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Graphene oxide/chitosan aerogel microspheres with honeycombcobweb and radially oriented microchannel structures for broad-spectrum and rapid adsorption of water contaminants Ruomeng Yu, Yongzheng Shi, Dongzhi Yang, Yaxin Liu, Jin Qu, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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ACS Applied Materials & Interfaces

Graphene oxide/chitosan aerogel microspheres with honeycomb-cobweb and radially oriented microchannel structures for broad-spectrum and rapid adsorption of water contaminants ⊥

⊥

Ruomeng Yu,a Yongzheng Shi,a Dongzhi Yang,*a Yaxin Liu,a Jin Qu,a Zhong-Zhen Yu*a,b a

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

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

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, China ⊥

These authors contributed equally to this work.

E-mail: [email protected] (D.-Z. Yang); [email protected] (Z.-Z. Yu)

ABSTRACT: Multifunctional graphene oxide/chitosan aerogel microspheres (GCAMs) with honeycomb-cobweb and radially oriented microchannel structures are prepared by combining electrospraying with freeze-casting to optimize adsorption performances of heavy metal ions and soluble organic pollutants. The GCAMs exhibit superior adsorption capacities of heavy metal ions of Pb(II), Cu(II) and Cr(VI), cationic dyes of methylene blue and Rhodamine B, anionic dyes of methyl orange and Eosin Y, and phenol. It takes only 5 min to reach 82% and 89% of equilibrium adsorption capacities for Cr(VI) (292.8 mg g-1) and methylene blue (584.6 mg g-1), respectively, much shorter than the adsorption equilibrium time (75 h) of a graphene oxide/chitosan monolith. More importantly, the GCAMs maintain excellent adsorption capacity for 6 cycles of adsorption-desorption. The broad-spectrum, rapid and reusable adsorption performance make the GCAMs promising for highly efficient water treatments.

ACS Paragon Plus Environment

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KEYWORDS: graphene oxide; aerogels; adsorption; water treatment; recyclability

1. INTRODUCTION With the rapid development of modern industries, water contamination resulted from leakages and spills of hazardous chemicals has been a growing global environmental concern.1,2 Contaminated water containing heavy metal ions and/or soluble organic pollutants causes serious environmental, health and safety risks.3-5 Till now, various methods have been researched to purify the contaminated water on the basis of adsorption,3,6 oxidation,7 biodegradation,8 filtration,9 distillation,10 and extraction.11 While adsorption is one of the most inexpensive, fast, and effective process for water treatment.12,13 Graphene oxide (GO) is a superior adsorbent for water purification since its large specific surface area, high hydrophilicity, numerous active adsorption sites with anionic properties, and low costs.14 Owing to electrostatic or π-conjugate interactions, GO shows fine affinity with many soluble cationic or aromatic pollutants.15 Moreover, the abundant oxygen-containing groups and large specific surface area allow GO to be chemically functionalized with lots of materials.16 Han et al. prepared a three-dimensional (3D) GO/polyethylenimine porous architecture by freezedrying, exhibiting high adsorption capacity for the anionic dye amaranth.17 Chen et al. reported that an agar/GO composite aerogel showed high adsorption capacity for methylene blue (MB).18 Fang et al. produced GO aerogels crosslinked by layered double hydroxides with adsorption capacities of 95.7 mg g-1 for Cd(II).19 Although GO-based adsorbents did exhibit high adsorption capacities for many contaminants, it took a long time to reach adsorption equilibriums. Since the equilibrium time is proportional to the diffusion path, the long time is reasonable for an elongated internal diffusion path in the disordered network of 3D GO-based architectures. Zhang


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et al. developed an oriented porous chitosan-gelatin/GO monolith, and the adsorption efficiency reached 88 wt% in 1 h.20 To further shorten the adsorption time, Bao et al. constructed millimeter-sized spherical GO beads for removing corrosive chemicals.21 Recently, Xia et al. fabricated graphene microspheres with radially oriented microchannel structure for oil adsorption, which greatly accelerated the adsorption rate.22 Although considerable progress has been made in the preparation of high-performance graphene- or GO-based porous adsorbents, it is still a challenge to develop an adsorbent with both broad-spectrum and fast adsorption performances while maintaining a high adsorption capacity. Herein, we have prepared GO/chitosan (CS) aerogel microspheres (GCAMs, ~200 µm in diameter) with honeycomb-cobweb and radially oriented microchannel structures for water purification by combining electrospraying with freeze-casting. As an environmentally friendly natural macromolecule with abundant sources, CS contains many amino and hydroxyl groups for efficient adsorption of metal ions and anionic organic compounds by chelating or electrostatic interactions. Interestingly, CS can be assembled with GO to form composite blocks by electrostatic interaction and chemical functionalization, thereby enhancing the stability of GO aerogels in aqueous solutions. Owing to the integration of CS with GO, the as-prepared hierarchical honeycomb-cobweb structured microspheres would exhibit high adsorption capacities of heavy metal ions, cationic dyes, anionic dyes, and phenol. The radially oriented microchannels inside the microspheres significantly shorten the internal diffusion path, thereby facilitating rapid diffusion and accelerating the adsorption rate. The GCAMs exhibit superior adsorption capacities of heavy metal ions, cationic dyes, anionic dyes and phenol with a satisfactory recyclability. The adsorption kinetics of GCAMs with the honeycomb-cobweb and


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radially oriented microchannel structures are investigated in details and the adsorption mechanisms for metal ions, dyes and phenol are confirmed.

2. Experimental Section 2.1 Materials Lead nitrate, copper nitrate, phenol, and CS (Mw 50 000) were provided by Adamas Reagent Co. Ltd. (China). Graphite flakes (300 mesh), potassium permanganate, sodium nitrate, sulfuric acid (98%), glutaraldehyde (50%), hydrochloric acid (37%), acetic acid, ammonium hydroxide (28%), acetic ether, n-hexane, and hydrogen peroxide (30%) were purchased from Beijing Chemical Works (Beijing, China). Potassium dichromate, MB, methyl orange (MO), Rhodamine B (Rh B), and Eosin Y were obtained from Aladdin Chemistry Co. Ltd. (Shanghai, China). 2.2 Preparation of GO/CS Suspensions GO was prepared by modified Hummers’ method,23 and 10 mg mL-1 of GO aqueous dispersion was obtained by ultrasonication for 0.5 h. 8 mg mL-1 of CS was prepared by dispersing CS powder in 2.5% aqueous acetic acid and stirring for 12 h. After the CS solution and the GO dispersion were mixed and sonicated for 30 min, the GO/CS suspension was formed by adding a few drops of ammonia hydroxide and sonicating for additional 30 min. 2.3 Fabrication of Hierarchical Honeycomb-Cobweb Structured Aerogel Microspheres The GO/CS suspension was added into the syringe, which was placed on an Ucalery electrospinning apparatus (Tianjin, China). The collector with n-hexane was maintained at -84 oC in an ethyl acetate solid-liquid mixture bath cooled by liquid nitrogen, in which the microdroplets sprayed from the needle were coagulated into ice microspheres. The flow rate of the suspension, the distance between the surface of the collector and the tip of syringe needle,


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and the electrospray voltage were set to 8 mL h-1, 10 cm, and +15 kV, respectively. After electrospraying, the ice microspheres were separated from n-hexane by filtration, and lyophilized at lower than -50 oC, and 20 Pa for 24 h. The freeze-dried microspheres were crosslinked with a glutaraldehyde vapor for 3 h to obtain GCAMs. A series of GCAMs with different CS contents of 0%, 5%, 10%, 20%, and 50% were designated as GCAM0, GCAM5, GCAM10, GCAM20, and GCAM50, respectively. 2.4 Characterization The morphology and microstructure of GCAMs with different CS contents were observed with a Nikon Eclipse Ci POL polarizing microscope (Japan) and a Hitachi S4700 field emission scanning electron microscope (SEM). X-ray diffraction (XRD) patterns were carried out on a Rigaku D/Max 2500 diffractometer (Japan) with CuKα radiation. X-ray photoelectron spectroscopy (XPS) curves were obtained with a Thermo VG RSCAKAB 250X high resolution X-ray photoelectron spectrometer. Raman spectra were measured with a Renishaw inVia Raman microscopy (UK) at an excitation wavelength of 514 nm. Fourier transform infrared spectroscopy (FT-IR) was carried on Nicolet Nexus 670 (US). Zeta potential was carried on a Malvern MS2000 particle size analyzer (UK). The concentrations of dyes and phenol were obtained by Shimadzu UV-2600 spectrophotometer. The concentrations of metals ions were analyzed using a Shimadzu ICPS-7500. 2.5 Adsorption Experiments Three metal ions (Pb(II), Cu(II) and Cr(VI)), four dyes (MB, Rh B, MO, and Eosin Y), and phenol were used to evaluate the adsorption behavior of the aerogel microspheres. Typically, 10 mg of GCAMs was dispersed into 500 mg L-1 of MB solution (50 mL) under mild stirring (25 o

C). the dispersion (1 mL) was taken out to measure with UV-Vis spectrometer at given time


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intervals. The adsorption capacities of GCAMs were calculated according to the following equation: qt =

ሺC0 - Ct ሻV m

(1)

where qt is the adsorption capacity (mg g-1) at t min, C0 and Ct are the concentrations of the adsorbate (mg L-1) after 0 and t min, respectively, V is the volume of the solutions (mL), and m is the mass of the adsorbent (g). To study the reusability of the adsorbent, the regeneration of used GCAMs was conducted by washing for several times with 0.1 M HCl for MB, Rh B, Pb(II) and Cu(II), or NaOH for MO, Eosin Y and Cr(VI), followed by washing with deionized water until a pH value near 7.

3. RESULTS AND DISCUSSION 3.1. Preparation and microstructures of GCAMs Three steps are required for the successful preparation of the hierarchical honeycomb-cobweb structured aerogel microspheres (Figure 1): (I) preparation of GO/CS composite suspensions, (II) low-temperature electrospraying, and (III) lyophilization. CS, a linear cationic polysaccharide with hydroxyl and amine groups,24 is used to crosslink negatively charged GO sheets to construct a 3D porous architecture. Upon adding the CS solution to the GO dispersion with violent shaking, the mixture immediately loses its fluidity and becomes a hydrogel because of self-assembly of CS chains and GO sheets.25 As shown in Figure S1, the acidic solution of CS is positively charged and GO is negatively charged. The electrostatic interaction and chemical functionalization compel the CS molecular chains to graft onto the GO sheets, which is substantiated with XPS and FT-IR data shown below. The polymer chains grafted on GO and free in the acidic solution are protonated and stretched to form the GO/CS hydrogel. Upon the


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addition of ammonium hydroxide, the CS chains grafted on GO sheets are deprotonated and rolled up to form GO/CS composite blocks.26 As the pH value increases, the zeta potential of GO/CS blocks changes from positive to negative (Figure S1), resulting in a homogeneous suspension of GO/CS composite blocks due to the electrostatic repulsion. In a typical procedure, the GO/CS suspension is loaded in the syringe of an electrospinning apparatus with the needle connected to a high voltage. The charged GO/CS suspension droplets at the tip of the needle are split into charged microdroplets by the high-voltage static electricity, and the microdroplets are collected in an n-hexane coagulation bath. Because the n-hexane collector had a high hydrophobicity and a low surface tension, and was kept at -84 oC by liquid nitrogen, the microdroplets remained spherical, forming the GO/CS ice microspheres.22 After removing the nhexane by vacuum filtration, the filtered ice microspheres were lyophilized to remove water and form micron-sized GCAMs with radially oriented pore structure, which were different form the reported graphene-CS aerogel monoliths, such as 3D interconnected graphene/CS aerogels,27 porous graphene oxide-chitosan aerogels,28 graphene/CS microporous composites.29 Very recently, Frindy et al. prepared the CS-GO microspheres with a disordered internal pore structure.30 The size distributions of GCAMs with different CS contents are shown in Figure S2a. GCAM5 with a low CS content of 5% (Figure S2b) has smaller diameters and narrower distributions than those of GCAM50 with a high CS content of 50% (Figure S2d), which is caused by the increased viscosity of the GO/CS suspension. As a cationic crosslinker, a high CS content leads to more CS chains grafted onto the GO sheets to form stickier GO/CS suspension (Figure S3), which is split up into millimeter-sized spheres by the electrospraying process.


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Figure 1. The preparation scheme of GCAMs. The content of CS has a great influence on the size, structural stability, and morphology of the microspheres (Figure 2, S4). In the absence of CS, no crosslinking in GCAM0 occurs, and the GO sheets form honeycomb-like microspheres (Figure S4a, S4b) via π-π stacking and hydrogen bonding. The random interaction of GO sheets results in fragile porous beehive walls and irregular microspheres with poor structural stability. When the crosslinking is promoted by the addition of CS, the GCAMs have robust intact hive walls and a regular spherical morphology (Figure 2a, b, S4c-h). As shown in Figure 2b for GCAM10, GO still maintains a honeycomb structure similar to that of GCAM0 (Figure S4b), but an additional cobweb structure is formed in the GO hives from the free CS molecular chains in the water, as illustrated in Figure 2e. This hierarchical honeycomb-cobweb structure is beneficial since it increases the surface area of the


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aerogel microspheres, thereby increasing their adsorption capacity. On the other hand, a high CS content leads to millimeter-sized composite spheres and large honeycomb-like pores (Figure

S4g, S4h) that elongate the diffusion path and hinder rapid adsorption.

Figure 2. SEM of (a, b) GCAM10, (c, d) the cross-section of GCAM10. (e) Schematic of GCAM10 with honeycomb-cobweb and radially oriented microchannel structures. The inner microstructures of GCAMs (Figure 2c and 2d) are observed by halving a GCAM10 ice microsphere before freeze-drying. It is observed a dandelion-like radially oriented microchannel structure in the GCAM, as illustrated in Figure 2e, which is ascribed to the uniform cooling of the GO/CS ice microsphere, in which the ice crystals grow in the radial direction and facilitate the orientations of GO/CS blocks.22 The size of the microchannel is about 10 µm and almost unchanged with increasing the CS/GO ratios. The radially oriented microchannel structure is retained by slowly removing the ice crystals during the lyophilization step. Such a microchannel structure shortens the diffusion path of adsorbates and facilitates the adsorption of contaminants with different surface charges in a short time.31


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Figure 3. (a) XRD patterns and (b) Raman spectra of GCAMs with different CS contents; XPS spectra of (c) GCAM0 and (d) GCAM10. To evaluate further the effect of CS chains on GO sheets, GCAMs with different CS contents are analyzed by XRD, XPS, FT-IR, and Raman spectroscopy. As seen in Figure 3a. XRD pattern of GCAM0 has a peak at 11.2o, which is in line with that of GO.32 With increasing the CS content, the peak shifts from 11.2o to 6.8o. Calculated by the Bragg equation, the d-spacing increases from 3.97 Å for GCAM0 to 6.48 Å for GCAM50 (Table S1), proving that the CS chains are indeed inserted into GO sheets to form robust hive walls,33 which is in accordance with the microstructure in TEM (Figure S2) and SEM (Figure 2 and S3) images. In addition, the


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ID/IG ratio from Raman spectra (Figure 3b and Table S2) increases with the CS contents. Since the ID/IG ratio is a parameter for the average size of the sp2 domains, the slight rise of the ID/IG ratio can be attributed to the increase in the number of small sp2 in-plane domains.34,35 GCAM10 exhibits an amide bond at 400.4 eV and a C-N bond at 286.2 eV (Figure 3d),33 which cannot be observed in the spectrum of GCAM0 (Figure 3c). The further enhancement of the amide peak and C-N bond peak, and the further reduction of the C-O bond in GCAM50 (Figure S5a) prove that more CS chains are grafted onto the GO sheets.33 The FT-IR spectra of GO, CS and GCAM (Figure S5b) show that the characteristic peak of CS at 1546 cm-1, corresponding to the bending vibration of N-H, red-shifts to 1527 cm-1, while the peak of GO at 1226 cm-1 attributed to the CO stretch vibration of epoxide group36 disappears in the spectrum of GCAMs, indicating the reaction between the amino group of CS and the epoxide group of GO occurs. Additionally, a cross-linker of glutaraldehyde is also used to enhance the structural stability of the microspheres. 3.2. Adsorption Behavior of GCAMs for Metal Ions To study the adsorption performances of GCAMs towards heavy metal ions, three types of ions are chosen, including Pb(II)37 (pH = 5.0), Cu(II)20 (pH = 4.9), and Cr(VI)38 (pH = 7.0). The adsorption kinetics are used to analyze the adsorption rates of the metal ions on GCAMs. Figure

4a is the adsorption kinetic curves of metal ions in GCAM10. It is seen that qt rises rapidly at the initial 5 min, and increases slowly to equilibrium. Pb(II) and Cu(II) reach the adsorption equilibrium in 40 min, whereas Cr(VI) is faster, because the adsorption capacities of Pb(II) (747.5 mg g-1) and Cu(II) (457.5 mg g-1) are higher than that of Cr(VI) (292.8 mg g-1).25


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Figure 4. (a) Adsorption kinetics of Pb(II), Cu(II) and Cr(VI) on GCAM10 (C0 = 500 mg L-1); (b) Fitting results of Pb(II) on GCAM10 for kinetic models; (c) Adsorption isotherms of Pb(II), Cu(II) and Cr(VI) on GCAM10 (t = 6 h); (d) Adsorption of Pb(II), Cu(II) and Cr(VI) on GCAMs with different CS contents (C0 = 500 mg L-1, t = 6 h). The adsorption kinetic curves are quantitatively analyzed using the pseudo-first-order and pseudo-second-order models (Figure 4b and S6a, b). The pseudo-first-order model assumes the internal diffusion process is the rate-controlling step, while the pseudo-second-order kinetic model considers that the chemisorption of the adsorbate molecules onto the active adsorption sites is the rate-controlling step. These two models are described by the following equations:


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(2)

(3) where k1 is the pseudo-first-order kinetic rate constant (min-1), k2 is the pseudo-second-order kinetic rate constant (g mg-1 min-1), and qe and qt are the adsorption capacities (mg g-1) at equilibrium and at t (min), respectively. In the adsorption experiment, magnetic stirring is used to eliminate the effect of external diffusion on the adsorption rate. Table 1 displays the fitting results of the adsorption kinetic curves. Comparing two correlation coefficients (R2), the pseudosecond-order kinetic model (R2 > 0.99) has a better correlation than the pseudo-first-order kinetic model (R2 < 0.99) for adsorption of the three types of metal ions with GCAM10. It is clear that the adsorption process of Pb(II) and Cr(VI) is mainly ruled by the chemisorption between the adsorbate ions and active sites of GCAMs, and the radially oriented microchannel structures of the aerogel microspheres greatly accelerate the internal diffusion rate. While for Cu(II), the R2 values of the pseudo-first-order kinetic model and the pseudo-second-order kinetic model are quite close, indicating that the adsorption of Cu(II) is controlled by both internal diffusion and chemisorption.

Table 1. Fitting results of contaminants on GCAM10 for kinetic models.

Adsorbate Pb(II) Cu(II) Cr(VI) MB Rh B MO Eosin Y

Pseudo-first-order qe k1 R2 -1 (mg g ) (min-1) 585.4 0.16 0.9752 314.2 0.10 0.9864 202.0 0.26 0.9617 597.6 0.22 0.9434 169.6 0.21 0.9902 136.3 0.21 0.9870 97.5 0.26 0.9499

Pseudo-second-order qe k2 -1 -1 (mg g ) (g mg min-1) 617.3 0.0004 326.8 0.0004 280.9 0.0034 271.0 0.0023 212.3 0.0023 110.3 0.0023 153.8 0.0072


R2 0.9963 0.9955 0.9927 0.9918 0.9989 0.9929 0.9974

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To quantitatively analyze the adsorption behaviors and mechanisms of the GCAM10 adsorbent, the adsorption data of metal ions are fitted by Langmuir (solid line) and Freundlich (dashed line) isotherm models (Figure 4c). The Langmuir and Freundlich isotherm model equations are:

Ce Ce 1 = + qe qm KL qm

(4)

1 lnCe n

(5)

lnqe = lnKf +

where Ce is the concentration (mg L-1) of the solution when the adsorption reaches equilibrium; qe is the equilibrium adsorption capacity (mg g-1); qm is the maximum adsorption capacity (mg g1

); KL is the Langmuir isothermal constant (L mg-1); KF is the Freundlich isothermal constant (mg

g-1) (L mg)1/n; and n is the factor relating to the adsorption intensity. The Langmuir isotherm model supposes adsorption occurs on a uniform surface with homogeneous active sites through monolayer adsorption. Hence, it can reach saturation. Whereas, the Freundlich isotherm adsorption model assumes that multilayer adsorption occurs on heterogeneous surfaces. The fitting results of the adsorption isotherm models are displayed in Table S2. All R2 of the Freundlich isotherm model are greater than 0.99, indicating that the Freundlich model has a better relativity than Langmuir model for experimental data. This suggests that the adsorption behavior occurs in the form of multilayers on the heterogeneous surfaces of the aerogel microspheres. Correlation to the Freundlich model supports the existence of electrostatic interactions between metal ions and functional groups of both GO honeycomb and CS cobweb, which will be discussed in detail in the adsorption mechanism of GCAM section below. It is noted that the two models for Pb(II) have close R2 values. To further analyze the process of Pb(II) adsorption, the adsorption isotherm at the low concentration is fitted for Langmuir model


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(Figure S6e). When the concentration of Pb(II) is low, some active sites with strong electrostatic interactions dominate the adsorption, and the adsorption tends to follow the Langmuir model with the assumption of monolayer adsorption. With increasing the concentration, the adsorption tends to be multilayer adsorption on heterogeneous surfaces. The influence of CS contents on the equilibrium adsorption capacities (qe) is investigated to further explore the adsorption performances of GCAMs (Figure 4d). The adsorption capacities of the three types of metal ions do not change significantly with increasing the CS contents. The oxygen-containing groups on GO have electrostatic interactions with Pb(II) and Cu(II). Simultaneously, abundant carboxyl groups of GO sheet present strong attractions to Cr(VI) by complexation and coordination.39 Since the chemical bonding between CS and GO is confirmed by XPS results (Figure 3d), the carboxylic adsorption sites of GO are reduced by the introduced CS molecules. The amino groups on CS molecular chains show a chelating effect with Pb(II) and Cu(II),40,41 and an electrostatic interaction with Cr(VI), which compensates for the loss of adsorption capacities. 3.3. Adsorption Behavior of GCAMs for Dyes and Phenol In addition to the effective adsorption of heavy metal ions, GCAMs are also efficient in adsorbing dyes, another type of ubiquitous contaminant,4,42 including cationic dyes (MB and Rh B) and anionic dyes (MO and Eosin Y). Figure 5a displays the adsorption kinetic curves of MB (pH = 6), Rh B (pH = 6.5), MO (pH = 7.5), and Eosin Y (pH = 7) on GCAM10. The adsorption capacities of the dyes increase with the contact time. All four dyes reach their adsorption equilibriums within 20 min. The equilibrium adsorption capacities of MB, Rh B, MO, and Eosin Y are 584.6, 492.8, 189.4, and 124.8 mg g-1, respectively, and adsorption of dyes is faster than adsorption of heavy metal ions. As analysis previously, metal ions are adsorbed on the active


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sites of GCAMs by chemisorption. The large hydration radii of metal ions cause the slow rate of chemical adsorption. The adsorption kinetic curves of MB, Rh B, MO, and Eosin Y in GCAM10 are also fitted to adsorption kinetic models (Figure 5b, S6c,d), and the fitting results are also listed in Table 1. Compared to the pseudo-first-order kinetic model, the pseudo-second-order kinetic model has a better correlation, indicating that the chemisorption of dye molecules on the active sites controls adsorption rather than internal diffusion. While the two models for MO have close R2 values, suggesting that the adsorption of MO is also controlled by both internal diffusion and chemisorption.

Figure 5. (a) Adsorption kinetics of MB, Rh B, MO, and Eosin Y on GCAM10 (C0 = 500 mg L1

); (b) Fitting results of MB on GCAM10 for kinetic models; (c) Adsorption isotherms of dyes on

GCAM10 (t = 6 h); (d) Adsorption of MB, Rh B, MO, and Eosin Y on GCAMs with different CS contents (C0 = 500 mg L-1, t = 6 h).


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Figure 5c depicts the adsorption isotherms of dyes on GCAM10. The adsorption data are fitted by Langmuir (solid line) and Freundlich (dashed line) models, and the fitting results are present in Table S2. For the cationic dyes of MB and Rh B, the results fit the Langmuir model. Since the adsorption of positively charged dyes mainly occurs on the oxygen-containing groups with negative charge of GO,43 the adsorption of dye molecules on the aerogel microspheres is a monolayer because the active sites are uniform, which is consistent with the Langmuir model. Also in agreement with the Langmuir model, we found that the maximum adsorption capacities of GCAM10 extrapolated from the line fitted to the Langmuir model in Figure 5c are 598.51 mg g-1 for MB and 507.39 mg g-1 for Rh B, which are consistent with the experimental data of 584.6 and 492.8 mg g-1, respectively. Differently, MO and Eosin Y fit the Freundlich model better. The π-conjugation between the dyes and non-oxidized regions of the GO sheets,44 and the electrostatic interaction between the anionic dyes and the positively charged amino of the CS chains are combined to produce heterogeneous adsorption sites.45 However, the two models for MO have close R2 values. The adsorption isotherm at the low concentration is fitted for Langmuir model (Figure S6f). The adsorption tends to fit the Langmuir model with the assumption of monolayer adsorption at the low concentration, while the adsorption tends to fit Freundlich model at the high concentration. In addition to the kinetics and adsorption isotherms for dyes, the adsorption capacities of different dyes on GCAMs are measured to facilitate the understanding of the adsorption mechanism (Figure 5d). The adsorption capacities of the cationic dyes decrease significantly when CS content increases. The qe of MB decreases from 636.0 to 183.2 mg g-1 and that of Rh B decreases from 583.1 to 225.6 mg g-1. While the adsorption capacities of the anionic dyes increase. The qe value increases from 111.9 to 554.7 mg g-1 (MO) and from 103.8 to 419.8 mg g-


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(Eosin Y). These results further prove that the adsorption of cationic dyes occurs on GO,

whereas that of anionic dyes occurs on CS. Some of the adsorption active sites are lost during the formation of the GO/CS composite blocks due to the stacking of the GO sheets or the charge neutralization of the CS.25 The adsorption capacities of cationic dyes on GCAMs with high CS contents are lower than those of neat GO aerogel microspheres. Phenol is a harmful pollutant present in many kinds of industrial wastewater.46 Figure S7 investigates the adsorption performance of phenol (pH = 6.0) on GCAMs. By π-conjugation, the partially non-oxidized regions of the GO sheets adsorb the aromatic phenol,47 thereby operating as active sites for adsorption. However, phenol is negatively charged in its aqueous solution, and the strong electrostatic repulsion of the negatively charged groups on GO hinder the adsorption. The adsorption capacity of phenol increases by grafting CS chains on the GO nanosheets. Therefore, the adsorption capacity of phenol increases from 9.5 mg g-1 for GCAM0 to 73.1 mg g1

for GCAM50.

3.4. Adsorption performances of GCAM10 towards mixed contaminates GCAMs can simultaneously adsorb a variety of dyes or metal ions, because of their plenty of positive and negative adsorption active sites. Figure S8 shows the adsorption performances of GCAM10 for mixed solutions containing MB-MO or Pb(II)-Cu(II). For the MB-MO solution, the adsorption rate of MB is faster than that of MO, due to the more negative active sites of MB. For the Pb(II)-Cu(II) solution, although the two metal ions are competing for adsorption, Cu(II) prevails because of its small hydration radius. 3.5. Adsorption Mechanisms of GCAMs The adsorption performances of GCAMs are compared to a GO/CS monolith (Figure S9a) fabricated by a freeze-drying method.20 The GO/CS monolith shows an irregular pore structure


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(Figure S9b) rather than the hierarchical honeycomb-cobweb structure of the GCAMs. Figure

S9a shows the adsorption kinetic curve of the monolith for MB, taking 75 h to reach its adsorption equilibrium, much longer than that of GCAMs. The acceleration of the adsorption rate is a result of the size effect and the radially oriented microchannels.48 The micro-sized spheres can be uniformly dispersed in a solution of contaminants by mild stirring. Compared to the monolith, the radially oriented microchannels inside the microspheres greatly reduce the diffusion resistance of the contaminants, and the micro-sized spheres have large apparent surface areas in contact with the contaminant solutions, reducing the internal diffusion path and thereby speeding up the adsorption rate. As reported above, the as-prepared GCAMs exhibit good adsorption performances for many pollutants, including cationic dyes, anionic dyes, heavy metal ions, and phenol. The oxygencontaining groups on GO sheets serve as negatively charged active adsorption sites, while the non-oxidized conjugated regions of the GO sheets perform as π-conjugation active adsorption sites (Figure 6). The amino groups on the CS chains act as positively charged active adsorption sites. The adsorption of cationic dyes on GCAMs is attributed to the electrostatic interaction between the dye and the oxygen-containing groups of GO sheets. Simultaneously, the πconjugate effect between the non-oxidized conjugated region and the benzene ring of the dye molecules may also contribute to the adsorption capacity.49 However, the adsorption of anionic dyes and phenol is mainly owing to the electrostatic interaction of the amino groups on CS chains. Yet the π-conjugation between anionic dyes and GO is hampered by the strong electrostatic repulsion of the oxygen-containing groups.


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Figure 6. A schematic illustrating the adsorption mechanisms of GCAMs for various pollutants. For the metal ions, the electrostatic interactions of the oxygen-containing groups on GO and the chelation of the amino groups of CS result in a high adsorption capacity for Pb(II) and Cu(II) on GCAMs. The adsorption of Cr(VI) is mainly due to the complexation and coordination of the carboxyl groups of GO sheets and the electrostatic attraction of the amino groups of the CS. Therefore, the combination of electrostatic interaction, π-conjugation, chelation, complexation, and coordination are the adsorption mechanisms that make the GCAMs a broad-spectrum adsorbent for water purification. GCAMs exhibit an excellent adsorption performance in both the maximum adsorption capacity and the adsorption rate (Figure 7 and Table S3).20,25,37,39,50-62 The significantly improved performances of GCAM10 over the reported graphene-based adsorbents are attributed to the radially oriented microchannels and the GO honeycomb-CS cobweb structures of the aerogel microspheres.


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Figure 7. Comparison of the adsorption performances of GCAM10 for MB with other graphenebased adsorbents. The GCAM adsorbents exhibit a satisfactory recyclability. The adsorption equilibrium times of MB and Pb(II) are not extended after 6 adsorption-desorption cycles (Figure 8a, c), and the adsorption capacities of MB and Pb(II) remain above 85% after the 6 cycles (Figure 8b, d). The slight decrease in adsorption capacities were caused by the partial collapse of the microchannel structure of the aerogel microspheres and the loss of CS, evidenced by the SEM images of the GCAM10 after the 6 cycles (Figure S10).


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Figure 8. Adsorption kinetics of (a) MB and (c) Pb(II) on GCAM10 in six cycles (C0 = 500 mg L-1); Cyclic utilizations of (b) MB, Rh B, MO, and Eosin Y, and (d) Pb(II), Cu(II), and Cr(VI) on GCAM10 (C0 = 500 mg L-1, t = 6 h).

4. CONCLUSION Micron-sized GCAMs were prepared by electrospraying-freeze-casting. On the surface of the GCAMs, the robust GO/CS blocks are self-assembled into honeycomb-like microspheres by electrostatic interaction and chemical bonding, with cobweb-like CS growing in the hive. Inside the GCAMs, the radially oriented microchannel structure is constructed owing to the uniform cooling of GO/CS ice microspheres from the surface to the interior. The strong electrostatic


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interaction and chemical bonding between GO and CS confer high structural stability and broadspectrum adsorption to aerogel microspheres. The hierarchical honeycomb-cobweb structure with radially oriented microchannels allows GCAMs to shorten the diffusion path and facilitate the rapid adsorption, as well as maintain a high adsorption capacity. The GCAMs possess superior adsorption capacities for heavy metal ions of Pb(II), Cu(II), and Cr(VI), cationic dyes of MB and Rh B, anionic dyes of MO and Eosin Y, and phenol due to the adsorption mechanisms of electrostatic interaction, π-conjugation, chelation, complexation, and coordination. The adsorption capacities of the prepared aerogel microspheres are up to 747.5 and 584.6 mg g-1 for Pb(II) and MB, respectively. Most importantly, the GCAMs can reach adsorption equilibrium within 20 min for most adsorbates, which is much more rapid than the GO/CS monolith and most of the previously reported graphene- or GO-based adsorbents. Interestingly, the GCAMs can maintain excellent adsorption capacity for 6 cycles of adsorption-desorption. Hence, the attractive performance of GCAMs makes it a promising candidate for efficient water treatment applications.

ASSOCIATED CONTENT Supporting Information Zeta potential curves of suspensions; size distributions of GCAMs; TEM images of GO/CS composite blocks; SEM images, d-spacing distances, FT-IR spectra, and ID/IG values of GCAMs; XPS spectrum of GCAM50; fitting results using isotherm and kinetic models; adsorption of phenol on GCAMs; UV-vis spectra of GCAM10; adsorptions of Pb(II) and Cu(II); adsorption kinetics of MB; SEM images of GO/CS composite monolith, and GCAM10 after 6 cycles; and comparison of the adsorption performances.


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AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (D.-Z. Yang); [email protected] (Z.-Z. Yu)

Author Contributions ⊥

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (51273015, 51533001, 51521062) is gratefully acknowledged.

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