Hydrothermal Modification of Coal Gangue for Cd2+ and Pb2+ Adsorption: Modelling and Optimization of Process Parameters by Response Surface Methodology

Hydrothermal treatment has been demonstrated as a highly effective approach to enhance the adsorption properties of coal gangue (CG) for Cd2+ and Pb2+. This study evaluated the influence of NaOH concentration (X1), hexadecyltrimethylammonium bromide dosage (X2) and hydrothermal duration (X3) on the modified CG’s composition and its adsorption capacities for Cd2+ (qe,Cd) and Pb2+ (qe,Pb). Response Surface Methodology (RSM) was utilized to explore the interactions among these variables, while XRD, SEM-EDX, FTIR and XPS techniques were employed to elucidate the adsorption mechanisms. The results reveal that hydrothermal treatment facilitated the formation of zeolite materials in the modified CG samples. Optimal conditions for Cd2+ adsorption (qe,Cd = 58.4 mg/g) were determined as X1 = 2.9 mol/L, X2 = 1 g and X3 = 16.8 h. For Pb2+ adsorption (qe,Pb = 233.6 mg/g), the optimal conditions were X1 = 2.4 mol/L, X2 = 0.57 g and X3 = 20.7 h. RSM analysis indicated that X3 significantly influences qe,Cd with the appropriate range of 14–18 h, while all three factors (X1, X2 and X3) notably affect qe,Pb, with suitable ranges identified as follows: X1 = 1.8–3 mol/L, X2 = 0.4–0.8 g and X3 = 16–24 h. The primary adsorption mechanism for Cd2+ and Pb2+ was determined to be ion exchange, with additional Pb2+ removal achieved through the precipitation of cerussite (PbCO3).