Unraveling Calcium Ion Effects on Water Adsorption of Acid-Treated Lignite via Experiments and DFT Calculation

High Resolution Image Download MS PowerPoint Slide This study investigated the mechanism of Ca 2+ incorporation on the water adsorption capacity of acid-treated lignite. The microstructure of samples was characterized using FTIR and XRD, while water adsorption was analyzed via 1 H NMR and density functional theory (DFT) calculations. The independent gradient model (IGM) was employed to visualize noncovalent interactions between lignite and water. FTIR analysis showed that Ca 2+ exchanges with carboxyl groups (−COOH) and alters the aromatic structure of lignite but minimally affects hydroxyl (−OH), aliphatic C–H, and substituted aromatic vibrations. Notably, Ca 2+ exhibits dual effects on graphitization: it enhances microcrystalline order at 0.05Ca ( d 002 = 0.360 ± 0.002 nm) while disrupting it at 0.2Ca ( d 002 = 0.363 ± 0.002 nm). 1 H NMR demonstrated that Ca 2+ enhances bound water adsorption ( T 2 < 10 ms) at 43% RH via Ca 2+ –carboxyl complexes and [Ca(H 2 O) 8 ] 2+ clusters, converting weakly to strongly bound states. DFT and IGM analyses further identified hydrogen bonding as the dominant interaction, with Ca 2+ creating new adsorption sites despite the reduction of single-water binding energy. This study clarifies the role of Ca 2+ in modifying the microstructure and surface hydrophilicity of acid-treated lignite, providing insights into optimizing lignite utilization and increasing its industrial value.