Mechanistic Insights into Hf(IV)/Zr(IV) Extraction by D2EHPA in Sulfate Media: A DFT Study Supported by Experiments with Industrial Sulfate Liquors

Abstract

The separation of zirconium and hafnium remains a major challenge in hydrometallurgical processing due to their similar chemical behavior and strong tendency toward hydrolysis and complexation in acidic media. In this work, a combined experimental and density functional theory (DFT) investigation was performed to elucidate the mechanism of Hf(IV)/Zr(IV) extraction by di(2-ethylhexyl) phosphoric acid (D2EHPA) in sulfuric acid solutions. DFT calculations indicate that both metals preferentially adopt 8-fold coordination in aqueous solution and undergo favorable first-stage hydrolysis even under strongly acidic conditions. Sulfate coordination was found to be highly exergonic, with bidentate sulfate complexes slightly more stable than monodentate species. Thermodynamic analysis demonstrates that extraction pathways involving concomitant sulfate transfer and proton exchange are significantly more favorable than purely hydroxide-based mechanisms, supporting the formation of neutral sulfate-containing metal–phosphonate complexes in the organic phase. The calculations further indicate a small energetic preference for Hf(IV) extraction relative to Zr(IV), consistent with the experimentally observed selectivity of D2EHPA in sulfuric media. Solvent extraction experiments were carried out using sulfate liquors produced directly from zircon concentrate supplied by Indústrias Nucleares do Brasil, reproducing realistic industrial conditions. Aging of the feed solution promoted increased Zr extraction and pronounced sulfate coextraction, indicating substantial changes in aqueous speciation over time. Addition of iodide reduced sulfur coextraction while maintaining relatively high metal extraction efficiencies, suggesting modification of the aqueous coordination environment. The combined theoretical and experimental results demonstrate that Hf/Zr separation in sulfate media is governed by tightly coupled hydrolysis, sulfate coordination, and proton-transfer equilibria, highlighting the critical role of sulfate-containing species in controlling extraction efficiency and selectivity under industrially relevant conditions. From a process-design perspective, the results suggest that tailored extractants capable of modulating the aqueous hydrolysis of Zr(IV) and Hf(IV), while preserving efficient coordination in the organic phase, may provide an effective strategy for enhancing extraction selectivity and developing more sustainable Hf/Zr separation processes.