The Synthesis and Phosphate Adsorptive Properties of Mg(II)–Mn(III) Layered Double Hydroxides and Their Heat-Treated Materials

Abstract 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 Å. 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 (Kd) 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− << 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.