Adsorption of heavy metals on silica-supported hydrophilic carbonaceous nanoparticles (SHNPs)

This paper reports experimental results on the removal of Cd(II), Ni(II) and Pb(II) ions using hydrophilic carbon nanoparticles (HNPs) supported over silica beads to enhance their separation from treated water. The supported HNPs (SHNPs) exhibit high removal efficiencies especially at neutral pH and low temperature (10 °C), conditions that commonly occur for natural water remediation and for some types of industrial wastewater. The maximum adsorption capacity of the SHNPs at a reference concentration of 0.2 mM is 0.042 mmol g−1, 0.027 mmol g−1 and 0.055 mmol g−1 for Cd(II), Pb(II) and Ni(II) ions, respectively. Modelling analysis on the adsorption isotherms revealed that the free Gibbs’ energy of interactions between the sorbent and Ni(II) and Pb(II) ions is higher than that of Cd(II) ions indicating that the sorbents are more affine to intermediate acids, as Ni(II) and Pb(II) ions, than to soft acids, as Cd(II) ions. The sorbents exhibit appreciable adsorption capacities per gram of active phase (0.54 mg g−1 for Cd(II) ions, 13.48 mg g−1 for Ni(II) ions and 8.87 mg g−1 for Pb(II) ions) at the corresponding quality limit admitted by Italian regulations on wastewater, suggesting their possible use in water treatment plants.