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Silver nano-particle coated hydroxyapatite nano-composite membrane for the treatment of palm oil mill effluent
Journal of Water Process Engineering 31 (2019) 100844
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Silver nano-particle coated hydroxyapatite nano-composite membrane for the treatment of palm oil mill effluent Fahmi Anwar, G. Arthanareeswaran
T
⁎
Membrane Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli, 620015, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Keywords: Polyphenylsulfone Palm oil mill effluent Hydroxyapatite Silver nano-particle
Nano-composite ultra-filtration membranes were developed for the treatment of Palm Oil Mill Effluent (POME). Silver nano-particle was synthesized and was used to coat hydroxyapatite nanotubes. This coated nano-filler was used as filler for the polyphenylsulfone (PPSU) membrane. The filler composition in the membrane was varied from 0 to 2.5 wt% to investigate the effect of fillers on the separation properties of the membrane. Both dead-end and cross-flow techniques were used for the separation. Characterization techniques such as contact angle analysis, particle size analyzer, XRD, SEM were used to analyze both nanoparticles as well as the nano-composite membranes. Parameters like COD, BOD, turbidity, suspended solids, pH, conductivity and TDS of the permeate were studied. Change in intrinsic membrane resistance with respect to filler composition was also studied. Based on the results the membrane having a filler composition of 2% was found to be exhibiting excellent properties compared to other membranes in terms of separation performances. The rejection percentage of PPSU membrane with a filler content of 2% was 89.74%. The composite membrane had shown promising results by removing all the organic matter present in the POME and can be further developed into a pilot plant.
1. Introduction
POME consists of 95–96% water, 0.6–0.7% oil and 4–5% solids [1]. It has high turbidity and color resulting from high organic matter content, suspended solids and a trace amount of minerals and heavy metals [7]. This organic content makes POME unfit for drinking if not treated. Therefore, a proper treatment method to remove the organic matter, minerals and heavy metals is required in the recycling of drinking water from POME. Currently, novel technologies such as microfiltration, ultrafiltration (UF), nanofiltration and reverse osmosis (RO) are showing high performance in the treatment of water compared to conventional treatments. The main advantages of membranes technology are energy efficiency, compact module, quality of the product, high selectivity, easy handling and cost efficiency [8]. UF membrane technology is recognized for its ability to reject organic matter in POME as well as microorganisms. [9]. Nevertheless, membrane technology faces challenges like fouling due to the organic matters and bacteria present in the effluent. To reduce fouling anti-bacterial fillers are incorporated into the polymer membranes [10]. Silver nanoparticles doped with hydroxyapatite have been reported showing exceptional anti-bacterial and anti-fouling properties when incorporated in polymer membranes [11–14]. The anti-bacterial property of silver combined with hydrophilicity of hydroxyapatite can improve the anti-fouling property of the membrane.
Water, essential to life is drying up around the world. Water scarcity is one of the alarming challenges that the modern world face today. Water shortages, deterioration of water quality, and environmental constraints have led to an augmented interest in recovering and recycling water in many parts of the world. This shortage of water drives people to recycle the effluents from different fields such as agricultural effluents, plantation effluents, industrial and farming effluents [1]. Plantation effluents include effluents from palm oil mills, rubber plantations, coconut mills etc. Palm oil is one of the world’s most rapidly expanding crops and is derived from the fleshy mesocarp of the fruit of oil palm (Elaeis gunineensis). One hectare of oil palm produces about 10–35 tonnes of fresh fruit bunches (FFB) per year [2]. While the palm oil industry has been known for development, it has also contributed to environmental pollution due to the production of huge quantities of byproducts from the oil extraction process [3]. The byproducts include Oil Palm Trunks (OPT), Oil Palm Fronds (OPF), Empty Fruit Bunches (EFB), Palm Pressed Fibres (PPF), and liquid discharge Palm Oil Mill Effluent (POME) [2,4,5]. According to Yacob et al., about 30 million tonnes of palm oil mill effluent (POME) was produced in Malaysia in the year 2004 [6]. ⁎
Corresponding author. E-mail address: [email protected] (G. Arthanareeswaran).
https://doi.org/10.1016/j.jwpe.2019.100844 Received 5 September 2018; Received in revised form 21 March 2019; Accepted 26 April 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 31 (2019) 100844
F. Anwar and G. Arthanareeswaran
Composite membranes were reported to exhibit enhanced permeation and separation properties [10]. Many polymers, as well as composite membranes, were reported showing excellent separation properties in the Ultrafiltration of POME. Out of these, Polyphenylsulfone (PPSU) was reported showing high stability during ultrafiltration studies [15]. Hwang et al studied different blend membranes of PPSU for the ultrafiltration applications [16]. Sulfonated PPSU was investigated for water treatment by Yang et al and reported that the hydrophilicity of the membrane was enhanced by the chemical modification [17]. In the present study silver nanoparticles were successfully synthesized, which were used for the coating of hydroxyapatite (Ca5(PO4)3(OH)) nanotubes. This coated nanofiller was incorporated into the PPSU and nanocomposite membranes were prepared by the phase inversion technique. The effect of filler on hydrophilicity, porosity, membrane morphology, and membrane separation performance was studied. These membranes were employed for the treatment of POME in dead-end and cross-flow membrane module.
Table 1 Composition of Silver-Hydroxyapatite incorporated PPSU membranes. Membrane Code
PPSU (%)
Silver-Hydroxyapatite (%)
NMP (mL)
PM-01 PM-02 PM-03 PM-04 PM-05 PM-06
100 99.5 99 98.5 98 97.5
0 0.5 1 1.5 2 2.5
20
solution were prepared by varying the filler concentration in the solution by dissolving corresponding amount filler and PPSU in 20 mL NMP. The different batches are shown in Table 1. For complete dissolution, the solution was kept for magnetic stirring for overnight to attain a homogeneous dope solution. The temperature was increased to 80 °C for 10 min and to avoid undissolved particles, the solution was later sonicated in a sonication bath for 15 min. For casting the membrane, wet phase inversion procedure was followed. The silver-hydroxyapatite incorporated PPSU film was immersed into a coagulation bath where the exchange of solvent and nonsolvent took place and the membrane was formed. The silver-hydroxyapatite incorporated PPSU solution was poured on a smooth glass plate and spread with the help of a metal rod to produce a thin film membrane of 250 μm thickness. After casting the membrane on the glass plate, it was immersed into a gelation bath. The gelation bath was prepared prior to the casting of the membrane. It consists of 4 wt. % SLS solution and during casting the bath temperature was brought down to 5 °C. Once they were immersed in gelation bath, the membranes got toughened and later they were peeled off the glass plate and were stored in distilled water for further use.
2. Experimental 2.1. Material PPSU (Radel R-5000) which is required for the preparation of composite membrane was provided by Solvay Advanced Polymer (Belgium). N-methyl-2-pyrrolidone (NMP) was the solvent used and it was purchased from Merck Life Science Private Limited, Mumbai, India. Silver nitrate and Sodium Boro-hydride which were used for the preparation silver nanoparticle were bought from Sigma-Aldrich Mumbai. Sodium Lauryl Sulfate (SLS) which was used for membrane casting was bought from Sigma-Aldrich Mumbai. Hydroxyapatite nanotubes were bought from Alfa Aesar USA. Methanol used for washing was bought from Titan Biotech Limited, Rajasthan, India. Acetone was purchased from Merck Specialities Private Limited, Mumbai, India.
2.5. Water content Membrane samples were cut into the desired size and soaked in water for 24 h and weighed immediately after blotting the free surface water. These wet membranes were dried for 12 h at 80 °C and the dry weights were determined. From the difference between dry and wet weights of the samples, the amount of absorbed water was calculated. From that, the percentage of water content was estimated. The porosity of the membranes was calculated by the formula given below:
2.2. Silver nanoparticle preparation A large excess of NaBH4 was needed to reduce the ionic silver and to stabilize the silver nanoparticles that were formed. 5 mL of 0.001 M silver nitrate was added dropwise to 30 mL of 0.002 M sodium borohydride solution that had been chilled in an ice bath. The reaction mixture along with the ice bath was stirred vigorously using a magnetic stirrer. The solution turned to light yellow after the addition of 2 mL of silver nitrate and to brighter yellow when all of the silver nitrate had been added and later to light yellow. The reaction is shown below.
=
( (
1
1 2 )/ d w
2)/ d w
+(
2 / dp )
(1)
Where Density of water dw = 1000 g/cc Density of polymer dp = 1370 g/cc Weight of wet polymer = w1 Weight of dry polymer = w2
AgNO3 + NaBH4 → Ag + H2+ B2H6 + NaNO3 After adding the entire silver nitrate, the stirring was stopped and the stir bar was removed. Reaction conditions including stirring time, relative quantities of reagents and reaction temperature must be carefully controlled to obtain stable colloidal silver. The colloidal silver particle solution was then centrifuged and dried in a vacuum oven to get the nanoparticles.
2.6. Characterization of nanofillers and membranes Various techniques were employed for the characterization of the fillers as well as the nanocomposite membranes. UV Spectroscopy was used to detect and confirm the presence of silver nanoparticles and the instrument used was Merck UV Spectrophotometer Spectroquant Model Prove 600 USA. X-ray Diffractometer (XRD) was used to confirm and study the microstructure of the silver nanoparticles as well as silver coated hydroxyapatite nanotubes. Perkin Elmer 1621 USA wide-angle X-ray diffractometer was the instrument used to study the microstructure of the nanofiller. Using Bragg’s law the average D-spacing of the nanoparticle was evaluated. It is as shown below, nλ = 2d sin θ, where n is an integral number, λ is the X-ray wavelength, d is for the inter-layer spacing and θ is the diffraction angle. Fourier transform infra-red (FTIR) was used to identify the functional
2.3. Silver-hydroxyapatite composite preparation About 3 mL of colloidal silver nanoparticle solution that had prepared was added to 3.6 mL of the commercially obtained 10% percent aqueous solution of hydroxyapatite nanotubes. The solution was left to dry at 50 °C overnight and the dried particles were collected and stored. 2.4. Polymer nano-composite membrane preparation The polymer that was chosen for the preparation was PPSU. The solvent used was N-Methyl-2-Pyrolidone. Different batches of polymer 2
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groups in nano-particle as well as in membranes. FTIR was also used to identify the functional groups in the fouled membrane after separation. It can be used for identifying the types of chemical bonds in a molecule by producing an IR absorption spectrum that is like a molecular fingerprint. The model used here for the analysis was Thermo Nicolet, Avatar 370 USA. Scanning Electron Microscopy (SEM) was used to study the surface as well as the cross-sectional morphology of the membranes. JEOL Model JSM - 6390 L V India was the model used for the analysis. The sample was coated with a thin layer of gold before analysis. Particle size analyzer was used to find the mean particle size and size distribution of the silver-coated hydroxyapatite nano-tubes. The model used here for analysis was HORIBA Laser Scattering Particle Size Distribution Analyzer LA-960. The contact angle of the membranes was measured by Goniometer to find out the change in hydrophilicity with respect to filler concentration. The instrument used for the analysis was ramé-hart Model 500 Goniometer USA. The mechanical strength of the membrane was found by Universal Testing Machine Mecmesin Model 2110 USA to perform stress-strain analysis of the membrane. It was performed to understand the change in the mechanical strength of the membrane with an increase in nanoparticles concentration.
2.7.2. Dead-end filtration set-up The ultrafiltration (UF) experiments were carried out in a 400 ml batch type stirred cell (ultrafiltration cell – S76-400-Model, Spectrum, USA) fitted with a Teflon coated magnetic paddle. The effective membrane area available for ultrafiltration was 38.5 cm2. The solution filled in the cell was stirred at 400 rpm using a magnetic stirrer. All the experiments were carried out at 26 °C and at 414 kPa transmembrane pressure. The permeating solution was collected from the bottom of the cell to a measuring cylinder. The schematic diagram of dead end module is shown in Fig. 2. The cell was pressurized using a nitrogen cylinder. The pure water flux was measured at every hour in order to monitor the compaction behavior. The membranes were compacted with time to attain the steady state flux value. Pure water flux was calculated over measured time intervals using the same equation used for cross-flow (Eq. (2)). The retentate coming out of the membrane module was recycled back to the feed tank. Water flux was determined to find out intrinsic membrane resistance of all the membranes. It is calculated by the formula given below (Eq. (4)).
Jk =
2.7. Ultra-filtration set-up
3.1. Confirmation silver nanoparticle Different analysis techniques were used to confirm the presence and characterize the prepared silver-nanoparticles. These confirmation analyses were performed before coating the hydroxyapatite nanotubes. They are described in detail as follows.
2.7.1. Cross-flow set-up UF experiments were carried out in a cross flow flat-frame membrane module (Model: PLEIADE Rayflows, Orelis Environment SAS, France) with a filtration area of 100 cm2 at a temperature of 26 °C and a controller to set the pressure at a transmembrane pressure of 200 kPa. The cross-flow ultrafiltration module is shown in Fig. 1. The experiments were conducted in a continuous mode where the permeate was continuously collected and measured in a measuring cylinder. The retentate coming out of the membrane module was recycled back to the feed tank. The permeate flux, as well as the water flux, was calculated by the equation given below (Eq. (2)).
Q A× t
3.1.1. UV–vis spectroscopy UV-Visible spectroscopy is one of the most widely used techniques for structural characterization of silver nanoparticles. It is quite sensitive to the presence of silver colloids because these nanoparticles exhibit an absorption peak due to the surface plasmon excitation. The absorption band in the 350 nm–450 nm region is typical for the silver nanoparticles [18]. The absorption spectra of the colloidal solution of silver are shown in Fig. 3. The spectra exhibit a plasmon absorption band at ˜ 400 nm which is the characteristic of silver nanoparticles. Such plasmon bands are unique physical properties of the nanoparticles themselves. Thus using UV Spectroscopy, the presence of silver nanoparticles was confirmed. The identification of silver nanoparticles was based on Surface Plasmon Resonance phenomenon. The penetration depth was found to be directly proportional to the exciting wavelength i.e., 325 nm because of decreased absorbance which is in accordance with Li et al [19]. Nanoparticles with absorbance in the range of 350 nm–450 nm were used for coating hydroxyapatite nanotubes.
(2)
where, Jw (L/m2h) is the membrane flux, A (m2) is the effective area of the membrane surface, Q (L) is the volume collected and Δt is the time interval of 10 min. The flux for POME was also calculated in a similar manner by calculating the effluent flux obtained through the membrane. The rejection of the POME effluent was found by using the equation given below (Eq. (3)).
R= 1-
CP × 100 CF
(4)
3. Results and discussions
Ultrafiltration studies have been carried out in cross-flow and deadend module
JW =
P Rm
(3)
3.1.2. X-ray diffraction (XRD) XRD was used as a secondary confirmation for the silver particles.
where CP and CF are the absorbances of permeate and feed respectively.
Fig. 1. UF Cross-Flow Filtration Set-up. 3
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Fig. 2. UF Dead-end Filtration Set-up.
hydroxyapatite particles was confirmed and their structure was studied based on the following studies. 3.2.1. X-ray diffraction (XRD) XRD was used as a secondary confirmation for the silver particles. Peaks were obtained according to reported data [20], this peak gave us further confirmation on the presence of nanoparticles. The interplanar distance D-spaces were measured using Bragg’s Law and are illustrated in Fig. 6. Their values are ranged between 1.5–3.5 Å. The pattern clearly shows the main peaks at (2θ) 26°, 33°, 49° and 54° corresponding to the 110, 202, 223 and 313 planes, respectively. These reported data clearly proved the presence of silver coated hydroxyapatite nanotubes. 3.2.2. Fourier transform infrared spectroscopy (FTIR) FTIR was used for added confirmation of the coated nanotubes and is shown in Fig. 7. The obtained spectra were compared with the standard IR frequencies table [20]. It showed peaks at 1735 (O-H) and 1020 (N = O) and by comparing with reported data, the particles that were prepared were confirmed to be silver nanoparticle coated hydroxyapatite nanotubes [21].
Fig. 3. UV Spectroscopy of Silver Nanoparticles.
Peaks were obtained according to reported data [18], these findings once again confirm the presence of nanoparticles. The interplanar distance D-spaces were measured using Bragg’s Law and are illustrated in Fig. 4. Their values ranged between 1–3 Å. The pattern clearly shows the main peaks at (2θ) 38°, 44°, 64° and 77° corresponding to the 111, 200, 220 and 311 planes, respectively. By comparing JCODS (file no: 89-3722), the typical pattern of chemical synthesized silver nanoparticles was found to possess an FCC structure [19].
3.2.3. Particle size analysis of nanotubes In order to find out the size distribution of the coated nano-tubes prepared, particle size analysis was carried out. Fig. 8 shows the size distribution of nanoparticles and they were having a Median and Mean size of 20.7 nm 33.16 nm respectively. Mode size and standard deviations were 12.378 μm and 32.5125 μm respectively. About 60% of particles were under the size of 20 μm.
3.1.3. Fourier transform infrared spectroscopy (FTIR) Further confirmation was carried out by FTIR. The FTIR spectra obtained was compared with the standard IR frequencies table [20]. The spectra of silver exhibited a high-intensity broad band at 1720 (OeH) due to the stretching of the hydrogen and oxygen bond and at 1230 (N]O) (Fig. 5) due to nitrogen-oxygen stretch bond. The functional groups obtained were in good agreement with what has been observed from reported data [18]. By this analysis, the particles that were prepared were confirmed to be silver nanoparticles
3.3. Effect of filler concentration on hydrophilicity and porosity of the membrane Contact angle analysis was performed to study the influence of filler concentration on the hydrophilicity of the membrane. Table 2 shows the contact angle of each membrane and how the filler concentration changes the angle. Data, the contact angle was decreased by almost 35° with an increase in filler composition, indicating the increase in the hydrophilic nature of the membrane (Fig. 9). Hydrophilicity helps to decrease the membrane fouling caused by the organic matter present in the effluent. Hence the effect of filler on the hydrophilicity of the composite membrane was tested to study the influence of nano-material in the membrane. From Table 2, it is clear
3.2. Confirmation of silver coated hydroxyapatite After confirming the presence of silver nanoparticles, it was used as a coating for hydroxyapatite nanotubes. The presence of coated 4
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Fig. 4. XRD of Silver Nanoparticle.
hydrophilic surfaces with contact angles less than 82. And PM-6 have the lowest contact angle and highest filler content. 3.4. Effect of filler concentration on the mechanical strength of the membrane The effect of filler concentration on the mechanical stability of the membrane is tabulated in Table 4. The material strength of the membranes prepared was studied by performing Stress-Strain tests. The samples of the membranes were cut into dimensions of length 30 mm and width 10 mm. The initial gauge length was set at 20 mm. The data was recorded for respective samples and tabulated in Table 4. It was observed that as the concentration of fillers in the membranes increased, the ultimate force the membranes could withstand first kept on increasing and then started to decrease. The membrane with a filler concentration of 1.5% was observed to be the most mechanically stable membrane. Initially, the presence of fillers made the membrane more mechanically stable, however, when the filler content went beyond 1.5%, the membrane became more porous compromising the structural stability of the membrane.
Fig. 5. FTIR of Silver Nanoparticle.
3.5. Confirmation of presence of nanomaterial in membrane
that as filler content was increased, the hydrophilic nature of the composite membranes was also improved which in turn results in a rise in the amount of water absorbed. This can be explained by the presence of hydroxyapatite group in the membrane which alters the membrane surface properties. The porosity of the membranes was calculated by the Eq. 1. The results of the test are tabulated in Table 3. The porosity kept on increasing with increase in filler content because the presence of fillers makes the membrane more porous by opening up its pores. From this test we can confirm that the membranes prepared were hydrophilic in nature and as the concentration of the inorganic fillers goes on increasing the membrane water absorption capacity and porosity also increases. Wettability of filler may aff ;ect the adsorbed fouling agent in POME and enhance the material interactions. Results indicate that the silver nanoparticle coated hydroxyapatite PPSU membranes have
Different analysis techniques were used to confirm the presence of nanomaterial in the membrane. Functional groups that were present in the silver nanoparticle coated hydroxyapatite are shown in Fig. 5. The spectra of silver nanoparticle coated hydroxyapatite PPSU membranes are shown in Fig. 10. At 1735 (OeH) oxygen-hydrogen stretch bond is present, which confirms the presence of silver coated hydroxyapatite nanotubes. Whereas the band at 1230 (N]O) nitrogen-oxygen double bond confirms the presence of silver nanoparticles. All the membranes except the virgin membrane exhibited these peaks. 3.5.1. SEM-EDAX of the composite membrane Fig. 11 (a) shows the top surface of PM-4 and Fig. 11 (b) shows the different elements present in that membrane. The SEM image shows a 5
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Fig. 6. XRD of Silver Coated Hydroxyapatite Nanotubes. Table 2 Contact Angle of Membranes. Membrane Code
Left Angle (o)
Right Angle (o)
Mean (o)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
88.6 80.6 74.8 70.4 67.2 61.6
96.3 83.2 75.2 72.8 63.0 58.5
92.4 81.9 75.0 71.6 65.1 60.1
As it is clear from the.
3.6. Morphological characteristics of the membrane The top surface and cross-sectional SEM images are shown in Figs. 12 and 13 respectively. It shows a smooth and defects free surface without any deformation. Compared to the composite membrane virgin membrane is less porous and the membrane formed is found to be very dense and the same can be inferred from Table 3, where the porosity of the membranes are tabulated. Virgin membrane i.e., PM-01 was found to be having a porosity of 0.768 and PM-06 was having a porosity of 0.921. This less porosity is the main factor that reduces the rejection percentage of the virgin membrane during POME treatment. Cross-sectional images show the honey-comb like structure of composite membranes (Fig. 4.11). The thickness of PM-04 was found to be 250 μm. The varying porosity which we had found earlier is picturized here. From Figs. 12 and 13 we can observe how the porosity is
Fig. 7. FTIR of Silver Coated Hydroxyapatite Nanotubes.
defect free, smooth surface of the membrane. Out of all the elements present in the membrane as observed in the result, Calcium and Phosphor are highest in content. These elements are present in the hydroxyapatite compound. With this analysis, we can conclude that the membrane is having the prepared fillers and they were not leached out of the membrane before they were used for separation.
Fig. 8. Particle Size Analysis. 6
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Fig. 9. Contact Angle of (a) PM-1, (b) PM-3, (c) PM-5 and (d) PM-6. Table 3 Water Content of Nanocomposite Membranes. Membrane Code
Percentage water content (%)
Porosity
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
70.8 79.59 84.14 84.79 90.81 91.23
0.768 0.842 0.879 0.884 0.901 0.921
varying from a less porous virgin membrane to a composite membrane which is rich in pores. This porosity helps to achieve a higher flux and permeability during separation processes. 3.7. Treatment of palm oil mill effluent Fig. 10. FTIR of Membranes.
Characteristics of the collected POME are tabulated in Table 4.4. The effluent was collected from Parison’s palm oil Kerala, India and was Table 4 Mechanical Strength of Membranes. Filler Concentration (%)
Break Distance (mm)
Ultimate force (N)
% Total Elongation
Ultimate Stress (MPa)
Yield Stress (Mpa)
Ultimate Strength (%)
0 0.5 1 1.5 2 2.5
00.833 1.03 1.35 1.94 2.72 1.10
3.53 4.97 5.10 8.23 6.23 5.17
8.33 10.3 13.5 19.4 27.2 11
0.707 0.993 1.02 1.65 1.25 1.03
0.707 0.993 1.02 1.65 1.25 1.03
4.42 7.56 7.14 15.7 26.8 7
7
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Fig. 11. SEM-EDAX of PM-4.
Fig. 12. Top Surface SEM Images of (a) PM-01, (b) PM-02, (c) PM-04 and (d) PM-05.
filtered using filter paper to remove the suspended solids. All the parameters were estimated after filtration (Table 5).
increase in porosity of the membrane as filler content makes the membrane more porous. The intrinsic resistance of the membranes was calculated and tabulated below (Table 6). Intrinsic resistance kept on decreasing from 19.178 kPahm2/L to 13.011 kPahm2/L with an increase in filler content. Due to higher porosity diffusion through pores require less activation energy and that makes the membrane less resistant and PM-6 was found to be having minimum intrinsic resistance and maximum porosity (0.921). After finding out the water flux, using POME as the feed, the permeate flux of the membrane was found and is shown in Fig. 15. With time the permeate flux kept decreasing and after some time it became
3.7.1. Ultrafiltration of POME in dead-end filtration module The transmembrane pressure was set at 8 bar and the temperature at 26 °C. The diameter of the membrane was 5 cm. Permeate was collected every 10 min. Water flux was found before separating effluent. Water flux is given in Fig. 14. Water flux was determined to find out intrinsic membrane resistance of all the membranes. Water flux of the membranes was observed to keep on increasing with increase in filler content and for PM-06 it reached up to 57 L/hm2. This is due to the 8
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Fig. 13. Cross-sectional SEM Images of (a) PM-01, (b) PM-02, (c) PM-03, (d) PM-04, (e) PM-05 and (f) PM-06.
Absorbance spectra of the feed, as well as the permeate, were taken to find out the wavelength at which the organic matter was present in POME. And they were found to be 230 and 270 nm. The absorbance of permeate collected at different time intervals, for each membrane, was noted at these two wavelengths. This can indirectly give us an insight into how much organic matter has been removed from the POME by membrane filtration. The absorbance of permeate at different times is shown in Fig. 16 (a) and (b). The absorbance of all the membranes kept on decreasing till PM-5. For PM-6, performance was reduced as the high porosity causes the selectivity to decrease. At both wavelengths, PM-5 and PM-4 were observed to be showing excellent separation properties. To study the separating performance of the membranes Rejection Percentage of the membrane was found. From Table 7 it is noticed that as the filler concentration was increased from 0 to 2.5%, the percentage rejection kept on increasing till PM-5. This
Table 5 Characteristics of Palm Oil Mill Effluent. Parameter
Value
Ph Conductivity (μS) TDS (mg/L) TS (mg/L) TSS (mg/L) COD (mg/L) BOD (mg/L) Turbidity (NTU)
4.2 977 2500 7500 5000 4300 3300 1060
constant. This is due to the clogging of pores due to fouling. The organic material deposited on the membrane reduces the POME flux over the passing of time. 9
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Fig. 14. Water Fluxes of Membranes. Table 6 Intrinsic Resistances of Nanocomposite Membrane. Membrane
Resistance (kPahm2/L)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
19.178 18.817 17.676 14.227 14.0 13.011
es
was improved by the addition of nanocoated hydroxyapatite in PPSU solution. Fig. 16. (a) Absorbance of permeate at 230 nm (b) Absorbance of permeate at 270 nm. Table 7 Membrane Rejection Percentage. Membrane
Rejection percentage (%)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
41 46.15 48.72 76.92 89.74 61.54
3.7.2. COD removal COD of permeate collected at different time intervals, for each membrane was plotted and shown below in Fig. 18. According to disposal standards, COD limit should be less than 100 mg/L [1]. For three membranes it was possible to achieve this range. Water reclamation for drinking is possible from POME if drinking water standards are achieved for the permeate collected after membrane treatment. If membrane modules are connected in series, COD can be brought down to zero. As the separation progressed, membranes got clogged and very less organic material permeated through the membrane causing a decrease in COD over time.
Fig. 15. POME fluxes of Membranes.
suggests that the adsorption capacity of the membranMaximum rejection percentage is obtained for membrane corresponding to a filler percentage of 2%. For PM-6, although the permeate flux is high, rejection was decreased due to larger pore size. Thus, the optimum filler percentage has to be fixed for better performances. More filler content can cause more openings in the membrane and less rejection percentage. Fig. 17 shows the feed and permeates samples.
3.7.3. BOD removal BOD of permeate collected at different time intervals, for each membrane was plotted and shown below in Fig. 19. According to disposal standards, BOD limit should be less than 25 mg/L [1]. For three 10
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Fig. 19. BOD of Permeate.
Fig. 17. Images of Feed and Permeate.
Fig. 20. Turbidity of Permeate. Fig. 18. COD of Permeate.
3.7.5. Total solids content The total solid content of permeate articulates the amount of organic content in the effluent. Fig. 21 shows how the total solid content is changing with different membranes. Total solid content consists of suspended solids and dissolved solids. This is the polluting agent in POME. Removal of total solids alone can make the permeate meets drinking water standards. For PM-6, the solids content was reduced to 450 mg/L, which is the lowest we achieved. The total solid content of permeate had decreased with time, as the organic content permeated through the membrane reduced with clogging of pores.
membranes it was possible to achieve this range. Due to the clogging of pores, less biological content was permeated over time and thus BOD decreased with time. Biological contents are the main reason for the pollution of water. If these agents are removed using membrane technology, water recovery from membrane treatment is a highly promising technology. 3.7.4. Turbidity measurement The turbidity of permeate collected at different time intervals, for each membrane, was plotted and shown in Fig. 20. The turbidity of drinking water should not be more than 5 NTU, and should ideally be below 1 NTU [22]. Results of PM-4 and PM-5 shows that drinking standards can be achieved by optimizing the filler content on the membrane.
3.7.6. Conductivity of permeate The conductivity of permeate collected at different time intervals, for each membrane, was plotted and shown in Fig. 22. The conductivity of drinking water should not be more than 50 μS [21]. The conductivity of the permeate is the indirect measure of pollutants in the permeate. Conductivity value can infer to the amount of ions in the permeate. From Fig. 22 it can be observed that the conductivity of the permeate 11
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Fig. 23. FTIR of Fouled Membrane. Fig. 21. Total Solids Content of Permeate.
Fig. 24. Images of PM-3 (a) Before Separation (b) After Separation.
Fig. 22. Conductivity of Permeate.
kept on decreasing with increase in filler content till PM-5. For PM-6, there is an exception due to the enlargement of pores. For PM-5 conductivity was reduced to less than 50 μS. 3.7.7. Fouling of membrane Fouling was one of the main problems encountered during POME treatment. After the separation, a fouling layer was developed on the membrane surface. Membranes were analyzed using FTIR for the changes in functional groups presented on the surface of the membrane before and after effluent separation. Fig. 23 shows FTIR of PM-3 before and after treatment. The changes in peaks were noted down to analyze the functional groups present in the cake deposited on the membrane surface. For the fouled membrane at 1753 (C]O) bond is observed which indicates the presence of fatty acid from palm oil. Fig. 24 shows the images of the membrane before and after separation. Visual examination can convey to us how much organic matter is deposited on the membrane during separation. The decline in flux was due to the development of this fouling layer.
Fig. 25. Pure water fluxes of membranes in Cross-flow module.
the modes. Permeate was collected every 10 min. Water flux was found before separating effluent. Water flux is given in Fig. 25. Water flux was evaluated to find out intrinsic membrane resistance of all the membranes. Water flux of the membranes was observed to keep on increasing with increase in filler content. This is due to the increase in porosity of the membrane as filler content makes the membrane more porous. However, more porosity can lead to a decrease in selectivity. Compared to dead-end filtration in cross-flow filtration, the water flux was more. This is due to the increase in effective area. The intrinsic resistance of the membranes was calculated and tabulated below (Table 8). Intrinsic resistance kept on decreasing with increase in filler content. Due to higher porosity diffusion through pores
3.7.7.1. Cross-flow filtration method. Separation has been conducted in the cross-flow mode to compare the performance of membrane in both 12
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F. Anwar and G. Arthanareeswaran
Table 8 Membrane Resistances. Membrane
Resistance (kPahm2/L)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
16.258 14.62 13.24 11.67 9.42 8.601
Fig. 27. Absorbance of Permeate at 230 nm.
Fig. 26. POME Fluxes of Membranes.
require less activation energy and that makes the membrane less resistant. Compared to dead-end filtration resistance was decreased as the flux is higher for cross-flow method. All the membranes were observed to be having less resistance in the case of cross-flow in comparison with dead-end resistances. After finding out the water flux, using POME as the feed, the permeate flux of the membrane was found and is shown in Fig. 26. With time the permeate flux kept decreasing and after some time it became constant. This is due to the clogging of pores due to fouling. The organic material deposited on the membrane reduced the flux. However, the reduction in flux due to clogging of pores was much less than that of what has been observed from dead-end filtration with the same membranes. Absorbance spectra of the feed, as well as permeate, was taken to find out the wavelength at which the organic matter is present in POME. And they were found to be 230 and 270 nm. The absorbance of permeate collected at different time intervals, for each membrane, was noted at these two wavelengths. This can indirectly give us an insight into how much organic matter has been removed from the POME by membrane filtration. The absorbance of permeate at different time period is shown in Figs. 27 and 28. The absorbance of all membranes kept on decreasing till PM-5. For PM-6, performance was reduced as the high porosity causes the selectivity to decrease. As the separation progressed, pores got clogged due to the presence of organic matter. This resulted in a highly selective permeation and the collected permeate thus had a low organic content corresponding to a lower absorbance. At both wavelengths, PM-5 and PM-4 were observed to be showing excellent separation properties. To study the separating performance of the membranes Rejection Percentage of the membrane was found. It is tabulated below (Table 9). Maximum rejection percentage was obtained for the membrane corresponding to a filler percentage of 2%. For PM-6, although the permeate flux is high, rejection is reduced. This can be as a result of high porosity. Thus, the optimum filler percentage has to be fixed for
Fig. 28. Absorbance of Permeate at 270 nm. Table 9 Membrane Rejection Percentage. Membrane
Rejection percentage (%)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
52.88 59.63 67.56 78.31 91.27 68.44
better performances. More filler content can cause more openings in the membrane and less rejection percentage. 4. Conclusions Advanced nanocomposite fillers from silver nanoparticles and hydroxyapatite nanotubes were developed by coating hydroxyapatite nanotubes with silver nanoparticles. Silver nanoparticles were chemically synthesized and were coated on commercially obtained hydroxyapatite nano-tubes for POME treatment. Composite membranes 13
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F. Anwar and G. Arthanareeswaran
fabricated from polyphenylsulfone and the prepared fillers were found to be exhibiting homogenous structures with satisfactory structural and morphological properties; indicating that membranes fit the prerequisites for further developments toward industrial applications. Treatment of POME as an approach to achieve high percentage reduction was accomplished by 91.3% of total solids and, 89.3% of BOD by the prepared membranes. The filler percentage and filtration mode had a direct effect on the permeate fluxes of membranes. The development of a fouling layer was observed from the decline in flux with filtration time. For all composite membranes, a 50–60% flux reduction was obtained. This phenomenon might be due to the existence of a fouling layer that acted as another resistance layer. The steady state of the permeate flux was reached after the fouling layer was fully established. Increasing filler concentration led to an increase in steady-state fluxes. For the percentage rejection study, mode of filtration and filler content had an influence on suspended solids rejection. However, in removing dissolved organic matter, analyzed through COD and BOD, the composite membrane illustrated a significant effect of filler content on the percentage rejection of COD and BOD ranged from 10% to 90%. High filler content resulted in a higher porosity of the membrane and membrane with a filler composition of 2% exhibited better results compared to the rest of the membranes Fillers were the key factors that altered the membrane characteristics, enhancing the separation efficiency of the membrane. They were in good agreement with what has been reported in the past. Composite membranes effectively removed all the organic matter present in the POME and the separation efficiency of the membrane was so high that it showed potential for industrial applications.
[6] S. Yacob, Y. Shirai, M.A. Hassan, M. Wakisaka, S. Subash, Start-up operation of semi commercial closed anaerobic digester for palm oil mill effluent treatment, Process. Biochem. 41 (2006) 962–964. [7] T.K. Hwang, S.M. Ong, C.C. Seow, H.K. Tan, Chemical composition of palm oil mill effluents, Planter 54 (1978) 749–756. [8] S. Xia, J. Nan, R. Liu, G. Li, Study of drinking water treatment by ultrafiltration of surface water and its application to China, Desalination 170 (2004) 41–47. [9] J.M. Arnal, M. Sancho, G. Verdú, J. Lora, J.F. Marín, J. Cháfer, Selection of the most suitable ultrafiltration membrane for water disinfection in developing countries, Desalination 168 (2004) 265–270. [10] J. Huang, G. Arthanareeswaran, K. Zhang, Effect of silver loaded sodium zirconium phosphate (nanoAgZ) nanoparticles incorporation on PES membrane performance, Desalination 285 (2012) 100–107. [11] M. Dıaz, F. Barba, M. Miranda, F. Guitian, R. Torrecillas, J.S. Moya, Synthesis and antimicrobial activity of a silver-hydroxyapatite nanocomposite, J. Nanomater. (2009) 6. [12] D. Predoi, C.L Popa, P. Chapon, A. Groza, S.L Iconaru, Evaluation of the Antimicrobial Activity of Different Antibiotics Enhanced with Silver-Doped Hydroxyapatite Thin Films. [13] G. Sahni, P. Gopinath, P. Jeevanandam, A novel thermal decomposition approach to synthesize hydroxyapatite–silver nanocomposites and their antibacterial action against GFP-expressing antibiotic resistant E. Coli, Colloids Surf. B Biointerfaces 103 (2013) 441–447. [14] G. Ciobanu, S. Ilisei, C. Luca, Hydroxyapatite-silver nanoparticles coatings on porous polyurethane scaffold, Mater. Sci. Eng. C35 (2014) 36–42. [15] S. Darvishmanesha, J.C. Jansenb, F. Tasselli, E. Tocci, P. Luis, J. Degrèvea, E. Drioli, B.V. der Bruggen, Novel polyphenylsulfone membrane for potential use in solvent nanofiltration, J. Membr. Sci. 379 (2011) 60–68. [16] L.L. Hwanga, H.H. Tseng, J.C. Chen, Fabrication of polyphenylsulfone/polyetherimide blend membranes for ultrafiltration applications: the effects of blending ratio on membrane properties and humic acid removal performance, J. Membr. Sci. 384 (2011) 72–81. [17] Y. Liu, S. Zhang, Z. Zhou, J. Ren, Z. Geng, J. Luan, G. Wang, Novel sulfonated thinfilm composite nanofiltration membranes with improved water flux for treatment of dye solutions, J. Membr. Sci. 394–395 (2012) 218–229. [18] M.U. Rashid, M.K.H. Bhuiyan, M.E. Quayum, Synthesis of silver nano particles (AgNPs) and their uses for quantitative analysis of vitamin C tablets, J. Pharm. Sci. 12 (2013) 29–33. [19] W.R. Li, X.B. Xie, Q.S. Shi, H.Y. Zeng, Y.S. Ou-Yang, Y.B. Chen, Antibacterial activity and mechanism of silver nanoparticles on Escherichia coli, Appl. Microbiol. Biotechnol. 85 (2010) 1115–1122. [20] A.S. Baharuddin, N.A.A. Rahman, U.K.M. Shah, M.A. Hassan, M. Wakisaka, Y. Shirai, Evaluation of pressed shredded empty fruit bunch (EFB)-palm oil mill effluent (POME) anaerobic sludge based compost using Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) analysis, Afr. J. Biotechnol. 10 (2011) 8080–8089. [21] G. Sahni, P. Gopinath, P. Jeevanandam, A novel thermal decomposition approach to synthesize hydroxyapatite–silver nanocomposites and their antibacterial action against GFP expressing antibiotic resistant E. Coli, Colloids Surf. B Biointerfaces 103 (2013) 441–447. [22] A.L. Ahmad, M.F. Chong, S. Ismail, S. Bhatia, Drinking water reclamation from palm oil mill effluent (POME) using membrane technology, Desalination 191 (2006) 35–44.
References [1] A.L. Ahmad, S. Ismail, S. Bhatia, Water recycling from palm oil mill effluent (POME) using membrane technology, Desalination 157 (2003) 87–95. [2] R.P. Singh, M.H. Ibrahim, E. Norizan, Composting of waste from palm oil mill: a sustainable waste management practice, review in environmental science and biotechnology, Rev. Environ. Sci. Biotechnol. 9 (2010) 331–344. [3] M.Y. Rosnah, A. Ghazali, W.D.W. Rosli, Y.M. Dermawan, Influence of alkaline peroxide treatment duration on the pulpability of oil palm empty fruit bunch, World Appl. Sci. J. 8 (2010) 185–192. [4] A. Aziz, B.T. Hanida, Reactive Extraction of Sugars From Oil Palm Empty Fruit Bunch Hydrolysate Using naphthalene-2-boronic Acid, Universiti Sains Malaysia, (2007) Thesis.. [5] A. Sulaiman, Accelerated start-up of a semi commercial digester tank treating palm oil mill effluent with sludge seeding for methane production, World Appl. Sci. J. 8 (2010) 247–258.
14
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Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Silver nano-particle coated hydroxyapatite nano-composite membrane for the treatment of palm oil mill effluent Fahmi Anwar, G. Arthanareeswaran
T
⁎
Membrane Research Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli, 620015, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Keywords: Polyphenylsulfone Palm oil mill effluent Hydroxyapatite Silver nano-particle
Nano-composite ultra-filtration membranes were developed for the treatment of Palm Oil Mill Effluent (POME). Silver nano-particle was synthesized and was used to coat hydroxyapatite nanotubes. This coated nano-filler was used as filler for the polyphenylsulfone (PPSU) membrane. The filler composition in the membrane was varied from 0 to 2.5 wt% to investigate the effect of fillers on the separation properties of the membrane. Both dead-end and cross-flow techniques were used for the separation. Characterization techniques such as contact angle analysis, particle size analyzer, XRD, SEM were used to analyze both nanoparticles as well as the nano-composite membranes. Parameters like COD, BOD, turbidity, suspended solids, pH, conductivity and TDS of the permeate were studied. Change in intrinsic membrane resistance with respect to filler composition was also studied. Based on the results the membrane having a filler composition of 2% was found to be exhibiting excellent properties compared to other membranes in terms of separation performances. The rejection percentage of PPSU membrane with a filler content of 2% was 89.74%. The composite membrane had shown promising results by removing all the organic matter present in the POME and can be further developed into a pilot plant.
1. Introduction
POME consists of 95–96% water, 0.6–0.7% oil and 4–5% solids [1]. It has high turbidity and color resulting from high organic matter content, suspended solids and a trace amount of minerals and heavy metals [7]. This organic content makes POME unfit for drinking if not treated. Therefore, a proper treatment method to remove the organic matter, minerals and heavy metals is required in the recycling of drinking water from POME. Currently, novel technologies such as microfiltration, ultrafiltration (UF), nanofiltration and reverse osmosis (RO) are showing high performance in the treatment of water compared to conventional treatments. The main advantages of membranes technology are energy efficiency, compact module, quality of the product, high selectivity, easy handling and cost efficiency [8]. UF membrane technology is recognized for its ability to reject organic matter in POME as well as microorganisms. [9]. Nevertheless, membrane technology faces challenges like fouling due to the organic matters and bacteria present in the effluent. To reduce fouling anti-bacterial fillers are incorporated into the polymer membranes [10]. Silver nanoparticles doped with hydroxyapatite have been reported showing exceptional anti-bacterial and anti-fouling properties when incorporated in polymer membranes [11–14]. The anti-bacterial property of silver combined with hydrophilicity of hydroxyapatite can improve the anti-fouling property of the membrane.
Water, essential to life is drying up around the world. Water scarcity is one of the alarming challenges that the modern world face today. Water shortages, deterioration of water quality, and environmental constraints have led to an augmented interest in recovering and recycling water in many parts of the world. This shortage of water drives people to recycle the effluents from different fields such as agricultural effluents, plantation effluents, industrial and farming effluents [1]. Plantation effluents include effluents from palm oil mills, rubber plantations, coconut mills etc. Palm oil is one of the world’s most rapidly expanding crops and is derived from the fleshy mesocarp of the fruit of oil palm (Elaeis gunineensis). One hectare of oil palm produces about 10–35 tonnes of fresh fruit bunches (FFB) per year [2]. While the palm oil industry has been known for development, it has also contributed to environmental pollution due to the production of huge quantities of byproducts from the oil extraction process [3]. The byproducts include Oil Palm Trunks (OPT), Oil Palm Fronds (OPF), Empty Fruit Bunches (EFB), Palm Pressed Fibres (PPF), and liquid discharge Palm Oil Mill Effluent (POME) [2,4,5]. According to Yacob et al., about 30 million tonnes of palm oil mill effluent (POME) was produced in Malaysia in the year 2004 [6]. ⁎
Corresponding author. E-mail address: [email protected] (G. Arthanareeswaran).
https://doi.org/10.1016/j.jwpe.2019.100844 Received 5 September 2018; Received in revised form 21 March 2019; Accepted 26 April 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Water Process Engineering 31 (2019) 100844
F. Anwar and G. Arthanareeswaran
Composite membranes were reported to exhibit enhanced permeation and separation properties [10]. Many polymers, as well as composite membranes, were reported showing excellent separation properties in the Ultrafiltration of POME. Out of these, Polyphenylsulfone (PPSU) was reported showing high stability during ultrafiltration studies [15]. Hwang et al studied different blend membranes of PPSU for the ultrafiltration applications [16]. Sulfonated PPSU was investigated for water treatment by Yang et al and reported that the hydrophilicity of the membrane was enhanced by the chemical modification [17]. In the present study silver nanoparticles were successfully synthesized, which were used for the coating of hydroxyapatite (Ca5(PO4)3(OH)) nanotubes. This coated nanofiller was incorporated into the PPSU and nanocomposite membranes were prepared by the phase inversion technique. The effect of filler on hydrophilicity, porosity, membrane morphology, and membrane separation performance was studied. These membranes were employed for the treatment of POME in dead-end and cross-flow membrane module.
Table 1 Composition of Silver-Hydroxyapatite incorporated PPSU membranes. Membrane Code
PPSU (%)
Silver-Hydroxyapatite (%)
NMP (mL)
PM-01 PM-02 PM-03 PM-04 PM-05 PM-06
100 99.5 99 98.5 98 97.5
0 0.5 1 1.5 2 2.5
20
solution were prepared by varying the filler concentration in the solution by dissolving corresponding amount filler and PPSU in 20 mL NMP. The different batches are shown in Table 1. For complete dissolution, the solution was kept for magnetic stirring for overnight to attain a homogeneous dope solution. The temperature was increased to 80 °C for 10 min and to avoid undissolved particles, the solution was later sonicated in a sonication bath for 15 min. For casting the membrane, wet phase inversion procedure was followed. The silver-hydroxyapatite incorporated PPSU film was immersed into a coagulation bath where the exchange of solvent and nonsolvent took place and the membrane was formed. The silver-hydroxyapatite incorporated PPSU solution was poured on a smooth glass plate and spread with the help of a metal rod to produce a thin film membrane of 250 μm thickness. After casting the membrane on the glass plate, it was immersed into a gelation bath. The gelation bath was prepared prior to the casting of the membrane. It consists of 4 wt. % SLS solution and during casting the bath temperature was brought down to 5 °C. Once they were immersed in gelation bath, the membranes got toughened and later they were peeled off the glass plate and were stored in distilled water for further use.
2. Experimental 2.1. Material PPSU (Radel R-5000) which is required for the preparation of composite membrane was provided by Solvay Advanced Polymer (Belgium). N-methyl-2-pyrrolidone (NMP) was the solvent used and it was purchased from Merck Life Science Private Limited, Mumbai, India. Silver nitrate and Sodium Boro-hydride which were used for the preparation silver nanoparticle were bought from Sigma-Aldrich Mumbai. Sodium Lauryl Sulfate (SLS) which was used for membrane casting was bought from Sigma-Aldrich Mumbai. Hydroxyapatite nanotubes were bought from Alfa Aesar USA. Methanol used for washing was bought from Titan Biotech Limited, Rajasthan, India. Acetone was purchased from Merck Specialities Private Limited, Mumbai, India.
2.5. Water content Membrane samples were cut into the desired size and soaked in water for 24 h and weighed immediately after blotting the free surface water. These wet membranes were dried for 12 h at 80 °C and the dry weights were determined. From the difference between dry and wet weights of the samples, the amount of absorbed water was calculated. From that, the percentage of water content was estimated. The porosity of the membranes was calculated by the formula given below:
2.2. Silver nanoparticle preparation A large excess of NaBH4 was needed to reduce the ionic silver and to stabilize the silver nanoparticles that were formed. 5 mL of 0.001 M silver nitrate was added dropwise to 30 mL of 0.002 M sodium borohydride solution that had been chilled in an ice bath. The reaction mixture along with the ice bath was stirred vigorously using a magnetic stirrer. The solution turned to light yellow after the addition of 2 mL of silver nitrate and to brighter yellow when all of the silver nitrate had been added and later to light yellow. The reaction is shown below.
=
( (
1
1 2 )/ d w
2)/ d w
+(
2 / dp )
(1)
Where Density of water dw = 1000 g/cc Density of polymer dp = 1370 g/cc Weight of wet polymer = w1 Weight of dry polymer = w2
AgNO3 + NaBH4 → Ag + H2+ B2H6 + NaNO3 After adding the entire silver nitrate, the stirring was stopped and the stir bar was removed. Reaction conditions including stirring time, relative quantities of reagents and reaction temperature must be carefully controlled to obtain stable colloidal silver. The colloidal silver particle solution was then centrifuged and dried in a vacuum oven to get the nanoparticles.
2.6. Characterization of nanofillers and membranes Various techniques were employed for the characterization of the fillers as well as the nanocomposite membranes. UV Spectroscopy was used to detect and confirm the presence of silver nanoparticles and the instrument used was Merck UV Spectrophotometer Spectroquant Model Prove 600 USA. X-ray Diffractometer (XRD) was used to confirm and study the microstructure of the silver nanoparticles as well as silver coated hydroxyapatite nanotubes. Perkin Elmer 1621 USA wide-angle X-ray diffractometer was the instrument used to study the microstructure of the nanofiller. Using Bragg’s law the average D-spacing of the nanoparticle was evaluated. It is as shown below, nλ = 2d sin θ, where n is an integral number, λ is the X-ray wavelength, d is for the inter-layer spacing and θ is the diffraction angle. Fourier transform infra-red (FTIR) was used to identify the functional
2.3. Silver-hydroxyapatite composite preparation About 3 mL of colloidal silver nanoparticle solution that had prepared was added to 3.6 mL of the commercially obtained 10% percent aqueous solution of hydroxyapatite nanotubes. The solution was left to dry at 50 °C overnight and the dried particles were collected and stored. 2.4. Polymer nano-composite membrane preparation The polymer that was chosen for the preparation was PPSU. The solvent used was N-Methyl-2-Pyrolidone. Different batches of polymer 2
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groups in nano-particle as well as in membranes. FTIR was also used to identify the functional groups in the fouled membrane after separation. It can be used for identifying the types of chemical bonds in a molecule by producing an IR absorption spectrum that is like a molecular fingerprint. The model used here for the analysis was Thermo Nicolet, Avatar 370 USA. Scanning Electron Microscopy (SEM) was used to study the surface as well as the cross-sectional morphology of the membranes. JEOL Model JSM - 6390 L V India was the model used for the analysis. The sample was coated with a thin layer of gold before analysis. Particle size analyzer was used to find the mean particle size and size distribution of the silver-coated hydroxyapatite nano-tubes. The model used here for analysis was HORIBA Laser Scattering Particle Size Distribution Analyzer LA-960. The contact angle of the membranes was measured by Goniometer to find out the change in hydrophilicity with respect to filler concentration. The instrument used for the analysis was ramé-hart Model 500 Goniometer USA. The mechanical strength of the membrane was found by Universal Testing Machine Mecmesin Model 2110 USA to perform stress-strain analysis of the membrane. It was performed to understand the change in the mechanical strength of the membrane with an increase in nanoparticles concentration.
2.7.2. Dead-end filtration set-up The ultrafiltration (UF) experiments were carried out in a 400 ml batch type stirred cell (ultrafiltration cell – S76-400-Model, Spectrum, USA) fitted with a Teflon coated magnetic paddle. The effective membrane area available for ultrafiltration was 38.5 cm2. The solution filled in the cell was stirred at 400 rpm using a magnetic stirrer. All the experiments were carried out at 26 °C and at 414 kPa transmembrane pressure. The permeating solution was collected from the bottom of the cell to a measuring cylinder. The schematic diagram of dead end module is shown in Fig. 2. The cell was pressurized using a nitrogen cylinder. The pure water flux was measured at every hour in order to monitor the compaction behavior. The membranes were compacted with time to attain the steady state flux value. Pure water flux was calculated over measured time intervals using the same equation used for cross-flow (Eq. (2)). The retentate coming out of the membrane module was recycled back to the feed tank. Water flux was determined to find out intrinsic membrane resistance of all the membranes. It is calculated by the formula given below (Eq. (4)).
Jk =
2.7. Ultra-filtration set-up
3.1. Confirmation silver nanoparticle Different analysis techniques were used to confirm the presence and characterize the prepared silver-nanoparticles. These confirmation analyses were performed before coating the hydroxyapatite nanotubes. They are described in detail as follows.
2.7.1. Cross-flow set-up UF experiments were carried out in a cross flow flat-frame membrane module (Model: PLEIADE Rayflows, Orelis Environment SAS, France) with a filtration area of 100 cm2 at a temperature of 26 °C and a controller to set the pressure at a transmembrane pressure of 200 kPa. The cross-flow ultrafiltration module is shown in Fig. 1. The experiments were conducted in a continuous mode where the permeate was continuously collected and measured in a measuring cylinder. The retentate coming out of the membrane module was recycled back to the feed tank. The permeate flux, as well as the water flux, was calculated by the equation given below (Eq. (2)).
Q A× t
3.1.1. UV–vis spectroscopy UV-Visible spectroscopy is one of the most widely used techniques for structural characterization of silver nanoparticles. It is quite sensitive to the presence of silver colloids because these nanoparticles exhibit an absorption peak due to the surface plasmon excitation. The absorption band in the 350 nm–450 nm region is typical for the silver nanoparticles [18]. The absorption spectra of the colloidal solution of silver are shown in Fig. 3. The spectra exhibit a plasmon absorption band at ˜ 400 nm which is the characteristic of silver nanoparticles. Such plasmon bands are unique physical properties of the nanoparticles themselves. Thus using UV Spectroscopy, the presence of silver nanoparticles was confirmed. The identification of silver nanoparticles was based on Surface Plasmon Resonance phenomenon. The penetration depth was found to be directly proportional to the exciting wavelength i.e., 325 nm because of decreased absorbance which is in accordance with Li et al [19]. Nanoparticles with absorbance in the range of 350 nm–450 nm were used for coating hydroxyapatite nanotubes.
(2)
where, Jw (L/m2h) is the membrane flux, A (m2) is the effective area of the membrane surface, Q (L) is the volume collected and Δt is the time interval of 10 min. The flux for POME was also calculated in a similar manner by calculating the effluent flux obtained through the membrane. The rejection of the POME effluent was found by using the equation given below (Eq. (3)).
R= 1-
CP × 100 CF
(4)
3. Results and discussions
Ultrafiltration studies have been carried out in cross-flow and deadend module
JW =
P Rm
(3)
3.1.2. X-ray diffraction (XRD) XRD was used as a secondary confirmation for the silver particles.
where CP and CF are the absorbances of permeate and feed respectively.
Fig. 1. UF Cross-Flow Filtration Set-up. 3
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Fig. 2. UF Dead-end Filtration Set-up.
hydroxyapatite particles was confirmed and their structure was studied based on the following studies. 3.2.1. X-ray diffraction (XRD) XRD was used as a secondary confirmation for the silver particles. Peaks were obtained according to reported data [20], this peak gave us further confirmation on the presence of nanoparticles. The interplanar distance D-spaces were measured using Bragg’s Law and are illustrated in Fig. 6. Their values are ranged between 1.5–3.5 Å. The pattern clearly shows the main peaks at (2θ) 26°, 33°, 49° and 54° corresponding to the 110, 202, 223 and 313 planes, respectively. These reported data clearly proved the presence of silver coated hydroxyapatite nanotubes. 3.2.2. Fourier transform infrared spectroscopy (FTIR) FTIR was used for added confirmation of the coated nanotubes and is shown in Fig. 7. The obtained spectra were compared with the standard IR frequencies table [20]. It showed peaks at 1735 (O-H) and 1020 (N = O) and by comparing with reported data, the particles that were prepared were confirmed to be silver nanoparticle coated hydroxyapatite nanotubes [21].
Fig. 3. UV Spectroscopy of Silver Nanoparticles.
Peaks were obtained according to reported data [18], these findings once again confirm the presence of nanoparticles. The interplanar distance D-spaces were measured using Bragg’s Law and are illustrated in Fig. 4. Their values ranged between 1–3 Å. The pattern clearly shows the main peaks at (2θ) 38°, 44°, 64° and 77° corresponding to the 111, 200, 220 and 311 planes, respectively. By comparing JCODS (file no: 89-3722), the typical pattern of chemical synthesized silver nanoparticles was found to possess an FCC structure [19].
3.2.3. Particle size analysis of nanotubes In order to find out the size distribution of the coated nano-tubes prepared, particle size analysis was carried out. Fig. 8 shows the size distribution of nanoparticles and they were having a Median and Mean size of 20.7 nm 33.16 nm respectively. Mode size and standard deviations were 12.378 μm and 32.5125 μm respectively. About 60% of particles were under the size of 20 μm.
3.1.3. Fourier transform infrared spectroscopy (FTIR) Further confirmation was carried out by FTIR. The FTIR spectra obtained was compared with the standard IR frequencies table [20]. The spectra of silver exhibited a high-intensity broad band at 1720 (OeH) due to the stretching of the hydrogen and oxygen bond and at 1230 (N]O) (Fig. 5) due to nitrogen-oxygen stretch bond. The functional groups obtained were in good agreement with what has been observed from reported data [18]. By this analysis, the particles that were prepared were confirmed to be silver nanoparticles
3.3. Effect of filler concentration on hydrophilicity and porosity of the membrane Contact angle analysis was performed to study the influence of filler concentration on the hydrophilicity of the membrane. Table 2 shows the contact angle of each membrane and how the filler concentration changes the angle. Data, the contact angle was decreased by almost 35° with an increase in filler composition, indicating the increase in the hydrophilic nature of the membrane (Fig. 9). Hydrophilicity helps to decrease the membrane fouling caused by the organic matter present in the effluent. Hence the effect of filler on the hydrophilicity of the composite membrane was tested to study the influence of nano-material in the membrane. From Table 2, it is clear
3.2. Confirmation of silver coated hydroxyapatite After confirming the presence of silver nanoparticles, it was used as a coating for hydroxyapatite nanotubes. The presence of coated 4
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Fig. 4. XRD of Silver Nanoparticle.
hydrophilic surfaces with contact angles less than 82. And PM-6 have the lowest contact angle and highest filler content. 3.4. Effect of filler concentration on the mechanical strength of the membrane The effect of filler concentration on the mechanical stability of the membrane is tabulated in Table 4. The material strength of the membranes prepared was studied by performing Stress-Strain tests. The samples of the membranes were cut into dimensions of length 30 mm and width 10 mm. The initial gauge length was set at 20 mm. The data was recorded for respective samples and tabulated in Table 4. It was observed that as the concentration of fillers in the membranes increased, the ultimate force the membranes could withstand first kept on increasing and then started to decrease. The membrane with a filler concentration of 1.5% was observed to be the most mechanically stable membrane. Initially, the presence of fillers made the membrane more mechanically stable, however, when the filler content went beyond 1.5%, the membrane became more porous compromising the structural stability of the membrane.
Fig. 5. FTIR of Silver Nanoparticle.
3.5. Confirmation of presence of nanomaterial in membrane
that as filler content was increased, the hydrophilic nature of the composite membranes was also improved which in turn results in a rise in the amount of water absorbed. This can be explained by the presence of hydroxyapatite group in the membrane which alters the membrane surface properties. The porosity of the membranes was calculated by the Eq. 1. The results of the test are tabulated in Table 3. The porosity kept on increasing with increase in filler content because the presence of fillers makes the membrane more porous by opening up its pores. From this test we can confirm that the membranes prepared were hydrophilic in nature and as the concentration of the inorganic fillers goes on increasing the membrane water absorption capacity and porosity also increases. Wettability of filler may aff ;ect the adsorbed fouling agent in POME and enhance the material interactions. Results indicate that the silver nanoparticle coated hydroxyapatite PPSU membranes have
Different analysis techniques were used to confirm the presence of nanomaterial in the membrane. Functional groups that were present in the silver nanoparticle coated hydroxyapatite are shown in Fig. 5. The spectra of silver nanoparticle coated hydroxyapatite PPSU membranes are shown in Fig. 10. At 1735 (OeH) oxygen-hydrogen stretch bond is present, which confirms the presence of silver coated hydroxyapatite nanotubes. Whereas the band at 1230 (N]O) nitrogen-oxygen double bond confirms the presence of silver nanoparticles. All the membranes except the virgin membrane exhibited these peaks. 3.5.1. SEM-EDAX of the composite membrane Fig. 11 (a) shows the top surface of PM-4 and Fig. 11 (b) shows the different elements present in that membrane. The SEM image shows a 5
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Fig. 6. XRD of Silver Coated Hydroxyapatite Nanotubes. Table 2 Contact Angle of Membranes. Membrane Code
Left Angle (o)
Right Angle (o)
Mean (o)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
88.6 80.6 74.8 70.4 67.2 61.6
96.3 83.2 75.2 72.8 63.0 58.5
92.4 81.9 75.0 71.6 65.1 60.1
As it is clear from the.
3.6. Morphological characteristics of the membrane The top surface and cross-sectional SEM images are shown in Figs. 12 and 13 respectively. It shows a smooth and defects free surface without any deformation. Compared to the composite membrane virgin membrane is less porous and the membrane formed is found to be very dense and the same can be inferred from Table 3, where the porosity of the membranes are tabulated. Virgin membrane i.e., PM-01 was found to be having a porosity of 0.768 and PM-06 was having a porosity of 0.921. This less porosity is the main factor that reduces the rejection percentage of the virgin membrane during POME treatment. Cross-sectional images show the honey-comb like structure of composite membranes (Fig. 4.11). The thickness of PM-04 was found to be 250 μm. The varying porosity which we had found earlier is picturized here. From Figs. 12 and 13 we can observe how the porosity is
Fig. 7. FTIR of Silver Coated Hydroxyapatite Nanotubes.
defect free, smooth surface of the membrane. Out of all the elements present in the membrane as observed in the result, Calcium and Phosphor are highest in content. These elements are present in the hydroxyapatite compound. With this analysis, we can conclude that the membrane is having the prepared fillers and they were not leached out of the membrane before they were used for separation.
Fig. 8. Particle Size Analysis. 6
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Fig. 9. Contact Angle of (a) PM-1, (b) PM-3, (c) PM-5 and (d) PM-6. Table 3 Water Content of Nanocomposite Membranes. Membrane Code
Percentage water content (%)
Porosity
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
70.8 79.59 84.14 84.79 90.81 91.23
0.768 0.842 0.879 0.884 0.901 0.921
varying from a less porous virgin membrane to a composite membrane which is rich in pores. This porosity helps to achieve a higher flux and permeability during separation processes. 3.7. Treatment of palm oil mill effluent Fig. 10. FTIR of Membranes.
Characteristics of the collected POME are tabulated in Table 4.4. The effluent was collected from Parison’s palm oil Kerala, India and was Table 4 Mechanical Strength of Membranes. Filler Concentration (%)
Break Distance (mm)
Ultimate force (N)
% Total Elongation
Ultimate Stress (MPa)
Yield Stress (Mpa)
Ultimate Strength (%)
0 0.5 1 1.5 2 2.5
00.833 1.03 1.35 1.94 2.72 1.10
3.53 4.97 5.10 8.23 6.23 5.17
8.33 10.3 13.5 19.4 27.2 11
0.707 0.993 1.02 1.65 1.25 1.03
0.707 0.993 1.02 1.65 1.25 1.03
4.42 7.56 7.14 15.7 26.8 7
7
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Fig. 11. SEM-EDAX of PM-4.
Fig. 12. Top Surface SEM Images of (a) PM-01, (b) PM-02, (c) PM-04 and (d) PM-05.
filtered using filter paper to remove the suspended solids. All the parameters were estimated after filtration (Table 5).
increase in porosity of the membrane as filler content makes the membrane more porous. The intrinsic resistance of the membranes was calculated and tabulated below (Table 6). Intrinsic resistance kept on decreasing from 19.178 kPahm2/L to 13.011 kPahm2/L with an increase in filler content. Due to higher porosity diffusion through pores require less activation energy and that makes the membrane less resistant and PM-6 was found to be having minimum intrinsic resistance and maximum porosity (0.921). After finding out the water flux, using POME as the feed, the permeate flux of the membrane was found and is shown in Fig. 15. With time the permeate flux kept decreasing and after some time it became
3.7.1. Ultrafiltration of POME in dead-end filtration module The transmembrane pressure was set at 8 bar and the temperature at 26 °C. The diameter of the membrane was 5 cm. Permeate was collected every 10 min. Water flux was found before separating effluent. Water flux is given in Fig. 14. Water flux was determined to find out intrinsic membrane resistance of all the membranes. Water flux of the membranes was observed to keep on increasing with increase in filler content and for PM-06 it reached up to 57 L/hm2. This is due to the 8
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Fig. 13. Cross-sectional SEM Images of (a) PM-01, (b) PM-02, (c) PM-03, (d) PM-04, (e) PM-05 and (f) PM-06.
Absorbance spectra of the feed, as well as the permeate, were taken to find out the wavelength at which the organic matter was present in POME. And they were found to be 230 and 270 nm. The absorbance of permeate collected at different time intervals, for each membrane, was noted at these two wavelengths. This can indirectly give us an insight into how much organic matter has been removed from the POME by membrane filtration. The absorbance of permeate at different times is shown in Fig. 16 (a) and (b). The absorbance of all the membranes kept on decreasing till PM-5. For PM-6, performance was reduced as the high porosity causes the selectivity to decrease. At both wavelengths, PM-5 and PM-4 were observed to be showing excellent separation properties. To study the separating performance of the membranes Rejection Percentage of the membrane was found. From Table 7 it is noticed that as the filler concentration was increased from 0 to 2.5%, the percentage rejection kept on increasing till PM-5. This
Table 5 Characteristics of Palm Oil Mill Effluent. Parameter
Value
Ph Conductivity (μS) TDS (mg/L) TS (mg/L) TSS (mg/L) COD (mg/L) BOD (mg/L) Turbidity (NTU)
4.2 977 2500 7500 5000 4300 3300 1060
constant. This is due to the clogging of pores due to fouling. The organic material deposited on the membrane reduces the POME flux over the passing of time. 9
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Fig. 14. Water Fluxes of Membranes. Table 6 Intrinsic Resistances of Nanocomposite Membrane. Membrane
Resistance (kPahm2/L)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
19.178 18.817 17.676 14.227 14.0 13.011
es
was improved by the addition of nanocoated hydroxyapatite in PPSU solution. Fig. 16. (a) Absorbance of permeate at 230 nm (b) Absorbance of permeate at 270 nm. Table 7 Membrane Rejection Percentage. Membrane
Rejection percentage (%)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
41 46.15 48.72 76.92 89.74 61.54
3.7.2. COD removal COD of permeate collected at different time intervals, for each membrane was plotted and shown below in Fig. 18. According to disposal standards, COD limit should be less than 100 mg/L [1]. For three membranes it was possible to achieve this range. Water reclamation for drinking is possible from POME if drinking water standards are achieved for the permeate collected after membrane treatment. If membrane modules are connected in series, COD can be brought down to zero. As the separation progressed, membranes got clogged and very less organic material permeated through the membrane causing a decrease in COD over time.
Fig. 15. POME fluxes of Membranes.
suggests that the adsorption capacity of the membranMaximum rejection percentage is obtained for membrane corresponding to a filler percentage of 2%. For PM-6, although the permeate flux is high, rejection was decreased due to larger pore size. Thus, the optimum filler percentage has to be fixed for better performances. More filler content can cause more openings in the membrane and less rejection percentage. Fig. 17 shows the feed and permeates samples.
3.7.3. BOD removal BOD of permeate collected at different time intervals, for each membrane was plotted and shown below in Fig. 19. According to disposal standards, BOD limit should be less than 25 mg/L [1]. For three 10
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Fig. 19. BOD of Permeate.
Fig. 17. Images of Feed and Permeate.
Fig. 20. Turbidity of Permeate. Fig. 18. COD of Permeate.
3.7.5. Total solids content The total solid content of permeate articulates the amount of organic content in the effluent. Fig. 21 shows how the total solid content is changing with different membranes. Total solid content consists of suspended solids and dissolved solids. This is the polluting agent in POME. Removal of total solids alone can make the permeate meets drinking water standards. For PM-6, the solids content was reduced to 450 mg/L, which is the lowest we achieved. The total solid content of permeate had decreased with time, as the organic content permeated through the membrane reduced with clogging of pores.
membranes it was possible to achieve this range. Due to the clogging of pores, less biological content was permeated over time and thus BOD decreased with time. Biological contents are the main reason for the pollution of water. If these agents are removed using membrane technology, water recovery from membrane treatment is a highly promising technology. 3.7.4. Turbidity measurement The turbidity of permeate collected at different time intervals, for each membrane, was plotted and shown in Fig. 20. The turbidity of drinking water should not be more than 5 NTU, and should ideally be below 1 NTU [22]. Results of PM-4 and PM-5 shows that drinking standards can be achieved by optimizing the filler content on the membrane.
3.7.6. Conductivity of permeate The conductivity of permeate collected at different time intervals, for each membrane, was plotted and shown in Fig. 22. The conductivity of drinking water should not be more than 50 μS [21]. The conductivity of the permeate is the indirect measure of pollutants in the permeate. Conductivity value can infer to the amount of ions in the permeate. From Fig. 22 it can be observed that the conductivity of the permeate 11
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Fig. 23. FTIR of Fouled Membrane. Fig. 21. Total Solids Content of Permeate.
Fig. 24. Images of PM-3 (a) Before Separation (b) After Separation.
Fig. 22. Conductivity of Permeate.
kept on decreasing with increase in filler content till PM-5. For PM-6, there is an exception due to the enlargement of pores. For PM-5 conductivity was reduced to less than 50 μS. 3.7.7. Fouling of membrane Fouling was one of the main problems encountered during POME treatment. After the separation, a fouling layer was developed on the membrane surface. Membranes were analyzed using FTIR for the changes in functional groups presented on the surface of the membrane before and after effluent separation. Fig. 23 shows FTIR of PM-3 before and after treatment. The changes in peaks were noted down to analyze the functional groups present in the cake deposited on the membrane surface. For the fouled membrane at 1753 (C]O) bond is observed which indicates the presence of fatty acid from palm oil. Fig. 24 shows the images of the membrane before and after separation. Visual examination can convey to us how much organic matter is deposited on the membrane during separation. The decline in flux was due to the development of this fouling layer.
Fig. 25. Pure water fluxes of membranes in Cross-flow module.
the modes. Permeate was collected every 10 min. Water flux was found before separating effluent. Water flux is given in Fig. 25. Water flux was evaluated to find out intrinsic membrane resistance of all the membranes. Water flux of the membranes was observed to keep on increasing with increase in filler content. This is due to the increase in porosity of the membrane as filler content makes the membrane more porous. However, more porosity can lead to a decrease in selectivity. Compared to dead-end filtration in cross-flow filtration, the water flux was more. This is due to the increase in effective area. The intrinsic resistance of the membranes was calculated and tabulated below (Table 8). Intrinsic resistance kept on decreasing with increase in filler content. Due to higher porosity diffusion through pores
3.7.7.1. Cross-flow filtration method. Separation has been conducted in the cross-flow mode to compare the performance of membrane in both 12
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Table 8 Membrane Resistances. Membrane
Resistance (kPahm2/L)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
16.258 14.62 13.24 11.67 9.42 8.601
Fig. 27. Absorbance of Permeate at 230 nm.
Fig. 26. POME Fluxes of Membranes.
require less activation energy and that makes the membrane less resistant. Compared to dead-end filtration resistance was decreased as the flux is higher for cross-flow method. All the membranes were observed to be having less resistance in the case of cross-flow in comparison with dead-end resistances. After finding out the water flux, using POME as the feed, the permeate flux of the membrane was found and is shown in Fig. 26. With time the permeate flux kept decreasing and after some time it became constant. This is due to the clogging of pores due to fouling. The organic material deposited on the membrane reduced the flux. However, the reduction in flux due to clogging of pores was much less than that of what has been observed from dead-end filtration with the same membranes. Absorbance spectra of the feed, as well as permeate, was taken to find out the wavelength at which the organic matter is present in POME. And they were found to be 230 and 270 nm. The absorbance of permeate collected at different time intervals, for each membrane, was noted at these two wavelengths. This can indirectly give us an insight into how much organic matter has been removed from the POME by membrane filtration. The absorbance of permeate at different time period is shown in Figs. 27 and 28. The absorbance of all membranes kept on decreasing till PM-5. For PM-6, performance was reduced as the high porosity causes the selectivity to decrease. As the separation progressed, pores got clogged due to the presence of organic matter. This resulted in a highly selective permeation and the collected permeate thus had a low organic content corresponding to a lower absorbance. At both wavelengths, PM-5 and PM-4 were observed to be showing excellent separation properties. To study the separating performance of the membranes Rejection Percentage of the membrane was found. It is tabulated below (Table 9). Maximum rejection percentage was obtained for the membrane corresponding to a filler percentage of 2%. For PM-6, although the permeate flux is high, rejection is reduced. This can be as a result of high porosity. Thus, the optimum filler percentage has to be fixed for
Fig. 28. Absorbance of Permeate at 270 nm. Table 9 Membrane Rejection Percentage. Membrane
Rejection percentage (%)
PM-1 PM-2 PM-3 PM-4 PM-5 PM-6
52.88 59.63 67.56 78.31 91.27 68.44
better performances. More filler content can cause more openings in the membrane and less rejection percentage. 4. Conclusions Advanced nanocomposite fillers from silver nanoparticles and hydroxyapatite nanotubes were developed by coating hydroxyapatite nanotubes with silver nanoparticles. Silver nanoparticles were chemically synthesized and were coated on commercially obtained hydroxyapatite nano-tubes for POME treatment. Composite membranes 13
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fabricated from polyphenylsulfone and the prepared fillers were found to be exhibiting homogenous structures with satisfactory structural and morphological properties; indicating that membranes fit the prerequisites for further developments toward industrial applications. Treatment of POME as an approach to achieve high percentage reduction was accomplished by 91.3% of total solids and, 89.3% of BOD by the prepared membranes. The filler percentage and filtration mode had a direct effect on the permeate fluxes of membranes. The development of a fouling layer was observed from the decline in flux with filtration time. For all composite membranes, a 50–60% flux reduction was obtained. This phenomenon might be due to the existence of a fouling layer that acted as another resistance layer. The steady state of the permeate flux was reached after the fouling layer was fully established. Increasing filler concentration led to an increase in steady-state fluxes. For the percentage rejection study, mode of filtration and filler content had an influence on suspended solids rejection. However, in removing dissolved organic matter, analyzed through COD and BOD, the composite membrane illustrated a significant effect of filler content on the percentage rejection of COD and BOD ranged from 10% to 90%. High filler content resulted in a higher porosity of the membrane and membrane with a filler composition of 2% exhibited better results compared to the rest of the membranes Fillers were the key factors that altered the membrane characteristics, enhancing the separation efficiency of the membrane. They were in good agreement with what has been reported in the past. Composite membranes effectively removed all the organic matter present in the POME and the separation efficiency of the membrane was so high that it showed potential for industrial applications.
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