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

Abstract

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 À and F À 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 À , NO 3 À 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 (À 48.7 kcal mol À 1). On the other hand, Hf(IV) emerges predominantly coordinated with NO 3 À and F À anions. The hafnium monomers [HfF 4 NO 3 ] À and [HfF 3 (NO 3) 2 ] À 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 (À 156.7 kcal mol À 1) correlating thermodynamic stability to the experimental (À 47 kcal mol À 1), for the formation reaction of [ZrF 5 ] À. 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.