Synthesis, characterization, and anti-fouling properties of cellulose acetate/polyethylene glycol-grafted nanodiamond nanocomposite membranes for humic acid removal from contaminated water

Synthesis, characterization, and anti-fouling properties of cellulose acetate/polyethylene... Polyethylene glycol-grafted nanodiamond (ND-PEG) was synthesized from pristine detonation NDs and utilized to prepare novel cellulose acetate/polyethylene glycol-grafted nanodiamond(CA/ND-PEG)nanocomposite membranes. Due to unique thermal, mechanical, and antibacterial properties and very easy cleaning of fouled ND-embedded CA nanocomposite mem- branes, we tried to investigate the performance of CA/ND-PEG membrane for humic acid (HA) removal from contaminated water. Surface functionalization was confirmed by Fourier transform infrared spectroscopy and thermogravimetry analysis. Pristine and functionalized ND with different concentration was added in the casting solution containing CA. The prepared membranes were characterized using contact angle, mechanical strength, scanning electron microscopy (SEM), transmis- sion electron microscopy, and permeation tests. SEM micrographs of the surface of the membranes depicted the increase in the number of pores by the addition of ND and especially ND-PEG into polymer matrix. The results indicated that the nanocomposite membrane with 0.5 wt% ND-PEG exhibited excellent hydrophilicity, mechanical properties, permeability, high rejection, high abrasion resistance, and good anti-fouling performance. The HA adsorption on the membrane surface −2 decreased from 2.85 to 2.15 mg cm when the ND-PEG content increased from 0 to 0.5 wt%. Most importantly, the HA filtration experiments revealed that the incorporation of ND and especially ND-PEG particles reduced membrane irrevers - ible fouling, dramatically. Meanwhile, the analysis of the fouling mechanism based on Hermia’s model revealed that cake formation is a prevailing mechanism for all membranes. Keywords Nanodiamond · Polyethylene glycol · Cellulose acetate · Nanocomposite membrane · Anti-fouling Introduction color of water [2–4]. Therefore, the removal of NOMs is extremely important and has become a challenging subject Membrane technology is widely used for water treatment of research in the current development of water purification and has obtained more attention than any other separation technologies. process due to low energy consumption, easy scale-up, less Cellulose acetate (CA) is one of the foremost among or no use of chemicals and no harmful by-product formation ultrafiltration (UF) polymer membranes. It has been widely [1]. Humic acid (HA), an important component of natural used in separation processes, due to its high hydrophilicity, organic matters (NOMs), is derived from the decomposition high biocompatibility, good desalting, non-toxic nature, per- of the plant and animal materials that are commonly found formance high potential flux, and relatively low cost [ 4–6]. in surface and ground water and strictly affect the taste and However, this membrane usually contains a dense skin layer and a low porous sub-layer, which result in an extremely low flux [ 7]. On the other hand, one of the serious problems * Reza Yegani arising during UF membrane filtration is membrane fouling ryegani@sut.ac.ir by NOMs [8]. Membrane fouling can be classified as revers - 1 ible fouling (which can be eliminated by physical cleaning) Faculty of Chemical Engineering, Sahand University and irreversible fouling (which cannot be entirely counter- of Technology, Tabriz, Iran 2 acted by physical cleaning and typically requiring chemical Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran Vol.:(0123456789) 1 3 382 Iranian Polymer Journal (2018) 27:381–393 cleaning) [9]. Hence, to improve the performance of CA membranes [11] in the current study, PEG-grafted ND parti- membranes, modification of polymer seems to be essential. cles were employed to fabricate ND-embedded CA nanocom- Recently, nanocomposite membranes have been applied posite membrane. As reported in the literature, nanocomposite in membrane technology for the removal of pollutants from membranes such as HDPE/SiO [11], polyethersulfone (PES)/ water to wastewater [10]. Polymer nanocomposite mem- Stöber silica [30], CA/organically modie fi d montmorillonite branes are formed by incorporation of nanoparticles into [7], polyamide (PA)/multi-wall carbon nanotube (MWCNT) polymer matrix. In many cases, a significant improvement in [31], polysulfone/GO-TiO [9], and PVDF/TiO [32] enhanced 2 2 mechanical, thermal, antibacterial, and anti-fouling proper- the performance of polymeric membranes in HA removal from ties has been achieved [11, 12]. However, the well dispersion contaminated water. In our previous work, pristine and carbox- of nanoparticles is still a challenging subject for researchers. ylated ND particles, i.e., ND and ND-COOH were added to Carbon-based nanomaterials are potentially useful due to the CA polymer to investigate the anti-fouling properties of the their unique physical and chemical properties. Among them, prepared nanocomposite CA membranes in HA removal from nanodiamond (ND) particles with a diamond core (sp car- contaminated water [33]. Obtained results were promising; bon–carbon bond) that is covered with multiple functional however, anti-fouling properties were far from our preliminary groups [13, 14] are of great interest. Due to interesting goal. It was concluded that more modified ND particles were characteristics such as hydrophilicity [15, 16], antibacterial required. activity [12, 17, 18], biocompatibility [19, 20], chemical In this work, due to the great performance of PEGylated stability [19], thermal stability [20] non-toxicity [19–21], NPs [11, 34], the anti-fouling properties of nanocomposite superior hardness and mechanical properties, resistance to CA membranes containing PEGylated ND particles were harsh environments [19], and ease of surface functionaliza- compared by CA-ND membranes during the filtration of HA. tion [20], ND particles have been nominated as reinforce- Nanocomposite membrane was synthesized using blending ment fillers in the fabrication of nanocomposite materials. method followed by phase inversion process using pristine Despite the advantages described above, pristine ND and PEG-grafted ND particles denoted as ND and ND-PEG, particles usually form micro-sized agglomerates [22]. The respectively. Several structural and operational analyses were presence of carbon impurities among the ND particles and carried out to identify enhancement in membrane surface consequently agglomeration of ND particles led to poor dis- hydrophilicity, abrasion resistance, and anti-fouling proper- persibility and weak interfacial interactions with polymer ties during the filtration of HA. The fouling mechanisms of matrix [22, 23]. Thus, surface functionalization is an essen- the fabricated membranes were also analyzed using classical tial method to reduce the aggregation of nanoparticles and Hermia model. improve their performance. According to our previous findings, polyethylene glycol Theory of fouling mechanism (PEG) embedded polyolefin membranes showed excellent hydrophilic properties and exhibited great anti-fouling To identify the possible dominant mechanism of the mem- performance [11]. PEG can enhance biological properties brane fouling under constant filtration pressure, Hermia mod- including high biocompatibility, weak interactions with pro- els were used. These models are written in a common math- tein, and antibacterial properties [24]. It is used as a head ematical form as follows [35]: group in nonionic surfactants to cover surfaces to prevent 2 m d t dt proteins and hydrophobic foulants in surface water to adhere = k (1) dV dV to the membrane surface [25]. In other words, PEG produces a high degree of steric exclusion and entropy at PEG–water where t is the filtration time, V is the filtrate volume, k is the interfaces due to the formation of hydrogen bond with water resistance coefficient, and m is a constant which character- molecules, which in turn results in protein repellency [11, izes the fouling model. The permeate flux (J ) is expressed 26]. The conjugated PEG chains on the nanoparticle surface as follows [36]: could reduce protein interactions with the nanoparticles and 1 dV consequently with the membrane surface [27]. According to J = . (2) A dt Wang et al. [24] and as well as Zhang et al. [28], PEGylated ND can improve dispersion and reduce interaction with pro- Equation (1) can be written as follows: teins. In addition, a literature survey showed that PEG is an dJ 2−m 3−m ideal polymer for the modification of nanostructured materi- =−kA J . (3) dt als to be used in polymeric membranes [11, 28, 29]. To combine the great mechanical and thermal properties According to this model, there are four fouling mecha- of ND particle-embedded CA membranes [12] with excel- nisms. In the complete pore blockage mode, under condition, lent anti-fouling performance of PEG containing polyolefin 1 3 Iranian Polymer Journal (2018) 27:381–393 383 2 −1 it is assumed that no particles are situated on top of other specific surface area of 282.8 m g , was used as nanoparti- particles or on membrane surface. The model was derived cle. N–N-dimethylformamide (DMF, 99.8%) was purchased by substituting m = 2 in Eq (3) and integrating with regard to from Merck (Germany) and used as a solvent to prepare the time of permeate flux, Eq (4 ) can be obtained [37, 38]: dope solution. Deionized (DI) water was used as the non- −1 solvent. PEG (M = 200 g mol ) and 1,3-phenylenediamine 1 1 ln = ln + K t (mPDA) as hydrophilic modifiers were purchased from (4) J J Sigma-Aldrich. Thionyl chloride (SOCl ), triethylamine, and tetrahydrofuran (THF) were obtained from Merck. where K and J are complete pore blocking coefficient and b 0 pure water flux, respectively. PEG‑grafted ND nanoparticles The internal pore blocking mode accounts for fouling that occurs in the internal structure of the membrane. Internal According to the literature [22], to remove the organic pore blocking is due to the constriction of membrane pores impurities and create carboxylic functional groups on the caused by small particles attached into pore walls. The expo- ND nanoparticles surface (denoted by ND-COOH), pristine nent is put with the value of m = 1.5 and integrating Eq (3), detonation NDs were dried at 80 °C for 2 h in a vacuum the permeate flux is decreased and is related to time using dryer and the process was continued by oxidation in air at a the following equation [37, 39]: temperature between 430 and 450 °C for 1.5 h. PEG-grafted 1 1 ND nanoparticles were synthesized according to the method = + K t √ √ (5) described by Wang et al. [24]. A sample of ND-COOH nan- oparticles (50.26 mg) was dried in a vacuum oven at 70 °C overnight and then suspended in 2 mL anhydrous DMF in where K is an internal pore blocking coefficient. an ultrasonic bath. The prepared suspension was immedi- The intermediate pore blockage mode is similar to the ately added to 20 mL SOCl and refluxed for 24 h at 70 °C complete pore blockage model, in addition to accounting 2 to convert the carboxylic acids to acyl chlorides (denoted for the possibility of particles bridging a pore by obstructing by ND-COCL). The mixture was evaporated with a rotary the entrance without completely blocking it [40]. This model evaporator to remove excess S OCl at 40 °C. After adding substitutes m = 1 in Eq (3), and by integrating, the following 2 20 mL anhydrous DMF, 50.48 mg PEG, and 2 mL triethyl- equation is obtained [41]: amine, the mixture was refluxed for 24 h at 90 °C, separated 1 1 by centrifugation, and washed with methanol and distilled = + K t i (6) J J water five times to remove excess PEG. The obtained pow - der was dried under vacuum at 60 °C, overnight. where K is an intermediate pore blocking coefficient. In the case of cake formation, particles do not participate Membrane preparation in changes in membrane pores. A cake layer is formed out- side the external membrane and increases hydraulic resist- The neat and nanocomposite CA membrane flat sheets were ance. The cake filtration model was obtained by substituting prepared by phase inversion via immersion precipitation the value of m = 0 and integrating Eq (3), the obtained equa- technique. The blend homogeneous solutions were prepared tion is expressed as follows [42]: by dissolving the 17.5 wt% of CA polymer in DMF as a sol- 1 1 vent with different concentrations of ND, and ND-PEG par - = + K t (7) 2 2 ticles ranged from 0 to 0.75 wt%. The rest of nanoparticles was added to DMF and stirred for 4 h at room temperature. where K is a cake pore blocking coefficient. The mixture was sonicated (Woson, China) at 50 kHz for 3 h to ensure the homogeneous spread of the nanoparticles and break up the agglomerates. Then, CA (17.5 wt%, by Experimental weight of the solution) was added to the initial mixture and dissolved in the solvent at 2000 rpm for 15 h. The casting Materials solution was then degassed, overnight without stirring to remove the gas bubbles, completely. The solution was then Cellulose acetate (M = 30,000) was used as polymer mate- cast onto a glass plate using automatic casting machine (Coa rial to prepare the CA membrane, supplied by Sigma- Test, Taiwan) to produce a flat sheet membrane (shear rate −1 Aldrich (Germany). The detonation ND nanoparticle pro- and thickness of 10 mm s and 200 μm, respectively) via cured from Nabond Technology Co., Ltd., China, with phase phase inversion method. The fabricated membranes were purity higher than 98%, an average diameter of 5 nm and a immersed in fresh distilled water for 24 h to remove all the 1 3 384 Iranian Polymer Journal (2018) 27:381–393 residual solvent. The synthesized membranes were washed the abrasion to record the loss in mass due to the abrasive thoroughly with distilled water and kept in DI water to make wearing. To carry out observatory analysis, SEM micro- them ready for further characterization. graphs of the surface of the original and the abraded samples were taken with an LEO 1455VP SEM (UK) at an accelerat- Membrane characterizations ing voltage of 10 kV. SEM Membrane porosity The morphology of the membrane samples was observed The membrane porosity ε(%) can be defined as the volume by a scanning electron microscope (LEO 1455VP, UK) of pores divided by the total volume of the porous mem- operating at 15 kV. For cross-sectional micrographs, the brane. The porosity of different membranes was calculated membranes were fractured in liquid nitrogen. All the sam- through Eq (8) [46, 47]: ples were gold-coated by sputtering to produce electrical (W − W ) D conductivity. w d w (%) = × 100 (8) (W − W ) D + W D w d w d p TEM where ε is the porosity of membrane (%), W is the wet TEM analysis was performed with a Philips CM120 trans- sample weight (g), W is the dry sample weight (g), and D d w −3 −3 mission electron microscope operating at 120  keV. The (0.998 g.cm ) and D (1.3 g.cm ) are the density of water sample membranes were embedded with epoxy, and cross (selected due to the hydrophilicity property of CA) and poly- sections of approximately 50 nm were obtained by section- mer at 25 °C, respectively. Three samples were measured for ing with a Leica Ultracut UCT ultramicrotome. each membrane and the average value of membrane porosity was reported. FTIR Contact angle measurement The chemical structure of nanoparticles was studied by Fou- rier transform infrared spectroscopy (FTIR) with a VERTEX The hydrophilicity of the membrane was determined by 70 FTIR spectrometer (Bruker, Germany) in the range of measuring the contact angle of the membrane surface with a −1 400–4000 cm . The sample pellet of nanoparticles for the contact angle goniometer (PGX, Thwing-Albert Instrument FTIR test was prepared by mixing the particles with KBr. Co., USA). At least five water contact angles at different locations on the membrane surface were recorded to get a Abrasion‑resistance testing reliable value. Abrasion is one of the most frequent wear mechanisms Water content on materials surface. It is defined as the material pull-out caused by the action of hard particles on the material surface Water content (WC) tests were conducted to study the which is subjected to the abrasive wear due to the relative adsorption of water to the membranes containing the ND movement between two surfaces [43]. The abrasion resist- and ND-COOH nanoparticles. Pieces of different membrane ance of pure and nanocomposite membranes was inves- samples were immersed in DI water at room temperature for tigated by stirred cell equipped with a fixture to keep the 24 h and the weight of wetted membrane (W ) was meas- wet membrane inside of abrasion suspension. Silicon carbide ured after mopping with a filter paper. The dry weight (W ) dry with 200–450 mesh (32–75 μm) particle size obtained from was determined after 48 h drying at 75 °C. The WC ratio was Sigma-Aldrich was used as the abrasive particle suspended calculated by the following equation [48, 49]: in DI water. To perform the abrasion test, 150 g of abrasive W − W slurry containing 10 wt% silicon carbide dispersed in DI wet dry WC(%) = × 100. (9) water was placed in a 200 mL beaker to allow sufficient wet movement of the slurry. The abrasion mixture was stirred at 400 rpm for 20 days according to the literature. Subse- Mechanical properties quently, the membrane sample was washed under ultrasoni- cation for 10 min to remove all debris worn away from the Tensile strength was performed by a universal testing membrane surface. A detailed description of the abrasion- machine (STAM-D, Santam, Iran) at room temperature at a −1 resistance test has been given by Ji et al. [44] and Lai et al. crosshead speed of 10 mm min . All samples for mechani- [45]. The membrane samples were weighed before and after cal testing were rectangular. The tensile strength was 1 3 Iranian Polymer Journal (2018) 27:381–393 385 extracted from the relevant stress–strain curves. The results performed for each experiment and the average value was were the average of at least three tests. reported. To evaluate the fouling resistance of the mem- branes, the flux recovery ratio (FRR) was calculated using TGA the following equation [33]: w2 FRR(%) = × 100. The Thermogravimetric analysis (TGA) of pristine and (11) w1 functionalized ND particles was determined using a ther- mogravimetric analyzer (Pyris Diamond TG/DTA, Perki- The flux loss caused by reversible (RFR), irreversible −1 nElmer, USA) at a heating rate of 10 °C min from room (IFR), and total fouling ratio (TFR) of the membrane in fil- temperature to 750 °C. tration was defined by the following equations [51]: J − J w2 HA HA adsorption experiments RFR(%) = × 100 (12) w1 Adsorption experiments were carried out by a batch method J − J at ambient temperature. The pure CA and nanocomposite w1 w2 IFR(%) = × 100 (13) flat sheet membranes were cut in the size of 1 × 1 cm and w1 −1 immersed into 5 mL of 100 mg L HA solution at pH 7 (adjusted with 0.1 N HCl solutions). Then, the samples con- J − J w1 HA TFR(%) = RFR(%) + IFR(%) = × 100. taining adsorbed HA were removed from the basic solution. w1 The vials were agitated with a shaker and the adsorption (14) experiments were carried out at the temperature of 25 °C for To measure the membrane rejection, the HA concentra- 24 h to reach equilibrium. The HA adsorptions were meas- tions of feed and permeate solutions were determined using ured at room temperature using a UV spectrophotometer a UV spectrophotometer (Bio Quest CE2501) and the HA (Bio Quest CE2501). The adsorption capacity was calcu- rejection of membrane was calculated using Eq (15) as fol- lated as follows [7, 50]: lows [52, 53]: C − C HA adsorption capacity = × V (10) R(%) = 1 − × 100 (15) where A (m ) is the membrane area, V is the total volume of the solution (5 mL), and C and C are the concentration of where R is the membrane rejection, and C and C are per- p f HA solution before and after contact with the membranes, meate and feed concentrations, respectively. respectively. Analyses of fouling resistances Results and discussion The effects of ND and ND-PEG nanoparticles on the removal efficiency of HA and the improvement of UF mem- Characteristics of functionalized ND nanoparticles brane fouling behavior were investigated. HA solution was prepared by dissolving 1 g of HA in 1 L of Milli-Q water. Figure  1 indicates the FTIR spectra of the pristine ND, The solution pH was adjusted at 6 by adding a small amount ND-COOH, and ND-PEG nanoparticles. For the ND pow- of either 0.1 M HCl or 0.1 M NaOH. The solution was fil- der, the main features in the spectrum are related to C–H tered through a 0.45 µm filter to remove particulates and −1 vibrations at 2981.7 and 2856.8 cm that correspond to stored in the refrigerator (4 °C) before use. Experiments the asymmetric and symmetric C–H-stretching vibrations, were performed in a dead-end filtration setup. respectively, and O–H-bending vibrations at 3486.2 and After pure water flux (J ) tests, the prepared HA solu- −1 w1 1642.3 cm . In addition, the spectrum revealed bonds at tion was performed and the permeate flux (J ) profile was −1 HA 1764.8 and 1177 cm which are attributed to the stretching recorded every 10 min to determine the dynamic fouling vibrations of carbonyl (C=O) and ether (C–O–C) groups, resistance of the membrane. After 240 min of HA filtra- respectively [54]. tion, the membrane was cleaned with distilled water under For the ND-COOH nanoparticles, it has almost the same magnetic stirring for 30 min, and then, pure water was again IR adsorptions as those of pristine ND, but C–H bonds in passed through the membrane to measure second pure water ND completely disappear and the absorption peak of C=O flux (J w ). The HA filtration measurement was carried out −1 are shifted to 1798  cm due to the conversion of some in transmembrane pressure of 1.5 bar. Three samples were 1 3 386 Iranian Polymer Journal (2018) 27:381–393 Fig. 2 TGA curves of pristine ND, ND-COOH, and ND-PEG nano- particles and ND-PEG nanoparticle concentration until 0.5  wt%. Then, the tensile strength decreased by adding nanoparticles. Fig. 1 FTIR spectra of pristine ND, ND-COOH, and ND-PEG nano- The tensile strength increased by 54.6% in the sample with particles 0.5 wt% of ND-PEG nanoparticles. An increase in the tensile strength was observed for the CA/ND-PEG membrane in oxygen containing groups like ketone, alcohol, and ester to comparison with CA/ND membrane, due to well dispersion carboxylic group [55]. of ND-PEG nanoparticles and well interaction between these In the characteristic peaks of ND-PEG nanoparti- nanoparticles and CA matrix. cles, C–H-stretching vibrations appeared at 2849.7 and However, the higher content of nanoparticles (> 0.5 wt%) −1 2921 cm corresponding to the –CH in PEG molecules. In will obviously cause a decrease in the mechanical properties addition, other characteristic peaks of PEG chains at 1479.3, of the nanocomposite membranes. When the content of ND −1 1259.4, 1136, 1095.5, and 825.5 cm were all observed, and ND-PEG nanoparticles was 0.75 wt%, the microstruc- indicating that PEG was indeed attached to the surface of ture of membranes obviously changed due to the aggrega- ND nanoparticles [27]. However, the intensity of the peak at tion of nanoparticles in CA matrix and this will result in the −1 1095.5 cm significantly increased. This increase is attrib- decline of mechanical properties [56]. uted to the C–O–C-stretching modes of ether groups in PEG molecules. Hydrophilicity, porosity, and water content of membranes Figure 2 shows the TGA curves of the pristine ND, ND- COOH, and ND-PEG nanoparticles. The weight loss per- The results of contact angle measurements for membranes centages of ND, ND-COOH, and ND-PEG nanoparticles are are given in Table 1. The contact angle of neat CA mem- 1.22, 3.67, and 18.81% at 500 °C, respectively, suggesting brane decreased from 65.4 to 58 and 52.2 by adding 0.5 wt% that more functional groups were introduced on the surface ND and ND-PEG nanoparticles, respectively. A decrease in of ND during PEG-grafting steps. The major weight loss of the water contact angle was corresponded with the improve- ND-PEG nanoparticles occurred between 180 and 750 °C. ment in membrane hydrophilicity. In comparison with the This weight loss may greatly be ascribed to the decompo- CA/ND membrane, CA membranes containing 0.5 wt% of sition of the PEG chains, since the onset was close to the ND-PEG nanoparticles show the lowest amount of water decomposition temperature of PEG (177 °C) [27]. contact angles. This is due to the existence of hydroxyl hydrophilic functional groups on the membranes structures Membrane characterization as well as the good dispersion of ND-PEG nanoparticles. Any further increase in ND nanoparticles up to 0.75 wt% Mechanical properties increased the water contact angle. According to the results reported by Xu et al., any increase in water contact angel at The mechanical properties of pure and nanocomposite mem- a higher ND nanoparticles content supports our speculation brane samples were measured and the results are shown in regarding the agglomeration and cluster formation of ND Table 1. It can be seen that the tensile strength of the nano- nanoparticles and consequently non-uniform dispersion of composite membranes increased with the increase of ND nanoparticles at higher concentrations [57]. 1 3 Iranian Polymer Journal (2018) 27:381–393 387 Table 1 Tensile strength, water contact angles, water content (%), porosity (%), HA adsorption capacity, and weight loss per unit area for pure CA and its nanocomposite membranes Sample code Tensile Water contact Water con- porosity (ε) (%) HA adsorption Weight loss −2 strength (MPa) angles (°) tent (%) capacity (mg cm ) per unit area −2 (mg m ) CA 9.7 65.4 71.6 76.7 2.85 7.2 CA/ND (0.25 wt%) 10 60.3 74.2 78.6 2.83 – CA/ND (0.5 wt%) 10.4 58 75 80.5 2.7 4.3 CA/ND (0.75 wt%) 9 59.1 73.5 78.5 2.8 – CA/ND-PEG (0.25 wt%) 11.8 55.8 79.2 82.5 2.5 – CA/ND-PEG (0.5 wt%) 15 52.2 79.8 82.8 2.15 2.5 CA/ND-PEG (0.75 wt%) 13.4 52.7 78.8 82.2 2.6 – The WC was calculated using Eq (9) and the value of membrane, the amount of adsorbed HA increased from 2.7 −2 the membranes are shown in Table  1. From Table  1, the to 2.8 mg cm , due to sever agglomeration of nanoparticles. WC of neat CA membrane showed a value of 71.6%. It As shown in Table 1, by increasing the ND-PEG nanopar- can be seen that all the nanocomposite membranes show ticles loading, the amount of adsorbed HA reached 2.5, 2.15, −2 higher WC value than neat CA. The highest value of 79.8% and 2.6 mg cm for loading of 0.25, 0.5, and 0.75 wt%, was appeared when the content of ND-PEG nanoparticles respectively. The reduction in HA adsorption on CA/ND- reached 0.5 wt%. The parameter of WC may be due to the PEG nanocomposite membranes (especially at 0.5 wt%) was detachment of polymer chains from the ND-PEG nanopar- ascribed to hydrophilic and well dispersion properties of ticles surface, causing interface voids. Furthermore, this led ND-PEG nanoparticles. In addition, due to the formation to an increase in void volume, resulting in the formation of of thin water film on the surface of CA/ND-PEG nanocom- bigger size pores on the membrane surface and an increase posite membranes, the reduction in HA adsorption occurred of WC in the pores. [7, 50]. The membrane porosities have been measured using gravimetric method and the results are reported in Table 1. Membrane morphology It is clear that increasing the concentration of pure and func- tionalized ND nanoparticles increased the membrane poros- The top surface and cross-sectional SEM micrographs of ity that attained its maximum value at the concentration of CA membrane and the selected nanocomposite membranes 0.5 wt%. Any further increase in the ND content decreased are presented in Fig. 3. As evidenced from these SEMs, the the porosity, which would contribute to particles’ agglom- porosity of membrane top surface increased due to the pres- eration. However, mixing hydrophilic nanoparticles with CA ence of both ND and ND-PEG nanoparticles. polymer could enhance the volume fraction between poly- The cross-sectional SEM micrographs in Fig. 4 show that mer chains, in addition, causing fast exchange of solvent all the membranes exhibit the typical asymmetric structure. and non-solvent during the phase inversion process [58]. It seems that macro-voids become longer when ND and ND- Moreover, during the phase inversion process of nanocom- PEG nanoparticles are added. The growth of macro-voids is posite membranes, especially CA/ND-PEG (0.5 wt%), the determined by the relative diffusion rates between solvent solvent and the non-solvent exchange rate increased due to and non-solvent. With the addition of ND and especially the hydrophilic nature and well dispersion of nanoparticles ND-PEG, the higher affinity between hydrophilic nanopar - additive. This led to the formation of membrane with more ticles and water accelerates the exchange rate of solvent and porous surface. non-solvent [59]. In the case of CA/ND-PEG (0.5  wt%) membrane, the hydrophilic functional groups on ND-PEG nanoparticles may facilitate the exchange rate of solvent and HA adsorption test non-solvent. These results indicated that the addition of ND and ND-PEG nanoparticles has bulky effects on the mem- Table  1 shows the results obtained from HA adsorption brane structure formation. test. Pure CA membrane sheet showed the maximum HA Information about the dispersion of nanoparticles cannot −2 adsorption about 2.85 mg cm . As the pristine ND parti- be obtained from the SEM micrographs. Therefore, TEM cles concentration increased from 0 to 0.5 wt%, the amounts was used to investigate the dispersion of nanoparticles in −2 of adsorbed HA decreased to 2.7 mg cm . However, as CA membranes, and the result is shown in Fig. 5. At low the ND content reached 0.75  wt% in the nanocomposite and especially high loading of ND nanoparticles, more 1 3 388 Iranian Polymer Journal (2018) 27:381–393 Fig. 4 SEM micrographs of the cross-sectional morphology of: a CA, Fig. 3 SEM micrographs of top surface of: a CA, b CA/ND b CA/ND (0.5 wt%), and c CA/ND-PEG (0.5 wt%) membranes (0.5 wt%), and c CA/ND-PEG (0.5 wt%) membranes 1 3 Iranian Polymer Journal (2018) 27:381–393 389 ND-PEG particles was established, as shown in Fig. 5c. In addition, this behavior has been reported elsewhere in which a smoother and rougher surface has been formed at low and high loading of nanoparticles into polymeric membranes, respectively [51, 59]. Abrasion resistance Table 1 represents the weight loss per unit area of pure CA and selected nanocomposite membranes after performing abrasion test. Obtained results show that in presence of ND- PEG nanoparticles, the lowest weight loss is occurred. In the presence of pristine ND nanoparticles as nanofiller, the weight loss is still lower than that of the pure CA membrane. Interestingly, the CA/ND-PEG membrane shows superior performance and lower weight loss than both neat and nano- composite CA membrane. This finding is mainly due to well distribution of ND nanoparticles as well as the contact rein- forcement/matrix effect in the CA matrix. A similar result was reported by Lai [60], who showed that the abrasion resistance of poly(vinylidene fluoride) membrane increased by adding nanoclay as reinforcing agent. Specific wear rate (W) can be calculated according to Ratner equation shown in Eq (16) as follows [61]: W = (16) where μ is the coefficient of friction, H is the indentation hardness, σ is the tensile strength, ε is the elongation at maximum load, and k is a proportionality constant. According to this equation and the results shown in Table  1, it is obvious that higher tensile strength as well as higher elongation-at-break results in lower abrasion rate. Therefore, CA/ND-PEG nanocomposite membrane with 0.5 wt% of ND nanoparticle content resists against the dam- age from the abrasive particles and exhibits lower abrasion rate due to the higher ductile strength. To better understand the effect of ND nanoparticles on the abrasion resistance of CA membrane, the abraded surfaces of the membranes were investigated by SEM analysis. Figure  6 shows the abraded surfaces of neat CA and selected CA/ND and CA/ND-PEG nanocomposite mem- branes. As shown in Fig. 6a, more ploughed furrows and pittings are formed on the abraded surface of CA membrane. By adding the ND nanoparticles, the number of pittings Fig. 5 TEM micrographs of distribution of: a 0.5  wt% of ND, b on the membrane surface remarkably decreases (Fig. 6b), 0.75  wt% of ND, and c 0.5  wt% of ND-PEG nanoparticles in CA implying that the presence of ND fillers in the nanocom- membranes posite membrane can enhance the abrasion resistance of CA membrane. Interestingly, in the presence of ND-PEG agglomerations are formed (Fig.  5a, b). While, in lower nanoparticles, the unenviable impact of abrasion test on the amounts of functionalized nanoparticles, i.e., 0.5 wt% ND- surface of the membrane was minimal and a smooth surface PEG, the construction of large aggregations was avoided, is observed (Fig. 6c). This result also implies that surface and therefore, a relatively uniform distribution of the modification of ND nanoparticles can improve the interfacial 1 3 390 Iranian Polymer Journal (2018) 27:381–393 Fig. 7 Flux-time behavior of pure and nanocomposite membrane dur- −1 ing filtration of 1 g L HA solution that membrane hydrophilicity played the vital role in the improvement of HA filtration. In an aqueous solution, the strong hydrogen bonding of water molecules to the oxygen atoms in PEG accumulates water molecules around PEG, which prevents hydrophobic foulants from approaching the surface. It is usually consid- ered as a steric stabilization effect [62, 63]. The character- istic features of the PEG coated layer on the ND nanopar- ticles, i.e., hydrophilicity, freedom of electrostatic charges, and the rapid motion of hydrated chains, have been of great help in terms of avoiding interaction between hydrophobic materials such as HA and the surface of ND nanoparticles. Therefore, ND-PEG nanoparticles improved the anti-fouling performance of CA membrane during HA filtration due to hydrophilicity, which prevents foulant attachments on the membrane surface. Figure 8 illustrates the fitting of the obtained experimen- tal data after using pure CA and CA/ND-PEG nanocompos- ite membranes at different predicted fouling mechanisms, including complete pore blocking, standard pore blocking, intermediate pore blocking, and cake formation. As shown Fig. 6 SEM micrographs of membrane surface after the abrasion test: in Fig. 8, it is observed that a cake filtration model provides a CA, b CA/ND (0.5 wt%) and c CA/ND-PEG (0.5 wt%) membranes the best fit for all membranes. To evaluate the anti-fouling performance of the fabricated bonding between ND-PEG nanoparticles and CA matrix, pure and nanocomposite CA membranes, flux recovery ratio resulting in more uniform dispersion of ND particles in the (FRR), total fouling ratio (TFR), and reversible (RFR) and bulk of polymer matrix (Fig. 5c) as well as on the surface of irreversible fouling ratio (IFR) were quantified during filtra - the CA membrane that improved the abrasion resistance of tion experiments and the calculated values are presented in the nanocomposite membrane. Table 2. It is obvious that the pure membrane had the high- est TFR (89.7%) and IFR (35%) values. While in the CA/ Fouling analysis and membrane performance ND-PEG (0.5 wt%) membranes, TFR and IFR values were considerably reduced. In the case of modified membranes, The flux decline behavior of the membrane during HA fil - the hydrophobic adsorption between the foulants and the tration is shown in Fig. 7. All filtration tests were carried surface of the modified membranes was decreased and the out until the flux reaches a stable value. The permeation attached foulants were easily removed during filtration. flux for CA/ND-PEG (0.5 wt%) nanocomposite membrane Since all membranes were tested under the same condi- was higher than that of the other membranes. It indicates tions, the surface property of membrane was a key factor in 1 3 Iranian Polymer Journal (2018) 27:381–393 391 determining the degree of TFR. Lower values of TFR show that less HA adsorption took place on the membrane surface or pore walls. Performance studies to determine the rejection of HA solution showed that the presence of ND-PEG nanopar- ticles increased rejection compared with the membranes containing pristine ND particles (Table 2). The addition of ND-PEG nanoparticle created a more hydrophilic surface for the modified membranes that prevented HA molecules from settling closer to the surface of the membrane and thus increased the HA rejection. Conclusion Polyethylene glycol-grafted nanodiamond (ND-PEG) embedded cellulose acetate (CA) membranes were pre- pared through solution casting by phase inversion method. The anti-fouling behavior of pure and nanocomposite CA membranes during the filtration of humic acid (HA) was investigated. The results of FTIR revealed that the PEG molecules were formed on the surface of ND nanoparticles. SEM analyses of the surfaces of membranes depicted the increase in the number of pores by the addition of ND and especially ND-PEG nanoparticles into the polymer matrix. The results revealed that nanocomposite CA membrane con- taining 0.5 wt% PEGylated ND nanoparticles showed high hydrophilicity, high porosity, low HA adsorption capacity, high abrasion resistance, and excellent anti-fouling prop- erties. It is due to the strong hydrogen bonding of water molecule to the oxygen atoms in PEG molecules, which pre- vents HA from approaching the surface of the membrane. According to the results obtained by Hermia’s model, it was concluded that cake formation mechanism is the pre- vailing fouling mechanism, which can be occurred for neat Fig. 8 Experimental data and Hermia fouling models for: a CA, b CA/ND (0.5  wt%), and CA/ND-PEG (0.5  wt%) membranes during and nanocomposite CA membranes. However, due to the filtration of HA solution enhancement of membrane wettability and surface hydro- philicity, CA/ND-PEG nanocomposite membrane showed more reversible fouling mechanism than that of the neat CA Table 2 Rejection and fouling parameters of the prepared membranes membrane. during HA filtration Sample code RFR IFR TFR FRR Rejection CA 54.7 35 89.7 65 94.5 CA/ND (0.5 wt%) 54 20.4 74.4 80 94.8 CA/ND-PEG (0.5 wt%) 50.7 8.4 59.1 91.6 95.4 1 3 392 Iranian Polymer Journal (2018) 27:381–393 Acknowledgements The authors gratefully acknowledge financial sup- 14. Haleem YA, Liu D, Chen W, Wang C, Hong C, He Z, Liu J, Song port from Sahand University of Technology (Grant No. 30/15975). P, Yu S, Song L (2015) Surface functionalization and structure characterizations of nanodiamond and its epoxy based nanocom- posites. Compos Part B Eng 78:480–487 Open Access This article is distributed under the terms of the Creative 15. 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Synthesis, characterization, and anti-fouling properties of cellulose acetate/polyethylene glycol-grafted nanodiamond nanocomposite membranes for humic acid removal from contaminated water

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Chemistry; Polymer Sciences; Ceramics, Glass, Composites, Natural Materials
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1026-1265
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10.1007/s13726-018-0616-7
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Abstract

Polyethylene glycol-grafted nanodiamond (ND-PEG) was synthesized from pristine detonation NDs and utilized to prepare novel cellulose acetate/polyethylene glycol-grafted nanodiamond(CA/ND-PEG)nanocomposite membranes. Due to unique thermal, mechanical, and antibacterial properties and very easy cleaning of fouled ND-embedded CA nanocomposite mem- branes, we tried to investigate the performance of CA/ND-PEG membrane for humic acid (HA) removal from contaminated water. Surface functionalization was confirmed by Fourier transform infrared spectroscopy and thermogravimetry analysis. Pristine and functionalized ND with different concentration was added in the casting solution containing CA. The prepared membranes were characterized using contact angle, mechanical strength, scanning electron microscopy (SEM), transmis- sion electron microscopy, and permeation tests. SEM micrographs of the surface of the membranes depicted the increase in the number of pores by the addition of ND and especially ND-PEG into polymer matrix. The results indicated that the nanocomposite membrane with 0.5 wt% ND-PEG exhibited excellent hydrophilicity, mechanical properties, permeability, high rejection, high abrasion resistance, and good anti-fouling performance. The HA adsorption on the membrane surface −2 decreased from 2.85 to 2.15 mg cm when the ND-PEG content increased from 0 to 0.5 wt%. Most importantly, the HA filtration experiments revealed that the incorporation of ND and especially ND-PEG particles reduced membrane irrevers - ible fouling, dramatically. Meanwhile, the analysis of the fouling mechanism based on Hermia’s model revealed that cake formation is a prevailing mechanism for all membranes. Keywords Nanodiamond · Polyethylene glycol · Cellulose acetate · Nanocomposite membrane · Anti-fouling Introduction color of water [2–4]. Therefore, the removal of NOMs is extremely important and has become a challenging subject Membrane technology is widely used for water treatment of research in the current development of water purification and has obtained more attention than any other separation technologies. process due to low energy consumption, easy scale-up, less Cellulose acetate (CA) is one of the foremost among or no use of chemicals and no harmful by-product formation ultrafiltration (UF) polymer membranes. It has been widely [1]. Humic acid (HA), an important component of natural used in separation processes, due to its high hydrophilicity, organic matters (NOMs), is derived from the decomposition high biocompatibility, good desalting, non-toxic nature, per- of the plant and animal materials that are commonly found formance high potential flux, and relatively low cost [ 4–6]. in surface and ground water and strictly affect the taste and However, this membrane usually contains a dense skin layer and a low porous sub-layer, which result in an extremely low flux [ 7]. On the other hand, one of the serious problems * Reza Yegani arising during UF membrane filtration is membrane fouling ryegani@sut.ac.ir by NOMs [8]. Membrane fouling can be classified as revers - 1 ible fouling (which can be eliminated by physical cleaning) Faculty of Chemical Engineering, Sahand University and irreversible fouling (which cannot be entirely counter- of Technology, Tabriz, Iran 2 acted by physical cleaning and typically requiring chemical Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran Vol.:(0123456789) 1 3 382 Iranian Polymer Journal (2018) 27:381–393 cleaning) [9]. Hence, to improve the performance of CA membranes [11] in the current study, PEG-grafted ND parti- membranes, modification of polymer seems to be essential. cles were employed to fabricate ND-embedded CA nanocom- Recently, nanocomposite membranes have been applied posite membrane. As reported in the literature, nanocomposite in membrane technology for the removal of pollutants from membranes such as HDPE/SiO [11], polyethersulfone (PES)/ water to wastewater [10]. Polymer nanocomposite mem- Stöber silica [30], CA/organically modie fi d montmorillonite branes are formed by incorporation of nanoparticles into [7], polyamide (PA)/multi-wall carbon nanotube (MWCNT) polymer matrix. In many cases, a significant improvement in [31], polysulfone/GO-TiO [9], and PVDF/TiO [32] enhanced 2 2 mechanical, thermal, antibacterial, and anti-fouling proper- the performance of polymeric membranes in HA removal from ties has been achieved [11, 12]. However, the well dispersion contaminated water. In our previous work, pristine and carbox- of nanoparticles is still a challenging subject for researchers. ylated ND particles, i.e., ND and ND-COOH were added to Carbon-based nanomaterials are potentially useful due to the CA polymer to investigate the anti-fouling properties of the their unique physical and chemical properties. Among them, prepared nanocomposite CA membranes in HA removal from nanodiamond (ND) particles with a diamond core (sp car- contaminated water [33]. Obtained results were promising; bon–carbon bond) that is covered with multiple functional however, anti-fouling properties were far from our preliminary groups [13, 14] are of great interest. Due to interesting goal. It was concluded that more modified ND particles were characteristics such as hydrophilicity [15, 16], antibacterial required. activity [12, 17, 18], biocompatibility [19, 20], chemical In this work, due to the great performance of PEGylated stability [19], thermal stability [20] non-toxicity [19–21], NPs [11, 34], the anti-fouling properties of nanocomposite superior hardness and mechanical properties, resistance to CA membranes containing PEGylated ND particles were harsh environments [19], and ease of surface functionaliza- compared by CA-ND membranes during the filtration of HA. tion [20], ND particles have been nominated as reinforce- Nanocomposite membrane was synthesized using blending ment fillers in the fabrication of nanocomposite materials. method followed by phase inversion process using pristine Despite the advantages described above, pristine ND and PEG-grafted ND particles denoted as ND and ND-PEG, particles usually form micro-sized agglomerates [22]. The respectively. Several structural and operational analyses were presence of carbon impurities among the ND particles and carried out to identify enhancement in membrane surface consequently agglomeration of ND particles led to poor dis- hydrophilicity, abrasion resistance, and anti-fouling proper- persibility and weak interfacial interactions with polymer ties during the filtration of HA. The fouling mechanisms of matrix [22, 23]. Thus, surface functionalization is an essen- the fabricated membranes were also analyzed using classical tial method to reduce the aggregation of nanoparticles and Hermia model. improve their performance. According to our previous findings, polyethylene glycol Theory of fouling mechanism (PEG) embedded polyolefin membranes showed excellent hydrophilic properties and exhibited great anti-fouling To identify the possible dominant mechanism of the mem- performance [11]. PEG can enhance biological properties brane fouling under constant filtration pressure, Hermia mod- including high biocompatibility, weak interactions with pro- els were used. These models are written in a common math- tein, and antibacterial properties [24]. It is used as a head ematical form as follows [35]: group in nonionic surfactants to cover surfaces to prevent 2 m d t dt proteins and hydrophobic foulants in surface water to adhere = k (1) dV dV to the membrane surface [25]. In other words, PEG produces a high degree of steric exclusion and entropy at PEG–water where t is the filtration time, V is the filtrate volume, k is the interfaces due to the formation of hydrogen bond with water resistance coefficient, and m is a constant which character- molecules, which in turn results in protein repellency [11, izes the fouling model. The permeate flux (J ) is expressed 26]. The conjugated PEG chains on the nanoparticle surface as follows [36]: could reduce protein interactions with the nanoparticles and 1 dV consequently with the membrane surface [27]. According to J = . (2) A dt Wang et al. [24] and as well as Zhang et al. [28], PEGylated ND can improve dispersion and reduce interaction with pro- Equation (1) can be written as follows: teins. In addition, a literature survey showed that PEG is an dJ 2−m 3−m ideal polymer for the modification of nanostructured materi- =−kA J . (3) dt als to be used in polymeric membranes [11, 28, 29]. To combine the great mechanical and thermal properties According to this model, there are four fouling mecha- of ND particle-embedded CA membranes [12] with excel- nisms. In the complete pore blockage mode, under condition, lent anti-fouling performance of PEG containing polyolefin 1 3 Iranian Polymer Journal (2018) 27:381–393 383 2 −1 it is assumed that no particles are situated on top of other specific surface area of 282.8 m g , was used as nanoparti- particles or on membrane surface. The model was derived cle. N–N-dimethylformamide (DMF, 99.8%) was purchased by substituting m = 2 in Eq (3) and integrating with regard to from Merck (Germany) and used as a solvent to prepare the time of permeate flux, Eq (4 ) can be obtained [37, 38]: dope solution. Deionized (DI) water was used as the non- −1 solvent. PEG (M = 200 g mol ) and 1,3-phenylenediamine 1 1 ln = ln + K t (mPDA) as hydrophilic modifiers were purchased from (4) J J Sigma-Aldrich. Thionyl chloride (SOCl ), triethylamine, and tetrahydrofuran (THF) were obtained from Merck. where K and J are complete pore blocking coefficient and b 0 pure water flux, respectively. PEG‑grafted ND nanoparticles The internal pore blocking mode accounts for fouling that occurs in the internal structure of the membrane. Internal According to the literature [22], to remove the organic pore blocking is due to the constriction of membrane pores impurities and create carboxylic functional groups on the caused by small particles attached into pore walls. The expo- ND nanoparticles surface (denoted by ND-COOH), pristine nent is put with the value of m = 1.5 and integrating Eq (3), detonation NDs were dried at 80 °C for 2 h in a vacuum the permeate flux is decreased and is related to time using dryer and the process was continued by oxidation in air at a the following equation [37, 39]: temperature between 430 and 450 °C for 1.5 h. PEG-grafted 1 1 ND nanoparticles were synthesized according to the method = + K t √ √ (5) described by Wang et al. [24]. A sample of ND-COOH nan- oparticles (50.26 mg) was dried in a vacuum oven at 70 °C overnight and then suspended in 2 mL anhydrous DMF in where K is an internal pore blocking coefficient. an ultrasonic bath. The prepared suspension was immedi- The intermediate pore blockage mode is similar to the ately added to 20 mL SOCl and refluxed for 24 h at 70 °C complete pore blockage model, in addition to accounting 2 to convert the carboxylic acids to acyl chlorides (denoted for the possibility of particles bridging a pore by obstructing by ND-COCL). The mixture was evaporated with a rotary the entrance without completely blocking it [40]. This model evaporator to remove excess S OCl at 40 °C. After adding substitutes m = 1 in Eq (3), and by integrating, the following 2 20 mL anhydrous DMF, 50.48 mg PEG, and 2 mL triethyl- equation is obtained [41]: amine, the mixture was refluxed for 24 h at 90 °C, separated 1 1 by centrifugation, and washed with methanol and distilled = + K t i (6) J J water five times to remove excess PEG. The obtained pow - der was dried under vacuum at 60 °C, overnight. where K is an intermediate pore blocking coefficient. In the case of cake formation, particles do not participate Membrane preparation in changes in membrane pores. A cake layer is formed out- side the external membrane and increases hydraulic resist- The neat and nanocomposite CA membrane flat sheets were ance. The cake filtration model was obtained by substituting prepared by phase inversion via immersion precipitation the value of m = 0 and integrating Eq (3), the obtained equa- technique. The blend homogeneous solutions were prepared tion is expressed as follows [42]: by dissolving the 17.5 wt% of CA polymer in DMF as a sol- 1 1 vent with different concentrations of ND, and ND-PEG par - = + K t (7) 2 2 ticles ranged from 0 to 0.75 wt%. The rest of nanoparticles was added to DMF and stirred for 4 h at room temperature. where K is a cake pore blocking coefficient. The mixture was sonicated (Woson, China) at 50 kHz for 3 h to ensure the homogeneous spread of the nanoparticles and break up the agglomerates. Then, CA (17.5 wt%, by Experimental weight of the solution) was added to the initial mixture and dissolved in the solvent at 2000 rpm for 15 h. The casting Materials solution was then degassed, overnight without stirring to remove the gas bubbles, completely. The solution was then Cellulose acetate (M = 30,000) was used as polymer mate- cast onto a glass plate using automatic casting machine (Coa rial to prepare the CA membrane, supplied by Sigma- Test, Taiwan) to produce a flat sheet membrane (shear rate −1 Aldrich (Germany). The detonation ND nanoparticle pro- and thickness of 10 mm s and 200 μm, respectively) via cured from Nabond Technology Co., Ltd., China, with phase phase inversion method. The fabricated membranes were purity higher than 98%, an average diameter of 5 nm and a immersed in fresh distilled water for 24 h to remove all the 1 3 384 Iranian Polymer Journal (2018) 27:381–393 residual solvent. The synthesized membranes were washed the abrasion to record the loss in mass due to the abrasive thoroughly with distilled water and kept in DI water to make wearing. To carry out observatory analysis, SEM micro- them ready for further characterization. graphs of the surface of the original and the abraded samples were taken with an LEO 1455VP SEM (UK) at an accelerat- Membrane characterizations ing voltage of 10 kV. SEM Membrane porosity The morphology of the membrane samples was observed The membrane porosity ε(%) can be defined as the volume by a scanning electron microscope (LEO 1455VP, UK) of pores divided by the total volume of the porous mem- operating at 15 kV. For cross-sectional micrographs, the brane. The porosity of different membranes was calculated membranes were fractured in liquid nitrogen. All the sam- through Eq (8) [46, 47]: ples were gold-coated by sputtering to produce electrical (W − W ) D conductivity. w d w (%) = × 100 (8) (W − W ) D + W D w d w d p TEM where ε is the porosity of membrane (%), W is the wet TEM analysis was performed with a Philips CM120 trans- sample weight (g), W is the dry sample weight (g), and D d w −3 −3 mission electron microscope operating at 120  keV. The (0.998 g.cm ) and D (1.3 g.cm ) are the density of water sample membranes were embedded with epoxy, and cross (selected due to the hydrophilicity property of CA) and poly- sections of approximately 50 nm were obtained by section- mer at 25 °C, respectively. Three samples were measured for ing with a Leica Ultracut UCT ultramicrotome. each membrane and the average value of membrane porosity was reported. FTIR Contact angle measurement The chemical structure of nanoparticles was studied by Fou- rier transform infrared spectroscopy (FTIR) with a VERTEX The hydrophilicity of the membrane was determined by 70 FTIR spectrometer (Bruker, Germany) in the range of measuring the contact angle of the membrane surface with a −1 400–4000 cm . The sample pellet of nanoparticles for the contact angle goniometer (PGX, Thwing-Albert Instrument FTIR test was prepared by mixing the particles with KBr. Co., USA). At least five water contact angles at different locations on the membrane surface were recorded to get a Abrasion‑resistance testing reliable value. Abrasion is one of the most frequent wear mechanisms Water content on materials surface. It is defined as the material pull-out caused by the action of hard particles on the material surface Water content (WC) tests were conducted to study the which is subjected to the abrasive wear due to the relative adsorption of water to the membranes containing the ND movement between two surfaces [43]. The abrasion resist- and ND-COOH nanoparticles. Pieces of different membrane ance of pure and nanocomposite membranes was inves- samples were immersed in DI water at room temperature for tigated by stirred cell equipped with a fixture to keep the 24 h and the weight of wetted membrane (W ) was meas- wet membrane inside of abrasion suspension. Silicon carbide ured after mopping with a filter paper. The dry weight (W ) dry with 200–450 mesh (32–75 μm) particle size obtained from was determined after 48 h drying at 75 °C. The WC ratio was Sigma-Aldrich was used as the abrasive particle suspended calculated by the following equation [48, 49]: in DI water. To perform the abrasion test, 150 g of abrasive W − W slurry containing 10 wt% silicon carbide dispersed in DI wet dry WC(%) = × 100. (9) water was placed in a 200 mL beaker to allow sufficient wet movement of the slurry. The abrasion mixture was stirred at 400 rpm for 20 days according to the literature. Subse- Mechanical properties quently, the membrane sample was washed under ultrasoni- cation for 10 min to remove all debris worn away from the Tensile strength was performed by a universal testing membrane surface. A detailed description of the abrasion- machine (STAM-D, Santam, Iran) at room temperature at a −1 resistance test has been given by Ji et al. [44] and Lai et al. crosshead speed of 10 mm min . All samples for mechani- [45]. The membrane samples were weighed before and after cal testing were rectangular. The tensile strength was 1 3 Iranian Polymer Journal (2018) 27:381–393 385 extracted from the relevant stress–strain curves. The results performed for each experiment and the average value was were the average of at least three tests. reported. To evaluate the fouling resistance of the mem- branes, the flux recovery ratio (FRR) was calculated using TGA the following equation [33]: w2 FRR(%) = × 100. The Thermogravimetric analysis (TGA) of pristine and (11) w1 functionalized ND particles was determined using a ther- mogravimetric analyzer (Pyris Diamond TG/DTA, Perki- The flux loss caused by reversible (RFR), irreversible −1 nElmer, USA) at a heating rate of 10 °C min from room (IFR), and total fouling ratio (TFR) of the membrane in fil- temperature to 750 °C. tration was defined by the following equations [51]: J − J w2 HA HA adsorption experiments RFR(%) = × 100 (12) w1 Adsorption experiments were carried out by a batch method J − J at ambient temperature. The pure CA and nanocomposite w1 w2 IFR(%) = × 100 (13) flat sheet membranes were cut in the size of 1 × 1 cm and w1 −1 immersed into 5 mL of 100 mg L HA solution at pH 7 (adjusted with 0.1 N HCl solutions). Then, the samples con- J − J w1 HA TFR(%) = RFR(%) + IFR(%) = × 100. taining adsorbed HA were removed from the basic solution. w1 The vials were agitated with a shaker and the adsorption (14) experiments were carried out at the temperature of 25 °C for To measure the membrane rejection, the HA concentra- 24 h to reach equilibrium. The HA adsorptions were meas- tions of feed and permeate solutions were determined using ured at room temperature using a UV spectrophotometer a UV spectrophotometer (Bio Quest CE2501) and the HA (Bio Quest CE2501). The adsorption capacity was calcu- rejection of membrane was calculated using Eq (15) as fol- lated as follows [7, 50]: lows [52, 53]: C − C HA adsorption capacity = × V (10) R(%) = 1 − × 100 (15) where A (m ) is the membrane area, V is the total volume of the solution (5 mL), and C and C are the concentration of where R is the membrane rejection, and C and C are per- p f HA solution before and after contact with the membranes, meate and feed concentrations, respectively. respectively. Analyses of fouling resistances Results and discussion The effects of ND and ND-PEG nanoparticles on the removal efficiency of HA and the improvement of UF mem- Characteristics of functionalized ND nanoparticles brane fouling behavior were investigated. HA solution was prepared by dissolving 1 g of HA in 1 L of Milli-Q water. Figure  1 indicates the FTIR spectra of the pristine ND, The solution pH was adjusted at 6 by adding a small amount ND-COOH, and ND-PEG nanoparticles. For the ND pow- of either 0.1 M HCl or 0.1 M NaOH. The solution was fil- der, the main features in the spectrum are related to C–H tered through a 0.45 µm filter to remove particulates and −1 vibrations at 2981.7 and 2856.8 cm that correspond to stored in the refrigerator (4 °C) before use. Experiments the asymmetric and symmetric C–H-stretching vibrations, were performed in a dead-end filtration setup. respectively, and O–H-bending vibrations at 3486.2 and After pure water flux (J ) tests, the prepared HA solu- −1 w1 1642.3 cm . In addition, the spectrum revealed bonds at tion was performed and the permeate flux (J ) profile was −1 HA 1764.8 and 1177 cm which are attributed to the stretching recorded every 10 min to determine the dynamic fouling vibrations of carbonyl (C=O) and ether (C–O–C) groups, resistance of the membrane. After 240 min of HA filtra- respectively [54]. tion, the membrane was cleaned with distilled water under For the ND-COOH nanoparticles, it has almost the same magnetic stirring for 30 min, and then, pure water was again IR adsorptions as those of pristine ND, but C–H bonds in passed through the membrane to measure second pure water ND completely disappear and the absorption peak of C=O flux (J w ). The HA filtration measurement was carried out −1 are shifted to 1798  cm due to the conversion of some in transmembrane pressure of 1.5 bar. Three samples were 1 3 386 Iranian Polymer Journal (2018) 27:381–393 Fig. 2 TGA curves of pristine ND, ND-COOH, and ND-PEG nano- particles and ND-PEG nanoparticle concentration until 0.5  wt%. Then, the tensile strength decreased by adding nanoparticles. Fig. 1 FTIR spectra of pristine ND, ND-COOH, and ND-PEG nano- The tensile strength increased by 54.6% in the sample with particles 0.5 wt% of ND-PEG nanoparticles. An increase in the tensile strength was observed for the CA/ND-PEG membrane in oxygen containing groups like ketone, alcohol, and ester to comparison with CA/ND membrane, due to well dispersion carboxylic group [55]. of ND-PEG nanoparticles and well interaction between these In the characteristic peaks of ND-PEG nanoparti- nanoparticles and CA matrix. cles, C–H-stretching vibrations appeared at 2849.7 and However, the higher content of nanoparticles (> 0.5 wt%) −1 2921 cm corresponding to the –CH in PEG molecules. In will obviously cause a decrease in the mechanical properties addition, other characteristic peaks of PEG chains at 1479.3, of the nanocomposite membranes. When the content of ND −1 1259.4, 1136, 1095.5, and 825.5 cm were all observed, and ND-PEG nanoparticles was 0.75 wt%, the microstruc- indicating that PEG was indeed attached to the surface of ture of membranes obviously changed due to the aggrega- ND nanoparticles [27]. However, the intensity of the peak at tion of nanoparticles in CA matrix and this will result in the −1 1095.5 cm significantly increased. This increase is attrib- decline of mechanical properties [56]. uted to the C–O–C-stretching modes of ether groups in PEG molecules. Hydrophilicity, porosity, and water content of membranes Figure 2 shows the TGA curves of the pristine ND, ND- COOH, and ND-PEG nanoparticles. The weight loss per- The results of contact angle measurements for membranes centages of ND, ND-COOH, and ND-PEG nanoparticles are are given in Table 1. The contact angle of neat CA mem- 1.22, 3.67, and 18.81% at 500 °C, respectively, suggesting brane decreased from 65.4 to 58 and 52.2 by adding 0.5 wt% that more functional groups were introduced on the surface ND and ND-PEG nanoparticles, respectively. A decrease in of ND during PEG-grafting steps. The major weight loss of the water contact angle was corresponded with the improve- ND-PEG nanoparticles occurred between 180 and 750 °C. ment in membrane hydrophilicity. In comparison with the This weight loss may greatly be ascribed to the decompo- CA/ND membrane, CA membranes containing 0.5 wt% of sition of the PEG chains, since the onset was close to the ND-PEG nanoparticles show the lowest amount of water decomposition temperature of PEG (177 °C) [27]. contact angles. This is due to the existence of hydroxyl hydrophilic functional groups on the membranes structures Membrane characterization as well as the good dispersion of ND-PEG nanoparticles. Any further increase in ND nanoparticles up to 0.75 wt% Mechanical properties increased the water contact angle. According to the results reported by Xu et al., any increase in water contact angel at The mechanical properties of pure and nanocomposite mem- a higher ND nanoparticles content supports our speculation brane samples were measured and the results are shown in regarding the agglomeration and cluster formation of ND Table 1. It can be seen that the tensile strength of the nano- nanoparticles and consequently non-uniform dispersion of composite membranes increased with the increase of ND nanoparticles at higher concentrations [57]. 1 3 Iranian Polymer Journal (2018) 27:381–393 387 Table 1 Tensile strength, water contact angles, water content (%), porosity (%), HA adsorption capacity, and weight loss per unit area for pure CA and its nanocomposite membranes Sample code Tensile Water contact Water con- porosity (ε) (%) HA adsorption Weight loss −2 strength (MPa) angles (°) tent (%) capacity (mg cm ) per unit area −2 (mg m ) CA 9.7 65.4 71.6 76.7 2.85 7.2 CA/ND (0.25 wt%) 10 60.3 74.2 78.6 2.83 – CA/ND (0.5 wt%) 10.4 58 75 80.5 2.7 4.3 CA/ND (0.75 wt%) 9 59.1 73.5 78.5 2.8 – CA/ND-PEG (0.25 wt%) 11.8 55.8 79.2 82.5 2.5 – CA/ND-PEG (0.5 wt%) 15 52.2 79.8 82.8 2.15 2.5 CA/ND-PEG (0.75 wt%) 13.4 52.7 78.8 82.2 2.6 – The WC was calculated using Eq (9) and the value of membrane, the amount of adsorbed HA increased from 2.7 −2 the membranes are shown in Table  1. From Table  1, the to 2.8 mg cm , due to sever agglomeration of nanoparticles. WC of neat CA membrane showed a value of 71.6%. It As shown in Table 1, by increasing the ND-PEG nanopar- can be seen that all the nanocomposite membranes show ticles loading, the amount of adsorbed HA reached 2.5, 2.15, −2 higher WC value than neat CA. The highest value of 79.8% and 2.6 mg cm for loading of 0.25, 0.5, and 0.75 wt%, was appeared when the content of ND-PEG nanoparticles respectively. The reduction in HA adsorption on CA/ND- reached 0.5 wt%. The parameter of WC may be due to the PEG nanocomposite membranes (especially at 0.5 wt%) was detachment of polymer chains from the ND-PEG nanopar- ascribed to hydrophilic and well dispersion properties of ticles surface, causing interface voids. Furthermore, this led ND-PEG nanoparticles. In addition, due to the formation to an increase in void volume, resulting in the formation of of thin water film on the surface of CA/ND-PEG nanocom- bigger size pores on the membrane surface and an increase posite membranes, the reduction in HA adsorption occurred of WC in the pores. [7, 50]. The membrane porosities have been measured using gravimetric method and the results are reported in Table 1. Membrane morphology It is clear that increasing the concentration of pure and func- tionalized ND nanoparticles increased the membrane poros- The top surface and cross-sectional SEM micrographs of ity that attained its maximum value at the concentration of CA membrane and the selected nanocomposite membranes 0.5 wt%. Any further increase in the ND content decreased are presented in Fig. 3. As evidenced from these SEMs, the the porosity, which would contribute to particles’ agglom- porosity of membrane top surface increased due to the pres- eration. However, mixing hydrophilic nanoparticles with CA ence of both ND and ND-PEG nanoparticles. polymer could enhance the volume fraction between poly- The cross-sectional SEM micrographs in Fig. 4 show that mer chains, in addition, causing fast exchange of solvent all the membranes exhibit the typical asymmetric structure. and non-solvent during the phase inversion process [58]. It seems that macro-voids become longer when ND and ND- Moreover, during the phase inversion process of nanocom- PEG nanoparticles are added. The growth of macro-voids is posite membranes, especially CA/ND-PEG (0.5 wt%), the determined by the relative diffusion rates between solvent solvent and the non-solvent exchange rate increased due to and non-solvent. With the addition of ND and especially the hydrophilic nature and well dispersion of nanoparticles ND-PEG, the higher affinity between hydrophilic nanopar - additive. This led to the formation of membrane with more ticles and water accelerates the exchange rate of solvent and porous surface. non-solvent [59]. In the case of CA/ND-PEG (0.5  wt%) membrane, the hydrophilic functional groups on ND-PEG nanoparticles may facilitate the exchange rate of solvent and HA adsorption test non-solvent. These results indicated that the addition of ND and ND-PEG nanoparticles has bulky effects on the mem- Table  1 shows the results obtained from HA adsorption brane structure formation. test. Pure CA membrane sheet showed the maximum HA Information about the dispersion of nanoparticles cannot −2 adsorption about 2.85 mg cm . As the pristine ND parti- be obtained from the SEM micrographs. Therefore, TEM cles concentration increased from 0 to 0.5 wt%, the amounts was used to investigate the dispersion of nanoparticles in −2 of adsorbed HA decreased to 2.7 mg cm . However, as CA membranes, and the result is shown in Fig. 5. At low the ND content reached 0.75  wt% in the nanocomposite and especially high loading of ND nanoparticles, more 1 3 388 Iranian Polymer Journal (2018) 27:381–393 Fig. 4 SEM micrographs of the cross-sectional morphology of: a CA, Fig. 3 SEM micrographs of top surface of: a CA, b CA/ND b CA/ND (0.5 wt%), and c CA/ND-PEG (0.5 wt%) membranes (0.5 wt%), and c CA/ND-PEG (0.5 wt%) membranes 1 3 Iranian Polymer Journal (2018) 27:381–393 389 ND-PEG particles was established, as shown in Fig. 5c. In addition, this behavior has been reported elsewhere in which a smoother and rougher surface has been formed at low and high loading of nanoparticles into polymeric membranes, respectively [51, 59]. Abrasion resistance Table 1 represents the weight loss per unit area of pure CA and selected nanocomposite membranes after performing abrasion test. Obtained results show that in presence of ND- PEG nanoparticles, the lowest weight loss is occurred. In the presence of pristine ND nanoparticles as nanofiller, the weight loss is still lower than that of the pure CA membrane. Interestingly, the CA/ND-PEG membrane shows superior performance and lower weight loss than both neat and nano- composite CA membrane. This finding is mainly due to well distribution of ND nanoparticles as well as the contact rein- forcement/matrix effect in the CA matrix. A similar result was reported by Lai [60], who showed that the abrasion resistance of poly(vinylidene fluoride) membrane increased by adding nanoclay as reinforcing agent. Specific wear rate (W) can be calculated according to Ratner equation shown in Eq (16) as follows [61]: W = (16) where μ is the coefficient of friction, H is the indentation hardness, σ is the tensile strength, ε is the elongation at maximum load, and k is a proportionality constant. According to this equation and the results shown in Table  1, it is obvious that higher tensile strength as well as higher elongation-at-break results in lower abrasion rate. Therefore, CA/ND-PEG nanocomposite membrane with 0.5 wt% of ND nanoparticle content resists against the dam- age from the abrasive particles and exhibits lower abrasion rate due to the higher ductile strength. To better understand the effect of ND nanoparticles on the abrasion resistance of CA membrane, the abraded surfaces of the membranes were investigated by SEM analysis. Figure  6 shows the abraded surfaces of neat CA and selected CA/ND and CA/ND-PEG nanocomposite mem- branes. As shown in Fig. 6a, more ploughed furrows and pittings are formed on the abraded surface of CA membrane. By adding the ND nanoparticles, the number of pittings Fig. 5 TEM micrographs of distribution of: a 0.5  wt% of ND, b on the membrane surface remarkably decreases (Fig. 6b), 0.75  wt% of ND, and c 0.5  wt% of ND-PEG nanoparticles in CA implying that the presence of ND fillers in the nanocom- membranes posite membrane can enhance the abrasion resistance of CA membrane. Interestingly, in the presence of ND-PEG agglomerations are formed (Fig.  5a, b). While, in lower nanoparticles, the unenviable impact of abrasion test on the amounts of functionalized nanoparticles, i.e., 0.5 wt% ND- surface of the membrane was minimal and a smooth surface PEG, the construction of large aggregations was avoided, is observed (Fig. 6c). This result also implies that surface and therefore, a relatively uniform distribution of the modification of ND nanoparticles can improve the interfacial 1 3 390 Iranian Polymer Journal (2018) 27:381–393 Fig. 7 Flux-time behavior of pure and nanocomposite membrane dur- −1 ing filtration of 1 g L HA solution that membrane hydrophilicity played the vital role in the improvement of HA filtration. In an aqueous solution, the strong hydrogen bonding of water molecules to the oxygen atoms in PEG accumulates water molecules around PEG, which prevents hydrophobic foulants from approaching the surface. It is usually consid- ered as a steric stabilization effect [62, 63]. The character- istic features of the PEG coated layer on the ND nanopar- ticles, i.e., hydrophilicity, freedom of electrostatic charges, and the rapid motion of hydrated chains, have been of great help in terms of avoiding interaction between hydrophobic materials such as HA and the surface of ND nanoparticles. Therefore, ND-PEG nanoparticles improved the anti-fouling performance of CA membrane during HA filtration due to hydrophilicity, which prevents foulant attachments on the membrane surface. Figure 8 illustrates the fitting of the obtained experimen- tal data after using pure CA and CA/ND-PEG nanocompos- ite membranes at different predicted fouling mechanisms, including complete pore blocking, standard pore blocking, intermediate pore blocking, and cake formation. As shown Fig. 6 SEM micrographs of membrane surface after the abrasion test: in Fig. 8, it is observed that a cake filtration model provides a CA, b CA/ND (0.5 wt%) and c CA/ND-PEG (0.5 wt%) membranes the best fit for all membranes. To evaluate the anti-fouling performance of the fabricated bonding between ND-PEG nanoparticles and CA matrix, pure and nanocomposite CA membranes, flux recovery ratio resulting in more uniform dispersion of ND particles in the (FRR), total fouling ratio (TFR), and reversible (RFR) and bulk of polymer matrix (Fig. 5c) as well as on the surface of irreversible fouling ratio (IFR) were quantified during filtra - the CA membrane that improved the abrasion resistance of tion experiments and the calculated values are presented in the nanocomposite membrane. Table 2. It is obvious that the pure membrane had the high- est TFR (89.7%) and IFR (35%) values. While in the CA/ Fouling analysis and membrane performance ND-PEG (0.5 wt%) membranes, TFR and IFR values were considerably reduced. In the case of modified membranes, The flux decline behavior of the membrane during HA fil - the hydrophobic adsorption between the foulants and the tration is shown in Fig. 7. All filtration tests were carried surface of the modified membranes was decreased and the out until the flux reaches a stable value. The permeation attached foulants were easily removed during filtration. flux for CA/ND-PEG (0.5 wt%) nanocomposite membrane Since all membranes were tested under the same condi- was higher than that of the other membranes. It indicates tions, the surface property of membrane was a key factor in 1 3 Iranian Polymer Journal (2018) 27:381–393 391 determining the degree of TFR. Lower values of TFR show that less HA adsorption took place on the membrane surface or pore walls. Performance studies to determine the rejection of HA solution showed that the presence of ND-PEG nanopar- ticles increased rejection compared with the membranes containing pristine ND particles (Table 2). The addition of ND-PEG nanoparticle created a more hydrophilic surface for the modified membranes that prevented HA molecules from settling closer to the surface of the membrane and thus increased the HA rejection. Conclusion Polyethylene glycol-grafted nanodiamond (ND-PEG) embedded cellulose acetate (CA) membranes were pre- pared through solution casting by phase inversion method. The anti-fouling behavior of pure and nanocomposite CA membranes during the filtration of humic acid (HA) was investigated. The results of FTIR revealed that the PEG molecules were formed on the surface of ND nanoparticles. SEM analyses of the surfaces of membranes depicted the increase in the number of pores by the addition of ND and especially ND-PEG nanoparticles into the polymer matrix. The results revealed that nanocomposite CA membrane con- taining 0.5 wt% PEGylated ND nanoparticles showed high hydrophilicity, high porosity, low HA adsorption capacity, high abrasion resistance, and excellent anti-fouling prop- erties. It is due to the strong hydrogen bonding of water molecule to the oxygen atoms in PEG molecules, which pre- vents HA from approaching the surface of the membrane. According to the results obtained by Hermia’s model, it was concluded that cake formation mechanism is the pre- vailing fouling mechanism, which can be occurred for neat Fig. 8 Experimental data and Hermia fouling models for: a CA, b CA/ND (0.5  wt%), and CA/ND-PEG (0.5  wt%) membranes during and nanocomposite CA membranes. However, due to the filtration of HA solution enhancement of membrane wettability and surface hydro- philicity, CA/ND-PEG nanocomposite membrane showed more reversible fouling mechanism than that of the neat CA Table 2 Rejection and fouling parameters of the prepared membranes membrane. during HA filtration Sample code RFR IFR TFR FRR Rejection CA 54.7 35 89.7 65 94.5 CA/ND (0.5 wt%) 54 20.4 74.4 80 94.8 CA/ND-PEG (0.5 wt%) 50.7 8.4 59.1 91.6 95.4 1 3 392 Iranian Polymer Journal (2018) 27:381–393 Acknowledgements The authors gratefully acknowledge financial sup- 14. Haleem YA, Liu D, Chen W, Wang C, Hong C, He Z, Liu J, Song port from Sahand University of Technology (Grant No. 30/15975). P, Yu S, Song L (2015) Surface functionalization and structure characterizations of nanodiamond and its epoxy based nanocom- posites. Compos Part B Eng 78:480–487 Open Access This article is distributed under the terms of the Creative 15. 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Journal

Iranian Polymer JournalSpringer Journals

Published: Apr 21, 2018

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