Photo-responsive halogen-doped carbon quantum dots enhance anaerobic co-digestion: Focusing on biogas yield and electron transfer

Abstract

Carbon quantum dots (CQDs) have been demonstrated to be effective accelerators in anaerobic co-digestion (AcoD) systems for enhancing digestion efficiency. Nevertheless, CQDs have greater potential when exposed to light illumination, and the synergistic interaction between CQDs and light in AcoD systems remains unexplored. In this study, different concentrations of halogen-doped CQDs (F/Cl/Br/I-CQDs) were added as accelerators to AcoD systems equipped with stationary illumination (2000–8000 Lux) to investigate the enhancing effects of halogen-doped CQDs on AcoD systems under light conditions. Four halogen-doped CQDs had different promotion effects on AcoD under 6000 Lux illumination, and F-CQDs showed the best promotion effect. The addition of F-CQDs (0.6 g/L) under 6000 Lux illumination greatly increased the biogas yield (541.2 mL/g VS), cumulative CH₄ yield (374.4 mL/g VS), and the degradation rate of total chemical oxygen demand (TCOD) (64.6 %), compared to those of the control group (367.9 mL/g VS, 168.5 mL/g VS, 36.7 %). These excellent enhancements are attributed to the good electron transfer properties of the halogen-doped CQDs and synergistic effect of light illumination, which improves direct interspecies electron transfer (DIET) among microorganisms. Halogen-doped CQDs and light can facilitate the enrichment of key anaerobic microbial communities. This study proposes a novel strategy that integrates halogen-doped CQDs and light irradiation to enhance biodegradation by enhancing the DIET, offering a sustainable approach for optimizing waste-to-energy conversion in AcoD systems with improved biogas productivity.