Insight into the key role of imine groups in polyaniline for adsorbing heavy metal ions: Density functional theory and experimental study

In the researches of polyaniline (PANI) or PANI-based adsorbents, views on which nitrogen group has the highest chemical affinity to heavy metal ions and dominates the adsorption process remain in debate. To address this problem, the interaction between different nitrogen groups and heavy metal ions was deciphered exactly via density functional theory (DFT) calculations and experiments. Theoretical calculations reveal that –N[dbnd] group, rather than other nitrogen groups within PANI, serves as the primary adsorption site. And the key step of PANI adsorption process is a ligand exchange reaction. In addition, H⁺ ions with a higher affinity to –N[dbnd] group can inhibit the adsorption of metal ions through the consumption of adsorption sites and ion exchange reaction. X-ray photoelectron spectra (XPS) spectra demonstrate that maintaining a reasonably high pH value (pH > 4) proven to be an effective strategy for mitigating H⁺ competition. This work provides a molecular-level understanding of PANI's adsorption sites, underlying mechanism, and H⁺ competition for metal ion adsorption by PANI, which is beneficial for advancing the development of PANI-based adsorbents.