DFT study on adsorption of Pb(Ⅱ) by biochar: focusing on molecular structure and alkali and alkaline earth metals

Abstract

To investigate the Pb²⁺ adsorption characteristics of biochar, DFT (Density functional theory) calculation was employed as an effective method to evaluate the adsorption behavior. The three-dimensional structure of biochar was determined as C₅₈H₄₀N₂O₉ through ¹³C NMR, FTIR, and XPS analyses, which consisted of multiple triphenylene and biphenylene units linked by aliphatic carbons, functionalized with carbonyl, carboxyl, hydroxyl, and pyrrole groups on the surface. The adsorption sites were determined by molecular ESP analysis. DFT calculation showed that the adsorption performance was not only related to the type of functional groups, but also to the molecular structure in which the adsorption site was located. Configuration 3 showed the strongest E_ads (−114 kJ mol⁻¹) that Pb²⁺ achieved synergistic binding through simultaneous coordination to the carbonyl oxygen O(50) and adjacent aromatic carbons C(11)/C(12). The ion exchange capacity contributed by alkali and alkaline earth metals (AAEMs) presented on biochar followed K > Ca > Na > Mg. The ΔG of the ion exchange reaction was predominantly lower than that of Pb²⁺ adsorbed on biochar molecule through OFGs complexation and cation-π interaction, indicating the significant effect of AAEMs on the adsorption performance. Moreover, AAEMs coordinated on O atoms could enhance Pb-O bond strength during adsorption. These findings suggested that K/Ca enriched biochar was recommended for Pb²⁺ removal.