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Remediation and Control Technologies
Electrically Switched Ion Exchange Based on Carbon-Polypyrrole Composite Smart Materials for the Removal of ReO4- from Aqueous Solutions Zizhang Guo, Mehnaz Shams, Chengzhou Zhu, Qiurong Shi, Yuhao Tian, Mark H. Engelhard, Dan Du, Indranil Chowdhury, and Yuehe Lin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04789 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019
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Electrically Switched Ion Exchange Based on Carbon-Polypyrrole
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Composite Smart Materials for the Removal of ReO4- from Aqueous
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Solutions
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Zizhang Guoa,b, Mehnaz Shamsc, Chengzhou Zhua, Qiurong Shia, Yuhao Tianc, Mark
5
H. Engelhard d, Dan Dua, Indranil Chowdhuryc,Yuehe Lina,*
6
a
7
Pullman, WA 99164-2920, United States.
8
b
9
Water Pollution Control and Resource Reuse, Shandong University, Jinan 250100,
School of Mechanical and Materials Engineering, Washington State University,
School of Environmental Science and Engineering, Shandong Key Laboratory of
10
China
11
c
12
Pullman, Washington 99164, United States
13
d
14
Laboratory, Richland, WA 99352, United States
Department of Civil and Environmental Engineering, Washington State University,
Environmental Molecular Sciences Laboratory, Pacific Northwest National
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Abstract
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A simple and rapid process of ReO4- (as a surrogate of TcO4-) removal from aqueous
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solutions based on the electrically switched ion exchange (ESIX) method has been
19
demonstrated in this work. Activated carbon-Polypyrrole (AC-PPy) was synthesized
20
from activated carbon and pyrrole by electrodeposition method which was served as an
21
electrically switched ion exchanger for ReO4- removal. The characterization results
22
show that the AC-PPy composite exhibited an excellent loading capacity and a high
23
stability for ions uptake and release. Chronoamperometric studies show that the ESIX
24
treatment could be completed within 60 s, demonstrating the rapid uptake and release
25
of ions. Uptake and release of ReO4- was verified by electrochemical quartz crystal
26
microbalance with dissipation shift (EQCMD) studies. By modulating the
27
electrochemical potential of the AC-PPy, the uptake and release of ReO4- ions can be
28
controlled. Similar trends of uptake and release of ReO4- were observed in cyclic
29
voltammetry (-0.4 V to 0.8 V) for 5 cycles with the EQCMD. X-ray photoelectron
30
spectroscopy (XPS) confirmed the process of ReO4- removal in the AC-PPy composite.
31
Conclusively, the smart material shows an excellent efficiency and selectivity for the
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removal ReO4- from aqueous solutions.
33 34
Keywords: electrically switched ion exchange, activated carbon, polypyrrole,
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composites, perrhenate removal.
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Introduction
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Radioactive 99Tc occupy a significant proportion of the radioactive activity of nuclear
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waste, which is a byproduct of the plutonium production process.1 They are present as
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TcO4- in the environment and has a very long half-life (2.13×105 yrs) and mobility.2, 3
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In addition, due to the high water solubility and a migrating ability in the earth's
42
environment, it has frequently caused water pollution. Accordingly, the removal of
43
TcO4- from radioactive-polluted water is a significant challenge. Generally, gravity
44
precipitation,4 solvent extraction5 and ion exchange6 were used to remove TcO4-.
45
Among these methods, the ion exchange method, with superiorities of high recovery
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rate and facile operation, was usually used in TcO4- removal, with perrhenate ReO4-
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anions typically being substituted for TcO4- in laboratory studies.7 Banerjee et al.8, 9
48
used functionalized hierarchically porous frameworks and a zirconium-based metal-
49
organic framework for removing TcO4- from wastewater; however, the ion exchange
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equilibrium, which was reached in 10 to 20 h, is time-consuming and exhibits poor
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efficiency. In this work, to improve the removal efficiency and effectiveness,
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electrochemical techniques were introduced to assist ion exchange processing, and the
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electrically switched ion exchange (ESIX) method was used for the removal of ReO4-
54
from aqueous solutions.
55
The ESIX technique was developed in our laboratory for selectively removal of Cs+
56
and ClO4-.10, 11 The main principles of this technique are that a smart material is used
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as the electrically switched ion exchanger to control the ion uptake and release by the
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modulation potential so that the adsorbed ions from the aqueous solution can be 3
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removed; in the meantime, the material can be reused. Thus, the smart materials with
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switch functions can possess a lower cost and are more environmentally friendly. In
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particular, the technique can reduce treatment costs and minimize secondary waste
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generation with the advantages of selectivity, reversibility and controllability.12
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The electrochemical active material plays a key role in the ESIX technique, which
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usually uses a conducting polymer composite with a high surface area because it is
65
desirable for electrochemical charge and target ion storage.13 The porous composite,
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due to the various sizes of the pores in its 3D structure, can provide the storage of
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different electrolyte ions. The micropores serve as ion sites for energy storage, the
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mesopores provide pathways for the energy transfer, while the macropores act as buffer
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reservoirs in the electrochemical process.14,
70
carbon (AC) as the in the synthesis of the conducting polymer composite. It is well
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established that AC possesses favorable conductivity,16 a particularly well-developed
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pore structure17 and a low cost of preparation.18 Consequently, it is more suitable for
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the synthesis of electrochemical active materials and for practical applications in water
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treatment.
15
In this work, we introduce activated
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The objectives of this work are (1) to study the feasibility of the electrosynthesized
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AC-PPy composite to remove ReO4- by the ESIX method, (2) to investigate the
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physicochemical and electrochemical properties of the AC-PPy composite, (3) and to
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explain the behavior and mechanism of ReO4- removal from aqueous solution using the
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smart material AC-PPy composite by electrochemical quartz crystal microbalance with
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dissipation shift (EQCMD) and X-ray photoelectron spectroscopy (XPS) techniques. 4
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Materials and methods
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Materials preparation and characterization
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The AC derived from biomass waste Phragmites australis (PA) was activated using
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potassium hydroxide at 750°C for 1 h.16 The PPy-functionalized AC was prepared by
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the method of electrolytic deposition, as described in our previous work11, 19 and the
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synthetic composite is referred to as AC-PPy. The morphological characteristics were
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verified using a scanning electron microscope (SEM) (JSM 7600F, Japan Electron
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Optics Lab.). The specific surface area and pore size distribution were calculated by the
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Brunauer-Emmett-Teller (BET) equation and the density functional theory (DFT)
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method according to nitrogen (N2) adsorption/desorption isotherms. The capacity of the
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electroactive materials was evaluated by cyclic voltammetry (CV). Amperometric i-t
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curves recorded the process of the ESIX for ReO4-. In this work, CV and i-t curves were
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measured by a standard three-electrode system similar to that used in PPy electrolytic
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deposition techniques. XPS was used to determine the changes in surface chemical
95
elements under different conditions.
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EQCMD study
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The uptake and release of ReO4- on the surfaces were investigated using an EQCMD
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(Q-sense E4, Biolin Scientific, Inc.) by monitoring the changes in frequency and
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dissipation shift. For an EQCMD, a negative frequency shift indicates uptake, and a
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positive frequency shift indicates release. Here, shifts in frequency and dissipation were
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monitored at the 3rd overtone. CV (-0.4 V to 0.8 V; Scan rate: 10 mV/s) measurement
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with 5 cycles was used on an EQCMD to reach the equilibrium condition. The ion 5
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exchange behavior between the conducting polymer and electrolyte can be verified
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from CV curves, as the charging and discharging is associated with the redox process.20
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To achieve a stable baseline, the sensor surface was rinsed with MilliQ (MQ) water for
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approximately 50 min. An Au electrode was used because it has a 3 times faster uptake
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rate than other electrodes.21, 22 The frequency shift is directly related to mass uptake, the
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change in the frequency shift, the higher the mass uptake, following the Sauerbrey
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equation:23
110
𝛥𝑚 = ―
𝐶𝛥𝑓𝑛
(1)
𝑛
111
where Δm is the deposited mass, Δfn is the shift in the overtone frequency, n is the
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overtone number (1, 3, 5, 7, ...), and C is the crystal constant (17.7 ng/Hz cm2 for the 5
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MHz crystal). Furthermore, mass uptake values can be obtained directly from the
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EQCMD. The values obtained from both the EQCMD and equation 1 show negligible
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difference. Using equation 1, the Sauerbrey equation is applicable to calculate the mass
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of a rigid layer of the deposited material from the frequency shift.
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Kinetics and selectivity study
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A 10 mL 0.01 M NaReO4 solution was added into a 20 mL glass vial. Then, the porous
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AC-PPy electrode was soaked in the solution for a desired time at room temperature.
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Then, the solution was filtered through a 0.22 μm filter. The filtrates were analyzed
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using ICP-MS to determine the concentration.
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The fully oxidized AC-PPy was inserted into the given solutions mainly Cl-, NO3- and SO42- under +0.8 V for selectivity study24: (a) 0.01 M ReO4-
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containing
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+ 2 M Cl- (NaCl); (b) 0.01 M ReO4- + 2 M NO3- (NaNO3); (3) 0.01 M ReO4- + 2 M 6
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SO42- (Na2SO4). The ReO4- relative amount adsorbed (%) was calculated as follows: 25 ReO4- relative amount adsorbed (%) = (𝐶0 ― 𝐶𝑒)/𝐶0 × 100
(2)
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where C0 and Ce are the initial and equilibrium concentration of ReO4- (mg/L).
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Results and Discussion
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Physicochemical and electrochemical properties of AC-PPy
(b)
(a)
6 μm
(d)
750
600
450
300
0.04
BET Surface Area= 2,559.64 m²/g Langmuir Surface Area= 3,099.72 m²/g Pore Volume= 1.11 cm³/g
0.02
150 0.00 0
0
5
10
15
20
Pore width (nm)
0.0
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AC AC-PPy
0.06
Volume (cm3/nm/g)
Volume adsorbed (cm3/g)
(c)
60 μm
0.2
0.4
0.6
0.8
392
1.0
P/P0
396
400
404
408
Binding energy (eV)
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Figure 1. SEM images of (a) AC and (b) the AC-PPy composite. (c) N2
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adsorption/desorption isotherm for the AC-PPy composite (the inset is the pore
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distribution). (d) N 1s spectra for AC and the AC-PPy composite.
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Figure 1a and b show the SEM images of AC and the AC-PPy composite. AC shows a
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morphology with a rough surface and an irregular porous structure, and typical PPy
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structures are presented on the surface of the composite26. The PPy film residing on the
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porous AC backbone to build the 3D porous structure will be highly beneficial for ion
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diffusion.27 The N2 adsorption/desorption isotherms of the AC-PPy composite are used
139
to calculate the BET surface area and are shown in Figure 1c. The AC-PPy composite
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reveals a high-surface-area value of 2559.64 m2/g with a pore volume of 1.11 cm3/g, as
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well as a type I isotherm without a hysteresis loop. The result indicates that the
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microporous structure inundates the AC-PPy composite smart material. 28, 29 In addition,
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the pore-size distribution of the AC-PPy composite is shown in the inset of Figure 1c
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and reveals microspores within the range below 2 nm. It is worth stating that the large
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BET surface area and developed microporous structure provided an abundance of
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storage sites for ReO4- ions.30 The N 1s spectra of AC and the AC-PPy composite were
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analyzed via XPS and are shown in Figure 1d. The AC-PPy composite shows a clear
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peak characteristic of nitrogen, further confirming the formation of PPy after
149
electrodeposition.
(b) GC electrode AC modified GC eletrode
Current (mA)
1.5
1.0
(c)
AC PPy AC-PPy
0.4
AC-PPy
0.4
0.2
Current (mA)
2.0
Current (mA)
(a)
0.0
0.2
0.0
0.5 -0.2
0.0
-0.4 0
150
-0.2
150
300
Times (s)
450
600
-0.4
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential (V)
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential (V)
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Figure 2 (a) Amperometric i-t curves for the electrodeposition of the PPy film on a bare
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glassy carbon (GC) electrode and AC-modified GC electrode. (b) CV curves (10 mV/s)
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of AC, the PPy film and the AC-PPy composite in a 0.2 M NaCl solution. (c) 50
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consecutive CV curves (10 mV/s) showing the effect of reduction-oxidation cycling on
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the capacity of the AC-PPy-composite-modified GC electrode in a 0.2 M NaCl solution.
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Amperometric i-t curves of the PPy film electrochemically deposited on the bare GC
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electrode and AC-modified GC electrode are shown in Figure 2a. Different from the
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bare GC electrode, a distinct current peak is observed at the beginning for the AC-
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modified GC electrode. These two curves show completely different trends because of
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the different nucleation processes early on in the electropolymerization.31 This finding
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suggests that the high surface area and electronic conductivity of the AC-modified GC
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electrode provided a larger number of nucleation sites for PPy deposition. Figure 2b
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shows the CV curves of AC, the PPy film, and the AC-PPy composite in a 0.2 M NaCl
164
solution, which were used to evaluate the electrically switched ion exchange capacity
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of the materials. In the potential range of -0.8 to +0.4 V, the oxidation and reduction
166
peaks in the CVs of AC, the PPy film, and the AC-PPy composite were gradually
167
enhanced, indicating that the capacities of the ions uptake and release were increasingly
168
powerful. The AC-PPy composite shows a higher oxidation peak and broader reduction
169
peak than does AC. The reason for this is the presence of the conductive polymer, which
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can exchange the surface charge at the time of the potential transition between the
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uptake and release of the ions in the aqueous solution. The same phenomenon also
172
occurs in the AC-PPy composite and PPy film; the possible reason is that the porous
173
structure of the AC-PPy composite facilitates ions to easily diffuse into or out of the
174
smart material.32 The effect of multiple voltammetric cycles on the AC-PPy composite
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deposited on the GC electrode is shown in Figure 2c. In the NaCl electrolyte solution,
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for 50 cycles, the AC-PPy composite retains the relatively sharp oxidation peak and
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broad reduction peak, indicating that the AC-PPy composite possessed a good recycling
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and regenerating capacity. The asymmetry characteristic of the peaks of the PPy film
179
was reported previously.33 However, the oxidative peak gradually shifted negatively
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due to the amount of the potential scanning contribution to the structural relaxation of
181
the AC-PPy composite, which makes it easier for the ions to move in and out.19
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Electrochemical uptake and release 9
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0.1
0.0
6
(c)
0.0
-0.5
Anodic treatment
4
Current (mA)
(b)
AC-PPy
Current (mA)
Current (mA)
(a) 0.2
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-1.0
Cathodic treatment -1.5
-2.0
-0.1 0
-2.5 -0.5
0.0
0.5
1.0
1.5
2.0
0
40
80
Potential (V)
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120
160
Time (s)
200
0
50
100
150
200
250
Time (s)
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Figure 3. (a) CV curves (10 mV/s) of the AC-PPy composite in a 0.1 M NaReO4
185
solution and i-t curves (b) during the anodic (1.8 V) and (c) cathodic (-0.4 V) treatments
186
in a 0.1 M NaReO4 solution.
187
The CV experiment was performed to certify the uptake and release of ReO4- by the
188
controlled oxidation and reduction under electrochemical potential and is shown in
189
Figure 3a. The presence of an oxidative peak is evidence that the ReO4- ions were
190
uptaken into the AC-PPy composite; conversely, the ions were released when a negative
191
potential was applied. To maintain the charge balance, the charges during the oxidation
192
and reduction of PPy are compensated with the same amount of ReO4- into and out of
193
PPy. The electroactive ion exchange can efficiently uptake and release ReO4- rapidly,
194
which is confirmed in the current-time transient curves for recording the anodic and
195
cathodic treatments of ReO4- on the AC-PPy-composite-modified electrode (Figure 3b
196
and c). The uptake and release of ReO4- were nearly finished within 60 s. The required
197
time is significantly less than that of the pure ion exchange in references.8, 9
198
EQCMD study
199
The whole process includes electrochemical deposition, adsorption and desorption took
200
place onto a gold-coated sensor through the EQCMD machine. This was followed a
201
background electrolyte solution of 0.2 M NaCl (Figure S1, stage i) until again stabilized.
202
The PPy films were electrochemically deposited on a gold-coated sensor under a +0.7 10
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V potential by flowing 0.2 M pyrrole dissolved in 0.2 M NaCl through the machine. A
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sharp decrease in the frequency shift detected on the Q-Sense software indicated the
205
deposition of PPy on the gold sensor (Figure S1, stage ii). Then, 0.2 M NaCl was flown
206
again through the module to remove loosely attached PPy from the surface (Figure S1,
207
stage iii). Following this, MiliQ (MQ) water was flown to the module as it is the
208
background solution of ReO4- solution (Figure S1, stage iv). ReO4- was deposited on
209
the PPy-coated gold sensor by flowing 0.1 M ReO4- in MQ water through the flow
210
channels by applying cyclic voltammetry (Figure S1, stage v).
211
The changes in the frequency shift while depositing ReO4- are plotted in Figure S2a.
212
It is found that during the flow of ReO4-, the change in the frequency shift is in the order
213
of AC-PPy > PPy > AC. Using the Sauerbrey equation, this frequency shift was
214
converted to mass change during uptake. Compared to only PPy and only AC, the mass
215
change values (Figure 4) also indicate that the highest mass uptake of ReO4- was
216
achieved for the AC-PPy composite. This finding clearly indicates the advantage of
217
introducing AC as a porous backbone structure that with PPy can provide storage places
218
for ions. Similarly, the release of ReO4- (Figure S2b) indicates that the highest changes
219
in frequency were observed for the AC-PPy composite. These results clearly indicate
220
that the active ion exchange sites are from PPy functional groups, not from AC.
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AC PPy AC-PPy
ReO4 deposition
800 700
700 600 500 400 300 200
600 500 400 300 200 100
100
0
0
221
AC PPy AC-PPy
ReO4 release
Areal mass release (ng/cm2)
Areal mass deposition (ng/cm2)
800
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1st Cycle
2nd Cycle
3rd Cycle
4th Cycle
5th Cycle
1st Cycle
2nd Cycle
3rd Cycle
4th Cycle
5th Cycle
222
Figure 4. Change in areal mass during the deposition of ReO4- on material surfaces
223
(negative to positive voltage applied) and release of ReO4- from material surfaces
224
(positive to negative voltage applied) under CV measurement.
225
Figure 4 also shows that for the AC-PPy composite, up to 3rd cycle, the release of
226
ReO4- from the material surfaces was lower than the uptake, which means that the
227
uptake is partially reversible and the recovery ratio is 82% for 1st cycle, 90% for the
228
2nd cycle and nearly 100% for the 3rd cycle. After the 3rd cycle, the uptake and release
229
phenomenon began reaching equilibrium. However, for only PPy, even after the 5th
230
cycle, the recovery ratio is just 92%. This finding implies that AC-PPy provides good
231
storage sites for ReO4- ions, and the 3D porous structure of the AC-PPy film facilitates
232
ions to easily diffuse into or out of the composite film.
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XPS evidence
C 1s O1s
N 1s
Na 1s
a
Re 4f
b
c
0
200
400
600
800
1000
1200
Binding energy (eV)
240 241
Figure 5. XPS survey scans for (a) the AC-PPy composite, (b) after an anodic potential
242
of 1.8 V and the uptake of ReO4- and then (c) a cathodic potential of -0.4 V was applied
243
to the AC-PPy composite.
244
XPS evidence for the uptake and release of ReO4- is shown in Figure 5. As shown in
245
Figure 5a, the appearance of the peaks of N 1s indicates that the PPy film was deposited
246
on AC-modified GC electrode during the electropolymerization. Figure 5b shows the
247
XPS result of the AC-PPy composite after an anodic potential for removing ReO4-; the
248
peak of Re can be observed, indicating that the ReO4- was uptook and reflecting its high
249
affinity. When the potential was transformed to cathodic (Figure 5c), the peak of Re
250
disappeared, which means that the ReO4- ions were ejected out of composite. The result
251
is consistent with electrochemical testing and EQCMD results and reflects the
252
feasibility of ESIX for the removal of ReO4- from water.
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256
Kinetics and selectivity study
257 258
Figure 6. (a) The kinetics of AC-PPy and PPy for ReO4- removal and (b) selectivity of
259
AC-PPy composite.
260 261
From Figure 6a, the kinetics show that the adsorption equilibrium of PPy was reached
262
after 60 s. AC-PPy showed a slightly slower response due to the larger surface area and
263
more adsorptive sites. AC-PPy has the electrical conductivity of PPy and large surface
264
area of AC, resulting in the largest adsorption capacity. The results of Figure 6b show
265
good selectivity and adsorption capacities for ReO4- despite the high concentration of
266
competing ions in solutions.
267 268
Supporting Information Available
269
Additional information mentioned was listed in Supporting Information. This material
270
is available free of charge via the Internet at http://pubs.acs.org. The frequency shift of
271
uptake and release of ReO4- on the AC-PPy by EQCMD.
272 273
Author information 14
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Notes
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The authors declare no competing financial interest
276
* Corresponding author:
277
Email:
[email protected] (Y. Lin)
278
Acknowledgement
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We thank EMSL, a national scientific user facility sponsored by the Department of
280
Energy’s Office of Biological and Environmental Research located at Pacific
281
Northwest National Laboratory, for providing the XPS measurement. Zizhang Guo
282
would like to acknowledge the fellowship from the China Scholarship Council
283
(201606220157). This work was also supported by US Geological Survey grant
284
(2016WA411B) via State of Washington Water Research Center.
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References
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