Anisotropic CoFe2O4@Graphene Hybrid Aerogels with High Flux and

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Functional Nanostructured Materials (including low-D carbon)

Anisotropic CoFe2O4@Graphene Hybrid Aerogels with High Flux and Excellent Stability as Building Blocks for Rapid Catalytic Degradation of Organic Contaminants in a Flow-Type Setup Xiao-Jie Yu, Jin Qu, Zuoying Yuan, Peng Min, Shu-Meng Hao, Zhong-Shuai Zhu, Xiaofeng Li, Dongzhi Yang, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10287 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Anisotropic CoFe2O4@Graphene Hybrid Aerogels with High Flux and Excellent Stability as Building Blocks for Rapid Catalytic Degradation of Organic Contaminants in a Flow-Type Setup Xiao-Jie Yu

1,2,

Jin Qu1,*, Zuoying Yuan1, Peng Min2, Shu-Meng Hao1, Zhong-Shuai Zhu2,

Xiaofeng Li2, Dongzhi Yang2, Zhong-Zhen Yu1,2* 1

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China 2 Beijing

Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China E-mails: [email protected] (J. Qu), [email protected] (Z.-Z. Yu)

ABSTRACT: Macroscopic three-dimensional catalytic materials could overcome the poor operability and avoid secondary pollution of common powdery counterparts, especially in flow-type setups. However, conventional isotropic graphene based aerogels and foams have randomly distributed graphene sheets, which may cause the stream erosion and reduce the flux seriously. Herein, for the first time, we design and fabricate a novel anisotropic CoFe2O4@graphene hybrid aerogel (CFO@GA-A) with a hydrothermal synthesis followed by directional-freezing and freeze-drying for a tube-like flow-type setup analogous to a wastewater discharge pipeline. The long and vertically aligned pores inside the aerogel provide an exceptional flux of 1100 L m-2 h-1, 450% higher than that of the rough and zigzag paths in the isotropic CoFe2O4@graphene hybrid aerogel (CFO@GA-I), and the leaching of metal ions is obviously inhibited by relieving the erosion of CoFe2O4. Besides, the CFO@GA-A could sustain the scour of high-speed flowing wastewater, and maintain its structural stability. Therefore, organic contaminants of indigo carmine, methyl orange, orange 1 ACS Paragon Plus Environment

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ΙΙ, malachite green, phenol and norfloxacin could readily flow over the nanocatalysts and be degraded rapidly within 7.5-12.5 mins at varied flowing rates from 60 to 120 mL h-1. The CFO@GA-A also exhibits a much better long-term stability with removal efficiencies towards indigo carmine at 100%, 91% and 85% for at least 30 h (60 mL h-1), 25 h (90 mL h1),

and 21 h (120 mL h-1), respectively. On the contrary, the CFO@GA-I exhibits

unsatisfactory removal efficiencies of less than 40%. Interestingly, CFO@GA-A could also serve as building blocks to stack each other for degrading intense flowing wastewater, exhibiting an outstanding composability. The high-flux and long-term stability makes the CFO@GA-A promise as an ideal catalytic material for wastewater treatments.

KEYWORDS: anisotropic graphene aerogel; CoFe2O4; peroxymonosulfate; catalytic degradation; organic contaminants

INTRODUCTION Wastewaters from industrial productions contain many organic pollutants and toxic substances, which seriously harm the environment and human health.1-3 Non-selective advanced oxidation processes (AOPs) could degrade these contaminants to CO2 and H2O.4-7 Particularly, peroxymonosulfate (PMS) has gained unprecedented attentions due to the generated SO4·- radicals with a superior oxidizing power.8,

9

PMS is usually activated by

transition metals, UV light, and thermal activation, and so on.10-12 Lots of powdery heterogeneous cobalt based nanomaterials are adopted due to their abundant varieties and remarkable PMS activation efficiencies.13-17 For example, Tian et al. designed novel N, Sdoped Co@C nanomaterials for efficient degradation of p-hydroxybenzoic acid and phenol.18 Lin et al. used cobalt nanosheets to fully remove caffeine in 20 min, faster than most common PMS activators.19 Li et al. synthesized a magnetic CoFe2O4 to degrade atrazine, exhibiting 2 ACS Paragon Plus Environment

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excellent recovery and high removal efficiency.20 However, powdery nanocatalysts have their own shortcomings for practical application. For example, powdery nanocatalysts are prone to agglomerate because of their high surface energy,21 and they are difficult to be separated from water, showing unsatisfactory recycling performances.22 Especially in most practical cases, the flowing state of wastewater makes separation and recycle of these powdery nanocatalysts more difficult.23 It is thus imperative to develop two-dimensional (2D) or three-dimensional (3D) macroscopic nanocatalysts feasible to flow-type setups. In flow-type setups, large flux and high catalytic efficiency of macroscopic nanocatalysts with satisfactory structural stability are essential for continuing treatment of flowing wastewater.24 For example, Luo et al. achieved a α-MnO2@CuO membrane with high water permeability, leading to an ultra-fast catalytic degradation of dyes.25 Recently, Zhu et al. fabricated a hydrophilic α-Fe2O3@bacterial cellulose membrane, which was combined in a flowing bed device to efficiently activate PMS for degrading dyes under dynamic process, exhibiting improved visible light catalytic performance and long-term stability.26 However, it is still an issue to increase the flux further. Functional graphene materials, especially those with 3D structures,27, 28 enable water to pass through quickly due to their large specific surface areas and high porosities.29-32 Compared to isotropic graphene aerogels with randomly distributed graphene sheets,33 anisotropic graphene aerogels with aligned graphene sheets could be advantageous for rapid catalytic degradation of organic contaminants with high fluxes due to their aligned porous flowing paths, which has not been reported before. Herein, by taking advantage of the outstanding catalytic ability of CoFe2O4 and the remarkable long-range porous structure of anisotropic graphene aerogel (GA), for the first time, we design and fabricate a novel anisotropic CoFe2O4@graphene aerogel (CFO@GA-A) with a hydrothermal synthesis followed by directional-freezing and subsequent freeze-drying for continuing treatment of flowing wastewater. Graphene hydrogel is firstly prepared by 3 ACS Paragon Plus Environment


reducing graphene oxide to graphene with ascorbic acid. By hydrothermal treatment of the hydrogel adsorbing ferric chloride hexahydrate, cobalt chloride hexahydrate, and sodium dodecyl sulfate, CoFe2O4 nanoparticles are generated and anchored on the graphene sheets, forming a CoFe2O4@graphene hydrogel. Subsequently, the directional-freezing process makes the CoFe2O4@graphene sheets vertically aligned in the hydrogel, while the subsequent freeze-drying of the aligned hydrogel ensures an anisotropic porous structure of the resultant hybrid aerogel by subliming of its ice component. CFO@GA-A could be set in a tube-like flow-type setup analogous to a wastewater discharge pipeline, and the long-range aligned pores are parallel to the flow direction. Therefore, the CFO@GA-A exhibits a pretty high flux of 1100 L m-2 h-1 corresponding to 182 mL h-1, 450% higher than that of an isotropic CoFe2O4@graphene aerogel (CFO@GA-I); while its leaching of metal ions is obviously decreased by relieving the erosion of CoFe2O4. It also shows excellent PMS-based degradation performances towards many kinds of dyes (indigo carmine, methyl orange, orange ΙΙ, and malachite green) and organics (phenol and norfloxacin) within 7.5-12.5 min at varied flowing rates of 60-120 mL h-1. Additionally, the CFO@GA-A exhibits satisfactory long-term stability and high catalytic activity for continuing treatment of flowing wastewaters. The removal efficiencies of indigo carmine could maintain at 100%, 91% and 85% for at least 30 h (60 mL h-1), 25 h (90 mL h-1) and 21 h (120 mL h-1), respectively; On the contrary, its isotropic counterpart, CFO@GA-I, presents low removal efficiencies of less than 40% after 14 h (60 mL h-1), 9 h (90 mL h-1), and 5 h (120 mL h-1) of continuing treatments. More uniquely, the CFO@GA-A could serve as building blocks to be stacked together for degrading high concentration dyes even at intense flowing rates, showing an extremely excellent efficiency. EXPERIMENTAL SECTION

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Materials. Ascorbic acid (Vc), ferric chloride hexahydrate (FeCl3·6H2O), cobalt chloride hexahydrate (CoCl2·6H2O), sodium dodecyl sulfate (SDS), sodium hydroxide (NaOH) were provided by Beijing Chemical Factory (China). Peroxymonosulfate (PMS), persulfate (PS), indigo carmine (IC), methyl orange (MO), orange ΙΙ (OII), and malachite green (MG) were purchased from Aladdin Bio-Chem. Technol. (China). Phenol and norfloxacin were supplied by Sigma-Aldrich. All reagents were used as received without further purification. Syntheses of CoFe2O4@Graphene Hybrid Hydrogels. Graphite oxide was prepared by the modified Hummers method.34 The GO suspension (5 mg L-1, 10 mL) was prepared by ultrasonication and mixed with ascorbic acid (1/1, w/w). The mixture was reacted at 70 oC for 4 h to obtain graphene hydrogel (GH). To prepare CoFe2O4@GH (CFO@GH) hybrid hydrogels, CoCl2·6H2O (1 mmol), FeCl3·6H2O (2 mmol), and SDS (1 mmol) were mixed and stirred to obtain a solution. After the GH was immersed in the solution for 12 h to achieve an adsorption equilibrium, sodium hydroxide (NaOH, 3mol L-1) was added to adjust the pH of the mixture to 11. The resultant mixture was then transferred into a Teflon-lined autoclave for 6 h at 120 oC for hydrothermal synthesis of CoFe2O4@GH hybrid hydrogel. Finally, the product was soaked in ultrapure water for three days to remove SDS. For comparison, CoFe2O4 was also synthesized using the same methodology in the absence of SDS and graphene hydrogel. Preparation

of

CoFe2O4@Graphene

Hybrid

Aerogels.

The

synthesized

CoFe2O4@GH hybrid hydrogel was placed in an iron box and semi-immersed in liquid nitrogen for directional-freezing treatment, and the frozen hydrogel was then freeze-dried at 54 oC in a FD-1C-50 freeze dryer for 2 days to obtain an anisotropic CoFe2O4@graphene aerogel (CFO@GA-A). For comparison, the synthesized CoFe2O4@GH hybrid hydrogel was completely immersed in liquid nitrogen for 10 min. The frozen hydrogel was freeze-dried at -



54 oC in a FD-1C-50 freeze dryer for 2 days to obtain the isotropic CoFe2O4@graphene aerogel (CFO@GA-I). Characterization. The as-prepared samples were analysed using a Rigaku D/Max 2500 X-ray diffractometer (XRD) with Cu Ka radiation at a generator voltage of 40 kV and a generator current of 40 mA. Morphology and microstructure were characterized with a JEOL JEM-3010 transmission electron microscope (TEM) and a Hitachi S4700 field-emission scanning electron microscope (SEM) along with an energy dispersive X-ray (EDX) detector. X-ray photoelectron spectra were measured using a Thermo VG RSCAKAB 250X high resolution X-ray photoelectron spectrometer (XPS). The functional groups presented in the samples were characterized using a Nicolet Nexus 670 Fourier-transform Infrared (FT-IR) spectrometer. Thermal stability of the samples was determined in an air atmosphere by a TA Instruments Q50 thermogravimetric analyser (TGA) at a heating rate of 10 oC min-1. The porosity, average macroporous diameter and volume of CFO@GAs were conducted using a mercury intrusion porosimeter with an AutoPore IV 9510 series mercury instrument. In addition to mercury intrusion porosimeter, the nanoscale pore sizes of CFO@GAs were measured by the Brunauer-Emmett-Teller (BET; JW-BK200C) method using N2 adsorption and desorption isotherms at 77.3 K, and the specific surface area was measured with the Barrett-Joyner-Halenda (BJH) method. The CoFe2O4@graphene hybrid aerogels were compressed from 0% to 50% to obtain stress-strain curves using an Instron 1185 testing machine at a constant rate of 10 mm min-1. The degradation abilities of the samples were evaluated using a Shimadzu UV-3600 UV-vis spectrophotometer and a high-performance liquid chromatography (HPLC). The Co and Fe elements after degradation experiments were analysed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Flux Calculation. The flux of pure water was measured with a flow model at the atmospheric pressure. The flux of initial pure water was obtained by filtering a volume of 6 ACS Paragon Plus Environment

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solution during a specific time interval. The flux (F) was calculated with the following equation:35 F = V/(S t), where F is the flux (L m-2 h-1), V is the water volume (L), S is the cross-sectional area (m2), and t is the operating time (h). Catalysis Experiments in a Flow-Type Setup. The diagram and digital photograph of the experimental device was shown in Figures 1 and S1, respectively. CFO@GA (165.1 mm2 x 11.4 mm) was placed at the bottom of the setup. IC solution (25-100 ppm) and PMS solution (0.2-0.6 g L-1) were injected by pumps at flow rates of 60, 90 or 120 mL h-1. For a given interval, 5 mL of the output solution was taken and tested with the UV-vis spectrophotometer. Similarly, MO, OII and MG (all in 25 ppm) were also degraded using the same process. Phenol and norfloxacin (all in 20 ppm) were characterized using a highperformance liquid chromatography. Meanwhile, the long-term catalytic degradation was also carried out with the same setup. RESULTS AND DISCUSSION Figure 1 schematically illustrates the preparation and application process of the CFO@GA-A hybrid aerogel. First, the graphene hydrogel (GH) is synthesized by chemical reduction of graphene oxide with ascorbic acid, and then immersed in a solution with CoCl2, FeCl3 and SDS for 12 h to achieve an adsorption equilibrium. It is seen that extending the adsorption time could not increase the loading amount of CoFe2O4 further (Figure S2). Second, CoFe2O4@GH hybrid hydrogel is synthesized by hydrothermal treatment in an alkaline medium, during which CoFe2O4 nanoparticles are formed in situ and anchored on the graphene sheets. Here, graphene hydrogel is chemically reduced by ascorbic acid, and further reduced by a hydrothermal process with the in situ growth of CoFe2O4 to consume the oxygen-containing groups.28,36-38 Therefore, the volume of CoFe2O4@GH hybrid hydrogel shrinks as compared to that of the pristine graphene hydrogel (Figure S3a, b), caused by the removal of oxygen-containing groups of the graphene oxide. Interestingly, vertical alignment 7 ACS Paragon Plus Environment


of the CoFe2O4-decorated graphene sheets is realized inside the hydrogel by directionalfreezing, during which the ice is nucleated from the bottom of the hydrogel adjacent to the liquid nitrogen and grown vertically to form numerous ice cylinders to expel the CoFe2O4decorated graphene sheets in the cylinders. Finally, a CFO@GA-A hybrid aerogel is formed by freeze-drying, during which the ice cylinders are removed by their subliming. The aligned pore walls are straight and flat, and the CoFe2O4 nanoparticles are well distributed on the inner walls of the long-range aligned pores. A self-designed tube-like flow-type setup analogous to a wastewater discharge pipeline is used to evaluate the fluxes of the CFO@GA hybrid aerogels and their catalytic degradation performances towards organic contaminants. Here, the long-range aligned pores are parallel to the flow direction. Such an anisotropic structure with long-range aligned pores could benefit high flux wastewater treatment, and effectively increase the contact efficiency between organic contaminants and CoFe2O4 nanoparticles for efficient catalytic degradation.

Figure. 1. Schematic illustrating the preparation of an anisotropic CoFe2O4@graphene hybrid aerogel and its application for continuing treatment of wastewater with a flow-type setup. 8 ACS Paragon Plus Environment

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Figure. 2. (a) Side-view, (b) top-view and (c) surface-view SEM images of CFO@GA-A. (d) Side-view, (e) top-view and (f) surface-view SEM images of CFO@GA-I. Elemental mapping images of (g) C, (h) Fe, and (i) Co of CFO@GA-A. Figure 2 shows the morphologies and inner structures of the anisotropic and isotropic CFO@GA aerogels. Clearly, the presence of CoFe2O4 nanoparticles does not affect the formation of the anisotropic structure with long-range aligned pores parallel to the axis of the aerogel. The CFO@GA-A has the same regular channel structure as the anisotropic pristine graphene aerogel (Figures 2a, b, S4a). The small CoFe2O4 nanoparticles of less than 45 nm (35 ± 8 nm) are well-distributed on the inner surface of the anisotropic pores (Figure 2c). The 9 ACS Paragon Plus Environment


CFO@GA-A just likes a reaction vessel for allowing wastewater to flow through quickly without blocking. Differently, CFO@GA-I shows an irregular porous structure (Figure 2d, e), although its size is nearly the same as that of its anisotropic counterpart (Figure S3c). The rough and zigzag paths in CFO@GA-I would cause the flow resistance and the erosion of CoFe2O4. It is noted that the CoFe2O4 nanoparticles remain uniformly distributed, and no agglomeration is observed on the surface of CFO@GA-I due to the presence of the SDS surfactant (Figure 2f). In fact, due to their high surface energy, the unmodified CoFe2O4 nanoparticles like to agglomerate once synthesized in the absence of the SDS surfactant (Figure S4b, S4c, S5a, S5b). They grow to large nanoparticles (82 ± 35 nm), and do not like to anchor onto the graphene sheets (Figure S4d-f). As reported, surfactants could prevent the aggregation of primary nanoparticles during their synthesis process.39 With the addition of SDS, the CoFe2O4 nanoparticles are uniformly distributed on the graphene sheets (Figure 2c, 2f, S5c, S5d), and the inner structure of the aerogel is not affected. The uniform distribution of CoFe2O4 nanoparticles in the CFO@GA-A is also proved by the apparent signals of C (red), Fe (blue) and Co (yellow) elements (Figures 2g-i, S6a, b). The EDX spectrum also proves the existences of C, O, Fe and Co elements (Figure S7). The Fe/Co ratio is approximately 2/1, proving the successful synthesis of CoFe2O4. The crystal structures of GA, CoFe2O4, and CFO@GA aerogels are evaluated with their XRD patterns (Figure 3a). The diffraction peaks of anisotropic and isotropic CFO@GA aerogels are almost identical. The peak at 22.8o is attributed to graphene component, while the peaks at 18.3o, 30.1o, 35.4o, 37.1o, 43.1o, 53.4o, 57.0o, 62.6o and 74.0o are indexed to (1 1 1), (2 2 0), (3 1 1), ( 2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) crystal planes of CoFe2O4 (PDF No. 22-1086), confirming the successful synthesis of CFO@GA. The chemical bonds in the hybrid aerogels are determined with the FT-IR curves (Figure 3b). The peaks at 2918 and 10 ACS Paragon Plus Environment

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2370 cm-1 result from reduced graphene oxide aerogels,40 while the peak at 583 cm-1 belongs to the overlap of Co-O and Fe-O bonds.41 In addition, the peaks at 3434 and 1625 cm-1 are attributed to the stretching and bending vibrations of O-H bond in the water molecules adsorbed on the surface.42

Figure. 3. (a) XRD patterns, (b) FT-IR spectra, and (c) XPS spectra of GA, CoFe2O4, and CFO@GAs. (d) C 1s spectra of GO, GA, and CFO@GAs; (e) O 1s of GA, CoFe2O4, and CFO@GAs; and (f) Fe 2p and Co 2p of CoFe2O4, and CFO@GAs. XPS spectra illustrate the chemical valences of elements and chemical compositions in the aerogels. Both CFO@GA-I and CFO@GA-A aerogels are composed of Fe, Co, O, and C elements, distinguishing from pristine graphene aerogel and CoFe2O4 (Figure 3c). The two CFO@GA aerogels also exhibit similar C1s and O1s curves, implying that the different freezing processes do not affect the surface chemical states (Figure 3d, e). In addition, the new peak of O1s at 530.2 eV is related to Co-O and Fe-O bonds, confirming the successful combination of GA and CoFe2O4 (Figure 3e).43 In the Fe 2p spectra, the peaks at 711.8 and 724.6 eV correspond to the electronic valence states of Fe 2p3/2 and Fe 2p1/2, respectively 11 ACS Paragon Plus Environment


(Figure 3f).44 The bimodal peaks at 781.6 and 796.7 eV in the Co 2p spectra (Figure 3f) are associated with Co 2p3/2 and Co 2p1/2, respectively.45 The peak positions of Fe and Co in the CFO@GA and the CoFe2O4 are completely consistent, confirming that the valence states of Fe and Co are not affected by the graphene aerogel substrates. Figure S8 shows the TGA curves of CoFe2O4 and CFO@GAs in an air atmosphere. The contents of CoFe2O4 in both the anisotropic and isotropic CFO@GAs remain 41.2 and 41.6 wt%, respectively. Since the different freezing processes do not affect the crystal structure, chemical constitution, valence state, and the loading of CoFe2O4, the effects of the anisotropic and isotropic structure on catalytic degradations in the same flow-type setup could be compared reasonably. Figure 4a shows the fluxes of CFO@GA aerogels in a self-designed flow-type setup (Figure 1). Generally, the fluxes of fresh CFO@GAs decrease with increasing the time. However, the flux of CFO@GA-A eventually stabilizes at 1100 L m-2 h-1 corresponding to 182 mL h-1, 450% higher than that of its isotropic counterpart (200 L m-2 h-1). When two CFO@GA-A aerogels are stacked together, the flux still retains at 915 L m-2 h-1, illustrating that the anisotropic hybrid aerogel is well suitable for high-flux wastewater treatment. The catalytic performances of CFO@GAs at different flow rates are investigated with IC (50 ppm) at 20 oC. As previously reported, CoFe2O4 could combine with PMS or persulfate (PS) to generate free radicals for degrading organic dyes.46-48 For comparison, equal amounts of PS and PMS are added to CFO@GA-A and CFO@GA-I (Figure 4b, c). Generally, PMS is activated by CoFe2O4 to produce both SO4·- and ·OH, while PS only produces SO4·-.11 SO4·has a better oxidizing power than ·OH, but sulfate radicals can be scavenged by another SO4·or transition metal ions (e.g. Fe2+ and Co2+).49 In other words, the catalytic activity would be inhibited largely if there were too much sulfate radicals in the solution. Since the catalytic effect of PMS is superior to that of PS here, PMS is chosen for further investigations.


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Figure. 4. (a) Fluxes of CFO@GA-I, CFO@GA-A, and stacked CFO@GA-A. Relative concentration changes (Ct/C0) of IC (50 ppm) at 120 mL h-1 without and with 0.2 g L-1 of PMS or PS for (b) CFO@GA-A, and (c) CFO@GA-I. Ct/C0 of IC (50 ppm) under different flow rates with 0.2 g L-1 of PMS for (d) CFO@GA-A, and (e) CFO@GA-I. (f) Plots of metal leaching at different flow rates. The CFO@GA-A at a flow rate of 60 mL h-1 undergoes simultaneous adsorption and catalytic process within 40 min to achieve 100% of degradation efficiency (Figure 4d). In fact, the catalytic degradation plays the predominant role during the whole process. As shown in Figure S9, the relative concentration (Ct/C0) of IC changes a little after 60 min in the absence of PMS. Upon adding of PMS, the Ct/C0 sharply decreases within 15 min. However, 0.2 g L-1 of PMS cannot meet the need of catalytic degradation, resulting in a decrease in degradation efficiency. Finally, the degradation rate of IC with the CFO@GA-A at 60 min is 79%. When the flow rate increases to 90 mL h-1, the time to reach the extremum is shortened and the final degradation efficiency reaches 74%. Increasing the flow rate to 120 mL h-1 shortens the time further for CFO@GA to reach the extremum, and the corresponding IC 13 ACS Paragon Plus Environment


degradation efficiency is 66%. Obviously, the flow rate has a giant impact on the catalytic degradation process. A higher flow rate causes a higher concentration gradient of the dye from the solution to the surface of CoFe2O4 nanoparticles. Therefore, high flow rates could promote simultaneous adsorption and catalytic process of IC, leading to a short time to reach the extremum. At the same time, a high flow rate would decrease the contact time between dyes and CoFe2O4 nanoparticles, and thus reduce the catalytic efficiency.26 Similarly, the CFO@GA-I exhibits the same trend. Although the CFO@GA-I is fast to reach the lowest Ct/C0 value (Figure 4e), it is difficult to remove dyes totally even at a low flow rate. Compared to the anisotropic pores, the rough and zigzag paths in the CFO@GA-I have a longer contact time with dyes to shorten the time to reach the extremum, but increases the flow resistance and the erosion possibility of CoFe2O4 component. A shown in Figure 4f, when the flow rate increases from 60, 90 to 120 mL h-1, the amount of cobalt ion leaching with the CFO@GA-I increases from 0.43, 0.61 to 0.86 ppm, while those of the iron ion leaching are 0.16, 0.18, 0.28 ppm, respectively. For comparison, the amount of cobalt ion leaching with the CFO@GA-A is just 0.29, 0.38 and 0.42 ppm, respectively, and the iron ions are hardly detected. Cobalt is the main active site to activate PMS, and iron could reduce cobalt leaching by Co-Fe bonds.50 As both the leaching of cobalt and iron in the CFO@GA-I are more serious, its degradation efficiency declines to below 50% at the flow rates of 60, 90, 120 mL h-1 (Figure 4e). Clearly, the CFO@GA-A is more suitable for continuous water treatment. Note that all the abovementioned amounts of leaching are lower than the standard (GB 3838-2002, Fe3+ < 0.3 ppm; GB 25476-2010, Co2+ < 1 ppm). To achieve optimal catalytic degradation performances, the dosage-dependent effect of PMS is investigated.51 The relative concentration changes of IC (50 ppm) in the presence of the CFO@GA-A with different amounts of PMS are shown in Figure 5a, and corresponding degradation rate constants (k) are given in Figure S11a. It is seen that the IC could be 14 ACS Paragon Plus Environment

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completely degraded within 40 min with 0.2 g L-1 of PMS (k = 1.06 × 10-1 min-1). When the dosage of PMS increases to 0.4 and 0.5 g L-1, IC could be removed completely within 15 min (k = 2.35 × 10-1 min-1) and 10 min (k = 5.12 × 10-1 min-1), respectively. Increasing the dosage of PMS to 0.6 g L-1, the degradation efficiency is not significantly improved further. Under the optimal PMS dosage of 0.5 g L-1, the CFO@GA-A could degrade many organic contaminants.

Figure. 5. (a) Plots of relative concentration change of IC (50 ppm) for CFO@GA-A with different dosages of PMS and at 120 mL h-1. (b) Plots of relative concentration change of MG, MO and OII (all in 25 ppm) for CFO@GA-A with 0.5 g L-1 of PMS and at 120 mL h-1. (c) Plots of relative concentration change of norfloxacin and phenol (both in 20 ppm) with 0.5 g L-1 of PMS and at 120 mL h-1. (d) Plots of relative concentration change of IC (25, 50, 75, 100 ppm) for CFO@GA-A with 0.5 g L-1 of PMS and at 120 mL h-1. (e) Plots of relative concentration change of IC (75 and 100 ppm) for 2 stacked CFO@GA-As with 0.5 g L-1 of PMS and at 120 mL h-1. (f) Real time catalytic experiments for degrading IC (100 ppm) in 60



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min with CFO@GA-A and stacked CFO@GA-As (one red arrow indicates one piece of CFO@GA-A). Figure 5b shows the relative concentration changes of MG, MO and OII (all in 25 ppm) in the presence of 0.5 g L-1 of PMS at a flow rate of 120 mL h-1. It is seen a typical non-selective advanced oxidation process, because the generated active species (such as SO4·-, ·OH) have superior radical oxidizing powers to organics.11 Although these dyes have different surface charges, all above could be totally degraded by the produced radicals after 10 min (Figure S11b). As SO4·- is electrophilic, its reaction with electron-donating groups such as hydroxyl (–OH), secondary amines (–NHR), π electrons and other unsaturated bonds is fast, while the reaction with electron-withdrawing groups such as tertiary amine (−NR2), sulfo group (−SO3) and carbonyl (C=O) is generally slow.11 As shown in Figure S10, OII has more electrondonating groups followed by MO and MG. Thus, the order of the degradation rate constants is 4.15×10-1 (MG) < 6.00×10-1 (MO) < 7.01×10-1 min-1 (OII). Additionally, the CFO@GA-A could also degrade phenol and norfloxacin. Phenol is a toxic substance,52 while norfloxacin is a common antibiotic drug with bacteriostatic effects. As shown in Figures 5c and S11c, at the flow rate of 120 mL h-1, norfloxacin (20 ppm) could be completely degraded in 7.5 min (k = 7.82 × 10-1 min-1), and phenol (20 ppm) could be totally removed in 12.5 min (k = 3.28 × 10-1 min-1). The high active secondary amines and large conjugated structure of norfloxacin make itself exhibit a higher reaction activity than phenol. The CFO@GA-A exhibits significant catalytic performances towards organic contaminants even at a high flow rate of 120 mL h-1, corresponding to a high flux of 725 L m-2 h-1. As shown in Table S2, the CFO@GA-A significantly outperforms many reported catalysts with steady stirring setups or flowing beds for removing organics, further confirming the remarkable structure advantage of the anisotropic hybrid aerogel.26,

53-62

The excellent

efficiency of various contaminant degradation is contributed to the synergistic effect between 16 ACS Paragon Plus Environment

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graphene

and

CoFe2O4.63,64

Firstly,

graphene

aerogels

could

adsorb

organic

contaminants/dyes by means of π-π interaction and auxiliary adsorption, including hydrogen bonding interaction, electrostatic interaction, hydrophobic interaction and pore-filling. Then, during the organic contaminant/dye solution containing PMS flows through the aerogel, PMS could fully contact CoFe2O4 and be activated into active species (such as SO4·-, ·OH), which could further degrade the contaminant molecules adsorbed in the aerogel.11, 65, 66 Thanks to the excellent catalytic degradation performances of the CFO@GA-A, its degradation characteristics at even higher concentrations of dyes are studied (Figure 5d). At a flow rate of 120 mL h-1, 25 and 50 ppm of IC could be completely degraded. When the concentration of IC increases to 75 ppm, the degradation efficiency could reach 100% and then gradually drop to 75%. Further increasing to 100 ppm corresponds to a final degradation efficiency of 60%, and the output solution is still in blue colour (Figure 5f). This illustrates that a single CFO@GA-A is not sufficient to degrade high concentration dyes. Fortunately, the macroscopic structure could provide more feasibilities. The CFO@GA-A could serve as building blocks to be stacked together to form a longer catalytic channel decorated with CoFe2O4. CoFe2O4 and PMS could fully contact with each other to produce more free radicals, promoting the degradation efficiency of IC at high concentrations. In addition, the stable flux of the stacked CFO@GA-A is still much higher than that of the CFO@GA-I (Figure 4a). Therefore, both 75 and 100 ppm of IC are fully degraded within a short time of 12.5 min (Figure 5e), and the output water is thus colourless (Figure 5f). These results further indicate a significant prospect of the CFO@GA-A for continuing wastewater treatment.



Figure. 6. (a) Plots of removal efficiencies of IC (25 ppm) for CFO@GA-A and CFO@GA-I with 0.5 g L-1 of PMS at different flow rates. (b) Side-view, (c) top-view and (d) surface-view SEM images of the used CFO@GA-A at a flow rate of 60 mL h-1 for 30 h. (e) Side-view, (f) top-view and (g) surface-view SEM images of the used CFO@GA-I at the flow rate of 60 mL h-1 for 14 h. In addition to the high flux and high degradation efficiency, the CFO@GA-A also has satisfactory catalytic stability and structural stability. Figure 6a shows the long-term catalytic 18 ACS Paragon Plus Environment

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performances of CFO@GA-A and CFO@GA-I at different flowing rates. For the CFO@GAA, IC could be completely degraded for at least 30 h at a flow rate of 60 mL h-1, indicating its superior catalytic stability. When the flow rate increases to 90 mL h-1, the degradation efficiency maintains at 100% for 16 h, and then slightly decreases and eventually stabilizes at ~91% from 17 to 25 h. With further increasing the flow rate to 120 mL h-1, the time with 100% of degradation degree is shortened to 12 h, and the degradation degree finally retains at 85% for 21 h. In a word, the degradation efficiency decreases with the increase of flow rates. Such a phenomenon has also been reported in our previous work due to the diffusion limitation affected by a high flow rate.26 For comparison, the removal extents with CFO@GA-I are much lower. For example, at the flow rate of 60 mL h-1, the removal efficiency of 100% could maintain for 4 h only, and then rapidly drops to 41% within subsequent 10 h. And even worse, at the flow rate of 120 mL h-1, the removal efficiency of IC decreases from the beginning, and sharply drops to 35% within 5 h. As shown in Table S1, although the specific surface area of CFO@GA-A is nearly the same as that of CFO@GA-I, the porosity and pore volume of CFO@GA-A are much higher than those of CFO@GA-I. In addition, the diameters of mesopores in both CFO@GA-A and CFO@GA-I are nearly the same, while the diameter of macropores in CFO@GA-A is much bigger than that of CFO@GA-I due to the different freezing processes. It means that CFO@GA-A could hold more solution and allow the solution rapidly flow through the anisotropic pore structure than CFO@GA-I, leading to a higher removal efficiency of dyes. The bad catalytic stability of CFO@GA-I should also be ascribed to its poor structural stability. Figure S12a shows the optical images of CFO@GAs after long-term catalytic reactions. There is no change in the appearance of the CFO@GA-A; while some crackles are present in the CFO@GA-I. Moreover, the used CFO@GA-A for 30 h still has the same longrange anisotropic porous structure as before (Figure 6b, c), and the CoFe2O4 nanoparticles are 19 ACS Paragon Plus Environment


still anchored on the graphene sheets (Figure 6d). The content of CoFe2O4 in the CFO@GAA aerogel is slightly lower than that of its fresh counterpart (Figure S13). However, the structure of the used CFO@GA-I is destroyed as compared to its fresh counterpart (Figure 6e, f), even if it is just used for 14 h. In addition, the rough and zigzag paths in the CFO@GA-I could not bear the high-speed and long-time scouring, which causes the erosion of CoFe2O4 (Figure 4d) and thus decreases the degradation efficiency. Figure S14 gives the axial and radial compressive stress-strain curves CFO@GAs. The stresses of both aerogels increase with strain, which is mainly attributed to the elastic bending and shearing deformation of graphene walls. Along the axial direction, the stress of CFO@GA-A at 50% strain is 58.2 kPa, much higher than that of CFO@GA-I (37 kPa), confirming the robust pore structure of the anisotropic aerogel. In the radial direction, the CFO@GAs exhibit similar curves. Therefore, CFO@GA-A shows an obvious mechanical anisotropy, indicating that it is suitable for wastewater treatment in a flow type setup. Figure 6g also shows that the loading of CoFe2O4 becomes less than that in the fresh CFO@GA-I (Figure 2f). The content of CoFe2O4 in the isotropic aerogel decreases largely to 27.5%, which is significantly less than the initial loading of 41.2% in the CFO@GA-I aerogel (Figure S13), confirming a much more serious erosion than the CFO@GA-A aerogel. Thanks to the excellent structural stability, the used CFO@GA-A could maintain its original shape after second freeze-drying or oven drying (Figure S12b, c). Apparently, the CFO@GA-A is well suitable for high-performance continuous water treatment with high-flux and excellent structural stability. CONCLUSION Anisotropic long-range aligned pores are well constructed in the macroscopic anisotropic CFO@GA hybrid aerogel by hydrothermal synthesis followed by directional-freezing and subsequent freeze-drying. The macroscopic 3D structure allow it be set handily in a tube-like flow-type setup analogous to a wastewater discharge pipeline. These aligned pores parallel to 20 ACS Paragon Plus Environment

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the axis of the aerogel provide straight and unhindered flow paths decorated with uniformly distributed CoFe2O4 nanocatalysts. Therefore, compared to the rough and zigzag paths in the isotropic CFO@GA aerogel, the aligned structure exhibits a 450% higher flux up to 1100 L m-2 h-1 and prevents the erosion of CoFe2O4 nanoparticles obviously due to the significantly decreased flow resistance. The CFO@GA-A effectively activates PMS continuously and outperforms many reported steady stirring setups or flowing beds for removing organics. The removal efficiencies of IC maintain at 100%, 91% and 85% for at least 30 h (60 mL h-1), 25 h (90 mL h-1) and 21 h (120 mL h-1), respectively, superior to those of CFO@GA-I aerogels. After long-term used, the microstructure and catalytic performances of the CFO@GA-A aerogel still stable. Moreover, the CFO@GA-A aerogels could be stacked together to meet complex application conditions due to its feasible operability. All these advantages make the CFO@GA-A ideal for practical continuous wastewater treatment in flow-type setups. ASSOCIATED CONTENT Supporting Information Digital photograph of a flow-type setup for catalytic experiments, TGA curves after soaking for different periods, photographs of graphene hydrogel, CFO@GAs, SEM image of anisotropic GA and anisotropic CFO@GA in the absence of SDS, TEM images of CoFe2O4 nanoparticles and CFO@GA-A with or without SDS, elemental mapping and EDX spectrum of CFO@GA-A, TGA curves of CoFe2O4 and CFO@GAs, porosity and surface area of CFO@GAs, stress−strain curves of CFO@GAs, Ct/C0 of IC (50 ppm) at 120 mL h-1 using CFO@GA-A without or with 0.5 g L-1 of PMS at different periods, molecular structures of organic pollutants, degradation rate constants (k) of CFO@GA-A with different PMS concentration and pollutants, photographs and TGA curves of CFO@GAs after long-term using, degradation performances of various catalysts under different condition. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. 21 ACS Paragon Plus Environment


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AUTHOR INFORMATION Corresponding authors: E-mail: [email protected] (J. Qu), [email protected] (Z. Z. Yu) ACKNOWLEDGEMENTS Financial support from the National Natural Science Foundation of China (51402012, 51533001),

the

National

Key

Research

and

Development

Program

of

China

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Anisotropic CoFe2O4@Graphene Hybrid Aerogels with High Flux and

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