High flux hyperbranched starch-graphene oxide piperazinamide composite nanofiltration membrane

Journal of Environmental Chemical Engineering 7 (2019) 103300

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High flux hyperbranched starch-graphene oxide piperazinamide composite nanofiltration membrane

T

Ambre Jyoti P.a, Dhopte Kiran B.a, Nemade Parag R.a,b, , Dalvi Vishwanath H.a ⁎

a b

Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai, Maharashtra, 400019, India Department of Oils, Oleochemicals and Surfactant Technology, Institute of Chemical Technology, Nathalal Parekh Marg, Mumbai, Maharashtra, 400019, India

ARTICLE INFO

ABSTRACT

Keywords: Hyperbranched starch-GO Nanofiltration Thin film nanocomposite High flux

Polypiperazinamide membranes are most commonly used membranes for nanofiltration (NF) of solution. These membranes suffer low flux due to the relatively flexible hydrophobic backbone. Hydrophilic graphene oxide (GO) was incorporated in the top layer of these membranes to improve flux. However, GO is expensive therefore, the GO was modified with starch, a benign and inexpensive hydrophilic matrix to minimize the economic burden. Starch functionalized GO was added in the polypiperazinamide network to give high-flux hyperbranched starch functionalized graphene oxide composite (HGOST) nanofiltration membranes. GO-starch composites were integrated in the polyamide (PA) top layer by esterification. Abundant oxygen functional groups in starch and graphene oxide decreased contact angle to 22.18°. Permeance of the membranes increased to 79.6 L m−2 h-1 (LMH) with the addition of HGOST to the top layer, with slight increase in the ionic rejection. The use of starch decreased the amount of graphene oxide needed to improve performance by about 80%. Hydrophilic nature of starch-GO composite, its compatibility with the polypiperazinamide matrix and enhance surface negative charge on the membrane in combination with a thin top layer are responsible for enhanced performance. Excellent stability was obtained due to bonding between starch, GO and polypiperazinamide layer. Membranes displayed excellent rejection of charged and neutral dyes as well. The inclusion of GO-starch composites shows good promise for enhancing the performance of polyamide membranes.

1. Introduction Access to clean drinking water is one of the sustainable development goals of the United Nations. However, this basic right has eluded mainly, due to scarcity of portable water. Water recycling by polluting industries will prevent the contamination of water bodies and go a long way towards fulfilment of the goal [1]. While the treatment of organic impurities can be addressed by advanced oxidation processes, dissolved salts must be removed by energy-intensive multi-effect evaporation or reverse osmosis. Low throughput of water recovery from water by reverse osmosis membrane can be overcome using nanofiltration membranes. A good separation of multivalent ions from water can be achieved at lower pressures and higher flux by nanofiltration [2]. Nanofiltration can also be used for the concentration of dyes and larger solutes from polluted water for subsequent treatment [3–5]. Nanofiltration membranes can be considered as “loose” reverse osmosis membranes and are commonly made by interfacial polymerization of diamine and acid chloride to give a polyamide separating layer. Use of

piperazine (PIP) in polyamide provides flexibility to the polymer chains, which improves the permeability, but the rejection of monovalent ions is poor. Thus, the trade-off between permeability and selectivity exists. Low fouling resistance of polymeric membranes is another limitation. Advances in nanofiltration membranes have been directed towards enhancing hydrophilicity, flux, fouling resistance and life span by (a) chemical modifications of polymers, (b) surface modification of membranes and (c) the use of composite materials [6–8]. Mixed matrix membranes containing nanoparticles dispersed in polymeric membranes overcome the material limitations. Membranes with high permeability, high rejection as well as good fouling resistance have been reported, particularly in ultrafiltration of fouling feeds [9]. Similar approach was extended to nanofiltration membranes through thin film nanocomposite (TFN) membranes. The nanoparticles are only dispersed in top layer in TFN membranes, while the support layer is polymeric. An advantage of this approach is that the amount of nanoparticles needed to be dispersed in greatly reduced since the top layer of the

⁎ Corresponding author at: Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai, Maharashtra, 400019, India. E-mail address: [email protected] (P.R. Nemade).

https://doi.org/10.1016/j.jece.2019.103300 Received 23 April 2019; Received in revised form 7 July 2019; Accepted 18 July 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Environmental Chemical Engineering 7 (2019) 103300

J.P. Ambre, et al.

membrane is very thin (< 1 μm in thickness) [10]. Over two-fold increase in permeability at similar or marginally lower rejection have been reported through the use of inorganic fillers, such as alumina nanoparticles, aluminosilicate nanotubes, metal-organic framework nanoparticles (ZIF-8, aminated MIL-53); zeolites, mesoporous silica, silver nanoparticles, TiO2, aminosilanized TiO2, mesoporous carbon nanofillers, functionalized multiwalled carbon nanotubes (MWCNT), etc. [3,11–17] and bio-inspired approaches such as biomimetic ion channels using zwitterionic nanoparticles, aquaporin Z based nanofiltration [18,19]. Other properties such as surface hydrophilicity, fouling resistance, smoothness were enhanced by the addition of external matrices. Recently, graphene-based nanocomposites have attracted interest due to higher hydrophilicity, increased stability and strength of thin film composite (TFC) membranes containing graphene. Piperazine-trimesoyl chloride (PIP-TMC) polyamide chains present in NF membranes are flexible, and therefore exhibit substantially higher flux than reverse osmosis membranes prepared using meta-phenylene diamine and trimesoyl chloride (TMC). Addition of graphene oxide into polyamide chains of NF membranes will increase stiffness of the chains due to cross-linking with functional groups on edges of graphene oxide sheets. Graphene oxide being an external matrix material may increase polymer free volume and therefore, improves permeates flux. 50% improvement in water permeability was obtained in NF membranes supported on GO-polysulphone supports without compromising on salt rejection [20]. PEG-modified GO nanosheets incorporated in polypiperazinamide (PA) nanofiltration membranes displayed increased rejection and lowering of contact angle with better antifouling properties though the permeate flux declined [21].Ganesh and co-workers obtained over 2-fold increase in rejection in Na2SO4 using mixed matrix GO-polysulfone membranes [22]. Use of reduced graphene oxide with TiO2(about 1.25% w/w polymer) in polypiperazinamide membranes gave a water permeability of 6.1 L m−2 h-1 bar-1 (LMH/bar) [23], while positively charged membranes made using layer by layer technique with good flux and excellent rejection was reported by Nan et al. [24]. While the use of GO imparts some exciting properties to the polymers, GO synthesis by modified Hummer’s methods uses large number of hazardous reagents such as KMnO4, conc. HNO3, conc. H2SO4 for intercalation and oxidation. Use of KMnO4 is also environmentally challenging. We have attempted to develop membranes by limiting the use of GO and increasing the use of benign materials to develop high throughput membranes. One such material is starch, a naturally abundant material with oxygen-rich functional sites. Starch is also significantly hydrophilic however is not charged in solution, and therefore of limited use in charged nanofiltration membranes. We used starch in addition to graphene oxide to synthesize thin film composite nanofiltration membranes. Graphene includes several moieties that acquire charge, therefore, GO-starch composite can increase both hydrophilicity as well as charge. Graphene oxide-starch composites were synthesized by esterification. Functionalization of graphene oxide with starch adds to the functional groups present on edges of GO sheets creating a hyperbranched structure in the polyamide membrane. Large number of hydroxyl groups on the starch increases the overall hydrophilicity of the membrane. Also, GO-starch membranes contain small proportion of expensive graphene oxide compared to the abundant starch in the composite, hence the GO-starch composite polyamide nanofiltration membranes are also economically attractive. Herein, we report mixed matrix nanofiltration membranes synthesized by esterification of starch functionalized graphene oxide with piperazine-trimesoyl chloride polyamide. The membranes developed in the current work have less than 1% (w/w polymer) GO in the top layer. GO-starch composites were characterized for assessment of chemical and functional changes. Membranes were characterized with various techniques for assessment of top TFN layer. Studies were conducted to determine the effect of solute charge on rejection and long-

time stability of the membrane. Finally, the effectiveness of the membrane for the removal of dyes was studied to provide insights into the applicability of the membranes. 2. Experimental Natural graphite powder (325 mesh, purity 99.9995%) was purchased from Alfa Aesar, UK. Potassium permanganate (purity 99%), Piperazine (PIP) (98% pure) and starch were procured from SD Fine Chemicals, Mumbai, India. Trimesoyl chloride (purity 98%) were procured from Sigma Aldrich, India. Polyethersulfone ultrafiltration (UF) membranes with 10,000 Da molecular weight cut-off was purchased from Nitto, US and was used as membrane supports. All other reagents, solutes, dyes, acids procured were of analytical grade. All experiments were performed using 18.5 MΩ Milli-Q water (Millipore, US). Graphene oxide (GO) was synthesized using modified Hummers method by exfoliation of natural graphite powder followed by oxidation with a mixture of KMnO4, H2SO4 and HNO3 [25–27]. The residue was washed with DI water and HCl thoroughly to remove traces of KMnO4 to give GO. Starch functionalized GO was synthesized by esterification of hydroxyl groups in starch and carboxylic groups in GO. The reaction was carried out by mixing desired amount of GO and 1 g starch dispersed in 50 mL of DMF in a round bottom flask at 150 °C for 5 h. The mixture was then cooled to room temperature and left overnight under stirring. The residue obtained on filtration was washed and dried in vacuum to afford GO-starch composite. Nanofiltration membranes were synthesized by interfacial polymerization of piperazine and trimesoyl chloride on 10 kDa MWCO Polyethersulfone ultrafiltration support (Fig. 1). UF support was saturated with an amine solution containing GO/GO-starch composite, PIP, TEA, and SDS in desired proportions. Any excess amine was removed with a roller and the support was air-dried for removal of surface water (10 min). Amine impregnated support was then immersed in a bath of TMC in n-hexane for one min. The membrane was washed with nhexane followed by annealing at 90 °C in DI water for eight min and then stored in DI water at room temperature. GO and GO-starch composites were characterized using X-ray diffraction (XRD, D8 Advance, Bruker, USA) for estimation of d-spacing in GO and the changes in GO stack after reaction with starch, infrared spectroscopy (Spectrum BX, Perkin Elmer, USA),and X-ray photoelectron spectroscopy (XPS, 5000 Versa probe II, Phi, USA) for assessment of functionalization. Thermogravimetric analysis (TGA/DSC 1, Star system, Mettler Toledo, USA) was carried out to ascertain the temperature stability of the composites. NF membranes were analyzed with Field emission Scanning Electron Microscope (MIRA3 TESCAN FEGSEM, Australia) to observe the cross-sectional overview of thin film composite. Membrane top layer was analyzed with Atomic force microscopy (Bruker Multimode 8 AFM) to evaluate surface roughness. NF membranes were further characterized for their confirmation of amide linkages using infrared spectroscopy, water contact angle, and surface zeta potential to confirm the negatively charged membrane surface. The water contact angle was measured in a goniometer (200-F4, ramé-hart Instrument Co, USA). Prior to the measurement, the membranes were dried for 24 h in a vacuum. 5 μL drop of deionized water was deposited on the surface and the contact angle was measured after 15 s. Averages of ten readings on the different positions of the membrane surface were carried out to determine the contact angle. Charge on the surface of MGO-ST2 was determined using Electro-Kinetic Analyser (Anton Paar, Austria). For measurement of streaming potential, 10 mm X 20 mm sized pieces of MGO-ST2 were dipped in electrolyte of 1 mM KCl solution and pressed in the adjustable gap of horizontal cylindrical cell equipped with two electrodes adjusted to build up the pressure of 400 mbar. Streaming potential over a pH range of 3–11 was measured. Membrane performance was studied by measuring the permeate flux and rejection of common salts such as NaCl, MgCl2, Na2SO4, MgSO4 in crossflow mode. The active membrane area was 14.7 cm2. The 2

Journal of Environmental Chemical Engineering 7 (2019) 103300

J.P. Ambre, et al.

Fig. 1. Schematic of synthesis of GO-starch nanocomposite membranes.

concentration of salt solutions was maintained so that the osmotic pressure of the solutions was 0.5 bar. Transmembrane pressure of 8 bar was maintained across the membrane. The permeate was collected in a vessel mounted on a weighing balance (Mettler-Toledo, USA, ME3002) connected to a personal computer. The data was acquired at a specific time interval using the Python program. The concentration of these ions from the solutions was estimated by ion conductivity measurements (AP2, Aquapro, India). Permeate flux (J, L m−2 h−1, LMH) was determined by Eq. (1) where ΔV is the volume of permeate (L) in time Δt (h) over an effective area of A (m2). Unless otherwise noted, the studies were repeated thrice and the errors represent plus and minus one standard deviation from the average of the three repeats.

J=

V A× t

(z) =

P (z ) dz = 1

z

z

P(z) dz

(2)

Where P (z ) is log-normally distributed and is given by

P (z ) =

z2 2

1 exp 2

(3)

and ¯

z=

log(a/ a ) log a

(4) ¯

Here, a is the Stokes radius of the solute, a is the geometric mean of the pore sizes (radius) and a is the geometric standard deviation of the pore sizes. Hence

(1)

(z ) = 1

The molecular weight cut off was estimated from the rejection of neutral sugars with progressively increasing molecular weights. The molecular weight cut off was estimated assuming the membrane pore size distribution is such that the solutes are rejected log normally [28] and assuming the fraction of the solute that permeates across the membrane ( )is proportional to the faction of the membrane pores that are permeable to that solute. This number in turn depends on the solute's Stokes-Einstein radius (a ). All pores with a pore size greater than a will be permeable to the solute. Hence (a) is the area under the normalized probability density function (PDF) of the pore size distribution ≥. Hence, if P (x ) is the PDF of the pore size distribution, then (a) is given by

1 2

z

exp

z2 dz = 1 2

CDF (z ) = 1

1 1 + erf 2

z 2

(5)

Where CDF (z ) is the cumulative distribution function,

(z) =

1 erfc 2

z 2

(6)

The membrane pore size distribution can be characterized by two ¯

parameters, a and a . These parameters can be evaluated graphically by inverting Eq. (6) as ¯

log(a) = log(a ) +

2 log( a ) erfc 1 [2 (z )]

(7)

The relationship between the Stokes radius and the molecular weight is given as [29] 3

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J.P. Ambre, et al.

Fig. 2. XRD of GO, starch, GO-starch mixture and HGOST.

logrs =

1.52517 + 0.47956 logMW

(8)

Fig. 3. FTIR of GO, starch, GO-starch mixture and HGOST.

Rejection of 200 ppm solutions of glucose, sucrose, raffinose, and βcyclodextrin in crossflow mode at a transmembrane pressure of 8 bar was used to estimate molecular weight cut-off. The concentration of solutions was estimated using TOC analyser (TOC, SGE ANATOC™ Series II Total Organic Carbon Analyser, USA). The effect of solute charge on the rejection was studied by filtration of (a) cationic dye, Safranin-O, (b) anionic dye, Acid Orange-2 and (c) neutral molecule, sucrose. The molecular weight of these solutes is similar and only the charge varies. The concentration of Safranin-O and Acid Orange-2 in their solutions was estimated by UV–vis spectrophotometry (Cary 50 UV–vis, Varian, Australia), while the concentration of sucrose was estimated from total organic carbon measurements. All filtration studies were performed on cross flow filtration setup except dye filtrations, which were performed on dead-end filtration cell under nitrogen pressure of 8 bar. The solute concentration during dye studies was 200 ppm. All studies were repeated for a minimum of three times unless stated otherwise. The standard error was less than 5% unless stated otherwise. Eq. (1) and (9) were used to determine the permeate flux and dye removal proficiency respectively.

% Dye removal =

C0

Ct C0

× 100

Stability of membranes was assessed in two ways: (a) carrying out long term permeation studies with Na2SO4 solutions; (b) exposing the membranes to solutions of different pH, and analysing flux and rejection of Na2SO4solutions. Low pH solution was prepared by addition of dilute HCl, while higher pH solution was made by the addition of NaOH. The membranes were exposed to a given pH solution by immersion for either 2 h or 24 h. Subsequently, the membranes were washed thoroughly and then evaluated for their performance. 3. Results and discussion 3.1. Characterization of HGOST HGOST were characterized for confirmation of esterification. To trace the changes in the stacking order of GO X-ray diffraction studies were carried out. X-ray diffractogram of graphene oxide synthesized by modified Hummers method is shown in Fig. 2. A sharp and intense peak at 9.1° corresponding to (002) planes in GO can be seen [30]. Sharp peaks at 17.4°, 22.7° and 24.2° in the XRD pattern of starch were attributed to B-type crystalline structure [31]. A combination of peaks due to GO and starch can be seen in the spectra of GO-starch mixture. In the spectra of HGOST, (002) peak due to GO at 9.1° broadened considerably over the parent GO peak, implying disruption in the lamellar structure of GO. The peak at 24.2° due to starch disappeared. The crystalline structure of both GO and starch was modified on formation of HGOST composite. Fourier transform infrared (FTIR) spectra of GO exhibited peaks at 3425 cm−1, 1720 cm−1 and 1099 cm−1 attributed to stretching vibrations of hydroxyl, carbonyl, and –C–O linkages (Fig. 3). Normalized

(9)

where, C0, and Ct (ppm) represent dye concentrations at initial, and at time t (min), respectively. Membranes were also evaluated for rejection of dyes commonly found in the effluents. We evaluated the rejection of reactive violet 1 and reactive green 19 dyes. The filtration was carried out in dead-end mode with 250 mL of feed solution under 8 bar transmembrane pressure. Approximately 50 mL of permeate was collected. The concentration of dyes was estimated using UV–vis spectrophotometer. 4

Journal of Environmental Chemical Engineering 7 (2019) 103300

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Fig. 4. XPS of (a) GO and (b) starch (c) GO-starch mixture, and (d) HGOST.

FTIR spectra of starch show a sharp peak at 2933 cm-1 and a broad shoulder hump in the region from 3420-3700 cm-1, which is the characteristic of aliphatic alkane and hydroxyl groups respectively. The vibrations in 950-1160 cm-1 region represents C–O bond stretching, whereas 950-730 cm-1 region represents ring vibrations in starch. On esterification of starch with graphene oxide, a sharpening of hydroxyl plateau is seen (3603 cm−1). Carbonyl peak due to −COOH in graphene oxide is shifted to lower wavenumber (1708 cm−1). The peak at 1404 cm−1 due to COO- in graphene is much lowered in intensity in HGOST [32]. An HGOST spectrum was compared with the spectra of GO-starch mixture prepared by simple physical mixing. The spectra of GO-starch mixture were an amalgamation of spectra of GO and starch. The changes in the peak positions and shapes of the peaks due to carbonyl and hydroxyl groups confirmed covalent attachment of starch to GO by esterification. Fig. 4 shows deconvoluted C1s XPS spectra of GO (a), starch (b) GOstarch Mixture (c) and GO-ST composite (d). The spectra of GO showed a distinct peak at 284.8 eV corresponding to (CeC) stretching. Peaks at 286.9 and 288.5 were also observed in the deconvoluted C1s spectra attributed to (C–OH) groups and (C]O) groups respectively. The binding energy region of (C–O–C) groups overlaps with those of (C]O) groups [33]. Deconvolution of XPS spectra of starch reveals three peaks at 284.8, 287.1 and 289 eV. Peak at 284.8 and 287.1 represent CeC and C–OH stretch respectively, whereas the peak at 289 eV is due C–O–C. XPS spectra of HGOST was deconvoluted into four peaks. Peaks at 284.9 eV, 286.2 eV and 288.9 eV corresponding to (CeC), (C–OH) and (C]O, C–O–C) respectively [34,35]. An additional peak at 291.0 eV attributed to (O = C–O) groups observed in HGOST XPS spectra is a strong evidence of covalent bonding between GO and starch [36,37]. Absence of (O = C–O) peak in the XPS spectra of GO-ST mixture that validated successful esterification of starch by GO. The intensity of (CeC) peak was highest in HGOST followed by that of (C–OH) peak, while the intensity of peak due to (C–OH) was higher than that due to

(CeC) in GO, starch and GO-starch mixture. Higher intensity of peak due to (C–O–C) in GO-starch composite than in GO indicated substantial increase in number of (C–O–C) linkages on esterification with starch. Thermo-gravimetric analyses (TGA) of GO and GO-starch composite (1:10 w/w) were carried out in order to determine the thermal stability. Substances were subjected to heating rate of 10°/min under nitrogen atmosphere with flow rate of 50 mL/min. are presented in Fig. 5. TGA curve of GO showed a gradual weight loss of approximately 23% until 140 °C, followed by 22% loss from 180 °C to 240 °C and a gradual weight loss thereafter until 900 °C to give a residue with 35% weight. TGA curve of GO-Starch composite showed about 14% loss until 130 °C, followed by a steep weight loss of 65% from 260 °C to 330 °C, and a gradual reduction in weight subsequently until 900 °C to give 18%

Fig. 5. Thermogravimetric analysis of GO, starch, GO-starch mixture and HGOST. 5

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Surface topography of M0 and MGOST membrane were analysed by AFM. Fig.6 (e, f) showed the three dimensional 5 μm scanned images for M0 and MGO10-ST2 membranes. Surface roughness of membranes was evaluated in terms of root average arithmetic roughness (Ra) and root mean surface roughness (Rq) values estimated from AFM images. Characteristic ridge and valley structure can be observed for both the membranes however, thorough analysis of AFM images reflect higher proportion of softer segment (white portion) in the HGOST modified membrane suggests smoother surface which is in accordance with the decreased value of root average arithmetic (Ra) roughness from M0 (10.4 nm) to MGO10-ST2 (4.51 nm). Smoother surface were thought to be the contributing parameters for enhanced flux value of MGO10-ST2 membrane along with higher hydrophilicity derived due to hydroxyl groups in HGOST composite incorporated membrane. Surface charge on the membrane is correlated to the hydrophilicity of the polymer surface [40]. The surface charge was determined as zeta potential of the membrane surface as a function of pH of the electrolyte solution. Fig. 7 demonstrates the measured zeta potential of thin film composite for pH range of 3–11. Surface potential of the membranes decreased with increasing pH [41]. MGO10-ST2 membranes exhibited significantly higher negative charge compared to GO membranes. The isoelectric point (IEP) of the control M0 membrane was at pH = 3.8, while that of MGO10-ST2 marginally increased to pH = 3.9. However, beyond IEP, the HGOST containing membranes displayed significantly higher negatively charged surface evident from the low zeta potential. Increase in surface charge also increases hydrophilicity. The higher hydrophilicity gives improved permeate flux. Any increase in permeate flux will also lead to increase in the salt flux and hence the salt rejection. The negative charges on the surface counter that phenomena giving both high flux and high salt rejection. In order to determine incorporation of composite in TFC, FTIR spectra of M0 and MGO10-ST2 freestanding membranes (without membrane support) were determined and is shown in Fig. 8. Polyamide groups can be confirmed with the characteristic NeH and C–N stretching 3434 cm−1, and 1247 cm−1 respectively. Characteristic peaks at 3531 cm-1, due to unreacted −OH groups from hydrolysed TMC, 2950 cm-1 and 2885 cm-1, due to alkyl (−CH2–) stretching vibrations were observed in spectra of M0 membrane. Stretching of interest was at 1629 cm-1 corresponding to the carbonyl of amide and supported by the peak at 1438 cm-1 due to the stretching vibrations of –C–N bond in amide, confirming polyamide formation. Addition of HGOST to the matrix of polyamide gave MG010-ST2 membrane. The spectra of MGO10-ST2 showed prominent hydroxyl stretching peak at 3435 cm-1 and alkyl group vibrations at 2920 cm-1. Peaks at 1726 cm-1 and 1620 cm-1 for carbonyl of ester and amide groups respectively were also visible. Further, a broad peak at 1450-1500 cm-1 attributed to alkane (–C–H) bending vibrations in starch in graphene oxide-starch composite was observed. The peak due to ester was not present in the spectra of M0 membrane, while the peak intensity of hydroxyl/amide peak was enhanced in the spectra of MGO10-ST2 membrane. HGOST is held in the PIP-TMC polyamide backbone with the ester bonds formed

residual weight. The weight loss at low temperature (< 140 °C) was attributed to the loss of free and bound moisture. Water intercalated in GO stacks and therefore evaporated at higher temperature. Based on the low-temperature weight loss, GO appeared to hold higher moisture content than HGOST composites. As the temperature increased above 200 °C, loss of oxygen functionalities was seen. GO is also an effective oxidizing agent [38,39]. Therefore, GO will catalyze oxidation and loss of oxygen functionalities in starch giving steep drop observed in GOstarch. The gradual weight loss at temperatures higher than 400 °C in both GO and HGOST was due to loss of refractory functional groups and charring at high temperatures. 3.2. Characterization of membranes The polyamide layer is formed by polymerization of piperazine in aqueous solution and trimesoyl chloride in organic layer of hexane at the interface. The propagation of the film is self-limiting as the diffusion of TMC in the polyamide layer is slow. Addition of GO-starch laminates in PIP solution increase the viscosity of the solution. Consequently, the mobility of TMC in the layer decreases, which gives a dense top layer whose thickness is self-limiting. The properties of this layer determine the permeation properties of the membrane. A thinner and more hydrophilic layer with larger polymer free volume improve water permeability. We synthesized several membranes to determine the origin of the performance of GO-starch composites (Table 1). Control NF membranes were synthesized by reacting PIP, starch-PIP, and GO-PIP with TMC. Additionally, membranes containing varying amount of HGOST as well as those with different proportion of GO in HGOST were synthesized to identify the effect of HGOST composition on the membrane performance. The thickness of the layer can be seen in the FEG-SEM images of the cross-section of the membranes (M0 and MGO10-ST2) in Fig. 6. Crosssectional SEM images of M0 and MGO-ST2 were captured by FEG-SEM. Membranes were dipped in liquid nitrogen, fractured and sputter coated with platinum prior to scanning under the electron beam. Finger-like voids with a porous top layer of the polysulfone support can be seen in the images of both the membranes. Polyamide layer in both the membranes is uniformly distributed over the support. The top layer of polyamide in M0 membranes appears rough and consists of an average thickness of 0.33 μm. In contrast, the polyamide layer of HGOST10-ST2 membrane appears smooth and dense with an average thickness of 0.261 μm. The denser surface appears due to lower mobility of TMC/PIP in aqueous/organic layers. The reason for lower mobility is presence of HGOST, which increase the viscosity of PIP solution. The mobility of TMC is decreased due to higher viscosity and hindrance due to HGOST laminates giving a thin dense layer. Although the interfacial polymerized layer in MGO10-ST2 is thinner than the control M0 membranes, the strength may not be affected as the inclusion of GO laminates is expected to improve the mechanical strength of the films. The membranes were not observed to be any less robust than the M0 membranes in general permeability studies. Table 1 Compositiona and properties of membranes synthesized. Membrane

PIP (%)

TMC (%)

Additive (GO/GO-ST) (%)

GO in Additive (%)

Pure water Permeance (L m−2 h−1 bar−1)

Na2SO4 Permeance (L m−2 h−1 bar−1)

Na2SO4 Rejection (%)

M0 MST1 MST2 MGO1 MGO2 MGO10-ST1 MGO10-ST2 MGO20-ST1 MGO20-ST2

1 1 1 1 1 1 1 1 1

0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8

None 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.2

None None None 100% 100% 10% 10% 20% 20%

5.5 ± 0.17 6.6 ± 0.35 8.4 ± 0.20 7.3 ± 0.15 9.3 ± 0.19 8.9 ± 0.26 10.0 ± 0.24 9.4 ± 0.18 10.1 ± 0.24

5.4 6.6 8.1 8.8 9.0 8.7 9.3 8.2 9.4

96.1 93.4 94.0 95.2 96.0 95.5 96.4 95.8 96.3

a

TEA (1%) and SDS (0.1%) were also added to PIP solutions. 6

± ± ± ± ± ± ± ± ±

0.31 0.48 0.27 0.17 0.73 0.67 0.34 0.23 0.25

± ± ± ± ± ± ± ± ±

0.82 1.04 0.96 0.18 0.8 0.8 0.38 0.68 0.96

Journal of Environmental Chemical Engineering 7 (2019) 103300

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Fig. 6. Cross-section SEM image of M0 (a, b) and MGO10-ST2 (c, d) at different magnifications focusing distinct layers of composite membrane and topography of M0 (e) and MGO10-ST2 (f) membranes using AFM.

Fig. 7. Surface zeta potential of M0 and MGO10-ST2 in 1 M KCl over a pH range of.3–11.

Fig. 8. FTIR of M0 polyamide membrane and MGO10-ST2 HGOST polyamide membrane.

between hydroxyl groups of graphene oxide composite and some unreacted trimesoyl chloride [42]. The position of ester peak is blue shifted in the membrane over the position in HGOST giving evidence of the incorporation of ester linkages in the polyamide backbone during membrane formation. Nanofiltration membranes synthesized with PIP-TMC give flexible polyamide backbone across which water, organic molecules and salt ions diffuse across into permeate. Increased polymer hydrophilicity assists in rapid transport of water across the membrane. Presence of oxygen functionalities enhances hydrophilicity. A measure of surface hydrophilicity is the water contact angle of the surface. Since the top surface of the membrane was not treated in any way, the water contact angle represents the hydrophilicity of top polypiperazinamide layer. The water contact angle of the membranes studied is shown in Fig. 9.

Addition of 0.2 g of GO to PIP matrix during MGO2 membrane synthesis, led to lowering of water contact angle to 24.8°. Addition of 0.2 g of GO-starch in MGO10-ST2 membrane further lowered the contact angle to 22.3°, a 39% decrease over control M0 membranes and about 12% lower than the water contact angle of MGO2 containing only GO. On the other hand, a decrease in HGOST composite used in MGO10-ST1 increased the contact angle to 26.1°. Thus, addition of external matrix, GO, as well as GO-starch composites led to an increase in hydrophilicity of the membrane and lowering of water contact angle attributed to the availability of abundant hydroxyl and carboxyl functional groups in GO and starch. GO and starch in HGOST composite were more effective in increasing membrane hydrophilicity than either starch or GO alone. 7

Journal of Environmental Chemical Engineering 7 (2019) 103300

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Fig. 9. Water contact angle of synthesized nanofiltration membranes.

3.3. Characterization of membrane performance Pure water flux and ionic rejection were measured under crossflow conditions under 8 bar transmembrane pressure (Fig. 10, bar graph). M0 membrane synthesized using only TMC and piperazine displayed a pure water flux of 44 LMH. Addition of 0.2 g of starch to the dope solution resulted in 36% increase in flux, while addition of same amount of GO led to increase of 68% increase. Therefore, the addition of GO has a greater effect on the flux than the addition of starch. GO-ST composites produced highest flux membranes among the membranes studied. The addition of same amount of GO-starch composite increased the flux by 81% over unmodified membranes to 80 LMH. The use of HGOST instead of GO alone reduced the amount of GO used in the membranes by 80% in addition to an improvement in the performance. Fig. 10 (scatter plot) also shows the salt rejection performance of the synthesized membranes under crossflow conditions. Nanofiltration membrane exhibit lower rejection of monovalent ions and high rejection of divalent ions and the same was observed. All synthesized membranes showed no significant variation in ionic rejection characteristics. About 50% rejection was observed for NaCl. Na2SO4, MgSO4 and MgCl2 rejection were studied to identify the nature of the membrane. Rejection for MgCl2 was lower at approximately 40% while Na2SO4 shows greater rejection (> 95%).MgSO4 with both divalent ions also exhibited slightly lower rejection (ca. 93%) than Na2SO4. The rejection of MGO10-ST2 membranes was higher than other membranes studied (96.4%). High rejection of Na2SO4 and low rejection of MgCl2 indicated fixed anionic charges on the membrane that attracted multivalent cations while excluding multivalent anions. Lower rejection of MgSO4 than Na2SO4 confirmed attraction of Mg2+ ions towards active membrane layer, and a corresponding leakage of SO42− ions across the membrane. The salt rejection trends, rejection of Na2SO4 > MgSO4 > NaCl > MgCl2, by synthesized membranes demonstrated negatively charged nanofiltration membranes. The molecular weight cut-off of the membranes was estimated based on the rejection of neutral solutes [28]. Carbohydrate oligomers, glucose, sucrose, raffinose, and β-cyclodextrin with molecular weight of 180, 342, 504 and 1135 respectively, were chosen as probes. Rejection of sucrose, raffinose and β-cyclodextrin was greater than 90% glucose rejection was about 73.3%. Membrane parameters of effective pore size and geometric mean standard deviation were obtained by plotting Eq. (7) in Fig. 11. MWCO was obtained from Eq. (8) using effective pore radius. Table 4 compares these parameters for M0 and MGO10-ST2 membranes. The MWCO of M0 and MGO10-ST2 membranes was found to be 310 Da and 330 Da, with mean pore radius of 0.46 nm and 0.52 nm respectively. The figure in the inset (Fig. 11b) shows the probability distribution curve for the two membranes. The two

Fig. 10. Permeate flux (a) and rejection of probe salts (b) across synthesized membranes. Conditions: transmembrane pressure: 8 bar; solution osmotic pressure Δπ: 0.5 bar.

Fig. 11. Plot of ln(a) vs 2 erfc 1 [2 (z )], shows the probability density function curves for the two membranes, M0 and MGO10-ST2.

membranes are quite similar in terms of pore distribution and MWCO, however, the permeability of MGO10-ST2 membranes is 80% higher than M0 membranes. Charged and uncharged solutes with higher molecular weight were used to confirm the charges on MGO10-ST2 membranes and the performance is listed in Table 2. Probes selected were Safranin O, Acid 8

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Table 2 Rejection of neutral, positively as well as negatively charged molecules across MGO10-ST2 nanofiltration membranes. Conditions: 200 ppm solutes, 8 bar transmembrane pressure. Probe Molecule

Molecular Formula

Type

Mol. Wt.

Solute Permeance (L m−2 h−1 bar−1)

% Rejection

Safranin O Acid orange 2 Sucrose Reactive violet 1 Reactive green 19

C20H19N4+Cl− C16H11N2NaO4S C12H22O11 C25H13Cl2N7Na4O14S4 C40H23Cl2N15Na6O19S6

Cationic Anionic Neutral Neutral Neutral

350.84 350.32 342.3 560 1418

8.80 ± 0.23 7.76 ± 0.51 9.13 ± 0.54 9.26 ± 0.46 8.4 ± 0.34

92.4 95.9 92.9 99.6 99.4

Orange 2 and Sucrose. These solutes have very similar molecular weights, but Safranin O is a cationic dye, Acid Orange 2 is an anionic dye while sucrose is a neutral solute. The filtrations were performed under transmembrane pressure of 8 bar with feed concentration being 200 ppm under dead-end conditions. The rejection of Acid orange 2, anionic dye was highest followed by sucrose and Safranin O respectively, confirming the negative nature of MGO10-ST2 membranes. The membrane had an apparent molecular weight cut-off of less than 342 Da, as the rejection of all three solutes was greater than 90%, which validated obtained MWCO of 330 Da. Higher molecular weight dyes, Reactive violet 1 and Reactive green 19 were almost completely rejected (> 99%). Thus MGO10-ST2 membranes can be used for rejection of salts, neutral solutes and dyes. Nanofiltration membranes can be considered as loose reverse osmosis membranes. The transport of water through these membranes can be explained with solution diffusion mechanism, in which, water first diffuses in to the polymer matrix and is then transported across the top layer of the membrane. Increasing hydrophilicity of the top layer improves permeate flux. The high permeability of the membranes is due to thinner top layer, increase in hydrophilicity of the HGOST-polypiperazinamide matrix from abundant oxygen functionalities in HGOST, and the compatibility of HGOST matrix with polyamide backbone, which was evident from low water contact angle of 21.5°. Therefore, ingress of water into the polyamide layer is facilitated. Though, with an increase in water flux, there is possibility of increased salt flux. However, presence of abundant charged functionalities in HGOST composite in polyamide layer evident from enhanced surface zeta potential helps to control the salt leakage. Negative zeta potential decreases the salt flux overcoming flux-rejection trade-off to give membranes with high flux as well as high rejection. Nanofiltration membranes can be used for treatment of industrial process fluids and effluents. Often these fluids involve extreme conditions of pH. Therefore, pH stability is an important property of nanofiltration membranes. The performance of MGO10-ST2 membranes was studied after exposure to high and low pH solutions. The membranes were immersed in the solution of desired pH in a beaker under continuous stirring for short (2 h) and long exposures (24 h). The effect of acid and alkali treatment on flux and rejection of 959 ppm Na2SO4 solution was estimated and is shown in Fig. 12. The membranes that were exposed to alkaline pH for 2 h, showed no significant change in their behavior. On increasing the exposure to pH 9 solution for 24 h, the pure water flux increased by 74% along with a 9.5% decrease in Na2SO4 rejection. The membranes contain amide linkage between PIP and TMC, ester linkages between TMC and starch, and GO and starch, which contributes to hyperbranched polymeric top layer [43]. Among these functionalities, ester bonds are likely to be hydrolyzed on long exposure to high pH which led to an increase in water flux with a corresponding decrease in Na2SO4 rejection. Pure water flux of MGO10-ST2 membrane was stable for short as well as long exposures to low pH solution. The flux of the membranes was marginally higher after exposure to low pH solutions. The membranes exposed to pH 3 for 24 h exhibited higher Na2SO4rejection of 98% in comparison to the rejection of control membranes at pH 7 (96.7%). The stability of MGO10-ST2 membranes was better in solution of low pH over that in solution of high pH. The potential for use of these

Fig. 12. Effect of different pH conditions on starch supported graphene oxide containing nanofiltration membranes for 2 h & 24 h at 8 bar.

membranes for the filtration of low pH solution can be explored further. Fig. 13 shows the long-term stability of MGO10-ST2 membrane. The membranes were evaluated by continuous crossflow filtration of Na2SO4 solution under transmembrane pressure of 8 bar for 50 h at pH 7. The flux and rejection remained stable for the period evaluated. The average flux during the period was about 80 LMH with an average rejection of 96%. Therefore, the membranes are robust and stable for long term continuous filtration, although, even longer studies may be needed for pilot-scale testing of the membranes. A comparison of synthesized membranes with similar nanofiltration membranes reported in literature is shown in Table 3. MGO10-ST2 membrane showed significantly higher permeability than membranes with greater than 90% rejection of Na2SO4. In fact, the permeability was substantially greater than commercial membrane (NT-7450) by Nitto Denko and N30 F by MICRODYN-Nadir, although Na2SO4rejection was lower by 3% [44]. Only entry with higher flux and > 90% salt rejection was bio-inspired aquaporin based positively charged membranes [19]. However, longterm stability of those membranes was not reported.

Fig. 13. Long-term stability of MGO10-ST2 membrane for Na2SO4filtration (Δπ: 0.5 bar, transmembrane pressure: 8 bar). 9

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Table 3 Comparison of mixed matrix nanofiltration membranes reported in the literature. Sr. No.

Material*

Matrix*

Rejection (Na2SO4)

Permeance (LMH/bar)

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Hydrophilized ordered mesoporous carbon Tea catechins/chitosan MWCNT-OH PMMA-MWCNT rGO/acid-MWCNT GO GO GO/PEI GO-COOH rGO/TiO2 Reduced preoxidized GO NTR7450 (produced by Nitto-Denko)(commercial) N30 F, a global membrane product (produced by MICRODYN-Nadir) M0 MG10-ST2

PA PSF PA PA PA PA PES PAN PSF PA PVDF Sulphonated PES Permanently hydrophilic PES

90 99 97.6 99 83.5 95 72 93.9 (MgCl2) 86 94 69 99 99.5

2.2 7.5 6.9 7.0 11.3 2.4 5 4.2 4.9 6.1 5.3 5.7 3.8

PA PA

96.1 96.4

5.5 10.0

14. 15.

MWCO

< 300 < 300 < 300 625

311 331

Reference [45] [46] [17] [44] [47] [20] [22] [24] [48] [23] [49] [44] [44] This work This work

* PA: polyamide; PEI: polyethylene imine; PMMA: polymethylmethacrylate; PAA-g-MWCNT: polyacrylic acid grafter MWCNT; rGO/acid-MWCNT: reduced graphene oxide-acid modified MWCNT; GO: graphene oxide; PES: polyethersulfone; PSF: polysulfone; PVDF: polyvinylidene difluoride, PAN: polyacrylonitrile; GO−COOH: glycine modified graphene oxide.

Appendix A. Supplementary data

Table 4 Table of MWCO, average pore radius and geometric standard deviation obtained from the plot of ln(a) vs 2 erfc 1 [2 (z)] (Fig. 11) for the two membranes, M0 and MGO10-ST2. Membrane

Permeance (LMH/bar)

MWCO (Da)

ap (nm)

M0 MGO10-ST2

5.5 10.0

310 330

0.46 0.52

a

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103300.

(nm)

References

1.63 1.66

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4. Conclusion We have developed mixed matrix nanofiltration membranes with high permeability and high rejection sidestepping permeability-rejection trade-off using Hyperbranched graphene oxide-starch additive in polyamide nanofiltration membranes. Starch was bound to graphene oxide by esterification and which was further linked to PIP-TMC polyamide matrix by ester linkages. The influence of graphene oxide on properties of GO-starch NF membranes was also evaluated and we have successfully decreased the amount of GO needed to achieve high performance by 80% and replaced it with an environmentally benign and inexpensive material, starch. The membranes are negatively charged, with a thinner top layer and exhibit high rejection of Na2SO4. MGO10ST2 membranes exhibited highest pure water flux of 10 Lm−2 h-1 bar1 and high Na2SO4 rejection (96%) with a molecular weight cut-off of 330 Da. The membranes exhibit high rejection of dyes and could find applications in the recovery of these solutes from lean solutions. Use of HGOST composites in polypiperazinamide membranes significantly improves the throughput of nanofiltration, along with excellent stability. The membranes developed show good potential for larger scale studies towards production of high-performance membranes. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We are thankful to the Board of Nuclear Sciences for funding under the YRSA program and University Grants Commission of Government of India for support under UGC-BSR Fellowships and UGC-Networking Resource Centre. 10

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