from complex vanadium solution using capacitive

Chemosphere 208 (2018) 14e20

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Recovery of V(V) from complex vanadium solution using capacitive deionization (CDI) with resin/carbon composite electrode Shenxu Bao a, b, *, Jihua Duan a, Yimin Zhang a, b, c, ** a

School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, PR China Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Wuhan, 430070, PR China c Hubei Provincial Engineering Technology Research Center of High Efficient Cleaning Utilization for Shale Vanadium Resource, Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University of Science and Technology, Wuhan, 430081, China b

h i g h l i g h t s
A novel resin-carbon composite electrode was fabricated for CDI treatment.
The composite electrode presents high selectivity for V(V).
The adsorption mechanism of ions on the composite electrode was illuminated.
V(V) can be separated and enriched by CDI with the composite electrode.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 April 2018 Received in revised form 23 May 2018 Accepted 24 May 2018 Available online 25 May 2018

The resin-activated carbon composite (RAC) electrodes were fabricated and applied in capacitive deionization for recovery of V(V) from complex vanadium solution. The adsorption capacity of the RAC electrode for V(V) is extremely low and the reduction of V(V) is significant in low pH solution, but the adsorbed V(V) on the electrode increases obviously and the reduction of V(V) gradually diminishes with the rise of pH. However, as the pH is increased to 10, the adsorbed V(V) on the RAC electrode declines. The higher applied potential is beneficial to the adsorption of V(V) and 1.0 V is appropriate for the adsorption. The impurities ions (Al, P and Si) are mainly adsorbed in the electric double layers on the RAC electrode and V(V) is dominantly adsorbed by the resins in the electrode. The adsorbed impurity ions can be easily removed by diluted H2SO4 and V(V) can be effectively eluted by 10% NaOH solution. The vanadium-bearing eluent can be recycled to recover and enrich vanadium from the complex solution. The performance of the RAC electrode keeps stable during the cyclic operation. This study may provide a promising and novel method for the recovery and separation of metals from aqueous solution. © 2018 Elsevier Ltd. All rights reserved.

Handling Editor: E. Brillas Keywords: Capacitive deionization Selectivity Vanadium Recovery Composite electrode

1. Introduction Capacitive deionization (CDI) is an emerging charge-based desalination technology where ions are separated under electrical field (Al Marzooqi et al., 2014). When an electrical potential is applied to the electrodes in CDI, charged ions migrate to the electrodes and are held in the electric double layers (EDLs) on the surface of the electrodes. Once the potential is removed, the

* Corresponding author. School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, PR China. ** Corresponding author. School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, PR China. E-mail addresses: [email protected] (S. Bao), [email protected] (Y. Zhang). https://doi.org/10.1016/j.chemosphere.2018.05.149 0045-6535/© 2018 Elsevier Ltd. All rights reserved.

adsorbed ions are quickly released back to the bulk solution (Kim and Choi, 2010). The CDI process commonly is conducted at a relatively low electric potential (typically 0.8e2.0 V), and it also does not produce contaminants during the treatment (Mossad and Zou, 2013; Porada et al., 2016). Hence, CDI technology has received a great deal of attention in the fields of separation and recovery of ions from aqueous solutions as an economical and environment friendly technique. The separation and removal of ions are achieved by adsorption of the ions in the EDLs on the electrode surface, which is highly dependent on the physicochemical properties of the electrode (Jia and Zhang, 2016). Thus, many studies have been conducted to screen or fabricate the electrode materials with outstanding characteristics, for example, excellent electrochemical properties,

S. Bao et al. / Chemosphere 208 (2018) 14e20

suitable pore structure and high specific surface area (SBET), to improve the performance of CDI. Carbon materials, such as activated carbon (AC) (Jande and Kim, 2013), carbon nanotube (CNT) (Liu et al., 2015; Wang et al., 2011), graphene (Li et al., 2010a) and carbon aerogel (CA) (Gabelich et al., 2002), due to their superior electrical conductivity and large SBET, are commonly used as electrode materials in CDI. The CDI process generally presents no or weak adsorption selectivity for ions because the ions are adsorbed by Coulomb force, resulting in the fact that CDI is only widely used to desalinate seawater or brackish water and is rarely applied in the fields needing selective removal of ions (Li et al., 2016), such as chemical engineering and environment. However, some researchers found that CDI can present selective adsorption capability for certain ions by using the composite or surface-modified materials as electrodes. Liu et al. (2013) used CDI technology to selectively adsorb Ca2þ from the solution containing Ca2þ, Mg2þ and Naþ by means of CNTs/Caselective zeolite composite electrodes. Xu et al. (2008) manufactured the composite carbon aerogel using a resorcinol/formaldehyde polymerization and pyrolization process and applied it in CDI to recover iodide from brackish water. Kim et al. (2017) synthesized sodium manganese oxide (Na0.44MnO2) via the solid-state reaction of Na2CO3 and Mn2O3 as the electrode in CDI and the CDI exhibited extremely high selectivity for Naþ than for Kþ, Mg2þ and Ca2þ in the electrolyte. Lee et al. (2017) fabricated a spinel type of LiMn2O4 as the lithium-selective electrode by using Li2CO3 and MnCO3 as reactants. Their studies verified the feasibility of the proposed concept that lithium can be recovered from aqueous solution containing lithium by using the CDI with LiMn2O4 electrode. Although these studies indicated the feasibility of selective adsorption of ions by CDI, there are very little reports on the separation and recovery of metals from the complex solution by CDI. Vanadium is an important versatile rare element, which is dispersedly distributed in the earth's crust. Separation and recovery of vanadium from the complex vanadium leaching solution is a necessary procedure for production of vanadium (He et al., 2007; Li et al., 2010b; Zhang et al., 2011). Moreover, the removal of vanadium from the wastewater in vanadium industry is also a problem which is widely concerned in environment (Kaczala et al., 2009). At present, the widely adopted methods for the recovery of vanadium from aqueous phases are solvent extraction and ion exchange. However, solvent extraction may cause potential threat to environment and ion exchange process usually needs high-pressure pumps and long operation time (Liang et al., 2016; Tang et al., 2017). CDI is hopeful to overcome these shortcomings and provide a novel and potential method for the separation and recovery of metals from complex solution. Our previous study demonstrated that the ion exchange resin-activated carbon composite electrode (RAC electrode) in CDI presents high affinity for vanadium ions (Duan et al., 2018). In this study, a novel RAC electrode was fabricated and applied in CDI for the separation and recovery of pentavalent vanadium (V(V)) from complex vanadium-bearing solution. This study may prove the feasibility of selective adsorption by CDI and give a promising alternative for the separation and removal of metals from complex solutions. 2. Materials and methods 2.1. Materials D201 anion exchange resin (mean diameter 0.15 mm) with high affinity for V(V) [Zhu et al., 2017], provided by Zhejiang Zhengguang Industrial Co. Ltd, and the AC with SBET of 1027 m2/g and mean diameter of 0.038 mm (Huang et al., 2014), purchased from Provedor de Laboratorios Co. Ltd., were used to prepare the RAC

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electrode in this study. The high-purity graphite flake with the dimensions of 100 mm (length)  50 mm (width)  1 mm (thickness) was supplied by Haimen Shuguang Carbon Industry Co. Ltd. N-dimethylacetamide (DMAC) with A.R. grade and Polyvinylidene fluoride (PVDF) with A.R. grade came from Sinopharm Chemical Reagent and Sigma Aldrich Co. Ltd., respectively. The simulated vanadium-bearing solution containing 1000 mg L1 V(V), 3000 mg/L Al, 230 mg L1 P and 35 mg L1 Si, referring to the common leaching solution in vanadium industry (Li et al., 2010a,b; Chen et al., 2010), was prepared by dissolving sodium metavanadate (NaVO3$xH2O), aluminum sulfate hexadecahydrate (Al2(SO4)3$18H2O), sodium phosphate (Na3PO4$12H2O) and sodium silicate (Na2SiO3$9H2O) in deionized water (Millpore Milli-Q®). The pure vanadium solution containing 1000 mg L1 V(V) was prepared by only dissolving sodium metavanadate in deionized water. The pH of solution was adjusted by sulfuric acid with A.R. grade in the experiments. Sodium metavanadate, ordered from Alfa Aesar (Tianjin) Chemical Co. Ltd. Aluminum sulfate hexadecahydrate, sodium phosphate and sodium silicate were obtained from Sinopharm Chemical Reagent Co. Ltd. All of these reagents are of C.P. grade.

2.2. Fabrication of electrodes The fabrication process of the RAC electrode is same to that described in our previous study (Duan et al., 2018). Briefly, D201 resins were firstly treated by soaking in 5% (v/v) HCl and 5% (m/v) NaOH solutions alternately to remove the remained monomers and other types of impurities which may be produced in the fabrication process, followed by washing with deionized water to neutral, and then were filtered and dried at 60  C in vacuum oven for 12 h before use. Subsequently, 2 g AC powder, 2 g pretreated resin and 0.4 g PVDF were mixed in 12 mL DMAC for 4 h to prepare the resin-AC composite material slurry, and then the slurry was uniformly smeared on the collector electrode, a high-purity graphite flake. Finally, the flake smeared with the slurry was put in vacuum oven at 65  C for at least 4 h to form the RAC electrode. PVDF was used in the fabrication process to bind the composite material with graphite flake and endow the electrode with appropriate mechanical stability. The preparation procedure for the AC electrode used in this study is same to that for the RAC electrode just no resin is added.

2.3. CDI adsorption The CDI equipment is made up of CDI cell, constant voltage direct-current (DC) power supply (IT6861A, ITECH Co. Ltd, China), peristaltic pump, tubes and beaker (Fig. SM-1). The CDI cell is assembled by three pairs of parallel electrodes which are accommodated in a polymethyl methacrylate tank. The space of each pair of electrodes is 3 mm, which can allow solution to flow freely and maintain suitable electric field intensity. Each pair of parallel electrodes is supplied a same constant voltage in the experiments. The RAC or AC electrodes were used as anode and the high-purity graphite flakes without the resin-AC composite material were used as cathode in CDI treatment. 200 mL feed solution was pumped into the CDI cell from a beaker through the inlet by peristaltic pump, and the effluent flowed into the beaker through the outlet for circular treatment. In the CDI adsorption experiments, the processing time was 120 min, and the flow rate was kept at 30 mL min1. The electrosorption capacity of electrode was calculated by Eq. (1).

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Q ¼ ðC0 V0  Ct Vt Þ=m

S. Bao et al. / Chemosphere 208 (2018) 14e20

(1)

where Q is the adsorption capacity of RAC electrode for ions (mg$g1). C0 and Ct are the initial concentration and the final concentration of ions in the solution, respectively (mg$L1). V0 and Vt are the initial and the final solution volume (L), respectively. m is the mass of resin-AC composite material on the electrode (g). The reduction rate of V(V) (m; %) after CDI treatment was calculated according to Eq. (2).

m ¼ CIV =CT

(2)

Where CIV is the V(IV) concentration in the solution after CDI adsorption (mg$L1). CT is the initial V(V) concentration in the solution (mg$L1).

2.4. CDI desorption The desorption experiments also were conducted in the same CDI cell as mentioned above. Firstly, the solution after adsorption in the cell was discharged. Then, 200 mL sulfuric acid solution with pH 2.5 was pumped into the cell to circulate for 10 min with the electrodes short-circuited (i.e. the cathodes and anodes are directly connected with wire). Subsequently, the acid solution was discharged and 200 mL 10 wt% NaOH solution was inlet into the cell to circulate for 30 min with three kinds of electrode connection modes, short-circuited, reversed (i.e. the cathodes and anodes are interchanged) and unchanged (i.e. the cathodes and anodes remain the same to those in the adsorption process), respectively. The solution after this procedure is vanadium-bearing pregnant solution. Finally, the electrodes were short-circuited and conditioned by 200 mL sulfuric acid solution with pH 2.5 for next adsorption. The desorption rate of vanadium (q, %) in this process was calculated by Eq. (3).

q ¼ Cd Vd =ðC0 V0  Ct Vt Þ

(3)

where Cd is the vanadium concentration in the vanadium-bearing pregnant solution (mg$L1). Vd is the volume of the pregnant solution (L).

2.5. Cyclic adsorption-desorption experiments In the cyclic adsorption-desorption experiments, the feed solution (simulated vanadium-bearing solution) was refreshed but the vanadium-bearing pregnant solution was cyclically used to achieve high vanadium content in the solution as described in section 2.4.

2.6. Measurements The ion concentrations in solution were analyzed by inductively coupled plasmaeoptical emission spectroscopy (ICPeOES, Optima 4300DV, PerkineElmer, USA). The V(V) content was determined by potentiometric titration using ammonium ferrous sulfate as reducing reagent (Bao et al., 2012), and the concentration of V(IV) was obtained by subtracting the V(V) content from the total vanadium concentration. The surface morphologies of the RAC electrode and the elements distribution on the electrode were investigated using a scanning electron microscope (SEM, JSMIT300, JEOL, Japan) equipped with an energy dispersive spectrometer (EDS, Oxford, UK).

3. Results and discussion 3.1. Effect of pH Under the constant electrode potential of 0.6 V, the adsorption capacities of the RAC electrode for V(V) in the pure vanadium solution at different pH are shown in Fig. 1. It can be found that the adsorption capability of the electrode is highly dependent on the pH of solutions. The RAC electrode shows extremely low adsorption capacity for V(V) at pH 1.0 (lower than 10 mg g1) but it is significantly elevated when pH is increased to 2.0 and reaches peak and keeps relative stable as about 110 mg g1 at pH 2.5, 3.0 and 4.0 after 120 min adsorption (Fig. 1c, d, e). However, when pH is increased to 10.0, the adsorption capacity sharply declines to about 45 mg g1 (Fig. 1f). The adsorption of vanadium on the RAC electrode may be closely related to its existing form in aqueous solution. It is well known that V(V) exists as various forms at different pH in solution as shown in Fig. SM-2. Most V(V) exists as cationic VOþ 2 in the so-

lution with pH 1.0. The cationic VOþ 2 only moves to the cathode and cannot be adsorbed under electric potential because the graphite flake was used as the cathode. Thus, almost no V(V) is adsorbed on the RAC electrode at pH 1.0 (Fig. 1a). As VOþ 2 moves to the surface of the cathode, the potential electrode reactions on the cathode and anode are presented as reaction (4) and reaction (5), respectively. þ VOþ þ e ¼ VO2þ þ H2 O 2 þ 2H

(4)

2H2 O  4e ¼ 4Hþ þ O2

(5)

The overall standard cell potential (i.e. the combined potential of reaction (4) and reaction (5)) is 0.238 V (Lide, 2008), which is by far lower than the applied potential in the experiments (0.6 V). Thus, reaction (4) is possible and nearly 70% of VOþ 2 is reduced to VO2þ after 120 min treatment at pH 1.0 (Fig. 1a). With the increase of pH, more and more VOþ 2 converts into the anionic forms, such as V10O26(OH)2 2 and V10O27(OH) (Olazabal et al., 1992; Fig. SM-2) and most V(V) exists as anionic forms at the solution with pH above 2.5. The anionic vanadium ions move to the anode under the applied potential and can react with the D201 resin embedded in the RAC electrode as Mn þ n(RN(CH3)3OH) ! nOH þ (RN(CH3)3)nM

(6)

where Mn is anionic vanadium ions, R is the resin's matrix. Therefore, the adsorption capacity of the RAC electrode for V(V) significantly increases with the pH and it keeps relative stable at the pH range of 2.5e4 (Fig. 1). Meanwhile, with the converting of VOþ 2 into anionic forms, the amount of vanadium which is forced to migrate to the cathode by Coulomb force gradually diminishes and then the reduction rate of vanadium also declines. The reduction of V(V) can be neglected when the pH is above 2.5 (Fig. 1c) because most V(V) exists as anionic forms. With the continuous increase of pH, reaction (6) gradually shifts to the left, resulting in the significant decline of the adsorption capacity for V(V) at pH 10 (Fig. 1f). Thus, pH 2.5 is selected as the appropriate condition in the following experiments. 3.2. Effect of potential As shown in Fig. 2, the adsorption capacity of the RAC electrode for V(V) is well positively related to the rise of the applied potential. High potential would enhance the migration speed of ions in

S. Bao et al. / Chemosphere 208 (2018) 14e20

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Fig. 1. The effect of pH on adsorptive property of RAC electrode for pure vanadium solution.

solution, and improve the storing capacity of EDLs for ions (Tsai and Doong, 2015). Thus, the adsorption capacity of the RAC electrode at 1.0 V increases by 16% than at 0.2 V for V(V), and the adsorption rate of V(V) on the electrode at 1.0 V is obviously faster than that at 0.2 V at the beginning of the experiments. When the applied potential is above 1.0 V, the electrolysis of water may become significant (Liang et al., 2017). Thus, 1.0 V is selected for the adsorption of V(V) on the RAC electrode in this study.

3.3. Adsorption mechanism on RAC electrode The adsorption capacities of different electrodes for the ions in the simulated vanadium solution are shown in Fig. 3. It can be seen that the AC electrode presents much higher adsorption capacity for Al and V(V) than for P and Si and the amount of adsorbed Al is more than that of adsorbed V(V) on the electrode (Fig. 3). The concentration of Al is higher than that of other ions in the solution and

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S. Bao et al. / Chemosphere 208 (2018) 14e20 Table 1 The adsorption of V(V) in different solution on electrodes (1.0 V, pH ¼ 2.5, 120 min). Adsorption capacity for V(V) (mg$g1)

Pure vanadium solution Simulated vanadium solution

Fig. 2. The effect of potential on adsorption capacity of RAC electrode for V(V).

AC electrode

RAC electrode

75.3 66.2

127.7 124.6

adsorption of V(V) on the RAC electrode than on the AC electrode, indicating that the impurity ions do not compete with V(V) for adsorbing on the RAC electrode. Generally, the adsorption selectivity for ions in EDLs of electrode during CDI treatment is weak (Mossad and Zou, 2012) but ion exchange resins possess strong affinity for certain ions. Thus, it is rational to deduce that most impurity ions may be adsorbed in the EDLs and V(V) are dominantly adsorbed by the resins embedded in the RAC electrode. The SEM-EDS images of the RAC electrode after treating the simulated vanadium solution are shown in Fig. 4. The RAC electrode exhibits rough surface and the relative big resin particles mingled with fine AC powder can be clearly seen (Fig. 4a). Based on the elemental distribution images recorded by EDS, the positioning of V(V) correlates well with the distribution of resin in the RAC electrode but Al, P and Si do not present obvious relevance to resin, which are distributed dispersedly in the electrode. It also verifies that V(V) is mainly adsorbed on the resin and most impurity ions are adsorbed in the EDLs of the electrode. Therefore, the adsorption processes of ions on the RAC electrode can be rationally deduced. Firstly, the ions in the solution migrate to the surface of the RAC electrode and are adsorbed in the EDLs under Coulomb force. Then, V(V) penetrates into the pores of the resin-AC composite material on the electrode and react with the resin according to reaction (6) due to the high affinity of resin for V(V). 3.4. The desorption of ions

Fig. 3. The adsorption capacities of different materials for ions in simulated vanadium solution (The data for resin were obtained at the same conditions for the reaction between the resin in the RAC electrode and ions in the solution).

some Al exists as the anionic form, AlðSO4Þ 2 in the solution at pH 2.5 (Puigdomench, 2015; Fig. SM-3) which can be adsorbed on the anode. Many studies indicated that AC electrode in CDI has weak adsorption selectivity for ions and the adsorption of ions is closely related to their concentrations on AC electrode (Al Marzooqi et al., 2014; Mossad and Zou, 2012). Thus, the AC electrode exhibits extremely high adsorption capacity for Al in comparison with V(V), P and Si due to their concentration discrepancies. As D201 resin was added to fabricate the RAC electrode, the adsorption of ions on the electrode can be significantly changed. The RAC electrode itself (at the condition with no power is applied) has the highest adsorption capacity for V(V) although the concentration of P is higher than that of V(V) because D201 resin owns the highest affinity for V(V) than for other ions (Fig. 3). As potential was applied on the RAC electrode, both the adsorption selectivity and adsorption capacity for V(V) have been largely improved, indicating that electric field can enhance the selective adsorption of V(V) on the RAC electrode. By comparing the adsorption capability of different electrodes in CDI for V(V) in the pure and simulated vanadium solution (Table 1), it can be seen that the impurity ions have less influence on the

As described in section 2.4, a complete desorption process after treating the simulated vanadium solution consists of H2SO4 desorption, NaOH desorption and H2SO4 conditioning. The H2SO4 desorption procedure is aimed to desorb the impurity ions adsorbed in the EDLs and NaOH solution is used to desorb the V(V) adsorbed on the resin in the RAC electrode. The desorption rates of V(V) from the RAC electrode using different connection modes for the electrodes are shown in Fig. SM-4. It can be seen that the unchanged connection mode causes higher desorption rate for V(V) than other two modes, which is different to the desorption process in the desalination using CDI where the ions commonly are desus orbed with the short-circuited or reversed electrodes (Brose et al., 2009). This can be explained by the separation mechanism of V(V) by the RAC electrode. As discussed above, V(V) are dominantly adsorbed on the resin in the RAC electrode and then the desorption of V(V) should be the reverse process of reaction (6) when NaOH is used as eluent reagent. Under the unchanged mode, OH is forced to enter the EDLs of the RAC electrode and then it exchanges with the adsorbed V(V) on the resins. However, there is no driving force for OH to migrate to the RAC electrode and react with the resin with the electrodes short-circuited and reversed. Therefore, the unchanged mode is the most appropriate one for the desorption of V(V) from the RAC electrode. Finally, the RAC electrodes were conditioned by H2SO4 for the next adsorption process. The ion concentrations of effluent in different procedures are listed in Table 2. It is observed that most impurity ions are desorbed in the H2SO4 desorption and most V(V) can be recovered in the vanadiumbearing pregnant solution. It needs to be emphasized that the desorbed V(V) in the H2SO4 desorption and conditioning

S. Bao et al. / Chemosphere 208 (2018) 14e20

Fig. 4. The SEM-EDS images of RAC electrode after adsorption.

Table 2 The concentration of ions in different desorption procedures. Concentration (mg$L1)

H2SO4 desorption NaOH desorption H2SO4 conditioning

V

Al

P

Si

24.04 287.8 12.65

80.0 19.64 6.5

6.64 2.95 0.64

1.17 1.29 1.03

procedures would not be lost. These effluents can be used in the acid leaching process for the extraction of vanadium (Li et al., 2010a,b).

3.5. The enrichment of vanadium Fig. 5 presents the changes of ions concentration in the pregnant solution during the cyclic absorption-desorption experiments. The V(V) content in the cyclic pregnant solution increases continuously

Fig. 5. Cyclic absorption-desorption experiments.

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with the cycles and it reaches 2164 mg L1 after 8 cycles, which is about 2.2 times higher than the initial V(V) concentration in the simulated vanadium solution. The concentrations of impurities (Al, P and Si) in the pregnant solution just slightly increase with the cycles (Fig. 5). Thus, V(V) in the simulated solution can be effectively separated and enriched by CDI with cyclic absorptiondesorption mode. In each adsorption process, the RAC electrode presents relatively stable adsorption capacity for V(V) and does not display obvious attenuation, indicating that the RAC electrode has stable physical and chemical characteristics and its performance also remains steady in the CDI running process. 4. Conclusions The adsorption of V(V) on the RAC electrode and the reduction of V(V) are closely related to the solution's pH during the CDI treatment process. V(V) cannot be adsorbed on the RAC electrode acting as anode because most V(V) exists as cationic VOþ 2 in the low pH solution. As the increase of pH, more and more V(V) converts into the anionic forms and the amount of adsorbed V(V) on the RAC electrode can be largely improved. As the pH increases to 10, the reaction between V(V) and the resin in the RAC electrode shifts to the reverse direction, resulting in the sharp decline of adsorbed V(V) on the electrode. The reduction of V(V) is significant for the low pH solution but it can be neglected in high pH solution owing to the fact that the anionic forms of V(V) is repelled from the cathode. The adsorption capacity of the RAC electrode for V(V) increases with the applied potential and 1.0 V is appropriate for the adsorption of V(V) on the RAC electrode by comprehensively considering the adsorption capacity and the electrodialysis of water. Most impurity ions are adsorbed in the EDLs of the RAC electrode and V(V) are dominantly adsorbed by the resins in the electrode when the simulated vanadium solution is treated by CDI with RAC electrode. The adsorbed vanadium can be effectively separated and recovered by eluting with NaOH solution after the impurity ions are removed with diluted H2SO4. The vanadiumbearing NaOH eluent can be recycled to separate and enrich vanadium from the complex vanadium-bearing solution. Acknowledgments This research was financially supported by the Major Technical Innovation Project of Hubei Province (2018ACA157) and National Key Science-Technology Support Programs of China (2015BAB03B05). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.chemosphere.2018.05.149. References Al Marzooqi, F., Al Ghaferi, A., Saada, I., Hilal, N., 2014. Application of capacitive deionisation in water desalination: a review. Desalination 342, 3e15. Bao, S., Zhang, Y., Huang, J., Yang, X., Hu, Y., 2012. Determination of vanadium valency in roasted stone coal by separate dissolve-potentiometric titration method. MRS Proc. 1380. us, R., Cigana, J., Barbeau, B., Daines-Martinez, C., Suty, H., 2009. Removal of Brose total dissolved solids, nitrates and ammonium ions from drinking water using

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