Novel dialkylphosphinic acid modified Merrifield resins: Synthesis, characterization, and their adsorption and separation behaviors for zirconium and hafnium

Deep separation of Zr and Hf has always been a great challenging due to their very similar physicochemical properties. In this study, we established a method to synthesize two novel extraction chromatographic resins which graft dialkylphosphinic acid groups through amide group. The dialkylphosphinic acid modified resins were characterized by FT-IR, EA, SEM-EDS, XPS, and BET. Their adsorption and separation behaviors for Zr and Hf were investigated, and the adsorption mechanism was studied. The Zr and Hf adsorption onto Mer-CON-POOH-272 and Mer-CON-POOH-HYY2 both fit pseudo-second order model and Langmuir isotherm model. Mer-CON-POOH-HYY2 has stronger affinity to Zr and Hf than Mer-CON-POOH-272. Mer-CON-POOH-272 has a theoretical maximum loading capacity of 5.663 mg·g−1 Zr and 11.111 mg·g−1 Hf, while Mer-CON-POOH-HYY2 has a higher theoretical maximum loading capacity of 8.376 mg·g−1 Zr and 13.157 mg·g−1 Hf. Dialkylphosphinic acids are the functional groups of Mer-CON-POOH-272 and Mer-CON-POOH-HYY2 that interact with Zr and Hf during adsorption. They adsorb Zr and Hf through cation exchange mechanism, which can be expressed as M(SO4)32– + 2HL ⇌ M(SO4)L2 +. 2H+ + 2SO42-. With increasing H2SO4 concentration, the Zr and Hf extraction percentages and their separation efficiency reduce. The DFT calculation reveals that the adsorption process is accompanied by charge transfer. Adsorption occurs mainly at P = O and P-O positions. The affinity of the two resins for Hf is higher than that of Zr, which is the reason why the resin preferentially adsorbs Hf.