1. Introduction
Nuclear energy is one of the most promising clean energy sources for mankind in the future, and spent fuel contains a large amount of raw material uranium, solid and gaseous radioactive elements produced by fission as well. During spent fuel reprocessing, processes such as shearing and dissolving, nitric acid recovery, process solution and liquid waste evaporation, waste calcination and melting, etc. will all produce process tail gas. The capture of the fission product129I in the nuclear fuel cycle is a growing priority for nuclear wasteform research and development. Due to its long half-life (t1/2-1.57×107 year) and high mobility in most geological environments, its removal is a difficult problem. 129I is of concern for spent nuclear fuel reprocessing facilities and in this regard a review of129I immobilization has recently been published1. Therefore, the capture and storage of gaseous iodine in off-gas has become the most important issue in the treatment of radioactive waste gas in the world2.
In the current PUREX process, radioiodine can be eliminated from the gaseous waste streams by counter-current scrubbing of the off-gas, including alkaline washing method, Mercurex method and Iodox method, etc3. But it has shortcomings such as high corrosivity, high fluidity, difficult storage, and secondary pollution3. Therefore, solid adsorption has been widely developed on iodine adsorption with the advantages of economical, convenient and highly effective. Moreover, using solid adsorbents to remove gaseous iodine can avoid the complication in system designing and high maintenance costs. The solid adsorbents mainly include activated carbon, inorganic porous materials, metal-organic frameworks, and porous organic materials. Although organic adsorption materials like metal-organic frameworks and porous organic materials have large adsorption capacity, they are not stable enough under water vapor conditions and are prone to adhesion, which are not suitable under actual working conditions. In addition, their manufacturing process is cumbersome. For these reasons, the currently promising solid adsorbents for gaseous iodine adsorption are mainly inorganic adsorbents, including activated carbon3-8, silver-loaded silica gel9-12, silver-loaded alumina13-15 and silver-exchanged zeolite16-22. Among them, the silver-exchanged zeolites are most commonly used for iodine capture, being considered as a benchmark sorbent.
Domestic and foreign scholars found that zeolites with silver nitrate can capture molecular iodine and iodine alkyl compounds, with decontamination factors higher than 10323-28. The saturated adsorption capacity was 196.6 mg g-1 for elemental iodine and the average utilization rate of silver atoms was greater than 86.5%29. But one important disadvantage of Ag-based zeolites is that only a portion of silver species participate in the I2 and CH3I capturing processes, but all of the spent solid sorbents need to be discarded after the adsorption process30. Although AgI and AgIO3 in the spent adsorbents can be converted back to Ag0 nanoparticles by the treatment with molecular hydrogen at high temperatures (ca. 773 K), the regenerated Ag nanoparticles undergo gradual sintering during the regeneration process, which leads to a significant decrease in their activity. Due to this deleterious feature, the expensive adsorbents are typically discarded after being recycled several times30. In addition, previous studies showed that the iodine adsorption performances of silver-containing zeolites were readily influenced by nitrogen oxides, which was due to the oxidation of metallic silver22,31-33. And increasing the maximum iodine adsorption capacity of the adsorbent can reduce the amount of secondary solid waste after the adsorption of radioactive iodine. There is an urgent need to find a zeolite adsorbent that is cheap, recyclable, especially suitable for industrial application.
All-silica zeolite has good hydrophobicity, acid resistance and thermal stability, which is suitable under actual spent fuel reprocessing conditions30,34,35. More importantly, it is cheap than Ag-loaded zeolite which shows a potential for iodine adsorption. Currently, the all-silica zeolites tested for iodine adsorption are poorly documented. A.Hijazi et.al studied the iodine adsorption on polyethyleneimine impregnated nanosilica sorbents. Several nanoporous silica of SBA-15 and Aerosil types were impregnated with branched polyethyleneimine with the aim to evaluate their iodine adsorption performance. The adsorption capacities of the adsorbents increased almost linearly with the N content provided by polyethyleneimine below certain threshold35. Tung Cao Thanh Pham et. al discussed power silicalite-1 all-silica zeolite on iodine adsorption performance. They observed silicalite-1 were stable in 5 M nitric acid and adsorbed iodine from highly acidic off-gas mixtures to much greater extents than does activated carbon. The iodine adsorption capacity for silicalite-1 was 480 mg g-1, and after hydrophobicity intensification, it performed better up to 530 mg g-130. However, in practical engineering applications, the adsorbent needs to be formed into spherical or strip-shaped materials with a size of micrometers or millimeters through a molding process, because powder materials can not be directly used in engineering applications. In the molding process, it is generally necessary to mix inorganic binders and adsorbent powders. High-temperature calcination is performed to enhance the mechanical strength of the spherical or strip-shaped materials and ensure that no pulverization occurs under actual application conditions after preliminary molding. Generally, inorganic materials used as binders do not have gas molecule adsorption capacity, thus the maximum adsorption capacity of the adsorbent after molding is generally lower than the maximum adsorption capacity of the adsorbent powder.
In this work, a new kind of shaped silicalite-1 all-silica zeolite was synthesized by a novel method with aluminum oxide as a binder which could be directly used in industrial applications. It was strip-shaped with a diameter of 3 mm and a length of about 1cm with a topological structure of MFI molecular sieve. The adsorption properties for iodine were investigated under the condition of reprocessing temperature about 348 K subsequently 36.