XAS investigation on the coordination structure and extraction mechanism of zinc(II) in ammoniacal solution

The extraction chemistry of zinc in ammoniacal solutions is very complicated. In this work, the extraction behaviors of zinc, H2O and NH3 in the organic phase have been investigated. The coordination structures of zinc species in both aqueous phase and organic phase have been examined by use of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The geometric configuration of zinc species in ammoniacal solution has been resolved from XANES spectra by principal component analysis and linear combination fitting. The results reveal that the formation of zinc ammine complexes can greatly depress the zinc extraction. It was interesting to find that the decrease trend of extraction efficiency can be attributed to the structural transform of zinc species in ammoniacal solution from octahedral configuration to tetrahedral configuration. The latter increases the stability of zinc species and decreasing their extraction reactivity. The fitting results of EXAFS data indicate that ammonia and water molecules can coordinate directly in the first shell of zinc extracts, thus decreasing the hydrophobicity of zinc extracts and inhibiting their distribution. This detailed structure information affords insights into extraction mechanism of zinc(II) in ammoniacal solution and is crucial to design ligands for separation of metal ions.

► Extraction of zinc in ammoniacal solution is sensitive to aqueous pH.
► The structures of zinc species have been examined by X-ray absorption spectroscopy.
► The configuration change of zinc ammine species affects their extraction reactivity.
► Ammonia and water molecules coordinate directly in the first shell of zinc extracts.