Mechanism of the anion exchange reactions of the layered double hydroxides (LDHs) of Ca and Mg with Al

The nitrate containing layered double hydroxide (LDH) of Ca with Al on reaction with aqueous solutions of Na2CO3 and Na3PO4 yields CaCO3 and Ca5(PO4)3OH respectively rather than the carbonate/phosphate containing LDHs. The LDH of Mg with Al on reacting with dissolved phosphate ions also leads to the formation of unitary phosphates. This shows that the anion exchange reactions of layered double hydroxides take place by the dissolution-reprecipitation mechanism rather than by the topotactic mechanism.