Na ion-exchanged zirconium phosphate crystal with high calcium ion selectivity.

We demonstrate hydrothermally grown sodium hydrogen zirconium phosphate ((Na,H)-ZrP) crystals exhibiting high calcium ion selectivity. The standard Gibbs free energies for Ca2+ exchange on (Na,H)-ZrP and γ-type ZrP were estimated to be -10.1 and -4.69 kJ mol-1, respectively. The high Ca2+ selectivity of (Na,H)-ZrP could be attributed to the size matching between the ion exchange site of (Na,H)-ZrP and Ca2+.