Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
Fluoride Removal from Water by Membrane Capacitive Deionization with a Monovalent Anion Selective Membrane Jiefeng Pan,†,§ Yu Zheng,†,§ Jincheng Ding,† Congjie Gao,† Bart Van der Bruggen,‡ and Jiangnan Shen*,† †
Center for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang University of Technology, Hangzhou 310014, P. R. China ‡ Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium S Supporting Information *
ABSTRACT: Contamination of groundwater by monovalent anions, for instance, fluoride (F−) and nitrite (NO2−), leaves an adverse impact on human health. This study aims at investigating the feasibility of the application of membrane capacitive deionization (MCDI) combined with a monovalent anion permselective exchange membrane (PSM) for the removal of fluoride from water. In this study, various parameters (anion composition and concentration, pH, operating voltage, flow rate, and time) were studied for the purpose of attaining the maximum selectivity. Evaluation of the selectivity for monovalent anions was performed with the help of a lab-made MCDI with a model aqueous system (F−/ SO42−). As revealed by the empirical findings, the removal of both F− and SO42− anions increased with the increase in anion concentration and pH, in addition to the increase in the selectivity for monovalent anions (F−). The removal of anions and selectivity also exhibited an increase with flow rate and operating time. Contrarily, the selectivity declined with the applied voltage. A monovalent anion selectivity of 1.43 was obtained subjected to the conditions of 1.0 V operating voltage, together with 10 min adsorption time and 30 mL/min feed flow rate. The results in this study are capable of helping develop the PSMCDI (permselective exchange membrane capacitive deionization) technology and expand its application for the removal of fluoride from drinking water.
1. INTRODUCTION Water is an essential material of economic development and human activities. Nevertheless, over the past few decades, owing to the growth of industrialization and urbanization, numerous water sources have become contaminated.1 Owing to diverse natural and man-made ecological factors, the groundwater of a numbers of areas has been contaminated by different kinds of hazardous compounds, such fluoride, nitrate, sulfate, pesticides, and heavy metals. Among these, monovalent anion pollution, which is represented by fluoride (F−) contamination, has been identified as a prominent inorganic pollutant, which impacts the human health in the impacted areas at the global scale, in accordance with the WHO.2 Fluoride is termed an integral component of both the teeth and bones, together with having a pivotal physiological function to the human body in drinking water. A specific amount of fluoride in drinking water or food is usually considered as safeguarding the occurrence of dental caries.3 Nonetheless, an excessive exposure to fluoride gives rise to dental fluorosis, skeletal fluorosis, and other diseases.4 For instance, more than 27 million people in China have been reported who suffer from skeletal fluorosis and dental fluorosis.5 Moreover, high fluorine content in drinking water is regarded as the key type of endemic fluorosis in China.6 Accordingly, it is considered to be © XXXX American Chemical Society
quite imperative to develop an effective technology for the selective removal of excess F− from drinking water. The conventional methodologies of defluorination from drinking water include adsorption,7,8 ion-exchange,9,10 membrane separation,11,12 and electrodialysis.13 Activated alumina and activated carbon are the most frequently employed adsorbents for the adsorption processes. Ku and Chiou7 made use of alumina for adsorbing the fluoride ion from aqueous solution. As revealed by the empirical findings, the process manifested high pH dependency and was merely effective in a narrow pH range (5−7). Making use of cation exchange/chelating resins, effective removal of fluoride is possible. Owing to the formation of hydrogen bonds, fluoride ions with stronger electronegativity are adsorbed prior to other anions.9 Nevertheless, this technique is costly for its reliance on expensive resin, a pretreatment for holding the pH, regeneration, and waste disposal.14 In recent times, membrane processes have garnered attention for their good performance at comparatively lower costs. For instance, using nanofiltration Received: Revised: Accepted: Published: A
March 1, 2018 April 26, 2018 May 8, 2018 May 8, 2018 DOI: 10.1021/acs.iecr.8b00929 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research membranes bearing a negative charge is capable of separating fluoride and borate from sulfate in the ash dam water.11 This membrane technique can be highly effective for fluoride removal. Either nanofiltration membranes or reverse osmosis membranes are capable of eliminating more than 98% of fluoride from the wastewater.12,15 Nonetheless, demerits, for instance, membrane fouling/degradation, make the membrane processes usually unfeasible. Electrodialysis is a well-known technique for the desalination of brackish water. Adhikary et al.13 studied the defluorination of saline water at various total dissolved solids (TDS) levels for the purpose of obtaining potable water with TDS below 1000 ppm and a fluoride content amounting to ca. 1.5 ppm. Nevertheless, the electrochemical techniques also suffer from a high cost during both installation and maintenance.16 Capacitive deionization (CDI), owing to the benefits of being energy efficient, cost-effective, and environmentally friendly, is considered to be an emerging technique for water desalination.17 A pair of oppositely placed porous electrodes with a separator in between constitutes a CDI cell. The feedwater flows through a spacer channel; in the meantime, the DC voltage is applied across the porous electrode pair, and salt ions in the feed migrate into the electrical double layers (EDLs) at the carbon/water interface (a process defined as “electrosorption”).18 Membrane capacitive deionization (MCDI) is a modified CDI technique that was, at first, scientifically demonstrated in the year 2006 by Lee et al.19 Since the charged membranes on the separator side of each electrode are capable of preventing co-ions (ions with the same charge as the local electrode), excess current is avoided, and the salt adsorption rate and capacity are substantially increased, in comparison with the CDI. Another benefit preferring MCDI to CDI suggests that the counterions (ions with the different charge as the local electrode) are more fully depleted from the electrodes all through the desorption.20 Consequently, the MCDI is a promising technology meant for the removal of charged species, for instance, sulfate,21 nitrate,22,23 phosphate,24 ammonium, calcium, and magnesium ions.25 This research work aims at defluorination of high fluoride water with the use of the MCDI. Previous studies demonstrated that the MCDI exhibits a higher selectivity for the divalent ions as compared with the monovalent ions.26 Accordingly, in this study, a monovalent permselective anion exchange membrane was prepared for the improvement of the removal of monovalent ions. Investigation of the impact of the composition of the feed solution in anions, the anion concentrations, and pH on the selectivity was carried out. Optimization of the applied voltage, flow rates, and operating times was also performed for the determination of the optimum operating conditions.
Figure 1. Schematic diagram of the experimental apparatus and MCDI cell configuration.
Glass, Japan), cathode electrode, and acrylic plate. (For more details see the Supporting Information.) 2.2. Experimental Methods. For the purpose of investigating the selectivity of the PSM in MCDI in a solution with sulfate and fluoride ions, the solution is pumped into the MCDI unit cell from the recycle reservoir with the help of a peristaltic pump and the effluent is returned to the recycle reservoir, which has been presented in Figure 1. The volume of solution is kept at 50 mL and the temperature is maintained at 298 K. All through the adsorption, application of a constant charging voltage is made to the MCDI unit cell for some time. During desorption, the electric field direction is reversed until the achievement of the initial performance in the recycle reservoir. On the basis of the above method, CDI is employed at first, followed by the conventional MCDI and eventually the PSMCDI, which is based on the monovalent anion permselective exchange membranes (ASV). The anion concentrations and pH of the feed solution are changed for the purpose of studying the impacts of water quality; in addition, various applied voltages, flow rates, and operating times are applied for the optimization of the operating conditions. Therein, only one parameter is changed in each of the tests. 2.3. Water Quality Analysis. Collection of the water samples was performed from the recycle reservoir following the adsorption operation, and determination of the ion concentrations was performed with the use of ion chromatography (ICS-1100, Thermo Fisher, USA). In this study, the removal rate (R) of each anion was calculated as follows,
2. MATERIALS AND METHODS 2.1. MCDI System. The schematic diagram, together with the detailed unit cell of the MCDI system in this study, is presented in Figure 1. The MCDI system is composed of a MCDI cell (Figure S2), a recycle reservoir, a DC power supply (E3634A, Keysight, USA), a peristaltic pump (BT100S, Lead Fluid, China), an electrical conductivity meter (DDSJ-308A, Rex, China), and a digital multimeter (DMM6000, PULIVA, China). The MCDI unit cell comprises an acrylic plate, anode electrode, cation exchange membrane (NeoceptaCMX, Tokuyama, Japan), nylon spacer, anion exchange membrane (NeoceptaAMX, Tokuyama, Japan, and SelemionASV, Asahi
R=
Cf − Ce Cf
× 100% (1)
where Cf (mM) is the anion concentration of the feedwater and Ce (mM) is the concentration of the effluent water after adsorption. The selectivity coefficient (S) was employed for the direct comparison of F− and SO42−removal rates, S= B
R F− RSO4 2−
(2) DOI: 10.1021/acs.iecr.8b00929 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Here RF− and RSO42− are the removal rates of F− and SO42−, respectively. The selectivity coefficient (S) was utilized for the evaluation of the anion removal selectivity.
3. RESULTS AND DISCUSSION 3.1. Anion Removal with Different Methods. The adsorption experiments were carried out with the use of CDI, conventional MCDI, and monovalent anion permselective exchange membrane based MCDI (PSMCDI). As evident from Figure 2, the selectivity coefficient (S) of CDI and conventional
Figure 3. F−/SO42− removal rates at three different feedwater compositions: solutions of NaF, Na2SO4, and a mixture of NaF and Na2SO4 at the same concentrations of 1.0 mM for PSMCDI. The applied voltages, feedwater flow rate, and operating time were set at 1.2 V, 30 mL/min, and 10 min, respectively.
dX), where Ji is the flux of ion i (mol m−2 s−1), Di is the diffusion coefficient of ion i (m2 s−1) (RSO42− = 1.07 × 10−9 m2 s−1, DF− = 1.48 × 10−9 m2 s−1), ci is the concentration of ion i (mM) in the spacer channel, and zi is the ion charge number. The value of JSO42−/JF− is calculated to be greater than 1; consequently, a preferential adsorption of SO42− over F− from the aqueous solution passing through the spacer channels in the conventional MCDI cells was expected.21,27 Furthermore, the removal rate of anions (i.e., F− and SO42−) was higher when only one kind of anion occurred in the feed solution. This phenomenon was also observed in a previous study, together with being associated with the competition of the two ions.28 3.2.2. Effect of pH. The pH value is among the major characteristics for the water system, directly impacting the water quality and further purifying the water process. Herein, the salt removal rate and ion selectivity coefficient (S) at the pH values of 4, 7, and 10 were measured by PSMCDI and shown in Figure 4. The removal rate of F− slightly increases with the increases of pH values. On the contrary, the removal rate of SO42− manifests a sharp decline. Subjected to the alkaline conditions, the selectivity coefficient manifests a slight increase as compared with the neutral pH. However, under acidic conditions, the selectivity coefficient sharply decreases. In particular, the removal rate of F− was 17.50% and the removal rate of SO42− was 23.41% at pH 4, so that the selectivity was totally lost. It could be that the H+ ions shielded the zeta potential,28 and consequently the electronegativity of the membrane surface reduced (see Figure S1 in the Supporting Information). 3.2.3. Anion Concentration. In a bid to further investigate the effect of concentrations, the PSMCDI experiments were carried out at different initial concentrations, i.e., 0.5 mM, 1.0 mM, and 1.5 mM. The removal rates of F− and SO42− ions and the selectivity coefficient of MCDI based on the PSM were measured as shown in Figure 5. Owing to the use of a monovalent anion selective exchange membrane, the F− removal efficiency was higher than that of SO42− under all applied concentrations. Additionally, the salt removal rate was lowered with the increase in the initial feed concentration. On
Figure 2. Removal rate of F−/SO42− and selectivity coefficient (S) of CDI, conventional MCDI, and PSMCDI system. The feedwater contained a mixture of NaF and Na2SO4 at the same concentration of 1.0 mM. The applied voltages, feedwater flow rate, and operating time were set at 1.2 V, 30 mL/min, and 10 min, respectively.
MCDI both amounted to be below 1.00. In detail, the removal rate of F− amounted to be 9.72% and the removal rate of SO42− was 12.62% in the CDI test; the corresponding removal rates of F− and SO42− were 15.85% and 24.29% in the conventional MCDI test, which suggested that the divalent anions were more efficiently removed by the anode. Quite substantially, the MCDI is termed as more efficient than CDI in terms of desalination. Contrarily, the selectivity coefficient (S) of the PSMCDI was larger than 1.00, and the removal rates of F− and SO42− were 20.22% and 17.41%, which indicated that the PSM could prevent the adsorption of SO42− to some extent, in addition to promoting the penetration of F−. Accordingly, the PSMCDI has the potential to efficiently remove fluoride from drinking water. The impact of each parameter on the selectivity coefficient (S) is further discussed in the following section. 3.2. Effects of Feedwater Quality on the Anion Removal Selectivity of PSMCDI. The feedwater quality holds the ability to impact the removal efficiency. In this study, the impact of solution chemistry on F−/SO42− removal efficiency was investigated through the change of the anion concentrations when all other operating conditions were kept constant. 3.2.1. Effect of Anion Composition. In this study, the removal rate of SO42− was measured to be 21.03% in Na2SO4 solutions and 17.41% in NaF/Na2SO4 mixed solutions. The removal rate of F− amounted to be 28.29% in the NaF solutions and 20.22% in a mixed NaF/Na2SO4 solution (Figure 3). Accordingly, the monovalent anions were favorably removed as compared with divalent anions, which leaves an opposite effect from what is predicted by the Nernst−Planck equation. Based on the Nernst−Planck equation: Ji = −Di (dCi/dX + ziCi dØ/ C
DOI: 10.1021/acs.iecr.8b00929 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
operating conditions were optimized. In particular, investigation of the applied voltage, flow rates, and operating times was carried out. In this part, the feedwater composition was kept constant. 3.3.1. Applied Voltage. Figure 6 shows the salt removal efficiency as well as the sorption capacity at different voltages.
Figure 4. Effect of pH on F−/SO42− removal selectivity in terms of selectivity coefficient (S) for PSMCDI. The feedwater contained a mixture of NaF and Na2SO4 at the same concentration of 1.0 mM. The pH of the feed solution was adjusted by adding NaOH or HCl. The applied voltage, feedwater flow rate, and operating time were set at 1.2 V, 30 mL/min, and 10 min, respectively. Figure 6. Effect of applied voltage on removal selectivity: removal rate of F−/SO42− and selectivity coefficient (S) as a function of the applied voltage for PSMCDI. The feedwater mixed NaF and Na2SO4 at the same concentration of 1.0 mM. The applied voltages were 1.0, 1.2, and 1.4 V; feedwater flow rate and operating time were set at 30 mL/min and 10 min, respectively.
As expected, the removal efficiency of F− increased by increasing the working voltages from 1.0 to 1.4 V (from 19.66% to 21.61%). The removal efficiency of SO42− showed the same trend (from 13.67% to 21.10%). Partial explanation of these kinds of results can be made by the higher electrosorption capacity of the activated carbon electrodes owing to the stronger electrostatic interaction, subjected to a higher voltage.30 However, the removal efficiency of F− did not grow faster in comparison with the removal efficiency of SO42− with a higher applied voltage, which resulted in a decrease in the selectivity coefficient. It should be noted that, when there were no exchange membranes or merely the conventional exchange membranes were utilized, the divalent ions adsorbed more easily onto the electrode surface as compared with the monovalent ions.31 Our results confirm the role of the PSM for blocking the divalent anions. In accordance with the removal rate, the removal selectivity exhibited a gradual decline with an increment of the applied voltage. Figure 6 presents the variation of the selectivity coefficient (S) at the applied voltages ranging from 1.0 to 1.4 V. For example, when applied voltages range from 1.0 to 1.2 V, the selectivity coefficient (S) decreased from 1.44 to 1.16, which suggested that more monovalent anions were removed than divalent anions at a certain applied voltage. When it was increased to 1.4 V, the applied voltages were too high to attain a satisfactory removal selectivity. In the meantime, the excessive voltage was capable of causing electrolysis and increasing the energy consumption.32 3.3.2. Flow Rate. Figure 7 sheds light on the removal rate and selectivity coefficient (S) under different operational flow rates. In theory, the lower the flow rate, the longer the residence time; accordingly, a more effective depletion of ions gives a lower average concentration in the cell. This would lead to an enhanced resistance for the ion transport and a lower salt
Figure 5. Effect of feedwater concentration on removal selectivity: removal rate of F−/SO42− and selectivity coefficient (S) as a function of the feedwater concentration for PSMCDI. The feedwater contained a mixture of NaF and Na2SO4 at the same concentration of 0.5 mM, 1.0 mM, and 1.5 mM. The applied voltages, feedwater flow rate, and operating time were set at 1.2 V, 30 mL/min, and 10 min, respectively.
the other hand, the removed amount of monovalent and divalent anions showed an increase. Similar results were reported in the literature.28 While increasing the initial concentration of anions, the higher concentration gradient is able to give rise to a stronger driving force for the ion migration. The anions are capable of migrating through the membranes more conveniently, followed by being adsorbed by the porous carbon electrodes. However, at the higher concentrations, the removal rates of the two anions were substantial lower owing to the saturation limit for adsorption,29 and the removal rate reduction of SO42− was faster than F−, leading to a change in the selectivity coefficient. As evident from Figure 5, when the concentration was 0.5 mM, the selectivity coefficient (S) amounted to be 1.06. As the initial concentration was increased to 1.5 mM, the selectivity coefficient (S) increased to 1.28. 3.3. Optimization of Operating Conditions. In a bid to obtain the maximum selectivity of the PSMCDI process, the D
DOI: 10.1021/acs.iecr.8b00929 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 7. Effect of flow rate on removal selectivity: removal rate of F−/ SO42− and selectivity coefficient (S) as a function of the flow rate for PSMCDI. The feedwater was mixed NaF and Na2SO4 at the same concentration of 1.0 mM. The flow rates were 10 mL/min, 30 mL/ min, and 50 mL/min; applied voltages and operating times were set at 1.2 V and 10 min, respectively.
Figure 8. Effect of operating time on removal selectivity: removal rate of F−/SO42− and selectivity coefficient (S) as a function of the operating time for PSMCDI. The feedwater was mixed NaF and Na2SO4 at the same concentration of 1.0 mM. The operating times were 5, 10, and 15 min; applied voltages and flow rate were set at 1.2 V and 30 min/mL, respectively.
removal rate. The phenomenon observed in this work also received support from a previous study, where it was concluded that, when the flow rate was low, the continuous ion removal in the CDI cells would be impeded owing to the lower circulation rate of the treated water.24 In addition to that, the low flow rate would create dead zones in the spacer region, resulting in a compromised performance.33 Accordingly, the total removal rate was increased with flow rate. On the other hand, a higher flow rate leads to a stronger turbulence of the water flow, capable of affecting the adsorption of anions on the electrode. Increasing the flow rate would also reduce the residence time, so that ions would have less contact time with the electrodes to adsorb onto the surface of the electrode.34 This may lead to a different trend of F− and SO42− removal rate. Accordingly, the selectivity coefficient (S) has increased from 1.05 (when flow rate was 10 mL/min) to 1.34 (when flow rate was 50 mL/min). 3.3.3. Operating Time. For the purpose of observing the impact of operating time on the removal selectivity, all other parameters were kept constant. As shown in Figure 8, the selectivity coefficient increased gradually with the increase in the operating time. Initially, the removal rate of F− (11.64%) was close to SO42− (10.98%). With the increase in the operation time, the monovalent selective anion exchange membrane effectively hindered the transport of divalent anions. This gave rise to an increase in the selectivity coefficient (S).
was not carried out with the use of high fluoride groundwater, but it can help in the development of the PSMCDI technology, in addition to expanding its application for the removal of fluoride from drinking water.
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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/acs.iecr.8b00929. Materials and preparation of electrode, zeta-potentials of ASV, and images of MCDI cell (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail address:
[email protected] (J. Shen). ORCID
Jiangnan Shen: 0000-0003-1384-6139 Author Contributions §
J.P. and Y.Z. contributed equally to the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (No. 21706232; No. 21676249), the National High Technology Research and Development Program 863 (No. 2015AA030502), the Public Welfare Project of the Science and Technology Committee of Zhejiang Province (No. 2016C31010), and the National key research and development plan (No. 2017YFC0403700).
4. CONCLUSIONS This work performed the investigation of the electrosorption of monovalent anions by membrane capacitive deionization. The results implied that the monovalent selective anion exchange membrane is capable of effectively hindering the passage of the divalent anions for the achievement of a selective removal of monovalent anions. It was also observed that the selectivity coefficient (S) manifests an increase with the increase in the feed solution concentration and longer operating time. A high operating voltage would lead to the water electrolysis, in addition to being insufficient for achieving an adequate separation efficiency. The water flow rate also determines the selectivity coefficient by affecting the migration of anions. Owing to the limitation of the empirical conditions, this study
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DOI: 10.1021/acs.iecr.8b00929 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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