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Low temperature pyrolysis of simulated spent anion exchange resins
Accepted Manuscript Title: Low temperature pyrolysis of simulated spent anion exchange resins Authors: Vittorio Luca, Hugo Luis Bianchi, Florencia Allevatto, Jorge O. Vaccaro, Agustin Alvarado PII: DOI: Reference:
S2213-3437(17)30368-8 http://dx.doi.org/doi:10.1016/j.jece.2017.07.064 JECE 1779
To appear in: Received date: Revised date: Accepted date:
20-3-2017 24-7-2017 26-7-2017
Please cite this article as: Vittorio Luca, Hugo Luis Bianchi, Florencia Allevatto, Jorge O.Vaccaro, Agustin Alvarado, Low temperature pyrolysis of simulated spent anion exchange resins, Journal of Environmental Chemical Engineeringhttp://dx.doi.org/10.1016/j.jece.2017.07.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Low Temperature Pyrolysis of Simulated Spent Anion Exchange Resins
Vittorio Luca 1*, Hugo Luis Bianchi 2,4, Florencia Allevatto 1, Jorge O. Vaccaro 3, Agustin Alvarado 3
1
Programa Nacional de Gestión de Residuos Radiactivos, Comisión Nacional de Energía
Atómica, Centro Atómico Constituyentes, Av. General, Paz 1499, 1650 San Martin, Provincia de Buenos Aires, República Argentina. 2
Gerencia de Química, Centro Atómico Constituyentes, Comisión Nacional de Energía
Atómica, Av. General, Paz 1499, 1650 San Martin, Provincia de Buenos Aires, República Argentina. 3
Centro Atómico Ezeiza, Comisión Nacional de Energía Atómica, Presbitero Juan González
y Aragón N° 15, 1804 Ezeiza, Provincia de Buenos Aires, República Argentina 4
Escuela de Ciencia y Tecnología, Universidad Nacional de General San Martín.
* Corresponding Author e-mail: [email protected], Tel. 54-11-6772 7018
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Abstract
Studies of the low temperature pyrolysis of simulated spent cation exchange resins have indicated that this process offers several advantages relative to more traditional approaches involving direct immobilization. However, little is known concerning the fate of long-lived radioisotopes that are typically concentrated in anion exchange resins. In this work anion exchange resins have been loaded with macroscopic quantities of I-, Cl- and CO32- spiked with 14
CO32- with the aim of simulating the spent radioactive anion exchange resins. The anion-
loaded resins were pyrolyzed at increasing temperatures up to 500 oC and the volatilization of 14
CO32-, I- and Cl- was monitored and quantified using radiotracer and mass spectrometric
methods. A significant fraction of the initial 14C and Cl- inventories were volatilized at temperatures in the range 200 – 300 oC. However, mass spectrometry of the gaseous releases indicated that at 300 oC most of the iodine remained within the pyropolymer product. This was confirmed through SEM-based energy dispersive X-ray analysis of the solid products. In the same temperature range, experiments indicated that water was expelled non-reversibly. Leaching experiments were conducted on the anion-loaded pyropolymer products generated at different temperatures in order to determine the quantity of iodide released to solution. For resin pyrolyzed below 400 oC the fractional iodine release to solution did not exceed 0.10 indicating that the major part of the iodine inventory in the pyropolymers was not leachable. The results indicate that low temperature pyrolysis could be used to convert spent anionic and cationic resins to stable pyropolymer products in which a large proportion of the initial radioisotope inventory could be rendered stable. Keywords: Spent resin; nuclear waste; radioactive waste; pyrolysis; immobilization; carbon14.
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1. Introduction Polymeric ion exchange resins are widely used in the nuclear power plants (NPPs) for the purification of water in the cooling system circuits of reactors. These resins are usually based on sulfonate- and quaternary amine-functionalized polystyrene divinyl benzene (PSdVB) copolymers, and once exhausted, represent the largest volume of radioactive waste that is generated during the operation of these plants. Many treatment and conditioning strategies have been investigated for this type of radiotoxic waste [1,2].
Typical strategies dealing with spent resins include direct immobilization in cements [3] or polymeric resins which can result in an expansion of waste volume by a factor of two to five or treatment using thermal or chemical processes that reduce waste volume. Such processes include incineration, pyrolysis, and chemical attack [4,5].
Pyrolysis represents an attractive alternative to incineration for the destruction of municipal organic wastes including biomass [6], waste plastics [7], and tyres [8-11]. The advantage of pyrolysis in these applications is that it is a low temperature, flameless process that can achieve significant volume reduction and at the same time yield valuable products such as fuel oils and biochars.
Because of many of the these same advantages, pyrolysis is also being considered, and even practiced, as a method for the treatment and conditioning of radioactive organic wastes such as solvents, oils and especially spent ion exchange resins that derive from the operation of NPPs. Therefore pyrolysis can potentially be used to treat multiple waste streams. A final potential advantage of pyrolysis is that it appears possible to retain a large part of the radio
4 isotope inventory in a stable waste product thus reducing the generation of secondary waste products [12,13].
To treat spent resins industrial scale pyrolysis-based processes have been developed by the likes of Nukem and Belgoprocess [14]. Being a flameless technology, pyrolysis has the advantage of employing lower operational temperatures, and of critical importance in a nuclear context, it has an enhanced safety profile. Thus, pyrolysis features as an innovative technology in a recent IAEA report on the thermal processing of radioactive wastes [15].
Pyrolysis is a possible means of reducing the volume and of stabilizing polystyrene-based polymeric spent radioactive ion exchange resins by conversion to stable pyropolymers or carbons that contain the greater part of the initial radio nuclide inventory. By retaining most of the radionuclide inventory within the pyrolysis products, the problem of dealing with secondary radiotoxic waste generation (especially gaseous waste) could be greatly mitigated, or potentially even eliminated.
In a previous study we attempted to shed light on the structure and microstructure of the waste products produced and quantify the leaching of Cs+, Sr2+ Co2+ and Ni2+ from cation exchange resins fully loaded with these cations after pyrolysis at temperatures in the range 200 - 600 oC [12,13]. 137Cs and 90Sr are fission products and 60Co and 59Ni activation products typically found in radioactive spent cation exchange resins [16]. Subsequently we have addressed similar questions only for a series of cation exchange resins with decreasing cation loading in order to better approximate real radioactive spent cationic resins where the cation loadings are extremely low [13].
5 In nuclear power plants however, cation and anion exchange resins are typically deployed as mixed beds and often in a 1:1 ratio. Anion exchange resins concentrate radionuclides such as 3
3
7
H (T1/2 = 12.33 years), 14C (T1/2 = 5.73x10 years), 129I (T1/2 = 1.57x10 years), 36Cl (T1/2 = 5
5
3.01x10 years), and 99Tc (T1/2 = 2.111x10 years) adsorbed in the form of simple and more complex oxo-anions. In this regard anion exchange resins might be considered even more problematic than their cation exchange resin counterparts. Aside from their high volatility these anionic species are extremely water soluble, are particularly long-lived and are biologically compatible. Although some 14C is released from NPPs during the normal course of operations, the bulk of the inventory is retained on the anion exchange resins in inorganic form as a carbonate and/or bicarbonate [16]. There has been considerable concern about the health impacts of gaseous reactor emissions and studies that have been undertaken have been quite controversial [17]. As a result the US Nuclear Regulatory Commission is sponsoring future detailed studies and analyses [18,19]. Therefore treating spent resins in a manner that releases this stored 14C into the environment would not be the optimum practice in a tightening regulatory regime.
In a very early Atomic Energy of Canada Limited report [20] on CANada Deuterium Uranium (CANDU) reactor resins doped with 14C it was observed that during heating of the resins in air only a relatively small fraction of the initial 14C inventory (<10%) was released at temperatures below 400 oC. At temperatures above 700 oC up to 80% of the inventory could be released. If however the resins were pretreated with CaCl2 solutions, significantly higher proportions of the inventory could be released at all temperatures. Little further work has been undertaken on this issue or the issue or resin rehydration that can result in enormous swelling pressures and fracturing of an immobilizing matrix [21,22].
6 The objectives of the present study were to understand the behaviour of volatile anionic species during the pyrolysis of simulated anion exchange resins as well as to shed light on their chemical speciation and stability within the resultant pyropolymer waste products. The present strategy depends on carrying out the pyrolysis of spent resins at a sufficiently low temperature as to keep most of the most radiotoxic radionuclides within the pyropolymer waste product. This is likely to be the case for cation exchange resins where metal species are most certainly retained at low temperatures (< 400 oC) but unlikely to be so for anion exchange resins where liberation of anionic species as gases is to be expected. If such species are liberated during low temperature processing, then they will need to be efficiently trapped and managed appropriately.
2. Material and Methods The anion exchange beads used throughout this study were Lewatit Monoplus M500 respectively. These are cross-linked polystyrene gel beads with quaternary amine functional groups. A flow chart of the preparation procedure is shown below. Briefly, the anion exchange resin beads were first exchanged with an alkaline silicate solution to incorporate anionic Si species as a non-volatile internal standard. Subsequently, these Si-exchanged beads were treated with Cl- and I--containing solutions at near neutral pH in order to saturate the ion exchange sites with these anions. Pyrolysis was undertaken in an inert gas flow either within a conventional tubular oven or using the thermal analysis system.
Leaching studies were conducted in a manner similar to the AS16.1 standard protocol. This involved immersion of 0.10 g of the whole resin beads in 10.0 mL volumes of milli-Q water. Following a given time interval the supernatant leachant was removed by decantation. In
7 order to prevent oxidation and volatilization of iodide an aliquot of the leachant solution was added to a 0.1% tetramethyl ammonium hydroxide solution (TMAH) in accordance with recent recommendations [23]. Using the same method, standard solutions (Chem-Lab NV) were prepared with concentrations 0.8, 2, 5, 10, 15 and 20 ng/mL. All solutions were analyzed for the 127I isotope using a Perkin-Elmer NexION 300X ICP-MS instrument using standard procedures. An RF power of 1250 W was used and the nebulizer, auxiliary and plasma gas flows were set to 0.85, 1.2 and 16.0 L/min respectively.
Secondary electron (SE) and back scattered electron (BSE) microscope images together with energy dispersive (EDS) X-ray analyses were obtained on an FEI Inspect F50 microscope.
Thermal analysis was conducted in both flowing air and nitrogen using on a TA Instruments Q600 system. Quantification of iodine present in the off-gas stream of the Q600 thermobalance was undertaken by capturing the off-gas in water traps and then analyzing the resultant solution using an Electrospray Ionization Mass Spectrometry (ESI-MS).
A gas mass spectrometer (OmniStar, Pfeiffer) attached to the Q600 was used to follow the behavior of iodine, chlorine, and carbon dioxide during the thermal treatment of the resin. Anionic resin fully loaded with NaHCO3 was labelled with 14C. The resin was washed and stored under water. Dewatered samples were weighed and heated in a 13 mm diameter quartz tube furnace under flowing nitrogen. A PID temperature controller was used to achieve a temperature ramp of 5 oC/min. A calibrated thermocouple was positioned near the sample to read a precise temperature during the thermal treatment.
8 Quantification of 14C release as a function of heating temperature was performed by low activity liquid scintillation analysis (TRI-CARB 3100TR), using the Ultima Gold AB cocktail. Flowing nitrogen gas was used for quantitative transfer of all the carbon dioxide released from the resin during heating and collected in traps containing NaOH solution and glass spheres. To the contents of the traps was added the scintillation cocktail and the full 14C inventory measured.
Leaching of the anion-loaded samples was undertaken in deionized water at 25 oC. Samples of the resins (0.050 g) treated at each temperature were weighed into six polyethyelene bottles and 10.0 mL of deionized water added to each bottle. The bottles were agitated periodically. At determined time intervals, aliquots of liquid were removed from each of the bottles and analyzed for I by ICP-MS.
3. Results 3.1. Thermal Analysis Figure 1 shows TGA traces of as-received and I-- and Cl--loaded anion exchange resin beads measured in flowing nitrogen. Three clear steps are observed in the TGA trace of the assupplied material (Figure 1a) with derivative peaks observed at around 56, 210 and 415 oC. In studying the anion exchange resin, Duolite A 101, Dubois [24] reported two mass losses at around 200 and 400 oC. The first weight loss was ascribed primarily to the loss of the functional amine group with some PSdVB backbone degradation and the second primarily to the degradation of the PSdVB backbone to CO2 and other organics respectively. The reason that no weight loss was observed at about 60 oC was presumably due to the fact that those resins were initially relatively dry.
9 For the present anion exchange resin in both the as-received and anion-loaded forms the weight loss between 25 and 150 oC as defined by the derivative peak at 56 oC can clearly be ascribed to dehydration. Between temperatures of 150 and 500 oC the two resins show markedly different behavior as is evident from shape of the weight loss curve and the change in number of derivative peaks. Whereas both resins show a weight loss defined by derivative peaks around 210 and 220 oC, the anion-loaded resin possess additional derivative peaks at 305 and 330 oC that do not exist in the as-supplied resin. It can be inferred therefore that these two additional mass loss events in the anion-loaded resin are due to the anions that were introduced, namely iodide and perhaps chloride. Anionic resins show overall lower thermal stability compared with cationic resin. Previous studies [25] have indicated that the weight loss at around 200 oC results from the loss of decomposition products associated with the quaternary ammonium functional groups of the resin. The ash contents for the two resins at 800 oC are 7 and 9 % respectively. The slightly higher yield in the case of the loaded material is probably associated with the Si that was included in order to act as an internal standard.
3.2. Iodine and Chlorine Volatility Although loss of I- and possibly Cl- between 300 and 350 oC can be inferred from the TGA, it does not permit the definitive identification of the species evolved. To identify definitively the products being evolved as a function of temperature, the products released to the off-gas stream were measured using ESI-MS of the gaseous effluents dissolved in water and through direct mass spectrometric analysis of the evolved gas.
The plot of iodine release as a function of pyrolysis temperature as measured by ESI-MS indicates that appreciable iodine is being released only as the pyrolysis temperature exceeded
10 about 300 oC (Figure 2). This is consistent with what was inferred from the TGA data. Thus, 300 oC represents the upper limit of temperature at which the major part of the iodine inventory could be retained on the anionic resins under pyrolysis conditions.
This result was entirely consistent with the direct MS analysis of evolved gas (Figure 2b). The analysis of the complex mixture of pyrolysis gases suggested that CH3I may form during pyrolysis of the anionic resin (see supplementary material Figure S1). Thus the experimental data from ESI-MS and gas MS detection of iodine are in broad agreement within experimental uncertainty with I (mass fragment 127) being detected above 300 oC.
Analysis of the chlorine behavior during the thermal treatment was similarly performed. Figure 3 shows that both natural isotopes of chlorine (35Cl and 37Cl) are released as function of temperature during the thermal ramp of the anion-loaded resin. The release of chlorine commenced at approximately at 220 oC, a much lower temperature than for iodine release as suggested from the observation of mass fragments at 50 and 52 corresponding to CH3Cl.
3.3. Microstructural Analysis of Anion Loaded Beads In order to monitor the iodine distribution and volatility using SEM, Si was introduced by ion exchange from alkaline silicate solution so as to act as an internal reference since Si is not expected to be volatile. SEM studies in combination with EDX analyses were conducted on samples heated in N2 flow to temperatures in the range 175 - 600 oC and the I/Si mole ratio monitored in bead cross-sections.
11 Low magnification SEM images (Figure S2) of sections of beads that were heated in flowing nitrogen at 175, 330, and 350 oC I/Si ratios of 1.59, 0.92 and 0.30 respectively suggesting a reduction in the iodine content with increasing temperature relative to the non-volatile Si. This variation in iodine content was roughly consistent with the curve of gaseous iodine release in Figure 2. Also consistent with this data was the fact that no iodine could be detected by EDX in anion-loaded resins heated beyond 400 oC.
Elemental maps of sectioned and polished samples heated to 200 oC in nitrogen indicated the distribution of iodine, chlorine and silicon within the surface layer to be relatively uniform (Figure 4a-d). For samples heated at 250 oC some surface cracking was apparent and the distribution of iodine appeared to be slightly less homogenous (Figure 6e-h).
Heating to 300 oC (Figure S3) did not provoke a significant change in the microstructure of the beads viewed in section with both iodine and chlorine detected by EDX: The I/Si ratios that were measured were somewhat reduced compared to beads heated to lower temperatures. Repeated EDX analysis of sections of the beads heated to 300 oC and the surfaces of the whole beads suggested that the surfaces had lower I/Si ratios.
Backscattered electron images, elemental maps and spot EDX analyses of anion-loaded resin heated to 330 oC in vacuum are shown in Figure 5. Once again the iodine distribution appeared relatively inhomogenous with beads having darker contrast being deficient in iodine and enriched in chlorine compared to those in lighter contrast as for the beads heated to 250 o
C. Thus, there are differences in composition from one bead to another and in many cases
within the same bead. Spot analyses confirmed the inhomogeneous distributions of these two elements in these sections (see supplementary materials section).
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Although residual I and Cl could still be detected by EDX in the sections of beads heated to 350 oC, the beads heated to 400 oC possessed no detectable iodine.
Our data confirms the results of Gay et al. [26] and suggests that heating in nitrogen to around 300 oC probably represents an upper limit for retention of a significant proportion of the initial iodine inventory under pyrolysis conditions. It is notable that even at temperatures as low as 330 oC the distribution of Cl was also relatively inhomogeneous compared with that of iodine.
3.4. Radiochemical Analysis of 14C Evolution Because it is well known that 14C in spent anionic resins is mainly found in inorganic form as CO32- or HCO3- [16] the resins were loaded with carbonate labelled with 14C tracer. Figure 6 shows the evolution of 14CO2 from the resin as a function of pyrolysis temperature. These results are broadly consistent with the results of gas MS that was also used to follow the evolution of the natural isotopes of the carbon dioxide (Figure S4).
The two experiments agree within experimental uncertainty and indicate that carbon dioxide sorbed as carbonate or bicarbonate on the anionic resin is expelled concomitantly with the decomposition of functional groups which commences at around 200 oC.
This result did not appear to agree with the observation of Speranzini and Buckley [20]. This could be due to the fact that these investigators did not heat their resins in nitrogen but rather in air, conditions that might have favored the fixation of the 14C tracer on the resin matrix irreversibly. They also did not report results below 200 oC a temperature we have now
13 observed to be sufficient to expel 99% of the 14C label. During our experiments, remnant 14C label was almost undetectable at temperature in excess of 400 oC or higher.
3.5. Capacity for Rehydration To study the propensity of the pyrolyzed resins for rehydration, samples were taken to various final temperatures on the thermal analysis system. Immediately following the heating step (arrows in Figure 7a) humidified air (75% RH) was introduced over the sample at a constant flow rate of about 20 mL/min and the weight change monitored over time.
The data of Figure 7a shows that the beads dried in flowing nitrogen at 175 oC sorb water at a rate of about 0.03 wt% per minute. At the end of a 250 minute period it would seem that a plateau is reached suggesting that although reabsorption of water has been severely reduced some water reabsorption could occur over the long term. For beads heated up to 300 oC the initial water adsorption rate is of the order of 0.006 wt% per minute and once again it appears that a plateau is reached. This water is most likely to be weakly physisorbed on bead surfaces. These data suggest that the beads become irreversibly dehydrated between 200 and 300 oC.
To investigate the ability of the thermally treated resins to re-adsorb water on complete immersion in bulk water over extended time frames the anion-loaded resin were heated in the thermobalance under flowing nitrogen at different temperatures and then they were immersed in deionized water for a period of several weeks. Following immersion in water, they were dried in air and the water content was estimated by heating the samples and determining the weight loss. Figure 7b shows that the samples treated at 250 and 275 oC, exhibit a very low weight loss suggesting little water had been taken up. The results of these resorption
14 experiments therefore indicate that it would be necessary to pyrolyze anionic resins beyond 250 oC in order to prevent water resorption.
3.6. Aqueous Leaching of Iodine
Low temperature pyrolysis of anionic resins containing radioisotopes such as 129I and 35Cl would have to render materials of sufficient stability that the release of these isotopes to the environment would be inhibited over long time frames. Thus it was of interest to gain an appreciation of the leachability of iodine in pure water from the most heavily anion loaded sample as a function of pyrolysis temperature. The initial iodine loading of the beads heated at various temperatures were determined by NAA and confirmed by TXRF. The loading of the beads at each pyrolysis temperature is shown in Table 1.
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From this table it is evident that as temperature was increased the loading of iodine in mmol/g initially increased due the expulsion of water and other organics and then decreased at 300 oC due to the volatilization of iodine as expected. In line with observations detailed in previous sections the remnant iodine content above 300 oC was particularly low. However, between 200 and 300 oC a large proportion of the initial iodine inventory remained in the beads.
Leaching of each pyrolyzed samples was then conducted in ultrapure water at ambient temperatures over a period of 120 hours.
As can be seen from Figure 8 leaching was lowest for the sample that had not been pyrolyzed. For resin pyrolyzed at 200 oC, where the functional group is being eliminated, the fractional iodine loss increased and then attained a steady state at 0.20. As the temperature increased to 300 oC the fractional loss initially increased slightly and then appeared to reduce over longer time frames, once again attaining a steady state close to detection limits. For beads heated to 400 oC, it can be seen that almost the entirety of the remaining iodine inventory was removed within the first 10 hours. However, in this case the data might be considered relatively unreliable given that the major part of the initial iodine inventory was volatilized leaving very small quantities of iodine in the pyropolymer product and hence in the leachates. In other words since the amount of iodine remaining in the beads after pyrolysis at 400 oC was extremely small, and the amount released to solution was also small, quantification of the released fraction was difficult and large uncertainties resulted.
Nevertheless, this experiment allows us to conclude that at least in ultra-pure water a large part of the iodine inventory can be retained provided the pyrolysis temperature is maintained at or below 300 oC. Considering that in this work we are dealing with anionic resin beads
16 loaded with macroscopic quantities of iodine and other anions many orders of magnitude beyond what would be expected in actual radioactive resins, the fractional leaching might be expected to be reduced as the loading level is reduced in a manner similar to what was observed in cationic beads [13].
4. Discussion In our previous work on the pyrolysis of cation exchange resins where volatility was not a consideration at least for pyrolysis temperatures below 600 oC, we observed very low leachability of all divalent cations in aqueous solutions and release of only a small fraction of the Cs inventory [12]. In subsequent work, the Cs leachability was observed to decrease dramatically to below detection limits as the initial Cs loading was reduced [13].
Anion exchange resins on the other hand present their own set of difficulties due to the high volatility of the radioisotopes involved, even if treatment temperatures are modest. In the present study we have observed that at high initial anion loadings most of the C-14 inventory is expelled when the temperature exceeds 200 oC. It is well known that most of this C-14 is concentrated on anion exchange resins and is present primarily in inorganic form as CO32and/or HCO3- [16,27,28].
In the present study iodine shows enhanced stability compared with 14C and Cl with almost the entire inventory being retained up to at least 300 oC. This result is consistent with very early work by Gay et al. [26] where only about 0.25% of the initial iodine inventory was released from iodine-saturated AmberliteTM anion exchange resins heated at 180 oC in air whereas close to 1% of the initial inventory was expelled by 265 oC. The consistency of results is despite the fact that in the Gay et al. study the heating was performed in air whereas
17 in the present study pyrolysis was conducted in nitrogen. However, the loss of even small part of the radioactive inventory to the off-gas stream during thermal process would not eliminate the need for the post pyrolysis capture of these radioisotopes. This could be achieved using methods and sorbents that are in widespread use in industry for CO2 and I2 capture. However, reduced radioisotope inventory in the flue gas, and hence reduced activity, would make offgas treatment less onerous.
The present study has shown that it is precisely in the temperature range 200 – 300 oC where irreversible dehydration and loss of the functional group occur. Pyrolysis processes that have been considered to date are typically carried out at 550 oC and above [29]. At such temperatures total destruction of the PSdVB occurs and a metal oxide ash remains. In the present work we have shown that temperatures as low as 300 oC are sufficient to irreversibly dehydrate the anion exchange resin while at the same time retaining much of the iodine. This generates a highly durable host material for radionuclides. However, carbonate and bicarbonate as well as chloride are expelled at this temperature. Our previous study of cation exchange resins showed that these were also irreversibly dehydrated in this temperature range and that stable products resulted [13]. Considering that the resin beads are normally deployed as mixed beds, and that fractionation into anionic and cationic components would imply an additional processing step, it would seem preferable to undertake pyrolysis of the entire mass in one step. In this case the upper temperature limit of a pyrolytic, or other thermal processes aiming to retain the major part of the radioisotope inventory, would be 300 oC, a limit that would be entirely determined by the anion exchange resins since no metal species are volatilized from cation exchange resins at such low temperatures.
18 We have observed that converting resin beads pyrolyzed at low temperatures (200 – 400 oC) to monoliths using a minimum of Epoxy resin so as to fill void space between the beads while at the same time reducing freeboard to a minimum results in monoliths with very good mechanical integrity that are unleachable even after long periods of immersion. Low temperature pyrolysis to generate irreversibly dehydrated pyropolymers followed by their encapsulation in a suitable polymer would eliminate dispensability and the generation radiolysis and gases as problems.
Thermal processes that generate a reduced volume of waste in a conditioned form would be considered superior to producing dispersable oxide-rich ash products at high temperatures. Pebble bed pyrolysis that can be applied to spent resins [14,30] and solvents [31] to produce materials that are subsequently bound in an inorganic or organic matrices would have significant advantages as would hot supercompaction in which a containerized consolidated waste form is generated directly in a simple operation [32,33]. Pebble bed pyrolysis is typically carried out at 500 oC while hot super compaction is carried out below 100 oC. Both processes are used on low activity resins in countries where volume reduction is paramount. However, hot supercompaction can have drawbacks such as a spring-back effect due to the resin elasticity.
Considering three options indicated in Figure S5 (SM section), our data suggests that option 2 in which low temperature pyrolysis is followed by immobilization in Epoxy, or other polymeric resin, is the most viable (see flow chart below). If only sufficient polymer is added so as to fill inter-bead voids and introduce a small freeboard, then the volume gain would be minimal. With respect to option 1 (Figure S5) that involves direct immobilization, option 2 has the advantage of an approximate two-fold volume reduction and enhanced waste product
19 stability resulting from the elimination of all water. The expulsion of C-14 in the form of CO2 below 300 oC necessarily implies a need to capture and store this C-14 and this could be viewed as a disadvantaged of the proposed low-temperature process. However, expulsion of 14
CO2 could also be viewed as an advantage in that the activity of the bulk of the remaining
mass of pyropolymer can be lowered significantly. Moreover, because the maximum proposed processing temperature is below the third weight loss observed in the TGA at about 400 oC where the polystyrene divinyl benzene backbone is destroyed, most of the CO2 evolved is expected to be from the carbonate and bicarbonate anions. In the present proposed process the evolved gases would first be destroyed at by an induction plasma system.
5. Conclusions The present study together with our previous work [13] suggest that low temperature pyrolysis of spent resins in the 200 to 300 oC temperature range to achieve an anhydrous and chemically stable pyropolymer followed by encapsulation in a thermosetting polymer such as Epoxy could represent a viable option for both radioactive anionic and cationic spent resins (option 2 of Figure S5). In contrast to cation exchange resins, the present study indicates that anion exchange resins treated at or above 300 oC would require some form of off-gas treatment system to destroy flue gases and capture primarily the small fraction of the 14CO2 inventory that would not be retained by the pyropolymer product at this temperature. The other major radioactive species in such resins, 129I, would be largely retained at such low temperature and was found to be relatively unleachable. Another major contributor to activity in such spent resins, tritium, would be emitted at temperatures below 200 oC and could be collected. A reduction in pyrolysis temperature would be beneficial in processing terms as well as simplifying equipment and reducing emissions.
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While our present and past studies on highly anion-loaded and cation-loaded resins offer guidance they are no substitute for careful experimentation with actual radioactive spent resins where the activity concentrations are many orders of magnitude lower. We have already observed in the case of cationic resins that cation leaching is to a large extent dependent on initial cation loading. There is no reason to suspect however, that the same situation would prevail with anionic resins where the contaminant anions are inherently more volatile and are not retained as strongly during the thermal transformations.
Acknowledgements The authors are grateful to Silvana Martín of the analytical facility, Centro Atómico Constituyentes, CNEA, for assistance with ICP-MS analyses. Funding for this research was exclusively provided by Comisión Nacional de Energía Atómica.
Supplementary Material Supplementary material including SEM images and other data associated with this article can be found in the online version at the following location:
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J. Wang and Z. Wan, Treatment and disposal of spent radioactive ion-exchange resins produced in the nuclear industry, Prog. Nucl. En. 78 (2015) 47-55. B. G. Place, Engineering Study for the Treatment of Spent Ion Exchange Resin Resulting From Nuclear Process Applications. 1990,WHC-EP-0375 J. F. Li and J. L. Wang, Advances in Cement Solidification Technology for Waste Radioactive Ion Exchange Resins: A Review, J. Haz. Mater. 135 (2006) 443-448. T.-H. Cheng, C.-P. Huang, Y.-H. Huang and Y.-J. Shih, Kinetic study and optimization of electro-Fenton process for dissolution and mineralization of ion exchange resins, Chem. Eng. J. 308 (2017) 954-962. T. L. Gunale, V. V. Mahajani, P. K. Wattal and C. Srinivas, Studies in liquid phase mineralization of cation exchange resin by a hybrid process of Fenton dissolution followed by wet oxidation, Chem. Eng. J. 148 (2009) 371-377. A. V. Bridgwater, Review of fast pyrolysis of biomass and product upgrading, Biomass Bioenergy 38 (2012) 68-94. A. Quek and R. Balasubramanian, Liquefaction of waste tires by pyrolysis for oil and chemicals - A review, J. Anal. Appl. Pyrolysis 101 (2013) 1-16. P. T. Williams, Pyrolysis of waste tyres: A review, Waste Manage. 33 (2013) 17141728. W. Kaminsky, C. Mennerich and Z. Zhang, Feedstock recycling of synthetic and natural rubber by pyrolysis in a fluidized bed, J. Anal. Appl. Pyrolysis 85 (2009) 334337. W. Kaminsky, Thermal recycling of polymers, J. Anal. Appl. Pyrolysis 8 (1985) 439448. W. Kaminsky and C. Mennerich, Pyrolysis of synthetic tire rubber in a fluidised-bed reactor to yield 1,3-butadiene, styrene and carbon black, J. Anal. Appl. Pyrolysis 5859 (2001) 803-811. V. Luca, H. L. Bianchi and A. C. Manzini, Cation immobilization in pyrolyzed simulated spent ion exchange resins, J. Nucl. Mater. 424 (2012) 1-11. F. Allevatto and V. Luca, Low temperature pyrolysis of simulated spent cation exchange resins: Leaching and microstructural changes as a function of cation loading, (2016) (submitted). S. Pettersson and G. Kemmler, Experience on resin pyrolysis, Waste Manage. Symp ., 2 (1984) 223-226 IAEA, Innovative Waste Treatment and Conditioning Technologies at Nuclear Power Plant , 2006,TECDOC 1504 S. D. Park, J. S. Kim, S. H. Han and K. Y. Jee, Distribution characteristics of 14C and 3H in spent resins from the Canada deuterium uranium-pressurized heavy water reactors (CANDU-PHWRs) of Korea, J. Radioanal. Nucl. Chem. 277 (2008) 503-511. I. Fairlie, Commentary: childhood cancer near nuclear power stations, Environ. Health 8 (2009) 1-12. NA/NRC, Analysis of cancer risks in populations near nuclear facilities: Phase I The National Academies Press, Washington DC, 2012. Stram D.O., Analysis of cancer risks in populations near nuclear facilities: Phase I. A report by the National Academies Nuclear and Radiation Studies Board., Health Phys. 106 (2014) 305-306. Speranzini R.A. and L.P. Buckley, Treatment of spent ion-exchange resins for disposal,
22
[21]
[22]
[23]
[24]
[25] [26] [27]
[28] [29] [30] [31] [32]
[33]
2. IAEA Research Coordination Meeting on Treatment of Spent Ion-Exchange Resins, Toronto, Ontario, Canada, (1981), 21-25. M. Neji, B. Bary, P. Le Bescop and N. Burlion, Swelling behavior of ion exchange resins incorporated in tri-calcium silicate cement matrix: I. Chemical analysis, J. Nucl. Mater. 467 (2015) 544-556. M. Neji, B. Bary, P. Le Bescop and N. Burlion, Swelling behavior of ion exchange resins incorporated in tri-calcium silicate cement matrix: II. Mechanical analysis, J. Nucl. Mater. 467 (2015) 863-875. J. Zheng, H. Takata, K. Tagami, T. Aono, K. Fujita and S. Uchida, Rapid determination of total iodine in Japanese coastal seawater using SF-ICP-MS, Microchem. J. 100 (2012) 42-47. M. A. Dubois, J. F. Dozol, C. Nicotra, J. Serose and C. Massiani, Pyrolysis and incineration of cationic and anionic ion-exchange resins Identification of volatile degradation compounds , J. Anal. Appl. Pyrolysis 31 (1995) 129-140. M. Matsuda, K. Funabashi, T. Nishi and M. Kikuchi, Decomposition of ion exchange resins by pyrolysis, Nucl. Tech. 75 (1986) 187-192. R.L. Gay, L.R. McCoy, K.M. Barclay, L.F. Grantham, Studies of the volatility of cesium and iodine, Waste Manage. Symp., Tucson, Arizona, 2 (1985) 357-361. Å. Magnusson, K. Stenström and P. O. Aronsson, 14C in spent ion-exchange resins and process water from nuclear reactors: A method for quantitative determination of organic and inorganic fractions, J. Radioanal. Nucl. Chem. 275 (2008) 261-273. M. S. Yim and F. Caron, Life Cycle and Management of Carbon-14 From Nuclear Power Generation, Prog. Nucl. Energy 48 (2006) 2-36. IAEA, Treatment of Spent Ion-Exchange Resins for Storage and Disposal, 1985, TRS 254. G. Brahler and R. Slametschka, Pyrolysis of spent ion exchange resins, Waste Manage. Conf., Phoenix, Arizona, USA, (2012) 12210. P. Luycx and J. Deckers, Pebble bed pyrolysis for the processing of alpha contaminated organic effluents, Waste Manage. Conf., Tucson, Arizona, (1999). J. Braet, D. Charpentier, B. Centner, S. Vanderperre, Radioactive spent ion-exchange resins conditioning by the hot supercompaction process at tihange NPP early experience, WasteManage. Conf., Phoenix, Arizona, (2012). H. Fehrmann, Hot resin supercompaction the development through the years, Atw. Inter. Zeitschrift Fuer Kernenergie 59 (2014) 228-234.
23
Figure Captions Figure 1. Thermogravimetric analyses in nitrogen of (a) as-received and (b) I- and Cl-loaded anion exchange beads. Figure 2. (a) ESI-MS analysis of the contents of the iodine traps from gas collected from the off-gas stream as a function of increasing pyrolysis temperatures and (b) iodine signal from Gas MS from the pyrolysis from anion-loaded resins.
Figure 3. Gas MS of the effluent of pyrolysis of chlorine related signals. (a) the plot shows signals form the natural isotopes of chlorine release as a function of temperature. 37 amu signal shows interference with pyrolysis products a high temperatures. (b) Signal from CH3Cl fragments as a function of temperature.
Figure 4. BSE image and elemental maps of anion-loaded beads heated in flowing nitrogen at 200 and 250 oC.
Figure 5. SEM images of cross-sections of beads heated in flowing nitrogen at 330 oC. Crosses indicate points at which spot EDX analysis were performed.
Figure 6. Determination of C-14 from bicarbonate-loaded anion exchange resins as a function of pyrolysis temperature.
Figure 7. (a) Mass change over time for anion-loaded beads heated at different temperatures and subsequently exposed to a constant flow of humidified air (75% RH). Vertical dashed lines mark the time points at which humidified air was introduced into the sample and (b)
24 weight loss of the samples immersed in water after heating at increasing temperature under flowing nitrogen.
Figure 8. Leached iodine fraction as a function of time for fully loaded anion exchange beads pyrolyzed at increasing temperatures.
25 100
100
a 415
56
80
Weight Loss (%)
Weight Loss (%)
80
b
60 210
40
20
0
410
330
60
305
40
52
218
20
0
100
200
300 400 500 o Temp. ( C)
600
700
800
0
0
100
200
300 400 500 o Temp. ( C)
600
700
800
Figure 1. Thermogravimetric analyses in nitrogen of (a) as-received and (b) I- and Cl-loaded anion exchange beads. Dashed curves are the derivatives calculated from the weight loss curves (solid lines).
Figure 2. (a) ESI-MS analysis of iodine signals (127 amu) from the contents of the iodine traps of gas collected from the off-gas stream as a function of increasing pyrolysis temperatures and (b) iodine signal from the direct MS analysis of the offgas stream from the pyrolysis of anion-loaded resins as a function of temperature.
26
a
b
Figure 3. Gas MS of the effluent of pyrolysis of chlorine related signals (a) the plot shows signals form the natural isotopes of chlorine release as a function of temperature. 37 amu signal shows interference with pyrolysis products a high temperatures. (b) Signal from CH3Cl fragments as a function of temperature.
a
c
Si
b
I
e
d
Cl
g
CI
f
I
h
Si
Figure 4. BSE image and elemental maps of anion-loaded beads heated in flowing nitrogen at 200 and 250 oC.
BSE 9 +
Si
1 +
8 + 2 + + 5+ +
I 10 +
7 + 6+
CI
27 Figure 5. SEM images of cross-sections of beads heated in flowing nitrogen at 330 oC. Crosses indicate points at which spot EDX analysis were performed.
Figure 6. Determination of C-14 from bicarbonate-loaded anion exchange resins as a function of pyrolysis temperature. 100
90
% Wt Change
0.029 %/min
a
o
175 C
b
80
70 o
60
300 C
0.0044 %/min
330 C
0.0056 %/min
50
40
0.0057 %/min
0
50
100
150
o
o
350 C
200 250 Time (min)
300
350
400
Figure 7. (a) Mass change over time for anion-loaded beads heated at different temperatures and subsequently exposed to a constant flow of humidified air (75% RH). Vertical dashed lines mark the time points at which humidified air was introduced into the sample and (b) weight loss of the samples immersed in water after heating at increasing temperature under flowing nitrogen.
28 1.4 25
Leached Fraction
1.2 1
200
0.8
300
0.6
400
0.4 0.2 0 0
10
20
30
40
50 60 70 Time (hours)
80
90
100 110 120
Figure 8. Leached iodine fraction as a function of time for fully loaded anion exchange beads pyrolyzed at increasing temperatures.
29 Flow chart 1 Resin
SijOm(OH)ni- - exchange pH ~ 11
I-, Cl- - exchange pH ~ 7
Resin Pyrolisis 25 – 600 oC
I Leaching Study
Gases MS Analysis
30 Flow chart 2 2H, 3H
Spent Resin Resin Dewatering
Recycle/Disposition
Low Temperature Pyrolysis
High Performance Plasma treatment or Post Combustion System
Polymer Encapsulation (Epoxy, phenolic resin…)
14C,129I…
Interim Storage/Repository
Radionuclide Capture in Selective Adsorbents
31 Table 1. Loading of the anion resins with iodine determined by NAA.
Pyrolysis
Loading
Temperature
(mmol/g)
25
0.1610
200
0.1870
300
0.0534
400
0.0033
S2213-3437(17)30368-8 http://dx.doi.org/doi:10.1016/j.jece.2017.07.064 JECE 1779
To appear in: Received date: Revised date: Accepted date:
20-3-2017 24-7-2017 26-7-2017
Please cite this article as: Vittorio Luca, Hugo Luis Bianchi, Florencia Allevatto, Jorge O.Vaccaro, Agustin Alvarado, Low temperature pyrolysis of simulated spent anion exchange resins, Journal of Environmental Chemical Engineeringhttp://dx.doi.org/10.1016/j.jece.2017.07.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Low Temperature Pyrolysis of Simulated Spent Anion Exchange Resins
Vittorio Luca 1*, Hugo Luis Bianchi 2,4, Florencia Allevatto 1, Jorge O. Vaccaro 3, Agustin Alvarado 3
1
Programa Nacional de Gestión de Residuos Radiactivos, Comisión Nacional de Energía
Atómica, Centro Atómico Constituyentes, Av. General, Paz 1499, 1650 San Martin, Provincia de Buenos Aires, República Argentina. 2
Gerencia de Química, Centro Atómico Constituyentes, Comisión Nacional de Energía
Atómica, Av. General, Paz 1499, 1650 San Martin, Provincia de Buenos Aires, República Argentina. 3
Centro Atómico Ezeiza, Comisión Nacional de Energía Atómica, Presbitero Juan González
y Aragón N° 15, 1804 Ezeiza, Provincia de Buenos Aires, República Argentina 4
Escuela de Ciencia y Tecnología, Universidad Nacional de General San Martín.
* Corresponding Author e-mail: [email protected], Tel. 54-11-6772 7018
2
Abstract
Studies of the low temperature pyrolysis of simulated spent cation exchange resins have indicated that this process offers several advantages relative to more traditional approaches involving direct immobilization. However, little is known concerning the fate of long-lived radioisotopes that are typically concentrated in anion exchange resins. In this work anion exchange resins have been loaded with macroscopic quantities of I-, Cl- and CO32- spiked with 14
CO32- with the aim of simulating the spent radioactive anion exchange resins. The anion-
loaded resins were pyrolyzed at increasing temperatures up to 500 oC and the volatilization of 14
CO32-, I- and Cl- was monitored and quantified using radiotracer and mass spectrometric
methods. A significant fraction of the initial 14C and Cl- inventories were volatilized at temperatures in the range 200 – 300 oC. However, mass spectrometry of the gaseous releases indicated that at 300 oC most of the iodine remained within the pyropolymer product. This was confirmed through SEM-based energy dispersive X-ray analysis of the solid products. In the same temperature range, experiments indicated that water was expelled non-reversibly. Leaching experiments were conducted on the anion-loaded pyropolymer products generated at different temperatures in order to determine the quantity of iodide released to solution. For resin pyrolyzed below 400 oC the fractional iodine release to solution did not exceed 0.10 indicating that the major part of the iodine inventory in the pyropolymers was not leachable. The results indicate that low temperature pyrolysis could be used to convert spent anionic and cationic resins to stable pyropolymer products in which a large proportion of the initial radioisotope inventory could be rendered stable. Keywords: Spent resin; nuclear waste; radioactive waste; pyrolysis; immobilization; carbon14.
3
1. Introduction Polymeric ion exchange resins are widely used in the nuclear power plants (NPPs) for the purification of water in the cooling system circuits of reactors. These resins are usually based on sulfonate- and quaternary amine-functionalized polystyrene divinyl benzene (PSdVB) copolymers, and once exhausted, represent the largest volume of radioactive waste that is generated during the operation of these plants. Many treatment and conditioning strategies have been investigated for this type of radiotoxic waste [1,2].
Typical strategies dealing with spent resins include direct immobilization in cements [3] or polymeric resins which can result in an expansion of waste volume by a factor of two to five or treatment using thermal or chemical processes that reduce waste volume. Such processes include incineration, pyrolysis, and chemical attack [4,5].
Pyrolysis represents an attractive alternative to incineration for the destruction of municipal organic wastes including biomass [6], waste plastics [7], and tyres [8-11]. The advantage of pyrolysis in these applications is that it is a low temperature, flameless process that can achieve significant volume reduction and at the same time yield valuable products such as fuel oils and biochars.
Because of many of the these same advantages, pyrolysis is also being considered, and even practiced, as a method for the treatment and conditioning of radioactive organic wastes such as solvents, oils and especially spent ion exchange resins that derive from the operation of NPPs. Therefore pyrolysis can potentially be used to treat multiple waste streams. A final potential advantage of pyrolysis is that it appears possible to retain a large part of the radio
4 isotope inventory in a stable waste product thus reducing the generation of secondary waste products [12,13].
To treat spent resins industrial scale pyrolysis-based processes have been developed by the likes of Nukem and Belgoprocess [14]. Being a flameless technology, pyrolysis has the advantage of employing lower operational temperatures, and of critical importance in a nuclear context, it has an enhanced safety profile. Thus, pyrolysis features as an innovative technology in a recent IAEA report on the thermal processing of radioactive wastes [15].
Pyrolysis is a possible means of reducing the volume and of stabilizing polystyrene-based polymeric spent radioactive ion exchange resins by conversion to stable pyropolymers or carbons that contain the greater part of the initial radio nuclide inventory. By retaining most of the radionuclide inventory within the pyrolysis products, the problem of dealing with secondary radiotoxic waste generation (especially gaseous waste) could be greatly mitigated, or potentially even eliminated.
In a previous study we attempted to shed light on the structure and microstructure of the waste products produced and quantify the leaching of Cs+, Sr2+ Co2+ and Ni2+ from cation exchange resins fully loaded with these cations after pyrolysis at temperatures in the range 200 - 600 oC [12,13]. 137Cs and 90Sr are fission products and 60Co and 59Ni activation products typically found in radioactive spent cation exchange resins [16]. Subsequently we have addressed similar questions only for a series of cation exchange resins with decreasing cation loading in order to better approximate real radioactive spent cationic resins where the cation loadings are extremely low [13].
5 In nuclear power plants however, cation and anion exchange resins are typically deployed as mixed beds and often in a 1:1 ratio. Anion exchange resins concentrate radionuclides such as 3
3
7
H (T1/2 = 12.33 years), 14C (T1/2 = 5.73x10 years), 129I (T1/2 = 1.57x10 years), 36Cl (T1/2 = 5
5
3.01x10 years), and 99Tc (T1/2 = 2.111x10 years) adsorbed in the form of simple and more complex oxo-anions. In this regard anion exchange resins might be considered even more problematic than their cation exchange resin counterparts. Aside from their high volatility these anionic species are extremely water soluble, are particularly long-lived and are biologically compatible. Although some 14C is released from NPPs during the normal course of operations, the bulk of the inventory is retained on the anion exchange resins in inorganic form as a carbonate and/or bicarbonate [16]. There has been considerable concern about the health impacts of gaseous reactor emissions and studies that have been undertaken have been quite controversial [17]. As a result the US Nuclear Regulatory Commission is sponsoring future detailed studies and analyses [18,19]. Therefore treating spent resins in a manner that releases this stored 14C into the environment would not be the optimum practice in a tightening regulatory regime.
In a very early Atomic Energy of Canada Limited report [20] on CANada Deuterium Uranium (CANDU) reactor resins doped with 14C it was observed that during heating of the resins in air only a relatively small fraction of the initial 14C inventory (<10%) was released at temperatures below 400 oC. At temperatures above 700 oC up to 80% of the inventory could be released. If however the resins were pretreated with CaCl2 solutions, significantly higher proportions of the inventory could be released at all temperatures. Little further work has been undertaken on this issue or the issue or resin rehydration that can result in enormous swelling pressures and fracturing of an immobilizing matrix [21,22].
6 The objectives of the present study were to understand the behaviour of volatile anionic species during the pyrolysis of simulated anion exchange resins as well as to shed light on their chemical speciation and stability within the resultant pyropolymer waste products. The present strategy depends on carrying out the pyrolysis of spent resins at a sufficiently low temperature as to keep most of the most radiotoxic radionuclides within the pyropolymer waste product. This is likely to be the case for cation exchange resins where metal species are most certainly retained at low temperatures (< 400 oC) but unlikely to be so for anion exchange resins where liberation of anionic species as gases is to be expected. If such species are liberated during low temperature processing, then they will need to be efficiently trapped and managed appropriately.
2. Material and Methods The anion exchange beads used throughout this study were Lewatit Monoplus M500 respectively. These are cross-linked polystyrene gel beads with quaternary amine functional groups. A flow chart of the preparation procedure is shown below. Briefly, the anion exchange resin beads were first exchanged with an alkaline silicate solution to incorporate anionic Si species as a non-volatile internal standard. Subsequently, these Si-exchanged beads were treated with Cl- and I--containing solutions at near neutral pH in order to saturate the ion exchange sites with these anions. Pyrolysis was undertaken in an inert gas flow either within a conventional tubular oven or using the thermal analysis system.
Leaching studies were conducted in a manner similar to the AS16.1 standard protocol. This involved immersion of 0.10 g of the whole resin beads in 10.0 mL volumes of milli-Q water. Following a given time interval the supernatant leachant was removed by decantation. In
7 order to prevent oxidation and volatilization of iodide an aliquot of the leachant solution was added to a 0.1% tetramethyl ammonium hydroxide solution (TMAH) in accordance with recent recommendations [23]. Using the same method, standard solutions (Chem-Lab NV) were prepared with concentrations 0.8, 2, 5, 10, 15 and 20 ng/mL. All solutions were analyzed for the 127I isotope using a Perkin-Elmer NexION 300X ICP-MS instrument using standard procedures. An RF power of 1250 W was used and the nebulizer, auxiliary and plasma gas flows were set to 0.85, 1.2 and 16.0 L/min respectively.
Secondary electron (SE) and back scattered electron (BSE) microscope images together with energy dispersive (EDS) X-ray analyses were obtained on an FEI Inspect F50 microscope.
Thermal analysis was conducted in both flowing air and nitrogen using on a TA Instruments Q600 system. Quantification of iodine present in the off-gas stream of the Q600 thermobalance was undertaken by capturing the off-gas in water traps and then analyzing the resultant solution using an Electrospray Ionization Mass Spectrometry (ESI-MS).
A gas mass spectrometer (OmniStar, Pfeiffer) attached to the Q600 was used to follow the behavior of iodine, chlorine, and carbon dioxide during the thermal treatment of the resin. Anionic resin fully loaded with NaHCO3 was labelled with 14C. The resin was washed and stored under water. Dewatered samples were weighed and heated in a 13 mm diameter quartz tube furnace under flowing nitrogen. A PID temperature controller was used to achieve a temperature ramp of 5 oC/min. A calibrated thermocouple was positioned near the sample to read a precise temperature during the thermal treatment.
8 Quantification of 14C release as a function of heating temperature was performed by low activity liquid scintillation analysis (TRI-CARB 3100TR), using the Ultima Gold AB cocktail. Flowing nitrogen gas was used for quantitative transfer of all the carbon dioxide released from the resin during heating and collected in traps containing NaOH solution and glass spheres. To the contents of the traps was added the scintillation cocktail and the full 14C inventory measured.
Leaching of the anion-loaded samples was undertaken in deionized water at 25 oC. Samples of the resins (0.050 g) treated at each temperature were weighed into six polyethyelene bottles and 10.0 mL of deionized water added to each bottle. The bottles were agitated periodically. At determined time intervals, aliquots of liquid were removed from each of the bottles and analyzed for I by ICP-MS.
3. Results 3.1. Thermal Analysis Figure 1 shows TGA traces of as-received and I-- and Cl--loaded anion exchange resin beads measured in flowing nitrogen. Three clear steps are observed in the TGA trace of the assupplied material (Figure 1a) with derivative peaks observed at around 56, 210 and 415 oC. In studying the anion exchange resin, Duolite A 101, Dubois [24] reported two mass losses at around 200 and 400 oC. The first weight loss was ascribed primarily to the loss of the functional amine group with some PSdVB backbone degradation and the second primarily to the degradation of the PSdVB backbone to CO2 and other organics respectively. The reason that no weight loss was observed at about 60 oC was presumably due to the fact that those resins were initially relatively dry.
9 For the present anion exchange resin in both the as-received and anion-loaded forms the weight loss between 25 and 150 oC as defined by the derivative peak at 56 oC can clearly be ascribed to dehydration. Between temperatures of 150 and 500 oC the two resins show markedly different behavior as is evident from shape of the weight loss curve and the change in number of derivative peaks. Whereas both resins show a weight loss defined by derivative peaks around 210 and 220 oC, the anion-loaded resin possess additional derivative peaks at 305 and 330 oC that do not exist in the as-supplied resin. It can be inferred therefore that these two additional mass loss events in the anion-loaded resin are due to the anions that were introduced, namely iodide and perhaps chloride. Anionic resins show overall lower thermal stability compared with cationic resin. Previous studies [25] have indicated that the weight loss at around 200 oC results from the loss of decomposition products associated with the quaternary ammonium functional groups of the resin. The ash contents for the two resins at 800 oC are 7 and 9 % respectively. The slightly higher yield in the case of the loaded material is probably associated with the Si that was included in order to act as an internal standard.
3.2. Iodine and Chlorine Volatility Although loss of I- and possibly Cl- between 300 and 350 oC can be inferred from the TGA, it does not permit the definitive identification of the species evolved. To identify definitively the products being evolved as a function of temperature, the products released to the off-gas stream were measured using ESI-MS of the gaseous effluents dissolved in water and through direct mass spectrometric analysis of the evolved gas.
The plot of iodine release as a function of pyrolysis temperature as measured by ESI-MS indicates that appreciable iodine is being released only as the pyrolysis temperature exceeded
10 about 300 oC (Figure 2). This is consistent with what was inferred from the TGA data. Thus, 300 oC represents the upper limit of temperature at which the major part of the iodine inventory could be retained on the anionic resins under pyrolysis conditions.
This result was entirely consistent with the direct MS analysis of evolved gas (Figure 2b). The analysis of the complex mixture of pyrolysis gases suggested that CH3I may form during pyrolysis of the anionic resin (see supplementary material Figure S1). Thus the experimental data from ESI-MS and gas MS detection of iodine are in broad agreement within experimental uncertainty with I (mass fragment 127) being detected above 300 oC.
Analysis of the chlorine behavior during the thermal treatment was similarly performed. Figure 3 shows that both natural isotopes of chlorine (35Cl and 37Cl) are released as function of temperature during the thermal ramp of the anion-loaded resin. The release of chlorine commenced at approximately at 220 oC, a much lower temperature than for iodine release as suggested from the observation of mass fragments at 50 and 52 corresponding to CH3Cl.
3.3. Microstructural Analysis of Anion Loaded Beads In order to monitor the iodine distribution and volatility using SEM, Si was introduced by ion exchange from alkaline silicate solution so as to act as an internal reference since Si is not expected to be volatile. SEM studies in combination with EDX analyses were conducted on samples heated in N2 flow to temperatures in the range 175 - 600 oC and the I/Si mole ratio monitored in bead cross-sections.
11 Low magnification SEM images (Figure S2) of sections of beads that were heated in flowing nitrogen at 175, 330, and 350 oC I/Si ratios of 1.59, 0.92 and 0.30 respectively suggesting a reduction in the iodine content with increasing temperature relative to the non-volatile Si. This variation in iodine content was roughly consistent with the curve of gaseous iodine release in Figure 2. Also consistent with this data was the fact that no iodine could be detected by EDX in anion-loaded resins heated beyond 400 oC.
Elemental maps of sectioned and polished samples heated to 200 oC in nitrogen indicated the distribution of iodine, chlorine and silicon within the surface layer to be relatively uniform (Figure 4a-d). For samples heated at 250 oC some surface cracking was apparent and the distribution of iodine appeared to be slightly less homogenous (Figure 6e-h).
Heating to 300 oC (Figure S3) did not provoke a significant change in the microstructure of the beads viewed in section with both iodine and chlorine detected by EDX: The I/Si ratios that were measured were somewhat reduced compared to beads heated to lower temperatures. Repeated EDX analysis of sections of the beads heated to 300 oC and the surfaces of the whole beads suggested that the surfaces had lower I/Si ratios.
Backscattered electron images, elemental maps and spot EDX analyses of anion-loaded resin heated to 330 oC in vacuum are shown in Figure 5. Once again the iodine distribution appeared relatively inhomogenous with beads having darker contrast being deficient in iodine and enriched in chlorine compared to those in lighter contrast as for the beads heated to 250 o
C. Thus, there are differences in composition from one bead to another and in many cases
within the same bead. Spot analyses confirmed the inhomogeneous distributions of these two elements in these sections (see supplementary materials section).
12
Although residual I and Cl could still be detected by EDX in the sections of beads heated to 350 oC, the beads heated to 400 oC possessed no detectable iodine.
Our data confirms the results of Gay et al. [26] and suggests that heating in nitrogen to around 300 oC probably represents an upper limit for retention of a significant proportion of the initial iodine inventory under pyrolysis conditions. It is notable that even at temperatures as low as 330 oC the distribution of Cl was also relatively inhomogeneous compared with that of iodine.
3.4. Radiochemical Analysis of 14C Evolution Because it is well known that 14C in spent anionic resins is mainly found in inorganic form as CO32- or HCO3- [16] the resins were loaded with carbonate labelled with 14C tracer. Figure 6 shows the evolution of 14CO2 from the resin as a function of pyrolysis temperature. These results are broadly consistent with the results of gas MS that was also used to follow the evolution of the natural isotopes of the carbon dioxide (Figure S4).
The two experiments agree within experimental uncertainty and indicate that carbon dioxide sorbed as carbonate or bicarbonate on the anionic resin is expelled concomitantly with the decomposition of functional groups which commences at around 200 oC.
This result did not appear to agree with the observation of Speranzini and Buckley [20]. This could be due to the fact that these investigators did not heat their resins in nitrogen but rather in air, conditions that might have favored the fixation of the 14C tracer on the resin matrix irreversibly. They also did not report results below 200 oC a temperature we have now
13 observed to be sufficient to expel 99% of the 14C label. During our experiments, remnant 14C label was almost undetectable at temperature in excess of 400 oC or higher.
3.5. Capacity for Rehydration To study the propensity of the pyrolyzed resins for rehydration, samples were taken to various final temperatures on the thermal analysis system. Immediately following the heating step (arrows in Figure 7a) humidified air (75% RH) was introduced over the sample at a constant flow rate of about 20 mL/min and the weight change monitored over time.
The data of Figure 7a shows that the beads dried in flowing nitrogen at 175 oC sorb water at a rate of about 0.03 wt% per minute. At the end of a 250 minute period it would seem that a plateau is reached suggesting that although reabsorption of water has been severely reduced some water reabsorption could occur over the long term. For beads heated up to 300 oC the initial water adsorption rate is of the order of 0.006 wt% per minute and once again it appears that a plateau is reached. This water is most likely to be weakly physisorbed on bead surfaces. These data suggest that the beads become irreversibly dehydrated between 200 and 300 oC.
To investigate the ability of the thermally treated resins to re-adsorb water on complete immersion in bulk water over extended time frames the anion-loaded resin were heated in the thermobalance under flowing nitrogen at different temperatures and then they were immersed in deionized water for a period of several weeks. Following immersion in water, they were dried in air and the water content was estimated by heating the samples and determining the weight loss. Figure 7b shows that the samples treated at 250 and 275 oC, exhibit a very low weight loss suggesting little water had been taken up. The results of these resorption
14 experiments therefore indicate that it would be necessary to pyrolyze anionic resins beyond 250 oC in order to prevent water resorption.
3.6. Aqueous Leaching of Iodine
Low temperature pyrolysis of anionic resins containing radioisotopes such as 129I and 35Cl would have to render materials of sufficient stability that the release of these isotopes to the environment would be inhibited over long time frames. Thus it was of interest to gain an appreciation of the leachability of iodine in pure water from the most heavily anion loaded sample as a function of pyrolysis temperature. The initial iodine loading of the beads heated at various temperatures were determined by NAA and confirmed by TXRF. The loading of the beads at each pyrolysis temperature is shown in Table 1.
15
From this table it is evident that as temperature was increased the loading of iodine in mmol/g initially increased due the expulsion of water and other organics and then decreased at 300 oC due to the volatilization of iodine as expected. In line with observations detailed in previous sections the remnant iodine content above 300 oC was particularly low. However, between 200 and 300 oC a large proportion of the initial iodine inventory remained in the beads.
Leaching of each pyrolyzed samples was then conducted in ultrapure water at ambient temperatures over a period of 120 hours.
As can be seen from Figure 8 leaching was lowest for the sample that had not been pyrolyzed. For resin pyrolyzed at 200 oC, where the functional group is being eliminated, the fractional iodine loss increased and then attained a steady state at 0.20. As the temperature increased to 300 oC the fractional loss initially increased slightly and then appeared to reduce over longer time frames, once again attaining a steady state close to detection limits. For beads heated to 400 oC, it can be seen that almost the entirety of the remaining iodine inventory was removed within the first 10 hours. However, in this case the data might be considered relatively unreliable given that the major part of the initial iodine inventory was volatilized leaving very small quantities of iodine in the pyropolymer product and hence in the leachates. In other words since the amount of iodine remaining in the beads after pyrolysis at 400 oC was extremely small, and the amount released to solution was also small, quantification of the released fraction was difficult and large uncertainties resulted.
Nevertheless, this experiment allows us to conclude that at least in ultra-pure water a large part of the iodine inventory can be retained provided the pyrolysis temperature is maintained at or below 300 oC. Considering that in this work we are dealing with anionic resin beads
16 loaded with macroscopic quantities of iodine and other anions many orders of magnitude beyond what would be expected in actual radioactive resins, the fractional leaching might be expected to be reduced as the loading level is reduced in a manner similar to what was observed in cationic beads [13].
4. Discussion In our previous work on the pyrolysis of cation exchange resins where volatility was not a consideration at least for pyrolysis temperatures below 600 oC, we observed very low leachability of all divalent cations in aqueous solutions and release of only a small fraction of the Cs inventory [12]. In subsequent work, the Cs leachability was observed to decrease dramatically to below detection limits as the initial Cs loading was reduced [13].
Anion exchange resins on the other hand present their own set of difficulties due to the high volatility of the radioisotopes involved, even if treatment temperatures are modest. In the present study we have observed that at high initial anion loadings most of the C-14 inventory is expelled when the temperature exceeds 200 oC. It is well known that most of this C-14 is concentrated on anion exchange resins and is present primarily in inorganic form as CO32and/or HCO3- [16,27,28].
In the present study iodine shows enhanced stability compared with 14C and Cl with almost the entire inventory being retained up to at least 300 oC. This result is consistent with very early work by Gay et al. [26] where only about 0.25% of the initial iodine inventory was released from iodine-saturated AmberliteTM anion exchange resins heated at 180 oC in air whereas close to 1% of the initial inventory was expelled by 265 oC. The consistency of results is despite the fact that in the Gay et al. study the heating was performed in air whereas
17 in the present study pyrolysis was conducted in nitrogen. However, the loss of even small part of the radioactive inventory to the off-gas stream during thermal process would not eliminate the need for the post pyrolysis capture of these radioisotopes. This could be achieved using methods and sorbents that are in widespread use in industry for CO2 and I2 capture. However, reduced radioisotope inventory in the flue gas, and hence reduced activity, would make offgas treatment less onerous.
The present study has shown that it is precisely in the temperature range 200 – 300 oC where irreversible dehydration and loss of the functional group occur. Pyrolysis processes that have been considered to date are typically carried out at 550 oC and above [29]. At such temperatures total destruction of the PSdVB occurs and a metal oxide ash remains. In the present work we have shown that temperatures as low as 300 oC are sufficient to irreversibly dehydrate the anion exchange resin while at the same time retaining much of the iodine. This generates a highly durable host material for radionuclides. However, carbonate and bicarbonate as well as chloride are expelled at this temperature. Our previous study of cation exchange resins showed that these were also irreversibly dehydrated in this temperature range and that stable products resulted [13]. Considering that the resin beads are normally deployed as mixed beds, and that fractionation into anionic and cationic components would imply an additional processing step, it would seem preferable to undertake pyrolysis of the entire mass in one step. In this case the upper temperature limit of a pyrolytic, or other thermal processes aiming to retain the major part of the radioisotope inventory, would be 300 oC, a limit that would be entirely determined by the anion exchange resins since no metal species are volatilized from cation exchange resins at such low temperatures.
18 We have observed that converting resin beads pyrolyzed at low temperatures (200 – 400 oC) to monoliths using a minimum of Epoxy resin so as to fill void space between the beads while at the same time reducing freeboard to a minimum results in monoliths with very good mechanical integrity that are unleachable even after long periods of immersion. Low temperature pyrolysis to generate irreversibly dehydrated pyropolymers followed by their encapsulation in a suitable polymer would eliminate dispensability and the generation radiolysis and gases as problems.
Thermal processes that generate a reduced volume of waste in a conditioned form would be considered superior to producing dispersable oxide-rich ash products at high temperatures. Pebble bed pyrolysis that can be applied to spent resins [14,30] and solvents [31] to produce materials that are subsequently bound in an inorganic or organic matrices would have significant advantages as would hot supercompaction in which a containerized consolidated waste form is generated directly in a simple operation [32,33]. Pebble bed pyrolysis is typically carried out at 500 oC while hot super compaction is carried out below 100 oC. Both processes are used on low activity resins in countries where volume reduction is paramount. However, hot supercompaction can have drawbacks such as a spring-back effect due to the resin elasticity.
Considering three options indicated in Figure S5 (SM section), our data suggests that option 2 in which low temperature pyrolysis is followed by immobilization in Epoxy, or other polymeric resin, is the most viable (see flow chart below). If only sufficient polymer is added so as to fill inter-bead voids and introduce a small freeboard, then the volume gain would be minimal. With respect to option 1 (Figure S5) that involves direct immobilization, option 2 has the advantage of an approximate two-fold volume reduction and enhanced waste product
19 stability resulting from the elimination of all water. The expulsion of C-14 in the form of CO2 below 300 oC necessarily implies a need to capture and store this C-14 and this could be viewed as a disadvantaged of the proposed low-temperature process. However, expulsion of 14
CO2 could also be viewed as an advantage in that the activity of the bulk of the remaining
mass of pyropolymer can be lowered significantly. Moreover, because the maximum proposed processing temperature is below the third weight loss observed in the TGA at about 400 oC where the polystyrene divinyl benzene backbone is destroyed, most of the CO2 evolved is expected to be from the carbonate and bicarbonate anions. In the present proposed process the evolved gases would first be destroyed at by an induction plasma system.
5. Conclusions The present study together with our previous work [13] suggest that low temperature pyrolysis of spent resins in the 200 to 300 oC temperature range to achieve an anhydrous and chemically stable pyropolymer followed by encapsulation in a thermosetting polymer such as Epoxy could represent a viable option for both radioactive anionic and cationic spent resins (option 2 of Figure S5). In contrast to cation exchange resins, the present study indicates that anion exchange resins treated at or above 300 oC would require some form of off-gas treatment system to destroy flue gases and capture primarily the small fraction of the 14CO2 inventory that would not be retained by the pyropolymer product at this temperature. The other major radioactive species in such resins, 129I, would be largely retained at such low temperature and was found to be relatively unleachable. Another major contributor to activity in such spent resins, tritium, would be emitted at temperatures below 200 oC and could be collected. A reduction in pyrolysis temperature would be beneficial in processing terms as well as simplifying equipment and reducing emissions.
20
While our present and past studies on highly anion-loaded and cation-loaded resins offer guidance they are no substitute for careful experimentation with actual radioactive spent resins where the activity concentrations are many orders of magnitude lower. We have already observed in the case of cationic resins that cation leaching is to a large extent dependent on initial cation loading. There is no reason to suspect however, that the same situation would prevail with anionic resins where the contaminant anions are inherently more volatile and are not retained as strongly during the thermal transformations.
Acknowledgements The authors are grateful to Silvana Martín of the analytical facility, Centro Atómico Constituyentes, CNEA, for assistance with ICP-MS analyses. Funding for this research was exclusively provided by Comisión Nacional de Energía Atómica.
Supplementary Material Supplementary material including SEM images and other data associated with this article can be found in the online version at the following location:
21
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23
Figure Captions Figure 1. Thermogravimetric analyses in nitrogen of (a) as-received and (b) I- and Cl-loaded anion exchange beads. Figure 2. (a) ESI-MS analysis of the contents of the iodine traps from gas collected from the off-gas stream as a function of increasing pyrolysis temperatures and (b) iodine signal from Gas MS from the pyrolysis from anion-loaded resins.
Figure 3. Gas MS of the effluent of pyrolysis of chlorine related signals. (a) the plot shows signals form the natural isotopes of chlorine release as a function of temperature. 37 amu signal shows interference with pyrolysis products a high temperatures. (b) Signal from CH3Cl fragments as a function of temperature.
Figure 4. BSE image and elemental maps of anion-loaded beads heated in flowing nitrogen at 200 and 250 oC.
Figure 5. SEM images of cross-sections of beads heated in flowing nitrogen at 330 oC. Crosses indicate points at which spot EDX analysis were performed.
Figure 6. Determination of C-14 from bicarbonate-loaded anion exchange resins as a function of pyrolysis temperature.
Figure 7. (a) Mass change over time for anion-loaded beads heated at different temperatures and subsequently exposed to a constant flow of humidified air (75% RH). Vertical dashed lines mark the time points at which humidified air was introduced into the sample and (b)
24 weight loss of the samples immersed in water after heating at increasing temperature under flowing nitrogen.
Figure 8. Leached iodine fraction as a function of time for fully loaded anion exchange beads pyrolyzed at increasing temperatures.
25 100
100
a 415
56
80
Weight Loss (%)
Weight Loss (%)
80
b
60 210
40
20
0
410
330
60
305
40
52
218
20
0
100
200
300 400 500 o Temp. ( C)
600
700
800
0
0
100
200
300 400 500 o Temp. ( C)
600
700
800
Figure 1. Thermogravimetric analyses in nitrogen of (a) as-received and (b) I- and Cl-loaded anion exchange beads. Dashed curves are the derivatives calculated from the weight loss curves (solid lines).
Figure 2. (a) ESI-MS analysis of iodine signals (127 amu) from the contents of the iodine traps of gas collected from the off-gas stream as a function of increasing pyrolysis temperatures and (b) iodine signal from the direct MS analysis of the offgas stream from the pyrolysis of anion-loaded resins as a function of temperature.
26
a
b
Figure 3. Gas MS of the effluent of pyrolysis of chlorine related signals (a) the plot shows signals form the natural isotopes of chlorine release as a function of temperature. 37 amu signal shows interference with pyrolysis products a high temperatures. (b) Signal from CH3Cl fragments as a function of temperature.
a
c
Si
b
I
e
d
Cl
g
CI
f
I
h
Si
Figure 4. BSE image and elemental maps of anion-loaded beads heated in flowing nitrogen at 200 and 250 oC.
BSE 9 +
Si
1 +
8 + 2 + + 5+ +
I 10 +
7 + 6+
CI
27 Figure 5. SEM images of cross-sections of beads heated in flowing nitrogen at 330 oC. Crosses indicate points at which spot EDX analysis were performed.
Figure 6. Determination of C-14 from bicarbonate-loaded anion exchange resins as a function of pyrolysis temperature. 100
90
% Wt Change
0.029 %/min
a
o
175 C
b
80
70 o
60
300 C
0.0044 %/min
330 C
0.0056 %/min
50
40
0.0057 %/min
0
50
100
150
o
o
350 C
200 250 Time (min)
300
350
400
Figure 7. (a) Mass change over time for anion-loaded beads heated at different temperatures and subsequently exposed to a constant flow of humidified air (75% RH). Vertical dashed lines mark the time points at which humidified air was introduced into the sample and (b) weight loss of the samples immersed in water after heating at increasing temperature under flowing nitrogen.
28 1.4 25
Leached Fraction
1.2 1
200
0.8
300
0.6
400
0.4 0.2 0 0
10
20
30
40
50 60 70 Time (hours)
80
90
100 110 120
Figure 8. Leached iodine fraction as a function of time for fully loaded anion exchange beads pyrolyzed at increasing temperatures.
29 Flow chart 1 Resin
SijOm(OH)ni- - exchange pH ~ 11
I-, Cl- - exchange pH ~ 7
Resin Pyrolisis 25 – 600 oC
I Leaching Study
Gases MS Analysis
30 Flow chart 2 2H, 3H
Spent Resin Resin Dewatering
Recycle/Disposition
Low Temperature Pyrolysis
High Performance Plasma treatment or Post Combustion System
Polymer Encapsulation (Epoxy, phenolic resin…)
14C,129I…
Interim Storage/Repository
Radionuclide Capture in Selective Adsorbents
31 Table 1. Loading of the anion resins with iodine determined by NAA.
Pyrolysis
Loading
Temperature
(mmol/g)
25
0.1610
200
0.1870
300
0.0534
400
0.0033