Choline Chloride-Based Deep Eutectic Solvents as an Alternative Decontamination Agent for Radioactive Metal Waste

Deep eutectic solvents (DES) are a mixture of a hydrogen bond acceptor (HBA), commonly quaternary ammonium salt, and a hydrogen bond donor (HBD), such as a carboxylic acid or alcohol. These solvents are readily synthesized from easily accessible materials. DES are characterized by their low cost, melting point, flammability, toxicity and wide electrochemical windows. Chemical and physical properties of DES can be tuned through judicious choice of HBA and HBD such as acidic HBD's which readily dissolves metal oxides. This illustrates novel advantages over currently used or proposed decontamination agents such as ionic liquid, molten salts or strong acids. In this research, we tested the feasibility of DES synthesized from choline chloride (ChCl) and ptoluenesulfonic acid (PtsA) as a decontamination agent. ChCl:PtsA was selected owing to its high metal oxide solubilizing power. Metal oxides such as Fe3O4, CoO, Cr2O3, and NiO present in the contaminated layer of stainless steel 304 were studied and were shown to have good solubilities in ChCl:PtsA. To simulate decontamination process, simulant contaminated stainless specimens were produced. Stainless steel 304 samples were oxidized at 800℃ for 30 minutes with constant steam supply and were cooled gradually in the furnace. The formation of layer was evident from SEM images, and the layer composition was studied by SEM-EDS. Laserinduced breakdown spectroscopy (LIBS) data was collected for oxide layer, which showed peaks that correspond to metals aforementioned. The simulant contaminated stainless specimens were leached, and their oxide layers were successfully removed in ChCl:PtsA. ChCl:PtsA solvent leaching kinetics and final concentration of metals were investigated using ICP-OES. SEM-EDS and LIBS results of specimen surfaces pre-leaching and postleaching were compared to assess decontamination ability. SEM images showed that the oxide layer was efficiently removed after the leaching. LIBS data of specimen pre-oxidation and post-leaching were almost identical, which shows that the metal oxide layer was removed and the base metal was exposed. These results corroborate that the eutectic mixture formed from two bio-compatible components has an excellent solubilizing power which efficiently dissolves and removes oxidation layer from the contaminated stainless steel. This work proposes and proves the possibility of using ChCl:PtsA as a decontamination agent. It was shown that while ChCl:PtsA with good metal oxide solubilizing power has numerous advantages over conventional leaching agents, it was also able to decontaminate simulant contaminated steels. The efficiency of the process is expected to be further improved when electrochemical process is combined. (author)