, O and N elements are uniformly distributed in the spherical carbon matrix. The Raman spectrum of the Fe@C-PDA was shown in Fig. S1A. The characteristic peaks of carbonaceous materials emerged at 1352 and 1600 cm-1, which corresponded to the sp3-graphitic (D band) and sp2-graphitic (G band) configurations, respectively [27]. The intensity ratio of the D to G (ID/IG) was 0.96, indicative of the 7
high disorder degrees of Fe@C-PDA, which meant that Fe@C-PDA had many defects and can afford abundant active sites [28-30]. TGA was employed to determine the change in the weight loss of Fe@C-PDA in air (Fig. S1B). It was stable at low temperature, and 66.39 weight loss occured at 330-525 ℃, which is presumably due to the oxidation of graphitic carbon [31]. The results demonstrated that the content of residual Fe2O3 is 32.44 wt%, corresponding to 22.71 wt% of Fe element (Eq. S1), indicative of high loading of Fe. The resulted Fe@C-PDA catalyst had a high surface area (343.02 m2/g, Fig. S1C) and ordered mesopores (4.09 nm, Fig. S1D) favoring the easy access of TC and PS, leading to enhanced catalytic properties [32]. The saturation magnetization value was measured to be 65.50 emu/g (Fig. S1E), the digital picture (Fig. S1F) indicated the easy separation of the catalyst from the solution by a magnet. XPS spectra were applied to study the composition of Fe@C-PDA. As displayed in Fig. 3A, Fe@C-PDA consisted of 84.34 at% of C, 8.27 at% of O, 4.90 at% of N, and 2.49 at% of Fe. Of note, the atomic percentage of Fe in Fe@C-PDA is a little lower than that in Fe/Fe3C@NC [2] and Fe3C@Fe/N-graphene [33]. Such small quantity of Fe was detected, suggesting that Fe3O4/Fe0/Fe3C nanoparticles are closely wrapped with graphitic carbon layers. Deconvolution spectrum of high resolution C 1s spectra indicated the existence of four peaks at 284.75 (C=C, 56.72 %), 285.57 (C=N&C-O, 11.77 %), 286.13 (C-O-C&C-N, 12.69 %), and 289.36 eV (-O=C-O, 18.82 %) (Fig. 3B) [33, 34]. As shown in Fig. 3C, N 1s spectrum can be separated into three portions at 398.70 (pyridinic N), 400.97 (graphitic N) and 403.33 eV 8
(oxidized N) [35]. As reported, graphitic N atoms (54.1% in our case) possessed higher electronegativity and a smaller atomic radius. Thus, benefiting from high graphitization degree and substantial graphitic N content of C-PDA matrix, it’s efficient to cause the breakage of O-O in PS quickly. 3.2. Catalytic performance of Fe@C-PDA in TC degradation Fig. 4A displayed the removal curve of TC under different reaction systems. For systems in the presence of only PS, almost no TC was removed during the reactions. After introducing C-PDA, the degradation efficiency of TC increased to 20.9%. However, rGO only achieved 4.45 % of removal efficiency under the same conditions (Fig. S3A, XRD patterns of rGO were exhibited in Fig. S3B), which indicated that C-PDA owned weak catalytic ability for activating PS. For Fe@C-PDA, 23.0 % of TC was removed by adsorption (Fig. S4A), demonstrating that TC was first enriched on the surface of catalyst. When Fe@C-PDA and PS were both existed in the reaction system, about 99.7% of TC was successfully decomposed after 60 min, indicating that the N doping and Fe encapsulation induced catalytic enhancement. Fig. 4B displayed the comparison of the degradation abilities in various initial concentrations (50.0, 100.0, and 150.0 mg/L of TC). In the Fe@C-PDA/PS system, with the initial TC concentrations rising, the reduction of degradation efficiencies from 99.7 to 92.7% was reached. The above experimental results indicated that the increase of TC concentration would lead to the competition of more target substances for active species. Impacts of some factors including catalyst dosage, PS concentration and solution’s pH were studied in Fig. 4C, 4D and Fig. S4B. With the 9
increase of catalyst amount from 0.10 to 0.30 g/L, the degradation efficiencies were measurably elevated, which was due to more catalytic active sites produced by enhanced catalyst’s amount. Of note, when PS concentration was 0.15 g/L, 93.9% of TC was removed, and the maximum removal efficiency was achieved at 0.20 g/L. Interestingly, when further increased catalyst dosage to 0.25 g/L, the efficiency remained stable. This observation can be explained that self-quenching reactions of PS can scavenge radicals and hinder the expression of Fe@C-PDA [14]. As showed in Fig. S4B, we could find that initial pH 9 had the best TC removal performance and initial pH 7.0 had the worst TC removal efficiency which can be explained that many of reactive oxygen species was generated under acid condition and SO4.− would react with ·OH (2SO4.− + 2·OH → 2HSO4- + O2↑) under neutral conditions and so that decreased the amount of SO4.− that reacted with TC [8]. The stability and reusability of Fe@C-PDA was further investigated by evaluating five cycling tests. Fig. 5A presented the degradation efficiency of TC after 60 min at each use was 99.7, 96.5, 92.5, 88.4 and 81.3%. Notably, puny decrease of degradation efficiency was observed after each use, which is maybe due to the active catalyst sites covered with intermediate degradation product of TC and the active Fe component of Fe@C-PDA consumed in each use [36]. The Fe dissolution concentrations were recorded by ICP during the five cycling tests (Fig. S4C). For fresh Fe@C-PDA, the accumulated Fe3+ concentration was 187 µg/L, and the Fe3+ concentration decreased as the recycle test continues, which are as lower than the European Union standard (200 µg/L). Therefore, as-developed Fe@C-PDA exhibited excellent stability and 10
recyclability as showed by the high degradation efficiency still above 80% after 5th cycles. 3.3. Catalytic mechanism for TC degradation PS activation was reported to generate SO4·− and ·OH via breaking the peroxo bond. To distinguish different radicals, DMPO was employed as the radical scavenging agent in EPR characterization [37]. As shown in Fig. 5B, for PS alone, some signals for DMPO-OH were detected. For the Fe@C-PDA and Fe@C-PDA/PS systems, strong signals appeared which can respond to the DMPOX adducts associated with the previous reports [38]. It has been reported that DMPO-SO4·− adduct was difficult to detect due to its lower sensitivity and short life-time. Thus, the DMPOX peaks could be originated from the DMPO-SO4·− adduct. Obviously, the intensity of DMPOX peaks was stronger in Fe@C-PDA/PS system, implying that PS activation enhanced obviously with the introduction of Fe@C-PDA. Moreover, the dominant radical that contributes to TC degradation was identified by free radical trapping experiments at EtOH/PS or TBA/PS molar ratio of 500 and 1000 in the Fe@C-PDA/PS system (Fig. 5C). TBA and EtOH were employed as the scavengers of ·OH and SO4·−, respectively [39]. It was observed that the degradation was inhibited in the presence of EtOH than TBA. In particular, a high removing efficiency with the adding the TBA and the weak degradation under the EtOH adding molar ratio of 1000, testified that Fe@C-PDA/PS system was controlled by radical pathway (SO4·− and ·OH radicals) rather than the non-radical active species [40], and the dominant is SO4·− radicals. Moreover, the possible reaction intermediates were also 11
explored in the SI (Fig. S6). FT-IR was used to characterize the structure variation of Fe@C-PDA catalyst before and after used (Fig. S4D). It can be seen that the intensity of Fe-O (500-600 cm-1) and C-H group (1398 cm-1) increased obviously and the C-O group (1252 cm-1) occurred after used. This demonstrated that Fe-based nanoparticles made function in TC degradation and TC molecules were adsorbed on the catalyst surface [41]. The XPS spectra of the Fe@C-PDA catalyst after the catalytic reaction were also discussed for full insight into the PS activation mechanisms. For N 1s after used (Fig. 6B), the relative content of pyridinic N decreased remarkably, manifesting that the N doping is important in PS activation. Besides, the introduction of N-contained groups from TC led to the increasing of the peak at ~401 eV (Fig. S4E). Since the nitrogen atom has higher electronegativity than carbon atom, thus increasing the negative charge density and the amount of higher basicity functionalities on the surface of catalyst, which activates the adjacent carbon atoms to make carbon matrix work as active sites for PS excitation and further enhance the catalytic performance, it’s believable that the N doping of C-PDA can also enhance the quantity of active sites. The O 1s core level has been fitted to four components, i.e., C=O, C-OH, C-C=O, and C-O-C (Fig. S5) [42]. The apparent increase of C-OH in O 1s after used provided dependable evidence for TC loaded on the surface of Fe@C-PDA hybrids. For Fe 2p spectra of the fresh Fe@C-PDA (Fig. 6C), Fe0 (708.72 eV), Fe2+ (710.59 eV), Fe3+ (712.16 eV), and shake-up satellite Fe 2p3/2 (718.64 eV) were identified [43-45]. After being used, the peak of Fe0 obviously reduced and there was a slightly positive shift 12
of Fe2+ component (710.59 → 710.65 eV). Notably, Fe0 could be firstly oxidized to Fe2+ in oxygenated and deoxygenated water (Eq. (1) and (2)), or reacted with PS by a direct electron transfer process (Eq. (3)). What's more, Fe0 could recycle Fe3+ to Fe2+ (Eq. (4)), and this reaction was thermodynamically favorable. 2Fe0 + O2 + H2O → 2Fe2+ + 4OH-
(1)
Fe0 + 2H2O → Fe2+ + 2OH- + H2↑
(2)
Fe0 + 2S2O82- → Fe2+ + 2SO42- + SO4.- (3) Fe0 + 2Fe3+ → 3Fe2+, ∆E0 = 1.21 V
(4)
The above results revealed that Fe3O4/Fe0/Fe3C nanoparticles took part in the redox reaction of electron transfer and acted as electron donors during the catalytic degradation reaction. As presented in Fig. 7, the excellent TC degradation properties in Fe@C-PDA/PS were mainly ascribed to the following: (i) TC and PS were first adsorbed on the Fe@C-PDA surface, which leaded to the enrichment of reactants around the catalyst, (ii) Fe3O4/Fe0/Fe3C nanoparticles with variable chemical valences were beneficial to transfer electrons and served as the active site to activate PS, (iii) the N doping sites also accelerated electron transfer for PS activation. Therefore, the synergistic effects between Fe3O4/Fe0/Fe3C nanoparticles and N doping of C-PDA afforded the Fe@C-PDA catalyst excellent catalytic activity for TC decomposition. 3.4. Effects of inorganic ions and practical water body Fig. 8A displayed the anti-interference of Fe@C-PDA for degrading TC with the presence of inorganic ions (HCO3- and Cl-) and humic acid (HA). The catalytic 13
degradation performance followed the order of Cl- (95.75 %) > HCO3- (83.92 %) > HA (77.48 %). Obviously, Cl- played the tiniest inhibitory impact on the TC degradation. This phenomenon can be explained that Cl- has inhibitory impact for SO4·−. In the PS system, Cl- could be converted into chloride radical (Cl·) [46], which consumes SO4·− and further results into the lower oxidation performance of TC. In comparison with Cl-, HCO3- has more inhibitory impact for TC degradation. HCO3can act as free-radical scavengers and has competitive effect in the degradation process [47]. For HA, which exhibits the maximum inhibitory impact, as we know that the HA with abundant phenolic hydroxyl and carboxyl groups can be adsorbed onto the surfaces of catalysts and further result in the restriction of activating PS. The TC degradation performance in real water bodies (tap water and lake water) was further explored and displayed in Fig. 8B. The TC removal efficiency in tap water and lake water were 90.66 and 66.97%, respectively. Of note, the TC degradation performance in lake water was lower than that obtained in tap water. It can be explained that the organic matters or pollutants in lake water would consume a portion of reactive radicals, thus punily interfere the degradation of TC.
4. Conclusions Through one-pot pyrolysis method combining Fe-PDA polymer as the precursor, we have successfully in-situ synthesized high performance catalysts with Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles encapsulated in porous carbonized polydopamine. The catalysts were then applied for TC degradation with the aid of PS as the active radical source, which exhibited the distinguished catalytic performance 14
and reusability. The analysis of degradation mechanisms indicated that the generated ·OH and SO4·− in the PS activation were the main active radicals and the change of Fe surface valences and N doping sites played important roles in the remarkable degradation performance. This finding is expected to shed light on rationally designing efficient Fe-based catalyst for the elimination of organic pollutants in water. Notes The authors declare no competing financial interests.
Acknowledgements Financial support from the National Natural Science Foundation of China (21477133), the Anhui Provincial Natural Science Foundation (1608085QB44), and Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences are acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.CEJ.2019.
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23
Figure caption Scheme 1. Preparation of Fe@C-PDA from Fe-PDA. Fig. 1 (A) XRD patterns of Fe-PDA complexes and Fe@C-PDA, and (B) room-temperature 57Fe Mössbauer spectrum of Fe@C-PDA. Fig. 2 (A) SEM image of Fe-PDA, (B, C) SEM, TEM images of Fe@C-PDA and (D, E) HRTEM images of Fe@C-PDA. (F) Schematic diagram of the Fe@N-doped C structure. (G) HAADF-STEM image and corresponding elemental mapping images of Fe, C, O and N atoms for Fe@C-PDA. Fig. 3 XPS survey of Fe@C-PDA (A), spectra of C 1s (B) and N 1s (C). Fig. 4 Comparison of the catalytic performance of as-obtained samples (A), the effects of initial TC concentration (B), different catalyst dosage (C), and different PS dosage (D) on TC removal. Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0. Fig. 5 Reusability of Fe@C-PDA for TC degradation (A), EPR spectra for DMPO adducts in different systems (B), degradation of BPA in the absence and presence of MeOH or TBA (C). Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0. Fig. 6 XPS survey of Fe@C-PDA after reaction (A), deconvolution results of N 1s after PS activation (B), and Fe 2p spectra of fresh (C) and used (D) Fe@C-PDA. Fig. 7 Possible reaction mechanism for TC degradation in the Fe@C-PDA/PS system. Fig. 8 Effects of chloride, bicarbonate ions, HA on the removal efficiency of TC (A), and real water body on TC degradation (B). Standard conditions: C[Catalyst] = 0.20 g/L, 24
C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0.
25
Pyrolysis
Fe-PDA
Fe@C-PDA
Scheme 1. Preparation of Fe@C-PDA from Fe-PDA.
26
A
B
♦
Fe3O4
0
♦ Fe ■
C 3 Fe3O4
Carbon
■
♥♥ ♥
♥ ♥
Transmission (a.u.)
Intensity (a.u.)
♥ Fe
Fe@C-PDA ■
■
♦
Fe
0
Fe3C
Fe-PDA
10
20
30
40
50
60
70
2 Theta (degree)
-10
-5
0 Velocity (mm/s)
5
10
Fig. 1 XRD patterns of Fe-PDA complexes and Fe@C-PDA (a), and (b) room-temperature 57Fe Mössbauer spectrum of Fe@C-PDA.
27
Fig. 2 (A) SEM image of Fe-PDA, (B, C) SEM, TEM images of Fe@C-PDA and (D, E) HRTEM images of Fe@C-PDA. (F) Schematic diagram of the Fe@N-doped C structure. (G) HAADF-STEM image and corresponding elemental mapping images of Fe, C, O and N atoms for Fe@C-PDA.
28
C: 84.34% N: 4.90% O: 8.27% Fe: 2.49%
(B)
(C)
G
(A)
)
Binding energy (eV)
3.62 % ic N (3
Oxidized N (12.29 %)
Pyridin
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
)
282 284 286 288 290 292 294 296
%
1000
09
800
C-O-C&C-N (12.69 %) -O-C=O (18.82 %)
4.
600
Binding energy (eV)
C=N&C-O (11.77 %)
(5
400
Fe 2p
N
O 1s N 1s
i t ic
C=C (56.72 %)
200
ph
ra
C 1s
396
398
400
402
Fig. 3 XPS survey of Fe@C-PDA (A), spectra of C 1s (B) and N 1s (C).
29
404
Binding energy (eV)
406
408
0.8
0.4
PS PS+C-PDA PS+Fe@C-PDA
0.2
(C) 1.0
10
20 30 40 Time (min)
50
0.0 -60 0
60
(D) 1.0
0.1 g/L 0.2 g/L 0.3 g/L
0.6
Ct/C0
Ct/C0
10 20 30 40 50 60 Time (min)
0.8
Adsorption
0.8
0.4
0.4
50 mg/L 100 mg/L 150 mg/L
0.2
0.0 -60 0
0.6
0.6
0.2 0.0 -60 0
0.4
Adsorption
C t /C 0
0.6
Adsorption
0.8
Ct/C0
(B) 1.0
Adsorption
(A) 1.0
0.15 g/L 0.20 g/L 0.25 g/L
0.2 0.0 -60 0
10 20 30 40 50 60
Time (min)
10 20 30 40 50 Time (min)
60
Fig. 4 Comparison of the catalytic performance of as-obtained samples (A), the effects of initial TC concentration (B), different catalyst dosage (C), and different PS dosage (D) on TC removal. Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0.
30
A
100
B
DMPO-OH
Ct/C0
60 40
20 0
Fe@C-PDA+PS
0.8
Without TBA/PS = 500 TBA/PS = 1000 EtOH/PS = 500 EtOH/PS = 1000
0.6
Ct/C0
Intensity (a.u.)
80
C
1.0
DMPOX
0.4
Fe@C-PDA
0.2
PS
0.0
1
2
3
4
5
Rounds
323.0
323.5
324.0
324.5
Magnetic field (G)
325.0
325.5
-60 0
10
20
30
40
50
60
Time (min)
Fig. 5 Reusability of Fe@C-PDA for TC degradation (A), EPR spectra for DMPO adducts in different systems (B), degradation of TC in the absence and presence of MeOH or TBA (C). Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0.
31
(A)
(B) Graphitic N + -NH- + -NH2
400
600
800
(12.7 ic N
396
1000
Oxidized N (9.24 %)
398
400
402
404
406
408
Binding energy (eV)
Binding energy (eV)
(C)
(D)
Fe
Fe
705
3+
Fe
Intensity (a.u.)
Fe
Intensity (a.u.)
(78.05 %)
Py r idin
Fe 2p
N 1s
200
Intensity (a.u.)
Intensity (a.u.)
O 1s
1% )
C 1s
2+
Shake-up
0
710
715
720
725
730
Fe
3+
2+
Shake-up Fe
705
0
710
715
720
725
730
Binding energy (eV)
Binding energy (eV)
Fig. 6 XPS survey of Fe@C-PDA after reaction (A), deconvolution results of N 1s after PS activation (B), and Fe 2p spectra of fresh (C) and used (D) Fe@C-PDA.
32
Fig. 7 Possible reaction mechanism for TC degradation in the Fe@C-PDA/PS system.
33
100
A
80
80
60
60
Ct/C0
Ct/C0
100
40 20 0
B
40 20
None
-
Cl
-
HCO3
0
HA
Milli-Q water Tap water
Lake water
Fig. 8 Effects of chloride, bicarbonate ions, HA on the removal efficiency of TC (A), and real water body on TC degradation (B). Standard conditions: C[Catalyst] = 0.20 g/L, C [PS] = 0.20 g/L, C [TC] = 100.0 mg/L, and pH = 7.0.
34
Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles encapsulated in porous carbonized polydopamine nanospheres (Fe@C-PDA) were fabricated and applied to activate persulfate toward degrading tetracycline.
35
Highlights
Porous carbonized polydopamine nanospheres (Fe@C-PDA) were fabricated.
Fe@C-PDA can activate persulfate toward degrading tetracycline.
Fe@C-PDA displayed an excellent efficiency and stability for tetracycline removal.
36
S1385-8947(19)30942-8 https://doi.org/10.1016/j.cej.2019.04.157 CEJ 21577
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
9 February 2019 22 April 2019 23 April 2019
Please cite this article as: K. Zhu, H. Xu, C. Chen, X. Ren, A. Alsaedi, T. Hayat, Encapsulation of Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles into carbonized polydopamine nanospheres for catalytic degradation of tetracycline via persulfate activation, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.04.157
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Encapsulation of Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles into carbonized polydopamine nanospheres for catalytic degradation of tetracycline via persulfate activation Kairuo Zhua,b,1, Huan Xua,1, Changlun Chena,c,d,*, Xuemei Rena, Ahmed Alsaedid, Tasawar Hayat d a
Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of
Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China b
University of Science and Technology of China, Hefei 230000, PR China
c
Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education
Institutions, Soochow University, Suzhou 215123, PR China d
NAAM Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia
1
These authors contribute equally to this work.
*Corresponding author: [email protected] (C.L. Chen); Phone: +86-551-65592788. Fax: 86-551-65591310.
1
Abstract Porous carbonized polydopamine nanospheres encapsulated with Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles (Fe@C-PDA) were feasibly fabricated by a one-step pyrolysis of the Fe-PDA polymer, which was employed to activate persulfate (PS) toward degrading tetracycline
(TC).
The
morphology,
structure,
chemical
compositions and textural properties of the resulting catalyst were characterized systematically, and effects of catalyst dosages, PS concentration, inorganic anions (HCO3- and Cl-), natural organic matter and practical water body were studied in detail. The N-doping derived from C-PDA not only enhanced TC adsorption capacity, but also worked as active sites for PS excitation. Moreover, Fe@C-PDA displayed an excellent efficiency and stability in the 5th cycle tests. Electron paramagnetic resonance, classical radical scavenging experiments and X-ray photoelectron spectroscopy analysis disclosed that the active species in the system were identified as sulfate radicals (SO4·−) and hydroxyl radicals (·OH), and the variable chemical valences of Fe3O4/Fe0/Fe3C nanoparticles as well as N-doping in the C-PDA nanospheres contributed to the outstanding catalytic activity. This work can provide new insight of making this kind of Fe-based materials as the promising catalysts for application in eliminating organic pollutants. Keywords: Fe3O4/Fe0/Fe3C nanoparticles; Catalysts; Tetracycline; Degradation
2
1. Introduction Water pollution arising from the abuse of multifarious aromatic hydrocarbon and its derivatives has raised remarkable environmental concerns because of their severe threat to human health and natural aquatic safety [1-3], among which, tetracycline (TC) is an antibiotic product widely used and is not biodegradable under natural conditions [4]. In the past decades, great efforts have been made to locate effective technologies for the disposal of tetracycline from aqueous media. Lately, persulfate (PS) activation was regarded as a potentially effective method degrading the various toxic organic contaminants by sulfate radicals (SO4·−) because of its higher standard redox potential (2.5-3.1 eV), long duration of time in solution and wide operative pH range [5, 6]. Transition metals (Fe, Co, Ni, and Mn, etc.) are commonly applied for PS activation. Among the various transition metals, Fe-based catalysts are widely implemented for persulfate activation due to its merits like the earth-abundance, eco-friendliness and cost-effectiveness [7-10]. However, the majority of them are prone to aggregate and subject to low catalytic activity [11]. Thus, increasing attention has been paid to the development of novel Fe-based materials for improving the activity and stability of catalyst. Recently, the encapsulation of transition metal into nitrogen doped carbon shells has been proposed to be a promising technique to synthesize the functional catalysts with high activity and stability, which not only prolongs their residence and but also promotes the catalytic reaction on a carbon surface owing to the electron transfer from encapsulated transition metal [12, 13]. For instance, Wen and his groups prepared the 3
Fe0/Fe3C-encapsulated in nitrogen doped carbon microcubes for the superior degradation of Bisphenol A [14]. Li et al. synthesized ordered mesoporous nitrogen doped carbon encapsulating Co nanoparticles via thermolysis [15], which exhibited high efficiency in HCOOH dehydrogenation. Wang et al. adopted one-pot pyrolysis route to synthesize the bamboo-like N-doped CNTs encapsulated with Ni nanoparticles [16], which showed the excellent catalytic stability toward removing sulfachloropyridazine. Of note, the direct carbonization of metal-coordinated polymers under an inert gas is seen as a straightforward route to prepare such catalysts [17], in which transition metal nanoparticles were confined in a carbonaceous matrix. Hence, the preparation of the high-performance catalyst based on the encased Fe-based
nanoparticles
and
nitrogen
doped
carbon
layer
derived
from
metal-coordinated polymers for organic contaminant degradation are amongst the ideal scenarios in this work. Herein, we synthesized porous carbonized polydopamine encapsulated with Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles (Fe@C-PDA) catalysts, in which Fe0 is generated by in-situ carbothermal reduction of the rationally designed Fe-PDA polymer precursor. The C-PDA shell not only prevented the Fe species from poisoning or leaching in the extremely harsh environment, but also enhanced catalytic property by its nitrogen-doping. The resulted Fe@C-PDA catalyst was employed to degrade TC by PS activation. The objectives of the present work were (i) to evaluate the catalytic performance of Fe@C-PDA toward TC degradation via PS activation; (ii) to investigate the impacts of some factors including initial TC concentration, different 4
catalyst dosage, solution’s pH, PS dosage, typical anions and practical water bodies on the TC degradation; (iii) to explore the mechanism of PS activation and TC degradation by electron paramagnetic resonance (EPR) and radical scavenging experiments.
2. Material and methods 2.1. Material synthesis. Fe@C-PDA hybrids were synthesized by a pyrolysis technique according to our previous report (seen in Supplementary Material) [18]. The possible preparation route of the materials was shown in Scheme 1. 2.2. Catalytic degradation of TC The TC catalytic degradation activities of Fe@C-PDA hybrids with PS were performed in a 250 mL beaker with continuous shaking under natural light condition. Briefly, Fe@C-PDA catalyst was dispersed into various concentrations of TC solution (150 mL) and then kept stirring for 1 h. The degradation reaction was initiated by introducing certain amounts of PS. At a regular time intervals, 3.0 mL of the suspension was taken out and then filtered (0.22 μm, PTFE) to remove the catalyst solids, following with immediately analyzing by UV-vis spectra (UV-2550, PerkinElmer) at 355 nm [19]. To detect reactive radicals, the catalysts and PS were immediately mixed with 5, 5-dimethyl-pyrroline-N-oxide (DMPO), filtered and analyzed using a Bruker EPR A200 spectrometer. For the recycle tests, the catalysts were magnetically separation and centrifugation, washed with Milli-Q water, dried and used in the following run with similar experimental conditions. Additionally, 5
HCO3-, Cl- or humic acid (HA) (5.0 mM) was added into the suspensions before initiating the reaction to ascertain their influences on TC degradation. Tap water and lake water were also collected to replace Milli-Q water and carried out the degradation experiment. To study the role of N doping, rGO was prepared by the pyrolysis of GO at 700 ℃ under Ar atmosphere and used to degrade TC. Reagents and characterization are also described in the Supplementary Material.
3. Results and discussion 3.1 Structural Characterizations The crystal structure and phase composition of Fe-PDA and Fe@C-PDA were performed by powder X-ray diffraction (XRD) (Fig. 1A). Typical diffraction peaks appeared in the Fe-PDA polymer confirmed that it was completely amorphous. After pyrolysis, the broad peak centered at 26.0° may be due to the successful incorporated nitrogen defects in the graphitic structure [20, 21]. Of note, Fe@C-PDA hybrids contained three Fe species, i.e. Fe0 (JCPDS no. 87-0722), Fe3O4 (JCPDS no. 79-0419) and Fe3C (JCPDS no. 85-1317) [18]. Among these, Fe0 showed the highest intensity, indicative of the maximum proportion of Fe0. 57
Fe Mössbauer spectroscopy was also employed to distinguish different Fe
species (Fig. 1B). The curve of Fe@C-PDA was fitted to four components: two sextet patterns shown in the figure in olive and magenta are assigned to the Fe3O4 phase [22]. The two remaining components-one doublets in blue and purple corresponded to the Fe0 and the Fe3C phases, respectively [23]. Obviously, the relative Fe amount in the Fe0 was higher than that of Fe3O4 and Fe3C (Fe0: 47.96 %; Fe3O4: 15.13 %; Fe3C: 6
36.91 %, Table S1), confirming that the high proportion of Fe0 in the Fe@C-PDA hybrid, in accordance with the XRD result. The scanning electron microscope (SEM) image (Fig. 2A and 2B) showed the structural change in the morphology from Fe -PDA to Fe@C-PDA. Typically, they all had a uniform spherical shape with a diameter of 100-150 nm. In contrast to the Fe-PDA with a smooth surface, the Fe@C-PDA nanosphere surface became rough and presented the obvious collapse of structure. The transmission electron microscope (TEM) images of Fe@C-PDA in Figs. 2C and 2D showed that the Fe3O4/Fe0/Fe3C nanoparticles appeared as black dots (5-20 nm) in the C-PDA nanospheres and were well encapsulated by graphitic carbon layers. The Fe3O4/Fe0/Fe3C nanoparticles were further characterized by high-resolution TEM (HRTEM) measurements. The lattice fringes with spacings of 0.21 nm corresponded to crystalline Fe0 [24, 25]. This geometric confinement of Fe@C (Fig. 2F) not only suppressed dissolution and agglomeration of the inner Fe3O4/Fe0/Fe3C nanoparticles in harsh conditions, but also enriched the electron density on the C-PDA surface, thus promoting the surface catalysis reaction [26]. Moreover, elemental mapping analysis of Fe@C-PDA (Fig. 2G) validated the strong Fe signals and showed that C, O and N elements are uniformly distributed in the spherical carbon matrix. The Raman spectrum of the Fe@C-PDA was shown in Fig. S1A. The characteristic peaks of carbonaceous materials emerged at 1352 and 1600 cm-1, which corresponded to the sp3-graphitic (D band) and sp2-graphitic (G band) configurations, respectively [27]. The intensity ratio of the D to G (ID/IG) was 0.96, indicative of the 7
high disorder degrees of Fe@C-PDA, which meant that Fe@C-PDA had many defects and can afford abundant active sites [28-30]. TGA was employed to determine the change in the weight loss of Fe@C-PDA in air (Fig. S1B). It was stable at low temperature, and 66.39 weight loss occured at 330-525 ℃, which is presumably due to the oxidation of graphitic carbon [31]. The results demonstrated that the content of residual Fe2O3 is 32.44 wt%, corresponding to 22.71 wt% of Fe element (Eq. S1), indicative of high loading of Fe. The resulted Fe@C-PDA catalyst had a high surface area (343.02 m2/g, Fig. S1C) and ordered mesopores (4.09 nm, Fig. S1D) favoring the easy access of TC and PS, leading to enhanced catalytic properties [32]. The saturation magnetization value was measured to be 65.50 emu/g (Fig. S1E), the digital picture (Fig. S1F) indicated the easy separation of the catalyst from the solution by a magnet. XPS spectra were applied to study the composition of Fe@C-PDA. As displayed in Fig. 3A, Fe@C-PDA consisted of 84.34 at% of C, 8.27 at% of O, 4.90 at% of N, and 2.49 at% of Fe. Of note, the atomic percentage of Fe in Fe@C-PDA is a little lower than that in Fe/Fe3C@NC [2] and Fe3C@Fe/N-graphene [33]. Such small quantity of Fe was detected, suggesting that Fe3O4/Fe0/Fe3C nanoparticles are closely wrapped with graphitic carbon layers. Deconvolution spectrum of high resolution C 1s spectra indicated the existence of four peaks at 284.75 (C=C, 56.72 %), 285.57 (C=N&C-O, 11.77 %), 286.13 (C-O-C&C-N, 12.69 %), and 289.36 eV (-O=C-O, 18.82 %) (Fig. 3B) [33, 34]. As shown in Fig. 3C, N 1s spectrum can be separated into three portions at 398.70 (pyridinic N), 400.97 (graphitic N) and 403.33 eV 8
(oxidized N) [35]. As reported, graphitic N atoms (54.1% in our case) possessed higher electronegativity and a smaller atomic radius. Thus, benefiting from high graphitization degree and substantial graphitic N content of C-PDA matrix, it’s efficient to cause the breakage of O-O in PS quickly. 3.2. Catalytic performance of Fe@C-PDA in TC degradation Fig. 4A displayed the removal curve of TC under different reaction systems. For systems in the presence of only PS, almost no TC was removed during the reactions. After introducing C-PDA, the degradation efficiency of TC increased to 20.9%. However, rGO only achieved 4.45 % of removal efficiency under the same conditions (Fig. S3A, XRD patterns of rGO were exhibited in Fig. S3B), which indicated that C-PDA owned weak catalytic ability for activating PS. For Fe@C-PDA, 23.0 % of TC was removed by adsorption (Fig. S4A), demonstrating that TC was first enriched on the surface of catalyst. When Fe@C-PDA and PS were both existed in the reaction system, about 99.7% of TC was successfully decomposed after 60 min, indicating that the N doping and Fe encapsulation induced catalytic enhancement. Fig. 4B displayed the comparison of the degradation abilities in various initial concentrations (50.0, 100.0, and 150.0 mg/L of TC). In the Fe@C-PDA/PS system, with the initial TC concentrations rising, the reduction of degradation efficiencies from 99.7 to 92.7% was reached. The above experimental results indicated that the increase of TC concentration would lead to the competition of more target substances for active species. Impacts of some factors including catalyst dosage, PS concentration and solution’s pH were studied in Fig. 4C, 4D and Fig. S4B. With the 9
increase of catalyst amount from 0.10 to 0.30 g/L, the degradation efficiencies were measurably elevated, which was due to more catalytic active sites produced by enhanced catalyst’s amount. Of note, when PS concentration was 0.15 g/L, 93.9% of TC was removed, and the maximum removal efficiency was achieved at 0.20 g/L. Interestingly, when further increased catalyst dosage to 0.25 g/L, the efficiency remained stable. This observation can be explained that self-quenching reactions of PS can scavenge radicals and hinder the expression of Fe@C-PDA [14]. As showed in Fig. S4B, we could find that initial pH 9 had the best TC removal performance and initial pH 7.0 had the worst TC removal efficiency which can be explained that many of reactive oxygen species was generated under acid condition and SO4.− would react with ·OH (2SO4.− + 2·OH → 2HSO4- + O2↑) under neutral conditions and so that decreased the amount of SO4.− that reacted with TC [8]. The stability and reusability of Fe@C-PDA was further investigated by evaluating five cycling tests. Fig. 5A presented the degradation efficiency of TC after 60 min at each use was 99.7, 96.5, 92.5, 88.4 and 81.3%. Notably, puny decrease of degradation efficiency was observed after each use, which is maybe due to the active catalyst sites covered with intermediate degradation product of TC and the active Fe component of Fe@C-PDA consumed in each use [36]. The Fe dissolution concentrations were recorded by ICP during the five cycling tests (Fig. S4C). For fresh Fe@C-PDA, the accumulated Fe3+ concentration was 187 µg/L, and the Fe3+ concentration decreased as the recycle test continues, which are as lower than the European Union standard (200 µg/L). Therefore, as-developed Fe@C-PDA exhibited excellent stability and 10
recyclability as showed by the high degradation efficiency still above 80% after 5th cycles. 3.3. Catalytic mechanism for TC degradation PS activation was reported to generate SO4·− and ·OH via breaking the peroxo bond. To distinguish different radicals, DMPO was employed as the radical scavenging agent in EPR characterization [37]. As shown in Fig. 5B, for PS alone, some signals for DMPO-OH were detected. For the Fe@C-PDA and Fe@C-PDA/PS systems, strong signals appeared which can respond to the DMPOX adducts associated with the previous reports [38]. It has been reported that DMPO-SO4·− adduct was difficult to detect due to its lower sensitivity and short life-time. Thus, the DMPOX peaks could be originated from the DMPO-SO4·− adduct. Obviously, the intensity of DMPOX peaks was stronger in Fe@C-PDA/PS system, implying that PS activation enhanced obviously with the introduction of Fe@C-PDA. Moreover, the dominant radical that contributes to TC degradation was identified by free radical trapping experiments at EtOH/PS or TBA/PS molar ratio of 500 and 1000 in the Fe@C-PDA/PS system (Fig. 5C). TBA and EtOH were employed as the scavengers of ·OH and SO4·−, respectively [39]. It was observed that the degradation was inhibited in the presence of EtOH than TBA. In particular, a high removing efficiency with the adding the TBA and the weak degradation under the EtOH adding molar ratio of 1000, testified that Fe@C-PDA/PS system was controlled by radical pathway (SO4·− and ·OH radicals) rather than the non-radical active species [40], and the dominant is SO4·− radicals. Moreover, the possible reaction intermediates were also 11
explored in the SI (Fig. S6). FT-IR was used to characterize the structure variation of Fe@C-PDA catalyst before and after used (Fig. S4D). It can be seen that the intensity of Fe-O (500-600 cm-1) and C-H group (1398 cm-1) increased obviously and the C-O group (1252 cm-1) occurred after used. This demonstrated that Fe-based nanoparticles made function in TC degradation and TC molecules were adsorbed on the catalyst surface [41]. The XPS spectra of the Fe@C-PDA catalyst after the catalytic reaction were also discussed for full insight into the PS activation mechanisms. For N 1s after used (Fig. 6B), the relative content of pyridinic N decreased remarkably, manifesting that the N doping is important in PS activation. Besides, the introduction of N-contained groups from TC led to the increasing of the peak at ~401 eV (Fig. S4E). Since the nitrogen atom has higher electronegativity than carbon atom, thus increasing the negative charge density and the amount of higher basicity functionalities on the surface of catalyst, which activates the adjacent carbon atoms to make carbon matrix work as active sites for PS excitation and further enhance the catalytic performance, it’s believable that the N doping of C-PDA can also enhance the quantity of active sites. The O 1s core level has been fitted to four components, i.e., C=O, C-OH, C-C=O, and C-O-C (Fig. S5) [42]. The apparent increase of C-OH in O 1s after used provided dependable evidence for TC loaded on the surface of Fe@C-PDA hybrids. For Fe 2p spectra of the fresh Fe@C-PDA (Fig. 6C), Fe0 (708.72 eV), Fe2+ (710.59 eV), Fe3+ (712.16 eV), and shake-up satellite Fe 2p3/2 (718.64 eV) were identified [43-45]. After being used, the peak of Fe0 obviously reduced and there was a slightly positive shift 12
of Fe2+ component (710.59 → 710.65 eV). Notably, Fe0 could be firstly oxidized to Fe2+ in oxygenated and deoxygenated water (Eq. (1) and (2)), or reacted with PS by a direct electron transfer process (Eq. (3)). What's more, Fe0 could recycle Fe3+ to Fe2+ (Eq. (4)), and this reaction was thermodynamically favorable. 2Fe0 + O2 + H2O → 2Fe2+ + 4OH-
(1)
Fe0 + 2H2O → Fe2+ + 2OH- + H2↑
(2)
Fe0 + 2S2O82- → Fe2+ + 2SO42- + SO4.- (3) Fe0 + 2Fe3+ → 3Fe2+, ∆E0 = 1.21 V
(4)
The above results revealed that Fe3O4/Fe0/Fe3C nanoparticles took part in the redox reaction of electron transfer and acted as electron donors during the catalytic degradation reaction. As presented in Fig. 7, the excellent TC degradation properties in Fe@C-PDA/PS were mainly ascribed to the following: (i) TC and PS were first adsorbed on the Fe@C-PDA surface, which leaded to the enrichment of reactants around the catalyst, (ii) Fe3O4/Fe0/Fe3C nanoparticles with variable chemical valences were beneficial to transfer electrons and served as the active site to activate PS, (iii) the N doping sites also accelerated electron transfer for PS activation. Therefore, the synergistic effects between Fe3O4/Fe0/Fe3C nanoparticles and N doping of C-PDA afforded the Fe@C-PDA catalyst excellent catalytic activity for TC decomposition. 3.4. Effects of inorganic ions and practical water body Fig. 8A displayed the anti-interference of Fe@C-PDA for degrading TC with the presence of inorganic ions (HCO3- and Cl-) and humic acid (HA). The catalytic 13
degradation performance followed the order of Cl- (95.75 %) > HCO3- (83.92 %) > HA (77.48 %). Obviously, Cl- played the tiniest inhibitory impact on the TC degradation. This phenomenon can be explained that Cl- has inhibitory impact for SO4·−. In the PS system, Cl- could be converted into chloride radical (Cl·) [46], which consumes SO4·− and further results into the lower oxidation performance of TC. In comparison with Cl-, HCO3- has more inhibitory impact for TC degradation. HCO3can act as free-radical scavengers and has competitive effect in the degradation process [47]. For HA, which exhibits the maximum inhibitory impact, as we know that the HA with abundant phenolic hydroxyl and carboxyl groups can be adsorbed onto the surfaces of catalysts and further result in the restriction of activating PS. The TC degradation performance in real water bodies (tap water and lake water) was further explored and displayed in Fig. 8B. The TC removal efficiency in tap water and lake water were 90.66 and 66.97%, respectively. Of note, the TC degradation performance in lake water was lower than that obtained in tap water. It can be explained that the organic matters or pollutants in lake water would consume a portion of reactive radicals, thus punily interfere the degradation of TC.
4. Conclusions Through one-pot pyrolysis method combining Fe-PDA polymer as the precursor, we have successfully in-situ synthesized high performance catalysts with Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles encapsulated in porous carbonized polydopamine. The catalysts were then applied for TC degradation with the aid of PS as the active radical source, which exhibited the distinguished catalytic performance 14
and reusability. The analysis of degradation mechanisms indicated that the generated ·OH and SO4·− in the PS activation were the main active radicals and the change of Fe surface valences and N doping sites played important roles in the remarkable degradation performance. This finding is expected to shed light on rationally designing efficient Fe-based catalyst for the elimination of organic pollutants in water. Notes The authors declare no competing financial interests.
Acknowledgements Financial support from the National Natural Science Foundation of China (21477133), the Anhui Provincial Natural Science Foundation (1608085QB44), and Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences are acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.CEJ.2019.
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Figure caption Scheme 1. Preparation of Fe@C-PDA from Fe-PDA. Fig. 1 (A) XRD patterns of Fe-PDA complexes and Fe@C-PDA, and (B) room-temperature 57Fe Mössbauer spectrum of Fe@C-PDA. Fig. 2 (A) SEM image of Fe-PDA, (B, C) SEM, TEM images of Fe@C-PDA and (D, E) HRTEM images of Fe@C-PDA. (F) Schematic diagram of the Fe@N-doped C structure. (G) HAADF-STEM image and corresponding elemental mapping images of Fe, C, O and N atoms for Fe@C-PDA. Fig. 3 XPS survey of Fe@C-PDA (A), spectra of C 1s (B) and N 1s (C). Fig. 4 Comparison of the catalytic performance of as-obtained samples (A), the effects of initial TC concentration (B), different catalyst dosage (C), and different PS dosage (D) on TC removal. Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0. Fig. 5 Reusability of Fe@C-PDA for TC degradation (A), EPR spectra for DMPO adducts in different systems (B), degradation of BPA in the absence and presence of MeOH or TBA (C). Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0. Fig. 6 XPS survey of Fe@C-PDA after reaction (A), deconvolution results of N 1s after PS activation (B), and Fe 2p spectra of fresh (C) and used (D) Fe@C-PDA. Fig. 7 Possible reaction mechanism for TC degradation in the Fe@C-PDA/PS system. Fig. 8 Effects of chloride, bicarbonate ions, HA on the removal efficiency of TC (A), and real water body on TC degradation (B). Standard conditions: C[Catalyst] = 0.20 g/L, 24
C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0.
25
Pyrolysis
Fe-PDA
Fe@C-PDA
Scheme 1. Preparation of Fe@C-PDA from Fe-PDA.
26
A
B
♦
Fe3O4
0
♦ Fe ■
C 3 Fe3O4
Carbon
■
♥♥ ♥
♥ ♥
Transmission (a.u.)
Intensity (a.u.)
♥ Fe
Fe@C-PDA ■
■
♦
Fe
0
Fe3C
Fe-PDA
10
20
30
40
50
60
70
2 Theta (degree)
-10
-5
0 Velocity (mm/s)
5
10
Fig. 1 XRD patterns of Fe-PDA complexes and Fe@C-PDA (a), and (b) room-temperature 57Fe Mössbauer spectrum of Fe@C-PDA.
27
Fig. 2 (A) SEM image of Fe-PDA, (B, C) SEM, TEM images of Fe@C-PDA and (D, E) HRTEM images of Fe@C-PDA. (F) Schematic diagram of the Fe@N-doped C structure. (G) HAADF-STEM image and corresponding elemental mapping images of Fe, C, O and N atoms for Fe@C-PDA.
28
C: 84.34% N: 4.90% O: 8.27% Fe: 2.49%
(B)
(C)
G
(A)
)
Binding energy (eV)
3.62 % ic N (3
Oxidized N (12.29 %)
Pyridin
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
)
282 284 286 288 290 292 294 296
%
1000
09
800
C-O-C&C-N (12.69 %) -O-C=O (18.82 %)
4.
600
Binding energy (eV)
C=N&C-O (11.77 %)
(5
400
Fe 2p
N
O 1s N 1s
i t ic
C=C (56.72 %)
200
ph
ra
C 1s
396
398
400
402
Fig. 3 XPS survey of Fe@C-PDA (A), spectra of C 1s (B) and N 1s (C).
29
404
Binding energy (eV)
406
408
0.8
0.4
PS PS+C-PDA PS+Fe@C-PDA
0.2
(C) 1.0
10
20 30 40 Time (min)
50
0.0 -60 0
60
(D) 1.0
0.1 g/L 0.2 g/L 0.3 g/L
0.6
Ct/C0
Ct/C0
10 20 30 40 50 60 Time (min)
0.8
Adsorption
0.8
0.4
0.4
50 mg/L 100 mg/L 150 mg/L
0.2
0.0 -60 0
0.6
0.6
0.2 0.0 -60 0
0.4
Adsorption
C t /C 0
0.6
Adsorption
0.8
Ct/C0
(B) 1.0
Adsorption
(A) 1.0
0.15 g/L 0.20 g/L 0.25 g/L
0.2 0.0 -60 0
10 20 30 40 50 60
Time (min)
10 20 30 40 50 Time (min)
60
Fig. 4 Comparison of the catalytic performance of as-obtained samples (A), the effects of initial TC concentration (B), different catalyst dosage (C), and different PS dosage (D) on TC removal. Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0.
30
A
100
B
DMPO-OH
Ct/C0
60 40
20 0
Fe@C-PDA+PS
0.8
Without TBA/PS = 500 TBA/PS = 1000 EtOH/PS = 500 EtOH/PS = 1000
0.6
Ct/C0
Intensity (a.u.)
80
C
1.0
DMPOX
0.4
Fe@C-PDA
0.2
PS
0.0
1
2
3
4
5
Rounds
323.0
323.5
324.0
324.5
Magnetic field (G)
325.0
325.5
-60 0
10
20
30
40
50
60
Time (min)
Fig. 5 Reusability of Fe@C-PDA for TC degradation (A), EPR spectra for DMPO adducts in different systems (B), degradation of TC in the absence and presence of MeOH or TBA (C). Standard conditions: C[Catalyst] = 0.20 g/L, C[PS] = 0.20 g/L, C[TC] = 100.0 mg/L, and pH = 7.0.
31
(A)
(B) Graphitic N + -NH- + -NH2
400
600
800
(12.7 ic N
396
1000
Oxidized N (9.24 %)
398
400
402
404
406
408
Binding energy (eV)
Binding energy (eV)
(C)
(D)
Fe
Fe
705
3+
Fe
Intensity (a.u.)
Fe
Intensity (a.u.)
(78.05 %)
Py r idin
Fe 2p
N 1s
200
Intensity (a.u.)
Intensity (a.u.)
O 1s
1% )
C 1s
2+
Shake-up
0
710
715
720
725
730
Fe
3+
2+
Shake-up Fe
705
0
710
715
720
725
730
Binding energy (eV)
Binding energy (eV)
Fig. 6 XPS survey of Fe@C-PDA after reaction (A), deconvolution results of N 1s after PS activation (B), and Fe 2p spectra of fresh (C) and used (D) Fe@C-PDA.
32
Fig. 7 Possible reaction mechanism for TC degradation in the Fe@C-PDA/PS system.
33
100
A
80
80
60
60
Ct/C0
Ct/C0
100
40 20 0
B
40 20
None
-
Cl
-
HCO3
0
HA
Milli-Q water Tap water
Lake water
Fig. 8 Effects of chloride, bicarbonate ions, HA on the removal efficiency of TC (A), and real water body on TC degradation (B). Standard conditions: C[Catalyst] = 0.20 g/L, C [PS] = 0.20 g/L, C [TC] = 100.0 mg/L, and pH = 7.0.
34
Fe0-dominated Fe3O4/Fe0/Fe3C nanoparticles encapsulated in porous carbonized polydopamine nanospheres (Fe@C-PDA) were fabricated and applied to activate persulfate toward degrading tetracycline.
35
Highlights
Porous carbonized polydopamine nanospheres (Fe@C-PDA) were fabricated.
Fe@C-PDA can activate persulfate toward degrading tetracycline.
Fe@C-PDA displayed an excellent efficiency and stability for tetracycline removal.
36