Unlocking anammox potential: functional group-optimized carbon quantum dots enhance nitrogen removal via...

Carbon quantum dots (CQDs) have attracted growing interest for enhancing anaerobic ammonium oxidation (anammox) due to their tunable surface chemistry and excellent electron transfer properties. In this study, a series of CQDs with different surface functional groups were prepared by adjusting microwave power and combining l-lysine modification. l-lysine modification introduced nitrogen-containing functional groups into CQDs and enhanced their surface polarity, structural ordering and electrochemical activity, which contributed to improved electron transfer characteristics. The reactor supplemented with l-lysine modified CQDs synthesized at 500 W (LCQD500) exhibited the best nitrogen removal performance. During the three loading stages, the average NH₄⁺-N removal efficiencies were as high as 100%, 96% and 97%, respectively. Particularly under high-load conditions (NH₄⁺-N and NO₂^--N: 100 mg/L), the removal efficiency was enhanced by 9% compared to the control group, demonstrating the strongest resistance to loading shock. From a microbiological perspective, LCQD500 optimized the structure of functional microbial communities. It increased the abundance of phylum Planctomycetota to 16.59% and raised the abundance of the core anammox genus Candidatus Brocadia to 12.69%. Meanwhile, compared with unmodified CQDs synthesized at 500 W without l-lysine modification (CQD500), this material increased the abundances of key anammox-related genes (hzsA, hzsB, hzsC), with increases of 28.73%, 24.21%, and 25.62%, respectively. It was the synergistic enhancement from microbiota to the key genes that ultimately established a multi-pathway, highly efficient nitrogen removal network centered on anammox. This study reveals the enhancing effect of surface functional groups of CQDs on anammox performance and the associated microbiological and genetic mechanisms.