Superlight Adsorbent Sponges Based on Graphene Oxide

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Applications of Polymer, Composite, and Coating Materials

Superlight Adsorbent Sponges Based on Graphene Oxide Crosslinked with Poly(Vinyl Alcohol) for Continuous Flow Adsorption Xianfeng Li, Tao Liu, Daohui Wang, Qing Li, Zhen Liu, Nana Li, Yufeng Zhang, Changfa Xiao, and Xianshe Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06802 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Applied Materials & Interfaces

Superlight Adsorbent Sponges Based on Graphene Oxide Crosslinked with Poly(Vinyl Alcohol) for Continuous Flow Adsorption Xianfeng Lia*, Tao Wanga, Daohui Wanga, Qing Lia, Zhen Liua, Nana Lia, Yufeng Zhanga, Changfa Xiaoa, Xianshe Fenga,b* a

State Key Laboratory of Separation Membranes and Membrane Processes, School of Materials Science and

Engineering, Tianjin Polytechnic University, Tianjin, 300387, China b

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

Email: [email protected] (X.F. Li), [email protected] (X. Feng). Tel: +86-22-83955055, Fax: +86-22-83955055.

ABSTRACT In this study, superlight adsorbent sponges (bulk density 0.016–0.049 g.cm-3) were developed based on graphene oxide (GO) crosslinked with poly(vinyl alcohol) (PVA). The interlayer spacing of the GO nanosheets was increased by the insertion of PVA, and good mechanical integrity was attained by the crosslinked structure. They showed excellent continuous flow adsorption capacity (CFAC) when methylene blue was used as a model contaminant; a water flux of 396 L.m-2.h-1 through a 2-cm thick adsorbent sponge was achieved at a hydraulic head of only 10 cm water, with an almost complete retention of methylene blue. The corresponded to a water permeability of 4.0×105 L.m-2.h-1.MPa-1, which was several orders of magnitudes higher than GO-based membranes for similar applications reported in the literature. The GO nanosheets were completely immobilized in the sponge by crosslinking with PVA, and thus there was no GO nanoparticle leaching or flushing out into the treated permeate water, which was another advantage over direct use of GO powders in water treatment. Because of the high water permeability and CFAC, the crosslinked GO/PVA sponges have a great potential for wastewater treatment. KEYWORDS: adsorbent sponge, graphene oxide (GO), polyvinyl alcohol (PVA), adsorption, methylene blue

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1. INTRODUCTION Due to pseudo two-dimensional structures, graphene-based nanomaterials exhibit excellent properties for filtration and separation applications1–8 along with their unique performance in electronics, energy storage, catalysis, and sensing.9–13 More recently, graphene oxide (GO) membranes have attracted considerable attention for potential uses in water and wastewater treatment because of their extremely high permeability to water resulting from the nanospaces between GO nanosheets.14–21 This is of particular interest in view of the serious scarcity of water resources globally. To better serve the application, efforts have been made to improve and optimize the microstructures and properties of GO based membranes14–21 including, for example, reducing the number of stacked GO nanolayers and increasing the nanopores on the graphene nanosheets, thereby lowering the mass transfer resistance for water permeation. However, the ultrathin GO layer is vulnerable to mechanical stress under transmembrane pressure differential,16 which compromises the stability of the membrane during the separation process.14–21 The stability of the GO membranes can be enhanced by crosslinking with multivalent cations16 and proper chemical crosslinking agents. Thus, the membrane preparation often involves sophisticated procedures in order to maintain a high flux and rejection. Especially, if the membrane is defective or has a broad pore size distribution, the solute rejection will be limited and a high flux will become meaningless. In our preliminary work with GO membranes for water filtration, the membrane rejection to various salt ions were rather low presumably due to membrane defects caused during the assembly of GO membranes. It is, however, surprising to notice that these GO membranes showed extremely high rejection to methylene blue (MB) for a short period of time during the initial stage of filtration 2


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experiments, while none of the ionic solutes tested (NaCl, MgCl2, Na2SO4 and MgSO4) were rejected. This suggests that the high rejection of the membrane to MB did not result from size sieving effects, but rather adsorption appears to be the dominating mechanism. Additional studies showed that the high rejection to MB mainly originated from the adsorption for MB on the membrane.22–33 The high adsorption rejection with only such a ultrathin layer of GO in the membrane further indicated that the GO layer has excellent adsorption capacity, thanks to its large specific surface area and the multitude of negatively charged oxygenic groups (i.e., carboxyl, enol and ether groups) on the basal surfaces.34 The total ion exchange capacity based on the quantity of acidic groups on the GO surface is 3-4 times higher than the ion exchange capacity of montmorillonite.34 It is hence no surprising that GO has been increasingly exploited as an adsorbent material35–60 in recent years for a variety of the adsorbate substances including radioelement ions35-46 (e.g., uranium and europium) and heavy metal ions (e.g., Cd,47,48 Co,47,48 Cr,49,50 Pb,51–54 Cu,51,55 Sb,56 Pd,57 Zn58). Due to the negatively charged oxygenic groups that causes electrostatic repulsion between the nanosheets, the raw GO is easy to disperse in water16 but difficult to remove, which represents a potential new water pollution issue. In addition, the continuous filtration/adsorption separation process requires a high porosity for easy flow and a good adsorption capacity under continuous flow. The continuous flow adsorption capacity (CFAC) is primarily determined by the adsorption rate and adsorption capacity. It is thus meaningful to develop GO-based filter materials that can be used in continuous flow adsorption in an operating mode similar to membrane filtration for adsorptive separation of different contaminates. The formation of such filter adsorbent is expected to be less challenging than GO membranes because the presence of micropores, which are otherwise defects in membranes, can now be tolerated to a much greater extent in the adsorbent. 3



In this work, we report on a simple method of preparing superlight microporous adsorbent based on graphene oxide (GO) crosslinked with poly(vinyl alcohol) (PVA) for continuous flow adsorption. The GO nanosheets were easily crosslinked with PVA, and the interlayer space between GO nanosheets was increased due to the crosslinking between GO and PVA. When tested with removal of MB from water, the adsorbent showed excellent adsorption capacity under continuous flow. In addition, the sorbent exhibited very low resistance to water flow. The hydraulic head of the feed solution alone was found to be adequate for continuous flow filtration/adsorption, and no additional pressure was required, making the process very energy-efficient.

2. EXPERIMENTAL SECTION 2.1. Preparation of crosslinked graphene oxide/poly(vinyl alcohol) adsorbent GO was produced using the improved Hummer’s method61 from graphite flakes supplied by Jinrilai Graphite Co. Ltd and was observed by transmission electron microscope (TEM, H-7650, Hitachi). PVA (Mw 95,000-110,000) was provided by Anhui Wanwei Group Co. Ltd. A GO slurry containing 1.2 wt% of GO was prepared by dispersing the GO flakes in distilled water, and PVA was dissolved in distilled water as a homogeneous solution at a concentration of 10 wt%. The two were mixed at predetermined proportions to achieve specific GO to PVA mass ratios, and the resulting GO/PVA slurry was further dispersed by agitation and ultrasonication at 60°C. Then, a predetermined amount of water-soluble starch (measuring 5 wt% of the GO/PVA slurry) or sodium dodecyl sulfate (measuring 1 wt% of the GO/PVA slurry, Tianjin Guangfu Fine Chemical Research), plus 3 mL of formaldehyde (CH2O, 40 wt%) and 7 mL of sulfuric acid (H2SO4, 50 wt%) were added to the slurry sequentially. The starch and sodium dodecyl 4


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sulfate were supplied by Tianjin Guangfu Fine Chemical Research Institute and they were used as pore forming agents, whereas CH2O and H2SO4 supplied by Tianjin Fengchuan Chemical Reagent Co., Ltd were used as intermediary agents. The mixed GO/PVA slurry was further stirred for 20 min at room temperature before being poured into a mold, which was then placed in an oven at 60 °C for 10 h during which period the GO/PVA slurry was solidified by crosslinking. The solidified sample was then rinsed thoroughly with distilled water to remove the pore-formers, followed by freeze-drying to yield a porous GO/PVA sponge. To study the relationship between the PVA content in the slurry and the properties of the GO/PVA sponges produced, different GO/PVA mass ratios were used in the experiments, and for convenience of discussion the crosslinked GO/PVA sponge samples were designated as SS-x (where x is the GO/PVA mass ratio; SS-0 means crosslinked PVA only). The crosslinked PVA (cPVA) was obtained via a same crosslinking condition with the crosslinked GO/PVA sponge under the absence of GO. The crosslinking mechanism could be understood based on an acetalation reaction, in which CH2O was used as an intermediary agent62–67 or an esterification reaction between GO and PVA.64 Besides the reported esterification reaction,64 another possible crosslinking mode of the GO/PVA adsorbent is shown in Scheme 1, where GO with -OH might be crosslinked with the -OH in PVA via the intermediary agent of CH2O.

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Scheme 1. Illustration of crosslinking mechanisms in GO/PVA sponge.

2.2. Characterization of GO/PVA sponges. X-ray diffractions (XRDs) were recorded on a diffractometer (Bruker D8 general area detector diffraction system) with Cu Kα radiation (λ=1.54 Å) at a generator voltage of 40 kV and a generator current of 40 mA. The binding energy of C1s was characterized by x-ray photoelectron spectroscopy (XPS) (Thermo Fisher, K-alpha), and the survey and detailed spectra were obtained on a probing spot size of 400 µm. Fourier transform infrared (FTIR) spectra were acquired using a FTIR spectrometer (Bruker Tensor 37), and Raman spectra were determined using a Horiba XploRA PLUS spectrometer. Thermogravimetry (TG) was performed on a Netzsch STA 409 PC thermal analyzer under a nitrogen flow over a temperature range of room temperature to 800°C at a heating rate of 10°C/min. Field emission scanning electron microscopy (FESEM) images were acquired using a Hitachi S-4800 cold-cathode field-emission scanning electron microscope, and the specimens were sputter coated with gold before SEM analysis. 6


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2.3. Capturing of MB from Waste Water. The apparatus used for continuous filtration/adsorption separation of MB from water is shown in Figure 1. The GO/PVA sponge sample (in a cylindrical shape, 26×20 mm) was lightly squeezed into a 26 mm-diameter plexiglass tube, and the MB solution (pH = 6.45) with an initial concentration of 30 mg/L was poured into the tube. The liquid level of the solution in the tube was kept at ca. 10 cm above the GO/PVA sponge, and continuous flow filtration/adsorption was accomplished under the hydraulic head.

Figure 1. Experimental setup for continuous flow through filtration/adsorption under hydraulic head.

For comparisons, cPVA, active carbon (AC) (Chengde LiJing Activated Carbon Manufacturing Co., Ltd, grade YH-19, particle size 10-24 mesh), and GO-loaded melamine sponge (GO/MS) were also used as adsorption materials for MB capture from water, in the tests, the volume of the adsorbent was the same as that used in GO/PVA adsorption tests. The preparation procedure and the structures of the GO/MS sponges can be found in the supporting information (SI).

2.4. Batch Adsorption Experiments. The adsorption capacity of GO/PVA sponge for MB was conducted in a batch system. Briefly, 20 7



mg GO/PVA sponge was immersed into 10 mL of MB solution at a known concentration under stirring until an adsorption equilibrium was reached. Then, the GO/PVA sponge was removed from the solution using tweezers, and the concentration of MB remaining in the solution was determined using a UV-1200 spectrophotometer (Shanghai Mapada Instruments Co., Ltd) at a wavelength of 664 nm. The equilibrium adsorption uptake (qe) of MB on GO/PVA and the MB removal efficiency (E) can be calculated using the following equations: (‫ܥ‬଴ − ‫ܥ‬ୣ )ܸ ݉ ‫ܥ‬଴ − ‫ܥ‬ୣ ‫=ܧ‬ × 100% ‫ܥ‬଴ ‫ݍ‬ୣ =

where qe is the quantity of adsorbed MB on the GO/PVA adsorbent at equilibrium (mg.g-1), C0 and Ce are the initial and equilibrium concentrations of MB in the solution (mg.L-1), respectively, V is the volume of the MB solution (L), and m is the mass of the GO/PVA sample (g). All the adsorption tests were carried out in triplicates, and the values reported were the average measurements.

3. RESULTS AND DISCUSSION 3.1. Adsorbent sponges and MB adsorption in flow through mode The resulting adsorbent sponge (crosslinked GO/PVA) was found to be superlight with a bulk density of 0.016–0.049 g.cm-3, which is shown in Figure 2 (a). By a comparison, the TEM images of the pristine GO are also shown in Figure 2 (b) and (c). The GO sheets could be seen obviously, which originated from the exfoliation of oxidized graphite. The mechanical integrity of adsorbent sponges was measured by a simple rolling method shown in SI. It can be seen from Figure S1 that, by contrast with the dispersibility of pure GO, the mechanical integrity of the sponges was clearly improved by the 8


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crosslinking of GO with PVA.

Figure 2. (a) Optical picture of a crosslinked GO/PVA sponge sample (SS-0.25); (b) and (c) the TEM images of pristine GO (at different magnifications).

Figure 1 shows the experimental setup for the treatment of simulated MB-containing waste water at a lab scale. The feed solution was poured into a plexiglass tube and a 10 cm hydraulic head above the GO/PVA sponge was maintained to induce continuous flow. Figure 3 shows the results of MB removal by these spongy materials. As one may expect, MB broke through the cPVA rather quickly because MB could hardly be adsorbed by cPVA. The permeate from AC or GO/MS was shown to be light initially but it became darker quickly. This indicates that both AC and GO/MS has a certain capacity of adsorbing MB, but their CFAC was not sufficiently high due to slow adsorption rate or quick saturation of adsorption. On the other hand, the GO/PVA sponge (SS-0.25) showed outstanding performance for adsorbing MB, and the permeate water was found to exhibit the same absorbance as the distilled water for at least 25 min of flow through filtration/adsorption, indicating complete capture of MB by the GO/PVA adsorbent in the continuous flow-through mode. It may be mentioned that the excellent adsorption performance was sustained for a much longer period of time, and this is remarkable in view that MB in the solution was 9



adsorbed completely through a single pass of the 2 cm-thick porous GO/PVA sponge layer. At a 10 cm hydraulic head, a water flux of 396 L.m-2.h-1 was attained, which corresponded to a water permeability of 4.0×105 L.m-2.h-1.MPa-1. As a comparison, Mi et al.2 reported a GO membrane, which was assembled via layer-by-layer deposition of GO nanosheets and cross-linked with 1,3,5-benzenetricarbonyl trichloride, for MB separation from water, and a MB rejection of 44–46% and water permeability of 80–276 L.m-2.h-1.MPa-1 were obtained at 0.34 MPa. The MB rejection was believed to originate mainly from the size sieving effects the GO membranes. The adsorption capacity of the GO/PVA adsorbent, determined from the batch adsorption experiment, was 135.4 mg g-1 and 182.6 mg g-1 for SS-0.25 and SS-0.50, respectively, which is lower than the adsorption capacity (240.6 mg g-1) of dispersed GO.30 This is understandable because PVA was used to crosslink the GO nanosheets in the adsorbent and thus the apparent adsorption capacity of the adsorbent in terms of sorption uptake per mass adsorbent was reduced. Nonetheless, the crosslinking with PVA resulted in a highly microporous 3-D sponge structure in the adsorbent, which was responsible for the good mechanical property of the adsorbent and the high adsorption rate. It should be mentioned that in the adsorption experiments with AC and GO/MS, filter papers were added to the top and and bottom of the packed AC and GO/MS adsorbent in order to slow down the water flow rate, otherwise the water flow would be so fast that MB breakthrough would occur almost instantaneously due to the large pores present in the adsorbent column. The influence of GO/PVA ratio in GO/PVA adsorbent on the breakthrough curve is shown in Figure 4; the diameter and height of the GO/PVA adsorbent sponges used in all the experiments were 26 and 20 mm, respectively. It can be seen that MB began to break through at an elution volume of 950 mL when SS-0.50 adsorbent was used, and the adsorbent was fully saturated with MB after 2450 mL of the feed 10


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water eluted through the adsorbent. As expected, a reduction in the GO/PVA amount (e.g., adsorbent SS-0.25 and SS-0.16) led to a quicker breakthrough in MB.

Figure 3. Eluted permeate water from GO/PVA sponge (a), AC (b) and cPVA (c). For comparison, the feed solution is shown in (d) (MB concentration 30 mg/L, pH 6.45, temperature 20°C).

Figure 4. Breakthrough curves for MB adsorption by the GO/PVA adsorbent sponges. (a) SS-0.16; (b) SS-0.25; (c) SS-0.50. Feed MB concentration 30 mg/L, pH 6.45, temperature 20°C. The reasons for good adsorption of GO had been discussed by many researchers.35–60 In this study, a primary reason should be the multitude of negatively charged oxygenic groups on the basal surfaces of 11



GO.34 The total ion exchange capacity originating from acidic groups on the basal surface is 3–4 times higher than the ion exchange capacity of montmorillonite.34 MB molecules are positively charged in nature, hence, the electrostatic interaction exists between the GO and the MB molecules, which led to the high adsorption capacity and rate of the sponges.31 In addition, the contribution of π–π stacking interaction to the adsorption capacity could not be excluded.31 Importantly, PVA with a large number of side hydroxyls on the molecular chain presents a good adsorption for dye and heavy metal,65–67 which should be a contributor of the high adsorption capacity although the adsorption capacity of GO could be influenced by the crosslinking with PVA. More importantly, the crosslinking of GO with a large amount of PVA led to an irregular stacking of GO sheets, which could be identified by the disappearing of the strong diffraction peak of the pristine GO (shown in the following XRD section). This should be an important reason to the high adsorption rate. 3.2. XRD. To better understand the excellent continuous flow adsorption capacity of the GO/PVA sponges produced using water soluble starch as a pore-forming agent, the GO/PVA adsorbent was characterized with XRD; for comparison purposes the cPVA and pristine GO nanosheets were characterized as well. As shown in Figure 5, a new wide and weak diffraction peak appeared at 5.9° (interlayer space 1.5 nm, calculated from Bragg equation) for all the GO/PVA adsorbent samples, which is a space of a dozen C-C or C-O bonds and the GO nanosheets displayed a strong diffraction peak at 10.6° with an interlayer space of 0.83 nm corresponded to a space of several C-C or C-O bonds. The diffraction strength of the GO/PVA adsorbent was obviously increased with an increase in the GO/PVA ratio. In the absence of GO, the cPVA (i.e., sample SS-0) showed a wide peak at ~18.0°. These results indicate that the interlayer space of the 12


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GO nanosheets was increased by crosslinking with PVA because the GO nanosheets in the crosslinked GO/PVA were not as well stacked as in the pristine GO sample due to the set-in of PVAs in the GO/PVA samples. It has also been reported that the diffraction peak at 10.6° for GO disappeared or became weaker when some inorganic (e.g., CdS, SnO2)9-11 or organic substances (e.g., amine monomers or ammonium ions )21,34,68 were set in the interlayers of GO nanosheets, which affected the stacking of the GO nanosheets. With increased interlayer space and the disordered stacks, the adsorption sites can be exposed to the sorbate molecules more easily, thereby enhancing the continuous flow adsorption capacity of the adsorbent.

Figure 5. XRDs of GO/PVA adsorbent with different GO/PVA ratios using soluble starch as the pore forming agent (a: cPVA; b: SS-0.16; c: SS-0.25; d: SS-0.5; e: GO).

Sodium dodecyl sulfate, a short-chain molecule, was also studied as a pore-forming agent. Its influence on the structures of resulting GO/PVA sponges was investigated. Figure 6 shows the XRD spectra of the GO/PVA. There was also a wide and weak diffraction peak located at ~5.8°, and the peak intensity increased with an increase in the GO/PVA ratio. This seems to suggest that sodium dodecyl sulfate affects the uniform stacking of GO nanosheets less significantly than the macromolecular starch. 13



However, the breakthrough tests showed that the crosslinked GO/PVA sponges with more uniform stacking tended to have a lower adsorption capacity. As a result, the interlayer space between the nanosheets was increased by the crosslinking.

Figure 6. XRDs of GO/PVA adsorbent with different GO/PVA ratios using sodium dodecyl sulfate as the pore forming agent (a: cPVA; b: SS-0.16; c: SS-0.25; d: SS-0.5; e: GO).

3.3. XPS. The changes in the oxygenic groups of GO after crosslinking were analyzed by XPS based on the C1s spectra of the GO, cPVA and crosslinked GO/PVA, and the results are shown in Figure 7. The C1s spectrum in GO may be divided into three combinations that correspond to the following functional groups: carbon sp2 (C=C, 285.5 eV), epoxy/hydroxyls (C-O, 287.5 eV) and carboxylates (O-C=O, 289.0 eV).69 For cPVA, the C1s spectrum may also be divided into three sections: carbon sp3 (C-C, 285.0 eV), alcoholic hydroxyl (C-O, 286.5 eV) and the groups formed by condensation polymerization (O-C-O, 288.0 eV). As one may expect, the C1s spectrum of crosslinked GO/PVA is more complicated because of the superposition of several C1s spectra. In all cases, a large number of carbons combining with oxygens were clearly observed, which result from the magnitude of oxygenic groups in GO and PVA. It is believed 14


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that the large number of oxygenic groups, bad nanosheet stacking and the increased interlayer spacing are the main reasons for the excellent continuous flow adsorption capacity.

Figure 7. C1s spectra of GO (a), cPVA (b) and crosslinked GO/PVA(c).

3.4. FTIR. Figure 8 shows the FTIR spectrum of crosslinked GO/PVA, and the spectrum of cPVA is also shown in the figure for comparisons. A stronger and broader peak at 3420 cm-1 was observed with the crosslinked GO/PVA, which is attributed to its much more hydroxyls (-OH) than in cPVA. In addition, some water was present in the crosslinked GO/PVA, which will be confirmed later by the thermal gravimetric analysis. For crosslinked GO/PVA, the peaks at 1620 cm-1 and 1430 cm-1 were attributed to the stretching vibration of C=C and the carbonyl groups (C=O). The peaks at 1750–1700 cm-1 confirm the stretching vibration of the carbonyl (C=O) in COOR and COOH groups originating from the oxidization of graphene and the condensation polymerization of carboxyl and hydroxy groups in GO and PVA. In addition, the stretching vibration of C-O-C appeared at 1190 cm-1, and this results from the acetalation of GO and PVA.

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Figure 8. FTIR spectra of cPVA (a), crosslinked GO/PVA (b), and pristine GO (c).

3.5. Raman spectrum. The Raman spectra of crosslinked GO/PVA are shown in Figure 9. Comparing to the two sharp peaks (i.e., the G peak at 1580–1600 cm-1 and the D peak at 1350 cm-1) of disordered graphite,69,70 the two main characteristic peaks of the crosslinked GO/PVA, located at 1579 cm-1 and 1355 cm-1, were broadened, and these peaks may be assigned to the G band (the vibration of sp2 carbon atoms) and the D band (the vibration of sp3 carbon atoms), respectively. After GO was crosslinked, there was no clear trend observed in the changes of the two peaks with a change in the GO/PVA ratio.61,69,70

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Figure 9. Raman spectra of crosslinked GO/PVA at different GO/PVA ratios (a: SS-0.5; b: SS-0.25; c: SS-0.16). 3.6. TG. Figure 10 shows the thermogravimetric curve of crosslinked GO/PVA. The weight loss of the samples can be divided into three parts. The first weight loss occurred at 100°C, which was attributed to the small amount of water contained in the hydrophilic samples. In the temperature range of 100–300°C, there was a slight weight loss in the crosslinked GO/PVA samples (a, b, and c in Figure 10), and no weight loss was observed for cPVA (d in Figure 10). The extent of the weight loss increased with an increase in the GO/PVA ratio. This seems to indicate that the main weight loss was from GO, primarily due to the oxidative decomposition of the oxygenic groups on the GO nanosheets.60 At 300°C, thermal decomposition of PVA chains began to occur. With a further increase in the temperature from 300 to 460°C, the decomposition of the GO body continued, and an overall weight loss as high as 80% was observed for the crosslinked GO/PVA samples. There was no significant weight loss in the samples at temperatures above 460°C, and the samples were carbonized almost completely.

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Figure 10. Thermogravimetric curves of crosslinked GO/PVA samples. (a) SS-0.5, (b) SS-0.25, (c) SS-0.16, and (d) SS-0 (i.e., cPVA).

3.7. FESEM. Figure 11 shows the FESEM images of the crosslinked GO/PVA samples with different GO/PVA ratios. Microporous structures were observed for all the samples, regardless of PVA content in the sample. This led to a high water permeability that yielded a good water flux even under a low hydraulic head of only 10 cm water. The difference in the morphologies between the crosslinked GO/PVA samples and the cPVA was that many stacked flakes can be found in crosslinked GO/PVA samples, and the stacked flakes became more and more apparent with an increase in the GO/PVA ratio.

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Figure 11. FESEM images of crosslinked GO/PVA samples with different PVA/GO ratios (a: SS-0; b: SS-0.16; c: SS-0.25; d: SS-0.5; (X)-high magnification; (X1)-low magnification).

4. CONCLUSIONS Superlight adsorbent sponges with excellent adsorption capacity under continuous flow were developed based on crosslinked GO/PVA. The interlayer space of the GO nanosheets was increased and the uniform stacking of GO nanosheets was disrupted by the insertion of PVA. The adsorbent was highly porous, having a bulk density of 0.016–0.049 g.cm-3. The adsorbent sponges were studied for removing MB from water under continuous flow dead-end filtration mode, and a water flux of 396 L.m-2.h-1 through a 2 cm thick adsorbent sponge was achieved at a low hydraulic head of only 10 cm water, with an almost 19



complete retention of methylene blue. Due to their high porosity, the adsorbent sponges exhibited a water permeability of 4.0×105 L.m-2.h-1.MPa-1, which is several orders of magnitudes higher than GO-based membranes for similar applications. Because the GO nanosheets were completely immobilized in the sponge by crosslinking with poly(vinyl alcohol), there were no GO nanoparticles leaching or flushing out into the treated permeate water stream, which is advantageous over the direct use of GO powders in water treatment. Because of the high water permeability and continuous flow adsorption capacity (CFAC), the crosslinked GO/PVA adsorbent sponges have a great potential for wastewater treatments.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at

AUTHOR INFORMATION Corresponding Author Email: [email protected] (X.F. Li), [email protected] (X. Feng). Tel: +86-22-83955055, Fax: +86-22-83955055 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support provided by the National Natural Science Foundation of China (No. 51273147, 51503144), the Scientific Research Foundation for Returned Overseas Chinese Scholars, and the State Key Laboratory of Separation Membranes and Membrane Processes (Z2-201528) was gratefully acknowledged.

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Superlight Adsorbent Sponges Based on Graphene Oxide

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