Adsorption and separation of zirconium and hafnium from sulfate media by a conveniently synthesized (2,4,4-trimethylpentyl)(2-methylpropyl)phosphinic acid functionalized Merrifield Resin: A combined study of experiments and DFT calculations

Abstract

A novel extractive resin grafting (2-methylpropyl)(2,4,4-trimethylpentyl)phosphinic acid groups through N atoms (marked as Mer-N-POOH-272), which is superior to most of the reported resins for hafnium(Hf) selective separation from zirconium (Zr), was synthesized and structurally characterized by FT-IR, SEM-EDS and XPS. The adsorption and separation behaviors of Mer-N-POOH-272 for Zr and Hf in sulphate media were investigated. The desorption behaviors of the adsorbed Zr and Hf by HCl, H₂SO₄, NaOH and Na₂CO₃ solutions were studied. Density functional theory (DFT) calculations were conducted to reveal the adsorption mechanism. Mer-N-POOH-272 preferentially adsorbs Hf over Zr in H₂SO₄ media, and the maximum Zr/Hf separation factor (β_Hf/Zr) reached to 6.5 $\pm$ 0.3. The Zr and Hf adsorption by Mer-N-POOH-272 fit pseudo-first kinetic model and Langmuir isothermal adsorptionc model. It is the intraparticle diffusion that probably determines the adsorption rate. The Zr and Hf adsorptions onto Mer-N-POOH-272 belong to monolayer adsorption, and their experimental maximum adsorption amounts were 16.8 $\pm$ 0.8 mg/g and 29.2 ± 1.0 mg/g, respectively. Sulfuric acid solution of 4.5 mol/L selectively desorbed the adsorbed Zr, and NaOH and Na₂CO₃ solutions of ≥ 1.0 mol/L efficiently desorbed the left Hf. The DFT calculations show it is the two O atoms of -POOH in Mer-N-POOH-272 that probably interact with Zr or Hf ions during adsorption in H₂SO₄ media, and Hf has lower adsorption energy than Zr, which explains the selectivity of Mer-N-POOH-272.