Unveiling the Zirconium and Hafnium Speciation in Fluoride‐Nitric Acid Solutions by Paper Spray Ionization Mass Spectrometry Combined with DFT Calculations

• Published in: European journal of inorganic chemistry Vol. 2021; no. 12; pp. 1175 - 1185
• Main Authors: Bertoli, Alexandre C., Miguita, Ana Gabriella C., Mingote, Raquel M., Augusti, Rodinei, Duarte, Hélio A.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 26.03.2021
• Subjects: Anions; Aqueous environments; DFT; Dimers; Fluorides; Free energy; Hafnium; Inorganic chemistry; Ionization; Mass spectrometry; Mathematical analysis; Nitric acid; Paper Spray Ionization; Scientific imaging; Speciation; Spectroscopy; Stability; Vapor phases; Zirconium
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.202001140

Paper spray ionization mass spectrometry (PSI‐MS) was used to detect a variety of species of hafnium and zirconium present in an aqueous acid solution containing NO3− and F− anions. This solution mimetics the acid waters used in separation/extraction processes. Hence, the information provided by PSI‐MS is combined with the results achieved from DFT calculations to confirm the nature and stability of each species. In the adopted experimental conditions, Zr(IV) appears coordinated with F−, NO3− and H2O to generate mainly the dimer [Zr2F3OH(NO3)2(H2O)5]2+, among other complexes, with remarkable thermodynamic stability (−48.7 kcal mol−1). On the other hand, Hf(IV) emerges predominantly coordinated with NO3− and F− anions. The hafnium monomers [HfF4NO3]− and [HfF3(NO3)2]− have thermodynamic stability related to the electron pairing of the valence layer. In the gas phase, the fluoroanions of Zr(IV) demonstrated a high electronic affinity and free energy via DFT (−156.7 kcal mol−1) correlating thermodynamic stability to the experimental (−47 kcal mol−1), for the formation reaction of [ZrF5]−. Besides, the presence of fluoride and nitrate in the reaction mixture induces the formation of dimers connected in μ2‐F bridges, with coordination 7 and 8 of the central atom. Finally, the results described herein are promising studies of chemical speciation of cationic metals in an aqueous environment, a challenging subject whose importance will grow in the coming years. The Zr/Hf separation is a challenge beyond mineral extraction processes and the lack of fundamental knowledge about their speciation in aqueous medium has to be fulfilled. The paper spray ionization mass spectrometry combined with density functional calculations were used to unveil important differences in the speciation of Zr(IV) and Hf(IV) at the presence of fluoride and nitric acid which can be explored for design new separation processes.