Quantitatively ion-exchange between Mg(II) and Pb(II)/Cd(II) during the highly efficient adsorption by MgO-loaded lotus stem biochar

Background: Toxic metals such as Pb(II) and Cd(II) have thrown great threat to human health. Adsorption has been drawn more attention due to its low-cost, easy process and extensive raw materials. However, high efficient and environmental-friendly adsorbents are still in great need. Furthermore, the quantitatively adsorption mechanisms of Pb(II) and Cd(II) remain unclear. MgO-loaded lotus stem biochar was proposed as a highly efficient adsorbent to removal aqueous Pb(II) and Cd(II).Methods: MgO-loaded biochar was prepared by using lotus stalk as a precursor to be pyrolyzed with MgCl2 impregnation. Adsorption isotherms, kinetics, thermodynamics experiments were carried out. SEM, XRD, FTIR, XPS, etc. were employed to characterize the adsorbent before and after adsorption.Significant findings: The adsorption capacities of MgO-loaded biochar for Pb(II) and Cd(II) reached 1379.54 mg/g and 404.04 mg/g, respectively, which were 5 and 20 times higher than that of the original biochar. The adsorption kinetics and thermodynamics indicated that the adsorption process was a chemisorption-based heat absorption reaction, and the characterization results showed the main adsorption mechanisms were ascribed to ion-exchange, precipitation, surface complexation between biochar and Pb(II)/Cd(II) ions, in which the ion exchange mechanism was quantitatively confirmed. Finally, MgO-loaded biochar has been proved to be an efficient adsorbent.