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

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.