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Electroactive Materials for Anion Separation—Technetium from Nitrate J. P. H. Sukamto, S. D. Rassat,(a) T. L. Hubler, M. A. Lilga,(a) G. M. Anderson,(b) S. A. Bryan,(a) W. H. Smyrl,(c) and J. McBreen(d) Supported by the Environmental Management Science Program, Office of Environmental Management, Department of Energy. (a) Environmental Technology Division. (b) Student. (c) Department of Chemical Engineering and Materials Science, University of Minnesota. (d) Brookhaven National Laboratory. The focus of the project is to develop a fundamental understanding of how the physical and chemical properties of electroactive ion exchange (EaIX) materials control their efficiency when used as mass separation agents. Specifically, the desirable characteristics of EaIX materials for separation applications are 1) high reversibility, 2) high selectivity, 3) acceptable physical and chemical stability, 4) rapid intercalation and de-intercalation rates, and 5) high capacity. Because of these requirements, EaIX materials share many properties in common with conventional ion exchangers and electroactive polymers. For example, EaIX materials require the selectivity typically found in ion exchangers; they also require the redox reversibility of electroactive polymers. The results of this work will allow the rational design of new materials and processes tailored for the separation of specific anions. For this work, the specific anion of interest is pertechnetate (ReO4-) and its separation from nitrate (NO3-). The specific objectives and their relevance are discussed. The first three objectives address specific material properties, and the last objective explores the scientific issues related to the preparation of high-capacity EaIX material. 1. Determine the effects of the distribution/ concentration and mobility of electroactive sites. A high concentration of electroactive sites is favorable for high capacity and fast electronic propagation through the material. However, increased site-to-site interactions can also be expected, which may reduce the intercalation rate and the utilization efficiency. Increasing the mobility of the electroactive sites will enhance charge propagation rates without increasing the site concentration. 2. Determine the effects of branching/cross-linking. Cross-linking makes the polymer less prone to swell. This can increase the selectivity for anions with lower hydration energy. Less swelling may also enhance the stability to delamination. But increased cross-linking can also reduce the penetrability of the polymer to all anions in addition to restricting segmental motion, which will, in turn, reduce the capacity and transport rates. 3. Determine the effects of polymeric chain length or molecular weight. A higher molecular weight polymer has an overall lower free energy of sticking to the electrode surface (provided that each segment is hydrophobic), and therefore, it is more stable. On the other hand, long polymeric chain length may result in entanglements. Entanglements may lead to restricted segmental motion and reduced free volume required for counter-ion diffusion. Ultimately, the intercalation/de-intercalation rates are reduced. Restricted segmental motion can also result in increased site-to-site interactions, which, as before, may reduce the intercalation rate and utilization efficiency. 4. Prepare and characterize high-surface-area EaIX materials using aerogel and phase-inversion techniques. If high-surface-area (based on the geometric or projected surface area) materials can be prepared, increased efficiency can be realized. We chose to initially study polyvinlyferrocene (PVF) because it is generally accepted as a model redox polymer, and therefore, its general characteristics are of interest in the study of other redox polymers. Therefore, the initial use PVF to achieve the stated objectives will yield results that are beneficial not only to the specific problem of pertechnetate removal, but also to the general topic of redox polymers. In addition, it will provide a firm basis to explore other ferrocene-containing redox polymers. In particular, polymers obtained from the polymerization of vinyl ether monomers will be studied. In contrast to PVF, the latter will contain ferrocenes that are appended to the polymer backbone such that the branch length and chemical property can be independently specified and controlled. In this way, more than by using PVF alone, a systematic study of electroactive site density and mobility, and the hydrophobicity of the polymer can be achieved. The preference of PVF for ReO4- over NO3- anions was measured in terms of the separation factor. The dynamic separation factor was determined using the electrochemical quartz crystal microbalance (EQCM). A PVF film potential cycled in solutions of 1) only NO3-, 2) only ReO4-, and 3) a mixture of NO3- and ReO4- are shown in Figures 5.2 and 5.3. The potential dependence of the current (CV), shown in Figure 5.2, shows that the oxidation and reduction of PVF is more facile in a solution of only ReO4- than in a solution of only NO3-. These results imply that ReO4- stabilizes the ferrocenium (Fc+) cation to a higher degree than NO3-. Equivalently, the complex formation constant of the (Fc+)(ReO4-) salt is larger than that of the (Fc+)(NO3-) salt (Inzelt and Horányi 1986). Gravimetric measurement results shown in Figure 5.3 were obtained simultaneously with the electrochemical results shown in Figure 5.2. Comparison of the data shown in Figures 5.2 and 5.3 shows that oxidation of the PVF film is accompanied with an increase in mass loading (represented as a decrease in the fundamental frequency of the quartz crystal). Furthermore, more mass loading accompanies the oxidation of the PVF film in a solution of only ReO4-, which is consistent with the higher molecular weight of ReO4- as compared to NO3-. Figure 5.2. CVs of PVF in different solutions. Figure 5.3. Measured frequency shifts corresponding to CVs in Figure 5.2. To be of practical use, EaIX materials need to be both physically and chemically stable. Chemical instability of ferrocenes and possible means to overcome it under alkaline conditions have been reported (Chambliss et al. 1998; Clark et al. 1996) and we are currently investigating these issues as they pertain to PVF. Physical instabilities (e.g., caused by swelling/ de-swelling of the PVF) during potential cycling are also issues that need to be addressed. The current-potential characteristics of a spin-coated PVF film in 0.5 M NaNO3 are shown in Figure 5.4 for the 3rd and 50th cycle. The integrated charge passed of the 50th cycle is 34% of that measured for the 3rd cycle. In contrast, a spin-coated PVF film that was exposed to a nitrogen plasma showed enhanced stability (see Figure 5.5); the charge measured in the 50th cycle is 62% of that measured for the 3rd cycle. The most likely explanation for the observed decrease in capacity with potential cycling (for both films) is the uncoiling and dissolution of individual PVF chains. PVF films typically swell during oxidation, and oxidation in the presence of highly hydrated anions (e.g., NO3-) will likely increase the degree of swelling. The degree of uncoiling and dissolution of individual PVF chains are likely to correlate directly with the degree of swelling. Exposure of a PVF film to a nitrogen plasma is expected to cross-link some of the chains, which, in turn, should reduce film swelling. Figure 5.4. CVs of spin-coated PVF. Figure 5.5. CVs of plasma-treated spin-coated PVF. Work has been initiated in developing techniques for obtaining good x-ray absorption (XAS) spectra for PVF under in-situ conditions in an electrochemical cell. Early work was done on spin-coated films of PVF on sputtered platinum on glass slides. However, preliminary ex-situ XAS studies on these films indicated that more concentrated samples were needed to obtain good XAS spectra. Electrochemical methods were developed to prepare samples with higher loadings per unit area. The most satisfactory method so far has been the electrodeposition of films on carbon cloth. Excellent ex-situ and in-situ XAS spectra were obtained at the Fe K edge (not shown). Good ex-situ XAS were obtained at the Re L3 edge. However, soluble ReO4-, entrained in the electrolyte in the pores of the carbon cloth, interfered with the spectra. This problem requires a new design for the electrochemical cell that will permit electrolyte exchange while maintaining the electrode under potential control. XAS spectra were also obtained on several standard compounds such as various iron oxides, ferrocene, ferrocenium hexafluorophosphate, and ferrocenium tetrafluoroborate. References Chambliss, C. K, M. A. Odom, C. M. L. Morales, C. R. Martin, and S. H. Strauss, "A Strategy for Separating and Recovering Aqueous Ions: Redox-Recyclable Ion-Exchange Materials Containing a Physisorbed, Redox-Active, Organometallic Complex," Analytical Chemistry. 70, 757 (1998). Clark, J. F., D. L. Clark, G. D. Whitener, N. C. Schroeder, and S. H. Strauss, "Isolation of Soluble 99Tc as a Compact Solid Using a Recyclable, Redox-Active, Metal-Complex Extractant," Environmental Science & Technology. 30, 3124 (1996). Inzelt, G., and G. Horányi, "Combined Cyclic Voltammetric and Radiometric Study of Polymer Film Electrodes," Journal of Electroanalytical Chemistry and Interfacial Electrochemistry. 200, 405 (1986). William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: June 12, 2001 Security & Privacy PNNL-13147