Mechanistic study of the adsorption capabilities of heavy metals on the surface of ferrihydrite: batch sorption, modeling, and density functional theory

Ferrihydrite (Fh), a widely distributed mineral in the environment, plays a crucial role in the geochemical cycling of elements. This study used experimental and computational approaches to investigate the adsorption behavior of seven heavy metal ions on Fh. The pH edge analysis revealed that the adsorption capacity followed the order: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+ > Co2+ > Mn2+, with Pb2+ showed the highest adsorption. Competitive adsorption was observed in multi-metal systems, and adsorption isotherms confirmed that Pb2+ and Cu2+ exhibited significantly higher equilibrium adsorption capacities than the other ions. Diffuse Layer Model (DLM) analysis indicated that for most heavy metals (HMs), [triple bond, length as m-dash]FesOM and [triple bond, length as m-dash]FewOM were the predominant adsorption species, while for Pb2+, [triple bond, length as m-dash]FesOPb dominated. Density Functional Theory (DFT) calculations were employed further to investigate the molecular interactions between HMs and Fh. The DFT results revealed that the distribution of surface iron sites on Fh strongly influences the adsorption process. Larger metal ions, such as Pb2+, form stronger coordination bonds with hydroxyl groups on the Fh surface, leading to distinct adsorption mechanisms compared to smaller ions. These findings, combining experimental and computational data, emphasize the critical role of surface iron site distribution and ion size in governing the adsorption behavior of HMs on Fh.