Adsorption of Pb(II) and Cd(II) hydrates via inexpensive limonitic laterite: Adsorption characteristics and mechanisms

Industrial heavy metal pollution has been a long-standing concern, and adsorption is an economical and effective measure for treating wastewater containing heavy metal ions. With its low price, large specific surface area, and high surface activity, limonitic laterite is an ideal low-cost adsorbent for the removal of heavy metal ions. In this paper, the adsorption behavior of heavy metal ions on limonitic laterite was studied, and then the mechanism of Pb2+ and Cd2+ hydrate adsorption was described in detail by combining density functional theory (DFT) calculations. The results showed that the removal ratios of Pb2+ and Cd2+ by laterite were 99.1 % and 86.17 %, respectively, which were much higher than the removal efficiencies of Pb2+ and Cd2+ by synthetic goethite. Furthermore, according to isothermal adsorption and kinetic analysis, Pb2+ and Cd2+ were chemically adsorbed on laterite as a single molecular layer. Further mechanistic analysis from DFT indicated that Pb2+ hydrate exhibits bidentate adsorption on the goethite (0 1 0) surface, while the Cd2+ hydrate displays monodentate adsorption. Partial density of states (PDOS) and differential charge density analyses indicate that the Pb6d orbital hybridizes with the O1s orbital and the Pb6s orbital with the O2p orbital, forming the Pb-O bond. The Cd5s orbital hybridizes with the O2p orbital, forming the Cd-O bond. Both Pb2+ and Cd2+ undergo stable chemical absorption on the goethite (0 1 0) surface.