Carbon nanotubes-mediated alleviation of diffusion limitation and syntrophic metabolic recoupling in anaerobic granular sludge under salinity stress.

Functional destabilization of anaerobic granular sludge (AnGS) under salinity stress remains a major bottleneck for anaerobic biotechnology, yet the link between diffusion limitation and syntrophic metabolic imbalance is poorly understood. By using a Spiral Symmetry Stream Anaerobic Bioreactor (SSSAB), this study elucidated salinity-induced diffusion limitation and carbon nanotubes (CNTs)-mediated syntrophic recoupling in AnGS. Under high salinity (>10 g/L), EPS composition and interfacial properties deteriorated, internal water fractions shifted, and diffusion resistance increased. Consequently, AnGS prematurely entered a diffusion-limited state, potentially restricting internal mass transfer and disrupting metabolic coordination between fermentative bacteria and methanogenic archaea, thereby contributing to a methanogenesis-limited state. Methanogenic archaeal abundance decreased from 10⁸ to 10⁴ copies/L, accompanied by alkalinity depletion and severe acidification. CNTs alleviated diffusion limitation and restored syntrophic metabolic coupling by simultaneously improving metabolite transport and electron exchange between syntrophic partners. The apparent drying rate constant increased from 0.071 to 0.137 min-1, while charge transfer resistance decreased from 3037 to 1299 Ω. Enhanced electron transfer and metabolite transport restored methane metabolism, particularly hydrogenotrophic methanogenesis, ultimately driving the system toward recoupled syntrophic metabolism. These findings link salinity-induced AnGS destabilization to internal diffusion limitation and suggest that CNTs alleviate this limitation by restoring mass transfer-electron transfer coupling and promoting syntrophic metabolic recoupling.