A technique for obtaining bentonite-based silver-containing sorbent for the uptake of radioiodine compounds
Abstract
The study is relevant to radioecology and is devoted to the protection of the environment from radioactive iodine isotopes. In radioactive waste (RW), iodine is present in anionic species I‑ and IO3‑. In these species, radioiodine migration into the environment is possible. The engineering barrier system in a nuclear waste repository should prevent the penetration of radionuclides outside the nuclear waste repository. They are supposed to be built from compacted bentonite clays, which have high waterproofing and cation-exchange properties, but do not absorb anions. Consequently, a modification of bentonites is required in order to make them capable of absorbing iodine, a promising option for which is the deposition of silver with its subsequent transfer into compounds suitable for fixing any mobile forms of radioiodine. There are a number of techniques for applying silver to various materials, however, they are either laborious or do not allow a uniform application of silver on porous materials.
In this study, a weighed portion of silver nitrate was dissolved in distilled water and a two-fold molar excess of sodium hydroxide was added. The precipitated silver oxide was dissolved by adding a concentrated ammonia solution in a dropwise manner. Then hexamethylenetetramine (HMTA) was added to the solution. The volume of the resulting solution was equal to the water capacity of the bentonite sample with a slight excess. The solution was introduced into a weighing bottle with a known weighed amount of bentonite and left for 25 hours for the complete swelling of the material. Then the bottle was kept for 24 hours in an oven at 90 °C. At elevated temperatures, the thermal hydrolysis of HMTA occurred with the release of formaldehyde, which reduced silver to metal. For the determination of the amount of deposited silver, bentonite was treated with 2 ml of 3M nitric acid and the resulting solution was titrated with a standardized solution of ammonium thiocyanate in the presence of Fe3+ions. According to the titration results, the yield of reduced silver deposited on the surface of bentonite was 95% of the applied amount. Electron microscopy and energy dispersive spectroscopy (EDS) were performed using a TESCAN VEGA 3 scanning electron microscope. The EDS spectrum had a distinct peak at an energy of 2.984 keV, indicating the presence of silver on the surface of the particles of the studied sample, which, as was shown by X-ray mapping, was evenly distributed over the bentonite surface.
An efficient technique has been developed for the uniform deposition of metallic silver on bentonite with a high yield (95%) for its subsequent conversion into compounds suitable for fixing any mobile forms of radioiodine, for example, the soluble anionic form of radioiodine in a nuclear waste repository.
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