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Self-assembled silica nanocrystal-based anti-biofouling nanofilter membranes3advances
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  Cite this:  RSC Advances , 2013,  3 , 458 Self-assembled silica nanocrystal-based anti-biofoulingnanofilter membranes 3 Received 7th June 2012,Accepted 29th October 2012 DOI: 10.1039/c2ra21135a www.rsc.org/advances Ajay K. Singh, Ravi P. Pandey, Amaranadh Jasti and Vinod K. Shahi* Herein, we report synthesis of organosiloxane 3-(2-((3-aminopropyl)diethoxysilyl)ethylthio)-5-(4-((3-aminopropyl)diethoxy silyl)phenyl)-4 H -1,2,4-triazol-4-amine (TS) by Barbiar–Grignard reaction. Hybridnanofiltration (NF) membranes were prepared from TS and poly(vinyl alcohol) (PVA)  via  sol–gel processfollowed by cross-linking and grafting of phosphonic acid groups. Physicochemical properties of thesemembranes revealed their stable (thermal, mechanical, and chemical) and chlorine tolerant nature. HRTEManalysis reveals homogenous silica distribution in the membrane matrix. The cross-link density of themembrane and the preparation conditions were studied in terms of NF performance. An  Escherichia coli bacterium was used to study antibacterial activity and anti-biofouling properties of the hybrid membrane.The short-term bacterial stability test showed that membrane TS-60a has good anti-fouling properties.Moreover TS-60a membrane showed excellent stability and anti-biofouling capability in long-timeoperation. Introduction Population growth, industrialization, and contamination(personal care products, agricultural pesticides, heavy metals,micro-pollutants, drugs, chemicals, and microbes) are seriousproblems for providing safe and clean drinking water. 1–4 Membrane technologies for water desalination/purificationhave a wide range of advantages and attract wide attention fordeveloping new types of stable and sustainable membranes. 4 Nanofiltration (NF) membranes show many advantages suchas low energy consumption, and high permeation flux performance over reverse osmosis (RO). 5,6 The surface chargeof NF membranes plays an important role due to Donnanexclusion and is responsible for high flux, salt rejection andfouling resistivity. 7,8 Generally, NF membranes have been derived from poly-amide, 9 sulphonated polysulfone, 10 and sulphonated polyphe-nylene oxide. 11  At present, NF membranes are prepared by different techniques such as phase inversion, 12 chemicalcross-linking, 13 photo-radiation plasma, 14 low-temperatureplasma, 15 and layer-by-layer deposition technique. 16 Polyamide membranes are chemically unstable in the pre-sence of sanitizing agents, especially chlorine. 17 Furthermore,the membrane is itself susceptible to fouling (organic fouling,inorganic fouling, colloidal fouling, and biofouling).Examination of fouled NF membranes exhibited more than50% (w/w) of dry fouling layer, which was of biologicalsrcin. 18 To avoid biofouling, generally anti-adhesion andantimicrobial strategies for NF/RO membranes have beenadopted. 19 In antimicrobial strategies generally silver nano-particles have been used, which does not comply with theregulations of the United States Environmental Protection Agency (USEPA) for water quality (silver content should be lessthan 3.4 ppb). Leaching of silver nanoparticles may pose aregulatory risk for applications in water treatment. 19 Chargedor hydrophilic modified NF membranes with a smooth surfaceare less sensitive to biofouling. 19  Anti-biofouling NF mem-branes are challenging because of their architecture (pore size,surface roughness and charged nature), hydraulic resistance,separation performance, mechanical stability, chemical stabi-lity, cost and flexibility of assembly. Attempts were made to develop organic/inorganic hybridNF membranes due to their rigidity, stability (cross-linking),and ease of tailoring the pore architecture and processabil-ity. 20 Thus, there is great opportunity for development of organic/inorganic hybrid membranes using polysiloxanes andpoly(vinyl alcohol) (PVA). Polysiloxanes are used for biomedi-cal applications, 21–25  while PVA forms chlorine-tolerant stableether-type linkages due to acetalisation with formaldehyde. 26  We reported 3-((4-(5-(2-((3-aminopropyl)diethoxysilyl)ethylthio)-1,3,4-oxadiazol-2-yl)phenyl)diethoxysilyl)propan-1-amine (APDSMO)-based quaternary ammonium-grafted hybrid NF membranes with antimicrobial activity. 27 But, the 1,2,4-triazole ring alsoshowed antibacterial and antifungalactivity. 28 Thus,the hybridNF membranewith1,2,4-triazoleringis alsoexpectedtoexhibit antimicrobial activity, good performance and stabilities.  Electro-Membrane Processes Division, Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G.B. Marg, Bhavnagar-364002 (Gujarat), India. E-mail: vkshahi@csmcri.org; vinodshahi1@yahoo.com; Fax: +91 278 2567562/6970; Tel: +91-9925125760 3  Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra21135a RSC Advances PAPER  458  |  RSC Adv. , 2013,  3 , 458–467 This journal is   The Royal Society of Chemistry 2013    D  o  w  n   l  o  a   d  e   d  o  n   0   2   M  a  r  c   h   2   0   1   3   P  u   b   l   i  s   h  e   d  o  n   0   6   N  o  v  e  m   b  e  r   2   0   1   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   2   R   A   2   1   1   3   5   A View Article Online View Journal | View Issue  Herein, we report phosphonic acid-grafted hybrid NFmembranes, containing 4-amino-1,2,4-triazole. Membranepreparation was achieved in aqueous media by acid-catalyzedsol–gel followed by cross-linking. Grafting of the phosphonicacid group, tailoring of pore texture and incorporation of antimicrobial active compounds, were responsible for anti-biofouling and high membrane performance. Experimental section Materials  Aminopropyltriethoxysilane (APTEOS) (99%) and polyethyleneglycol (PEG; different molecular weights) were obtained fromSigma-Aldrich chemicals. Poly(vinyl alcohol) (Mw: 125 000),formaldehyde (37% in water), phosphorous acid, hydrochloricacid, sulphuric acid,  p -chlorobenzoic acid, hydrazine hydrate,carbon disulphide, sodium hydroxide, sodium chloride, sucrose,magnesium chloride, sodium hypochlorite,  N  -methylpyrrolidone,dimethylformamide, hexane, iodine crystals, magnesium turn-ings, tetrahydrofuran (THF), streptomycin,fluconazole, acetoneand methanol were obtained from SD. Fine Chemicals, India.Solvents were used after proper distillation and double-distilled water was used for experiments. Synthesis of organosiloxane (TS) Synthesis of 3-(2-chloroethylthio)-5-(4-chlorophenyl)-4  H  -1,2,4-triazole-4-amine (TC) was achieved through multistep synth-esis (Scheme 1; details of TC synthesis are included in sectionS1, ESI 3 ). Synthesis of organosiloxane (TS) has been reportedearlier. 20,27,29 In a general preparation method, to a three-necked round-bottom flask (250 ml, equipped with a magneticstirrer, nitrogen inlet, drying tube, condenser, and additionfunnel), a mixture of Mg turnings (1.5 g) and APTEOS (44.4 ml;0.2 mol) in THF (30 ml) and iodine crystals were added underconstant stirring conditions. After 30 min, 20 mL solution of 3-(2-chloroethylthio)-5-(4-chlorophenyl)-4  H  -1,2,4-triazol-4-amine(TC) (2 mmol) in THF was added dropwise over 30 min, thenfurther refluxed at 10 h. After 10 h the clear solution turned toa greenish yellow liquid. The reaction mixture was cooled at room temperature and THF was removed by vacuum rotary evaporator. Viscous liquid was washed with 30 ml hexane anddried under vacuum.Yield: 80% (yellow coloured transparent semisolid). IR (KBr)cm 2 1 : 3395 (–NH), 2981 (–Ar–H), 1590 (C L N triazole), 1528(NH), 1317 (N–N), 1013 (Si– O –C), 760 (Si–Ph), 681(C–Si);  1 HNMR (D 2 O):  d  7.67 (Ph–Si), 7.28 (Ph–triazole), 3.47, 1.02 (– O –CH 2 –CH 3 ), 3.43 (–OCH 2 CH 3 ), 2.63, 1.47 (–SCH 2 CH 2 ), 2.57,(–CH 2 –CH 2 –Si–) 0.99(–CH 2 Si–Ph), 0.43,0.45 (–CH 2 Si);  13 C NMR (DMSO-d 6 ):  d  174.68, 173.55, 136.3, 130.54, 128.18, 17.66,57.23, 42.49, 30.25, 23.07, 16.91, 10.27. Anal. calcd for[C 24 H 46 N 6 O 4 SSi 2 ] (570.91): C, 50.45; H, 8.15; N, 14.76; S, 5.64,Si, 9.87%. Found: C, 50.5; H, 8.13; N, 14.71; S, 5.61; Si, 9.84%. Preparation of hybrid NF membrane PVA was dissolved in distilled water (10 wt%) under stirring conditions for 24 h. Further, TS (desired amount) was added toPVA solution under constant stirring (300 rpm) for 6 h.Obtained solutions were transformed into a gel by adding 1 MHCl and pH was maintained according to Table 1. Theresultant white viscous gels were cast on a PVC sheet and thedesired thickness was maintained with a doctor blade. Afterpartial drying (10 min), thin films were gelated (precipitated)in hexane at 10  u C for 20 min. The membrane was furtherdried at room temperature for 24 h (slow evaporation of solvent) followed by in a vacuum oven at 80  u C for 24 h. Theobtained membrane was cross-linked by formal reaction under varied cross-linking conditions (Table 1). Cross-linked mem-branes were subjected to phosphorylation using 1 : 1 (w/w)formaldehyde and phosphorus acid, for 3 h at 70  u C. Different prepared membranes were designated as TS-  X  (  y ), where  X   isthe weight percentage of TS, and  y  denotes different cross-linking conditions, as described in Table 1. Instrumental characterization of the membranes FTIR spectra of dried membrane samples were obtainedSpectrum GX series 49 387 spectrometer in the range of 4000–450 cm 2 1 . The IR spectrum for a synthesized intermediate was Scheme 1  Schematic reaction steps involved in preparation of organosiloxane(TS). This journal is   The Royal Society of Chemistry 2013  RSC Adv. , 2013,  3 , 458–467  |  459 RSC Advances Paper     D  o  w  n   l  o  a   d  e   d  o  n   0   2   M  a  r  c   h   2   0   1   3   P  u   b   l   i  s   h  e   d  o  n   0   6   N  o  v  e  m   b  e  r   2   0   1   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   2   R   A   2   1   1   3   5   A View Article Online  obtained by the KBr pellet method. Wide-angle X-ray diffracto-grams of the nanocomposite membranes were recorded using Philips Xpert X-ray diffractometer with Cu-K a  (1.54056)radiation.  1 H and  13 C were used to characterize the synthe-sized material recorded by an NMR spectrometer (Bruker 500MHz) in a D 2 O and d 6 -DMSO solvents.The thermal degradation processes and stabilities of themembranes were investigated using a thermo gravimetricanalyzer (Mettler Toledo TGA/SDTA851 with  Star   software)under a nitrogen atmosphere with a heating rate of 10  u Cmin 2 1 from 30 to 450  u C. Differential scanning calorimetry (DSC) measurements were carried out in a temperature rangeof 30–450  u C with a heating rate of 5  u C min 2 1 . The dynamicmechanical stabilities of the composite membranes wereevaluated by using a Mettler Toledo dynamic mechanicalanalyzer 861 instruments with Star software under nitrogen with a heating rate of 10  u C min 2 1 from 30 to 320  u C to verify the effect of the silica content in the form of a polymericmembrane.The surface morphology of thoroughly dried membranes was studied by a JEOL 1200EX transmission electron micro-scope (TEM). The JEOL 1200EX transmission electron micro-scope with tungsten electron source was operated at anaccelerating voltage of up to 120 kV. Atomic force microscopy (AFM) images of dried membranes were recorded using anNTEGRA AURA (NTMDT). Semi-contact mode SPM NSG 01 tip was used to determine the surface roughness. The tip usedhad a radius of curvature of approximately 10 nm and thenatural frequency for the cantilever was 300 kHz. In thismeasurement mode, the cantilever where the tip is locatedoscillates with its natural frequency and the sample topogra-phy is obtained from the subsequent changes in the oscillationamplitude. Differences in viscoelastic properties can bedetected from the changes in the oscillation phase. Forscanning electron microscopy (SEM), gold sputter coatings were carried out on desired membrane samples at pressuresranging between 1 and 0.1 Pa. The sample was loaded in themachine, which was operated at 10 2 2 to 10 2 3 Pa with EHT15.00 kV with 300 V collector bias using Leo microscope torecord SEMs. The optical densities of microbial solutions wereevaluated by using a VARIAN 50 bio UV-Vis spectrophotometerinstruments. The Total Organic Count (TOC) content in thefeed and permeate solutions was obtained by employing acustomized acid digestion method, followed by analysis with aliquid TOC elementar. Membrane permeation measurements  Water flux across hybrid membranes was measured in a two-compartment cell separated by circular membrane disk (4.0cm 2 ) placed at the bottom of the cell with top active layertowards the feed solution. Feed compartment was well stirredby a mechanical stirrer (300 rpm). Electrolyte concentration inthe feed and permeate was determined by conductivity measurements (Toshniwal TCM 15). 30,31 For the neutralorganic molecule solutions, a customized total organic carbon(TOC) digestion method was used to quantitatively measurethe concentration of neutral organic solutes in the permeateand in the retentate. Calibration plots for all three types of analyses were run with standard feed solutions prior to thestudies to ensure accuracy of the measurements. Water flux and solute rejection were estimated by following equations: Flux (l m { 2 h { 1 ) ~  permeate (l)membrane area (m 2 ) | time (h) (1)Rejection ~  1 { permeate concentrationfeed concentration    (2) The chlorine-tolerant nature of the membranes was assessedin terms of flux and rejection performance, after membranetreatment in sodium hypochlorite (pH 5–6) aqueous solution(5000 ppm) solution for 24 h. Table 1  Cross-linking densities and preparation conditions for different membranes MembraneMembrane preparation conditionsCross-linking density (mol g  2 1 )Formaldehyde concentration (wt%) Time (h) pH  T  / u CTS-30 2.5 3 2 60 0.009TS-40 2.5 3 2 60 0.0302TS-50 2.5 3 2 60 0.061TS-60a 2.5 3 2 60 0.0808TS-60b 2.0 3 2 60 0.0696TS-60c 1.5 3 2 60 0.0454TS-60d 1.0 3 2 60 0.0062TS-60e 2.5 4 2 60 0.0542TS-60f 2.5 2 2 60 0.0122TS-60g 2.5 1 2 60 0.0818TS-60h 2.5 3 3 60 0.0622TS-60i 2.5 3 4 60 0.0316TS-60j 2.5 3 5 60 0.0102TS-60k 2.5 3 6 60 0.0002TS-60l 2.5 3 2 70 0.082TS-60m 2.5 3 2 50 0.011TS-60n 2.5 3 2 40 0.0068Error limits: Formaldehyde concentration ( ¡ 0.1 wt%), time ( ¡ 0.05 h), pH ( ¡ 0.05), temperature ( ¡ 2  u C), and cross-linking density ( ¡ 0.0002). 460  |  RSC Adv. , 2013,  3 , 458–467 This journal is   The Royal Society of Chemistry 2013 Paper RSC Advances    D  o  w  n   l  o  a   d  e   d  o  n   0   2   M  a  r  c   h   2   0   1   3   P  u   b   l   i  s   h  e   d  o  n   0   6   N  o  v  e  m   b  e  r   2   0   1   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   2   R   A   2   1   1   3   5   A View Article Online   Antibacterial activity   Antibacterial properties of hybrid (TS-X) NF membranes weretested against   Escherichia coli   bacteria by using reportedmethods. 20,27 Standard testing protocol for water-insolubleantimicrobials was used. 32,33  Antibacterial tests were per-formed in triplicate to insure reproducibility. Results and discussion Membrane structure The monomer precursor (TS) was synthesized by Barbier–Grignard reaction using TC and APTEOS (Scheme 1).  1 H NMR spectra for TS showed two peaks at 7.26 and 7.67 ppm due tophenyl ring protons (Fig. S1, ESI 3 ). The presence of triazoleand phenyl rings were confirmed by   13 C NMR signals at   d  value174.6, 173.5 and 136.3, 130.5, 128.1, 117.6 ppm (Fig. S2, ESI 3 ). A carbon peak for –OCH 2 – (57.23 ppm) confirmed the presenceof a –OCH 2 CH 3  group in TS.Guide peaks in FT-IR spectra of TC (Fig. S3, ESI 3 ) confirmedreaction between 5-(4-chlorophenyl)-1,2,4-triazole-4-amine-2-thiol and 1,2-dichloroethane in the presence of weak base.TC showed two medium intensity bands at   ca.  745 and 661cm 2 1 ( n (Ph–Cl) and  n (C–Cl)). After Barbiar–Grignard reactionthese two bands disappeared and new bands appeared at   ca. 776 and 691 cm 2 1 ( n (Si–Ph) and  n (Si–C) vibration band).Presence of triazole ring in TC and TS was confirmed by thestrong intensity band at   ca.  1080 and 1095 cm 2 1 (C–N–Cstretching vibration) (Fig. 1). TS showed medium intensity bands at 3433 cm 2 1 and 1552 cm 2 1 ( n (N–H), while the band at  ca.  1596 cm 2 1 ( n (C L N) was assigned to the triazole ring. 33 Inthe case of TS-60a membrane, a broad band at   ca.  1400–1445cm 2 1 and a strong band at   ca.  1649 cm 2 1 confirmedphosphorylation of the –NH group. The absorption band at around 1260 cm 2 1  was a characteristic band of the –Si– O –Si– Fig. 1  FT-ATR spectra of TS-60a and TS-30 membrane. This journal is   The Royal Society of Chemistry 2013  RSC Adv. , 2013,  3 , 458–467  |  461 RSC Advances Paper     D  o  w  n   l  o  a   d  e   d  o  n   0   2   M  a  r  c   h   2   0   1   3   P  u   b   l   i  s   h  e   d  o  n   0   6   N  o  v  e  m   b  e  r   2   0   1   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   2   R   A   2   1   1   3   5   A View Article Online  asymmetrical stretching vibration. This confirmed molecularlevel hybridisation between organic and inorganic segments. 34 The band at   ca.  1021 cm 2 1 (–C– O –C– stretching vibration foracyclic ether ring of PVA) showed formal cross-linking withformaldehyde. These results confirmed formation of aphosphonic acid-functionalized cross-linked hybrid mem-brane. On the basis of spectral and elemental analysis,structures for TS and hybrid NF membranes were proposed(Scheme 1 and Scheme S1 (ESI)). Surface morphologies of hybrid NFM TEM images (TS-60a membrane as representative case) (Fig. 2)revealed homogeneous silica distribution in the membranematrix. HRTEM (Fig. 2a&b) analysis confirmed 2–5 nm particlesize of silica and inter-planar distance was below 0.24 nmconfirming the crystalline silica in the membrane matrix. 35  WXRD analysis showed an increase in the amorphous region with TS content (reduced PVA content), which is a favourablecondition for formation of Si– O –C bonds (availability of moresilica) (Fig. S4, ESI 3 ). The absence of evident peaks in TSshowed its amorphous nature.Scanning electron microscopy results are systematized inFig. 3 and Fig. S5, (ESI 3 ). Surface analysis of membrane TS-60a& TS-30 showed the upper surface is dense (Fig. 3a&c), whilethe bottom surface in the membrane has a porous nature(Fig. 3b&d). The bottom surface of the TS-60a membrane lookslike a honeycomb structure (Fig. 3b). Membrane cross-sectionSEM images showed that the membranes are arranged with alayer morphology and the upper surface in dense in nature while lower surface is porous in nature (Fig. 3c&f). The surfacemorphology of the synthesised organosiloxane looks like acrushed stone (Fig. S5a, ESI 3 ) and the elemental mapping datasupport their elemental analysis (Fig. S5b, ESI 3 ). The success-ful grafting of the membrane surface by phosphonic acid wasconfirmed by the phosphorous mapping (green dot) in TS-60aand TS-30 membrane (Fig. S5c&d, ESI 3 ). The synthesizedorganosiloxane (TS) elemental analysis is further supported by the EDX analysis (Fig. S5e, ESI 3 ).Membrane fouling is controlled by interaction between themembrane surface and solute such as protein, carbohydratesand bacteria. Thus surface roughness of the membrane isimportant in determining the susceptibility to membranefouling. Previously, a number of studies suggested that surfaceroughness may be the most important factor to avoidmembrane fouling. 19 Membrane surface images were obtainedin semi-contact mode of AFM study and the pictures aresystematized in Fig. 4. For the surface roughness ana-lysis, measurement of a small portion of the membrane(1.4  6 1.4  m m) were selected and NOVA P9 INTEGRA software was used for analysis. Obtained data concluded that mem-brane roughness decreased with increasing organosiloxane(TS) content. TS-60a membrane has the lowest surface rough-ness 55 nm. Thermal, mechanical, and chemical stability  Thermal stabilities of pristine and hybrid membranes wereanalysed by thermogravimetric analysis (TGA) (Fig. S6, ESI 3 ).The NF membranes showed two-steps of weight loss. The first step (50–170  u C) occurred due to dehydration, while thesecond step (280–450  u C) occurred due to degradation of theorganic segment. After 450  u C, no significant weight loss wasobserved. TS-60a membrane showed higher weight loss (60%)in 300–400  u C, indicating that more phosphoric acid iscontained in this membrane. At 500  u C, the remaining weight indicates that membrane TS-60a was thermally stable incomparison to other synthesized membranes. DSC thermo-grams (Fig. S7, ESI 3 ) revealed dependence of   T  g   on organosi-loxane (TS) content. The increase in glass transitiontemperature ( T  g  ) (85–114  u C) from TS-30 to TS-60a membraneindicates that cross-linking and ionic interactions betweenrespectively organic and inorganic segments increased. The  T  g   value of pristine PVA membrane was 78  u C. 36 Mechanical stability is another problem for NF membranes. We determined Young’s modulus with DMA analysis. With thehelp of the Young’s modulus, membranes’ cross-linking densities were determined by following equation. 37 Fig. 2  TEM images of NF membrane (TS-60a): (a) low resolution and (b) high resolution (linear inter-planar lines were separated by 0.24 nm). 462  |  RSC Adv. , 2013,  3 , 458–467 This journal is   The Royal Society of Chemistry 2013 Paper RSC Advances    D  o  w  n   l  o  a   d  e   d  o  n   0   2   M  a  r  c   h   2   0   1   3   P  u   b   l   i  s   h  e   d  o  n   0   6   N  o  v  e  m   b  e  r   2   0   1   2  o  n   h   t   t  p  :   /   /  p  u   b  s .  r  s  c .  o  r  g   |   d  o   i  :   1   0 .   1   0   3   9   /   C   2   R   A   2   1   1   3   5   A View Article Online
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