Production of Layered Double Hydroxides for Anion Capture and Storage
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1215-V11-04
Production of Layered Double Hydroxides for Anion Capture and Storage Jonathan D. Phillips1 and Luc J. Vandeperre1 1 Department of Materials, Imperial College London, Prince Consort Road, London, SW7 2AZ, United Kingdom. ABSTRACT Technetium has a long half life of up to 2.13x105 years. It is separated from liquid waste streams with tetraphenylphosphonium bromide [1], which upon degradation releases Tc as the pertechnetate anion, TcO4-. Pertechnetate is highly mobile in groundwater and it is therefore highly desirable to capture and immobilise this anion within a solid for interim and ultimately long term storage. Layered Double Hydroxide (LDH) materials are known to possess excellent anion sorption capabilities due to their structure which consists of ordered positively charged sheets intercalated with interchangeable hydrated anions. The composition can be tailored to produce suitable precursors for ceramic phases by varying the divalent and trivalent cations and the anions. LDHs with the general formula Ca1-x (Fe1-y, Aly)x (OH)2 (NO3)x . nH2O were produced by a co-precipitation method from a solution of mixed nitrates. Calcination leads to the formation of Brownmillerite Ca2(Al,Fe)2O5 like compounds for temperatures as low as 400°C, this is close to the lowest temperature at which Tc is known to volatilise (310.6 °C Tc2O7). It was shown that after calcining up to 600°C, the LDH structure is recovered in water allowing rapid ion capture to occur. This suggests that these materials have potential for both capture and as a storage medium for Tc. INTRODUCTION In the United Kingdom, one of the waste streams produced during the reprocessing of spent nuclear fuel at the Sellafield site is a medium active liquor rich in radionuclides including technetium. To convert this liquor to a solid the radionuclides are precipitated by the addition of sodium hydroxide, a flocculating agent and tetraphenylphosphonium bromide (TPPB) [2]. The resultant floc is separated from solution by ultrafiltration and incorporated into a cementitious waste form, before being stored in a repository. The addition of TPPB allows technetium to be removed from the waste stream with an efficiency of 97%.[1] However it is known that in alkaline environments, such as those found in the cement, TPPB can degrade by alkaline hydrolysis resulting in the release of the pertechnetate anion [2]. The pertechnetate anion does not bind well with soils and as such is highly mobile in groundwater [3]. Technetium-99 is of significant environmental concern because it is a weak beta emitter produced in significant quantity (6% of all fission products) and has a long half life of 2.13x105 years. Layered double hydroxides (LDHs) are family of materials renowned for their anion exchange and capture properties [4]. Furthermore they can be synthesized with a broad range of compositions which could make them ideal as precursors for other phases. LDHs are structurally similar to the minerals Brucite, Mg(OH)2, and Portlandite, Ca(OH)2, in which a central divalent
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