Influence of pH on the synthesis of the Zn-Al-nitrate layered double hydroxide and the exchange of nitrate by phosphate ions

The effect of pH on the preparation of the Zn-Al-NO3 precursor and on the exchange of nitrate by orthophosphate ions was investigated. Both the precursors and the phosphate-exchanged phases, prepared at different pH values, were characterized by X-ray powder diffraction and Fourier transform infrared spectroscopy. At low pH values, the crystallinity decreased due to the partial dissolution of the layered double hydroxides in the acidic solutions. The interlayer space was observed to decrease with increased pH, due to the predominant decrease in the size of the phosphate species, leading to stronger attractions between the anions and the positively charged layers.