Selective Extraction of Zr(IV) over Hf(IV) from Aqueous Hydrochloric Acid with p-tert-Butylcalix[4]arenediphosphonic Acid

Abstract p-tert-Butylcalix[4]arenediphosphonic acid (L) extracts Zr(IV) and Hf(IV) from aqueous HCl, H2SO4, and H3PO4. In the extraction from aqueous HCl of different concentrations, the percentage of extraction (E%) of Hf(IV) decreases notably with an increase in acid concentration (96% at 0.5 M HCl and 12% at 7 M HCl), whereas the E% of Zr(IV) remains at a moderate level (90% at 0.5 M HCl and ∼60% at 7 M HCl). Thus, selective extraction of Zr(IV) over Hf(IV) is achieved from high-concentration HCl solutions. The separation factor (SZr) reaches 24 when using two molar equiv of L to Zr(IV) at 7 M HCl. Mechanistic studies indicate that L extracts Zr(IV) from high-concentration HCl solutions by a solvation mechanism by forming a 1:1 complex, LZrCl4. Back extraction of Zr(IV) proceeds quantitatively by treating the organic phase with 5 M H3PO4 after the extraction of Zr(IV).