The fate of reverse micellar aggregation in irradiated diglycolamide in n-dodecane solution

Abstract

Diglycolamides (DGAs) are susceptible to organic phase splitting, known as third phase formation, during the course of solvent extraction of trivalent metal ions from high-level liquid waste (HLLW). Third phase formation was due to the extensive reverse micellar aggregation of the strongly polar extracted species of diglycolamide in the non-polar n-dodecane (n-DD) medium, which was conventionally employed as a diluent in a solvent extraction procedure. Since DGA/n-DD can undergo hydrolytic and radiolytic degradation during the course of solvent extraction from nuclear waste, the fate of aggregation and third phase formation behaviour of degraded DGA/n-DD is unknown. To unravel this feature, N,N,N′,N′-tetraoctyldiglycolamide (TODGA) in n-dodecane was irradiated to various absorbed gamma dose levels (up to 500 kGy) and the irradiated organic phase was subjected to dynamic light scattering and FTIR spectroscopic investigations. The organic phase composed of 0.1 M TODGA/n-DD was contacted either with nitric acid or Nd(III) solution in nitric acid to simulate the actual feed condition before subjecting the organic phase to irradiation. The DLS studies showed that the aggregate size and its distribution in the organic phase decreased with an increase of absorbed gamma dose, due to the decrease in the concentration of TODGA upon irradiation and also due to the formation of polar degradation products, which disperses the aggregates in the organic phase. Since the aggregation was significantly lowered in the irradiated TODGA/n-DD solution, the third phase formation, characterised by the limiting organic phase concentration (LOC) of nitric acid and Nd(III), as well as the limiting aggregate size (LAS) for third phase formation were shifted to much higher concentration levels in the feed solution. In view of this, the study pre-empted the widely conceived notion of early third phase formation in the irradiated solvent systems containing diglycolamide in n-DD.