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(SO 4 ) 3 2– + 2HL ⇌ M(SO 4 )L 2 +. 2H + + 2SO 4 2- . With increasing H 2 SO 4 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.