Supplementary Material Graphene Oxide-Embedded Thin-Film Composite Reverse Osmosis Membrane with High Flux, Anti-biofouling, and Chlorine Resistance Hee-Ro Chae a , Jaewoo Lee a , Chung-Hak Lee a* , In-Chul Kim b , Pyung-Kyu Park c* a School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea b Research Center for Biobased Chemistry, Korea Research Institute of Chemical Technology, P.O. Box 107, Daejeon 305-600, Korea c Department of Environmental Engineering, Yonsei University, Wonju, Gangwon-do 220-710, Korea Surface morphologies of TFC membranes (without GO and with non-fractionated and with fractionated GO)
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Supplementary MaterialGraphene Oxide-Embedded Thin-Film Composite Reverse Osmosis Membrane with High Flux, Anti-biofouling, and Chlorine Resistance
a School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea
b Research Center for Biobased Chemistry, Korea Research Institute of Chemical Technology, P.O. Box 107, Daejeon 305-600, Korea
c Department of Environmental Engineering, Yonsei University, Wonju, Gangwon-do 220-710, Korea
Surface morphologies of TFC membranes (without GO and with non-fractionated and with fractionated GO)
Fig. S1. SEM surface images of three TFC membranes: TFC membranes prepared (a, a', and a") without GO and with (b, b', and b") non-fractionated and (c, c', and c") fractionated GO.
The surface of the TFC membrane without GO is bright and homogeneous over the entire area (Fig. S1a) with a ridge and valley structure similar to a typical TFC membrane(Fig. S1a' and a"). However, the surface of the GO-TFC
membrane prepared with non-fractionated GO was not homogeneous and had dark spots with sizes ranging from tens to hundreds of micrometers (Fig. S1b). The darkness of the spots indicated that the regions contained only few and small ridges (Fig. S1b' and b") [S1], and the smooth surface reduced the water flux of the membrane [S2]. The abnormal surface morphology, the dark spots, might originate from the GO because the morphology was only observed on the GO-TFC membrane and the wrinkles in the morphology (Fig. S1b") were very similar to those of graphene. Because the size of the abnormal surface morphology was more than approximately ten micrometers, it was considered that large-size GO (> 10 μm) caused the abnormal surface morphology. Then, the ridge formation might be inhibited because large-size GO (> 10 μm) could block the diffusion of m-phenylenediamine (MPD) during interfacial polymerization. Therefore, GO was fractionated using a 5-μm track-etch membrane. As a result, the surface of the GO-TFC membrane prepared with fractionated GO recovers a homogeneous and bright surface morphology with ridge and valley structures (Fig. S1c); however, a few micro-scale dark spots were still observed (Fig. S1c' and c"). Furthermore, the water flux of the membrane was improved because of the increasing surface roughness.
Characterization of GO before and after fractionation
The fractionated and non-fractionated GO (500 ppm each) were evaluated in terms of the size distribution and zeta potential. The size distributions were measured using a laser diffraction particle size analyzer (LS 13 320, Beckman Coulter, USA). The numbers of GO at various size ranges were counted over the range of 4–120 μm using a particle counter (Multisizer 4, Beckman Coulter, USA) to observe the trace amounts of large-size GO. The change in the zeta potential of GO with fractionation was measured using a zeta-potential and particle-size analyzer (ELS-Z, Otsuka Electronics, Japan).
GO was fractionated with a track etch membrane with a pore size of 5 μm to eliminate large-size GO (> 10 μm). After fractionation, the fraction of smaller GO increased (the size distribution of GO shifted to the left), and the average size of GO was reduced from 1.0 μm to 0.4 μm (Fig. S2a). Moreover, the total number of large-size GO was reduced by approximately 74 % (Fig. S2b). In addition, the discrepancy in the particle size results between SPM and particle analyzer might originate from the difference not only in the measurement mechanism but also in the number of measurements. Even though GO was fractionated with a 5-μm filter, undesired rejection of GO smaller or passage of GO larger than 5 μm was unavoidable. The inhibition to the passage of small-size GO might be attributed to the secondary dynamic membrane layer [S3] formed by large-size GO. In addition, GO with lengths greater than 5 μm could pass through the membrane because of its planer and elliptical shape. Note that the zeta potential of GO also changed from -33.8 mV to -45.5 mV after fractionation. In other words, GO has a more negative zeta potential as its average size decreases, which could be caused by increasing the “edge” of GO to which numerous negatively charged functional groups are attached (Fig. 1). The augmentation of the negative zeta potential is considered to result in two advantages: (1) the surface zeta potential of the GO-TFC membrane became more negative with the same GO dosage and (2) the dispersion stability of GO in water increased [S4], resulting in a more homogeneous dispersion of GO in the PA layer.
Fig. S2. (a) Size distributions in volume fraction and (b) number concentration at each given size of fractionated and non-fractionated GO.
Oxygen weight fraction of TFC and GO-TFC membranes measured by energy dispersive spectroscopy.
The oxygen weight fractions of the 0, 15, 38 and 76-GO-TFC membranes were measured using an energy dispersive spectroscope (EDS, Xflash Detector 4010, Bruker, USA). The EDS measurements were conducted using a field-emission scanning electron microscope (FE-SEM, SUPRA 55VP, Carl Zeiss, Germany) after coating the membrane surface with platinum using a platinum sputter coater (SCD 005, BAL-TEC, Germany). The FE-SEM was operated at an acceleration voltage of 15.0 kV under a magnification of 3000x. Fig. S3 shows that the oxygen weight fraction increased with the addition of GO. The enhancement of the oxygen fraction could originate from GO, which contained oxygen functional groups.
Fig. S3. Oxygen weight fraction of the 0, 15, 38, and 76-GO-TFC membranes measured by EDS.
Measurement method for PA layer thickness
Cross-sectional SEM image analysis was one of the methods used for the measurement of the thickness of the PA layer (Fig. S4) [S5,S6].
Fig. S4. (a, b) Cross-sectional SEM images for measuring the thickness of the PA layer [S5,S6].
Fig. S5. (a) Cross-sectional SEM image, (b) PA layer part of the SEM image, and (c) silhouetted PA layer part.
If the PA layer has a high roughness (non-uniform thickness), the thickness of the PA layer is not the same as the height of the ridges. Therefore, the thickness of the PA layer in the cross-sectional SEM images was evaluated using a new method. Using the tilting and rotating option, the exact cross-sectional view of the horizontal membrane surface was obtained by SEM (Fig. S5a). That image was cut along the line that seemed to be the base line of the PA layer (Fig. S5b). Then, the area of the PA layer (the red region in Fig. S5c) was estimated using a computer program “Image J.” Lastly, that area was divided by the length of the SEM image, which represented the average thickness of the PA active layer.
Surface morphology of the TFC and GO-TFC membranes measured by SPM
Fig. S6. SPM surface images for the (a) 0, (b) 15, (c) 38, and (d) 76-GO-TFC membranes. The ridge heights and surface roughness were observed to decrease with increasing GO content.
Table S1
The water flux and salt rejection of the 0, 15, 38, and 76-GO-TFC membranes.
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