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Highly Conductive Ultrafiltration Membrane via Vacuum Filtration Assisted Layer-by-Layer Deposition of Functionalized Carbon Nanotubes Farah Rahman Omi, Mahbuboor Rahman Choudhury, Nawrin Anwar, Ahmed Refaat Bakr, and Md. Saifur Rahaman Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017

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Industrial & Engineering Chemistry Research

Highly Conductive Ultrafiltration Membrane via Vacuum Filtration Assisted Layer-byLayer Deposition of Functionalized Carbon Nanotubes Author(s): Farah R. Omia, Mahbuboor R. Choudhurya, Nawrin Anwara, Ahmed R. Bakra, Md. Saifur Rahamana* a

Department of Building Civil and Environmental Engineering, Concordia University, 1455 de

Maisonneuve Blvd. West, Montreal H3G 1M8, Quebec, Canada *

Corresponding Author: Tel: +1-514-848-2424 ext 5058. E-mail: [email protected]

Abstract: Conductive membranes can offer innovative solutions for membrane-fouling control while maintaining enhanced filtration performance. Here, an emerging technique, vacuum filtration assisted layer-by-layer deposition of functionalized multi-walled carbon nanotubes (MWNTs), was used to prepare conductive surfaces on polysulfone (PSf) ultrafiltration membranes. PSf membranes were functionalized with oxygen-containing negatively charged functional groups, through oxygen plasma treatment. MWNT-PSf membranes were prepared with 5, 10, 15, and 20 bilayers with amine- and carboxylic-functionalized MWNTs. The prepared membranes were characterized by the thickness, contact angle, and conductivity of membranes. SEM images of the membranes confirmed uniform MWNT distribution across the membrane surface. MWNT-PSf membranes exhibited slightly reduced permeability, improved selectivity and greater conductivity with increasing number of MWNT bilayers and demonstrated almost complete inactivation of E. coli at low applied DC potential (1-3V). Furthermore, significant (around 99%) degradation of methyl orange during electrofiltration was observed, supporting an expected reduction in organic fouling of the membrane.

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1. INTRODUCTION Membrane biofouling, caused by the deposition of organic matter or bio-film formation on the surface of a membrane, is one of the major limiting factors for widespread adoption of membrane filtration 1. Biofouling has become a major challenge for the membrane industry as it both hinders the filtration rate and makes water flux recovery very difficult. Biofouling is a major weakness for any kind of membrane as it affects the permeability, increases energy consumption and therefore limits performance reproducibility. There are two types of foulant attachment: reversible fouling (weak attachment, removable with strong shear force or backwashing) and irreversible fouling (strong attachment, not removable with physical cleaning). Biofouling, or organic biofouling, is an irreversible process that requires a cleaning step for continuous use; so there remains a tradeoff between the membrane fouling and the membrane performance. Polysulfone (PSf) membranes are the most commonly used membranes in ultrafiltration due to their chemical and mechanical robustness, however they are prone to fouling. As ultrafiltration membranes are low-pressure membranes and fouling causes flux declination, focus should be given to new ideas for the prevention or control of fouling during operation. Inactivation of microorganisms can prevent biofouling by reducing the strength of microorganismal attachment, thus discouraging the formation of colonies. Researchers have studied the effect of incorporating biocidal nanoparticles, such as titanium dioxide (TiO2) 2, zinc oxide (ZnO) 3, silver nanoparticles (Ag) 4, magnesium hydroxide Mg(OH)2 nanoparticles

6–8

5

and carbon based

, into the membrane matrix and have seen promising antifouling properties.

Carbon based nanoparticles include single-walled carbon nanotubes (SWNT), multi-walled carbon nanotubes (MWNTs), and graphene oxide (GO) at different sizes, as well as differing percentages of carbon content

9–11

. Most metal oxide nanoparticles (e.g. silver oxide, copper

oxide, titanium oxide etc.), which exhibit biocidal properties usually dissociate into ions during the oxidation process or biocidal activity, thus the available quantity decreases over time. On the other hand, carbon nanotubes (CNTs) don’t dissociate and thus keep the loaded quantity available for electrooxidation. Therefore, while other nanomaterials lose their specific capacity during biocidal activity, carbon nanomaterials provide electrical conductivity with enhanced charge transport, not compromising specific capacity 12.


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Where the application of low DC potential would result in inactivation of microorganisms and degradation of organic matters, conductive membranes can also control fouling. Conductive nanocomposite materials have been widely applied in fuel cells and batteries, especially in microbial fuel cells (MFCs)

13–15

, where they are used as electrocatalysts to accelerate the

electrochemical reaction. MWNTs, consisting of multiple one-atom-thick sp2-bonded carbon cylindrical nanostructures, have been closely investigated in several studies as they provide greater surface area, higher electrical conductivity, antimicrobial properties and are relatively inexpensive 16. In one study, Vecitis et al. (2011) deposited MWNTs on polytetrafluoroethylene (PTFE) filters and found that MWNT modified filters exhibited greater than 75% bacterial and 99.6% viral inactivation at low DC potentials (2V and 3V) 16. In a follow-up study, Rahaman et al. (2012) reported complete removal (log(5.8) to log(7.4)) and significant inactivation of viral particles when 2V or 3V of potential was applied to an electrochemical MWNT filter

17

. In

another study, Gao et al. (2014) developed a conductive CNT-PVDF membrane that shows high (>99%) reduction of organic nitrobenzene through sequential reduction oxidation processes

18

.

Duan et al. (2016) fabricated a highly conductive and anodically stable composite polyanilinecarbon nanotube ultrafiltration electrically conductive membranes, which showed enhanced electrical conductivity, increased surface hydrophilicity while not impacting its selectivity or permeability, and greatly improved stability under anodic conditions 19. Along with the MWNT deposition method, embedding of MWNTs into the bulk polymer matrix has also been studied and has been shown to cause polymer wrapping of the MWNTs. Such wrapping limits the contact-based interaction, which is more effective in reducing fouling. In order to achieve contact-based interaction between the MWNTs and the foulant, post-treatment of the membrane surface can cause the MWNTs to remain on the membrane surface. Layer-bylayer (LBL) deposition is a post-fabrication technique suitable for a variety of substrates in which films are formed by depositing (dip coating, spin coating, or spray coating) alternating layers of oppositely charged electrolytes with wash steps in between. The LBL method has been widely applied to develop mechanically and chemically strong membranes with surface functionalities 20 due to higher control of layer thickness. There are four types of interactions that hold the polyelectrolyte layers to the substrate membrane: electrostatic interactions hydrophobic interactions

22

, hydrogen bonding

23–25

21

,

, and attraction through Van Der Waal’s



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forces

26

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. LBL self-assembly has been studied with a variety of metal oxides

27–30

, multi-walled

carbon nanotubes 31, graphene oxide 32 and silver nanoparticles 33. A major downside of dip coating is that it is very time-consuming process and the reproducibility and homogeneity of the coatings are influenced by process parameters, with consistency very difficult to maintain 34. On the other hand, spin coating is not suitable for large-scale coating as large substrates cannot be spun at a sufficiently high enough rate to form a uniform layer and the non-uniform distribution of materials along the radial direction becomes worse as dimensions are scaled up

35

. Spray coating techniques require expensive instrumentation to ensure

reproducibility and overcome problems associated in the process

36,37

. Vacuum filtration (VF)

assisted LBL self-assembly can offer quick deposition and requires the least costly instrumentation to develop the film deposition method 35. In this study, VF assisted LBL self-assembly of functionalized MWNTs on PSf membranes was carried out for the development of an electrically conductive surface. The VF assisted LBL deposition approach has been suggested due to its easy application and the simple instrumentation requirements needed to achieve the requisite modification of PSf membranes, thus allowing facile fabrication of highly conductive ultrafiltration membranes. The plasma treated polysulfone membrane was subjected to chemical cross-linking to ensure stable oxygen-containing functional groups. Amine functionalized MWNTs were then deposited on a negatively charged polysulfone surface with the assistance of vacuum filtration, progressing to the deposition of carboxylic functionalized MWNT layer. VF assisted LBL deposition might also induce more MWNTs (in addition to those deposited by electrostatic LBL interactions) to pressure accumulate on the PSf membrane due to forces including hydrophobic interactions and Van Der Waal’s interactions. This may lead to a thicker MWNT layer due to additional accumulation, with electrostatic interaction (as in LBL) existing at the monolayer between the two different MWNTs. The work provided a simple and efficient method of VF assisted LBL self-assembly to prepare a conductive surface on polysulfone membranes, a combination showing greater potential to reduce fouling in electrofiltration processes than polysulfone ultrafiltration membranes alone.


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2. MATERIALS AND METHOD 2.1. Chemical Reagents and Membranes Polysulfone membranes were purchased from SEPRO separation solutions (PS20, 165µm) while triton X 100, N-Ethyl-N′-(3-dimethylaminopropyl) carbodimide hydrochloride (EDC, assay ≥ 98%), MES monohydrate (BioXtra, assay ≥ 99% (T)) and N-Hydroxysuccinimide (NHS, 98%) were purchased from Sigma Aldridge. Reagent grade hydrochloric acid, potassium hydroxide, methyl orange, sodium chloride, and sodium hydroxide were also purchased from Sigma Aldridge. Amine and carboxylic acid-functionalized multi-walled carbon nanotubes (MWNTNH2 and MWNT-COOH) were purchased from Cheap Tubes (99% purity, 8-13 nm diameter and 3-30 µm length). 2.2. Bacterial Cells and Nutrients For antimicrobial tests, Escherichia coli (E. coli, Top10, pGEN-GFP, LVA) was used in the experiments. Agar (Microbiology grade) and Ampicillin, used in the present research, were purchased from Sigma Aldridge. 2.3. Functionalization of the Substrate Membrane The commercial polysulfone membranes were cleaned with a solution prepared by dissolving 0.5% non-ionic surfactant triton X 100 in DI water. The membranes were then air dried with an air knife for 15 min and stored in desiccators prior to plasma treatment. The dry membranes were placed in a plasma chamber (PICO, Diener Electronic, Ebhausen, Germany) connected to an O2 gas cylinder. The membranes were allowed to rest under the O2 gas stream for 10 min in order to eliminate the impurities in the plasma chamber before activating the plasma generator. The flow rate of O2 gas was maintained at 20 sccm (pressure range of 0.8-1.0 mbar) and the power was set to 100W. The membranes were treated for 30s, 60s, 90s, 120s, and 180s to functionalize the surface with negatively charged functional groups (carboxyl, carbonyl, and hydroxyl groups). After plasma treatment, O2 gas was allowed to flow through the plasma chamber for 30 min to avoid any potential reaction between the remaining free radicals and air 38. Plasma treatment is a simple and efficient method for surface functionalization but the stability of these functional groups is subject to aging effects

39

. Therefore, EDC/NHS cross-linking

chemistry was employed to create amine reactive esters that facilitate the reaction between the carboxylic functional groups present on the membrane and the amine-functionalized MWNT.



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This cross-linking chemistry ensures the activation of amine reactive esters on negatively charged functional groups of the plasma treated polysulfone membranes. A recently reported protocol by Perreault et al. was followed in this study for EDC/NHS crosslinking on the membrane surface

32

. The membranes were taped in glass Petri dishes with

waterproof tape leaving the active side exposed to a solution of 4 mM EDC, 10 mM NHS, and 0.5 M NaCl in 10 mM MES buffer (adjusted pH to 5) for 1 hr. The membranes were rinsed twice with DI water to remove excess EDC/NHS solution from the surface and then stored in DI water for further use. 2.4. Solution chemistry of the MWNT-NH2 and MWNT-COOH for vacuum filtration assisted LBL self-assembly The MWNTs (positively charged MWNT-NH2 and negatively charged MWNT-COOH) were dispersed in DI water to prepare a 0.05mg/mL electrolyte solution. The suspensions of MWNTNH2 and MWNT-COOH were probe sonicated for 1h using a Branson 3510 ultrasonic cleaner at 50% amplitude to form a stable dispersion. Figure S1 (supporting information) represents the zeta potential of the MWNT-COOH and MWNT-NH2 solutions at different pH ranges. For this study, the pH of the MWNT-NH2 and MWNT-COOH solutions were adjusted to 2.5 and 3.5, respectively, with the aid of 1M, 100mM and 10mM hydrochloric acid and sodium hydroxide. The pH-adjusted solutions were sonicated for 15 min prior to use in vacuum filtration assisted LBL assembly and an ice bath was used to avoid overheating of the solution. A positively charged MWNT (MWNT-NH2, pH 2.5) solution was filtered through a 47mm diameter functionalized polysulfone membrane using a vacuum filtration assembly. The adsorption of MWNTs initiated electrostatic interaction, which was facilitated by the transport of MWNTs through convection force. The LBL deposition continued with the incorporation of a layer of negatively charged MWNT (MWNT-COOH, pH 3.5) on the layer already containing positively charged MWNT. pH values of 2.5 and 3.5 were chosen for the dispersed solutions of MWNT-NH2 and MWNT-COOH, respectively. These values were chosen from a previous study by Lee et al. (2008)

40

, which observed LBL assembly of all carbon nanotube ultrathin films

(comprising of MWNT-NH2 and MWNT-COOH). The surface charges on MWNTs are not only vital to sustaining stable dispersions of CNTs, but are also critical in achieving LBL assembled films. From the measurements in Figure S1, it can be seen that the zeta potential values of MWNT-NH2 decrease sharply with increased pH beyond pH 6. The zeta potential values below


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pH 6 did not vary much with decreasing pH, and was observed to be between 38-41 mV. In this study, pH 2.5 was selected for the solution of MWNT-NH2 to ensure a sufficient positive surface charge (at pH 2.5 zeta potential ~39.5 mV, close to the observed maximum). On the other hand, the zeta potential of MWNT-COOH decreases with decreasing pH due to protonation of the COOH group on the MWNTs. So a pH 8 for the MWNT-COOH solution would have ensured the greatest difference in zeta potential between the two MWNTs. However, Lee et al. (2008) 40 demonstrated the effect of changing pH of the MWNT-COOH solution on the thickness of ultrathin films formed by LBL assembly of MWNT-NH2 and MWNT-COOH. In this previous study, it was observed that for an MWNT-COOH solution pH of 2.5 the thickness of thin films increased considerably due to the significant charge decrease of carboxylic acid functionalized MWNTs. However for pH values of 3.5 and higher, the thickness variation was not very significant

40

. Hence, a solution pH value of 3.5 was chosen for MWNT-COOH in the present

study. After deposition of each bilayer, consisting of one positively charged MWNT layer and one negatively charged MWNT layer, 5mL of DI water was filtered through the membrane to wash away any residual pH solutions. No washing was performed between depositions of the two layers comprising a bilayer. Hyder et al. (2014)

41

indicated that the rinsing step between

depositions of layers was not necessary for the LBL assembly since the process relies on electrostatic interaction coupled with convective force. Finally, the modified membrane was washed twice with DI water to ensure there was no trace of pH solutions left on the membrane. 2.5 Characterization of the Modified Membrane The zeta potential of the MWNT-NH2 and MWNT-COOH suspensions as a function of the pH was measured with a Zeta potential analyzer (Brookhaven Instruments Corp., US). A scanning electron microscope (JEOL, JSM-7600 TFE, Japan) was used to take images of the membrane’s surface and the membrane in cross-section. Cross-sectional SEM images (Figure S2) were taken by separating the polysulfone polymer layer from the PET support after placing it in liquid nitrogen and breaking it. Attenuated total reflection-Fourier transform infrared spectroscopy (Nicolet 6700 / Smart iTR, Thermo Scientific, US) was used for a qualitative analysis of the functional groups on the plasma-treated PSf membranes where the intensity of the peaks at certain specified wavelengths confirmed presence of certain functional groups. To observe the



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change in percentage of oxygen content after plasma treatments of different durations (i.e., 30s, 60s, 90s and 120s), X-ray photoelectron spectroscopy analysis (XPS, SK-Alpha, Thermo Scientific, US) was also performed on the plasma-treated membranes. Contact angle measurements were taken using a VCA Optima Contact Angle Surface Analysis System (AST Products, Inc., Billerica, MA, USA) to investigate the wettability of the membranes modified with MWNTs and comparing this to that of the pristine and plasma-treated PSf membranes. During contact angle analysis, images were taken in a dynamic mode for a 1µL water droplet on the sample surface and then analyzed by software (AST Products, Inc., Billerica, MA, USA) that examined water droplet shape profile on the sample surface. For each sample, seven consecutive measurements were taken and the average values are reported as representative measurements. The stability of the MWNT layers was assessed by exposing them to harsh physical and chemical stresses

42

. Physical stress was applied to the membrane by immersing it in 10 mL of

DI water and bath sonicating (Branson 8510R-MTH) it for 2 minutes. To amplify the different chemical stresses, samples of the modified membranes were immersed in either an acidic solution of pH 2 (0.01M HCl), a basic solution of pH 12 (0.01M NaOH) or a saline solution of 5M NaCl for 15 minutes with a DI wash step afterwards. The membrane wettability was analyzed by contact angle measurements to evaluate the stability of the MWNT layers on the modified membranes. 2.6 Membrane Performance Evaluation The electrical resistivity of the membranes was analyzed with the Van der Pauw method (4points-4TS, Sigmatone-302, USA) and conductivity was determined using the following equation: =

1

The sheet resistance was measured by a 4-point probe tip that penetrated through the crosssection, which was then multiplied by the thickness of the MWNT layer to obtain the resistivity of the membrane. The water flux through the membrane was measured with an Amicon 10-mL stirred ultrafiltration cell (Amicon 8010, Millipore, Cole Permer, US). A 2.54 cm coupon was cut using a membrane cutter (Power punch maxi set, Spearhead 130) and pre-compacted for 30 minutes


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under a pressure of 30 psi to obtain a steady flow rate. The permeability of the ultrafiltration membrane, Lp was calculated using the following equation 43, =

Where Jv is the volumetric filtrate flux (volume flow rate per membrane area) or hydraulic permeability as the solvent is water and ∆P is the trans-membrane pressure driving force. In order to determine the membrane selectivity (Molecular Weight Cut-off), rejection tests were performed using a polyethylene glycol (PEG, MW 20KD) solution. Briefly, 1g/L (Polymer source, Montreal, Quebec, Canada) PEG solutions were prepared and filtered through the modified membrane via the stirred ultrafiltration cell after pre-compaction for 30 min. The feed and permeate solutions were analyzed for total organic carbon (TOC) using a TOC analyzer (TOC VCPH/CPN, Shimadzu corp., Japan). Rejection was then calculated with the concentration of TOC present in permeate and feed solution using the following equation and expressed in percentages: Rejection %! = 1 −

C$%&'%()% C*%%+

2.7 Organic matter degradation and bacterial inactivation of conductive membranes in an electrofiltration cell To investigate the efficiency of organic matter degradation, methyl orange (14µM) was used as a model organic compound in a background solution of 10 mM NaCl 44. The modified membrane was placed in an electrofiltration setup 16,17 with the active side facing towards a thin perforated stainless steel sheet. The stainless steel sheet served as the cathode and the modified membrane acted as the anode. A brief description of the electrochemical setup (Figure S5) has been provided in the supporting information. DI water was used to flush the tubing and calibrate the flow rate, which was set to 1.5 mL/min after calibration. The influent was filtered through the modified VF assisted LBL membranes (5, 10, 15 and 20 bilayers) consecutively by means of a peristaltic pump (Masterflex L/S 77800-60, US). To operate the electrochemical filtration, the external wires of the electrofiltration unit’s anode and cathode were connected to a DC power supply (Agillent, Germany). The cell was operated at an applied potential of 3V and the effluent was analyzed with an UV-Visible-NIR spectrophotometer (Perkin Elmer, Lambda 750) to determine the concentration of methyl orange



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from the absorption values at a wavelength of 464 nm. UV/Vis (Ultraviolet-visible spectroscopy) method was used to evaluate the changes in concentration of methyl orange before and after electrooxidation followed by filtration. Methyl Orange is a standard organic matter and very suited for electrooxidation experimentation. UV/Vis method has been used commonly and reliably to monitor the changes in concentration of Methyl Orange

45,46

. The drop in

concentration of methyl orange in the effluent was used to calculate the percent removal of methyl orange by the membrane, as compared to the influent concentration. Bacterial inactivation experiments were carried out by following the procedure of Vecitis et al. 16

. The procedure includes several steps starting from an overnight bacterial culture, then

preparation of the E. coli solution for antimicrobial testing and finally preparation of plates for determination of active bacterial cell concentration through plate counting. The detailed procedure (Procedure S4) has been provided in the supporting information.

3. RESULTS AND DISCUSSION 3.1 Optimization of the plasma duration for membrane functionalization 3.1.1 ATR-FTIR analysis of the membranes The chemical changes on the surface of the pristine polysulfone membrane after plasma treatment was revealed by Attenuated Total Reflectance-Fourier Transform Infrared (ATRFTIR) spectroscopy. The pristine polysulfone membrane surface contained different functional groups such as C-C stretch, asymmetric S=O, symmetric S=O, C=C, and C-O-C stretch. After plasma-treatment, some carbonyl stretch C=O and the acid functional group O-H were expected to appear in particular wavenumbers (Table S3). Figure 1 shows the ATR-FTIR spectra for the pristine and plasma-treated polysulfone membranes for different plasma duration. The spectra for the polysulfone membrane clearly depicted peaks at 1100 cm-1 (C-C stretch), 1300 cm-1 (asymmetric S=O stretch), 1232 cm-1 (C-O-C stretch), 1800 cm-1 (C=C stretch) and 1143 cm-1 (symmetric stretch) while the O2 plasma treated membrane showed additional significant peaks for the carbonyl functional group (C=O) at wave frequencies around 1740 cm-1. No polymeric O-H bend was observed on the pristine PSf membrane at the wave frequency of 1540 cm-1 and after plasma treatment, a very small level of O-H bend was observed at the same frequency in the FTIR spectra. In general, the peak broadens with the increase in plasma treatment time. However, some exceptions might be observed due to the polysulfone membrane 10 ACS Paragon Plus Environment

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matrix heterogeneity even though the polysulfone has been reported to be resistant to etching with mass losses of 2 mg cm-2s-1 for a high energy plasma 47.

FIGURE 1: Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra for pristine and plasma treated polysulfone membrane with peak identification

3.1.2 Quantification of the membrane oxygen content, membrane wettability and water permeability for different plasma treatment duration Figure 2 shows variations in (A) percentage of membrane surface oxygen content (%), (B) contact angle (o), and (C) water permeability through the membrane (after 30 minute precompaction) after different plasma treatment time intervals. X-ray photoelectron spectroscopy (XPS) analysis was carried out to measure the percentage of oxygen content as compared to sulfur and carbon. The pristine PSf membrane had an oxygen content of 18.5% while the 30s plasma treated membrane exhibited an oxygen content of about 29.5%. The curve reached an oxygen content plateau after 30s of plasma treatment and continued to increase only in small, gradual increments for the 60s, 90s and 120s plasma treated membrane.



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FIGURE 2: (A) Percentage of membrane surface atomic oxygen content compared to sulfur and carbon as a function of O2 plasma treatment time, analyzed by XPS (B) Water contact angle for different plasma treatment time durations (C) Permeability for different plasma duration after 30 minute precompaction

The water contact angle of the plasma-treated PSf membranes was found to be inversely proportional to the increase in plasma duration. The pristine PSf membrane showed an average contact angle of 68.6º, supporting previously documented observations, with a reduction to 44.2º 12 ACS Paragon Plus Environment

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after being plasma treated for 180s. This decrease in contact angle confirmed the increase of oxygen-containing functional groups on the membrane surface. Oxygen-containing functional groups increased the membrane hydrophilicity, which leads to a gradual decrease in water contact angle (i.e., increased wettability). The water permeability of the membranes increased with greater plasma treatment time, indicating the positive relationship with surface hydrophilicity. It has been reported that plasma treatment improves water permeability of polymeric membranes 42. As the number of functional groups increased after 30s of plasma treatment (Figure 1), the membrane should have been more hydrophilic than the pristine membrane. However, results from present study indicated a drop in water permeability as compared to the pristine membrane after 30s of plasma treatment. This drop in permeability, from pristine membranes, after 30s of plasma treatment was a deviation from the expected behavior. Drying of the membrane during plasma treatment could be responsible for the reduced permeability, as sufficient wetting time was not provided to the membrane after 30s of plasma treatment 48. As a result, the effect of enhanced hydrophilicity was not realized. However, for any further increase in plasma treatment time (e.g., 60s to 180s), the water permeability increased due to increasing pore diameters. The permeability increased from 463 LMH/bar (for pristine PSf membrane) to 966 LMH/bar (for 180s plasma-treated membrane).” 3.1.3 Surface morphology of the PSf membrane for optimized plasma duration Scanning electron microscopy (SEM) was used to monitor the effect of O2 plasma treatment on PSf membrane surface morphology. The SEM images showed an increase in pore size and density with increasing plasma treatment time (Figure 3). Four consecutive images were taken at different positions to visualize the uniformity of the pore size and distribution. The average pore diameter of the pristine PSf membrane was determined to be 26.2nm while the average for the 60s plasma-treated PSf membrane was 38nm. This increase in pore size can be attributed to oxygen absorption in the pores and the creation of new pores through surface oxidization. Based on the analysis, the plasma treatment time for the PSf membrane was optimized to be 60s. Although 30s of plasma treatment showed a significant addition of functional groups on the membrane surface, the decrease in permeability restrained this study from using 30s as the optimal plasma treatment time. Longer plasma duration was avoided as the increased plasma



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intensity on the dense top layer of the PSf membrane could oxidize and therefore, could compromise the overall selectivity of the membrane.

FIGURE 3: SEM images of (A) pristine PSf membrane, (B) pristine PSf membrane with marked pore diameter, (C) 60s plasma-treated PSf membrane and (D) 60s plasma-treated PSf membrane with marked pore diameter

3.2 VF assisted LBL MWNT modified PSf membrane characterization 3.2.1 SEM images of the MWNT modified PSf membranes Figure 4 clearly depicts the CNTs on the VF assisted LBL MWNT modified PSf membrane surface. The modified membrane surface morphology illustrates a uniform distribution of CNTs. It was difficult to differentiate between different functionalized CNTs in the SEM images due to the high aspect ratio of CNTs. 14 ACS Paragon Plus Environment

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Figure 4: SEM images of the MWNT-NH2/MWNT-COOH modified VF assisted LBL selfassembled membrane

3.2.2 Water contact angle and thickness measurement of the VF assisted LBL MWNT modified PSf membranes The contact angle of the modified membranes, as a function of the surface modification and number of bilayers, is presented in Figure 5(A). The 60s plasma-treated PSf membrane, with an observed contact angle of 56.3˚, serves as a control in the comparison, as it was used for the surface modification through VF assisted LBL assembly. The EDC/NHS treatment converted the unstable oxygen-containing functional groups to stable amine reactive ester groups and made the membrane more hydrophilic. After EDC/NHS treatment, the contact angle decreased to 48.1˚, which indicated an improved hydrophilicity as compared to the control, the plasma-treated membrane. The incorporation of CNTs through LBL self-assembly made the membrane surface more hydrophobic due to the higher hydrophobicity of the CNTs as it lacked sufficient polar groups to enhance water affinity. The 5-bilayer MWNT membrane showed a contact angle of 92.8˚, which was almost double the contact angle of the EDC/NHS, modified membrane. The contact angle increased to 102.9˚, 114.3˚, and 116.5˚ with 10, 15, and 20 bilayers, respectively, indicating even higher hydrophobicity. Though hydrophobic membranes are reported to be more 15 ACS Paragon Plus Environment


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prone to fouling 49, the modified membrane in the present study were used in an electrofiltration cell where applied voltage is expected to play a significant role in controlling fouling events.

FIGURE 5: (A) Water contact angle for the VF assisted LBL MWNT modified membranes; (B) Thickness of the VF assisted LBL MWNT modified membrane with increasing number of bilayers It was expected that the incorporation of MWNT bilayers would increase the thickness of the modified membrane and the resulting thickness of the modified membrane has been shown in Figure 5(B). The average thicknesses for 5, 10, 15 and 20 bilayers were 3.02µm, 6.6µm, 10.3µm and 14.3µm, respectively, with an average single layer thickness of 302nm, 330nm, 343.3nm and 357.5nm. The atomic layer thickness of the horizontally aligned CNTs should have been around 10-15nm as the diameter of the nanotubes was 8-13nm. On the other hand, the average length of the CNTs was 3-30µm. The high aspect ratio of the CNTs could have made it very difficult to align them horizontally or even vertically. The nanotubes might have curled during LBL deposition and oriented themselves in a variety of patterns making it nearly impossible to obtain an atomic level layer thickness. Nonetheless, the linearity of the curve in Figure 5(B) suggested that the number of bilayers was proportional to the thickness of the MWNT LBL film.


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The non-atomic layer distribution could also be explained by the fact that vacuum filtration might have forced all MWNTs to deposit on the membrane surface, whereas, for other LBL techniques (dip, spray and spin) the electrostatic force dominates the deposition. Here, the electrostatic interactions, as well as several other interactions (hydrophobic interaction, Van Der Waals attraction, and hydrogen bonding) may have played a principal role in holding the bilayers to the base polysulfone membrane. The escape of excess nanotubes, those which were not bound by electrostatic interaction, could have been prevented by vacuum filtration causing complete deposition of all nanotubes, leading to a thicker layer. 3.2.3 Electrical conductivity of the VF assisted LBL MWNT modified PSf membranes The mean conductivity of the membrane increased (i.e., resistivity decreased) with the increasing number of bilayers (Figure 6). The total amount of MWNTs (per unit cm2) incorporated into 5, 10, 15, and 20 bilayer modified membranes were 0.26mg, 0.52mg, 0.78mg, and 1.04mg, respectively. The increase in mean conductivity indicates an improvement in the MWNT network, which in turn improved the electron transfer capacity. LBL modification for conductive membranes was shown to be the best method for investigating the effect of increased MWNT concentration on electrical conductivity while controlling the thickness of the layers. Oher methods, such as cross-linking, have been reported to encounter difficulties in depositing MWNTs uniformly across the membrane surface 50. The data provided in Figure 6 in regards to the change in conductivity showed non-significant differences beyond 10 bilayers, however, the trend may remain relevant and may only exist in terms of average conductivity. For 10 or less bilayers, the trend of incremental increases in conductivity with increasing number of bilayers holds true.



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FIGURE 6: Electrical conductivity of the VF assisted LBL MWNT modified membrane as a function of number of MWNT bilayer

3.3 VF assisted LBL MWNT modified PSf membrane performance evaluations 3.3.1 Water flux and selectivity of the modified PSf membranes Water permeability and selectivity (solute rejection) are the two most important parameters in determining the performance of the filtration membrane. A trade-off is always noticed between permeability and selectivity; as selectivity increases, permeability decreases and vice versa. Pure water flux of the membranes in respect to different stages of modification has been determined and the permeability (%) has been presented in Figure 7(A) to demonstrate the effect of LBL modification with respect to the pristine PSf membrane. A 5% increase in permeability was observed for the 60s plasma treated membrane when compared to the pristine PSf membrane of the same condition. Incorporation of MWNT lowered permeability of the membranes when compared to the pristine membrane, except in the case of the 5-bilayer modified membrane. The permeability decreased 22%, 30%, and 37% for 10, 15, and 20-bilayer modified membranes, respectively. The reduction in permeability occurred due to increase in membrane thickness that provided additional hydraulic resistance. Moreover, the intrinsic hydrophobicity of the carbonbased nanomaterial contributed to the permeability reduction.


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FIGURE 7: (A) Permeability (%) of the modified membranes with different quantities of MWNT bilayers. The control membrane was the 60s plasma-treated PSf membrane without carbon nanotube (CNT) bilayers. Here the experiments were conducted with a constant transmembrane pressure of 30 psi at room temperature and all results were compared to the pristine PSf membrane; (B) Comparison of the selectivity (20KD PEG) of pristine PSf membrane with that of VF assisted LBL MWNT modified PSf membranes.

The selectivity of the membranes was investigated using low molecular weight (20KD) polyethylene glycol (PEG) in a procedure for molecular weight cutoff (MWCO) analysis. The average PEG rejections (%) for different numbers of bilayers indicate an increasing trend of rejection (%) with increasing number of bilayers (Figure 7(B)). However, this trend is nonsignificant, and the increasing numbers of bilayers do not necessarily indicate a substantial increasing trend in rejection (%). Nonetheless, incorporation of MWNT bilayers exhibited significant improvement in PEG selectivity of the modified membrane. As shown in Figure 7(B), the pristine membrane achieved only an average of 19% rejection of the PEG molecules for this specific molecular weight. The 5-bilayer modified membrane showed an average of 71% PEG rejection, while the 20-bilayer modified membrane demonstrated the highest average rejection, 76%. The higher solute rejection of MWNT modified membranes may be attributed to the adsorption of PEG on the surface of the CNTs; the functional groups of CNTs offer more 19 ACS Paragon Plus Environment


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adsorption sites for PEG and thus the concentration reduces on the permeate side. The results indicate that higher rejection can be achieved with this modified membrane. Even superior rejection could be possible for compounds with higher molecular weight. 3.3.2 Model organic compound removal by VF assisted LBL MWNT modified PSf membrane in an electrofiltration system Electrochemical filtration for organic matter degradation has been defined by three essential steps: mass transfer, physical adsorption, and the electron transfer mechanism 44. The membrane was tested with 14µM methyl orange at a flow rate of 1.5 ml/min. The flow rate of 1.5 ml/min was chosen based on a previous study by Liu and Vecitis (2012) 44. For a fixed flow rate of 1.5 ml/min, the resulting permeate flux of pristine, 5-bilayer, 10-bilayer, 15-bilayer, and 20-bilayer were observed to be 7.71×10-5, 6.02×10-5, 4.82×10-5, 4.33×10-5 and 3.80×10-5 m3/m2.s respectively. With 1.5 ml/min flow rate through the experimental electrofiltration set up, the pristine polysulfone ultrafiltration membrane would take 5.2 hours to filter 1 liter of water. For experimentation, the flux is satisfactory, however, for a continuous electrooxidation process, a lower flow rate is desirable to allow sufficient residence time for a reaction to occur. The pH of influent water (14µM methyl orange in a background solution of 10mM NaCl) was 6.0. However, identification of organic matter degradation pathways was not performed in the current study. The different number of bilayers in the membrane produced different conductivities and therefore, the electron transfer rate was different. Also there will be substantial adsorption of methyl orange to the CNTs even without any applied voltage. Methyl orange removal by 5bilayer, 10-bilayer, 15-bilayer, and 20-bilayer modified membranes was 36%, 39%, 40%, and 43%, respectively at 0V. A previous study by Yao et al. (2011) 51 reported an adsorption of 35.4 – 64.7 mg methyl orange per gram of MWNTs at equilibrium. Another study by Zhao et al. (2013)

52

also reported an equilibrium adsorption of 44.2 – 54.1 mg methyl orange per gram of

MWNTs. For this study, the electrical potential was kept constant to visualize the effects of the varying quantities of bilayers in the membrane. The methyl orange degradation results are shown in Figure 8 and are very promising. The pristine membrane, for which the mechanisms were only physical adsorption and sieving, showed a 21% removal rate. Although the carbon nanotube concentration was low for the 5-bilayer modified membrane, a 98% removal of organic matter was observed. The membranes with the higher numbers of bilayers achieved 99% removal. The 20 ACS Paragon Plus Environment

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results proved that 98-99% removal was achievable with an applied electrical potential of 3V, which indicated that sufficient electron transfer rate was available for methyl orange (having 14µM concentration) degradation. The methyl orange electro-oxidation has been explained by both direct and indirect oxidation depending on the anode potential

44

. The direct oxidation of

methyl orange occurs when physical adsorption of methyl orange takes place on CNT anode and rapid electron transfer accelerates oxidation as a function of anode potential. In the present study, anode potential corresponding to applied potential of 1V, 2V, and 3V were determined to be 0.33V, 0.74V, and 1.4V, respectively for modified membranes in 10mM NaCl (pH 6). The anode current density for applied potential values of 1V, 2V, and 3V were 0.17 mA/cm2, 0.40 mA/cm2, and 0.75 mA/cm2, respectively. Previous work by Gao and Vecitis (2011)

53

indicated cyclic

voltammetry (CV) curve for methyle orange oxidation on conductive MWNT modified filters. The CV curve indicated an irreversible oxidation peak of methyl orange around 0.8V vs. Ag/AgCl, and water oxidation around 1.2V vs. Ag/AgCl

53

. Methyl orange oxidation potential

has been reported to increase with decreasing pH: 0.3 V at pH 7 to 0.7 V at pH 3

54

, which is

well below the observed anode potential measured in this study. A previous study by Bakr and Rahaman (2016)

55

investigated possible electrooxidation of CNTs due to an applied anodic

potential of ≤3V. The study performed characterization of CNTs (both before and after the filtration and voltage application) and did not notice any surface corrosion or oxidation of the CNTs in the applied anodic potential range55.



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FIGURE 8: Removal of methyl orange (concentration 14µM) through electrochemical filtration at a flow rate of 1.5 ml/min and applied potential of 3V.

3.3.3 Inactivation of E. coli (antimicrobial activity) by VF assisted LBL MWNT modified PSf membrane in an electrofiltration cell To examine the inactivation of microorganisms at different applied potentials, the 10-bilayer modified membrane was used as the control membrane as it shows consistent performance with a modest concentration of MWNT. As indicated in Figure 9, the baseline loss of E. coli due to MWNT toxicity was determined to be 31.2%. This was due to the needle like structure of the MWNT rupturing the cell membrane of the microorganisms, inactivating them due to physical lysis. The bacterial cells were physically adsorbed and then further oxidized by electrical current leading to bacterial inactivation. The inactivation of E. coli reached 91.9%, 94.3% and 100% at applied potentials of 1V, 2V and 3V, respectively. It can be observed that major inactivation took place when adsorption of bacteria was enhanced by electrostatic attraction and thus cellular rupture by MWNT was enhanced. The present study only focused on observing the loss of bacterial viability due to application of different levels of potential (0V, 1V, 2V, and 3V) on the modified membrane (10 bilayers). Baseline bacterial viability on PSf membrane, without MWNT deposition and no applied potential, was not assessed in the present study. Surface analysis of the modified PSf membrane showed that the membrane was completely covered with


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MWNT bilayers. Hence, interactions with bacteria, on the modified membrane, were assumed to be with the MWNT bilayers. Even if some interactions took place between the bacteria and PSf membrane, the PSf membrane can only retain some microorganisms and cannot inactivate microorganisms.

FIGURE 9: Electrochemical loss of E. coli viability versus applied potential for a 10-bilayer modified membrane

3.3.4 Stability of VF assisted LBL MWNT films on modified PSf membrane under physical and chemical stresses The stability of the VF assisted LBL MWNT film on modified PSf membrane was examined by physical (bath sonication) and chemical (low pH, high pH, and salt concentration) stress tests. The contact angles measured after exposure to different stress conditions are shown in Figure 10. There was a slight increase in contact angle with the harsh physical test, which can be caused by etching of the modified surface. Though the physical stress was able to change the orientation of the top layer that leads to higher hydrophobicity as well as an increase in the contact angle, there was no visible change or distortion in the MWNT layer. At pH 2 the H+ ions were able to decrease the charge density of the MWNT-COOH and the same effect was observed in pH 12 where OH- ions affected the charge density. The control membrane initially had a contact angle of 102.9˚, which changed to 82.9˚ after being subjected to NaCl. This can be explained by the dissociation into Na+ and Cl- ions that can combine with the MWNT-COOH, thus affecting the charge density of the charged MWNT-COOH top layer. The membrane demonstrated 23 ACS Paragon Plus Environment


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insignificant changes under harsh conditions because of the charge distribution of the MWNTCOOH layer.

FIGURE 10: Contact angle of the 10-bilayer modified membrane after physical (sonication) and chemical stress (immersion in solutions having pH 2, pH 12, and 5M NaCl, respectively for 15 minutes)

4. CONCLUSIONS The PSf membrane was successfully functionalized with oxygen-containing functional groups via plasma treatment; providing a platform for the VF assisted LBL self-assembly of MWNTs. Contact angle analysis showed improved wettability while several other analyses such as SEM, ATR-FTIR, XPS, and permeability measurement confirmed the successful functionalization of the polysulfone membrane. The plasma-induced, EDC/NHS treated PSf membrane was shown to have a lower contact angle, and was therefore more hydrophilic. Stable amine reactive ester groups were also created on the membrane surface using EDC/NHS cross-linking chemistry. After successful membrane functionalization, the VF assisted LBL selfassembly of MWNT-NH2 and MWNT-COOH was carried out to generate the desired number of MWNT bilayers. The characterization of the modified membrane surface showed a highly hydrophobic membrane surface due to the inherent hydrophobicity of the CNTs. The thickness 24 ACS Paragon Plus Environment

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profile of membranes with differing numbers of bilayers presented a linear behavior, while the atomic layer deposition was difficult to ensure due to the high aspect ratio and the varying orientation of the CNTs. The modified membrane surface exhibited excellent conductivity (4.1x 103 s/m), at a similar order of magnitude as graphite. A slight increase in conductivity was observed with the increasing number of bilayers. The permeability trend of the modified membranes was slightly downward with the increasing numbers of bilayers. While selectivity improved significantly after the introduction of MWNT bilayers (5-bilayers) and gradually increased in proportion with the increasing numbers of bilayers. The membranes developed in this study showed high levels of organic matter degradation and inactivation of almost 100% of bacteria when aided by electrochemical filtration at very low levels of electrical potential application. The modified membrane was also found to be very stable against physical and chemical stresses. Overall, these results advocate for the potential application of such a modified conductive ultrafiltration membrane in large-scale water treatment plants for the production of high quality water free of any pathogenic microorganisms.

5. ACKNOWLEDGEMENT The authors gratefully acknowledge the Natural Sciences and Engineering Research Council (NSERC – NSERC Discovery Grant # N01397) of Canada for providing financial support for this project, and Concordia University for providing the extensive laboratory facilities required to perform the experimental work reported in this manuscript.

6. SUPPORTING INFORMATION Zeta potential of MWNT-COOH and MWNT-NH2 at different pH (Figure S1); SEM images of the thickness profile for (A) 5 bilayer, (B) 10 bilayer, (C) 15 bilayer and (D) 20 bilayer polysulfone-CNT membrane developed by vacuum filtration assisted LBL assembly (Figure S2); Experimental and literature frequencies for specific functional groups (Table S3); Bacterial inactivation experimental procedure (Procedure S4); (a) Schematics of the electrochemical filtration setup, (b) Pictures of the electrochemical filtration setup used in the present study (Figure S5).

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