Research - (2021) Volume 10, Issue 5
The development of nanocomposite membranes for desalination and wastewater treatment has attracted significant interest in recent years. This exceptional growth is owed to the enhanced multi functionalities of the nanocomposite membranes in terms of permselectivity, thermal stability, electrical conductivity, and antifouling propensities. In the present work, we fabricated novel thin film nanocomposite (TFN) polyamide membranes using indium tin oxide (ITO) nanoparticles. The thin film composite (TFC) membranes were synthesized by interfacial polymerization (IP) reaction using m-phenylenediamine (MPD)-aqueous solution and trimesoyl chloride (TMC)-ITO heptane solution. The addition of ITO nanoparticles to the polyamide layer significantly improved the water flux from 25.3 LMH for the pristine TFC membrane to 43.5 LMH for the ITO-modified TFN membrane with a slight reduction in salt rejection from 98% to 97% during the treatment of saline water (2000 ppm NaCl). Also, TFN membranes demonstrated superior antifouling propensity than the pristine and commercial membranes during filtration of oil sands produced water to remove organic matter. The improved fouling resistance of TFN membranes was mainly attributed to their enhanced surface wettability as the contact angle decreased from ~82° for the pristine TFC to ~65° for TFN membranes.
Thin film nanocomposite membranes , Reverse osmosis , Polyamide , Indium tin oxide nanoparticles , Antifouling membranes
Clean water has become a vital global commodity for many municipal, agricultural, and industrial applications due to world-wide climate change, exponential population growth, and rapid industrialization over the last decade [1,2]. It is projected that about 50% of the world’s population will experience water scarcity in their living area by 2025 . Furthermore, the current large dependency on conventional energy resources has exacerbated this problem as most of the energy based industries (e.g., oil and gas) require water for their activities. The current inefficient water treatment techniques enhance demands on finite freshwater resources, pushing the limits of environmental sustainability. Hence, to relieve the water shortage stress, global efforts have been accelerated to develop new technologies to produce clean water from contaminated resources . Membrane based separation processes, especially reverse osmosis (RO), are proved as attractive techniques for water and wastewater treatment due to their distinct advantages over conventional methods, e.g., higher quality of permeate and smaller footprint . The most broadly utilized RO membranes are thin film composite (TFC) polyamide membranes . TFC membranes consist of a thin film of a polyamide film (~100-200 nm) on top of a microporous substrate . Although significant advancement has been made in the development of high performance TFC membranes, there is still high interest in the fabrication of innovative membranes with enhanced permeation properties and antifouling propensity for diverse applications, including industrial and municipal waste treatment and seawater desalination [8–12]. This demand can be met by modifying the TFC membranes mainly by chemical grafting of functionalized macromolecules to the surface and integrating multifunctional nanofillers into the active polyamide layer. Nanomaterials offer several advantages like easier scale up and environmental friendliness due to the stronger entrapment of nanomaterials within the polymer matrix compared to the anchored functional materials via chemical grafting to the membrane surface. In addition, nanomaterials can substantially alter the surface and bulk physicochemical properties of the composite polymers owing to their enhanced area to volume ratio [13,14]. The first report on the modification of polymer properties with nanomaterials is a study by Paul and Kemp in 1973. They added zeolite nanofillers into polydimethylsiloxane (PDMS) polymer to improve its permeation performance for the gas separation application . Later, the zeolite modified membranes were tested for water treatment by Jeong et al. in 2007. They synthesized the first polyamide thin film nanocomposites (TFN) by integrating zeolite A with the polyamide layer of a conventional TFC membrane . Jeong et al. reported a substantial enhancement in water flux, with almost the same salt removal percentage as the base TFC membrane. This modification approach has become a focus for the development of novel TFN membranes with enhanced permselectivity, thermomechanical durability, electrical conductivity, and antibacterial propensity [17–20]. These functionalities have been established by the incorporation of a large family of nanomaterials, including carbon based nanofillers (such as carbon nanotubes and graphene derivatives), metal and metal oxides (such as silver and alumina), and minerals (such as silicon oxide and zeolite) [21–25]. Despite the vast number of published reports, this stream of research and development is advancing with high momentum to explore novel nano-enabled membranes for water treatment and gas separation [26–28].
Our objective in the present work is to investigate the impact of incorporating indium tin oxide (ITO) NPs into the selective polyamide film of TFC membranes on the permeation and antifouling performance of the resulting TFN membranes. Based on an earlier report by Khorshidi et al. , the bulk integration of ITO to a polyethersulfone membrane via phase inversion method has led to the formation of nanocomposite membrane with enhanced water permeability, electrical conductivity, thermal stability, and antifouling propensity. However, the additional benefit of integrating ITO NPs within the active polyamide layer needed further investigation. To overcome the challenge of nanoparticle aggregation, we prepared a monodisperse suspension of nano-sized ITO particles in an organic solvent (heptane) through a systematic study of eight dispersion parameters . The stable ITO suspension was added to the organic monomer solution and used to synthesize polyamide film via interfacial polymerization reaction . Various surface and bulk characterization techniques were used to evaluate the structural morphology, permeability, and physicochemical characteristics of the membranes. The fouling resistance of the membranes was also assessed by the filtration of process-affected water generated in Canada’s oil sands industry.
InCl3 and SnCl4 were purchased from Strem Chemicals Inc. (MA, USA). BYK-106 dispersing chemical was purchased from ALTANA (Wesel, Germany). Polyethersulfone (PES) microfiltration membrane was obtained from Sterlitech Co. (WA, USA). 1,3-Benzenediamine (MPD), 1,3,5-Benzenetricarbonyl trichloride (TMC), ammonium hydroxide (NH4OH), triethylamine (TEA), camphorsulfonic acid (CSA), and sodium dodecyl sulfate (SDS) were all supplied from Sigma Aldrich (USA). Fouling tests were conducted using the produced water of steam assisted gravity drainage (SAGD) operation, a common bitumen recovery method in Alberta, Canada. In the SAGD process, steam is pumped to the production well to lower heavy oil viscosity and facilitate the recovery process. To recycle the process affected water and minimize freshwater consumption, the produced water is treated by a series of chemical treatment methods, including warm lime softener (WLS) and ion exchange (IX) resins [31–34]. Table 1 presents the main properties of the WLS inlet water.
|Silica, dissolved (ppm)||90-130|
Synthesis of ITO NPs
The ITO NPs were prepared following the procedure outlined in our previous study . In summary, InCl3 and SnCl4, with a molar ratio of ten to one for indium and tin, were first reacted in an aqueous solution containing NH4OH to produce a mixture of SnO2-In (OH)3 and NH4Cl. After several centrifuging/ washing cycles (for eliminating the NH4Cl), the resulting white SnO2-In (OH)3 compound (Figure 1a) was treated at 700°C for crystallization and removal of excess water, where yellow ITO powder was generated (Figure 1b). Finally, the yellow crystals were hydrogenated in a tube furnace at 350 °C to yield blue conductive ITO NPs (Figure 1c). Figure 1d shows the X-ray powder diffraction (XRD) profile and high resolution scanning electron microscopy (SEM) image of the synthesized ITO NPs. The size of the NPs was obtained to be 15±0.3 nm using the Debye Scherer equation [ 29].
Figure 1. (a) White SnO2-In(OH)3 compound synthesized with the reaction of InCl3 and SnCl4 in the presence of NH4OH, followed by centrifuging and washing of reaction product; (b) Synthesized yellow ITO crystals after crystallization of SnO2-In(OH)3 compound at 700 Â°C; (c) Conductive blue ITO crystals after hydrogenation with gas streams of 10%/90% H2/Ar; (d) XRD and FESEM of the ITO NPs.
Fabrication of TFC and TFN membrane
The composite membranes were fabricated by interfacial polymerization (IP) reaction using an aqueous MPD solution and an organic-TMC solution, as shown schematically in Figure 2. First, the PES support was mounted onto a plexiglass frame, and then an aqueous solution containing MPD (2 wt.%), TEA (1 wt.%), CSA (2 wt.%), and SDS (0.2 wt%) was poured on the surface of PES support. After a soaking time of 8 minutes, the excess aqueous solution was removed. Then, 0.2 wt.% TMC solution was poured over the MPD-impregnated PES support to allow the polymerization reaction to proceed for 30 seconds. Finally, to remove the residual organic solvent, the membrane was dried for 4 minutes in an air circulated oven at 60 ºC. The fabrication procedure of the TFN membranes is shown in Figure 3. First, a mother NP suspension was prepared by dispersing 0.6 g ITO, and 150 μl BYK106 in 30 ml heptane solution based on an optimal synthesis step, reported in our previous work . Then, two different volumes of 1 ml and 2 ml from the mother suspension were added to the TMC-heptane solution and agitated for 4 minutes in a bath sonicator. The prepared ITO- TMC solution was employed in IP reaction to prepare ITO-modified TFN membranes. The TFN membranes were labeled as TFN1 and TFN2, which were synthesized using the dose of 1 ml and 2 ml of the ITO NP suspension in the TMC solution, respectively.
Characterization of membranes physicochemical properties
The surface morphology of the membranes was analyzed using field emission scanning electron microscopy (FESEM, Zeiss Sigma, Germany). The cross section of the membranes was observed using transmission electron microscopy (TEM, Philips/FEI Morgagni, The Netherlands). The chemical composition of the membranes was analyzed using energy dispersive X-ray (EDX) spectroscopy (Bruker, Billerica, Massachusetts, USA). The surface wettability of the membranes was characterized using a drop shape analyzer (Krüss, Hamburg, Germany). A stripe of membrane sample was placed on a microscope slide, and a drop of DI water was placed on the active (polyamide) side of the membrane. The drop was allowed to rest for 30 seconds to ensure the equilibrium contact angle was reached. The test was repeated four times at different locations of the sample, and the average data was reported. The zeta potential of the membranes was measured by an electrokinetic analyzer (SurpassTM 3, Anton Parr, Graz, Austria). Thermal gravimetric analysis (TGA) was conducted on the fabricated membranes to measure their thermal stability. TGA measures the change in weight of the sample in relation to a changing temperature. 10 mg of the membrane sample was placed in the sample holder of the TGA Q50 (TA Instrument, Newcastle, Delaware, USA). The temperature was then increased to 700°C with a rate of 10°C/min, and the change in weight of the samples was recorded. Based on the literature, the decomposition temperature of the TFN membrane was defined as the temperature after a 3% weight loss .
Characterization of permeation performance of the membranes
The synthesized membranes were tested in RO crossflow filtration setup (Sterlitech Co, WA, USA). The performance of membranes was evaluated at steady state with a transmembrane pressure of 220 psi, and a feed flow rate of 1 Lmin-1 at 25 °C. Equation 1 was used to calculate the water flux (Jw).
where Δm is the mass of the permeate water over the measurement time (Δt), divided by the effective surface area of the membrane (Am). The salt rejection percentage (R) was obtained using Equation 2.
The salt concentrations of permeate water (CP) and the feed solution (Cf) were measured using a conductometer (Accumet AR50, Fisher Scientific).
Characterization of the antifouling properties of the membranes
Fouling tests were conducted using the RO filtration setup. WLS feed water was filtered for 360 min at different transmembrane pressures to obtain a similar initial flux (25 LMH) for all membranes. The total organic carbon (TOC) and the total dissolved solids (TDS) of the permeate (Cp) were measured at the end of filtration. The rejection percentage of TOC and TDS was measured using Equation 2, and the TOC and TDS of feed solution (Cf) were presented in Table 1. Also, three commercial membranes, namely Hydranautics NF ESNA, RO ESPA, and FilmTec NF270, were used for benchmarking our synthesized membranes. The properties of these membranes are provided in detail elsewhere .
In the present study, we integrated ITO NPs into the polyamide layer of a TFC membrane to enhance its permeation and antifouling properties. The synthesized ITO-TFN membranes demonstrated superior water flux compared to the pristine TFC membrane without a significant reduction in salt removal. Furthermore, the ITO-modified TFN membranes demonstrated enhanced fouling resistance owing primarily to their improved surface wettability toward water compared to base TFC membrane when tested with WLS feed water of the SAGD process. The present study presents the promising application of ITO NPs to develop novel energy efficient nanocomposite membranes with enhanced permeation and antifouling performance. It is suggested as a continuation of this study to evaluate the impact of higher concentrations of ITO NPs on the permselectivity, high temperature resistance, and electrical conductivity of the ITO-TFN membranes. This objective needs appropriate tuning of the interfacial polymerization reaction in terms of the concentrations of ITO NPs, the reacting monomers, and the chemical additives to fabricate robust and defect free TFN membranes.
The authors gratefully appreciate the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Suncor Energy, Devon Energy, and ConocoPhillips.
B.K. and Z.A conducted the experiments, analyzed and validated the data, and wrote the original draft. G.M. synthesized and characterized the ITO nanoparticles. M.S. reviewed and edited the manuscript and played an advisory role. All authors contributed to the writing of the manuscript.
The authors declare no competing interests.
All data generated or analyzed during this study are included in this published article.