Letter pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Fabric-Based Ion Concentration Polarization for Pump-Free Water Desalination Biao Ma, Junjie Chi, and Hong Liu* State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Sipailou 2#, Nanjing, Jiangsu Province 210096, P. R. China S Supporting Information *
ABSTRACT: We report on water desalination on fabric based on ion concentration polarization (ICP). The use of fabric as the substrate reduces the cost and simplifies the fabrication procedure for water desalination. On the fabric, water is transported and recovered by capillary force without any external pump. Filtration based on the porous structure of the fabric and electrocoagulation purification induced by anodic electrochemical reaction are also integrated. By the fabric-based water desalination, 85% salt in 50 mM NaCl solution is rejected with an applied voltage of 15 V, which demonstrates its potential in water desalination for remote or underdeveloped areas.
KEYWORDS: Desalination, Ion concentration polarization, Fabric, Electrocoagulation, Water treatment
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INTRODUCTION The insufficient supply of fresh water is a worldwide issue, which is especially acute in developing countries and resource-limited areas.1,2 One attractive solution to this issue is desalination of sea or brackish water.3 Conventional desalination techniques, such as reverse osmosis (RO)4 and thermal distillation,5 have been well developed for large-scale supply of fresh water. However, the expensive infrastructure and high energy consumption limit their application in remote or underdeveloped areas. Therefore, a few efforts have been devoted to develop alternative desalination techniques, such as solar steam generation using plasmonic nanoparticles,6,7 salt absorption by nonporous membrane,8 and desalination batteries.9,10 These techniques rely on complicated material synthesis processes, making them less robust and expensive. Ion concentration polarization (ICP) is an emerging desalination technique with high salt rejection capacity, which was first reported by Han and co-workers in 2010.11 This electrokinetic phenomenon occurs near an ion-selective membrane with voltage application. The selective transport of ions associated with the maintenance of electroneutrality results in an ion depletion zone near the membrane.12 The depletion zone can be used as a virtual ion filter to repel charged species.13,14 ICP desalination can also be accomplished based on bipolar electrochemistry. 15 The applied voltage can be significantly reduced with the use of a bipolar electrode which decreases the energy consumption.16 However, all previously reported ICP desalination was performed in expensive and delicate microfluidic PDMS devices. Tedious microfabrication processes, such as photolithography, bonding, and channel © XXXX American Chemical Society
pretreatment, as well as cleanroom facilities have to be involved. Moreover, the small microfluidic device has to be linked with a cumbersome pumping system to drive the flow, which not only increases the cost and energy consumption but also limits its portability. Recently, paper-based materials have been increasingly used for fabrication of various low-cost fluidic devices for a range of interesting applications.17−19 ICP on paper-based substrates has also been reported for preconcentration of analyte in a sample.20,21 The open, free-standing paper capillary channel makes ICP easy to operate. Moreover, the porous paper substrate can suppress undesirable vortical flow,22,23 so the ICP process on paper is quite stable.24 However, due to the poor mechanical strength of a paper substrate, paper-based microfluidic devices are often disposable. For fabrication of more robust devices, fabric substrates are often used.25 Here, we report a fabric-based fluidic device for water desalination based on ICP. It offers several major advantages. First, the use of fabric as the substrate material remarkably decreases the cost and simplifies the fabrication procedures. Only two steps are needed, namely, cutting and lamination of the fabric. Second, because of the capillary force of the fabric, no pump is required for either water desalination or collection. Third, the porous fabric substrate material has inherent filtration ability which can be used for water pretreatment. Fourth, an Received: October 11, 2017 Revised: December 7, 2017 Published: December 11, 2017 A
DOI: 10.1021/acssuschemeng.7b03679 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering electrocoagulation effect induced by the anodic electrochemical reaction can be used for further water purification.
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RESULTS AND DISCUSSION As shown in Figure 1, the fabric-based fluidic device consisted of a “Y”-shaped channel fabricated by cutting a piece of polyester
Figure 2. (a) Experimental setup of the fabric-based desalination device. Scale bar: 5 mm. (b) Capillary force-based water collector using glass slides. Scale bar: 3 mm. (c) Desalted water can be easily collected by tilting the slides. Scale bar: 5 mm.
Figure 3 shows the development of the ICP phenomenon near the Nafion membrane within 180 s. A focusing plug (indicated by Figure 1. Schematic illustration showing the fabric-based device for pump-free water desalination.
fabric, a piece of Nafion membrane as the cation exchange nanoporous material, and a capillary force-based water collector. Water can spontaneously flow on the fabric by capillary action, with no pump required. With voltage application, only cations in the salt water can migrate across the Nafion membrane. To maintain the local electroneutrality near the anodic side of the membrane, an ion depletion zone was thus induced, acting as a virtual ion filter for desalination. To collect the water after desalination, two glass slides were used to create a capillary cavity so that the desalted water flowed into and was stored in the cavity. The device fabrication process is depicted in Figure S1a. Briefly, a commercially available CO2 laser cutting machine was used to cut the fabric into a designed shape. This method is rapid (∼6 s per device), easy to operate, and highly reproducible. The fabric was adhered to the laminating film with double-sided tape, followed by placing a piece of Nafion membrane on the desalted fabric near the bifurcation. Then, the unwanted part of the laminating film was removed. After lamination, the membrane and fabric were well bonded; even a 100 g weight was used to pull the membrane, as shown in Figure S1b. The entire fabrication time for one device is about 5 min, much less than that for conventional PDMS devices. The cost for each unit was about $0.05. Figure 2a is a photo of the fabric-based desalination device. Aluminum (Al) wire was immersed into the salt water (100 μL, spiked with brilliant blue dye) for voltage application. Another role of the Al wire was taken as the sacrificial anode to induce an electrocoagulation effect, which we discuss later. To collect the water from the fabric, part of each bifurcated fabric (∼7 mm long) was sandwiched between two glass slides. Once the fabric was wetted, the capillary force generated from the gap between the glass slides and gravity continuously absorbed the water out of the fabric. As shown in Figure 2b, a droplet of the absorbed water is generated at the end of the fabric, and the droplet then flows into the cavity between the slides. The water between the two glass slides can be easily collected by simply tilting the sides, as shown in Figure 2c. To show the ICP effect, anionic fluorescein dye (100 μg/mL) was added to the salt water (100 mM NaCl). The fabric was prefilled with the salt solution, and the applied voltage was 15 V.
Figure 3. (a) Fluorescence images showing the ICP phenomenon near the Nafion membrane. The applied voltage was 15 V. Feed water was 100 mM NaCl spiked with 100 μg/mL sodium fluorescein.
yellow arrow) can be observed at the anodic side of the membrane in 10 s, indicating the rapid formation of the ICP effect. Moreover, the fluorescent intensity in the desalted channel gradually decreased. So, the ICP effect in this simple fabric was similar to that in a conventional PDMS-based water desalination device.11 The performance of the device, including the salt rejection ratio and energy consumption, was characterized. Here, NaCl with different concentrations (50, 100, and 200 mM) were used for desalination. After 15 min desalination, the volume of the collected water was measured with a microsyringe to obtain the flow rate. Then, 10 μL of the desalted water was diluted to 2000 μL with deionized water so that a conductivity meter can be used for conductivity measurement. The conductivity was converted to salt concentration through a calibration curve, which was obtained by measuring the conductivity of a series of diluted NaCl solutions at known concentrations (Figure S2). Figure 4a shows the salt rejection ratio as a function of the applied voltage ranging from 5 to 20 V. Each data was the average of four parallel test results. The salt rejection capacity was monotonously increased with increasing applied voltage for each concentration of NaCl. This is reasonable because desalination at a high voltage enhances the ion transport across the Nafion membrane. Specific to the 50 mM NaCl, the salt concentration of the desalted water reached the potable level14 (below 8.6 mM NaCl) when the B
DOI: 10.1021/acssuschemeng.7b03679 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
Along with water desalination, it is essential to integrate water purification steps for practical application. Water purification not only improves the quality of water but also mitigates membrane fouling. Here, two water treatment processes, filtration and electrocoagulation, were integrated to the desalination system without adding the device complexity. Since the fabric has a porous matrix, undesired impurities can be removed by sizedependent filtration. To show this filtration effect, 0.5% (w/v) polystyrene monodisperse fluorescent microspheres (4 μm in diameter) were added into the feedwater. As shown in Figure 5a,
Figure 4. (a) Salt rejection ratio as a function of applied voltage. (b) Typical current−time response for 50 mM NaCl at 15 V. (c, d) Averaged current level and the power consumption as a function of the applied voltage, respectively.
applied voltage was 15 V with the rejection ratio of 85%. With voltage of 20 V, salt rejection ratios of 91%, 89%, and 85% corresponding to 50, 100, and 200 mM NaCl were obtained, respectively. To measure the energy consumption, the current−time response during the desalination process was recorded. Figure 4b is a typical current−time response for desalination of 50 mM NaCl at 15 V. A current drop was observed in the first 45 s, which was due to the formation of the low conductive ion depletion zone. Afterward, the current reached a steady state, representing a stable ICP process.26 Figure 4c shows the averaged current level under each voltage. The flow rates for desalted and brine water were ∼1.1 and ∼0.80 μL/min, respectively. Based on these parameters, the power consumption was obtained, as shown in Figure 4d. For each concentration of NaCl, almost exponential growth in power consumption was observed when the applied voltage was increased. In addition, the throughput of desalination can be improved by stacking multiple layers of fabric. To this end, we stacked two layers of fabric, and each layer was separated by double-sided tape. The flow rate in the desalted channel increased to 1.9 μL/min, which was 73% higher than that using one layer fabric (1.1 μL/min). The effect of divalent cations on desalination efficiency was also investaged. Desalination of the salt solution containing 100 mM NaCl, 11 mM MgCl2, and 2.2 mM CaCl2 was performed. The molar ratio of Na+, Mg2+, and Ca2+ ions is similar to that in seawater.27 Here, a 70% salt rejection ratio was achieved with an applied voltage of 15 V, very close to that for 100 mM NaCl with a rejection ratio of 74%. This result indicates that the presence of divalent ions does not significantly affect the desalination efficiency. The overall desalination cost, including the capital and operating cost was also estimated. The method for cost calculation was based on the work previously reported by Kim et al.,28 and the details are provided in the Supporting Information. Note that the cost was estimated assuming the water throughput was 1 L/min using 106 water desalination units. The desalination cost is plotted as a function of applied voltage for NaCl with various concentrations in Figure S3. The cost ranges from $12/m3 to $42/m3 depending on the applied voltage and salt concentration.
Figure 5. Photographs showing the filtration and electrocoagulation effect. To show the filtration effect, 0.5% (w/v) monodisperse fluorescent microspheres (4 μm in diameter) was added the feedwater (50 mM NaCl). After 15 min desalination, 5 μL desalted water and 5 μL feedwater was dropped on a glass slide. (a, b) Bright field and fluorescent images of the drops, respectively. (c) Electrocoagulation effect was demonstrated by adding brilliant blue dye (100 μg/mL) to the feedwater. After desalination, the rest of feedwater was centrifuged. The flocs were observed at the bottom of the tube.
the collected desalted water was clarified. The fluorescence image also confirmed that the particles were removed by filtration (Figure 5b). Even with the mean pore size of the fabric (16 μm) being larger than the diameter of microspheres, effective filtration was still achieved. This is due to the inhomogeneous distribution of the pore size of the fabric and the long filtration length, which is 25 mm, 6250-times longer than the diameter of the microspheres. The fabric can also effectively filtrate hydrophilic microspheres of SiO2 (4 μm in diameter), as shown in Figure S4a. However, smaller microspheres (1 μm in diameter) cannot be effectively filtrated by the fabric, as shown in Figure S4b. In this work, low cost Al wire was used as the anode. With voltage application, the Al anode induced an accompanying purification process of electrocoagulation along with the desalination. Electrocoagulation is a widely used electrochemical water treatment technique for removing a number of substances such as suspended particles, heavy metals, and organic dyes.29,30 In an electrocoagulation process, the electrolytic oxidation of a metal anode (Al or Fe) generates metallic hydroxides, which are taken as the coagulant. The target species are removed from water by allowing them to coagulate with the metallic hydroxides.31 To demonstrate the electrocoagulation effect, 100 μg/mL of anionic dye (brilliant blue) was added into the feedwater. After desalination for 15 min, the rest of the feedwater was centrifuged, and the flocs were observed in the tube, as shown in Figure 5c. The supernatant was taken to analyze using a UV−vis spectrophotometer. For quantitative assessment of the dye removal, a calibration curve was generated with standard solutions (Figure S5). Here, 67% of the dye was removed by electrocoagulation. C
DOI: 10.1021/acssuschemeng.7b03679 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
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In conclusion, we demonstrated water desalination with lowcost materials and simple operation. Water pretreatment processes, filtration, and electrocoagulation were also integrated.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03679. Information about chemicals, materials, experimental setup, desalination cost calculation, device fabrication, and additional figures. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Junjie Chi: 0000-0002-0779-9287 Hong Liu: 0000-0002-9841-1603 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Chinese Recruitment Program of Global Experts, Innovative and Entrepreneurial Talent Recruitment Program of Jiangsu Province, the National Natural Science Foundation of China (21405014, 21327902, and 21635001), the Natural Science Foundation of Jiangsu (BK20140619), the Science and Technology Development Program of Suzhou (ZXY201439), State Key Project of Research and Development (2016YFF0100802).
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REFERENCES
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DOI: 10.1021/acssuschemeng.7b03679 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acssuschemeng.7b03679 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
