Weak-reduction graphene oxide membrane for improving water purification performance

Journal Pre-proof Weak-reduction graphene oxide membrane for improving water purification performance Hao Yu, Yi He, Guoqing Xiao, Yi Fan, Jing Ma, Yixuan Gao, Ruitong Hou, Jingyu Chen

PII:

S1005-0302(19)30361-5

DOI:

https://doi.org/10.1016/j.jmst.2019.08.024

Reference:

JMST 1764

To appear in:

Journal of Materials Science & Technology

Received Date:

29 May 2019

Revised Date:

2 August 2019

Accepted Date:

17 August 2019

Please cite this article as: Yu H, He Y, Xiao G, Fan Y, Ma J, Gao Y, Hou R, Chen J, Weak-reduction graphene oxide membrane for improving water purification performance, Journal of Materials Science and amp; Technology (2019), doi: https://doi.org/10.1016/j.jmst.2019.08.024

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Reserch Article Weak-reduction graphene oxide membrane for improving water purification performance Hao Yu a,b, Yi He a,b,*, Guoqing Xiao b,*, Yi Fan a,c, Jing Ma a,b, Yixuan Gao a,b, Ruitong Hou a,b, Jingyu Chen d,* a

State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest

Petroleum University, Chengdu 610500, China. College of Chemistry and Chemical Engineering, Southwest Petroleum University,

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b

Chengdu 610500, China. c

Chengdu Graphene Application Institute of Industrial Technology, Chengdu 611130,

China

Deakin University, Institute for Frontier Materials, VIC 3220, Australia

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d

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[Received 29 May 2019, Received in revised form 2 August 2019; Accepted 17 August 2019]

* Corresponding author.

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E-mail address: [email protected] (Y. He).

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Graphene oxide membrane was reduced controllably by UV-irradiating. The reduction process was realized without any additional agent. More pristine sp2 domains in rGO membrane promoted the transport of water molecules. A better separation performance was obtained with the weak-reduced GO membrane.

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Highlights:

Two-dimensional (2D) materials are promising candidates for advanced water purification membranes. In this work, UV reduced GO (UrGO) membranes on the support of polyvinylidene fluoride (PVDF) had been fabricated for wastewater treatment. The reduction degree of GO membrane on the effect of water purification performance was investigated, and it was found that the weak-reduction GO membrane exhibited the optimal performance of removing pollutant from wastewater

than GO membrane. Besides, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectra and contact angle tests were used to characterize the physical and chemical properties of the UrGO membranes, and the permeance and rejection ability of the as-prepared filtration membranes were determined. Due to the weak-reduction of GO, pristine graphitic sp2 domains increased with slightly decreasing D-spacing. Thus, the UrGO membrane showed a higher water flux of 38.27 L m-2 h-1 bar-1, which was improved more than 270% compared to GO membrane, and dyes rejection increased. Those outstanding performances indicated

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that the UrGO membrane could effectively regulate the contradiction of the trade-off balance between flux and rejection, and hold great potential in real-world waste-water purification.

Introduction

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1.

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Key words: Graphene oxide; Membrane; UV irradiation; Weak-reduction; Purification

With increasing economic and population, the shortage of fresh water was

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becoming more serious[1–3]. In order to solve the current situation of water shortage, membrane separation technology of sewage treatment and seawater desalination were

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regarded as a simple and efficient methods[4–6]. An ideal membrane should be as thin as possible to optimize the solution flux, enough mechanical strength in case the membrane structure be damaged, and have appropriate homogeneous pore sizes to

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sieve pollutant from the solution[7]. Graphene oxide (GO) emerged as a potential effective nanofiltration membrane has attracted much interest due to its

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one-atom-thick nanosheet structure. Compared with traditional polymer nanofiltration membrane, GO and its derivatives were excellent materials for developing size-selective composite membrane[8–13] owing to the atomic thickness, homogeneous nanochannels and facile preparation process. The reason for these performances was that GO was an excellent 2D lamellar structure material, and the D-spacing of adjacent GO layers was ˂ 2 nm[14,15]. This D-spacing could be employed to separate molecules and metal ions effectively[16–21]. However, GO membrane had

shortcomings since the in-plane oxygen functional groups could hinder the water transport by hydrogen bond interaction[22]. Meanwhile, the GO membrane was prone to swelling or re-dispersing in aqueous condition because of oxygen functional groups, and this instability seriously affects the performance of the membrane [21]. On the other hand, numerous scientific studies pointed out that water flow confined in nanochannels put up abnormal behavior. For example, Mashl et al.[22] using molecular dynamics simulations show us that with decreasing nanotube size, the water adopts more arrangement, in the nanotube with a width of 8.6 Å, water phase

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changed as hexagonal ice crystal. Subsequently, Wu et al.[23] calculated the migration energy barriers for hexagonal ice monolayer sliding, which were not more than 30

meV in between the GO flakes based on previous studies. Wu et al.[24,25] established a simple and effective model to calculate water flow rate that confined in nanochannels,

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and all of these works were based on the concept of effective slip. Accordingly, these

results implied that confined water molecules could transmit through nanochannels

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ultrafast and frictionless, and the pristine sp2 graphene domains provided fast low-friction channels for water flow. But, the graphene (or completely reduced GO)

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membrane suffers from low water permeance because D-spacing between adjacent nanosheets is so small that water molecules can hardly enter the interlayer. More

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importantly, graphene cannot be well dispersed in aqueous solution. In order to improve the dispersibility of graphene, several oxygen functional groups are needed. All told, moderate reduction degree of GO might be helpful. Goh et al.[26] intercalated

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multi-walled carbon nanotubes (MWCNTs, 10 nm in diameter) into GO layers to make GO-MWCNTs composite membrane, then reduced with 57 wt% hydrogen

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iodide (HI) vapor at 90 ℃ for 2‒5 min. The water flux reached 52.7 L m-2 h-1 bar-1, and rejection for three organic dyes with different charges attained almost 100%. Besides, Huang et al.[27] reduced GO dispersion by hydrazine to prepare rGO membrane, and the flux of acetone was 215 L m-2 h-1 bar-1. The membrane was also stable in organic solvents, strong acidic, alkaline, and oxidative media. Zhang et al.[14] reduced GO dispersion by hydrothermal reduction method. The results showed that heat treated reduction GO (HTrGO) membranes showed a well stability in water, and

weak reduction HTrGO membrane increased the separate performance. In this work, with the polyvinylidene fluoride (PVDF) supporting substrate, GO membrane was fabricated by vacuum filtration method. Subsequently, we proposed a controllable way to prepare the reduced GO separation membrane by UV irradiation (UrGO) without any additional chemical additives. The schematic diagram of the fabrication of reduction GO membranes is described in Fig. 1. Then, the UrGO membranes had been employed for wastewater treatment, and the reduction degree of UrGO membrane on the effect of water purification performance was investigated.

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Besides, scanning elecrton microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectra and contact angle tests were used to characterize the physical and chemical properties of the UrGO membranes, and the permeance and rejection

ability of the as-prepared filtration membranes were determined on a dead-end

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pressure filtration equipment. It was found that the weak-reduction GO membrane exhibited the optimal performance of removing pollutant from waste water than GO

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membrane, and the reduction degree of the UrGO was highly correlated with the UV irradiation time. Furthermore, different microstructures, element contains and

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separation performance were observed with the variation of reduction time. As a result, a moderately reduced GO membrane could significantly improve the water flux

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and removal ability of contaminants at the same time, breaking the trade-off effect of traditional nanofiltration membrane to a certain extent. On the other hand, the as-prepared UrGO membrane also presented excellent mechanical and chemical

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stability. Those outstanding performances make our UrGO hold great potential in

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real-world waste water treatment. 2.

Experimental

2.1. Materials GO was purchased from the sixth element (Changzhou, China) Materials Technology Co., Ltd. The specifications for GO: oxygen content: ≤ 40.0%, average lateral dimension: ˂ 4 µm, monolayer probability: ˃ 90%. HCl (36.0%-38.0%),

Eriochrome black T (EBT, negatively charged, Mw 461.38), Crystal violet (CV, positively charged, Mw 407.99), Rhodamine B (Rh B, positively charged, Mw 479.01), Methylene blue (MB, positively charged, Mw 319.85) were purchased from Kelong Chemical Co., Ltd. (Chengdu, China). Commercial PVDF membrane (pore size: 0.05 µm) was purchased from Zhongkeruiyang Membrane Technology Co., Ltd. (Beijing, China). 2.2. GO membrane fabrication

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1.5 mL GO dispersion (0.05 mg mL-1) was added into 500 mL beaker, and diluted with 200 mL deionized water, then the obtained 0.375 mg L-1 GO dispersion was ultrasonicated with stirring for 10 min for sufficient dispersion. Took the beaker

out and poured the GO dispersion slowly into filter cup. Finally, GO membranes were

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prepared by vacuum filtration on PVDF ultrafiltration membrane (0.05 μm). The PVDF membranes were used as substrate of GO-based membranes to enhance the

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mechanical properties, in which, the mass loading of GO was 53.03 mg m-2. We also fabricated thick GO and UV-irradiation treated rGO (UrGO) membranes above 2 g

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m-2 to analyze the X-ray diffraction (XRD) and SEM.

Before the experiment, we also investigated the effects of concentration of GO

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dispersion and drying mode of the membrane on the flux. In this experiment, we diluted 1.5 mL 0.05 mg/mL GO dispersion with 20 mL and 200 mL pure water, respectively. Results showed that when the mass loading was 53.03 mg m-2, the flux

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of pure water was decreased from 28.64 L m-2 h-1 bar-1 to 19.76 L m-2 h-1 bar-1. It was shown that the compacted GO membrane attributed to low concentration of GO

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dispersion. Furthermore, the flux of GO membrane decreased from 18.35 L m-2 h-1 bar-1 to 11.98 L m-2 h-1 bar-1 after drying in a vacuum at 60 ℃ for 12 h. Because GO layers compacted more tightly without water molecule, the drying process was essential for the preparation of compacted GO nanofiltration membranes. 2.3. UV-irradiating reduction of GO membranes The obtained GO membranes were treated with UV irradiation[28–30] (365 nm and

253.7 nm ZF-2 Triple Ultraviolet Analyser, height: ~1 cm) for required time. Reaction is as follows[30]: UV

GO + H2 O + O2 → rGO + CO2 + H2 + H2 O 2.4. Materials characterization UV-vis absorption spectroscopy (UV-1800, SHIMADZU) was used to analysis the dye concentration in feed and permeate solutions over 400‒800 nm. SEM (FEI Inspect F50) was used to characterize the morphology and thickness of GO/UrGO

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membranes. The D-spacing of GO/UrGO membranes were determined by X-ray diffractometer (XRD, PANalytical, The Netherlands, CuKα radiation source). LabRAM HR800 (532 nm laser, spot size ~1 µm2) was taken to characterize Raman

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spectra. Contact angle goniometer (Beijing Hake, XED-SPJ) was used to analyze

hydrophilicity of the GO/UrGO membranes. The content of chemical composition

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was measured by XPS (Kratos, XSAM800, Al Kα excitation source).

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2.5. Permeance and rejection properties of the membranes In this work, the permeance and rejection properties of the GO/UrGO membranes were measured on a homemade dead-end pressure filtration equipment (1

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bar) with an effective area of 7.07 cm2 at room temperature. The concentration of feed dye solution was 10 mg L-1, the volume of dye solution is 50 mL, and we did six

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cycles for each data. The permeance 𝐽 (L m-2 h-1 bar-1) and rejection 𝑅 (%) were calculated on the basis of Eqs. (1) and (2), respectively. 𝑉

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𝐽 = 𝐴∆𝑡𝑃

𝐶p

𝑅 = 1 − 𝐶 × 100% f

(1) (2)

where 𝑉 (L) was the volume of feed solution, 𝐴 (m2) was the effective area of membrane, ∆𝑡 (h) was the permeate time, 𝑃 (bar) was the applied pressure, and 𝐶p and 𝐶f were the concentrations of permeate and feed solutions, respectively. 3.

Results and discussion

According to literature reports, usually the rGO membranes were filtrated by GO dispersion that reduced by chemical reductant[31–34] or hydrothermal reduction[14,20] methods, but there was a fatal flaw that the dispersion property of rGO decreased sharply[29,35]. Meanwhile, the additional chemical additives intercalated GO layers to enlarged D-spacing and further decreased rejection property of membranes. In this work, GO membranes were fabricated firstly, it crafty avoided the fatal flaw of adding additives, then be reduced by UV irradiating. Fig. 2(a) shows the color of membranes changed from light-yellow to dark-brown. This result indicated indirectly a restoring

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of the sp2-bonded carbon network. It also showed that the water contact angle (CA) changed with increasing UV irradiation time in Fig. 2(b), and the water CA of GO/UrGO membranes were 52°, 65°, 70.3° and 73.1°, respectively, owing to the decrease of oxygen containing functional groups content. After that, we filtrated

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another thick GO membrane (> 2g m-2) to characterize the XRD of GO and UrGO

membranes (wet- and dry-states) in Fig. 2(c). Compared with dry-state GO membrane

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(2θ = 11.23°), the XRD peak of 6-UrGO, 12-UrGO and 18-UrGO dry-state membranes (11.27°, 11.39° and 11.40°, respectively) shifted right slightly. Something

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similar happened with wet-state membranes, and the XRD peaks of GO, 6-UrGO, 12-UrGO and 18-UrGO membranes were 7.11°, 7.22°, 7.28° and 7.32°, respectively.

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These results indicated that the interlayer distance of rGO membranes decreased with the increase of UV irradiation time. Because oxygen functional groups between GO layers were removed, D-spacing of rGO layers got closer by π-π configuration effect

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(noncovalent interaction). And compared with dry-state GO or rGO membranes, all XRD peaks of wet-state GO or rGO membranes shifted left enormously, because in

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the water environment, water molecules entered the adjacent GO nanosheets and enlarged the D-spacing. SEM was used to characterize the surface morphologies of the GO, 6-UrGO,

12-UrGO and 18-UrGO membranes (53.03 mg m-2) in top view and cross-section view, respectively. As shown in Fig. 3(a, e, i), the fabricated GO membrane exhibited a typical wrinkled structure without any defects. And the cross-section view indicated a 1.70 μm effective thickness GO layers on the PVDF support. Furthermore, the GO

layers stacked compact without any gaps as shown in Fig. 3(i). Compared with GO membrane, the surface morphologies of UrGO membranes exhibited more wrinkles in the top-view, and the degree of wrinkles deepened on the deepening of reduction as shown in Fig. 3(a‒d), respectively. In addition, with the increase of UV reduction time, the thickness of UrGO membranes continued to decrease, the thickness of GO, 6-UrGO, 12-UrGO, 18-UrGO membranes were 1.70, 1.15, 1.09 and 0.51 μm, respectively. It might due to the number of oxygen functional groups on GO layers continued decline, and the D-spacing of GO membrane compacted without large

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number of oxygen functional groups. And it could be observed directly in the cross-section of GO and UrGO membranes after magnify 50,000 times. As shown in

Fig. 3(j), after UV irradiated 6 h, the rGO layers crinkled slightly, and gaps appeared

as marked in the red boxes. As UV irradiation time went on, the gaps enlarged as

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shown in Fig. 3(k). After 18 h UV irradiation in Fig. 3(l), the 18-UrGO membranes not only appeared large gaps, but also exhibited severe wrinkles. These results might

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be due to the accumulative pressure of CO2 that produced during the reduction process[30,36].

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Then we used XPS to characterize the chemical composition and bonding states of the GO and UrGO membranes as shown in Fig. 4(a‒d). The O atomic ratio of the

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GO, 6-UrGO, 12-UrGO, 18-UrGO membranes were 39.44%, 31.73%, 26.99% and 20.09%, respectively. It could be found that the intensity of C-O bonds (286.6 eV, including hydroxyl and epoxy groups) decreased sharply by comparing the 6-UrGO

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membrane (Fig. 4(a)) to the GO membranes (Fig. 4(b)), and C=O bonds (287.4 eV) decreased slightly in the same case. With longer UV irradiation time, there was no

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significant change be observed in XPS curves. These results revealed that during the reduced process, the GO membrane was partially reduced, and the reduction degree could be controlled by UV irradiation time. Fig. 4 exhibits the Raman spectroscopic curves of the GO, 6-UrGO, 12-UrGO and 18-UrGO membranes. The peak at 1347 cm-1 was known as the D band (disordered band) due to first-order zone boundary phonons caused by the graphene edges[36,37], and the different oxygen functional groups (including -OH, -COOH and epoxy) would increase the intensity of the D peak.

Whereas the G peak arised at 1616 cm-1 correspond to the in-phase vibration of defect-free graphene lattice because of in-plane optical vibrations, and this peak would be enhanced by the highly ordered configuration of graphene. As shown in Fig. 4(e), compared with the GO membrane, the ID/IG of the 6-UrGO membrane increased slightly, since the degree of order had not changed much. In conjunction with Fig. 3(j) and Fig. 4(b), during this process, large amounts of hydroxyl groups were oxidized firstly[30], and the numerous reproduced sp2 construction declined the ratio of ID/IG, but the generated-gaps raised the ratio of ID/IG, all above resulted the degree of order

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shiftless, whereas, the ID/IG of the 12-UrGO membrane increased a lot due to the degree of order changed significant. During this process, -COOH on the edges of GO sheets were oxidized to produce CO2, and CO2 pressure

[30,36]

separated stacked GO

layers to engender gaps and wrinkles as shown in Fig. 3(k). The disorder degree of

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GO were enlarged. Interestingly, the ID/IG of the 18-UrGO membrane decreased dramatically, indicating that more oxygen groups were removed and UrGO layers

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stacked serious, meanwhile, more sp2 domains generated. All those reasons above improved the order degree of the 18-UrGO membrane.

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The initial flux of the GO membranes was not steady due to swelling characteristics in pure water[38], and in ordered to get an effective flux value, we tested

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a series fluxes of different filtrated time. In this work, the PVDF membranes were used as substrate, and the mass loading was 53.03 mg m-2. As shown in Fig. 6(a), the initial flux of GO membrane was 27.24 L m-2 h-1 bar-1, but decreased fast in 0.5 h to

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15.83 L m-2 h-1 bar-1. 1.5 h later, the flux reached steady state about 12.23 L m-2 h-1 bar-1, while the steady flux value dropped to about 1/2 of the initial flux. For further

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study of membranes stability, the GO membranes (53.03 mg mL-1 on PVDF substrate, and the stability of GO on PVDF membranes was better than that on cellulose acetate membranes[39] for stronger interaction force between GO and support membranes.) which filtrated 1.5 h (compared with 0 h) were placed into petri dish and immersed in pure water. Then the membranes suffered 10 min ultrasonic at 400 W. As shown in Fig. 5(a), the GO membrane without filtrating decomposed within 1 min. Following time, the membrane was quickly broken away from PVDF substrate and just little GO

remained on the PVDF membrane surface after 10 min. Interestingly, the GO membrane after filtrated (Fig. 5(b)) put up excellent stability after ultrasound treated 10 min. It was not decomposed and barely changed since beginning. On one hand, the electrostatic[40–42] and hydrogen bond[43,44] interaction (Fig. S2) existed between GO and PVDF support membranes. On the other hand, the D-spacing of adjacent GO layer was compacted under the pressure filtrated for 1.5 h. And due to the two reasons, the stability of GO membrane with filtrated was improved greatly. We also tested the stability of 6-UrGO membrane, and the membrane remained the original appearance

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after 10 min ultrasound treated (Fig. S3). Next, we studied how the mass loading on PVDF affects the properties of GO membranes. As shown in Fig. 6(b), the curves indicated the pure water flux with different GO loadings ranging from 7.58 mg m-2 to 100 mg m-2. The lowest loading of GO (7.58 mg m-2) showed the highest water flux

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under steady state about 162.48 L m-2 h-1 bar-1, with increasing loading mass. The flux dropped dramatically to 37.82 L m-2 h-1 bar-1 (15.15 mg m-2), continued to increase

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GO loading mass to 53.03 mg m-2, and the flux decreased from 37.82 to 14.1 L m-2 h-1 bar-1 stably. These results indicated that it was inability to form a completely

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membrane when the GO loading mass was less than 15.15 mg m-2, and defects still existed in the range from 15.15 to 53.03 mg m-2. Subsequently, the flux went down

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slowly when the mass loading was more than 53.03 mg m-2, and the value was about 14.1 L m-2 h-1 bar-1. Methylene blue (MB) solution was used to investigate separation performance of the GO/UrGO membranes with different loading mass. To all

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membranes, the flux of MB was slightly lower than pure water but still maintaining above 92.77%, the rejection of MB increased from 99.32% and reached the highest

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value of 99.99% when the mass loading was 22.73 mg m-2 as shown in Fig. 6(c). It was indicated that, when the GO loading mass was less than 22.73 mg m-2, the defects of GO membrane still existed that MB molecules could pass though. We then evaluated how the degree of reduction affects the properties of the GO/UrGO membranes through dead-end pressure filtration as Fig. shown in 6(e, f). The membranes were all fabricated with same GO/ UrGO area density of 53.03 mg m-2 to exclude the influence of membrane loading mass. As shown in Fig. 6(e), the water

flux of GO membrane was 14.10 L m-2 h-1 bar-1, which was similar to previously reported (range from 3 to 22 L m-2 h-1 bar-1), but faster than the result reported by Han et al.[20] (< 3 L m-2 h-1 bar-1) with the same area density, because Han et al. screened out and used smaller size of GO sheets[20], and more wrinkles can be avoided. After 6 h UV irradiation, the water flux of 6-UrGO membrane showed a significant improvement, about 38.27 L m-2 h-1 bar-1. Corresponding of Fig. 6(d), the D-spacing (calculated through Bragg formulation as follow: 𝜆 = 2𝑑 sin 𝜃 , where 𝜆 is the wavelength of the Cu X-ray beam about 0.154056 nm, d is the interlayer spacing

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between contiguous GO/UrGO layers and 𝜃 is the diffraction angle.) and oxygen content decreased from 12.44 Å to 12.24 Å and 39.44% to 31.73%, respectively. Lead more frictionless water channels of pristine sp2 domains formed and adjacent GO

layers stacked, in addition, the generated micro gaps (Fig. 3(j)) promoted water flux,

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under the combined effect of the above reasons, the flux of 6-UrGO membrane

obtained great improvement. Meanwhile, the rejection of Rhodamine B (Rh B) (MB

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also has been studied as shown in Fig. S4) improved from 85.31% to 99.00%, as shown in Fig. 6(f). This result indicated that, during the filtrated processing, the Rh B

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molecules were too large to pass through the 6-UrGO membrane and be blocked in the gaps, so that the flux ratio of Rh B solution decreased sharply to pure water.

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Furthermore, the water flux of UrGO membranes decreased with longer UV irradiation time, and 12-UrGO membranes and 18-UrGO membranes showed impermeable ability to water and the flux, which was 18.51 L m-2 h-1 bar-1 and 11.64 L

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m-2 h-1 bar-1, respectively, as show in Fig. 6(e). At the same time, the flux of Rh B solution was close to that of pure water (Fig. 6(f)). It might be due to lack of carboxyl

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groups on GO sheets edges and smaller D-spacing that fewer Rh B even water molecules could entrance the compacted GO sheets[45], but according to Fig. 3(l), the enlarged gaps and wrinkles let water and Rh B molecules to pass through, and lead low Rh B solution flux and rejection as shown in Fig. 6(f). In ordered to characterize the organic molecules separation performance of 6-UrGO membranes with area density of 53.03 mg m-2, we tested four dyes with different charges and molecule sizes, including eriochrome black T (EBT, negatively

charged, Mw 461.38, 10 mg L-1), crystal violet (CV, positively charged, Mw 407.99, 10 mg L-1), rhodamine B (Rh B, positively charged, Mw 479.01, 10 mg L-1), methylene blue (MB, positively charged, Mw 319.85, 10 mg L-1). The content of dyes was tested by UV-vis analysis. As shown in Fig. 7(a), the flux of EBT, CV, Rh B and MB were 12.27, 13.5, 13.36 and 13.44 L m-2 h-1 bar-1, respectively, and the flux of dyes solution was more than 60% of that of pure water. Meanwhile, the 6-UrGO membrane showed an excellent rejection performance: the rejections of EBT, CV, Rh B and MB were 99.3%, 96.13%, 99.00% and 99.99%, respectively. As shown in Fig. S5, each dye

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feed has its own unique absorption peak, and the absorbance of relevant permeat toward to zero. It indicates that the rejection of each dye is nearly 100%. Considering the molecule sizes and charges of these dyes, we believed that the steric hindrance

effect played a vital role in the excellent dye rejection performance. Furthermore, Fig.

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7(b) showed GO membranes and 6-UrGO membranes (on PVDF substrates) after

continual shaking in different pH conditions (pH= 1, 7, 13) at 100 rpm for 1 month,

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respectively. both GO membranes and 6-UrGO membranes kept the original appearance under neutral and acidic conditions. It indicated that GO or 6-UrGO

Conclusion

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membranes put up excellent acid-base stability.

In conclusion, we used UV irradiation method to prepare weak-reduction GO membranes. Studied the effect of different reduction degree on the structure and

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properties. It was found that weak-reduction GO membranes remained the compacted Layer-By-Layer microstructure without any gaps; meanwhile, it increased the pristine

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graphitic sp2 domains with lower D-spacing. Therefore, the 53.03 mg m-2 weak-reduction GO membrane showed near 270% higher water flux (38.27 L m-2 h-1 bar-1) than GO membrane with the same area density, and the rejection of Rh B increased from 85.31% to 99.0%, and other dyes’ rejections were all above 96.3%. Those outstanding performances indicated that the weak-reduction GO membrane could effectively regulate the contradiction of the trade-off balance between flux and rejection, and hold great potential in real-world waste water purification.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51774245), Applied Basic Research Program of Science and Technology Department of Sichuan Province (No. 2018JY0517), Open Fund (No. PLN161) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation and

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Research Center of Energy Polymer Materials (Southwest Petroleum University).

References [1]

Q. Schiermeier, Nature 452 (2011) 260–261.

[2]

K. Rahaoui, L.C. Ding, L.P. Tan, W. Mediouri, F. Mahmoudi, K. Nakoa, A. Akbarzadeh, Energy Proc. 110 (2017) 414–419.

[3]

M.A. Eltawil, Z. Zhengming, L. Yuan, Renew. Sustain. Energy Rev. 13 (2009) 2245–2262. V. Freger, Science 348 (2015) 1317–1318.

[5]

Z. Jiang, A.G. Livingston, K. Santanu, Science 348 (2015) 1347–1351.

[6]

J. Kamcev, H.B. Park, B.D. Freeman, L.M. Robeson, M. Elimelech, Science

ro of

[4]

356 (2017) eaab0530. [7]

S.P. Koenig, L. Wang, J. Pellegrino, J.S. Bunch, Nat. Nanotechnol. 7 (2012) 728–732.

H.H. Huang, R.K. Joshi, K.K.H. De Silva, R. Badam, M. Yoshimura, J. Memb.

-p

[8]

Sci. 572 (2019) 12–19.

Y. Wei, Y. Zhang, X. Gao, Z. Ma, X. Wang, C. Gao, Carbon 139 (2018) 964–

re

[9]

981.

lP

[10] J. Abraham, K.S. Vasu, C.D. Williams, K. Gopinadhan, Y. Su, C.T. Cherian, J. Dix, E. Prestat, S.J. Haigh, I. V. Grigorieva, P. Carbone, A.K. Geim, R.R. Nair,

na

Nat. Nanotechnol. 12 (2017) 546–550.

[11] M.Y. Lim, Y.S. Choi, J. Kim, K. Kim, H. Shin, J.J. Kim, D.M. Shin, J.C. Lee, J. Memb. Sci. 521 (2017) 1–9.

ur

[12] Q. Yang, Y. Su, C. Chi, C.T. Cherian, K. Huang, V.G. Kravets, F.C. Wang, J.C. Zhang, A. Pratt, A.N. Grigorenko, F. Guinea, A.K. Geim, R.R. Nair, Nat.

Jo

Mater. 16 (2017) 1198–1202.

[13] J. Ma, D. Ping, X. Dong, Membranes 7 (2017) 52. [14] Q. Zhang, X. Qian, K.H. Thebo, H.M. Cheng, W. Ren, Sci. Bull. 63 (2018) 788–794. [15] W.S. Hung, T.J. Lin, Y.H. Chiao, A. Sengupta, Y.C. Hsiao, S.R. Wickramasinghe, C.C. Hu, K.R. Lee, J.Y. Lai, J. Mater. Chem. A 6 (2018) 19445–19454.

[16] L. Huang, J. Chen, T. Gao, M. Zhang, Y. Li, L. Dai, L. Qu, G. Shi, Adv. Mater. 28 (2016) 8669–8674. [17] H.B. Park, H.W. Yoon, Y.H. Cho, Graphene Oxide Fundam. Appl. 61 (2017) 296–313. [18] P. Sun, M. Zhu, K. Wang, M. Zhong, J. Wei, D. Wu, Z. Xu, H. Zhu, ACS Nano 7 (2013) 428–437. [19] P. Sun, R. Ma, H. Deng, Z. Song, Z. Zhen, K. Wang, T. Sasaki, Z. Xu, H. Zhu, Chem. Sci. 7 (2016) 6988–6994.

ro of

[20] Y. Han, Z. Xu, C. Gao, Adv. Funct. Mater. 23 (2013) 3693–3700. [21] L. Qiu, X. Zhang, W. Yang, Y. Wang, G.P. Simon, D. Li, Chem. Commun. 47 (2011) 5810–5812.

[22] R.J. Mashl, S. Joseph, N.R. Aluru, E. Jakobsson, Nano Lett. 8 (2003) 589–592.

-p

[23] K. Wu, Z. Chen, J. Li, X. Li, J. Xu, X. Dong, Proc. Natl. Acad. Sci. 114 (2017) 3358–3363.

re

[24] K. Wu, Z. Chen, J. Li, X. Li, J. Xu, X. Dong, Proc. Natl. Acad. Sci. 114 (2017) 3358–3363.

lP

[25] H. Zeng, K. Wu, X. Cui, Z. Chen, Nano Today 16 (2017) 7–8. [26] K. Goh, W. Jiang, H.E. Karahan, S. Zhai, L. Wei, D. Yu, A.G. Fane, R. Wang,

na

Y. Chen, Adv. Funct. Mater. 25 (2015) 7348–7359. [27] L. Huang, J. Chen, T. Gao, M. Zhang, Y. Li, L. Dai, L. Qu, G. Shi, Adv. Mater. 28 (2016) 8669–8674.

ur

[28] P. Sun, Q. Chen, X. Li, H. Liu, K. Wang, M. Zhong, J. Wei, D. Wu, R. Ma, T. Sasaki, H. Zhu, NPG Asia Mater. 7 (2015) e162-e168.

Jo

[29] Y.H. Ding, P. Zhang, Q. Zhuo, H.M. Ren, Z.M. Yang, Y. Jiang, Nanotechnology 22 (2011) 215601.

[30] G. Heit, A. Neuner, P. Saugy, M. Braun, J. Phys. Chem. A 102 (1998) 5551– 5561. [31] C.K. Chua, M. Pumera, Chem. Soc. Rev. 43 (2014) 291–312. [32] J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Chem. Commun. 46 (2010) 1112–1114.

[33] A.A. Nezhad, M. Alimoradi, M. Ramezani, Mater. Res. Express 5 (2018) 025508. [34] E.F. Sheka, N.A. Popova, Phys. Chem. Chem. Phys. 15 (2013) 13304–13322. [35] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 3 (2008) 101–105. [36] A.I.P. Advances, T. Madras, AIP Adv. 2 (2012) 032183. [37] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud’homme, I.A. Aksay, R. Car, Nano Lett. 8 (2008) 36–41.

ro of

[38] S. Zheng, Q. Tu, J.J. Urban, S. Li, B. Mi, ACS Nano 11 (2017) 6440–6450. [39] J. Hou, C. Bao, S. Qu, X. Hu, S. Nair, Y. Chen, Appl. Surf. Sci. 459 (2018) 185–193.

[40] C. Sun, X. Feng, Sep. Purif. Technol. 185 (2017) 94–102.

-p

[41] N.G.P. Chew, S. Zhao, C. Malde, R. Wang, J. Memb. Sci. 563 (2018) 31–42.

[42] M. Ping, X. Zhang, M. Liu, Z. Wu, Z. Wang, J. Memb. Sci. 570–571 (2019)

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286–293.

(2019) 5971–5987.

lP

[43] Z. Cui, X. Tang, W. Li, H. Liu, J. Zhang, H. Wang, J. Li, J. Mater. Sci. 54

[44] G.E. Chen, W.G. Sun, Y.F. Kong, Q. Wu, L. Sun, J. Yu, Z.L. Xu, Polym. Plast.

na

Technol. Eng. 57 (2018) 108–117.

[45] D.W. Boukhvalov, M.I. Katsnelson, Y.W. Son, Nano Lett. 13 (2013) 3930–

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Figure list:

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Fig. 1. Schematic diagram of the fabrication of weak-reduction GO membranes.

Fig. 2. Characterization of the membranes. (a) photographs of GO/UrGO membranes

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under different UV irradiation time. (b) Water CAs on GO, 6-UrGO, 12-UrGO, 18-UrGO membranes with different UV irradiation time (0 h, 6 h, 12 h, 18 h). (c)

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XRD patterns of GO, 6-UrGO, 12-UrGO, 18-UrGO membranes (wet and dry).

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Fig. 3. SEM images of the GO and UrGO membranes. (a, e, i) GO membranes, (b, f, j)

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The area density of all membranes was 2 g m-2.

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6-UrGO membranes, (c, g, k) 12-UrGO membranes, (d, h, l) 18-UrGO membranes.

Fig. 4. High-resolution C 1s XPS spectra of the (a) GO, (b) 6-UrGO, (c) 12-UrGO and (d) 18-UrGO membranes. (e) Raman spectra of the GO, 6-UrGO, 12-UrGO and 18-UrGO membranes.

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Fig. 5. Digital photos of GO membranes during ultrasonic treatment 10 min (a) before

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and (b) after filtrated 1.5 h.

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Fig. 6. (a) Relationship between flux of GO membranes (53.03 mg m-2 on PVDF, 50 nm) and filtrated (1 bar) time. (b) The water flux of GO membranes with different

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loading mass. (c) The flux ratio of MB solution to pure water and MB rejection of GO membranes with different loading mass. (d) The D-spacing and oxygen content of GO, 6-UrGO, 12-UrGO and 18-UrGO membranes. (e) The water flux of GO, 6-UrGO, 12-UrGO and 18-UrGO membranes (53.03 mg m-2). (f) The flux ratio of Rh B solution to pure water and Rh B rejection of GO, 6-UrGO, 12-UrGO and 18-UrGO membranes (53.03 mg m-2).

Fig. 7. (a) Flux and rejection of different dyes of 6-UrGO membrane. (b) Stability of

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GO and 6-UrGO membranes with different pH conditions.