Removal of cadmium and lead from aqueous solutions by thermal activated electrolytic manganese residues

Electrolytic manganese residues (EMR) is produced from the electrolysis manganese industry. In this study, the thermal activated EMRs (T-EMR) were used to adsorb cadmium and lead from aqueous solution. X-ray diffractometer (XRD), scanning electron microscope-Energy Dispersive Spectrometer (SEM-EDS), X-ray photoelectron spectroscopy (XPS) were adopted to characterize EMR before and after the modification, and the performance and adsorption mechanisms of T-EMR for cadmium and lead were determined. Results show that the pH has a strong influence on the adsorption of cadmium and lead and the maximum adsorption capacity can be achieved at pH 6. The adsorption of Cd(II) can be better fitted by the Lagergren pseudo-first-order dynamic model, while that of Pb(II) fits the pseudo-second-order kinetic model better. The Freundlich isotherm model fits the adsorption of two metals better than Langmuir model. The thermodynamic results demonstrate that the adsorption of Cd(II) or Pb(II) on T-EMR is endothermic and spontaneous. As the nitric acid with pH 0.5 was used, nearly all of the adsorbed Cd(II) and 75% Pb(II) can be desorbed from the loaded T-EMR. It is concluded that the adsorption of Cd(II) and Pb(II) on T-EMR is in virtue of electrostatic attraction, ion-exchange and surface precipitation. The heavy metals are mainly adsorbed on ferric and manganese oxides and silicate minerals in T-EMR by electrostatic attraction. In addition, cadmium and lead also can be adsorbed via the ion exchange reaction. Moreover, some Pb(II) are adsorbed by forming lead sulfate. Thus, T-EMR may be an environmentally-friendly, effective adsorbent for the removal of heavy metals from aqueous solution.