Organically-bound forms of alkali and alkaline earth metals in high-alkali coals and their geological indications

Authors

WANG Xiaoling, School of Geology and Mining Engineering, Xinjiang University, Urumqi 830017, ChinaFollow
WANG Shaoqing, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, ChinaFollow
ZHAO Yungang, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China; CCTEG China Coal Research Institute, Beijing 100013；China
MU Ruifeng, School of Civil Engineering and Transportation, Beihua University, Jilin 132013, China
SHAO Yan, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China

Abstract

Objective and Method Alkali and alkaline earth metals (AAEMs) in coals adversely affect coal processing and utilization. Among the AAEMs, water-soluble and mineral-hosted metals can be largely removed through water washing, flotation, and heavy liquid separation, while organically-bound AAEMs are difficult to eliminate. In this study, high-alkali coal samples with varying vitrinite/inertinite ratios (V/Is) collected from the Zhundong coalfield in Xinjiang were selected to investigate the organically-bound forms of AAEMs. Through sequential chemical extraction, combined with experimental techniques such as ²³Na cross-polarization magic-angle spinning nuclear magnetic resonance (²³Na CP/MAS NMR) and transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS), this study systematically analyzed the contents, distribution patterns, and occurrence forms of AAEMs in coals.Results AAEMs in coals in the Zhundong coalfield occur primarily in water-soluble and ion-exchangeable forms. Among them, Na⁺ and Ca²⁺ are more prone to undergo interactions with coal molecules via ion exchange due to their significantly higher concentrations than other AAEM ions. Both AAEM forms bind with oxygen-containing functional groups (e.g., carbonyl, carboxyl, and phenolic hydroxyl) in coal organic matter primarily through adsorption. Quantum chemical calculations indicate that Na⁺ forms sodium carboxylate or coordination structures (−C=ONa⁺) predominantly by binding with carboxylate ions or carbonyl groups. In contrast, Ca²⁺ binds with organic molecules largely through chemical adsorption, demonstrating strong ionic bonds. With an increase in the vitrinite content, the content of ion-exchangeable Na increased from 28.3% to 30.9%, while that of ion-exchangeable Ca rose from 62.3% to 70.1%, indicating an enhanced affinity of vitrinite for AAEMs. AAEM ions migrate with groundwater flow and are then adsorbed near oxygen-containing functional groups, with the adsorption capacity controlled by coal macerals. Such cascade control by macerals, functional groups, and adsorption in sequence serves as the underlying reason for the differential enrichment of AAEMs.Conclusions The occurrence patterns of organically-bound AAEMs such as Na and Ca hold defined geological implications, providing an essential coal petrological basis for predicting the vertical AAEM enrichment horizons in coal seams. Besides, AAEM ions pose adverse impacts on coalbed methane (CBM) adsorption by occupying adsorption sites on oxygen-containing functional groups. Future research should focus on the molecular mechanisms by which AAEM ions restrict the competitive adsorption of CBM, providing theoretical foundations for the rational, clean utilization of high-alkali coals.

Keywords

high-alkali coal, alkali and alkaline earth metals (AAEMs), water-soluble form, organically-bound form, geological implication

DOI

10.12363/issn.1001-1986.25.05.0355

Recommended Citation

WANG Xiaoling, WANG Shaoqing, ZHAO Yungang, et al. (2025) "Organically-bound forms of alkali and alkaline earth metals in high-alkali coals and their geological indications," Coal Geology & Exploration: Vol. 53: Iss. 12, Article 16.
DOI: 10.12363/issn.1001-1986.25.05.0355
Available at: https://cge.researchcommons.org/journal/vol53/iss12/16

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