Journal of NUCLEAR SCIENCE and TECHNOLOGY, 24[2], pp. 124~128 (February 1987) Influence of Functional Sulfonic Acid Group on Pyrolysis Characteristics for Cation Exchange Resin Masami MATSUDA, Kiyomi FUNABASHI, Hideo YUSA, Energy Research Laboratory, Hitachi Ltd. * Makoto KIKUCHI Hitachi Works, Hitachi Ltd. ** Received May 16, 1986 Revised August 19, 1986 Pyrolysis of spent ion exchange resins is used to reduce radioactive waste volume and to make the final waste form more stable. The weight loss of cation exchange resin after pyrolysis is only 50w/0 while that of anion exchange resin is 90 w/0. Fundamental experiments were performed to investigate the reason for the small weight loss of the former, The cation resin consists of base polymer and functional sulfonic acid groups. Chemical analyses of the pyrolysis products showed that 65% of the functional groups decomposed at about 300dc and generated SO2 gas. However, only a small amount of the base polymer was pyrolyzed even at 600dc and the weight loss was only 50 w/0. The IR and XPS studies on the residue showed that 35% of the functional sulfonic acid groups was converted to sulfonyl and sulfur bridges between the base polymers during pyrolysis. These bridges made the base polymers thermally stable. Therefore, the small weight loss of the cation resin was attributed to formation of bridges, which originated from the functional groups. KEYWORDS: bridge, cations, chemical composition, functional group, infrared spectroscopy, ion exchange materials, pyrolysis, radioactive wastes, resins, sulfonic acids, X-ray photoelectron spectroscopy I. INTRODUCTION Ion exchange resins are widely used for water treatment in nuclear power plants because of their ability to remove impurities from reactor water. Spent resins comprise a major fraction of the total radioactive waste, and several decomposition techniques(1)(2) are being developed with a view towards reducing the waste volume and also making the final waste form more stable. Pyrolysis is one of the most effective methods. It is characterized by a low operating temperature and low gas flow rate in the pyrolysis vessel(3)(4). A pyrolysis system for spent resins has been developed and the results of its pilot plant tests were reported by Pettersson & Kemmler(3). In their system, the resins were pyrolyzed at about 500dc and the residue after pyrolysis was solidified with cement to produce the final waste package. It was shown that the final waste volume was reduced to about 1/4 compared with a normal cementation process and the final package was stable during long term storage. Both anion and cation exchange resins consist of base polymer and functional groups. The former is a copolymer of styrene and divinylbenzene and the latter is sulfonic acid (cation resin) or quaternary ammonium (anion one). The weight loss after pyrolysis is known to change with resin types, that is, for cation resin the loss is only 50w/0 after pyrolysis at 500dc, while * Moriyama -cho, Hitachi-shi 316. ** Saiwai-cho, Hitachi-shi 317. 36
Vol. 24, No. 2 (Feb. 1987) 125 that of the anion resin is 90w/0(4). Neely(5) investigated the pyrolysis characteristics of the cation resin by thermogravimetric analyses and residual elemental analyses. His main objective was to produce polymer carbon (carbonaceous adsorbent) with good yield. The small weight loss of the cation resin, namely good yield of the polymer carbon, was presumed to reflect some of the functional sulfonic acid groups (-SO3H) producing sulfur bridges during pyrolysis. This suggests that the functional group is a key factor leading to the small weight loss (50w/0 at 500dc) for the cation resin. Therefore, the influence of functional sulfonic acid group on pyrolysis characteristics of the cation resin was investigated. At first, the residue and off gas were subjected for chemical analyses. Secondly, infrared spectroscopy and X-ray photoelectron spectroscopy were applied to examine chemical structure of the residue. II. EXPERIMENTAL Cation exchange resin (DIAION SKN1), with mean particle size of 500 mm, was dried in an oven at about 100dc to provide samples with water content of less than 2w/0. The resin consists of base polymer and functional group. The former was styrene-divinylbenzene (ST-DVB) copolymer with 8 w/0 DVB and the latter was sulfonic acid (-SO3H). The ST-DVB copolymer with the same DVB ratio (DIAION HP50), namely base polymer without the functional sulfonic acid group, was also prepared to examine the effect of the functional group on the resin pyrolysis. Samples (1 g each) were pyrolyzed at selected temperatures from 200~ 600dc for 2 h in a quartz tube under N2 atmosphere at a flow rate of 20 ml/min, as is shown in Fig. 1. The pyrolysis characteristics were examined by the following methods. 1. Characterization of Pyrolysis Products The residual weight was first measured and weight loss was calculated as a function of pyrolysis temperature. Subsequently, pyrolysis residue was examined by elemental analyses. Each residue was completely burned in an air flow to give such gases as H2O, CO2 and SO2. Elemental ratios in the residue, namely H, C and S contents, were examined from these gas compositions. Oxygen content was evaluated by subtracting weights of these three elements (H, C and S) from the initial weight of the residue, because the cation resin contains only four elements. Off gas analyses were also performed. The H2O, SO2 and various hydrocarbon gases were generated during pyrolysis. The H2O was condensed in a cold trap (coolant : 5dc water) together with high boiling hydrocarbon gases. The quantity of H2O was determined by Karl Fischer titration(6). The SO2 was adsorbed into 3% H2O2 solution (volume : 400 ml) and its amount was determined by BaSO4 turbidimetry. Low boiling hydrocarbons, such as methane, benzene and styrene, were analyzed using gas chromatography (detector : FID). 2. Characterization of Chemical Structure Both infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) were applied to determine chemical structure of the residues. The residue was ground to a powder and mixed with KBr (potassium bromide) powder in the weight ratio of 1:20. Then, IR spectra were examined by the diffuse reflection method(7) using a Fourier transform infrared (FT-IR )spectrometer. The binding energies of sulfur 2p were also measured by an XPS spectrometer(8) utilizing Al-Ka radiation (1,486 ev). III. RESULTS AND DISCUSSION Fig. 1 Flow diagram of experimental method 1. Characterization of Pyrolysis Products Figure 2 compares the changes in the weight of residual resin for cation exchange resin and ST-DVB copolymer without the functional sulfonic acid group. The latter decomposed in the temperature range of 300~400dc and its weight loss was about 90w/0 above 400dc. On the other hand, the pyrolysis of the cation resin started 37
126 J. Nucl Sci. Technol., between 200 and 300dc but the weight loss was limited only to 50w/0 even at 600dc. These results indicated that cation resin was thermally more stable than ST-DVB copolymer. Table 1 Off gas compositions for cation exchange resin pyrolyzed at 300 and 500dc Fig. 2 Changes in weight of residual resin for cation exchange resin and styrene-diyinylbenzene (ST-DVB) copolymer without functional group Figure 3 summarizes the residual elements and off gas compositions of the cation resin as a function of pyrolysis temperature, and Table 1 lists the detailed constituents of off gas. The functional sulfonic acid groups decomposed at 300dc, because SO2 and H2O gases were generated and S and O contents in the residue decreased at this temperature as indicated in Fig. 3. On the other hand, only a small amount of hydrocarbon gases were generated, and most of the carbon, which forms the base polymer (ST-DVB copolymer), remained in the residue. These facts indicated that only a small fraction of the ST-DVB copolymer was pyrolyzed in the cation resin, while most of the copolymer was pyrolyzed in sulfonic acid group free case (Fig. 2). In Fig. 4, the weight changes of polyethylene and polyacrylonitrile are compared as a function of pyrolysis temperature(9). The former was completely decomposed in the temperature range of 300~400dc and hydrocarbon gases were generated as is shown below: (1) Fig. 4 Comparison polyethylene of weight loss between and polyacrylonitrile(9) Fig. 3 Residual elements and off gas compositions of cation exchange resin Polyacrylonitrile consists of polyethylene (mainchain) and nitrile group (side-chain), namely its base polymer is in common with polyethylene. However, the weight loss was only 40w/0 at 500dc (Fig. 4). The IR study showed that some of the nitrile groups formed bridges between the main-chains as follows(10): 38
Vol. 24, No. 2 (Feb. 1987) 127 (2) It is generally known that such bridges make polymers thermally stable, causing their weight loss to decrease(9). We reasoned analogously that the small weight loss of the cation resin was also due to a formation of bridges during pyrolysis. 2. Characterization of Chemical Structure The IR spectra before and after pyrolysis are shown in Fig. 5(a)~(c). For the sample before pyrolysis, five peaks (1,230, 1,180, 1,130, 1,030 and 660 cm-1) were observed corresponding to the functional sulfonic acid group (-SO3H)(11)(12). However, they vanished and two new peak groups were detected in the spectra after pyrolysis at 300dc. One group, at (a) before pyrolysis (b) pyrolyzed at 300dc (c) pyrolyzed at 500dc Fig. 5 FT-IR spectra of cation exchange resin with and without pyrolysis 760 and 700 cm-1, corresponded to benzene mono-substitution and the other, at 1,310, 1,180 and 1,160 cm-1, was due to a sulfonyl bridge (-SO2- ). Hence, some of the functional sulfonic acid groups changed into gaseous SO2, leaving benzene mono-substitutions behind. The rest formed the sulfonyl bridges due to a dehydration reaction. The conversion ratio from sulfonic acid to SO2 gas was about 65%, because 35% of the sulfur remained in the residue at 300dc as in Fig. 3. Based on these results, the main pyrolysis routes for the cation resin can be expressed by (3) Most of the functional sulfonic acid groups were pyrolyzed at about 300dc and such off gases as SO2 and H2O were generated. However, 35% of the functional groups formed sulfonyl bridges (-SO2-) between base polymers. These bridges made the base polymer thermally stable and the weight loss was small, just as with polyacrylonitrile pyrolysis. Peaks corresponding to a sulfonyl bridge (1,310, 1,180 and 1,160 cm-1) vanished for the IR spectra after pyrolysis at 500dc as in Fig. 5(c). This suggested that chemical structure of the bridge changed in this temperature range. Figure 6 shows the XPS spectra of sulfur 2p for the samples before and after pyrolysis(8)(13). Before pyrolysis, the binding energy of sulfur 2p was 168.8 ev which was equal to the value for benzenesulfonic acid (C6H5SO3H)(13). Two new peaks, at 168.0 and 164.2 ev, were detected in the spectra after pyrolysis at 300dc. The former peak was similar to the value of 167.9 ev measured for diphenyl sulfone (C6H5)2SO2 and the latter to the value of 164.0 ev for phenyl sulfide polymer (C6H5S)n. That is, the functional sulfonic acid groups produced not only sulfonyl bridges (-SO2-) but also sulfur bridges (-S-) at 300dc. The latter was not detected by IR spectroscopy, because it has only small IR absorptions(12). Based on these 39
128 J. Nucl. Sci. Technol., Fig. 6 Changes in XPS spectra of sulfur 2p for cation exchange resin results, the changes in chemical structure of the bridge can be expressed by the following reaction: ( 4 ) The functional groups first formed sulfonyl bridges between base polymers after dehydration reaction. Subsequently, some of the sulfonyl bridges formed sulfur bridges due to a deoxidation reaction. The peak at 168.0 ev vanished and only the 164.2 ev peak was observed for the spectra pyrolyzed at 500dc. The same spectra were also obtained for the residue at 400dc, showing that most of the sulfonyl bridges changed into sulfur bridges between 300 and 400dc. These spectral changes were compatible with the data of elemental analyses, that is, little oxygen was detected in the residue pyrolyzed above 400dc as in Fig. 2. Therefore, the small weight loss (50w/0 at 600dc) of the cation resin was reasonably attributed to the formation of sulfonyl and sulfur bridges between base polymers during pyrolysis. On the other hand, the ST-DVB copolymer was easily decomposed (Fig. 2), because it contained no sulfonic acid group. IV. CONCLUSION Pyrolysis characteristics for cation exchange resin were investigated to examine influence of the functional sulfonic acid group. At first, the residue and off gas were subjected for chemical analyses. Secondly, chemical structure of the residue was examined by IR and XPS. The following results were obtained: (1) The cation resin composes of ST-DVB copolymer (base polymer) and functional sulfonic acid group. Weight loss of the cation resin was only 50w/0 at 600dc. However, 90w/0 of ST-DVB copolymer without the functional group decomposed at 400dc, showing that the functional sulfonic acid group was a key factor leading to the small weight loss of the cation resin. (2) Sixty-five percent of the functional group in the cation resin decomposed at about 300dc and generated SO2 gas. However, only a small amount of the base polymer was pyrolyzed and most remained in the residue. (3) Thirty-five percent of the functional sulfonic acid groups formed sulfonyl and sulfur bridges between the base polymers. These bridges made the base polymer thermally stable. Therefore, the small weight loss of the cation resin was attributed to formation of bridges, which originated from the functional sulfonic acid group, during pyrolysis. REFERENCES (1) KARITA, Y.: J. At. Energy Soc. Jpn. (in Japanese), 26[41, 277 (1984). (2) MATSUDA, M., et al.: J. Nucl. Sci. Technol., 23[31, 244 (1986). (3) PETTERSSON, S., KEMMLER, G.: "Waste Management '84", Vol. 2, 223 (1984), Arizona Board of Regents. (4) MATSUDA, M., et al.: Submitted to Nucl. Technol. (5) NEELY, J. W.: Carbon, 19, 27 (1981). (6) YASUMORI, Y., et al,: Jpn. Analyst, 14, 871 (1965). (7) WANG, S. H.: Fuel, 64, 229, (1985). (8) LINDBERG, B. J., et al.: Phys. Sci., 1, 286 (1970). (9) BAMFORD, C. H., TIPPER, C. F. H.: "Comprehensive Chemical Kinetics", Vol. 14, Degradation of Polymer, Chap. 1 (1975), Elsevier, Amsterdam. (10) SCHURZ, J.: J. Polym, Sci., 28, 438 (1958). (11) KOENIG, J. L.: Appl. Spectra., 29, 293 (1975). (12) SILVERSTEIN, R. M., et al.: "Spectrometric Identification of Organic Compounds", Chap. 3 (1974), John Wiley & Sons, New York. (13) CHANG, C. H.: Carbon, 19, 175 (1981). 40