Ag-exchanged mesoporous chromium terephthalate with sulfonate for removing radioactive methyl iodide at extremely low concentrations in humid environments

Highlights

• Ag-exchanged mesoporous MIL-101 materials with a sulfonate group are prepared for the effective removal of methyl iodide.

• Enhanced hydrophobicity of Ag-MIL-101 functionalized by alkyl sulfonate group is confirmed by the in situ FT-IR and HI.

• Outstanding performance for CH₃I removal is achieved even under RH 95% and at the ppb CH₃I concentration level.

Abstract

The development of efficient adsorbents to remove radioactive methyl iodide (CH₃I) in humid environments is crucial for air purification after pollution by nuclear power plant waste. In this work, we successfully prepared a post-synthetic covalent modified MIL-101 with a sulfonate group followed by the ion-exchange of Ag (I), which is well characterized by diffuse reflectance FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and the hydrophobic index (HI). After modification of the MOFs, we applied functionalized MIL-101 obtained by either one-pot synthesis (MIL-101-SO₃Ag) or a post-synthetic modification process (MIL-101-RSO₃Ag, R = NH(CH₂)₃) to remove the CH₃I at an extremely low concentration (0.31 ppm) in an environment with very high relative humidity (RH 95%). Enhanced hydrophobicity of the surface-modified MIL-101 was evaluated by examining the HI with the competitive adsorption of water and cyclohexane vapor, with a high surface area maintained, as confirmed by Ar physisorption. Interestingly, the post-synthetically modified MIL-101-RSO₃Ag showed exceptional adsorption performance as determined by its decontamination factor (DF = 195,350) at 303 K and RH 95%. This performance was in comparison to Ag (I)-exchanged 13X zeolite and MIL-101-SO₃Ag, which include much higher amounts of Ag. Furthermore, MIL-101-RSO₃Ag retained ~94–100% of its fresh adsorbent performance during five cycle repetitions.

Introduction

The effective management of several fission products released from nuclear power plants is one of the major issues related to the safe use of nuclear energy (Nandanwar et al., 2016). Among radionuclides, isotopes of radioactive iodine (¹²⁹I and ¹³¹I) have a critical impact on the human body and on the environment due to nuclear fission properties, especially considering the high radioactivity of ¹³¹I with a short half-life of 8.05 days and the accumulation and persistence of ¹²⁹I in the environment given its long half-life of 1.52 × 10⁷ years (Nandanwar et al., 2016, Dickman et al., 2003). Gaseous radionuclides in elemental (I₂) and organic iodine (CH₃I) forms can be released not only under accidental conditions but also even during normal power operations (Nandanwar et al., 2016, Nenoff et al., 2014, Reyerson and Cameron, 1936, Choi et al.,). Specifically, gaseous radioactive iodine released under normal power operations is of great concern owing to the low concentration of radionuclides as well as the typically high humid of these environments (Nenoff et al., 2014, Choi et al.,). Elemental iodine (I₂) can easily be removed by conventional adsorbents such as activated carbon and silver-doped materials, even under extremely humid conditions. However, it is known that the removal of organic iodides by the adsorbents is more difficult than for the elemental iodine and that the removal efficiency can be steeply decreased in humid environments (Tietze et al., 2013, Hughes et al., 2013). As a results, an enormous research effort has been invested to develop novel adsorbent materials that can efficiently remove organic iodides, methyl iodide (CH₃I) in particular, in humid environments.

ASZM-TEDA (an activated carbon impregnated with Ag, Cu, Zn, and Mo), triethylenediamine (TEDA) (Park et al., 1995, Aneheim et al., 2018, Park et al., 2001), and silver-exchanged porous materials such as silica gel (Asmussen et al., 2019) or zeolites (Nenoff et al., 2014, Pham et al., 2016, Chebbi et al., 2017, Azambre and Chebbi, 2017, Chebbi et al., 2016, Chapman et al., 2010) are representative adsorbents used for CH₃I removal. However, these materials exhibited limitations when used for competitive adsorption at a relative humidity (RH) of 95% at 303 K and extremely low concentration (0.31 ppm) of CH₃I. The commercial adsorbent ASZM-TEDA provided sufficient amine adsorption sites (in TEDA), but the force binding ASZM and TEDA was not high enough for effective competitive adsorption of CH₃I in humid environments. In this experimental condition, the molar ratio of methyl iodide to water is 1: 130,000. Accordingly, the presence of active sites that can more selectively bind to CH₃I becomes crucial. Alternatively, porous materials with Ag (I) incorporated, such as zeolites capable of strong alkylation leading to the formation of AgI, have been applied to increase the adsorption performance of CH₃I removal. Although Ag-exchanged zeolite was applied to remove CH₃I, limitations such as the high loading amount of Ag (~23 wt%) and lower adsorption capacity at RH 95% are still unresolved (Chebbi et al., 2017, Azambre and Chebbi, 2017, Chebbi et al., 2016).

Metal-organic frameworks (MOFs) with framework structures comprising metal- containing nodes connected by organic bridging ligands, have proven to offer advantages as promising adsorbents. This is due to their high surface area, exceptional porosity, tunable pore size, and simple modulation. In general, MOFs has been extensively studied due to the diversity of their framework designs, which provide a great variety of pore structures and properties using the post-synthetic modification (PSM) approach (Cohen, 2012). MOFs are receiving close attention in the fields of gas sorption (Rosi et al., 2003, Yoon et al., 2017, Kaye et al., 2007), gas separation (Couck et al., 2009, Liu et al., 2018, Li et al., 2009), sensing (Kreno et al., 2012), and catalysis (Lee et al., 2009, Farrusseng et al., 2009, Valerkar et al., 2016). The PSM process for the synthesis of MOF has been regarded as a very promising method for the introduction of functionality in MOF frameworks without causing structural damage to the parent MOF. Moreover, post-synthetically modified MOFs have been used as adsorbents in the removal of toxic industrial gases, chemical warfare agents, and radioactive gases (Sava et al., 2011, Zeng et al., 2010, Banerjee et al., 2016, Sava et al., 2013, Li et al., 2017). Recently, MOFs including square forms of HKUST-1 and MIL-53, and the hexagonal window geometry of MIL-120, have been reported to exhibit the best CH₃I adsorption capacities (425, 164, and 127 mg/g) at 308 K and 1333 ppm of CH₃I (Chebbi et al., 2018).

Very recently, Li et al. reported the post-synthetic functionalization of MIL-101 with different molecules having amine groups. Among the various amine-functionalized forms of MIL-101, MIL-101-TEDA showed a higher loading capacity (120 wt%) of CH₃I within 10 min and reached its maximum uptake amount of 160 wt% by 120 min at 313 K (Li et al., 2017). The authors have also evaluated the efficiency of adsorbents with the “decontamination factor” (DF), as the ratio of radioactivity before and after decontamination procedures. For a low CH₃I concentration of 50 ppm, the DF of MIL-101-TEDA has been achieved within the range 4800–6300 at 423 K. However, the adsorption data of MIL-101-TEDA has not been determined at an extremely low concentration (0.31 ppm) of CH₃I, which is a critical point related to the off-gas management of the facilities of a nuclear power plant under normal operating conditions.

In the work reported herein, we prepared a series of Brønsted acid-functionalized MIL-101s, made using either one-pot synthesis (Akiyama et al., 2011) or the sequential PSM method (Andriamitantsoa et al., 2016). By introducing sulfonic acid groups through two strategies, they could be utilized for the ion exchange of Ag, which can strongly interact with CH₃I. MIL-101-SO₃H was synthesized in a one-pot fashion using a ligand with a sulfonate group, while MIL-101-RSO₃H (R = NH(CH₂)₃) was prepared via consecutive PSM processes (MIL-101-NO₂ to -NH₂, to -NH-RSO₃H) incorporating more hydrophobic alkyl sulfonate to minimize the effect of humidity. Both samples were further modified using the Ag ion-exchange process, and the CH₃I removal efficiency of MIL-101-RSO₃Ag was markedly improved (by more than 99.99%) at 303 K, even at RH 95%. Furthermore, we demonstrated that the Ag ion-exchanged MIL-101-RSO₃H could be a promising candidate for use under the real conditions in a nuclear power plant.