Sorption of Pb²⁺ and Zn²⁺ions on Zr-Ca-Mg phosphates: Kinetics and mechanism studies

The kinetics study of Pb²⁺ and Zn²⁺ ions adsorption by low-cost mixed Zr-Ca-Mg phosphates with a different Zr/(Ca + Mg) ratio was carried out, for the first time. The effect of chemical composition on adsorption characteristics of obtained adsorbents was established. It was shown that an increase of Ca²⁺ and Mg²⁺ cations content in the composition of Zr-Ca-Mg phosphate adsorbents contributed to an increase in the adsorption capacity to Pb²⁺ (3.82–4.76 mmol/g) and Zn²⁺ (2.60–4.13 mmol/g) ions. Such improvements in the adsorption capacities were due to the amelioration of the textural properties of adsorbents, so that A_BET of adsorbents was in the range of 68–96 m²/g and V_{des BJH} of adsorbent was in the range of 0.19–0.24 cm³/g. On the other hand, increase of Ca²⁺ and Mg²⁺ cations led to a high difference in solubility products of initial phosphates and spent adsorbents. Kinetic data of Pb²⁺ and Zn²⁺ ions adsorption revealed that the adsorption rate is controlled by film diffusion, intraparticle diffusion and chemical interaction, which was confirmed by the results of X-ray diffraction analysis of spent adsorbents and described by the pseudo-second-order model. The increase of zirconium content caused an increase in the rate constant of the pseudo-second-order of Pb²⁺ and Zn²⁺ ions adsorption. Intraparticle diffusion had a significant effect on Zn²⁺ ions adsorption. Due to the suitable kinetics parameters and sorption capacities of composite Zr-Ca-Mg phosphates for divalent metal ions, these sorbents can be recommended as effective materials for heavy metals removal.