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Development of graphene oxide-based membrane as a pretreatment for thermal seawater desalination
Desalination 465 (2019) 13–24
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Development of graphene oxide-based membrane as a pretreatment for thermal seawater desalination
T
Bassel A. Abdelkadera, Mohamed A. Antara, , Tahar Laouib,a, , Zafarullah Khana ⁎
a b
⁎⁎
Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Desalination Pretreatment Graphene oxide Polyethersulfone Nanofiltration Top brine temperature
Scaling constitutes a major concern in seawater desalination. In thermal desalination, scaling is limiting the top brine temperature (TBT). Brine temperature limitation depends on the concentration of divalent ions. This paper reports the synthesis of graphene oxide (GO)/polyacrylamide (PAM)/polyethersulfone (PES) membrane, in which GO layer is deposited on the surface of PES substrate using PAM as an adhesive layer, to be used as a pretreatment step to filter the divalent ions in thermal seawater desalination application. GO/PAM-PES membrane is prepared via spin coating technique, then reduced to rGO/PAM-PES by subjecting the membrane to hydrogen iodide (HI). PAM is used as an adhesive layer onto PES membrane (an ultrafiltration membrane) to improve rGO layer attachment onto its surface. The results indicate that increasing pressure increases both the rejection and the permeate flux. Using seawater, rGO/PAM-PES yields a higher rejection for Mg2+ and Ca2+ ions compared with commercial nanofiltration membrane (NF270). The pretreatment step allows the increase in TBT to 148 °C using NF, 160 °C using NF-GO and 166 °C using rGO membranes.
1. Introduction One of the major problems in water desalination technologies, whether thermal or membrane, is the relatively high cost associated with its operation due to scaling and fouling. Scaling depends on temperature, pH, total dissolved salt (TDS), and the concentration of divalent ions. Scaling occurs at supersaturation, proceeds with ⁎
nucleation and crystal growth up to scale precipitation [1]. Supersaturation is a state of a solution that contains more ions than what could be dissolved by the solvent (e.g. water) at a given temperature and pressure. Top brine temperature (TBT) is usually about 112 °C in multi-stage flash (MSF) and 66 °C in multiple-effect distillation (MED). Scaling in thermal desalination can be divided into two types: soft scale (magnesium hydroxide and calcium carbonate) and hard scale (calcium
Corresponding author. Correspondence to: T. Laoui, Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates. E-mail addresses: [email protected] (M.A. Antar), [email protected] (T. Laoui).
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https://doi.org/10.1016/j.desal.2019.04.028 Received 22 December 2018; Received in revised form 30 March 2019; Accepted 23 April 2019 Available online 01 May 2019 0011-9164/ © 2019 Elsevier B.V. All rights reserved.
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thermal desalination process. NF membranes were previously studied as pretreatment step in several desalination processes. Membrane fouling, pore size and raw material measurement, membrane performance and modeling were previously addressed [3]. Llenas et al. [18,19] examined the performance of several NF membranes. Their results showed that the sulfate had the highest rejection. Mabrouk et al. [20] investigated numerically and experimentally a NF-MSF system heated by solar concentrators. The system had four solar concentrators in series with a tracking system. NF permeate was heated by hot oil using a heat exchanger. The unit was investigated at different TBT varying from 60 to 100 °C. GOR increased with TBT and the highest achieved GOR was 15 at 100 °C. Aghigh et al. [17] discussed the development in graphene used in water desalination by means of novel methods such as nano-porous graphene (NG) sheets as well as capacitive deionization (CDI). They found that uniform NG sheets could be used for water filtration and desalination. Ali et al. [21] studied thin film composite membranes prepared from m-phenylenediamine (MPD) and 1,3,5-benzene tricarbonyl chloride or trimesoyl chloride (TMC) as displayed in Table 2 by interfacial polymerization on the surface of a polysulfone substrate, graphene oxide (GO) was embedded into the membrane to improve the performance. A portion of the available data for GO reinforced MPD/ TMC membranes is reported in Table 2. The ion rejection percentage of the GO membrane depends on the operating parameters, ions composition and membrane characteristics. It is clear that eliminating divalent ions increases TBT, and consequently increases the gain output ratio (GOR). Therefore, the objective of this paper is to develop and investigate the performance of a graphene oxide (GO)/polyacrylamide (PAM)/polyethersulfone (PES) membrane, in which a GO layer is deposited on the surface of a commercial PES substrate (UF membrane) using PAM as an adhesive layer, and compare it with a commercially used NF membrane (NF270). Moreover, NF-GO is produced by depositing a GO layer on NF 270 to enhance the antifouling and antibacterial properties, with NF270 being the active (also called selective) layer. However, for 2rGO/PAM-PES membrane (2rGO means two deposited layers of rGO), the rGO is the active layer, beside its high antifouling and antibacterial properties. The performance of the developed membrane is assessed via cross flow filtration test using magnesium sulfate (MgSO4) solution, calcium
Table 1 Hydrated diameter for different ions in seawater. Ion
Ca2+
Mg2+
SO42−
Na+
K+
Cl−
Hydrated diameter [nm]
– 0.824 0.824 0.82 0.824
– 0.856 – 0.86 0.856
– – 0.758 – –
0.716 0.716 – 0.72 0.716
0.602 0.662 0.662 0.66 0.662
0.582 0.664 0.664 0.66 0.664
Reference [5] [6] [7] [8] [9]
sulfate) [2,3]. Therefore, eliminating Mg, Ca and SO4 will enable us to increase the TBT in thermal seawater desalination plants safely. The composition of seawater with salinity of 36,000 ppm includes monovalent ions that represent almost 87%, whereas the divalent ions contribute to 13% of the composition [4]. To increase TBT, all divalent ions (calcium, sulfate, magnesium) should be removed from seawater. It is important to state that the smallest hydrated diameter of divalent ion (sulfate) is 0.758 nm, and the largest diameter of monovalent ion (sodium) is 0.716 nm as shown in Table 1. A common method used as a pretreatment step for different desalination processes is nanofiltration (NF) since it has high rejection to the divalent ions, which are the reason of scale formation. Sulfate scaling is a crucial problem to any desalination process as it precipitates on the tubes or membranes. Accordingly, the energy consumption and cost of desalination processes increase and, it may also lead to re-tubing for thermal plants or changing the membranes for membrane type of desalination [4]. Graphene oxide (GO) is very thin; only one carbon atom thick, and successfully used for molecular-sieving yielding a high flux [10]. GO layers are known to be chemically and thermally stable [11–13]. Thus, it could potentially be used to reject divalent ions. GO is an oxygen, hydrogen and carbon compound of different proportions [14]. GO coated membrane is basically GO flakes stacked over each other. Water passes between these flakes as shown in Fig. 1. Layer spacing between GO layers is higher than layer spacing of graphite [15]. By using reduced graphene oxide (rGO), the interlayer spacing can be controlled [16]. Thus, by reducing the interlayer spacing to < 0.75 nm, corresponding to sulfate hydrated diameter (Table 1), we can eliminate most of the divalent ions, responsible for brine temperature limitation in
Fig. 1. Transport of ions and molecules in GO membrane [17]. 14
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Table 2 Performance of graphene oxide membranes. Membrane
GO concentration ppm
Pressure (bar)
NaCl conc. (ppm)
Water flux (L/m2·h)
Salt rejection (%)
Water contact angle (°)
Reference
MPD/TMC MPD/TMC MPD/TMC MPD/TMC MPD/TMC MPD/TMC
76 100 1000 2000 20,000 50
15 15 55 15 15.5 15
2000 2000 32,000 2000 2000 2000
16.6 29.6 28 22 14 43.3
99 98 98 88 96 98.87
47 56 55.5 65 26 56.5
[22] [23] [24] [25] [26] [27]
sulfate (CaSO4) solution and actual seawater. Furthermore, the effect of pretreatment on TBT was evaluated using PHREEQC software.
Table 3 Concentration of the prepared solutions. PAM GO MgSO4 CaSO4
1.34 mg/ml DI water 0.5 mg/ml in DI water 2500 ppm 2500 ppm
2. Experimental work 2.1. Material PES membrane with a pore size of 30 nm, and 47 mm diameter was purchased from Sterlitech, USA. NF270 was acquired from Dow water & process solutions, USA. A highly concentrated graphene oxide dispersion in water (60 ml) with concentration of 5.5 mg/ml was purchased from Graphene Supermarket Calverton, USA, and polyacrylamide (PAM) from Polyscience, USA. Magnesium sulfate heptahydrate extra pure, calcium sulfate, hydriodic acid 55% AR with stabilizer, and hydrogen iodide (HI) solution were acquired from Loba Chemie, India. A conductivity meter supplied by eDAQ was used. The conductivity meter was calibrated for MgSO4 and for CaSO4 solutions. The calibration graphs generated from a series of standard solutions were produced using MgSO4 and the corresponding conductivity values were measured in millisiemens (mS). The surface of the coated GO layers was analyzed by optical microscopy, whereas the surface of the GO layers was observed using a field emission scanning electron microscope (FESEM), Xray diffraction (XRD), and Raman spectroscopy.
Table 4 Procedure used for developing rGO/PAM-PES membrane. Solutions
GO/PAM-PES membrane rGO/PAM-PES membrane
PAM
0.2 g PAM - 150 ml of DI water
GO
0.5 mg/ml
Spin coating
200 rpm for 120 s - 3000 rpm for 360 s Deposition 1 ml PAM – > washing – > 1 ml GO GO/PAM-PES membrane subjected to HI vapor
Fig. 2. Sequence followed for developing rGO/PAM-PES membrane.
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Fig. 3. Schematic diagram of the cross-flow test.
Fig. 4. Cross-flow permeation test system used for membrane testing.
2.2. Preparation of solutions GO solution was prepared from highly concentrated GO (5.5 mg/ ml) and deionized (DI) water to obtain a 0.5 mg/ml solution, which was stirred for 20 min to be homogeneous. PAM solution was prepared by adding 0.2 g of PAM to 150 ml DI water, and was stirred for 30 min. Magnesium sulfate and calcium sulfate solutions were prepared by adding 12.5 g of MgSO4 or CaSO4 to 5 L of DI water followed by stirring for 30 min to produce a 2500 ppm MgSO4/CaSO4 solution. Table 3 presents the solutions concentration used in this work. 2.3. GO/PAM-PES and rGO/PAM-PES
Fig. 5. Effect of pressure using MgSO4 solution on the a) rejection, b) permeate flux.
The spin coater used is SCS G3 Spin Coater. The PES membrane with a diameter of 47 mm was washed by DI water, then placed in the spin coater. Then, 1 ml of PAM solution was added and left to spin at 200 rpm for 2 min. Then spinning was increased to 3000 rpm for 6 min. It was then washed by 2 ml of DI water and placed on the spinner at 3000 rpm for 6 min. Then, 1 ml of GO solution was deposited over the membrane at 200 rpm for 2 min, rotation speed was then increased to 3000 rpm for 6 min. The last step was repeated twice. Table 4 and Fig. 2 present the procedure of coating the ultrafiltration membrane. The interlayer spacing is controlled in reduced graphene oxide (rGO) membranes by subjecting the GO membrane to HI vapor.
2.4. Membrane performance The performance of the developed membranes was evaluated by a cross flow test using MgSO4 solution, CaSO4 solution and real seawater. The cross-flow system consists of a test cell, variable speed pump, pressure gauge and a relief valve. The variable speed pump was purchased from Eldex, California, USA with speed ranging between 0.01 and 10 ml/min and handling a pressure up to 40 bar. The membrane effective area of the cell used was 4.906 cm2, the pressure was 16
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Fig. 7. Effect of pressure using CaSO4 solution on the a) rejection, b) permeate flux.
Fig. 6. Effect of inlet flow rate using MgSO4 solution on the a) rejection, b) permeate flux.
controlled by a relief valve as shown in Fig. 3 and Fig. 4. There is a safety relief valve calibrated at 40 bar. The solution was pumped through the system for 1.5 h to reach steady state. The effect of pressure and the inlet flow rate on the rejection and permeate flux were studied for MgSO4 solution, CaSO4 solution and real seawater. The rejection and flux were measured at five values of pressure starting with 5 bar with a step of 2.5 and four value of inlet flow rate starting with 2.5 ml/ min with a step of 2.5. The rejection for the MgSO4 and CaSO4 solutions were measured by the conductivity meter while for the seawater the cations were measured using inductively coupled plasma mass spectrometry (ICP-MS) while the anions were measured using ionic chromatography (IC). On the other hand, the permeate flux was calculated by dividing the measured output volume over time and membrane area. The developed membranes were compared with a commercial NF membrane. Three samples of each membrane were tested for a given condition.
for 6 min. It was then washed by 2 ml of DI water and placed on the spinner at 3000 rpm for another 6 min period. Afterwards, 1 ml of GO solution (0.5 mg/ml) was deposited over the membrane at 200 rpm for 2 mins then at 3000 rpm for 6 min. The difference between NF-GO and 2rGO/PAM-PES membranes is that GO on the NF 270 membrane was used to enhance the antifouling and antibacterial properties, since NF270 has a thin polyamide layer acting as the active (selective) layer. However, for 2rGO/PAM-PES membrane, the rGO is the active layer beside its high antifouling and antibacterial properties. The effect of pressure and the inlet flow rate on the rejection and the permeate flux were studied. The rejection and flux were measured at five values of pressure starting with 5 bar with a step of 2.5 bar for four value of inlet flow rate starting with 2.5 ml/min with step of 2.5 ml/ min. Rejection was measured by the conductivity meter, whereas, the permeate flux was calculated by dividing the measured output volume over time and membrane effective area.
2.5. Membrane testing
2.6. TBT model using PHREEQC
The developed membrane 2rGO/PAM-PES, commercial nanofiltration (NF270), and NF-GO membrane were tested. The NF270 membrane with an area 4*4 cm was washed by DI water and then placed in a spin coater. Then, 1 ml of PAM solution was added and left to spin at 200 rpm for 2 min, then the rotation speed was increased to 3000 rpm
PHREEQC is a widely used geochemical modeling software available from the USGS, PHREEQC. It is based on an ion-association aqueous model. It has the ability to calculate speciation and saturation index (SI). SI indicates the state of equilibrium with respect to a given mineral. SI is expressed as 17
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Fig. 8. Effect of inlet flow rate using CaSO4 solution on the a) rejection, b) permeate flux. Table 5 Composition of the used seawater. Ions +
Na Mg2+ K+ Ca2+ Cl− SO42−
SI = log
IAP K
ppm 13905 1564 400 520 27917 3312
(1)
When a mineral is in equilibrium within a solution, SI is zero: a negative SI indicates undersaturation, and a positive SI indicates supersaturation. The ion activity product (IAP) is calculated as
IAP =
C cD d Aa B b
Fig. 9. Effect of pressure using seawater on the rejection a) NF270, b) NF-GO, c) 2rGO/PAM-PES.
(2)
where A, B, C and D are activities of the ions and a, b, c and d denote the respective stoichiometric values. The IAP involves the actual activities. For example, to get the IAP for CaSO4, we multiply the actual activity of Ca by the actual activity of SO4. The interpretation of IAP is the following:
1. IAP > K: The reaction is progressing from left to right, producing more products. This state is also described as supersaturation. 2. IAP = K: The reaction is in equilibrium, and there is equal flow of reaction to the right and to the left 18
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Fig. 11. Effect of a) pressure and b) inlet flow rate on the permeate flux using seawater.
3. IAP < K: The reaction is progressing from right to left, producing more reactants. This state is also described as undersaturation. With the SI approach, it is possible to predict the reactive mineralogy from the water composition. If the SI for a mineral is less than zero, the aqueous solution is undersaturated with respect to that mineral - which corresponds to the state where the mineral may dissolve to reach equilibrium concentration. If the SI is greater than zero, then the mineral may precipitate from the aqueous solution (oversaturated). To conclude, when the SI is close to zero, the water is in near- saturation with respect to that mineral [28]. 3. Results and discussion The performance of the developed membranes was investigated by studying the effect of operating parameters (pressure and inlet flow rate) on the ion rejection and permeate flux. The performance of different membranes was investigated and compared. The ion rejection and permeate flux were compared for MgSO4, CaSO4 and seawater. Moreover, the permeate was analyzed to estimate the TBT for MSF desalination system.
Fig. 10. Effect of inlet flow rate using seawater on the rejection a) NF270, b) NF-GO, c) 2rGO/PAM-PES.
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Fig. 12. SEM micrographs for GO/PAM-PES and 2rGO/PAM-PES along with water contact angle (inset).
3.1. Membrane performance using MgSO4 solution
rejection at high pressure (> 10 bar). 2rGO/PAM-PES exhibits almost the same rejection for both MgSO4 and CaSO4 solutions. Furthermore, NF-GO provides the highest permeate flux as shown in Fig. 7b. The effect of inlet flow rate on the rejection and permeate flux was evaluated for 2rGO/PAM-PES, and compared with NF270 and NF-GO membranes at 10 bar using CaSO4 solution, as displayed in Fig. 8. Increasing inlet flow rate induces only a slight increase in rejection rate, but has more influence on permeate flux particularly at 10 ml/min inlet flow rate. 2rGO/PAM-PES yields the highest rejection for Ca2+ and SO42− ions. NF-GO yields also a high rejection close to that of 2rGO/ PAM-PES. The rate of increase in the permeate flux increases as the inlet flow rate increases as displayed in Fig. 8.
The effect of operating pressure on the ion rejection and permeate flux using MgSO4 is shown in Fig. 5. The developed membrane 2rGO/ PAM-PES was tested and compared with the commercial nanofiltration membrane (NF270), and the prepared NF-GO membrane. The operating pressure was varied from 5 to 15 bar at constant inlet flow rate of 10 ml/min. The results indicate that increasing pressure increases both rejection and permeate flux. This can be explained using the established transport mechanisms through NF membranes, namely convection and diffusion: increasing pressure, the contribution of convection overpowers diffusion. This is because of the high water flux; leading to an increase in the rejection. NF270 yields the highest permeate flux and rejection for Mg2+ and SO42− ions, whereas, NF-GO has more ion rejection than 2rGO/PAM-PES except at 15 bar. The ion rejection of 2rGO/PAM-PES membrane increases significantly with pressure compared to NF270 and NF-GO. For instance, increasing pressure from 5 to 15 bar increases the ion rejection by 40% for 2rGO/PAM-PES and only about 5% for NF270 and NF-GO. The NF270 and NF-GO membranes have the same ion rejection trend. This is due to the fact that both have the same active layer. The antifouling properties are enhanced by adding GO layer to the NF270 membrane as GO has good antifouling properties. However, for the 2rGO/PAM-PES membrane, the active layer is the rGO. The effect of inlet flow rate on the rejection and permeate flux using MgSO4 was evaluated as shown in Fig. 6. The inlet flow rate is varied from 2.5 to 10 ml/min, with a step of 2.5 ml/min at 10 bar. Increasing inlet flow rate, slightly increases the rejection. NF270 yields the highest permeate flux and rejection for Mg2+ and SO42− ions. The rate of increase in the permeate flux decreases as the inlet flow rate increases.
3.3. Membrane performance using seawater The effect of pressure and the inlet flow rate on the rejection and permeate flux was studied using actual seawater. The rejection of the cations was measured using ICP-MS, and the anions using IC. The permeate flux was calculated by dividing the measured output volume over time and membrane effective area. The seawater composition is presented in Table 5. A comparison of the three membranes was performed at 10 ml/min inlet flow rate. The membrane 2rGO/PAM-PES yields the highest rejection for Mg2+ and Ca2+ ions while NF270 has the highest rejection for SO42− ions. On the other hand, NF-GO has more Ca2+ rejection than NF270, while for Mg2+ both membranes exhibit a similar rejection. The membrane 2rGO/PAM-PES ion rejection increases significantly with increasing pressure as shown in Fig. 9. The reported mechanisms for the solute transport through NF membrane are convection, diffusion, and electro-migration [29–31]. The low rejection for Ca2+ ions using NF270 is due to the concentration polarization, where a high Ca2+ concentration layer forms at the membrane surface, leading to more Ca2+ ions diffusing through the membrane. For GO and rGO membranes, water passes between interlayer spacing. Therefore, ion rejection depends on the interlayer spacing and electrostatic interactions [17]. The membrane 2rGO/PAM-PES has almost the same rejection for Mg2+ and Ca2+ ions, while SO42− ions has a higher rejection due electrostatic interaction since GO and SO42− ions are both negatively charged. However, at high pressure the Mg2+, Ca2+
3.2. Membrane performance using CaSO4 solution The membrane 2rGO/PAM-PES was tested at different pressures (5–15 bar) with CaSO4 solution and compared with commercial nanofiltration (NF270) and NF-GO membranes at 10 ml/min inlet flow rate, as displayed in Fig. 7. As expected, increasing pressure increases both the rejection and the permeate flux. NF270 has the lowest rejection for Ca2+ and SO42− ions. Whereas, NF-GO has more ion rejection than 2rGO/PAM-PES at low pressure, while, 2rGO/PAM-PES has the highest 20
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flux increases as the pressure and inlet flow rate increase as shown in Fig. 11. 3.4. Membrane surface characterization Interlayer spacing can be controlled through the reduction process. Subjecting GO membrane to HI vapor removes some of the hydroxyl, carbonyl and carboxyl groups, yielding a decrease in the interlayer spacing. Therefore, the characterization of GO/PAM-PES and 2rGO/ PAM-PES are discussed. Fig. 12 presents the SEM images for GO/PAMPES and 2rGO/PAM-PES. The effect of water contact angle on reduction was studied for 2rGO/PAM-PES membrane. It was found that rGO had a higher contact angle compared to that of GO. The oxygen function group increases the hydrophilicity of the membrane, which increases water flux. XRD results indicate that GO/PAM-PES membrane (under dry condition) has a peak at 2Ɵ of 10.10 whereas the 2rGO/PAM-PES membrane has a peak at 2Ɵ of 26.020 as shown in Fig. 13-a. The interlayer spacing may be calculated using Bragg equation,
= 2dsin( )
(3)
where λ is the wavelength of the X-ray beam, d the interlayer spacing, and Ɵ the diffraction angle. GO in GO/PAM-PES membrane has a d-spacing of 0.854 nm, whereas rGO in 2rGO/PAM-PES membrane a d-spacing of 0.342 nm. However, the d-spacing of GO and rGO membranes in wet condition can be significantly larger than that in dry condition [32,33]. The structure of GO was investigated by Raman spectroscopy. GO exhibits two main bands labelled/called D and G. The D band is related to the degree of order/disorder because of a breathing k-point phonon of A1g symmetry. On the other hand, the G band is related to the stacking structure [34]. Raman spectroscopy was performed to comprehend the difference between GO, and rGO. The results are presented in Fig. 13-b. The intensity ID/IG ratio relates to sp3/sp2 carbon ratio. For 2rGO/PAM-PES, it has less hydroxyl, carbonyl and carboxyl groups, which explains the increase in ID/IG ratio. The cross-sectional SEM images for 2rGO/PAM-PES membrane show that the active layer (rGO) thickness is about 70 nm as shown in Fig. 14. Fig. 15 displays the SEM images and EDX analysis for the membrane surface before and after the cross-flow test using seawater. The SEM images indicate that there is no significant change in the surface after performing the test, which indicates a stable coating. On the other hand, the EDX results confirm the presence of some ions that are attached to the surface; specifically, chloride, potassium and magnesium. However, there is no calcium and sulfate ions attached on the surface due to high rejection rate of calcium and sulfate ions that are carried away by the flowing stream during the cross-flow experiment.
Fig. 13. a) XRD patterns b) Raman spectra for GO/PAM-PES and 2rGO/PAMPES.
and SO42− ions have almost a similar rejection as shown in Fig. 9. The effect of inlet flow rate on the rejection and permeate flux was measured for 2rGO/PAM-PES and compared with NF270 and NF-GO membranes at 10 bar. Increasing inlet flow rate slightly increases rejection. 2rGO/PAM-PES membrane yields the highest rejection for Ca2+ ions while for Mg2+ the rejection increases significantly with inlet flow rate. NF-GO exhibits a high Mg2+ rejection close to that of NF270, which has the lowest Ca2+ rejection as displayed in Fig. 10. Permeate
Fig. 14. Cross-sectional SEM micrograph for 2rGO/PAM-PES membrane. 21
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Fig. 15. SEM micrographs for 2rGO/PAM-PES membrane a) before and, b) after cross flow test, along with corresponding EDX analysis.
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Fig. 17. Effect of pressure on top brine temperature.
NF, NF-GO and 2rGO/PAM-PES permeates was calculated using PHREEQC at different pressures (5–15 bar) as shown in Fig. 16. When the saturation index is zero, the solution is in equilibrium and the corresponding temperature is considered as the top TBT. The TBT before treatment was 102 °C without antiscalant. After treatment, TBT reached 148 °C using NF, 160 °C using NF-GO and 166 °C using rGO membrane as displayed in Fig. 17. Increasing TBT, the performance ratio (PR) increases monotonically with number of stages in MSF and MED systems [35]. 4. Conclusion A membrane composed of graphene oxide (GO)/polyacrylamide (PAM) over polyethersulfone (PES) substrate is developed to be utilized as a pretreatment step for thermal seawater desalination for preventing the passage of divalent ions. The GO/PAM-PES membrane is prepared via spin coating technique, then reduced to 2rGO/PAM-PES by subjecting the membrane to hydrogen iodide. The advantage of using 2rGO/PAM-PES compared to NF membrane is that similar high rejection rates (for calcium, magnesium and sulfate ions) could be achieved. In fact, the rejection rate achieved for calcium and magnesium ions is considerably higher than that of NF membrane. In general, increasing pressure increases both the rejection and the permeate flux. Using seawater, the 2rGO/PAM-PES membrane yields the highest rejection rate for Mg2+ and Ca2+ ions compared with NF270. The 2rGO/PAM-PES membrane has almost the same rejection rate for Mg2+ and Ca2+ ions, while a higher rejection for SO42− ions is due to electrostatic interaction, GO and SO42− ions being both negatively charged. However, at high pressure the Mg2+, Ca2+ and SO42− ions have similar rejection rates. On the other hand, the low rejection for Ca2+ ions using NF270 membrane is due to the concentration polarization, a high Ca2+ concentration develops in the layer near the membrane surface. Thus, more Ca2+ ions tend to diffuse through the membrane. The ultrafiltration (UF) membrane (the PES substrate) coated with rGO yields the highest TBT without scaling for MSF. Using a membrane as a pretreatment step increases TBT to 148 °C using NF, 160 °C using GO-NF and 166 °C using 2rGO/PAM-PES. Therefore, one could postulate that UF membrane coated with rGO is an adequate pretreatment step that would be a potential replacement for the use of NF filtration in thermal seawater desalination.
Fig. 16. Effect of pressure on the saturation index for a) NF270, b) NF-GO, c) 2rGO/PAM-PES.
3.5. Top brine temperature analysis Brine temperature limitation in thermal seawater desalination technologies depends on the presence and concentration of divalent ions. TBT is usually about 112 °C in MSF. In order to calculate the TBT for the permeate of the three membranes, the saturation index for the 23
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Acknowledgement
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The support provided by KFUPM to carry out this work is gratefully acknowledged. References [1] N. Voutchkov, Desalination Engineering Planning and Design, (2013) no. 9. [2] J. Cotruvo, N. Voutchkov, J. Fawell, P. Payment, D. Cunliffe, S. Lattemann, Desalination Technology Health and Environmental Impacts, (2010). [3] B.A. Abdelkader, M.A. Antar, Z. Khan, Nanofiltration as a pretreatment step in seawater desalination: a review, Arab. J. Sci. Eng. (2018), https://doi.org/10.1007/ s13369-018-3096-3. [4] H.T. El-Dessouky, H.M. Ettouney, Fundamentals of Salt Water Desalination, vol. 1, (2002). [5] M.-H. E.Y, Physical Chemistry (Book II). – M, Foreign literature Press, 1962. [6] B. Zhu, Z. Hong, N. Milne, C.M. Doherty, L. Zou, Y.S. Lin, A.J. Hill, X. Gu, M. Duke, Desalination of seawater ion complexes by MFI-type zeolite membranes: temperature and long term stability, J. Membr. Sci. 453 (2014) 126–135. [7] O. Bakajin, A. Noy, F. Fornasiero, H.G. Park, J.K. Holt, S. Kim, Membranes With Functionalized Carbon Nanotube Pores for Selective Transport, (2011). [8] K. Xu, B. Feng, C. Zhou, A. Huang, Synthesis of highly stable graphene oxide membranes on polydopamine functionalized supports for seawater desalination, Chem. Eng. Sci. 146 (2016) 159–165. [9] A.G. Volkov, S. Paula, D.W. Deamer, Two mechanisms of permeation of small neutral molecules and hydrated ions across phospholipid bilayers, Bioelectrochem. Bioenerg. 42 (2) (1997) 153–160. [10] H. Li, Z. Song, X. Zhang, Y. Huang, S. Li, Y. Mao, H.J. Ploehn, Y. Bao, M. Yu, Ultrathin, molecular-sieving graphene oxide membranes for selective hydrogen separation, Science 342 (6154) (2013) 95–98 Oct. [11] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, S.T. Nguyen, R.S. Ruoff, Preparation and characterization of graphene oxide paper, Nature 448 (7152) (2007) 457–460 Jul. [12] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (5887) (2008) 385–388 Jul. [13] S. Chen, L. Brown, M. Levendorf, W. Cai, S.-Y. Ju, J. Edgeworth, X. Li, C.W. Magnuson, A. Velamakanni, R.D. Piner, J. Kang, J. Park, R.S. Ruoff, Oxidation resistance of graphene-coated Cu and Cu/Ni alloy, ACS Nano 5 (2) (2011) 1321–1327 Feb. [14] W.H. Jr, R. Offeman, Preparation of graphitic oxide, J. Am. Chem. 80 (6) (1958) 1339. [15] S. Bano, A. Mahmood, S.-J. Kim, K.-H. Lee, Graphene oxide modified polyamide nanofiltration membrane with improved flux and antifouling properties, J. Mater. Chem. A 3 (5) (2015) 2065–2071. [16] A. Altaee, G. Zaragoza, H.R. van Tonningen, Comparison between forward osmosisreverse osmosis and reverse osmosis processes for seawater desalination, Desalination 336 (2014) 50–57. [17] A. Aghigh, V. Alizadeh, H.Y. Wong, M.S. Islam, N. Amin, M. Zaman, Recent advances in utilization of graphene for filtration and desalination of water: a review,
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Development of graphene oxide-based membrane as a pretreatment for thermal seawater desalination
T
Bassel A. Abdelkadera, Mohamed A. Antara, , Tahar Laouib,a, , Zafarullah Khana ⁎
a b
⁎⁎
Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Desalination Pretreatment Graphene oxide Polyethersulfone Nanofiltration Top brine temperature
Scaling constitutes a major concern in seawater desalination. In thermal desalination, scaling is limiting the top brine temperature (TBT). Brine temperature limitation depends on the concentration of divalent ions. This paper reports the synthesis of graphene oxide (GO)/polyacrylamide (PAM)/polyethersulfone (PES) membrane, in which GO layer is deposited on the surface of PES substrate using PAM as an adhesive layer, to be used as a pretreatment step to filter the divalent ions in thermal seawater desalination application. GO/PAM-PES membrane is prepared via spin coating technique, then reduced to rGO/PAM-PES by subjecting the membrane to hydrogen iodide (HI). PAM is used as an adhesive layer onto PES membrane (an ultrafiltration membrane) to improve rGO layer attachment onto its surface. The results indicate that increasing pressure increases both the rejection and the permeate flux. Using seawater, rGO/PAM-PES yields a higher rejection for Mg2+ and Ca2+ ions compared with commercial nanofiltration membrane (NF270). The pretreatment step allows the increase in TBT to 148 °C using NF, 160 °C using NF-GO and 166 °C using rGO membranes.
1. Introduction One of the major problems in water desalination technologies, whether thermal or membrane, is the relatively high cost associated with its operation due to scaling and fouling. Scaling depends on temperature, pH, total dissolved salt (TDS), and the concentration of divalent ions. Scaling occurs at supersaturation, proceeds with ⁎
nucleation and crystal growth up to scale precipitation [1]. Supersaturation is a state of a solution that contains more ions than what could be dissolved by the solvent (e.g. water) at a given temperature and pressure. Top brine temperature (TBT) is usually about 112 °C in multi-stage flash (MSF) and 66 °C in multiple-effect distillation (MED). Scaling in thermal desalination can be divided into two types: soft scale (magnesium hydroxide and calcium carbonate) and hard scale (calcium
Corresponding author. Correspondence to: T. Laoui, Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates. E-mail addresses: [email protected] (M.A. Antar), [email protected] (T. Laoui).
⁎⁎
https://doi.org/10.1016/j.desal.2019.04.028 Received 22 December 2018; Received in revised form 30 March 2019; Accepted 23 April 2019 Available online 01 May 2019 0011-9164/ © 2019 Elsevier B.V. All rights reserved.
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thermal desalination process. NF membranes were previously studied as pretreatment step in several desalination processes. Membrane fouling, pore size and raw material measurement, membrane performance and modeling were previously addressed [3]. Llenas et al. [18,19] examined the performance of several NF membranes. Their results showed that the sulfate had the highest rejection. Mabrouk et al. [20] investigated numerically and experimentally a NF-MSF system heated by solar concentrators. The system had four solar concentrators in series with a tracking system. NF permeate was heated by hot oil using a heat exchanger. The unit was investigated at different TBT varying from 60 to 100 °C. GOR increased with TBT and the highest achieved GOR was 15 at 100 °C. Aghigh et al. [17] discussed the development in graphene used in water desalination by means of novel methods such as nano-porous graphene (NG) sheets as well as capacitive deionization (CDI). They found that uniform NG sheets could be used for water filtration and desalination. Ali et al. [21] studied thin film composite membranes prepared from m-phenylenediamine (MPD) and 1,3,5-benzene tricarbonyl chloride or trimesoyl chloride (TMC) as displayed in Table 2 by interfacial polymerization on the surface of a polysulfone substrate, graphene oxide (GO) was embedded into the membrane to improve the performance. A portion of the available data for GO reinforced MPD/ TMC membranes is reported in Table 2. The ion rejection percentage of the GO membrane depends on the operating parameters, ions composition and membrane characteristics. It is clear that eliminating divalent ions increases TBT, and consequently increases the gain output ratio (GOR). Therefore, the objective of this paper is to develop and investigate the performance of a graphene oxide (GO)/polyacrylamide (PAM)/polyethersulfone (PES) membrane, in which a GO layer is deposited on the surface of a commercial PES substrate (UF membrane) using PAM as an adhesive layer, and compare it with a commercially used NF membrane (NF270). Moreover, NF-GO is produced by depositing a GO layer on NF 270 to enhance the antifouling and antibacterial properties, with NF270 being the active (also called selective) layer. However, for 2rGO/PAM-PES membrane (2rGO means two deposited layers of rGO), the rGO is the active layer, beside its high antifouling and antibacterial properties. The performance of the developed membrane is assessed via cross flow filtration test using magnesium sulfate (MgSO4) solution, calcium
Table 1 Hydrated diameter for different ions in seawater. Ion
Ca2+
Mg2+
SO42−
Na+
K+
Cl−
Hydrated diameter [nm]
– 0.824 0.824 0.82 0.824
– 0.856 – 0.86 0.856
– – 0.758 – –
0.716 0.716 – 0.72 0.716
0.602 0.662 0.662 0.66 0.662
0.582 0.664 0.664 0.66 0.664
Reference [5] [6] [7] [8] [9]
sulfate) [2,3]. Therefore, eliminating Mg, Ca and SO4 will enable us to increase the TBT in thermal seawater desalination plants safely. The composition of seawater with salinity of 36,000 ppm includes monovalent ions that represent almost 87%, whereas the divalent ions contribute to 13% of the composition [4]. To increase TBT, all divalent ions (calcium, sulfate, magnesium) should be removed from seawater. It is important to state that the smallest hydrated diameter of divalent ion (sulfate) is 0.758 nm, and the largest diameter of monovalent ion (sodium) is 0.716 nm as shown in Table 1. A common method used as a pretreatment step for different desalination processes is nanofiltration (NF) since it has high rejection to the divalent ions, which are the reason of scale formation. Sulfate scaling is a crucial problem to any desalination process as it precipitates on the tubes or membranes. Accordingly, the energy consumption and cost of desalination processes increase and, it may also lead to re-tubing for thermal plants or changing the membranes for membrane type of desalination [4]. Graphene oxide (GO) is very thin; only one carbon atom thick, and successfully used for molecular-sieving yielding a high flux [10]. GO layers are known to be chemically and thermally stable [11–13]. Thus, it could potentially be used to reject divalent ions. GO is an oxygen, hydrogen and carbon compound of different proportions [14]. GO coated membrane is basically GO flakes stacked over each other. Water passes between these flakes as shown in Fig. 1. Layer spacing between GO layers is higher than layer spacing of graphite [15]. By using reduced graphene oxide (rGO), the interlayer spacing can be controlled [16]. Thus, by reducing the interlayer spacing to < 0.75 nm, corresponding to sulfate hydrated diameter (Table 1), we can eliminate most of the divalent ions, responsible for brine temperature limitation in
Fig. 1. Transport of ions and molecules in GO membrane [17]. 14
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Table 2 Performance of graphene oxide membranes. Membrane
GO concentration ppm
Pressure (bar)
NaCl conc. (ppm)
Water flux (L/m2·h)
Salt rejection (%)
Water contact angle (°)
Reference
MPD/TMC MPD/TMC MPD/TMC MPD/TMC MPD/TMC MPD/TMC
76 100 1000 2000 20,000 50
15 15 55 15 15.5 15
2000 2000 32,000 2000 2000 2000
16.6 29.6 28 22 14 43.3
99 98 98 88 96 98.87
47 56 55.5 65 26 56.5
[22] [23] [24] [25] [26] [27]
sulfate (CaSO4) solution and actual seawater. Furthermore, the effect of pretreatment on TBT was evaluated using PHREEQC software.
Table 3 Concentration of the prepared solutions. PAM GO MgSO4 CaSO4
1.34 mg/ml DI water 0.5 mg/ml in DI water 2500 ppm 2500 ppm
2. Experimental work 2.1. Material PES membrane with a pore size of 30 nm, and 47 mm diameter was purchased from Sterlitech, USA. NF270 was acquired from Dow water & process solutions, USA. A highly concentrated graphene oxide dispersion in water (60 ml) with concentration of 5.5 mg/ml was purchased from Graphene Supermarket Calverton, USA, and polyacrylamide (PAM) from Polyscience, USA. Magnesium sulfate heptahydrate extra pure, calcium sulfate, hydriodic acid 55% AR with stabilizer, and hydrogen iodide (HI) solution were acquired from Loba Chemie, India. A conductivity meter supplied by eDAQ was used. The conductivity meter was calibrated for MgSO4 and for CaSO4 solutions. The calibration graphs generated from a series of standard solutions were produced using MgSO4 and the corresponding conductivity values were measured in millisiemens (mS). The surface of the coated GO layers was analyzed by optical microscopy, whereas the surface of the GO layers was observed using a field emission scanning electron microscope (FESEM), Xray diffraction (XRD), and Raman spectroscopy.
Table 4 Procedure used for developing rGO/PAM-PES membrane. Solutions
GO/PAM-PES membrane rGO/PAM-PES membrane
PAM
0.2 g PAM - 150 ml of DI water
GO
0.5 mg/ml
Spin coating
200 rpm for 120 s - 3000 rpm for 360 s Deposition 1 ml PAM – > washing – > 1 ml GO GO/PAM-PES membrane subjected to HI vapor
Fig. 2. Sequence followed for developing rGO/PAM-PES membrane.
15
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Fig. 3. Schematic diagram of the cross-flow test.
Fig. 4. Cross-flow permeation test system used for membrane testing.
2.2. Preparation of solutions GO solution was prepared from highly concentrated GO (5.5 mg/ ml) and deionized (DI) water to obtain a 0.5 mg/ml solution, which was stirred for 20 min to be homogeneous. PAM solution was prepared by adding 0.2 g of PAM to 150 ml DI water, and was stirred for 30 min. Magnesium sulfate and calcium sulfate solutions were prepared by adding 12.5 g of MgSO4 or CaSO4 to 5 L of DI water followed by stirring for 30 min to produce a 2500 ppm MgSO4/CaSO4 solution. Table 3 presents the solutions concentration used in this work. 2.3. GO/PAM-PES and rGO/PAM-PES
Fig. 5. Effect of pressure using MgSO4 solution on the a) rejection, b) permeate flux.
The spin coater used is SCS G3 Spin Coater. The PES membrane with a diameter of 47 mm was washed by DI water, then placed in the spin coater. Then, 1 ml of PAM solution was added and left to spin at 200 rpm for 2 min. Then spinning was increased to 3000 rpm for 6 min. It was then washed by 2 ml of DI water and placed on the spinner at 3000 rpm for 6 min. Then, 1 ml of GO solution was deposited over the membrane at 200 rpm for 2 min, rotation speed was then increased to 3000 rpm for 6 min. The last step was repeated twice. Table 4 and Fig. 2 present the procedure of coating the ultrafiltration membrane. The interlayer spacing is controlled in reduced graphene oxide (rGO) membranes by subjecting the GO membrane to HI vapor.
2.4. Membrane performance The performance of the developed membranes was evaluated by a cross flow test using MgSO4 solution, CaSO4 solution and real seawater. The cross-flow system consists of a test cell, variable speed pump, pressure gauge and a relief valve. The variable speed pump was purchased from Eldex, California, USA with speed ranging between 0.01 and 10 ml/min and handling a pressure up to 40 bar. The membrane effective area of the cell used was 4.906 cm2, the pressure was 16
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Fig. 7. Effect of pressure using CaSO4 solution on the a) rejection, b) permeate flux.
Fig. 6. Effect of inlet flow rate using MgSO4 solution on the a) rejection, b) permeate flux.
controlled by a relief valve as shown in Fig. 3 and Fig. 4. There is a safety relief valve calibrated at 40 bar. The solution was pumped through the system for 1.5 h to reach steady state. The effect of pressure and the inlet flow rate on the rejection and permeate flux were studied for MgSO4 solution, CaSO4 solution and real seawater. The rejection and flux were measured at five values of pressure starting with 5 bar with a step of 2.5 and four value of inlet flow rate starting with 2.5 ml/ min with a step of 2.5. The rejection for the MgSO4 and CaSO4 solutions were measured by the conductivity meter while for the seawater the cations were measured using inductively coupled plasma mass spectrometry (ICP-MS) while the anions were measured using ionic chromatography (IC). On the other hand, the permeate flux was calculated by dividing the measured output volume over time and membrane area. The developed membranes were compared with a commercial NF membrane. Three samples of each membrane were tested for a given condition.
for 6 min. It was then washed by 2 ml of DI water and placed on the spinner at 3000 rpm for another 6 min period. Afterwards, 1 ml of GO solution (0.5 mg/ml) was deposited over the membrane at 200 rpm for 2 mins then at 3000 rpm for 6 min. The difference between NF-GO and 2rGO/PAM-PES membranes is that GO on the NF 270 membrane was used to enhance the antifouling and antibacterial properties, since NF270 has a thin polyamide layer acting as the active (selective) layer. However, for 2rGO/PAM-PES membrane, the rGO is the active layer beside its high antifouling and antibacterial properties. The effect of pressure and the inlet flow rate on the rejection and the permeate flux were studied. The rejection and flux were measured at five values of pressure starting with 5 bar with a step of 2.5 bar for four value of inlet flow rate starting with 2.5 ml/min with step of 2.5 ml/ min. Rejection was measured by the conductivity meter, whereas, the permeate flux was calculated by dividing the measured output volume over time and membrane effective area.
2.5. Membrane testing
2.6. TBT model using PHREEQC
The developed membrane 2rGO/PAM-PES, commercial nanofiltration (NF270), and NF-GO membrane were tested. The NF270 membrane with an area 4*4 cm was washed by DI water and then placed in a spin coater. Then, 1 ml of PAM solution was added and left to spin at 200 rpm for 2 min, then the rotation speed was increased to 3000 rpm
PHREEQC is a widely used geochemical modeling software available from the USGS, PHREEQC. It is based on an ion-association aqueous model. It has the ability to calculate speciation and saturation index (SI). SI indicates the state of equilibrium with respect to a given mineral. SI is expressed as 17
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Fig. 8. Effect of inlet flow rate using CaSO4 solution on the a) rejection, b) permeate flux. Table 5 Composition of the used seawater. Ions +
Na Mg2+ K+ Ca2+ Cl− SO42−
SI = log
IAP K
ppm 13905 1564 400 520 27917 3312
(1)
When a mineral is in equilibrium within a solution, SI is zero: a negative SI indicates undersaturation, and a positive SI indicates supersaturation. The ion activity product (IAP) is calculated as
IAP =
C cD d Aa B b
Fig. 9. Effect of pressure using seawater on the rejection a) NF270, b) NF-GO, c) 2rGO/PAM-PES.
(2)
where A, B, C and D are activities of the ions and a, b, c and d denote the respective stoichiometric values. The IAP involves the actual activities. For example, to get the IAP for CaSO4, we multiply the actual activity of Ca by the actual activity of SO4. The interpretation of IAP is the following:
1. IAP > K: The reaction is progressing from left to right, producing more products. This state is also described as supersaturation. 2. IAP = K: The reaction is in equilibrium, and there is equal flow of reaction to the right and to the left 18
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Fig. 11. Effect of a) pressure and b) inlet flow rate on the permeate flux using seawater.
3. IAP < K: The reaction is progressing from right to left, producing more reactants. This state is also described as undersaturation. With the SI approach, it is possible to predict the reactive mineralogy from the water composition. If the SI for a mineral is less than zero, the aqueous solution is undersaturated with respect to that mineral - which corresponds to the state where the mineral may dissolve to reach equilibrium concentration. If the SI is greater than zero, then the mineral may precipitate from the aqueous solution (oversaturated). To conclude, when the SI is close to zero, the water is in near- saturation with respect to that mineral [28]. 3. Results and discussion The performance of the developed membranes was investigated by studying the effect of operating parameters (pressure and inlet flow rate) on the ion rejection and permeate flux. The performance of different membranes was investigated and compared. The ion rejection and permeate flux were compared for MgSO4, CaSO4 and seawater. Moreover, the permeate was analyzed to estimate the TBT for MSF desalination system.
Fig. 10. Effect of inlet flow rate using seawater on the rejection a) NF270, b) NF-GO, c) 2rGO/PAM-PES.
19
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Fig. 12. SEM micrographs for GO/PAM-PES and 2rGO/PAM-PES along with water contact angle (inset).
3.1. Membrane performance using MgSO4 solution
rejection at high pressure (> 10 bar). 2rGO/PAM-PES exhibits almost the same rejection for both MgSO4 and CaSO4 solutions. Furthermore, NF-GO provides the highest permeate flux as shown in Fig. 7b. The effect of inlet flow rate on the rejection and permeate flux was evaluated for 2rGO/PAM-PES, and compared with NF270 and NF-GO membranes at 10 bar using CaSO4 solution, as displayed in Fig. 8. Increasing inlet flow rate induces only a slight increase in rejection rate, but has more influence on permeate flux particularly at 10 ml/min inlet flow rate. 2rGO/PAM-PES yields the highest rejection for Ca2+ and SO42− ions. NF-GO yields also a high rejection close to that of 2rGO/ PAM-PES. The rate of increase in the permeate flux increases as the inlet flow rate increases as displayed in Fig. 8.
The effect of operating pressure on the ion rejection and permeate flux using MgSO4 is shown in Fig. 5. The developed membrane 2rGO/ PAM-PES was tested and compared with the commercial nanofiltration membrane (NF270), and the prepared NF-GO membrane. The operating pressure was varied from 5 to 15 bar at constant inlet flow rate of 10 ml/min. The results indicate that increasing pressure increases both rejection and permeate flux. This can be explained using the established transport mechanisms through NF membranes, namely convection and diffusion: increasing pressure, the contribution of convection overpowers diffusion. This is because of the high water flux; leading to an increase in the rejection. NF270 yields the highest permeate flux and rejection for Mg2+ and SO42− ions, whereas, NF-GO has more ion rejection than 2rGO/PAM-PES except at 15 bar. The ion rejection of 2rGO/PAM-PES membrane increases significantly with pressure compared to NF270 and NF-GO. For instance, increasing pressure from 5 to 15 bar increases the ion rejection by 40% for 2rGO/PAM-PES and only about 5% for NF270 and NF-GO. The NF270 and NF-GO membranes have the same ion rejection trend. This is due to the fact that both have the same active layer. The antifouling properties are enhanced by adding GO layer to the NF270 membrane as GO has good antifouling properties. However, for the 2rGO/PAM-PES membrane, the active layer is the rGO. The effect of inlet flow rate on the rejection and permeate flux using MgSO4 was evaluated as shown in Fig. 6. The inlet flow rate is varied from 2.5 to 10 ml/min, with a step of 2.5 ml/min at 10 bar. Increasing inlet flow rate, slightly increases the rejection. NF270 yields the highest permeate flux and rejection for Mg2+ and SO42− ions. The rate of increase in the permeate flux decreases as the inlet flow rate increases.
3.3. Membrane performance using seawater The effect of pressure and the inlet flow rate on the rejection and permeate flux was studied using actual seawater. The rejection of the cations was measured using ICP-MS, and the anions using IC. The permeate flux was calculated by dividing the measured output volume over time and membrane effective area. The seawater composition is presented in Table 5. A comparison of the three membranes was performed at 10 ml/min inlet flow rate. The membrane 2rGO/PAM-PES yields the highest rejection for Mg2+ and Ca2+ ions while NF270 has the highest rejection for SO42− ions. On the other hand, NF-GO has more Ca2+ rejection than NF270, while for Mg2+ both membranes exhibit a similar rejection. The membrane 2rGO/PAM-PES ion rejection increases significantly with increasing pressure as shown in Fig. 9. The reported mechanisms for the solute transport through NF membrane are convection, diffusion, and electro-migration [29–31]. The low rejection for Ca2+ ions using NF270 is due to the concentration polarization, where a high Ca2+ concentration layer forms at the membrane surface, leading to more Ca2+ ions diffusing through the membrane. For GO and rGO membranes, water passes between interlayer spacing. Therefore, ion rejection depends on the interlayer spacing and electrostatic interactions [17]. The membrane 2rGO/PAM-PES has almost the same rejection for Mg2+ and Ca2+ ions, while SO42− ions has a higher rejection due electrostatic interaction since GO and SO42− ions are both negatively charged. However, at high pressure the Mg2+, Ca2+
3.2. Membrane performance using CaSO4 solution The membrane 2rGO/PAM-PES was tested at different pressures (5–15 bar) with CaSO4 solution and compared with commercial nanofiltration (NF270) and NF-GO membranes at 10 ml/min inlet flow rate, as displayed in Fig. 7. As expected, increasing pressure increases both the rejection and the permeate flux. NF270 has the lowest rejection for Ca2+ and SO42− ions. Whereas, NF-GO has more ion rejection than 2rGO/PAM-PES at low pressure, while, 2rGO/PAM-PES has the highest 20
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flux increases as the pressure and inlet flow rate increase as shown in Fig. 11. 3.4. Membrane surface characterization Interlayer spacing can be controlled through the reduction process. Subjecting GO membrane to HI vapor removes some of the hydroxyl, carbonyl and carboxyl groups, yielding a decrease in the interlayer spacing. Therefore, the characterization of GO/PAM-PES and 2rGO/ PAM-PES are discussed. Fig. 12 presents the SEM images for GO/PAMPES and 2rGO/PAM-PES. The effect of water contact angle on reduction was studied for 2rGO/PAM-PES membrane. It was found that rGO had a higher contact angle compared to that of GO. The oxygen function group increases the hydrophilicity of the membrane, which increases water flux. XRD results indicate that GO/PAM-PES membrane (under dry condition) has a peak at 2Ɵ of 10.10 whereas the 2rGO/PAM-PES membrane has a peak at 2Ɵ of 26.020 as shown in Fig. 13-a. The interlayer spacing may be calculated using Bragg equation,
= 2dsin( )
(3)
where λ is the wavelength of the X-ray beam, d the interlayer spacing, and Ɵ the diffraction angle. GO in GO/PAM-PES membrane has a d-spacing of 0.854 nm, whereas rGO in 2rGO/PAM-PES membrane a d-spacing of 0.342 nm. However, the d-spacing of GO and rGO membranes in wet condition can be significantly larger than that in dry condition [32,33]. The structure of GO was investigated by Raman spectroscopy. GO exhibits two main bands labelled/called D and G. The D band is related to the degree of order/disorder because of a breathing k-point phonon of A1g symmetry. On the other hand, the G band is related to the stacking structure [34]. Raman spectroscopy was performed to comprehend the difference between GO, and rGO. The results are presented in Fig. 13-b. The intensity ID/IG ratio relates to sp3/sp2 carbon ratio. For 2rGO/PAM-PES, it has less hydroxyl, carbonyl and carboxyl groups, which explains the increase in ID/IG ratio. The cross-sectional SEM images for 2rGO/PAM-PES membrane show that the active layer (rGO) thickness is about 70 nm as shown in Fig. 14. Fig. 15 displays the SEM images and EDX analysis for the membrane surface before and after the cross-flow test using seawater. The SEM images indicate that there is no significant change in the surface after performing the test, which indicates a stable coating. On the other hand, the EDX results confirm the presence of some ions that are attached to the surface; specifically, chloride, potassium and magnesium. However, there is no calcium and sulfate ions attached on the surface due to high rejection rate of calcium and sulfate ions that are carried away by the flowing stream during the cross-flow experiment.
Fig. 13. a) XRD patterns b) Raman spectra for GO/PAM-PES and 2rGO/PAMPES.
and SO42− ions have almost a similar rejection as shown in Fig. 9. The effect of inlet flow rate on the rejection and permeate flux was measured for 2rGO/PAM-PES and compared with NF270 and NF-GO membranes at 10 bar. Increasing inlet flow rate slightly increases rejection. 2rGO/PAM-PES membrane yields the highest rejection for Ca2+ ions while for Mg2+ the rejection increases significantly with inlet flow rate. NF-GO exhibits a high Mg2+ rejection close to that of NF270, which has the lowest Ca2+ rejection as displayed in Fig. 10. Permeate
Fig. 14. Cross-sectional SEM micrograph for 2rGO/PAM-PES membrane. 21
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Fig. 15. SEM micrographs for 2rGO/PAM-PES membrane a) before and, b) after cross flow test, along with corresponding EDX analysis.
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Fig. 17. Effect of pressure on top brine temperature.
NF, NF-GO and 2rGO/PAM-PES permeates was calculated using PHREEQC at different pressures (5–15 bar) as shown in Fig. 16. When the saturation index is zero, the solution is in equilibrium and the corresponding temperature is considered as the top TBT. The TBT before treatment was 102 °C without antiscalant. After treatment, TBT reached 148 °C using NF, 160 °C using NF-GO and 166 °C using rGO membrane as displayed in Fig. 17. Increasing TBT, the performance ratio (PR) increases monotonically with number of stages in MSF and MED systems [35]. 4. Conclusion A membrane composed of graphene oxide (GO)/polyacrylamide (PAM) over polyethersulfone (PES) substrate is developed to be utilized as a pretreatment step for thermal seawater desalination for preventing the passage of divalent ions. The GO/PAM-PES membrane is prepared via spin coating technique, then reduced to 2rGO/PAM-PES by subjecting the membrane to hydrogen iodide. The advantage of using 2rGO/PAM-PES compared to NF membrane is that similar high rejection rates (for calcium, magnesium and sulfate ions) could be achieved. In fact, the rejection rate achieved for calcium and magnesium ions is considerably higher than that of NF membrane. In general, increasing pressure increases both the rejection and the permeate flux. Using seawater, the 2rGO/PAM-PES membrane yields the highest rejection rate for Mg2+ and Ca2+ ions compared with NF270. The 2rGO/PAM-PES membrane has almost the same rejection rate for Mg2+ and Ca2+ ions, while a higher rejection for SO42− ions is due to electrostatic interaction, GO and SO42− ions being both negatively charged. However, at high pressure the Mg2+, Ca2+ and SO42− ions have similar rejection rates. On the other hand, the low rejection for Ca2+ ions using NF270 membrane is due to the concentration polarization, a high Ca2+ concentration develops in the layer near the membrane surface. Thus, more Ca2+ ions tend to diffuse through the membrane. The ultrafiltration (UF) membrane (the PES substrate) coated with rGO yields the highest TBT without scaling for MSF. Using a membrane as a pretreatment step increases TBT to 148 °C using NF, 160 °C using GO-NF and 166 °C using 2rGO/PAM-PES. Therefore, one could postulate that UF membrane coated with rGO is an adequate pretreatment step that would be a potential replacement for the use of NF filtration in thermal seawater desalination.
Fig. 16. Effect of pressure on the saturation index for a) NF270, b) NF-GO, c) 2rGO/PAM-PES.
3.5. Top brine temperature analysis Brine temperature limitation in thermal seawater desalination technologies depends on the presence and concentration of divalent ions. TBT is usually about 112 °C in MSF. In order to calculate the TBT for the permeate of the three membranes, the saturation index for the 23
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Acknowledgement
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