Baiyan Li, et al., report that effective capture of radioactive organic iodides from nuclear waste remains a significant challenge due to the drawbacks of current adsorbents such as low uptake capacity, high cost, and non-recyclability. We report here a general approach to overcome this challenge by creating radioactive organic iodide molecular traps through functionalization of metal-organic framework materials with tertiary amine-binding sites.
The molecular trap exhibits a high CH3I saturation uptake capacity of 71 wt% at 150 °C, which is more than 340% higher than the industrial adsorbent Ag0@MOR under identical conditions. These functionalized metal-organic frameworks also serve as good adsorbents at low temperatures. Furthermore, the resulting adsorbent can be recycled multiple times without loss of capacity, making recyclability a reality. In combination with its chemical and thermal stability, high capture efficiency and low cost, the adsorbent demonstrates promise for industrial radioactive organic iodides capture from nuclear waste. The capture mechanism was investigated by experimental and theoretical methods.
Currently, nuclear power provides ~11% of the world’s electricity offering a cost-effective option compared to other energy sources1. Rapidly increasing global energy needs will likely increase the demand for nuclear energy in the future. Under normal operating conditions, fuel rods used in nuclear power plants need to be reprocessed. This procedure involves the production of complex off-gas mixtures consisting of HNO3, NO2, and N2O5 along with radioactive molecular iodine and organic iodides (ROIs, e.g., methyl iodide and ethyl iodide) at elevated temperatures (e.g., 150 °C). Radioactive I2 and ROI species must be selectively captured and sequestered to ensure safe nuclear energy usage. ROI species are known to be particularly difficult to capture, and a recent study has shown that the CH3I adsorption rate is only half of that for I2. It thus tends to leak into the environment more easily. Among various current capture technologies, solid sorbent-based fixed-bed methods have proven superior due to their simplicity and relatively low cost.
Examples of solid adsorbents for capturing ROIs from off-gas mixtures include triethylenediamine (TED) impregnated activated carbon (AC), and silver impregnated/exchanged solid supports such as silica, alumina, and zeolites. Typically the temperature of the off-gas is performed at high temperature (such as ~150 °C) in order to accelerate the chemical reactions and to remove adsorbed water from the narrow pores of the supports. AC-based adsorbents, however, can only be used under 120 °C and are limited to specific applications absent of NOx because of low ignition temperatures and the risk of formation of explosive compounds. Silver functionalized porous materials are capable of performing at higher temperatures, but the high costs associated with the noble metal limits their widespread application. Additionally, much to their detriment, chemical adsorption of CH3I—necessary for its high uptake at high temperature in such systems—makes the silver-based adsorbents poorly recyclable. Based on these considerations, and the fact that the uptake capacity of all existing adsorbents remains insufficient, new types of adsorbent materials that are noble metal free, highly efficient, cost effective, recyclable, and safe to use, are much needed for ROIs capture.
To tackle the aforementioned challenges, a desired ROI adsorbent must possess the following features: extraordinarily high adsorption capacity at the reprocessing temperature; high tolerance toward nitrogen-based oxides, acidity, and humidity; high thermal stability (≥150 °C) that meets the required reprocessing conditions; high efficiency well above reprocessing facility regulatory requirements and low cost and excellent recyclability. To fulfill this goal, we investigate a different type of crystalline porous materials, metal-organic frameworks (MOFs), as tunable and recyclable solid adsorbents for ROIs capture. They reason that such materials can potentially offer the following advantages: large and adjustable surface area and pore size enable accommodation of a large amount of ROI molecules and thus result in high ROI capacity; modular nature allows for rational design and tailoring of structural topology and functional sites; multivariate syntheses offer possibilities for obtaining topologically identical yet functionally diverse crystalline frameworks; modifiable open metal sites (OMSs) that form reversible coordination bonds with tertiary amines provide an effective means for recyclability. While numerous previous investigations have illustrated that MOFs serve as an excellent platform for capture of radioactive molecular iodine, their use as adsorbent materials to effectively trap ROI species remains unexploited to this date.
Here they demonstrate that highly efficient ROI molecular traps can be obtained via tertiary amine grafting to binding sites within a MOF framework. One such designed molecular trap, MIL-101-Cr-TED, exhibits an exceptionally high uptake capacity of 71 wt% for methyl iodide at 150 °C. Under identical capture conditions, this performance is more than 340% higher than that of the industrial adsorbent Ag0@MOR, a leading material in the United States for ROIs capture. Furthermore, the pristine MOF sample can be regenerated and reused multiple times without decrease in uptake capacity. This is a notable advancement as high-temperature recyclable adsorbent materials have been pursued since the 1980s but with very little success to this date. Coupled with its high chemical and thermal stability, relatively low cost, and high capture efficiency at both room and high temperatures, amine functionalized MIL-101-Cr-TED demonstrates a considerable potential for use as MOF-based adsorbent for the ROIs capture technology. Employing both experimental and theoretical methods, we further carried out an in-depth study to investigate and understand the mechanism of CH3I capture in the MIL-101-Cr-TED system. Our findings show that the construction and optimization of such molecular traps can be expanded to other MOFs by a suitable combination of robust frameworks with strong binding sites and tertiary amine molecules, which may lead to a large number of ROI capture materials. This strategy thus paves the way for further research and advancement on MOF-based molecular traps for their ultimate utility in ROIs capture from nuclear waste.
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