Influence of calcination on the adsorptive removal of phosphate by Zn-Al layered double hydroxides from excess sludge liquor

The influence of calcination of Zn-Al layered double hydroxides (LDHs) on their phosphate adsorption capacity was studied in order to improve phosphorus removal from an excess sludge liquor. Powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermogravimetry-differential scanning calorimetry (TG-DSC) and nitrogen adsorption-desorption were employed to characterize the raw Zn-Al and the calcined products. The results reveal that the Zn-Al LDHs evolved to a phase of mixed metal oxides with the calcination temperature increasing to 300 deg. C and finally to spinel ZnAl2O4 at 600 deg. C. When the Zn-Al was calcined at 300 deg. C, the interlayer carbonate ions were removed and the greatest BET surface area of 81.20 m2/g was achieved. The tested phosphate adsorption capacities of the raw and calcined Zn-Al were closely related to the evolution of physicochemical properties of the LDHs during the calcination. The Zn-Al-300 (Zn-Al LDHs calcined at 300 deg. C) exhibited the highest P uptake of 41.26 mg P/g in 24 h. The phosphate adsorption by the raw Zn-Al and the Zn-Al-300 both follows a pseudo-second-order kinetic model; the adsorption isotherms show a good fit with a Langmuir-type equation.