Silver-functionalized silica aerogels and their application in the removal of iodine from aqueous environments

Accepted Manuscript Title: Silver-Functionalized Silica Aerogels and Their Application in the Removal of Iodine from Aqueous Environments Authors: R. Matthew Asmussen, Josef Maty´asˇ, Nikolla P. Qafoku, Albert A. Kruger PII: DOI: Reference:

S0304-3894(18)30336-4 https://doi.org/10.1016/j.jhazmat.2018.04.081 HAZMAT 19364

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

29-9-2017 1-4-2018 29-4-2018

Please cite this article as: Asmussen RM, Maty´asˇ J, Qafoku NP, Kruger AA, Silver-Functionalized Silica Aerogels and Their Application in the Removal of Iodine from Aqueous Environments, Journal of Hazardous Materials (2010), https://doi.org/10.1016/j.jhazmat.2018.04.081 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.

Silver-Functionalized Silica Aerogels and Their Application in the Removal of Iodine from Aqueous Environments

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R. Matthew Asmussen1, ⃰ Josef Matyáš1, Nikolla P. Qafoku1, Albert A. Kruger2

1

Pacific Northwest National Laboratory; P.O. Box 999; Richland, WA, USA 99352

2

U.S. Department of Energy, Office of River Protection, P.O. Box 450; Richland, WA, USA 99352

⃰ Corresponding Author: Ph: 509-372-6023; Fax:509-372-5997; E-mail:[email protected]

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Graphical abstract

Highlights Silver-functionalized silica aerogel (AgAero) is novel material for iodine capture. AgAero completely and fast removed I- from different aqueous environments. AgAero exhibited a preferred removal of I- over Br- and Cl-. AgAero was able to remove IO3- in DIW through reduction to I-.

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Abstract

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One of the key challenges for radioactive waste management is the efficient capture and immobilization of radioiodine, because of its radiotoxicity, high mobility in the environment, and long half-life (t1/2 = 1.57 × 107 years). Silver-functionalized silica aerogel (AgAero) represents a strong candidate for safe sequestration of radioiodine from various nuclear waste streams and subsurface environments. Batch sorption experiments up to 10 days long were carried out in oxic and anoxic conditions in both deionized water (DIW) and various Hanford Site Waste Treatment Plant (WTP) off-gas condensate simulants containing from 5 to 10 ppm of iodide (I-) or iodate (IO3-). Also tested was the selectivity of AgAero towards I- in the presence of other halide anions. AgAero exhibited fast and complete removal of I- from DIW, slower but complete removal of I- from WTP off-gas simulants, preferred removal of I- over Br- and Cl-, and it demonstrated ability to remove IO3- through reduction to I-.

1. Introduction

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Keywords: Nuclear waste management; Iodine; Capture; Aqueous environments; Silverfunctionalized silica aerogel

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Iodine-129 (129I) is a radionuclide of concern at many nuclear waste storage sites around the world.1-5 It is also released into aqueous solutions and gas streams during the reprocessing of used nuclear fuel6-8 and nuclear accidents9. Due to its long half-life of 1.57 × 107 years and high mobility in the environment, best evidenced by its absence in the Oklo natural reactor in the Gabon10, iodine species (in the gaseous state as molecular iodine or organic iodides, or in the aqueous state as iodide [I-] or iodate [IO3-]) must be captured and immobilized into a stable matrix for safe and long-term disposal.11 At the United States Department of Energy (DOE) Hanford Site in Eastern Washington, I plumes covering an area of more than 50 km2 exist, resulting from radioactive liquid discharges into unlined surface cribs and leaking from the nuclear waste tanks on the site.12-13 This contamination highlights the need for development of novel sorbent materials for efficient sequestration of iodine from liquid wastes and contaminated ground water. In addition, joule129

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heated ceramic melter technology will be used at the Hanford Tank Waste Treatment and Immobilization Plant (WTP) for immobilization of low-activity radioactive wastes (LAW) into a borosilicate glass. This waste stream is a highly caustic (pH ~ 13.6) and high ionic strength aqueous solution containing Na+ (5-10 M), K+, Al(OH)4-, Cl-, F-, NO2-, NO3-, OH-, CO32-, organics, and minor species including dissolved metals (Cr, Ni, Cd, Pb) and radionuclides such as 99Tc and 129I14. High temperature (~ 1150 °C) required for vitrification of LAW poses a challenge for incorporation of several radionuclides, including 129I, into the glass due to their high volatility.15-16 This volatility results in the accumulation of the radionuclides in an aqueous off-gas condensate. Instead of recycling this secondary-waste stream back to the vitrification facility, treatment with a sorbent is being considered to capture the radionuclides and to immobilize them with an alternative immobilization process. Doing so would shorten the duration of the vitrification campaign and decrease the quantity of waste glass produced.

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Previously, several sorbent materials for capturing iodine from the gas and aqueous phases have been reported, such as metal sulfides17, hydrotalcites18, silicates18 and coals17-19, as well as chalcogels20, natural organic material21, metal oxides22, layered double hydroxides23,composite absorbents24, nano-materials25-26, zeolites27 and aerogels28-32. Materials containing Ag, to facilitate the precipitation of AgI (Ksp = 8 × 10-17)33 as the stable matrix for I, usually have the highest performance for iodine removal.19 A desirable sorbent for efficient removal of iodine should have a high surface area and easy-to-access active sites. Of the previously discussed materials zeolites are the standard benchmark for I treatment.34 The porous framework of the zeolites provides high surface area with low density and easy access to Ag within the zeolite. However, extensive studies at Oak Ridge National Laboratory indicated that the iodine sorption for silver-containing zeolite mordenite decreased markedly when it was exposed to gas streams containing H2O and NOx.35-36 In addition, release of I from loaded zeolites does not meet the requirements for long term disposal.37 Further immobilization of the iodine containing materials is required, such as in a cement or grout based waste form38 or a lowtemperature glass composite material39-40.

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Silica-based aerogels present a promising alternative to iodine treatment because they possess a high surface area and mesoporous pore volume, and can be functionalized with Ag.4144 . The major advantage of silver-functionalized silica aerogel (AgAero) is its ability to be sintered/densified after capturing iodine, producing a silica-based waste form with encapsulated inclusions of AgI. The release of iodine from consolidated aerogel is then controlled by the dissolution of the durable silica matrix to expose the AgI. Commercially available technologies such as hot uniaxial pressure, hot isostatic pressing and spark plasma sintering have all been shown to be effective methods for consolidation of iodine-loaded AgAero.32,45 In addition, the high long-term stability, high selectivity and high sorption capacity of AgAero toward gaseous I2 with iodine capacities up to 48 mass% was demonstrated under prototypical off-gas conditions.43, 46,47

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As previous work focused on gas-phase capture of iodine, the study reported here investigates efficiency of AgAero for removing iodine, as iodide (I-) and iodate (IO3-), from aqueous streams. The sorption performance of the AgAero was tested in neutral, low ionic strength environments as well as simulated secondary-waste streams. The selected samples were characterized with X-ray diffraction (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). 2. Experimental 2.1. Ag0-Functionalized Silica Aerogel (AgAero)

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Granules of AgAero were synthesized using a previously developed procedure.41 Figure1A shows in detail functional groups installed on a silica aerogel support. The silver nanoparticles were anchored to a propylthiol monolayer. Figure1B shows the physical appearance of the granules. The granules were black with yellow spots, larger than 0.85 mm, and had a bulk density of 460 kg/m3. Figure 2 shows TEM images of silver nanoparticles on silica aerogel. These nanoparticles were less than 10 nm big and uniformly distributed on pore surfaces. The composition of the AgAero as determined with inductively coupled plasma optical emission spectroscopy (ICP-OES) was 35.5 mass % of Ag, 1.0 mass of % S and 51.5 mass % of SiO2. The remaining 12 mass % consisted of physically sorbed water and organic moiety (installed on pore surfaces during functionalization), the quantity of which was determined by mass loss during heating at 5°C/min from room temperature to 350°C and holding at this temperature for 10 min. Figure 3 shows type IV isotherm for AgAero which is typical for mesoporous materials (pore sizes from 2 to 50 nm). The relatively small degree of adsorption/desorption hysteresis indicates that the energetics of the pore-filling and -emptying processes are similar in nature. Figure 4 shows adsorption differential pore volume dV/d(logd) vs. pore size for AgAero. Brunauer-Emmett-Teller (BET) characterization of the granules revealed a surface area (S) of 85 m2/g, pore volume (V) of 0.22 × 10-6 m3/g, and adsorption/desorption (ads/des) pore size of 15/7.5 nm. This indicates a high degree of pore surface functionalization when compared to raw silica aerogel (S = 923 m2/g, V = 7.1 × 10-6 m3/g, and ads/des = 60/15 nm). The reduction capacity of the AgAero was 4230 ± 350 microequivalents of electrons per gram as measured with a Ce(IV) method48.

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2.2. Solutions and WTP Off-Gas Simulants All solutions were prepared using deionized water (DIW) (18.2 MΩ∙cm at 25°C) delivered from a Millipore® system. Chemicals such as KCl, KBr, KI, KIO3, and those listed in Table 1 (Sigma-Aldrich, reagent grade) were all used as received. The secondary-waste simulant, based on a WTP off-gas condensate waste stream, was developed by Hanford Tank Waste Operation Simulator49. The simulant, WTP off-gas condensate, was prepared by adding the components to DIW in the order and amounts presented in Table 1. The final pH of the simulant was 6. Two additional simulants were prepared to investigate the effect of Cr and pH

on sorption performance: 1) “WTP off-gas condensate no Cr” (Na2Cr2O7•2H2O was omitted) and 2) “WTP off-gas condensate pH 9.5” (NaOH was added to adjust the pH to 9.5). No solids or precipitates were present in any of the prepared simulants. 2.3. Batch Experiments

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In each case, 104-mL solutions were prepared in 125 mL polytetrafluoroethylene (PTFE) bottles. This volume allowed collection of two blank samples (2 mL each) prior to addition of the solid sorbent. The solutions were spiked with the species of interest (iodide, iodate, chloride, and bromide) from a 10,000 ppm stock solutions prepared with Cl- (282 mmol/L as KCl), Br(125 mmol/L, as KBr), I- (78 mmol/L as KI) , and IO3- (57 mmol/L as KIO3) in DIW. Then, two 2-mL samples were collected from the spiked solutions to determine the initial concentration of the species of interest (blank samples). Following collection of blank samples, the aerogel granules were introduced into solutions.

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The batch experiments in DIW were performed in an anaerobic chamber, containing N2 with a small amount of H2 (0.7%). This setup prevented drastic pH drifts due to ingress of CO2 into a solution. The batch reactors were placed in an anaerobic chamber for 24 h, after which the solutions were spiked with concentrated IO3- stock (10 000 ppm IO3- (57 mmol/L) from NaIO3) or I- stock (10 000 ppm I- (78 mmol/L) from NaI) solutions. The testing in the WTP off-gas condensate simulant was performed on the benchtop in open atmosphere to simulate realistic conditions at a treatment facility. It was expected that the WTP off gas condensate simulant would act as a buffer and therefore be resistant to the ingress of CO2. In DIW the target concentration of iodide of 5.6 ppm (0.044 mmol/L) was determined as 10 times the value predicted in LAW with a Na concentration of 6.5 mol/L, based on the HTWOS model. The measured concentration of this solution was 7 ppm (0.05 mmol/L). In the WTP off-gas condensate simulant, a higher concentration of 10 ppm (0.078 mmol/L) was selected to improve detection in the more complex simulant matrix and the measured concentration was identical to the target of 0.078 mmol/L.

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For all batch tests, following introduction of 1 g of the AgAero to 100-mL solution, sampling occurred at regular intervals, with care taken not to remove any of the AgAero granules from the solution. Samples of 2 mL were collected at each interval and filtered with a 0.2 µm filter membrane syringe.

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Following sampling the solutions were analyzed for total iodine quantitatively at mass 127 using a Perkin Elmer ELAN DRC II (or Thermo Scientific X-Series 2) quadrupole Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) and an Elemental Scientific SC2 (SC4 on the X-Series) DX FAST auto-sampler interface. While ion chromatography (IC) measurements were made for other halides on a Dionex DX-600 instrument. The analytical process followed for the ICP-MS and IC is given here, using ICP-MS as an example. The instrument was calibrated using standards made by the High-Purity Standards Corporation to generate calibration curves. The seven calibration standards ranged from 0.05ppb to 5ppb. This calibration was verified

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immediately with an initial calibration verification (ICV) and during sample analysis with continuing calibration verifications (CCV) which are run every ten samples at a minimum as per Hanford Analytical Quality Assurance Requirements Document (HASQARD, available online at http://www.hanford.gov/page.cfm/AnalyticalServices) requirements. Calibration blanks were also analyzed after each calibration verification to ensure background signals and potential carryover effects were not a factor. The calibration was independently verified using standards made by Inorganic Ventures. All measured calibration verification values must be within ±10% of their known concentrations to comply with the QA/QC requirements as defined in HASQARD Volumes 1 and 4 (Note: Conducting Analytical Work in Support of Regulatory Programs is the name of the document that PNNL uses to remain in compliance with HASQARD). A 10ppb Rh and Sb solution was added as an on line internal standard to all samples, standards and blanks during the analysis to demonstrate the stability of the instrument and sample introduction system.

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A Method Detection limit (MDL) of 0.0126 ppb was established by running the lowest calibration standard (0.05 ppb) seven consecutive times and multiplying the standard deviation of those seven replicates by 3.142 (student t-test value for n=7) to establish an Instrument Detection Limit (IDL) and then multiplying that number by 5 to get the Method Detection Limit (MDL). This process was repeated three times on non-consecutive days and averaged to establish a working MDL in ppb. This resulted in a method detection limit 25 µg/L for total iodine and method detection limit of 2.5 µg/L Cl and 5 µg/L Br using IC.

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All samples and standards were diluted with a solution of 0.5% Triethylene Tetramine and twice deionized water with resistivity no lower than 18.0 MΩ-cm. The Triethylene Tetramine is made by Inorganic Ventures (product # UNS-2B). All Instrument blanks used the same solution.

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2.4. Scanning Electron Microscopy and Energy Dispersive Spectroscopy

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A JEOL JSM-7001F/TTLS scanning electron microscope (JEOL USA, Inc., Peabody, MA) equipped with a field emission gun was used to examine granules and powders of selected samples in low-vacuum mode at an accelerating voltage of 15 kV to minimize beam penetration. An EFlash® 6-60 Si-drift detector (Bruker, Madison, WI) was used to conduct EDS for elemental spot analysis.

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2.5. X-ray Diffraction Selected samples were hand-ground to a fine powder with an agate mortar and pestle. This powder was dispersed in a couple of drops of ethanol before being collected with a pipette and deposited on the surfaces of zero-background XRD sample holders. The samples were scanned with an X-ray diffractometer (Bruker D8 Advanced, Bruker AXS Inc., Madison, WI) configured with a Cu Kα target (λ = 1.5406 Å) set to a power level of 40 kV and 40 mA,

goniometer radius of 250 mm, 0.3° fixed divergence slit, and a LynxEyeTM position-sensitive detector with a collection window of 3° 2θ. The scan parameters were 0.03° 2θ step size, 4 s dwell time, and 5 to 80° 2θ scan range. Bruker AXS DIFFRACplus EVA software was used to identify crystalline phases. 2.6. X-ray Photoelectron Spectroscopy

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Selected powder samples were mounted using carbon tape on a silicon substrate and analyzed using a Kratos Analytical AXIS Ultra X-ray photoelectron spectrometer. Survey scans and regional scans for I- and IO3- were collected. Data were analyzed using CasaXPS. 3. Results and Discussion 3.1. Iodide Removal

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The AgAero was first tested for I- removal in DIW and WTP off-gas condensate simulants. Figure 5 shows the time resolved percentage of total I- removed from different solutions in the batch experiments. In DIW (performed in anaerobic conditions), the AgAero removed 98% of the initial 5.6 ppm (0.044 mmol/L) I- after the first hour of contact; by 9 days of contact time there was no I- (below detection limit of 25 µg/L) left in the solution. In the WTP off-gas condensate simulant (in oxic conditions), the removal of the initial 10 ppm (0.078 mmol/L) of I- was not as fast: ~ 40% of total I- was removed after 7 h of exposure. However, the AgAero was still highly successful in sequestering I- from the WTP off-gas condensate simulant by removing all the iodine (below detection limit) after 7 days of contact.

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A separate batch experiment was performed to determine maximum removal capacity of the AgAero. Approximately 1 g of the AgAero was added to either 20 mL of 17 000 ppm (133 mmol/L) solution of I- in DIW or 100 mL of a 3.8 g/L I- (0.030 mmol/L) solution in DIW and allowed to sorb I- for 120 h. The determined maximum removal capacity of the AgAero was 88.0 ± 6.8 mg I-/g AgAero, which corresponds to 21% silver utilization in the aqueous environment based on the total Ag content. This low utilization might be caused by hydrophobic nature of the sorbent. This would prevent a full contact of silver nanoparticles inventory with an iodine laden solution. Figure 6 shows XRD patterns for AgAero as prepared and after the maximum I- sorption test. The only identified phases were unreacted silver metal and AgI (product of sorption).

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The Cr(VI) that is present in the WTP off gas condensate simulant is a redox active element which may interfere with I- removal and may interact with the sulfide component of the AgAero. However, nearly identical I- removal rates were observed for the AgAero in the WTP off-gas condensate simulant prepared without Cr; 39.2% and 99.5% of the I- was removed after 7 h and 7 days, respectively. The pH may also affect the ability of the AgAero to remove I- from solution. However, increase of pH to 9.5 for the WTP off-gas condensate simulant had little effect on overall I- removal; 33.2 % of I- was removed after 7 h and 99.4 % removed during 7 days. Figure 7 shows that first-order sorption rates for iodide removal from the various WTP

off-gas condensate simulants were about the same, 0.029–0.031 h-1. These results show the AgAero has a high affinity for I- in neutral waste streams such as WTP off-gas condensate and its sorption performance is not affected by Cr(VI) or by an increase of pH (pH <9.5).

𝐾𝑑 =

𝑐𝑖 − 𝑐𝑡 𝑉 × 𝑐𝑡 𝑚

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The modified distribution coefficient, Kd, is commonly used to compare the transfer of species from solution to a solid material. It is determined using the following equation:

where ci is the initial concentration of species in solution, ct is the concentration after a time interval, V is the volume of solution (mL) and m is the mass of solid (g). A higher Kd translates to a greater amount of the species being transferred to the solid surface. A comparison of the Kds for AgAero and other materials (the highest Kd in each report) for iodide removal from nearly neutral solutions is given in Table 2. The AgAero is among the best-performing materials.

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3.2. Iodate Removal

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Iodate may be generated as a nonvolatile product in nuclear waste off gas streams 50 and is the dominant species of the iodine subsurface contamination at the Hanford site51. In both scenarios, efficient removal of IO3- from the aqueous phase would reduce the environmental risk of iodine contamination. The ability of AgAero to remove IO3- was investigated in DIW and WTP off-gas condensate simulant in batch experiments. The results are given in the Figure 8. In DIW, the AgAero successfully removed IO3-, although the rate was not as rapid as the one observed for iodide (shown in Figure 5). After 4 h of contact, ~16% of total IO3- was removed from DIW solution. After 5 days, 75% of the IO3- was removed by the AgAero. An additional 7 days decreased the total concentration of IO3- by 94 %. Figure 9 shows that first-order sorption rate for iodide removal from DIW was about 9.35 × 10-3 h-1. In the WTP off-gas condensate simulant, the IO3- removal by AgAero was drastically decreased, with only 7% of the IO3removed after 7 days of contact. The origin of the decrease in IO3- removal becomes evident from XPS spectra shown in Figure 10. Following the batch experiment in DIW with I-, the AgAero spectrum shows two peaks in the I3d/7 region at 631 eV and 620 eV, corresponding to the expected bands for I-.52 Following the batch experiment in DIW with IO3- , the XPS spectrum from the AgAero displays bands in the same position as for AgAero tested for I- removal. The expected positions for iodate bands are ~635 eV and 624 eV.53 Therefore, the end product of IO3- removal by the AgAero in DIW is I-. In order for the I- to form, the IO3- must first be reduced. The reduction potential of IO3- is 1.19 V (vs. standard hydrogen electrode (SHE) )54. Using the Ce(IV) method,48 the reduction capacity of the AgAero was measured at 4230 ± 350 microequivalents of electrons per gram of AgAero, which is high relative to other highly active reductants55. Two elements in the AgAero system contribute to high reduction capacity: i) Ag (the reduction potential of Ag+ is 0.80 V vs SHE) and ii) sulfur (with multiple oxidation pathways).33 Both of these species can serve as reducing species for conversion of IO3- to I-. However, in the WTP off-gas condensate system there are two primary sources of interference. First, the Cr(VI) present in the WTP off –gas condensate simulant will be preferentially reduced

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before the IO3- with a standard reduction potential of 1.33 V vs. SHE. This reduction of Cr(VI) would use up the available reductive capacity of the AgAero, thus limiting the amount of IO3that can be reduced. As well the WTP off-gas condensate simulant contains a competitive halide, chloride, which may preferentially precipitate with any available Ag. This competition is discussed in Section 3.3 below. Figure 11 shows an SEM image of AgAero after a sorption test in DIW containing 5 ppm of I- (0.039 mmol/L) and IO3- (0.029 mmol/L). Table 3 shows the EDS analysis of three different areas marked in Figure 11, revealing presence of AgI (product of sorption), SiO2 (silica aerogel support), and S (from functionalization). 3.3. Selectivity of AgAero

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In the batch experiments with WTP off gas condensate simulants, the rate of I- removal was slower than that in DIW. It is possible that competition was occurring between the halide anions present in the simulants and the I-. Competing anions present in the waste streams may hinder removal of iodine species. A test was performed to investigate the selectivity of the AgAero towards halides. Approximately 0.5 mmol/L of I- (0.45 mmol/L, 57.1 ppm), Cl- (0.49 mmol/L, 17.4 ppm) and Br- (0.48 mmol/L, 38.4 ppm) were spiked into the same bottle containig100 mL DIW, followed by contact with 1 g of AgAero. The change in concentration of each anion over time is shown in Figure 12. The AgAero was capable of removing all three anions from solution, with I- being removed at a much faster rate (0.038 h-1 for I- vs 0.026 and 0.016 h-1 for Br- and Cl-, respectively), as shown in Figure 13. After 4 h of contact the AgAero removed 40% of I-, 24% of Br-, and 19% of Cl-. After 48 h, the concentration of I- dropped to 0.06 mmol/L (7.6 ppm, ~14 % of its original value), while the Br- concentration had decreased to 0.13 mmol/L (10.4 ppm, ~28 % of its original value), and that of Cl- to 0.22 mmol/L (7.8 ppm, ~46 % of its original value). This difference prevailed after 120 h of sorption with 94% of I-, 86% of Br-, and 70% of Cl- removed from solution. The AgAero exhibited a preferential removal of I- in the presence of other halide species. These results demonstrate the ability of AgAero sorbent to function in complex environments, such as various waste streams and subsurface environments. 4. Conclusion

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The AgAero was tested for its ability to remove I- and IO3- from a variety of aqueous environments. In neutral, less-complex media, the AgAero demonstrated a rapid removal of Ifrom solution, preferred removal of I- over Br- and Cl-, and ability to remove IO3- through a conversion to I-. Coupled with its strong performance in gaseous iodine removal and ability to be sintered into a high-iodine-loaded waste form, the ability of AgAero to remove both iodide and iodate from aqueous environments strengthens the candidacy of AgAero to be used as an iodine capture technology in various nuclear waste streams and subsurface environments.

Acknowledgement

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The authors gratefully acknowledge the financial support of the U.S. Department of Energy Federal Project Office for the Hanford Tank Waste Treatment and Immobilization Plant. The authors would also like to acknowledge Steven R. Shen for laboratory assistance and help with experiments and Yingee Du for XPS analyses. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830.

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15. Riley, B. J.; Schweiger, M. J.; Kim, D.-S.; Lukens Jr, W. W.; Williams, B. D.; Iovin, C.; Rodriguez, C. P.; Overman, N. R.; Bowden, M. E.; Dixon, D. R.; Crum, J. V.; McCloy, J. S.; Kruger, A. A., Iodine solubility in a low-activity waste borosilicate glass at 1000 °C. Journal of Nuclear Materials 2014, 452 (1–3), 178-188. 16. Yang, J. H.; Shin, J. M.; Park, J. J.; Park, G. I., Waste Form of Silver Iodide (AgI) with Low-Temperature Sintering Glasses. Separation Science and Technology 2014, 49 (2), 298-304. 17. Ikeda, Y.; Sazarashi, M.; Tsuji, M.; Seki, R.; Yoshikawa, H., Adsorption of I Ions on Cinnabar for 129-I Waste Management. Radiochimica Acta 1994, 65, 195-198. 18. Mattigod, S. V.; Serne, R. J.; Fryxell, G. E., Selection and Testing of "Getters" for Adsorption of Iodine-129 and Technetium-99: A Review. PNNL-14208 2003, Pacific Northwest National Laboratory, Richland, Washington. 19. Krumhansl, J. L.; Pless, J. D.; Chwirka, J. B.; Holt, K. C., Yucca Mountain Project Getter Program Results (Year 1). SAND2006-3869 2006, Sandia National Laboratory, Albuquerque, New Mexico. 20. Riley, B. J.; Chun, J.; Um, W.; Lepry, W. C.; Matyas, J.; Olszta, M. J.; Li, X.; Polychronopoulou, K.; Kanatzidis, M. G., Chalcogen-Based Aerogels As Sorbents for Radionuclide Remediation. Environmental Science & Technology 2013, 47 (13), 7540-7547. 21. Steinberg, S.; Schmett, G.; Kimble, G.; Emerson, D.; Turner, M.; Rudin, M., Immobilization of fission iodine by reaction with insoluble natural organic matter. J Radioanal Nucl Chem 2008, 277 (1), 175-183. 22. Zhang, X.; Stewart, S.; Shoesmith, D. W.; Wren, J. C., Interaction of Aqueous Iodine Species with Ag2O ∕ Ag Surfaces. Journal of The Electrochemical Society 2007, 154 (4), F70F76. 23. Kentjono, L.; Liu, J. C.; Chang, W. C.; Irawan, C., Removal of boron and iodine from optoelectronic wastewater using Mg–Al (NO3) layered double hydroxide. Desalination 2010, 262 (1–3), 280-283. 24. Zhang, H.; Gao, X.; Guo, T.; Li, Q.; Liu, H.; Ye, X.; Guo, M.; Wu, Z., Adsorption of iodide ions on a calcium alginate–silver chloride composite adsorbent. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2011, 386 (1–3), 166-171. 25. Mnasri, N.; Charnay, C.; de Ménorval, L.-C.; Moussaoui, Y.; Elaloui, E.; Zajac, J., Silver nanoparticle-containing submicron-in-size mesoporous silica-based systems for iodine entrapment and immobilization from gas phase. Microporous and Mesoporous Materials 2014, 196 (0), 305-313. 26. Madrakian, T.; Afkhami, A.; Zolfigol, M. A.; Ahmadi, M.; Koukabi, N., Application of Modified Silica Coated Magnetite Nanoparticles for Removal of Iodine from Water Samples. Nano-Micro Letters 2012, 4 (1), 57-63. 27. Asmussen, R. M.; Neeway, J. J.; Lawter, A. R.; Wilson, A.; Qafoku, N., Silver Based Getters for 129-I Removal from Low Activity Waste. Radiochimica Acta 2016, Accepted, DOI:http://dx.doi.org/10.1515/ract-2016-2598. 28. Sánchez-Polo, M.; Rivera-Utrilla, J.; Salhi, E.; von Gunten, U., Removal of bromide and iodide anions from drinking water by silver-activated carbon aerogels. Journal of Colloid and Interface Science 2006, 300 (1), 437-441. 29. Sánchez-Polo, M.; Rivera-Utrilla, J.; von Gunten, U., Bromide and iodide removal from waters under dynamic conditions by Ag-doped aerogels. Journal of Colloid and Interface Science 2007, 306 (1), 183-186.

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30. Yang, D. J.; Zheng, Z. F.; Zhu, H. Y.; Liu, H. W.; Gao, X. P., Titanate Nanofibers as Intelligent Absorbents for the Removal of Radioactive Ions from Water. Advanced Materials 2008, 20 (14), 2777-2781. 31. Lu, Y.; Liu, H.; Gao, R.; Xiao, S.; Zhang, M.; Yin, Y.; Wang, S.; Li, J.; Yang, D., Coherent-Interface-Assembled Ag2O-Anchored Nanofibrillated Cellulose Porous Aerogels for Radioactive Iodine Capture. ACS Applied Materials & Interfaces 2016, 8 (42), 29179-29185. 32. Gao, R.; Lu, Y.; Xiao, S.; Li, J., Facile Fabrication of Nanofibrillated Chitin/Ag(2)O Heterostructured Aerogels with High Iodine Capture Efficiency. Scientific Reports 2017, 7, 4303. 33. Haynes, W. M., CRC handbook of chemistry and physics. CRC press: 2014. 34. Chapman, K. W.; Chupas, P. J.; Nenoff, T. M., Radioactive Iodine Capture in SilverContaining Mordenites through Nanoscale Silver Iodide Formation. Journal of the American Chemical Society 2010, 132 (26), 8897-8899. 35. Jubin, R.; RAMEY, D.; SPENCER, B.; Anderson, K.; Robinson, S., Impact of Pretreatment and Aging on the Iodine Capture Performance of Silver-Exchanged Mordenite. Waste Management 2011. 36. Bruffey, S. H.; Patton, K. K.; Walker Jr, J.; Jubin, R. T. Complete NO and NO2 Aging Study for AgZ; Oak Ridge National Laboratory: Oak Ridge, TN, 2015. 37. Scheele, R.; Wend, C.; Buchmiller, W.; Kozelisky, A.; Sell, R., Preliminary Evaluation of Spent Silver Mordenite Disposal Forms Resulting from Gaseous Radioiodine Control at Hanford’s Waste Treatment Plant. PNWD-3225, WTP-RPT-039 2002, Pacific Northwest National Laboratory (Richland, WA, USA). 38. Asmussen, R. M.; Pearce, C. I.; Lawter, A. R.; Miller, B. W.; Neeway, J. J.; Lawler, B.; Smith, G.; Serne, J.; Swanberg, D. J.; Qafoku, N., Preparation, Performance and Mechanism of Tc and I Getters in Cementitious Waste Forms Proceedings of Waste Management Symposium 2017, 2017, Paper 17124. 39. Garino, T. J.; Nenoff, T. M.; Krumhansl, J. L.; Rademacher, D. X., Low‐Temperature Sintering Bi–Si–Zn‐Oxide Glasses for Use in Either Glass Composite Materials or Core/Shell 129I Waste Forms. Journal of the American Ceramic Society 2011, 94 (8), 2412-2419. 40. Nenoff, T. M.; Krumhansl, J. L.; Garino, T. J.; Ockwig, N. W., Low sintering temperature glass waste forms for sequestering radioactive iodine. Google Patents: 2012. 41. Matyas, J.; Fryxell, G.; Busche, B.; Wallace, K.; Fifield, L. In Functionalised silica aerogels: advanced materials to capture and immobilise radioactive iodine, Ceramic Engineering and Science Proceedings, American Ceramic Society, Inc., 735 Ceramic Place Westerville OH 43081 United States: 2011; pp 23-32. 42. Akimov, Y. K., Fields of Application of Aerogels (Review). Instruments and Experimental Techniques 2003, 46 (3), 287-299. 43. Matyas, J.; Canfield, N.; Silaiman, S.; Zumhoff, M., Silica-based waste form for immobilization of iodine from reprocessing plant off-gas streams. Journal of Nuclear Materials 2016, 476, 255-261. 44. Amonette, J. E.; Matyáš, J., Functionalized silica aerogels for gas-phase purification, sensing, and catalysis: A review. Microporous and Mesoporous Materials 2017. 45. Matyáš, J.; Engler, R., Assessment of methods to consolidate iodine-loaded silverfunctionalized silica aerogel. Pacific Northwest National Laboratory, PNNL-22874 2013. 46. Strachan, D. M.; Chun, J.; Matyas, J.; Lepry, W. C.; Riley, B. J.; Ryan, J. V.; Thallapally, P. K. Summary report on the volatile radionuclide and immobilization research for FY2011 at

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Figure Captions

Figure 1 - Schematic of AgAero showing the Si-O network, propylthiol monolayer and silver nanoparticle (A) and an optical image of as prepared granules (B).

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Figure 2 - TEM images of silver nanoparticles on silica aerogel support. Figure 3 - Adsorption/Desorption Isotherms for AgAero.

Figure 4 – Adsorption/Desorption differential pore volume dV/(logd) vs. pore size for AgAero.Figure 5 - Contact-time curves for removal of iodide from DIW (7 ppm (0.055 mmol/L) of I-) and different WTP off-gas condensate simulants (10 ppm (0.078 mmol/L) of I-).

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Figure 6 - XRD patterns and identified phases for as-prepared and iodine-loaded AgAero (after maximum sorption test).

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Figure 7 - First-order sorption rates (k) for iodide removal from WTP off-gas condensate simulants.

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Figure 8 - Contact-time curves for removal of iodate from DIW (7 ppm (0.055 mmol/L) of IO3-) and WTP off-gas condensate (10 ppm (0.078 mmol/L) of IO3-).

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Figure 9 - First-order sorption rate for iodate removal from DIW (rate constant of 9.35 × 10-3 h1 ).

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Figure 10 - XPS spectra of the I3d/7 region following the batch experiments with DIW containing 7 ppm of I- or IO3-.

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Figure 11 - SEM backscattered electron image of AgAero after sorption testing in iodide/iodate solution (5 ppm). Elemental composition of highlighted areas 1–3 is summarized in Table 3.

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Figure 12 - Selectivity of the AgAero towards halides in DIW spiked with ~0.5 mmol/L of I-, Br-, and Cl-.

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Figure 13 - First-order sorption rates for I-, Br-, and Cl- removal from DIW.

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Figure 6 As prepared AgAero Iodine-loaded AgAero

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Figure 10 I 3d/7

IO3- in DIW

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Figure 13

Table 1 - Composition of the WTP off gas condensate simulant (pH ~6). Table 1 Concentration, ppm

Aluminum nitrate nonahydrate

Al(NO3)3•9H2O

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Sodium dichromate dihydrate

Na2Cr2O7•2H2O

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Potassium chloride Sodium chloride Sodium fluoride Ammonium nitrate

KCl NaCl NaF NH4NO3

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Sodium nitrate

NaNO3

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Sodium nitrite

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Ammonium sulfate Dibasic sodium phosphate dihydrate

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Table 2 - Comparison of Kd values for iodide removal from aqueous solutions. Table 2

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Material Argentite Ag-C Hg-thiol SAMMS Ag-Z AgAero in WTP off-gas condensate simulant AgAero in DIW Hydrotalcite (Cu6Al2(OH)18) Chalcopyrite Cu metal Galena Sub-bituminous coal S-cinnabar Illite Silica coated magnetite nanoparticles Mg-Al layered double hydroxide TiO2 Calcium alginate AgCl composition absorbent

Kd 6.20E+05 3.70E+05 1.00E+05 9.10E+04 1.46E+04 1.50E+04 9.54E+03 7.00E+02 1.90E+02 1.37E+02 132 103 24 20.40 10.1 mg/g 3 1.15

Solution pH 7 7 7.24 7 6 7 7 - 8.5 7.2 7.2 6.14 7 6 7.9 7 9.2 9.5 6

Reference 27 27 18 27

19 56 57 58 59 17 60 26 23 61 62

Table 3 - Concentrations of Si, O, S, Ag, and I in mass% for three areas from Figure 8. Table 3 O 41.51 35.14 38.95

S 8.21 7.66 8.58

Ag 13.73 14.59 10.49

I 0.33 0.43 0.32

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Si 36.22 42.19 41.67

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