Carrier mediated transport of actinides using hexa–n-hexylnitrilotriacetamide (HHNTA)

Highlights

• SLM containing HHNTA (a novel tripodal amide) was used for U(VI), Np(IV) and Pu(IV) transport.

• The transport studies indicated relatively fast mass transfer as 90 % transport was seen in 5 h.

• The transport efficiency showed the trend: U(VI) << Np(IV) < Pu(IV).

• Efficient mass transfer was seen for both Np(IV) and Pu(IV) at 3 M HNO₃.

• The SLM stability studies were also carried out which showed degradation with time.

Abstract

Solutions of N,N,N’,N’,N”,N”-hexa-n-hexylnitrilotriacetamide (HHNTA) in 20 % isodecanol-80 % n-dodecane (v/v) were employed for the supported liquid membrane (SLM) transport of U(VI), Np(IV), and Pu(IV) from nitric acid feed solutions using PTFE (polytetrafluoroethylene) flat sheet membranes. Preliminary transport studies revealed poor transport of U(VI) when compared with that of the tetravalent ions Np(IV) and Pu(IV) which may find application for their separation from acidic radioactive feeds. In view of this, all subsequent studies were carried out using Np(IV) and Pu(IV). The SLM studies performed with 0.08 M HHNTA as the carrier extractant indicated increased transport of the tetravalent actinide ions with increasing nitric acid and carrier extractant concentrations, while using 0.5 M HNO₃ + 0.5 M oxalic acid as the strip (or receiver) phase composition. However, while > 80 % transport of Np(IV) and Pu(IV) was obtained for a feed phase of 3 M HNO₃, a sharp decrease in the transport rate was seen at a feed acidity of 6 M HNO₃ probably due to anionic hexanitrato complex formation of the metal ions and also because of lower availability of the free ligand in the membrane phase at higher nitric acid concentrations. The membrane stability was poor suggesting the need for continuous replenishment for long term use.

Introduction

One of the major factors which concern the global acceptance of nuclear energy as an alternative to the conventional fossil fuel-based energy is the safe management of the radioactive wastes [1]. Spent nuclear fuel reprocessing using the PUREX (Plutonium Uranium Reduction Extraction) process leaves behind large volumes of raffinate, which, when concentrated, gives rise to high level waste (HLW) containing long-lived minor actinides (Am, Cm, Np), small fractions of the unextracted uranium and plutonium, fission products and structural materials [2]. The safe management of the HLW involves a strategy for the partitioning [[3], [4], [5]] of the long-lived actinides and their subsequent ‘burning’ in high flux reactors [6], now famously known as the ‘P&T’ strategy. It is now clear that the major radiotoxicity of the HLW is due to the long-lived actinides, i.e., the long-lived isotopes of U, Np, Pu, Am and Cm. Therefore, remediation of HLW necessarily involves selective separation of these actinide elements; the better the separation efficiency, the more successful the radioactive waste management program [7].

Though extractants such as CMPO (carbamoylmethylphosphine oxide) [8], TRPO (trialkylphosphine oxide) [9] and DIDPA (diisodecyl phosphoric acid) [10] have been thoroughly evaluated for the extraction of actinide ions from HLW simulants, these are not considered for large scale applications due to the fact that large volumes of solid waste are generated. On the other hand, CHON extractants such as malonamides have been found to be quite promising [11]. The European Union has proposed actinide partitioning [12] schemes based on DMDOHEMA (N,N’-dimethyl-N,N’-di-n-octyl hexylethoxy malonamide), a malonamide extractant. During the turn of the century, diglycolamide extractants were proposed by Sasaki et al. [13], which turned out to be even better than the malonamides and hence, have been extensively investigated by various researchers, globally [[14], [15], [16]]. One of the key features of the diglycolamide-based extractants is their preference for trivalent lanthanides vis-à-vis the trivalent actinides. Sasaki et al. reported a separation factor (SF = D_Eu/D_Am) value of ca. 10, using 0.1 M TODGA (N,N,N’,N’-tetra-n-octyldiglycolamide) as the extractant and 1 M HNO₃ as the aqueous phase [13]. The same authors reported a reversal in this selectivity using a tripodal amide of nitrilotriacetic acid, termed as NTA amide [17]. This unique feature of this class of extractants has fascinated many researchers and there are a flurry of research activities using these ligands, of late [[18], [19], [20]]. We have recently studied the extraction of actinide ions such as UO₂²⁺, Np(IV), Pu(IV), etc. using HHNTA (N,N,N’,N’,N”,N”-hexa-n-hexylnitrilotriacetamide) and the results are quite promising [21].

Supported liquid membrane (SLM) based separations have many advantages over solvent extraction techniques [[22], [23], [24]], which include low inventory of the carrier ligands, simultaneous extraction and stripping of metal ions and alleviation of tricky issues such as formation of third phase, phase entrainment, phase disengagement limitations, etc., which are often encountered with solvent extraction processes. While using rather exotic ligands such as the NTA derivatives, it is imperative to develop separation methods with very low ligand inventory and hence, SLM transport studies were taken up during this study.

The present study involves studies on the transport of U(VI), Np(IV) and Pu(IV) from nitric acid feeds using N,N,N’,N’,N”,N”-hexa-n-butylnitrilotriacetamide (HBNTA) and N,N,N’,N’,N”,N”-hexa-n-hexylnitrilotriacetamide (HHNTA) in a mixture of isodecanol and n-dodecane (Fig. 1) across PTFE (polytetrafluoroethylene) flat sheet supports. To the best of our knowledge, it is the first report on the use of these two NTA ligands as carrier ligands in SLM for the separation of any of the actinide ions from acidic feed solutions.