Accepted Manuscript Carbon adhered iron oxide hollow nanotube on membrane fouling Krishnan Muthukumar, D. Shanthana lakshmi, Mayank Saxena, Santlal Jaiswar, Saravanan Natarajan, Amitava Mukherjee, H.C. Bajaj PII:
S0254-0584(18)30169-X
DOI:
10.1016/j.matchemphys.2018.03.014
Reference:
MAC 20417
To appear in:
Materials Chemistry and Physics
Received Date: 19 January 2018 Revised Date:
1 March 2018
Accepted Date: 4 March 2018
Please cite this article as: K. Muthukumar, D. Shanthana lakshmi, M. Saxena, S. Jaiswar, S. Natarajan, A. Mukherjee, H.C. Bajaj, Carbon adhered iron oxide hollow nanotube on membrane fouling, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.03.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Carbon Adhered Iron Oxide Hollow Nanotube on membrane fouling Krishnan Muthukumar1, D.Shanthana lakshmi2,5*, Mayank Saxena2, Santlal Jaiswar3, Saravanan Natarajan4, Amitava Mukherjee4, H.C.Bajaj1
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Inorganic Materials and Catalysis Division1, Reverse Osmosis Division2, Marine
Biotechnology and Ecology Division3 Council of Scientific and Industrial Research (CSIR), Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), 4
Academy of Scienctific and Innovative Research (AcSIR), CSIR-CSMCRI, Bhavnagar, G. B.
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5
Centre for Nanobiotechnology, VIT University, Vellore 632014, India
Marg, Gujarat 364 002, India, Ph +91 278 2471793
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E-mail:
[email protected],
[email protected]
Abstract
Ultrafiltration membrane’s efficiency and antifouling properties can be enhanced by blending with benign tailor made inorganic nano-materials. Carbon adhered Fe3O4 hollow nanotube, nanosheet synthesized from iron alkoxide using iron (III) acetate source refluxed with methanol. The prepared Fe3O4 (nanotubes & nanosheets) were characterised by PXRD, FESEM, TEM, FT-IR and elemental analysis. Polymer nanocomposite membranes were prepared by blending PVDF (14 wt%) / Fe3O4 (2 wt%) / DMF and characterised in detail.
ACCEPTED MANUSCRIPT PVDF / Fe3O4 (hollow nanotube) composite membrane showed enhanced flux property and antifouling efficiency confirmed by BSA rejection and molecular weight cutoff (MWCO). The biofilm formation studies of PVDF/ Fe3O4 hollow nanotube composite membrane were evaluated using E.coli, fresh water Bacillus subtilis and marine Bacillus bacteria showed excellent control of biofilm growth by PVDF/ Fe3O4 (hollow nanotube) membrane. The
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antifouling studies of PVDF blank and PVDF/Fe3O4 (holow nanotube) with B. subtilis and P. aeruginosa showed 0.943± 0.02, 0.932± 0.01 A.U for PVDF blank and 0.88± 0.02 and 0.802± 0.02 A.U, and this indicates the significant antifouling control arise from the Fe nanomaterial
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Keywords
PVDF ultrafiltration membrane: Inorganic cluster molecule: Iron alkoxide: carbon contain
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Fe3O4: magnetite Introduction
The Ultrafiltration (UF) method has wide application in areas like water treatment [1] macromolecule and bacterial removal from various sources [2]. The advantage of polymeric membrane based separation is that, it can be tailor made according to the need of specific
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application [3]. UF widely used in size based separation industries like dairy and human health. Different research group reported UF membrane based on different polymers, i.e. poly
ethyl
sulfone
[4],
poly
vinylidene
fluoride
(PVDF),
polysulfone
(PSf),
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polyetheretherketone (Weert and Peek), etc. Among those PVDF has enormous potential, due to its thermal and mechanical stability. However, PVDF membrane’s hydrophobic nature facilitates the protein adsorption and excess bacterial growth on membrane surface which
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dampened the membrane efficiency [5, 6]. And this can be surmounted by well established methods like modifying hydrophobicity, surface and surface pores.(chemical grafting) [7, 8]. In addition, incorpoaration of suitable inorganic additive materials also control the biofouling property which reduces protein and other organic molecule deposition over membrane surface even after prolonged use [6, 9]. The success of any research technology depends mainly on; how it overcomes limitations with superior quality. Existing primary problems are, modifying hydrophobic nature and antibacterial property using different materials [10] thereby increase flux and molecular rejection intact [11]. Carbon contains one-dimensional hollow nanotube
where, surface
ACCEPTED MANUSCRIPT voids (defects) were developed from less benign metal sources may improve limitations of fouling. The idea behind the hollow nanotube with carbon content is; it may work as an additional surface pore and acts as a micro connecting channel on membrane surface to facilitate the flux without pore blocking. The presence of carbon in the nanomaterials control leaching effect, act as antimicrobial material and increase hydrophilic nature of the
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membrane [12-14]. The combination of benign Fe3O4 with carbon content have some additional benefits of more defects due to different oxidation state of metals. Rasel Das et al, reported surface modified carbon nanotube acts as nanochannel to increase flux and selfcleaning property. The immobilization of hydrophilic modified carbon nanotube may reject
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the hydrophilic pollutants. Ultrafiltration is widely used for macromolecule separation and metal ions removal and membrane performance degrades due to clogging of pores by biomolecules. The hydrophobic nature of protein easily binds with hydrophobic polymer
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surface results in to flux decline. By enhancing the hydrophilicity and increases nanopores using hollow materials with carbon can give solution to existing limitations in Ultrafiltration [15]. Based on the above-mentioned assumptions, we have chosen the iron metal source to produce one-dimensional Fe3O4 nanotube with predefined property i.e metal oxide with carbon content. Fe3O4 is biocompatible less hazardous and active against E.Coli bacteria and
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affordable scale up [16]. In the present investigation, the preparation of PVDF composite membranes by incorporating carbon contain magnetic Fe3O4 nanoparticles are discussed. Materials and Method
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Materials
Iron (III) acetate procured from Sigma-Aldrich. Methanol, Dimethyl formamide (DMF) was
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purchased from Merck and used without further purification. Polyvinylidene fluoride purchased from Solvay, Poly Ethylene Glycol (PEG-1000000Da), Bovine serum albumin (BSA) purchased from S.D Fine chemical Preparation of iron oxide hollow nanotube & nanosheetIron oxide nanotube is prepared as follows: Iron (III) acetate (1gm) taken in a 250ml RB flask with 60ml methanol and refluxed at 60°C for 5h. Solution colour changes from reddish brown to greenish yellow and then reaction mixture was allowed to reach room temperature, filtered and dried in a vacuum desiccator for 24 h. The obtained residue was calcined in tubular furnace at 400 °C with / without nitrogen atmosphere for 1h at the rate of 10 °C per
ACCEPTED MANUSCRIPT min ramp. The nitrogen atmosphere calcined material INP-1 and oxygen atmosphere material INP-3 are coded as such and INP-3 used to compare the role of carbon’s presence in material property. Fe3O4 nanosheets were obtained by solvothermal decomposition of iron alkoxide in methanol; in brief 1g of iron acetate dissolved in methanol and refluxed at 60 °C for 1 h and then transferred to teflon lined autoclave reactor(65ml capacity- ¾ filled) and heated to 200
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°C for 5 h in a pre-heated oven. The prepared Fe3O4 was filtered, washed with methanol and dried at 100 °C for 3h and coded as INP-2.
The Powder X-Ray Diffraction of prepared Fe3O4 materials analyzed by (Philips X’pert MPD system) using Cu Kα1 radiation (λ= 0.15406 nm). The X-ray diffraction pattern measured in
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the 2θ range of 5-80° with the operating voltage and current were 30kV and 15mA, respectively. Fourier transform Infrared Spectra (FT-IR) recorded using Perkin Elmer-
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Spectrum GX spectrometry from 400-4000cm-1 with the resolution of 4cm-1 using KBr pellets. Iron alkoxide characterized by the microscope (optical images- 40X lens), scanning electron microscope (Leo VP 1430) and transition electron microscope (JEOL, JEM 2100). The amount of carbon's presence was analysed by CHNS elemental analyer (Elementar, Vario micro cube).
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Preparation of polymer solution and membrane casting
Polyvinylidene fluoride (PVDF) 14 % solution was prepared in DMF at 50 °C to this ~ 2.8 Wt. % Fe3O4 hollow nanotube (INP-1) (w.r.t polymer weight) was added and sonicated for 10 min followed by 2 h stirring. The amount of the nanomaterial fixed based on the
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preliminary experiment results. The resultant solution was kept static for 1h, and PVDF membranes were casted over the polystyrene support using indigenous membrane casting
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instrument by phase inversion method at controlled humidity (25 %) and temperature (25 °C). The prepared membrane without nanomaterial coded as M-1 and with Fe3O4 hollow nanotube coded as M-2 were twice washed and soaked in water for five days to remove the solvents and then characterized. Similarly same procedure followed to prepared PVDF membrane with Fe3O4 nanosheet (INP-2) and nanorod (INP-3), the prepared membrane coded as M-3 and M-4. To ensure the homogeneous nanomaterial distribution, nanomaterial dispersed in DMF and sonicate using high energy sonicator for even distribution and it dispersed in the polymer solution. Further, mechanical stirring and followed by high sonication used.
ACCEPTED MANUSCRIPT Membrane characterization The viscosity of the prepared PVDF/iron nanomaterial solution was measured by Brookfield DV-II + Pro Viscometer, using LV3 spindle at different RPM (10 to 80). The observed viscosity is stable with different shear rate (Rotation per minute, RPM); the average value
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was taken. Membrane hydrophilicity is measured by change of contact angle using KRUSS-DSA 100 instrument. The water droplet kept on the membrane surface, and measurements were made after droplet stabilization. These experiments were repeated for six times, and average values
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were reported.
Attenuated Total Reflectance- Infrared Reflectance Spectroscopy (ATR-IR) was used to
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analyse the polymer backbone’s atomic level interaction with nanomaterial. The ATR-IR spectra were taken from 400 to 4000 cm-1 with 4 cm-1 resolution, Ge crystal probe directly contacts with membrane surface. The glass transition temperature of the membrane was analyzed by Differential Scanning Calorimeter (NETZSCH DSC 204F1 Phoenix 240-120239-L) at 60ml/N2 gas purging condition. Thermogravimetric Analysis (TG-209 Libra NETZSCH) was carried to find the residual mass of the membrane, from 50 to 800 °C with
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10 °C per min temperature increase.
Membrane surface roughness was analysed by atomic force microscopy (AFM) - Scanning Probe Microscope (NT-MDT, Netgra AURA) in semi-contact mode. About 1cmxcm size
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membrane was fixed on glass plate and scanned. The surface charge of the prepared PVDF/ Fe3O4 membrane was measured by Zeta potential
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(Zeta CAD Keithley ZC 1200 S/N 178) using 2mmol KCl solution at 40 bar nitrogen pressure. The analysis conducted up to the equilibrium plot. Porosity of the prepared PVDF blank/ Fe3O4 membranes measured by, 3 X 3 cm size membranes pieces dipped in water for about 16 h. After that surface water was gently sorbed by tissue paper and weighed. Same portions of the membrane were dried in an oven at 70 °C till it attained constant weight. The membrane’s porosity calculated by the following equation % =
− × 100 × ×ρ
ACCEPTED MANUSCRIPT Where Wwet and Wdry are the weight of the membranes in wet and dry conditions respectively, A is the membrane area; d is the density of water and ρ is the thickness of the membranes.[17]
Bacterial culture preparation and biofilm adhesion test on membrane
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Bacteria cultivation done (Escherichia coli (E.coli), Bacillus Subtilis (B. Subtilis)in 30ml of culture tube with LB (Luria-Bertani) and ZMB (Zobell Marine Broth) broth procured from Himedia at 30ºC with shaking speed of 180 rpm. The bacteria culture was harvested in their exponential growth phase by using centrifuge (Sigma, 3K30, UK) at 5000rpm for 10 min at 4
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°C. The obtained pellet washed carefully with phosphate buffer and the pellet again gently redispersed in to 10ml of lake water and sea water (filtered through 0.45µm membrane).[18]
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Bacterial adhesion on the membrane surfaces tested by shake flask method. Prior to experiment membranes were sterilized using ultraviolet radiation in the laminar air cabinet for 30 min (Esco, USA), four to five pieces of 1cm square size were placed in all the bacterial suspensions at 30 ºC with shaking at180rpm for 24 h. Fluorescence microscopic (Axioimager M1, Zeiss, Germany) method used to check the biofilm growth on the membranes using
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alcian blue staining method [19].
Observation of live cells through fluorescent microscope The presence of physical adherence of live bacterial cells was tested by LIVE/DEAD
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BacLight Bacterial Viability Kit (L-7007, Molecular Probes), method.[20] In brief, membranes were removed from bacterial suspension and washed 3 times with mili-Q water and placed on the microscopic slide. For staining bacteria, equal volumes of two components
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(Propidium iodide (PI) + SYTO 9) were mixed. 3 µl of this mixture carefully added to each ml of bacterial suspension as per user instruction. Then, 5 µl of this mixture spread over the membrane surface and incubated for 20 min in dark for stain adsorption by microorganism. Finally, the unbounded stain washed by water and analysed through fluorescence microscope. The filter (Green fluorescent protein) used for fluorescence microscopy was BP 450-490, FT 510, LP 520 and images were captured through AxioCam HRC camera. Biofilm Growth formation studies by Alcian Blue The membrane in the incubation medium removed after 24h contact time, washed with miliQ water to remove the merely adhered bacteria on the surface. Membranes were tested for the
ACCEPTED MANUSCRIPT presence of biofilm formation on the surface. The membranes were then incubated in 0.5 % of the alcian blue solution for about 30 min, after that it was washed with distilled water to remove excess stains. The blue colour development identified using microscope with 100X oil immersion lens at the bright field.[21]
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Slime formation test Slime formation on PVDF nanocomposite membranes was evaluated according to protocol adopted by [22]. PVDF (Blank) (M1), Fe-Nanosheet (M2), Fe-Nanorod (M3), and FeNanotube (M4) incorporated PVDF membranes were interacted with gram positive (B. subtilis) and gram negative Pseudomonas aeruginosa (P. aeruginosa) bacterial strains for 24
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h. After the 24 h of interaction, the bacterial slime adhesion on the surface of PVDF membrane was measured using sequence of staining and washing steps. The interacted PVDF
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membrane was immersed in carnoy’s solution in the ratio of absolute ethanol: chloroform: glacial acetic acid (6:3:1) was added and incubated for 30 mins. The membrane was further stained using toluidine blue (0.1%) for 1 h follwed by rinsing with 3 ml Milli-Q water, then 0.2M of NaOH was added and heated for 1 h at 85°C. The sample was then cooled in room temprature and the absorbance was measured at 590 nm using UV-Vis spectrophotometer.
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Statistical analysis
Experiments values are mentioned as mean ± SE. The significance difference between the various exposure conditions and various strains was determined by two-way ANOVA. The
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statistical significance difference was accepted at a level of P >M-2(̴ 77.5%) >M-3 & M-4 (̴ 72%) in the order. The formation of pores during phase inversion was controlled by the polymer concentration, solution viscosity and inorganic fillers solid –liquid exchange rate. It is well known fact that material with higher viscosity provide more dense structure, due to slow exchange (kinetics) of solvent molecules with coagulation bath solution than less The viscosity results (Table-1) support the porosity and pore
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viscous solution [34] .
formation process in M-3 & M-4. The ironic result in M-2 indicates that there may be some other reason than polymer solution’s viscosity for the porosity and pore formation. During the phase inversion process liquid-liquid mixing and solid liquid mixing (inorganic material)
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alters the solvent exchange rate and size of the nano material also affects the morphology of the membrane. [35] reported the effect of inorganic material size, property based solvent exchange and morphology change in the phase inversion process [36] . Further to understand
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the relation between pore formation, porosity and nanomaterial during the phase inversion,
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membranes were characterised by FE-SEM (Fig. 7).
Fig. 7. FE-SEM Image of M-1, M-2, M-3 and M-4 Membrane.
ACCEPTED MANUSCRIPT Fig. 7. FE-SEM images of all membranes show less surface porous nature, M-1 membrane (blank PVDF) with 40-60 nm surface pore size. Whereas, Fe3O4 nanomaterial incorporated M-2 & M-3 membrane showed same surface porosity, in particular, M-2 membrane, showed non-uniform porous nature and M-3 showed uniform porous nature. As discussed above, viscosity changes and nanomaterial size and shape might be a reason for the surface
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morphology was substantiated by FE-SEM image. For an instant, to know the impact of different shape nanomaterial incorporated into membrane, FE-SEM cross section images (See- supportive information) indicates less significant changes in cylindrical and micropores in membrane. But the layer thickness is different for M-3 in comparison with M-2 & M-4 and
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at higher magnification cross section image of M-3 showed more tiny nanopores exists in the surface layer. From the FE-SEM image, the role of nanomaterial size and shape in surface pore formation and morphology was confirmed. In order to find the nanomaterial distribution
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inside the membrane, elemental mapping was carried out for M-2 & 3 membranes (M-4 having the similar type of nanostructure).
The elemental distribution data Figure 8 showed an even distribution of Fe in M-3 membrane rather than the M-2 and this variation may be due to size and shape of nanomaterial. Also, the larger distribution of nanosheet inside the membrane may be the reason for the uniform
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formation of surface pores and asymmetric cylindrical pore morphology. But in the case of M-2, Fe3O4 hollow nanotube spreads randomly inside the membrane and elemental mapping confirms it in-between pores. This type of distribution may act as an additional connection/
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interlink between pores and may enhance the water permeation of the membrane.
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Fig. 8. Cross-Sectional Elemental Mapping of M-2 & 3 Membrane.
Flux property
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Plain water flux
Fig. 9. shows the plain water flux permeability in the following order M- 2 > M- 3 > M-4 > M-1, higher flux observed for M-2 may be due to more pores present on membrane surface and the incorporated hollow nanotube (INP-1) works as a connecting channels as mentioned in elemental mapping. Moreover, the presence of hollow structure in the
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membrane facilitates the water permeability as compare to the other nanomaterial incorporated membrane. Whereas, the decreased water flux observed in M-3 may be due to the morphology change by nanomaterial in the phase inversion, and it elaborately discussed. On the other hand, the solid-liquid diffusion during the phase inversion process also plays a
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vital role in the membrane morphology [37]. In addition, the presence of additives the surface tension of the polymer solution and this impacts the membrane morphology [28]. In nutshell,
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the incorporated nanomaterial property have a great impact in the membrane morphology, and its typical behaviour is the reason for the decreased plain water permeation behaviour of the M-3 and M-4 membrane. In line, the plain water permeation test reveals the surface pore based plain water flux, and it evincing the nanomaterial size and shape induced surface morphology.
ACCEPTED MANUSCRIPT 10 psi
20 psi
30 psi
40 psi
LMH
90
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60
M-1
M-2
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30 M-3
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Membrane
M-4
Fig. 9. Plain Water Flux of M-1, 2, 3 & 4 Membrane.
BSA rejection
Fig. 10. BSA rejection studies of M-1, 2, 3 & 4 composite membrane gives higher flux values with increasing pressure (Fig. 11.) indicates anti-fouling nature. Though all composite
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membranes (M-2,3& 4) showed a slight variation in BSA flux, CA measurement of M-2 membrane showed fractional changes after
BSA contact
which indicates the higher
hydrophilic nature of M-2 surface. Fig. 11. showed the molecular weight cut-off (MWCO) and the observed flux increase was 100 % with very small amount of rejection observed in
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the case of M-2. The rest of the membranes (M-3 & 4) also showed the higher flux than M-1 & M-2 with more rejection. The smaller rejection and higher flux of M-2 membrane may be
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due to the presence of additional pores generated by the hollow nanotubes (Fig. 11.) / increased hydrophilic nature that restricts the protein absorption over the membrane surface. Here, the observed changes in M-2 membrane reveals its property retained without much changes. The observed MWCO, BSA rejection, and plain water flux results all provide additional evidence for nanomaterial structure and shape based hydrophilicity changes, so based on the appreciable results the prepared composite membranes were tested against the microbial biofouling control.
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M2
M3
M4
40
LMH
30
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20
10
20psi
30psi Pressure
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10psi
40psi
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Fig. 10. BSA (500 ppm) Solution Flux of M-1, 2, 3 & 4 Membrane.
70
90
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LMH
60
100
50
70
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80
R(%)
80
30
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20
M-1
60 M-2
M-3
M-4
Sample
Fig. 11. Flux at 25 psi, Polyethylene Glycol Molecular Weight -100000.
Biofilm formation studies The prepared membrane used to study the biofilm formation using E.coli (Fig. 12) and it reveals the good control for M-2, 3 & 4 membrane rather than M-1 (blank membrane) The observed live and dead cell discrimination studies substantiate biofilm results with less bacteria adherence on membrane surface, in particular nanomaterial incorporated membranes.
ACCEPTED MANUSCRIPT Generally bacterial adherence takes place via formation of extracellular polysaccharide (EPS) formation over the substrate and followed by the anchoring of bacteria/colony formation. [38].This EPS formation in membrane surface mostly depends on surface hydrophobicity, surface morphology, chemical nature and available nutrient medium [39-41]. The material incorporated in the PVDF membrane is an iron oxide which is well known for E. coli
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bacterial growth control.[42] So, to find out the reason, fresh water Bacillus subtilis and marine B. subtilis bacteria were also tested for one week in nutrient medium (ESI- S4 & S5). The results showed a similar trend for M-2 membrane and slight growth observed in M-3 & 4. But, the overall biofilm control for all the membrane was better than M-1 (blank PVDF).
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The enhanced anti bio-fouling property of M-2 membrane may be due to the hydrophilic nature and CA values of M-2 after BSA treatment also supports the same. So, this may be the
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reason for the enhanced anti- bio-fouling property of the M-2 membrane than others.
Fig. 12. Biofilm studies, live and dead cell discrimination studies of PVDF membrane with
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E.coli, medium –Lauria-Bertani Broth, time-7 days, Temperature 37 °C. Slime formation study
The antifouling activity of Fe NPs incorporated PVDF membranes against B. subtilis and P. aeruginosa were evaluated under dark and UV- C condition (Fig.13). PVDF Blank membrane (M1) showed significantly higher slime formation under dark and UV-C condition against B.subtilis (P0.05). For P.aeruginosa, there was no significant difference was observed between blank and Fe incorporated film under UV-C condition (P>0.05), whereas under dark condition M3
ACCEPTED MANUSCRIPT (Fe-Nanosheet) and M4 (Fe-Nanorod) showed significant difference (P0.05). Decrease in bacterial slime formation in Fe incorporated PVDF films indicates its anti-biofouling property as compared to that of pristine PVDF membrane. Impregnation of Fe on PVDF membrane effectively prevented the formation of biofilms. Similar to our observation, (Li et al. 2013) reported that the Ag/PVDF
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membranes efficiently prevented the formation of biofilms of E. coli.
Fig. 13. Slime formation in PVDf composite membrane with B.subtilis and P. aeraginose.
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Conclusion
In conclusion, we have successfully prepared carbon contain magnetic Fe3O4 hollow nanotube and nanosheet by thermal decomposition of iron alkoxide in nitrogen atmosphere at 400 °C and solvothermal decomposition in methanol at 200 °C for 5 h. The prepared
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magnetic Fe3O4 was confirmed by different analytical methods like PXRD FT-IR and SEM, TEM and other techniques. PVDF ultrafiltration membranes were prepared using 14 wt %
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PVDF with different shape Fe3O4 nanomaterials. Among the prepared membranes Fe3O4 hollow nanotube contain (M-2) showed enhanced flux, good BSA rejection (fouling controlling property). Also M-2 membrane showed enhanced anti-bio-fouling property towards E.coli, freshwater and seawater Bacillus subtilis for 7 days. The results reveal the excellent biofilm growth control of M-2 membrane.
The antifouling studies of PVDF
Blank/composite membrane (M1) showed higher slime formation under dark and UV-C condition against B.subtilis and P.aeruginosa (P
