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Nanomaterials-Enhanced Electrically Switched Ion Exchange Process for Water Treatment
CHAPTER
Nanomaterials-Enhanced Electrically Switched Ion Exchange Process for Water Treatment
17
Yuehe Lin1, Daiwon Choi2, Jun Wang2 and Jagan Bontha2 1 Chemistry Department, King Abdulaziz University, Jeddah, Saudi Arabia; School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA 2 Pacific Northwest National Laboratory, Richland, WA, USA
17.1 Introduction ...................................................................................................271 17.2 Principle of the electrically switched ion exchange technology ........................272 17.3 Nanomaterials-enhanced electrically switched ion exchange for removal of radioactive cesium-137..................................................................................273 17.4 Nanomaterials-enhanced electrically switched ion exchange for removal of chromate and perchlorate...............................................................................276 17.5 Conclusions...................................................................................................279 Acknowledgments ...................................................................................................280 References .............................................................................................................280
17.1 Introduction Electrically switched ion exchange (ESIX) technology combines ion exchange and electrochemistry to provide a selective, reversible method for the removal of target species from wastewater. In this technique, an electroactive ion exchange layer is deposited on a conducting substrate, and ion uptake and elution are controlled directly by modulation of the potential of the layer. ESIX offers the advantages of no secondary waste generation. We have improved upon the ESIX process by modifying the conducting substrate with carbon nanotubes (CNTs) prior to the deposition of the electroactive ion exchanger. The nanomaterial-enhanced electroactive ion exchange technology will remove cesium-137, chromate, and perchlorate rapidly from wastewater. The high porosity and high surface area of the electroactive ion Street, Sustich, Duncan and Savage. Nanotechnology Applications for Clean Water, 2nd Edition. © 2014 Elsevier Inc. All rights reserved. DOI: http://dx.doi.org/10.1016/B978-1-4557-3116-9.00017-2
271
272
CHAPTER 17 Nanomaterials-Enhanced ESIX Process
exchange nanocomposites results in high loading capacity and minimizes interferences for nontarget species. Since the ion adsorption/desorption is controlled electrically without generating a secondary waste, this electrically active ion exchange process is a novel technology that will greatly reduce operating costs.
17.2 Principle of the electrically switched ion exchange technology The ESIX technology, being developed at the Pacific Northwest National Laboratory (PNNL), provides a more economical remediation alternative [15]. It is a novel technique that combines both the principles of electrochemistry and ion exchange for the removal of toxic ions from waste effluents. This technology utilizes the redox reaction of electrically conductive material to regulate the uptake or elution of various ions to/from and into aqueous solution to maintain charge neutrality. By modulating the potential and time of the ESIX material, selective ion exchange of various cations or anions can be achieved. The main advantage of ESIX over conventional systems in wastewater treatment applications is in selectivity and reversibility for ion separation that lowers costs and minimizes secondary waste generation typically associated with conventional regenerated ion exchange processes. For example, cation separation is done by applying a cathodic potential to the film sufficient to induce electrochemical reduction of an electroactive species, X, which forces a cation from the waste solution into the film following Equation 17.1. Cation unloading is done by applying an anodic potential to the electrodes; reoxidation of the electroactive film instigates cation out of the film and into the external elution solution following reversed direction of Equation 17.1 [14]. A similar mechanism applies to anion removal following Equation 17.2. Thus, the use of regenerant chemicals and the multiple steps following regeneration, each resulting in a huge amount of secondary waste that needs to be disposed of, is eliminated. Electrically switched ion exchange enables the reuse of the ion exchange medium up to fifteen hundred cycles as opposed to conventional ion exchange where inorganic ion exchangers are more often used only once and discarded. v
e2 1 X 1 M1 2 jX2 M1 j 2 v
X 1 M 2 jX1 M2 j 1 e2
(17.1) (17.2)
As in conventional ion exchange, selectivity of X for the metal ion of interest is important whenever the waste solution contains more than one cation. If X has a greater selectivity for the cation of interest M11 than a second cation M21, the film may first be “activated” by reduction in the presence of a solution of M21. Introducing the waste solution results in uptake of M11 by ion exchange for M21 where M21 is displaced into the waste solution Equation 17.3. v
1 1 M21 X2 1 M1 1 2 jM1 X 2 j1M3
(17.3)
17.3 ESIX for removal of radioactive cesium-137
Competition for binding sites will occur, and loading will be driven by thermodynamics. Therefore, to successfully remove the target cation, films must bind that ion preferentially, which depends on the electroactive property of X.
17.3 Nanomaterials-enhanced electrically switched ion exchange for removal of radioactive cesium-137 Significant amounts of cesium-137, an important fission by-product, are present in radioactive liquid waste generated during the reprocessing of nuclear fuel by the Purex process. Removing the long-lived (T1/2 5 32 years) radioisotope of cesium from the radioactive waste is a challenging project and several techniques, such as ion exchange, sorption, solvent extraction, and precipitation processes, have been investigated. One such process is ESIX using electroactive nickel hexacyanoferrates (NiHCF) developed at PNNL. Transition metal hexacyanoferrates (MHCFs) are an important class of inorganic polynuclear mixed-valence compounds because of their interesting properties including electrocatalysis, electrochromicity, ion exchange selectivity, sensing, and magnetism. Therefore, preparation and application of Prussian blue (PB) and its analogous forms such as samarium hexacyanoferrate, NiHCF, and cobalt hexacyanoferrate have been extensively studied. Also, thin films of PB and related analogous complex salts are also promising candidates for separating membranes capable of ion sieving and electrochemical-controllable ion exchange because of their zeolitic structure. Hexacyanoferrates are well-known ion exchangers with high selectivity over cesium in concentrated sodium solutions but hexacyanoferrates used in conventional packed column ion exchange are difficult to elute [15]. Therefore, columns are normally discarded once used. However, when hexacyanoferrates are applied as electroactive films, depending on the film deposition formulation and substrate, highly reversible uptake and elution of cesium can be achieved from fifteen hundred to over four thousand cycles [2,4]. When a cathodic potential is applied to the NiHCF film, Fe31 (ferricyanide) is reduced to the Fe21 state (ferrocyanide); thus, a cation must be intercalated into the film to maintain its charge neutrality. In practice, the reduction step is usually conducted in a sodium solution as shown in Equation 17.4. The film is then exposed to the waste solution containing cesium, which loads into the film via ion exchange for sodium following Equation 17.5. When an anodic potential is applied, a cesium cation is released from the film as shown in Equation 17.6. Therefore, NiHCF has been coated on the electrode as shown in Fig. 17.1 for ESIX of Cs [2]. CsNiFeIII ðCNÞ6 1 Na1 1 Na1 1 e2 -CsNaNiFeII ðCNÞ6
(17.4)
CsNaNiFeII ðCNÞ6 1 Cs1 -Cs2 NiFeII ðCNÞ6 1 Na1
(17.5)
Cs2 NiFeII ðCNÞ6 -CsNiFeIII ðCNÞ6 1 Cs1 e2
(17.6)
273
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CHAPTER 17 Nanomaterials-Enhanced ESIX Process
Load cycle
+ Cs+
CsNiFe(III)(CN)6
Cs2 NiFe(II)(CN)6
–
+e
Conductive substrate
Unload cycle
Cs2NiFe(II)(CN)6
– Cs+ CsNiFe(III)(CN)6 – e–
Conductive substrate
FIGURE 17.1 Electrically switched ion exchange (ESIX) mechanism of nickel hexacyanoferrate thin film electrode for Cs1 loading and unloading.
(A)
(B)
Clean water
(C)
Wash Contaminated water
ESIX
Concentrated eluant
Waste form
FIGURE 17.2 Schematic illustration of (A) a bench-scale electrically switched ion exchange (ESIX) system based on (B) nickel hexacyanoferrate coated nickel foam electrodes, and (C) fiveelectrode cell stacks used as the ESIX system.
Figure 17.2 shows bench-scale flow cell testing of ESIX systems developed at PNNL using NiHCF films deposited on high surface area nickel foam electrodes with a nominal surface area per volume of 40 cm2/cm3 (60 ppi) [4]. The flow tests using NiHCF material were performed to evaluate the feasibility of the approach as an alternative to conventional ion exchange processes.
17.3 ESIX for removal of radioactive cesium-137
Loading NaNiFe(III)(CN)6
CsNaNiFe(II)(CN)6 Unloading
+ Cs+ + e–
Loading
Unloading
– Cs+
– e–
FIGURE 17.3 Schematic illustration for electrically switched ion exchange based on carbon nanotubes, polyanline, and nickel hexacyanoferrate nanocomposite. Data from [5] with permission.
Bench-scale ESIX flow system studies showed no change in capacity or performance of the ESIX films at a flow rate up to 113 bed volumes/h (BV/h). A comparison of results for a stacked five-electrode cell versus a single-electrode cell showed enhanced breakthrough performance. In the stacked configuration, breakthrough began at approximately 120 BV for a feed containing 0.2 ppm cesium at a flow rate of 13 BV/h. A case study for the KE Basin (a spent nuclear fuel storage basin) on the Hanford Site demonstrated that wastewater could be processed continuously with minimal waste generation, reduced disposal costs, and lower capital expenditures. Novel nanocomposite materials comprised of high surface area of CNTs, chemically stable polyaniline (PANI), and ion exchange properties of NiHCF have been synthesized through electrodeposition [5,6]. Figure 17.3 illustrates the concept of electrically switched ion exchange based on CNT/PANI/NiHCF nanocomposites. High surface area of the porous CNT film leads to the high loading capacity for the NiHCF nanoparticles, which in turn leads to the high ion exchange capacity of the NiHCF/CNT nanocomposite film. The presence of the PANI polymer film further improves the stability of the nanocomposite film, which undergoes more than five hundred cycles retaining 92 percent of its initial capacity. Recently, NiHCF nanotubes have been fabricated by an electrokinetic method based on the distinct surface properties of porous anodic alumina [7]. By this method, nanotubes formed rapidly with the morphologies replicating the nanopores in the template. The electrochemical measurements show that the NiHCF nanotubes exist only in the form of K2Ni[Fe(CN)6] with excellently stable cesium-selective ion exchange ability due to single composition and unique nanostructure. NiHCF nanotubes modified electrodes retains 95.3 percent of its initial value after five hundred potential cycles. Even after fifteen hundred and three thousand cycles, the NiHCF nanotubes still retain 92.2 percent and 82.9 percent, respectively, of their ion exchange capacity.
275
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17.4 Nanomaterials-enhanced electrically switched ion exchange for removal of chromate and perchlorate Wide use of heavy metals and its related compounds by industries has resulted in the pollution of the environment. These inorganic toxins are of considerable concern since they are nonbiodegradable, highly toxic with possible carcinogenic effect. Among the heavy metals, chromium is one of the most widely used metals in many of the industrial processes such as tanning, electroplating, making printed circuit boards, painting, and steel fabrication. Most of the chromium is discharged into aqueous waste as Cr(III) and Cr(VI). Cr(VI), which is the more toxic of the two, is present as dichromate (Cr2O722), chromate (CrO422), or hydrogen chromate (HCrO42). The current maximum contamination level (MCL) for total chromium in drinking water in the United States is regulated by the United States Environmental Protection Agency (EPA) at 100 μg/L. Perchlorate ion also poses significant health concern since it can block the uptake of iodine in the thyroid gland and thereby affect the production of thyroid hormones. Perchlorate anion is a critical component in combat and training munitions. In addition, perchlorate salts are extensively used as chemical reagents in the production of leather, rubber, fabrics, paints, and aluminum. As a result, perchlorate contamination is now recognized as a widespread concern affecting many water utilities. Recently, EPA, based on a recommendation by the National Research Council (NRC), set the safe dose for perchlorate at 0.70 μg/kg of body weight per day. However, due to its solubility and nonreactivity, perchlorate is a very stable substance in aquatic systems and is, therefore, difficult to remove. Conventional chromium and perchlorate removal methods include adsorption, chemical precipitation, biological degradation, ion exchange, and electrochemical method. However, these technologies have technical limitations, generate substantial secondary wastes, and are costly. Therefore, innovative, cost-effective, and green technology for the treatment of perchlorate needs to be developed. Electrochemical ion exchange treatment is a promising technique that offers several advantages over other techniques for remediation of metals from contaminated effluents or wastewaters. For chromate removal, arc-assisted carbon (AC), activated carbon (RC), and carbon aerogel have been used for ion exchange electrode [8,9]. Hexavalent chromium, [Cr(VI)] may exist in the aqueous phase in different anionic forms, such as Cr2O722, CrO422, or HCrO42, with total chromate concentrations and pH dictating which particular chromate species is predominant. Equation 17.7 shows that equilibrium relationship between the different chromium anions: 2
2
2 2CrO24 1 2H2ð2HCrO2 4 Þ2Cr2 O7 1 H2 O
(17.7)
Carbon obtained by the arc-assisted evaporation of graphite rods (AC) exhibited a high selectivity toward anions of hexavalent chromium from aqueous solutions in the pH range of 39 [8]. There is almost no removal of trivalent
17.4 ESIX for removal of chromate and perchlorate
chromium or other metal cations such as Pb1 and Zn1 at low pH values (i.e., ,9) since trivalent chromium is present as a cation in solution whereas hexavalent chromium exists as an anion. This is consistent with the hypothesis that the surface of AC carries positive charges that facilitate the removal of the anion of the hexavalent chromium but prevent the removal of metal cations. At high pH, the positive surface charges are neutralized by the presence of OH1 ; hence, the metal cation removal efficiency is increased by an ion exchange adsorption mechanism. Commercial carbon (RC), on the other hand, has a negatively charged surface due to the presence of oxygen functional groups of “oxo” (CxOy) type produced during its the activation at a very high temperature. Therefore, metal cation removal is facilitated. An increase in the pH of the solution contributes to an increase of the surface negative charge density, resulting in an even greater removal efficiency of lead, zinc, and trivalent chromium metal cations. Recently, carbon aerogel electrodes were applied for chromium removal from wastewater since carbon aerogel possess unique thermal, mechanical, and electrical properties, which are directly related to their unusual nanostructure composed of interconnected particles with microscopic interstitial pores [9]. Carbon aerogel is an ideal electrode material because of its low electrical resistivity (#40 mΩcm), high specific surface area (4001100 m2/g), and controllable pore size distribution (50 nm). The effect of pH (27), concentration 28 mg/l, and charge 0.31.3 A h was investigated where the chromium ion removal was significantly increased at reduced pH and high charge conditions. The metal concentration in the wastewater can be reduced by 98.5 percent under high charge (0.8A h) and acidic conditions (pH 5 2). Also, the elution solution can be used repeatedly, thereby minimizing secondary wastes at the largest degree and reducing costs greatly. For perchlorate removal, electrically conductive polypyrrole (PPy) polymer as an electroactive ion exchange layer has been deposited onto a conducting substrate [10,11]. Figure 17.4 shows the schematic illustration of PPy preparation and the electrically controlled anion exchange for the separation of the perchlorate ion
− +
A–
Polymerization +
e–
A– e–
–
−
+ +
−
−
+
−
+
+ −
− +
A– Reduction Oxidation
FIGURE 17.4 Schematic illustration for the polymerization of electrically conductive polypyrrole (PPy) and anion intake and elution with the oxidation and reduction of PPy film. Three or four pyrrole units involving one positive charge are considered in oxidized PPy.
e–
277
CHAPTER 17 Nanomaterials-Enhanced ESIX Process
(A)
(B)
Current/1e-4A
278
1.8 1.6 (i) 1.4 PPy on CNT 1.2 1.0 0.8 0.6 0.4 (ii) 0.2 0 PPy Film on GC -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 Potential/V
FIGURE 17.5 (A) Scanning electron microscope image of polypyrrole (PPy) film on carbon nanotube (CNT)/ glassy carbon (GC) electrode surface prepared by controlling the potential at 0.7 V, 100 seconds, and (B) a comparison of the tenth cycle behavior of (i) CNTPPy and (ii) GC/PPy. Data from [11] with permission.
from wastewaters using PPy. The pyrrole monomer polymerizes into PPy during oxidation in supporting electrolyte with anions. After electrode has been coated with PPy, anions can be reversibly ion exchanged by applying anodic and cathodic potentials. Therefore, the ClO42 ion uptake occurs during electrochemical oxidation of the electroactive species by applying an anodic potential on the film in the solution containing ClO42, which forces the ClO42 from the waste solution into the film. Elution occurs when the potential is switched to cathode, forcing the ClO42 out of the film into the elution solution. Therefore, the eluant can be repeatedly used to significantly reduce the quantity of secondary waste generated. Although the ion exchange property of PPy has been realized, its capacity is limited by only one positive charge per three or four pyrrole units. 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. At PNNL, in order to improve the mass transfer properties of the PPy, and thereby the ion exchange capacity, PPy was deposited on to high surface area CNT substrates. Carbon nanotubes are one of the novel nanostructure forms of carbon with very high surface area and good conductivity that offers an idea matrix for depositing PPy film. According to a survey by Cientifica, the world’s leading nanotechnology information company, CNT costs are expected to decrease by a factor 10100 in the next 5 years making CNTcomposite a more practical solution for large-scale waste processing. Figure 17.5 shows the microstructure and cyclic voltammogram of CNT/PPy nanocomposite electrode. Also shown in Fig. 17.5B is cyclic voltammogram of PPy deposited on a glassy carbon (GC) electrode. It can be seen that significantly higher electrochemical current density was delivered by creating higher active area using CNT substrate. The CNT/PPy electrode showed higher perchlorate ion
17.5 Conclusions
4500 2p3/2
4000 3500
2p1/2
c/s
3000
a Cl−
2500 2000
2p3/2 2p1/2
1500 1000 500 0
b
−
ClO4 c
215
210
205
200
195
190
Binding energy (eV)
FIGURE 17.6 High-resolution X-ray photoelectron spectroscopy Cl spectrum of polypyrrole (PPy) film prepared (a) in 0.2 M NaCl, (b) after control of the electrode at 0.4 V for 300 seconds in a solution containing 0.02 M NaClO4 and 0.2 M NaCl, and (c) after a cathode potential at 20.8 V was applied on the film for 300 seconds in a solution of 0.2 M NaCl. Data from [10] with permission.
selectivity over chlorine ion and X-ray photoelectron spectroscopy analysis (Fig. 17.6) indicated reversible elution of absorbed ions, making PPy a promising candidate material for perchlorate removal [10,11]. When ion-exchange capacity and reversibility of pure PPy and CNT/PPy nanocomposite thin films were compared using electrochemical quartz microbalance (EQCM) in NaCl, NaClO4, NaNO3, Na2SO4, and NaHCO3 electrolyte solution, the results indicated that the ion exchange kinetics of PPy improved by more than ten times when CNT substrate was used. In addition, the ion exchange reached saturation within 100 seconds for CNT/PPy. Finally, the anion loading capacity (mole percent) of CNT/ PPy was in order of Cl2 . ClO42 . NO32 . SO422 . CO322 whereas selectivity of anions was in order of ClO42 . NO32 . SO422 . Cl2 . CO322.
17.5 Conclusions Electrically switched ion exchange technology has recently been applied for removing cesium, chromate, and perchlorate ions from wastewater. The efficiency of these ESIX systems can be significantly improved through nanotechnology by providing better electrochemical stability and a larger contact area with wastewater. Therefore, the combination of novel electroactive ion exchange material with
279
280
CHAPTER 17 Nanomaterials-Enhanced ESIX Process
nanotechnology will lead to the development of more efficient and economic wastewater treatment systems based on ESIX technology.
Acknowledgments This work is supported by the US Department of Defense Strategic Environmental Research and Development Program (Project No ER-1433). The research described in this chapter was performed at the Environmental Molecular Science Laboratory, a national scientific 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-AC05-76L01830. The authors would like to acknowledge Dr. Mike A. Lilga for helpful discussion.
References [1] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.M. Haight, D.T. Schwartz, Metal ion separations using electrically switched ion exchange, Sep. Purif. Technol. 11 (1997) 147158. [2] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.D. Rassat, J.D. Genders, R. Gopal, Cesium separation using electrically switched ion exchange, Sep. Purif. Technol. 24 (2001) 451466. [3] S.D. Rassat, J.H. Sukamto, R.J. Orth, M.A. Lilga, R.T. Hallen, Development of an electrically switched ion exchange process for selective ion separations, Sep. Purif. Technol. 15 (1999) 207222. [4] M.A. Lilga, R.J. Orth, and J.P.H. Sukamto. Electrically switched cesium ion exchange FY98 Final Report, PNNL-12002, Pacific Northwest National Laboratory. 1998. [5] Y. Lin, X. Cui, Novel hybrid materials with high stability for electrically switched ion exchange: carbon nanotubes/polyaniline/nickel hexacyanoferrate nanocomposites, Chem. Commun. 17 (2005) 22262228. [6] Y. Lin, X. Cui, Electrosynthesis, characterization, and application of novel hybrid materials based on carbon nanotube-polyaniline-nickel hexacyanoferrate nanocomposites, J. Mater. Chem. 16 (2006) 585592. [7] W. Chen, X.H. Xia, Highly stable nickel hexacyanoferrate nanotubes for electrically switched ion exchange, Adv. Funct. Mater. 17 (2007) 29432948. [8] S.B. Lalvani, T. Wiltowski, A. Hubner, A. Weston, N. Mandich, Removal of hexavalent chromium and metal cations by a selective and novel carbon adsorbent, Carbon 36 (78) (1998) 12191226. [9] P. Rana, N. Mohan, C. Rajagopal, Electrochemical removal of chromium from wastewater by using carbon aerogel electrodes, Water Res. 38 (2004) 28112820. [10] Y. Lin, X. Cui, J. Bontha, Electrically controlled anion exchange based on polypyrrole and carbon nanotubes nanocomposite for perchlorate removal, Environ. Sci. Technol. 40 (2006) 40044009. [11] X. Cui, M.H. Engelhard, Y. Lin, Preparation, characterization and anion exchange properties of polypyrrole/carbon nanotube nanocomposites, J. Nanosci. Nanotechnol. 6 (2006) 547553.
Nanomaterials-Enhanced Electrically Switched Ion Exchange Process for Water Treatment
17
Yuehe Lin1, Daiwon Choi2, Jun Wang2 and Jagan Bontha2 1 Chemistry Department, King Abdulaziz University, Jeddah, Saudi Arabia; School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, USA 2 Pacific Northwest National Laboratory, Richland, WA, USA
17.1 Introduction ...................................................................................................271 17.2 Principle of the electrically switched ion exchange technology ........................272 17.3 Nanomaterials-enhanced electrically switched ion exchange for removal of radioactive cesium-137..................................................................................273 17.4 Nanomaterials-enhanced electrically switched ion exchange for removal of chromate and perchlorate...............................................................................276 17.5 Conclusions...................................................................................................279 Acknowledgments ...................................................................................................280 References .............................................................................................................280
17.1 Introduction Electrically switched ion exchange (ESIX) technology combines ion exchange and electrochemistry to provide a selective, reversible method for the removal of target species from wastewater. In this technique, an electroactive ion exchange layer is deposited on a conducting substrate, and ion uptake and elution are controlled directly by modulation of the potential of the layer. ESIX offers the advantages of no secondary waste generation. We have improved upon the ESIX process by modifying the conducting substrate with carbon nanotubes (CNTs) prior to the deposition of the electroactive ion exchanger. The nanomaterial-enhanced electroactive ion exchange technology will remove cesium-137, chromate, and perchlorate rapidly from wastewater. The high porosity and high surface area of the electroactive ion Street, Sustich, Duncan and Savage. Nanotechnology Applications for Clean Water, 2nd Edition. © 2014 Elsevier Inc. All rights reserved. DOI: http://dx.doi.org/10.1016/B978-1-4557-3116-9.00017-2
271
272
CHAPTER 17 Nanomaterials-Enhanced ESIX Process
exchange nanocomposites results in high loading capacity and minimizes interferences for nontarget species. Since the ion adsorption/desorption is controlled electrically without generating a secondary waste, this electrically active ion exchange process is a novel technology that will greatly reduce operating costs.
17.2 Principle of the electrically switched ion exchange technology The ESIX technology, being developed at the Pacific Northwest National Laboratory (PNNL), provides a more economical remediation alternative [15]. It is a novel technique that combines both the principles of electrochemistry and ion exchange for the removal of toxic ions from waste effluents. This technology utilizes the redox reaction of electrically conductive material to regulate the uptake or elution of various ions to/from and into aqueous solution to maintain charge neutrality. By modulating the potential and time of the ESIX material, selective ion exchange of various cations or anions can be achieved. The main advantage of ESIX over conventional systems in wastewater treatment applications is in selectivity and reversibility for ion separation that lowers costs and minimizes secondary waste generation typically associated with conventional regenerated ion exchange processes. For example, cation separation is done by applying a cathodic potential to the film sufficient to induce electrochemical reduction of an electroactive species, X, which forces a cation from the waste solution into the film following Equation 17.1. Cation unloading is done by applying an anodic potential to the electrodes; reoxidation of the electroactive film instigates cation out of the film and into the external elution solution following reversed direction of Equation 17.1 [14]. A similar mechanism applies to anion removal following Equation 17.2. Thus, the use of regenerant chemicals and the multiple steps following regeneration, each resulting in a huge amount of secondary waste that needs to be disposed of, is eliminated. Electrically switched ion exchange enables the reuse of the ion exchange medium up to fifteen hundred cycles as opposed to conventional ion exchange where inorganic ion exchangers are more often used only once and discarded. v
e2 1 X 1 M1 2 jX2 M1 j 2 v
X 1 M 2 jX1 M2 j 1 e2
(17.1) (17.2)
As in conventional ion exchange, selectivity of X for the metal ion of interest is important whenever the waste solution contains more than one cation. If X has a greater selectivity for the cation of interest M11 than a second cation M21, the film may first be “activated” by reduction in the presence of a solution of M21. Introducing the waste solution results in uptake of M11 by ion exchange for M21 where M21 is displaced into the waste solution Equation 17.3. v
1 1 M21 X2 1 M1 1 2 jM1 X 2 j1M3
(17.3)
17.3 ESIX for removal of radioactive cesium-137
Competition for binding sites will occur, and loading will be driven by thermodynamics. Therefore, to successfully remove the target cation, films must bind that ion preferentially, which depends on the electroactive property of X.
17.3 Nanomaterials-enhanced electrically switched ion exchange for removal of radioactive cesium-137 Significant amounts of cesium-137, an important fission by-product, are present in radioactive liquid waste generated during the reprocessing of nuclear fuel by the Purex process. Removing the long-lived (T1/2 5 32 years) radioisotope of cesium from the radioactive waste is a challenging project and several techniques, such as ion exchange, sorption, solvent extraction, and precipitation processes, have been investigated. One such process is ESIX using electroactive nickel hexacyanoferrates (NiHCF) developed at PNNL. Transition metal hexacyanoferrates (MHCFs) are an important class of inorganic polynuclear mixed-valence compounds because of their interesting properties including electrocatalysis, electrochromicity, ion exchange selectivity, sensing, and magnetism. Therefore, preparation and application of Prussian blue (PB) and its analogous forms such as samarium hexacyanoferrate, NiHCF, and cobalt hexacyanoferrate have been extensively studied. Also, thin films of PB and related analogous complex salts are also promising candidates for separating membranes capable of ion sieving and electrochemical-controllable ion exchange because of their zeolitic structure. Hexacyanoferrates are well-known ion exchangers with high selectivity over cesium in concentrated sodium solutions but hexacyanoferrates used in conventional packed column ion exchange are difficult to elute [15]. Therefore, columns are normally discarded once used. However, when hexacyanoferrates are applied as electroactive films, depending on the film deposition formulation and substrate, highly reversible uptake and elution of cesium can be achieved from fifteen hundred to over four thousand cycles [2,4]. When a cathodic potential is applied to the NiHCF film, Fe31 (ferricyanide) is reduced to the Fe21 state (ferrocyanide); thus, a cation must be intercalated into the film to maintain its charge neutrality. In practice, the reduction step is usually conducted in a sodium solution as shown in Equation 17.4. The film is then exposed to the waste solution containing cesium, which loads into the film via ion exchange for sodium following Equation 17.5. When an anodic potential is applied, a cesium cation is released from the film as shown in Equation 17.6. Therefore, NiHCF has been coated on the electrode as shown in Fig. 17.1 for ESIX of Cs [2]. CsNiFeIII ðCNÞ6 1 Na1 1 Na1 1 e2 -CsNaNiFeII ðCNÞ6
(17.4)
CsNaNiFeII ðCNÞ6 1 Cs1 -Cs2 NiFeII ðCNÞ6 1 Na1
(17.5)
Cs2 NiFeII ðCNÞ6 -CsNiFeIII ðCNÞ6 1 Cs1 e2
(17.6)
273
274
CHAPTER 17 Nanomaterials-Enhanced ESIX Process
Load cycle
+ Cs+
CsNiFe(III)(CN)6
Cs2 NiFe(II)(CN)6
–
+e
Conductive substrate
Unload cycle
Cs2NiFe(II)(CN)6
– Cs+ CsNiFe(III)(CN)6 – e–
Conductive substrate
FIGURE 17.1 Electrically switched ion exchange (ESIX) mechanism of nickel hexacyanoferrate thin film electrode for Cs1 loading and unloading.
(A)
(B)
Clean water
(C)
Wash Contaminated water
ESIX
Concentrated eluant
Waste form
FIGURE 17.2 Schematic illustration of (A) a bench-scale electrically switched ion exchange (ESIX) system based on (B) nickel hexacyanoferrate coated nickel foam electrodes, and (C) fiveelectrode cell stacks used as the ESIX system.
Figure 17.2 shows bench-scale flow cell testing of ESIX systems developed at PNNL using NiHCF films deposited on high surface area nickel foam electrodes with a nominal surface area per volume of 40 cm2/cm3 (60 ppi) [4]. The flow tests using NiHCF material were performed to evaluate the feasibility of the approach as an alternative to conventional ion exchange processes.
17.3 ESIX for removal of radioactive cesium-137
Loading NaNiFe(III)(CN)6
CsNaNiFe(II)(CN)6 Unloading
+ Cs+ + e–
Loading
Unloading
– Cs+
– e–
FIGURE 17.3 Schematic illustration for electrically switched ion exchange based on carbon nanotubes, polyanline, and nickel hexacyanoferrate nanocomposite. Data from [5] with permission.
Bench-scale ESIX flow system studies showed no change in capacity or performance of the ESIX films at a flow rate up to 113 bed volumes/h (BV/h). A comparison of results for a stacked five-electrode cell versus a single-electrode cell showed enhanced breakthrough performance. In the stacked configuration, breakthrough began at approximately 120 BV for a feed containing 0.2 ppm cesium at a flow rate of 13 BV/h. A case study for the KE Basin (a spent nuclear fuel storage basin) on the Hanford Site demonstrated that wastewater could be processed continuously with minimal waste generation, reduced disposal costs, and lower capital expenditures. Novel nanocomposite materials comprised of high surface area of CNTs, chemically stable polyaniline (PANI), and ion exchange properties of NiHCF have been synthesized through electrodeposition [5,6]. Figure 17.3 illustrates the concept of electrically switched ion exchange based on CNT/PANI/NiHCF nanocomposites. High surface area of the porous CNT film leads to the high loading capacity for the NiHCF nanoparticles, which in turn leads to the high ion exchange capacity of the NiHCF/CNT nanocomposite film. The presence of the PANI polymer film further improves the stability of the nanocomposite film, which undergoes more than five hundred cycles retaining 92 percent of its initial capacity. Recently, NiHCF nanotubes have been fabricated by an electrokinetic method based on the distinct surface properties of porous anodic alumina [7]. By this method, nanotubes formed rapidly with the morphologies replicating the nanopores in the template. The electrochemical measurements show that the NiHCF nanotubes exist only in the form of K2Ni[Fe(CN)6] with excellently stable cesium-selective ion exchange ability due to single composition and unique nanostructure. NiHCF nanotubes modified electrodes retains 95.3 percent of its initial value after five hundred potential cycles. Even after fifteen hundred and three thousand cycles, the NiHCF nanotubes still retain 92.2 percent and 82.9 percent, respectively, of their ion exchange capacity.
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17.4 Nanomaterials-enhanced electrically switched ion exchange for removal of chromate and perchlorate Wide use of heavy metals and its related compounds by industries has resulted in the pollution of the environment. These inorganic toxins are of considerable concern since they are nonbiodegradable, highly toxic with possible carcinogenic effect. Among the heavy metals, chromium is one of the most widely used metals in many of the industrial processes such as tanning, electroplating, making printed circuit boards, painting, and steel fabrication. Most of the chromium is discharged into aqueous waste as Cr(III) and Cr(VI). Cr(VI), which is the more toxic of the two, is present as dichromate (Cr2O722), chromate (CrO422), or hydrogen chromate (HCrO42). The current maximum contamination level (MCL) for total chromium in drinking water in the United States is regulated by the United States Environmental Protection Agency (EPA) at 100 μg/L. Perchlorate ion also poses significant health concern since it can block the uptake of iodine in the thyroid gland and thereby affect the production of thyroid hormones. Perchlorate anion is a critical component in combat and training munitions. In addition, perchlorate salts are extensively used as chemical reagents in the production of leather, rubber, fabrics, paints, and aluminum. As a result, perchlorate contamination is now recognized as a widespread concern affecting many water utilities. Recently, EPA, based on a recommendation by the National Research Council (NRC), set the safe dose for perchlorate at 0.70 μg/kg of body weight per day. However, due to its solubility and nonreactivity, perchlorate is a very stable substance in aquatic systems and is, therefore, difficult to remove. Conventional chromium and perchlorate removal methods include adsorption, chemical precipitation, biological degradation, ion exchange, and electrochemical method. However, these technologies have technical limitations, generate substantial secondary wastes, and are costly. Therefore, innovative, cost-effective, and green technology for the treatment of perchlorate needs to be developed. Electrochemical ion exchange treatment is a promising technique that offers several advantages over other techniques for remediation of metals from contaminated effluents or wastewaters. For chromate removal, arc-assisted carbon (AC), activated carbon (RC), and carbon aerogel have been used for ion exchange electrode [8,9]. Hexavalent chromium, [Cr(VI)] may exist in the aqueous phase in different anionic forms, such as Cr2O722, CrO422, or HCrO42, with total chromate concentrations and pH dictating which particular chromate species is predominant. Equation 17.7 shows that equilibrium relationship between the different chromium anions: 2
2
2 2CrO24 1 2H2ð2HCrO2 4 Þ2Cr2 O7 1 H2 O
(17.7)
Carbon obtained by the arc-assisted evaporation of graphite rods (AC) exhibited a high selectivity toward anions of hexavalent chromium from aqueous solutions in the pH range of 39 [8]. There is almost no removal of trivalent
17.4 ESIX for removal of chromate and perchlorate
chromium or other metal cations such as Pb1 and Zn1 at low pH values (i.e., ,9) since trivalent chromium is present as a cation in solution whereas hexavalent chromium exists as an anion. This is consistent with the hypothesis that the surface of AC carries positive charges that facilitate the removal of the anion of the hexavalent chromium but prevent the removal of metal cations. At high pH, the positive surface charges are neutralized by the presence of OH1 ; hence, the metal cation removal efficiency is increased by an ion exchange adsorption mechanism. Commercial carbon (RC), on the other hand, has a negatively charged surface due to the presence of oxygen functional groups of “oxo” (CxOy) type produced during its the activation at a very high temperature. Therefore, metal cation removal is facilitated. An increase in the pH of the solution contributes to an increase of the surface negative charge density, resulting in an even greater removal efficiency of lead, zinc, and trivalent chromium metal cations. Recently, carbon aerogel electrodes were applied for chromium removal from wastewater since carbon aerogel possess unique thermal, mechanical, and electrical properties, which are directly related to their unusual nanostructure composed of interconnected particles with microscopic interstitial pores [9]. Carbon aerogel is an ideal electrode material because of its low electrical resistivity (#40 mΩcm), high specific surface area (4001100 m2/g), and controllable pore size distribution (50 nm). The effect of pH (27), concentration 28 mg/l, and charge 0.31.3 A h was investigated where the chromium ion removal was significantly increased at reduced pH and high charge conditions. The metal concentration in the wastewater can be reduced by 98.5 percent under high charge (0.8A h) and acidic conditions (pH 5 2). Also, the elution solution can be used repeatedly, thereby minimizing secondary wastes at the largest degree and reducing costs greatly. For perchlorate removal, electrically conductive polypyrrole (PPy) polymer as an electroactive ion exchange layer has been deposited onto a conducting substrate [10,11]. Figure 17.4 shows the schematic illustration of PPy preparation and the electrically controlled anion exchange for the separation of the perchlorate ion
− +
A–
Polymerization +
e–
A– e–
–
−
+ +
−
−
+
−
+
+ −
− +
A– Reduction Oxidation
FIGURE 17.4 Schematic illustration for the polymerization of electrically conductive polypyrrole (PPy) and anion intake and elution with the oxidation and reduction of PPy film. Three or four pyrrole units involving one positive charge are considered in oxidized PPy.
e–
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(A)
(B)
Current/1e-4A
278
1.8 1.6 (i) 1.4 PPy on CNT 1.2 1.0 0.8 0.6 0.4 (ii) 0.2 0 PPy Film on GC -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 Potential/V
FIGURE 17.5 (A) Scanning electron microscope image of polypyrrole (PPy) film on carbon nanotube (CNT)/ glassy carbon (GC) electrode surface prepared by controlling the potential at 0.7 V, 100 seconds, and (B) a comparison of the tenth cycle behavior of (i) CNTPPy and (ii) GC/PPy. Data from [11] with permission.
from wastewaters using PPy. The pyrrole monomer polymerizes into PPy during oxidation in supporting electrolyte with anions. After electrode has been coated with PPy, anions can be reversibly ion exchanged by applying anodic and cathodic potentials. Therefore, the ClO42 ion uptake occurs during electrochemical oxidation of the electroactive species by applying an anodic potential on the film in the solution containing ClO42, which forces the ClO42 from the waste solution into the film. Elution occurs when the potential is switched to cathode, forcing the ClO42 out of the film into the elution solution. Therefore, the eluant can be repeatedly used to significantly reduce the quantity of secondary waste generated. Although the ion exchange property of PPy has been realized, its capacity is limited by only one positive charge per three or four pyrrole units. 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. At PNNL, in order to improve the mass transfer properties of the PPy, and thereby the ion exchange capacity, PPy was deposited on to high surface area CNT substrates. Carbon nanotubes are one of the novel nanostructure forms of carbon with very high surface area and good conductivity that offers an idea matrix for depositing PPy film. According to a survey by Cientifica, the world’s leading nanotechnology information company, CNT costs are expected to decrease by a factor 10100 in the next 5 years making CNTcomposite a more practical solution for large-scale waste processing. Figure 17.5 shows the microstructure and cyclic voltammogram of CNT/PPy nanocomposite electrode. Also shown in Fig. 17.5B is cyclic voltammogram of PPy deposited on a glassy carbon (GC) electrode. It can be seen that significantly higher electrochemical current density was delivered by creating higher active area using CNT substrate. The CNT/PPy electrode showed higher perchlorate ion
17.5 Conclusions
4500 2p3/2
4000 3500
2p1/2
c/s
3000
a Cl−
2500 2000
2p3/2 2p1/2
1500 1000 500 0
b
−
ClO4 c
215
210
205
200
195
190
Binding energy (eV)
FIGURE 17.6 High-resolution X-ray photoelectron spectroscopy Cl spectrum of polypyrrole (PPy) film prepared (a) in 0.2 M NaCl, (b) after control of the electrode at 0.4 V for 300 seconds in a solution containing 0.02 M NaClO4 and 0.2 M NaCl, and (c) after a cathode potential at 20.8 V was applied on the film for 300 seconds in a solution of 0.2 M NaCl. Data from [10] with permission.
selectivity over chlorine ion and X-ray photoelectron spectroscopy analysis (Fig. 17.6) indicated reversible elution of absorbed ions, making PPy a promising candidate material for perchlorate removal [10,11]. When ion-exchange capacity and reversibility of pure PPy and CNT/PPy nanocomposite thin films were compared using electrochemical quartz microbalance (EQCM) in NaCl, NaClO4, NaNO3, Na2SO4, and NaHCO3 electrolyte solution, the results indicated that the ion exchange kinetics of PPy improved by more than ten times when CNT substrate was used. In addition, the ion exchange reached saturation within 100 seconds for CNT/PPy. Finally, the anion loading capacity (mole percent) of CNT/ PPy was in order of Cl2 . ClO42 . NO32 . SO422 . CO322 whereas selectivity of anions was in order of ClO42 . NO32 . SO422 . Cl2 . CO322.
17.5 Conclusions Electrically switched ion exchange technology has recently been applied for removing cesium, chromate, and perchlorate ions from wastewater. The efficiency of these ESIX systems can be significantly improved through nanotechnology by providing better electrochemical stability and a larger contact area with wastewater. Therefore, the combination of novel electroactive ion exchange material with
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nanotechnology will lead to the development of more efficient and economic wastewater treatment systems based on ESIX technology.
Acknowledgments This work is supported by the US Department of Defense Strategic Environmental Research and Development Program (Project No ER-1433). The research described in this chapter was performed at the Environmental Molecular Science Laboratory, a national scientific 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-AC05-76L01830. The authors would like to acknowledge Dr. Mike A. Lilga for helpful discussion.
References [1] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.M. Haight, D.T. Schwartz, Metal ion separations using electrically switched ion exchange, Sep. Purif. Technol. 11 (1997) 147158. [2] M.A. Lilga, R.J. Orth, J.P.H. Sukamto, S.D. Rassat, J.D. Genders, R. Gopal, Cesium separation using electrically switched ion exchange, Sep. Purif. Technol. 24 (2001) 451466. [3] S.D. Rassat, J.H. Sukamto, R.J. Orth, M.A. Lilga, R.T. Hallen, Development of an electrically switched ion exchange process for selective ion separations, Sep. Purif. Technol. 15 (1999) 207222. [4] M.A. Lilga, R.J. Orth, and J.P.H. Sukamto. Electrically switched cesium ion exchange FY98 Final Report, PNNL-12002, Pacific Northwest National Laboratory. 1998. [5] Y. Lin, X. Cui, Novel hybrid materials with high stability for electrically switched ion exchange: carbon nanotubes/polyaniline/nickel hexacyanoferrate nanocomposites, Chem. Commun. 17 (2005) 22262228. [6] Y. Lin, X. Cui, Electrosynthesis, characterization, and application of novel hybrid materials based on carbon nanotube-polyaniline-nickel hexacyanoferrate nanocomposites, J. Mater. Chem. 16 (2006) 585592. [7] W. Chen, X.H. Xia, Highly stable nickel hexacyanoferrate nanotubes for electrically switched ion exchange, Adv. Funct. Mater. 17 (2007) 29432948. [8] S.B. Lalvani, T. Wiltowski, A. Hubner, A. Weston, N. Mandich, Removal of hexavalent chromium and metal cations by a selective and novel carbon adsorbent, Carbon 36 (78) (1998) 12191226. [9] P. Rana, N. Mohan, C. Rajagopal, Electrochemical removal of chromium from wastewater by using carbon aerogel electrodes, Water Res. 38 (2004) 28112820. [10] Y. Lin, X. Cui, J. Bontha, Electrically controlled anion exchange based on polypyrrole and carbon nanotubes nanocomposite for perchlorate removal, Environ. Sci. Technol. 40 (2006) 40044009. [11] X. Cui, M.H. Engelhard, Y. Lin, Preparation, characterization and anion exchange properties of polypyrrole/carbon nanotube nanocomposites, J. Nanosci. Nanotechnol. 6 (2006) 547553.