Dual roles of multi-walled carbon nanotubes in anammox systems: Conductive enhancement or toxic burden?

Integrating multi-walled carbon nanotubes (MWCNTs) into anaerobic ammonium oxidation (anammox) systems offers a promising nitrogen removal strategy, yet the balance between conductive stimulation and nanotoxicity remains poorly understood. This study elucidated the dose-dependent mechanisms of MWCNTs on anammox consortia. A distinct biphasic response was observed: low doses (optimal 30 mg·L-1) enhanced specific anammox activity by 18.82 ± 1.73% (p < 0.001), whereas high doses (≥ 100 mg·L-1) caused severe inhibition. Mechanistically, 30 mg·L-1 MWCNTs stimulated EPS to physically intercept nanoparticles while triggering an adaptive antioxidant response. Metatranscriptomic revealed that MWCNTs acted as "conductive bridges" to reprogram microbial electron flow, thereby selectively favoring AnAOB, promoting endogenous electron production and transfer, and ultimately enhancing the turnover of key terminal enzymes (hzs, hdh). Conversely, excessive exposure overwhelmed EPS and antioxidant defenses, causing reactive oxygen species accumulation and cellular disruption. Long-term operation confirmed sustained performance, expanding the AnAOB ecological niche (21.72 ± 9.62% increase) while suppressing competing denitrifiers. These findings provide a comprehensive molecular framework for precisely regulating nanomaterial dosing to optimize advanced wastewater treatment while mitigating ecological risks.