PREPARATION AND PHOSPHATE ION-EXCHANGE PROPERTIES OF A HYDROTALCITE-LIKE COMPOUND

A Cl--intercalated hydrotalcite-like compound (HTAL) was prepared by neutralizing magnesium and aluminum chlorides with sodium hydroxide. The HTALs were characterized by chemical analyses, X-ray diffraction (XRD), differential thermal (DTA)/thermogravimetry (TG) analyses, and scanning electron microscopic (SEM) observation. The HTAL crystal with a single unit cell, a0 = b0 = 0.31 and c0 = 2.33 nm as the lattice constant, was formed by aging at 353 K. The crystal growth proceeded for all three axes a, b, and c up to 5 h of aging, while the stacking along the c axis was preferentially observed by aging beyond 5 h. Phosphate ion-exchange properties of HTAL were investigated by a batch method. The HTAL obtained by longer aging time showed a slightly larger phosphate uptake, owing to better crystallization of the hydrotalcite structure. The pH dependence of the phosphate ion exchange showed that phosphate uptake has a maximum around pH 7. Chemical analyses of HTALs before and after the phosphate loading showed that phosphate ions are mostly ion-exchanged with interlayer Cl- ions, and the composition of phosphate ions in the solid phase is similar to that in the solution phase. The isotherm for phosphate uptake followed the Langmuir equation at pH 7.53 and 310 K; it gave an ion-exchange capacity of 2.37 mmol of P/g for phosphate.