Separation of trivalent actinides from lanthanides using hollow fiber supported liquid membrane containing Cyanex-301 as the carrier

doi:10.1016/j.memsci.2007.12.012

Journal of Membrane Science

Volume 312, Issues 1–2, 1 April 2008, Pages 1-5

https://doi.org/10.1016/j.memsci.2007.12.012 Get rights and content

Abstract

The transport as well as separation behaviour of trivalent actinides viz. Am³⁺ and lanthanides viz. La³⁺, Eu³⁺, Tb³⁺, Ho³⁺, Yb³⁺ and Lu³⁺ were investigated using polypropylene (PP) hollow fiber-based supported liquid membrane containing Cyanex-301 (bis(2,4,4-trimethyl pentyl)dithiophosphinic acid) in n-dodecane as the carrier. The transport rate of the trivalent actinide ion was much faster as compared to those of the trivalent lanthanide ions investigated in the present study. The transport rates were found to be affected by the carrier concentration. High decontamination factors (DF) as well as throughputs suggested possibility of application of this method for real waste solutions.

Introduction

‘Actinide partitioning’ is emerging as a strategy to separate minor actinides from the fission products which may help in long term remediation of high level nuclear waste [1]. Several processes such as ‘TRUEX Process’ (using CMPO) and ‘DIAMEX Process’ (using tetra-alkyl malonamides) recommended for ‘actinide partitioning’ and have been successfully demonstrated at the pilot scale [2], [3]. Subsequent step of burning/transmutation of long-lived minor actinides in high flux reactors/accelerator driven sub-critical systems (ADSS) requires the separation of trivalent lanthanides from trivalent actinides [4]. The presence of lanthanides, which act as neutron poisons due to their large absorption cross sections, could affect this ‘burning’ of actinides in high flux reactors. However, it is quite a challenge for the chemical separation of trivalent lanthanides from the trivalent actinides due to their similar chemical behaviour. It is desirable, therefore, to develop suitable ligands which can selectively complex with either of these two groups of elements from their mixtures. Soft donor ligands containing coordinating atoms like N and S are found promising in the SANEX (selective actinide extraction) processes [4]. Substituted dithiophosphinic acid extractants viz. Cyanex-301 (bis(2,4,4-trimethyl pentyl)dithiophosphinic acid and bis-chlorophenyl dithiophosphinic acid have been investigated in detail for this purpose [5], [6]. Large volumes of organic waste arising from the degradation of the thio-compounds can be avoided if the ligand inventory can be reduced significantly using separation techniques such as liquid membrane or extraction chromatography. Such methods have been reported to operate with favourable economical as well as environmental issues [7].

Our earlier report on the liquid membrane-based separation studies of trivalent lanthanides viz. La³⁺, Eu³⁺, Tb³⁺, Ho³⁺, Yb³⁺, Lu³⁺ from actinides viz. Am³⁺, Cm³⁺ using Cyanex-301 as the carrier extractant was very encouraging with decontamination factors (DF, defined as the ratio of product to impurity in the receiver divided by that in the feed) as high as 100 [8]. However, the time required for quantitative Am and Cm transport was about 4 h and 20 h, respectively. Moreover, in actual high-level waste (HLW), the concentration of lanthanides is much higher (about 2 g/L for a burn up of 33,000 MWD/tonne). In order to enhance the transport efficiency and throughput, hollow fiber supported liquid membranes have a clear edge in view of their large surface area to volume ratios. Geist et al. [9], [10] have used the hollow fiber membrane technique for lab-scale separation of actinides from lanthanides using BTP (2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine) and bis-chlorophenyl dithiophosphinic acid as the extractants operated in the non-dispersive extraction mode. However, application of hollow fiber-based separation of lanthanides from trivalent actinides using Cyanex-301 which gave the highest separation factors (D_Am/D_Eu) in the solvent extraction is not reported to our knowledge. Moreover, application of hollow fibre supported liquid membrane (HFSLM)-based separation method will drastically reduce the extractant inventory. Some literature reports on HFSLM applications for metal ion pre-concentration/recovery appear promising [11], [12]. Therefore, it was only pertinent to carry out studies with hollow fiber supported liquid membranes with lanthanide concentration present in HLW (1–2 g/L) using Cyanex-301 and to evaluate the possible larger scale applications. The present communication highlights the excellent separation data obtained using Cyanex-301 loaded HFSLMs. To our knowledge, the present work is the only report on lanthanide–actinide separation using HFSLM method.

Section snippets

Reagents

Commercial Cyanex-301 (Cytec Canada Inc.) was purified by the reported method [5] and the purity was checked by ³¹P NMR, GC–MS and elemental analysis. Sulphanilic acid was procured from Sisco Research Laboratory (SRL), Mumbai and was used as such. All other reagents were of AR grade. Suprapur nitric acid (Merck) was used for preparing the tracer solutions. Europium oxide, lanthanum oxide and neodymium oxide (>99.99%), obtained from Alpha Biochem were used in the studies involving macro

Results and discussion

Our solvent extraction studies [18] had indicated the following two-phase extraction equilibria for the extraction of trivalent lanthanides and actinides from aqueous nitrate medium using Cyanex-301:An³⁺ + 3HA_(o) ⇄ AnA_3(o) + 3H⁺Ln³⁺ + 2HA_(o) + NO₃⁻ ⇄ Ln(A)₂(NO₃)_(o) + 2H⁺where HA represents the undissociated form of Cyanex-301 and the species with subscript ‘(o)’ represent those in the organic phase while those without any subscript represent species present in the aqueous phase. The relative

Conclusions

The group separation of the trivalent actinides from lanthanides represented by Am³⁺, La³⁺, Eu³⁺, Tb³⁺, Ho³⁺, Yb³⁺ and Lu³⁺ and containing equivalent amount of lanthanide carrier as present in high level wastes was successfully carried out using hollow fiber supported liquid membrane separation method using Cyanex-301 in n-dodecane as the carrier. The average DF for the purification of Am with respect to the rare earth elements was ∼90 when operated for 10 min using 0.5 L of feed solution. The

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Rapid communicationSeparation of trivalent actinides from lanthanides using hollow fiber supported liquid membrane containing Cyanex-301 as the carrier

Abstract

Introduction

Section snippets

Reagents

Results and discussion

Conclusions

Sep. Purif. Technol.

Membr. Technol.

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

TRUEX process and the management of liquid TRU waste

Sep. Sci. Technol.

Recovery of minor actinides from HLLW using the DIAMEX process

Radiochim. Acta

Rapid communication
Separation of trivalent actinides from lanthanides using hollow fiber supported liquid membrane containing Cyanex-301 as the carrier