Redox-mediated Zn electrode for hydrogen supplementation to enhance microbial electrolysis reduces toxicity and improved nitrate removal.

Microbial electrolysis cells (MECs) offer a promising approach for nitrate-contaminated sewage wastewater treatment oxidative stress-induced damage to electroactive biofilms, and inadequate hydrogen availability for complete denitrification. This study presents a novel redox-mediated Zn electrode designed to Zn²⁺/Zn⁰ redox couple selectively scavenges highly reactive oxidative species, including hydroxyl radicals (•OH) and peroxynitrite (ONOO⁻), thereby mitigating oxidative stress and protecting electroactive microorganisms and redox-active enzymes. The Zn-redox electrode demonstrated remarkable antioxidative capacity, reducing •OH and ONOO⁻ levels by 88.3 ± 2.8% and 89.1 ± 3.2%, respectively. This radical scavenging activity translated into significant improvements in cellular health and metabolic function. Geobacter sulfurreducens and mixed denitrifying consortia exhibited 12.51 ± 0.53 higher survival rates after 48-hour exposure, accompanied by a 78% increase in succinate levels (72.4 mM/mg protein), an elevated NADH/NAD⁺ ratio (4.98 ± 0.43), and a tripling of ATP content (72.3 ± 2.23 nmol/mg protein). Nitrate removal efficiency reached Hydrogen96% within 12 h using the Zn-redox cathode, compared to only 58% for conventional carbon felt controls. Current density peaked at 15.1 A/m² (control 3.7 A/m²), while maximum power density reached 3.9 W/m² (control 0.6 W/m²). Biomass density on the electrode surface increased threefold, from 0.18 to 0.78 mg protein/cm². Notably, nitrate removal was 98 ± 1, nitrite accumulation was reduced to <2 mg/L (control: 28 mg/L), and N₂O emissions decreased by 90%, addressing two major concerns in biological denitrification. The Zn-redox electrode selectively enriched Proteobacteria and Actinobacteriota, while promoting dominant electroactive and denitrifying genera including Geobacter, Pseudomonas, and Trichococcus. Concurrently, the system promoted efficient denitrification through enhanced extracellular electron transfer and hydrogen-assisted nitrate reduction, enabling rapid nitrate-to-N₂ conversion with minimal nitrite accumulation and markedly reduced N₂O emissions. The Zn-redox electrode maintained stable performance over 10 operational cycles with only 20% mass loss, demonstrating practical applicability.