Competitive adsorption of Cd(II), Pb(II) and Cu(II) ions from acid mine drainage with zero-valent iron/phosphoric titanium dioxide: XPS qualitative analyses and DFT quantitative calculations

In order to treat heavy metal pollutants in acid mine drainage, zero-valent iron/phosphoric titanium dioxide (PTO-3nZVI and PTO-nZVI) was developed with phosphoric acid treatment and loading of zero valent iron and showed excellent adsorption of heavy metals. PTO-3nZVI and PTO-nZVI greatly improved the adsorption capacity for the targeted heavy metal Cd(II) (308 mg·g−1 for PTO-3nZVI and 206 mg·g−1 for PTO-nZVI) through complexation and coprecipitation. Additionally, competitive adsorption occurred between Cd(II) and coexisting heavy metal ions (Pb(II) and Cu(II)), resulting in different adsorption effects for PTO-3nZVI and PTO-nZVI, and their adsorption efficiencies decreased in the order Cu(II) > Pb(II) > Cd(II) for PTO-3nZVI and Pb(II) > Cu(II) > Cd(II) for PTO-nZVI. Based on these two aspects, X-ray photoelectron spectrometer (XPS) analyses and Density Functional Theory (DFT) calculations were used to characterize the adsorption behaviors of PTO-3nZVI and PTO-nZVI. In particular, competitive relationships for different heavy metals were clearly revealed by binding energies, Frontier Molecular Orbitals (FMOs) analyses, electrostatic potentials, and total potential and differential charge analyses of adsorption sites, binding sites and binding strengths.