The synthesis and phosphate adsorptive properties of Mg(II)-Mn(III) layered double hydroxides and their heat-treated materials

Hydrotalcite-like layered double hydroxides (LDHs) of Mg(II) and Mn(III) were prepared by a co-precipitation/air oxidation method using MgCl2 and MnCl2 as starting materials. Syntheses were studied at different precipitation temperatures and variable starting Mg/Mn ratios. MgMn-LDH were obtained at a preparation temperature below 333 K. Samples prepared at a Mg/Mn ratio of 3 showed a crystalline LDH phase having a basal spacing of 7.85 Angstrom. The LDH prepared at lower Mg/Mn ratios had slightly less crystalline phase, with lower basal spacing. Calcination of MgMn-LDH at 573 K for 4 h in air resulted in a transformation from a layered to an amorphous phase, accompanied by a loss of interlayer water and a partial loss of carbonate ions. Chemical analysis and IR studies showed that oxidation from Mn(III) to Mn(IV) progressed during the calcinations, and that HCO3- was formed in the calcined solid. Equilibrium distribution coefficients (K-d) of different kinds of anions were measured for the uncalcined and calcined samples. The selectivity sequences were Cl- < NO3- < SO42- < HPO42- and Cl-, NO3-, SO42- much less than HPO42- for the uncalcined and calcined ones, respectively. The latter had specific selectivity for phosphate ions. The calcined sample showed a maximum phosphate uptake of 1.1 mmol-P/g at a pH around of 8. The pH titration study of the supernatant solution suggested that the phosphate adsorption progresses mainly through the HCO3-/HPO42- ion exchange reaction.