PHASE-TRANSITION OF CL--INTERCALATED HYDROTALCITE-LIKE COMPOUND DURING ION-EXCHANGE WITH PHOSPHATES

A Cl--intercalated hydrotalcite-like compound (HTAL) was prepared by neutralizing magnesium and aluminum chlorides with sodium hydroxide. The phosphate/Cl- ion exchange on HTAL was studied by X-ray diffraction (XRD), differential thermal (DTA)-thermogravimetry (TG) analyses, and nitrogen adsorption-desorption at 77 K. The process of phosphate/Cl- ion exchange could be divided into three stages depending on the amount (P(load)) of phosphate loading. In stage I (0% < P(load) < 6%), the Cl--intercalated phase (interlayer distance = 0.30 nm) alone was observed. In stage II (6% < P(load) < 80%), the phase transition from the Cl--intercalated phase to the phosphate-exchanged phase (interlayer distance = 0.36 run) took place. In stage III (80% < P(load) < 100%), the phosphate-exchanged phase alone was observed; the interlayer distance increased slightly (from 0.36 to 0.37 nm) with an increase in the phosphate loading, indicating a formation of solid solution. DTA-TG analyses showed the presence of three kinds of water (surface, interlayer, and structural water) in the phosphate-loaded HTAL. The amount of surface and interlayer water increased with an increase in the phosphate loading. Nitrogen adsorption study showed relatively small surface areas and pore volumes for the original HTAL (50 m2/g and 0.15 cm3/g) and phosphate-loaded HTAL (49 m2/g and 0.10 CM3/g). This suggested that nitrogen molecules could not enter the interlayer space and were adsorbed on the surface of the microcrystals. A schematic model was proposed for the phosphate/Cl- ion exchange on HTAL.