Article pubs.acs.org/est
Ambivalent Effect of Thermal Reduction in Mass Rejection through Graphene Oxide Membrane Jin-Hyeok Jang,† Ju Yeon Woo,† Jaeyeol Lee,‡ and Chang-Soo Han*,† †
School of Mechanical Engineering, Korea University, Seoul, Republic of Korea Composite Laboratory, Agency for Defense Development, Daejeon, Republic of Korea
‡
S Supporting Information *
ABSTRACT: We report ambivalent rejection behavior of a graphene oxide membrane (GOM) having a reduced interlayer spacing. Ultrathin GOMs having a thickness of 50 nm were fabricated using a vacuum filtration method followed by subjecting the samples to thermal reduction at 162 °C. The interlayer spacing of GOMs was reduced by 1 Å on thermal reduction as compared with that of the natural GOMs. The rejection rate with dye molecules was tested using dyes having three different types of charges in a dead-end filtration instrument. Rejection rate of the reduced GOM with the dyes having an opposite charge was improved up to 99.7%, indicating the dominant effect of the physical sieving diameter. In contrast, in the case of ion permeation of natural GOM, a higher rejection rate for several metal ions was observed as compared with that of GOMs having 1 Å smaller interlayer spacing, indicating the dominant effect of surface charges on the GOM samples.
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INTRODUCTION The scarcity of safe drinking water has emerged as a serious issue due to environmental pollution and climate changes caused by drastic industry acceleration, urbanization, and population growth.1,2 Currently, safe drinking water is no more an expendable resource and is being regarded as an essential resource. Novel and advanced water purification technologies as compared to the existing purification technologies must be discovered. Among various water treatment technologies, membrane technology is rapidly growing and is widely used in water industry such as for desalination and wastewater treatment owing to the advantages of the method such as the low energy consumption, cost effectiveness, use of less chemicals, and reduction in the processing steps.3 This process requires an ultrafast permeation in the water purification system consuming a low energy as well as a high rejection against unwanted matters.1,4−6 Recently, graphene oxide membrane (GOM) has attracted a great attention as a promising membrane material as compared with polymeric, ceramic, and other carbon-based membranes owing to its several advantages such as the nanoscale pore size, large-area facile production in aqueous solution, thermal stability, and outstanding mechanical strength having a high flexibility.7−10 Graphene oxide (GO) synthesized by oxidizing pure graphitic flakes using the modified Hummers’ method has two-dimensional sheet like structure containing both sp2 C−C and sp3 C−O bonds and several oxygen-containing groups such as epoxy, hydroxyl, and carboxyl groups at the basal planes and the edges of the sheet.7 Individual GO sheets can be © 2016 American Chemical Society
interlocked in a parallel fashion using various methods such as drop-casting,11 spray or spin coating, 12 Langmuir− Blodgett,13 and vacuum filtration14 on a target substrate. Vacuum filtration is a common and efficient method for fabricating closely and uniformly stacked GO membrane structures.15−17 The stacked structure of GO membrane results from several interactions, such as electrostatic repulsion, van der Waals forces, and hydrogen bonding, exhibiting hydrophilic property. Previous studies suggested that water molecules rapidly permeate through GOM nanochannels owing to the low friction regions of nonoxidized GO sheets like CNTs.14,16,18,19 The separation mechanism of charged ions and molecules is based on physical sieving as well as the electrostatic interaction between the charged target materials and the negatively charged GO sheets.11,16,20−23 The membrane and filtration properties of GOMs could be controlled by tuning conditions such as acidity of the solvent, the type of solutions used, concentration of electrolytes in the feed solution, and the applied pressure. Many studies have focused on understanding of fluids in the nanoscale channels in GOMs 16,24 and improving the purification performance by modifying GOMs using various methods such as reducing the functional groups and combining GOMs with nanomaterials.22,25−29 However, the exact Received: Revised: Accepted: Published: 10024
June 8, 2016 August 20, 2016 August 26, 2016 August 26, 2016 DOI: 10.1021/acs.est.6b02834 Environ. Sci. Technol. 2016, 50, 10024−10030
Article
Environmental Science & Technology
dye solutions using an Optizen 2120UV (Mecasys) UV−vis spectrophotometer. XRD measurements were carried out with a SmartLab (Rigaku), equipped with Cu Kαradiation (9 kW). Raman spectra of GOM and trGOM were recorded using a LabRam ARAMIS IR2 (Horiba Jobin Yvon), equipped with a 633 nm laser source. ICP-MS measurements were performed on a NexION 300D (PerkinElmer). The zeta-potential was analyzed using a Zeta-potential & Particle size Analyzer ELSZ-2 (Photo OTSUKA ELETRONICS).
mechanism of separation of ions and molecules should be investigated further. Even though several studies regarding water permeation through reduced GOMs have been reported, the results are inconsistent in nature and the permeation mechanism has not yet been well explained. Herein, we demonstrate that the reduction of interlayer distance by 1 Å in thermally reduced ultrathin (50 nm) GOMs considerably impacts the water separation. For this study, we used three types of charged (positive, negative, neutral) molecules, namely, methyl blue (MB), Congo red (CR), and rhodamine B (RB) and heavy metal ions, viz. Cu2+, Cd2+, Pb2+, Hg2+, and Mg2+ for the separation test. We find that the rejection mechanism of GOMs and thermally reduced GOMs depends on the physical sieving diameter and the electric charges present on the membrane. Finally, we suggest a plausible rejection mechanism for the thermally reduced GOMs.
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RESULTS AND DISCUSSION The two-dimensional hydrated GO sheet has a thickness of about 1.3 nm. (Supporting Information, Figure S2) We designed ultrathin GOMs as the membranes used for filtration or purification must be as thin as possible for achieving a high permeability for targeting organic solvents or water.21,22,30 The concentration of the GO solution was diluted to as low as 0.01 mg mL−1 with distilled water in order to obtain GOMs having a uniform and smooth surface. A porous anodized aluminum oxide (AAO) filter was used as the supporting substrate for stacking the GO sheets having a high mechanical strength.17 GOMs on the porous AAO filter was fabricated by vacuum filtration for obtaining close packed and flattened GO sheets. The uniformity and the flatness of GOMs are crucial for obtaining uniform nanochannels having a high permeability for water molecules.31 We fabricated GOM samples having a thickness of 50 nm. Figure 1A shows a photograph of the GOM bent on the porous AAO filter. The samples were characterized using a scanning electron microscope (SEM). Figure 1B illustrates the surface morphology of GOMs exhibiting an average pore size of 200 nm. The GOM appears transparent because of its ultrathin nature (50 nm). Figure 1C shows the cross section morphology of the GOM. Uniformly stacked GOMs having no visible voids are observed on the sample subjected to vacuum filtration. Thicker GOM samples exhibit wrinkles on the surface due to the presence of tightly stacked GO layers. (Supporting Information, Figure S3) The thickness of the GOMs was measured using an atomic force microscope (AFM). Figure 1D shows the phase image of the GOM and the porous AAO filter. The height was measured using line profiling. Average thickness of GOMs on using 5 μL of the GO solution (5 g L−1) is about 40 nm. Considering the AAO filter area, we fabricated 50 nm thick membranes. Figure 1E shows the experimental setup used for the permeation or the filtration test and the photographs of GOMs and thermally reduced graphene oxide membranes (trGOMs). We performed a tight sealing process using epoxy at the circumference of the filter. The color of the GOM changes from yellow to brown (Figure 1E) indicating thermal reduction. The interlayer distance (2D-nanochannels) of the natural GOM and trGOMs was measured by X-ray diffraction (XRD). Figure 2A shows XRD patterns of GOMs and the trGOM samples treated at 130 and 162 °C. Thermal processing causes an increase in the 2θ values in the samples, indicating a decrease in the interlayer distance.22,29 An intense peak for the GOM samples observed at 2θ = 11° corresponds to an interlayer distance of 8 Å (Table 1). The thermal reduction leads to a gradual shift of the XRD peaks (for samples treated at 130 and 162 °C). The trGOM samples exhibit broad peaks in the XRD patterns as compared with GOMs, indicating that not only a gradual or drastic vaporization of the intercalated/ adsorbed water molecules between the GO sheets, but also the reduction of oxygen-containing functional groups on the GO
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EXPERIMENTAL METHODS Materials. Graphene oxide was prepared from natural graphite powder using the modified Hummer’s method. Anodized aluminum oxide (AAO) filter (Anodic 47) with a pore size of 200 nm was purchased from Whatman. All other reagents were purchased from Sigma-Aldrich and were used as received. Fabrication of GOMs. The GO dispersion was prepared by diluting the GO solution (5 g L−1) with DI water. GOMs were fabricated by carrying out vacuum filtration of an extremely dilute GO dispersion through the AAO filter. The thickness of the GOM samples was controlled by changing the volumes of the GO solutions and the diameter of the AAO filter used in the process. After carrying out the filtration, the sample was dried in an oven for 24 h at 60 °C. Before installing in a deadend-filtration instrument, the circumference of the GOM samples was sealed with epoxy. For AFM characterizations, GOM was filtrated on the AAO filter having a diameter of 42 mm. All other GOM samples were fabricated on an AAO filter having a diameter of 37.5 mm. Thermal Reduction of GOMs. Before sealing the circumference of the membrane, GOMs were thermally reduced by a rapid thermal processing system. GOMs were placed in a vacuum chamber followed by annealing the samples at 130 or 162 °C for 1 h. The vacuum chamber was maintained at a vacuum pressure of ∼3 × 10−3 Torr in nitrogen atmosphere during thermal processing for preparing thermally reduced GOM. Water Flux Test and Rejection Test. The water flux test and the rejection test were carried out on a dead-end filtration instrument driven by nitrogen gas. (Supporting Information, Figure S1) The effective area of the membrane is 8 cm2. A photograph of the instrument is given in Supporting Information, Figure S1. The membranes and the components of the filtration instrument were perfectly assembled by silicon O-ring and gasket. In the water test, 100 mL DI water was filled on the membranes. The water flux was obtained by measuring the mass of the water collected every minute. In the rejection test with the dye molecules, 5 mL of the feed solution was used. In the rejection test of heavy metal ions, the volume of the feed solution used was 25 mL. Characterization. SEM images were obtained with a Nova Nano SEM 200 (FEI) at 1 kV. AFM images were recorded using a XE-100 (Park Systems), under the tapping mode. UV− vis spectra were recorded for determining the concentration of 10025
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Figure 1. (A) Photograph of GOMs having a thickness 50 nm bent a porous AAO filter. (B) SEM image (top view) of the surface of GOM. (C) SEM image (cross section view) of the cross-section of GOM. (D) AFM image of the exfoliated GOM having a thickness of 40 nm on a porous AAO filter along with the height images using line profiling. (red and green line). (E) Schematic of the dead-end filtration instrument used for the water purification test and the photographs of GOM and thermally reduced GOM.
Figure 2. (A) XRD patterns of GOMs and GOMs thermally reduced at 130 and 162 °C in vacuum. (B) Raman spectra of GOMs and trGOMs treated at 162 °C. XPS C 1s spectra of (C) GOMs and (D) trGOMs. The inset shows water contact angles of GOMs and trGOMs. (E) Schematic of the predictable effect of the reduction of 1 Å in the interlayer distance in trGOMs treated at 162 °C.
sheets occurs with thermal reduction. The elimination of water molecules and oxygen-containing functional groups results in an increase of the surface roughness of the membrane. (SSupporting Information, Figure S4) Furthermore, the interlayer distance in the samples is reduced to about 7 Å from the initial distance of 8 Å. Before performing the thermal reduction of ultrathin GOMs, the XRD patterns of GOMs having a thickness of about 400 nm were examined. (Supporting Information, Figure S5) No changes in the intensity ratios of G and D bands of the graphite (Figure 2B) are observed in the Raman spectra of GOM and trGOM, indicating that the size of sp2 clusters in the graphite sheets is unchanged. X-ray photoelectron spectroscopy (XPS) was employed to monitor changes in the chemical bonding and the atomic concentration of GOMs after performing the thermal reduction at 162 °C. The C 1s peak of GOM or trGOM samples can be divided into four peaks (Figure 2C, 2D), corresponding to C−C/CC (aromatic/aliphatic), C−
O−C (epoxy carbon), O−CO (carboxylate carbon), and OC−OH (carboxyl group) observed at 284.6, 286.6, 288.4, and 289 eV, respectively. The area of C−C/CC peaks increases and the area of C−O peaks (except for OC−OH peak) noticeably decreases along with thermal reduction (Figure 2D), implying the removal of oxygen-containing groups. The loss of oxygen-containing groups results in the reduction of the interlayer distance of GOMs. The charge effect of negatively charged GOMs diminishes after thermal reduction carried out at 162 °C. Carboxyl groups (OC−OH) are not present in GOMs owing to ionization (OC−O−) of the carboxyl groups (OC−OH) on the edges of GO sheet. The contact angles of GOMs and trGOMs are 55° and 90°, respectively (insets of Figure 2C and D), indicating a higher hydrophobicity of trGOMs.32 We expect that the efficiency of water purification can be improved using trGOMs because of the reduction in the pore size (interlayer distance) and the 10026
DOI: 10.1021/acs.est.6b02834 Environ. Sci. Technol. 2016, 50, 10024−10030
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Environmental Science & Technology Table 1. Water Flux and Rejection Performance for GOMs and trGOMs dye rejection rate [%]
ion rejection rate [%]
membrane
interlayer distance [Å]
water fluxc [L m−2 h−1 bar−1]
MBd
CRd
RBd
Cue
Pbe
Cde
GOMa trGOMb
8 7
0.32 0.27
93.5 99.7
99.2 99.7
87.7 96.3
97.1 65.4
95.9 42.7
81.6 52
Dried GOMs at 60 °C. bThermally reduced GOMs at 162 °C. cThe initial water flux. dThe volume of feed solution for dyes was 5 mL. eThe volume of feed solution for heavy metal ions was 25 mL. a
Figure 3. UV−vis absorbance changes of (A) MB, (B) CR, and (C) RB solutions using GOMs and (A) MB, (B) CR, (C) RB using trGOMs. Five mL of the feed solution was used with an applied constant pressure of 2 bar. The inset shows photographs of the feed solution (left) and the permeate solution (right). The color change indicates the rejection performance of the dye solution (RG is the rejection rate of the dye solution for GOMs and RtrG is the rejection rate of the dye solution for trGOMs).
Figure 3A−F show changes in the absorbance spectra of the feed and permeate solutions after the rejection test. The insets in Figure 3A−F indicate color changes of the feed and permeate solutions after the test. We evaluated the rejection performance of GOMs (Figure 3A−C) and trGOMs (Figure 3D−F) for MB, CR, and RB. For GOMs having a thickness of 50 nm, the rejection rate of the charged dyes, MB and CR, is over 90%. The rejection rate of the electro-neutral RB is 87.7%, which is lower than that of MB and CR. To our knowledge, negative charges are intrinsically present on the GO surfaces. The presence of surface charges on the hydrated GO sheets was confirmed by measuring the zeta potential (−30.34 mV at 25 °C) of the GO solution. (Supporting Information, Figure S7) Thus, the charged dye molecules are affected by charges present on the GO surface during permeation of the molecules through the membranes. The negatively charged CR molecules are subjected to an electrostatic repulsion due to the presence of negative charges on the GO sheets, whereas the case of positively charged MB, an attraction force occurs. Therefore, more molecules are intercalated and intervened between the GO layers, which are difficult to escape from GOMs. On the contrary, the neutral dye, RB, is only affected by the physical sieving diameter of the GOM. Figure 4A shows XRD patterns indicating changes in the interlayer distance between the GO layers after performing the rejection test. The intercalation and intervention of dye molecules between the GO layers result in a left-shift in the 2θ value, corresponding to an increase in the interlayer distance. We examined the number of the positively charged dye molecules intercalated between the GO sheets.
presence of more hydrophobic channels in the membrane. As illustrated in Figure 2E, in the case of separation of ions and molecules, the physical sieving effect of trGOMs might be higher than that of GOM. We tested the pure water flux of GOMs and trGOMs. A low pressure of 2 bar was applied for preventing the damage of GOMs. Our GOMs are well stacked without having any voids and are tightly sealed using epoxy. Variations in the pure water flux with GOMs and trGOMs were measured (Supporting Information, Figure S6). The water flux decreases with a decrease in the interlayer distance, indicating that the reduction in the pore size (interlayer distance) results in an enhancement of the water transport resistance. In addition, the water flux of GOMs and trGOMs is slightly decreased at a low pressure of 2 bar.22,33 We tested the rejection performance using GOMs and trGOMs for three dye molecules having different types of charges. The negatively charged MB, positively charged CR, and the electroneutral RB were used for the dead-end filtration test under a constant pressure of 2 bar. UV−vis analysis was conducted in order to compare the dye molecule concentrations in the feed and permeate solutions. The rejection rate of organic dye molecules in an aqueous solution can be calculated using the following equation: Cfeed − Cpermeate Cfeed
× 100
(1)
, where Cfeed is the dye concentration in the feed solution and Cpermeate is the dye concentration in the permeate solution. 10027
DOI: 10.1021/acs.est.6b02834 Environ. Sci. Technol. 2016, 50, 10024−10030
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molecules are accumulated on the circumference area because of the vertically applied pressure and water permeation. We also tested the rejection performance of heavy metal ions (Cu2+, Cd2+, Pb2+, Hg2+, and Mg2+) having a concentration of 0.1 mM under a pressure of 2 bar. According to the Donnan theory, negatively charged GOMs exclude anions and cations owing to the electrostatic repulsion and the charge neutrality requirements.34,35 Additionally, the heavy metal ions (hydrated radius value of ∼4−4.5 Å) have sizes similar to the interlayer distance (8 Å) of the GOMs. Hence, we used heavy metal ions in the feed solutions in order to demonstrate the thermal reduction effects. The permeate solution was quantitatively analyzed using an inductively coupled plasma mass spectrometer (ICP-MS). The rejection performance indicates a high rejection rate (>80%) with GOMs (Figure 5A). Thus, the
Figure 4. (A) XRD patterns of GOMs after performing the purification test with charged or uncharged organic dye molecules. (B), (C), and (D) SEM images (top view) of the surface of GOMs after performing the purification test with MB, CR, and RB solutions. (E), (F), and (G) SEM images (top view) of the surface of trGOMs after performing the purification test with MB, CR, and RB solutions.
With trGOMs, the rejection rates for all molecules are over 95%, as shown in Figure 4D−F and Table 1. After thermal reduction, the effects of negative charges on the GO sheets could be reduced. Therefore, the repulsion by the negative charges on the GO sheet could be decreased, resulting in a relatively less improvement in the rejection rate for the negatively charged dye molecules. Therefore, two effects can be considered for explaining the changes in the rejection rate occurring after thermal reduction, namely, the charges on the GO sheet and the interlayer distance. Apparently, the physical sieving effect seems more dominant, as the rejection rate for all the dye molecules is improved. We also examined the GOM surface by SEM after performing the purification test with the dyes (Figure 4B−G). With GOM, only CR molecules are mostly remained on the GOM surface (Figure 4C). In the different types of dye molecules, no residues were observed on the surface. Clean surfaces of GOMs were observed with MB and RB as shown in Figure 4B and D, indicating that the molecules are well permeated through GOMs without sticking on the surface. We observed molecules remaining on the trGOM surface after performing the purification test, different from those observed in the case of GOM samples. The molecules are remained on the surface likely due to the high rejection rate, as the rejected molecules are not washed away after the test. The insets shown in Figure 4B−G indicate the stacking of dye molecules around the circumference of GOM after performing the test, indicating that the rejected dye
Figure 5. (A) Rejection rate of heavy metal ions and chloride ions for GOMs (0.1 mM of feed solution). (B) XRD patterns of GOMs after performing the purification test with heavy metal ions. (C) Rejection rate of the heavy metal ions (Cu2+, Cd2+, and Pb2+) for GOMs and trGOMs.
negatively charged GOMs effectively reject the cationic heavy metal ions. In addition, the average rejection rate (46.2%) of chloride ions present in the heavy metal solutions indicates that the physical sieving effect contributes to the permeation of ions as the dye molecules are separated by GOMs. The interlayer distance of the GOM is relatively wide for Cl− ions to penetrate through the membrane. XRD patterns shown in Figure 5B indicate changes in the interlayer distance between the GO layers after performing the rejection test with heavy metal ions. The intercalation of heavy metal ions between the GO layers results in a left-shift in the 2θ value, indicating an increase in the interlayer distance. In addition, all the heavy metal ions induce similar changes in the 2θ values and the interlayer distance in GOMs is increased from 8 Å to about 9 Å. All the heavy metal ions are rejected from GOMs owing to the same mechanism resulting from the Donnan potential (electrostatic interaction) as well as physical sieving. In order to confirm the reduction effect of the heavy metal ions, we examined the rejection performance of trGOMs for three types of heavy metal ions (Cu2+, Cd2+, Pb2) (Figure 5C). The rejection rate is deceased (∼30−50%) with the three heavy metal ions, unlike in the case 10028
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of dye molecules (Table 2). This result indicates that the Donnan potential for heavy metal ions is deceased with thermal reduction. Thus, the heavy metal ions having a size as small as the interlayer distance (∼7−8 Å) of the GOMs experienced a relatively dominant electrostatic interaction rather than the effect of the physical sieving diameter. In conclusion, we demonstrate the ambivalent effect of GOM having an interlayer distance reduced by 1 Å. A physical sieving effect with the dye molecules was observed irrespective of the kind of charges in the molecules, and a less rejected effect with metal ions (Cu2+, Cd2+, and Pb2+) was observed. One Å reduction in the interlayer spacing resulted in a decrease in the mass flow rate. On using three types of charged dyes (MB, CR, and RB) for examining the mass rejection, charged dyes exhibited different rejection behaviors due to the combined effect of the physical sieving diameter and the surface charges on the GOMs. However, the reduction of interlayer spacing of GOM seemed to depend more on the physical sieving effect since even though the charges on the samples were reduced with thermal reduction, the rejection rates of all dyes were increased. On the contrary, natural GOMs exhibited high rejection rates (∼81.6−97.1%) for metal ions (Hg2+, Mg2+, Cu2+, Cd2+, and Pb2+), while the reduced GOMs exhibited much lower rejection rates (around 50%). Thus, the intrinsic charge on the GOM surface is a dominant factor in determining the rejection of metal ions. We expect that these results could be useful in the membrane filtration of water and chemicals in the future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b02834. Additional information as noted in the text (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 82-2-3290-3354. E-mail:
[email protected]. Author Contributions
J.H.J., J.Y.L., and C.S.H. conceived the idea and designed the experiments. J.H.J. performed the experiments. J.H.J. and J.Y.W. fabricated the membrane and analyzed the data. J.H.J., J.Y.L., and C.S.H. discussed and wrote the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful for the financial support of the Defense Acquisition Program Administration and Agency for Defense Development and Basic Science Research Program (201501004751) and Nano Material Fundamental Research (2012M3A7B4049863) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (MSIP) in Korea.
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REFERENCES
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DOI: 10.1021/acs.est.6b02834 Environ. Sci. Technol. 2016, 50, 10024−10030
Article
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DOI: 10.1021/acs.est.6b02834 Environ. Sci. Technol. 2016, 50, 10024−10030
