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Fabrication of sewage sludge-derived magnetic nanocomposites as heterogeneous catalyst for persulfate activation of Orange G degradation
Accepted Manuscript Title: Fabrication of sewage sludge-derived magnetic nanocomposites as heterogeneous catalyst for persulfate activation of Orange G degradation Authors: Ying Li, Zhiquan Yang, Hongguo Zhang, Xiaowen Tong, Jinna Feng PII: DOI: Reference:
S0927-7757(17)30606-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.06.043 COLSUA 21730
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21-2-2017 15-6-2017 16-6-2017
Please cite this article as: Ying Li, Zhiquan Yang, Hongguo Zhang, Xiaowen Tong, Jinna Feng, Fabrication of sewage sludge-derived magnetic nanocomposites as heterogeneous catalyst for persulfate activation of Orange G degradation, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.06.043 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.
Fabrication of sewage sludge-derived magnetic nanocomposites as heterogeneous catalyst for persulfate activation of Orange G degradation
Ying Li a,b, Zhiquan Yang a, b *, Hongguo Zhang c，Xiaowen Tong a,b, Jinna Fenga,b
a
School of Environment and Energy, South China University of Technology, Guangzhou Higher
Education Mega Centre, Guangzhou 510006, PR China
b
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of
Education, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, PR China
c
School of Environmental Science and Technology, Guangzhou University, Guangzhou Higher
Education Mega Centre, Guangzhou 510006, PR China
* Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, PR China Tel./ fax: +83 20 18620507019. E-mail address: [email protected] (Z. Yang)
1
Graphical abstract
Highlights
·Sewage sludge-derived magnetic nanocomposites was synthesized with a facile method.
·Orange G could be effectively degraded by MnFe2O4-SAC/persulfate process.
·MnFe2O4-SAC exhibited excellent catalytic activity, reusability and stability.
·The catalytic reaction might happen in the boundary layer near the subsurface of the catalyst.
Abstract
An increasing amount of sewage sludge is produced by wastewater treatment plants worldwide. Therefore, the disposal of sewage sludge is one of the vital environmental issues currently. In the present work, we developed an alternative strategy of converting sewage sludge into porous activated carbon (SAC), and then synthesized a magnetically separable 2
MnFe2O4-SAC heterogeneous catalyst through an environmentally friendly method. Several characterization techniques such as XRD, FTIR, SEM and XPS indicated the evidence of MnFe2O4 nanoparticles successfully sticking to surface of porous SAC. The MnFe2O4-SAC composites exhibited stable and excellent catalytic activity in activating persulfate (PS) for the degradation of Orange G (OG) throughout the wide pH range of 3-9, and could be recycled by a magnet without any loss. The activation energy was calculated to be 14.94 kJ·mol -1, indicating that the OG degradation in MnFe2O4-SAC/PS process required moderate activation energy. The results of heavy metals leaching experiments revealed an excellent stability of the MnFe2O4-SAC catalyst. Possible reaction mechanism in MnFe2O4-SAC/PS system was also discussed in detail.
Keywords: Sewage sludge; MnFe2O4; Magnetic nanomaterial; Sulfate radical; Heterogeneous degradation.
1. Introduction
The quantity of sewage sludge produced in conventional wastewater treatment plants increases remarkably as a result of rapid industrialisation and urbanization. The annual amount of dry sewage sludge in the United States, the European Union and China were 5.6-7, 9.8 and 8.0 million tons, respectively [1,2]. Furthermore, it is estimated that the production of sewage sludge will continue to increase at an annual growth rate of about 2 vol.% [3,4]. The presence of organic compounds, pathogenic microorganisms and potentially toxic elements in sewage sludge are at the risk of contaminating the environment and threatening human
3
health [5]. Several traditional options have been utilized to dispose of municipal sewage sludge such as land filling and incineration treatments, which may cause secondary pollution and are no longer acceptable [6]. Converting waste material into valuable products offers an initiative way to dispose of sludge from an environmental standpoint. Some new processes such as sewage sludge composting, pyrolysis and gasification are being developed [7,8]. Since sewage sludge is materially carbonaceous [9], pyrolysis is considered as an effective method to convert sewage sludge into porous activated carbon (AC) [10].
As a good adsorbent with high surface area, sewage sludge-derived activated carbon (SAC) was reported recently as an emerging catalyst support for advanced oxidation processes (AOPs) such as photo-Fenton reaction and persulfate (PS) activation [11-13]. Sulfate radical (SO4-·) which is generated from PS through the activation processes has attracted an increasing interest as an alternative to hydroxyl radical (·OH) [14]. It has been reported that SO4-· is more powerful and selective than ·OH for the degradation of recalcitrant organic pollutants. Furthermore, PS has a lower energy input than H 2O2, and it is an environmentally friendly and sustainable oxidant [15]. In general, SO4-· can be effectively generated from PS under controlled conditions such as heat, transition metals and ultraviolet irradiation [16,17]. Transition metal activation is regarded as the most feasible one among these methods due to reducing high energy demand and complicated operation [18]. It has also been found that the surface oxygen functional groups on AC may act as an activator to catalyze PS to release SO4-· and ·OH [19] .
4
Spinel ferrites nanoparticles, which are expressed as MFe2O4 (M=Mn, Co, Ni and Cu), exhibit high catalytic performance in SO4-· based catalytic oxidation [15]. However, the unique magnetic properties of the MFe2O4 nanoparticles have led to their aggregation, and the lower surface area significantly reduces its catalytic efficiency [20].
To solve this problem and to find an effective approach for SAC application, in this paper, a novel heterogeneous catalyst including magnetic MnFe2O4 and SAC composites have been firstly explored and applied for the PS activation. Orange G (OG), a typical and poisonous azo dye which widely exist in textile wastewater, was chosen as our target model pollutant [21]. The properties of MnFe2O4-SAC hybrids were characterized and its catalytic performances were investigated by altering the particular parameters including temperature and initial pH. Besides, we evaluated the stability and recyclability of the as-synthesized catalysts and the possible catalytic mechanism was also put forward through a series of radicals quenching experiments. Our study proposed an environmentally beneficial approach for reusing of sewage sludge, and provided a stable and efficient heterogeneous catalyst for sulfate radicalsbased advanced oxidation process.
2. Experimental
2.1. Reagents and materials
The dewatered sewage sludge sample was obtained from the wastewater treatment plant located in Guangzhou, China. The collected sample was initially dried in an oven at 105℃
5
for 24 h to achieve constant weight, then it was ground and sifted through 100 meshes, and properly stored for further studies.
Ferric chloride hexahydrate (FeCl3·6H2O), manganese dichloride tetrahydrate (MnCl2·4H2O), sodium persulfate(PS, Na2S2O8), tert-butyl alcohol (TBA) and Orange G were purchased from Aladdin Industrial Co. Ltd., (Shanghai, China). Other reagents including Sodium acetate anhydrous (NaAc, >99%), HNO3, ethanol, methanol, phenol were obtained from Guangzhou Chemical Reagent Co. Ltd. All of the organic and inorganic reagents were of analytical grade and used without any further purification. All of the solutions in this study were prepared with deionized water from an ultrapure water system.
2.2. Preparation of sludge based activated carbon (SAC)
10 g dried sewage sludge was firstly added into a 40 ml HNO3 (20%) solution and stirred for 24 h at room temperature. The impregnated sludge suspension was then centrifuged, and the precipitate was dried in an oven at 105℃overnight. The dried sludge was pyrolyzed at 700℃ for 1 h in the presence of nitrogen at a heating rate of 10℃/min to form activated carbon materials. After pyrolysis, the product was cooling to room temperature in nitrogen gas atmosphere. Finally, the product was ground and washed with 3 mol/L HCl to remove inorganic impurities; then it was rinsed with deionized and dried in air at 105 ℃for 24 h. The obtained product was denoted as SAC.
2.3. Fabrication of MnFe2O4-SAC nanocomposites
6
The MnFe2O4-SAC composites were prepared by a facile one-pot solvothermal method[23]. In a typical synthesis, a mixture of 0.2 g SAC, 3.7 mmol FeCl 3·6H2O and 1.85 mmol MnCl2·4H2O were dissolved in 30 mL ethylene glycol under ultrasonication for 3 h. Later, 3 g of NaAc were added and stirred vigorously for 30 min at room temperature. After stirring, the mixture was transferred to a Teflon-lined stainless steel autoclave (50 ml capacity) and heated to 200℃for 10 h to get the solid product. After the autoclave cooled to room temperature, the resulting precipitate was collected by centrifugation and repeatedly washed with ethanol and deionized water, and finally dried under vacuum at 60℃overnight. For comparison, pure MnFe2O4 nanoparticles were also synthesized by a similar method without SAC [23].
2.4. Characterization of the as-synthesized catalyst
All samples must be completely dried and ground into tiny particles before characterization. The structures and crystal phases of the as-synthesized catalyst were characterized via X-ray diffraction (XRD) patterns, which were performed with a Bruker D8Advance diffractometer using Cu Kα radiation source from 5 ° to 80 ° of
2θ. X-ray
photoelectron spectroscopic (XPS) patterns were recorded with a Thermo Escalab 250Xi instrument to analyze chemical species of Mn, Fe, O and C. XPS PEAK4.1 software was employed for background subtraction and peak-fit processing. The surface functional groups of catalyst were determined by Fourier Infrared Spectrometer (FTIR; Nexus Por Euro, Thermo Nicolet Co.) using KBr pellet method at room temperature. Morphological observations were conducted using high resolution field emission scanning electron microscope(SEM; Merlin Compact; Carl zeiss Co.) . 7
Total organic carbon (TOC) was determined by a TOC analyzer (LiquiTOC, Elementar, Germany). Before measuring TOC, 15 mL sample was extracted, filtrated and quenched with 1mL of 100mM sodium nitrite solution. The pH was measured with a pH meter (PHSJ-4F, Leici, China). The leaching concentrations of heavy metals were determined by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 720ES, USA). The magnetic properties of the asobtained catalysts were measured by vibrating sample magnetometer (VSM; LakeShore 740) at 300 K. The zeta potential was measured using a zeta potential analyzer (Malvern, Zetasizer Nano, ZS90).
2.5. Catalytic degradation experiments
Batch experiments of OG degradation were carried out in a series of 250 mL conical flasks, which placed in a shaking table with fixed rotation speed of 160 rpm. The reaction temperature was set at 25±0.1℃during the experiments, unless otherwise specified. In a typical procedure, 0.02g catalyst was added into the OG solution (100 ml, 20 mg/L) and shook for 30 min to reach the adsorption equilibrium. The initial pH of the solution was adjusted by H2SO4 (0.1 M) or NaOH (0.1 M). Then the catalytic reaction was initiated once the oxidation agent (PS) was added. Solution samples (2.5 mL) were taken at predetermined time internals and immediately filtered through 0.22 μm membrane to remove the catalyst and quench the reaction. Subsequently, the residual concentration of OG was analyzed using a UV-2100 spectrometer (Unico, Shanghai) at its maximum absorption wavelength (478nm).
8
To evaluate the stability of the MnFe2O4-SAC product, the catalyst was collected by a magnet after each run of test, washed, dried and reused in a fresh solution of OG for 4 times. All experiments were performed in triplicate within accepting error range (±5%), and all results were expressed as mean value with error bar.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. X-ray diffraction
Fig. 1.
XRD analysis was employed to identify crystallinity and purity of as-synthesized samples of SAC, MnFe2O4 and the MnFe2O4-SAC composites. As shown in Fig.1, the main peaks of MnFe2O4-SAC and MnFe2O4 samples exhibited similar XRD patterns. The diffraction peaks of MnFe2O4-SAC and MnFe2O4 at 2θ=18.289º, 30.134º, 35.415º, 36.480º, 43.082º, 53.554º, 57.032º, 62.575º and 74.033º were associated with (111), (220), (311), (222), (400), (422), (511), (440) and (533) reflections of standard XRD pattern of MnFe2O4 nanoparticles (JCPDS No. 10-0319) [22,23]. Proof of successfully synthesized MnFe2O4-SAC composites was evidenced by another two diffraction peaks of MnFe2O4-SAC sample at 2θ=20.852º and 26.658º, which could be perfectly indexed to the typical quartz (SiO2) crystallite structures [13,24].
3.1.2. FTIR analysis
9
Fig. 2.
The surface functional groups and chemical bonds on the samples were characterized by the FTIR spectra in the range of 4000-350 cm-1 (Fig. 2). The broad band in the region of 33003650 cm-1 which was presented in all samples could be designated as the stretching of O-H groups and surface adsorbed water [22]. The aqueous dispersible stability of the magnetic composites was greatly improved by high density of hydrophilic O-H groups [25]. The band at 1630 cm-1 was related to C=O stretching of COOH groups, whilst the small peak at 1430 cm -1 could be assigned to the symmetrical deformations of CH 2 [13]. MnFe2O4-SAC hybrid composites showed a characteristic peak at 1035 cm -1, which was attributed to Si-O bond stretching vibration of Si-O-Si groups [26]. Moreover, two absorption peaks observed at around 584 and 390 cm-1 corresponded to the Fe-O and Mn-O bond vibrations of tetrahedral and octahedral sites, respectively [27]. These results clearly indicated the formation of chemical bonds between the inorganic constituent (SiO2) in SAC and loaded MnFe2O4 nanoparticles.
3.1.3. Morphological analysis
Fig. 3.
The morphology and structures of bare MnFe2O4 nanoparticles, SAC and MnFe2O4-SAC composites were obtained by SEM observations (Fig. 3). As indicated from Fig.3a, the surface morphology of SAC was relatively a rough structure with irregular cavities which was in the form of flake. These cavities could be due to the nitric acid activation, carbonization of organic
10
molecules and biomacromolecules during the pyrolysis process [13]. Fig. 3b evidenced that the bare MnFe2O4 particles were well-defined spherical morphology with average particle size in the range of 350-600 nm. Besides, the severe aggregation was also observed. In comparison, the image of MnFe2O4-SAC hybrids shown that MnFe2O4 nanoparticles anchoring to the surface of SAC, while surface area and pore structures of virgin SAC were still exposed. It might be important for the hybrid material to keep high specific surface area and to prevent the agglomeration of MnFe2O4 particles[28,29].
3.1.4. XPS analysis
Fig. 4.
The chemical composition and oxidation state of the as-synthesized MnFe2O4-SAC composites were further characterized by XPS. Fig. 4A illustrated the wide scan XPS spectrum of MnFe2O4-SAC, revealing the existence of elements C, O, Mn and Fe. The C element was originated from the sewage sludge-based carbon. The binding energy of the C1s peak (Fig. 4B) centered at 284.6 eV was used as the reference for calibration, and another two peaks centered at energy 287.4 and 288.8 eV were ascribed to the binding of C=O and O=C-O bonds, respectively [30]. The O1s peak centered at 530.2 eV (Fig. 4C) was primarily assigned to the contributions of oxygen in metal oxides, such as Fe-O and Mn-O of MnFe2O4 [29,31]. And the other peak at 531.6 eV might be ascribed to the surface adsorbed oxygen groups in H 2O and organic compounds such as ethylene glycol [13]. The Fe 2p (Fig. 4D) spectrum shown two main peaks at binding energies of 710.8 and 724.4 eV, which were assigned to Fe 2p3/2 and Fe 2p1/2
11
[32], respectively, indicating completely oxidation state of iron (Fe 3+) on the surface of the MnFe2O4-SAC composites [33]. From the Mn 2p spectrum in Fig. 4E, the peaks at 652.2 and 653.9 eV were caused by Mn 2p1/2. While the peak at 641.4 eV was corresponded to the binding energy for Mn 2p3/2, accompanied by a satellite peak at 646 eV, which deviated about 5 eV towards higher binding energy from the peak of the Mn 2p3/2. This result proved the presence of Mn2+ in MnFe2O4 [32,34]. The above analysis further confirmed the MnFe2O4-SAC hybrid composites have been successfully synthesized.
3.1.5. VSM study
Fig. 5.
Magnetic properties of the as-synthesized MnFe2O4 and MnFe2O4-SAC were obtained by field dependent magnetization measurements at 300 K. As can be seen from Fig. 5, the hysteresis loops of both samples exhibited no remnant magnetization or coercivity, suggesting that the as-obtained nano-materials may be superparamagnetic at room temperature. The saturation magnetization (Ms) value of pure MnFe2O4 was 49.62emu/g, nevertheless, the Ms of MnFe2O4-SAC composites was 43.64 emu/g. The Ms value of MnFe2O4-SAC was much higher comparing to magnetic catalysts in other literatures reported previously. For instance, Ms of MnFe2O4-graphite photocatalyst was 30.34emu/g [32], Ms of graphite spheres with magnetic Fe3O4 nanoparticles was 3.1 emu/g [44], and Ms of CuFe2O4/AC magnetic adsorbents was 5.93 emu/g [28]. Previous studies have shown that the Ms values of the magnetic particles could
12
be affected by particle size, crystallinity, and coupling [45]. The reduced M s value of the MnFe2O4-SAC might be due to the decrease in particle size as revealed by SEM results.
The excellent dispersability and rapid magnetic responsiveness of MnFe2O4-SAC were verified by a preparatory experiment as shown in the insert of Fig. 5. The result indicated that the MnFe2O4-SAC composites could be easily manipulated by magnetic separation, which has a potential superiority for recycling and reusing.
3.2 Catalytic evaluation of MnFe2O4-SAC
Fig. 6.
Fig. 6 displayed the degradation profiles of OG under different experimental processes, including PS alone, MnFe2O4/PS, SAC/PS and MnFe2O4-SAC/PS, Mn2+ (0.867mM) /PS, Fe3+ (1.734mM) /PS, as well as experiments of the adsorption abilities of MnFe 2O4-SAC. It can be seen that in the absence of PS, MnFe2O4-SAC exerted little OG adsorption and the removal efficiency was only about 2%, indicating the influence of adsorption by catalysts could be negligible. No obvious OG degradation was detected after 30 min with PS alone either, suggesting that the oxidant itself failed to decompose the dye. However, with the co-existence of PS and MnFe2O4-SAC, the concentration of OG decreased dramatically and more than 95% decolorization efficiency can be achieved within 30. The catalytic oxidation of OG could be attributed to the synergistic function of MnFe2O4-SAC and PS. MnFe2O4-SAC could facilitate the decomposition of persulfate to produce SO4-·. Furthermore, the removal efficiency of OG in homogeneous Mn2+/PS and Fe3+/PS processes was found to be slight (<15%), indicating that
13
the heterogeneous catalytic reaction played a dominant role in OG degradation with MnFe2O4-SAC/PS process.
It was noteworthy that the removal rate in SAC/PS system was 60.7% during 30 min, but the rate constant of MnFe2O4-SAC/PS(0.176 min-1) was much better than that of SAC/PS (0.025min-1), suggesting that intense interfacial interaction of SAC and MnFe2O4 enhanced the ability of electron transport and reaction sites [35]. The TOC removal in MnFe2O4-SAC/PS system was found to be about 18.47% after 30 min reaction, demonstrating most of Orange G has transformed into the reaction intermediates. Higher mineralization was achieved in 60 min(26.21%) and 90 min (30.50%), indicating that extending reaction time was favourable to enhance mineralization rate.
3.3. Effect of temperature
Fig. 7.
The effect of temperature on OG degradation with MnFe2O4-SAC/PS system was investigated. It can be clearly seen from Fig. 7 that increasing temperature significantly accelerated the degradation of OG. As the temperature rose from 15 to 55℃, the OG degradation rate constant increased from 0.07 to 0.247 min-1. This result could be ascribed to the thermal activation of PS and PS decomposed faster at higher temperature [36]. According to the first order kinetics, the activation energy for OG degradation by the MnFe 2O4-SAC/PS process was calculated using the Arrhenius equation. As demonstrated in the inset of Fig. 7, lnK and 1/T followed a good linear relationship (R2 = 0.996) with Eq.(1)
14
lnK=lnA-Ea/RT
(1)
Where K is the reaction rate constant; A is the preexponential factor; Ea is the activation energy (kJ·mol-1); R is the ideal gas constant (8.314 J·mol -1K-1); T is the absolute temperature (K). The activation energy was calculated to be 14.94 kJ·mol-1, indicating that the OG degradation in MnFe2O4-SAC/PS process required moderate activation energy. Previous investigations reported that the values of activation energy on various heterogeneous catalysts could be different, such as 49.5 kJ·mol -1 on Co-graphene/PMS- Orange II [37], 69.23 kJ·mol-1 on mesoporous Fe2O3-Rhodamine B [38] and 31.7 kJ·mol-1 on MnFe2O4/PMS-Orange II processes [39]. The low activation energy signified the MnFe2O4-SAC/PS oxidation process was easy to occur and was very promising for heterogeneous catalytic reaction.
3.4. Effect of initial pH
Fig. 8.
Experiments of the initial pH influence on the OG degradation with MnFe 2O4-SAC/PS process were also carried out. As presented in Fig. 8, the fastest OG degradation rate occurred at pH=3 and its corresponding rate constant was 0.195min-1. As the pH value increased, a decrease of OG degradation was observed, indicating that the acidic and near neutral conditions were more beneficial to the OG degradation than strong alkaline condition. Surface properties of MnFe2O4-SAC and the species of PS in solution might account for this result. Catalyst displayed a positive surface charge when solution pH was lower than the potential of zero charge (pHpzc), and the surface was negatively charged when pH (solution) was greater
15
than pHpzc [40]. The pHpzc of MnFe2O4-SAC could be found in Supplementary Material Fig. S1, it was about 4.85. OG is an anionic dye, revealing surface repulsive force existed between the MnFe2O4-SAC and OG when solution pH>4.85. As the solution pH further increased, the increasing electrostatic repulsive force among MnFe2O4-SAC, OG and SO4-· led to a decline of OG degradation rate. In addition, when solution pH exceed 10, ·OH would supersede SO4-· and become predominant active free radical. The lifetime of ·OH (20ns [41]) is much shorter than SO4-· (4s [42])and ·OH is apt to react with other ions from the aqueous solution, which would result in a rapid quenching of ·OH [43]. It was worth mentioning that as shown in Fig. 8, OG could be effectively degraded throughout the pH range of 3-9, demonstrating MnFe2O4SAC/PS combined process had a great prospect in the application of water treatment.
3.5. Stability and reusability of MnFe2O4-SAC catalyst
Fig. 9.
In general, the stability and reusability of the catalysts are the most important aspects in the practical application. To evaluate the catalytic stability of spent catalyst in the PS oxidation process, the catalysts were recovered by an external magnet and repeatedly reused for degradation of fresh Orange G solution. As shown in the Supplementary Material Table. S1, the MnFe2O4-SAC catalyst showed excellent catalytic performance during the successive five cycles. Although a slight reduction was observed, the decolorization efficiency of OG was more than 94% even after five continuous cycles. Activated sewage sludge accumulates high concentration of heavy metals, higher concern should be paid to the issue of metals leaching
16
during the application of sewage sludge-based heterogeneous catalysts. In this study, ICP-MS was performed to determine the leaching concentration of heavy metals. As can be seen from Fig. 9, the corresponding concentration values of the residual supernatant solution after reaction were quite low, except for Zn. The leaching concentration of Zn was 0.082 mg/L, which was also less than the limits in National Recommended Water Quality Criteria issued by US Environmental Protection Agency [46]. In addition, the contents of heavy metals in MnFe2O4-SAC catalyst haven’t changed remarkably after the catalytic reaction, and the degradation efficiency was almost unaffected by such a low concentration of leaching metals, which further verified the heterogeneous catalytic reaction took place in the MnFe2O4-SAC/PS system.
3.6. Reaction mechanism
As mentioned above, PS can be activated by free transition metal ions and the surface oxygen functional groups to generate SO4-· and ·OH, which play significant roles in the oxidative degradation of organic contaminants [15]. The predicted reaction mechanisms could be described as follows, respectively [47-49]:
M3++S2O82-→M2++S2O8-· (M=Fe,Mn)
M2++S2O82-→SO4-·+M3++SO42- (M=Fe,Mn)
(2)
(3)
MnFe2O4-SACsurface-OOH+S2O82-→SO4-·+MnFe2O4-SACsurface-OO·+HSO4- (4)
MnFe2O4-SACsurface-OH+S2O82-→SO4-·+MnFe2O4-SACsurface-O·+HSO4- (5)
17
Additional SO4-· and other radicals can be produced by the radical chain reactions, which involved S2O8-·, the product of the reaction between M3+ and S2O82- [49]. And the presence of SO4-·can result in radical conversion reactions that lead to the generation of ·OH as shown in Eq.(6) and Eq.(7) [50].
Alkaline pH conditions:
SO4-·+OH- →·OH+SO42-
(6)
All pH conditions:
SO4-·+H2O →·OH+H++SO42-
(7)
It is well accepted that alcohols containing alpha hydrogen can easily react with SO4-· and ·OH [19]. The broad applications of methanol (MA) as a quenching agent of both SO4-· and ·OH, because MA can react with both radicals at high rates (kSO4-·/MA=1.6×107~7.7×107M-1s-1, k·OH/MA=1.2×109~2.8×109M-1s-1) [51,52]. Tert-butyl alcohol (TBA) is also an effective quencher for ·OH, but the reaction rate with SO4-·(kSO4-·/TBA=4×105~9.1×105M-1s-1) is considerably slower than that with ·OH (k·OH/MA=3.8×108~7.6×108M-1s-1 ) [51,52]. However, as seen from Fig. S2, no obvious inhibiting effects of OG degradation were observed with the addition of neither MA (100mM) nor TBA (100mM) in the MnFe2O4-SAC/PS system.
This result contrasted sharply with the presence of phenol (100mM), which is another strong scavenger of SO4-·and·OH (kSO4-·/phenol=8.8×109M-1s-1, k·OH/phenol =6.6×109M-1s-1) [53]. More than 85% of the decolorization efficiency was inhibited in the MnFe2O4-SAC/PS/phenol system, exerting the powerful inhibiting effect of phenol. As MA and TBA both are hydrophilic 18
compounds, which are difficult to accumulate on the MnFe2O4-SAC catalyst surface. MA and TBA could only serve as liquid phase quenchers for SO4-·and·OH. Previous studies had reported that phenol could be eliminated by AC adsorption [54], which means phenol is more approachable to the surface of the catalyst. Therefore, phenol could be a contender for OG molecules to react with free radicals which existed in the boundary layer near the MnFe2O4/SAC surface.
Furthermore, the oxygen surface functional groups such as carboxyl, hydroxy and lactones groups, which acted as crucial roles in catalytic decomposition of PS, were on the marginal carbon atoms of the aromatic ring rather than on the surface of catalyst. And the M2+ (M=Fe,Mn) species on MnFe2O4 surface binding with hydroxyl groups (M-OH) through adsorption of dissociative water molecules would also produce surface-bound SO4-· [55]. In consequence, OG degradation reaction might occur in the boundary layer and subsurface of MnFe2O4-SAC catalyst [53,56].
4. Conclusion In this study, a magnetically separable MnFe2O4-SAC hybrid catalyst has been successfully synthesized using a facile solvothermal method. The crystal phase structures and surface properties of the material were characterized using various techniques such as XRD, FTIR, SEM, XPS and VSM. The MnFe2O4-SAC catalyst exhibited excellent catalytic activity in PS activation for OG degradation, in the meantime, the as-prepared catalyst maintained high catalytic performance after repeated five runs, and the degradation efficiency of OG could still be more than 94%. The reaction of OG degradation might happen in the boundary layer near 19
the subsurface of the catalyst. In addition, the solidification of heavy metals in the catalyst demonstrated favorable stability of the catalyst. The results presented in this paper provided an environmentally friendly method for the reuse of sewage sludge. The outstanding catalytic activity, reusability and stability of the MnFe2O4-SAC catalyst also implied a promising potential for practical application of refractory organic pollutants degradation in wastewater treatment.
Acknowledgements This work was supported by Fundamental Research Funds for the Central Universities, and Science and Technology Program of Guangzhou, China.
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Figure captions Fig. 1. XRD patterns of the as-synthesized SAC (a), MnFe2O4-SAC (b) and bare MnFe2O4 nanoparticles (c).
Fig. 2. FTIR spectra of SAC, MnFe2O4-SAC and MnFe2O4.
Fig. 3. SEM images of (a) SAC, (b) MnFe2O4 microspheres, (c) MnFe2O4-SAC composites.
Fig. 4. Wide scan XPS spectra of MnFe2O4-SAC composites (A) and deconvoluted XPS spectra of MnFe2O4-SAC (B) C1s, (C) O1s, (D) Fe 2p, (E) Mn 2p.
Fig. 5. Hysteresis loops of pure MnFe2O4 and MnFe2O4-SAC composites. The inset reveals the magnetic separation property of the MnFe2O4-SAC composites.
Fig. 6. Degradation of OG under different conditions. (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
Fig. 7. Effect of reaction temperature on OG degradation (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=15~55℃).
Fig. 8. Effect of initial pH on OG degradation (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
Fig. 9. Leaching concentrations of heavy metals in the residual solution, and metals content in MnFe2O4-SAC composites before and after reaction.
29
Fig. 1. XRD patterns of the as-synthesized SAC (a), MnFe2O4-SAC (b) and bare MnFe2O4 nanoparticles (c).
30
Fig. 2. FTIR spectra of SAC, MnFe2O4-SAC and MnFe2O4.
31
Fig. 3. SEM images of (a) SAC, (b) MnFe2O4 microspheres, (c) MnFe2O4-SAC composites.
32
33
34
Fig. 4. Wide scan XPS spectra of MnFe2O4-SAC composites (A) and deconvoluted XPS spectra of MnFe2O4-SAC (B) C1s, (C) O1s, (D) Fe 2p, (E) Mn 2p.
35
Fig. 5. Hysteresis loops of pure MnFe2O4 and MnFe2O4-SAC composites. The inset reveals the magnetic separation property of the MnFe2O4-SAC composites.
Fig. 6. Degradation of OG under different conditions. (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
36
Fig. 7. Effect of reaction temperature on OG degradation (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=15~55℃).
Fig. 8. Effect of initial pH on OG degradation (Experimental conditions: [OG]=20mg/L,
37
100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
Fig. 9. Leaching concentrations of heavy metals in the residual solution, and metals content in MnFe2O4-SAC composites before and after reaction.
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S0927-7757(17)30606-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.06.043 COLSUA 21730
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21-2-2017 15-6-2017 16-6-2017
Please cite this article as: Ying Li, Zhiquan Yang, Hongguo Zhang, Xiaowen Tong, Jinna Feng, Fabrication of sewage sludge-derived magnetic nanocomposites as heterogeneous catalyst for persulfate activation of Orange G degradation, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.06.043 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.
Fabrication of sewage sludge-derived magnetic nanocomposites as heterogeneous catalyst for persulfate activation of Orange G degradation
Ying Li a,b, Zhiquan Yang a, b *, Hongguo Zhang c，Xiaowen Tong a,b, Jinna Fenga,b
a
School of Environment and Energy, South China University of Technology, Guangzhou Higher
Education Mega Centre, Guangzhou 510006, PR China
b
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of
Education, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, PR China
c
School of Environmental Science and Technology, Guangzhou University, Guangzhou Higher
Education Mega Centre, Guangzhou 510006, PR China
* Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, PR China Tel./ fax: +83 20 18620507019. E-mail address: [email protected] (Z. Yang)
1
Graphical abstract
Highlights
·Sewage sludge-derived magnetic nanocomposites was synthesized with a facile method.
·Orange G could be effectively degraded by MnFe2O4-SAC/persulfate process.
·MnFe2O4-SAC exhibited excellent catalytic activity, reusability and stability.
·The catalytic reaction might happen in the boundary layer near the subsurface of the catalyst.
Abstract
An increasing amount of sewage sludge is produced by wastewater treatment plants worldwide. Therefore, the disposal of sewage sludge is one of the vital environmental issues currently. In the present work, we developed an alternative strategy of converting sewage sludge into porous activated carbon (SAC), and then synthesized a magnetically separable 2
MnFe2O4-SAC heterogeneous catalyst through an environmentally friendly method. Several characterization techniques such as XRD, FTIR, SEM and XPS indicated the evidence of MnFe2O4 nanoparticles successfully sticking to surface of porous SAC. The MnFe2O4-SAC composites exhibited stable and excellent catalytic activity in activating persulfate (PS) for the degradation of Orange G (OG) throughout the wide pH range of 3-9, and could be recycled by a magnet without any loss. The activation energy was calculated to be 14.94 kJ·mol -1, indicating that the OG degradation in MnFe2O4-SAC/PS process required moderate activation energy. The results of heavy metals leaching experiments revealed an excellent stability of the MnFe2O4-SAC catalyst. Possible reaction mechanism in MnFe2O4-SAC/PS system was also discussed in detail.
Keywords: Sewage sludge; MnFe2O4; Magnetic nanomaterial; Sulfate radical; Heterogeneous degradation.
1. Introduction
The quantity of sewage sludge produced in conventional wastewater treatment plants increases remarkably as a result of rapid industrialisation and urbanization. The annual amount of dry sewage sludge in the United States, the European Union and China were 5.6-7, 9.8 and 8.0 million tons, respectively [1,2]. Furthermore, it is estimated that the production of sewage sludge will continue to increase at an annual growth rate of about 2 vol.% [3,4]. The presence of organic compounds, pathogenic microorganisms and potentially toxic elements in sewage sludge are at the risk of contaminating the environment and threatening human
3
health [5]. Several traditional options have been utilized to dispose of municipal sewage sludge such as land filling and incineration treatments, which may cause secondary pollution and are no longer acceptable [6]. Converting waste material into valuable products offers an initiative way to dispose of sludge from an environmental standpoint. Some new processes such as sewage sludge composting, pyrolysis and gasification are being developed [7,8]. Since sewage sludge is materially carbonaceous [9], pyrolysis is considered as an effective method to convert sewage sludge into porous activated carbon (AC) [10].
As a good adsorbent with high surface area, sewage sludge-derived activated carbon (SAC) was reported recently as an emerging catalyst support for advanced oxidation processes (AOPs) such as photo-Fenton reaction and persulfate (PS) activation [11-13]. Sulfate radical (SO4-·) which is generated from PS through the activation processes has attracted an increasing interest as an alternative to hydroxyl radical (·OH) [14]. It has been reported that SO4-· is more powerful and selective than ·OH for the degradation of recalcitrant organic pollutants. Furthermore, PS has a lower energy input than H 2O2, and it is an environmentally friendly and sustainable oxidant [15]. In general, SO4-· can be effectively generated from PS under controlled conditions such as heat, transition metals and ultraviolet irradiation [16,17]. Transition metal activation is regarded as the most feasible one among these methods due to reducing high energy demand and complicated operation [18]. It has also been found that the surface oxygen functional groups on AC may act as an activator to catalyze PS to release SO4-· and ·OH [19] .
4
Spinel ferrites nanoparticles, which are expressed as MFe2O4 (M=Mn, Co, Ni and Cu), exhibit high catalytic performance in SO4-· based catalytic oxidation [15]. However, the unique magnetic properties of the MFe2O4 nanoparticles have led to their aggregation, and the lower surface area significantly reduces its catalytic efficiency [20].
To solve this problem and to find an effective approach for SAC application, in this paper, a novel heterogeneous catalyst including magnetic MnFe2O4 and SAC composites have been firstly explored and applied for the PS activation. Orange G (OG), a typical and poisonous azo dye which widely exist in textile wastewater, was chosen as our target model pollutant [21]. The properties of MnFe2O4-SAC hybrids were characterized and its catalytic performances were investigated by altering the particular parameters including temperature and initial pH. Besides, we evaluated the stability and recyclability of the as-synthesized catalysts and the possible catalytic mechanism was also put forward through a series of radicals quenching experiments. Our study proposed an environmentally beneficial approach for reusing of sewage sludge, and provided a stable and efficient heterogeneous catalyst for sulfate radicalsbased advanced oxidation process.
2. Experimental
2.1. Reagents and materials
The dewatered sewage sludge sample was obtained from the wastewater treatment plant located in Guangzhou, China. The collected sample was initially dried in an oven at 105℃
5
for 24 h to achieve constant weight, then it was ground and sifted through 100 meshes, and properly stored for further studies.
Ferric chloride hexahydrate (FeCl3·6H2O), manganese dichloride tetrahydrate (MnCl2·4H2O), sodium persulfate(PS, Na2S2O8), tert-butyl alcohol (TBA) and Orange G were purchased from Aladdin Industrial Co. Ltd., (Shanghai, China). Other reagents including Sodium acetate anhydrous (NaAc, >99%), HNO3, ethanol, methanol, phenol were obtained from Guangzhou Chemical Reagent Co. Ltd. All of the organic and inorganic reagents were of analytical grade and used without any further purification. All of the solutions in this study were prepared with deionized water from an ultrapure water system.
2.2. Preparation of sludge based activated carbon (SAC)
10 g dried sewage sludge was firstly added into a 40 ml HNO3 (20%) solution and stirred for 24 h at room temperature. The impregnated sludge suspension was then centrifuged, and the precipitate was dried in an oven at 105℃overnight. The dried sludge was pyrolyzed at 700℃ for 1 h in the presence of nitrogen at a heating rate of 10℃/min to form activated carbon materials. After pyrolysis, the product was cooling to room temperature in nitrogen gas atmosphere. Finally, the product was ground and washed with 3 mol/L HCl to remove inorganic impurities; then it was rinsed with deionized and dried in air at 105 ℃for 24 h. The obtained product was denoted as SAC.
2.3. Fabrication of MnFe2O4-SAC nanocomposites
6
The MnFe2O4-SAC composites were prepared by a facile one-pot solvothermal method[23]. In a typical synthesis, a mixture of 0.2 g SAC, 3.7 mmol FeCl 3·6H2O and 1.85 mmol MnCl2·4H2O were dissolved in 30 mL ethylene glycol under ultrasonication for 3 h. Later, 3 g of NaAc were added and stirred vigorously for 30 min at room temperature. After stirring, the mixture was transferred to a Teflon-lined stainless steel autoclave (50 ml capacity) and heated to 200℃for 10 h to get the solid product. After the autoclave cooled to room temperature, the resulting precipitate was collected by centrifugation and repeatedly washed with ethanol and deionized water, and finally dried under vacuum at 60℃overnight. For comparison, pure MnFe2O4 nanoparticles were also synthesized by a similar method without SAC [23].
2.4. Characterization of the as-synthesized catalyst
All samples must be completely dried and ground into tiny particles before characterization. The structures and crystal phases of the as-synthesized catalyst were characterized via X-ray diffraction (XRD) patterns, which were performed with a Bruker D8Advance diffractometer using Cu Kα radiation source from 5 ° to 80 ° of
2θ. X-ray
photoelectron spectroscopic (XPS) patterns were recorded with a Thermo Escalab 250Xi instrument to analyze chemical species of Mn, Fe, O and C. XPS PEAK4.1 software was employed for background subtraction and peak-fit processing. The surface functional groups of catalyst were determined by Fourier Infrared Spectrometer (FTIR; Nexus Por Euro, Thermo Nicolet Co.) using KBr pellet method at room temperature. Morphological observations were conducted using high resolution field emission scanning electron microscope(SEM; Merlin Compact; Carl zeiss Co.) . 7
Total organic carbon (TOC) was determined by a TOC analyzer (LiquiTOC, Elementar, Germany). Before measuring TOC, 15 mL sample was extracted, filtrated and quenched with 1mL of 100mM sodium nitrite solution. The pH was measured with a pH meter (PHSJ-4F, Leici, China). The leaching concentrations of heavy metals were determined by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 720ES, USA). The magnetic properties of the asobtained catalysts were measured by vibrating sample magnetometer (VSM; LakeShore 740) at 300 K. The zeta potential was measured using a zeta potential analyzer (Malvern, Zetasizer Nano, ZS90).
2.5. Catalytic degradation experiments
Batch experiments of OG degradation were carried out in a series of 250 mL conical flasks, which placed in a shaking table with fixed rotation speed of 160 rpm. The reaction temperature was set at 25±0.1℃during the experiments, unless otherwise specified. In a typical procedure, 0.02g catalyst was added into the OG solution (100 ml, 20 mg/L) and shook for 30 min to reach the adsorption equilibrium. The initial pH of the solution was adjusted by H2SO4 (0.1 M) or NaOH (0.1 M). Then the catalytic reaction was initiated once the oxidation agent (PS) was added. Solution samples (2.5 mL) were taken at predetermined time internals and immediately filtered through 0.22 μm membrane to remove the catalyst and quench the reaction. Subsequently, the residual concentration of OG was analyzed using a UV-2100 spectrometer (Unico, Shanghai) at its maximum absorption wavelength (478nm).
8
To evaluate the stability of the MnFe2O4-SAC product, the catalyst was collected by a magnet after each run of test, washed, dried and reused in a fresh solution of OG for 4 times. All experiments were performed in triplicate within accepting error range (±5%), and all results were expressed as mean value with error bar.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. X-ray diffraction
Fig. 1.
XRD analysis was employed to identify crystallinity and purity of as-synthesized samples of SAC, MnFe2O4 and the MnFe2O4-SAC composites. As shown in Fig.1, the main peaks of MnFe2O4-SAC and MnFe2O4 samples exhibited similar XRD patterns. The diffraction peaks of MnFe2O4-SAC and MnFe2O4 at 2θ=18.289º, 30.134º, 35.415º, 36.480º, 43.082º, 53.554º, 57.032º, 62.575º and 74.033º were associated with (111), (220), (311), (222), (400), (422), (511), (440) and (533) reflections of standard XRD pattern of MnFe2O4 nanoparticles (JCPDS No. 10-0319) [22,23]. Proof of successfully synthesized MnFe2O4-SAC composites was evidenced by another two diffraction peaks of MnFe2O4-SAC sample at 2θ=20.852º and 26.658º, which could be perfectly indexed to the typical quartz (SiO2) crystallite structures [13,24].
3.1.2. FTIR analysis
9
Fig. 2.
The surface functional groups and chemical bonds on the samples were characterized by the FTIR spectra in the range of 4000-350 cm-1 (Fig. 2). The broad band in the region of 33003650 cm-1 which was presented in all samples could be designated as the stretching of O-H groups and surface adsorbed water [22]. The aqueous dispersible stability of the magnetic composites was greatly improved by high density of hydrophilic O-H groups [25]. The band at 1630 cm-1 was related to C=O stretching of COOH groups, whilst the small peak at 1430 cm -1 could be assigned to the symmetrical deformations of CH 2 [13]. MnFe2O4-SAC hybrid composites showed a characteristic peak at 1035 cm -1, which was attributed to Si-O bond stretching vibration of Si-O-Si groups [26]. Moreover, two absorption peaks observed at around 584 and 390 cm-1 corresponded to the Fe-O and Mn-O bond vibrations of tetrahedral and octahedral sites, respectively [27]. These results clearly indicated the formation of chemical bonds between the inorganic constituent (SiO2) in SAC and loaded MnFe2O4 nanoparticles.
3.1.3. Morphological analysis
Fig. 3.
The morphology and structures of bare MnFe2O4 nanoparticles, SAC and MnFe2O4-SAC composites were obtained by SEM observations (Fig. 3). As indicated from Fig.3a, the surface morphology of SAC was relatively a rough structure with irregular cavities which was in the form of flake. These cavities could be due to the nitric acid activation, carbonization of organic
10
molecules and biomacromolecules during the pyrolysis process [13]. Fig. 3b evidenced that the bare MnFe2O4 particles were well-defined spherical morphology with average particle size in the range of 350-600 nm. Besides, the severe aggregation was also observed. In comparison, the image of MnFe2O4-SAC hybrids shown that MnFe2O4 nanoparticles anchoring to the surface of SAC, while surface area and pore structures of virgin SAC were still exposed. It might be important for the hybrid material to keep high specific surface area and to prevent the agglomeration of MnFe2O4 particles[28,29].
3.1.4. XPS analysis
Fig. 4.
The chemical composition and oxidation state of the as-synthesized MnFe2O4-SAC composites were further characterized by XPS. Fig. 4A illustrated the wide scan XPS spectrum of MnFe2O4-SAC, revealing the existence of elements C, O, Mn and Fe. The C element was originated from the sewage sludge-based carbon. The binding energy of the C1s peak (Fig. 4B) centered at 284.6 eV was used as the reference for calibration, and another two peaks centered at energy 287.4 and 288.8 eV were ascribed to the binding of C=O and O=C-O bonds, respectively [30]. The O1s peak centered at 530.2 eV (Fig. 4C) was primarily assigned to the contributions of oxygen in metal oxides, such as Fe-O and Mn-O of MnFe2O4 [29,31]. And the other peak at 531.6 eV might be ascribed to the surface adsorbed oxygen groups in H 2O and organic compounds such as ethylene glycol [13]. The Fe 2p (Fig. 4D) spectrum shown two main peaks at binding energies of 710.8 and 724.4 eV, which were assigned to Fe 2p3/2 and Fe 2p1/2
11
[32], respectively, indicating completely oxidation state of iron (Fe 3+) on the surface of the MnFe2O4-SAC composites [33]. From the Mn 2p spectrum in Fig. 4E, the peaks at 652.2 and 653.9 eV were caused by Mn 2p1/2. While the peak at 641.4 eV was corresponded to the binding energy for Mn 2p3/2, accompanied by a satellite peak at 646 eV, which deviated about 5 eV towards higher binding energy from the peak of the Mn 2p3/2. This result proved the presence of Mn2+ in MnFe2O4 [32,34]. The above analysis further confirmed the MnFe2O4-SAC hybrid composites have been successfully synthesized.
3.1.5. VSM study
Fig. 5.
Magnetic properties of the as-synthesized MnFe2O4 and MnFe2O4-SAC were obtained by field dependent magnetization measurements at 300 K. As can be seen from Fig. 5, the hysteresis loops of both samples exhibited no remnant magnetization or coercivity, suggesting that the as-obtained nano-materials may be superparamagnetic at room temperature. The saturation magnetization (Ms) value of pure MnFe2O4 was 49.62emu/g, nevertheless, the Ms of MnFe2O4-SAC composites was 43.64 emu/g. The Ms value of MnFe2O4-SAC was much higher comparing to magnetic catalysts in other literatures reported previously. For instance, Ms of MnFe2O4-graphite photocatalyst was 30.34emu/g [32], Ms of graphite spheres with magnetic Fe3O4 nanoparticles was 3.1 emu/g [44], and Ms of CuFe2O4/AC magnetic adsorbents was 5.93 emu/g [28]. Previous studies have shown that the Ms values of the magnetic particles could
12
be affected by particle size, crystallinity, and coupling [45]. The reduced M s value of the MnFe2O4-SAC might be due to the decrease in particle size as revealed by SEM results.
The excellent dispersability and rapid magnetic responsiveness of MnFe2O4-SAC were verified by a preparatory experiment as shown in the insert of Fig. 5. The result indicated that the MnFe2O4-SAC composites could be easily manipulated by magnetic separation, which has a potential superiority for recycling and reusing.
3.2 Catalytic evaluation of MnFe2O4-SAC
Fig. 6.
Fig. 6 displayed the degradation profiles of OG under different experimental processes, including PS alone, MnFe2O4/PS, SAC/PS and MnFe2O4-SAC/PS, Mn2+ (0.867mM) /PS, Fe3+ (1.734mM) /PS, as well as experiments of the adsorption abilities of MnFe 2O4-SAC. It can be seen that in the absence of PS, MnFe2O4-SAC exerted little OG adsorption and the removal efficiency was only about 2%, indicating the influence of adsorption by catalysts could be negligible. No obvious OG degradation was detected after 30 min with PS alone either, suggesting that the oxidant itself failed to decompose the dye. However, with the co-existence of PS and MnFe2O4-SAC, the concentration of OG decreased dramatically and more than 95% decolorization efficiency can be achieved within 30. The catalytic oxidation of OG could be attributed to the synergistic function of MnFe2O4-SAC and PS. MnFe2O4-SAC could facilitate the decomposition of persulfate to produce SO4-·. Furthermore, the removal efficiency of OG in homogeneous Mn2+/PS and Fe3+/PS processes was found to be slight (<15%), indicating that
13
the heterogeneous catalytic reaction played a dominant role in OG degradation with MnFe2O4-SAC/PS process.
It was noteworthy that the removal rate in SAC/PS system was 60.7% during 30 min, but the rate constant of MnFe2O4-SAC/PS(0.176 min-1) was much better than that of SAC/PS (0.025min-1), suggesting that intense interfacial interaction of SAC and MnFe2O4 enhanced the ability of electron transport and reaction sites [35]. The TOC removal in MnFe2O4-SAC/PS system was found to be about 18.47% after 30 min reaction, demonstrating most of Orange G has transformed into the reaction intermediates. Higher mineralization was achieved in 60 min(26.21%) and 90 min (30.50%), indicating that extending reaction time was favourable to enhance mineralization rate.
3.3. Effect of temperature
Fig. 7.
The effect of temperature on OG degradation with MnFe2O4-SAC/PS system was investigated. It can be clearly seen from Fig. 7 that increasing temperature significantly accelerated the degradation of OG. As the temperature rose from 15 to 55℃, the OG degradation rate constant increased from 0.07 to 0.247 min-1. This result could be ascribed to the thermal activation of PS and PS decomposed faster at higher temperature [36]. According to the first order kinetics, the activation energy for OG degradation by the MnFe 2O4-SAC/PS process was calculated using the Arrhenius equation. As demonstrated in the inset of Fig. 7, lnK and 1/T followed a good linear relationship (R2 = 0.996) with Eq.(1)
14
lnK=lnA-Ea/RT
(1)
Where K is the reaction rate constant; A is the preexponential factor; Ea is the activation energy (kJ·mol-1); R is the ideal gas constant (8.314 J·mol -1K-1); T is the absolute temperature (K). The activation energy was calculated to be 14.94 kJ·mol-1, indicating that the OG degradation in MnFe2O4-SAC/PS process required moderate activation energy. Previous investigations reported that the values of activation energy on various heterogeneous catalysts could be different, such as 49.5 kJ·mol -1 on Co-graphene/PMS- Orange II [37], 69.23 kJ·mol-1 on mesoporous Fe2O3-Rhodamine B [38] and 31.7 kJ·mol-1 on MnFe2O4/PMS-Orange II processes [39]. The low activation energy signified the MnFe2O4-SAC/PS oxidation process was easy to occur and was very promising for heterogeneous catalytic reaction.
3.4. Effect of initial pH
Fig. 8.
Experiments of the initial pH influence on the OG degradation with MnFe 2O4-SAC/PS process were also carried out. As presented in Fig. 8, the fastest OG degradation rate occurred at pH=3 and its corresponding rate constant was 0.195min-1. As the pH value increased, a decrease of OG degradation was observed, indicating that the acidic and near neutral conditions were more beneficial to the OG degradation than strong alkaline condition. Surface properties of MnFe2O4-SAC and the species of PS in solution might account for this result. Catalyst displayed a positive surface charge when solution pH was lower than the potential of zero charge (pHpzc), and the surface was negatively charged when pH (solution) was greater
15
than pHpzc [40]. The pHpzc of MnFe2O4-SAC could be found in Supplementary Material Fig. S1, it was about 4.85. OG is an anionic dye, revealing surface repulsive force existed between the MnFe2O4-SAC and OG when solution pH>4.85. As the solution pH further increased, the increasing electrostatic repulsive force among MnFe2O4-SAC, OG and SO4-· led to a decline of OG degradation rate. In addition, when solution pH exceed 10, ·OH would supersede SO4-· and become predominant active free radical. The lifetime of ·OH (20ns [41]) is much shorter than SO4-· (4s [42])and ·OH is apt to react with other ions from the aqueous solution, which would result in a rapid quenching of ·OH [43]. It was worth mentioning that as shown in Fig. 8, OG could be effectively degraded throughout the pH range of 3-9, demonstrating MnFe2O4SAC/PS combined process had a great prospect in the application of water treatment.
3.5. Stability and reusability of MnFe2O4-SAC catalyst
Fig. 9.
In general, the stability and reusability of the catalysts are the most important aspects in the practical application. To evaluate the catalytic stability of spent catalyst in the PS oxidation process, the catalysts were recovered by an external magnet and repeatedly reused for degradation of fresh Orange G solution. As shown in the Supplementary Material Table. S1, the MnFe2O4-SAC catalyst showed excellent catalytic performance during the successive five cycles. Although a slight reduction was observed, the decolorization efficiency of OG was more than 94% even after five continuous cycles. Activated sewage sludge accumulates high concentration of heavy metals, higher concern should be paid to the issue of metals leaching
16
during the application of sewage sludge-based heterogeneous catalysts. In this study, ICP-MS was performed to determine the leaching concentration of heavy metals. As can be seen from Fig. 9, the corresponding concentration values of the residual supernatant solution after reaction were quite low, except for Zn. The leaching concentration of Zn was 0.082 mg/L, which was also less than the limits in National Recommended Water Quality Criteria issued by US Environmental Protection Agency [46]. In addition, the contents of heavy metals in MnFe2O4-SAC catalyst haven’t changed remarkably after the catalytic reaction, and the degradation efficiency was almost unaffected by such a low concentration of leaching metals, which further verified the heterogeneous catalytic reaction took place in the MnFe2O4-SAC/PS system.
3.6. Reaction mechanism
As mentioned above, PS can be activated by free transition metal ions and the surface oxygen functional groups to generate SO4-· and ·OH, which play significant roles in the oxidative degradation of organic contaminants [15]. The predicted reaction mechanisms could be described as follows, respectively [47-49]:
M3++S2O82-→M2++S2O8-· (M=Fe,Mn)
M2++S2O82-→SO4-·+M3++SO42- (M=Fe,Mn)
(2)
(3)
MnFe2O4-SACsurface-OOH+S2O82-→SO4-·+MnFe2O4-SACsurface-OO·+HSO4- (4)
MnFe2O4-SACsurface-OH+S2O82-→SO4-·+MnFe2O4-SACsurface-O·+HSO4- (5)
17
Additional SO4-· and other radicals can be produced by the radical chain reactions, which involved S2O8-·, the product of the reaction between M3+ and S2O82- [49]. And the presence of SO4-·can result in radical conversion reactions that lead to the generation of ·OH as shown in Eq.(6) and Eq.(7) [50].
Alkaline pH conditions:
SO4-·+OH- →·OH+SO42-
(6)
All pH conditions:
SO4-·+H2O →·OH+H++SO42-
(7)
It is well accepted that alcohols containing alpha hydrogen can easily react with SO4-· and ·OH [19]. The broad applications of methanol (MA) as a quenching agent of both SO4-· and ·OH, because MA can react with both radicals at high rates (kSO4-·/MA=1.6×107~7.7×107M-1s-1, k·OH/MA=1.2×109~2.8×109M-1s-1) [51,52]. Tert-butyl alcohol (TBA) is also an effective quencher for ·OH, but the reaction rate with SO4-·(kSO4-·/TBA=4×105~9.1×105M-1s-1) is considerably slower than that with ·OH (k·OH/MA=3.8×108~7.6×108M-1s-1 ) [51,52]. However, as seen from Fig. S2, no obvious inhibiting effects of OG degradation were observed with the addition of neither MA (100mM) nor TBA (100mM) in the MnFe2O4-SAC/PS system.
This result contrasted sharply with the presence of phenol (100mM), which is another strong scavenger of SO4-·and·OH (kSO4-·/phenol=8.8×109M-1s-1, k·OH/phenol =6.6×109M-1s-1) [53]. More than 85% of the decolorization efficiency was inhibited in the MnFe2O4-SAC/PS/phenol system, exerting the powerful inhibiting effect of phenol. As MA and TBA both are hydrophilic 18
compounds, which are difficult to accumulate on the MnFe2O4-SAC catalyst surface. MA and TBA could only serve as liquid phase quenchers for SO4-·and·OH. Previous studies had reported that phenol could be eliminated by AC adsorption [54], which means phenol is more approachable to the surface of the catalyst. Therefore, phenol could be a contender for OG molecules to react with free radicals which existed in the boundary layer near the MnFe2O4/SAC surface.
Furthermore, the oxygen surface functional groups such as carboxyl, hydroxy and lactones groups, which acted as crucial roles in catalytic decomposition of PS, were on the marginal carbon atoms of the aromatic ring rather than on the surface of catalyst. And the M2+ (M=Fe,Mn) species on MnFe2O4 surface binding with hydroxyl groups (M-OH) through adsorption of dissociative water molecules would also produce surface-bound SO4-· [55]. In consequence, OG degradation reaction might occur in the boundary layer and subsurface of MnFe2O4-SAC catalyst [53,56].
4. Conclusion In this study, a magnetically separable MnFe2O4-SAC hybrid catalyst has been successfully synthesized using a facile solvothermal method. The crystal phase structures and surface properties of the material were characterized using various techniques such as XRD, FTIR, SEM, XPS and VSM. The MnFe2O4-SAC catalyst exhibited excellent catalytic activity in PS activation for OG degradation, in the meantime, the as-prepared catalyst maintained high catalytic performance after repeated five runs, and the degradation efficiency of OG could still be more than 94%. The reaction of OG degradation might happen in the boundary layer near 19
the subsurface of the catalyst. In addition, the solidification of heavy metals in the catalyst demonstrated favorable stability of the catalyst. The results presented in this paper provided an environmentally friendly method for the reuse of sewage sludge. The outstanding catalytic activity, reusability and stability of the MnFe2O4-SAC catalyst also implied a promising potential for practical application of refractory organic pollutants degradation in wastewater treatment.
Acknowledgements This work was supported by Fundamental Research Funds for the Central Universities, and Science and Technology Program of Guangzhou, China.
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Figure captions Fig. 1. XRD patterns of the as-synthesized SAC (a), MnFe2O4-SAC (b) and bare MnFe2O4 nanoparticles (c).
Fig. 2. FTIR spectra of SAC, MnFe2O4-SAC and MnFe2O4.
Fig. 3. SEM images of (a) SAC, (b) MnFe2O4 microspheres, (c) MnFe2O4-SAC composites.
Fig. 4. Wide scan XPS spectra of MnFe2O4-SAC composites (A) and deconvoluted XPS spectra of MnFe2O4-SAC (B) C1s, (C) O1s, (D) Fe 2p, (E) Mn 2p.
Fig. 5. Hysteresis loops of pure MnFe2O4 and MnFe2O4-SAC composites. The inset reveals the magnetic separation property of the MnFe2O4-SAC composites.
Fig. 6. Degradation of OG under different conditions. (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
Fig. 7. Effect of reaction temperature on OG degradation (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=15~55℃).
Fig. 8. Effect of initial pH on OG degradation (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
Fig. 9. Leaching concentrations of heavy metals in the residual solution, and metals content in MnFe2O4-SAC composites before and after reaction.
29
Fig. 1. XRD patterns of the as-synthesized SAC (a), MnFe2O4-SAC (b) and bare MnFe2O4 nanoparticles (c).
30
Fig. 2. FTIR spectra of SAC, MnFe2O4-SAC and MnFe2O4.
31
Fig. 3. SEM images of (a) SAC, (b) MnFe2O4 microspheres, (c) MnFe2O4-SAC composites.
32
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Fig. 4. Wide scan XPS spectra of MnFe2O4-SAC composites (A) and deconvoluted XPS spectra of MnFe2O4-SAC (B) C1s, (C) O1s, (D) Fe 2p, (E) Mn 2p.
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Fig. 5. Hysteresis loops of pure MnFe2O4 and MnFe2O4-SAC composites. The inset reveals the magnetic separation property of the MnFe2O4-SAC composites.
Fig. 6. Degradation of OG under different conditions. (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
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Fig. 7. Effect of reaction temperature on OG degradation (Experimental conditions: [OG]=20mg/L, 100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=15~55℃).
Fig. 8. Effect of initial pH on OG degradation (Experimental conditions: [OG]=20mg/L,
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100mL, [PS]=0.5g/L, [catalyst]=0.2g/L, T=25±0.1℃).
Fig. 9. Leaching concentrations of heavy metals in the residual solution, and metals content in MnFe2O4-SAC composites before and after reaction.
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