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

Abstract

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 °C and finally to spinel ZnAl 2O 4 at 600 °C. When the Zn–Al was calcined at 300 °C, the interlayer carbonate ions were removed and the greatest BET surface area of 81.20 m 2/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 °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.