Unveiling the Role of N-Doped Carbon Quantum Dots in Relieving Humic Acid Inhibition and Promoting Methanogenesis via Enhanced Electron Transfer

Humic acid (HA) is an inhibitor that can diminish bioconversion during anaerobic digestion (AD). In this study, a non-metallic nanomaterial, N-doped carbon quantum dots (NCQD), was synthesized to alleviate the HA inhibition, followed by exploring its mechanism. Adding 500   mg/L NCQD prevented HA inhibition by binding to HA or microorganism surfaces, thus avoiding contact between them. This phenomenon, known as shielding inhibition, involves the formation of hydrogen bonds and chemical bonds. The intermolecular force and dissociation constant ( K D ) between NCQD and HA were determined to be 112.83 ± 3.98 nN and 1.0 ± 0.07 × 10 -7   M, respectively. NCQD could promote electron transfer in AD systems. According to the in-depth microbial analysis, NCQD significantly enhanced the metabolic pathways of methanogenesis and biosynthesis of coenzyme F 420 . This groundbreaking research demonstrates the dual functionality of NCQD by effectively shielding against HA inhibition and promoting electron transfer. The study also unravels the mechanism of interface interaction, electron transfer, and metabolic pathways, leading to significant breakthroughs in addressing HA inhibition.