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Identifying pore wetting thresholds of surfactants in direct contact membrane distillation
Accepted Manuscript Identifying Pore Wetting Thresholds of Surfactants in Direct Contact Membrane Distillation Coral R. Taylor, Pejman Ahmadiannamini, Sage R. Hiibel PII: DOI: Reference:
S1383-5866(18)32189-0 https://doi.org/10.1016/j.seppur.2019.01.061 SEPPUR 15291
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
23 June 2018 23 January 2019 23 January 2019
Please cite this article as: C.R. Taylor, P. Ahmadiannamini, S.R. Hiibel, Identifying Pore Wetting Thresholds of Surfactants in Direct Contact Membrane Distillation, Separation and Purification Technology (2019), doi: https:// doi.org/10.1016/j.seppur.2019.01.061
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Identifying Pore Wetting Thresholds of Surfactants in Direct Contact Membrane Distillation
Coral R. Taylor1, Pejman Ahmadiannamini2, Sage R. Hiibel2*
1
Civil and Environmental Engineering, University of Nevada, Reno
2
Chemical and Material Engineering, University of Nevada, Reno
*Corresponding author Sage R. Hiibel Chemical and Materials Engineering University of Nevada, Reno 1664 N. Virginia Street, MS 0388 Reno, NV 89557 USA [email protected]
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Abstract Direct contact membrane distillation (DCMD) is a potential on-site water treatment option for water reuse at solar energy facilities (SEF) that require water for cooling and cleaning of panels/mirrors. Surfactants, which are often used as part of the panel washing, can negatively impact the hydrophobic DCMD membranes and diminish the quality of the treated water. In this work, surfactant effects on DCMD were evaluated to assist SEF in developing standard operating procedures for the reuse of wash waters by on-site DCMD systems. A non-ionic surfactant and an anionic surfactant were tested in a bench-scale DCMD system using commercially available hydrophobic membranes of varying materials and pore sizes. The surfactant concentration at which pore wetting occurred was determined using a novel graphical method developed for this work and verified analytically. All membranes evaluated had reduced hydrophobicity after being exposed to the surfactants, and all membrane materials evaluated were determined to have lower pore wetting concentrations for the non-ionic surfactant than of the anionic surfactant. Pore size had no significant effect on pore wetting concentrations. Overall, it was determined that the membrane material had the most significant effect on membrane performance, with PTFE membranes being able to tolerate higher concentrations of both surfactant types before pore wetting.
Keywords Pore wetting; Critical surfactant concentration; Membrane distillation; Water-energy nexus; Solar panel washing; Water reuse
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Abbreviations
CSP
concentrated solar power
DCMD
direct contact membrane distillation
DI
deionized
ECTFE
ethylene chlorotrifluoroethylene
EIS
electrochemical impedance spectroscopy
HLB
hydrophilic-lipophilic balance
LEP
liquid entry pressure
MD
membrane distillation
NaCl
sodium chloride
PP
polypropylene
PS80
polysorbate80
PTFE
polytetrafluoroethylene
PV
photovoltaic
PVDF
polyvinylidene fluoride
SDS
sodium dodecyl sulfate
SEF
solar energy facilities
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1. Introduction As the world population and the scarcity of adequate freshwater supplies continue to increase, many areas are looking to wastewaters as potential alternative sources for potable reuse. With the quality of source waters decreasing, the amount of energy required to treat those waters also increases. The water-energy nexus is the concept that energy and water are inextricably linked; water is used to produce energy and energy is used to produce water (Scott et al., 2011; Stillwell et al., 2011; Vakilifard et al., 2018). To counter adverse environmental impacts associated with added energy requirements, treatments that can utilize renewable energy sources are of interest. For locations with high amounts of solar irradiance, solar photovoltaic (PV) or concentrated solar power (CSP) installations may be the preferred renewable energy source (Klise et al., 2013; Moh and Ting, 2016); however, many areas with abundant solar resources have limited water availability, which can hinder the development of solar power (Bukhary et al., 2018).
Two major factors that lead to a decrease in the efficiency of solar PV and CSP installations are elevated temperatures and soiling of panels and mirrors. Solar panel efficiency can decrease by 0.25-0.50% per °C, thus water-induced cooling can increase panel efficiency by 6-30% (Moh and Ting, 2016; Nižetić et al., 2016; Smith et al., 2014). Energy outputs can also be reduced from environmental dust, pollen, pollution, bird excrement, and other particles on panels and mirrors (Appels et al., 2013; Khonkar et al., 2014; Massi Pavan et al., 2011; Vivar et al., 2010), especially in dry climates with limited rainfall. Accumulation of dust and other particles decreases the irradiation that reaches the solar cell, resulting in a loss of generated power up to 20% daily energy loss in arid environments (Zorrilla-Casanova et al., 2011). Cleaning the solar panels is required to restore efficiency (Appels et al., 2013; Khonkar et al., 2014; Massi Pavan et
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al., 2011; Zorrilla-Casanova et al., 2011); for large-scale PV arrays, 1-10% improvements were noted after washing (Haeberlin and Graf, 1998; Massi Pavan et al., 2011), while up to 26% improvements were found for CSP facilities (Vivar et al., 2010). Unfortunately, washing can require significant amounts of water, 0.08 to 0.15 m3 water per MWh, with cleaning frequency dependent on site-specific characteristics and panel/mirror materials and orientation (Bukhary et al., 2018). The use of anionic or non-ionic surfactants can minimize water usage while still effectively removing deposited particles (Abd-Elhady et al., 2011; Appels et al., 2013; Khonkar et al., 2014; Moharram et al., 2013), and can help preserve solar cell efficiency at a constant level over time.
Membrane distillation (MD) is an emerging water separation technology that shows promise for reducing fossil fuel energy requirements and addressing the water-energy nexus. MD is a thermal process that utilizes a microporous, hydrophobic membrane that allows the transport of water vapor while rejecting non-volatile contaminants such as salts and minerals. A temperature difference between the warm, ‘dirty’ feed water and the cool, clean product water creates a partial vapor pressure difference across the membrane that drives the passage of water vapor from the feed solution to the distillate stream. The primary advantages of MD are that it operates at low pressures, can maintain efficacy at high salt concentrations, and has a high rejection of non-volatile compounds (Cath et al., 2004; Khayet and Matsuura, 2011). Because MD is a thermally-driven process, another advantage is that low-grade heat sources, such as industrial waste heat or solar heat, can be utilized to minimize the electrical energy needed for feed water heating (Dow et al., 2016; Elzahaby et al., 2016). Solar energy can be used to generate both the
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thermal energy that drives the phase-change process, and electricity to operate pumps and other equipment (Chafidz et al., 2014; Selvi and Baskaran, 2015).
The advantages of MD make it an ideal technology to couple with solar energy, specifically for the recovery and reuse of panel wash water (Fig. 1). Water and surfactants applied to the surface of the dirty panel can serve the dual purpose of removing dust particles and reducing panel temperatures, resulting in a clean, cooled panel with rejuvenated efficiency. The warm, dirty panel rinse water containing dust particles and surfactants can be used as a pre-heated feed solution for MD. The cooler distillate water produced from MD can then be reused for panel washing and to remove additional heat from the panel surfaces. It should be noted that a small amount of make-up freshwater will be required, and a small, concentrated waste stream containing surfactants and material removed from the solar panels may require further treatment before discharge.
Make-Up Water
Waste Retentate Stream
Fig. 1. Schematic depiction of the interaction between DCMD, solar panels, and the associated water cycle. 6
Previous studies have utilized direct contact MD (DCMD) to treat water contaminated with surfactants. Chew et al. investigated the roles of different types and concentrations of oils and surfactants as emulsions on the fouling and wetting behaviors of a polyvinylidene fluoride (PVDF) membrane in DCMD treating produced water (Chew et al., 2017). They found that oils in the emulsion delayed the onset of membrane wetting and that the time of wetting onset for different surfactants depended on the surfactants’ hydrophilic-lipophilic balance (HLB) values; surfactants with lower HLB values were adsorbed more onto the membrane surface and caused earlier wetting (Chew et al., 2017). Eykens et al. found that wetting from surfactant-oil emulsions can be predicted by surface tension and water contact angle measurements, with less hydrophobic membranes being more susceptible to wetting than more hydrophobic ones (Eykens et al., 2017). Chen et al. found that superhydrophobic PVDF membranes had strongly negative surface charges and stable MD performance for an anionic surfactant, but experienced severe wetting with a cationic surfactant (Ying Chen et al., 2017).
Membrane hydrophobicity is critical for MD performance as it prevents the liquids in the feed solution from permeating to the distillate side (Khayet and Matsuura, 2011; Rezaei et al., 2017). A critical failure of MD is pore wetting, which occurs when the pores of the hydrophobic MD membrane are filled with liquid water such that the solutes on the feed side leak through the membrane and contaminate the distillate water (Warsinger et al., 2017). Surfactants have been reported to decrease the effective membrane hydrophobicity by reducing the pore liquid entry pressure (LEP) of the membrane, leading to pore wetting and ultimately MD failure (Camacho et al., 2013; Chew et al., 2017; Franken et al., 1987; Lin et al., 2015). Early detection of pore
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wetting is difficult to determine, although once the membrane is partially wetted, the distillate conductivity will reflect the passage of salt through the wetted membrane pores (Resaei et al., 2018). Recently, a method utilizing in situ electrochemical impedance spectroscopy (EIS) was used to identify when real-time pore wetting started to occur (Yuanmiaoliang Chen et al., 2017); however, to date this method has been limited to a single study (Yuanmiaoliang Chen et al., 2017) and has not been validated for use with anionic surfactants. Another recent development in pore wetting detection is the creation of electrically conductive MD membranes (Ahmed et al., 2017); given that such membranes are not commercially available and require fabrication, this option was not considered to be viable for this work.
In this work, a bench-scale DCMD system was used to treat water with an anionic or non-ionic surfactant, and a novel graphical method was developed to determine the surfactant concentration threshold at the onset of pore wetting. The effect of different surfactant types and the surfactant concentration threshold where pore wetting occurs were determined for different membrane materials and pore sizes. From this work, we establish that MD is a valid method for producing high-quality reuse water from surfactant-laden solar panel wash water, and we can begin to establish protocols for acceptable surfactant loads that can be used when cleaning solar panels. The work also opens the potential of applying MD to a broader range of waters requiring treatment, such as greywater and other reuse waters.
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2. Materials and Methods 2.1.
Membranes and chemicals
Hydrophobic, microporous membranes were obtained from Osmonics (Minnetonka, MN), Clarcor IA (Overland Park, KS), and 3M (Davidson, NC) (Table 1, Table 2). Sodium chloride (NaCl) was purchased from Fisher Scientific (Hampton, NH) and pool salt from Diamond Crystal (Wayzata, MN); polysorbate80 (PS80; trade name Tween®80) and 85% pure sodium dodecyl sulfate (SDS) were purchased from Acros Organics (Pittsburgh, PA). HPLC-grade benzene, buffer solution – sulfate type, and detergents reagent powder pillows were purchased from Hach Company (Loveland, CO).
2.2.
Bench-scale DCMD system
Coupons from ten different membranes were tested in a bench-scale DCMD system (Fig. 2), using a custom-made, crossflow flat sheet acrylic membrane module with an effective membrane surface area of 78.31 cm2. Two plastic, mesh-type spacers were used on either side of the membrane to generate turbulence, reduce polarization effects, and promote mass transfer. The module was operated horizontally with the warm feed side on top. Two gear pumps (ColeParmer; Vernon Hills, IL) circulated the feed and distillate solutions counter-currently. A water bath (Precision, ThermoFisher Scientific; Pittsburgh, PA) was used for heating the feed solution; the distillate water was cooled via a heat exchanger connected to a chiller (ThermoFlex 1400, ThermoFisher Scientific; Pittsburgh, PA) circulating ethylene glycol. Real-time data were collected every 30 or 60 seconds using LabView 13.0.1 software (National Instruments; Austin, TX) and a T7 data acquisition module (LabJack; Lakewood, CO). Temperatures were monitored using thermocouples (LabJack; Lakewood, CO) at the feed and distillate inlet and outlet ports of
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the membrane module. The concentration of NaCl in the distillate was monitored using an electrical conductivity probe (Cole-Parmer; Vernon Hills, IL) and transmitter (Eutech Instruments; Vernon Hills, IL). Water flux was determined by the mass gained in the distillate loop using a balance (Mettler Toledo; Columbus, OH), as described in (Rao et al., 2015).
Fig. 2. Process schematic of bench-scale DCMD system. C – conductivity probe/meter, T – thermocouple.
2.3.
Experimental set-up and procedure
2.3.1. Flux testing Bench-scale DCMD experiments were performed with a warm feed solution composed of 35.0 g/L NaCl and deionized (DI) water (no surfactant) to determine the flux of the commercial membranes. The feed and distillate streams were maintained at 40.9 ± 2.6 and 24.1 ± 0.5 °C, respectively, and were circulated at 0.5 L/min (17.2 cm/s linear flow velocity). The system was operated for ~2 hr for each membrane, with an initial period of stabilization (typically 20 – 40 min), followed by a minimum of 60 min of stable operation; the flux for each membrane was calculated as the average flux during stable operation.
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2.3.2. Surfactant tests All bench-scale DCMD experiments were performed with a feed solution containing 17.5 g/L NaCl in DI water and one of the two surfactants at varying concentrations. The initial surfactant concentrations for both the anionic SDS and non-ionic PS80 ranged from 90 – 6,500 mg/L and 50 – 250 mg/L, respectively. All membranes were tested with SDS, and then a select group of five membranes were also tested with PS80. The feed solution was heated to 41.1 ± 5.6 °C and circulated at 0.5 L/min. The distillate was maintained at 21.6 ± 2.1 °C and circulated at 0.5 L/min. MD was operated beyond the point where the membrane was wetted by the surfactant, as determined by a large increase in distillate conductivity. After DCMD was terminated, the system was flushed with DI water on both the feed and distillate sides to remove salt and surfactants from the system, and the membrane was removed for characterization. A minimum of three tests for each membrane and surfactant were performed, and the average value along with the standard deviation are reported in Table 2.2 as [SDS]PW and [PS80]PW “calculated”.
2.4.
Pore wetting determination and calculation
A novel graphical method was developed to determine the surfactant concentration threshold at the onset of pore wetting for the multiple membranes. Others have used an increase in distillate conductivity and/or water flux as an indication of membrane pore wetting (Chew et al., 2017); however, in this work the two indicators did not provide consistent results with each other for the various membranes evaluated (Figs. 3a, S1). Ideally, the membrane offers 100% rejection and no salt passes through the membrane from the feed to the distillate sides prior to the onset of pore wetting. In actuality, membranes are imperfect and a small amount salt passes through the membrane even before pore wetting; during this time the specific salt flux (Js/Jw) is stable and
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the feed side salt concentration will increase gradually as water vapor is the primary species passing through the membrane. When pore wetting occurs, membrane selectivity is lost and water vapor along with both liquid water and salt can freely pass through the wetted pores, resulting in an increase in specific salt flux. However, depending on the nature of the membrane, pore wetting can occur gradually or rapidly, and may occur only in localized locations on the membrane or across the entire membrane. Thus, at the system level, membrane selectivity may be slowly compromised and the increase in specific salt flux alone may not be initially apparent due to partial salt rejection via the remaining dry pores. In such cases, the feed-side salt concentration continues to increase (Fig. S1), in some cases quite rapidly depending on how rapidly the membrane wetting process occurs, resulting in an increased concentration difference driving force between the feed and distillate through the wet pores. By plotting the specific salt flux normalized to the feed-side salt concentration, all the phenomena occurring during the pore wetting process is captured in a single graphical representation (Fig. 3b), and can be used to compare multiple membranes. In this work, membrane wetting was considered to occur at a threshold of 5% of the normalized specific salt flux. This value was selected as the graphical representation of these data, which is reminiscent of an adsorption isotherm, and 5% is a typical value used in adsorption theory (Seader and Henley, 2006), although the threshold can be adjusted as appropriate based on the sensitivity of any downstream applications of the product water.
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Fig. 3. Development of novel graphical pore wetting method: (a) water flux and distillate conductivity increase after pore wetting¸ but are not consistent indicators of where pore wetting occurs; (b) specific salt flux normalized to the feed-side salt concentration provides a graphical method to determine pore wetting. Data are shown for 3M PP 0.2 membrane with SDS.
2.5.
Calculations
The mass of the distillate was measured continuously and used to calculate the water flux using Eq. (1): (1)
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where Jw (L/m2·hr) is the water flux, Mw,d (g) is the mass of the distillate, ρ is the density of distillate (assumed to be equal to the density of pure water), A is the effective membrane area (7.831x10-4 m2), and Δt (hr) is the sampling time.
The conductivity of the distillate side was continuously measured and used to calculate the distillate salt concentration using Eq. (2): (2) where [NaCl]d (g/L) is the distillate salt concentration, EC (μS/cm) is the electrical conductivity of the distillate, and 1560 is the conversion factor between μS/cm and g/L (Peet and Foundation, 2001). The conductivity was validated before and after DCMD trials, using known conductivity standards.
The salt flux across the membrane was then calculated using Eq. (3): (3) where Js (g/m2·hr) is the salt flux and Ms,d (g) is the mass of salt in the distillate.
The specific salt flux ( ) was calculated using Eq. (4): (4) The concentration of NaCl in the feed solution was calculated using the mass balance in Eq. (5): (5) where [NaCl]f (g/L) is the salt concentration in the feed solution, Ms,i (g) is the initial salt mass in the feed solution, and Vf,i (L) is the initial feed volume.
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The surfactant concentration at the time of pore wetting, determined when
/ [NaCl]f = 0.05,
was calculated using Eq. (6), assuming 100% rejection of the surfactant prior to pore wetting: (6) where [Surf]PW (mg/L) is the concentration of the surfactant at the time of pore wetting, [Surf]i (mg/L) is the known initial concentration of the surfactant, and Vd,PW (L) is the volume of distillate water at the time of pore wetting.
2.6.
Surfactant measurement
Samples of the feed solution were taken during the DCMD runs and stored at 4 °C until they were analyzed for surfactant concentrations. The concentrations of both SDS and PS80 were measured utilizing Hach Method 8028 “Surfactants, Anionic (Detergents)”, using a spectrophotometer (DR 6000, Hach Company; Loveland, CO). Due to the difference between the non-ionic surfactant (PS80) and the anionic surfactant (SDS), a PS80-specific calibration curve was created in the range of Method 8028 (0.002 – 0.275 mg/L) to accurately measure the concentration of the non-ionic surfactant. These values are shown in Table 2.2 as [SDS]PW and [PS80]PW “measured”. No standard deviations are reported as only the single sample nearest to the calculated time of pore wetting onset was measured.
2.7.
Membrane characterization
To investigate the effect of the different surfactants on the membranes, the hydrophobicity of the membranes before and after exposure to surfactants was evaluated. The contact angle with water (θ) was used as an indication of membrane hydrophobicity; hydrophobic membranes are
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considered to have 90° < θ < 150° (Ashoor et al., 2016). Dry membrane samples were used for all contact angle measurements, which were performed using an optical tensiometer (OneAttension, model Theta, Biolin Scientific; Stockholm, Sweden) using the sessile drop method at ambient temperature. DI water droplets (~10 μL) were placed on the membrane surface by the automated syringe attachment and the mean contact angle value was calculated from the measured right and left contact angles over 10 s. A minimum of five measurements at different locations on the feed-side surface were made for each membrane, and the average θ ± standard deviation is reported.
3. Results 3.1.
Membrane hydrophobicity
Membrane contact angles before and after exposure to surfactants were measured to determine how membrane hydrophobicity was affected in DCMD systems treating surfactant-laden waters (Table 1). The water flux for membranes treating a saline solution without surfactants was also measured (Table 1). Table 1: Contact angles of various commercial membranes before (virgin) and after pore wetting by SDS or PS80. PP – polypropylene; ECTFE – ethylene chlorotrifluoroethylene; PVDF – polyvinylidene fluoride; PTFE – polytetrafluoroethylene. Nominal Membrane Contact Angle (°) Pore Water Size Flux After SDS After PS80 Supplier Material (μm) (L/m2-hr) Virgin Pore Wetting Pore Wetting 3M PP 0.1 12.2 144.8 ± 1.6 140.3 ± 3.4 3M PP 0.2 17.7 146.3 ± 2.4 136.3 ± 3.5 3M PP 0.2 17.7 126.8 ± 2.1 120.4 ± 2.2 3M ECTFE 0.2 16.4 111.1 ± 1.7 103.1 ± 2.3 3M PP 0.45 17.4 128.2 ± 2.2 122.6 ± 4.0 Osmonics PP 0.2 13.3 143.2 ± 1.0 133.0 ± 6.9 119.9 ± 6.5 14.8 134.0 ± 2.6 Osmonics PVDF 0.45 131.4 ± 4.7 116.1 ± 4.4 Clarcor IA PTFE 0.11 9.2 135.5 ± 2.8 120.5 ± 4.3 113.5 ± 3.8 Clarcor IA PTFE 0.22 15.4 129.3 ± 1.6 113.6 ± 5.2 110.2 ± 2.7 Clarcor IA PTFE 0.45 25.2 134.4 ± 2.1 123.8 ± 4.4 111.8 ± 4.0
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No correlation between pore size and hydrophobicity was observed for the virgin membranes or for the surfactant-exposed membranes. However, strong positive correlations were found for the virgin contact angles and the post-surfactant contact angles (SDS: R2=0.85; PS80: R2=0.80), suggesting that membranes with higher initial hydrophobicity retain a higher level of hydrophobicity even after pore wetting. It should be noted that all membranes remained hydrophobic (θ > 90°) after contact with both SDS and PS80.
3.2.
Pore wetting surfactant concentrations
The surfactant concentrations at pore wetting were calculated as described above and compared to the measured surfactant concentrations of the feed stream after the DCMD test was terminated (Table 2). The evaluated membranes responded differently to the anionic SDS compared to the non-ionic PS80, with much lower [PS80]PW values observed in all cases.
Table 2: Calculated and measured SDS and PS80 concentrations at pore wetting for various commercial membranes. PP – polypropylene; ECTFE – ethylene chlorotrifluoroethylene; PVDF – polyvinylidene fluoride; PTFE – polytetrafluoroethylene. Nominal [SDS]PW (mg/L) [PS80]PW (mg/L) Pore Size Calculated Measured Calculated Measured Supplier Material (μm) 3M PP 0.1 223.8 ± 6.4a 195 3M PP 0.2 120.7 ± 5.9a 117 3M PP 0.2 110.4 ± 2.9a 115 3M ECTFE 0.2 121.3 ± 13.5a 141 3M PP 0.45 136.5 ± 2.8a ** Osmonics PP 0.2 187.8 ± 0.7b ** 88.0 ± 25.2 49 Osmonics PVDF 0.45 793.7 ± 149.1c 718 230.0 ± 24.7 262 Clarcor IA PTFE 0.11 16,588.4 ± 165.0a 20,925 298.0 ± 87.3 271 Clarcor IA PTFE 0.22 2,696.4 ± 240.7b 3,600 181.0 ± 63.0 114 Clarcor IA PTFE 0.45 3,923.8 ± 195.6a 7,200 198.1 ± 49.4 91 ** – measurements could not be taken due to insufficient sample volume Number of replicates: a = 3, b = 4, c = 6.
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No apparent correlation between pore size and pore wetting concentration was observed for either SDS or PS80. The PTFE membranes exhibited higher [SDS]PW than the PP, PVDF, or ECTFE membranes evaluated, which agrees with previous results in the literature (Eykens et al., 2017; Thomas et al., 2017). However, these membranes also had the most variable results in our work. The calculated [PS80]PW values were similar between the PTFE and PVDF membranes, while the PP membrane had a slightly lower calculated [PS80]PW.
4. Discussion DCMD performance varied depending on surfactant type and membrane material. The initial membrane hydrophobicity also varied between suppliers and materials. All membranes tolerated higher surfactant concentrations than expected; in one study, wetting of a PTFE membrane occurred with a concentration as low as 0.1 mM SDS (Lin et al., 2014), while others found wetting of a PVDF membrane occurred with concentrations of 50 mg/L SDS, Tween20 and Span20 (Chew et al., 2017). However, a decrease in hydrophobicity was observed for all membranes after exposure to both surfactant types. The [PS80] PW values were much lower than the corresponding [SDS]PW values for a given membrane (Table 2), indicating that different wetting regimes may be present for the non-ionic surfactant and the anionic surfactant. Cationic surfactants were not considered in this study, as they have been shown to be less effective at removing deposited sand particles from solar panels in previous work (Abd-Elhady et al., 2011).
An important parameter for DCMD is the LEP, which describes a membrane’s ability to resist wetting, and is the pressure that the feed water must overcome to penetrate the membrane pores
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resulting in wetting (Warsinger et al., 2017). To prevent wetting, the total pressure (hydrostatic pressure plus partial vapor pressures, where hydrostatic pressure is generally much lower than the partial vapor pressures) must be less than the LEP. The LEP (Pa) can be calculated using the Laplace (Cantor) equation in Eq. (7): (7) where B is the dimensionless geometric factor for the pore shape (0 < B ≤ 1, B = 1 for perfectly cylindrical pores), (N/m) is the surface tension of the feed solution liquid, (°) is the contact angle between the membrane and the feed solution liquid, and rmax (m) is the pore radius (Camacho et al., 2013; Yuanmiaoliang Chen et al., 2017). Surfactants can impact MD performance by accumulating at the membrane surface with their hydrophilic head groups aligning into solution and their hydrophobic tail groups aligning with the membrane surface (Yuanmiaoliang Chen et al., 2017). The presence of surfactants at the membrane surface lowers the LEP by reducing both and (Yuanmiaoliang Chen et al., 2017; El-Abbassi et al., 2013; Franken et al., 1987; Garcia-Payo et al., 2000).
Despite its role in LEP (based on Eq. 7), the pore size did not appear to play a role in pore wetting for the membranes tested in this work. This may be attributed, in part, to the narrow pore size range evaluated (0.1 – 0.45 m; Table 1). However, a clear reduction in θ was observed for all membranes exposed to surfactants in this work (Table 1), resulting in the feed solution intruding into the pores and a corresponding increase in distillate conductivity (Fig. 3a). It should be noted that the contact angle measurements were taken from membrane samples collected after the pore wetting experiments were completed; these membranes were exposed to SDS or PS80 concentrations higher than the reported [SDS]PW and [PS80] PW. All membranes were also rinsed 19
with DI water and dried prior to testing with the tensiometer. Although any of these factors may have introduced error to the contact angle results, since all membranes were treated in the same fashion this potential error was systemic and is not expected to have affected comparative results.
One of the challenges of this work was how to consistently identify the onset of pore wetting prior to catastrophic performance failure and to determine the pore wetting surfactant concentrations. A recent effort successfully used EIS to determine real-time pore wetting (Yuanmiaoliang Chen et al., 2017), although the EIS instrumentation costs are high compared to the simple analytical needs of the graphical method that relies on conductivity and mass data to make simple calculations. The novel graphical method developed addresses this problem by incorporating water flux, salt flux, and salt accumulation on the feed side into a single plotted parameter (Fig. 3b). The method was successfully used to identify the onset of pore wetting and calculate pore wetting concentrations for several different membranes and different surfactant types. It is important to note that this graphical method is only valid for the onset of pore wetting and does not provide any guidance regarding partially wetted or long-term operation of the membrane in the presence of surfactant. This is clear from Figures 3b and S1, where maximum values of the specific salt flux normalized to the feed-side salt concentration are observed, which corresponds to the rapid increase in the feed side NaCl concentration while the membrane is partially wetted.
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To verify the calculated [SDS]PW and [PS80] PW values, grab samples of the feed were collected and analyzed using a spectrophotometric method. The observed differences between measured and calculated values can be attributed to the challenge of collecting the samples at the onset of pore wetting, which was unknown at the time of experiment. Samples were collected based on distillate side conductivity, which was an inconsistent indicator of pore wetting (Fig. S1). Regardless, the measured and calculated pore wetting concentrations were strongly correlated (SDS: R2=0.99; PS80: R2=0.76), validating the calculated values (Table 2). The establishment of [SDS]PW and [PS80] PW for commercially available DCMD membranes provides important guidance for treating waters containing surfactants, including wash water from solar energy facilities as well as other reuse waters. Results of this study can assist the solar energy industry in developing standard operating procedures for the selection of surfactant type and concentrations used for cleaning solar panels/mirrors that will enable on-site DCMD systems to operate without pore wetting. This, in turn, may be a step towards addressing some concerns of the water-energy nexus.
5. Conclusions The overall results of this work indicate that membrane material and surfactant type can affect DCMD performance when treating surfactant-containing waters. Key conclusions from this work are:
The threshold surfactant concentrations at which pore wetting occurred was determined using a novel graphical method developed for this work and verified analytically;
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[SDS]PW and [PS80]PW values were determined for commercially available hydrophobic, microporous membranes commonly used for MD;
[SDS]PW values were significantly higher than [PS80]PW values; and
[SDS]PW and [PS80] PW values were much higher for PTFE membranes than for other membrane materials tested.
6. Acknowledgements This material is based upon work supported by the National Science Foundation under Grant No. IIA-1301726. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. A portion of the membrane samples used were generously provided by Clarcor IA and 3M.
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Supplemental Figures:
Fig. S1. Representative behavior of three different membranes illustrating the need for the novel graphical method to determine the onset of pore wetting that was used to calculate [SDS] PW and [PS80] PW. Data are shown for 3M ECTFE 0.2 membrane with SDS (a-c), Clarcor IA PTFE 0.45 membrane with SDS (d-f), and Osmonics PVDF 0.45 membrane with PS80 (g-i). Symbol legend: ♦ JW; ● Conductivity; ▲ JS/JW; ▬ [NaCl]Feed; ■ (JS/JW)/[NaCl]Fee d
29
Highlights
DCMD was able to treat saline water containing up to 1.66 wt% SDS
PTFE demonstrated the highest pore wetting resistance in the presence of surfactants
Membranes tolerated higher anionic SDS concentrations than non-ionic PS80
Novel graphical method to determine onset of pore wetting developed
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Graphical abstract
31
S1383-5866(18)32189-0 https://doi.org/10.1016/j.seppur.2019.01.061 SEPPUR 15291
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
23 June 2018 23 January 2019 23 January 2019
Please cite this article as: C.R. Taylor, P. Ahmadiannamini, S.R. Hiibel, Identifying Pore Wetting Thresholds of Surfactants in Direct Contact Membrane Distillation, Separation and Purification Technology (2019), doi: https:// doi.org/10.1016/j.seppur.2019.01.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Identifying Pore Wetting Thresholds of Surfactants in Direct Contact Membrane Distillation
Coral R. Taylor1, Pejman Ahmadiannamini2, Sage R. Hiibel2*
1
Civil and Environmental Engineering, University of Nevada, Reno
2
Chemical and Material Engineering, University of Nevada, Reno
*Corresponding author Sage R. Hiibel Chemical and Materials Engineering University of Nevada, Reno 1664 N. Virginia Street, MS 0388 Reno, NV 89557 USA [email protected]
1
Abstract Direct contact membrane distillation (DCMD) is a potential on-site water treatment option for water reuse at solar energy facilities (SEF) that require water for cooling and cleaning of panels/mirrors. Surfactants, which are often used as part of the panel washing, can negatively impact the hydrophobic DCMD membranes and diminish the quality of the treated water. In this work, surfactant effects on DCMD were evaluated to assist SEF in developing standard operating procedures for the reuse of wash waters by on-site DCMD systems. A non-ionic surfactant and an anionic surfactant were tested in a bench-scale DCMD system using commercially available hydrophobic membranes of varying materials and pore sizes. The surfactant concentration at which pore wetting occurred was determined using a novel graphical method developed for this work and verified analytically. All membranes evaluated had reduced hydrophobicity after being exposed to the surfactants, and all membrane materials evaluated were determined to have lower pore wetting concentrations for the non-ionic surfactant than of the anionic surfactant. Pore size had no significant effect on pore wetting concentrations. Overall, it was determined that the membrane material had the most significant effect on membrane performance, with PTFE membranes being able to tolerate higher concentrations of both surfactant types before pore wetting.
Keywords Pore wetting; Critical surfactant concentration; Membrane distillation; Water-energy nexus; Solar panel washing; Water reuse
2
Abbreviations
CSP
concentrated solar power
DCMD
direct contact membrane distillation
DI
deionized
ECTFE
ethylene chlorotrifluoroethylene
EIS
electrochemical impedance spectroscopy
HLB
hydrophilic-lipophilic balance
LEP
liquid entry pressure
MD
membrane distillation
NaCl
sodium chloride
PP
polypropylene
PS80
polysorbate80
PTFE
polytetrafluoroethylene
PV
photovoltaic
PVDF
polyvinylidene fluoride
SDS
sodium dodecyl sulfate
SEF
solar energy facilities
3
1. Introduction As the world population and the scarcity of adequate freshwater supplies continue to increase, many areas are looking to wastewaters as potential alternative sources for potable reuse. With the quality of source waters decreasing, the amount of energy required to treat those waters also increases. The water-energy nexus is the concept that energy and water are inextricably linked; water is used to produce energy and energy is used to produce water (Scott et al., 2011; Stillwell et al., 2011; Vakilifard et al., 2018). To counter adverse environmental impacts associated with added energy requirements, treatments that can utilize renewable energy sources are of interest. For locations with high amounts of solar irradiance, solar photovoltaic (PV) or concentrated solar power (CSP) installations may be the preferred renewable energy source (Klise et al., 2013; Moh and Ting, 2016); however, many areas with abundant solar resources have limited water availability, which can hinder the development of solar power (Bukhary et al., 2018).
Two major factors that lead to a decrease in the efficiency of solar PV and CSP installations are elevated temperatures and soiling of panels and mirrors. Solar panel efficiency can decrease by 0.25-0.50% per °C, thus water-induced cooling can increase panel efficiency by 6-30% (Moh and Ting, 2016; Nižetić et al., 2016; Smith et al., 2014). Energy outputs can also be reduced from environmental dust, pollen, pollution, bird excrement, and other particles on panels and mirrors (Appels et al., 2013; Khonkar et al., 2014; Massi Pavan et al., 2011; Vivar et al., 2010), especially in dry climates with limited rainfall. Accumulation of dust and other particles decreases the irradiation that reaches the solar cell, resulting in a loss of generated power up to 20% daily energy loss in arid environments (Zorrilla-Casanova et al., 2011). Cleaning the solar panels is required to restore efficiency (Appels et al., 2013; Khonkar et al., 2014; Massi Pavan et
4
al., 2011; Zorrilla-Casanova et al., 2011); for large-scale PV arrays, 1-10% improvements were noted after washing (Haeberlin and Graf, 1998; Massi Pavan et al., 2011), while up to 26% improvements were found for CSP facilities (Vivar et al., 2010). Unfortunately, washing can require significant amounts of water, 0.08 to 0.15 m3 water per MWh, with cleaning frequency dependent on site-specific characteristics and panel/mirror materials and orientation (Bukhary et al., 2018). The use of anionic or non-ionic surfactants can minimize water usage while still effectively removing deposited particles (Abd-Elhady et al., 2011; Appels et al., 2013; Khonkar et al., 2014; Moharram et al., 2013), and can help preserve solar cell efficiency at a constant level over time.
Membrane distillation (MD) is an emerging water separation technology that shows promise for reducing fossil fuel energy requirements and addressing the water-energy nexus. MD is a thermal process that utilizes a microporous, hydrophobic membrane that allows the transport of water vapor while rejecting non-volatile contaminants such as salts and minerals. A temperature difference between the warm, ‘dirty’ feed water and the cool, clean product water creates a partial vapor pressure difference across the membrane that drives the passage of water vapor from the feed solution to the distillate stream. The primary advantages of MD are that it operates at low pressures, can maintain efficacy at high salt concentrations, and has a high rejection of non-volatile compounds (Cath et al., 2004; Khayet and Matsuura, 2011). Because MD is a thermally-driven process, another advantage is that low-grade heat sources, such as industrial waste heat or solar heat, can be utilized to minimize the electrical energy needed for feed water heating (Dow et al., 2016; Elzahaby et al., 2016). Solar energy can be used to generate both the
5
thermal energy that drives the phase-change process, and electricity to operate pumps and other equipment (Chafidz et al., 2014; Selvi and Baskaran, 2015).
The advantages of MD make it an ideal technology to couple with solar energy, specifically for the recovery and reuse of panel wash water (Fig. 1). Water and surfactants applied to the surface of the dirty panel can serve the dual purpose of removing dust particles and reducing panel temperatures, resulting in a clean, cooled panel with rejuvenated efficiency. The warm, dirty panel rinse water containing dust particles and surfactants can be used as a pre-heated feed solution for MD. The cooler distillate water produced from MD can then be reused for panel washing and to remove additional heat from the panel surfaces. It should be noted that a small amount of make-up freshwater will be required, and a small, concentrated waste stream containing surfactants and material removed from the solar panels may require further treatment before discharge.
Make-Up Water
Waste Retentate Stream
Fig. 1. Schematic depiction of the interaction between DCMD, solar panels, and the associated water cycle. 6
Previous studies have utilized direct contact MD (DCMD) to treat water contaminated with surfactants. Chew et al. investigated the roles of different types and concentrations of oils and surfactants as emulsions on the fouling and wetting behaviors of a polyvinylidene fluoride (PVDF) membrane in DCMD treating produced water (Chew et al., 2017). They found that oils in the emulsion delayed the onset of membrane wetting and that the time of wetting onset for different surfactants depended on the surfactants’ hydrophilic-lipophilic balance (HLB) values; surfactants with lower HLB values were adsorbed more onto the membrane surface and caused earlier wetting (Chew et al., 2017). Eykens et al. found that wetting from surfactant-oil emulsions can be predicted by surface tension and water contact angle measurements, with less hydrophobic membranes being more susceptible to wetting than more hydrophobic ones (Eykens et al., 2017). Chen et al. found that superhydrophobic PVDF membranes had strongly negative surface charges and stable MD performance for an anionic surfactant, but experienced severe wetting with a cationic surfactant (Ying Chen et al., 2017).
Membrane hydrophobicity is critical for MD performance as it prevents the liquids in the feed solution from permeating to the distillate side (Khayet and Matsuura, 2011; Rezaei et al., 2017). A critical failure of MD is pore wetting, which occurs when the pores of the hydrophobic MD membrane are filled with liquid water such that the solutes on the feed side leak through the membrane and contaminate the distillate water (Warsinger et al., 2017). Surfactants have been reported to decrease the effective membrane hydrophobicity by reducing the pore liquid entry pressure (LEP) of the membrane, leading to pore wetting and ultimately MD failure (Camacho et al., 2013; Chew et al., 2017; Franken et al., 1987; Lin et al., 2015). Early detection of pore
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wetting is difficult to determine, although once the membrane is partially wetted, the distillate conductivity will reflect the passage of salt through the wetted membrane pores (Resaei et al., 2018). Recently, a method utilizing in situ electrochemical impedance spectroscopy (EIS) was used to identify when real-time pore wetting started to occur (Yuanmiaoliang Chen et al., 2017); however, to date this method has been limited to a single study (Yuanmiaoliang Chen et al., 2017) and has not been validated for use with anionic surfactants. Another recent development in pore wetting detection is the creation of electrically conductive MD membranes (Ahmed et al., 2017); given that such membranes are not commercially available and require fabrication, this option was not considered to be viable for this work.
In this work, a bench-scale DCMD system was used to treat water with an anionic or non-ionic surfactant, and a novel graphical method was developed to determine the surfactant concentration threshold at the onset of pore wetting. The effect of different surfactant types and the surfactant concentration threshold where pore wetting occurs were determined for different membrane materials and pore sizes. From this work, we establish that MD is a valid method for producing high-quality reuse water from surfactant-laden solar panel wash water, and we can begin to establish protocols for acceptable surfactant loads that can be used when cleaning solar panels. The work also opens the potential of applying MD to a broader range of waters requiring treatment, such as greywater and other reuse waters.
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2. Materials and Methods 2.1.
Membranes and chemicals
Hydrophobic, microporous membranes were obtained from Osmonics (Minnetonka, MN), Clarcor IA (Overland Park, KS), and 3M (Davidson, NC) (Table 1, Table 2). Sodium chloride (NaCl) was purchased from Fisher Scientific (Hampton, NH) and pool salt from Diamond Crystal (Wayzata, MN); polysorbate80 (PS80; trade name Tween®80) and 85% pure sodium dodecyl sulfate (SDS) were purchased from Acros Organics (Pittsburgh, PA). HPLC-grade benzene, buffer solution – sulfate type, and detergents reagent powder pillows were purchased from Hach Company (Loveland, CO).
2.2.
Bench-scale DCMD system
Coupons from ten different membranes were tested in a bench-scale DCMD system (Fig. 2), using a custom-made, crossflow flat sheet acrylic membrane module with an effective membrane surface area of 78.31 cm2. Two plastic, mesh-type spacers were used on either side of the membrane to generate turbulence, reduce polarization effects, and promote mass transfer. The module was operated horizontally with the warm feed side on top. Two gear pumps (ColeParmer; Vernon Hills, IL) circulated the feed and distillate solutions counter-currently. A water bath (Precision, ThermoFisher Scientific; Pittsburgh, PA) was used for heating the feed solution; the distillate water was cooled via a heat exchanger connected to a chiller (ThermoFlex 1400, ThermoFisher Scientific; Pittsburgh, PA) circulating ethylene glycol. Real-time data were collected every 30 or 60 seconds using LabView 13.0.1 software (National Instruments; Austin, TX) and a T7 data acquisition module (LabJack; Lakewood, CO). Temperatures were monitored using thermocouples (LabJack; Lakewood, CO) at the feed and distillate inlet and outlet ports of
9
the membrane module. The concentration of NaCl in the distillate was monitored using an electrical conductivity probe (Cole-Parmer; Vernon Hills, IL) and transmitter (Eutech Instruments; Vernon Hills, IL). Water flux was determined by the mass gained in the distillate loop using a balance (Mettler Toledo; Columbus, OH), as described in (Rao et al., 2015).
Fig. 2. Process schematic of bench-scale DCMD system. C – conductivity probe/meter, T – thermocouple.
2.3.
Experimental set-up and procedure
2.3.1. Flux testing Bench-scale DCMD experiments were performed with a warm feed solution composed of 35.0 g/L NaCl and deionized (DI) water (no surfactant) to determine the flux of the commercial membranes. The feed and distillate streams were maintained at 40.9 ± 2.6 and 24.1 ± 0.5 °C, respectively, and were circulated at 0.5 L/min (17.2 cm/s linear flow velocity). The system was operated for ~2 hr for each membrane, with an initial period of stabilization (typically 20 – 40 min), followed by a minimum of 60 min of stable operation; the flux for each membrane was calculated as the average flux during stable operation.
10
2.3.2. Surfactant tests All bench-scale DCMD experiments were performed with a feed solution containing 17.5 g/L NaCl in DI water and one of the two surfactants at varying concentrations. The initial surfactant concentrations for both the anionic SDS and non-ionic PS80 ranged from 90 – 6,500 mg/L and 50 – 250 mg/L, respectively. All membranes were tested with SDS, and then a select group of five membranes were also tested with PS80. The feed solution was heated to 41.1 ± 5.6 °C and circulated at 0.5 L/min. The distillate was maintained at 21.6 ± 2.1 °C and circulated at 0.5 L/min. MD was operated beyond the point where the membrane was wetted by the surfactant, as determined by a large increase in distillate conductivity. After DCMD was terminated, the system was flushed with DI water on both the feed and distillate sides to remove salt and surfactants from the system, and the membrane was removed for characterization. A minimum of three tests for each membrane and surfactant were performed, and the average value along with the standard deviation are reported in Table 2.2 as [SDS]PW and [PS80]PW “calculated”.
2.4.
Pore wetting determination and calculation
A novel graphical method was developed to determine the surfactant concentration threshold at the onset of pore wetting for the multiple membranes. Others have used an increase in distillate conductivity and/or water flux as an indication of membrane pore wetting (Chew et al., 2017); however, in this work the two indicators did not provide consistent results with each other for the various membranes evaluated (Figs. 3a, S1). Ideally, the membrane offers 100% rejection and no salt passes through the membrane from the feed to the distillate sides prior to the onset of pore wetting. In actuality, membranes are imperfect and a small amount salt passes through the membrane even before pore wetting; during this time the specific salt flux (Js/Jw) is stable and
11
the feed side salt concentration will increase gradually as water vapor is the primary species passing through the membrane. When pore wetting occurs, membrane selectivity is lost and water vapor along with both liquid water and salt can freely pass through the wetted pores, resulting in an increase in specific salt flux. However, depending on the nature of the membrane, pore wetting can occur gradually or rapidly, and may occur only in localized locations on the membrane or across the entire membrane. Thus, at the system level, membrane selectivity may be slowly compromised and the increase in specific salt flux alone may not be initially apparent due to partial salt rejection via the remaining dry pores. In such cases, the feed-side salt concentration continues to increase (Fig. S1), in some cases quite rapidly depending on how rapidly the membrane wetting process occurs, resulting in an increased concentration difference driving force between the feed and distillate through the wet pores. By plotting the specific salt flux normalized to the feed-side salt concentration, all the phenomena occurring during the pore wetting process is captured in a single graphical representation (Fig. 3b), and can be used to compare multiple membranes. In this work, membrane wetting was considered to occur at a threshold of 5% of the normalized specific salt flux. This value was selected as the graphical representation of these data, which is reminiscent of an adsorption isotherm, and 5% is a typical value used in adsorption theory (Seader and Henley, 2006), although the threshold can be adjusted as appropriate based on the sensitivity of any downstream applications of the product water.
12
Fig. 3. Development of novel graphical pore wetting method: (a) water flux and distillate conductivity increase after pore wetting¸ but are not consistent indicators of where pore wetting occurs; (b) specific salt flux normalized to the feed-side salt concentration provides a graphical method to determine pore wetting. Data are shown for 3M PP 0.2 membrane with SDS.
2.5.
Calculations
The mass of the distillate was measured continuously and used to calculate the water flux using Eq. (1): (1)
13
where Jw (L/m2·hr) is the water flux, Mw,d (g) is the mass of the distillate, ρ is the density of distillate (assumed to be equal to the density of pure water), A is the effective membrane area (7.831x10-4 m2), and Δt (hr) is the sampling time.
The conductivity of the distillate side was continuously measured and used to calculate the distillate salt concentration using Eq. (2): (2) where [NaCl]d (g/L) is the distillate salt concentration, EC (μS/cm) is the electrical conductivity of the distillate, and 1560 is the conversion factor between μS/cm and g/L (Peet and Foundation, 2001). The conductivity was validated before and after DCMD trials, using known conductivity standards.
The salt flux across the membrane was then calculated using Eq. (3): (3) where Js (g/m2·hr) is the salt flux and Ms,d (g) is the mass of salt in the distillate.
The specific salt flux ( ) was calculated using Eq. (4): (4) The concentration of NaCl in the feed solution was calculated using the mass balance in Eq. (5): (5) where [NaCl]f (g/L) is the salt concentration in the feed solution, Ms,i (g) is the initial salt mass in the feed solution, and Vf,i (L) is the initial feed volume.
14
The surfactant concentration at the time of pore wetting, determined when
/ [NaCl]f = 0.05,
was calculated using Eq. (6), assuming 100% rejection of the surfactant prior to pore wetting: (6) where [Surf]PW (mg/L) is the concentration of the surfactant at the time of pore wetting, [Surf]i (mg/L) is the known initial concentration of the surfactant, and Vd,PW (L) is the volume of distillate water at the time of pore wetting.
2.6.
Surfactant measurement
Samples of the feed solution were taken during the DCMD runs and stored at 4 °C until they were analyzed for surfactant concentrations. The concentrations of both SDS and PS80 were measured utilizing Hach Method 8028 “Surfactants, Anionic (Detergents)”, using a spectrophotometer (DR 6000, Hach Company; Loveland, CO). Due to the difference between the non-ionic surfactant (PS80) and the anionic surfactant (SDS), a PS80-specific calibration curve was created in the range of Method 8028 (0.002 – 0.275 mg/L) to accurately measure the concentration of the non-ionic surfactant. These values are shown in Table 2.2 as [SDS]PW and [PS80]PW “measured”. No standard deviations are reported as only the single sample nearest to the calculated time of pore wetting onset was measured.
2.7.
Membrane characterization
To investigate the effect of the different surfactants on the membranes, the hydrophobicity of the membranes before and after exposure to surfactants was evaluated. The contact angle with water (θ) was used as an indication of membrane hydrophobicity; hydrophobic membranes are
15
considered to have 90° < θ < 150° (Ashoor et al., 2016). Dry membrane samples were used for all contact angle measurements, which were performed using an optical tensiometer (OneAttension, model Theta, Biolin Scientific; Stockholm, Sweden) using the sessile drop method at ambient temperature. DI water droplets (~10 μL) were placed on the membrane surface by the automated syringe attachment and the mean contact angle value was calculated from the measured right and left contact angles over 10 s. A minimum of five measurements at different locations on the feed-side surface were made for each membrane, and the average θ ± standard deviation is reported.
3. Results 3.1.
Membrane hydrophobicity
Membrane contact angles before and after exposure to surfactants were measured to determine how membrane hydrophobicity was affected in DCMD systems treating surfactant-laden waters (Table 1). The water flux for membranes treating a saline solution without surfactants was also measured (Table 1). Table 1: Contact angles of various commercial membranes before (virgin) and after pore wetting by SDS or PS80. PP – polypropylene; ECTFE – ethylene chlorotrifluoroethylene; PVDF – polyvinylidene fluoride; PTFE – polytetrafluoroethylene. Nominal Membrane Contact Angle (°) Pore Water Size Flux After SDS After PS80 Supplier Material (μm) (L/m2-hr) Virgin Pore Wetting Pore Wetting 3M PP 0.1 12.2 144.8 ± 1.6 140.3 ± 3.4 3M PP 0.2 17.7 146.3 ± 2.4 136.3 ± 3.5 3M PP 0.2 17.7 126.8 ± 2.1 120.4 ± 2.2 3M ECTFE 0.2 16.4 111.1 ± 1.7 103.1 ± 2.3 3M PP 0.45 17.4 128.2 ± 2.2 122.6 ± 4.0 Osmonics PP 0.2 13.3 143.2 ± 1.0 133.0 ± 6.9 119.9 ± 6.5 14.8 134.0 ± 2.6 Osmonics PVDF 0.45 131.4 ± 4.7 116.1 ± 4.4 Clarcor IA PTFE 0.11 9.2 135.5 ± 2.8 120.5 ± 4.3 113.5 ± 3.8 Clarcor IA PTFE 0.22 15.4 129.3 ± 1.6 113.6 ± 5.2 110.2 ± 2.7 Clarcor IA PTFE 0.45 25.2 134.4 ± 2.1 123.8 ± 4.4 111.8 ± 4.0
16
No correlation between pore size and hydrophobicity was observed for the virgin membranes or for the surfactant-exposed membranes. However, strong positive correlations were found for the virgin contact angles and the post-surfactant contact angles (SDS: R2=0.85; PS80: R2=0.80), suggesting that membranes with higher initial hydrophobicity retain a higher level of hydrophobicity even after pore wetting. It should be noted that all membranes remained hydrophobic (θ > 90°) after contact with both SDS and PS80.
3.2.
Pore wetting surfactant concentrations
The surfactant concentrations at pore wetting were calculated as described above and compared to the measured surfactant concentrations of the feed stream after the DCMD test was terminated (Table 2). The evaluated membranes responded differently to the anionic SDS compared to the non-ionic PS80, with much lower [PS80]PW values observed in all cases.
Table 2: Calculated and measured SDS and PS80 concentrations at pore wetting for various commercial membranes. PP – polypropylene; ECTFE – ethylene chlorotrifluoroethylene; PVDF – polyvinylidene fluoride; PTFE – polytetrafluoroethylene. Nominal [SDS]PW (mg/L) [PS80]PW (mg/L) Pore Size Calculated Measured Calculated Measured Supplier Material (μm) 3M PP 0.1 223.8 ± 6.4a 195 3M PP 0.2 120.7 ± 5.9a 117 3M PP 0.2 110.4 ± 2.9a 115 3M ECTFE 0.2 121.3 ± 13.5a 141 3M PP 0.45 136.5 ± 2.8a ** Osmonics PP 0.2 187.8 ± 0.7b ** 88.0 ± 25.2 49 Osmonics PVDF 0.45 793.7 ± 149.1c 718 230.0 ± 24.7 262 Clarcor IA PTFE 0.11 16,588.4 ± 165.0a 20,925 298.0 ± 87.3 271 Clarcor IA PTFE 0.22 2,696.4 ± 240.7b 3,600 181.0 ± 63.0 114 Clarcor IA PTFE 0.45 3,923.8 ± 195.6a 7,200 198.1 ± 49.4 91 ** – measurements could not be taken due to insufficient sample volume Number of replicates: a = 3, b = 4, c = 6.
17
No apparent correlation between pore size and pore wetting concentration was observed for either SDS or PS80. The PTFE membranes exhibited higher [SDS]PW than the PP, PVDF, or ECTFE membranes evaluated, which agrees with previous results in the literature (Eykens et al., 2017; Thomas et al., 2017). However, these membranes also had the most variable results in our work. The calculated [PS80]PW values were similar between the PTFE and PVDF membranes, while the PP membrane had a slightly lower calculated [PS80]PW.
4. Discussion DCMD performance varied depending on surfactant type and membrane material. The initial membrane hydrophobicity also varied between suppliers and materials. All membranes tolerated higher surfactant concentrations than expected; in one study, wetting of a PTFE membrane occurred with a concentration as low as 0.1 mM SDS (Lin et al., 2014), while others found wetting of a PVDF membrane occurred with concentrations of 50 mg/L SDS, Tween20 and Span20 (Chew et al., 2017). However, a decrease in hydrophobicity was observed for all membranes after exposure to both surfactant types. The [PS80] PW values were much lower than the corresponding [SDS]PW values for a given membrane (Table 2), indicating that different wetting regimes may be present for the non-ionic surfactant and the anionic surfactant. Cationic surfactants were not considered in this study, as they have been shown to be less effective at removing deposited sand particles from solar panels in previous work (Abd-Elhady et al., 2011).
An important parameter for DCMD is the LEP, which describes a membrane’s ability to resist wetting, and is the pressure that the feed water must overcome to penetrate the membrane pores
18
resulting in wetting (Warsinger et al., 2017). To prevent wetting, the total pressure (hydrostatic pressure plus partial vapor pressures, where hydrostatic pressure is generally much lower than the partial vapor pressures) must be less than the LEP. The LEP (Pa) can be calculated using the Laplace (Cantor) equation in Eq. (7): (7) where B is the dimensionless geometric factor for the pore shape (0 < B ≤ 1, B = 1 for perfectly cylindrical pores), (N/m) is the surface tension of the feed solution liquid, (°) is the contact angle between the membrane and the feed solution liquid, and rmax (m) is the pore radius (Camacho et al., 2013; Yuanmiaoliang Chen et al., 2017). Surfactants can impact MD performance by accumulating at the membrane surface with their hydrophilic head groups aligning into solution and their hydrophobic tail groups aligning with the membrane surface (Yuanmiaoliang Chen et al., 2017). The presence of surfactants at the membrane surface lowers the LEP by reducing both and (Yuanmiaoliang Chen et al., 2017; El-Abbassi et al., 2013; Franken et al., 1987; Garcia-Payo et al., 2000).
Despite its role in LEP (based on Eq. 7), the pore size did not appear to play a role in pore wetting for the membranes tested in this work. This may be attributed, in part, to the narrow pore size range evaluated (0.1 – 0.45 m; Table 1). However, a clear reduction in θ was observed for all membranes exposed to surfactants in this work (Table 1), resulting in the feed solution intruding into the pores and a corresponding increase in distillate conductivity (Fig. 3a). It should be noted that the contact angle measurements were taken from membrane samples collected after the pore wetting experiments were completed; these membranes were exposed to SDS or PS80 concentrations higher than the reported [SDS]PW and [PS80] PW. All membranes were also rinsed 19
with DI water and dried prior to testing with the tensiometer. Although any of these factors may have introduced error to the contact angle results, since all membranes were treated in the same fashion this potential error was systemic and is not expected to have affected comparative results.
One of the challenges of this work was how to consistently identify the onset of pore wetting prior to catastrophic performance failure and to determine the pore wetting surfactant concentrations. A recent effort successfully used EIS to determine real-time pore wetting (Yuanmiaoliang Chen et al., 2017), although the EIS instrumentation costs are high compared to the simple analytical needs of the graphical method that relies on conductivity and mass data to make simple calculations. The novel graphical method developed addresses this problem by incorporating water flux, salt flux, and salt accumulation on the feed side into a single plotted parameter (Fig. 3b). The method was successfully used to identify the onset of pore wetting and calculate pore wetting concentrations for several different membranes and different surfactant types. It is important to note that this graphical method is only valid for the onset of pore wetting and does not provide any guidance regarding partially wetted or long-term operation of the membrane in the presence of surfactant. This is clear from Figures 3b and S1, where maximum values of the specific salt flux normalized to the feed-side salt concentration are observed, which corresponds to the rapid increase in the feed side NaCl concentration while the membrane is partially wetted.
20
To verify the calculated [SDS]PW and [PS80] PW values, grab samples of the feed were collected and analyzed using a spectrophotometric method. The observed differences between measured and calculated values can be attributed to the challenge of collecting the samples at the onset of pore wetting, which was unknown at the time of experiment. Samples were collected based on distillate side conductivity, which was an inconsistent indicator of pore wetting (Fig. S1). Regardless, the measured and calculated pore wetting concentrations were strongly correlated (SDS: R2=0.99; PS80: R2=0.76), validating the calculated values (Table 2). The establishment of [SDS]PW and [PS80] PW for commercially available DCMD membranes provides important guidance for treating waters containing surfactants, including wash water from solar energy facilities as well as other reuse waters. Results of this study can assist the solar energy industry in developing standard operating procedures for the selection of surfactant type and concentrations used for cleaning solar panels/mirrors that will enable on-site DCMD systems to operate without pore wetting. This, in turn, may be a step towards addressing some concerns of the water-energy nexus.
5. Conclusions The overall results of this work indicate that membrane material and surfactant type can affect DCMD performance when treating surfactant-containing waters. Key conclusions from this work are:
The threshold surfactant concentrations at which pore wetting occurred was determined using a novel graphical method developed for this work and verified analytically;
21
[SDS]PW and [PS80]PW values were determined for commercially available hydrophobic, microporous membranes commonly used for MD;
[SDS]PW values were significantly higher than [PS80]PW values; and
[SDS]PW and [PS80] PW values were much higher for PTFE membranes than for other membrane materials tested.
6. Acknowledgements This material is based upon work supported by the National Science Foundation under Grant No. IIA-1301726. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. A portion of the membrane samples used were generously provided by Clarcor IA and 3M.
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Supplemental Figures:
Fig. S1. Representative behavior of three different membranes illustrating the need for the novel graphical method to determine the onset of pore wetting that was used to calculate [SDS] PW and [PS80] PW. Data are shown for 3M ECTFE 0.2 membrane with SDS (a-c), Clarcor IA PTFE 0.45 membrane with SDS (d-f), and Osmonics PVDF 0.45 membrane with PS80 (g-i). Symbol legend: ♦ JW; ● Conductivity; ▲ JS/JW; ▬ [NaCl]Feed; ■ (JS/JW)/[NaCl]Fee d
29
Highlights
DCMD was able to treat saline water containing up to 1.66 wt% SDS
PTFE demonstrated the highest pore wetting resistance in the presence of surfactants
Membranes tolerated higher anionic SDS concentrations than non-ionic PS80
Novel graphical method to determine onset of pore wetting developed
30
Graphical abstract
31