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Evaluation of Humic Acid and Tannic Acid Fouling in Graphene Oxide-coated Ultrafiltration Membranes Kyoung Hoon Chu, Yi Huang, Miao Yu, Namguk Her, Joseph Raymond V. Flora, Chang Min Park, Suhan Kim, Jaeweon Cho, and Yeomin Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08020 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016
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ACS Applied Materials & Interfaces
Evaluation of Humic Acid and Tannic Acid Fouling in Graphene Oxide-coated Ultrafiltration Membranes
Kyoung Hoon Chu†, Yi Huang‡, Miao Yu‡, Namguk Her¶, Joseph R.V. Flora†, Chang Min Park†, Suhan Kim§, Jaeweon Cho₸, and Yeomin Yoon†,*
†
Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC 29208, USA ‡
Department of Chemical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA
¶
Department of Civil and Environmental Engineering, Korea Army Academy at Young-Cheon, 495 Hogook-ro, Kokyungmeon, Young-Cheon, Gyeongbuk 38900, South Korea
§
Department of Civil Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 608-737, Republic of Korea ₸
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan 689-798, South Korea *
Corresponding author: phone: +1-803-777-8952; fax: +1-803-777-0670; e-mail:
[email protected] (Y. Yoon)
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ABSTRACT Three commercially available ultrafiltration (UF) membranes (polyethersulfone, PES) that have nominal molecular weight cut-offs (5, 10, and 30 kDa) were coated with graphene oxide (GO) nanosheets.
Field-emission
scanning
electron
microscopy,
Fourier-transform
infrared
spectroscopy, confocal laser scanning microscopy, water contact angle measurements, and X-ray photoelectron spectroscopy, were employed to determine the changed physicochemical properties of the membranes after GO coating. The water permeability and single-solute rejection of GO-coated (GOC) membranes for humic acid (HA) molecules were significantly higher by approximately 15% and 55%, respectively, compared to those of pristine UF membranes. However, the GOc membranes for single-solute tannic acid (TA) rejection showed similar trends of higher flux decline versus pristine PES membranes, because the relatively smaller TA molecules were readily adsorbed onto the membrane pores. When the mixed-solute of HA and TA rejection tests were performed, in particular, the adsorbed small TA molecules resulted in irreversible membrane fouling due to cake formation and membrane pore blocking on the membrane surface for the HA molecules. Although both membranes showed significantly higher flux declines for small molecules rejection, the GOc membranes showed better performance than the pristine UF membranes in terms of the rejection of various mixed-solute molecules, due to higher membrane recovery and antifouling capabilities.
KEYWORDS: membrane surface modification, graphene oxide coating, antifouling, humic acid and tannic acid, surface water treatment
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ACS Applied Materials & Interfaces
1. INTRODUCTION Membrane technology in the water and wastewater treatment areas has increasingly grown due to public demands for purer water quality and stricter regulations. Based on this technology, lowpressure membrane processes with ultrafiltration (UF) are commonly used in the drinking water treatment. However, membrane fouling due to both
irreversible and reversible deposition of
retained various solutes is one of major restrictive factors in membrane filtration processes.1, 2 Natural organic matter (NOM) has been commonly identified as a key solute in membrane fouling during UF membrane filtration in water treatment, which also leads to the cake layer formation on the membrane surface and/or the pore blocking in the membrane pores.3, 4 Various studies have investigated reducing membrane fouling associated with NOMs, which include the feed water pre-treatment, membrane cleaning methods, and membrane surface modification.5-9 Recently, the modification of the membrane surface has attracted great interest in the surface water treatment fields, because the hydrophobic properties of membranes readily induce NOMs to deposit during treatment. Accordingly, increasing membrane hydrophilicity, leading to strongly negatively charged membrane surfaces, has used to reduce the accumulated foulant influence and cake formation that cause membrane reversible fouling.2,
10
To improve
permselectivity, hydrophilicity, and membrane fouling resistance, various studies have reported that the membranes were blended and/or coated with nano-sized inorganic nanoparticles such as TiO2, carbon nanotubes (CNTs), and graphene oxides (GOs).11-20 Additionally, several other studies were performed to synthesize various nano-sized carbon materials with different polymers to enhance their properties.21-30 However, there are no or not much oxygen functional groups on the surface of TiO2 or oxidized CNTs without inventive techniques, resulting in the unsatisfied hydrophilicity, permeability, and specific surface area. Thus, the oxygen-rich
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functional groups of nano-sized carbon materials are predictable to gain the relatively high hydrophilic membrane as well as enhanced pure water flux, antifouling, solute rejection, and mechanical performance capability. GO nanosheets with the oxygen-rich functional groups, which have great chemical stability, exceptional transport properties, and excellent mechanical stiffness and strength,18, 31,
32
have
appeared as a good material for enhanced performance membrane operations in the treatment of surface water. These GO nanosheets can also form stable dispersions in water due to their stackable single-atom layer nature, strong hydrogen bonding structures, and extreme hydrophilicity. According to these properties, drop-casting, spin-coating and filtration methods have been used to fabricate flat GO membranes.12-14, 16-18, 32 Moreover, the functional groups in the GO structure lead to gaps (approximately 1-nm) with nanochannels of GO sheets33 for the sieving mechanism of membranes. During the performance of surface water treatment, water is allowed to pass through the membrane nanochannels while larger solutes of NOMs are rejected. However, NOMs represent a complex mixture of biomolecular solutes, composed of humic acid (HA) substances and non-HA materials that cause interactions between intermolecular and intramolecular solutes or between these solutes and the membrane during water treatment. These interactions, by hydrophobic attraction, van der Waals force, and hydrogen bonds, and, in membrane filtration can result in incomplete separation of solutes along with their external structural characteristics (e.g., surface functionalities, shape, and size), because small molecular solutes are possibly to pass through the membrane pores than larger molecular solutes.34 Moreover, adsorption of these small molecular solutes onto the membrane pores can cause irreversible membrane fouling, which can lead to the membrane pore blocking and/or gel/cake layer formation of larger molecular solutes.
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However, no performance evaluation as to which molecular sizes of NOM may possibly cause reversible and irreversible membrane fouling during the filtration of GO-coated (GOc) membrane has yet been reported. In addition, one interesting study reported that GOc membranes delayed the permeation of methanol and ethanol molecules, whereas water molecules could pass through readily due to the hydrophilic characteristics of the material.35 To date, it is somewhat uncertain if the nanochannels within a GOc membrane can provide a sufficient pore size for the removal of NOM materials with different hydrophobicities. Thus, in this study, we deposited GO layers on the surface of three commercially available polyethersulfone (PES) UF membranes (nominal molecular weight cut-offs (MWCOs; 5, 10, and 30 kDa) through vacuum filtration based on our preliminary study.36 As a first attempt to answer the questions above, we characterized the physicochemical properties of the pristine and GOc UF membranes using Xray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared (FTIR) spectroscopy, water contact angle measurements, and confocal laser scanning microscopy (CLSM). Then, we confirmed the changes in pure water flux and MWCOs for the resulting membranes after GO coating. In addition, tests were conducted with pristine and GOc membranes to evaluate the effects of the NOM fouling with different molecular sizes and hydrophobicities using both HA and tannic acid (TA) substances as single solutes. Lastly, the rejection performance of mixed solutes was compared for TA and HA materials with various ratios and foulants causing reversible and irreversible fouling for the pristine and GOc membranes.
2. EXPERIMENTAL SECTION
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2.1. Membrane Fabrication. Commercially available flat-sheet PES UF membranes (Koch Membrane Systems Inc., Wilmington, MA, USA) were used as support membranes having different nominal MWCOs of 5, 10, and 30 kDa for GO coating. To use as raw material for GO deposition, single-layered GO powders prepared with a modified Hummer method37 were purchased from Cheap Tubes, Inc. (Brattleboro, VT, USA). The support PES membranes with 5, 10, and 30 kDa MWCOs were modified by depositing/coating a 10-nm-thick GO multilayer onto the surface via simple vacuum filtration based on phase inversion method. Figure 1 illustrates the general membrane synthesis procedure, while details have been described in in our preliminary study.36 In briefly, functionalized GOs were sonicated in deionize (DI) as a solvent for good dispersion. After dispersing GOs, large aggregates were removed by centrifugation for the complete and homogeneous dissolution. The subsequent GO dispersion was used as a stock GO solution. In order to control the GOc membrane thickness, the stock GO dispersion solution’s concentration and stable GO suspensions with preferred concentrations were determined by the same method described previously.17 GO deposition was finished by the GO suspension filtration with the membrane via the Millipore filtration unit. The coated membrane was dried in a vacuum drier to evaporate the remained solvent for the stabilization of GO deposition, and then the membranes were soaked in DI water until use.
2.2. Membrane Characterization. All membrane samples investigated in this study were dried at room temperature (20oC) to evaluate the characterization of membrane after GO coating. The chemical compositions, surface hydrophilicity, and
functional groups of pristine and GOc
membranes were characterized by various techniques including XPS, water contact angle measurements, and FTIR spectra, respectively. FTIR spectra were obtained on a Nicolet 6700
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FTIR spectrometer (Thermo Scientific, Waltham, USA) with a Smart iTR attenuated total reflectance (ATR) sampling equipment. The surface chemical compositions of all membranes were determined via XPS (Kratos, Axis Ultra DLD, UK) with a monochromated Al Kα X-ray source and hemispherical equipment. The contract angles of water were determined with a VCA Optima system (Optima XE, AST Products, Inc., USA). The water droplet size of 3-5 µL, the optimal volume for producing reproducible contact angle measurements with a nominal error of ± 2.3°, was cropped cautiously onto the pristine or GOc membranes. The surface morphology of all membranes was observed using FE-SEM, and surface roughness was characterized by CLSM. The FE-SEM images (Zeiss, UltraPlus, Germany) were taken at low and high magnifications (2–20 µm) from the of the pristine membrane and GOc membrane surfaces. In order to gain satisfactory images, a low voltage of 2–5 kV, a small working distance of
