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

This study investigated the mechanism of Ca²⁺ 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 ¹H 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 Ca²⁺ 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, Ca²⁺ exhibits dual effects on graphitization: it enhances microcrystalline order at 0.05Ca (<i>d</i> ₀₀₂ = 0.360 ± 0.002 nm) while disrupting it at 0.2Ca (<i>d</i> ₀₀₂ = 0.363 ± 0.002 nm). ¹H NMR demonstrated that Ca²⁺ enhances bound water adsorption (<i>T</i> ₂ < 10 ms) at 43% RH via Ca²⁺-carboxyl complexes and [Ca-(H₂O)₈]²⁺ clusters, converting weakly to strongly bound states. DFT and IGM analyses further identified hydrogen bonding as the dominant interaction, with Ca²⁺ creating new adsorption sites despite the reduction of single-water binding energy. This study clarifies the role of Ca²⁺ in modifying the microstructure and surface hydrophilicity of acid-treated lignite, providing insights into optimizing lignite utilization and increasing its industrial value.