Liquid–liquid extraction and flat sheet supported liquid membrane studies on Am(III) and Eu(III) separation using 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine as the extractant
Highlights
► n-Pr-BTP was used for Am3+/Eu3+ separation. ► 70% n-dodecane + 30% 1-octanol was found to be suitable diluent. ► The SLM studies indicated about 85% Am3+ and 2–3% Eu3+ transport in 6 h. ► The stability of the reagent and the liquid membrane were not satisfactory.
Introduction
One of the major drawbacks of nuclear energy programme is the generation of high level liquid wastes (HLLW) containing long lived radionuclides (t1/2 = 102–106 years) such as minor actinides (237Np, 241,243Am and 245Cm) and fission products (93Zr, 99Tc, 129I, 135Cs, etc.). Therefore, safe management of HLLW is required for the public acceptability of the nuclear energy programme. The strategy of vitrification and storage in deep geological repositories is now accepted worldwide for the management of HLW. It, however, requires surveillance over millions of years to monitor deformation of glass and the migration of the long lived radionuclides to aquatic environment under natural calamities such as earth quake, volcanic eruption, etc. An alternative strategy is to separate the long-lived radionuclides followed by their transmutation in high flux reactors or accelerator driven sub-critical systems (ADSS) and is known as the ‘Partitioning and Transmutation’ strategy [1]. The partitioning step proposed to selectively extract the minor actinides co-extracts the lanthanides. Many of the lanthanide isotopes have high neutron capture cross-sections resulting in adverse effect on the transmutation process. It is, therefore, required to carry out separation of lanthanides from the trivalent minor actinides immediately after the partitioning step [2].
Lanthanide–actinide separation is, therefore, a key step in the back end of the fuel cycle. Amongst the early processes employed for the Ln(III)/An(III) separation, the TRAMEX (tertiary amine extraction) process and the TALSPEAK (trivalent actinide lanthanide separation by phosphorus extractants aqueous komplexes) process using Alamine-336 and di-2-ethylhexylphosphoric acid (D2EHPA), respectively as the extractants were found to be promising [3], [4], [5]. Due to similar charge and ionic size (ionic potential), trivalent actinides and lanthanides show similar chemical behaviour towards the hard ‘O’ donor ligands. Actinides, however, can form stronger covalent bond with the soft donor ligands because of higher spatial distribution of the ‘5f’ valence orbitals of the actinides as compared to the valence ‘4f’ orbitals of the lanthanides and this property is being exploited for the separation of trivalent actinides and lanthanides. Dithiophosphinic acids with ‘S’ donor atoms such as bis(2,4,4-trimethyl pentyl)dithiophosphinic acid (Cyanex-301) and bis-(chlorophenyl)dithiophosphinic acid have also been used as selective extractants for the trivalent actinides [6], [7]. In spite of very high selectivity for the trivalent actinides over the lanthanides, the major disadvantage of Cyanex-301 is its poor extractability at low pH region (pH < 3) [8], [9]. Bis-(chlorophenyl)dithiophosphinic acid, however, can extract the trivalent actinides from acidic medium with poor selectivity over the lanthanides [7]. Recently, several ‘N’ donor reagents such as 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (n-Pr-BTP) ligand have been proposed for the selective extraction of trivalent actinides from acidic feeds (0.1–1.0 M HNO3) with reasonably high S.F. (∼150) values with respect to the trivalent lanthanides [10], [11]. Softer nature of this aromatic ‘N’ donor ligand is responsible for the selectivity of n-Pr-BTP towards trivalent actinides over the lanthanides. A number of reports appeared in the literature on the complexation studies of n-Pr-BTP with trivalent actinides and lanthanides and its selectivity towards the actinides over lanthanides was explained on the basis of formation of ML3·[NO3]3 type of species in higher extent in case of actinides [12] due to higher stability constant of this complex in case of actinides [13].
Most of the separation studies of trivalent actinides and lanthanides involving n-Pr-BTP as the extractant were carried out using solvent extraction [14]. The major drawback of solvent extraction is the requirement of large solvent inventory and formation of third phase. These problems can be alleviated by the use of alternative methods such as extraction chromatography and liquid membrane. There are very limited reports available using BTP derivatives for lanthanide–actinide separation by extraction chromatography [15], [16], [17], [18] and liquid membrane [19], [20]. Supported liquid membrane (SLM) based separation methods have received considerable attention by the researchers for various separations and purification of metal ions [21], [22], [23]. SLM based techniques are particularly attractive as they involve low inventory of organic extractants and simultaneous extraction and stripping of the metal ion [21]. It is, therefore, pertinent to evaluate supported liquid membranes (SLMs) using n-Pr-BTP as the carrier extractant for the selective transport of trivalent actinides. The present study involves the synthesis of n-Pr-BTP derivative and its evaluation for the extraction and supported liquid membrane transport studies of Am(III) and Eu(III) using PTFE flat sheets of different pore size and a variety of feed and strip conditions, viz. varying the NaNO3 concentration in the feed solution and using different stripping agents (pH 2.0 and 0.01 M EDTA). Effect of organic diluent was also studied on the extraction and transport behaviour of Am3+ and Eu3+.
Section snippets
Synthesis of n-Pr-BTP
The synthesis of the bis-triazinyl pyridine 4 is depicted in Scheme 1. Stetter reaction of butanal in the presence of 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride and triethylamine in ethanol under reflux gave the hydroxy ketone 1 (68%) which under Swern oxidation conditions gave the diketone 2 in very good yield (87%). The required dihydrazide partner 3 was prepared from commercially available 2,6-dicyanopyridine by treating with excess of hydrazine hydrate at room temperature in a
Effect of equilibrium time
The attainment of equilibrium of Am3+ extraction by n-Pr-BTP was investigated as a function of equilibration time and the results are plotted in Fig. 1. A continuous increase in the DAm value was observed up to 60 min beyond which no change was seen suggesting that 1 h equilibration time is sufficient. All subsequent experiments were carried out with 2 h equilibration time to ensure attainment of equilibrium. It has been reported earlier that Am3+ extraction rate is linearly dependent on the free n
Conclusions
From the solvent extraction studies highest distribution ratio of Am3+ was observed by n-Pr-BTP in dodecane/nitrobenzene mixture. The SLM study, however, shows promising result in dodecane/octanol mixture, where ∼85% transport of Am3+ was observed in 6 h, which was accompanied by ∼6% of Eu3+, using 1 M NaNO3 at pH 2 as feed and pH 2 solution as the strippant. The distribution ratio and permeability coefficient values were found to be increasing with the ligand as well as aqueous NaNO3
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