Enhanced adsorption and separation of zirconium and hafnium under mild conditions by phosphoric acid based ligand functionalized silica gels: Insights from experimental and theoretical investigations

Separation of zirconium and hafnium is one of the challenging problems due to their similar chemical behaviors and in general, their separation has been carried out under very high acidic conditions. Herein, we have proposed novel synthesis and application of modified phosphoric acid based ligands, viz., mono-phosphate functionalized silica gel (MPFSG), bi-phosphate functionalized silica gel (BPFSG) and amine, amido pyridyl phosphate functionalized silica gel (AAPPFSG). Important outcome of the present work is that there is a significant enhancement of adsorption and separation of the ions Zr and Hf by employing the above ligands and more importantly, we can achieve the above separation process under mild conditions (pH 0.5-3.0) which is in contrary to the usage of very harsh conditions followed in the existing solid phase extraction based methods. The kinetics of the adsorption is found to be fast and takes place within 10 min with the adsorbent dose while the selectivity for zirconium over hafnium increases in the order, MPFSG < BPFSG < AAPPFSG under mild conditions. The results based on the quantum chemical calculations reveal that the enhanced separation process is manifested by the differences in the electronic properties (dipole moment and polarizability) of the metal ion complexes and functionalized phosphate ligands.