The Hydrolysis of Hydroxamic Acid Complexants in the Presence of Non-Oxidizing Metal Ions 2: Neptunium (IV) Ions

Abstract

Hydroxamic acids are salt free, organic compounds with affinities for cations such as Fe 3+ , Np 4+ and Pu 4+ and have been identified as suitable reagents for the control of Pu and Np in advanced nuclear fuel reprocessing. The results of a UV-visible-near IR spectrophotometric study of the 1:1 and 2:1 complexes formed between formo-and aceto-hydroxamic acids (FHA, AHA) and Np(IV) ions are interpreted using speciation diagrams for the identification of the species present at diferent pH and ligand to metal ratios. A kinetic model that describes the instability of the complex due to the hydrolysis of the hydroxamate moeity, previously developed for the Fe(III)-AHA complexes [1], is tested here against experimental Np(IV)-FHA data. Consequently, the complexation constant for formation of the 1:1 Np(IV)-FHA complex in nitric acid is estimated at K 1 = 2715, and indications are that complexation protects the ligand against hydrolysis at 0.1 > pH >-0.1.

Determination of the Constants of Formation of Complexes of Iron(III) and Acetohydroxamic Acid

“…The work carried out in this study supports the wider understanding of the behaviour of hydroxamic acids as chelating ligands for hard cations such as Fe(III), Pu(IV) and Np(IV), in particular the kinetics of hydrolysis of these ligands in the presence of such cations in the context of nuclear reprocessing and specifically the development of advanced reprocessing flowsheets. As such the determination of the thermodynamic equilibrium constants for the formation of the various Fe-AHA complexes, allows for our model of the kinetics of hydrolysis of hydroxamic acids in the presence of non-oxidising metal cations (Andrieux et al,2007) to be extended to temperatures away from room temperature and determine the activation energy and pre-exponential factor associated with the hydrolysis processes, giving access to valuable mechanistic information. This work is currently ongoing in our laboratories and will be the subject of a forthcoming publication.…”

“…Whilst there have been many studies of hydroxamic acid hydrolysis and their complexation reactions with metal ions, there have been relatively few studies of the stability of the metal-hydroxamate complexes towards hydrolysis. We have previously reported on the kinetics of the hydrolysis of hydroxamic acids both in free solution and when bound to metal ions and developed a kinetic model describing this process at room temperature (Andrieux et al,2007;Carrott et al,2008). However, the applications of hydroxamic acids in biological-related fields requires an understanding of the behaviour of these systems at temperatures of biological interest, for example in vivo (310K) or temperatures at which proteins start to denature (∼ 325K) (Roos,2006).…”

The effect of acetohydroxamic acid on stainless steel corrosion in nitric acid

“…As part of an Advanced PUREX process (Adv-PUREX), the use of acetohydroxamic acid (AHA) has been proposed as a means to hold back Pu and Np in the aqueous phase, thus allowing U(VI) to be extracted into the TBP without Pu/Np contamination [2][3][4][5][6]. This has led to the development of a variety of advanced fuel cycle concepts that incorporate AHA, such as URanium EXtraction Plus (UREX+) [7,8] and Grouped ActiNide Extraction (GANEX) [9,10], which do not extract pure plutonium but rather a mix of Pu, Am and Np, providing a non-proliferation advantage over the traditional PUREX process [11].…”

The effect of SO3-Ph-BTBP on stainless steel corrosion in nitric acid

“…Considering the ability of said ligands to complex with An(III) and Ln(III) species [13,14], the potential stripping via complexation of either Fe 3+ or Cr 3+ from the process steel and subsequent destabilization of the protective oxide fi lm is a concern.…”