The dynamic process of radioactive iodine removal by ionic liquid 1-butyl-3-methyl-imidazolium acetate: discriminating and quantifying halogen bonds versus induced force

Abstract

With the increasing demand for nuclear energy and the Fukushima Daiichi nuclear disaster in 2011, the removal of radioactive and hazardous iodine has attracted more and more attention. Here, we investigate the dynamic process of radioactive iodine sorption in a representative acetate-based ionic liquid (AcIL), 1-butyl-3-methyl-imidazolium acetate [BMIM][Ac], via in situ UV-Vis spectroscopy in combination with a two-dimensional correlation technique. More importantly, the halogen bonds (including interior and exterior types) and induced force (only possessing an exterior form) resulting in iodine sorption in [BMIM][Ac] at specific time points are discriminated and quantified. The results show that the iodine sorption in [BMIM][Ac] can be divided into three zones. In the first 140 min, only halogen bonds occur (Zone 1). From 140 to 240 min, (exterior) halogen bonds and induced force occur simultaneously (Zone 2). After 240 min, only induced force occurs (Zone 3). Specifically, Zone 1 consists of two subzones, i.e., Zone 1a (before 90 min) and Zone 1b (90–140 min), corresponding to interior and exterior halogen bonds, respectively. Zone 2 is composed of three subzones, i.e., Zone 2a (140–180 min), Zone 2b (180–200 min), and Zone 2c (200–240 min), with (exterior) halogen bonds taking up the majority, approximately one half, and a small part of the total iodine sorption, respectively. The proportion of halogen bonds and induced force resulting in iodine sorption by [BMIM][Ac] can be approximately derived as 100% and 0% within 140 min, 96% and 4% within 240 min, and 91% and 9% within 570 min, respectively. Furthermore, the proportion of interior and exterior halogen bonds resulting in iodine sorption by [BMIM][Ac] could be approximately derived as 85% and 15% within 140 min, 80% and 20% within 240 min, and 80% and 20% within 570 min, respectively. These processes and quantifications can provide insight into the radioactive iodine removal by ILs in addition to the [BMIM][Ac] that we investigated here, and may motivate further experimental or theoretical studies on the application of halogen bonds for removal of iodine by designing new types of ILs.