titanate nanotubes

Journal of Water Process Engineering 33 (2020) 101098

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Novel polyethersulfone ultrafiltration membranes modified with Cu/titanate nanotubes

T

Kacper Szymańskia,*, Dominika Darownaa, Paulina Sienkiewicza, Manu Josea, Karolina Szymańskab, Michał Zgrzebnickia, Sylwia Moziaa,* a

West Pomeranian University of Technology in Szczecin, Faculty of Chemical Technology and Engineering, Department of Inorganic Chemical Technology and Environment Engineering, Pułaskiego 10, 70-322 Szczecin, Poland b West Pomeranian University of Technology in Szczecin, Nanomaterials Physicochemistry Department, Piastów 45, 70-311 Szczecin, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Titanate nanotubes Copper Polyethersulfone membrane Fouling Antibacterial

The influence of addition of titanate nanotubes modified with copper (Cu/TNTs) on the physicochemical, antifouling and antibacterial properties of polyethersulfone (PES) membranes obtained under various conditions is presented. The effect of polymer concentration (15 and 16 wt%) in casting dope and temperature of coagulation bath (10 and 20 °C) was investigated. Furthermore, the influence of Cu content in the Cu/TNTs (1.86 vs 12.23 wt %) on the properties of the membranes was evaluated. The physicochemical characterization of the membranes was based on scanning electron microscopy, atomic force microscopy, isoelectric point and contact angle measurements. The permeability of the mixed matrix membranes was higher compared to the neat ones, and the improvement was the most significant for 15 wt% PES content in the casting dope and Cu/TNTs with higher Cu loading. On the opposite, the best antifouling performance exhibited the membranes characterized by the lowest pure water flux (16 % PES, 10 °C) and a lower Cu content. The addition of Cu/TNTs improved the antibacterial activity of the membranes. The inhibition of the growth of Staphylococcus epidermidis was depended on the kind of Cu/TNTs used, while no significant influence in case of Escherichia coli was observed.

1. Introduction Polyethersulfone (PES) membranes are widely applied during water purification using pressure driven techniques such as microfiltration (MF) and ultrafiltration (UF). They are characterized by a good mechanical resistance, as well as high thermal, chemical and oxidative stability [1]. Despite these advantages, they suffer from membrane fouling due to the relatively low hydrophilicity of PES. Fouling is one of major challenges related to MF and UF operation. This undesired phenomenon contributes to a decrease of membranes permeability or even their damage, thus increasing the process costs. Currently, modern methods for improving membrane antifouling properties are being searched. Amongst numerous attempts to fouling minimization, modification of membranes with nanomaterials seems a very promising solution. Various modifying agents have been proposed, including zerodimensional TiO2 nanoparticles (NPs) or one-dimensional carbon nanotubes (CNTs), halloysite nanotubes (HNTs) and titania/titanate nanotubes (TNTs). Moreover, different types of metallic nanoparticles such as Ag and Cu or metal oxides (e.g. iron oxides, ZrO2, Al2O3, ZnO [2]) or their composites have been examined over the years [3]. In ⁎

general, incorporation of these nanomaterials was found to increase membrane hydrophilicity and contribute to improvement of permeability and (bio)fouling resistance. Amongst various NPs, the TNTs are still poorly explored as membrane modifying agents. Subramaniam et al. [4] obtained PVDF ultrafiltration membrane loaded with TNTs with enhanced fouling resistance. Wan Azelee et al. [5] presented that the polyamide (PA) thin film nanocomposite (TFN) membranes with addition of multi-walled carbon nanotube-titania nanotube (MWCNT-TNT) hybrid NPs were characterized by a higher water permeability compared to the neat PA membranes. An increase of hydrophilicity, permeability and anti(bio) fouling resistance of polysulfone (PSU) membranes modified with sulfonic acid-functionalized TiO2 nanotubes was also reported by Alsohaimi [6]. Furthermore, Emadzadeh et al. [7] prepared reverse osmosis (RO) membrane modified by amino-functionalized titanate nanotubes (NH2-TNTs) with enhanced antifoulingproperties. The pervaporation membranes for separation of water-isopropanol mixtures based on poly (vinyl alcohol) (PVA) incorporated with TNTs and cross-linked with glutaraldehyde were synthesized by Raeisi et al. [8]. Ng et al. [9] obtained nanocomposite membranes using TNTs modified with

Corresponding authors. E-mail addresses: [email protected] (K. Szymański), [email protected] (S. Mozia).

https://doi.org/10.1016/j.jwpe.2019.101098 Received 16 September 2019; Received in revised form 28 November 2019; Accepted 6 December 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 33 (2020) 101098

K. Szymański, et al.

Nomenclature

NF nanofiltration NH2 amino-functionalized titanate nanotubes NPs nanoparticles PA polyamide PBS phosphate buffered saline PECVD plasma-enhanced chemical vapour deposition PES polyethersulfone pH(I) isoelectric point PSU polysulfone PVA poly(vinyl alcohol) PVDF poly(vinylidene fluoride) R rejection coefficient Ra average membrane roughness RO reverse osmosis ROS reactive oxygen species SCA static contact angle SPAES sulfonated polyarylethersulfone t time TEM transmission electron microscope TFN thin film nanocomposite TMP transmembrane pressure TNRs titanate nanoribbons TNTs titanate nanotubes/titania nanotubes TOC total organic carbon UF ultrafiltration UHR FEFE-SEM ultra-high resolution field emission scanning electron microscope V volume XRD X-ray diffraction

List of symbols A AFM Ag/TNTs B BHI BSA BSE C Cf CFU CNTs Cp Cu/TNT Cu/TNT Cu/TNTs D DMF HEMA HFBA HNTs ICP

membrane area atomic force microscope Ag-modified TNTs number of bacteria colonies brain heart infusion bovine serum albumin back-scattered electrons amount of bacteria in control sample concentration of BSA in feed colony forming unit carbon nanotubes concentration of BSA in permeate composite Cu/TNTs obtained from Cu(CH3COO)2 composite Cu/TNTs obtained from Cu(NO3)2 Cu-modified TNTs dilution factor N,N-dimethylformamide hydroxyethyl methacrylate hexafluorobutyl acrylate halloysite nanotubes Inductively Coupled Plasma-Optical Emission Spectrometry J permeate flux MF microfiltration MWCNT multi-walled carbon nanotube NA nutrient agar

surface area and thus enhancement of water permeability. It was also reported that the modified membranes possessed antibacterial properties. Cu NPs incorporated to a membrane structure have been reported to be relatively easily released from the polymeric matrix [13], leading to a decrease of the positive effects of the modification as well as contamination of feed or permeate. Copper species present in the treated wastewater discharged to the environment can act as toxic agents for the aquatic organisms due to the induction of the oxidative stress and membrane cell damage [16]. In case of application of the permeate as a potable water the elevated Cu level could have a negative impact on human health [17]. Therefore, prior to application in membranes fabrication, the Cu NPs should be immobilized on organic or inorganic carriers to reduce release of metal ions from the membrane structure [14]. There are some reports concerning immobilization of Cu on other nanomaterials such as halloysite [18], alumina [19], magnesium [20], graphene [21] or TiO2 [22]. Alternatively, titanate nanotubes can be regarded as one of the inorganic carrier used in this approach. However, there are only few reports related to preparation of Cu-modified TNTs [23–25]. Umek et al. [23] prepared copper doped sodium titanate nanotubes and nanoribbons (TNRs) by two different methods, utilizing in situ and ex situ approach. In the first method the TNTs and TNRs were prepared from anatase TiO2 doped with Cu2+ from Cu(CH3COO)2, while in the second one, the nanotubes and nanoribbons were impregnated with copper by stirring in CuSO4 solution. Doong et al. [24] obtained Cu-modified TiO2/TNT nanocomposite by photodeposition of copper from Cu(NO3)2 solution on TNTs prepared hydrothermally from TiO2 and calcined at 500 °C. The NPs were applied for photocatalytic removal of bisphenol A. Similar photodeposition approach, using CuSO4 as a precursor, was applied by Joshi et al. [25] for fabrication of Cu-modified TNTs (Cu/TNTs) with antibacterial properties. The authors reported that under dark conditions the Cu/TNTs exhibited similar antibacterial activity towards S. aureus compared to Ag-modified TNTs

hexafluorobutyl acrylate (HFBA) or hydroxyethyl methacrylate (HEMA) by plasma-enhanced chemical vapour deposition (PECVD). The above overview clearly shows that the number of reports on the TNTs-loaded UF polymeric membranes is very limited. Nonetheless, the present state of the art in the area of UF polymeric membranes modified with TNTs functionalized with metallic NPs such as Ag or Cu is also very scarce [10,11]. The application of Ag or Cu was proposed to improve the resistance of membranes to biofouling. Although Ag is well known as an excellent antibacterial agent, there are also reports showing superior activity of Cu with reference to some bacteria species. For example, Ruparelia et al. [12] compared the antibacterial activity of Ag and Cu NPs towards various strains of Escherichia coli, Staphylococcus aureus and Bacillus subtilis. In case of E. coli, silver was for circa 40–50 % more effective in inhibition of their growth compared to copper, in case of S. aureus Ag was more effective for only circa 10–15 %, and for B. subtilis Cu exhibited almost 90 % higher activity than Ag. The application of Cu NPs for modification of UF membranes made of PES blended with sulfonated polyarylethersulfone (SPAES) was reported by Zhang et al. [13]. The addition of Cu NPs into the membrane casting dope suppressed macropores formation, and the increasing content of the nanomaterial resulted in an increase in pore density and decrease in pore size. The Cu-modified membranes exhibited higher hydrophilicity and water permeability compared to the neat membrane. Moreover, the modified membranes were characterized by an improved overall antifouling performance, although the higher content of Cu NPs led to a higher reversible fouling. The mixed matrix membranes exhibited also antibacterial properties, as was found on a basis of inhibition zone measurement with application of E. coli [14]. Cherif et al. [15] prepared thin film nanocomposite (TFN) membranes for nanofiltration (NF), by incorporation of Cu NPs in the PA layer. They reported that the membranes containing Cu NPs exhibited much rougher surface than the unmodified one, which led to an increase of the membrane 2

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2 h in a glass reactor. Subsequently, the suspension was irradiated with a low-pressure mercury vapour lamp (TNN 15/32, Heraeus Noblelight GmbH, 15 W, λmax =254 nm) for 2 h with continuous stirring. The obtained nanomaterial was centrifuged, washed with pure water to remove the excess of Cu2+ ions and dried at 80 °C in an oven for 12 h. The samples were denoted later as Cu/TNT-N and Cu/TNT-Ac, where the extensions N and Ac correspond to Cu(NO3)2 or Cu(CH3COO)2, respectively.

(Ag/TNTs). The literature on application of Cu/TNTs for membranes fabrication is even more limited. To the best of our knowledge there is only one report, by Sumisha et al. [26] on that subject. Nonetheless, the authors prepared an NF membrane containing Cu/TNTs for improvement of salt rejection. There are, however, no reports on the preparation of UF PES membranes modified with hybrid Cu/TNTs, characterized by enhanced permeability and hydrophilicity as well as antibacterial properties. In view of the above, in the present paper the investigations on the influence of Cu/TNTs on the physicochemical and antibacterial properties as well as performance of PES membranes obtained under various preparation conditions is presented and discussed. Especially, the effect of polymer concentration and temperature of coagulation bath was investigated. Furthermore, the influence of the content of copper in the Cu/titanate nanotubes on the properties of the obtained membranes was examined.

2.4. Preparation of membranes The membranes were fabricated by the wet phase inversion method. The casting solution for preparation of the unmodified membranes was obtained by dissolution of a proper amount (15 or 16 wt%) of PES in 50 mL of DMF and stirring in a tightly closed bottle overnight. Then the casting dope was degassed by leaving it at rest to get a bubble-free solution and casted on a glass plate using an automatic film applicator (Elcometer 4340, Elcometer Ltd., UK) equipped with a knife. The knife gap was set at 0.1 mm, which corresponded to the membrane thickness in the range of 35−38 μm. The casted film was immersed in the nonsolvent bath at 10 °C or 20 °C and kept in pure water for 24 h. The temperature of coagulation bath of 20 °C was selected as an ambient temperature, commonly used during phase inversion process. Higher temperature was not selected because it is well known from literature that with increasing temperature of coagulation bath, the membranes exhibit higher pure water flux and, typically, lower resistance to fouling (which is a flux-driven phenomenon). On the opposite, lower temperature of non-solvent leads to formation of a denser membrane. Hence, the membranes under decreased temperature of coagulation bath (10 °C) with a hypothesis that it would be beneficial in terms of fouling mitigation, were prepared. In case of the modified membranes the fabrication procedure was similar to the abovementioned, however, an additional step of Cu/TNTs sonication was applied. At this stage 1 wt% of Cu/TNTs in relation to PES was dispersed in 10 mL of DMF using ultrasonic liquid processor (Vibra-cell VCX-130, Sonics, USA; output power 130 W, frequency 20 kHz, amplitude 80 %) equipped with a 6 mm ultrasonic probe. The obtained suspension was then added into the solution of PES in DMF, subsequently sonicated (Sonic-6D, Polsonic, Poland; output power 320 W, frequency 40 kHz) for 15 min (at 20−25 °C), and finally stirred (200 rpm) for additional 15 min at temperature of 55−60 °C. The sonication and stirring steps were carried out by turns for 2 h. A summary of the conditions applied during preparation of the membranes is presented in Table 1.

2. Experimental 2.1. Materials and methods Polyethersulfone was provided by BASF, Germany. N,N-dimethylformamide (DMF) solvent was purchased from Avantor Performance Materials Poland S.A., Poland. Pure water (Elix 3, Millipore) was applied as a non-solvent. Titanate nanotubes were prepared hydrothermally from anatase TiO2 powder (Sigma Aldrich, USA), NaOH and HCl (Avantor Performance Materials Poland S.A., Poland). Bovine serum albumin (BSA) used as a model foulant was purchased from Merck Millipore, Germany. Nutrient Agar (NA) and Brain Heart Infusion Agar (BHI) for preparation of agar plates were purchased from Biomaxima S.A. (Poland). KCl, Na2HPO4, KH2PO4, HCl (Avantor Performance Materials, Poland) and NaCl (Merck, Germany) were used for preparation of phosphate buffered saline (PBS, pH 7.2). Gram-negative Escherichia coli (strain K12, ATCC 29425, USA) and Gram-positive Staphylococcus epidermidis (ATCC 49461, USA) were used as model microorganisms. The initial concentration of E. coli suspension in 0.85 wt% NaCl solution or S. epidermidis in PBS was adjusted to 0.5 according to McFarland scale (McFarland standards, bioMérieux, France). 2.2. Preparation of titanate nanotubes Titanate nanotubes were prepared using hydrothermal method [27] in the BLH-800 pressure reactor (Berghof, Germany). First, 2 g of anatase TiO2 powder and 60 mL of 10 M NaOH were added into 75 mL Teflon vessel and sonicated for 1 h. Then the vessel was mounted in the reactor and the hydrothermal reaction was performed for 24 h at the temperature of 140 °C. The obtained suspension was washed with ultrapure water (Simplicity®, Millipore) and 0.1 M HCl, and subsequently dried (80 °C for 24 h) and ground using agate mortar.

2.5. Characterization of composite Cu/TNTs The morphological analysis of Cu/TNTs was carried out using transmission electron microscope (TEM) FEI Tecnai F20. The samples were prepared by sonication in ethanol followed by adding a drop of the suspension on a nickel grid (300 mesh). The phase composition of Cu/TNTs was determined based on X-ray diffraction (XRD) method (PANalytical Empyrean X-ray diffractometer) using CuKα radiation (λ =1.54056 Å). The isoelectric point and zeta potential of the TNTs and Cu/TNTs dispersions in ultrapure water were determined using Zetasizer Nano-ZS (Malvern Instruments Ltd.) equipped with Multi

2.3. Preparation of composite Cu/TNTs The hybrid Cu/TNTs were prepared by photodeposition method. In the first step, 0.5 g of TNTs were dispersed in 50 mL of Cu(NO3)2 or Cu (CH3COO)2 solution (100 mM) and magnetically stirred (250 rpm) for Table 1 Membranes preparation conditions. Membrane

M1

M2

M3

M4

M5

M6

M7

M8

M9

PES concentration [wt%] Coagulation bath temperature [°C] Nanofiller

15 20 –

15 20 Cu/ TNT-N

15 20 Cu/ TNT-Ac

16 20 –

16 20 Cu/ TNT-N

16 20 Cu/ TNT-Ac

16 10 –

16 10 Cu/ TNT-N

16 10 Cu/ TNT-Ac

3

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were used to mark pores on the previously recorded 1 μm × 1 μm AFM images. The water contact angles were measured using a sessile water drop method with application of the goniometer model 260 (ramé-hart instruments co., USA). The static contact angle (SCA) values were calculated as an average of at least 10 measurements from 10 different membrane pieces. The volume of the ultrapure water drop in this set of experiments was 10 μL. The isoelectric point (pH(I)) and zeta potential of the membranes were measured using SurPASS™ 3 analyzer (Anton Paar GmbH, Austria). The 0.001 M KCl solution in ultrapure water was applied as an electrolyte, while the pH was adjusted using HCl and KOH solutions. The stability of membranes was evaluated on a basis of Cu and Ti release in 0.85 wt% NaCl solution. According the procedure, the samples (0.00156 m2) of modified membranes were inserted into glass bottles containing 25 mL of saline solution and a stirrer bar. The bottles were stirred at 250 rpm and 37 °C for 7 days. The liquid samples were collected after 24, 72 and 168 h, and analyzed using Optima 5300DV ICP-OES spectrometer.

Purpose Titrator MPT-2 and a degasser. The pH was adjusted using HCl and NaOH solutions. The Cu content in the NPs was determined based on Inductively Coupled Plasma-Optical Emission Spectrometry (ICPOES) using Optima 5300DV ICP-OES spectrometer (Perkin Elmer, USA). Before measurement the sample was dissolved in a hot solution of (NH4)2SO4 in a concentrated H2SO4. After cooling down the solution was diluted with ultrapure water. 2.6. Characterization of membranes The morphology of the membranes cross-sections was examined using Hitachi (Japan) SU8020 Ultra-High Resolution Field Emission Scanning Electron Microscope (UHR FE-SEM). Prior to SEM analysis, the pieces of the membranes, previously dewatered using ethanol solutions and broken in liquid nitrogen, were coated with a chromium layer (Q150 T ES, Quorum Technologies Ltd., UK). To examine the dispersion of Cu/TNTs in the membranes matrix the SEM images were recorded in the back-scattered electrons (BSE) mode. The accelerating voltage during SEM analysis was 15 kV. The AFM surface images of the membranes were collected using NanoScope V Multimode 8 scanning probe microscope (Bruker Corp., USA). The silicon nitride ScanAsystAir probe and the ScanAsyst mode were used. The scanned surface was 10 μm × 10 μm. The average membrane roughness (Ra) was evaluated using the NanoScope Analysis software. The Ra results represent a mean value calculated for five randomly selected areas. The size of the membrane pores in the top layer was calculated using the Gwyddion software package, slightly modifying the method described by Khanukaeva et al. [28]. Namely, both Watershed and Threshold modes

2.7. Membrane filtration The measurements of membranes permeability and antifouling performance were realized in a laboratory scale cross flow membrane system. The installation was equipped with a feed tank, from which the feed was pumped using a plunger pump with a pressure dampener to two parallel stainless steel membrane modules. The working membrane area was 0.0025 m2. A 1.194 mm feed spacer was applied in all experiments. The transmembrane pressure (TMP) in the range of 0.10.3 MPa was controlled using manometers and needle valves. The

Fig. 1. TEM images of the Cu/TNTs. 4

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0.85 wt% NaCl and expressed as colony forming unit (CFU) per mL values (CFU mL−1). In brief, 0.3 mL of a diluted suspension was spread on an appropriate agar plate. The plates were incubated at 37 °C for 24 h. After that time, the colonies visible on the plates were counted using counter (LKB 2002, POL-EKO, Poland). The results were calculated according to the Eq. (3):

temperature was maintained at 20 ± 1 °C. The changes of the permeate flux were evaluated by measuring the volume of the permeate passing through the membrane during a fixed period of time and calculated according to the Eq. (1):

J=

V A× t

(1) 2

where: V-volume of permeate [L], A-membrane area [m ], t-time during which the permeate was collected [h]. Each experiment was repeated at least three times in order to confirm the reproducibility of the results. The antifouling properties of the membranes were evaluated by ultrafiltration of BSA solution at a concentration of 1 g L−1 under TMP of 0.2 MPa and feed cross flow velocity of 1 m s−1. The concentration of BSA in feed (Cf) and permeate (Cp) was evaluated using the total organic carbon (TOC) analyzer (multi N/C 3100, Analytik Jena, Germany). The rejection coefficient (R) was calculated on a basis of the Eq. (2):

R=

Cf − Cp Cf

× 100%

logCFU= log

B× D V

(3)

where: B-number of bacteria colonies visible on Petri dish, D-dilution factor, V-volume of bacteria suspension placed on agar plates (0.3 mL). The log reduction of bacterial growth with reference to the control (blank) sample was calculated on a basis of Eq. (4):

C logreduction = log ⎛ ⎞ ⎝M⎠

(4) −1

where: C-amount of bacteria in control sample (CFU mL of bacteria in the presence of a membrane (CFU mL−1).

(2)

), B-amount

3. Results and discussion

2.8. Antibacterial study

3.1. Characterization of Cu/TNTs

First, 100 mL of suspension of E. coli in 0.85 wt% NaCl or S. epidermidis in PBS at a concentration of 0.5 according to McFarland scale were added to glass bottles. Then, an ellipsoidal stirring bar and a piece of a dry membrane (12.5 cm × 4.5 cm) were introduced inside each bottle. Additionally, control samples (i.e. without a membrane) were prepared. The bottles containing the samples were incubated at 37 °C for 24 h with continuous stirring at 250 rpm. Then, the amount of surviving bacteria was determined by the serial decimal dilution in

During our preliminary studies (data not shown) we have observed that the antibacterial properties of Cu/TNTs are strongly dependent on Cu loading. Since one of the issues discussed in the present manuscript is antibacterial performance of the mixed-matrix membranes, on a basis of these investigations the Cu/TNT-N with low (1.86 wt%) and the Cu/ TNT-Ac with high (12.23 wt%) Cu content were selected for the membranes fabrication. Such an attempt gives an opportunity to evaluate the effect of Cu loading in the hybrid NPs on the membranes

Fig. 2. SEM-BSE microphotographs of the cross-sections of the investigated membranes (the Cu/TNTs agglomerates are circled). 5

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3.2. Characterization of membranes

performance. Based on TEM analysis of Cu/TNTs (Fig. 1) it was found that the nanotubes exhibited multi-walled and open-ended structure. The length of TNTs was in the range of 50−200 nm and inner/outer diameter amounted to circa 5/13 nm, respectively. On the outer surface of the TNTs some small Cu NPs can be observed with dominant diameters in the range of circa 2−3 nm (Fig. 1). The XRD patterns (Fig. S1) of the prepared Cu/TNTs exhibited four broad peaks positioned at 2θ = 9.5, 24.5, 28.5, and 48.5°, which correspond with (020), (110), (130), and (200) reflections of the layered titanates (e.g., orthorhombic H2Ti2-x/ 4□x/4O4, a = 0.3643, b = 1.8735 and c =0.2978 nm) [29]. Moreover, the absence of peaks corresponding to anatase TiO2 indicates that after the hydrothermal treatment the precursor was completely converted to layered titanates [30,31]. However, no visible reflections representing metallic Cu or copper oxides were observed in the XRD patterns, which is possibly due to the quite low Cu content in the nanocomposites or the small size of Cu NPs. The presence of copper in the prepared Cu/TNTs was confirmed on a basis of ICP-OES analysis. The Cu/TNT-N contained 1.86 wt% of Cu and Cu/TNT-Ac exhibited six times higher amount of this metal (12.23 wt%). The isoelectric point of Cu/TNT-Ac was 4.1(0.1), and the pH(I) of Cu/TNT-N was 3.9(0.1), which could also indicate lower amount of Cu in these NPs, as the unmodified TNTs had the isoelectric point of 3.1(0.1). Above these pH values the surface of the nanoparticles was negatively charged.

3.2.1. Membranes morphology SEM was applied for analysis of membranes morphology and distribution of nanomaterials within the membranes structure. The crosssections of all investigated UF membranes (Fig. 2) exhibit typical asymmetric porous structure with a dense skin layer, a porous sub-layer containing well-developed narrow finger-like pores, and larger fingerlike pores in the bottom part with a spongy structure between them. The finger-like pores widened towards the bottom section of the membranes. In some places some spherical macrovoids can be also observed. In case of all modified membranes the Cu/TNTs agglomerates imbedded in the polymeric matrices are visible (Fig. 2). Nevertheless, the structure of the membranes prepared from the casting dope containing 16 wt% of PES at the non-solvent bath temperature of 20 °C (M4-M6) exhibit lower number of macropores and richer spongy structure compared to 15 wt% of PES (M1-M3), which is related to a higher polymer content. Moreover, the decrease of temperature of nonsolvent bath to 10 °C (M7-M9) caused the enhancement of this effect, since under lower temperature solvent diffusion is slower, and viscosity of polymer solution increases. Low temperature of the non-solvent bath diminishes the mutual diffusivity between the solvent and non-solvent during phase inversion and in result - instantaneous demixing, hence the number of macropores is reduced and the membrane structure is much denser [32].

Fig. 3. AFM images of the surface of the investigated membranes. 6

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especially formed at 10 °C. At low surface porosity and smaller pores the flow of water into membrane pores due to capillary forces was inhibited resulting in higher SCA values. On the opposite, in case of the membranes obtained using dopes containing 15 wt% of PES, the pores were larger which allowed water to infiltrate more easily thus reducing the SCA [32,34]. To investigate the stability of the membranes the release of Cu and Ti in 0.85 wt% NaCl solution at 37 °C was evaluated. Such conditions represented the parameters applied during evaluation of the antibacterial properties of the membranes. It was found that no Ti was released from the membranes during 7 days of experiments. Moreover, the Cu concentration in the solution did not exceed 0.02 mg L−1 after 7 days, being circa 4–5 times higher in case of Cu/TNT-Ac, containing higher amount of Cu, compared to Cu/TNT-N. The obtained results revealed good stability of the prepared membranes containing Cu/TNTs nanocomposites under the applied conditions.

3.2.2. Surface properties of the membranes Fig. 3 presents the three-dimensional AFM images of the membranes surfaces. Both large mountain-like agglomerates as well as small needle-like structures are visible on the skin of all modified membranes. The diameters and heights of the larger species range from 0.5 μm to 6.35 μm and from 60 to 180 nm, respectively, while in case of the small aggregates the values are 0.1-0.5 μm and 19−50 nm, respectively. The presence of the Cu/TNTs-based species resulted in an increase of the surface roughness (6.0–8.9 nm) compared to the neat membranes (4.7–5.1 nm), as can be found from the Ra values summarized in Table 2. It can be seen that the temperature of the non-solvent bath had an influence on the membrane surface properties. All membranes prepared at 20 °C have higher surface roughness (5.1–8.9 nm) than the samples prepared at 10 °C (4.7–6.4 nm). This observation could also be made by comparing the Ra of samples fabricated at 20 °C using the same nanofillers but different PES concentration (8.9 nm or 8.2 nm for Cu/ TNT-N and 15 wt% or 16 wt% of PES, respectively, and 6.4 or 6.8 nm for Cu/TNT-Ac and 15 wt% or 16 wt% of PES, respectively). It can be seen that these results are similar to each other and yet they are higher than Ra of the samples casted at 10 °C (6.4 nm for Cu/TNT-N and 6.0 nm for Cu/TNT-Ac). It can be noted that the addition of Cu/TNT-N as a nanofiller had a higher impact on Ra (6.4–8.9 nm) than Cu/TNT-Ac (6.0–6.9 nm). The reason can be that Cu/TNT-N nanoparticles contained lower amount of copper, which could influence the interactions between NPs and cause different agglomeration than in case of Cu/ TNT-Ac. The effect of coagulation bath temperature on the membranes properties can be also seen when analysing their surface porosity. In Fig. S2 the pores on the surface of the neat M1, M4 and M7 membranes marked and calculated using Gwyddion software are presented. Both samples casted at 20 °C (M1 and M4) exhibited slightly larger pores (mean pore size of 7.4 nm and 7.0, respectively) and higher porosity (8.1 % and 7.8 %, respectively) than the sample fabricated at 10 °C (5.9 nm and 6.8 %, respectively). Although the porosity was the lowest in case of the lowest non-solvent temperature, the number of pores was for that membrane the highest (Fig. S2). These findings are in agreement with the literature data. Li et al. [33] proved that the increase of the coagulation bath temperature increases the porosity and pore size of the PES membranes. They attributed these results to the changes of solvent-water exchange rates. The isoelectric points (Table 2) of the prepared membranes shifted slightly from 2.7 to 2.8 for the neat membranes to 2.9–3.1 for the membranes with the nanofiller indicating that the Cu/TNTs, which have the isoelectric point of 4.1 and 3.9 (for Cu/TNT-Ac and Cu/TNT-N, respectively) are present on the surface. At pH above the isoelectric point the membranes possess a negative charge, while at pH < pH(I) their surface is positively charged. The highest static water contact angles were observed for the unmodified PES membranes (Table 2). The water contact angles of the composite membranes were reduced since their hydrophilicity was improved with the addition of Cu/TNTs. The most hydrophilic membranes were those prepared from 15 wt% of PES (Table 1). In general, the SCA increased with the increase of polymer concentration and decrease of temperature of non-solvent bath. The explanation could be a much denser skin layer of the membranes casted from the dope containing 16 wt% of polymer,

3.2.3. Water permeability of membranes The pure water permeability of the examined membranes is summarized in Table 2. It can be seen that the addition of Cu/TNTs into PES matrix improved the permeability regardless of the applied parameters of membranes fabrication. The highest improvement was noted for the membranes obtained with application of 15 wt% PES concentration in the casting solution. For both modified membranes the permeability increased for about 31–32 % in comparison with that of the unmodified M1. The membranes casted from the solutions containing 16 wt% of PES exhibited lower permeability compared to the M1-M3 membranes, due to a denser structure resulting from a higher polymer content. However, despite that, also the improvement of membranes permeability associated with the incorporation of Cu/TNTs was significantly less remarkable than observed for 15 wt% of PES in the casting dope. The permeability of M5 and M6 increased for only 9 and 13 %, respectively, compared to M4. Further, in case of the membranes casted at the coagulation bath temperature of 10 °C the Cu/TNTs modification led to a less significant improvement compared to the M1-M3 series, reaching 11–12 %. From the data presented in Table 2 it can be also observed that a higher temperature of the coagulation bath contributed to an increase of membranes permeability, however, the differences between M4-M6 and M7-M9 series were not as large as in case of the membranes prepared with different PES concentration. The main explanation can be a much denser skin layer of the membranes fabricated using higher PES concentration. The skin, formed at the very early stage of the coagulation bath - casting dope interaction, was limiting further diffusion of non-solvent in and solvent out of the underneath layer [35]. As a result of the thicker skin the membrane resistance for water transportation increased leading to lower permeate fluxes. Nevertheless, the observed positive effect of the addition of the nanofillers into all types of the PES membranes matrices was caused by hydrophilic properties of the NPs and the additional contribution of microchannels in the Cu/TNTs structure that provided new flow paths for water molecules [36,37]. These statements are confirmed by the higher hydrophilicity of the modified membranes and the presence of Cu/TNTs agglomerates within their matrices. The more hydrophilic modified membranes could attract

Table 2 Selected physicochemical properties of the membranes. Membrane

M1

M2

M3

M4

M5

M6

M7

M8

M9

Isoelectric point, pH(I) Surface roughness, Ra [nm]

2.7 (0.1) 5.1 (0.3)

2.9 (0.1) 8.9 (2.6)

2.9 (0.1) 6.4 (2.1)

2.8 (0.3) 5.1 (0.2)

3.0 (0.4) 8.2 (3.8)

2.9 (0.1) 6.8 (3.4)

2.8 (0.1) 4.7 (0.5)

3.1 (0.1) 6.4 (1.6)

Static water contact angle (SCA) [°]

53 (1) 147 (9)

50 (1) 212 (8)

49 (1) 215 (9)

55 (1) 128 (19)

51 (1) 141 (6)

51 (1) 147 (8)

58 (1) 107 (7)

52 (0) 121 (3)

3.1 (0.2) 6.0 (2.2) 52 (0) 120 (8)

Pure water permeability [L m−2 h-1 bar-1]

7

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the presence of Cu/TNTs in the membranes matrix the zeta potential values were only slightly lower compared to those measured for the neat membranes. Moreover, no noticeable effect of the type of Cu/TNTs on the zeta potential was observed. These results confirm again that the permeability, closely associated with membranes porosity is an important issue to explain the fouling behaviour. As was discussed in Section 3.2.2, the membranes casted at higher non-solvent temperature exhibited higher porosity and pore size compared to those obtained at 10 °C, thus relatively higher amount of BSA molecules could enter and block their pores [42]. The data presented in Fig. 4 show that the Cu/TNT-N-modified membranes exhibited the highest fouling resistance amongst all the prepared samples. These results could be explained by different Cu content in the hybrid NPs. As was already reported in Section 3.1, in case of the Cu/TNT-N the Cu loading was about six times lower compared to Cu/TNT-Ac. Taking into account that copper ions can form complexes with amino acid residues of the BSA molecules [43,44], the

easily water molecules and facilitate their transport through the pores. A slightly higher permeability was observed in case of the membranes modified with Cu/TNT-Ac compared to Cu/TNT-N, which is especially visible for the membranes prepared at the non-solvent temperature of 20 °C (Table 2). That can be related to the significantly higher content of copper in Cu/TNT-Ac (12.23 wt%) than in Cu/TNT-N (1.86 wt%) [37,38]. The obtained results demonstrate that the application of composite Cu/TNTs as a nanofiller can be regarded as a method for improvement of membranes permeability. 3.2.4. Antifouling properties of membranes Fig. 4 presents changes of permeate fluxes during ultrafiltration of BSA solution through the examined membranes. It can be observed that the effect of the Cu/TNTs presence on the antifouling properties of the membranes was dependent on the parameters of their fabrication. The permeate fluxes in case of the M2 and M3 modified membranes (Fig. 4a) were noticeably lower than that measured for the unmodified M1. During ultrafiltration of BSA solution through the M5 and M6 the permeate fluxes were comparable to that observed for the unmodified M4 membrane (Fig. 4b). Finally, in case of the M8 and M9 membranes, prepared at the lowest coagulation bath temperature (10 °C), the incorporation of Cu/TNTs resulted in an improvement of their antifouling properties. From the very beginning of the ultrafiltration of the BSA solution the permeate fluxes through the modified M8 and M9 membranes were higher than that of the neat M7 (Fig. 4c). Moreover, the flux measured for the M8 membrane modified with Cu/TNT-N was the highest amongst the fluxes measured for all the examined membranes. After 2 h of experiment the permeate fluxes through the Cu/TNT-Nmodified membranes reached the values of 153 L m−2 h-1 in case of M8, 132 L m−2 h-1 for M2 and 127 L m−2 h-1 for M5. The M2 and M3 were characterized by the highest water permeability from all the examined membranes (Table 2). Thus, one explanation for the significant flux decline in their case can be the enhanced deposition of BSA molecules on the membranes surface and within their pores due to the enhanced transport of feed components towards the membrane at higher flux during the initial stage of the process [39]. For the M5 and M6 membranes the increase of permeability due to the modification was not as high as in case of M1-M3 series (Table 2), hence the fouling intensity was less severe. The lowest water permeability was obtained for the membranes casted at 10 °C, which corresponded with their superior resistance to BSA fouling. Generally, the improvement of antifouling properties of the coppermodified membranes is attributed to the hydrophilic nature of a Cubased nanofiller [38,40]. For example, Nasrollahi et al. [38] explained in such a way the positive effect of CuO NPs addition into PES membrane on its enhanced fouling resistance. However, in the present studies, the lower fouling intensity in case of M5 and M6 compared to the M2 and M3, was not reflected by the contact angle values (Table 2). This indicates that some other factors are responsible for the observed results, e.g. relatively lower water permeability and a denser skin layer, which prevented from the enhanced deposition of BSA molecules during ultrafiltration process. Furthermore, despite the lowest hydrophilicity of M8 and M9 membranes among all the modified ones, these membranes exhibited the best antifouling properties. Except from the water permeability issue, the explanation of the observed results could be related to the membranes surface roughness. The Ra values of M8 and M9 membranes were relatively low (6.4 and 6.0 nm, respectively), therefore the deposition of BSA on the membranes surfaces was hindered to some extent. In order to evaluate whether the surface charge of the membranes under conditions applied during BSA fouling studies was important for their antifouling properties, the zeta potential at pH 6.85 was also determined. A more negative zeta potential of the membranes might help to mitigate their fouling by the negatively charged BSA, since the deposition of negatively charged foulants is hindered by forming higher electrostatic double layer repulsion [41]. However, it was found that in

Fig. 4. Changes of permeate fluxes during ultrafiltration of 1 g L−1 BSA solution through a) M1, M2 and M3 membranes (PES concentration: 15 wt%, coagulation bath temperature: 20 °C); b) M4, M5 and M6 membranes (PES concentration: 16 wt%, coagulation bath temperature: 20 °C); c) M7, M8 and M9 membranes (PES concentration: 16 wt%, coagulation bath temperature: 10 °C). 8

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peptidoglycan layer found in Gram-positive bacteria that could possibly act as a protective layer. Thus, the cell wall destruction due to physical interaction between NPs and the cell wall or the penetration of copper ions released from the NPs into the cell of Gram-negative bacteria is facilitated [49]. Analysis of the results presented in Fig. 5 revealed also that the membranes preparation conditions (PES content, non-solvent temperature) had no significant effect on the antibacterial performance of membranes. This can be explained by the same content (1 wt%) of Cu/ TNT-Ac or Cu/TNT-N in the two types of membranes.

high Cu content can promote binding of the foulant with the membrane surface. Such a phenomenon was much more possible for the membranes modified with Cu/TNT-Ac which exhibited stronger binding affinities due to a higher Cu content compared to the samples modified with Cu/TNT-N. The interactions between metal ions present in Cu/ TNTs could contribute to the membrane fouling thus increasing its severity [39]. In view of the above, it is clear that a balance between high water permeability and fouling resistance is a key issue when designing the Cu/TNTs-modified membranes. No significant influence of the modification on the rejection of BSA was found. For all the examined membranes the retention coefficients ranged from 99.4(0.2)% to 99.6(0.2)%, regardless of membrane preparation conditions.

4. Conclusions Mixed matrix PES membranes fabricated under different conditions and modified with two various Cu/TNTs have been thoroughly studied, with particular emphasis on determination of anti(bio)fouling properties. The permeability of the Cu/TNTs-modified membranes was higher compared to the neat ones, and the difference was especially significant in case of 15 wt% PES content in the casting dope. A slightly higher pure water permeability was observed in case of the membranes modified with Cu/TNT-Ac compared to Cu/TNT-N, which was explained by a higher Cu content in the former NPs. The fouling by BSA was the most significant in case of the membranes characterized by the highest water permeability (i.e. casted at 20 °C from a solution containing 15 wt% PES), while the lowest permeate flux decline was observed for the membranes exhibiting the lowest permeability (i.e. prepared at 10 °C from the casting dope containing 16 wt% PES). The BSA membrane fouling was slightly higher in case of the membranes containing Cu/TNT-Ac compared to Cu/TNT-N, which could be explained by a higher binding affinity resulting from complex formation between Cu2+ in NPs present in membranes matrix and amino acid residues of the BSA in case of higher Cu content. The addition of Cu/TNTs improved the antibacterial activity of the prepared membranes. A higher biocidal ability was observed for E. coli than S. epidermidis, which was explained by the differences in the cell wall structure. Moreover, the inhibition of the growth of S. epidermidis was depended on the kind of Cu/TNTs used for membrane modification, being more effective for the NPs with higher Cu content. On the opposite, no significant influence of Cu amount in the hybrid NPs on the antibacterial ability against E. coli was observed.

3.3. Antibacterial performance of membranes Fig. 5 shows antibacterial ability of the prepared membranes against S. epidermidis and E. coli. Generally, it was noticed that the addition of Cu/TNTs into membranes matrices improved their antibacterial properties against both bacteria. Nevertheless, for all examined membranes, higher log reduction values were obtained in case of E. coli compared to S. epidermidis (Fig. 5). Moreover, no significant differences between the antibacterial activity of the membranes modified with two various types of Cu/TNTs against E. coli was observed. On the opposite, in case of S. epidermidis the membranes containing Cu/TNT-Ac (M3, M6, M9) exhibited significantly higher antimicrobial action compared to Cu/ TNT-N, regardless of the conditions of membrane fabrication. These differences can be attributed to the higher Cu content in case of Cu/ TNT-Ac compared to Cu/TNT-N. The antibacterial mechanism of Cu is not fully understood. Taking into account that metallic Cu is very sensitive to air and that Cu oxides are thermodynamically more stable than metallic Cu, the formation of an oxide layer on the surface of the Cu NPs typically occurs [45]. Therefore, the antibacterial properties of Cu NPs should be discussed in terms of the activity of Cu oxides. Taking this into account, Applerot et al. [46] based on their investigations on CuO antibacterial action towards Gram-negative E. coli and Gram-positive S. aureus, reported that: (i) smaller NPs (diameter of circa 2 nm) possessed higher antibacterial activity and higher affinity to bacterial cells in case of both examined bacteria compared to large NPs (circa 800 nm); (ii) CuO NPs not only attached to the bacterial surface but also penetrated the cell membrane; (iii) the reduction of bacterial growth was more efficient in case of E. coli than S. aureus, (iv) CuO NPs produced reactive oxygen species (ROS) being responsible for oxidative stress leading to bacterial cells damage. Furthermore, based on analysis of the action of Cu2+ ions and CuO NPs they concluded that the antibacterial properties are mainly due to attachment of CuO NPs to bacterial cell and ROS production by the CuO NPs rather than the action of soluble copper ions. Both phenomena cause an increase in membrane cell permeability resulting in an uncontrolled transport of CuO NPs into the bacterial cell and ultimately cells death [46]. However, in case of the mixed matrix membranes fabricated in the present study, the Cu NPs were deposited on TNTs and additionally fixed in the porous polymeric structure. Thus, the mechanism based on penetration of Cu NPs is little probable. Nonetheless, since the Cu/TNTs were present on membranes surface, as was found from AFM analysis, the mechanism can be related to a direct contact between Cu species and bacteria cells, similarly as in case of copper surfaces [47,48]. The data presented in Fig. 5 revealed that the antibacterial action of the prepared membranes was more efficient in case of E. coli compared to S. epidermidis. These results are in agreement with literature reports [46] showing higher inhibition of bacterial growth in case of Gramnegative compared to Gram-positive bacteria. This phenomenon is commonly explained by the differences in the bacterial cell wall structure. The Gram-negative bacteria do not possess a thick

Declaration of Competing Interest The authors declare that there are no conflicts of interest.

Acknowledgements This work was supported by the National Science Centre, Poland under project No. 2016/21/B/ST8/00317.

Fig. 5. Antibacterial properties of membranes against S. epidermidis and E. coli. 9

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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jwpe.2019.101098.

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