Molecular transformation of dissolved organic matter during landfill leachate treatment by microbial electrolysis cell-anaerobic digestion system.

The complex dissolved organic matter (DOM) in landfill leachate posed significant challenges for conventional biological treatment. In this study, a microbial electrolysis cell-anaerobic digestion (MEC-AD) system was constructed to explore DOM transformation pathways. An orthogonal experimental design was employed to evaluate the effects of applied voltage, anode-to-cathode area ratio, and stainless-steel cathode mesh size. Under an applied voltage of 1.2 V, an anode-to-cathode area ratio of 1:1, and a cathode mesh size of 200 mesh, the highest overall COD removal efficiency of 60.2% was observed. Spectroscopic analysis showed a reduction in biodegradable soluble microbial byproducts during treatment. GC-MS showed changes in the profiles of representative aliphatic and aromatic compounds. FT-ICR MS analysis revealed compositional reorganization of DOM, characterized by the disappearance of a fraction of protein- and lipid-like DOM and the persistence or relative enrichment of more unsaturated and aromatic constituents. Microbial community analysis revealed the relative enrichment of hydrolytic and fermentative bacteria (e.g., norank_f__Anaerolineaceae) as well as bacteria potentially involved in electron transfer (e.g., Thiobacillus). Metagenomic analysis suggested functional potential for amino acid metabolism, fatty acid degradation, glycolysis, and pyruvate metabolism. In contrast, the relatively low abundance of aromatic compound degradation functions was consistent with the persistence of refractory aromatic DOM. This study revealed that MEC-AD treatment was associated with differential transformation and molecular reorganization of landfill leachate DOM, and provided molecular-level insights beyond conventional bulk organic matter removal.