Unraveling the mechanistic effects of oxidation and ionization on polydopamine-Pb(Ⅱ) interaction: MD and DFT study

Abstract

Polydopamine (PDA), recognized for its straightforward synthesis and multifunctional groups, serves as an effective adsorbent for lead ions (Pb(II)). The structures of PDA are affected by different oxidation and ionization degrees, consequently influencing the adsorption performance for Pb(Ⅱ). Despite its extensive use, the detailed microscopic mechanisms by which these modifications affect Pb(II) adsorption remain poorly understood. In this study, all-atom molecular dynamics (AAMD) and density functional theory (DFT) simulations were employed to elucidate the interactions between PDA and Pb(II) under varying conditions of PDA’s ionization and oxidation. Enhancing PDA's ionization and oxidation degrees can effectively boost Pb(II)'s adsorption capacity. Specifically, heightened ionization significantly amplifies adsorption, as evidenced by the transfer of PDA monomer to the 6p vacant orbital of Pb(II), fostering strong coordination bonds through increased electron density. Meanwhile, escalated oxidation levels of PDA facilitate enhanced aggregation among polydopamine monomers, forming a more porous structure. Furthermore, oxidation activates inner orbital electrons within the polydopamine monomer, elevating electron transfer efficiency between PDA and Pb(II). These insights clarify the role of ionization and oxidation in optimizing PDA's performance as a Pb(II) adsorbent and provide valuable guidance for the design of advanced materials for heavy metal removal.