Adsorption of aqueous Pb(II), Cu(II), Zn(II) ions by amorphous tin(VI) hydrogen phosphate: an excellent inorganic adsorbent

Amorphous tin(VI) hydrogen phosphate (ATHP) was synthesized using the liquid phase precipitation method and served as an adsorbent to remove Pb(II), Cu(II), and Zn(II) from aqueous solutions. The ATHP was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption–desorption techniques. Adsorption properties were evaluated as a function of pH, reaction time, concentration of reactants, and salinity. Their equilibrium adsorption data were modeled using Freundlich, Langmuir, and Dubinin–Kaganer–Radushkevich isotherms, respectively. The results revealed that adsorption equilibrium reached within 180 min. ATHP indicated good adsorption even below the pH_ZPC, and best adsorption at pH 5 for Pb(II) and Cu(II) and at pH 5.5 for Zn(II) was observed. Equilibrium data fitted better to the Langmuir model for Pb(II) and Cu(II) and fitted better to the Freundlich model for Zn(II). The saturated adsorption capacities deduced from the Langmuir model were 2.425, 1.801, and 0.600 mmol/g for Cu(II), Pb(II), and Zn(II), respectively, indicating an adsorption affinity order of Cu > Pb > Zn. There is a negative correlation between the concentration of NaCl and adsorption capacity of ATHP, yet ATHP still exhibited excellent adsorption having an adsorption capacity of 19.35, 15.16, 6.425 mg/g when the concentration of NaCl was 0.6 mol/L. The free energy (E) was 12.33, 10.70, and 14.74 kJ/mol for Pb(II), Cu(II), and Zn(II), respectively. An adsorption mechanism based on ion exchange between heavy metal ions and H⁺ in the ATHP is proposed. Furthermore, the used ATHP was regenerated by HCl solution and the adsorbent was used repeatedly.