From Fossil to Green: A Low-Carbon Electrochemical Pathway for Lignite Depolymerization

Abstract

The high-value utilization of lignite is pivotal for the clean transformation of the coal industry. However, traditional methods for preparing humic acid struggle to simultaneously achieve high efficiency, environmental friendliness, and economic viability. This study proposes a mild, clean electrochemical depolymerization strategy to efficiently convert Zhaotong lignite slurry into humic acid under alkaline constant-current conditions. Comprehensive multiscale analyses elucidated the structural and chemical evolution of residual coal throughout the electrolysis process. Based on these findings, the reaction mechanism for humic acid generation driven by electrochemical processes was inferred. Results indicate that within 20 min of alkaline constant-current electrolysis, the dry-basis humic acid yield from Zhaotong lignite reached 57.04%. The residual coal’s oxygen content significantly increased from 23.60% to 35.27%. Aliphatic components fragmented into shorter chains, while the relative distribution of oxygen-containing functional groups underwent dynamic changes. The dominance of C–O bonds strengthened and established a central role, whereas the dominance of COO– bonds weakened. The aromaticity of solid residual coal decreased significantly from 48.28% to 24.33%. This efficient conversion stems from the selective cleavage of Cal-O type bridging bonds and aliphatic side chains via electrochemical means. This process promotes the dissolution and oxidation of low-condensation-degree aromatic clusters into water-soluble humic acids, while higher-condensation-degree bicyclic and polycyclic aromatic clusters remain retained in the residual coal. This work systematically reveals the depolymerization mechanism of lignite macromolecules into humic acids via weak bond cleavage under electric field drive, providing theoretical support for the electro-synthesis of high-value coal-based chemicals.