Adsorptive removal of phosphate by Mg–Al and Zn–Al layered double hydroxides: Kinetics, isotherms and mechanisms

Abstract

Phosphate removal is important for the natural water or wastewater treatment, and adsorption is an efficient treatment process. In this study, Mg–Al and Zn–Al layered double hydroxides (LDHs) were synthesized by co-precipitation method at a constant pH of 9–10. The prepared LDHs were characterized by X-ray diffraction (XRD) and BET surface area determination. The XRD patterns showed the characteristic basal reflections of hydrotalcite-like LDH materials. BET surface area of Zn–Al LDH was larger than that of Mg–Al LDH. Adsorption experiments were carried out as a function of LDHs dosage, contact time and initial pH of phosphate solution. The adsorption of phosphate reached equilibrium quickly at about 40min. The experimental data showed a good compliance with the pseudo-second-order kinetic model. The Freundlich and Langmuir models both described the adsorption isotherm data well (R2>0.98). Zeta potential and fourier transform infrared spectroscopy (FTIR) analyses were used to elucidate adsorption mechanisms. The results indicated that phosphate species were adsorbed via electrostatic attraction, ligand exchange and ion exchange. The removal ratio of phosphate onto Zn–Al LDH was higher than Mg–Al LDH and reached 95% respectively, suggesting that Mg–Al and Zn–Al LDHs were excellent adsorbents for phosphorus removal from aqueous solution.