A comparative study on lead sorption by amorphous and crystalline zirconium phosphates

In the present study amorphous and crystalline zirconium phosphate (ZrP) were synthesized to explore their different behavior towards lead sorption. Both sorbents were characterized by X-ray diffraction (XRD), pH-titration and X-ray photoelectron spectroscopy (XPS). Lead sorption onto both sorbents was found to be pH-dependent due to the ion-exchange mechanism. Sorption isotherms onto both sorbents can be well correlated by Langmuir model. By comparison with crystalline ZrP, the amorphous one exhibited more favorable sorption of lead ion particularly in terms of high capacity and selectivity, as indicated by the qm values from Langmuir model (1.50 mequiv./g for amorphous ZrP and 0.20 mequiv./g for crystalline ZrP) and the distribution coefficients even several orders higher than crystalline ZrP when calcium ion coexisted at a high level in solution. Besides, rapid sorption kinetics were observed for both sorbents, and both lead-laden sorbents were amenable to an efficient regeneration by dilute HCl solution. Results indicated that amorphous ZrP can be selected as an ideal inorganic exchanger in the preparation of inorganic-polymer composites for heavy metals removal from water.