Unveiling the zirconium and hafnium speciation in fluoride-nitric acid solutions by paper spray ionization mass spectrometry combined with DFT calculations

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 NO${}_3^{-}$ 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${}^{-}$, NO${}_3$${}^{-}$ and H${}_2$O to generate mainly the dimer [Zr${}_2$F${}_3$OH(NO${}_3$)${}_2$(H${}_2$O)${}_5$]${}^{2+}$, among other complexes, with remarkable thermodynamic stability (-48.7 kcal mol${}^{-1}$). On the other hand, Hf(IV) emerges predominantly coordinated with NO${}_3$${}^{-}$ and F${}^{-}$ anions. The hafnium monomers [HfF${}_4$NO${}_3$]${}^{-}$ and [HfF${}_3$(NO${}_3$)${}_2$]${}^{-}$ 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 [ZrF${}_5$]${}^{-}$. Besides, the presence of fluoride and nitrate in the reaction mixture induces the formation of dimers connected in µ${}_2$-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. (© 2021 Wiley‐VCH GmbH)