Hydrogel-enhanced bioelectrochemical nitrate reduction for ammonium recovery from dilute nitrate via S <i>hewanella oneidensis</i> MR-1

Abstract Bioelectrochemical reduction of dilute nitrate (NO₃⁻; sub-mM to low-mM range) to ammonium (NH₄⁺) offers a promising route toward circular nitrogen management from contaminated groundwater and environmental waters. However, on-site application of bioelectrochemical systems remains challenging due to low reduction rates and poor electron transfer efficiency of naturally formed biofilm electrodes. Here, we constructed a hydrogel biocathode by applying a carbon black/riboflavin/sodium alginate/cellulose hydrogel incorporating Shewanella oneidensis MR-1 cells to a graphite felt electrode via brush coating. The hydrogel electrode achieved NH₄⁺ production rates of 0.16–0.19 mol m⁻³ h⁻¹ without NO₂⁻ accumulation, and these rates were maintained without significant performance loss across three consecutive cycles with medium exchange over 1.5 days of total operation. The hydrogel electrode increased the current density by more than 5-fold compared with a conventional S. oneidensis biofilm electrode, indicating enhanced electron transfer efficiency per unit biomass, which directly contributed to the high NH₄⁺ production rates. The electricity consumption for NH₄⁺ production of 1.68–2.39 × 10² kJ g-N⁻¹ was substantially lower than that of metal catalyst systems at comparable NO₃⁻ concentrations (typically, >10 4 kJ g-N⁻¹). These findings demonstrate that the hydrogel electrode design represents an energy-efficient, and readily fabricated platform for bioelectrochemical NH₄⁺ production from dilute NO₃⁻.