Adsorption mechanism of Pb2+ in montmorillonite nanopore under various temperatures and concentrations.

Adsorption of lead (Pb2+) onto the montmorillonite (Mt) surface is one of the key approaches to remove Pb2+ in geological and environmental engineering. Temperature and initial Pb2+ concentration are two essential factors that influence the adsorption capacity of Mt on absorbing Pb2+. However, the nanoscale governing mechanism of temperature and initial concentration on Pb2+ adsorbing of Mt is still unclear. This research performed comprehensively molecular dynamics (MD) simulations to investigate how temperature and initial concentration affect the dynamic Pb2+ adsorption of Mt nanopore. The Pb2+ removal ratio shows a two-stage variation with the increase of initial Pb2+ concentration. Temperature controls the maximum initial Pb2+ concentration for complete Pb2+ removal by changing the maximum adsorption energy of Mt. Temperature also influences the maximum adsorption capacity and Pb2+ removal ratio of Mt nanopore indirectly by changing diffusion and hydration state of Pb2+. The initial Pb2+ concentration corresponding to the maximum adsorption energy coincides with the maximum initial Pb2+ concentration determined by the Pb2+ removal ratio. Lower adsorption energy and higher level of hydration and diffusion make Pb2+ absorbing on Mt surface become more difficult, reducing the Pb2+ adsorbing capacity of Mt. The initial Pb2+ concentration influences adsorption capacity and Pb2+ removal ratio not only via altering the quantity of Pb2+ but also through controlling the adsorption energy of Mt, as well as the diffusion and hydration state of Pb2+. With the increase of initial Pb2+ concentration, the hydration of Pb2+ is weakened while the adsorption energy of Mt and diffusion of Pb2+ are enhanced.