Adsorption mechanism of Cd(II) by calcium-modified lignite-derived humin in aqueous solutions

Lignite-derived humin (CHM) was extracted from raw coal in Heihe City, China, producing calcium-modified lignite-derived humin (Ca-CHM) by Ca(OH) 2 . The physical and chemical performances of CHM and Ca-CHM were analyzed with SEM, 13 C spectra and XPS techniques. The results show that Ca-CHM exhibited weaker aliphatic, more aromatic polar compared with CHM, which improves the adsorption capacity for Cd(II). XPS analysis indicates that Ca(II) has been loaded onto Ca-CHM successfully after modification. This batch adsorption experiments report the adsorption performance of CHM and Ca-CHM for Cd(II). The adsorption process of CHM and Ca-CHM for Cd(II) conform to pseudo-second-order model, which is chemical adsorption, and the adsorption data presented good fits to the Langmuir model. The maximum adsorption amount ( Q m ) of Cd(II) onto CHM and Ca-CHM by the Langmuir model is 15.29 mg/g and 41.84 mg/g, respectively. Based on the results of SEM, 13 C spectra, and XPS analysis, we concluded that the main adsorption mechanism of Ca-CHM on Cd(II) was ion exchange of Cd(II) for Ca(II), static-adsorbed and surface complexation. Therefore, Ca(II) can be loaded on the surface of Ca-CHM by chemical modification, improving the adsorption capacity of materials in aqueous solutions. Graphical abstract