Choline chloride-hexanoic acid deep eutectic solvent: structure, stability and aggregation behaviour in light rare earth extraction.

Deep eutectic solvents (DESs) formed by mixing a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) in specific molar ratios have emerged as green solvents because of their advantageous physicochemical properties, biodegradability and low toxicity. In the present work, a hydrophobic DES (HDES) composed of choline chloride (ChCl) and hexanoic acid (HexA) in a molar ratio of 1 : 4 was prepared and characterised by FTIR and 1H NMR spectral studies. Physico-chemical property measurements showed a gradual decrease in density and refractive index as the temperature increased from 298.15 K to 328.15 K. The conductivity and dielectric constant values increased in the same temperature range. The 1 : 4 ChCl : HexA DES was further used for the solvent extraction of lanthanum (La), cerium (Ce), praseodymium (Pr), and neodymium (Nd) from aqueous nitrate solutions. The effects of various parameters, such as pH, HNO3 molarity, shaking time, DES/aqueous phase volume ratio, and temperature, on the efficiency of light rare-earth extraction were investigated. Good extraction efficiencies of 83.03%, 88.30%, 91.20% and 78.78% were obtained for La(iii), Ce(iii), Pr(iii) and Nd(iii), respectively, from 3 M HNO3 at a DES/aqueous phase ratio of 3 : 1. The radiation stability of the DES and its subsequent effect on the extraction behaviour of LREEs were examined. The aggregation behaviour of DESs before and after irradiation was also investigated using dynamic light scattering.