Comparison of adsorption behavior studies of Cd2+ by vermicompost biochar and KMnO4-modified vermicompost biochar.

Cd2+ pollution in aquatic environments can pose a serious threat to human health. Biochar can remove Cd2+ from aquatic environments, but the Cd2+adsorption capacity of conventional biochar is low, therefore, we focused on exploring the Cd2+ adsorption capacity of modified biochar. In this study, KMnO4 was used to modify vermicompost biochar (VBC), and static adsorption tests for Cd2+ were carried out. The biochar properties and its adsorption efficiency toward Cd2+ before and after modification were studied by kinetics and isotherm model fitting, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). Additionally, an adsorption mechanism was discussed. The results showed that the KMnO4-modification resulted in a successful loading of the vermicompost biochar with MnO2, which greatly improved its adsorption capacity for Cd2+. The adsorption of Cd2+ by VBC and MVBC was a spontaneous, endothermic, and monolayer chemical adsorption process. Mineral precipitation mechanism accounted for the largest proportion, and CdCO3 was the main precipitate. After modification the proportion of surface precipitation and other mechanisms (π-electron coordination and the inner/outer sphere surface coordination) increased，while adsorption via cation exchange, oxygen-containing functional groups, physical adsorption and electrostatic attraction reduced. Hence, KMnO4 modification has a significant effect on the Cd2+ adsorption behavior of vermicompost biochar.