Water vapor transport properties of interfacially polymerized thin film nanocomposite membranes modified with graphene oxide and GO-TiO2 nanofillers
Graphical abstract
GO and GO-TiO2 incorporation in TFN membrane by interfacial polymerization for excellent water vapor separation performance.
Introduction
Water vapor is a dominant greenhouse gas. In clear skies, the contribution of water vapor to the greenhouse effect is approximately 60%, followed by carbon dioxide (∼25%), ozone (∼8%) and other trace gases [1]. With the amount of carbon dioxide that exists in the earth’s atmosphere, increasing water vapor content (relative humidity) amplifies the heat trapping effect of CO2, thus resulting in a potent impact on global warming. An enormous amount of water vapor is released into the atmosphere by a number of industries including power plants and chemical factories. This gradual moistening of the earth’s troposphere has increased the global warming effect of CO2 by two folds [2]. With the increased use of greener technologies to curb CO2 emissions, there is also a greater need to address the water vapor emissions from man-made sources. In addition to large industrial units such as cooling towers, removal of water vapor is also crucial in applications such as dehumidification and air conditioning of buildings [3].
Currently, water vapor is removed from industrial cooling towers and air dehumidification units by a number of techniques such as desiccant drying [4], gas condensation [5], and membrane technology [6]. Among these, the polymeric membrane technology has been commercially implemented since the 1970′s due to its smaller footprint and relatively simple operation [6]. Separation is achieved using membranes that have a dense top layer and a porous support. Thin film composite (TFC) and nanocomposite (TFN) membranes meet these requirements. These composite membranes are mostly prepared through an interfacial polymerization (IP) reaction of two or more monomers on the surface of a porous polymeric support. Research on TFC membranes for gas separation surged after Cadotte et al. [7] proposed interfacial polymerization (IP) as a feasible method to prepare a selective polyamide layer [7]. The polyamide layer possesses desirable characteristics such as mechanical strength, chemical resistance, and thermal stability coupled with high selectivity which is essential in gas separation [8]. The major aim in gas separation membranes is to overcome the upper limit of permeance-selectivity trade-off curve, commonly known as the ‘Robeson Plot’. With the optimum reaction conditions already known for TFC membranes, [9], [10], [11] it is becoming increasingly difficult to overcome this upper bound.
A new path towards developing the next generation of composite membranes was introduced when Jeong et al. successfully demonstrated that adding Zeolite A nanoparticles in the polyamide matrix during the IP reaction can give highly permselective thin film nanocomposite membranes for reverse osmosis application [12]. Nanoparticles inclusion in the polyamide layer makes it much easier to control its properties because of the high surface area and surface energy of nanoparticles [13]. Highly permeable and selective gas separation membranes can be produced by simply tuning the type of nanoparticles, their size, and surface chemical properties.
Wide variety of nanomaterials have been used as nano-fillers for gas separation membranes. A filler material shall ideally have a small size (<100 nm) because most polyamide layers are of hundred nanometers thick. Furthermore, it should also be compatible with the polyamide matrix by forming covalent bonds. Recently, Gao et al. synthesized polyethyleneimine grafted ZIF-8 particles for CO2 separation [14]. The modified porous ZIF-8 particles were found to have better compatibility with the polymer matrix while maintaining high permselectivity. Shamsabadi et al. proposed TiO2 nanoparticles as nano-fillers for CO2/N2 separation membranes [15]. Similarly, other nanomaterials such as carbon nanofibers and nanotubes [16], [17], silica [18], and metal organic frameworks [19], [20] have been used as filler materials for gas separation membranes because of their tunable properties and pore structures. Recently, researchers have shown that incorporating graphene oxide in the polyamide layer for CO2 separation results in an acceptable permeance and selectivity trade-off [21], [22]. Apart from above mentioned applications, membranes are widely used in solar cells and batteries [23], [24]. Applying phase inversion method Li et al. prepared porous membranes using polyetherimide with tunable morphology and for lithium-ion batteries (LIBs) [25]. Recently researchers have also developed different eggshell membranes for lithium-ion battery along with membranes for vanadium redox flow battery applications [26], [27].
However, the use of nanoparticles in TFN membranes is mainly limited to conventional gas separation applications such as CO2/N2, CO2/CH4 etc. In our previous work, we have demonstrated that functionalized silicon nanoparticles (15–20 nm) are resistant to agglomeration at low concentrations and exhibit high water vapor permeance and selectivity [28]. Similarly, in previous research, used TiO2 nanoparticles and acid activated bentonite clay to fabricate highly hydrophilic nanocomposite membranes for improved water vapor permeance [29], [30]. Use of new nanomaterials for gas separation applications is essential in order to overcome the trade-off between permeance and selectivity.
In this work, we have improved the water vapor permeance and selectivity of TFN membranes by utilizing two types of nano-fillers. We have incorporated graphene oxide (GO) and Graphene oxide-TiO2 (GT) as a filler material in the polyamide selective layer. GO has gained wide spread attention because of its abundant hydroxyl and carboxyl groups that form hydrogen networks with water molecules [31]. Furthermore, graphene oxide has a better dispersion in water and polar solvents which makes it easier to be used in IP reaction [32]. GO based TFN membranes have already been shown to increase the water permeability for nanofiltration applications [33], [34], [35]. In addition, when coupled with TiO2 nanoparticles, GO-TiO2 composites have further increased the membrane separation performance [36]. The synergistic effects of using two nanomaterials at once can lead to an improved water vapor separation. Herein, we examine how GO and GO-TiO2 affects the polyamide layer and its bonding chemistry. The aim is to increase the water vapor permeance and selectivity while adding only minimum amount of nano-fillers. It is hoped that the findings of this research will contribute to a deeper understanding of the nanomaterials and their interaction with water vapor.
Section snippets
Materials
Polysulfone ultrafiltration hollow fiber membranes, having an outer diameter of 1400 µm and inner diameter of 1000 µm, were obtained from Guiyang Shidai Huitong Film Technology Co. Ltd. China. The molecular weight cut-off of the purchased membranes was 8000 Da. Graphene oxide dispersion solution in H2O (2 mg/ml, GO), TiO2 nanoparticles (average particle size ∼ 70 nm), m-Phenylenediamine (99%, MPD) and 1,3,5-Benzenetricarbonyl trichloride (98%, TMC) were purchased from Sigma-Aldrich. n-Hexane
ATR-FTIR
FTIR spectra of the pristine Polysulfone ultrafiltration membrane and the thin film nanocomposite membranes are presented in Fig. 1. Typical infrared spectra showing the characteristic peaks of Polysulfone (PSf) were obtained in accordance with the reported literature [41]. The peaks at 1580 cm−1 and 1486 cm−1 are assigned to the aromatic CC stretching of the sulfone group, 1323 cm−1 and 1293 cm−1 are characteristic peaks of asymmetric SO2 stretching vibration, the peak at 1250 cm−1 can be
Conclusions
Thin film nanocomposite membranes were prepared using graphene oxide (GO) and graphene oxide-TiO2 (GT) composites as filler materials for the polyamide layer. The nano-fillers were added to the aqueous phase monomer in different concentrations. The interfacial polymerization reaction with the organic phase monomer resulted in a dense polyamide nanocomposite membrane on a porous PSf hollow fiber support. It was found that GO and GT are bonded to the polyamide layer via hydrogen bonds. This
Declaration of Competing Interest
There is no conflict of interest to declare.
Acknowledgements
This work was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CRC-15-07-KIER). PGI would like to acknowledge Council of Scientific and Industrial Research (CSIR), New Delhi in-house Project No. OLP-2021.
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