Adsorption mechanisms of lead and cadmium by alteration residues of coal slime-based activated materials: Theory and experiment.

To address uncertainties regarding the environmental fate and functionality of alteration residues derived from coal slime-based activated materials (insoluble alteration residues) after soil application, this study investigated their immobilization capacity and atomic-scale mechanisms toward Pb2+ and Cd2+ using adsorption experiments, multiscale characterization, and density functional theory (DFT) calculations. The alteration product comprises muscovite and amorphous aluminosilicate phases with abundant exposed edge-site functional groups. Adsorption of both metals follows a spontaneous monolayer chemisorption process. Pb2+ shows a higher adsorption capacity and a faster adsorption rate than Cd2+. Mechanistic analyses reveal distinct fixation pathways: Pb2+ removal involves surface precipitation (PbCO3 and lead silicates) and strong inner-sphere complexation, while CdCO3 precipitation with weaker complexation dominates Cd2+ immobilization. DFT calculations further demonstrate that Pb binds more strongly than Cd on both crystalline edge surfaces and amorphous phases. These findings clarify the initial immobilization potential of coal slime-based material alteration products for heavy metals, providing a theoretical basis for assessing their environmental effects and for developing solid-waste-derived passivation materials.