Environ. Sci. Technol. 2006, 40, 4004-4009
Electrically Controlled Anion Exchange Based on Polypyrrole and Carbon Nanotubes Nanocomposite for Perchlorate Removal Y U E H E L I N , * ,† X I A O L I C U I , †,‡ A N D JAGAN BONTHA† Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, Richland, Washington 99352, and Department of Materials Science, Fudan University, Shanghai, 200433, China
A simple and highly effective process for perchlorate removal based on electrically switched ion exchange (ESIX) was developed by using polypyrrole (PPy) deposited on high surface area carbon nanotubes. The redox switching of conducting polymers such as polypyrrole is accompanied by the exchange of ions into or out of the polymer. This effect could be used for the development of an electrically switchable ion-exchanger for water purification, particularly for the removal of anions. In the research presented in this paper, the anion-exchange behavior and ion-exchange capacity of electrochemically prepared polypyrrole on glassy carbon electrodes with and without carbon nanotube (CNT) backbones are characterized using cyclic voltammetry and X-ray photoelectron spectroscopy. It has been found that the presence of carbon nanotube backbone results in an improvement in the anion exchange stability of polypyrrole, which may be due to the stronger interaction between carbon nanotubes and polypyrrole. Chronoamperometric studies show that the process of electrically switched anion exchange could be finished within 10 s. The selectivity of PPy/CNTs films for the perchlorate ion is demonstrated using cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). The results of the present study point to the possibility of developing a green process for removing ClO4- from wastewater using such a novel nanostructured PPy/CNT composite thin film through an electrically switched anion exchange.
Introduction It is well-known that perchlorate ion poses significant health concern since it can block the uptake of iodine in the thyroid gland and thereby affect the production of thyroid hormones. Recently, the U.S. Environmental Protection Agency (EPA), based on a recommendation by the National Research Council (NRC), has set the safe dose for perchlorate at 0.7 µg per kg of body weight per day (1). Perchlorate anion is a critical component in combat and training munitions. In addition, perchlorate salts are also extensively used in various chemical productions such as those of leather, rubber, fabrics, paints, and aluminum. As * Corresponding author phone: 509-376-0529; fax: 509-376-5106; e-mail:
[email protected]. † Pacific Northwest National Laboratory. ‡ Department of Materials Science, Fudan University. 4004
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a result, perchlorate contamination is now recognized as a widespread concern affecting many water utilities. Thus, the removal of perchlorate and the treatment of perchloratecontaminated groundwater (2-4) and milk, and the determination of perchlorate ion (5, 6), are of special importance. Due to its solubility and nonreactivity, perchlorate is a very stable substance in aquatic systems and is, therefore, difficult to remove. Several approaches, such as ion exchange based on culture (2), selective anion exchange (7, 8), microbial and biological reduction (9, 10), electrochemical and chemical reduction (11), have been evaluated for the treatment of perchlorate-contaminated wastewaters. All of the technologies investigated so far, for the treatment of perchlorate in waters, have technical limitations, generate substantial secondary wastes, and are costly. Therefore, it is necessary to develop an innovative, cost-effective, and green technology for the treatment of perchlorate. In recent years, there has been a considerable interest in conducting polymers because of their extensive potential applications in areas such as energy-storage and separations (12-16), catalysts, and chemical sensing (17, 18). Of all the conducting polymers, polypyrrole (PPy) has a much higher chemical stability and has no toxicity. PPy is a particularly interesting ion exchange material in the development of electrochemically controlled delivery devices and separation systems for charged species. Depending upon the polypyrrole polymerization conditions, both anions and cations can be exchanged with the polymer (19). With PPy films deposited by the oxidation of pyrrole monomer at an electrode surface, the electrochemical reduction and oxidation of the polymer is accompanied by the elution and uptake, respectively, of dopant anions for charge balance in the polymer films. This is because the oxidized conducting polymeric macromolecular chain has positive charges, and the charge must be balanced by the dopant (counterion). When it is reduced, the dopant will be released from the polymer. In this way, PPy could be used as an electrically switched ion exchanger, and it has been successfully used in solid-phase microextraction of inorganic anions (13). Recently, a comparative study of the behavior of anions in PPy film showed that the motilities of these anions have a well-defined order: ClO4< Br- < Cl- < NO3- (20). A voltammetric and energydispersive X-ray study for perchlorate interchange during the redox process of PPy/PVS [poly(vinylsulfonate)] films in an acetonitrile medium was also reported (21). Based on such an electrically switched ion exchange (ESIX) property of conducting polymers such as PPy, a novel technique for removing the target ion seems feasible by using high surface area conducting polymer nanocomposite. ESIX using nickel hexacyanoferrate for removing cesium has been successfully developed at Pacific Northwest National Laboratory (22-25). In ESIX, an electroactive ion exchange layer is deposited onto a conducting substrate, and ion uptake and elution are controlled directly by the modulation potential of the film, resulting in a highly efficient use of electrical energy. Also, the elution solution can be used repeatedly, thereby minimizing secondary wastes at the largest degree and reducing costs greatly. Figure 1 shows the schematic illustration of preparation of PPy and the electrically controlled anion exchange for the separation of the perchlorate ion from wastewaters using PPy film as the electroactive ion exchanger. Anions are doped into the PPy film during the polymerization. The ClO4- ion uptake occurs when the electrochemical oxidation of the electroactive species is performed by applying an anodic potential on the film in the solution containing ClO4-, which forces the ClO410.1021/es052148u CCC: $33.50
2006 American Chemical Society Published on Web 05/18/2006
FIGURE 1. Schematic illustration for the polymerization of PPy and anion intake and elution with the oxidation and reduction of polypyrrole film, the anion exchange process occurs when another suitable anion coexisted in the solution. Three or four pyrrole units involving one positive charge are considered in oxidized PPy. from the waste solution into the film. Elution occurs when the potential is switched to cathode, which forces the ClO4out of the film and into the elution solution. This process should be considered as a green one since the elution is controlled by the applied potential rather than by the concentration of the eluting solution. Therefore, the eluant can be repeatedly used to significantly reduce the quantity of secondary waste generated. Although the natural ion exchange property of PPy has been realized, its capacity is limited since there is only one positive charge per three or four pyrrole units (21). Besides, it is difficult for the dopant anions to diffuse in and out of the polymer due to the poor mass transfer properties of the PPy films. One possible way to improve the mass transfer properties of the PPy deposits, and thereby the ion exchange capacity, is to increase the surface area of electrode by depositing PPy on a porous matrix. Carbon nanotubes (CNTs) are one of the novel nanostructure forms of carbon materials with very high surface area and good conductivity which provide an idea matrix for depositing PPy film. Nanocomposite materials based on carbon nanotubes and conductive polymer offer superior characteristics for developing energy storage and conversion devices and chemical sensors. According to a survey by Cientifica, the world’s leading nanotechnology information company, CNT costs are expected to decrease by a factor 10-100 in the next five years (26). This will make it more practical to use CNT-composite for large scale waste processing. In this paper, nanostructured composite thin films of polypyrrole and CNTs were electrosynthesized, characterized, and evaluated as an electrically switched ion exchanger for removing perchlorate ion from aqueous solution.
Experimental Section Chemicals. Multiwalled carbon nanotubes (CNT, >95% purity, diameter 20∼50 nm, length 1∼5 µm) were purchased from NanoLab, Inc. (Newton, MA). Pyrrole (98%) was obtained from Aldrich. NaCl was purchased from Sigma and ultrapure water (18.3 MΩ cm) was used to prepare the solutions. High purity argon gas was used in the preparation of PPy experiments. Instruments. Polypyrrole electrodeposition and cyclic voltammetric studies of the resultant films were performed
with a CHI 660 electrochemical workstation (CH Instruments Inc, Austin, Texas). All experiments were carried out with a conventional three-electrode system. The working electrode was a bare glassy carbon (GC, 3 mm in diameter, BAS, West Lafayette, IN) electrode or GC electrode modified with CNT film in advance. Coiled platinum wire was used as counter electrode (MW-1033, BAS). The reference electrode was Ag/ AgCl saturated by KCl, and all the potential of the working electrode was measured against this reference. All the electrochemical experiments were carried out at room temperature. The XPS measurements were performed using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe. This system uses a focused monochromatic Al KR X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 16 element multichannel detection system. A 105 W X-ray beam focused to 100 µm diameter was rastered over a 1.4 mm × 0.2 mm rectangle on the sample. The X-ray beam was incident normal to the sample and the X-ray photoelectron detector was at 45° off-normal. Data were collected using a pass energy of 46.95 eV. For the Ag 3d5/2 line, these conditions produced a full width at halfmaximum (fwhm) of 0.98 eV. Although the binding energy (BE) scale was calibrated using the Cu 2p3/2 feature at 932.62 ( 0.05 eV and Au 4f at 83.96 ( 0.05 eV for known standards, the samples can experience variable degrees of charging depending on the conductivity of the films. 1 eV, 20 µA electrons and low energy Ar+ ions were used to minimize this charging, and the BE positions were referenced using the C 1s line at 285 eV. XPS measurements were performed on one sample under three different conditions to demonstrate the electrically switched anion exchange and perchlorate selectivity of the PPY films. The first measurement was made with the asprepared PPy film on a CNT/GC electrode surface in 0.2 M NaCl solution. The second measurement, to show the intake of ClO4- with the presence of Cl-, was made after the sample was held at a controlled potential of 0.4 V for 300 s in a solution containing 0.02 M NaClO4 and 0.2 M NaCl. The third measurement, to illustrate the electrically switched elution of ClO4- out of the PPy film, was made after the sample was held at a cathode potential at -0.8 V for 300 s in a solution of 0.2 M NaCl. VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Typical cyclic voltammograms indicate the sensitivity of ClO4- over Cl- in PPy film on CNT with different concentrations of NaClO4, ClO4-/Cl- 0:1 (a); 1:10 (b); 1:2.5 (c). The scan rate is 10 mV/s. The potential of the working electrode was versus Ag/AgCl saturated by KCl.
FIGURE 2. Consecutive cyclic voltammograms showing the effect of reduction-oxidation cycling in a 0.2 M NaCl solution on the capacity for films on bare glassy carbon electrode (A), and glassy carbon electrode modified with CNT (B). The cycle numbers of 1, 3, 5, 10, 50 are shown. The scan rate is 5mV/s. The potential of the working electrode was versus Ag/AgCl saturated by KCl. Preparation of Working Electrodes. CNT were dispersed in N,N-dimethylformamide (DMF) with the aid of ultrasonic agitation. The black solutions of CNT-DMF (5 mg/mL, 5 µL) were introduced onto the surface of the GC electrode substrate using a dropper, and the solvent was evaporated at room temperature. Before the modification, the GC was polished carefully with a 0.3 and 0.05 µm alumina slurries, washed with water and finally ultrasonically cleaned for 5 min in ultrapure water and dried under a nitrogen stream. Before the electrodeposition, cyclic voltammograms were recorded in K3[Fe(CN)6] at different scan rates in order to estimate the effective electrode area. The electrochemical polymerization of polypyrrole was carried out in a three-electrode cell from an argon-purged aqueous solution containing 0.1 M pyrrole monomer and 0.2 M NaCl. The PPy film was directly prepared on the surface of the working electrode by applying a constant deposition potential of 0.7 V vs Ag/AgCl for a period of 10 min. After electropolymerization, the polymer-coated working electrode was completely rinsed with purified water. Cyclic voltammetric characterizations were performed pyrrole monomerfree electrolyte solution of the same salt concentration as that used for the film formation.
Results and Discussion Cyclic Voltammetric Characterization of PPy. The anion exchange stability of as-prepared PPy films was characterized 4006
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and demonstrated using the cyclic voltammetry method in 0.2 M NaCl solution. All scans were performed at a scan rate of 5 mV/s between the potential of 0.4 V to -0.8 V. Figure 2 show the effect of multiple voltammetric cycles on PPy films deposited on a bare GC (A) and on GC/CNT (B) electrodes. For both cases, well-defined and reversible cyclic voltammetric responses were obtained in NaCl electrolyte solution as indicated by the relatively sharp oxidation peak and a broad and flat reduction peak. Such an asymmetry characteristic of the oxidation and reduction peaks of PPy films has been previously reported (27, 28). In Figure 2A and B it can be seen that the first cycle is different from the others. In cyclic voltammetry, this is known as the first scan effect and has been reported by other researchers (21). In successive cycles, the redox event observed around -0.2 V in Figure 2 has been associated with the oxidation of PPy, which requires uptake of anions into the film; while the reduction forces the release of the anions from the film. The cyclic voltammograms in Figure 2 show that the process of oxidation and reduction are chemically reversible, and that anions loading and unloading can be controlled by modulating the electrode potential. With the consecutive cycling, the capacity for anions loading/unloading decays little by little, which may be caused by structural relaxation in the films. As seen in Figure 2, the oxidative peak gradually shifted negatively for the PPy film both on bare GC surface or on CNTs surface. This phenomenon can be explained by the structural relaxation of PPy film with potential scanning, which makes it easier for the anions to move in and out of the film. The substrates exert great influence to the reductionoxidation cycling stability of the PPy film. The loss of activity occurs quickly on repeated cycling for the films on the bare GC electrode. After 50 cycles, this feature of PPy film was almost lost completely for the bare GC/PPy electrode, while the stability of the PPy films on porous CNT matrix has been improved greatly. After 50 cycles of redox switching, the cyclic voltammetric curve still keeps the feature of PPy film. This fact indicates that the PPy film is very stable when it was prepared on the surface of CNT. The peak current of PPy film decreases little from cycle 50 to cycle 100. The oxidative peak current decreases little after 100 cycles which highlights the stability of electrically switched anion exchange. The improvement of ion exchange capacity and stability of PPy/ CNT film is repeatable with the presence of different anions, and this behavior may result from the 3-D structure of the nanocomposite (29).
FIGURE 5. XPS survey scans for PPy film prepared in 0.2 M NaCl (a), after control the electrode at 0.4 V for 300 s in a solution containing 0.02 M NaClO4 and 0.2 M NaCl (b) and a cathode potential at -0.8 V was applied on the film for 300 s in a solution of 0.2 M NaCl (c).
FIGURE 4. Current-time transients curve recorded during the anodic (A) and cathodic (B) treatment in 0.2 M NaCl solution. The inset shows the beginning period. The Electrically Switched Anion Exchange. It is well established that PPy films can be used to extract perchlorate ion from aqueous solution. By modulating the electrochemical potential of the film, uptake and release of perchlorate ions can be controlled (15). Previous studies of the influence of the properties of small anions on the electrochemical behavior of PPy also showed that the mobility of ClO4- is smaller than that of Cl-. The Cl- ion is spherical and with a crystallographic radium of 0.181 nm, while the shape of ClO4- is tetrahedral and with a size of 0.240 nm (20). Such differences in shape and size will exert an influence on their strength of interaction with polymer chains and also on their mobility in and out of the polymer films. The fraction of the polymer involved in the loading and elution steps depends on the mobility of the doping anions in and out of the polymer. A series of cyclic voltammetric experiments were performed to quantify the preferential ClO4- selectivity over Clfor the films on the two different substrates being tested. Figure 3 shows the typical cyclic voltammograms for a series mixtures solution containing NaCl and NaClO4. With the addition of NaClO4 into the solution, the oxidative peak moved positively to a higher potential which reflects that ClO4- replaced Cl- partly in the PPy films. In this way, ClO4was taken into the PPy films. When a negative potential was applied on the electrode, perchlorate ions were released from the PPy/CNT film. Both of the electrodes tested have similar cyclic voltammetric performance with the addition of NaClO4. The cyclic voltammetric responses in Figure 3 are similar to those reported in the literature (20) for PPy-NO3 film in
contact with the solution of Cl- and ClO4-. The NO3- ion in PPy-NO3 film was replaced by Cl- or ClO4- because the difference of mobility for these anions in PPy (20). The CV peak moves to a higher potential which also indicates the relative selectivity of ClO4- over Cl-. When high anion affinity causes ion flow to be hindered, cyclic voltammogram peaks broaden because of slow charge compensation within the film. The more positive oxidation potential for PPy-ClO4is also in agreement with the previous findings (20). The process of electrically switched anion exchange may proceed with high rate, which can be confirmed from current-time transient curve shown in Figure 4. This figure shows the current-time transients curve recorded during the anodic or cathodic treatment for PPy film on GC surface. The process of electrically switched anion exchange almost finished within 10 s. The cations may enter the PPy film if the PPy film electrode is controlled at cathode potential for too long. From Figure 5 it can be seen the peaks of sodium which indicates that sodium enter the film during the cathodic potential treatment. At the same time, PPy may be over oxidized if the PPy film electrode is controlled at anode potential for a long time (30). X-ray Photoelectron Spectroscopy Evidence. To confirm the feasibility of anion exchange between ClO4- and Cl- and the higher affinity of PPy film to ClO4- than Cl-, X-ray photoelectron spectroscopy (XPS) was used to provide further information on the composition of the PPy film after electrically switched anion exchange. Figure 5 depicts survey scans of PPy film prepared in the solution of 0.2 M NaCl (a) and after different electrochemical treatments (b, c). As shown in Figure 5a, the appearance of peaks of Cl2s and Cl2p indicates that Cl- was doped into PPy film during the polymerization. Figure 5b shows the XPS results of PPy film after an applied anodic potential of 0.4 V for 300 s in a solution containing 0.2 M NaCl and 0.02 M NaClO4. Two peaks of Cl2p can be distinguished, which means that the ClO4- has been taken into the film and the percent of Cl- decreased, although the concentration of ClO4- is 10 times lower than VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for DOE under contract DE-AC05-76RL01830. The authors would like to thank Mr. Mark H. Engelhard for the XPS test.
Literature Cited
FIGURE 6. High-resolution Cl spectrum of PPy film prepared in 0.2 M NaCl (a) after control the electrode at 0.4 V for 300s in a solution containing 0.02 M NaClO4 and 0.2 M NaCl (b) and a cathode potential at -0.8 V was applied on the film for 300 s in a solution of 0.2 M NaCl (c).
TABLE 1. Surface Composition in (in at. %) of PPy at Different State samples
C1s
N1s
O1s
PPy film as-prepared 76.0 15.2 5.5 in 0.2 M NaCl 0.4 V, 300 s, in 0.2 M 72.3 13.5 12.9 NaCl + 0.02 M NaClO4 -0.8 V, 300 s, 70.1 12.6 15.0 in 0.2 M NaCl
Na1s
Cl2p/ Cl2p/ Cl- ClO4-
0.0
3.30
0.00
0.0
1.80
0.70
2.2
0.13
0.00
that of Cl-, reflecting the higher affinity of ClO4-. After the test, the PPy film containing both ClO4- and Cl- was catholically polarized at -0.8 V for 300 s in a solution of 0.2 M NaCl (c). As shown in Figure 5c, the peaks of Cl2p disappeared, which indicates that almost all the anions including ClO4- and Cl- were ejected out of film, which reflects the feasibility of electrically controlled anion exchange. All the changes can be seen much clearer from the high resolution of Cl for the PPy film before and after the anion exchange as shown in Figure 6. Two peaks appeared after applied an anodic potential at 0.4 V for 300 s in a solution containing 0.2 M NaCl and 0.02 M NaClO4. It is easier to see the replacement of ClO4- by Cl-. This fact clearly confirms the relative high affinity of ClO4- in the film of PPy. Table 1 shows the atomic percents of composition of PPy film after different treatment. Comparing of the atomic percents of Cl2p for Cl- and ClO4-, the ratio of ClO4-/Cl- is 0.37 in the PPy film while it is 10 in the solution, which indicating the high affinity of PPy film to ClO4-. This translates to anion exchange selectivity on the order of 27 for ClO4- over Cl-. Implications for Wastewater Treatment. The results from this work have demonstrated the feasibility and potential application of PPy/CNTs nanocomposite for removing perchlorate from aqueous solutions through electrically switched anion exchange. Such a novel and stable hybrid material show promise for the development of a new green route for removing perchlorate from contaminated water through electrically switched ion exchange, while minimizing the production of secondary wastes.
Acknowledgments This work is supported by the U.S. Department of Defense Strategic Environmental Research and Development Program. The research described in this paper was performed 4008
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Received for review October 27, 2005. Revised manuscript received March 17, 2006. Accepted April 12, 2006. ES052148U
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