Study on adsorption of Cr(Ⅵ) in aqueous solution by polyethyleneimine modified magnetic coal gangue

Coal gangue is a large amount of solid waste generated during coal mining. Its resource utilization is of great significance for environmental protection and sustainable development of resources.This study used coal gangue as raw material, which was subjected to acid washing, thermal activation, Fe₃O₄ coprecipitation magnetization, 3-aminopropyl triethoxysilane (KH-550) grafting modification, and polyethyleneimine (PEI) functionalization, successfully preparing polyethyleneimine-modified magnetic coal gangue adsorption material (PEI/KH-550/MCG), which was used for the removal of Cr(Ⅵ). Characterization by SEM, FTIR, XRD, XPS, etc. confirmed that multi-step modification significantly improved the surface activity and functional group distribution of the material. Adsorption experiments show that under optimal conditions, this adsorption material can remove up to 99.33% of Cr(Ⅵ) and has a maximum adsorption capacity of 105.77 mg/g. The adsorption behavior conforms to the Langmuir isotherm model and quasi-second-order kinetic model, revealing a single-molecule-layer adsorption mechanism dominated by chemical adsorption. Thermodynamic analysis indicates that the adsorption process is spontaneous and exothermic, with adsorption capacity exhibiting a trend of “initial enhancement followed by weakening” as temperature increases. XPS analysis and mechanism discussions indicate that the material achieves adsorption through the electrostatic adsorption of protonated amino groups; coordination complexation and ion exchange between surface functional groups such as amino and hydroxyl groups and Cr(Ⅵ); PEI and Fe₃O₄ reduce Cr(Ⅵ) to Cr(Ⅲ) and further fix it, along with the physical adsorption effect of the pore structure, among other mechanisms, to achieve efficient removal of Cr(Ⅵ), providing a new pathway for the resource-efficient high-value utilization of coal gangue.