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carbon composite electrode and application in selective adsorption of vanadium(IV) by capacitive deionization
Accepted Manuscript Title: The characteristics of resin/carbon composite electrode and application in selective adsorption of vanadium(IV) by capacitive deionization Author: Jihua Duan Shenxu Bao Yimin Zhang PII: DOI: Reference:
S0263-8762(18)30023-6 https://doi.org/doi:10.1016/j.cherd.2018.01.021 CHERD 2995
To appear in: Received date: Revised date: Accepted date:
13-5-2017 22-10-2017 9-1-2018
Please cite this article as: Duan, J., Bao, S., Zhang, Y.,The characteristics of resin/carbon composite electrode and application in selective adsorption of vanadium(IV) by capacitive deionization, Chemical Engineering Research and Design (2018), https://doi.org/10.1016/j.cherd.2018.01.021 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.
The characteristics of resin/carbon composite electrode and
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application in selective adsorption of vanadium(IV) by capacitive
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deionization
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Jihua Duana, Shenxu Baoa, b*, Yimin Zhanga, b, c
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a. School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR
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China
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b. Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology,
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Wuhan 430070, PR China
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c. Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University
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* Corresponding author: [email protected] Tel/Fax: +86-27-87212127
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Abstract: The characteristics and adsorption performances of three kinds of resin/mineral active
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carbon (Resin/AC-m) composite electrodes, the pure AC-m electrode and the resins were
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comparatively investigated. The Resin/AC-m composite electrodes have lower specific
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capacitance and higher resistance than the AC-m electrode, but their hydrophilia, specific surface
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areas and microspores content can be largely improved due to the addition of the resins, which is
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beneficial to the ions adsorption on the electrodes. The hydrated radius of VO2+ is smaller than
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that of Fe3+ and Al3+, then VO2+ is easier to pass through the microspores of the D860/AC-m
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electrode and adsorbed by the composite electrode. Thus, the D860/AC-m electrode presents the
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highest adsorption capacity and the strongest selectivity for VO2+, which may be resulted from the
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combined actions of the ion-sieve effect of the composite electrode material and the affinity of the
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resin for VO2+. Multiple adsorption-regeneration cycles proved that the performance of the
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D860/AC-m electrode can remain stable in long-term operation, indicating it may be suitable for
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cycle use in CDI. This study may provide a promising method for separation and recovery of ions
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from complex solutions.
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Key words: Capacitive deionization; Selectivity; Vanadium; Ion exchange resin; Composite
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electrode
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power supply and the salt ions are adsorbed onto the surface of charged electrodes due to
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electrostatic forces [Nie et al., 2012]. After the electrodes reach adsorption saturation, the
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adsorbed ions can be desorbed by shortening the electrodes or reversing the potential and the
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electrodes also are regenerated [Benjamin et al., 2015]. The potential required for desalination
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1. Introduction
Capacitive deionization (CDI), as an emerging charge-based desalination method, was first
proposed by Murphy and Caudle in the 1960s [Murphy et al., 1965]. During the desalination process, the salinity solution is pumped through the electrode pairs which are connected with DC
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process is minimal, usually less than 1.23 V to prevent water electrolysis and it also does not
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produce contaminants during the desalination and regeneration processes [Mossad et al., 2013].
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Hence, CDI technology is characterized by low energy consumption and environmental
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friendliness.
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To date, CDI is mainly adopted as a promising desalination technique to treat brackish water
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or seawater and most research in CDI field focuses on the screening and fabrication of electrode
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materials with outstanding features, such as excellent electrochemical properties, high specific
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surface area (SBET) and suitable pore structures to improve the desalination performance. Carbon
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materials, activated carbon (AC) [Jande et al., 2013], carbon aerogel (CA) [Gabelich et al., 2002],
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carbon nanotubes (CNTs) [Wang et al., 2011], carbon nanofibers (CNFs) [Gao et al., 2009] and
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graphene [Li et al., 2010] for example, are commonly used as the electrode materials in CDI
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system due to their excellent electrical conductivity and large SBET. However, some researchers
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found that CDI may present selective adsorption ability for certain ions by using composite
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materials or surface-modified materials as electrodes. Liu et al. [Liu et al., 2013] prepared the
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CNTs/Ca-selective zeolite composite electrodes using electrophoretic deposition method to selectively adsorb Ca2+ from the solution containing Ca2+, Mg2+ and Na+ in CDI process. Pan et al.
[Pan et al., 2009] fabricated carbon nanotube and nanofiber (CNT-CNF) composite film as CDI electrode material and found it presented better adsorption capacity for smaller hydrated anions.
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Kim and Choi [Kim et al., 2012.] prepared the nitrate-selective composite carbon electrodes by
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coating the surface of an AC electrode with a nitrate-selective anion exchange resin and this
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electrode showed slight adsorption selectivity and greater adsorption capacity for nitrate compared
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with the counterpart ion exchange membrane capacitive deionization (MCDI). Although these 3
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studies indicate the feasibility of selective adsorption by CDI, the current research is by far
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insufficient for the application of CDI in the selective adsorption of certain metal ions from
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complex solutions. Moreover, the mechanism of selective adsorption needs intensive research to
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obtain reasonable explanation.
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Vanadium is an important rare element, which is dispersedly distributed in the earth’s crust.
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Acid leaching is an effective method to extract vanadium from the vanadium-bearing resources,
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such as stone coal [Liang et al., 2016], wasted catalysts [Hu et al., 2006.] and fly ashes [Navarro et
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al., 2007]. Thus, separation and recovery of vanadium from the complex acid leaching solutions is
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vital for production of vanadium. Currently, the vanadium-containing solution is commonly
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treated by ion exchange (IX) and solvent extraction (SX). IX process is easy to operate and control,
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and it is also regarded as an environmental friendly separation method. However, the mass transfer
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of ions in the resin is relatively slow and it takes long time for the resin to achieve adsorption
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equilibrium. For example, Fan et al. [Fan et al., 2013] found that the equilibrium time of the
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vanadium adsorption on D314 resin is 14h. SX process is characterized by high selectivity and
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high adsorption capacity, but this process is complex and the reagents consumption is high. Besides, the emulsification and loss of extractants also cause potential threat to environment [Liang et al., 2016]. Thus, the CDI using resin/carbon composite electrodes was investigated for selective and efficient separation of V(V) from simulated vanadium-leaching solution in this study
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to overcome some intrinsic shortcomings of IX and SX. Three kinds of Resin/AC composite
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electrodes were fabricated by using different IX resins and minerals active carbon (AC-m), and
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their physicochemical and electrochemical properties and the adsorption characteristics for the
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ions in the simulated solution were comparatively studied with the pure AC-m electrode in CDI 4
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processes. Furthermore, the performance of the composite electrodes in the multiple
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adsorption-regeneration cycles was also investigated. This study may provide a novel promising
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method for separation and purification of ions from complex solutions.
2. Materials and Methods
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2.1. Materials
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Three kinds of cation exchange resins, D001, D840 and D860, which are usually used for
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water treatment and metals recovery, are selected to prepare the Resin/AC-m composite electrodes.
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D001 is widely used to adsorb rare and precious metal ions. D840 and D860, as the chelate resins
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with thiourea and amino phosphonic acid functional groups respectively, have high affinity for
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certain metal ions [Luo et al., 2011; Zeng et al., 2012]. These resins possessing the same matrix
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but different functional groups, were provided by Zhejiang Zhengguang Industrial Co. Ltd., China
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and their properties are listed in Table 1.
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Table 1 Properties of the resins used
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D840
D860
-SO3H
-CH2-S-C-HNH2
-CH2NHCH2P(O)(OH)2
≥4.35 H+ Poly(styrene-divinyl benzene) Brown, opaque 0.15
≥3.18 H+ Poly(styrene-divin ylbenzene) Lacteous, opaque 0.15
≥3.18 H+ Poly(styrene-divinylbenz ene) Beige, opaque 0.15
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D001
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Provedor de Laboratorios Co. Ltd., Mexico, were selected to prepare electrodes because it owns
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big SBET and excellent electrochemical properties [Huang et al., 2014]. The high-purity graphite
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flake with the dimensions of 100 (length) × 50 (width) × 1 (thickness) mm was supplied by
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Haimen Shuguang Carbon Industry Co. Ltd., China, Polyvinylidene fluoride (PVDF) (A.R.) and
Functional groups Total capacity (mmol/g) Ion form Matrix
Appearance Size (mm)
The AC-m with SBET of 1027 m2/g and mean diameter of 0.038 mm, purchased from
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N-dimethylacetamide (DMAC) (A.R.) came from Sigma Aldrich, United States, and Sinopharm
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Chemical Reagent Co. Ltd., China, respectively. The acid leaching solution of stone coal commonly contains vanadium(IV), iron(III) and
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aluminum(III) where vanadium mainly exists as VO2+ in the acid leaching solution [Xiong et al.,
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2017]. Thus, the simulated vanadium leaching solution containing 300 mg/L VO2+, 300 mg/L Fe3+
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and 300 mg/L Al3+ was prepared by dissolving vanadyl sulfate (VOSO4·xH2O), iron(III) sulfate
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hydrate (Fe2(SO4)3·9H2O) and aluminum sulfate hexadecahydrate (Al2(SO4)3·16H2O) in deionized
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water (Milli-Q® Millipore). The pure vanadium solution with 300 mg/L VO2+ was prepared by
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only dissolving vanadyl sulfate in deionized water. Vanadyl sulfate, ordered from Alfa Aesar
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(Tianjin) Chemical Co. Ltd., China, and iron sulfate hydrate and aluminum sulfate
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hexadecahydrate, which were obtained from Sinopharm Chemical Reagent Co. Ltd., China, are all
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C.P. grade.
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2.2. Fabrication of electrodes
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The AC-m slurry was prepared by mixing 4 g AC-m powder and 0.4 g PVDF in 12 mL
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DMAC for 4 h. Then, the slurry was uniformly casted on a collector electrode, a high-purity graphite flake, and the graphite was put in vacuum oven at 65 ◦C for at least 4 h to make the organic solvent (DMAC) entirely volatilized to produce the pure AC-m electrode. As for the fabrication of Resin/AC-m composite electrodes, the resins were firstly treated by
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soaking in 5% (m/v) NaOH and 5% (v/v) HCl solutions alternately to remove the remained
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monomers and other types of impurities which may be produced in the fabrication process,
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followed by washing with deionized water to neutral, and then were filtered and dried at 60 ◦C in
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vacuum oven for 12 h before use. Subsequently, the Resin/AC-m composite slurry was prepared 6
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by mixing 2 g AC-m powder, 2 g resin and 0.4 g PVDF in 12 mL DMAC for 4 h. Then, the
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composite slurry was casted on the high-purity graphite flake to fabricate the Resin/AC-m
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composite according to the preparation steps for the pure AC-m electrode. The composite
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electrode containing D001, D840 or D860 resin was referred as D001/AC-m, D840/AC-m or
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D860/AC-m electrode, respectively. The PVDF is used in the fabrication process to bind the slurry
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with graphite flake and endow the electrodes with appropriate mechanical strength.
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2.3. Static adsorption of resins
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The pH of simulated solution and pure vanadium solution were adjusted to 2 by sulphuric
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acid for static adsorption of resin and CDI process because this pH is suitable for the adsorption of
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vanadium on the resins used in this study. 0.50 g resin was introduced into a conical flask
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containing 100 mL solution, then the conical flask was shaken (100 rpm) at 298K for 6 h to attain
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equilibrium. The adsorption capacity of resin for ions, is calculated by
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Q=(C0 V0 -Ct Vt )/m
(1)
where Q is the adsorption capacity of resin (mg/g). C0 and Ct are the initial concentration and
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where C1 and C2 are the concentrations of vanadium on the resin (mg/g) and in the solution
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after adsorption (mg/L), respectively. C3 and C4 are the concentrations of the counterpart metal,
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i.e. Fe3+ or Al3+on the resin (mg/g) and in the solution (mg/L) after adsorption, respectively. The
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experimental conditions for the static adsorption of AC-m were same to those for the resins.
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final concentration of ions in the solution, respectively (mg/L). V0 and Vt are initial and final
solution volume (L), respectively. m is the mass of resin (g). The separation factor of vanadium(IV) to iron(III) (βV/Fe) or aluminum(III) (βV/Al) is
expressed as
βV/metal=(C1 /C2 )/(C3 /C4 )
(2)
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2.4. CDI ad dsorption
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The CDI equuipment is mainly m made up u of a CDI ceell, peristalticc pump and DC D supply (Fiig. 1).
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C supply cam me from Natonng Peristalticc Pump Manuufacture Co. Ltd., The peristaltic puump and DC
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ment Co. Ltdd., China, reespectively. The T CDI ceell is Chinna, and Zhaaoxin Electroonic Equipm
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assembled by three t pairs of o parallel electrodes which w are accommodated in polym methyl
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methhacrylate. Th he space betw ween two eleectrodes is 3 mm. The feed f solution with pH off 2 is
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pum mped into the CDI cell from a beaker at a 30 mL/minn through a loower inlet by y peristaltic pump,
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f into thee beaker thro ough an upperr outlet for 60 min circulaar treatment. Each and the effluent flows
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pair of parallel electrodes iss supplied wiith 1.5 V byy the DC suppply in the experiments. The
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electtrosorption caapacity of eleectrode can be b calculated by equation 1 and the sep paration factoors of
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b analyzed acccording to equation 2. diffeerent electroddes also can be
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F 1. Fig.
Diaggram of the CDI C equipmennt
2.5. Adsorp ption-regenerration cycle experiments e
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The perform mance of Ressin/AC-m com mposite electtrode in long-term use waas investigateed by
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treatting the simuulated leachinng solution foor 50 times addsorption-reg generation cyccles. One cyccle in
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a off ions and the t regenerattion of electrodes. Afterr the CDII process inccludes the adsorption
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com mpletion of adsorption, a thhe treated soolution was discharged and a then thee electrodes were 8
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shortened and dilute sulphuric acid (8% v/v) was introduced into the CDI cell to strip the ions
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adsorbed on the electrodes for 10 min. Finally, the electrodes were washed by deionized water to
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the pH of effluent above 2 to achieve regeneration and the simulated leaching solution was fed for
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next cycle.
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2.6. Measurements
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2.6.1. Scanning electron microscope (SEM)
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The surface morphologies of the fabricated electrodes were investigated using a SEM
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(JSM-IT300, Japan).
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2.6.2. SBET and pore structure
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The debris was carefully scraped from the prepared electrodes using a small blade for the
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analysis of SBET and pore structure. SBET of the material was determined by a surface and pore size
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analyzer (ASAP 2020M, Micromeritics Instruments Co. Ltd., United States) using the Brunauer–
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Emmett–Teller (BET) method. The pore size distribution was obtained from the isotherm
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adsorption branch based on the Barrett–Joyner–Halenda (BJH) method [Hou et al., 2014].
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2.6.3. Contact angle
Contact angle of the electrodes was determined using Wilhelmy plate method with the help of
fully automatic surface tension meter K100 (KRUSS Co. Ltd., Germany). 2.6.4. Electrochemical measurement
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Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were
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conducted by VersaSTAT 4 (AMETEK, United States) working station with a three-electrode cell
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including a working electrode (the electrode to be analyzed) with dimension of 10 mm × 10 mm, a
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counter electrode (platinum wire) and a reference electrode (Ag/AgCl) at 25 °C. The specific 9
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capacitance was determined by conducting CV in the potential range of −0.3 to 0.9 V at a specific
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scan rate of 20 mV/s by equation 3. C=∫ dE/(Ms(∆E))
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(3)
where C is specific capacitance (F/g). i is the instantaneous current (A). M is the effective mass of
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electrode material (g). s is the scan rate (V/s). ∆E is the potential window width (V).
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The EIS measurements were carried out using AC perturbation amplitude of 5 mV around the
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equilibrium potential (0 V). The data were collected in the frequency range from 105 Hz to 0.1 Hz.
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0.5 mol/L KCl solution was used as the electrolyte in the CV and EIS measurements.
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2.6.5. Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy
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The electrodes before and after treating the pure vanadium solution (300 mg/L) were
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analyzed by FTIR to probe the adsorption of vanadium on them. After adsorption, the electrode
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was washed fully by deionized water and then dried in vacuum oven at 65 °C. After drying, the
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subsample for FTIR analysis was obtained by scraping the electrode with a small blade. The FTIR
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spectra were recorded using a Nicolet 6700 spectrometer (Thermo Fisher Scientific Co. Ltd.,
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United States) at room temperature.
3. Results and discussion
3.1. Adsorption characteristics of resins
The adsorption capacity and the separation factor of three resins and AC-m for VO2+, Fe3+
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and Al3+ are listed in Table 2. It can be seen that the resins present distinct different affinity for the
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ions. D001 presents good adsorption capacity for Fe3+and Al3+ but low capacity for VO2+. D840
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almost do not adsorb Al3+ and also shows extremely low adsorption capacity for VO2+ and Fe3+.
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D860 can adsorb VO2+ as the following way [Xiong et al., 2002]: 10
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(4)
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The amino phosphonic p a functionaal group contaaining O and N can exert a concerted effect acid e
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of ioon exchange and coordinnation with V VO2+, so D8660 owns strong affinity for fo VO2+ andd also
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presents the bestt adsorption selectivity s forr VO2+ (i.e. the t maximum m βV/Fe and βV/Al) among three
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resinns.
T Table 2
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VO
5.98 2.99 22.16 4.00
d
Adsoorption capaccity (mg/g) 2+
Fe
Ac ce p
D001 D D D840 D D860 A AC-m
Adsoorption capaccity of resins and AC-m foor different ioons 3+
te
R Resins
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M
for Fe F 3+, indicatinng this materiial owns bad selectivity too VO2+ (Tablee 2).
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h low adsoorption capaciity for VO2+ and Al3+ butt moderate addsorption cappacity The AC-m has
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13.21 1.65 10.31 24.30
Separatiion factor All
3+
21.88 8.118 0.998
βV/Fe
βV/Al
0.42 1.99 3.12 0.15
0.52 4.23 5.09
3.2. Surfacee morphologgy and wettab bility of electtrodes
As shown inn Fig. 2, the Resin/AC-m m composite electrodes e preesent more ro ough surface than
the A AC-m electroode, indicatinng that the ad dditon of ressin powder innto the AC-m m slurry can form
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moree holes or fraactures for thee composite electrodes, e whhich may be caused by thee fact that thee size
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of thhe resins (0.155 mm) is largger comparedd to that of thee AC-m (0.0338 mm).
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×2000
10μm
(c)
×2000
(d)
10μm
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(b)
(a)
×2000
10μm
×2000
Fig. 2. SEM im mages of (a) AC-m A electrodde; (b) D001//AC-m electrrode; (c) D840 0/AC-m electtrode
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and (d) D860/AC-m D e electrode.
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It is found from f Table 3 that the wetttability of thee electrodes also a was chaanged after addding
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the rresins. Becauuse IX resin ggenerally is more m hydrophhilic than AC C-m [Lee et al., 2009], thu us the
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t compositee electrodes and results in n the introoduction of resin can incrrease the hyddrophilia of the
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her hydrophiilia of electrode is benefficial to the ionic decline of contact angles (Taable 3). High e [P Park and Choi, 2010], and d then condduction at thee interface beetween the eleectrode and electrolyte is coonducive to thhe adsorptionn of ions durinng the CDI prrocess. Table 3
Co ontact angles of electrodess
Electrod des
AC-m
D001/AC-m
D84 40/AC-m
D860/AC-m m
Contact ang gles (°)
107.6
97.11
103.4
93.4
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3.3. SBET an nd pore struccture of electtrodes
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It can be obviously o seeen from Tabble 4 that thee SBET of th he AC-m eleectrode (42 m2/g)
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matically decrreases compaared to that off the AC-m ppowder (1027 7 m2/g), indiccating that thee fine dram
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m may be bllocked during g the AC--m powder iss easy to be wrapped andd most poress in the AC-m 12
Page 12 of 25
fabrication process because PVDF was added as the blinder. The SBET of three composite
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electrodes are significantly higher than that of the AC-m electrode (Table 4), suggesting that the
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relatively coarse resins contribute to keep relatively high SBET for the electrodes. The difference
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among the SBET of three composite electrodes may be resulted from the variation in the SBET of
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three resins.
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SBET also can be reflected by pore size distribution to some extent. It can be seen from Fig. 3
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that the SBET of electrodes (Table 4) are approximately related to the content of microspores in the
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electrodes. The composite electrodes own significantly abundant microspores (< 2 nm) than the
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AC-m electrode (Fig. 3), indicating that the introduction of relatively coarse resin powder is also
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beneficial to keep the pore structure for the composite electrodes. Table 4
269 Electrodes
AC-m
2
SBET of four electrodes D001/AC-m
D840/AC-m
D860/AC-m
164
209
228
42
d
SBET (m /g)
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AC-m D001/AC-m D840/AC-m D860/AC-m
0.016
0.012
3
dV/dD Pore volume/cm /g-nm
Ac ce p
0.014
0.010 0.008 0.006 0.004 0.002 0.000
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2
3
4
5
6
7
8
Pore diameter/nm
9
10
Fig. 3. Pore size distribution of materials
272 273
3.4. Electrochemical characteristics
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As shown in Fig. 4 and Table 5, it is clear that the specific capacitance of electrodes is on the
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order of AC-m D860/AC-m D001/AC-m D840/AC-m. The electrode with high specific
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capacitance generally owns large adsorption capacity for ions in CDI process [Zhang et al., 2012].
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IX resin, with styrene-divinylbenzene copolymer matrix, is the insulator compared with AC-m,
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thus the specific capacitance of the electrodes was reduced after the additon of resins because the
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capacitance of material positively related to its electrical conductivity [Yang et al., 2013]. The
280
order of the specific capacitance is in accordance with that of hydrophilia (Table 3) and the
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micropores distribution (Fig. 3) for three composite electrodes, indicating that the abundant
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micropores and strong hydrophilia are beneficial to the the specific capacitance of the electrodes
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[Yoon et al., 2004; Chmiola et al., 2006].
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1.5
M
1.0
0.0
-0.5
te
-1.0
d
Current/A·g
-1
0.5
AC-m D001/AC-m D840/AC-m D860/AC-m
-1.5
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential v.s (Ag/AgCl)/V
288
Ac ce p
284 285
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small potential amplitude. In the EIS Nyquist plot, the impedance at a certain frequency is
290
indicated by one point on the plot. The real impedance characterizes the solution resistance,
291
electrode charge transfer and ionic diffusion resistance, and the imaginary impedance reflects the
292
capacitive characteristics of the electrode.
286
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Fig. 4. Cyclic voltammograms of electrodes Table 5 Specific capacitance of electrodes
Electrodes
AC-m
D001/AC-m
D840/AC-m
D860/AC-m
Specific capacitance (F·g-1)
62.0
37.1
32.1
40.3
In contrast to CV analysis, EIS is deemed to have explicit and dynamic capacitive and
resistive analysis for the electrode [Lufrano and Staiai, 2010.]. Generally, EIS is conducted by
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In Fig. 5, the inset shows the amplified impedance spectroscopy at high frequency region. It
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can be seen that the composite electrodes present the similar semicircle shape, which is larger than
295
that of the AC-m electrode (Fig. 5). The semicircle at high frequency region represents the
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existence of the interface resistance of the electrode [Shaijumon et al., 2008]. The corresponding
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semicircle diameter is related to the contact resistance and the smaller semicircle diameter
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indicates the lower contact resistance for the charge transfer, which contributes to the ionic
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conduction at the interface between the electrode and electrolyte [Ruffo et al., 2009; Portet et al.,
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2004]. It includes the impedance at the interface between the electrode materials, the interface
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between the collector electrode and the electrode materials, and the interface between the
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electrode and solution [Liu et al., 2016]. Therefore, the Nyquist plot indicates that the AC-m
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electrode has the lowest internal resistance and the highest electric conductivity among the
304
electrodes becasuse there is no non-conducting resin added. At intermediate frequency region, it is
305
found that the curves of all electrodes extend with a short Warburg line (slope ~45°) and then early
306
develop as a straight vertical line, suggesting that these electrodes present the nearly ideal
308 309 310
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307
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293
capacitor behavior and allow the ions to be electro-adsorbed more efficiently and easily in the solution [Gao et al., 2013].
Thereby, it can be rational deduced that the electrochemical performance of the composite
electrodes is dependent on the resin and AC-m. Because D860 resin has satisfactory adsorption
311
capacity and selectivity for VO2+, and D860/AC-m electrode also exhibits good hydrophilia and
312
excellent electrochemical properties which is beneficial to ions adsorption in CDI process [Seo et
313
al., 2010] among three composite electrodes, D860/AC-m electrode was chosen for the selective
314
adsorption of VO2+. 15
Page 15 of 25
10
AC -m D001/AC-m D840/AC-m D860/AC-m
2
9
7
5
1
0 2
3
4
5
6
7
8
9
10
Z'/ohm
4
ip t
-Z''/ohm
6
-Z''/ohm
8
3 2
0 2
3
4
5
6
7
8
9
10
Z'/ohm
315
cr
1
Fig. 5.
317
3.5. FTIR spectra
318
Fig. 6 compares the FTIR spectra of D860/AC-m electrode before and after treating the pure
319
vanadium solution. For the electrode before the adsorption, the characteristic adsorption band of
320
the functional groups in D860 resin can be easily distinguished: the absorption band at 3011 cm-1
321
comes from the stretching vibration of N-H bond, and the absorption band at 927 cm-1 is ascribed
322
to the stretching vibration of P-O bond [Yang et al., 2013]. In the spectrum of the electrode after
323
adsorbing vanadium, it can be seen that the characteristic absorption bands of the functional
325 326 327 328
an
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324
Nyquist plot for the impedance response of electrodes
us
316
groups are changed: the absorption band of N-H bond shifts from 3011 cm-1 to 3025 cm-1, and that of the P-O bond moves from 927 to 919 cm-1 (Fig. 6), which are caused by the adsorption of
vanadium on D860 in the composite electrode [Yin et al., 2013; Zhang et al., 2016]. This result proves that vanadium can be absorbed by reaction with D860 in the composite electrode because the electrode was fully wash with water after adsorption.
16
Page 16 of 25
After adsorption
Transmittance/%
3025 (N-H)
919
ip t
(P-O)
927
3500
3000
Before adsorption
2500
329
2000
1500
1000
cr
3011
500
-1
Wave number/cm
us
Fig. 6. FTIR spectra of D860/AC-m electrode before and after adsorption
330
3.6. Adsorption capacity and selectivity
332
The adsorption capacity and selectivity of D860 resin and the electrodes for different ions are
333
compared in Table 6 and Fig. 7. The AC-m used in this study presents low adsorption capacity for
334
VO2+ and Al3+ and somewhat selectivity for Fe3+ (Table 2), but the adsorption capacity of the
335
AC-m electrode for Fe3+ significantly reduces and it shows similar adsorption capacity for VO2+,
336
Fe3+ and Al3+ (Table 6). The SBET of AC-m was largely reduced (Table 4) when it was fabricated as
337
the electrode in CDI, thus the adsorption sites for the ions also greatly reduce and then the
339 340 341
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an
331
adsorption capacity declines. The AC-m electrode presents similar adsorption capacity for VO2+, Fe3+ and Al3+, indicating it has no selectivity for these ions. D860 resin has by far the higher
adsorption capacity for ions and stronger selectivity for VO2+ than the AC-m electrode (Table 6). When they were composited to form the composite electrode, the adsorption capacity of the
342
composite electrode for VO2+ was largely declined (from 52.88 mg/g to 26.63 mg/g in Table 6)
343
due to the addition of the AC-m with low adsorption capacity and the blinder, PVDF, which may
344
impede the reaction between the ion and D860 resin. Although the specific capacitance of the
345
D860/AC-m electrode is lower than that of the AC-m electrode (Table 5), the addition of D860 17
Page 17 of 25
346
resin can enhance the adsorption capacity of the AC-m electrode in CDI (Table 6), indicating that
347
most vanadium may be adsorbed by the resin other than in the electric double layer (EDL) on the
348
AC-m surface which is commonly considered as the reservoir for storing ions (Bian et al., 2015). It is interesting to find that the selectivity of the composite electrode for VO2+ is much higher
350
than that of D860 resin and the pure AC-m electrode (Fig. 7), suggesting that the combination of
351
D860 resin and AC-m can create the preferential adsorption for VO2+ in the CDI process. From the
352
FTIR spectra (Fig. 6) and the adsorption capacity values (Table 6), it can be rational concluded
353
that most vanadium may be adsorbed on the resin. However, if just the resin works in the
354
adsorption, it cannot present such high selectivity for VO2+. It is generally acknowledged that ions
355
are surrounded by hydration shell of water molecule in solution, which leads to the fact that the
356
effective size of the hydrated ions is greater than that of the bare ions [Driesner et al., 1998]. The
357
ions with lower ionic radius and higher charge can by surrounded by more water molecules and
358
have bigger hydration radius in aqueous solution [Tansel et al., 2006]. The hydrated ion forms of
359
VO2+, Fe3+ and Al3+ are VO(H2O)52+, Fe(H2O)63+ and Al(H2O)63+ [Persson, 2010], respectively.
361 362 363
cr
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360
ip t
349
According to the distance of metal ions and oxygen (M-O), and the structural configurations, the radii of VO(H2O)52+, Fe(H2O)63+ and Al(H2O)63+ can be estimated to be 0.330 nm, 0.480 nm and
0.469 nm, respectively. In the CDI process, the smaller VO(H2O)52+ can pass through the pores of
the composite electrode material and react with the resin or be adsorbed in the EDL more easily,
364
which is accordance with the adsorption preferentiality of ions in the CDI process with
365
CNTs-CNFs composite electrode [Gao et al., 2009]. Moreover, the D860/AC-m composite
366
electrode has more abundant micropores than the AC-m electrode (Fig. 3) and the former is more
367
hydrophilic than the latter (Table 3), which is conducive to the ions transfer and can strengthen the 18
Page 18 of 25
ion-sieve effect for the ions in the solution. Thus, the selective adsorption of VO2+ on the
369
D860/AC-m composite electrode can be speculated. Firstly, VO2+, Fe3+ and Al3+ are transferred to
370
the electrode under electrical field. The smaller VO2+ ions can penetrate the micropores of the
371
composite electrode material more easily and react with the resin. Then, D860 resin preferentially
372
adsorbs the penetrated VO2+ due to its relatively high affinity for VO2+, which also enhances the
373
transfer of VO2+ conversely. Consequently, the D860/AC-m composite electrode presents the
374
extremely high selectivity for VO2+ than the pure AC-m electrode and D860 resin under the
375
combined actions of ion-sieve effect of the electrode material and the preferential adsorption of
376
the resin, and the ion-sieve effect may dominantly determine the selective adsorption of VO2+. Adsorption capacity and selectivity of D860 resin and electrodes Adsorption capacity (mg/g)
Materials
VO
2+
3+
22.16 (52.88*) 4.48 (12.11*) 15.67 (26.63*)
te
d
D860 resin AC-m electrode D860/AC-m electrode
Separation factors 3+
Fe
Al
10.31 4.86 1.73
8.18 5.76 2.38
βV/Fe
βV/Al
3.12 1.09 14.80
4.23 0.99 11.56
* The values in parentheses were obtained by treating the pure vanadium solution.
Ac ce p
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Table 6
377
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cr
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368
Adsorption capacity/mg·g-1
25
20
15
10
5
0 2+
VO
60 D8
+
+
m C/A 60 e D8 trod c ele
3 Al
379 Fig. 7.
380
als teri Ma
in
-m de AC tro c ele
ns Io
s re
3 Fe
Adsorption capacity and selectivity of different materials
19
Page 19 of 25
3.7. Multiple adsorption-regeneration cycles
382
Fig. 8 shows the change of adsorption performance of the D860/AC-m electrode for
383
long-term cycle use. It can be found that the adsorption capacity of the composite electrode for
384
VO2+ almost remains stable during the 50 times adsorption-regeneration cycles, and its selectivity
385
(i.e. βV/Fe and βV/Al) declines slightly during the first 10 times, and then also tends to be stable in
386
the following cycles, indicating that the performance of the Resin/AC-m composite electrode can
387
keep stable and it is suitable for long-term operation in CDI process.
us
cr
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381
30
2+
an
Adsorption capacity for VO βV/Fe βV/Al
M
20
10
5
388
392 393
20
Cycles
15
10
5
30
40
50
0 60
Ac ce p
391
10
20
Fig. 8. Performance of composite electrode in multiple adsorption-regeneration cycles
389 390
0
te
0 -10
25
β
15
d
2+
Adsorption capacity for VO (mg/g)
25
30
4. Conclusions
Three kinds of IX resins were used to fabricate the Resin/AC-m composite electrode with
AC-m. As the IX resins are not conductive, the composite electrode has lower specific capacitance and higher internal resistance than the pure AC-m electrode. However, the hydrophilia, SBET and
394
the content of micropores can be largely improved due to the addition of the resins with bigger
395
size and stronger hydrophilia than the AC-m powder, which is beneficial to the ions adsorption in
396
the CDI process. The adsorption capacity and selectivity of the AC-m electrode in CDI can be largely
397
20
Page 20 of 25
improved by adding IX resin to fabricate Resin/AC-m composite electrode. VO2+ has smaller
399
hydrated ionic radius than Fe3+ and Al3+ in the solution, then VO2+ can be easier to pass through
400
the micropores of the D860/AC-m composite electrode and selectively adsorbed by D860 resin
401
with strong affinity for VO2+. Thus, the D860/AC-m electrode has stronger selective adsorption
402
capacity for VO2+ than the pure AC-m electrode and D860 resin, which may be resulted from the
403
combined actions of the ion-sieve effect of the composite electrode material and the affinity of the
404
resin for VO2+.
us
cr
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398
Multiple adsorption-regeneration tests proved that the performance of the D860/AC-m
406
electrode can remain stable in the long-term operation, indicating that the Resin/AC-m electrode is
407
suitable for multiple cycles use in CDI process. The CDI with resin/carbon composite electrode
408
may be a feasible and potential technique for the separation and recovery of ions from complex
409
solutions.
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Acknowledgements
411
This research was supported by National Natural Science Foundation of China (51404177),
412 413 414 415 416 417 418 419 420 421 422 423 424
Ac ce p
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the Fundamental Research Funds for the Central Universities (WUT: 2017II34GX) and National
Key Science-Technology Support Programs of China (2015BAB03B05).
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Research Highlights The resin/carbon composite electrode was fabricated and applied in CDI
Vanadium was selectively adsorbed by the composite electrode in CDI
The mechanism of selective adsorption of vanadium was proposed
The physical and chemical properties of composite electrode was investigated
The performance of resin/carbon composite electrode for long time run was monitored
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S0263-8762(18)30023-6 https://doi.org/doi:10.1016/j.cherd.2018.01.021 CHERD 2995
To appear in: Received date: Revised date: Accepted date:
13-5-2017 22-10-2017 9-1-2018
Please cite this article as: Duan, J., Bao, S., Zhang, Y.,The characteristics of resin/carbon composite electrode and application in selective adsorption of vanadium(IV) by capacitive deionization, Chemical Engineering Research and Design (2018), https://doi.org/10.1016/j.cherd.2018.01.021 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.
The characteristics of resin/carbon composite electrode and
2
application in selective adsorption of vanadium(IV) by capacitive
3
deionization
4
Jihua Duana, Shenxu Baoa, b*, Yimin Zhanga, b, c
5
a. School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, PR
6
China
7
b. Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology,
8
Wuhan 430070, PR China
9
c. Hubei Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Wuhan University
cr
11
* Corresponding author: [email protected] Tel/Fax: +86-27-87212127
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of Science and Technology, Wuhan 430081, PR China
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Page 1 of 25
Abstract: The characteristics and adsorption performances of three kinds of resin/mineral active
31
carbon (Resin/AC-m) composite electrodes, the pure AC-m electrode and the resins were
32
comparatively investigated. The Resin/AC-m composite electrodes have lower specific
33
capacitance and higher resistance than the AC-m electrode, but their hydrophilia, specific surface
34
areas and microspores content can be largely improved due to the addition of the resins, which is
35
beneficial to the ions adsorption on the electrodes. The hydrated radius of VO2+ is smaller than
36
that of Fe3+ and Al3+, then VO2+ is easier to pass through the microspores of the D860/AC-m
37
electrode and adsorbed by the composite electrode. Thus, the D860/AC-m electrode presents the
38
highest adsorption capacity and the strongest selectivity for VO2+, which may be resulted from the
39
combined actions of the ion-sieve effect of the composite electrode material and the affinity of the
40
resin for VO2+. Multiple adsorption-regeneration cycles proved that the performance of the
41
D860/AC-m electrode can remain stable in long-term operation, indicating it may be suitable for
42
cycle use in CDI. This study may provide a promising method for separation and recovery of ions
43
from complex solutions.
44
Key words: Capacitive deionization; Selectivity; Vanadium; Ion exchange resin; Composite
45
electrode
d te
46
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30
52
Ac ce p
47
53
power supply and the salt ions are adsorbed onto the surface of charged electrodes due to
54
electrostatic forces [Nie et al., 2012]. After the electrodes reach adsorption saturation, the
55
adsorbed ions can be desorbed by shortening the electrodes or reversing the potential and the
56
electrodes also are regenerated [Benjamin et al., 2015]. The potential required for desalination
48 49 50 51
1. Introduction
Capacitive deionization (CDI), as an emerging charge-based desalination method, was first
proposed by Murphy and Caudle in the 1960s [Murphy et al., 1965]. During the desalination process, the salinity solution is pumped through the electrode pairs which are connected with DC
2
Page 2 of 25
process is minimal, usually less than 1.23 V to prevent water electrolysis and it also does not
58
produce contaminants during the desalination and regeneration processes [Mossad et al., 2013].
59
Hence, CDI technology is characterized by low energy consumption and environmental
60
friendliness.
ip t
57
To date, CDI is mainly adopted as a promising desalination technique to treat brackish water
62
or seawater and most research in CDI field focuses on the screening and fabrication of electrode
63
materials with outstanding features, such as excellent electrochemical properties, high specific
64
surface area (SBET) and suitable pore structures to improve the desalination performance. Carbon
65
materials, activated carbon (AC) [Jande et al., 2013], carbon aerogel (CA) [Gabelich et al., 2002],
66
carbon nanotubes (CNTs) [Wang et al., 2011], carbon nanofibers (CNFs) [Gao et al., 2009] and
67
graphene [Li et al., 2010] for example, are commonly used as the electrode materials in CDI
68
system due to their excellent electrical conductivity and large SBET. However, some researchers
69
found that CDI may present selective adsorption ability for certain ions by using composite
70
materials or surface-modified materials as electrodes. Liu et al. [Liu et al., 2013] prepared the
72 73 74
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cr
61
CNTs/Ca-selective zeolite composite electrodes using electrophoretic deposition method to selectively adsorb Ca2+ from the solution containing Ca2+, Mg2+ and Na+ in CDI process. Pan et al.
[Pan et al., 2009] fabricated carbon nanotube and nanofiber (CNT-CNF) composite film as CDI electrode material and found it presented better adsorption capacity for smaller hydrated anions.
75
Kim and Choi [Kim et al., 2012.] prepared the nitrate-selective composite carbon electrodes by
76
coating the surface of an AC electrode with a nitrate-selective anion exchange resin and this
77
electrode showed slight adsorption selectivity and greater adsorption capacity for nitrate compared
78
with the counterpart ion exchange membrane capacitive deionization (MCDI). Although these 3
Page 3 of 25
studies indicate the feasibility of selective adsorption by CDI, the current research is by far
80
insufficient for the application of CDI in the selective adsorption of certain metal ions from
81
complex solutions. Moreover, the mechanism of selective adsorption needs intensive research to
82
obtain reasonable explanation.
ip t
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Vanadium is an important rare element, which is dispersedly distributed in the earth’s crust.
84
Acid leaching is an effective method to extract vanadium from the vanadium-bearing resources,
85
such as stone coal [Liang et al., 2016], wasted catalysts [Hu et al., 2006.] and fly ashes [Navarro et
86
al., 2007]. Thus, separation and recovery of vanadium from the complex acid leaching solutions is
87
vital for production of vanadium. Currently, the vanadium-containing solution is commonly
88
treated by ion exchange (IX) and solvent extraction (SX). IX process is easy to operate and control,
89
and it is also regarded as an environmental friendly separation method. However, the mass transfer
90
of ions in the resin is relatively slow and it takes long time for the resin to achieve adsorption
91
equilibrium. For example, Fan et al. [Fan et al., 2013] found that the equilibrium time of the
92
vanadium adsorption on D314 resin is 14h. SX process is characterized by high selectivity and
94 95 96
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93
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high adsorption capacity, but this process is complex and the reagents consumption is high. Besides, the emulsification and loss of extractants also cause potential threat to environment [Liang et al., 2016]. Thus, the CDI using resin/carbon composite electrodes was investigated for selective and efficient separation of V(V) from simulated vanadium-leaching solution in this study
97
to overcome some intrinsic shortcomings of IX and SX. Three kinds of Resin/AC composite
98
electrodes were fabricated by using different IX resins and minerals active carbon (AC-m), and
99
their physicochemical and electrochemical properties and the adsorption characteristics for the
100
ions in the simulated solution were comparatively studied with the pure AC-m electrode in CDI 4
Page 4 of 25
101
processes. Furthermore, the performance of the composite electrodes in the multiple
102
adsorption-regeneration cycles was also investigated. This study may provide a novel promising
103
method for separation and purification of ions from complex solutions.
2. Materials and Methods
105
2.1. Materials
106
Three kinds of cation exchange resins, D001, D840 and D860, which are usually used for
107
water treatment and metals recovery, are selected to prepare the Resin/AC-m composite electrodes.
108
D001 is widely used to adsorb rare and precious metal ions. D840 and D860, as the chelate resins
109
with thiourea and amino phosphonic acid functional groups respectively, have high affinity for
110
certain metal ions [Luo et al., 2011; Zeng et al., 2012]. These resins possessing the same matrix
111
but different functional groups, were provided by Zhejiang Zhengguang Industrial Co. Ltd., China
112
and their properties are listed in Table 1.
te
d
M
an
us
cr
ip t
104
Table 1 Properties of the resins used
113
D840
D860
-SO3H
-CH2-S-C-HNH2
-CH2NHCH2P(O)(OH)2
≥4.35 H+ Poly(styrene-divinyl benzene) Brown, opaque 0.15
≥3.18 H+ Poly(styrene-divin ylbenzene) Lacteous, opaque 0.15
≥3.18 H+ Poly(styrene-divinylbenz ene) Beige, opaque 0.15
114
Ac ce p
D001
115
Provedor de Laboratorios Co. Ltd., Mexico, were selected to prepare electrodes because it owns
116
big SBET and excellent electrochemical properties [Huang et al., 2014]. The high-purity graphite
117
flake with the dimensions of 100 (length) × 50 (width) × 1 (thickness) mm was supplied by
118
Haimen Shuguang Carbon Industry Co. Ltd., China, Polyvinylidene fluoride (PVDF) (A.R.) and
Functional groups Total capacity (mmol/g) Ion form Matrix
Appearance Size (mm)
The AC-m with SBET of 1027 m2/g and mean diameter of 0.038 mm, purchased from
5
Page 5 of 25
119
N-dimethylacetamide (DMAC) (A.R.) came from Sigma Aldrich, United States, and Sinopharm
120
Chemical Reagent Co. Ltd., China, respectively. The acid leaching solution of stone coal commonly contains vanadium(IV), iron(III) and
122
aluminum(III) where vanadium mainly exists as VO2+ in the acid leaching solution [Xiong et al.,
123
2017]. Thus, the simulated vanadium leaching solution containing 300 mg/L VO2+, 300 mg/L Fe3+
124
and 300 mg/L Al3+ was prepared by dissolving vanadyl sulfate (VOSO4·xH2O), iron(III) sulfate
125
hydrate (Fe2(SO4)3·9H2O) and aluminum sulfate hexadecahydrate (Al2(SO4)3·16H2O) in deionized
126
water (Milli-Q® Millipore). The pure vanadium solution with 300 mg/L VO2+ was prepared by
127
only dissolving vanadyl sulfate in deionized water. Vanadyl sulfate, ordered from Alfa Aesar
128
(Tianjin) Chemical Co. Ltd., China, and iron sulfate hydrate and aluminum sulfate
129
hexadecahydrate, which were obtained from Sinopharm Chemical Reagent Co. Ltd., China, are all
130
C.P. grade.
te
d
M
an
us
cr
ip t
121
2.2. Fabrication of electrodes
132
The AC-m slurry was prepared by mixing 4 g AC-m powder and 0.4 g PVDF in 12 mL
133 134 135 136
Ac ce p
131
DMAC for 4 h. Then, the slurry was uniformly casted on a collector electrode, a high-purity graphite flake, and the graphite was put in vacuum oven at 65 ◦C for at least 4 h to make the organic solvent (DMAC) entirely volatilized to produce the pure AC-m electrode. As for the fabrication of Resin/AC-m composite electrodes, the resins were firstly treated by
137
soaking in 5% (m/v) NaOH and 5% (v/v) HCl solutions alternately to remove the remained
138
monomers and other types of impurities which may be produced in the fabrication process,
139
followed by washing with deionized water to neutral, and then were filtered and dried at 60 ◦C in
140
vacuum oven for 12 h before use. Subsequently, the Resin/AC-m composite slurry was prepared 6
Page 6 of 25
by mixing 2 g AC-m powder, 2 g resin and 0.4 g PVDF in 12 mL DMAC for 4 h. Then, the
142
composite slurry was casted on the high-purity graphite flake to fabricate the Resin/AC-m
143
composite according to the preparation steps for the pure AC-m electrode. The composite
144
electrode containing D001, D840 or D860 resin was referred as D001/AC-m, D840/AC-m or
145
D860/AC-m electrode, respectively. The PVDF is used in the fabrication process to bind the slurry
146
with graphite flake and endow the electrodes with appropriate mechanical strength.
us
cr
ip t
141
2.3. Static adsorption of resins
148
The pH of simulated solution and pure vanadium solution were adjusted to 2 by sulphuric
149
acid for static adsorption of resin and CDI process because this pH is suitable for the adsorption of
150
vanadium on the resins used in this study. 0.50 g resin was introduced into a conical flask
151
containing 100 mL solution, then the conical flask was shaken (100 rpm) at 298K for 6 h to attain
152
equilibrium. The adsorption capacity of resin for ions, is calculated by
d
M
an
147
Q=(C0 V0 -Ct Vt )/m
(1)
where Q is the adsorption capacity of resin (mg/g). C0 and Ct are the initial concentration and
159
Ac ce p
154
te
153
160
where C1 and C2 are the concentrations of vanadium on the resin (mg/g) and in the solution
161
after adsorption (mg/L), respectively. C3 and C4 are the concentrations of the counterpart metal,
162
i.e. Fe3+ or Al3+on the resin (mg/g) and in the solution (mg/L) after adsorption, respectively. The
163
experimental conditions for the static adsorption of AC-m were same to those for the resins.
155 156 157 158
final concentration of ions in the solution, respectively (mg/L). V0 and Vt are initial and final
solution volume (L), respectively. m is the mass of resin (g). The separation factor of vanadium(IV) to iron(III) (βV/Fe) or aluminum(III) (βV/Al) is
expressed as
βV/metal=(C1 /C2 )/(C3 /C4 )
(2)
7
Page 7 of 25
2.4. CDI ad dsorption
175
The CDI equuipment is mainly m made up u of a CDI ceell, peristalticc pump and DC D supply (Fiig. 1).
176
C supply cam me from Natonng Peristalticc Pump Manuufacture Co. Ltd., The peristaltic puump and DC
177
ment Co. Ltdd., China, reespectively. The T CDI ceell is Chinna, and Zhaaoxin Electroonic Equipm
178
assembled by three t pairs of o parallel electrodes which w are accommodated in polym methyl
179
methhacrylate. Th he space betw ween two eleectrodes is 3 mm. The feed f solution with pH off 2 is
180
pum mped into the CDI cell from a beaker at a 30 mL/minn through a loower inlet by y peristaltic pump,
181
f into thee beaker thro ough an upperr outlet for 60 min circulaar treatment. Each and the effluent flows
182
pair of parallel electrodes iss supplied wiith 1.5 V byy the DC suppply in the experiments. The
183
electtrosorption caapacity of eleectrode can be b calculated by equation 1 and the sep paration factoors of
184
b analyzed acccording to equation 2. diffeerent electroddes also can be
176 177 178
Ac ce p
te
d
M
an
us
cr
ip t
165
F 1. Fig.
Diaggram of the CDI C equipmennt
2.5. Adsorp ption-regenerration cycle experiments e
182
The perform mance of Ressin/AC-m com mposite electtrode in long-term use waas investigateed by
183
treatting the simuulated leachinng solution foor 50 times addsorption-reg generation cyccles. One cyccle in
184
a off ions and the t regenerattion of electrodes. Afterr the CDII process inccludes the adsorption
185
com mpletion of adsorption, a thhe treated soolution was discharged and a then thee electrodes were 8
Page 8 of 25
shortened and dilute sulphuric acid (8% v/v) was introduced into the CDI cell to strip the ions
183
adsorbed on the electrodes for 10 min. Finally, the electrodes were washed by deionized water to
184
the pH of effluent above 2 to achieve regeneration and the simulated leaching solution was fed for
185
next cycle.
ip t
182
2.6. Measurements
187
2.6.1. Scanning electron microscope (SEM)
188
The surface morphologies of the fabricated electrodes were investigated using a SEM
us
(JSM-IT300, Japan).
an
189
cr
186
2.6.2. SBET and pore structure
191
The debris was carefully scraped from the prepared electrodes using a small blade for the
192
analysis of SBET and pore structure. SBET of the material was determined by a surface and pore size
193
analyzer (ASAP 2020M, Micromeritics Instruments Co. Ltd., United States) using the Brunauer–
194
Emmett–Teller (BET) method. The pore size distribution was obtained from the isotherm
195
adsorption branch based on the Barrett–Joyner–Halenda (BJH) method [Hou et al., 2014].
197 198 199
d
te
Ac ce p
196
M
190
2.6.3. Contact angle
Contact angle of the electrodes was determined using Wilhelmy plate method with the help of
fully automatic surface tension meter K100 (KRUSS Co. Ltd., Germany). 2.6.4. Electrochemical measurement
200
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were
201
conducted by VersaSTAT 4 (AMETEK, United States) working station with a three-electrode cell
202
including a working electrode (the electrode to be analyzed) with dimension of 10 mm × 10 mm, a
203
counter electrode (platinum wire) and a reference electrode (Ag/AgCl) at 25 °C. The specific 9
Page 9 of 25
204
capacitance was determined by conducting CV in the potential range of −0.3 to 0.9 V at a specific
205
scan rate of 20 mV/s by equation 3. C=∫ dE/(Ms(∆E))
206
(3)
where C is specific capacitance (F/g). i is the instantaneous current (A). M is the effective mass of
208
electrode material (g). s is the scan rate (V/s). ∆E is the potential window width (V).
cr
ip t
207
The EIS measurements were carried out using AC perturbation amplitude of 5 mV around the
210
equilibrium potential (0 V). The data were collected in the frequency range from 105 Hz to 0.1 Hz.
211
0.5 mol/L KCl solution was used as the electrolyte in the CV and EIS measurements.
an
us
209
2.6.5. Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy
213
The electrodes before and after treating the pure vanadium solution (300 mg/L) were
214
analyzed by FTIR to probe the adsorption of vanadium on them. After adsorption, the electrode
215
was washed fully by deionized water and then dried in vacuum oven at 65 °C. After drying, the
216
subsample for FTIR analysis was obtained by scraping the electrode with a small blade. The FTIR
217
spectra were recorded using a Nicolet 6700 spectrometer (Thermo Fisher Scientific Co. Ltd.,
219 220 221
d
te
Ac ce p
218
M
212
United States) at room temperature.
3. Results and discussion
3.1. Adsorption characteristics of resins
The adsorption capacity and the separation factor of three resins and AC-m for VO2+, Fe3+
222
and Al3+ are listed in Table 2. It can be seen that the resins present distinct different affinity for the
223
ions. D001 presents good adsorption capacity for Fe3+and Al3+ but low capacity for VO2+. D840
224
almost do not adsorb Al3+ and also shows extremely low adsorption capacity for VO2+ and Fe3+.
225
D860 can adsorb VO2+ as the following way [Xiong et al., 2002]: 10
Page 10 of 25
227 234 235 236
238
(4)
ip t
237
The amino phosphonic p a functionaal group contaaining O and N can exert a concerted effect acid e
237
of ioon exchange and coordinnation with V VO2+, so D8660 owns strong affinity for fo VO2+ andd also
238
presents the bestt adsorption selectivity s forr VO2+ (i.e. the t maximum m βV/Fe and βV/Al) among three
239
resinns.
T Table 2
245
VO
5.98 2.99 22.16 4.00
d
Adsoorption capaccity (mg/g) 2+
Fe
Ac ce p
D001 D D D840 D D860 A AC-m
Adsoorption capaccity of resins and AC-m foor different ioons 3+
te
R Resins
244
M
for Fe F 3+, indicatinng this materiial owns bad selectivity too VO2+ (Tablee 2).
239
240
an
h low adsoorption capaciity for VO2+ and Al3+ butt moderate addsorption cappacity The AC-m has
238 239
us
cr
236
13.21 1.65 10.31 24.30
Separatiion factor All
3+
21.88 8.118 0.998
βV/Fe
βV/Al
0.42 1.99 3.12 0.15
0.52 4.23 5.09
3.2. Surfacee morphologgy and wettab bility of electtrodes
As shown inn Fig. 2, the Resin/AC-m m composite electrodes e preesent more ro ough surface than
the A AC-m electroode, indicatinng that the ad dditon of ressin powder innto the AC-m m slurry can form
246
moree holes or fraactures for thee composite electrodes, e whhich may be caused by thee fact that thee size
247
of thhe resins (0.155 mm) is largger comparedd to that of thee AC-m (0.0338 mm).
11
Page 11 of 25
×2000
10μm
(c)
×2000
(d)
10μm
247
an
us
cr
10μm
246
ip t
(b)
(a)
×2000
10μm
×2000
Fig. 2. SEM im mages of (a) AC-m A electrodde; (b) D001//AC-m electrrode; (c) D840 0/AC-m electtrode
249
and (d) D860/AC-m D e electrode.
M
248
It is found from f Table 3 that the wetttability of thee electrodes also a was chaanged after addding
255
the rresins. Becauuse IX resin ggenerally is more m hydrophhilic than AC C-m [Lee et al., 2009], thu us the
256
t compositee electrodes and results in n the introoduction of resin can incrrease the hyddrophilia of the
258 259 255
te
Ac ce p
257
d
254
her hydrophiilia of electrode is benefficial to the ionic decline of contact angles (Taable 3). High e [P Park and Choi, 2010], and d then condduction at thee interface beetween the eleectrode and electrolyte is coonducive to thhe adsorptionn of ions durinng the CDI prrocess. Table 3
Co ontact angles of electrodess
Electrod des
AC-m
D001/AC-m
D84 40/AC-m
D860/AC-m m
Contact ang gles (°)
107.6
97.11
103.4
93.4
256
3.3. SBET an nd pore struccture of electtrodes
259
It can be obviously o seeen from Tabble 4 that thee SBET of th he AC-m eleectrode (42 m2/g)
260
matically decrreases compaared to that off the AC-m ppowder (1027 7 m2/g), indiccating that thee fine dram
261
m may be bllocked during g the AC--m powder iss easy to be wrapped andd most poress in the AC-m 12
Page 12 of 25
fabrication process because PVDF was added as the blinder. The SBET of three composite
260
electrodes are significantly higher than that of the AC-m electrode (Table 4), suggesting that the
261
relatively coarse resins contribute to keep relatively high SBET for the electrodes. The difference
262
among the SBET of three composite electrodes may be resulted from the variation in the SBET of
263
three resins.
cr
ip t
259
SBET also can be reflected by pore size distribution to some extent. It can be seen from Fig. 3
265
that the SBET of electrodes (Table 4) are approximately related to the content of microspores in the
266
electrodes. The composite electrodes own significantly abundant microspores (< 2 nm) than the
267
AC-m electrode (Fig. 3), indicating that the introduction of relatively coarse resin powder is also
268
beneficial to keep the pore structure for the composite electrodes. Table 4
269 Electrodes
AC-m
2
SBET of four electrodes D001/AC-m
D840/AC-m
D860/AC-m
164
209
228
42
d
SBET (m /g)
M
an
us
264
te
270
AC-m D001/AC-m D840/AC-m D860/AC-m
0.016
0.012
3
dV/dD Pore volume/cm /g-nm
Ac ce p
0.014
0.010 0.008 0.006 0.004 0.002 0.000
271
2
3
4
5
6
7
8
Pore diameter/nm
9
10
Fig. 3. Pore size distribution of materials
272 273
3.4. Electrochemical characteristics
274
As shown in Fig. 4 and Table 5, it is clear that the specific capacitance of electrodes is on the
275
order of AC-m D860/AC-m D001/AC-m D840/AC-m. The electrode with high specific
13
Page 13 of 25
capacitance generally owns large adsorption capacity for ions in CDI process [Zhang et al., 2012].
277
IX resin, with styrene-divinylbenzene copolymer matrix, is the insulator compared with AC-m,
278
thus the specific capacitance of the electrodes was reduced after the additon of resins because the
279
capacitance of material positively related to its electrical conductivity [Yang et al., 2013]. The
280
order of the specific capacitance is in accordance with that of hydrophilia (Table 3) and the
281
micropores distribution (Fig. 3) for three composite electrodes, indicating that the abundant
282
micropores and strong hydrophilia are beneficial to the the specific capacitance of the electrodes
283
[Yoon et al., 2004; Chmiola et al., 2006].
an
us
cr
ip t
276
1.5
M
1.0
0.0
-0.5
te
-1.0
d
Current/A·g
-1
0.5
AC-m D001/AC-m D840/AC-m D860/AC-m
-1.5
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential v.s (Ag/AgCl)/V
288
Ac ce p
284 285
289
small potential amplitude. In the EIS Nyquist plot, the impedance at a certain frequency is
290
indicated by one point on the plot. The real impedance characterizes the solution resistance,
291
electrode charge transfer and ionic diffusion resistance, and the imaginary impedance reflects the
292
capacitive characteristics of the electrode.
286
287
Fig. 4. Cyclic voltammograms of electrodes Table 5 Specific capacitance of electrodes
Electrodes
AC-m
D001/AC-m
D840/AC-m
D860/AC-m
Specific capacitance (F·g-1)
62.0
37.1
32.1
40.3
In contrast to CV analysis, EIS is deemed to have explicit and dynamic capacitive and
resistive analysis for the electrode [Lufrano and Staiai, 2010.]. Generally, EIS is conducted by
14
Page 14 of 25
In Fig. 5, the inset shows the amplified impedance spectroscopy at high frequency region. It
294
can be seen that the composite electrodes present the similar semicircle shape, which is larger than
295
that of the AC-m electrode (Fig. 5). The semicircle at high frequency region represents the
296
existence of the interface resistance of the electrode [Shaijumon et al., 2008]. The corresponding
297
semicircle diameter is related to the contact resistance and the smaller semicircle diameter
298
indicates the lower contact resistance for the charge transfer, which contributes to the ionic
299
conduction at the interface between the electrode and electrolyte [Ruffo et al., 2009; Portet et al.,
300
2004]. It includes the impedance at the interface between the electrode materials, the interface
301
between the collector electrode and the electrode materials, and the interface between the
302
electrode and solution [Liu et al., 2016]. Therefore, the Nyquist plot indicates that the AC-m
303
electrode has the lowest internal resistance and the highest electric conductivity among the
304
electrodes becasuse there is no non-conducting resin added. At intermediate frequency region, it is
305
found that the curves of all electrodes extend with a short Warburg line (slope ~45°) and then early
306
develop as a straight vertical line, suggesting that these electrodes present the nearly ideal
308 309 310
cr
us
an
M
d
te
Ac ce p
307
ip t
293
capacitor behavior and allow the ions to be electro-adsorbed more efficiently and easily in the solution [Gao et al., 2013].
Thereby, it can be rational deduced that the electrochemical performance of the composite
electrodes is dependent on the resin and AC-m. Because D860 resin has satisfactory adsorption
311
capacity and selectivity for VO2+, and D860/AC-m electrode also exhibits good hydrophilia and
312
excellent electrochemical properties which is beneficial to ions adsorption in CDI process [Seo et
313
al., 2010] among three composite electrodes, D860/AC-m electrode was chosen for the selective
314
adsorption of VO2+. 15
Page 15 of 25
10
AC -m D001/AC-m D840/AC-m D860/AC-m
2
9
7
5
1
0 2
3
4
5
6
7
8
9
10
Z'/ohm
4
ip t
-Z''/ohm
6
-Z''/ohm
8
3 2
0 2
3
4
5
6
7
8
9
10
Z'/ohm
315
cr
1
Fig. 5.
317
3.5. FTIR spectra
318
Fig. 6 compares the FTIR spectra of D860/AC-m electrode before and after treating the pure
319
vanadium solution. For the electrode before the adsorption, the characteristic adsorption band of
320
the functional groups in D860 resin can be easily distinguished: the absorption band at 3011 cm-1
321
comes from the stretching vibration of N-H bond, and the absorption band at 927 cm-1 is ascribed
322
to the stretching vibration of P-O bond [Yang et al., 2013]. In the spectrum of the electrode after
323
adsorbing vanadium, it can be seen that the characteristic absorption bands of the functional
325 326 327 328
an
M
d
te
Ac ce p
324
Nyquist plot for the impedance response of electrodes
us
316
groups are changed: the absorption band of N-H bond shifts from 3011 cm-1 to 3025 cm-1, and that of the P-O bond moves from 927 to 919 cm-1 (Fig. 6), which are caused by the adsorption of
vanadium on D860 in the composite electrode [Yin et al., 2013; Zhang et al., 2016]. This result proves that vanadium can be absorbed by reaction with D860 in the composite electrode because the electrode was fully wash with water after adsorption.
16
Page 16 of 25
After adsorption
Transmittance/%
3025 (N-H)
919
ip t
(P-O)
927
3500
3000
Before adsorption
2500
329
2000
1500
1000
cr
3011
500
-1
Wave number/cm
us
Fig. 6. FTIR spectra of D860/AC-m electrode before and after adsorption
330
3.6. Adsorption capacity and selectivity
332
The adsorption capacity and selectivity of D860 resin and the electrodes for different ions are
333
compared in Table 6 and Fig. 7. The AC-m used in this study presents low adsorption capacity for
334
VO2+ and Al3+ and somewhat selectivity for Fe3+ (Table 2), but the adsorption capacity of the
335
AC-m electrode for Fe3+ significantly reduces and it shows similar adsorption capacity for VO2+,
336
Fe3+ and Al3+ (Table 6). The SBET of AC-m was largely reduced (Table 4) when it was fabricated as
337
the electrode in CDI, thus the adsorption sites for the ions also greatly reduce and then the
339 340 341
M
d
te
Ac ce p
338
an
331
adsorption capacity declines. The AC-m electrode presents similar adsorption capacity for VO2+, Fe3+ and Al3+, indicating it has no selectivity for these ions. D860 resin has by far the higher
adsorption capacity for ions and stronger selectivity for VO2+ than the AC-m electrode (Table 6). When they were composited to form the composite electrode, the adsorption capacity of the
342
composite electrode for VO2+ was largely declined (from 52.88 mg/g to 26.63 mg/g in Table 6)
343
due to the addition of the AC-m with low adsorption capacity and the blinder, PVDF, which may
344
impede the reaction between the ion and D860 resin. Although the specific capacitance of the
345
D860/AC-m electrode is lower than that of the AC-m electrode (Table 5), the addition of D860 17
Page 17 of 25
346
resin can enhance the adsorption capacity of the AC-m electrode in CDI (Table 6), indicating that
347
most vanadium may be adsorbed by the resin other than in the electric double layer (EDL) on the
348
AC-m surface which is commonly considered as the reservoir for storing ions (Bian et al., 2015). It is interesting to find that the selectivity of the composite electrode for VO2+ is much higher
350
than that of D860 resin and the pure AC-m electrode (Fig. 7), suggesting that the combination of
351
D860 resin and AC-m can create the preferential adsorption for VO2+ in the CDI process. From the
352
FTIR spectra (Fig. 6) and the adsorption capacity values (Table 6), it can be rational concluded
353
that most vanadium may be adsorbed on the resin. However, if just the resin works in the
354
adsorption, it cannot present such high selectivity for VO2+. It is generally acknowledged that ions
355
are surrounded by hydration shell of water molecule in solution, which leads to the fact that the
356
effective size of the hydrated ions is greater than that of the bare ions [Driesner et al., 1998]. The
357
ions with lower ionic radius and higher charge can by surrounded by more water molecules and
358
have bigger hydration radius in aqueous solution [Tansel et al., 2006]. The hydrated ion forms of
359
VO2+, Fe3+ and Al3+ are VO(H2O)52+, Fe(H2O)63+ and Al(H2O)63+ [Persson, 2010], respectively.
361 362 363
cr
us
an
M
d
te
Ac ce p
360
ip t
349
According to the distance of metal ions and oxygen (M-O), and the structural configurations, the radii of VO(H2O)52+, Fe(H2O)63+ and Al(H2O)63+ can be estimated to be 0.330 nm, 0.480 nm and
0.469 nm, respectively. In the CDI process, the smaller VO(H2O)52+ can pass through the pores of
the composite electrode material and react with the resin or be adsorbed in the EDL more easily,
364
which is accordance with the adsorption preferentiality of ions in the CDI process with
365
CNTs-CNFs composite electrode [Gao et al., 2009]. Moreover, the D860/AC-m composite
366
electrode has more abundant micropores than the AC-m electrode (Fig. 3) and the former is more
367
hydrophilic than the latter (Table 3), which is conducive to the ions transfer and can strengthen the 18
Page 18 of 25
ion-sieve effect for the ions in the solution. Thus, the selective adsorption of VO2+ on the
369
D860/AC-m composite electrode can be speculated. Firstly, VO2+, Fe3+ and Al3+ are transferred to
370
the electrode under electrical field. The smaller VO2+ ions can penetrate the micropores of the
371
composite electrode material more easily and react with the resin. Then, D860 resin preferentially
372
adsorbs the penetrated VO2+ due to its relatively high affinity for VO2+, which also enhances the
373
transfer of VO2+ conversely. Consequently, the D860/AC-m composite electrode presents the
374
extremely high selectivity for VO2+ than the pure AC-m electrode and D860 resin under the
375
combined actions of ion-sieve effect of the electrode material and the preferential adsorption of
376
the resin, and the ion-sieve effect may dominantly determine the selective adsorption of VO2+. Adsorption capacity and selectivity of D860 resin and electrodes Adsorption capacity (mg/g)
Materials
VO
2+
3+
22.16 (52.88*) 4.48 (12.11*) 15.67 (26.63*)
te
d
D860 resin AC-m electrode D860/AC-m electrode
Separation factors 3+
Fe
Al
10.31 4.86 1.73
8.18 5.76 2.38
βV/Fe
βV/Al
3.12 1.09 14.80
4.23 0.99 11.56
* The values in parentheses were obtained by treating the pure vanadium solution.
Ac ce p
378
M
Table 6
377
an
us
cr
ip t
368
Adsorption capacity/mg·g-1
25
20
15
10
5
0 2+
VO
60 D8
+
+
m C/A 60 e D8 trod c ele
3 Al
379 Fig. 7.
380
als teri Ma
in
-m de AC tro c ele
ns Io
s re
3 Fe
Adsorption capacity and selectivity of different materials
19
Page 19 of 25
3.7. Multiple adsorption-regeneration cycles
382
Fig. 8 shows the change of adsorption performance of the D860/AC-m electrode for
383
long-term cycle use. It can be found that the adsorption capacity of the composite electrode for
384
VO2+ almost remains stable during the 50 times adsorption-regeneration cycles, and its selectivity
385
(i.e. βV/Fe and βV/Al) declines slightly during the first 10 times, and then also tends to be stable in
386
the following cycles, indicating that the performance of the Resin/AC-m composite electrode can
387
keep stable and it is suitable for long-term operation in CDI process.
us
cr
ip t
381
30
2+
an
Adsorption capacity for VO βV/Fe βV/Al
M
20
10
5
388
392 393
20
Cycles
15
10
5
30
40
50
0 60
Ac ce p
391
10
20
Fig. 8. Performance of composite electrode in multiple adsorption-regeneration cycles
389 390
0
te
0 -10
25
β
15
d
2+
Adsorption capacity for VO (mg/g)
25
30
4. Conclusions
Three kinds of IX resins were used to fabricate the Resin/AC-m composite electrode with
AC-m. As the IX resins are not conductive, the composite electrode has lower specific capacitance and higher internal resistance than the pure AC-m electrode. However, the hydrophilia, SBET and
394
the content of micropores can be largely improved due to the addition of the resins with bigger
395
size and stronger hydrophilia than the AC-m powder, which is beneficial to the ions adsorption in
396
the CDI process. The adsorption capacity and selectivity of the AC-m electrode in CDI can be largely
397
20
Page 20 of 25
improved by adding IX resin to fabricate Resin/AC-m composite electrode. VO2+ has smaller
399
hydrated ionic radius than Fe3+ and Al3+ in the solution, then VO2+ can be easier to pass through
400
the micropores of the D860/AC-m composite electrode and selectively adsorbed by D860 resin
401
with strong affinity for VO2+. Thus, the D860/AC-m electrode has stronger selective adsorption
402
capacity for VO2+ than the pure AC-m electrode and D860 resin, which may be resulted from the
403
combined actions of the ion-sieve effect of the composite electrode material and the affinity of the
404
resin for VO2+.
us
cr
ip t
398
Multiple adsorption-regeneration tests proved that the performance of the D860/AC-m
406
electrode can remain stable in the long-term operation, indicating that the Resin/AC-m electrode is
407
suitable for multiple cycles use in CDI process. The CDI with resin/carbon composite electrode
408
may be a feasible and potential technique for the separation and recovery of ions from complex
409
solutions.
te
d
M
an
405
Acknowledgements
411
This research was supported by National Natural Science Foundation of China (51404177),
412 413 414 415 416 417 418 419 420 421 422 423 424
Ac ce p
410
the Fundamental Research Funds for the Central Universities (WUT: 2017II34GX) and National
Key Science-Technology Support Programs of China (2015BAB03B05).
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Research Highlights The resin/carbon composite electrode was fabricated and applied in CDI
Vanadium was selectively adsorbed by the composite electrode in CDI
The mechanism of selective adsorption of vanadium was proposed
The physical and chemical properties of composite electrode was investigated
The performance of resin/carbon composite electrode for long time run was monitored
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Page 25 of 25