A new function of spent activated carbon in BAC process: Removing heavy metals by ion exchange mechanism.

To investigate the potential of the spent activated carbon (AC) on removing heavy metals, the spent ACs used 5 years were collected from a full-scale BAC water treatment plant of southern China. The study found that the spent ACs had very good adsorption capacity for Pb(II) and Cd(II) at low concentrations (about 200 μg/L or less) with the maximum removal rates of more than 95% and 86% respectively (only 10-15% for virgin ACs), which will provide the theoretical basis for the disposal of spent AC (the hazardous waste) in BAC process or the combination reuse of spent AC and the virgin AC. Surface properties analyses showed that compared to virgin AC, the pH and PZC in the spent AC significantly decreased, and the relative abundance of surface carboxyl increased by 81% on average, which are essential for the adsorption of metals. To explore the adsorption mechanism, take Pb(II) for example, the adsorption isotherm and kinetics fittings were carried out, which can be well described by Freundlich model (R2 = 0.9356) and the pseudo-second-order kinetic model (R2 = 0.9276), respectively. Analyses of influencing factors, FT-IR and XPS before and after Pb(II) adsorption confirmed the ion exchange mechanism of spent AC for the removal of heavy metals.