Removal of Cd2+ and Pb2+ from an Aqueous Solution Using Modified Coal Gangue: Characterization, Performance, and Mechanisms

The impact of various modification methods on enhancing the adsorption performance of coal gangue (CG) for hazardous heavy metals has not been thoroughly investigated. In this study, three CG samples were first modified by calcination, followed by acid washing, alkali washing, and hydrothermal treatment, to obtain modified CG samples. The adsorption performance was assessed based on the adsorption capacities for Cd2⁺ and Pb2⁺ (i.e., qe,Cd and qe,Pb), and the kinetics of the adsorption processes were analyzed using kinetic equations. XRD, SEM-EDX, FTIR, and N2 adsorption–desorption isotherms were used to elucidate the adsorption mechanisms. Results indicated that qe,Cd and qe,Pb of raw CG samples were approximately 10 and 25 mg/g, respectively, with only slight changes observed after calcination, acid washing, and alkali washing. In contrast, hydrothermal treatment yielded NaP and NaA zeolites, which significantly enhanced qe,Cd and qe,Pb to values of 48.5–72.7 and 214.9–247.5 mg/g, respectively. The hydrothermally treated CG samples primarily adsorbed Cd2⁺ and Pb2⁺ through ion exchange with Na⁺ within the zeolite structure, facilitating the entry of these ions into the zeolite’s pore channels. The adsorption processes were effectively described by the pseudo-second-order kinetic model. By optimizing the conditions of hydrothermal modification, the adsorption performance of CG samples is anticipated to further improve due to the creation of additional adsorption sites.