Quantitative analysis on the mechanism of Cd2+ removal by MgCl2-modified biochar in aqueous solutions

In this study, a pristine biochar (BC) and MgCl2-modified biochar (MBC) were prepared using Pennisetum sp. straw for removing Cd2+ from aqueous solutions. Scanning electron microscope (SEM) imaging combined with energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), as well as the surface area and porosity analyses were used to reveal the physico-chemical characteristics of the pristine and modified adsorbents. Results suggested that MgCl2 impregnation during the synthesis had enhanced the specific surface area and pore volume of the biochar. Batch adsorption experiments indicated that the Cd2+ adsorption data of MBC fitted the Langmuir isothermal and pseudo-second order kinetic models, indicating a chemical adsorption was undergoing in the system. The maximum adsorption capacity of Cd2+ on MBC was 763.12 mg/g, which was 11.15 times higher than that of the pristine BC. The Cd2+ removal by MBC was mainly attributed to the mechanisms in an order: Cd(OH)2 precipitation (73.43%) > ion exchange (22.67%) > Cd2+-π interaction (3.88%), with negligible contributions from functional group complexation, electrostatic attraction and physical adsorption. The MBC could thus be used as a promising adsorbent for Cd2+ removal from aqueous solutions.