Understanding the coordination behavior of quaternary ammonium and phosphonium nitrate ionic liquids under gamma irradiation: A combined spectroscopic investigation

Abstract

In the process of spent nuclear fuel reprocessing, it is very essential to investigate the radiation stability of the extracting phase.From the ionic liquid (IL) family, quaternary ammonium or phosphonium based ILs: methyltrioctylammonium nitrate ([N1888][NO3]) or trihexyl(tetradecyl)phosphonium nitrate ([P66614][NO3]) have many commendable features while being used as the solvent for the extraction and separation of the target metal ions from nuclear waste solution. In this context, a comparative investigation on the irradiation effect on the stability of [N1888][NO3] and [P66614][NO3] and their coordination ability towards acid and metal ion was furnished systematically. A marginal change in the extraction ability was acquired upon irradiation. The spectroscopictechniques such as UV–Visible, ATR-FTIR and fluorescence spectroscopy were employed to deduce the intrinsic role of the applied dose. The peak at ∼ 260 nm in UV–Visible spectra is broadened and red shifted upon irradiation for both ILs with substantial shift in [P66614][NO3]. FTIR spectra suggests the appearance of new peaks at 752 cm−1 upon radiation exposure for both ILs indicating the formation of degraded products of NO3– ion (bending vibration of NO2. radical) and - CH2 group (cleavage of - H. radical). A remarkable difference in the peak intensity and shape for [P66614][NO3] as compared to [N1888][NO3] confirms the radiation tolerance level of two ILs. For both irradiated and unirradiated ILs, the presence of water coordination was confirmed due to the peak at (bending mode of H2O), which later showed blue shift upon metal (or acid) loading.Luminescence patterns of Eu(III) in the irradiated ILs realizes an adequate quenching of Eu(III) emission in [P66614][NO3] in comparison to [N1888][NO3] which was less affected by the radiation. In all the spectroscopic evidences specify the vital role of NO3– anion in altering the radiation stability of ILs.