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Potential Use of Nanofiltration Like-Forward Osmosis Membranes for Copper Ion Removal
Accepted Manuscript Potential Use of Nanofiltration Membranes for Copper Ion Removal
Like-Forward
Osmosis
Wan Nur Ain Shuhada Abdullah, Sirinan Tiandee, Woei-Jye Lau, Farhana Aziz, Ahmad Fauzi Ismail PII: DOI: Reference:
S1004-9541(19)30791-8 https://doi.org/10.1016/j.cjche.2019.05.016 CJCHE 1522
To appear in:
Chinese Journal of Chemical Engineering
Received date: Revised date: Accepted date:
13 March 2019 12 May 2019 22 May 2019
Please cite this article as: W.N.A.S. Abdullah, S. Tiandee, W.-J. Lau, et al., Potential Use of Nanofiltration Like-Forward Osmosis Membranes for Copper Ion Removal, Chinese Journal of Chemical Engineering, https://doi.org/10.1016/j.cjche.2019.05.016
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ACCEPTED MANUSCRIPT Potential Use of Nanofiltration Like-Forward Osmosis Membranes for Copper Ion Removal Wan Nur Ain Shuhada Abdullaha,b, Sirinan Tiandeec, Woei-Jye Laua,b,*, Farhana Aziza,b, Ahmad Fauzi Ismaila,b
Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi
SC RI PT
a
Malaysia, 81310 Skudai, Johor, Malaysia b
School of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 Skudai,
Johor, Malaysia c
Faculty of Science and Industrial Technology, Prince of Songkla University, Surat Thani
Campus, 84000 Surat Thani, Thailand
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*Corresponding author: [email protected]; [email protected]
ABSTRACT
The discharge of industrial effluent containing heavy metal ions would cause water pollution
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if such effluent is not properly treated. In this work, the performance of emerging nanofiltration (NF) like-forward osmosis (FO) membrane was evaluated for its efficiency to
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remove copper ion from water. Conventionally, copper ion is removed from aqueous solution via adsorption and/or ion-exchange method. The engineered osmosis method as proposed in
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this work considered four commercial NF membranes (i.e., NF90, DK, NDX and PFO) where their separation performances were accessed using synthetic water sample containing 100 mg/L copper ion under FO and pressure retarded osmosis (PRO) orientation. The
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findings indicated that all membranes could achieve almost complete removal of copper regardless of membrane orientation without applying external driving force. The high removal rates were in good agreement with the outcomes of the membranes tested under pressure-driven mode at 10 bar. The use of appropriate salts as draw solutes enabled the NF membranes to be employed in engineered osmosis process, achieving a relatively low reverse solute flux. The findings showed that the best performing membrane is PFO membrane in which it achieved >99.4% copper rejection with very minimum reverse solute flux of <1 g/m2. h.
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ACCEPTED MANUSCRIPT Keywords: Nanofiltration, forward osmosis, copper, divalent salt, water flux. 1. Introduction
Over the years, the negative impact of heavy metals has been a major concern for the environment and public health. Copper is one of the heavy metal ions commonly found in wastewater discharged from industries such as mining, smelting, semiconductor and
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metallurgy [1,2]. The concentration of copper in the industrial wastewater varies substantially depending on its process. In most of the cases, copper with concentration in the range of 10– 150 ppm is detected in the effluent [3]. In order to reduce its impacts towards environment, wastewater containing copper needs to be treated at an end-of-pipe before discharging to any receiving water bodies.
Although copper is not classified as a carcinogenic material by the United States
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Environmental Protection Agency (US EPA) [4,5], the excessive ingestion of it could bring serious toxicological concerns such as vomiting, cramps, convulsions, nausea, diarrhea,
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epigastric pain, dizziness and possible death. According to the World Health Organisation (WHO), the concentration of copper should be acceptable at the health-based guideline value of 2 mg/L [6]. It is undeniable that copper is an essential nutrient for human, but only very
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small quantity of it is needed on a daily basis [7,8]. USA and Canada recently established a recommended dietary allowance (RDA) for human. For examples, RDA for an adult is 900
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μg/day while child with less than 3-year old is only 340 μg/day [7]. Conventional treatment methods for copper ions removal such as sorption [9–11], ion
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exchange [12], chemical precipitation [13], flocculation settling [14] and flotation technique [15] are always associated with drawbacks. These include sludge production from the
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chemicals added during treatment, incomplete removal of ions and long treatment period. Over the last decade, membrane technologies have been widely used for water and wastewater treatment process. Nanofiltration (NF) membrane in particular offers a good alternative for separating multi-valent ions from water source [16–18]. Typically, NF membranes are negatively charged at neutral and alkaline conditions. Thus, the separation mechanism of ions by NF membrane is governed by both steric effect (sieving) and charge effects (Donnan exclusion) [19,20]. Since operating pressure of NF process is lower than reverse osmosis (RO) process, it consumes significantly lower energy to achieve the same water flux as RO membrane [21]. With respect to the capability of NF membrane
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ACCEPTED MANUSCRIPT for copper ions removal, previous studies demonstrated that NF process could attain >98% removal rate [22,23]. There is a potential to use NF membrane in engineered osmosis process that does not require an external driving force during operation, provided an ideal solute is used as draw solute. Similar to the RO-like forward osmosis (FO) membranes which are widely used in treating brackish water and desalination of seawater [24–26], the NF-like FO membranes
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could in principle be used for water treatment. Compared to the RO-like FO membranes, NFlike FO membranes is able to achieve higher water flux owing to the relatively looser selective layer that offers lower water transport resistance. Previously, Abdullah et al. [27] used the commercial NF membranes for FO and PRO process and reported the membrane performance for treating aerobically-treated palm oil mill effluent (AT-POME). The findings showed that the AT-POME's colour compounds could be completely removed by the NF-like
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FO membranes tested using magnesium chloride (MgCl2) as draw solute with relatively high flux and low reverse solute flux.
On the other hand, Su et al. [28] evolved cellulose acetate (CA)-based NF membranes
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used for the engineered osmosis application. It was reported that water fluxes in the range of 1.8–5.0 L/m2.h and 2.7–7.3 L/m2.h could be achieved by the NF membranes tested under FO
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and PRO mode, respectively, using 0.5–2.0 M MgCl2 draw solutions. Furthermore, Setiawan et al. [29] discovered that different types of in-house made NF-like FO hollow fiber membranes could also be used to achieve higher water flow. When pure water and 0.5 M
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MgCl2 solution were used as feed and draw solution, respectively, these newly developed membranes achieved FO water flux of 9.74 L/m2.h. When a large amount of solute (up to 5 M
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MgCl2) was employed in the draw solution, the water flux of the NF-like FO membrane was further increased [30].
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The primary objective of the present study is to evaluate the potential of NF membranes in engineered osmosis process for copper ion removal under FO and PRO orientation. Four commercial membranes, i.e., NF90, DK, NDX and PFO membranes were characterized and assessed under FO and PRO orientation and their performances were further compared with the findings obtained from pressure-driven process. 2. Methodology 2.1 Nanofiltration membranes
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ACCEPTED MANUSCRIPT Table 1 shows the information of four NF membranes used in this work for the copper ion removal. Four flat sheet membranes were purchased from different manufacturers and were in the dry form. These membrane were then stored in water at room temperature for several days before being used for evaluation. The pore size and polymer material of each membrane are given by the manufacturer, except for PFO membrane which remains
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unknown.
Table 1 Properties of membranes used in this work for copper ion removal Membrane
Membrane
Polymer
NF90
Dow Filmtec™
Polyamide
TFC
~200–400
~184–214
NDX
Synder™
Polyamide
TFC
~800–1,000
~125–165
DK
GE Osmonics™
Polyamide
TFC
~150–300
~156–184
PFO
Porifera Inc
Nomex®
n/a
~30–100
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Structure
Asymmetric
thickness (m)
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Designation
MWCO (Da)
Overall
Manufacturer
2.2. Pressure-driven membrane filtration
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Before proceeding to FO/PRO tests, all the membranes were evaluated with respect to water flux and solute rejection in dead-end permeation cell (Sterlitech, HP4750) following the procedure described in our previous publication [27]. Firstly, the loaded membrane
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(effective surface area: 14.62 cm2) was compacted at 11 bar for 30 min to achieve stable flux conditions. After completing the compaction, the membrane pure water flux, 𝐽𝑣 (L/m2.h) and
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respectively [31]:
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water permeability, 𝐴 (L/m2.h.bar) was determined at 10 bar using Equation (1) and (2),
∆V 𝐴𝑚 × ∆t 𝐽 A= 𝑣 ∆P 𝐽𝑣 =
(1) (2)
where 𝐴𝑚 is the effective membrane area and ∆𝑃, ∆𝑡 and ∆𝑉 is trans-membrane pressure difference, time interval and permeate volume difference, respectively The membrane rejections against MgCl2 (Sigma-Aldrich) and copper ions (Fisher Scientific) were also determined by subjecting the membrane to the filtration of 100 ppm
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ACCEPTED MANUSCRIPT single solute (either MgCl2 or copper ions) at 10 bar for 30 min. The rejections of MgCl2 and copper ion are important as it could indicate whether the membranes are suitable for engineered osmosis process. The water flux of membrane during salt solution filtration process was calculated using Equation (1) while its solute rejection, R (%) was calculated based on the following equation [26]:
𝑐𝑓−𝑐𝑝 𝑐𝑓 × 100
(3)
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𝑅=
where 𝑐𝑓 and 𝑐𝑝 is the salt concentration of feed and permeate sample, respectively. The concentration solution samples (for MgCl2 rejection) were evaluated by concentration conversion based on the conductivity against salt concentration calibration curve. Before the
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concentration conversion, the conductivity of the solution samples was first evaluated by a benchtop conductivity meter (Jenway, 4520). The concentration of copper ions in the aqueous solutions meanwhile was determined using atomic absorption spectroscope (AAS,
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were reported for each membrane.
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Shimadzu, AA-7000). The average water flux and rejection obtained in 30-min experiment
2.3 Osmotically-driven membrane filtration
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For FO and PRO process, the customized lab-scale cross-flow filtration setup described in our previous publication was used [27]. Both cross-flow velocities of the feed and draw
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solution was fixed at 32.72 cm/s. Two high-precision gear pumps (LongerPump, WT3001JA) were used to circulate the feed and draw solutions, respectively. The designed FO
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system has a total effective membrane area of 20.02 cm2 (in rectangular dimension) in the cross-flow cell. A digital weight balance (Amput, 457A) was placed at the draw solution tank (2-L capacity) in order to precisely record the amount of water drawn from the feed solution (either pure water or copper aqueous solution) to the draw solution. All the membranes were evaluated under two different orientations, i.e., FO mode (active-layer-facing-feed-solution, AL-FS) and PRO mode (active-layer-facing-draw-solution, AL-DS). The 𝐽𝑣 obtained from different mode could be measured using Equation 3 [26]: ∆V 𝐽𝑣 = 𝐴 × ∆t 𝑚
(4)
5
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where 𝐴𝑚 is the effective membrane area, 𝜌 is the feed solution density, and ∆𝑉, ∆𝑚 and ∆𝑡 is the volume change of draw solution, weight change of draw solution and time interval, respectively. The average of three replications with 20-min time interval for each run was used to determine the membrane water flux. The following equation was used to calculate the reverse solute flux, 𝐽𝑠 (g/m2.h) of membrane under FO or PRO mode [26]:
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∆(𝐶𝑡𝑉𝑡) 𝐽𝑠 = 𝐴 × 𝑚 ∆t
(5)
where 𝐴𝑚 is the effective membrane area, ∆𝑡 is the time interval, 𝐶𝑡 and 𝑉𝑡 are the change of feed concentration and change of feed solution volume measured at the beginning and end of the time interval, respectively. The benchtop conductivity meter (Jenway, 4520) was used to
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determine the change in the conductivity of feed solution.
Prior to copper ion removal test, the membranes were evaluated using DI water as feed
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solution and 2 M MgCl2 as draw solution. After completion of preliminary experiment, the feed DI water was changed to copper aqueous solution in which the effect of copper ion concentration (i.e., 50, 100 and 150 ppm) on the membrane performance was studied. The
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concentration of copper ions in the draw solution was determined using AAS (Shimadzu,
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AA-7000).
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2.4 Membrane characterization
The top and bottom surface of the membrane wetting characteristics were analysed using
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contact angle goniometer (DataPhysics, OCA) by operating the static contact angle (CA) measurement with using pure water as the probe liquid. The average value of ten measurements were taken at different locations for each membrane at room temperature was reported. A Thermo Scientific Nicolet 5700 was used to record Fourier transform infrared (FTIR) spectra of the top and bottom surface of membranes. The attenuated total reflection (ATR) mode was used to collect and record the spectra in the 600–4000 cm-1 wavenumber with an average of 16 scans and at a resolution of 4 cm-1. For the TFC membranes, the analysis was performed without peeling out the polyamide layer from the non-woven polyester fabric. The cross-sectional structure and surface morphology (top and bottom
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ACCEPTED MANUSCRIPT surface) of the membranes were using scanning electron microscope (SEM) (Hitachi, TM 3000). 3. Results and Discussion 3.1 Pressure-driven membrane process
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Fig. 1 shows the pure water flux and permeability of each membrane. As can be seen, the NF90 membrane exhibits the highest pure water flux and permeability compared to other
Pure Water Flux
Permeability
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80
20
0
NF90
8
4
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40
10
6
MA
60
PT
Pure Water Flux (L/m2. h )
100
2
Permeability (L/m2. h bar)
membranes.
0 NDX
DK
PFO
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Fig. 1. Pure water flux (L/m2.h) of membranes tested at 10 bar and their respective water
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permeability (L/m2.h.bar).
The membrane pure water flux is in order of NF90 (82.3 L/m2.h) > DK (46.5 L/m2.h) > PFO (35.9 L/m2.h) > NDX (16.4 L/m2.h). Quite surprisingly, the NDX membrane that possesses the largest molecular weight cut-off as claimed by the manufacturer produces the lowest water flux. According to Back et al. [32], the unfavorable design of the NDX membrane support, which presents a high resistance for water molecule to flow is the main factor leading to low water permeability. The high flux of NF90 membrane as obtained in this work is in good agreement with the previous studies in which pure water fluxes in the range of 5.63–9.44 L/m2.h.bar were reported [27,33,34]. The water fluxes recorded by DK and PFO membrane meanwhile are similar to the values reported by Al-Moudi et al. [35] and 7
ACCEPTED MANUSCRIPT Motsa et al. [36], respectively. Previous work also indicated that there was no clear correlation between the PFO membrane structural factors and other parameters which led to its high pure water permeability and salt rejection [36].
Water Flux (Salt Solution) Water Flux (Copper Solution) Rejection (Magnesium Chloride) Rejection (Copper)
80
60
40
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20
0 NF90
NDX
80
60
40
Rejection (%)
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100
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Water Flux (L/m2. h)
100
20
0 DK
PFO
Fig. 2. Water flux (L/m2.h) and rejection (%) of membranes evaluated. (Experimental
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conditions: operating pressure: 10 bar, stirring speed: 200 rpm and feed solution
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concentration: 100 ppm MgCl2 or copper ions) Figure 2 displays the properties of membranes with respect to water flux, MgCl2
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rejection and copper rejection. The trend of membrane water flux in filtering salt and copper solution is consistent with the results shown in Fig. 1, i.e., NF90 > DK > PFO > NDX. There
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is a slight reduction in the membrane water flux compared to the pure water flux, owing to the presence of dissolved ions that negatively affects the driving force as a result of concentration polarization (CP) effect [37]. The convective permeate flow usually causes a buildup of solute at the membrane active layer surface and produces higher resistance for water molecules to pass through during pressure-driven membrane process [38]. Therefore, it reduces the water permeability. With respect to separation efficiencies, NF90 membrane shows the highest rejection rates against both types of solutes tested, recording 95.7% and 97.7% for MgCl2 and copper ions, respectively. It is followed by PFO membrane that shows 96.8% rejection for MgCl2 and 99.4% rejection for copper ions. DK membrane can achieve similar copper rejection as 8
ACCEPTED MANUSCRIPT PFO membrane (i.e., 98.6%), but its rejection against MgCl2 is relatively lower, i.e., 86.4%. Among of all the membranes evaluated, NDX membrane demonstrates the lowest separation capabilities with 13.4% and 29.7% rejection for MgCl2 and copper ions, respectively. Generally, the higher rejection rate of NF membranes against copper ion compared to the MgCl2 is mainly due to the large atomic weight (AW) of copper, i.e., 63.55 g/mol. MgCl 2 when dissolved in aqueous solution would dissociate into Mg2+ and Cl- with AW of 24.3 and
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35.5 g/mol, respectively. As the size of the ions is significantly different, the separation mechanism by the NF membrane could also be governed by the sieving effect, in addition to Donnan exclusion effect [39].
The filtration data obtained from the pressure-driven process is critical for the successful implementation of NF membranes for osmotically-driven process. Since MgCl2 will be used as draw solute to draw water molecules from feed solution containing copper ions during
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FO/PRO process, high rejection rate of each solute (~ 90% rejection) is required to ensure only water molecule is permeated through the semi-permeable membrane. The low MgCl2 rejection as shown by the NDX membrane (<20% rejection) makes the membrane not
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suitable to be used for FO/PRO process as high reverse solute flux (permeation of draw solute from draw solution to feed solution) that is expected to take place could affect osmotic
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pressure difference between two solutions. In view of this, only three commercial membranes – NF90, DK and PFO membranes will be further evaluated for their FO/PRO performance in
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the following section.
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3.2 Osmotically-driven membrane process 3.2.1 Pure water as feed solution
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In this experiment, MgCl2 is used as draw solute because it can create higher osmotic pressure difference from its dissociated ions (3 mol of ions (1 mol of Mg2+ and 2 mol of Cl-) from 1 mol of MgCl2), leading to higher water flux [27]. Figure 3 shows the performance of commercial membranes during FO/PRO process using pure water as feed solution and 2M MgCl2 as draw solution. For both FO and PRO experiments, it is found that PFO membrane achieves the highest water flux, Jv followed by NF90 and DK membranes. Furthermore, PFO membrane also shows significantly lower reverse solute flux, J s (0.16–0.27 g/m2.h) than that of NF90 (0.85–1.44 g/m2.h) and DK membranes (0.61–0.84 g/m2.h). The water flux and reverse salt flux of membrane are strongly dependent on the membrane orientation, but
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ACCEPTED MANUSCRIPT generally membrane tested under PRO mode would achieve greater water flux that is associated with an increase in reverse solute flux. This phenomenon can be explained based on the internal concentration polarization (ICP) effect. The ICP effect is less severe in the PRO mode as the feed solution is contacted with the substrate layer instead of dense active layer of membrane in FO mode [40,41]. Referring to the results shown in Fig. 1, it can be seen that PFO membrane outperforms NF90 and DK membrane in terms of water flux in the
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osmotically-driven process. These results further reveal that the entire structure of membrane (top and bottom surface) is important in the FO/PRO process. The significantly lower water flux as experienced by NF90 and DK membranes could be attributed to the existence of nonwoven fabric as the supporting layer for both TFC membranes.
5
3.0
1.5
1.5
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2.0
4
3
3
2
2
1
1
Jv (L/m2h)
2.0
4
Js (g/m2h)
2.5
MA
Jv (L/m2h)
2.5
1.0
5
Jv Js
Jv Js
Js (g/m2h)
3.0
1.0
0.5
0.0 NF90
DK
Membrane
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0.5
0.0
PFO
0 NF90
DK
PFO
Membrane
(b) PRO mode
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(a) FO mode
0
Fig. 3. Performance of three commercial membranes with respect to water flux (J v) and
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reverse solute flux (Js), (a) FO mode and (b) PRO mode (Testing conditions: RO water as feed solution and 2 M MgCl2 as draw solution).
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Figure 4 compares the FTIR spectrum of top and bottom surface of three membranes. As can be clearly seen, no broad peak is detected at 3700–3100 cm-1 on the bottom surface of NF90 and DK membranes in comparison to their respective top surface. This indicates the low hydrophilicity degree of bottom surface compared to the top polyamide layer. Since both membrane surfaces are contacted directly with aqueous solutions (either draw or feed solution depending on membrane orientation), their hydrophilic surfaces are important to minimize ICP impact. Meanwhile, the presence of peaks at wavenumber of 1459 cm-1 (C=C aromatic ring stretching), 1503 cm-1 (CH3-C-CH3 stretching), 1242 cm-1 (asymmetric C-O-C stretching), 1151 cm-1 (symmetric O=S=O stretching) and 1293 cm-1 (asymmetric O=S=O
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ACCEPTED MANUSCRIPT stretching) are corresponded to the specific functional groups of the interlayer of TFC membrane that is made of polysulfone [42,43]. On the other hand, the peaks at 1585 cm-1 and 1552 cm-1 on top surface corresponded to the C-N stretching and C=O stretching, respectively which indicate the characteristics of polyamide layer [44–46]. Peaks detected at 1711, 1241 and 718 cm-1 on the bottom surface confirm the material (polyester) that is used for non-woven fabric fabrication.
N-H and -OH
C=C PFO
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DK
Transmittance (%)
Transmittance (%)
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C-N and C=O
O=S=O C-O-C
N-H and -OH
C-H3-C-CH3 PFO
DK
NF90 4000
3500
3000
2500
2000
1500
500
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Wavenumber (cm-1)
1000
(a)
4000
NF90 3000
2000
1000
Wavenumber (cm-1)
(b)
PT
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Fig. 4. ATR-FTIR spectra of NF90, DK and PFO membrane, (a) bottom surface and (b) top surface.
For the PFO membrane, it is found that both top and bottom surfaces show broad
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peak at 3700–3100 cm-1, owing to overlapping of stretching vibration of N–H and carboxylic groups in the polyamide layer and groups such as O–H [47–49]. The broad peak centred at
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about 3300 cm-1 with much higher intensity is due to the over-abundance of O–H groups in the layers [48,50]. The only difference is the lower peak intensity on the bottom surface in comparison to the top surface. The presence of the O–H groups in the layer indicates the clear improvement in the membrane hydrophilicity which is likely to achieve higher water flux. Looking at the peaks on the both surfaces of PFO membrane could reveal that the entire membrane is made of same polymeric materials. Based on the patent filed by Porifera Inc. [51], the materials used for PFO membrane fabrication are aramid polymers such as meta-aramids and mixture of meta-aramids, e.g., porous Nomex® (poly-meta-phenylene isophthalamide). These kinds of polymers offer several advantages such as excellent membrane formability and flexibility, improved hydrophilicity and enhanced chemical
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ACCEPTED MANUSCRIPT resistance and structural stability, which could result in enhanced anti-fouling property and enhanced water flux [51]. Nevertheless, it must be pointed out that the exact fabrication conditions and properties of PFO membrane are still remained unknown to public owing due to the trade secret in manufacturing the membrane products.
50 � �
(d)
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_1 � �
50 � �
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(c)
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(b)
(a)
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50 � �
AC
CE
PT
(e)
50 � �
(f)
_1 � �
50 � �
50 � �
Fig. 5. SEM images of top (left) and bottom surface (right) of membranes, (a,b) NF90, (c,d) DK and (e,f) PFO (Scale bar: 50 𝜇 m). Higher image resolution (Scale bar: 0 𝜇 m) was provided as the inset in (c) and (f).
Figure 5 presents the SEM images of the top and bottom surface of NF90, DK and PFO membranes. As can be seen, NF90 and PFO membrane had relatively smooth top surfaces compared to DK membrane. The significant difference of the bottom surface of NF90 and DK membrane in comparison to the PFO membrane is due to the presence of non-
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ACCEPTED MANUSCRIPT woven support (loose fibers) that is used to fabricate TFC membrane. Although the existence of non-woven support has minimal impact on the membrane water flux during pressuredriven filtration process, it has great impact on the membrane performance during osmotically-driven filtration process as a result of ICP effect (see Fig. 3). The driving force for osmosis is negatively affected with the presence of thick non-woven support that hinders the water molecules to transfer through membrane [53]. Because of this, the PFO membrane
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that is not supported by non-woven fabric outperformed the TFC membranes to achieve greater water flux as evidenced in this work.
The cross-sectional SEM images of three membranes are also examined and the results are presented in Fig. 6. As can be clearly seen, NF90 and DK membranes have completely different exhibits in structure compared to the PFO membrane. As NF90 and DK membranes are known as TFC membranes with ultrathin polyamide layer deposited on the
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UF substrate, the observation of the spongy-like structure is the typical characteristics of UF substrate made of phase inversion technique. Since irregular voids are found in the NF90 membrane, there is the reason to believe they could play a role in reducing water transport
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resistance, leading to higher water flux compared to the DK membrane as evidenced from Fig. 3. The presence of large micro-voids in the PFO membrane meanwhile is the main factor
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contributing to the remarkable high water flux of this membrane. Its high structural porosity
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CE
PT
could allow water molecules to permeate easily and improve water production rate.
Fig. 6. SEM cross-sectional image of (a) NF90, (b) DK and (c) PFO membrane (Scale bar: 10 𝜇m).
Figure 7 compares the contact angle of the top and bottom surface of three membranes. As can be seen, the contact angle of NF90 top surface (98.1o) is significantly higher compared to the rest of the membranes evaluated (42.2o–50.7o). The high contact angle of NF90 membrane is usually reported in the literature [27,33,48,50,54], but it does not
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ACCEPTED MANUSCRIPT really indicate that such membrane is of hydrophobic. This is because the surface roughness of the membrane could also affect the contact angle obtained due to the Wenzel or Cassie effect as previously reported [55]. With respect to the bottom surface contact angle, all three membranes display very similar value, i.e., 72.5–75.4.
Top Surface
120
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Contact Angle (º)
100
Bottom Surface
80
60
40
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20
NF90
DK
MA
0
PFO
Fig. 7. Contact angle of the membrane top and bottom surface
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3.2.2 Copper aqueous solution as feed solution
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The performance of all membranes are further evaluated by replacing the pure water with 100 ppm copper aqueous solution and the results are shown in Fig. 8. When the copper is
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used as feed solution, the water fluxes of membranes (compared to Fig. 3) are reduced. For instance, PFO membrane displays FO and PRO flux of 1.75 and 2.90 L/m2.h, respectively
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when tested using copper aqueous solution. These values are lower than the results achieved by the same membrane (2.55 and 4.20 L/m2.h) when pure water is used as feed solution. The reduced water flux is mainly because of the presence of ions in the feed solution that negatively affects the osmotic pressure difference between the feed and draw solution [27]. With respect to reverse solute flux, the membranes generally show < 0.5 and < 1.0 g/m2.h when tested under FO and PRO mode, respectively. The minimum J s values indicate the suitable use of NF membranes for osmotically-driven process. In addition, the solution samples randomly taken from the draw solutions are found to contain less than 3 ppm copper ions, regardless of membrane type and orientation. This confirms the promising performance of NF membranes in retaining copper ions from permeating through the membrane from feed
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ACCEPTED MANUSCRIPT to draw solution. The finding is also in line with the high rejection rates achieved by the membranes as shown in Fig. 1 (>97%).
3.0
5
2.5
2.5
4
4
2.0
2.0
3
3
1.5
1.5
2
2
1.0
1.0
0.5
0.5
5
Jv Js
0.0 NF90
DK
PFO
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0.0
Jv (L/m2h)
Js (g/m2h)
Jv (L/m2h)
Jv Js
1
0
NF90
Membrane
Js (g/m2h)
3.0
1
0
DK
PFO
Membrane
(a) FO mode
(b) PRO mode
Fig. 8. Performance of three commercial membranes with respect to water flux (J v) and
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reverse solute flux (Js), (a) FO mode and (b) PRO mode (Testing conditions: 100 ppm copper
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solution as feed solution and 2 M MgCl2 as draw solution). Table 2 Performance of PFO membranes tested with various concentration of copper
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solution. Copper ion
pH
100 ppm
Js/Jv
Js
Js/Jv
(g/m2.h)
(g/L)
(g/m2.h)
(g/L)
6.1
0.279
0.302
0.357
0.244
5.7
0.362
0.202
0.643
0.218
4.3
0.945
0.448
1.384
0.415
AC
150 ppm
CE
50 ppm
PRO mode
Js
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concentration (feed)
FO mode
The effect of copper ions concentration on the performance of PFO membrane is further evaluated and the results are tabulated in Table 2. As can be seen, the increase in the copper ion concentration in the feed solution has negatively increased the reverse solute flux. With increasing copper ion concentration from 50 to 150 ppm, Js value of PFO membrane is increased from 0.279 to 0.945 g/m2.h and from 0.357 to 1.383 g/m2.h for FO and PRO mode, respectively. On the other hand, it is also reported that J s/Jv ratios of membrane are larger at the highest copper ion concentration (150 ppm). Larger ratios of Js/Jv reflect a decrease in the selectivity of the overall membrane and lower efficiency of the process. Nevertheless,
15
ACCEPTED MANUSCRIPT compared to the findings reported in the literature (with Js/Jv ratio > 0.4 [56–59]), our results are satisfactory.
4
Conclusion
In this work, the performances of commercial NF membranes were evaluated for their
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efficiency in engineered osmosis process for copper ion removal under engineered osmosis process. The experimental findings showed that NF90, DK and PFO membranes were able to exhibit promising results in removing copper ions, but the PFO membrane outperformed others with respect to the water permeability. This is mainly due to its good structural integrity coupled with excellent surface properties. Using MgCl2 as draw solutes enabled the PFO membranes to achieve low reverse solute flux without compromising copper ions
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separation efficiency. Higher water flux was able to be achieved by the PFO membrane in PRO mode in comparison to the experiment run in FO mode, owing to the reduced ICP
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effect. By increasing the concentration of the copper in the feed solution from 50 to 150 ppm, it was found that the PFO membrane exhibited increasing Js value for both FO and PRO mode, causing an increase in Js/Jv ratios. Such ratios however were at a satisfactory level.
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With respect to the water sample quality, it was found that the PFO membrane could almost completely retain the copper ion regardless of membrane orientation and copper
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concentration (in the feed solution), offering an alternative solution to treat the industrial
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wastewater without applying external driving force.
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Acknowledgements
The authors gratefully acknowledge the financial support provided by the Malaysian Ministry of Education (MoE) under the Fundamental Research Grant Scheme (Grant no.: R.J130000.7851.5F017) and Universiti Teknologi Malaysia (UTM) under the UTMSHINE Signature Grant (Grant no.: Q.J130000.2451.07G79).
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Like-Forward
Osmosis
Wan Nur Ain Shuhada Abdullah, Sirinan Tiandee, Woei-Jye Lau, Farhana Aziz, Ahmad Fauzi Ismail PII: DOI: Reference:
S1004-9541(19)30791-8 https://doi.org/10.1016/j.cjche.2019.05.016 CJCHE 1522
To appear in:
Chinese Journal of Chemical Engineering
Received date: Revised date: Accepted date:
13 March 2019 12 May 2019 22 May 2019
Please cite this article as: W.N.A.S. Abdullah, S. Tiandee, W.-J. Lau, et al., Potential Use of Nanofiltration Like-Forward Osmosis Membranes for Copper Ion Removal, Chinese Journal of Chemical Engineering, https://doi.org/10.1016/j.cjche.2019.05.016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Potential Use of Nanofiltration Like-Forward Osmosis Membranes for Copper Ion Removal Wan Nur Ain Shuhada Abdullaha,b, Sirinan Tiandeec, Woei-Jye Laua,b,*, Farhana Aziza,b, Ahmad Fauzi Ismaila,b
Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi
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a
Malaysia, 81310 Skudai, Johor, Malaysia b
School of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 Skudai,
Johor, Malaysia c
Faculty of Science and Industrial Technology, Prince of Songkla University, Surat Thani
Campus, 84000 Surat Thani, Thailand
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*Corresponding author: [email protected]; [email protected]
ABSTRACT
The discharge of industrial effluent containing heavy metal ions would cause water pollution
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if such effluent is not properly treated. In this work, the performance of emerging nanofiltration (NF) like-forward osmosis (FO) membrane was evaluated for its efficiency to
PT
remove copper ion from water. Conventionally, copper ion is removed from aqueous solution via adsorption and/or ion-exchange method. The engineered osmosis method as proposed in
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this work considered four commercial NF membranes (i.e., NF90, DK, NDX and PFO) where their separation performances were accessed using synthetic water sample containing 100 mg/L copper ion under FO and pressure retarded osmosis (PRO) orientation. The
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findings indicated that all membranes could achieve almost complete removal of copper regardless of membrane orientation without applying external driving force. The high removal rates were in good agreement with the outcomes of the membranes tested under pressure-driven mode at 10 bar. The use of appropriate salts as draw solutes enabled the NF membranes to be employed in engineered osmosis process, achieving a relatively low reverse solute flux. The findings showed that the best performing membrane is PFO membrane in which it achieved >99.4% copper rejection with very minimum reverse solute flux of <1 g/m2. h.
1
ACCEPTED MANUSCRIPT Keywords: Nanofiltration, forward osmosis, copper, divalent salt, water flux. 1. Introduction
Over the years, the negative impact of heavy metals has been a major concern for the environment and public health. Copper is one of the heavy metal ions commonly found in wastewater discharged from industries such as mining, smelting, semiconductor and
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metallurgy [1,2]. The concentration of copper in the industrial wastewater varies substantially depending on its process. In most of the cases, copper with concentration in the range of 10– 150 ppm is detected in the effluent [3]. In order to reduce its impacts towards environment, wastewater containing copper needs to be treated at an end-of-pipe before discharging to any receiving water bodies.
Although copper is not classified as a carcinogenic material by the United States
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Environmental Protection Agency (US EPA) [4,5], the excessive ingestion of it could bring serious toxicological concerns such as vomiting, cramps, convulsions, nausea, diarrhea,
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epigastric pain, dizziness and possible death. According to the World Health Organisation (WHO), the concentration of copper should be acceptable at the health-based guideline value of 2 mg/L [6]. It is undeniable that copper is an essential nutrient for human, but only very
ED
small quantity of it is needed on a daily basis [7,8]. USA and Canada recently established a recommended dietary allowance (RDA) for human. For examples, RDA for an adult is 900
PT
μg/day while child with less than 3-year old is only 340 μg/day [7]. Conventional treatment methods for copper ions removal such as sorption [9–11], ion
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exchange [12], chemical precipitation [13], flocculation settling [14] and flotation technique [15] are always associated with drawbacks. These include sludge production from the
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chemicals added during treatment, incomplete removal of ions and long treatment period. Over the last decade, membrane technologies have been widely used for water and wastewater treatment process. Nanofiltration (NF) membrane in particular offers a good alternative for separating multi-valent ions from water source [16–18]. Typically, NF membranes are negatively charged at neutral and alkaline conditions. Thus, the separation mechanism of ions by NF membrane is governed by both steric effect (sieving) and charge effects (Donnan exclusion) [19,20]. Since operating pressure of NF process is lower than reverse osmosis (RO) process, it consumes significantly lower energy to achieve the same water flux as RO membrane [21]. With respect to the capability of NF membrane
2
ACCEPTED MANUSCRIPT for copper ions removal, previous studies demonstrated that NF process could attain >98% removal rate [22,23]. There is a potential to use NF membrane in engineered osmosis process that does not require an external driving force during operation, provided an ideal solute is used as draw solute. Similar to the RO-like forward osmosis (FO) membranes which are widely used in treating brackish water and desalination of seawater [24–26], the NF-like FO membranes
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could in principle be used for water treatment. Compared to the RO-like FO membranes, NFlike FO membranes is able to achieve higher water flux owing to the relatively looser selective layer that offers lower water transport resistance. Previously, Abdullah et al. [27] used the commercial NF membranes for FO and PRO process and reported the membrane performance for treating aerobically-treated palm oil mill effluent (AT-POME). The findings showed that the AT-POME's colour compounds could be completely removed by the NF-like
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FO membranes tested using magnesium chloride (MgCl2) as draw solute with relatively high flux and low reverse solute flux.
On the other hand, Su et al. [28] evolved cellulose acetate (CA)-based NF membranes
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used for the engineered osmosis application. It was reported that water fluxes in the range of 1.8–5.0 L/m2.h and 2.7–7.3 L/m2.h could be achieved by the NF membranes tested under FO
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and PRO mode, respectively, using 0.5–2.0 M MgCl2 draw solutions. Furthermore, Setiawan et al. [29] discovered that different types of in-house made NF-like FO hollow fiber membranes could also be used to achieve higher water flow. When pure water and 0.5 M
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MgCl2 solution were used as feed and draw solution, respectively, these newly developed membranes achieved FO water flux of 9.74 L/m2.h. When a large amount of solute (up to 5 M
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MgCl2) was employed in the draw solution, the water flux of the NF-like FO membrane was further increased [30].
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The primary objective of the present study is to evaluate the potential of NF membranes in engineered osmosis process for copper ion removal under FO and PRO orientation. Four commercial membranes, i.e., NF90, DK, NDX and PFO membranes were characterized and assessed under FO and PRO orientation and their performances were further compared with the findings obtained from pressure-driven process. 2. Methodology 2.1 Nanofiltration membranes
3
ACCEPTED MANUSCRIPT Table 1 shows the information of four NF membranes used in this work for the copper ion removal. Four flat sheet membranes were purchased from different manufacturers and were in the dry form. These membrane were then stored in water at room temperature for several days before being used for evaluation. The pore size and polymer material of each membrane are given by the manufacturer, except for PFO membrane which remains
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unknown.
Table 1 Properties of membranes used in this work for copper ion removal Membrane
Membrane
Polymer
NF90
Dow Filmtec™
Polyamide
TFC
~200–400
~184–214
NDX
Synder™
Polyamide
TFC
~800–1,000
~125–165
DK
GE Osmonics™
Polyamide
TFC
~150–300
~156–184
PFO
Porifera Inc
Nomex®
n/a
~30–100
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Structure
Asymmetric
thickness (m)
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Designation
MWCO (Da)
Overall
Manufacturer
2.2. Pressure-driven membrane filtration
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Before proceeding to FO/PRO tests, all the membranes were evaluated with respect to water flux and solute rejection in dead-end permeation cell (Sterlitech, HP4750) following the procedure described in our previous publication [27]. Firstly, the loaded membrane
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(effective surface area: 14.62 cm2) was compacted at 11 bar for 30 min to achieve stable flux conditions. After completing the compaction, the membrane pure water flux, 𝐽𝑣 (L/m2.h) and
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respectively [31]:
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water permeability, 𝐴 (L/m2.h.bar) was determined at 10 bar using Equation (1) and (2),
∆V 𝐴𝑚 × ∆t 𝐽 A= 𝑣 ∆P 𝐽𝑣 =
(1) (2)
where 𝐴𝑚 is the effective membrane area and ∆𝑃, ∆𝑡 and ∆𝑉 is trans-membrane pressure difference, time interval and permeate volume difference, respectively The membrane rejections against MgCl2 (Sigma-Aldrich) and copper ions (Fisher Scientific) were also determined by subjecting the membrane to the filtration of 100 ppm
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ACCEPTED MANUSCRIPT single solute (either MgCl2 or copper ions) at 10 bar for 30 min. The rejections of MgCl2 and copper ion are important as it could indicate whether the membranes are suitable for engineered osmosis process. The water flux of membrane during salt solution filtration process was calculated using Equation (1) while its solute rejection, R (%) was calculated based on the following equation [26]:
𝑐𝑓−𝑐𝑝 𝑐𝑓 × 100
(3)
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𝑅=
where 𝑐𝑓 and 𝑐𝑝 is the salt concentration of feed and permeate sample, respectively. The concentration solution samples (for MgCl2 rejection) were evaluated by concentration conversion based on the conductivity against salt concentration calibration curve. Before the
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concentration conversion, the conductivity of the solution samples was first evaluated by a benchtop conductivity meter (Jenway, 4520). The concentration of copper ions in the aqueous solutions meanwhile was determined using atomic absorption spectroscope (AAS,
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were reported for each membrane.
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Shimadzu, AA-7000). The average water flux and rejection obtained in 30-min experiment
2.3 Osmotically-driven membrane filtration
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For FO and PRO process, the customized lab-scale cross-flow filtration setup described in our previous publication was used [27]. Both cross-flow velocities of the feed and draw
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solution was fixed at 32.72 cm/s. Two high-precision gear pumps (LongerPump, WT3001JA) were used to circulate the feed and draw solutions, respectively. The designed FO
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system has a total effective membrane area of 20.02 cm2 (in rectangular dimension) in the cross-flow cell. A digital weight balance (Amput, 457A) was placed at the draw solution tank (2-L capacity) in order to precisely record the amount of water drawn from the feed solution (either pure water or copper aqueous solution) to the draw solution. All the membranes were evaluated under two different orientations, i.e., FO mode (active-layer-facing-feed-solution, AL-FS) and PRO mode (active-layer-facing-draw-solution, AL-DS). The 𝐽𝑣 obtained from different mode could be measured using Equation 3 [26]: ∆V 𝐽𝑣 = 𝐴 × ∆t 𝑚
(4)
5
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where 𝐴𝑚 is the effective membrane area, 𝜌 is the feed solution density, and ∆𝑉, ∆𝑚 and ∆𝑡 is the volume change of draw solution, weight change of draw solution and time interval, respectively. The average of three replications with 20-min time interval for each run was used to determine the membrane water flux. The following equation was used to calculate the reverse solute flux, 𝐽𝑠 (g/m2.h) of membrane under FO or PRO mode [26]:
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∆(𝐶𝑡𝑉𝑡) 𝐽𝑠 = 𝐴 × 𝑚 ∆t
(5)
where 𝐴𝑚 is the effective membrane area, ∆𝑡 is the time interval, 𝐶𝑡 and 𝑉𝑡 are the change of feed concentration and change of feed solution volume measured at the beginning and end of the time interval, respectively. The benchtop conductivity meter (Jenway, 4520) was used to
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determine the change in the conductivity of feed solution.
Prior to copper ion removal test, the membranes were evaluated using DI water as feed
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solution and 2 M MgCl2 as draw solution. After completion of preliminary experiment, the feed DI water was changed to copper aqueous solution in which the effect of copper ion concentration (i.e., 50, 100 and 150 ppm) on the membrane performance was studied. The
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concentration of copper ions in the draw solution was determined using AAS (Shimadzu,
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AA-7000).
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2.4 Membrane characterization
The top and bottom surface of the membrane wetting characteristics were analysed using
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contact angle goniometer (DataPhysics, OCA) by operating the static contact angle (CA) measurement with using pure water as the probe liquid. The average value of ten measurements were taken at different locations for each membrane at room temperature was reported. A Thermo Scientific Nicolet 5700 was used to record Fourier transform infrared (FTIR) spectra of the top and bottom surface of membranes. The attenuated total reflection (ATR) mode was used to collect and record the spectra in the 600–4000 cm-1 wavenumber with an average of 16 scans and at a resolution of 4 cm-1. For the TFC membranes, the analysis was performed without peeling out the polyamide layer from the non-woven polyester fabric. The cross-sectional structure and surface morphology (top and bottom
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ACCEPTED MANUSCRIPT surface) of the membranes were using scanning electron microscope (SEM) (Hitachi, TM 3000). 3. Results and Discussion 3.1 Pressure-driven membrane process
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Fig. 1 shows the pure water flux and permeability of each membrane. As can be seen, the NF90 membrane exhibits the highest pure water flux and permeability compared to other
Pure Water Flux
Permeability
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80
20
0
NF90
8
4
ED
40
10
6
MA
60
PT
Pure Water Flux (L/m2. h )
100
2
Permeability (L/m2. h bar)
membranes.
0 NDX
DK
PFO
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Fig. 1. Pure water flux (L/m2.h) of membranes tested at 10 bar and their respective water
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permeability (L/m2.h.bar).
The membrane pure water flux is in order of NF90 (82.3 L/m2.h) > DK (46.5 L/m2.h) > PFO (35.9 L/m2.h) > NDX (16.4 L/m2.h). Quite surprisingly, the NDX membrane that possesses the largest molecular weight cut-off as claimed by the manufacturer produces the lowest water flux. According to Back et al. [32], the unfavorable design of the NDX membrane support, which presents a high resistance for water molecule to flow is the main factor leading to low water permeability. The high flux of NF90 membrane as obtained in this work is in good agreement with the previous studies in which pure water fluxes in the range of 5.63–9.44 L/m2.h.bar were reported [27,33,34]. The water fluxes recorded by DK and PFO membrane meanwhile are similar to the values reported by Al-Moudi et al. [35] and 7
ACCEPTED MANUSCRIPT Motsa et al. [36], respectively. Previous work also indicated that there was no clear correlation between the PFO membrane structural factors and other parameters which led to its high pure water permeability and salt rejection [36].
Water Flux (Salt Solution) Water Flux (Copper Solution) Rejection (Magnesium Chloride) Rejection (Copper)
80
60
40
MA
20
0 NF90
NDX
80
60
40
Rejection (%)
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100
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Water Flux (L/m2. h)
100
20
0 DK
PFO
Fig. 2. Water flux (L/m2.h) and rejection (%) of membranes evaluated. (Experimental
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conditions: operating pressure: 10 bar, stirring speed: 200 rpm and feed solution
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concentration: 100 ppm MgCl2 or copper ions) Figure 2 displays the properties of membranes with respect to water flux, MgCl2
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rejection and copper rejection. The trend of membrane water flux in filtering salt and copper solution is consistent with the results shown in Fig. 1, i.e., NF90 > DK > PFO > NDX. There
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is a slight reduction in the membrane water flux compared to the pure water flux, owing to the presence of dissolved ions that negatively affects the driving force as a result of concentration polarization (CP) effect [37]. The convective permeate flow usually causes a buildup of solute at the membrane active layer surface and produces higher resistance for water molecules to pass through during pressure-driven membrane process [38]. Therefore, it reduces the water permeability. With respect to separation efficiencies, NF90 membrane shows the highest rejection rates against both types of solutes tested, recording 95.7% and 97.7% for MgCl2 and copper ions, respectively. It is followed by PFO membrane that shows 96.8% rejection for MgCl2 and 99.4% rejection for copper ions. DK membrane can achieve similar copper rejection as 8
ACCEPTED MANUSCRIPT PFO membrane (i.e., 98.6%), but its rejection against MgCl2 is relatively lower, i.e., 86.4%. Among of all the membranes evaluated, NDX membrane demonstrates the lowest separation capabilities with 13.4% and 29.7% rejection for MgCl2 and copper ions, respectively. Generally, the higher rejection rate of NF membranes against copper ion compared to the MgCl2 is mainly due to the large atomic weight (AW) of copper, i.e., 63.55 g/mol. MgCl 2 when dissolved in aqueous solution would dissociate into Mg2+ and Cl- with AW of 24.3 and
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35.5 g/mol, respectively. As the size of the ions is significantly different, the separation mechanism by the NF membrane could also be governed by the sieving effect, in addition to Donnan exclusion effect [39].
The filtration data obtained from the pressure-driven process is critical for the successful implementation of NF membranes for osmotically-driven process. Since MgCl2 will be used as draw solute to draw water molecules from feed solution containing copper ions during
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FO/PRO process, high rejection rate of each solute (~ 90% rejection) is required to ensure only water molecule is permeated through the semi-permeable membrane. The low MgCl2 rejection as shown by the NDX membrane (<20% rejection) makes the membrane not
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suitable to be used for FO/PRO process as high reverse solute flux (permeation of draw solute from draw solution to feed solution) that is expected to take place could affect osmotic
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pressure difference between two solutions. In view of this, only three commercial membranes – NF90, DK and PFO membranes will be further evaluated for their FO/PRO performance in
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the following section.
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3.2 Osmotically-driven membrane process 3.2.1 Pure water as feed solution
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In this experiment, MgCl2 is used as draw solute because it can create higher osmotic pressure difference from its dissociated ions (3 mol of ions (1 mol of Mg2+ and 2 mol of Cl-) from 1 mol of MgCl2), leading to higher water flux [27]. Figure 3 shows the performance of commercial membranes during FO/PRO process using pure water as feed solution and 2M MgCl2 as draw solution. For both FO and PRO experiments, it is found that PFO membrane achieves the highest water flux, Jv followed by NF90 and DK membranes. Furthermore, PFO membrane also shows significantly lower reverse solute flux, J s (0.16–0.27 g/m2.h) than that of NF90 (0.85–1.44 g/m2.h) and DK membranes (0.61–0.84 g/m2.h). The water flux and reverse salt flux of membrane are strongly dependent on the membrane orientation, but
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ACCEPTED MANUSCRIPT generally membrane tested under PRO mode would achieve greater water flux that is associated with an increase in reverse solute flux. This phenomenon can be explained based on the internal concentration polarization (ICP) effect. The ICP effect is less severe in the PRO mode as the feed solution is contacted with the substrate layer instead of dense active layer of membrane in FO mode [40,41]. Referring to the results shown in Fig. 1, it can be seen that PFO membrane outperforms NF90 and DK membrane in terms of water flux in the
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osmotically-driven process. These results further reveal that the entire structure of membrane (top and bottom surface) is important in the FO/PRO process. The significantly lower water flux as experienced by NF90 and DK membranes could be attributed to the existence of nonwoven fabric as the supporting layer for both TFC membranes.
5
3.0
1.5
1.5
NU
2.0
4
3
3
2
2
1
1
Jv (L/m2h)
2.0
4
Js (g/m2h)
2.5
MA
Jv (L/m2h)
2.5
1.0
5
Jv Js
Jv Js
Js (g/m2h)
3.0
1.0
0.5
0.0 NF90
DK
Membrane
ED
0.5
0.0
PFO
0 NF90
DK
PFO
Membrane
(b) PRO mode
PT
(a) FO mode
0
Fig. 3. Performance of three commercial membranes with respect to water flux (J v) and
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reverse solute flux (Js), (a) FO mode and (b) PRO mode (Testing conditions: RO water as feed solution and 2 M MgCl2 as draw solution).
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Figure 4 compares the FTIR spectrum of top and bottom surface of three membranes. As can be clearly seen, no broad peak is detected at 3700–3100 cm-1 on the bottom surface of NF90 and DK membranes in comparison to their respective top surface. This indicates the low hydrophilicity degree of bottom surface compared to the top polyamide layer. Since both membrane surfaces are contacted directly with aqueous solutions (either draw or feed solution depending on membrane orientation), their hydrophilic surfaces are important to minimize ICP impact. Meanwhile, the presence of peaks at wavenumber of 1459 cm-1 (C=C aromatic ring stretching), 1503 cm-1 (CH3-C-CH3 stretching), 1242 cm-1 (asymmetric C-O-C stretching), 1151 cm-1 (symmetric O=S=O stretching) and 1293 cm-1 (asymmetric O=S=O
10
ACCEPTED MANUSCRIPT stretching) are corresponded to the specific functional groups of the interlayer of TFC membrane that is made of polysulfone [42,43]. On the other hand, the peaks at 1585 cm-1 and 1552 cm-1 on top surface corresponded to the C-N stretching and C=O stretching, respectively which indicate the characteristics of polyamide layer [44–46]. Peaks detected at 1711, 1241 and 718 cm-1 on the bottom surface confirm the material (polyester) that is used for non-woven fabric fabrication.
N-H and -OH
C=C PFO
NU
DK
Transmittance (%)
Transmittance (%)
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C-N and C=O
O=S=O C-O-C
N-H and -OH
C-H3-C-CH3 PFO
DK
NF90 4000
3500
3000
2500
2000
1500
500
MA
Wavenumber (cm-1)
1000
(a)
4000
NF90 3000
2000
1000
Wavenumber (cm-1)
(b)
PT
ED
Fig. 4. ATR-FTIR spectra of NF90, DK and PFO membrane, (a) bottom surface and (b) top surface.
For the PFO membrane, it is found that both top and bottom surfaces show broad
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peak at 3700–3100 cm-1, owing to overlapping of stretching vibration of N–H and carboxylic groups in the polyamide layer and groups such as O–H [47–49]. The broad peak centred at
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about 3300 cm-1 with much higher intensity is due to the over-abundance of O–H groups in the layers [48,50]. The only difference is the lower peak intensity on the bottom surface in comparison to the top surface. The presence of the O–H groups in the layer indicates the clear improvement in the membrane hydrophilicity which is likely to achieve higher water flux. Looking at the peaks on the both surfaces of PFO membrane could reveal that the entire membrane is made of same polymeric materials. Based on the patent filed by Porifera Inc. [51], the materials used for PFO membrane fabrication are aramid polymers such as meta-aramids and mixture of meta-aramids, e.g., porous Nomex® (poly-meta-phenylene isophthalamide). These kinds of polymers offer several advantages such as excellent membrane formability and flexibility, improved hydrophilicity and enhanced chemical
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ACCEPTED MANUSCRIPT resistance and structural stability, which could result in enhanced anti-fouling property and enhanced water flux [51]. Nevertheless, it must be pointed out that the exact fabrication conditions and properties of PFO membrane are still remained unknown to public owing due to the trade secret in manufacturing the membrane products.
50 � �
(d)
MA
_1 � �
50 � �
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(c)
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(b)
(a)
ED
50 � �
AC
CE
PT
(e)
50 � �
(f)
_1 � �
50 � �
50 � �
Fig. 5. SEM images of top (left) and bottom surface (right) of membranes, (a,b) NF90, (c,d) DK and (e,f) PFO (Scale bar: 50 𝜇 m). Higher image resolution (Scale bar: 0 𝜇 m) was provided as the inset in (c) and (f).
Figure 5 presents the SEM images of the top and bottom surface of NF90, DK and PFO membranes. As can be seen, NF90 and PFO membrane had relatively smooth top surfaces compared to DK membrane. The significant difference of the bottom surface of NF90 and DK membrane in comparison to the PFO membrane is due to the presence of non-
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ACCEPTED MANUSCRIPT woven support (loose fibers) that is used to fabricate TFC membrane. Although the existence of non-woven support has minimal impact on the membrane water flux during pressuredriven filtration process, it has great impact on the membrane performance during osmotically-driven filtration process as a result of ICP effect (see Fig. 3). The driving force for osmosis is negatively affected with the presence of thick non-woven support that hinders the water molecules to transfer through membrane [53]. Because of this, the PFO membrane
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that is not supported by non-woven fabric outperformed the TFC membranes to achieve greater water flux as evidenced in this work.
The cross-sectional SEM images of three membranes are also examined and the results are presented in Fig. 6. As can be clearly seen, NF90 and DK membranes have completely different exhibits in structure compared to the PFO membrane. As NF90 and DK membranes are known as TFC membranes with ultrathin polyamide layer deposited on the
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UF substrate, the observation of the spongy-like structure is the typical characteristics of UF substrate made of phase inversion technique. Since irregular voids are found in the NF90 membrane, there is the reason to believe they could play a role in reducing water transport
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resistance, leading to higher water flux compared to the DK membrane as evidenced from Fig. 3. The presence of large micro-voids in the PFO membrane meanwhile is the main factor
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contributing to the remarkable high water flux of this membrane. Its high structural porosity
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CE
PT
could allow water molecules to permeate easily and improve water production rate.
Fig. 6. SEM cross-sectional image of (a) NF90, (b) DK and (c) PFO membrane (Scale bar: 10 𝜇m).
Figure 7 compares the contact angle of the top and bottom surface of three membranes. As can be seen, the contact angle of NF90 top surface (98.1o) is significantly higher compared to the rest of the membranes evaluated (42.2o–50.7o). The high contact angle of NF90 membrane is usually reported in the literature [27,33,48,50,54], but it does not
13
ACCEPTED MANUSCRIPT really indicate that such membrane is of hydrophobic. This is because the surface roughness of the membrane could also affect the contact angle obtained due to the Wenzel or Cassie effect as previously reported [55]. With respect to the bottom surface contact angle, all three membranes display very similar value, i.e., 72.5–75.4.
Top Surface
120
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Contact Angle (º)
100
Bottom Surface
80
60
40
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20
NF90
DK
MA
0
PFO
Fig. 7. Contact angle of the membrane top and bottom surface
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3.2.2 Copper aqueous solution as feed solution
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The performance of all membranes are further evaluated by replacing the pure water with 100 ppm copper aqueous solution and the results are shown in Fig. 8. When the copper is
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used as feed solution, the water fluxes of membranes (compared to Fig. 3) are reduced. For instance, PFO membrane displays FO and PRO flux of 1.75 and 2.90 L/m2.h, respectively
AC
when tested using copper aqueous solution. These values are lower than the results achieved by the same membrane (2.55 and 4.20 L/m2.h) when pure water is used as feed solution. The reduced water flux is mainly because of the presence of ions in the feed solution that negatively affects the osmotic pressure difference between the feed and draw solution [27]. With respect to reverse solute flux, the membranes generally show < 0.5 and < 1.0 g/m2.h when tested under FO and PRO mode, respectively. The minimum J s values indicate the suitable use of NF membranes for osmotically-driven process. In addition, the solution samples randomly taken from the draw solutions are found to contain less than 3 ppm copper ions, regardless of membrane type and orientation. This confirms the promising performance of NF membranes in retaining copper ions from permeating through the membrane from feed
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ACCEPTED MANUSCRIPT to draw solution. The finding is also in line with the high rejection rates achieved by the membranes as shown in Fig. 1 (>97%).
3.0
5
2.5
2.5
4
4
2.0
2.0
3
3
1.5
1.5
2
2
1.0
1.0
0.5
0.5
5
Jv Js
0.0 NF90
DK
PFO
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0.0
Jv (L/m2h)
Js (g/m2h)
Jv (L/m2h)
Jv Js
1
0
NF90
Membrane
Js (g/m2h)
3.0
1
0
DK
PFO
Membrane
(a) FO mode
(b) PRO mode
Fig. 8. Performance of three commercial membranes with respect to water flux (J v) and
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reverse solute flux (Js), (a) FO mode and (b) PRO mode (Testing conditions: 100 ppm copper
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solution as feed solution and 2 M MgCl2 as draw solution). Table 2 Performance of PFO membranes tested with various concentration of copper
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solution. Copper ion
pH
100 ppm
Js/Jv
Js
Js/Jv
(g/m2.h)
(g/L)
(g/m2.h)
(g/L)
6.1
0.279
0.302
0.357
0.244
5.7
0.362
0.202
0.643
0.218
4.3
0.945
0.448
1.384
0.415
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150 ppm
CE
50 ppm
PRO mode
Js
PT
concentration (feed)
FO mode
The effect of copper ions concentration on the performance of PFO membrane is further evaluated and the results are tabulated in Table 2. As can be seen, the increase in the copper ion concentration in the feed solution has negatively increased the reverse solute flux. With increasing copper ion concentration from 50 to 150 ppm, Js value of PFO membrane is increased from 0.279 to 0.945 g/m2.h and from 0.357 to 1.383 g/m2.h for FO and PRO mode, respectively. On the other hand, it is also reported that J s/Jv ratios of membrane are larger at the highest copper ion concentration (150 ppm). Larger ratios of Js/Jv reflect a decrease in the selectivity of the overall membrane and lower efficiency of the process. Nevertheless,
15
ACCEPTED MANUSCRIPT compared to the findings reported in the literature (with Js/Jv ratio > 0.4 [56–59]), our results are satisfactory.
4
Conclusion
In this work, the performances of commercial NF membranes were evaluated for their
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efficiency in engineered osmosis process for copper ion removal under engineered osmosis process. The experimental findings showed that NF90, DK and PFO membranes were able to exhibit promising results in removing copper ions, but the PFO membrane outperformed others with respect to the water permeability. This is mainly due to its good structural integrity coupled with excellent surface properties. Using MgCl2 as draw solutes enabled the PFO membranes to achieve low reverse solute flux without compromising copper ions
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separation efficiency. Higher water flux was able to be achieved by the PFO membrane in PRO mode in comparison to the experiment run in FO mode, owing to the reduced ICP
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effect. By increasing the concentration of the copper in the feed solution from 50 to 150 ppm, it was found that the PFO membrane exhibited increasing Js value for both FO and PRO mode, causing an increase in Js/Jv ratios. Such ratios however were at a satisfactory level.
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With respect to the water sample quality, it was found that the PFO membrane could almost completely retain the copper ion regardless of membrane orientation and copper
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concentration (in the feed solution), offering an alternative solution to treat the industrial
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wastewater without applying external driving force.
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Acknowledgements
The authors gratefully acknowledge the financial support provided by the Malaysian Ministry of Education (MoE) under the Fundamental Research Grant Scheme (Grant no.: R.J130000.7851.5F017) and Universiti Teknologi Malaysia (UTM) under the UTMSHINE Signature Grant (Grant no.: Q.J130000.2451.07G79).
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