KHCO3 activated biochar supporting MgO for Pb(II) and Cd(II) adsorption from water: Experimental study and DFT calculation analysis

A new strategy that simultaneous use of KHCO3 activated biochar and nano-MgO incorporation for Pb2+ and Cd2+ removal from water was raised. After activating by KHCO3, the BC showed a higher surface area and could carry more MgO nanoparticles the BC owned. The synthesized MgO-K-BC had a large adsorption capacity for Pb2+ (1625.5 mg/g) and Cd2+ (480.8 mg/g). Multiple characterization and comparative test have been performed to demonstrate that ion-exchange, precipitation, and complexation are the main mechanisms for Pb2+ and Cd2+ removal by MgO-K-BC. In order to further explore the adsorption mechanism in-depth, the density functional theory (DFT) calculation combined with experimental results were performed. The O-top of MgO was the most stable adsorption site for Pb2+/Cd2+ adsorption compared with other adsorption sites (Mg-top, bridge, and hollow). In addition, the results of charge density maps and projected density of state (PDOS) showed that the overlap of electron cloud and orbits between MgO and Pb 2+ were denser than Cd2+, indicating that MgO-KBC had a stronger affinity for Pb2+ than Cd2+, so that, MgO-K-BC had a higher adsorption capacity for Pb2+ than Cd2+. This work provides a deep understand of the mechanism for heavy metals adsorption by metal oxide and a practical and theoretical guidance for adsorbent preparation with high adsorption ability for heavy metal.