Silver-Loaded Aluminosilicate Aerogels As Iodine ... - ACS Publications

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Silver-Loaded Aluminosilicate Aerogels As Iodine Sorbents Brian J. Riley,* Jared O. Kroll, Jacob A. Peterson, Josef Matyás,̌ Matthew J. Olszta, Xiaohong Li, and John D. Vienna Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *

ABSTRACT: In this paper, aluminosilicate aerogels were used as scaffolds for silver nanoparticles to capture I2(g). The starting materials for these scaffolds included Na−Al−Si−O and Al−Si−O aerogels, both synthesized from metal alkoxides. The Ag0 particles were added by soaking the aerogels in aqueous AgNO3 solutions followed by drying and Ag+ reduction under H2/Ar to form Ag0 crystallites within the aerogel matrix. In some cases, aerogels were thiolated with 3(mercaptopropyl)trimethoxysilane as an alternative method for binding Ag+. During the Ag+-impregnation steps, for the Na− Al−Si−O aerogels, Na was replaced with Ag, and for the Al− Si−O aerogels, Si was replaced with Ag. The Ag-loading of thiolated versus nonthiolated Na−Al−Si−O aerogels was comparable at ∼35 atomic %, whereas the Ag-loading in unthiolated Al−Si−O aerogels was significantly lower at ∼7 atomic % after identical treatment. Iodine loadings in both thiolated and unthiolated Ag0-functionalized Na−Al−Si−O aerogels were >0.5 mI ms−1 (denoting the mass of iodine captured per starting mass of the sorbent) showing almost complete utilization of the Ag through chemisorption to form AgI. Iodine loading in the thiolated and Ag0-functionalized Al−Si−O aerogel was 0.31 mI ms−1. The control of Ag uptake over solution residence time and [Ag] demonstrates the ability to customize the Ag-loading in the base sorbent to regulate the loading capacity of iodine. KEYWORDS: iodine, aerogel, silver, thiolation

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INTRODUCTION The capture of radioiodine (e.g., 131I, 129I) has been a topic of focus for many decades by the community studying the capture and immobilization of potential nuclear fuel reprocessing wastes and legacy nuclear weapons production wastes.1−3 Iodine capture technologies discussed in the literature include various types of solution-based processes like Mercurex,4,5 Iodox,5 electrolytic scrubbing,6,7 and caustic scrubbing5 as well as solid sorbents including metal-exchanged ceramics such as mordenite (AgZ) or faujasite (AgX), 8 chalcogen-based aerogels,9−11 metal−organic frameworks,12,13 granular activated carbon,14 graphene powders/aerogels,15 copper metal,16 Bicompounds,17 Ag0-functionalized silica aerogels,18,19 Ag-impregnated Al2O3,20 and Ag-impregnated SiO2.21 Many different metals have been evaluated with solid sorbents as getters for I2(g) including Ag, Bi, Cd, Cu, Hg, Mn, Pb, Pd, Sb, Sn, and Tl,9,10,17,22−28 although Ag is one of the most effective getters tested to date due to strong chemisorption with iodine. In the current work, a new form of iodine sorbent is evaluated, which is an Ag0-functionalized aluminosilicate (Al−Si−O) aerogel. Oxide-based aerogels are semirigid, highly porous solids that can have enormous specific surfaces areas (SSA) of >1000 m2 g−1. These can be made in a variety of ways and involve a wide range of reactant precursors, solvents, and water to initiate and sustain the hydrolysis and polycondensation processes. During © XXXX American Chemical Society

hydrolysis reactions of metal alkoxides, the typical precursor for oxide aerogels, organic functional groups are removed and replaced with −OH groups. Here, various hydroxylated moieties link together through polycondensation at different rates depending on solution pH,29−32 catalyst used,29 precursor concentrations,33 reaction temperatures,33 and the molar ratio of water to the precursor(s).34,35 A schematic showing the process for making alcogels, xerogels, and aerogels is shown in Figure S1 (Supporting Information, SI). Examples of how these process reactions occur for one of the common Si-alkoxides used in these processes, tetraethyl orthosilicate [TEOS, Si(OC2H5)4], are shown in Figure S2 (SI).30 The hydrolysis process involves a nucleophilic attack of the oxygen (from the water) on the Si atom of the TEOS molecule, which was confirmed by Khaskin using 18O-enriched water.36 In the current work, the formulation of an aerogel scaffold material with high SSA was evaluated by controlling a variety of process variables. All materials were fabricated using metal alkoxides (see Table S1, SI), but for the materials containing Na, different Na-alkoxide precursors were used (i.e., NaOCH3 and NaOC2H5). Additional variables were evaluated including Received: July 17, 2017 Accepted: September 5, 2017

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DOI: 10.1021/acsami.7b10290 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Table 1. Summary of Gelation Time (tgel) for Alcogels As Well As the Measured Specific Surface Areas (SSA), Pore Volumes (Vp), and Pore (Meso and/or Micro) Sizes (sp) for the As-Made (AM) and/or Treated Aerogels (i.e., Ag0, AgI) and Xerogels (NAS-7)a sample ID NAS-1 NAS-7 NAS-8 NAS-9 NAS-11 NAS-11a NAS-12a NAS-12a-Ag0 NAS-12a-AgI NAS-12b NAS-13b NAS-14c NAS-15d AS-1

description

tgel (h)

SSA (m2 g−1)

Vp (cm3 g−1)

sp (nm)

AM AM Xerogel AM AM AM AM AM Ag0 AgI AM AM AM AM AM

∼72 16 16 17 ∼310 22 1000 for vessel ventilation off-gas, cell off-gas, and melter off-gas;58−60 (see Riley et al.1 for descriptions of these processes). For the current work, the Na−Al−Si−O aerogels NAS-11aAgI, NAS-11a-SH-AgI, and NAS-12a-SH-AgI showed high Iloadings of 0.56, 0.50, and 0.55 mI ms−1, respectively (Table 3). For the comparison of NAS-11a aerogels that were thiolated and unthiolated, the unthiolated showed a 10% higher iodine loading. Iodine loading in AS-1-SH-AgI (Al−Si−O aerogel) was notably less at 0.31 mI ms−1. Also, the Ag+-impregnated NAS-11a sorbents, NAS-11a-Ag++I and NAS-11a-SH-Ag++I, showed I-loadings of 0.52 and 0.40 mI ms−1, respectively, revealing that it might not be necessary to functionalize (i.e., Ag+ → Ag0). While these sorbents show high capacities under I2(g) saturation conditions, although it was not the focus of the current work, future work will require demonstration under more realistic conditions to what might be found in a reprocessing facility as described above. General Comparisons between Na−Al−Si−O and Al− Si−O Sorbents. The Na−Al−Si−O and Al−Si−O gels behaved quite differently at all stages in the process where each had advantages and disadvantages. From a processing point of view, the Al−Si−O gels were easier to make and work with. While both sets of gels were rigid in a matter of a few hours, the Al−Si−O alcogels and aerogels were more robust (mechanically rigid) than the Na−Al−Si−O gels and easier to handle, suggesting a more structurally sound network was formed in the Al−Si−O gels. Also, the Al−Si−O gels were far more resilient to pore structure collapse during the 350 °C heat treatment process than the Na−Al−Si−O with a final SSA of 754 m2 g−1 for AS-1 versus 291 m2 g−1 for NAS-11a, respectively. However, from a sorbent perspective, the Na−Al−Si−O gels were more effective than the Al−Si−O gels at binding I2(g). Since the Na was not present in the Al−Si−O gels for Ag ion exchange, Ag+ impregnation proved far more difficult if

Figure 12. (a) BSE-SEM and (b) TEM/SAD micrographs of I-loaded NAS-12a-SH-AgI.

Figure 13. Summary of (a) SSA and (b) Vp for NAS-11a aerogels subjected to different treatments where “AM”, “HT”, “SH”, “Ag+”, and “Ag0” denote as-made, heat treated (350 °C, 30 min), thiolated, Ag+impregnated, and Ag0-functionalized aerogels, respectively.

DF = 1/(1 − efficiency) = 1/breakthrough

(3)

While the iodine uptake data presented here only focused on static tests, the true test of sorbent performance should be evaluated under the conditions where the environment is closer H


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ACS Applied Materials & Interfaces thiolation was not used. Also, Ag+ impregnation was still not as effective even when thiolation was performed beforehand as the Ag0-functionalized Al−Si−O gels were extremely heterogeneous in appearance and in Ag-distribution as compared to the Na−Al−Si−O gels, as determined from EDS results (see Tables S6−S8, SI). This is, at least partially, attributed to the incongruent way that the 3-(mercaptopropyl)trimethoxysilane was distributed across the AS-1 aerogel granules prior to thiolation, whereas for the Na−Al−Si−O gels (i.e., NAS-11a, NAS-12a), the thiolation did not notably interrupt the network of Na present within the gel structure; a drop in Na concentration was observed between the transition of NAS11a-HT and NAS-11a-SH (see Figure S16, SI), suggesting at least some Na loss during the thiolation process. Thus, during the Ag+-impregnation process, the evenly distributed Na present in the gel was replaced with Ag ions creating a more dispersed apportioning of Ag throughout the gel than could be achieved in Al−Si−O (AS-1) gels where the primary method of adding Ag to the gel was through the S−Ag bond. It is this heterogeneity and uneven distribution of Ag in the AS-1-SHAg0 versus NAS-11a-SH-Ag0 that likely resulted in the notably lower iodine capacities in these sorbents at 0.31 and 0.50 mI ms−1, respectively.

in Teflon beakers with Teflon-coated magnetic stir bars. Following precursor mixing (Figure S1a,b, SI), most gels were cast into polypropylene vials, and the vials were capped and then left upright to undergo gelation in air at ambient temperature. For the first few formulations, mixtures were left in the Teflon mixing beaker to undergo gelation. The gelation time (tgel) was determined by visual observations of the rigidity of the mixture either in the beaker or in the casted vials by shaking or agitating the partially filled vial in each batch. Following gelation, the samples were removed from their vials, placed into a 50/50 solution of DIW/EtOH, and chopped into smaller pieces of ∼2−8 mm with a stainless steel razor to undergo aging for ∼7−14 d. Following aging, the gels were solvent exchanged in pure ethanol ≥10 times to remove water in the pore structure and replace it with ethanol (alcogel stage; Figure S1e, SI). Following this stage, to create aerogels, the alcogels were then loaded into a critical point dryer apparatus (E3100, Polaron Range, UK), the device was cooled to 10 °C, and then a series of 10 rinses in liquid CO2 from a compressed CO2 bottle with a siphon-draw tube (PC050S) at 5.8 MPa (∼850 PSI) were conducted to replace the ethanol in the gel matrix with liquid CO2. After the final exchange, the device was taken to ∼40 °C and 9.6 MPa (∼1400 PSI) where the liquid CO2 became a supercritical fluid. After allowing the temperature to equilibrate for ∼15 min, the CO2 was slowly vented as a gas over the course of ∼30−45 min, and the resultant product was an aerogel (Figure S1g, SI). To create xerogels (see Figure S1f-2, SI) the solvent matrix in the alcogels was allowed to evaporate at room temperature and pressure for several days followed by several days in a vacuum desiccator until the gel collapsed and no more mass loss was observed. A summary of the main variables for each sample batch is presented in Table S2 (SI) where the “NAS” and “AS” designations were given to Na−Al−Si−O and Al−Si−O formulations, respectively. The processes for fabricating NAS-1, NAS-11, NAS-12a, and AS-1 are described below in more detail as they are the most representative examples covering the range of processing conditions and were the most studied from the full sample matrix. NAS-1. For NAS-1, a 1× batch was made using NaOMe along with IPA at a 3:1 (V V−1) mixture with Al(OBus)3 and without prehydrolysis or the HOAc catalyst. Two separate solutions were made in Teflon beakers. Solution (1) contained 4.763 mL of TEOS mixed with an equal volume of EtOH. For solution (2), 5.546 mL of Al(OBus)3 was mixed with 16.638 mL IPA and, once that solution was homogeneous, 4.829 mL of NaOMe was added to that, all within the nitrogen glovebox. When the NaOMe was added to solution (2), white particles were observed suspended in the solution and these dissolved after a period of ∼1 h. Approximately 80 min after NaOMe was added to solution (2), solution (1) was brought into the glovebox and slowly added to solution (2). This mixture of solutions (1) and (2) was stirred for 3 h, at which point 1.522 mL of DIW was added, the mixture was stirred for 16 h, and then the stir bar was removed and the container was sealed with Parafilm to undergo gelation. NAS-11. For NAS-11, a 1× batch was made using NaOMe along with IPA at a 3:1 (V V−1) mixture with Al(OBus)3 using prehydrolysis and the HOAc catalyst. Here, 1.588 mL of EtOH, 0.127 mL of DIW, 1.588 mL TEOS, and 0.020 mL of HOAc were added to a Teflon beaker and mixed for 6.5 h (i.e., 1:1 V V−1 of TEOS:EtOH; 1:1 TEOS:DIW, by mole; 1:0.05 TEOS:HOAc, by mole), at which point it was moved to the glovebox and mixing was resumed. Then, 5.54 mL of IPA was added to this solution immediately followed by 1.849 mL of Al(OBus)3 and approximately 2/3 of the way through this addition, the solution became cloudy; after mixing for another ∼16 h, the solution was almost completely clear. At this point, 1.61 mL of NaOMe was slowly added dropwise to this mixture over the course of ∼12 min. Approximately 3.5 h later, the solution was removed from the glovebox, and DIW was added. This addition was split up into two different volumes, where the first (0.254 mL) was added immediately after removal from the glovebox, and the second (0.253 mL) was added 1.5 h later for a total volume of 0.507 mL of DIW. Then, 2.75 h after the second addition of water was added, the solution was cast in

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CONCLUSIONS In this study, Na−Al−Si−O and Al−Si−O aerogels were synthesized using a variety of precursors and processing conditions. While some as-made aerogels had moderate specific surface areas (≤300 m2 g−1), others had very large values of >700 m2 g−1. During Ag+-impregnation studies with Na−Al− Si−O aerogels, it was observed that the Na was replaced with Ag, providing a route for adding Ag+ without prior thiolation. Ag-loading was comparable in thiolated and unthiolated Na− Al−Si−O aerogels at ∼35 atomic % but was much lower in unthiolated Al−Si−O aerogels (∼7 atomic %). This shows the added benefit of having the Na present in the structure for getting higher Ag-loadings at this stage of the process. The technique of thiolating the aerogels adds another step to the process and notably reduced the specific surface area when compared to unthiolated Na−Al−Si−O aerogels. However, for Al−Si−O aerogels, the thiolating process drastically improved the Ag-loading. This study demonstrates how thiolation can be avoided when an ion, such as Na+, is present in the aerogel structure and is exchanged for Ag+. Iodine loadings in thiolated and unthiolated Na−Al−Si−O aerogels were >0.50 mI ms−1 compared with that of thiolated Al−Si−O aerogels at 0.31 mI ms−1. Additionally, the Ag+impregnated NAS-11a sorbents, NAS-11a-Ag++I and NAS-11aSH-Ag++I, showed I-loadings of 0.52 and 0.40 mI ms−1, respectively, revealing that it might not be necessary to reduce the Ag (i.e., Ag+ → Ag0). The benchmark standard for iodine sorbents is silver mordenite (AgZ), which was also evaluated during this study and showed values of 0.12−0.13 mI ms−1. Thus, these Ag0-loaded aluminosilicate aerogels showed iodine capacities more than 4-fold higher than the baseline material and can be made in a wide variety of chemistries.

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MATERIALS AND METHODS

Making Base Materials. A summary of the additives and shorthand notation used in this study are shown in Table S1 (SI). All solutions were measured and added with graduated cylinders and/ or micropipettes. The batch sizes varied between samples where a 1× batch targeted 1 g of oxide aerogel product. All solutions were mixed I


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Table 4. Experimental Parameters for Ag+ Impregnation Including Batch Size (×), Mixing Time in Solution (tm), Sample Mass (ms), AgNO3 Mass (mAgNO3), Solution Volume (Vsol), and Ag+ Concentration ([Ag+])a

a

sample ID

batch size (×)

tm (min)

ms (g)

mAgNO3(g)

Vsol (mL)

[Ag+] (mmol L−1)

−1 ms m−1 AgNO3(g g )

−1 mAgNO3 V−1 sol (g L )

NAS-11a-90m-0×-60 NAS-11a-90m-1×-60 NAS-11a-90m-2×-60 NAS-11a-90m-4×-60 NAS-11a-90m-8×-60 NAS-11a-1m-8×-60 NAS-11a-5m-8×-60 NAS-11a-25m-8×-60 NAS-11a-90m-8×-180 NAS-11a-SH-90m-8×-180 NAS-12a-SH-90m-8×-360 AS-1-90m-0×-60 AS-1-90m-1×-60 AS-1-90m-2×-60 AS-1-90m-4×-60 AS-1-90m-8×-60 AS-1-5m-8×-60 AS-1-25m-8×-60 AS-1-SH-90m-8×-180

0 1 2 4 8 8 8 8 8 8 8 0 1 2 4 8 8 8 8

90 90 90 90 90 1 5 25 90 90 90 90 90 90 90 90 5 25 90

0.0626 0.0626 0.0606 0.0668 0.0623 0.0628 0.0644 0.0633 0.1895 0.1887 0.6502 0.0639 0.0643 0.0639 0.0634 0.0641 0.0639 0.0635 0.1894

− 0.2216 0.4423 0.8828 1.7669 1.7676 1.7670 1.7671 5.2994 5.2994 10.5996 − 0.2210 0.4419 0.8832 1.7667 1.7666 1.7666 5.3008

60 60 60 60 60 60 60 60 180 180 360 60 60 60 60 60 60 60 180

− 21.7 43.4 86.6 173.4 173.4 173.4 173.4 173.3 173.3 173.3 − 21.7 43.4 86.7 173.3 173.3 173.3 173.3

− 0.282 0.137 0.076 0.035 0.036 0.036 0.036 0.036 0.036 0.062 − 0.291 0.145 0.072 0.036 0.036 0.036 0.036

− 3.7 7.4 14.7 29.4 29.5 29.5 29.5 29.4 29.4 29.4 − 3.7 7.4 14.7 29.4 29.4 29.4 29.4

All solutions contained 5:1 DIW:MeOH (V V−1).

∼4 mL polypropylene vials and each vial was tightly capped with a lid and left to undergo gelation. NAS-12a. For NAS-12a, a 1× batch was made using NaOEt along with EtOH at a 6:1 (V V−1) mixture with Al(OBus)3 using prehydrolysis and the HOAc catalyst. Here, 1.588 mL of EtOH, 0.127 mL of DIW, 1.588 mL TEOS, and 0.020 mL of HOAc were added to a Teflon beaker and mixed for 6.5 h (i.e., 1:1 V V−1 of TEOS:EtOH; 1:1 TEOS:DIW, by mole; 1:0.05 TEOS:HOAc, by mole), at which point it was moved to the glovebox and mixing was resumed. Then, 11.10 mL of EtOH was added to this solution immediately followed by 1.849 mL of Al(OBus)3; the solution turned cloudy white when the Al(OBus)3 was added. After mixing overnight (∼15 h), 2.628 mL of NaOEt was slowly added dropwise to this mixture over the course of 10 min and about halfway through this addition, the solution appeared a yellow-brown color. Then, 3 h later, the solution was taken out of the glovebox and a 0.507 mL of DIW was added. (Note that for NAS-12, the DIW addition was originally split up like it was in NAS-11, but the second addition did not appear to fully incorporate into the gel and was present as a clear layer on top of the alcogel within the vials so this approach was changed for NAS12a where all of the DIW was added at the same time.) Finally, after 1.6 h of stirring, the mixture was cast into 4 mL polypropylene vials, capped with tightly fitting lids and left to undergo gelation. AS-1. For AS-1, a 5× batch was made using with EtOH at a 6:1 (V V−1) mixture with Al(OBus)3 using prehydrolysis and the HOAc catalyst. Here, 7.94 mL of EtOH, 0.634 mL of DIW, 7.94 mL TEOS, and 0.101 mL of HOAc were added to a Teflon beaker and mixed for 6 h (i.e., 1:1 V V−1 of TEOS:EtOH; 1:1 TEOS:DIW, by mole; 1:0.05 TEOS:HOAc, by mole), at which point it was moved to the glovebox and mixing was resumed. Then, 67.0 mL of EtOH was added to this solution immediately followed by 9.24 mL of Al(OBus)3; the solution turned cloudy white when the Al(OBus)3 was added. (Note: additional EtOH was added compared to NAS-12a to make up for the additional liquid volume not added since the Na-precursor was not used in this batch.) After mixing overnight (∼16.5 h), the solution was taken out of the glovebox and a 2.536 mL of DIW was added; no visible change was observed during the DIW addition. Finally, after 7 h of mixing, still no visible change was noted, and the mixture was then cast into 4 mL polypropylene vials, capped with tightly fitting lids and left to undergo gelation. Solution viscosity started to increase notably during casting so likely should be cast after mixing times of ≤6 h.

Preparing the Gels with Isothermal Heat Treatments. Subsets of samples from NAS-1, NAS-11a, NAS-12a, and AS-1 were heat treated at different temperatures from 100−1000 °C for different times (15−60 min) to desorb hydrocarbons, reduce hydroxyl groups, reduce residual moisture, and increase mechanical rigidity. Following these heat treatments, some specimens were analyzed with FTIR spectroscopy, described below in more detail. Then, N2(g) adsorption/ desorption isotherms were collected. From these isotherms, SSA, Vp, and sp were determined using the Brunauer−Emmett−Teller (BET)62 and the Barret−Joyner−Halenda (BJH)63 methods. Sample mass (ms) was also tracked before (ms,i) and after (ms,f) the heat treatments. Thiolation. Some samples were thiolated including subsets of NAS-11a, NAS-12a, and AS-1 with the intent of adding −SH tethers to the surfaces of the aerogel granules using a method previously demonstrated by Matyás ̌ et al.18,19 Prior to thiolation, each gel was heat treated at 350 °C for 30 min. Then, the gels were hydrated to make the subsequent processes more effective by placing the gels in a glass vessel containing a separate beaker full of DIW and sealing the vessel for up to 24 h. Following hydration, 3-(mercaptopropyl)trimethoxysilane [3MPTMS; HS(CH2)3Si(OCH3)3, 95%, Sigma-Aldrich, St. Louis, MO] was added dropwise to the sample using a syringe at ∼1.5 mL per 1 g of unhydrated sample. Once this was added, the material was loaded into a 100 mL high-pressure autoclave at 150 °C, the lid was secured. The vessel was charged with liquid CO2 at 24 MPa (3.5 kPSI) and left for up to 44 h. The chamber was then vented slowly, and the sample was removed. For NAS-11a, 0.6473 g of sample was used, resulting in 1.2078 g of hydrated sample (87% uptake) after 24 h, ∼1 mL of 3-MPTMS was added, and the gels were left in the autoclave for ∼44 h before venting. For NAS-12a, 0.6615 g of sample was used, resulting in 0.8810 g of hydrated sample (33% uptake) after 4 h, ∼1 mL of 3-MPTMS was added, and the gels were left in the autoclave for ∼4 h before venting. For AS-1, 0.2152 g of sample was used, resulting in 0.3178 g of hydrated sample (48% uptake) after 28 h, ∼0.5 mL of 3-MPTMS was added and the gels were left in the autoclave for 23 h before venting. Ag+ Impregnation (General). Silver impregnation (or Ag+ impregnation) was performed by placing gels in solutions of 5:1 DIW:MeOH with different concentrations of AgNO3 (≥99%, SigmaAldrich) and mixing the gels in the solution for different times using a magnetic stir bar on a Cimarec i stir plate (Poly 15; Fisher Scientific, Hampton, NH). A comprehensive list of these experiments is J


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ACS Applied Materials & Interfaces I‐loading = mI /ms (or mIms−1)

presented in Table 4 with the sample naming convention of NAS##-(SH/noSH)-t-c-s-(-,M,L) for Na−Al−Si−O samples and AS##-(SH/noSH)-t-c-s-V for Al−Si−O samples where ##, (SH/ noSH), t, c, s, and (-,M,L) denote the sample ID, whether the sample was thiolated (“SH”) or not (left blank), the residence time in solution (in min), the AgNO3 concentration (where 1× = 3.7 g L−1), and batch volume (in mL), respectively. For the experiments where Vsol = 60 mL, the gels were suspended within custom-made polytetrafluoroethylene plastic mesh baskets (ET8500−18P; Industrial Netting, Minneapolis, MN) to make it easier to retrieve them out of the solutions at the end of the experiment (see Figures S3 and S4, SI). For 180 and 360 mL experiments, the gels were placed in a Pyrex beaker. Some aerogel granules floated at first but once they were pushed under the surface with a glass rod, they sank to the bottom and remained out of the way of the magnetic stir bar. Following the soak times (±1 s), the gels were removed of most remaining liquid through capillary forces on a dry paper towel and then placed into a vacuum desiccator to dry to constant mass over the course of several days. Ag+-Impregnation Study on Na−Al−Si−O aerogels (NAS11a). A separate Ag+-impregnation study was performed with NAS11a aerogels. Here, a consistent mass (0.0632 ± 0.0018 g) of aerogel granules heat treated at 350 °C for 30 min were soaked in different concentrations of AgNO3 (0−173 mmol L−1) in a DIW/MeOH (5:1, V V−1) solution for different times (1−90 min). The sample matrix for this work is shown in Table 4. These samples were placed in Teflon baskets shown in (see Figure S3 and Figure S4, SI) and run through the procedure described above. Ag+-Impregnation Study on Al−Si−O Aerogels (AS-1). A separate Ag+-impregnation study was performed with AS-1 aerogels. Here, a consistent mass (0.0638 ± 0.0003 g) of aerogel granules heat treated at 350 °C for 30 min were soaked in different concentrations of AgNO3 (0−173 mmol L−1) in a DIW/MeOH (5:1, V V−1) solution for different times (5−90 min). The sample matrix for this work is shown in Table 4. These samples were placed in Teflon baskets shown in (see Figure S3 and Figure S4, SI) and run through the procedure described above. Ag0 Functionalization. Silver functionalization (denoted as Ag0) was performed by heating the Ag+-impregnated gels at 125 °C under a 25 mL/min stream of 2.7%H2 in Ar in a glass fixture shown schematically in Figure S5 (SI). For samples NAS-11a-Ag+, NAS-11aSH-Ag+, NAS-12a-SH-Ag+, and AS-1-SH-Ag+, the H2/Ar purge was done for 22, 22, 18, and 22 h, respectively. Iodine Capacity and Ag Utilization. Iodine loading was assessed for some of the samples using the apparatus shown in Figure S6 (SI). Experimental details are provided in Table S3 (SI). Samples were placed into preweighed glass vials and then inserted into the top portion of the apparatus, iodine (99.99%, Alfa Aesar) was placed into a separate glass vial and inserted into the bottom portion of the apparatus, and the apparatus was sealed. Then, the vessel was placed into an oven at 150 °C overnight (19−22 h residence time, tr, see Table S3, SI). Following this soak, the vials were carefully removed from the apparatus and placed directly into the same oven at 150 °C for ∼1 h so that any physisorbed iodine could be desorbed. During each experiment, the iodine capacity of silver mordenite (AgZ) obtained from Oak Ridge National Laboratory was also evaluated in parallel (in the same apparatus at the same time), i.e., AgZ +I-1, AgZ+I-2, and AgZ+I-3. These samples were stored in a vacuum desiccator prior to the experiment to help reduce aging artifacts and water adsorption. Following the experiments, eqs 4−6 were used to evaluate the iodine capacity where ms was the starting mass of the sample, ms+I was the final mass following iodine capture, mI was the mass of iodine captured by the sample [by mass difference from ms, see eq 4], m%I was the mass % of iodine in the final sample, and mI ms−1 is the term denoting the mass of iodine captured per starting mass of the sorbent (loading). mI = ms + I − ms

(4)

m%I = 100 × (mI /ms + I)

(5)

(6)

Finally, the Ag-utilization was calculated as the ratio of I:Ag (atomic %) in the iodine-sorbed materials shown in eq 7. Ag utilization >1 suggests that some physisorbed iodine remained within the sample or iodine is binding to a different location within the sorbent; it is possible that that iodine can adsorb directly to the aerogel surface or the −SH tethers.

Ag utilization = 100 × [I]/[Ag](in atomic %)

(7)

Specific Surface Area, Pore Volume, and Pore Size. The SSA, Vp, and sp analyses were measured on most of the as-made aerogels as well as some Ag+-impregnated, Ag0-functionalized, and thiolated aerogels with nitrogen [N2(g)] adsorption and desorption isotherms collected with a Quantachrome Autosorb-6B (Quantachrome Instruments, Boynton Beach, FL) gas sorption system on degassed samples. Samples were loaded in a glass cell and degassed at 25 °C for 8 h while under vacuum. The degassed samples were analyzed with nitrogen adsorption and desorption at a constant temperature of 77.4 K (−195.75 °C), the temperature of liquid nitrogen. The surface area was determined from the isotherm with the BET method.62 The BJH method63 was used for the porosity and pore size analyses. Fourier Transform Infrared Spectroscopy. The FTIR analysis of as-made and heat treated samples was used to track the residual hydrocarbons within the aerogel network using the C−H stretch64 region at ∼2974 cm−1 for reference and to track the O−H and H−O− H stretch envelopes65 in the range of 3663−2549 cm−1. The analysis was performed with a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Inc.) using pelleted samples in transmission mode. Here, a small aliquot of powdered sample was added to dry KBr powder (Alfa Aesar, FTIR grade) at 0.5 mass% and a total mass of ∼0.4 g. The two powders were then mixed together in a small agate mortar and pestle, loaded into a 20 mm diameter stainless steel die, and a pellet was formed using a uniaxial press under a load of 275 MPa (40 kPSI). These pellets were loaded into an upright pellet holder in the sample chamber of the FTIR and scanned using 64 coadds with a resolution of 4 cm−1. Each pellet was scanned three times. Thermo OMNIC (v7.2a) software was used to first normalize and then average the spectra for each pellet. Thermo TQ Analyst software (v7.1.0.32) was used to perform O−H peak area quantification. Powder X-ray Diffraction. Powder X-ray diffraction (P-XRD) was performed by grinding select samples in an agate mortar and pestle and pressing the ground powders into a 10-mm zerobackground (off-axis) quartz holder or by placing onto a 25-mm zero-background silicon holder by first suspending in EtOH, dropping onto the holder, and allowing it to dry prior to analysis. Samples were then analyzed with a D8 Advance (Bruker AXS Inc., Madison, WI) diffractometer with Cu Kα emission. The instrument was equipped with 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 2-s dwell at each step. Bruker AXS DIFFRACplus EVA (v14) and TOPAS (v4.2) software programs were used to identify and quantify the crystalline phases, respectively. Whole pattern fitting was done according to the fundamental parameters approach.66 Scanning Electron Microscopy. Imaging of the specimens was performed using JSM-7600 or JSM-7001F field-emission gun scanning electron microscopes (SEM, JEOL USA, Inc., Peabody, MA). For the JSM-7600 analysis, the samples were prepared by placing a small (pinhead) amount of powder between two glass slides and crushing to separate any clumping. A drop of IPA was then placed onto the finest powder, and then the drop was drawn up using a pipet and placed onto a small chip of Si adhered to an Al stub. The IPA was then allowed to evaporate, thus leaving behind a finely spread powder, which allowed examination of single particles (or groups, which showed little to no clumping). The Si chip was then lightly coated (∼5 nm) with amorphous carbon to provide electron conductivity in the SEM. Imaging was performed at 5 kV to examine the very surface of the samples using both backscattered electron (BSE) and secondary electron (SE) imaging detectors. For the JSM-7001F analysis, samples were adhered to double-stick carbon tape and sputter-coated with Pt K



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(Polaron Range SC7640, Quorum Technologies Ltd., East Sussex, England) to create a conductive coating. Energy-Dispersive Spectroscopy. The EDS area analysis was performed on powder mounts (samples on carbon tape) with a JSM7001F field-emission gun SEM (JEOL) coupled with a Bruker xFlash 6|60 (Bruker AXS Inc., Madison, WI) using ESPRIT (v2.0). The conditions used for data collection were 15 kV and ∼50−80 k counts per second. A minimum of three regions were analyzed and then averaged. Transmission and Scanning Transmission Electron Microscopies and Selected Area Diffraction. Samples were prepared for transmission and scanning transmission electron microscopies (TEM and STEM, respectively) using the drop method onto lacy carbon grids. A pinhead amount of powder of each sample was placed between two glass slides, which were rubbed vigorously to further separate the powders into the smallest possible form. A lacy carbon grid was then placed onto the sheared powders to adhere the particles to the lacy carbon. Samples were analyzed in both TEM and STEM modes in a JEOL ARM200CF aberration-corrected (probe) TEM. For the majority of samples, a ≤ 70 μm condenser aperture was used in order to minimize beam damage. For the iodine-containing samples, it was determined that the AgI crystals would quickly agglomerate and then recrystallize upon exposure to the beam except for when a 10-μm condenser aperture was utilized. Images with condensed beams and uncondensed beams were taken in an attempt to collect artifact-free images. Selected area diffraction (SAD) was also conducted on various samples.

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REFERENCES

(1) Riley, B. J.; Vienna, J. D.; Strachan, D. M.; McCloy, J. S.; Jerden, J. L., Jr Materials and processes for the effective capture and immobilization of radioiodine: A review. J. Nucl. Mater. 2016, 470, 307. (2) Jubin, R. T. A Literature Survey of Methods to Remove Iodine from Off-Gas Streams using Solid Sorbents, ORNL/TM-6607; Oak Ridge National Laboratory, 1979. (3) Burger, L. L.; Scheele, R. D. HWVP Iodine Trap Evaluation, PNNL-14860; Pacific Northwest National Laboratory, 2004. (4) Collard, G. E. R.; Hennart, D.; Van Dooren, J.; Goosens, W. R. A. In 16th DOE Nucl. Air Clean. Conf.; First, M. W., Ed.; United States Nuclear Regulatory Commission: San Diego, CA, 1980; p 552. (5) Trevorrow, L. E.; Vandegrift, G. F.; Kolba, V. M.; Steindler, M. J. Compatibility of Technologies with Regulations in the Waste Management of H-3, I-129, C-14, and Kr-85. Part I. Initial Information Base, ANL-8357; Argonne National Laboratory, 1983. (6) Mailen, J. C.; Horner, D. E. Removal of Radioiodine from Gas Streams by Electrolytic Scrubbing. Nucl. Technol. 1976, 30, 317. (7) Horner, D. E.; Mailen, J. C.; Posey, F. A. Electrolytic Trapping of Iodine from Process Gas Streams. US patent 4004993, 1977. (8) Haefner, D. R.; Tranter, T. J. Methods of Gas Phase Capture of Iodine from Fuel Reprocessing Off-Gas: A Literature Survey, INL/EXT07-12299; Idaho National Laboratory, 2007. (9) Riley, B. J.; Chun, J.; Um, W.; Lepry, W. C.; Matyás,̌ J.; Olszta, M. J.; Li, X.; Polychronopoulou, K. Chalcogen-Based Aerogels as Sorbents for Radionuclide Remediation. Environ. Sci. Technol. 2013, 47, 7540. (10) Riley, B. J.; Pierce, D. A.; Chun, J.; Matyás,̌ J.; Lepry, W. C.; Garn, T.; Law, J.; Kanatzidis, M. G. Polyacrylonitrile-Chalcogel Hybrid Sorbents for Radioiodine Capture. Environ. Sci. Technol. 2014, 48, 5832. (11) Riley, B. J.; Chun, J.; Ryan, J. V.; Matyás,̌ J.; Li, X. S.; Matson, D. W.; Sundaram, S. K.; Strachan, D. M.; Vienna, J. D. Chalcogen-Based Aerogels as a Multifunctional Platform for Remediation of Radioactive Iodine. RSC Adv. 2011, 1, 1704. (12) Sava, D. F.; Garino, T. J.; Nenoff, T. M. Iodine Confinement into Metal−Organic Frameworks (MOFs): Low-Temperature Sintering Glasses to Form Novel Glass Composite Material (GCM) Alternative Waste Forms. Ind. Eng. Chem. Res. 2012, 51, 614. (13) Sava, D. F.; Chapman, K. W.; Rodriguez, M. A.; Greathouse, J. A.; Crozier, P. S.; Zhao, H.; Chupas, P. J.; Nenoff, T. M. Competitive I2 Sorption by Cu-BTC from Humid Gas Streams. Chem. Mater. 2013, 25, 2591. (14) Radioactive Waste Management Vol. 7: Management Modes for Iodine-129; Hebel, W., Cottone, G., Eds.; Harwood Academic Publishers: Brussels, 1982. (15) Scott, S. M.; Hu, T.; Yao, T.; Xin, G.; Lian, J. Graphene-Based Sorbents for Iodine-129 Capture and Sequestration. Carbon 2015, 90, 1. (16) Metalidi, M. M.; Beznosyuk, V. I.; Kalinin, N. N.; Kolyadin, A. B.; Fedorov, Y. S. Treatment of Gas−Air Flows to Remove Radioiodine Using Metallic Copper. Radiochemistry 2009, 51, 409. (17) Yang, J. H.; Shin, J. M.; Park, J. J.; Park, G. I.; Yim, M. S. Novel Synthesis of Bismuth-Based Adsorbents for the Removal of 129I in OffGas. J. Nucl. Mater. 2015, 457, 1. (18) Matyás,̌ J.; Fryxell, G. E.; Busche, B. J.; Wallace, K.; Fifield, L. S. In Ceramics Engergy Sci.ence: Ceramic Materials for Energy Applications; Lin, H.-T., Katoh, Y., Fox, K. M., Belharouak, I., Widjaja, S., Singh, D., Eds.; Wiley−The American Ceramic Society: Daytona Beach, FL, 2011; Vol. 32, p 23. (19) Matyás,̌ J.; Robinson, M. J.; Fryxell, G. E. In Ceramics Materials Energy Applications II; Lin, H., Katoh, Y., Fox, K. M., Belharouak, I., Eds.; American Ceramic Society, 2012; Vol. 33, p 121. (20) Kondo, Y.; Sugimoto, Y.; Hirose, Y.; Fukasawa, T.; Furrer, J.; Herrmann, F. J.; Knoch, W. In 22nd DOE/NRC Nuclear Air Cleaning Conference, NUREG/CP-0130; First, M. W., Ed.; The Harvard Air Cleaning Laboratory: Portland, OR, 1993; Vol. 1, p 118. (21) Mineo, H.; Gotoh, M.; Iizuka, M.; Fujisaki, S.; Hagiya, H.; Uchiyama, G. Applicability of a Model Predicting Iodine-129 Profile in

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10290. Aerogel reactions and schematics; experimental and characterization details including precursor information, Ag+ impregnation, Ag0 functionalization, iodine-loading, P-XRD, SEM, EDS, consolidation experiments; thermogravimetric analysis, sample images (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Brian J. Riley: 0000-0002-7745-6730 Author Contributions

The manuscript was written through contributions of all authors./All authors have given approval to the final version of the manuscript. Funding

This work was funded by the Department of Energy Office of Nuclear Energy. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute for the DOE under contract DEAC05-76RL01830. We are thankful to Ken Marsden, Mike Goff, and Steven Frank for programmatic support, Carmen Rodriguez for help with the TGA measurements, and Stephanie Bruffey at Oak Ridge National Laboratory for providing the AgZ materials used for comparison purposes. L


Research Article


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