Deep Insight into Fine-Tuning the Structure Affecting the Extraction Sequence of Metal Cations by...

In order to avoid the wastage of reagents during the industrialized process, three small-sized substituents are introduced into (2-diethoxy phosphoryl) butyric acid (PBA) to fine-tune the structure and regulate the extraction sequence of the impurity metal elements. PBA extracts the Fe³⁺ cation first, then the Y³⁺ cation, and finally, the Al³⁺ cation. The introduction of alkoxy between the phosphoryl and carboxyl groups reverses the extraction sequence between the Y³⁺ cation and Fe³⁺ cation, while sulfhydryl and amidogen groups do not cause a change. The decomposition of Gibbs free energy changes provides a novel insight into the fact that the solubility, ligand deformation, and chemical bonding play important roles in determining the extraction sequence. It was discovered that the strong hydrophilicity of Al³⁺ is the main factor for the last extraction sequence compared to Fe³⁺ and Y³⁺, and the difference in the hydrophilicity of Fe³⁺ and Y³⁺ is the premise for regulating the extraction selectivity. Although the introduced substituents modify the ligand deformation and chemical bonding to facilitate the control of the extraction sequence, only the alkoxy group reverses the order of the ΔG values for extracting Fe³⁺ and Y³⁺. Further analysis reveals that ligand deformation has a greater effect than chemical bonding on preserving the extraction sequence.