Significant role of counterion for lead(Ⅱ) ion adsorption on carbon pore surface

We have presented a predictive tool for describing Pb2+ adsorption on activated carbon (AC). Based on the correlation between the G/D ratio and the adsorbed amount of Pb2+ obtained in our previous study (Kuroki et al., 2019), we have further validated the predictive tool by carefully analyzing the results of lead ion adsorption on different AC samples in the present study. Our results show that the method can predict the amount of Pb2+ adsorption within 10%, which corroborates our previous interpretation based on the hard and soft acids and bases theory. Molecular dynamic simulations were carried out to resolve the role of the counter anion and how it affects the configuration and the amount adsorbed of Pb2+ ions on the non-polar graphite surface of AC. The simulation results showed that when the counterion is NO3−, the adsorption of Pb2+ is enhanced by the electrostatic interactions of lead ions with adsorbed NO3− because NO3− is attracted strongly to the surface of graphite. This unique feature of adsorbed NO3− is substantiated with the use of Cl− as the counter anion, which did not show any significant adsorption of lead ions because chlorine ions are not strongly attracted to the surface of graphite. This mechanism is presented for the first time in the literature, and it can pave the path to exploring and understanding the complex phenomena of ions in the aqueous phase onto activated carbon.