Decomposition of uranyl nitrate in the silica gel matrix under the action of microwave radiation

Abstract

Decomposition of aqueous solutions of uranyl nitrate in a matrix of granulated silica gel of KSKG grade under the action of microwave radiation (MWR) was studied. Microwave irradiation leads not only to formation of solid decomposition products UO₃, UO₂(OH)NO₃, and their hydrates in pores of KSKG granules, but also to accumulation of gaseous NO _x and H₂O. The presence of NO _x in KSKG pores leads to HNO₃ formation in the course of washing of sorbent granules with water. This prevents hydrolysis of uranyl nitrate and formation of UO₂(OH)₂·H₂O in KSKG pores. Washout of uranium with water and HClO₄ solutions from the KSKG fraction containing products of decomposition of 2 and 10 g of the initial UO₂(NO₃)₂·6H₂O under the action of MWR (hereinafter denoted as KSKG-P-I) was studied. Upon ∼7-day contact of the solid and liquid phases at the total ratio S : L = 1 : 20, from 5 to 14% of U passes into the aqueous phase from KSKG-P-I samples obtained in experiments with 10 and 2 g of UO₂(NO₃)₂·6H₂O, respectively. In the course of repeated treatments of KSKG-P-I with water, pH of the wash water increased from 3 to 6, owing to removal of NO _x from KSKG pores. Then an insoluble phase of uranyl hydroxide UO₂(OH)₂·H₂O, which can also be presented as hydroxylated uranium trioxide UO₃·2H₂O, is gradually formed from the solution obtained by treatment of KSKG-P-I with water. On treatment of KSKG-P-I with HClO₄ solutions (pH 1–2), virtually all uranium species formed by MWR treatment of aqueous uranyl nitrate solutions in KSKG matrix dissolve (at a contact time of the solid and liquid phases of ∼21 days, the amount of U that passed into HClO₄ solutions is ∼90%). The amount of the U form that is not extracted with HClO₄ solutions and remains in KSKG granules is ∼12% of its initial amount. X-ray phase analysis suggests that the uranium species remaining in KSKG are silicate compounds formed by sorbent saturation with a uranyl nitrate solution and subsequent MWR treatment.