Electropositive interface engineering accelerates start-up and enhances nitrate removal in microbial electrochemical...

Under low C/N conditions, microbial electrochemical denitrification systems (MEDS) are attractive because they do not require external organic carbon inputs. However, practical deployment is often limited by slow start-up and modest nitrate removal rates, largely due to insufficient enrichment of electroautotrophic denitrifiers on cathodes and inefficient electron transfer within biofilms. Here, we present a facile, effective, and scalable electrode/interface engineering strategy for denitrifying biocathodes. Three representative cationic functional materials were comparatively employed to tailor the cathode surface, thereby strengthening interfacial properties and microbe/electrode interactions. Relative to unmodified controls, all modified reactors achieved higher nitrate removal rates in the initial operational stage and shortened the time required to reach stable nitrate removal. Mechanistically, the cationic surface treatment simultaneously increased surface electropositivity and hydrophilicity, promoting the adhesion of negatively charged electroactive cells and likely increasing nitrate accessibility in the immediate cathode/biofilm interfacial region. Electrochemical analyses showed reduced charge-transfer resistance and a greater contribution of surface-confined, direct electron transfer dominated electron-transfer pathways. Consistent with these findings, microbial community profiling and functional-gene prediction revealed higher relative abundances of key denitrification genes and electron-transfer markers, collectively improving biofilm electron uptake and nitrate reduction capacity. Among the three modifiers, chitosan modified cathode system exhibited the best overall denitrification performance, which was associated with a more favorable balance among surface electropositivity, hydrophilicity, biofilm formation, and microbial functional enrichment. Overall, this work establishes a cathode-oriented and scalable interface-engineering route to accelerate start-up and improve MEDS performance under carbon-limited conditions, providing design guidance for electroautotrophic denitrifying biocathodes in engineered water treatment systems.