Polyacrylonitrile-Chalcogel Hybrid Sorbents for Radioiodine Capture

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Polyacrylonitrile-Chalcogel Hybrid Sorbents for Radioiodine Capture Brian J. Riley,*,† David A. Pierce,† Jaehun Chun,† Josef Matyás,̌ † William C. Lepry,† Troy G. Garn,‡ Jack D. Law,‡ and Mercouri G. Kanatzidis§ †

Pacific Northwest National Laboratory, Richland, Washington 99352, United States Idaho National Laboratory, Idaho Falls, Idaho 83401, United States § Northwestern University, Evanston, Illinois 60208, United States ‡

S Supporting Information *

ABSTRACT: Powders of a Sn2S3 chalcogen-based aerogel (chalcogel) were combined with powdered polyacrylonitrile (PAN) in different mass ratios (SnS33, SnS50, and SnS70; # = mass% of chalcogel), dissolved in dimethyl sulfoxide, and added dropwise to deionized water to form pellets of a porous PAN-chalcogel hybrid material. These pellets, along with pure powdered (SnSp) and granular (SnSg) forms of the chalcogel, were then used to capture iodine gas under both dynamic (dilute) and static (concentrated) conditions. Both SnSp and SnSg chalcogels showed very high iodine loadings at 67.2 and 68.3 mass%, respectively. The SnS50 hybrid sorbent demonstrated a high, although slightly reduced, maximum iodine loading (53.5 mass%) with greatly improved mechanical rigidity. In all cases, X-ray diffraction results showed the formation of crystalline SnI4 and SnI4(S8)2, revealing that the iodine binding in these materials is mainly due to a chemisorption process, although a small amount of physisorption was observed. maximize the specific surface area of the final product; however granules and monoliths can also be fabricated.1,6,10,11 Riley et al.5,6 demonstrated high capture efficiencies for I2(g) with chalcogels of various compositions, which is of interest when considering that these sorbents could be used to capture the long-lived radioactive isotope 129I2(g) with a half-life of 1.6 × 107 y. This isotope is evolved from process off-gas streams during nuclear fuel reprocessing, and its release into the environment is highly regulated in the U.S. Several other iodine sorbents and storage materials have been investigated internationally that include metal−organic frameworks (MOFs),12−15 silver-exchanged zeolites,16−19 apatites,20−22 zeolite−apatite composites,23 AgI−Ag2O−P2O5 glass,24 and silver-functionalized silica aerogels.25 The Environmental Protection Agency requires that 99.4% of 129 I must be captured;26 although, based on the processing conditions, the fuel burn-up, and cooling time, it is expected that >99.9% will need to be captured.27 In a spent fuel reprocessing facility, the iodine capture would typically be conducted at elevated temperatures of ≥125 °C and at parts per million (ppm) concentrations of iodine or lower with highvelocity air as a carrier gas. However, the friability of the chalcogels would limit the air flow rate in a reprocessing facility to unacceptably low values because higher flow rates would

1. INTRODUCTION In recent years, aerogels of various types have been demonstrated to have a wide range of utility in different areas of research. Chalcogen-based aerogels, called chalcogels, have shown promise for the remediation of heavy metals (e.g., Pb2+, Cd2+, Hg2+),1−4 radioactive ions (e.g., UO22+, TcO4−),5 and gaseous species, such as I2(g).5,6 However, when considering these materials for applications involving flowing environments such as air or water streams, their low mechanical rigidity limits their applicability because they are naturally friable. The different methods for making chalcogels are described elsewhere.1−11 The chalcogels discussed here are made by mixing at least two precursor solutions together, one of which contains a chalcogen-based building block (e.g., Sn2S62−, MoS42−) and the other contains at least one dissolved interlinking metal (e.g., Sn2+, Co2+, Ni2+) in a solvent (e.g., water, formamide). Upon mixing, certain combinations of these precursors undergo gelation, which can take hours to several weeks, depending on the formulation. Through a series of solvent exchanges, the unreacted byproducts can be removed from the gel. This is followed by rinses with liquid CO2 inside an autoclave, which is then converted into a supercritical fluid by increasing the temperature inside the vessel, and then CO2 is vented as a gas to prevent collapse of the scaffolding within the chalcogel. Solvent exchange becomes increasingly inefficient when preparing chalcogels of increasing dimensions because residual solvent can be trapped within the porous network. Thus, it is easiest to make these types of aerogels as powders to © 2014 American Chemical Society

Received: Revised: Accepted: Published: 5832

December 31, 2013 April 15, 2014 April 16, 2014 April 29, 2014 dx.doi.org/10.1021/es405807w | Environ. Sci. Technol. 2014, 48, 5832−5839

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Figure 1. Pictures of (left) the synthesis process as a function of time (∼5 min) during precursor mixing and (right) the supercritically dried chalcogel powder (vessel width is 27 mm).

Table 1. Recipes for Making PAN-Chalcogel Hybrid Sorbents sample

mCg (g)

mPAN (g)

mtotal (g)

m%Cg (mass%)

mCg:mPAN

DMSO (mL)

SnS33 SnS50 SnS70

0.20 0.13 0.19

0.40 0.13 0.08

0.60 0.26 0.27

33 50 70

0.33 0.50 0.70

6.00 2.50 1.20

water (by volume) for 24 h to increase their rigidity. Then, the gels were rinsed 10 times over the course of 2 days, each time with fresh 50:50 solution to remove the water-soluble byproducts, followed by 10 rinses with pure ethanol to remove the water. The gels were loaded into a stainless steel mesh basket in a 450 mL autoclave (4762Q, Parr Instrument Company, Moline, IL), fresh ethanol was added to the top of the gels, and the vessel was pressurized to 1 × 107 Pa (1500 pounds per square inch) with CO2 at ambient temperature. The autoclave was flushed with several liters of liquid CO2 over the course of a 24 h period to remove the ethanol. Then, the vessel was heated to 50 °C to convert the CO2 into a supercritical fluid that was then released as a gas while preserving the pore structure of the chalcogel. The product from this process was a mixture of chalcogel granules (SnSg) and powders (SnSp). A picture of the dried Sn2S3 chalcogel powder is shown in Figure 1. 2.2. PAN-Chalcogel Hybrid Synthesis. Three PANchalcogel hybrid pellets were made with different masses of SnSp (mCg) in PAN (mPAN) (Dralon X 100, Dralon GmbH, Dormagen, Germany) with final mass percentages (m%Cg) of 33% (SnS33), 50% (SnS50), and 70% (SnS70) as shown in Table 1. To make each of the pellets, a known mass of SnSp was added to a known volume of dimethyl sulfoxide (DMSO, Sigma-Aldrich) while stirring (Table 1). Second, a known mass of PAN fibers was added with heating to 40 °C to dissolve the PAN (Table 1). Third, the mixture was added dropwise from a pipet into a bath of water while stirring to form solid beads of consistent volume. Finally, the beads were rinsed with water and dried overnight in an oven at 50 °C, during which time they decreased in size. 2.3. Specific Surface Area. Specific surface areas were measured for the chalcogels with N2(g) adsorption and desorption isotherms collected with a gas sorption system on degassed samples at liquid N2 temperature (Autosorb-6B, Quantachrome Instruments, Boynton Beach, FL). Samples were loaded into a glass sample holder and degassed at 60 °C for 8 h while under vacuum. The specific surface areas were determined from the isotherm with the Brunauer−Emmett− Teller (BET) method.33 2.4. Iodine Capture Experiments under Dynamic Conditions. A schematic of the experimental setup used to capture iodine at low concentrations is shown elsewhere.5 In this apparatus, air was flowed through a column in a 100 °C oven and then through a DYNACAL iodine permeation tube

cause attrition of the chalcogel and subsequent loss from the sorption bed. Additionally, the stream of air will likely contain nitrogen oxide gases (e.g., NO, NO2), organic iodides (CH3I), and HI in addition to water vapor so the sorbents need to be selective for the targeted iodine-based species as well as stable under acidic conditions.27,28 Thus, the goal of the current work was to assess a method of making the chalcogels more mechanically stable for I2(g) by imbedding them in polyacrylonitrile [PAN, (C3H3N)n], a porous polymer, at different loadings using a process recently developed for other sorbents like hydrogen mordenite and MOFs.29−31 Then, the various PAN-chalcogel hybrid sorbents were used to study the relationship of chalcogel loadings in the sorbents to I2(g) capture efficiency and their adsorption behaviors were evaluated in dynamic (dilute) and static (concentrated) iodine conditions. Furthermore, thermal analysis and X-ray diffraction (XRD) were used to evaluate the nature of the iodine binding in these materials. The chalcogel studied here was a Sn2S3 formulation that was selected based on the high iodine capture efficiency shown in a previous study.5

2. EXPERIMENTAL SECTION 2.1. Chalcogel Synthesis. The precursors used to make the Sn2S3 chalcogel (Cg) were Na4Sn2S6·14H2O (P1) and Sn(CH3COO)2 (P2) (eq 1). The Sn(CH3COO)2 was purchased from Sigma-Aldrich (St. Louis, MO), and the Na4Sn2S6·14H2O was made fresh from Na2S·9H2O and SnCl4·5H2O (Sigma-Aldrich) according to procedures described in the literature (eq 1).5,11,32 Equation 1 provides the millimolar ratios required for a simplified 1× mixture; for the current work, a 100× batch was processed. Both P1 and P2 were dissolved in separate 1 L beakers containing 200 mL of formamide. Then, P2 was slowly added to P1 with vigorous stirring. 0.1Na4Sn2S6·14H 2O(P1) + 0.2Sn(CH3COO)2 (P2) → 0.2Sn 2S3(Cg) + 0.4Na + + 0.4CH3COO− + 1.4H 2O (1)

Once P1 and P2 were fully combined as shown in Figure 1, they were mixed for 10 min, cast into polypropylene vials (10 mm ⌀ × 50 mm long), and left to undergo gelation for 37 days. The gels were then removed from their vials, cut into ∼2−5 mm pieces, and placed in a solution of 50:50 ethanol/ 5833

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Figure 2. Cross-sectional SEM micrographs of the pure PAN and chalcogel-PAN sorbents SnS33, SnS50, and SnS70.

2.6. Electron Microscopy. Scanning electron microscopy (SEM) was performed with a JSM-7001F field-emission gun microscope (JEOL USA, Inc. Peabody, MA). Energy dispersive spectroscopy (EDS) was performed with an EDAX Si-drift detector (Apollo XL, AMETEK, Berwyn, PA). Samples were analyzed as powders on carbon tape and as monoliths that were mounted in resin and polished. 2.7. X-ray Diffraction. For analysis of the PAN hybrid sorbents, carbon tape was adhered to a zero-background quartz disc, a few pellets were placed on the tape, and the pellets were cut into small pieces. Then, a glass slide was used to flatten the sample to evenly cover the tape. For the pure chalcogels, the sorbents were packed into a cavity on the zero-background quartz holder without any adhesive. Powder diffraction patterns were obtained with a Bruker D8 Advance (Bruker AXS Inc., Madison, WI) XRD with Cu Kα emission and a LynxEye position-sensitive detector with a collection window of 3° 2θ. Scan parameters were 5−70° 2θ with a step of 0.009° 2θ and a 1 s dwell at each step. 2.8. Thermal Analysis. In order to study the effect of composition versus heat-treatment temperature for the SnSp+I material, small quantities (60−80 mg) were loaded into alumina crucibles and heated to different temperatures within an SDT Q-600 (TA Instruments, New Castle, DE) for thermogravimetric analysis (TGA). Individual samples were heated to temperatures of 100, 200, 300, 400, 500, and 600 °C at a heating rate of 10 K min−1 with an argon cover gas to prevent oxidation. Following these heat-treatments, the specimens were analyzed with XRD and SEM-EDS so the phase distributions and compositions, respectively, could be determined.

(107-039-0004-C100, Valco Instruments Co., Inc., Poulsbo, WA) with an iodine permeation rate of 22.8597 ng s−1. The flow rate of air was adjusted to obtain an iodine concentration of 4.2 ppm (by volume). This mixture was passed into a second column in a 125 °C oven where the sample holder, a 10 mL glass pipet, was attached to the column. After passing through the pipet, either empty or containing the sorbent, the gas mixture was bubbled through a solution of 0.1 M NaOH, which was periodically replaced with a fresh solution. Aliquots of 20 mL from the scrubber solution were analyzed with inductively coupled plasma mass spectrometry (ICP-MS), and the iodine evolution rate (μg L−1 s−1) was determined. Prior to the experiments, calibration runs were performed without a sorbent in the sample holder to determine the iodine concentration. Then, 1−5 mL of chalcogel granules were lightly packed into the sample holder. The scrubber solutions were collected at different evolution times and analyzed with ICPMS to determine the concentration of iodine in each sample (Cs). The capture efficiency of the sorbent was then determined with eq 2 where Cc was the iodine concentration calculated from the calibration runs (without sorbent) for the same evolution time. %efficiency = 100 × (Cc − Cs)/Cc

(2)

2.5. Iodine Capture Experiments under Static Conditions. Specimens of SnSp, SnSg, PAN, SnS33, and SnS50 were placed in separate tared 20 mL glass scintillation vials. All of the sorbents had been aged in air for 1 y prior to these experiments. The SnS70 was not tested under these static conditions. These vials, along with an empty tared vial, were placed inside a glass vacuum desiccator at ambient temperature along with pure iodine (99.9999%, Alfa Aesar, Ward Hill, MA) in a separate container. The vials were periodically removed and weighed to measure sorbent mass uptake until no further mass gain was observed (saturation). The (normalized) mass% of iodine (m%I) was calculated based on eq 3, where mt was the total mass of the sorbent with adsorbed iodine, and ms was the mass of the original sorbent. The mass of the vial was normalized out using the data collected from the blank vial, which was negligible at 99.3% iodine capture,5 and this is credited to the higher available surface area without any PAN diluting the sorbent. Phase analysis of the sorbents following these experiments under dynamic conditions did not show any iodide complex formation due to the low amounts of iodine captured over the short duration of the experiments. While the SnSg sorbents remained stable in the experimental configuration here, it is likely that, when subjected to increased air flow rates, these granules could be carried downstream in the column. Thus, the PAN-chalcogel hybrid approach, or something comparable, will be needed to help reduce this mobility. 3.3. Iodine Capture Experiments under Static Conditions. The results for maximum iodine adsorption under static conditions are presented in Figure 5 and Table 2. The iodine loadings of the SnSp+I and SnSg+I were both very high and similar at 67.2 and 68.3 mass%, respectively. The rate of adsorption was higher for SnSp+I than for SnSg+I with maximum adsorptions (saturation) occurring at 7.8 and 24.6 d, respectively. This difference is thought to be attributed to a difference in pore accessibility between the powdered and granular forms of the chalcogel. In order to normalize the iodine capture data, the adsorption rate, ra, was calculated with eq 4, where mI is the total mass (in g) of adsorbed iodine and t is the time (in days) to reach saturation (Table 2). The hybrid sorbents captured I2(g) at average rates that were approximately ten times lower than those for SnSp and SnSg (Table 2 and Figure S1 in the Supporting Information, SI). This is likely a result of the significant reduction in available surface area from the chalcogel contribution in the hybrid sorbents (13−23 m2/g) versus those of the as-made chalcogel (270 m2/g). Additionally, the

Figure 3. Pictures of (left) PAN, SnS33, and SnS50 and (right) PAN+I, SnS33+I, and SnS50+I.

different from one another in size, shape, and color. The crosssectional views of the pellets in Figure 2 showed variations in the porosity as well as the distributions of pore sizes and shapes. The lower chalcogel-loaded PAN sorbents (SnS33 and SnS50) had high porosity with small pores toward the perimeters and large pores toward the centers of the pellets. The open porosity of the SnS70 sorbent was significantly lower than that of the other sorbents. The pure PAN pellets were solid white and generally spherical. The SnS33 pellets were a light tan in color, less spherical than the pure PAN sorbents, and larger (∼2×) than the SnS50 and SnS70 sorbents. Following the maximum iodine adsorption experiments, the PAN, SnS33, and SnS50 pellets were darker yellow (for PAN) or orange (for SnS33 and SnS50) due to the capture of pure iodine (PAN) or the formation of SnI4 (SnS33 and SnS50) (Figure 3). The PAN significantly improved the rigidity of the hybrid sorbents over the SnSg sorbent. While the powdered and granular forms of the chalcogel are friable, the PAN hybrid sorbents were much more mechanically stable and had higher densities due to collapsed pore structure making them much easier to handle. 3.2. Iodine Capture Experiments under Dynamic Conditions. The results of the iodine capture experiments under dynamic conditions (4.2 ppm by volume) are presented in Figure 4. These results show that the hybrid sorbents can effectively capture iodine from air at low concentrations. However, the capture efficiencies for all of the PAN-chalcogel 5835

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Figure 6. The XRD patterns for (a) aged SnSg (“Amor.” denotes an amorphous phase) (b) SnSg+I, (c) SnS33+I, and (d) SnS50+I. The measured, calculated, difference, and reference diffraction patterns. (Note that some peaks could not be identified.)

Figure 5. Iodine adsorption under static conditions for (a) PAN hybrid sorbents as well as (b) SnSg and SnSp.

originated from other Sn−S or Sn−O phases (Figure 6a). The SnSg+I was primarily composed of SnI4 with low-intensity peaks originating from SnI4(S8)2 and S8 (Figure 6b). The SnI4(S8)2 compound is believed to be SnI4 in the proximity of S8 chains or a complex formed as the result of iodine reacting with the crystalline SnS in the original starting material. The diffraction patterns for the SnS33+I and SnS50+I both showed peaks for SnI4, but the SnS50+I showed additional low-intensity peaks from SnI4(S8)2 (Figure 6c,d). 3.5. Phase Analysis with TGA and SEM-EDS. The trends from the thermal analysis are presented in Figure 7 and in the SI. The trends of composition shown in Figure 7a indicate that the sulfur fraction remained relatively constant while the Sn fraction increased and the iodine fraction decreased over the temperature range. Also, the iodine content remained relatively constant up to 200 °C, after which point the iodine losses increased sharply. A decreasing iodine concentration with increasing Sn suggested that SnI4 decomposed between the melting (143 °C) and boiling (364 °C) temperatures (SI Table S1). When this occurred, the Sn/S increased from near the target of 2.47 to 6.56 (Figure 7b). This suggests that near its boiling temperature (445 °C), sulfur was preferentially volatilized and the Sn remained. Also, it is worth noting that, if all of the Sn in the chalcogel was converted to SnI4, the I/Sn should be 4.28; the observed I/Sn prior to heating was 2.36. This suggests that ∼60% of the Sn in the starting chalcogel was converted to SnI4 during iodine adsorption.

maximum iodine adsorption of the sorbents evaluated here followed a general trend where the sorbents with higher fractions of the Sn2S3 chalcogel adsorbed consistently more iodine (SI Figure S2). This is to be expected considering that the contribution of iodine adsorption by the pure PAN material was very low (6.60 × 10−5 g d−1, Table 2) and the Sn2S3 chalcogel appears to solely provide the iodine binding sites.

ra = mI /t

(4)

The time to reach saturation for SnS50+I was more than 3 times faster than with SnS33+I at 6.9 and 19.5 days, respectively (Table 2). The difference in these times is attributed to a larger fraction of accessible binding sites with the increased SnS chalcogel loading. Regardless, the ra values for both SnS33 and SnS50 sorbents were very similar at 1.97 × 10−3 and 3.62 × 10−3 g d−1, respectively (Table 2). It is worth noting that less time was needed to reach maximum adsorption with SnS50+I (6.9 d) than both SnSp+I (7.8 d) and SnSg+I (24.6 d) (Table 2). The reason for this is not currently understood. 3.4. Phase Analysis with XRD. The XRD pattern for SnSg showed amorphous structure (broad peaks) and diffraction peaks that matched SnS (Figure 6a). Some additional peaks were found but could not be identified within the known chemistry. However, structural fits suggest that these peaks

Table 2. Summary of Iodine Adsorption Data under Static Conditions, Where ra Is the Mass of Iodine Adsorbed Per Day on Average until Saturation Was Achieved (Eq 4)a

a

sample

SnS (mass%)

saturation (days)

sample mass (ms)

sample+I2 (mt)

mass I2 (mI)

mass% I2 (m%I)

PAN+I SnS33+I SnS50+I SnSg+I SnSp+I

0 33 50 100 100

22.7 19.5 6.9 24.6 7.8

0.0505 0.0790 0.0216 0.6178 0.1755

0.0520 0.1175 0.0464 1.9318 0.5292

0.0015 0.0385 0.0248 1.3140 0.3537

2.9 32.7 53.4 68.3 67.2

ra (g d−1) 6.60 1.97 3.62 5.35 4.54

× × × × ×

10−5 10−3 10−3 10−2 10−2

The mass% of PAN = 100 − m%Cg. 5836

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4. ENVIRONMENTAL APPLICATIONS Chalcogels have been demonstrated to have high affinities for a variety of heavy metals and radioactive contaminants,1−5 in addition to I2(g).5,6 These tests have been carried out in experiments with powdered and granular forms of different chalcogel compositions. However, for application in the field, a more robust and durable form is needed. Thus, with the approach discussed here to make polymer-chalcogel hybrid sorbents, the range of applications for chalcogels becomes broader because this technique produces a sorbent with significantly increased mechanical integrity as compared to the friable powdered or granular chalcogel material and, hence, provides a material with greater utility. Additionally, these materials were made with Sn and S, neither of which is regulated by the Resource Conservation and Recovery Act.42 This process has not yet been optimized for the chalcogels discussed here, but various parameters in the synthesis process could be adjusted to yield a more effective sorbent, including the ratio of mCg/mPAN, the PAN-chalcogel pore structure, the precursor mixing order, and the chalcogel chemistry. The ratio of mCg/mPAN was not optimized and could be further adjusted to determine the maximum possible sorbent loading in the binder before the pore structure of the binder collapses. The pore structure of the hybrid pellets as per the cross-sectional views in Figure 2 showed a wide distribution of pore sizes from the center to the exterior of the pellets, with pore size increasing toward the center of the pellets. More uniform poresize distribution is needed. Regarding the precursor mixing order, in some instances, the PAN was dissolved in DMSO solution prior to adding the chalcogel precursor and this appeared to improve the final surface area of the resulting hybrid sorbents so this aspect of the process should be further investigated. Finally, the only chalcogel discussed here is the Sn2S3 chalcogel, but other chalcogels are possible candidates that could lead to better adsorption properties and higher capacities. Additionally, this approach could be extended to capture substances above maximum contaminant levels, such as hazardous air pollutants or toxic water pollutants in drinking or groundwater.

Figure 7. (a) A graph of composition for SnSp+I after heat treatment to various temperatures and (b) elemental ratios as a function of temperature. The targeted ratios of Sn2S3 (starting chalcogel material) and SnI4 (compound observed with XRD) are also shown as dotted lines in (b).

3.6. Binding Mechanism. In previous accounts of environmental remediation with chalcogels,1−6 the chalcogel affinity for various sorbates was explained in terms of the chemical hardness (η),34−36 which can be used to classify species as hard or soft Lewis acids or bases. The backbone of these chalcogels is composed of sulfur, a soft Lewis base that interacts with sorbates of concern that are often soft Lewis acids. However, the iodine binding mechanism for SnSg, SnSp, and the hybrid sorbents appears to be mostly due to a chemical reaction (chemisorption) between Sn and iodine to form SnI4 (Figure 6), which is a spontaneous reaction with ΔGf° = −215.1 kJ mol−1 at 298 K (SI Table S2). Similar phenomena are observed with iodine in other Ag-containing sorbents where the iodine is chemisorbed to make AgI.19,25 It is worth noting that, when left in the vacuum desiccator following maximum iodine uptake, the SnS50+I sorbent lost a total of ∼5.6 mass% iodine over the course of 15 days; this suggests the removal of physisorbed iodine and provides evidence for a binding mechanism that is not entirely based on chemisorption. 3.7. Post-Sorption Options. Once these sorbents have been used to capture the species of interest, the final waste form must meet disposal requirements for chemical durability. One possibility is that they could be imbedded in a low-temperature secondary material like Cast Stone, geopolymers, or resins,37,38 an approach shown to encapsulate zeolites, MOFs, and other radiological sorbents.39−41 It is possible that the sorbate could be released from the chalcogel through heating, although it is likely that heating the PAN-chalcogel hybrid to these temperatures would destroy the polymer matrix. Additionally, if the sorbate is ionic, it might be possible to desorb certain species by submerging the pellets in water and changing solution pH.

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ASSOCIATED CONTENT

* Supporting Information S

Additional data on iodine adsorption rates, thermal analysis data, compositional correlations, and analysis for heat-treated chalcogels containing iodine. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (509)372-4651; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Department of Energy Office of Nuclear Energy under the Fuel Cycle Research and Development Program. Pacific Northwest National Laboratory is operated by the U.S. Department of Energy under Contract Number DE-AC05-76RL01830. At Northwestern University, this work was supported by the NEUP program of DOE (M.G.K.). The authors thank Denis Strachan for comments on this document. The authors also thank Benjamin Yuhas of 5837

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Northwestern University for providing the Na4Sn2S6·14H2O precursor that was used to make the Sn2S3 chalcogel, Xiaohong Li for help with BET, Maura Zimmerschied for technical editing, and Michael Perkins for help with the graphics.

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ABBREVIATIONS BET Brunauer−Emmett−Teller EDS energy dispersive spectroscopy ICP-MS inductively coupled plasma mass spectrometry PAN polyacrylonitrile SEM scanning electron microscopy SnSg granular Sn2S3 chalcogel SnSp powdered Sn2S3 chalcogel SnS33 PAN-chalcogel hybrid with 33 mass% Sn2S3 SnS50 PAN-chalcogel hybrid with 50 mass% Sn2S3 SnS70 PAN-chalcogel hybrid with 70 mass% Sn2S3 TGA thermogravimetric analysis XRD X-ray diffraction

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