Article pubs.acs.org/cm
Chalcogenide Aerogels as Sorbents for Radioactive Iodine K. S. Subrahmanyam,† Debajit Sarma,† Christos D. Malliakas,† Kyriaki Polychronopoulou,‡ Brian J. Riley,§ David A. Pierce,§ Jaehun Chun,§ and Mercouri G. Kanatzidis*,† †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Department of Mechanical Engineering, Khalifa University of Science, Technology, and Research, 127788 Abu Dhabi, United Arab Emirates § Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡
S Supporting Information *
ABSTRACT: Iodine (129I and 131I) is one of the radionuclides released in nuclear fuel reprocessing and poses a risk to public safety due to its involvement in human metabolic processes. In order to prevent the release of hazardous radioactive iodine into the environment, its effective capture and sequestration is pivotal. In the context of finding a suitable matrix for capturing radioactive iodine, several sulfidic chalcogels were explored as iodine sorbents including NiMoS4, CoMoS4, Sb4Sn3S12, Zn2Sn2S6, and K0.16CoSx (x = 4−5). All of the chalcogels showed high uptake, reaching up to 225 mass % (2.25 g/g) of the final mass owing to strong chemical and physical iodine−sulfide interactions. Analysis of the iodine-loaded specimens revealed that the iodine chemically reacted with Sb4Sn3S12, Zn2Sn2S6, and K0.16CoSx to form the metal complexes SbI3, SnI4, and, KI, respectively. The NiMoS4 and CoMoS4 chalcogels did not appear to undergo a chemical reaction with iodine since iodide complexes were not observed with these samples. Once heated, the iodine-loaded chalcogels released iodine in the temperature range of 75 to 220 °C, depending on the nature of iodine speciation. In the case of Sb4Sn3S12 and Zn2Sn2S6, iodine release was observed around 150 °C mainly in the form of SnI4 and SbI3, respectively. The NiMoS4, CoMoS4, and K0.16CoSx released elemental iodine at ∼75 °C, which is consistent with physisorption. Preliminary investigations on consolidation of iodine-loaded Zn2Sn2S6 chalcogel with Sb2S3 as a glass forming additive produced glassy material whose iodine content was around 25 mass %.
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INTRODUCTION The rapidly growing global demand for energy has engendered interest for clean, efficient, low cost, and environmentally friendly alternative energies. Among the alternative energies, nuclear power is the most prominent and feasible source and produces around 11% of the world’s electricity production.1 However, the control of waste products and their safe disposal in a viable waste form is one of the main concerns in nuclear power production.2−4 The nuclear fission of uranium fuel generates volatile radionuclides such as tritium ( 3 H), technetium (99Tc), krypton (85Kr), iodine-129 (129I), and iodine-131 (131I).5 Among them, 129I poses a major long-term risk due to its long half-life (t1/2 = 1.6 × 107 years) and its adverse health effects in humans (also the case with 131I). Currently, there is a strong interest in the nuclear energy community to find effective means to capture these volatile radionuclides, but the focus of this article is on iodine. An effective iodine sorbent should be stable during the treatment processes and capable of selectively capturing large amounts of iodine. In this regard, several materials have been investigated over the last several decades, and the most successful were silver-loaded zeolites.6 On exposure to iodine, silver incorporated within the zeolite reacts to form AgI. Similar concepts (i.e., strong interactions with Ag) with different © 2015 American Chemical Society
materials have also been investigated. Silver-functionalized silica aerogels show high iodine affinity, and the iodine release is minimal during consolidation.7,8 Recently, we have demonstrated that silver-laden polymeric porous organic frameworks absorb a significant amount of I2 vapor and stabilize it in the form of AgI as well as molecular I2.9 Prior to the disposal of the aforementioned AgI-embedded host matrices, it is optimal that they be converted into more viable and environmentally stable waste forms such as being imbedded in a secondary waste form like glass or cement.10−12 The main aim of the present work is the evaluation of different chalcogen-based aerogels called “chalcogels” as sorbents for the capture and immobilization of 129I. Previously, we employed the metathesis chemistry between transition metal linkers and chalcogenide clusters to give a broad and functionally diverse family of metal chalcogenides with high surface areas.13,14 The main advantage of this synthesis method is the capability to tune the properties of chalcogels through appropriate selection of anions and cations. Chalcogels are highly porous and have potential applications in catalysis,15−19 Received: February 2, 2015 Revised: March 11, 2015 Published: March 16, 2015 2619
DOI: 10.1021/acs.chemmater.5b00413 Chem. Mater. 2015, 27, 2619−2626
Article
Chemistry of Materials gas separation,20−24 and environmental remediation.25−28 Furthermore, chalcogels show excellent affinity toward Hg2+ because their chalcogenide surfaces have strong soft Lewis basic properties.13 On the basis of the hard soft acid base (HSAB) principle, it has been predicted that chalcogels might also show good affinity toward I2 since iodine is a soft Lewis acid.26,29 Recently, Riley et al.26−28,30 demonstrated the utility of some chalcogels as effective iodine sorbents in granular as well as polymer hybrid forms. Here, we have extended these studies to new chalcogels, we show that the mechanism of capture is chemisorption, and we demonstrate that they can capture even larger quantities of iodine. Herein, we present a detailed investigation that demonstrates the efficacy of several chalcogels of a wide variety of compositions to adsorb iodine. The chalcogels with nominal compositions NiMoS4, CoMoS4, Sb4Sn3S12, Zn2Sn2S6, and K0.16CoSx were selected to provide insights on how iodine interacts with the respective chalcogenide species. Our results reported here show that the I2−chalcogel interactions extend beyond soft Lewis acid−base chemistry and into redox chemistry between sulfide ions and the structure and I2. Xray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS), and nitrogen adsorption−desorption isotherm measurements were utilized to determine the nature of iodine binding. Furthermore, desorption of iodine from the iodine-loaded chalcogels was studied as a function of time. To illustrate the viable path as a waste form, promising preliminary results on the consolidation of iodine-loaded Zn2Sn2S6 are also reported.
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EXPERIMENTAL SECTION
Co2 + + MoS4 2 − → CoMoS4
(3)
2Zn 2 + + Sn2S6 4 − → Zn2Sn2S6
(4)
K + Co2 + + S52 − → KCoSx
(5)
After gelation, the monolithic gels were soaked in an ethanol/water (1:1 V/V) mixture followed by 100% ethanol for 4 days each. During the soaking period, solvents were exchanged by fresh solvents every 12 h. Then, the residual solvent in the gel framework was removed using supercritical CO2 at 42 °C and a pressure of 9.65 × 106 Pa (1400 psi). Wet gels were cut into pieces and placed in a custom-built metal basket in the glovebox that was quickly transferred into the supercritical drying chamber from the glovebox. In supercritical drying, the remnant alcohol in the gel was exchanged with liquid carbon dioxide (CO2) over a period of 6 h where fresh CO2 was introduced into the chamber every 20 min. Different soaking and exchange times were employed depending on the nature and amount of the gel. At the end, gaseous CO2 was bled very slowly to yield high porosity aerogels, called chalcogels. The drying of gels without using supercritical CO2 but, rather, under vacuum or ambient conditions yielded xerogels with a significant reduction in volume. After preparation, both the chalcogels and xerogels were stored in an inert atmosphere glovebox for further characterization as specified later. Iodine Adsorption Experiments. Two different types of adsorption experiments were performed, and these are denoted by the temperature at which the experiments were conducted as described below in more detail. Type-I Iodine Adsorption Experiments at Elevated Temperatures in Nitrogen Atmosphere. The adsorption testing was carried out in a nitrogen filled glovebox. For these tests, commercially obtained highpurity (Sigma-Aldrich, ≥99.8%, solid) nonradioactive iodine (surrogate) was used. In a typical experiment, 100−600 mg of iodine was placed in a glass vial, and 20−100 mg of sorbent was placed into a conical-shaped filter paper. The filter paper was attached at the top of the vial in such a way that the cone tip was hanging toward the bottom of the vial (see Figure S4 in Supporting Information for a schematic). This setup was placed in another larger vial that was closed with a screw cap and placed in a sand bath with a steady temperature of 60 °C overnight for adsorption testing. Type-II Iodine Adsorption Experiments at Room Temperature under Vacuum. Specimens of ZnSnS and SbSnS were placed in separate tared 20 mL glass scintillation vials. These vials, along with an empty tared vial and a tared vial with pure iodine (99.9999%, Alfa Aesar, Ward Hill, MA) were placed inside a glass vacuum desiccator at ambient temperature (22 ± 1 °C). The other chalcogels were not used in this particular experiment because their inherent static charge made it difficult to keep them in their respective scintillation vials. A dynamic vacuum was applied to the vessel, the vials were periodically removed, and the mass was recorded to measure sorbent mass uptake for the samples and mass loss for the iodine vessel. This process was carried out iteratively over the course of several days until no further mass gain was observed. The mass of iodine uptake (mI) was calculated using eq 6, where mt was the total mass of the sorbent with adsorbed iodine, and ms was the mass of the original sorbent and the mass of adsorbed iodine = mt − ms. The mass of the vial was normalized out using the data collected from the blank vial, which was negligible at 0.8). The adsorption was attributed to micropores in the low pressure regime and both mesopores and macropores in the higher pressure regime. The hysteresis could be attributed to percolation effects that are generally observed in disordered porous materials. The xerogels of NiMoS, SbSnS, ZnSnS, and KCoS exhibited very low specific surface areas typically in the 2621
DOI: 10.1021/acs.chemmater.5b00413 Chem. Mater. 2015, 27, 2619−2626
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Chemistry of Materials
Table 1. BET Specific Surface Areas, Iodine Mass-Uptake Obtained from Mass Difference, and Mass Loss of Iodine Loaded Chalcogels from TGA
sample
BET specific surface area (m2 g−1)
NiMoS CoMoS SbSnS ZnSnS KCoS
490 360 240 400 350
iodine uptake (Type-I) obtained by mass difference after 2 days (total mass/mass of initial chalcogel in g/g·100)
mass loss of iodineloaded samples obtained from TGA (mass %)
residence time (days)
225 200 200 225 160
50 65 70 70 35
42 25 21 21 30
violet in both outer and interior regions of the gel indicating that iodine adsorption occurred throughout the gel. As NiMoS, CoMoS, and KCoS were initially black in color and SbSnS is red, no noticeable color change could be observed for these materials after iodine adsorption. It appears that the high porosity of the gels facilitates the high uptake and diffusion throughout the gel. The iodine-loaded samples were stored at ambient temperature under nitrogen for 3−4 weeks and then examined with XRD, TGA, and SEM-EDS analyses. The XRD patterns of iodine-loaded chalcogels revealed that the iodine-loaded NiMoS and CoMoS chalcogels were amorphous, whereas the iodine-loaded SbSnS, ZnSnS, and KCoS chalcogels had sharp crystalline diffraction peaks that were indexed to SbI3, SnI4, and KI, respectively (Figure 2). The presence of KI in iodine-loaded CoS is attributed to the reaction of iodine with the residual potassium in the KCoS chalcogel. This chalcogel formally is an anionic [CoxSy]n− network that balances the residual anionic charge with K+ ions as was reported previously.32 This confirms that the iodine undergoes mainly physisorption with the NiMoS and CoMoS chalcogels while it reacts chemically with the SbSnS, ZnSnS, and KCoS chalcogels, oxidizing the sulfide ions to form the iodide ions which combine with the Sb3+, Sn4+, and K+ cations. TGA of iodine-loaded chalcogels was carried out to assess the nature of iodine binding and the thermal stability of chalcogels with and without iodine loading. Figure 3 presents the TGA traces of the both pristine and iodine-loaded samples of NiMoS, CoMoS, SbSnS, and ZnSnS. Among the pristine chalcogels, ZnSnS and SbSnS were most stable with mass losses
Figure 2. Powder X-ray diffraction patterns of pristine and iodineloaded (a) ZnSnS, (b) SbSnS, and (c) KCoS chalcogels. The high diffuse background is caused by X-ray fluorescence from the incident radiation used (Cu Ka). The sharp crystalline peaks in iodine-loaded ZnSnS, SbSnS, and KCoS represent SbI3, SnI4, and KI, respectively.
conditions employed in our experiments for the I2 vapor capture may be different from those used in other classes of materials reported in the literature. The high iodine uptake values of the chalcogels are indeed noteworthy and imply a high potential to act as host matrices to capture radioactive iodine. Among the chalcogels evaluated here, ZnSnS and NiMoS showed the highest iodine uptake, whereas KCoS exhibited the lowest (Table 1). Interestingly, SbSnS had the smallest specific surface area but performed better than KCoS in the adsorption experiments suggesting a higher reactivity. NiMoS exhibited better uptake than its cobalt analogue following the same order as their specific surface areas. The above observations clearly imply that iodine sorption capacity mainly depends on the nature of chalcogel chemistries and their porosity. On adsorption, the pristine fluffy aerogels became relatively dense in comparison to their initial state. The ZnSnS chalcogel showed a color change from pale yellow to
Figure 3. TGA traces of pristine and iodine-loaded (a) NiMoS, (b) CoMoS, (c) SbSnS, and (d) ZnSnS. 2622
DOI: 10.1021/acs.chemmater.5b00413 Chem. Mater. 2015, 27, 2619−2626
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Chemistry of Materials of ∼10 mass % after heating to 600 °C. The mass loss at the early stage in ZnSnS up to 200 °C corresponds to the loss of solvent residue from the chalcogel synthesis process. The mass loss in both the ZnSnS and SbSnS chalcogels from 200 to 450 °C is attributed to the removal of sulfur and thermal decomposition at ≥500 °C. The NiMoS and CoMoS chalcogels were less thermally stable relative to ZnSnS and SbSnS showing a loss of up to 20 mass %. Both chalcogels showed a sharp mass loss around 300 °C followed by a gradual mass loss up to 600 °C, which correspond to the thermal decomposition and loss of sulfur, respectively. Among the chalcogels employed, KCoS was the least thermally stable and started decomposing even after 150 °C with a total mass loss of about 40 mass % (Figure S5, Supporting Information). This behavior is consistent with the presence of polysulfide units in the structure which are prone to thermal decomposition [Sx2− → S2− + (x − 1)S]. Depending on the composition, the iodine-loaded chalcogels showed mass losses at various temperatures above 75 °C. Iodine-loaded ZnSnS and SbSnS showed a mass drop at ∼150 °C, whereas iodine-loaded NiMoS, CoMoS, and KCoS started to lose mass starting at 75 °C. The gradual mass losses at temperatures >200 °C in iodine-loaded NiMoS, CoMoS, and KCoS follow a trend similar to that of their pristine counterparts. The mass losses at
