Effect of oscillating temperature and crystallization on graphene oxide composite pervaporation membrane for inland brine desalination

Journal of Membrane Science 588 (2019) 117210

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Effect of oscillating temperature and crystallization on graphene oxide composite pervaporation membrane for inland brine desalination

T

Withita Cha-Umponga, Guangxi Donga, Amir Razmjoua,b,∗, Vicki Chena,c a

UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales, Sydney NSW, 2052, Australia Department of Biotechnology, Faculty of Advanced Sciences and Technologies, University of Isfahan, Isfahan, 73441-81746, Iran c School of Chemical Engineering, University of Queensland, Queensland, 4072, Australia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Oscillating temperature pervaporation Crystallization Concentrated inland brine Graphene oxide

Membrane distillation (MD) for desalination experienced technical challenges when treating concentrated inland brine with high salinity under industrially-relevant oscillating temperature condition or solar desalination. In this study, we fabricated a composite membrane using graphene oxide (GO) and used it in a pervaporation (PV) process to tackle the issues encountered in the MD processes. The resultant GO composite membranes demonstrated fast water vapor transport through nano-channels between the GO interlayers. In addition, the smoother and negatively charged surface offered by the GO layer hindered the undesired crystal deposition on the membrane surface, and therefore giving a substantially improved performance than the MD process when operated under oscillating temperature. More importantly, the presence of divalent cations in inland brine led to a naturally-occurring crosslinking between the GO nanosheets that offered molecular sieving functionality to the GO layer, preventing the transport of salt ions, as well as GO swelling and therefore providing exceptional performance sustainability, especially under oscillating temperature condition. Interestingly, it was found that GO composite membrane in a PV process for desalination offered greater overall water productivity when operated under the oscillating temperature compared to the constant temperature. These GO composite membranes also demonstrated good performance stability after chemical cleaning and when exposed to minor contaminants found in inland brine such as humic acid.

1. Introduction Desalination technologies such as multi-effect distillation (MED), multi-stage flash distillation (MSF) and reverse osmosis (RO) have been implemented globally to alleviate the water shortage issues [1]. The well-established RO process has a water recovery of 80–92% for brackish water [2]. However, its recovery is limited when treating concentrated brines due to higher osmotic pressure and energy consumption. Highly concentrated brine is commonly found as a by-product of the RO process, which needs to be further treated or disposed to minimize its environmental implications [3]. For instance, for the inland RO process, injecting the concentrated brine back to the aquifer will inevitably increase the salinity of the groundwater. Furthermore, leakage between aquifers can also cause contamination of freshwater. The ideal solution is to reduce the volume of the concentrated brine [4]. Recently, two membrane-based desalination technologies — membrane distillation (MD) and pervaporation (PV) have been considered for

treating high salinity solution because they are cost-effective, unaffected by the osmotic pressure, and suffer less fouling than RO [5,6]. Moreover, these thermally-driven membrane processes can be coupled with low grade heat sources to assemble into portable desalination unit, ideal for fresh water supply in rural communities [7–9]. For the MD process, vacuum membrane distillation (VMD) can be used for desalination due to its higher permeate flux than the direct contact membrane distillation (DCMD) [10–12]. Moreover, a high thermal efficiency is possible to achieve in VMD due to negligible heat lost by conduction [13,14]. In comparison to the conventional crossflow VMD configuration, submerged VMD has lower heat loss and energy consumption required to pump feed solution through the system. Furthermore, the membranes in submerged configuration can be accessed easily for cleaning [15]. In submerged VMD, the porous hydrophobic membranes are submerged in the feed vessel with the feed solution in direct contact with the membrane. The water evaporates at the feed-membrane interface and passes through membranes, which is then

∗ Corresponding author. UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales, Sydney NSW, 2052, Australia. E-mail address: [email protected] (A. Razmjou).

https://doi.org/10.1016/j.memsci.2019.117210 Received 3 April 2019; Received in revised form 24 June 2019; Accepted 24 June 2019 Available online 25 June 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

Journal of Membrane Science 588 (2019) 117210

W. Cha-Umpong, et al.

based membranes were focused on seawater desalination using low NaCl concentration. The deposited salt was easily washed off or dissolved after the addition of feed solution [41,42]. For inorganic scaling on GO-based membranes, only gypsum scaling in RO process was studied [43]. The use of complex inland brine as feed solution and crystallization of salts on GO composite pervaporation membrane have not been investigated. In this work, stacked GO sheets were deposited on the PP hydrophobic hollow fiber membrane by vacuum filtration. These GO composite membranes were used in a PV process for concentrated inland brine desalination. Their stability and response to heterogeneous crystallization were investigated using different electrolyte solutions and model concentrated inland brine under oscillated temperatures. The performances of the GO composite membrane in submerged PV mode were compared against submerged VMD process using the PP membranes under identical operating conditions. In addition, chemical cleaning was implemented to examine the reusability of the GO composite membrane, and organic foulant was used to test the wetting resistance of the GO composite membranes.

condensed outside the membrane module by a condenser and being collected [16]. One major concern is that high feed salinity lead to nucleation and crystallization of salt on the membrane surface resulting in a rapid flux decline, and a reduced salt rejection [17]. Low salt rejection was reported to be more severe with intermittent MD operations due to constantly changed feed temperature [18,19]. To tackle salt crystallization in MD, one approach is membrane surface modification such as combining nanomaterials with membrane technology to enhance the rejection efficiency and performance sustainability of a MD process [20,21]. Unlike this approach, the current study explored an alternative strategy of depositing a dense layer of graphene oxide (GO) nanosheets on porous polypropylene (PP) membranes for the desalination of concentrated brine through a so-called PV process. In a typical PV process, an asymmetric membrane with a dense skin layer on a porous support is usually used. Owing to its unique morphology, the dense skin layer can effectively prevent salt ions to permeate through the membrane, whilst the water vapor can pass through the dense skin and the subsequent porous support with minimal mass transfer resistance [8]. The substrate PP membranes in this study are commonly used in conventional MD due to their high porosity and hydrophobicity. Through GO deposition, these hydrophobic PP membranes can be converted into asymmetric composite membranes suitable for the PV processes [22,23]. The GO nanosheet is an ultrathin two-dimensional material, which consists of oxygen-containing functional groups such as carboxylic, hydroxyl, and epoxy at its edges and basal plane [24]. It was found that the amphiphilic GO nanosheet allowed water molecules to attach on hydroxides (hydrophilic side) and diffuse through the hydrophobic carbon core [25]. Therefore, the deposition of GO nanosheets provided nano-capillary effect for rapid water transport, selective molecular sieving and blocked the passage of salt ions [26]. Furthermore, the stacked GO nanosheets also reduced membrane surface roughness, which impeded crystallization due to the reduced available nucleation sites [27]. Previous studies have demonstrated the advantages of GO-based membranes for water purification due to their hydrophilicity, surface charge, and controllable interlayer spacing [28–32]. However, most of them were used in desalination with pressure or concentration gradient as the driving force for the filtration process [25], which did not involve the transport of water vapor through the GO layer. Although many benefits have been identified for the GO nanosheets, its swelling propensity when in contact with water at low temperature could create flow paths for liquid water and salt ions, and therefore caused membrane wetting [33]. In industry, intermittent operation with frequent system shut-down and restart is usually required, especially in solar-driven desalination plants and mobile brackish groundwater desalination system. Although researchers reported the effect of intermittent operation on membranes for MD which led to the precipitation of salt crystals and membrane wetting, the study was mainly focused on the feed residues dry-out on the membrane surface during shut-down period [18]. The salt nucleation and crystallization behavior during the MD process with oscillating temperature was not considered. In addition, for pervaporation, the stability of the GO composite membranes in oscillating temperature has not been investigated. At low temperatures, the GO stacks are exposed to liquid water, which results in an increase in the interspacing between the GO nanosheets up to 11 Å, large enough for salt ions to travel through [34–36]. On the other hand, cations have been reported to interact with the GO nanosheets via cation-π interactions and intercalation with oxygen functional groups, which could minimize the undesired swelling of the GO layers [37].The abundant presence of alkaline and alkaline earth cations in concentrated brine could lead to crosslinking of GO nanosheets with cations which prevents the swelling of the GO layers in the composite membranes [32,37–40]. However, the studies were based on nanofiltration and at ambient temperature condition. The effect of crosslinking on the GO composite membrane for brine desalination via a PV process, especially under oscillated temperatures has not been evaluated. Moreover, most studies on the GO-

2. Materials and methods 2.1. Chemicals and model inland brine Hydrophobic PP hollow fiber membranes (Accurel PP S6/2, from Membrana GmbH, Germany) were used as the substrate of the GO composite membranes and in the submerged VMD tests. The porosity, pore size, wall thickness and inner diameter of the PP membrane were 73%, 0.2 μm, 450 μm, 1800 μm, respectively. GO powder and humic acid sodium salt were purchased from Sigma Aldrich. Sodium chloride (NaCl, 99.7%), calcium chloride (CaCl2, 99–105%), magnesium chloride hexahydrate (MgCl2·6H2O, 98–101%), sodium sulfate (Na2SO4, 99%), and sodium bicarbonate (NaHCO3, 99.7–100.3%) were purchased from Chem Supply. Table 1 lists the compositions of different electrolyte solutions and concentrated model inland RO brine. The composition of the concentrated model inland brine was based on approximately 60% water recovery ratio of initial inland RO brine [44–46]. 2.2. Fabrication of GO composite membranes The protocol for the fabrication of GO composite membrane was adopted from a previous study [47]. The PP membrane was firstly wetted by filtering small volume of 30% ethanol in water through the membrane before vacuum filtration. GO powders (2 mg/mL) were dispersed in Milli-Q water through probe sonication for 10 min at 20% amplitude with the interval of 30-sec pulse on and 5-sec pulse off. The GO suspension solution was then diluted to the concentration of 0.02 mg/mL (0.002 wt% GO suspension) and sonicated for 10 min. Then, the GO suspension solution was filtered through the PP hollow fiber membrane on the shell side under vacuum filtration with various volumes to obtain different GO layer thickness. The membranes were Table 1 Feed compositions of concentrated model inland RO brine and different electrolyte solutions. Feed

Concentrated model inland brine NaCl NaCl + CaCl2 NaCl + CaCl2+ MgCl2

2

Concentrations (ppm) NaCl

MgCl2·6H2O

CaCl2

Na2SO4

NaHCO3

TDS

17520

9750

1050

2700

2190

33210

33210 32160 22410

– – 9750

– 1050 1050

– – –

– – –

33210 33210 33210

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W. Cha-Umpong, et al.

Fig. 1. Lab-scale set up for submerged VMD and pervaporation.

(ScanAsyst-Air) with 5 μm × 5 μm scan size.

dried at room temperature overnight, followed by 30 min in the oven at 50 °C. These GO composite membranes were labeled as GO/PP membrane hereafter.

2.4.3. X-ray diffraction analysis (XRD) X-ray diffraction measurement (The Empyrean PANalytical thinfilm XRD) was carried out to measure layer-to-layer distance (d-spacing) of dry and wet GO composite membrane. Scanning was conducted from 2θ = 2°–20° at a scan speed of 1.23°/min. The d-spacing was calculated using Bragg's equation as follows [48]:

2.3. Submerged vacuum membrane distillation and pervaporation The experimental setup for the submerged VMD is shown in Fig. 1, same as the pervaporation setup. All the desalination tests were carried out in submerged and vacuum mode. Two hollow fiber membranes were used in each experiment with effective membrane length of 9.8–10.2 cm, and the membranes were vertically submerged in the feed vessel. Prior to the desalination test, Milli-Q water was used as feed solution to obtain pure water flux of the membranes at 70 °C with and without stirring. The stirring rate was 350 RPM. Different salt concentrations were used to represent concentrated inland RO brine and electrolyte solutions. Milli-Q water was added to the feed solution every hour to compensate the water vapor loss, maintain the feed concentration and the feed volume at 2 L. During the experiment, vacuum pressure was applied on the permeate side using a peristaltic pump (Masterflex®, Cole Palmer) with a maximum vacuum of −98 kPa. The vacuum pressure, the weight of permeate and feed temperature were recorded using a data acquisition software (LabView) with a 10s interval. Salt rejection were measured by recording the permeate conductivity every 30 min throughout the tests using waterproof conductivity meter (WP-81, TPS). For constant temperature operation, the feed temperature was kept at 70 °C. For oscillating temperature operation, feed temperature was set at 70 °C for 7 h, and then water bath was turned off for 8 h to cool down the feed solution in each cycle.

nλ = 2dsinθ

(1)

where λ, d, θ and n are the X-ray wavelength, interlayer spacing between GO nanosheets (d-spacing), the angle of incidence and an integer, respectively.

2.4.4. Fourier transform infrared (FT-IR) FT-IR (Bruker FT-NIR/IR spectrometer) was used to determine the functional group of GO powder, GO/PP and PP membranes in the range from 400 to 4000 cm−1.

2.4.5. Surface zeta potential The surface zeta potential of the GO powder and precipitated crystals collected in the feed solution were measured using dynamic light scattering with zeta potential cell kit (DLS, Zetasizer nano, Malvern) using NaCl as electrolyte solution. The surface zeta potentials of the GO/PP and PP membranes were measured using SurPASS Electro-Kinetic Analyzer (Anton Paar Corportation).

2.4. Characterization 2.4.6. Contact angle measurement and surface free energy of membranes The contact angle of the GO/PP and PP membranes were measured using Attension theta (Biolin Scientific) to indicate hydrophilicity and hydrophobicity of the membrane surface by sessile drop method. The liquid drop size was 2 μL. The reported values were the average of at least 3 measurements. Surface free energy of the membranes was calculated using acid-base (Van Oss) approach adopted from a previous study [49]. In this method, the contact angles of the GO/PP and PP membranes with Milli-Q, glycerol, and di-iodomethane were recorded.

2.4.1. Scanning electron microscopy (SEM), energy dispersive X-Ray spectroscopy (EDS) elemental analysis and focused ion beam (FIB) Field-emission electron microscope (FEI Nova NanoSEM 230 and 450 with Bruker EDS detector) was used to characterize membrane surface morphology. The crystal composition on the membrane surface and cross-sectional area were identified by EDS. High-Resolution focused ion beam (FIB-SEM, Dualbeam FEI Nova Nanolab 200) was used to measure the GO layer thickness on the porous substrate. To minimize the beam damage, platinum gas was deposited on the GO surface with the thickness of 2 μm. A thin protective layer was formed on top of the region of interest. Then, the thickness of 3 μm was milled away by FIB to obtain the cross-sectional image and measured membrane thickness (tilted 52°). Samples for FIB and SEM were prepared by carbon coating using DCT turbo-pump desktop carbon evaporator.

2.4.7. Inductively coupled plasma optical emission spectrometry (ICP-OES) ICP-OES (PerkinElmer Optima 7300) was used to identify the concentration of sodium, calcium and magnesium in the permeate solution. The ICP-OES detection limit is 0.2 ppm.

2.4.2. Atomic force microscope (AFM) Atomic force microscopy (Bruken Dimension ICON SPM) was used to measure surface morphology, size of single GO nanosheets and roughness of GO/PP and PP membrane using peak force tapping mode

2.4.8. Liquid chromatography-organic carbon detection (LC-OCD) LC-OCD Model 8 (DOC-LABOR, Karlsrushe, Germany) was used to measure humic acid in some permeate samples. 3

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Fig. 2. (a) AFM image of (b) single layer and (c) random stacking multi-layer thickness of GO nanosheets. (d) FT-IR curves, (e) surface zeta potential, (f) average surface roughness from AFM, and (g) water contact angles and surface free energy of PP and GO/PP membranes fabricated using different GO loading.

3. Results and discussions

the GO/PP membrane surface remained negatively charged as the solution concentration increased to 0.2 M. Negative charges on the GO surface indicated anion rejection functionality. The average zeta potential of the GO powders and CaCO3 crystals were −13.53 mV and −5.70 mV respectively in 0.5 M NaCl solution, similar to the concentration of the feed solution used in the subsequent desalination tests. The negative zeta potential of CaCO3 particles was also reported in other studies [27,54]. The average surface roughness of the PP hollow fiber membrane was 314 nm for the scan size of 5 × 5 μm2 (Fig. 2f). The deposition of GO nanosheets on the PP membrane significantly reduced the surface roughness by 65%–75%. Smoother surface was also observed from the surface FIB-SEM images of the GO/PP membrane compared to the pristine PP membrane (Fig. 3). The thickness of the GO layer on the PP membrane is shown in Fig. 3. In addition, the water contact angle of the GO/PP membrane with 0.112 mg/cm2 GO loading was 79.8°, in agreement with a recent study [30]. This value was 50° lower than that of the PP membrane (Fig. 2g). The reduction of water contact angle was due to an increase in surface free energy from 2.4 to 39 mN/m. These results suggested that the GO layer increased the hydrophilicity and decreased surface roughness due to the presence of oxygen-functional groups and 2D structure of GO nanosheet [55,56]. An increase in GO

3.1. Characterization of PP and GO/PP membranes SEM images, XRD and FT-IR were used to confirm the deposition of GO nanosheets on the PP membrane surface. AFM images of single and multiple layer GO nanosheets showed the individual GO nanosheet thickness of about 1.3 nm (Fig. 2 a-c), consistent with literature [50,51]. The oxygen-containing functional groups of the GO nanosheets were observed using FT-IR spectra as shown in Fig. 2d. The characteristic peaks at approximately 3350, 1720 and 1620 cm−1 represent C–OH (hydroxyl), C=O (carboxyl) and C=C (carbon lattices) groups, respectively. The peaks at 1360, 1220 and 1050 cm−1 are from epoxy and alkoxy groups [52,53]. The surface zeta potential analysis was conducted at pH 8 with increasing solution concentration to obtain the surface charge of the PP and GO/PP membranes in high salinity environment (Fig. 2e). K+ ion was used to represent monovalent ion in the feed solution. As the concentration increased, the surface charge of the PP membrane became more positive (−51 mV at 0.001 M to 39 mV at 0.2 M of KCl solution). This indicated that the PP membrane had a positive surface charge during the submerged VMD desalination test. On the other hand, 4

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Fig. 4. The average d-spacingof GO/PP membrane (0.112 mg/cm2 GO loading) using X-ray diffraction in dry condition as well as after 24 h of pervaporation in Milli-Q water and electrolyte solutions.

11.31 ± 0.24 Å, 8.70 ± 0.19 Å, and 9.89 ± 0.06 Å for Milli-Q water, NaCl, NaCl + CaCl2, and NaCl + CaCl2+MgCl2 as the feed solution, respectively. The d-spacing of the GO/PP membrane in NaCl + CaCl2 feed was closed to that of the dry GO/PP membrane, resulting in the lowest degree of swelling compared to the other feed solutions. The GO/PP membranes in Milli-Q, NaCl, and NaCl + CaCl2 solution showed swelling percentage of 5.17%, 7.12% and 3.82%, respectively, indicating the Ca2+ ions bonding reduced swelling in the GO/PP membranes. Na+, Ca2+ and Mg2+ are common ions found in inland RO brine. The presence of these cations affected salt rejections of the GO composite membranes during desalination due to the interaction between the GO nanosheets and the cations. The interaction of monovalent cation (Na+) and the GO nanosheets is by cation-π interaction rather than interaction with oxygen functional group on the GO nanosheets, such as hydroxyl and carboxylate groups [58,59]. However, the bonding between Na+ and GO nanosheets was not strong enough resulting in the expansion of interlayer spacing and higher degree of penetration during pervaporation. The weak bonding of Na+ ion and GO was due to the lower cation- π interaction energy compared to the divalent cations because cation- π interaction energy increased with charge density of cations. In hydration, Na+ has a larger affinity toward water molecules than aromatic rings on the GO nanosheets [60]. The first hydration shell of Na+ remains intact making it difficult for π clusters to replace the water molecules [61]. In contrast, Mg2+ and Ca2+ have higher interaction energy with aromatic rings than water molecules [60]. Concurrently, they also cross-link with oxidized groups on the GO surfaces between basal planes and at the edges of GO sheets [40]. As a result, the interactions between the hydrated Ca2+ or Mg2+ within the GO nano-channels are stronger than that of the hydrated Na+ resulting in a better GO membrane stability, lower swelling and d-spacing.

Fig. 3. Surface SEM images of (a) PP membrane and GO/PP membrane fabricated with GO loading of (c) 0.112 mg/cm2; (e) 0.168 mg/cm2 and (g) 0.224 mg/cm2. Cross-section FIB-SEM images of (b) GO/PP hollow fiber membrane. Cross-sectional FIB-SEM images of GO/PP membrane with GO loading of (d) 0.112 mg/cm2; (f) 0.168 mg/cm2 and (h) 0.224 mg/cm2. GO composite membranes were coated with platinum to protect the membrane from focused ion beam damage during FIB-SEM analysis.

loading from 0.168 to 0.224 mg/cm2 slightly increased the surface roughness and water contact angle due to the aggregation of the GO nanosheets, as shown in Fig. 3. The XRD analysis on the GO/PP membrane identified the d-spacing of the GO layer using Bragg equation [48], which is the interlayer spacing between the GO laminates. The d-spacing of dry GO/PP membrane was approximately 8.18 ± 0.27 Å, with a diffraction peak at 2 θ = 10.8° ± 0.37° (Fig. 4). The value was comparable to the dspacing of the GO membrane reported in literature [37,57]. Hydrophilic GO nanosheets were easily hydrated in liquid due to the oxygencontaining functional groups. To test the stability of the GO/PP membrane for desalination through a PV process, the structural changes of wet GO/PP membrane in Milli-Q water, NaCl, NaCl + CaCl2 and NaCl + CaCl2+MgCl2 solutions were studied to reveal the causes for the variation in salt rejection. The GO/PP membranes with 0.112 mg/ cm2 GO loading were used in the pervaporation tests for 24 h at 70 °C without stirring. The interlayer spacing expanded to 11.48 ± 0.25 Å,

3.2. Effect of sodium chloride solution and oscillating temperature on submerged VMD and pervaporation performance NaCl is the most common and abundant mineral substance found in brackish groundwater and seawater. However, the interaction between the GO nanosheets and Na+ is weak as mentioned earlier. To study the stability of the GO/PP membrane for the thermally-driven desalination process, NaCl was used as the feed solution. The initial flux of 11.5 and 10.9 L/m2h was achieved at an operating temperature of 70 °C for the 0.112 mg/cm2 GO/PP membrane and the PP membrane, respectively 5

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Fig. 5. Temperature profile, permeate flux and conductivity from GO/PP pervaporation membrane with different GO loading and PP membrane (Feed: NaCl) in VMD without stirring: (a) and (b) are constant temperature operation at 70 °C; (c) and (d) are oscillating temperature operation.

(Fig. 5a). The difference in permeate flux between NaCl and Milli-Q water feed was negligible. In addition, the permeate flux observed in this study was similar to those reported by other VMD studies using Accurel S6/2 PP membrane [62,63]. This indicated that the GO layer did not create extra resistance during this PV process. Regarding the permeate conductivity in a constant temperature operation, the permeate conductivity increased from 2 to 374 μS/cm after 20 h of operation for the PP membrane. On the other hand, the permeate conductivity of the GO/PP membrane with 0.112 mg/cm2 GO loading showed negligible increase for the first 30 h of operation, followed by an increase after 30 h (Fig. 5b). The imperfect stacking of the GO nanosheets by vacuum filtration led to an inconsistent GO layer thickness and the GO/PP membrane with 0.112 mg/cm2 GO loading had thin GO layer in some areas, as shown in Fig. 3d, where NaCl solution penetrated the membrane more easily after swelling. In addition, the increased permeate conductivity of the GO/PP membrane with 0.112 mg/ cm2 GO loading after 30 h operation could be due to the instability of the GO membrane structure in NaCl solution as suggested in other studies [38,64]. Cations have been reported to interact with pristine regions of the GO nanosheets via cation-π interactions, which are noncovalent interactions between aromatic π-electron cloud and cations. However, the presence of Na+ did not prevent swelling of the membrane as aforementioned. Due to partial wetting, flux decline in the GO membrane with 0.112 mg/cm2 was evident. On the other hand, the vacuum pressure applied on the permeate side caused the compaction of the GO nanosheets after swelling resulting in fluctuated water flux (Fig. 5a). When increasing the GO loadings, the flux was relatively constant with less than 8% deviation in NaCl solution. This indicated that some water molecules evaporated to vapor phase inside the GO nanochannel resulting in a rapid vapor transport and low resistance against the nanocapillary wall. Despite having similar flux during constant temperature pervaporation, the GO/PP membrane with 0.168 mg/cm2 and 0.224 mg/cm2 GO loading had higher NaCl rejection compared to the GO/PP membrane with 0.112 mg/cm2 GO loading (Fig. 5b). The GO layer with 0.168 mg/cm2 and 0.224 mg/cm2 GO loading completely covered the porous PP membranes as shown in Fig. 3 e and g. On the otherhand, small pores were visible from the GO layer with 0.112 mg/

cm2 GO loading (Fig. 3c) The GO layer became loosely packed in NaCl solution, which promoted the salt penetration through the GO layer with the 0.112 mg/cm2 GO loading. Based on these results, for constant temperature PV, the thickness of the GO layer played an important role in achieving high distillate quality. Industrial membrane separation process requires frequent system shutdown and restart. Previous studies reported an increased permeate conductivity when restarting the MD operation [65,66]. Such behavior is also expected for the PV process when operating intermittently. For the PP membrane in VMD, the permeate flux fluctuated during the second cycle of temperature oscillation (Fig. 5c) resulting in a rapid increase in the permeate conductivity (Fig. 5d). For the GO/PP membrane, oscillating temperature led to a lower NaCl rejection compared to the constant temperature system. More importantly, the increased conductivity happened when the heat was not supplied. At low temperature, surface diffusion was higher than the rate of desorption and evaporation. Hydrophilic GO nanosheets allowed water molecules to adsorb into the GO layer by surface diffusion and flow through the membrane [33], and the mechanism of salt removal became molecular sieving or size-exclusion [37]. At high temperature, the evaporation rate increased and some of the water molecules inside the GO composite membrane were in vapor phase. Consequently, the higher evaporation rate prevented the movement of salt solution to the permeate side of the membrane [33]. The negative charge on the GO surface also promoted electrostatic repulsion between anions and GO nanosheets according to the Donnan exclusion theory [67]. Once the water vapor was condensed, the addition of pure water decreased permeate conductivity. For the GO/PP membrane with 0.168 mg/cm2 GO loading (Fig. 5d), the conductivity decreased from 254 to 161 μS/cm2 after the feed temperature reached the target temperature during the second cycle. The reduction in conductivity was also observed on the GO/PP membrane with GO loading of 0.112 mg/cm2 at the second cycle. However, partial wetting occurred in the third cycle. The permeate flux dropped and conductivity increased drastically (Fig. 5c and d). These results indicated that the GO/PP membrane cannot deliver stable performance for desalination of NaCl solution by PV with oscillating temperature.

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Fig. 6. Permeate flux and conductivity from GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) with different electrolyte solution under the constant temperature (70 °C).

3.3. Effect of different electrolyte solutions on structure of GO/PP pervaporation membrane

Fig. 7. Feed temperature profile, permeate flux, and conductivity from GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) with different electrolyte feed solution under oscillating feed temperature.

As mentioned earlier, swelling of the GO layer is one of the key factors affecting salt rejection in the GO composite membranes, and divalent alkaline earth metal ions such as Mg2+ and Ca2+ commonly found in inland brine could function as crosslinking agents to control interlayer spacing and prevent the swelling of the GO layers. In this study, CaCl2 and MgCl2 were added into the NaCl solution to evaluate their role in GO interlayer stabilization. As shown in Fig. 6, stable water flux was achieved for the GO/PP membrane with 0.112 mg/cm2 GO loading throughout 40 h of operation. Moreover, the permeate conductivity remained at 2 μS/cm when the feed conductivity of the GO/PP membrane in NaCl solution was 300 μS/cm. As discussed in Section 3.2, the salt rejection of the GO/PP membrane under oscillating temperature was significantly lower than that of the constant temperature operation, particularly during the shutdown period. Thus, the effect of cations cross-linking on performance sustainability of the GO composite membranes under oscillating temperature were investigated by using NaCl + CaCl2 and NaCl + CaCl2 + MgCl2 mixtures as feed solutions. From the ICP-OES (Table 2) and permeate conductivity results (Fig. 7), it was clear that divalent cations significantly minimized the transport of Na+ through the membrane, suggesting the GO composite membrane was suitable to remove salt ions in the Ca2+ and Mg2+ rich inland brine under intermittent operation. The hydrated diameter of Na+, Ca2+ and Mg2+ were reported to be 7.2, 8.2, and 8.6 Å, respectively [42]. The effective interlayer channel spacing was estimated from d-spacing of the GO/PP membrane (Fig. 4) minus the effective thickness of one graphene layer

(3.4 Å) [32,68]. This resulted in the effective channel size of 5.3 Å and 6.49 Å for the GO/PP membranes in NaCl + CaCl2 and NaCl + CaCl2+MgCl2 feed solution, respectively, which were smaller than the hydration radius of Na+. As a result, the hydrated cations needed to lose some of its surrounding water molecules to penetrate through the nano-channel. Furthermore, these nano-scale interlayer channels also led to higher energy barrier for larger molecules to penetrate through due to higher ion charge and stronger attraction to water molecules [32]. In addition, the passage of Na+ can also be blocked due to (i) steric hindrance when Ca2+ and Mg2+ formed cation-π interactions with the aromatic ring, and (ii) hydrodynamic interaction with the nano-capillary walls [69,70]. Calcium and humic acid typically formed complexes that bound humic acid tightly to the membrane surface and induced rapid aggregation of complex precipitates. From a previous VMD study using NaCl + CaCl2+humic acid feed mixture, the presence of calcium and humic complexes affected the transport of vapor resulting in a rapid decrease in flux within 20 h of operation, with the permeate conductivity reaching 500 μS/cm [71]. For the GO/PP membrane tested in this study (Fig. 8), partial wetting and flux decline was not observed after 3 cycles of oscillating temperature, even though the adhesion of humic acid on the GO surface was evident. This was further supported by the LC-OCD results that showed nearly complete rejection of humic acid.

3.4. Crystallization of concentrated model inland brine on PP and GO/PP membranes under oscillating temperature

Table 2 ICP-OES results of the permeate composition from the GO/PP membrane under the oscillating temperature. Elements (ppm)

Ca2+ Mg2+ Na+

One major issue with the thermally-driven desalination process is scaling, which results in low permeate flux. From previous studies with similar model inland brine, calcium carbonate was the major precipitate formed in bulk and crystallized on the membrane surface [6,72]. In this work, the effect of oscillating temperature on crystallization was studied using both PP and GO/PP membranes. Fig. 9 compares the salt crystals formed on membrane surface with and without stirring. The crystals on the surface of PP and GO/PP

Feed solution NaCl + CaCl2

NaCl + CaCl2+ MgCl2

NaCl

0.13 0.00 0.24 ± 0.002

0.00 0.00 0.13

0.00 0.00 247 ± 2.47

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were nucleated from heterogeneous nucleation. The higher amount of crystals on the PP membrane can be explained by its hydrophobicity. From Young's equation, a hydrophobic surface has higher liquid-substrate interfacial energy due to higher static water contact angle and lower substrate free energy [73]. According to classical nucleation theory for heterogeneous nucleation, high liquid-substrate interfacial energy resulted in lower nucleation free energy and promoted heterogeneous nucleation on the surface [74]. In terms of the hydrophilic/ hydrophobic characteristic of the membrane surface, lower water contact angle of the surface resulted in less adhesion force between CaCO3 and the surface [75]. This also explained the lower salt crystals deposition on the GO/PP membrane surface compared to the PP membrane. Moreover, salt particles had a high chance of getting trapped in the valleys on the rougher membrane surface. The lateral movement of particles were restricted inside the valleys, which favored particle deposition [76]. In addition, both negatively charged GO surface and calcium carbonate led to strong electrostatic repulsive which suppressed crystals deposition. It was found that the size of salt crystals on the GO/PP membrane was reduced after stirring (Fig. 9c and d), in agreement with a previous study [15]. This can be ascribed to the shear force near the membrane surface which reduced the temperature polarization and disrupted the deposition of crystals [77]. The cross-sectional SEM images, shown in Fig. 10, indicated a thicker layer of salt crystals on the PP membrane than on the GO/PP membrane. This confirmed that the GO coating reduced salt precipitation on the membrane surface. The crystal deposition on both membrane surfaces was mostly calcium salts due to inverse solubility with respect to temperature. The large magnesium-based crystals on the outermost part of the scaling layer were magnesium carbonate in flower-like structure. Smaller magnesium-based crystals were also

Fig. 8. Permeate flux and conductivity from GO/PP membrane (0.112 mg/cm2 GO loading) using 100 g/L NaCl, 1.26 g/L CaCl2 and 10 mg/L humic acid.

membranes were identified as calcium and magnesium elements from the SEM/EDS analysis. Scaling study on the GO/PP membrane showed that the nucleation rate of salt crystals on the GO surface was slower than those on the PP membrane surface. A continuous layer of crystals was observed on the PP membrane with and without stirring. As a result, most membrane pores were blocked for water vapor transport (Fig. 9a and b). On the other hand, GO surface was still visible after 24 h of oscillating temperature PV without stirring, as shown in Fig. 9c. In addition, the calcium-based crystals formed on the GO/PP membrane was larger than those on the PP membrane surface without stirring. Some of the larger crystals could form in the bulk feed solution and adhered on GO surface [27]. For the PP membrane, the smaller crystals

Fig. 9. Surface SEM images of PP membrane in VMD with stirring at (a) 0 RPM and (b) 350 RPM in VMD; GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) with stirring at (c) 0 RPM and (d) 350 RPM for concentrated model inland brine treatment under oscillating feed temperature at 24 h. 8

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Fig. 10. SEM/EDS cross sectional mapping images of (a) GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) and (b) PP membrane in VMD for concentration model inland brine treatment with 350 RPM stirring and oscillating temperature at 60 h.

operation with the concentrated model inland brine. After chemical cleaning, the GO/PP membranes were rinsed with Milli-Q water to remove any remaining cleaning acid. After the first cleaning cycle, the GO/PP membranes were dried before submerging in a new batch of model concentrated inland brine and tested for 48 h. The chemical cleaning was able to remove all crystals on the membrane surface and fully restored the initial permeate flux of 13 L/m2h (Fig. 12a). From the SEM images, (Fig. 12b), some GO nanosheets were detached after the first cleaning cycle. However, the membrane performance was comparable to the previous cycle. The water contact angle of the GO/PP membrane increased to 80.42° due to the exposure of hydrophobic substrate. The sign of flux decline was observed after the second acid cleaning cycle. The average permeate flux dropped by 21% compared to the previous cleaning cycle and the water contact angle of the membrane increased to 91.07°. The SEM images showed further removal of the GO nanosheets on the membrane surface (Fig. 12c). Nevertheless, the GO/PP membranes still delivered slower rate of flux decline and produced more pure water compared to the PP membranes in Fig. 11b.

found at the bottom of the scaling layer, which could penetrate the gaps between the calcium crystals and caused pore blocking. In terms of the effect of stirring on permeate flux, the permeate flux of the PP membrane reduced to zero after two cycles of temperature oscillation (Fig. 11a) without stirring, while the flux decline for the GO/ PP membrane was much lower than the PP membrane. With stirring, the water flux of the GO/PP membrane was 50% higher than that of the PP membrane at the beginning of the second and third oscillating cycle (Fig. 11b). This indicated that GO surface benefited the membrane performance by reducing mass transfer resistance. The nucleation and adhesion of salt crystals on the GO surface were affected by surface roughness due to lower surface area for crystals to grow or attach on the surface [78]. 3.5. Effect of chemical cleaning on the performance of GO/PP membrane under oscillating temperature Although the GO/PP membranes showed better anti-scaling property than the PP membrane, the overall flux recovery could not be achieved with Milli-Q water cleaning. To test reusability of the membranes after chemical cleaning, the GO/PP membranes were cleaned with 2 wt% citric acid for 30 min under constant stirring after 48 h of

Fig. 11. Permeate flux from GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) and PP membrane in VMD with (a) 0 RPM and (b) 350 RPM feed stirring for concentrated model inland brine treatment under oscillating temperature. 9

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Fig. 12. Permeate flux (a) from GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) in concentrated model inland brine treatment with 350 RPM stirring, oscillating feed temperature and cleaning every 48 h. Dotted line indicated cleaning. Surface SEM images of GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) after (b) the first cleaning and (c) second cleaning with 2 wt% citric acid and continuous stirring for 30 min.

volume at 40 h was 320 L/m2, 11% higher than that of the constant temperature system. Even though lower feed temperature during shutdown led to a reduced permeate flux, the oscillating temperature operation introduced a relaxation period to the system, which slowed down the flux decline and heterogeneous nucleation. Thus, higher overall water production was obtained. Scale deposits were minimized as temperature decreased during the shutdown period due to the inverse temperature-solubility relationship of the alkaline scales [79].

3.6. Performance comparison between constant and oscillating feed temperature of the GO/PP membrane using concentrated model inland brine The overall permeate volumes obtained from both constant and oscillating feed temperature operations were compared in Fig. 13. The PV desalination tests on the GO/PP membrane were conducted until the permeate flux at 70 °C feed temperature approached zero, which was around 40 h and 67 h for constant and oscillating temperature, respectively. For oscillating temperature, the accumulated permeate

4. Conclusions This study explored the use of GO nanosheets to easily convert inexpensive PP membrane that is only suitable for MD process to the GO/ PP composite pervaporation membrane for oscillating temperature process. Although both processes can be used for inland brine desalination purpose, the PV process with the GO/PP membrane delivered higher water productivity and improved performance stability than the submerged VMD process using PP membranes. More importantly, under an industrially-relevant condition with oscillated temperatures, the GO/PP composite membrane showed significantly enhanced performance sustainability than the PP membrane. Through a comprehensive investigation, it was found that the presence of Ca2+ and Mg2+ in the inland brine enabled cation- π interaction and cross linking with oxygen functional groups between GO nanosheets, which prevented GO layer swelling and gave an ideal interlayer spacing between the GO nanosheets that led to molecular

Fig. 13. Accumulated permeate volume from GO/PP pervaporation membrane (0.112 mg/cm2 GO loading) with 350 RPM stirring for concentrated model inland brine treatment under constant and oscillating feed temperature. 10

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sieving functionality during shut-down period. Therefore, the GO layer impeded the transport of salt ions, particularly under oscillating temperature. Furthermore, the GO/PP membranes offered greater resistance to salt crystallization than the PP membrane. This was largely due to the smoother, hydrophilic and negatively charged surface of the GO/PP composite membranes, which increased electrostatic repulsion and energy barrier for heterogeneous nucleation. Even though chemical cleaning removed a small fraction of the GO nanosheets on the membrane surface, the desalination performance of the GO/PP composite membranes after two cleaning cycles was still much better than that of the PP membrane. In addition, the GO/PP membranes prevented undesired membrane wetting with the feed solution containing humic acid. Interestingly, it was also found that the GO/PP membrane produced more water in an oscillating temperature mode than the same membrane under the constant temperature system. This study demonstrated that the PV process with the GO/PP composite membrane had great potential for concentrated inland brine desalination, especially when the process required frequent shutdown and restart for maintenance or temperature oscillation from solar energy and portable system. However, further research on improving the robustness of the GO/PP composite membranes in harsh chemicals is needed.

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