Selective extraction of Zr(IV) over Hf(IV) from aqueous hydrochloric acid with p-tert-Butylcalix[4]arenediphosphonic acid

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 ³60% at 7 M HCl). Thus, selective extraction of Zr(IV) over Hf(IV) is achieved from high-concentration HCl solutions. The separation factor (S_Zr) 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).