Organic association of alkali and alkaline earth metals in a high alkali coal and its geological significance

objective The occurrence of alkaline and alkaline earth metals (AAEMs) in coal significantly impacts its processing and utilization. Water-soluble and mineral-hosted AAEMs can be largely removed by water washing, flotation, or heavy-liquid separation, AAEMs organically bound to the coal matrix, however, is difficult to eliminate. To investigate the organic AAEMs in the high-alkali coal, the Zhundong coals with varying vitrinite/inertinite ratios were selected in this work. Using integrating sequential chemical extraction with ²³Na CP/MAS NMR and TEM-EDS, the AAEMs’ content, distribution patterns, and occurrences were systematically analyzed.

Results (1) AAEMs in Zhundong coals primarily exist as water-soluble and ion-exchangeable species. Na⁺ and Ca²⁺ exhibit significantly higher concentrations than other AAEM ions and demonstrate stronger interactions with coal molecules via ion exchange. (2) Both types of AAEMs are adsorbed via oxygen-containing functional groups (e.g., carbonyl, carboxyl, and phenolic hydroxyl) in the organic matrix. Quantum chemical calculations indicate that Na⁺ predominantly binds to carboxylate or carbonyl groups, forming carboxylate-Na complexes or coordination structures (−C=ONa⁺), whereas Ca²⁺ chemically adsorbs via robust ionic bonds. As vitrinite content increases, the ion-exchanged Na increased from 28.3% to 30.9%, and the ion-exchanged Ca increased from 62.3% to 70.1%, indicating that the vitrinite enhanced the affinity for AAEMs. (3) Groundwater flow drives AAEMs’ migration, with an adsorption preferentially occurring at oxygen functional groups. This adsorption capacity is controlled by macerals, establishing a macerals-functional groups-adsorption cascade as the fundamental mechanism for differential occurrence.

Conclusions The organic occurrence of AAEMs (Na, Ca) has clear geological significance, thus providing an essential coal petrological basis for predicting vertical enrichment layers in coal seams. Besides, AAEMs may adversely affect the coalbed methane adsorption by occupying adsorption sites. Future research should prioritize elucidating molecular inhibition mechanisms of AAEMs in CBM competitive adsorption, providing theoretical foundations for the clean utilization of high-alkali coals.