Hydrolysis of Hydroxamic Acid Complexants in the Presence of Non-Oxidizing Metal Ions
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Hydrolysis of Hydroxamic Acid Complexants in the Presence of Non-Oxidizing Metal Ions Scott Edwards1, Fabrice Andrieux1, Colin Boxall1, Robin Taylor2, David Woodhead2 Engineering Deparment, Lancaster University, Lancaster, Lancashire, LA1 4YW, U.K. 2 The National Nuclear Laboratory, Central Laboratory, Sellafield, Cumbria, CA20 1PG, U.K. 1
ABSTRACT Simple Hydroxamic acids (XHAs) are salt free, organic compounds with affinities for hard cations such as Fe3+, Np4+, Pu4+ and have been identified as suitable reagents for the control of Pu and Np in advanced nuclear fuel reprocessing. Building upon previous work on the neptunium(IV)-formohydroxamic(FHA) acid system [1], a model that describes the hydrolysis of the acetohydroxamate moiety has been extended to include hydrolysis of bishydroxamatoneptunium(IV) complex. The model has been used to determine the rate constants for hydrolysis of mono- and bis-acetohydroxamatoneptunium(IV) at 25 °C, which were found to be 1.0×10-5 dm3 mol-1 s-1 and 5.0×10-5 dm3 mol-1 s-1, respectively. INTRODUCTION All commercial nuclear fuel reprocessing plants currently use the hydrometallurgical plutonium Uranium Extraction (PUREX) process to separate uranium and plutonium from used nuclear fuel. This process results in the isolation of plutonium from the reprocessing liquor, posing proliferation issues. Additionally, neptunium an actinide decay product that is not currently recycled as fuel, is found in a number of PUREX process streams including the separated U and Pu streams and the highly active waste stream. The former raises questions of process efficiency while the latter raises disposal issues. Several adaptations to PUREX have been suggested, one such adaptation is Advanced PUREX (or UREX+), which uses simple hydroxamic acids (XHAs) to coextract plutonium and neptunium from the reprocessing liquor leaving uranium unaffected [2], thereby addressing the main issues of PUREX. Such XHAs are known to be useful due to their ability to complex hard cations such as Fe3+, Np4+ and Pu4+ as well as their ability to reduce metal ions; for example, they rapidly reduce Np(VI) to Np(V) [3]. These properties may allow for redox control of Np and Pu in the U/Pu separation column and remove the need for organic salts, which can create disposal problems further downstream. However, hydroxamic acids undergo hydrolysis in acidic media, which could have negative implications in advanced nuclear fuel reprocessing. Hydrolysis of hydroxamates in acidic solutions is known to take place on the free ligand and when bound to a metal centre, although only the former is well characterized [4]. This paper seeks to better understand the kinetics of hydrolysis of the metal bound ligand, ultimately leading to the design of meaningful Advanced PUREX flowsheets. Previous modeling work focused on hydrolysis of the monohydroxamatoneptunium(IV) complex using a semi analytical model [1]. Extension of the previous model to conditions where the bishydroxamatoneptunium(IV) complex is also present is non-trivial. The present w
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