Selective separation of Nd from La/Ce/Pr using phosphate-based ionic liquids: Solvent extraction studies and density functional theory

Since neodymium (Nd) has similar physicochemical properties to lanthanum (La), cerium (Ce) and praseodymium (Pr), their efficient separation and purification is very difficult. Three new phosphate-based ionic liquids (ILs): N,N-dimethyloctylamine bis(2-ethylhexyl)phosphate ([N-1,N-1,N-8,N-H][DEHP]), N,N-dimethyldecylamine bis(2-ethylhexyl)phosphate ([N-1,N-1,N-10,N-H][DEHP]), N,N-dimethyldodecylamine bis(2-ethylhexyl)phosphate ([N-1,N-1,N-12,N-H] [DEHP]) were synthesized and evaluated for selective separation of Nd(III) from aqueous solution. The influences concentration of Nd(III), phase volume ratio (O/A), cation chain length of ILs, extraction time, extraction temperature, salt concentration, solution acidity have on the performance of ILs for extraction separation of Nd(III) from aqueous solution were systematically investigated. It was found that the extraction efficiency (E) of Nd(III) using [N-1,N-1,N-8,N-H][DEHP] was close to 100% at pH of 4, and that it only required 10 min to reach extraction equilibrium at 303 K. Meanwhile, the separation factors (beta) values of beta Nd/La, beta Nd/Ce, beta Nd/pr using [N-1,N-1,N-8,N-H][DEHP] were all higher than 3. The density functional theory (DFT) results indicated that the interaction between [N-1,N-1,N-8,N-H][DEHP] and Nd were stronger than that between La, Ce and Pr. In addition, almost 100% of Nd(III) could be recycled from the loaded [N-1,N-1,N-8,N-H][DEHP] phase using 0.16 mol center dot L-1 hydrochloric acid via one step, and the E of Nd(III) by regenerated [N-1,N-1,N-8,N-H][DEHP] remained about 97% after eight cycles. Moreover, the extraction mechanism of Nd(III) using [N-1,N-1,N-8,N-H][DEHP] was complexation mechanism based on infrared spectroscopy and slope analysis. This work furnishes a strategy for selective separation of Nd(III) using phosphate-based ILs without diluent.